Dissertations / Theses on the topic 'Solar fuel production'

Uno de los retos actuales de la ciencia y la tecnología es el desarrollo y utilización de fuentes de energías limpias, sostenibles y seguras, con el fin de sustituir el uso de combustibles fósiles. La energía solar, única alternativa viable, puede convertirse y almacenarse en forma de enlaces moleculares, mimetizando el proceso de fotosíntesis de las plantas, para la obtención de combustibles u otros productos de valor añadido. Para ello se requieren materiales semiconductores que puedan absorber y transformar la energía solar en energía química de manera eficiente. En la presente tesis doctoral se abordó el estudio de materiales semiconductores empleados en la obtención fotoelectrocatalítica de combustibles solares. Dicha investigación se realizó desde diferentes enfoques, que inlcuyen: la modificación de fotoelectrodos con recubrimientos catalíticos obtenidos a partir de marcos metal-orgánicos; la implementación de un nuevo método para la comprensión de los mecanismos de operación de fotoelectrodos; la integración de dispositivos electrocatalíticos y fotovoltaicos; y la evaluación y establecimiento de nuevos sistemas con potencial aplicación en procesos foto-electrocatalíticos.
The development and use of clean, sustainable and safe energy sources, in order to substitute the use of fossil fuels, is a current challenge of science and technology. Solar energy, the only viable alternative, can be converted and stored in the form of molecular bonds, mimicking the photosynthesis process in green plants, to obtain fuels or other added-value products. This process requires semiconductor materials that can efficiently harvest and transform solar into chemical energy. In the present doctoral thesis, the study of semiconductor materials for photo-electrocatalytic applications was addressed from different approaches. That includes: the modification of photoelectrodes with catalytic coatings, obtained from a metal-organic framework; the implementation of a new method for the understanding of the photoelectrodes operating mechanisms; the integration of electrocatalytic and photovoltaic devices from Earth-abundant materials; and, finally, the investigation of new systems with potential application in photo-electrocatalytic processes. (Signatura
Programa de Doctorat en Ciències

Increasing energy consumption and its environmental impacts make it necessary to look for alternative energy sources. Solar energy as huge energy source which is able to cover the terms sustainability is considered as a favorable alternative. Solar cells and solar fuels are two kinds of technologies, which make us able to harness solar energy and convert it to electricity and/or store it chemically. Metal oxide semiconductors (MOSs) have a major role in these devices and optimization of their properties (composition, morphology, dimensions, crystal structure) makes it possible to increase the performance of the devices. The light absorption, charge carriers mobility, the time scale between charge injection, regeneration and recombination processes are some of the properties critical to exploitation of MOSs in solar cells and solar fuel technology. In this thesis, we explore two different systems. The first one is a NiO mesoporous semiconductor photocathode sensitized with a biomimetic Fe-Fe catalyst and a coumarin C343 dye, which was tested in a solar fuel device to produce hydrogen. This system is the first solar fuel device based on a biomimetic Fe-Fe catalyst and it shows a Faradic efficiency of 50% in hydrogen production. Cobalt catalysts have higher Faradic efficiency but their performance due to hydrolysis in low pH condition is limited. The second one is a photoanode based on the nanostructured hematite/magnetite film, which was tested in a photoelectrochemical cell. This hybrid electrode improved the photoactivity of the photoelectrochemical cell for water splitting. The main mechanism for the improvement of the functional properties relies with the role of the magnetite phase, which improves the charge carrier mobility of the composite system, compared to pure hematite, which acts as good light absorber semiconductor. By optimizing the charge separation and mobility of charge carriers of MOSs, they can be a promising active material in solar cells and solar fuel devices due to their abundance, stability, non-toxicity, and low-cost. The future work will be focused on the use of nanostructured MOSs in all-oxide solar cell devices. We have already obtained some preliminary results on 1-dimensional heterojunctions, which we report in Chapter 3.3. While they are not conclusive, they give an idea about the future direction of the present research.

With the issue of the rise of anthropogenic CO2, global warming and rise of the primary energy demand, strong measures for the energy transition and the diversification with renewables and existing fossil-based infrastructure are required. Also, carbon capture and utilization of CO2 would also be needed. In that sense, thermochemical redox cycles gain particular interest to produce synthetic fuels, which can be used for energy generation and production of chemicals. In a two-step redox cycles, metal oxides acts as oxygen carriers and undergo looping between two reactors. In the reduction reactor, metal oxide is reduced with release of oxygen (solar-thermal) or produces syngas (for fuel reduction) whereas, in oxidation, CO2/H2O splits for form syngas when in contact with the metal oxide. Ceria being readily available at large scale and due to its nature of undergoing reduction non-stoichiometrically at low temperature makes it a good candidate. In the present thesis, a detailed investigation of thermochemical dissociation of CO2 and H2O considering solar thermal and fuel reduction with a focus on non-structured reactors is carried out. For the solar-driven cycle, an assessment of counter-current flow moving bed reactors for reduction and oxidation is performed and a chemical looping (CL) unit is added to a 100 MW power plant. With an operating temperature of 1600oC and 10-7 bar pressure, a maximum power output of 12.9 MW with solar to electricity efficiency of 25.4% is calculated. This additional power would bring down the efficiency loss due to carbon capture from 11.3 to 6%. Even though a considerable efficiency is obtained on very optimistic operating conditions, it still requires a huge solar field. Economics revealed that with a carbon tax of $40/tone of CO2 the levelized cost of electricity (LCOE) achieved is 17.8 times higher than the existing market price (without carbon capture). If a higher carbon tax of 80$/MWh is considered that it would still be 6.28 times higher for a plant with a carbon tax. As an alternative, methane-driven CL unit is integrated into a power plant to access the overall system efficiency and amount of efficiency regain after carbon capture. Since there exists no solid-state kinetic model in the literature for methane driven CO2/H2O splitting cycle, an experimental investigation was performed which revealed that an Avrami-Erofe’ev (AE3) model fit best to both oxidation and reduction, with activation energies of 283 kJ/mol and 59.7 kJ/mol, respectively. A comparative assessment was performed to investigate the influence of kinetics. A CL unit based on thermodynamics and kinetics (with moving bed reactors) were tested in a power plant. A drop of 20% in the efficiency of the CL unit was observed when the kinetic-based CL unit is considered. However, due to thermal balance within the system, a similar thermal efficiency of the overall plant was achieved as 50.9%. However, when the thermodynamic-based CL unit layout is considered there exists an excess heat which predicts the possibility of improving the efficiency. An economic assessment revealed a specific overnight capital cost of 2455$/kW, a levelized cost of CO2 savings of 96.25 $/tonneCO2, and a LCOE of 128.01 $/MWh. However, with a carbon tax of 6 $/tonneCO2, the LCOE would drop below 50 $/MWh. The methane-driven CL unit is later integrated as an add-on unit to a polygeneration plant that produces electricity and dimethyl ether. The results showed that the plant can produce 103 MWe and 2.15 kg/s of DME with energy and exergy efficiency of 50% and 44%, respectively. The capital investment required for the plantis about $534 million. With the carbon tax of $40/tonne of CO2, a current DME price of $18/GJ and an electricity price of $50/MWh would be achieved. Overall, the integration of the CL unit as an add-on unit to the power plant is more suitable than polygeneration with respect to the existing market price.
El aumento del CO2 antropogénico y el calentamiento global y el aumento de la demanda de energía primaria hace que se requieran medidas para la transición energética y la diversificación con energías renovables e infraestructuras existentes basadas en combustibles fósiles. Además de implementar medidas para la captura y el secuestro de carbono, también se necesita desarrollar métodos para la utilización de CO2. En ese sentido, los ciclos redox termoquímicos son particularmente interesantes para producir combustible sintético que, a su vez, pueden utilizarse para la producción de otras substancias químicas. La rotura de CO2 / H2O (CL) mediante una vía termoquímica de dos pasos está compuesta por dos reacciones redox con un óxido metálico. El primer paso es la reducción de los óxidos metálicos al perder oxígeno y crear vacantes en la red a una temperatura más alta y convertirse en óxido de metal de valencia más baja. Durante la etapa de oxidación, los gases reactivos CO2 / H2O reaccionan con el óxido metálico reducido formando CO y H2. Se ha investigado el uso de diferentes óxidos metálicos en función de su capacidad de transporte de oxígeno y sus propiedades para realizar ciclos redox continuos a distintos valores de temperatura y presión. Después de un examen cuidadoso, se ha seleccionado a la ceria para la división de CO2 / H2O a gran escala. En el presente trabajo, se investigan las divisiones termoquímicas de CO2 / H2O impulsadas por energía solar y la reducción de metano para la producción de gas de síntesis, con especial atención a su aplicación en reactores no estructurados. Se evalúa el uso de reactores de lecho móvil basado en flujo contracorriente y reactores de lecho fluidizado que funcionan en diferentes regímenes de fluidización. Es un reactor de lecho móvil tanto para la etapa de reducción como para la etapa de oxidación se obtienen altas selectividades de CO y H2 con volúmenes óptimos del reactor, mientras que en un reactor de lecho fluidizado el volumen requerido es mucho más alto, lo que lo hace inviable. Los modelos de reactor se han desarrollado en Aspen plus y se validan a partir de la literatura. Un análisis de sensibilidad ha revelado que la unidad CL depende en gran medida de la temperatura y la presión. El análisis se ha ampliado integrando la unidad desarrollada de CL como una unidad adicional a una central eléctrica de 100 MW con captura de carbono. La eficiencia de la planta se ha investigado considerando sólo la división de CO2, sólo la del H2O y la mezcla de CO2 y H2O como alimentación al reactor de oxidación de la unidad CL. El resultado es de una potencia máxima de 12.9 MW con una eficiencia de energía solar a eléctrica de 25.4%. Esta potencia adicional reduciría la pérdida de eficiencia debido a la captura de carbono de 11.3 a 6%. Para lograr esto, el reactor de reducción de la unidad CL debe funcionar a 1600 ° C y 10-7 bar de presión. Estas condiciones necesitarían un enorme campo solar y la operación, en ausencia de almacenamiento térmico, se limitaría a unas pocas horas durante el día. El análisis técnico-económico ha revelado que el coste nivelado de la electricidad es de 1321 $/MWh sin incluir incentivos ni impuestos sobre el carbono. Posteriormente, se ha considerado la reducción del metano como una alternativa a la reducción térmica. Al principio, se realizaron análisis termodinámicos de la unidad de CL impulsada por metano. A partir del análisis, se ha demostrado que la temperatura mínima requerida es de 900°C con 50% de exceso de metano para la reducción, lo que supone una eficiencia de la unidad CL de 62% con un rendimiento óptimo de CO y H2. La división de CO2/H2O en el reactor de oxidación a una mayor temperatura de salida beneficiaría considerablemente la eficiencia energética del ciclo redox CL completo. La variación de la relación H2/CO en la salida con respecto a los parámetros de entrada variables que incluyen la composición del gas al reactor de oxidación se ha estudiado con el fin de especificar las condiciones operativas idóneas. Posteriormente, la unidad CL impulsada por metano se ha integrado como una unidad adicional a una central eléctrica de 500 MW alimentada por oxígeno. Se ha investigado el rendimiento de un sistema con un ciclo combinado de gas natural convencional con o sin captura de carbono. Se ha obtenido una eficiencia de sistema y eficiencia energética de 50.7 y 47.4%, respectivamente. La eficiencia del sistema podría mejorarse a 61.5%, sujeto a la optimización del sistema. La evaluación tecno-económica ha revelado un coste de capital durante la noche de 2455 $/kW con un coste de ahorro de CO2 de 96.25 $/tonelada CO2 y un LCOE de 128.01 $/MWh. Sin embargo, con créditos de carbono de 6 $/tonelada CO2, el LCOE caería por debajo de 50 $/MWh.
Con l'aumento delle emissioni di CO2 antropogenica che contribuiscono al riscaldamento globale e l'incremento della domanda mondiale di energia primaria, sono richieste significative misure per favorire la diversificazione delle fonti e la transizione energetica tramite fonti rinnovabili a partire dalle infrastrutture esistenti basate su combustibili fossili. Prima ancora degli interventi per la cattura e il sequestro dell’anidride carbonica, anche l’utilizzo della CO2 rappresenta una misura necessaria al raggiungimento degli obiettivi di decarbonizzazione. In questo senso, i cicli redox termochimici hanno acquisito particolare interesse per la produzione di combustibile sintetico da utilizzare come intermedio nella produzione di altri prodotti chimici. La separazione chimica di CO2/H2O attraverso un ciclo termochimico – chemical looping splitting (CL) – in due fasi è composta da due reazioni redox con un ossido di metallo. La prima fase del ciclo avviene alla temperatura più elevata e consiste nella riduzione dell’ossido di metallo, che cede ossigeno creando vacanze nel reticolo e diventando ossido di metallo a bassa valenza. Durante la fase di ossidazione, i gas reagenti CO2/H2O reagiscono con l'ossido di metallo ridotto che forma CO e H2. Una mappatura dettagliata dei diversi ossidi di metallo è stata effettuata in base alla loro capacità di trasporto dell’ossigeno e alle proprietà nei cicli di ossido-riduzione a funzionamento continuo in condizioni di variazione di temperatura e pressione. Dopo un attento esame, l’ossido di Cerio - ceria - è stato selezionato per l'applicazione che può essere disponibile per la scissione CO2 / H2O su larga scala. In questo lavoro, sia la separazione termochimica di CO2/H2O alimentata tramite energia solare, sia i cicli con riduzione tramite metano, entrambi finalizzati all produzione di syngas sono stati studiati con particolare attenzione ai reattori non strutturati. Per il ciclo termochimico basato su energia solare, è stata effettuata la valutazione dei reattori a letto mobile a flusso in controcorrente e a letto fluido che operano in diversi regimi di fluidizzazione. Il reattore a letto mobile è stato individuato come il più performante sia per la riduzione che l’ossidazione, con elevate selettività verso CO e H2 e volumi ottimali del reattore, mentre una resa analoga con reattori a letto fluidizzato potrebbe essere ottenuta solo con volumi di reattore molto alti, rendendo questa scelta irrealizzabile nella pratica. I modelli di reattore sono stati sviluppati in Aspen plus e sono stati validati dalla letteratura. Un'analisi di sensitività ha rivelato che la performance dell'unità CL è in larga misura dipendente dalla temperatura e dalla pressione di riduzione. L'analisi è stata estesa integrando l'unità CL sviluppata come unità aggiuntiva di una centrale elettrica a ossicombustione da 100 MW con cattura di carbonio. L'efficienza dell'impianto è stata studiata considerando di alimentare il reattore di ossidazione dell'unità CL sia con CO2, sia con H2O, sia con una miscela di CO2 e H2O. I risultati indicano una potenza massima di 12,9 MW con un rendimento da solare a elettricità del 25,4% generabile grazie all’unità di CL. Questa potenza aggiuntiva ridurrebbe la perdita di efficienza dovuta alla cattura di carbonio dall'11,3 al 6%. Per ottenere ciò, il reattore di riduzione dell'unità CL deve operare a 1600 ° C con una pressione di 10-7 bar. Queste condizioni avrebbero bisogno di un enorme campo solare e l'operazione sarebbe limitata a poche ore durante il giorno senza l’integrazione di un accumulo termico. L'analisi tecno-economica ha rivelato che il costo livellato (levelizad cost) dell'elettricità era di 1321 $ / MWh, senza includere incentivi o tassazione sul carbonio. Successivamente, è stata considerata la riduzione della ceria con metano come alternativa alla riduzione termica. Inizialmente, sono state condotte analisi termodinamiche dell'unità CL con riduzione a metano. Dall'analisi è emerso che la temperatura minima richiesta era 900 °C per la riduzione con un eccesso di metano del 50%, che ha prodotto un'efficienza dell'unità CL del 62% con una resa ottimale di CO e H2. In questo caso, la scissione di CO2/H2O nel reattore di ossidazione consisteva nell'ossidazione completa esotermica della ceria, per cui una temperatura di uscita più elevata avrebbe notevolmente migliorato l'efficienza energetica del ciclo CL redox completo. La variazione del rapporto H2 / CO all'uscita rispetto ai vari parametri di input, compresa la composizione del gas inviato al reattore di ossidazione, è stata studiata per specificare le condizioni operative necessarie. Successivamente, l'unità CL a metano è stata integrata come unità aggiuntiva in una centrale elettrica a ossicombustione da 500 MW. Sono state studiate le prestazioni del sistema in una valutazione comparativa con un ciclo combinato convenzionale a gas naturale, un ciclo a ossicombustione con cattura di carbonio e l'impianto proposto. Sono stati ottenuti per l’impianto rispettivamente un rendimento del sistema e un'efficienza energetica del 50,7% e del 47,4%. L'efficienza del sistema potrebbe essere migliorata fino al 61,5% tramite l'ottimizzazione del recupero termico del sistema, valutata attraverso la pinch analysis del sistema. Una dettagliata valutazione tecno-economica ha rivelato un costo specifico del capitale di 2455 $ / kW (overnight cost), un costo livellato delle emissioni di CO2 evitate 96,25 $ / tonnellata di CO2, e un costo dell’elettricità (LCOE) di 128,01 $ / MWh. Tuttavia, considerando un incentivo di 6 $ / tonnellata di CO2 evitata, il LCOE scenderebbe sotto i 50 $ / MWh. L'unità CL a metano viene successivamente integrata come unità aggiuntiva in un impianto di poligenerazione che produce elettricità e dimetil-etere. I risultati hanno mostrato che l'impianto può produrre 103 MWe e 2,15 kg/s di DME con un’efficienza energetica ed exergetica del 50% e del 44% rispettivamente. L'investimento di capitale richiesto per l'impianto ammonta a 534 M$. Con un valoré per la carbon tax di $ 40 / tonnellata di CO2, il DME e l’elettricità raggiungerebbero la parità con gli attuali prezzi di mercato, pari a $18/GJ per il DME e $50/MWh per l’elettricità. I costi risultanti sono dovuti all'unità di separazione dell'aria richiesta per la centrale elettrica a ossicombustione e può essere ridotta sostituendo l'unità di separazione dell'aria con una tecnologia a membrana per la separazione dell'ossigeno. Poiché in letteratura non esiste un modello completo per cinetica dello stato solido che descriva la riduzione con metano della ceria, esso è stato ricavato per via sperimentale. Sono stati condotti esperimenti in un reattore tubolare orizzontale a letto fisso in un intervallo di temperatura di 900-1100 °C. E’ stata studiata la cinetica della scissione della CO2, essendo una reazione più complessa rispetto alla scissione dell'acqua, la cui cinetica è stata invece ottenuta dalla letteratura. In base all’analisi sperimentale condotta, il modello cinetico Avrami-Erofe'ev (AE3) è risultato essere il migliore per entrambe le reazioni, con le rispettive energie di attivazione ottenute rispettivamente come 283 kJ/mol e 59,68 kJ/mol. L'ordine della reazione è stato ricavato come relazione tra temperatura e concertazione dei reagenti. L'analisi è stata effettuata seguendo un approccio termodinamico, ma la reazione eterogenea dell'ossido di metallo e dei gas reagenti limita il raggiungimento dell'equilibrio durante la reazione e dipende sempre dal tipo di reattore scelto per x l'applicazione. Pertanto, un modello di reattore a letto mobile è stato sviluppato considerando la riduzione del metano ottenuta sperimentalmente e la cinetica di splitting della CO2 è stata incorporata per valutare i due impianti proposti: la centrale elettrica e l'impianto di poligenerazione. È stata osservata una riduzione del 20% nell'efficienza dell'unità CL. Tuttavia, grazie all’integrazione termica interna al sistema, l’efficienza termica dell'impianto complessivo è molto simile a quella raggiunta nell’analisi termodinamica, con un valore del 50,9%. Tuttavia, a differenza del layout termodinamico, non è disponibile calore in eccesso per migliorare ulteriormente l'efficienza del sistema. Oltre al riciclo e all'utilizzo della CO2, come criteri di valutazione della sostenibilità per il layout proposto sono stati analizzati anche l’occupazione del suolo terreno e il fabbisogno idrico. Sia il fabbisogno di terra che di acqua aumentano di 2,5 volte rispetto ad una centrale convenzionale a ciclo combinato a gas naturale. Inoltre, anche l’impianto di poligenerazione con produzione di energia elettrica e dimetil etere (DME) è stato studiato considerando un modello dell’unità CL basato sulla cinetica e ha rilevato che la produzione di DME scenderebbe da 2,15 kg/s a 1,48 kg/s e la potenza elettrica prodotta da 103 a 72 MW. Pertanto, la cinetica ha una forte influenza sulla prestazione complessiva del sistema, e considerarla nell’analisi porta a ridurre la produzione di energia e DME di circa il 30% con un aumento di costo del 30%. Complessivamente, l'integrazione dell'unità CL come unità aggiuntiva ad una centrale elettrica a ossicombustione risulta più adatta rispetto alla poligenerazione, considerando il prezzo di mercato attuale per le commodities prodotte.

In order to optimise the efficiency of solar fuel devices, development of cheap, active and stable reduction electrocatalysts for solar fuel production is crucial. To this end, ligand stabilised nickel nanoalloys of around 10 nm with relatively small size distributions, have been synthesised for a variety of compositions utilising first row transition metals (Cr, Fe, Co and Cu). Bi- and trimetallic nanoalloys have been synthesised and good control over composition was demonstrated. Synthesised nanoalloys were electrochemically tested to assess their proton and CO2 reduction activities. All nanoalloys showed higher hydrogen evolution reaction (HER) activity than pure nickel. For bimetallic nanoalloys, in pH 1, a general increase in HER activity with increased electron negativity was observed. The Ni0.5Cu0.3Co0.2 nanoalloy showed the highest HER activity at pH 1, whereas the Ni0.5Co0.3Fe0.2 nanoalloy was most active for HER in pH 13. Little difference between the activities for all nanoalloys was observed at pH 7. The nanoalloys showed differing selectivity’s for CO2 reduction products. Solution based CO2 reduction products were detected at low overpotentials (below -0.789 V vs RHE, pH 6.8), although low faradaic efficiencies (< 1%) were observed. High resolution scanning electron microscopy (HR-SEM) was used to attempt to analyse the nanoalloys after deposition onto the electrodes and after electrochemical testing. The results indicated the presence of sub-monolayer coverage, therefore increasing the nanoalloy coverage without large amounts of agglomeration occurring could result in the observation of higher current densities at lower overpotentials. The stability of the nanoalloy electrodes was also investigated and no decrease in HER activity was observed over 12 hours at -0.5 V vs RHE. Moreover, repeated cycling resulted in an increase in activity being observed. This may be due to leaching of elements overtime. A procedure has been developed using a range of techniques to analyse nanoalloy composition, test proton and CO2 reduction activities and assess stability. This has not only allowed for direct comparison between different materials studied, it also provides a framework for future investigations of nanoalloys for (photo)electrochemical proton and CO2 reduction.

The transition of primary energy supply from fossil fuels to renewable and clean energy sources has become critical in the wake of concerns over ever increasing global energy demand and the urgent need to reduce carbon dioxide emissions. One promising and effective way of minimising carbon emissions is to convert abundant solar energy into storable and transportable fuels, e.g. solar fuels. In this context, solar-driven thermochemical water splitting represents an alternative clean and sustainable route to produce hydrogen (H2) from water. In a typical thermochemical solar energy conversion process, thermal reduction and water dissociation take place in separate steps. A metal oxide based catalyst is used to decrease the required high processing temperature and prevent mixing of the O2 and H2 produced by the process. The overall performance and energy conversion efficiency of a thermochemical water splitting cell is largely dependent on the inherent catalytic characteristics of the catalysts.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
Griffith School of Environment
Science, Environment, Engineering and Technology
Full Text

Thesis advisor: Udayan Mohanty
Among the existing strategies to direct solar energy harvesting and storage, solar fuel production by photoelectrocatalysis promises a comparatively simple, low-cost route. The science behind this process is straightforward: stable semiconductors absorb sunlight and use the energy to excite charges, which then drive redox reactions at the surface. Careful studies of the photoelectrode surface provide important considerations in building a high-performance photoelectrode. Specifically, I focused on controlling the surface band alignment of Cu2O photocathode|water for hydrogen evolution reaction. A ZnS buried heterojunction is formed to improve the photovoltage. Then I focused on understanding the influence of chemical species on surface kinetics and energetics for water oxidation reaction. Two hematite photoanodes with preferably exposed {001} and {012} facets were examined. Further, I systematically studied three different types of surfaces, bare hematite, hematite with a heterogenized Ir water oxidation catalyst (WOC), and a heterogeneous IrOx WOC. While both WOCs improve the performance of hematite by a large margin, their working mechanisms are found to be fundamentally different. I also focused on utilizing surface photoexcited species to control product selectivity. Selective CO production by photoelectrochemical methane oxidation is successfully demonstrated. Detailed experimental investigations revealed that a synergistic effect by adjacent Ti3+ sites is the key to CO formation
Thesis (PhD) — Boston College, 2018
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Chemistry

The world is currently faced with the enormous challenge of slowing down human triggered global warming. As the global energy demand increases, there is an urgent need for renewable and carbon-neutral fuel-sources. Isoprene and isobutene are crude-oil derived, short, volatile and reactive hydrocarbons that can be polymerised into longer chains to be used as jet fuel. Isoprene has previously been produced from the cyanobacterial strain Synechocystis sp. PCC 6803 but there has been no reported isobutene synthesis from any photosynthetic organism. This work aimed to synthesise isobutene in Synechocystis using a cytochrome P450 from Cystobasidium minutum with reported isobutene production capability. Substrate availability was to be provided through the insertion of two heterologous enzymes, IpdC from Salmonella typhimurium and PadA from Escherichia coli. Both IpdC and PadA were successfully expressed in Synechocystis but the functional activities of IpdC, PadA and the cytochrome P450 in Synechocystis remains undetermined. This project also had the aim to design and construct a photo-bioreactor and gas collection system capable of producing and harvesting isoprene directly from an engineered Synechocystis strain. Herein lies a description of a closed system photobioreactor connected to a cold-trap that was able to concentrate isoprene produced from Synechocystis to measurable amounts.

Doctor of Philosophy
Department of Chemical Engineering
Peter H. Pfromm
Approximately 45% of the global hydrogen production (from fossil fuels such as natural gas or coal totaling 2% of the global energy generation) is absorbed as feedstock in the synthesis of over 130 million metric tons ammonia (NH[subscript]3) annually. To achieve food security for a growing world population and to allow for additional uses of the nitrogen-fertilizer for production of bio-energy feedstock or as combustion fuel or H[subscript]2 carrier - demand for NH[subscript]3 is projected to increase. This work pursues the synthesis of ammonia at atmospheric pressure and without fossil fuel. Conceptually, concentrated solar radiation is utilized to transfer electrons from the lattice oxygen of a transition metal oxide to the metal ion. This yields a metallic reactant that provides the reducing power for the subsequent six-electron reductive cleavage of N[subscript]2 forming a transition metal nitride. In a second reaction, the generated lattice nitrogen is hydrogenated with hydrogen from H[subscript]2O to NH[subscript]3. This furnishes the transition metal oxide for perpetuated NH[subscript]3 synthesis. Theory and experimentation identified manganese nitride as a promising reactant with fast diffusion characteristics (8 ± 4 x 10[superscript]-9 cm[superscript]2 s [superscript]-1 apparent nitrogen diffusion constant at 750 degree C) and efficient liberation of 89 ± 1 mol% nitrogen via hydrolysis at 500 degree C. Opposed to only 2.9 ± 0.2 mol% NH[subscript]3 from manganese nitride, 60 ± 8 mol% of the nitrogen liberated from molybdenum nitride could be recovered as NH[subscript]3. Process simulation of a Mo-based NH[subscript]3 synthesis at 500-1200 degree C estimates economically attractive production under fairly conservative process and market conditions. To aid the prospective design of a Mn or Mo-based reactant, correlating the diffusion constants for the hydrolysis of seven nitrides with the average lattice nitrogen charge (9.96-68.83%, relative to an ideal ionic solid) indicates the utility of first-principle calculations for developing an atomic-scale understanding of the reaction mechanism in the future.

Thesis (M.S.Ch.E.)--University of Toledo, 2007.
Typescript. "Submitted as partial fulfillment of the requirements for The Master of Science degree in Chemical Engineering." Bibliography: leaves 72-77.

One of today’s most pressing scientific challenges is the conception, development and deployment of renewable energy technologies that will meet the demands of a rapidly increasing population. The motivation is not only dwindling fossil fuel reserves, but also the necessary curtailment of emissions of the greenhouse gas carbon dioxide (a product of burning fossil fuels). The sun provides a vast amount of energy (120,000 TW globally), and one major challenge is the conversion of a fraction of this energy into chemical energy, thereby allowing it to be stored. Dihydrogen (H₂) that is produced from water is an attractive candidate to store solar energy (a ‘solar fuel’), as are high energy carbon-containing molecules (such as CO) that are formed directly from carbon dioxide. One key aspect is the development of catalysts that are able to offer high rates and efficiencies. In biology, some microbes acquire energy from the metabolism of H₂ and CO. The biological catalysts - enzymes - that are responsible are hydrogenases (for the oxidation of H₂ to protons); and carbon monoxide dehydrogenases (CODHs, for the oxidation of CO to CO₂). These redox enzymes, containing nickel and iron as the only metals, are extraordinary in terms of their catalytic characteristics: many are fully reversible catalysts and offer very high turnover frequencies (thousands per second are common), with only tiny energy input requirements. This Thesis uses a hydrogenase from the bacterium Escherichia coli, and two CODHs from the bacterium Carboxydothermus hydrogenoformans, as the catalysts in H2 production and CO₂ reduction systems. Chapter 3 describes the concept and development not of a solar fuel system, but of a device that catalyses the water-gas shift reaction (the reaction between CO and water to form H₂ and CO₂) - a process of major industrial importance for the production of high purity H₂. Chapters 4, 5 and 6 detail photochemical CO₂ reduction systems that are driven by visible light. These systems, operating under mild, aqueous conditions, involve CODHs attached either to TiO₂ nanoparticles that are sensitised to visible light by the co-attachment of a ruthenium-based dye complex, or to cadmium sulfide nanomaterials that, having a narrow band gap, are inherently photoexcitable by visible light. The motivation here is not the construction of technological devices; indeed, the enzymes that are used are fragile, highly sensitive to oxygen, and impossible to scale to industrial levels. Rather, the drivers are those of scientific curiosity (can the incorporation of these remarkable biological catalysts enable the creation of outstanding solar fuel devices?), and of producing systems that serve as benchmarks and inspiration for the development of fully synthetic systems that are robust and scalable.

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Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

This study focuses on upgrading biomass derived syngas for the synthesis of liquid fuels using Fischer-Tropsch synthesis (FTS). The process includes novel gasification of biomass via a tri-reforming process which involves a synergetic combination of CO2 reforming, steam reforming, and partial oxidation of methane. Typical biomass-derived syngas H2:CO is 1:1 and contains tars that deactivate FT catalyst. This innovation allows for cost-effective one-step production of syngas in the required H2:CO of 2:1 with reduction of tars for use in the FTS. To maximize the performance of the tri-reforming catalyst, an attempt to control oxygen mobility, thermal stability, dispersion of metal, resistance to coke formation, and strength of metal interaction with support is investigated by varying catalyst synthesis parameters. These synthesis variables include Ce and Zr mixed oxide support ratios, amount Mg and Ni loading, and the preparation of the catalyst. Reaction conditions were also varied to determine the influences reaction temperature, gas composition, and GHSV have on the catalyst performance. Testing under controlled reaction conditions and the use of several catalyst characterization techniques (BET, XRD, TPR, XAFS, SEM-EDS, XPS) were employed to better explain the effects of the synthesis parameters. Applications of the resulting data were used to design proof of concept solar powered BTL plant. This paper highlights the performance of the tri-reforming catalyst under various reaction conditions and explains results using catalyst characterization.

Compte tenu du récent taux alarmant d'épuisement des réserves de combustibles fossiles et de l'augmentation drastique des niveaux de CO2 dans l'atmosphère qui a conduit au réchauffement de la planète et à des changements climatiques sévères, l'exploitation de toutes sortes d'énergies renouvelables a été la Parmi les principales priorités de la recherche Champs à travers le monde. L'une des nombreuses voies de ce genre est la réduction du CO2 aux combustibles utilisant des énergies renouvelables, plus communément appelées cellules photoélectro-catalytiques (PEC). Des essais expérimentaux sur la réduction du CO2 ont été réalisés sur différents types de catalyseurs dans les deux cellules (Conçu par un laboratoire) afin de comprendre la sélectivité, la productivité et les produits de réaction obtenus. Des essais expérimentaux ont été réalisés sur différents types de catalyseurs à la fois dans les cellules en phase gazeuse et en phase liquide pour comprendre la sélectivité, la productivité et les produits de réaction obtenus. Pour les études sur la réduction EC du CO2 en phase gazeuse, une série d'électrodes (à base de nanoparticules (NPs) de Cu, Fe, Pt et CuFe déposées sur des nanotubes de carbone ou de noir de carbone puis placées à l'interface entre une membrane Nafion et Une électrode à couche de diffusion de gaz). Les résultats démontrent le type divers de produits formés et leurs productivités. Dans des conditions sans électrolyte, la formation de produits ≥C1 tels que l'éthanol, l'acétone et l'isopropanol a été observée la plus élevée étant pour Fe et suivie de près par Pt. Pour améliorer Combustibles nets, un ensemble différent d'électrodes a été préparé sur la base de revêtements MOF de type imidazolate de type zéolitique substitué (SIM-1) (Fe-CNT, Pt-CNT et CuFe-CNT basés sur MOF) Et Pt-MOF a montré des carburants améliorés. En se reportant aux études sur la réduction EC du CO2 dans une cellule en phase liquide, un ensemble similaire d'électrodes a été prepare (NP - Cu, Fe, Pt, Ru, Co déposées sur des nanotubes de carbone ou du noir de carbone ont). Pour les conditions de phase liquide, en termes de produits C nets, les électrodes catalytiques à base de Pt sont en tête de la catégorie, suivies de près par Ru et Cu, tandis que Fe a obtenu la position la plus basse. Le mécanisme réactionnel sous-jacent probable a également été fourni. Afin d'améliorer encore les performances, on a synthétisé des NP de metal (Ru, Fe, Pt et Cu) de différentes tailles en utilisant différentes techniques de synthèse: (i) l'itinéraire d'imprégnation (ImR) pour obtenir des NP dans la plage de tailles de 10 à 50 nm; (Ii) Approche organométallique (OM) pour synthétiser des NPs uniformes et ultrafines dans la plage de tailles de 1-5 nm. Fe ont été synthétisés par une nouvelle voie de synthèse et des conditions pour atteindre des NP de 1 à 3 nm. (Iii) Approche de haut en bas de Nanowire pour obtenir des NP de cuivre ultrafin dans la plage de taille de 2-3,8 nm. Les améliorations apportées à la productivité du carburant se sont révélées être de 5 à 30 fois plus élevées pour les petites NP sur les NP plus importantes et, en outre, une charge réduite de 10 à 1-2% en poids. Un autre ensemble d'électrodes à base de nano-mousses (Cu NF et Fe NF sur Feuille de Cu, Feuille de Foie, Al Foil, Inconel foil et Al grid / mesh) préparés par électrodéposition ont également été étudiés afin d'améliorer encore la conversion de CO2 / carburant. Après, l'optimisation du dépôt et de la tension à l'aide de la voltamétrie cyclique, les carburants se sont améliorés de 2 à 10 fois par rapport aux combustibles nets les plus élevés obtenus à l'aide d'électrodes CNT dopées à base de NP
In view of the recent alarming rate of depletion of fossil fuel reserves and the drastic rise in the CO2 levels in the atmosphere leading to global warming and severe climate changes, tapping into all kinds of renewable energy sources has been among the top priorities in the research fields across the globe. One of the many such pathways is CO2 reduction to fuels using renewable energies, more commonly referred as photo-electro-catalytic (PEC) cells. Experimental tests were carried out on various types of catalysts in both the gas and liquid phase cells (lab-designed) to understand the different selectivity, productivity and the reaction products obtained. For the studies on the EC reduction of CO2 in gas phase cell, a series of electrodes (based on Cu, Fe, Pt and Cu/Fe metal nanoparticles – NPs - deposited on carbon nanotubes – CNTs - or carbon black and then placed at the interface between a Nafion membrane and a gas-diffusion-layer) were prepared. Under gas phase, the formation of ≥C1 products (such as ethanol, acetone and isopropanol) were observed, the highest being for Fe and closely followed by Pt, evidencing that also non-noble metals can be used as efficient catalysts under these conditions. To enhance the net fuels, a different set of electrodes were also prepared based on substituted Zeolitic Imidazolate (SIM-1) type MOF coatings (MOF-based Fe-CNTs, Pt-CNTs and Cu/Fe-CNTs) and Pt-MOF showed improved fuels. Moving to the studies on the EC reduction of CO2 in liquid phase cell, a similar set of electrodes were prepared (metal NPs of Cu, Fe, Pt, Ru and Co deposited on CNTs or carbon black). For liquid phase conditions, in terms of net C-products, catalytic electrodes based on Pt topped the class, closely followed by Ru and Cu, while Fe got the lowest position. The probable underlying reaction mechanism was also provided. In order to improve further the performances, varied sized metal NPs (Ru, Fe, Pt and Cu) have been synthesized using different techniques: (i) impregnation (ImR) route to achieve NPs in the size range of 10-50 nm; (ii) organometallic (OM) approach to synthesize uniform and ultrafine NPs in the size range of 1-5 nm (i.e., Fe NPs were synthesized through a novel synthesis route to attain 1-3 nm NPs); (iii) Nanowire (NW) top-down approach to obtain ultrafine copper metal NPs in the size range of 2-3.8 nm. The enhancements in the fuel productivity were found to be 5-30 times higher for the smaller metal NPs over the larger metal NPs and moreover, with reduced metal loading from 10 to 1-2 wt %. A different set of electrodes based on nano-foams (Cu NF and Fe NF on Cu foil, Fe foil, Al foil, Inconel foil and Al grid/mesh) prepared via electro-deposition were also investigated, to further improve CO2 to fuels conversion. After, optimization of deposition and voltage using cyclic voltammetry, the fuels improved by 2-10 times over the highest net fuels achieved using metal NPs doped CNT electrodes

The excessive consumption of non-renewable energy sources such as fossil fuels has lead the world to a global climate change, urging for new energy consumption habits together with developing cost- effective alternative renewable technologies. Photoelectrochemical (PEC) water splitting allows for direct conversion of solar light and water into hydrogen and oxygen, storing energy into chemical bonds, solving the storage problem of photovoltaic technology. It has demonstrated to produce pure hydrogen and oxygen in significant efficiencies, although this technology is not ready for market implementation due to lack of efficient, stable and scalable photoelectrodes. In this work, we undertake a journey from improving the efficiency of stable metal-oxide-based photoanodes to stabilizing efficient photovoltaic materials by the introduction of protective, transparent, conductive and catalytic layers. Efforts have focused on using cost-effective and scalable materials and techniques. Metal oxide candidate TiO2 is reported stable in alkaline electrolytes and at anodic potentials, but they present low photon to current conversion efficiencies. This is due to excessively large band gap, absorbing small part of the visible spectra, and small electron and hole mobility. Its efficiency is increased both by microstructuring the substrate and nanostructuring the thin film into nanorods, and by modifying the electronic structure with a reductive H2 treatment, enhancing potential drop inside the nanorods. The strategy is shifted into stabilizing highly efficient short band gap semiconductor materials used by the photovoltaic industry. Silicon based photocathodes are protected from acidic electrolyte corrosion by TiO2 overlayers grown by atomic layer deposition (ALD). Temperature is found to play a key role for both efficient film conductivity and stability, being this caused by polycrystalline films formation. ALD enabled high thickness control and pinhole-free layers, together with lower crystallization temperatures than other techniques. Copper-indium-gallium-selenide (CIGS) solar cells fabricated on flexible stainless steel substrates are also protected from corrosion by TiO2 ALD protective layers. The transparent conductive oxide (TCO) already used in solar cells is found necessary for efficient p-n junction formation and charge transport to the hydrogen evolution reaction. Copper-zinc-tin- sulfide/selenide (CZTS/Se) solar cells, where scarce indium and gallium are substituted by tin and zinc, are implemented for PEC devices with TiO2 overlayers too. By modifying the S/Se ratio, band gap can be tuned, an especially interesting characteristic to design tandem PEC devices. ALD deposited protective layers are also studied in anodic polarizations and alkaline electrolytes. By varying the deposition temperature of TiO2, completely amorphous, mixed amorphous and crystalline and fully crystalline films are deposited, and a clear conductivity increase is observed correlated to crystallization. Preferential conductivity paths are observed inside crystalline grains, proposed to be related to crystalline defects and grain boundaries. Few hundred hours stability tests reveals significant photocurrent decrease, with no observed dissolution of the Si photoabsorber. This is attributed to oxidative potentials and electrolyte hydroxides diminishing the n-type semiconductor behavior of TiO2 and forming a barrier to charge injection into the oxygen evolution reaction. UV superimposed illumination partially recovered conductivity. NiO films are ALD-deposited on Si photoanodes and conductivity is found to decrease when temperature is increased from 100 to 300 ºC, simultaneous to a change in preferential crystal growth direction. Higher stoichiometric film, being formed when increasing temperature, decreases Ni2+ vacancies, responsible of the p-type semiconductor behavior. Impressive 1000 hours stability measurements are obtained. Although, this is only attained under periodic cyclic voltammetries, avoiding partial deactivation of the photoanodes. This is attributed to chemical modifications at the surface in such highly oxidative conditions.

Hydrogen energy is an ideal energy resource owing to its clean and efficient utilization. As an energy carrier without natural abundance, the limited reserve makes the high consumption a big challenge. In the meantime, fossil fuels, e.g., coal, oil and gas, have been important carbon carriers in the long-term carbon cycle, but their upgrading is restricted to conventional thermocatalysis. Solar energy with the advantages of large abundance, widespread distribution, and high flux appeals extensive attention, but unfortunately is underutilized at the moment. Photocatalysis initiated with semiconductors is a promising pathway towards the conversion and storage of solar energy into chemical stocks, and has been studied for several decades. However, due to the low photoresponse capacity and solar energy conversion efficiency of the existing photocatalysts, the prospect of their industrialization is still unclear. Photothermal catalysis integrating photocatalysis and thermocatalysis into one unit has been proposed in the past several years. Although its quantum efficiency and reaction turnover frequency were significantly improved, the reaction mechanisms have not yet been well illustrated. This PhD study is to develop photo assisted catalysis to obtain high performances for energy preparation and fossil fuels upgrading, and to have a deep insight into their reaction mechanisms. First, in-plane heterostructured graphene/carbon nitride photocatalyst was prepared via a hydrogeninitiated chemical epitaxial growth strategy. With the insert of nano-graphene into the porous carbon nitride, the quantum efficiency of the water splitting reaction for hydrogen generation was significantly enhanced (Chapter 3). Considering the unsatisfied incident light to electron efficiency, the study unveiled the potential difference as the internal electrical field affecting the separation, transfer and output of photoinduced charge carriers. Meanwhile, the quantum efficiency and utilization of solar light were both improved via the optimization of potential differences in photocatalytic systems (Chapter 4). In addition, the active sites (Chapter 5) and relationships between photocatalysis and thermocatalysis (Chapter 6) in photothermal catalytic systems were both in-depth studied. With the available reaction mechanism and optimization of reaction conditions, the photothermal catalytic performances in the upgrading of fossil fuels are increased to a industrialization level. This PhD project contributes to the improvements of quantum efficiency via catalyst modification, reaction optimization and mechanism investigation and then expects to provide both technological and scientific knowledge for the full storage and conversion of solar energy into fuels.

Growing global emission of carbon dioxide gas (CO2) reflects the world’s energy dependence on fossil fuels. The conversion of CO2 emission into value-added products, like fuels completes a circular CO2 economy which requires a renewable energy conversion and storage system. Amongst a few, photo/electrochemistry has been particularly appealing thanks to its energy efficiency and enormous potential for industrial applications. Formic acid (HCOOH) production from CO2 reduction appears as an alternative energy storage option based on the commercialization of this process. Herein, stable and selective catalysts working at low overpotential are needed to reduce CO2. Likewise, cell design is critical to have improved CO2 mass transport for obtaining high conversion efficiencies and to achieve feasible production yields. The initial work was conducted on the design and understanding of operational parameters of an electrochemical flow cell (ECf-cell) such as flow rates and electrode potentials. For CO2 reduction at the cathode site, two different gas diffusion electrodes were produced by electrodeposition: Sn-GDE and Cu-GDE. An optimum potential range was established to control HCOOH selectivity. The complementing reaction at the anode site, oxygen evolution reaction (OER), was studied using Mn-Co oxide nanoparticles to replace expensive DSA: Ir-Ta oxide catalyst. Subsequent efforts were devoted on the assembly of a photoelectrochemical flow cell (PECf-cell) which enabled coupling of Sn-GDE as cathode vs. TiO2 nanorods as photoanode. This led to nearly 1/3 reduction in overall cell voltage reaching an energy efficiency up to 70 %. The solar-to-fuel (STF) conversion efficiency was 0.25% which was one of the highest efficiencies reported amongst the data obtained from a cell in device level. The results proved that optimized system efficiency could be achieved with a large bandgap photoanode having superior stability and a GDE cathode with improved CO2 mass transfer. The deployment of renewable energy sources will require new technologies to emerge. The photoelectrochemical flow cell developed in this work can store energy from intermittent electricity sources (i.e. wind and solar) in a sustainable manner. This may pave the way for commercialization of this process and moving towards a circular CO2 economy.
La conversión de CO2 en productos de valor añadido con energías renovables resulta interesante para mitigar las emisiones de este. La conversión foto/electroquímica es atractiva por su eficiencia energética y su enorme potencial para aplicaciones industriales. La producción de ácido fórmico (HCOOH) a partir de la reducción de CO2 aparece como una vía alternativa para su comercialización. Sin embargo, se requieren catalizadores estables y selectivos que trabajen a bajo sobre potencial. Además, el diseño de la celda es crítico para mejorar el transporte de masa de CO2 y obtener elevadas eficiencias de conversión. En este trabajo se estudió en un primer lugar el diseño y la comprensión de los parámetros operativos de una celda de flujo electroquímica: caudales y potenciales de electrodo. Para la reducción de CO2 sobre el cátodo, se emplearon dos electrodos diferentes de difusión de gas preparados por electrodeposición: Sn-GDE y Cu-GDE. Se estableció un valor de operación óptimo para controlar la selectividad a HCOOH. Se estudió también la reacción complementaria en el ánodo (evolución de O2), empleando nanopartículas de óxido de Mn-Co para reemplazar el elevado coste del catalizador de óxido de Ir-Ta. Finalmente, se montó una celda fotoelectroquímica de flujo que permitió la inclusión de TiO2 nanorods como fotoánodo. El voltaje total de la celda se redujo alrededor 1/3 alcanzando una eficiencia energética del 70 %. El rendimiento de conversión de energía solar a combustible (STF) fue de 0,25%. Los resultados demuestran que se puede lograr una eficiencia optimizada del sistema con un fotoánodo que tiene una buena estabilidad y un cátodo que favorece la transferencia de masa de CO2. La celda de flujo fotoelectroquímica desarrollada en este trabajo permite almacenar energía de fuentes de electricidad intermitentes (eólica y/o solar) de una manera sostenible, con el consiguiente avance en una economía circular de CO2.

Cette étude se focalise sur le développement de procédés de dissociation de H2O et CO2 par voie thermochimique utilisant des oxides métalliques non-stœchiométriques et l’énergie solaire concentrée pour la production de carburants solaires. Les procédés redox se décomposent en deux réactions distinctes : tout d’abord, une réduction thermique à haute température de l’oxyde métallique avec la création de lacunes en oxygène dans la structure cristallographique, entrainant une production d’oxygène ; puis, une réoxydation de l’oxyde métallique par H2O et/ou CO2, conduisant à la production de H2 et/ou CO. La cérine et les pérovskites ont été étudiées comme matériaux réactifs pour les cycles thermochimiques. Pour augmenter l’efficacité des cycles thermochimiques, différents paramètres ont été étudiés, comme la composition chimique et la morphologie de l’oxyde réactif, les conditions opératoires, ainsi que la configuration du réacteur solaire. Dans un premier temps, les activités redox, la cinétique et la thermodynamique de différentes pérovskites ont été étudiées expérimentalement pour les cycles redox. Par la suite, les performances thermochimiques de différents matériaux réactifs sous forme de structures poreuses ou de particules ont été étudiées dans des réacteurs solaires (configuration monolithique ou lit fixe) permettant de réaliser des cycles thermochimiques en deux étapes. Une étude paramétrique détaillée a été effectuée pour déterminer les taux et vitesses de production. La vitesse de production de CO la plus élevée (9.9 mL/min/g) a été obtenue avec des mousses réticulées en cérine. Enfin, un réacteur solaire membranaire a été développé pour produire en isotherme et en continu du CO (ou H2) par dissociation de CO2 (ou H2O) avec une membrane réactive et perméable à l’oxygène. La vitesse de production la plus élevée atteint 0.133 µmol/cm2/s à 1550 °C en utilisant une membrane en cérine avec un revêtement en pérovskite
This study is focused on the development of thermochemical H2O and CO2 splitting processes using non-stoichiometric metal oxides and concentrated solar energy to produce solar fuels. The redox process is composed of two distinct reactions: first, a thermal reduction at high temperature of the metal oxide with creation of oxygen vacancies in the crystallographic structure, resulting in released oxygen; second, the re-oxidation of the metal oxide by H2O and/or CO2, leading to H2 and/or CO production. Ceria and perovskite materials have been investigated as reactive oxides for thermochemical cycles. To increase the thermochemical process efficiency, different aspects were investigated, such as chemical composition and morphology of the metal oxide, operating parameters, and solar reactor configuration. The redox activities, kinetics and thermodynamics of different perovskite materials were first experimentally investigated for two-step thermochemical cycles. Then, the thermochemical performances of various reactive materials shaped as porous structures or particulate media were investigated in solar reactors (monolithic or packed-bed configurations) able to perform two-step thermochemical cycles. A detailed parametric study was performed to determine fuel production rates and yields. The highest CO production rate (9.9 mL/min/g) was achieved with ceria reticulated foams. Finally, a solar membrane reactor was developed for isothermal and continuous production of CO (or H2) by CO2 (or H2O) splitting with a reactive and oxygen-permeable membrane. The highest CO production rate reached 0.133 µmol/cm2/s at 1550 °C using a perovskite-coated ceria membrane

Les procédés thermochimiques solaires étudiés concernent la conversion de charges hydrocarbonées solides ou gazeuses en syngas, ainsi que la réduction d’oxydes en métaux en utilisant l’énergie solaire concentrée pour effectuer les réactions endothermiques, permettant ainsi le stockage de l’énergie solaire intermittente en carburants sans émissions de CO2. Ce travail a pour objectif l’étude expérimentale de trois procédés solaires incluant la gazéification de biomasse, le reformage de méthane en boucle chimique, et la carboréduction de ZnO et MgO. La gazéification et le reformage permettent la valorisation de biomasse bois et de méthane en syngas, tandis que la carboréduction permet de produire Zn et Mg à partir de ZnO et MgO. Ces procédés ont été étudiés dans des réacteurs solaires de 1.5 kWth, en utilisant le rayonnement concentré fourni par des systèmes à concentration du laboratoire PROMES, Odeillo, France. L’impact des paramètres opératoires de chaque procédé sur les mécanismes réactionnels, conversion, rendement, et performances énergétiques a été évalué en détail. Ces procédés ont permis d’améliorer la conversion chimique, les rendements en syngas, les efficacités énergétiques tout en permettant un stockage de l’énergie solaire en combustibles transportables, avec des performances globales supérieures aux procédés conventionnels. De plus, leur faisabilité, fiabilité et robustesse pour la conversion de méthane et biomasse en syngas et la production de Mg et Zn en fonctionnement batch ou continu sous pression réduite ou atmosphérique en conditions solaires réelles ont été démontrés
The investigated solar thermochemical processes consist of the thermochemical conversion of solid and gaseous carbonaceous feedstocks into syngas as well as metal oxides reduction into metal commodities utilizing concentrated solar energy to drive endothermic chemical reactions, thereby enabling intermittent solar energy storage into solar fuels and avoiding CO2 emissions. This work aims to experimentally investigate three key solar thermochemical conversion approaches regarding biomass gasification, chemical looping reforming of methane, and carbothermal reduction of ZnO and MgO. Solar gasification and solar chemical looping reforming allowed valorizing wood biomass and methane into syngas, while solar carbothermal reduction was applied to produce Zn and Mg from ZnO and MgO. Such solar thermochemical processes were performed in 1.5 kWth prototype solar chemical reactors, utilizing highly concentrated sunlight provided by a solar concentrator at PROMES laboratory, Odeillo, France. The impact of controlling parameters of each process on the reaction mechanism, conversion, yields, and process performance, during on-sun testing was investigated and evaluated thoroughly. Such processes were proved to significantly improve the chemical conversion, syngas yields, energy efficiency, with solar energy storage into transportable fuels, thereby outperforming the conventional processes. Moreover, their feasibility, reliability, and robustness in converting both methane and biomass feedstocks to syngas as well as producing Mg and Zn metals in batch and continuous operation under vacuum and atmospheric conditions during on-sun operation were successfully demonstrated

The modernized world is over-consuming low-cost energy sources that strongly contributes to pollution and environmental stress. As a consequence, the interest for environmentally friendly alternatives has increased immensely. One such alternative is the use of solar energy and water as a raw material to produce biohydrogen through the process of photosynthetic water splitting. In this work, the relation between H2-production and photosynthesis in the green algae Chlamydomonas reinhardtii was studied with respect to three main aspects: the establishment of prolonged H2-production, the involvement of PSII in H2-production and the electron pathways associated with PSII during H2-production. For the first time, this work reveals that PSII plays a crucial role throughout the H2-producing phase in sulfur deprived C. reinhardtii. It further reveals that a wave-like fluorescence decay kinetic, before only seen in cyanobacteria, is observable during the H2-producing phase in sulfur deprived C. reinhardtii, reflecting the presence of cyclic electron flows also in green algae.

Inexpensive petroleum is the cornerstone of the modern global economy despite its huge environmental costs and its nature as a non-renewable resource. While ninety percent of petroleum is ultimately used as fuel and can in principle be replaced by sources of renewable electricity, ten percent is used as a feedstock to produce societally important chemicals that cannot currently be made at a reasonable cost through alternative processes. In this dissertation, I will discuss my efforts, together with several colleagues, to apply synthetic biology approaches to the challenge of producing renewable petrochemical replacements. In Chapter 2, I discuss our efforts to engineer E. coli to produce fatty acids with a wide range of chain lengths at high yield, thereby providing an alternative platform for the production of diverse petrochemicals. In Chapter 3, I describe a novel method of DNA assembly that we developed to facilitate synthetic biology efforts such as those in Chapter 2. This method is capable of simultaneously assembling multiple DNA pieces with substantial sequence homology, a common challenge in synthetic biology. In Chapter 4, I discuss the development of a "bionic leaf": a hybrid microbial-inorganic catalyst that marries the advantages of photovoltaic-based light capture and microbial carbon fixation to achieve solar biomass yields greater than those observed in terrestrial plants. This technology offers a potentially low-cost alternative to photosynthesis as a source of biomass and derived chemicals and fuels. The work described in this dissertation demonstrates the capacity of synthetic biology to address the problem of renewable chemical production, and offers proof of principle demonstrations that both the scope and efficiency of biological chemical production may be improved.

Although concentrated solar power can be used to produce power using traditional electricity generation, energy storage has become a problem due to the intermittent supply of solar energy. By using solar energy in chemical production processes, the solar energy can be stored in a useful chemical product. The purpose of this thesis will be to examine the possibilities of a new solar chemical cycle the produces iron and ethylene from hematite (a form of iron oxide) and ethane using concentrated solar power. These two products are important stepping stones in the production of steel and polymers. This process could allow for the current process of steel production to move away from processes using coal and towards a more sustainable process using the hydrogen formed from the ethane cracking process and solar energy. The thesis will include: (1) the development of a new solar powered iron and ethylene combined cycle, (2) a feasibility study of a Concentrated Solar Heat Supply System (CSHSS) being developed at Georgia Tech, and (3) an assessment of the proposed cycle. The assessment will include an estimate of production including a thermodynamic ASPEN model, assessment of research to realize actualization of the theoretical cycle, an exergy analysis, and a heat exchanger analysis for the exchange of heat between the CSHSS and the chemical process.

Ce travail de thèse porte sur l’étude de la réduction de CO 2 et H 2 O en CO et H 2 au moyen de cycles thermochimiques. Ces cycles utilisent des oxydes métalliques pour réaliser ces réductions en deux étapes, permettant de diminuer la température nécessaire. Dans une première étape endothermique, l’oxyde métallique est réduit à haute température (>1200°C) grâce à un apport d’énergie solaire concentrée. Dans une seconde étape exothermique réalisée à plus basse température (<1200°C), cette espèce réduite est ré-oxydée en présence d’eau ou de CO 2 , produisant H 2 ou CO et régénérant l’oxyde métallique pour un autre cycle. Le mélange de H 2 et CO (syngas), ainsi produit uniquement grâce à de l’énergie solaire peut ensuite être transformé en carburant liquide conventionnel par un procédé catalytique de type Fischer-Tropsch. Cette étude s’intéresse particulièrement aux cycles à base d’oxydes volatiles, ZnO/Zn et SnO 2 /SnO, dont le produit de la première étape de réduction est sous forme gazeuse à la température de réaction, puis se condense sous forme de nanoparticules. Tout d’abord, des moyens et méthodes ont été développés pour l’étude de la cinétique des réactions de réduction à hautes températures, en particulier une méthode inverse utilisant la mesure en ligne de l’oxygène produit dans un réacteur solaire, et un dispositif de thermogravimétrie solaire. Par ailleurs, différents moyens de diminuer la température des réactions de réduction ont été étudiés, à savoir la diminution de la pression et l’emploi d’un agent réducteur carboné. L’impact de la diminution de la pression sur la cinétique de réduction a été quantifié pour SnO 2 et ZnO.Une étude de l’évolution physico-chimique de poudres de SnO durant la deuxième étape d’oxydation du cycle a ensuite été réalisée, montrant l’importance de la réaction de dismutation de SnO en Sn et SnO 2 sur la réactivité des poudres dans la gamme de température étudiée
This PhD thesis focuses on the study of the CO2 and H2O reduction into CO and H2 using thermochemical cycles. These cycles use metal redox pairs for stepwise reduction at lower temperature. The first step consists of the endothermic high temperature reduction of the metal oxide (>1200°C) using concentrated solar energy. The second step, operated at a lower temperature (<1200°C), uses the reduced specie to reduce CO2 or H2O, yielding CO or H2 and regenerating the metal oxide. The CO and H2 mixture (syngas), produced using solar energy, can then be converted into liquid fuel using a conventional Fischer-Tropsch catalytic process. The study considers more specifically the volatile oxide cycles, ZnO/Zn and SnO2/SnO, for which the reduced specie is obtained in gaseous phase at the reaction temperature, and is then condensed as nanoparticles. First, means and methods for studying the kinetics of reduction reactions at high temperatures were developed, namely an inverse method based on the online analysis of O2 production in a solar reactor and a solar-driven thermogravimeter. In addition, the study of reduced pressure operation and the use of a carbonaceous reducer were considered as efficient means to decrease the operating temperature and to promote a fast reaction. The impact of reduced pressure was quantified for SnO2 and ZnO reduction. A study of the evolution of the morphology and chemistry of the SnO powder during the second oxidation step was then conducted, emphasizing the importance of SnO disproportionation on the powder reactivity

Sistemas híbridos consistem de duas ou mais fontes geradoras de eletricidade, normalmente uma ou mais fontes convencionais e uma ou mais fontes renováveis e, objetivam promover a economia de combustível e obter uma fonte confiável de suprimento de energia, podendo estar ou não conectados a rede de distribuição. Este trabalho objetiva avaliar através do software HOMER, a viabilidade técnica, econômica e ambiental de implantação de um sistema híbrido de geração de eletricidade. Este sistema é composto por gerador movido a biogás, gerador movido a biodiesel e captação de energia solar. Todo o sistema está localizado no município de Serafina Corrêa onde há elevada concentração de suinocultores que, através do tratamento dos resíduos suinícolas poderá levar a produção de biogás para ser aproveitado como combustível para geração de energia elétrica. Diversas configurações foram avaliadas sob aspecto econômico e ambiental. A configuração ótima da estrutura do sistema híbrido foi a composta por geração elétrica a partir de painéis fotovoltaicos com 172,4 kW, gerador a biogás 55 kW e inversor de frequência de 110 kW. Neste cenário, o capital inicial soma R$ 1.150.055,00, valor presente líquido de R$ 1.150.004,00 e o custo da energia (COE) é de R$ 0,22/kW. O payback definido pelo software é de 7,1 anos, mostrando-se economicamente viável. Neste contexto, o software HOMER apresenta-se como importante ferramenta a tomada de decisões configurando-se como método de avaliação quanto ao melhor cenário para instalação de sistemas híbridos.
Hybrid systems consist of two or more electricity generating sources, usually one or more conventional sources and one or more renewable sources, and aim to promote fuel economy and obtain a reliable source of energy supply, off-grid or grid-connected to the distribution network. This work aims to evaluate through the HOMER software the technical, economic and environmental feasibility of implementing a hybrid electricity generation system. This system consists of a biogas generator, biodiesel generator and solar energy capture. The entire system is located in the municipality of Serafina Corrêa where there is a high concentration of swine farmers that, through the treatment of pig waste, can lead to the production of biogas to be used as fuel for electric power generation. Several configurations were evaluated under economic and environmental aspect. The optimum configuration of the hybrid system structure is composed of electric generation from photovoltaic panels with 172,4 kW, 55 kW biogas generator and 110 kW inverter. In this scenario, the initial capital amounts to R$ 1.150.055,00, net present value of R$ 1.150.004,00 and the cost of energy (COE) is R$ 0.22. The payback defined by the software is 7.1 years, proving to be economically viable. In this context, the HOMER software presents itself as an important decision-making tool, being configured as an evaluation method for the best scenario for the installation of hybrid systems.

Concentrated Solar Thermal technology (CST) is a very promising renewable energy technology and has a broad range of use. Conventionally, CST systems are mostly used for power generation according to the Rankine cycle and thus often referred to as Concentrated Solar Power (CSP). In this present study, the solar heat is utilized to drive a thermochemical redox cycle of a metal-oxide in order to produce synthetic gas, a combination of hydrogen and carbon monoxide. Later, the synthetic gas is converted into usable liquid fuel whereas the production pathway is CO2 free. This thesis focuses on the process modeling and economic evaluation of solar-driven future fuels production plants. Four future fuels have been selected and modeled using commercial simulation software Aspen Plus®. These 4 future fuels are syngasoline, syndiesel, ethanol and methanol where they can be seen as a very good substitute for current transportation fuels. The heat required at high temperature is delivered using concentrated solar thermal technology with tower configuration for which the heliostat field is designed using in-house software HFLCAL developed by DLR. Syngas is converted into aforementioned fuels using either Fischer-Tropsch or plug-flow reactor. The reactor is modeled taking into account the kinetic of reaction for each fuel, while in case of the absence of kinetic, a stoichiometric approach is implemented. To analyze the hourly plant’s performance, a quasi-steady state analysis is done within MATLAB® environment. The metric used to evaluate the plants are production cost in €/L and overall thermal efficiency. The results show that aforementioned conversion pathway yields higher production costs compared to current market while the lowest production cost is obtained for Methanol at 1.42 €/L. It is shown that solid to solid heat exchanger (STS) efficiency plays a major role in order to make the plant more economically viable. Combining electricity supply of Photovoltaic (PV) and CSP is also shown to be one way to reduce the production cost. If the plant combines PV-CSP is used as the electricity source, syngasoline emerges to be the closest proposed plant to current market fuel production cost with a production cost of 5.99 €/L at the base case scenario which corresponds to 622% relative difference with current market’s production cost and 2.87 €/L at the best case scenario which corresponds to 245% relative difference with current market’s production cost. At the base case scenario, the highest overall thermal efficiency is obtained for the syngasoline plant (4.05%) and at the best case scenario for the ethanol plant (9.2%).

Mixed-metal supramolecular complexes containing one or two RuII light absorbing subunits coupled through polyazine bridging ligands to a RhIII reactive metal center were prepared for use as photocatalysts for the production of solar H2 fuel from H2O. The electrochemical, photophysical, and photochemical properties upon variation of the monodentate, labile ligands coordinated to the Rh reactive metal center were investigated. Bimetallic complexes [(Ph2phen)2Ru(dpp)RhX2(Ph2phen)]3+ (Ph2phen = 4,10-diphenyl-1,10-phenanthroline; dpp = 2,3-bis(2-pyridyl)pyrazine; X = Br- or Cl-) were prepared using a building block approach, allowing for selective component choice. The identity of the halide coordinated to Rh did not impact the light absorbing or excited state properties of the structural motif. However, the o-donating ability of the halides modulated the Rh-based cathodic electrochemistry and required the use of multiple pathways to explain the reduction of Rh by two electrons. Regardless of halide identity, the bimetallic complex possessed a Ru-based HOMO (highest occupied molecular orbital) and Rh-based LUMO (lowest unoccupied molecular orbital) important for photoinitiated electron collection at Rh. As a photocatalyst for H2 evolution, the X = Br- complex produced nearly 30% more H2 than the X = Cl- analogue. H2 production experiments with added halide suggested that ion pairing with halides played a major role in catalyst deactivation, which provided evidence for the importance of component selection for photocatalyst design. New trimetallic complex [{(bpy)2Ru(dpp)}2Ru(OH)2](PF6)5 (bpy = 2,2'-bipyridine) was prepared for comparison to halide analogues [{(bpy)2Ru(dpp)}2RhX2](PF6)5 (X = Br- or Cl-). The synthesis of a halide-free supramolecule containing OH- ligands afforded an ideal system to further examine the impact of the ligands at the reactive metal center on H2 photocatalysis. Electrochemistry results revealed that while the identity of the ligands at Rh did modulate the Rh-based reduction potential, all three complexes possessed a Ru-based HOMO and Rh-based LUMO. The light absorbing properties were not impacted by the identity of the monodentate ligands at Rh; however, the excited state properties did vary upon changing the ligands at Rh. The hydroxo trimetallic complex functioned as a photocatalyst for H2 production in organic solvent, producing nearly double the amount of H2 as the highest performing Br-' trimetallic complex in DMF solvent. Interestingly, H2 production studies in high dielectric aqueous solvent revealed no discrepancies in H2 evolution upon variation of the ligands at Rh, which further supported the ion pairing phenomenon realized for the bimetallic motif. Variation of the labile ligands coordinated to the Rh reactive metal center in RuIIRhIII multimetallic supramolecules provided important insight about the large impact of small structural variation on H2 photocatalysis. Electrochemical, photophysical, and photochemical studies of new RuIIRhIII complexes afforded a deeper understanding of the molecular processes important for the design of new complexes applicable to solar fuel production schemes.
Ph. D.

Développer de nouvelles sources d’énergie respectueuses de l’environnement est un des enjeux majeur de nos sociétés développées. Pour espérer la pérennité de notre espèce sur cette planète, il est indispensable de développer les sources d'énergie renouvelable ; permettant de nous affranchir de la dépendance aux énergies fossiles polluantes et dont les stocks s’épuisent. Il appartient aux scientifiques d’apporter leurs contributions à cet important défi que l’on appelle la transition énergétique et pour ça d’aider à développer une énergie idéale qui ne produirait pas de déchet polluant, serait très efficace et largement disponible. L'énergie solaire représente un excellent candidat car elle est de loin la plus abondante et prometteuse source d’énergie propre. D'importants efforts sont donc menés pour développer les technologies solaires, notamment la photosynthèse artificielle.La photosynthèse artificielle a vu le jour il y a une centaine d’années et fait l’objet de beaucoup d’intérêt et de recherche. Cette technologie cherche à imiter la photosynthèse naturelle réalisée par les plantes; et cela afin de stocker l’énergie provenant du Soleil dans des composés utilisables par l’Homme. La photosynthèse artificielle consiste en l’élaboration de systèmes synthétiques capables sous impulsion lumineuse de réaliser la décomposition de l’eau de manière catalytique, pour générer du dihydrogène ou des produits issus de la réduction du CO2, que l’on appelle combustibles solaires car à haut potentiel énergétique. En effet, la photosynthèse débute par la photo-catalyse de l’oxydation de l’eau, qui permet d’extirper les électrons et les protons des molécules d’eau. Ce sont ces électrons et protons qui seront utilisés par un catalyseur pour produire les combustibles solaires.Depuis peu, une véritable volonté de comprendre les mécanismes qui ont lieu lors de ces réactions catalysées semble apparaitre. Ces réactions mettent en jeu des transferts électroniques multiples photo-induits et cela rend leur étude assez compliquée. Grâce à des avancées technologiques importantes, nous avons étudié de manière plus approfondies plusieurs systèmes photo-catalytiques afin d’en tirer des savoirs permettant de rationaliser le design et d’améliorer les capacités des futurs systèmes développés. Ces avancées techniques ont été possibles grâce à des collaborations interdisciplinaires entre des chimistes et des physiciens et ont permis de développer un montage d’absorption transitoire « double-pump» afin de caractériser les espèces transitoires formées et de retracer les mécanismes lors de deux transferts électroniques photo-induits successifs.Dans la seconde partie de ce travail, de nouveaux catalyseurs ont été développé pour la réaction de photo-catalyse de l’oxydation de l’eau. La majorité des études menées jusqu’ici sur le sujet ont porté sur des systèmes moléculaires, mais le manque de robustesse et de réutilisabilité des catalyseurs homogènes a poussé la recherche vers le domaine des matériaux. Ainsi depuis une quarantaine d’années des systèmes photo-catalytiques hétérogènes ont été développé. Nous avons explorés deux types de matériaux, des nanoparticules catalyseurs dans des systèmes photo-catalytiques, et des polymères qui à eux seuls sont capables de réaliser l’ensemble des fonctions nécessaires à la photo-catalyse d’une réaction telle que l’oxydation de l’eau sous irradiation de lumière visible.Ainsi au cours de cette thèse nous avons tenté par deux approches d’avancer les connaissances et le développement de la photosynthèse artificielle. Une solution encore peu développée au problème énergétique auquel notre société fait face est le recours aux combustibles solaires, et il est grand temps que la recherche avance et que la transition énergétique s’impose plus efficacement et largement
Developpment of environment-friendly sources of energy is one of the stakes major for our societies. To hope for the sustainability of Humans on Earth, it is essential to change our consumer habits on energetics by breaking our dependance on fossil fuels, which use leads to ecological desasters and which stocks are running out. The key of this important challenge is the growth of renewable energy sources, and this is called energy transition. The ideal energy would not produce any polluting waste, would be efficient and widely available. Solar energy is an excellent candidate because it is by far the most abundant and promising source of clean energy. Thus, important efforts are made to developp the solar technologies, including artificial photosynthesis.Artificial photosynthesis was created a century ago and is the focus of many interests and researchs. This technology aims at mimicking the natural photosynthesis realized by plants ; and that in order to store the energy coming from the Sun irriadiation in compounds that can be used at demand. Artificial photosynthesis consists in the elaboration of synthetic systems able under light impulsion to realize the water splitting/decomposition reactions in a catalytique way, generating hydrogène or CO2 reduction products, which are called solar fuels thanks to their high energetic potentials. Indeed, photosynthesis begins with the photo-catalysis of water oxidation, which extirpates the electrons and protons of water molecules. And it is these electrons and protons which will be used to produce the solar fuels.Recently, a real commitment to understand deaply the mechanisms that take place during these catalysed reactions seems to appear. These transformations involve multiple photo-induced electron transfers and it returns their study relatively complicated. Thanks to technological breakthroughs, we studied in a thorough way several photocatalytic systems to draw knowledges ; allowing the rationalisation of the design and then the efficiency improvement of future developped systems. These technical advances were possible thanks to interdisciplinary collaborations between chemists and physicists and led to the developpment of a set-up of « double-pump » transient absorption, that enables to characterize the transient species formed and to track down the pathways during two successive photoinduced electron transfers.In the second part of this work, new catalysts were developped for the photocatalysis of water oxidation reaction. The big majority of the studies led so far on this subject concerned molecular systems, but the lack of robustness and reusability of homogeneous catalysts pushed the research towards materials area. Since about forty years, heterogeneous systems were developped for photocatalysis of several reactions. We explored two types of materials, nanoparticules as catalyst in photocatalytic systems ; and polymers that are able on their own to realize all the functions required for the photocatalysis of a reaction such as water oxidation under visible light irradiation.Thus, during this PhD we tried by two approaches to increase the knowledges and the development of artificial photosynthesis. A solution that is still under-developped to fix the energetic issue our society is facing to, is the use of solar fuels ; and it’s imperative for the research to move forward and that energy transition prevails more effectively and widely

La photocatalyse est une voie de synthèse prometteuse de l’hydrogène comme carburant solaire. La production photocatalytique est un moyen, à la fois de stocker l’énergie solaire sous forme d’énergie chimique et de produire des carburants de manière renouvelables en utilisant l’eau ou des alcools biosourcés comme matière première. L’objectif de cette thèse est d’étudier la déshydrogénation photocatalytique d’alcools à l’aide de sulfures de métaux de transitions, supportés sur TiO2 (MSx/TiO2). Ces sulfures de métaux de transitions ont des propriétés d’activation de l’hydrogène, des propriétés électrochimiques et des propriétés optiques intéressantes. Une série de sept MSx/TiO2 (M = Co, Ni, Cu, Mo, Ru, Ag, Hg) ont été étudiés. La réaction de déshydrogénation photocatalytique du propan-2-ol est utilisée comme réaction modèle. Des corrélations sont établies entre les propriétés intrinsèques de ces MSx/TiO2 et leur activité photocatalytique. De plus, la mesure d’énergie d’activation d’apparente apporte une compréhension supplémentaire sur les mécanismes photocatalytiques. Cette dernière montre que la production photocatalytique d’hydrogène est principalement limitée par les phénomènes de séparation et de transfert de charges dans les photocatalyseurs. Ainsi, une méthodologie combinant la spectroscopie de photoélectrons UV et la spectroscopie d’absorption UV-Visbile a été mis en place pour déterminer la structure électronique des poudre photocatalytiques. Ce travail conclue sur le caractère central de la structure électronique en photocatalyse. Dans le cas du photocatalyseur RuS2/TiO2, le transfert électronique est l’étape cinétiquement déterminante pour la déshydrogénation photocatalytique du propan-2-ol
Photocatalysis is a promising way to synthesize H2 as a solar fuel. On one hand, the photocatalytic H2 production stores solar energy under chemical energy. On the other hand, it produces H2 with a renewable process using water and bio-based alcohols as a feedstock. This Ph.D thesis aims to study the photocatalytic dehydrogenation of alcohols with transition metal sulfides supported on TiO2 (MSx/TiO2). Those transition metal sulfides have versatile and highly tunable properties. They can activate H2, they have promising electrochemical behavior and optical properties. Seven MSx/TiO2 (M = Co, Ni, Cu, Mo, Ru, Ag, Hg) are therefore studied. The photocatalytic dehydrogenation of propan-2-ol is used as a model reaction. Structure-activity relationships are found between the intrinsic properties of the MSx/TiO2 and their photocatalytic activity. Measuring an apparent activation energy provides additional mechanistic insights. It shows that the photocatalytic production of hydrogen is mostly limited by the charge carrier separation and by the electronic transfer. Therefore a method combining the UPS and the UV-Visbile absorption spectroscopies has been develop to establish the electronic structure of photocatalytic powders. This work concludes that the electronic structure plays a crucial role in photocatalysis. With RuS2/TiO2 photocatalyst, the electron transfer is evidenced as the rate-determining step of the photocatalytic dehydrogenation of propan-2-ol

Greenhouse gas (GHG) emissions from liquid fuel consumption account for nearly one third of the global anthropogenic emissions. Sustainable carbon-neutral fuel production is imperative to meet the global GHG emissions reduction targets set by the Paris Agreement. Thermochemical conversion of biomass can provide carbon-neutral high calorific-value fuels for subsequent conversion to liquid fuels. Supercritical water gasification (SCWG) is such a process, and converts wet biomass and carbonaceous waste. Compared to conventional gasification, SCWG offers more flexibility in terms of feedstock, lower char/tar formation, higher yield, and reduced feedstock-drying costs. It can be argued that algae are the ideal feedstocks for the SCWG process. Algae, as a renewable biomass source, is not a seasonal crop, has a high growth rate, can be cultivated even in brackish water, and contains high fixed carbon. Integrating the SCWG process with a concentrated solar thermal (CST) heat source offers a renewable and carbon-free replacement to traditional routes, with higher energy efficiency and syngas yield. Production of syngas via SCWG of algae typically results in methane which requires a reforming step before the syngas is suitable for downstream Fischer-Tropsch (FT) or methanol synthesis (MS) process. There is a lack of analysis of the system-level challenges and economic feasibility of these concepts in the literature. Motivated by this research gap, this thesis investigates the techno-economic performance of the proposed solar fuel plant. Steady-state physical models of the upstream solar-SCWG-reforming plant at a fixed CST input of 50 MWth are developed in Aspen Plus software. The methane reforming techniques considered are steam methane reforming, autothermal reforming and partial oxidation/dry reforming. In order to obtain a compatible composition of syngas for a downstream application, two scenarios are evaluated here: (i) discarding carbon in the form of carbon dioxide from the SCWG reactor, and (ii) supplying renewable hydrogen from PV-electrolysis into the algae-derived syngas. Optimal process parameters are determined through an exergy-based optimisation of the integrated plant. The subsequent conversions of solar-syngas to synthetic gasoline/diesel and to methanol are modelled in Aspen Plus software. In order to mitigate the influence of solar fluctuations, there is an on-site syngas storage acting as a buffer between the upstream gasification and downstream FT/MS units. To explore the dynamic behaviour of the plant under the variable solar resource throughout the year, a system-level energy model is developed using polynomial performance curves of the gasification/FT/MS units along with control logic for cut-off points, syngas storage dynamics, and predictive dispatch. The cost analysis of the captured scenarios includes the system capital, operating and maintenance costs, from which the levelised cost of fuel (LCOF) is calculated. Single- and multi-objective genetic algorithms have been formulated to investigate the economic and overall system performance objectives individually and collectively, and to speculate how each impacts the optimal design of the considered configurations. There is a detailed uncertainty analysis with respect to key economic parameters and ramping times of SCWG-reforming and FT/MS reactors at the optimal design of the plant from the economic and system performance perspectives. A range of optimal configurations are found, which vary greatly based on the available cost of renewable hydrogen for both gasoline/diesel and methanol processes. Although the LCOF of the proposed solar fuel plant is relatively high compared with conventional petroleum-based fuels, further opportunities to lower the LCOF are foreseen through cost savings in algae farming and hydrogen production, upscaling the solar field, and moving the plant to an area of higher DNI.

In order to realize energy independence and substantially combat global climate change, renewable and sustainable energy technologies must be developed. Solar energy is the most readily abundant, and if converted into a chemical fuel, could be stored and transported easily. Solar-driven thermochemical cycling is a method of chemical fuel production that shows great promise, but current state-of-the-art systems have very low efficiencies. This work discusses new reactor designs and cycling techniques using nonstoichiometric oxides that will enable more efficient solar to fuel energy conversion. Practical aspects of the reactor design are explored – specifically, thermochemical expansion of the reactive oxide, and morphologies aimed at enhancing the reaction kinetics. Additionally, doped fluorite- and perovskite-structured materials are evaluated for thermodynamic behavior and in-situ thermochemical cycling performance. Oxide morphology and new doped compounds show little improvement over previously established neat ceria due to thermodynamic limitations. The thermodynamic limit is explored in new reactor geometries and is shown to demonstrate significantly more efficient fuel production. Finally, different nonstoichiometry thermodynamics are explored to provide guidance for further material exploration, as well as applicable methodologies.

The overall solar-to-fuel efficiency of the synthesis gas production via solar-driven thermochemical splitting of CO2 and H2O reactions is highly dependent on the energy required to break down the strong molecules such as CH4, CO2 and H2O. To maximize the syngas production yields, designing new redox materials and optimizing the reactor designs and receiver models are of great importance. Redox materials mediate the thermochemical process by exchanging oxygen with the reactant gases and their performance is mainly assessed by the oxygen exchange capacity, syngas yields and structural stability. In this thesis, a range of redox materials including LaSrMnO3 perovskites and cerium-vanadium mixed/doped metal oxides are studied for syngas production via cyclic H2O and CO2 splitting coupled with methane partial oxidation and high temperature inert gas reduction. The effects of reducing atmospheres such as Ar and CH4 on the structural features in LaxSr1-xMnO3 perovskites are investigated. The La0.5Sr0.5MnO3 powders composed of nano-crystalline particles are considered as the best performing Perovskites with premium structural stability and a 117% higher initial syngas production rate than that of pure SrMnO3 and LaMnO3 structures. The overall syngas production rates are 9 times faster during the chemical looping reforming of methane when compared to those of inert gas reduction. It is demonstrated that lanthanum incorporation prevents the structural breakdown caused by CH4 and up to 65-100% of the initial perovskite structure is regenerated. Notably, H2 purity of up to 93% is achieved by lanthanum-rich LSM structures during the H2O splitting redox cycles coupled with an efficient methane reforming reaction. These findings provide a robust set of physiochemical properties of LaSrMnO3 systems that can be utilized for enhanced solar fuel production via thermochemical redox cycles. The effects of vanadium (V) and cerium (Ce) concentrations (each varying in the 0-100% range) in CeO2-CeVO4 mixed-phase, Ce4+-doped V2O5 and V5+-doped CeO2 redox materials are explored for synthesis gas production via thermochemical redox cycling of CO2 and H2O splitting coupled to methane partial oxidation reactions. In particular, an optimum mixture of CeO2 and CeVO4 is achieved by 25 wt% of vanadium incorporation in the CeO2 powders, which produce up to 68% higher syngas yields than that of pure ceria. It is observed that V5+ provides more reducing states for the hydrocarbon oxidation, while cerium ions act as an oxygen buffer for the re-oxidation reaction. Notably, doping of vanadium increases the cycle capacity of ceria by 400% and the activation temperature of the methane reforming reaction is lowered by up to 178C, while doping the V2O5 lattice with large cerium cations results in a V2O5-to-V2O3 phase transition and produces up to 100 times higher syngas production rates when compared to the pure V2O5. Finally, these findings suggest that a facile combination of the extraordinary catalytic properties of vanadia and superior oxygen ion mobility of ceria can be a powerful approach for an efficient and effective solar thermochemical fuel production.

Two-step solar thermochemical water splitting is a promising pathway for renewable fuel production due to its potential for high solar-to-fuel efficiency via full-spectrum sunlight utilization. However, such a promise critically relies on simultaneous innovation in the redox materials and the reactor systems that utilize them. The present research aims to gain a fundamental understanding of the process thermodynamics and transport phenomena of the solar thermochemical systems. This will help guide material development and reactor design towards achieving an unprecedentedly high solar-to-fuel thermal efficiency. With more materials and reactors being developed, thermodynamic analysis serves as a critical starting point to explore the maximum efficiencies of their various combinations. Materials under study are the state-of-the-art metal oxides including pure ceria, Zr-doped ceria and doped lanthanum manganite perovskites, while reactors of interest are the conventional and membrane counterflow types. Previous studies typically employed a simplified equilibrium approach that could overpredict the fuel output as well as the thermal efficiency. Herein, a revised model is developed to offer more accurate performance upper limits based on the first and second laws of thermodynamics. It is found that the conventional counterflow reactor system is far more efficient than the membrane type, while pure and Zr-doped ceria outperform the perovskites under most scenarios. In addition, the effects of hypothetical materials are investigated in order to guide future material design. A global efficiency map is presented for all redox materials, revealing important tradeoffs due to competing effects such as thermodynamic favorability, heat losses, sweep gas and oxidizer supply, as well as metal oxide preheating. An optimal material regime is thus identified for a set of system conditions, leading to peak efficiency that could reach 46%. The conventional counterflow reactor is further studied from the transport phenomena viewpoint to offer more realistic performance. A single tube reactor composed of a downward particle flow against an upward inert gas flow is employed as the model system with ceria reduction being the model reaction. Coupled phenomena of mass and momentum transfer as well as chemical kinetics are simulated based on Euler-Lagrange approach in the dilute particle flow regime under isothermal operation. The model predicts the reduction extent under a variety of design and operational conditions, with the critical conversion-limiting factors also being identified. This numerical work corroborates the first-stage thermodynamic counterflow model and paves the way for the development of a two-phase heat transfer model in the near future.

abstract: Developing a system capable of using solar energy to drive the conversion of an abundant and available precursor to fuel would profoundly impact humanity's energy use and thereby the condition of the global ecosystem. Such is the goal of artificial photosynthesis: to convert water to hydrogen using solar radiation as the sole energy input and ideally do so with the use of low cost, abundant materials. Constructing photoelectrochemical cells incorporating photoanodes structurally reminiscent of those used in dye sensitized photovoltaic solar cells presents one approach to establishing an artificial photosynthetic system. The work presented herein describes the production, integration, and study of water oxidation catalysts, molecular dyes, and metal oxide based photoelectrodes carried out in the pursuit of developing solar water splitting systems.
Dissertation/Thesis
Ph.D. Chemistry 2013

Direct solar energy conversion is one of few sustainable energy resources able to wholly satisfy global energy demand; however, utility scale adoption and reliance are currently limited by the lack of a cost effective energy storage technology. The production of fuel from sunlight (solar fuels) enables solar energy storage in chemical bonds, a volumetrically and gravimetrically dense form compatible with current infrastructure worldwide. Hydrogen production via water splitting is a first generation solar fuel targeted herein that is currently used for hydrocarbon up-grading and fertilizer production and could further be utilized in combustion cycles and/or fuel cells for electricity and heat production and transportation.

This thesis presents achievements that form the foundation for Si microwire array based solar water splitting devices beginning with a tandem junction device design using Si microwire arrays as the architectural motif and one of many active components. Si microwire arrays have potential advantages over two dimensional planar device architectures such as minimized resistance losses, lower semiconductor material usage, and embedment in a polymeric membrane enabling a flexible device.

Experimental fabrication and characterization of this tandem junction device design was realized in the form of a np⁺-Si microwire array coated by either tungsten oxide (WO₃) or titanium dioxide (TiO₂) as the second tandem semiconductor. The Si/TiO₂ device demonstrated the highest performance with an expected solar-to-hydrogen efficiency of 0.39%. To achieve these demonstrations new processing methods were needed and developed for formation of the np⁺-Si microwire array homojunction and formation of a low resistance contact between the p⁺-Si and second semiconductor using sputtered tin- doped indium oxide (ITO) and spray pyrolyzed fluorine-doped tin oxide (FTO).

Another achievement includes demonstration of the longest known (>2200 hours) photoanode stability for water oxidation using a np⁺-Si microwire array coated with an in-house developed amorphous TiO₂ protection layer and NiCrO_x electrocatalyst. Additionally, the Si microwire array architecture was used to enable decoupling of semiconductor light absorption and catalytic activity, two performance metrics that ideally are maximized simultaneously. However, all previous demonstrations have shown anti-correlation between these performance metrics because planar architectures are subject to a trade-off where adding electrocatalyst increases catalytic activity, but decreases semiconductor light absorption and vice versa.

Finally, a techno-economic analysis of solar water splitting production facilities was performed to assess economic competitiveness because this is the ultimate metric by which all energy production technologies are currently evaluated. This analysis suggests that a hydrogen production facility that is cosmetically similar to current solar panel installations with hydrogen collection from distributed tilted panels is unlikely to achieve cost competitiveness with fossil fuel derived hydrogen due to the balance of systems costs alone. A cost of CO₂ greater than ~$800 (ton CO₂)^-1 was estimated to be necessary for the least expensive base-case solar-to-hydrogen system to reach price parity with hydrogen derived from steam reforming of methane priced at $3 (MM BTU)^-1 ($1.39 (kg H₂)^-1). Direct CO₂ reduction systems were also explored and resulted in even larger challenges than hydrogen production. Accordingly, major facility wide breakthroughs are required to obtain viable economic costs for solar hydrogen production, but the barriers to achieve cost-competitiveness with existing large-scale thermochemical processes for CO₂ reduction are even greater.

Immobilization of photosynthetic cultures has been used to generate biofuels and high value compounds through direct conversion of CO2 and water using sunlight. Compared with suspended cultures, immobilized bacteria can achieve much higher densities resulting in greater areal productivity. Limitations exist however, on the density that can be reached without compromising access to light and other nutrients. In this thesis an optofluidic approach to overcoming the challenge of light delivery to high density cultures of cyanobacteria is described and proof of concept experiments presented. This approach uses optical waveguides to deliver light to cells through bacterial interaction with the evanescent field and is tailored to meet each cell's need for light and nutrients. Experiments presented here demonstrate biofilm proliferation in the presence of evanescent fields. Illumination of surfaces by surface plasmon enhanced evanescent fields is also shown to be an effective and potentially useful technique to grow biofilms within optofluidic architectures.

Artificial photosynthesis is regarded as the best way to protect the environment while producing carbon-based fuels, because it closes the anthropogenic carbon cycle. Herein we simulate a Photovoltaics-Electrochemical (PV-EC) system capable of converting CO2 into usable carbon-based fuels, in order to analyse the implementation of synergetic techniques such as intermediate electronic regulation and thermal coupling on the improvement of the energetic performance. We proved that, when thermally coupled, the two cells of the system exhibit a symbiotic behaviour: the solar-to-fuel efficiency stays almost temperature-independent due to improved reaction kinetics which compensates for photovoltaic thermal losses. The electronic regulation is equally important to enhance efficiency because it guarantees that we make use of the full PV power output to the EC load. These solutions are tested in two pathways for methane production: 1-step, CO2→CH4, and 2-step, CO2→Syngas→CH4, exhibiting solar-to-fuel efficiency gains up to 586% and 43%, respectively, when compared with the systems without both the thermal coupling and the DC-to-DC converter. Lastly, an energetic comparison of the two pathways was made. The direct production (1-step) of methane showed to provide 20% less energy than the second path, where syngas is produced and converted to methane through a Fischer-Tropsch synthesis at 350 °C and 10 atm.

Solar energy has the potential to meet all of society’s energy demands, but challenges remain in storing it for times when the sun is not shining. Electrolysis is a promising means of energy storage which applies solar-derived electricity to drive the production of chemical fuels. These so-called solar fuels, such as hydrogen gas produced from water electrolysis, can be fed back to the grid for electricity generation or used directly as a fuel in the transportation sector. Solar fuels can be generated by coupling a photovoltaic (PV) cell to an electrolyzer, or by directly converting light to chemical energy using a photoelectrochemical cell (PEC). Presently, both PV-electrolyzers and PECs have prohibitively high capital costs which prevent them from generating hydrogen at competitive prices. This dissertation explores the design of membraneless electrolyzers and PECs in order to simplify their design and decrease their overall capital costs. A membraneless water electrolyzer can operate with as few as three components: A cathode for the hydrogen evolution reaction, an anode for the oxygen evolution reaction, and a chassis for managing the flows of a liquid electrolyte and the product gas streams. Absent from this device is an ionically conducting membrane, a key component in a conventional polymer electrolyte membrane (PEM) electrolyzer that typically serves as a physical barrier for separating product gases generated at the anode and cathode. These membranes can allow for compact and efficient electrolyzer designs, but are prone to degradation and failure if exposed to impurities in the electrolyte. A membraneless electrolyzer has the opportunity to reduce capital costs and operate in non-pristine environments, but little is known about the performance limitations and design rules that govern operation of membraneless electrolyzers. These design rules require a thorough understanding of the thermodynamics, kinetics, and transport processes in electrochemical systems. In Chapter 2, these concepts are reviewed and a framework is provided to guide the continuum scale modeling of the performance of membraneless electrochemical cells. Afterwards, three different studies are presented which combine experiment and theory to demonstrate the mechanisms of product transport and efficiency loss. Chapter 3 investigates the dynamics of hydrogen bubbles during operation of a membraneless electrolyzer, which can strongly affect the product purity of the collected hydrogen. High-speed video imaging was implemented to quantify the size and position of hydrogen gas bubbles as they detach from porous mesh electrodes. The total hydrogen detected was compared to the theoretical value predicted by Faraday’s law. This analysis confirmed that not all electrochemically generated hydrogen enters the gas phase at the cathode surface. In fact, significant quantities of hydrogen remain dissolved in solution, and can result in lower product collection efficiencies. Differences in bubble volume fraction evolved along the length of the cathode reflect differences in the local current densities, and were found to be in agreement with the primary current distribution. Overall, this study demonstrates the ability to use in-situ HSV to quantitatively evaluate key performance metrics of membraneless electrolyzers in a non-invasive manner. This technique can be of great value for future experiments, where statistical analysis of bubble sizes and positions can provide information on how to collect hydrogen at maximum purity. Chapter 4 presents an electrode design where selective placement of the electrocatalyst is shown to enhance the purity of hydrogen collected. These “asymmetric electrodes” were prepared by coating only one planar face of a porous titanium mesh electrode with platinum electrocatalyst. For an opposing pair of electrodes, the platinum coated surface faces outwards such that the electrochemically generated bubbles nucleate and grow on the outside while ions conduct through the void spacing in the mesh and across the inter-electrode gap. A key metric used in evaluating the performance of membraneless electrolyzers is the hydrogen cross-over percentage, which is defined as the fraction of electrochemically generated hydrogen that is collected in the headspace over the oxygen-evolving anode. When compared to the performance of symmetric electrodes – electrodes coated on both faces with platinum – the asymmetric electrodes demonstrated significantly lower rates of cross-over. With optimization, asymmetric electrodes were able to achieve hydrogen cross-over values as low as 1%. These electrodes were then incorporated into a floating photovoltaic electrolysis device for a direct demonstration of solar driven electrolysis. The assembled “solar fuels rig” was allowed to float in a reservoir of 0.5 M sulfuric acid under a light source calibrated to simulate sunlight, and a solar to hydrogen efficiency of 5.3% was observed. In Chapter 5, the design principles for membraneless electrolyzers were applied to a photoelectrochemical (PEC) cell. Whereas an electrolyzer is externally powered by electricity, a PEC cell can directly harvest light to drive an electrochemical reaction. The PEC reactor was based on a parallel plate design, where the current was demonstrated to be limited by the intensity of light and the concentration of the electrolyte. By increasing the average flow rate of the electrolyte, mass transport limitations could be alleviated. The limiting current density was compared to theoretical values based off of the solution to a convection-diffusion problem. This modeled solution was used to predict the limitations to PEC performance in scaled up designs, where solar concentration mirrors could increase the total current density. The mass transport limitations of a PEC flow cell are also highly relevant to the study of CO2 reduction, where the solubility limit of CO2 in aqueous electrolyte can also limit performance.

As the cost of solar energy continues to drop, the major hurdle limiting the widespread use of intermittent renewable solar energy is the lack of efficient and cost-effective energy storage. Electrochemical technologies, such as electrolyzers, photoelectrochemical cells, and fuel cells, have the potential to compensate for solar energy intermittency on a large scale, by converting excess solar energy into storable solar fuels, such as hydrogen (H2), which can be converted back to electrical energy at a later time. However, improvements in the efficiency and lifetime of these technologies, in particular the electrocatalysts, are necessary for their commercialization. During operation, efficiency losses result from energetic penalties (overpotentials) associated with several processes occurring at or near the electrocatalyst/electrolyte (ohmic resistance, kinetic barriers, and mass transport limitations). These losses can be further exacerbated due to electrocatalyst durability issues such as dissolution, agglomeration, detachment, and poisoning. A major challenge in electrocatalysis field is developing methods to mitigate these losses without adversely affecting the electrocatalytic stability, selectivity, and/or activity. One promising solution is an oxide-encapsulated electrocatalyst architecture, which has been shown to improve electrocatalyst durability and provide mechanisms for controlling reaction pathways. Previous studies on oxide-encapsulated electrocatalysts, in which metal catalysts are fully or partially covered by ultrathin layers of permeable oxide films, have mostly focused on supported nanoparticles because of their high electrochemically active surface area per catalyst loading. However, these nanoparticle-based architectures tend to have poorly defined and/or non-uniform structures which make it difficult to understand and elucidate structure-property-relationships. This dissertation investigates well-defined oxide-coated electrocatalysts, which serve as model platforms for gaining a fundamental understanding of kinetic and transport phenomena that underlie their operation. This dissertation presents three studies which highlight the versatile functionalities of oxide-encapsulated electrocatalysts to improve the electrocatalyst stability, selectivity, and activity in different electrochemical systems. This dissertation demonstrates the ability of room temperature synthesized silicon oxide (SiOx)-encapsulated Pt electrocatalysts to: i) stabilize nanoparticles and improve electron transfer, ii) mitigate catalyst poisoning and control reaction pathways through selective transport, and iii) alter reaction energetics associated with catalysis at the buried interface. First, this dissertation establishes the ability of room temperature synthesized SiOx coatings to stabilize nanoparticle electrocatalysts by mitigating electrocatalyst migration, coalescence, and detachment on metal-insulator-semiconductor (MIS) photoelectrodes for solar-driven water splitting. Metallic Pt nanoparticles are inherently unstable on the insulating support due to poor physical adhesion and electronic coupling between Pt and SiO2. To overcome this issue, a room temperature UV ozone synthesis process was used to deposit 2-10 nm thick SiOx overlayers on top of electrodeposited Pt nanoparticles to stabilize Pt on the electrode surface. The photoelectrodes containing oxide-encapsulated electrocatalysts exhibit superior durability and electron transfer (ohmic) properties compared to the photoelectrode that lacked the SiOx encapsulation. While this study demonstrates that the oxide-encapsulated electrocatalyst architecture improves the stability of electrocatalytic nanoparticles deposited on insulating materials, it does not elucidate how reactants and products transport through the SiOx barrier to reach the Pt surface. In order to gain a better understanding of kinetic and transport phenomena that govern performance of oxide-encapsulated electrocatalysts, the following studies investigate model electrodes consisting of continuous SiOx overlayers of uniform thickness deposited onto smooth Pt thin films. This planar electrode geometry allows for simple and unambiguous characterization of structure-property relationships. The next study systematically evaluates the influence of SiOx thickness on the HER performance to understand species transport through SiOx. Through detailed characterization and electroanalytical tests, it is shown that proton and H2 transport occur primarily through the SiOx coating such that the HER occurs at the buried Pt|SiOx interface. Importantly, the SiOx nanomembranes were found to exhibit high selectivity for proton and H2 transport compared to Cu2+, a model HER poison. Leveraging this property, it is shown that SiOx–encapsulation can enable poison-resistant operation of Pt HER electrocatalysts. This oxide-encapsulated architecture offers a promising approach to enhancing electrocatalyst stability while incorporating advanced catalytic functionalities such as poison resistance or tunable reaction selectivity. The final study demonstrates ability of SiOx overlayers to alter reaction energetics associated with catalysis at the buried interface. Carbon monoxide (CO), methanol, and ethanol oxidation reactions are studied for their relevance in direct alcohol fuel cell applications. Oxide-supported catalysts have been shown to enhance alcohol oxidation by promoting CO oxidation at metal/oxide interfacial regions through the so-called bifunctional mechanism, in which hydroxyls on the oxide facilitate the removal of adsorbed CO−intermediates from active sites. A key advantage of the oxide-encapsulated electrocatalyst design compared to oxide–supported nanoparticles is that the former maximizes the density of metal/oxide interfacial sites. This study shows that the SiOx overlayer provides proximal hydroxyls, in the form of silanol groups, which can enhance CO and alcohol oxidation through unique interactions at the buried Pt|SiOx interface. Overall, this dissertation highlights the potential of using oxide-encapsulated electrocatalysts for stable, selective, and efficient electrochemical production and use of solar fuels.

Sunlight is one of the few renewable resources that can meet global energy demand. Unfortunately, while solar energy has grown in the past few years, several economic and scientific constraints have hindered mass adoption. One of the main obstacles solar energy faces is the lack of economically competitive storage technologies. Artificial photosynthesis is a potential solution in which solar energy is directly converted into energy dense chemical bonds that can be easily stored and transported.

One impediment facing the commercialization of artificial photosynthesis is the use of expensive and rare precious metals as catalysts. This dissertation focuses on the achievements of the past five years in characterizing novel, earth-abundant, acid-stable hydrogen evolution catalysts. While nickel alloys have long been known as catalysts for the hydrogen evolution reaction in basic media, it has only been in the past decade that earth abundant catalysts that are stable in acidic media have been reported. These discoveries are critically important as the many proposed artificial photosynthetic devices require the use of acidic media.

In this dissertation we examine two families of hydrogen evolution catalysts: transition metal chalcogenides (namely molybdenum and cobalt selenide) as well as transition metal phosphides (cobalt phosphide). In addition to the electrochemical characterization of these catalysts, spectroscopic characterizations were performed in order to carefully examine the chemical compositions of these catalysts before, after and during the hydrogen evolution reaction. This analysis elucidated both chemical, and structural changes that occurred after the catalysts had been subject to the hydrogen evolution reaction conditions.

The final chapter in this thesis delves into the techno-economic realities of energy transportation via different fuels. Due to the strong interest in renewable energy, several future energy transportation scenarios, including 100% grid electrification and widespread installation of hydrogen pipelines, have been proposed. In order to get a fuller understanding of such potential infrastructure alternatives, we report their differing energy transportation costs.

Energy storage by chemical looping steam/dry reforming is a promising alternative for the utilization of solar energy in the industrial and transport sectors. Efficient oxygen carriers with a facile and scalable synthesis method are crucial to achieve economic competitiveness for this solar thermochemical process. In this thesis, a comprehensive overview of solar chemical looping reforming is provided and the state of the art research of its associated oxygen carriers is discussed. Improvement in syngas yields and production rates in solar chemical looping reforming were then explored via morphological and structural enhancements of the oxygen carriers. Firstly, the impact of ceria structural features on its syngas production performance during two-step isothermal redox cycles for four different nano and micro morphologies was investigated. Highly porous flame-made agglomerates composed of small crystalline particles were determined as the best performing morphology with initial production rates of H2 and CO up to 167% higher than that of commercial sub-micro ceria. Upon 10 isothermal redox cycles at 1173 K, these flame-made structures still maintained at up to 57% faster production rates. It was shown that the high porosity of the flame-made agglomerates was important in inhibiting sintering and grain growth. Notably, higher specific surface area flower-like morphologies collapsed and densified rapidly, and exhibited the slowest kinetics. These findings provide a robust set of structural properties to engineer efficient materials for enhanced solar fuel production by high temperature thermochemical cycles. Secondly, a first-time investigation of using an earth-abundant manganese-based oxygen carrier in solar chemical looping dry methane reforming was demonstrated. It revealed a manganese carbide/oxide redox cycle that resulted in high mass-specific syngas yields and production rates when the oxygen carrier's matrix was incorporated with fractional amount of cerium ions. In particular, 15 times higher CO2 splitting rates than the undoped manganese oxide, and also 8 times higher CO yields than cerium oxide was achieved. The long-term performance with 100 cycles revealed that this is not a short-lived enhancement and that the synergetic contribution by cerium ions were highlighted. A thorough investigation of this manganese carbide/oxide redox mechanism was experimentally pursued further with a series of custom-synthesized Ce-Mn oxygen carriers via solar chemical looping steam methane reforming cycles. Interesting discoveries of 3% Ce suggest the intense surface distribution of ceria-rich nanoparticles that efficiently dissociate the chemisorbed species into H2 and CO during methane reforming and water splitting. Additionally, the abundance of Ce ions in the bulk lattice effectively unlock the oxygen carrier’s reversible diffusion of oxygen and carbon in this vacancy-based redox mechanism.

To mitigate the emissions from the widely studied and even applied coal to FT liquid (FTL) fuels systems, two kinds of promising renewable energy, biomass and solar energy, have been proposed and assessed as a partial or total substitute for coal feed. The concept of a solar hybridized FTL fuels production system has the potential to obtain higher productivity with lower greenhouse gas emissions, when compared with a conventional system. However, less attention has been paid to the comprehensive system analysis of this topic. Hence, the aim of the present thesis is to achieve the annual performance of the solar hybridized solid fuels to FTL fuels processes with novel configurations. A novel solar hybridized dual fluidized bed (SDFB) gasification process for FTL fuels production is proposed and investigated in the present thesis for cases with high reactivity solid fuels as the feedstock. The concept offers sensible thermal storage of the bed material and a process that delivers a constant production rate and quality of syngas despite solar variability. As a reference scenario for this concept, the proposed solar hybridized coal-to-liquids (SCTL) process is simulated for the case with lignite as the feedstock using a pseudo-dynamic model that assumes steady state operation at each time step for a one-year, hourly integrated solar insolation time series. For a solar multiple of 3 and bed material storage capacity of 16 h, the calculated annual solar share is 21.8%, assuming that the char conversion in the steam gasification process is 100%. However, the solar share is also found to be strongly dependent on the char conversion in the steam gasification process, so that the solar share is calculated to decrease to zero as the conversion is decreased to 57%. New configurations of the solar hybridized solid fuels (biomass and/or coal) to FTL fuels process are proposed and assessed, which are characterized with a novel SDFB gasifier with char separation, the incorporation of carbon capture and sequestration (CCS) and/or the use of FT reactor tail-gas recycle. Montana lignite and spruce wood have been chosen as the studied coal and biomass, respectively. Assessed using the pseudo-dynamic model, the annual solar share of the SCTL system can be increased from 12.2% to 20.3% by the addition of the char separation, for a char gasification conversion of 80%. To achieve well-to-wheel greenhouse gas emissions for FT liquid fuels parity with diesel derived from mineral crude oil, a biomass fraction of 58% is required for the studied non-solar coal and biomass-to-liquids system with a dual fluidized bed (DFB) gasifier. This biomass fraction can be reduced to 30% by the addition of carbon capture and sequestration and further reduced to 17% by the integration of solar energy with a solar multiple of 2.64 and a bed material storage capacity of 16 h. This reduction of the biomass fraction is very important given that biomass is typically more expensive than coal. As the biomass fraction is increased from 0% to 100%, the specific FT liquids output is decreased from 59.6% to 48.3% due to the increasing light hydrocarbons content. These two outputs (for biomass fractions of 0% and 100%, respectively) can both be increased to 71.5% and 70.9%, respectively, by integrating a tail-gas recycling configuration. Co-gasification of biomass with coal has the potential to further reduce the GHG emission from the SCTL systems, as discussed above. The application of biomass is usually limited by some properties (e.g., high moisture, low heating value and so on), which can be improved by torrefaction, as proved by previous work. Previous work also found that torrefaction can impact the bio-char gasification reactivity. In the present thesis, to better understand the influence of torrefaction on the bio-char gasification reactivity, further investigations were carried out on the char physicochemical characteristics that can influence the gasification reactivity, i.e., the char specific surface area, the char carbonaceous structure and the catalytic effect of inorganic matter in the char. The present experimental investigation showed that the influence of the torrefaction on the char gasification reactivity depended strongly on the biomass species and char preparation conditions. For a pyrolysis temperature of 800 ºC, the gasification reactivity of the chars from both the torrefied grape marc and the torrefied macroalgae were found to be lower than that of the chars from their corresponding raw fuels. This is mainly due to a lower specific surface area and a lower content of alkali metals (sodium and/or potassium) in the chars produced from both the torrefied grape marc and the torrefied macroalgae than for those chars produced from their corresponding raw fuels. However, the opposite influence of torrefaction was found for the macroalgae char when the pyrolysis temperature was increased to 1000 ºC. This is mainly due to a higher sodium concentration and a more amorphous carbonaceous structure for the torrefied macroalgae char than for the raw macroalgae char. In the present thesis, the process modelling results can be used for further economic analysis of the proposed novel configurations of solar hybridized coal and/or biomass to FTL fuels system via an SDFB gasifier. In addition, according to the experimental results of this study, the investigation of the influence of torrefaction on the bio-char characteristics can help to better understand the influence of torrefaction on the bio-char gasification reactivity.
Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Chemical Engineering, 2017

abstract: Thermodynamic development and balance of plant study is completed for a 30 MW solar thermochemical water splitting process that generates hydrogen gas and electric power. The generalized thermodynamic model includes 23 components and 45 states. Quasi-steady state simulations are completed for design point system sizing, annual performance analysis and sensitivity analysis. Detailed consideration is given to water splitting reaction kinetics with governing equations generalized for use with any redox-active metal oxide material. Specific results for Ceria illustrate particle reduction in two solar receivers for target oxygen partial pressure of 10 Pa and particle temperature of 1773 K at a design point DNI of 900 W/m2. Sizes of the recuperator, steam generator and hydrogen separator are calculated at the design point DNI to achieve 100,000 kg of hydrogen production per day from the plant. The total system efficiency of 39.52% is comprised of 50.7% hydrogen fraction and 19.62% electrical fraction. Total plant capital costs and operating costs are estimated to equate a hydrogen production cost of $4.40 per kg for a 25-year plant life. Sensitivity analysis explores the effect of environmental parameters and design parameters on system performance and cost. Improving recuperator effectiveness from 0.7 to 0.8 is a high-value design modification resulting in a 12.1% decrease in hydrogen cost for a modest 2.0% increase in plant $2.85M. At the same time, system efficiency is relatively inelastic to recuperator effectiveness because 81% of excess heat is recovered from the system for electricity production 39 MWh/day and revenue is $0.04 per kWh. Increasing water inlet pressure up to 20 bar reduces the size and cost of super heaters but further pressure rises increasing pump at a rate that outweighs super heater cost savings.
Dissertation/Thesis
Doctoral Dissertation Mechanical Engineering 2018

The rise of Earth’s atmospheric CO2 levels, primarily due to combustion of fossil fuels, has affected its ecosystems. A way to combat this is by mimicking the plants photosynthesis by capturing CO2 from the atmosphere and convert it to usable hydrocarbon fuels, such as methane (CH4), because of the easy adaptability to the well-established infrastructure for natural gas (NG) storage, distribution and consumption. The denominated “solar methane”, very similar to NG, can be produced by converting solar energy from photovoltaic (PV) panels into electricity to power a 1-step reaction on electrochemical flow cell(s), using CO2 and water as the feedstock. Here, we simulate solar methane production and storage and apply it to address the energetic needs of concept buildings that have space and domestic hot water heating requirements. A combination of solar thermal collectors (STCs) and PV panels is optimized for buildings in different European locations, in which the heating needs that cannot be fulfilled by the STCs are satisfied by the combustion of methane synthesized by the PV-powered electrolyzers. Various combinations of situations for a whole year were studied and it was found that this auxiliary system can produce, per m2 of PV area, in the worst case scenario 23.6 g/day (0.328 kWh/day) of methane in Stockholm and in the best case scenario 47.4 g/day (0.658 kWh/day) in Lisbon.