Dissertations / Theses on the topic 'Metal-Air Batteries'

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 67-78).
Metal-air batteries boast high theoretical energy densities, but negative electrode corrosion can severely reduce their usable capacity and commercial utility. Most methods to mitigate corrosion focus on electrode and electrolyte modification such as electrode alloying, electrolyte additives, and gel and nonaqueous electrolytes. These methods, however, either insufficiently suppress the parasitic reaction or compromise power and energy density. This thesis focuses on a different approach to corrosion mitigation involving electrolyte displacement from the electrode surface. Multiple electrolyte-displacement concepts were generated and investigated. The most promising of the concepts was the reversible displacement of the electrolyte from the electrode surface with an oil. To enable this method, the fundamental physics of underwater oil-fouling resistant surfaces was investigated, tested, and characterized. Design equations that aid in the appropriate selection of electrodes, displacing oils, and separator membranes were also developed. The oil displacement method was demonstrated in a primary (single-use) aluminum-air (Al-air) battery that achieved a 420% increase in useable energy density and was estimated to enable pack-level energy densities as high as 700 Wh 1- and 900 Wh kg-1. This method could, in principle, be used in any of the metal-air batteries, aqueous or nonaqueous, or in other energy storage systems that suffer from corrosion if appropriate displacing oils and separator membranes are found using the discussed design principles. With the oil displacement method, aqueous metal-air batteries that rely on abundant, broadly dispersed materials could provide safe, low-cost, sustainable primary and secondary (rechargeable) batteries for many applications including grid-storage, off-grid storage, robot power, and vehicular propulsion.
by Brandon J. Hopkins.
Ph. D.

The rise in wind, solar and tidal renewable power generation presents a new challenge for the future stability of electrical networks on the national and international scale. The modal nature of renewable power and its incompatibility with consumer demand necessitates a means for largescale energy storage with high efficiency and relatively low cost. Zinc-air flow batteries represent one possible solution to this problem. The energy is stored in the metallic zinc, and reversed with the oxidation to form zincate releasing the energy on demand. The majority of energy losses in the zinc-air battery are for the O2 evolution and reduction reactions on the air electrode. A stable, durable and low-cost bi-functional air electrode would allow the introduction of zinc-air flow batteries to support the power grids of the future. The work in this thesis will investigate the activity of NiCo2O4 electrocatalysts prepared by various methods, for their use as bi-functional electrocatalysts in the air-electrode. The electrocatalyst prepared on to a gas diffusion electrode, to determine activity in lab-scale half-cells. Improvements to catalyst activity are then considered through the addition of metal nanoparticles to the surface of NiCo2O4, with in-situ X-ray absorbance measurements to determine the oxidation states of ruthenium during the O2 evolution reaction. The activity of NiCo2O4 was compared to alternative perovskite mixed metal oxide electrocatalysts.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2014.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 93-100).
Electric vehicles ("EVs") require high-energy-density batteries with reliable cyclability and rate capability. However, the current state-of-the-art Li-ion batteries only exhibit energy densities near ~150 Wh/kg, limiting the long-range driving of EVs with one charge and hindering their wide-scale commercial adoption.1-3 Recently, non-aqueous metal-O₂ batteries have drawn attention due to their high theoretical specific energy.2, 4-6 Specifically, the issues surrounding battery studies involve Li-O₂ and Na-O₂ batteries due to their high theoretical specific energies of 3.5 kWh/kg (assuming Li 20 2 as a discharge product in Li-O₂ batteries) and 1.6 and 1.1 kWh/kg (assuming Na₂O₂ and NaO₂ as discharge products, respectively, in Na-O₂ batteries). Since the potential of Li-O₂ batteries as an energy storage system was first proposed in 1996,1 various studies have criticized and verified their shortcomings, such as their low power density, poor cyclability, and poor rate capability. ₇, ₈ Substantial research attempts have been made to identify the cause of the high overpotentials and electrolyte decomposition and to search for better cathode/electrolyte/anode and/or catalyst material combinations. However, Li-O₂ battery technology remains in its infancy primarily due to the lack of understanding of the underlying mechanisms. Therefore, we investigate the charging mechanism, which contributes to the considerable energy loss using first-principles calculations and propose a new charging mechanism based on experimental observations and knowledge concerning Li-ion and Na-ion batteries. Most studies on metal-O₂ batteries have mainly focused on Li-O₂ batteries. However, recently, the promising performance of Na-O₂ systems has been reported.₉, ₁₀ Although Na-O₂ batteries exhibit slightly lower theoretical specific energies than those of the Li-O₂ batteries as specified above, the chemical difference between the two alkali metals substantially distinguishes the electrochemistry properties of Na-O₂ and Li-O₂. In the Na-O₂ system, both NaO₂ and Na₂O₂ are stable compounds, while in the Li-O system, LiO₂ is not a stable compound under standard state conditions (300 K and 1 atm).₁₁, ₁₂ Presumably, due to this chemical difference, the Na-O₂ system has exhibited a much smaller charging overpotential, as low as 0.2 V, when NaO₂ is formed as a discharge product, compared with that in Li-O₂ system, >1 V. Such a low charging overpotential in Na-O₂ batteries demonstrates their potential as a next generation electrochemical system for commercially viable EVs .₉,₁₀ In this thesis, we study the thermodynamic stability of Na-O compounds to identify the phase selection conditions that affect the performance of Na-O₂ batteries.
by ShinYoung Kang.
Ph. D.

The electrocatalysis of oxygen reduction and evolution reactions (ORR and OER, respectively) on the same catalyst surface is among the long-standing challenges in electrochemistry with paramount significance for a variety of electrochemical systems including regenerative fuel cells and rechargeable metal-air batteries. Non-precious group metals (non-PGMs) and their oxides, such as manganese oxides, are the alternative cost-effective solutions for the next generation of high-performance bifunctional oxygen catalyst materials. Here, initial stage electrocatalytic activity and long-term durability of four non-PGM oxides and their combinations, i.e. MnO₂, perovskites (LaCoO₃ and LaNiO₃) and fluorite-type oxide (Nd₃IrO₇), were investigated for ORR and OER in alkaline media. The combination of structurally diverse oxides revealed synergistic catalytic effect by improved bifunctional activity compared to the individual oxide components. Next, the novel role of alkali-metal ion insertion and the mechanism involved for performance promotion of oxide catalysts were investigated. Potassium insertion in the oxide structures enhanced both ORR and OER performances, e.g. 110 and 75 mV decrease in the OER (5 mAcm-²) and ORR (-2 mAcm-²) overpotentials (in absolute values) of MnO₂-LaCoO₃, respectively, during galvanostatic polarization tests. In addition, the stability of K⁺ activated catalysts was improved compared to unactivated samples. Further, a factorial design study has been performed to find an active nanostructured manganese oxide for both ORR and OER, synthesized via a surfactant-assisted anodic electrodeposition method. Two-hour-long galvanostatic polarization at 5 mAcm-² showed the lowest OER degradation rate of 5 mVh-¹ for the electrodeposited MnOx with 270 mV lower OER overpotential compared to the commercial γ-MnO₂ electrode. Lastly, the effect of carbon addition to the catalyst layer, e.g. Vulcan XC-72, carbon nanotubes and graphene-based materials, was examined on the ORR/OER bifunctional activity and durability of MnO₂ LaCoO₃. The highest ORR and OER mass activities of -6.7 and 15.5 Ag-¹ at 850 and 1650 mVRHE, respectively, were achieved for MnO₂-LaCoO₃-multi_walled_carbon_nanotube-graphene, outperforming a commercial Pt electrode. The factors affecting the durability of mixed-oxide catalysts were discussed, mainly attributing the performance degradation to Mn valence changes during ORR/OER. A wide range of surface analyses were employed to support the presented electrochemical results as well as the proposed mechanisms.
Applied Science, Faculty of
Chemical and Biological Engineering, Department of
Graduate

Non-aqueous metal-air (Li-O2 and Na-O2) batteries have been emerging as one of the most promising high-energy storage systems to meet the requirements for demanding applications due to their high theoretical specific energy. In the present thesis work, advanced characterization techniques are demonstrated for the exploration of metal-O2 batteries. Prominently, the electrochemical reactions occurring within the Li-O2 and Na-O2 batteries upon cycling are studied by in operando powder X-ray diffraction (XRD). In the first part, a new in operando cell with a combined form of coin cell and pouch cell is designed. In operando synchrotron radiation powder X-ray diffraction (SR-PXD) is applied to investigate the evolution of Li2O2 inside the Li-O2 cells with carbon and Ru-TiC cathodes. By quantitatively tracking the Li2O2 evolution, a two-step process during growth and oxidation is observed. This newly developed analysis technique is further applied to the Na-O2 battery system. The formation of NaO2 and the influence of the electrolyte salt are followed quantitatively by in operando SR-PXD. The results indicate that the discharge capacity of Na-O2 cells containing a weak solvating ether solvent depends heavily on the choice of the conducting salt anion, which also has impact on the growth of NaO2 particles. In addition, the stability of the discharge product in Na-O2 cells is studied. Using both ex situ and in operando XRD, the influence of sodium anode, solvent, salt and oxygen on the stability of NaO2 are quantitatively identified. These findings bring new insights into the understanding of conflicting observations of different discharge products in previous studies. In the last part, a binder-free graphene based cathode concept is developed for Li-O2 cells. The formation of discharge products and their decomposition upon charge, as well as different morphologies of the discharge products on the electrode, are demonstrated. Moreover, considering the instability of carbon based cathode materials, a new type of titanium carbide on carbon cloth cathode is designed and fabricated. With a surface modification by loading Ru nanoparticles, the titanium carbide shows enhanced oxygen reduction/evolution activity and stability. Compared with the carbon based cathode materials, titanium carbide demonstrated a higher discharge and charge efficiency.

La charge rapide des batteries métal-air représente un des verrous technologiques auquel cette technologie est confrontée. Pour répondre à cette problématique, l'électrolyte est soumis à un écoulement pour évacuer les bulles d’oxygène sources d'affaiblissement de l'efficacité de ces batteries. L’écoulement de l'électrolyte permet une réduction du potentiel des électrodes à dégagement de gaz. L’électrode présente une surface active plus élevée, réduisant son potentiel électrique pour un courant donné. La microscopie optique met en évidence le caractère bimodal de la répartition de la taille de bulles qui tend vers une répartition monomodal lorsque le débit augmente. Ces caractérisations électrochimiques et optiques apportent les informations pour développer un modèle analytique pour la prédiction du comportement dynamique de ces systèmes. Ce modèle est complété par une simulation numérique qui met en évidence les phénomènes oscillatoires mesurés à forts courants. L’optimisation énergétique du procédé est réalisée par le choix d’un débit optimal qui concilie le gain en puissance électrique et les pertes de charges hydrauliques. La diminution des pertes par l'adaptation de la géométrie de la cellule d’écoulement a été abordée. La cellule à configuration triangulaire permet une double optimisation énergétique. Ces cellules ont été testées expérimentalement et présentent de meilleures caractéristiques en termes d’évacuation naturelle et forcée des bulles. Une étude préliminaire et les perspectives de l’effet de l’écoulement sur les dendrites de zinc sont présentées. L'écoulement de l'électrolyte dans la cellule augmente le temps de court-circuit
The fast charge of metal-air batteries represent one of the main scientific and technical challenges facing this technology. Oxygen bubbles formed during the charge process has a negative impact on the performances of the cells. Using flowing electrolyte for the evacuation of oxygen bubbles leads to a decrease of the electric potential of the gas evolving electrodes. For a given current, the electrode has more active surface, decreasing its potential. Optical measurement under microscope shows the bimodal distribution of the bubbles sizes. This repartition trends to a uni-modal distribution when the flow rate of the electrolyte increases. Those electrochemical and optical characterizations bring information to develop an analytical modelling for the predictions of the dynamic behavior of these systems. A numerical simulation is also proposed to complete the analytical model. This simulation is able to reproduce the oscillatory behavior at high currents. The optimization of the energy efficiency of the process is done by calculating and choosing an optimal flow rate, corresponding to the best balance between the power gained and the hydraulic power consumed by the flow. The decrease of the hydraulic power needed is done by the adaptation of the geometry of the flow cells. Triangular configuration for the inlet and outlet zones of the flow are tested and shows better characteristics for natural and forced evacuation of the bubbles. A preliminary study and outlooks of the effect of flowing electrolyte on zinc dendrites are presented. Flowing electrolyte increase the time before a short-circuit occurs

The power requirement for the soldier's equipment is largely supplied by batteries. Situational awareness is critical for a soldier to perform his tasks. Therefore the radio used by the soldier is a key element in situational awareness and also consumes the most power. The South African National Defence Force (SANDF) uses the A43 tactical radio specifically designed for them. The radios are regarded as old technology but will be in use for about another five years. The radios still use non-rechargeable alkaline batteries which do not last very long and are not cost effective. The purpose of this study is to research the new generation secondary batteries as a possible replacement for the alkaline battery packs. The new generation batteries investigated in this study are the latest rechargeable batteries, also called secondary batteries. They include nickel cadmium, nickel metal hydride, lithium ion, rechargeable alkaline manganese and zinc air. The main features of rechargeable cells are covered and the cell characteristics are defined to allow the technology to be matched to the user requirement. Li-ion technology was found to be the best choice. This research also showed that international trends in battery usage are towards Li-ion. A new Li-ion battery was designed based on commercial cells. Tests showed that commercial Li-ion cells can be used in the radio and that they outperform the current battery by far. The study also examined the design of a New Generation Battery System consisting of an intelligent battery, a charger which uses a Systems Management Bus and a battery 'state of health" analyser to assist the user to maintain the batteries. Tests were done to demonstrate that the battery can withstand typical military environmental conditions. Expected military missions for a battery system were defined and used to compare the cost between the existing batteries and the new batteries system. Important usage factors which will influence the client when using a New Generation Battery System were addressed. To summarise, this study showed that by using a New Generation Battery System, the SANDF could relieve the operational cost of the A43 radio while saving on weight and enabling the soldier to carry out longer missions.
Thesis (M.Ing. (Electronical Engineering))--North-West University, Potchefstroom Campus, 2008.

Ce travail porte sur la synthèse et la caractérisation de membranes polymères échangeuses d'anions, destinées à la protection de l'électrode à air dans une batterie lithium-air (en vue d'une application pour véhicule électrique). Ces matériaux à architecture de réseaux interpénétrés de polymères (RIP) associent un réseau polyélectrolyte cationique hydrocarboné, la poly(épichlorohydrine) (PECH), à un réseau de polymère neutre qui peut être soit hydrocarboné, soit fluoré. Tout d'abord, la synthèse du réseau polyélectrolyte et son assemblage sur l'électrode à air ont été optimisés. Une première série de RIP associant ce réseau PECH à un réseau de poly(méthacrylate d'hydroxyéthyle) a été synthétisée. Une seconde série de matériaux combinant ce même réseau PECH à un réseau de polymère fluoré a été développée. L'ensemble de ces matériaux a été caractérisé, et pour chaque série de RIP, la méthode de synthèse et la composition ont été optimisées. Les membranes RIP présentent des propriétés améliorées par rapport au réseau simple de PECH. L'électrode à air protégée par ces nouvelles membranes échangeuses d'anions présente une stabilité améliorée dans les conditions de fonctionnement de la batterie lithium-air. Plus précisément, une durée de vie de 1000 h est obtenue lorsque l'électrode à air a été modifiée avec un RIP fluoré, soit une augmentation d'un facteur 20 de la durée de vie de l'électrode non modifiée
This work focuses on the synthesis and characterization of polymer membranes to be used as anion exchange membranes for protection on an air electrode in a new lithium–air battery for electric vehicle. In these materials showing interpenetrating polymer networks (IPN) architecture, a hydrogenated cationic polyelectrolyte network, the poly(epichlorohydrin) (PECH), is associated with a neutral network, which can be either hydrogenated or fluorinated. First, the synthesis of the polyelectrolyte network and the membrane/electrode assembly were optimized. Second, a first IPN series associating the PECH network with a poly(hydroxyethyl methacrylate) network was synthesized. Third, the same PECH network was associated with a fluorinated polymer network. All the materials were characterized, and optimal synthesis methods as well as an optimal composition were determined for each association. The IPNs show improved properties compared with the single PECH network. The air electrode protected by these new anion exchange membranes shows improved stability in the working conditions of the lithium-air battery. Specifically, a lifetime of 1000 h was obtained when the electrode was modified with a fluorinated IPN, a 20-fold increase in the lifetime of the non-modified electrode

A reação de redução de oxigênio (RRO) foi estudada em eletrocatalisadores formados por nanopartículas de óxidos puros e mistos de metais de transição de Mn, Co e Ni, além de estrutura tipo espinel, e por nanopartículas de Ag, Au e Ag3M (M= Au, Pt, Pd e Cu) suportadas em carbono Vulcan, em eletrólito alcalino. Os óxidos de metais de transição foram sintetizados por decomposição térmica de seus respectivos nitratos e as nanopartículas a base de prata e ouro foram sintetizadas por redução química com borohidreto. Os eletrocatalisadores foram caracterizados por Difratometria e Espectroscopia de Absorção de Raios X (somente para os óxidos de transição). Os materiais a base de óxidos de manganês, mostraram-se com alta atividade para a RRO, para os quais os resultados espectroscópicos in situ evidenciaram a ocorrência da redução do Mn(IV) para Mn(III), na região de início da RRO. Assim, as atividades eletrocatalíticas foram associadas à ocorrência da transferência de elétrons do Mn(III) para o O2. Entretanto, apresentaram forte desativação após ciclagem potenciodinâmica, o que foi associado à formação da fase Mn3O4, conforme indicado por difratometria de Raios X, após os experimentos eletroquímicos, que é eletroquimicamente inativa. Já o material formado pela estrutura do tipo espinel de MnCo2O4 apresentou alta atividade e estabilidade frente à ciclagem e à RRO. A alta atividade eletrocatalítica foi relacionada a ocorrência do par redox CoII/CoIII em maiores valores de potencial em relação ao CoOx e MnOx, devido a interações entre os átomos de Co e Mn no reticulo espinélico. Contrariamente ao observado nos óxidos com maior quantidade de manganês, o espinel mostrou-se altamente estável, o que foi associada à não alteração de sua estrutura no intervalo de potenciais que a RRO ocorre. Para os materiais bimetálicos a base de prata e ouro, os experimentos eletroquímicos indicaram maior atividade eletrocatalítica para o material de Ag3Au/C. Neste caso, a alta atividade foi associada a dois efeitos principais: (i) a um efeito sinergético, no qual os átomos de ouro atuam na região de ativação, favorecendo a adição de hidrogênio e os átomos vizinhos de prata proporcionam a quebra da ligação O-O, conduzindo a RRO pelo caminho de quatro elétrons por molécula de O2; (ii) ao aumento força da ligação Ag-O, devido à interação da Ag com o Au, resultando em maior atividade para a quebra da ligação O-O, aumentando a atividade da Ag para a RRO, em relação à atividade da Ag pura. Assim, a RRO apresentou menor sobrepotencial e maior número de elétrons em Ag3Au/C, quando comparado com as demais nanopartículas bimetálicas.
The oxygen reduction reaction (ORR) was studied on electrocatalysts composed by pure and mixed transition metal oxides of Mn, Co, and Ni, including spinel-like structures, and by Ag, Au, and Ag3M/C (M= Au, Pt, Pd e Cu) bimetallic nanoparticles, in alkaline electrolyte. The transition metal oxides were synthesized by thermal decomposition of their nitrates, and the silver and gold-based nanoparticles by chemical reduction using borohydride. The electrocatalysts were characterized by X-Ray Diffraction and X-Ray Absorption Spectroscopy (in the case of the metal oxides). The manganese-based oxide materials showed high activity for the ORR, in which the in situ spectroscopic results evidenced the Mn(IV) to Mn(III) reduction, in the range of the ORR onset. In this case, the electrocatalytic activities were correlated to the transfer of electron from Mn(III) to O2. However, they presented strong deactivation after several potentiodynamic cycles, which was ascribed to the formation of the electrochemically inactive phase of Mn3O4, as indicated by the XRD results, after the electrochemical experiments. On the other hand, the MnCo2O4 spinel-like material showed high activity and stability for the ORR. Its high electocatalytic activity was attributed to the CoII/CoIII redox pair, taking place at higher potentials, in relation to that of the CoOx e MnOx pure phases, due to the Co and Mn interactions in the spinel lattice. Contrarily to the behavior observed for the manganese-based materials, the spinel oxide presented high stability, which was ascribed to the non alteration of its crystallographic structure in the range of potentials tha the ORR takes place. For the Au and Ag-based materials, the electrochemical experiments indicated higher electrocatalytic activities for Ag3Au/C. In this case, its higher activity as associated to two main aspects: (i) to a synergetic effect, in which the gold atoms act in the activation region, facilitating the hydrogen addition, and the neighboring Ag atoms promoting the O-O bond breaking, leading the ORR to the 4-electrons pathway; (ii) to the increased Ag-O bond strength, due to the electronic interaction between Ag and the Au atoms, resulting in a faster O-O bond breaking, enhancing the electrocatalytic activity of the Ag atoms in the Ag3Au/C nanoparticle, in relation to that on the pure Ag. Therefore, the ORR presented lower overpotential and higher number of electrons in the Ag3Au/C electrocatalyst, when compared to the other investigated bimetallic nanoparticles.

Metal air battery has captured the spotlight recently as a promising class of sustainable energy storage for the future energy systems. Metal air batteries offer many attractive features such as high energy density, environmental benignity, as well as ease of fuel storage and handling. In addition, wide range of selection towards different metals exists where different energy capacity can be achieved via careful selection of different metals. The most energy dense systems of metal-air battery include lithium-air, aluminum-air and zinc-air. Despite the choice of metal electrode, oxygen reduction (ORR) occurs on the air electrode and oxidation occurs on the metal electrode. The oxidation of metal electrode is a relatively facile reaction compared to the ORR on the air electrode, making latter the limiting factor of the battery system. The sluggish ORR kinetics greatly affects the power output, efficiency, and lifetime of the metal air battery. One solution to this problem is the use of active, affordable and stable catalyst to promote the rate of ORR. Currently, platinum nanoparticles supported on conductive carbon (Pt/C) are the best catalyst for ORR. However, the prohibitively high cost and scarcity of platinum raise critical issues regarding the economic feasibility and sustainability of platinum-based catalysts. Cost reduction via the use of novel technologies can be achieved by two approaches. The first approach is to reduce platinum loading in the catalyst formulation. Alternatively platinum can be completely eliminated from the catalyst composition. The aim of this work is to identify and synthesize alternative catalysts for ORR toward metal air battery applications without the use of platinum re other precious metals (i.e., palladium, silver and gold). Non-precious metal catalysts (NPMC) have received immense international attentions owing to the enormous efforts in pursuit of novel battery and fuel cell technologies. Different types of NPMC such as transition metal alloys, transition metal or mixed metal oxides, chalcogenides have been investigated as potential contenders to precious metal catalysts. However, the performance and stability of these catalysts are still inferior in comparison. Nitrogen-doped carbon materials (NCM) are an emerging class of catalyst exhibiting great potential towards ORR catalysis. In comparison to the metal oxides, MCM show improved electrical conductivity. Furthermore, NCM exhibit higher activity compared to chalcogenides and transition metal alloys. Additional benefits of NCM include the abundance of carbon source and environmental benignity. Typical NCM catalyst is composed of pyrolyzed transition metal macrocycles supported by high surface area carbon. These materials have demonstrated excellent activity and stability. However, the degradation of these catalysts often involves the destruction of active sites containing the transition metal centre. To further improve the durability and mass transport of NCM catalyst, a novel class of ORR catalyst based on nitrogen-doped carbon nanotubes (NCNT) is investigated in a series of studies. The initial investigation focuses on the synthesis of highly active NCNT using different carbon-nitrogen precursors. This study investigated the effect of using cyclic hydrocarbon (pyridine) and aliphatic hydrocarbon (ethylenediamine) towards the formation and activity of NCNT. The innate structure of the cyclic hydrocarbon promotes the formation of NCNT to provide higher product yield; however, the aliphatic hydrocarbon promotes the formation of surface defects where the nitrogen atoms can be incorporated to form active sites for ORR. As a result, a significant increase in the ORR activity of 180 mV in half-wave potential is achieved when EDA was used as carbon-nitrogen precursor. In addition, three times higher limiting current density was observed for the NCNT synthesized from ethylenediamine. Based on the conclusion where highly active NCNT was produced from aliphatic hydrocarbon, similar carbon-nitrogen precursors with varying carbon to nitrogen ratio in the molecular structure (ethylenediamine, 1, 3-diaminopropane, 1, 4-diaminobutane) were adapted for the synthesis of NCNT. The investigation led to the conclusion that higher nitrogen to carbon ratio in the molecular structure of the precursors benefits the formation of active NCNT for ORR catalysis. The origin of such phenomena can be correlated with the higher relative nitrogen content of the resultant NCNT synthesized from aliphatic carbon precursor that provided greater nitrogen to carbon ratio. As the final nitrogen content increased in the molecular structure, the half-wave potential of the resultant NCNT towards ORR catalysis was increased by 120 mV. The significant improvement hints the critical role of nitrogen content towards ORR catalysis. To further confirm the correlation between the nitrogen content and ORR activity, another approach was used to control the final nitrogen content in the resultant NCNT. In the third investigation, a carbon-nitrogen precursor (pyridine) was mixed with a carbon precursor (ethanol) to form an admixture. The relative proportion of the two components of the admixture was varied to produce NCNT with different nitrogen content. By adopting this methodology, potential effect of different carbon-nitrogen precursors on the formation of NCNT can be eliminated since the same precursors were used for NCNT synthesis. Based on the electrochemical evaluations, the nitrogen content can be positively correlated to ORR activity. Among the NCNT samples, 41% higher limiting current density was achieved for 0.7 at. % increase in overall nitrogen content. Furthermore, the selectivity of the NCNT catalyst with higher nitrogen content favours the production of water molecule—the favourable product in metal-air battery by 43%. ORR catalyst is an outer-sphere electron transfer reaction whereby the reactants interact with the surface of catalysts. Consequently, the surface structure can be a determining factor towards the ORR activity of the NCNT in addition to the nitrogen content. In the forth investigation, the surface structure of NCNT was tailored to differentiate the ORR activity of smooth and rugged surface while controlling the overall nitrogen content to be similar. NCNT having different surface structures but similar nitrogen content (approximately 2.7 to 2.9 at. %) were successfully synthesized using different synthesis catalysts. Comparison of the two NCNT catalysts showing different surface structure resulted in a 130 mV increased in half-wave potential favouring the NCNT with more rugged surface structure. This study provided insights to the potential effects of synthesis catalyst towards directing the surface structure and the ORR activity of NCNT. Through a series of studies, the important parameters affecting the ORR performance of NCNT were elucidated and the most active NCNT catalyst synthesized was used for testing in a prototype zinc-air battery. The fifth study evaluated the performance of NCNT catalyst in different concentrations of alkaline electrolyte and at different battery voltage. An increase in the electrolyte’s alkaline strength improved the battery performance to a certain degree until the increasing viscosity impeded the performance of the battery system. The zinc-air battery employing NCNT as ORR catalyst produced a maximum battery power density of 69.5 mWcm-2 in 6M potassium hydroxide. The fifth study illustrated the great potential of NCNT towards the ORR catalysis for metal-air batteries. In combination, the series of investigations presented in this document provide a comprehensive study of a novel material and its application towards ORR catalysis in metal air batteries. Specifically, this report provides insights into the fundamentals of NCNT synthesis; the origins of ORR activity and the optimal operating conditions of NCNT in a prototype zinc-air battery. The excellent performance of NCNT warrants further studies of this material in greater details, and the information presented in this document will create a basis for future investigations towards ORR catalysis.

Global warming and depletion in fossil fuels have forced the society to search for alternate, clean sustainable energy sources. An obvious solution to the aforesaid problem lies in electrochemical energy storage systems like fuel cells and batteries. The desirable properties attributed to these devices like quick response, long life cycle, high round trip efficiency, clean source, low maintenance etc. have made them very attractive as energy storage devices. Compared to many advanced battery chemistries like nickel-metal hydride and lithium - ion batteries, metal-air batteries show several advantages like high energy density, ease of operation etc. The notable characteristics of metal - air batteries are the open structure with oxygen gas accessed from ambient air in the cathode compartment. These batteries rely on oxygen reduction and oxygen evolution reactions during discharging and charging processes. The efficiency of these systems is determined by the kinetics of oxygen reduction reaction. Platinum is the most preferred catalyst for many electrochemical reactions. However, high cost and stability issues restrict the use of Pt and hence there is quest for the development of stable, durable and active electrocatalysts for various redox reactions. The present thesis is directed towards exploring the electrocatalytic aspects of titanium carbonitride. TiCN, a fascinating material, possesses many favorable properties such as extreme hardness, high melting point, good thermal and electrical conductivity. Its metal-like conductivity and extreme corrosion resistance prompted us to use this material for various electrochemical studies. The work function as well as the bonding in the material can be tuned by varying the composition of carbon and nitrogen in the crystal lattice. The current study explores the versatility of TiCN as electrocatalyst in aqueous and non-aqueous media. One dimensional TiC0.7N0.3 nanowires are prepared by simple one step solvothermal method without use of any template and are characterized using various physicochemical techniques. The 1D nanostructures are of several µm size length and 40 ± 15 nm diameter (figure 1). Orientation followed by attachment of the primary particles results in the growth along a particular plane (figure 2). (a) (b) (c) Figure 1. (a) SEM images of TiC0.7N0.3 nanowires (b) TEM image and (c) High resolution TEM image showing the lattice fringes. (a) (b) (d) Figure 2. Bright field TEM images obtained at different time scales of reaction. (a) 0 h; (b) 12 h; (c) 72 h and (d) 144 h. The next aspect of the thesis discusses the electrochemical performance of TiC0.7N0.3 especially for oxygen reduction. Electrochemical oxygen reduction reaction (ORR) reveals that the nanowires possess high activity for ORR and involves four electron process leading to water as the product. The catalyst effectively converts oxygen to water with an efficiency of 85%. A comparison of the activity of different (C/N) compositions of TiCN is shown in figure 3. The composition TiC0.7N0.3 shows the maximum activity for the reaction. The catalyst is also very selective for ORR in presence of methanol and thus cross-over issue in fuel cells can be effectively addressed. Density functional theory (DFT) calculations also lead to the same composition as the best for electrocatalysis, supporting the experimental observations. Figure 3. Linear sweep voltammetric curves observed for different compositions of titanium carbonitride towards ORR. The next chapter deals with the use of TiC0.7N0.3 as air cathode for aqueous metal - air batteries. The batteries show remarkable performance in the gel- and in liquid- based electrolytes for zinc - air and magnesium - air batteries. A partial potassium salt of polyacrylic acid (PAAK) is used as the polymer to form a gel electrolyte. The cell is found to perform very well even at very high current densities in the gel electrolyte (figures 4 and 5). Figure 4 Photographs of different components of the gel - based zinc - air battery. (a) (b) Figure 5. a) Discharge curves at different current densities of 5, 20, 50 and 100 mA/cm2 for zinc-air system with TiC0.7N0.3 cathode b) Charge – discharge cycles at 50 mA/cm2 for the three electrode configuration with TiC0.7N0.3 nanowire for ORR and IrO2 for OER and Zn electrode (2h. cycle period). Similarly, the catalytic activity of TiC0.7N0.3 has also been explored in non-aqueous electrolyte. The material acts as a bifunctional catalyst for oxygen in non- aqueous medium as well. It shows a stable performance for more than 100 cycles with high reversibility for ORR and OER (figure 6). Li-O2 battery fabricated with a non-aqueous gel- based electrolyte yields very good output. (a) (b) (c) Figure 6. Galvanostatic charge –discharge cycles. (a) at 1 mA/cm2 (b) specific capacity as a function of no. of cycles (c) photographs of PAN-based gel polymer electrolyte. Another reaction of interest in non –aqueous medium is I-/I3-. redox couple. TiC0.7N0.3 nanowires show small peak to peak separation, low charge transfer resistance and hence high activity. The catalyst is used as a counter electrode in dye sensitized a solar cell that shows efficiencies similar to that of Pt, state of the art catalyst (figure 7). (a) (b) (c) Figure 7 (a) Cyclic voltammograms for I-/I3 - redox species on TiC0.7N0.3 nanowires (red), TiC0.7N0.3 particle (black) and Pt (blue). (b) Photocurrent density - voltage characteristics for DSSCs with different counter electrodes. TiC0.7N0.3 nanowire (black), TiC0.7N0.3 particle (blue), Pt (red). (c) Photograph of a sample cell. (a) (b) (c) (d) Figure 8 a) Comparison ORR activity for (i) NiTiO3(black), (ii) N-rGO (red), (iii) NiTiO3 – N-rGO (green) and (iv) Pt/C (blue) (b) Linear sweep voltammograms for OER observed on NiTiO3 – N-rGO composite (black), NiTiO3 (brown), N-rGO (blue), glassy carbon (red) in 0.5 M KOH. (c) Galvanostatic discharge curves of NiTiO3 – N-rGO as air electrode (d) Charge – discharge cycle at 5 mA/cm2 for the rechargeable battery with 10 min. cycle period. The last part of the thesis discusses about a ceramic oxide, nickel titanate. The electrocatalytic studies of the material towards ORR and OER reveal that the catalyst shows remarkable performance as a bifunctional electrode. A gel - based zinc - air battery fabricated with nickel titanate – reduced graphene oxide composite shows exceptional performance of 1000 charge-discharge cycles in the rechargeable mode (figure 8). Of course, the primary battery configuration works very well too The thesis contains seven chapters on the aspects mentioned above with summary and future perspectives given as the last chapter. An appendix based on TiN nanotubes and supercapacitor studies is given at the end.

碩士
國立高雄大學
應用化學系碩士班
103
Oxygen reduction reactions ( ORR ) at the air cathode in non-aqueous electrolytes are well-known to influence the performance of Li-air batteries. In this work, highly ordered mesoporous metal oxide composites were designed as electrocatalysts and porous air cathode material in the Li-air battery. The highly ordered mesoporous MnO2/C and TiO2/C composites were synthesized by soft template method and hydrothermal method, combining solvent-evaporation-induced self-assembly and the in situ carbothermal reduction reaction and using the triblock copolymer F127 as the structure-directing agent and resol as the carbon source. In summary, the XRD patterns show metal oxide can be attributed to a pure and well-crystallized MnO2 and TiO2 phase. The metal oxide composites with large specific surface area 424 m2/g ( MnO2/C ) and 599 m2/g ( TiO2/C ). The porous structure of metal oxide composites provides high electrocatalytic active sites and sufficient transmission paths for O2 and electrolyte. Both ordered metal oxide composites show good elecrcatalytic activity toward Oxygen Reduction Reactions ( ORR ) / Oxygen Evolution Reactions ( OER ) in non-aqueous electrolytes. Employing the ordered mesoporous metal oxides as electrocatalyst in Li-air batteries, the Li-air batteries display lower overpotential and good discharge capacity. This result demonstrates ordered mesoporous metal oxide composites are promising cathode electrocatalysts for non-aqueous Li-air batteries.