Academic literature on the topic 'Oxygen Electrocatalysts'

Philosophiae Doctor - PhD<br>The widespread use of fossil energy has been most convenient to the world, while they also cause environmental pollution and global warming. Therefore, it is necessary to develop clean and renewable energy sources, among which, hydrogen is considered to be the most ideal choice, which forms the foundation of the hydrogen energy economy, and the research on hydrogen production and fuel cells involved in its production and utilization are naturally a vital research endeavor in the world. Electrocatalysts are one of the key materials for proton exchange member fuel cells (PEMFCs) and water splitting. The use of electrocatalysts can effectively reduce the reaction energy barriers and improve the energy conversion efficiency.

Nowadays, under the background of environmental pollution and energy crisis, with the continuous development of various forms of new energy, energy conversion and storage devices are essential to the utilization of renewable energy. Among them, clean battery technology is developing rapidly. Compared with traditional batteries including lithium batteries, Zn-air batteries have unique advantages and face significant development opportunities due to their high theoretical energy density, safety and environmental protection. However, as a secondary battery, the rechargeable Zn-air battery is charged and discharged at the air cathode. The high overpotential and slow kinetic process of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which occur respectively, seriously affect the actual efficiency of the battery, which has become an essential obstacle for the commercial use of Zn-air batteries. In order to explore the excellent theoretical performance of Zn-air batteries, it is an essential mean to use electrocatalysts with ORR/OER dual catalytic function to improve the cathode reaction efficiency. Based on this, this thesis reviews the structure, reaction mechanism, structure and performance characteristics of various oxygen reaction electrocatalysts of Zn-air batteries, and analyzes the material factors affecting the performance of catalysts. Utilizing strategies such as porous carbon, heteroatom doping, transition metal oxides, single-atom modulation, and defect control, a series of composite materials with high ORR/OER dual catalytic activity were prepared. The structure, electrochemical properties and catalytic performance of the materials in Zn-air batteries were studied. In addition, this thesis also investigates the application of catalysts in miniature wearable solid-state Zn-air batteries combined with various new battery configuration designs. In this dissertation, a series of transition metal carbon-based bifunctional catalysts with excellent performance were prepared based on various building elements. However, the analysis and research in this thesis revealled the fine materials science contents such as the electronic state distribution of the adsorption and bonding of heteroatom-doped carbon to oxygen reaction intermediates, and the regulation pathway of adsorption sites caused by defects. Understanding these contents is very important for explaining the structure-activity relationship of catalytic materials. Studying a bifunctional catalyst for Zn-air batteries that can work stably and maintain a terrific constant charge-discharge potential gap, withstanding high current density, will eventually lead to a revolution in the battery industry. However, most of the reported electrocatalysts in high-rate Zn-air batteries to date do not have enough durability, suffering from unstable nanostructures, poor electrical conductivity, low active sites, and high overpotentials. In view of this, it is the ultimate proposition of the road to Zn-air batteries applications to pursuit ultrastable and cheap catalysts that can alleviate particle aggregation, have abundant active sites and low resistance. In addition, compared with liquid Zn-air batteries, the performance of all-solid-state Zn-air batteries in various reports, is often poor, which originates from the insufficient water retention and conductivity of solid-state electrolytes, improper catalyst loading method and battery configuration. Various factors such as design, especially the research and development of new solid-state electrolytes, are of great significance for the development of wearable rechargeable Zn-air batteries with both flexible mechanical properties and charge-discharge efficiency. In order to solve the above problems and these drawbacks of Zn-air batteries, different characterization techniques are used to determine the similarity or commonality of composite electrocatalysts with high efficiency and activity. The main research contents are as follows: (1) Spinel-type metal oxides, as a group of the transition metal oxides, are considered as one of the most promising bifunctional oxygen electrocatalysts due to its unique electronic structure, mixed metalic valence centres with redox behaviours, abundance and environmental friendliness. In the first work, a facile one-step hydrothermal method is reported for the synthesis of a high-performance bifunctional oxygen electrocatalyst, cobalt-doped Mn3O4 nanocrystals supported on graphene nanosheets (Co-Mn3O4/G). Compared to pristine Mn3O4, this Co-Mn3O4/G exhibits greatly enhanced electrocatalytic activity, delivering a half-wave potential of 0.866 V for the ORR and a low overpotential of 275 mV at 10 mA cm−2 for the OER. The Zn-air battery built with Co-Mn3O4/G shows a reduced charge–discharge voltage of 0.91 V at 10 mA cm−2, a peak power density of 115.24 mW cm−2 and excellent stability without any degradation after 945 cycles (315 h), outperforming the state-of-the-art Pt/C–Ir/C catalyst-based device. This work offers an efficient strategy to synthesize spinel-type complex oxide materials in high-performance bifunctional oxygen electrocatalyst areas. (2) In order to make the Zn-air batteries work well at a high current, structural optimization is imperative. In the second work, a rapid seeding synthesis strategy is reported for the fabrication of impregnated Co3O4-based carbon ultra-thin nanosheets (Co3O4/C-NS) architecture induced by CoMOF as a bifunctional electrocatalyst. The impregnated ultra-thin nanosheets network would provide prolific pathways for efficient mass transfer, which allows the inner active sites to be accessible to electrolyte and oxygen. Additionally, the MOF-derived carbon matrix would suppress the aggregation of Co3O4 nanoparticles and increase the stability of the catalyst during the high-density charge/discharge cycling. Our Co3O4/C-NS exhibits uniform morphology, high specific area, low internal resistance, and superior ORR and OER activity to the benchmark Pt/C and Ir/C, respectively. Furthermore, the Zn-air batteries fabricated with the assynthesized electrocatalyst afford remarkably stable charge/discharge at a high current density of 25 mA cm-2, surpassing most of the previously reported catalysts. The material engineering approach highlighted herein exemplifies a facile yet effective avenue towards stable, efficient and robust non-noble metal-based electrocatalysts. (3) Single-atom catalysts (SACs) have attracted great interest in the field of catalysis, mainly because SACs not only possess the advantages of homogeneous and heterogeneous catalysts, but also possess some unique properties. In the third work, NiCo-LDH with electrocatalytic Ni and Co were grown on Ni, Co-codoped, hierarchically ordered macroporous carbon (NiCo-LDH@NiCo-SAs/OPC) derived from pyrolysis of ZIFs via a facile method. The strong coupling between NiCo-LDH and NiCo- SAs/OPC not only sharply facilitates the electron transfer but also result in high chemical stability against the corrosion during charging and discharging processes. Additionally, interconnected hierarchically porous structures were involved in NiCo-SAs/OPC via introducing removable templates, which would serve as channels to accelerate mass transport (O2 and electrolytes) during electrochemical steps. The obtained hierarchically porous NiCo-LDH@NiCo-SAs/OPC possesses abundant atomically dispersed Ni-Nx, Co-Nx sites and continuous species/charge transport channels, and exhibits good bifunctional ORR/OER electrocatalytic performance, which is superior to the corresponding noble metals Pt/C and RuO2 catalysts. More importantly, the rechargeable Zn metal-air batteries assembled with NiCo-LDH@NiCo-SAs/OPC also exhibited good charge-discharge performance and long-term stability. (4) Alloy-based electrocatalysts have been studied as bifunctional catalysts for ORR/OER for a long-time. In the fourth work, NiCo bi-alloy particles are used to embed onto the carbonized MF framework (NiCo@CMF), which has shown excellent performance, providing a new idea for designing other non-precious metal ORR/OER bifunctional electrocatalysts. More importantly, NiCo@CMF electrode can be processed into various shapes, furthermore, the assembled Zn-air battery shows pretty good flexibility during application as well as an appreciable charge-discharge voltage gap. While maintaining high-efficiency battery performance, the battery exhibits excellent bending mechanical properties. These works provide the power supply for nextgeneration smart wearable devices. The unique machinable NiCo@CMF electrode will have many potential applications, providing more possibilities for the design of wearabletype Zn-air batteries, and the cost-effectiveness of the NiCo@CMF electrode allows it to be fabricated on a large scale, providing a more economically viable avenue to the Zn-air batteries technology. This strategy can even be extended to other wearable devices for wider promotion.<br>Thesis (PhD Doctorate)<br>Doctor of Philosophy (PhD)<br>School of Environment and Sc<br>Science, Environment, Engineering and Technology<br>Full Text

The increasing energy demand in the context of population explosion excites human efforts to explore more renewable power sources. Among various systems for sustainable energy producing, Microbial Fuel Cells (MFCs) are considered as a promising alternative to generate renewable energy, being an environmental biotechnology that turns the treatment of organic wastes into electricity. However, the high - and further increasing - cost of materials to build up devices, especially precious platinum catalyst at the cathode side, hinders MFCs being popular in the practical applications. This research aimed to study non-noble catalysts for oxygen reduction reaction (ORR) in order to substitute state-of-art platinum. In particular, three different synthetic strategies were explored to fabricate iron-based catalysts with low-cost and high catalytic activity towards ORR. Inorganic iron-based catalysts were obtained from a two-step deposition of i) iron from inorganic source and ii) nitrogen from ammonia gas on carbon nanotubes (CNTs). Iron was impregnated on CNTs by a reduction of iron nitrate in ethylene glycol. After that, these FeCNTs compounds were treated under ammonia gas at 700°C for 2 h. Two Fe:CNTs ratios, 0.1:10 and 1:10, were investigated resulting two catalysts, labeled as FeCNTs 0.1:10 700 and FeCNTs 1:10 700. Iron chelate-based catalysts were obtained from ethylenediamine-N,N’-bis(2- hydroxyphenylacetic acid), nitrilotriacetic acid and diethylene triamine pentaacetic acid as iron - nitrogen precursors. Iron chelates were dispersed uniformly on both carbon Vulcan and carbon nanotubes by mixing these materials in water and drying at 70°C. The catalyst activation was carried out by annealing the mixture under argon gas at 800°C for 1.5 h. The catalysts are labeled as FeEDDHA, FeNTA, FeDTPA on C/CNTs. Polyindole-based catalysts were prepared by polymerization reaction of indole on either carbon Vulcan or carbon nanotubes in the presence of iron phthalocyanine (FePc), this latter being a macrocycle complex that has been widely used as ORR catalyst. The reaction was carried out in methanol which was completely evaporated in a water bath and in a vacuum oven at 70°C, and two different FePc:(PID/CNTs) ratios were explored, 1:1 and 3:1, obtaining samples labeled as FePc-PID-CNTs 1:1 and FePc-PID-CNTs 3:1. A catalyst prepared by mechanically mixing of polyindole on CNTs and FePc, was also prepared (PIDCNTs + FePc). In both cases, no further heat treatment at high temperature was applied. Morphology of prepared catalysts was examined by means of scanning electron microscopy and transmission electron microscopy. The results showed the uniform distribution of iron catalysts on the surface of carbon substrate. Total surface area as well as total pore volume was evaluated by nitrogen physisorption experiments, demonstrating IV that the catalysts supported on CNTs had a higher surface area and pore volume than those of catalysts supported on carbon Vulcan. X-ray photoelectron spectroscopy and neutron activation analysis were used to analyze the surface and bulk content of iron, respectively and revealed active sites in coordination with nitrogen. The electrochemical activity towards ORR of these samples was assessed by cyclic voltammetry in phosphate buffer electrolyte solution at pH 7. The results indicated that these iron-based catalysts are active with oxygen. Carbon nanotubes based catalysts had a greater oxygen reduction activity than that of carbon Vulcan based catalysts due to the higher total surface and pore volume. This preliminary characterization allowed selecting the most performing catalysts: FeCNTs 1:10 700, FeEDDHA/CNTs, and FePc-PID-CNTs. The performance for electricity production of the selected electrocatalysts was verified by means of test in air-cathode single-chamber MFCs fed either with domestic wastewater or phosphate buffer solution containing acetate. Polarization and power density curves of MFC based on FeCNTs 1:10 700, FeEDDHA/CNTs, and FePc-PIDCNTs 1:1 as cathode catalysts were similar or even improved with respect to those obtained by using platinum. FePc-PID-CNTs 1:1 cathode showed power density of 796 mW/ m2 and maximum current density of 4280 mA/m2, while a standard Pt catalyst produced 705 mW/m2 and 3972 mA/m2. The stability of the catalysts was evaluated by means of durability tests during the cell functioning over 700 h. The cost of prepared iron-based catalysts was calculated in laboratory scale and they were much lower than commercial platinum catalyst, allowing for a cost reduction up to 78.8 %. In conclusion, some inexpensive and effective methods to prepare iron-based materials for ORR were developed. MFC tests indicated the prepared iron-based catalysts The increasing energy demand in the context of population explosion excites human efforts to explore more renewable power sources. Among various systems for sustainable energy producing, Microbial Fuel Cells (MFCs) are considered as a promising alternative to generate renewable energy, being an environmental biotechnology that turns the treatment of organic wastes into electricity. However, the high - and further increasing - cost of materials to build up devices, especially precious platinum catalyst at the cathode side, hinders MFCs being popular in the practical applications. This research aimed to study non-noble catalysts for oxygen reduction reaction (ORR) in order to substitute state-of-art platinum. In particular, three different synthetic strategies were explored to fabricate iron-based catalysts with low-cost and high catalytic activity towards ORR. Inorganic iron-based catalysts were obtained from a two-step deposition of i) iron from inorganic source and ii) nitrogen from ammonia gas on carbon nanotubes (CNTs). Iron was impregnated on CNTs by a reduction of iron nitrate in ethylene glycol. After that, these FeCNTs compounds were treated under ammonia gas at 700°C for 2 h. Two Fe:CNTs ratios, 0.1:10 and 1:10, were investigated resulting two catalysts, labeled as FeCNTs 0.1:10 700 and FeCNTs 1:10 700. Iron chelate-based catalysts were obtained from ethylenediamine-N,N’-bis(2- hydroxyphenylacetic acid), nitrilotriacetic acid and diethylene triamine pentaacetic acid as iron - nitrogen precursors. Iron chelates were dispersed uniformly on both carbon Vulcan and carbon nanotubes by mixing these materials in water and drying at 70°C. The catalyst activation was carried out by annealing the mixture under argon gas at 800°C for 1.5 h. The catalysts are labeled as FeEDDHA, FeNTA, FeDTPA on C/CNTs. Polyindole-based catalysts were prepared by polymerization reaction of indole on either carbon Vulcan or carbon nanotubes in the presence of iron phthalocyanine (FePc), this latter being a macrocycle complex that has been widely used as ORR catalyst. The reaction was carried out in methanol which was completely evaporated in a water bath and in a vacuum oven at 70°C, and two different FePc:(PID/CNTs) ratios were explored, 1:1 and 3:1, obtaining samples labeled as FePc-PID-CNTs 1:1 and FePc-PID-CNTs 3:1. A catalyst prepared by mechanically mixing of polyindole on CNTs and FePc, was also prepared (PIDCNTs + FePc). In both cases, no further heat treatment at high temperature was applied. Morphology of prepared catalysts was examined by means of scanning electron microscopy and transmission electron microscopy. The results showed the uniform distribution of iron catalysts on the surface of carbon substrate. Total surface area as well as total pore volume was evaluated by nitrogen physisorption experiments, demonstrating IV that the catalysts supported on CNTs had a higher surface area and pore volume than those of catalysts supported on carbon Vulcan. X-ray photoelectron spectroscopy and neutron activation analysis were used to analyze the surface and bulk content of iron, respectively and revealed active sites in coordination with nitrogen. The electrochemical activity towards ORR of these samples was assessed by cyclic voltammetry in phosphate buffer electrolyte solution at pH 7. The results indicated that these iron-based catalysts are active with oxygen. Carbon nanotubes based catalysts had a greater oxygen reduction activity than that of carbon Vulcan based catalysts due to the higher total surface and pore volume. This preliminary characterization allowed selecting the most performing catalysts: FeCNTs 1:10 700, FeEDDHA/CNTs, and FePc-PID-CNTs. The performance for electricity production of the selected electrocatalysts was verified by means of test in air-cathode single-chamber MFCs fed either with domestic wastewater or phosphate buffer solution containing acetate. Polarization and power density curves of MFC based on FeCNTs 1:10 700, FeEDDHA/CNTs, and FePc-PIDCNTs 1:1 as cathode catalysts were similar or even improved with respect to those obtained by using platinum. FePc-PID-CNTs 1:1 cathode showed power density of 796 mW/ m2 and maximum current density of 4280 mA/m2, while a standard Pt catalyst produced 705 mW/m2 and 3972 mA/m2. The stability of the catalysts was evaluated by means of durability tests during the cell functioning over 700 h. The cost of prepared iron-based catalysts was calculated in laboratory scale and they were much lower than commercial platinum catalyst, allowing for a cost reduction up to 78.8 %. In conclusion, some inexpensive and effective methods to prepare iron-based materials for ORR were developed. MFC tests indicated the prepared iron-based catalysts The increasing energy demand in the context of population explosion excites human efforts to explore more renewable power sources. Among various systems for sustainable energy producing, Microbial Fuel Cells (MFCs) are considered as a promising alternative to generate renewable energy, being an environmental biotechnology that turns the treatment of organic wastes into electricity. However, the high - and further increasing - cost of materials to build up devices, especially precious platinum catalyst at the cathode side, hinders MFCs being popular in the practical applications. This research aimed to study non-noble catalysts for oxygen reduction reaction (ORR) in order to substitute state-of-art platinum. In particular, three different synthetic strategies were explored to fabricate iron-based catalysts with low-cost and high catalytic activity towards ORR. Inorganic iron-based catalysts were obtained from a two-step deposition of i) iron from inorganic source and ii) nitrogen from ammonia gas on carbon nanotubes (CNTs). Iron was impregnated on CNTs by a reduction of iron nitrate in ethylene glycol. After that, these FeCNTs compounds were treated under ammonia gas at 700°C for 2 h. Two Fe:CNTs ratios, 0.1:10 and 1:10, were investigated resulting two catalysts, labeled as FeCNTs 0.1:10 700 and FeCNTs 1:10 700. Iron chelate-based catalysts were obtained from ethylenediamine-N,N’-bis(2- hydroxyphenylacetic acid), nitrilotriacetic acid and diethylene triamine pentaacetic acid as iron - nitrogen precursors. Iron chelates were dispersed uniformly on both carbon Vulcan and carbon nanotubes by mixing these materials in water and drying at 70°C. The catalyst activation was carried out by annealing the mixture under argon gas at 800°C for 1.5 h. The catalysts are labeled as FeEDDHA, FeNTA, FeDTPA on C/CNTs. Polyindole-based catalysts were prepared by polymerization reaction of indole on either carbon Vulcan or carbon nanotubes in the presence of iron phthalocyanine (FePc), this latter being a macrocycle complex that has been widely used as ORR catalyst. The reaction was carried out in methanol which was completely evaporated in a water bath and in a vacuum oven at 70°C, and two different FePc:(PID/CNTs) ratios were explored, 1:1 and 3:1, obtaining samples labeled as FePc-PID-CNTs 1:1 and FePc-PID-CNTs 3:1. A catalyst prepared by mechanically mixing of polyindole on CNTs and FePc, was also prepared (PIDCNTs + FePc). In both cases, no further heat treatment at high temperature was applied. Morphology of prepared catalysts was examined by means of scanning electron microscopy and transmission electron microscopy. The results showed the uniform distribution of iron catalysts on the surface of carbon substrate. Total surface area as well as total pore volume was evaluated by nitrogen physisorption experiments, demonstrating IV that the catalysts supported on CNTs had a higher surface area and pore volume than those of catalysts supported on carbon Vulcan. X-ray photoelectron spectroscopy and neutron activation analysis were used to analyze the surface and bulk content of iron, respectively and revealed active sites in coordination with nitrogen. The electrochemical activity towards ORR of these samples was assessed by cyclic voltammetry in phosphate buffer electrolyte solution at pH 7. The results indicated that these iron-based catalysts are active with oxygen. Carbon nanotubes based catalysts had a greater oxygen reduction activity than that of carbon Vulcan based catalysts due to the higher total surface and pore volume. This preliminary characterization allowed selecting the most performing catalysts: FeCNTs 1:10 700, FeEDDHA/CNTs, and FePc-PID-CNTs. The performance for electricity production of the selected electrocatalysts was verified by means of test in air-cathode single-chamber MFCs fed either with domestic wastewater or phosphate buffer solution containing acetate. Polarization and power density curves of MFC based on FeCNTs 1:10 700, FeEDDHA/CNTs, and FePc-PIDCNTs 1:1 as cathode catalysts were similar or even improved with respect to those obtained by using platinum. FePc-PID-CNTs 1:1 cathode showed power density of 796 mW/ m2 and maximum current density of 4280 mA/m2, while a standard Pt catalyst produced 705 mW/m2 and 3972 mA/m2. The stability of the catalysts was evaluated by means of durability tests during the cell functioning over 700 h. The cost of prepared iron-based catalysts was calculated in laboratory scale and they were much lower than commercial platinum catalyst, allowing for a cost reduction up to 78.8 %. In conclusion, some inexpensive and effective methods to prepare iron-based materials for ORR were developed. MFC tests indicated the prepared iron-based catalysts as good candidates for platinum substitution.