Dissertations / Theses 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.

Increasing energy demand and environmental awareness have promoted the development of efficient and environment-friendly hydrogen technologies. Water electrolysis (2𝐻2𝑂→2𝐻2+𝑂2) is a promising way to store renewable electricity generated by solar or wind energy into chemical fuel in the form of H2. Water electrolysis is comprised of a hydrogen evolution reaction (HER) on the cathode and an oxygen evolution reaction (OER) on the anode. For both HER and OER, highly catalytic active electrocatalysts are required to lower the overpotentials and to speed up the sluggish kinetics. To date, noble metal catalysts are still the most efficient electrocatalysts for these two reactions, but their high cost and low abundance on Earth limit the scalable application of water electrolysis. Therefore, investigation of alternative catalysts with low cost and high electrocatalytic activity is urgently needed. This thesis focuses on alkaline electrocatalytic HER, as well as related reactions such as OER, and hydrazine oxidation(HzOR)-assistant HER. In terms of material design, the components are introduced to improve conductivity and mass transfer, as well as boost the intrinsic catalytic activity. Moreover, the mechanism was investigated through exploring the link between structure and performance, as well using density functional theory (DFT) calculations. The first two experimental chapters employed a two-dimensional (2D) material, MXene, as support. In Chapter 2, ruthenium single atoms were incorporated onto ultrathin Ti3C2Tx MXene nanosheets to unlock its electrocatalytic activity. The RuSA@Ti3C2Tx presented a 1 A cm−2 HER current density with an over potential of 425.7 mV, outperforming the commercial Pt/C benchmark. Operando Raman test under HER potential showed the different protonation level between RuSA@Ti3C2Tx and Ti3C2Tx, suggesting the different hydrogen absorption energy of the oxygen terminal on the Ti3C2Tx basal plane. Finally, the theoretical calculations confirmed that the RuSA not only facilitates water dissociation, but also modulates the hydrogen After increasing the Ru content and conducting electroreduction, RuTi alloy nanoclusters were constructed on the surface of Ti3C2Tx. Surprisingly, the RuTi@Ti3C2Tx showed better performance in HER, and excellent hydrazine oxidation reaction (HzOR) performance. The overpotential to attain a current density of 10 mA cm−2 for HER was only 14 mV, lower than that of the commercial Pt/C. The HzOR catalytic activity also outperformed most reported work. In addition, the overall hydrazine spitting was conducted in an H-type electrolytic cell, demonstrating superior thermodynamic advantage and good stability. Defect-abundant active carbon (AC-DCD) as support was prepared by the hydrothermal reaction with dicyanamide. Then, the Ru nanoparticles were grown on the surface. Compared to the catalyst with pristine AC as support prepared under same conditions, Ru600@AC-DCD presented a larger electrochemical special area with strain-abundant Ru nanoparticles. Ru600@AC-DCD delivered excellent HER performance in alkaline media, and good catalytic properties in acidic and neutral media. Finally, another novel metal@carbon composite, Ni nanoparticles encapsulated in graphite carbon layers, was synthesized by directly annealing the Ni-imidazole framework precursors at 350 °C in H2/Ar. By tuning the annealing time under H2/Ar flow, Ni nanoparticles with different crystalline phases were synthesized. These Ni@C samples are di-function electrocatalysts for HER and OER in alkaline condition. The mixed-phase catalyst mix2-Ni@C delivered the highest activity to catalyze HER, while the pure hcp phase catalyst hcp-Ni@C showed best OER activity. This work provided a practical method to prepare low-cost difunctional electrocatalysts for overall water electrolysis. In summary, the thesis innovatively contributes to the knowledge in material science and water electrolysis in the aspects of: (i) designing novel supported composite electrocatalysts with high catalytic activity for HER, OER, and HzOR; (ii) monitoring the changing of surface terminal by operando Raman spectroscopy to verify the HER mechanism; (iii) development of metal nanostructures, like RuTi alloy, hcp phase Ni and mixed-phase Ni, via facile methods, and investigation of their unique properties; and (iv) application of large current HER and exploration of the kinetics under different potentials.<br>Thesis (PhD Doctorate)<br>Doctor of Philosophy (PhD)<br>School of Environment and Sc<br>Science, Environment, Engineering and Technology<br>Full Text

In the last few years the quest towards a hydrogen based economy has intensified interest for effective and less expensive catalysts for fuel cell applications. Due to its slow kinetics, alternative catalysts for the oxygen electroreduction reaction are actively researched. Platinum alloys with different transition metals (for example: Ni, Co and Fe) have shown improved activity over pure Pt. The design of a Pt-free catalysts is also highly desirable, and different alternatives including metalloporphyrins and Pd-based catalysts are being researched. Pd-based catalysts constitute an attractive alternative to Pt alloys in some fuel cell applications, not only because of lower costs but also because of the lower reactivity of Pt alloys towards methanol, which is important for improved methanol crossover tolerance on direct methanol fuel cells. In this work we apply density functional theory (DFT) to the study of four catalysts for oxygen electroreduction. The electronic structure of these surfaces is characterized together with their surface reconstruction properties and their interactions with oxygen electroreduction intermediates both in presence and absence of water. The energetics obtained for the intermediates is combined with entropy data from thermodynamic tables to generate free energy profiles for two representative reaction mechanisms where the cell potential is included as a variable. The study of the barriers in these profiles points to the elementary steps in the reaction mechanisms that are likely to be rate-determining. The highest barrier in the series pathway is located at the first proton and charge transfer on all four catalytic surfaces. This is in good agreement with observed rate laws for this reaction. The instability of hydrogen peroxide on all surfaces, especially compared with the relatively higher stability of other intermediates, strongly points at this intermediate as the most likely point where the oxygen bond is broken during oxygen reduction. This adds to the argument that this path might be active during oxygen electroreduction. A better understanding behind the reaction mechanism and reactivities on these representative surfaces will help to find systematic ways of improvement of currently used catalysts in the oxygen electroreduction reaction.

The oxygen evolution reaction (OER) is kinetically slow and hence a significant efficiency loss in electricity-driven water electrolysis. Understanding the relationships between architecture, composition, and activity in high-performing catalyst systems are critical for the development of better catalysts. This dissertation discusses areas both fundamental and applied that seek to better understand how to accurately measure catalyst activity as well as ways to design higher performing catalysts. Chapter I introduces the work that has been done in the field to date. Chapter II compares various methods of determining the electrochemically active surface area of a film. It further discusses how pulsed and continuous electrodepostition techniques effect film morphology and behavior, and shows that using a simple electrodeposition can create high loading films with architectures that outperform those deposited onto inert substrates. The reversibility of the films, a measure of the films transport efficiency, is introduced and shown to correlate strongly with performance. Chapter III uses high energy x-ray scattering to probe the nanocrystalline domains of the largely amorphous NiFe oxyhydroxide catalysts, and shows that significant similarities in the local structure are not responsible for the change in performance for the films synthesized under different conditions. Bond lengths for oxidized and reduced catalysts are determined, and show no significant phase segregation occurs. Chapter IV seeks to optimize the deposition conditions introduced in Chapter II and to provide a physical representation of how tuning each of the parameters affects film morphology. The deposition current density is shown to be the most important factor affecting film performance at a given loading. Chapter V highlights the different design considerations for films being used in a photoelectrochemical cell, and how in situ techniques can provide information that may otherwise be unobtainable. Chapter VI serves as a summary and provides future directions. This dissertation contains previously published coauthored material.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2013.<br>Cataloged from PDF version of thesis.<br>Includes bibliographical references.<br>Increases in global energy demand and rising levels of atmospheric carbon dioxide demand renewable alternatives to fossil fuels as the primary energy sources of the 21st century. Solar energy is by far the most abundant renewable energy resource, yet its widespread use has been hampered by a lack of suitable methods to store energy from sunlight in a cheap and efficient manner. Solar driven water splitting is a promising method of storing solar energy, but a critical bottleneck in developing efficient photoelectrochemical (PEC) water splitting systems lies in the kinetic sluggishness of the water splitting reactions, particularly the oxygen evolution reaction (OER). In this thesis the structural and mechanistic underpinnings for the activity of a promising nickel-based oxygen evolving catalyst (OEC) are discussed. The catalyst is particularly attractive as a result of the simplicity of its preparation as a thin film from aqueous borate-buffered solutions of Ni₂ . Electrochemical and in situ X-ray absorption near-edge structure (XANES) studies of this nickel-borate (Ni-Bi) catalyst indicate that upon initial electrodeposition, Ni centers in the film exist predominantly in the +3 oxidation state and the as-deposited material is largely inactive towards the OER. Catalytic activation is achieved by anodization of the as-deposited material in concentrated borate buffer, pH 9.2, a process which serves to oxidize the nickel centers to a mixed-valence Ni(II/IV) state. Extended X-ray absorption fine structure (EXAFS) spectroscopy studies indicate that Ni-Bi is comprised of nanometer-sized clusters of edge sharing NiO₆ octahedra. A structural transformation is observed during anodization that is akin to that observed in the [beta]-NiOOH-[gamma]-NiOOH transformation, challenging the long-held view that the phase that is the most catalytically active towards the OER is the all-Ni(III) [rho]-NiOOH. Electrokinetic studies indicate that the as-deposited Ni-Bi exhibits a Tafel slope close to 2.3 x 2RT/F, consistent with a turnover-limiting electron transfer (ET) from the geometrically distorted low-spin d⁷ Ni(III) state. Upon anodization to the mixed valence Ni(III/IV) state and elimination of geometric distortion, ET from the resting state becomes more facile resulting in a low Tafel slope of 2.3 x RT/2F, indicative of a rapid two-electron pre-equilibrium followed by a rate limiting chemical step, likely O₂ formation. Anodized Ni-Bi also exhibits an inverse third order dependence in proton activity and inverse first order dependence in borate anion. This kinetically-relevant two-electron, three-proton proton-coupled electron transfer (PCET) equilibrium prior to rate limiting O₂ formation forms the mechanistic basis for the pHdependent difference in activity between Ni-Bi and its cobalt-based analog, which contrarily mediates oxygen evolution via a kinetically-relevant one-electron, one-proton PCET transformation. The difference in catalytic O₂ evolution mechanism is a principal factor in the determination of the overall solar-to-fuels efficiency of PEC water splitting systems.<br>by Daniel Kwabena Bediako.<br>S.M.

Dealloying carbon supported Pt alloy nanoparticles has been shown to particles with a Pt rich outer shell surrounded by an alloy rich core that are highly active electrocatalysts for the oxygen reduction reaction, which is of interest for use in fuel cell cathodes. The structure of these materials as well as how the size, elemental distribution and composition changes during fuel cell operation is important. The catalysts were subjected to an accelerated stability test under similar conditions to those experienced in fuel cell cathodes. At various points throughout the test, the ECSA was recorded and samples taken for ex situ analysis. A variety of x-ray based spectroscopic techniques including XPS, XRD and XAS were used to investigate how the catalyst structure has been affected by the test. TEM will also be used. The 5 nm Pt/C and equivalent alloy catalysts were shown to be stable under these conditions with no significant change in structure or surface area. This shows that the protocol used here does not fully represent the conditions experienced in the fuel cell where degradation is observed. In comparison, the ECSA of 2 nm Pt/C sample was greatly decreased. Further testing for either a longer duration or using higher acid concentration is required to differentiate between the stability of the 5 nm nanoparticle samples. Additionally, as measurements of the electrocatalytic activity made using the RDE technique differ significantly to the performance obtained in an MEA an alternative method was proposed. The GDE combines the advantages of the RDE system in terms of speed of testing and the quantity of catalyst required, with a more accurate representation of the conditions experienced in a fuel cell i.e ability to access the high current density regime. This method was shown to compare favourably with other electrode configurations from the literature such as floating electrodes in terms of ease of use and similarity to results from testing in PEMFC MEAs. Several issues remain with the system, notably quantifying the amount of the catalyst actually utilised, although this does also allow the study of fuel cell related phenomena such as flooding of catalyst layers.

We are facing an energy crisis because of the limitation of the fossil fuel and the pollution caused by burning it. Clean energy technologies, such as fuel cells and metal-air batteries, are studied extensively because of this high efficiency and less pollution. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential in the process of energy storage and conversion, and noble metals (e.g. Pt) are needed to catalyze the critical chemical reactions in these devices. Functionalized carbon nanomaterials such as heteroatom-doped and molecule-adsorbed graphene can be used as metal-free catalysts to replace the expensive and scarce platinum-based catalysts for the energy storage and conversion. Traditionally, experimental studies on the catalytic performance of carbon nanomaterials have been conducted extensively, however, there is a lack of computational studies to guide the experiments for rapid search for the best catalysts. In addition, theoretical mechanism and the rational design principle towards ORR and OER also need to be fully understood. In this dissertation, density functional theory calculations are performed to calculate the thermodynamic and electrochemical properties of heteroatom-doped graphene and molecule-adsorbed graphene for ORR and OER. Gibb's free energy, overpotential, charge transfer and edge effect are evaluated. The charge transfer analysis show the positive charges on the graphene surface caused by the heteroatom, hetero-edges and the adsorbed organic molecules play an essential role in improving the electrochemical properties of the carbon nanomaterials. Based on the calculations, design principles are introduced to rationally design and predict the electrochemical properties of doped graphene and molecule-adsorbed graphene as metal-free catalysts for ORR and OER. An intrinsic descriptor is discovered for the first time, which can be used as a materials parameter for rational design of the metal-free catalysts with carbon nanomaterials for energy storage and conversion. The success of the design principle provides a better understanding of the mechanism behind ORR and OER and a screening approach for the best catalyst for energy storage and conversion.

Solar water-splitting is a potentially transformative renewable energy technology. Slow kinetics of the oxygen evolution reaction (OER) limit the efficiency of solar-water-splitting devices, thus constituting a hurdle to widespread implementation of this technology. Catalysts must be stable under highly oxidizing conditions in aqueous electrolyte and minimally absorb light. A grand goal of OER catalysis research is the design of new materials with higher efficiencies enabled by comprehensive understanding of the fundamental chemistry behind catalyst activity. However, little progress has been made towards this goal to date. This dissertation details work addressing major challenges in the field of OER catalysis. Chapter I introduces the current state-of-the-art and challenges in the field. Chapter II highlights work using ultra-thin films as a platform for fundamental study and comparison of catalyst activity. Key results of this work are (1) the identification of a Ni0.9Fe0.1OOH catalyst displaying the highest OER activity in base to date and (2) that in base, many transition-metal oxides transform to layered oxyhydroxide materials which are the active catalysts. The latter result is critical in the context of understanding structure-activity relationships in OER catalysts. Chapter III explores the optical properties of these catalysts, using in situ spectroelectrochemistry to quantify their optical absorption. A new figure-of-merit for catalyst performance is developed which considers both optical and kinetic losses due to the catalyst and describes how these factors together affect the efficiency of composite semiconductor/catalyst photoanodes. In Chapter IV, the fundamental structure-composition-activity relationships in Ni1-xFexOOH catalysts are systematically investigated. This work shows that nearly all previous studies of Ni-based catalysts were likely affected by the presence of Fe impurities, a realization which holds significant weight for future study of Ni-based catalyst materials. Chapter V discusses the synthesis of tin-titanium oxide nanoparticles with tunable lattice constants. These materials could be used to make high-surface-area supports for thin layers of OER catalysts, which is important for maximizing catalyst surface area, minimizing the use of precious-metal catalysts, and optimizing 3D structure for enhanced mass/bubble transport. Finally, Chapter VI summarizes this work and outlines directions for future research. This work contains previously published and unpublished co-authored material.<br>2015-03-29

A class of novel nickel cobalt oxide hollow nanosponges were synthesized through a sodium borohydride reduction strategy. Due to their porous and hollow nanostructures, and synergetic effects between their components, the optimized nickel cobalt oxide nanosponges exhibited excellent catalytic activity towards oxygen evolution reaction.

Electrolysis of water to form H2 and O2 and electrocatalytic reduction of CO2 to CO have attracted increasing attention these years. To realize a large-scale production of H2 and CO, it is critical to develop efficient and earth-abundant catalysts that could overcome the slow kinetics of the O2 evolution reaction in water splitting and selectively reduce CO2 over the competing H2 evolution reaction. This thesis describes the synthesis, characterization, and evaluation of nickel-based heterogeneous and homogeneous (molecular) electrocatalysts for water oxidation and CO2 reduction, respectively. The first project describes the Fe incorporated ultrathin Ni(OH)2 nanosheets which exhibit dramatically enhanced performance in electrocatalytic O2 evolution. The second project focus on a molecular Ni complex with a pyridyl biscarbene ligand for electrocatalytic CO2 reduction. Compared to those typical Ni complexes with cyclam-derivatized and/or -analogous tetradentate ligands, our Ni complex showed high selectivity for CO2 reduction over the competing H2 evolution reaction.

This thesis mainly focuses on the rational design and preparation of bi-transition metal oxide materials for high-performance electrochemical catalysis, such as hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). To address the challenges of sluggish kinetics and large overpotentials in HER and OER, the effective strategy of morphology engineering, introducing a secondary metal element and supporting on carbon-based materials were carried out and discussed.

Transition metal-nitrogen complex have shown promising electrocatalytic activity towards the oxygen reduction reaction (ORR) that can potentially replace the platinum-based electrocatalysts in fuel cell, which generally suffer from scarcity and instability issue. Iron and cobalt have been reported to posses the best electrocatalytic performance in comparison with other transition metals due to the nature of their d-electron configuration that fulfill the prerequisite strong back-bonding for the activation of oxygen molecule. Apart from the metal active centre, other factors such as catalyst support, electrode thickness and surface-nitrogen content have also been considered play important roles to improve the catalytic performance of transition-metal-nitrogen complex materials. In this study we integrated those factors and approaches to create non-noble metal-based electrocatalysts for proton exchange membrane fuel cell (PEMFC) with improved catalytic activity. Iron and cobalt were used as ORR metal active centers and different type of carbon supports were employed as electrocatalysts supports. Three different electrocatalysts were developed in this project, including ironcobaltnitrogen complex supported carbon nanotubes that were grown on carbon paper substrate, iron-cobalt-nitrogen complex incorporated vertically aligned carbon nanotubes and iron-cobalt-nitrogen complex incorporated vertically aligned nitrogen-doped carbon nanotubes. The electrochemical performances of those electrocatalysts were compared with platinum-based electrocatalyst, which is the most common commercial electrocatalysts recently. The results show that the developed non-noble metal-based electrocatalysts posses improved electrocatalytic properties in terms of electrochemical surface area, electron transfer number, kinetic rate constant, durability and methanol fuel tolerance.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.<br>Cataloged from PDF version of thesis.<br>Includes bibliographical references.<br>Searching for active and cost-effective catalysts for oxygen electrocatalysis is essential for the development of efficient clean electrochemical energy technologies. Perovskite oxides are active for surface oxygen exchange at evaluated temperatures and they are used commonly in solid oxide fuel cells (SOFC) or electrolyzers. However, the oxide surface chemistry at high temperatures and near ambient oxygen pressure is poorly understood, which limits the design of highly active catalysts. This work investigates heterostructured interfaces between (Lai. xSrx)CoO 3-3 (where x = 0.2 and 0.4, LSC80-2011 3 and LSC60-40 113 respectively) and (Lao. 5 Sro.5 )2CoO 4 ,3 (LSC2 14) enhanced ORR catalytic activity 1) via electrochemical impedance spectroscopy, atomic force microscopy, scanning electron microscopy, scanning transmission electron microscopy, and high resolution X-ray diffraction (HRXRD) and 2) using in situ ambient pressure X-ray photoelectron spectroscopy (APXPS) and in situ HRXRD. Here we show that the ORR of epitaxial LSC80-20 1 3 and LSC60-40113 is dramatically enhanced (~3-4 orders of magnitude above bulk LSC113) by surface decorations of LSC214 (LSC 1 31214) with coverage in the range from ~0.1 to ~15 nm. Such high surface oxygen kinetics (~ 110-5 cm-s1 at 550 C) are among the most active SOFC cathode materials reported to date. Although the mechanism for ORR enhancement is not yet fully understood, our results to date show that the observed ORR enhancement can be attributed to highly active interfacial LSCn 13/LSC214 regions, which were shown to be atomically sharp. Using in situ HRXRD and APXPS we show that epitaxial LSC80-20n3 thin films have lower coverage of surface secondary phases and higher Strontium enrichment in the perovskite structure, which is attributed to its markedly enhanced activity relative to LSC80-20113 powder. APXPS temperature cycling of epitaxial LSC80-20113 APXPS reveled upon heating to 520 *C the initial Sr enrichment which is irreversible, however subsequent temperature cycling demonstrates a small amount of reversible Sr enrichment. With applied potentials LSC80- 2013/214 shows significant Sr enrichment greater then LSC80-20 113, and the ability to stabilize high concentrations of both lattice and surface Sr which we hypothesize is a very important factor governing LSC80-2011 3214 enhanced ORR activity.<br>by Ethan Jon Crumlin.<br>Ph.D.

In the search for alternative energy technologies, low temperature fuel cells continue to feature as technologies with the most promise for mass commercialization. Among the low temperature fuel cells, alkaline and proton exchange membrane fuel cells are the most popular. Alkaline fuel cells have typically been used for water generation as well as auxiliary power for space shuttles. Their bulkiness however makes them undesirable for other applications, especially in automobiles, where there is a great demand for alternative technologies to internal combustion engines. Proton exchange membrane fuel cells on the other hand possess numerous qualities including their compact size, high efficiency and versatility. Their mass implementation has however been delayed, because of cost among other reasons. Most of this cost is owed to the Pt/C catalyst that accounts for about half of the price of the PEM Fuel Cell. This catalyst is used to drive the sluggish oxygen reduction reaction that occurs at the cathode of the PEM Fuel Cell. To overcome this obstacle, which is to make PEM Fuel Cell technology more affordable, reducing the amount Pt has traditionally been the approach. Another approach has been to find new ideal catalyst-support combinations that increase the intrinsic activity of the supported material. One more strategy has been to find lower cost alternative materials to Pt through synthetic and kinetic manipulations to rival or exceed the current oxygen reduction reaction activity benchmark. To this end, Palladium has garnered significant interest as a monometallic entity. Its manipulation through synthetic chemistry to achieve different morphologies - which favor select lattice planes - in turn promotes the oxygen reduction reaction to different degrees. In bimetallic or, in more recent times multimetallic frameworks, geometric and ligand effects can be used to form ideal compositions and morphologies that are synergistic for improved oxygen reduction reaction kinetics. In this dissertation, we have explored three different approaches to make contributions to the catalysis and electrocatalysis body of literature. In the first instance, we look at the influence of ligand effects through the active incorporation of a PVP capping agent on the stability of ~3nm Pt NPs. Washed (no capping agent) and unwashed (with capping agent) batches of NPs were evaluated via cyclic voltammogram analyses to evaluate differences there might be between them. It was found that the current density measurements for unwashed particle batches were higher. This increase in current density was attributed to the monodentate and bidentate ligand bonding from the PVP, which increased as a function of cycle number and plateaued when the PVP was completely decomposed. The complete decomposition of PVP during the CV experiment was estimated to occur around 200 cycles. The remaining portion of the dissertation explores the electrocatalytic properties of Palladium based NPs. The first instance, a monometallic study of Palladium cubes and dendrites was aimed at building on a recent publication on the enhanced ORR activity that was achieved with a PdPt bimetallic dendrite morphology. In our work, we sought to isolate the dendritic morphology properties of the monometallic Pd composition in order to understand what advantages could be achieved via this morphology. Pd cubes were used as a comparison, since they could be generated through the combination of a similar set of reagents simply by switching the order of addition. It was found that while there was no significant variation in the ORR activity as a function of morphology / shape, there was an interesting interaction between hydrogen and the palladium NPs in the hydrogen oxidation region that varied as a function of shape. This led to further sorption and ethylene hydrogenation studies, which suggested that, the interaction between hydrogen and Pd depended on the environment. Within the electrochemical environment, the ECSA measured, suggested that hydrogen was being reversibly absorbed into the sub-surface octahedral sites of Pd. The higher ECSA for Pd cubes corroborated with higher sorption for Pd cubes as well. However ethylene hydrogenation showed that the fringes of the Pd dendrites provided additional sites for reaction, which in turn translated to higher conversion. Furthermore, through a Koutecky-Levich analysis, it was found out that the Pd dendrites while exhibiting slightly lower activity, favored the 4-electron oxygen reduction process more than the Pd cubes. In the last part of this dissertation we explored the electrocatalytic properties of Pd-based bimetallic NPs under different morphologies including nanocages and sub-10nm alloys. With the inclusion of Ag, it was found out, through Koutecky-Levich analysis that the 4-electron process was better observed under alkaline conditions using a 0.1M NaOH(aq) electrolyte solution instead of a 0.1M HClO4 (aq) for acidic media testing. It was found that, for PdAg nanocage morphologies, where the Pd galvanically replaced the Ag to form cages, the four-electron process was suited to thinner Pd shells. Indeed the average electron numbers measured for Ag nanocubes coated with a 6nm shell was in agreement, within reason of literature values for bulk Ag. However, since the binding energy that both metals have for OH is so close, the potential for contributions to the ORR kinetics in alkaline media by Pd is a potential consideration.

This dissertation the syntheses of Pt-based binary and ternary alloy electrocatalysts using the transition metals of Co and Ni are presented. These electrocatalysts were synthesised by an impregnation-reduction procedure at high temperature whereby Pt supported on carbon, (Pt/C (40 percent), was impregnated with the various metal and mixtures thereof and reduced at high temperatures in a H2 atmosphere. The procedure was also designed in such a way so as to prevent the oxidation of the support material (carbon black) during the alloy formation. The resultant nanoparticles (9-12 nm) of Pt3Co/C, Pt3Ni/C and Pt3Co0.5Ni0.5/C were also subjected to a post treatment procedure by acid washing (denoted AW) to produce electrocatalysts of Pt3Co/C-AW, Pt3Ni/C-AW and Pt3Co0.5Ni0.5/C-AW to study the effect of acid treatment on these electrocatalysts. The synthesised electrocatalysts were then characterised by a number of physical and electrochemical techniques and compared to that of commercial Pt/C (Pt/C-JM, HiSpec 4000) as well as Pt/C catalysts (Pt/C-900 and Pt/C-900-AW) treated under the same conditions used for the alloy synthesis. The electrocatalysts were then used to fabricate MEAs that were loaded into commercial single test cells and characterised by means of polarisation curves and Electrochemical Impedance Spectroscopy (EIS). The extensive physical characterisation included Powder X-Ray Diffraction (PXRD) analysis, Transmission Electron Microscopy (TEM), elemental analysis by Energy Dispersive Spectroscopy (EDS) and metal loading by Thermo-Gravimetric Analysis (TGA). These studies showed that Pt-based alloy electrocatalysts were successfully synthesised with particle sizes ranging from 9 - 12 nm, within their respective atomic ratios and whereby no significant loss of carbon support occurred. This indicated that significant sintering or electrocatalyst particles occurred when compared to that of the starting Pt/C catalyst (3 – 4 nm). From the combined results of the physical characterisation procedures, it was also shown that leaching as a result of acid washing was catalyst dependent with Ni containing catalysts showing a significant degree of leaching compared to that of Co containing catalysts. Electrochemical characterisation in terms of Electrochemical Active Surface Area (ECSA) by Cyclic Voltammetry (CV) and ORR activity by Rotating Disc Electrode (RDE) analysis revealed that a significant decrease in the ECSA resulted from the increase in particle size and this had a major influence on the ORR activity. Furthermore it was found that a significant improvement in the ORR activity was achieved by the synthesis of Pt-based alloys. It was also found that catalytic properties of the acid washed electrocatalysts were substantially different from that of non-acid washed electrocatalysts. The experimental data confirmed that it was possibly to achieve better catalytic performance as compared to that of Pt/C at a lower material cost when Pt is alloyed with base transition metals. The trend observed from the ORR activity studies by RDE was successfully repeated in the in-situ fuel cell testing in terms of mass activity of the electrocatalysts. Of the electrocatalysts studied under „real‟ fuel cell conditions Pt/C-JM had the best performance compared to the others, with the ternary Pt3Co0.5Ni0.5/C showing better catalytic performance compared to the Pt3Co/C electrocatalyst. This was found to be due to a higher charge transfer resistance observed in Pt3Co/C as compared to that of Pt3Co0.5Ni0.5/C which was similar than that of the commercial Pt/C-JM catalyst with both Pt3Co/C and Pt3Co0.5Ni0.5/C-AW having similar but higher ohmic resistances than that of Pt/C-JM as determined by electrochemical impedance spectroscopy. The results showed that a great potential exist to improve the catalytic performance of low temperature PEM fuel electrocatalysts at a reduced cost as compared to that of pure Pt provided a method of controlling the particle size was established.

付記する学位プログラム名: 京都大学大学院思修館<br>京都大学<br>新制・課程博士<br>博士(総合学術)<br>甲第23346号<br>総総博第19号<br>京都大学大学院総合生存学館総合生存学専攻<br>(主査)教授 寶 馨, 教授 内本 喜晴, 特定教授 橋本 道雄<br>学位規則第4条第1項該当<br>Doctor of Philosophy<br>Kyoto University<br>DFAM

<p>Electrocatalysts based on IrO₂ have been synthesised and characterised using a wide range of techniques. These oxide materials were primarily developed as oxygen evolution electrocatalysts for proton exchange membrane (PEM) water electrolysis. This development has enabled high performances to be achieved in a PEM water electrolysis cell. Overall the best result was obtained with an Ir_0.6Ru_0.4O₂ anode and 20 wt% Pt/C cathode, with a cell voltage of 1.567 V at 1 A cm⁻² and 80 °C when using Nafion 115 as the electrolyte membrane. This represents a cell efficiency of 76 % (ε_ΔG) and an energy consumption of 3.75 kWhr Nm⁻³ H2 at 1 A cm⁻².</p><p>Initial results showed that previous synthesis methods used for PEM water electrolysis electrocatalysts, were not suitable for multi element oxides, due to the formation of multiple oxide phases, and general controllability problems. This lead to the development of a new method, based on the thermal oxidation of metallic colloids. This method (modified polyol method) was then used to prepare Ir_xSn_1−xO₂ and Ir_xRu_0.5−_xSn_0.5O₂. </p><p>The effect of annealing temperature on Ir_xSn_1−xO₂was examined and showed that the crystallinity increases, and the active area decreases, with increasing annealing temperature. The specific activity of this material however was seen to be constant over the entire temperature range, with the performance losses only due to the reduction of active surface area. At high temperatures, the solid solution of Ir_xSn_1−xO₂ was seen to become unstable, with segregation of SnO2 from the oxide lattice, and formation of metallic iridium.</p><p>The electrochemical properties of the oxides prepared at 500 °C, showed that the addition of SnO₂ to IrO₂ particles had no beneficial effect. Cyclic voltammetry showed that the active area decreases as the tin content increases, with this related to the crystallinity increase and dilution of the active iridium oxide sites. Additions of up to 20 mol% tin may be acceptable as little change in the active area occurs at low tin contents, however there was still a 40 mV increase in cell voltage at 1 A cm₋₂ and 80 °C in a PEM water electrolyser. The specific activity of the oxides prepared by the polyol method remains constant until 50–60 mol% tin whereafter the activity decreases. Overall, the electrocatalytic properties in 0.5 M H₂SO₄ and a PEM cell are similar, however there is evidence to suggest that there are extra resistance issues in the PEM cell due to the poor conductivity of some of the prepared oxides. The effect of anode composition on the PEM cell ohmic resistance confirmed that high tin contents causes high performance losses due to poor layer conductivity. </p><p>Ir_xRu_0.5−xSn_0.5O₂ electrocatalysts showed that additions of 15–25 mol% ruthenium improved the overall oxygen evolution performance at low current densities. However due to agglomeration of metallic ruthenium during the colloid synthesis stage, the electrochemically active area of this oxide, decreased with ruthenium content. XPS revealed that the reduction in active area was directly related to the concentration of noble metals at the surface of the powders. The specific activity of the iridium species at the electrode surface was seen to increase as the ruthenium content of the bulk increased. This finding maybe due to a “support” affect, in which the underlying ruthenium influences the structure and electronic properties of the surface iridium. This reduced active surface area and the increase specific electrocatalytic activity resulted in an optimum in the electrocatalytic performance towards the oxygen evolution reaction at 15–25 mol% Ru. </p><p>Ir_xRu_yTa_zO₂ nanocrystalline and amorphous powders were prepared by a hydrolysis method. These oxides exhibited high active surface areas and supercapacitor like properties with the electrode capacitance typically around 200-300 F g⁻¹. No evidence was found to suggest that tantalum oxide forms solid solutions with iridium or ruthenium oxides. It was found however, that the lattice parameters of the rutile oxide in Ir_xRu_yTa_zO₂samples, can be explained by a solid solution between IrO₂ and RuO₂. Electrochemical measurements showed that additions of tantalum decreased the electrochemically active surface area, as did high levels of ruthenium. At intermediate ruthenium contents, it was shown that both the low current and high current performance in a PEM water electrolysis cell was very high. Additions of up to 20 mol% Ta are possible without significantly decreasing the performance.</p><p>X-ray absorption spectroscopy was used to examine a range of oxide electrocatalysts using both ex-situ and in-situ measurements. The in-situ measurements were performed on oxide electrodes polarised in aqueous 0.5 M H₂SO₄ electrolyte. The ex-situ measurements on Ir_xSn_1−xO₂ showed that it is unlikely that Ir and Sn are atomically mixed as there was no evidence to suggest that Sn is within the first few coordination shells of the Ir. In contrast, for Ir_xRu_1−xO₂, Ru was found in the first few Ir–metal coordination shells. Electrochemical measurements showed that for amorphous IrO₂, increasing the electrode potential from 0.5 to 1.4 V causes there to be more d-orbital vacancies in the iridium atoms. A valence change (probably Ir³⁺ → Ir⁴⁺) also occurred between 0.9 and 1.0 V vs RHE and there was no evidence to suggest that the iridium was present in any higher oxidation state. From the EXAFS analysis, it was found that the Ir–O bond length decreased by 0.05°A as the potential increased from 0.5 to 1.4 V vs RHE. This is in good agreement with the expected bond length change due to valence change.</p>

Electrocatalysis contributes to a huge extent in a large array of research fields and applications, including corrosion science, electroanalytical sensors, wastewater treatment, electro-organic synthesis and more importantly, energy conversion applications. Of the many electrocatalytic processes, the oxygen evolution reaction (OER) and triiodide reduction reaction (IRR) are of widespread importance in electrochemical cells and dye-sensitised solar cells (DSSCs). OER is a key half reaction in electrochemical water splitting, direct solar-to-electricity driven water splitting and metal-air batteries. The high cost of efficient benchmark electrocatalysts, such as RuO2 or IrO2, however, is a major drawback of OERs. While, IRR plays a significant role in DSSCs, which must be electrocatalysed at the counter electrode to complete the external circuit in real devices and thereby successfully convert solar energy to electricity. Traditionally, Pt is accepted as an ideal benchmark electrocatalyst for IRR, but its high cost and scarcity limits broad application of DSSCs. Thus, extensive effort has been made to find active alternative electrocatalysts with low-cost, high electrocatalytic activity and excellent stability for OER and IRR to the noble metals (Ru, Ir and Pt). Therefore, a rational design of earth-abundant and low-cost electrocatalysts for OER and IRR maintains a paramount significance for energy conversion applications to meet the constantly growing demand for energy supply.<br>Thesis (PhD Doctorate)<br>Doctor of Philosophy (PhD)<br>Griffith School of Environment<br>Science, Environment, Engineering and Technology<br>Full Text

Two major challenges that impede fuel cell technology breakthrough are the insufficient activity of the electrocatalysts for the oxygen reduction reaction and their degradation during operation, caused by the potential-induced corrosion of their carbon-support upon fuel cell operation. Unsupported electrocatalysts derived from tailored noble-metal nanostructures are superior to the conventional carbon-supported Pt nanoparticle catalysts and address these barriers by fine-tuning the surface composition and eliminating the support. Herein, recent efforts and achievements in the design, synthesis and characterization of unsupported electrocatalysts are reviewed, paying special attention to noble-metal aerogels, nano/meso-structured thin films and template-derived metal nanoarchitectures. Their electrocatalytic performances for oxygen reduction are compared and discussed, and examples of successful catalyst transfer to polymer electrolyte fuel cells are highlighted. This report aims to demonstrate the potential and challenges of implementing unsupported catalysts in fuel cells, thereby providing a perspective on the further development of these materials.

Oxygen reduction reaction (ORR) is one of the critical reactions in many energy storages and conversion systems, such as fuel cells and metal-air batteries. Cost-effective and high-performance electrocatalysts for oxygen reduction reactions are needed for many energy storage and conversion devices. This project will prove that whey powder, a cheap by-product in the production of cheese and casein, can be used as a sustainable precursor to produce heteroatom-doped carbon electrocatalysts for ORR. The compounds rich in N and S elements in whey powder can produce plentiful catalytic active sites. However, the reactants in ORR cannot easily reach these sites. A dual-template method was used to create a hierarchical and interconnected porous structure with micropores created by Zinc chloride (ZnCl2) and large mesopores generated by fumed Silicon dioxide(SiO2) particles. At the optimum mass ratio of whey power: ZnCl2: SiO2 at 1:3:0.8, the resulting carbon material has a large specific surface area at 1944.7 m2 g–1, containing 4.6 at.% of N with 50.4% as pyridinic N. This carbon material shows superior catalytic activity for ORR, with an electron transfer number of 3.88 and a large kinetic limiting current density of 45.40 mA cm–2. They were employed as ORR catalysts to assemble primary zinc-air batteries, which deliver a power density of 84.1 mW cm−2 and a specific capacity of 779.5 mAh g−1, outperforming batteries constructed using the commercial Pt/C catalyst. Our findings open new opportunities to use an abundant biomaterial, whey powder, to create high-value-added carbon electrocatalyst for emerging energy applications.

The aim of this thesis is to develop more active catalysts for the oxygen reduction reaction whilst decreasing the metal content to drive forward the emergence of the fuel cell technology on the market. Chapter 3 presents the preparation of Ni modified Pt/C catalysts (Ni(acac)2 and Ni(Cp)2) using the controlled surface modification technique. The resulting catalysts were heat treated at 200, 500, 750 and 900 °C and the catalysts were characterised by ICP-OES, TEM, EDX, CV, RDE, EXAFS and XPS. The catalysts exhibited up to 8-fold increase in specific activity and up to 9-fold increase in mass activity. The increase in activity was assigned to (a) the synergistic effect of Ni on Pt and (b) the degree of alloying which has two consequences: (a) decrease of the Pt d-band centre and (b) change of the arrangement of the Pt and Ni atoms at the surface of the particles. The decrease in the Pt d-band centre resulted in the lowering of the adsorption strength of the oxide species which in turn led to a lower Pt-O coverage. This was supported by the decrease of the reduction potential of the oxide reduction peak as the heat treatment temperature increased. In addition, as the heat treatment temperature increased, the Pt surface concentration increased due to the diffusion of the Ni atoms inwards and the diffusion of the Pt atoms towards the outside of the particle. This led to larger and more well-defined Pt crystal planes. The presence of more well-defined Pt crystal planes seemed to provide more suitable adsorption site for the dual-site adsorption of the oxygen, thus increasing the activity. Last but not least, the highest increase in catalytic activity was exhibited by the catalysts heat treated at 500 °C. This demonstrated the importance of the choice of the secondary metal and the importance of the arrangement of the atoms at the surface of the particles. Chapter 4 presents what is believed to be the first attempt to prepare Pt modified Ni/C catalysts (Pt(acac)2) using the controlled surface modification technique. The catalysts were characterised by TEM, CV and RDE. The deposition of the Pt precursor was shown to be incomplete; however, the catalysts still had a Pt content of ~ 4 wt%. Despite the low Pt content, the catalysts exhibited up to 8-fold increase in specific activity. The increase in activity was assigned to the synergistic effect between Ni and Pt which was shown by the decrease in the lattice parameter and the decrease of the overpotential of the oxide reduction peak. Chapter 5 offers a summary of the thesis as well as a list of the strategies employed to date to increase the catalytic activity of the cathode catalysts. It also includes some suggestion for future work including underpotential deposition (UPD), MEA testing and stability testing

The PhD project has been performed in the Surfaces and Catalysts group active in the Department of Chemical Sciences, within the frame of the grant “A rational approach to the optimization of efficient electrocatalysts for the next generation Fuel Cells”, funded by CARIPARO foundation. The project has been focused on the preparation and characterization of new carbon-based materials for applications in Polymer Electrolyte Membrane Fuel Cells (PEMFCs), also known as oxygen-hydrogen FCs. The preparation of the materials has been performed using different techniques, depending on the type of the target material and on the possible applications that these materials can offer. With reference to the studied model systems (Highly Oriented Pyrolytic Graphite (HOPG) and Glassy Carbon (GC)), the introduction of doping heteroatoms has been performed by ion implantation, while the study of new chemical functionalities has been allowed by the use of Wet Chemistry techniques, in particular derived from the electrochemical synthesis. The deposition of thin films or nanoparticles (metal or oxides of transition metals) on the ion-modified materials has been carried out in-situ by using advanced techniques under Ultra High Vacuum conditions (UHV), such as Physical Vapor Deposition (PVD). Within the study of the model systems, PVD was chosen because of its ability to provide an atomic scale control of the metal deposition. In a second time, conventional deposition techniques such as chemical or electrochemical reduction of suitable metal precursors have been performed, in a synergistic combination between Surface Science and Electrochemistry-derived techniques. The characterization of these materials has been performed using the facilities of the Surface Science group, such as the X-ray and Ultraviolet Photoelectron Spectroscopy (XPS - UPS), Scanning Tunneling and Atomic Force Microscopy (STM - AFM), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDX) and Low Energy Electron Diffraction (LEED). To get a deeper insight in the chemistry/structure/properties of the prepared systems, synchrotron light-based techniques such as HR-XPS, NEXAFS, ARPES, ResPES and PEEM have been extensively used. The study of the electro-catalytic activity has been performed using conventional Electrochemistry techniques, in particular Cyclic and Linear Sweep Voltammetry (CV - LSV), as well as electro-dynamic techniques such as Rotating Disk Electrode (RDE). Finally, in order to support the experimental data or to bring their understanding at a deeper level, simulations using Density Functional Theory (DFT) have been performed in collaboration with the group coordinated by Prof. Cristiana Di Valentin (University of Milano Bicocca). During the course of the doctorate, several collaborations have been pursued with other research groups operating in the Department of Chemical Sciences or abroad, such as the "Interfaces and Energy Conversion E19" research unit, Technical University of Munich (TUM, Germany), coordinated by Profs. O. Schneider and J. Kunze-Liebhäuser.<br>Il progetto di dottorato nasce all’interno del gruppo di ricerca di Superfici e Catalizzatori operante nel dipartimento di Scienze Chimiche, nell’ambito della borsa a titolo vincolato “Un approccio razionale alla ottimizzazione di elettrocatalizzatori efficienti per le celle a combustibile di nuova generazione”, finanziata da fondazione CARIPARO. Le tematica è stata focalizzata sulla preparazione e caratterizzazione di nuovi materiali a base di carbonio utilizzabili per applicazioni in celle a combustibile di tipo PEMFCs (Polymer Electrolyte Membrane Fuel Cells) ad ossigeno-idrogeno. La preparazione dei materiali è avvenuta facendo uso di differenti tecniche, in relazione al tipo di materiale oggetto di studio ed alle applicazioni che tali materiali possono offrire. Con riferimento allo studio dei sistemi modello (grafite pirolitica altamente orientata, HOPG, e carbonio vetroso, GC), il drogaggio degli stessi mediante l’introduzione di eteroatomi (in particolare azoto) è avvenuto ricorrendo alla tecnica dell’impiantazione ionica, mentre lo studio di nuove funzionalità chimiche è stato permesso dall’utilizzo di tecniche di Wet Chemistry, in particolare mutuate dalla sintesi elettrochimica. La deposizione di film sottili o di nanoparticelle (metalliche o a base di ossidi di metalli di transizione) su tali materiali modificati è stata effettuata facendo uso di tecniche avanzate come la deposizione fisica da fase vapore (PVD) in condizioni controllate di Ultra Alto Vuoto (UHV), in grado di offrire un controllo su scala atomica della deposizione di tali film. Sono state utilizzate anche tecniche di deposizione tradizionali quali la riduzione chimica o elettrochimica di opportuni precursori metallici: l‘utilizzazione di una siffatta combinazione sinergica tra tali differenti tecniche di preparazione ha permesso di ottenere materiali caratterizzati da strutture e proprietà peculiari. La caratterizzazione di tali materiali è svolta utilizzando le facilities del gruppo di Scienza delle Superfici, come la spettroscopia di fotoelettroni (XPS) o della banda di valenza (UPS), la microscopia ad effetto tunnel o a forza atomica (STM - AFM), la microscopia elettronica e la dispersione energetica dei raggi X indotta dagli elettroni (SEM-EDX), la diffrazione di elettroni lenti (LEED). Allo scopo di caratterizzare maggiormente in dettaglio la struttura e le proprietà chimiche dei materiali preparati sono state usate estensivamente le tecniche di indagine offerte dalla luce di sincrotrone (HR-XPS, NEXAFS, ARPES, ResPES, PEEM), mentre lo studio della reattività catalitica si basa su tecniche derivate dall’analisi elettrochimica, in particolare la voltammetria ciclica ed a scansione lineare del potenziale applicato, nonchè tecniche elettro-dinamiche come la voltammetria su elettrodo rotante. Infine, allo scopo di supportare i dati sperimentali o portare la comprensione delle proprietà dei materiali ad un livello più profondo, simulazioni mediante teoria del funzionale densità (DFT) sono state adottate per un approccio critico allo studio dei materiali preparati (in collaborazione con il gruppo coordinato dalla prof. Cristiana Di Valentin, Università di Milano Bicocca). Durante il corso del dottorato, diverse collaborazioni sono state perseguite con gruppi interni al Dipartimento di Scienze Chimiche o anche Esteri, come l’unità di ricerca “Interfaces and Energy Conversion E19”, dell’università tecnica di Monaco di Baviera (TUM, Technische Universität München, Germania), coordinata dai proff. O. Schneider e J. Kunze-Liebhäuser.

This project focuses on the computational design of novel catalyst for artificial synthesis: converting sunlight into fuels. With the atomic-scale insight of catalysts obtained by theoretical calculations, many efficient and optimum catalysts for these processes have been designed and engineered. The outcomes of this thesis are expected to provide theoretical solutions for current global energy and environmental challenges.