Academic literature on the topic 'Oxygen Electrocatalysts'

Oxygen reduction reaction (ORR) has attracted considerable attention for clean energy conversion technologies to reduce traditional fossil fuel consumption and greenhouse gas emissions. Although platinum (Pt) metal is currently used as an electrocatalyst to accelerate sluggish ORR kinetics, the scarce resource and high cost still restrict its further scale-up applications. In this regard, biomass-derived carbon electrocatalysts have been widely adopted for ORR electrocatalysis in recent years owing to their tunable physical/chemical properties and cost-effective precursors. In this minireview, recent advances of the optimization strategies in biomass-derived carbon electrocatalysts towards ORR have been summarized, mainly focusing on the optimization of pore structure and active site. Besides, some current challenges and future perspectives of biomass-derived carbon as high-performance electrocatalysts for ORR have been also discussed in detail. Hopefully, this minireview will afford a guideline for better design of biomass-derived carbon electrocatalysts for ORR-related applications.

Hydrogen is regarded as a key renewable energy source to meet future energy demands. Moreover, graphene and its derivatives have many advantages, including high electronic conductivity, controllable morphology, and eco-friendliness, etc., which show great promise for electrocatalytic splitting of water to produce hydrogen. This review article highlights recent advances in the synthesis and the applications of graphene-based supported electrocatalysts in hydrogen evolution reaction (HER). Herein, powder-based and self-supporting three-dimensional (3D) electrocatalysts with doped or undoped heteroatom graphene are highlighted. Quantum dot catalysts such as carbon quantum dots, graphene quantum dots, and fullerenes are also included. Different strategies to tune and improve the structural properties and performance of HER electrocatalysts by defect engineering through synthetic approaches are discussed. The relationship between each graphene-based HER electrocatalyst is highlighted. Apart from HER electrocatalysis, the latest advances in water electrolysis by bifunctional oxygen evolution reaction (OER) and HER performed by multi-doped graphene-based electrocatalysts are also considered. This comprehensive review identifies rational strategies to direct the design and synthesis of high-performance graphene-based electrocatalysts for green and sustainable applications.

The commercialization of metal–air batteries requires efficient, low-cost, and stable bifunctional electrocatalysts for reversible electrocatalysis of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER).

Multifunctional electrocatalysts are vastly sought for their applications in water splitting electrolyzers, metal-air batteries, and regenerative fuel cells because of their ability to catalyze multiple reactions such as hydrogen evolution, oxygen evolution, and oxygen reduction reactions. More specifically, the application of single-atom electrocatalyst in multifunctional catalysis is a promising approach to ensure good atomic efficiency, tunability and additionally benefits simple theoretical treatment. In this review, we provide insights into the variety of single-site metal catalysts and their identification. We also summarize the recent advancements in computational modeling of multifunctional electrocatalysis on single-site catalysts. Furthermore, we explain each modeling step with open-source-based working examples of a standard computational approach.

Durability and degradation are in the focus of modern electrocatalysis research. Before moving to real applications, e.g. fuel cells in transportation or water electrolyzers for production of green hydrogen, novel electrocatalytic materials must prove acceptable stability, but “how to test the stability of electrocatalysts”? In the relatively mature proton exchange membrane fuel cell (PEMFC) research, stability is evaluated using various accelerated stress tests (ASTs). Unfortunately, even for the most studied Pt/C electrocatalysts, degradation processes like carbon corrosion and Pt dissolution that occur during common ASTs are not easily distinguishable [1]. Moreover, advanced electrocatalysts such as different shape-controlled Pt alloy nanostructures, showing promising stability in ASTs performed in model aqueous systems, are often rendered useless when moved to real applications [2]. Catalysts free of platinum-group-metals, e.g. FeNC, demonstrate different degradation extents if tested in oxygen or argon [3]. Iridium oxides, the state of the art oxygen evolution reaction (OER) electrocatalysts, are prone to dissolution in aqueous media but much more stable in solid electrolyte based electrolyzers [4]. These examples demonstrate the need for rethinking current approaches to test electrocatalyst stability. This work highlights our recent results on using coupled electrochemical techniques and tuned gas diffusion electrode (GDE) and membrane electrode assembly (MEA) cells in fuel cell and water electrolysis research. It shows that by hyphenating GDE with inductively coupled plasma mass spectrometry (ICP-MS) it is possible to investigate dissolution of electrocatalysts, such as Pt/C for PEMFC and Fe-N-C for anion exchange membrane fuel cells (AEMFC), in-operando at conditions closely resembling those in real devices [5, 6]. As another representative example, the use of model MEAs to address the discrepancy of Ir dissolution in aqueous and solid polymer electrolytes is given [7]. Based on these examples, new strategies to test and understand electrocatalysts’ degradation are discussed. References: [1] E. Pizzutilo et al., On the need of improved accelerated degradation protocols (ADPs): Examination of platinum dissolution and carbon corrosion in half-cell tests, J. Electrochem. Soc., 163 (2016) F1510-F1514. [2] K. Kodama et al., Challenges in applying highly active Pt-based nanostructured catalysts for oxygen reduction reactions to fuel cell vehicles, Nature Nanotechnology, 16 (2021) 140-147. [3] K. Kumar et al., On the influence of oxygen on the degradation of Fe-N-C catalysts, Angew. Chem. Int. Ed., 59 (2020) 3235-3243. [4] S. Geiger et al., The stability number as a metric for electrocatalyst stability benchmarking, Nature Catalysis, 1 (2018) 508-515. [5] K. Ehelebe et al., Platinum dissolution in realistic fuel cell catalyst layers, Angew. Chem. Int. Ed., 60 (2021) 8882-8888. [6] Y.-P. Ku et al., Oxygen reduction reaction causes iron leaching from Fe-N-C electrocatalysts, (2021) Submitted, DOI: 10.21203/rs.3.rs-1171081/v1. [7] J. Knöppel et al., On the limitations in assessing stability of oxygen evolution catalysts using aqueous model electrochemical cells, Nature Communications 12 (2021) 2231.

Electrochemical energy storage and conversion technologies based on electrocatalysis have been attracting more and more attention addressing increasing concerns on fossil fuel crisis and environmental deterioration. Fuel cells, zinc-air batteries, and water electrolyzer are believed to be promising candidates due to the environmental friendliness and high efficiency. These systems are associated with key reactions including oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Due to slow kinetics of these reactions, efficient electrocatalysts, e.g., Pt for ORR and RuOx/IrOx for OER, are usually required to overcome the energy barrier in electrochemical reactions to increase the reaction rate. However, the most advanced electrocatalysts are still based on above-mentioned noble metals with high cost and scarcity, which inevitably retards the large-scale commercialization of these noble metal-based energy systems. It is of great significance to replace noble metal catalysts with earth-abundant, cost-effective, and highly efficient catalysts. Here, we reported the controlled synthesis of ultrafine Co9S8 nanocrystals embedded in N, S-codoped multilayer-assembled carbon nanoplates (Co9S8/NSCP) for highly efficient oxygen electrocatalysis. The bifunctional Co9S8/NSCP electrocatalyst displays a high half-wave potential for ORR, and a low overpotential for OER in 0.1M KOH at a current density of 10 mA cm -2, much better than those of single component counterparts (Co9S8 or carbon) and comparable to noble metal catalysts. The high performance of Co9S8/NSCP can be attributed to the rationally designed hierarchical architecture with nanosized Co9S8 nanocrystals, rich N, S-codopants, highly exposed surface area, and protective graphitic layers, providing abundant active sites with full utilization and stable carbon support towards fast catalytic kinetics and durability. This work will promote further research on the development of highly efficient and stable non-noble metal electrocatalysts for ORR and OER.

Zinc-air battery technology is gaining recognition as a promising energy storage device to be used in portable electronics and electric vehicles. Despite possessing high theoretical energy density, environmental and operational safety, and easy accessibility of zinc reservoirs, the successful commercialization of zinc-air batteries suffers due to the poor oxygen electrocatalysis kinetics at the air cathode. The kinetically inept oxygen reduction and oxygen evolution reactions at the cathode lead to a large overpotential barrier and poor charge-discharge cyclic performance of the rechargeable zinc-air battery. This work demonstrates designing a multi-principal metal bifunctional electrocatalyst that is directly deposited on conductive, porous, and flexible substrates to eliminate the necessity of polymeric binders. The flexible bifunctional oxygen electrocatalyst used for the cathode of solid-state ZAB is assembled with gel polymer electrolyte and zinc anode giving excellent charge-discharge cyclic stability and constant discharge voltage (close to 1.65 V). These multi-principal metal electrocatalysts constituting quasi-equimolar concentration, provide numerous combinations of surface functionality, multiple adsorption sites, and electronic environments thus enabling better optimization of the catalytic performance.

Zinc–air batteries are one of the most excellent of the next generation energy devices. However, their application is greatly hampered by the slow kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) of air electrode. It is of great importance to develop good oxygen electrocatalysts with long durability as well as low cost. Here, A-site deficient (SmSr)0.95Co0.9Pt0.1O3 perovskites have been studied as potential OER electrocatalysts prepared by EDTA–citrate acid complexing method. OER electrocatalytic performance of (SmSr)0.95Co0.9Pt0.1O3 was also evaluated. (SmSr)0.95Co0.9Pt0.1O3 electrocatalysts exhibited good OER activities in 0.1 M KOH with onset potential and Tafel slope of 1.50 V and 87 mV dec−1, similar to that of Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF-5582). Assembled rechargeable Zn–air batteries exhibited good discharge potential and charge potential with high stability, respectively. Overall, all results illustrated that (SmSr)0.95Co0.9Pt0.1O3 is an excellent OER electrocatalyst for zinc–air batteries. Additionally, this work opens a good way to synthesize highly efficient electrocatalysts from A-site deficient perovskites.

Polyoxometalates (POMs), as carbon-free metal-oxo-clusters with unique structural properties, are emerging water-splitting electrocatalysts. Herein, we explore the development of cobalt-containing polyoxometalate immobilized over the carbon nanotube fiber (CNTF) (Co4POM@CNTF) towards efficient electrochemical oxygen evolution reaction (OER). CNTF serves as an excellent electron mediator and highly conductive support, while the self-activation of the part of Co4POM through restructuring in basic media generates cobalt oxides and/or hydroxides that serve as catalytic sites for OER. A modified electrode fabricated through the drop-casting method followed by thermal treatment showed higher OER activity and enhanced stability in alkaline media. Furthermore, advanced physical characterization and electrochemical results demonstrate efficient charge transfer kinetics and high OER performance in terms of low overpotential, small Tafel slope, and good stability over an extended reaction time. The significantly high activity and stability achieved can be ascribed to the efficient electron transfer and highly electrochemically active surface area (ECSA) of the self-activated electrocatalyst immobilized over the highly conductive CNTF. This research is expected to pave the way for developing POM-based electrocatalysts for oxygen electrocatalysis.

Hydrogen generation through water electrolysis is an efficient technique for hydrogen production, but the expensive price and scarcity of noble metal electrocatalysts hinder its large-scale application. Herein, cobalt-anchored nitrogen-doped graphene aerogel electrocatalysts (Co-N-C) for oxygen evolution reaction (OER) are prepared by simple chemical reduction and vacuum freeze-drying. The Co (0.5 wt%)-N (1 wt%)-C aerogel electrocatalyst has an optimal overpotential (0.383 V at 10 mA/cm2), which is significantly superior to that of a series of M-N-C aerogel electrocatalysts prepared by a similar route (M = Mn, Fe, Ni, Pt, Au, etc.) and other Co-N-C electrocatalysts that have been reported. In addition, the Co-N-C aerogel electrocatalyst has a small Tafel slope (95 mV/dec), a large electrochemical surface area (9.52 cm2), and excellent stability. Notably, the overpotential of Co-N-C aerogel electrocatalyst at a current density of 20 mA/cm2 is even superior to that of the commercial RuO2. In addition, density functional theory (DFT) confirms that the metal activity trend is Co-N-C > Fe-N-C > Ni-N-C, which is consistent with the OER activity results. The resulting Co-N-C aerogels can be considered one of the most promising electrocatalysts for energy storage and energy saving due to their simple preparation route, abundant raw materials, and superior electrocatalytic performance.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 143-160).
Understanding and mastering the kinetics of oxygen electrocatalysis is instrumental to enabling solar fuels, fuel cells, electrolyzers, and metal-air batteries. Non-precious transition metal oxides show promise as cost-effective materials in such devices. Leveraging the wealth of solid-state physics understanding developed for this class of materials in the past few decades, new theories and strategies can be explored for designing optimal catalysts. This work presents a framework for the rational design of transition-metal perovskite oxide catalysts that can accelerate the development of highly active catalysts for more efficient energy storage and conversion systems. We describe a method for the synthesis of X-ray emission, absorption, and photoelectron spectroscopy data to experimentally determine the electronic structure of oxides on an absolute energy scale, as well as extract key electronic parameters associated with the material. Using this approach, we show that the charge-transfer energy - a parameter that captures the energy configuration of oxygen and transition-metal valence electrons - is a central descriptor capable of modifying both the oxygen evolution kinetics and mechanism. Its role in determining the absolute band energies of a catalyst can rationalize the differences in the electron-transfer and proton-transfer kinetics across oxide chemistries. Furthermore, we corroborate that the charge-transfer energy is one of the most influential parameters on the oxygen evolution reaction through a statistical analysis of a multitude of structure-activity relationships. The quantitative models generated by this analysis can then be used to rapidly screen oxide materials across a wide chemical space for highthroughput materials discovery.
by Wesley T. Hong.
Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2011.
Cataloged from PDF version of thesis.
Includes bibliographical references.
The electrocatalytic conversion of water to O₂ is the key efficiency-determining reaction for the storage of electrical energy in the form of liquid fuels. In this thesis, the simple preparation of a cobalt-based catalyst for the oxygen evolution reaction (OER) is described and details of its structure, valency, mechanism of action, and mechanism of formation at intermediate pH are elaborated. The catalyst is obtained as an electronically conductive, porous thin film by electrolysis of Co2 in aqueous phosphate, methylphosphonate, or borate electrolyte. Catalyst films prepared from phosphate are comprised of Co oxo/hydroxo clusters of molecular dimensions, as determined by X-ray absorption spectroscopy. The clusters consist of edge-sharing CoO6 octahedra arranged in a sheet-like pattern. The average cluster nuclearity increases as the film thickness increases. X-ray absorption near edge structure (XANES) spectra, EPR spectra, and electrochemical data support a catalyst film consisting predominately of Co(III) in the absence of an applied bias with minor populations of Co(II) and Co(IV) centers. As the film is polarized in the region of water oxidation, the population of Co(IV) centers increases systematically. The mechanism of the OER mediated by the catalyst was studied at neutral pH by electrokinetic and 180 isotope experiments. The catalyst exhibits an OER Tafel slope approximately equal to 2.3 x RT/F, an inverse first order dependence on proton activity, and a zeroth order dependence on phosphate for buffer strengths > 0.03 M. In the absence of phosphate, the Tafel slope increases ~3 fold and the overall activity is greatly diminished. These data point to an OER mechanism involving a rapid, one electron, one proton, equilibrium between Co"'-OH and CoWv-O in which a phosphate species is the proton acceptor, followed by a chemical turnover-limiting process involving oxygen-oxygen bond coupling. The mechanisms of nucleation, steady-state growth, and repair of the catalyst were studied at intermediate pH by electrokinetic, AFM and NMR methods. Catalyst nucleation is progressive with a non-zero-order nucleation rate law. Steady-state growth exhibits a Tafel slope approximately equal to 2.3 x RT/F, an inverse third order dependence on proton activity, and an inverse first order dependence on buffer strength. Together, the electrokinetic studies point to a mechanism involving a rapid one-electron, three-proton equilibrium oxidation of Co2+ coupled to phosphate dissociation from the catalyst surface, which is followed by a chemical rate-limiting process involving Co binding to the growing clusters. Consistent with the disparate pH profiles for the OER and catalyst formation, functional stability and repair are operative at pH > 6 whereas catalyst corrosion prevails at lower pH.
by Yogesh Surendranath.
Ph.D.

Proton exchange membrane (PEM) water electrolysers are forecast to become an important intermediary energy storage technology between renewable power sources and energy distribution/usage. This is because they offer a production route to high purity H2 that is both non-polluting and efficient. Energy stored as H2 can be converted back to electricity for use in the national grid, pumped into existing natural gas networks or used as a fuel for hydrogen-powered vehicles. The majority of the energy losses in a PEM water electrolyser are associated with the high overpotential that is required for the electrochemical evolution of O2 that occurs at the anode. The highly oxidising conditions of this reaction coupled to the low pH of the PEM environment restrict electrocatalyst selection to expensive noble metal oxides. Thus to enhance the commercial viability of PEM electrolysers, the goal of electrocatalyst development for the O2 evolution reaction is to (i) increase the catalytic performance, (ii) increase the catalyst stability and (iii) reduce the cost of the catalyst components. In this work a range of iridium-based electrocatalysts with reduced Ir contents have been prepared. Two methods are employed to reduce the Ir content: (i) mixing the Ir with ruthenium to form a binary metal oxide and (ii) dispersing the active Ir phase on an indium tin oxide (ITO) support. Investigation of the electrocatalysts via a combination of different physical and electrochemical characterisation techniques, including a novel in-situ X-ray absorbance experiment, indicates that both approaches produce electrocatalysts with comparable or improved O2 evolution activity compared to the state-of-the-art iridium oxide (IrO2) material. However selection of the most appropriate catalyst for PEM electrolysis may ultimately be a compromise between activity, stability and cost.

Novel clean energy conversion and storage technologies, such as electrochemical water splitting and metal-air battery, play significant roles in the future clean energy society. Oxygen evolution reaction (OER), as the fundamental reaction of these technologies, is crucial for their practical application. However, OER process is sluggish since the complex reaction process (multi-electron and multi-intermediate involved reaction). Developing efficient and affordable OER electrocatalysts remains a great challenge. Recently, the multimetal incorporation strategy has aroused extensive research interest since it can effectively enhance the catalytic performance of the catalysts. Nevertheless, there are still many scientific questions to be answered for such materials systems, such as the reaction mechanism and the optimum element composition. In this thesis, earth-abundant transition metals Cobalt and iron were selected as the basic elements. Cheap and abundant metals Vanadium, Chromium, and Tungsten were chosen as the incorporation elements respectively because of their unique d orbital structure in oxidation state. Their oxides/(oxy)hydroxides were elaborately designed and synthesised. The OER performance of the incorporated materials display a huge improvement. A variety of characterisations were employed to investigate the electrochemical properties of the materials. Theoretical calculations were also applied and combined with the characterisation observation to explain the reaction mechanism and the role of the incorporation element. Practical electrical water electrolyser devices were built up to determine the synthesised OER electrocatalysts in a real situation. Specifically, a facile electrodeposition catalysts synthesis method was developed, which can rapidly manufacture electrodes with efficient OER electrocatalysts on a large scale.

Philosophiae Doctor - PhD
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.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environment and Sc
Science, Environment, Engineering and Technology
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.

Multi-walled carbon nanotubes (MWNTs) have been synthesized by the pyrolysis of acetylene using hydrogen decrepitated Mischmetal (Mm) based AB3 alloy hydride catalyst. MWNTs have been characterized by SEM, TEM, Raman and XRD studies. Pt-supported MWNTs (Pt/MWNTs) have been prepared by chemical reduction method using functionalized MWNTs. Composites of Pt/MWNTs and Pt/C have been used as electrocatalysts for oxygen reduction reaction in Proton Exchange Membrane Fuel Cell (PEMFC). Cathode catalyst with 50% Pt/MWNTs and 50% Pt/C gives the best performance because of the better dispersion and good accessibility of MWNTs support and the Pt electrocatalysts in the mixture for the oxygen reduction reaction in PEMFC. The paper emphasizes that Pt/C and Pt/MWNTs composites have good potential as catalyst support material in PEMFC.

A number of carbon supported bi/multi-metallic Pt-based electrocatalysts with a metal particle-size and shape controllable in nanoscale and a narrow size distribution were prepared by the improved polyol method. Among the electrocatalysts prepared in-house, PtSn/C showed a high direct ethanol fuel cell performance and PtPd/C exhibited a favorable methanol-tolerant property and oxygen-reduction activity. Several MEA fabrication methods such as direct-spray, decal and screen-printing were developed, through which the pore structure and hydrophilic/hydrophobic properties in the MEAs could be controlled desirably. With multi-layer structured electrodes, the maximum power density of 300 mW/cm2 and 240 mW/cm2 for the single cells were achieved at 90 °C under 0.2 MPa pressures of oxygen and air, respectively. Several demonstrations of active and passive compact DMFC systems ranging from sub-watts to 200 watt were fabricated. Some of them were demonstrated in PDA, toy cars, mobile phones and laptop computers.

Fuel cells are a promising technology to deal with energy sustainability, especially for mobility purposes the Proton Exchange Membrane Fuel Cell and hydrogen produced from biomass could be coupled to overcome the amount of CO2 emissions. In order to improve fuel cells performances the search for new electrocatalysts has a great importance in this technology the challenge for a fuel cell catalyst that is less poisoned by CO is one of the most important field in low temperature fuel cell developments that use alcohol and hydrocarbons as primary fuels. In this work PtSm, PtTb, PtDy, PtU, PtRuMo and PtRuDy systems have been synthesized by the colloid method, investigated by the following techniques: X-rays fluorescence analysis (XFA), X-rays powder diffraction (XRD), X-rays photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), cyclic voltammetry (CV) and polarization curves (Exi). The results obtained in this work shows that PtRuMo is the best choice for direct methanol oxidation. For direct ethanol oxidation the higher activity was found in PtRuDy system. PtU system was investigated and showed an interesting behaviour in ethanol oxidation. After two cycles of H2/O2 and ethanol/O2 the catalyst was able to reach the initial figures on hydrogen/oxygen oxidation which means that no degradation of the catalyst was indentified.

The copper-chlorine (Cu-Cl) thermochemical cycle uses both heat and electricity to carry out a series of chemical and electrochemical reactions with the net reaction being the splitting of water into hydrogen and oxygen. The process forms a closed loop with all intermediate chemicals being recycled. All of the chemical and electrochemical reactions can be carried out at temperatures that do not exceed about 530°C. Thus, the heat requirement of this process can be satisfied by intermediate temperature nuclear reactors such as the Super Critical Water Reactor (SCWR) developed in Canada by Atomic Energy of Canada Limited (AECL). AECL is particularly interested in developing the electrochemical reactions that comprise the Cu-Cl cycle. There are two variations on the Cu-Cl cycle. In the original cycle copper metal is produced electrochemically by the disproportionation of cuprous chloride (CuCl), which is dissolved in hydrochloric acid (HCl) electrolyte. It is expected that this reaction will be carried out at a temperature that is below 100°C. Hydrogen gas is then produced by a chemical reaction that takes place between the copper metal and gaseous HCl at a temperature of 430–475°C. It was recognized by AECL that these two reaction steps could be replaced by a single electrochemical reaction that generates hydrogen directly. It is expected that this step will also be carried out at a temperature below 100°C. In this process, referred to as the CuCl/HCl electrolysis step, hydrogen gas is produced at the cathode of an electrochemical cell by the reduction of protons that are supplied by aqueous 6 M HCl while cupric chloride (CuCl2) is produced at the anode by the oxidation of CuCl, which is dissolved in 6 M HCl. The CuCl2 that is formed is recycled and is used in a reaction with steam at 400°C to produce a copper oxychloride. This reaction is common to both versions of the Cu-Cl cycle. It is the purpose of this paper to present electrochemical results from both half-cell and single-cell studies carried out to verify and understand the CuCl/HCl electrolysis step. Half-cell electrochemical data is presented that demonstrates the practicality of the electrode reactions. Electrochemical data is presented to show that the CuCl/HCl electrolysis step can be carried out in a single-cell. In both the half-cell and single-cell experiments platinum electrocatalysts are used to carry out the desired reactions.