Splitting water into hydrogen and oxygen by electrolysis using electricity from intermittent ocean current, wind, or solar energies is one of the easiest and cleanest routes for high-purity hydrogen production and an effective way to store the excess electrical power without leaving any carbon footprints. The key dilemma for efficient large-scale production of hydrogen by splitting of water via the hydrogen and oxygen evolution reactions is the high overpotential required, especially for the oxygen evolution reaction. Hence, engineering highly active and stable earth-abundant oxygen evolution electrocatalysts with three-dimensional hierarchical porous architecture via facile, effective and commercial means is the main objective of the present PhD study. Finally, we developed two kinds of good performance oxygen evolution electrocatalysts through two different way combined with in situ electrochemical activation.For the first oxygen evolution electrocatalyst, we report a codoped nickel foam by nickel crystals, tricobalt tetroxide nanoparticles, graphene oxide nanosheets, and in situ generated nickel hydroxide and nickel oxyhydroxide nanoflakes via facile electrolytic codeposition in combination with in situ electrochemical activation as a promising electrocatalyst for oxygen evolution reaction. Notably, this hybrid catalyst shows good electrocatalytic performance, which is comparable to the state-of-the-art noble catalysts. The hybrid catalyst as an electrocatalytically-active and robust oxygen evolution electrocatalyst also exhibits strong long-term electrochemical durability. Such a remarkable performance can be benefiting from the introduced active materials deposited on nickel foam, in situ generated nickel oxyhydroxide nanoflakes and their synergistic effects. It could potentially be implemented in large-scale water electrolysis systems.For the second oxygen evolution electrocatalyst, a facile and efficient means of combining high-velocity oxy-fuel spraying followed by chemical activation, and in situ electrochemical activation based on oxygen evolution reaction has been developed to obtain a promising self-supported oxygen evolution electrocatalyst with lattice-distorted Jamborite nanosheets in situ generated on the three-dimensional hierarchical porous framework. The catalyst developed in this work exhibits not only exceptionally low overpotential and Tafel slope, but also remarkable stability. Such a remarkable feature of this catalyst lies in the synergistic effect of the high intrinsic activity arising from the lattice-dislocated Jamborite nanosheets as the highly active substance, and the accelerated electron/ion transport associated with the hierarchical porous architecture. Notably, this novel methodology has the potential to produce large-size-electrode for alkaline water electrolyzer, which can provide new dimensions in design of highly active and stable self-supported electrocatalysts.Furthermore, we have also initially developed good hydrogen evolution electrocatalysts upon in situ electrochemical activation, coupled with the obtained superior oxygen evolution electrocatalysts forming two-electrode configurations, respectively, both of which rivalled the integrated state-of-the-art ruthenium dioxide-platinum electrode in alkaline overall water splitting.In summary, a methodology of fabricating easy-to-commercial, high performance catalytic electrodes by combining general coating processes with in situ electrochemical activation has been realized and well developed. The in situ electrochemical activation mentioned above is a dynamic self-optimization behavior which is facile, flexible, effective and eco-friendly, as a strategy of fabricating self-supported electrodes for efficient and durable overall water splitting. We hope our work can promote advanced development toward large-scale hydrogen production using excess electrical power whenever and wherever available.

Research on alternative energy harvesting technologies, conversion and storage systems with high efficiency, cost-effective and environmentally friendly systems, such as fuel cells, rechargeable metal-air batteries, unitized regenerative cells, and water electrolyzers has been stimulated by the global demand on energy. The conversion between oxygen and water plays a key step in the development of oxygen electrodes: oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), processes activated mostly by precious metals, like platinum. Their scarcity, their prohibitive cost, and declining activity greatly hamper large-scale applications. This issue reports on novel non-precious metal electrocatalysts based on the innovative design in chemical compositions, structure, and morphology, and supports for the oxygen reaction.

The book starts with a theoretical understanding of electrocatalysis in the framework of density functional theory followed by a vivid review of oxygen reduction reactions. A special emphasis has been placed on electrocatalysts for a proton-exchange membrane-based fuel cell where graphene with noble metal dispersion plays a significant role in electron transfer at thermodynamically favourable conditions. The latter part of the book deals with two 2D materials with high economic viability and process ability and MoS2 and WS2 for their prospects in water-splitting from renewable energy.

Electrocatalysis plays a crucial role in a wide range of renewable and sustainable energy technologies, which are required to create a carbon-neutral or carbon-negative energy ecosystem, ultimately fostering the long-term prosperity of humankind. The oxygen reduction reaction (ORR) is key in electrochemical energy conversion and storage technologies such as fuel cells and metal-air batteries, which, for instance, have the potential to help decarbonize transportation and provide clean intermittent renewable energy storage. However, cheaper electrocatalyst materials with improved ORR activity, 4e--product selectivity, and stability are needed to deploy these promising technologies at a large scale. Anion exchange membrane fuel cells (AEMFCs) have emerged as a promising complementary alternative to the more mature proton exchange membrane fuel cells (PEMFCs) because the alkaline environment in AEMFCs allows for improved ORR kinetics and wider material stability compared to in the acidic conditions in PEMFCs. Moreover, diversifying ORR catalysts beyond conventional Pt-based materials is crucial for H2 FCs to achieve large scale deployment and thrive as a resilient and robust energy technology. Ag, which is two orders of magnitude cheaper than Pt, has emerged as a promising active, stable, and selective non-precious metal alkaline ORR catalyst. Moreover, Ag-bimetallics are an interesting class of materials for which density functional theory (DFT) modeling has predicted the possibility of intrinsically enhanced ORR kinetics. Ag-Cu, for instance, has already been shown to yield enhanced ORR active sites at certain surface compositions. Studying Ag-bimetallics in a well-controlled and systematic fashion is, therefore, crucial to developing material-property relationships that would aid in the design of optimal catalysts for the ORR and other important electrocatalytic reactions. In addition to catalyst material engineering, it is also important to study the electrolyte effects on electrocatalytic performance to design optimal electrochemical microenvironments. In this dissertation, I employ a wide range of complementary physical and electrochemical methods, in conjunction with DFT, to understand how to engineer high performing electrocatalysts. Specifically, I systematically synthesize, characterize, and test Ag-Pd and Ag-Mn alkaline ORR electrocatalysts, ultimately, establishing the fundamental material-property relationships attributed to the measured intrinsic catalyst performance as a function of composition and structure. The use of physical vapor deposition (PVD) is crucial for the systematic catalyst design and development in my work. Moreover, using PVD as a bridge between fundamental rotating disk electrode (RDE) and applied FC device studies, I systematically fabricate model ionomer-free Ag-Pd gas diffusion electrodes (GDEs) to investigate the performance of this material system in H2-O2 AEMFCs. Varying only the Ag:Pd alloy ratio, I find good agreement between the performance trends measured in the RDE and AEMFC configurations. In terms of electrocatalyst material engineering, in this work I develop Ag-based ORR electrocatalysts with tuned oxygen-adsorbate binding, affording state-of-the-art AEMFC performance. In addition, I also investigate the role of acid electrolyte anions on the ORR performance of Ag and Pd, as well as on the hydrogen and oxygen electrocatalysis performance of Pt. I find that performance varies as a function of electrolyte, that acid electrolyte anions effects are potential dependent, and that nitric acid affords improved electrolyte microenvironments conducive to improved performance compared to certain acids. By fundamentally understanding the interfacial processes responsible for the measured electrochemical performance, herein, I engineer high performing Ag-based ORR electrocatalysts and establish fundamental engineering principles to design optimal catalyst materials and catalyst--electrolyte microenvironments.

Understanding the fundamentals of electrochemical interfaces will undoubtedly reveal a path forward towards a society based on clean and renewable energy. In particular, it has been proposed that hydrogen can play a major role as an energy carrier of the future. To fully utilize the clean energy potential of a hydrogen economy, it is vital to produce hydrogen via water electrolysis, thus avoiding co-production of CO2 inherent to reformate hydrogen. While significant research efforts elsewhere are focused on photo-chemical hydrogen production from water, the inherent low efficiency of this method would require a massive land-use footprint to achieve sufficient hydrogen production rates to integrate hydrogen into energy markets. Thus, this research has primarily focused on the water splitting reactions on base-metal catalysts in the alkaline environment. Development of high-performance base-metal catalysts will help move alkaline water electrolysis to the forefront of hydrogen production methods, and when paired with solar and wind energy production, represents a clean and renewable energy economy. In addition to the water electrolysis reactions, research was conducted to understand the de-activation of reversible hydrogen electrodes in the corrosive environment of the hydrogen-bromine redox flow battery. Redox flow batteries represent a promising energy storage option to overcome the intermittency challenge of wind and solar energy production methods. Optimization of modular and scalable energy storage technology will allow higher penetration of renewable wind and solar energy into the grid. In Chapter 1, an overview of renewable energy production methods and energy storage options is presented. In addition, the fundamentals of electrochemical analysis and physical characterization of the catalysts are discussed. Chapter 2 reports the development of a Ni-Cr/C electrocatalyst with unprecedented mass-activity for the hydrogen evolution reaction (HER) in alkaline electrolyte. The HER kinetics of numerous binary & ternary Ni-alloys and composite Ni/metal-oxide/C samples were evaluated in aqueous 0.1 M KOH electrolyte. The highest HER mass-activity was observed for Ni-Cr materials which exhibit metallic Ni as well as NiOx and Cr2O3 phases as determined by ex-situ XRD and in-situ XAS analysis. The on-set of the HER is significantly improved compared to numerous binary and ternary Ni-alloys - including state-of-the-art Ni-Mo materials. It is likely that at adjacent Ni/NiOx sites, the oxide site facilitates formation of adsorbed hydroxide (OHads) from the reactant (H2O) thus minimizing the high activation energy of cleaving the H-OH bond to form the Hads HER intermediate on the metallic Ni site. This is confirmed by in-situ XAS studies which show that the synergistic HER enhancement is due to NiOx content and that the Cr2O3 appears to stabilize the composite NiOx component under HER conditions (where NiOx would typically be reduced to metallic Ni0) Furthermore in contrast to Pt, the Ni(Ox)/Cr2O3 catalyst appears resistant to poisoning by the anion exchange ionomer (AEI), a serious consideration when applied to an anionic polymer electrolyte interface. Furthermore a model of the double layer interface is proposed, which helps explain the observed ensemble effect in the presence of AEI. In Chapter 3, Ni-Fe and Ni-Fe-Co mixed-metal-oxide (MMO) films were investigated for oxygen evolution reaction (OER) activity in 0.1M KOH on high surface area Raney-Nickel supports. During investigations of MMO activity, aniline was identified as a useful "capping agent" for synthesis of high-surface area MMO-polyaniline (PANI) composite materials. A Ni-Fe-Co/PANI-Raney-Ni catalyst was developed which exhibits enhanced mass-activity compared to state-of-the-art Ni-Fe OER electrocatalysts reported to date. Furthermore, in-situ XAS analysis revealed charge-transfer effects of MMOs in which the average oxidation state of the OER-active NiOx(OH)y sites is affected by the binary or ternary components (Fe &/or Co). Cyclic voltammetry results show changes in the potential of the Ni2+/3+ transitions in the presence of binary or ternary metals. In-situ XAS analysis confirms that the redox peaks can be attributed to the Ni sites and the shifts in the XANES peak as a function of applied potential indicates that Fe acts to stabilize Ni in the 2+ oxidation state, while Co facilitates oxidation to the 3+ state. The enhanced OER activity of the ternary Ni-Fe-Co/PANI-Raney catalyst is likely due to "activation" of the conductive Ni(III)OOH phase at lower overpotential due to the charge-transfer effects of the cobalt component. The morphology of the MMO catalyst film on PANI/Raney-Ni support provides excellent dispersion of active-sites and should maintain high active-site utilization for catalyst loading on gas-diffusion electrodes. In Chapter 4, the de-activation of reversible-hydrogen electrode catalysts was investigated and the development of a Pt-Ir-Nx/C catalyst is reported, which exhibits significantly increased stability in the HBr/Br2 electrolyte. Initial screening of Rh- and Ru-chalcogenides (oxides, sulfides and selenides) indicates that these non-Pt catalysts do not exhibit sufficient hydrogen reaction kinetics for use in the hydrogen electrode of a H2-Br2 redox flow battery (RFB). However, a standard Pt/C catalyst suffered from rapid and irreversible de-activation upon high-voltage cycling or exposure to Br2. In contrast a Pt-Ir/C catalyst exhibited increased tolerance to high-voltage cycling and in particular showed recovery of electrocatalytic activity after reversible de-activation (presumably from bromide adsorption and subsequent oxidative bromide stripping). Under the harshest testing conditions of high-voltage cycling or exposure to Br2 the Pt-based catalyst showed a trend in stability: Pt