Due to continuously increasing energy demands, particularly with the emergence of electric vehicles (EV), smart energy grids, and portable electronics, advanced energy conversion and storage systems such as fuel-cells and metal-air batteries have drawn tremendous research and industrial attention. Even though the lithium-ion battery technology is the most developed and widely distributed energy device for a wide range of applications, some researchers view its energy density insufficient for fulfilling the ultimate requirements of highly energy intensive applications such as EVs. Recently, zinc-air batteries have re-gained research attention since the initial development in the 1970s due to their remarkably highly energy density and the potential to be electrically rechargeable. However, some technological hurdles such as low charge/discharge energy efficiency, and insufficient cycle stability have hampered commercialization and introduction of rechargeable zinc-air batteries to the market. The mentioned hurdles are currently the main challenges of rechargeable zinc-air battery developed, and they stem from the fact that the reaction kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are intrinsically very sluggish. The two are the main electrochemical reactions that govern the charge and discharge processes of a rechargeable metal-air battery at the air electrode, and these oxygen reactions must be facilitated by active electrocatalysts in order to progress them at practically viable and stable rates. Currently, the best known catalysts for ORR and OER are carbon supported platinum (Pt/C) and iridium (Ir/C), respectively. However, the use of these precious metal based catalysts for large scale applications like EVs and energy storage systems is prohibitively expensive. Additionally, the durability of these catalysts have been reported to be insufficient for long-term usage under normal device operating conditions. Perhaps most importantly, the precious metal based catalysts are strongly active towards only one of the two oxygen reactions required for rechargeable applications. For example, Pt/C is a strong ORR active catalyst, while Ir/C is a strong OER active catalyst. Recently in the literature, a simple physical mixture of these two catalysts have been used to render bi-functionality, but this method is very rudimentary and still requires two separate syntheses for each catalyst. This suggests that future bi-functionally active catalysts must not only be non-precious (inexpensive), but also a single active material capable of catalyzing both ORR and OER over the same active surface.Having said above, non-precious catalyst research, specifically for bi-functional ORR and OER electrocatalyses, has increased dramatically beginning in the 90’s with a very popular and positive belief in the energy community that rechargeable lithium-air batteries could potentially replace lithium-ion batteries. This wave of interest has also picked up research in rechargeable zinc-air batteries since the electrochemical oxygen reactions that take place at the air electrodes are fundamentally very similar. Additionally, the use of zinc metal as the anode, which is one of Earth’s most abundant elements, and the water-based (aqueous) solutions as the electrolyte (as opposed to organic ones) made the rechargeable zinc-air battery development very attractive and seemingly easy to scale-up. Moreover, primary (non-rechargeable) zinc-air batteries have already been commercialized and are available in the market as hearing aid batteries, leading many researchers to believe that a simple tuning of the current technology would lead to a successful secondary (rechargeable) zinc-air battery development. However, there are a set of technical difficulties specific to rechargeable zinc-air batteries that have slowed the development for the past few decades. Therefore, the work presented in this thesis aims to address the challenges of rechargeable zinc-air batteries particularly from the active bi-functional electrocatalyst standpoint to make them as commercially viable as possible.In the first study, a facile hydrothermal materials synthesis technique has been employed to synthesize a non-precious metal cobalt oxide bi-functional catalyst. Microscopic characterizations have revealed the morphology of this material to be mesoporous hexagonal nanodisks, a high surface area catalyst compared to simple granular nanoparticles which enhances active site exposure and transport of reactants during the electrochemical reactions. This unique nanostructure has been made possible with the addition of surface-active agents that played a role of capping agent, binding to specific crystal faces and allowing growth of cobalt oxide only in certain directions. Additionally, the adsorbed capping agent has been found to leave mesopores on the nanodisks as it decomposes during the heat treatment following the hydrothermal process. Compared to randomly shaped nanoparticle catalyst of the same atomic composition, the mesoporous nanodisks greatly outperformed in terms of both charge and discharge performance of a rechargeable zinc-air battery. In the second study, the bi-functional capabilities of the cobalt oxide catalyst towards the ORR and OER in the first study have been improved by introducing nickel metal substituents into the spinel crystal lattice, as well as adapting a highly conductive nano-structured carbon support. The bi-functional activity enhancements have been attributed to an increase in electrical conductivity of spinel cobalt oxide with the insertion of nickel atoms into specific interstitial sites of the spinel lattice, as well as the high surface area nano-carbon support which helped to disperse the active spinel oxide catalyst and facilitate charge transfer during the electrochemical reactions.In the third study, the effect of nickel and manganese insertion into the spinel cobalt oxide lattice on the bi-functional catalytic activity has been studied more in detail. Spinel oxide catalysts with different atomic compositions, including cobalt oxide (un-doped), nickel cobalt oxide, and manganese cobalt oxide, have been synthesized as nanocrystals that self-assembled into high surface area porous spheres. Based on the electrochemical evaluation, the best overall bi-functional catalytic activity has been observed with nickel-substituted cobalt oxide, while the least has been observed with manganese cobalt oxide, with pristine cobalt oxide in the middle. Interestingly, computational modelling of these catalysts has resulted in the same activity trend, confirming the importance of choosing an appropriate metal substituent depending on the level of bi-functional activity required.In the last study, the knowledge gained from the high surface area nanostructured spinel oxide catalysts has been transferred to the fabrication of active catalyst/current collector assemblies. Specifically, cobalt oxide nanowire array has been directly grown on stainless steel mesh, a typical current collector used for zinc-air batteries. This unique active electrode assembly design greatly simplified battery architecture and the preparation steps required to produce a rechargeable air electrode, which usually involve physical deposition techniques such as spray-coating to deposit as-synthesized catalysts on gas diffusion layers. During this step, catalyst is mixed with ancillary materials such as carbon black and polymer ionomer, which corrode during battery charging. The direct coupling of active cobalt oxide catalyst onto the current collector completely eliminated the use of any additional material, and a gas diffusion layer was simply attached to the active assembly to form a rechargeable air electrode. Without any corrosion, the advanced electrode has demonstrated a remarkable durability during rechargeable zinc-air battery testing, lasting over 600 hours of operation, which has never been reported in the literature.There are still a plenty of opportunities to further leverage the knowledge and experience gained from this thesis work to improve the performance of electrically rechargeable zinc-air batteries. For example, the cobalt oxide nanowire arrays can be doped with other metals such as nickel and manganese to precisely tune the bi-functional catalytic activity depending on specific requirements for the battery application. Also, the idea of high surface area nano-carbon support can be used to fabricate an interfacial layer between the cobalt oxide array and stainless steel mesh to improve charge transfer during the reactions. Graphitized carbon, such as graphene nanosheets and carbon nanotubes, that are stable in rechargeable zinc-air battery conditions are great candidates for this purpose and is likely to significantly improve both the activities of ORR and OER.
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Nanostructured Spinel Oxides as Bi-functional Electrocatalysts for Rechargeable Metal-Air Batteries
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