Heterogeneous Catalysts for the Oxygen Evolution Reaction and the Hydrolysis of Ammonia Borane
Open Access
- Author:
- Mondschein, Jared Scott
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 10, 2018
- Committee Members:
- Raymond Edward Schaak, Dissertation Advisor/Co-Advisor
Raymond Edward Schaak, Committee Chair/Co-Chair
John B Asbury, Committee Member
Benjamin James Lear, Committee Member
Michael John Janik, Outside Member - Keywords:
- Oxygen Evolution Reaction
Electrocatalysis
Ammonia Borane
Water Splitting
Hydrogen - Abstract:
- Recent developments in photovoltaic technologies have enabled the cost-effective and energy efficient generation of power from sunlight. However, a key limitation preventing the widespread adoption of solar power is the inherent intermittent nature of sunlight and the current lack of a viable energy storage solution. A promising biomimetic approach is the storage of sunlight in chemical bonds, similar to the photosynthetic process employed by autotrophs. This scheme relies on the photoelectrolysis of water into oxygen gas at the anode via the oxygen evolution reaction (OER) and hydrogen gas at the cathode via the hydrogen evolution reaction (HER). The produced H2(g) is a gravimetrically dense energy carrier that could be consumed in fuel-cell powered technologies or used as a raw material in a wide variety of industrial processes ranging from ammonia production to food processing. Electrocatalysts for the HER and OER are needed to overcome poor reaction kinetics at their respective electrodes. To achieve economies of scale and to enable industrial applications, OER and HER catalysts should be capable of producing large current densities with minimal overpotentials for prolonged time periods and should be composed solely of Earth-abundant elements. The hydrogen evolution reaction can be catalyzed in strongly acidic, pH neutral, and strongly alkaline electrolytes, and several highly active corrosion-resistant electrocatalysts composed of Earth-abundant elements have been discovered in recent years. The oxygen evolution reaction can also be catalyzed in a wide pH range, but few highly active and stable Earth-abundant electrocatalysts for the OER have been discovered for operation in strongly acidic electrolytes. Indeed, the advantages of operating photoelectrochemical water splitting cells with strongly acidic electrolytes, including improved device component compatibilities, higher achievable current densities and longer periods of operation, motivates the discovery of novel acid-stable water oxidation catalysts. We first discovered that cobalt oxide thin films supported on fluorinated tin oxide were promising water oxidation catalysts in strongly acidic electrolytes, producing industrially relevant current densities at moderate overpotentials. The low dissolution rate of the Co3O4/FTO catalyst was a significant advance over previously reported Earth-abundant OER catalysts. Importantly, we found that the electrode preparation method employed in this study was critical to the observed activity and stability. We then found that intermetallics of 3d transition metals with tantalum, specifically Ni2Ta, Fe2Ta, and Co2Ta, could serve as pre-catalysts for active and stable water oxidation catalysts for use in strongly acidic electrolytes. These water-splitting anodes were synthesized as stand-alone electrodes via arc melting and polycrystalline solid-state reactions. These intermetallics displayed improved corrosion and water splitting properties over their monometallic end members. Expanding on this work, we developed a water-oxidizing anode based on the crystalline oxide Li3Co2TaO6 with demonstrated capability for prolonged operation in strongly acidic electrolytes. The durability of this catalyst could be improved by adjusting the chemical composition of the conductive support, with a platinum-supported catalyst operating for ~2x the operation time of a gold-supported catalyst. Developing synthetic routes toward high surface area Li3Co2TaO6 anodes shows promise for decreasing the overpotentials required to produce industrially relevant current densities. The transportation of photoelectrochemically-produced hydrogen gas is resource-intensive, and the development of dense solids capable of the storage and on-demand release of hydrogen is therefore of high scientific interest. Ammonia borane is a promising hydrogen storage material, capable of releasing three molar equivalents of hydrogen via catalytic hydrolysis. Unfortunately, the most active catalysts for the hydrolysis of ammonia borane are composed of noble metals, motivating the discovery of highly active, recyclable catalysts with diluted noble metal content. To this end, we discovered a class of metal ruthenate perovskites that are active catalysts for the hydrolysis of ammonia borane, a promising hydrogen storage material. These metal ruthenates exhibited turnover frequencies that are comparable to emerging Earth-abundant ammonia borane hydrolysis catalysts. Importantly, these materials expand the scope of active oxide-based catalysts, potentially motivating the synthesis of complex metal oxides as discovery targets.