Groves, Alexandra Rose; (2021) High Throughput Synthesis and Discovery of Sustainable Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER) Catalysts. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Hydrogen as a fuel for electrochemical cells has many benefits. The chemical energy stored in hydrogen is much higher than that found in traditional battery materials and the only emissions produced from hydrogen utilisation devices such as fuel cells are heat and water. However, using hydrogen as a fuel and generating hydrogen from water require opposing reactions, namely the Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER). Both reactions have kinetically complex four electron reaction pathways that require expensive noble metal catalysts, reducing the commercial feasibility of hydrogen fuel cells. Low-cost, non-precious-metal catalysts are promising choices to replace traditional noble metal counterparts. However, there are extensive challenges in developing alternative catalysts with comparable performance to noble metals, whilst ensuring sufficient cost reduction. The primary goal of this thesis was to use Continuous Hydrothermal Flow Synthesis (CHFS) to produce libraries of candidate materials for use as ORR/OER catalysts in aqueous electrolytes. Initially, a library of ATiO3 perovskites (where A = Ba, Ca and Sr) were synthesised and evaluated as ORR catalysts demonstrating a catalytic dependence on the chemical composition. Increased performance was seen in Ba rich areas of the phase space. A series of AMnO3 perovskites (where A = La, Y, Sm and Ca) were then synthesised with the aim of investigating the effect of A site substitution on ORR catalysis. LaMnO3 was shown to be an excellent candidate material with a low overpotential (0.31 V) and high limiting current density (−6.2 mA cm−2). Further electrocatalytic studies on LaxMnyNizO3 showed that enhanced bifunctional activity can be achieved in a region of La sub-stoichiometry with an optimum composition of La0.83Mn0.85Ni0.32O3 with a bifunctional overpotential of 0.69 V. Lastly, a spinel phase diagram consisting of NixMnyFezO4 was investigated for bifunctional oxygen activity and enhanced ORR catalysis was seen in a Mn rich Ni/Fe poor region of the phase space.

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