An investigation into the effect A and B-site doping of precious metal free perovskite oxides on the material structure and the electrocatalytic facilitation of the Oxygen Evolution Reaction (OER) as alkaline electrolyser anodes

Abstract

Climate change is arguably the single most important issue facing humanity in the modern day. Increasing global temperatures due to the greenhouse effect arising from carbon emissions are predicted to have devasting effects on oceanic seawater levels, disease, and agriculture if current levels of emission are not reduced. In 2020, fossil-fuels (coal, oil, and natural gas) were responsible for around 32 billion tonnes of CO2 emission worldwide. Therefore, there is a strong push for scientists and engineers to develop sustainable energy alternatives to the current fossil fuel based systems that are currently in place. Although there are many fields of interest where sustainable energy processes would be highly initiative, one of the largest uses of fossil fuels is in the production of combustible fuels. It has long been known that hydrogen gas has the highest energy density of any fuel (120 MJ/kg) where the gasoline contains 44 MJ/kg, and when combusted produces water vapour instead of carbon dioxide. Currently, alongside the known dangerous of storing hydrogen gas, the main source of hydrogen currently used for fuels are carbon containing natural gasses such as methane, so this is not a long-term sustainable solution to reducing carbon emissions. However, processes such as electrolysis of water into oxygen and hydrogen gas provide carbon free fuels as well as a safer hydrogen storage method. Specifically, alkaline electrolysers (AE) offer a promise as a sustainable green energy producer due to higher durability, relatively inexpensive electrolytes and electrocatalysts, and lower dissolution of the anodic electrocatalyst into the electrolyte solution. Alkaline electrolyser systems utilise alkaline electrolyte solutions to increase the facilitation of the oxygen evolution reaction (OER) at the anodic electrocatalyst. The oxygen evolution reaction is the limiting reaction in water splitting due to the sluggish kinetics of the 4- electron catalytic reaction, so increasing the facilitation of this reaction will in turn lead to an increase in the amount of hydrogen fuel generated at the cathode. The current state-of-the-art alkaline electrolyser systems use precious metal oxide catalysts such as IrO2 or PdO2 as anode materials. The selection of these catalysts is considered a limitation due to poor ionic conductivity and economically unfavourable materials used. This produces an opportunity into developing new alternative catalytic materials which have high performance and are economically and environmentally sustainable. This material development is the primary focus of the work contained in this thesis. Precious metal free, transition metal-based perovskite oxides have been observed to have these desirable qualities when utilised as oxygen evolution reaction electrocatalysts. In this PhD project, various perovskite oxide material series with the formula ABO3−δ, with selected dopants A’ and B’ were synthesised through wet chemical methods, and the electrocatalytic performance was measured. In particular, the manipulation of the atomic structure of perovskite oxides through various cation doping to alter the levels of electrocatalytic facilitation of the oxygen evolution reaction were explored mainly to control the oxidation state of the transition metals occupying the B lattice site and the oxygen vacancy content (δ). This was then combined with investigations into altered grain morphologies through alternative material synthesis pathways to provide a thorough investigation into the level of electrocatalytic activity of the materials and further optimise from the atomic into grain-size level, with respect to the oxygen evolution reaction in alkaline media. The initial investigation of the thesis is understanding the effects of increased addition of Sr2+ aliovalent dopant ions into the crystal lattice of Pr1-xSrxFeO3−δ (PSFO) perovskites via A-site substitution on the OER. The addition of the Sr2+ ions was found to drastically improve the OER catalytic activity of the material, as undoped PrFeO3 catalyst was not electrocatalytically active up to 1.70 VRHE in 0.1 M KOH electrolyte, whereas the x = 0.5 material measured a lower onset potential and the highest level of catalytic activity. This improvement in the electrocatalytic performance was associated with the increase in oxygen vacancies in the material due to the change imbalance of the Pr3+ and Sr2+ ions, which was shown through changed to the iron oxidation state. Building on these findings, Sr2+ was replaced with Ba2+ inducing an ordered double perovskite structure, and the stoichiometry of the B-site was altered to produce a series of PrBaCo2(1-y)Fe2yO6 (PBCF) double perovskites. From the results of the double perovskite study, the ideal stoichiometric ratio of Co/Fe cations was determined to be y = 0.2. From this point a material using the combined findings was developed with the formula Pr(BaSr)Co2(1-y)Fe2yO6 (PBSCF). This material had similar results to the strontium free double perovskite and was theorised to be limited due to poor phase purity, as phases such as Ruddlesden-Popper (An+1BnO3n+1), single perovskite (ABO3), and spinel (AB2O4) were discovered alongside the primary double perovskite phase. All materials in this study were comprehensively characterised through powder X-Ray diffraction (XRD) and refining the resulting powder diffraction patterns using Rietveld method to determine factors such as space group, lattice parameters, lattice volume, and bond lengths. The surface structure was investigated using X-Ray photoelectron spectroscopy (XPS), and the electrocatalytic activity was characterised through techniques such as cyclic voltammetry (CV), electrical impedance spectroscopy (EIS) and Tafel slope analysis.