Abstract
As the world’s power demand swells even bigger every year, the already worrying CO2 emissions expand with it. If our future generations are going to inherit a healthy planet, measures must be taken to reduce the reliance on fossil fuels. A carbon free fuel that has great potential and is predicted to have a substantial role in the future sustainable energy mix, is hydrogen. Since it only emits water upon combustion, hydrogen is ideal in order for industries with large carbon footprints to transition towards lower emissions. Still, almost all hydrogen produced today comes from natural gas and is accompanied by enormous amounts of CO2 emissions. Electrolysis of water is a green way of producing hydrogen, when the electricity comes from renewable energy sources. However, a limiting factor of water electrolysis today is the slow kinetics of the anodic oxygen evolution reaction (OER). Earth-abundant, highly efficient and low cost electrocatalysts are therefore needed to reduce the energy losses and total costs of water electrolysis. In this work, we synthesised 17 oxygen evolution electrocatalysts based on the parent oxide perovskite Ba0.5Gd0.8La0.7Co2O6-δ (BGLC587) through the sol-gel citrate synthesis route. We substituted Co on B-site with different amounts of Fe and Ni, having the general formula Ba0.5Gd0.8La0.7Co2-x-yFexNiyO6-δ. Furthermore, we substituted Ba with K, Ca and Cs and attempted nitrogen doping on the oxygen site. Physiochemical characterisation with XRD, SEM, TEM and Brunauer-Emmet-Teller surface area resulted in a thorough structural, elemental and morphological characterisation of the B-site substituted electrocatalysts. All materials revealed complex phase compositions and significant variations in physical surface area. Substitution with Fe increased the surface area, whereas Ni had less impact. The major LaCoO3 (R3 ̅c) phase in BGLC587 was found to be gradually replaced by the double perovskite, BaGdCo2O6-δ (Pmmm), phase and the single perovskite, LaFeO3-δ/Gd0.8La0.2O3-δ (Pbnm/Pnma), phases with increasing Fe content. Ni substitution resulted in secondary phases of both NiO and La3Ni2O7 (Ruddlesten-Popper type structure). The electrocatalytic performance of the catalysts was assessed at room temperature in alkaline solution using linear sweep voltammetry, cyclic voltammetry, chronopotentiometry, chronoamperometry and electrochemical impedance spectroscopy. The A-site substituted materials had less significant impact on the OER activity and hence the extensive electrochemical characterisation was not carried out for these. On the contrary, we found that substitution with Fe generally increased the OER performance and that a 30-70% Fe content gave the lowest overpotentials (428-439 mV) and Tafel slopes (64-73 mV dec-1), however not as low as the state-of-the-art IrO2 (330-450 mV). Based on the oxygen content in the Fe substituted electrocatalysts, we argue that the increased activity of these is due to a higher concentration of electron holes. However, no indication of higher valent Fe4+ was found by XPS or in situ Raman spectroscopy, thus we concluded that the electron holes are delocalised in the bulk of the structure or at Co. Alloying with Ni had less impact on the activity, however we found that a combination of Fe and low Ni content showed outstanding activity. Particularly, substitution with 20% Fe and 10% Ni (BGLCFN2010) resulted in the overall highest specific activity (i_s) of 1.77 mA cm-2oxide at 400 mV overpotential. The overpotential at 10 mA cm-2geo was 451 mV. This was followed by the best performing Fe substituted materials with i_s from 1.04-1.21 mA cm-2oxide. The N-doped sample (BGLCN-NH3) also showed high catalytic activity with an overpotential of 429 mV at 10 mA cm-2geo and Tafel slope of 54 mV dec-1. However, we explained this to be due to an exsolved Co layer on the surface and not a successful N-doping on oxygen site. From Tafel slopes and in situ Raman spectroscopy, we found that the rate determining step of the OER in all catalysts is likely a proton-exchange reaction. We also found indication of oxy-hydroxide intermediates on Ni containing catalysts during OER, whereas Fe catalysts had only oxide termination. Based on this, we concluded that the OER is likely to progress through the adsorbate evolution mechanism (AEM) in the Fe-based electrocatalysts. Faradaic efficiency measurements of selected catalysts indicated close to 100% efficiency. An increased stability was found from galvanostatic measurements at 10 mA cm-2geo with Fe substitutions of 50% and 70% (BGLCF50 and BGLCF70). These electrocatalysts experienced a 20% degradation after ∼50 h, which was twice the duration of the pristine BGLC587. Since the activity of these were among the highest, it was concluded these materials do not follow the typically reported inverse trend that high activity is usually followed by poor stability. The best stability was found with 30% Fe and 20% Ni (BGLCFN3020), which endured for 84 h before reaching 20% degradation all the while operating at lower overpotentials than both BGLCF50 and BGLCF70. On the contrary, the electrocatalyst with 50% Ni (BGLCN50) showed the lowest stability with mediocre activity. Hence, we concluded that there must be a synergy in electrocatalysts with both Fe and low Ni content giving higher stability and activity. This master’s thesis hence contributes to further understanding of perovskites as earth-abundant oxygen evolution catalysts (OECs) and how their catalytic activity can be enhanced through tuning elemental composition, structure, electronic properties and reaction intermediates.