Improving electrocatalytic OER activity of perovskites through insertion of ternary B-site metals

Abstract

Anion exchange membrane water electrolysers (AEMWEs) demonstrate promise for efficient and cost-effective hydrogen production. However, the sluggish kinetics of the oxygen evolution reaction (OER) hinder their deployment, requiring the development of cheap and efficient electrocatalysts. In this work, lanthanum nickelate-based perovskites have been prepared via sol-gel synthesis and modified with ternary Co and Mn B-site metals, increasing their catalytic activity. X-ray diffraction (XRD) data indicates that a rhombohedral crystal structure has been achieved, with strain increasing as the percentage of ternary metal increases. The best performing cobalt-doped material, with a Ni:Fe:Co atomic ratio of 6:2:2, displayed a Tafel slope of 38.4 mV dec−1, with an OER overpotential of 348 mV at 10 mA cm−2, while the best performing manganese-doped material had a Ni:Fe:Mn atomic ratio of 8:1.5:0.5 and achieved 10 mA cm−2 at an OER overpotential of 361 mV and a Tafel slope of 42.7 mV dec−1. Electrochemical surface area (ECSA) calculations and particle size analysis suggest that the surface area and physical structure of the materials is not the most influential factor in determining catalytic activity; rather, electronic structure appears to play a defining role. XPS analysis has shown that both Co- and Mn-doped catalysts with an average Fe oxidation state around 2.5 and average Ni oxidation state around 2.4 demonstrate the best performance, likely due to an increased number of oxygen vacancies, which are necessary for achieving charge neutrality within the perovskite structure. Furthermore, ternary metal insertion results in a negative shift in the Ni2+ binding energy, facilitating electrochemical oxidation of Ni(II) to Ni(III), which is an essential prerequisite to the OER.