Tungsten-Induced Operando Transformations of the Perovskite Ba_0.5Sr_0.5Ni_1-xW_xO_3-δ Series during the Oxygen Evolution Reaction

Water electrolysis provides a means for long-term energy storage in the form of hydrogen gas, helping to offset seasonal variations in renewable energy production. Developing non-noble metal catalysts for the anodic oxygen evolution reaction (OER) is a key step in reducing the cost and increasing the energy efficiency and sustainability of this vital technology. Flame-spray synthesized perovskite oxides of the composition Ba_0.5Sr_0.5Ni_1-xW_xO_3-_δ exhibit excellent OER activity in alkaline conditions. In particular, Ba_0.5Sr_0.5Ni_0.5W_0.5O_3-_δ, with a catalytic turn over frequency (TOF) five-times that of NiO, displays a competitive Tafel slope of 44.7 mV/dec. The perovskite structure, with its tunable ABO₃ formula, provides a unique opportunity to enhance the intrinsic activity of Ni sites with high-valence metal co-doping, and explore the relationship between structural properties and electrocatalytic activity.<br/><br/>B-site W doping strongly influences the oxidation state of surface Ni sites, as revealed by <i>ex-situ</i> soft X-ray absorption spectroscopy (XAS) at the Ni L-edge. A higher concentration of W⁴⁺ increases the Ni²⁺/Ni³⁺ ratio, enhancing the intrinsic activity of the Ni active sites. In addition, analysis of the O K-edge indicates a positive correlation between W content and oxygen vacancy concentration, activating Ba_0.5Sr_0.5Ni_0.5W_0.5O_3-_δ for the lattice oxygen mechanism (LOM), whereby mobile oxygen atoms can move through vacant lattice sites to participate directly in OER catalysis.¹ This pathway is largely considered to be more efficient than the traditional adsorbate evolution mechanism (AEM), leading to enhanced reaction kinetics and decreased OER overpotentials.¹<br/><br/>Akin to the the well-known surface reconstruction processes of NiO during alkaline OER (Ni^IIO → Ni^II(OH)₂ → Ni^IIIOOH), NiW perovskites experience complex, electrochemically-induced transformations under OER conditions.² Therefore, catalyst ‘pre-activation’ using cyclic voltammetry is essential to develop their high-performance final state<i> in-situ</i>. <i>Operando</i> hard XAS at the Ni K-edge and W L₃-edge revealed that NiW perovskites experience significant oxidation state changes at the catalytic Ni sites during cyclic voltammetry, with a strong overall trend of Ni²⁺ → Ni³⁺ oxidation. However, both the extent and rate of this oxidation are greatly influenced by the B-site W concentration. Furthermore, <i>operando</i> X-ray diffraction revealed significant surface reconstruction that facilitates a strong growth in electrochemically active surface area, but this process is also heavily dependent on the W content. It is evident that the ‘pre-activation’ of Ba_0.5Sr_0.5Ni_1-xW_xO_3-_δ catalysts is a function of the Ni:W ratio; elucidating the ways in which high valence metal doping influences the catalyst activation mechanism will guide the development of future Ni-based OER catalysts, for which this process is intrinsically coupled with OER activity.<br/><br/>1. J. S. Yoo, X. Rong, Y. Liu and A. Kolpak, <i>ACS Catal.</i>, 2018, <b>8</b>, 5, 4628–4636<br/>2. V. der Ven, D. Morgan, Y. S. Meng, and G. Ceder, J. Electrochem. Soc., 2006, <b>153</b>, 2, A210-A215