Li and Na substituted (Fe,Co,Ni,Cu,Zn)O high entropy oxide catalysts for oxygen evolution reaction

Abstract

We report the stability landscape of (FeCoNiCu,Zn)O, (Li,FeCo,Ni,Cu,Zn)O, and (Na,FeCo,Ni,Cu,Zn)O rocksalt HEOs, synthesized via a solution combustion route, and investigated their intrinsic electrocatalytic activity for the oxygen evolution reaction (OER). The inclusion of lithium and sodium tends to stabilize the rocksalt solid solution at lower temperatures by increasing the configurational entropy through the creation of vacancies and the oxidation of transition metal cations, most notably Co and Ni, from +2 to +3 state. The formation of an intermediate spin state of Co³⁺ (t⁵_2ge¹_g) on the surface combined with electron deficiency results in optimum adsorption energy for O* or OH*, supporting the oxygen evolution reaction (OER). A low OER overpotential of 322 mV at 10 mA cm⁻² current density, accompanied by a Tafel slope of 63 mV dec⁻¹, is demonstrated for Li¹⁺ added (Fe,Co,Ni,Cu,Zn)O HEO. Chronopotentiometry at 10 mA cm⁻² showed stable OER kinetics for at least 30 hours in an alkaline solution. A clear volcano trend in d- and p-band centers supports the adsorbate evolution mechanism (AEM) as the dominant pathway for the OER in the obtained HEO electrocatalysts. The catalytic performance achieved in this study is among the most efficient OER activity reported for rocksalt high-entropy oxides to date.