Establishing the oxygen evolution reaction pathway on iron-oxy-hydroxide through electro-kinetic study

Abstract

Electrokinetic analyses harnessing intrinsic reaction parameters of the electrocatalytic oxygen evolution reaction (OER) shed light on the reaction mechanism. Given the superior stability of the iron oxy-hydroxide under alkaline OER conditions, α-FeO(OH) and γ-FeO(OH) are often found to be the active catalyst. Herein, nanocrystalline α-FeO(OH) and γ-FeO(OH) materials are used as catalysts to perform alkaline OER and detailed electrokinetic studies are conducted to establish the reaction pathway. The intrinsic parameters like anodic transfer coefficient (α_a), specific exchange current density (j_0,s), activation energy ($E_a^0$), and reaction order (m) are experimentally determined for both FeO(OH) phases. To obtain these important parameters, OER is performed with α-FeO(OH) and γ-FeO(OH) deposited on nickel foam as anode while varying the cell temperature from 298 K to 343 K and electrolyte concentrations from 0.05 M to 2.0 M KOH. The j_0,s values for α-FeO(OH) and γ-FeO(OH) are almost comparable 2.5 ± 0.5 × 10⁻³ mA cm⁻² highlighting a similar rate of electron transfer. The activation energy barrier for OER on α-FeO(OH) and γ-FeO(OH) is identified to be 9.45 kJ mol⁻¹ and 8.06 kJ mol⁻¹, respectively and the values are manyfold less compared to that observed for previously reported IrO₂ or NiFeO_x materials emphasizing a faster kinetics on the FeO(OH) surface. The first-order reaction is determined from the electrolyte concentration variation suggesting the dissociation of O-H could be the rate-determining step (RDS) which is contrary to the mechanism proposed for IrO₂ or NiFeO_x where the O-O bond formation was found to be rate-limiting. Extracting the intrinsic reaction parameters from the electro-kinetics study, the OER pathway on the FeO(OH) surface has been established here.