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

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, alpha-FeO(OH) and gamma-FeO(OH) are often found to be the active catalyst. Herein, nano- crystalline alpha-FeO(OH) and gamma-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 (alpha a), specific exchange current density (j0,s), activation energy (E0a), and reaction order (m) are experimentally determined for both FeO(OH) phases. To obtain these important parameters, OER is performed with alpha-FeO(OH) and gamma-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 alpha-FeO(OH) and gamma-FeO(OH) are almost comparable 2.5 +/- 0.5 x 10-3 mA cm-2 highlighting a similar rate of electron transfer. The activation energy barrier for OER on alpha-FeO(OH) and gamma-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 IrO2or NiFeOx 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 NiFeOx 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.