Slow O-H Dissociation in the First-Order Oxygen Evolution Reaction Kinetics on Polycrystalline gamma-FeO(OH)

To unravel the reaction mechanism of the electrochemical oxygen evolution reaction (OER), analysis of the intrinsic activity parameters obtained via electrokinetic investigation remains effective. In this study, polycrystalline gamma-FeO(OH), synthesized at room temperature, was used as a stable, albeit reactive, anode for OER, and electrokinetic studies were performed to unravel the oxygen evolution reaction pathway. The cell temperature, hydroxyl ion concentration, and the cation of the supporting electrolyte were varied, and the influence of external bias on the OER activity was recorded. Changes in the cell temperature from 30 to 65 degrees C lead to an enhanced OER activity due to small overpotential, Tafel slope, and charge-transfer resistance (Rct) values at high temperatures, which unambiguously highlights the influence of the thermodynamic barrier and electron-transfer kinetics. The calculated anodic charge-transfer coefficient (alpha a) and specific exchange current density (j0,s) values are 0.65 and 2.24 x 10-5 mA cm-2, respectively. The faster OER kinetics on polycrystalline gamma-FeO(OH) can also be attributed to an appreciably low activation energy of 8.26 +/- 2 kJ mol-1, where variation of the electrolyte concentration indicated a first-order dependence on [OH]-. Experimentally obtained intrinsic kinetic parameters such as the Tafel slope, anodic charge-transfer coefficient, exchange current density, activation energy (Ea), reaction order (m), and deuterium isotope effect implicate the dissociation of hydroxyl ions on the polycrystalline gamma-FeO(OH) as the rate-determining step. The direct effect of cations such as Li+, Na+, and K+ of the electrolyte on OER highlights a weak interaction of the cations with the surface-active [FeIII-OH] species. A change in the electrode substance from three-dimensional highly conductive nickel foam (NF) to two-dimensional less conductive carbon cloth (CC) resulted in a change in the slow step, leading to the formation of a high-valency iron oxo intermediate from the iron hydroxy species.