Optimizing Ni–Fe Oxide Electrocatalysts for Oxygen Evolution Reaction by Using Hard Templating as a Toolbox

A specific investigation was carried out to study the influence of the Ni/Fe ratio for oxygen evolution reaction (OER) by using the hard templating method as a toolbox. Various compositions of homogeneously blended Ni-Fe oxide nanoparticles with a primary particle size of around 8 nm were simply prepared by using pore confinement of the tea leaves template. Based on the similar physical properties, including particle size and surface area, for all samples, it was verified that the OER activity in alkali electrolyte was mainly governed by the metal stoichiometry, where a maximum current density was obtained with a Ni/Fe ratio of 32/1. The higher catalytic performance of Ni32Fe oxide was attributed to lower reaction resistance and higher intrinsic activity, which are confirmed by electrochemical impedance spectroscopy and surface area analysis, respectively. The lowest overpotential (0.291 V-RHE at 10 mA/cm(2)) as well as the highest current density (over 600 mA/cm(2) at 1.7 V-RHE) was achieved with Ni/Fe = 32/1 loaded on nickel foam due to (i) an uniform distribution of Fe into NiO, (ii) a high conductivity, and (iii) an activation of Ni by neighboring Fe under applying bias. The environmentally benign surfactant-free synthetic procedure and the electrocatalytic system consisting of earth-abundant elements only (Fe, Ni, and O) should be attractive for the development of practical and economical energy conversion devices to split water.