A Stable and Electrocatalytic Iron Electrode for Oxygen Evolution in Alkaline Water Electrolysis

Abstract

Iron electrodes disintegrate in alkali at the anodic potentials needed for oxygen evolution. Hence, relatively expensive nickel-based electrodes and substrates have been the workhorse of materials for industrial water electrolysis. We report here a novel surface-modification method that transforms iron into an electrocatalytically-active and robust oxygen evolution electrode for alkaline water electrolysis. This surface modification consists of an electrochemical oxidation step followed by a thermal decomposition step. The electrochemical oxidation of the iron substrate in alkali transformed the surface to high surface area iron(II) hydroxide with a coral-like morphology that converted to magnetite upon heat treatment. On this high-surface area substrate an electrocatalytic layer of amorphous nickel hydroxide was produced by the thermal decomposition step. Such a surface-modified iron electrode exhibited an extremely low overpotential of 195 mV at 10 mA/cm2 and a low Tafel slope of 43 mV/decade. The electrochemical oxidation step during preparation results in doubling of the electrochemically active area. Even after 1500 h of oxygen evolution at 10 mA/cm2, there was no noticeable change in the electrode potential, morphology, or surface composition of these iron electrodes. This durability was attributed to the conductive oxide layer of magnetite that protects iron surface at the anodic potentials of oxygen evolution. The remarkable electrocatalytic activity was attributed to the amorphous layer of nickel hydroxide. Development of such a robust and electrochemically-active iron electrode presents a unique opportunity for the advancement of alkaline water electrolysis.