Optimization of Ni−Co−Fe-Based Catalysts for Oxygen Evolution Reaction by Surface and Relaxation Phenomena Analysis

Trimetallic double hydroxide NiFeCo−OH is prepared by coprecipitation, from which three different catalysts are fabricated by different heat treatments, all at 350 °C maximum temperature. Among the prepared catalysts, the one prepared at a heating and cooling rate of 2 °C min⁻¹ in N₂ atmosphere (designated NiFeCo−N₂-2 °C) displays the best catalytic properties after stability testing, exhibiting a high current density (9.06 mA cm⁻² at 320 mV), low Tafel slope (72.9 mV dec⁻¹), good stability (over 20 h), high turnover frequency (0.304 s⁻¹), and high mass activity (46.52 A g⁻¹ at 320 mV). Stability tests reveal that the hydroxide phase is less suitable for long-term use than catalysts with an oxide phase. Two causes are identified for the loss of stability in the hydroxide phase: a) Modeling of the distribution function of relaxation times (DFRT) reveals the increase in resistance contributed by various relaxation processes; b) density functional theory (DFT) surface energy calculations reveal that the higher surface energy of the hydroxide-phase catalyst impairs the stability. These findings represent a new strategy to optimize catalysts for water splitting.