The role of Cr doping in Ni–Fe oxide/(oxy)hydroxide electrocatalysts for oxygen evolution

Efficient and earth-abundant electrocatalysts for water oxidation are essential for renewable and sustainable energy conversion technologies. And Ni(Fe)O_xH_y especially attractive as a state-of-the-art best candidate catalyst for efficient electrochemical oxygen evolution reaction (OER). In previous research, Cr has been reported could benefit the Ni(Fe)O_xH_y catalysts with conflicting mechanism. Here, a series of ternary (Ni, Fe and Cr) amorphous metal oxide catalysts for OER are synthesized via a simple thermal decomposition method. We show that Ni_0.6Fe_0.3Cr_0.1O_x has a turnover frequency of 0.046 ± 0.004 s⁻¹ at 300 mV overpotential which is ∼31% more active than an analogous Ni_0.6Fe_0.4O_x film, 0.035 ± 0.007 s⁻¹, in 0.1 M KOH media. Using electrochemical voltammetry and AC impedance analysis, we demonstrate that Cr increases the number of electrochemically available active sites, as a pore forming agent, but does not affect the intrinsic per metal atom activity. We find that the Cr begins to leach immediately upon electrochemical testing, and the Cr is almost completely depleted after a 24 h stability test. Importantly, along with the decreased content of Cr, the catalyst activity is further promoted. Although the Cr itself may not be responsible for the improvement, its dissolution results in an ideal type of pore and/or active sites. We further optimize the deposition of high-surface-area and high-mass-loading Ni_0.6Fe_0.3Cr_0.1O_x on carbon-cloth electrodes and demonstrate an overpotential as low as 251 mV at 10 mA cm⁻² in 1 M KOH.