Precipitating Metal Nitrate Deposition of Amorphous Metal Oxyhydroxide Electrodes Containing Ni, Fe, and Co for Electrocatalytic Water Oxidation

We report here a facile, one-step precipitating metal nitrate deposition (PMND) method to prepare amorphous metal oxyhydroxide films containing Fe, Co, and Ni as efficient electrocatalysts for water oxidation. The unique synthesis technique allows easy control of the metal composition over a wide range on various substrates. A series of unary and binary metal oxyhydroxides of 30 compositions are synthesized by PMND on fluorine-doped tin oxide (FTO) substrate as water oxidation electrocatalysts. The activity of the metal oxyhydroxide films is represented by a volcano plot as a function of a single experimental descriptor, i.e., the fraction of hydroxide in the surface oxygen species. The optimum compositions for binary metal oxyhydroxide (NiFe, NiCo, and CoFe) are determined on conductive substrates of FTO, nickel foam (NF), nickel mesh (NM), and carbon felt (CF), and the best NiFe (2:8) electrocatalyst on NF exhibits a water oxidation current density of 100 mA/cm2 with only 280 mV of overvoltage, which outperforms conventional noble metal catalysts like IrOx and RuOx in an alkaline medium. Finally, we demonstrate a tandem PV–electrolysis system by using a c-Si PV module with a power conversion efficiency of 13.71% and an electrochemical cell composed of NiFe (2:8)/NF anode and a bare NF cathode with a conversion efficiency of 71.8%, which records a solar-to-hydrogen conversion efficiency of 9.84%.