Boosting oxygen evolution activity of nickel iron hydroxide by iron hydroxide colloidal particles

Nickel iron hydroxides (NiFeOH) have been drawing enormous attention as effective catalysts for oxygen evolution reaction (OER), a key process in water splitting. Herein, we report that negatively charged iron(III) hydroxide colloidal particles, can significantly enhance the OER activity of NiFeOH in alkaline media. NiFeOH is grown on nickel foam in a supersaturated iron(III) salt solution, which also contains a high content of Fe(OH)₃ colloidal nanoparticles, forming free-standing NiFeOH@C_x electrodes (with x being the Fe(OH)₃ concentration). The interface between NiFeOH and Fe(OH)₃ colloidal particles, as manifested by the unique volcano-like holes on the NiFeOH@C_x surface, is likely the OER active sites. In comparison to Fe(OH)₃-free NiFeOH, NiFeOH@C₁₀₀₀ exhibits a 40-fold enhancement of the OER activity, confirming the significant effect of Fe(OH)₃ colloidal nanoparticles in boosting the OER activity, likely as a result of enhanced charge transfer from Ni²⁺ to Fe³⁺ that facilitates the adsorption of key reaction intermediates. Furthermore, by coupling the free-standing NiFeOH@C₁₀₀₀ electrode with commercial Pt/C, full water splitting can occur and reach a current density of 10 mA cm^-2 under a cell voltage of 1.51 V, which is lower than that (1.59 V) based on noble metal catalysts of RuO₂ + Pt/C.