Synergistic Activity of Co and Fe in Amorphous Cox-Fe-B Catalyst for Efficient Oxygen Evolution Reaction

Water splitting has been greatly limited by the sluggish kinetics of the oxygen evolution reaction (OER). High-oxidation-state metal species are required as the favorable active sites in OER. Here, amorphous Cox-Fe-B (x is the molar ratio of Co/Fe), Co-B, and Fe-B compounds were successfully synthesized as the oxygen evolution electrocatalysts. The calculation of turnover frequency (TOF) indicates that both the Co and Fe sites are active for OER. Cyclic voltammetry, X-ray photoelectron spectroscopy, and long-term stability curves were used to demonstrate that Fe can stabilize Co in a higher oxidation level and meanwhile promote the generation of OOH-like species (the key intermediates for OER). The reduced impedance for Co2-Fe-B (compared with that for Fe-B and Co-B) obtained from the electrochemical impedance spectra confirms the enhanced conductivity for the Co2-Fe-B. This optimal sample on Cu substrate shows a low overpotential of 0.298 V at the current density of 10 mA cm^-2 with a decreased overpotential of 42 mV compared to that of Co-B. The Co2-Fe-B catalyst also exhibits a small Tafel slope of 62.6 mV/dec and good stability. The enhanced performance could be attributed to the synergistic effect of the increased population of high-oxidation-state metal-OOH species and the promoted conductivity of the catalyst. A solar-to-hydrogen energy conversion efficiency of 4.2% and a Faradaic efficiency of 97.2% can be achieved by connecting the HER and as-prepared OER electrodes to a crystalline silicon solar cell.