Enhanced Catalysis of the Electrochemical Oxygen Evolution Reaction by Iron(III) Ions Adsorbed on Amorphous Cobalt Oxide

The oxygen evolution reaction (OER) is the bottleneck in the efficient production of hydrogen gas fuel via the electrochemical splitting of water. In this work, we present and elucidate the workings of an OER catalytic system which consists of cobalt oxide (CoOx) with adsorbed Fe3+ ions. The CoOx was electrodeposited onto glassy-carbon-disk electrodes, while Fe3+ was added to the 1 M KOH electrolyte. Linear sweep voltammetry and chronopotentiometry were used to assess the system’s OER activity. The addition of Fe3+ significantly lowered the average overpotential (η) required by the cobalt oxide catalyst to produce 10 mA/cm2 O2 current from 378 to 309 mV. The Tafel slope of the CoOx + Fe3+ catalyst also decreased from 59.5 (pure CoOx) to 27.6 mV/dec, and its stability lasted ～20 h for 10 mA/cm2 O2 evolution. Cyclic voltammetry showed that oxidation of the deposited CoOx, from Co2+ to Co3+ occurred at a more positive potential when Fe3+ was added to the electrolyte. This could be attributed to interactions between the Co and Fe atoms. Comprehensive X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy were conducted. The in situ XANES spectra of Co sites in the CoOx, CoOx + Fe3+, and control Fe48Co52Ox catalysts were similar during the OER, which indicates that the improved OER performance of the CoOx + Fe3+ catalyst could not be directly correlated to changes in the Co sites. The XANES spectra of Fe indicated that Fe3+ adsorbed on CoOx did not further oxidize under OER conditions. However, Fe’s coordination number was notably reduced from 6 in pure FeOx to 3.7 when it was adsorbed on CoOx. No change in the Fe–O bond lengths/strengths was found. The nature and mechanistic role of Fe adsorbed on CoOx are discussed. We propose that Fe sites with oxygen vacancies are responsible for the improved OER activity of CoOx + Fe3+ catalyst