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

Abstract

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 (CoO x ) with adsorbed Fe 3+ ions. The CoO x was electrodeposited onto glassy-carbon-disk electrodes, while Fe 3+ 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 Fe 3+ significantly lowered the average overpotential (η) required by the cobalt oxide catalyst to produce 10 mA/cm 2 O 2 current from 378 to 309 mV. The Tafel slope of the CoO x + Fe 3+ catalyst also decreased from 59.5 (pure CoO x ) to 27.6 mV/dec, and its stability lasted ∼20 h for 10 mA/cm 2 O 2 evolution. Cyclic voltammetry showed that oxidation of the deposited CoO x, from Co 2+ to Co 3+ occurred at a more positive potential when Fe 3+ 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 CoO x, CoO x + Fe 3+, and control Fe 48 Co 52 O x catalysts were similar during the OER, which indicates that the improved OER performance of the CoO x + Fe 3+ catalyst could not be directly correlated to changes in the Co sites. The XANES spectra of Fe indicated that Fe 3+ adsorbed on CoO x did not further oxidize under OER conditions. However, Fe’s coordination number was notably reduced from 6 in pure FeO x to 3.7 when it was adsorbed on CoO x . No change in the Fe–O bond lengths/strengths was found. The nature and mechanistic role of Fe adsorbed on CoO x are discussed. We propose that Fe sites with oxygen vacancies are responsible for the improved OER activity of CoO x + Fe 3+ catalyst.