Unravelling faradaic electrochemical efficiencies over Fe/Co spinel metal oxides using surface spectroscopy and microscopy techniques.

Cobalt and iron metal-based oxide catalysts play a significant role in energy devices. To unravel some interesting parameters, we have synthesized metal oxides of cobalt and iron ( FeO, CoO, CoFeO and CoFeO), and measured the effect of the valence band structure, morphology, size and defects in the nanoparticles towards the electrocatalytic hydrogen evolution reaction (HER), the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). The compositional variations in the cobalt and iron precursors significantly alter the particle size from 60 to <10 nm and simultaneously the shape of the particles (cubic and spherical). The Tauc plot obtained from the solution phase ultraviolet (UV) spectra of the nanoparticles showed band gaps of 2.2, 2.3, 2.5 and 2.8 eV for FeO, CoO, CoFeO and CoFeO, respectively. Further, the valence band structure and work function analysis using ultraviolet photoelectron spectroscopy (UPS) and core level X-ray photoelectron spectroscopy (XPS) analyses provided better structural insight into metal oxide catalysts. In the CoO system, the valence band structure favors the HER and FeO favors the OER. The composites CoFeO and CoFeO show a significant change in their core level (O 1s, Co 2p and Fe 2p spectra) and valence band structure. CoO shows an overpotential of 370 mV against 416 mV for FeO at a current density of 2 mA cm for the HER. Similarly, FeO shows an overpotential of 410 mV against the 435 mV for CoO at a current density of 10 mA cm for the OER. However, for the ORR, CoO shows 70 mV improvement in the half-wave potential against FeO. The composites (CoFeO and CoFeO) display better performance compared to their respective parent oxide systems (, CoO and FeO, respectively) in terms of the ORR half-wave potential, which can be attributed to the presence of the oxygen vacancies over the surface in these systems. This was further corroborated in density functional theory (DFT) simulations, wherein the oxygen vacancy formation on the surface of CoFeO(001) was calculated to be significantly lower (∼50 kJ mol) compared to CoO (001). The band diagram of the nanoparticles constructed from the various spectroscopic measurements with work function and band gap provides in-depth understanding of the electrocatalytic process.