In operando Raman investigation of Fe doping influence on catalytic NiO intermediates for enhanced overall water splitting

Transition metal iron (Fe)-incorporated Ni oxide and oxyhydroxide compounds generally show an enhanced activity for alkaline water splitting. However, the role of Fe for this enhanced activity is not fully elucidated, especially under hydrogen evolution reaction (HER). Herein, we combine electrochemical and spectroscopic techniques to investigate the Fe doping effect on self-standing NiO nanosheets for enhanced activities for both HER and oxygen evolution reaction (OER) in overall water splitting. The results show that the presence of Fe suppresses Ni self-oxidation and adjusts the Ni–O local environment and its ability to form surface phases. In operando Raman spectroscopy is utilized to explore the active intermediates present under catalytic conditions. Apart from a slight suppression of grain size, our results show that Fe incorporation into NiO enhances in-situ formation of active layered intermediates Ni_xFe_1-xOOH with a phase transformation of FeOOH layers into γ-NiOOH layers containing Ni⁴⁺ at potentials approaching OER in contrast to undoped NiO electrodes with a dominating conversion of NiO to β-NiOOH, with persisting Ni³⁺. In addition, the work function on the electrode surface is reduced by 90 meV upon Fe doping, revealing a higher intrinsic Fermi-level and thus a lower requirement for added bias during HER. Together with the lower resistance for electron transport beneficial for both HER and OER, this leads to improved OER and HER efficiency upon Fe-doping. The study shows how Fe doping influences the active catalytic NiO intermediates for both HER and OER. Specifically, in operando vibrational spectroscopy utilized in parallel with electrochemical characterization can shed light on enhancement mechanisms and influence of doping for catalytic intermediates under any chosen bias at the respective electrode under full water splitting.