Influence of Electrolyte Cations on the Oxygen Evolution Reaction of Nanochanneled Manganese Oxide

The electrochemical oxygen evolution reaction (OER) is a critical reaction to produce chemical fuels from electricity. So far, many studies have reported the influence of electrolytes on the OER efficiency. However, enhancing the activity by strategically tuning the electrolyte remains challenging, because the interaction between the electrocatalyst and electrolyte ions are still ambiguous. Here, we show that the OER activity of nanochanneled manganese oxide (α-MnO 2 ) is enhanced in the presence of K + compared to Na + in a pH 13 alkaline electrolyte. We attribute the pronounced electrolyte dependence to cation intercalation within the nanochannels of α-MnO 2, because the cation dependence was markedly suppressed for γ-MnO 2 whose channels are too small for intercalation. Ultraviolet photoelectron spectroscopy (UPS) analysis revealed that the valence band position of α-MnO 2 shifts depending on the cation present during electrolysis. However, no changes could be observed using gradient-incidence X-ray diffraction (GI-XRD) or conversion electron yield-extended X-ray absorption fine structure (CEY-EXAFS) analysis, despite these techniques being more surface-sensitive (probe depth: approximately 100 nm) than standard XRD or EXAFS. These results indicate that structural changes are localized to the surface of α-MnO 2, occurring only at scales near the UPS probe depth of approximately 10 nm. The shift of the valence band position within the UPS spectra, along with the minimal change in the average oxidation state based on X-ray photoelectron spectroscopy (XPS), indicate that the Mn 3d orbitals of α-MnO 2 are stabilized in the presence of K + . This would enhance the electrophilicity and reactivity of surface oxyl species, thus promoting O–O bond formation. Our study highlights the importance of optimizing not only the ions in the electrolyte but also their interaction with nanoscale structures in the electrode material.