Charge state manipulation induced through cation intercalation into MnO2 sheet arrays for efficient water splitting

The ability of the catalysts to catalyze oxygen evolution reaction (OER) is closely related to the electronic state of its active site. Herein, the electronic state of the MnO2 sheets assembled on nickel foams by hydrothermal reaction was tuned by the intercalated transition metal ions including Fe, Co and Ni. Cationic intercalation leads to the convert of manganese ions from a stable d3 (Mn4+) state to an unstable high-spin d4 state (Mn3+), and the reduction of the average chemical state of Mn species greatly increases the density of oxygen vacancies in the material, thereby improving the adsorption capacity of the material for free –OH when performing a catalytic reaction. Due to its higher valence state and oxidation activity, Fe3+ showed higher diffusion control contribution and produced more redox active sites in MnO2, which is in an advantageous position among all ions embedded in the MnO2 layer. Then achieving current density of 20 mA cm−2 on Fe-MnO2/NF only requires a driving voltage of 330 mV for OER. This work not only established a method to activate the self-generated oxygen vacancies by using electronic effects, but also provided new ideas for improving the OER performance of MnO2.