Fig. 1: Electrode material characterization.

a Schematic illustration of the electrode configuration of the nickel-decorated Mn3O4 NPs (Ni–Mn3O4 NPs). b TEM image of the monodispersed 10-nm Mn3O4 NPs. c HR-TEM EELS Ni L-edge spectra from the bottom (black) to the surface (orange) of Ni–Mn3O4NPs/NiO. The spot size for the EELS spectra is 10 nm. d Cross-sectional HR-TEM image of Ni–Mn3O4NPs/NiO. Each cross symbol corresponds to the EELS spectra in c.