Figure 1.

a) Schematic sketch of NiO₂H_x layered structure showing bonded hydrogen and interlayer species (ions and solvent molecules). b) Scanning electron microscope (SEM) images of NiO₂H_x thin film model electrode. c) TEM image of the scraped NiO₂H_x thin film, d) Raman spectra of NiO₂H_x thin film model electrode under and polarization configurations. The corresponding band assignments (E_g and A_1g) and the sketches of the two bands are also shown. e) UV–vis absorption spectrum of NiO₂H_x thin film model electrode marked with available excitation laser wavelengths. f) Raman spectra of NiO₂H_x thin film model electrode using different excitation lasers with equivalent laser power of 4 mW.