a Energy level transition diagram of CaZn3.6VO and O2. b Optical image of the designed galvanic cell. c V 2p XPS spectra of the CaVO electrodes at different states and the CaZn3.6VO electrodes after being oxidized by O2 in 4 M Zn (CF3SO3)2 solution for different times. d Comparison of XPS spectra of V 2p before and after oxidation. e XRD patterns and f calculational interlayer spacing of (002) plane of CaZn3.6VO electrodes after being oxidized by O2 for different times.