Ni/La reaches 3.5:1 in LNO-Vo' while it is only 0.92:1 in LNO-Vo, indicating the formation of a Ni-rich surface when annealing the LNO in the H2 atmosphere. The nickel-rich surface is also another important factor contributing to the superior OER performance of LNO-Vo'. More importantly, since the surface roughness and the morphology of the samples change after introduction of oxygen defects as observed in the AFM results, the morphology significantly affects the OER activity. To eliminate the influence of morphological change on the OER performance, we normalized the current density of the CV plots by using the electrochemical active surface area (ECSA), as shown in Figure . The results showed that introducing oxygen defects was still beneficial to the activity of OER when normalizing the current on the ECSA. We also carried out EIS measurements to investigate the charge transfer at the solid-liquid interface. Figure shows the Nyquist plots of LNO, LNO-Vo and LNO-Vo' at a potential of +1.7 V (vs RHE). The Nyquist plots are simulated by the Randles equivalent circuit, the fitted values of charge transfers resistance (Rct) are summarized in Table . The Rct of LNO-Vo and LNO-Vo' is much smaller than that of LNO, indicating oxygen vacancy can facilitate the charge transfer process at LNO and electrolyte interface. Next to activity, stability is aso an important metric for evaluating OER performance of a catalyst material. We measure the CV polarization plot after 10 CV and 1000 CV cycles with the potential range of +1.0 V to 1.5 V vs RHE at 100 mV/s scan rate, as shown in Figure . To evaluate the stability of the catalysts, we measured the current density at +1.7 V vs RHE (Figure ) after the cycling treatment.