As shown in Fig. 1a, the Ni-MH battery is first charged in the battery charging step (Step 1), in which the Ni(OH)2 is oxidized to NiOOH (Eq. (1)) and the hydrogen storage alloy (M) is reduced from M to MH (Eq. (2)). In HER step (step 2), MH is oxidized to M (Eq. (3)), accompanied by the reduction of water to H2 (Eq. (4)). In OER step (step 3), the NiOOH oxidized in step 1 is reduced back to Ni(OH)2 (Eq. (5)), accompanied by the oxidation of water to oxygen (Eq. (6)). Step1:NiOH2+OH-=NiOOH+H2O+e-
