context: The mechanism of the one-cell, two-electrode decoupled water splitting using Ni(OH)2 as the relay is shown in Fig. 1a. For the H2 evolution step, H2O is reduced on the GEE and the Ni(OH)2 electrode is oxidized to NiOOH simultaneously. By reversing the current polarity, O2 is produced on the GEE by an anodic oxidation of OH−; meanwhile the NiOOH electrode is reduced to Ni(OH)2. The cyclic voltammogram (CV) curve of the Ni(OH)2 electrode tested at a scan rate of 1 mV s−1 using Ag/AgCl as the reference electrode is shown in Fig. 1b. One pair of redox peaks located at 0.423 V/0.238 V (vs. Ag/AgCl) can be indexed to the reversible Ni(OH)2/NiOOH processes, which is consistent with the potential of the Ni(OH)2 electrode in the HER and OER processes. In operation, the one-cell, two-electrode design requires a cell voltage of 0.17 V for the OER step and 1.52 V for the HER step (Fig. 1c). The different operating voltages of the cell for HER and OER steps are caused by the redox potential of the RE (NiOOH/Ni(OH)2), which is much closer to the potential for the OER than the HER. As shown in Fig. 1d, at a cell current of 100 mA, the two-step water splitting process can be reversibly cycled for 24 rounds in 8 hours without noticeable decay. The duration for each OER and HER step was 10 min and the duration can be extended up to 3.5 h before the RE was fully consumed (Fig. S6†). Interestingly, in each gas evolution cycle, after the OER step, there is always a potential drop in the following HER step (inset in Fig. 1d). Meanwhile the HER process does not show this kind of peak. Since the ORR requires lower potential than the HER, after the OER step, there is residual O2 trapped on the GEE or dissolved in the electrolyte, and the ORR occurs to consume this O2, which temporarily reduces the cell voltage.