Hydrogen is essential to several key industrial processes and could play a major role as an energy carrier in a future 'hydrogen economy'. Although the majority of the world's hydrogen supply currently comes from the reformation of fossil fuels, its generation from water using renewables-generated power could provide a hydrogen source without increasing atmospheric CO₂ levels. Conventional water electrolysis produces H₂ and O₂ simultaneously, such that these gases must be generated in separate spaces to prevent their mixing. Herein, using the polyoxometalate H₃PMo₁₂O₄₀, we introduce the concept of the electron-coupled-proton buffer (ECPB), whereby O₂ and H₂ can be produced at separate times during water electrolysis. This could have advantages in preventing gas mixing in the headspaces of high-pressure electrolysis cells, with implications for safety and electrolyser degradation. Furthermore, we demonstrate that temporally separated O₂ and H₂ production allows greater flexibility regarding the membranes and electrodes that can be used in water-splitting cells.