Solution-phase decoupled water electrolysis in a flow cell with a simple size exclusion membrane separator

• Decoupled electrolysis allows H 2 and O 2 to be produced separately by electrolysis. • This relaxes the need for gas-impermeable perfluorosulfonic acid-based separators. • Herein, a simple cellulose-based separator was used in a decoupled system. • The crossover of H 2 into the O 2 stream was negligible, even at 500 mA/cm 2. • Extended operation will require more robust and more ionically-conductive membranes. Perfluorosulfonic acid membranes are currently the state-of-the-art in terms of electrolytes for proton exchange membrane electrolysers for the production of green hydrogen using renewably-generated power. This is because such materials are chemically robust, have low resistance, and importantly because they greatly reduce the mixing of the hydrogen and oxygen products of electrolysis. However, these materials are also acknowledged to have a number of drawbacks for large-scale use, including high cost, supply shortages and the fact that perfluoro compounds are “forever chemicals” that persist in the environment and are difficult to recycle. In decoupled electrolysis, the hydrogen and oxygen products can be generated in entirely different spaces at entirely different times, and so (at least in theory), gas-impermeable perfluorosulfonic acid membranes are not required in order to prevent gas mixing. However, the use of alternative membranes in solution-phase decoupled electrolysis has received very little attention to date. Herein, we show that a (gas-permeable) simple cellulose-based membrane can be employed in a solution-phase decoupled electrolysis flow system across a range of current densities (25–500 mA/cm 2 ) for 5 h, without evidence for any significant gas mixing. Although optimisation of the membranes for more extended operation is required, this work serves to show that cheap and simple size exclusion membranes are viable for safe water electrolysis in a decoupled system, potentially allowing the replacement of perfluorosulfonic acid membranes in a number of electrolysis applications.