Decoupled Water Electrolysis at High Current Densities Using a Solution-Phase Redox Mediator

The electrolysis of water using renewably generated power to give "green" hydrogen is a key enabler of the putative hydrogen economy. Conventional electrolysis systems are effective for hydrogen production when steady power inputs are available, but tend to handle intermittent or low-power inputs much less well, in particular because it becomes very difficult to ensure separation of the hydrogen and oxygen products under intermittent or low-power regimes. Decoupled electrolysis offers one potential solution to the problem of interfacing electrolyzers with intermittent and low-power inputs: by allowing the hydrogen and oxygen products of electrolysis to be produced in separate devices to each other, systems in which gas mixtures are inherently much less likely to form can be designed. However, in general, decoupled electrolysis systems operate at rather low current densities (up to a few hundred mA/cm²), which detracts somewhat from their suitability for applications. Herein, we constructed a flow system device for decoupled hydrogen production using a solution of the polyoxometalate silicotungstic acid as a liquid-phase decoupling agent. This mediator has been explored as a mediator for decoupled hydrogen evolution before, but in this work, we significantly expanded the range of current densities over which decoupling is demonstrated, from 50 mA/cm² up to 1.35 A/cm², the latter of which exceeds the current densities at which commercial alkaline electrolyzers operate and which begins to approach those achievable with proton exchange membrane electrolyzers. Essentially complete decoupling of the hydrogen and oxygen generation processes is achieved across this full range of current densities, suggesting that rapid oxygen production with coupled redox mediator reduction is possible without compromising on decoupling efficiency.