Abstract
The storage of renewably-generated energy as hydrogen via the electrolysis of water is a fundamental cornerstone of a sustainable hydrogen economy. Conventional electrolysers usually require stable power inputs in order to operate effectively and safely and so may be unsuited to harnessing renewable power, which is often intermittent and diffuse. Electrolysis mediated by Electron-Coupled-Proton Buffers has the potential to overcome some of the challenges surrounding electrolysis using low and/or sporadic power inputs (especially those related to gas crossover) as the use of Electron-Coupled-Proton Buffers allows the oxygen and hydrogen evolution reactions to be completely decoupled from one another. Herein, we show that silicotungstic acid can be used as an Electron-Coupled-Proton Buffer in a proton exchange membrane cell, decoupling the hydrogen and oxygen evolution reactions at steady state current densities as high as 500 mA cm−2. O2 and H2 can be produced continuously by this system by cycling a fixed volume of the Electron-Coupled-Proton Buffer solution. Even at current densities as low as 25 mA cm−2, the level of hydrogen in the oxygen stream is <0.4%, whereas a conventional proton exchange membrane electrolyser operating at this current density produces oxygen containing nearly 2% hydrogen (unacceptable for most applications). Furthermore, using silicotungstic acid as an Electron-Coupled-Proton Buffer also confers greater tolerance to non-deionised water inputs and reduces fluoride release from the perfluorosulfonated membrane (a marker for membrane degradation) relative to a conventional proton exchange membrane electrolyser. Together, these results highlight the promise and potential advantages of Electron-Coupled-Proton Buffers (and silicotungstic acid in particular) for the electrolytic production of hydrogen and oxygen over a wide range of current densities, such as might be delivered by renewable power inputs.
Highlights
The electrolysis of water to generate hydrogen and oxygen is seen as an increasingly attractive route by which to store intermittent, renewably-generated electricity, as hydrogen is an excellent transportable fuel [1e3]
Through the use of a second electrochemical cell to re-oxidise the reduced ECPB, we show that this system can almost completely decouple the oxygen and hydrogen evolution reactions from each other under continuous and stable steady-state operation at an imposed current density of 500 mA cmÀ2, with cell voltages only a little in excess of 2 V
We have demonstrated the operation of an electrolyser using silicotungstic acid as an electron-coupled-proton buffer where complete decoupling of the oxygen evolution and hydrogen evolution reactions can be achieved at current densities of up to 500 mA cmÀ2 under steady state conditions
Summary
The electrolysis of water to generate hydrogen and oxygen is seen as an increasingly attractive route by which to store intermittent, renewably-generated electricity, as hydrogen is an excellent transportable fuel [1e3] Consumption of this fuel (either by combustion or in an electrochemical device such as a fuel cell) releases only energy and water, meaning that electrolyticallygenerated hydrogen has tremendous potential as a sustainable energy vector [4,5]. At low current densities, the rates at which H2 and O2 are produced in an electrolyser may be slower than the rates at which these gases permeate the membrane [10] Such gas crossover would, at the very least, reduce the amount of hydrogen that could be harvested from such a device and the efficiency of electrolysis, and could in extreme cases lead to the creation of hazardous mixtures of hydrogen and oxygen. Gas crossover of this nature would be expected to lead to the production of reactive oxygen species that degrade cell components (in particular the membrane) and shorten the electrolyser’s lifespan [11,12]
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