Abstract
AbstractElectrolytic water splitting to generate hydrogen and oxygen is one of the most promising ways in which to harness intermittent renewable power sources and store the energy these provide as a clean‐burning and sustainable fuel. In recent years, this has led to an explosion in reports on electrochemical water splitting, most of them focused on improving the efficiency of the electrochemical reactions themselves. However, efficient generation of hydrogen and oxygen is of little use if these products cannot be kept separate and the community is now coming to realize that there are considerable challenges associated with maintaining adequate separation between H2 and O2 during electrolysis driven by intermittent renewable sources. Decoupled electrolysis (whereby oxygen production occurs with reduction of a suitable mediator and hydrogen production is then paired with the reoxidation of this mediator) offers a solution to many of these challenges by allowing O2 and H2 to be produced at different times, at different rates, and even in completely different electrochemical cells. In this review, an overview of recent progress in the field of decoupled electrolysis for water splitting is given and the potential that this approach has for enabling a range of other sustainable chemical processes is explored.
Highlights
Renewable energy sources, such as wind, most promising ways in which to harness intermittent renewable power sources and store the energy these provide as a clean-burning and sustainable fuel
The oxygen may be vented to the atmosphere whilst the hydrogen tial that this approach has for enabling a range of other sustainable chemical is stored as a fuel
It is becoming increasingly clear that greenhouse gases that are formed during combustion of these fuels are linked to oceanic sustainably sourced hydrogen could be used to reduce CO2 or N2 from the atmosphere to generate carbon-neutral fuels and commodity chemicals such as hydrocarbons and ammonia
Summary
In its simplest form, water electrolysis will occur under the influence of a direct current between two electrodes in a single compartment. A current density of 10 mA cm–2 is considered a useful benchmark for solar-driven electrolyzers, as this is the approximate current density expected of a water splitting device operating at 10% solar-to-fuels efficiency under “1 Sun” illumination (AM 1.5, 100 mW cm–2).[23] In this scenario, crossover of hydrogen into the anodic chamber would be a real possibility and would be hazardous, as the lower explosion limit of hydrogen in oxygen is only 4 mol% H2 in O2.[24–27] even if efficient and safe gas separation could be achieved, any solar-to-hydrogen device in which the half-reactions of water splitting remain coupled (as in a conventional electrolyzer, see Figure 1) will suffer from the fact that the rate of the relatively facile HER would still be limited by the more sluggish OER. On account of its ability to accept both electrons and protons in this manner, the term “Electron-Coupled-Proton Buffer” (ECPB) was coined to describe this and other redox mediators with similar capabilities
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