Decoupled water electrolysis: Flexible strategy for pure hydrogen production with small voltage inputs

Hydrogen gas is widely regarded as an ideal green energy carrier and a potential alternative to fossil fuels for coping with the aggravating energy crisis and environmental pollution. Currently, the vast majority of the world's hydrogen is produced by reforming fossil fuels; however, this hydrogen-making technology is not sustainable or environmentally friendly because of its high energy consumption and large carbon emissions. Renewables-driven water splitting (2H2O → 2H2 + O2) becomes an extensively studied scheme for sustainable hydrogen production. Conventional water electrolysis requires an input voltage higher than 1.23 V and forms a gas mixture of H2/O2, which results in high electricity consumption, potential safety hazards, and harmful reactive oxygen species. By virtue of the auxiliary redox mediators (RMs) as the robust H+/e– reservoir, decoupled electrolysis splits water at a much lower potential and evolves O2 (H2O + RMsox → O2 + H-RMsred) and H2 (H-RMsred → H2 + RMsox) at separate times, rates, and spaces, thus producing the pure target hydrogen gas safely. Decoupled electrolysis has accelerated the development of water electrolysis technology for H2 production. However, it is still lack of a comprehensive and in-depth review in this field based on different types of RMs. This review highlights the basic principles and critical progress of this emerging water electrolysis mode over the past decade. Several representative examples are then displayed in detail according to the differences in the RMs. The rational choice and design of RMs have also been emphasized. Subsequently, novel applications of decoupled water splitting are briefly discussed, including the manufacture of valuable chemicals, Cl2 production, pollutant degradation, and other half-reactions in artificial photosynthesis. Finally, the key characteristics and disadvantages of each type of mediator are summarized in depth. In addition, we present an outlook for future directions in decoupled water splitting. Thus, the flexibility in the design of mediators provides huge space for improving this electrochemical technology.