Construction and Progress of Small Molecule‐Based Coupled Electrolyzers

Coupled electrolyzer is a desirable way to realize efficient energy conversion from electricity to chemical energy. Using coupled electrolyzers highly valuable chemicals (e.g., H₂, CH_xCOO⁻, nitrile, S, NH₃, CO) can be obtained at low voltages, environmental pollutants can be alleviated, and wastewater (e.g., ammonia, urea, hydrazine) can be recycled. They are even helpful to realize the goal of carbon peaking and carbon neutrality. Compared to traditional chemical methods, small molecule‐based coupled electrolyzers are more cost‐efficient. This review summarizes state‐of‐art of coupled electrolyzers, mainly the replacement of oxygen reduction reaction with oxidation reactions of small molecules and their further coupling with cathodic reduction reactions such as hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), CO₂ reduction reaction (CO₂RR), N₂ reduction reaction (NRR), and other reduction reactions of matching small molecules. In terms of oxidation reactions of small molecules, two types of reactions are covered: sacrificial agent oxidation reaction (SAOR) and electrochemical synthesis reaction (ESR). After detailing the design principle of coupled electrolyzers and several oxidation reactions of small molecules, construction, characterization, and performance of coupled electrolyzers are systematically overviewed along with discussion and outline of current challenges and prospects of this appealing strategy.