Supporting-electrolyte-free electrosynthesis of hydroquinone in solid polymer electrolyte electrolyzers.

The electrification of chemical manufacturing requires electrochemical processes that are not only selective, but also simple in composition, energy-efficient, and scalable. In organic electrosynthesis, however, the use of supporting electrolytes often complicates product isolation and reduces process practicality. Herein, we demonstrate the supporting-electrolyte-free electrochemical synthesis of hydroquinone (HQ) from benzoquinone (BQ) in solid polymer electrolyte (SPE) electrolyzers coupled with water oxidation. In a proton exchange membrane (PEM) electrolyzer, BQ was efficiently reduced to HQ without any added supporting electrolyte, and Ketjenblack outperformed carbon-supported metal catalysts as the cathode material. This result indicates that outer-sphere electron transfer at the carbon surface is sufficient for quinone reduction, whereas supported metals instead may promote competing hydrogen evolution. The PEM electrolyzer also enabled substantially lower-voltage electrolysis than a conventional batch-type divided cell, and gram-scale synthesis of HQ was achieved in 97% gravimetric yield with >99% purity by simply removing solvent after the electrolysis. In addition, the operating current density could be tuned according to the desired balance between productivity and energy efficiency. By contrast, an anion exchange membrane (AEM) electrolyzer showed markedly different behaviour: less hindered benzoquinones underwent side reactions under the basic interfacial environment, causing membrane fouling and cell-voltage increase, whereas a sterically protected quinone was reduced smoothly. These results demonstrate that SPE electrolyzers provide a practical platform for supporting-electrolyte-free organic electrosynthesis and, more importantly, that the membrane-defined reaction environment is a decisive factor governing substrate compatibility.