Organic Electrosynthesis in Solid Polymer Electrolyte Electrolyzers: Interfacial Reaction Engineering and Operando...

Organic electrosynthesis has re-emerged as an attractive strategy for electrifying chemical industries without stoichiometric redox reagents. However, the practical value of an electrochemical transformation is not determined by yield and selectivity alone. Energy consumption, supporting-electrolyte removal, and downstream separation must also be considered when translating laboratory-scale reactions into chemical production. Solid polymer electrolyte (SPE) electrolyzers, which have been developed extensively in fuel cells and water electrolysis, offer a promising architecture for addressing these practical requirements in organic electrosynthesis. In an SPE electrolyzer, an ion-exchange membrane is sandwiched between electrodes to form a membrane electrode assembly with a zero-gap configuration. This structure shortens the ion-conduction path, lowers ohmic losses, and eliminates the need for dissolved supporting electrolytes. Within the electrode, the electrocatalyst, ionomer, and organic phase form a triple-phase boundary where electron transfer, ion transport, substrate adsorption, and chemical conversion occur in close proximity. Thus, the SPE architecture provides not only energetic and process advantages but also a platform for engineering the local environment in which electrosynthesis takes place. This Account is organized around three general advances enabled by this architecture. First, the zero-gap configuration can lower cell resistance, while supporting-electrolyte-free operation can simplify product isolation. Second, integration of the electrocatalyst and ionomer converts the triple-phase boundary into a tunable interfacial reaction field. The electrocatalyst controls substrate adsorption and the formation and reactivity of surface intermediates, whereas the ionomer governs the local ionic environment. Membrane selection is also important for broadening the range of compatible substrates and electrocatalysts. Together, the ionomer, electrocatalyst, and membrane provide complementary means of controlling activity and selectivity. Third, SPE electrolyzers offer a useful platform for operando characterization because the working ionomer-catalyst interface can be retained during spectroscopic measurements. Operando infrared and X-ray absorption spectroscopies reveal adsorbed hydrogen species, substrate adsorption geometries, and catalyst oxidation states that cannot be reliably inferred from ex situ characterization alone. These observations show that the catalytically relevant interface is dynamic and that rational catalyst and ionomer design must be based on the states present under operating conditions. After outlining the energetic basis and historical development of SPE electrosynthesis, we discuss representative hydrogenation and oxidation studies that establish these design principles. By integrating low-voltage and supporting-electrolyte-free operation with interfacial reaction engineering and operando mechanistic analysis, SPE electrolyzers provide a framework for developing selective organic transformations driven by renewable electricity. Further advances in membrane and ionomer stability, mass transport, reactor durability, and scale-up will be required to translate this framework into broadly applicable chemical manufacturing processes.