Substrate Diffusion Electrodes for Electrochemical Hydrogenation: Influence of Material Choice and Process Conditions

Abstract Synthetic electrochemistry enables selective chemical transformations without the need for stoichiometric reactants, yet the reliance on electrolytes and substrate dilution often limits its feasibility. Addressing this, we previously introduced the substrate diffusion electrode (SDE) as a flexible and modular platform for the conversion of pure substrate feeds. Herein, we examine the impact of the separation layer material, adjacent electrolyte, process conditions, and catalyst properties on electrochemical hydrogenation (EChH) efficiency and crossover rates. A linear correlation between the choice of hydrophobic transport layers and local substrate concentration was demonstrated, highlighting substrate concentration as a key factor for semi‐hydrogenations. Our results reveal that the ion exchange capacity (IEC) of the ionomer transport layer significantly influences semi‐hydrogenation efficiency, with a notable improvement in faradaic efficiency (FE) when switching from lower‐IEC to higher‐IEC membranes. However, the trade‐off between high FE and low electrolyte crossover persists, with the optimal conditions varying based on electrolyte pH and cation concentrations. Additionally, we pioneered operando spectroscopy in this context confirming that the local substrate concentration is a dynamic, tunable factor. The presented results reveal how the investigated parameters influence the local substrate‐electrolyte concentration balance and how they can be tuned to obtain optimum conditions for the valorization of discrete substrate feeds.