Harnessing the Electrochemical Hydrogen Storage Capability of N-Doped Carbons for Metal-Free Hydrogenations

Electrochemically driven organic reactions present an appealing alternative to traditional catalytic methods, which often involve harsh conditions. To date, commercial (noble) metal electrodes have dominated the field, with the development of effective, cheap, and stable electrode materials being overlooked. Nitrogen-doped carbons (NDCs) are widely used in electrocatalysis, energy storage, and even electrochemical hydrogen storage, which can be potentially beneficial in hydrogenation reactions, yet their potential in organic electrosynthesis has remained underexplored. In this study, we synthesized a nanoporous NDC from 7,7,8,8-tetracyanoquinodimethane via the salt-melt method and employed it for the electrochemical hydrogenation of maleic acid to succinic acid (SA). The NDC demonstrated high SA yield rates with nearly 100% Faradaic efficiency, with its performance being comparable or better than conventional (photo)catalytic methods, while using milder conditions, with water as the hydrogen source, and without any metal catalysts. Owing to the NDC’s chemical structure, which causes specific adsorptive interactions, the reaction mechanism resembles that of noble metals, where protons reduced in the Volmer step recombine with coadsorbed maleic acid in a subsequent chemical step. Additionally, due to these adsorptive interactions, the reaction could be directed at will toward the nonredox electrochemical isomerization to fumaric acid, by simply adjusting the potential and electrolyte acidity.