Competing pathways in electrocarboxylation on copper electrodes: defining reactivity regimes through substrate...

Electrocarboxylation of organic substrates offers a promising route for CO₂ utilization in sustainable synthesis, but its practical application is limited by poor selectivity arising from competing electrochemical pathways. Here, we investigate the electrocarboxylation of benzyl bromide, benzaldehyde, and styrene on polycrystalline copper electrodes in acetonitrile to elucidate how substrate structure and applied potential govern reaction selectivity. By combining cyclic voltammetry, bulk electrolysis, and in situ FTIR spectroscopy, we found electrocarboxylation does not follow a single mechanistic pathway but is instead controlled by the competition between substrate activation, CO₂ reduction, and parallel side reactions. Benzyl bromide exhibits the highest electrocarboxylation efficiency, consistent with facile substrate activation and rapid coupling with CO₂. In contrast, benzaldehyde forms a ketyl intermediate that predominantly undergoes dimerization and hydrogenation, limiting carboxylation. For styrene, the more negative potentials required for substrate activation overlap with CO₂ reduction, resulting in a regime dominated by CO₂-derived products and minimal electrocarboxylation. These findings demonstrate that the relative alignment of substrate and CO₂ reduction potentials defines distinct reactivity regimes that determine electrocarboxylation selectivity. This mechanistic framework contributes to the basis for rational design of electrochemical systems for efficient CO₂ utilization.