Quaternary Ammonium Additives as Dual Inhibitors of Hydrogen Evolution and Cathodic Corrosion in Aqueous Electrosynthesis

In organic electrosynthesis, the hydrogen evolution reaction (HER) is a parasitic process that significantly diminishes the faradaic efficiency (FE) of aqueous electrochemical reductions and contributes to the cathodic corrosion of widely used metals such as lead and tin. Developing strategies that selectively suppress HER without hindering desired electrochemical transformations is therefore crucial. In this study, we demonstrate that various quaternary ammonium salts (QAS) suppress HER on lead cathodes even under acidic conditions (pH 1). These QAS electrostatically self-assemble at the negatively charged lead surface, forming a cationic barrier that hinders hydronium ion (H₃O⁺) diffusion to the surface, thereby mitigating HER. Chronoamperometry (CA) at -1.8 V vs Ag/AgCl for 1 h revealed stark differences in QAS performance depending on molecular structure. H12MS (<i>N</i>,<i>N</i>,<i>N</i>,<i>N'</i>,<i>N'</i>,<i>N'</i>-hexamethyl-1,12-dodecanediammonium methyl sulfate) was the most effective salt, suppressing hydrogen evolution from ∼0.76 to ∼0.11 mmol cm^-2 (an 85% decrease), even at concentrations as low as 1 μM. CA also showed that the monotonic increase in current over time for blank lead electrodes, which is due to corrosion and surface roughening, was also suppressed in the presence of QAS, underscoring their dual role as inhibitors of both HER and cathodic corrosion. Moreover, during the electrochemical hydrogenation of fumaric acid at -1.7 V vs Ag/AgCl, the addition of 1 mM H12MS enhanced the faradaic efficiency from 7.3% to 38.5% (a 5.3-fold increase) without affecting the yield of succinic acid. These findings highlight the effectiveness of QAS additives in tailoring the boundary layer to improve the efficiency and durability of electrochemical processes.