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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 (N,N,N,N′,N′,N′-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. 

With an estimated global cost of $2.5 trillion per year, metal corrosion represents a major challenge across all industrial sectors. Numerous inorganic and organic corrosion inhibitors have been developed, but there are growing concerns about their toxicity and impact on the environment. Here, superior organic corrosion inhibitors based on indole-3-carboxaldehyde, a compound commonly found in the digestive system, and thiosemicarbazones, a safe class of ligands, were designed and studied for mild steel in pH 1 sulfuric acid solutions. Electroanalytical techniques and gravimetric tests revealed inhibition efficiencies as high as 98.9% at 30 °C. Models using Langmuir isotherms gave adsorption equilibrium constants Kads of 2 to 9 × 104 M–1 and corresponding Gibbs free energies of adsorption (ΔGads) as high as −41.44 kJ mol–1, indicating their chemisorption. SEM images confirmed the efficacy of these corrosion inhibitors, as surface features showed limited to no changes after tests. Surface analysis by XPS and LC-MS revealed inhibitor concentrations on the order of 0.7 to 1.8 μg cm–2 for the best compounds, further underlining their performance at low concentrations. Mapping of the surface by MALDI-MS further confirmed the homogeneous coating of the steel surface, with no visible fluctuations in concentrations. As all inhibitors shared the same indole thiosemicarbazone platform, unique structure–performance relationships were drawn from theoretical calculations. Notably, DFT and AIMD explained the differences in performance, highlighting the role of side groups in the distribution of the molecular orbitals and the role of water molecules in enhancing the electronic properties of the organic corrosion inhibitors and promoting their chemisorption.