Mitigating the HER-Promoted Corrosion of Lead Cathodes during Electrosynthesis

Abstract

The hydrogen evolution reaction (HER) is a significant side reaction that lowers the faradaic efficiency of electrochemical reductions in aqueous electrolytes. HER has also been identified as a primary contributor to cathodic corrosion for metals like lead, tin, and mercury—commonly employed as cathodes in electrosynthesis. Consequently, the suppression of HER is of paramount importance, not only for enhancing product faradaic efficiencies but also for mitigating corrosion and extending the lifetime of metal cathodes.In the present study, we demonstrate the suppression of HER over lead electrodes in an acidic electrolyte through the addition of the quaternary ammonium salt N,N,N,N’,N’,N’-hexamethyl-1,12-dodecanediammonium methyl sulfate (H12MS). The quaternary ammonium ions electrostatically interact with the surface of the negatively charged lead electrode, creating a cationic layer that protects the cathode’s surface by repelling hydroniums, thereby suppressing HER. The amount of evolved hydrogen was estimated by conducting chronoamperometry (CA) experiments at a potential of -1.8 V vs. Ag/AgCl (after the onset of HER) for 1 hour in a 0.1 M H2SO4 solution (pH 1). It is observed that the hydrogen produced dropped from ~1.62 mmol/cm2 to ~0.26 mmol/cm2 in the presence of 1 mM H12MS. Further, we notice that the current increased monotonically with time for the blank lead electrode during CA (-43.76 to -114.57 mA/cm2), indicating an increase in the exposed surface area due to corrosion. In contrast, the current remained almost constant (-13.2 to -13.8 mA/cm2) in the presence of 1 mM of H12MS, indicating that cathodic corrosion is mitigated. This mitigation in cathodic corrosion is further supported by the scanning electron microscopy images acquired after 1 hour of vigorous HER process. Without H12MS, the lead coupon exhibited significant surface disintegration, indicative of cathodic corrosion. Conversely, in the presence of 1mM H12MS, the surface of the lead coupon remained intact after HER, affirming the successful mitigation of the corrosion process. Our findings for electrochemical hydrogenation will be extended to electrocatalytic transformations, for example, to favor electro-hydrogenation over HER on PGM catalysts.