Stability of Quaternary Akyl Ammonium Cations during the Hydrogen Evolution Reduction: A Differential Electrochemical Mass Spectrometry Study

Abstract

Organic cations, such as R4N+ (R = CH3–, C2H5–, and n-C3H7−), are suggested as potential candidates to tune the activity and product selectivity of the CO2 reduction reaction (CO2RR). The stability of R4N+ species during the hydrogen evolution reaction (HER) is systematically examined by differential electrochemical mass spectrometry (DEMS), since the HER always occurs in parallel with the CO2RR in aqueous systems. Possible reaction mechanisms of R4N+ are investigated systematically by changing the composition of electrodes materials (CuOx, glassy carbon, and polycrystalline Pt), size of cations, types of anions, and their concentration. Our DEMS results and combined ex situ H1 NMR analysis reveal that during the HER at all three electrodes the following occur: (i) R4NOH and R4NClO4 (R = CH3–, C2H5–, and n-C3H7−), as well as (CH3)4NCl, are stable, while (C2H5)4NCl and (n-C3H7)4NCl are not. (ii) Widely distributed mass signals (from m/e = 2 to 150) are detected in solutions containing (C2H5)4NCl and (n-C3H7)4NCl, which come probably from (C2H5)3N and (n-C3H7)3N produced by the HER induced chemical reaction of R4N+. (iii) Small quantities of (C2H5)3NH+Cl– and (n-C3H7)3NH+Cl– are produced in (C2H5)4NCl and (n-C3H7)4NCl solution, respectively, probably through Hoffman elimination. The (C2H5)3NH+Cl– and (n-C3H7)3NH+Cl– will react with OH– produced from the HER and releases R3N, giving the false appearance that R4N+ is not stable under HER conditions.