Atoms vs. Ions: Intermediates in Reversible Electrochemical Hydrogen Evolution Reaction

Abstract

We present a critical analysis of the mechanism of reversible hydrogen evolution reaction based on thermodynamics of hydrogen processes considering atomic and ionic species as intermediates. Clear distinction between molecular hydrogen evolution/oxidation (H2ER and H2OR) and atomic hydrogen evolution/oxidation (HER and HOR) reactions is made. It is suggested that the main reaction describing reversible H2ER and H2OR in acidic and basic solutions is: H3O++2e−⇌(H2+)adH2+OH− and its standard potential is E0 = −0.413 V (vs. standard hydrogen electrode, SHE). We analyse experimentally reported data with models which provide a quantitative match (R.J.Kriek et al., Electrochem. Sci. Adv. e2100041 (2021)). Presented analysis implies that reversible H2 evolution is a two-electron transfer process which proceeds via the stage of adsorbed hydrogen molecular ion H2+ as intermediate, rather than Had as postulated in the Volmer-Heyrovsky-Tafel mechanism. We demonstrate that in theory, two slopes of potential vs. lg(current) plots are feasible in the discussed reversible region of H2 evolution: 2.3RT/F≈60 mV and 2.3RT/2F≈30 mV, which is corroborated by the results of electrocatalytic hydrogen evolution studies reported in the literature. Upon transition to irreversible H2ER, slowdown of H2+ formation in the first electron transfer stage manifests, and the slope increases to 2.3RT/0.5F≈120 mV; R,F,T are the universal gas, Faraday constants and absolute temperature, respectively.

Alkaline Electro-Sorption of Hydrogen Onto Nanoparticles of Pt, Pd, Pt80Pd20 and Cu(OH)2 Obtained by Pulsed Laser Ablation

“…Juodkazytè and their colleagues propose that the electro-sorption of platinum results in the formation of intermediate molecular hydrogen ions (H 2 + ) as a precursor to the H 2 molecule [ 41 ]. This assumption is justified because in the acqueous phase water makes the adsorption of H atoms unfavorable [ 42 ].…”

Prediction of Limits of Solar‐to‐Hydrogen Efficiency from Polarization Curves of the Electrochemical Cells

“…Equation (4) describes the voltage efficiency of an EC cell with a Faradaic efficiency of 100%, which is realistic for optimized experimental EC cells. [8,47,48] Although details of the hydrogen and oxygen evolution reactions during water splitting are still under debate, [2,49] we have assumed ΔE ¼ 1.23 V, which is widely used in the literature, [1,[6][7][8][9][10][11][19][20][21][22][23][31][32][33][34][35][43][44][45][46][47][48][50][51][52][53][54][55] to calculate the EC voltage efficiency. In this section, we address the coupling and STH limit problems using the absolute currents of both PV and EC devices as well as the absolute total irradiance P in , which is practical if both devices have predefined areas.…”

The Effect of Electrolytes on the Kinetics of the Hydrogen Evolution Reaction

“…Contrary to the longstanding assumption involving H ad atoms as intermediates, the study proposed that the adsorbed molecular ion (H 2 + ) ad serves as the intermediate in both HER and HOR reactions, aligning better with contemporary experimental observations. Moreover, [24] analyzed experimentally reported data with models which provide a quantitative match [25]. The analysis suggests that the reversible electrochemical hydrogen evolution (H 2 ER) occurring in acidic, neutral, and alkaline aqueous solutions can be described by one common reaction (Equation ( 14)), which involves an intermediate stage of the adsorbed hydrogen molecular ion (H 2 + ) instead of H ad as postulated in the typical Volmer-Heyrovsky-Tafel mechanism.…”