Although electrochemical hydrogen evolution and oxidation are arguably the best-understood reactions in electrocatalysis, the anomalous effect of pH on hydrogen reaction kinetics has defied simple explanation for decades. This longstanding puzzle exposes gaps in the fundamental understanding of electrocatalysis by showing that singular adsorption descriptors (e.g., the hydrogen binding energy) cannot describe kinetic effects across electrolytes. In this Perspective, we discuss the strengths and shortcomings of binding energies as HER/HOR activity descriptors across different electrolytes and catalyst surfaces, with a special emphasis on the bifunctional mechanism, and identify several "beyond adsorption" descriptors for chemical dynamics in the double layer, including the potential of zero (free/ total) charge, the binding energy of coadsorbed spectator species, transition state barrier heights, and the solvation strength of electrolyte cations. Recent evidence for and against the importance of these phenomena is assessed in the context of hydrogen electrocatalysis to determine their feasibility to accurately predict catalyst behavior. Finally, we propose paths forward for improving the mechanistic understanding of how specific interactions between the surface and species in solution affect macroscopic rates, which include combining single-crystal voltammetry, electroanalytical chemistry, in-operando spectroscopy, atomic-scale DFT calculations, and molecular "double-layer dopants".
The slow kinetics of the hydrogen oxidation and hydrogen evolution reactions (HER and HOR) in alkaline compared to acidic media remain a fundamental conundrum in modern electrocatalysis. Recent efforts have proposed that OH, as well as H, must bind optimally for improved kinetics, but the exact role of adsorbed OH is not yet known. In this work, we combine steady-state single-crystal voltammetry and microkinetic modeling to determine the roles of adsorbed hydroxide and the so-called bifunctional mechanism in alkaline HER and HOR kinetics. We consider both a direct Volmer mechanism, in which H and OH compete for sites on Pt (110), and an OH-mediated mechanism, in which Pt ( 111) adsorbs H while transition metal clusters adsorb OH. Our experimental and computational results show that on a thermodynamic coverage basis, increasing OH adsorption strength cannot promote faster HER/HOR kinetics. Only changes to the kinetic rate constants can explain experimental observations. We speculate that adequate electrocatalyst design in alkaline media additionally requires manipulation of interfacial water structure to lower energetic barriers for HER and HOR.
The pH-dependent
kinetics of the hydrogen oxidation and evolution
reactions (HERs and HORs) remain a fundamental conundrum in modern
electrochemistry. Recent efforts have focused on the impact of the
interfacial water network on the reaction kinetics. In this work,
we quantify the importance of interfacial water dynamics on the overall
hydrogen reaction kinetics with kinetic isotope effect (KIE) voltammetry
experiments on single-crystal Pt(111) and Pt(110). Our results find
a surface-sensitive KIE for both the HER and the HOR that is measurable
in base but not in acid. Remarkably, the HOR in KOD on Pt(111) yields
a KIE of up to 3.4 at moderate overpotentials, greater than any expected
secondary KIE values, yet the HOR in DClO4 yields no measurable
KIE. These results provide direct evidence that solvent dynamics play
a crucial role in the alkaline but not in the acidic hydrogen reactions,
thus reinforcing the importance of “beyond adsorption”
phenomena in modern electrocatalysis.
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