“Beyond Adsorption” Descriptors in Hydrogen Electrocatalysis

• Published in: ACS catalysis Vol. 10; no. 24; pp. 14747 - 14762
• Main Authors: Rebollar, Luis, Intikhab, Saad, Oliveira, Nicholas J, Yan, Yushan, Xu, Bingjun, McCrum, Ian T, Snyder, Joshua D, Tang, Maureen H
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 18.12.2020; American Chemical Society (ACS)
• Subjects: Chemistry; solvent effects; bifunctional; DFT; spectroscopy; electrocatalysis; electrochemical double layer
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.0c03801

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".