Against the backdrop of global energy transition and growing demand for carbon-neutral technologies, green hydrogen production via renewable-electricity-driven water electrolysis is emerging as an important pathway toward a sustainable energy future. A collaborative team led by Xiaobin Zhong and Jinchao Dong from the School of Energy and Power Engineering at North University of China, together with Gen Huang from the College of Energy and the State Key Laboratory of Physical Chemistry of Solid Surfaces at Xiamen University, has systematically reviewed key intermediates and interfacial processes involved in the electrochemical hydrogen evolution reaction (HER). Focusing on three critical species—adsorbed hydrogen (H*), hydroxyl (OH*), and interfacial water (H₂O*)—the review summarizes their in situ identification, structure–activity relationships on single-crystal surfaces, and the translation of mechanistic insights from model systems to practical nanocatalysts, providing valuable perspectives for understanding HER mechanisms and rationally designing high-performance catalysts.
Although HER appears macroscopically simple—producing hydrogen gas—its microscopic reaction pathways are considerably more complex. In acidic media, protons are first adsorbed on catalytic sites to form H*, followed by hydrogen generation through either the Heyrovsky or Tafel pathway. Under alkaline conditions, however, water molecules must first be activated and dissociated, accompanied by the formation and removal of OH*. Consequently, how H adsorbs, how H₂O is organized and activated, and how OH* forms and desorbs** collectively determine the overall reaction kinetics.
Conventional electrochemical measurements and density functional theory calculations provide essential thermodynamic and kinetic information, but they cannot fully capture the rapid evolution of dynamic electrochemical interfaces. The review therefore highlights advanced in situ techniques, including electrochemical surface-enhanced infrared absorption spectroscopy (EC-SEIRAS), electrochemical surface-enhanced Raman spectroscopy (EC-SERS), electrochemical scanning tunneling microscopy (EC-STM), and electrochemical atomic force microscopy (EC-AFM). By combining the chemical specificity of spectroscopy with the spatial resolution of microscopy, researchers are increasingly able to determine what the intermediates are, where they are located, and how they evolve during a reaction .
Structurally well-defined single-crystal surfaces provide ideal model platforms for uncovering fundamental HER mechanisms. On Pt(111), Pt(100), and related surfaces, for example, researchers can distinguish hydrogen adsorption behaviors associated with specific crystal facets and catalytic sites. Advanced approaches such as shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) further enable direct observation of H* and other interfacial species while minimizing perturbation to the intrinsic single-crystal surface. These studies reveal that HER activity depends not only on the binding strength of H*, but also on interfacial water structures, OH* adsorption, and the local electric field.
Interfacial water is particularly important in alkaline HER and should not be regarded merely as a passive reaction medium. Water molecules, ions, and hydrogen bonds near the electrode form a complex interfacial network. The identity of electrolyte cations, electrolyte composition, and even the orientation of water molecules can influence water dissociation and proton-transfer kinetics. The review further emphasizes that proton transport is not simply the diffusion of isolated species but is closely associated with collective motions and relay-like transfer processes within the hydrogen-bond network. This understanding expands HER catalyst design beyond the conventional optimization of hydrogen adsorption toward the deliberate engineering of the entire electrode–electrolyte microenvironment .
Another central challenge is translating knowledge obtained from ideal single-crystal systems into practical nanocatalysts. The review systematically discusses single-crystal surface modification as an important bridge between these two regimes. Introducing species such as Ni(OH)₂ or Ru onto Pt single crystals can create well-defined heterogeneous interfaces, where oxophilic sites facilitate water dissociation and regulate OH*, while neighboring metallic sites promote H* formation and H₂ release. Combined with facet engineering, heteroatomic interfaces, grain-boundary engineering, and strain modulation, these strategies offer a route to transfer the “structure–intermediate–activity” relationships established on single crystals to structurally complex nanocatalysts.
Despite substantial progress, major challenges remain. Real electrochemical interfaces are highly dynamic, and many intermediates exist only for extremely short periods. Current characterization methods still face limitations in sensitivity, chemical specificity, and temporal and spatial resolution. Future advances may therefore rely on emerging approaches such as ultrafast time-resolved vibrational spectroscopy and electrochemical cryogenic electron microscopy. Integrating Raman and infrared spectroscopy with X-ray techniques, microscopy, electrochemical measurements, and theoretical calculations will also be essential. Meanwhile, ab initio molecular dynamics, machine-learning potentials, and artificial intelligence could provide powerful tools for deciphering increasingly complex and multimodal datasets.
Importantly, mechanistic studies must also move beyond idealized laboratory conditions toward realistic operating environments. Bridging the gap between low-current-density studies using single crystals or rotating disk electrodes and high-current-density operation in gas-diffusion electrodes and membrane electrode assemblies will be critical. Mass transport, local pH, temperature, and other operating parameters need to be incorporated into a unified framework for understanding HER under technologically relevant conditions.
From observing a single H* intermediate to understanding an interfacial water layer and ultimately engineering an efficient nanoscale catalytic interface, this review outlines a clear pathway from microscopic mechanistic understanding to rational catalyst design. As advanced in situ characterization, multiscale theoretical modeling, and studies under practical operating conditions become increasingly integrated, precise knowledge of reaction intermediates is expected to evolve into actionable design principles, providing a stronger scientific foundation for the development of efficient electrocatalysts for sustainable hydrogen production.
Title: In Situ Probing of Electrochemical Hydrogen Evolution Reaction Intermediates: From Single-Crystal Models to Nano-Catalysts
Journal Name: Nano-Micro Letters
DOI: https://doi.org/10.1007/s40820-026-02286-6
URL: https://scholarcast.cast.org.cn/top-news/2101495750262734848
Nano-Micro Letters
Experimental study
Not applicable
In Situ Probing of Electrochemical Hydrogen Evolution Reaction Intermediates: From Single-Crystal Models to Nano-Catalysts
6-Jul-2026