SMART framework for single-atom HER catalysts

Single-atom catalysts (SACs) are assumed to remain structurally static during catalysis, while dynamic behavior under operating conditions is rarely explored. We propose a Structure-Mobility-Activity-Restructuring-Thermodynamics (SMART) framework to replace the static structure-activity framework, establishing atomic mobility as a critical parameter. In this work, under identical conditions, we conduct a comprehensive comparison of the behavior of SACs based on Ni, Pd and Pt in the hydrogen evolution reaction (HER), expanding the fundamental knowledge about the dynamic behavior of heterogeneous systems. Here, we show that Pt, Pd, and Ni atoms on graphite do not remain isolated during HER but migrate and thermodynamically self-assemble into nanoclusters. Using identical-location SEM, HAADF-STEM, XPS, electrochemical analysis, DFT modeling, and machine learning analysis, we demonstrate that catalytic behavior is governed not only by initial atomic dispersion, but by metal-specific atomic mobility. Pd atoms exhibit rapid migration with the lowest Tafel slope (37 mV dec -1 ), Pt shows intermediate mobility and performance (53 mV dec -1 ), while Ni remains dispersed longest but is less active (76 mV dec -1 ). DFT reveals that diffusion barriers correlate with cohesive and binding energies, providing a quantitative explanation for the observed restructuring trends.