Pre-catalysts for the hydrogen evolution reaction: from dynamic reconstruction to rational design.

The hydrogen evolution reaction (HER) is central to green hydrogen production, yet its sluggish kinetics impose a fundamental limit on energy conversion efficiency. Catalyst design is undergoing a paradigm shift from static optimization toward dynamic control, as the catalytically active phase often emerges in situ at reductive potentials, giving rise to the concept of the "pre-catalyst". While oxidative reconstruction in the oxygen evolution reaction (OER) has been extensively studied, pre-catalyst reductive reconstruction under HER conditions involves metal-hydrogen (M-H) chemistry and remains mechanistically elusive. This review presents a "dynamic reconstruction to rational design" framework for HER pre-catalysts, surveying reconstruction pathways, dissecting extrinsic-intrinsic driving forces (potential, pH vs. electronic structure, and defects), and distilling strategies (defect engineering, chemical state modulation, etc.) that pre-encode the target active phase. We further highlight the methodological synergy of multimodal operando characterization and multiscale computation and argue that HER reconstruction is a thermodynamically driven, chemically pre-programmable evolutionary process rather than an incidental phenomenon. This reconstruction-centered view offers a design logic that extends beyond the HER to other reductive electrocatalytic systems.