Construction of High-Surface-Area NiMo Electrocatalysts and the Evolution of HER Dynamics

Maximizing the electrochemically active surface area (ECSA) through three-dimensional structuring remains a prevailing strategy for advancing hydrogen evolution reaction (HER) electrocatalysts. Yet how an enlarged active area ultimately governs catalytic performance, especially through its adverse side effects, has received insufficient attention. Here, a series of three-dimensional NiMo catalysts with high specific surface areas were fabricated on nickel foam (NF) substrates via interface-charge modulation for controlled particle deposition. Both the apparent and intrinsic activities of the resulting electrodes were evaluated systematically. Enriched active sites boosted apparent catalytic performance. The optimized electrode (NiMo/NF-1000) delivered an ultralow Tafel slope of 33 mV dec–1 and overpotentials of 36 mV at 10 mA cm–2 and 134 mV at 100 mA cm–2. However, its turnover frequency is 4.0-fold lower than NiMo/NF-200 in turnover frequency (TOF), revealing diminished single-site efficiency. In situ electrochemical impedance spectroscopy and optical imaging of bubble dynamics revealed that excessive proliferation of active sites drives structural complexity and disorder. This obstructs bubble detachment pathways, persistently blocks catalytic sites, and ultimately suppresses the TOF of active centers. This work confronts the active-site gains conferred by three-dimensional structural construction alongside their concomitant influence on bubble behavior, providing guidance for the rational design of catalytic electrodes.