Co‐Implementing Optimized Volmer and Heyrovsky Reactions in Hydrogen Production via Composite Active Sites on Electron‐Withdrawing Carriers

The trade-off of the Volmer reaction for *H formation and the Heyrovsky/Tafel Reaction for *H desorption is a crucial challenge in alkaline hydrogen evolution reaction (HER). Thus, aside from the conventional construction of bifunctional active sites to accelerate the water dissociation step, the electronic retrimming of each metal site is also non-negligible. Herein, an efficient HER electrocatalyst combining Pt single-atoms and Pt nanoparticles on the etched boron nitride (eBN) coated carbon nanotube (Pt-eBN@CNT) is synthesized, where both the Volmer and Heyrovsky steps are promoted by the synergistic adsorption of the composite active sites and the electron-withdrawing effect of eBN carrier, respectively. The proposed catalyst shows impressive performance in alkaline media with an overpotential of 25.1 ± 1.7 mV at 10 mA cm^-2, and a TOF of 17.1 ± 1.3 s^-1 at 0.15 V vs RHE under extremely low metal loading of 6 µg_Pt cm^-2 (catalyst loading of 0.77 mg cm^-2), significantly better than those for the commercial benchmark Pt/C (44.2 ± 4.1 mV, 0.9 ± 0.1 s^-1 at catalyst loading of 0.77 mg cm^-2; 156.1 ± 0.6 mV, 1.6 ± 0.1 s^-1 at metal loading of 6 µg_Pt cm^-2) under the same conditions. Moreover, a membrane-electrode-assembly (MEA) electrolyzer with Pt-eBN@CNT as the cathodic catalyst exhibits a notable charge transfer amount (stands for a long service life) of 2.0 × 10⁶ C·cm^-2 within high-temperature electrocatalysis at 1.0 A·cm^-2, significantly higher than those of state-of-the-art Pt-based electrocatalysts. This work demonstrates the important role of active-site combination coupled with carrier effect in optimizing the kinetics of multi-step electrocatalytic reaction.