Quantum size–induced orbital splitting drives d–p hybridization for enhanced hydrogen evolution in anion-exchange membrane water electrolyzers

Electronic modulation in quantum-size catalysts plays a critical yet underexplored role in governing the hydrogen evolution reaction (HER) in anion-exchange membrane water electrolyzers (AEMWEs). Here, cobalt nanoparticles with tunable diameters (5-15 nm) encapsulated in nitrogen-doped carbon nanotubes (Co@NCNTs) were engineered to elucidate how size-dependent orbital evolution dictates HER activity in alkaline media. We found that a smaller particle diameter results in a better charge transfer capability. Besides, the in-situ Raman spectroscopy shows that Co@NCNTs with an optimized diameter preserve a stable Co-N-C vibration at 690 cm^-1 throughout HER, while transiently forming surface C-OH species-key intermediates that promote water dissociation. Inverse and ultraviolet photoelectron spectroscopy (IPES/UPS) further reveal that the quantum-size-effect splits the d-band and gives rise to a stronger Co 3d-O 2p hybridization. This enhanced hybridization facilitates electronic delocalization across Co-N-C/O interfaces, enabling dynamic charge transfer that favors the formation and subsequent reduction of C-OH intermediates during HER. As a result, the optimized Co@NCNTs catalyst achieves an overpotential of only 33 mV at 10 mA cm^-2 and delivers 550 h operational stability in a full AEMWE. These findings provide direct experimental evidence that quantum-size-tailored orbital coupling governs interfacial OH/H₂, thereby linking spectroscopic insight with catalytic function and establishing a rational design principle for high-performance non-noble-metal catalysts.