Fine-Tuning HER Activity Through the Interplay Between Nanoparticle Size and Ligand Coordination Mode in Surface-Functionalized Nanocatalysts

Metallic nanoparticles (NPs) are state-of-the-art catalysts for the hydrogen evolution reaction (HER), and surface functionalization provides a powerful tool to tune both their physical properties and catalytic performance. This work explores the influence of stabilizing ligands on the HER performance of Ru NPs. We report Ru NPs functionalized with controlled amounts of 2,2′-bipyridine (bpy). The ligand loading has the usual influence on the NP size, as higher bpy concentrations produce smaller NPs. In contrast, the NP size affects the HER activity in a counterintuitive manner: the smallest NPs are the least active, whereas the largest ones achieve catalytic performance comparable to that of commercial Pt/C. Detailed compositional, spectroscopic, and electrochemical characterization, including analysis of bpy coordination via N 1s XPS, enabled the construction of quasi-realistic computational models. DFT calculations combined with the Yang and Saidi electrochemical model reveal that HER activity depends on both NP size and ligand coordination mode. Importantly, we provide direct evidence that σ- versus π-coordination of bpy controls the intrinsic HER catalytic activity of distinct Ru sites, as only the DFT-calculated exchange current density of σ-coordinated sites matches experimental trends. These results establish ligand coordination mode as a decisive descriptor in surface-functionalized nanocatalysts and introduce an additional design principle for developing highly efficient HER electrocatalysts.