Different role of ruthenium and platinum defective sites on the catalytic activity for the hydrogen evolution reaction

The reduction of protons to H2 (the hydrogen evolution reaction, or HER) is one of the two half reaction of water electrolysis. It produces H2 that can be used as energy vector or sustainable feedstock for other compounds. Platinum electrodes are the “golden standard” catalysts. However, due to Pt cost, there is a need for cheaper alternatives. Indeed, highly active ruthenium nanoparticles are promising alternative. In this contribution, we use DFT (PBE-D2) calculations to understand the differences in the catalytic activities of Ru and Pt materials by considering slab models containing crystalline close packed planes, terraces, steps, islands and adatoms. Results reveal that the strong adsorption of isolated H atoms on either material and on various sites prevents the hydrogen evolution reaction from taking place. Regardless of the model considered, the formation of a monolayer is also too favorable, thus suggesting that higher coverages are needed before HER onset. The adsorption energies of H atoms exceeding the monolayer on platinum's crystalline surfaces, defective steps and terraces are very close to the ideal value, consistent with platinum's high catalytic activity. These results outline that the presence of lowly coordinated metal centers has little effect on the reactivity of Pt. The adsorption energy of extra H on Ru surfaces depends on the adsorption site: On non-defective sites, the adsorption of the extra H is unfavorable, leading to inefficient catalysts. In contrast, the adsorption on defective sites is favorable and close to the ideal value, thus suggesting an enhancement of the catalytic activity when Ru coordination decreases. These results outline that the reactivity toward hydrogen of Ru materials is more sensitive to surface morphology than Pt-based ones. Indeed, the presence of low coordinated centers is larger on Ru nanoparticles, thus rationalizing their high HER catalytic activity.