Hydrogen evolution reaction catalyzed by ruthenium ion-complexed graphitic carbon nitride nanosheets

The development of cost-effective, high-performance electrocatalysts for hydrogen evolution reaction (HER) is urgently needed. In the present study, a new type of HER catalyst was developed where ruthenium ions were embedded into the molecular skeletons of graphitic carbon nitride (C₃N₄) nanosheets of 2.0 ± 0.4 nm in thickness by refluxing C₃N₄ and RuCl₃ in water. This took advantage of the strong affinity of ruthenium ions to pyridinic nitrogen of the tri-s-triazine units of C₃N₄. The formation of C₃N₄-Ru nanocomposites was confirmed by optical and X-ray photoelectron spectroscopic measurements, which suggested charge transfer from the C₃N₄ scaffold to the ruthenium centers. Significantly, the hybrid materials were readily dispersible in water and exhibited apparent electrocatalytic activity towards HER in acid and their activity increased with the loading of ruthenium metal centers in the C₃N₄ matrix. Within the present experimental context, the sample saturated with ruthenium ion complexation at a ruthenium to pyridinic nitrogen atomic ratio of ca. 1:2 displayed the best performance, with an overpotential of only 140 mV to achieve the current density of 10 mA cm^-2, a low Tafel slope of 57 mV dec^-1, and a large exchange current density of 0.072 mA cm^-2. The activity was markedly lower when C₃N₄ was embedded with other metal ions such as Fe³⁺, Co³⁺, Ni³⁺ and Cu²⁺. This suggests minimal contributions from the C₃N₄ nanosheets to the HER activity, and the activity was most likely due to the formation of Ru-N moieties where the synergistic interactions between the carbon nitride and ruthenium metal centers facilitated the adsorption of hydrogen. This was strongly supported by results from density functional theory calculations.