The Central Role of Nitrogen Atoms in a Zeolitic Imidazolate Framework-Derived Catalyst for Cathodic Hydrogen Evolution.

ABSTRACT

Platinum usually offers the most effective active center for hydrogen evolution reaction (HER), because of the optimal trade-off between the adsorption and desorption of hydrogeN atoms (H*) on Pt atoms. Herein, we report an unusual result regarding the active center of a HER catalyst, which was synthesized by electrodepositing traces of Pt nanoparticles (NPs) into a porous nitrogen-rich dodecahedron matrix derived from zeolitic imidazolate framework ZIF-8. With an ultra-low Pt loading of 2.76 µg cm-2 , the N-Pt-bonded catalyst can produce a current density of 117 mA cm-2 for the HER in 1.0 m H2 SO4 at an overpotential of 50 mV, whereas the commercial Pt/C (300 µg cm-2 Pt) can only reach 50 mA cm-2 under the same conditions. Cyclic voltammetry demonstrates that both the H* adsorption and the Pt oxidation are not allowed to occur on this catalyst, due to a full surface coverage of the trace Pt NPs by imidazole. The results from the specially designed experiments indicate that the imidazole N atoms may act as proton anchor-sites for the HER due to their electron donor nature. Density functional theory calculations also support a catalytic HER mechanism centered at the Pt-supported N active center, which needs a Gibbs free energy of H* absorption (ΔGH* ) significantly smaller than the absolute value of ΔGH* on the Pt(111) surface. We hope that the results of this study will encourage the research on novel N-centered catalysts for the HER.