A pyrazine-incorporated graphdiyne nanofilm as a metal-free electrocatalyst for the hydrogen evolution reaction

Abstract

A graphdiyne (GDY) analogue that features pyrazine as the aromatic core (PR-GDY) was synthesized under air using a liquid/liquid interfacial synthesis. Optical microscopy, SEM, and AFM revealed the sheet morphology of PR-GDY with a typical thickness of 20 nm in AFM, while TEM disclosed its amorphous nature. It served as a metal-free electrocatalyst for the hydrogen evolution reaction (HER) from water. The presence of the butadiynyl linker and terminal ethynyl group, respectively, was confirmed by Raman spectroscopy. Polymerization to PR-GDY afforded optical bandgap narrowing compared with the corresponding monomer. XPS provided a ratio of 2 : 1 for the sp and sp² carbons. Moreover, a single Gaussian was fitted with the N 1s peak, ascribable to the pyridinic nitrogen. Such selective doping of heteroatoms is hard to realize in carbon materials. PR-GDY was subjected to TGA, such that its thermal stability up to 300 °C was confirmed. N₂ adsorption–desorption isotherms featured a typical type I adsorption behavior with micropores, with a Brunauer–Emmett–Teller surface area and mean pore size of 408 m² g⁻¹ and 0.8 nm, respectively. A series of electrochemical measurements were conducted to evaluate the HER activity of PR-GDY. The HER performance of GDY was only slightly better than that of glassy carbon, while that of PR-GDY was improved significantly in both acidic and basic aqueous media with an onset potential of −275 mV (vs. the reversible hydrogen electrode, RHE), and −475 mV to achieve a current density of 10 mA cm⁻² in 0.5 M H₂SO₄, and –270 and −710 mV in 0.1 M NaHCO₃ + 0.1 M Na₂CO₃. The present work demonstrates experimentally that PR-GDY serves as a metal-free HER electrocatalyst, and manifests that pyridinic nitrogen enhances the electrocatalytic activity of carbon materials.