Defective nitrogen-doped carbon materials have shown a promising application as metal-free electrocatalysts in the oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER). However, there are still some challenges in the tuning of metal-free electrocatalysts and in understanding the roles of various nitrogen species in their electrocatalytic performance. Herein, we design a covalent triazine framework (CTF)-based material as an effective metal-free bifunctional electrocatalyst. We chose BINOL-CN (2,2'-dihydroxy-[1,1'-binaphthalene]-6,6'-dicarbonitrile) as both a carbon and a nitrogen source for the fabrication of N-containing CTF-based materials. Four BINOL-CTFs with varying N-functionalities (pyridinic-N/triazine-N, pyrrolic-N, quaternary-N, and pyridine-N-oxide) were successfully obtained. These materials were evaluated in the ORR and the HER in basic and acidic conditions, respectively. The best material has an onset potential of 0.793 V and a half-wave potential of 0.737 V, and it follows first-order kinetics in a 4e- pathway in the ORR reaction. The same material shows an impressive HER activity with an overpotential of 0.31 V to achieve 10 mA/cm2 and a small Tafel slope of 41 mV/dec, which is comparable to 31 mV/dec for Pt/C, making it a potential bifunctional electrocatalyst. We showed that the ORR and HER reactivity of CTF-based materials depends exclusively on the amount of quaternary-N species and on the available surface area and pore volume. This work highlights the engineering of CTF materials with varying amounts of N species as high-performance bifunctional electrocatalysts.
Keywords: covalent triazine framework; electrocatalysis; hydrogen evolution reaction; metal-free catalysis; oxygen reduction reaction.