DFT investigation of the oxygen reduction reaction over nitrogen (N) doped graphdiyne as an electrocatalyst: the importance of pre-adsorbed OH* and the solvation effect

Abstract

Searching for novel electrocatalysts that can replace precious platinum for oxygen reduction reaction (ORR) is important for developing fuel cells. Recently, nitrogen (N) doped graphdiyne (GDY) has been synthesized and proved that the ORR electrocatalytic activity catalyzed by N-doped GDY is significantly improved, however, the roles of sp-N (including sp-N1 and sp-N2) and pyridinic (Pyri)-N dopants in mediating the ORR are still unclear. To clarify which sp-N or Pyri-N creates the active site for ORR, we systematically studied the ORR mechanism on sp-N1GDY and pyri-NGDY supported on graphene (G) with the solvation effect, which was performed using density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. Firstly, we found that the dissociative mechanism is preferred on sp-N1GDY/G and the surface is easily terminated by the OH* intermediates, while the OH* pre-adsorbed surface (sp-N1GDY(OH)/G) prefers the associative mechanism. Pyri-NGDY/G also prefers the associative mechanism without any termination. Then, the solvation effect stabilizes all ORR intermediates in both cases. From the calculated free energy diagram, a model with water solvent gives a more appropriate estimation of the overpotential than the one without the water solvent, and sp-N1GDY/G with OH* pre-adsorbed has a lower overpotential (0.46 V), which is close to the experimental value (0.36 V), compared with Pyri-NGDY/G (0.75 V). Our study provides useful information for understanding the reaction mechanisms of ORR at the solid/liquid interface on the N-doped GDY surface.