P-Doped Graphene/Graphitic Carbon Nitride Hybrid Electrocatalysts: Unraveling Charge Transfer Mechanisms for Enhanced Hydrogen Evolution Reaction Performance

Recently, hybrid electrocatalyst systems involving an active layer of g-C₃N₄ on a conductive substrate of N-doped graphene (g-C₃N₄@NG) have been shown to achieve excellent efficiency for the hydrogen evolution reaction (HER) [e.g., Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Du, A.; Jaroniec, M.; Qiao, S. Z. Nat. Commun. 2014, 5, 3783 ]. We demonstrate here through first principle calculations examining various hybrid g-C₃N₄@MG (M = B, N, O, F, P. and S) electrocatalysts that the N-doped case may be regarded as an example of a more general modulation doping strategy - by which either electron donating or electron withdrawing features induced in the substrate can be exploited to promote the HER. Despite the intrinsically cathodic nature of the HER, our study reveals that all of the graphene substrates have an increasingly electron withdrawing influence on the g-C₃N₄ active layer as H atom coverage increases, modulating binding of the H atom intermediates, the overpotential, and the likely operational coverage. In this context, it is not surprising that p-doping of the substrate can further enhance the effect. Our calculations show that B is the most promising doping element for g-C₃N₄@MG (M = B, N, O, F, P, and S) electrocatalysts due to the predicted overpotential of 0.06 eV at full coverage and a large interfacial adhesion energy of -1.30 eV, offering prospects for significant improvement over the n-dopant systems such as g-C₃N₄@NG that have appeared in the literature to date. These theoretical results reveal a more general principle for rational design of hybrid electrocatalysts, via manipulation of the Fermi level of the underlying conductive substrate.