First-Principles Investigation of Phosphorus-Doped 2D C ₆ N ₇ as a Metal-Free Electrocatalyst for the Hydrogen Evolution Reaction

Producing green hydrogen on a large scale requires electrocatalysts that are not only active and durable but also abundant enough to realistically replace platinum in the hydrogen evolution reaction (HER). Here, using density functional theory (DFT) calculations, we reveal that substitutional phosphorus doping of two-dimensional heptazine-based C 6 N 7 can show near-optimal hydrogen adsorption thermodynamics when used as an HER electrocatalyst. Phosphorus, boron, sulfur, and silicon dopants were systematically examined at chemically compatible substitution sites with single substitution corresponding to ∼4 atom % doping concentration. Notably, phosphorus incorporation at nitrogen positions (P@C 6 N 7 ) delivers a near-thermoneutral Gibbs free energy of hydrogen adsorption (Δ G H* = −0.104 eV). P@C 6 N 7 drastically outperforms pristine C 6 N 7 (Δ G H* ≈ +1.97 eV) and all other nonmetal-doped variants. A moderate kinetic barrier of only 0.94 eV for the rate-determining Volmer–Heyrovsky step is observed. P substitution induced a downward shift in the p-band center as well as pronounced electronic rearrangements in adjacent carbon edges with a reduction in the work function, which correlates with the enhanced HER descriptors. Ab initio molecular dynamics simulations (10 ps at 300 K) indicate that the P@C 6 N 7 framework retains its structural integrity under thermal fluctuations. The present results are based on first-principles thermodynamic and kinetic descriptors under idealized conditions. P@C 6 N 7 achieves favorable hydrogen adsorption while using only carbon, nitrogen, and phosphorus elements, which are mostly earth-abundant and compatible with biological cycles. Implicit solvation effects were considered by using the VASPSol model, which results in a slightly increased overpotential while preserving the observed activity trend. As a result, this material emerges as a promising metal-free HER catalyst. Apart from our specific system, this study also paves the way toward a practical design strategy for activating polymeric 2D materials through site-specific nonmetal substitutions