Potential-Dependent Interfacial Proton Dynamics Promote NRR on Mo- Anchored Holey Graphene: Success of Dynamics and Failure of Static Models

DS Teja and BS Mallik, JOURNAL OF PHYSICAL CHEMISTRY C (2025).

DOI: 10.1021/acs.jpcc.5c06123

Electrocatalytic nitrogen reduction reaction (eNRR) is the major concern because of its lower activity and poor selectivity, which prevent it from large-scale ammonia synthesis. Herein, we explored the N2 reduction on Mo-anchored N-doped graphene/holey graphene (Mo-N3 and Mo-N3-2Vc) by using the computational hydrogen electrode model. The hybrid solvation model is incorporated to represent the aqueous environment. Free energy calculations show that the potential-determining step for Mo-N3 and Mo-N3-2Vc (two carbon vacancy) is the formation of NH3 (*NH2 -> *NH3) and *NNH (*N2 -> *NNH), with Gibbs free energy changes of 0.437 and 0.248 eV, respectively. The charge capacitor model (CCM) formulated by Chan-Norskov and constant potential ab initio molecular dynamics (CP- AIMD) with the addition of a slow growth approach, is used to calculate the potential-dependent activation free energy barrier (Gact). In the CCM, the G act computation requires only the activation barrier with the corresponding work functions and surface charges for reaction states. The resultant activation and reaction energies via Cl-NEB are used in the CCM formulation and extrapolated up to the desired potential from 0.0 V vs RHE. In contrast, CP-AIMD simulates reactions under constant potential by dynamically exchanging electrons with an external bath, capturing realistic interfacial charge fluctuations. Experimental studies show that the NRR occurs at an applied potential of -0.25 V vs RHE with an NH3 yield of 31.5 mu gh-1mgcat -1 on Mo-N3 and at -0.05 V vs RHE with a yield of 3.6 mu gh-1mgcat -1 on Mo-N3-2Vc. The CCM demonstrated that Mo-N3 has a reaction barrier of 0.761 eV, whereas Mo-N3-2Vc shows 1.063 eV at its respective potentials. Through CP-AIMD, a significant reduction in the reaction barrier is observed due to dynamic modulation of the charged surface, consistent with experimental trends. It emphasizes the importance of studying the electrochemical reactions at the electrode-electrode interface with the application of a constant potential condition. Our study provides valuable insights into examining the reaction at a constant potential, both with and without altering the background electrons.

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