Breaking the Barrier: How Alkali Cations Promote Enhanced N₂ Adsorption and *NNH Formation

Abstract

Alkali metal cations and hydronium ions significantly modulate the reaction pathways and kinetics of the nitrogen reduction reaction (NRR) at the electrode–electrolyte interface, but it lacks atomic-level insights. Herein, by employing constant potential ab initio molecular dynamics simulations, we examined how alkali cation, hydronium ion, and interfacial charge distribution influence N 2 reduction at the Ni single-atom-anchored N-doped graphene. We explored the reaction barrier of (i) N 2 protonation, (ii) N 2 activation, and (iii) hydrogen adsorption (*H) to evaluate the NRR selectivity in acidic and alkaline media. Alkali cations (K + ) are found to interact with adsorbed *N 2, elongating the N≡N bond, enhancing charge transfer, and facilitating the *NNH formation step kinetically and thermodynamically. In an acidic medium, the *H reaction barrier is 0.398 eV, while in an alkaline medium, it increases to 0.543 eV as alkali cations disrupt the hydrogen-bond network near the interface (K + hydration shell), inhibiting proton transfer, whereas the free H + ion (Eigen and Zundel cations) in an acidic medium facilitates the hydrogen evolution reaction. We identified the NRR selectivity (*N 2 ) in an alkaline medium. Charge analysis reveals that K + and Ni play synergistic roles in activating the *N 2 . Experimental studies show a high Faradaic efficiency for NRR of 11% at −0.3 V vs RHE of pH = 13 in an alkaline medium, compared to 1.3% at −0.1 V vs RHE (pH = 2) in an acidic medium. These findings align with experimental results and provide extensive atomic-level insights into cation–adsorbate interaction in reaction activity and the selectivity of NRR.

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