Si₃C Monolayer as an Efficient Metal-Free Catalyst for Nitrate Electrochemical Reduction: A Computational Study

Nitrate electroreduction reaction to ammonia (NO₃ER) holds great promise for both nitrogen pollution removal and valuable ammonia synthesis, which are still dependent on transition-metal-based catalysts at present. However, metal-free catalysts with multiple advantages for such processes have been rarely reported. Herein, by means of density functional theory (DFT) computations, in which the Perdew-Burke-Ernzerhof (PBE) functional is obtained by considering the possible van der Waals (vdW) interaction using the DFT+D3 method, we explored the potential of several two-dimensional (2D) silicon carbide monolayers as metal-free NO₃ER catalysts. Our results revealed that the excellent synergistic effect between the three Si active sites within the Si₃C monolayer enables the sufficient activation of NO₃^- and promotes its further hydrogenation into NO₂^*, NO^*, and NH₃, making the Si₃C monolayer exhibit high NO₃ER activity with a low limiting potential of -0.43 V. In particular, such an electrochemical process is highly dependent on the pH value of the electrolytes, in which acidic conditions are more favorable for NO₃ER. Moreover, ab initio molecular dynamics (AIMD) simulations demonstrated the high stability of the Si₃C monolayer. In addition, the Si₃C monolayer shows a low formation energy, excellent electronic properties, a superior suppression effect on competing reactions, and high stability, offering significant advantages for its experimental synthesis and practical applications in electrocatalysis. Thus, a Si₃C monolayer can perform as a promising NO₃ER catalyst, which would open a new avenue to further develop novel metal-free catalysts for NO₃ER.