Hf₂B₂O₂-TM_x (x = 1, 2, 3) as highly active and selective electrocatalysts for nitric oxide reduction to ammonia

The electrochemical reduction of nitric oxide (NORR) offers a promising strategy for converting hazardous NO into valuable NH₃. However, discovering electrocatalysts with high activity and selectivity remains challenging, hindered by inherent linear scaling relationships (LSRs) and competitive side reactions. Herein, we employ the hexagonal MBene (h-MBene) Hf₂B₂O₂ as a substrate to construct a series of atomically dispersed catalysts—Hf₂B₂O₂-TM₁, Hf₂B₂O₂-TM₂ and Hf₂B₂O₂-TM₃—featuring single, double, and triple transition metal (TM) atoms embedded in defective Hf₂B₂O₂. The density functional theory (DFT) and constant-potential DFT results demonstrate that Hf₂B₂O₂-Co₂ and Hf₂B₂O₂-Fe₃ emerged as promising NORR candidates due to their ultra-low limiting potentials (U_L = −0.10 V and −0.05 V, respectively), excellent NO adsorption selectivity over a wide potential range, and effective suppression of byproducts. Notably, Hf₂B₂O₂-Co₂ maintains low ΔG_max (0.10 ∼ 0.27 eV) across a broad range of pH and electrode potentials. Furthermore, the ternary active centers composed of two different metals (i.e. Hf and TM in Hf₂B₂O₂-TM₁ and Hf₂B₂O₂-TM₂) exhibit distinct adsorption and activation behaviors compared to those with three identical doped metals (Hf₂B₂O₂-TM₃). Spontaneous N[sbnd]O bond cleavage occurs during the initial hydrogenation step, resulting in unique reaction mechanisms and enhanced NH₃ selectivity. What's more, Hf₂B₂O₂-TM₁ and Hf₂B₂O₂-TM₂ effectively circumvent the LSRs in NORR, allowing the independent optimization of key steps. This work not only provides valuable insights into the design of efficient NORR electrocatalysts but also expands the catalytic applications of h-MBenes.