Vacancy-Anchored Single-Atom Nb₂CO₂ MXene: Electronic Origins of Multi-Site Cooperative Trifunctional...

The rational design of trifunctional electrocatalysts capable of driving the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER) remains a central challenge in renewable energy conversion. In particular, it remains insufficiently understood whether these reactions proceed on a universal active site or arise from reaction-dependent site specialization and multisite cooperation in single-atom catalysts. Herein, we perform a systematic density functional theory (DFT) screening of transition-metal (TM) single atoms anchored at oxygen vacancy sites of Nb₂CO₂ MXenes to identify stable and experimentally viable trifunctional SACs. Among the candidates, Pt-Nb₂CO₂ exhibits favorable conductivity, thermodynamic stability, and competitive trifunctional electrocatalytic activity, with overpotentials of 0.42 V for OER, 0.60 V for ORR, and -0.07 V for HER, comparable to benchmark catalysts such as Pt(111) and IrO₂(110). Detailed electronic structure analyses reveal that the trifunctional activity originates from multisite cooperative catalysis. The d orbitals of the TM atoms dominate the activity of oxygen-related reactions (OER/ORR), where the d-band center, modulated by bandwidth effects, correlates well with the activity trends. In contrast, the HER activity is governed by TM-induced charge transfer and site-specific hydrogen binding characteristics. This work clarifies the electronic origin of multisite cooperative trifunctional electrocatalysis in TM-MXene SACs and provides a rational theoretical framework for the design of experimentally accessible multifunctional electrocatalysts.