Tuning quantum capacitance in the MA2Z4 family: Transition-metal-doped MoN2B2X2 (X = S, Se, Te) as high-performance electrode materials

Quantum capacitance (C Q ) is a key electronic factor that limits the charge-storage capability of two-dimensional electrodes in electric double-layer capacitors. Emerging MA 2 Z 4 -derived monolayers exhibit great potential for high-performance supercapacitor electrodes, yet systematic modulation of C Q in MoN 2 B 2 X 2 (X = S, Se, Te) via vacancy defects and transition-metal doping remains understudied. Herein, first-principles calculations were performed to systematically elucidate how vacancy defects and vacancy-site 3d transition-metal doping regulate the electronic structure, work function, and quantum capacitance of MoN 2 B 2 X 2 monolayers. Pristine MoN 2 B 2 Te 2 exhibited the highest C Q among the three pristine systems, reaching 196.61 μF/cm 2 at −0.6 V. Vacancy introduction further enhanced C Q to 241.38, 230.74, and 298.31 μF/cm 2 for V S -MoN 2 B 2 S 2 , V Se -MoN 2 B 2 Se 2 , and V Te -MoN 2 B 2 Te 2 , respectively. More importantly, the Cr-to-Cu dopant series enabled a systematic evaluation of the effect of progressive 3d-electron occupation on C Q regulation. Co-doped MoN 2 B 2 X 2 systems delivered strong negative-bias C Q responses of 296.39–333.00 μF/cm 2 , whereas Cu-doped systems exhibited enhanced positive-bias C Q behavior, with Cu@V Te -MoN 2 B 2 Te 2 reaching 315.02 μF/cm 2 . The enhanced C Q was attributed to the combined effects of dopant-induced charge redistribution, work-function modulation, and orbital hybridization near the Fermi level, as supported by Bader charge, work-function, and PDOS analyses. Compared with previously reported two-dimensional electrode materials, vacancy- and dopant-engineered MoN 2 B 2 X 2 systems exhibit competitive C Q values, establishing a systematic, mechanism-based strategy for designing MA 2 Z 4 -derived electrodes with high quantum capacitance.