Functionalization-driven modulation of electronic structure and quantum capacitance in o-MXene Y2VC2Tx (Tx = O2, F2, and OF) for high-performance energy storage

In this study, first-principles calculations based on density functional theory (DFT) combined with the pseudopotential method are employed to investigate and compare the electronic properties, structural stability, and energy-storage potential of novel two-dimensional binary transition metal systems, Y 2 VC 2 T x (T x = O 2 , F 2 , and OF) o-MXenes. Structural, mechanical, and dynamical analyses confirm the potential stability of all Y 2 VC 2 T x o-MXenes. Among them, the oxygen-terminated Y 2 VC 2 O 2 layer exhibits superior energetic and mechanical stability compared to the pristine Y 2 VC 2 , fluorinated Y 2 VC 2 F 2 , and mixed-functionalized Y 2 VC 2 OF structures. The calculated B/G ratios for Y 2 VC 2 , Y 2 VC 2 F 2 , Y 2 VC 2 O 2 , and Y 2 VC 2 OF are above 1.75, indicating ductile mechanical characteristics in these systems. Electronic structure analyses conducted using different exchange-correlation schemes (GGA and GGA+HSE06) reveal that the studied o-MXenes behave metallically within the GGA approximation, while under the GGA+HSE06 functional, the materials Y 2 VC 2 , Y 2 VC 2 F 2 , Y 2 VC 2 OF, and Y 2 VC 2 O 2 exhibit energy gaps of 1.62, 2.94, 2.27, and 2.48 eV, respectively, confirming their semiconducting nature. The pseudogap positioned closer to the Fermi level in Y 2 VC 2 O 2 implies higher electronic robustness compared to other variants. In aqueous media, oxygen-terminated surfaces exhibit the strongest electrochemical activity, while pristine Y 2 VC 2 shows the weakest response. Y 2 VC 2 O 2 and Y 2 VC 2 OF consistently function as anodic electrodes, whereas Y 2 VC 2 and Y 2 VC 2 F 2 act as cathodic electrodes under aqueous conditions. In ionic/organic environments, Y 2 VC 2 , Y 2 VC 2 O 2 , and Y 2 VC 2 OF maintain anodic behavior, while Y 2 VC 2 F 2 remains cathodic. Moreover, Y 2 VC 2 O 2 demonstrates remarkable quantum capacitance and highly tunable electrochemical performance, largely driven by surface functionalization. These enhancements significantly improve the energy-storage capabilities of the materials, positioning o-MXenes as strong candidates for next-generation supercapacitors and battery electrodes.