Quantum Capacitance and Stability of Mo a Ti b C a + b –1 , Mo a V b C a + b –1 , and Ti a V b C a + b –1 Double Transition Metal MXenes: A DFT Study

Combining high energy and power density makes supercapacitors promising candidates for high-power applications. However, their energy density is still lower than that of traditional batteries. One of the key challenges in enhancing their energy density is related to the quantum capacitance of the electrodes. Due to their unique electronic properties, double transition metal (DTM) MXenes may possess great potential as electrode materials for supercapacitors. In the present study, density functional theory (DFT) was employed to evaluate the potential of Mo a Ti b C a + b –1, Mo a V b C a + b –1, and Ti a V b C a + b –1 DTM MXenes─featuring two, three, and four transition metal layers─as electrode materials for electrochemical double-layer capacitors (EDLCs). Structural stability was assessed using phonon dispersion and ab initio molecular dynamics (AIMD) simulations, confirming the dynamic and thermal stability of all examined DTM MXenes. Mechanical stability was established through elastic constant evaluations and Young’s modulus assessments, indicating superior stiffness compared to monotransition metal MXenes. Electronic structure analysis showed that all MXenes, except semimetallic TiVC, exhibit good conductivity. Quantum capacitance calculations revealed that Mo 2 V 2 C 3, Ti 2 V 2 C 3, and V 2 Mo 2 C 3 with maximum integrated quantum capacitance ( C q int ) higher than 1350 μF/cm 2 exhibit the highest quantum capacitance in ionic/organic electrolytes among the considered DTM MXenes. In aqueous electrolytes, Ti 2 V 2 C 3 shows the maximum C q int value of 1323.85 μF/cm 2, indicating the highest quantum capacitance in this environment. Comparison of the quantum capacitance of the examined DTM MXenes with the corresponding mono–transition-metal MXenes and known materials highlights their strong potential for use as supercapacitor electrode materials.