Unlocking High Energy Density in Ionic Liquid Supercapacitors via Termination-Free Mo₂C MXene

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Abstract
The growing demand for high-energy supercapacitors necessitates electrode–electrolyte systems that can operate across wide voltage windows without compromising stability. Here, we report symmetric supercapacitors based on termination-free Mo₂C MXene films synthesized via chemical vapor deposition (CVD) and paired with ionic liquid (IL) electrolytes. Unlike conventionally etched MXenes, the CVD-grown Mo₂C features a clean, termination-free surface that enables a stable and compatible interface with ILs. Using [BMIM]BF₄, the devices deliver energy densities of 114-121 Wh kg⁻¹ and power densities up to 1.8 MW kg⁻¹ within a voltage window of ~2.2-2.3 V, while retaining up to 96% capacitance after 10,000 cycles. When paired with STFSI, the devices achieve the highest specific power density, reaching 2.9 MW kg⁻¹. These results highlight the large surface area of Mo₂C, its excellent electrical conductivity, and strong compatibility with ionic liquid electrolytes. Comparative studies with filter paper and Celgard separators further reveal a trade-off between ionic accessibility and long-term durability, underscoring the critical role of electrolyte-separator-electrode synergy. Collectively, this work establishes termination-free CVD-grown Mo₂C MXene as a promising design pathway for next-generation IL-based high-energy supercapacitors.

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