Investigation of Ionic Radius Effects on Aqueous Electrolyte Performance in Mo₂C-based MXene Supercapacitors

Samaneh Vaez,^a,b Ahmad Bagheri,^a,b Hossein Beydaghi,^a Sebastiano Bellani,^a,c Teresa Gatti, and Francesco Bonaccorso ^a

^a BeDimensional S.p.A., Lungotorrente Secca 30R, 16163 Genoa, Italy

^b Department of Applied Science and Technology (DISAT), Politecnico di Torino, 10129 Torino, Italy

^c Antares Electrolysis S.r.l., Piazza della Vittoria 14/19, 16121 Genova, Italy

This study proposes an effective strategy for designing advanced electrode materials, featuring a molybdenum carbide chloride/few-layer graphene hybrid (Mo₂CCl₂/FLG) as a high-performance negative electrode, paired with curved graphene/few-layer graphene (CG/FLG) as the positive electrode for aqueous asymmetric supercapacitors (ASCs). The Mo₂CCl₂/FLG hybrid exhibits outstanding electrochemical performance due to the synergistic interaction between pseudocapacitive Mo₂CCl₂ and the highly conductive FLG, which enhances charge-transfer kinetics, accelerates ion diffusion, and improves electrochemical reversibility. ^{[1, 2]} To investigate the impact of electrolyte composition, ASCs were assembled in three aqueous electrolytes: 3 M H₂SO₄, 2 M NaCl, and 8 m NaNO₃. Among these, the 3 M H₂SO₄-based device achieved the highest specific capacitance of 29.57 F/g at 1 A/g, attributed to rapid proton transport and favorable redox reactions at the Mo₂CCl₂ interface. ^[3] However, despite its high ionic conductivity, the acidic electrolyte is limited by a narrow electrochemical stability window, restricting safe operating voltage and energy density due to water decomposition. ^[4] In contrast, sodium-based electrolytes provided wider potential windows and improved voltage stability. The ASC with 8 m NaNO₃ delivered an energy density of ~8 Wh kg⁻¹ and excellent cycling stability, retaining 94% of its initial capacitance after 12000 cycles. The 2 M NaCl electrolyte showed stable long-term performance, indicating suitability for durable aqueous energy-storage systems. ^[5] Electrochemical impedance spectroscopy revealed distinct electrolyte-dependent behaviors. The 3 M H₂SO₄ system exhibited the lowest series resistance (Rₛ ≈ 0.04 Ω), consistent with rapid ionic transport, whereas 8 m NaNO₃ displayed higher resistance due to slower diffusion of larger hydrated ions. Overall, the Mo₂CCl₂/FLG // CG/FLG ASC configuration demonstrates strong potential for high-performance, scalable aqueous supercapacitors when coupled with optimized electrolytes. These results provide key design principles for tuning electrode–electrolyte interactions and offer valuable guidance for developing next-generation aqueous energy-storage devices with balanced energy and power characteristics.