Novel δ-MnO₂/MXene Heterostructures as Cathode Materials for Zinc-Ion Hybrid Supercapacitors

Although manganese-based oxide is regarded as a promising cathode material for zinc-ion hybrid supercapacitors (ZHSCs), its practical application is hindered by slow zinc ion diffusion and the instability of MnO₂. To overcome this obstacle, a δ-MnO₂/MXene heterostructure was created using a simple one-step process under gentle condition. The ZHSC was assembled using this heterostructure as the cathode, activated carbon (AC) as the anode and 2 mol·L⁻¹ ZnSO₄ as the electrolyte. The resulting δ-MnO₂/MXene//ZnSO₄//AC ZHSC shows a maximum specific capacitance of 97.4 F·g⁻¹ and an energy density of 32.27 W·h·kg⁻¹ at the best cathode-to-anode mass ratio. Ex situ characterizations reveal the reversible energy storage mechanism combing Zn²⁺ insertion/extraction in the cathode, ion adsorption and desorption on the anode surface, and partial reversible formation and dissolution of Zn₄SO₄(OH)₆·5H₂O (ZHS) components on both electrodes. Adding of Mn²⁺ to the electrolyte reduced Mn dissolution, improving the ZHSC’s specific capacitance and energy density to 140.4 F·g⁻¹ and 49.36 W·h·kg⁻¹, respectively, while also enhancing its rate performance and cyclability. The improved electrochemical reaction kinetics was verified through various tests. The results suggest that the δ-MnO₂/MXene heterostructure has great potential as a high-performance cathode material for ZHSCs.