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 zincion 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 Wh·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 Wh·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.