Scalable synthesis of K⁺/Na⁺ pre-intercalated -MnO₂ via Taylor fluid flow-assisted hydrothermal reaction for high-performance asymmetric supercapacitors

In this work, a two-step synthetic method: Couette-Taylor flow mixing, and hydrothermal method have been used to synthesize alpha-manganese oxide pre-intercalated with sodium and potassium ions (Na+/K+@alpha-MnO2) for supercapacitor application. In addition, the effects of different aqueous electrolytes on the energy storage properties of the Na+/K+@alpha-MnO2 nanowires are investigated. The results show that KOH is a better electrolyte than NaOH and Na2SO4, with Na+/K+@alpha-MnO2 nanowires exhibiting a higher specific capacitance of 124 Fg(-1) in KOH electrolyte. This is attributed to the high ionic conductivity and smaller hydrate ion of K+ ions. DFT calculations reveal that K+ ions have a higher adsorption energy and a higher partial density of states than Na+ ions, indicating more favourable pseudocapacitive behaviour for K+ ions compared to Na+ ions. Furthermore, the asymmetric capacitor device based on N-CNTs@CF//Na+/K+@alpha-MnO2 delivers a specific energy density of 21 Wh kg(-1) and a specific power density of 450 W kg(-1), with excellent cycling performance (similar to 94 % capacity retention after 5,000 cycles). Our findings demonstrate that the pre-intercalation of alpha-MnO2 nanowires account for enhanced cycling stability, as well as higher capacitance.