Excellent power and cycling stability of Mn2PO4F-based pseudocapacitor electrode: Ceria-induced electron-starvation of surface Mn2+ species

Manganese fluorophosphate, Mn 2 PO 4 F (MPF), is investigated as a potential candidate electrode material for aqueous asymmetric supercapacitor systems. CeO 2 coatings at concentrations of different weights are introduced, along with a 10 wt% carbon black (CB) coating serving as a conducting agent. The 5 % CeO 2 (MPF/C/CeO 2 -5) composite was identified as a stable, high-power positive electrode material in an asymmetric supercapacitor device using nitrogen-doped hollow carbon spheres (NHCS) as the negative electrode. These electrodes are coupled with a 17 m water-in-salt NaClO 4 electrolyte, yielding a sustainable high-power pseudocapacitive device. The device achieves a specific capacitance of 38 F/g, and a maximum energy density of 21.0 Wh/kg at a power density of 390.9 W/kg. Remarkably, at a current density of 3 A/g the device achieved a maximum power density of 11.632 kW/kg at an energy density of 11.2 Wh/kg. Power densities of this magnitude are generally only achieved at higher current densities compared to other manganese-based materials in the recent literature. Favourable cycling stability was achieved and attributed to the electronic interaction between CeO 2 and Mn 2+ as a similar device using pure MPF achieved a capacity retention of only 64.9 % under the same experimental conditions. • Pseudocapacitance of CeO 2 -coated Mn 2 PO 4 F in water-in-salt electrolyte investigated. • No structural effect on the triplite structure in the presence of CeO 2 or carbon. • CeO 2 transforms surface Mn 2+ to Mn 3+ to enhance the cyclability. • Asymmetric cell achieved high power and energy densities. • Pseudocapacitor exhibits excellent cycling stability of 89 % after 15 000 cycles.