High capacity Mo₇S₈ nanosheets with electrodeposited alpha-Co(OH)₂ thin layer as positive electrode in hybrid potassium dual-ion supercapacitors

Alpha-Co(OH)₂ nanoflakes are grown on Mo₇S₈ electrodes by galvanostatic electrodeposition. The influence of various electrodeposition time durations on the electrochemical properties of alpha-Co(OH)₂@Mo₇S₈ electrodes is compared. When the optimized deposition time is 1440 s, alpha-Co(OH)₂@Mo₇S₈ composite electrode obtains the highest specific capacity of 484.8 C g⁻¹, the lowest internal resistance of 1.96 Ω, the shortest time constant of 2.15 s, and the maximum diffusion coefficient of 2.27 × 10⁻¹⁴ cm² s⁻¹. The kinetic analysis of this electrode reveals the highest diffusion and the lowest non-diffusion contributions of 76.25% and 23.75% at 0.5 mV s⁻¹, respectively. Ex-situ X-ray photoelectron spectroscopy shows that chemical valences of Mo and Co increased from +2 to +4 and from +2 to +3 after OH⁻ insertion, respectively. The hybrid potassium dual-ion supercapacitor is assembled with alpha-Co(OH)₂@Mo₇S₈ cathode (positive electrode) and molybdenum-phosphate-based MoPO@Mo₇S₈ anode (negative electrode). The device exhibits remarkable capacitance characteristics with a high specific energy of 59.4 Wh kg⁻¹ at a specific power of 734 W kg⁻¹ and a capacity-maintaining retention rate of 97.3% after 10,000 charge–discharge cycles.