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

•Co(OH)2@Mo7S8 was synthesized by hydrothermal and electrodeposition.•Specific capacity of Co(OH)2@Mo7S8 electrode is 561 C g−1 at 1 A g−1.•Storage of OH− in Co(OH)2@Mo7S8 is dominated by diffusion control.•Chemical valences of Mo and Co increase after OH− insertion.•Hybrid capacitors manifest remarkable specific energy and brilliant lifespan. 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. [Display omitted]