In-situ construction of amorphous–crystalline NiCo bimetallic oxyhydroxide for low-temperature supercapacitor device

As rechargeable energy storage device, supercapacitor (SC) cell is facing a new challenge for low-temperature application since the low-temperature environment seriously suppresses the reaction kinetics of the electrode materials. In this work, NiCo bimetallic oxyhydroxides (Ni_xCo_yOOH, Ni/Co = x/y) composed of amorphous-crystal structure were in-situ synthesized using electrodeposition, which are capable of operating at temperatures as low as − 4 °C, totally different from the pure Ni or Co oxyhydroxides counterparts. The microstructures of the Ni_xCo_yOOH were characterized by XRD, SEM, TEM, and XPS, finding an obviously transit from microparticles (NiOOH) and porous arrays (CoOOH) to flower-like nanospheres and a complicated amorphous-nanocrystalline structure. Electrochemical tests show that the Ni₂Co₁OOH cathode presents a super-high specific capacitance of 3720 and 2311F g⁻¹ at low and high charging current densities of 1 and 20 A/g at 23 ℃ in aqueous electrolyte, respectively. The assembled device of Ni₂Co₁OOH//activated carbon (AC) delivers a high energy density of 22.8 Wh kg⁻¹ at 7466 W kg⁻¹ in polyvinyl alcohol (PVA)-KOH hydrogel at 0 ℃, and offers an exceptional stability of 92.5 % of capacity after 8000 cycles at 10 A/g and 0 ℃. To evaluate the serviceability of the as-prepared electrode materials, the devices in parallel and series connections encapsulated by 3D printing exhibits a stable discharging performance at 0 °C, demonstrating a potential application in cold environment. Heteroatomic intercalating promotes the co-adsorption of hydroxyls on the double active sites of the Ni and Co, resulting in an improved supercapacitance.