Morphology Control and Capacitive Properties of In Situ Ni-Doped CoMoO₄ Electrode Materials

Energy, environment and industrial production are interconnected with the rapid development of the industrialization process, conventional fossil fuels are close to exhaustion, and the environment on which mankind depends for survival is seriously damaged. The development of efficient, clean and sustainable energy conversion and storage technology is thus an important research direction in scientific community. In this paper, in-situ Ni-doped CoMoO₄ electrode materials were synthesized on acidified pretreated nickel foam with nickel chloride formed on hydrochloric acid pretreated nickel foam as a reactive nickel source. The capacitive properties of in-situ Ni-doped CoMoO₄ electrode materials synthesized at different hydrothermal reaction temperatures were investigated.

In this work, 1 mmol Co(NO₃)₂·6H₂O and 1 mmol Na₂MoO₄·2H₂O were dissolved in 60 mL of deionized water (DI) and stirred for 30 min, and then the conventionally cleaned nickel foams and four pieces of acidified pretreated nickel foams were immersed into five beakers of the solutions above, respectively. The solutions were then transferred into a hydrothermal high-pressure reactor lined with polytetrafluoroethylene. The reactor was kept in an oven at different temperatures (i.e.,180, 120, 150, 180 ℃, and 200 ℃) for 12 h, respectively. The nickel foam was removed from the cooled reactor, washed and dried in a vacuum oven at 60 ℃ for 12 h. The synthesized materials were labeled as CoMoO₄ (3.6 mg), Ni-CoMoO₄-120 (3.93 mg), Ni-CoMoO₄-150 (3.46 mg), Ni-CoMoO₄-180 (4.88 mg), and Ni-CoMoO₄-200 (7.54 mg), respectively.

The XRD pattern of CoMoO₄ shows that the diffraction peaks at 13.1°, 23.3°, 26.4°, 27.3°, 33.7°, 36.6°, 38.9°, 41.7°, 47.4°, and 54.5° correspond to the crystal planes (001), (021), (002), (112), (222), (400), (040), (422), (222), (421), and (440), respectively, of CoMoO₄ (JCPDS 21–0868), indicating the effective synthesis of CoMoO₄. However, CoMoO₄ electrode material synthesized by in-situ Ni doping shows an overall shift of the XRD diffraction peak to the left. The SEM images show that compared to the undoped CoMoO₄, Ni-doped CoMoO₄ nanosheets at different reaction temperatures are uniformly nucleated and grown on the surface of Ni foam due to Ni ions provided by the nickel foam as a crystallization core, and there is almost no pore space between the nanosheets. The nanosheets are stacked together with mutual reliance and supported. The results of specific surface area measurements indicate that in-situ Ni-doped CoMoO₄ has the maximum specific surface area at a hydrothermal temperature of 180 ℃. The results of CV and GCD tests show that the optimum specific capacitance of Ni-doped CoMoO₄ electrode material can be obtained at 180 ℃.

In-situ Ni-doped CoMoO₄ electrode materials were synthesized on acidified pretreated nickel foam by a hydrothermal method. The morphology of Ni-doped CoMoO₄ electrode materials was controlled via modulating the hydrothermal temperatures (i.e., 120, 150, 180 ℃, and 200 ℃), in turn modulating the capacitive properties of CoMoO₄ electrode materials. Compared to undoped CoMoO₄, Ni-doped CoMoO₄ nanosheets with different reaction temperatures uniformly nucleated and grew on the surface of nickel foam due to Ni ions provided by nickel foam as a crystallization core, and the pores between nanosheets became smaller. The nanosheets rely on each other to stack together and support. Ni-CoMoO₄ was less prone to some problems like structural collapse and volume deformation when subjected to long-cycle charging and discharging, showing a better cycling performance. The optimum capacitance performance of Ni-doped CoMoO₄ could be obtained at 180 ℃, with a high specific capacitance of 4680 mF·cm^–2 at 5 mA·cm^–2. The capacitance retention of the ASC device after 10 000 cycles was 96%.