M2+-Doped Nickel Selenide Nanoflowers (M = Co, Cu, and Zn) for Supercapacitors

Transition-metal doping engineering is an important strategy to adjust the structures of electrode materials (i.e., nickel selenides) for supercapacitors (SCs), and there remain great challenges to seek rational methods for realizing systematical doping. Herein, based on the open-channel structures of Ni0.85Se and NiSe, a simple hydrothermal process combined with a universal ion exchange reaction was designed to fabricate Ni0.85–xSe and NiSe nanoflowers doped by a series of 3d-transition-metal M2+ ions (M = Co, Cu, and Zn). Structure, morphology, and spectroscopic characterizations as well as Rietveld refinement were employed to research the phases, morphologies, and compositions of Ni0.85–xSe, NiSe, MxNi0.85–xSe, and MxNi1–xSe. It was demonstrated that the unique structures of Ni0.85Se and NiSe reduce the activation energies of M2+ ions transported through the interstitial lattice position, and the formation of Ni2+ vacancies decreases the steric hindrance of the insertion of divalent cations. Thus, Ni2+ was easily substituted by M2+ ions via cation exchange reaction at room temperature in water, realizing a 5–7% doping amount. Such transition-metal doping effects, from crystal structure modulation to electronic conductivity improvement, can effectively enhance the supercapacitor properties of Ni0.85Se and NiSe. The CoxNi1–xSe electrode delivers specific capacitances of 918.8, 770.5, 617.5, 496.9, and 437.5 F g–1 at 1, 2, 5, 7.5, and 10 A g–1, respectively, which is the best among the eight electrodes. Besides, the "kick-out" cation exchange mechanism for the synthesis of MxNi0.85–xSe and MxNi1–xSe was discussed in detail. This work gives us feasible guidance to fabricate desired nickel selenides for SCs; moreover, the facile and universal cation exchange route can be expanded to purposefully design other cation-doped transition-metal selenides for energy storage.