Biomass-derived C/N co-doped Ni(OH)2/NixSy with a sandwich structure for supercapacitors

Development of sustainable biomass-derived materials is an emerging trend to alleviate the consumption of non-renewable resources. Herein, we present a simple one-step hydrothermal method to obtain carbon and nitrogen co-doped nickel hydroxide/nickel sulfide (C/N–Ni(OH)2/NixSy) with a sandwich structure by using egg white as a precursor. Due to the abundant elements (C, N, and S) and strong ability to associate with metal ions, egg white can not only act as a source of carbon and nitrogen for doping, but can also serve as a sulfur source for NixSy. The C/N co-doping could well improve the electrical conductivity and create more active sites for C/N–Ni(OH)2/NixSy. Moreover, the NixSy nanoparticles dispersedly and tightly anchored on Ni(OH)2 nanoflakes, forming a particle/flake sandwich structure. This unique sandwich structure effectively improves the specific surface area and creates an unimpeded path for electrolyte ion migration. Taking advantage of the C/N co-doping and sandwich structure, the as-prepared C/N–Ni(OH)2/NixSy exhibits a high capacitance of 1731.2 F g−1 at 0.5 A g−1. Besides, the fabricated hybrid supercapacitor, C/N–Ni(OH)2/NixSy//reduced graphene oxide hydrogel, shows a remarkable energy density of 38.98 W h kg−1 and an excellent cycling stability (10 000 cycles, 107.6%). Therefore, this work presents a novel strategy for heteroatom-doping and structural control via an extremely facile route by using biomass.