Self-etching Ni–Co hydroxides@Ni–Cu nanowire arrays with enhancing ultrahigh areal capacitance for flexible thin-film supercapacitors

Flexible thin-film supercapacitors with high specific capacitance are highly desirable for modern wearable or micro-sized electrical and electronic applications. In this contribution, Ni–Co hydroxides (NCH) nanosheets were deposited on top of Ni–Cu alloy (NCA) nanowire arrays forming a freestanding thin-film composite electrode with hierarchical structure for supercapacitors. During electrochemical cycling, the dissolution of Cu into Cu ions will create more active sites on NCA, and the re-deposited copper oxide can be coated onto NCH, giving rise to substantial increase in specific capacitance with cycling. Meanwhile, NCA and NCH have excellent conductivity, thus leading to excellent rate performance. This flexible thin-film electrode delivers an ultrahigh initial specific capacitance of 0.63 F·cm⁻² (or 781.3 F·cm⁻³). During charge–discharge cycles, the specific capacitance can increase up to 1.18 F·cm⁻² (or 1475 F·cm⁻³) along with the “self-etching” process. The electrode presents a better specific capacitance and rate capability compared with previously reported flexible thin-film electrode, and this novel design of etching technique may expand to other binary or ternary materials.