Fractal (NixCo1−x)9Se8 Nanodendrite Arrays with Highly Exposed () Surface for Wearable, All‐Solid‐State Supercapacitor

Abstract Hierarchical nanostructures with highly exposed active surfaces for high‐performance pseudocapacitors have attracted considerable attention. Herein, a one‐step growth of (Ni x Co 1− x ) 9 Se 8 solid solution series in various conductive substrates as advanced electrodes for flexible, foldable supercapacitors is developed. The formation of (Ni x Co 1− x ) 9 Se 8 solid solution is confirmed by Vegard's law. Interestingly, the as‐grown (Ni x Co 1− x ) 9 Se 8 solid solution series spontaneously crystallized into nanodendrite arrays with hierarchical morphology and fractal feature. The optimized (Ni 0.1 Co 0.9 ) 9 Se 8 nanodendrites deliver a specific capacitance of 3762 F g −1 at a current density of 5 A g −1 and remains 94.8% of the initial capacitance after 5000 cycles, owing to the advantage from fractal feature with numerous exposed ( ) surface as well as fast ion diffusion. The as‐assembled flexible (Ni 0.1 Co 0.9 ) 9 Se 8 @carbon fiber cloth (CFC)//PVA/KOH//reduced graphene oxide@CFC device exhibits an ultrahigh energy density of 17.0 Wh kg −1 @ 3.1 kW kg −1 , outperforming recently reported pseudocapacitors based on nickel‐cobalt sulfide and selenide counterparts. This study provides rational guidance toward the design of fractal feature with superior electrochemical performances due to the significantly increased electrochemical active sites. The resulting device can be easily folded, pulled, and twisted, enabling potential applications in high‐performance wearable and gadget devices.