Rational design of cobalt-nickel double hydroxides for flexible asymmetric supercapacitor with improved electrochemical performance.

Rational design of micro-nano morphology and suitable crystalline structure are highly desired for metal hydroxides to achieve overall high-performance in the advanced electrodes for flexible supercapacitors. Herein, a novel wisteria flower-like microstructure of cobalt-nickel double hydroxide (CoNi-DH) is successfully constructed on carbon cloth (CC) using an in-situ hydrolysis-induced exchange process between hydroxide ions and organic ligands of the Co-MOF in four different kinds of solutions containing Ni2+. The as-prepared wisteria flower-like microstructure grown on CC shows vertically aligned arrays with high specific area and abundant active sites, which not only guarantee the CoNi-DH active materials to be thoroughly exposed in the electrolyte, resulting in highly effective pseudocapacitive energy storage, but also are beneficial to rapid and reversible redox kinetics and thus give rise to high-rate capability. In addition, compared to Ni(NO3)2, NiCl2, and Ni(CH3COO)2 solutions, the Ni2SO4 solution is found to facilitate the formation of the most regular morphology and the largest interlayer spacing on (003) plane of the layered nickel hydroxide phase in the resultant CoNi-DH. As a result, the optimal CoNi-DH-S@CC (CoNi-DH prepared in Ni2SO4) serves as an advanced electrode to show high-rate capability (only 13% Cs decay after a 15-fold current elevation) and a superior specific capacity (Cs) of 929.4 C g-1, which remarkably exceeds those of CoNi-DH-N (823.1 C g-1, in Ni(NO3)2), CoNi-DH-Cl (798.4 C g-1, in NiCl2), CoNi-DH-C (803.8 C g-1, in Ni(CH3COO)2), and other similar metal hydroxides. Moreover, with this CoNi-DH-S electrode as the positive electrode, the as-prepared asymmetric supercapacitor (ASC) delivers an impressive capacity of 204.8 C g-1, a superior energy density of 42.5 Wh kg-1, and satisfactory cycle life (81.5% reservation after 7500 cycles). As a proof-of-concept application, a quasi-solid-state ASC is further successfully fabricated based on the CoNi-DH-S electrode to exhibit encouraging application potential.