Solid State Flexible Asymmetric Supercapacitor Based on Carbon Fiber Supported Hierarchical\nCo(OH)_<i>x</i>CO₃ and Ni(OH)₂

Abstract

Conducting\nflexible carbon fiber (CF) cloth was used as a substrate\nfor the hydrothermal growth of nickel hydroxide (Ni­(OH)₂) and cobalt hydroxy carbonate [Co­(OH)_<i>x</i>CO₃] with unique hierarchical flowery architecture and\nthen was used as a flexible supercapacitor electrode. In a three-electrode\nconfiguration in 6 M KOH aqueous electrolyte, the CF–Ni­(OH)₂ and CF–Co­(OH)_<i>x</i>CO₃ electrode showed the maximum specific capacitance of 789 F/g and\n550 F/g, respectively, at 2A/g current accompanied by outstanding\ncycle stability by retaining 99.9% and 99.5% specific capacitance\nover 1500 consecutive charge–discharge cycles at 5 A/g. However,\nthe low cell voltage (0.4 V) restricted the respective specific energy\nto 4.38 and 3.05 Wh/kg at a specific power of 100 W/kg. To overcome\nthe issue, two solid state flexible asymmetric supercapacitors were\nfabricated using the CF–Ni­(OH)₂ and CF–Co­(OH)_<i>x</i>CO₃ as the anode and sonochemically\ndeposited CNT over carbon fiber as the cathode material in PVA-KOH\ngel electrolyte. The as-fabricated flexible supercapacitors CF–Ni­(OH)₂//CF–CNT and CF–Co­(OH)_<i>x</i>CO₃//CF–CNT were able to deliver high specific\nenergy of 41.1 and 33.48 Wh/kg, respectively, at high specific power\nof 1.4 kW/kg accompanied by excellent cycle stability (retaining 98%\nand 97.6% specific capacitance, respectively, over 3000 charge–discharge\ncycle at 5 A/g).