Bilateral Growth of Monoclinic Tungsten Oxide and 2D Ti3C2Tx-MXene on 3D Free-Standing Multilayer Hollow Graphene Foam for Flexible All-Solid-State Pseudocapacitor

Abstract

To overcome the subordinate electrochemical performance of the pseudocapacitor, it is essential to design two-dimensional (2D) electroactive nanostructured material with enormous electroactive sites on the highly conductive current collector for enriching the rates of reversible charge transfer reactions. Herein, a scalable strategy was employed to prepare the electrodes by depositing pseudocapacitive materials on a three-dimensional (3D) free-standing highly conductive hollow multilayered graphene foam (HGF) synthesized through a template assisted-chemical vapor deposition (TA-CVD) method for the first time. The stable nanostructured monoclinic tungsten oxide (m-WO3) nanoparticles and 2D transition metal carbide (Ti3C2Tx-Mxene) sheets were bilaterally loaded on inner and outer sides of 3D HGF by a unipolar electrodeposition (UPED) method and a drop casting method, respectively. This free-standing electrode exhibited exceptional specific capacitance (Cs) of 487 F/g (439 mF/cm2) at 2 mA/cm2, outstanding rate performance and extraordinary cycling stability of 92.9 % over 5000 cycles owing to its significant metallic conductivity and reversible redox reactions of hydrophilic 2D Ti3C2Tx-Mxene. To assemble asymmetric pseudocapacitor, a MWCNT/RuO2 electrode with tiny RuO2 was also synthesized through a simple and inexpensive layer-by-layer (LBL) method. The obtained flexible asymmetric solid-state pseudocapacitor (FASC) illustrated 2-fold wider potential of 1.4 V in an acidic electrolyte when compared with Mxene based symmetric supercapacitor. This FASC displayed excellent Cs of 152.2 F/g at 5 mV/s, high energy density of 18 Wh/kg at 497 W/kg and an outstanding capacity retention of 93.3 % over 10000 GCD cycles with superb coulombic efficiencies of 99 ~ 100 %. In addition, the extraordinary demonstration performance of this FASC device by driving light emitting diodes (LEDs) and digital stopwatch signified its real-world applicability in electronic gadgets.