Flexible freestanding sandwich type ZnO/rGO/ZnO electrode for wearable supercapacitor

Highlights: • Flexible electrode of freestanding sandwich ZnO/rGO/ZnO paper is fabricated. • The effects of ZnO sol-gel parameters on sandwich morphology are investigated. • Best sol parameter is 0.2 M in 1-PrOH and TeA stabilizer deposited at 30 mm min. • ZnO/rGO/ZnO composite electrode exhibits capacitance value of 60.63 F g⁻¹. - Abstract: The development of flexible supercapacitors with high energy and power density as one of the main components of wearable electronics is in an enormous interest. In this report, a unique flexible electrode based on freestanding sandwich type ZnO/rGO/ZnO paper is fabricated by a simple low cost sol-gel method for utilizing in flexible supercapacitor. ZnO layers are deposited on both sides of rGO paper which is prepared by a modified Hummer's method and evaporation induced assembly. The uniform and densely packed ZnO layers are formed on graphene oxide paper and the paper is simultaneously reduced. Structural analysis reveals the formation of ZnO thin films on both sides of rGO nanosheets, which leads to the sandwich architecture. Also, the effect of ZnO sol-gel process parameters on microstructure of sandwich paper are investigated and the most suitable condition for highest supercapacity performance is the solvent of 1-PrOH, stabilizer of TeA, sol concentration of 0.2 M, deposition speed of 30 mm min⁻¹, and 10 deposited layers. The results of electrochemical impedance spectroscopy, galvanostatic charge-discharge, and cyclic voltammetry confirm that the incorporation of ZnO improves the capacitive performance of rGO electrode. Moreover, ZnO/rGO/ZnO flexible electrode exhibits suitable capacitance value of 60.63 F g⁻¹ at scan rate of 5 mV/s.