Augmenting energy storage capacity of aqueous-electrolyte-based supercapacitors: Integrating multivalent vanadium into freestanding ZnO/carbon nanofiber electrodes

V/ZIF-7/polymer nanofibers were fabricated by electrospinning and subsequently converted into highly conductive carbon nanofiber (CNF) composites by high-temperature annealing in an Ar environment. Using polyacrylonitrile, a polymer with a high carbon yield, a flexible supercapacitor (SC) containing a freestanding composite fiber electrode was fabricated for application in wearable electronics. This study leveraged the fact that ZIF-7-derived ZnO and multivalent elements such as V can be embedded into CNFs as excellent electrochemical materials to fabricate green energy-storage devices. Owing to the synergistic effects of V, ZnO, and carbon, the V(0.2)ZC specimen (prepared using 0.2 wt% V salt in the precursor solution) exhibited a high areal capacitance of 780.5 mF center dot cm(-2) at a current density of 1 mA center dot cm(-2). Even with a minimal quantity of V in the composite fibers, a 2.5-fold increase in energy density was observed compared with that of the ZnO/CNF electrode. Specifically, an energy density of 212.5 mu Wh center dot cm(-2) was achieved over a potential window of 0-1.4 V using an aqueous electrolyte (6 M KOH). Furthermore, the V(0.2)ZC electrode exhibited a capacitance retention of 97 % after 10,000 galvanostatic charge-discharge cycles. Additionally, the practical viability of a flexible SC containing the freestanding electrode for energy-storage applications was demonstrated through bending tests, prolonged cycling experiments, and light-emitting diode charging.