Capacitance performance of carbon paper supercapacitor using redox-mediated gel polymer electrolyte

A novel redox-mediated gel polymer electrolyte (GPE) including polyvinyl alcohol, phosphoric acid and phosphomolybdic acid (PVA-H3PO4-PMo12) and symmetric graphite carbon paper electrodes was applied to construct a solid-state carbon paper supercapacitor. The ionic conductivity increased from 3.85 mS cm−1 for PVA-H3PO4 GPE to 5.45 mS cm−1 for PVA-H3PO4-PMo12 GPE with optimal PMo12/H3PO4 mass ratio of 0.5, presenting the improved ionic transportation. The specific capacitance of carbon paper supercapacitors obviously increased from 12.16 mF cm−2 (or 296.6 mF cm3) for PVA-H3PO4 GPE to 28.65 mF cm−2 (or 698.8 mF cm−3) for PVA-H3PO4-PMo12 GPE at a current density of 0.2 mA cm−2, presenting highly improved capacitance performance. The capacity retention of carbon paper supercapacitors was also enhanced from 21.4% for PVA-H3PO4 GPE to 45.9% for PVA-H3PO4 -PMo12 GPE when the current density increased from 0.2 to 2.0 mA cm−2, presenting highly improved rate capability. The PMo12 with redox activity could contribute to the pseudocapacitance through fast and reversible multi-electron transfer process at the interface of carbon paper electrode and GPE electrolyte. However, the capacitance retention obviously decreased from almost above 100% for PVA-H3PO4 GPE-based supercapacitor to 50% for PVA-H3PO4-PMo12 GPE-based supercapacitor after 2000 charge−discharge cycles at 0.5 mA cm−2. The gradually descended reversibility of PMo12 transformed products mostly caused a lowered cycling stability for PVA-H3PO4-PMo12 GPE in long-term cycling redox process. Accordingly, PMo12 could act as a potential redox mediator in GPE electrolyte to improve the capacitance performance of the carbon paper supercapacitors.