Preparation Of The Redox-mediator-doped Gel Polymer Electrolyte For Quasi-solid-state Supercapacitor And Study On Its Electrochemical Performance

As the new generation of electrolyte for supercapacitor, gel polymer electrolyte（quasi-solid-state） has great prospect of development because of its advantages overhigh stability, reliability and flexibility, which can not only be free of the potentialproblems of liquid electrolytes, such as leakage and blast, but also provide minitypeand superthin devices. However, there are several factors negatively affecting itsapplications in supercapacitor as follows:（1） Gel polymer electrolyte exhibitsrelatively lower ionic conductivity （¹0-3S cm1, at room temperature） than that ofliquid system, which will limit the charge-discharge rate performance ofsupercapacitor system.（2） The supercapacitor with gel polymer electrolyte maintainpoor interfacial contact between the electrode and electrolyte due to its quasi-solid-state, which leads the device to display large resistance and restricts the performanceof system improving.In this paper, novel redox-active gel polymer electrolytes were prepared bydoping redox-mediators into the conventional gel polymer electrolytes to attempt torealize the objective of enhancing the electrochemical performance of the preparedgel polymer electrolyte based on its safety and reliability. Herein, a study on theelectrochemical performance of supercapacitor with redox-active gel polymerelectrolyte and the role played by the redox-mediator in the system was given. Theexperiment content and results are as follows:1. PVA-PVP-H₂SO₄-MB systemA redox-active gel electrolyte （PVA-PVP-H₂SO₄-MB） was prepared by solutionmixing method using methylene blue （MB） as a redox mediator and PVA-PVP hybirdas a polymer host. With the addition of MB additive, the ionic conductivity of gelelectrolyte increased by56%up to36.3mS·cm1, and the electrode specificcapacitance （Cs） of the supercapacitor with PVA-PVP-H₂SO₄-MB electrolyte is328F·g^-1, increasing by164%compared to that of the PVA-PVP-H₂SO₄system. Theenergy density of the supercapacitor increase from3.2to10.3Wh kg–1. In addition, the supercapacitor exhibits small resistance and excellent cycle-life stability.2. PAA-PVA-KOH-K₄[Fe（CN）₆] systemA redox-active gel electrolyte （PAA-PVA-KOH-K₄[Fe（CN）₆]） was prepared bytwo-step method: Firstly, a PAA-PVA polymer host was prepared by chemical cross-linking. Secondly, the host absorbed a liquid solution mixed by KOH aqueoussolution and potassium hexacyanoferrate （K₄[Fe（CN）₆]） redox-mediator to form agel system. In the optimized conditions, the conductivity of PAA-PVA-KOH-K₄[Fe（CN）₆] electrolyte reaches417mS cm1, the Csand energy density of thecorresponding quasi-solid-state supercapacitor are328F·g^-1and11.5Wh kg–1,respectively. Moreover, the supercapacitor with PAA-PVA-KOH-K₄[Fe（CN）₆]electrolyte displays an excellent cyclical stability.3. PVA-H₂SO₄-PySH systemA novel and high-effective gel electrolyte （PVA-H₂SO₄-PySH）, which exhibitsan ionic conductivity of43.3mS cm1, was prepared by solution mixing method using2-mercaptopyridine （PySH） as a redox-mediator. Due to the hihgly redox-active ofPySH, the Csof the corresponding supercapacitor is as high as to394F·g^-1,increasing by188%compared to that of PySH-undoped system. The energy densityand power density of the supercapacitor reach12.4Wh·kg^-1and496W·kg^-1,respectively. Furthermore, it exhibits outstanding cyclical stability and low self-discharge performance.The suitable redox-mediator can accelerate the transfer diffusion processes ofions in gel polymer electrolyte, which may result in high-conductivity. The propertiesof supercapacitors with the three above-mentioned redox-mediators are improvedenormously for the following reason: In the new redox-active electrolyte system, thesupercapacitor can be regarded as a novel hydrid supercapacitor application thatcombines two energy-storage processes: the double-layer formation characteristic ofcarbon-based material and the faradaic reaction characteristic of redox mediator at theelectrode|electrolyte interface.