Construction And Electrochemical Performance Of Aquous Redox Electrolyte Zinc-based Hybrid Supercapacitors

In order to meet the requirements of clean energy conversion and storage,supercapacitors,as one of the most promising electrochemical energy storage devices,have attracted extensive attention.However,the energy density of the traditional aqueous double electric layer capacitors cannot satisfy the needs of increasingly updated electronic equipment.Therefore,the hybrid capacitor,as a new type of energy storage device combining high-energy battery and high-power supercapacitor,can provide both high energy density and power density.Recently,zinc-based hybrid capacitors（ZHS）have been proposed in 2018 by Feiyu Kang et al,which is considered to be an emerging energy storage device in hybrid capacitors and gradually become a research hotspot in energy storage field.This is mainly due to the suitable redox potential of Zn/Zn²⁺at-0.76 V（vs.SHE）and its satisfactory theoretical capacity of 820mAh g^-1 in aqueous solution,making ZHS get more attention.For purpose of improving the electrochemical performance of ZHS,most of recently reported work focus on reasonable design and structure optimization of their electrode materials,but this improvement is limited.Introducing extra Faraday contribution of redox additives in electrolyte on the basis of the original energy storage mechanism is considered to be a simple and effective approach to improve the electrochemical performance,which has already been applied in traditional double electric layer capacitors.However,for these ZHS,especially for different types of ZHS,the introduction of redox additives into electrolyte to promote their energy storage performance is rarely explored.This paper mainly focused on design/optimization of electrode materials and introduction of redox active additives into electrolyte to study their electrochemical performance,the main work contents and research results are as follows:1.B,N dual-doped carbon tubes were prepared by pyrolysis and carbonization of in-situ N included polypyrrole tubes and boric acid in inert gas at high temperature.It was found that the B,N dual-doped carbon tube（BN-CMT-5）with excellent surface pseudo capacitance and good electron conduction was obtained by adjusting boric acid addition during the preparation process.And the ZHS with Zn//2 M Zn SO₄//BN-CMT-5construct can achieve highest capacity(138.5 mAhg^-1,1A g^-1).Based on this ZHS,the redox pairs of I^-/I₂（I₃^-）were introduced in electrolyte to form Z-IHS hybrid capacitor with better performance(416.6 mAh g^-1,1A g^-1),showing excellent cycle stability.According to the analysis of the control mechanism of electrochemical reaction,the Faraday redox reaction of I^-/I₂（I₃^-）is controlled by diffusion.This is due to the introduction of I^-/I₂（I₃^-）redox reaction occupying part of the active sites of BN-CMT-5 and restraining part of capacitive-control,thus making Z-IHS exhibit more diffusion-control contribution than ZHS.2.The optimized B,N dual-doped MOF derived carbon（BN-ZIC-3）was prepared by in-situ N included MOF and boric acid.But enhancing electrochemical energy storage of ZHS via B,N dual-dopping in carbon electrodes to promote pseudocapacitance effect is limited.It was found that the conductivity of 2 M KOH solution electrolyte(344 m S cm^-1)was higher than that of 2 M Zn SO₄ solution electrolyte(49.4 m S cm^-1),and the alkaline environment also can provide stable faradaic oxidation/reduction at the zinc negative electrode(Zn+4OH^--2e^-=Zn（OH）₄^2-,E₀=-1.199 vs SHE).Therefore,the A-ZCHS of Zn//2 M KOH//BN-ZIC-3 showed excellent electrochemical performance(162.6 mAh g^-1,1 A g^-1)compared with that of ZHS(92.7 mAh g^-1,1 A g^-1).In addition,based on A-ZCHS,the redox pairs of Fe（CN）₆^3-/Fe（CN）₆^4-were introduced in 2 M KOH electrolyte to form A-ZFHS-2/A-ZFHS-5 with better electrochemical performance(187.1 mAh g^-1/223.1 mAh g^-1 at 2 A g^-1).According to the analysis of the control mechanism of electrochemical reaction,A-ZCHS exhibits more capacitive-control contribution than ZHS;With the increase of K₃Fe（CN）₆ content in the 2 M KOH electrolyte,the diffusion-control contribution of A-ZFHS is more obvious than that in A-ZCHS.3.Pruschblue analogues（Zn-PBA,Mn-PBA and Co-PBA）were prepared by facile one-step method and considered as Zn²⁺intercalating/deintercalating battery-type materials.The electrochemical properties of these pruschblue analogues were evaluated in 2 M Zn SO₄ aqueous electrolyte and the corresponding control mechanism were analyzed.Compared with Zn-PBA and Mn-PBA,Co-PBA exhibited highest capacity in 2 M Zn SO₄(67.71 mAh g^-1,0.2A g^-1),and Zn²⁺intercalating/deintercalating processes in Co-PBA display faster capacitance-control contribution.With Zn Br₂gradually added into electrolyte,the capacity of Co-PBA in 2 M Zn SO₄+0.01 M Zn Br₂electrolyte is higher than that in 2 M Zn SO₄+0.005 M Zn Br₂ electrolyte.According to the analysis of control mechanism of electrochemical reaction,in 2 M Zn SO₄+0.005 M Zn Br₂ electrolyte Co-PBA shows relatively excellent Co²⁺/Co³⁺redox reaction(Zn²⁺intercalating/deintercalating process),and relatively fast surface Br^-/Br₃^-redox reaction.When Co-PBA was applied in AC//2 M Zn SO₄+0.005 M Zn Br₂//Co-PBA（CB-ZHS）,its energy storage performance is better than AC//2 M Zn SO₄//Co-PBA（C-ZHS）.4.The organic polymer frameworks of hydrogel were prepared via in-situ polymerization of functional monomers acrylic acid（AA）and acrylamide（AM）in sodium alginate（SA）main natural polymer backbone.With a certain amount of Zn SO₄and Zn SO₄+Zn Br₂ introduced in hydrogel,the SA-Zn and SA-Zn-Br hydrogel electrolytes for flexible ZHS were prepared successfully.The prepared hydrogel electrolytes exhibit excellent mechanical properties,water retention rate,ionic conductivity and relatively high tolerance voltage.Thus,the assembled flexible SA-Zn-ZHS/SA-Zn-Br-ZHS showed excellent electrochemical performance.Due to the introduction of Br^-/Br₃^-redox pairs in hydrogel electrolyte,SA-Zn-Br-ZHS exhibit higher power density and energy density,with an energy density of 605 Wh kg^-1 at power density of 1848 W kg^-1.In the bending test,SA-Zn-Br-ZHS shows excellent stability.In addition,due to the interaction and coordination between Zn²⁺and charged functional groups in the polymer chain of hydrogel electrolyte,homogeneous nucleation and zinc layered deposition can be formed on the surface of Zn foil negative electrode,ensuring its stability and safety.