| Electrochemical double-layer capacitors exhibit high power density(>1 kW/kg)and excellent cycling stability(>100 k cycles),but their energy density has been limited to less than 10 W h/kg by the specific surface area of the activated carbon.One solution to increase the energy storage of EDLCs is to add soluble redox couples to the electrolyte,so as to use the "dead weight" of the electrolyte to add faradaic charge,such type of EDLCs is also known as redox ECs or dual-redox ECs,the latter referring to EDLCs using redox couples for both positive and negative electrodes.Among these dual-redox ECs,the system with the highest energy density at small current is SnSO4/VOSO4(75 W h/kg at 55 mA/g),and the system with the highest energy density at moderate current is EVBr2(64 W h/kg at 1 A/g).To further improve the specific energy of the dual-redox ECs,we should choose new redox couples with larger voltage gap,fast electrochemical kinetics as well as high solubility.Br-is attractive redox-active species due to fast electrochemical kinetics,highly positive redox potential as well as low cost.When pairing with redox couple of highly negative redox potentials,bromide-based dual-redox EC is expected to have a higher energy density than ever before.This article focuses on the high energy density bromide-based dual-redox ECs and systematically analyzes the electrochemical performances of these systems.The following research progress has been achieved:1.Cu/Br dual-redox EC was first proposed.We found that Cu/Br dual-redox EC suffers from excessive self-discharge rate and the dissolution/diffusion of Br3-and the erosion of Cu by Br-are the primary causes.In this study,tetrapropyl ammonium cation(TPA+)was used to immobilize Br3-and inhibit Br-erosion.The experimental results show that the self-discharge rate decreases with the increase of TPA+concentration,and 50 mM TPA+can improve the cycling stability of Cu/Br dual-redox EC.The newly configured Cu/TPA/Br redox EC exhibits a specific energy of 24 W h/kg at 1 A/g,a high power density(2 kW/kg)and stable cycling over 5000 cycles.2.Zn/Br Dual-Redox EC was first proposed.The theoretical voltage gap between the two redox couples is larger than 1.7 V,which is an advantage for high specific energy.However,poor reversibility of Zn2+/Zn redox couple and the soluble nature of Br3-,result low coulombic efficiency,poor cycleability and high self-discharge.The experimental results show that TPA is an effective complex agent for Br3-,however,TPA+ will also react with ZnBr42-(one of the main species in the Zn/Br electrolyte)to form a precipitate,[TPA]2[ZnBr4],resulting in the incompatibility of TPA+in the Zn/Br dual-redox EC.Our experiments show that the compatibility of TPA+can be improved by adding appropriate amount of Zn(OTf)2.Raman characterization has been used to investigate the function of Zn(OTf)2 to the system,and the results show that the change of the molar ratio of Zn2+/Br-is the key parameter leading to the improved compatibility of TPA+.The performance of the Zn/OTf/TPA/Br dual-redox EC was then evaluated and compared with Zn/Br dual-redox EC.After adding these two additives,the coulombic efficiency of the dual-redox EC has been increased from 90%to 99%,and the open-circuit voltage loss(24 h)has been dropped from 31%to 5%.The newly configured Zn/OTf/TPA/Br redox EC exhibits a specific energy of 110 W h/kg at 1 A/g,a high power density(7 kW/kg)and stable cycling over 6000 cycles.3.1,1’-Bis[3-(trimethylammonio)propyl]-4,4’-bipyridinium/bromide dual-redox EC(NV/Br)was first proposed.As NV4+provides redox potential as low as-0.72 V(vs.SHE)and high charge capacity(two-electron storage),NV/Br dual-redox EC is expected to have a higher discharge plateau voltage and higher specific discharge capacity,comparing with the reported one-electron storage viologen/Br dual-redox ECs.However,charging products(NV3+,NV2+and Br3-)are water-soluble,which make them easy to diffuse across the porous membrane and react with counter electrode,causing low coulombic efficiency and severe self-discharge behavior.To effectively prevent the shuttle of NV3+and NV2+,quaternized polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene Br-exchange membrane(SEBS-QA)was synthesized as a separator.The experimental results show that the penetration rate of NV3+ and NV2+ through SEBS-QA membrane was slow,due to high Br-selective permeability of SEBS-QA membrane,and large hydrated ion radius and high charge number of NV3+and NV2+.In addition,Although part of Br3-can be complexed with NV4+,there still is a large amount of free Br3-,due to insufficient complexation by NV4+.To improve the Br3-complexing capacity of catholyte,we added tetrapropylammonium cation(TPA+)as a strong complexing agent to inhibit Br3-dissolution/diffusion.With the help of SEBS-QA membrane separator and TPA+,the CE of NV/Br dual-redox EC has been increased from 64%to 99%,and the open-circuit voltage loss(24 h)has been dropped from 83%to 34%,showing improved self-discharge characteristic.In addition,this system produced a promising specific energy of 87 W h/kg at 1 A/g,a high power density greater than 4.8 kW/kg,and stable cycling over 4000 cycles. |
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