Electrochemical Properties Of Electrolytes For Energy Storage Zinc-iodine Liquid Flow Batteries

Global energy supply and environmental issues are becoming increasingly prominent problems to human beings.These lead to continuous efforts in promoting energy conservation and emission reduction as well as exploring the development of renewable energies.However,application of renewable energies is subject to intermittent and decentralized instability.Thus,the large-scale energy storage offers a solution for stable and deep energy use.Compared with other large-scale energy storage technologies,electrochemical energy storage technology can increase energy efficiency and improve the structure of energy supply due to its outstanding advantages.Therefore,liquid flow battery based on electrochemical principles is developing rapidly in recent years.The zinc-iodine liquid flow battery was studied through probing the electrochemical properties of key materials.A total of 11 kinds of ambient liquid polyiodine substances were prepared and developed.Furthermore,the polyiodine substances suitable for being the new electrolyte in liquid flow batteries were also proposed.Compared with conventional electrolytes with zinc-iodide（Zn I₂）as the electrolyte,three types of optimized electrolytes are described as follows:1)using zinc-bromide（Zn Br₂）as stabilizer on top of the conventional electrolyte;2)replacing Zn I₂with sodium-iodide（Na I）and zinc-chloride（Zn Cl₂）as stabilizer;3)replacing Zn I₂with potassium-iodide（KI）and Zn Br₂as stabilizer with the addition of potassium-chloride（KCl）as supporting agent.It is obvious that three parameters as density,conductivity,and thermal stability of the polyiodide substances can effectively affect performance of the cell,the reactive ions in the electrolytes,and operating temperature range of the electrolytes.The physicochemical properties of the prepared polyiodide substances were first measured or determined in this study.The macroscopic liquid flow characteristics of prepared electrolytes were observed,the thermal stability of prepared polyiodine substances were measured using TG-DSC technology,the polyiodine reactive ions present in the prepared electrolyte were determined by UV photometry,the density of the substances was determined using two different methods based on methods of the Chinese national standard and the conductivity was also studied.It can be concluded from above-mentioned experiments that the prepared polyiodides with thermogravimetric properties greater than 150°C,lower density,higher conductivity,and better fluidity should be proposed to be new electrolytes for prepared zinc-iodine liquid flow battery with different electrochemical advantage.The electrochemical performances of conventional electrolytes,three types of optimized electrolytes and new newly proposed electrolyte systems were further measured by the cyclic voltammetry（CV）,linear sweep voltammetry（LSV）and electrochemical impedance spectrum（EIS）technologies in an electrochemical workstation.This means that a total of 28 different concentrations of electrolytes were studied in the paper.And some results were obtained for the effect of varying single-factor conditions on the electrochemical properties.The CV experimental results indicated that five types of prepared electrolyte systems exhibited different cycling mechanisms.Three optimized and newly proposed electrolyte systems showed better performance than the conventional ones.In addition,no significant change in cycling curves after 50 cycles of cyclic voltammetry measurement can be observed in the newly prepared electrolytes.The LSV experimental results showed that different peak patterns can be observed for different electrolyte systems.However,the abundance of reactive substances in the prepared polyiodide and the newly proposed electrolyte can be confirmed.It can also be concluded that the electrode processes in the conventional and optimized electrolyte systems were controlled by diffusion,while the newly proposed electrolyte system was mainly controlled by kinetics under the condition of changing the single-factor conditions.The results of peak currents and peak potentials of all electrolyte systems showed that concentration of electrolyte was more important than sweep speed for the increasing trend of peak currents and peak potentials.The cathodic polarization resistance of the optimized electrolyte systems was decreased with increasing concentration of active substances.Three types of optimized electrolyte systems,which had lower cathodic polarization resistance than conventional one,had better stability for electrochemical reactions due to easily producing electrochemical reactions.The newly proposed electrolyte system displayed an average decrease in polarization resistance of 22.90%.The electrochemical kinetic behaviors of different electrolyte systems were also studied using EIS technology through fitting experimental curves of the equivalent circuit impedance spectrum for evaluating the equivalent circuit components.The above-mentioned experimental results demonstrated that two types of optimized electrolyte systems have better electrochemical behavior than the conventional one.However,the newly proposed electrolyte system displayed higher resistance in terms of dual-interface charge-electron transfer capacity.The single-factor experimental studies also confirmed that increasing the concentration of electrolyte system can enhance not only the substance transfer process controlled by the diffusion step but also increase the charge transfer rate controlled by the electrochemical reaction step.