Optimization And Design For Electrolyte Of Low-cost Metal-based Flow Batteries

All-vanadium redox flow batteries use vanadium compounds as reaction raw materials for both positive and negative electrolytes.However,due to the surge in vanadium prices,the development of all-vanadium redox flow batteries was severely limited by the raw material costs.Nevertheless,the low-cost flow batteries meet the chances to achieve further investigation and commercialization such as iron-based flow batteries.This dissertation develops two ways to solute the cost issue of metal-based flow batteries.Firstly,it is highly crucial to identity the key factors that limits the cost of all-vanadium flow battery and solving them.Then,we investigated the novel all-iron flow battery which can be a potential alternative to all-vanadium flow battery.Due to the best comprehensive performance for all-vanadium flow battery,it occupies the leading position among various flow battery systems in terms of operating power and energy efficiency.However,cost has become a key factor restricting its development at present.lacking the polarization analysis and internal resistance analysis of the all-vanadium redox flow battery system has hinders the further improvement of its operating current density and energy efficiency.Therefore,in this dissertation,we adopted the conventional design of the all-vanadium redox flow battery as an example,in which the battery polarization curve was used as an analyzing method.When the current density ranging from 25 to 250 mA cm^-2,the ohmic polarization accounts for the largest proportion of the total polarization.Furthermore,it can be demonstrated that the electrolyte resistance accounts for the largest proportion compared to other components,exceeding 70%.In order to reduce the ohmic polarization caused by the electrolyte,the ionic conductivity of electrolyte was improved by optimizing the composition of the electrolyte,so that the all-vanadium redox flow battery with a conventional design can realize 80%energy efficiency at 250 mA cm^-2.According to the results,the effect of electrolyte dilution on reducing the cost and operating conditions of the stack was discussed,and a low-cost application strategy of all-vanadium redox flow batteries was proposed.The capacity of the metal-based flow battery is an equally vital standard to the operating power,which can be effectively determined by the capacity attenuation,thus determining the maintenance cost of the flow battery.Various flow battery loss factors,such as ion diffusion and side reactions,have been reported in the open literature before,and related suppression measures have been proposed.However,the effect and mechanism of capacity loss caused by electrolyte transfer have not been systematically explored.Therefore,in this dissertation,the electrolyte transfer caused by the viscosity difference between the positive and negative electrolytes is proved through modeling analysis and experimental verification.The main determined factors of electrolyte transfer:electrolyte viscosity and flow rate are revealed and discussed in batteries with a Nafion membrane.Correspondingly,the optimization strategy of electrolyte flow rate is proposed.Compared with the mixed electrolyte method,this method can be applied to varied flow battery systems.Due to the surge in the raw material cost of the all-vanadium flow battery,researchers have begun to actively explore other systems of flow battery systems in recent years.Among them,the all-iron flow battery possesses the advantage of using low-cost neutral iron salts as the electrolyte.Therefore,the all-iron flow battery will ensue a great potential to become a low-cost,high-performance metal-based flow battery,as long as the issue of poor reversibility of its negative half-cell can be handled.Fundamentally,the poor reversibility originated from the inherent solvation structure of Fe²⁺in the water.To delicately tune the Fe²⁺ solvation structure,we proposed sodium citrate and DMSO,which can intimately coordinate the Fe²⁺and can change the preferred crystal plane,respectively.In addition,both of them can alter the solvation structure of Fe[（H₂O）₆]²⁺,greatly improving the reversibility of the plating/stripping reaction at the negative half-cell,thereby extending the operating life of the all-iron flow battery and providing the possibility for the further commercialization of the all-iron flow battery.