A Study On Performances Of Lead Redox Flow Batteries With Lead Fluoroborate Electrolyte

In order to solve the energy shortage and smog pollution issues during current national economy and social development in the“Thirteenth Five”period,we urgently need to change the energy framework and develop novel technologies for energy storage.Compared with other electrochemical energy storage methods,redox flow batteires have some advantages,e.g.site conditions are not restricted,flexible scale design,and reliable performances.Lead redox flow batteries are deposit style redox flow batteries using Pb as the active material.Lead redox flow batteries still maintain some positive characteristics of traditional lead acid batteries,such as low cost,mature industry chains,and without membranes.In this thesis,lead fluoroborate electrolytes were adopted to study the performances.Based upon previous research outcomes of our group,the research methods of this thesis are half-cell analysis,Hull-cell test,single cell performance test,and numerical modelling.The influences of factors?e.g.concentration distributions and charge-dischage current densities?on the battery performances were studied.Approaches for improving battery performances were further discussed,with practical cell test and modelling.The thesis contains three main topics:1.The influences of compositions of the lead fluoroborate electrolytes on their electrochemical propertiesWith the rotating disk electrode?RDE?,which incorporates forced convection,the diffusion coefficient of Pb²⁺in the electrolytic solution was determined to be 1.74±0.5×10^-6cm²·s^-1,calculated by Levich Equation.The electrolytic composition has essential influences on the physical and electrochemical properties of lead electrolyte.Increasing the contents of Pb²⁺and HBF₄ resulted in higher kinematic viscosity of the electrolyte and when Pb²⁺was about 1 mol·dm^-3,the ionic conductivity exhibited a peak value and decreased with further increasing Pb²⁺content.Considering kinematic viscosity and conductivity optimal composition of electrolyte was 1 mol·dm^-3 HBF₄ and 1.5 mol·dm^-3 Pb?BF₄?₂,of which the exchange current density was 4.2×10^-2 A·cm^-2.2.The influence of surfactants on the micromorphologies of Pb and PbO2 depositsThe?-PbO₂ and?-PbO₂ co-exist in PbO₂ deposit layer of the positive electrode during charging lead redox flow battery.Increasing the current density resulted in that the?-PbO₂ratio gradually decreased and when current density was 100 mA·cm^-2,?-PbO₂ ratio was hardly identified in the deposits.The surfactant additives have apparent influences on the electrochemical properties of lead redox flow batteries.For example,adding sodium lignosulfonate resulted in more positive equilibrium potentials and smaller exchange current densities.However,effects of TritonX-100 were just the inverse of adding sodium lignosulfonate.The addition of sodium lignosulfonate to the electrolyte can improve the flatness of Pb and PbO₂ deposits,and adding TritonX-100 decreased the compactness.3.Multiphysic numerical modelling and performance test of lead redox flow batteryCOMSOL software was used to study the multiphysics modelling.The procedure was from 0D model of the electrode kinetics,2D model of the dynamic process of the battery charge and discharge,and the 3D model of the three-phase boundary of electrode interface.The partial differential equtions were solved by finite element numerical analysis,in order to explain the the dynamic processes of battery oepration and to optimize the operation parameters.Uniformity of flow rate distribution of the electrolyte could be improved by applying serpentine type flow field,which is beneficial for cell performances.The inlet flow rate may directly affects the electrode reactions.When the flow rate was too small,there was almost no active materials at the outlet,and the current density was almost zero.In practical battery charge and discharge tests,with the serpentine flow field,batteries could stably operate for more than 1000 charge-discharge cycles at a current density of 50 mA·cm^-2,of which the average energy efficiency was greater than 80%.In this study,the influences of electrolytic factors such as ionic composition on the performances of lead redox flow batteries were explained.The influences of adding surfactants on tuning the micromorphologies of electrode deposits were identified.Multiphysics modelling were studied to understand the dynamic processes of battery charge-discharge and the operation parameters?flow rate,etc.?on battery performances.