Investigation On Mass-Charge Transfer And Electrochemical Reaction Process In Non-aqueous Redox Flow Battery

Energy is the core of human civilization,and the supply of abundant fossil energy is the primary driving force for human progress in the past 100 years.With the consumption of fossil fuel reserves and serious environmental problems,there is a growing demand for abundant,clean and safe renewable energy.The use of renewable energy not only reduces the risk of environmental pollution,but also promotes the harmonious coexistence between human and nature.However,since renewable energy sources（such as wind,solar,etc.）are inherently intermittent,the substantial mismatch between supply and demand limits their widespread use.Therefore,it is necessary to make full use of the renewable energy and develop efficient large-scale energy storage systems.Among various energy storage technologies,rechargeable batteries have become one of the most important technologies for many years.In particular,redox flow batteries（RFBs）have received intense attention due to their high energy efficiency,excellent scalability and exceptional design flexibility,and are considered to be the most promising technologies for large-scale energy storage.However,the main constraints limiting the widespread commercialization of RFB appears to be their low energy density.One way to overcome this drawback is to use non-aqueous electrolyte solvents that can provide a wider potential window of operation as well as increasing the energy capacity of the system.Moreover,compared with aqueous electrolytes,the choice of redox couples is broadened and the space for optimizing energy density and electrode dynamics is expanded.In this thesis,low-cost ionic liquid analogs,namely deep eutectic solvents（DESs）,are used as potential applications for non-aqueous electrolytes in flow batteries.The effects of additives on the physical and electrochemical properties of the electrolyte and the mass transfer characteristics wherein have been investigated,in order to benefit the future development of non-aqueous flow batteries.Firstly,The effects of CO₂ on the physical and electrochemical properties of DES containing Fe（II）/Fe（III）redox couple were investigated.It was found that the addition of CO₂ had no significant effect on the reaction kinetics of Fe（II）/Fe（III）on DES,but it could reduce the viscosity and ohmic resistance of the electrolyte solution.This is because the CO₂ dissolved in the electrolyte increases the holes in the solution,and the compressibility of gas makes the interstices of ions changeable,accelerating the movement of ions.Therefore,adding CO₂ to DES can reduce the internal loss of the battery to some extent.Antimony(Sb³⁺)ion with low cost and high catalytic activity was added to the electrolyte solution as an additive to study the influence on the physical and electrochemical properties of the electrolyte solution.The results showed that the electrochemical reaction kinetics of V（III）/V（II）redox couple is enhanced by the addition of Sb³⁺（up to 22.6%）,the diffusion coefficient of vanadium ions also increases（up to 63.3%）and the charge transfer resistance decreases（up to 11.9%）,which indicates that it can play a positive role in the mass transfer process inside the battery.In view of the fact that Sb³⁺ions can effectively enhance the physical and electrochemical properties of electrolytes,the homogeneous ion exchange membrane and Nafion membrane were used for the assembly of Fe/V batteries,respectively.The charge and discharge performance and power density of the batteries before and after the addition of additives were investigated.It was found that the Nafion membrane can transport ions more efficiently.Moreover,the battery after adding Sb³⁺ions has better charge and discharge performance,the energy density and power density of the battery are also improved,and the performance is optimal when the concentration of added Sb³⁺ion is 15 mM.The field emission scanning electron microscope（FE-SEM）shows that Sb ions are electrodeposited on the surface of graphite felt,which contributes a catalytic effect on the electrochemical reaction so as to improve the electrochemical performance.However,too more deposition of Sb³⁺ion will cover the active sites of the electrode,causing the decline of performance.Therefore,the trade-off between positive and negtive effects results in an optimal concentration of Sb³⁺ion additive in this DES-electrolyte redox flow battery.