A New Tin-iron Flow Battery:Electrochemical Characteristics And Performance Optimization

Large-scale and high-efficient electrical energy storage is one of the key issues of new energy ultilization.Among various electrical energy storage technologies,the flow battery recently has received much attention because of decoupling in energy and power,ease in scaling,flexibility of location and high energy-conversion efficiency.As one of the most developed flow battery technologies,all vanadium redox flow battery is limited by its high electrolyte cost of vanadium.Iron-chromium redox flow battery has lower electrolyte cost,but it requires higher operating temperature or additional catalysts to improve the reaction kinetics of Cr²⁺/Cr³⁺.In addition,its low energy density limits its large-scale application.Hybrid flow batteries are featured by at least one redox couple species is not fully soluble and might be either a metal or gas,in this sense,reducing the amount of electrolyte used and thus improves the volume energy density of the system.Most of the existing hybrid systems currently suffering from low power density and poor cycle performance resulted from the low reaction kinetics and parasitic reactions.Hence,developing a hybrid system with excellent rate and cycle performance is of great significance.To mitigate above issues,here we report a tin-iron hybrid flow battery in which the stannous chloride anolyte is separated from the ferric/ferrous chloride catholyte.Firstly,cycle voltammetry based on the chosen electrolyte and the assembled battery was tested respectively.Secondly,the morphology and crystal form of the reaction product were characterized.In order to have a better understanding of the properties of Sn-Fe flow battery,key battery components like electrode,flow field,membrane and electrolyte were synthetically discussed for their impact on the battery performance.The main contents are as follows:In galvanostat charging/discharging test,Sn-Fe RFB exhibited excellent rate performance,demonstrating a voltage efficiency and columbic efficiency of 79%and98%respectively at a current density of 200 mA/cm² and can stably operate over 700cycles.The deposition products on negative electrode were characterized by XRD and EDS,proved that the production is merely?-tin,with no other by-product.SEM images of the electrode was used to calculated the mean diameter of the products and the particle distribution,result shows that mean diameter of the products is decreased with increasing charging current density and the sizes of products would grow with higher SoC.The conductivity of the solid matrix is higher than that of the solution phase,thereby creating a distribution of the reaction current biased toward the membrane,which cause the tin mainly deposited near to the membrane.On the basis of this system,different flow fields and electrolyte flow rate were adopted in order to further understand the relationship between mass transfer and battery performance.While increasing the electrolyte flow rate,voltage efficiency could have an 8%improvement and maximum power density could be improved by43%.However,the crossover of active ion would be more significant under higher flow rate,which causing more severe capacity decay.The uniform distribution of electrolyte in the electrode could help voltage distribute more evenly in the electrode,thus improves the battery long-term stability.It could also improve the utilization of active size of electrode,in this case,playing a key role in battery performance.With different mass transfer mechanism of serpentine flow field?SSF?and interdigitated flow field?IFF?,result shows that IFF outperforms SSF under low electrolyte flow rates for it force electrolyte flow through electrode,minimized the concentration overpotential.While SSF could distributes electrolyte more uniformly under higher flow rate,thereby exhibited a better performance.The positive and negative electrolytes were updated separately for the battery after the cycling test.The result shows that the decrease of Fe²⁺concentration in the positive electrolyte is the main reason for capacity decay of the battery during the cycling test.The voltage efficiency might mainly affected by the property of negative electrolyte.Materials such as graphite felt,carbon paper,carbon cloth and carbon foam have different porosity and density,in this study,we utilized these material as negative electrode and carbon cloth as positive electrode for battery test.Remaining the same initial electrode thickness and compression ratio,the result shows that the voltage efficiency of the battery is positively correlated with the conductivity and material transport capacity of the electrode material.With higher electronic conductivity and unique microstructure that benefit to mass transfer,battery with carbon cloth has highest voltage efficiency,reached 81.8%at a current density of 200 mA/cm².Since the plate electrode?empty chamber?is limited by a low specific surface area,it has minimum reaction sites and its voltage efficiency is the lowest among several electrodes,with only 73.5%.By using a mixed electrolyte,the concentration difference between the anolyte and catholyte owing to the difference in active ion concentration is effectively balanced,the crossover of the active material is thus alleviated,improved the cycle life of the battery.Set 67 mAh charging capacity as testing cut-off condition,battery could operate 220 more cycle.