Study On Energy Density Improvement Mechanism Of Alkaline Zinc-based Flow Battery

Flow battery,as an advanced large-scale energy storage technology,can make up for the shortcomings of intermittent energy generation and maximize the use of renewable energy to generate electricity.It can be used in all aspects of the power system to improve the reliability and stability of the power system.Traditional Aqueous flow batteries（all-vanadium flow batteries,iron-chrome flow batteries,etc.）are limited by open circuit voltage and active material concentration,and have low energy density（<50 Wh/L）.Therefore,it is important to develop a new type of high-energy-density flow battery system.In this research,two kinds of high-energy-density zinc-based flow batteries were developed with potassium ferricyanide（K₃[Fe（CN）]₆）and sodium permanganate（Na Mn O₄）as the main research objects.Alkaline zinc-iron flow battery（Zn/Fe RFB,Zinc-Iron Redox Flow Battery）and alkaline zinc-manganese flow battery（Zn/Mn RFB,Zinc-Manganese Redox Flow Battery）.The influence of different factors on the performance of single batteries was explored,which laid the theoretical and experimental basis for the development of low cost,high energy density flow batteries.Basic Zn/Fe RFB uses K₃[Fe（CN）]₆ as the positive active material,zinc plate as the negative active material,and sodium hydroxide（Na OH）as the supporting electrolyte.Sulfonated poly（ether ether ketone）membranes with different degrees of sulfonation are prepared by solution casting were used as the ion exchange membrane.The results show that the SPEEK-Na type membrane assembled with a sulfonation degree of 57 has the best comprehensive performance;Adjusting the concentration of Na OH can make the solubility of K₃[Fe（CN）]₆ in Na OH as high as 1 M.At a current density of 60 m A cm^-2,the actual energy density of a single battery assembled with a SPEEK-Na membrane with a degree of sulfonation of 57 is 47.74 Wh/L,the energy efficiency is maintained at about 81.78%and the capacity retention rate is as high as 94% within 245 cycles,and the cost of the electrolyte is only about 1/5 of that of the all-vanadium flow battery.However,this method can only improve the solubility of K₃[Fe（CN）]₆ in alkaline solution to a certain extent,but it is still limited by the solubility of K₃[Fe（CN）]₆ itself.The assembled basic Zn/Fe RFB energy density is still low.The positive electrode of basic Zn/Mn RFB uses Mn O₄^-/Mn O₄^2-redox couple and Nafion212 as ion exchange membrane.The results show that under alkaline conditions,the pair has good electrochemical reversibility and a suitable electrode potential of 0.5V（vs.Hg/Hg O）,and solubility up to 3.92 M.Considering the high concentration of the negative electrolyte Na₂[Zn（OH）]₄ is difficult to prepare.The actual energy density of alkaline Zn/Mn RFB assembled based on Na₂Mn O₄ and Na₂[Zn（OH）]₄ can only reach41.1 Wh/L,while the alkali based on the assembly of Na Mn O₄ and Zn plate The actual energy density of the Zn/Mn RFB can be as high as 184.8 Wh/L.At a current density of30 m A cm^-2,the battery’s Coulombic efficiency is maintained at about 97.54%and the capacity retention rate is 81.44% at 72 charge and discharge cycles.