Study On Structure Control And Performance Of Hybrid Ion Exchange Membrane For Vanadium Redox Flow Battery

To fully use the intermittent renewable energy（e.g.,wind and solar energy）,a large-scale and reliable energy storage system is critical essential.All-vanadium redox flow battery（VRFB）is regarded as one of the promising large scale energy storage systems because of its long lifetime,fast response,high reliability,depth discharge,high efficiency,low cost,decoupled power and capacity.The commonly using ion exchange membrane for VRFB is Nafion series membranes（Du Pont）,due to its high proton conductivity,good chemical and mechanical stability.However,severe vanadium ions permeability and high cost of the Nafion membrane are greatly hindering the further development of VRFB.This study focuses on the structure control and performance of ion exchange membranes with lignin and TiO₂ nanotubes to reduce the vanadium ion permeability,enhance the ion selectivity,and promote the electrochemical performance of the membrane.The main research contents are as follows.Sulfonated poly（ether ether ketone）（SPEEK）with a low degree of sulfonation is blended with lignin to prepare a SPEEK/Lignin hybrid membrane.The lignin with rich hydroxyls contributes it uniform dispersing in the SPEEK matrix to tune the proton conductivity,ion selectivity and mechanical stability of the SPEEK membrane.The VRFB single cell with an optimized SPEEK/Lignin hybrid membrane exhibits extremely high Coulomb efficiency（up to 99.5%）and excellent energy efficiency（up to 83.5%）.Moreover,the optimized hybrid membrane showed good discharge capacity retention（64.1%）after 350 charge-discharge cycles.It is attributed to the lignin uniform dispersing in the SPEEK matrix to reduce the vanadium ion permeability（lower one order than the Nafion 212）,while maintains good proton conductivity.It provides a new strategy for the further development of non-fluoride ion exchange membranes for VRFB.Lignin is employed as additive in Nafion matrix to form a uniform membrane via solution cast method for VRFB application.The lignin controls the ion channel of the Nafion matrix to reduce the vanadium ion permeability,and enhance the ion selectivity.The device performance of an optimized Nafion/Lignin hybrid membrane is more excellent than that of the Nafion 212.Due to the good block effect of lignin for vanadium ion permeability,the open circuit voltage maintain time of the optimized Nafion/Lignin hybrid membrane is over 7 times of the Nafion 212 under 75%state of charge.The lignin benefiting for the mechanical stability of the Nafion membrane,the tensile strength of the optimized Nafion/Lignin hybrid membrane（29.40 MPa）is higher than that of the Nafion 212（27.71 MPa）.Additionally,at high current density(120 m A cm^–2),the VRFB single cell with an optimized Nafion/lignin hybrid membrane exhibits high Coulombic efficiency（～97.4%）and energy efficiency（～82.7%）.The cell with the optimized Nafion/Lignin hybrid membrane displays prominent cyclability（52.8%discharge capacity retention after 1 000 cycles（361 h）vs.34.8%of the Nafion 212 only 150 cycles）.In addition,the FT-IR results of the Nafion/Lignin membrane after long-term cycle test demonstrate the hybrid membrane possessing good chemical stability.The superhydrophilic TiO₂ nanotube is prepared by electrochemical anodizing and blended with Nafion to fabricate a Nafion/TiO₂ nanotube hybrid membrane by casting solution method.Compared with the commercial Nafion 212,the Nafion/TiO₂nanotube hybrid membranes exhibit low water uptake and swelling ratio because of the TiO₂ nanotube uniform dispersing in the Nafion matrix to control the ion cluster and limit the membrane expansion.The nanotubes uniform embedding into the Nafion matrix can lengthen or block the ion channel to reduce the vanadium ion permeability.The interaction between the Ti⁴⁺of TiO₂ nanotube and the sulfonic acid group of Nafion matrix aligns the ion exchange groups（sulfonic acid groups）on the surface of the nanotube that enhances the protons transport.The VRFB single cell with an optimized Nafion/TiO₂ nanotube hybrid membrane exhibits impressive performance with high Coulombic efficiency（CE,～98.3%）and outstanding energy efficiency（EE,～84.4%）at 120 m A cm^–2,being higher than that of the commercial Nafion 212membrane（CE,～94.5%;EE,～79.2%）.In addition,the cell maintains a discharge capacity of～55.7%after 1 400 cycles（518 h）,in obvious contrast to that of～20.0%after only 410 cycles for the one with commercial Nafion 212.