基于电渗析膜组件的全钒液流电池阳离子交换膜的钒离子迁移特性研究

Four kinds of vanadium ions will change its valence states due to cross-mixing of positive and negative electrolytes, so it is difficult to measure the transfer of vanadium ions in the charge-discharge process of vanadium flow battery (VFB). In order to facilitate the measurement, a new VFB with the composite electrolyte instead of ion exchange membrane is designed to control the process of vanadium ion transfer and make the concentration of crossover vanadium ions measurable. The composite electrolyte is composed of five parts which are two pieces of cation exchange membranes, a piece of anion exchange membrane and two liquid electrolyte like H2SO4 solution, and those five parts are assembled according to the structure of electrodialysis. With the help of the new VFB, several interesting results will be present. Firstly, the process of vanadium ion transfer is stopped before cross-mixing and stable vanadium ions after permeation of cation exchange membrane are obtained. Therefore, the permeation rate and permeability coefficient of vanadium ions in membranes of Nafion® and sulfonated poly ether ether ketone will be determined during a charge-discharge process of VFB, and a common method will be proposed to measure permeability coefficient of vanadium ions based on the new VFB. Secondly, it will be found that capacity fading caused by migration of vanadium ions can be divided into two parts, one is capacity loss by the loss of vanadium ions, and the other is capacity loss caused by cross-mixing. Thirdly, water transfer of positive and negative electrolytes will be investigated in terms of the loss of vanadium ions and cross-mixing of vanadium ions. Finally, it is wonderful that quantitative investigation will be available for migration of vanadium ions in charge-discharge cycling of VFB, which will provide valuable data and technique for the development of VFB membrane and study of capacity fading. To conclude, the project is worth studying.

针对全钒液流电池(VFB)运行过程中透过离子交换膜的四种价态钒离子因共混变价而无法定量研究的问题，本项目基于电渗析膜组件而设计固液组合电解质（阳离子交换膜+硫酸溶液+阴离子交换膜+硫酸溶液+阳离子交换膜）原位替换单一固体离子交换膜，从而构建VFB运行过程中四种价态钒离子迁移过程可调控、浓度可测量的电池结构。通过调控钒离子迁移共混变价的连续过程，使钒离子停留在透过膜之后、共混之前的不变价状态，测定充放电循环工况下Nafion®膜和磺化聚醚醚酮膜中四种价态钒离子的渗透速率与渗透系数，建立钒离子渗透系数测定的普适性方法，揭示钒离子迁移造成的容量衰减包含钒离子迁出和钒离子迁入共混变价两部分容量损失的本质，阐明钒离子迁出与迁入共混时正负极水迁移规律，从而解决全钒液流电池运行过程中无法定量研究四种价态钒离子迁移的难题，为VFB专用膜的研发和容量衰减机理研究提供数据支持与技术支撑，具有重要研究价值。

全钒液流储能电池（VFB）是一种大规模高效储能系统，被认为是解决可再生能源发电并网问题的有效途径。本项目针对VFB运行过程中透过离子交换膜的四种价态钒离子因共混变价而无法定量研究的问题，基于电渗析膜组件而设计了固液组合电解质原位替代单一固体离子交换膜，旨在使钒离子停留在透过膜之后、共混之前的不变价状态，从而可以测定在线工况下阳离子的渗透机理。按照项目方案执行过程中发现：固体组合电解质电阻太大，导致无法对电池进行有效的充放电循环，实验方案无法执行。于是提出了另一种在线调控电池的技术：利用聚酯框热压技术制备了膜的有效面积可调、多种不同类型隔膜继承到同一膜组件的VFB专用膜组件。得出以下结论：①利用该膜组件成功地实现了VFB隔膜两个功能区（组装区和离子传输区）的分离，大幅度降低VFB隔膜机械强度的要求，使机械强度较低的隔膜能够应用于VFB；②同一张膜，其有效离子传输区面积越小，电池的库伦效率越高，电池电阻越大；③Nafion 212能够承受的最高电流密度是200 mA cm-2；④ 成功地使多种不同类型的隔膜集成到统一膜组件中。此外，研究者基于VFB关键材料如隔膜和电极材料均为柔性材料的事实，创新性地提出了一种螺旋卷式结构的液流储能电池的设计，可以大幅度提升VFB的体积能量密度和质量能量密度。目前为止，已经申请六项专利，其中两项已授权。