液流电池多孔电极内的气相析出现象及其对流动与传质的影响

Carbon fiber based porous electrodes (e.g. carbon felt) have been widely used in redox flow batteries (RFBs), in which the electrodes are not only reaction sites but also act as both electron conductor and reactant transport path. When the liquid electrolyte flow through a porous electrode, the variation of temperature (or pressure) or side reactions often lead to gas evolutions and create complicated gas-liquid two-phase flow and transport. The entrapped gas bubbles are obstacles along the flow path, which not only impede both ionic transport and uniform feeding of electrolyte but also reduce the effective reaction area. Concerning this gas-liquid two-phase flow coupled with diffusion, migration and electrochemical reaction processes, the gas saturation, relative permeability and local mass transfer coefficient will be measured experimentally. Base on both experimental and numerical approaches, the following investigation will be performed. (1) Hydrodynamic characteristics of two-phase flow in porous electrodes induced by heating. (2) Hydrodynamic characteristics of two-phase flow in porous electrodes induced by the hydrogen evolution reaction. (3) Effect of the hydrogen evolution reaction on the mass transport and the performance of porous electrodes. The distributions and the dynamic behavior of gas bubbles in porous electrodes will be demonstrated in this work. What is more, the key parameters affecting gas babble entrapment as well as their impact on the electrolyte transport and the electrochemical reactions will also be revealed. As a result, all these output will be useful in the design and optimization of both electrode material and RFB systems with high energy efficiencies.

以碳毡为代表的碳纤维多孔电极在液流电池中应用广泛，是电化学反应的主要场所，同时承担着电子传导和反应物传输的任务。液相电解质流经多孔电极时，温度、压力的变化或副反应的发生常会导致气相的析出，诱发复杂的气液两相流动和传质现象。滞留在多孔电极内的气泡，不仅会阻碍液相电解质的流动和均匀配送，还会降低离子的传输能力和电极的有效反应面积。围绕多孔介质内耦合流动、扩散、迁移和电化学反应的气液两相传输问题，运用实验手段测量气相饱和度、相对渗透率和局部传质系数等关键参数，并结合数值模拟研究如下内容：(1) 多孔电极内热致析气诱导的两相流动特性；(2) 多孔电极内析氢诱导的两相流动特性；(3) 析氢副反应对多孔电极内传质和电化学性能的影响。上述研究将展现多孔电极内析出气泡的分布和动态行为，揭示气泡滞留的主导因素及其对流动、传质和反应的影响，为电极材料、电池系统的优化设计和高效运行提供理论指导和实验依据。

金属离子水基液流电池(MARFB)，如：全钒液流电池(VRFB)和铁铬液流电池(ICRFB)等，凭借功率和容量可独立设计、布置灵活、响应快、运行安全可靠等特点，成为大规模储能应用的重要选择。但由于溶剂水电化学活性窗口的限制，该类液流电池的运行过程伴随着少量的负极侧析氢及正极侧析氧副反应。此外，当电解液流经以碳毡为代表的多孔电极时，其温度与压力的变化也会导致内部溶解气相的过饱和析出。上述多孔电极内的原位析气现象，及其带来的电极内部气相滞留和气液两相流动，会进一步引发液流电池系统储能容量不平衡、泵功损失上升、各单池间供液不均和单池内局部传质恶化等多种问题。.本课题研究围绕液流电池多孔电极内部的气相析出现象展开，通过实验和电池模型模拟的研究手段获得主要成果如下：.1）.使用透明流动池研究了热致析气诱发两相流动过程中，电极润湿性和工质流动方向对气相含率、气相分布和液相相对渗透率动态演化规律的影响，并构建了气相含率与液相相对渗透率的实验关联式。研究发现对于润湿性良好的活化电极，微量析气仍可引发显著的流阻上升。同时，不同流向条件下浮升力作用的差异会改变气相在电极内部的分布模式，进而影响流动阻力与其相含率的依变关系。.2）.通过调整负极电解液的组成成分，研究了电流与电解液流速可独立控制的原位电化学析氢现象。发现了低流速、高析氢速率条件下两相流动的不稳定性。同时，8mm/s表观流速可有效去除无析气条件下电池内部滞留的气相。当电解液流速提升至16mm/s时，气相吹扫所需时间减半。.3）.建立VRFB二维电堆模型，模拟气相阻塞造成局部电解液供给不足时，单池内充、放电过程中副反应的发生情况。结果显示，单池供液不足对电堆电压的影响有限，但缺液单池下游量级可观的副反应，会影响到电池的储能容量及电堆材料的运行寿命。.4）.针对控制电堆内析氢副反应及实施容量再平衡的实际需求，申请了发明专利：“具备催化剂管理与电解液容量再平衡的液流电池子系统”。