基于EQCM技术原位动态研究添加剂对钒电池电极和隔膜界面电化学过程的影响

Adding the representative additive such as hydrochloric acid, ammonium biphosphate, D-sorbitol or methyl sulfonic acid into the V(V) electrolyte of a vanadium redox flow battery can significantly improve its stability and reduce the negative impact of V2O5 and improve the performance of battery. However, after a long time operation for the battery, the additive will flow with electrolyte to the surface of the electrode and membrane, which would change the surface micrographs of the electrode and membrane and affect their interface electrochemical processes such as migration, diffusion and adsorption of molecular or ions and electron transfer and further affect their electrochemical properties and the performance of battery, and the reported papers rarely refer to these effects and corresponding mechanisms. This project is carried out based on the previous research of our group. Simulating the battery operation, the technology of electrochemical quartz crystal microbalance (EQCM) is employed to dynamic study the effect of the several representative additives above in the flowing electrolyte on the interface electrochemical processes of graphite felt and Nafion membrane with an in situ cooling/heating treatment based on the molecular or ion level and micro level. And the relationship among the important parameters of additives such as type, structure and amount of additives, the temperature, the corresponding interface electrochemical processes and electrochemical properties of the electrode and membrane are stated and the effect mechanisms of these additives on electrochemical properties of the electrode and membrane are discussed. The research results is advantageous to develop the in situ study methods for the interface electrochemical processes in the dynamic battery and perfect the theory system of V(V) electrolyte additives.

向V(V)电解液中加入代表性添加剂，如盐酸、磷酸二氢铵、D-山梨醇或甲磺酸，可以显著提高其稳定性，降低V2O5沉淀的不良影响，同时提升钒电池性能。但电池运行时，添加剂会随电解液流动至电极和隔膜表面，改变其微观形貌，影响其分子离子迁移、扩散和吸附以及电子转移等界面电化学过程，进而影响其电化学性能和电池性能，文献对此影响及机理报道较少。本项目结合前期的研究基础，模拟电池运行动态，基于电化学石英晶体微天平（EQCM）技术通过冷热原位动态方法从分子离子水平、微观层次上研究流动电解液中以上几种代表性添加剂对石墨毡电极和Nafion膜界面电化学过程的影响，阐述添加剂重要理化参数（如种类、结构和用量）、温度、电极与隔膜的界面电化学过程和电化学性能之间的关系，探讨添加剂对电极和隔膜电化学性能的影响机理。研究结果有助于发展动态钒电池中界面电化学过程的原位研究方法和完善钒电池V(V)电解液添加剂的理论体系。

本项目针对钒电池V(V)电解液稳定性较差的问题，结合前期的研究基础，构建了V(V)电解液添加剂参数库，初步建立了V(V)电解液添加剂理论体系，同时自组装搭建了静态和动态EQCM变温电化学测试平台并总结了EQCM技术在电化学领域的应用。采用EQCM原位定量技术，结合SEM、EDS、XRD和XPS等表征结果，确定了金电极表面球形颗粒沉淀的成分主要为V2O5和V2O5•3H2O，初步获得了V2O5在金电极表面的沉淀过程：随温度的升高、CV扫描圈数的增加，沉淀量均呈线性增加，添加剂的引入对沉淀量的影响也较大；同时初步获得了金电极表面钒离子、水、氢气和氧气等的吸脱附行为：V(V)还原为V(IV)时吸脱附不明显，没有出现吸脱附峰，继续还原为V(III)和V(II)时，出现一个明显的吸脱附峰，同时开始析氢，V(II)氧化为V(III)时，再次出现一个明显的吸脱附峰，继续氧化为V(IV)和V(V)时，没有再出现吸脱附峰，同时开始析氧，随温度的升高、CV扫描圈数的增加，沉淀量逐渐增大，但金电极表面净质量的累积先由于钒离子、水、氢气和氧气的吸脱附净量大于V2O5沉淀量，净质量开始减小，经过一定时间后，净质量开始逐渐增大，直到累积至检测上限。添加剂的引入会对V2O5沉淀量及沉淀过程造成一定的影响，金电极表面累积的沉淀量不能作为判断添加剂性能好坏的唯一依据。Nafion膜层形成的气孔有利于V2O5沉淀的成核、生长和累积，这一现象有助于预测钒电池隔膜的性能和寿命；含磷酸二氢铵添加剂电解液样品的电池能量效率高于空白样，这与文献报道基本一致。本项目将EQCM原位定量技术首次应用于钒电池电解液研究领域，获得的研究成果具有较高的创新性，将进一步完善钒电池V(V)电解液理论体系，通过本项目积累的原位定量研究经验将为其它能源材料及器件领域的研究提供借鉴和参考。