Lutz Christian, Hampel Sven, Beuermann Sabine, Turek Thomas, Kunz Ulrich, Garrevoet Jan, Falkenberg Gerald, Fittschen Ursula
Institute of Inorganic and Analytical Chemistry, Clausthal University of Technology, Arnold-Sommerfeld-Straße 4, Clausthal-Zellerfeld 38678, Germany.
Institute of Technical Chemistry, Clausthal University of Technology, Arnold-Sommerfeld-Straße 4, Clausthal-Zellerfeld 38678, Germany.
J Synchrotron Radiat. 2021 Nov 1;28(Pt 6):1865-1873. doi: 10.1107/S160057752100905X. Epub 2021 Nov 3.
Vanadium-ion transport through the polymer membrane results in a significant decrease in the capacity of vanadium redox flow batteries. It is assumed that five vanadium species are involved in this process. Micro X-ray absorption near-edge structure spectroscopy (micro-XANES) is a potent method to study chemical reactions during vanadium transport inside the membrane. In this work, protocols for micro-XANES measurements were developed to enable through-plane characterization of the vanadium species in Nafion 117 on beamline P06 of the PETRA III synchrotron radiation facility (DESY, Hamburg, Germany). A Kapton tube diffusion cell with a diameter of 3 mm was constructed. The tube diameter was chosen in order to accommodate laminar flow for cryogenic cooling while allowing easy handling of the cell components by hand. A vertical step size of 2.5 µm and a horizontal step size of 5 µm provided sufficient resolution to resolve the profile and good statistics after summing up horizontal rows of scan points. The beam was confined in the horizontal plane to account for the waviness of the membrane. The diffusion of vanadium ions during measurement was inhibited by the cryogenic cooling. Vanadium oxidation, e.g. by water radiolysis (water percentage in the hydrated membrane ∼23 wt%), was mitigated by the cryogenic cooling and by minimizing the dwell time per pixel to 5 ms. Thus, the photo-induced oxidation of V in the focused beam could be limited to 10%. In diffusion experiments, Nafion inside the diffusion cell was exposed on one side to V electrolyte and on the other side to VO. The ions were allowed to diffuse across the through-plane orientation of the membrane during one of two short defrost times (200 s and 600 s). Subsequent micro-XANES measurements showed the formation of VO from V and VO inside the water body of Nafion. This result proves the suitability of the experimental setup as a powerful tool for the determination of the profile of vanadium species in Nafion and other ionomeric membranes.
钒离子通过聚合物膜的传输会导致钒氧化还原液流电池的容量显著下降。据推测，该过程涉及五种钒物种。微X射线吸收近边结构光谱（micro-XANES）是研究膜内钒传输过程中化学反应的有效方法。在这项工作中，开发了用于micro-XANES测量的方案，以便在PETRA III同步辐射装置（德国汉堡DESY）的P06光束线上对Nafion 117中的钒物种进行面内表征。构建了一个直径为3毫米的Kapton管扩散池。选择该管直径是为了在进行低温冷却时实现层流，同时便于手动操作池组件。垂直步长为2.5微米，水平步长为5微米，在对扫描点的水平行进行求和后，提供了足够的分辨率来解析轮廓并获得良好的统计数据。光束在水平平面内受限，以考虑膜的波纹度。测量过程中，低温冷却抑制了钒离子的扩散。通过低温冷却并将每个像素的停留时间最小化至5毫秒，减轻了钒的氧化，例如由水辐射分解（水合膜中的水含量约为23 wt%）引起的氧化。因此，聚焦光束中V的光致氧化可限制在10%以内。在扩散实验中，扩散池内的Nafion一侧暴露于V电解质，另一侧暴露于VO。在两个短除霜时间（200秒和600秒）之一期间，允许离子在膜的面内方向上扩散。随后的micro-XANES测量表明，在Nafion水体内部由V和VO形成了VO。该结果证明了该实验装置作为测定Nafion和其他离子交换膜中钒物种分布的有力工具的适用性。
