Halter J, Gloor T, Amoroso B, Schmidt T J, Büchi F N
Electrochemistry Laboratory, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland.
Phys Chem Chem Phys. 2019 Jun 28;21(24):13126-13134. doi: 10.1039/c9cp02149c. Epub 2019 Jun 6.
Wetting properties of phosphoric acid in porous materials of high temperature fuel cells (HT-PEFC), operating at around 160 °C, are important for cell performance and durability, but the underlying wetting parameters have been unknown so far. Therefore, the influence of phosphoric acid temperature and concentration on the wetting behavior of porous HT-PEFC materials is investigated. The acid filling of gas diffusion and catalyst layers as function of capillary pressure is monitored with X-ray tomographic microscopy under the well defined conditions of an ex situ set-up at temperatures up to 160 °C. For the wetting of gas diffusion layers, with pore sizes in the order of few 10 μm, two opposing trends are shown. With increasing phosphoric acid concentration, less capillary pressure is required, while with increasing temperatures, higher capillary pressures are needed for filling up to a given saturation. The same trends are also found for the contact angle of phosphoric acid on PTFE. A higher contact angle is observed with increasing temperature while increasing the phosphoric acid concentration decreases the contact angle. As both trends are of a similar order of magnitude, the wetting behavior of concentrated (113 wt%) phosphoric acid at 160 °C is astonishingly similar to the wetting behavior of water at room temperature. Another important property for HT-PEFC operation is the filling of cracks in the catalyst layer, which have widths up to 100 μm. For large cracks (>60 μm), a capillary pressure of only 15 mbar was deduced from the measurement, increasing to 30 mbar for cracks between 20 and 60 μm. This, for the first time, allows for assessing the membrane phosphoric acid pressure during fuel cell operation. This can guide the development of improved porous materials for HT-PEFC.
在约160°C下运行的高温燃料电池（HT - PEFC）多孔材料中磷酸的润湿性对电池性能和耐久性很重要，但迄今为止其潜在的润湿参数尚不清楚。因此，研究了磷酸温度和浓度对HT - PEFC多孔材料润湿行为的影响。在高达160°C的非原位设置的明确条件下，用X射线断层显微镜监测气体扩散层和催化剂层的酸填充量随毛细管压力的变化。对于孔径在几十微米量级的气体扩散层的润湿，呈现出两种相反的趋势。随着磷酸浓度的增加，所需的毛细管压力降低，而随着温度升高，填充到给定饱和度需要更高的毛细管压力。在磷酸与聚四氟乙烯的接触角方面也发现了相同的趋势。随着温度升高观察到接触角增大，而增加磷酸浓度则接触角减小。由于这两种趋势的量级相似，160°C下浓（113 wt%）磷酸的润湿行为与室温下水的润湿行为惊人地相似。HT - PEFC运行的另一个重要特性是催化剂层中宽度达100μm的裂缝的填充情况。对于大裂缝（>60μm），测量得出的毛细管压力仅为15毫巴，对于20至60μm之间的裂缝，毛细管压力增加到30毫巴。这首次使得能够评估燃料电池运行期间膜内的磷酸压力。这可为开发用于HT - PEFC的改进型多孔材料提供指导。
