Wetting properties of porous high temperature polymer electrolyte fuel cells materials with phosphoric acid

Abstract

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.