Comparison of Ex-Situ and In-Situ Synchrotron Imaging Measurements of the Water Distribution in PEM Fuel Cell Materials

Kurzfassung

This work investigates distinctive water distributions in the aged and fresh GDL samples of the SGL29 BC and its forerunner, the SGL25 BC. In this regard, all the GDL samples are tested in a point injection device during tomographic ex-situ measurements. On the other hand, fresh SGL25 BC samples at anode and cath-ode of the fuel cell are probed within a fuel cell operation via radiographic in-situ and tomographic quasi in-situ measurements. Through the ex-situ measurements the distribution of the injected water in the GDL is analyzed in 3D. Likewise, the distributions of liquid water in the cathodic and anodic GDL are examined in 3D by the assistance of the quasi in-situ measurements. The in-situ measurements yield the dynamic characteristics of water transport in the fuel cell, so the effect of different electric loads on water formation and water discharge is investigated. At the end, water distribution resulting from the fuel cell operation is compared to the water distribution provided by the point injection device. In the light of the ex-situ measurements, it is found out that the aged SGL25 BC sample has greater water content than the fresh SGL25 BC sample. Likewise, the water amount occupying the pore spaces in the aged SGL29 BC sample is less than the water amount detected in the aged SGL25 BC sample. The in-situ measurements indicate that water discharge rate reduces if current density is increased such that it pro-vides a transition from the activation polarization zone to the ohmic zone. The quasi in-situ measurements reveal that water amount at cathode is greater than the water amount at anode which is mostly induced by the back diffusion through the polymer electrolyte membrane. Furthermore, water amount underneath the ribs is larger than the water amount under the channel configuration in the anodic GDL. However, water amounts under the channel and ribs do not differ from each other in the cathodic GDL. Based on the point injection test, promising results are achieved to enhance water transport during fuel cell operation. According to these results, the injected water prefers a nearly straight pathway while mi-grating through XY-plane from the MPL zone to the upper water outlet. It means that a limited diffusion in the XY-plane takes place within the water transport, decreasing remaining water content in the GDL. This finding of point injection can be adapted to the fuel cell operation by producing GDLs consisting of elongated pore spaces or elongated holes through XY-plane. By this means, water discharge through the GDL can be accelerated, avoiding water flooding. As a result of this improvement, the efficiency of fuel cell operation is enhanced especially for aged GDL samples in which water accumulations are formed more easily.