(168d) Experimental Measurements of Water Transport in Proton Exchange Membrane Fuel Cells Via in-Situ Performance Testing and Ex-Situ Synchrotron X-Ray Radiography

Proton Exchange Membrane fuel cells are an electrochemical energy conversion device offering the potential for high power density and no harmful emissions at the point of use. As hydrogen and oxygen react, the byproducts are electricity, heat, and water. Managing that product water is essential to maintain high performance and efficiency. Excessive water in the flow channels or porous electrode materials can increase the system pressure drop and lower power output. An impediment to studying water in the fuel cell itself is that the fuel cell materials are opaque, hindering direct optical visualization. The current study addresses this issue via novel performance testing methods of an operating (in-situ) fuel cell and Synchrotron X-ray radiography testing of the porous materials in a non-operating (ex-situ) setup. In the in-situ testing, the fuel cell is initially operated at typical operating conditions (75^oC, fully humidified inlet gases, inlet pressure at 206.9 kPag, and an operating current density of 1000 mA/cm²). To initiate the testing, the anode (hydrogen) flow is switched to a dry inlet, creating a concentration gradient that favors water transport to the anode. At two minute intervals, the anode flow rate is increased in stepwise fashion over a range of stoichiometries from 1.5-10, further increasing the water removal capacity of the anode at each step. At each interval, the cell voltage, the anode pressure drop, and the cathode pressure drop are measured. Importantly, a calibration curve of the anode pressure drop at several relative humidity values is used to determine the relative humidity of the anode throughout this operation. This relative humidity measurement, and knowing the temperature and pressure, thus gives a quantitative estimate of the amount of water transported to the anode at each flow interval. Knowing this value, estimates can be made related to the magnitude of water accumulating in the porous layer and the amount accumulating in the cathode (air) flow channels. Tests showing greater water removal also show substantial increases in voltage during the test, indicative of the porous layer being saturated. To corroborate the data related to saturation of the porous layer, Synchrotron X-ray radiography at the Canadian Light Source is used with an acrylic setup that contains commercially available graphite porous layers. Water is injected through a plenum below the porous layer while heated air flows through channels above, mimicking fuel cell operation. The acrylic limits attenuation, providing a more accurate measurement of water accumulated in the porous layer. By considering a range of flow rates and temperatures, the removal rate of water from the porous layer is quantified through 2D radiographs and 3D computed tomography scans.