Diffusion layer saturation analysis (DLSA) is introduced in order to further investigate water transport within the cell. The analysis relies on two separate experimental processes. First, an ex-situ investigation of the relative humidity of the gas streams and their resulting pressure drop is performed. Next, multiple variables of the cathode and anode gas streams are manipulated in-situ to create an evaporative driving force to remove water out of the porous layers. Multiple gas stream settings are investigated as well as the temperature set point of the cell. The ex-situ pressure drop data is used to infer how much water is being removed from the system and to begin to estimate an overall water balance. Multiple cathode and anode GDL configurations are tested in order to further investigate initial saturation conditions of the GDLs. By coupling the voltage results to the calculated water removed from the system, it can be seen which configurations had low initial voltage due to GDL oversaturation. The potential for DLSA both as a diagnostic tool and an investigative technique into multiphase flow in the porous layer is demonstrated.

Abstract:A relative humidity (RH) measurement based on pressure drop analysis is presented as a diagnostic tool to experimentally quantify the amount of excess water on the cathode side of a polymer electrolyte membrane fuel cell (PEMFC). Ex-situ pressure drop calibration curves collected at fixed RH values, used with a set of well-defined equations for the anode pressure drop, allows for an estimate of in-situ relative humidity values. During the in-situ test, a dry anode inlet stream at increasing flow rates is used to create an evaporative gradient to drive water from the cathode to the anode. This combination of techniques thus quantitatively determines the changing net cell water flux. Knowing the cathodic water production rate, the net water flux to the anode can explain the influence of liquid and vapor transport as a function of GDL selection. Experimentally obtained quantified values for the water removal rate for a variety of cathode gas diffusion layer (GDL) setups are presented, which were chosen to experimentally vary a range of water management abilities, from high to low performance. The results show that more water is transported to the anode when a GDL with poor water management capabilities is used, due to the higher levels of initial saturation occurring on the cathode. At sufficiently high concentration gradients, the anode removes more water than is produced by the reaction, allowing for the quantification of excess water saturating the cathode. The protocol is broadly accessible and applicable as a quantitative diagnostic tool of water management in PEMFCs.