Tuesday, 3 October 2017
Prince George's Exhibit Hall D/E (Gaylord National Resort and Convention Center)
In this study, we conducted an experimental test to evaluate the performance of a PEM single cell at different humidity level of the reactants (i.e. hydrogen and air). The motivation was to use the test dataset to validate a simple numerical model which predict the I-V curve of the cell when the humidity is varied. We conducted the test using a 50cm2 single cell, five-layer membrane electrode assembly composed of a Nafion 211 membrane and carbon paper as GDL. Relative humidity (RH) was varied alternatively between 50 and 100 percent both at anode and cathode side. We found that the best cell performance was obtained when RH was fixed at 100% both at anode and cathode side. On the other hand, cell performance was particularly affected when relative humidity was reduced at the cathode side. This can be explained by the decrement of water diffusion from the cathode to anode and therefore a loss in electrical conductivity. The test also showed that humidity mainly affects Ohmic losses rather than activation or overpotential losses. Ohmic losses are mainly related to water hydration in polymer electrolyte membranes.
The cell performance model considers only relationships for electro-osmotic drag, membrane ionic conductivity and diffusion coefficient of water in the membrane. Hydraulic permeability was neglected as the pressure difference between anode and cathode side was minimal during the test. Temperature, mass flow rate, and pressure were also important to estimate the water and mass balance. We fitted the model parameters to the test results and find out that cell ohmic resistance was the main relationship to be modulated at different humidity levels. In fact, there is a close link between water uptake, cell resistance, and membrane conductivity and thickness.
The model simplicity was essential because the code was aimed to be implemented in a larger fuel cell system model. The model will come useful to develop a strategy for water management.