Air-breathing polymer electrolyte membrane fuel cells (ABFCs) use free convection airflow to supply oxygen to their cathodes. These cells are typically characterized by low output power densities compared with forced-convection fuel cells. Because ABFC designs rely on natural convection air delivery, cathode performance is often the performance bottleneck. This paper specifically examines the tradeoff between mass transport losses and ohmic electrical resistance losses for an optimal ABFC cathode design. Optimization is nontrivial because the simultaneous requirements for excellent cell compression, current collection, and gas access are often in contradiction. Simple scaling analysis and experimental observations suggest that the tradeoff between lateral mass transport resistance losses and cathode/gas diffusion layer (GDL) contact resistance losses determines the optimal ABFC cathode design. In order to quantitatively study these effects, we have tested a series of different cathode geometries in a standardized ABFC. Using high frequency resistance measurements and fast-scan polarization measurements, we have been able to interrogate both the ohmic and mass transport losses associated with each cathode configuration. We have also used pressure sensitive foils to examine the pressure distribution for representative configurations, providing a quantitative link between pressure distribution and cell resistance. Finally, we have studied the effect of deploying a current collecting contact layer between the cathode and the GDL. Results indicate that the deployment of a sufficiently stiff yet highly porous contact layer significantly reduces contact resistance losses while imposing minimal additional mass transport losses. A stiff yet porous contact layer reduces the contact resistance losses by increasing the total contact surface area and providing a more even distribution of pressure across the face of the cell. By minimizing contact resistance losses, this strategy enables the deployment of ABFC cathode structures with greater than 90% open area, thereby leading to an enhanced ABFC performance, particularly at high current densities.
Impact of Hydrophobic Coating on Mass Transport Losses in PEFCs
