(376a) Impact of Ionomer Resistance in Nanofiber-Nanoparticle Electrodes for Ultra-Low Platinum Fuel Cells

Recently, in our laboratory, a new simultaneous electrospinning and electrospraying (E/E) process was developed to produce unique nanofiber-nanoparticle electrodes for polymer electrolyte membrane (PEM) fuel cells that resulted in high power densities at ultra-low platinum (Pt) loadings. These results demonstrated the impact of electrode (catalyst layer) morphology on fuel cell performance. Although promising, these previous results did not explore the impact of all of the electrode morphology parameters (e.g., fiber/catalyst content, fiber size, fiber/catalyst structure, porosity, etc.) on power density (kW/cm²), Pt loading (mg/cm²), and subsequently Pt utilization (kW/mg). In this study, E/E electrodes were systematically fabricated to fundamentally investigate the impact of the ionomer (Nafion) in the catalyst layer on transport resistances. To our knowledge, previous studies have not been able to independently study mass transfer resistance, proton transfer resistance, and porosity of the catalyst layer on fuel cell performance, because of their coupled effects within the catalyst layer. Utilizing the E/E process, the effect of the ionomer in the electrodes on mass transfer and proton transfer resistances can be investigated separately, which will significantly impact the design of fuel cell catalyst layers and elucidate various transport phenomena effects in the catalyst layer for ultra-low Pt loadings. Overall, the results show an increase in fuel cell power density with increasing ionomer content in nanofiber-nanoparticle electrodes until a specific ionomer content is reached. After this optimal ionomer content, the fuel cell power density decreases with increasing ionomer content. Furthermore, results show that at zero to low ionomer content, there is insignificant difference in power density, yet the catalyst layer resistance decreases due to the increase of triple phase boundary in the catalyst layer, i.e., decrease in proton transfer resistance. As the ionomer content increases further, the catalyst layer resistance begins to increase because of the oxygen mass transfer resistance due to the ionomer thin film resistance. Therefore, there is an optimum ionomer content to minimize proton transfer resistance and oxygen mass transfer resistance. Furthermore, the oxygen mass transfer resistance seems to have more of an impact on the power density than the proton transfer resistance.