The performance of PEMFCs has been linked to the porosity of the catalyst/carbon/ionomer composite in the cathode catalyst layer (CL), and this can be controlled to some degree by the equivalent weight (EW) of the ionomer.(1) Here, we study impact on PEMFC performance when the cathode CL contains a low equivalent weight (LEW) ionomer having short-side-chains (SSCs). Inks for the CLs are prepared using Pt/C electrocatalysts and either 27 wt.% Aquivion® SSC perfluorosulfonic acid ionomers (i.e. Aquivion® ionomer 980 EW, 830 EW and 700 EW) or a standard 33 wt.% long-side chain (LSC) Nafion® 1100 EW ionomer. The Pt/C/ionomer ink is directly deposited onto the Nafion proton exchange membrane by direct ultrasonic spray deposition to form catalyst-coated membranes (CCMs). The effects of ionomer EW on the surface area, pore structure and morphology of the prepared cathode CLs are surveyed by nitrogen adsorption and scanning electron microscopy (SEM). The CCMs are assembled into membrane electrode assemblies (MEAs), and the performance of the resulting MEAs are analyzed by I-V polarizations at different cell operating temperatures (i.e. 95, 80, 70 and 40°C) and relative humidity (i.e. 100, 70, 50 and 25% RH) in both air and oxygen environments. Cyclic voltammetry is used to determine the cathode CLs Pt ECSA and Pt utilization. In situ electrochemical impedance spectroscopy is used to understand the polarization losses in the cells, particularly in the mass transport region where electrode porosity has a large impact on flooding.
Figure 1 compares the average polarization curves measured for 5 MEAs employing the SSC ionomer in their cathode CLs (i.e. Aquivion® 830 EW) to 5 MEAs employing the LSC ionomer in their cathode CLs (i.e. Nafion® 1100EW) at 80°C fed with ambient pressure air humidified at 100 % and 50 % RH. In the kinetic region (i.e. E ≥ 0.80 V) of the polarization curves, the cell performance of both set of MEAs are identical at both 100 and 50% RH. In the mass transport region at high current densities, the MEAs containing the SSC ionomer in their cathode CLs perform better than those containing the LSC ionomer in their cathode CLs under both moderate (50%) and high (100%) relative humidity. Our results appear to be unique because others have only seen improvement in PEMFCs with SSC LEW ionomers under low RH conditions, while we see performance improvements from the LEW SSC ionomers across the full range of RH. (1, 2)
This presentation will discuss how the SSC ionomer affects the physical attributes of the MEA (porosity, impedance), resulting in higher performance at high current densities independent of the RH.
Figure 1. Comparison of the average polarization curves measured for 5 Pt/VC MEAs employing the SSC ionomer (Aquivion® 830EW) in their cathode CLs to 5 Pt/VC MEAs employing the LSC ionomer (Nafion® 1100EW) in their cathode CLs at 80°C fed with ambient pressure air humidified at 50% and 100% RH.
1. C. Lei, D. Bessarabov, S. Y. Ye, Z. Xie, S. Holdcroft and T. Navessin, J Power Sources, 196, 6168 (2011).
2. Y. C. Park, K. Kakinuma, H. Uchida, M. Watanabe and M. Uchida, J Power Sources, 275, 384 (2015).