Investigation of Membrane Chemical Degradation As a Function of Catalyst Platinum Loading
Authors: Spears André , Rockward Tommy , Mukundan Rangachary , Garzon Fernando H 
Published: February 2020

Membrane chemical degradation is one of many factors that can impact fuel cell durability. The fuel cell’s lifetime heavily depends on the membrane and its ability to maintain chemical and mechanical integrity. Studies indicate degradation is due to the formation of hydroxyl radicals that attack the polymer structure resulting in membrane thinning, and the release of fluoride and sulfate ions1. This consequently causes mechanical failures to transpire, e.g. cracks, tears, perforations and pinhole formation, leading to an increase in membrane degradation rate. Previous research employed supported and non-supported platinum (Pt) catalysts and PtCo alloy catalysts in their studies. Their findings showed a lower membrane degradation rate when using the alloy catalyst at low RH indicating that catalyst activity may be an important factor in membrane degradation.2

In this study, we investigated how varying the amount of Pt-supported catalyst affects membrane degradation. In order to expedite degradation, we used the DOE’s accelerated stress test (AST) for chemical stability to induce the production of free radicals.3 Free radicals are known to degrade membranes and other membrane electrode assembly (MEA) components. The focus of this research is to probe membrane degradation for MEAs with ultra-low Pt loadings (i.e. ≤ 0.1 mgPt cm-2) at each electrode to contribute to existing knowledge of membrane degradation mechanisms.
Twenty-five cm2 catalyst coated membranes (CCMs) were made by spraying catalyst inks on to Nafion® membranes (NR211, Ion Power, Inc.) using a Sono-Tek Exactacoat spray coater. The catalyst inks were comprised of 50% Pt/C (TKK) catalyst mixed with 5% ionomer solution (Dupont D521). Fuel Cell Technologies current collection endplates were used to assemble the cell, which utilized SGL’s Sigracet 25 BC gas diffusion layers and two graphite plates with triple serpentine flow channels sealed by Furon® gaskets.
The AST test conditions require the fuel cell to be held at open circuit voltage while at 90oC, 30% relative humidity and a back pressure of 150kPa. H2 and air flows were set at 10 stoics at 0.2 A/cm2.
We evaluated the fuel cell performance every 12 hours using various diagnostic tests to probe the membrane degradation. Polarization (VI) curves were measured to analyze overall fuel cell performance; electrochemical impedance spectroscopy (EIS) provided useful information on the membrane resistance, electrode kinetics and transport properties, and cyclic voltammetry allowed tracking electrochemical active surface area (ECSA). We also assessed the hydrogen permeation through the membrane using linear sweep voltammetry. In addition, a performance recovery technique similar to Zhang et al. was applied to the fuel cell to mitigate impacts of recoverable catalyst poisoning4. We also collected the effluent water for further scrutiny of membrane degradation by probing for fluoride and sulfate ions using Ion Chromatography (IC). Fluoride emission rates (FER) and sulfate emission rates (SER) were used to gauge the condition of the MEA during its lifetime.
Figure 1A shows VI curves in 24 hour increments for a fuel cell exposed to AST conditions with 0.1 mgPtcm-2 at each electrode. The results show recoverable voltage losses within the first 48 hours. This indicates that the performance losses were due to catalyst poisoning rather than membrane pinhole formation. IC results for FER are shown in Figure 1B. Although FER increased during AST conditions, fluoride loss is not impactful on performance in the initial 48 hours of testing. After 72 hours, the voltage losses are irreversible which suggest the onset of membrane failure. SER results also increased during recovery, and degradation is also evident by irreversible open circuit voltage losses and a lower ECSA. EIS results are consistent with the polarization curve data in that the charge transfer resistances remained constant over the initial 48 hours after recovery. The ECSA gradually decays during the testing with only partial surface area restoration after the recovery protocol was applied. H2 crossover results showed a constant increase during fuel cell testing further signifying membrane failure. This talk will summarize the results of these tests at various catalyst loadings and illustrate the influence of catalyst loading on membrane degradation.

Acknowledgements:

The authors wish to acknowledge the financial support of the LANL African American Partnership Program and NNSA Minority Serving Institutes Program Managers: David Canty, Jennifer Kline and Dave Rude.

References:

(1) Coms, F. D. ECS Transactions; 2008. https://doi.org/10.1149/1.2981859..
(2) Mittal, V. O.; Kunz, H. R.; Fenton, J. M.. J. Electrochem. Soc.
2006. https://doi.org/10.1149/1.2219708.
(3) United States Department of Energy Fuel Cells Technologies Office Multi-Year Research, Development, and Demonstration Plan. https://www.energy.gov/sites/prod/files/2017/05/f34/fcto_myrdd_fuel_cells.pdf
(4) Zhang, J.; Litteer, B. A.; Coms, F. D.; Makharia, R. J. Electrochem. Soc. 2012. https://doi.org/10.1149/2.063207jes.

Figure 1