Impact of Cationic Impurities on Low-Pt Loading PEFC Cathodes

Many demanding issues still need to be addressed before full commercialization of PEFC can take place, and cationic contamination is one of the major obstacles. Foreign cations diminish proton conductivity and affect water content of the polymer electrolyte membrane, increasing the cell performance losses. Foreign cations may originate from the soil, road salts, corrosion of the cell, stack, components, as well as coolant fluids, and balance of plant (BoP) components. Among the cations, Ca²⁺ receives special attention because it is one of the most prevalent elements within our planet. In this work, sulfate (SO4 ²⁻) is selected as the anion due to its compatibility with the PEFC, considering sulfuric acid is a common catalyst and electrode characterization solution. The cation solution injection at cathode was employed to better control on dosage and to emulate cationic contamination of fuel cells under actual conditions [1].

To test the real world condition and obtain the consistent data, the commercially available the low loading catalyst (Pt: 0.1 mg /cm²) coated membrane (GORE Inc.) was used. During the contaminant test, cell was maintained at 80^◦C with anode H₂ and cathode air flow rates of 1.75 slpm and 1.66 slpm, respectively. The CaSO₄ solution of 5 ppm of total concentration was injected (130 µl/min) at the cathode inlet using nebulizer [1]. Overall, the anode/cathode RH was maintained at 25%/125%, and the anode/cathode outlet back pressure was maintained at 10 kPa/100 kPa gauge pressure (∼1.5 psig/15 psig, ambient pressure 1 atm), respectively. Figure 1 shows the cell voltage decay during the constant-current hold tests (1 A cm^-2) with and without Ca ²⁺ solution injection. It is observed that during the initial injection of water there is no significant performance loss, whereas while injecting the CaSO₄ solution (5 ppm) steep fall in cell voltage is observed. In order to get a clear understanding on factors that contribute to the severe voltage loss during the CaSO₄ injection, pressure of the cathode inlet and outlet are monitored (Figure 1 (b)). These data reveal that the increased pressure over the time is due to salt precipitation in the cathode gas diffusion layer and the flow field, and the pressure gain trend is correlated with the voltage degradation. The cell test was stopped after 350 h of CaSO₄injection due to blocking of the cathode outlet.

The polarization curves were obtained before and after the injection of CaSO₄(Figure 2), a sharp fall in voltage was observed in the case of  end of test (EOT ) curve, which indicates a very severe mass transport loss due to blocking of the cathode outlet [1-3]. These studies confirm that apart from the contaminant effect in polymer electrolyte membranes and catalyst layer’s ionomer, there is a severe impact that comes from the salt precipitation at gas diffusion layer and flow field plates. In addition, the effect of contaminant on catalyst support corrosion, membrane conductivity loss over the time also performed.

Acknowledgement

Financial support from the Department of Energy (DOE)-EERE, DE-EE0000467 (University of Hawaii, prime contractor, Jean St-Pierre, PI) is greatly acknowledged. 

Reference

1. X. Wang, J. Qi, O. Ozdemir, A. Uddin, U. Pasaogullari, L. Bonville, and T. Molter, J. Electrochem. Soc., 161, F1006 (2014).
2. M. A. Uddin, X. Wang, J. Qi, M. O. Ozdemir, L. Bonville, U. Pasaogullari, and T. Molter, ECS Trans., 61(12), 49 (2014).
3. M. A. Uddin, J. Qi, M. O. Ozdemir, L. Bonville, U. Pasaogullari, and T. Molter, ECS Trans., 61(12), 37 (2014).