Polymer electrolyte membrane fuel cell contamination

FIGURE 8.14 Voltage versus time curves with various levels of toluene at different current densities. Cell temperature = 80°C, Relative humidity = 80%, 30 psi back pressure, stoichiometry 1.5/3.0 for Hj/air. (Reprinted from Journal of Power Sources, 185, Li, H. et al. Polymer electrolyte membrane fuel cell contamination Testing and diagnosis of toluene-induced cathode degradation, 272-279, Copyright (2008), with permission from Elsevier.)... 

Li, H., Zhang, J. L., Fatih, K. et al. 2008. Polymer electrolyte membrane fuel cell contamination Testing and diagnosis of toluene-induced cathode degradation. Journal of Power Sources 185 272-279. 

The influence of a variety of contaminants in reactants and in the fuel cells themselves on the polymer electrolyte membrane fuel cell lifetime, as well as the mechanisms of this influence, have been examined in a review by Cheng et al. (2007). 

Zamel, N. and Li, X. (2011) Effect of contaminants on polymer electrolyte membrane fuel cells. Prog. Energy Com- 28. bust. Sci., 37, 292-329. 

The scientific community has made great progress in increasing the durability of polymer electrolyte membrane fuel cell (PEMFC) systems, but durability must further increase before we can consider fuel cells economically viable [1]. As durability increases, new modes of fuel cell contamination and failure are exposed. We expect state of the art PEMFC systems to run for thousands of hours. This means that each sulfonate group in typical per-fluorosulfonic acid (PFSA) membranes used in today s PEMFC systems will associate with several million protons over the lifetime of the systems. Even if other cations replace only a small fraction of the protons entering the electrolyte membrane, these contaminant cations can build up in the system and degrade the fuel cell system performance over time. 

Kienitz, B. 2009. The effects of cationic contamination on polymer electrolyte membrane fuel cells. PhD disser.. Case Western Reserve University, Cleveland, OH. 

Imamura, D. and Yamaguchi, E. 2009b. Effect of air contaminants on electrolyte degradation in polymer electrolyte membrane fuel cells. ECS Transactions 25 813-819. 

PEM fuel cells use a solid proton-conducting polymer as the electrolyte at 50-125 °C. The cathode catalysts are based on Pt alone, but because of the required tolerance to CO a combination of Pt and Ru is preferred for the anode [8]. For low-temperature (80 °C) polymer membrane fuel cells (PEMFC) colloidal Pt/Ru catalysts are currently under broad investigation. These have also been proposed for use in the direct methanol fuel cells (DMFC) or in PEMFC, which are fed with CO-contaminated hydrogen produced in on-board methanol reformers. The ultimate dispersion state of the metals is essential for CO-tolerant PEMFC, and truly alloyed Pt/Ru colloid particles of less than 2-nm size seem to fulfill these requirements [4a,b,d,8a,c,66j. Alternatively, bimetallic Pt/Ru PEM catalysts have been developed for the same purpose, where nonalloyed Pt nanoparticles <2nm and Ru particles <1 nm are dispersed on the carbon support [8c]. From the results it can be concluded that a Pt/Ru interface is essential for the CO tolerance of the catalyst regardless of whether the precious metals are alloyed. For the manufacture of DMFC catalysts, in... 

The contaminants in a fuel cell system, if not effectively mitigated, can result in fuel cell performance degradation and premature failure of the fuel cell system. The adverse effect of certain contaminants (e.g., CO, H2S, NH4) on fuel cell performance can be detected quickly. These effects tend to be somewhat reversible. The effects of other contaminants are not easily detected, and tend to be irreversible. For example, certain trace metallic ions (Fe" % Cu ) can catalyze the decomposition of polymer electrolyte membranes without affecting ionic conductivity or the gas crossover rate of the membrane until... 

Cationic contaminants may emanate from many sources. Metals, such as iron and copper, in system components may ionize due to corrosion exchange with protons in the membrane. Metallic salts, such as sodium and calcium, may enter the fuel cell from coastal water or from deicing agents. The most likely source of cationic contaminants is from the fuel line. Hydrogen from reformed hydrocarbons usually contain parts per million (ppm) of ammonia. This ammonia can be oxidized to ammonium ions and enter the polymer electrolyte. 

Cationic contaminants tend to build up in the polymer electrolyte. This is because the sulfonate sites have a higher affinity for most other cations than protons and because most other cations do not partake in a suitable reaction to exit the polymer electrolyte phase [2,3]. In the case of ammonia, there is a suitable reaction at the cathode to remove ammonium ions from the system, but this reaction is likely slower than proton reduction. Some other metal ions, such as copper and cobalt, are electrochemically active in the fuel cell potential window and tend to "plate out" of the system. In general, once a cationic contaminant is in the polymer electrolyte phase it tends to stay there until the membrane has an acid treatment. 

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