Long Term Effects of Airborne Contaminants in PEMFC Cathode

Air pollution is a challenge for the commercialization of proton exchange membrane fuel cells (PEMFCs) [1]. Recently, twenty-one species were selected from a list of more than 200 airborne pollutants (US Environmental Protection Agency database) for single cell accelerated tests [2]. Results showed that most contaminants differently affected cell performance. Seven of these twenty-one species were down selected for detailed studies based on quantitative criteria [2]. The seven contaminants are acetonitrile, acetylene, bromomethane, iso-propanol, methyl methacrylate, naphthalene and propene [2]. All seven contaminants can adsorb on the Pt surface, reduce the effective electrochemical active area for the oxygen reduction reaction (ORR), and even impact the ORR pathway and increase the formation of hydrogen peroxide (H₂O₂) [3] that promotes ionomer decomposition [4]. Acetonitrile also decreases the proton conductivity of the membrane. The larger H₂O₂ production rate determined with a rotating ring/disc electrode maintained at 30 °C is not expected to be representative of a PEMFC. Therefore, it is currently unclear if long term operation of a PEMFC exposed to a contaminant will lead to faster catalyst layer ionomer and membrane degradation.

A long term test with acetonitrile injected in the cathode was conducted. Acetonitrile was selected because it has several impacts on cell performance and promotes the formation of H₂O₂. Electrochemical impedance spectroscopy (EIS) was intermittently used to obtain information on cell processes with minimal disturbance to the test. Cyclic voltammetry (CV) was applied before and after the long term test to measure the decrease in electrochemically active Pt sites. Transmission electron microscopy (TEM) was employed to analyze Pt particles agglomeration and Pt deposition in the electrolyte membrane. Inductively coupled plasma mass spectroscopy (ICP-MS) was utilized to detect Pt dissolution in exhaust water. Ion chromatography (IC) was exploited to measure the F^– emission rate in the exhaust water and identify other ions including NH₄⁺. Scanning electron microscopy (SEM) was used to investigate thickness changes of catalyst layers and membrane.

Figure 1 shows the cell voltage and high frequency resistance responses during the long term acetonitrile contamination experiment. Initially, the cell voltage at 0.4 A cm^–2 was 0.667 V and the high frequency resistance (HFR) was ~0.1 Ω cm². Within the 300 h pre-poisoning period, the cell voltage decreased with a degradation rate of 1.18 x 10^–4 V h^–1 and the HFR stabilized at 0.088 Ω cm² at 220 h. During the first 20 h of acetonitrile exposure, the cell voltage dropped by 0.13 V and the HFR increased by 0.021 Ω cm2. Interestingly, the cell performance was stable during the following ~600 h even though the HFR continued its slow increase. The cell voltage eventually decreased by ~0.02 V during the last 400 h of exposure to acetonitrile. For the recovery phase, the cell voltage and HFR were respectively restored to 0.551 V and 0.109 Ω cm² during the first 20 h. After that period, an accidental interruption occurred leading to a 4 min OCV event that subsequently increased the voltage and decreased the HFR. After that event, the cell voltage resumed its decay with a degradation rate of 1.40 x 10^–4 V h^–1. At around 425 h of the recovery period, a second interruption occurred resulting in a cell voltage jump of ~0.02 V which was followed by degradation with a similar rate after the first interruption. The long term exposure to acetonitrile appears to mitigate the regular degradation of cell performance; despite the initial performance loss due to acetonitrile adsorption on Pt and the continuing HFR increase attributed to the effect of acetonitrile reaction products on ionomer electrolyte. The OCV operation may also accelerate the cell performance recovery.

The presence of long term contaminant effects was ascertained by focusing on Pt catalysts and ionomer electrolyte degradation. The Pt catalysts’ degradation was probed by comparing EIS, CV, ICP-MS and TEM results with control samples. The ionomer degradation was explored by comparing EIS, IC and SEM results with control samples.

ACKNOWLEDGMENTS

This work is supported by the Department of Energy (DE-EE0000467). Authors are also grateful to the Hawaiian Electric Company for their ongoing support to the operations of the Hawaii Sustainable Energy Research Facility.

REFERENCES

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