Abstract
Air pollution is a challenge for the commercialization of proton exchange membrane fuel cells (PEMFCs) due to the detrimental impact of pollutants on cell performance and durability (1-5). Some of these pollutants adsorb onto the Pt surface and compete with the crucial oxygen reduction reaction (ORR). Contaminants such as chloride anions also undergo an irreversible complexing reaction with Pt (5).
We have previously reported how chlorobenzene, a halocarbon and commodity production intermediate inhibits the ORR and causes rapid and significant performance loss when introduced into the cathode of a PEMFC (6). Chlorobenzene adsorption, reactions and molecular orientation on the Pt surface depend on the electrode potential. Cl- is created and remains in the membrane/electrode assembly (MEA). Cl-binds to the Pt surface much more strongly than chlorobenzene but is slowly flushed out by liquid water.
We have recently carried out PEMFC-cathode poisoning studies with bromomethane, another halocarbon, is a solvent and a chemical manufacture precursor (7). Chlorobenzene and bromomethane have an electron withdrawing halogen moiety. The Br- adsorbate on Pt is more strongly bound than Cl- in HClO4(8). These observations suggest that bromomethane and chlorobenzene may share similar features during PEMFC contamination, although methyl and aromatic group adsorption differ. A comparison between these 2 species supports the development of the bromomethane contamination mechanism.
Bromomethane contamination was investigated with a 50 cm2 active area single cell and a constant current of 1 A cm-2. Electrochemical impedance spectroscopy, cyclic voltammetry, linear sweep voltammetry and polarization measurements were used to characterize the temporary effect and the permanent performance loss. X-ray absorption spectroscopy was carried out in a spectro-electrochemical cell with similar procedures as for the chlorobenzene contamination study, to identify chemical changes to Pt and bromomethane and adsorbates on Pt (6,7). A contamination mechanism and a performance-recovery method were derived from these results.
While chlorobenzene poisoned the PEMFC within an hour and performance could be largely recovered with voltammetric cycling, poisoning with bromomethane took days and the losses to the PEMFC were less reversible. We surmised that unlike chlorobenzene, bromomethane is hydrolyzed to bromide and methanol before reaching the Pt surface, as shown in Figure 1a. Bromomethane also permeates through the ionomer film and is subsequently hydrolyzed on the Pt electrode. The methanol product is readily oxidized on the Pt surface, thus favoring the hydrolysis. Bromide anions outside the ionomer film cannot access the Pt surface due to Donnan exclusion. Bromide anions created at the Pt interface are difficult to remove for the same reason.
X-ray absorption near edge structure analysis revealed identical mechanisms for bromomethane and chlorobenzene adsorption on Pt above 0.3 V vs SHE. However, figure 1b illustrates that during bromomethane and bromide adsorption, the Br atom is in contact with the Pt surface within the accessible cathode potential range (0–1 V vs SHE). In contrast, for chlorobenzene, the aromatic ring is lying on the Pt surface below 0.3 V vs SHE. At cathode potentials near or lower than the point of zero charge (PZC), bromomethane, bromide and chloride desorb from the Pt surface which facilitate their removal by dissolution into liquid water in the catalyst layer. These considerations were synthesized into an effective method to recover the remaining cell performance loss after contamination injection was interrupted.
We conclude that contamination research is still an important research field as such two seemingly similar poisoning compounds behave so differently in a practical PEMFC.
Acknowledgments
The authors are grateful to the United States Department of Energy (award DE-EE0000467) and the Office of Naval Research (award N00014-13-1-0463) for financial support of this project. The 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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Fig. 1. a) Proposed bromomethane and Br- transport paths and bromomethane hydrolysis reaction, b) adsorption configurations of bromomethane and chlorobenzene on the cathode catalyst of a PEMFC under different potentials.
Figure 1