Degradation Mitigation in PEM Fuel Cells Using Metal Nanoparticle

Downloaded by UNIV OF PITTSBURGH on December 24, 2014 | http://pubs.acs.org Publication Date (Web): March 11, 2010 | doi: 10.1021/bk-2010-1034.ch013

Chapter 13

Degradation Mitigation in PEM Fuel Cells Using Metal Nanoparticle and Metal Oxide Additives Panagiotis Trogadas,1 Javier Parrondo,2 and Vijay Ramani*,1 1Department

of Chemical and Biological Engineering, Illinois Institute of Technology, Chicago, IL 60616 2Facultad de Ciencia y Technologia, University of the Basque Country, Leioa, Spain 48940 *[email protected]

The efficacy of added metal oxides and metal nanoparticles in mitigating free radical induced polymer electrolyte membrane (PEM) degradation was investigated. Freestanding and silica supported platinum, palladium, silver and gold nanoparticles, cerium oxide and manganese oxide supports, and ceria supported platinum nanoparticles were prepared. The nanoparticles were characterized by TEM and XRD to determine the particle and crystallite size. Their radical scavenging tendency was estimated by UV-vis spectroscopy using a model free radical (DPPH) as a test species. Composite membranes were prepared by adding 3 wt% of the freestanding or supported metal nanoparticles to Nafion®, followed by solvent casting. The fluoride emission rate (FER) was ascertained for each membrane from accelerated tests. The addition of Au, Pd, Pt and Ag nanoparticles led to lowering of FER by an order of magnitude, 75%, 60% and 35% respectively while the addition of MnO2, CeO2 and Pt on CeO2 nanoparticles resulted in an order of magnitude FER reduction, indicating effective radical scavenging by the nanoparticles. Hence, the addition of metal nanoparticles and metal oxides with radical scavenging abilities is a promising route to mitigate PEM degradation.

© 2010 American Chemical Society In Functional Polymer Nanocomposites for Energy Storage and Conversion; Wang, Q., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Introduction Membrane degradation is one of the most important factors limiting the lifetime of polymer electrolyte fuel cells (PEFCs) (1–10). Membrane degradation in a PEFC occurs via a multistep mechanism. The two major steps are: (i) formation of reactive oxygen radicals by reaction of hydrogen peroxide (generated in situ) with trace metal ions in the membrane electrode assembly (MEA) (11), and (ii) attack of weak links in the polymer backbone or side-chains by the radicals. Hydroxyl radicals (·OH) have been identified as the highly aggressive oxidative species responsible for the propagation of chemical degradation of perfluorosulfonic acid membranes (2–4, 11–15). Mechanisms by which radicals attack the polymer have been proposed by various researchers (11, 16, 17). For Nafion® membranes, the commonly proposed mechanism is hydrogen abstraction from the reactive end groups (such as COOH) in the proton exchange membrane (PEM), leading to membrane decay (5). Three approaches can be used to minimize the effect of reactive oxygen species in a fuel cell: (i) the use of free radical scavengers in the electrolyte (17, 18); (ii) the use of dispersed peroxide decomposition catalysts within the electrolyte (19); and (iii) the use of dispersed peroxide decomposition catalysts in the electrodes (20–23). The first approach was used in our previous work (18) using CeO2 nanoparticles as the free radical scavenger. We showed that the incorporation of these nanoparticles within a recast Nafion® membrane led to lowering the fluoride emission rate (obtained from degradation accelerated tests) by more than 1 order of magnitude, suggesting that CeO2 nanoparticles have tremendous potential to greatly enhance membrane durability. In this study, we investigate the efficacy of metal nanoparticle based free radical scavengers, namely, platinum, palladium, gold and silver nanoparticles, in a PEFC environment. There are previous reports about the incorporation of metal nanoparticles or metal oxides within a polymer electrolyte membrane (PEM) (24–26). Watanabe and coworkers (24–26) proposed new PEMs (Pt-PEM, TiO2-PEM, Pt-TiO2-PEM) with highly dispersed nanometer-size Pt and/or metal oxides. The Pt particles were expected to inhibit the crossover by the catalytic recombination of crossover H2 and O2. The hygroscopic oxide particles were expected to adsorb the water produced at Pt particles together with that produced as reaction product at the cathode and to release the water under low humidity conditions. The PEFCs with these new PEMs demonstrated the superior performances and suppression of the crossover even under non-humidified condition. It has been reported that the incorporation of hydrophilic metal oxide particles such as SiO2, TiO2, ZrO2, Al2O3 (27–37) within PEM leads to an enhancement of the water retention properties and resulting proton conductivities of PEMs under high temperature operating conditions. However, the excessive incorporation of these nonconductive inorganic compounds in PEM results in a decrease of proton conductivity (29, 35–37). To minimize the loss of proton conductivity caused by the addition of the inorganic compound, sulfonated groups are often grafted onto the surfaces of these inorganic compounds (38–41). 188 In Functional Polymer Nanocomposites for Energy Storage and Conversion; Wang, Q., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Several reports are available regarding the antioxidant function of metal nanoparticles (42–46). Esumi and coworkers (43) demonstrated that gold-chitosan nanocomposites had a hydroxyl radical elimination activity that was 80 times higher than that of ascorbic acid, which is a known antioxidant (47). The same research group (44) prepared gold-dendrimer nanocomposites in the presence of poly(amidoamine) (PAMAM) dendrimer. The catalytic activities of these nanocomposites for hydroxyl radical scavenging was 85 times higher than that of ascorbic acid and was found to be independent of dendrimer concentration. Endo and coworkers (46) investigated the catalytic activity of gold-platinum, gold-palladium and platinum-palladium dendrimer nanocomposites for scavenging 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals. The bimetallic-dendrimer nanocomposites exhibited higher catalytic activity for the scavenging reaction of DPPH radicals than that of monometallic nanocomposites. Additionally, cerium and manganese ions have been reported to be highly effective mitigants of polymer membrane chemical degradation (17). Ce3+ and Mn2+ undergo redox reactions with hydroxyl radical (·OH) to produce H2O and the corresponding oxidized cation (Ce4+ and Mn3+). The greater effectiveness of Ce3+ relative to Mn2+ is tied to its faster rate of reaction with hydroxyl radicals. The oxidized cations must be reduced in order to capture additional hydroxyl radicals. Both Ce4+ and Mn3+ are reported to oxidize H2O2 to oxygen (17, 48–50) since both are very strong oxidizing agents with high reduction potentials of 1.72 and 1.54 V (vs NHE). The present study is aimed at examining and potentially applying the free radical scavenging properties of selected metal nanoparticles (Au, Ag, Pt and Pd) to mitigate free radical induced PEM degradation. Nafion® is used as a model PEM and the fluoride emission rate (FER) is used as the primary metric to monitor PEM degradation. In an attempt to minimize the potential of electrical shorting of the membrane, silica (SiO2) has been used to support the metal nanoparticles. Moreover, the effect of ceria (CeO2) particle size on its scavenging properties was investigated as ceria nanoparticles with varying particle sizes were incorporated within the PEM. Ceria supported Pt nanoparticles were also studied. The presence of platinum (based on the degradation mitigation reaction mechanism) should enhance the reduction of Ce+4 to Ce+3 ions, increasing the amount of hydroxyl radical scavengers available on CeO2 surface and hence reducing the amount of harmful radicals present on composite Nafion® membrane.

Experimental Reagents Concentrated ammonia (28% w/w aqueous solution), silver nitrate (AgNO3), palladium chloride (PdCl2), tetraethoxysilane (99.9%), concentrated ammonium hydroxide (28% w/w aqueous solution) and trimetylammonium bromide (CTAB) were purchased from Alfa Aesar. Arabic gum, D-glucose 189 In Functional Polymer Nanocomposites for Energy Storage and Conversion; Wang, Q., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


(C6H12O6), tetraammineplatinum(II) chloride hydrate (Pt(NH3)4Cl2·xH2O), sodium citrate (HOC(COONa)(CH2COONa)2·2H2O), chloroauric acid (HAuCl4), tetraoctylammonium bromide (CH3(CH2)7]4N(Br)) in toluene solution, sodium borohydride (NaBH4), dodecanethiol (CH3(CH2)11SH), hexachloroplatinic acid (H2PtCl6), hexamethylenetetramine, cerium nitrate hexahydrate, cerium oxide nanopowder (CeO2,

Degradation Mitigation in PEM Fuel Cells Using Metal Nanoparticle

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