Abstract
The impact of model membrane degradation compounds on the relevant electrochemical parameters for the oxygen reduction reaction (i.e. electrochemical surface area and catalytic activity), was studied for both polycrystalline Pt and carbon supported Pt electrocatalysts. Model compounds, representing previously published, experimentally determined polymer electrolyte membrane degradation products, were in the form of perfluorinated organic acids that contained combinations of carboxylic and/or sulfonic acid functionality. Perfluorinated carboxylic acids of carbon chain length C1 – C6 were found to have an impact on electrochemical surface area (ECA). The longest chain length acid also hindered the observed oxygen reduction reaction (ORR) performance, resulting in a 17% loss in kinetic current (determined at 0.9 V). Model compounds containing sulfonic acid functional groups alone did not show an effect on Pt ECA or ORR activity. Greater than a 44% loss in ORR activity at 0.9 V was observed for diacid model compounds DA-Naf (perfluoro(2-methyl-3-oxa-5-sulfonic pentanoic) acid) and DA-3M (perfluoro(4-sulfonic butanoic) acid), which contained both sulfonic and carboxylic acid functionalities.
Highlights
Adsorption Impact of Perfluorosulfonic acids (SA1 and SA2).— Electrochemical results for the model compound SA1, shown in Figure 2, overlapped with the baseline indicating that under the experimental conditions tested, the compound is non-adsorbing on polycrystalline Pt in the studied potential window (0.04–1.2 V)
Low concentrations and different structural nature of the compound used in this study may contribute to differences in the observations. Results of this SA1 study showed that the sulfonic acid functional group, along with the attached fluorocarbon chain, is very weakly or non-adsorbing on the polycrystalline Pt surface when SA1 is present at low concentrations (≤0.1 mM)
DA-Naf and DA-3M are experimentally determined compounds derived from the decomposition of Nafion and 3M polymer electrolyte membranes, respectively, Due to the diacid nature of compounds, several additional model compounds were selected to better isolate adsorption effects of the individual functional groups as well as perfluorocarbon chain length
Summary
Numerous studies focusing on the performance impact of impurities found in the anode fuel stream (e.g. CO, CO2, H2S, NH3, CH4 and HCOOH), as well as common airborne contaminants present in the cathode stream (e.g. SOx and NOx) have been conducted.[3,4,5,6,7,8,9,10,11] In addition, various research groups have examined the impact of aromatic contaminants and environmentally common anionic and cationic species.[12,13,14,15,16,17,18] Additional studies have investigated the performance impact of foreign metal ions (e.g. Fe3+, Ni2+, Cu2+, Cr3+, Al3+ and Co2+), originating from the bipolar plates and catalyst layer.[19,20,21,22,23,24,25] Results of these studies show, in many cases, severe effects on fuel cell performance due in large to anion adsorption and irreversible chemisorption of poisoning compounds on the catalyst layer, as well as foreign cation uptake in the membrane. Much of the previous literature mentioned has focused on insitu experiments monitoring the effects contaminating species have on overall fuel cell performance While these studies are useful for providing information on performance degradation resulting from realistic and/or real world operating conditions, they lack insight into which specific PEMFC components are affected and to what extent. Through ex-situ experiments can the effect from anode, membrane cross-over, and oxygen diffusion limitations be eliminated and specific adsorption and kinetics occurring at the catalyst surface be determined.[9] there have been numerous ex-situ studies investigating the impact common anions (e.g. Cl− and Br−) have on ORR activity, only a small number of studies have examined other possible fuel cell contaminants.[8,9,36,37,38,39] This work utilizes ex-situ cyclic voltammetry (CV) and linear sweep voltammetry (LSV) using a rotating disk electrode (RDE) to investigate the effects of seven model compounds (see Figure 1) on electrocatalyst performance. DA-Naf and DA-3M were obtained in their lithium salt forms from collaborators at 3M at a reported purity of >95%, with the
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