Yu, Ted H. and Sha, Yao and Liu, Wei-Guang and Merinov, Boris V. and Shirvanian, Pezhman and Goddard, William A. III (2011) Mechanism for Degradation of Nafion in PEM Fuel Cells from Quantum Mechanics Calculations. Journal of the American Chemical Society, 133 (49). pp. 19857-19863. ISSN 0002-7863 http://resolver.caltech.edu/CaltechAUTHORS:20120214-070031098
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We report results of quantum mechanics (QM) mechanistic studies of Nafion membrane degradation in a polymer electrolyte membrane (PEM) fuel cell. Experiments suggest that Nafion degradation is caused by generation of trace radical species (such as OH^●, H^●) only when in the presence of H_2, O_2, and Pt. We use density functional theory (DFT) to construct the potential energy surfaces for various plausible reactions involving intermediates that might be formed when Nafion is exposed to H_2 (or H^+) and O_2 in the presence of the Pt catalyst. We find a barrier of 0.53 eV for OH radical formation from HOOH chemisorbed on Pt(111) and of 0.76 eV from chemisorbed OOH_(ad), suggesting that OH might be present during the ORR, particularly when the fuel cell is turned on and off. Based on the QM, we propose two chemical mechanisms for OH radical attack on the Nafion polymer: (1) OH attack on the S–C bond to form H_2SO_4 plus a carbon radical (barrier: 0.96 eV) followed by decomposition of the carbon radical to form an epoxide (barrier: 1.40 eV). (2) OH attack on H_2 crossover gas to form hydrogen radical (barrier: 0.04 eV), which subsequently attacks a C–F bond to form HF plus carbon radicals (barrier as low as 1.00 eV). This carbon radical can then decompose to form a ketone plus a carbon radical with a barrier of 0.86 eV. The products (HF, OCF_2, SCF_2) of these proposed mechanisms have all been observed by F NMR in the fuel cell exit gases along with the decrease in pH expected from our mechanism.
|Additional Information:||© 2011 American Chemical Society. Received: August 8, 2011. Publication Date (Web): October 21, 2011. This work was supported with funding from the Ford Motor Company.|
|Official Citation:||Mechanism for Degradation of Nafion in PEM Fuel Cells from Quantum Mechanics Calculations Ted H. Yu, Yao Sha, Wei-Guang Liu, Boris V. Merinov, Pezhman Shirvanian, and William A. Goddard III Journal of the American Chemical Society 2011 133 (49), 19857-19863|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||14 Feb 2012 15:36|
|Last Modified:||26 Dec 2012 14:50|
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