Side-chain degradation of perfluorosulfonic acid membranes: An ab initio study
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Chemical degradation of the side-chain of perfluorosulfonic acid (PFSA) membranes by hydroxyl radicals (dOH) is examined with electronic structure calculations. The energetics associated with homolytic bond cleavage and for the sequence of reactions involved in the degradation was determined. Results show that the degradation of side-chain begins with the cleavage of the C–S bond. The sequence of reactions of the side-chain with dOH indicates scission of the backbone yielding reactive end-groups. The kinetics of the C–S bond cleavage was studied via: (i) reaction of anionic fragment with a dOH; and (ii) decomposition of fragment radical. The activation energy for the second pathway was calculated to be ;11 kcal/mol lower requiring a change in symmetry of the molecular geometry of the sulfonate group from trigonal pyramidal to trigonal planar. This suggests that although the C–S bond may be the weakest in the side chain of a PFSA ionomer, its cleavage is kinetically hindered.
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.191
CFð½OCF2 CFðCF3 Þm O ½CF2 n SO3 H; where m 5 1 and n 5 2 for NafionÒ, m 5 0 and n 5 4 for 3M, and m 5 0 and n 5 2 for the SSC ionomer. The cost and the lifetime of a PEM fuel cell depend on the components of the MEA. Aging of MEA affects the performance (current density and cell voltage) of the cell.1 The cell shows a gradual decline in performance after ;5000 h of operation which mainly occurs due to the degradation of the membrane. The lifetime of a membrane can be affected mechanically, thermally, and by the chemical environment in which the PEM fuel cell operates.4,5 Mechanical degradation can occur due to various factors, such as microcracks or tears. The operation of PEM fuel cells at temperatures greater than 90 °C can lead to membrane breakdown, pinhole formation or morphological change in the membrane.6–9 The operation of these cells at low relative humidity also adversely affects the membrane.6–8,10 Membrane degradation can accelerate due to the presence of ionic impurities.7,11–18 The exposure of the membrane to an oxidizing medium on the anode side and to a reducing medium on the cathode side of the PEM cell can cause chemical degradation of the membrane. Researchers have speculated that the crossover of H2 or O2 to the opposite side of the cell might cause physical degradation of the membrane and affect the efficiency of the cell. But, it has been proven that crossing of the gasses is relatively slow and the loss in efficiency is only about 1–3%.7,19–24 The crossover of gases might lead to the exothermic combustion of H2, which can cause pinhole formation in the membrane.7 Chemical degradation of a membrane due to the presence of hydrogen peroxide (H2O2) in the fuel cell has been discussed in the literature.15,25,26 The formation
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Ó Materials Research Society 2012
I. INTRODUCTION
A variety of commercially available fuel cells have potential application in automobiles, portable devices (mobile phones and laptops), and station
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