Radical (HO_•, H_• and HOO_•) Formation and Ionomer Degradation in Polymer Electrolyte Fuel Cells

Abstract

Formation of radicals, such as HO , H and HOO , in the membrane of the polymer electrolyte fuel cell and their attack on perfluoroalkylsulfonic acid (PFSA) and poly(styrenesulfonic acid) (PSSA) ionomers was simulated based on a kinetic framework with H 2 O 2 as "parent" molecule and with contaminating Fe as parameter. Analysis under quasi-steady state conditions yielded radical concentrations of around 10 À19 M for H , 10 À16 M for HO and 10 À10 M for HOO . H is formed via the reaction of HO with H 2 dissolved in the membrane. The attack of the PFSA ionomer was assumed to proceed via weak carboxylic end-groups. The corresponding calculated fluoride emission rate (FER) showed good agreement with experimental data under ex situ Fenton test conditions. The predicted FER under fuel cell operating conditions was underestimated by 2-3 orders of magnitude. It is likely that degradation via side-chain attack is prevalent during open circuit voltage hold tests. The oxidative degradation of PSSA ionomer follows an entirely different pathway, because, in addition to a-hydrogen abstraction by HO , the aromatic ring effectively scavenges HO to form an OH-adduct. Follow-up reactions lead to chain scission and formation of a stable hydroxylated degradation product.The proton conducting membrane in the polymer electrolyte fuel cell (PEFC) is exposed to considerable oxidative stress due to the presence of reactive intermediates formed in the membrane electrode assembly (MEA), which attack the electrolyte membrane and lead to chain scission, loss of polymer constituents, membrane thinning and, eventually, failure of the cell. 1,2 The chemical breakdown of the polymer additionally causes loss of mechanical integrity with ensuing mechanical failure of the membrane due to pinhole or crack formation. 3 The nature of the reactive intermediates formed during electrochemical H 2 and O 2 conversion in the PEFC has been discussed for many years in many articles. The insight that H 2 O 2 is involved in membrane degradation was already gained in the 1960s. 4,5 The observation that iron or other redox-active metal ions appeared to accelerate the degradation led to the suggestion that oxygen radicals are produced through the metal-ion-catalyzed decomposition of hydrogen peroxide (Fenton reaction). The working hypothesis that hydroxyl radicals (HO ) and hydroperoxyl radicals (HOO ) are the responsible species for membrane degradation has been accepted for a long time. Direct detection of radical intermediates in fuel cells was accomplished only recently by introducing spin-trapping agents into the MEA and placing a miniature fuel cell into an electron spin resonance (ESR) spectrometer. Using this approach, Roduner et al. proposed that HO and polymer derived carbon centered radicals are formed. 6 More recently, Schlick et al., using a similar approach, proved the presence of carbon centered radicals, HO , HOO as well as H in an operating fuel cell. 7 The reaction mechanisms leading to the formation of radical intermediates have been the subject ...

In-Situ Modeling of Chemical Membrane Degradation and Mitigation in Ceria-Supported Fuel Cells

“…22,23 The hydroxyl radical formed is highly reactive and can attack the ionomer membrane in terms of side chain cleavage and unzipping 18,20,21,24,25 which is responsible for the deteriorated physicochemical properties observed in degraded membranes. Radical scavenging is therefore proposed to mitigate the chemical damage by quenching the radicals before they attack the ionomer membrane.…”

Effect of CeOx Crystallite Size on the Chemical Stability of CeOx Nanoparticles

“…Fe 2+ /Fe 3+ ) greatly accelerates the decomposition of H 2 O 2 into aggressive free radicals. [6][7][8] These free radicals are known to attack PFSA membranes through an "unzipping" mechanism in which decomposition is initiated at one end of a PFSA unit and continues until complete decomposition into HF, CO 2 and low molecular weight compounds has occurred. [3][4][5]9 As a result of free radical attack, membrane thinning, and eventually complete failure (due to pinhole formation), can occur.…”

Local Impact of Pt Nanodeposits on Ionomer Decomposition in Polymer Electrolyte Membranes

“…Chemical degradation is linked to the attack of the weak bonds in ionomer side chains by radical species. The main type of radical species, hydroxyl radicals, is formed by the reaction of hydrogen peroxide, H 2 O 2 , at impurities such as Fe 2+ [12,13], which are present in small amounts due to the dissolution of end plates in contact with humidified oxygen and hydrogen [14][15][16]. H 2 O 2 -the main chemical compound responsible for chemical PEM degradation-is the product of surface reactions of H 2 and O 2 at PITM [17][18][19][20][21][22][23].…”

“…The net rate of H 2 O 2 formation at PITM depends on the size, shape, and distribution of Pt nanoparticles in the membrane and local reaction conditions around particles, determined by temperature, electrical potential, reactant concentrations, pH, and RH [12][13][14][15][16][17][18][24][25][26][27][28][29][30][31].…”