Fluoride Emission Rate Research Articles

Modeling the Effect of Pt Precipitation on PEM Degradation

Polymer electrolyte membranes (PEM) are critical components of the fuel cell that enables conduction of protons while preventing the flow of electrons through it. Additionally, the membrane separates the fuel and the oxidant streams that react electrochemically at the anode and cathode. The degradation and failure of proton exchange membranes (PEMs) in fuel cells remain a challenge for long-life applications. For many applications, a high durability of the PEM is required, often 5–20 khr. Accurate lifetime predictions are critical for the commercial viability of these applications because such long times are not usually available to assure the lifetime of the cell-stack design before fielding significant numbers of commercial units. The membrane in fuel cell conditions is subjects to chemical and mechanical degradation. The chemical degradation is catalyzed by Pt particles precipitated in the membrane during operation. Pt is a commonly used catalyst in the fuel cell electrodes. Instability of Pt at high potentials is greater and is aggravated by potential cycling. The Pt ions dissolved at the cathode, diffuse through the membrane towards the anode and precipitates in the membrane forming narrow band of Pt particles. The location of the Pt band in the membrane is governed by potential of Pt particles in the membrane, which is determined by competition of oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) at the particles [1]. The potential of Pt particle is high near the cathode and abruptly drops in some point X0 in the membrane bulk. Dissolved in the membrane Pt ions are reduced to Pt atoms in vicinity of X0 and then are agglomerated into Pt particles, which form Pt band [2]. Cross-overs of oxygen and hydrogen in the membrane react with each other at the Pt in the membrane. At high potential, catalytic oxygen reduction reaction (ORR) proceeds with H2O formation, while ORR involves H2O2 formation as by product at low potential. Direct formation of OH radicals by ORR is possible at intermediate potentials [3], which holds in the Pt band in the membrane. Therefore, oxygen cross-over is reduced in the membrane with generation of free OH radicals. The radicals attack the polymer molecules of the membrane causing chemical degradation which is accompanied by generation of hydrogen fluoride (HF). The rate of chemical degradation of the membrane is measured experimentally by fluoride emission rate (FER). Intensive chemical degradation leads to pinholes formation in vicinity of Pt band in the membrane [2,4]. A three-layer 1D model, including the anode, cathode, and membrane, was developed to describe the chemical degradation of the membrane. The model predicts dependence of FER on key factors that impact membrane degradation: amount of platinum, membrane thickness, the hydration level of the membrane, the concentration of the crossover reactants, and the temperature. Unexpected dependences of FER on Pt particles size and membrane thickness are predicted by the model. For low Pt loading in the membrane (small spacing between particles) dependences of FER on the Pt particle size and the membrane thickness have a maximum. That is due to twofold role of Pt in the membrane. On the one hand, the Pt particles are sources for free OH radicals in the membrane. On the other hand, the Pt particles are sinks for OH radicals due to diffusion-controlled quenching of the radicals at Pt surface. Competition of these two processes results in non-monotonic dependence of membrane degradation rate on Pt loading. The low loading of small Pt particles in membrane causes fast degradation of the membrane. In contrast, the high loading of large Pt particles in the membrane mitigates membrane from chemical degradation.

Fuel Cell Durability Enhancement with Cerium Oxide under Combined Chemical and Mechanical Membrane Degradation

A CeO2 supported membrane electrode assembly (MEA) was fabricated by hot-pressing CeO2-coated electrodes and a PFSA ionomer membrane. Upon application of a combined chemical and mechanical accelerated stress test (AST), the CeO2 supported MEA showed six times longer lifetime and 40 times lower fluoride emission rate than a baseline MEA without cerium. The membrane in the CeO2 supported MEA effectively retained its original thickness and ductility despite the highly aggressive AST conditions. Most of the cerium applied on the anode migrated into the membrane and provided excellent mitigation of joint chemical and mechanical membrane degradation.

Influence of Membrane Thickness on Membrane Degradation and Platinum Agglomeration under Long-term Open Circuit Voltage Conditions

Membrane chemical degradation and platinum catalyst agglomeration under long-term open circuit voltage (OCV) conditions were investigated using three types of composite membranes with various membrane thicknesses. Hydrogen permeation increases as membrane thickness decreases, which has a significant influence on proton exchange membrane and platinum catalysts. Higher Hydrogen permeation accelerated the membrane degradation, resulting in the thinning of membrane which can be verified by fluoride emission rates (FERs). Carbon-supported platinum catalysts also experienced agglomeration under OCV conditions. The statistics of platinum size distribution demonstrated catalysts size growth, ranging from 3.83 to 6.02nm in diameter along with the increasing hydrogen crossover

Mitigation of PEM Fuel Cell Electrolyte Degradation with Metal Oxide/Nafion Nanofiber Interlayers

A new nanofiber-network material enriched with cerium oxide nanoparticles (NFCeOx) has been developed as radical trap at the membrane electrode interface. Mitigation properties of NFCeOx, oriented to the anode side and to the cathode side, were investigated in in situ OCV hold test conditions at low relative humidity (50 %) and high temperature (90 °C). The results of OCV hold testing under these conditions show that whereas MEAs integrating non-modified Nafion®-212, show a marked drop in OCV with time, and end of life at < 200 hours, an MEA comprising a NFCeOx at the anode side gave very stable OCV and a lifetime close to 1000 hours. Post mortem analysis of aged MEAs indicated lower degree of degradation for a membrane protected by NFCeOx than for the reference material. These results are in agreement with OCV profile and fluoride emission rate.

Chemistry and Degradation Mitigation Effect of Cerium Oxide in Polymer Electrolyte Membranes

One of the main obstacles to wide-spread commercialization of polymer electrolyte membrane fuel cells (PEMFC) is the short operational lifetime of perfluorosulfonic acid (PFSA) membranes used in these devices. Various radical oxygen species are considered to be the main degrading species for PFSA membranes.[1]Radical scavenging properties of cerium oxide Ce(III)/Ce(IV) redox couple in the biological environment are well-known and hence its application in PEMFC, as a degradation mitigator, has received considerable attention.[2, 3] In this work, two cerium oxide nanoparticle formulations with an order of magnitude difference in average size (2-5nm and 20-150nm) were added to 1100 EW PFSA membranes in 0.5, 1.0 and 2.0 wt% loading. We will discuss that ceria, in the acidic environment of heated/humidified membranes, is reduced to Ce(III) ions which negatively impacts proton conductivity. Both liquid and gas Fenton tests were conducted. These tests showed that the fluoride emission is reduced by an order of magnitude with the polymer end groups as the main point of attack for liquid Fenton tests and additional side-chain attack for gas Fenton tests. The extent of durability improvement is found to be independent of cerium oxide particle sizes.[4] The membranes were subjected to 94 and 500 h open-circuit voltage hold accelerated durability tests. The open-circuit voltage decay rate was shown to be decreased by half and seven-fold for the 94 h and 500 h tests respectively. In 500 h tests, ceria-containing MEAs demonstrated at least two orders of magnitude reduction in fluoride emission rates with minimal change in the integrity of the MEA while the baseline MEA underwent catastrophic failure. In the presence of cerium oxide nanoparticles, the platinum deposition in the membrane was found at previously reported distance from cathode; however, the platinum particles showed an increase in size and caused a broadening with particles reaching further into the membrane.[5][1] J.M. Fenton, M.P. Rodgers, D.K. Slattery, X. Huang, V.O. Mittal, L.J. Bonville, H.R. Kunz, Membrane Degradation Mechanisms and Accelerated Durability Testing of Proton Exchange Membrane Fuel Cells, ECS Trans. 25 (2009) 233-247.[2] P. Trogadas, J. Parrondo, V. Ramani, Degradation mitigation in polymer electrolyte membranes using cerium oxide as a regenerative free-radical scavenger, Electrochemical and Solid State Letters 11 (2008) B113-B116.[3] F.D. Coms, H. Liu, J.E. Owejan, Mitigation of perfluorosulfonic acid membrane chemical degradation using cerium and manganese ions, ECS Trans. 16 (2008) 1735-1747.[4] B.P. Pearman, N. Mohajeri, D.K. Slattery, M.D. Hampton, S. Seal, D.A. Cullen, The chemical behavior and degradation mitigation effect of cerium oxide nanoparticles in perfluorosulfonic acid polymer electrolyte membranes, Polymer Degradation and Stability 98 (2013) 1766-1772.[5] B.P. Pearman, N. Mohajeri, R.P. Brooker, M.P. Rodgers, D.K. Slattery, M.D. Hampton, D.A. Cullen, S. Seal, The degradation mitigation effect of cerium oxide in polymer electrolyte membranes in extended fuel cell durability tests, Journal of Power Sources 225 (2013) 75-83.

The Chemistry of Membrane Degradation in PEM Fuel Cells

The lifetime of proton exchange membrane (PEM) fuel cells depends largely on the durability of the PEM and is a critical issue in the commercialization of PEM fuel cell technology (1, 2). Perfluorosulfonated acid (PFSA) membranes, such as Nafion®, are the most widely used electrolyte material for PEM fuel cells.Many factors can cause PEM failure, including manufacturing and design issues, material properties, and fuel cell operation conditions. Each factor results in failure through a particular degradation mode. These modes can be classified as thermal, mechanical, and chemical modes (3-7). Because fuel cells typically operate at temperatures below that at which Nafion thermally degrades, chemical and mechanical degradation are generally considered the most important modes of membrane degradation.Although there has been a lot of progress in the area, the exact chemical membrane degradation mechanisms, the most susceptible points, and the impact of test conditions on degradation are not completely understood. The formation of radicals and their reactivity during the fuel cell operation have been confirmed as the main cause of PEM chemical degradation (8-11). These radicals are formed as a result of the reaction of crossover H2 and O2 on the surface of Pt or other metal ion contaminants (4). In addition to the electrodes, Pt particles have been observed to migrate into the membrane, producing another site for radical formation. Radical attack leads to loss of fluorine, sulfur, and organofluorine compounds from the PEM, and PEM thinning, all resulting in loss of mechanical integrity. The radical formation mechanisms and the impact of test conditions on radical formation are also not completely understood.It has been suggested that earlier versions of Nafion membranes chemically degrade through radical attack on carboxylic acid groups end groups (12-14) or at the C−S or O−C bonds in the side chain and main chain (12, 14-18), as shown in Figure 1. Chemically stabilized Nafion, e.g., Nafion 211, has been developed where the concentration of terminal carboxylic acid groups was decreased to negligible levels and, as a result, degradation decreased substantially (13). The use of radical scavengers such as ceria have increased the durability of PFSA membranes by as much as 1000-fold compared to unmitigated membranes (19). Despite the recent durability improvements, there is evidence that PFSA membranes are still susceptible to chemical damage.Typical indicators for chemical degradation of Nafion membranes include increased H2crossover, decreased ion exchange capacity, decreased proton conductivity, and decreased PEM thickness. Localized crossover tests have shown that membrane damage occurs preferably at the perimeter of the active area. The quantitative method of choice used for detecting chemical degradation is typically detection of fluoride in fuel cell outlets, reported as a fluoride emission rate (FER).Figure 1. Proposed locations of radical attack in NafionTo improve PEM chemical durability, a true understanding of the decay mechanisms is required. Models of some of the decay modes have been developed and they show a consistent but incomplete understanding of membrane decay. Through a combination of specific modeling and directed testing, an overall decay model can be developed.

The degradation study of Nafion/PTFE composite membrane in PEM fuel cell under accelerated stress tests

Accelerated stress tests (ASTs) were performed to study the degradation mechanism of Nafion/PTFE composite membrane in PEM fuel cell with intensive RH cycling and load cycling. It was recognized that the edge of membrane electrode assembly (MEA) should be carefully treated to prevent the immediate failure for excessive or non-uniform mechanical stress mainly caused by RH cycling in the early period of ASTs. A long accelerated life (over 1000 h) was obtained for MEA with an edge protection and a hot-pressing process along with a low hydrogen permeation current. In addition, the decay of open circuit voltage, the fluoride emission rate (FER) from cathode side and the polarization curves were also monitored during the test. It was verified that the chemical degradation of membrane occurred inevitably caused by radical attack (HO, HO2 and H2O2) due to the intensification of gas mutual permeation. Membrane thinning, Pt particles gathering along the interfaces, even ionomer disappearing at cathode side could be observed from TEM and SEM results. Besides, open circuit voltage under H2/N2 atmosphere of MEA was confirmed to be a rapid diagnose tool of membrane physical condition.

PEMFC에서 전극 열화가 전해질 막 열화에 미치는 영향

Until a recent day, degradation of PEMFC MEA (membrane and electrode assembly) has been studied, separated with membrane degradation and electrode degradation, respectively. But membrane and electrode were degraded coincidentally at real PEMFC operation condition. During simultaneous degradation, there was interaction between membrane degradation and electrode degradation. The effect of electrode degradation on membrane degradation was studied in this work. We compared membrane degradation after electrode degradation and membrane degradation without electrode degradation. I-V performance, hydrogen crossover current, fluoride emission rate (FER), impedance and TEM were measured after and before degradation of MEA. Electrode degradation reduced active area of Pt catalyst, and then radical/ evolution rate decreased on Pt. Decrease of radical/ reduced the velocity of membrane degradation.

Degradation behaviors of perfluorosulfonic acid polymer electrolyte membranes for polymer electrolyte membrane fuel cells under varied acceleration conditions

AbstractThe degradation of perfluorosulfonic acid (PFSA) membranes (e.g., Nafion membranes) in polymer electrolyte membrane fuel cells has caused wide widespread concern. However, their degradation behaviors, which lead to the damage of fuel cells, need to be investigated under alternative accelerating environments by the simulation of fuel‐cell operating conditions. Nafion membranes showed a homogeneous degradation behavior during hydrogen peroxide (H2O2) aging, whereas a nonhomogeneous (or crack‐type) degradation behavior occurs for Nafion membranes aged in an H2O2/Fe2+ system (Fenton's reagent), where plenty of the typical microcracks appeared. Interestingly, in the case of nonhomogeneous degradation, the membrane presented a lower fluoride emission rate than that with the homogeneous degradation; this indicates a possible selective attack model of free radicals to both CF2 and the defect end groups in PFSA membranes. In addition, the effects of the different degradation behaviors on the thermal stability and water uptake of membranes were examined by thermogravimetric analyses. H2 crossover and single‐fuel‐cell tests were carried out to evaluate the influence of the degradation behaviors on the fuel‐cell performance. These showed that the membrane with a nonhomogeneous degradation behavior had a higher hydrogen crossover and was more destructive than that with a homogeneous behavior. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Decreasing Membrane Degradation through Heteropolyacid Sub-layers

Heteropolyacids (HPAs) were adsorbed onto high surface area carbon, and incorporated in polymer electrolyte membrane fuel cells (PEMFCs) between the electrodes and membrane, forming “sub-layers.” The addition of HPAs to these sub-layers significantly decreased membrane degradation in cells subjected to accelerated stress tests, as evidenced by a 50% reduction in fluoride emission rate (FER) and open circuit voltage (OCV) decay. Evidence of increased HPA and platinum concentration within the sub-layer indicates substantial HPA mobility. Modeling of the voltage profile through the sub-layer and membrane indicated that platinum precipitation was possible within the sub-layer, and that this is dependent on both the reduction potential and concentration of the HPA.

The degradation mitigation effect of cerium oxide in polymer electrolyte membranes in extended fuel cell durability tests

In this work, two formulations of cerium oxide nanoparticles were incorporated into perfluorosulfonic acid membrane electrode assemblies (MEAs) and their ability to improve the in-situ membrane durability was studied by subjecting them to 94 and 500 h open-circuit voltage hold accelerated durability tests. In the shorter test the open circuit voltage decay rate was reduced by half and the fluoride emission by at least one order of magnitude, though no effect on hydrogen crossover or performance on the baseline MEAs was measured. The presence of the additive increased the particle size but decreased the number of platinum catalyst particles that were deposited in the membrane. The main Pt band was found at the predicted location; however, the incorporation of ceria caused a broadening with particles reaching further into the membrane. In 500 h tests, ceria-containing MEAs demonstrated a seven-fold decrease in open-circuit voltage decay and three orders of magnitude reduction in fluoride emission rates with unchanged performance and hydrogen crossover, remaining effectively pristine whilst the baseline MEA underwent catastrophic failure.

PEMFC에서 전극과 전해질 막의 열화 가속 시험

최근까지 대부분의 PEMFC MEA (Membrane and Electrode Assembly) 열화 연구는 전극과 전해질막 각각 분리되어 연구되었다. 그런데 실재 PEMFC 운전조건에서는 전극과 막은 동시에 열화된다. 그래서 본 연구에서는 전극과 막열화가 동시에 일어나는 조건에서 열화 가속시험을 하였다. 실험결과 전극과 막 열화가 서로 상호작용함을 보였다. 막열화는 촉매의 활성면적 감소폭을 줄였고, 전극열화는 막의 수소투과 전류와 불소유출속도(FER) 증가폭을 감소시켰다. Until a recent day, degradation of PEMFC MEA (membrane and electrode assembly) has been studied, separated with membrane degradation and electrode degradation, respectively. But membrane and electrode were degraded coincidentally at real PEMFC operation condition. Therefore in this work, AST (Accelerated Stress Test) of MEA degradation was done at the condition that membrane and electrode were degraded simultaneously. There was interaction between membrane degradation and electrode degradation. Membrane degradation reduced the decrease range of catalyst active area by electrode degradation. Electrode degradation reduces increase range of the hydrogen crossover current and FER (Fluoride Emission Rate) by membrane degradation.

Synthesis of Nafion/CeO2 hybrid for chemically durable proton exchange membrane of fuel cell

Abstract Nafion/CeO 2 hybrid composites prepared through a self-assemble route are proposed for chemically durable electrolyte membrane of fuel cell. Self-assembly of Nafion and CeO 2 nanoparticles between the positively charged CeO 2 and the negatively charged SO 3 − end groups of Nafion ionomers gives the composite structure a good compatibility on the interface. As a result, the Nafion/CeO 2 hybrid proton exchange membranes present conductivities slightly lower than that of the pristine Nafion membrane at 100 RH% condition, whereas much higher than the later when humidification is below 75 RH%. The Nafion/CeO 2 hybrid membranes also display excellent radical scavenge ability under hydroxyl radical corrosion condition. With Fenton reagent of 30 wt% H 2 O 2 solution containing 20 ppm FeCl 2 at 85 °C, the fluoride emission rates for the self-assembled Nafion/CeO 2 membranes with ceria content of 1, 3, 5, 10 wt% are 43.05, 8.67, 6.01, 4.47 mg h −1 , respectively, much lower than that for pristine Nafion membrane (55.78 mg h −1 ) and Nafion/CeO 2 membrane prepared from the conventional sol–gel method (11.64 mg h −1 ). Another important aspect of the self-assembled Nafion/CeO 2 membrane contributed to the durability is the low humidity-induced stress and dimensional stability. The humidity-induced stress of the self-assembled Nafion/CeO 2 membranes with ceria content of 1–10 wt% was 1.22–1.93 MPa, in comparison to 2.25 MPa of pristine Nafion membrane. Thus, the proton exchange membranes prepared from self-assembled Nafion/CeO 2 present high durability under the fuel cell operating conditions. In the open circuit voltage accelerated test under in situ accelerating RH cyclic test, the OCV reduction rate of self-assembled Nafion/CeO 2 membrane with ceria content of 5 wt% is 1.13×10 −4 mV/s, much lower than 11.7×10 −4 mV/s of the pristine Nafion membrane and 5.78×10 −4 mV/s of the Nafion/CeO 2 hybrid membrane prepared from the conventional sol–gel method.

A strategy for facile durability improvement of perfluorosulfonic electrolyte for fuel cells: Counter ion-assisted decarboxylation at elevated temperatures

A strategy for improving the chemical stability of perfluorosulfonic acid is proposed by hydrothermal decarboxylation of defect groups with the assistance of counter ions and ethylene glycol at elevated temperatures. An apparent decrease in the carboxyl end groups is observed at decarboxylation temperatures of 180–220 °C. Decarboxylation of perfluorosulfonic acid significantly improves the chemical stability of the perfluorosulfonic electrolyte toward hydroperoxyl radicals, resulting in high concentrations of S 2p and F 1s after radical corrosion for 12 h. The decarboxylation of the electrolyte at elevated temperature also improves the side chain stability of the perfluorosulfonic acid. The fluoride emission rate decreases with increased decarboxylation temperature, implying a low side chain cleavage of the perfluorosulfonic acid polymer.

Radicals in Fuel Cell Membranes: Mechanisms of Formation and Ionomer Attack

Radical intermediates formed during polymer electrolyte fuel cell (PEFC) operation cause degradation of the proton exchange membrane. Generation and decay of the radicals HO•, H•, and HOO• in the membrane was simulated based on a kinetic framework with H2O2 as precursor. The degradation of the PFSA membrane was assumed to proceed via end-group attack. Two chemical environments were considered, a fuel cell test and an ex situ Fenton test. It was found that the reaction pathways and corresponding radical concentrations differ considerably. In particular, no H• is formed in a Fenton test due to the absence of H2. The influence of the concentration of Fe-ion impurities and end-groups on the radical concentration and rate of ionomer attack was studied. The calculated fluoride emission rate shows fairly good agreement with experimental results reported in the literature.

Radical (HO•, H• and HOO•) Formation and Ionomer Degradation in Polymer Electrolyte Fuel Cells

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 H2O2 as “parent” molecule and with contaminating Fe as parameter. Analysis under quasi-steady state conditions yielded radical concentrations of around 10−19 M for H•, 10−16 M for HO• and 10−10 M for HOO•. H• is formed via the reaction of HO• with H2 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 α-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.

Degradation mitigation in PEM fuel cells using metal nanoparticle additives

The efficacy of added freestanding and silica-supported metal (Pt, Pd, Ag, and Au) nanoparticles in mitigating polymer electrolyte membrane (PEM) degradation in an operating fuel cell was investigated. The metal nanoparticles to be added were chosen based on their ability to scavenge free radicals, as confirmed through ex situ measurements using a model free radical 1,1-diphenyl-2-picrylhydrazyl as a test species. Composite membranes were prepared by adding 3 wt% of the freestanding or supported metal nanoparticles to Nafion®. Membrane electrode assemblies prepared using these membranes were subjected to accelerated degradation tests to determine the fluoride emission rate (FER), a key measure of the macroscopic rate of PEM degradation in an operating fuel cell. The results were as follows: the addition of Ag, Pt, Pd and Au freestanding nanoparticles lowered FER by 35%, 60%, 80% and 90%, respectively, when compared with the recast Nafion® membrane (control). In the case of silica-supported nanoparticles, more modest reductions were achieved: 40% and 60% in the case of Au on SiO2 and Pd on SiO2, respectively. Ag or Pt on SiO2 resulted in similar FER values as the control. These results suggest that the addition of selected metal nanoparticles with radical scavenging abilities is a promising approach to mitigate PEM degradation in an operating fuel cell.

10.2478/s11814-009-0212-9

An accelerated single cell test and single electrode cell test were carried out to investigate membrane degradation by Pt dissolved/deposited on the membrane. For a cell operating under accelerated conditions (OCV, 90°C, anode RH 0%, cathode O2 supply), MEA analyses revealed that Pt particles were deposited in the membrane at the anode side, with a decrease in F, O, and C content near the anode side of the membrane. Dissolved Pt from the cathode showed that Pt existed mainly in the form of Pt2+ ionic species. Oxygen and hydrogen helped Pt dissolution from the cathode and Pt deposition in the membrane, respectively. Radical formation on deposited Pt in the membrane was detected by electron spin resonance (ESR). Fluoride emission rate (FER, an indicator of membrane degradation rate) increased with an increase in the amount of Pt in the membrane.

The effect of platinum in a Nafion membrane on the durability of the membrane under fuel cell conditions

The effect of platinum on free radical generation and membrane degradation in proton exchange membrane (PEM) fuel cells is investigated using three typical cell configurations. Examinations of the fluoride emission rates (FERs) under different testing conditions indicate that platinum deposited in the membrane plays an important role as a catalytic center for the formation of H 2O 2 and HO free radicals, leading to PEM degradation. The chemical durability of the membranes is tested in accelerated Fenton tests. It confirms the formation of free radicals in the presence of platinum in the decomposition of H 2O 2 by colorimetric method with dimethyl sulfoxide (DMSO) as the trapping agent. In addition, structural and morphological changes of the membranes are characterized using FT-IR spectroscopy and scanning electron microscopy (SEM).

In Situ Detection of Reactive Oxygen Species in PEM Fuel Cells

Preliminary results are presented in this study concerning i) the ex-situ identification of reactive oxygen species (ROS) within a model Fe+2/H2O2 solution using fluorescence spectroscopy; and ii) the effect of cerium oxide particle size on its radical scavenging properties and hence on the fluoride emission rate (FER) of composite Nafion® / CeO2 membranes. On the degradation diagnostics front, it is demonstrated that fluorescence fiber-optic measurements can sense the ex-situ formation of hydroperoxyl (HOO*) radicals in a Fe+2/H2O2 solution through a decrease in the fluorescence intensity in the presence of the radicals. Work to extend these studies for hydroxyl radicals and to achieve in-situ estimation is ongoing. On the degradation mitigation front, it is seen that lowering the CeO2 particle size leads further reduces the FER from composite membranes by 30-50%.