Vadim V. Atrazhev

Degradation of Polymer-Electrolyte Membranes in Fuel Cells I. Experimental

cRussian Academy of Sciences, Moscow, Russia In this work, chemical degradation is studied using highly controlled measurements of the fluoride ion release from subscale cells in degrading environments using perfluorosulfonic-acid-based membrane electrode assemblies, primarily with cast, 25 m 1m il thick membranes. Effects of key variables, such as oxygen concentration, relative humidity RH, temperature, and membrane thickness on the fluoride ion emission rate FER are described under open-circuit decay conditions. Some of the observed trends are expected or consistent with previous observations, such as decreasing FER with decreasing temperature and increasing RH. Other trends observed are not expected, such as a logarithmic decrease of FER with oxygen concentration and increasing FER with increasing membrane thickness. Cross-sectional transmission electron microscopy analysis of decayed membranes indicates a surprisingly homogeneous distribution of small Pt particles 3 to 20 nm in diameter, presumably from dissolution and migration from the cathode. The experimental results are consistent with radical generation at these Pt particles from crossover hydrogen and oxygen, subsequent radical migration, and polymer attack. The response of the FER to new experimental conditions in this study suggests that the attack can exist at any plane within the membrane, not just the “Xo” plane of maximum Pt precipitation.

Degradation of Polymer-Electrolyte Membranes in Fuel Cells II. Theoretical model

A physics-based theoretical model that predicts the chemical degradation of the perfluorosulfonic acid polymer electrolyte membrane during fuel cell operation is developed. The model includes the transport and reaction of crossover gases, hydrogen and oxygen, to produce radicals in the membrane that subsequently react with the polymer to release hydrogen fluoride. The model assumes that a uniform distribution of nanometer-sized platinum deposits in the membrane (as a model input) originating from cathode dissolution provides the sites for radical generation. The degradation rate, measured by the release of hydrogen fluoride, depends on the net radical generation sites in the membrane, the concentration of the crossover gases, the hydration level of the membrane, the operating temperature, the operating voltage, and the thickness of the membrane. The model-predicted trends agree well with those reported and with our experimental results reported in the first article of this series by Madden et al. [J. Electrochem. Soc., 156, B657 (2009)]. Furthermore, the model provides insight to the factors that affect radical generation vs radical quenching, which aids in explaining the experimentally observed nonlinear trends of fluoride emission with reactant concentration and membrane thickness.

A Carbon Corrosion Model to Evaluate the Effect of Steady State and Transient Operation of a Polymer Electrolyte Membrane Fuel Cell

A carbon corrosion model is developed based on the formation of surface oxides on carbon and platinum of the polymer electrolyte membrane fuel cell electrode. The model predicts the rate of carbon corrosion under potential hold and potential cycling conditions. The model includes the interaction of carbon surface oxides with transient species like OH radicals to explain observed carbon corrosion trends under normal PEM fuel cell operating conditions. The model prediction agrees qualitatively with the experimental data supporting the hypothesis that the interplay of surface oxide formation on carbon and platinum is the primary driver of carbon corrosion. Carbon is commonly used as material for catalyst supports, gasdiffusion media and bipolar plates in Proton Exchange Membrane (PEM) fuel cells, though it is well-known that carbon is thermodynamically unstable in PEM fuel cell cathode environments. The current PEM fuel cell cathodes typically operate at temperatures in the range of 60 ◦ C–85 ◦ C and potentials in the range of 0.5–0.95 V (vs. RHE), which is significantly more anodic than the equilibrium potential for carbon oxidation to carbon dioxide (0.207 V vs. RHE). However, the kinetics of carbon oxidation under PEM operational conditions is relatively slow. This slow oxidation kinetics makes carbon an appropriate material for PEM electrodes. Carbon corrosion occurs at different rates under various fuel cell operating conditions. A few distinct conditions that can lead to extensive carbon corrosion and catastrophic performance decay in a short period of time have been identified. One such condition occurs during start/stop operations when air leaks into or is present in the anode gas channels and creates potential variation in the planform. 1–3 Another condition that leads to carbon corrosion is fuel starvation induced by flow mal-distribution in the individual cells of the stack. 4 In both cases, undesirable oxygen reduction reaction (ORR) at the anode decreases the potential of ionomer keeping high potential difference between carbon and ionomer (close to equilibrium potential of ORR). That results in high potential difference (∼1.4 V) between carbon and ionomer at the opposite electrode and accelerated carbon corrosion at this potential. 1–4 The corresponding system strategies to mitigate have

The Dynamics of Platinum Precipitation in an Ion Exchange Membrane

Microscopy of polymer electrolyte membranes (PEMs) that have undergone operation under fuel cell conditions, have revealed a well defined band of platinum in the membrane. Here, we propose a physics based model that captures the mechanism of platinum precipitation in the polymer electrolyte membrane. While platinum is observed throughout the membrane, preferential growth of the platinum at the band of platinum is dependent on the electrochemical potential distribution in the membrane. In this paper, the location of the platinum band is calculated as a function of gas concentration at the cathode and anode, gas diffusion coefficients, and solubility constants of the gases in the membrane, which are functions of relative humidity. Potential cycling the PEM fuel cell under H 2 /N 2 conditions that resulted in the platinum band is near the cathode-membrane interface. As the oxygen concentration in the cathode gas stream increases and/or the hydrogen concentration in the anode gas stream decreases, the band moves toward the anode. The model developed in this paper agrees with the set of experimental data on the platinum band location and the platinum particle distribution and size.

Direct Mechanism of OH Radicals Formation in PEM Fuel Cells

The generation of hydroxyl and/or peroxyl (·OOH) radicals on fuel cell catalysts has direct consequences for the durability of the polymer electrolyte membrane. Whether radical generation occurs from hydrogen peroxide decomposition vs. direct generation on the Pt catalyst surface is of key interest. Ab initio calculations were performed with Gaussian software employing both cluster models of a platinum surface, as well as single / double Pt atom ensembles. Although complete four-electron reduction to water on Pt is desired, hydrogen peroxide is also formed at potentials below 0.6 Vrhe. In this work, we show that the main pathway of H2O2 formation on platinum involves the ·OH free radical as an intermediate. ·OH radicals may be released from the Pt surface either by dissociation of the adsorbed OOH intermediate at high potentials or by dissociation of one-site adsorbed peroxide intermediate at low potentials. Adsorption of hydrogen peroxide transported to the Pt surface to achieve this state, though possible, is unfavored. These results indicate that the direct formation of hydroxyl radicals on Pt surfaces is possible under certain conditions and need not proceed through a peroxide intermediate.

Surface tension model for surfactant solutions at the critical micelle concentration.

A model for the limiting surface tension of surfactant solutions (surface tension at and above the critical micelle concentration, cmc) was developed. This model takes advantage of the equilibrium between the surfactant molecules on the liquid/vacuum surface and in micelles in the bulk at the cmc. An approximate analytical equation for the surface tension at the cmc was obtained. The derived equation contains two parameters, which characterize the intermolecular interactions in the micelles, and the third parameter, which is the surface area per surfactant molecule at the interface. These parameters were calculated using a new atomistic modeling approach. The performed calculations of the limiting surface tension for four simple surfactants show good agreement with experimental data (~30% accuracy). The developed model provides the guidance for design of surfactants with low surface tension values.

Stalled phase transition model of high-elastic polymer

The microscopic model of semi-crystalline polymer in a high-elastic state is proposed. The model is based on the assumption that, below the melting temperature, the semi-crystalline polymer comprises crystal nuclei connected by stretched chain segments (SCS) with a random configuration of monomers. The key factor that stalls the phase transition below the melting temperature is the tension of the SCS. External stress applied to the polymer also shifts the equilibrium and causes unfolding of the nuclei, which enables large reversible deformation of the polymer without loss of integrity. The simple 1D model predicts a plateau in the stress?strain curve of the high-elastic polymer above the yield stress, which agrees with experimental observations. The model prediction for the temperature dependence of polytetrafluoroethylene (PTFE) yield stress in the high-elastic state is in satisfactory agreement with experiment.

Molecular dynamics simulation of chains mobility in polyethylene crystal

The mobility of polymer chains in perfect polyethylene crystal was calculated as a function of temperature and chain length through Molecular dynamics in united atom approximation. The results demonstrate that the chain mobility drastically increases in the vicinity of the phase transition from the orthorhombic to quasi-hexagonal phase. In the quasi-hexagonal phase, the chain mobility is almost independent on temperature and inversely proportional to the chain length.