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.

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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. Adsorptio n 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.

Molecular-dynamics simulations were used to study the structure of water clusters in different perfluorinated ionomers. The ionomers included in this work were Perfluorosulfonic Acid (PFSA) and Perfluoroimide Acid (PFIA). Different shapes and sizes of water domains in PFSA and PFIA were observed. In PFSA, ionomer water domains have spherical-like shape while water domains have complicated branch structure in PFIA ionomer. The mean water-cluster size for PFSA and PFIA ionomers as a function of water content lie on one universal curve, which indicates that the size of water clusters depends primarily on water content and weakly on ionomer chemistry. In PFSA, almost all hydronium ions are located predominantly close to the water cluster/backbone interface for both large and small values of water content; while in PFIA a relatively large fraction of hydronium ions are distributed within the bulk of the water clusters. A similar distribution of acidic groups was observed in both ionomers. In PFSA ionomer, all sulfonic groups are located in the surface water layer at the distance from 3 Å to 6 Å from the backbone. In PFIA ionomer, approximately 60% of sulfonic groups are located in the surface water layer and 40% of sulfonic groups are immersed into the bulk of water cluster. Perfluorinated ion-exchange membranes are widely used in both fuel cells and flow-battery cells. Flow batteries, such as vanadium redox batteries (VRBs), are promising devices for the large-scale Electrical Energy Storage (EES) applications 1 and are receiving increasing interest due to the demand for grid-scale EES solutions and recent improvements in VRB cell performance.2 The ion-exchange membrane is a critical component in these different types of electrochemical flow cells, which conducts the charge-carrier ions (e.g., protons) while preventing the flow of electrons through it. Additionally, in a flow-battery cell, the membrane separates the positive and negative liquid electrolytes containing the active redox-species ions. For example, in a VRB cell, it is desirable to prevent the crossover of vanadium ions through the membrane since the result is a decrease in both the energy efficiency and energy capacity of the VRB system. 2,3 In VRBs, this energy-capacity loss is recoverable, since the vanadium can be simply rebalanced by pumping some of the electrolyte with the excess V to the other electrolyte tank (albeit with an additional energy loss due to self-discharging that results from mixing the two reactants). In flow-battery chemistries with dissimilar active-species ions (e.g., Fe-Cr systems), crossover can also cause contamination of the electrolytes and/or electrodes, which can result in additional adverse effects on the flow-battery system. 1 In any case, the activespecies ions are transported through the membrane in the hydrophilic phase (i.e., the water clusters) of the membrane. Therefore, the study of the water-cluster structure is required to understand and model the transport of different ions in the membrane. One of the ultimate...

We use molecular dynamics to predict the ionic conductivities of lithiated Nafion perfluorinated ionomeric membranes swelled in dimethyl sulfoxide (DMSO) and acetonitrile (ACN). The experimental conductivity of lithiated Nafion swollen with DMSO is two orders of magnitude higher than with ACN. Conversely, the mobility of Li + ions in a solution of LiPF 6 in ACN is approximately six times higher than in DMSO. In this work, we demonstrate that the ionic conductivity of Nafion is substantially governed by the concentration of free Li + ions, i.e. by the degree of dissociation of the Li + and SO 3 − pairs, and that the inherent mobility of Nafion is a cation-exchange membrane that has been extensively studied and used in aqueous electrochemical systems. In this work, we seek to understand the applicability of Nafion to flow batteries that use nonaqueous solvents and lithium salts. A flow battery is an electrochemical device that stores and releases energy by oxidizing and reducing redox molecules that remain dissolved in electrolytes. The active molecules are stored in external vessels and pumped to reactors to charge or discharge the battery. This arrangement provides a separation of energy and power that is absent from conventional enclosed batteries like lead acid. The all-vanadium redox flow battery, for example, is an aqueous system that relies on the V 2+ /V 3+ and VO 2+ /VO 2 + couples at the negative and positive electrodes and H + as the primary charge carrier. Nonaqueous electrolytes are being considered for redox flow batteries to enable electrochemical couples with potential differences substantially exceeding the aqueous stability limit of 1.23 V, and, consequently, increase energy density. Two key considerations in reactor design are power density and Coulombic efficiency. The membrane must possess high ionic conductivity, low electronic conductivity, and low permeability of active species (the various vanadium ions in the aqueous example mentioned above) to simultaneously achieve high area-specific power density and high Coulombic efficiency. In this work, Li + is the primary charge carrier, and the unspecified active species are assumed to be absent from the membrane. Future work could examine the behavior of active molecules in lithiated membranes containing different solvents, with a specific focus on the charge on the active molecule.Many experimental 1-9 and numerical 10-21 works discussing ion diffusion and conductivity in Nafion swollen with water have been published during the past two decades. The dependence of conductivity on water content and temperature is well understood. Furthermore, a number of theoretical 22,23 and experimental works consider Nafion swollen with methanol or water/methanol mixtures in the context of direct methanol fuel cells. 24,25 It has been demonstrated that the diffusion coefficients of water, methanol, and hydronium decrease with increasing methanol concentration. There have been far fewer studies on Nafion containing solvents other than water and methanol. A nota...