Advances in the design of materials for energy storage and conversion (i.e., "energy materials") increasingly rely on understanding the dependence of a material's performance and longevity on three-dimensional characteristics of its microstructure. Three-dimensional imaging techniques permit the direct measurement of microstructural properties that significantly influence material function and durability, such as interface area, tortuosity, triple phase boundary length and local curvature. Furthermore, digital representations of imaged microstructures offer realistic domains for modeling. This article reviews state-of-the-art methods, across a spectrum of length scales ranging from atomic to micron, for three-dimensional microstructural imaging of energy materials. The review concludes with an assessment of the continuing role of three-dimensional imaging in the development of novel materials for energy applications.

Anion exchange membranes (AEMs) are being developed for potential use in fuel cell systems which include portable power applications. In a fuel cell, these membranes transport hydroxide ions from the cathode to the anode. If carbon dioxide is present, carbonate and bicarbonate ions can form, displacing the hydroxide ions. Among the challenges this presents, the carbonate and bicarbonate are less mobile than the hydroxide and therefore the ionic conductivity of the membrane suffers. A procedure is outlined to take data from a permeation based water flux experiment and determine diffusion coefficients and the ionic conductivity of the membrane. The water-membrane diffusion coefficients can be measured from a water flux experiment. Using principles from kinetic theory, the water-membrane diffusion coefficient can be converted to an appropriate ion-membrane diffusion coefficient. Finally, an equation derived from the dusty fluid model can be used to calculate the ionic conductivity of the membrane in different counter ion forms. The calculated ionic conductivities have been shown to agree well with reported values for proton and anion exchange membranes.Anion exchange membrane fuel cells (AEMFCs) have received increased attention in recent years. The AEMFC typically operates at low temperatures, below 80 • C, and can utilize alcohol fuels; making it of possible appeal for portable power applications. Operating in a high pH environment allows for favorable alcohol oxidation kinetics and the ability to use non-platinum oxygen reduction catalysts. 1,2 Despite recent improvements, there are still several challenges confronting the technology. The low hydroxide ionic conductivity of the AEM and the formation of carbonate and bicarbonate species which further reduce the membrane's ionic conductivity are two such challenges that are examined in this study. 3,4 Current AEMs often use a polymer hydrocarbon backbone with benzyl-trimethylammonium fixed side chain groups. This cation is a strong base (pK b ≈ 1) which allows for reasonable dissolution of the hydroxide ions (OH − ) from the membrane and easy transport through the membrane. 5 The polymer backbone can range from several polymers including poly(ethylene-co-tetrafluoroethylene) (ETFE), poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP), polypropylene, and polysulphone. 6-10 In one study, a fully hydrated AEM with an ETFE backbone was reported to have an ionic conductivity of roughly 30 mS/cm at 30 • C. When comparing this to Nafion 115 proton exchange membrane (PEM), which has a similar IEC, the PEM has a much higher ionic conductivity around 90 mS/cm. 7 If carbon dioxide (CO 2 ) is present, then the formation of carbonate (CO 3 −2 ) and bicarbonate (HCO 3 − ) ions can affect the membrane in several ways. One effect is the decrease in pH, which might actually work to increase the stability of the membrane. 6 However, the same ion exchange process also reduces the ionic conductivity of the membrane. This happens because the carbonate species displace the hydroxide ...

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High temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) are being studied due to a number of benefits offered versus their low temperature counterparts, including co-generation of heat and power, high tolerance to fuel impurities, and simpler system design. Approximately 90% of the literature on HT-PEM is related to the electrolyte and, for the most part, these electrolytes all use free phosphoric acid, or similar free acid, as the ion conductor. A major issue with using phosphoric acid based electrolytes is the free acid in the electrodes. The presence of the acid on the catalyst sites leads to poor oxygen activity, low solubility/diffusion, and can block electrochemical sites through phosphate adsorption. This review will focus on these issues and the steps that have been taken to alleviate these obstacles. The intention is this review may then serve as a tool for finding a solution path in the community.

Water transport properties in polymer electrolyte membranes are a subject of interest in studying low temperature fuel cells. This is because these membranes require the presence of water to be conductive for ion transport. One important water transport mechanism is water diffusion. This mechanism is characterized by the water diffusion coefficient which is strongly dependent on local water content. There have been many approaches proposed to measure or calculate the water diffusion coefficient for different membranes. Some of the more established techniques include sorption, permeation, and nuclear magnetic resonance (NMR). Unfortunately, disagreement between reported values exists. The technique used in this work involves the use of a permeation experiment where the diffusion coefficient is determined through coupling a numerical model with the data from the experimental rig. The process was first applied to Nafion, a well-known and widely used proton exchange membrane (PEM), for validation purposes. The calculated diffusion coefficient as a function of water content agreed well with NMR data from the literature with such agreement being rare, especially across different techniques. Once the process was validated it was then extended to SnowPure Excellion, an anion exchange membrane (AEM).

This study quantitatively describes the carbonate-and bicarbonate-forming reaction mechanisms in an operating alkaline exchange membrane fuel cell that occur as a result of carbon dioxide in the cathode gas stream. A transient, spatially-averaged theoretical model was created for this study and validated to experimental data from the literature. Results present the prediction of the membrane's ionic conductivity as a function of operating conditions and membrane properties. The self-purging phenomenon was observed and studied, as well as the emission of carbon dioxide from the membrane during operation. Following the conductivity study, suggestions can be made for optimal operating conditions and membrane properties to improve fuel cell performance in the presence of carbon dioxide.

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The CO 2 transport in the proton form Nafion membranes (E.W. 1100) under various hydration and temperature conditions were studied by using an IR based CO 2 detector to monitor both the transient and steady state CO 2 permeate flux through the membranes. A time-lag method was used to extract the CO 2 diffusivity and solubility from the CO 2 permeation-time curve. It was found that the CO 2 diffusivity in a dry membrane is very low, but it increases rapidly with the membrane-hydration level. A high CO 2 solubility was observed in dry membranes as compared to those in humidified membranes, for which the CO 2 solubility remains nearly invariant to the membrane hydration level at a given temperature. By comparing the measured data with those calculated for the CO 2 transport solely in the water channels, it is concluded that the CO 2 transport in Nafion membranes is dominated by its transport in the water channels. CO 2 and methanol crossovers in a direct methanol fuel cell (DMFC) configuration were also examined. By operating the DMFC configuration in methanol electrolysis mode, one can measure the CO 2 crossover rate, which can be used to correct the methanol crossover measurements from measuring CO 2 flux in the DMFC cathode effluent stream, and to derive the local CO 2 partial pressure at the DMFC anode catalyst layer.