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We describe a quantitative neuroimaging method to estimate the macromolecular tissue volume (MTV), a fundamental measure of brain anatomy. By making measurements over a range of field strengths and scan parameters, we tested the key assumptions and the robustness of the method. The measurements confirm that a consistent, quantitative estimate of macromolecular volume can be obtained across a range of scanners. MTV estimates are sufficiently precise to enable a comparison between data obtained from an individual subject with control population data. We describe two applications. First, we show that MTV estimates can be combined with T1 and diffusion measurements to augment our understanding of the tissue properties. Second we show that MTV provides a sensitive measure of disease status in individual patients with multiple sclerosis. The MTV maps are obtained using short clinically appropriate scans that can reveal how tissue changes influence behavior and cognition.

Two mathematical models of gas-diffusion electrodes, one for liquid electrolytes and one for ion-exchange polymer electrolytes, are presented to investigate the effects of mass-transport limitations on the polarization characteristics of a reaction obeying Tafel kinetics. The focus is on low-temperature fuel-cell cathodes, and in particular; contrasting two limiting cases that may be encountered at high current densities: control by kinetics and dissolved oxygen mass transport vs. control by kinetics and ionic mass transport. It is shown that two distinct double Tafel slopes may arise from these two limiting cases. The former is first order, and the latter is half-order with respect to oxygen concentration. How the modeling results may be applied to diagnose the performance of fuel-cell cathodes is also presented. Since the ionic-masstransport-limited case has generally been neglected in previous gas-diffusion electrode models, specific examples of fuelcell cathode data from the literature which display the behavior predicted by the models in this case are given and briefly discussed.* Electrochemical Society Student Member.

© 2002 The Electrochemical Society.

Flow batteries with flow-through porous electrodes are compared to cells with porous electrodes adjacent to either parallel or interdigitated channels. Resistances and pressure drops are measured for different configurations to augment the electrochemical data. Cell tests are done with an electrolyte containing VO 2+ and VO 2 + in sulfuric acid that is circulated through both anode and cathode from a single reservoir. Performance is found to depend sensitively on the combination of electrode and flow field. Theoretical explanations for this dependence are provided. Scale-up of flow through and interdigitated designs to large active areas is also discussed.

Redox-flow batteries are entering a period of renaissance, buoyed by both the increasing need for affordable large-scale energystorage solutions, as well as leveraging the advancements in flow-cell technology, mainly in polymer-electrolyte fuel cells. This perspective highlights the research-and-development avenues and opportunities for redox-flow-battery cells and materials. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. Redox-Flow Batteries (RFBs) are an Electrical-Energy-Storage (EES) technology that was first developed by NASA during the energy crisis of the 1970's. Unlike traditional batteries, RFBs utilize redox couples that can be stored in independent tanks and only brought together when power is needed through an energy-conversion cell stack as shown in Fig. 1. 1 It has long been recognized that stationary EES systems could save substantial quantities of energy, as well as provide other potential benefits, such as improved reliability and reduced emissions. However, to date, most electrical grids utilize minimal storage since EES technologies have not been economically viable and proven. There is a growing need for grid-and urban-scale EES, due to a number of factors (e.g., the growth in stochastic renewable energy-generation systems, "smart-grid" initiatives, time-of-use rates, aggregation of generation resources, etc.), but this growth in demand does not necessarily lead to more attractive cost targets for EES systems. The competing technology that dictates cost is essentially rapid-response power generation, such as spinning reserves of various gas turbines. Grid-scale EES must still reach lower cost in order to be widely deployed. The cost targets for grid-scale EES are typically more aggressive than those for portable or transportation applications; however, the typical size of the EES system is also orders of magnitude larger, as shown in Fig. 2. Clearly, an EES technology that can scale up in a cost-effective manner is highly desirable and necessary.RFBs are inherently well suited for large applications since they scale-up in a more cost-effective manner than other batteries. Since the energy and power capacities of a RFB system are independent variables, the required capacities for any application can be met using correctly-sized energy and power modules. RFBs also possess other compelling attributes for stationary EES applications, especially longduration applications. For example, the RFB architecture enables long lifetimes since the electrodes are not inherently required to undergo physiochemical changes during charge/discharge cycles. Because of the growing demand for grid-scale EES and the inherent attractive attributes of RFBs, the technology appears to be undergoing a renaissance period. This growth in RFB research is readily evident...

Transport of active species through the ion-exchange membrane separating the electrodes in a redox-flow battery is an important source of inefficiency. Migration and electro-osmosis have significant impacts on the crossover of reactive anions, cations, and neutral species. In this paper, these phenomena are theoretically and experimentally explored for commercial cation-exchange membranes. The theoretical analysis indicates that plotting the cumulative Coulombic mismatch between charge and discharge as a function of time can be used to assess crossover rates. The relative importance of migration and electro-osmosis over diffusion is quantified and shown to increase with increasing current density and membrane thickness because the contributions of migration and electro-osmosis to ionic flux are independent of membrane thickness and proportional to current density, while diffusion is inversely proportional to membrane thickness and independent of current density. Redox-flow batteries (RFBs) possess compelling attributes for stationary energy storage. In particular, the inherent decoupling of energy and power in RFBs enables the cost effective use of active materials with low energy density.1,2 Energy is stored in redox-active molecules and power is generated by oxidizing and reducing different redox couples at the positive and negative electrodes.3-5 Two promising technologies are the all-vanadium RFB (VRB) and the hydrogen/bromine RFB, where good performances have been demonstrated recently. [6][7][8][9][10][11][12][13][14] Protons in a cation-exchange membrane (CEM) typically carry charge between the two electrodes in these two RFBs. In both systems, movement of reactant or product species from one electrode to the other through the membrane results in inefficiency and reversible capacity loss. 8,15,16 For these reasons it is worthwhile to investigate the causes of crossover, and examination of the two chosen RFBs allows for the study of transport of active cations, anions, and neutral species.The reactions at the negative and positive electrodes in a VRB are:andrespectively. Protons are the primary ionic charge carriers. Because the active species at both electrodes are vanadium ions, transfer from one electrode to the other is less detrimental than it is in other RFBs that have dissimilar active materials at the two electrodes. Vanadium that traverses the separator reacts to form a discharged ion at the opposite electrode. 17 Although imbalances that occur after repeated cycling can be recovered by mixing the electrolytes, vanadium that crosses the membrane represents an inefficiency that is detrimental in applications like grid-scale energy storage where high efficiency is required for economic success.For the hydrogen/bromine RFB, the reactions at the negative and positive electrodes areBr 2 (aq) + 2e 18-21 Br species that enter the negative electrode can be separated from the negative electrolyte and returned to the positive electrolyte without causing permanent decay, in a similar fashion to vanadium in V...

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