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Review Article

J. Electrochem. En. Conv. Stor.. 2017;15(1):010801-010801-21. doi:10.1115/1.4037248.

The utilization of intermittent renewable energy sources needs low-cost, reliable energy storage systems in the future. Among various electrochemical energy storage systems, redox flow batteries (RFBs) are promising with merits of independent energy storage and power generation capability, localization flexibility, high efficiency, low scaling-up cost, and excellent long charge/discharge cycle life. RFBs typically use metal ions as reacting species. The most exploited types are all-vanadium RFBs (VRFBs). Here, we discuss the core components for the VRFBs, including the development and application of different types of membranes, electrode materials, and stack system. In addition, we introduce the recent progress in the discovery of novel electrolytes, such as redox-active organic compounds, polymers, and organic/inorganic suspensions. Versatile structures, tunable properties, and abundant resources of organic-based electrolytes make them suitable for cost-effective stationary applications. With the active species in solid form, suspension electrolytes are expected to provide enhanced volumetric energy densities.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2017;15(1):010802-010802-10. doi:10.1115/1.4037785.

The shuttle effect and poor conductivity of the discharge products are among the primary impediments and scientific challenges for lithium–sulfur batteries. The lithium–sulfur battery is a complex energy storage system, which involves multistep electrochemical reactions, insoluble polysulfide precipitation in the cathode, soluble polysulfide transport, and self-discharge caused by chemical reactions between polysulfides and Li metal anode. These phenomena happen at different length and time-scales and are difficult to be entirely gauged by experimental techniques. In this paper, we reviewed the multiscale modeling studies on lithium–sulfur batteries: (1) the atomistic simulations were employed to seek alternative materials for mitigating the shuttle effect; (2) the growth kinetics of Li2S film and corresponding surface passivation were investigated by the interfacial model based on findings from atomistic simulations; (3) the nature of Li2S2, which is the only solid intermediate product, was revealed by the density functional theory simulation; and (4) macroscale models were developed to analyze the effect of reaction kinetics, sulfur loading, and transport properties on the cell performance. The challenge for the multiscale modeling approach is translating the microscopic information from atomistic simulations and interfacial model into the meso-/macroscale model for accurately predicting the cell performance.

Commentary by Dr. Valentin Fuster

Research Papers

J. Electrochem. En. Conv. Stor.. 2017;15(1):011001-011001-8. doi:10.1115/1.4037582.

Ionic liquids are considered promising electrolytes for developing electric double-layer capacitors (EDLCs) with high energy density. To identify optimal operating conditions, we performed molecular dynamics simulations of N-methyl-N-propyl pyrrolidinium bis(trifluoromethanesulfonyl)imide (mppy+ TFSI) ionic liquid confined in the interstices of vertically aligned carbon nanostructures mimicking the electrode structure. We modeled various surface charge densities as well as varied the distance between nanotubes in the array. Our results indicate that high-density ion storage occurs within the noninteracting double-layer region formed in the nanoconfined domain between charged nanotubes. We determined the specific arrangement of these ions relative to the nanotube surface and related the layered configuration to the molecular structure of the ions. The pitch distance of the nanotube array that enables optimal mppy+ TFSI storage and enhanced capacitance is determined to be 16 Å.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2017;15(1):011002-011002-10. doi:10.1115/1.4037766.

This work presents a comparison between carbon felt-type and paper-type gas diffusion layers (GDLs) for polymer electrolyte membrane (PEM) fuel cells in terms of the similarities and the differences between their microstructures and the corresponding manner in which liquid water accumulated within the microstructures during operation. X-ray computed tomography (CT) was used to investigate the microstructure of single-layered GDLs (without a microporous layer (MPL)) and bilayered GDLs (with an MPL). In-operando synchrotron X-ray radiography was used to visualize the GDL liquid water accumulation during fuel cell operation as a function of current density. The felt-type GDLs studied here exhibited a more uniform porosity in the core regions, and the carbon fibers in the substrate were more prone to MPL intrusion. More liquid water accumulated in the felt-type GDLs during fuel cell operation; however, when differentiating between the microstructural impact of felt and paper GDLs, the presence of an MPL in bilayered GDLs was the most influential factor in liquid water management.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2017;15(1):011003-011003-9. doi:10.1115/1.4037583.

A perspective on emergent phase formation is presented using an interdisciplinary approach gained by working at the “interface” between diverse application areas, including solid oxide fuel cells (SOFCs) and ionic membrane systems, solid state lithium batteries, and ceramics for nuclear waste immobilization. The grain boundary interfacial characteristics of model single-phase materials in these application areas, including (i) CeO2, (ii) Li7La3Zr2O12 (LLZO), and (iii) hollandite of the form BaxCsyGa2x+yTi8-2x-yO16, as well as the potential for emergent phase formation in composite systems, are discussed. The potential physical properties resulting from emergent phase structure and distribution are discussed, including an overview of existing three-dimensional (3D) imaging techniques recently used for characterization. Finally, an approach for thermodynamic characterization of emergent phases based on melt solution calorimetry is outlined, which may be used to predict the energy landscape including phase formation and stability of complex multiphase systems.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2017;15(1):011004-011004-12. doi:10.1115/1.4037942.

In this study, a three-dimensional (3D) agglomerate model of an anion exchange membrane (AEM) fuel cell is proposed in order to analyze the influence of the composition of the catalyst layers (CLs) on overall fuel cell performance. Here, a detailed comparison between the agglomerate and a macrohomogeneous model is provided, elucidating the effects of the CL composition on the overall performance and the individual losses, the effects of operating temperature and inlet relative humidity on the cell performance, and the CL utilization by the effectiveness factor. The results show that the macrohomogeneous model overestimates the cell performance compared to the agglomerate model due to the resistances associated with the species and ionic transports in the CLs. Consequently, the hydration is negatively affected, resulting in a higher Ohmic resistance. The activation overpotential is overpredicted by the macrohomogeneous model, as the agglomerate model relates the transportation resistances within the domain with the CL composition. Despite the higher utilization in the anode CL, the cathode CL utilization shows a significant drop near the membrane–CL interface due to a high current density and a low oxygen concentration. Additionally, the influences of operating temperature and relative humidity at the flow channel inlet have been analyzed. Similar to the macrohomogeneous model, the overall cell performance of the agglomerate model is enhanced with increasing operating temperature due to the better electrochemical kinetics. However, as the relative humidity at the inlet is reduced, the overall performance of the cell deteriorates due to the poor hydration of the membrane.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2017;15(1):011005-011005-5. doi:10.1115/1.4038020.

We report a simple novel annealing technique for the synthesis of NbS2 nanoflakes. The synthesized NbS2 flakes were characterized well with different spectroscopic and microscopic techniques and confirmed they are in 3R-NbS2 polymorph structure, which is semiconducting in nature. Later, they were successfully deposited onto carbon cloth (CC) and tested for Li–S cell. Lithium–sulfur batteries suffer from polysulfide (PS) shuttling effects which hinder the performance of the cell. High capacity fade, slow redox kinetics, and the low cyclability of cells are just some of the many problems caused by the shuttling effect that hinder the viability of the battery. Herein, we utilized the catalytic nature of NbS2 along with the high conductivity of CC for better PS adsorption, their liquid to solid conversion, fast PS redox kinetics which substantially enhanced the overall Li–S performance.

Commentary by Dr. Valentin Fuster

Expert View

J. Electrochem. En. Conv. Stor.. 2017;15(1):014701-014701-7. doi:10.1115/1.4037244.

The microstructure of a fuel cell electrode largely determines the performance of the whole fuel cell system. In this regard, tomographic imaging is a valuable tool for the understanding and control of the electrode morphology. The distribution of pore- and feature-sizes within fuel cell electrodes covers several orders of magnitude, ranging from millimeters in the gas diffusion layer (GDL) down to few nanometers in the catalyst layer. This obligates the application of various tomographic methods for imaging every aspect of a fuel cell. This perspective evaluates the capabilities, limits, and challenges of each of these methods. Further, it highlights and suggests efforts toward the integration of multiple tomographic methods into single multiscale datasets, a venture which aims at large-scale, and morphologically fully resolved fuel cell reconstructions.

Commentary by Dr. Valentin Fuster

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