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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
J. Electrochem. En. Conv. Stor.. 2017;15(1):011006-011006-7. doi:10.1115/1.4038075.
FREE TO VIEW

This paper for inclusion in the special issue provides a brief synopsis of lithium-ion battery safety research efforts at the Naval Research Laboratory (NRL) and presents the viewpoint that lithium-ion battery safety is a growing research area for both academic and applied researchers. We quantify how the number of lithium-ion battery research efforts worldwide has plateaued while publications associated with the safety aspect of lithium-ion batteries are on a rapid incline. The safety challenge creates a unique research opportunity to not only understand basic phenomena but also enhance existing fielded system through advanced controls and prognostics. As the number of lithium-ion battery safety research contributions climbs, significant advancements will come in the area of modeling across multiple time and length scales. Additionally, the utility of in situ and in operando techniques, several performed by the NRL and our collaborators, will feed the data necessary to validate these models. Lithium-ion battery innovations are no longer tied to performance metrics alone, but are increasingly dependent on safety research to unlock their full potential. There is much work to be done.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2017;15(1):011007-011007-10. doi:10.1115/1.4038244.

Enzymatic electrochemical cells (EECs) are a candidate for providing “green” solutions to a plethora of low-power, long-lifetime applications. A prototype three-electrode biobattery configuration of an EEC has been designed and fabricated for neutron imaging and electrochemical testing to characterize cell performance. The working electrode (WE) was catalyzed by a polymer ink-based biocatalyst with carbon felt (CF) serving as the supporting material. Results of both ex situ and in operando neutron imaging are presented as methods for relating fuel distribution, the distribution of the enzymes, and cell electrochemical performance. Neutron radiography (NR) was also performed on fuel solutions of varied concentrations to calibrate fuel solution thickness and allow for transient mapping of the fuel distribution. The calibration data proved useful in mapping the thickness of fuel solution during transient radiography. When refueled after electrochemical testing and neutron imaging, the cell surpassed its original performance, indicating that exposure to the neutron beam had not detrimentally affected enzyme activity. In operando mapping of the fuel solution suggests that increased wetting of the catalyst region increases cell performance. The relation of this performance increase to active region wetting is further supported by fuel distributions observed via the ex situ tomography. While useful in mapping aggregate solution wetting, the calibration data did not support reliable mapping of detailed glucose concentration in the WE. The results presented further demonstrate potential for the application of neutron imaging for the study of EECs, particularly with respect to mapping the distribution of aqueous fuel solutions.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2017;15(1):011008-011008-8. doi:10.1115/1.4038258.

Thermal management of Li-ion battery packs is a critical technological challenge that directly impacts safety and performance. Removal of heat generated in individual Li-ion cells into the ambient is a considerably complicated problem involving multiple heat transfer modes. This paper develops an iterative analytical technique to model conjugate heat transfer in coolant-based thermal management of a Li-ion battery pack. Solutions for the governing energy conservation equations for thermal conduction and convection are derived and coupled with each other in an iterative fashion to determine the final temperature distribution. The analytical model is used to investigate the dependence of the temperature field on various geometrical and material parameters. This work shows that the coolant flowrate required for effective cooling can be reduced significantly by improving the thermal conductivity of individual Li-ion cells. Further, this work helps understand key thermal–electrochemical trade-offs in the design of thermal management for Li-ion battery packs, such as the trade-off between temperature rise and energy storage density in the battery pack.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2017;15(1):011009-011009-6. doi:10.1115/1.4037907.

There is a growing interest in minimizing the energy and cost associated with desalination. To do this, various new desalination systems and approaches are being explored. One growing area of interest revolves around electrochemical separations for deionization. Electrochemical separations primarily consist of technologies which either intercalate or electroadorb species of interest from a bulk mixture. This can be conducted through polarizing a battery electrode, or more commonly a capacitive electrode. One example is the technology capacitive deionization (CDI). CDI is being investigated as a means to augment the current state of the art, and as a stand-alone brackish water treatment technology. Despite the potential of this technology, there is still much that is not known regarding the energetics and efficiency of both the desalination and brine formation process. Here, blue refrigeration is a term used to broadly describe desalination cycles and processes. The analogy aims to compare the energetics associated with a desalination cycle to the energetics well studied in thermal refrigeration cycles. This perspective aims to evaluate some of the emerging energetic issues associated with CDI, and to describe how new system architectures may play a role in achieving more ideal energy and desalination performance.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2017;15(1):011010-011010-9. doi:10.1115/1.4038518.

Thermal runaway is a phenomenon that occurs due to self-sustaining reactions within batteries at elevated temperatures resulting in catastrophic failure. Here, the thermal runaway process is studied for a Li-ion and Na-ion pouch cells of similar energy density (10.5 Wh, 12 Wh, respectively) using accelerating rate calorimetry (ARC). Both cells were constructed with a z-fold configuration, with a standard shutdown separator in the Li-ion and a low-cost polypropylene (PP) separator in the Na-ion. Even with the shutdown separator, it is shown that the self-heating rate and rate of thermal runaway in Na-ion cells is significantly slower than that observed in Li-ion systems. The thermal runaway event initiates at a higher temperature in Na-ion cells. The effect of thermal runaway on the architecture of the cells is examined using X-ray microcomputed tomography, and scanning electron microscopy (SEM) is used to examine the failed electrodes of both cells. Finally, from examination of the respective electrodes, likely due to the carbonate solvent containing electrolyte, it is suggested that thermal runaway in Na-ion batteries (NIBs) occurs via a similar mechanism to that reported for Li-ion cells.

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

Errata

Commentary by Dr. Valentin Fuster

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