Deep eutectic solvents (DESs) are an emerging class of mixtures characterized by significant depressions in melting points compared to those of the neat constituent components. These materials are ...promising for applications as inexpensive “designer” solvents exhibiting a host of tunable physicochemical properties. A detailed review of the current literature reveals the lack of predictive understanding of the microscopic mechanisms that govern the structure–property relationships in this class of solvents. Complex hydrogen bonding is postulated as the root cause of their melting point depressions and physicochemical properties; to understand these hydrogen bonded networks, it is imperative to study these systems as dynamic entities using both simulations and experiments. This review emphasizes recent research efforts in order to elucidate the next steps needed to develop a fundamental framework needed for a deeper understanding of DESs. It covers recent developments in DES research, frames outstanding scientific questions, and identifies promising research thrusts aligned with the advancement of the field toward predictive models and fundamental understanding of these solvents.
To achieve high temperature operation of proton exchange membrane fuel cells (PEMFC), preferably under ambient pressure, acid–base polymer membranes represent an effective approach. The phosphoric ...acid-doped polybenzimidazole membrane seems so far the most successful system in the field. It has in recent years motivated extensive research activities with great progress. This treatise is devoted to updating the development, covering polymer synthesis, membrane casting, physicochemical characterizations and fuel cell technologies. To optimize the membrane properties, high molecular weight polymers with synthetically modified or N-substituted structures have been synthesized. Techniques for membrane casting from organic solutions and directly from acid solutions have been developed. Ionic and covalent cross-linking as well as inorganic–organic composites has been explored. Membrane characterizations have been made including spectroscopy, water uptake and acid doping, thermal and oxidative stability, conductivity, electro-osmotic water drag, methanol crossover, solubility and permeability of gases, and oxygen reduction kinetics. Related fuel cell technologies such as electrode and MEA fabrication have been developed and high temperature PEMFC has been successfully demonstrated at temperatures of up to 200
°C under ambient pressure. No gas humidification is mandatory, which enables the elimination of the complicated humidification system, compared with Nafion cells. Other operating features of the PBI cell include easy control of air flow rate, cell temperature and cooling. The PBI cell operating at above 150
°C can tolerate up to 1% CO and 10
ppm SO
2 in the fuel stream, allowing for simplification of the fuel processing system and possible integration of the fuel cell stack with fuel processing units. Long-term durability with a degradation rate of 5
μV
h
−1 has been achieved under continuous operation with hydrogen and air at 150–160
°C. With load or thermal cycling, a performance loss of 300
μV per cycle or 40
μV
h
−1 per operating hour was observed. Further improvement should be done by, e.g. optimizing the thermal and chemical stability of the polymer, acid–base interaction and acid management, activity and stability of catalyst and more importantly the catalyst support, as well as the integral interface between electrode and membrane.
Rechargeable redox flow batteries are being developed for medium and large-scale stationary energy storage applications. Flow batteries could play a significant role in maintaining the stability of ...the electrical grid in conjunction with intermittent renewable energy. However, they are significantly different from conventional batteries in operating principle. Recent contributions on flow batteries have addressed various aspects, including electrolyte, electrode, membrane, cell design,
etc.
In this review, we focus on the less-discussed practical aspects of devices, such as flow fields, stack and design considerations for developing high performance large-scale flow batteries. Finally, we provide suggestions for further studies on developing advanced flow batteries and large-scale flow battery stacks.
This review article summarizes the development of flow fields and stacks, and design considerations for next-generation flow batteries.
Redox flow batteries with flow field designs have been demonstrated to boost their capacities to deliver high current density and power density in medium and large-scale energy storage applications. ...Nevertheless, the fundamental mechanisms involved with improved current density in flow batteries with serpentine flow field designs have been not fully understood. Here we report a three-dimensional model of a serpentine flow field over a porous carbon electrode to examine the distributions of pressure driven electrolyte flow penetrations into the porous carbon electrodes. We also estimate the maximum current densities associated with stoichiometric availability of electrolyte reactant flow penetrations through the porous carbon electrodes. The results predict reasonably well observed experimental data without using any adjustable parameters. This fundamental work on electrolyte flow distributions of limiting reactant availability will contribute to a better understanding of limits on electrochemical performance in flow batteries with serpentine flow field designs and should be helpful to optimizing flow batteries.
•A three-dimensional model of serpentine flow fields in a flow battery.•Flow distributions at the flow field-porous carbon electrode interface.•Quantified electrolyte penetration into the porous carbon electrode.•Maximum current density estimate based on reactant availability.•Effects of electrode thickness and porosity on maximum current density.
Iron chloride in a deep eutectic solvent containing high concentrations of iron with choline chloride and ethylene glycol have been synthesized. It was found that physical properties of the ...electrolytes, as well as the nature of the electroplated iron are greatly influenced by electrolyte composition. This is not surprising in that electrochemical reactivity of the solute ions as well as the physical properties of the electrolyte are controlled by speciation of the metals in solution. When the chloride to iron ratio is ≥4:1, complexes such as FeCl4− and FeCl42− were shown to be the dominant species using X-ray absorption near edge structure measurements coupled with Raman spectroscopy. However, when the chloride to iron ratio falls below 4:1, the ethylene glycol was found to complex the iron; the presence of this complex hinders fluid properties as shown by an order of magnitude decrease in solution conductivity as well as alter the iron deposition mechanism.
•We propose a mathematical model of vanadium flow battery with flow field design.•The flow and pressure distributions in the flow channel and electrode are examined.•Comparisons of ideal plug flow ...and ideal parabolic flow inlet boundary conditions.•The volumetric flow rate penetration through the carbon paper electrode is studied.•Maximum current density is estimated based on 100% of stoichiometry reaction
In this work, a two-dimensional mathematical model is developed to study the flow patterns and volumetric flow penetrations in a segment of flow channel over porous electrode layered system in vanadium flow battery with flow field design. The flow distributions at the interface between the flow channel and porous electrode are examined. It is found that the non-linear pressure distributions can distinguish the interface flow distributions under the ideal plug flow and ideal parabolic flow inlet boundary conditions. Nevertheless, the volumetric flow penetration within the porous electrode beneath the flow channel through the integration of interface flow velocity reveals that this value is identical under both ideal plug flow and ideal parabolic flow inlet boundary conditions. The volumetric flow penetrations under the advection effects by both flow channel and landing/rib are estimated. The maximum current density achieved in a flow battery with a flow field architecture can be predicted based on the 100% amount of electrolyte flow reactant consumption through the porous electrode beneath flow channel and landing/rib. The corresponding theoretical maximum current densities achieved in vanadium flow battery with one and three layers of SGL 10AA carbon paper electrode have reasonable agreement with experimental results under a proper permeability.
Slurry electrodes are investigated in order to decouple the energy storage capacity from the power delivery capability in an all-iron flow battery. For the slurry electrode to perform effectively, ...the battery negative reaction must occur on the slurry particles at reasonably high current densities. Mathematical modeling is used to investigate the current distribution in a slurry electrode as a function of the slurry specific area and electrical conductivity in order to achieve >95% plating in the slurry electrode (not on the flat plate) at > 200 mA cm−2. From the mathematical modeling, MWCNTs are selected to demonstrate slurry electrode performance. The experimental performance of all-iron batteries charged using the MWCNT slurry electrodes is found to improve while increasing the battery state-of-charge. Two possible mechanisms contributing to this effect are an increase in electronic conductivity of the slurry and an increase in plating kinetics. After cycling the battery, <5% of the total battery charge was observed to have plated onto the current collector.
•Slurries are investigated as negative electrodes in all-iron flow batteries.•Current distribution is modeled as a function of area and electrical conductivity.•MWCNTs slurries contained >95% of battery charge (as opposed to current collector).•Slurry electrodes can be effectively used to decouple all-iron flow batteries.
For large-scale energy storage, flow batteries present many advantages. These benefits include, but are not limited to, decoupling power rating from energy capacity and projected lower cost energy ...storage and long cycle life. Several reviews and a comprehensive handbook have come available recently describing the various flow battery chemistries and the details of cell and stack construction. However, none of the chemistries proposed are perfect and the commercialization path to widespread adoption remains challenging. For the purposes of this article, we will take a closer look at the concept and development of an all-iron slurry flow battery and the intellectual property (IP) protection and commercialization process that occurred within an academic environment.
A simple analytical model of a layered system comprised of a single passage of a serpentine flow channel and a parallel underlying porous electrode (or porous layer) is proposed. This analytical ...model is derived from Navier–Stokes motion in the flow channel and Darcy–Brinkman model in the porous layer. The continuities of flow velocity and normal stress are applied at the interface between the flow channel and the porous layer. The effects of the inlet volumetric flow rate, thickness of the flow channel and thickness of a typical carbon fiber paper porous layer on the volumetric flow rate within this porous layer are studied. The maximum current density based on the electrolyte volumetric flow rate is predicted, and found to be consistent with reported numerical simulation. It is found that, for a mean inlet flow velocity of 33.3 cm s−1, the analytical maximum current density is estimated to be 377 mA cm−2, which compares favorably with experimental result reported by others of ∼400 mA cm−2.
•An analytical model for vanadium RFBs with serpentine flow channel is developed.•The flow velocity distributions in the porous electrode is analyzed.•Effects of geometry parameters on flow rate penetrating in electrode are studied.•The maximum current density achieved based on reactant flow rate is predicted.
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