Composition and Conductivity of Membranes Equilibrated with Solutions of Sulfuric Acid and Vanadyl Sulfate

In a flow battery, the salient impact of the electrolyte velocity on the mass transfer coefficient in carbon felt electrodes is demonstrated and quantified. A lab-scale flow battery, fed with identical electrolyte solutions containing Fe 2+ /Fe 3+ as active substances in both the anode and the cathode, is used to realize stable tests free from side reactions in a broad range of current densities. The electrolyte velocities ranging from 2.5 to 15 mm s −1 are selected in this work, which are typical in flow through electrodes in most flow batteries. By measuring limiting currents at various flow rates, a correlation between the mass transfer coefficient and the velocity in dimensionless form is obtained as Sh = 1.68 Re 0.9 . Meanwhile, a 2-D numerical model incorporating this correlation and the experimentally measured electrolyte conductivity is proposed. Voltage losses of the battery fed with adequate reactants at different velocities are both experimentally measured and numerically simulated. The agreement between simulated results and experimental data verifies the applicability of this correlation under normal operating conditions below limiting currents. Owing to the exclusive advantage of decoupling power generation and energy storage, redox flow batteries (RFBs) have been considered as a critical candidate for large-scale electrical energy storage (EES). [1][2][3][4] Unlike the electrodes of conventional secondary batteries, the electrodes of RFBs do not participate in reactions but only provide electrochemical reaction sites for active ions and support electrons transfer. For numerous kinds of developed RFBs, the porous carbon felts, composed of randomly interlaced carbon fibers, are generally selected as electrodes. [5][6][7] Obtaining high voltage efficiency at high current densities is one of the major challenges in the commercialization of RFBs. Much efforts have been devoted to diminishing the activation loss by modifying the carbon fiber surface. [8][9][10][11][12] In addition, ohmic loss and mass transfer loss also lead to the reduction of the voltage efficiency. It is generally recognized that supplying high flow rate of electrolyte for the RFB is the most feasible method to minimizing mass transfer loss, but more pumping power is inevitably required for increasing flow rates. Tang et al. 6 and Ma et al. 7 carried out special research on optimizing the operating strategy of electrolyte flow rate for kilowatts class allvanadium flow battery (VRFB) systems, in which the significant impact of flow rate on cell voltage was detected during the whole charge/discharge process. In a carbon felt electrode, the mass transfer loss is caused by the species transport between the bulk solution in the pore and the carbon fiber surface, which is usually quantified by the mass transfer coefficient k m . 13 For cells operating under high current densities, the mass transfer loss also contributes a significant proportion to the voltage loss, especially when approaching the end of charge/discharge process. Considerin...

Section: Resultsmentioning

confidence: 99%

The Dependence of Mass Transfer Coefficient on the Electrolyte Velocity in Carbon Felt Electrodes: Determination and Validation

You

Cheng

2017

“…Similar pore size constriction has been observed for the protonated N117 with the increase of sulfuric acid concentration in aqueous system. 21 Figure 5b presents the through-plane ionic conductivity (κ m ) of Li-N117 in various electrolytes. At the same LiPF 6 concentration, the order of κ m always follows: PC < PC : EC < DMSO, which is consistent with that of the in-plane conductivities of Li-N117 in the same set of pure solvents.…”

Section: Resultsmentioning

confidence: 99%

“…The same soaking time was used for protonated N117 in sulfuric acid solution. 21 The solvent weight uptake (ω) is defined as: ω = (m n − m 0 )/m 0 , where n = 1 or 2. The thickness change (σ) is defined as: σ = (l n -l 0 )/l 0 , where n = 1 or 2.…”

Section: Methodsmentioning

confidence: 99%

An Investigation of the Ionic Conductivity and Species Crossover of Lithiated Nafion 117 in Nonaqueous Electrolytes

Darling

Gallagher

et al. 2015

Nonaqueous redox flow batteries are a fast-growing area of research and development motivated by the need to develop lowcost energy storage systems. The identification of a highly conductive, yet selective membrane, is of paramount importance to enabling such a technology. Herein, we report the swelling behavior, ionic conductivity, and species crossover of lithiated Nafion 117 membranes immersed in three nonaqueous electrolytes (PC, PC : EC, and DMSO). Our results show that solvent volume fraction within the membrane has the greatest effect on both conductivity and crossover. An approximate linear relationship between diffusive crossover of neutral redox species (ferrocene) and the ionic conductivity of membrane was observed. As a secondary effect, the charge on redox species modifies crossover rates in accordance with Donnan exclusion. The selectivity of membrane is derived mathematically and compared to experimental results reported here. The relatively low selectivity for lithiated Nafion 117 in nonaqueous conditions suggests that new membranes are required for competitive nonaqueous redox flow batteries to be realized. Large scale energy storage for the electricity grid is widely considered to be necessary for enabling the use of intermittent renewables as primary power sources, for stabilizing power delivery in developing economies, and for facilitating a range of high-value grid services aimed at improving efficiency and lifetime.1 To date, broad market penetration of energy storage systems has been hindered primarily by high installation and operation costs, resulting in only ∼2.5% of the total electricity production in the US relying on grid energy storage predominantly in the form of pumped hydro-electric.2 However, the rapid and sustained growth of renewable electricity generation (e.g., solar photovoltaic, wind) continues to drive the need for advanced low cost energy storage.3-5 While certain energy storage technologies, such as lithium-ion (Li-ion) batteries, are currently at a price point to fulfill niche markets, 6 present electrical energy storage technologies, in general, are still not economically viable for wide-scale deployment, spurring research and development efforts worldwide. 7Redox flow batteries (RFBs) are electrochemical systems wellsuited for multi-hour energy storage and offer several key advantages over enclosed batteries (e.g., Li-ion, Lead-acid) including independent scaling of power and energy, long service life, improved safety, and simplified manufacturing. [8][9][10][11] Since their advent in the 1970s, 12 numerous aqueous redox chemistries have been developed but none has experienced widespread commercial success. The use of nonaqueous electrolytes in flow batteries is a nascent, yet burgeoning, concept.Transitioning from aqueous to nonaqueous electrolytes offers an extended window of electrochemical stability (> 3 V) and an enriched selection of redox materials due to the broader design space. However, this approach is not without technical hurdles such as the identificati...

“…As previously discussed, the ion conduction mechanism in QDAPP may be strongly related to the acid/water uptake behavior. Thus, it follows that when SoC changes, which can affect acid uptake, 23 the membrane conductivity may also change. In contrast, the SDAPP membranes do not appear to have the same sensitivity to 100% SoC solutions.…”

Section: Resultsmentioning

confidence: 99%

“…The membrane conductivity was measured with a four probe conductivity cell in a protocol described elsewhere. 23 Thicknesses of membrane samples were taken by a Mitutoyo 369-350 digital micrometer after soaking in DI water.…”

Section: Methodsmentioning

confidence: 99%

Full Cell Study of Diels Alder Poly(phenylene) Anion and Cation Exchange Membranes in Vanadium Redox Flow Batteries

Pezeshki

Tang

Fujimoto

et al. 2015

In this work, we report on the performance of Diels Alder poly(phenylene) membranes in vanadium redox flow batteries. The membranes were functionalized with quaternary ammonium groups to form an anion exchange membrane (QDAPP) and with sulfonic acid groups to form a cation exchange membrane (SDAPP). Both membrane classes showed similar conductivities in the battery environment, suggesting that the ion conduction mechanism in the material is not strongly affected by the moieties along the polymer backbone. The resistance to vanadium permeation in QDAPP was not improved relative to SDAPP, further suggesting that the polarity of the functional groups do not play a significant role in the membrane materials tested. Both QDAPP and SDAPP outperformed Nafion membranes in cycling tests, with both achieving voltage efficiencies above 85% while maintaining 95% coulombic efficiency while at a current density of 200 mA/cm 2 . © The Author The need for grid-scale energy storage is well-established as demand for intermittent renewable energy sources increases. Among grid-scale energy storage technologies, vanadium redox flow batteries (VRFBs) present an attractive option, as has been extensively discussed elsewhere.1-4 One of the key components in VRFBs is the membrane. As the separator between the positive and negative sides, the membrane is responsible for limiting undesired crossover of vanadium species and water while enabling ion conduction. The membrane's ability to restrict crossover is the key factor in cell coulombic efficiency and is important in slowing capacity fade. The conductivity determines the ohmic drop in the membrane, which is the major source of ohmic resistance in the battery and therefore plays a key role in determining voltage loss.5 Thus, it is desirable to engineer membranes that have a high selectivity, i.e., low vanadium permeability with high conductivity for charge carrying ions, predominantly protons or bisulfate. Commonly used membrane materials in the laboratory consist of perfluorinated polymers that provide chemical-mechanical stability, with polar side chains to provide ion conduction pathways; however, materials such as Nafion are relatively expensive and exhibit poor selectivity. Modified Nafion membranes can provide improved selectivity, 6,7 but are still based on the expensive Nafion material. Several studies have investigated alternative hydrocarbon backbone polymers as a means to decrease cost and improve selectivity. Proton exchange membranes in the form of sulfonated aromatic polymers have received the most attention 1,9-13 due to the relative ease of preparation and promising VRFB performance. However, issues of low membrane cyclic lifetimes have been reported and suggest membrane oxidation through the VO 2 + ion. 14 Therefore, anion exchange membranes (AEM) have been presented as an attractive option because vanadium permeation is expected to be limited by the Donnan potential generated by the positively charged functional groups, which would not only reduce vanadium crossover, bu...