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

Self Cite

Nonaqueous redox flow batteries (NAqRFBs) are a promising, but nascent, concept for cost-effective grid-scale energy storage. While most studies report new active molecules and proof-of-concept prototypes, few discuss cell design. The direct translation of aqueous RFB design principles to nonaqueous systems is hampered by a lack of materials-specific knowledge, especially concerning the increased viscosities and decreased conductivities associated with nonaqueous electrolytes. To guide NAqRFB reactor design, recent techno-economic analyses have established an area specific resistance (ASR) target of <5 cm 2 . Here, we employ a state-of-the-art vanadium flow cell architecture, modified for compatibility with nonaqueous electrolytes, and a model ferrocene-based redox couple to investigate the feasibility of achieving this target ASR. We identify and minimize sources of resistive loss for various active species concentrations, electrolyte compositions, flow rates, separators, and electrode thicknesses via polarization and impedance spectroscopy, culminating in the demonstration of a cell ASR of ca. 1.7 cm 2 . Further, we validate performance scalability using dynamically similar cells with a ten-fold difference in active areas. This work demonstrates that, with appropriate cell engineering, low resistance nonaqueous reactors can be realized, providing promise for the cost-competitiveness of future NAqRFBs. Grid-scale energy storage has emerged as a critical technology for alleviating the intermittency of renewables, 1 improving the efficiency of the existing grid infrastructure, and offering frequency or voltage regulation.2 Redox flow batteries (RFBs) are particularly attractive storage devices for energy-intensive grid applications.2,3 In a typical RFB, charge-storing active species are dissolved in liquid electrolytes, which are housed in large and inexpensive tanks. The electrolytes are pumped through a power-generating electrochemical reactor, where the active species undergo reduction or oxidation on the surfaces of porous electrodes to charge or discharge the battery. [3][4][5][6] This unique architecture offers several advantages over conventional battery systems, including decoupled power and energy scaling, long operational lifetimes, easy maintenance, simplified manufacturing, and high active-to-inactive materials ratio (particularly at long storage durations). Despite many attractive features, state-of-the-art RFBs are currently too expensive for widespread adoption.7 While the majority of RFB electrolytes are aqueous, 6 transitioning to nonaqueous chemistries could enable higher cell potentials via wider electrolyte stability windows, subsequently increasing the electrolyte energy density and reducing chemical costs. 6,[8][9][10][11] Furthermore, the use of nonaqueous solvents enables a broader palette of potentially inexpensive active materials, which could significantly lower RFB costs. 7,12 In contrast to their aqueous counterparts, nonaqueous redox flow batteries (NAqRFBs) are a nascent technology,...

mentioning

confidence: 99%

Towards Low Resistance Nonaqueous Redox Flow Batteries

Milshtein

Barton

Carney

et al. 2017

Self Cite

“…Note that highly reversible redox couples were recorded for V(acac) 3 in the presence of tetraethylammonium hexafluoro phosphate (TEAPF 6 ) or TEAB supporting electrolyte in AN on a GC electrode. 5,7 On the other hand, the two vanadium redox couples exhibited quasi-reversible behavior under these given conditions, particularly in the TBAP and NaClO 4 -supporting electrolytes. In addition, the V(acac) 3 redox behavior varied according to the supporting electrolyte used, 31 which means that V(acac) 3 shows sluggish electron transfer in the both the TBAP and NaClO 4 supporting electrolytes, which might have influenced the cell potential variations.…”

Section: Resultsmentioning

confidence: 90%

Na-β-Alumina as a Separator in the Development of All-Vanadium Non-Aqueous Tubular Redox Flow Batteries: An Electrochemical and Charging-Discharging Examination Using a Prototype Tubular Redox Flow Cell

Muthuraman

Boyeol

Moon

2018

The non-aqueous redox flow battery (N-ARFB) is in the development stages with aim to produce high power density storage systems. In addition to the development of N-ARFBs, this study examined the applicability of a sodium beta alumina (Na-β-Al 2 O 3 ) membrane in the development of a N-ARFB through an analysis of the electrochemical processes, redox active species migration, and charging/discharging at room temperature (25 ± 3 • C). Through impedance analysis, the ionic conductivity of the Na-β-Al 2 O 3 was 2.97 × 10 −2 S cm −1 which is slightly higher than the literature value. UV-Visible analysis showed no migration of the vanadium acetylacetonate (V(acac) 3 ) ion from one compartment to another, either during the charging or discharging process. In addition, the lack of a change in the morphology of the spent membrane revealed not only stability, but also confirmed no permeation of V(acac) 3 species. The maximum applied current density for charging and discharging was 0.01 mA cm −2 and 0.0015 mA cm −2 , respectively. The charging/discharging of V(acac) 3 enables voltage and current efficiencies of almost 16% and 11% respectively, at the state of charge of 15%. This demonstrates that the Na-β-Al 2 O 3 membrane can be improved for use in N-ARFB after optimizing the conditions. © The Author The non-aqueous redox flow battery (N-ARFB) is in the development stages with the strong potential for high power density (>2 V) N-ARFB that can store energy from non-conventional sources, such as wind mills and solar power. 1 Although the development of N-ARFBs has attracted considerable interest, such as redox active species selection, 2 electrolyte development, 2,3 and solvent selection, 4 the identification of a suitable membrane is one of the crucial factors because of the crossover of redox active species. [4][5][6] The oxidation state of most redox active metal organic complexes (RAMOCs) is negative, which means that there is theoretically less migration on membranes with the same polarity but the transport of oppositely charged negative ions is allowed. Using this approach, many anionic selective polymeric membranes (AEM) were developed and made commercially available for N-ARFBs, such as Neosepta AHA (Astom, Japan), FAP4 (FuMaTech Co.), and AMI-7001 (Membrane International Inc., USA), which were investigated in N-ARFB systems using a conventional H-type cell. [7][8][9] On the other hand, they showed poor chemical and mechanical stability in organic electrolytes. 1 Through a zero-gap (electrodes coated on the membrane) redox flow battery prototype design, the modified Nafion 1035 membrane could be operated in all VRFB with columbic and energy efficiencies of 91% and 80%, respectively. 10 A polymeric membrane prepared by a simultaneous polymerization and quarternization method showed low vanadium permeability while retaining good dimensional and chemical stability in N-ARFB. 11 In addition to crosslinking, a pore-filled polyvinyl chloride (PVC) AEM exhibited high ion conductivity and V(III)(acetylacetonate) 3 (V(acac) ...

“…6,8,12,13 Most commercially developed RFBs use IEMs, 14 such as perfluorinated Nafion, 12 due to its robustness, stability and suitable performance in aqueous environments. IEMs are typically the membrane of choice since these are non-porous sheets of crosslinked polyelectrolytes capable of exchanging ions at its interface with low levels of solution mixing.…”

mentioning

confidence: 99%

“…13,16 Nonetheless, even with a highly conductive IEM, challenges associated with material crossover and broad chemical compatibility remain. Here, we validate an alternate approach for enabling the widespread implementation of NRFBs in the form of size-exclusion cells that utilize commercial-off-the-shelf membranes in combination with soluble redox active polymers (RAPs).…”

mentioning

confidence: 99%

“…[6][7][8][9][10][11] Yet, their wide-scale adoption has been hampered by the lack of chemically-suitable ion-exchange membranes (IEMs) that decrease areal resistance while simultaneously preventing material crossover between compartments. 6,8,12,13 Most commercially developed RFBs use IEMs, 14 such as perfluorinated Nafion, 12 due to its robustness, stability and suitable performance in aqueous environments. IEMs are typically the membrane of choice since these are non-porous sheets of crosslinked polyelectrolytes capable of exchanging ions at its interface with low levels of solution mixing.…”

mentioning

confidence: 99%

See 1 more Smart Citation

Redox Active Polymers for Non-Aqueous Redox Flow Batteries: Validation of the Size-Exclusion Approach

Montoto

Gavvalapalli

Moore

et al. 2017

Self Cite

101

108

Non-aqueous redox flow batteries (NRFBs) are emerging technologies that promise higher energy densities than aqueous counterparts. Unfortunately, cell resistance and redox component crossover observed when using ion-exchange membranes (IEMs) hinders NRFB development. The size exclusion approach for polymer-based NRFBs addresses these issues by using macromolecular design to mitigate crossover. Here, we highlight the benefits of this approach using inexpensive nano-porous separators (PS) (Celgard and Daramic). We evaluated these along with an IEM (Fumasep) in a flow cell configuration using a classical redox couple of viologen and ferrocene, both in monomer and polymer forms. High Coulombic efficiencies above 98% and access up to 80% of capacity were observed for the polymer cells. These displayed better performance with PS than with the IEM, which exhibited lower energy efficiencies from higher overpotentials. The monomer equivalent cells with PS resulted in lower efficiencies and rapid decrease in depth of discharge. Post-cycling analysis by ultramicroelectrode voltammetry showed that the small molecules freely crossed PS and to a lesser degree through the IEM. Therefore, here we demonstrate that the combination of redox active polymers and simple PS enables a potential next-generation NRFB system that provides a competitive alternative to the use of IEMs in NRFBs. Renewable energy sources, such as wind and solar, are emerging inputs in the electrical grid.1 However, these sources are inherently intermittent, thus strategies designed to decouple energy production from its use are highly desirable. Redox flow batteries (RFBs) promise to address these challenges by offering convenient charge storage in fluid form with straightforward scalability and deployment, and long-term durability.2-4 In particular, non-aqueous redox flow batteries (NRFBs) promise higher energy densities than aqueous RFBs owing to a wider electrochemical window. 5,6 NRFBs provide a wide range of molecular and electrolyte designs with attractive redox potentials, solubilities and chemical tunability.6-11 Yet, their wide-scale adoption has been hampered by the lack of chemically-suitable ion-exchange membranes (IEMs) that decrease areal resistance while simultaneously preventing material crossover between compartments. 6,8,12,13 Most commercially developed RFBs use IEMs, 14 such as perfluorinated Nafion, 12 due to its robustness, stability and suitable performance in aqueous environments. IEMs are typically the membrane of choice since these are non-porous sheets of crosslinked polyelectrolytes capable of exchanging ions at its interface with low levels of solution mixing.12 Crossover in those cases is typically ascribed to varying levels of ion selectivity 12,14 in the membranes. However, recent reports suggest low conductivity of these membranes in typical non-aqueous battery solvents (e.g. propylene carbonate, acetonitrile, etc.).13,15 A possible solution is to explore membrane chemistries that provide highly-conductive IEMs tailored to the...