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Tuning the Perfluorosulfonic Acid Membrane Morphology for Vanadium Redox Flow Batteries Vijayakumar Murugesan, Qingtao Luo, Ralph Lloyd, Zimin Nie, Xiaoliang Wei, Bin Li, Vincent L. Sprenkle, J-David Londono, Murat Unlu, and Wei Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10744 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 25, 2016

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ACS Applied Materials & Interfaces

Tuning the Perfluorosulfonic Acid Membrane Morphology for Vanadium Redox Flow Batteries M. Vijayakumar,a Qingtao Luo,a Ralph Lloyd,b Zimin Nie,a Xiaoliang Wei,a Bin Li,a Vincent Sprenkle,a J-David Londono,c Murat Unlu,d and Wei Wanga* aPacific

Northwest National Laboratory, 902 Battelle Blvd, Richland, WA 99354, USA Chemicals and Fluoroproducts, 22828 Hwy 87 South, Fayetteville, NC 28306, USA cDuPont Central Research and Development, 700-707 Powder Mill Rd, Wilmington, DE 19880, USA dThe Chemours Company, P.O. Box 8352, Wilmington, DE 19803, USA bDuPont

Correspondence: Dr W Wang, Pacific Northwest National Laboratory, 902 Battelle Blvd, Richland, WA 99354, USA. E-mail: [email protected]

ABSTRACT: The microstructure of the perfluorinated sulfonic acid proton exchange membranes such as Nafion® significantly affects their transport properties and performance in a vanadium redox flow battery (VRB). In this work, Nafion® membranes with various equivalent weights (EW) ranging from 1000 to 1500 are prepared and the morphology-property-performance relationship is investigated. Nuclear magnetic resonance (NMR) and small-angle X-ray scattering (SAXS) studies revealed their composition and morphology variances, which lead to major differences in key transport properties related to proton conduction and vanadium ion permeation. Their performances are further characterized as VRB membranes. Based on those understanding, a new perfluorosulfonic acid membrane is designed with optimal pore geometry and thickness, leading to higher ion selectivity and lower cost compared with the widely used Nafion® 115. Excellent VRB single-cell performance (89.3% energy efficiency at 50mA∙cm-2) was achieved along with a stable cyclical capacity over prolonged cycling.

Keywords: Nafion, Redox Flow Battery, ion exchange membrane, 19F NMR, SAXS

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INTRODUCTION Nafion® membranes are the state-of-art perfluorosulfonic acid-based (PFSA) proton exchange membrane because of their excellent chemical stability and high proton conductivity. They are widely used in fuel cells and other electrochemical devices, where high ionic conductivity, long-term durability, and good selectivity are often demanded in strongly acidic and highly corrosive environments.1 In addition, Nafion® membranes are also predominantly used in all-vanadium redox flow battery (VRB), which has recently attracted a great deal of research and development interests as a promising candidate for large-scale stationary energy storage.2-4 Extensive research has been undertaken to understand the morphology and structure of Nafion® membranes and their performance.1 However, most of those studies have been focused on the performance of Nafion® membrane in chloro-alkali cells and proton exchange membrane fuel cells. The properties of the Nafion® membranes in VRBs are relatively unexplored. The working mechanisms and environment of Nafion® membrane in a redox flow battery, VRB for example, are drastically different from those in other applications, such as in fuel cells. Significant knowledge gaps therefore remain on the fundamental understanding of the morphology-property-performance relationship of the Nafion® membrane under VRB operating conditions, which has largely hindered the efforts to tailor the Nafion® membrane structure for the vanadium redox flow batteries.

In particular, vanadium ions readily cross through the Nafion® membrane,5-6 leading to substantial capacity decay with a significant and potentially devastating risk of vanadium precipitation during long-term battery operation.7 The poor capacity retention of the Nafion® membrane along with its high price has seriously slowed the development of VRB technology.2, 8 A few reports thus have focused on the vanadium-ion permeability issue by adopting blocking agents through post modification of Nafion® membranes, such as the Nafion®/SiO2,9 Nafion®/TiO2,10 and


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Nafion®/ polyvinylidene fluoride hybrid membranes.11 Yet, a clear picture of Nafion’s structure in relationship with its behavior in VRB cells has not emerged. The morphology of the Nafion® membrane is a two-phase structure, in which the tetrafluoroethylene (TFE) forms the hydrophobic backbone while the pendant sulfonic side chains comprise the hydrophilic region. Figure 1a represents schematic view of Nafion® molecule with backbone, side chain and terminal sulfonic group. Various models have been developed to explain the proton transport mechanism through these hydrophilic pore.1 In general, the sulfonic acid sites (-SO3) are aggregated into network of pore structure, which act as water channels and promote excellent proton conductivity, while the TFE backbone affords superior chemical and mechanical stabilities. In view of its two-phase structure, a rational design to improve its selectivity is to reduce the content of sulfonic side chains, thereby limiting the accessible water channels for vanadium-ion permeation to improve the capacity retention capability of the Nafion® membrane. The weight of dry polymer in grams containing one mole of sulfonic acid groups is traditionally defined as equivalent weight (EW).1 Recent studies were focused on the role of various macroscopic physical properties such as thickness, thermal treatment of Nafion® membranes on vanadium diffusion and subsequent VRFB cell performance12-14. However, the vanadium diffusion through the Nafion® membranes is a molecular level process, and hence optimal membrane design would require fundamental knowledge about the evolution of pores with molecular structural changes. Systematic investigations of the morphological changes and transport characteristics for Nafion® membranes with various EW value are hereby conducted, based on which a highly selective Nafion® membrane is developed for VRB application with promising performance in terms of both the capacity retention and cost reduction.

EXPERIMENTAL PROCEDURE Membrane Preparation The EW value of the Nafion® perfluorosulfonic acid copolymer is controlled by adjusting the ratio of monomers during copolymerization of TFE and perfluorinated



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vinyl ether that contains sulfonyl fluoride15. For this study the co-monomer ratio was adjusted to achieve polymers with EW values of 1000, 1200, and 1500 and designated as NDM 220, NDM 223 and NDM 221-1 respectively. These polymers were then isolated, extruded into film with controlled thickness, hydrolyzed to convert the sulfonyl fluoride pendant group into the sulfonate form, and then acid exchanged to achieve sulfonic acid pendant groups. The thicknesses of the membranes are kept at around 2mil (~50 µm), while the EW of the membranes increased from 1000 for NDM220 to 1500 for NDM221-1. In addition, a thinner membrane (~1mil) with EW value of 1500 was prepared and designated as NDM221-2. It should be noted that NDM220 has the same EW value, but one fifth of a thickness compared with commercially available N115 membrane (EW value of 1000 and thickness ~5mil), which is currently the most widely used membrane in various VRB systems. As purchased N115 membrane is used as a control sample and tested under identical conditions.

Nuclear Magnetic Resonance (NMR) Analysis A Varian 500 Inova spectrometer (B0= 11.1T and 19F Larmor frequency of 470.6 MHz) was used for the 19F NMR measurements. The quantitative 19F magic MASNMR spectra were recorded at a spinning speed of 13.5 kHz under single-pulse measurements with a recycle delay of 60 seconds. The membranes were cut into small pieces and tightly packed in 4-mm zirconia rotors for the MAS-NMR measurements. The 19F chemical shifts were externally referenced to an aqueous solution of sodium trifluoroacetate (δiso=-75.4ppm). The line width and peak area for each component were obtained by fitting the line-shapes of both resonance lines and spinning side bands using the DMfit program. The estimated uncertainties in fitted parameters were typically less than 3%.

Small Angle X-ray scattering (SAXS) Analysis Small Angle X-ray Scattering (SAXS) measurements were performed on a conventional three pinhole laboratory based instrument. X-rays were produced by


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a Rigaku rotating Cu anode operating at 40 KV and 30 mA. A multilayer optic from Osmic and three consecutive pinholes were used to produce a collimated and monochromatic 200 mm x-ray beam of CuKα wavelength (l=1.54Å). The distance between the source and the sample was 2400mm, and between the sample and the detector was 340mm. A Vantec 2000 detector from Bruker was used to measure the 2D-SAXS radiation. The azimuthal averaging of the data was performed with Irena16 in IGOR from Wavemetrics.17 A 3 mm diameter home-made beamstop with a pin diode detector was used to integrate the incident intensity during the period of the measurement and also to measure the sample transmission. A glassy carbon secondary intensity standard, the sample transmission and the sample thickness, were used to scale the intensity to absolute units of cm-1. The intensity were expressed as a function of the scattering vector magnitude or scattering coordinate within the range 0.08 NDM220 at current density of 50 mA cm-2 and in the order of NDM223 > NDM220 > NDM221-1 at 150 mA∙cm-2. The trend is mainly due to the different increase of CEs with increasing current


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densities. When the current density increased from 50 mA∙cm-2 to 150 mA∙cm-2, the CE of NDM220 increased by 4.4%, while those of NDM223 and NDM221-1 only increased by 1.3% and 0.5%.

Figure 4. Efficiencies and capacity of VRB flow cell with different PFSA membrane: (a) columbic efficiencies; (b) voltage efficiencies; (c) energy efficiencies; (d) cyclic capacity behavior at 50 mA∙cm-2. Evidently, the functional properties greatly depend on the morphologies and thus the EW values of Nafion membranes, which provide a tunable parameter to design optimal membrane for VRFB application. It is generally known that the proton is transported through a ‘hopping’ mechanism while other cation species transported by a ‘vehicle’ mechanism.28 The impact of the chemical composition change and thus the morphological variation on the two types of transport mechanisms is clearly different. As EW value increase, the decrease in the vanadium-ion diffusion coefficient is much greater than the decrease in proton conductivity as shown in



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Table 1. By employing EW tuning, we can capitalize on the tradeoff between two competing factors of proton conductivity and vanadium ion permeability to achieve high selectivity and conductivity simultaneously. As discussed in the VRB performance, although the high EW membrane (NDM221-1) diminishes the crossover of vanadium ions achieving an improved CE, it also impedes the proton transport and results in a low VE in VRB operation. To minimize the membrane contribution to cell resistance, a thinner membrane (31µm of NDM221-2) was therefore designed to compensate for the decrease of ionic conductivity. As a result, the NDM221-2 membrane presents a comparable area resistance to that of N115 with an extremely low VO2+ ion flux even at almost five times smaller thickness. Therefore, the NDM221-2 membrane, at ~1/5 of thickness of the widely used Nafion115 membrane, exhibits a higher ion selectivity (97.6 vs. 64.9) with a lower VO2+ diffusion coefficient and areal resistance than those of N115. Critical membrane properties are summarized in Table 2, which validate our design strategy that the crossover of vanadium ions through perfluorosulfonic acid membranes can be significantly mitigated without sacrificing conductivity by tailoring its microstructure and macro dimension. Another advantage of this new design is the potential to achieve lower manufacturing costs of the NDM221-2 membrane, resulting from the reduction in the membrane thickness, which provides a viable solution to the lingering conflicting goals of achieving high chemical stability and low membrane cost.

Table 2. Properties of the NDM221-2 and N115 Membranes EW

Thickness (µm)

NDM221-2

1500

N115

1000

16

Area Resistance (mΩ∙cm2) 157

Diffusion Coefficient of VO2+ (10-6 cm2∙min-1) 0.17

VO2+ Ion Flux (*10-7 mol∙cm2 ∙min-1) 0.53

Selectivity between H+ and VO2+ 98

77.9

179.1

1.20

0.89

64.9

Conductivity (mS∙cm-1)

31

Water Uptake (%) 3.2

135

17.8

Figure 5(a) presents typical charge-discharge voltage curves of a VRB single cell assembled with the NDM221-2 and N115 membranes under a current density of 50mA∙cm-2, in which the VRB with the NDM221-2 membrane exhibits a similar


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charge-discharge voltage but a longer discharge time because of its slightly lower area resistance and extremely low vanadium-ion diffusion coefficient. These advantages are readily observable in the comparison of efficiencies shown in Table 3. As expected, the cell with the NDM221-2 membrane shows higher columbic efficiencies than the cell with the N115 membrane over the range of the current densities from 50 to 100mA∙cm-2. Under all three current densities, the voltage efficiencies of VRBs with NDM221-2 membranes are comparable to those with N115 membranes. As a result, approximately 1 to 2% higher energy efficiencies were obtained with the NDM221-2 membrane.

Table 3. Cell Performance of VRBs with NDM221-2 and N115 Membranes Current density (mA∙cm-2) 50 75 100

NDM221-2 CE (%) 99.2 99.1 99.1

EE(%) 89.3 84.6 79.8

VE (%) 90.0 85.4 80.5

N115 CE(%) 97.5 97.9 97.9

EE(%) 87.7 83.5 78.4

VE(%) 90.0 85.3 80.1

In addition to higher energy efficiency, another more prominent advantage of the NDM221-2 membrane is the excellent capacity retention during cycling. Figure 5 (b) shows the capacity changes for VRBs with NDM221-2 and N115 membranes along cycling. The VRB test cell with the NDM221-2 membrane exhibits stable capacity over 200 cycles, while the VRB with the N115 membrane shows substantial capacity decay from the first cycle. The stable capacity behavior is of great importance for the long-term operation of VRB. In Nafion®-based systems, capacity fading over charge/discharge cycles is mostly attributed to the net transfer of vanadium-ion species from one half-cell to the other and subsequent self-discharge reactions between the transferred and native vanadium ions.2, 7 Although the electrolyte chemistry can be restored with different rebalance technologies, the rudimental solution to this problem would be to develop a membrane with low vanadium ion-permeability. In this regard, the stable capacity of VRB with NDM2212 membranes demonstrates extremely low net transfer of vanadium ions during long-term cycling because of its high selectivity. The unique advantage of the



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NDM221 membrane makes it possible to achieve long-term operation of VRBs without complicated electrolyte maintenance.

Figure 5. (a) Voltage profile of VRBs with NDM221-2 and N115 membranes at a current density of 50mA∙cm-2. (b) Cyclic capacity behavior of VRB with NDM221-2 and N115 membranes at a current density of 50mA∙cm-2.

CONCLUSIONS The impact of the EW value on the membrane morphologies and properties and VRB flow cell performance is studied, based on which a tailored perfluorosulfonic acid membrane with a higher EW value and smaller thickness was specifically designed, manufactured, and evaluated for VRB applications. The change of the chemical composition of the PFSA membrane was found to have significant influence on the membrane properties and performance. The increase of EW value improves the selectivity of the membrane. In addition, the thinner membrane was found to successfully compensate for the decrease of ionic conductivity and achieve reasonable area resistance. Stable cyclical capacity behavior was demonstrated, thereby eliminating the need for complicated electrolyte rebalancing and enabling long-term stable and maintenance-free operation. With the acceptable membrane resistance, extremely low vanadium ions permeability, and more important, the combination of excellent chemical stability and low-cost potential, the new


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membrane described in this paper is expected to be a promising choice for VRB applications.

CONFLICT OF INTEREST The authors declare no conflict of interest.

ACKNOWLEDGMENTS The authors would like to acknowledge financial support from the U.S. Department of Energy’s (DOE) Office of Electricity Delivery and Energy Reliability (OE) under contract number 57558. Pacific Northwest National Laboratory is a multi-program national laboratory operated by Battelle for DOE under Contract DE-AC0576RL01830.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Details of the Kinning and Thomas model for fitting SAXS data; Conductivity measurement; permeability of VO2+; Flux and selectivity.

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Abstract graphic


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