Elsevier

Journal of Membrane Science

Volume 563, 1 October 2018, Pages 552-560
Journal of Membrane Science

Highly ion selective hydrocarbon-based membranes containing sulfonated hypercrosslinked polystyrene nanoparticles for vanadium redox flow batteries

https://doi.org/10.1016/j.memsci.2018.06.022Get rights and content

Highlights

  • Sulfonated hypercrosslinked polystyrene nanoparticles (sHCP) have been prepared.

  • sHCP are used as filler (up to 10 wt%) to fabricate composite ion-exchange membranes.

  • Composite membranes maintain high conductivity and V+5 permeability decreases.

  • Composite membranes reduce battery capacity fading of 33% as compared to Nafion 212.

Abstract

Composite membranes based on sulfonated poly(phenylene sulfide sulfone) loaded with sulfonated hypercrosslinked polystyrene nanoparticles are here proposed for redox flow battery applications. Nanoparticles have been synthesized via Friedel-Craft alkylation and subsequent post-functionalization, and their chemical structure and morphology has been fully deconvoluted. The membranes, obtained by solution casting, have been characterized ex-situ and in operando to shed light on their performances as compared to commercial Nafion 212.

The use of organic fillers offers different advantages over inorganic ones, such as better compatibility with the polymer matrix and easier tunability of the degree of sulfonation, and thus of the ion exchange capacity. Composite membranes displayed a significant lowering of the vanadium-ion crossover, yet maintaining high proton conductivity. This result greatly impacted on flow battery self-discharge and on its capacity fading. The first increased from 2 to 7.5 days, while the latter decreased (over 25 cycles) from 8% to 5% for composite membranes with respect to Nafion 212.

Introduction

Renewable resources are attracting increasing attention due to the continuous growth of energy request and to the issues linked to environmental pollution associated with the use of fossil fuels. Do to their inherent intermittence, the so-called green energies, such as solar and wind, require storage systems capable of stocking large amounts of charge to be given back according to demand [1]. To this purpose, many efforts have been voted to the development of energy storage systems such as redox flow batteries, capacitors and lithium ion batteries [2], [3], [4], [5], [6].

In terms of large-scale energy storage, redox flow batteries (RFBs) are considered one of the most promising options especially for energy-intensive applications [7], as they allow for independent scaling of energy (tank size) and power (reactor size), and display improved safety and simplified manufacturing with respect to enclosed batteries. All-vanadium RFBs (VRFBs), firstly studied in the early 1980s by Skyllas-Kazacos [8], are the most advanced technology. In sulfuric acid solution, vanadium can assume four different oxidation states (II, III, IV, V) and therefore it can be used both as catholyte (VO2+/VO2+) and anolyte (V3+/V2+). Catholyte and anolyte are divided by a membrane (or separator), whose main function is to facilitate the transport of charge carriers (high conductivity) while hampering the cross-mixing of active species (low permeability). Due to its great influence on battery performances, the membrane represents a key component of VRFB and it is one of the most expensive constituents [9], [10]. Besides high selectivity, defined as conductivity to permeability ratio, an ideal membrane should possess good chemical and mechanical resistance during operational conditions and as low as possible manufacturing costs [11].

Nafion, a sulfonated perfluorinated polymer commercialized by Du Pont, has been the most widely used membrane because of its high proton conductivity and chemical stability. However, various drawbacks, like high cost and low selectivity to vanadium species, limit battery service life [12]. To overcome the above mentioned Nafion constraints, different approaches have been adopted. On the one hand, inorganic or inorganic-organic nanoparticles have been added to Nafion matrix. Nanocomposite membranes containing several inorganic filler such as SiO2, TiO2, organic silica modified TiO2, zeolites, graphene-oxide and ZrO2 nanotubes displayed improved selectivity with respect to the pristine membrane [13], [14], [15], [16], [17], [18], [19]. On the other hand, other cation-exchange membranes have been realized by employing hydrocarbon-based polymers, such as sulfonated poly(ether ether ketone) (SPEEK), sulfonated poly(fluorenyl ether ketone), sulfonated poly(aryl ether sulfone ketone), sulfonated poly(aryl ether sulfone) and sulfonated poly(imide) [10], [20], [21], [22], [23], [24], [25], [26], [27]. Indeed, the use of non-fluorinated monomers may reduce the membrane costs, and the presence of highly rigid macromolecular backbones causes the formation of less interconnected ionic clusters, which, in turn, are expected to decrease electrolyte permeation. In addition, this peculiar microstructure does not usually affect proton conductivity, which must be preserved to sustain high current densities. Both these factors contribute to improve battery performances, due to higher membrane selectivity. However, the lack of long term experiments poses unresolved questions on their chemical stability [28].

In a different approach, the use of porous nanofiltration membranes has been also proposed. For these membranes, ion selectivity is guaranteed by a size-exclusion mechanism that permits proton exchange, while hampering vanadium ion diffusion. Various membranes based on polyacrylonitrile (PAN) [29], [30], poly(ether sulfone) (PES) [31], [32] and polybenzimidazole (PBI) [33], [34] have been studied for this purpose.

Among hydrocarbon-based ionomers, sPEEK is probably the most extensively studied, as due to its interesting features, such as low crossover rate and simple synthetic process. To enhance its already good selectivity, the addition of nanofillers such as nanoxides, graphene or graphene oxide, and carbon nanotubes has been also evaluated [35], [36], [37], [38], [39].

Although significant improvement has been made, sPEEK displays intrinsic poor oxidation stability due to ether bond cleavage in V+5 that limits membrane life time [40]. As a more stable alternative to sPEEK, we have recently reported on the use sulfonated poly(phenylene sulfide sulfone)-based membranes (sPSS) [41]. In the presence of V+5, sPSS undergoes oxidation of the thioether linkage, which prevents any polymer degradation, i.e. decrease of its molecular weight.

In this contribution, composite sPSS-based membranes have been prepared and studied for VRFB applications. With the aim of improving sPSS selectivity, sulfonated hypercrosslinked polystyrene (sHCP) nanoparticles have been introduced into sPSS. The use of fully organic nanoparticles is expected to offer different advantages over inorganic fillers, such as a better stabilization of the polymer hydrophobic domains, improved particle dispersion due to the higher filler/matrix compatibility, and easier tuning of the acid groups content through post-functionalization.

sHCP nanoparticles showing similar ion exchange capacity (IEC) with pristine sPSS have been added to the polymer matrix to obtain composite membranes. Different filler loadings have been considered and the performances of composite membranes have been tested both ex-situ as well as in operando, and compared to those of Nafion 212 (N212) and neat sPSS.

Section snippets

Materials

All used materials are analytical grade Sigma-Aldrich products. The reagents used for polymer synthesis, i.e. α,α’-dichloro-p-xylene (DCX), anhydrous ferric trichloride (FeCl3), potassium carbonate anhydrous, disodium 3,3’-disulfonate-4,4’-dichlorodiphenylsulfone (SBCPS), 4,4’-dichlorodiphenylsulfone (BCPS), and 4,4’-thiobisbenzenethiol (TBBT), were dried overnight at T = 90 °C in a vacuum oven prior to use. Vanadium oxide sulfate hydrate (V(IV)), magnesium sulfate anhydrous, chlorosulfonic

Hypercrosslinked polystyrene nanoparticle characterization

The structure of HCP and sHCP was verified by FT-IR spectroscopy (Fig. 2A). Both samples show a clear band positioned at about 2920 cm−1 due to the stretching of methylene groups, meaning that the monomer structure is preserved after the synthetic process. Moreover, the signals at 670 and 1280 cm−1 ascribable to CH2–Cl stretching and wagging vibrations are respectively not visible and very weak, indicating the success of the crosslinking procedure [43].

As compared to HCP, sHCP spectrum contains

Conclusions

Fully organic nanofillers based on sulfonated hypercrosslinked polystyrene particles have been successfully synthesized and employed for the first time in the fabrication of composite membranes for redox flow battery applications. The prepared sHCP particles, with a diameter of about 10–30 nm, displayed a IEC of 0.92 meq g-1 and a very high surface area, above 1000 m2 g-1. When introduced in sPSS matrix, sHCP-containing membranes highlighted a much higher selectivity with respect to N212 and

Acknowledgements

The present work was carried out with the support of the “European Union's Horizon 2020 Research And Innovation Programme”, under H2020-FTIPilot-2015-1 (Grant agreement no. 720367-GREENERNET). The valuable technical support of Mrs. C. D’Ottavi is gratefully acknowledged.

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