Tuning the ion channel network of perfluorosulfonated membranes via a facile sacrificial porogen approach
Graphical abstract
Introduction
Ion exchange membranes are a key component in a number of electrochemical applications that involve selective permeation of ions or protons such as proton exchange membrane fuel cells (PEM-FCs), (reverse) electrodialysis (RED/ED), pressure retarded osmosis (PRO), redox flow batteries (RFB) and electrokinetic energy conversion (EKEC) processes [1], [2], [3], [4], [5], [6], [7]. The transport properties of such membranes depend on the characteristics of the solvent swollen ion channel network (i.e. average channel diameter, porosity and tortuosity factor) and immobile charge density [8].
With respect to selectivity, immobile charges on the channel walls, e.g. sulfonate (SO3-) and other negatively charged groups in cation conductive membranes, exclude co-ions (anions) allowing the partitioning and permeation of mainly counter-ions (cations) [9]. The screening potential of the immobile charges in the proximity of the channel walls is an increasing function of the surface charge density and decays steeply towards the center of the ion channel, hence it is only effective up to a certain critical size [10], [11], [12], [13], [14], [15]. In a simplified picture the ion channel provides an optimal permselectivity () toward co-ions until the Debye screening length () of the solution inside the channel is larger or similar to the average channel radius (rp). In these conditions the electrical double layers (EDLs) overlap and the potential profile is almost constant in the radial direction [16], [17], [18]. However, high permselectivity is a trade-off with the membrane conductivity () that increases with rp and the external solution concentration for a fixed surface charge density. Furthermore, the solution permeability and volume flux () is an increasing function of rp [14], [19], [20]. Additionally, and are extensive variables (i.e. dependent on the system geometry) and will for this reason increase with the porosity ( and decrease with the tortuosity factor () at a fixed average ion channel dimension, while is an intensive variable and therefore independent of porosity and tortuosity factors.
The choice of membrane, as separator for a specific electrochemical application, is a trade-off between selectivity, membrane conductivity and the solution flux/permeability [21]. The membrane resistance (proportional to ) should be kept as low as possible to minimize the parasitic ohmic resistances, still providing the required co-ion selectivity, while high can either be an unwanted feature because volumetric crossover unbalances the reservoirs or, in other applications, required to maximize the power output [22], [23]. For instance in RFBs a high improves the power density, cycle efficiency and even the capacity, however, volumetric crossover and crossover of redox active species must be kept at a minimum. On the other hand, in membrane-based EKEC a hydrostatic pressure load is converted directly into electrical energy [12], [24] and a combination of high permselectivity and large ion channel radius has been shown to enhance the efficiency and power density significantly [25].
Nafion is a highly stable state-of-art cation exchange membrane because of its fluorinated backbone [9]. The phase separation between the highly hydrophobic fluorocarbon (PTFE based) and the highly hydrophilic ionic phase is very distinct [8], [9]. Nonetheless in the solvent-swollen membrane the reorganization of these phases at a microscopic level is still under debate and several structural models have been proposed since the 1970s. These include Gierke's cluster-network model [8], parallel cylindrical nanochannels model [26], and network models [27], [28], among others. As a consequence, only approximate structural features can be sketched: in particular, in fully hydrated conditions, the channel network is believed to have a characteristic rp around 1–2 nm, relatively small porosity (0.3–0.4) and high tortuosity factor (10–20) [8], [14], [26].
In the following a facile sacrificial porogen approach to tune the nanosized network structure of Nafion is described. The polymer precursors were fabricated by casting the commercially available Nafion solution together with sulfonated polystyrene (SPS). The modified membranes were then obtained via a facile selective SPS leaching by swelling the cast membranes in methanol/water mixture. The successful modification was confirmed via SAXS/WAXS experiments and the transport properties were evaluated. At low SPS concentration the porogen mainly increased the apparent porosity and interconnectivity of the network thus enhancing the membrane conductivity by up to 60% with respect to the pristine membrane preserving the same permselectivity toward co-ions. At higher SPS content the porogen resulted in an evident modification confirmed by the characteristic spacing of the hydrophilic domains (up to approximately 7–8 nm from SAXS data). This resulted in enhanced membrane conductivities up to 300% with respect to the pristine membrane in LiCl with only a slight decrease in the membrane permselectivity in diluted LiCl solution. This ultimately indicates the effectiveness of SPS as sacrificial porogen to increase the apparent porosity and channel sizes of the hydrophilic network, still being in the nanoscopic range.
Section snippets
Chemicals
Dimethylacetamide (DMAc, anhydrous 98.9%), diethyl ether (99.7%), hydrogen peroxide (30 wt% in H2O) and methanol ( 99.9%) were all used as received from Sigma Aldrich. Lithium chloride, sodium chloride and sulfuric acid (95%) were purchased from VWR and used as received. Sodium hydroxide (32%, technical grade) was obtained from Applichem. The alcohol-based Nafion solution (D2020, EW1000, 20 wt% in 1-propanol-water solution), obtained from Ion Power, was pre-cast in Teflon petri dishes overnight
Membrane characteristics
In the following the membranes will be referred to as N-SPS where represents the percentage mass ratio of the SPS with respect to Nafion i.e. . To get a basic understanding of the structural features of the synthesized membranes they were initially characterized by apparent membrane porosity, fraction of SPS present in the membrane after the porogen removal step and the characteristic dimensions of the hydrophilic network. In this study the porosity is defined as
Conclusions
In this work an effective and facile approach to increase the apparent porosity and ion channel size of the Nafion nanostructure has been achieved by adding sulfonated polystyrene (SPS) as a sacrificial porogen. The modification resulted in larger ion channels, yet nanoscopic in dimension, while at the same time maintaining good transport properties in terms of high permselectivity and membrane conductivity. Below a threshold value of added SPS around 30% the porogen mainly influenced the
Acknowledgments
The authors wish to thank the Villum Foundation (Grant no. VKR022356, Young Investigator Programme) and The Aarhus University Research Foundation for funding. On the part of New Technologies - Research Centre, the result was developed within the CENTEM project, Reg. no. CZ.1.05/2.1.00/03.0088, cofunded by the ERDF as part of the Ministry of Education, Youth and Sports OP RDI programme and, in the follow-up sustainability stage, supported through CENTEM PLUS (LO1402) by financial means from the
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