Electrospun tri-layer membranes for H2/Air fuel cells
Graphical abstract
Introduction
One focus area for fuel cell R&D has been the synthesis of high ion exchange capacity (IEC) perfluorinated sulfonic acid ionomers and their fabrication into high performance proton-exchange membranes (PEMs). By increasing the concentration of ion-conducting sites in the ionomer [1] or by adding inorganic proton conductors (i.e. heteropolyacids or sulfonated inorganic nanoparticles [2]), the ionic conductivity of a membrane can be increased. Increased proton conductivity is particularly important during fuel cell operation under reduced feed gas humidification conditions. Increasing membrane ion-exchange capacity, however, also leads to greater ionomer swelling by water [3], [4]. While proton conductivity is normally viewed as the single most important property of a fuel cell membrane, low in-plane water swelling is also required to maintain the mechanical integrity of the membrane during on/off fuel cell cycling [5], [6]. Excessive in-plane expansion and contraction of a membrane under wet/dry (on/off) conditions, when the membrane becomes part of a membrane-electrode-assembly (MEA) in a fuel cell stack, will cause membrane cracks and pinholes to form, leading to the mixing of hydrogen fuel and air and significant power losses [6], [7], [8], [9], [10].
A number of methods have been employed to restrict the swelling behavior of fuel cell PEMs. Such methods include the use of: (i) block copolymers [6], [11], (ii) polymer blends of an ionomer and uncharged polymer [12], [13], [14], [15], [16], and (iii) composite membranes where an ionomer is impregnated into a porous uncharged polymer support [17], [18], [19]. For the latter two approaches, the introduction of an uncharged polymer into the membrane results in a decrease in proton conductivity due to ionomer dilution and percolation effects [13], [20], [21], [22]. A variant of method (iii) is the use of dual fiber electrospinning [18], [22], [23] which was introduced by Pintauro and coworkers to simplify the procedure of fabricating polymer-reinforced cation exchange membranes [24]. Membranes were made by simultaneously electrospinning ionomer and uncharged polymer fibers from separate spinnerets, followed by mat hot pressing or exposure to organic vapor, to transform the porous mat into a dense and defect-free membrane. Using such an approach, two nanofiber composite membrane structures were made from the same dual fiber mat: (1) uncharged reinforcing polymer fibers embedded in an ionomer matrix (this structure works well for hydrogen/air fuel cells [18]) or (2) ionomer fibers surrounded by an uncharged polymer (which is the preferred membrane morphology for an alkaline fuel cell [25], [26] and a H2/Br2 regenerative/redox flow battery [27]). For proton conducting membranes, poly(phenyl sulfone) or poly(vinylidene fluoride) was typically employed as the reinforcing component, and a perfluorosulfonic acid ionomer (Nafion® or a 3 M Co. polymer) was used as the cation-exchange material. Nanofiber composite membranes typically exhibited a proton conductivity of 0.02–0.07 S/cm with an in-plane swelling of 4–7% (both were measured for samples equilibrated in room temperature water) [18].
An unexplored extension of the dual fiber electrospinning technique is its use to create multi-layered membranes, where the distribution of reinforcing fibers varies in the membrane thickness direction. Such structures can be easily made during dual fiber electrospinning by changing the flow rates of the ionomer and reinforcing polymer solutions. Reinforced and layered films cannot be made by conventional ionomer impregnation into a porous support because the uncharged support material is typically of uniform porosity. Nevertheless, some impregnated composite films possess a tri-layer structure due to the presence of charged polymer overlayers, created inadvertently or by design [28], [29], [30], [31], [32]. Additionally, there are a limited number of reports in the literature of multi-layer fuel cell membranes which were prepared by sequential solution casting of blended polymer films, e.g., the study by Feng and coworkers [33] who prepared a tri-layer film by spreading a layer of sulfonated carbon nanotubes onto a preformed solution cast film of sulfonated poly(arylene ether nitrile) (SPEN), followed by the casting of a second SPEN layer. The resulting tri-layer film, which contained 1 wt% sulfonated carbon nanotubes, exhibited less in-plane water swelling as compared to a conventional blend [33] with a homogeneous distribution of nanotubes (8.4% vs. 16.5%), but the conductivity (0.073 S/cm) was too low for commercial fuel cell applications.
In the present paper, we report on the fabrication of single-layer and multi-layer composite membranes that were prepared by dual fiber electrospinning of 825 EW PFSA ionomer and polyamide-imide (PAI). PAI was selected as the reinforcing material because of its excellent chemical stability and superior mechanical properties (in particular, a tensile strength of 117 MPa [34] after sufficient curing above its glass transition temperature, which is greater than 285 °C [35]). Two types of composite PFSA-PAI membranes were fabricated: (1) Single-layer membranes with a uniform distribution of PFSA and PAI in the thickness direction, and (2) Multi-layer membranes with a high PFSA content in the surface layers and a low PFSA content in the inner layers.
Three types of multi-layer membrane structures were studied. The first utilized neat PFSA in the outer layers and 70 wt% PFSA (30 wt% PAI) in the inner layers, where the total number of layers was 3, 5, 7, or 9. This structure is shown schematically in Fig. 1a. Such a film with 3 sub-layers simulates the structure of commercially available reinforced fuel cell membranes, e.g., Gore-SELECT, where neat ionomer is impregnated into and overlays a pre-fabricated porous support. The second type of multi-layer membrane utilized a tri-layer structure with outer layers that contained 95 wt% PFSA (5 wt% PAI to reduce the swelling of the outer layers), with an inner layer that contained a uniform PFSA content of 75, 60, or 40 wt%. This structure is shown schematically in Fig. 1b. The third membrane type was a tri-layer design with surface layers containing 95 wt% PFSA and a low PFSA content inner layer with a symmetric gradient in PFSA content. This structure (shown in Fig. 1c) was chosen to eliminate the step-change in ionomer content between sub-layers, thus reducing the swelling gradient within the membrane. While conventional impregnation methods of a porous mat can lead to fiber-reinforced membranes similar to structure #1 and multiple laminated reinforced sub-layers could, in principle, be used to fabricate a multi-layer membrane (Fig. 1a and Fig. 1b), dual fiber electrospinning is the only way to create a tri-layer membrane with a reinforcing layer of variable composition (Fig. 1c).
A single layer composite membrane had a PFSA content of 80 wt% and thickness of ~ 20 µm. This composition and thickness were selected to obtain low area-specific resistance (ASR, 0.022 Ω cm2 in water at 25 °C), which would provide good performance (a low IR drop) during fuel cell operation. In order to reduce the membrane's in-plane swelling, the PAI was redistributed in the thickness direction to form alternating multi-layers. A series of such membranes was prepared with 3, 5, 7, and 9 layers, where the PFSA content alternated from neat PFSA at the outer layers to a PFSA-PAI nanofiber mixture. The overall PFSA content in each membrane was kept constant at 80 wt%. Thus, the effective IEC of the composite membranes (0.97 mmol/g, given that the IEC of 3 M's 825 EW ionomer is 1.21 mmol/g) was greater than that of 1100 EW Nafion (0.91 mmol/g). An additional series of tri-layer films were fabricated with ~5 µm surface layers containing 5 wt% PAI and an inner layer containing 25 wt%, 40 wt%, or 60 wt% PAI. These membranes had thicknesses of 17–23 µm and their inner layer thickness was adjusted so that the overall membrane composition was held constant at 80 wt% PFSA and 20 wt% PAI. All the single-layer, tri-layer, and multi-layer membranes were compared in terms of proton conductivity, gravimetric water uptake, in-plane water swelling, and mechanical properties.
Section snippets
Materials and solution preparation
825 EW perfluorosulfonic acid (PFSA) from 3 M Co. and Torlon® 4000T (polyamide-imide, PAI) from Solvay Specialty Polymers were both received as dry powders. Dimethylformamide (DMF), dimethylacetamide (DMAc), tetrahydrofuran (THF), and poly(ethylene oxide) (PEO) with a molecular weight of 600 kDa were obtained from Sigma-Aldrich, and used without further purification. N-propanol was obtained from Fisher Scientific and was used without further purification.
Dispersions of 825 EW PFSA and solutions
Electrospun single-layer and solution cast PFSA/PAI composite membranes
A top-down SEM image of a dual fiber mat (80 wt% PFSA and 20 wt% PAI) is shown in Fig. 3a. PFSA and PAI fibers are indistinguishable, with an average fiber diameter of 290 nm ± 90 nm. Top-down and freeze-fractured cross-section SEMs of the dense membrane created from the 80/20 PFSA/PAI mat are shown in Fig. 3b and Fig. 3c, respectively. The processed membrane surface is featureless (no fibers are present) due to thin layers of neat PFSA at the membrane surfaces; this is a consequence of
Conclusions
A series of single and multi-layer nanofiber composite membranes, for possible use in a proton exchange membrane fuel cell, was fabricated where a low equivalent weight 3 M ionomer (825 EW perfluorosulfonic acid, PFSA) was reinforced by Torlon® polyamide-imide (PAI) nanofibers. Three different types of composite membranes were prepared by dual fiber electrospinning: (1) a single-layer membrane (80 wt% PFSA), where PAI fibers were homogeneously distributed throughout the membrane in the
Acknowledgements
The authors would like to thank the U.S. Department of Energy (EERE Cooperative Agreement no. DE-EE0006362) for funding this work, as well as Dr. Michael Yandrasits at 3 M for providing the 825 EW PFSA used in this study.
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