Balancing dimensional stability and performance of proton exchange membrane using hydrophilic nanofibers as the supports
Graphical abstract
Proton exchange membrane with high dimensional stability, good proton conductivity and fast dynamic response was developed by anchoring perfluorosulfonic acid into the hydrophilic poly(lactic-co-glycolic acid) (PLGA) nanofibrous network.
Highlights
► Stable proton exchange membrane. ► hydrophilic poly(lactic-co-glycolic acid) nanofibers as membrane supports. ► Low humidity-induced stress to the yield strength. ► High physical stability.
Introduction
The proton exchange membrane fuel cell (PEMFC) attracts much attention due to its high efficiency, high power density, zero emission, and low-temperature start-up as a power source suitable for both stationary and mobile application [1], [2]. As the key material of PEMFC, proton exchange membranes (PEMs) largely determine the performance and life of PEMFC. State of art proton exchange membranes such as Nafion perfluorosulfonic acid (PFSA) membrane have low dimensional stability at a humidity cycling environment. The stresses induced by dimensional changes can lead to crack formation, gas crossover and ultimately failure of the fuel cell [3], [4], [5], [6]. For the homogeneous Nafion membranes, the dynamic humidity in the fuel cell always changes the water content in the electrolyte membranes, generates humidity-induced stress greater than 2.3 MPa toward free-standing PEMs, and causes membrane degradation [4].
The humidity-induced stress can be controlled by impregnating the polymer electrolyte into a hydrophobic porous membrane, such as porous polytetrafluoroethylene (PTFE) membrane [7], [8], [9], [10], or incorporating inorganic metal oxide nanowires or nanoparticles such as SiO2, ZrO2, TiO2, and zirconium phosphate into Nafion to form the composite proton exchange membranes [11], [12], [13], [14], [15]. In the case of PTFE/Nafion composite membrane, the humidity-induced stress was controlled (reduced from 2.3 MPa of the homogeneous Nafion211 membrane to 0.4 MPa of PTFE/Nafion composite membranes and the mechanical strength also increased from 18 MPa to 34 MPa), which effectively improved the mechanical stability of the proton exchange membranes [4], but a crucial challenge in the rational design of new PEMs is thus a fundamental understanding of the structural and water transport characteristics needed to achieve high proton conductivity. A thinner membrane enhances the back diffusion of water to the anode, which facilitates water balance at high current densities under conditions of low humidity [16]. Nevertheless, as reducing the thickness of the membrane, the typical membrane currently used for the PEMFC, such as Nafion perfluorosulfonic acid (PFSA) membrane, shows a significant loss in the dimensional stability at a humidity cycling environment [3]. The PTFE/Nafion composite membranes balanced the mechanical stability and the thickness of the PEMs, but the proton conductivity of the proton exchange membranes (PEMs) depends strongly on the host polymer structure and water transport property of the membrane. The hydrophobic and non-conducting PTFE substrate, to some degree, restricted proton conductivity and water transport property of the composite membranes. And the same situation also happened to the Nafion/inorganic nanowires or particles composite membrane, The inorganic nanowires or particles improved the mechanical strength, thermal stability, selective permeability and water retention capacity under the low humidity or dry condition, and the humidity-induced stress was also reduced from 2.1 MPa of homogeneous Nafion membrane to 1.0 MPa of 8% TiO2 nanowires Nafion composite membranes but the lack protogenic inorganic nanocomponent restricted the proton exchange channel of Nafion structure and decreased the proton conduction property of the composite membrane [15]. Thus, finding hydrophilic and conductive reinforced fibers is an important issue for developing the high dimensional stability and conductivity of the PEMs for fuel cells application.
The electrospinning could be regarded as a contemporary top-down approach for synthesis of meso-, micro- and nanosized fibrous materials with high length-to-diameter ratio and porosity, high specific surface area and a controllable mean grain size [17], [18], [19], [20], [21]. Recently, multiple studies have demonstrated that electrospun polyelectrolyte fibers, such as Nafion, sulfonated polyimide or the compound of Nafion and carrier polymer (e.g., poly(ethylene oxide) (PEO), poly(vinyl alcohol) (PVA), or poly(acrylic acid) (PAA), etc.), could create effective proton-transport pathways on the axial direction of fibers [22], [23], [24]. It is generally accepted that conductive polymer fibrosis can improve the conductivity of the bulk material. But to date, there are no reports focusing on the mechanical stability and water transport property of Nafion membrane anchored by using conductive polymer fibers, which not only reduce the humidity-induced stress, but also improve the water transport under low or dry condition. In this study, electrospinning was proposed to prepare conducting poly(lactic-co-glycolic acid) (PLGA) nanofibrous-network structure, which could anchor perfluorinated sulfonic acid resin (Nafion) to form a compact PLGA/Nafion composite proton exchange membranes. The preparation procedure and properties of the composite membranes were studied in detail. The results showed that PLGA nanofibers-reinforced Nafion composite membranes possessed excellent interfacial properties, very low humidity-induced stress, outstanding water transport property under the low humidity, and shorter dynamic response time compared to Nafion211 membranes.
Section snippets
Fabrication of the PLGA fiber network
A poly(lactic-co-glycolic acid) solution was prepared by dissolving 1.5 g of PLGA (L/G ratio = 75:25, Mw = 8 K) into 5 mL of an admixture solvent of tetrahydrofurane (THF) and dimethylformamide (DMF) (v/v ratio = 1:1) and the admixture was stirred for 18 h. Then, the solution could be filled into a 10 mL syringe with a needle (inner diameter of 1.0 mm) and the syringe was fixed on the electrospinning system. Then the homogeneous mixture solution was ejected continuously with a programmable
Characterization of PLGA fiber network and the PLGA/Nafion composite membranes
At the optimized electrospinning conditions described in the section of experiment, the PLGA microfibrous network was prepared and the SEM images of the prepared PLGA fibers are shown in Fig. 1(a and b). It can be seen in Fig. 1(a) that continuous ultrafine fibers of PLGA possessed the common feature of being bead free, randomly arrayed, and porous. The majority of the PLGA fibers had diameters ranging from 150 to 500 nm and an average diameter of 300 nm. And Fig. 1(a) illustrated, in the
Conclusions
We have developed the novel composite membranes by anchoring the Nafion resin into hydrophilic and protogenic PLGA nanofiber network. The PLGA nanofibers had been synthesized by the electrospinning method. It was clear that the PLGA/Nafion composite membranes possessed high impregnated Nafion loading, excellent dimensional stability and proton diffusion capacity. When the humidity of the membranes changed from soaking in water to 25 RH% at 90 °C, the PLGA fiber network effectively controlled
Acknowledgment
This work is financially supported by the National Nature Science Foundation of China (51272200), Program for New Century Excellent Talents in University (NCET-12-0911) and Wuhan “Cheng-guang” Project (201150431089).
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