Reinforcement of Nafion Membranes with Electrospun PVDF-HFP Fibers for PEM Water Electrolyzers

Abstract

In this work, we present a procedure for the fabrication of Nafion composite membranes reinforced with electrospun PVDF-HFP fibers to enhance the mechanical stability of thin membranes for the PEMWE application. The presented fabrication procedure combines the spray coating with the electrospinning technique by successively depositing thin Nafion layers and PVDF-HFP fiber mats. The fabrication approach allows a high freedom of design in terms of the final membrane thickness, the position of the fiber mat and the fiber to Nafion ratio. Furthermore, the approach enables the integration of a reinforcement layer into the novel direct membrane deposition (DMD) MEA design by depositing the reinforced membrane directly on the cathode electrode.The mechanical properties were analysed by means of a tensile test. Thin unreinforced and reinforced Nafion membranes with a thickness of approximately 60 µm with varying fiber loadings were fabricated. The best mechanical strength was determined for reinforced membranes with a fiber loading of 0.12 mgfiber/cm2. This yielded an average E-modulus of 215 MPa and break strain of 40 % compared to unreinforced membranes with an average E-modulus of 198 MPa and break strain of 18 %. The mechanical strength did not further improve with an increased fiber loading, but instead was found to decrease strongly. By use of confocal light microscopy and Raman microscopy, it was shown that sufficient infiltration of the PVDF-HFP fiber mats with Nafion was limited due to a low material compatibility of the highly hydrophobic fibers with Nafion. Subsequently, insufficient infiltration resulted in the entrapment of air voids inside the porous fiber structure. The percentage of air voids drastically increases with an increasing fiber loading. As a result, non-continuous, defective membranes were obtained which explained the observed decrease of mechanical strength for higher fiber loadings.It was found that an effective method to reduce the number of air voids and thus circumvent infiltration limitations could be achieved by hot pressing the membrane as post-fabrication treatment at a temperature close to or above the glass transition temperature of Nafion. Simultaneously, the mechanical strength of the Membrane is improved. In a tensile test it was shown that the hot pressing of unreinforced membranes improved the mechanical stability with an average E-modulus of 220 MPa and a break strain of 38 %. Using confocal Raman microscopy it was determined that the higher mechanical strength was attributed to a lower water uptake of hot pressed membranes as compared to the untreated membranes. This is likely due to a realignment of the mechanically stable PTFE backbone and the hydrophilic sidechain units during hot pressing.In order to electrochemically characterize composite membranes and test them in their actual application, PEMWE cell tests were conducted. Reinforced DMD MEAs with a membrane thickness of 50 µm and a fiber loading of 0.12 mgfiber/cm2, corresponding to a fiber/Nafion ratio of 1.4 wt.%, were fabricated. In addition to untreated reinforced DMD MEAs, reinforced DMD MEAs were subjected to post-fabrication hot pressing. It was proven that the fiber reinforcement approach presented in this work is applicable to real operating PEMWE systems. Furthermore, it was shown that DMD MEAs can be subjected to post-fabrication treatments without impairing the performance. The robustness and handling of MEAs was found to be improved through both the reinforcement layer and the treatments. Figure 1