PVA/Polyelectrolyte Bipolar Membranes for Fuel Cell Applications

In the last few years, direct borohydride/hydrogen peroxide fuel cells (DB/HPFC) have attracted considerable attention due to their potential use as air-independent power sources in undersea vehicles. In general, a DB/HPFC system can be built using either a cation exchange membrane (CEM) or an anion exchange membrane (AEM) as a polymer electrolyte. Unfortunately, to date, these systems exhibited low efficiency, mainly due to some problems related to the intrinsic properties of the polymer electrolyte themselves. Bipolar membranes are promising candidates to improve the efficiency of DB/HPFC systems, since these incorporate advantageous features of both AEM and CEM, and also mitigate the loss of fuel.

The present work focuses on the fabrication and characterization of a novel bipolar membrane for DB/HPFC applications. The first goal is to fabricate water-soluble PVA-based AEM and CEM with both high ionic conductivity and excellent mechanical stability. For this purpose, a variety of AEMs are prepared via polymer blending of PVA and poly(diallyldimethylammonium chloride) (PDDA) or poly(acrylamide-co-diallyldimethylammonium chloride) (PACoDDA). Similarly, CEMs are fabricated via polymer blending of PVA and acid polymer ((poly (styrene sulfonic acid) (PSSA) or poly (acrylic acid) (PAA)). Subsequently, the membranes are cross-linked using a chemical or physical method. The chemical cross-linking is performed at different concentrations of glutaraldehyde (GA), whereas the physical cross-linking is performed at different temperatures. The electrical, mechanical and morphological properties of these membranes are evaluated as a function of PVA content and cross-linking process conditions. Some of the material characterization techniques used in this study include, but are not limited to: Fourier transform infrared spectroscopy (FTIR), four-electrode impendence measurement, transmission electron microscopy (TEM), and tensile tests. Preliminary results indicate that PVA-PDDA and PVA-PSS membranes have excellent mechanical and alkaline stability. Also, the maximum OH⁻ conductivity of 8.57E-3 S cm⁻¹ was achieved for physically cross-linked PVA/PDDA membrane with a polymer composition of 70/30 as shown in Figure 1a. In the case of CEM, the high H⁺ conductivity of 1.77E-2 S cm⁻¹ corresponds to the chemically cross-linked PVA-PSS membrane with a polymer composition of 70:30 (see Fig. 1b).

Figure 1. Ionic conductivity as a function of PVA content for (a) PVA/PDDA membranes and (b) PVA/PSS membranes.