Preparation and characterization of carbon molecular sieve (CMS) / SPEEK bilayer membranes and SPEEK / polyimide (PI) blend membranes for direct alcohols fuel cells / (DAFC) performance

This research work is based on the preparation of membranes from SPEEK with reduced fuel cross-over following two different approach: 1) the preparation of bilayer membranes from in-home sulfonated PEEK (Vitrex) and a thin layer of in-home prepared carbon molecular sieve (CMS) from polyimide (Matrimid 5218); 2) blending of SPEEK with polyimide polymers. In both ways, it was possible to decrease the fuel cross-over in both direct methanol fuel cell (DMFC) and direct ethanol fuel cell (DEFC) without compromise on proton conductivity. In this research work, a SPEEK with 56 % sulfonation degree was prepared and then used for making polymeric membranes. The specific modifications made to SPEEK polymeric membranes can be summarized as follows: (1) CMS coated SPEEK bilayer membranes: The idea was to take advantage of molecular sieving effects of the CMS for the rejection of bigger fuel molecules from the fuel mixture (water/alcohols) during pervaporation, DMFC and DEFC tests. The CMS layer was prepared by pyrolysis at (500°C-1000°C), under inert (N2 gas) atmosphere, of a thin film prepared from a polyimide (Matrimid 5218) solution in chloroform. The complete pyrolysis to carbon was confirmed by FTIR-ATR analysis. CMS prepared at 800 °C for 8 hours were chosen for further characterization because of high water/alcohols selectivity and easily detached from the quartz glass without any damage to them. Based on the thickness of CMS layer resulted from polyimide solutions with different concentration (1 and 2 wt. %), two types of bilayer membranes (CMS/SPEEK) were prepared. i. SPEEK coated with 180 nm CMS thick layer (from 1 wt. % solution) (CMS/SPEEK 1). ii. SPEEK coated with 400 nm CMS thick layer (from 2 wt. % solution) (CMS/SPEEK 2). Another approach was the preparation of a membrane electrode assembly (MEA) by the incorporation of platinum as catalyst into the CMS layer in order to take advantage of both fuel barrier and electron conducting properties of the CMS layer. In order to determine fuel-crossover through plain SPEEK and CMS/SPEEK bilayer membranes, pervaporation experiments were carried out with methanol solutions at different concentrations (5, 10 and 20 wt. %) as well pervaporation of ethanol, n-propanol and iso-propanol solutions each with different concentrations (5 and 10 wt. %) were also carried out to investigate the pore size of the thin CMS layer. Table 1 shows the pervaporation results using both methanol and ethanol solutions with the same concentration (5 wt. %) and the proton conductivity measured at 60°C and 100% relative humidity (R.H.). It can be seen that fuel cross-over through the CMS/SPEEK bilayer membranes is quite low as compared to plain SPEEK and at the same time the proton conductivity remains almost constant. These results highlight the fuel barrier properties of the CMS layer. Table 7.1: Methanol and Ethanol permeability at 55 °C and proton conductivity at 60 °C and 100% R.H. Membrane CMS layer thickness (nm) P (10-10 Kg m s-1 m-2) Proton conductivity (mS cm-1) MeOH EtOH SPEEK 0 28 19 34 CMS/SPEEK 1 180 18 8.5 33 CMS/SPEEK 2 400 10 3.1 35 (2) SPEEK/PI Blends membranes: The idea was to take advantage of hydrophobic property of polyimide by incorporating it into SPEEK matrix for rejection of alcohols/water mixture during pervaporation, DMFC and DEFC performances. Three types of homogeneous SPEEK/PI blends were prepared at high temperatures (80°C, 90°C, 100°C, 110°C, 120°C and 130°C) based on the amount of polyimide (Matrimid 5218) added to the blend solution using dimethyl sulfo-oxide (DMSO) as solvent.  SPEEK/PI (90/10)  SPEEK/PI (80/20)  SPEEK/PI (70/30) It was observed that homogeneous membranes were obtained at casting temperatures higher than 110 °C independent of the PI concentration in the SPEEK/PI blend solution. The same solutions rendered non-homogeneous membranes when cast below this temperature. For further characterization, homogeneous membranes from the above mentioned SPEEK/PI blend solutions were prepared by casting the solution at 130°C. The methanol and ethanol permeation through these membranes was determined by the pervaporation of methanol and ethanol solutions (5 wt % concentrations) at 55°C (Table 2). Table 7.2: Methanol and Ethanol permeability at 55 °C and proton conductivity at 60 °C and 100% R.H. Membrane P (10-10 Kg m s-1 m-2) Proton conductivity (mS cm-1) MeOH EtOH SPEEK 28 19 34 SPEEK/PI (90/10) 7.21 4.4 33 SPEEK/PI (80/20) 2.61 1 28 SPEEK/PI (70/30) 0.55 0.4 25 The hydrophobic role of PI towards fuel rejection through the blend membranes as compared to plain SPEEK membranes is the reason for the low permeability values when increasing the concentration of PI in the blend solution. The proton conductivity (mS cm-1) at 60 °C is also shown in Table 2. About the effect of the blend composition onto the proton conducting properties of the membranes, just a slight decrease of the proton conductivity has been observed when increasing the PI concentration. Other functional characterizations of all the CMS/SPEEK and SPEEK/PI membranes prepared in this work included water and mixture uptake, direct methanol fuel cell (DMFC) and direct ethanol fuel cell (DEFC) performance tests. The morphology of the CMS/SPEEK and SPEEK/PI membranes (both homogeneous and non-homogeneous) was analysed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). While attenuated total reflectance (ATR-FTIR), dynamic mechanical thermal analysis (DMTA), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed for the SPEEK/PI blend membranes to analyse the homogeneity of these membranes as compared to the pure constituents (polyimide and SPEEK).