SPEEK membranes

PEMFCs have a solid ion exchange membrane made of sulfonated fluoropolymer, or a sulfonated polyetheretherketone (SPEEK) which is the electrolyte, and for the most part uses platinum catalysts. There are other materials in use for combination type membranes. It should be mentioned that currently SPEEK membranes do not hold up as well as the fluoropolymers such as Nafion , but research is ongoing to produce a more reliable and longer lasting membrane. 

Fig. 7 Polarisation characteristics of sPEEK and sPEEK-ZrP (50 (Am) prepared by ion-exchange/precipitation with sPEEK membranes. The electrodes employed in these MEAs were standard ImgPtcm" E-TEK, Nafion dispersion was used at the mem-brane/electrode interface, and the cell was pressurised to 2.6barabs. sPEEK at 85 °C (A) sPEEK-ZrP at 85 °C ( ) and sPEEK-ZrP at 100°C ( )...

In the case of a layered Nafion/sPEEK/Nafion membrane [132], the liquid uptake from methanol solution (up to 5 M) was, as expected, in between the values for the corresponding Nafion and sPEEK membranes. 

There are several sPEEK membranes with very good relative selectivity and several sPEEK composites with silica, zirconia and their mixtures with heteropolyacids [370, 371, 377] which exhibit remarkable selectivities with high proton conductivities. Composites of sPAEK with epoxy resin [383] and crosslinked with PVA [386] also show excellent selectivities and high conductivities. Unfortunately, the performances of these promising membranes in DMFC have not been reported yet. 

Maab H, Pereira Nunes S (2010) Modified SPEEK membranes for direct ethanol fuel cell. J Power Sources 195 4036-4042... 

Yang and Manthiram studied SPEEK membranes for use in a DMEC (Yang and Manthiram 2003). It was reported that a membrane with 50% degree of sulfonation (DS) exhibited comparable DMEC performance to Naflon 115 at 65°C, while the methanol crossover was two times lower than the Naflon membrane. Sulfonated poly(ether ketone) (SPEK), sulfonated poly(ether ketone ketone) (SPEKK), and sulfonated poly(ether ether ketone ketone) (SPEEKK) come from the same family as SPEEK and therefore their characteristics are quite similar. 

The sulfonation of poly(ether ether ketone) is similar to the sulfonation technique for PSSA using sulfonation agents such as high concentration of sulfuric acid. The DS of SPEEK can be controlled by varying the reaction time and temperature (Xing et al. 2004). The work of Lee et al. showed that the proton conductivity of the SPEEK membrane increased with DS (Lee et al. 2007). The increase in DS will increase the hydrophilicity and water uptake and, consequently, gives an opportunity for the formation of water-mediated pathways for protons. Methanol permeability measurement indicates that its rate increases with the increase of DS but at a much lower rate than the Naflon membrane. This is due to the difference in their microstructures (Li et al. 2003a). 

Water uptake of the SPEEK membrane is known to increase with the increase in DS due to enhancement of the hydrophilicity when introdncing the snlfonic acid groups in the aromatic PEEK polymer. These sulfonic acid groups, in the presence of water molecules, facilitate proton transfer through the membrane, thus increasing the proton conductivity. Therefore, water uptake is a crucial characteristic of the SPEEK membrane, since it is proportional to DS and proton conductivity. However, as methanol transfer is also facilitated by the water molecules, increasing the water uptake normally increases the methanol permeability as well. 

The proton conductivity of SPEEK membrane can be improved by incorporating a fast proton conductor in the polymer matrix as studied by Silva et al. (2005). They utilized zirconium phosphate (ZrPh) pretreated with -propylamine and then with PBI. The pretreatment was to improve the compatibility between ZrPh and SPEEK. Even though the compatibility of the inorganic was good, the proton conductivity and water uptake of the composite membrane were reduced, consequently reducing the methanol permeability. 

A new polymer-inorganic nanocomposite SPEEK membrane filled with Cloisite 15A clay by introducing 2,4,6-triaminopyrimidine (TAP) as the compatibilizer for DMFC applications has been successfully prepared. The organoclay Cloisite is a material prepared from MMT and the cation ditallow. Tallow is a mixture of octa-decyl, hexadecyl, and tetradecyl, wherein octadecyl is the major component (>60%). Cloisite is a very promising additive in nanocomposite membranes, since it is already being organically modified from the natural MMT, which is expected to enhance the compatibility with organic polymers (Cervantes-Uc et al. 2007). 

Ren et al. introduced a layer technique where they immersed the SPEEK membrane in a Naflon casting solution, as a result of which a layer membrane of Nafion-SPEEK-Naflon was prepared (Ren et al. 2005). The proton conductivity of the modified membrane was greater than the original SPEEK, but, unfortunately, the methanol permeability also increased. This shows that Nation, which has high methanol permeability and is on the top and bottom layers of the modified SPEEK, has some influence in ino easing the methanol permeability. 

J.M. Woudstra, K.J. Ooms, Investigating the water in hydrated sPEEK membranes using multiple quantum filtered NMR spectroscopy, J. Phys. Chem. B 116 (2012) 14724-14730. 

It has been suggested that instead of membranes made from perfluorinated sulfonic acids (PFSAs), membranes be used made from other, heat-resistant polymers containing the sulfonic acid group. One could think here of sulfonated polyimides (SPIs) and sulfonated polyether ether ketones (SPEEKs). Membranes made from such polymers have a sufficiently high protonic conductivity at elevated temperatures but are less sensitive to lower humidities and water loss. 

The increase in porosity may be directly confirmed by high-resolution images of an unmodified PSU membrane and a PSU/SPEEK membrane with 5% of the charged polymer (Figure 6.23). 

The adhesion of a range of colloid probes, both inorganic and biological, is greatly reduced at the PSU/SPEEK membranes, as shown by the data in Table 6.5. 

Table 6.5 Normalized adhesion forces for a range of colloid probes at a PSU/SPEEK membrane (S5-20).

SPEEK polymer is supposed to be a noble substitute to Nafion membranes as far as its use in DMFC is concerned. SPEEK has potential to substitute Nafion membranes but only after modification of certain properties. Mainly these properties include protonic conductivity of SPEEK membranes as compared with Nafion membranes otherwise, SPEEK has comparable methanol permeation, high temperature stability, and low cost as compared with commercial Nafion membranes. The... 

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