Swelling acid doping

As mentioned earlier, a variety of membrane materials have been investigated for possible use in hydrogen/alr fuel cells and DMFCs, including sulfonated polyimides, sulfonated poly(arylene ether)s, sulfonated poly(ether ether ketone)s, and phosphoric acid-doped polybenzimidazole [5,321-331]. Many of these membrane materials suffer from poor oxidative stability, excessive swelling when wet and brittleness when dry, and only moderately high proton conductivity. Thus, none of the polymers has yet emerged as the preferred proton exchange membrane material for fuel cell... 

Halik and coworkers subsequently extended this process to pattern conducting polymer electrodes, such as poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid (PEDOT-PSS) [11]. In this process, PEDOT-PSS is uniformly coated on a substrate that has prepattemed photoresist. The PEDOT-PSS-coated substrate is then immersed in acetone to swell the photoresist underneath the... 

Figure 7 showed that neither NaCl nor CaCl2 affected the swellability of poly (PVA-MA) at pH 5 and below. However, NaCl suppressed the expansion significantly between pH 5 and 7. It has been reported that urea enhanced the pH-induced swelling of poly-HEMA hydrogel doped with methacrylic acid. However, we have found fhat urea suppressed the expansion of poly(PVA-MA). 

The water and phosphoric acid uptake cause dimensional changes of the membrane (swelling). The swelling is normally calculated on the dry undoped membrane volume basis according to (6.15), where Uvmdopcd and Udoped are the volume of the undoped and doped membrane, respectively. 

The most commonly used membrane material for HT-PEM MEAs (PBI based) has to be soaked or doped with phosphoric acid to get sufficient proton conductivity. This doped or soaked membrane shows a lowering of the mechanical strength, due to the fact that the uptake of phosphoric acid leads to swelling of the PBI. 

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