Membranes @ MMI
IN A NUTSHELL: A new type of proton exchange membrane (PEM) for use in hydrogen or methanol fuel cells has been developed. The ideal PEM has high proton conductivity across a range of humdities (from 0 to 100% RH), can function at high temperature (90°C and above) and is dimensionally stable (does not swell) in the presence of water. In this technology, polyhedral oligosilsesquioxane (POSS), a form of nanosilica, carries proton-conducting groups and is used to enhance the performance of existing proton-conducting polymer membranes, and to achieve a combination of good conductivity and good dimensional stability. MMI’s membranes have superior conductivity and dimensional stability to the industry standard fluoropolymer, Nafion®, particularly at low relative humidity and when used in multilayered membrane structures (e.g., 100% sulfonated polyphenylsulfone (S-PPSU) / 80% S-PPSU+20% S-POSS / 100% S-PPSU as shown in Figure).
THE PROBLEM: In order to achieve maximum catalyst efficiency, PEMs in hydrogen fuel cells should function above 90°C. The current industry standard PEM, Nafion®, fails above 90°C. In addition, in order to have a high enough sulfonation level to conduct protons, conventional fuel cell membranes are also prone to swelling, mechanical failure and poor performance in humidity cycling conditions.
THE STATE OF THE TECHNOLOGY: Right now, PEMs are based on either fluoropolymers such as Nafion® or upon sulfonated aromatics. Both classes of polymer carry hydrophilic sulfonic acid groups, and are prone to swelling. Other proton-conducting groups such as phosphonic acid and imidazole have been studied, but have been found to have inferior conductivity to sulfonic acid systems. Addition of liquids such as phosphoric acid, heteropolyacids (HPAs) or ionic liquids enhances conductivity, but creates a potential for the liquids to leach out of the membrane. The addition of micro-additives such as silica improves mechanical properties but is detrimental to proton conductivity. Hydrophilic-hydrophobic block copolymer architectures have been used to create proton channels and minimize swelling, but these materials require multi-step polymer and monomer syntheses.
OUR NOVELTY: MMI has developed a sulfonated polyhedral oligosilsesquioxane (S-POSS) nanoadditive for fuel cell proton exchange membranes (PEMs) that has a unique combination of proton conductivity and reinforcing properties, that can be easily produced, processed and cast, and that can used to enhance the performance of a variety of proton-conducting materials. An S-POSS nanoadditive formulated into a sulfonated polyphenylsulfone (S-PPSU) gives a membrane with superior conductivity and dimensional stability to the industry standard Nafion® at 90°C and 25% relative humidity. Both S-POSS and S-PPSU are easily produced in one step from commercially available starting materials.