Section: Batteries & Fuel Cells

Membranes

Fig. 1 PEM Fuel CellFuel cells are highly efficient devices that convert the chemical energy stored in a fuel directly intoelectricity. Within a fuel cell, the polymer electrolyte membrane (PEM) serves as the conducting interface between the anode and cathode, transporting the ions (Figure 1). As a result, the PEM is a central, and often performance-limiting, component of the fuel cell. Nafion, the most widely used PEM material due to its remarkable proton conductivity and mechanical strength, combines a hydrophobic Teflon-like backbone with hydrophilic ionic side groups. Upon hydration, these components self-assemble into a bicontinuous nanostructure comprised of a hydrophobic matrix containing ionic nanochannels (Figure 2).1 The free movement of protons through these ionic nanochannels is the source of Nafion's impressive conductivity.

Fig. 2 Nafion nanostructureThere is currently significant interest in the development of hydroxide conducting PEMs, also known as alkaline anion exchange membranes (AAEMs), for fuel cells operating under basic conditions.2 The critical advantage of hydroxide conducting fuel cells, relative to proton conducting fuel cells, is enhanced kinetics; permitting the use of cheaper, non-noble metal catalysts as opposed to platinum.

Fig. 3 Procedure for preparing AAEMsWe have recently developed a ring opening olefin metathesis route (Figure 3) to AAEMs via the copolymerization of a tetraalkylammonium-functionalized norbornene (Fxnal-NBE) with dicylocpentadiene (DCPD).3 The thin films generated are robust, exhibit good hydroxide conductivites and exceptional methanol tolerance. Future work will focus on studying the efficacy of these materials under operating conditions using in house fuel cell test stations. Additionally we are developing new materials that retain excellent mechanical properties, but display much greater hydroxide conductivity.

 

References
(1) Chen, Q.; Schmidt-Rohr, K. Nat. Mater. 2008, 7, 75.
(2) Varcoe, J.; Slade, R.; Wright, G.; Chen, Y. J. Phys. Chem. B 2006, 110, 21041.
(3) Clark, T.; Robertson, N.; Kostalik IV, H.; Lobkovsky, E.; Mutolo, P.; Abruña, H.; Coates, G. Am. Chem. Soc. 2009, 131, 12888.