(Invited) Thin Robust Anion Exchange Membranes for Fuel Cell Applications

Advanced anion exchange membranes have the potential to enable new electrochemical devices based on base catalysis such as fuel cells or electrolyzers. However, little is known about transport in these newer materials. Alkaline fuel cells (AFCs) have been widely investigated since the 1960s. The advantage of alkaline electrolytes (e.g., KOH solution) is that non precious metal catalysts can be deployed that can also offer fuel flexibility. However, liquid alkaline electrolytes have  disadvantages such as electrolyte leakage, component corrosion, reaction with CO₂ to form insoluble carbonates that can block the electrodes, which must be scrubbed out necessitating the use of pumps and dramatically lowering system power density. There is currently great interest in using anion exchange membranes (AEMs) in electrochemical devices, as carbonate is mobile, the elimination of the liquid electrolyte increases system simplicity, and dramatically higher power densities can be achieved.

Two limitations of AEMs must be overcome to enable practical applications: their inherent low conductivity (compared to proton exchange membranes) and the chemical stability of the organic cations that are susceptible to nucleophile attack by hydroxide in the operating device.  A large number of new chemistries have been proposed to overcome the two issues of needing high ionic conductivity and chemical stability. However, in order to design next generation AEMs we must correlate the cation chemistry with the other membrane properties: transport, morphology, water absorption, etc. in order to better understand AEM performance.  Simple quaternium ammonium cations have been studied extensively as they provide good model systems and in certain membranes have been shown to have adequate stability, but they do not provide a route to the thousands of hours of transient operation required in a real device.  Various 2^nd generation cations have been proposed for enhanced chemical stability including phosphonium, pydridinium, sulfonium, imidazolium, guanidinium, and complex metal cation (bis-terpyridine Ru). Of particular interest are the imidazolium functionalized AEMs, which provide the following potential benefits: 1. Conjugated structures generated from five heterocyclic ring help to delocalize positive charges, thus preventing nucleophilic attack by OH^- groups through Hofmann or S_N2 elimination. 2. Imidazolium functionalized membranes may more favorably generate phase-separated morphologies. 3. The imidazolium caion is thermally more stable than the ammonium cation.  Other approaches include large phosphonium cations and perethyl cobaltocenium cations.  All of these large stable cations will perturb the morphology and transport properties of the material.  In this talk we will discuss the implicationsof the usr of large cations for fuel cell reday membranes.