Understanding Polymer Ion Clustering and Its Implications for Fast Ion Transport in Polymer Electrolyte Membranes

Advanced polymer electrolyte membranes have the potential to enable new electrochemical devices, as acid functionalized membranes are aimed at higher and drier operating conditions and anion exchange membranes (AEMs)  allow catalysis in alkaline media. However, little is known about transport in these newer materials. Alkaline fuel cells (AFCs) have been widely investigated since the 1960s. The perceived advantages of alkaline electrolytes (e.g., KOH solution) used in AFCs include the applications of non precious metal catalysts and increased fuel flexibility. However, the use of liquid alkaline electrolytes have disadvantages such as maintaining containment of the electrolyte, component corrosion, and reaction with CO₂ to form insoluble carbonates that can block the electrodes, which must be scrubbed, necessitating the use of pumps and dramatically lowering system power density. There is currently great interest in using AEMs in electrochemical devices, as the carbonate anion is mobile in these materials, 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 other membrane properties, e.g. anion and water transport, morphology, water absorption, in order to better understand the performance of the AEM. Simple quaternary 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 many real devices.  Various 2^nd generation cations have been proposed for enhanced chemical stability including phosphonium, pydridinium, sulfonium, imidazolium, guanidinium, and complex metal cations, e.g. (bis-terpyridine Ru).  In all cases it is rare, especially when the membranes are not water saturated, to observe simple Arrhenius behavior for ionic conductivity in these systems.  Generally the ion pairs in the material are not fully dissociated and the ionic movement is coupled either to the relaxations of the side group, VTF behavior, or the polymer backbone.  In addition we are increasingly observing that the ions on the polymer backbone relax from a distributed configuration to a thermodynamically proffered clustered state in which ion transport becomes hindered.  Very Often this transition as observed by broadband electric spectroscopy, electrochemical impedance spectroscopy, or Pulsed Field Gradient Spin Echo NMR spectroscopy are below or at the temperature of operation of the devices in which the materials are designed to function.  Obviously, understanding these processes and designing materials that do not undergo such limitations is crucial to the design and implementation of next generation electrochemical energy conversion devices.  In this talk we will show examples of how ionic conductivity is affected by the properties of the polymer matrix.