Kyle T. Clark

Correlating Humidity-Dependent Ionically Conductive Surface Area with Transport Phenomena in Proton-Exchange Membranes

The objective of this effort was to correlate the local surface ionic conductance of a Nafion 212 proton-exchange membrane with its bulk and interfacial transport properties as a function of water content. Both macroscopic and microscopic proton conductivities were investigated at different relative humidity levels, using direct-current voltammetry and current-sensing atomic force microscopy (CSAFM). We were able to identify small ion-conducting domains that grew with humidity at the surface of the membrane. Numerical analysis of the surface ionic conductance images recorded at various relative humidity levels helped determine the fractional area of ion-conducting active sites. A simple square-root relationship between the fractional conducting area and observed interfacial mass-transport resistance was established. Furthermore, the relationship between the bulk ionic conductivity and surface ionic conductance pattern of the Nafion membrane was examined.

Blend Membranes of Highly Phosphonated Polysulfone and Polybenzimidazoles for High Temperature Proton Exchange Membrane Fuel Cells

Blend Membranes of Highly Phosphonated Polysulfone and Polybenzimidazoles for High Temperature Proton Exchange Membrane Fuel Cells R. A. Potrekar † , K. T. Clark †‡ , X. Zhu † , and J. B. Kerr †* Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, CA 94720 Department of Materials Science and Engineering, University of California Berkeley, Berkeley, CA 94720 The aim of the presented work is to develop a polymer electrolyte having the specific composition of polysulfone (PSU) with phosphonic acid groups in the tethered form and polybenzimidazoles, doped with phosphoric acid, which facilitates self-ion transfer. The blended membranes show high proton conductivity (3.8 x 10 -2 S/cm at 170°C at 25% RH) and have good thermal and mechanical properties. Introduction Proton conduction in polymer membranes in fuel cells is due to the vehicular, Grotthuss (hopping or structure diffusion), and segmented motion mechanisms. These mechanisms are dependent on the morphology of the polymers as well as the proton conducting functional groups. It is observed that polymers containing acid groups mainly facilitate structure diffusion and vehicular transport mechanisms for proton transport. However, the presence of these acid groups imposes a limitation on the operating temperature (<90°C) (1). For the vehicular proton transport mechanism, an aqueous media is essential to act as a mobile phase to facilitate the conduction of protons. Nevertheless, the presence of an aqueous media acts as a plasticizer and adversely affects the mechanical properties (swelling), increases the permeability of the fuel and oxidant, etc (2). On the other hand, the segmental proton transfer mechanism depends on the segmental motion of the polymer backbone, which requires a very low glass transition temperature that limits its mechanical properties, especially at high temperatures (3). In the Grotthuss mechanism, the proton, or protonic defects, diffuse through the hydrogen bond network by formation of cleavage of bonds and the proton is transferred through self or auto ionization (4). For example, at high temperatures in imidazole, protonic defects form and the proton is conducted through hydrogen bonding. Considering the limitations and advantages of all of the above three mechanisms, the transport phenomena involved, conformational, morphological contrast, mechanical strength, permeability of fuel/oxidant, and its temperature dependency etc., a polymer material needs to be designed and developed that can satisfy all the necessary properties and provide a robust material which can sustain the harsh oxidation/reduction environment.