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We seek to understand the factors that most strongly govern the processes of ion transport and charge separation across synthetic membranes using scanning electrochemical microscopy (SECM).  Our ultimate goal is to identify new or optimized materials that would afford more efficient and cost-effective fuel cells. By systematically varying membrane composition and environmental conditions, the permeability, conductivity, and operating range of candidate materials can be evaluated.

SECM is well suited for studying ion transport across membranes. Specifically, the intrinsic two-electrode design allows for electrochemical interrogation on both sides of a suspended surface and the establishment of a potential gradient across a membrane. Prior to the direct measurement of hydrogen transport, it is essential that we establish a reliable method by which we can modify our substrates with a uniform and stable membrane morphology. Initial studies have employed naturally self-assembling phospholipid bilayers as a model system. These lipids are easily deposited onto a gold substrate in a conventional SECM cell set-up. In order to allow for diffusion-controlled transport as well as to create a continuously fluid environment, the lipid bilayers must be suspended on the gold substrate. To that end, surfaces are first incubated with a thiol moiety, which serves as a cushion for subsequent bilayer formation and prevents unwanted redox activity at the substrate electrode.

Work in the second year of this project has primarily focused on optimizing a protocol for reproducible membrane formation via the direct deposition of lipid vesicles onto modified surfaces. Extruded solutions of phosphatidylcholine and 1,2-dioleoyl-sn-glycero-3-phospho-L-serine are introduced to a surface that has been passivated with mercaptoundecanoic acid and allowed to self-assemble for varying lengths of time. Evidence of bilayer formation is qualitatively assessed by a combination of cyclic voltammograms, approach curves, and current density area scans. Preliminary results show consistent and thorough surface coverage with step-wise incubation of the passivating agent and lipids. Current efforts are focused on characterizing these self-assembled membranes with atomic force microscopy and quartz crystal microbalance.  Future work will employ both phospholipid membranes and Nafion-based materials in a custom-made electrochemical cell for the direct detection of hydrogen transport. Environmental conditions such as temperature and humidity will be varied, and their influence on the rate of transport evaluated.

During the past academic year, undergraduates Camille Gandara (‘11) and Michelle Sykes (’12) worked on different aspects of this PRF-funded project as they completed research for credit. Michelle now continues to work on this project in pursuit of her senior honors thesis and will submit an abstract for presentation at the ACS National Meeting next spring.

Support from the ACS PRF has been instrumental to the advancement of this project and to the overall development of our research program.