Fundamental studies of electroomosis and its applications for miniature fuel cells

This thesis presents (1) molecular dynamics simulation studies of electroosmosis and (2) experimental investigations on electroosmotic pumps applied for miniature fuel cells. Major results of molecular dynamics simulation include layering and ordering of water molecules near solid surfaces, non-uniformity of permittivity and diffusivity, and, especially, lowered ion and water mobilities near rough surfaces. The lowered mobilities result in significantly lowered electroosmotic flows and zeta potential in rough wall nanochannels. Molecular dynamics studies also find the existence of electroosmotic flows on uncharged surface, first hypothesized by Dukhin et al. (2005). Among many possible explanations, polarization of water due to layering and near-wall molecular ordering are perhaps the mechanisms for this poorly understood phenomenon. Asymmetric interactions of positive and negative ions with water molecules (shown by three-dimensional density distributions) may also contribute to this phenomenon. The second half of this thesis presents the development and characterization of electroosmotic pumps for the following miniature fuel cells applications: Air delivery via liquid electroosmotic pumping and fuel delivery in direct methanol fuel cells. An important figure of merit for these applications is the flow rate per power. An analytical model, based on the Possion-Boltzmann solutions, suggests the use of low ion density solvents, such as organic solvents, for high flow rate per power pumping. Experiments done at a negligible pump load condition show that acetone enables about ten times greater flow rate per power than dilute aqueous buffers (e.g., at 1 mM sodium concentration). Utilizing these low ion density solvents, the proposed oxygen delivery was demonstrated in proof-of-concept experiments. The experiments confirmed the feasibility of the scheme but the pump performance should be improved in order to achieve more energetically favorable pumping. Electroosmotic pumping of methanol/water mixtures was also characterized as a study of the fundamental limitations and capabilities of pumping methanol for fuel cell applications. The experiments with a fuel cell integrated with a pump show that the pump can deliver methanol to the cell and yet consume only 1-4% of the fuel cell power.