A two-dimensional, two-phase, multicomponent, transient model was developed for the cathode of the proton exchange membrane fuel cell. Gas transport was addressed by multicomponent diffusion equations while Darcy's law was adapted to account for the capillary flow of liquid water in the porous gas diffusion layer. The model was validated with experimental results and qualitative information on the effects of various operating conditions and design parameters and the transient phenomena upon imposing a cathodic overpotential were obtained. The performance of the cathode was found to be dominated by the dynamics of liquid water, especially in the high current density range. Conditions that promote faster liquid water removal such as temperature, dryness of the inlet gas stream, reduced diffusion layer thickness, and higher porosity improved the performance of the cathode. There seems to be an optimum in the diffusion layer thickness at the low current density range. The model results showed that for a fixed electrode width, a greater number of channels and shorter shoulder widths are preferred. The transient profiles clearly showed that liquid water transport is the slowest mass-transfer phenomenon in the cathode and is primarily responsible for mass-transfer restrictions especially over the shoulder.

This communication reports the design and characterization of an air-breathing laminar flow-based microfluidic fuel cell (LFFC). The performance of previous LFFC designs was cathode-limited due to the poor solubility and slow transport of oxygen in aqueous media. Introduction of an air-breathing gas diffusion electrode as the cathode addresses these mass transfer issues. With this design change, the cathode is exposed to a higher oxygen concentration, and more importantly, the rate of oxygen replenishment in the depletion boundary layer on the cathode is greatly enhanced as a result of the 4 orders of magnitude higher diffusion coefficient of oxygen in air as opposed to that in aqueous media. The power densities of the present air-breathing LFFCs are 5 times higher (26 mW/cm2) than those for LFFCs operated using formic acid solutions as the fuel stream and an oxygen-saturated aqueous stream at the cathode ( approximately 5 mW/cm2). With the performance-limiting issues at the cathode mitigated, these air-breathing LFFCs can now be further developed to fully exploit their advantages of direct control over fuel crossover and the ability to individually tailor the chemical composition of the cathode and anode media to enhance electrode performance and fuel utilization, thus increasing the potential of laminar flow-based fuel cells.

A two-dimensional model available in the literature for conventional gas distributors was expanded to account for the dimension along the length of the channel. The channel was discretized into control volumes in series that were treated as well mixed. An iterative solution procedure was incorporated in each control volume to determine the average current density and the corresponding oxygen consumption and water generation rates. Down stream channel concentrations were calculated based on stoichiometric flow rates and the solution obtained from the preceding control volumes. Comparison of the model results with experimental data and the existing two-dimensional model showed that accounting for the oxygen concentration variations along the channel and its effect on the current density is critical for accurately predicting the cathode performance. Variations in the current density along the channel were strongly influenced by the changes in oxygen concentration caused by consumption due to reaction and dilution caused by water evaporation. Operating parameters that facilitated better water removal by evaporation like higher temperature and stoichiometric flow rates and lower inlet stream humidity resulted in higher net current. Operating conditions that resulted in minimal loss in oxygen concentrations resulted in a more uniform current density distribution along the channel.

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