Membraneless Liquid-Fuel Microfluidic Fuel Cells: A Computational Study

Presented in this paper is a computational analysis of a membraneless microfluidic fuel cell that uses the laminar nature of microflows to maintain the separation of fuel and oxidant streams. The fuel cell consists of a T-shaped microfluidic channel with liquid fuel and oxidant entering at separate inlets and flowing in parallel without turbulent or convective mixing. Electrodes are placed along the walls, and the resulting redox reactions provide the cell voltage and current. A concise electrochemical model of the key reactions and appropriate boundary conditions for the computational fluid dynamic (CFD) modelling of this system are developed and implemented into the numerical model. The coupled flow, species transport and chemical aspects of the microfluidic fuel cell are simulated. The effects of geometry and flow rates on fuel cell performance are investigated. Results indicate that the microfluidic fuel cell performance is limited by the transport of reactants through the concentration boundary layer to the electrodes. Three typical geometries were simulated, and it was found that increasing the aspect ratio of the channel cross-section from a square geometry to a rectangular one leads to more than a two-fold increase in fuel utilization. The two rectangular geometries simulated consist of a design with a high aspect ratio in the direction perpendicular to the plane of cross-stream diffusion as well as a design with a high aspect ratio in the direction parallel to the plane of cross-stream diffusion. The electrode placement and geometry play key roles with respect to mixing and fuel utilization. The design with a high aspect ratio in the direction perpendicular to the plane of cross-stream diffusion demonstrated relatively less cross-stream mixing compared to the other rectangular geometry, and had the potential for improved fuel utilization with appropriate electrode design. In addition, results suggest that fuel utilization can be increased from previous values by a factor of two or more. Decreasing the inlet velocity from 0.1 m/s to 0.02 m/s caused the fuel utilization to increase non-linearly from 8 % to 23 %, and only caused an increase of 3 % in cross-stream mixing at the outlet.