Improved fuel utilization in 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. Recent experimental studies have established proof-of-concept of such fuel cells and have also shown that their performance is greatly limited by poor fuel utilization. Improving fuel utilization while minimizing fuel-oxidant mixing in microfluidic fuel cells is the focus of this study. A concise electrochemical model of the key reactions and appropriate boundary conditions are presented in conjunction with the development of a computational fluid dynamic (CFD) model of this system that accounts for coupled flow, species transport and reaction kinetics. 3D numerical simulations show that the fuel cell is diffusion limited, and both microchannel and electrode geometry play key roles in system performance. Three cross-sectional geometries are investigated, and a high aspect ratio rectangular geometry results in a two-fold increase in fuel utilization compared to a square geometry with the same hydraulic diameter. By tailoring the flow rate to the axial length of the fuel cell, fuel utilization is increased to 23%. Using the numerical simulation to guide the electrode design process, an extended tapered-electrode design is proposed. Simulations of the tapered-electrode microfluidic fuel cell demonstrate a fuel utilization of over 50%.