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Porous electrodes in microfluidic fuel cell (MFC) operate with nonuniform reaction rate. It is intriguing to improve the utilization degree of the porous electrodes. In this work, a three-dimensional computational model is developed for MFC with flow-through porous electrodes. Characteristics of the reaction rate distributions under different electrode geometries are examined. The results show that reaction rate varies noticeably along the electrode width direction, but minimally along the electrode length direction. High reaction rate region locates in the vicinity of the interface between the porous electrode and the middle channel. A relatively high aspect ratio, defined as the ratio of the electrode length to width, is beneficial to improve the utilization degree of the porous electrodes. Yet, concentration losses increase due to the decreased fluid velocity. Considering the cell performance, optimal electrode aspect ratios are derived for the anode and cathode, respectively.