Planar and three-dimensional microfluidic fuel cell architectures based on graphite rod electrodes

We propose new membraneless microfluidic fuel cell architectures employing graphite rod electrodes. Commonly employed as mechanical pencil refills, graphite rods are inexpensive and serve effectively as both electrode and current collector for combined all-vanadium fuel/oxidant systems. In contrast to film-deposited electrodes, the geometry and mechanical properties of graphite rods enable unique three-dimensional microfluidic fuel cell architectures. Planar microfluidic fuel cells employing graphite rod electrodes are presented here first. The planar geometry is typical of microfluidic fuel cells presented to date, and permits fuel cell performance comparisons and the evaluation of graphite rods as electrodes. The planar cells produce a peak power density of 35 mW cm−2 at 0.8 V using 2 M vanadium solutions, and provide steady operation at flow rates spanning four orders of magnitude. Numerical simulations and empirical scaling laws are developed to provide insight into the measured performance and graphite rods as fuel cell electrodes.We also present the first three-dimensional microfluidic fuel cell architecture with multiple electrodes. The proposed fuel cell architecture, consisting of a hexagonal array of graphite rods, enables scale-up/integration of microfluidic fuel cell technology as well as power conditioning flexibility beyond that of the traditional fuel cell stack. When provided the same flow rate as the planar cell, the array cell generated an order of magnitude higher power output. The array architecture also enabled unprecedented levels of single pass fuel utilization, up to 78%.