461692 Nature-Inspired Fractal Flow Field for PEM Fuel Cells
Jason In Sung Cho1,2, Toby Neville1,2, Jeffrey Marquis3, Panagiotis Trogadas1, Dan Brett1,2 and Marc-Olivier Coppens1
1 Department of Chemical Engineering, University College London, London WC1E 7JE, United Kingdom, email@example.com
2 Electrochemical Innovation Lab, Department of Chemical Engineering, University College London, London WC1E 7JE, United Kingdom
3 Isermann Department of Chemical & Biological Engineering, Rensselaer Polytechnic Institute, 110 8th St., Troy New York, United States
One of the ongoing challenges faced by polymer electrolyte membrane (PEM) fuel cells for broader commercialisation is the accomplishment of uniform gas distribution across the catalyst layer. Ensuring reactant homogeneity is essential to improve fuel cell efficiency and to forestall adverse side reactions that are detrimental to fuel cell longevity . Within the confines of the conventional flow fields, such as serpentine channels, this is an unavoidable consequence, due to reactant depletion across the channel and over the active area as a whole .
This work aims to address the issue by implementing a novel 3D flow field inspired by the fractal geometry of the upper respiratory tract of the human lung. The human lung has been shown to have an optimal architecture that uniformly distributes oxygen from the trachea to the alveoli. Furthermore, the human lung transitions between two flow regimes: 14-16 upper generations of branches dominated by convection, and 7-9 lower generations of space-filling acini dominated by diffusion[3, 4]. The upper generations of branches are designed to slow gas flow to a rate compatible with the rate in the diffusional regime (Pe≈1)  and the entropy production was shown to be uniformly distributed in both regimes [3, 4]. Furthermore, an attractive aspect is the scalability of fractal networks. They can bridge multiple length scales by the addition of further generations, while preserving the building units and microscopic function of the system . These characteristics that make the lung an optimal transport structure are all desirable for a PEM fuel cell. However, rather than uniformly distributing oxygen in a given volume, we wish to distribute the gases uniformly over a plane to ultimately deliver a uniform concentration of reactant throughout the catalyst layer .
The fractal pattern consists of repeating “H” shapes and, by employing a fractal dimension of 2, where daughter “H’s” are half the parent’s size, a plane-filling geometry is attained. Furthermore, the three-dimensional branching structure allows only the outlets of the fractal inlet channel to be exposed to the MEA, eliminating the reactant depletion along the channel, and thus providing uniform local conditions on the surface of the catalyst layer.
In this study, the reactant flow and cell performance were modelled for different numbers of fractal branching generation. Optimal performance was reached for branching levels N ³ 5. Based on the simulation results, three fractal flow field prototypes were produced via selective laser sintering (SLS), a «3D printing» method, each representing a branching generation level of N = 3, 4 and 5, and validated against a standard serpentine flow field.
The results suggest improved mass transport for the fractal flow fields at high current density, due to the homogeneous gas distribution. The fractal prototype with N = 4 generations demonstrated improved performance over the serpentine at all operating conditions investigated in this study and a good correlation with the results predicted by the model. The worse-than-expected performance of for N = 5 generations at higher humidity level reveals insufficient convective liquid water removal due to the slow air velocity from each fractal outlet hampering effective diffusion within the porous medium. The issues with flooding were alleviated by operating the fuel cell at lower humidity levels, which led to improved mass transport for the prototype with N = 5 generations. However, fuel cells should ideally be operated under fully humidified condition to minimise local membrane drying and subsequent Ohmic losses from reduced proton transport.
The findings elucidate the need for implementation of active water management systems to alleviate flooding for the fractal flow fields, especially at higher generation levels where flooding is projected to be more pronounced.
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The authors wish to thank the National Science Foundation Fuel Cell IGERT at Rensselaer, DGE-0504361 for supporting initial work. Financial support from an EPSRC “Frontier Engineering” Award (EP/K038656/1) and a UCL Faculty of Engineering Sciences Dean’s Scholarship are also gratefully acknowledged.