(373a) Nature-Inspired Gas Distributor Design with Application to Low-Temperature PEM Fuel Cells

Abstract: This work investigates a novel design for a low-temperature proton exchange membrane fuel cell (PEMFC) flow field¹. Typical, existing flow field designs suffer from the same fundamental deficiency, namely the non-uniform distribution of reactant gases over the surface of the catalytically active area. This leads to inefficient usage of the expensive, platinum-based catalyst material within the fuel cell.

The proposed, new design is inspired by the architecture of the human lung, which has been shown to be an extremely efficient gas transport network as a result of displaying equipartition of entropy production^{2, 3}. Furthermore, the fractal, self-similar geometry of the human lung is capable of performing two other key tasks: evenly distributing oxygen throughout the lung, and slowing down the gas velocity, so that, when the oxygen reaches the alveoli, the oxygen’s convective flux is equal to its diffusive flux. These two properties are highly desirable in a fuel cell gas distributor as well.

By evenly distributing reactant gases over the surface of the catalytically active material all the platinum catalyst can be efficiently used. As a result, less platinum can be used to achieve the same power output. The novel gas distributor design is guided by the geometry of the bronchial tree. It has one inlet and 4ⁿ outlets, where n is the number of branching generations. Similar fractal geometries have been used in applications ranging from fluidization and irrigation to electronics cooling^4-6. A variety of product (liquid water, excess air) removal flow paths are also being studied, including a second fractal network, as well as more conventional serpentine and parallel flow fields.

Simulations of this gas distributor geometry have been conducted using COMSOL Multiphysics 3.5a. The results of these simulations have shown that 8 branching generations are needed to achieve an equality of convective and diffusive fluxes at the outlet of the gas distributor when operating at standard conditions. However, as few as 4 generations, in conjunction with a typical gas diffusion layer will provide a nearly uniform distribution of reactant gases over the catalytically active surface.

As a result of these simulations a prototype design of the fractal gas distributor has been generated using the computer-aided-design package NX 7.5. This design has been successfully prototyped by using a high resolution sterolithography machine. These gas distributors are assembled along with other necessary fuel cell components to form a test cell. The performance of this gas distributor is tested using the fuel cell test equipments available within the Center for Automation Technologies & Systems (CATS) at Rensselaer. Key performance characteristics, including average current density, average power output, and platinum usage, will be discussed, and compared to systems that utilize more conventional flow field geometries (serpentine, parallel, and parallel-serpentine).

References:

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2)      S. Gheorghiu, S. Kjelstrup, P. Pfeifer, M.-O. Coppens, Fractals in Biology and Medicine, Vol. 4 (2005).

3)      E. Johannessen, S. Kjelstrup, Chemical Engineering Science 60, 3347-3361 (2005).

4)      M.-O. Coppens, Ind. Eng. Chem. Res. 44, 5011-5019 (2005).

5)      W. Escher, B. Michel, D. Poulikakos, International Journal of Heat & Mass Transfer 52, 1421-1430 (2009).

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