(712a) Nature-Inspired Fractal Gas Distributor for Low-Temperature PEM Fuel Cell Cathodes

This work examines a novel approach to the design of a fuel cell gas distributor by mimicking the structure and functionality of the human lung¹. A fractal flow field geometry is proposed to address the fundamental shortcomings of conventional flow field designs, namely the non-uniform distribution of reactant gases over the surface of the catalytically active area. The latter lead to highly non-uniform transport and reaction conditions throughout the cathode catalyst layer.

The proposed design is inspired by ability of the fractal geometry of human lungs to uniformly distribute oxygen throughout the lung from a single inlet (trachea) through many outlets (alveoli). Our novel flow field design utilizes a similar fractal branching structure to provide nearly uniform coverage of reactant oxygen gas across the cathode gas diffusion layer. This results in an almost constant oxygen concentration boundary condition across the entire gas diffusion layer - cathode catalyst layer interface. This eliminates concentration depletion along the flow paths, seen in conventional flow geometries².

The fractal geometry for the gas inlet is a repeated “H” shape that has one inlet and 4ⁿ outlets, where n is the number of branching generations. Fractal geometry has been applied to fuel cell components (flow field / current collector) in the past; however, this design is fundamentally different, as it is a truly three-dimensional branching structure as compared with previously reported two-dimensional designs^3,4.

Simulations of this gas distributor geometry have been conducted using COMSOL Multiphysics 3.5a. The simulation results provide insight into the necessary spacing between adjacent outlets to deliver a uniform concentration of oxygen across the catalyst layer interface.

Three gas distributor prototypes were fabricated, each representing an increased number of generations (more tightly packed outlets). The prototypes were fabricated via sterolithography (one resin, two in stainless steel) and tested against a standard single serpentine channel. Results indicate that the higher generation fractal prototype outperforms the serpentine flow field when operating at elevated current densities (above 0.90 A/cm²) with air as the cathode gas. Enhanced performance is attributed to uniform distribution of reactant gases in the cathode catalyst layer.

References:

1)      S. Kjelstrup, M.-O. Coppens, J. G. Pharoah, P. Pfeifer, Energy & Fuels, 24, 5097 (2010)

2)      A. Su, Y. C. Chiu, F. B. Weng, Int. J. Energy Res., 29, 409-425 (2005)

3)      K. Tuber, A. Oedegaard, M. Hermann, C. Hebling, J. Power Sources, 131, 175-181 (2004)

4)      J.-Y. Chang, Y.D. Kuan, S.-M. Lee, S.-R. Lee, J. Power Sources, 184,180-190 (2008)