(433d) Nature-Inspired Flow Fields for PEM Fuel Cells

Jason In Sung Cho^1,2, Toby Neville^1,2, Panagiotis Trogadas¹, Billy Wu³, Dan Brett^1,2 and Marc-Olivier Coppens¹

¹ Department of Chemical Engineering, University College London, London WC1E 7JE, United Kingdom

² Electrochemical Innovation Lab, Department of Chemical Engineering, University College London, London WC1E 7JE, United Kingdom

³ Department of Mechanical Engineering, Imperial College London, UK

Ensuring uniform gas distribution across the catalyst layer remains one of the on-going challenges impeding broader commercialisation of polymer electrolyte membrane fuel cells (PEMFCs). This is an inevitable consequence within the framework of the conventional flow field designs, such as serpentine channel, predominantly due to reactant depletion along the channel and over the active area of the electrode ^[1].

The novel design of the proposed nature-inspired flow field draws inspiration from the fractal structure and functionality of the upper respiratory tract of the human lung to address this fundamental shortcoming. The fractal geometry of the human lung has been shown to ensure uniform distribution of air from a single outlet (trachea) to multiple outlets (alveoli). The convection driven air flow rate in trachea is reduced until it is equal to the diffusion driven air flow rate in alveoli (Pe ≈1)^[2], resulting in uniform distribution of entropy production in both regimes^[3,4].

By employing a 3D fractal structure as flow field inlet channel, we aim to yield similar benefits from replicating these characteristics of the human lung. The fractal pattern comprises repeating “H” shapes and a fractal dimension of 2, where daughter “H’s” are half the parent’s size, leading to a plane-filling geometry. Fractal geometry has previously been invoked in flow field designs. However, this design is fundamentally different, as the three-dimensional branching structure allows only the outlets of the fractal inlet channel to be exposed to the membrane-electrode assembly (MEA), eliminating the reactant depletion along the channel, and thus providing uniform local conditions on the surface of the catalyst layer.

Numerical simulations were performed to obtain a detailed insight into the effect of the number of fractal branching generations on the reactant distribution in the catalyst layer. Based on the simulation results, three fractal flow field prototypes (10 cm²surface area), each representing a branching generation of N = 3, 4 and 5, were manufactured via selective laser sintering (SLS), and experimentally validated against conventional serpentine flow fields. Results indicate that the N = 4 and N = 5 fractal flow fields outperform the serpentine flow fields at high current densities (at 50% and 75% RH) due to more uniform reactant distribution across the catalyst layer. At higher humidity levels (100% RH), though, the performance of N = 5 fractal flow fields significantly deteriorates; the reduced air flow rate within the fractal flow fields hampers effective gas diffusion within the porous medium resulting in insufficient convective liquid water removal.

Another important characteristic of the fractal flow field design is that uniform gas distribution across the catalyst layer is preserved when these fractal flow fields are scaled-up (27 cm² surface area). The fuel cell performance of the large-scale fractal flow field remained almost unchanged on a per area basis, compared to its smaller active area counterpart. On the contrary, larger, conventional serpentine flow fields (27 cm²surface area) exhibited what appears to be improved performance as a result of an order-of-magnitude higher pressure drop than that of a fractal flow field, because this induces faster reaction kinetics and better liquid water removal. However, such excessive pressure drop renders the use of a large scale serpentine flow field prohibitive, thus favouring the fractal flow field. Implementation of effective water management strategies could circumvent the remaining problems of high-generation fractal flow fields.

References

[1] Tüber, K., Oedegaard, A., Hermann, M., and Hebling, C., Investigation of fractal flow-fields in portable proton exchange membrane and direct methanol fuel cells. Journal of Power Sources, 2004. 131(1–2): p. 175-181.

[2] Coppens, M.-O., A nature-inspired approach to reactor and catalysis engineering. Current Opinion in Chemical Engineering, 2012. 1(3): p. 281-289.

[3] Kjelstrup, S., Coppens, M.-O., Pharoah, J., and Pfeifer, P., Nature-inspired energy-and material-efficient design of a polymer electrolyte membrane fuel cell. Energy & Fuels, 2010. 24(9): p. 5097-5108.

[4] Trogadas, P., Ramani, V., Strasser, P., Fuller, T.F., and Coppens, M.-O., Hierarchically Structured Nanomaterials for Electrochemical Energy Conversion. Angewandte Chemie International Edition, 2016. 55(1): p. 122-148.

[5] Coppens, M.-O., A nature-inspired approach to reactor and catalysis engineering. Current Opinion in Chemical Engineering, 2012. 1(3): p. 281-289.