InterPore2022

Minimal surfaces in gas diffusion layers

2 Jun 2022, 13:30

15m

Oral Presentation (MS01) Porous Media for a Green World: Energy & Climate MS01

Speaker

Mohammad Javad Shojaei (Imperial College London)

Description

Gas diffusion layers in fuel cells need to allow the simultaneous flow of gases and water. Empirically this is achieved by coating the naturally hydrophilic carbon fibres with a hydrophobic coating, usually PTFE. We have used high-resolution X-ray imaging to observe the configuration of water within gas diffusion layers with different degrees of PTFE coating, and to quantify curvature and contact angle.
We observe that the fluid menisci form minimal surfaces. These surfaces occur in equilibrium at pinned contact lines at the boundary between hydrophilic and hydrophobic parts of the fibres. Minimal surfaces have two desirable features for multiphase flow. Firstly, the pressure difference between the water and gas is zero, which means that no additional water pressure is required, preventing retention and clogging of the pore space. Secondly, from topological principles, minimal surfaces ensure well-connected phases: the water clusters contain many redundant loops which helps maintain the continuity of flow under operando conditions.
In our work we found minimal surfaces for gas diffusion layers with 5, 20 and 40% coating with PTFE. The layer with a 60% coating had a markedly lower porosity and was more characteristically hydrophobic with higher average contact angles and a negative capillary pressure – that is the water pressure was higher than the surrounding air. The presence of minimal surfaces suggests that the water and gas pressures are equal, allowing water to flow readily without a pressure build-up. From topological principles, the negative Gaussian curvature of the menisci implies that the fluid phases are well connected. The implication of the design of porous materials where the simultaneous flow of two phases over a wide saturation range is explored.

References

Reference
(1) Blunt, M. J. Multiphase flow in permeable media: A pore-scale perspective; Cambridge University Press, 2017.
(2) Lin, Q.; Bijeljic, B.; Berg, S.; Pini, R.; Blunt, M. J.; Krevor, S. Minimal surfaces in porous media: Pore-scale imaging of multiphase flow in an altered-wettability Bentheimer sandstone. Physical Review E 2019, 99 (6), 063105.
(3) AlRatrout, A.; Raeini, A. Q.; Bijeljic, B.; Blunt, M. J. Automatic measurement of contact angle in pore-space images. Advances in Water Resources 2017, 109, 158-169.
(4) Liu, C. P.; Saha, P.; Huang, Y.; Shimpalee, S.; Satjaritanun, P.; Zenyuk, I. V. Measurement of Contact Angles at Carbon Fiber–Water–Air Triple-Phase Boundaries Inside Gas Diffusion Layers Using X-ray Computed Tomography. ACS Applied Materials & Interfaces 2021, 13 (17), 20002-20013.

Presentation materials

abstract_interpore_2022.docx