Novel Electrospun Gas Diffusion Layers with Spatially Graded Pore Sizes for Polymer Electrolyte Membrane Fuel Cells

Abstract

High current density performance of a polymer electrolyte membrane (PEM) fuel cell is severely limited by the accumulation of liquid water in the gas diffusion layer (GDL) of the cell. The accumulation of water blocks pathways for oxygen transport and leads to reduced cell performance. Graded GDL properties, such as increasing porosity and decreasing hydrophobicity are strategies for expelling excess water and improving cell performance [1,2]. A powerful tool to manufacture GDL materials with tailored, graded properties is electrospinning [3]. In this study, novel electrospun, nano-fibrous, graphitic, and hydrophobic GDLs with increasing pore-sizes from the catalyst layer (CL) to the flow fields were designed and fabricated to improve the high current density performance of PEM fuel cells. The fiber diameters, pore-sizes, material composition, and electrical conductivity of the novel materials where quantified using a combination of scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and 4-point probe measurements. The effects of the electrospun GDLs (eGDLs) with graded pore-sizes on cell performance were evaluated via fuel cell performance testing and electrochemical impedance spectroscopy (EIS). A solution of polyacrylonitrile (PAN) and N,N-dimethylformamide was used to electrospin nano-fibrous polymer substrates. Higher PAN concentrations in the polymer solution led to larger fibre diameters and pore sizes. Graded pore structures were manufactured by spinning subsequent layers with increasing PAN concentrations. The electrospun substrates were converted to graphitic carbon substrates via heat treatment at 1400 oC and rendered hydrophobic without structural morphological alterations via an oxygen plasma and trichloro(1H,1H,2H,2H-perfluorooctyl)silane treatment. Two uniform eGDLs were manufactured for base line comparison, and two graded eGDLs were manufactured to test the effect of graded pore-sizes on cell performance. The two uniform eGDLs were called s-eGDL and l-eGDL and comprised of small fibres and large fibres, respectively. The two graded structures were called the Bi-Layer eGDL and Tri-Layer eGDL and had increasing fibre diameters and pore-sizes from the catalyst layer to the flow fields. The fuel cell tests were conducted at 60oC and 50, 75,and 100% RH conditions. The graded eGDL materials led to improved high current density performance compared to the uniform eGDLs at all test conditions with the most significant improvements at 100% RH (≥ 27% increase in peak power output). EIS measurements at 2.0 A/cm2 and 100% RH indicated a substantial decrease (≥ 23%) in mass transport resistance for the graded eGDLs compared to the uniform eGDLs. The reduction in mass transport losses is attributed to enhanced liquid water removal due to the graded structures. The increasing pore-sizes from the CL to the flow fields leads to a capillary pressure gradient favourable for liquid water removal which leads to improved oxygen transport. Additionally, the high frequency resistance (HFR) of the cell at 100% RH did not vary significantly between the s-eGDL and the graded eGDL materials at 2.0 A/cm2 (≤ 8%). However, the l-eGDL material, which had relatively larger pores at the CL compared to the other three materials, had a significantly higher measured HFR value compared to the s-eGDL (31% increase) at the same conditions. The higher HFR value for the l-eGDL material may be attributed to reduced diffusion of water from the CL towards the membrane due to the larger pore-sizes, leading to reduced membrane hydration and increased ohmic losses. In this study, novel electrospun GDLs with graded pore sizes were manufactured for PEM fuel cells. The results provide valuable insight into the effects of GDL pore structure and material properties on cell performance. The work presented can aid in the design of the next generation of GDL materials for improved PEM fuel cell performance. REFERENCES Kitahara, T., Nakajima, H., Inamoto, M., Shinto, K. (2014). Triple microporous layer coated gas diffusion layer for performance enhancement of polymer electrolyte fuel cells under both low and high humidity conditions. Journal of Power Sources, Vol. 248, 1256-1263. doi:https://doi.org/10.1016/j.jpowsour.2013.10.066Ko, D., Doh, S., Park, H. S., Kim, M. H. (2018). The effect of through plane pore gradient GDL on the water distribution of PEMFC. International Journal of Hydrogen Energy, Vol. 42, 2369-2380. doi: https://doi.org/10.1016/j.ijhydene.2017.12.007Chevalier, S., Lavielle, N., Hatton, B. D., Bazylak, A. (2017). Novel electrospun gas diffusion layers for polymer electrolyte membrane fuel cells: Part I. Fabrication, morphological characterization, and in situ performance. Journal of Power Sources, Vol. 352, 272–280. doi: http://dx.doi.org/10.1016/j.jpowsour.2017.03.098