Experimental Study of Thermal Conductivity and Compression Measurements of the GDL-MPL Interfacial Composite Region

Abstract

Through-plane thermal conductivity and thickness variation under different compaction pressures were measured for a composite region of commercial gas diffusion layer (GDL), namely Freudenberg H1410 GDL, as received as well as saturated with a custom-made carbon ink/microporous layer (MPL). 3D X-Ray pictures were taken of the materials. Thermal conductivity for the composite region is higher than for the GDL alone. The compressibility of the composite region is in the same order of magnitude as pure GDL material. The found results motivate further investigation of this composite region that has gotten very little attention in the literature. A PEMFC consists of several components, i.e. the membrane electrolyte assembly (MEA) sandwiched between a thin MPL and a somewhat thicker GDL on each side. The MEA consists of a membrane (PEM) coated with catalyst layers (CL) on each side. MPL and GDL are often treated as separate layers in the literature. However, there exists a considerable interfacial region where the two different materials are intertwined. The fine material of the MPL can intrude considerably into the fiber structure of the GDL material. It is this region we are interested in. The temperature difference across the PEMFC can reach several °C despite being less than a millimeter thick between the gas flow plates. Temperature differences arise mainly across the GDL. Several research efforts have led to a good understanding of the thermal conductivity of the GDL and how it changes with compression, temperature, PTFE content, different fabrics, and water content. This work presents an effort to determine the thermal conductivity as well as the compressibility of the aforementioned composite region. [2] At a compaction pressure of 9.2 bar the thermal conductivity of untreated Freudenberg H1410 (114 μm), was found to be 0.111±0.009 W K-1 m-1 and for the custom-MPL-drenched Freudenberg H1410 material it was 0.124±0.005 W K-1 m-1. The untreated Freudenberg H1410 material was compacted to 87% of its original thickness at 9.2 bar compaction pressure. The MPL-treated H1410 material was compacted to 77% of its original thickness at 9.2 bar compaction pressure. The MPL-treated Freudenberg H1410 has a larger compression from atmospheric pressure to first compaction pressure, but a less steep compression gradient with rising compaction pressure than the original H1410 material, see figure. The thermal conductivity at 9.2 bar compaction pressure of another untreated Freudenberg GDL, namely H2315 (182 μm), was reported earlier to be 0.15±0.02 W K-1 m-1, roughly 50% higher than for the H1410 we measured. This complies with a trend seen in Toray paper, where the thermal conductivity at 9.3 bar compaction increases from 0.53±0.03 W K-1 m-1 for Toray TGP-H-060 (165 μm) to 0.65±0.02 W K-1 m-1 for Toray TGP-H-090 (265 μm) and to 0.81±0.03 W K-1 m-1for Toray TGP-H-120 (333 μm). [1] SIGRACET GDL 10 AA, also an untreated GDL, was measured to have a thermal conductivity of 0.38±0.03 W K-1 m-1 at 9.2 bar compaction pressure, also much higher than the Freudenberg GDL. A SolviCore PTL had a very similar thermal conductivity of 0.36±0.08 W K-1 m-1 at 9.2 bar compaction pressure. [1] The way the GDL is produced, the Teflon content, and the binder content all have influence on the final value of the thermal conductivity for a GDL material. Toray paper has a higher thermal conductivity when the thickness increases. An increase in PTFE content will lower the thermal conductivity. In general, Freudenberg GDLs have a lower thermal conductivity than other commercially available GDLs. By depositing MPL material into the GDL, thermal contact between GDL fibers is enhanced, hence the overall thermal conductivity of the material increases. This could be a means to improve thermal conductivity of GDL materials with low thermal conductivity to achieve better temperature management in the fuel cell. Acknowledgment The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors would like to acknowledge assistance of Dula Parkinson with the experimental set-up at the beamline.