Impact of Contact Pressure Cycling on Non-Woven GDLs of HT-PEM Fuel Cells

The Gas Diffusion Layer (GDL) plays an important and critical role within High Temperature Polymer Electrolyte Membrane fuel cells (HT-PEMFC). GDLs usually consist of woven (cloth) or non-woven (e.g. felt or paper) arrays of carbon fibers. The main tasks of a GDL are:

• Gas transport and distribution of reactants and reaction products
• Electrical conductivity
• Heat transportation
• Water management
• Support of Membrane-Electrode-Assemblies (MEA).

The influence of the GDL on the performance of a fuel cell is an extensively discussed topic nowadays (1, 2). The impact on the durability also needs attention. Physical damages, like for example fiber intrusion through the catalyst layer into the membrane, due to high contact pressures have been confirmed prior to this (3, 4). For this reason, ex-situ measurements with an ad hoc designed compression device (5) have been performed. In this work we will examine in more detail the dependency on compression and the behavior of non-woven carbon cloth GDL material. Therefore the compression device, used before, had to be optimized and rescaled furthermore. This allows a better geometrical resolution while utilizing the tool in micro-computed tomography (µ-CT). Results with the new device will be shown within this work.

In addition to the ex-situ investigations of the GDL defects based on compression forces, in-situ electrochemical measurements of MEAs like polarization curves, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and linear sweep voltammetry (LSV) will be carried out. Therefore, MEAs were physically stressed with specific contact pressure cycling tests. One cycle starts at 0.2 MPa up to 1.5 MPa (steps: 0.2, 0.5, 1.0 and 1.5 MPa) and will be repeated 3 times with conditioning or relaxation time over night for each contact pressure and electrochemical characterization during day time.

Supplementary, GDLs will be analyzed separately with the help of the new compression device and the µ-CT. Focus of this work lies on the investigation of the porosity of the non-woven GDL material, which is utilized in the investigated MEAs. Within bipolar plates (BPP) with their various flow fields, the porosity of a GDL typically shows a non-uniform behavior (6). Due to the channel and land areas of the serpentine flow fields utilized in this study, the compression on the carbon fibers is locally different. Under the land areas the GDL will be squeezed and within the channels a dilation of the GDL material will be observed (4, 7). To analyze reversible and irreversible defects, the in-situ contact pressure cycling will be imitated completely ex-situ with pure non-woven GDL material. With help of the reconstructed and binary µ-CT data, the porosity of the GDL will be determined and illustrated within 3-dimensional models for each contact pressure (Figure 1).

Figure 1: 3D-Model of an uncompressed non-woven GDL

References:

1.            N. Parikh, J.A. Allen, R.S. Yassar, Fuel Cells 12, 3, 382-390 (2012).

2.            J. Kleemann, F. Finsterwalder, W. Tillmetz, Journal of Power Sources, 190, 92-102 (2009)

3.            A. Diedrichs, M. Rastedt, F.J. Pinar, P. Wagner, J. Appl. Electrochem, 43, 1079-1099 (2013).

4.            M. Rastedt, F.J. Pinar, N. Bruns, A. Diedrichs, P. Wagner, ECS Transactions, 58, 443-452 (2013).

5.            M. Karwey, Bachelor Thesis, Fachhochschule Südwestfalen (2012).

6.            Z. Shi, X. Wang, O. Draper, ECS Transactions, 11, 637-646 (2007)

7.            H.-W. Mindt, M. Megahed, ESC Transactions, 40, 25-34 (2012).