The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Inhomogeneous Compression of PEMFC Gas Diffusion Layers

Iwao Nitta

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Faculty of Information and Natural Sciences for public examination and debate in Auditorium AS1 at Helsinki University of Technology (Espoo, Finland) on the 14th of March, 2008, at 12 noon.

Overview in PDF format (ISBN 978-951-22-9233-2)   [4226 KB]
Dissertation is also available in print (ISBN 978-951-22-9232-5)

Abstract

Proton exchange membrane fuel cells (PEMFCs) are electrochemical devices which convert the chemical energy of reactants directly into electrical energy. This technology enables high efficiency and high energy density compared to internal combustion engines and current batteries, thereby making the technology attractive for broad range of applications. Furthermore, the only exhaust from PEMFCs is water, which makes them favorable from the environmental point of view.

Gas diffusion layer (GDL) is one of the most important components in a fuel cell, whose functions cover a wide range of operations: to provide a passage for reactant access and product water removal, to conduct electricity and heat between adjacent components, and to provide mechanical support for the MEA. The properties of GDL are strongly dependent on compression pressure. Of particular importance is the fact that the compression pressure on GDL is inhomogeneous because of the rib/channel structure of bipolar plate. However, previous theoretical studies have typically neglected this effect, and thus they inherently contained errors in the modeled results. Therefore, the aim of this study is to obtain insight into the actual effects of the inhomogeneous compression of GDL using experimental and theoretical approaches.

The experimentally evaluated properties are GDL mechanical properties, gas permeability, in-plane and through-plane electric conductivities, electric contact resistances between fuel cell components, thermal bulk conductivity and thermal contact resistance. All parameters are evaluated as a function of compressed GDL thickness. It was found that compression increases electric GDL conductivities but does not affect the thermal conductivity. Both electric and thermal contact resistances and gas permeability are decreased nonlinearly by the compression.

The modeling study was performed by applying the experimentally evaluated parameters for the systematic investigation of the effect of inhomogeneous compression. It was found that the inhomogeneous compression does not significantly affect the polarization behavior and gas-phase mass transport. However, the effects on the current density distribution were evident. This was caused by the changes in the selective current path, which is determined by the combinations of conductivities of components and contact resistance between them. Despite the highly uneven current distribution and variation in material parameters by the inhomogeneous compression, the temperature profile was fairly even over the active area, contrary to the predictions in previous studies.

This study suggests that high current density distribution caused by the inhomogeneous compression of GDL has a significant effect on the local cell performance and cell durability. The insight obtained from this study is highly beneficial for development and construction of fuel cells, as well as predicting their performance and life time.

This thesis consists of an overview and of the following 5 publications:

  1. Iwao Nitta, Tero Hottinen, Olli Himanen, and Mikko Mikkola, 2007, Inhomogeneous compression of PEMFC gas diffusion layer: Part I. Experimental, Journal of Power Sources, volume 171, number 1, pages 26-36.
  2. Tero Hottinen, Olli Himanen, Suvi Karvonen, and Iwao Nitta, 2007, Inhomogeneous compression of PEMFC gas diffusion layer: Part II. Modeling the effect, Journal of Power Sources, volume 171, number 1, pages 113-121.
  3. Iwao Nitta, Olli Himanen, and Mikko Mikkola, 2008, Contact resistance between gas diffusion layer and catalyst layer of PEM fuel cell, Electrochemistry Communications, volume 10, number 1, pages 47-51.
  4. Iwao Nitta, Olli Himanen, and Mikko Mikkola, 2008, Thermal conductivity and contact resistance of compressed gas diffusion layer of PEM fuel cell, Espoo, Finland, 20 pages, Helsinki University of Technology Publications in Engineering Physics, TKK-F-A853, Fuel Cells, submitted for publication, 4th October 2007. © 2008 by authors.
  5. Iwao Nitta, Suvi Karvonen, Olli Himanen, and Mikko Mikkola, 2008, Modeling the effect of inhomogeneous compression of GDL on local transport phenomena in a PEM fuel cell, Espoo, Finland, 23 pages, Helsinki University of Technology Publications in Engineering Physics, TKK-F-A854, Fuel Cells, submitted for publication, 8th November 2007. © 2008 by authors.

Keywords: PEMFC, gas diffusion layer, inhomogeneous compression, mathematical modeling, contact resistance, conductivity

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© 2008 Helsinki University of Technology

Last update 2011-05-26