Diffusion layer parameters influencing optimal fuel cell performance

https://doi.org/10.1016/S0378-7753(99)00489-9Get rights and content

Abstract

The performance of polymer electrolyte fuel cells (PEFCs) is substantially influenced by the morphology of the gas diffusion layer. Cells utilising sintered gas diffusion layers made with a low pore volume Acetylene Black carbon, at an optimised thickness, showed better performance compared with cells containing Vulcan XC-72R carbon. The cells were optimised using both oxygen and air as oxidants showing that different conditions were required in each case to achieve optimum cell performance. A model, in which the hydrophobicity and porosity of the diffusion layer affect water impregnation and gas diffusion through the gas diffusion layer, is presented to explain the influence of the diffusion layer morphology on cell performance.

Introduction

The diffusion layer in a polymer electrolyte fuel cell (PEFC) consists of a thin layer of carbon black mixed with polytetrafluoroethylene (PTFE) that is coated onto a sheet of macro-porous carbon backing paper. This diffusion layer provides a physical micro-porous support for the catalyst layer while allowing gas transport to and from the catalyst layer. Although the diffusion layer is a seemingly minor component in a fuel cell, it has been shown that altering the composition of the diffusion layer can lead to substantial improvements in the performance of the cell [1], [2], [3], [4], [5]. The improvements reported relate to the PTFE content, the effect of sintering the diffusion layer, the thickness of the layer, and the morphology of the carbon black used in the fabrication of the diffusion layer.

Paganin et al. [1] have investigated the performance of PEFCs containing Vulcan XC-72R carbon black diffusion layers with varying PTFE contents and diffusion layer thickness. The content of PTFE (10, 15, 20, 30, 40%) in the diffusion layer was tested and found to have a small effect on the cell performance, a diffusion layer containing 15% PTFE giving the best performance. A possible explanation, given by Giorgi et al. [2], is that the content of PTFE affects the porosity of the diffusion layer, an increase in the PTFE content resulting in a decrease in diffusion layer porosity. The thickness of the diffusion layer was reported to have a much greater influence compared with the PTFE content. When diffusion layers of various thickness (15, 25, 35, 50, 65 μm) were investigated, a diffusion layer thickness of 50 μm was found to give the best performance, corresponding to a ∼25% increase in maximum power density compared with the worst-performing 15-μm diffusion layer. The effect of diffusion layer thickness was attributed to a decrease in the electrical resistance of the gas diffusion electrode as the diffusion layer thickness was increased. The decrease in performance with the highest-loading 65-μm diffusion layer was attributed to a long gas diffusion path or with flooding problems.

The effect of carbon morphology in the diffusion layer has recently been studied by comparing the performance of PEFCs made with Vulcan XC-72R and Acetylene Black carbon diffusion layers [3]. The morphology of these two types of carbon differs in surface area and in pore size distribution. Acetylene Black has a lower surface area of ∼50 m2 g−1 and lower porosity compared with Vulcan XC-72R which has a surface area of ∼250 m2 g−1 and a higher volume of micro-pores [3], [6]. A ∼15% improvement in maximum power density was observed when Acetylene Black carbon was used in place of Vulcan XC-72R. The increase in performance was attributed to Acetylene Black being better able to remove water from the cell, resulting in a reduction in cathode flooding.

In the present study, the conditions leading to an optimised diffusion layer are reported in more in detail and proposed mechanisms for the improvement in performance, observed when the diffusion layer parameters were varied, are suggested.

Section snippets

Experimental

To make the membrane electrode assemblies (MEAs), a homogeneous suspension, referred to as carbon ink, of 10 wt.% PTFE and carbon (Vulcan XC-72R or Acetylene Black) was prepared in cyclohexane. Electrodes with various diffusion layer loadings (0.7–2.5 mg cm−2) were prepared by brushing the carbon ink onto a wet-proofed TPG-H-120 carbon paper followed by drying and weighing. The carbon backing paper with the diffusion layer was also sintered by placing it in a furnace at 350°C for 30 min, to

Carbon loading

Cell potential–current density plots for cells of varying carbon loadings are shown in Fig. 1a and b for cells operated using oxygen and air oxidants, respectively. The maximum power density derived from the cell potential–current density plots, along with optimum cell operating conditions, are shown in Fig. 1 and Table 1. As can be determined from these, the two intermediate loading diffusion layers performed noticeably better than either the lowest or highest diffusion layer loading. This is

Conclusion

The diffusion layer can substantially affect the performance of PEFCs. Parameters that are critical to cell performance include the diffusion layer thickness, the PTFE content of the diffusion layer, and the diffusion layer morphology. Finally, the operating temperature and gas humidification conditions may need to be optimised for particular diffusion layers. All these parameters are thought to affect the water retention/removal properties of the PEFC which are vital in maintaining a balance

References (7)

  • L. Giorgi et al.

    Electrochim. Acta

    (1998)
  • M. Wohr et al.

    Int. J. Hydrogen Energy

    (1998)
  • V.A. Paganin et al.

    J. Appl. Electrochem.

    (1996)
There are more references available in the full text version of this article.

Cited by (0)

View full text