Optimal composition of polymer electrolyte fuel cell electrodes
determined by the AC impedance method
J.M. Song
a
, S.Y. Cha
b
, W.M. Lee
a,*
a
Department of Molecular Science and Technology, Ajou University, Suwon 442-749, South Korea
b
Department of Energy Systems Engineering, Ajou University, Suwon 442-749, South Korea
Received 22 February 2000; received in revised form 3 July 2000; accepted 9 October 2000
Abstract
A proton exchange membrane fuel cell (PEMFC) electrode having a modi®ed morphology of conventional Te¯on (PTFE) bonded
electrodes was studied using the AC impedance method. The electrode differs from other types of electrodes in the presence of a thin
catalyst-supporting layer between the gas diffusion backing and the catalyst layer. The thickness and composition of the supporting layer
were optimized on the basis of the information from AC impedance measurements. The optimal thickness of the supporting layer and its
PTFE content turned out to be approximately 3.5 mg cm
À2
and 30 wt.%, respectively. The catalyst layer was cast on top of the supporting
layer, from solution that has the proper ratio of ionomer Na®on and Pt/C catalyst. The optimal amount of the ionomer in the catalyst layer
was approximately 0.8 mg cm
À2
when Pt loading was kept at 0.4 mg cm
À2
. These values are rationalized in terms of the catalyst active area
and the transport of the involved species for the electrode reaction. # 2001 Elsevier Science B.V. All rights reserved.
Keywords: AC impedance method; PEMFC; Pt/C electrode; PTFE; Na®on
1. Introduction
The economic future of proton exchange membrane fuel
cells (PEMFC) for applications such as transportation [1,2]
critically depends on ef®cient utilization of the platinum
catalyst. Major breakthroughs have been made with respect
to lowering the amount of the catalyst loaded on the PEMFC
electrode. They include the method for impregnating the
catalyst side of the electrode with a solubilized form of the
ionomer Na®on [3±6]. More recently, a drastic reduction in
the Pt loading was achieved by fabricating separately the
catalyst layer and PTFE (polytetra¯uoroethylene) contain-
ing diffusion layers [7±12]. Low Pt loading was also
achieved by a process that features cross-linking carbon-
supported Pt (Pt/C) with per¯uorosulfonate ionomer during
its coagulation from the colloid [13,14]. Improvements in
utilization were also made by methods such as sputter-
deposition of the catalyst directly on the carbon surface
[15,16] or the membrane surface [17,18], or electrodeposi-
tion of the catalyst through the Na®on onto the electrode
[19]. These methods aimed at minimizing the catalyst
loading must also satisfy other criteria, which are proton
access, gas access, and electronic path continuity. A com-
mon strategy to meet these requirements is to separate PTFE
from the catalyst layer. Although PTFE plays the role of
hydrophobicity in the gas diffusion regime of the electrode,
it does not have to be present in the immediate vicinity of the
catalyst sites [8]. The hydrophobic treatment of a carbon
paper or cloth, which is used as the starting material of the
gas diffusion backing, often includes the in®ltration of a
PTFE/carbon mixture into the porous body of the backing
and subsequent baking and sintering [2].
In our study the gas diffusion backing of the cell electrode
was treated in two steps, a small modi®cation of conven-
tional methods [20]. The carbon paper ®rst underwent the
hydrophobic treatment, through Te¯on resin wetting and
sintering. Then a thin PTFE/carbon ®lm was cast on its
treated surface, from an alcoholic suspension. This ®lm
plays the role of supporting the catalyst layer such that
the catalyst particles do not fall into the pores in the backing.
This layer, then called the catalyst-supporting layer, will be
actively involved with the electrode reaction, imparting
hydrophobicity to and maintaining a proper water balance
in the interface. The interface between the carbon backing
and the polymer electrolyte, with or without the supporting
layer, is depicted in Fig. 1. The insertion of the thin
supporting layer between the backing surface and the cat-
alyst ®lm can be advantageous not only in supporting the
Journal of Power Sources 94 (2001) 78±84
*
Corresponding author. Fax: 82-31-219-2969.
E-mail address: wmlee@madang.ajou.ac.kr (W.M. Lee).
0378-7753/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0378-7753(00)00629-7