Analysis Method of Oxygen Permeation Resistance of Ionomer in Cathode Catalyst Layer in PEFC
Authors: Ogawa Hikaru , Kageyama Miho , Gyoten Hisaaki , Kawase Motoaki 
Published: November 2022

Introduction

The oxygen permeation resistance of ionomer determines the limiting current density, that is, cell performance. In order to analyze governing factors of the oxygen reduction reaction (ORR) rate, oxygen permeation resistance should be formulated[1]. In this study, instead of commercial supported catalyst, platinum slits were prepared by coating the platinum slabs with ionomer. Oxygen permeance per side area of platinum slits catalyst was determined by measuring current and analyzing other mass transfer resistances. The objective of this study is to establish measurement and analysis methods of oxygen permeation resistance of ionomer.

Theory

In our previous study[2], the current density is expressed as

i = 4F
δ
(C)
k
vcm

p
Oc

F
e [A/m2], (1)

where δ
(C) is the cathode catalyst layer (CCL) thickness, k
vc is the reaction rate constant per unit volume of CCL which does not include through-plane mass transport resistances but oxygen permeation resistance of ionomer [mol/(Pa·m3·s)]. p
O is the oxygen partial pressure [Pa]. The subscript m and c denote PEM–CCL boundary and CCL–gas diffusion layer boundary, respectively. F
e is effectiveness factor, a function of the following 4 dimensionless moduli, M
Om
, M
pm
, P
Om
, and y
Oc
. F
e is defined as the ratio of the observed reaction rate to the reaction rate which does not have the influence of through-plane mass transfer resistance which is in parallel with reaction resistance as shown in Fig. 1. F
e includes the influence of oxygen permeation resistance of ionomer, which is in series with reaction resistance as follows:

1/k
vcm
= 1/k
°
vcm
+ 1/a
v
k
(I)
pO (2)

where k
°
vcm
denotes the reaction rate constant which does not include the influence of neither parallel resistances nor series resistance with reaction resistance. a
v
k
(I)
pO denotes the oxygen permeance. These two parameters are per unit volume of CCL [mol/(Pa·m3·s)]. In this study, new dimensionless moduli, O
O
(I) and F
u, are proposed in order to separate a
v
k
(I)
pO and k
°
vcm
from k
vcm
. Define O
O
(I) as the ratio of the oxygen permeation resistance to the reaction resistance and the platinum utilization factor, F
u, as the ratio of observed reaction rate to the reaction rate which does not have the influence of mass transfer resistance through plane nor oxygen permeation resistance of ionomer. In order to explain the influence of the oxygen permeation resistance on the ORR rate, F
u and k
°
vcm
were applied to Eq. (1) instead of F
e and k
vcm
. The current density is expressed as

i = 4F
δ
(C)
k
°
vcm

p
Oc

F
u = 4F
δ
(C)
k
vcm

p
Oc
(1+O
O
(I)) F
u [A/m2]. (3)

Results and Discussion

F
u and F
e can be calculated by solving dimensionless equations of 1D isothermal homogeneous medium model with assuming no condensation[3]. Fig. 2 shows the calculation results of F
u at fixed Peclet number P
Om
, y
Oc
and dimensionless oxygen transport resistance M
Om
. Increasing dimensionless proton transfer resistance M
pm
and oxygen permeation resistance O
O
(I) leads to the decrease in F
u.When δ
(C), E
cm
, temperature, ionomer thickness and relative humidity are fixed, F
u and M
pm
is proportional to i/p
Oc
and p
Oc

1/2, respectively. M
pm
and O
O
(I) can be determined just by fitting experimental data of i/p
Oc
plotted against p
Oc

1/2 to the theoretical curves. To determine O
O
(I), the current should be measured at fixed P
Om
, y
Oc
and M
Om
. Since it is difficult to fix these three parameters in case of using Pt supported catalyst, platinum slits catalyst coated with ionomer film is applied to the CCL alternatively. The Pt slits catalyst structure which has the high voidage as shown in Fig. 3 enables oxygen to transfer fast through plane compared with Pt supported catalyst layer. That means M
Om
and P
Om
can be regarded as 0 because the oxygen diffusion is much faster than oxygen reduction reaction and convection. If P
Om
and M
Om
are approximately zero, the influence of y
Oc
on F
u is negligible. Fig. 4 shows the measured ORR rate dependency on RH. The current density at fixed E
cm
increases by raising RH due to the increase in proton conductivity, oxygen permeability[4] and catalytic activity[5]. By measuring the ORR rate dependency on p
Oc
in the same way, O
O
(I) can be determined. k
(I)
pO can be obtained when O
O
(I) is determined.

Conclusions

Determination method of the ratio of the oxygen permeation resistance to the reaction resistance, O
O
(I), using platinum slits to obtain oxygen permeance through ionomer is proposed. By measuring the ORR rate dependency on oxygen partial pressure, M
pm
and O
O
(I) can be obtained.

Acknowledgement

This work was supported by the FC-Platform Program: Development of design-for-purpose numerical simulators for attaining long life and high performance project (FY 2020–2024) conducted by the New Energy and Industrial Technology Development Organization (NEDO), Japan.

Figure 1