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_vk^(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_vk^(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_vk^(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.