Determination Method of Reaction Rate and Mass Transfer Parameters in Cathode Catalyst Layer in PEFC
Authors: Ogawa Hikaru , Kageyama Miho , Gyoten Hisaaki , Murakami Shuka , Kawase Motoaki 
Published: December 2024

Introduction

It is important to optimize the cathode catalyst layer (CCL) because the oxygen reduction reaction (ORR) in the CCL is sluggish. Our previous study showed that the mass transfer resistance in the CCL can be quantified by measuring the ORR rate dependency on the oxygen partial pressure at fixed temperature[1,2]. In this study, polarization curves were measured and analyzed at varied oxygen partial pressure and temperature. The objective of this study is to establish the quantification methods of mass transfer and reaction rate parameters, which explain the behavior of polarization curves at various conditions.

Experimental

A cell whose active area was 20 mm × 20 mm was used in experiments. The membrane electrode assembly (MEA, Eiwa Corp.) consists of proton exchange membrane (PEM, Chemours NafionTM, NR-212) and two catalyst layers of each side of the PEM. Platinum catalyst supported on Ketjen Black (Pt/C weight ratio: 1.0) and a NafionTM ionomer (ionomer/carbon weight ratio: 1.0) consists of the catalyst layer. Oxygen and nitrogen were supplied to the cathode in order to vary the oxygen partial pressure. H2, N2, and O2 were humidified in bubblers to keep the inlet relative humidity (RH) at 0.8. The cell temperature was varied at 50, 60, 70, and 80 °C. The Oxygen mole fraction at the cathode was also varied at 1.5, 3, 6, 10, and 21 % on a dry basis.

Results and Discussion

Polarization curves at varied oxygen partial pressure, p
Oc
and temperature are shown in Figs. 1–4. Since the reaction and mass transfer rate are improved at higher temperature, the current density at fixed cathode electromotive force, E
cm
increases when temperature increases as far as RH is identical. Arrhenius plot at the E
cm
of 0.2 V is shown in Fig. 5. Since the reaction rate is proportional to the oxygen partial pressure[3,4], the current divided by the oxygen partial pressure on the vertical axis in Fig. 5 is proportional to the observed reaction rate constant. Fig. 5 shows that the apparent activation energy decreases as temperature increases at the p
Oc
range of 2.5 to 7.7 kPa. This result indicates that the reaction rate saturates when temperature increases due to the effect of oxygen permeation resistance of the ionomer layer. The observed reaction rate constant decreases at higher p
Oc
, particularly at 15.9 kPa, due to the effect of proton transfer resistance. Fig 6. shows the apparent activation energies plotted against E
cm
. Both apparent activation energies of the ORR did not depend on E
cm
. Tafel slope, b
c should be the function of temperature, T as expressed in Eq. (1):

b
c=RT/4aF, (1)

where R, a, and F is the gas constant, transfer coefficient, and Faraday constant, respectively. The ORR rate constant, k
vc
° is the exponential function of the inverse of the temperature. Considering Eq. (1), k
vc
° should be expressed as follows:

k
vc° = k
vc0exp(-E/RT-E
c/b
c) = k
vc0exp(-(E+4aFE
c)/RT), (2)

where E is the activation energy. Although the apparent activation energy of the ORR should increase when E
cm
increases, the measurement results were independent of E
cm
, especially below 0.65 V. It indicates that the Tafel slope does not depend on temperature. The apparent activation energies of the ORR and of the oxygen permeance through the ionomer layer were obtained as 52 kJ/mol and 15 kJ/mol, respectively.

Reaction rate constant, effective oxygen diffusion coefficient, effective proton conductivity, and oxygen permeance through the ionomer layer can be determined by applying the dimensionless isothermal 1D cathode model[1, 2, 4] to the obtained polarization curves.

Conclusions

The polarization curves at varied oxygen partial pressure and temperature were measured and analyzed. As the activation energy of the reaction is higher than that of diffusion and permeation, the observed activation energy in the Arrhenius plot at varied cathode electromotive force decreased when temperature increases. The observed activation energy of the ORR did not depend on the cathode electromotive force.

Acknowledgements

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 2020–2023) conducted by the New Energy and Industrial Technology Development Organization (NEDO), Japan. This work was supported by Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (KAKENHI) Grant Number 22KJ1899.

References

[1] Ogawa et al., ECS Trans. 112(4), 121–133 (2023).

[2] Kageyama et al., ECS Trans., 98(9), 163–175 (2020).

[3] Nagayoshi et
al., Proc. 9th WCCE & 15th
APCChE
Congr. (Seoul, August 18–23, 2013), MoP-T1-170

[4] Kawase et
al., AIChE J. 63(1), 249–256 (2017).

Figure 1