Reaction Characteristics around Triple Phase Boundary in Anode Including Proton Conductor for Solid Oxide Fuel Cell

Introduction

    Recently, a new anode including proton conductor BaCe_0.8Y_0.2O_3-δ (BCY) was proposed by the authors^1,2 to increase a power density of SOFCs and it was found that the anode overpotential was reduced by adding BCY particles to the conventional Ni/GDC cermet anode¹. Moreover, it was found that BCY particles might contribute to supply the adsorbed hydrogen to the triple phase boundary (TPB) in anodic reaction².

    In this study, a modified reaction model around the TPB was proposed to express the mechanism of reduction of overpotential in the anode including the proton conductor. Based on the reaction model, analytical expressions of current density or anode overpotential were obtained. The analytical results were compared with those obtained by experiments^1,3.

Reaction Model

    Figure 1 shows a schematic diagram for three kinds of TPB consisting of the nickel metal, the oxide ion conductor, and the proton conductor particles, and gas phases. The following assumption was introduced: (i) Chemical species was adsorbed on a finite narrow area on each material around the TPB. (ii) Hydrogen and Oxygen were adsorbed mainly on the surface areas (Area 1) and (Area 2), respectively.  (iii) The reaction rate in the anode was controlled by the surface reaction between H_ad and O_ad, while all other reaction took place under the condition of chemical equilibrium.

    Based on the model, under the condition that the pure oxygen (P_O2=1atm) is used in the cathode side, a simple expression for current density i and anode overpotential η_a could be obtained explicitly as a function of an oxygen activity at the anode a_O and a current density i respectively. The latter is described as follows.

Eq. [1]

Here,

Eq. [2]

Eq. [3]

Here, P_H2 and P_H2O are partial pressure of hydrogen and water vapor at the anode side, respectively. K_i, K'_i  (i = species) and k_i (i = anodic or cathodic reaction for (I) H_ad(1)+H_ad(1)+O_ad(2) → H₂O_ad(1)+V_ad(1)+V_ad(2) and (II) H_ad(1)+H_ad(1)+O_ad(2) → H₂O_ad(2)+V_ad(1)+V_ad(1), Here, 1 and 2 represent Area 1 and 2 respectively) express the equilibrium constant and the rate constant, respectively. Herein, the effect of adding proton conductor particles is expressed by substituting a larger value of K_H instead of the summation of K_H and Eq. [4] for adsorption on the surfaces of Area 1 and Area P. A unique combination of fitting parameters, such as equilibrium constants and reaction rate constants were obtained from a comparison between analytical and experimental results for the cases of Ni/GDC, Ni/GDC including BCY, Ni/ScSZ and Ni/YSZ anodes.

Analytical Results and Discussion

    Figure 2 shows the effect of current density on the anode overpotential in the case of Ni/GDC and Ni/GDC including BCY anodes. The experimental results under the condition of operating temperature of 1073 K are also shown¹. In the case of Ni/GDC anode, the overpotential decreases with increasing humidity from 3% to 20%. On the other hand, the overpotential does not change by Ar gas dilution of 70% under the condition of 3% humid. The most striking feature is that the anode overpotential of the Ni/GDC including BCY becomes smaller than that of the conventional Ni/GDC under the condition of 3% humid, which is explained by using a greater equilibrium constant by a factor of 1.8.

Conclusion

    Using a modified reaction model, analytical results agreed well with experimental results.

References

1. S. Yano, et al., Journal of Thermal Science and Technology, Vol.4, No.3, pp.431-436 (2009)

2. K. Masuda, et al., Proceedings of the 4th International Conference on Heat Transfer and Fluid Flow in Microscale, 4-9 September, 2011, Fukuoka, Japan

3. M. Ihara, et al., J. Electrochem. Soc., 148(3) A209-A219 (2001)