Overpotential and Interfacial Conductivity of Ni/YSZ Anode Based on Species Territory Adsorption Model in SOFCs

Introduction

   Recently, a new anode incorporating a BaCe_0.8Y_0.2O_3-δ (BCY) proton conductor was proposed by the authors to increase a power density of SOFCs, and it was found that the anode overpotential was reduced by incorporating 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. Based on these results, a modified reaction model around the TPB was proposed to explain the mechanism of overpotential reduction in the anode incorporating the proton conductor, and analytical expression of current density with oxygen activity and anode overpotential with current density were obtained³.

   In this study, the analytical expression of interfacial conductivity of anode (the inverse of the anode polarization resistance) was obtained based on the reaction model³. In addition, the analytical results of overpotential or interfacial conductivity of conventional Ni/YSZ anode were compared with those obtained from experiments⁴, and the validity of the reaction model was discussed.

Reaction model

   The details of the reaction model were shown in our previous work³. In this model, some finite areas, which contribute to species adsorption, are assumed around TPB, and hydrogen and oxygen are assumed to be adsorbed mainly on the surface areas of Ni and oxide ion conductor, respectively (We define this idea as “species territory adsorption model”). In addition, it is also assumed that the reaction rate in the anode is controlled by the surface reaction between H_ad and O_ad, while all other reaction takes 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. Here, a_O is expressed as a_O=exp(2FE/RT) (E is anode potential).

   Using the analytical expression of i as a function of a_O, the interfacial conductivity (σ_E) of anode, which is described as the inverse of the anode polarization resistance, is derived as follows.

Eq. [1]

Here, θ_V(1) and θ_V(2) means the coverage of vacant adsorption site on the surface areas of Ni and oxide ion conductor, respectively. These are described as follows.

Eq. [2, 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_aj, k_cj (a, c = anodic or cathodic reaction for 2H_ad+O_ad → H₂O_ad+2V_ad) show the equilibrium constant and the rate constant, respectively. A unique combination of fitting parameters, such as equilibrium constants and reaction rate constants were obtained from a comparison between analytical and experimental results⁴ for the case of Ni/YSZ anode.

Analytical results and discussions with experiments

   Figure 1 shows the analytical and experimental⁴ results of anode overpotential of Ni/YSZ with current density at 1273K. From Figure 1, it was disclosed that the anode overpotential decreased with increasing humidity from 3% to 20%, while, it increased with increasing humidity from 20% to 40%. These results are in good agreement with those of experiments. Moreover, Figure 2 shows the analytical and experimental⁴ results of interfacial conductivity of Ni/YSZ as a function of water vapor pressure at 1273K under the open-circuit condition. In this case, the hydrogen partial pressure P_H2 was equal to 1-P_H2O atm. The analytical results were calculated from Eq. [1]. From Figure 2, it was disclosed that the interfacial conductivity had a local maximum value in the range of P_H2O=0.15-0.25 atm in the experiments. These results successfully followed the analytical results. From the analytical results, it was disclosed that the term ∂i/∂a_O decreases with increasing P_H2O, while ∂a_O/∂E increases. As a result, σ_E has a local maximum value.

Conclusions

   Based on the reaction model of SOFC anode proposed by the authors, the overpotential and interfacial conductivity of conventional Ni/YSZ anode was discussed. The experimental results of humidity dependence of anode overpotential or interfacial conductivity is in good agreement with those obtained by the analytical results based on the proposed model.

 

References

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2. K. Masuda, M. Ihara, and K. Hanamura, Mech. Eng. J., 1(3), 14-00017 (2014).

3. T. Nagasawa and K. Hanamura, ECS Trans., 64(2), 117 (2014).

4. T. Matsui, R. Kishida, J-Y. Kim, H. Muriyama, and K. Eguchi, J. Electrochem. Soc., 157(5) B776 (2010).