Reaction Characteristics around Triple Phase Boundary in Anode Including Proton Conductor for Solid Oxide Fuel Cell
Recently, a new anode including proton conductor BaCe0.8Y0.2O3-δ (BCY) was proposed by the authors1,2 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 experiments1,3.
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 Had and Oad, while all other reaction took place under the condition of chemical equilibrium.
Based on the model, under the condition that the pure oxygen (PO2=1atm) is used in the cathode side, a simple expression for current density i and anode overpotential ηa could be obtained explicitly as a function of an oxygen activity at the anode aO and a current density i respectively. The latter is described as follows.
Here, PH2 and PH2O are partial pressure of hydrogen and water vapor at the anode side, respectively. Ki, K'i (i = species) and ki (i = anodic or cathodic reaction for (I) Had(1)+Had(1)+Oad(2) → H2Oad(1)+Vad(1)+Vad(2) and (II) Had(1)+Had(1)+Oad(2) → H2Oad(2)+Vad(1)+Vad(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 KH instead of the summation of KH and Eq.  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.
Using a modified reaction model, analytical results agreed well with experimental results.
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