Comprehensive Kinetics Model with Competitive Adsorption Reaction on Water Electrode in Reversible Solid Oxide Fuel Cell / Electrolysis

Abstract

Hydrogen generation technologies by electrolysis (EC) and the reversible operation with fuel cells (FC) are rapidly increased their attention as a large-scale energy storage to solve the gap between the unsteady power supply and the electricity demand due to the expanding installation of renewable energy. For example, if solar cells are installed into a certain microgrid to supply 30% of their electricity demand, large amount of surplus power generated in almost every sunny day. Thus, large scale energy storage by water electrolysis or a reversible FC/EC system will be strongly desired. To apply the reversible FC/EC with unsteady operations, the appropriate and quantitative understanding of the reversible reaction kinetics is required. The kinetics of hydrogen oxidation reaction in solid oxide fuel cell (SOFC) on Ni/ yttria stabilized zirconia (YSZ) have been developed and a number of kinetics models based on the chemical and/or electrochemical reactions have been proposed [1-5]. The rate determining step was described as a surface chemistry of neutral absorbents on triple phase boundary (TPB) under the local equilibrium between the oxygen atom on TPB, oxide ion and electron [1, 2], or the charge transfer reaction of neutral absorbent, surface ion and electron between Ni and YSZ [3-5]. The rate of anode reaction in each model is described chemically such as Langmuir type surface reaction [2] and electrochemically such as Butler-Volmer equation [5, 6], respectively. By applying those models, there are a number of discussion about the reaction of electrolysis, but most of them have suggested that the water reduction reaction on TPB cannot be described as a simple reverse reaction of hydrogen oxidation reaction [1, 5, 6]. Thus, in order to establish the comprehensive redox reaction model on the water electrode of reversible SOFC/EC, it is required to clarify the mechanism of the water reduction reaction and to describe its kinetics model, but there are a limited number of discussion which focus on the mechanism of the electrolysis reaction. To establish the comprehensive redox kinetics model on the water electrode of SOFC/EC, in this work we applied the existing reaction model of SOFC anode which we previously developed and shown in Figure (a) [2]. The model is described the kinetics by competitive adsorption reaction at TPB with oxygen activity (aO). aO is calculated from anode potential (Ea= ohmic free potential difference between anode working electrode and cathode reference electrode). The coverage ratio of adsorbing oxygen on TPB is fixed under the local equilibrium against O2- in YSZ, aO and electron at Ni and YSZ, and the rate determining step was described as Langmuir type surface chemical reaction on TPB. We can separately discuss the effect of the chemical/electrochemical property and the electrode porous structure. This time we applied the model to the reversible SOFC/EC with the series of H2/H2O gas components and discuss the SOEC anode reaction. Electrolyte support type cell were prepared with a YSZ disc as electrolyte, Ni/YSZ with 3:2 weight ratio as a fuel electrode and La0.85Sr0.15MnO3/ScSZ as an air electrode. Reversible SOFC/EC measurement were carried out in 1-5 kPa H2O / 30-99 kPa H2 / Ar balance and the current density was measured under 10-13 < aO < 10-7. As a result, the current density of SOEC was much larger than the extrapolated value by SOFC data at any H2/H2O gas conditions. By reinvestigating the local equilibrium of adsorbing oxygen atoms, oxide ion and electron on Ni as shown in Figure (right), we proposed a modified kinetics model by defining the “electron activity ratio between Ni and YSZ” as another independent variable. The kinetics is discussed with the change of the surface components on TPB by the shift of local oxygen equilibrium due to their shift of the charge balance. Acknowledgement: a part of this work is supported by the New Energy and Industrial Technology Development Organization (NEDO) [1] J. Mizusaki et al., Solid State Ionics, 70/71, 52 (1994). [2] M. Ihara et al., J. Electrochem. Soc., 148(3), A209 (2001). [3] A. Bieberle et al., J. Electrochem. Soc., 148(6), A646 (2001). [4] S. P Jiang et al., Solid State Ionics 123, 209 (1999). [5] D. G. Goodwin et al., J. Electrochem. Soc., 156(9), B1004 (2009). [6] M. Vogler et al., Journal of The Electrochemical Society, 156(5) B663 (2009). Figure 1