1. The Electrochemical and Thermal Performances of Ca3Co4O9-δ as a cathode material for IT-SOFCs
UCCS
K. Nagasawaa, O. Mentreb, S. Daviero-Minaudb, N. Preuxb, A. Rolleb, and H. Nakatsugawaa
 a Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya Word, Yokohama, Kanagawa,　 , Japan
b UCCS - Unite de Catalyse et de Chimie du Solide – UMR CNRS 8181, Ecole Nationale Superieure de Chimie de Lille, Batiment C7a – BP.90, 108, 59652, Villeneuve d'Ascq cedex, France
Abstract
Introduction
Layered calcium cobalt oxide Ca3Co4O9-δ is most known for thermoelectric material due to the high figure of merit. From the oxygen deficient nature of the rock-salt layers and from the excellent electronic transport of similar [CoO2] layers, Ca3Co4O9-δ could be considered as a natural intergrowth between electronic and potential O2- conducting units. We surmise that this new candidate for IT-SOFC cathode is well from the point of view of electrochemistry, thermal expansion and chemical stability[1]. And finally, we proved the electrochemical power generation tests using Ca3Co4O9-δ cathode.
[1] K.Nagasawa, S.Daviero-Minaud, N.Preux, A.Rolle, P.Roussel, H.Nakatsugawa, O.Mentré, Chemistry of Materials, (2009), in press.
Fig.2. Crystal structure of Ca3Co4O9-δ.
Fig.1. Construction of SOFC.
Results and discussion
-Chemical Reactivity-
-Thermal Dilatometory-
The comparison of TEC between electrode and electrolyte is constitutive attribute along with electrochemical properties. Figure 4 is shown the linear thermal expansion and TEC for Ca3Co4O9-δ. It yields TEC values of about ×10-6 K-1 in the investigated region. On the other hand, TEC value for CGO electrolyte is ×10-6. In fact, this apparently good mechanical compatibility is a contributing factor for IT-SOFC cathode application.
Fig.3. X-ray diffraction pattern in reactivity tests.
Fig.4. Liner thermal expansion and thermal expansion coefficient.
We carried out preliminary chemical reactivity tests between layered Ca3Co4O9-δ and several electrolytes. Ca3Co4O9-δ. No impurity phase was observed between Ca3Co4O9-δ.and CGO electrolyte, confirming a good compatibility between them.
-Impedance Spectra-
-SEM Observations-
Fig.6. Oxygen partial pressure dependence of impedance spectra of Ca3Co4O9-δ (a) and 30%CGO-Ca3Co4O9-δ (b) deposited CGO pellet.
Fig.5. Scanning electron micrograph of cutting cross section (left) and surface (right) of cell.
We carried out the impedance spectroscopy measurements for Ca3Co4O9-δ electrode deposited CGO electrolyte. Figure 6 shows Nyquist plots in dependence of several oxygen partial pressures or temperatures. In ambient air, the resulting ASR values are 3.0 and 24.5 Ωcm2 at 700 and 600 ºC respectively.
Typical SEM image of the cross section is shown on Figure 5. It was observed that the adherence between the electrode and electrolyte is good without any evidence for surficial cracks. The porosity appears of medium quality. The thickness of cathode cross section is estimated to about μm.
Fig.7. Calculated activation energy.
-Single Cell Power Generation Tests-
We attempted the power generation tests using Ca3Co4O9-δ cathode, NiO-Gd0.1Ce0.9Ox anode and Ce0.9Gd0.1O1.95 electrolyte. Fabricated SOFC single cell showed values of [V] in open circuit voltage and demonstrated the generation of electricity as Ca3Co4O9-δ cathode material for SOFC. Furthermore, relatively low polarization resistance in electrode ware obtained.
Fig.8. Power generation tests results. Open circuit voltage (left), I-V characteristics (middle), Impedance spectra (right).
Conclusions
Postscript
These results are considered to apply for US patent now. We are looking for the US company that can make a license agreement with us. If you have an interest in this matter, please contact following address.
-Good chemical compatibility with CGO (TEC, Chemical reactivity).
-Good adherence between electrode and electrolyte, no crack surface.
-Relatively Low ASR value of Ca3Co4O9 and 30%CGO-Ca3Co4O9 cell.
-Proof of power generation in SOFC single cell using Ca3Co4O9-δ cathode.