Elsevier

Ceramics International

Volume 42, Issue 9, July 2016, Pages 11239-11247
Ceramics International

Characteristics of Cu and Mo-doped Ca3Co4O9−δ cathode materials for use in solid oxide fuel cells

https://doi.org/10.1016/j.ceramint.2016.04.037Get rights and content

Abstract

In this study, Cu and Mo ions were doped in Ca3Co4O9−δ to improve the electrical conductivity and electrochemical behavior of Ca3Co4O9−δ ceramic and the performance of a solid oxide fuel cell (SOFC) single cell based on NiO-SDC/SDC/doped Ca3Co4O9−δ-SDC were examined. Cu substitution in the monoclinic Ca3Co4O9−δ ceramic effectively enhanced the densification, slightly increased the grain size, and triggered the formation of some Ca3Co2O6; however, no second phase was found in porous Mo-doped Ca3Co4O9−δ ceramics even when the sintering temperature reached 1050 °C. Substitution of Cu ions caused slight increase in the Co3+ and Co4+ contents and decrease in the Co2+ content; however, doping with Mo ions showed the opposite trend. Doping the Ca3Co4O9−δ ceramic with a small amount of Cu or Mo increased its electrical conductivity. The maximum electrical conductivity measured was 218.8 S cm−1 for the Ca3Co3.9Cu0.1O9−δ ceramic at 800 °C. The Ca3Co3.9Cu0.1O9−δ ceramic with a coefficient of thermal expansion coefficient of 12.1×10−6 K−1 was chosen as the cathode to build SOFC single cells consisting of a 20 μm SDC electrolyte layer. Without optimizing the microstructure of the cathode or hermetically sealing the cell against the gas, a power density of 0.367 Wcm−2 at 750 °C was achieved, demonstrating that Cu-doped Ca3Co4O9−δ can be used as a potential cathode material for IT-SOFCs.

Introduction

Among various fuel cell systems, solid oxide fuel cells (SOFCs) have drawn considerable attention in large power facilities, in residential and commercial properties for heat and power generation, and in auxiliary power units for transportation applications. SOFC systems have several major advantages, including their high energy conversion efficiencies, self-reforming abilities, compatibilities with common hydrocarbon fuels, use of solid materials, and no need for noble metal catalysts [1], [2]. Electrolytes with high ionic conductivities and electrodes with low polarization resistances (Rp) are considered to be the key for the effective performance of SOFCs [3]. Therefore, alternative electrolytes that have higher ionic conductivities at low temperatures, such as La0.9Sr0.1Ga0.8Mg0.2O3−δ (LSGM), Sm0.2Ce0.8O2−δ (SDC), and doped Bi2O3, or the use of a thin yttria-stabilized zirconia (YSZ) electrolyte film [4], have been studied. In addition, the oxygen reduction reaction must be activated on SOFCs at normal operating temperatures, and this is generally regarded as the most challenging task for researchers. Therefore, the cathode is the limiting resistance of the cell because of its large overpotential [5], [6].

At high temperatures (>850 °C), perovskite La1−xSrxMnO3 (LSM) is commonly used as the cathode in conjunction with a YSZ electrolyte. However, the performance of LSM cathodes tends to decrease as the operating temperatures decline; this occurs because of the low oxygen ion conductivity and high activation energy for oxygen disassociation in LSM [5], [7]. Mixed ionic/electronic conductivity (MIEC) materials such as La0.6Sr0.4Co0.8Fe0.2O3−δ (LSCF), Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF), and SmCoO3 have been used as alternative cathodes for SOFCs to operate at temperatures below 800 °C [8], [9]. These MIEC materials can improve cathode performance by expanding the three-phase boundary (TPB) to the entire cathode–gas interface [10], [11]. However, these cathodes are chemically incompatible with zirconia-based electrolytes; furthermore, there is a thermal expansion mismatch with common electrolytes [12], [13].

Recently, misfit-layered cobaltite, Ca3Co4O9−δ (δ≈0.37 for the ideal structure), which is extensively used as a thermoelectric material, has been investigated as a potential cathode material for SOFCs [14], [15]. The Ca3Co4O9−δ cathode has good electrical conductivity (≈100 S cm−1 at 700 °C), good chemical stability with ceria-based electrolytes, and a matching thermal expansion coefficient of 9–10×10−6 K−1 [13], [16], [17]. This Ca3Co4O9−δ compound can be represented as [Ca2CoO3−δ]0.62[CoO2] and is built up of alternating layers of an oxygen-deficient rock-salt Ca2CoO3−δ′ slab and a CdI2-type CoO2 slab. Mixed ionic and electronic conduction observed in this material has been reported to originate from ionic conduction in the Ca2CoO3−δ′ layers and electronic conduction in the CoO2 layers [13]. To optimize the performance of the Ca3Co4O9−δ cathode in SOFCs, composite cathodes of Ca3Co4O9−δ–Ce0.9Gd0.1O1.95 and Ca3Co4O9−δ–La0.7Sr0.3CoO3 have been used to reduce the polarization resistance and increase the cell durability [14], [17], [18]. To date, few reports have been made concerning the effect of doping on the electrical properties of the Ca3Co4O9−δ cathode for use in SOFCs [19], [20], and only a few studies concerning dopant effects on Ca3Co4O9−δ for thermoelectric applications have been made [21], [22], [23], [24], [25].

In this study, Cu and Mo ions were doped in Ca3Co4O9−δ, taking the positions of the Co ions, to alter the electrical conductivity and electrochemical behavior of Ca3Co4O9−δ ceramics. Ca3Co4−xCuxO9−δ (x=0–0.15) and Ca3Co4−xMoxO9−δ (x=0–0.15) ceramics were prepared via solid state reactions. In addition, to examine the densification, microstructure, and electrical properties of the Cu- and Mo-doped Ca3Co4O9−δ oxides as potential SOFC cathode materials, we assessed the performance of a single cell based on a NiO-SDC/SDC/doped Ca3Co4O9−δ-SDC.

Section snippets

Experimental procedure

Powders of doped Ca3Co4O9−δ materials used in this study were prepared by solid state reactions. Highly pure (>99.9% purity) CaCO3 (Alfa Aesar, reagent grade), Co3O4 (Alfa Aesar, reagent grade), MoO3 (Showa, reagent grade), and CuO (Showa, reagent grade) were used as raw materials. Oxides comprising Ca3Co4−xCuxO9−δ (x=0–0.15) and Ca3Co4−xMoxO9−δ (x=0–0.15) were mixed and milled in methyl alcohol solution in polyethylene jars with zirconia balls for 24 h. The solids were then oven-dried at 80 °C

Results and discussion

Fig. 1(a) shows the densities of Ca3Co4−xCuxO9−δ ceramics sintered at various temperatures. The sintered densities of all Ca3Co4−xCuxO9−δ ceramics increased gradually with the sintering temperature. The densification was significantly enhanced by the substitution of Cu ions at the Co-ion sites. For the pure Ca3Co4O9−δ ceramic, maximum densification was not achieved even after the sintering temperature reached 1050 °C. In contrast, greater than 98% of the theoretical density was obtained for the

Summary

The characteristics of the Cu- and Mo-doped Ca3Co4O9−δ oxides were examined, and the performance of a single cell based on NiO-SDC/SDC/doped Ca3Co4O9−δ-SDC was assessed. The results can be summarized as follows:

  • Cu substitution was more effective to enhance the densification of Ca3Co4O9−δ ceramic compared to Mo substitution. A slight increase in grain size with Cu content was observed and a small quantity of Ca3Co2O6 was found to be present in the dense, monoclinic Ca3Co4−xCuxO9−δ ceramics.

References (35)

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  • Electrochemical characteristics of Ca<inf>3</inf>Co<inf>4</inf>O<inf>9+δ</inf> oxygen electrode for reversible solid oxide cells

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    CCO is also a MIEC material with alternate Ca2CoO3-δ oxygen deficient rock-salt type layers and CoO2 hexagonal layers [21,23]. In consideration of its high ionic and electronic conductivities and appropriate thermal performance, CCO has been used as cathode for SOFCs [24–26]. The performance of Ca3Co4O9-δ cathode on Sm0.075Nd0.075Ce0.085O2-δ (SNDC) electrolyte was evaluated by Zhu et al. [27].

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