Elsevier

Journal of Power Sources

Volume 218, 15 November 2012, Pages 307-312
Journal of Power Sources

Nanostructured Gd–CeO2 electrolyte for solid oxide fuel cell by aqueous tape casting

https://doi.org/10.1016/j.jpowsour.2012.07.005Get rights and content

Abstract

Gadolinia-doped ceria (Ce0.9Gd0.1O1.95, GDC) electrolyte was fabricated by aqueous-based tape casting method for solid oxide fuel cells (SOFCs). The ceramic powder prepared by combustion synthesis was used with poly acrylic acid (PAA), poly vinyl alcohol (PVA), poly ethylene glycol (PEG), Octanol, 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate and double distilled water as dispersant, binder, plasticizer, defoamer, surfactant and solvent respectively, to prepare stable GDC slurry. The conditions for preparing stable GDC slurries were studied and optimized by sedimentation, zeta potential and viscosity measurements. Green tapes with smooth surface, flexibility, thickness in the range of 0.35–0.4 mm and 45% relative green density were prepared. Conventional and flash sintering techniques were used and compared for densification which demonstrated the possibility of surpassing sintering at high temperatures and retarding related grain growth.

Highlights

► Fabrication of GDC electroyte for SOFC by tape casting. ► Optimization of electrolyte slurry formulation. ► Reproducibility of green scrap tapes. ► Densification by novel flash sintering technique.

Introduction

Nanostructured materials with enhanced desired characteristics have attracted great attention for many niche applications in recent years. The nanostructured ceramics with improved ionic conductivity have recently become one of the popular fields of research since they can be used in emerging energy conversion devices such as SOFCs [1].

SOFCs have gained widespread attention due to their high-energy conversion efficiency and low pollution. However, high temperature operation would incur materials selection issues; expensive balance of plant (BoP) and fabrication cost are the barriers towards the commercialization of SOFC. Therefore, lowering the operating temperature of SOFCs and developing the low-cost and environmentally friendly fabrication techniques are the focus of recent studies [2].

High-ionic conducting solid oxide electrolytes based on rare earth doped ceria have been intensively investigated [3]. In order to reduce the operation temperature from 1000 °C to lower temperatures, gadolinium doped ceria (GDC) solid solution formed by replacing the Ce4+ sites of the CeO2 lattice by Gd3+ cations, has been considered as a candidate solid electrolyte material for intermediate and low temperature SOFCs [4], [5].

In the case of electrolyte-supported cells, the fabrication of the electrolyte is dominated by tape casting which is a well established and cost-effective method in the electroceramics industry and is scalable for mass production [6]. Tape casting often uses a slurry containing ceramic powder, solvent, binder, plasticizer, and additives such as de-foamer, surfactant and dispersant [7]. Traditionally, tape casting was done using organic solvents. Due to environmental, health, safety and economic reasons, aqueous-based tape casting method using water as solvent is adopted to replace the traditional organic-based tape casting method [5]. Despite these advantages, several critical challenges should be overcome which include slow drying rate of the tape, high crack sensitivity, flocculation and poor wetting of the slip due to the high surface tension of water [8], [9]. Therefore, one of the main challenges in aqueous tape casting is to formulate a suitable suspension composition to avoid the defects which may occur during the drying process.

Ceramic powder as main constituent of slurry plays an important role in suspension composition. Colloidally stable nano-sized powder suspensions are known to have a markedly lower volume loading compared to suspensions with larger particle sizes [10]. There are a wide variety of processes for the synthesis of ceramic powders, which provide different properties such as particle size, shape, surface area and morphology. For particles in the nanometres range, adhering layer becomes significant and the maximum volume loading for a stable suspension is decisively lower. As a consequence of this, the green bodies resulting from nano powders usually have low green densities and are difficult to sinter to dense ceramics [11], [12].

A number of sintering methods rely on the assistance of applied electric field, such as microwave sintering, electric discharge compaction (EDC), and spark plasma sintering have been used for superfast densification of ceramic powders despite each of these techniques has some limitations [13], [14], [15]. Recently, flash sintering emerged as a potential technique for fast sintering is under exploration. This is achieved by a sudden sintering event when the sample reaches a certain temperature under a given applied electrical field, thus enhancing the sintering rate and retarding grain growth [16], [17], [18], [19], [20].

Besides organic-based tape casting of commercially produced Ce0.1Gd0.9O1.95 powder [21], [22], aqueous-based tape casting of Ce0.8Gd0.2O1.9 synthesized by a co-precipitation method [23] and commercially produced Ce0.1Gd0.9O1.95 powder [5] have been reported.

We have earlier reported the combustion synthesis of rare earth doped ceria nano powders [24], [25]. In this investigation, aqueous-based tape casting of Ce0.1Gd0.9O1.95 powder prepared by combustion synthesis was used for fabricating the SOFC electrolyte. The characteristics of tape casting ceramic slurry, green and sintered tapes were also studied.

Section snippets

Starting materials

Ce0.9Gd0.1O1.95 (GDC) powders were synthesized by the nitrate-fuel combustion method. Citric acid was used as an organic fuel and high purity (>99.9%) cerium nitrate [Ce(NO3)3·6H2O] and gadolinium nitrate [Gd(NO3)3·6H2O] were used as precursor reagents. The details of the method have been reported elsewhere [24]. For preparing the slurry, distilled water was used as the solvent. Poly acrylic acid (PAA) with molecular mass of 1800 g mol−1 was used as an electrosteric dispersant. Poly ethylene

Results and discussion

To describe the surface charge behaviour of the particles in the suspensions, zeta potential measurements were performed to identify the optimum pH value and dispersant content of GDC suspension. As shown in Fig. 1, the maximum zeta potential (absolute value) does not exceed 30 mV for the pH value range of 2–10, indicating that the electrostatic repulsion between GDC particles is insufficient for stabilizing suspension in the absence of dispersant. For the suspension containing more than 1 wt.%

Conclusions

High density nanostructured GDC electrolyte was fabricated by aqueous tape casting and novel flash sintering technique. The slurry composition was optimized by using poly acrylic acid, poly vinyl alcohol, poly ethylene glycol and double distilled water as dispersant, binder, plasticizer and solvent, respectively. The green tape displays very smooth surface and flexibility with thickness in the range of 0.35–0.4 mm and a relative green density of 45%. Microstructures of the conventionally

Acknowledgements

The authors acknowledge FONDECYT, Government of Chile (Project No.: 1100349) Direction of Investigation, University of Concepcion, Chile for the support to carry out this project.

References (27)

  • A.U. Chavan et al.

    Ceram. Int.

    (2012)
  • L.H. Luo et al.

    Mater. Sci. Eng. A

    (2006)
  • F. Tietz et al.

    Solid State Ionics

    (2002)
  • T. Tian et al.

    J. Eur. Ceram. Soc.

    (2007)
  • C. Fu et al.

    Int. J. Hydrogen Energy

    (2010)
  • M.D. Snel et al.

    J. Eur. Ceram. Soc.

    (2007)
  • J.V. Herle et al.

    Ceram. Int.

    (1998)
  • R. Orru et al.

    Mater. Sci. Eng. R

    (2009)
  • M. Cologna et al.

    J. Eur. Ceram. Soc.

    (2011)
  • X. Hao et al.

    J. Power Sources

    (2012)
  • L.P. Meier et al.

    J. Eur. Ceram. Soc.

    (2004)
  • T. Klemensø et al.

    Ceram. Int.

    (2010)
  • Y. Fu et al.

    Ceram. Int.

    (2009)
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