Preparation of NiO–YSZ tubular support with radially aligned pore channels

doi:10.1016/S0167-577X(02)01002-9

Materials Letters

Volume 57, Issue 8, February 2003, Pages 1428-1434

https://doi.org/10.1016/S0167-577X(02)01002-9 Get rights and content

Abstract

A porous NiO–YSZ tubular support with radially aligned pore channels was prepared by freeze-drying of a water-based slurry. The slurry was frozen, controlling the growth direction of the ice, and it was sublimated at a reduced pressure. After sintering, the support tube showed radially aligned large pore channels parallel to the ice growth direction, and fine pores are formed around the outer surface of the tube. Bilayer consisting of dense thin electrolyte film of YSZ onto the support has been successfully fabricated using a slurry-coating process followed by co-firing. It is regarded that such a unique bilayer structure is suitable for constructing electrode-supported type electrochemical cells.

Introduction

Oxygen ion-conducting electrolyte films can be applied to various kinds of electrochemical devices such as oxygen pumps, electrochemical cell for NO decomposition and solid oxide fuel cells (SOFC), since the ohmic loss across the electrolyte is lowered as its thickness is reduced. This allows the electrochemical cell to operate at lower temperatures and higher efficiency. In order to prepare the electrolyte film, various thin-film processes have been investigated such as electrochemical vapor deposition [1], sputtering [2], physical vapor deposition [3], electrophoretic deposition [4] and slurry coating [5]. Among these processes, the colloidal deposition process, such as slurry coating, is considered to be cost-effective and scalable.

The electrolyte film should be maintained by the electrode as a support. Thus, the electrode should work as a structural support and gas transport passage, as well as an electrochemical reaction site. Electrochemical activity of an electrode is generally proportional to the amount of triple-phase boundary, which indicates the interface of pore, electrode and electrolyte (pore/electrode/electrolyte). Therefore, the support must be processed to yield high surface area with fine microstructure. However, reducing the pore size may increase the tortuosity of gas transport passage and this gives rise to increase in concentration polarization [6]. In case of using slurry-coating process, the support should meet more complicated requirements, since it is difficult to get a dense electrolyte film by sintering a colloidal deposit on non-shrinking substrate.

Co-firing a support and a slurry-coating layer is widely used way to obtain bilayers with dense electrolyte films on porous supports. However, in order to achieve sufficient amount of open pores, mixing a burnable pore former such as starch or carbon with an electrode powder in the forming step of support is required, because the co-firing temperature is fairly higher than the post-firing temperature of a conventional thick-film electrode. Such processes have been widely used in thin-film SOFC researches [4], [5] due to their simplicity, but they have many drawbacks to be solved. The percolation of the pore formers is essential in the process, thus, fairly large amount of pore former is required. Homogeneous distribution of the pore former and the electrode powder is also not a simple problem due to their large differences in density and particle size. Therefore, cracking or wrapping of the substrate frequently occurs after burning out the pore former.

Another well-known method to fabricate the porous support for SOFC is a coat-mix® process [7]_. Instead of using particulate pore former, the majority of open pores in the coat-mix® process are formed between the intentionally coagulated agglomerates, which are coated with thermosetting resin. The production of the green support is preformed by warm pressing of the resin-coated coagulates in a metallic die at 120 °C with a pressure of 1 MPa. However, burning out the thermosetting resin such as phenolformaldehyde, which is used as a binder, may bring about serious environmental problems.

This study reports an approach to fabricate a tubular type NiO–YSZ support with radially aligned pore channels, which is considered to be ideal for constructing SOFC. This may reduce the effective path length necessary for gas phase diffusion. A method of preparing a porous Al₂O₃ disk with vertically aligned pore channels by the freeze-drying of its water-based slurry has been reported [8]. It has been regarded as environmentally friendly process for preparing porous ceramics. The aligned large pores with 100-μm scales in the report are expected to be very effective for gas permeation through the support. However, to obtain a defect-free slurry-coating layer on the support with such large pores has been considered to be difficult. A major portion of slurry would penetrate into the support instead of building up a continuous coating layer on the surface of the support, because well-dispersed slurry for coating generally consists of submicron range flocks. Recently, we have found out that the pore size can be changed according to the freezing rate and ice growth direction. Based on this phenomenon, it was possible to fabricate tubular NiO–YSZ supports, which have radially aligned pore channels and the layer with fine pores around their outer surfaces. The dense films of YSZ were successfully deposited on the tubular supports by slurry coating followed by co-firing.

Section snippets

Experimental procedure

The water-based slurry of NiO–YSZ with different solid loadings was prepared. The NiO (High Purity Chemicals, Japan) and YSZ (8 mol% Y₂O₃-stabilized ZrO₂, TZ8YS, Tosho, Japan) powder were mixed with a small amount of dispersant (Alon A-6114, Toa-Gosei Chemicals, Japan) by ball-milling. The slurry was de-aired by stirring under vacuum, and then it was poured into the container designed as shown in Fig. 1. The container consists of a metal cylinder with high thermal conductivity, and Teflon rod

Results and discussion

Fig. 2 shows the relationship between the porosity and solid loading of the slurry. The porosities of the freeze-dried specimens were greater than that of the cold isostatically pressed specimen, and the porosity of both green and sintered body increased linearly with increasing water content (inversely proportional to the solid loading) in the slurry. This indicates that we can easily control the porosity of the support by changing the solid loading. NiO–YSZ tubular support with extremely high

Conclusions

Freeze-drying of water-based slurry is an attractive forming process for fabricating porous support, because it is friendly to the environment, scalable and the porosity is easy to control. The tubular supports prepared by freeze-drying process consisted of radially aligned pore channels parallel to the ice growth direction and fine pores around their outer surfaces. Forming a layer with fine pores in the vicinity of the outer surface is considered to be a critical step to slurry-coating

Acknowledgments

This work has been supported by AIST, METI, Japan, as part of the Synergy Ceramics Project. Part of the work has been supported by NEDO. The authors are members of the Joint Research Consortium of Synergy Ceramics.

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Preparation of NiO–YSZ tubular support with radially aligned pore channels

Abstract

Introduction

Section snippets

Experimental procedure

Results and discussion

Conclusions

Acknowledgments

Solid State Ionics

J. Mater. Process. Technol.

J. Electrochem. Soc.

J. Am. Ceram. Soc.