Fabrication of a dense La_0.2Sr_0.8CoO_3−δ/CoO composite membrane by utilizing the electroless cobalt plating technique

Abstract

Fabrication of dense ceramic electrolyte membranes on porous supports is a key step towards performing gas separations (H₂ or O₂) through the electrochemical pathway. This research develops an approach by making use of the electroless plating method for the preparation of metal-ceramic composite membrane, which is used as the precursor to a metal-oxide composite membrane. As a model of the composite membrane, metallic cobalt is incorporated into a powder-packed layer of La_0.2Sr_0.8CoO_3−δ (LSCO-80), which is pre-coated on a porous MgO disk. When this composite membrane is subjected to sintering at 1000 °C in air, an interpenetrating laminar structure consisting of CoO and LSCO-80 phases is formed according to the cross-section EDX profiles. The oxidation of Co during sintering causes a structure expansion, which exerts a compressive stress on LSCO-80 phase, thus effectively buffering a tensile stress applied by the support. As a result, the composite membrane LSCO-80/CoO can achieve almost gas-tight at ambient temperature.

Introduction

The perovskite-type oxide La_1−xSr_xCoO_3−δ (LSCO-x) is a typical high-temperature mixed conductor [1], resulting in its potential application as membrane material for oxygen separation [2] and catalytic oxidation [3]. The mixed-conductive (e⁻/O²⁻) property makes it suitable to be used for electrochemical separation, which involves the reduction of O₂ at one side of a dense LSCO-x membrane and subsequent re-oxidation of O²⁻ at the other side of the membrane. A pressure gradient of O₂ across the membrane provides the driving force for migration of O²⁻ ions in the lattice of LSCO-x. The electronic conductivity provides a return path for the electrons so as to balance the current of oxygen ions in the absence of an external electrical field. In such a separation system the oxygen permselectivity is infinite as long as the membrane is absolute gas-tight. To set up a LSCO-x membrane separation device, a porous support is required to maintain the mechanical integrity of the membrane and allow the membrane to be accessible by the reacting species. The support normally has a very different composition than the membrane material in order to achieve great mechanical strength, good thermal stability and cost-effectiveness. However, the use of such an asymmetric support gives rise to the inevitable cracking in the supported membrane upon sintering due to a large shrinkage mismatch between the membrane and the underlying support. Thus, the primary challenge to devise a solid electrochemical oxygen separation apparatus is to overcome the cracking problem in LSCO-x membrane, and subsequently to pursue a dense bulk for allowing only O²⁻ to go through.

This research employed La_0.2Sr_0.8CoO_3−δ (LSCO-80) as the membrane material, which was prepared as the form of very fine particles by using the Pichini method [4], [5]. A colloidal suspension was then formulated by dispersing the LSCO-80 powder into an organic liquid medium. The colloidal suspension is well suited for the deposition of a powder packed thin layer onto porous substrates by dip coating [6], [7]. The MgO disk with a specific porosity was used as the support in this work because, besides its excellent mechanical and thermal properties, it does not react with the LSCO-x composition at high temperature. It also has a similar thermal expansion coefficient (TEC=14.7×10⁻⁶ K⁻¹) with that of LSCO-80 (TEC=14.2×10⁻⁶ K⁻¹) in the temperature range of 500–900 °C, which will minimize the magnitude of stress built up in the membrane upon thermal cycles.

We found recently [6] that a silver-topping layer on the powder packed layer is effective in buffering the tensile stress that exists within the membrane during the sintering of the powder assembly. However, this approach cannot avoid the generation of a porous structure in the membrane [8]. The present work employs electroless cobalt plating [9] to deposit cobalt metal onto the packed LSCO-80 powder layer. By this metalization means, Co metal can penetrate into the powder-packed layer through the inter-particulate space, resulting in a Co/LSCO-80 composite. As expected, an almost-dense CoO/LSCO-80 dual phase composite membrane has been realized after sintering. Although the presence of non-conductive CoO phase in the membrane will sacrifice the oxygen flux, it furnishes a practical way to fabricate a LSCO-x membrane on an asymmetric porous support.