Pt–perovskite cermet cathode for reduced-temperature SOFCs

doi:10.1016/S0167-2738(02)00116-9

Solid State Ionics

Volume 148, Issues 3–4, 2 June 2002, Pages 551-555

https://doi.org/10.1016/S0167-2738(02)00116-9 Get rights and content

Abstract

In order to develop a cathode for reduced-temperature solid oxide fuel cells, cermetting effect of LSCF with Pt was investigated. The effect of cermetting was observed. However, the obtained cathode activity was only an order of 0.5 S/cm² at 973 K. The activation energy at low and high LSCF/Pt ratios was smaller than that at medium LSCF/Pt ratio region. The cathode interfacial conductivity, σ_E, of the very low Pt containing LSCF cermet cathode increased with increasing cathode thickness, whereas with high Pt content, σ_E showed optimum. These results suggested that Pt has both enhancing and obstructing functions. Based on SEM observation, the main reason of these complicated results was considered to be due to poor porosity and narrow surface area due to high electrode sintering temperature. Pt may enhance sintering of LSCF. So reduced-temperature cathode sintering should be investigated for LSCF/Pt cathode preparation.

Introduction

Reduced-temperature solid oxide fuel cells (SOFCs) are attracting extensive interests because of their potential applications to automobile power units and compact-sized cogeneration units [1].

In order to reduce operation temperature, a highly active cathode is one of the indispensable requirements. The reduction of operation temperature leads to large electrode overpotential because of decrease in electrode activity. As a candidate cathode material, lanthanum cobaltite has been investigated widely [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14] due to its mixed conductivity. Pt is another probable candidate for cathode because of its high catalytic activity for reduction/oxidation of oxygen. Pt can be recycled even though it is expensive. Recently, we found that Pt–(Sc_0.10Ce_0.01) Zr_0.89O₂ (SSZ) and PtAg–SSZ cermet cathodes have high cathodic activity of 6.7 and 12 S/cm², respectively, at 973 K in air [15], [16]. However, SSZ/Pt and PtAg/SSZ cermet cathodes need Pt of 40 and 19 mg/cm² for the optimum activity, respectively. It will require large initial investment and recycle loss of Pt. The purpose of this paper is to examine the possibility of further improvement of noble metal–SSZ cathodes by replacing SSZ with mixed conductor, (La_0.8Sr_0.2)(Co_0.6Fe_0.4)O₃ (LSCF), to reduce the required Pt. In this case, 20% Gd-doped ceria (GDC) was used as electrolyte instead of yttria stabilized zirconia (YSZ) because of mutual reaction between LSCF and YSZ.

Section snippets

Experimental

Commercially available powders, Pt-AY1010 (TANAKA Kikinzoku Kogyo K.K.), LSCF powder (Powlex) and YSZ powder (Tohso), were used as Pt, LSCF and YSZ powder sources, respectively. LSCF and Pt powders were mixed by a high-energy ball mill in vacuum in LSCF/Pt ratios from 20:80 to 95:5 (wt) and made into Pt–LSCF cermet pastes using ethyl-cellulose and organic solvent. Pt–LSCF cermet electrodes of 6 mm in diameter were prepared by painting Pt–LSCF pastes via screen-printing technique on

Morphology

SEM micrographs of surface of the sintered cathodes are shown in Fig. 1. Because of high cathode sintering/joining temperature of 1473 K, Pt–LSCF particles lost porosity and became somehow round maybe due to melting at the particle surface. It was difficult to lower the joining temperature because of delamination of cathode from GDC electrolyte.

AC impedance arc on GDC electrolyte

The AC impedance spectra showed two arcs at 1123 and 1173 K, and one arc below 1073 K. The arcs that appeared in the low-frequency region were one order

Discussion

Our purpose is to reduce the required amount of Pt and to improve electrode activity of LSCF by mixing them as cermet cathode. Even though the effect of mixing on cathodic activity could be recognized, it was not so remarkable (Fig. 2). The mixing increased the activation energy of cathodic interfacial conductivity (Fig. 3). The effect of cathode thickness was also recognized, but the effect was different with mixing ratio of LSCF to Pt (Fig. 4). When we compare the LSCF–Pt cathode and SSZ–Pt

Conclusions

In order to develop a cathode for reduced temperature, LSCF/Pt cermet cathode was investigated. The cermetting effect was observed. However, the obtained cathode activity was far from satisfactory. The activation energy at low and high LSCF/Pt ratios was smaller than that at medium LSCF/Pt ratio region. The σ_E of the very low Pt containing LSCF cermet cathode increased with increasing cathode thickness, whereas with high Pt content, σ_E showed optimum. Based on SEM observation, the main reason

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