Preparation and performance of a Cu–CeO2–ScSZ composite anode for SOFCs running on ethanol fuel

doi:10.1016/j.jpowsour.2006.10.056

Journal of Power Sources

Volume 164, Issue 1, 10 January 2007, Pages 203-209

https://doi.org/10.1016/j.jpowsour.2006.10.056 Get rights and content

Abstract

Solid oxide fuel cells (SOFCs) that can operate directly on hydrocarbon fuels, without external reforming, have the potential of greatly speeding up the application of SOFCs for transportation and distributed-power supplies. In this paper, a dual tape casting method for fabricating an anode-supported thin-electrolyte (scandia stabilized zirconia (ScSZ)) film and SOFCs that are active for the oxidation of wet ethanol was presented. The fabrication method relies upon the inclusion of ammonium oxalate ((NH₄)₂C₂O₄·H₂O) pore formers in the anode green tape in order to produce a porous ScSZ matrix, which forms the anode after wet impregnation with aqueous solutions of Cu(NO₃)₂ and Ce(NO₃)₃, and firing. Anodes with different ratios of copper to ceria but with the same total loading were fabricated and measured. The performance characteristics for such cells were studied in both H₂ and C₂H₅OH + H₂O, for comparison, and the long-term performance of the cells in C₂H₅OH stream at 800 °C was also presented.

Introduction

Due to their high efficiencies, low cost and fuel flexibility, solid oxide fuel cells (SOFCs) are considered to be promising candidates for electrical power generation. Many fuels have been suggested as potentially applicable to SOFCs and among them, ethanol is considered to be an attractive “green” fuel due to the following reasons: ethanol is a liquid fuel which is easy and safe in storage, handing and delivery; ethanol is renewable from various biomass sources including energy plants, waste materials from agriculture, forestry residue materials and even organic fractions from solid wastes; ethanol can be easily mixed with the necessary amount of water and vaporized simultaneously for reforming; ethanol nowadays is widely available and there seems to be no difficulty in its supply infrastructure.

It is well known that Ni–YSZ (yttria stabilized zirconia) cermet anodes of solid oxide fuel cell have excellent catalytic properties and stability for the H₂ oxidation at SOFC operation conditions [1], [2]. However, since Ni is a good catalyst for the hydrocarbon cracking reaction, the use of hydrocarbon fuels in an SOFC with Ni-based anode results in carbon deposition and rapid, irreversible cell degradation [3], [4], [5], [6], [7]. Ni–YSZ cermet anodes can only be directly used for hydrocarbon fuels if excess steam is present to ensure complete fuel reforming and to suppress carbon deposition [4], [8].

Gorte and his co-workers focused their attention on developing carbon resistant anodes by replacing Ni with Cu and CeO₂ [9], [10], [11], [12], [13]. Compared to Ni, Cu is not catalytically active for carbon deposition but is effective as a current collector, while ceria provides a high catalytic activity for hydrocarbon reforming due to its special mixed conductivity. Since the melting points of both copper and copper oxide are significantly less than the sintering temperature of, ca.1500 °C which is necessary for the densification of electrolytes, it is not possible to prepare Cu–YSZ cermets by high temperature sintering technology. Therefore, an alternative method for preparing Cu–YSZ cermets was developed in which a porous YSZ matrix was prepared first, and then both Cu and CeO₂ were added through wet impregnation [10], [12].

In this paper, we present a tape casting method for preparing SOFCs with a Cu–CeO₂–ScSZ anode, and study the cells’ performance in ethanol with steam for a long time. Tape casting is favorable for preparing very thin electrolyte films, and multi-layer structures. It is also easy in scaling up. We adopted this process to prepare a thin dense film of scandia stabilized zirconia (ScSZ) electrolyte, supported by a thick porous ScSZ layer which serves as the framework for the final anode. Ammonium oxalate ((NH₄)₂C₂O₄·H₂O) was used as the pore former. Although wet impregnation of Cu and CeO₂ was carried out with a similar process as in the YSZ system [10], [12], the method adopted in this study is able to prepare large area cells, which are necessary for real application of SOFCs. In previous work, on Cu–CeO₂–YSZ composite anodes by others, the ratios of Cu:CeO₂ were not stable, so the influence was not clear. In order to study the influence of different amounts of copper and ceria on the fuel oxidation, we fabricated, measured and compared anodes with different ratios of copper to ceria. We also demonstrated the feasibility and stability of using liquid fuels such as ethanol on Cu–CeO₂–ScSZ composite anodes.

Section snippets

Preparation of porous anode matrix

Fine ScSZ powders (99.99% pure, Daiichi Kigenso, Japan) and ammonium oxalate reagent (99.8% pure) were used to prepare the porous supporter of ScSZ by a tape casting and subsequent sintering process. The solvent system used in this paper was an azeotropic mixture of butanone and ethyl alcohol. Triethanolamine as a kind of zwitterionic dispersant was used as the dispersant. Poly-vinyl-butyl (PVB) and a mixture of polyethylene glycol (PEG 200) and dibutyl-o-phthalate (DOP) were used as the binder

Porous anode matrix characterization

The effect of the amount of pore formers on the porosity of the anode film was initially studied. The porosity, defined as the void volume divided by the total volume of the sintered anode film, is plotted in Fig. 2 as a function of the weight percent of pore formers in the green tape. It is noted that the weight percent of pore formers is based on the mass of ScSZ powders in the green tape. We can find that the porosity of the sintered porous ScSZ layer increases linearly with the weight

Conclusions

We successfully fabricated a Cu–CeO₂–ScSZ composite anode by impregnating solutions of copper and cerium nitrates into a porous ScSZ matrix made by a tape casting method and we also produced unit cells with different ratio of copper versus ceria, of which cell 1 with 21.5 wt.% Cu–8.5 wt.% CeO₂–ScSZ anode showed the best performance. The cell performances were good while using H₂, C₂H₅OH + H₂O as fuels at 800 °C and 750 °C, but poor at 700 °C partly due to the poor cathode performance. The Cu–CeO₂–ScSZ

Acknowledgements

The authors thank the financial support from Chinese Government High Tech Developing Project (2003AA517010) and the Postdoctoral Foundation of Shanghai (Grant No. 06R214156), and also thank Lin Xiong for providing PCM powders.

References (17)

R.T.K. Baker
Carbon
(1989)
B.C.H. Steele
Solid State Ionics
(1996)
K. Hernadi et al.
Appl. Catal.
(2000)
J.H. Koh et al.
Solid State Ionics
(2002)
R. Farrauto et al.
Appl. Catal.
(1992)
S.P. Jiang et al.
Mater. Sci. Technol.
(2004)
S.P. Jiang et al.
J. Mater. Sci.
(2004)
C.H. Bartholomew
Catal. Rev. Sci. Eng.
(1982)

There are more references available in the full text version of this article.

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Preparation and performance of a Cu–CeO2–ScSZ composite anode for SOFCs running on ethanol fuel

Abstract

Introduction

Section snippets

Preparation of porous anode matrix

Porous anode matrix characterization

Conclusions

Acknowledgements

Carbon

Solid State Ionics

Appl. Catal.

Solid State Ionics

Appl. Catal.

Mater. Sci. Technol.

J. Mater. Sci.

Catal. Rev. Sci. Eng.

Preparation and performance of a Cu–CeO₂–ScSZ composite anode for SOFCs running on ethanol fuel