Technical Communication
Development of a novel type of composite cathode material for proton-conducting solid oxide fuel cells

https://doi.org/10.1016/j.ijhydene.2011.11.096Get rights and content

Abstract

A high-performance solid oxide fuel cell La1−xSrxMnO3 (LSM) cathode/metallic interconnect contact material Ni1−xCoxO, added with the mixed ionic-electronic conducting Sm0.2Ce0.8O2−δ (SDC), was proposed as a novel composite cathode for proton-conducting solid oxide fuel cells (H-SOFCs) with BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) as the electrolyte. The X-ray diffraction (XRD) results indicated that the maximum doped ratio of Ni1−xCoxO was Ni0.7Co0.3O (NC3O), also shown that NC3O was chemically compatible with SDC at temperatures up to 1400 °C. The TEC of NC3O was also measured to check its thermal compatibility with other components. Laboratory-sized tri-layer cells of NiO–BZCYYb/BZCYYb/NC3O-SDC were fabricated and tested with humidified hydrogen (∼3% H2O) as fuel and static air as oxidant, respectively. A maximum power density of 204 mW cm−2 and a low interfacial polarization resistance Rp of 0.683 Ω cm2 were achieved at 700 °C. The results have indicated that the NC3O-SDC composite is a simple, stable and cost-effective cathode material for H-SOFCs.

Introduction

In recent years, the proton-conducting solid oxide fuel cells (H-SOFCs) have several advantages over the oxygen-ion conducting solid oxide fuel cells, including simpler fuel-recycling instruments and being able to avoid the dilution of the fuel [1], [2]. More importantly, the low activation energy of proton transport is favorable for the H-SOFCs to operate at intermediate-to-low temperatures, thus leading to a prolonged operational lifetime and improved materials compatibility [3], [4]. While in a power generation system, a number of single cells are stacked together to produce substantial voltage and power output. Individual fuel cells are electrically connected by interconnects which not only provide the electrical conduction path but also separate the fuel at the anode side from the air at the cathode side. Finding a suitable cathode material is very challenging that is not the only one requiring a lower polarization resistance of the cathode–electrolyte interface [5], [6], [7], [8], [9], [10]. Besides, the requirements for cathode materials contacting with the interconnect materials include high chemical and microstructural stability, high electrical conductivity, compatibility with the interconnect materials, reasonable thermal compatibility match with other cell components, and high sinter ability to ensure high mechanical strength and good bonding with the adjacent components.

Previous studies in this area have focused on tailoring the existing cathode materials, such as La1−xSrxMnO3, LaNi1−xFexO3, and La1−xSrxCo1-yFeyO3 [11], [12], [13], [14], unfortunately, a major challenge for these materials is that they exhibit very limited sintering [13], [14] and therefore low bond strength at the final stack fabrication temperature, which is usually below 950 °C. Recently, Ni1−xCoxO was reported to developed for solid oxide fuel cell (SOFC) cathode/interconnect contact applications, which have a higher average mechanical strengths and a lower area specific resistance (ASR) observed after 1200 h exposure in air at 800 °C [15].Therefore, employing doped NiO as cathode materials will be promising, which is usually used as anode precursor, especially in cell stacks. In order to enhance conductivity and catalytic property, mixed ionic–electronic conducting Sm0.2Ce0.8O2−δ (SDC) is added into doped NiO to form a composite cathode. To the best of our knowledge, the performance of NC3O-SDC as a novel composite cathode has not been reported to date.

Section snippets

Powders synthesis

Ni1−xCoxO (NCO, 0.0 < x ≤ 0.5), Ce0.8Sm0.2O2−δ (SDC) and BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) powders were all synthesized by auto ignition process. Taking the synthesis of BZCYYb as an example: Ba(NO3)2·9H2O, Zr(NO3)4·4H2O, Ce(NO3)3·6H2O, Y2O3 and Yb2O3 as raw materials were dissolved in distilled water at stoichiometric ratio, citric acid and EDTA were then added as complexing agents, with the molar ratio of citric acid: EDTA: metal cations of 1.5:1:1. Under heating and stirring, the solution

Results and discussion

Fig. 1a is the XRD patterns of the Ni1−xCoxO (NCO, 0.0 < x ≤ 0.5) cathode powders after calcining at 1000 °C for 2 h. The cubic phase structure of the Pm3m (225) space group is maintained up to x = 0.3, whereas, a slight second phase of NiCo2O4 is observed when x > 0.3.There is also a gradual shift in the XRD peaks’ position toward lower diffraction angle with the substitution of Ni by Co, indicating an increase in the lattice parameter as x changes from 0.1 to 0.4. The NCO (x = 0.1) powder’s

Conclusions

In this work, a high-performance solid oxide fuel cell La1−xSrxMnO3 (LSM) cathode/metallic interconnect contact material Ni1−xCoxO, which could improve contact and the good bonding of the interfaces between the contact and the cathode and between the contact and the interconnect materials, added with the mixed ionic–electronic conducting Sm0.2Ce0.8O2−δ (SDC), was proposed as a novel composite cathode for proton-conducting solid oxide fuel cells (H-SOFCs) with BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb)

Acknowledgments

The authors would like to thank the financial support from Chinese Natural Science Foundation on contract No. 51102107, No. 50730002, and the financial support from the National High-tech R&D Program of China (No.: 2007AA05Z157).

References (21)

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These authors contribute equally to this work.

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