Cobalt-free Ba0.5Sr0.5Fe0.8Cu0.1Ti0.1O3−δ as a bi-functional electrode material for solid oxide fuel cells
Introduction
Ceramic fuel cells, which are more frequently called solid oxide fuel cells (SOFCs), belong to a category of green energy conversion technology that may play an important role in future energy systems. The interest in SOFCs is related to their advantages, such as high efficiency, low emissions, fuel and size flexibility, long lifetime, and high quality of exhaust heat. In the early development stage, SOFCs were primarily composed of a thick yttria-stabilized zirconia (YSZ) electrolyte, a nickel cermet anode and a La0.8Sr0.2MnO3−δ (LSM) cathode and were operated at elevated temperatures of approximately 1000 °C [1]. Based on the numerous drawbacks associated with operation at elevated temperatures, many studies have focused on decreasing the operation temperature of SOFCs to the intermediate range, i.e., 500–800 °C [2], [3], [4], [5], [6], [7]. To maintain a high power output at a reduced temperature, thinning of the electrolyte layer has been widely applied to compensate for the decrease in the ionic conductivity to ensure low ohmic resistance of the cells. It was estimated by Steele that YSZ and Ce0.9Gd0.1O1.95 electrolytes could still maintain sufficiently low ohmic resistance at temperatures down to 500 and 700 °C, respectively, when their thickness in an SOFC was reduced to 15 μm [8]. Fortunately, mass production of thin-film electrolytes with a thickness down to 10 μm has become a reality due to the use of mature advanced fabrication techniques, such as spray deposition [9], tape casting [10], dip casting [11] and screen printing [12]. However, in practice, the power outputs of cells with thin-film electrolytes are still low, which is primarily due to significantly reduced activity for the oxygen reduction reaction (ORR) of the conventional La0.8Sr0.2MnO3−δ cathode [13], [14]. Therefore, the development of alternative cathode materials or advanced architectures with improved performance at reduced temperatures is critical. In addition, if the same material can be used for both the anode and the cathode, the cell fabrication process can be significantly simplified, which would make the SOFCs a more economically attractive technology [15], [16], [17].
The introduction of oxygen-ionic conducting pathways into the electrode material is an effective method for improving the electrode activity for the ORR (cathodic reaction), as well as fuel oxidation (anodic reaction), at reduced temperatures because the number of reaction sites may significantly increase, extending from the triple phase boundary region of a typical electrode with a pure electronic conductor to the whole exposed surface of a mixed oxygen-ionic and electronic conducting electrode [18], [19]. During the past decade, cobalt-containing perovskite oxides have been primarily exploited as new cathode materials for SOFCs [2], [3], [4], [5], [20], [21], [22], [23]. The facile reduction of cobalt ions facilitates the formation of oxygen vacancies inside the oxide lattice, which are charge carriers for oxygen ions that introduce oxygen-ion conductivity under operation conditions (i.e., intermediate temperature, open circuit or current polarization). A typical material is the Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) perovskite oxide, which exhibits outstanding catalytic activity for the ORR even at a relatively low temperature of 500 °C [2]. However, additional investigations demonstrated that many cobalt-based perovskite oxide electrodes have several disadvantages, such as the high price and environmental unfriendliness of cobalt, a large thermal expansion coefficient (TEC), and poor phase stability due to the high reducibility of cobalt ions [24], [25]. More recently, the development of cobalt-free high-performance cathode materials for intermediate temperature (IT) SOFCs has received considerable attention [26], [27], [28], [29], [30].
Iron-based perovskite oxides are a type of the most attractive alternative cathode materials for SOFCs due to their inexpensive cost, environmental friendliness, versatile coordination numbers of iron ions with oxygen, appropriate cation size of iron for the B-site of perovskite, and high electrical conductivity. Recently, BaFeO3−δ with multiple phase structures exhibited interestingly high activity for the ORR at intermediate temperatures [31]. In addition, the stabilization of the cubic phase of BaFeO3−δ via doping has been widely applied because the phase transition may be accompanied by a change in the lattice parameter, which could induce a large internal strain, which is detrimental to the operational stability of the electrode [32], [33], [34]. Both A-site and B-site doping of perovskite materials can significantly affect their catalytic activity and phase stability. Recently, Ba0.5Sr0.5Fe0.8Cu0.2O3−δ was reported to exhibit favorable catalytic activity for the ORR at intermediate temperatures [35], [36], [37]. Both Fe and Cu have relatively low bond energies with oxygen (i.e., Fe–O 407.0 ± 1.0 kJ mol−1 and Cu–O 287.4 ± 11.6 kJ mol−1) [38]. Although the low bond energies between the cations and oxygen in the perovskite oxide lattice benefit oxygen diffusion, this low bond energy may also have a detrimental effect on the chemical stability of the oxide. Typically, doping of the perovskite lattice with a cation possessing a large bond energy with oxygen could improve the phase stability. Therefore, the partial substitution of the B-site cations of Ba0.5Sr0.5Fe0.8Cu0.2O3−δ (Cu or Fe) with cations that have a much larger bond energy with oxygen, such as, Zr and Ti (i.e., Zr–O 766.1 ± 10.6 kJ mol−1 and Ti–O 666.5 ± 5.6 kJ mol−1) [38], may be an effective method for improving the phase stability.
In this study, we report a perovskite-type oxide with a nominal composition of Ba0.5Sr0.5Fe0.8Cu0.1Ti0.1O3−δ (BSFCuTi) as a new cobalt-free cathode material in SOFCs for operation at intermediate temperatures. Here, titanium was selected for the partial substitution of the iron in Ba0.5Sr0.5Fe0.9Cu0.1O3−δ (BSFCu). The material exhibits comparable performance to that of BSFCu for the ORR but with improved phase stability. In addition, this material can be used as an anode material in SOFCs, making it a suitable bi-functional electrode material. Its favorable catalytic activity and phase stability highly support its use in practical SOFCs.
Section snippets
Powder preparation
A sol–gel route using EDTA and citrate as the complexing agents was applied to synthesize the Gd0.2Ce0.8O1.9 (GDC), Ba0.5Sr0.5Fe0.9Cu0.1O3−δ (BSFCu) and Ba0.5Sr0.5Fe0.8Cu0.1Ti0.1O3−δ (BSFCuTi) powders. For GDC or BSFCu, stoichiometric amounts of analytical grade Gd(NO3)3 and Ce(NO3)3, or Ba(NO3)2, Sr(NO3)2, Fe(NO3)3 and Cu(NO3)2 were dissolved in deionized water. Afterwards, the EDTA and citric acid were added to the nitrate solution with a molar ratio of 1:1:2 for the total metal nitrates,
Basic properties
The incorporation of Ti into BSFCu was performed to create a stable backbone for the perovskite lattice, which improved the phase stability of the oxide under fuel cell operating conditions. The effect of Ti doping on the phase structure of the BSFCu oxide was first investigated by XRD. The typical XRD patterns of the BSFCu and BSFCuTi oxides, which were prepared by an EDTA-citrate complexing method and calcined in air at 950 °C, are shown in Fig. 1. To obtain further structural information,
Conclusions
In the present study, cobalt-free Ba0.5Sr0.5Fe0.8Cu0.1Ti0.1O3−δ perovskite oxide was developed as a potential electrode material for IT-SOFCs. Doping titanium into the perovskite structure resulted in a higher structure stability. BSFCuTi exhibited a low area specific resistance of 0.088 Ω cm2 at 600 °C and a long-term stability of 200 h for the symmetrical cell. The peak power density of the single cell reached 1166 mW cm−2 at 600 °C with the GDC electrolyte using the BSFCuTi cathode. Under a
Acknowledgments
This research was financed by the Australian Research Council Future Fellowship (No. FT100100134) and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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