Electrochemical study of lithiated transition metal oxide composite as symmetrical electrode for low temperature ceramic fuel cells
Graphical abstract
Introduction
Fuel cells that operate at low temperatures between 300 and 600 °C are of particular interest, since this temperature range allows for the use of less precious cell components and hydrocarbon fuel, and facilitates simpler cell/stack/module assembly and operational durability [1], [2], [3], [4], [5]. The recent studies of doped ceria-carbonates composite electrolytes, offering a combination of high ionic conductivity and fuel cell performance, have open the avenue of realizing ceramic fuel cell operating at this low temperature range [6], [7], [8], [9], [10], [11], [12]. In addition, a growing interest is attracted to symmetrical fuel cell (SFC) [13], [14], [15], [16], [17], [18] for its number of advantages, such as high tolerance to sulfur poisoning and carbon deposition, the simpler cell manufacture process and subsequent improved stability and reliability. Currently, the used symmetric electrode materials are perovskite oxides [13], [16]. However, most of them suffer from one or several drawbacks: high processing cost, inadequate electrical conductivity both in oxidation and reduction atmospheres, and unsatisfactory electro-catalytic activity as electrode both for oxygen reduction and fuel oxidation even at extreme high temperature, and so on [17]. For example, the perovskite oxide Sr2Fe1.5Mo0.5O6−δ was thought to be the most promising electrode for symmetrical SOFC in the term of its electrical conductivity and catalytic activity [16], [18]. It, however, was found to react with water below 800 °C, causing degradation during thermal cycling [19]. Therefore, it is highly desirable to develop novel symmetrical electrode materials for low temperature SFC.
As a continuation of research into the electrode materials for low temperature ceramic fuel cell (LT-CFC) [20], [21], [22], [23], [24], [25], in this study, we further choose CuO and ZnO modified lithiated NiO (denoted as LNCZO) as the special electronic component and characterize its electrochemical performance as a symmetrical electrode for ceria-based composite based CFCs. The choice of such composition has the following considerations: Lithiated NiO has been employed as the electrodes for SFC and shows adequate catalytic activity [20]. For the anode, the formation of Cu/Ni alloy can reduce the electrode polarization resistance and enhance the tolerance to carbon deposition [26]. Besides, the Cu/ZnO composite, a commercial methanol steam reforming catalyst, partially fills the requirement of the direct hydrocarbon fueled CFCs [23]. Moreover, the chemically benign and structurally stable n-type semiconductor of ZnO oxide can enhance the dispersion of metal or alloy and thus improve the electrode redox stability [27]. From the cathode aspect, the transition oxide composite was employed as cathode materials for molten carbonate fuel cell [28]. The lithiated NiO have also been demonstrated to show the potentiality for oxygen reduction in low temperature FC [29]. Furthermore, the introduction of the metal oxide solid solution to NiO lattice can effectively improve the oxygen reduction reaction rate and also the dissolution tolerance in molten carbonate [30]. Therefore, integrating with these multi-functionalities, low cost and the novel SFC configuration, it is expected that LNCZO symmetrical electrodes will show promising prospect for low temperature application.
On the other side, though there are several reports on the development of novel electrode for LTCFC, especially the lithiated transition metal oxide composite [21], [23], few has examined the electrode catalytic losses in these novel LTCFC systems [22], [24], [31]. It is unclear why these LTCFC systems performed so well while the catalytic activity of traditional cathodes at these temperatures (≤600 °C) is the key barrier for high performance. This work illustrates the potential of lithiated transition metal oxide as potential electrode for ceria-based composite low temperature CFCs by (1) proving high catalytic activity both in air and in reducing atmospheres and (2) identifying the hydrate effect of transition metal oxide using symmetrical cells and by means of impedance spectroscopy in different applied gas atmospheres, i.e. in air, hydrogen gas and humidified air.
Section snippets
Experimental
Composite electrolyte with the composition of Ce0.8Sm0.2O2−δ (SDC)-(Li/Na)2CO3 was prepared as our previous work [10]. LNCZO nanocomposite was synthesized by a solid-state reaction method with Li2CO3, Ni(NO3)2∙6H2O, Cu(NO3)2∙6H2O and ZnO as the raw materials. All the chemicals are analytical grade and purchased from the Guangfu Reagent Company, Tianjin. The molar ratio of Li: Ni: Cu: Zn is 2:2:1:2. The raw materials were first ball milling mixed and subsequently sintered at 700 °C for 180 min
Results and discussion
The crystal structure of the lithiated transition metal oxide and the hydrogen reduced sample are shown in Fig. 1, the standard diffraction pattern of NiO (PDF No. 73-1519), ZnO (PDF No. 89-0510) and Ni/Cu alloy (PDF No. 65-9048) are also given for identification. In the oxidation state, the oxide is identified to NiO and ZnO respectively. The peaks belonged to CuO and lithium oxide are not observed; both of them may be doped into the crystal lattice of NiO and ZnO to form the solid solution,
Conclusions
In summary, LNCZO oxides showed adequate electro-catalytic activity both for HOR and ORR between 500 °C and 600 °C. The interface resistances for ORR were 1.63, 2.79 and 3.48 Ω cm2 at 600, 550 and 500 °C, respectively, with a noticeable activation energy of 42 kJ mol−1. A prominent electrode polarization resistance of 0.094 Ω cm2 was achieved at 600 °C for hydrogen oxidation. Anode supported fuel cells with LNCZO composite symmetrical electrodes and ceria-carbonate electrolyte presented a total
Acknowledgments
The European Commission FP7 TriSOFC Project (No. 303454), Swedish Research Council (VR, No. 621-2011-4983), the Swedish VINNOVA (the Swedish Agency for Innovation) Systems, Chinese Scholarship Council (CSC, No.2010625060) and KIC Innoenergy are recognized for the financial support. This work is also partially supported by the National Basic Research Program of China (No. 2012CB720302) and the Natural Science Foundation of Tianjin City of China (Key program, No. 12JCZDJC27000). XRD measurement
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