Abstract
To improve the performance and overcome the processing difficulties of La0.99Ca0.01NbO4 proton-conducting ceramic oxide, external and internal strategies were used, respectively, to modify La0.99Ca0.01NbO4 with NiO. The external strategy refers to the use of the NiO as a sintering aid. The NiO was added to the synthesized La0.99Ca0.01NbO4 powder as a secondary phase, which is the traditional way of using the NiO sintering aid. The internal strategy refers to the use of NiO as a dopant for the La0.99Ca0.01NbO4. Both strategies improve the sinterability and conductivity, but the effect of internal doping is more significant in enhancing both grain growth and conductivity, making it more desirable for practical applications. Subsequently, the influences of different concentrations of NiO were compared to explore the optimal ratio of the NiO as the dopant. It was found that the sample with 1 or 2 wt.% NiO had similar performance, while with 5 wt.%, NiO doping content hampered the grain growth. In addition, the inhomogeneous distribution of the element in the high-NiO content sample was found to be detrimental to the electrochemical performance, suggesting that the moderate doping strategy is suitable for La0.99Ca0.01NbO4 proton-conducting electrolyte with improved performance. Furthermore, first-principle calculations indicate the origin of the enhanced performance of the internally modified sample, as it lowers both oxygen formation energy and hydration energy compared with the un-modified one, facilitating proton migration.
Highlights
Solid oxide fuel cells (SOFCs) can be divided into two types depending on the properties of the electrolyte, including oxygen–ion conducting solid oxide fuel cells [1]and proton-conducting solid oxide fuel cells [2]
The NiO source was added externally to obtain a composite of La0.99 Ca0.01 NbO4 (LCNO) and 1 wt.% NiO, which means that the NiO peak should be shown in the X-ray diffraction (XRD)
The absence of the NiO peak is probably due to the very low amount of NiO used in the current case, which leads to undefined NiO peaks in the XRD pattern
Summary
Proton-conducting solid oxide fuel cells [2]. Classical SOFCs (oxygen-ion conducting electrolyte) require high working temperatures (>700 ◦ C), which results in many problems, such as electrode sintering, diffusion at the interface, and difficulty in the preparation of seals and interconnection [3]. Classical SOFCs produce water at the anode side (fuel side), which would dilute the fuel and reduce fuel efficiency. Proton-conducting SOFCs would permit a reduction in working temperatures due to the lower activation energy for proton migration than that for oxygen-ions [4,5]. Water is formed at the cathode side, so the fuel is not diluted, and the anode avoids the danger of being oxidized even at high current conditions [6]. Proton-conducting SOFCs are currently a popular topic in the field of SOFCs [7,8]
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