Abstract
Recently, appreciable ionic conduction has been frequently observed in multifunctional semiconductors, pointing out an unconventional way to develop electrolytes for solid oxide fuel cells (SOFCs). Among them, ZnO and Li-doped ZnO (LZO) have shown great potential. In this study, to further improve the electrolyte capability of LZO, a typical ionic conductor Sm0.2Ce0.8O1.9 (SDC) is introduced to form semiconductor-ionic composites with LZO. The designed LZO-SDC composites with various mass ratios are successfully demonstrated in SOFCs at low operating temperatures, exhibiting a peak power density of 713 mW cm−2 and high open circuit voltages (OCVs) of 1.04 V at 550 °C by the best-performing sample 5LZO-5SDC, which is superior to that of simplex LZO electrolyte SOFC. Our electrochemical and electrical analysis reveals that the composite samples have attained enhanced ionic conduction as compared to pure LZO and SDC, reaching a remarkable ionic conductivity of 0.16 S cm−1 at 550 °C, and shows hybrid H+/O2− conducting capability with predominant H+ conduction. Further investigation in terms of interface inspection manifests that oxygen vacancies are enriched at the hetero-interface between LZO and SDC, which gives rise to the high ionic conductivity of 5LZO-5SDC. Our study thus suggests the tremendous potentials of semiconductor ionic materials and indicates an effective way to develop fast ionic transport in electrolytes for low-temperature SOFCs.
Highlights
Ca0.04 Ce0.80 Sm0.16 O2-δ (SCDC) at 600 ◦ C, while a similar high ionic conductivity was observed in a composite based on Ni0.8 Co0.15 Al0.05 LiO2-δ (NCAL) and Ce0.8 Sm0.2 O2-δ -Na2 CO3 (NSDC) [22,23]. These high ionic conduction examples are widely regarded as a result of hetero-interface superionic transport, since highly disordered oxygen plane can be created at semiconductor/ionic conductor interface as reported by previous study when Garcia-Barriocanal et al detected a colossal ionic conductivity of ~0.1 S cm−1 at 200 ◦ C at the Y2 O3 -stabilized ZrO2 (YSZ)/SrTiO3 interface [25]
It is evident that no peaks of the metal nitrates can be observed in the pattern of Li-doped ZnO (LZO), and there are no extra signal corresponding with the Zn, Li, or Li2 O related secondary phases, reflecting that Li is completely incorporated into the crystal lattice of ZnO through occupying an interstitial site or a substitution site of Zn
For a conventional solid oxide fuel cells (SOFCs), semiconductor LZO is not suitable to be used as the electrolyte layer, since the electrons produced at the anode could transfer to the semiconductor electrolyte to induce short-circuiting issues [35,36], and the used SDC is commonly reduced in a H2 rich atmosphere, resulting in unfavorable deterioration of open circuit voltages (OCVs)
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Ca0.04 Ce0.80 Sm0.16 O2-δ (SCDC) at 600 ◦ C, while a similar high ionic conductivity was observed in a composite based on NCAL and Ce0.8 Sm0.2 O2-δ -Na2 CO3 (NSDC) [22,23] These high ionic conduction examples are widely regarded as a result of hetero-interface superionic transport, since highly disordered oxygen plane can be created at semiconductor/ionic conductor interface as reported by previous study when Garcia-Barriocanal et al detected a colossal ionic conductivity of ~0.1 S cm−1 at 200 ◦ C at the YSZ/SrTiO3 interface [25]. Our approach provides a feasible way to develop LT electrolyte for SOFCs
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