Molybdate crystals tend to precipitate in nuclear waste glasses, significantly compromising their chemical and thermal stability, thereby rendering them unsuitable for long-term storage. However, the mechanisms by which glass composition influences the precipitation of molybdate crystals remain poorly understood. This study investigated this influence by preparing three series of molybdenum-doped sodium–calcium mixed aluminum borosilicate glasses using the melt-quenching technique. Solid-state nuclear magnetic resonance (SSNMR) spectroscopy, supplemented by Raman spectroscopy, was utilized to examine the glass structure at the atomic scale to reveal composition-dependent structural impacts on crystallization, while transmission electron microscopy (TEM) and X-ray diffraction (XRD) were employed to identify the precipitated crystals. The results demonstrate that increasing the Al2O3 content effectively suppresses molybdate crystal precipitation. It has been proven that high-valence cations differ in their ability to capture free oxygen, with the order of strength being Al3+ > Mo6+ > B3+ and Si4+. It is the strong ability of Al3+ to capture free oxygen and the formation of Al[4]–Ca2+–Mo[6] linkages that are responsible for inhibiting molybdate crystallization in the glass. An intriguing and important abnormal crystallization behavior was observed: a slight substitution of Na2O with CaO resulted in CaMO4 crystal precipitation, whereas larger substitutions paradoxically suppressed it. The findings reveal that in CaO–Na2O mixed aluminum borosilicate glasses, Al[4] preferentially attracts Na+ over Ca2+ to compensate for its negative charge. Meanwhile, Ca2+ ions are capable of forming an Al[4]–Ca2+–Mo[6] linkage, which Na+ ions cannot achieve. This fundamental difference results in the abnormal precipitation of CaMO4 crystals.
Chenshuo Yuan,Dongwang Feng,Hui Ye,Pengkai Shan,Lin Ge,Han Chen,Yifeng Zheng,Sheng Cui
Reversible solid oxide cells (RSOCs) represent a promising technology for efficient energy conversion and storage. However, the performance is often limited by the sluggish oxygen kinetics and poor structural stability of the air electrode. The cobalt-free Bi0.5Sr0.5FeO3−δ (BSF) air electrode exhibits a comparatively desirable performance and is expected to be further optimized. Herein, we propose a B-site co-doping strategy by incorporating high valence Nb5+ and Ta5+ into BSF to synergistically optimize the oxygen transport capacity and surface reactivity. The optimized Bi0.5Sr0.5Fe0.8Nb0.1Ta0.1O3−δ (BSFN0.1T0.1) demonstrates exceptional oxygen ion diffusivity (Dchem = 4.378 × 10−4 cm2 s−1) and exchange kinetics (Kchem = 1.330 × 10−3 cm s−1), leading to a 69% reduction in polarization resistance (from 0.195 to 0.06 Ω cm2) at 750 °C. Notably, the BSFN0.1T0.1 air electrode maintains stable performance under 3–10% CO2 atmosphere, demonstrating superior CO2 tolerance. In fuel cell mode, the single cell delivers a peak power density of 616 mW cm−2 (98% enhancement over BSF) at 750 °C. In electrolysis cell mode, a current density of 1370 mA cm−2 is obtained at 750 °C and 1.5 V in 70% CO2/30% CO atmosphere (116% enhancement over BSF). The synergistic effect of Nb5+ and Ta5+ co-doping arises from their similar ionic radii, stable high valence states, and electronegativity differences, which stabilize the perovskite lattice and facilitate oxygen migration, thereby optimizing oxygen reduction/evolution reaction (ORR/OER) activity and electrochemical performance. This work provides a rational design strategy for advanced RSOC air electrodes.
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