[en] Proton-conducting oxide materials are interesting objects from both fundamental and applied viewpoints due to the origination of protonic defects in a crystal structure as a result of their interaction with hydrogen-containing atmospheres at elevated temperatures. The high mobility of such defects at temperatures between 400 and 700 °C leads to superior ionic conductivity. As a result, some perovskite-type proton-conducting oxides have been proposed as electrolytes for solid oxide fuel and electrolysis cells. Barium cerate (BaCeO${}_3$), barium zirconate (BaZrO${}_3$), and barium cerate-zirconates (BaCeO${}_3$-BaZrO${}_3$) have been widely studied in terms of the parent phases of proton-conducting electrolytes. Among them, Y and Yb co-doped Ba(Ce,Zr)O${}_3$ can be identified as one of the most promising systems so far. This review discloses key functional properties of such phases and explains the increased attention of researchers to this system. (© 2023 Wiley‐VCH GmbH)

[en] Silicon and related materials have recently received considerable attention as potential anodes in Li-ion batteries for their high theoretical specific capacities. To overcome the problem of volume variations during the Li insertion/extraction process, in this work, Si/C composites with low carbon content were synthesized from cheap coarse silicon and citric acid by simple ball milling and subsequent thermal treatment. The effects of ball milling time and calcination temperature on the structure, composition and morphology of the composites were systematically investigated by the determination of specific surface area (BET) and particle-size distribution, X-ray diffraction (XRD), O₂-TPO, and scanning electron microscopy (SEM). The capacity and cycling stability of the composites were systematically evaluated by electrochemical charge/discharge tests. It was found that both the initial capacity and the cycling stability of the composites were dependent on the milling and calcination conditions, and attractive overall electrochemical performance could be obtained by optimizing the synthesis process.

[en] The formation mechanism of a spinel-type lithium titanate Li₄Ti₅O₁₂ with TiO₂ anatase as raw material, in both a conventional solid-state reaction (SSR) and a cellulose-assisted glycine-nitrate combustion (cellulose-GN) process are comparatively studied. XRD characterization demonstrates high-purity Li₄Ti₅O₁₂ forms at 750 ^oC by the cellulose-GN synthesis, which occurs at a temperature at least 100 ^oC lower than that via SSR. The solid-phase reaction between TiO₂ and lithium compounds to form Li-Ti-O spinel and the phase transition of TiO₂ from anatase to 'inert' rutile phase occur competitively during both synthesis processes. SEM results suggest that the solid precursor from the cellulose-GN process has a smaller particle size and a more homogenous mixing of the reactants than that in the SSR. Temperature-programmed oxidation experiments demonstrate that cellulose thermal pyrolysis creates a reducing atmosphere, which may facilitate the oxygen-ion diffusion. Both factors facilitate the formation of Li-Ti-O spinel, while the TiO₂ anatase transforms to TiO₂ rutile during the SSR, which has slow lithium-insertion kinetics. As a result, a high calcination temperature is necessary to obtain a phase-pure Li₄Ti₅O₁₂. Charge-discharge and EIS tests demonstrate the Li₄Ti₅O₁₂ obtained by the cellulose-GN process shows much better low-temperature electrochemical performance than that obtained by standard SSR. This improvement attributes to the reduced particle size due to the lower synthesis temperature.

[en] Pure-phase spinel-type lithium titanate, Li₄Ti₅O₁₂, was successfully fabricated by cellulose-assisted glycine-nitrate (cellulose-GN) combustion process at reduced temperature using anatase TiO₂ solid as raw material of titanium. The influence of cellulose and impregnation sequence on the phase purity, particle size and electrochemical performance of Li₄Ti₅O₁₂ was investigated. The sequence of preparation was found to have big effect on the phase formation and electrochemical performance of the oxides. High-purity and well-crystallized Li₄Ti₅O₁₂ oxides were obtained at a calcination temperature of 750 deg. C via the sequence III, for which the cellulose was first adsorbed by the mixed solution of LiNO₃ and glycine followed by the impregnation of TiO₂ suspension. Compared with solid-state reaction, the cellulose-GN process produced the Li₄Ti₅O₁₂ oxides with smaller particle size and higher specific capacity due to the lower synthesis temperature. A reversible capacity of 103 mAh/g at 20 C rate and fairly stable cycling performance even at 40 C was achieved.

[en] There is increasing interest in flexible, safe, high-power thin-film lithium-ion batteries which can be applied to various modern devices. Although TiO₂ in rutile phase is highly attractive as an anode material of lithium-ion batteries for its high thermal stability and theoretical capacity of 336 mA h g⁻¹ and low price, its inflexibility and sluggish lithium intercalation kinetics of bulk phase strongly limit its practical application for particular in thin-film electrode. Here we show a simple way to prepare highly flexible self-standing thin-film electrodes composed of mesoporous rutile TiO₂/C nanofibers with low carbon content (<15 wt.%) by electrospinning technique with outstanding electrochemical performance, which can be applied directly as electrodes of lithium-ion batteries without the further use of any additive and binder. The atmosphere during calcination plays a critical role in determining the flexible nature of thin film and particle size of TiO₂ in as-fabricated nanofibers. Big size (10 cm × 4 cm), flexible thin film is obtained after heat treatment under 10%H₂–Ar at 900 °C for 3 h. After optimization, the diameter of fibers can reach as small as ∼110 nm, and the as-prepared rutile TiO₂ films show high initial electrochemical activity with the first discharge capacity as high as 388 mA h g⁻¹. What is more, very stable reversible capacities of ∼122, 92, and 70 mA h g⁻¹ are achieved respectively at 1, 5 and 10 C rates with negligible decay rate within 100 cycling times.

[en] Co-free oxides with a nominal composition of LnBaFe₂O_5+δ, where Ln = La, Pr, Nd, Sm, Gd, and Y, were synthesized and phase structure, oxygen content, electronic conductivity, oxygen desorption, thermal expansion, microstructure and electrochemical performance were systematically investigated. Among the series of materials tested, LaBaFe₂O_5+δ oxide showed the largest electronic conductivity and YBaFe₂O_5+δ oxide had the smallest thermal expansion coefficient (TEC) of 14.6 × 10⁻⁶ K⁻¹ within a temperature range of 200–900 °C. All LnBaFe₂O_5+δ oxides typically possess the TEC values smaller than 20 × 10⁻⁶ K⁻¹. The oxygen content, electronic conductivity and TEC values are highly dependent on the cation size of the Ln³⁺ dopant. The lowest electrode polarization resistance in air under open circuit voltage condition was obtained for SmBaFe₂O_5+δ electrode and was approximately 0.043, 0.084, 0.196, 0.506 and 1.348 Ω cm² at 800, 750, 700, 650 and 600 °C, respectively. The SmBaFe₂O_5+δ oxide also demonstrated the best performance after a cathodic polarization. A cell with a SmBaFe₂O_5+δ cathode delivered peak power densities of 1026, 748, 462, 276 and 148 mW cm⁻² at 800, 750, 700, 650 and 600 °C, respectively. The results suggest that certain LnBaFe₂O_5+δ oxides have sufficient electrochemical performance to be promising candidates for cathodes in intermediate-temperature solid oxide fuel cells.

[en] Zn-air batteries (ZABs) have been recognized as one of the most efficient, cost effective and environmentally benign energy storage devices that may play an important role in future sustainable energy system. Air electrode is a key part of ZABs, where oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) appear during discharge and charge processes which largely determine the performance, such as durability, rate performance and round-trip efficiency. However, the sluggish kinetics of ORR/OER may cause high cell overpotential, thus requiring certain electrocatalyst to speed up both reactions. Based on several considerations, such as cost, activity, stability, and conductivity, metal-free carbon materials have received considerable attentions as electrocatalysts of air electrode in ZABs to boost both OER and ORR. In this review article, the recent progress in applying metal-free carbon materials as air electrode in ZABs is summarized. The two main ways to tune the properties of carbon materials and eventually the catalytic performance for ORR and OER, i.e., heteroatom doping (N, P, B, S, F, etc.) and defect engineering, are focused. The performance indicators of ZABs based on the carbon materials were also tabulated. Finally, the latest challenges and future opportunities associated with metal-free carbon materials as air electrode in ZABs are discussed.

[en] Three kinds of carbon conductive additives, i.e. multi-walled carbon nanotubes (MWNTs), vapour grown carbon fibres (VGCFs) and acetylene carbon blacks (AB), were investigated to improve the electrochemical performance of activated carbon (AC) used as electrode materials for supercapacitors. Galvanostatic charge/discharge and cyclic voltammetric measurements demonstrate that MWNTs are the most effective additive to improve the electrochemical performance of AC under the same conditions. To get the same results for AC in a symmetric supercapacitor, the desired additive amount was 3.0 wt% MWNTs, ∼5.0 wt% VGCFs and ∼9.0 wt% AB, respectively. X-ray diffraction analysis results demonstrate MWNTs have the sharpest (002) peak and highest graphitic degree. Scanning electron microscopy images show MWNTs have a vimineous fibre shape and VGCFs have a stubbed virgate shape. MWNTs run across AC particles and VGCFs distribute discretionarily among AC particles. High-resolution transmission electron microscopy demonstrates the microstructural difference between MWNTs and VGCFs. Some mechanisms were then developed to explain the different performance of the three kinds of carbon conductive additives

[en] (Ba_0.5Sr_0.5)_1+xCo_0.8Fe_0.2O_3-δ, or BSCF(1 + x), (0 ≤ x ≤ 0.3) oxides were synthesized and investigated as cathodes for intermediate-temperature solid-oxide fuel cells. The A-site cation excess in BSCF(1 + x) resulted in a lattice expansion and the creation of more active sites for oxygen reduction reaction due to the lowered valence states of the B-site ions and the increased oxygen vacancy concentration, which improved the oxygen adsorption process. On the other hand, the A-site excess could also result in higher resistances for oxygen adsorption (due to the formation of BaO and/or SrO impurities), and oxygen-ion transfer (by facilitating the solid-phase reaction between the cathode and the electrolyte). By taking all these factors into account, we found BSCF1.03 to be the optimal composition, which lead to a peak power density of 1026.2 ± 12.7 mW cm^-2 at 650 deg. C for a single cell

[en] New mixed conducting oxides with the composition of Sr_1-xY_xCo_1-yY_yO_3-δ (x = 0.0-0.8, y = 0.0-0.1) were exploited and synthesized. The resulted materials were investigated by X-ray diffraction, four-probe dc conductivity, temperature-programmed desorption characterization, and oxygen permeability measurement. As compared with the oxides with only one-site (A or B) being Y³⁺-doped, i.e., Sr_1-xY_xCoO_3-δ and SrCo_1-yY_yO_3-δ, the double-site Y³⁺-doped ones show improved phase stability, higher electrical conductivity under reduced atmosphere, and higher oxygen permeability and stability. Particularly, Sr_0.95Y_0.05Co_0.95Y_0.05O_3-δ oxide demonstrates stable cubic perovskite phase in air, oxygen and nitrogen, high electrical conductivity of ∼110 S cm^-1 in air and ∼50 S cm^-1 in nitrogen, and a maximum permeation flux of 1.35 x 10^-6 mol cm^-2 s^-1 at 900 deg. C under an air/helium gradient. Long-term permeation study at 850 deg. C indicates that Sr_0.95Y_0.05Co_0.95Y_0.05O_3-δ can operate stably as oxygen semi-permeable membrane