Chunyu Zhu

Microencapsulation of Metal-based Phase Change Material for High-temperature Thermal Energy Storage

Latent heat storage using alloys as phase change materials (PCMs) is an attractive option for high-temperature thermal energy storage. Encapsulation of these PCMs is essential for their successful use. However, so far, technology for producing microencapsulated PCMs (MEPCMs) that can be used above 500°C has not been established. Therefore, in this study, we developed Al-Si alloy microsphere MEPCMs covered by α-Al2O3 shells. The MEPCM was prepared in two steps: (1) the formation of an AlOOH shell on the PCM particles using a boehmite treatment, and (2) heat-oxidation treatment in an O2 atmosphere to form a stable α-Al2O3 shell. The MEPCM presented a melting point of 573°C and latent heat of 247 J g−1. The cycling performance showed good durability. These results indicated the possibility of using MEPCM at high temperatures. The MEPCM developed in this study has great promise in future energy and chemical processes, such as exergy recuperation and process intensification.

Cotton derived porous carbon via an MgO template method for high performance lithium ion battery anodes

Porous carbon has received great attention for electrochemical energy storage devices. In this study, we proposed a novel and scalable method to fabricate porous carbon, which contained macro and mesopores, from sustainable biomass raw material of cotton cellulose. A MgO template, which acted as a pore creator, was incorporated into the cellulose-derived carbon by absorbing a Mg(NO3)2 solution into cellulose fibers with subsequent drying and carbonization processes. After removing the MgO template by acid leaching, porous carbon was produced with a specific surface area as high as 1260 m2 g−1. The sample showed attractive electrochemical performances as the anode material for Li ion batteries (LIBs). The carbon anode delivered a high reversible capacity of 793 mA h g−1 at a current density of 0.5 A g−1 after 500 cycles. The carbon anode also showed a high-rate capability and a capacity of 355 mA h g−1 can be obtained at a current density of 4 A g−1. A widespread comparison with the literature also showed that the cotton-derived porous carbon was among the most promising carbon-based anodes for LIBs.

A new CaCO3-template method to synthesize nanoporous manganese oxide hollow structures and their transformation to high-performance LiMn2O4 cathodes for lithium-ion batteries

This paper presents a new CaCO3-template synthesis of highly nanoporous manganese oxide hollow structures and their transformation to high-performance LiMn2O4 cathodes for lithium-ion batteries via the facile coprecipitation of Mn–Ca-carbonates and temperature-controlled decomposition of MnCO3 and CaCO3, followed by the selective removal of the carbonates by washing with HCl. The as-prepared Mn2O3 nanostructures showed very high specific surface area with their subunit particle size of 100 nm. The transformation to uniformly porous LiMn2O4 hollow structures was successfully achieved by the facile impregnation of LiOH into the porous Mn2O3 hollow precursors, followed by a conventional solid-state reaction. The LiMn2O4 hollow structures deliver a discharge capacity of about 120 mA h g−1 at a 1 C rate and 115 mA h g−1 at a 10 C rate with excellent cycling stability. The capacity retention approached 94% after up to 800 cycles of charging–discharging at a 10 C rate.

MnO nanoparticles embedded in a carbon matrix for a high performance Li ion battery anode

Manganese oxides are promising anode materials for lithium ion batteries based on conversion reactions. In this paper, MnO nanoparticles that were embedded in a carbon matrix were directly produced by a facile glycine–nitrate-based solution combustion synthesis (SCS) process with subsequent calcination treatment under an inert atmosphere. The effect of the amount of glycine used in the SCS process and the calcination temperature on the composite products as well as their electrochemical properties were investigated. The carbon content in the composite can be controlled by changing the amount of glycine, while the crystallinity, and morphology of the MnO particles, phase composition, and the characteristics of the carbon materials were quite dependent on the calcination temperature. The sample calcined at 700 °C with a composite carbon content of around 27.7% provided the best electrochemical performance. This sample delivered a reversible specific capacity of 437.6 mA h g−1 at a high current density of 500 mA g−1 after 300 cycles. The enhanced electrochemical properties can be ascribed to the formation of a MnO nanoparticle/carbon composite. The carbon matrix offered a connected structure for fast Li ion and electron transportation, and worked as a buffer to accommodate the volume change upon lithium insertion/extraction.

Designed synthesis of LiNi0.5Mn1.5O4 hollow microspheres with superior electrochemical properties as high-voltage cathode materials for lithium-ion batteries

LiNi0.5Mn1.5O4 hollow microspheres with designed subunits as high-voltage cathode materials for lithium-ion batteries were synthesized using dense or hollow Mn2O3 porous-spheres as self-templates. Dense Mn2O3 porous-spheres with mesosized subunits (>100 nm) were prepared by the direct decomposition of microspherical MnCO3, while the hollow and porous Mn2O3 microspheres with nanosized subunits (<30 nm) were synthesized by temperature-controlled decomposition of Mn–Ca bicarbonates followed by the selective removal of carbonates with HCl. Through a solid state reaction between Li/Ni precursors with meso/nanoporous Mn2O3, LiNi0.5Mn1.5O4 hollow spheres with different cavity size and wall structure were prepared. The as-synthesized hollow microspheres exhibited superior cyclability and high-rate capability. The best sample delivered a high and reversible discharge capacity of around 130 mA h g−1 with a capacity retention efficiency of 98.6% after 60 cycles at 1 C rate. The sample also showed high reversible capacities of 100.5 mA h g−1 even at a high current rate of 5 C. As a comparison, LiNi0.5Mn1.5O4 powders were also produced by a conventional solid state process using ball-milled Li–Ni–Mn hydroxide-oxide precursors, which showed low capacities of around 110 mA h g−1 at 1 C and greatly degraded capacities at higher current rates.

Improved electrochemical properties of LiMn2O4 with the Bi and La co-doping for lithium-ion batteries

A series of LiBixLaxMn2−2xO4 (x = 0, 0.002, 0.005, 0.010, 0.020) samples were synthesized by solution combustion synthesis in combination with calcination. The phase structure and morphology of the products were characterized by X-ray diffraction, scanning electron microscopy, and transition electron microscopy. The results demonstrated that a single-phase LiMn2O4 spinel structure was obtained for the LiBixLaxMn2−2xO4 (x = 0, 0.002, 0.005) samples, whereas impurities were observed for the LiBixLaxMn2−2xO4 (x = 0.010, 0.020) samples as a result of the doping limit. The electrochemical properties were investigated by galvanostatic charge–discharge cycling and cycling voltammetry in a voltage range of 3.2–4.4 V. The substitution of Mn3+ by equimolar Bi3+ and La3+ could significantly improve the structural stability and suppress the Jahn–Teller distortion, thereby resulting in improved electrochemical properties for the Bi and La co-doped samples in contrast with the pristine LiMn2O4 sample. In particular, the LiBi0.005La0.005Mn1.99O4 sample delivered a high initial discharge capacity of 130.2 mA h g−1 at 1C, and following 80 cycles, the capacity retention was as high as 95.0%. Moreover, it also presented the best rate capability among all the samples, in which a high discharge capacity of 98.3 mA h g−1 was still maintained at a high rate of 7C compared with that of 75.8 mA h g−1 for the pristine LiMn2O4 sample.

MnO nanocrystals incorporated in a N-containing carbon matrix for Li ion battery anodes

In this study, MnO nanocrystals incorporated in a N-containing carbon matrix were fabricated by the facile thermal decomposition of manganese nitrate-glycine gels. MnO/C composites with different carbon contents were prepared by controlling the initial ratio of manganese to glycine. The composition, phase structure and morphology of the composites were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning and transmission electron microscopy, and thermogravimetric analysis. The results indicated that MnO nanocrystals were uniformly embedded in the N-doped carbon matrix. The carbon matrix could effectively enhance the electrical conductivity of MnO and alleviate the strain arising from the discharge/charge cycling. The composite materials exhibited high discharge/charge capacities, superior cycling performance, and excellent rate capability. A high reversible capacity of 556 mA h g−1 was obtained after 110 cycles of discharging and charging at a current rate of 0.5 A g−1. Even at a high current rate of 3 A g−1, the sample still delivered a capacity of around 286 mA h g−1. The easy production and superior electrochemical properties enables the composites to be a promising candidate as an anode alternative for high-performance lithium-ion batteries.

Controlled synthesis of LiNi0.5Mn1.5O4 cathode materials with superior electrochemical performance through urea-based solution combustion synthesis

High-voltage LiNi0.5Mn1.5O4 cathode materials were synthesized using urea-based solution combustion synthesis combined with a calcination treatment. The morphology and particle size distribution of the products were considerably dependent on the amount of urea fuel. The electrochemical characterization illustrated that the sample that was produced with a fuel ratio of ϕ = 0.5 had a homogenous particle size distribution of approximately 8 μm, and showed the best cycling and rate performance. LiNi0.5Mn1.5O4 with two different structures of disordered Fdm and ordered P4332 were obtained by controlling the calcination process. The samples, which were calcined at 800 °C with fast cooling, presented a disordered structure of Fdm, and the samples, which were calcined at 800 °C with slow cooling and reannealing at 600 °C, demonstrated an ordered structure of P4332. The sample with a disordered structure exhibited a better electrochemical performance than the sample with an ordered structure. The disordered sample produced at ϕ = 0.5 presented a discharge capacity of 130.73 mA h g−1 and a capacity retention of 98.43% after 100 cycles at 1 C. Even at a higher current rate of 3 C, the sample still showed a high discharge capacity of 117.79 mA h g−1 and a capacity retention efficiency of 97.63% after 300 cycles.

Low-Temperature Oxygen Storage of CrIV–CrV Mixed-Valence YCr1–xPxO4−δ Driven by Local Condensation around Oxygen-Deficient Orthochromite

The oxygen storage capability and related defect structure of tetrahedral orthochromite(V) compound YCr1-xPxO4 (x = 0, 0.3, 0.5, and 0.7) were investigated by employing thermal gravimetry and in situ X-ray spectroscopy for reversible oxygen store/release driven by heating-cooling cycles in the temperature range from 50 to 600 °C. YCr1-xPxO4 started releasing oxygen as heated from 50 °C under ambient atmosphere, with reduction of CrV to CrIV, while the reduced YCr1-xPxO4-δ phase was significantly reoxidized via absorbing oxygen by cooling to 50 °C under ambient atmosphere, recovering the original stoichiometric phase. Operando X-ray adsorption spectroscopy and first-principles calculations demonstrate that nonstoichiometric YCr1-xPxO4-δ phases were stabilized by forming linking polyhedral CrIV2O76- via corner sharing between oxygen-deficient CrIVO32- and adjacent CrIVO44-. YCr1-xPxO4 was found to have an extremely low reduction enthalpy of about 20 kJ mol-1 probably due to the relatively high reduction potential of high-valence-state Cr(V)/Cr(IV) redox pairs, thereby resulting in reversible oxygen storage in such a low-temperature region.

Evaluation of thin film fuel cells with Zr-rich BaZrxCe0.8−xY0.2O3−δ electrolytes (x ≥ 0.4) fabricated by a single-step reactive sintering method

This paper reports a survey of power generation characteristics of anode-supported thin film fuel cells with Zr-rich BaZrxCe0.8−xY0.2O3−δ (x = 0.4, 0.6, 0.7, and 0.8) proton-conducting electrolytes, which were fabricated by single step co-firing with Zn(NO3)2 additives at a relatively low temperature (1400 °C). The grain sizes significantly increased to several μm for x = 0.4 and 0.6, whereas the grain sizes remained in the sub-μm ranges for x = 0.7 and 0.8, which resulted in large gaps of the fuel cell performances at x over and below 0.6. The cells for x = 0.4 and 0.6 exhibited efficient power generation, yielding peak powers of 279 and 336 mW cm−2 at 600 °C, respectively, which were higher than those of the corresponding cells previously reported. However, the performances abruptly deteriorated with the increasing x to more than 0.7 because the electrolyte films were highly resistive due to the coarse-grained microstructures. Impedance spectroscopy for the dense sintered BaZrxCe0.8−xY0.2O3−δ discs confirmed that the total proton conductivity of BaZr0.6Ce0.2Y0.2O3−δ was higher than that of BaZr0.4Ce0.4Y0.2O3−δ at temperatures above 500 °C despite relatively small grain sizes. In addition, BaZr0.6Ce0.2Y0.2O3−δ cells could gain a stable current throughout a continuous run for a few days under CO2-containing fuel supply, which was due to high fraction of thermodynamically stable BaZrO3 matrices. It was demonstrated that BaZr0.6Ce0.2Y0.2O3−δ is a promising electrolyte for proton-conducting ceramic fuel cells with excellent proton conductivity and CO2 tolerance at intermediate temperatures.