Zhihong Du

MoS2 Nanosheets Vertically Grown on Graphene Sheets for Lithium-Ion Battery Anodes

A designed nanostructure with MoS2 nanosheets (NSs) perpendicularly grown on graphene sheets (MoS2/G) is achieved by a facile and scalable hydrothermal method, which involves adsorption of Mo7O24(6-) on a graphene oxide (GO) surface, due to the electrostatic attraction, followed by in situ growth of MoS2. These results give an explicit proof that the presence of oxygen-containing groups and pH of the solution are crucial factors enabling formation of a lamellar structure with MoS2 NSs uniformly decorated on graphene sheets. The direct coupling of edge Mo of MoS2 with the oxygen from functional groups on GO (C-O-Mo bond) is proposed. The interfacial interaction of the C-O-Mo bonds can enhance electron transport rate and structural stability of the MoS2/G electrode, which is beneficial for the improvement of rate performance and long cycle life. The graphene sheets improve the electrical conductivity of the composite and, at the same time, act not only as a substrate to disperse active MoS2 NSs homogeneously but also as a buffer to accommodate the volume changes during cycling. As an anode material for lithium-ion batteries, the manufactured MoS2/G electrode manifests a stable cycling performance (1077 mAh g(-1) at 100 mA g(-1) after 150 cycles), excellent rate capability, and a long cycle life (907 mAh g(-1) at 1000 mA g(-1) after 400 cycles).

High-Performance Anode Material Sr2FeMo0.65Ni0.35O6−δ with In Situ Exsolved Nanoparticle Catalyst

A metallic nanoparticle-decorated ceramic anode was prepared by in situ reduction of the perovskite Sr2FeMo0.65Ni0.35O6-δ (SFMNi) in H2 at 850 °C. The reduction converts the pure perovksite phase into mixed phases containing the Ruddlesden-Popper structure Sr3FeMoO7-δ, perovskite Sr(FeMo)O3-δ, and the FeNi3 bimetallic alloy nanoparticle catalyst. The electrochemical performance of the SFMNi ceramic anode is greatly enhanced by the in situ exsolved Fe-Ni alloy nanoparticle catalysts that are homogeneously distributed on the ceramic backbone surface. The maximum power densities of the La0.8Sr0.2Ga0.8Mg0.2O3-δ electrolyte supported a single cell with SFMNi as the anode reached 590, 793, and 960 mW cm(-2) in wet H2 at 750, 800, and 850 °C, respectively. The Sr2FeMo0.65Ni0.35O6-δ anode also shows excellent structural stability and good coking resistance in wet CH4. The prepared SFMNi material is a promising high-performance anode for solid oxide fuel cells.

Investigation of In-doped BaFeO3−δ perovskite-type oxygen permeable membranes

Cobalt-free BaFe1−xInxO3−δ perovskites, with Fe partially substituted by indium at the B-site, were synthesized by a conventional solid state reaction and systematically characterized in terms of their phase composition, crystal structure, thermal reducibility, oxygen permeability, as well as structural stability in order to evaluate their application as oxygen permeation membranes. Introduction of more than 10 at.% of In into BaFe1−xInxO3−δ causes the formation of a single phase material with a cubic perovskite structure, which exhibits no phase transition during the cooling process. The thermal reducibility and thermal expansion coefficient are effectively reduced by indium doping, owing to the less changes of concentration of the oxygen vacancies in these compounds. However, the In occupying B-site breaks the B–O–B double exchange mechanism, and thus results in a gradual decrease of the electrical conductivity upon doping. Rietveld refinement and first principles calculation were performed to get an insight into the In influence on the lattice structure, oxygen migration energy and electron conduction behaviour of BaFe1−xInxO3−δ. When using He/Air as sweep/feed gas, the BaFe0.9In0.1O3−δ dense membrane with 1.0 mm thickness features a high oxygen permeation flux of 1.11 mL cm−2 min−1 at 950 °C. The observed good performance is attributed to the relatively high concentration of oxygen vacancies and low energy barrier for oxygen ion migration. It is also found that for membranes thinner than 0.8 mm, the oxygen flux is no longer limited by the bulk diffusion, while the oxygen surface exchange process becomes the dominant factor.

Evaluation of La0.3Sr0.7Ti1−xCoxO3 as a potential cathode material for solid oxide fuel cells

Perovskites La0.3Sr0.7Ti1−xCoxO3 (LSTCs, x = 0.3–0.6) are systematically evaluated as potential cathode materials for solid oxide fuel cells. The effects of Co substitution for Ti on structural characteristics, thermal expansion coefficients (TECs), electrical conductivity, and electrochemical performance are investigated. All of the synthesized LSTCs exhibit a cubic structure. With Rietveld refinement on the high-temperature X-ray diffraction data, the TECs of LSTCs are calculated to be 20–26 × 10−6 K−1. LSTC shows good thermal cycling stability and is chemically compatible with the LSGM electrolyte below 1250 °C. The substitution of Co for Ti increases significantly the electrical conductivity of LSTC. The role of doping on the conduction behavior is discussed based on defect chemistry theory and first principles calculation. The electrochemical performances of LSTC are remarkably improved with Co substitution. The area specific resistance of sample La0.3Sr0.7Ti0.4Co0.6O3 on the La0.8Sr0.2Ga0.8Mg0.2O3−δ (LSGM) electrolyte in symmetrical cells is 0.0145, 0.0233, 0.0409, 0.0930 Ω cm2 at 850, 800, 750 and 700 °C, respectively, and the maximum power density of the LSGM electrolyte (400 μm)-supported single cell with the Ni–GDC anode, LDC buffer layer and LSTC cathode reaches 464.5, 648, and 775 mW cm−2 at 850 °C for x = 0.3, 0.45, and 0.6, respectively. All these results suggest that LSTC are promising candidate cathode materials for SOFCs.

Design and synthesis of a 3-D hierarchical molybdenum dioxide/nickel/carbon structured composite with superior cycling performance for lithium ion batteries

Molybdenum dioxide is an attractive material for anodes of lithium ion batteries due to its high theoretical capacity, more than twice that of graphite. However, slow electrode reaction kinetics and structural degradation caused by large volume changes and phase separation during cycling hinder its practical application. To solve these problems, we design and fabricate a novel, 3-D hierarchical MoO2/Ni/C architecture by a combination of a hydrothermal method with chemical vapor deposition. The nickel nanoparticles are in situ formed and disperse uniformly with flower-like MoO2 particles, which are coated by thin carbon layers. The Ni particles act as a catalyst during the carbon coating process to promote the in situ growth of graphene in the carbon layer. Together, MoO2 and nickel nanoparticles, as well as amorphous carbon and graphene sheets build a 3-D hierarchical robust MoO2/Ni/C structure with a good electronically conductive network and lots of void space. Such a 3-D hierarchical structure combines multiple advantageous features, including an enhanced 3-D electronically conductive network, plenty of tunnels for electrolyte solution penetration, void space for volume change accommodation, and more surface areas for the electrode reaction. The manufactured MoO2/Ni/C composite exhibits a high reversible capacity, and excellent rate capability of 576 and 463 mA h g−1 at current densities of 100 and 1000 mA g−1, respectively. The excellent cycling performance is recorded with a capacity of 445 mA h g−1 maintained at 1000 mA g−1 after 800 cycles. The proposed synthesis process is simple and the design concept can be broadly applied, providing a novel, general approach towards manufacturing of metal oxide/metal/carbon (graphene) composites for high energy density storage or other electrochemical uses.

Novel cobalt-free BaFe1−xGdxO3−δ perovskite membranes for oxygen separation

A cobalt-free perovskite-type mixed ionic and electronic conductor (MIEC) is of technological and economic importance in many energy-related applications. In this work, a new group of Fe-based perovskite MIECs with BaFe1−xGdxO3−δ (0.025 ≤ x ≤ 0.20) compositions was developed for application in oxygen permeation membranes. Slight Gd doping (x = 0.025) can stabilize the cubic structure of the BaFe1−xGdxO3−δ perovskite. The Gd substitution of BaFe1−xGdxO3−δ materials increases the structural and chemical stability in the atmosphere containing CO2 and H2O, and decreases the thermal expansion coefficient. The BaFe0.975Gd0.025O3−δ membrane exhibits fast oxygen surface exchange kinetics and a high bulk diffusion coefficient, and achieves a high oxygen permeation flux of 1.37 mL cm−2 min−1 for a 1 mm thick membrane at 950 °C under an air/He oxygen gradient, and can maintain stability at 900 °C for 100 h. Compared to the pristine BaFeO3−δ and the well-studied Ba0.95La0.05FeO3−δ membranes, a lower oxygen permeation activation energy and higher oxygen permeability are obtained for the 2.5 at% Gd-doped material, which might be attributed to the expanded lattice by doping large Gd3+ cations and a limited negative effect from the strong Gd–O bond. A combination study of first principles calculation and experimental measurements was further conducted to advance the understanding of Gd effects on the oxygen migration behavior in BaFe1−xGdxO3−δ. These findings are expected to provide guidelines for material design of high performance MIECs.

(101) Plane-Oriented SnS2 Nanoplates with Carbon Coating: A High-Rate and Cycle-Stable Anode Material for Lithium Ion Batteries

Tin disulfide is considered to be a promising anode material for Li ion batteries because of its high theoretical capacity as well as its natural abundance of sulfur and tin. Practical implementation of tin disulfide is, however, strongly hindered by inferior rate performance and poor cycling stability of unoptimized material. In this work, carbon-encapsulated tin disulfide nanoplates with a (101) plane orientation are prepared via a facile hydrothermal method, using polyethylene glycol as a surfactant to guide the crystal growth orientation, followed by a low-temperature carbon-coating process. Fast lithium ion diffusion channels are abundant and well-exposed on the surface of such obtained tin disulfide nanoplates, while the designed microstructure allows the effective decrease of the Li ion diffusion length in the electrode material. In addition, the outer carbon layer enhances the microscopic electrical conductivity and buffers the volumetric changes of the active particles during cycling. The optimized, carbon coated tin disulfide (101) nanoplates deliver a very high reversible capacity (960 mAh g-1 at a current density of 0.1 A g-1), superior rate capability (796 mAh g-1 at a current density as high as 2 A g-1), and an excellent cycling stability of 0.5 A g-1 for 300 cycles, with only 0.05% capacity decay per cycle.

Effective Ca-doping in Y1-xCaxBaCo2O5+δ Cathode Materials for Intermediate Temperature Solid Oxide Fuel Cells

Ca-doping at the Y-site of Y1−xCaxBaCo2O5+δ (YCBC) double perovskites is shown as an effective strategy to develop a highly efficient, stable, lanthanide-free cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). The proposed Ca-doping has a beneficial influence on the structural stability, thermal expansion coefficient, electronic and ionic transport, and electrochemical properties of YCBC oxides. The phase stability and durability at evaluated temperature are greatly enhanced by Ca-doping. The thermal expansion coefficients of Y1−xCaxBaCo2O5+δ are calculated to be 18.1–18.7 × 10−6 K−1. At 800 °C, the conductivity is as high as 220 S cm−1 for the Y0.8Ca0.2BaCo2O5+δ sample. Area specific resistances as low as 0.010, 0.018, 0.032, 0.068 and 0.142 Ω cm2 at 850, 800, 750, 700 and 650 °C, respectively, are delivered by the Y0.8Ca0.2BaCo2O5+δ cathode in a La0.8Sr0.2Ga0.8Mg0.2O3−δ electrolyte supported symmetric cell. The maximum power densities of a full cell with the Y0.8Ca0.2BaCo2O5+δ/Ce0.9Gd0.1O2−δ composite cathode are registered to be 1066, 841, 634 and 430 mW cm−2 at 850, 800, 750 and 700 °C, respectively. All the results clearly demonstrate that Ca-doped Y1−xCaxBaCo2O5+δ double perovskites are highly stable and effectively working candidate cathodes for IT-SOFCs.

Novel ReBaCo1.5Mn0.5O5+δ (Re: La, Pr, Nd, Sm, Gd and Y) perovskite oxide: influence of manganese doping on the crystal structure, oxygen nonstoichiometry, thermal expansion, transport properties, and application as a cathode material in solid oxide fuel cells

In this work, a novel series of Mn-containing ReBaCo1.5Mn0.5O5+δ (Re: selected rare earth elements) perovskite-type oxides is studied, with systematic measurements of physicochemical properties being reported. Comparison with the very well-studied, parent ReBaCo2O5+δ allows determination of the role of the introduced manganese concerning modification of the crystal structure at room temperature and its evolution at high temperatures, variation of the oxygen content, thermal stability of the materials, and total electrical conductivity, as well as thermal and chemical expansion. Generally, the presence of Mn cations does not affect the tendency for A-site cation ordering, resulting in an increased unit cell volume of the compounds, as well as causing an increase of the oxygen content. Reduced thermal expansion, together with high values of electrical conductivity and suitable thermal stability, makes the compounds containing larger Re3+ cations attractive from the point of view of application as cathode materials in solid oxide fuel cells. Chemical compatibility studies reveal the sufficient stability of the considered perovskites in relation to Ce0.8Gd0.2O2−δ solid electrolyte, while unexpected, somewhat increased reactivity towards La0.8Sr0.2Ga0.8Mg0.2O3−δ and La0.4Ce0.6O2−δ is also reported. Furthermore, the electrochemical tests of the symmetric cells show strong dependence of the polarization resistance of the electrode on the synthesis and sintering temperatures. For the selected and optimized NdBaCo1.5Mn0.5O5+δ layer employed in the electrolyte-supported (LSGM) symmetric cell with a CGO buffer layer, the cathodic polarization resistance is 0.043 Ω cm2 at 900 °C. A wet hydrogen-fuelled button-type cell with the NdBaCo1.5Mn0.5O5+δ-based cathode is also prepared, delivering the maximum power density exceeding 1.3 W cm−2 at 850 °C.

Superior High-Rate and Ultralong-Lifespan Na3V2(PO4)3@C Cathode by Enhancing the Conductivity Both in Bulk and on Surface

Na3V2(PO4)3 has shown great promise in next-generation cathode materials for sodium-ion batteries owning to its fast Na+ diffusion in the three-dimensional open NASICON framework and high theoretical energy density. However, Na3V2(PO4)3 suffers from undesirable rate performance and unstable cyclability arising from low electronic conductivity. Herein, we propose a facile approach for significantly enhancing the electrochemical properties of Na3V2(PO4)3 by Ti doping at V site and constructing nanoparticle@carbon core-shell nanostructure. This material design provides fast electron conduction network within the whole active particles because of the mixed valence Ti4+/3+ in bulk and highly conductive carbon shell on the surface. Lattice doping and carbon coating reduce the electrode polarization and facilitate the electrode reaction kinetics, while the nanostructure enhances the ionic conduction by shortening the diffusion distance and offers sufficient contact of active particles with organic electrolyte. The multiple synergetic effects enable a superior electrochemical performance. The optimized Na3V1.9Ti0.1(PO4)3@C cathode shows a high specific capacity (116.6 mAh g-1 at 1C), an unprecedented rate performance (93.4 mAh g-1 at 400C), and an exceptional long-term high-rate cycling stability (capacity retention of 69.5% after 14 000 cycles at 100C, corresponding to 0.0002% decay per cycle).