ABSTRACT
A novel, facile, non-hazardous, low temperature/pressure microwave solvothermal method of producing pure copper, silver, and nickel metal nanofoams is presented. The nanofoams have been produced using inexpensive metal acetates and polyglycol solvent. The nanofoam formation proceeds in two steps within a single-pot synthesis: formation of metal nanoparticles, followed by the sintering of nanoparticles into nanofoams. The nanofoams have many potential uses in clean energy applications, particularly lithium-ion batteries.
ABSTRACT
Herein, we report the synthesis, structure, and electrochemistry of the first Na(+)-ion cathode with two distinct types of polyanions: Fe3P5SiO19. The Fe-based cathode has a reversible capacity of ca. 70 mA h g(-1); ca. 1.7 Na(+) ions per formula can be inserted/extracted at an average voltage of 2.5 V versus Na(+)/Na.
ABSTRACT
A series of carbon-supported core-shell nanoparticles with Pd(x)Cu(y)-rich cores and Pt-rich shells (Pt@Pd(x)Cu(y)/C) has been synthesized by a polyol reduction of the precursors followed by heat treatment to obtain the Pd(x)Cu(y)/C (1 ≤ x ≤ 3 and 0 ≤ y ≤ 5) cores and the galvanic displacement of Pd(x)Cu(y) with [PtCl(4)](2-) to form the Pt shell. The nanoparticles have also been investigated with respect to the oxygen reduction reaction (ORR) in proton-exchange-membrane fuel cells (PEMFCs). X-ray diffraction (XRD) analysis suggests that the cores are highly alloyed and that the galvanic displacement results in a certain amount of alloying between Pt and the underlying Pd(x)Cu(y) alloy core. Transmission electron microscopy (TEM) images show that the Pt@Pd(x)Cu(y)/C catalysts (where y > 0) have mean particle sizes of <8 nm. Compositional analysis by energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) clearly shows Pt enrichment in the near-surface region of the nanoparticles. Cyclic voltammograms show a positive shift of as much as 40 mV for the onset of Pt-OH formation in the Pt@Pd(x)Cu(y)/C electrocatalysts compared to that in Pt/C. Rotating disk electrode (RDE) measurements of Pt@PdCu(5)/C show an increase in the Pt mass activity by 3.5-fold and noble metal activity by 2.5-fold compared to that of Pt/C. The activity enhancements in RDE and PEMFC measurements are believed to be a result of the delay in the onset of Pt-OH formation.
ABSTRACT
Pt-encapsulated Pd(x)Co(100-x) nanoalloy electrocatalysts supported on carbon have been synthesized by a rapid microwave-assisted solvothermal (MW-ST) method within 15 min at as low as 300 degrees C. Subsequently, the samples have been heat treated at 900 degrees C in a reducing gas atmosphere to obtain Pt-Pd-Co nanoalloys. X-ray diffraction (XRD) analysis of the as-synthesized and 900 degrees C heat-treated samples reveals interesting changes in phase compositions and degree of alloying with Co and Pt contents and heat treatment. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) data of the as-synthesized samples confirm Pt enrichment on the surface of the Pd-Co nanoparticles. Rotating disk electrode (RDE) and single cell proton exchange membrane fuel cell measurements reveal that the as-synthesized Pt-encapsulated Pd(80)Co(20) (i.e., 75 wt % Pd(80)Co(20) + 25 wt % Pt) with 20 wt % total metal loading on carbon or 5 wt % Pt exhibit higher catalytic activity for the oxygen reduction reaction (ORR) compared to Pt with 20 wt % Pt loading on carbon. Significant changes in the catalytic activity for ORR occur on heat treatment at 900 degrees C as a result of changes in the phase composition and increase in particle size. This study demonstrates that the encapsulation of Pd-Co alloys with Pt offers a significant enhancement in activity for ORR per unit mass of Pt, offering a significant cost savings.
ABSTRACT
We demonstrate an efficient and rapid microwave irradiated solvothermal method to prepare nanostructured lithium metal phosphates LiMPO(4) (M = Mn, Fe, Co, and Ni) within a short reaction time (5-15 min) at temperatures as low as 300 degrees C without requiring any post annealing at elevated temperatures. The highly viscous, high-boiling tetraethyleneglycol used as the solvent not only provides a reducing atmosphere to prevent the oxidation of M(2+) to M(3+) but also inhibits the agglomeration of the nanoparticles formed. The enhanced reaction rates facilitated by the dielectric volumetric heating of the microwave absorbing reactants led to the formation of highly crystalline, phase-pure LiMPO(4) powders. The samples are characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy (TEM), and electrochemical measurements in lithium cells. High-resolution TEM studies reveal the formation of single-crystalline LiMPO(4) with nano-thumblike shapes. The dimensionally modulated nano-thumblike shapes with the lithium diffusion direction (b axis) along the shorter dimension are particularly beneficial to achieve high-power capability in lithium ion cells. Subsequent networking of the single-crystalline LiMPO(4) nano-thumps with multiwalled carbon nanotubes by a simple solution-based mixing at ambient temperatures to overcome the electronic conductivity limitations offers excellent electrochemical performance in lithium ion cells.
ABSTRACT
Reaction of nickel chloride with sodium dithionite in aqueous solutions at ambient temperature has been investigated systematically to obtain nickel sulfides. The products are characterized by X-ray diffraction, thermogravimetric analysis, and electrical resistivity and magnetic susceptibility measurements. It is found that the compositions and structures of the products are controlled by the reaction pH and the amount of the reactants. While reactions under highly acidic (pH < or = 2) and basic (pH > or = 7) conditions yield crystalline sulfur and amorphous or poorly crystalline NiySx, respectively, those at intermediate 3 < or = pH < or = 6 give crystalline NiySx. Both crystalline Ni3S2 (heazlewoodite structure) and Ni3S4 (spinel structure) have been obtained at room temperature. Additionally, NiS (millerite structure) is obtained by carefully heating Ni3S4 at 200 degrees C in a mixture of 90% Ar and 10% H2. Ni3S4 is found to be metastable, and it begins to disproportionate above 100 degrees C. Both Ni3S2 and Ni3S4 show metallic behavior. While Ni3S2 exhibits temperature-independent magnetic susceptibility, Ni3S4 shows ferrimagnetic ordering below 20 K.
ABSTRACT
A systematic investigation of the reduction of aqueous Na(2)WO(4) with aqueous NaBH(4) at ambient temperatures reveals the formation of several lower valent tungsten oxides such as the tetragonal (x < 0.38) and cubic (x > 0.43) tungsten bronzes Na(x)()WO(3) and the binary oxides WO(2) and W(24)O(68). The nature of the product formed is influenced both by the (i) reducing power of NaBH(4), which is controlled by the volume and concentration of the borohydride and the reaction pH, and (ii) the degree of condensation of the tungstate ions, which is controlled by the reaction pH. Although the reducing power of NaBH(4) increases with decreasing pH, an increasing degree of condensation of the tungstate tends to lower the degree of reduction in many instances. The as-prepared samples are amorphous as revealed by X-ray diffraction and crystallize around 450 degrees C as revealed by differential scanning calorimerty. The tungsten bronzes undergo interesting crystal-chemical changes with the temperature of heating.
ABSTRACT
Reduction of aqueous K(2)MoO(4) with aqueous KBH(4) at ambient temperatures has been investigated systematically to obtain lower valent molybdenum oxides. Several lower valent oxides such as MoO(2), Mo(4)O(11), K(0.26)MoO(3) (red bronze), K(0.30)MoO(3) (blue bronze), and K(0.85)Mo(6)O(17) are formed during the reduction process; however, only MoO(2) has been obtained as single-phase product. The nature of the product formed is strongly influenced by the reducing power of KBH(4). The reducing power increases with decreasing pH or increasing concentration and volume of KBH(4). The as-prepared samples are amorphous as revealed by X-ray diffraction and transmission electron microscopy. They crystallize sharply at around 350-500 degrees C as revealed by differential scanning calorimetry. Since the products formed are amorphous in nature, they may become particularly attractive for battery electrodes and catalysis.
