Yuandao Chen

Hydrothermal synthesis of porous Co(OH)2 nanoflake array film and its supercapacitor application

Porous α-Co(OH)2 nanoflake array film is prepared by a facile hydrothermal synthesis method. The α-Co(OH)2 nanoflake array film exhibits a highly porous net-like structure composed of interconnected nanoflakes with a thickness of 15 nm. The pseudo-capacitive behaviour of the Co(OH)2 nanoflake array film is investigated by cyclic voltammograms (CV) and galvanostatic charge–discharge tests in 2 M KOH. The α-Co(OH)2 nanoflake array film exhibits high capacitances of 1017 F g − 1 at 2 A g − 1 and 890 F g − 1 at 40 A g − 1 as well as rather good cycling stability for supercapacitor application. The porous architecture is responsible for the enhancement of the electrochemical properties because it provides fast ion and electron transfer, large reaction surface area and good strain accommodation.

High performance Li4Ti5O12/CN anode material promoted by melamine–formaldehyde resin as carbon–nitrogen precursor

A carbon–nitrogen coating approach using melamine–formaldehyde resin as carbon–nitrogen source is introduced in this work with the aim of getting high rate Li4Ti5O12/CN composite. Li4Ti5O12/CN composite, particle size of 50–100 nm in diameter, is well dispersed and the carbon–nitrogen layers are 2 nm in thickness. The composite delivers much higher electrochemical performance than those of Li4Ti5O12. At 0.2 C and 10 C, it exhibits a discharge capacity of 172 mA h g−1 and 160 mA h g−1, respectively, and after 250 cycles at 10 C, 97.4% of its initial capacity is retained. The superior electrochemical performance can be attributed to the improved ionic and electronic conductivity in the electrode due to the uniform and ultrathin carbon–nitrogen coating layer.

New reduction mechanism of CO dimer by hydrogenation to C2H4 on a Cu(100) surface: theoretical insight into the kinetics of the elementary steps

A systematic DFT study that examines the role of the kinetics of the elementary reaction steps during the course of the reduction of a CO dimer, OCCO*, to C2H4 on Cu(100) is presented for the first time in the present study, and a new mechanism is introduced. Kinetic analysis of the elementary reaction steps has suggested that the further reduction of CO is the key selectivity-determining step for the formation of C2H4 and CH4 on Cu(100) and Cu(111), respectively. The main reaction pathway on Cu(111) proceeds through the reduction of CO to a CHO* intermediate, which may eventually result in CHx species by the breaking of a C–O bond and production of CH4. On Cu(100), OCCO* is first formed by CO dimerization, which is the first step and a more favorable pathway than the further hydrogenation of CO. This explains why only C2 species and not C1 species are observed experimentally on Cu(100). For the formation of C2H4 on Cu(100), the results suggest that the hydrogenation of OCCO* to the OCCHO* intermediate is the most likely reaction path, followed by the formation of intermediate OHCCHO* through further hydrogenation of the OCCHO* intermediate. The formation of OCCO* may be the rate-determining step in the reduction mechanism of the CO dimer. Kinetic analysis of the elementary steps gives a different mechanistic explanation for the selectivity of C2H4 production, which is in contrast to a previous suggested thermodynamic theoretical study on the reduction mechanisms of a CO dimer to C2H4. This present reduction pathway is consistent with the latest experimental results and explains the experimental uncertainty regarding the reaction intermediates. At present, it appears that the mechanism proposed in this study is most agreeable with the present experimental results.

Mechanical Properties of Elastomeric Terpolymer Composites Containing Carbon Nanotubes

Nanocomposites were obtained by mixing terpolymer and carbon nanotubes (CNTs) in a twin-screw extruder at 135–155°C. The results indicated that increasing the amount of PSt, the molecular weight of PSt macromonomer of terpolymers and CNTs charged resulted in improving the tensile strength and modulus at 300% elongation of nanocomposites.

Na+ and Zr4+ co-doped Li4Ti5O12 as anode materials with superior electrochemical performance for lithium ion batteries

Na+ and Zr4+ co-doped lithium titanates with the formula Li4−xNaxTi5−xZrxO12 (x = 0, 0.01, 0.03, 0.05, 0.10, 0.15 and 0.20) were successfully synthesized via a solid-state reaction in air. XRD analysis indicates that Na+ and Zr4+ co-doping does not change the spinel structure of Li4Ti5O12 and the lattice parameter slightly increases with the enhancement of doping amount. Smaller particle size and larger BET surface areas are obtained for Na+ and Zr4+ co-doped Li4Ti5O12 samples. The four-point probe method and electrochemical impedance spectroscopy (EIS) results demonstrate that the Na+ and Zr4+ co-doped Li4Ti5O12 samples possess higher electrical conductivity and smaller charge transfer resistance compared with undoped Li4Ti5O12, resulting in improved electrochemical performance. Particularly, Li3.97Na0.03Ti4.97Zr0.03O12 exhibits the best rate capability and cycling stability. Even at 20C, it delivers a discharge capacity of 140 mA h g−1, and after 100 cycles at 10C, 97.7% of its initial capacity is retained.

Theoretical insight into effect of doping of transition metal M (M = Ni, Pd and Pt) on CO2 reduction pathways on Cu(111) and understanding of origin of electrocatalytic activity

The effect of the doped transition metal M (M = Ni, Pd and Pt) on CO2 reduction pathways and the origin of the electrocatalytic activity are investigated systematically by focusing on the CH4 and CH3OH formation pathways based on DFT calculations associated with the computational hydrogen electrode model. Our studies show that the doping of Ni, Pd and Pt can promote CO2 reduction into hydrocarbons and influence the selectivity of reduction pathways, in which the doping of Pt may be able to lead to the strongest catalytic activity. The adsorption behavior between reaction intermediates and surfaces is crucial and the interactions of intermediates with the catalysts should be moderate in order to efficiently catalyze CO2 reduction into CH4 and CH3OH, and avoid OH surface poisoning. The enhanced electrocatalytic activity of transition metal-doped Cu(111) surfaces may be owing to decreased overpotential and moderate electronic interactions between Cu and the doped transition metals. The doped Ni, Pd and Pt atoms can considerably decrease the overpotential and remove surface OH poisoning, in which the doped Pt can simultaneously reduce overpotential for CO formation and further reduction, and most easily remove OH, thus suggesting the best electrocatalytic activity. The moderate electron interaction between Cu and Pt and moderate upshift of the d-band center of Pt also explain why the Pt-doped Cu(111) surface has the best electrocatalytic activity for CO2 reduction. Two possible descriptors can be proposed in order to scale the electrocatalytic activity of Cu-based electrocatalysts for CO2 reduction, in which an ideal Cu-based electrocatalyst should be able to reduce barriers for CO formation and further reduction, and should have moderate electron interactions between Cu and the doped transition metals, and a moderate upshift of d-band center of the doped transition metals. In these ways, CO2 reduction pathways can be facilitated and the yield of hydrocarbons CH4 and CH3OH can be enhanced.

Theoretical insights into the alkaline metal M (M = Na and Cs) promotion mechanism for CO2 activation on the Cu(111) surface

A systematic study on the alkaline metal M (M = Na and Cs) promotion mechanism for CO2 activation on the Cu(111) surface is presented for the first time based on self-consistent density functional theory calculations. The results show that CO2 adsorbs weakly and molecularly on a clean Cu(111) surface, whereas a drastic influence on the bonding, structure, and reactivity of adsorbed CO2 through the formation of CO2δ− radical anions was exerted by the presence of alkaline metals Na and Cs adatoms on the Cu(111) surface. This is the main physical origin of alkaline metals Na and Cs promotion in CO2 activation. The Na and Cs adatoms lower the dissociation activation barrier of CO2, in which the effect of Cs on CO2 dissociation is significantly larger than that of Na. Thus, the promotion effect of alkaline metals for the CO2 reduction into hydrocarbons on Cu catalysts can be attributed to a reduction of the dissociation activation barrier of CO2. The results also show that the origin of this promotion effect is predominately a direct electronic interaction between the Na and Cs-promoted Cu(111) surface and CO2 molecules. The presence of Na and Cs on the Cu(111) resulted in a decrease in the work function of the surface. The reduced amount of the work function on the Cs-promoted Cu(111) surface is more than that on the Na-promoted Cu(111) surface, explaining why the dissociation activation barrier of CO2 is lower on the Cs-promoted Cu(111) surface. The strong work function decrease of Na and Cs-promoted Cu(111) surface is corroborated by the presence of charge transfer. The charge transfer leads to the formation of a partially negative species, CO2δ−, which can be ascribed to the enhancement of back-donation of electrons from Na and Cs-promoted Cu(111) surface into an empty π orbital of CO2 compared to that on the clean Cu(111) surface. However, the back-donation mechanism of electrons is different between the Na and Cs-promoted Cu(111) surfaces, in which Na is an effective electron donor, whereas Cs is an electron acceptor, thus leading to the difference between promoting mechanisms of alkaline metals Na and Cs on CO2 activation. Cu as an electron donor in the Cs-promoted Cu(111) surface may result in a more reduced amount of the work function of Cs-promoted Cu(111) surface.