gui Li

Mesoporous N-Doped Carbons Prepared with Thermally Removable Nanoparticle Templates: An Efficient Electrocatalyst for Oxygen Reduction Reaction

Thermally removable nanoparticle templates were used for the fabrication of self-supported N-doped mesoporous carbons with a trace amount of Fe (Fe-N/C). Experimentally Fe-N/C was prepared by pyrolysis of poly(2-fluoroaniline) (P2FANI) containing a number of FeO(OH) nanorods that were prepared by a one-pot hydrothermal synthesis and homogeneously distributed within the polymer matrix. The FeO(OH) nanocrystals acted as rigid templates to prevent the collapse of P2FANI during the carbonization process, where a mesoporous skeleton was formed with a medium surface area of about 400 m(2)/g. Subsequent thermal treatments at elevated temperatures led to the decomposition and evaporation of the FeO(OH) nanocrystals and the formation of mesoporous carbons with the surface area markedly enhanced to 934.8 m(2)/g. Electrochemical measurements revealed that the resulting mesoporous carbons exhibited apparent electrocatalytic activity for oxygen reduction reactions (ORR), and the one prepared at 800 °C (Fe-N/C-800) was the best among the series, with a more positive onset potential (+0.98 V vs RHE), higher diffusion-limited current, higher selectivity (number of electron transfer n > 3.95 at +0.75 V vs RHE), much higher stability, and stronger tolerance against methanol crossover than commercial Pt/C catalysts in a 0.1 M KOH solution. The remarkable ORR performance was attributed to the high surface area and sufficient exposure of electrocatalytically active sites that arose primarily from N-doped carbons with minor contributions from Fe-containing species.

Improving performance of polymer photovoltaic devices using an annealing-free approach via construction of ordered aggregates in solution

Low crystalline order has been proved to be one of the main hindrances for achieving high performance devices based on thin films composed of crystallizable polymer. In this work, we use a facile method to substantially improve crystallinity of poly(3-hexylthiophene) (P3HT) in its pure or composite film via the construction of ordered precursors in the solution used for thin film deposition. These improvements have been confirmed by bright-field transmission electron micrography, electron diffraction, UV-Vis absorption and wide-angle X-ray diffraction. The electrical conductivity of thus obtained P3HT films is increased by almost two orders of magnitude. Polymer solar cells based on P3HT:PCBM ([6,6]-phenyl C61 butyric acid methyl ester) composite fabricated using this method achieve power conversion efficiencies (PCEs) as high as 3.9%, which is almost four times that of pristine devices and also higher than thermally-annealed devices under the same measurement conditions. This simple method paves the way for the fabrication of high performance devices with an “annealing-free” approach, and enriches the ways to improve crystalline order in thin films comprising crystallizable polymers.

Three-Dimensional Hierarchical Frameworks Based on MoS2 Nanosheets Self-Assembled on Graphene Oxide for Efficient Electrocatalytic Hydrogen Evolution

Advanced materials for electrocatalytic water splitting are central to renewable energy research. In this work, three-dimensional (3D) hierarchical frameworks based on the self-assembly of MoS2 nanosheets on graphene oxide were produced via a simple one-step hydrothermal process. The structures of the resulting 3D frameworks were characterized by using a variety of microscopic and spectroscopic tools, including scanning and transmission electron microscopies, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman scattering. Importantly, the three-dimensional MoS2/graphene frameworks might be used directly as working electrodes which exhibited apparent and stable electrocatalytic activity in hydrogen evolution reaction (HER), as manifested by a large cathodic current density with a small overpotential of -107 mV (-121 mV when loaded on a glassy-carbon electrode) and a Tafel slope of 86.3 mV/dec (46.3 mV/dec when loaded on a glassy-carbon electrode). The remarkable performance might be ascribed to the good mechanical strength and high electrical conductivity of the 3D frameworks for fast charge transport and collection, where graphene oxide provided abundant nucleation sites for MoS2 deposition and oxygen incorporation led to the formation of defect-rich MoS2 nanosheets with active sites for HER.

MoO2 nanobelts@nitrogen self-doped MoS2 nanosheets as effective electrocatalysts for hydrogen evolution reaction

Advanced materials for electrocatalytic water splitting are central to renewable energy research. In this study, MoO2 nanobelts@nitrogen self-doped MoS2 nanosheets are produced by nitridation and sulfuration treatments of MoO3 nanobelts. The material structures are characterized by a variety of techniques including scanning electron microscopy, transmission electron microscopy, Raman scattering, X-ray photoelectron spectroscopy, and X-ray diffraction spectroscopy. It is found that because of nitrogen doping and the abundance of exposed active edges, the heterostructures exhibit high electronic conductivity, and more importantly, enhanced and stable electrocatalytic activity in hydrogen evolution reaction (HER), as manifested in electrochemical studies. The onset potential is found to be only −156 mV (vs. RHE), which is 105 mV more positive than that of pure MoS2 under identical experimental conditions. The corresponding Tafel slope is estimated to be 47.5 mV dec−1, even slightly less than that of commercial 10 wt% Pt/C (49.8 mV dec−1), suggesting that the reaction dynamics is largely determined by the electrochemical desorption of hydrogen. This is accounted for by nitrogen doping that leads to an enhanced electronic conductivity of the heterostructures as well as a high density of spinning electron states around the N and Mo atoms in MoS2 nanosheets that are the active sites for HER, as manifested in density functional theory studies of a N-doped MoS2 monolayer.

Precise construction of PCBM aggregates for polymer solar cells via multi-step controlled solvent vapor annealing

Polymer solar cells based on poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric-acid methyl ester (P3HT/PCBM) composite are one of state-of-the-art polymer photovoltaic devices in terms of performance. In this work, we applied two-step controlled solvent vapor annealing (C-SVA) to achieve an optimized morphology for the photoactive layer with both an appropriate size of PCBM aggregates and an improved crystallinity of P3HT. As revealed by bright-field transmission electron microscopy (TEM), and atomic force microscopy (AFM), X-ray diffraction (XRD) and UV-Vis spectroscopy, PCBM forms aggregates with sizes of ca. 30 nm during the first step C-SVA in tetrahydrofuran vapor. The second step treatment using carbon disulfide vapor on one hand reduces the large size of these PCBM aggregates to ca. 20 nm, and on the other hand substantially increases the crystallinity of P3HT. The polymer solar cells employing a thus-treated composite film gave a power conversion efficiency as high as 3.9%, in contrast to 3.2% for the thermally annealed device under the same characterization conditions. This result shows the importance of a precisely controlled morphology of the photoactive layer in device performance.

MoS2 nanosheet-coated CoS2 nanowire arrays on carbon cloth as three-dimensional electrodes for efficient electrocatalytic hydrogen evolution

The design and engineering of low-cost and high-efficiency electrocatalysts for the hydrogen evolution reaction (HER) has attracted increasing interest in renewable energy research. Herein, MoS2 nanosheet-coated CoS2 nanowire arrays supported on carbon cloth (MoS2/CoS2/CC) were prepared by a two-step procedure that entailed the hydrothermal growth of Co(OH)2 nanowire arrays on carbon cloth followed by reaction with (NH4)2MoS4 to grow an overlayer of MoS2 nanosheets. Electrochemical studies showed that the obtained 3D electrode exhibited excellent HER activity with an overpotential of −87 mV at 10 mA cm−2, a small Tafel slope of 73.4 mV dec−1 and prominent electrochemical durability. The results presented herein may offer a new methodology for the design and engineering of effective multilevel structured catalysts for the HER based on earth-abundant components.

Solvent-soaking treatment induced morphology evolution in P3HT/PCBM composite films

Morphology of the active layer has been proven to play an important role in determining the final device performance of photovoltaic devices. Herein, we present a facile mixed solvents soaking approach to tailor the morphology of the active layer, in which not only the crystallinity of poly(3-hexylthiophene) (P3HT) in its composite film with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) has been substantially improved, but also an interpenetrating network composed of highly crystalline P3HT and PCBM nanoaggregates is constructed as confirmed by transmission electron microscopy. Furthermore, X-ray photoelectron spectroscopy analysis reveals that P3HT chains enrich at the active layer/anode interface while more PCBM are found to present on the active layer/cathode interface along the vertical direction of the composite films, which is beneficial for charge carrier transport and will contribute to better device performance. The power conversion efficiency of the device using this method is improved to 3.23%, in contrast to 1.45% for a pristine device and 2.79% for a thermally annealed device. Therefore, this simple technique can simultaneously optimize lateral and vertical nanoscale phase separation of crystalline P3HT and PCBM, and shows high potential application in the preparation of high performance cost-effective polymer solar cells.

Nitrogen and sulfur co-doped porous carbon derived from human hair as highly efficient metal-free electrocatalysts for hydrogen evolution reactions

Design and engineering of low-cost and high-efficiency electrocatalysts for hydrogen evolution reactions (HER) has attracted increasing interest in renewable energy research. Herein, a highly active and stable metal-free electrocatalyst, N and S co-doped porous carbon derived from human hair, was developed for HER for the first time, with an electrocatalytic performance comparable to that of state-of-the-art commercial 20 wt% Pt/C catalysts. SEM, TEM and nitrogen adsorption–desorption measurements showed that the resultant carbon exhibited a porous structure with a high specific surface area (up to 830.0 m2 g−1) and rich porosity. XPS measurements showed that N and S were co-doped into the carbon molecular skeletons. Importantly, electrochemical measurements showed high activity for hydrogen evolution with a low overpotential of only −12 mV, a Tafel slope of 57.4 mV dec−1, a current density of 10 mA cm−2 at −0.1 V vs. RHE, and remarkable durability. The results highlight a unique paradigm for the preparation of highly efficient electrocatalysts for HER based on abundant biowastes.

N-doped carbon-coated cobalt nanorod arrays supported on a titanium mesh as highly active electrocatalysts for the hydrogen evolution reaction

N-doped carbon-coated cobalt nanorod arrays supported on a Ti mesh were prepared by a two-step procedure involving hydrothermal synthesis of Co3O4 nanorods followed by thermal reduction to metallic cobalt. The nanocomposites exhibited a remarkable catalytic activity that was comparable to that of leading commercial Pt/C catalysts.

Solvent-Induced Crystallization of Poly(3-dodecylthiophene): Morphology and Kinetics

We report on the self-assembly of poly(3-dodecylthiophene) (P3DDT) into nanowhiskers for the first time via addition of the marginal solvent anisole into its well-dissolved solution. By controlling the solvent composition and aging time, we observed a morphology evolution from nanowhiskers to two-dimensional nanoribbons and foliated aggregates, which was ascribed to diverse driving forces for self-assembly in the process of crystallization. UV-vis absorption spectroscopy and dynamic lighting scattering (DLS) measurements were employed to in situ monitor crystallization kinetics of P3DDT induced by mixed solvents. It has been shown that conformational transition serves as a critical factor for P3AT to perform π-π stacking to form nanowhiskers. From a thermodynamic point of view, P3AT dispersion dissolved in mixed solvents is actually not a thermodynamic equilibrium system, but a multicomponent and multiphase case whose phase composition and properties evolve with time. The understanding in morphology transition mechanisms and crystallization kinetics of P3DDT can provide guidelines for optimization of processing parameters and enhance performance of photovoltaic devices.