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In this work, we, for the first time, observed the remarkable thermoelectric properties of a few high-quality PtSe2 layered films fabricated by a post selenization of Pt thin films. An excellent po...

Exploring electrocatalysts composed of earth-abundant elements for a highly efficient hydrogen evolution reaction (HER) is scientifically and technologically important for electrocatalytic water splitting. In this work, we report HER properties of acid treated pyrolyzed vitamin B12 supported on reduced graphene oxide (B12/G800A) that shows an extraordinarily enhanced catalytic activity with low overpotential (115 mV vs. RHE at 10 mA cm−2), which is better than that of most traditional nonprecious metal catalysts in acidic media. Stability tests through long-term potential cycles and at a constant current density confirm the exceptional durability of the catalyst. Notably, the B12/G800A catalyst exhibits extremely high turnover frequencies per cobalt site in acid, for example, 0.85 and 11.46 s−1 at overpotentials of 100 and 200 mV, respectively, which are higher than those reported for other scalable non-precious metal HER catalysts. Moreover, it has been conjectured that the covalency of Co–C and Co–N bonds affects HER activities by comparing the extended X-ray absorption fine structure (EXAFS) spectra of the B12/G800A. High-temperature treatment can modify the Co-corrin structure of B12 to form Co–C bonds along with Co–N, which broadens the band of cobalt, essentially lowering the d-band center from its Fermi level. The lower d-band center leads to a moderate hydrogen binding energy, which is favorable for hydrogen adsorption and desorption.

Applications of lithium ion battery have been hampered by a lack of ideal anode materials in terms of capacity and stability. The emergence of metal chalcogenide as a candidate material has reinvigorated the search of a low cost and high capacity material system. However, debate about the underlying mechanisms and overall appraisal of its usage in lithium ion battery system remains. Here, a comprehensive study on the energy storage mechanism of copper zinc tin sulfide (CZTS) nanowalls possessing ultrahigh rate capability (500 mAh g−1 charged within 60 s) is reported. Structural evolutions along with the accompanying changes in the oxidation state upon charge/discharge were monitored by ex-situ X-ray diffraction and X-ray photoelectron spectroscopy. During lithiation, lithium ion reacted with CZTS to form lithium sulfides. At the same time, a sequential conversion reactions of copper, zinc and tin sulfides enabled the CZTS nanowalls to achieve excellent electrochemical performance (1400 mAh g−1 at a current density of 1000 mA g−1 over 400 cycles). Multi-element metal chalcogenides in conjunction with an adhesion-enhancing seed layer and a rational nanostructure design hold the key to such ultrahigh capacity and stable anode materials for next generation energy storage devices.

Understanding the role of the oxidation state of the Cu surface and surface-adsorbed intermediate species in electrochemical CO2 reduction is crucial for the development of selective CO2-to-fuel electrocatalysts. In this study, the electrochemical CO2 reduction mechanism over the Cu catalysts with various oxidation states was studied by using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS), in situ soft X-ray absorption spectroscopy (Cu L-edge), and online gas chromatography measurements. The atop-adsorbed CO (COatop) intermediate is obtained on the electrodeposited Cu surface which primarily has the oxidation state of Cu(I). COatop is further reduced, followed by the formation of C1 product such as CH4. The residual bridge-adsorbed CO (CObridge) is formed on the as-prepared Cu surface with Cu(0) which inhibits hydrocarbon formation. In contrast, the CV-treated Cu electrode prepared by oxidizing the as-prepared Cu surface contains different amounts of Cu(I) and Cu(0) states. The major theme of this work is that in situ SEIRAS results show the coexistence of COatop and CObridge as the reaction intermediates during CO2 reduction and that the selectivity of CO2-to-ethylene conversion is further enhanced in the CV-treated Cu electrode. The Cu catalysts modulated by the electrochemical method exhibit different oxidation states and reaction intermediates as well as electrocatalytic properties.

Understanding the role of the oxidation state of the Cu surface and surface-adsorbed intermediate species in electrochemical CO

The controlled growth of metal oxide nanostructures within hierarchically porous conductive carbon-based frameworks is critically important to achieving high volumetric performance and appropriate channel size for energy storage applications. Herein, we grow cobalt oxide (Co3O4) nanoflakes, using a sequential-electrodeposition process, into spherically porous sponge-like few-layer partially reduced graphene oxide (SrGO) synthesized by template-directed ordered assembly. Maximum specific/volumetric capacitances of 1112 F gCo3O4−1 (at 3.3 A gCo3O4−1), 178 F cm−3 (at 2.6 A cm−2), and 406 F gtotal−1 (at 1 A gtotal−1) and sensible rate capability (80% retention by increasing the charge/discharge current from 1 A g−1 to 16 A g−1) are obtained for the Co3O4 nanoflakes@SrGO hybrid electrodes. Besides, an asymmetric supercapacitor is made with the Co3O4[63%]@SrGO[37%] hybrid and activated carbon as a positive and a negative electrode, respectively. Electrochemical results indicate an energy density of 23.3 W h kg−1 at a high power density of 2300 W kg−1 (discharge time of about 42 s) and 62% retention even at a remarkable power density of 36 600 W kg−1 (discharge time of 1.6 s). Moreover, it shows excellent cycling stability with no decay after 20 000 charge/discharge cycles. This performance is attributed to the unique pore-sizes for an ion to channel into the porous structures, conductivity, and mechanical stability of the SrGO framework, which makes it promising for next-generation supercapacitors.

Herein, an electrochemical technique as a cost-effective and one-step approach was utilized to fabricate graphene quantum dots (GQDs). Different amounts of GQDs (0, 0.2, 0.4, 0.8, and 1.2 wt %) were decorated uniformly on the surface of anodized ZnO nanowires (NWs) forming GQD/ZnO NWs. Transmission electron microscopy and atomic force microscopy confirmed formation of GQDs on the ZnO NWs, 12–22 nm in width and 1–3 graphene layers thick. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy were employed to verify the functional groups on the surface of GQDs, and the results indicated that GQDs readily anchored on the surface of ZnO NWs. The GQD/ZnO NWs exhibited a considerable improvement on the photocatalytic degradation of methylene blue under solar irradiation, due to efficient light absorption. In addition, the results indicated that the optimized GQD (0.4 wt %)/ZnO NWs showed the highest photoactivity with about 3-fold enhancement as compared to pure ZnO NWs. Finally, a mechani...

A novel and simple strategy for the synthesis of cobalt oxide decorated reduced graphene oxide (CoO-rGO) nanocomposites was developed as a high performance anode for lithium ion battery (LIB). The CoO-rGO nanocomposites were prepared by simultaneous reduction of GO and Co2+ in the presence of NaBH4. Addition of 15 wt% rGO produced a narrow size distribution of ultra-fine CoO nanoparticles with nano-dimensional contact between CoO and rGO leading to an excellent electrochemical performance. The reversible capacity of CoO-rGO nanocomposites is between 1140 and 1260 mAh/g at the current density of 150 mA/g and sustained at 473 mAh/g at high current density of 2400 mA/g. In addition to the excellent rate capability of 15 wt% CoO-rGO, a reversible capacity of 690 mAh/g is achieved at 600 mA/g after 60 cycles. In addition, XPS spectra and XRD patterns of CoO-rGO after cycling clearly indicate the involvement of electrolyte into CoO nanoparticles during intercalation-deintercalation cycling, and lead to the transformation of Co species from Co2+ to Co3+ for the enhanced electrochemical performance at high current density.

Kesterite-type Cu 2 ZnSnS 4 (CZTS) has been attracting a lot of attention in thin-film solar cells due to its relatively low cost, earth abundant and environmentally benign nature compared to its analog Cu(In, Ga)Se 2 materials. Until now, solution-based processes are considered as promising methodologies for mass production of CZTS materials for industrial demands. However, most material sources are highly toxic as well as dangerous. In this study, we proposed a facile and green synthesis strategy for the CZTS absorber by taking advantage of an ultrasonic spray technique where we adopted metallic nanopowders and ethanol as our nontoxic precursors as well as solvents, respectively. The phase formation and transition mechanism of the synthesized CZTS film are investigated through varying sulfurization temperatures and analyzed using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. The solar cell device can be made at a scale as large as 20 × 20 mm 2 , and the cell shows a power conversion efficiency of 1.5%.

Tin-based composites are promising anode materials for high-performance lithium-ion batteries (LIBs); however, insufficient conductivity, as well as fatal volume expansion during cycling lead to poor electrochemical reversibility and cycling stability. In this work, we demonstrate the lithium-ion storage behaviors of SnS2 anode material deposited on different electrode supports. The SnS2 grown on 3D hierarchical carbon nanotube-carbon cloth composites (SnS2-CNT-CC) shows superior capacity retention and cycle stability, compared to that on planar Mo sheets and carbon cloth. The specific capacity of SnS2 on Mo, CC, and CNT-CC is around 240, 840, and 1250 g−1, respectively. The SnS2-CNT-CC electrode outperforms in the cyclic performance and rate capability compared to other electrode configurations due to the multi-electron pathway and high surface area derived from 3D hierarchical CNT-CC electrode support. Furthermore, a significant decrease in the charge transfer resistance is observed by utilizing 3D hierarchical CNT-CC electrode support. The use of 3D hierarchical structures as electrode support could be the best alternative to enhance the electrochemical performances for the next generation LIBs.

Gas sensors, which play an important role in the safety of human life, cover a wide range of applications including intelligent systems and detection of harmful and toxic gases. It is known that graphene is an ideal and attractive candidate for gas sensing due to its high surface area and excellent mechanical, electrical, optical, and thermal properties. However, in order to fully realize its potential as a commercial gas sensor, demand for a graphene-based device of low-limit detection, high sensitivity, and fast response time needs to be met. Here, we demonstrate a metal/insulator/semiconductor (MIS) based gas sensor consisting of as-grown epitaxial graphene nanowalls (EGNWs)/silicon carbide (SiC)/silicon (Si) structure. The unique edge dominant three-dimensional (3D) EGNWs based MIS device achieved an extraordinarily low limit of detection (0.5 ppm) and unprecedented sensitivity (82 μA/ppm/cm

A promising strategy for the fabrication of hierarchically porous carbon materials (HPCMs) using sugarcane bagasse as the scaffold was developed for the application of supercapacitors with enhanced electrochemical performance. HPCMs were fabricated by evaporation-induced self-assembly using resol polymer and triblock copolymer F127 as the carbon precursor and structural directing agent, respectively. The HPCMs show a 2-dimensional hexagonally ordered structure with pore size distribution centered at 3.4 nm. After activation with carbon dioxide at 850 °C for 12 h to remove residual carbons and/or impurities, the specific surface area increases to 1638 m2 g−1 with micropore surface area of 1058 m2 g−1, which can facilitate the fast electrolyte and electron transport. The CO2-activated HPCM electrodes exhibit excellent specific capacitances of 144–232 F g−1 at the scan rates of 10–50 mV s−1, and the specific capacitance can be up to 8.8–14.2 μF cm−2. In addition, the activated HPCMs show a good cycling stability, low IR drop, and the energy and power densities are in the range 12.3–26.1 Wh kg−1 and 1.05–4.91 kW kg−1, respectively. Results obtained in this study clearly demonstrate a cost-effective and environmentally benign method for synthesis of bagasse-based HPCMs, which can potentially apply to supercapacitor with enhanced electrochemical performance and improved energy density.

Production of hydrogen from water electrolysis has stimulated the search of sustainable electrocatalysts as possible alternatives. Recently, cobalt phosphide (CoP) and molybdenum phosphide (MoP) received great attention due to their superior catalytic activity and stability towards the hydrogen evolution reaction (HER) which rivals platinum catalysts. In this study, we synthesize and study a series of catalysts based on hybrids of CoP and MoP with different Co/Mo ratio. The HER activity shows a volcano shape and reaches a maximum for Co/Mo = 1. Tafel analysis indicates a change in the dominating step of Volmer–Hyrovský mechanism. Interestingly, X-ray diffraction patterns confirmed a major ternary interstitial hexagonal CoMoP2 crystal phase is formed which enhances the electrochemical activity.

The thickness-dependent surface states of MoS2 thin films grown by the chemical vapor deposition process on the SiO2-Si substrates are investigated by X-ray photoelectron spectroscopy. Raman and high-resolution transmission electron microscopy suggest the thicknesses of MoS2 films to be ranging from 3 to 10 layers. Both the core levels and valence band edges of MoS2 shift downward ∼0.2 eV as the film thickness increases, which can be ascribed to the Fermi level variations resulting from the surface states and bulk defects. Grainy features observed from the atomic force microscopy topographies, and sulfur-vacancy-induced defect states illustrated at the valence band spectra imply the generation of surface states that causes the downward band bending at the n-type MoS2 surface. Bulk defects in thick MoS2 may also influence the Fermi level oppositely compared to the surface states. When Au contacts with our MoS2 thin films, the Fermi level downshifts and the binding energy reduces due to the hole-doping characteristics of Au and easy charge transfer from the surface defect sites of MoS2. The shift of the onset potentials in hydrogen evolution reaction and the evolution of charge-transfer resistances extracted from the impedance measurement also indicate the Fermi level varies with MoS2 film thickness. The tunable Fermi level and the high chemical stability make our MoS2 a potential catalyst. The observed thickness-dependent properties can also be applied to other transition-metal dichalcogenides (TMDs), and facilitates the development in the low-dimensional electronic devices and catalysts.

Three-dimensional, hierarchically ordered, porous carbon (HOPC) with designed porous textures, serving as an ion-buffering reservoir, an ion-transport channel, and a charge-storage material, is expected to be advanced an energy material for high-rate supercapacitors. Herein, HOPC without/with partially graphitic nanostructures have been directly synthesized by means of a simple one-pot synthesis procedure. The designed porous textures of the 3D HOPC materials are composed of highly ordered, fcc macroporous (300 nm), interconnected porous structures, including macroporous windows (170 nm), hexagonally ordered mesopores (5.0 nm), and useful micropores (1.2 nm). 3D HOPC-g-1000 (g=graphitic, 1000=pyrolysis temperature of 1000 °C) with partially graphitic nanostructures has a low specific surface area (296 m(2) g(-1)) and a low gravimetric specific capacitance (73.4 F g(-1) at 3 mV s(-1)), but improved electrical conductivity, better rate performance, higher electrolyte accessibility (24.8 μF cm(-2) at 3 mV s(-1)), faster frequency response (≈1 Hz), and excellent cycling performance (>5400 cycles). The specific capacitance per surface area is higher than that of conventional porous carbons, carbon nanotubes, and modified graphene (10-19 μF cm(-2)).

Water splitting by electrocatalysis into hydrogen via the hydrogen evolution reaction (HER) offers a promising solution for green renewable energy generation1. In past several years, Pt and Pt based nanomaterials are the mostly served as an efficient electrocatalyst for HER; however, due to high-cost and low abundancy of Pt limits its vast utility for the commercial HER application. Therefore, it's highly needed to develop nonprecious metal based electrocatalyst for HER towards cost effective commercial application2. Over the past decades, tremendous efforts have been made for replacing the Pt based electrocatalysts for HER. Recent advances in carbon nanomaterials (RGO, CNT, etc.,) have shown their promising future in energy-related electrocatalytic reactions, such as ORR, OER, and HER. After heteroatom (such as N, B, P, and S) doping, the enhanced catalysis activity of carbon nanomaterials has been widely illustrated for ORR. Among all the reported electrocatalysts, the co-doping of trace transition metals on heteroatoms doped carbon materials leads to form metal complexes (Ex. Co-Nx), showing promising HER activity in the aqueous electrolyte. In addition, the reported density functional theory (DFT) simulation shows that when combining both coordinations into one complex, the optimized charge distribution results in an ideal value of ΔGH in Metal-Carbon-Nitrogen (M-C-N) hybrid system and which is much better than the single or mixture system of M-C and M-N3. In this presentation, we will demonstrate a new HER electrocatalyst based on N-Co-C system. The N-Co-C catalysts are prepared by a one-step pyrolysis process at the high temperature in which vitamin-B12 used as a metal precursor together with reduced graphene oxide (RGO). We observed that the electrocatalytic activities of N-Co-C catalysts are strongly related with the pyrolysis temperature and metal loading. The N-Co-C catalyst pyrolyzed at 800oC showed the highest HER activity with lowest overpotential of 110 mV at a current density of 10 mA cm-2. It shows the optimum Tafel slope of 65 mV decade-1 and excellent stability over 20 h in acidic condition. The as synthesized catalyst was characterized by XRD, XPS and XAS. The results indicate that Co-corrin complexes (Vitamin B12) together with RGO have been decomposed at the high temperature to form N-Co-C structure. This conjugation induces that, downshift of the d-band center of cobalt, and thereby decreases its hydrogen binding energy. This, in turn, favors the electrochemical desorption of Hads and leads to a relatively moderate Co−H binding strength, resulting in the enhancement of the hydrogen evolution reaction. The comparison of HER activity and stability of N-Co-C electrocatalyst in alkaline electrolytes will be discussed at the meeting.

A promising energy storage material, MnO2 /hierarchically porous carbon (HPC) nanocomposites, with exceptional electrochemical performance and ultrahigh energy density was developed for asymmetric supercapacitor applications. The microstructures of MnO2 /HPC nanocomposites were characterized by transmission electron microscopy, scanning transmission electron microscopy, and electron dispersive X-ray elemental mapping analysis. The 3-5 nm MnO2 nanocrystals at mass loadings of 7.3-10.8 wt % are homogeneously distributed onto the HPCs, and the utilization efficiency of MnO2 on specific capacitance can be enhanced to 94-96 %. By combining the ultrahigh utilization efficiency of MnO2 and the conductive and ion-transport advantages of HPCs, MnO2 /HPC electrodes can achieve higher specific capacitance values (196 F g(-1) ) than those of pure carbon electrodes (60.8 F g(-1) ), and maintain their superior rate capability in neutral electrolyte solutions. The asymmetric supercapacitor consisting of a MnO2 /HPC cathode and a HPC anode shows an excellent performance with energy and power densities of 15.3 Wh kg(-1) and 19.8 kW kg(-1) , respectively, at a cell voltage of 2 V. Results obtained herein demonstrate the excellence of MnO2 /HPC nanocomposites as energy storage material and open an avenue to fabricate the next generation supercapacitors with both high power and energy densities.

Correction for ‘Multi-porous Co3O4 nanoflakes@sponge-like few-layer partially reduced graphene oxide hybrids: towards highly stable asymmetric supercapacitors’ by Mohammad Qorbani et al., J. Mater. Chem. A, 2017, DOI: 10.1039/c7ta00694b.

Graphene-based materials, as promising supports for electrocatalysts, attract lots of attention due to its good conductivity, high surface area and chemical stability. However, the influence of such support to the behavior of the catalyst still lacks a unified understanding. Herein, we report, for the first time, a systematic study on the strain effect of substrate toward the catalysts. In this study, we fabricated epitaxial graphene nanowalls (EGNWs) with controllable strain (from tensile, EGNWs-T, to compressive, EGNWs-C) by taking advantages of microwave plasma enhanced chemical vapor deposition, and Pt nanoparticles deposited on EGNWs by RF magnetron sputtering systems, denoted as Pt/EGNWs. It is worth noting that the catalytic properties of Pt on methanol oxidation reactions are highly dependent on the strain of graphene substrates. The catalytic properties of Pt/EGNWs-T shows 100 times higher than that of Pt/EGNWs-C, and the mass activity of Pt/EGNWs-T is 876 A/g, which is almost 10 times higher than that of commercial Pt/C catalysts. In addition to catalytic properties, XANES analysis also gave clear understanding on the electronic state of Pt. We believed that EGNWs provides an excellent opportunity to improve the performance of catalysts by applying different strain on catalysts supports.