Toward the Improvement of Silicon-Based Composite Electrodes via an In-Situ Si@C-Graphene Composite Synthesis for Li-Ion Battery Applications

Using Si as anode materials for Li-ion batteries remain challenging due to its morphological evolution and SEI modification upon cycling. The present work aims at developing a composite consisting of carbon-coated Si nanoparticles (Si@C NPs) intimately embedded in a three-dimensional (3D) graphene hydrogel (GHG) architecture to stabilize Si inside LiB electrodes. Instead of simply mixing both components, the novelty of the synthesis procedure lies in the in situ hydrothermal process, which was shown to successfully yield graphene oxide reduction, 3D graphene assembly production, and homogeneous distribution of Si@C NPs in the GHG matrix. Electrochemical characterizations in half-cells, on electrodes not containing additional conductive additive, revealed the importance of the protective C shell to achieve high specific capacity (up to 2200 mAh.g^-1), along with good stability (200 cycles with an average Ceff > 99%). These performances are far superior to that of electrodes made with non-C-coated Si NPs or prepared by mixing both components. These observations highlight the synergetic effects of C shell on Si NPs, and of the single-step in situ preparation that enables the yield of a Si@C-GHG hybrid composite with physicochemical, structural, and morphological properties promoting sample conductivity and Li-ion diffusion pathways.

Sparsely Pillared Graphene Materials for High-Performance Supercapacitors: Improving Ion Transport and Storage Capacity

Graphene-based materials are extensively studied as promising candidates for supercapacitors (SCs) owing to the high surface area, electrical conductivity, and mechanical flexibility of graphene. Reduced graphene oxide (RGO), a close graphene-like material studied for SCs, offers limited specific capacitances (100 F·g^-1) as the reduced graphene sheets partially restack through π-π interactions. This paper presents pillared graphene materials designed to minimize such graphitic restacking by cross-linking the graphene sheets with a bifunctional pillar molecule. Solid-state NMR, X-ray diffraction, and electrochemical analyses reveal that the synthesized materials possess covalently cross-linked graphene galleries that offer additional sites for ion sorption in SCs. Indeed, high specific capacitances in SCs are observed for the graphene materials synthesized with an optimized number of pillars. Specifically, the straightforward synthesis of a graphene hydrogel containing pillared structures and an interconnected porous network delivered a material with gravimetric capacitances two times greater than that of RGO (200 F·g^-1 vs 107 F·g^-1) and volumetric capacitances that are nearly four times larger (210 F·cm^-3 vs 54 F·cm^-3). Additionally, despite the presence of pillars inside the graphene galleries, the optimized materials show efficient ion transport characteristics. This work therefore brings perspectives for the next generation of high-performance SCs.

Non-conductive ferromagnets based on core double-shell nanoparticles for radio-electric applications

Two fabrication schemes of magnetic metal-polymer nanocomposites films are described. The nanocomposites are made of graphene-coated cobalt nanoparticles embedded in a polystyrene matrix. Scheme 1 uses non-covalent chemistry while scheme 2 involves covalent bonding with radicals. Preservation of the net-moment of cobalt and electrical insulation are achieved by means of a core double-shell structure of cobalt–graphene–polystyrene. The graphene shell has two functions: it is a protective layer against metal core oxidation and it serves as the functionalization surface for polymer grafting as well. The polystyrene shell is used as an insulating layer between nanoparticles and improves nanoparticles dispersion inside the polystyrene matrix. The theoretical maximum volume filling ratio estimated at ~30 % is almost reached. The nanocomposites are shown to undergo percolation behavior but retain low conductivity (<1 S/m) at the highest filling ratio reached ~25 % leading to extremely low losses (10⁻³) at high frequency. Such low conductivity values are combined with large magnetization, as high as 0.9 T. Ability for radiofrequency applications is discussed in regards to the obtained magnetization.

Evidence for Charge Transfer at the Interface between Hybrid Phosphomolybdate and Epitaxial Graphene

The interfacing of polyoxometalates and graphene can be considered to be an innovative way to generate hybrid structures that take advantage of the properties of both components. Polyoxometalates are redox-sensitive and photosensitive compounds with high temperature stability (up to 400 °C for some), showing tunable properties depending on the metal incorporated inside the complex. Graphene has a unique electronic band structure combined with good material properties for electrical and optical applications. The spontaneous, rather than electrochemical, functionalization of epitaxial graphene on SiC with Keggin phosphomolybdate derivative TBA3[PMo11O39{Sn(C6H4)C≡C(C6H4)N2}] (named K(Mo)Sn[N2(+)]) bearing a phenyl diazonium unit is investigated. Graphene decoration is evidenced by means of AFM, Raman, XPS, and cyclic voltammetry, indicating a successful immobilization of the polyoxomolybdate. The covalent bonding of the polyoxometalate to the graphene substrate can be deduced from the appearance of a D band in the Raman spectra and from the loss of mobility in the electrical conduction. High-resolution XPS spectra reveal an electron transfer from the graphene to the Mo complex. The comparison of charge-carrier density measurements before and after grafting supports the p-type doping effect, which is further evidenced by work function UPS measurements.

Core double–shell cobalt/graphene/polystyrene magnetic nanocomposites synthesized by in situ sonochemical polymerization

Core double–shell cobalt/graphene//polystyrene nanocomposites (Co/C//PS) were synthesized byin situsonochemical polymerization technique.

From gold porphyrins to gold nanoparticles: catalytic nanomaterials for glucose oxidation

Au(iii) porphyrin was synthesized and evaluated for electrocatalytic oxidation of glucose. These Au(III) porphyrins, immobilized on a multiwalled carbon nanotube matrix, oxidized glucose at low overpotentials. Furthermore, AuNPs were electrogenerated by reduction of the Au(III) porphyrins. The electrocatalytic properties of these compounds towards glucose oxidation were compared and characterized by electrochemistry, electron microscopy and XPS.

Highly stable tetrathiafulvalene radical dimers in [3]catenanes

Two [3]catenane 'molecular flasks' have been designed to create stabilized, redox-controlled tetrathiafulvalene (TTF) dimers, enabling their spectrophotometric and structural properties to be probed in detail. The mechanically interlocked framework of the [3]catenanes creates the ideal arrangement and ultrahigh local concentration for the encircled TTF units to form stable dimers associated with their discrete oxidation states. These dimerization events represent an affinity umpolung, wherein the inversion in electronic affinity replaces the traditional TTF-bipyridinium interaction, which is over-ridden by stabilizing mixed-valence (TTF)2•+ and radical-cation (TTF•+)2 states inside the 'molecular flasks.' The experimental data, collected in the solid state as well as in solution under ambient conditions, together with supporting quantum mechanical calculations, are consistent with the formation of stabilized paramagnetic mixed-valence dimers, and then diamagnetic radical-cation dimers following subsequent one-electron oxidations of the [3]catenanes.