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.

Local Degradation of PEDOT:PSS on Silicon Nanostructures Using Scanning Electrochemical Microscopy.

Conducting polymers show attractive characteristics as electrode materials for micro-electrochemical energy storage (MEES). However, there is a lack of characterization techniques to study conjugated/conducting polymer-based nanostructured electrodes. Here, scanning electrochemical microscopy (SECM) is introduced as a new technique for in situ characterization and acceleration of degradation processes of conducting polymers. Electrodes of PEDOT:PSS on flat silicon, silicon nanowires (SiNWs) and silicon nanotrees (SiNTrs) are analyzed by SECM in feedback mode with approach curves and chronoamperometry. The innovative degradation method using SECM reduces the time required to locally degrade polymer samples to a few thousand seconds, which is significantly shorter than the time usually required for such studies. The degradation rate is modeled using Comsol Multiphysics. The model provides an understanding of the phenomena that occur during degradation of the polymer electrode and describes them using a mathematical constant A0 and a time constant τ.

New PEDOT Derivatives Electrocoated on Silicon Nanowires Protected with ALD Nanometric Alumina for Ultrastable Microsupercapacitors.

This work deals with electroactive conducting polymers (ECPs) used as a complementary component on purely capacitive silicon nanowires protected by a 3 nm alumina layer. Accordingly, in this work, we use a fast and simple deposition method to create a pseudocapacitive material based on the electropolymerization in aqueous micellar media (SDS and SDBS 0.01 M) of hydroxymethyl-EDOT (EDOT-OH) onto 3 nm alumina-coated silicon nanowires (Al3@SiNWs). The composite material displays remarkable capacitive behavior with a specific capacitance of 4.75 mF·cm-2 at a current density of 19 µA·cm-2 in aqueous Na2SO4 electrolyte.

Atomic Layer Deposition Alumina-Passivated Silicon Nanowires: Probing the Transition from Electrochemical Double-Layer Capacitor to Electrolytic Capacitor.

Silicon nanowires were coated by a 1-5 nm thin alumina layer by atomic layer deposition (ALD) in order to replace poorly reproducible and unstable native silicon oxide by a highly conformal passivating alumina layer. The surface coating enabled probing the behavior of symmetric devices using such electrodes in the EMI-TFSI electrolyte, allowing us to attain a large cell voltage up to 6 V in ionic liquid, together with very high cyclability with less than 4% capacitance fade after 106 charge/discharge cycles. These results yielded fruitful insights into the transition between an electrochemical double-layer capacitor behavior and an electrolytic capacitor behavior. Ultimately, thin ALD dielectric coatings can be used to obtain hybrid devices exhibiting large cell voltage and excellent cycle life of dielectric capacitors, while retaining energy and power densities close to the ones displayed by supercapacitors.

3D hierarchical assembly of ultrathin MnO2 nanoflakes on silicon nanowires for high performance micro-supercapacitors in Li- doped ionic liquid.

Building of hierarchical core-shell hetero-structures is currently the subject of intensive research in the electrochemical field owing to its potential for making improved electrodes for high-performance micro-supercapacitors. Here we report a novel architecture design of hierarchical MnO2@silicon nanowires (MnO2@SiNWs) hetero-structures directly supported onto silicon wafer coupled with Li-ion doped 1-Methyl-1-propylpyrrolidinium bis(trifluromethylsulfonyl)imide (PMPyrrBTA) ionic liquids as electrolyte for micro-supercapacitors. A unique 3D mesoporous MnO2@SiNWs in Li-ion doped IL electrolyte can be cycled reversibly across a voltage of 2.2 V and exhibits a high areal capacitance of 13 mFcm(-2). The high conductivity of the SiNWs arrays combined with the large surface area of ultrathin MnO2 nanoflakes are responsible for the remarkable performance of these MnO2@SiNWs hetero-structures which exhibit high energy density and excellent cycling stability. This combination of hybrid electrode and hybrid electrolyte opens up a novel avenue to design electrode materials for high-performance micro-supercapacitors.

Enhanced charge separation in ternary P3HT/PCBM/CuInS2 nanocrystals hybrid solar cells.

Geminate recombination of bound polaron pairs at the donor/acceptor interface is one of the major loss mechanisms in organic bulk heterojunction solar cells. One way to overcome Coulomb attraction between opposite charge carriers and to achieve their full dissociation is the introduction of high dielectric permittivity materials such as nanoparticles of narrow band gap semiconductors. We selected CuInS2 nanocrystals of 7.4 nm size, which present intermediate energy levels with respect to poly(3-hexylthiophene) (P3HT) and Phenyl-C61-butyric acid methyl ester (PCBM). Efficient charge transfer from P3HT to nanocrystals takes place as evidenced by light-induced electron spin resonance. Charge transfer between nanocrystals and PCBM only occurs after replacing bulky dodecanethiol (DDT) surface ligands with shorter 1,2-ethylhexanethiol (EHT) ligands. Solar cells containing in the active layer a ternary blend of P3HT:PCBM:CuInS2-EHT nanocrystals in 1:1:0.5 mass ratio show strongly improved short circuit current density and a higher fill factor with respect to the P3HT:PCBM reference device. Complementary measurements of the absorption properties, external quantum efficiency and charge carrier mobility indicate that enhanced charge separation in the ternary blend is at the origin of the observed behavior. The same trend is observed for blends using the glassy polymer poly(triarylamine) (PTAA).

Micro-ultracapacitors with highly doped silicon nanowires electrodes.

Highly n-doped silicon nanowires (SiNWs) with several lengths have been deposited via chemical vapor deposition on silicon substrate. These nanostructured silicon substrates have been used as electrodes to build symmetrical micro-ultracapacitors. These devices show a quasi-ideal capacitive behavior in organic electrolyte (1 M NEt4BF4 in propylene carbonate). Their capacitance increases with the length of SiNWs on the electrode and has been improved up to 10 µFcm-2 by using 20 µm SiNWs, i.e., ≈10-fold bulk silicon capacitance. This device exhibits promising galvanostatic charge/discharge cycling stability with a maximum power density of 1.4 mW cm-2.

Delineating poly(aniline) redox chemistry by using tailored oligo(aryleneamine)s: towards oligo(aniline)-based organic semiconductors with tunable optoelectronic properties.

The simple and elegant Buchwald-Hartwig cross-coupling reaction has been used to synthesise a designed range of new aniline-based tetramers in one step, and without the need for protecting groups. Variation of the central aromatic ring has provided the opportunity to carefully tune the optoelectronic properties in this series, thus enabling a structure-activity relationship study by using a range of photophysical and electrochemical techniques. As a result, the long-proposed sequences of electron-electron (EE) and electron-chemical (EC) processes that support the complex redox and proton-transfer reactions involved in the well-known switching of redox states of poly- and oligo(aniline)s are revealed here for the first time. We also present the initial results from time-dependent DFT calculations to clarify the optoelectronic behaviour of these oligomers. The dc-conductivity measurements of conducting thin films of this series, doped with the prototypical poly(aniline) protonating agent D,L-camphor-10-sulfonic acid (CSA), externally plasticised with triphenyl phosphate (TPP), and processed from m-cresol (MC) solutions, are also presented.

Poly(bisthiophene-carbazole-fullerene) double-cable polymer as new donor-acceptor material: preparation and electrochemical and spectroscopic characterization.

A new donor-acceptor dyad, namely, a 3,6-bis(thiophen-2-yl)carbazole derivative bearing a C(60) fullerene as a side group (BTC-F), was prepared and characterized. Electropolymerization of BTC-F leads to the formation of a donor-acceptor double-cable polymer (PBTC-F) with high fullerene content (63 wt %) corresponding to one C(60) per polymer repeat unit. The electronic properties of BTC-F and PBTC-F were studied by electrochemical and spectroscopic techniques. Photoluminescence quenching is observed in diluted solutions of BTC-F compared to the nongrafted monomer BTC indicating that an intramolecular charge transfer takes place between the two components of the dyad. The positions of the HOMO and LUMO levels of the monomer and the polymer were accurately determined by differential pulse voltammetry (DPV). The LUMO energy level of the fullerene moiety in BTC-F lies at 3.7 eV below the vacuum level, i.e., slightly higher than corresponding levels of C(60) and PCBM. DPV characterization of PBTC-F indicates little ground state interaction between the pi-conjugated main chain and the C(60) side groups and a high donor HOMO-acceptor LUMO gap of 1.47 eV.

Molecular weight dependent charge carrier mobility in poly(3,3' '-dioctyl-2,2':5',2' '-terthiophene).

Poly(3,3' '-dioctyl-2,2':5',2' '-terthiophene), a polymer recently used for the fabrication of organic field effect transistors, has been fractionated into five fractions distinctly differing in their molecular weights (Mn), with the goal of determining the influence of the degree of polymerization (DPn) on its principal physicochemical parameters. It has been demonstrated that within the Mn range studied (from 1.5 kDa to 10.5 kDa by SEC), corresponding to DPn from 10 to 38, the polymer band gap steadily decreases with growing molecular weight, which is clearly manifested by an increasing bathochromic shift of the band originating from the pi-pi* transition. The same trend is observed for the HOMO level, determined from the onset of the p-doping in cyclic voltammetry, which shifts from -5.10 eV to -4.90 eV for the lowest and the highest molecular weight fractions, respectively. The most pronounced influence of DPn has been found for the charge carriers' mobility-one of the most important parameters of field effect transistors (FETs) fabricated from this polymer. A fourfold increase in DPn results in an increase of the carriers' mobility by more than 3 orders of magnitude. Comparison of these results with those obtained for fractionated regioregular poly(3-hexylthiophene) shows a strikingly similar behavior of both polymers with respect to the molecular weight.

Size and ligand effects on the electrochemical and spectroelectrochemical responses of CdSe nanocrystals.

The electrochemical properties of CdSe quantum dots with electrochemically inactive surface ligands (TOPO) have been investigated in comparison with the analogous nanocrystals containing electrochemically active oligoaniline ligands. The TOPO-capped nanocrystals have been studied in a wide size range (from 3 to 6.5 nm) with the goal to amplify the influence of the quantum confinement effect on the electrochemical response. The determined HOMO and LUMO levels have been found in good agreement with the ones obtained from photoluminescence studies and those predicted theoretically. Ligand exchange with aniline tetramer significantly influences the voltammetric peaks associated with the HOMO oxidation and the LUMO reduction of the quantum dots, which are shifted to higher and lower potentials, respectively. These shifts are interpreted in terms of the positive ligand charging which precedes the oxidation of the nanocrystals and the insulating nature of the ligand in the case of the nanocrystal reduction. The ligand-nanocrystal interactions have also been studied by UV-Vis-NIR and Raman spectroelectrochemistry in comparison with a specially prepared model compound which, apart from the anchoring function is identical to the grafted oligoaniline ligand. Both spectroelectrochemical techniques clearly indicate the same nature of the oxidation/reduction pathway for both the model compound and the grafted ligand. The influence of the grafting is manifested by a shift in the onset of the ligand oxidation as compared to the case of the "free" model compound. Since both components (ligands and nanocrystals) mutually influence their electrochemical and spectroelectrochemical properties, the newly developed system can be considered as a true molecular hybrid. Such hybrids are of interest because the potential zone of the ligand electroactivity is well separated from that of the nanocrystals and, as a result, the organic part can be electrochemically switched between the semiconducting and the conducting states with no change in the oxidation state of the nanocrystal. The newly developed system offers therefore the possibility of an electrical addressing of individual nanocrystals via the conducting ligands.