Electrografting of Phenyl Phosphate Layers onto Glassy Carbon for Tuning Catalytic Activity toward the Hydrogen Evolution Reaction.

In this study, we explored the surface modification of a glassy carbon electrode through the electrografting of 4-Aminophenyl phosphate, which features heteroatoms and ionic properties. The electrochemical grafting process involves reducing in situ-generated diazonium derivatives. The primary objective of this research was to immobilize organic layers and assess their electrochemical and surface properties. Subsequently, the generated surface serves as a template for the electrochemical growth of Pd and Co nanoparticles on functionalized electrodes. The electrocatalytic performances of these hybrid electrodes in driving the hydrogen evolution reaction were investigated. The obtained results indicate an enhancement in the electrocatalytic activity of the modified electrodes, where lower overpotential and higher stability were observed when the catalyst was electrodeposited onto the attached ionic layer. These findings highlight the synergistic effect between the attached phenyl phosphate moieties and electrocatalysts.

Investigating Localized Electrochemical of Ferrocenyl-Imidazolium in Ionic Liquid Using Scanning Electrochemical Microscopy Configuration.

In the present work, the localized electrochemical behavior of redox molecule in ionic liquid has been investigated using scanning electrochemical microscopy. The electrochemical response of ferrocenyl-imidazolium redox mediator was studied by recording approach curves over a conducting and insulating substrate in an undiluted ionic liquid. The SECM approach curve over the conducting substrate displays a positive feedback, as observed in classical solvent. However, in the case of the insulating substrate, the approach curve reveals different shapes, depending on the used approach speed. In this configuration, low approach speed is necessary to reach the expected negative feedback. Interestingly, at a very close distance between the UME and the insulating substrate, a thin film behavior is revealed. In addition, the approach curves on both insulator and conducting substrates can be reconstructed from punctual responses at different distance tip-substrate. The latter match perfectly with the expected theoretical curves over conducting and insulating under diffusion control.

Surface Initiated Immobilization of Molecules Contained in an Ionic Liquid Framework.

A simple and general route for the immobilization of molecules containing ionic liquids framework was described. The proposed approach is inspired from the classical synthesis of ionic liquid and labeled surface-initiated synthesis of molecules bearing ionic liquid components. In the first step, bromide end layer was electrochemically grafted onto the electrode surface followed by its reaction with imidazole derivatives. The generated modified materials were characterized by electrochemistry and by X-ray photoelectron spectroscopy (XPS). As a result, molecule-based ionic liquids were successfully attached onto electrode material. The possibility to perform an anion-exchange reaction within the layer was demonstrated. Furthermore, the proposed surface functionalization approach was successfully performed without requiring the synthesis of any intermediate. The generated structures provide multifunctional systems containing ions, immobilized cation and mobile anion, and redox species.

Formation of a bifunctional redox system using electrochemical reduction of platinum in ferrocene based ionic liquid and its reactivity with aryldiazonium.

The electrochemical reduction of platinum in ferrocene based ionic liquid, 1-ferrocenylethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([FcEMIM][TFSI]), has been investigated on both macro and micro platinum electrodes. The electrochemical and X-ray photoelectron spectroscopy (XPS) analyses of the reduced platinum evidence the presence and the intercalation of the ferrocene imidazolium into the platinum surface. The resulting metal-organic phase was anticipated to be of the general formula [Ptn(δ-), FcEMIM(δ+)]. Next, the spontaneous reaction of the electrogenerated Pt phases as reducing agent with phenyl diazonium salts was performed. The "reduced platinum phase" reacts with nitrophenyl diazonium salts, without applying any external potential, and conduces to the attachment of nitrophenyl groups onto the surface. Detailed experiments based on cyclic voltammetry and XPS were carried out to follow the occurrence of the grafting. Indeed, both experiments evidence the presence of ferrocene and nitrophenyl groups on the Pt surface. As a result, a bifunctional active surface containing two redox systems has been obtained.

When electron transfer meets electron transport in redox-active molecular nanojunctions.

A scanning electrochemical microscope (SECM) was used to arrange two microelectrodes face-to-face separated by a micrometric gap. Polyaniline (PANI) was deposited electrochemically from the SECM tip side until it bridged the two electrodes. The junctions obtained were characterized by following the current through the PANI as a function of its electrochemical potential measured versus a reference electrode acting as a gate electrode in a solid-state transistor. PANI nanojunctions showed conductances below 100 nS in the oxidized state, indicating control of the charge transport within the whole micrometric gap by a limited number of PANI wires. The SECM configuration makes it possible to observe in the same experiment and in the same current range the electron-transfer and electron-transport processes. These two phenomena are distinguished here and characterized by following the variation of the current with the bias voltage and the scan rate. The electron-transfer current changes with the scan rate, while the charge-transport current varies with the bias voltage. Finally, despite the initially micrometric gap, a junction where the conductance is controlled by a single oligoaniline strand is achieved.

Oxidative and stepwise grafting of dopamine inner-sphere redox couple onto electrode material: electron transfer activation of dopamine.

The immobilization of dopamine, a neurotransmitter, onto macroelectrode and microelectrode surfaces has been performed following two strategies. The first consists of a one-step grafting based on electrochemical oxidation of an amino group in acidic media. The second is a stepwise process starting with electrochemical grafting of diazonium, leading to the attachment of aryl layer bearing an acidic headgroup, followed by chemical coupling leading to immobilized dopamine molecules onto the electrode surface. Electrochemical, infrared (IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) analyses evidence that both methods are suitable for the immobilization of dopamine onto millimetric and micronic electrodes. The electrochemical responses of modified electrodes demonstrate that the electroactivity of the attached dopamine layer appears unaffected by the nature of the spacer, alkyl or aryl layers, suggesting that the communication, through tunneling, between the attached dopamine and the electrode is possible. More interestingly, the dopamine-modified electrode exhibits electron transfer activation toward dopamine in solution. As a result, not only does the dopamine modified electrode yield a fast electron transfer with lower ΔE(p) (30 mV) than the majority of pretreatment procedures but also the ΔE(p) is as small as that observed for more complex surface treatments.

Formation of mixed organic layers by stepwise electrochemical reduction of diazonium compounds.

This work describes the formation of a mixed organic layer covalently attached to a carbon electrode. The strategy adopted is based on two successive electrochemical reductions of diazonium salts. First, bithiophene phenyl (BTB) diazonium salt is reduced using host/guest complexation in a water/cyclodextrin (ß-CD) solution. The resulting layer consists of grafted BTB oligomers and cyclodextrin that can be removed from the surface. The electrochemical response of several outer-sphere redox probes on such BTB/CD electrodes is close to that of a diode, thanks to the easily p-dopable oligo(BTB) moieties. When CD is removed from the surface, pinholes are created and this diode like behavior is lost. Following this, nitrophenyl (NP) diazonium is reduced to graft a second component. Electrochemical study shows that upon grafting NP insulating moieties, the diode-like behavior of the layer is restored which demonstrates that NP is grafted predominately in the empty spaces generated by ß-CD desorption. As a result, a mixed BTB/NP organic layer covalently attached to a carbon electrode is obtained using a stepwise electrochemical reduction of two diazonium compounds.

Electrochemical fabrication of highly stable redox-active nanojunctions.

Redox-gated molecular junctions were obtained starting with a relatively large gap between two electrodes, in the micrometer range, followed by electrochemical polymerization of aniline. Polyaniline (PANI) grows from the tip side until it bridges the two electrodes. The resulting junctions were characterized electrochemically by following the variation of the tip-substrate current as a function of the electrochemical gate potential for various bias voltages and by recording their I(V) characteristics. The two electrodes make contact through PANI wires, and microjunctions with conductances around 10(-3) S were obtained. On the basis of a similar setup, PANI nanojunctions with conductances between 10(-7) and 10(-8) S were made, where the current appears to be controlled by fewer than 10 oligoaniline strands. Despite the small number of strands connecting the two electrodes, the junctions are highly stable even when several successive potential sweeps are performed. Comparison of the conductance measured in the oxidized and reduced states leads to an on/off ratio of about 70-100, which is higher than that reported for a single aniline heptamer bridging two electrodes, highlighting the interest of connecting a few tens of molecules using the scanning electrochemical microscopy (SECM) configuration. In some cases, the switching of the PANI takes place in several individual conductance steps close to that obtained for a single oligoaniline. Finally, starting with a microjunction and mechanically withdrawing the tip shrinks it down to the nanometer scale and makes it possible to reach the regime where the conductance is controlled by a limited number of strands. This work presents an easy method for making redox-gated nanojunctions and for probing the conductance of a few oligoanilines despite an initially large tip-substrate gap.

Host-guest complexation: a convenient route for the electroreduction of diazonium salts in aqueous media and the formation of composite materials.

Electrochemical grafting of a water-insoluble diazonium salt in aqueous media onto an electrode surface was achieved by host-guest complexation. 1-(2-Bisthienyl)-4-aminobenzene (BTAB) was solubilized in a water/beta-cyclodextrin solution (beta-CD). The corresponding diazonium salt was generated in situ then electroreduced. This process leads to the attachment of bithiophene or short oligothiophene groups to the electrode surface. The modified surfaces were analyzed by cyclic voltammetry (CV), scanning electrochemical microscopy (SECM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), and atomic force microscopy (AFM). The electrochemical investigations show that the water-based modified surface is similar to one generated in acetonitrile without beta-CD. Thus, the attached organic layer behaves like an electrochemical switch (above some threshold potential, a soluble external probe is oxidized, but the oxidized form cannot be reduced). The modified surfaces consist of grafted bisthienylbenzene (BTB) and cyclodextrins that can be removed from the surface. This procedure may be considered as a new means of creating a surface made of submicrometric holes in an organic semiconducting layer.

Giant plasmon resonance shift using poly(3,4-ethylenedioxythiophene) electrochemical switching.

Herein, we report the variation of localized surface plasmon resonance (LSPR) of gold nanoparticle (NP) arrays covered by poly(3,4-ethylenedioxythiophene) (PEDOT) as a function of the electronic state of the polymer. Giant shifts and fine-tuning of the LSPR of gold NPs surrounded by PEDOT/sodium docecyl sulfate have been achieved. The color variations of plasmonic/conducting polymer (CP) devices are given not only by changes of the optical properties of the CP upon doping but also by a close synergy of the optical properties of CP and NP. Such systems can considerably extend the field of CP-based electrochromic devices.

Ionic liquid viscosity effects on the functionalization of electrode material through the electroreduction of diazonium.

The electrochemical reduction of 4-nitrophenyl diazonium, NPD, in different ionic liquids presenting different viscosities has been investigated. The electrochemical studies show that the reduction of diazonium leading to the formation of its corresponding radical occurs whatever the viscosity of the grafting media. Following that, the presence of an organic layer attached to the electrode after electrochemical treatment was evidenced by cyclic voltammetry (CV) in acidic media thanks to the presence of nitro groups. Moreover, infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) confirm the presence of a nitrophenyl (NP) layer attached to the electrode material. Next, the examination of the electrochemical data through the measurement of the charge, corresponding to the reduction of the attached nitrophenyl (NP) moieties, shows that the surface concentration of NP, Γ(NP), decreases when the viscosity, η, of the grafting media increases. Additionally, in the case of the more viscous ionic liquid, N-tributyl-N-methylammonium bis(trifluoromethylsulfonyl)imide [Bu(3)MeN] [NTf(2)], a cosolvent has been added leading to fine decrease of the viscosity. The IR and CV investigations of the modified electrodes demonstrate the decrease of the amount of the attached molecules when the viscosity of the grafting media increases. In addition, a correlation between Γ(NP) as function of 1/η was observed. Finally, XPS and AFM experiments lead to an estimate of the thickness of the attached layer. As a result, both methods are in perfect agreement and thicknesses of 4 and 1 nm are measured after grafting in acetonitrile and in pure ionic liquid [Bu(3)MeN] [NTf(2)], respectively. By comparison with classical solvent, the use of viscous ionic liquid for the grafting leads to a decrease in the amount of the attached molecules and conduce to the formation of thinner or less dense layer.

Recent Advances in the Development of Organic and Organometallic Redox Shuttles for Lithium-Ion Redox Flow Batteries.

In recent years, redox flow batteries (RFBs) and derivatives have attracted wide attention from academia to the industrial world because of their ability to accelerate large-grid energy storage. Although vanadium-based RFBs are commercially available, they possess a low energy and power density, which might limit their use on an industrial scale. Therefore, there is scope to improve the performance of RFBs, and this is still an open field for research and development. Herein, a combination between a conventional Li-ion battery and a redox flow battery results in a significant improvement in terms of energy and power density alongside better safety and lower cost. Currently, Li-ion redox flow batteries are becoming a well-established subdomain in the field of flow batteries. Accordingly, the design of novel redox mediators with controllable physical chemical characteristics is crucial for the application of this technology to industrial applications. This Review summarizes the recent works devoted to the development of novel redox mediators in Li-ion redox flow batteries.

The Effect of Heteroatom Doping on Nickel Cobalt Oxide Electrocatalysts for Oxygen Evolution and Reduction Reactions.

The synthesis of nickel cobalt oxide materials and their electrocatalytic performance toward the oxygen reduction and evolution reactions are reported. Nickel cobalt oxides were synthesized in a sol-gel process with different precursors, namely nitrate, sulfate, and chloride. Structural analyses show that the structures have mesoporous morphologies and indicate the formation of nickel cobalt oxide spinel structures with a size ranging from 35 to 65 nm. Furthermore, the physicochemical properties differ depending on the nature of the selected precursors, including the materials' morphology and the chemical composition. Electrocatalytic investigations demonstrate that the catalytic activity toward the oxygen reduction reaction (ORR) could be modulated between two- and four-electron pathways, depending on the precursors used. The Cl-NiCoO sample displays a selective two-electron reduction of O2 , with H2 O2 production higher than 90 %. The sample prepared using sulfate displays the highest performance toward the oxygen evolution reaction (OER), with a low overpotential value (0.34 V) to drive a current density of 10 mA.cm-1 . Overall, these results confirm that the chemical composition of the precursor used during the nanomaterials synthesis can be used to tune the electrocatalytic performances toward ORR and OER.

Grafting oligothiophenes on surfaces by diazonium electroreduction: a step toward ultrathin junction with well-defined metal/oligomer interface.

The functionalization of electrode materials through diazonium electroreduction using a heteroaromatic compound, without phenyl groups, has been investigated for the first time. The electrochemical reduction of 2-aminoterthiophenyldiazonium cation, generated in situ, coats the electrode (glassy carbon (GC), gold or platinum) with an ultrathin organic layer, shown by X-ray photoelectron spectroscopy (XPS) of that deposited on gold to consist of terthiophene or oligothiophene. The coating is electroactive at potential close to that of terthiophene in solution. The electrochemical response of the modified GC electrode in the presence of various reversible redox couples shows that the attached layer acts as a conductive switch. It behaves as a barrier to electron transfer when the standard redox potential is below 0.5 V/SCE; in this case diode-like behavior is observed. However, for more oxidizing redox probes the layer can be considered as transparent and no barrier effect is observed. The layer deposited on a platinum ultramicroelectrode (UME) behaves similarly to that obtained on the large GC electrode. Scanning electrochemical microscopy (SECM) can be performed using this electroswitchable modified platinum UME which can act as a filter toward competitive redox exchange pathways.

Electrochemical Growth of Metallic Nanoparticles onto Immobilized Polymer Brush Ionic Liquid as a Hybrid Electrocatalyst for the Hydrogen Evolution Reaction.

Platinum and palladium are the first choice electrocatalysts to drive the hydrogen evolution reaction. In this report, surface modification was introduced as a potential approach to generate hybrid electrocatalyst. The immobilized polymer brush, poly(1-allyl-3-methylimidazolium) (PAMI), was used as a nanostructured template for guiding the electrochemical deposition of metallic nanoparticles (Pd, Pt). The intrinsic properties of the polymer brush in term of nanostructured architecture and the anions mobility within the polymer was exploited to generate a hybrid electrocatalyst. The latter was generated using two different approaches including the direct electrochemical deposition of Pd or Pt metal and the indirect approach through the anion exchange reaction followed by the electrochemical deposition under self-electrolytic conditions. The hybrid structure based on the polymer/metallic NP exhibits an enhancement of the catalytic performance toward hydrogen evolution reaction with a low Tafel slope and overpotential. Interestingly, the indirect approach leads to decrease the metal loading by two orders of magnitude, when compared to those generated in the absence of the polymeric layer, while retaining the electrocatalytic performance.

Conducting ferrocene monolayers on nonconducting surfaces.

The redox activity of a ferrocenyl monolayer grafted on an n-type Si111 substrate was investigated by scanning electrochemical microscopy (SECM) in conditions where the substrate plays the role of an insulator. This approach permits the differentiation between the different possible electron-transfer and mass-transport pathways occurring at the interface. As an exciting result, the thin ferrocenyl monolayer behaves like a purely conducting material, highlighting very fast electron communication between immobilized ferrocenyl headgroups in a 2D-like charge-transport mechanism.

Variations of diffusion coefficients of redox active molecules in room temperature ionic liquids upon electron transfer.

In ionic liquids, the diffusion coefficients of a redox couple vary considerably between the neutral and radical ion forms of the molecule. For a reduction, the inequality of the diffusion coefficients is characterized by the ratio gamma = D(red)/D(ox), where D(red) and D(ox) are the diffusion coefficients of the electrogenerated radical anion and of the corresponding neutral molecule, respectively. In this work, measurements of gamma have been performed by scanning electrochemical microscopy (SECM) in transient feedback mode, in three different room temperature ionic liquids (RTILs) sharing the same anion and with a series of nitro-derivative compounds taken as a test family. The smallest gamma ratios were determined in an imidazolium-based RTIL and with the charge of the radical anion localized on the nitro group. Conversely, gamma tends to unity when the radical anion is fully delocalized or when the nitro group is sterically protected by bulky substituents. The gamma ratios, standard potentials of the redox couple measured in RTILs, and those observed in a classical organic solvent were compared for the investigated family of compounds. The stabilization energies approximately follow the gamma ratios in a given RTIL but change considerably between ionic liquids with the nature of the cation.

Determining Li+-Coupled Redox Targeting Reaction Kinetics of Battery Materials with Scanning Electrochemical Microscopy.

The redox targeting reaction of Li+-storage materials with redox mediators is the key process in redox flow lithium batteries, a promising technology for next-generation large-scale energy storage. The kinetics of the Li+-coupled heterogeneous charge transfer between the energy storage material and redox mediator dictates the performance of the device, while as a new type of charge transfer process it has been rarely studied. Here, scanning electrochemical microscopy (SECM) was employed for the first time to determine the interfacial charge transfer kinetics of LiFePO4/FePO4 upon delithiation and lithiation by a pair of redox shuttle molecules FcBr2+ and Fc. The effective rate constant keff was determined to be around 3.70-6.57 × 10-3 cm/s for the two-way pseudo-first-order reactions, which feature a linear dependence on the composition of LiFePO4, validating the kinetic process of interfacial charge transfer rather than bulk solid diffusion. In addition, in conjunction with chronoamperometry measurement, the SECM study disproves the conventional "shrinking-core" model for the delithiation of LiFePO4 and presents an intriguing way of probing the phase boundary propagations induced by interfacial redox reactions. This study demonstrates a reliable method for the kinetics of redox targeting reactions, and the results provide useful guidance for the optimization of redox targeting systems for large-scale energy storage.

Plasmon-Induced Conductance Switching of an Electroactive Conjugated Polymer Nanojunction.

A plasmonic molecular electronic device, consisting of poly(3,4-ethylenedioxythiophene) (PEDOT) nanowires bridging an ultramicroelectrode and an indium tin oxide (ITO) substrate covered by gold nanoparticles (Au NPs), has been developed. Light irradiation of this device has a dramatic impact on its conductance. Polymer strands, maintained electrochemically in their oxidized, conducting state, reversibly switch to their insulating state upon irradiation by visible-wavelength light, resulting in a sharp decrease in the conductance. The high-conductance state is restored when the light is turned off. Switching depends on the wavelength and the intensity of the incident light. It is due to reversible reduction of the nanosized region of PEDOT nanowires in contact with a gold NP and is attributed to plasmon-induced hot-electron injection into the PEDOT. The high/low conductance ratio can be as great as 1000, and switching requires low light intensity (220 W/m2). These results could open the way to the design of a new family of optoelectronic switches.

Copper Nanowires through Oriented Mesoporous Silica: A Step towards Protected and Parallel Atomic Switches.

The formation of copper atomic contacts has been investigated. Copper nanowires were grown by electrochemical deposition, in the scanning electrochemical microscopy (SECM) configuration, from a platinum microelectrode to an indium tin oxide (ITO) substrate. Self-termination leaves copper filaments between the two electrodes with an atomic point contact at the ITO electrode. Histogram analysis shows that the conductance of this contact is close to, or less than, 1 G0. Atomic contacts were also fabricated on ITO electrodes covered with vertically-aligned mesoporous silica films. Scanning Transmission Electron Microscopy images show that copper filaments occupy individual isolated nanopores. Contacts generated on bare ITO break down rapidly in sodium salicylate, whereas those generated in ITO/nanopores are unaffected; the nanopores protect the copper filaments. Finally, atomic switch behaviour was obtained using these ITO and ITO/nanopores electrodes.