Voltammetry at graphite electrodes: The oxidation of hexacyanoferrate (II) (ferrocyanide) does not exhibit pure outer-sphere electron transfer kinetics and is sensitive to pre-exposure of the electrode to organic solvents

Reductive Inner-Sphere Electrosynthesis of Ammonia via a Nonelectrocatalytic Outer-Sphere Redox

Electrochemistry of outer-sphere redox molecules involves an essentially intact primary coordination sphere with minimal secondary sphere adjustments, resulting in very fast electron transfer events even without a noble metal-based electrocatalyst. Departing from conventional electrocatalytic paradigms, we incorporate these minimal reaction coordinate adjustments of outer-sphere species to stimulate the electrocatalysis of energetically challenging inner-sphere substrates. Through this approach, we are able to show an intricate 8e- and 9H+ transfer inner-sphere reductive electrocatalysis at almost half the energy input of a conventional inner-sphere electron donor. This methodology of employing outer-sphere redox species has the potential to notably improve the cost and energy benefits in electrochemical transformations involving fundamental substrates such as water, CO2, N2, and many more.

Carbon Thin-Film Electrodes as High-Performing Substrates for Correlative Single Entity Electrochemistry

Correlative methods to characterize single entities by electrochemistry and microscopy/spectroscopy are increasingly needed to elucidate structure-function relationships of nanomaterials. However, the technical constraints often differ depending on the characterization techniques to be applied in combination. One of the cornerstones of correlative single-entity electrochemistry (SEE) is the substrate, which needs to achieve a high conductivity, low roughness, and electrochemical inertness. This work shows that graphitized sputtered carbon thin films constitute excellent electrodes for SEE while enabling characterization with scanning probe, optical, electron, and X-ray microscopies. Three different correlative SEE experiments using nanoparticles, nanocubes, and 2D Ti3C2Tx MXene materials are reported to illustrate the potential of using carbon thin film substrates for SEE characterization. The advantages and unique capabilities of SEE correlative strategies are further demonstrated by showing that electrochemically oxidized Ti3C2Tx MXene display changes in chemical bonding and electrolyte ion distribution.

Electrochemistry needs electrochemists: “goodbye to rotating discs”

The essential need for expert, fully trained electrochemists in the successful application of the subject is illustrated with several examples including the use of rotating electrodes and impedance spectroscopy where the use of the techniques in “black box” mode non-experts is likely to lead to disappointment or embarrassment.

The Influence of Spectator Cations on Solvent Reorganization Energy Is a Short-Range Effect

In this manuscript, we use classical molecular dynamics simulation to explore the origin of specific cation effects on the rates of bulk-phase aqueous electron transfer (ET) reactions. We consider 0.6 M solutions of Cl^- and a series of different cations: Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺. We evaluate the collective electrostatic fluctuations that drive Marcus-like ET and find that they are essentially unaffected by changes in the cationic species. This finding implies that the structure making/breaking properties of various cations do not exert a significant influence on bulk-phase ET reactions. We evaluate the role of ion pairing in these specific cation effects and find, unsurprisingly, that model redox anions that are more highly charged tend to pair more effectively with spectator cations than their monovalent counterparts. We demonstrate that this ion pairing significantly affects local electrostatic fluctuations for the anionic redox species and thus conclude that ion pairing is one of the likely sources of rate-dependent cation effects in aqueous ET reactions.

Ultrasonic exfoliation of carbon fiber: electroanalytical perspectives

Abstract

Electrochemical anodisation techniques are regularly used to modify carbon fiber surfaces as a means of improving electrochemical performance. A detailed study of the effects of oxidation (+ 2 V) in alkaline media has been conducted and Raman, XPS and SEM analyses of the modification process have been tallied with the resulting electrochemical properties. The co-application of ultrasound during the oxidative process has also been investigated to determine if the cavitational and mass transport features influence both the physical and chemical nature of the resulting fibers. Marked discrepancies between anodisation with and without ultrasound is evident in the C1s spectra with variations in the relative proportions of the electrogenerated carbon-oxygen functionalities. Mechanisms that could account for the variation in surface species are considered.

Graphic abstract

Linkage Effect in the Heterogenization of Cobalt Complexes by Doped Graphene for Electrocatalytic CO2 Reduction

Immobilization of planar Co^II -2,3-naphthalocyanine (NapCo) complexes onto doped graphene resulted in a heterogeneous molecular Co electrocatalyst that was active and selective to reduce CO₂ into CO in aqueous solution. A systematic study revealed that graphitic sulfoxide and carboxyl dopants of graphene were the efficient binding sites for the immobilization of NapCo through axial coordination and resulted in active Co sites for CO₂ reduction. Compared to carboxyl dopants, the sulfoxide dopants further improved the electron communication between NapCo and graphene, which led to the increase of turnover frequency of the Co sites by about 3 times for CO production with a Faradic efficiency up to 97 %. Pristine NapCo in the absence of a graphene support did not display efficient electron communication with the electrode and thus failed to serve as the electrochemical active site for CO₂ reduction under the identical conditions.

MoS2 versatile spray-coating of 3D electrodes for the hydrogen evolution reaction

A facile one-step MoS₂ spray-coating method was applied to a range of rigid, flexible, porous and 3D printed carbon-based surfaces, yielding high loadings in MoS₂ flakes. The characterization of MoS₂ flakes from a commercial lubricant spray reveals up to micron-scale bulk sheets of the layered material, constituted in its majority by the semiconducting 2H polymorph, in the presence of the metallic 1T phase. Consequently, the process generates MoS₂ spray-coated surfaces with improved hydrogen evolution reaction (HER) catalytic performance. In the case of carbon-based screen printed electrodes (SPE), a short-term thermal post-treatment of the MoS₂ spray-coated SPE had a further beneficial effect in the HER overpotential. The MoS₂ spray-coated 3D metallic meshes held the lowest HER overpotential of the series. Finally, MoS₂ spray-coated 3D printed electrodes yielded improved heterogeneous charge transfer and a 500 mV shift in the required overpotential at a current density of -10 mA cm^-2. The MoS₂ spray-coated 3D printed electrode displayed an abundant coverage at the inner, external and planar zones of electrodes by the MoS₂ sheets, even after long-term operation conditions. These outcomes can be beneficial for future tailoring of MoS₂ spray-coated surfaces and their implementation in energy conversion technologies.

Electrochemical degradation of pyridine by Ti/SnO2-Sb tubular porous electrode

Diffusion in electrochemistry is a critical issue for water purification. Electrocatalytic reactor system in improving water quality is a useful way to induce convection to enhance diffusion. This study focuses on the preparation and the characterization of Ti/SnO2-Sb tubular porous electrode for degrading pyridine wastewater. The electrode as an anode in reactor system is prepared by coating SnO2-Sb as an electro-catalyst via Pechini method on the tubular porous Ti. Scanning Electron Microscopy, Energy Dispersive Spectrum, X-ray Diffraction and Pore Distribution are employed to evaluate the structure and morphology of the electrodes coatings, and Linear Sweep Voltammetry and Cyclic Voltammetry are used to illustrate the electrochemical properties of the electrodes coatings. Furthermore, the electrochemical oxidation performance of Ti/SnO2-Sb tubular porous electrode is characterized by degrading pyridine wastewater. The effects of flow and static pattern, initial pyridine concentration, supporting electrolyte concentration, current density and pH on the performance of the reactor were investigated in the electrocatalytic reactor system. The results indicated that the removal ratio of pyridine reaches maximum which is 98% under the optimal operation conditions, that are 100 mg L(-1) initial pyridine concentration, 10 g L(-1) supporting electrolyte concentration, 30 mA cm(-2) current density and pH 3. Transition state calculation based on the density function theory was combined with High Performance Liquid Chromatography, Gas Chromatography and Ionic Chromatography results to describe the pathway of pyridine degradation.

Interfacial electron-shuttling processes across KolliphorEL monolayer grafted electrodes

Covalently grafted KolliphorEL (a poly(ethylene glycol)-based transporter molecule for hydrophobic water-insoluble drugs; MW, ca. 2486; diameter, ca. 3 nm) at the surface of a glassy-carbon electrode strongly affects the rate of electron transfer for aqueous redox systems such as Fe(CN)6(3-/4-). XPS data confirm monolayer grafting after electrochemical anodization in pure KolliphorEL. On the basis of voltammetry and impedance measurements, the charge transfer process for the Fe(CN)6(3-/4-) probe molecule is completely blocked after KolliphorEL grafting and in the absence of a "guest". However, in the presence of low concentrations of suitable ferrocene derivatives as guests, mediated electron transfer across the monolayer via a shuttle mechanism is observed. The resulting amplification of the ferrocene electroanalytical signal is investigated systematically and compared for five ferrocene derivatives. The low-concentration electron shuttle efficiency decreases in the following sequence: (dimethylaminomethyl)ferrocene > n-butyl ferrocene > ferrocene dimethanol > ferroceneacetonitrile > ferroceneacetic acid.

Amplified electron transfer at poly-ethylene-glycol (PEG) grafted electrodes

Electron transfer at pegylated electrode surfaces is suppressed for Fe(CN)₆^3−/4− and then recovered in the presence of ferrocene-dimethanol.

Kinetics of reduction of aqueous hexaammineruthenium(III) ion at Pt and Au microelectrodes: electrolyte, temperature, and pressure effects

Rate constants kel obtained by impedance spectroscopy for the reduction of Ru(NH3)6(3+) at polycrystalline Pt and Au ultramicroelectrodes depend strongly on the identity and concentration of the anion present in the order CF3SO3(-) < Cl(-) < ClO4(-), but not on the cation of the supporting electrolyte (Na(+), K(+), H(+)). For Cl(-) as the sole anion present, kel is directly proportional to the total [Cl(-)], such that kel would be zero if Cl(-) were hypothetically absent, indicating that Cl(-) is directly involved in mediation of the Ru(NH3)6(3+/2+) electron transfer. For CF3SO3(-) as the sole counterion, the dependence of kel on the total [CF3SO3(-)] is not linear, possibly because blocking of the available electrode surface becomes dominant at high triflate concentrations. Volumes of activation ΔVel(⧧) for reduction of Ru(NH3)6(3+) at an electrode in presence of Cl(-) or CF3SO3(-) are much more negative than predictions based on theory (Swaddle, T. W. Chem. Rev.2005, 105, 2573) that has been successful with other electron transfer reactions but which does not take into account the involvement of the anions in the activation process. The strongly negative ΔVel(⧧) values probably reflect solvation increases peculiar to activation processes of Ru(III/II) am(m)ine complexes, possibly together with promotion of desorption of surface-blocking Cl(-) or CF3SO3(-) from electrodes by applied pressure. Frumkin corrections for Ru(NH3)6(3+) within the diffuse double layer would make ΔVel(⧧) even more negative than is observed, although the corrections would be small. The strongly negative ΔVel(⧧) values are inconsistent with reduction of Ru(NH3)6(3+) in direct contact with the metallic electrode surface, which would entail substantial dehydration of both the electrode and Ru(NH3)6(3+). Reduction of Ru(NH3)6(3+) can be regarded as taking place in hard contact with adsorbed water at the outer Helmholtz plane.