Electrochemical Metal Recycling: Recovery of Palladium from Solution and In Situ Fabrication of Palladium-Carbon Catalysts via Impact Electrochemistry.

Recycling of critical materials, regeneration of waste, and responsible catalyst manufacture have been repeatedly documented as essential for a sustainable future with respect to the environment and energy production. Electrochemical methods have become increasingly recognized as capable of achieving these goals, and "impact" electrochemistry, with the advantages associated with dynamic nanoelectrodes, has recently emerged as a prime candidate for the recovery of metals from solution. In this report, the nanoimpact technique is used to generate carbon-supported palladium catalysts from low-concentration palladium(II) chloride solutions (i.e., a waste stream mimic) as a proof of concept. Subsequently, the catalytic properties of this material in both synthesis (Suzuki coupling reaction) and electrocatalysis (hydrogen evolution) are demonstrated. Transient reductive impact signals are shown and analyzed at potentials negative of +0.4 V (vs SCE) corresponding to the onset of palladium deposition in traditional voltammetry. Direct evidence of Pd modification was obtained through characterization by environmental scanning electron microscopy/energy-dispersive X-ray spectroscopy, inductively coupled plasma mass spectrometry, X-ray photoelectron spectroscopy, transmission electron microscopy, and thermogravimetric analysis of impacted particles. This showed the formation of deposits of Pd0 partially covering the 50 nm carbon black particles with approximately 14% Pd (wt %) under the conditions used. This material was then used to demonstrate the conversion of iodobenzene into its biphenyl product (confirmed through nuclear magnetic resonance) and the successful production of hydrogen as an electrocatalyst under acidic conditions (under cyclic voltammetry).

Nanoparticle electrochemistry.

This perspective article provides a survey of recent advances in nanoscale electrochemistry, with a brief theoretical background and a detailed discussion of experimental results of nanoparticle based electrodes, including the rapidly expanding field of "impact electrochemistry". Following this, recent advances in experimental study of exotic nanoparticle electrodes are reviewed.

Modular construction of size-selected multiple-core Pt-TiO2 nanoclusters for electro-catalysis.

Size-selected binary platinum-titanium dioxide (Pt-TiO2) clusters have been generated using a magnetron sputtering gas condensation cluster source and imaged using a Scanning Transmission Electron Microscope (STEM) in High Angle Annular Dark Field (HAADF) mode. The core-shell clusters exhibit a Pt core of preferred size 30 ± 6 atoms (1 nm), embedded in an oxidised Ti shell, independent of the overall cluster size (varied between 2 nm and 5 nm). Smaller clusters, with mass ≤50 000 Daltons, show a single Pt core while larger clusters, ≥55 000 Daltons, feature multiple Pt cores, either isolated or aggregated within the TiO2 shell. These clusters may have applications in solar hydrogen production; preliminary work indicates catalytic active in the hydrogen evolution reaction.

Electrochemistry of nickel nanoparticles is controlled by surface oxide layers.

The kinetics of proton reduction are reported for Ni and NiO surfaces and compared to that measured at Ni@NiO nanoparticles. Kinetic acceleration is found to occur by virtue of oxide overlayers and not due to size effects on the nanoscale.

Nanoparticle-electrode impacts: the oxidation of copper nanoparticles has slow kinetics.

The electrochemical oxidation of copper nanoparticles in aqueous solution was studied via their electrolysis upon impacting a carbon electrode held at a suitable anodic potential. The oxidations were found to be quantitative such that complete oxidation of the particle took place allowing their sizing. Experiments were performed in 1.0 M HNO(3) and in 1.0 M HNO(3)-0.1 M KCl. In the former case a two electron oxidation to Cu(2+) was seen at a formal potential of +0.11 V (vs. SCE). In the latter case two separate one-electron oxidations at -0.01 V and +0.26 V were seen. In addition, theoretical results were derived for the analysis of impact-charge vs. potential data for reversible and irreversible charge transfer kinetics for nanoparticle oxidation. This enabled the inference that overpotential is required for the oxidations and Butler-Volmer transfer coefficients to be determined. The latter are compared with literature data seen for macroscopic copper.

The charge transfer kinetics of the oxidation of silver and nickel nanoparticles via particle-electrode impact electrochemistry.

The electro-oxidation of silver and nickel nanoparticles in aqueous solution was studied via their collisions with a carbon electrode. The average charge passed per impact varies with electrode potential and was analysed to determine that AgNPs display an electrochemically fast ("reversible") one-electron oxidation, whilst the NiNPs exhibit slow ("irreversible") 2-electron kinetics. Kinetic parameters are reported.

Impact Electrochemistry of MoS2: Electrocatalysis and Hydrogen Generation at Low Overpotentials.

MoS2 materials have been extensively studied as hydrogen evolution reaction (HER) catalysts. In this study nanoparticulate MoS2 is explored as a HER catalyst through impact voltammetry. The onset potential was found to be -0.10 V (vs RHE) at pH 2, which was confirmed to be due to HER by scale-up of the impact experiment to generate and collect a sufficient volume of the gas to enable its identification as hydrogen via gas chromatography. This is in contrast to electrodeposited MoS2, which was found to be stable in pH 2 sulfuric acid solution with an onset potential of -0.29 V (vs RHE), in good agreement with literature. XPS was used to categorize the materials and confirm the chemical composition of both nanoparticles and electrodeposits, with XRD used to analyze the crystal structure of the nanoparticles. The early onset of HER was postulated from kinetic analysis to be due to the presence of nanoplatelets of about 1-3 trilayers participating in the impact reactions, and AFM imaging confirmed the presence of these platelets.

Nanoparticle-electrode collision processes: the underpotential deposition of thallium on silver nanoparticles in aqueous solution.

The electrochemistry of collisions between metal nanoparticles (NPs) and electrode surfaces has been of recent interest with the development of anodic particle coulometry as a characterisation method. For the first time the underpotential deposition of metal ions from solution onto metal nanoparticles during collisions between the NPs and an inert electrode is reported.

Towards the electrochemical quantification of the strength of garlic.

A simple but sensitive technique has been demonstrated towards the electroanalytical quantification of the strength of garlic. This technique can also be used to quantify dialkyldisulfides. The cyclic voltammetry of bromide was found to be a sensitive electrochemical probe, electrogenerated bromine reacting with dialkyldisulfides to catalytically regenerate bromide, resulting in a significant increase in peak current. A linear response of current vs. concentration was observed between 0.1 and 15 mM dipropyldisulfide at edge plane pyrolytic graphite (EPPG) electrodes; a smaller range up to ca. 5 mM was available at screen printed carbon electrodes (SPCEs), with a detection limit (from 3σ) of 0.067 mM. The response of diallyldisulfide was found to be essentially identical. Shaking garlic puree in acetonitrile for 5 minutes, followed by dilution with water then recording the voltammetry at the cheap, disposable SPCE gave a linear trend in current with respect to the quantity of garlic present, corresponding to the diallyldisulfide extracted. This has potential applications in monitoring the garlic content of medicinal supplements, batch-to-batch variation and the stability of garlic during storage.

Cisplatin adducts of DNA as precursors for nanostructured catalyst materials.

The synthesis and characterisation of novel metal-modified DNA precursors for fuel cell catalyst development are described. Material precursors in the form of metal-DNA complexes were prepared through the reaction of DNA with cisplatin at various loadings and spectroscopically tested to confirm the platinum binding mode and the degree of complexation. The surface morphology of the DNA-metal material was analysed by Scanning Transmission Electron Microscopy (STEM), which revealed the extent of platinum nanocluster formation, with low metal loadings leading to observation of individual platinum atoms. Electrochemical measurements showed a greater electrocatalytic activity for the hydrogen evolution reaction (HER) with increased platinum loadings, shifting the half wave potential, E 1/2, away from the glassy carbon limit towards that of a bulk Pt electrode. This is explained further by Tafel plots, from which a change in the mechanism of the apparent rate limiting step for proton reduction from a Volmer to a Heyrovsky mechanism is postulated as the platinum loading increases.

A photoelectrochemical method for tracking the motion of Daphnia magna in water.

We present the novel use of photoelectrochemistry to detect and monitor the motion of a single Daphnia magna swimming in a confined volume of water. Using an array of individually-addressable electrodes under illumination and potentiostatted so that a photocurrent is generated, the motion of the daphnid is detected by means of measuring "dark" transients as the shadow cast by the moving sphere passes over each electrode. The method can be used to determine the size and speed of a single daphnid.

Investigating the concept of diffusional independence. Potential step transients at nano- and micro-electrode arrays: theory and experiment.

Microelectrode arrays find broad application in electroanalysis offering the enhanced sensitivity associated with microelectrodes, but with a high total current output. Such arrays are often constructed to make the electrodes 'diffusionally independent'. To emphasize that this is a time dependent property, a two-dimensional simulation, in conjunction with the diffusional domain approach, is used to model potential step transient currents at microelectrode arrays. Two types of array, hexagonal and cubic, are considered. In both cases the absolute (not relative) microelectrode separation distance has a significant effect on transient current. Three different regimes of transient current versus time can be observed at microelectrode arrays. At short times the transient response of isolated microelectrodes is seen, then at intermediate times the steady-state response of independent electrodes can be observed. At longer times planar diffusion to the entire array takes over. It follows that only at timescales corresponding to the first two regimes can the electrodes be considered as diffusionally independent. To verify the theory the potential step experiment is performed at a regularly spaced hexagonal iridium microdisk array. Theory is found to be in a good agreement with the experimental results.

A method for the positioning and tracking of small moving particles.

On the move: Electrochemistry has been used to detect and monitor the motion of a single 330 microm sphere in both time and space (see picture). The motion was recorded simultaneously by video and chronoamperometry, which showed an excellent correlation. The ability to fabricate electrode arrays capable of spatial resolution at the sub-micrometer scale opens the possibility of using this technique to monitor considerably smaller particles.

Alkali metal reductions of organic molecules: why mediated electron transfer from lithium is faster than direct reduction.

Lithium metal reductions are widely employed in organic synthesis, where it is common to employ a "mediator" to speed up the electron transfer kinetics. We present experimental data for the electrode kinetics of the reduction of the most common mediator, 4,4'-di-tert-butyl-1,1'-biphenyl (DBB) in tetrahydrofuran (THF) over a range of temperatures. Using corresponding data for the oxidation of lithium we present quantitative estimates of the kinetic advantage for the use of DBB as a mediator in lithium reductions, over, in particular, direct reduction using lithium metal.

Enhancement of the Hydrogen Evolution Reaction from Ni-MoS2 Hybrid Nanoclusters.

This report focuses on a novel strategy for the preparation of transition metal-MoS2 hybrid nanoclusters based on a one-step, dual-target magnetron sputtering, and gas condensation process demonstrated for Ni-MoS2. Aberration-corrected STEM images coupled with EDX analysis confirms the presence of Ni and MoS2 in the hybrid nanoclusters (average diameter = 5.0 nm, Mo:S ratio = 1:1.8 ± 0.1). The Ni-MoS2 nanoclusters display a 100 mV shift in the hydrogen evolution reaction (HER) onset potential and an almost 3-fold increase in exchange current density compared with the undoped MoS2 nanoclusters, the latter effect in agreement with reported DFT calculations. This activity is only reached after air exposure of the Ni-MoS2 hybrid nanoclusters, suggested by XPS measurements to originate from a Ni dopant atoms oxidation state conversion from metallic to 2+ characteristic of the NiO species active to the HER. Anodic stripping voltammetry (ASV) experiments on the Ni-MoS2 hybrid nanoclusters confirm the presence of Ni-doped edge sites and reveal distinctive electrochemical features associated with both doped Mo-edge and doped S-edge sites which correlate with both their thermodynamic stability and relative abundance.

The electrochemical detection of tagged nanoparticles via particle-electrode collisions: nanoelectroanalysis beyond immobilisation.

The use of particle-impact coulometry in identifying and quantifying nanoparticles tagged (or labelled) with electroactive molecules is demonstrated via the detection of 1,4-nitrothiophenol-tagged silver nanoparticles in aqueous dispersion at potentials more negative than -0.17 V (vs. Ag/AgCl, the reduction potential of nitrothiophenol) via monitoring of particle-electrode collisions.

Gold nanoparticles show electroactivity: counting and sorting nanoparticles upon impact with electrodes.

Gold nanoparticles (AuNPs) in aqueous 0.10 M HCl are shown to be electroactive at oxidising potentials greater than 1.0 V (vs. Ag/AgCl) by means of voltammetric monitoring of AuNP-electrode collisions. The method promises the use of anodic particle coulometry for the detection and characterisation of the AuNPs.