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Green hydrogen production can only be realized via water electrolysis using renewable energy sources. Proton exchange membrane water electrolyzers have been demonstrated as the technology of choice for mass production of green hydrogen due to their scalability and potential high efficiency. However, the technology is still relatively expensive due to the catalyst materials cost and operational limitations due to mass transfer and activation polarizations. During the oxygen evolution reaction, oxygen bubbles stick to the electrode surface and this causes a low reaction rate and high mass transfer losses. In this study, the commonly used electrocatalyst for oxygen evolution reactions; IrO2, is modified by intro-ducing magnetic Fe3O4 to achieve greater bubble separation at the anode during operation. The prepared composite catalysts were characterized using Scanning Electron Microscope, Energy Dispersive X-Ray Analysis, X-Ray Powder Diffraction, X-ray photoelectron spec-troscopy and Brunauer-Emmett-Teller characterization methods. The modified composite electrocatalyst samples are magnetized to investigate the magnetic field effect on oxygen evolution reaction performance in proton exchange membrane water electrolyzers. 90% IrO2-10% Fe3O4 and 80% IrO2-20% Fe3O4 samples are tested via linear sweep voltammetry both ex-situ and in-situ in a proton exchange membrane water electrolyzer single cell. According to the linear sweep voltammetry tests, the magnetization of the 80% IrO2-20% Fe3O4 sample resulted in 15% increase in the maximum current density. Moreover, the single cell electrolyzer test showed a four-fold increase in current density by employing the magnetized 80% IrO2-20% Fe3O4 catalyst. (C) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Iridium-based oxides, currently the state-of-the-art oxygen evolution reaction (OER) electrocatalysts in acidic electrolytes, are cost-intensive materials which undergo significant corrosion under long-term OER operation. Thus, numerous researchers have devoted their efforts to mitigate iridium corrosion by decoration with corrosion-resistant metal oxides and/or supports to maximize OER catalyst durability whilst retaining high activity. Herein a one-step, facile electrochemical route to obtain improved IrOx thin film OER stability in acid by decorating with amorphous tungsten sulphide (WS3−x) upon electrochemical decomposition of a [WS4]2− aqueous precursor is proposed. The rationale behind applying such WS3−x decoration stems from the generation of a tungsten oxide phase, a well-known corrosion-resistant photoactive OER catalyst. The study demonstrates the viability of the proposed WS3−x decoration, allowing the tailoring of experimental parameters responsible for WS3−x nanoparticle size and surface coverage. OER stability tests coupled by ex situ SEM and XPS corroborate the beneficial effect of WS3−x decoration, yielding improved OER specific activity metrics along with minimized Ir surface roughening, a characteristic of electrodissolution. Iridium decoration with electrodeposited, corrosion-resistant oxides is consequently shown to be a promising route to maximize OER stabilities.

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, E1/2, 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.

We report the first investigation into a quaternary-doped graphene catalyst material for the oxygen reduction reaction (ORR) under alkaline conditions. The material was synthesized via direct pyrolysis of graphene oxide (GO) with boron, nitrogen, phosphorus and sulfur precursors. The resulting BNPS-Gr catalyst was thoroughly characterized and its composition found via XPS to be 6.4% (B), 6.1% (N), 2.6% (P) and 0.5% (S), while HR-TEM imaging shows a few-layered graphene structure. Electrochemical investigations into the catalytic activity towards the ORR show improved performance of the quaternary-doped graphene with the effective number of electrons transferred being ca. 3.7, compared to single-doped graphenes (2.9–3.2) and significantly lower peroxide production. This demonstrates BNPS-Gr to be a promising alternative to current Pt-group catalysts for the ORR, especially in methanol-fuelled fuel cells, where the presence of methanol in the cathode side from the fuel crossover is shown not to affect the performance of the quaternary-doped graphene catalyst.

The commercialisation of polymer electrolyte fuel cells (PEFCs) has been hampered by the high cost of platinum metal. Due to its high durability and catalytic activity, platinum is widely used to catalyse the oxygen reduction and hydrogen oxidation reactions essential to the operation of these cells. Core@shell nanoparticles with thin layers of platinum deposited on cores composed of cheaper materials have offered an attractive route towards the reduction of overall loading of platinum, with the retention of active catalyst surface area. This review surveys approaches taken to prepare idealised active and durable core@shell nanocatalysts by tweaking core compositions. A critical reflection on the current status of the field, as well as predictions as to likely directions for future developments, are discussed as a conclusion to the review.

A hydrogen economy powered by fuel cells is emerging as an alternative to the current fossil-fuel based energy system where hydrogen, produced through renewable sources, is used to generate electricity via fuel cells. A commonly investigated fuel cell, the Polymer Electrolyte Fuel Cell (PEMFC), usually uses platinum and other platinum group metal nanomaterials to catalyze the rate limiting Oxygen Reduction Reaction (ORR) of this process. The high prices and durability limitations of these catalysts have prevented their mass commercialization. Biosynthesis of nanomaterials has emerged as a potentially attractive “eco-friendly” alternative to conventional chemical synthesis methods. Various attempts have been made to biosynthesize nanoparticles for use in fuel cells. However, the processing methods used during and post synthesis increase their costs and limit their overall efficacies. We report bimetallic Pt/Pd nanoparticles (NPs) biosynthesized by E. coli [E. coli-Pt/Pd (10 wt%:10 wt%)] that shows promise for direct use as a PEMFC catalyst. This catalyst outperformed single metal versions of the same, i.e., E. coli-Pt (20 wt%) and E. coli-Pd (20 wt%) when tested as an electrocatalyst ex-situ. Direct use of E. coli-synthesized nanoparticles in PEMFCs is limited by the inherent resistances of the bacteria and the internal localization of nanoparticles. Transmission electron microscopy images and impedances (resistivity) tests showed that by initially synthesizing Pd nanoparticles on the E. coli cells, followed by Pt, gave a cell surface-localized metallic shell that improved conductivity of the catalyst. Catalyst Pt synthesis is likely mediated by initially-formed Pd-NPs reducing Pt (IV) under H2 resulting in alloying; this was evidenced by XRD data that showed XRD peaks for E. coli-Pt/Pd (10%:10%) which lie in between XRD peaks for Pt-only and Pd-only nanoparticles on the same planes. Protrusions of agglomerated nanoparticles were seen on the cell surface to form sites for catalytic activity. The catalyst, used in the ORR without optimization, performed significantly worse (~100 times) than a commercial catalyst (extensively developed for purpose) but which contained 5 times as much Pt. This serves as a starting point for a more engineered approach to bio-synthesizing nanoparticles for PEMFC catalysts.

Some carbon materials commonly used as electronically-conductive supports for catalysts in fuel cell research are Carbon Black (CB), multi-walled carbon nanotubes (MWCNT), Graphene Oxide (GO) and reduced graphene oxide (rGO). Here we present a comparative study into the relative effects of each of these on the performance towards the oxygen reduction reaction (ORR) in alkaline media. For the purposes of comparing the supports, a simple Pt catalyst is used and the performance is evaluated via Koutecky–Levich analysis and direct measurement of peroxide by rotating ring-disk electrode (RRDE) to determine the number of electrons (n) transferred in the ORR. It is found that Pt/CB follows a quasi 4-electron mechanism due to that the ORR takes place mainly on the active Pt particles, whereas Pt/MWCNT, Pt/GO and Pt/rGO exhibit a mixed behaviour between the two proposed mechanisms due to the higher activity of the graphene-derived supports towards the peroxide formation compared to CB. The effect of the oxide groups of GO and the metal impurities of MWCNT on the catalytic performance is also studied.

The electrochemical reduction of oxygen in a range of six polar aprotic solvents is investigated via linear sweep voltammetry at platinum, gold, and carbon fibre microelectrodes. Values for the standard heterogeneous electron transfer rate constant (k0) are reported, and in all cases follow the trend with electrode material of k0(C) > k0(Pt) ~ k0(Au). The variation in k0 with solvent is discussed in terms of the Debye model and a dependence on the longitudinal dielectric relaxation constant (τL) is found of the form k0 ∝ τL−θ. Static solvent effects are accounted for via consideration of both spherical and connected-spheres models of solvation, and it is found that θ ≈ 0.65 indicating that the reduction of dioxygen in these solvents is a non-adiabatic outer-sphere electron transfer.

Herein, we demonstrate strongly enhanced visible light photoelectrocatalytic hydrogen evolution reaction (HER) in few-layer MoS2 grown on a mesoporous TiO2(B) nanobelt (NB) by selective decoration of platinum (Pt) nanoparticles (NPs) on the edge/defect sites of the MoS2 layer. Three catalytically active components are anchored together to increase the photoelectrocatalytic HER activity synergistically, beyond that of commercial Pt/C electrodes (20 wt% Pt). An extremely low concentration of Pt NPs (1.4 wt%) with average size ∼3.8 nm was decorated over the preferentially edge-site-exposed few-layer MoS2, with lateral sizes 130–350 nm, as evidenced from high-angle annular dark-field STEM imaging. During the heterojunction formation, S is doped in the TiO2 layer causing a high density of electrons in TiO2 that migrate to the MoS2 layer inducing n-type doping in it and thus TiO2 acts as an efficient photocathode in photoelectrocatalysis. Quantitative XPS analysis reveals that the catalytically active bridging S22−/apical S2− increases up to ∼72% after the formation of the ternary system Pt@MoS2/TiO2(B). S-enriched MoS2/TiO2(B) selectively loaded with Pt NPs on the edge sites of MoS2 exhibits a giant enhancement in the HER activity in an acidic medium under light. We record a nearly 16 fold higher exchange current density (0.296 mA cm−2) for the ternary system as compared to that of the MoS2/TiO2 binary system under visible light excitation. The marginally Pt loaded ternary system exhibits an extremely low charge transfer resistance (14 Ω) and a low overpotential as well as Tafel slope (−74 mV and 30 mV dec−1, respectively) boosting the overall HER performance under visible light. Chronopotentiometric measurements reveal the high stability of binary and ternary systems to sustain a 10 mA cm−2 cathodic current up to 12 hours. The results show that the marginally loaded Pt NPs activate the inert basal plane, edge sites of MoS2 and porous sites of TiO2, forming an integrated network where the photogenerated electrons can easily be injected from the TiO2 to MoS2 and then to Pt NPs, presenting a feasible approach to boost the HER activity under visible light.

High purity hydrogen has many applications one of which is in the hydrogen fuel cell industry. Hydrogen can be easily produced from water electrolysis; however, the most economical method is steam reforming of methane. This delivers a mixture of gaseous compounds from which hydrogen can be extracted. Besides various techniques such as pressure swing adsorption and cryogenic distillation, dense metal membranes offer an energy efficient and highly selective method for separating hydrogen from a hot gas mixture achieving high purity levels. This review article covers the fundamentals of hydrogen selective membranes for both the porous and dense kind. An in-depth look at dense and porous membranes is taken to establish their current development and a comparison is drawn between both types showing that dense metal membranes have the best hydrogen flux and selectivity. A variety of commercial dense metal membranes are compared revealing the Group V elements such as vanadium (V), niobium (Nb) and tantalum (Ta) to have the highest hydrogen permeability. A major limitation with these metals is their tendency to form a stable oxide layer under ambient conditions. Palladium (Pd) does not suffer this problem at typical membrane operating conditions and with relatively high hydrogen permeability is a suitable alternative as a dense metal membrane. Over the years it has been discovered that alloying Pd with elements such as silver (Ag), yttrium (Y) and copper (Cu) results in marked improvements in hydrogen permeability, mechanical durability and in some cases resistance to contamination by sulphur containing compounds. Nevertheless, there are still opportunities to improve the performance of the existing commercial Pd-based membranes by investigating the endless scope of unexplored Pd binary and ternary alloys.

The gas diffusion layer (GDL) has a vital role in the operation of a polymer electrolyte fuel cell (PEFC). Therefore, studying GDL characteristics and their effect on the cell performance is fundamental for the development of more efficient PEFCs. The work presented covers a selection of commercially available GDL types used in fuel cell development. It highlights some key GDL properties and their influence on PEFC performance. The results show that GDL materials have a direct effect on the ohmic and mass transport losses in the membrane electrode assembly (MEA). They also show that studying the effect of GDL properties on the performance is rather complex, due to the many interrelated properties. The study shows that GDL thickness has a significant effect on the mass transport properties of MEA, but has minimal effect on the ohmic losses. The bulk density of the substrate has a significant effect on the water transport properties and the maximum current density achieved. It is also found that woven and non-woven GDLs achieve comparable performance at optimized operating conditions. Moreover, the felt fiber structure has higher ohmic resistance but achieves better performance than that of straight fiber carbon paper due to its enhanced water transport ability.

The determination of total arsenic through As(V) anodic stripping voltammetry (ASV) is, in some cases, preferable over As(III) ASV. The As(V) ASV procedure has no chemical reduction step from As(V) into As(III), which results in decreased analysis time and no contamination from reducting reagents. A simple and reliable procedure of As(V) determination is proposed. Anodic stripping determination of trace As(V) at gold microelectrode ensembles in diluted HCl solution in the presence of dissolved oxygen is shown. The electrode is based on a carbon black (30%)-polyethylene composite. The sensor was prepared by gold electrodeposition on the surface of the composite electrode. The given sensor is cheap, reliable and stable, especially when electrochemical activation is employed. The experimental parameters for the electrochemical determination were optimized, namely 0.005M HCl as the background electrolyte, the deposition potential -2.2V (versus Ag/AgCl in 1M KCl) and 180mVs-1 linear scan rate. Calibration curves were obtained and were linear in [As(V)] over the 1.5-45 μgL-1 range, with a LOD of 0.5 μgL-1. The effect of common interfering species is studied. The electrochemical behaviour of As(III) form is studied in the same experimental conditions. It was found that As(III) is deposited at lower potentials (starting at -0.6 V) and the sensitivity of As(III) detection is higher, but dependant on the presence of dissolved oxygen. The speciation of inorganic forms of arsenic is discussed. © 2013 Taylor and Francis.

The influence of near wall hindered diffusion of nanoparticles on anodic particle coulometry (APC) is investigated. In an APC experiment, an electrode in a solution of nanoparticles is subject to a potential step from a value where no oxidation of nanoparticles occurs, to one where oxidative dissolution of nanoparticles is transport controlled. Nanoparticle-electrode impacts are then observed as spikes in the measured current. The area under these spikes corresponds to the charge transferred during the nanoparticle oxidation. A computational model is developed to simulate APC experiments, including the effect of near wall hindered diffusion. It is shown that this new, more complete picture is able to successfully simulate the experimental APC of nickel nanoparticles. It is also shown that a detailed, quantum mechanical tunnelling model is not required to describe the destructive oxidation of nanoparticles; an alternative simple impact model can successfully simulate experimental data. © 2013 Elsevier B.V. All rights reserved.

Rigorous and approximate analytical expressions are deduced for Differential Pulse Voltammetry at spherical electrodes of any size, including microelectrodes, when the electrogenerated species is soluble in the electrolytic solution. From these, we examine the utility of DPV for the determination of diffusion coefficients and formal potentials, establishing the optimum conditions for this purpose. The experimental validation of the theoretical results is reported for mercury microelectrodes of ca. 50 and 10 μm diameters both in aqueous and ionic liquid media. © 2009 Elsevier B.V. All rights reserved.

The design, construction, and application of a miniature channel electrode operating with very high flow conditions, of up to ca. 10 cm3 s-1, are reported. The channel cell has approximate cross-section dimensions of 0.2 cm × 100 μm so that the axial velocity of the solution through the channel can reach values as high as 75 m s-1 (270 km h-1, 167 mi h-1) at the channel center. The electrode takes the form of a microband electrode presently of length ca. 5-10 μm. With this arrangement voltammetry can be performed under flow conditions where the mass transport coefficient approaches 1.0 cm s-1. Voltammetric measurements utilizing the new cell are reported, and the mass transport is shown to be quantitatively consistent with a model in which the dominant transport is by diffusion normal to the electrode and by convection axially through the cell. Axial diffusion effects are relatively insignificant. The cell is applied to the study of ECE processes, and it is shown that homogeneous rate constants as high as 105 s-1 are readily measureable from steady-state experiments. © 1995 American Chemical Society.

The uptake of molecular species by spherical particles is diverse in nature ranging from amalgamation processes at mercury droplets through ion transfer through the spherical interface between two immiscible electrolyte solutions (ITIES) (droplets, vesicles, etc.) to monitoring events in and around individual biological cells. Such processes can be controlled and monitored using voltammetry. We report the full characterization of such systems with reverse and differential pulse voltammetry. In particular, the former is a very useful diagnostic tool because of the appearance of a unique peak-shaped response in the reverse pulse voltammetry curves that arises solely as a consequence of the depletion of the species inside the particle. The greater this effect is, the smaller the size of the particle. © 2009 American Chemical Society.

Critical analysis of the Butler-Volmer and Marcus-Hush-Chidsey kinetic models is carried out through the study of the first one-electron reduction of cyclooctatetraene in DMSO at mercury hemispherical microelectrodes. This is an atypically slow process due to the conformational distortion taking place from the tub-shaped neutral species to the planar radical anion. Cyclic and square wave voltammetries are employed with the aim of examining the consistency of the results extracted from each model. Whereas the experimental results in cyclic voltammetry are adequately fitted by both models, the fit of square wave voltammograms at high frequency are not satisfactory when the Marcus-Hush-Chidsey formalism is employed. © 2011 Elsevier B.V. All rights reserved.

The application of fast-scan linear sweep voltammetry methods to a high-speed wall-tube electrode (HWTE) is reported. Experiments are reported for the oxidation of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) in propylene carbonate solution containing 0.10 M tetrabutylammonium perchlorate for a 24 μm radius platinum microdisk electrode housed within the HWTE using a range of scan rates from 200 to 3000 V s-1 and average flow jet velocities from 0.24 to 9.4 m s-1 (corresponding to volume flow rates of 0.003-0.12 cm3 s-1, and centre-line jet velocities from 0.5 to 18.9 m s-1). Linear sweep voltammograms (LSVs) are analysed for a simple electron transfer under high volume flow rates, by curve fitting. Analysis of the transient LSVs yielded values for k0, α, and Ef0 for TMPD of (5.9±2.4)×10-2 cm s-1, 0.46±0.08 and 0.217±0.019 V (vs. Ag), respectively. This is in good agreement with independent experiments conducted using the high-speed channel electrode which yielded the results: k0=(6.3±0.4)×10-2 cm s-1, α=0.52±0.01, and Ef0=0.234±0.005 V (vs. Ag). The range of applicability of this method for measuring k0 was also investigated and compared with existing channel electrode techniques. © 2003 Elsevier B.V. All rights reserved.

The study of the variation of the kinetics of electrode processes with temperature is performed considering the asymmetric version of the Marcus theory for the first time. This kinetic model introduces a new symmetry parameter that takes into account possible differences between the inner-sphere force constants of the oxidized and reduced electroactive species. Variable temperature measurements in fast-scan cyclic voltammetry are employed to gain further insights into the molecular properties of the electroreduction of 1-nitropentane in acetonitrile on mercury hemispherical microelectrodes of 28.5 μm radius. Thus, the values of the reorganization energy of the electron transfer process and the symmetry parameter as well as the degree of adiabaticity of the electron transfer reaction are evaluated. The results are also analyzed in terms of the ability of the different kinetic models employed in Electrochemistry (Butler-Volmer, symmetric Marcus-Hush and asymmetric Marcus-Hush) to fit the experimental voltammograms with consistent values for the kinetic parameters in a range of temperatures and scan rates. © 2012 Elsevier B.V. All rights reserved.

The application of steady-state and fast-scan linear sweep voltammetry to a high-speed wall-tube electrode (HWTE) is reported in different solvents to investigate the response of the HWTE over a wide range of Reynolds' numbers (Re). Experiments are reported for the oxidation of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) in propylene carbonate (PC), water, butyronitrile (BN), acetonitrile (AN), and acetonitrile-water mixture solutions containing 0.10 M supporting electrolyte for a 24 μm radius platinum microdisk electrode housed within the HWTE using a range of average flow jet velocities from 0.03 to 19.8 m s-1 (corresponding to volume flow rates of 0.003-0.25 cm3 s-1 and center-line jet velocities from 0.05 to 39.5 m s-1). Fast scan linear sweep voltammetry is presented for the oxidation of TMPD in PC and of 9,10-diphenylanthracene (DPA) in AN. Theoretical results are derived using finite element methods for both one- and two-dimensional mass transport models. It is found that, for solvents with a kinematic viscosity above ca. 7.5 × 10-3 cm2 s-1, the hydrodynamic behavior for Re < 2000 is as expected with current responses in accordance with those predicted for a laminar, parabolic inlet flow profile. In low viscosity solvents, where Re < 2000, currents are lower than expected, indicating a departure from laminar flow in practical cells even at low Re. The HWTE is compared to the channel electrode in the light of the experimental results, theoretical limits of electron-transfer rate detectable, and conclusions drawn that the channel electrode is more reliable for kinetic measurements.

Recent work on faradaic processes occurring during thermal nanoparticle-electrode collisions contrasts significantly from analogous research using ultrasonically-driven microparticles, where no faradaic signals were found. It is suggested that this might be explained by the differences in both particle size and contact time. To investigate this, we present results from adapted Monte Carlo random walk simulations. Using the underpotential deposition of thallium onto silver nanoparticles as a model system, it is found that an estimated minimum contact time of ca. 10-4 s is required to deposit a complete monolayer (from a 10 mM solution) onto a nanoparticle of radius 45 nm. © 2011 Elsevier B.V. All rights reserved.

We present experimental results to determine the proportion of nanoparticle (NP) impacts that result in adsorbed NPs, using gold and nickel nanoparticles (AuNPs/NiNPs) in collision with a glassy carbon electrode. Results are given for NP radii of 10 nm (Au) and 26 nm (Ni), as well as a range of electrode potentials. No significant systematic trends were found in either case, and the sticking coefficients were found to be s = 0.19 ± 0.03 for Au and s < 0.01 for Ni. © 2012 Elsevier B.V. All rights reserved.

In this study, variable temperature experiments have been used in conjunction with numerical fitting of cyclic and square wave voltammogram responses via the asymmetric version of Marcus theory to provide molecular insight into the electroreduction of the 3-nitrophenolate anion in DMSO on mercury hemispherical microelectrodes of 23 μm radius. As a result, the reorganization energy, differences between the inner-sphere force constants of the electroactive species, and the degree of adiabaticity have been evaluated. Furthermore, a comparison between the quality of fitting obtained through the Butler-Volmer and asymmetric Marcus-Hush models is also presented to examine the relative ability of the kinetic models in predicting voltammetric responses in a range of temperatures and scan rates. © 2013 Elsevier Ltd. All rights reserved.

We present an experimental comparative evaluation of the Marcus-Hush (MH) and Butler-Volmer (BV) kinetic formalisms. Numerical simulations using both kinetic models are used to fit experimental voltammetry of the one-electron oxidation of 9,10-diphenylanthracene (DPA) and the one-electron reduction of 2-nitropropane (NP) at a high-speed channel microband electrode. For DPA the Butler-Volmer and Marcus-Hush formalisms yield indistinguishable fits, as expected for a system with fast electrode kinetics. For the BV formalism best fits were obtained using k0 = 0.83 cm s-1 and α = 0.49; for MH the best fit parameters were k0 = 0.85 cm s-1 and λ = 0.58 eV. For NP neither Butler-Volmer nor Marcus-Hush models are able to obtain very accurate fits to experiment, although it was possible for the Butler-Volmer model to yield more accurate fits if the transfer coefficients, α and β, are not required to sum to unity, which is possibly justifiable due to the very large difference between oxidative and reductive peak potentials. The best fit obtained using MH kinetics used k 0 = 7.0 × 10-6 cm s-1 and λ = 1.0 eV, while BV kinetics was able to fit using k0 = 9.5 × 10 -4 cm s-1 and α = β = 0.24. © 2011 American Chemical Society.

"Bamboo-like" multiwalled (b-MWCNT), "hollow-tube" multiwalled (h-MWCNT) and single-walled carbon nanotubes (SWCNT), chemically modified with 1-anthraquinonyl (AQ) or 4-nitrophenyl (NP) groups, are characterized using voltammetric, electron microscopic and Raman spectroscopic techniques. The pKa values of the AQ-modified CNTs are found to be shifted by greater than three units when compared to the pKa values of anthrahydroquinone (AHQ, the reduced form of AQ) in aqueous solution to beyond pH 14. These large changes in the surface pKa values of the modified CNTs are explored further by comparing the pKa values of CNTs modified with an anthraquinonyl-2-carboxylic acid group. These groups are attached to the CNT surface via the formation of an amide bond with an aminophenyl "spacer" unit derived from the chemical reduction of NP modified CNTs. The location of reactive sites on the CNT surface is investigated and their influence on the pKa of the modified materials is discussed. Comparison with modified pyrolytic graphite electrodes exposing pure edge-plane or pure basal-plane crystal faces indicates that the modifying aryl groups are predominantly located on edge-plane like defects at the tube ends of MWCNTs. The effect of polymer formation on electron transfer kinetics of b-MWCNTs and h-MWCNTs is also discussed. In contrast SWCNTs show both significant side-wall functionalisation and fast electron transfer kinetics which is attributed to their different electronic structure. This journal is © The Royal Society of Chemistry.

Double potential step chronoamperometry for the two-electron transfer amalgam-forming deposition and stripping of cadmium at a hemispherical mercury electrode in aqueous solution containing different concentrations of supporting electrolyte is compared with simulated results using the Nernst-Planck-Poisson system of equations. The latter were generated using an approximation of zero electric field at the interface [J. Phys. Chem. C 2008, 112, 13716-13728]; the approximation gives a good agreement between theory and experiment, so validating its use for the description of macroelectrode and microelectrode (but not nanoelectrode) problems involving low or negligible levels of supporting electrolyte. © 2009 American Chemical Society.

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.

Iron is an abundant element in the environment which plays an important role in environmental and biological systems. In particular, its essential function in photosynthesis has been seen as a limiting factor for phytoplanktons in ocean waters. Thus, sensitive speciation and determination of iron is of major interest, and many techniques have been established for analytical purposes. Electrochemical methods have been commonly explored due to their inexpensive, simple and rapid nature, with adsorptive stripping voltammetry (AdSV) being widely used due to its ability to complex and preconcentrate iron ions for ultrasensitive detection. This paper aims to present a review of recent determinations of trace iron using electrochemical methods. © 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

We report an experimental comparative evaluation of the Butler-Volmer (BV), symmetric and asymmetric Marcus-Hush (MH) kinetic formalisms. Numerical simulations using these kinetic models are employed to fit experimental cyclic voltammetry of the one-electron oxidation of tetraphenylethylene in dichloromethane under conditions of full electrolyte support at a platinum microdisk electrode. When compared with the BV formalism, the symmetric MH model is seen to give rise to an inferior quality of fit determined by calculating the mean scaled absolute deviation (MSAD) between theory and experiment. This can be traced to its inherent assumption of identical force constants of the reagent and product, rendering it unable to address the asymmetry that exists between the forward and reverse sweeps of the cyclic voltammetry due to the difference in force constants. Where the asymmetric MH formalism is used, cyclic voltammograms with comparable quality of fit to that of BV are obtained. The best-fit parameters obtained for each kinetic model (25 °C) are: Butler Volmer: k 0 = 0.17 cm s -1, α = 0.65 Symmetric Marcus-Hush: k 0 = 0.15 cm s -1, λ ≥ 0.53 eV Asymmetric Marcus-Hush: k 0 = 0.15 cm s -1, λ ≥ 0.53 eV, β/λ = 0.55 eV -1. © 2012 Elsevier B.V. All rights reserved.

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.

The application of the Marcus-Hush-Chidsey (MHC) formalism to heterogeneous electron transfer processes is reviewed and compared with the commonly employed Butler-Volmer (BV) approach. The MHC model is potentially more powerful than BV since it enables us to analyze the electrode kinetics in terms of the microscopic nature of the electroactive species, electrode and solvent. Despite having been studied fairly extensively in the case of surface-bound redox couples, its application to heterogeneous electron transfer reactions has been limited. Here, we review the use of the MHC approach for the analysis of both surface-bound and solution phase redox systems and its suitability as a model for the parameterisation of electrode kinetics. Results obtained for several slow electrode reactions using cyclic voltammetry and reverse scan square wave voltammetry are reviewed which reveal that the simple MHC model does not always give a satisfactory fit to experimental data in contrast to BV parameterisation. Possible physical reasons for the experimental deviations from simple MHC theory are examined. © 2011 Elsevier Ltd.

We report a theoretical study of steady-state voltammetry at the channel electrode comparing the Butler-Volmer and Marcus-Hush (both symmetric and asymmetric) models of electrode kinetics. The latter model is shown, for small values of both reorganisation energy and standard rate constant, to exhibit a kinetically-limited steady-state current which is smaller than the mass-transport limiting current given by the Levich equation. However, the combination of parameters required to observe this kinetically-limited current is unlikely to occur in a 'real' system. We conclude that the Butler-Volmer model is sufficient to accurately model voltammetry under these conditions. © 2012 Elsevier B.V. All rights reserved.

The electrochemical reduction of triphenylethylene (TriPE) was studied in dimethylsulfoxide (DMSO) containing 0.1 M tetra-n-butylammonium perchlorate supporting electrolyte through the use of cyclic voltammetry. Experiments were conducted using two different mercury hemisphere electrode sizes (radii 3 and 25 μm) and with TriPE concentrations of 0.50 and 2.00 mM so as to span fully the transition from convergent to linear diffusion and steady state to transient voltammetry. Voltammetric responses obtained display two sucessive reduction waves with formal potentials separated by ca. 290 mV. Taking the diffusion coeffient of TriPE, TriPE − and TriPE2− to be 2.8 × 10−10 m2 s−1, excellent fits between simulation and experiments were obtained across all experimental conditions, with the following DISP2 reaction scheme: Heterogeneous reactions: TriPE + e - ⇌ TriPE - k 1 0 = 0.15 cm s - 1 , α 1 = 0.5 TriPE - + e - ⇌ TriPE 2 - k 2 0 = 0.032 cm s - 1 , α 2 = 0.5 ( E f , 2 0 - E f , 1 0 = 290 mV ) Homogeneous reactions: TriPE − + TriPE − ⇌ TriPE + TriPE2− KDISP = 1.3 × 10−5, kf,DISP = 5.0 × 104 mol−1 dm3 s−1 TriPE 2 - → products K eq > 1.5 × 10 6 , k f > 1.0 × 10 6 s - 1 where k0 denotes the standard electrochemical rate constant, α the transfer coefficient, E f 0 the standard formal potential, KDISP the equilibrium constant of the disproportionation reaction, kf,DISP the rate of the forward reaction of disproportionation, Keq the equilibrium constant of the decay of TriPE2−, and kf the rate of the forward reaction of the decay of TriPE2−.

We review the joint use of ionic liquids and electrochemical methods in the separation of carbon dioxide from other gases and transformation into useful organic feedstocks. © 2011 The Royal Society of Chemistry.

Experimental cyclic voltammetry at a hemispherical mercury microelectrode in acetonitrile solution, containing 3 mM cobaltocenium hexafluorophosphate and different concentrations of supporting electrolyte, is compared with theoretical simulations using the Nernst-Planck-Poisson system of equations, without the assumption of electroneutrality, and is found in to be in good agreement. Deviations from diffusion-only theory are analyzed in terms of migration and potential drop in the solution as a function of the concentration of supporting electrolyte. We are unaware of previous reports in which non-steady-state cyclic voltammetry without supporting electrolyte has been quantitatively and fully simulated, so this work opens up a new area for voltammetry. © 2010 Published by Elsevier B.V.

Chronoamperometric transients are recorded in different quantities of supporting electrolyte, from full to self-support, for one electron transfer reactions of redox systems carrying diverse charges, and are studied to dissect migrational and diffusional contributions to mass transport under different conditions (applied overpotential, electrode radius, kinetics and charge of electroactive species). The one-electron redox systems studied are the reductions of Ru3+ (hexaammineruthenium(III)/(II)), Co3+ (cobaltocenium/cobaltocene), and Fe3+ (hexacyanoferrate(III)/(II)). The mass transport (diffusion and migration) is simulated using the Nernst-Planck-Poisson (NPP) model, with good agreement between experiment and theory. The effect of the charge of the electroactive species on its mass transport to the electrode is considered when only small amounts of supporting electrolyte are added to the solution. The mass transport to the electrode is increased by "attractive" migration when a positively charged electroactive species is reduced or a negatively charged species is oxidized. Conversely, when a positively charged electroactive species is oxidized or a negatively charged species is reduced, the migration is "repulsive" and the mass transport to the electrode is decreased. Copyright © 2010 American Chemical Society.

A rigorous analytical solution for Reverse Pulse Voltammetry (RPV) at spherical electrodes of all sizes is presented, including the case where the diffusion coefficients of the electroactive species are different. From this solution, the value of RPV for the determination of both diffusion coefficients and the formal potential of the redox couple is demonstrated. The experimental application of the theoretical results is carried out both in ionic liquids and in aqueous solution with mercury microelectrodes of ca. 50 and 10 μm diameters. The former solvent system is of particular interest due to the great importance of these new liquids in which notably unequal diffusion coefficients are commonly found. © 2009 Elsevier B.V. All rights reserved.

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.

Chronoamperometry is reported on the one electron oxidation of 3 mM ferrocene in acetonitrile using a 300 (± 5 μm radius gold hemisphere electrode. Varying concentrations of supporting electrolyte (tetra- nbutylammonium perchlorate) are used ranging from the fully supported (100 mM) to the almost completely unsupported (0.1 mM). The response is simulated using the Nernst-Plank-Poisson system of equations and excellent agreement between theory and experiment noted, so vindicating a recent theory (J. Phys. Chem. C, 2008, 112, 13716) for voltammetry in weakly supported media. © 2009 American Chemical Society.

A theoretical model for electrochemical processes in resistive media is applied to interpret the current measured for the stripping of thallium from a Tl/Hg amalgam. Chronoamperometry is presented for a potential step experiment at a 12.5 μm radius hemispherical mercury drop in which thallium is first deposited and then stripped. Unusual features are observed in the transient stripping current, and it is proposed that these occur when the concentration of thallium(I) cations in solution is so great that the inert electrolyte salt is no longer in excess and the system is only partially supported. The theoretical model uses the Nernst-Planck-Poisson equations, which take into account the effects of the electrical potential in the aqueous phase, and avoids making the approximation of electroneutrality. The numerically simulated current accurately predicts the unusual experimental behavior. © 2008 American Chemical Society.

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.

“Nanosecond” electrochemistry is used to investigate the impacts of various solid particles (alumina and graphite suspended in aqueous perchloric acid and copper suspended in aqueous sodium perchlorate) at a polycrystalline gold electrode under insonation. Current spikes of microsecond duration, attributed to individual impacts, are observed under potentiostatic conditions, with a polarity that inverts at the potential of zero charge of the electrode/electrolyte system. It is found that there is no significant change in the magnitude of these transients as the sizes of alumina particles range from 0.3 to 25 Im. However, appreciable changes are observed in the transients when using electroactive particles, specifically graphite powder surface modified with adsorbed N,N′-diphenyl-1,4-phenylenediamine, and evidence is presented for the occurrence of electron transfer during the impact event which occurs on the sub-microsecond time scale.

Rate constants depend on molecular size: Results are presented to validate the relationship between molecular size and the standard electrochemical rate constant predicted by Marcus theory (see graphic). Measurement of hydrodynamic radii and electron-transfer rates for substituted 1,4-phenylenediamines have been made under steady-state conditions using both hydrodynamic and stationary electrodes.

The electrochemical oxidation of N,N-diethyl-p-phenylenediamine in dimethylformamide has been studied at platinum and gold microdisk electrodes of various radii between 6.7 and 66 μm. The voltammetric responses revealed two electrochemically reversible waves the second of which becomes larger at higher concentrations and bigger electrode radii. The voltammetric signals have been modelled and the electrochemical oxidation reaction is not inconsistent with an ECrevECE reaction. Kinetic parameters are reported.

The physical processes giving rise to sustained electrolytic currents within sono-emulsions are explored and shown to arise from: (i) the 'thinning' of the diffusion layer at the active electrode and in particular (ii) the release of dissolved material from the suspended phase within the diffusion layer. © 2002 Elsevier Science B.V. All rights reserved.

The theory for homogeneous ECrevE, ECrevCE and ECECE mechanisms at macroscopically large channel electrodes is derived. The photoelectrochemical reductions of para-bromonitrobenzene and 2,4-dibromonitrobenzene in acetonitrile solutions and at macroscopic platinum channel electrodes are studied using irradiation at 330 and 470 nm, corresponding to absorption bands in the corresponding radical anions. The former compound is shown to follow a homogeneous ECrevCE pathway in acetonitrile solutions containing tetrabutylammonium-based supporting electrolytes; by changing the supporting electrolyte to a salt of the tetramethylammonium cation, the mechanism is changed qualitatively and follows an ECEE pathway. The photoelectrochemical reduction of 2,4-dibromonitrobenzene in acetonitrile solution containing supporting electrolytes derived from the tetrabutylammonium cation is shown to follow an overall ECECE mechanism, with both chemical steps being chemically-reversible, and with the loss of the ortho-bromo in the dark. In the presence of chloride supporting electrolytes, it is shown that light-induced rupture of a C-Br bond occurs reversibly with the competing formation of a C-Cl bond. Unoptimised bulk photoelectrosynthesis indicates that some halogen exchange occurs, demonstrating the viability of a novel approach to halex reactions. © 2002 Elsevier Science B.V. All rights reserved.

The oxidative dimerisation reaction of N,N-dimethyl-p-toluidine (DMT) in acetonitrile was studied by cyclic voltammetry using a range of concentrations from 1 to 70 mM at microdisk electrodes of radii 10, 12.5, and 50 μm using scan rates from 20 mV s-1 to 6 V s-1. Theory was developed to simulate the following mechanism using the ADI method and comparison made to experimental voltammograms.The mechanism was found to be consistent with the observed data under all experimental conditions employed. The following rate constants were inferred: k1=(5.9±2.2)×105 cm3 mol-1 s-1, k-1≈0, and k2=(9.2±3.8)×108 cm3 mol-1 s-1. © 2002 Elsevier Science B.V. All rights reserved.