Rapid-scan Voltammetry Research Articles

Edge-Sharing Octahedrally Coordinated NiFe Dual Active Sites on ZnFe2 O4 for Photoelectrochemical Water Oxidation.

The structural properties of octahedral sites (BOh ) in spinel oxides (AB2 O4 ) play vital roles in the electrochemical performance of oxygen-related reactions. However, the precise manipulation of AB2 O4 remains challenging due to the complexity of their crystal structure. Here, a simple and versatile molten-salt-mediated strategy is reported to introduce Ni2+ in Boh sites intentionally on the surface of zinc ferrite (ZnFe2 O4 , ZFO) to promote the active sites for photoelectrochemical (PEC) water splitting. The as-created photoanode (ZFO-MSNi) shows a remarkable cathodic shift of ≈ 450mV (turn-on voltage of ≈ 0.6 VRHE ) as well as three times the 1-sun photocurrent density at 1.23 VRHE for PEC water oxidation in comparison with bare ZFO. A comprehensive structural characterization clearly reveals the local structure of the introduced Ni2+ in ZFO-MSNi. Fewer surface trapping states are observed while the precisely introduced Ni2+ and associated neighboring Fe(3-σ)+ (0<σ<1) sites unite in an edge-sharing octahedral configuration to function as NiFe dual active sites for PEC water oxidation. Moreover, open circuit potential measurements and rapid-scan voltammetry investigation give further insight into the enhanced PEC performance. Overall, this work displays a versatile strategy to regulate the surface active sites of photoelectrodes for increasing performance in PEC solar energy conversion systems.

Understanding the Roles of NiOx in Enhancing the Photoelectrochemical Performance of BiVO4 Photoanodes for Solar Water Splitting.

Solar water oxidation is considered as a promising method for efficient utilization of solar energy and bismuth vanadate (BiVO4 ) is a potential photoanode. Catalyst loading on BiVO4 is often used to tackle the limitations of charge recombination and sluggish kinetics. In this study, amorphous nickel oxide (NiOx ) is loaded onto Mo-doped BiVO4 by photochemical metal-organic deposition method. The resulting NiOx /Mo:BiVO4 photoanodes demonstrate a two-fold improvement in photocurrent density (2.44 mA cm-2 ) at 1.23 V versus reversible hydrogen electrode (RHE) compared with the uncatalyzed samples. After NiOx modification the charge-separation and charge-transfer efficiencies improve significantly across the entire potential range. It is further elucidated by open-circuit photovoltage (OCP), time-resolved-microwave conductivity (TRMC), and rapid-scan voltammetry (RSV) measurements that NiOx modification induces larger band bending and promotes efficient charge transfer on the surface of BiVO4 . This work provides insight into designing BiVO4 -catalyst assemblies by using a simple surface-modification route for efficient solar water oxidation.

The effect of pH and ligand exchange on the redox properties of blue copper proteins.

A study of the structure and redox properties of the copper site in azurins by means of EXAFS, NMR, redox titrations, potentiometry, equilibrium cyclic voltammetry and rapid scan voltammetry on protein films is reported. The results are discussed in light of existing theories on structure and function of type-1 copper sites. The exit and entry of electrons take place through the C-terminal histidine ligand of the copper. The hydrophobic patch through which this residue penetrates the protein surface plays an important role in partner docking (cf. The rim of the porphyrin ring sticking through the surface of the cytochromes-c). We find no experimental evidence for strain around the metal site. The active centre is able to maintain ET activity even in the presence of fairly gross disturbances of the site structure. The analysis of the thermodynamics of the redox reaction shows that the protein matrix and the solvent play an important role in 'tuning' the redox potential around a "design" value of around 300 mV at room temperature. The metal site appears "designed" to stabilise the Cu(II) instead of the Cu(I) form. The remarkable evolutionary success of the blue copper proteins is ascribed to the sturdy overall beta-sandwich structure of the protein in combination with a metal site that is structurally adaptable because three of its four ligands are located on a loop. The electronic "gate" that occurs in the middle of a hydrophobic patch allows for fine tuning of the docking patch for recognition purposes.

Schedule-induced polydipsia and the nucleus accumbens: electrochemical measurements of dopamine efflux and effects of excitotoxic lesions in the core

The efflux of dopamine (DA) in the nucleus accumbens (NAcc) core during the acquisition of schedule-induced polydipsia (drinking in response to intermittent food presentation) was measured using rapid scan voltammetry. DA efflux increased throughout the SIP sessions, always reaching a peak after the session had terminated. There was, however, no relationship between the acquisition of the drinking response to intermittent food presentation and DA efflux. When water was absent from the test chamber, DA efflux still increased and reached a peak after food delivery was terminated, dissociating drinking and increased DA efflux. Taken in conjunction with previously presented data, these results suggest that the presence of DA in the NAcc core might be necessary for the development of SIP but that its efflux does not bear a systematic relationship to the acquisition of adjunctive behaviour. In a second experiment the effects of NMDA-induced lesions of the NAcc core on the acquisition and performance of SIP were examined. Lesioned rats did not differ to controls in terms of water intake, mean drinking bout length, latency to panel press for food or to begin drinking. The number of drinking bouts/min was reduced in lesioned rats, but did not reach statistical significance; the number of panel presses/min was significantly reduced in lesioned rats. These data demonstrate that the NAcc core is not necessary for the development of SIP but that elements of performance are affected. This suggests that the development of SIP can be fractionated and that different neural elements control different aspects of its expression. These data are used to support the hypothesis that the NAcc core is involved in focusing behaviour and regulating switching between response options.

Hydrogen adsorption–desorption and oxide formation–reduction on polycrystalline platinum in unbuffered aqueous solutions

The properties of microelectrodes are exploited to characterise hydrogen adsorption–desorption and oxide formation–reduction at polycrystalline platinum in several aqueous electrolytes, especially unbuffered aqueous media including water without the addition of electrolyte. In unbuffered solutions, the peaks for hydrogen adsorption–desorption on a cyclic voltammogram are separated by several hundred millivolts. This separation results principally from pH changes within the solution close to the electrode, induced by the surface reactions themselves, although there is also a contribution from the slow kinetics of the electron-transfer processes. The contribution from the irreversibility of the electron-transfer reactions has been estimated using rapid-scan voltammetry in both acid and alkaline solutions. The influence of the mass-transport regime on the response is also discussed.

Rapid scanning voltammetry under steady-state conditions in a flow through thin layer cell with a microelectrode

A microelectrode based detector system has been developed for measurement of steady state voltammetric curves in flowing solutions. Two microprocessors operating in parallel allow the direct transfer of collected data to a floppy diskette. Long term experiments can then be performed, with individual voltammograms being rapidly obtained, recorded and stored. The system can be used with scan rates up to 10 V s-1 and with 1 mV resolution over a potential range of 2.5 V. When a 10 μm diameter micro-disk platinum electrode serves as the working electrode, rapid scan voltammetry (scan rate 1 to 10 V s-1) can be undertaken under steady state conditions for reversible processes with a flow rate in the range of 1 to 3 ml min-1 as evidenced by the observation of sigmoidal rather than the peak shaped curves obtained with previously described rapid scan systems. That is, complete voltammograms can be obtained with minimal distortion due to uncompensated resistance and charging current which is not the case when conventionally sized electrodes are used or when microelectrodes are used at excessively high scan rates where linear diffusion terms become important. The working microelectrodes were developed to suit a conventional thin-layer cell design and therefore permit ready adaptation to existing flow through electrochemical detection systems. The detection limits for the determination of ferrocene in methanol at flow rates up to 3 ml min-1 were 10-6 mol dm-3 after background correction, and the response was found to be linear over the concentration range 10-3 to 10-6 mol dm-3. Three-dimensional methods of data treatment and contour plots can be used to interpret results obtained from steady state or near steady state voltammograms of incompletely resolved chromatograms as demonstrated with range of biologically important compounds.

Convection/diffusion- and diffusion-controlled rapid-scan voltammetry in liquid chromatographic systems with a large-volume wall-jet detector

On-line voltammetric measurements under liquid chromatographic (LC) conditions yield voltammograms which are either purely diffusion- or purely convection/diffusion-controlled, or a combination of both. The dependence of this behaviour on flow-rate, scan-rate and electrode diameter is investigated for a large-volume wall-jet detector. It is shown that for 4 V s −1 scan-rates at 1 ml min −1 flow-rates (conventional LC conditions), S-shaped (convection/diffusion-controlled) voltammograms can be obtained with macro-electrodes (⩾ 1 mm diameter). Distortion of voltammogram shape by the cell time constant is discussed for macro-electrodes. The behaviour of the cell in microbore and micro-LC applications is demonstrated. The advantage of being able to change from convection/diffusion-controlled to diffusion-controlled behaviour is discussed.

Rapid-scan voltammetry of Ag(I) and Pb(II) in molten sodium nitrate—barium nitrate eutectic

The reduction of Ag(I) and Pb(II) in molten sodium nitrate-barium nitrate eutectic was studied at platinum wire micro-electrodes. The reduction of Ag(I) occurs reversibly and uncomplicated by any reaction of the deposited metal with either the platinum electrode or the nitrate solvent. The reduction of Pb(II), on the other hand, results in the deposition of a liquid metal which reacts with the nitrate solvent to form the oxide. The technique of rapid scan voltammetry was employed to study the above electrode reactions. Scan rates of 20 mV–130 V s −1 were used. It was shown that the reaction between the lead metal and nitrate solvent was negligible at fast scan rates (20–130 V s −1 ).

Rapid-scan voltammetry of Ag(I) and Pb(II) in molten sodium nitrate-barium nitrate eutectic

The reduction of Ag(I) and Pb(II) in molten sodium nitrate-barium nitrate eutectic was studied at platinum wire micro-electrodes. The reduction of Ag(I) occurs reversibly and uncomplicated by any reaction of the deposited metal with either the platinum electrode or the nitrate solvent. The reduction of Pb(II), on the other hand, results in the deposition of a liquid metal which reacts with the nitrate solvent to form the oxide. The technique of rapid scan voltammetry was employed to study the above electrode reactions. Scan rates of 20 mV–130 V s−1 were used. It was shown that the reaction between the lead metal and nitrate solvent was negligible at fast scan rates (20–130 V s−1).

Voltammetry of nickel in molten lithium fluoride-sodium fluoride-potassium fluoride

Results are presented on the rapid scan voltammetry and anodic stripping voltammetry of nickel in molten LiF-NaF-KF. The technique of anodic stripping voltammetry is promising as a means of determining trace quantities of nickel directly in the melt; as little as 1 p.p.m. can be detected. The half-wave potential for the reduction of Ni(II) → Ni(0) occurs at approximately −0.2 V vs. the platinum quasi-reference electrode. Current-voltage curves were recorded at a pyrolytic graphite indicator electrode at voltage scan rates from 50 mV/min-10 V/min. At the faster scan rates, peak-shaped curves resulted which indicated that the transport process was mainly diffusion-controlled. A diffusion coefficient of 1·10 −6 cm 2/sec at 500° was calculated. An activation energy of about 18 kcal/mole was calculated for the current-limiting process which corresponds to the reduction of Ni(II) to the metal.

Rapid scan voltammetry and chronopotentiometric studies of iron in molten fluorides: Fabrication and use of a pyrolytic graphite indicator electrode

Voltammetric and chronopotentiometric studies of iron(II) in molten LiF-NaF-KF and LiF-BeF 2 were conducted. The current-voltage curves were recorded by means of a controlled-potential polarograph capable of measuring cell currents up to 5 mA and providing scan-rates from 50 mV/min to 10 V/min. At faster scan-rates (~1 V/min and greater) peak-shaped curves resulted which indicated that the transport process to the electrode was diffusion-controlled. The diffusion coefficient was calculated to be approximately 1 · 10 −6 cm 2/sec and 5 · 10 −6cm 2/sec in molten LiF-NaF-KF and LiF-BeF 2, respectively. The diffusion coe LiF-BeF 2, calculated from chronopotentiometric data, was too large. This was later found to be due to an alternate current path within the cell. Pyrolytic graphite electrodes prepared from commercially available plates, when properly oriented and insulated in boron nitride, appear to be well suited for electrochemical measurements in corrosive melts.