Slow Charge Transfer Processes Research Articles

Influence of O–O formation pathways and charge transfer mediator on lipid bilayer membrane-like photoanodes for water oxidation

Inspired by the function of crucial components in photosystem II (PSII), electrochemical and dye-sensitized photoelectrochemical (DSPEC) water oxidation devices were constructed by the self-assembly of well-designed amphipathic Ru(bda)-based catalysts (bda = 2,2ʹ-bipyrdine-6,6ʹ-dicarbonoxyl acid) and aliphatic chain decorated electrode surfaces, forming lipid bilayer membrane (LBM)-like structures. The Ru(bda) catalysts on electrode-supported LBM films demonstrated remarkable water oxidation performance with different O–O formation mechanisms. However, compared to the slow charge transfer process, the O–O formation pathways did not determine the PEC water oxidation efficiency of the dye-sensitized photoanodes, and the different reaction rates for similar catalysts with different catalytic paths did not determine the PEC performance of the DSPECs. Instead, charge transfer plays a decisive role in the PEC water oxidation rate. When an indolo[3,2-b] carbazole derivative was introduced between the Ru(bda) catalysts and aliphatic chain-modified photosensitizer in LBM films, serving as a charge transfer mediator for the tyrosine-histidine pair in PSII, the PEC water oxidation performance of the corresponding photoanodes was dramatically enhanced.

A kinetic study of La0.75Sr0.25Cr0.5Mn0.5O3-δ nano-structured electrodes for intermediate temperature symmetric solid oxide fuel cells

La0.75Sr0.25Cr0.5Mn0.5O3-δ (LSCM) mixed conducting has been studied as nanostructured air and fuel electrode, for intermediate temperature symmetric Solid Oxide Fuel cell (S–SOFC). The possible mechanisms involved in the oxygen reduction and hydrogen oxidation reactions of these LSCM nanostructures porous electrodes deposited on La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) electrolyte, were analyzed by electrochemical impedance spectroscopy (EIS) at 700 °C varying the oxygen partial pressure (pO2) and the hydrogen partial pressure (pH2). This analysis was complemented by the study of the electrical conductivity by the four-probe DC technique in the range between 300 and 800 °C in flowing dry atmospheres of air or H2. Results suggested that the O2 reduction reaction mechanism involves the O2- dissociative adsorption and O- ion migration near the surface region, while the H2-oxidation reaction limiting-step is controlled by a slow charge transfer process, and H2- dissociative adsorption on the ultimate O-layer.

Rational Design of Cathode Structure in Li-S Batteries

As one of the most intensely investigated technologies in the electrochemical energy storage beyong Lithium-ion, lithium-sulfur (Li-S) batteries have observed rapid improvements in their properties in recent years. This presentation reports recent progresses in the understanding and development of Li-S cathode design. One of the technological bottlenecks in Li-S batteries is the insulating nature of elementary sulfur and the discharging products Li2S2 and Li2S. The slow charge transfer process in the cathode structure thus imposes restrictions on the degree of sulfur utilization, initial capacity and the rate performance.The nano-size effect in Li-S cathodic charge transfer has been thoroughly investigated. S nanoparticles with diameters below 50 nm were fabricated via a membrane-assisted precipitation method. With conductive PEDOT coating, these S nanoparticles show high specific capacity and improved cycle stability. The study indicated that nanosized sulfur required shorter charge diffusion distance and thus showed higher capacity (Sci.Rep.2013,3,1910). Further understanding of the nano-size effect needs better size-controlled S materials, which were obtained using the sulfur-amine chemistry recently developed (Chem. Commun., 2014,50,1202). A series of mono-dispersed S nanoparticles with diameters ranging from 5 nm to 150 nm were employed to investigate the size-dependent charge transfer process and the influence on device performance. Experimental results revealed much improved specific capacity, rate performance and cycle stability upon the reduction of S nanoparticle size. Importantly, the 5 nm S particles showed an initial capacity of 1670 mAh/g, which is the theoretical limit of sulfur (Nano Lett. 2015, 15, 798). This work demonstrates that the high theoretical performance can indeed be realized. The understanding of S cathode charge transfer will help future cathodic design in the Li-S technology.

Optical demagnetization in defect-mediated ferromagnetic ZnO:Cu films

Optical demagnetization effect has been observed in a defect-rich ZnO:Cu film. Phenomenally, UV illumination can effectively quench dual-donor oxygen vacancy (VO) and interstitial zinc (Zni) mediated magnetic ordering (90% loss) with carrier recapture in ionized Zni (Zni2+ rather than Zni+) traps. The involvement of carriers by means of metal Zn vapor annealing provides a bound carrier related magnetic scenario, stressing a more significant place of bound carrier rather than free carrier. Using light correlated electron spin resonance and magnetic measurements, the magnetic “on” and “off” state can be continuously manipulated via UV light, which is explained by the reversible process of [Cus+/Cus2+] + Zni2+ ↔ [Cus+, h] + Zni+ (s and h denote the substitutional Cu ions on Zn sites in ZnO lattice and hole carrier, respectively). A slow charge-transfer process (∼15 μs) from Zni+ bound carriers to [Cus+, h] states determines the unexpected optical demagnetization, well corresponding to control experiments.

Platinum-graphene counter electrodes for dye-sensitized solar cells

This paper describes the photovoltaic performance of dye-sensitized solar cells (DSSCs) containing graphene-incorporated counter electrodes (CEs). The location and thickness of graphene in CEs are optimized to improve the photovoltaic performance of DSSCs, compared with typical Pt CEs. The DSSC, with a Pt/few-layer graphene (FLG) CE, achieved 8% in short-circuit current density and 13% in power conversion efficiency (PCE). Electrochemical impedance spectroscopy shows that the DSSC, with a Pt/FLG CE, exhibits a series resistance lower than that with a Pt CE. The lower series resistance is attributed to the contact resistance at the interface of platinum and fluorine doped tin oxide. The contact resistance is reduced by the formation of the thin platinum-carbon composite layer. It is demonstrated that the consumption of Pt could be reduced with a Pt/FLG CE. However, graphene/Pt CEs resulted in a slow charge-transfer process and consequently a worse photovoltaic performance of DSSCs.

On the meaning of the diffusion layer thickness for slow electrode reactions

A key concept underpinning electrochemical science is that of the diffusion layer - the zone of depletion around an electrode accompanying electrolysis. The size of this zone can be found either from the simulated or measured concentration profiles (yielding the 'true' diffusion layer thickness) or, in the case of the Nernst ('linear') diffusion layer by extrapolating the concentration gradient at the electrode surface to the distance at which the concentration takes its bulk value. The latter concept is very well developed in the case of fast (so-called reversible) electrode processes, however the study of the linear diffusion layer has received scant attention in the case of slow charge transfer processes, despite its study being of great interest in the analysis of the influence of different experimental variables which determine the electrochemical response. Analytical explicit solutions for the concentration profiles, surface concentrations and real and linear diffusion layers corresponding to the application of a potential step to a slow charge transfer process are presented. From these expressions the dependence of the diffusion layer thickness on the potential, pulse time, heterogeneous rate constant and ratio of bulk concentrations of electroactive species and of diffusion coefficients is quantified. A profound influence of the reversibility degree of the charge transfer on the diffusion layer thickness is clear, showing that for non-reversible processes the real and linear diffusion layers reveal a minimum thickness which coincides with the equilibrium potential of the redox couple in the former case and with the reversible half-wave potential in the latter one.

Additive Differential Pulse Voltammetry for the Study of Slow Charge Transfer Processes at Spherical Electrodes

The application of additive differential pulse voltammetry to the study of the kinetics of a charge transfer process is studied. A simple analytical solution is presented, valid for spherical electrodes of any size and for electrode processes of any reversibility. From this solution, valuable diagnostic criteria for the elucidation of the electrochemical reversibility are established based on the variation of the ADPV signal with the duration of the potential pulses, the electrode radius and the pulse height. Working curves for the determination of the kinetic parameters are also given. The value of the ADPV technique is experimentally demonstrated by studying the kinetics of the reduction of 3-nitrophenolate− and europium3+ at mercury hemispherical microelectrodes.

Analytical solution for Reverse Pulse Voltammetry at spherical electrodes: A remarkably sensitive method for the characterization of electrochemical reversibility and electrode kinetics

Reverse Pulse Voltammetry (RPV) is a powerful double pulse technique for kinetic studies. The theory of this technique for slow charge transfer processes at spherical electrodes is developed. An explicit analytical solution is deduced applicable to electrodes of any size, whatever the reversibility of the system and the length of the two potential pulses. From this solution we examine the influence on the RPV response of the electrode sphericity and the kinetics of the electrode process. It is shown that visual inspection of the curves provides immediate valuable information on electrochemical kinetics. Moreover, simple diagnosis criteria are established and general working curves derived for the quantitative extraction of the kinetic parameters and the formal potential. Some particularities in the morphology of the anodic branch of RPV curves for slow charge transfer processes when the second potential pulse is sufficiently large are also described. The merits of RPV as an exquisitely sensitive measure of electrochemical reversibility and electrode kinetics are advocated.

Application of double pulse theory for hemispherical microelectrodes to the experimental study of slow charge transfer processes

Differential Pulse Voltammetry and Reverse Pulse Voltammetry are applied to the study of the electrode kinetics of slow charge transfer processes at hemispherical microelectrodes of ca. 25 μm radius. The electrochemical reversibility of three redox systems: 3-nitrophenolate −/2− , 3-nitrophthalate 2−/3− and europium 3+/2+, are evaluated with both techniques by determining the heterogeneous rate constant, the electron transfer coefficient as well as the formal potential for each. The experimental results show the value of these techniques for the characterization of the electrode kinetics by means of simple diagnostic criteria and single-point fit to general working curves. Further, theory developed for double potential pulse under radial diffusion conditions is validated.

Characterization of slow charge transfer processes in differential pulse voltammetry at spherical electrodes and microelectrodes

Abstract The use of differential pulse voltammetry at spherical electrodes for the study of the kinetic of charge transfer processes is analyzed. An analytical solution is presented, valid for any value of the electrode radius, the heterogeneous rate constant and the transfer coefficient. Several reversibility criteria are established based on the variation of DPV peak with the duration of the potential pulses and the electrode radius. Moreover, general working curves for extraction of kinetic parameters from DPV experiments are given. The anomalous shape of DPV curves for quasireversible processes with small values of the transfer coefficient is reported. The effect of the presence of both electroactive species on DPV curves for quasireversible and irreversible systems is also studied.

Cyclic Reciprocal Derivative Chronopotentiometry with Exponential Time Currents in the Study of Slow Charge Transfer Processes between Electrodes and Redox Adsorbates

Theoretical expressions are presented corresponding to the response of adsorbed molecules which exhibit a quasi-reversible or totally irreversible charge transfer in cyclic reciprocal derivative ch...

A semiclassical analysis of orbiting resonances in slow charge transfer processes

The orbiting charge transfer resonances appearing at very low energies in a theoretical study of the collision system N 3+ + H have been analysed semiclassically and were found to depend strongly on structural properties. The semiclassical analysis also gave new insight into the dynamics of the charge transfer process.

The occlusion of methionine and cystine during copper electrodeposition

A radioisotopic technique has been used to measure the extents to which the addition agents methionine and cystine are occluded in copper electrodeposits. Increase of the current density from 2.5 to 60 mA cm−2 resulted in a decrease in the volume density of occluded radioisotopes. This behaviour would be expected if the net adsorption rates of additives vary only slightly relative to the change in the growth rates of the metallic lattice. The mean occlusion rates of methionine exhibited a maximum at about 50 mA cm−2, which may be due to a potential-dependent adsorption process. An empirical variation in the mean occlusion rates for cystine is interpreted on the assumption that cystine is reduced by a slow charge-transfer process, and is occluded either as cysteine or as a copper-cysteine complex.

The occlusion of methionine and cystine during copper electrodeposition

Summary A radioisotopic technique has been used to measure the extents to which the addition agents methionine and cystine are occluded in copper electrodeposits. Increase of the current density from 2.5 to 60 mA cm −2 resulted in a decrease in the volume density of occluded radioisotopes. This behaviour would be expected if the net adsorption rates of additives vary only slightly relative to the change in the growth rates of the metallic lattice. The mean occlusion rates of methionine exhibited a maximum at about 50 mA cm −2 , which may be due to a potential-dependent adsorption process. An empirical variation in the mean occlusion rates for cystine is interpreted on the assumption that cystine is reduced by a slow charge-transfer process, and is occluded either as cysteine or as a copper-cysteine complex.