Faradaic mediators

The surface states at the semiconductor electrolyte interface under illumination for the electrochemical redjgtion of carbon dioxide has been determined to be 10 cm. Surface states are induced by adsorbed ions and act as faradaic mediators for the photo-electrochemical reduction of carbon dioxide. It is shown that CO is adsorbed on platinum and adsorbed C0 is the intermediate radical. The rate determining step involves further reduction of CO to give the final products. Adsorption of NH, ions on p-GaP has been studied using FTIRRAS. At cathodic potentials adsorbed ammonium ions are reduced and the reduced ammonium radical desorbs. The structure of adsorbed ammonium is investigated. 

Surface states act as faradaic mediators for the photoelectro-chemical reduction of CO ... 

The charge propagates in the film by electron hopping between the polymer Red/Ox couples. This is controlled by the electrode potential only in a close proximity of the electrode in more distant sites, the charge transport is driven by a concentration gradient of reduced or oxidized mediators. The observed faradaic current density, jF, is a superposition of... 

The anodic reaction used is an indirect oxidation of benzene by Ag(I)/Ag(II) as redox mediator, because of its high faradaic yield. The high yield of BQ of 84% (of the theoretical yield) compared to the yield of the direct oxidation on the Pb02 anode of 62% indicates that some mechanism to minimize side reactions such as formation of o-BQ is operative. The highest yields are achieved with AgC104, (cf. Table 2, Ref. [66]). The use of AgC104 excludes its application in large scale synthesis. 

Because the oxidation of an analyte is effected with a mediator, the potential of the working electrode need not be as anodic as the potential needed for the first type of sensor, implying that anions such as ascorbate (see above) will remain unnoticed as electro-inactive contaminants. By remaining unoxidized, the magnitude of the non-faradaic current is kept to a minimum. 

First, C02 reduction at metal electrodes in both aqueous and nonaqueous media, as well as in systems coupled with electron-mediating complexes are detailed. The faradaic efficiency of such a system can be used as a measure of efficiency and selectivity. For a specific, electrochemically generated product, the faradaic efficiency is the ratio of the actual and theoretical amounts of product formed within the same time interval, based on charge passed. An efficient and selective system will lead to a 100% faradaic yield for a single product in other words, all of the charge passed in the system has gone into the production of that product. 

The rhenium complexes described in Section 11.2 have also been studied as electron mediators for C02 reduction at metal electrodes. Hawecker et al. used the complex Re(bpy)(CO)3Cl in DMF/water (9 1) at glassy carbon electrodes at a potential of-1.44V (versus SCE) to produce CO with 98% faradaic efficiency [15, 87]. Likewise, Sullivan et al. reported the production of CO with similar efficiency at a platinum electrode at -1.5 V (versus SCE) by using the complex fac-Re(bpy) (CO)3Cl [88]. Ruthenium complexes that have been used in photochemical... 

Fisher and Eisenberg showed that Ni(II) and Co(II) tetra-azamacrocyclic complexes in water-acetonitrile mixtures at Hg electrodes reduced C02 to CO, H2, and formic acid at -1.5V (versus SCE) [97]. Here, the total faradaic efficiencies approached 100%, but the ratios for CO H2 were 1 1. Tinnemans et al. also investigated several Ni and Co tetra-azamacrocyclic complexes as electron mediators at Hg electrodes [33]. As an example, in DMF with 5% water at -1.3 V (versus SCE), up to 66% faradaic efficiency was achieved for CO with the complex Co(II)[Me2-Pyo(14)trieneN4] 2+, using Hg electrodes. 

Beley et al. and others studied tetra-azamacrocyclic Ni complexes, similar to those presented in Section 11.2, in aqueous and organic solvents for the mediated reduction of C02 [98-100], In aqueous 0.1 M KN03 solution at a potential of-1.2 V (versus SCE), Ni(II)-cyclam dichloride (cyclam = 1,4,8,11-tetra-azatetradecane) reduced C02 to CO with 96% faradaic efficiency at Hg electrodes. The mechanism involved a first electron reduction of the species which coordinated C02, followed by C02 protonation, and a second electron transfer to yield CO and OH (as dis-... 

Recently, interest has been expressed in natural enzymes that effect the reduction of C02 to various products (see Section 11.4). For example, Reda et al. used a tungsten-containing formate dehydrogenase 1 enzyme derived from Syntropho-bacterjumaroxidans in the mediated electroreduction of C02 to formate [103]. The enzyme, which is either adsorbed onto the graphite electrode surface or is free in solution, was observed to reduce C02 to formate with near-100% faradaic efficiency. Although a minimal overpotential for the process was required (-0.4V of applied bias), the current densities were rather low. 

Similarly, when Bockris and Wass used various organic mediators in a DMF solution with 5% water and p-CdTe as the photocathode [120], the bare p-CdTe produced a faradaic efficiency for CO of 92%. Subsequently, by using their best catalysts, namely 15-crown-5 ether and 18-crown-6 ether, these authors were able also to produce methanol with faradaic efficiencies of 14% and 13%, respectively, with the remaining current going to CO production. Although the potential at which these electrolyses were carried out was not reported, the onset potential for C02 reduction was shifted some 400 mV anodically in the presence of the organic mediators. 

More recently, the use of a pyridinium mediator in an aqueous p-GaP photo-electrochemical system illuminated with 365 nm and 465 nm light has been reported [125], In this case, a near-100% faradaic efficiency was obtained for methanol production at underpotentials of 300-500 mV from the thermodynamic C02/methanol couple. Moreover, quantum efficiencies of up to 44% were obtained. The most important point here, however, was that this was the first report of C02 reduction in a photoelectrochemical system that required no input of external electrical energy, with the reduction of C02 being effected solely by incident fight energy. 

Label-free detection of ligand-aptamer interaction was also demonstrated by means of impedance spectroscopy technique [52,53]. Simultaneously, Radi et al. [52] and Rodriguez et al. [53] reported application of Faradaic impedance spectroscopy (FIS) in detection of interaction of proteins with DNA aptamers. The detection method is based on the measurement of resistance in presence of redox mediator Fe(CN)6-In absence of target protein, the negatively charged aptamer repulse the redox mediator molecules from the sensor surface. In a paper by... 

In -> polarography sometimes faradaic currents are observed which cannot be attributed to diffusion-limited reduction of electroactive species under investigation. Sometimes substances (which are not necessarily electroactive themselves) lower the hydrogen overpotential of the mercury electrode in various ways (by adsorption, by acting as a redox mediator), thus a hydrogen evolution current (a catalytic hydrogen wave) is observed [ii—iv]. See also - Mairanovskii. 

In this section, we first consider a general model of the faradaic processes occurring at the semiconductor-electrolyte interface due to Gerischer [11]. From Gerischer s model, using the potential distribution at the interface, we may derive a Tafel-type description of the variation of electron transfer with potential and we will then consider the transport limitations discussed above. We then turn to the case of intermediate interactions, in which the electron transfer process is mediated by surface states on the semiconductor and, finally, we consider situations in which the simple Gerischer model breaks down. 

Many of these approaches rely on the differential hybridization of target DNAs that are perfectly matched with probe sequences to achieve highly specific and accurate target selection. Other assays use the sensitivity of DNA-mediated charge transport to duplex structure in order to signal the presence of a sequence of interest. Efforts also have been directed toward exploiting non-faradaic processes unique to DNA-modified surfaces as the basis for electrochemical readout. This chapter discusses each of these methods, and is intended to highlight how the DNA/electrode interface can... 

The probe in SECM produces a signal that must be transduced and amplified prior to recording. At a voltammetric tip, electrolysis of either a mediator or a substrate-produced substance produces a faradaic current signal. At a potentiometric tip, the activity of a solution phase species generates a voltage signal. 

Alternatively the second barrel can be filled with the electrolyte and used in the same manner as in ion-conductance microscopy (39). It is possible to relate the solution conductance between tip and counterelectrode to the normalized tip-to-substrate distance L. In fact, after normalizing the conductance G(L) by the value with the tip far from the surface G,, the distance dependence of the conductance is identical to that for faradaic currents in feedback SECM with a redox mediator. 

By definition, mediated electron transfer of the type discussed in Section 14.4.2(c) cannot occur in blocking films. However for very thin films, e.g., self-assembled monolayers (SAM) of alkane thiols or oxide films, electrons can tunnel through the film and cause faradaic reactions. This phenomenon is important in electronic devices, in passivation of metal surfaces, and in fundamental studies of the distance dependence of the rate of electron transfer. 

The hydrodynamic voltammogram does not yield a plateau current. A possible reason is that a second faradaic process begins to add to the backgound current this is better seen by cyclic voltammetry, Fig.6. This process will cause a decrease in the current due to the mediated oxidation of cystine in the event that the further-oxidized product of the background process is a less effective mediator than the parent species. 

The electrochemical impedance for surface state-mediated charge transfer has been computed recently [78]. The key results are summarized in Fig. 16. Figure 16(a) contains the proposed equivalent circuit for the process and features a parallel connection of the impedance for the Faradaic process [Zf( )] (co = angular frequency, 2nf) and the capacitance of the semiconductor depletion layer, Csc- The... 

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