Faradaic electrochemical techniques

Faradaic techniques are those in which oxidation or reduction of analyte species occurs at the electrodes and therefore a measurable current is passed through the electrochemical cell. This discussion will be limited to controlled-potential techniques, primarily volta-metry and amperometry, coupled to liquid chromatography. While other Faradaic electrochemical techniques have been developed and electrochemical techniques in bulk solution are common, the use of liquid chromatography employing these two detection strategies is by far the most common electroanalytical technique in pharmaceutical studies. 

For successful application of Faradaic electrochemical techniques, it is necessary to understand the fundamental processes that occur at the surface of the electrode in electrolyte solution. When a potential is applied between two electrodes in solution, a narrow interphase region, the electrical double-layer, develops at the surface of the electrodes. All oxidative or reductive electrochemical reactions between the electrode and the analyte occur in this interphase region between the electrode surface and bulk solution. The bulk solution will remain at electroneutrality because the potential drop between the electrodes only exists in the interphase region. Therefore, molecules in the bulk solution cannot feel the presence of the electrodes... 

A physical model for film transport that accommodates the available experimental observations is a conventional film permeation model [10]. Candidate permeant molecules partition into a film from solution via an equilibrium process and then diffuse through the film to the electrode surface where they undergo oxidation or reduction. The redox product molecules then diffuse to the film/solution boundary and exit the film via a reverse partitioning step that, again, is sufficiently rapid to occur at equilibrium. Faradaic electrochemical techniques are especially powerful for interrogating such processes - in part because they require permeants to traverse films essentially completely in order for signals to be observed. (Recall that interfacial electron transfer can occur with reasonable efficiency only over distances of ca 10, or perhaps 20, A. Depending on the compound, these distances correspond to one or two film monolayers.)... 

How can one explain such a huge Faradaic efficiency, A, value As we shall see there is one and only one viable explanation confirmed now by every surface science and electrochemical technique, which has been used to investigate this phenomenon. We will see this explanation immediately and then, in much more detail in Chapter 5, but first let us make a few more observations in Figure 4.13. It is worth noting that, at steady-state, the catalyst potential Uwr, has increased by 0.62 V. Second let us note that upon current interruption (Fig. 4.13), r and UWr return to their initial unpromoted values. This is due to the gradual consumption of Os by C2H4. 

This value represents the upper limit of a first order reaction rate constant, k, which may be determined by the RHSE. This limit is approximately one order of magnitude smaller that of a rotating electrode. One way to extend the upper limit is to combine the RHSE with an AC electrochemical technique, such as the AC impedance and faradaic rectification metods. Since the AC current distribution is uniform on a RHSE, accurate kinetic data may be obtained for the fast electrochemical reactions with a RHSE. 

Lithography With the STM Electrochemical Techniques. The nonuniform current density distribution generated by an STM tip has also been exploited for electrochemical surface modification schemes. These applications are treated in this paper as distinct from true in situ STM imaging because the electrochemical modification of a substrate does not a priori necessitate subsequent imaging with the STM. To date, all electrochemical modification experiments in which the tip has served as the counter electrode, the STM has been operated in a two-electrode mode, with the substrate surface acting as the working electrode. The tip-sample bias is typically adjusted to drive electrochemical reactions at both the sample surface and the STM tip. Because it has as yet been impossible to maintain feedback control of the z-piezo (tip-substrate distance) in the presence of significant faradaic current (vide infra), all electrochemical STM modification experiments to date have been performed in the absence of such feedback control. 

There are a few electrochemical techniques in which the working electrode is moved with respect to the solution (i.e. either the solution is agitated or the electrode is vibrated or rotated). Under these conditions, the thickness of the diffusion layer decreases so that the concentration gradient increases. Since the rate of the mass transport to an electrode is proportional to the concentration gradient (Chapter 1, Section 4.2.2), the thinning of the diffusion layer leads to an increase of the mass transport, and hence to an increase of the faradaic currents. 

I) Faradaic electrochemical methods. From a general analytical point of view, electrochemical techniques are very sensitive methods for identifying and determining the electroactive species present in the sample and, in addition, they also are able to carry out speciation studies, providing a complete description of the states of oxidation in which the ionic species are present in the object. Other applications and improvements obtained by their hyphenation with other instrumental techniques, such as atomic force microscopy (AFM), will be described in the following chapters. 

U) Non-Faradaic electrochemical methods. Conductometric methods have been extensively used by scientists and conservators for monitoring the content of salts removed during water immersion treatments of ancient tiles and archaeological ceramic remains. In a different manner to IC, this technique provides the total ionic... 

In this section the use of amperometric techniques for the in-situ study of catalysts using solid state electrochemical cells is discussed. This requires that the potential of the cell is disturbed from its equilibrium value and a current passed. However, there is evidence that for a number of solid electrolyte cell systems the change in electrode potential results in a change in the electrode-catalyst work function.5 This effect is known as the non-faradaic electrochemical modification of catalytic activity (NEMCA). In a similar way it appears that the electrode potential can be used as a monitor of the catalyst work function. Much of the work on the closed-circuit behaviour of solid electrolyte electrochemical cells has been concerned with modifying the behaviour of the catalyst (reference 5 is an excellent review of this area). However, it is not the intention of this review to cover catalyst modification, rather the intention is to address information derived from closed-circuit work relevant to an unmodified catalyst surface. 

Both faradaic and non-faradaic currents may flow when an electrode reaction occurs. Thus, the total current which flows is often the sum of the faradaic and non-faradaic contributions to the current. Most often, it is the faradaic current that is of interest. Many electrochemical techniques have been developed which minimize or eliminate this non-faradaic contribution to the current, but discussion of these is beyond the scope of the present chapter. 

Two electrochemical techniques are directly based on the expression for the faradaic current density jF, namely chronoamperometry and normal pulse polarography. A third technique, named chronocoulometry, deals with the integral of jF, giving the charge transferred per unit area via the faradaic process as a function of time. The general expression obtained... 

It can be assumed, at least approximately, that the current for any electrochemical technique can be expressed as the sum of a pure faradaic current because of the... 

Double Potential Pulse Electrochemical Techniques combine the faradaic currents at two successive potential pulses recovering then the initial equilibrium conditions (in the case of a DME the two successive potentials are applied to the same drop). 

In the last 30 years, the manufacturing and use of micrometer- and nanometer-sized electrochemical interfaces, microelectrodes, and micro-ITIES have been widely extended. The main advantages associated with the reduction of the size of the interface are the fast achievement of a time-independent current-potential response (independent of the electrochemical technique employed), the decrease of the ohmic drop, the improvement of the ratio of faradaic to charge current, and the enhancement of the mass transport. Their small size has played an important role in... 

In this section, the subtractive multipulse techniques DMPV and SWV are applied to reversible ion transfer across different liquid-liquid systems with one or two polarizable interfaces. These electrochemical techniques allow the accurate and easy determination of standard potentials directly from the peak potentials of the current-potential curves since non-faradaic and background currents are minimized [12, 35-40]. 

As stated in Sect. 6.4.1, in the theoretical treatment of the electrochemical responses of surface-bound molecules, it has been assumed that the measured experimental currents and converted charges when a potential Ep is applied can be considered as the sum of a pure faradaic contribution, given by Eqs. (6.130) and (6.131), and a non-faradaic one, Qp nf and fpnt (given by Eqs. (6.150) and (6.157)). The correction of this non-faradaic component of the response can be done simply when subtractive electrochemical techniques are used). We assume the parallel capacitors model introduced by Damaskin [78], for which CAnf can be written as... 

The phenomenon of EPOC or NEMCA effect was first reported in solid electrolyte systems [23, 195-205], but several NEMCA studies already exist using aqueous electrolyte systems [23, 30, 31,145] or Nafion membranes [23]. The EPOC phenomenon leads to apparent Faradaic efficiencies, A, well in excess of 100% (values up to 105 have been measured in solid-state electrochemistry and up to 102 in aqueous electrochemistry). This is due to the fact that, as shown by a variety of surface science and electrochemical techniques [23, 40, 195-198, 206-209], the NEMCA effect is due to electrocatalytic (Faradaic) introduction of promoting species onto catalyst-electrode surfaces [23, 196], each of these promoting species being able to catalyze numerous (A) catalytic turnovers. 

The similarities between ITIES and conventional electrode electrochemistry provide an arsenal of electrochemical techniques that have been previously tested in the more common electroanalytical chemistry and physical electrochemistry. To understand the similarities between ITIES and electrode electrochemistry, it is more useful to look at the differences first. Faradaic current flow through an electrochemical cell is associated with redox processes that occur at the electrode surface. The functional analog of an electrode surface in ITIES is the interface itself. However, the net current observed when the interface is polarized from an outside electric source is not a result of a redox process at the interface rather, it is an effect that is caused by an ion transport through the interface, from one phase to another. 

As in all potentiostatic techniques, the double layer charging is a parallel process to the faradaic reaction that can substantially attenuate the photocurrent signal at short-time scale (see Section 5.3)" . This element introduces another important difference between fully spectroscopic and electrochemical techniques. Commercially available optical instrumentation can currently deliver time resolution of 50 fs or less for conventional techniques such as transient absorption. On the other hand, the resistance between the two reference electrodes commonly employed in electrochemical measurements at the liquid/liquid interfaces and the interfacial double layer capacitance provide time constants of the order of hundreds of microseconds. Consequently, direct information on the rate of heterogeneous electron injection from/to the excited state is not accessible from photocurrent measurements. These techniques do allow sensitive measurements of the ratio between electron injection and decay of the excited state under pho-tostationary conditions. Other approaches such as photopotential measurements, i.e. relative changes in the Fermi levels in both phases, can provide kinetic information in the nanosecond regime. 

The charging current of the electrical double layer at the electrode interface hmits the electrode reaction rate measurements hy traditional electrochemical techniques. The potential-modulated ER spectroscopic technique, on the other hand, involves the measurement of the faradaic current as a difference in the spectrum between oxidized and reduced forms at the electrode surface generated by an ac modulation of the electrode potential. This technique enables the measurement of electrode reaction rates up to approximately 10" s because the effect of the double-layer charging current can be minimized [28]. 

Adsorption is a fairly common phenomenon in electrochemical measurement of faradaic activity, especially in the analysis of organic and biological systems. Quantitative theories concerning the effects of adsorption on the current and potential responses obtained by a variety of electrochemical techniques have been published by Laviron [204]. The adsorption of the test substance during the preconcentration step may be successfully exploited in stripping techniques (see section 7) for their determination. The aim of this method, however, is not the determina-... 

In a typical OEMS experiment, the itm current corresponding to a given species of interest is recorded in parallel to the faradaic electrode current during the potential sweep (cyclic voltammogram), yielding the so-called mass spectrometric voltammograms (MSCV). Other electrochemical techniques, such as potentiostatic [4, 5] or galvanostatic ones [6] and even pulsed voltammetry for short time (1 s), have also been combined with OEMS. 

Similar to EIS, SWV (square-wave voltammetry) is another frequency-dependent electrochemical technique that could also be used in label-free Faradaic immunosensing [167]. In this case, a train of potential pulses is superimposed on a staircase potential signal with the latter centered between a cathodic pulse and an anodic pulse of the same amplitude. During each cathodic pulse, the analyte diffuses to the electrode surface and it is immediately reduced. During the anodic pulse, analyte that was just reduced is reoxidized. The current is sampled just before and at the end of each pulse and the current difference between these two points is then plotted against the staircase potential in a SW voltammogram. A linear potential scan in SWV is faster than EIS record and a familiar peak-shaped signal is more easily interpreted. 

Potential-step method is an electrochemical technique in which the potential of the working electrode is either held at constant or stepped to a predetermined value, and the resulting current due to Faradaic processes and double-layer charging processes occurring at the electrode (caused by the potential step) is monitored as a function of time. Especially for practical electrochemical sensor in real-world application, chronoamperometry is preferred due to its simplicity and low cost of instrumentation. Besides its wide use in most electrochemical sensor systems, chronoamperometry has been used in the understanding of the kinetics of the electrochemical processes as well. 

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