Faradaic potential step

CS = coulostatic method, CV = cyclic voltamogram, FD = Faradaic distortion method, FR = Faradaic rectification, GD = galvanostatic double pulse method, IP = impedance method, PS = potential step method. See also list of... 

In this section, we will present and discuss cyclic voltammetry and potential-step DBMS data on the electro-oxidation ( stripping ) of pre-adsorbed residues formed upon adsorption of formic acid, formaldehyde, and methanol, and compare these data with the oxidative stripping of a CO adlayer formed upon exposure of a Pt/ Vulcan catalyst to a CO-containing (either CO- or CO/Ar-saturated) electrolyte as reference. We will identify adsorbed species from the ratio of the mass spectrometric and faradaic stripping charge, determine the adsorbate coverage relative to a saturated CO adlayer, and discuss mass spectrometric and faradaic current transients after adsorption at 0.16 V and a subsequent potential step to 0.6 V. 

In order to distinguish more clearly between effects induced by the varying potential and kinetic contributions, the continuous oxidation of the three Cj molecules was followed at a constant potential after the potential step. The corresponding faradaic and mass spectrometric (m/z = 44) current transients recorded after 3 minutes adsorption at 0.16 V and a subsequent potential step to 0.6 V (see Section 13.2) are reproduced in Figs. 13.5-13.7. In all cases, the faradaic current exhibits a small initial spike, which is associated with double-layer charging when stepping the electrode potential to 0.6 V. 

Figure 13.5 Potential-step electro-oxidation of formic acid on a Pt/Vulcan thin-film electrode (7 p,gptcm, geometric area 0.28 cm ) in 0.5 M H2SO4 solution containing 0.1 M HCOOH upon stepping the potential from 0.16 to 0.6 V (electrol)Te flow rate 5 p,L s at room temperature). (a) Solid line, faradaic current transients dashed line, partial current for HCOOH oxidation to CO2. (b) Solid line, m/z = 44 ion current transients gray line, potential-step oxidation of pre-adsorbed CO derived upon HCOOH adsorption at 0.16 V, in HCOOH-ftee H2SO4 solution.

Formaldehyde Oxidation The faradaic current for formaldehyde oxidation (solid line in Fig. 13.6a) is largely suppressed during the first minute after stepping to 0.6 V, and then increases with time, resulting in an S-shaped transient. It reaches its maximum current about 4-5 minutes after the potential step, followed by a slow and nearly linear decay of the faradaic current with time (solid line in Fig. 13.6a). In comparison with formic acid oxidation (solid line in Fig. 13.5a), three major differences can be noted ... 

The current efficiencies for the different reaction products CO2, formaldehyde, and formic acid obtained upon potential-step methanol oxidation are plotted in Fig. 13.7d. The CO2 current efficiency (solid line) is characterized by an initial spike of up to about 70% directly after the potential step, followed by a rapid decay to about 54%, where it remains for the rest of the measurement. The initial spike appearing in the calculated current efficiency for CO2 formation can be at least partly explained by a similar artifact as discussed for formaldehyde oxidation before, caused by the fact that oxidation of the pre-formed COacurrent efficiency. The current efficiency for formic acid oxidation steps to a value of about 10% at the initial period of the measurement, and then decreases gradually to about 5% at the end of the measurement. Finally, the current efficiency for formaldehyde formation, which was not measured directly, but calculated from the difference between total faradaic current and partial reaction currents for CO2 and formic acid formation, shows an apparently slower increase during the initial phase and then remains about constant (final value about 40%). The imitial increase is at least partly caused by the same artifact as discussed above for CO2 formation, only in the opposite sense. 

Figure 4. Plot of poly-I conductivity as a function of potential. A series of potential step of 20mV were employed on a sandwich electrode. Each potential was held until Faradaic current ceased, where upon a DC conductivity measurement, AE = 60MV, was taken, before proceding with the next potential. The results are for 0.05 M II electrolyte in acetonitrile vs. Ag+/Ag.

When a potential step is applied at a flat electrode, the time dependence of Faradaic current I is well described by the Cottrell equation119... 

Overlapping of Double-Layer Charging and Faradaic Currents in Potential Step and Double Potential Step Chronoamperometry. Oscillating and Nonoscillating Behavior... 

The faradaic current response to a single potential step... 

In this technique, introduced by Christie et al. [46] and developed extensively by Anson [47], essentially the current flowing after a potential step is integrated electronically to give the charge Q(t) transported between t = 0 and t. The function q(t) = Q(t)/A vs. tin has the shape depicted in Fig. 12 and is the sum of the contributions of double-layer charging and the faradaic process... 

In spite of its advantages and the simplicity of its performance, chrono-coulometry is seldom used for studying charge transfer kinetics. In fact, the method is much more popular because of its suitability for the study of reactant adsorption at the initial potential, which is manifested as an extra time-independent amount of faradaic charge involved in the potential step. This will be considered in more detail later. 

In this section, a description of the state of the art is attempted by (i) a review of the most fundamental types of reaction schemes, illustrated by some examples (ii) formulation of corresponding sets of differential equations and boundary conditions and derivation of their solutions in Laplace form (iii) description of rigorous and approximate expressions for the response in the current and/or potential step methods and (iv) discussion of the faradaic impedance or admittance. Not all the underlying conditions and fundamentals will be treated in depth. The... 

At a double electrode, such as the rotating ring—disc electrode, a potential step at the disc will produce a ring current transient, the form of which is affected only by Faradaic current components at the disc. This fact can be very useful in separating Faradaic and non-Faradaic processes. 

The excitation signal in chronoamperometry is a square-wave voltage signal, as shown in Figure 3.3A, which steps the potential of the working electrode from a value at which no faradaic current occurs, E , to a potential, Es, at which the surface concentration of the electroactive species is effectively zero. The potential can either be maintained at Es until the end of the experiment or be stepped to a final potential Ef after some interval of time t has passed. The latter experiment is termed double-potential-step chronoamperometry. The reader is referred to Section II. A for a detailed description of the resulting physical phenomena that occur in the vicinity of the electrode. 

Figure 5.4 Current response for a 12.5 pm platinum microelectrode modified with a [Os(bpy)2 py(p3p)]2+ monolayer following a potential step where the overpotential rj was —100 mV the supporting electrolyte is 0.1 M TBABF4 in acetonitrile. The inset shows ln[fp(f)] versus f plots for the Faradaic reaction when using a 12.5 pm (top) and 5 pm (bottom) radius platinum microelectrode. Reprinted with permission from R. J. Forster, Inorg. Chem., 35, 3394 (1996). Copyright (1996) American Chemical Society...

The study of the variation of the current response with time under potentiostatic control is chronoamperometry. In Section 5.4 the current resulting from a potential step from a value of the potential where there is no electrode reaction to one corresponding to the mass-transport-limited current was calculated for the simple system O + ne-— R, where only O or only R is initially present. This current is the faradaic current, If, since it is due only to a faradaic electrode process (only electron transfer). For a planar electrode it is expressed by the Cottrell equation4... 

Fig. 10.1. Evolution of current with time on applying a potential step at a stationary electrode. 7f is the faradaic current and 7C the capacitive current.

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