Steady-state voltammetry ultramicroelectrodes

The evidence that the 1 couple can diffuse freely in the liquid domains entrapped by the three-dimensional network of the gelators has also been found in the case of a PVDF-HFP gel via steady-state voltammetry at ultramicroelectrodes. Quite surprisingly the voltammogramms of the liquid and of the gel are almost perfectly superimposable (Fig. 17.14) and the diffusion coefficient of the redox ions could be calculated to be 3.6 x 10 cm2/s and 4.49 x 10-6 cm2/s for I- and I3, respectively, using Equation 17.15,... 

Figure 17.14 Steady-state voltammetry of a liquid and polymer (PVDF-HFP) gel electrolyte at a Pt ultramicroelectrode. Scan speed lOmV/s. Reprinted by permission from Mac Millan Publishers Ltd Nature Materials, 2003, 2, 402.

ABSTRACT. Several aspects of electrochemistry at ultramicroelectrodes are presented and discussed in relevance to their application to the analysis of chemical reactivity. The limits of fast scan cyclic voltammetry are examined, and the method shown to allow kinetic investigations in the nanosecond time scale. On the other hand, the dual nature of steady state at ultramicroelectrodes is explained, and it is shown how steady state currents may be used, in combination with transient chronoamperometry, for the determination of absolute electron stoichiometries in voltammetric methods. Finally the interest of electrochemistry in highly resistive conditions for discussion and investigation of chemical reactivity is presented. 

Finally, to conclude this introduction, and to avoid any possible confusion in the terminology, we wish to define briefly what is an ultramicroelectrode (at least in our sense ). When their interfacial properties are to be considered identical with those of any other electrode of a larger dimension, ultramicroelectrodes must remain much larger than the double layer thickness. This sets a lower dimension of a few tens of A for ultramicroelectrodes.On the other hand, if diffusional steady state voltammetry has to be observed without significant interference of convection, they must be smaller than convective layers, which sets an upper limit of a few tens of /im. Between these limits, all ultramicroelectrodes possess identical intrinsic physico-chemical properties. However, their behavior (viz. ohmic drop, steady state or transient currents, etc) obviously depends on the medium and the time-scale considered. ... 

C60 with strong acids, a C q solution was generated by controlled potential electrolysis and titrated with concentrated triflic acid. UV/visible-near IR spectra as well as steady state voltammetry with ultramicroelectrodes were used to monitor 50 and C60H. Addition of the triflic acid led to a decrease of absorbance and steady state oxidation current of Cgo". At the same time, a reduction current appeared at potentials more negative than the half wave potential of Cgg oxidation. This current was attributed to CeoH reduction. With some simple assumptions, the current changes were used to calculate the equilibrium constant of the reaction ... 

The time domain on a window accessed by a given experiment or technique, e.g., femtosecond, picosecond, microsecond, millisecond. The time scale (or domain) is often characterized by a set of physical parameters associated with a given experiment or technique, e.g., r2 ]/1) (for - ultramicroelectrode experiments) - thus if the electrode radius is 10-7 cm and the - diffusion coefficient D = 1 x 10-5 cm2/s-1 the time scale would be 10 9s. Closely related to the operative kinetic term, e.g., the time domain that must be accessed to measure a first-order -> rate constant k (s-1) will be l//ci the time domain that must be accessed to measure a given heterogeneous rate constant, k willbe /)/k2. In - cyclic voltammetry this time domain will be achieved when RT/F v = D/k2 with an ultramicroelectrode this time domain will be achieved (in a steady-state measurement when r /D = D/k2 or ro = D/k at a microelectrode [i-ii]. 

The development of ultramicroelectrodes with characteristic physical dimensions below 25 pm has allowed the implementation of faster transients in recent years, as discussed in Section 2.4. For CA and DPSC this means that a smaller step time x can be employed, while there is no advantage to a larger t. Rather, steady-state currents are attained here, owing to the contribution from spherical diffusion for the small electrodes. However, by combination of the use of ultramicroelectrodes and microelectrodes, the useful time window of the techniques is widened considerably. Compared to scanning techniques such as linear sweep voltammetry and cyclic voltammetry, described in the following, the step techniques have the advantage that the responses are independent of heterogeneous kinetics if the potential is properly adjusted. The result is that fewer parameters need to be adjusted for the determination of rate constants. 

Although the use of ultramicroelectrodes is not restricted to any specific measurement technique [125,143,178,181], only applications in the context of cyclic voltammetry at high sweep rates are considered here (see also Sec. IV). For the studies of reaction kinetics using ultramicroelectrodes under steady-state conditions the reader is referred to the original literature [182]. 

The origins of SECM homogeneous kinetic measurements can be found in the earliest applications of ultramicroelectrodes (UMEs) to profile concentration gradients at macroscopic (millimeter-sized) electrodes (1,2). The held has since developed considerably, such that short-lived intermediates in electrode reactions can now readily be identified by SECM under steady-state conditions, which would be difficult to characterize by alternative transient UME methods, such as fast scan cyclic voltammetry (8). 

Usual conditions for LSV or CV experiments require a quiet solution in order to allow undisturbed development of the diffusion layer at the electrode. Some groups, however, have purposely used the interplay between diffusion and convection in electrolytes flowing in a channel or similar devices [23]. In these experiments (see also Chapter 2.4), mass transport to the electrode surface is dramatically enhanced. A steady state develops [54] with a diffusion layer of constant thickness. Thus, such conditions are in some way similar to the use of ultramicroelectrodes. Hydro-dynamic voltammetry is advantageous in studying processes (heterogeneous electron transfer, homogeneous kinetics) that are faster than mass transport under usual CV or LSV conditions. A recent review provides several examples [22]. 

The diffusion coefficient and concentration of oxygen in neat ionic liquids can be determined by cyclic voltammetry using an ultramicroelectrode and chronoamperometry using a macroelectrode [23]. The steady-state current, ijs. with an ultramicroelectrode having radius tq is given by the following equation ... 

The considerable decrease of ohmic drop at ultramicroelectrodes has allowed significant electrochemical data to be obtained from voltammetry in highly resistive media. Thus voltammetry can be performed in usual electrochemical solvents, but without purposefully added supporting electrolyte." Also, steady state voltanunograms can be obtained in solvents with very low constants,such as alcanes or arenes, if a small concentration of an inert electrolyte is added. In the following we want to present two examples of application of voltammetry under these conditions, to the unravelling of organometallic reactivity. 

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