Electrochemical methods time resolution

Potential or current step transients seem to be more appropriate for kinetic studies since the initial and boundary conditions of the experiment are better defined unlike linear scan or cyclic voltammetry where time and potential are convoluted. The time resolution of the EQCM is limited in this case by the measurement of the resonant frequency. There are different methods to measure the crystal resonance frequency. In the simplest approach, the Miller oscillator or similar circuit tuned to one of the crystal resonance frequencies may be used and the frequency can be measured directly with a frequency meter [18]. This simple experimental device can be easily built, but has a poor resolution which is inversely proportional to the measurement time for instance for an accuracy of 1 Hz, a gate time of 1 second is needed, and for 0.1 Hz the measurement lasts as long as 10 seconds minimum to achieve the same accuracy. An advantage of the Miller oscillator is that the crystal electrode is grounded and can be used as the working electrode with a hard ground potentiostat with no conflict between the high ac circuit and the dc electrochemical circuit. 

The electrochemical techniques do not differ significantly with respect to time resolution. Pseudo first order rate constants ranging from about 0.1 to 10 S can be measured by techniques which monitor the response of the intermediate and LSV and electrocatalysis can give estimates of rate constants as high as 10 s . In the opinion of the author, the factors of most importance to be considered in selecting a measurement method of the first style are (i) the selectivity of the response, (//) the ease of obtaining reliable data, and (ill) the kinetic or thermodynamic information content of the data. Another factor of utmost importance to the non-specialist is (iv) the availability of instrumentation. 

The empirical approach adopted here integrates classical electrochemical methods with modem surface preparation and characterization techniques. As described in detail elsewhere, the actual experimental procedure involves surface analysis before and after a particular electrochemical process the latter may vary from simple inunersion of the electrode at a fixed potential to timed excursions between extreme oxidative and reductive potentials. Meticulous emphasis is placed on the synthesis of pre-selected surface alloys and the interrogation of such surfaces to monitor any electrochemistry-induced changes. The advantages in the use of electrons as surface probes such as in X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), high-resolution... 

Such a technique is possibly even more severely limited to reversible electrochemical systems than EMIRS. Using this method, the authors obtained time-domain FTIR spectra for the reduction of adsorbed TCNE anion radical at a reflective Pt electrode in MeCN/TBAF, stepped from a base potential of +0.6V to -0.8 V vs. Ag/Ag+, as shown in Fig. 57. The positive band near 2083 cm-1 corresponds to the adsorbed TCNE radical nion and the negative feature near 2065 cm-1 to the C=N stretch of the solution-soluble dianion. The time resolution was reported to be 1 ms between spectra with the use of a rapid-scan FTIR, capable of acquiring spectra at almost 100 s-1, the authors postulated temporal resolutions of the order of 10 /is. 

A significantly improved time resolution on the picosecond scale has been reported [307, 308]. The method has been applied to study the potential jump at the electrochemical interface platinum/aqueous solution of 0.1 M HCIO4 saturated with carbon monoxide induced by pulse irradiation with visible light. Position and intensity of the infi ared absorption caused by the CO stretching mode were measured as a function of time after the 532 nm pump pulse. 

Past efforts allow us to formulate three objectives for the present work. First we would like a technique that is roughly 2 to 4 (or more) orders of magnitude more sensitive than existing spectro-electrochemical methods. If this were achieved, the techniques could be applied to high-sensitivity analysis where one has a complex mixture and one makes use of the selectivity of spectroelectrochemis-try. Second, it would be valuable to lower the usable time scale of spectroelectrochemistry down into the microsecond region for a variety of chemical systems. With an optically transparent electrode and virtually all spectroelectrochemical methods, the response is limited by an effective path length which decreases with the time scale. Therefore, it is very difficult to monitor species on a microsecond time scale simply due to the low sensitivity of the techniques. The third objective is spatial.resolution of the diffusion layer. It would be very informative from both fundamental and practical standpoints to be able to accurately observe concentration vs. distance profiles. 

Various electrochemical methods have been appfied for the analysis of NAs, including DPP [5, 11] and DPV[13, 269, 270], linear sweep and CV [13, 271] square wave [138] and a.c. voltammetry [272-274], and recently constant current chronopotentiometry [249, 255-257, 275, 276] and elimination voltammetry [139, 277-279]. DPP was applied for the analysis of DNA in 1966 [280], and in a short time, it replaced OP and d.c. po-larography used in the early NA studies [4, 5]. The main advantage of DPP is its better sensitivity and resolution of peaks. Calf thymus ssDNA produced a well-developed DPP peak III (Fig. 6d) at concentrations of about 10 to 20 igml while dsDNA was inactive at the same concentration. At higher concentrations (hundreds of pgml ), dsDNA produced peak II at potentials by about 70 mV more positive than peak III (Fig. 6c). For years, DPP was the most sensitive instrumental method of determination of traces of ssDNA in dsDNA samples [5]. 

We will show that the ILIT method eliminates some of the problems associated with an electrical perturbation and, not surprisingly, creates new, interesting, and challenging problems. Improved electronics developed at the time of the termination of this program, coupled with picosecond or subpicosecond laser pulses, should, in principle, allow the ILIT method to probe interfacial relaxations of the order of 1 ns or less (our published work has used a slower system with a response function of the order of 15 ns). Of course, really dramatic improvement in response time will be achieved only with a pump-probe approach. Nevertheless, even at its present stage of development, ILIT effects significant improvement in time resolution over methods using conventional electrochemical perturbations where the time resolution is limited by solution resistance and interfacial capacitance (see Ref. 41) ... 

Table 7.1 A comparison of the sensitivity, and energy, time and spatial resolution of electrochemical methods, Raman spectroscopy and scanning tunneling microscopy (STM) in practical electrochemistry study...

The principle of most electrochemical methods is based upon the competition between transport to and from the electrode and creation or disappearance via chemical reactions. Indeed the electrons are supplied at, or taken from, the solution/electrode interface whereas the substrate and products evolve in the solution volume adjacent to the interface. Thus the principle of the direct electrochemical methods consists in opposing the rate of transport, usually by means of diffusion-migration, to that of chemical reactions. Extraction of the sought information then requires the resolution of the pertinent transport-reaction, time and space dependent equations. A more serious limitation of direct electrochemical methods is that the interfacial nature of the electron exchange limits the time scale domain accessible, when the kinetics have to be studied in conditions matching those encountered in homogeneous chemical situations. Indeed, because of the potential difference between the electrode and the solution, an interfacial charged... 

To conduct proton conductivity measurements, Buchi et al. [3] designed a current interruption device that used an auxiliary current pulse method and an instrument for generating fast current pulses (i.e. currents > 10 A), and determined the time resolution for the appropriate required voltage acquisition by considering the relaxation processes in the membrane of a PEM fuel cell [3]. They estimated that the dielectric relaxation time, or the time constant for the spontaneous discharge of the double-layer capacitor, t, is about 1.4 x 10 ° s. They found that the potential of a dielectric relaxation process decreased to <1% of the initial value after 4.6r (6.4 x 10 s) and that the ohmic losses almost vanished about half a nanosecond after the current changes. Because there is presently no theory about the fastest electrochemical relaxation processes in PEM fuel cells, the authors assumed a conservative limit of 10 s, based on observations of water electrolysis membranes. They concluded that the time window for accurate current interruption measurements on a membrane is between 0.5 and 10 ns. Another typical application of the current interruption method was demonstrated by Mennola et al. [1], who used a PEM fuel cell stack and identified a poorly performing individual cell in the stack. 

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