Spectroelectrochemical flow cell

Many different designs have been described for transmission spectroelectrochemical cells (21). To increase the sensitivity, long path length cells, in which the light beam passes parallel to the electrode surface, have been used (22). In spectroelectrochemical flow cells, the solution flows in a thin channel between a working and counter electrodes, which are outside of the field of view of the spectrometer (23). 

Fig. 11.6.10. In situ UV/Vis/NIR spectroelectrochemical flow cell design which allows spectra to be recorded in transmission and in reflection mode...

The chaimel-flow electrode has often been employed for analytical or detection purposes as it can easily be inserted in a flow cell, but it has also found use in the investigation of the kinetics of complex electrode reactions. In addition, chaimel-flow cells are immediately compatible with spectroelectrochemical methods, such as UV/VIS and ESR spectroscopy, pennitting detection of intennediates and products of electrolytic reactions. UV-VIS and infrared measurements have, for example, been made possible by constructing the cell from optically transparent materials. 

Chen YX, Heinen M, Jusys Z, Behm RB. 2006. Kinetics and mechanism of the electrooxidation of formic acid—Spectroelectrochemical studies in a flow cell. Angew Chem Int Ed 45 981-985. 

Fig. 6. Spectroelectrochemical circulating flow cell. WE, Working electrode C, electrical contact to WE SP, solution propeller M, magnet to drive SP CE, counter electrode PE, potentiometric electrode RE, reference electrode PV, porous vycor plug to isolate CE, RE V, valve FO, fiber optic for absorption spectra FL, focusing lens W, quartz window RC, Raman capillary for 90° excitation. (Reproduced with permission from ref. 16.)...

The experiment was based on a channel flow cell system (see Fig. II.6.2) with UVMs detection downstream of the working electrode (Fig. n.6.2d). Switching the electrode potential from the potential region with no faradaic current into a region with diffusion-limited faradaic current allowed the transient change in the UVAfls absorption to be monitored. The data analysis for this transient UVAds response was based on a computer simulation model, which allowed T>(TMPD) and T)(TMPD ) to be varied independently. Interestingly, the difference in Z)(TMPD) and T)(TMPD ) was relatively high in water and ethanol (15% slower diffusion of the radical cation) and considerably lower (5%) in the less polar solvent acetonitrile. This example demonstrates the ability of transient spectroelectrochemical experiments, in conjunction with computer simulation-based data analysis, to unravel even complex processes. 

Vibrational spectroelectrochemical techniques, particularly Fourier transform infrared (FTIR), with the advent of less-expensive and more sensitive spectrometers, have enjoyed a great increase in popularity over the last few years. The advantages over UV-vis spectroscopy include greater specificity and enhanced information content. An IR OTTLE transmission cell with IR-transparent windows can be built by modifying a commercial liquid cell. Raman spectroscopy is also very easily carried out with an OTTLE cell, but a flow cell is more useful for SERS studies of adsorption, since the solution can be replaced without disturbing the optical alignment. 

Bond and coworkershave developed a small-volume (0.2 ml) variable-temperature EPR spectroelectrochemical cell that enables simultaneous rapid-scan voltammetry and EPR measurements to be made. The performance of this cell is compared to that of a flow-through cell designed by Coles and Compton. The small-volume cell permits cyclic voltammetric studies at variable temperatures but has significantly lower sensitivity compared to the flow-through cell, which is not amenable to low-temperature work. 

The spectroelectrochemical cell based on the LIGA-OTE can be employed in a flow-through-type system. Then, this cell allows the rapid renewal of the solution inside the cell after a spectroelectrochemical experiment. Experiments can be repeated rapidly and with a small volume of sample. A series of experiments can be conducted by varying the conditions without the need to open the cell. The use of optical waveguides to connect the cell with a light source and the spectrometer offers further scope for improvements in the experimental methodology. 

The in situ UVAds/NIR spectroelectrochemical cells shown in Figs. II.6.6 and II.6.7 permit solution to flow through the cell. However, the flow is only used to... 

Figure 16 Example of a cell used for IR-spectroelectrochemical experiments. A flow of cold nitrogen gas is maintained for low-temperature experiments (reproduced by courtesy J. Chem. Soc., Dalton Trans. 1996,...

Figure la (solid line) shows the absorbance spectrum in the VIS/NIR of the RC in the spectroelectrochemical cell, with FeCy as a mediator, equilibrated at an electrode potential of 0 V (vs. Ag/AgCl). When the potential is switched to +0.4 V, a current is observed which decays over several minutes effectively to zero (see inset the potential was switched to +0.4 V at t=0). The positive sign of the current indicates electron flow out of the solution. After four minutes, the spectrum of the electrogenerated species was recorded (dashed line in fig la). The decrease of the absorbance band at 866 nm, the blue shift of the 803 nm absorbance and the absorbance increase above ca. 930 nm clearly indicate the transformation of (BChl)2 into its 7r-cation radical form. 

Fig. 11.6.6 a, b. Capillary slit in situ UV/Vis/NIR spectroelectrochemical cells with an optically transparent electrode prepared from a metal mesh, grid or gauze in a left cuvette cell and in a right flat cell with outlet allowing the solution to flow through the slit... 

When the spectroelectrochemical experiments are performed to characterize a reduced species, the oxygen dissolved in the solution has to be removed by bubbling a flow of nitrogen or argon through the sample solution and, after that, the cell has to be perfectly sealed. 

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