Channel flow cell

Peter and Wang [266] invented a channel flow cell for rapid growth of CdTe films they showed that 2 p,m Aims can be deposited in less than 20 min, as opposed to the 2-3 h normally required in the conventional stirred single batch cells. The as-deposited films were structurally more disordered than the conventional ones, but after annealing and type conversion they became suitable for fabrication of efficient solar cells. A test cell with an AMI.5 efficiency approaching 6% was fabricated using a film prepared in the channel cell. 

The mechanism of the iodide formation at platinum immersed in aqueous electrode was recently studied by laser-activated voltammetry in a channel flow cell system [161]. In this technique, solid deposits of iodine are removed from the electrode continuously by short nanosecond high-power laser pulses. By removing deposits on electrode surfaces within a channel flow cell, the voltammetric measurements becomes time independent and data can be analyzed and modeled quantitatively. Laser activation using a 10-Hz pulsed Nd YAG 532-nm laser was shown to remove bulk iodine from the electrode surface so that under sustained pulsed... 

The channel flow cell has been used to study a wide variety of liquid-solid reactions. It consists of a tube of rectangular cross section, typically 4.5 cm long by 0.6 cm wide by 0.1 cm deep (Fig. 5.20). The reactive solid is embedded in the base of the cell and a detector downstream of the reactive solid is used to monitor reactant and product concentrations. 

Using an alternative geometry Evans et al. [16] developed the channel stopped flow method (CSFM). This technique, to date, has been used to measure solution diffusion coefficients (independent of knowledge of the concentration of the electroactive species) and crystal dissolution kinetics. The channel flow cell consisted of a rectangular electrode, typical dimensions 2.5 mm long and 6.25 mm wide, situated in a rectangular duct, 10 mm wide and 0.25-1.0 mm high. The electrode was placed a suitable distance... 

Fig. 10.15. Experimental (—) and theoretical (—) chronoamperometric response for the diffusion-limited oxidation of 2 x 10-3 mol dm-3 Fe(CN)S in 0.1 mol dm-3 KC1 at a rectangular electrode, 2.5 mm long and 6.25 mm wide, in a 0.5 mm high channel flow cell under channel stopped flow conditions. The initial volume flow rate of the solution was 0.197 cm3 s-1, which gave a limiting current at the channel electrode, defined as / . At time, f.top, solution flow was retarded (Evans et al., in preparation). The theoretical data has been simulated assuming Df (cn)2 = 6.5 x 10-6 cm2 s l.

Channel flow cell -> electrochemical cell (practical aspects) (subentry channel flow electrochemical cell), and... 

The characteristics of laminar flow can allow mathematical prediction of the solution velocity and this has led to a range of hydrodynamic devices which use forced convection as a transport component under laminar flow conditions, examples include, the -> rotating disk electrode [i],-> wall jet electrode [ii], and channel flow cell (see -> flow cell). 

Flow profile is perturbed by electrodes which are not flush or smooth Generally steady state. Apart from RDE, models are two-dimensional due to convection EPR, spectroscopic and photochemical methods are easily incorporated into the small, transparent channel flow cell. RDE is less versatile... 

Finally, mention should be made of a new generation of channel flow cells, in which the electrochemically generated species are transported by a laminar flow from the working electrode to an optically transparent part of the cell [316-318]. 

The channel-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, channel-flow cells are immediately compatible with spectroelectrochemical methods, such as UVA IS and ESR spectroscopy, permitting detection of intermediates and products of electrolytic reactions. UV-VIS and infrared measurements have, for example, been made possible by constructing the cell from optically transparent materials. 

The combination of electrochemistry and photochemistry is a form of dual-activation process. Evidence for a photochemical effect in addition to an electrochemical one is normally seen in the form of photocurrent, which is extra current that flows in the presence of light [88, M and M] In photoelectrochemistry, light is absorbed into the electrode (typically a semiconductor) and this can induce changes in the electrode s conduction properties, thus altering its electrochemical activity. Alternatively, the light is absorbed in solution by electroactive molecules or their reduced/oxidized products inducing photochemical reactions or modifications of the electrode reaction. In the latter case electrochemical cells (RDE or channel-flow cells) are constructed to allow irradiation of the electrode area with UVWIS light to excite species involved in electrochemical processes and thus promote further reactions. 

A dual-channel flow cell has been designed and built for use in protein adsorption and ex vivo shunt studies. The current cell incorporates several design modifications intended to minimize the poor liquid flow and difficult operational character stics of two previous generations of flow cells (3-6). 

Voltammetric techniques may be broadly divided into steady-slate techniques, such as channel flow cell [44 6], rotating disc [47, 48], or microelectrode [49] voltammetry at sufficiently low potential scan rate to give a current response independent of time, and transient techniques, such as cyclic voltammetry or chronoamperometry, giving a current response which is dependent on time. 

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. 

A very elegant approach overcoming this problem has been proposed based on a channel flow cell geometry with downstream detection (Fig. II.6.2d). The potential of the electrode is stepped during steady-state flow of the solution across the electrode. A downstream UVA is detector system is then employed to measure the time dependence of the concentration profile formation at the electrode surface. A computer program is employed to relate the time-dependent absorbance signal to the concentration profile of reactant and product at the electrode surface. Alternatively, direct measurement of the concentration profiles at the electrode surface has also been reported based on confocal Raman spectroelectrochemistry [16]. 

The relatively long equilibration time of the stagnant thin layer of the OTTLE or reflection cell does not allow one to study the kinetics of fast reactions. Alternatively, the forced convection regimes of the RDE and the channel-flow cell allow production of steady state currents, and both have been investigated with optical detection of electrogenerated species. Further details can be found in Chapter 2.4 in this volume. The rotating OTE was investigated early on and the theory has recently been expanded [204]. The electrode is used... 

The channel-flow cell technique may also be used for transient absorption measurements (chronoabsorptometry) with numerical simulation of the data. The kinetics of the dimerization of TMPD+ and MV+ were both on the order of 10" s k The technique differs from the usual OTE method in that it allows the determination of the diffusion coefficient of the product. Dr [209]. 

The fully implicit BI method was used by Anderson [45, 46] to simulate the steady state current response to flow rate within a channel flow cell. Subsequently, this was implemented by Fisher and Compton [47] to evaluate the time-dependent convective-diffusion problem... 

Fig. 18 Schematic illustration of the in situ channel flow cell. 

Cell Design Albery and coworkers [9-14] used tubular electrodes for ex situ electrochemical EPR experiments. The tubular electrode is equivalent to the channel electrode in all respects, except that the cross section is circular rather than rectangular [82, 137]. Like the later-developed channel flow cell, this setup (shown in Fig. 23) permits the interrogation of electrode reaction mechanisms of relatively long-lived radical species, [9-14] since the convective-diffusion equations are mathematically well defined, which at steady state are given by Eq. (37)... 

Cell Design Waller and Compton [85] effectively mimicked the Allendo-erfer coaxial design vide supra), whilst simultaneously maintaining the mathematically well-defined laminar flow of the channel flow cell. This improved cell for electrochemical EPR [85] allowed an improvement in the channel cell regarding lifetimes of radicals amenable to study, whilst retaining the hydrodynamic flow that is essential for the investigation of electrode reaction mechanisms. 

Since the hydrodynamics are analogous to those experienced in the channel flow cell, it follows that steady state EPR signals should behave analogously, that is. 

EC Waller and coworkers employed the channel flow cell for the study of a catalytic mechanism [68]. The (preequilibrium) EC mechanism can be defined by the following kinetic scheme. 

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