Electrochemical cell general requirements

Product Recovery. Comparison of the electrochemical cell to a chemical reactor shows the electrochemical cell to have two general features that impact product recovery. CeU product is usuaUy Uquid, can be aqueous, and is likely to contain electrolyte. In addition, there is a second product from the counter electrode, even if this is only a gas. Electrolyte conservation and purity are usual requirements. Because product separation from the starting material may be difficult, use of reaction to completion is desirable ceUs would be mn batch or plug flow. The water balance over the whole flow sheet needs to be considered, especiaUy for divided ceUs where membranes transport a number of moles of water per Earaday. At the inception of a proposed electroorganic process, the product recovery and refining should be included in the evaluation to determine tme viabUity. Thus early ceU work needs to be carried out with the preferred electrolyte/solvent and conversion. The economic aspects of product recovery strategies have been discussed (89). Some process flow sheets are also available (61). 

Air quality measuring systems that detect for example the leading substance C02 (refer to chapter 53.3.3) do not require the high accuracy of expensive measuring systems. Therefore, not much effort needs to be put in the development and construction of C02 detectors. In general the accuracy of such systems amounts to 10%, which is achievable for the cost of an electrochemical cell. Fast measurements aren t needed but nevertheless averaging to increase accuracy is possible and recommended. 

Generally, irrespective of the technique for which they are used, electrochemical cells are constructed in a way which minimizes the resistance of the solution. The problem is particularly accentuated for those techniques which require high current flows (large-scale electrolysis and fast voltammetric techniques). When current flows in an electrochemical cell there is always an error in the potential due to the non-compensated solution resistance. The error is equal to / Rnc (see Chapter 1, Section 3). This implies that if, for example, a given potential is applied in order to initiate a cathodic process, the effective potential of the working electrode will be less negative compared to the nominally set value by a amount equal to i Rnc. Consequently, for high current values, even when Rnc is very small, the control of the potential can be critical. 

Electrode preparation can be a significant problem for some catalyst systems. In general, it is necessary to sinter the catalyst to ensure adherence to the electrolyte substrate. This does not present much of a problem for metal electrodes but for oxides, where a particular phase may be required, the need for sintering can cause difficulties. This review will not, however, deal with the details of electrode preparation rather the reader should refer to an original article for details of preparation of a particular electrode (a short review of electrode preparation in solid electrolyte electrochemical cells can be found in reference S). 

Electrolysis cells for BDD electrodes are commercially available, e.g. a modular electrochemical cell from CSEM (now Adamant Technologies, Switzerland) (Haenni et al. 2002). Unfortunately, they are designed for wastewater treatment and therefore generally not applicable to organic media. Furthermore, these cell dimensions require relatively large electrodes. In order to work on a smaller scale and to apply various reaction conditions for preparative purposes, a novel cell geometry was developed. 

Cell separators (or dividers) are generally required in an electrochemical cell in order to prevent both intermixing of anolyte and catholyte, and, possibly, shorting between anode and cathode.In many cases, without a separator, the cell either does not work at all or works at a much lower efficiency and with a shorter cell life. This is particularly true for the chlor-alkali celP and the Fe-Cr redox cell," both of which require membranous separators. 

Understanding electrochemical instrumentation requires a basic knowledge of electricity and basic electronics. Coverage of these fundamentals is impossible in a text of this size. The student is advised to review the concepts of electricity learned in general physics, and to understand the definitions of current, voltage, resistance, and similar basic terms. The texts by Kissinger and Heineman, Malmstadt et al., or Diefenderfer and Holton, listed in the bibliography, are excellent sources of information on electronics used in instrumentation. The electrochemical cell is one circuit element with specific electrical properties in the complete instrumentation circuit. 

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