Parallel-electrode electrolysis

Electrochemical reactors (cells, tanks) are used for the practical realization of electrolysis or the electrochemical generation of electrical energy. In developing such reactors one must take into account the purpose of the reactor as well as the special features of the reactions employed in it. Most common is the classical reactor type with plane-parallel electrodes in which positive and negative electrodes alternate and all electrodes having the same polarity are connected in parallel. Reactors in which the electrodes are concentric cylinders and convection of the liquid electrolyte can be realized by rotation of one of the electrodes are less common. In batteries, occasionally the electrodes are in the form of two long ribbons with a separator in between which are wound up as a double spiral. 

It is not usual to talk about the resistance of electrolytes, but rather about their conductance. The specific conductance (K) of an electrolyte is defined as the reciprocal of the resistance of a part of the electrolyte, 1 cm in length and 1 cm2 in cross-sectional area. It depends only on the ions present and, therefore it varies with their concentration. To take the effect of concentration into account, a function called the equivalent conductance, A, is defined. This is more commonly (and conveniently) used than the specific conductance to compare quantitatively the conductivities of electrolytes. The equivalent conductance A is the conductance of that volume of the electrolyte which contains one gram equivalent of the ions taking part in the electrolysis and which is held between parallel electrodes 1 cm apart (units ohm-1 cm4). If V cubic centimeters is the volume of the solution containing one gram equivalent, then the value of L will be 1 cm and the value of A will be V square centimeters, so that... 

In aqueous solutions, concentrations are sometimes expressed in terms of normality (gram equivalents per liter), so that if C is concentration, then V = 103/C and a = 103 K/C. To calculate C, it is necessary to know the formula of the solute in solution. For example, a one molar solution of Fe2(S04)3 would contain 6 1CT3 equivalents cm-3. It is now clear as to why A is preferred. The derivation provided herein clearly brings out the fact that A is the measure of the electrolytic conductance of the ions which make up 1 g-equiv. of electrolyte of a particular concentration - thereby setting conductance measurements on a common basis. Sometimes the molar conductance am is preferred to the equivalent conductance this is the conductance of that volume of the electrolyte which contains one gram molecule (mole) of the ions taking part in the electrolysis and which is held between parallel electrodes 1 cm apart. 

With minor modifications, the setup can also be used with a solid working electrode, or for nonaqueous electrolyte solutions. H-cells with solid plane parallel electrodes of the same area are frequently utilized for work in anhydrous media, also since they provide a uniform current distribution. A small distance between the electrodes, not only for this cell design, makes them suitable for work in media of low electrical conductivity. The cell design can be used for electrolysis in liquid ammonia, if a connection between the anode and cathode compartment above the solution level is ensured, to equilibrate the pressure in the system [iii]. 

Marken and co-workers accomplished electrolysis without an intentionally added electrolyte by using a simple microflow electrochemical cell having a parallel electrode configuration [37]. Two electrodes are placed facing each other at a distance of the order of micrometers, and a substrate solution flows through the chamber. In this system, the liquid flow and the current flow are perpendicular. 

Hydrobromic acid electrolysis is carried out in two different separate reactors supplied by different companies and run in parallel Both reactors use undivided, vertical, parallel electrode cells. 

Design possibilities for electrolytic cells are numerous, and the design chosen for a particular electrochemical process depends on factors such as the need to separate anode and cathode reactants or products, the concentrations of feedstocks, desired subsequent chemical reactions of electrolysis products, transport of electroactive species to electrode surfaces, and electrode materials and shapes. Cells may be arranged in series and/or parallel circuits. Some cell design possibiUties for electrolytic cells are... 

When the temperature is raised to 75 °C a decrease in the rate of carbon monoxide reduction is observed with a parallel decrease in the faradaic efficiency. When the electrode is used a second time for carbon monoxide reduction at 60 °C, after it was used for electrolysis at 75 °C, (last entry in Table I) it shows considerable deactivation. The reduction of carbon dioxide also shows a similar... 

Recently flow coulometry, which uses a column electrode for rapid electrolysis, has become popular [21]. In this method, as shown in Fig. 5.34, the cell has a columnar working electrode that is filled with a carbon fiber or carbon powder and the solution of the supporting electrolyte flows through it. If an analyte is injected from the sample inlet, it enters the column and is quantitatively electrolyzed during its stay in the column. From the peak that appears in the current-time curve, the quantity of electricity is measured to determine the analyte. Because the electrolysis in the column electrode is complete in less than 1 s, this method is convenient for repeated measurements and is often used in coulometric detection in liquid chromatography and flow injection analyses. Besides its use in flow coulometry, the column electrode is very versatile. This versatility can be expanded even more by connecting two (or more) of the column electrodes in series or in parallel. The column electrodes are used in a variety of ways in non-aqueous solutions, as described in Chapter 9. 

Figure 19.16. Basic designs of electrolytic cells, (a) Basic type of two-compartment cell used when mixing of anolyte and catholyte is to be minimized the partition may be a porous diaphragm or an ion exchange membrane that allows only selected ions to pass, (b) Mercury cell for brine electrolysis. The released Na dissolves in the Hg and is withdrawn to another zone where it forms salt-free NaOH with water, (c) Monopolar electrical connections each cell is connected separately to the power supply so they are in parallel at low voltage, (d) Bipolar electrical connections 50 or more cells may be series and may require supply at several hundred volts, (e) Bipolar-connected cells for the Monsanto adiponitrile process. Spacings between electrodes and membrane are 0.8-3.2 mm. (f) New type of cell for the Monsanto adiponitrile process, without partitions the stack consists of 50-200 steel plates with 0.0-0.2 ram coating of Cd. Electrolyte velocity of l-2 m/sec sweeps out generated Oz.

Sometimes the potentials are measured by the commutator method during which the electrolyzing current is suddenly interrupted and the value of the EMF of the cell shown by the brief reverse deflection of the voltmeter is quickly read. The voltmeter is connected in an electric circuit parallel with the electrodes. The deflection is caused by products accumulating at the electrodes in the course of electrolysis. The system acts for a short period as a galvanic cell. 

The electrolysis is carried out in a set of steel tanks arranged in cascades (for a 9000 to 12 000 A load the tank measures 3 000.2 000. 1 400 mm). They are lined with acid resisting bricks. Suspended in each tank are 144 anodes and 28 steel sheets acting as cathodes arranged in parallel rows. The magnetite electrodes shaped as narrow, hollow tiles closed at the bottom (120.50. ... 

As for the acid red 14 removal by EC, 95% color removal and 85% COD removal were obtained when the pH ranged from 6 to 9, time of electrolysis was approximately 4min and current density was approximately 80 Am-2. The results also showed that an EC cell with monopolar electrodes had higher color removal efficiency than an EC cell with bipolar electrodes. Furthermore, within an EC cell, the series connection of the monopolar electrodes was more effective for the treatment process than the parallel connection in color removal (Daneshvar et al. 2004). 

Optically transparent electrode — (OTE), the electrode that is transparent to UV-visible light. Such an electrode is very useful to couple electrochemical and spectroscopic characterization of systems (- spectroelectro-chemistry). Usually the electrodes feature thin films of metals (Au, Pt) or semiconductors (In203, SnCb) deposited on transparent substrate (glass, quartz, plastic). Alternatively, they are in a form of fine wire mesh minigrids. OTE are usually used to obtain dependencies of spectra (or absorbance at given wavelengths) on applied potentials. When the -> diffusion layer is limited to a thin layer (i.e., by placing another, properly spaced, transparent substrate parallel to the OTE), bulk electrolysis can be completed in a few seconds and, for -> reversible or - quasireversible systems, equilibrium is reached for the whole solution with the electrode potential. Such OTEs are called optically transparent thin-layer electrodes or OTTLE s. 

Parallel plate and frame cell — This - electrolysis cell type has been developed since the 1970s, and reliable cells from laboratory up to industrial production scale in divided or undivided form are commercially available. Parallel plate and frame cells comprise interelectrode gaps of 1 to 12 mm. They can be used as monopolar (A) or -> bipolar electrode (B) arrangement, see figure. 

Stuart cell — Monopolar water - electrolysis tank cell employing plate electrodes with those of the same polarity connected in parallel resulting in a cell voltage of 1.7-2 V. Cells are connected in series, the inherent drawbacks of cells of the filterpress design (e.g., complicated sealing and interconnect devices) are avoided. 

In a vessel for electrolysis three parallel nickel plates were installed. The inner nickel plate was the working electrode (anode), and the two outer nickel plates were counter electrodes (cathode). A 1.2 liter mixture consisting of fluorene (0.01 mol) and LiPFs (0.1 mol) dissolved in propylene carbonate were then added to the vessel. The three nickel plates were immersed in the mixture to a depth of 90 mm. Two lithium metal sheets were used as reference electrodes, with each sheet placed between the anode and the cathode. The electrolysis was carried out by a potential-sweep method for 4 hours under a potential width of 4.5 to 6.7 V with a sweep time of 50 mV/s. The inner... 

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