Conventional electrolyzer

Figure 7.3 Performance of a conventional electrolyzer as a function of time. Data from [21].

Figure 7.17 shows a summary of the available conditions of water electrolysis [72]. For each configuration there exists a range of performance. Conventional electrolyzers, which nevertheless are still the most common in the current production of H 2 on the intermediate and small scale, show high overpotential and a relatively small production rate. Membrane (SPE) and advanced alkaline electrolyzers show very similar performance, with somewhat lower overpotential but a much higher production rate. Definite improvements in energy consumption would come from high temperature (steam) electrolysis, which is, however, still far from optimization because of a low production rate and problems of material stability. 

In the planning stage is another 500-kW electrolyzer that is to furnish the fuel for a hydrogen-powered bus in the German city of Karlsruhe. Unlike conventional electrolyzers, which have to operate at a fairly constant rate, the GHW machine adjusts quickly and automatically to wide swings in the power supply—from 15 percent to 120 percent of rated capacity. It permits power plant operators to operate at close to 100 percent of rated capacity without fears of mismatch and frequency fluctuations (which cause electric clocks to show the wrong time and tape recorders to run at... 

A particular approach adopted by General Electric In U.S.A. is the solid polymer electrolyte (SPE) cell in which the porous cloth-type separator is replaced by a polymeric ion exchange membrane which is conductive to cations (Figure 5). The particular membrane employed, NAFION, is a perfluorsulphonlc acid pol3nner which is extremely stable in both acid alkaline solution. Appropriate electrocatalysts are coated on each face of the polymer sheet and these are contacted by a metal mesh current collector. Further research is aimed at reducing the cost and improving the electrical efficiency of the system to make it competitive with conventional electrolyzers. 

For a long time, conventional alkaline electrolyzers used Ni as an anode. This metal is relatively inexpensive and a satisfactory electrocatalyst for O2 evolution. With the advent of DSA (a Trade Name for dimensionally stable anodes) in the chlor-alkali industry [41, 42[, it became clear that thermal oxides deposited on Ni were much better electrocatalysts than Ni itself with reduction in overpotential and increased stability. This led to the development of activated anodes. In general, Ni is a support for alkaline solutions and Ti for acidic solutions. The latter, however, poses problems of passivation at the Ti/overlayer interface that can reduce the stability of these anodes [43[. On the other hand, in acid electrolysis, the catalyst is directly pressed against the membrane, which eliminates the problem of support passivation. In addition to improving stability and activity, the way in which dry oxides are prepared (particularly thermal decomposition) develops especially large surface areas that contribute to the optimization of their performance. 

Finally, metal objects can sometimes be fabricated in their entirety by electrodeposition (electroforming), with much the same considerations as electroplating. Conversely, portions of a metal specimen can be selectively electrolyzed away (electrochemical machining). This technique is especially useful where the metal to be shaped is too hard or the shape to be cut is too difficult for conventional machining. The sample is made the anode, a specially shaped tool the cathode, and electrolyte solution (e.g., aqueous NaCl) is fed rapidly but uniformly over the surface to be machined. Current densities may reach several hundred amperes per square centimeter across the electrolyte gap of a millimeter or so. Excellent tolerances can be achieved in favorable circumstances.16... 

The process for electrowinning of copper is schematically shown in Fig. 12.9. If copper(I) sulfate in AN-H20-H2S04 solution is electrolyzed using a platinum electrode as anode and a copper electrode as cathode, one-electron processes occur at the two electrodes (Cu1 —> Cu2 at the anode and Cu1 —> Cu° at the cathode). Compared with the conventional electrowinning from the aqueous acidic solution of copper(II) sulfate (water oxidation at the anode and Cu2+ —> Cu° at the cathode), the electric power consumed is only about 10% and high-quality copper can be obtained. It is of coruse necessary to return Cu2+, generated at the anode, to Cu+. But various methods are applicable to it, e.g., by the contact with coarse copper. 

In the electrorefining of copper, copper(I) sulfate in the AN-H20-H2S04 solution is electrolyzed using a coarse copper electrode as anode and a pure copper electrode as cathode. The reaction at the anode is Cu° —> Cu+, while that at the cathode is Cu+ —> Cu°. Compared to the conventional process which uses aqueous acidic solutions of copper(II) sulfate, this method is advantageous in that the quantity of electricity is one-half and the electric power is also small. Moreover, the low quality AN used in this method is available at low price and in large quantity as a by-product of chemical industries. 

The proposed optimization strategy will replace the traditional method of controlling the release of Oz. Today, the rate of 02 released is controlled to maintain the d/p between the electrolyte chambers in order to limit the force that the separation diaphragm has to withstand. When the pressure differential is detected and controlled by conventional d/p cells, the measurement cannot be sensitive or accurate therefore, the diaphragm has to be strong, and the electrolyzer (or fuel cell) must be bulky and heavy. In this optimized design (if a liquid electrolyte design is selected), differential level control (ALC-12) will be used, which can control minute differentials. 

When different kinds of ions are in the solution they undergo the electro-lytical action of the current at different electrode potentials. Consequently their oxidation or reduction will occur in stages with increasing terminal voltage of the electrolyzer. With regard to the conventional marking of electrolytic potentials the following rules could be derived for the succession of the individual partial processes. 

Because all electrochemical devices such as batteries, fuel cells, sensors, and electrochromics require an electrolyte, the potential applications for ionic conductors are enormous. In addition to these more conventional applications, solid electrolyte materials are investigated for use as electrochemical memory devices, oxygen pumps, gas phase electrolyzers, and thermoelectric generators. ... 

The brute force approach to achieve this goal is to employ a solid-state photovoltaic cell to generate electricity that is subsequently passed into a commercial-type water electrolyzer. Although efficiencies obtained are relatively high, i.e., close to 8 %, these devices are very expensive. Hence the price of hydrogen produced this way cannot compete with conventional sources. The long-term outlook is better for systems that borrow their principles from natural photosynthesis (see Section 1.4.1 above). 

Conventional hydrochloric acid electrolyzers consist of 30-36 individual cells connected in series (bipolar arrangement). The cells are formed from vertical electrode plates manufactured from graphite, between which there are diaphragms (for instance made from PVC fabric, distance to the plates ca. 6 mm). The feed with hydrochloric acid (22 wt%, identical for anode and cathode compartment) and the removal of the gases produced take place according to the filter press principle (see Chapter 2). Chlorine leaves the cell with the anolyte, hydrogen with the catholyte. 

Fig. 22 Current density - voltage diagrams of a conventional and an advanced water electrolyzer [52, p. 467],...

With the aim to avoid the utilization of VOCs and/or bases, an alternative electrochemical procedure has been studied. To the solutions of thiazolium salts in organic solvents or in RTILs, previously electrolyzed under galvanostatic conditions, were added of aldehydes and l-buten-3-one and stirred at controlled temperature for a prefixed interval of time. Conventional workup of the resulting solutions afforded 1,4-dicarbonyl compounds and a-hydroxyketones as by-products (Scheme 16.25) [159]. 

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