Electrolyzers Alkaline

In alkaline electrolyzers, hydrogen is obtained at the cathode with a purity of approximately 98 vol%, with oxygen and water vapor as the only impurities. Hydrogen may be further purified to almost 100% by the removal of oxygen in a catalytic deoxidizer and the subsequent removal of water vapor in a dryer. In the purification step, 5-10% of the produced hydrogen may be lost therefore, the use of electrolytic hydrogen without purification should always be considered in priority for each application. 

An example of an alkaline electrolyzer operating under variable power input is shown in Figure 5.4, where the power output of the wind turbine (Pwt) is depicted versus the power... 

The Hydrogen Research Institute in Canada has developed and tested a stand-alone renewable energy system composed of a 10 kW wind turbine, a 1 kWpeak photovoltaic array, a 5 kW alkaline electrolyzer, and a 5 kW PEM fuel cell. The components of the system are electrically integrated on a 48 V DC bus [50]. 

In the "PURE" project on the Shetland Islands, the wind-hydrogen system is composed of two wind generators of 15 kW power each, a 15 kW advanced alkaline electrolyzer operating at 55 bars, a 16-cylinder stack of 44 Nm3 H2 capacity at the same pressure, and a 5 kW PEM fuel cell [54],... 

In alkaline electrolyzers, Ni is the only elemental cathode that can be used. It is generally considered as a fairly good electrocatalyst, but in facts it exhibits two shortcomings (i) its activity decreases with time [cf. the AVtterm in Equation (7.16)] especially under conditions of intermittent electrolysis and (ii) shutdown of industrial cells (for maintenance) leads to Ni dissolution at the cathode since this electrode is driven to more positive potentials by short-circuit with the anode. These shortcomings can be alleviated if Ni cathodes are activated, that is, if they are coated with a thin layer of more active and more stable materials. Activation has been attempted with a variety of materials from sulfides to oxides, from alloys to intermetallic compounds. 

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. 

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. 

As the name indicates alkaline electrolyzers use high pH electrolytes like aqueous sodium hydroxide or potassium hydroxide. This is the oldest, most developed and most widely used method of water electrolysis. Hydrogen evolution takes place at the cathode, and oxygen evolution takes place at the anode. The cathodic reaction can be represented by the following steps [26,27]... 

Fig. 2.2 Schematics of hydrogen production using an alkaline electrolyzer unit with bipolar electrode geometry.

See color insert following page 140.) Unipolar (left) and bipolar (right) alkaline electrolyzers. 

Both Italy and Switzerland have work in progress on advanced alkaline electrolyzers. The DeNora Corporation and Brown Boveri of Italy and Switzerland, respectively, have built some of the largest industrial electrolysis plants and are each supporting in-house R D efforts, as well as working on contracts from their respective governments. 

Electrolysis of water — This is a process of electrochemical decomposition of water into -> hydrogen and -> oxygen. Apart from alkaline electrolyzers using 25% KOH solution [i], devices with polymer, or ceramic ion-conducting -> membranes have been developed for industrial applications [ii]. 

The alkaline electrolyzer is a well-established technology that typically employs an aqueous solution of water and 25-30 wt.% potassium hydroxide (KOH). However, sodium hydroxide (NaOH), sodium chloride (NaCl) and other electrolytes have also been used. The liquid electrolyte enables the conduction of ions between the elec... 

PEM technology was originally developed as part of the Gemini space program.16 In a PEM electrolyzer, the electrolyte is contained in a thin, solid ion-conducting membrane rather than the aqueous solution in the alkaline electrolyzers. This allows the H+ ion (proton) or hydrated water molecule (HsO+) to transfer from the anode side of the membrane to the cathode side, and separates the hydrogen and oxygen... 

For harsh alkaline electrolyzer environments, the ideal is to have no more than a 0.15v potential difference between the various metals used. In practice, however, you may have to use metals that are more galvanically incompatible than this ideal. The important thing is to be aware of this issue and its possible impact on the performance of the device in question and in terms of maintenance required. 

Solid electrolyte PEM (proton exchange membrane) electrolyzers can be used in systems to avoid use of caustics as an added safety factor and where no one is available to frequently monitor a fluid electrolyte system. PEM electrolyzers are much more expensive, and do not have the track record that alkaline electrolyzers have in use. Although they are reportedly almost trouble free during use, they do pose problems in terms of cost of replacement parts when they become inoperable. Failures in PEM electrolyzers are usually membrane blow-outs or catalyst degeneration. Both problems are costly to service with replacement parts. 

Alkaline electrolyzers, on the other hand, use a very inexpensive electrolyte and low cost electrodes such as nickel which are easy to obtain... 

An alkaline electrolyzer, as used in this system, is a simple electrochemical device that disassociates water into its constituent molecules, oxygen and hydrogen. This is accomplished by the application of very low voltage and high amperage DC (direct current) electricity in an alkaline electrolyte solution consisting of potassium hydroxide (KOH) and distilled water. 

The use of woven asbestos in alkaline electrolyzers gave one of the best performances for temperatures below 100°C. However, the increase in the performance of both anodes and cathodes at higher temperatures leads to the search for novel membranes to be used in electrolyzers. One of the most attractive membranes is Nation in 20% of sodium hydroxide at 120°C-160°C, but it works better in acid media [22,36-38]. 

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