Water electrolysis,

Water was first electrolyzed in 1801, as soon as the Volta pile had become known. Grove s discovery of a fuel cell (described in 1839) was the result of research 

Fuel Cells Problems and Solutions, Second Edition. Vladimir S. Bagolsky. 

The structure of membrane electrolyzers differs little from that of the membrane fuel cells described in Chapter 3 their work differs only in the direction of current flow through the device and in the directions of all electrochemical reactions. Certain structural differences between fuel cells and electrolyzers arise from the intended function and properties of the gas-diffusion layers (GDLs). In a fuel cell, the GDLs must provide uniform access of the reactant gases to the catalytically active layer of the MEA. A GDL is built with highly hydrophobic materials to keep out any water that could interfere with the gas supply. In an electrolyzer, water is the reactant, and the corresponding layer of the MEA must be hydrophilic to secure a water supply. On the other hand, gases are the reaction products and must be able to get away from the MEA. For these reasons, the GDL in an electrolyzer needs to have a carefully balanced ratio of hydrophilic to hydrophobic pores. 

3 Hydrogen-Oxygen Systems for Storing Electrical Energy 

In the development of URFCs that conld work as both an electrolyzer and a fuel cell, depending on the demand, two difQculties became apparent. One of them was associated with the gas-diffusion layer. The way to overcome it is along 

Water electrolysis is a route to both very pure hydrogen and very pure oxygen, although in most applications it is the hydrogen which is considered the principal product. The overall cell reaction is  

From thermodynamic considerations, this reaction requires, at room temperature, a potential of —1.23 V at all pH. (The reversible potentials for both hydrogen evolution and oxygen formation shift by 60 mV pH unit towards negative potentials as the pH is increased. 

Therefore, water electrolysis is used particularly where high purity is essential (e.g. for foodstuffs or where catalyst poisoning is a problem) and/or cheap hydroelectric power is available. Moreover, it should be remembered that hydrogen is a by-product in the chlor-alkali industry. 

Small cells are utilized to electrolyse deuterium or tritium containing water. There are two applications for these cells (1) they may be operated in a similar fashion to conventional water electrolysers but producing deuterium or tritium gas (in place of pure hydrogen) from D2O, DHO or HTO (2) (and more commonly), the cells may be used to concentrate the amount of deuterium or tritium in the electrolyte (DTO, DHO, HTO or D2O). This is made possible by kinetic factors which determine that hydrogen is evolved more rapidly than deuterium or tritium, e.g. hydrogen is evolved from 2 to 10 times faster than deuterium. The natural abundance of deuterium in water is very low (c. 150 mg dm ). Hence, extensive electrolysis is required to produce a significant level of heavy (deuteriated) water. 

There are two typical types of design for deuteriated or tritiated water electrolysis the filterpress and specialized low-volume cells. Examples are shown in Fig. 5.3. 

Water electrolysis has also been used as a method of producing heavy water. Kinetic factors determine that hydrogen is evolved more rapidly than deuterium (by a factor of 2—10) and hence the deuterium concentrates in the electrolyte. 

The natural abundance of deuterium in water is, however, only 150 p.p.m. and hence much electrolysis is necessary to product a significant percentage of heavy water. In practice, it is best achieved by using a cascade of cells through which the water passes as it becomes enriched. This would now only be an economic procedure if cheap electricity was available and the hydrogen gas could be used fruitfully. 

In this connection, several recent research programmes have sought to reduce the electricity consumed in water electrolysis by supplying some of the energy required to split water either thermally or photochemically. An example of the former is the following scheme for the production of hydrogen the electrolysis step is 

Hydrogen will possibly play a major role among prospeetive energy earners, and the most suitable method for industrial hydrogen produetion is water electrolysis. Membrane cells provide much better efficiency in comparison with other methods.  

Values of Some Fundamental Constants [1,1-1, Chapter 10, Table 10.1] 

Note Uncertainty of the constants are shown by digits in brackets. 

However, electric resistivity of pure water is very high due to low concentration of ions. To decrease resistivity between electrodes, either a strong acid or a strong base can be added. A basic aqueous solution is usually less corrosive than acidic, and, therefore, KOH(aq) solution is used in commercial electrolyzers to decrease the solution resistivity. The total electrochemical reaction, which takes place in a water electrolyzer, is as follows  

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The porous electrodes in PEFCs are bonded to the surface of the ion-exchange membranes which are 0.12- to 0.25-mm thick by pressure and at a temperature usually between the glass-transition temperature and the thermal degradation temperature of the membrane. These conditions provide the necessary environment to produce an intimate contact between the electrocatalyst and the membrane surface. The early PEFCs contained Nafton membranes and about 4 mg/cm of Pt black in both the cathode and anode. Such electrode/membrane combinations, using the appropriate current coUectors and supporting stmcture in PEFCs and water electrolysis ceUs, are capable of operating at pressures up to 20.7 MPa (3000 psi), differential pressures up to 3.5 MPa (500 psi), and current densities of 2000 m A/cm. ... 

Parameter Steam reforming (SR) Partial oxidation (POX) Texaco gasification (TG) Water electrolysis... 

Eig. 6. Comparison of current density and cell voltage characteristics of the electrolysis systems where lines A and B represent steam electrolysis and the use of SPE, respectively, the conventional KOH water electrolysis, and, 2ero-gap cell geometry employing 40% KOH, at 120—140°C. 

Multistep Thermochemical Water Splitting. Multistep thermochemical hydrogen production methods are designed to avoid the problems of one-step water spHtting, ie, the high temperatures needed to achieve appreciable AG reduction, and the low efficiencies of water electrolysis. Although water electrolysis itself is quite efficient, the production of electricity is inefficient (30—40%). This results in an overall efficiency of 24—35% for water electrolysis. 

Two of these cycles have an electrolysis step. Although one of the purposes of the thermochemical cycles is to avoid electrolysis and the associated iaefftciencies of electricity production, the electrolysis steps proposed use much less electrical energy than water electrolysis. The Mark 13 is regarded as the most advanced thermochemical cycle, with overall efficiency of about 40%, including the electrolysis step (164). 

A detailed discussion of thermochemical water splitting is available (155,165—167). Whereas many problems remain to be solved before commercia1i2ation is considered, this method has the potential of beiag a more efficient, and hence more cost-effective way to produce hydrogen than is water electrolysis. 

Hydrogen peroxide can be dissociated over a catalyst to produce oxygen, water, and heat. It is an energetic reaction, and contaminants can spontaneously decompose the hydrogen peroxide. Oxygen from water electrolysis is used for life support on submarines. 

Synthesis gas preparation consists of three steps ( /) feedstock conversion, (2) carbon monoxide conversion, and (2) gas purification. Table 4 gives the main processes for each of the feedstocks (qv) used. In each case, except for water electrolysis, concommitant to the reactions shown, the water-gas shift reaction occurs. 

Natural gas Naphtha Fuel oil Coal gasification Water electrolysis... 

Overcharge Reactions. Water electrolysis during overcharge is an irreversible process. Oxygen forms at the positive electrode ... 

Chemical Production. Electrolytic production of chemicals is conducted either by solution (water) electrolysis or fused-salt electrolysis. Fluorine, chlorine, chlorate, and manganese dioxide are Hberated from water solutions magnesium and sodium are generated from molten salt solutions. 

Stress corrosion can arise in plain carbon and low-alloy steels if critical conditions of temperature, concentration and potential in hot alkali solutions are present (see Section 2.3.3). The critical potential range for stress corrosion is shown in Fig. 2-18. This potential range corresponds to the active/passive transition. Theoretically, anodic protection as well as cathodic protection would be possible (see Section 2.4) however, in the active condition, noticeable negligible dissolution of the steel occurs due to the formation of FeO ions. Therefore, the anodic protection method was chosen for protecting a water electrolysis plant operating with caustic potash solution against stress corrosion [30]. The protection current was provided by the electrolytic cells of the plant. 

Where low-cost electricity is available, water electrolysis is used to produce hydrogen. In water electrolysis... 

Wurster, R. Water Electrolysis and Solar Hydrogen Demonstration Projects 27... 

A process involving water electrolysis is the production of heavy water. During cathodic polarization the relative rates of deuterium discharge and evolution are lower than those of the normal hydrogen isotope. Hence, during electrolysis the solution is enriched in heavy water. When the process is performed repeatedly, water with a D2O content of up to 99.7% can be produced. Electrochemical methods are also used widely in the manufacture of a variety of other inorganic and organic substances. 

With solid (and particularly polymeric) electrolytes which at the same time function as separators, one can appreciably reduce the distance between the electrodes and hence increase the electrode area per unit of reactor volume. Very compact equipment for water electrolysis which has no liquid electrolyte has been designed. 

I It must be noted that already a decade earlier the Dutchmen R van Troostwijk and J.R. Deim [7. Phys. 2,130 (1790)] showed that during spark discharge a (short-time) process of water electrolysis is achieved. These results were known by Nicholson and Carlisle when (using the then new Volta pile) they reported on long-time water electrolysis, but in their publication these results were not mentioned [R. de Levie, 7. Electroanal. Chem., 476, 92 (1999)]. 

One effect of the electrochemical reactions in an aqueous system is a local pH change around the electrodes. By water electrolysis, hydronium ions (H30+) are generated at the anode, while hydroxyl ions (OH ) are produced at the cathode. These changes have been utilized for controlling the permeability of polyelectrolyte gel membrane or on-off solute release via ion exchange or surface erosion of interpolymer complex gels. 

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