Electrolysis: of water

Water electrolysis is currently only of interest where favorable conditions hold (accounts for 3% of hydrogen production), but could become important in a future hydrogen-based economy 

Hydrogen production by the electrolysis of water is currently less important, accounting for less than 3% of the hydrogen produced, due to the low overall efficiency of 20 to 25% including the electricity production. Large plants are only constructed where favorable conditions obtain, mainly near dams e.g. in Egypt (a plant at the Assuan dam has an ammonia capacity of 33 000 m- /h), India, Peru and in countries with low electricity prices or where there is a favorable demand for the byproduct oxygen e.g. in Norway. 

Hydrogen produced by electrolysis is also used where particularly pure hydrogen is required such as in food technology (margarine production) or for small users. The process eould in the long term beeome important in a hydrogen-based economy in the post-oil era. 

At present, electrolysis cells basically consist of two electrodes separated by an asbestos diaphragm impermeable to gases. 20 to 30% potassium hydroxide is dissolved in the electrolyte to increase its conductivity. The electrolysis is carried out at 80 to 85°C. The theoretical decomposition potential is 1.24 V with 1.9 to 2.3 V being used in practice due to overvoltage effects etc. Oxygen is produced at the anode and hydrogen at the cathode  

The specific energy consumption per m of hydrogen (and 0.5m oxygen) produced is about 4.5 to 5.45 kWh. 

Water electrolysis is the opposite process to the fuel cell reaction 

If an electrolyzer works at 100% efficiency (i.e., at 1.48 V under the standard condition), the heat generated from the electrolyzer will be equal to the heat needed for the electrolysis to proceed, and therefore, a thermoneutral situation is achieved. In other words, the electrolyzer neither releases nor absorbs heat to or from the environment. The voltage, 1.48 V under this condition, is called the thermoneutral voltage. Since AH increases with temperature, then the thermoneutral voltage increases slightly with temperature (please refer to Table 2.1 for water in vapor phase). Also, since the difference between AH and AG increases with temperature, the heat needed for the electrolysis to proceed increases with the temperature. 

Reaction 3.15 is composed of two half reactions, as shown by Reactions 3.16 and 3.17, respectively. 

Reaction 3.16 produces O2 and is called the oxygen evolution reaction (OER). The protons transport through the PEM from the positive electrode to the negative electrode, where they combine with the electrons from the power source to form H2. 

If we assume fhaf fhe loss due to the mass transport resistance is negligible, then at an external voltage of 1.6-1.7 V, the electrolysis current densities will range from 50 fo 500 mA cm with an efficiency aroimd 85-90%. 

Under ordinary atmospheric conditions (1 atm and 25°C), water will not spontaneously decompose to form hydrogen and oxygen gas because the standard free-eneigy change for the reaction is a large positive quantity  

Remember that at 25 C pure water has only a very low concentratkin of ions [K4 Section 16.2]  

On burning hydrogen with oxygen to form liquid water H30 (1) or water vapour HaO(g) at 25 °C (298.1 °K) and 1 atm. pressure a great amount of heat is evolved  

Jjjhen water is electrically decomposed hydrogen is evolved at the cathode and oxygen at the anode. In order to increase the low specifio electrical conductance of chemically pure water, suitable electrolytes which possess a higher decomposition voltage than mere water are dissolved in it. Sodium or potassium hydroxides are most suitable for this purpose acids or salts which may corrode the electrolytical installation should bo avoided. 

On electrolyzing a diluted solution of sodium hydroxide the current is transferred to the anode by hydroxyl ions which are discharged on the anode 

As no new H+ ions migrate towards the cathode there must occur a dissociation of water molecules  

for each molecule of hydrogen evolved two molecules of water are dissociated. 

Kevin Harrison and Johanna Ivy Levene NREL, Golden, CO 

Ammonia and fertilizer manufacture Synthesis of methanol Sorbitol production General pharmaceuticals and vitamins 

Polysilicon production Epitaxial deposition Fiber optics 

Petroleum refinement Liquid rocket fuel Some use in fuel cells 

Haldane, in his talk entitled, Daedalus or Science and the Future, Cambridge University, 1923.13 

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In the laboratory it can be prepared by the electrolysis of water or by heating potassium chlorate with manganese dioxide as a catalyst. 

Chloiine is pioduced at the anode in each of the three types of electrolytic cells. The cathodic reaction in diaphragm and membrane cells is the electrolysis of water to generate as indicated, whereas the cathodic reaction in mercury cells is the discharge of sodium ion, Na, to form dilute sodium amalgam. 

Since 1960, about 95% of the synthetic ammonia made in the United States has been made from natural gas worldwide the proportion is about 85%. Most of the balance is made from naphtha and other petroleum Hquids. Relatively small amounts of ammonia are made from hydrogen recovered from coke oven and refinery gases, from electrolysis of salt solutions, eg, caustic chlorine production, and by electrolysis of water. In addition there are about 20 ammonia plants worldwide that use coal as a hydrogen source. 

The electrolytic processes for commercial production of hydrogen peroxide are based on (/) the oxidation of sulfuric acid or sulfates to peroxydisulfuric acid [13445-49-3] (peroxydisulfates) with the formation of hydrogen and (2) the double hydrolysis of the peroxydisulfuric acid (peroxydisulfates) to Caro s acid and then hydrogen peroxide. To avoid electrolysis of water, smooth platinum electrodes are used because of the high oxygen overvoltage. The overall reaction is... 

For well over 100 years after its discovery and initial preparation, oxygen was made either chemically or by the electrolysis of water. Early in the twentieth century, Linde and Claude introduced processes for the Hquefaction and distillation of air that have since grown into a mature and highly competitive industry. In 1991, over 13.4 X 10 (4.7 x 10 ft ) of oxygen was produced in the United States. About 70 X 10 (24.7 x 10 ft ) was... 

Hydrogen produced by the electrolysis of water is used in special circumstances only where electric power is plentiful, inexpensive, and light hydrocarbons are not available. 

In appHcatioa, the automobile or other article to be coated is made the cathode ia an electro deposition system. A current differential on the order of 250 to 400 V is appHed, which attracts the positively charged coating aggregates to the cathode. At the cathode, hydroxide ions from the electrolysis of water precipitate the aggregates on the surface of the metal. As the conveyor removes the coated product from the bath, residual Hquid is tinsed off with water and the article is conveyed iato a bakiag ovea for a high temperature bake. 

Electrolysis. For reasons not fiiUy understood (76), the isotope separation factor commonly observed in the electrolysis of water is between 7 and 8. Because of the high separation factor and the ease with which it can be operated on the small scale, electrolysis has been the method of choice for the further enrichment of moderately enriched H2O—D2O mixtures. Its usefiilness for the production of heavy water from natural water is limited by the large amounts of water that must be handled, the relatively high unit costs of electrolysis, and the low recovery. 

It has been claimed that the D-D fusion reaction occurs when D2O is electroly2ed with a metal cathode, preferably palladium, at ambient temperatures. This claim for a cold nuclear fusion reaction that evolves heat has created great interest, and has engendered a voluminous titerature filled with claims for and against. The proponents of cold fusion report the formation of tritium and neutrons by electrolysis of D2O, the expected stigmata of a nuclear reaction. Some workers have even claimed to observe cold fusion by electrolysis of ordinary water (see, for example. Ref. 91). The claim has also been made for the formation of tritium by electrolysis of water (92). On the other hand, there are many experimental results that cast serious doubts on the reahty of cold fusion (93—96). Theoretical calculations indicate that cold fusions of D may indeed occur, but at the vanishingly small rate of 10 events per second (97). As of this writing the cold fusion controversy has not been entirely resolved. 

Electrochemical systems convert chemical and electrical energy through charge-transfer reactions. These reactions occur at the interface between two phases. Consequendy, an electrochemical ceU contains multiple phases, and surface phenomena are important. Electrochemical processes are sometimes divided into two categories electrolytic, where energy is supplied to the system, eg, the electrolysis of water and the production of aluminum and galvanic, where electrical energy is obtained from the system, eg, batteries (qv) and fuel cells (qv). 

Total production of hydrogen in the United States in 1988 was 61.5 x 10 (49). Total hydrogen production by electrolysis of water in 1988 was... 

Explosion Hazards. The electrolysis of aqueous solutions often lead to the formation of gaseous products at both the anode and cathode. Examples are hydrogen and chlorine from electrolysis of NaCl solutions and hydrogen and oxygen from electrolysis of water. The electrode reactions. 

Electric current passing through a metal joint having a moisture content causes electrolysis of water vapour. Copper, being a galvanic metal, forms an electrolytic circuit with other metals and decomposes the joint. Decomposition is corroding and erodes the aluminium metal. 

Later, Du Pont in America developed its own ionically conducting membrane, mainly for large-scale electrolysis of sodium chloride to manufacture chlorine, Nafion , (the US Navy also used it on board submarines to generate oxygen by electrolysis of water), while Dow Chemical, also in America, developed its own even more efficient version in the 1980s, while another version will be described below in connection with fuel cells. Meanwhile, Fenton et al. (1973) discovered the first of a... 

Deuterium discovered specU oscopically and enriched by gaseous diffusion of hydrogen and by electrolysis of water (H. C. Urey, F. G. Brickwedde and G. M. Murphy). 

The products obtained from the electrolysis of water-free fatty acids in pyridine are similar, although the total yields of alkylated products are considerably lower (Table V), presumably because here the radicals are formed in a high concentration on the anode... 

Electrolysis of water (H2O). Tfie volume of hydrogen (ty formed in the tube at the right is twice the volume of oxygen (02) formed in the tiiie at the left, in accordance with the equation 2mi)---- 2H2(g)+02(g). 

A balloonist wants to fill a balloon with hydrogen gas. How long must a current of 12.0 A be used in the electrolysis of water to fill the balloon to a volume of 10.00 L and a pressure of 0.924 atm at 22°C ... 

Choose the figure below that best represents the results after the electrolysis of water. (Circles represent hydrogen atoms and squares represent oxygen atoms.)... 

Similar considerations apply of course to the opposing electromotive forces of polarisation during electrolysis, when the process is executed reversibly, since an electrolytic cell is, as we early remarked, to be considered as a voltaic cell working in the reverse direction. In this way Helmholtz (ibid.) was able to explain the fluctuations of potential in the electrolysis of water as due to the variations of concentration due to diffusion of the dissolved gases. It must not be forgotten, however, that peculiar phenomena—so-called supertension effects—depending on the nature of the electrodes, make their appearance here, and com-... 

W. Doenitz, and E. Erdle, High-temperature electrolysis of water vapor - Status of development and perspectives for application, International Journal Hydrogen Energy 10, 291-295(1985). 

Self-Test 4.13B Students collecting hydrogen and oxygen gases by electrolysis of water failed to separate the two gases. If the total pressure of the dry mixture is i 720. Torr, what is the partial pressure of each gas ... 

In the course of the electrolysis of water, hydrogen gas was collected at one electrode over water at 20.°C when the external pressure was 756.7 Torr. The vapor pressure of water at 20.°C is 17.54 Torr. The volume of the gas was measured to... 

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