Water decomposition electrolysis

The potential required to split water into and O, i.e., (E - E is equal to 1.229 V. Though the theoretical potential is 1.23 V for water electrolysis, in practice the actual water decomposition will occur only above 1.7 V. The extra potential, which is essential for the water decomposition, is called overpotential. Overvoltages are composed of activation or charge transfer overvoltage, concentration or diffusion or mass transfer overvoltage and resistance overvoltage. Overvoltage is evaluated mainly as a function of current and temperature (Viswanathan, 2006). 

Like other non-oxidic semiconductors in aqueous solutions, surface oxidized and photocorrosive InP is a poor photoelectrode for water decomposition [19,27,32,33], To enhance properties several efforts have focused on coupling of the semiconductor with discontinuous noble metal layers of island-like topology. For example, rhodium, ruthenium and platinum thin films, less than 10 nm in thickness, have been electrodeposited onto p-type InP followed by a brief etching treatment to achieve an island-like topology on the surface [27,28]. In combination with a Pt counter electrode, under AM 1.5 illumination of 87 mW/cm the metal (Pt, Rh, Ru) functionalized p-InP photocathodes [27] see a reduction in the threshold voltage for water electrolysis from 1.23 V to 0.64 V, and in aqueous HCl solution a photocurrent density of 24 mA/cm with a photoconversion efficiency of 12% [27]. 

The ready availability of electricity following the invention by Alssandro Volta of his famous pile in 1800 prompted, from an early date, the study of its effects on condensed matter and, most particularly, the decomposition of water by electrolysis involving chemical reactions at the electrodes. Work developed to the point where, by the middle of the second half of the nineteenth century, well-established industrial processes for the manufacture of aluminium and chlorine gas operated by electrolysis. 

The decomposition voltage necessary for decomposing water by electrolysis can be calculated from the heat of formation which is 68,400 calories. Since 1 joule or 1 volt-coulomb1 is equivalent to 0 239 calorie, the electrical energy necessary for decomposing 1 gram-molecule of... 

Water decomposition combined with nuclear energy appears to be an attractive option. Low temperature electrolysis, even if it is used currently for limited amounts is a mature technology which can be generalised in the near future. However, this technology, which requires about 4 kWh of electricity per Nm3 of hydrogen produced, is energy intensive and presents a loiv efficiency. 

The standard formation enthalpy for water is equal to 286 kj/mole H2 relative to the formation of liquid water and corresponding to (HHV) of H2. The theoretical voltage for pure water decomposition is 1.23 V. However, the majority of conventional electrolysis devices need at least 2.0 V when economically reasonable current densities are maintained. This value translates into a water electrolysis Faraday s efficiency of about 74%. If a thermal-to-electric conversion efficiency of 45% is assumed, the total equivalent heat requirement corresponds to a heat input of 859 kj/mole H2. 

On the other hand, several experimental parameters have been defined to quantify the destruction of an organic species in aqueous medium by anodic oxidation (38-44). Because the main side reaction is 02 evolution due to water decomposition, the instantaneous current efficiency (ICE) at a given time t for its oxidation can be determined from the 02 flow rate during electrolysis in the absence (V0) and the presence (F,>org) of the selected pollutant as follows ... 

Temperatures up to 1500 °C reduce the reversible thermodynamic potential for water decomposition from a room temperature value of 1.23 V to 0.7 V (43%). The cost of electrolytic hydrogen varies linearly with the potential of the cell at the current density being used, since cost of the electricity is the dominating item in the cost of electrolytic hydrogen, high-temperature steam electrolysis would greatly improve the economics. Heat is needed to maintain the temperature of the system, but heat costs only a third of the cost of electricity. So far, very high temperature cells are research items, but 1000 °C cells have been developed in Europe under the nickname Hot Elly. ... 

Hydrogen can be produced from splitting of water through various processes ranging from water electrolysis, photo(solar)-electrolysis, photo-biological production to high-temperature water decomposition. 

Russell, J.H., Sedlak, Dr. J.M., General Electric Company, Direct Energy Conversion Programs, Economic Comparison of Hydrogen Production Using Solid Polymer Electrolyte Technology for Sulfur Cycle Water Decomposition and Water Electrolysis, EPRI Research Project 1086-3, Final Report, December 1978. 

Farbman, G. H. Krasicki, B. R. Hardman, C. C. Lin, S. S. Parker, G. H. EPRI-EM-789 "Economic Comparison of Hydrogen Production Using Sulfuric Acid Electrolysis and Sulfur Cycle Water Decomposition" March 1978 prepared by Westinghouse AESD for Electric Power Research Institute Palo Alto, California 94304. 

Thermodynamic and over potential region For a terminal voltage smaller than the water decomposition potential Ud (Ud 2 V), no significant electrolysis happens and no current flows between the electrodes. 

If the terminal voltage U is higher than the water decomposition potential Ud, electrolysis takes place and the current is related to U according to (3.40) and (3.22) by ... 

The water cleavage can be easily performed in the laboratory via electrolysis by the use of electrical energy instead of visible light. For the electrolytical water decomposition into its isolated elements two operational units are required a) anelectrical... 

The Gibbs-Helmholtz equation dG = dH — T dS yields the thermodynamics of chemical reactions. In the case of a negative free enthalpy dG, a spontaneous reaction occurs. Water splitting means a positive dH and dS, respectively. So it is only for very large T that spontaneous water decomposition occurs. As mentioned above, this means temperatures of about 2000 °C. For electrolysis, an electric potential in line with the free enthalpy dG is applied so the reaction can take place. The equation demonstrates that the required voltage sinks with higher temperatures. 

The decomposition of water through electrolysis comprises two partial reactions which take place at the respective electrodes, the anode and the cathode, and which are connected through an ion-conducting electrolyte. Depending on the electrolytes used, there are three relevant processes for water electrolysis. They are summarized in Fig. 11.2 with their partial reactions for the hydrogen evolution reaction (HER)... 

Molten salt electrolysis (MSB) is a high temperature electrowinning operation for producing metals that cannot be electrowon due to water decomposition, which promotes hydrogen evolution at the cathode before metal deposition occurs. Metals produced or recovered using this technique are Al, Mg, Be, Ce, iVa, K, Li, U, Pu, etc.. 

Consider the decomposition of water by electrolysis as shown in Figure 2.3b. An electrical power source is connected to two electrodes, or two plates, typically made of inert metal such as platinum, which are placed in the water. Electrolysis of pure water is very slow and can only occur because of the selfionization of water. Pure water has an electrical conductivity approximately one-millionth that of seawater. It is sped up dramatically by adding an electrolyte (such as a salt, an acid, or a base). When the power is applied, hydrogen will appear at the cathode where electrons are pumped into the water, and oxygen will appear at the anode. With aqueous electrolytes such as sulfuric acid in water, the reactions that occur at each electrode are as follows ... 

---