Thermodynamic water decomposition

Although the rate of these reactions is slow, according to its thermodynamic situation the lead dioxide electrode is not stable. Since a similar situation applies to the negative electrode, the lead-acid battery system as a whole is unstable and a certain rate of water decomposition cannot be avoided. 

Early work on peroxo compounds of molybdenum has been reviewed.286 The stoichiometri-cally simplest Mo peroxo complex is the red-brown Mo(02)4 ion formed by the reaction of MoO2- with H202. Although the complex is not stable in solution and decomposes slowly with the evolution of 02, the anion can be crystallized as the Zn(NH3)4+ salt whose structure has been determined. In a sense, Mo(02)4 is an intermediate in the thermodynamically favored decomposition of hydrogen peroxide to water and oxygen. 

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. ... 

The reversible potential ofBr2 + 2e —> 2 Br is 1.08 V atm. The pH of sea water is near 7 and 25 °C hence the reversible potential of 2H+ + 2e —> Hj is about -0.42 V. The reversible cell potential would be then about 1.5 V. The idea here is to reduce the potential below the thermodynamically reversible value by photoillumination ofthe anode (photoassisted water decomposition Szklarczyk, 1983). 

Noteworthy exanples are sensitizers such as Ru(bipy)j (29). porphyrins derivatives (30) and acridine dyes, e.g., proflavin (31). Among the electron relay compounds investigated it is worth mentioning the viologens (32) Eu , and their respective salicylate complexes ( ), Ru(bipy)j (3U) and cobalt complexes (35). Thus, at present a considerable choice of sensitizer/relay pairs is available that fulfill the photochemiical and thermodynamic requirements for water decomposition. 

For catalytic water decomposition, it is therefore necessary to work in acidic solution where, although Fe is not thermodynamically capable of reducing water and Fe is not thermodynamically capable of oxidizing water, both reactions can be driven photochemically. Because of this, two photons are used for the reaction and a large portion of the solar spectrum can in theory be collected. In practice this is not the case as both Fe" and Fe absorb mainly in the UV region of the spectrum. 

The explanation of this behavior, which has paramount importance to the nuclear industry, was given by Allen [3]. After World War II, Allen depicted a model of water decomposition under radiation that considers the production and consumption of Hj. The key role of OH, H2 and O2 involved in the chain as a carrier or a breaker is clear (see Inset). Within this chain reaction, the reaction between H2 and O2 (which is thermodynamically favorable) takes place in water at high temperature only in the presence ofa catalyst such as copper or silver cations. In the radiolysis of water, the reaction can take place at room temperature in the presence of free radicals which form the molecular products H2, H2O2 and O2 at the first step inside the tracks or the spurs. Subsequently in the bulk of the solution, the free radicals which have escaped recombination in the tracks recombine as molecular products into water. The molecular products are formed in the nanosecond range and their recombination takes place in the millisecond range. 

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. 

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. 

Water decomposition forming oxygen should be preferred according to thermodynamics, because its standard potential is lower ... 

Table 1.3 Thermodynamic data of lead acid and nickel/cadmium batteries and water decomposition.

Physical Properties. Thionyl chloride [7719-09-7], SOCI2, is a colorless fuming Hquid with a choking odor. Selected physical and thermodynamic properties are Hsted in Table 6. Thionyl chloride is miscible with many organic solvents including chlorinated hydrocarbons and aromatic hydrocarbons. It reacts quickly with water to form HCl and SO2. Thionyl chloride is stable at room temperature however, slight decomposition occurs just... 

---