Electrochemical stability water electrolysis

Fig. 13M PotentiallpH diagrams for water. Only the equilibria for water electrolysis and for self ionization are shown. The partial pressures of oxygen and hydrogen are taken as unity. The shaded area is the region of thermodynamic stability of water. Data from Pourbaix in "Atlas of Electrochemical Equilibria in Aqueous Solutions", Pergamon Press, 1966.

Ionomer membranes based on perfluorocarbon polymers became available In the late 196O s. These materials have excellent chemical resistance, thermal stability, mechanical strength and strong acid strength, A number of functionalities have been studied. Including carboxylate, sulfonate and sulfonamide, but only the first two are available as commercial materials. Ferfluorlnated lonomers have been evaluated as membranes In a variety of applications, such as water electrolysis, fuel cells, air driers, Donnan dialysis In waste metal recovery, and acid catalysts, but the primary interest in these materials is for the permselective membrane In electrochemical processes such as In the production of chlorine and caustic (58). 

The monomer XXVIII is copolymerized with tetralluoroethylene to give a polymer-containing pendant fluorosulfonyl groups that are then hydrolyzed and acid exchanged to produce Nafion (XXIX) The resulting polymer combines the chemical, thermal, and oxidative stability of perfluorinated polymers such as polytetra-fluoroethy lene with the properties ofahighly acidic fluorinated sulfonic acid. Nafion is used in a variety of electrochemical applications such as the synthesis of chlorine and caustic and as the conductive membrane of many modern fuel cells. It has also been used in water electrolysis and as an acid catalyst in many proprietary commercial processes. 

Conductive boron-doped polycrystalline CVD diamond, identified as BDD (boron doped diamond) electrodes, exhibit good electrochemical stability, together with the largest overpotentials for water electrolysis among well-known electrodes. In particular, this combination of properties is the reason for increasing efforts with the aim of developing highly efficient electrochemical processes with BDD electrodes [1,2], Applications such as disinfection [3,4] and electroljd ic oxidation [5-9] are areas where electrolysis based on BDD electrodes can offer new or improved solutions. 

ILs are thought to be electrochemically stable. They possess large electrochemical potential windows from 2V to 6V, but these data are the result of rather short cyclic voltammetry experiments (Galinski et al., 2006). It has been demonstrated that different reaction products can be found when longer electrolysis experiments are performed (Kroon et al., 2006). The differences in the electrochemical stabilities of ILs results from the presence of impurities. In the presence of water the potential window of ILs narrows dramatically (Islam Ohsata, 2008 Welton, 1999). Moreover, some reactions, e.g. the generation of a superoxide ion or a hydroxide radical, which limits the chemical stability of ILs, may be affected by the presence of water in ILs (Barnes et al., 2008). 

Fig. 16. Photovoltaic and electrolysis charge transfer for thermal electrochemical solar driven water splitting.90 Photocurrent is shown for one, two or three 1.561 cm2 HECO 335 Sunpower Si photovoltaics in series at 50 suns. Photovoltaics drive 500-°C molten NaOH steam electrolysis using Pt gauze anode and cathodes. Inset electrolysis current stability.

The mono reduced complex have also been generated electrochemically. A highly efficient (98 % faradic yield) and selective (less than 1 % H2 produced) electrocatalytic system has been discovered by using the same fac-[Re(bpy)(CO)3Cl] complex. It displayed high chemical stability and no decrease in electrocatalytic activity was observed versus electrolysis time [72]. The system showed a marked dependence on the amount of water present, proving the crucial need of protons (oxygen acceptor from the CO2) during the catalysis. 

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