O2 evolution

The most favorable conditions for equation 9 are temperature from 60—75°C and pH 5.8—7.0. The optimum pH depends on temperature. This reaction is quite slow and takes place in the bulk electrolyte rather than at or near the anode surface (44—46). Usually 2—5 g/L of sodium dichromate is added to the electrolysis solution. The dichromate forms a protective Cr202 film or diaphragm on the cathode surface, creating an adverse potential gradient that prevents the reduction of OCU to CU ion (44). Dichromate also serves as a buffering agent, which tends to stabilize the pH of the solution (45,46). Chromate also suppresses corrosion of steel cathodes and inhibits O2 evolution at the anode (47—51). 

Demmig, B. Bjorkman, O. (1987). Comparison of the effect of excessive light on chlorophyll fluorescence (77 K) and photon yield of O2 evolution in leaves of higher plants. Planta, 171,171-84. 

The major problem in accomplishing water splitting via the pathway of Scheme 4 is how to suppress the back recombination reaction + A -> D + A, which is a simple exothermic bimolecular process and therefore typically proceeds much more rapidly than complex catalytic reactions of H2 and O2 evolution. 

The membrane-bound catalyst for water oxidation to O2 can be prepared via oxidation of Mn(Il) and Co(ll) salts to Mn(IV) and Co(Ill) hydroxides, respectively, in the presence of lipid vesicles. Using these catalysts and photogenerated Ru(bipy)j complex as an oxidant, it is possible to oxidize water to O2 in vesicle systems. One of such systems for O2 evolution is schematically represented in Fig. 4. 

Fig.4. Photocatalytic S O -Ru(bipy) -- (OH)x -Co0(3 .,y2 lipid vesicle system for O2 evolution.

Cadmium oxide, CdO, is a semiconductor with a band gap of 2.3 eV. Irradiation of CdO powder suspended in alkaline solution resulted in the formation of 0 when an electron acceptor such as ferricyanide was present in the solution. When RuOj was deposited onto the surface of the CdO particles the yield of decreased relative to naked CdO. In this respect CdO differs from Ti02 where RUO2 is mandatory if O2 evolution is to be observed Colloidal CdO has not been known until recently. It can... 

Substantial research efforts should be dedicated also to the development of low cost multiple-electron transfer catalysts for oxygen production. Electrochemical losses related to O2 evolution are a considerable part of the overall inefficiencies. 

Of note is the apparent lack of chloride in the first coordination shell of Mn in either the S or the S2 state as revealed by EXAFS studies of Mn (35). This observation is of particular interest because chloride is required for optimal O2 evolution rates (39) and has been proposed to act as a bridging ligand in a polynuclear Mn complex (40). Recent EPR studies, however, also suggest that chloride is not bound to Mn in the S or S2 state (41). 

A remarkable O2 evolution chemistry was observed with these complexesWhen one equivalent of HPFe is added to CH3CN solutions of either cis- or tra i-[Os (tpy)(Cl)2 N(0)SC6H3Me2 ], O2 is produced in a very rapid reaction (Equation (68)). When a stoichiometric amount of bpy in air-saturated CH3CN is added the osmium(IV) sulfoximido is regenerated (Equation (69)). This O2 evolution can be made catalytic by adding large excess of HPFe in the presence of MesNO (Equation (70)) ... 

Other orchid metabolites suchs as batatasin 1, inhibited the growth of liverworts, algae and oat coleoptiles. Batatasin 1 also inhibited the CO2 dependent O2 evolution and the flow of electrons from water to methylvi-ologen in spinach chloroplasts, and it inhibited the succinate-dependent O2 uptake in potato tuber mitochondria. Other phenanthrenes such as orchinol, which has a free hydroxyl at the 7-position, inhibit indole-3-acetic acid (lAA) oxidation catalyzed by horseradish peroxidase. 

In fact, in 1972, Fujishima and Honda (10) demonstrated that O2 evolution on n-type Ti02 occurs as a photocurrent, proportional to the light intensity (Figure 1) of wavelengths less than 415 nm, i.e. for photon energies equal to or greater than the band gap of Ti02 3.0 eV. In this work and that of Ohnishi et al. (11) a platinum black metal cathode was connected in an external circuit to an indium contact on the back side of the photo-anode (see... 

Fourth, the favoured anodic reaction at the semiconductor must be O2 evolution from water, rather than some anodic dissolution process in which the semiconductor breaks down, as happens with Ge 9), GaP (14,15,16) CdS or even ZnO (13). 

O2 evolution was measured with a Clark-type electrode. Specific activity of the untreated controls averaged 40 9 ymoles 0 evolved/mg Chi h. Data shown are arithmetic averages SD of determinations made with a minimum of three different isolations of chloroplasts. 

The SPE technology solves some problems but it poses others. In particular, the strong acid environment developed on the membrane calls for a complete change of electrode materials from those used in the conventional alkaline electrolysis. More specifically, especially the requirements for electrode materials for O2 evolution are stringent since the anodic conditions are especially aggressive for corrosion problems. 

As a demanding reaction, it is very sensitive to the structural and compositional details of the anode materials. For this reason, research on anodes for O2 evolution calls for close characterization of electrocatalysts, especially from the point of view of materials chemistry and physics. 

The intermediate energy depends on the interaction with the electrode surface. Materials binding OHads weakly as a rule show high overpotentials for O2 evolution. Their mechanism is dominated by step (7.25) as the rate-determining step and the Tafel slope is close to 120 mV. 

What makes the approach to electrocatalysis much more complex is the fact that after step (7.25) there is never just one more step, but at least two more. As a consequence of the complexity of the surface chemistry of OH species, several mechanisms have been proposed for O2 evolution, differing in small and sometimes speculative details. However, most of the experimental observations can be interpreted on the basis of three simplified schemes. 

Although the most active O2 evolution electrocatalysts exhibit Tafel slopes in the range 30-40 mV, in some cases slopes close to 60 mV have been reported. For these cases, a third mechanism has been proposed in which the formation of Oads is preceded by the acid-base dissociation of OHads- This mechanism is known for the name of its proposer, but it could be defined as follows. 

As implied in the schemes of the mechanisms above, a surface oxide is formed during O2 evolution. Since M-0 is a much stronger bond than M-H, absorption of O is much more probable than that of H. Thus, whereas H2 evolution can be treated as occurring on bare metal surfaces, O2 evolution cannot. In the end, after O2 evolution, a metal surface turns out coated by an oxide layer electrolytically... 

Electrolytic oxides are responsible for the passivity of corroding metals, for example Ti02 and NiO. However, this is not generally the case under O2 evolution conditions. If an oxide passivates a surface, it is not a good electrocatalyst for O2 evolution. On the other hand, oxides that are good catalysts for O2 evolution very often are unstable under O2 evolution and dissolve. For instance, NiO passivates Ni in alkali and is also a good electrocatalyst for O2 evolution. However, it dissolves in acids and the metal cannot be used for water electrolysis at low pH. Similarly,... 

Ru is easily oxidized anodically but the oxide is not stable and dissolution occurs under O2 evolution both in acid and in base [43, 56]. Nevertheless, if Ru oxide is electrodeposited during anodic polarization of aqueous solutions of RUCI3, the electrodeposited Ru oxide is catalytically active for O2 evolution, as shown by the decrease in anodic overpotential. However, such a configuration is impractical for water electrolysis since the liquid phase should contain RUCI3, which would be deposited everywhere in the cell circuit. 

For a long time, conventional alkaline electrolyzers used Ni as an anode. This metal is relatively inexpensive and a satisfactory electrocatalyst for O2 evolution. With the advent of DSA (a Trade Name for dimensionally stable anodes) in the chlor-alkali industry [41, 42[, it became clear that thermal oxides deposited on Ni were much better electrocatalysts than Ni itself with reduction in overpotential and increased stability. This led to the development of activated anodes. In general, Ni is a support for alkaline solutions and Ti for acidic solutions. The latter, however, poses problems of passivation at the Ti/overlayer interface that can reduce the stability of these anodes [43[. On the other hand, in acid electrolysis, the catalyst is directly pressed against the membrane, which eliminates the problem of support passivation. In addition to improving stability and activity, the way in which dry oxides are prepared (particularly thermal decomposition) develops especially large surface areas that contribute to the optimization of their performance. 

Pt is, of course, not a good electrocatalyst for the O2 evolution reaction, although it is the best for the O2 reduction reaction. However, also with especially active oxides of extended surface area, the theoretical value of E° has never been observed. For this reason, the search for new or optimized materials is a scientific challenge but also an industrial need. A theoretical approach to O2 electrocatalysis can only be more empirical than in the case of hydrogen in view of the complexity of the mechanisms. However, a chemical concept that can be derived from scrutiny of the mechanisms mentioned above is that oxygen evolution on an oxide can be schematized as follows [59] ... 

Figure 7.11 Electrocatalytic activity of thermal oxides for O2 evolution as a function of heat of oxide formation [59].

Another typical example is given in Figure 7.13. In this case, Ru + Ir oxides are prepared using three different procedures [62]. It is readily evident that the mechanism of O2 evolution varies with composition differently depending on the preparation method. Scrutiny of the data reveals that thermal decomposition... 

Figure 7.12 Dependence of the mechanism (Tafel slope) of O2 evolution on the electrode surface area (or 1/particle size) [60].

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