Electrochemical reactions involving oxygen

When, after the attainment of zero surface concentration, a constant current density is maintained artificially from outside, the electrode potential will shift to a value such that a new electrochemical reaction involving other solution components can start (e.g., in aqueous solution, the evolution of hydrogen or oxygen). It follows from Eq. (11.9) that at a given concentration Cy the product is constant and is... 

When the layer has electronic in addition to ionic conductivity, the electrochemical reaction will be partly or completely pushed out to its outer surface. In addition, other electrochemical reactions involving the solution components, particularly anodic oxygen evolution, can occur on top of the layer. 

For these low-temperature fuel cells, the development of catalytic materials is essential to activate the electrochemical reactions involved. This concerns the electro-oxidation of the fuel (reformate hydrogen containing some traces of CO, which acts as a poisoning species for the anode catalyst methanol and ethanol, which have a relatively low reactivity at low temperatures) and the electroreduction of the oxidant (oxygen), which is still a source of high energy losses (up to 30-40%) due to the low reactivity of oxygen at the best platinum-based electrocatalysts. 

Yttria stabilized zirconia formed by this reaction fills the air electrode pores. The dynamics of this CVD stage has been modeled by Carolan and Michaels [120], who observed that films produced in this manner penetrate the substrate no more than 2-3 pore diameters from the chloride face of the substrate. It has also been shown that the penetration depth is independent of water concentration. The second step of this method is the EVD step. Once pore closure is achieved, the reactants are not longer in contact. Electrochemical semipermeability related to the existence of small electronic conductivity and large gradient of oxygen partial pressure leads to oxygen transport from the steam side to the chloride side through the deposited electrolyte. The electrochemical reactions involved are ... 

One of the main problems of low-temperature (20°C-80°C) PEMFCs is the relatively low kinetics rate of the electrochemical reactions involved, e.g., the oxygen reduction at the cathode and fuel (hydrogen from a reformate gas, or alcohols) oxidation at the anode. Another problem, particularly crucial for the electric vehicle, is the relatively low-working temperature (70°C-80°C), which prevents efficient exhaust of the excess heat generated by the power fuel cell. 

Hydrogen—Oxygen Cells. The hydrogen—oxygen cell can be adapted to function as a rechargeable battery, although this system is best known as a primary one (see Fuel cells). The electrochemical reactions involve ... 

Except for the electrochemical reaction (2.6), all other reactions depend on the pH of the solution. A number of electrochemical reactions proceed in this system, which form different electrode systems, depending on lead ion valency, solution composition and pH, and electrode potential. These reactions cover a potential range of 2.0 V. Table 2.4 summarises the electrochemical reactions involving Pb, lead oxides, PbS04 and basic lead sulfates, and the equilibrium potentials of the respective electrode systems. The reactions and the equilibrium potentials for the hydrogen and oxygen electrodes are also given in the table. Several chemical reactions in which basic lead sulfates take part are also included in Table 2.4. 

Tab. 2 Standard redox potentials for various electrochemical reactions involving molecular oxygen and its reduced forms...

The reactions at the electrodes are given below. The oxygen is reduced by an electrochemical reaction involving either two electrons (equation [10.9]) or four electrons (equations [10.9] and [10.12]). 

Mechanistic smdies are needed on a select nnmber of electrochemical reactions, particularly those involving oxygen. These smdies are far from routine and reqnire advances in knowledge of molecular interactions at electrode surfaces in the presence of an electrolyte. Recent achievements in surface science under ultrahigh vacuum conditions snggest that a comparable effort in electrochemical systems would be equally fmitful. 

Current flow at electrode surfaces often involves several simultaneous electrochemical reactions, which differ in character. For instance, upon cathodic polarization of an electrode in a mixed solution of lead and tin salt, lead and tin ions are discharged simultaneously, and from an acidic solution of zinc salt, zinc is deposited, and at the same time hydrogen is evolved. Upon anodic polarization of a nonconsumable electrode in chloride solution, oxygen and chlorine are evolved in parallel reactions. 

Electrochemical corrosion involves one of three major cathodic reactions. The first occurs in aerated, acid to neutral solutions (e. g. in seawater and under conditions of atmospheric weathering) and involves reduction of oxygen. 

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