Cathodes Direct conversion

Direct Anodic and Cathodic Electrochemical Conversions of Organic Substrates... 

Research on the direct conversion of chemical energy to electricity via fuel cells has received considerable attention in the past decades. Fuel cells are indeed attractive alternatives to combustion engines for electrical power generation in transportation applications and also as promising future power sources, especially for mobile and portable applications. Thus, the search for excellent electrocatalysts for the electro-catalytic oxygen reduction and methanol oxidation reactions, which are the two important cathodic and anodic reactions in fuel cells, is intensively pursued by scientists... 

In fuel cells hydrocarbons are dehydrogenated by active catalysts. The main purpose of these cells is the direct conversion of combustion energy into electrical energy. At the fuel cell cathode the hydrocarbon is in must cases oxidized to carbon dioxide (equation 26a), because the intermediates are more easily oxidized than the starting hydrocarbon. Only in few cases are substitution, dehydrogenation or coupling products of the hydrocarbon obtained. At the fuel cell anode oxygen is reduced to water (equation 26b). 

The thermodynamics of electrochemical reactions can be understood by considering the standard electrode potential, the potential of a reaction under standard conditions of temperature and pressure where all reactants and products are at unit activity. Table 1 Hsts a variety of standard electrode potentials. The standard potential is expressed relative to the standard hydrogen reference electrode potential in units of volts. A given reaction tends to proceed in the anodic direction, ie, toward the oxidation reaction, if the potential of the reaction is positive with respect to the standard potential. Conversely, a movement of the potential in the negative direction away from the standard potential encourages a cathodic or reduction reaction. 

The end point of the reaction is conveniently determined electrometrically using the dead-stop end point procedure. If a small e.m.f. is applied across two platinum electrodes immersed in the reaction mixture a current will flow as long as free iodine is present, to remove hydrogen and depolarise the cathode. When the last trace of iodine has reacted the current will decrease to zero or very close to zero. Conversely, the technique may be combined with a direct titration of the sample with the Karl Fischer reagent here the current in the electrode circuit suddenly increases at the first appearance of unused iodine in the solution. 

The band edges are flattened when the anode is illuminated, the Fermi level rises, and the electrode potential shifts in the negative direction. As a result, a potential difference which amounts to about 0.6 to 0.8 V develops between the semiconductor and metal electrode. When the external circuit is closed over some load R, the electrons produced by illumination in the conduction band of the semiconductor electrode will flow through the external circuit to the metal electrode, where they are consumed in the cathodic reaction. Holes from the valence band of the semiconductor electrode at the same time are directly absorbed by the anodic reaction. Therefore, a steady electrical current arises in the system, and the energy of this current can be utilized in the external circuit. In such devices, the solar-to-electrical energy conversion efficiency is as high as 5 to 10%. Unfortunately, their operating life is restricted by the low corrosion resistance of semiconductor electrodes. 

Further, it can be seen from Fig. 1.1 that under all conditions prevailing Cu is the positive and Zn the negative pole however, in case (b) Cu is the cathode (reduction) and Zn the anode (oxidation). Considering the flow direction within the electrolyte, one usually finds that the anode is upstream and the cathode downstream. It is also clear that by the electrochemical conversions the original galvanic cell is depleted in case (b), but can be restored by the external electrical energy source in case (c). 

---