Ethanol oxidation open-circuit potential

Promotion and deactivation of unsupported and alumina-supported platinum catalysts were studied in the selective oxidation of 1-phenyl-ethanol to acetophenone, as a model reaction. The oxidation was performed with atmospheric air in an aqueous alkaline solution. The oxidation state of the catalyst was followed by measuring the open circuit potential of the slurry during reaction. It is proposed that the primary reason for deactivation is the destructive adsorption of alcohol substrate on the platinum surface at the very beginning of the reaction, leading to irreversibly adsorbed species. Over-oxidation of Pt active sites occurs after a substantial reduction in the number of free sites. Deactivation could be efficiently suppressed by partial blocking of surface platinum atoms with a submonolayer of bismuth promoter. At optimum Bi/Ptj ratio the yield increased from 18 to 99 %. 

Fig. 5.43. Current-potential curves for ethanol oxidation and oxygen reduction on smooth and platinized platinum. OCP=Open Circuit potential (catalyst potential at which no net current flows).

Bruhsett et al. [34] examined the performance of air-breathing membraneless LFFC operated with ethanol and methanol imder acidic (H2SO4) and alkaline (KOH) conditions. Methanol and ethanol showed improved open-circuit potential and maximum power density in alkaline media (1.2 and 0.7 V, 17.2 and 12.1mWcm ) compared with acidic conditions (0.93 and 0.41V, 11.8 and 1.9 mW cm ). The improved performance in alkaline media was the result of the enhanced alcohol oxidation kinetics and oxygen reduction kinetics compared with acidic media. 

Another important source of power loss performance in direct alcohol fuel cells is so-called crossover effect, which is the passage of the fuel from the anode to the cathode through the membrane. This crossover effect is responsible for a large part of power losses of DAFCs, especially in the open circuit region. Besides, ethanol can be oxidized at the cathode electrode resulting in mixed potentials that also contribute to the decreasing of the efficiency of the fuel cell. As a rule of thumb the crossover effect increases with the concentration of ethanol and with the increasing temperature and decreases with the thickness of the membrane thus... 

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