Electrochemistry of carbon monoxide and hydrogen

In order to understand the behavior of a PEMFC in the presence of CO, the electrochemistry of CO and hydrogen on the surface of platinum must be understood. This is because platinum or its alloys are invariably used and needed in low-temperature acid electrolyte fuel cells to facilitate the electrochemical reaction for the production of electric power. The electrochemistry of CO in an acidic environment has been studied extensively [95] and reviewed by Baschuk and Li [96]. CO oxidation and adsorption occurs on the (1 0 0) and (1 1 0) sites of platinum in an acid electrolyte. The adsorption of CO involves CO linearly bonded to platinum, and the adsorption isotherm is that of a Tempkin isotherm, which can be written as [97]  

The interaction parameter and the free energy of adsorption were found to be functions of temperature, and in addition, the interaction parameter was found to be highly dependent on catalyst structure. Also, fliis relation was only valid for relatively high temperatures of greater than 130 °C for low temperatures, such as those encountered in a PEMFC, the coverage appeared to be a function of the anode potential as well as the concentration, in addition to the temperature and catalyst structure. 

The oxidation of hydrogen and the effect of CO in an acid environment have also been extensively studied. The mechanism for electrochemical hydrogen oxidation over smooth platinum in an acid electrolyte is the slow dissoeiation of adsorbed hydrogen molecules to hydrogen atoms, known as the Tafel reaetion, followed by the fast electroehemical oxidation of the adsorbed hydrogen atoms, known as the Volmer reaction [99]. This is illustrated below  

It was also diseovered that the dissociative ehemisorption of hydrogen on platinum was independent of both surfaee geometry and erystallite size, leading to the result that the exehange current density for the hydrogen reaction was independent of catalyst structure [100]. 

the coverage of CO on the platinum eleetrode surfaee becomes a significant parameter in the performance of an electrode. In the experimental studies cited earlier, it was concluded that the coverage of CO was a function of potential as 

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