Quantification of formaldehyde production during alkaline methanol electrooxidation
The alkaline methanol electrooxidation reaction (MOR) in alkaline direct methanol fuel cells is still very little understood with regard to its electrochemical behavior. Theoretically, when using a rotating disk (RDE) as working electrode, the limiting current from an electrochemical reaction increases with the rotation rate as described by Levich. Contrary to this principle, the current resulting from the alkaline MOR does not increase, but decreases with rotation rate. In this work, we investigate the reason for this phenomenon using the method described by Nash and modified by Belman to quantify formaldehyde, a reaction intermediate of the alkaline methanol electrooxidation. The amount of formaldehyde is in direct relation to the rotation rate, proving that the current density loss can originate from an intensified removal of formaldehyde into the bulk solution. We analyse the influence of the electrolyte and methanol concentration on the formation of formaldehyde in order to investigate which conditions support the complete oxidation pathway and suppress the incomplete oxidation to formaldehyde. The concentration ratio as well as the absolute concentrations are of great importance for the pathways taking place. A low electrolyte concentration leads to an increase of the formaldehyde but decreasing the methanol concentration results in an absence of formaldehyde in the bulk solution.