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Molecules 2021, 26, 2144 8 of 37 In alkaline media, both carbonate ions, as shown in reaction (4), and formate ions (reaction (5)) has been detected as a product of methanol oxidation [83]: CH3OH+5OH− →HCOO− +4H2O+4e− (5) Reaction 4 takes place in a few steps with adsorbed species [83]: CH3OH+4OH− →(CO)ads +4H2O+4e− (6) (CO)ads + 2 (OH)ads → CO2 + H2O (7) and adsorbed hydroxide is present on the electrode surface as a result of a reaction (7) [83]: OH− → (OH)ads + e− (8) As two adsorbed OH species are required for oxidization, one adsorbed CO molecule exchanges electrons to sum up to 6, which agrees with reaction (4). Carbonate ions are present in a solution as a result of the following reaction [34,83]: CO2 +OH− →CO32− +H+ (9) This reaction is particularly hazardous because the released carbonate ions can com- petitively adsorb on the electrode surface instead of methanol molecules and hydroxide ions. Additionally, the released CO32− leads to a decrease in the medium pH, which causes more sluggish reaction kinetics. For these reasons, alkaline DMFCs, despite their successful operation in space programs, have not been widely used on an industrial scale on Earth [83]. Complete methanol electrooxidation is a 6-electron pathway, reaction (4), with differ- ent steps and intermediate species, such as carbon oxide (reactions (1) and (5)), formalde- hyde and formic acid (reaction (5)) depending on the process conditions. The formation of these intermediates causes slow reaction kinetics and low efficiency because the formation of every final product that is different from CO2 lowers the number of exchanged electrons (reaction (4) vs. reaction (5)), leading to a lower efficiency for the whole process [38,83]. The reactions shown above represent a so-called bifunctional mechanism that assumes that methanol adsorbs only on the surface of platinum, while on the surface of ruthenium, only water splitting takes place. This theory simplifies some phenomena, such as water oxidation on platinum in a higher potential range [96] and the adsorption of methanol on ruthenium at a higher temperature [96,108]. Additionally, adsorbed species can move on the surface and occupy Ru atoms even though they are primarily formed on the Pt surface [96]. This simplification is possible because the best results are observed if the final step of MOR takes place exactly, as shown in reaction (3)–between carbon monoxide adsorbed on the surface of platinum and hydroxide adsorbed on the surface of ruthenium. For this reason, the diffusion rate of (CO)ads on the catalyst surface limits this reaction because this species must migrate towards the adsorbed hydroxide for the reaction to take place [96]. Commercially available catalysts are made of Pt and Ru in an atomic ratio of 1:1, but the proposed methanol oxidation mechanism suggests that three free platinum sites are needed for methanol adsorption and that one ruthenium free site is necessary for water splitting (reaction (2)), so the best results should be observed for catalysts containing platinum and ruthenium in atomic ratios from 3:1 to 1:1 [108]. Methanol oxidation on Pt–Ru reaches a maximum peak current for ruthenium contents between 15 and 45%. A further increase in Ru content leads to a decrease in the reaction current because too few platinum reaction centers are available for methanol molecules to adsorb [105]. The oxidation of the adsorbed carbon monoxide derivative reaction (7) is the limiting step for the MOR, especially on platinum-based catalysts [83,109]. This reaction strongly depends on the amount of adsorbed OH species because if most of the reaction sites arePDF Image | Effect of Anode Material on Electrochemical Oxidation of Alcohols
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