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Molecules 2021, 26, 2144 12 of 37 Unfortunately, for ethanol oxidation, most catalytic materials are not selective enough to break the C–C bond, and instead, the C2 mechanism reaction takes place, where the main products are acetic acid (reaction (14)) and acetaldehyde (reaction (15)) that provide only 2 and 4 electrons, respectively [7,135,137–139]: CH3CH2OH+3H2O→CH3COOH+4H+ +4e− (14) CH3CH2OH → CH3CHO + 2 H+ + 2 e− (15) The possible products of ethanol oxidation are carbon dioxide, acetaldehyde and acetic acid, but the carbon dioxide distribution is low than that of methanol oxidation (where it is approximately 90 to 100%) [135]. Mechanisms of ethanol oxidation in acidic and in the alkaline environment are shown in Figure 4. Figure 4. Mechanism of ethanol oxidation in (a) alkaline and (b) acidic media reprinted with permission of Elsevier [136]. Additionally, a subsequent reaction between ethanol and ethanal, which leads to the formation of diethyl acetal, is possible and results in lowering the process efficiency [135]: CH3CHO + 2CH3CH2OH → CH3CH(OCH2CH3)2 + H2O (16) The oxygen necessary for CO2 formation is provided by water molecules, which are strongly visible in reaction (13). Increasing the participation of water in the feed stream is beneficial only to the maximum, limiting value. For water-to-ethanol mole ratios higher than 5:1, the ethanol partial pressure can decrease, which can lead to mass transport problems [135]. The presence of water promotes more current efficient reactions (reaction (13) and (14)) of ethanol oxidation, and, in its presence, only trace amounts of diethyl acetal have been found in the product stream. According to the Le Chatelier–Braun rule, in the presence of water, the equilibrium of reaction (16) is strongly shifted to the left side; therefore, very little or no ethanol diethyl acetal is present in the product stream. Temperature also has an impact on the reaction rate; at elevated temperatures (above 150 ◦C), reactions have been noted to be more current efficient [135]. From a fuel efficiency point of view, ethanol oxidation to carbon dioxide is the desired reaction in direct ethanol fuel cells, so research towards electrocatalytic materials for DEFCs anodes should focus on developing materials that catalyze the C1 reaction mechanism. However, the C2 mechanism of EOR, leading to obtaining acetaldehyde as a reaction product, can also be used in DEFC because such product would not accumulate in the environment since Mammals and yeasts are capable of producing enzymes that biologically degrade acetaldehyde to acetic acid and acetates. This amplifies the idea that acetaldehyde emissions from fuel cells will not cause acetaldehyde accumulation in the environment or in living species. However, acetaldehyde’s environmental impact would still be higher than the impact of carbon dioxide emitted from methanol-fed fuel cells [135]. Similar to the MOR, electrooxidation of ethanol can take place in the acidic and alkaline media. Anodic materials must be properly chosen and adapted to the reaction conditions. More researchers have focused on developing anodic materials for alkalinePDF Image | Effect of Anode Material on Electrochemical Oxidation of Alcohols
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