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Molecules 2021, 26, 2144 9 of 37 taken by hydroxide species, too few free reaction sites are available for methanol particles; however, if too few hydroxide anions are adsorbed, the reaction cannot be optimized [83]. The potential at which methanol oxidation takes place can strongly influence the contribution of each possible product. For the Pt (111) catalyst in alkaline media, at 0.4 V, formate is observed to be the main product, but at approximately 0.5 V, carbonate is detected as the dominant product. Additionally, in the low potential range, the amount of adsorbed OH species is so high that the reaction rate decreases as a result of oxidation of adsorbed OH ions into electrochemically inactive oxide [83]. Because MOR is very sensitive to the catalyst surface structure [96] for platinum catalysts with structures other than (111), potential values of approximately 0.6 V have led mainly to the formation of formate [83]. This difference is related to the varying coverage of the platinum surface with CO species depending on the catalyst structure. The slowest methanol dehydrogenation (reaction (6)) takes place on a Pt(111) surface, which makes it the least covered with CO species among basal structures—Pt(100) and Pt(110). This phenomenon is related to the number of defects on the platinum surface since defects are the active sites for methanol dehydrogenation. Different types of defects promote different reactions; for example, kink-type defects on Pt(111) structures promote CO oxidation, and step-type defects on the same structure promote methanol dehydrogenation. This means that by controlling the surface structure, we can control the CO coverage of the electrode surface and, by that, the reaction path and its rate [81,83,96]. Additionally, at higher temperatures and methanol concentrations, dimethoxymethane and methyl formate have been identified as products of methanol and formaldehyde reac- tions. The current efficiency and ratio of different intermediates strongly depend on the reaction conditions and type of electrode or catalyst [96]. Platinum- and platinum-based nanomaterials have been widely used as anode ma- terials for methanol oxidation in DMFCs [38,54,83]. In acidic media, the electroactivity is the highest, but at the low temperatures at which DMFCs function, carbon monoxide poisoning readily occurs. To overcome this disadvantage, Pt-based electrodes have been improved with the addition of p- or d-orbital elements such as Ru [21,96,97,105], Pd [74], Ni [21,99], Fe [42,66], Cu [19,94], B [77], Au [64], Nb [78] and Co [12,21,54,62] Mo [21,96] or Sn [21,97,105] as co- catalysts. Among the platinum-based alloys, Pt–M intermetallic compounds are attracting attention because of their high activity as fuel cell anodes, especially methanol [110–113]. The addition of these metallic elements lowers the onset MOR potential and boosts the peak current density, which leads to higher reaction yields and eventually higher DMFC efficiency. Additionally, additives can change the CO adsorption sites and thus prevent CO poisoning. Not only bimetallic but also ternary systems have been studied, and the presence of a third metallic element can significantly improve the kinetics of the MOR, i.e., Pt–Cu–Fe/C electrodes show lower CO adsorption and lower MOR onset poten- tial than their disoriented counterparts and Pt/C catalysts [94]. Platinum properties can be changed not only by doping pure metals but also by the presence of their oxides. If metal oxides (i.e., RuO2, MnO2, MoO2, and IrO2) are used for platinum embedding, they alter the electrochemical features of the obtained electrode (electronic effect), which changes the adsorption energy of methanol and thus improve the kinetics of the MOR. This effect is caused by changing the conditions of proton and electron transfer by metal oxide hydration [34,112,114,115]. Because catalysis is a surface process, there is a chance of lowering the overall cost of catalysts using different materials underneath the active layer, for example, by using core– shell nanostructures. The core material should be resistant to the process conditions and cheaper than the shell material. Many core–shell materials have been studied; for example, Pt@Ru and PtRu@IrNi core–shell materials have been tested as methanol oxidation catalysts in acidic media. The proposed materials show better catalytic properties towards MOR than commercially available materials, even without the preferred surface composition (Pt:Ru 3:1) [108].PDF Image | Effect of Anode Material on Electrochemical Oxidation of Alcohols
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