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Molecules 2021, 26, 2144 13 of 37 feed streams because in this kind of medium, with the chemical activity of ethanol, the oxidation rate is higher [32,136]. Additionally, a high pH inside the fuel cell is beneficial from a corrosion point of view because, in basic solutions, a wide range of anodic materials is more immune towards corrosion than in acidic ones [11]. Anodic materials for ethanol oxidation can be divided into two main groups: materials based on platinum [83] and materials in which palladium is the main ingredient [11]. Platinum, as an electrocatalytic material, provides a high number of active coordina- tion sites and shows relatively high selectivity towards breaking the inter-carbon bonds of alcohols [138]. As mentioned, after adsorbing on the active sites of platinum, ethanol molecules can react through various pathways. They can either succumb to dissocia- tion (C1 mechanism), which leads to strong adsorption of carbon monoxide derivatives (CO)ads and CHx intermediates on the surface of the electrode, or oxidation (C2 mech- anism), which results in acetic acid and acetaldehyde. To further oxidize the adsorbed carbon species, the presence of adsorbed hydroxide ions is necessary. This complicated cascade of reactions is why ethanol oxidation is so problematic. The adsorption of ethanol and the further breakage of its C–C bonds are inhibited by the presence of adsorbed car- bon species at low overpotentials and by (OH)ads at high overpotentials, which is why the EOR regime should be strictly controlled by the electrode polarization potential and environmental composition [138]. Platinum-based nanocompounds are one of the most promising nanomaterials for elec- trocatalytic ethanol oxidation. Their main advantages are stability and predictable surface composition leading to predictable distribution of active centers. Unfortunately, Pt-based materials are very prone to carbon monoxide poisoning and lose their reactivity due to nanoparticle migration and agglomeration [138,140,141] and have a sluggish reaction rate. The doping of oxophilic elements, such as Sn or Ru, in platinum electrodes significantly improves their catalytic performance by enabling the adsorption of hydroxide ions at low overpotentials thanks to the bifunctional effect [138,140]. They also change the electronic structure of the electrode by decreasing the d-band center, which weakens the adsorption of CO intermediates [15,140], so they enable both bifunctional and electronic (ligand) ef- fects [15,138,140]. Unfortunately, doping of this kind of element leads to the lowering of the catalytic selectivity towards the oxidation of ethanol to carbon dioxide [138]. Similar to PtRu in the case of methanol, platinum electrodes doped with tin are very popular materials for the electrooxidation of ethanol in acidic environments. Ruthenium- doped electrodes do not work in the case of complete ethanol oxidation because they are unable to break the C–C bond [16,136,139,142,143]. The incorporation of tin into a platinum catalyst changes the electrode geometric and electronic structure, providing conditions required for complete ethanol oxidation to carbon dioxide [15,16,143,144]. Due to natural differences in their electronegativity, charge transfer from less elec- tronegative tin towards more electronegative platinum takes place. As a result, the unoccu- pied platinum 5d orbital is partially filled with 2d Sn electrons, and an electronic (ligand) effect takes place [15,16,144]. Modification of the unoccupied platinum d band leads to a lower affinity of carbon species towards platinum, which causes a decrease in catalyst poisoning by COads. Weaker bonding occurs not only between the Pt and carbon interme- diates but also with all electroactive species; however, the decrease in platinum catalytic properties is balanced by tin catalytic properties and a weaker poisoning effect [15,16]. Additionally, CO can adsorb on the surface of Pt (111) in various forms, such as linear or bridged, but on the surface of Pt3Sn (111), because of the incorporation of tin atoms into the lattice, CO can adsorb only in linear form, which decreases the amount of CO adsorbed [15,16]. The presence of tin promotes the oxidation of alcohols by providing adsorbed OH species from water dissociation taking place at low potentials due to the presence of tin hydroxides [144]. Therefore, PtSn catalysts show a bifunctional oxidation mechanism, as shown in Figure 5, and an enhanced ability to break the C–C bonds in simple alcohols, such as ethanol [15,16,143,144]. The optimum tin content provides an optimal number of surface oxygen derivatives that are capable of oxidizing the adsorbedPDF Image | Effect of Anode Material on Electrochemical Oxidation of Alcohols
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