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Molecules 2021, 26, 2144 21 of 37 reaction product from its surface [173]. The composition of multimetallic electrodes is a very sophisticated process because catalyst composition strongly influences the properties of the final material. For example, in the case of a PtRu electrode, the activation energy for EGOR on a PtRu catalyst strongly depends on the Ru content; if too little ruthenium is doped into the electrode material, unfavorable adsorption kinetics take place. On the other hand, the addition of too much Ru strongly inhibits electrode activation [173,174]. Moreover, the addition of too much dopant, such as ruthenium or tin, may lead to the formation of separate metallic phases [168]. The maximum ruthenium content varies depending on the electrolysis environment. In acidic environments, with increasing Ru content, the onset potential for EGOR decreases. This phenomenon is probably caused by the optimum Pt:Ru ratio, which oscillates from approximately 15–20%, enabling the optimal ratio of three platinum adsorption sites to one ruthenium adjacent atom providing the optimal ratio of adsorbed hydroxide ions for oxidation of adsorbed ethylene glycol molecules [174]. In alkaline environments, the maximum ruthenium content is 50% because a further increase in its content leads to a decrease in the EGOR to CO2 reaction efficiency, which is related to ruthenium’s influence only on the main EGOR reaction but also on the parallel reactions [162]. The formation of ternary systems with catalytic activity towards EGOR is more com- plicated than in the case of ethanol or methanol oxidation. The addition of popular doping elements, such as nickel or palladium, has not influenced the activity of PtRu electrodes [49]. Different results are observed after the addition of tungsten into the Pt-Ru system. Usage of such ternary electrodes has led to higher peak current and lower reaction onset poten- tial [49,162]. Such behavior has been explained by a bifunctional mechanism, in which tungsten is responsible for enhanced water dissociation [49,162]. It has not only naturally low activation energy towards water split reaction [49] but also thanks to its ability to form oxides with different oxidation states-WO2, W2O5 and WO3. Changing the oxidation states of tungsten can render active sites for water dissociative adsorption [49,162]. A similar situation takes place in the case of ruthenium and platinum oxides. De- pending on the oxidation state of the metal, such molecules show different behaviors in electrolytic systems. They can either catalyze or inhibit the reaction. Ruthenium and platinum molecules containing metals in higher (RuIV and PtIII and PtIV) oxidation states slow the reaction. They are either very inactive towards EGOR or even prevent alcohol oxidation. Additionally, molecules containing these metals in lower oxidation states (0, I and II) are considered active species towards alcohol oxidation. For this reason, the stability of the electrode material is extremely important because its oxidation leads to lower activity and thus to an overall drop in the reaction efficiency [162]. Because platinum resources are limited, optimal usage of this strategic metal is nec- essary to lower the cost of anodic materials. One of the best strategies to obtain the best catalytic properties with the least platinum is the formation of core–shell structures. Be- cause electrolytic oxidation is a surface process, the usage of platinum as a shell material enables favorable catalytic properties with the lowest possible platinum usage. In the core–shell particles, the core must consist of a material that is immune to a harsh fuel cell environment and cheaper than platinum so that the overall cost of the catalyst can be reduced [108]. Different metal combinations have been studied, such as Pt cores at Ru shells [85], Pt@Ru, PtRu@Ni or PtRu@IrNi [108], and PtRu or PtNi [163]. The composition of core–shell particles is even more prudent than that of classic electrodes. For example, preparation of Pt@Ru core–shell nanoparticles with almost no ruthenium present in the shell layer leads to structures that are less active than metallic PtRu catalysts because, without ruthenium, a bifunctional mechanism cannot take place, and thus, the current efficiency of the overall reaction decreases [85]. Core–shell particles are not the only catalytic nanomaterials that can be used for the electrooxidation of organic molecules, such as ethylene glycol. Furthermore, one- dimensional materials, such as nanowires [25] or nanofibers [15], can be applied. Their strong advantage is the fact that they are characterized by a very large electroactive surfacePDF Image | Effect of Anode Material on Electrochemical Oxidation of Alcohols
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