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Molecules 2021, 26, 2144 10 of 37 Not only the composition of the electrode surface but also its roughness affects the MOR. Smooth Pt electrodes show an enhanced yield of partial MOR leading to formic acid and formaldehyde. The same effect is observed for enhanced mass transport conditions and ambient temperatures. This also shows that laboratory results from experiments on smooth electrodes and under ideal conditions are poorly related to the reaction characteristics under DMFC operational conditions [93]. Although platinum-based catalysts yield the best results for MOR in acidic media, Pt shortages and high prices have compelled researchers to look for nonplatinum electrocat- alytic materials. In addition, Pt-based electrodes may catalyze the formation of HCOOCH3, either by catalyzing the esterification reactions between formic acid and adsorbed methanol or as a result of the nucleophilic attack of CH3O− on adsorbed HC = O [94]. Increasing the Pt content increases the HCOOCH conversion efficiency linearly [94]. For the mentioned reasons, attempts to obtain a nonplatinum electrode material for acidic media have been made. For example, Co–Pd/Sn/RGO active electrocatalysts obtained by electroless deposi- tion of cobalt nanoparticles on RGO can be cost-effective nonnoble electrodes for methanol oxidation for DAFC with the use of acidic media [10]. Better results for the usage of nonnoble metals as anode materials for MOR have been reported for alkaline media [54]. Alkaline media not only enable better kinetics of methanol oxidation than acidic ones but also are coupled to weaker poisoning effects [54,83]. Even more important, alkaline media enable the usage of cheaper materials, such as nickel [116–118] or copper [119–121], as anodes because under these conditions and MOR leads to fewer intermediates than in acidic media. The alkaline environment is much less corrosive for nonnoble electrode materials, and thus, the overall process costs are lower [54,83]. Additionally, in alkaline media, anion adsorption is weaker, enhancing the main oxidation reaction because more free active sites on the surface of the electrode are accessible for methanol particles [83]. For alkali media, Pt can also be used as a catalyst material, but similar to acidic media; some additives have been used to enhance its catalytic activity under these conditions. Because of the alkaline environment, water splitting is no longer necessary for the pres- ence of (OH)ads, which leads to the usage of different metals for doping Pt electrodes. In contrast to acidic media, the addition of Ru does not improve platinum catalyst proper- ties towards MOR [83], but the presence of Ni significantly improves platinum catalyst performance [122]. Other metal dopants, such as gold [83] or silver [50], have been tested as platinum catalyst additives. In the case of gold, a synergic effect has been observed, leading to obtaining the same electrode activity of 75% Au and 25% Pt on carbon support as Pt/C has been reported [83]. Regardless of their excellent performance, platinum-based catalysts are too expensive when large-scale applications are considered. Additionally, if platinum shortages are taken into account, it becomes clear that other electrocatalytic materials for DMFC anodes must be found [34,108]. The most popular anodic materials for alkaline media are nickel [106,118,123–126] and cobalt-based electrodes [10,106,107,123,126,127], but other metals, such as gold [35,57], have also been studied. Similar to platinum, their properties can be changed by doping with other elements, both metallic, such as the Pd–Co [128] system and Ni–Cr2O3 [129], and nonmetallic, such as borides [123] and phosphates [106]. The slow kinetics of methanol electrooxidation lead to lower power densities of direct methanol fuel cells. Using anode materials that indirectly oxidize methanol (such as nickel or cobalt) accelerates MOR kinetics and results in a higher power density in fuel cells [130,131]: NiOOH + CH3OH + 1.25 O2 → Ni(OH)2 + CO2 + 1.5 H2O (10) CoOOH+CH3OH+2.5O2 →2Co(OH)2 +2CO2 +3H2O (11)PDF Image | Effect of Anode Material on Electrochemical Oxidation of Alcohols
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