Alcohols, electrooxidation

Simultaneously with the investigations on Pt, the reactivity of the primary C3 alcohols on gold elecrodes in acid media (0.5 mol dm H2SO4) was studied by the same authors. The results obtained, in agreement with previous observations, confirm once more that no reactions with n-propanol occur at a gold electrode in acid medium. Bulk allyl alcohol oxidation leads to the formation of CO2 and acrolein, while propargyl alcohol electrooxidation produces CO2 and propargyl aldehyde. Some of the main conclusions are as follows ... 

An efficient ethanol electrooxidation catalyst should combine at least two features (i) high tolerance to CO and other intermediate species generated over the surface of the electrocatalyst during alcohol electrooxidation and (ii) ability to break the C-C bond of the ethanol molecule under mild conditions. The most relevant features for the designing of CO tolerant electrocatalysts have been described above namely, Pt modification with more oxophilic metals such as Ru, Mo or Sn renders the best electrocatalysts. This is because such oxophilic atoms promote the formation of -OfT. species (involved in the CO j oxidation reaction) at potentials that are more negative than that on pure Pt (Eq. 9.17). Among those, Sn-modified Pt electrocatalysts are the most active formulations. There is also widespread consensus that the PtsSn phase is the most active one in the CO reaction and early stages of the ethanol electrooxidation process. ... 

The faster kinetics of alcohol oxidation and oxygen reduction reactions in alkaline direct alcohol fuel cells opens up the possibility of using less expensive Pt-free catalysts, as nickel, gold, palladium and their alloys [30]. Thus, the cost of ADAFC could be potentially lower compared to the acid DAFC technology if non-precious metal alloys are used for the alcohol electrooxidation, being the nanoparticulated Ni-Fe-Co alloys developed by Acta (Italy) with the trade name of HYPERMEC a good example. 

As mentioned above, the alcohol crossover from the anode to the cathode is a important problems to be overcome to improve the DAFC performance. This is due to the fact that the commonly used Pt-based cathode electrocatalysts are also active for the adsorption and oxidation of methanol [1]. So, in addition to the resulting mixed potential at the cathode, there is a decrease in the fuel utilization. Therefore, considering the above exposed reactions for the alcohol electrooxidation, and the features that govern the ORR electrocatalytic activity, as discussed in the Sect. 5.2, it is ready to conclude the importance of the modification of the active ORR electrocatalyst surfaces in order to inhibit the methanol or ethanol oxidative adsorption steps. In the next sections, some recent materials being developed to overcome the problems caused by the alcohol crossover will be presented. 

Unlike the relative abrmdance of mechanistic studies in acidic media, very few works dealing with alcohol electrooxidation on Pt-based systems in alkaline media have been published in the relevant literatirre, while studies involving Pd-based catalysts are virtrrally absent rmless one cortsiders some papers... 

Y. (2013) Highly ordered mesoporous carbons as the support for Pt catalysts towards alcohol electrooxidation the combined effect of pore size and electrical conductivity. Int. J. Hydrogen Energy, 38, 1405-1412. 

Xu CW, Tian ZQ, Shen PK, Jiang SP (2008) Oxide (Ce02, NiO, Co(3)0(4) and Mn304)-promoted Pd/C electrocatalysts for alcohol electrooxidation in alkaline media. Electrochim Acta 53(5) 2610-2618... 

Palladium-Based Nanocatalysts for Alcohol Electrooxidation in Alkaline Media... 

The research and development of nanostructured electrode materials for improved performance of the direct alcohol fuel cells (DAFCs) in alkaline electrolytes has continued to grow. Palladium-based nanocatalysts, in particular, have continued to receive much research attention because of their unique properties in alcohol electrooxidation in alkaline media compared to their platinum-based counterparts [1]. 

Palladium is more abundant in nature and sells at half the current market price of platinum. Unlike Pt, the Pd-based electrocatalysts are more active towards the oxidation of a plethora of substrates in alkaline media. The high activity of Pd in alkaline media is advantageous considering that non-noble metals are sufficiently stable in alkaline for electrochemical applications. Importantly, it is believed that the integration of Pd with non-noble metals (as bimetallic or ternary catalysts) can remarkably reduce the cost of the membrane electrode assemblies (MEAs) and boost the widespread application or commercialization of DAFCs [1]. Palladium has proved to be a better catalyst for alcohol electrooxidation in alkaline electrolytes than Pt [2]. Palladium activity towards the electrooxidation of low-molecular weight alcohols can be enhanced by the presence of a second or third metal, either alloyed or in the oxide form [3]. 

Much effort has been made to enhance the performance of Pd—M (where M = second metal) for alcohol electrooxidation. These efforts were persuaded by the bifunctional mechanism and the electronic/Ugand effect, which can be explained by either the electron density, electronegativity, density functional theory, or d-band theory [6-17]. For example, according to the bifunctional theory of electrocatalysis, the oxidation of a primary alcohol to CO2 and R-COOH (or and R-COO in... 

Yan Z, He G, Zhang G, Meng H, Shen PK (2010) Pd nanoparticles suppmted on ultra high surface area honey-comb-Uke carbon for alcohol electrooxidation, hit J Hydrogen Energy 35 3263-3269... 

Lu J, Lu S, Wang D, Yang M, Liu Z, Xu C, Jiang SP (2009) Nanostructured PdxPtl-x/Ti anodes prepared by electrodeposition for alcohol electrooxidation. Electrochim Acta 54 ... 

Ciszevski and Milczarek reported roughly similar oxidation onset potentials for methanol and ethanol in 0.1 M NaOH using Ni(II) tetrakis(3-methoxy-4-hydroxyphenyl) porphyrin film (poly-NiTMHPP). However, they explored catalytic oxidations up to 0.5 M methanol concentration [207]. Since the alcohol electrooxidation occurs at potentials more positive than the redox potential of Ni(III)/Ni(II) (i.e., Ni(III) is the electrocatalytically active form), a direction to improve the performance would be to find macrocycles that negatively shift the Ni(III)/Ni(II) potential. Another variant would be to develop mixed macrocycle catalysts by incorporating elements that promote OHad formation, since on Ni polymer films the methanol oxidation was strongly dependent on flie OH" concentration [207]. 

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