Electrooxidation of Ethanol

Ethanol is considered as the ideal fuel for the so-called direct alcohol fuel cells (DAFCs). This is because ethanol has a number of advantages over methanol it can be produced in a sustainable manner, easily stored and transported, and is less toxic or corrosive than methanol. The theoretical mass energy of ethanol is 8.0 kWh kg compared to 6.1 kWh kg for methanol. The complete oxidation of ethanol releases 12 electrons per molecule its standard electromotive force E° q =1145V, is similar to that of methanol. 

However, a number of issues, particularly those related to the performance of the electrocatalyst, should be improved in order to implement direct ethanol fuel-cell (DEFC) technology. Among them, the m or challenge is developing electrocatalysts that can break the G-C bond at relative low potentials. 

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.  

Although PtSn/C are the best binary electrocatalysts for the electrooxidation of ethanol, the main reaction products are acetic add (AA) and acetaldehyde (AAL).  

Several approaches to the designing of efficient electrocatalysts for the electrooxidation of ethanol have been reported. Some of them focused on the structure of the catalytic particles. For instance, by generating high-index faceted Pt particles, the catalytic activity and selectivity to COg of Pt/C can be improved.The same is true for PtNi/C catalysts.However, high-index planes are not stable in nanoparticles, hence the mass activity of those catalysts is low. Most studies, however, deal with bimetallic particles, typically Pt-M (M = W, Pd, Rh, Re, with PtSn 

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Gold is generally considered a poor electro-catalyst for oxidation of small alcohols, particularly in acid media. In alkaline media, however, the reactivity increases, which is related to that fact that no poisoning CO-hke species can be formed or adsorbed on the surface [Nishimura et al., 1989 Tremihosi-Filho et al., 1998]. Similar to Pt electrodes, the oxidation of ethanol starts at potentials corresponding to the onset of surface oxidation, emphasizing the key role of surface oxides and hydroxides in the oxidation process. The only product observed upon the electrooxidation of ethanol on Au in an alkaline electrolyte is acetate, the deprotonated form of acetic acid. The lack of carbon dioxide as a reaction product again suggests that adsorbed CO-like species are an essential intermediate in CO2 formation. 

I,eung LWH, Chang SC, WeaverMJ. 1989. Real-time FTTR spectroscopy as an electrochemical mechanistic probe—Electrooxidation of ethanol and related species on well-defined Pt(l 11) surfaces. J Electroanal Chem 266 317-336. 

Tremiliosi-Eilho G, Gonzalez ER, Motheo AJ, Belgsir EM, Leger JM, Lamy C. 1998. Electrooxidation of ethanol on gold Analysis of the reaction products and mechanism. J Electroanal Chem 444 31-39. 

Other electrocatalysts were considered for the electrooxidation of ethanol, such as rhodium, iridium or gold, " " leading to similar results in acid medium. The oxidation of ethanol on rhodium proceeds mainly through the formation of acetic acid and carbon monoxide. Two types of adsorbed CO are formed, i.e., linearly-bonded and bridge-bonded, in a similar amount, at relatively low potentials, then leading rapidly to carbon dioxide when the rhodium surface begins to oxidize, at 0.5-0.7 V/RHE. On gold in acid medium the oxidation reaction leads mainly to the formation of acetaldehyde. " " ... 

In-situ FTIR studies on the electrooxidation of ethanol on polycrystalline Pt [97-99] as well as on single-crystal Pt electrodes [lOO/lOl] have shown the formation of acetaldehyde and acetic acid in addition to cmbon dioxide as soluble products. Figure 29 shows the typical features for thesq/products, which were assigned according to Ihble 1. 

Oliveira Neto A, Linardi M, dos Anjos DM, Tremiliosi-Filho G, Spinace EV (2009) Electrooxidation of ethanol on PtSn/Ce02-C electrocatalyst. J Appl Electrochem 39 1153-1156... 

In particular, the electrooxidation of ethanol over Pt in acidic media has two major hmitations which prevent its viability as it was discussed by Koper [109]. The first relates to the fact that reaction predominantly produces acetate and acetic acid intermediates thus resulting in only 2 and 4 electrons respectively which are only very minor contributions to the possible current. Both are thus unwanted side products for fuel cell applications. The second limitation is that the path to CO2 is rather difficult in that it requires the activation of C-C bond as well as the oxidation of both the CHx and CO intermediates that form. Both of these intermediates tend to inhibit or poison metal surfaces at lower potentials [110]. 

Lai SCS, Kleyn SEF, Rosea V, Koper MTM (2008) Mechanism of the dissociation and electrooxidation of ethanol and acetaldehyde on platinum as studied by SERS. J Phys Chem C 112 19080-19087... 

Shao MH, Adzic RR (2005) Electrooxidation of ethanol on a Pt electrode in acid solutions in situ ATR-SEIRAS study. Electrochim Acta 50 2415-2422... 

Shieh DT, Hwang BJ (1995) Kinetics for electrooxidation of ethanol on thermally prepared ruthenium oxide in alkaline-solution. J Electrochem Soc 142(3) 816-823... 

Same as the Pd-Ag/vulcan C. Pd-Ag/Nb Tii xOi exhibited excellent performance for the oxidation of ethanol and higher durability in alkaline solution compared with Pd-Ag/C and Pd-Ag/commercial Ti02 Compared with those traditional Pd catalysts (i.e., Pd nanoparticles, Pd nanowire, and arrays), this new Raney-like nanoporous Pd nanocatalyst shows superior electrocatalytic activity and stability towards electrooxidation of ethanol... 

NPPd exhibits high catalytic activity for electrooxidation of ethanol... 

Wang Y, Nguyen TS, Liu X, Wang X (2010) Novel palladium-lead (Pd-Pb/C) bimetallic catalysts for electrooxidation of ethanol in alkaline media. J Power Sources 195 2619-2622... 

He Q, Chen W, Mukerjee S, Chen S, Laufek F (2009) Carbon-supported PdM (M=Au and Sn) nanocatalysts for the electrooxidation of ethanol in high pH media. J Power Sources 187 298-304... 

Li G, Pickup PG (2006) The promoting effect of Pb on carbon supported Pt and Pt/Ru catalysts for electrooxidation of ethanol. Electrochim Acta 52 1033-1037... 

Abd-El-Latif AA, Mostafa E, Huxter S, Attard G, Baltruschat H (2010) Electrooxidation of ethanol at polycrystalline and platinum stepped single crystals a study by differential electrochemical mass spectrometry. Electrochim Acta 55(27) 7951-7960... 

As the complete electrooxidation of ethanol in an acid medium yields two molecules of CO2 and 12 electrons per ethanol molecule and involves the cleavage of the C-C bond, which requires rather high activation energy, the anodie eflianol electrooxidation on Pt is very sluggish, especially at low temperatures [99]. Despite significant efforts and numerous studies, the mechanism of flie ethanol electrooxidation reaction still remains unclear some studies are even contradictory. Nevertheless, electrooxidation of ethanol often does not proceed to completion, yielding adsorbed intermediates such as acetaldehyde [100,101] ... 

Much of the effort on the electrooxidation of ethanol has been devoted mainly to identifying the adsorbed intermediates on the electrode and elucidating the reaction mechanism by means of various techniques, as differential electrochemical mass spectrometry, in situ Fourier transform infrared spectroscopy, and electrochemical thermal desorption mass spectroscopy. The established major products include CO2, acetaldehyde, and acetic acid, and it has been reported that methane and ethane have also been detected. Surface-adsorbed CO is still identified as the leading intermediate in ethanol electrooxidation, as it is in the methanol electrooxidation. Other surface intermediates include various Ci and C2 compounds such as ethoxy and acetyl [102]. There is general agreement that ethanol electrooxidation proceeds via a complex multi-step mechanism, which involves a number of adsorbed intermediates and also leads to different byproducts for incomplete ethanol oxidation, as shown in Figure 1.22. 

Figure 1.22. A probable reaction pathway for the electrooxidation of ethanol [103]. (Reproduced by permission of ECS— The Electrochemical Society, from Oliveira Neto A, Giz MJ, Perez J, TicianeUi EA, Gonzalez ER. The eleetrooxidation of ethanol on Pt-Ru and Pt-Mo particles supported on high-surface-area carhon.)...

Vitvitskaya GV, Daniel-Bek VS. Electrooxidation of ethanol in basic media at low values of anodic polarization. Zh Prikl Khim 1965 38 1043-8. 

Delime F, Leger J-M, Lamy C. Enhancement of the electrooxidation of ethanol on a Pt-PEM eleetrode modified by tin. Part I Half cell study. J Appl Electrochem 1999 29 1249-54. 

Wu G, Swaidan R, Cui G. Electrooxidations of ethanol, acetaldehyde and acetic acid using PtRuSn/C catalysts prepared by modified alcohol-reduction process. J Power Sources 2007 172 180-8. 

For the electrooxidation of alcohols in alkaline medium (see Section 6.4), Shen et al. (2006) suggested a platinum or palladium catalyst promoted with 25 wt% of nickel oxide NiO deposited on a carbon-black support. According to then-data, this additive substantially accelerates the electrooxidation of methanol in an alkaline medium. Tarasevich et al. (2005) suggested a Ru-Ni catalyst deposited on carbon black for the electrooxidation of ethanol in an alkaline medium it is considerably more active than pure ruthenium. Under the operating conditions of fuel cells in acidic media as well as in contact with proton-conducting membranes of the Nafion type, the use of nonplatinum catalysts is highly restricted, owing to corrosion problems. 

Remarkably, the scarce pioneering studies of the ethanol electrooxidation reaction already identified acetic acid (AA) and acetaldehyde (AAL) as the major products of the electrooxidation of ethanol in acid medium with a minority production of CO2 [9,10]. In spite of the enormous body of work published during the last years, and of the availability of powerful in situ diffraction and spectroscopy methods coupled to electrochemical techniques (EC-FTIR, OEMS, Raman, X-ray) features such as the actual electrooxidation mechanism, reaction kinetics, the nature of the active site(s) and accurate identification reaction intermediates of the electrooxidation of small organic molecules remain elusive, especially when molecules containing C—C bonds such as ethanol are involved. 

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