Electrocatalytic Oxidation of Small Carbohydrate Fuels at Pt−Sn Modified Electrodes

Abstract

The electrocatalytic oxidations of water-soluble alcohols and diols and some other small organic molecules (MeOH, EtOH, 2-fluoroethanol, 2,2-difluoroethanol, 2,2,2-trifluoroethanol, CH2(OH)2, ethylene glycol, n-PrOH, i-PrOH, 1,4-butanediol, 1,3-propanediol, 1,2-propanediol, t-BuOH, neopentyl alcohol, benzyl alcohol, 2-Me-1-PrOH, formic acid, acetaldehyde, acetic acid, propionaldehyde, propionic acid, acetone, glycolaldehyde, glyoxal, glycolic acid, glyoxylic acid, oxalic acid) have been compared in aqueous 0.5 M H2SO4 at C and Au electrodes modified with an electrodeposited Pt−Sn catalyst at low (−0.1 to 0.5 V vs SCE) potentials. The Pt−Sn catalyst is electrochemically deposited and forms as a smooth deposit on Au electrodes. At a catalyst loading of ∼0.5 mg/cm2 the electrodeposition of the Pt−Sn catalyst results in formation of adherent ∼0.8 μm size particles on C electrodes. In general, for organic molecules containing only C, H, and O with two or more carbon atoms, the presence of H atoms on both the α- and β-carbon results in a relatively negative potential for onset of catalytic current (usually between −0.1 and −0.2 V vs SCE) at Pt−Sn compared to Pt alone. Of the alcohols and diols studied, formaldehyde (which exists in aqueous solutions as the hydrated form, CH2(OH)2) shows the highest electrocatalytic currents at the Pt−Sn catalyst. The ultimate product, via HCOOH, is CO2. EtOH is oxidized only to acetic acid on Pt−Sn electrodes. The EtOH and n-PrOH oxidation yields, determined by exhaustive electrolysis, are 4 e- per molecule, while i-PrOH yields only 2 e- per molecule. We confirmed the generation of acetic acid, propionic acid, and acetone as the final products for the oxidations of EtOH, n-PrOH, and i-PrOH, respectively, by 13C NMR and/or GCMS. The electron yield of the oxidation of ethylene glycol at Pt−Sn surfaces is only 4 e- per molecule instead of the value of 8 e- per molecule expected for the oxidation of ethylene glycol to oxalic acid. Glycolic acid (CHOCOOH) is the oxidation product by GCMS. This substance is not electrocatalytically oxidized on Pt−Sn at potentials negative of ∼ +0.4 V vs SCE in comparison to the −0.1 V vs SCE onset for ethylene glycol oxidation. MeOH, the molecule with the highest electron yield on Pt−Sn (6 e- per molecule), unfortunately shows the most positive potential onset for oxidation among the group of alcohols compared in this study, while the two intermediates along the path of oxidation to CO2, formaldehyde, and formic acid are oxidized on Pt−Sn at very negative potentials compared to MeOH. Thus, the conversion of MeOH to formaldehyde is the efficiency-determining step in the oxidation of MeOH to CO2.

Novel Nanoscale Ceria–Platinum Composite Electrodes for Direct Alcohol Electro-Oxidation

“…The DAFC is in general a PEM fuel cell in which the alcohol is directly fed into the anode without the requirement of a fuel reformer [3]. The simplicity of the DAFCs has been shown with methanol [8][9][10][11][12][13] and ethanol [14][15][16][17][18][19][20] as well as with other small alcohols and carbohydrates [21]. Although DAFCs offer great promise, they also show several drawbacks.…”

“…The most common alcohol for DAFCs is methanol (Direct Methanol Fuel Cells, DMFCs), although ethanol has been recently gaining considerable attention due to its greater availability and higher energy storage capacity (per volume) than methanol [19,21]. The best performing anode material for the electrochemical oxidation of methanol is platinum, whose high cost puts a great economic challenge into the DAFC design.…”

Electrodeposited MnO2 as electrocatalyst for carbohydrate oxidation

“…Carbohydrates can be considered to be potentially useful in fuel cell application [18] due to the presence of the oxidisable carbonyl group in the molecule, along with the multiple hydroxyl groups. Moreover, the relative abundance of the most important monosaccharide, dextrose, is so high that it has been estimated that there are more dextrose units in nature than the total number of all other organic groups [19].…”

“…The materials used for this purpose include bulk metals (Au, Pt) with different surface treatments [23][24][25][26], metal (Pt, Cu) alloys [18,27], microparticles of metals (Pt, Au, Cu) dispersed on different substrates or incorporated into polymeric matrices [28][29][30][31][32], metal (Ni, W, Ru) oxides [33][34][35], metal (Ru) complex [36] and different types of carbon based electrodes [32,[37][38][39]. However, to qualify as an electrocatalyst for fuel oxidation the electrochemical response needs to remain stable over a considerable period of time.…”

Catalysts for Methanol Oxidation

“…As shown by Gonzalez et al [42], carbon supported nanocatalysts have been studied for the oxidation of methanol and other fuels. There is, however, a problem in the comparison of results, because the catalysts have been prepared by a variety of different methods.…”