Ethanol electrocatalysts

Lamy C, Rousseau S, Belgsir EM, Coutanceau C, Leger JM. 2004. Recent progress in the direct ethanol fuel cell Development of new platinum-tin electrocatalysts. Electrochim Acta 49 3901-3908. 

Leger JM, Rousseau S, Coutanceau C, Hahn E, Lamy C. 2(X)5. How bimetallic electrocatalysts does work for reactions involved in fuel cells Example of ethanol oxidation and comparison to methanol. Electrochim Acta 50 5118-5125. 

The alcohol tolerance of O2 reduction by bilirubin oxidase means that membraneless designs should be possible provided that the enzymes and mediators (if required) are immoblized at the electrodes. Minteer and co-workers have made use of NAD -dependent alcohol dehydrogenase enzymes trapped within a tetraaUcylammonium ion-exchanged Nafion film incorporating NAD+/NADH for oxidation of methanol or ethanol [Akers et al., 2005 Topcagic and Minteer, 2006]. The polymer is coated onto an electrode modified with polymethylene green, which acts as an electrocatalyst... 

The need to develop alternative electrocatalysts with low or, even better, without noble metals (platinum), to decrease the costs due to Pt shortage. New nano-structured electrocatalysts (HYPERMEC by ACTA SpA for example, http // www.acta-nanotech.com) [51, 52] have been developed, which are based on non-noble metals, preferentially mixtures of Fe, Co, Ni at the anode, and Ni, Fe or Co alone at the cathode. With ethanol, power densities as high as 140 mW cm-2 at 0.5 V have been obtained at 25 °C with self-breathing cells containing commercial anion-exchange membranes. 

For these low-temperature fuel cells, the development of catalytic materials is essential to activate the electrochemical reactions involved. This concerns the electro-oxidation of the fuel (reformate hydrogen containing some traces of CO, which acts as a poisoning species for the anode catalyst methanol and ethanol, which have a relatively low reactivity at low temperatures) and the electroreduction of the oxidant (oxygen), which is still a source of high energy losses (up to 30-40%) due to the low reactivity of oxygen at the best platinum-based electrocatalysts. 

In the case of ethanol, Pd-based electrocatalysts seem to be slightly superior to Pt-based catalysts for electro-oxidation in alkaline medium [87], whereas methanol oxidation is less activated. Shen and Xu studied the activity of Pd/C promoted with nanocrystalline oxide electrocatalysts (Ce02, C03O4, Mn304 and nickel oxides) in the electro-oxidation of methanol, ethanol, glycerol and EG in alkaline media [88]. They found that such electrocatalysts were superior to Pt-based electrocatalysts in terms of activity and poison tolerance, particularly a Pd-NiO/C electrocatalyst, which led to a negative shift of the onset potential ofthe oxidation of ethanol by ca 300 mV compared... 

It must be stressed that there is still a fair chance of arriving at electrocatalysts that would achieve reasonable anode potentials (say +0.3 V vs RHE) at technically acceptable current densities (say 0.3 to 0.4 A/cm2). The situation for the next higher alcohol, ethanol, however, is almost hopeless. Any work aimed at developing catalysts for anodic oxidation of the much more inert hydrocarbons at low temperatures will certainly be frustrated. 

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 conclusion, the presence of both poisoning species (mainly CO) and intermediate reaction products (AAL, AA) decreases correspondingly the useful energy density of the fuel, and also the power density, since the oxidation current densities are lower than those obtained with the oxidation of methanol and above all of hydrogen. To improve the kinetics of ethanol oxidation would require the development of new electrocatalysts able to break the C-... 

Hable and Wrighton were the first to study the electrocatalytic oxidation of ethanol on Pt-Ru and Pt-Sn catalyst particles in PAni [46]. They found that dispersion of Pt, Pt-Ru, and Pt-Sn in PAni greatly enhanced the oxidation current of ethanol, the Pt-Sn electrocatalyst being far superior to the two others, with oxidation current... 

Figure 11 Tafel plots for the electro-oxidation of 0.1 M ethanol in 0.1 M HCIO4 on different Pt-based electrodes dispersed in a 0.5-pm PAni film containing 600 pgcm of electrocatalysts. 

Fuel cells, due to their higher efficiency in the conversion of chemical into electrical energy vhth respect to thermo-mechanical cycles, are another major area of R D that has emerged in the last decade. Their effective use, ho vever, still requires an intense effort to develop ne v materials and catalysts. Many relevant contributions from catalysis (increase in efficiency of the chemical to electrical energy conversion and the stability of operations, reduce costs of electrocatalysts) are necessary to make a step for vard in the application of fuel cells out of niche areas. This objective also requires the development of efficient fuel cells fuelled directly vith non-toxic liquid chemicals (ethanol, in particular, but also other chemicals such as ethylene glycol are possible). Together vith improvement in other fuel cell components (membranes, in particular), ethanol direct fuel cells require the development of ne v more active and stable electrocatalysts. 

PtSn/C electrocatalysts prepared by different methods for direct ethanol fuel cell... 

PtSn/C electrocatalysts with R Sn atomic ratios of 50 50 and 90 10 were prepared by alcohol-reduction process, using ethylene glycol as solvent and reducing agent, and by borohydride reduction. The electrocatalysts were characterized by EDX, XRD and cyclic voltammetry. The electro-oxidation of ethanol was studied by cyclic voltammetry using the thin porous coating technique. The electrocatalysts performance depends greatly on preparation procedures and R Sn atomic ratios. 

Direct alcohol fuel cells (DAFC) are very attractive as power sources for mobile and portable applications. The alcohol is fed directly into the fuel cell without any previous chemical modification and is oxidized at the anode while oxygen is reduced at the cathode. Methanol has been considered the most promising fuel because it is more efficiently oxidized than other alcohols. Among different electrocatalysts tested in the methanol oxidation, PtRu-based electrocatalysts were the most active [1-3]. In Brazil ethanol is an attractive fuel as it is produced in large quantities from sugar cane and it is much less toxic than methanol. On the other hand, its complete oxidation to CO2 is more difficult than that of methanol due to the difficulty in C-C bond breaking and to the formation of CO-intermediates that poison the platinum anode catalysts. Thus, more active electrocatalysts are essential to enhance the ethanol electrooxidation [3],... 

Recently, Lamy and co-workers [4,5] described that PtSn/C electrocatalysts were more active than PtRu/C electrocatalysts for ethanol oxidation. For electrocatalysts prepared by co-impregnation-H2 reduction and Bonneman methods, they found that the optimum tin composition was in the range of 10-20 at.%. In these conditions, the electrode activity was enhanced and the CO-intermediates coming from ethanol dissociative chemisorption were reduced. Xin and co-workers [6-9] prepared PtRu/C and PtSn/C electrocatalysts by a polyol method and tested for ethanol oxidation. It was observed that the addition of some elements, like W, could improve the PtRu/C electrocatalyst activity. However, the activities of the PtRu/C electrocatalysts were inferior to those of PtSn/C electrocatalysts. It was also found that PtSn/C electrocatalysts with Pt Sn atomic ratios of 60 40 and 50 50 were more active than electrocatalysts with 75 25 and 80 20 atomic ratios. Thus, it seems that the performance of PtSn/C electrocatalysts depends greatly on their preparation procedure. 

In this work RSn/C electrocatalysts with Pt Sn atomic ratios of 50 50 and 90 10 were prepared by two different methods and tested for ethanol oxidation using cyclic voltammetry. 

The performances of PtSn/C electrocatalysts for ethanol eletro-oxidation are shown in Fig. 3. The anodic cyclic voltammetry responses were plotted after subtracting the backgrounds currents [12,13] and the currents values were normalized per gram of platinum, considering that ethanol adsorption and dehydrogenation occur only on platinum sites at room... 

Figure 3 Cyclic voltammetry of PtSn/C and PtRu/C electrocatalysts in 0.5 mol L H2SO4 containing 1.0 mol L of ethanol with a sweep rate of 10 mV s , considering only the anodic sweep.

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