Ethanol crossover

An additional problem arises from ethanol crossover through the proton exchange membrane. It results that the platinum cathode experiences a mixed potential, since both the oxygen reduction and ethanol oxidation take place at the same electrode. The cathode potential is therefore lower, leading to a decrease in the cell voltage and a further decrease in the voltage efficiency. 

The Direct Ethanol Fuel Cell (DEFC) is an attractive system since this fuel can be produced by fermentation of sugar-containing raw materials from agriculture. However, despite several advances in recent years, the existing electrocatalyst still possesses very low electrochemical conversion efficiency to CO2 at ambient temperature, particularly at high concentration values of the alcohol molecule (>0.1 mol L ) [3]. In addition to this, the ethanol crossover seems to be even more prominent in the DEFC, causing more severe losses in the fuel cell power. 

In the case of DEFC, Piipadchev and co-authors [28] developed platinum-based (Pt/C) cathode catalysts modified with S, P, and Bi that were highly tolerant to ethanol crossover into the cathode compartment of the fuel cell. In the case of doping Pt/C with these elements, the researchers used ttiphenylphosphine, bismuth nitrate, and thiourea as dopant sources. The catalysts were synthesized by high-temperature treatment of the catalyst-dopant element mixture in an inert atmosphere (further details see Ref. [28]). 

Different from the results on Pt, the curves demonstrate the very high tolerance of the Pd-based catalysts to ethanol during the ORR. According to the authors, this phenomenon might be related to the inactivity of Pd and/or Co for ethanol oxidation reaction, and this might be due to the absence of adsorption of the alcohol and/or intermediate reaction species onto the catalyst surface. So, these results indicate that the utilization of Pd-aUoys as cathode catalysts in a DEFC will help in reducing the high increase in cathode overpotential observed on Pt catalysts due to the ethanol crossover. 

Song et al. [286] studied ethanol crossover through an expanded Nafion 115 membrane at 75 °C, observing a considerable increase of the crossover rate when the ethanol concentratimi rises from 1.5 to 7.5 M. An estimation of the permeability coefficients, neglecting contribution from the hydraulic permeability contribution yields to values from 2 to 7.10 cm. s , which are much lower than those reported for methanol in Fig. 6.18. 

Leykin AY, Ahkrebko OA, Tarasevich MR (2009) Ethanol crossover through alkali-doped polybenzoimidazole membrane. J Membr Sci 328 86-89... 

CFD models of DEFC have been also proposed [188]. Suresh and Jayanti developed an one-dimensional, single phase, isothermal mathematical model for a liquid-feed DEFC, taking into account mass transport and electrochemical phenomena on both the anode side and the cathode side [189]. Tafel kinetics expressions have been used to describe the electrochemical oxidation of ethanol at the anode and the simultaneous ethanol oxidation and ORR at the cathode. The model in particular accounts for the mixed potential effect caused by ethanol cross-over at the cathode and is validated using the data from the literature. Model results show that ethanol crossover can cause a significant loss of cell performance. 

Fig. 8.18 (a) Effect of current density on ethanol crossover rate at different ethanol concentrations at 90 °C. (b) Effect of both current density and ethanol feed eoneentration on ethanol crossover rate at 90 C (Sotffce [190] reprodueed with permission of Elsevier)... 

Andreadis G, Tsiakaras P (2006) Ethanol crossover and direct ethanol PEM fuel cell performance modeling and experimental validation. Chem Eng Sci 61 7497-7508... 

Proton conducting composite membranes with low ethanol crossover... 

FeCo-CNF FD 100-200 nm FeCo-CNF catalysts possess the comparable electrocatalytic activity and better tolerance to ethanol crossover than Pt/C in the oxygen reduction [36]... 

Leykin, A.Y., Shkrebko, O.A., Tarasevich, M.R. (2009) Ethanol crossover through alkali-doped polybenzimidazole membrane. Journal of Membrane Science, 328, 86-89. 

Figure 5.31 Concentration dependence of CD spectra at 205 nm (top) and specific heat (bottom) of DCg.gPC tubules prepared in ethanol-water (70 30) at lipid concentrations of (a) 0.2, (b) 0.5, (c) 1.0, (d) 2.0, and (e) 5.0 mg/ml. These results show crossover from continuous to discontinuous melting as lipid concentration is increased. Reprinted with permission from Ref. 137. Copyright 1997 by the American Chemical Society.

Finally, an additional approach to using hydrocarbon fuels with Ni-based anodes involves using methanol and ethanol, molecules that carry sufficient oxygen to avoid carbon formation.Unlike the case with low-temperature fuel cells, methanol crossover is not an issue with ceramic membranes. Since methanol decomposes very readily to CO and H2. SOFC can operate with a very high performance using this fuel. ° ° In addition, recent work has shown promising performance levels with limited carbon deposition using dimethyl ether as fuel. ° ° ... 

However, solvent viscosity, rather than polarity, has been a useful tool for mechanistic purposes. Although the quantum yield of the ortho-rearranged product of 4-methylphenyl acetate (20) does not change with viscosity of the medium, the formation of 4-methylphenol (22) is highly sensitive to this factor. Thus, its quantum yield is 0.45 in ethanol (1.00 cP) but only 0.02 in Carbowax 600 (109 cP) (Scheme 8 Table 3) [13], This clearly supports the mechanism involving caged radical pairs. A related aspect is the intramolecular nature of the process confirmed by the lack of cross-coupling products in crossover experiments with mixtures of different esters [10]. 

Several AFM studies examined the effect of buffer conditions on the formation of motifs with DNA molecules. For instance, it has been foimd that the multivalent cations induce the condensation of DNA molecules into higher ordered structures, including toroids and rods [122], More specifically, Zn and Mg ions induce the formation of DNA kinked and perfect circles, respectively [123] (Fig. 16). Also, higher concentrations of spermidine induce the formation of complex flower-shaped structures with single crossover points [122] and increased concentrations of ethanol lead to complex and looped structures [ 124] (Fig. 17). 

Solvated crystals are also common in the chemical and pharmaceutical industries. Figure 2-18 shows the room temperamre solubility curve of ibuprofen-lysinate as a function of water content in ethanol. As shown in the figure, the crossover point between anhydrous solid and monohydrate is < 5% water. At room temperamre, ibuprofen-lysinate remains anhydrous when the water content is below 5% and transforms into monohydrate when the water content is above 5%. In this example, solvate and anhydrous materials also have different crystal habits, as shown in Fig. 2-19. 

---