Alloys of platinum and ruthenium have become common electrocatalysts for fuel cells, because it is believed that alloying ruthenium with platinum will help increase the carbon monoxide tolerance of the platinum catalysts. Alloys of platinum and ruthenium have also been used extensively for DMFC fuel cells, along with hydro — gen/oxygen fuel cells that employ hydrogen gas formed from a reformation process that may have carbon monoxide or carbon monoxide-like by-products. Although

extensive research was done on Pt/Ru alloys on carbon supports and platinum on carbon supports, there was no statistical difference between the selectivity of the two catalysts for ethanol electrooxidation [13]. Figure 10.2 shows a comparison of fuel cell performance for different alcohol fuels employing Pt/Ru alloys as catalysts. It is apparent that methanol performance is better at high current densities (at a current density of 250mA/cm2, the cell voltages are 0.354V for methanol, 0.305V for ethanol, 0.174V for 1-propanol, and 0.054V for 2-propanol [13]), but ethanol performance is better at low current densities (>0.05V at low current densities). The excellent performance of ethanol at low current density is likely due to a decrease in crossover of ethanol versus methanol to the cathode. It is also interesting to note that propanol performance is significantly worse than methanol and ethanol. 1-propanol oxidation forms carbon dioxide and propionaldehyde, but 2-propanol oxidation forms carbon dioxide and acetone [14]. The direct alcohol fuel cells studied in Figure 10.2 are being operated at a temperature of 170°C [13]. This temperature is extremely high (harsh enough that Nafion is not particularly stable and another polymer electrolyte (polybenzimidazole) was used) and is above the temperatures that are realistic for portable power applications, but they provide a benchmark for comparing the 4 alcohols.

PtSn Catalysts

Zhou and coworkers have studied the effect of other alloys on ethanol electro­oxidation. Figure 10.3 shows representative cyclic voltammograms of alloys of

platinum with ruthenium, tungsten, palladium, and tin. These voltammograms were taken at room temperature in solutions that contain 1.0 M ethanol and 0.5 M sulfuric acid. The voltammograms show the largest catalytic activity (current density at the oxidation peak) for PtSn on carbon, but the PtRu on carbon has the lowest over­potential for the ethanol oxidation peaks (0.23 V lower than pure platinum on carbon) [15]. Figure 10.4 shows voltage-current curves and power curves for the same catalysts in a direct ethanol fuel cell at 90°C. The results indicate that Sn, Ru, and W increase the catalytic activity for ethanol oxidation on platinum (max­imum power density of 52.0 mW/cm2, 28.6 mW/cm2, and 16.0mW/cm2, respec­tively, compared to 10.8 mW/cm2 for pure platinum on carbon [15]). Tin and ruthenium are believed to have a bifunctional mechanism to supply surface oxygen containing species for the oxidative removal of carbon monoxide like species that typically passivate the surface of pure platinum [16]. The proposed mechanism for ethanol oxidation at Pt/Sn alloys is shown below [17]:

C2H5OH + Pt(H2O) ^ Pt(C2H5OH) + H2O Pt(C2H5OH) + Pt ^ Pt(CO) + Pt(res) + xH++ xe — Pt(C2H5OH) ^ Pt(CH3CHO) + 2H + + 2e-

Pt(CH3CHO) + SnCl4(OH)2- ^ CH3COOH + SnCl2- + 2H2O + e — + H+ Pt(CO), Pt(res) + SnCl4(OH)2- ^ CO2 + SnCl2- + H2O + Pt H2O + Pt = Pt(OHU + e — + H+

2Pt(OH)a* + SnCl4- = SnCl4(OH)2- + 2Pt

where Pt(res) is an oxidized residue adsorbed to the surface of platinum, Pt(H2O) is water adsorbed to the surface of platinum, Pt(CO) is carbon mon­oxide adsorbed to the surface of platinum, Pt(C2H5OH) is ethanol adsorbed to the surface of platinum, and Pt(CH3CHO) is acetaldehyde adsorbed to the surface of platinum.

After Pt/Sn alloys were determined to be the optimal elemental alloy, Zhou and coworkers examined the importance of tin content and temperature on the fuel cell power curves. Figure 10.5 shows the effect of altering the tin content on the direct ethanol fuel cell performance at a temperature of 60°C. The figure shows both current voltage curves and power curves. The results clearly show that Pt3Sn2 on carbon is the best catalyst choice for 60°C [18]. Figure 10.6 shows the effect of altering tin catalyst content on the fuel cell performance at a tem­perature of 90°C. The results clearly show that Pt2Sn1 on carbon is best for temperatures that are greater than 75°C [18]. Figure 10.5 and Figure 10.6 show that tin content does affect fuel cell performance and temperature affects the catalytic activity of each fuel cell differently. The operating temperature for DEFC

is a function of application. Most portable power applications need to operate between room temperature and 50°C, but performance tends to increase with temperature until crossover and/or polymer electrolyte membrane degradation take over. At 30°C, DEFC have maximum power densities that range from 2 to 10 mW/cm2 [19]. Figure 10.7 shows the effect of a wider range of temperatures (50°C-110°C) for a DEFC with a Pt-Sn (9:1)/C anode. It is important to note that fuel cell performance is a function of temperature and a degradation is not seen at high temperatures [5]. Open circuit potentials do not vary significantly with temperature, but maximum power ranges from 6 to 26 mW/cm2 [5].

Catalyst loading and catalyst supports have also been investigated as para­meters that may affect DEFC performance. Studies in hydrogen/oxygen and DMFC have shown that loading of the catalyst can affect fuel cell performance. If the catalyst loading of the DEFC in Figure 10.5 is changed from 30% metal on vulcanized carbon XC-72 to 60% metal on vulcanized carbon XC-72, the maximum power can increase to 28 mW/cm2 and the open circuit potential can increase from 0.72V to 0.75 V [5]. Research has also shown that transitioning from vulcanized carbon supports (XC-72) to multiwall carbon nanotubes (MWNTs) increases both the open circuit potential and the maximum power density of a DEFC with a platinum/tin alloy catalyst [9]. This is shown in Figure 10.8 where the open power curve shows an increase from 30 mW/cm2 to 38 mW/cm2 with an increase of 80 mV in open circuit potential.

DEFCs can be fabricated by methods similar to DMFCs. The most common format is the membrane electrode assembly (MEA). A MEA is a single assembly that contains the anode, the cathode, and the polymer electrolyte membrane

in ionic contact with each other. MEAs are formed most commonly using a conventional heat pressing method, but they can also be fabricated using a decal transfer method. The conventional method involves sandwiching the PEM between an anode and cathode and heat pressing the sandwich at a temperature above the glass transition temperature of the PEM to melt the electrodes into ionic contact with the PEM. The conventional method shows a 34% decrease in power density over a 10-hour period and delamination of the electrodes from the Nafion [20]. This is likely due to the increased swelling of Nafion in the presence of ethanol, but the decal transfer method only shows a 15% decrease and no delamination, along with no change in resistance [20]. Therefore, the decal transfer method is a better method for forming DEFC MEAs. The decal transfer method involves spray painting the catalyst layer directly onto the polymer electrolyte membrane, instead of onto an electrode support (such as carbon paper) and then heat pressing into the polymer electrolyte membrane.

Researchers have also studied tertiary catalyst systems, but the fuel cell performance has not been greatly affected by adding a third component to the system for alloys containing platinum and ruthenium with a third component of tungsten, tin, or molybdenum [15]. Tertiary catalysts with tungsten and tin did show a measurable increase in power compared to pure Pt/Ru alloys, but both power densities are less than pure Pt/Sn alloys under the same operating condi­tions [15].

CONCLUSIONS

Direct ethanol fuel cells are a relatively new technology for portable power generation. Results have concluded that electrochemical oxidation of ethanol on platinum-based catalysts is not significantly lower than for methanol [21] and the intermediate products of ethanol oxidation are less toxic than methanol oxidation. Although catalytic performance with pure platinum catalysts is low, the perfor­mance of Pt/Sn and Pt/Ru alloys is good. Future research will focus on the development of electrocatalysts that show improved catalytic activity and lower electrode fouling at low and moderate temperatures (room temperature to 50°C). Research on fuel cell lifetimes will also be conducted to study the long-term effects of continuous operation on the catalysts, electrode support, and polymer electrolyte membranes. Improved lifetimes are an issue for both methanol and ethanol, because both oxidation processes produce carbon monoxide and carbon monoxide-like products that adsorb/passivate the catalyst and both alcohols swell the polymer electrolyte membrane, which typically decreases the lifetime and stability of the membrane. Overall, DEFCs are a relatively new technology compared to DMFCs, but ethanol has advantages over methanol in decreased toxicity and environmental issues. From a catalytic perspective, the catalytic rates are similar between ethanol and methanol oxidation, but methanol oxidation is more efficient (produces a larges percentage of carbon dioxide (the complete oxidation product)).