Alcohol Fuel Cell Active Systems

Direct Alcohol Fuel Cell Active Systems 

In the temperature interval from 25 to 40°C, the performance trend exhibited by the three DAFCs is analogous to that found for the passive cells, with the DEFC being superior to both the DMFC and the DGFC. Unexpectedly, however, increasing the eell temperature to 60°C leads to a reverse order of activity with the highest power density delivered by the DMFC (peak power density of 95 mW cm at 80°C) and the lowest peak power density supplied by the DEFC (73 mW cm at 80°C). In partieular, the latter cell is clearly featured by a sudden drop of voltage after 300 mA cm, which is consistent with a strong contribution of the concentration polarization to the overall cell polarizatioa Below 300 mA cm at 80°C, the DEFC is competitive with the other cells. 

Bianchini and cowoikers have suggested that the MWCNT support, rich of surface caiboxylic acid groups, may be responsible for the observed activity trend of the cells wiA the Pd/MWCNT anode through a specific control of substrate dif- 

Indirect support to the role of the carbon support on the DAFC performance has been provided by following studies in the same laboratory showing that ethanol is a better fuel than methanol or glycerol at any temperatme from 20 to 80°C in identical cells except for containing anodes coated with either Pd-(Ni-Zn) or Pd-(Ni-Zn-P) supported on Vnlcan XC-72.  

The polarization and power density curves of the DAFCs with Pd-(Ni-Zn)/C anodes are shown in Fig. 32. Only slightly inferior results have been obtained for DAFCs containing Pd-(Ni-Zn-P)/C anodes.  

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Direct Alcohol Fuel Cell Active Systems... 

The same problem is encountered during electrosynthesis by the oxidation of a chemical compound. In an electrochemical reactor, the oxidation reaction has to be counterbalanced with a reduction reaction in order to close the electrical circuit. Under these conditions, it is better for industrial applications to use oxygen from air, which is free, as the oxidative agent. Such a system then becomes very close to a fuel cell system, apart from the oxidation reaction that has to be controlled here, whereas the complete oxidation of alcohol into CO2 is sought in direct alcohol fuel cells (DAFCs). For this reason, fuel cell systems will be considered in this chapter to illustrate the important problem of oxygen activation for electrochemical processes. 

There are different kinds of DAFC operation conditions depending of the way the fuel and the oxidant (oxygen/air) are fed into the cell. In complete active fuel cells the liquid fuel (neat alcohol or aqueous solution) is pumped and gas is compressed, using auxiliary pumps and blowers, in order to improve mass transport and reduce concentration polarization losses in the system. On the other hand, in complete passive DAFC the alcohol reaches the anode catalyst layer by natural convection and the cathode breathes oxygen directly from the air. A number of intermediate options have been also studied and tested. 

In conclusion, all of these observations indicate that there is still much room to improve ADAFC performance by developing novel materials and, on the other hand, by optimizing the operational conditions of the fuel cell. Future work should look into a wider range of potential low-cost materials and composites with novel structures and properties, presenting catalytic activity comparable to that of noble metals. The development of new catalyst systems is more likely in alkaline media because of the wide range of options for the materials support and catalyst, as compared to acidic media which offer more limited materials choice. Moreover, efforts have to be addressed to meet the durability targets required for commercial application. More work is needed to optimize the operational fuel cell conditions, by achieving suitable chemical (OH concentration, hydroxyl/alcohol ratio in the fuel stream) and physical (temperature, pressure, flow rate) parameters. 

Yet, the most interesting results for ORR on Fe-Pd/WC catalyst are those obtained in the presence of alcohol. The electrode response for Fe-Pd/WC system in oxygen saturated acid has not been affected even at high concentrations of methanol, whereas the ORR on conventional catalyst Pt/C was completely restrained since the dominant reaction was basically methanol oxidation rally. Such results suggested that Pd-Fe/WC/C could be an excellent candidate for direct methanol fuel cell (DMFC) cathode because of its inert activity toward methanol oxidation as seen in Fig. 23.3. The utilization of a completely inert catalyst to methanol oxidation is one of the important criteria for DMFCs to operate at higher power densities since the issue of methanol crossover is unavoidable and can cause dramatic loss in cell performance, especially with common catalysts that are active... 

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