Alcohol oxidation DMFCs

DMFCs and direct ethanol fuel cells (DEFCs) are based on the proton exchange membrane fuel cell (PEM FC), where hydrogen is replaced by the alcohol, so that both the principles of the PEMFC and the direct alcohol fuel cell (DAFC), in which the alcohol reacts directly at the fuel cell anode without any reforming process, will be discussed in this chapter. Then, because of the low operating temperatures of these fuel cells working in an acidic environment (due to the protonic membrane), the activation of the alcohol oxidation by convenient catalysts (usually containing platinum) is still a severe problem, which will be discussed in the context of electrocatalysis. One way to overcome this problem is to use an alkaline membrane (conducting, e.g., by the hydroxyl anion, OH ), in which medium the kinetics of the electrochemical reactions involved are faster than in an acidic medium, and then to develop the solid alkaline membrane fuel cell (SAMFC). 

Amraig various anode catalysts developed, ft-Ru alloys are generally cmisidered as the best candidates for H2/CO and alcohol oxidation these alloy catalysts show high CO tolerance and acceptable durability under FC operating conditions. Several commercial ft-Ru alloy nanoparticles supported on carbon black have been available for applications in PEMFCs, DMFCs, and DEFCs. Efforts to improve the activity and stability of ft-Ru alloy catalysts continuously are being made. Recently, the nanocapsule method has been successfully employed to synthesize ft-Ru nanoparticles with... 

Considering (Mily the thermodynamics of the DMFC (used here as a representative of direct alcohol fuel cells), methanol should be oxidized spraitaneously when the potential of the anode is above 0.05 V/SHE. Similarly, oxygen should be reduced spontaneously when the cathode potential is below 1.23 V/SHE, identical to a H2-O2 fuel cell. However, kinetic losses due to side reactions cause a deviation of ideal thermodynamic values and decrease the efficiency of the DMFC. This is presented in Fig. lb, which includes various limiting effects as kinetics, ohmic resistance, alcohol crossover, and mass transport. The anode and cathode overpotentials for alcohol oxidation and oxygen reduction reduce the cell potential and together are responsible for the decay in efficiency of approximately 50 % in DMFCs [13, 27]. 

Alcohol oxidation from Pt is one of the most studied in PCs because it is the basis of the anode reaction in direct methanol PCs. Methanol is oxidized in a six-electron reaction to COj, H and HjO on nanoscale Pt at around 0.4 V to 0.5 vs RHE. Methanol is oxidized more efficiently at lower potentials on PtRu alloys and related materials. The mechanism of the oxidation process on alloy catalysts and the form of the catalysts has been the subject of hundreds of papers and is beyond the scope of this paper. When alcohol is present on a catalyst in low concentrations as an impurity, it can be oxidized by exposing the PC to open-circuit conditions, or implementing various air starvation and transient operation methods discussed in the patent literature cited above. Methanol oxidation to COj and H completes its removal from the catalyst surface. Methanol poisoning at the cathode can also be a problem at DMFC cathodes, as methanol can easily cross over the Nafion. If the PC becomes severely contaminated, even open-circuit potentials might be too low to fully oxidize the methanol, requiring aggressive procedures to oxidize the methanol from the catalysts. 

In fuel cells working with a liquid fuel, usually an alcohol such as methanol (a direct methanol fuel cell - DMFC), ethanol (a direct ethanol fuel cell - DEFC), glycerol (a direct glycerol fuel cell - DGEC), etc., in addition to the necessity to activate the ORR at the cathode, the alcohol oxidation reaction at the anode also involves a high overpotential. This high overpotential is mainly due to the formation, after dissociative adsorption of the alcohol at the catalyst surface, of poisoning species which block the catalytic surface the main one adsorbed is carbon monoxide. ... 

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

Hydrogen is a secondary fuel and, like electricity, is an energy carrier. It is the most electroactive fuel for fuel cells operating at low and intermediate temperatures. Methanol and ethanol are the most electroactive alcohol fuels, and, when they are electro-oxidized directly at the fuel cell anode (instead of being transformed in a hydrogen-rich gas in a fuel processor), the fuel cell is called a DAFC either a DMFC (with methanol) or a DEFC (with ethanol). 

Methanol is the most electro-reactive organic fuel, and, when it is electro-oxidized directly at the fuel anode (instead of to be transformed by steam reforming in a hydrogen-rich gas), the fuel cell is called a DMFC. More generally if the direct oxidation of a given fuel (alcohols, borohydrides, etc.)... 

Shukla et al. [39] developed a MR-DMFC using carbon supported RuSe cathode and PtRu anode, which reached 45 mW.cm at 0.2 V. However, there were little efforts in the open literature reporting improvements in this type of cells, probably because of the difficulty of obtaining selective catalysts for ORR and MOR and also for the oxidation of higher alcohols. 

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... 

Our work on the electrooxidation of alcohols with homogeneous heterobimetallic catalysts was initially motivated by the advances in DMFC anodes 8,21-23). The strategy initially employed for this project was to utilize the bifunctional oxidation mechanism observed in DMFC anodes to design discrete bimetallic complexes as catalysts for the electrooxidation of renewable fuels. These complexes can be viewed as utilizing all of the metal as opposed to bulk metal anodes, where the reaction occurs only at active surface sites. Although bimetallic complexes are not accurate models of the proposed surface binding site on bulk metal anodes, our investigations address the question of whether it is possible to reproduce the essential functions of the proposed electrooxidation mechanism in discrete heterobinuclear complexes 24-26). 

Alcohol fuel cells, such as direct ethanol fuel cell (DMFCs) and direct ethanol fuel cells (DEFCs), become attractive because of high conversion efficiency, low pollution, lightweight, high power density, and applications from small power supplies for electronic devices such as PCs, notebooks, and cellular phones. In a typical alcohol fuel cell, oxidation reaction of... 

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