Specific activity, platinum fuel cell

For fuel-cell technology development, it has been important to understand the characteristics and operation of highly dispersed platinum and platinum alloy electrocatalysts. A series of papers on platinum crystallite size determinations in acid environments for oxygen reduction and hydrogen oxidation was published together by Bert, Stonehart, Kinoshita and co-workers.5 The conclusion from these studies was that the specific activity for oxygen reduction on the platinum surface was independent of the size of the platinum crystallite and that there were no crystallite size effects. 

The most important operational features are the relationships between the crystallite sizes of the platinum electrocatalysts to the specific (A.real m"2 Pt) and the mass (Ag 1 Pt) activities. These features are most directly applicable to the efficiency and utilization of the catalyst in operating fuel-cells. 

As part of the early work to find alloys ofplatinum with higher reactivity for oxygen reduction than platinum alone, International Fuel Cells (now UTC Fuel Cells, LLC.) developed some platinum-refractory-metal binary-alloy electrocatalysts. The preferred alloy was a platinum-vanadium combination that had higher specific activity than platinum alone.25 The mechanism for this catalytic enhancement was not understood, and posttest analyses26 at Los Alamos National Laboratory showed that for this binary-alloy, the vanadium component was rapidly leached out, leaving behind only the platinum. The fuel- cell also manifested this catalyst degradation as a loss of performance with time. In this instance, as the vanadium was lost from the alloy, so the performance of the catalyst reverted to that of the platinum catalyst in the absence of vanadium. This process occurs fairly rapidly in terms of the fuel-cell lifetime, i.e., within 1-2000 hours. Such a performance loss means that this Pt-V alloy combination may not be important commercially but it does pose the question, why does the electrocatalytic enhancement for oxygen reduction occur ... 

An EFC consists of two electrodes, anode and cathode, connected by an external load (shown schematically in Figure 5.1). In place of traditional nonselective metal catalysts, such as platinum, biological catalysts (enzymes) are used for fuel oxidation at the anode and oxidant reduction at the cathode. J udicious choice of enzymes allows such reactions to occur under relatively mild conditions (neutral pH, ambient temperature) compared to conventional fuel cells. In addition, the specificity of the enzyme reactions at the anode and cathode can eliminate the need for other components required for conventional fuel cells, such as a case and membrane. Due to the exclusion of such components, enzymatic fuel cells have the capacity to be miniaturized, and consequently micrometer-dimension membraneless EFCs have been developed [7]. In the simplest form, the difference between the formal redox potential (F ) of the active site of the enzymes utilized for the anode and cathode determines the maximum voltage (A ) of the EFC. Ideally enzymes should possess the following qualities. 

In a catalytic reaction, all steps do not equally depend on the surface structure. So, for example, the rate of simple desorption processes is often not markedly affected by the structure of the surface. In catalysis, therefore, reactions are classified into "structure sensitive" and "structure insensitive" [5], usually on the basis of the variation of reactivity with particle size. As an example, the electrocatalytic oxygen reduction at platinum (which is of importance for fuel cells) will be mentioned, where a decrease of specific activity with increasing particle size was reported [6,7]. In a theoretical analysis [8], the kinetics was treated on the (111), (10 0), and (211) facets of several transition metals, and the results were combined with simple models for the geometries of catalytic nanoparticles. Thus, the experimentally observed trend could be well reproduced. 

Platinum metals are an example of highly active catalysts for a great number of electrochemical reactions (i.e., practically, they have no selectivity). Enzymes described in Section 9.2.1 are an example of highly selective catalysts, but they require specific working conditions, which, for the moment, limit their application in technical devices. It can be anticipated that in the future they will be used primarily in commercialized biological fuel cells working with neutral electrolyte solutions (pH near 7) at temperatures of 20 to 40°C. 

Fujitsu has developed a MEA based on an aromatic hydrocarbon solid electrolyte material, coated with a high density of highly active platinum-based nano-particle catalyst, having less than one-tenth of the methanol crossover rate encountered with typical fluorinated polymers. Fig. 10 shows Fujitsu s 15 W micro fuel cell based on a new hydrocarbon solid electrolyte material that enables use of 30% methanol powering a note book PC. The basic specification of a 3.78 W prototype micro fuel cell is given in Table 2. 

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