Direct methanol fuel cells operation methods

Fuel cells incorporating lithographic methods and masking/deposition/etching protocols have been fabricated on Si wafers and thereby satisfy two critical needs in a standard fuel cell collection of electrons (current collectors) and controlling the flow field of fuel and oxidant. Kelley et al. produced a miniature direct methanol fuel cell (DMFC) with a current— voltage and fuel utilization performance that matched standard-sized DMFCs prepared in-lab.A working volume for the miniature DMFC of 12 mm was reported, with an operational performance of 822 W h kg at 70 °C. ... 

The methanol crossover rates of operating direct methanol fuel cells possessing PSSA-PVDF membranes has been measured at different temperatures and is shown in Fig. 1.83. This was accomplished by measuring the CO2 content of the cathodic exit stream with an on-line CO2 analyzer. The methanol crossover values obtained with this technique correlated well with results obtained with a chemical analytical method involving the reaction of CO2 with Ba(OH)2 to produce BaCOs. The methanol crossover rates observed with a MEA fabricated with a PSSA-PVDF membrane (MEA 2) and using a 1 x 1 ... 

In the case of direct methanol fuel cells, compared with oxygen reduction, methanol oxidation accounts for the main activation loss because this process involves six-electron transfer per methanol molecule and catalyst self-poison when Pt alone was used from the adsorbed intermediate products such as COads-From the thermodynamic point of view, methanol electrooxidation is driven due to the negative Gibbs free energy change in the fuel cell. On the other hand, in the real operation conditions, its rate is obviously limited by the sluggish reaction kinetics. In order to speed up the anode reaction rate, it is necessary to develop an effective electrocatalyst with a high activity to methanol electrooxidation. Carbon-supported (XC-72C, Cabot Corp.) PtRu, PtPd, PtW, and PtSn were prepared by the modified polyol method as already described [58]. Pt content in all the catalysts was 20 wt%. 

At the beginning of the 1980s, the interest in DMFC arose and methanol was used as fuel for high-temperature fuel cells [12]. Nowadays, PEM fuel cells with Nafion as electrolyte appear to be well suited for the direct oxidation of methanol. There are, however, some major problems in adapting a PEMFC to operate with methanol. The catalytic material of the anode has to be improved in order to avoid the loss of activity because of the formation of by-products. As in the case of a PEMFC using hydrogen from a reformer as fuel, a method to improve the anodic material is the use of Pt-Ru or Pt-Ru-Sn mixtures [8]. A particular problem of the DMFC is cross-diffusion of methanol through the electrolyte. 

PANI-NTs synthesized by a template method on commercial carbon cloth have been used as the catalyst support for Pt particles for the electro-oxidation of methanol [501]. The Pt-incorporated PANl-NT electrode exhibited excellent catalytic activity and stabUity compared to 20 wt% Pt supported on VulcanXC 72R carbon and Pt supported on a conventional PANI electrode. The electrode fabrication used in this investigation is particularly attractive to adopt in solid polymer electrolyte-based fuel cells, which arc usually operated under methanol or hydrogen. The higher thermal stabUity of y-Mn02 nanoparticles-coated PANI-NFs on carbon electrodes and their activity in formic acid oxidation pomits the realization of Pt-free anodes for formic acid fuel cells [260]. The exceUent electrocatalytic activity of Pd/ PANI-NFs film has recently been confirmed in the electro-oxidation reactions of formic acid in acidic media, and ethanol/methanol in alkaline medium, making it a potential candidate for direct fuel cells in both acidic and alkaline media [502]. 

The accuracy of the A a method depends on the availability of a good reference spectrum, most favourably a spectrum obtained from the blank, adsorbate free metallic snrface. Unfortunately, for in-situ measurements in a working fuel cell the situation is not so straightforward, as it is difficult to find an appropriate reference spectram where no species, or at least a controlled amount, are adsorbed on the surface. The specific measmes taken for methanol and reformate operation in a working fuel cell, i.e., when there is CO, H and 0[H] co-adsorption, are described below. More specifically, we illustrate the Ap XANES technique to follow the coverage of specific adsorbates on carbon snpported Pt and different Pt/Rn bimetallic systems in pnre H2, in simulated reformate (150 ppm CO in H2) and in direct methanol operation as a function of current in an operating PEM fuel cell. 

Platinum-based materials (Pt or Pt alloys) are by far the best catalysts for the hydrogen and methanol oxidation (Eqs. (5) and (7)) as well as oxygen reduction reactions (Eqs. (6) and (8)). Unfortunately, Pt is a precious, very expensive metal, which limits the widespread commercialization of Pt-based fuel cells. Besides, the stability of Pt and Pt alloys becomes a serious problem for long-term operation of the cells. Hence, extensive research is underway to overcome these difficulties. The works are going in two directions. Firstly, the new methods to ensure consumption of smaller amounts of platinum and at the same time providing a more stable and effective catalyst are developed. And secondly, the new... 

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