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Arrangement of engine room

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Arrangement of engine room

(8) Integrated experiment on fuel supply circuit

For evaluating the dynamic characteristics of the fuel supply circuit, and for further verifying the measures adopted for enhancing load-following capability, an exeriment is currently under way on a model fuel supply system representing a 10-kW class PEFC unit fueled with methanol-water mixture, provided with combined plate type reformer and shift converter, and with CO removal ensured by selective oxidation process. The model is not equipped with fuel cell, and hydrogen gas consumption is simulated by adjusting the rate of gas discharge to system exterior.

POLYMER ELECTROLYTE DIRECT METHANOL FUEL CELLS: AN OPTION FOR TRANSPORTAION APPLICATIONS

S. Gottesfeld, S. J. C. Cleghorn, X. Ren, T. E. Springer, M. S. Wilson and T. A. Zawodzinski Materials Science and Technology Division, MS D429 Los Alamos National Laboratory, Los Alamos, NM 87545

Introduction

In the last few years polymer electrolyte fuel cell (PEFC) technology has advanced to the point of being considered a viable option for primary power sources in electric vehicles. The systems most frequently considered in this context have been based on either hydrogen carried on board the vehicle, or steam-reforming of methanol on board to generate a mixture of hydrogen and CO2. Direct methanol fuel cells (DMFCs), which use a liquid methanol fuel feed, completely avoid the complexity and weight penalties of the reformer. Yet until recently DMFCs have not been considered a serious option for transportation applications, primarily because of the much lower power densities achieved compared with operation on hydrogen rich gaseous feeds. Recent advancements in DMFC research and development have been quite dramatic, however, with the DMFC reaching power densities which are significant fractions of those provided by the reformate/air fuel cell (RAFC). The use of established Pt-Ru anode electrocatalysts and Pt cathode electrocatalysts in polymer electrolyte DMFCs has resulted in very significant enhancements in DMFC performance particularly when such cells are operated at temperatures above 100'C and when catalyst layer composition and structure are optimized. The higher DMFC power densities recently achieved provide a new basis for consideration of DMFCs for transportation applications.

DMFC fabrication and testing at LANL

Thin film catalysts bonded to the membrane by the decal method [1,2] provided our best results in terms of catalyst utilization and DMFC performance. Unsupported Pt-RuOx (Pt:Ru = 1:1) or supported PtRu/C catalysts were used for the anode catalyst and Pt-black or Pt/C was used for the cathode catalyst. Unsupported Pt-Ru anode catalysts yielded the highest overall anode performances. Catalyst inks were prepared by adding 5% Nation solution to the water-wetted metal catalysts. To prepare the membrane/electrode assemblies (MEAs), appropriate amounts of anode and cathode inks were uniformly applied to Teflon decal blanks to give metal catalyst loadings of approximately 2 mg/cm2. The single-cell fuel cell hardware, cell testing and high-frequency resistance measurement [3] systems have been described previously.

Figure 1 shows polymer electrolyte DMFC performances under conditions that take advantage of the significant increase in DMFC performance with temperature but may still be amenable to transportation applications. Air cathodes at 3 atm were used and the cell temperatures were set at 110'C. Figure 1 shows that, with the Nafion 112 MEA, a current of 370 mA/cm2 at 0.5 V cell voltage was obtained with a 1M methanol feed. The low cell resistances measured at 110'C (Figure 1) are apparently brought about by the liquid anode feed in contact with the membrane. The polymer electrolyte DMFC may thus be easier to operate at temperatures above 100 C than the hydrogen/air PEFC. Figure 2 summarizes DMFC power outputs we obtained with oxygen and with air cathodes at 130'C and 110*C, respectively, and shows peak power outputs for this type of DMFC at almost 400 mW/cm2 for the case of an oxygen cathode at 130"C and about 250 mW/cm2 for the air cathode at 110'C.

DMFC vs. Reformer -t- RAFC: A Comparative Evaluation:

The significant increase in demonstrated DMFC performance, as shown above and by other research groups [4,5], has brought the peak power density of the polymer electrolyte DMFC to a level which is only 2-3 times lower than that of a reformate/air fuel cell (RAFC). Consequently, at this point, some simple calculations reveal that the two options, (i) a DMFC stack and, (ii) a methanol reformer + RAFC stack, show comparable overall system characteristics.

CURRENT DENSITY (A cm'2)

Figure 1. Polarization and high frequency resistance curves for a 110'C, 3 atm air cathode DMFC based on thin-film catalyzed membranes. Anodes: 2.2 mg/cm2 Pt-RuOx, 1 M. methanol at 2 ml/min and 1.8 atm. Cathodes: 2.3 mg/cm2 Pt-black.

CURRENT DENSITY (A cm'2)

Figure 1. Polarization and high frequency resistance curves for a 110'C, 3 atm air cathode DMFC based on thin-film catalyzed membranes. Anodes: 2.2 mg/cm2 Pt-RuOx, 1 M. methanol at 2 ml/min and 1.8 atm. Cathodes: 2.3 mg/cm2 Pt-black.

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