PEFCs most frequently considered for electric vehicles have been based on either hydrogen carried aboard, or steam-reforming of methanol on board to produce H2 + CO2. Direct methanol fuel cells (DMFCs), which use a liquid methanol fuel feed, completely avoid the complexity and weight penalties of the reformer, but have not been considered a serious option until recently, because of much lower power densities. Recent advances in DMFCs have been dramatic, however, with the DMFC reaching power densities which are significant fractions of those provided by reformate/air fuel cells. Use of established Pt-Ru anode electrocatalysts and Pt cathode electrocatalysts in polymer electrolyte DMFCs has resulted in enhanced DMFC performance, particularly when operated above 100 C and when catalyst layer composition and structure are optimized. The higher DMFC power densities recently achieved provide a new basis for considering DMFCs for transportation applications

Cleghorn, S. J. C.

Gottesfeld, S.

Ren, X.

Springer, T. E.

Wilson, M. S.

Zawodzinski, T. A.

UNT Digital Library

LA-UR- 96-3234 Title: Author(s) : Subrnittec !0: POLYMER ELECTROLYTE DIRECT METHANOL FUEL CELLS: AN OPTIONE'OR TRANSPORTATION APPLICATIONS S. Gottesfeld, S. J. C. Cleghor, X. Ren, T. E. Springer M. S. Wilson, T. A. Zawodzinski Fuel Cell Siminar, Kissinme, F1, November 1996 L O ~  Alarnos National Laboratory, an affirmative actionlequal opportunity ernpldyer, is operated by the University of California for the US. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for U.S. Government purposes. The Los Alarnos National Laboratory requests that the publisher identify this article as work performed under the auspices of the U.S. Department of Energy. Form ST No. 2629 836 1 OB1 R5DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, process, or service by trade name, trademark, manufac- turer, or otherwise does not necessarily constitute or impiy its endorsement, recom- mend;rtion, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. - 3 2 3 6  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 C02. 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 [1,21 provided our best results in terms of catalyst utilization and DMFC performance. Unsupported Pt-RuO, (Pt:Ru = 1 : 1) or supported PtRdC 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% Nafion solution to the water-wetted metal catalysts. To prepare the membrandelectrode 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 hydrogedair PEFC. Figure 2 summarizes DMFC power outputs we obtained with oxygen and with air cathodes at 130°C and 1 lOT, 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 + 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 GOTTESFELD ET AL - 2 -  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 + RAFT stack, show comparable overall system characteristics. 1 .o N 1 1 5  u N1 1 7  0.8 P~-RUOX, 2.2 mg cm2 3 0.6 5 D 0.4 sN h v 0.2 0 0 0.2 0.4 0.6 0.8 1.0 1.2 CURRENT DENSITY (A cm") Figure 1. Polarization and high frequency resistance curves for a 1 10°C, 3 atm air cathode DMFC based on thin-film catalyzed membranes. Anodes: 2.2 mg/cm2 Pt-RuO,, 1 M methanol at 2 ml/min and 1.8 atm. Cathodes: 2.3 mg/cm2 Pt-black. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 CURRENT DENSITY (A/cm2) Figure 2. Power density curves for the thin-film catalyzed Nafion 112 assembly, operating at 130°C on oxygen and at 1 10°C on air. GOTTESFELD ET AL, - 3 -  rable I. Calculation of Energy Conversion Efficiencies for DMFC and FL4FC Systems AssumDtions Methanol is either: (case 1) Converted directly in DMFC, or (case 2) Steam reformed to H2 and converted in RAFC Efficiency calculation (case 1, DMFC) (case 2, RAFC) Total Eff. = IV (Ifuel-fch = [(Vce11)1N0MeOWCO2I (Ilfuel,fc)l Total Eff. = qref qprox vv (1fuel,fc)2 = [(Vcell)2NoMeO~C021 (Ifuel,fc)2 qref Iprox. Conclusion To achieve equal overall conversion efficiencies (MeOH to dc power): (Vcell)l= (Vcellh [(1fue1,fc>2/(1fuel,fc)ll qref Iprox- Assuming (vhel,fc)l = 0.90, qref = 0.72 and qprox = 0.97, then (VceIOl = 0.79 (Vcellh. i.e., same overall energy conversion for DMFC operating at 0.55 V and RAFC at 0.70 V. ubscripts: v = voltage, fuel.fc = fuel use in the fuel cell, ref = reforming + shift reactors, prox = preferential oxidation reformer. Table I shows a comparison of overall system efficiencies. The calculation shows that the overall energy conversion efficiencies (methanol chemical energy to DC power) of the two systems are comparable (close to 40%) when the DMFC is operating at 0.55V and the RAFC is operating at 0.70V. This is true assuming: (a) fuel efficiencies of 90% can be reached in the DMFC (100% fuel efficiency assumed for the RAFC), (b) the methanol reformer efficiency (hydrogen energy out/methanol energy in), is 72%, and (c) the overall energy efficiency of the preferential oxidation reactor (PROX) is 97%. The main DMFC parameter that needs significant improvement to reach the DMFC performance level assumed in Table I is the fuel efficiency, which has been significantly smaller (around 50% at 80°C) in DMFCs employing NafionB 1100 membranes. However, recent efforts to fabricate and evaluate DMFC membranes of significantly lower methanol “cross-over’’ rates (yet good protonic conductivity) have provided some promising results. The probability of reaching high DMFC fuel efficiencies following further similar efforts seems significant. The conclusion of a comparable overall energy conversion efficiency for the two systems at the typical operation voltage per cell in each case (DMFC at 0.55V and RAFC at 0.70V) is significant yet obviously depends on the trade-off between the lower DMFC voltage efficiency and the efficiency losses of methanol reforming. Thus, the typically lower DMFC operation voltage is not to be taken as an immediate indication of an inferior overall system energy efficiency. GOTTESFELD ET AL -4- Table 11. Projected DMFC and RAFC Stack And Systems Characteristics ~~ DMFCa R A F C ~  Stack Power Density (kW/kg) 0.25 1 .o Projectedc 200 45 Stack Materials Cost ($/kW) Today 2500 1500 System Energy Densityd ( W a g )  750 750 a) Advanced fuel cell stack is 50 - 75% of total weight. b) Advanced fuel cell stack is 10 - 30% of tota weight. c) Assuming drop in PFSA membrane cost by order of magnitude. d) Assuming, in each case, a 50 kW; 300 kwh system of overall weight 400 kg and overall energy conv. efficiency 40%. Table 11 uses some very rough estimates to compare three other key parameters for the two systems (DMFC and reformer + RAFC): the power densities, stack materials costs and system energy densities. The stack power density for the reformate/& fuel cell may be a factor of three higher, but the stack is only 10-3096 of the total weight of the system vs. 50-75% of the weight expected in the case of the DMFC. The stack materials costs per kW are projected to be four times higher for the DMFC, mainly due to the lower power density, however, the DMFC stack cost is a much larger percentage of the total systems cost. The total system energy density for a 50 kW vehicle with a range of 300 miles assuming average speed of 50 mph (80 kph) is estimated to be similar for both options, around 750 Wh/kg. These estimates are very rough, but it seems that the DMFC could become a serious candidate for transportation applications, provided the following requirements are met: (1) Catalyst loadings are further reduced, (2) Long term stable performances are demonstrated, and (3) Fuel efficiencies are actually increased to the 90% level. ACKNOWLEDGMENT This work was supported by the U. S .  Dept. of Energy, Office of Transportation Technology. REFERENCES 1. M. S. Wilson and S. Gottesfeld, "Thin-film Catalyst Layers for Polymer Electrolyte Fuel Cell Electrodes," J. Appl. Electrochem., Vol. 22, pp. 1-7, 1992. 2. M. S. Wilson, J. Valerio and S .  Gottesfeld, "Low Platinum Loading Electrodes for Polymer Electrolyte Fuel Cells Fabricated Using Thermoplastic Ionomers," Electrochim. Acta, Vol. 40, p. 355, 1995. 3. T. E. Springer, M. S. Wilson and S. Gottesfeld, "Modelling and Experimental Diagnostics in Polymer Electrolyte Fuel Cells," J. Electrochem. SOC., Vol. 140, No. 12, pp. 3513-3526, Dec. 1993. 4. S. Surampudi, S. R. Narayanan, E. Vamos, H. Frank, G. Halpert, A. LaConti, J. Kosek, G. K. Surya Prakash and G. A. Olah, "Advances in Direct Oxidation Methanol Fuel Cells," J. Power Sources, Vol. 47, p. 377, 1994. 5. H. Grune, G. Euft, K. Mund and M. Waidhas, "Research and Development of Low Temperature Fuel Cells at Siemens," Program and Abstracts, Fuel Cell Seminar, p. 474, San Diego, CA, Nov. 28 - Dec. 1, 1994. 

Polymer electrolyte direct methanol fuel cells: an option for transportation applications

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Polymer electrolyte direct methanol fuel cells: an option for transportation applications

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