Gc

ai O

CURRENT DENSITY (A/cnT)

Figure 2. Power density curves for the thin-film catalyzed Nafion 112 assembly, operating at 130°C on oxygen and at 110°C on air.

CURRENT DENSITY (A/cnT)

Figure 2. Power density curves for the thin-film catalyzed Nafion 112 assembly, operating at 130°C on oxygen and at 110°C on air.

Table I. Calculation of Energy Conversion Efficiencies forDMFC and RAFC Systems

Assumptions

(case 1) Converted directly in DMFC, or

(case 2) Steam reformed to H2 and converted in RAFC

Efficiency calculation

Total Eff. =iiv • (nfuel-fc)l = [(Vcell)l/VMe0H/C02] • (tlfuel,fc)l

Total Eff. = Tlref • T|pr0x • *lv • (T|fuel,fc)2

= [(Vccll)2/VMe0H/C02] • (nfuel,fc)2 ' Tlref • ilprox-

Conclusion

To achieve equal overall conversion efficiencies (MeOH to dc power):

(VCell)l = (VCell)2 • [Cnfuel,fc)2/(nfuel,fc)l] * Tlref • %rox-Assuming (tlfUel,fc)l = 0.90, iiref = 0.72 and Tiprox = 0.97, then (VCell)l=0.79(Vcell)2. i.e., same overall energy conversion for DMFC operating at 0.55 V and RAFC at 0.70 V.

Subscripts: 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 Nafion® 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.

Table n. Projected DMFC and RAFC Stack And Systems Characteristics

Stack Power Density (kW/kg) Stack Materials Cost (S/kW)

DMFCa 0.25

RAFCb 1.0

Projected0

0 0

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