Mergel, J; Janssen, H; Mueller, M; Wilhelm, J; Stolten, D
Direct methanol fuel cells (DMFCs) are attractive for
various applications, above all, however, as replacements for batteries or accumulators. They may
be used in different power classes. A market analysis indicated that the use of a DMFC energy
system in the kW class had the best chance of commercial realization if applied in forklift
trucks for material handling in large distribution centers or warehouses. An advantage of such
energy systems is that there is no need for the relatively time-consuming recharging of the
lead-acid batteries, nor is it necessary to have spare batteries available for multishift
operation. This calls for DMFC energy systems that are capable of replacing the existing Pb
accumulators in terms of space requirements and energy. However, this requires considerable
improvements to be made in terms of power and stability over time of DMFC systems and, in
comparison to their present status, an increase of overall efficiency. Recent cost analyses for
the overall system; for example, show that for the DMFC stack, a durability of at least 5000 h
must be achieved with an overall efficiency for the DMFC system of at least 30%, with the
constraint that the system can be operated in a water-autonomous manner up to an ambient
temperature of 35 degrees C. As part of a joint R&D project with industrial partners, two systems
were constructed and each subjected to long-term testing for 3000 and more than 8000 h,
respectively, with realistic load profiles from driving cycles. In this test, the stack from the
first system, DMFC V 3.3-1, displayed an aging rate of approximately 52 mu V h(-1) at a current
density of 100 mA cm(-2). This corresponds to a performance degradation of 25% over a period of
3,000 h. The DMFC V 3.3-2 system, a modified and optimized version of the first system, also
underwent long-term testing. In this case, the aging rate of the stack was only approximately 9
mu V h(-1) at a current density of 100 mA cm(-2). The system has thus been operated to date for
more than 8000 h under realistic load profiles. [DOI:10.1115/1.4006490]