Discussions of performance targets and efficiencies for DMFCs are complicated due to the wide-ranging conditions, fuel and oxidant sources, and intended appli­cations for DMFCs. In this section, a survey of performance data and examples of target system requirements listed by government agencies are used to give a sense of the state of the art. Also, targets set by researchers in the literature are discussed.

In 2002, Jorissen et al. suggested DMFC performance targets to compete in terms of efficiency with reformate fed PEFCs [24]. The target they set for a DMFC is a power density of 250 mW cm-2 at a cell voltage of 500 mV and that furthermore, parasitic power loss due to methanol crossover should be no more than 50 mA cm-2 at a power density of 250 mW cm-2.

In a 1999 review of advanced electrode materials for use in DMFCs, Lamy and Leger discussed the suitability of a number of energy systems in relation to DMFCs for use in automobiles [17]. Secondary batteries (e. g., Li-ion) are limited by recharge time and power density (100-150 Wh kg-1 at maximum). PEFCs are attractive with specific power densities on the order of 1000 W kg-1 and specific energy density >500 Wh. Energy density of pure H2 is 33 kWh kg-1 but storage concerns make it less attractive and less efficient. Performance characteristics of DMFCs circa 1999 is 200 mA cm-2 at 0.5 V, or 100 mW cm-2 with electrocatalyst loadings under 1 mg cm-2.

Performance targets for a complete DMFC power system were posted in the Spring of 2005 by the U. S. Army Operational Test Command (OTC). The spec­ifications are target requirements for a ruggedized DMFC power plant for use in the field on armored and other military vehicles [25]. The specifications outline threshold requirements and objective targets for the power system. A summary of the requirements are listed in Table 9.1. In an effort to meet the objectives listed in Table 9.1, a 300-W prototype DMFC power plant was developed by T. Valdez and his team at the Jet Propulsion Laboratory [26]. The demonstration

power plant was designed for 100 hours of continuous operation and used 80 cells with active areas of 80 cm2. The electrocatalyst was PtRu at the anode and cathode. The plant generated 370 W during bench testing and had a start-up time of 18 minutes. The plant was operated continuously for 8 hours, generating a lower than expected power of 50 W. The continuous operation test was ended due to water accumulation in the stack exhaust manifold.

Subsequent to testing of the prototype power plant, the stack was torn down and components evaluated. The wettability of the cathodes of the MEAs had increased and evidence of the ruthenium migration was observed. These obser­vations were the impetus to study of the long-term stability of DMFC MEAs. The team at JPL individually ran four MEAs on a single-cell test stand for 250 hours. All of the MEAs showed irreversible voltage decay ranging from 0.2 to 0.6 mV hr-1 at a current density of 100 mA cm-2 that resulted in an average decline in power of 20%. However, unlike when the MEAs were run as compo­nents of the stack in the prototype power plant, the individually run MEAs showed no evidence of electrocatalyst migration. The important issue of electrocatalyst migration will be addressed again in the final section of this chapter.

According to Knights et al. at Ballard Power Systems, fuel cell power plants used in automobile, bus, and stationary applications require operational lifetimes on the order of 4000, 20,000, and 40,000 hours, respectively [27]. The degradation rate of the power supply is set by the beginning-of-life (BOL) and end-of-life (EOL) performances; a degradation rate on the order of 10 to 25 p V hr-1 is common for DMFCs. The group studied the strategy of load cycling in DMFCs to reduce performance degradation caused by water build-up at the cathode with time.

Ball Aerospace is developing a personal DMFC power system to meet the needs of the U. S. foot soldier [28]. It was developed under the Defense Advanced Research Projects Agency (DARPA) Palm Power program and produces average power of 20 W at 12 V and has a 30-W peak power. The unit operates for 50 hours on the fuel provided by one fuel cartridge, and is ten times lighter than the equivalent battery power plant; weighing in at three pounds with full fuel complement.

Yi et al. characterize the changes in MEA morphology of a single-cell DMFC run for a little longer than three days [29]. Long-term stability of the cell and electrocatalyst are important questions. The cell was run at 100 mA cm-2 and suffered from irrecoverable performance degradation, degrading at the rate of 1.0 to 1.5 mV hr-1. Following the run, Yi and his group found signs of delamination between the layers of the MEA and that both of the carbon-supported electrocat­alysts, PtRu/C on the anode and Pt/C on the cathode, had undergone a particle size redistribution resulting in larger particle sizes on average. The redistribution for the PtRu electrocatalyst was more pronounced than for Pt and more severe in the anode.

An assessment of the state of the art in DMFC performance can be made from relevant data from references in this chapter; data are listed in Tables 9.2 and 9.3. Where possible, the data listed from a particular reference includes data for the “best” test cell and the associated control cell. The best test cell is considered the one with highest maximum power density. The control cell is usually of a typical Nafion MEA construction consisting of carbon-supported PtRu on the anode, car­bon-supported Pt on the cathode and a Nafion 115 membrane as the separator. Efforts have been made to include operating conditions and loadings. Where an entry is listed as “n/a” the value for that parameter is not available. That is, the reference does not explicitly state the value of that parameter.