30-cell DMFC stack

As will be reported, as result of continuous efforts to optimize cell components and cell structure in developing DMFCs for portable power applications, we were able to demonstrate 30-cell DMFC stacks operated at 60 °C and fed with ambient air at 2-3 times faradaic stoichiometry, generating a power density of 320 W (active stack volume) during at least the initial week of testing in our laboratory. 

Figure 2.2 A photograph of the 30-cell DMFC stack shown together with a US Quarter for comparison of dimensions. More detailed stack properties and performance parameters for this current stack design are listed in Table 2.3.

From the foregoing discussion, it is clear that, in a DMFC, the air cathode has to be operated under rather challenging conditions, that is, with a low air feed rate at nearly full water saturation. This type of operating conditions can easUy lead to cathode flooding and thus poor and unstable air cathode performance. To secure better air cathode performance, we have made great efforts to improve the ell cathode structure and cathode flow field design to facilitate uniform air distribution and easy water removal. The performance of our 30-cell DMFC stacks operated with dry air feed at low stoichiometry is reported in the following section. 

Three 30-cell DMFC stacks with 45 cm active electrode area were tested under various conditions to evaluate their performance for portable power applications. All three stacks showed nearly identical performance and the results from one stack that had gone through the most complete set of tests are presented here. 

Figure 2.9 Steady-state V-l curves for a 30-cell DMFC stack operated at various temperatures with a 0.5 M methanol solution feed at 125 ml min at the anode and with 0.76 atm dry air feed at 7.35 SLPM at the cathode. The steady state of the stack performance was verified by comparing the V—l curves with that of stack performance over 30 min for each given stack voltage listed in Table 2.2.

Figure 20. Performance characteristics of the International Fuel Cells 10-cell DMFC stack. (After Ref. 112 reproduced by permission of The Electrochemical Society, Inc.)...

Figure 11. Voltage-current curves of a 28-cell DMFC stack with 22 cm electrode area at 60 °C using air as oxidant and methanol as fuel with various concentrations. The results are obtained by simulation from DMFC single cell data.

Table 25. Peak system power, energy efficiency, energy density and open circuit voltages of a DMFC system that contains a 28-cell DMFC stack with 22 cm electrode area using l.OM methanol as fuel and O2 as oxidant operated at different temperatures.

For a net drag coefficient of 4.0, no hydraulic permeation effects, and an electro-osmotic drag coefficient of 3.0, calculate the water crossover and equivalent f)ower lost per day for a 10 M methanol solution at idle in the anode of a 10-cell, 10-cm /cell DMFC stack. 

Fuel cells can run on fuels other than hydrogen. In the direct methanol fuel cell (DMFC), a dilute methanol solution ( 3%) is fed directly into the anode, and a multistep process causes the liberation of protons and electrons together with conversion to water and carbon dioxide. Because no fuel processor is required, the system is conceptually vei"y attractive. However, the multistep process is understandably less rapid than the simpler hydrogen reaction, and this causes the direct methanol fuel cell stack to produce less power and to need more catalyst. 

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

In this paper we report the effect of varying loads on a small size DMFC stack (10 cells with 9 cm active-area each). The transient responses of the stack voltage have been investigated upon variable current load conditions to obtain the information on the dynamic characteristics of the stack. Also, the transient responses of the stack current upon changing fuel flow rates have been monitored to obtain the optimal operating conditions for the staek. 

The Jet Propulsion Laboratory and Giner Inc. have an on-going collaboration to develop electrochemical DMFC stacks. A 5-cell stack (with an active area of the electrode of 25 cm ) was designed and constructed for operation with unpressurized air. " The performance characteristics of the stack at two operating temperatures (60 and 90 °C) and two 1 M methanol flow rates (5 and 2 liter/min), are rather good 2 V at 250 mA/cm at 90 C. The variation in cell-to-cell performance was very small. Efforts are being made at several other laboratories (e.g., LANL, H-Power) to design, construct, and test DMFC stacks. 

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