DMFC, cell voltage

The theoretical cell voltage of a DMFC at standard conditions is 1.20 V. The materials used in DMFCs are similar to those in PEMFCs. Pt, PtRu, and Nafion membrane are used as cathode catalyst, anode catalyst, and proton transfer membranes, respectively. However, the catalyst loading in a DMFC is much higher than the loading used in H2/air fuel cells, because both side reactions are slow (Pt loadings 4 mg/cm2 for a DMFC, 0.8 mg/cm2 for a H2/air fuel cell). 

United Technologies Fuel Cells is engaged in DMFC development, in competition with Ballard/Johnson Matthey. It is a part in the project by Renault to develop the Scenic vehicle fuel cell. Neither for its PEFC, nor for its DMFC (and MCFC), does UTC Fuel Cells offer product-coloured illustrations. Moreover, its literature or listed web site does not deal with the cell voltage reversal problem, mentioned in Ballard patents above in connection with fuel cell bus operation. Accordingly it is not possible for the author to portray the UTC Fuel Cells scheme of things. 

Figure 3.5 shows a plot of cell voltage and power density for the micro-DMFC at the flow rate of 80 pL min" . A maximum power density of 1.4 mW cm at 0.25 V was obtained for the 2 M methanol-saturated oxygen. In the voltage-current curve, an abrupt drop of the cell voltage at approximately 8 mAcm was confirmed, suggesting... 

The cell voltage in air at 1.0 M concentration of methanol solution was stable during 300 hours of operation (510 mV at 100 mA/cm ). That is 50 mV higher than the performance of commercially produced DMFCs at the same current density. 

Figure 9. Plots of cell voltage, Faradic efficiency and energy efficiency versus discharge current for a DMFC single cell at 60 °C using air as oxidant. The methanol concentrations are A, 0.5M B, l.OM C, 2.0M and D, 3.0M, respectively.

Figiire 20. Comparison of power-current curves among DMFC single cell, stack and system. The simulated DMFC system contains a 28-cell stack with 22 cm electrode area using l.OM methanol at 60 °C. The points are obtained from a DMFC single cell result. The cell voltage was limited at 0.28V. 

---