DMFC, anode polarization voltage

Much worse is the situation with a direct methanol fuel cell (DMFC) anode. For this electrode, jcrit is very low, about 20mAcm (Table 23.1). This means that at a working current of 100 mAcm , a typical DMFC anode operates in the double-Tafel regime, which dramatically increases the anode polarization voltage. 

Regardless of fuel cell type, in a certain range of polarization voltages this rate is well approximated by the Butler-Volmer or Tafel equations. A better (non-Tafel) approximation of the reaction rate for DMFC anode is considered in Section 2.7. 

Suppose that both sides of the DMFC operate in a low-current regime. The half-cell polarization voltages are then determined by Eq. (2.29), where on the anode side ct = c is given by (3.16) and on the cathode side we should set Ct = Ct (Eq. (3.25)). 

This mixed potential is explained in Fig. 5 through an Evans diagram. In an operating fuel cell, along with this polarization close to open circuit voltage (OCV), there are losses due to hydrogen permeation into cathode electrode from anode chambers in PEMFC and methanol crossover in direct methanol fuel cell (DMFC). In a half-cell system, the crossover losses do not exist, but the polarization due to the carbon oxidation or any other contaminant participating in a side-reaction depresses the OCV. 

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