DMFC, half-cell reactions

The other member of the family of low-temperature fuel cells utilizing liquid methanol as a fuel and oxygen/air as oxidant is the direct methanol fuel cell (DMFC). The half-cell electrochemical reactions in DMFC are... 

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. 

Methanol is one of the few alcohols that can be fed directly into a fuel cell and can be converted electrochemically at the anode. The DMFC can be fed with a gaseous or liquid fuel feed. The liquid DMFC generally uses a diluted methanol in water mixture (ty pically 1-2 molar) and only a fraction of the methanol is used at the anode (Collins, 2001). The DMFC, like an ordinary battery, provides DC electricity according to the following half reactions. 

The left half of Figure 15.3, thus, depicts the catalytic processes that different fuels imdergo, such as reforming, high-temperature shift and low-temperatore shift, and preferential oxidation (PrOx), which are individually conducted at different temperatures so as to make the most of their thermodynamic limitations and kinetics. In direct fuel ceUs with relatively complex fuels, such as methanol, in fact, many of these reaction steps occur electrocatalylicaUy within the fuel cell, thus accounting for the large oveipotentials, e.g., in a DMFC. 

A direct methanol fuel cell (DMFC) uses liquid methanol (CH3OH) as fuel. The electrochemical half and overall reactions in a DMFC is summarized in Table 4.1. The overall reaction in a DMFC is given as... 

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