Kinetic Limitations

Considering only the thermodynamics of the DMFC, in principle methanol should be oxidized spontaneously when the potential of the anode is above 0.046 V with respect to the reversible hydrogen electrode (RHE) (see the discussion of electrochemical thermodynamics in Chapter 3). Similarly, oxygen should be reduced spontaneously when the cathode potential falls below 1.23 V vs. RHE. Hence, the DMFC would produce a cell voltage of 1.18 V at 100% voltage efficiency, independent of the current demand. In reality, the reactions shown in Eqs. (7.1) and (7.2) are both highly activated, and hence poor electrode kinetics (kinetic losses) cause the electrode reactions to deviate from their ideal thermodynamic values in such a way as to bring about a serious penalty on the operational efficiency of the DMFC. This is demonstrated in Fig. 7.4, which breaks down the various limiting effects, including kinetics, resistance, methanol crossover, and mass transport. Ohmic effects relating to the electrolyte and the electrodes have been discussed in Chapter 4.

1.2t

Cathode:

~25% Efficiency Loss

Cathode:

~25% Efficiency Loss

Anode:

~25% Efficiency Loss

Anode:

~25% Efficiency Loss

Kinetic Losses

Electrode Ionic/

Ohmic Resistance

Fuel

Mass Transport n

Electrolyte

Ionic Resistance □

Current (A) Observed Cell Voltage

FIGURE 7.4 Breakdown of anode-, cathode-, and electrolyte-related performance losses in a DMFC.

In order to draw a current from the DMFC, a far more positive potential (overpotential) is required at the anode and a more negative potential (overpotential) at the cathode to accelerate the reactions to a reasonable rate (i.e., to produce a cell current). These are shown in Fig. 7.4 as the dark gray areas (kinetic losses), and their effect on the efficiency of the cell is interpreted as a reduction in the light gray area, which represents the observed voltage from the cell. As Fig. 7.4 demonstrates, the anode and cathode overpotentials reduce the cell potential by approximately similar amounts. Together, they may be responsible for a loss of DMFC efficiency of approximately 50%.

The poor electrode kinetics at the anode and cathode arise from the fact that the electrochemical processes are substantially more complex than they appear in Eqs. (7.1) and (7.2). The kinetics of the oxygen reduction reaction (ORR) are discussed in more detail in Chapters 3, 4 and 6.

A simple argument shows why ORR is a highly activated process: Each O2 molecule requires the transfer of four electrons for complete reduction, the simultaneous transfer of these electrons being highly unlikely. In fact, partial electron transfer takes place leading to the formation of surface intermediates such as superoxide. The application of a platinum electrocatalyst allows the stabilization of these intermediates and allows the reaction to proceed at a reasonable and useful rate (see Chapter 6). In addition, the catalyst may accelerate the reaction by opening up new reaction pathways.

In the case of methanol electro-oxidation at the DMFC anode, the picture is less clear. The electro-oxidation of methanol only occurs at a reasonable rate in the presence of platinum or a platinum-based electrocatalyst. This reaction has remained an active focus of research, and substantial studies into this process have been reported in the literature (Parsons and Vandernoot, 1988). However, a great deal of discrepancy exists among experimental data; this may be due to the wide range of experimental conditions used in the studies.

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Responses

  • Louise
    How do you transport hydrogen?
    1 month ago

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