Reaction
The DMFC relies upon the oxidation of methanol on a catalyst layer to form carbon dioxide. Water is consumed at the anode and is produced at the cathode. Protons (H+) are transported across the proton exchange membrane - often made from Nafion - to the cathode where they react with oxygen to produce water. Electrons are transported through an external circuit from anode to cathode, providing power to connected devices.
The half-reactions are:
| Equation | |
|---|---|
| Anode | |
| Cathode | |
| Overall reaction |
Methanol and water are adsorbed on a catalyst usually made of platinum and ruthenium particles, and lose protons until carbon dioxide is formed. As water is consumed at the anode in the reaction, pure methanol cannot be used without provision of water via either passive transport such as back diffusion (osmosis), or active transport such as pumping. The need for water limits the energy density of the fuel.
Currently, platinum is used as a catalyst for both half-reactions. This contributes to the loss of cell voltage potential, as any methanol that is present in the cathode chamber will oxidize. If another catalyst could be found for the reduction of oxygen, the problem of methanol crossover would likely be significantly lessened. Furthermore, platinum is very expensive and contributes to the high cost per kilowatt of these cells. During the methanol oxidation reaction carbon monoxide (CO) is formed, which strongly adsorbs onto the platinum catalyst, reducing the surface area and thus the performance of the cell. The addition of another components, such as ruthenium or gold, to the catalyst tends to ameliorate this problem because, according to the most well-established theory in the field, these catalysts oxidize water to yield OH radicals: H2O → OH• + H+ + e-. The OH species from the oxidized water molecule oxidizes CO to produce CO2 which can then be released as a gas: CO + OH• → CO2 + H+ + e-.
Using these OH groups in the half reactions, they are also expressed as:
| Equation | |
|---|---|
| Anode | |
| Cathode | |
| Overall reaction |
Read more about this topic: Direct Methanol Fuel Cell
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