The Direct Methanol Fuel Cell Process
The thermodynamic potential is created through the use of a polymer electrolyte membrane that allows only certain chemical species to pass through it. On one side of this membrane, a methanol and water mixture is fed to an anode catalyst that separates the methanol molecule into hydrogen atoms and carbon dioxide. The separated hydrogen atoms are then typically stripped of their electron, and passed through the membrane to the cathode side of the cell. At the cathode catalyst, the protons (hydrogen atoms without an electron) react with the oxygen in air to form water minus an electron. By connecting a conductive wire from the anode to the cathode side, the electrons stripped from the hydrogen atoms on the anode side can travel to the cathode side and combine with the electron deficient species.
From a thermodynamic perspective, the electrons "want" to travel to the cathode side a specific "amount," that can be quantified as the open circuit voltage. The open circuit voltage is measured with a voltmeter across the cell set to an extremely high resistance. The thermodynamic favoring of reacting the methanol and O 2 into carbon dioxide and water forces a difference in energy to build across the membrane until the system reaches equilibrium. Once this level is reached the components stop reacting, and no additional useful energy is produced.
Useful energy is produced by lowering the voltage across the membrane below the equilibrium value. This is done by placing a resistance on the wire connecting the two sides that is weak enough that current can flow through it. The smaller the voltage difference that is imposed on the fuel cell in this manner, the more current is produced until a proton transport rate limit is reached, after which no additional energy is produced.
The overall reaction occurring in the DMFC is the same as that for the direct combustion of methanol,
i.e; CH 3 OH + 3/2O 2 CO 2 + 2H 2 O
The figure above shows a schematic of the operating principles of fuel cell utilizing methanol as fuel, i.e., a Direct Methanol Fuel Cell (DMFC). When providing current, methanol is electrochemically oxidized at the anode electrocatalyst to produce electrons which travel through the external circuit to the cathode electrocatalyst where they are consumed together with oxygen in a reduction reaction. The circuit is maintained within the cell by the conduction of protons in the electrolyte.