Method of measuring methanol concentration in an arqueous solution

#The present invention relates to a method of measuring methanol concentration in an aqueous solution, and more particularly to methods and apparatus for monitoring methanol concentration in a direct methanol fuel cell system.

#A fuel cell is a galvanic cell that generates electrical energy by converting chemical energy, derived from a fuel supplied to the cell, directly into electrical energy by an electrochemical process in which the fuel is oxidized in the cell. A typical fuel cell includes an anode, a cathode, electrocatalysts and an electrolyte housed in a casing. The fuel material and oxidant are continuously and independently supplied to the anodes and cathodes, respectively, where the fuel and oxidant react electrochemically to generate a useable electric energy. The reaction by-products are withdrawn from the cell.

#A great advantage of a fuel cell is that it converts chemical energy directly to electrical energy without the necessity of undergoing any intermediate steps, for example, combustion of a hydrocarbon or carbonaceous fuel as takes place in a thermal power station. A fuel cell reactor may comprise a single-cell, or a multi-cell stack. In either case, the membrane/electrode assembly (MEA), comprising the proton-conducting membrane (the electrolyte), the anode and cathode catalyst layers, and gas diffusion layers, is typically sandwiched between two highly electrically conductive flow field plates.

#The preferred embodiments of the present invention will be described with reference to the accompanying drawings in which like numerals refer to the same parts in the several views and in which:

#FIG. 1 is a schematic diagram of a direct methanol fuel cell (DMFC) stack system with a methanol concentration measurement sensor.

#FIG. 2 is a schematic diagram of a DMFC single cell as a sensor.

#The preferred embodiments of the present invention will now be described with reference to the accompanying figures.

#In a DMFC, an aqueous methanol solution is directly used as the fuel. The methanol solution is fed to the anode side of the cell, while air is introduced at the cathode. It has been found that when the fuel cell is operated at an open circuit state, that is, the fuel cell is not connected to any external circuit, the methanol concentration in the aqueous methanol solution fed to the anode is related to the open circuit voltage. The methanol introduced at the anode will diffuse (crossover) through the proton conducting membrane to the cathode. Because the fuel cell is at an open circuit state, there is no electrochemical reaction taking place at the anode (at an electrothermodynamic equilibrium state). However, the methanol diffused from the anode will be oxidized at the cathode by oxygen in the air stream fed to the cathode chamber.

#FIG. 1 shows, in a schematic way, a DMFC system. A dilute aqueous methanol solution in reservoir 1 is pumped through a pump 2 into the anode chamber of the DMFC stack 3. Unreacted aqueous methanol solution is recycled back to reservoir 1 through tubing 4. A small portion of the aqueous methanol solution is fed into the sensor fuel cell through tubing 6. Unreacted solution from the sensor fuel cell is recycled back into the recycle tubing 4 through tubing 7. The byproduct, carbon dioxide, is vented through tuning 8 to a residual methanol recovery device (not shown). At the same time, air is pumped through pump 9 into the cathode side of the fuel cell stack. Residual air and by-product water flow into a condenser 10. The residual air is vented through tubing 11 and water is recycled back to a water reservoir 12 through tubing 13. A small portion of air from pump 9 flows into the cathode chamber of the sensor fuel cell through tubing 14 and the residual air is vented through tubing 15.