2-Propanol exhibits a substantially higher cell voltage than methanol in a direct liquid fuel cell at a current density less than ca. 100   mA / cm 2 . Although the performance increases with cell temperature, the cell can deliver 690 mV at a current density of 20   mA / cm 2 at room temperature. These features could make 2-propanol an attractive fuel for portable power applications. © 2002 The Electrochemical Society. All rights reserved.

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A reliable, fast-responding, and low-cost methanol concentration sensor has been constructed. The sensor electrochemically measures the concentration of methanol solution used in a direct methanol fuel cell ͑DMFC͒. The novelty of the sensor lies in that it comprises a flexible and bendable composite of several layers, which are wrapped around a tube having an aperture that provides fluid contact between the sensing component and the reactant flow stream. Such a sensor can be easily incorporated into the methanol solution flow loop in a DMFC system.In recent times, there has been appreciable effort to develop and commercialize direct methanol fuel cells ͑DMFCs͒. 1-6 When a DMFC is under operation, methanol is consumed at the anode resulting in a decline in its concentration. Hence, for a DMFC system in practical applications, a sensor is required to monitor the concentration of the methanol solution continuously so that pure methanol is added when the methanol concentration declines to a predetermined value.Sensing methanol concentration is not a trivial task. A few sensing mechanisms have been explored to date. Kumagai et al. proposed to use the open-circuit voltage change caused by the presence of methanol as a means to determine the methanol concentration. 7 The structure of the sensing device is similar to a regular DMFC. When methanol diffuses through the membrane to the air cathode side, it causes a decrease in the cathode open-circuit voltage. The higher the methanol concentration, the more methanol that diffuses through the membrane and, consequently, the cathode open-circuit voltage is lower. We tried to use this method to determine methanol concentration, but found that reproducibility of this technique was relatively poor.Capacitance was also used as a means of monitoring methanol concentration in a mixture of gasoline and methanol. 8,9 Due to the difference in dielectric constants between methanol and gasoline, the capacitance between two electrodes changes with the methanol concentration. Unfortunately, because the dielectric constant difference between water and methanol is much less than that between gasoline and methanol, 10 and the methanol concentration used in a DMFC is normally less than 5 wt %, such a method can hardly provide a satisfactory measure of methanol concentration in water.Recently, the limiting currents from the electrochemical oxidation of methanol have been used by Narayanan et al. [11][12][13]14 respectively, to measure the methanol concentration for DMFCs. Their difference relies on where the oxidation of methanol is controlled to occur. In Barton's design, methanol is oxidized after it diffuses through a Nafion membrane. It was demonstrated that a strong correlation existed between the limiting current and methanol concentration ͑up to 4 M͒ over a temperature range from 40 to 80°C. Primarily depending on the temperature, the transient current response time to the change of methanol concentration was about 10-50 s. In Narayanan's design, methanol is oxidized directly at the met...

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