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Fig. 1. Methanol concentration expression conversion at 20 °C. Different concentration expressions can be converted based on the molar mass of both solute and solvent and the density of the solutions. The conversion relationships between the molarity of a methanol concentration and the mole fraction, as well as the volume fraction, weight percent and molality of methanol concentration in aqueous solution at 20°C are calculated and plotted in Fig. 1.

Figure 1 Methanol concentration expression conversion at 20 °C. Different concentration expressions can be converted based on the molar mass of both solute and solvent and the density of the solutions. The conversion relationships between the molarity of a methanol concentration and the mole fraction, as well as the volume fraction, weight percent and molality of methanol concentration in aqueous solution at 20°C are calculated and plotted in Fig. 1.

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Fig. 2. Schematic of the electrochemical methanol sensor. The fundamental construction of the electrochemical sensor is the same as a conventional polymer electrolyte membrane-based direct methanol fuel cell, as shown in Fig. 2. The sensing cell consists of a polymer electrolyte membrane (usually Nafion) coated with Pt/Ru (anode) and Pt (cathode) on each side, respec- tively. A porous electrode backing substrate (diffusion layer) such as porous carbon fibre paper, is hot-pressed onto each side of the coated membrane, forming a membrane electrode assem- bly (MEA). The anode perforated conductive plate serves as a mechanical support and a current collector, and also acts as a fuel supplier. The cathode perforated conductive plate supports the cathode side of the MEA and is configured for air access to the cathode side or hydrogen by-product exhaust from the cathode, and also serves as a current collector. A dilute aque- ous methanol solution is fed into the anode side of the sensor and diffuses through the diffusion layer to oxidize at the anode Pt/Ru—Nafion interface of the MEA. On the cathode side, air diffuses through the perforated plate and diffusion layer to the Pt—Nafion interface of the MEA, where water is produced by
Fig. 2. Schematic of the electrochemical methanol sensor. The fundamental construction of the electrochemical sensor is the same as a conventional polymer electrolyte membrane-based direct methanol fuel cell, as shown in Fig. 2. The sensing cell consists of a polymer electrolyte membrane (usually Nafion) coated with Pt/Ru (anode) and Pt (cathode) on each side, respec- tively. A porous electrode backing substrate (diffusion layer) such as porous carbon fibre paper, is hot-pressed onto each side of the coated membrane, forming a membrane electrode assem- bly (MEA). The anode perforated conductive plate serves as a mechanical support and a current collector, and also acts as a fuel supplier. The cathode perforated conductive plate supports the cathode side of the MEA and is configured for air access to the cathode side or hydrogen by-product exhaust from the cathode, and also serves as a current collector. A dilute aque- ous methanol solution is fed into the anode side of the sensor and diffuses through the diffusion layer to oxidize at the anode Pt/Ru—Nafion interface of the MEA. On the cathode side, air diffuses through the perforated plate and diffusion layer to the Pt—Nafion interface of the MEA, where water is produced by
Fig. 3. Details of construction of the methanol sensor, reproduced by permission of The Electrochemical Society, Inc., courtesy of [4,8]. Depending on the reaction mechanism, the electrochemical sensors function by measuring the methanol oxidation current of a sensing cell under a constant potential (oxidation current type) or by measuring the operating characteristics of a small capacity unit cell such as open-circuit voltage, short-circuit current, or operating voltage under a constant load (fuel cell-based type).
Fig. 3. Details of construction of the methanol sensor, reproduced by permission of The Electrochemical Society, Inc., courtesy of [4,8]. Depending on the reaction mechanism, the electrochemical sensors function by measuring the methanol oxidation current of a sensing cell under a constant potential (oxidation current type) or by measuring the operating characteristics of a small capacity unit cell such as open-circuit voltage, short-circuit current, or operating voltage under a constant load (fuel cell-based type).
Fig. 4. Polarization curve for methanol oxidation, reproduced by permission of The Electrochemical Society, Inc., courtesy of [4].
Fig. 4. Polarization curve for methanol oxidation, reproduced by permission of The Electrochemical Society, Inc., courtesy of [4].
Fig. 5. Open-circuit voltage of a unit cell vs. methanol concentration, courtesy of [9].
Fig. 5. Open-circuit voltage of a unit cell vs. methanol concentration, courtesy of [9].
Fig. 6. Short-circuit current vs. methanol concentration, courtesy of [11]. Rte eet ae i a ee Sapreties Tn US patent 6,488,837 a methanol electro- oxidation anod and an air electro-reduction cathode were shorted togethe through an external electrical connection [11]. Methanol acces: from the aqueous solution to the anode catalyst side of the MEA was limited by diffusion through a physical barrier consistin; of an aperture opening and a porous medium. In this senso all the methanol that reaches the anode catalyst is completel consumed at the anode (no cross-over) with concomitant oxy gen reduction at the cathode. The measured short-circuit curren response shows a good linear relationship with the methano concentration up to 1 M, as shown in Fig. 6.
Fig. 6. Short-circuit current vs. methanol concentration, courtesy of [11]. Rte eet ae i a ee Sapreties Tn US patent 6,488,837 a methanol electro- oxidation anod and an air electro-reduction cathode were shorted togethe through an external electrical connection [11]. Methanol acces: from the aqueous solution to the anode catalyst side of the MEA was limited by diffusion through a physical barrier consistin; of an aperture opening and a porous medium. In this senso all the methanol that reaches the anode catalyst is completel consumed at the anode (no cross-over) with concomitant oxy gen reduction at the cathode. The measured short-circuit curren response shows a good linear relationship with the methano concentration up to 1 M, as shown in Fig. 6.
Fig. 7. Fuel cell current vs. methanol concentration, courtesy of [13].
Fig. 7. Fuel cell current vs. methanol concentration, courtesy of [13].
Fig. 8. Density of aqueous methanol solutions vs. molarity of methanol at 20 °C [21]. Enoksson and Sparks developed a MEMS density sensor based on the resonant vibration of a U-shaped silicon micro- tube [30,31]. For this type of sensor, the tube etched on a silicon wafer is continuously loaded with the fluid. A change of the liq- uid density in the tube changes the mass loading of the vibrating
Fig. 8. Density of aqueous methanol solutions vs. molarity of methanol at 20 °C [21]. Enoksson and Sparks developed a MEMS density sensor based on the resonant vibration of a U-shaped silicon micro- tube [30,31]. For this type of sensor, the tube etched on a silicon wafer is continuously loaded with the fluid. A change of the liq- uid density in the tube changes the mass loading of the vibrating
Fig. 10. Relative dielectric constant of aqueous methanol solutions vs. molarity of methanol in aqueous solution at 23 °C [21]. The dielectric properties of aqueous methanol solutions have been well investigated by Bao, Mashimo, Sato and Bolund [17-20]. The static dielectric constant of these solutions varies in accordance with the methanol concentration, as shown in Fig. 10. The dielectric constant (or the methanol concentra-
Fig. 10. Relative dielectric constant of aqueous methanol solutions vs. molarity of methanol in aqueous solution at 23 °C [21]. The dielectric properties of aqueous methanol solutions have been well investigated by Bao, Mashimo, Sato and Bolund [17-20]. The static dielectric constant of these solutions varies in accordance with the methanol concentration, as shown in Fig. 10. The dielectric constant (or the methanol concentra-
Fig. 9. Cross-sectional illustration of a MEMS density sensor, © 2005 IEEE, courtesy of [30].
Fig. 9. Cross-sectional illustration of a MEMS density sensor, © 2005 IEEE, courtesy of [30].
Fig. 11. Measured dielectric spectra of aqueous methanol solution at 20°C, © 2005 IEEE, courtesy of [37].
Fig. 11. Measured dielectric spectra of aqueous methanol solution at 20°C, © 2005 IEEE, courtesy of [37].
Fig. 13. Viscosity of aqueous methanol solutions vs. molarity of methanol at 20°C [21].
Fig. 13. Viscosity of aqueous methanol solutions vs. molarity of methanol at 20°C [21].
Fig. 12. Structure of the capacitive methanol sensor, © 2005 IEEE, courtesy of [37].
Fig. 12. Structure of the capacitive methanol sensor, © 2005 IEEE, courtesy of [37].
Fig. 14. Methanol concentration sensor by measuring the viscosity, courtesy of [41].
Fig. 14. Methanol concentration sensor by measuring the viscosity, courtesy of [41].
Fig. 15. The speed of sound in aqueous methanol solutions vs. methanol con- centration in weight percent [22].
Fig. 15. The speed of sound in aqueous methanol solutions vs. methanol con- centration in weight percent [22].
34 9.43 9.52 9.61 9.70 9.80 9.90 10.00 10.10 10.20 jum wavelength Fig. 17. Peak absorption of IR spectra for aqueous methanol solutions, courtes} of [43]. For low concentrations of aqueous methanol solutions, the change of the absorption frequency or wavenumber is not observable in the specific band of the IR spectrum. However, changes in the amplitude of absorption can be observed and related to small changes in the methanol concentration, as shown in Fig. 17. This can be used to analyze the methanol concentration. The transmittance of IR radiation was detected by an infrared sensor and used for analysis of the methanol
34 9.43 9.52 9.61 9.70 9.80 9.90 10.00 10.10 10.20 jum wavelength Fig. 17. Peak absorption of IR spectra for aqueous methanol solutions, courtes} of [43]. For low concentrations of aqueous methanol solutions, the change of the absorption frequency or wavenumber is not observable in the specific band of the IR spectrum. However, changes in the amplitude of absorption can be observed and related to small changes in the methanol concentration, as shown in Fig. 17. This can be used to analyze the methanol concentration. The transmittance of IR radiation was detected by an infrared sensor and used for analysis of the methanol
Fig. 16. Methanol sensor based on the speed of sound, courtesy of [42].
Fig. 16. Methanol sensor based on the speed of sound, courtesy of [42].
Fig. 18. Refractive index vs. methanol concentration [21].
Fig. 18. Refractive index vs. methanol concentration [21].
Fig. 19. Optical methanol concentration sensor, courtesy of [45].
Fig. 19. Optical methanol concentration sensor, courtesy of [45].
Fig. 20. Molar excess isobaric heat capacity of aqueous methanol solutions vs. mole fraction of methanol [24,25].
Fig. 20. Molar excess isobaric heat capacity of aqueous methanol solutions vs. mole fraction of methanol [24,25].
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EngineeringHigh Energy Density PhysicsDirect methanol fuel cell
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