Measurement of Water Vapor Concentration in Narrow Channel of PEFC Using Fiber-Optic Sensor Based on Laser Absorption Spectroscopy

Abstract

To achieve high performance and long-term operation of polymer electrolyte fuel cell (PEFC), it is essential to understand and control water transports in operating fuel cell. In the previous studies, water distributions along flow channels were investigated using micro gas chromatograph (mGC) [1-2]. Although GC method enables to analyze water vapor concentration with high accuracy, it takes a long time (at least two minutes) to collect a gas sample and identify a species. On the other hand, several researchers have applied tunable diode laser absorption spectroscopy (TDLAS) technique to gas analysis in PEFCs [3-4]. TDLAS is one of the high-sensitive absorption spectroscopy techniques which measure the absorbance of a species and identify its concentration using infrared diode laser. Since laser beam is directly injected into gas channel of PEFC, in-situ monitoring of water vapor concentration can be performed on operational fuel cells with a high time resolution of millisecond. The authors have now developed a fiber-optic gas sensor based on TDLAS to apply it to the measurement of water vapor concentration in flow channels of working PEFCs. The introduction of small-size fiber-optic sensors enables in-situ laser diagnostics within millimeter-scale narrow channels. In this study, a transmission/reception integrated fiber-optic probe is newly developed to detect absorption spectra of water in gas channels of PEFCs. This novel probe is compactly composed of one light-projecting fiber and six receiving fibers, which are coaxially arranged. Since multiple output signals detected by using six photodiodes are synthesized, the optical fringe noise in them can be effectively canceled. Even if the fringe noise tends to be generated inside narrow channels, the high-resolution absorption spectra with better S/N ratio can be obtained by the signal synthesizing. Furthermore, the TDLAS system with the above-mentioned fiber-optic probe is constructed as shown in Fig. 1, and the concentration measurement of water vapor was tentatively attempted using a simulated flow cell. In this system, the triangular-wave laser beam modulated at the high frequency of 10 kHz is emitted from a DFB laser diode (wavelength: 1392 nm) and injected into the straight gas flow channel (30 mm length, 1.5 mm width, 1.5 mm depth) of the simulated cell through the fiber-optic probe. Some reflected lights from the gas channel are detected at six photodiodes and converted to the electric signals. Subsequently, the synthesized output signal is transmitted to a lock-in amplifier and the high-sensitive harmonic spectra are obtained by phase-sensitive detection. The method to quantify the water vapor concentration in the channel from the measured spectrum data is also proposed. The simulated flow cell is fabricated by stainless steel plates. The humidified oxygen is fed into the gas channel at the flow rate of 200 mL/min. Fig. 2 presents the second-harmonic (2f) spectra of water obtained from the TDLAS measurement in the simulated flow cell at 70 deg. C. The mole fraction of water vapor is changed from 0.3 to 25.7%. The vertical axis shows the intensity of 2f signal, and the horizontal axis shows the wavelength. At the low water concentration of 0.3-8.9 mol%, the peak-valley height of 2f absorption spectrum increases with an increase in water concentration. On the other hand, when the concentration of water vapor is higher than 14.3 mol%, the peak-valley height of 2f signal gradually decreases with increasing water mole fraction. Since the relationship between the water mole fraction and the peak-valley height of 2f spectrum is not monotonous, the water concentration cannot be quantitatively estimated only by the 2f spectrum data. The authors propose the improved method for the accurate and effective quantification of water vapor concentration with the use of 2f and 4f absorption spectra. As a result of the spectral analysis, it was found that the peak-valley ratio of 2f and 4f spectra exponentially increases with water mole fraction. The water vapor concentration can be estimated from the 2f/4f peak-valley ratio. In addition, it was revealed that the measurement accuracy of water vapor mole fraction is less than 0.5% at 70 deg. C. These results suggest that the fiber-optic TDLAS sensor has the ability to accurately monitor in-situ water transports within narrow channels of actual fuel cells.