Evaluation of Dry-Wet Transition in PEFC Under Load Changes Based on Laser Spectroscopy Using Fiber-Optic Probe

Abstract

The low-humidity operations of a polymer electrolyte fuel cell (PEFC) unfortunately induce the membrane dehydration and deteriorate the proton conductivity through the membrane. Especially, when the output current rapidly rises in steps under load-change conditions, the remarkable decrease of the water concentration on the anode side temporarily occurs and it causes the sudden drop of cell voltage called the “undershoot”. To alleviate this issue, it’s necessary to grasp the dry-wet transition in a PEFC operated under low-humidity and load-change conditions. In the previous studies, the transient water distribution along flow fields of operating PEFCs was vigorously investigated using gas chromatography (GC) [1]. Although GC techniques provide more accurate gas concentrations in flow channels, it takes a long time (at least two minutes) to collect a gas sample and identify a species. As an alternative approach, several researchers have applied tunable diode laser absorption spectroscopy (TDLAS) to the measurement of the water transport in working PEFCs [2,3]. TDLAS is one of the high-speed and high-sensitive absorption spectroscopy techniques which detect the absorbance of a chemical species and identify its concentration using infrared diode laser. Since the laser beam is directly injected into gas channels of PEFCs, the in-situ monitoring of water vapor concentration can be performed on operational fuel cells with a high time resolution of millisecond. However, it’s difficult to sufficiently secure the laser beam path within millimeter-scale narrow channels. In addition, the injection of laser beam into narrow channels tends to generate optical fringe noise due to the multiple reflection. To further enhance the sensitivity and accuracy of gas sensing, it’s necessary to improve the optical system of TDLAS inside fuel cell channels. The authors developed a new TDLAS-based gas sensing system with a single-ended fiber-optic probe to highly accurately measure the water vapor distribution in narrow flow channels of fuel cells at high speed. Fig. 1 shows the schematic diagram of the gas sensor developed in this study. The small-size single-ended probe with pitch and catch fibers enables the in-situ laser diagnostics within millimeter-scale narrow channels. In this measurement system, wavelength modulation spectroscopy (WMS) technique is adopted for high-sensitive gas detection. Furthermore, multiple detection signals are synthesized to cancel the fringe noise and improve the S/N ratio. The fiber-optic probe is directly inserted into the serpentine flow field (241 mm length, 1.0 mm width, 1.0 mm depth) of the experimental fuel cell, and the triangular modulated beam emitted from a DFB laser diode (Wavelength: 1392.5 nm) is injected into a channel through the single-mode pitch fiber. The back-reflection lights from the electrode surface are captured by six multi-mode catch fibers and converted electric signals by six photodiodes (PDs). After six signals are synthesized, the composite signal is transmitted to a lock-in amplifier and the second-harmonic (2f) spectra are obtained by phase-sensitive detection. The mole fraction of water vapor can be estimated from the peak height ratio of 2f and 4f spectra. This gas sensor was applied to evaluate the dry-wet transition in the anode channel of an operating PEFC under low-humidity and load change conditions. Fig. 2 presents the dynamic response of cell voltage and the transient water concentrations along the anode channel of a PEFC during the low-humidity and current-cycling operation. The repetitive current cycling was performed between 0.2 A/cm2 for 23 s, and 0.6 A/cm2 for 23 s. The inlet gas humidifications of the anode and cathode are 45 and 0% RH, respectively. When the current density is rapidly increased from 0.2 to 0.6 A/cm2, the large undershoot of cell voltage is observed because the water content at the anode side of the membrane is suddenly decreased by the strong electro-osmotic effect and the proton conductivity is deteriorated. It can also be seen that the water vapor concentration in the anode channel temporarily and largely drops in the downstream section (x/L=0.5-0.8). After the voltage undershoot, the cell voltage is gradually recovered because the back-diffusion of product water hydrates the membrane from the cathode to anode and reduces its ohmic resistance. The water concentration in the anode channel is gradually increased by the back-diffusion effect. It was found that the TDLAS-based gas sensing technique enables to detect the instantaneous fluctuation of water concentration in flow channels of PEFCs during load-change operations.