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The development, implementation, and demonstration of in-fiber Bragg grating ͑FBG͒ sensors for a simultaneous measurement of temperature and relative humidity ͑RH͒ in an operating polymer electrolyte membrane fuel cell ͑PEMFC͒ are presented. Etched fiber, polymer-coated FBG sensors with fast response and high sensitivity are installed in situ in a minimally invasive manner in the cathode unipolar plate of a single PEMFC with serpentine flow fields. The performance of the fuel cell under transient operating conditions is monitored.Step increases in current induce significantly larger increases in RH near the outlet than near the inlet of the cell, and associated transients within the fuel cell are found on a time scale approaching the sensor response time ͑ϳ1 Hz͒. The improved response of the technique, together with the significantly improved temperature and RH resolution, provides useful information on the dynamics of heat and water management. The technique is well suited for distributed measurements, and its relatively low cost and nonintrusive character make it a good candidate for practical multipoint monitoring of complete stacks.The temperature and relative humidity ͑RH͒ inside a polymer electrolyte membrane fuel cell ͑PEMFC͒ have a significant impact on performance, particularly due to the effect these coupled parameters have on the water balance within the cell. 1-5 Optimal water balance is achieved when the membrane is well hydrated, ensuring good proton conductivity, and the electrodes are relatively free of liquid water to maintain effective gas transport to the reaction sites.Water produced at the cathode during cell operation is effective in hydrating the membrane at lower cell temperatures. But at higher operating temperatures, which are preferred for reduced activation polarization, the cathode RH is insufficient and the membrane dries out. Membrane dry-out not only causes an increase in ohmic overpotential but can also accelerate membrane degradation. 6 To avoid this dry operation, gas streams are typically humidified before entering the cell. Excessive humidification, however, can cause flooding and can severely decrease the cell performance due to mass transport losses. Nonuniformities in the temperature and water distribution are often present 7-9 and add to the challenges of thermal and water management.Further adding to the difficulty of achieving controlled water balance in a PEMFC are dynamic operating conditions, which are common in transportation applications. Recent experimental and modeling studies further elucidate the roles of temperature and humidity on cell performance under these conditions. 10,11 As predictive models are extended to two and three dimensions, the ability to measure distributed temperature and RH in situ becomes increasingly useful for validation. Techniques developed for these purposes also supplement conventional bulk diagnostic techniques.Distributed measurements of RH and temperature inside a PEMFC have been performed using electrical and optical techniques wit...

A relative humidity sensor based on polymer-coated optical fibre Bragg gratings is presented. This fully functional sensor has response time and resolution comparable to the current capacitive relative humidity (RH) sensors, but with greater applicability. Numerical and experimental methods are used to determine the effects of coating thickness and fibre diameter on the response time and sensitivity of Bragg gratings coated with Pyralin. Transient results indicate that coating thicknesses of less than 4 μm are needed to achieve a response time of 5 s, competitive with commercial capacitive RH sensors. Using thin coatings of ∼2 μm, for a short response time, sensors with reduced fibre diameter were fabricated and tested under steady-state, transient and saturated conditions. By chemical etching from 125 to 20 μm, the sensitivity increased by a factor of 7. Such an increase in sensitivity allows for the resolution and response time of the Pyralin-coated sensor to be comparable to commercial capacitive RH sensors. These characteristics, in addition to the sensor’s rapid recovery from saturation in liquid water, indicate good potential for use of this sensor design in applications where electronic RH sensors are not suitable.

Water management is an important factor influencing the performance of Proton Exchange Membrane fuel cells (PEMFC). This study investigates the effects of a magnetic field (MF) on water transport in PEMFC in terms of performance enhancement. The results show that the performance of the test fuel cell improved by increasing the MF strength. The temperature difference between the anode and the cathode increases when the MF strength increases (B > 0 T). On the other hand, the RH difference decreases with an applied MF. Dynamic behaviors of the fuel cell in terms of voltage undershoot and overshoot is improved at high MF. In addition, the cell performance increases due to that some water is pushed out from the high magnetic field area to the lower field strength and making more pore space of porous layer for oxygen reactant. Moreover, measurement of temperature and relative humidity (RH) inside the cell during such operation using FBG sensors provides valuable information to the field.

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