Simultaneous In Situ Measurement of Temperature and Relative Humidity in a PEMFC Using Optical Fiber Sensors

Abstract

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...

Measurements of Electrode Temperatures, Heat and Water Fluxes in PEMFCs: Conclusions about Transfer Mechanisms

“…Internal temperature measurement.-Among various temperature sensors available (thin film temperature sensor used by He et al, 15 infiber Bragg grating used by David et al, 16,17 thermocouples used by Vie and Kjelstrup, 10 Lee and al. 18 and Zhang and al.…”

An experimental investigation of temperature distribution and flooding phenomena of cathode flow fields in a proton exchange membrane (PEM) fuel cell

“…Infra-red thermal imaging techniques [18] are frequently used with the advantages of providing temperature distribution of the whole surface. Also, there are other non-conventional methods such as optical fiber temperature sensors [19], in-fiber Bragg grating sensors [20], band-gap sensors [21], and tunable diode laser absorption spectroscopy [22].…”

Detection of liquid water in the flow channels of PEM fuel cell using an optical sensor

“…While a promising technique, the authors had presented the operation of the sensor in an experimental cell that was non-functional but represented a PEMFC. David et al [2] developed an optical sensor for the measurement of humidity in PEMFCs with the use of Bragg optical fibers. While being able to measure relative humidity in the cell, these sensors were not capable of point measurement or detection of liquid water.…”