In situ simultaneous measurements of temperature and water partial pressure in a PEM fuel cell under steady state and dynamic cycling
Introduction
Proton exchange membrane (PEM) fuel cells generate electricity directly through two electrochemical reactions, which take place at the interface between a proton conductive membrane and catalyst electrodes. In a PEM fuel cell, controlled hydration of the membrane is required for proper operation. The hydrogen and oxygen feed streams are typically hydrated to bring water vapor into the cell, but several transport processes are responsible for nonhomogeneous distribution of water across the cell cross-section, including diffusion due to partial pressure gradients and electro-osmotic drag of water by protons through the membrane [1], [2]. In addition, the cathode reactions produce water that may condense depending on local temperature and partial pressure. The fuel cell's overall performance can be very sensitive to water management since excessive water can lead to flooding and limit the rate of reactant transport to the electrodes and a reduction in water can decrease the protonic conductivity of the membrane. Nafion©, which is the most common membrane material exhibits a protonic conductivity change of an order of magnitude due to variation of relative humidity between 35 and 85% [3].
Similarly, the temperature of a PEM fuel cell impacts performance of the catalyst electrodes and impacts water transport and liquid/vapor balance. For these reasons, understanding of the distribution of water and local temperatures within operating fuel cells is important for optimizing system operation and design. Accurate, fast, in situ measurements of water concentration would enable both better understanding of water transport for improved cell design and advanced control strategies. In this paper, we describe an optical technique for simultaneously measuring water partial pressure and temperature in PEM cells based on water absorption of laser transmission through the flow passages in the bipolar plate. This approach permits non-intrusive in situ measurements and extends the capabilities of existing measurement techniques. The measurement approach is validated in steady state operation of a PEM cell with controlled humidity of incoming gas streams and cell temperature. The optical measurement is then applied to the PEM cell undergoing cyclic loading to simulate the conditions that might be present in transportation applications where instantaneous power requirements fluctuate. The measurements of water partial pressure and temperature in the cathode flow passages detail the time response of the system to transient events.
Section snippets
Background
Development of tools for sensing of temperature and chemical species in fuel cells is a relatively new area of research. Prior to the last 5 years, most measurements in fuel cell systems were limited to global measurements of electrical cell performance. Polarization curve measurements, for example, are routinely used to track cell performance and can be combined with simple models to diagnose component problems in the cell [4]. More recent refinement of global measurement techniques has
Experimental approach
The experimental methodology is based on that reported by Basu et al. [22]. Tunable diode laser absorption spectroscopy was used to measure water vapor absorption profiles as a function of excitation wavelength. However, the laser was changed from the previous experiments to enable measurement of water transitions in a different wavelength regime. In this work, the fiber pig-tailed output of a distributed feedback (DFB) diode laser (NEL # NLK1S5G1AA) at a wavelength of 1470 nm in the near-IR
Spectral simulations
The experimental study reported by Basu et al. [22] showed that the ro-vibrational transitions available at 1491 nm, which was the wavelength of the laser used in that study, were not sufficiently sensitive to temperature in the range from 60 to 90 °C. The absorption intensity varied by only 10% and the spectral width varied by only 2.2% and did not allow extracting temperature in addition to water partial pressure. Thus, for the current work a more exhaustive analysis of the water spectrum from
System calibration
Measurements were first made in the fuel cell without external loading or hydrogen flow to calibrate the measured halfwidths and spectral intensities against the simulations presented in the previous section. As established from the HITRAN simulation, the experimental profiles are predominately Lorentzian and in the spectral window available with the current laser, five water transitions contribute significantly to the overall absorption shape. The data analysis procedure consists of fitting
Conclusions
The measurement of partial pressure and temperature during steady state and dynamic cycle operation of a PEM fuel cell was demonstrated with a novel diagnostic technique by monitoring laser transmission and water absorption through a flow passage in the bipolar plate. Simulations of water absorptivity were utilized to select a laser wavelength that optimized sensitivity to temperature and partial pressure for conditions relevant to a PEM fuel cell. Calibration measurements in a non-operating
Acknowledgments
The research reported here was funded by the U.S. Army RDECOM, CERDEC (Fort Belvoir, VA) through the Connecticut Global Fuel Cell Center (CGFCC). The authors would like to acknowledge the useful discussions with Dr. Frano Barbir over the course of this work.
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Present address: Yildiz Technical University, Istanbul, Turkey.