LOCALISED RELATIVE HUMIDITY AND TEMPERATURE MEASUREMENTS IN A PEM HYDROGEN FUEL CELL G. Hinds, M. Stevens, J. Wilkinson, M. de Podesta and S. Bell National Physical Laboratory Teddington, Middlesex, TW11 0LW, United Kingdom Abstract Miniature relative humidity and temperature sensors have been used to measure the evolution of conditions along gas flow channels within a polymer electrolyte membrane (PEM) hydrogen fuel cell. Des capteurs miniatures de l'humidité relative et de température ont été utilisés pour mesurer l'évolution des conditions le long des cours de débit de gaz dans une pile à combustible à membrane d'échange de protons. Introduction Hydrogen fuel cells provide a portable power source which is pollution-free at the point of use, and can be low- carbon if the fuel (hydrogen) is generated by low-carbon methods. However, the performance, cost and durability of fuel cells need to be significantly improved if this technology is to be widely adopted. Hence there is considerable interest in research that can improve the performance and lifespan of the critical component – the membrane electrode assembly (MEA). In operation, a polymer electrolyte membrane fuel cell (PEMFC) consumes hydrogen and emits water. The hydrogen is split electrochemically at the anode to produce protons and electrons. The fuel cell produces electricity because the polymer membrane conducts protons but not electrons, which are forced around an external circuit. At the cathode protons, electrons and oxygen atoms combine to form water. Since the proton conductivity of the membrane increases with water content, humidification of reactant gases is often used to ensure membrane hydration. However, if conditions are too wet, transport of the gases to the electrode surface is impeded. Therefore, monitoring of water vapour is a powerful tool in understanding fuel cell performance, and provides essential data for modelling of fuel cell operation. Measurements of localised humidity and temperature conditions within fuel cells can chart the localised generation of water vapour, the extent of localised heating, and the combined impact on relative humidity in the flowing gas. These can also usefully be correlated with localised current density within the cell. In-situ humidity measurements in this application are difficult because of the inaccessibility and small size of the flow channels. The conditions of temperature up to 90 °C and humidity up to saturation can be challenging for some measurement methods. In addition, sensors can be at risk of drift due to chemical interferences or permanent contamination. Workers have previously tried various measurement techniques to study water distribution in the gas channels of PEM fuel cells. Mench et al. [1] used gas chromatography (GC) to measure the water vapour content in the anode and cathode gas channels. However, the drawback of the GC technique is that the measurements can only be made about every 2 minutes. Partridge et al. [2] demonstrated that mass spectrometry could be used to measure relative humidity by sampling gas from a PEMFC stack through fine glass capillaries. Chen et al. [3] used neutron radiography to detect liquid water in the gas channels of a single cell PEMFC of active area 100 cm 2 . Unfortunately, in order to differentiate between water content in the cathode and the anode, the cathode and anode channels were shifted by one channel width, which meant that they did not overlap over the majority of the active area. Basu et al. [4] explored the use of absorption spectroscopy with a fibre optic coupled diode laser sensor in a single cell PEMFC. Nishikawa et al. [5] measured the relative humidity in the cathode flow channel of a single cell PEMFC of active area 289 cm 2 , by extracting the gas through a stainless steel tube to a humidity sensor of diameter 4 mm located outside the cell. An increase in relative humidity from inlet (30 %rh) to outlet (70 %rh) was observed with a humidifier temperature of 40 °C and a cell temperature of 80 °C. While these techniques provide useful information about the variation in water content in the gas channels, they lack a simultaneous measurement of temperature at the point of interest in order to enable accurate calculation of the dew-point temperature. Lee et al. [6] used a dual temperature/humidity probe to measure the relative humidity in a micro-fuel cell but no information on the spatial variation of relative humidity was obtained. In the work reported here, 32 miniature combined humidity and temperature sensors were installed in the air and hydrogen flow channels of a functioning laboratory fuel cell. The study was aimed at obtaining in-situ measurements in real time, and at demonstrating that the humidity and temperature results can be related to the localized operation within the cell. The ultimate goal is to provide input data for validation of models of water distribution and transport in PEMFC membranes, and to support fuel cell design and optimisation.