The Effect of Nitrogen Cross-Over on Water Balance Measurements in Proton Exchange Membrane Fuel Cell Using Constant Temperature Anemometry
Authors: Al Shakhshir Saher , Berning Torsten , Kær Søren Knudsen 
Published: February 2020

R&D on Proton exchange membrane fuel cells (PEMFC’s) technology has been accelerated in the last few years to reduce the dependency on fossil fuel. PEMFC’s operate by internally combining oxygen from air with hydrogen to form water and generate electricity and waste heat. The most common hurdle for enhanced PEMFC durability and performance is still the water management: the proton exchange membrane in the center of these fuel cells has to be hydrated in order to keep its ability to conduct protons from anode to cathode side while on the other hand excessive liquid water can lead to cell flooding and increased degradation rates of the cell. Thus, a detailed understanding of all aspects of water management in PEMFC is important. This includes the fuel cell water balance, i.e. the question which fraction of the product water leaves the fuel cell via the anode channels versus the cathode channel. Our research group is currently developing a state of art technology to obtain an ad-hoc and real time electrical signal of the fuel cell water balance by employing a constant temperature hot wire anemometry [1]. The hot wire sensor is placed in the anode outlet of PEMFC, and the voltage signal received gives valuable insight into heat and mass transfer phenomena which can be interpreted directly to real time PEMFC water balance. So far, ex-situ experiments have been conducted to measure the voltage signal of the hot wire anemometer for a known gas stream that contains a binary mixture of hydrogen and water vapour as would ideally leave the fuel cell anode. However, in an operating fuel cell there are two main uncertainties in this method: (i) there is some internal hydrogen crossover that does not load to an external current, thus the exact amount of hydrogen leaving the fuel cell anode is not known, and (ii) that there is nitrogen crossing over from the cathode side to the anode side, and this nitrogen does slighty falsify the voltage signal that the hot wire yields. The effects of nitrogen-cross over on the hot wire voltage signal at the anode outlet and consequently on the measured water balance (rd
) is studied in this work.

In our previous work it was shown that the only unknown in the determination of the hot wire voltage signal is the equation that determines the heat transfer around the hot wire, and we have shown that it is necessary to employ a power-law equation as suggested by Hilpert:

Nu = C Pr0.33 Rem

where Re is the Reynolds number and Pr
is the

Prandtl number are calculated at film temperature. The constants C and m usually depend on the Re, and they will be determined out of the experimental data for the gas mixture, by plotting the measured (Nu/ Pr0.333
) versus the Re number of the gas mixture stream. The missing C and m from Hilpert equation can then be easily obtained from a power-fit of the general form Y = CXm
. Also, it was shown somewhere else that in fact only the exponent to the Reynolds number, of the gas mixture stream is important when determining the fuel cell water balance out of the hot wire signal [1].

 The nitrogen-cross over is experimentally demonstrated by introducing 1% of nitrogen to the dry hydrogen molar flow. The 99%H2
+1%N2
of the dry mixture is humidified with water vapor by controlling the relative humidity (RH) of the dry mixture with in the range of (0-100)%RH, simulating the PEMFC anode outlet. The hot wire voltage is measured with and without nitrogen and it was slightly lower with the presence of 1 % nitrogen in the flow. The effect of the voltage reduction on the measured water balance can be neglected. This because the effect of 1% nitrogen on power law constant’s m which is used in determining the water balance as explained somewhere else is extremely low. It might be concluded that, the hot wire technique for measuring the water balance is still accurate and can be used for ad-hoc and real time water balance measurements and the nitrogen cross-over affect can be neglected, knowing that the 1% N2 is one order of magnitude higher than the actual concertation of 0.02 m2
active area cell used for lab testing.

 

[1] Berning, T., & Al Shakhshir, S., Applying hot wire anemometry to directly measure the water balance in a proton exchange membrane fuel cell–Part 1: Theory. International Journal of Hydrogen Energy, 40(36) (2015), 12400-12412.