A Comparison of the Proton Exchange Membrane Fuel Cell Water Balance Measured Simultaneously with the Water Knock-out Method and Hot Wire Anemometry
Authors: Al Shakhshir Saher , Berning Torsten , Liso Vincenzo , Kær Søren Knudsen 
Published: February 2020

The water balance of a proton exchange membrane fuel cell (PEMFC) can provide valuable insight into the state of the fuel cell and may even limit the current density due to anode dry-out. Typically, commercial fuel cell test stations can be equipped with a method of detecting the fuel cell water balance using the water knock-out method. In this method, the anode and/or cathode exhaust gas streams are led through a condensing unit and the liquid water that is collected over time is back-calculated in terms of the water balance, usually denoted rd
and defined below. Thus, it is a dimensionless property that indicates how much product water has been transported through the membrane from anode to cathode, and negative values indicate overall back-transport from cathode to anode. The definition is also such that when multiplying with the factor 2 one obtains the percentage of the product water that has crossed the membrane, i.e. a value of -0.02 indicates that 4% of the product water has been back-transported from cathode to anode.

Adding the option to measure the water balance to fuel cell test stations increases the cost significantly because additional equipment has to be added to the system such as heat exchangers, through-liquid drainers, beakers, mass scales, control valves and all the hardware and software for controlling and integrating the water knock-out system. After the gas stream escapes from the cathode and anode outlet it passes through the heat exchanger in order to cool the gas stream and condense the water in the stream. The liquid drainer delivers the product water to a beaker on a mass balance scale. The software monitors the mass until the water level reaches a high point, and then drains the water automatically through a solenoid (GREENLIGHT INNOVATION, Canada). Because there is no external cooling reservoir, the condensing of water occurs at room temperature and some water vapor escapes the measurement. In addition, such an experiment has to run for an extended period of time in order to collect a sufficient amount of water for an accurate result.

Meanwhile, our research group has developed a novel method to measure the fuel cell water balance directly and in real time using hot wire anemometry. This method yields a voltage signal that can be directly converted into the fuel cell water balance such that the fuel cell water balance can be a direct output signal similar to voltage and current density. Because, this method requires the hot wire and a heating coil to control the temperature as an extra added hardware in the anode outlet. In addition, it provides an ad-hoc real time electrical signal of the PEMFC water balance, thus the cell size and the current density have no significant effect on the accuracy of measured water balance. The Ad-hoc real time rd
measurements allow this method to be part of FCV systems and/or their test equipment. Behind the test section the anode exhaust gas is just led through the fume hood and o drainage equipment is required.

In this work, the water balance for 5 KW water-cooled PEMFC stack is simultaneously measured using the two aforementioned methods. The stack is run under anode and cathode wet conditions at different current densities and stoichiometric ratios. The hot-wire method has shown a higher sensitivity and an instant response for the measured rd
when changing the running conditions such as the current density. Meanwhile, the knock-out method showed less sensitivity and much slower time response for the measured rd. In addition, it suffers from higher inaccuracies due the loss water vapor with the exhausted gases. However, Figure 1 shows that the overall comparison between both methods is very good and may serve as validation of the hot wire method.

Figure 1