Polymer Electrolyte Membrane Fuel Cell as a Hydrogen. Flow Rate Monitoring Device. S. Giddey* and S.P.S. Badwal. CSIRO Energy Technology

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1 The final definitive version of this manuscript was published in Ionics. The final publication is available at Ionics short communication Polymer Electrolyte Membrane Fuel Cell as a Hydrogen Flow Rate Monitoring Device S. Giddey* and S.P.S. Badwal CSIRO Energy Technology Private Bag 33, Clayton South 3169, Victoria, Australia *Corresponding author Sarb.Giddey@csiro.au Phone: (61 3) , fax: (61 3) Key words: Polymer Electrolyte Membrane Fuel Cell, hydrogen flow rate monitor, hydrogen flow sensor, PEMFC limiting current flow sensor Abstract A normal polymer electrolyte membrane (PEM) fuel cell has been demonstrated to be useful for measuring accurate flow rates of hydrogen or hydrogen containing gases. The concept involves applying a constant voltage to a PEM fuel cell to oxidise entirely the hydrogen supplied to the anode compartment of the fuel cell and observing limiting current values attained. The limiting current is directly proportional to the amount of hydrogen being available for consumption in the fuel cell per unit time. PEM fuel cells with an active area of 50cm 2 were constructed and used to measure the flow rates of hydrogen containg gases up to 175ml/min. It has been demonstrated that hydrogen flow rates can be measured accurately by using a polymer electrolyte fuel cell. The fuel cell size and flow field design can be optimised for a given hydrogen flow rate range and fast response time.

2 Introduction Total global hydrogen production is over 60 million metric tons per annum. Hydrogen is mainly used in ammonia (mainly fertiliser) production, petroleum industry for conversion of heavy oils to lighter oil fractions and in other industrial processes (chemical, metallurgical, medical and aerospace) [1, 2]. Only a very small percentage of the total global hydrogen produced is currently used as an energy carrier. Hydrogen is also considered as an energy currency and storage media especially for intermittent renewable energy sources. It is also considered as a clean fuel for future transport vehicles either for combustion in an internal combustion engine or consumption in a fuel cell [3-5]. There is a general need for hydrogen sensing / monitoring devices for safety and control systems. A number of electrochemical sensors for detecting hydrogen have been reported in the literature and are used widely in industry [6-10]. Hydrogen sensors based on solid polymer electrolyte membranes (PEM) are typically amperometric devices and are based on measuring limiting current to calculate hydrogen concentration in the gas phase [8-10]. Hydrogen supplied to the anode can be oxidised to generate protons and electrons by applying voltage to the cell. Protons transported through the membrane to the cathode are converted back to hydrogen in the absence of oxygen, and to water in the presence of oxygen. If sufficient voltage is applied to a PEM fuel cell, the limiting current will be achieved due to diffusion limitation of the reactant reaching the anode/membrane interface. This, using the Faraday law, can be directly related to the concentration of hydrogen in the gas stream. Alternatively, the limiting current can be directly related to the flow rate of hydrogen for a pure hydrogen 2

3 stream or to a hydrogen containg gas stream with known percentage of hydrogen. In this paper, the use of a PEM fuel cell as a hydrogen flow rate measuring device has been explored and discussed. Experimental Details PEM fuel cells with an active area of 50cm 2 were constructed and experiments were conducted using dry hydrogen and hydrogen/nitrogen mixtures with known hydrogen concentration supplied to the hydrogen (anode) electrode, and humidified air supply to the oxygen (cathode) electrode of the cell. Membrane electrode assemblies (MEAs) used for this purpose consisted of hydrogen and air electrodes on both sides of a Nafion N115 proton conducting membrane (PEM) with a thickness of 125µm. Each electrode was fabricated by screen printing 20wt% Pt/C catalyst layer on a 0.35mm thick 25 wt% wet proofed (to make it partially hydrophobic) carbon paper (TGPH-120 Toray Carbon paper). Typical Pt catalyst loading was 0.4mg/cm 2 /electrode. The cell was assembled in the test fixture with the help of resin impregnated graphite plates on both sides of the cell. The hydrogen side graphite plate consisted of a parallel double serpentine flow field to ensure complete utilisation of hydrogen and air side flow field consisted of a cross-channel parallel flow field. The channel cross section (width x depth) on the anode as well as cathode side graphite plates was 2mm x 2mm. The complete cell fabrication and assembly details, and test station employed to carry out the experiments are reported elsewhere in literature [11, 12]. 3

4 A DC power supply (Powerbox PBX ) in a voltage limiting and current compliance mode was used to apply voltage to the cell. Steady state values of limiting currents were measured for different values of hydrogen inlet flow rates controlled by using a pre-calibrated Bronkhorst HI-TEC mass flow controller (MFC). Hydrogen flow rates were measured for different setting of the MFC by employing a volumetric flow measuring device (Bios Definer 220). The data acquisition system, based on Doric data logger (Digitrend 235) was used to continuously record the current and cell voltage at a rate of 6 samples per minute. The hydrogen based gases employed were industrial grade >99.5% H 2, 10% H 2 (in N 2 ) and 5% H 2 (in N 2 ). Humidified air was supplied to the cathode chamber of the cell for the purpose of humidification of the membrane, and dry hydrogen containing gases were supplied to the anode chamber of the cell. The measurements were made at room temperature (~ 22 O C). Figure 1 shows schematic of the experimental set-up employed for monitoring flow rates of hydrogen containing gas based on limiting current measurements. In this set-up on the cell exit side, two gas bubblers had to be used to visually make sure that the entire hydrogen gas is utilised in the cell, and also to prevent any air being sucked back into the fuel chamber from the atmosphere on the fuel exit side, due to vacuum being created by complete utilisation of hydrogen when the limiting current was achieved. The limiting current values were obtained by setting the gas flow rate in the range of 20 to 175 ml/min. The limiting current condition was attained by controlling the cell voltage such that in the second bubbler the fluid level in the submerged tube stays levelled with that of fluid in the main body of the bubbler. In the case of measurements on 5% and 10% H 2 in nitrogen gas mixtures, the set-up did not require the presence of any gas bubbler on the fuel chamber exit as there was always balance of the unconsumed gas (impurities and 4

5 nitrogen) flowing from the exit, after the consumption of hydrogen (in the gas mixture) inside the cell. Results and Discussion In Figure 2 experimental results of hydrogen flow rates calculated from limiting current measurements have been plotted against the actual set flow rates (symbols) for hydrogen flow rates between 20 and 175ml/min for a single PEM fuel cell with an active area of 50cm 2. The relationship between hydrogen flow rates determined from the limiting current values and actual hydrogen flow rates is linear. Also shown in the diagram are the theoretical flow rates expected for different limiting currents (solid line) assuming 100% hydrogen consumption. In general, it is clear that there is an excellent agreement between the calculated and experimental results. The slight deviation for the experimental values of measured flow rates is likely to be due to the presence of moisture and other impurities up to 0.5% in the industrial grade hydrogen supply including 0.2% O 2 and some errors in hydrogen flow rate measurements. These experiments were repeated with a different fuel cell and results for both calculated and experimental data are given in Table 1. The level of error or deviation from theoretical data reported in Table 1 and Figure 2 were similar. PEM fuel cells have been used as hydrogen sensing devices in the past [6-10]. From above results, it appears that such systems can also be used as a hydrogen flow rate measuring device with ease. When a voltage is applied to a PEM fuel cell, hydrogen supplied to the anode is oxidised to protons and electrons, protons transported through the membrane to the cathode are converted back to hydrogen in the 5

6 absence of oxygen (pumping mode), and to water in the presence of oxygen in the fuel cell mode. Provided design parameter and operating conditions are set correctly, with time, a limiting current condition would be achieved. At this point, the limiting current value is directly related to the flow rate of hydrogen or hydrogen containing gases by the Faraday law. For pure hydrogen, the simplified relationship at 22 o C is as follow: Hydrogen flow rate (ml/min) = x Limiting current value (A) The applied voltage must be kept below the thermo-neutral voltage of 1.48V to avoid electrolysis of water present in the fuel chamber either due to some moisture in supplied gases or through diffusion of water from the air side where it is formed in the electrochemical reaction. In order to extend the work further and evaluate the capability of the PEM fuel cell as a continuous monitoring device for measuring flow rates of hydrogen and gases containing lower concentration of hydrogen, further experiments were performed with both >99.5% hydrogen, and 10% and 5% hydrogen in nitrogen gas mixtures. In these experiments the values of currents were continuously logged in real time as the flow rates of hydrogen and hydrogen/nitrogen gas mixtures were changed at regular intervals. The results of these measurements are shown in Figures 3, 4 and 5. In all three cases there was a very good agreement between the calculated and theoretical data. The response of the fuel cell limiting current to changing flow rate was reasonably fast for >99.5% and 10% H 2 /N 2 gases, however, the fuel cell response 6

7 time was quite sluggish for the 5% H 2 /N 2 gas mixture mainly resulting from the dilution effect of nitrogen and hydrogen distribution in the flow fields. In order to make the PEM fuel cell work as a reliable hydrogen flow-monitoring device, the following conditions must be met: The membrane electrode assembly needs to be conditioned in a humid environment. The applied voltage should not exceed 1.4V, as there are risks that above this safe voltage limit (1.48V is the absolute upper limit), the water in the membrane itself or in the gas may start dissociating to produce oxygen and protons. This would produce erroneously high currents and may also lead to drying of the membrane and damage to the cell. The active area of the cell has to be sufficient to cater for the hydrogen flow rates to be measured, so that current densities are not excessively large to avoid cell degradation and to prolong device lifetime. The design of the cell and stack has to be such that all the supplied hydrogen must be consumed within the cell. If the supplied gas contains CO or CO 2, these may have to be removed before entry to the cell, otherwise cell performance degradation would occur leading to false limiting current densities. In cases where other combustible gases such as CO and CH 4 etc. (or even H 2 O and / or CO 2 ) are present, it may be necessary to incorporate a thin Pd sheet upstream to allow only hydrogen to go through for detection by the 7

8 device. However, for accurate measurement of hydrogen flow rates, the hydrogen flux through the Pd membrane should not be rate limiting. It appears that the double serpentine design of the fuel flow field used here in this study is sufficient to allow total hydrogen consumption over the 50cm 2 active area of the fuel cell for at least up to 175 ml/min flow rate for 100% hydrogen. However, further work can be performed to optimise the fuel cell or stack size, active area, design of the fuel channels, cell configuration and operating conditions to cater for wide ranges of flow rates of hydrogen and hydrogen containing gases and to minimise response time. Conclusions A concept of measuring hydrogen flow rates using PEM fuel cell as a hydrogen flow sensing monitor device has been demonstrated using 50cm 2 active area fuel cells. The results showed that the flow rates of hydrogen or hydrogen containing gases with known hydrogen concentration can be measured accurately. The double serpentine design of the fuel flow field is sufficient to allow total hydrogen consumption over the 50cm 2 active area of the fuel cell for at least up to 175 ml/min flow rate for 100% hydrogen. Further work can be performed to optimise the fuel cell size, active area, fuel distribution channel design and operating conditions for higher flow rates of hydrogen and hydrogen containing gases. Acknowledgements 8

9 The work described in the paper was carried out as part of CSIRO Energy Transformed Flagship activity. The authors would like to thank Daniel Fini with general technical assistance and Aniruddha Kulkarni for reviewing the manuscript. References 1. Clarke RE, Giddey S, Badwal SPS (2010) Stand alone PEM water electrolysis system for fail safe operation with a renewable energy source. International J Hydrogen Energy 35: Ramachandran R, Menon RK (1998) An overview of industrial uses of hydrogen. International Journal of Hydrogen Energy 23 (7): White CM, Steeper RR, Lutz AE (2006) The hydrogen-fueled internal combustion engine: a technical review. International Journal of Hydrogen Energy 31: Sorensen B (2005) Hydrogen and Fuel Cells, Emerging technologies and applications, A volume in the "Sustainable World" series, Elsevier Academic Press. 5. Marbán G, Valdés-Solís T (2007) Towards the hydrogen economy?. International Journal of Hydrogen Energy 32: Ghenadii Korotcenkov G, Han SD, Stetter JR (2009) Review of Electrochemical Hydrogen Sensors. Chem. Rev. 109: Alber KS, Cox JA, Kulesza PJ (1997) Solid-state amperometric sensors for gas phase analytes: A review of recent advances. Electroanalysis 9(2):

10 8. Ramesh C, Velayutham G, Murugesan N, Ganesan V, Dhathathreyan KS, Periaswami G (2003) An improved polymer electrolyte based amperometric hydrogen sensor. J. Solid State Electrochem 7: Lu X, Wu S, Wang L, Su Z (2005) Solid-state amperometric hydrogen sensor based on polymer electrolyte membrane fuel cell, Sensors and Actuators B 107: Sakthivel M, Weppner W (2006) Development of a hydrogen sensor based on solid polymer electrolyte membranes. Sensors and Actuators B 113: Giddey S, Ciacchi FT, Badwal SPS (2004) Design, assembly and operation of polymer electrolyte membrane fuel cell stacks to 1 kw e capacity. Journal of Power Sources 125: Giddey S, Badwal SPS, Ciacchi FT, Fini D, Sexton BA, Glenn F, et al. (2010) Investigations on fabrication and lifetime performance of self - air breathing direct hydrogen micro fuel cells. International Journal of Hydrogen Energy 35:

11 Figure captions Fig.1 Schematic of the experimental set-up employing a PEM fuel cell for monitoring flow rates of hydrogen containing gases based on limiting current measurements. Fig. 2 A graph showing the experimental (circles) and theoretical (solid line) values of hydrogen flow rates measured using a 50cm 2 active area PEM fuel cell. Fig. 3 The response of the 50 cm 2 active area PEMFC to changing flow rates of 100% hydrogen. Fig. 4 The response of the 50 cm 2 active area PEMFC to changing flow rates of 10% H 2 / N 2 gas mixture. Fig. 5 The response of the 50 cm 2 active area PEMFC to changing flow rates of 5% H 2 / N 2 gas mixture. 11

12 Table 1: Actual and theoretical (assuming 100% hydrogen consumption) hydrogen flow rates for different limiting currents obtained for different hydrogen flow rates into fuel chamber of a 50cm 2 active area PEM fuel cell. Limiting Current, H 2 flow rate, ml/min A Actual Theoretical

13 Data Logger 100 mω shunt Humidified Air To exhaust Power Supply + _ O 2 electrode Polymer membrane H 2 electrode Fig. 1 Dry H 2 or H 2 gas mixture Flow Meter To exhaust

14 Flow Rate fromm Limiting Current, ml/min Fig Actual H 2 Flow Rate, ml/min

15 H2flow rate, ml/min Cell voltage, mv >99.5% H Limiting current, A 2 0 Fig Time, mins. 40 0

16 Gas flow rate, ml/min 10% H 2 /N 2 mixture Limiting current, A Fig Time, mins. 20 0

17 Lim miting current, A % H 2 /N 2 mixture Gas f flow rate, ml/min Time, mins. 0 Fig. 5