Factors Affecting the Performance of a Polymer Electrolyte Fuel Cell

Presented to: Shadi Taghavi

Group BBLaboratory Projects IIDepartment of Chemical Engineering Queens University

January 30, 2015We do hereby verify that this written report is our own individual work and contains our own original ideas, concepts and designs. No portion of this report has been copied in whole or in part from another source, with the possible exception of properly referenced material.______________________________________________ _______________________________________________________________ _______________________________________________________________ ____________________________________________________________________ ___________________

1.0 IntroductionThis experiment analyzes the factors affecting the performance of a polymer electrolyte membrane (PEM) fuel cell by varying the cell operating conditions. A fuel cell converts chemical energy into electrical energy. 1.1 Principles of Polymer Electrolyte Fuel Cells

Fuel cells consists of three main components: an anode, an electrolyte membrane and a cathode as seen in Figure 1.

Figure 1: Schematic of a polymer electrolyte membrane fuel cell (PerkinElmer Inc., 2010).In this experiment hydrogen gas is used as a fuel and is fed to the anode, while oxygen is used as the oxidant and is fed to the cathode. Equation 1 is the reaction that occurs in PEM fuel cell power generation: (1) At the anode, dihydrogen dissociates into hydrogen ions and electrons as seen in equation 2. (2)The hydrogen ions can then move through the electrolyte membrane towards the cathode and the electrons are forced outside the cell to produce an electric current. At the cathode, hydrogen and oxygen react to form water as seen in Equation 3 (U.S. Department of Energy, 2000). (3)1.2 Fuel Source and Oxidant

Hydrogen gas is used as the fuel source because of its reactivity towards an appropriate catalyst, the convenience of production via hydrocarbons and its ability to be cryogenically stored while maintaining an increased energy density allowing for easy storage. Oxygen is chosen as an oxidant since it is easily obtained from air and, like hydrogen gas, is easily stored (U.S. Department of Energy, 2000). 1.3 Electrolytic Membrane

The electrolyte is located between the two electrodes and is responsible for the transfer of protons. PEM fuel cells require a solid polymer electrolyte (SPE) membrane that meets the following criteria: high conductivity of protons, low loses from resistance, no electric conductivity, minimal fuel and oxygen transport, durable material and cost-efficient. There are several types of SPE membranes, such as perfluorinated isomers, partially fluorinated polymers and non-fluorinated membranes with aromatic backbone (Smitha, Sridhar, & Khan, 2005).This type of cell has only one liquid source resulting in minimized corrosion of equipment. Water produced at the cathode is evaporated based on a calculated humidifier temperature. The temperature is chosen such that water does not evaporate too quickly so that the membrane remains hydrated. Water traps are also included in the system to prevent flooding, as seen in Figure 2 (U.S. Department of Energy, 2000).

Figure 2: Schematic of the ProFC-XU series stacks (H2 Economy, 2008).1.4 Performance Graph of a Typical Fuel Cell

The equilibrium potential () of a PEM fuel cell is calculated via the following equation:

(4)Where is the change in Gibbs free energy calculated by the half-cell reactions at the anode and cathode (assuming an ideal system). The number of electrons is denoted by and Faradays constant by . If this were an ideal process, open circuit voltage (OCV) could be used to determine the potential of the fuel cell by measuring the potential difference between the anode and cathode with no current. Since there are losses associated with real fuel cells this approach is inaccurate. To account for these losses the following equation is used to calculate the operational potential (): (5)Where accounts for the activation losses, accounts for the ohmic losses and accounts for the mass transport losses. A general performance graph for a fuel cell can be seen in Figure 3 (Department of Chemical Engineering, 2015).

Figure 3: Performance graph for a fuel cell (U.S. Department of Energy, 2000).1.5 Sources of Losses

Activation polarization is a result of energy barriers preventing electron flow and is determined by the rate at which electrochemical reactions are occurring. By increasing the temperature and surface area of the catalyst the effects of activation polarization are minimized. This can be seen at low current densities on the performance graph in Figure 3 (U.S. Department of Energy, 2000).Ohmic polarization is caused by the resistance of ions through the electrolyte membrane as well as the resistance of electrons in the electrode. The effects of ohmic polarization can be minimized by decreasing the separation between the anode and cathode as well as increasing the conductive capabilities of the electrolyte. The severity of ohmic losses occurs as current density increases, as seen in Figure 3 (U.S. Department of Energy, 2000). Concentration polarization occurs as a result of a concentration gradient which prevents the transport of hydrogen to the cathode. Losses can be reduced by increasing the output humidity to remove excess water, increasing the rate at which hydrogen enters the system and using porous electrodes. Concentration polarization occurs at high current density, as seen in Figure 3, due to blocked reaction sites by excess water and humidification (U.S. Department of Energy, 2000).

1.6 Advancements in Fuel Cell Technology

Currently, the leading source of energy used in the production of electricity is fossil fuels. Fossil fuels are transformed into a useful energy via combustion reactions. However, 50% of the energy produced in this process is lost as heat and pollutant gasses are produced. The conversion of chemical energy directly into electrical energy is thus an active area of study. PEM fuel cells are of interest since hydrogen as a fuel is obtained from a sustainable source, the energy produced per unit volume of gas is high and the process does not require combustion. Fuel cells can operate at 60% efficiency and can reach up to 80% in systems that involve cogeneration (Calay & Mustafa, 2013). In the past 10 years fuel cells have been introduced as a potential alternative to internal combustion engines in vehicles. Honda released the first fuel cell car commercially in 2008. Fuel cell applications in public transportation have also received government funding in hopes of reducing the production of pollutants. The first fuel cell buses were released in 1994. The application of fuel cell technology has also been seen in vehicles used for material handling such as forklifts (Calay & Mustafa, 2013). Platinum catalysts are most commonly used in PEM fuel cells, such as in this experiment. However, there have been recent efforts to implement a more cost-efficient alternative, such as an iron catalyst. Iron catalysts are less expensive but have lower power density. They possess weaker mass-transport properties (Proietti, et al., 2011). To increase the efficiency catalytic and conductive electrodes are required. The rate at which reactions occur is increased by the use of porous electrodes that have a greater surface area (U.S. Department of Energy, 2000). 1.6 Limitations of Fuel Cell TechnologyIn order to overcome commercial barriers associated with fuel cells the technology would have to become as reliable as the internal combustion engine. Fueling stations as well as qualified mechanics would also be required in order for the fuel cell technology to be successful. In order for the fuel cell to compete with the current combustion engine the cost of the system must be decreased by a factor of two and the life time of the system increased by a factor of two. The predicted fuel cell cost in transport applications is $49 per kilowatt. This cost must be reduced to at least $39 per kilowatt to be in competition with the combustion engine. Reducing costs requires a cut back on materials (such as platinum), power density enhancement, and increase in durability of the system (Calay & Mustafa, 2013). 2.0 Experimental2.1 Equipment UsedThe experiment was conducted using an H2 ECOnomy ProFC series fuel cell. This model has 3 cells in the stack, and an N-112 perfluorosulfonic acid polymer membrane (H2 Economy, 2008). In order to control the water content of the air entering the cell, a (insert name/model of humidifier used) was used at a temperature of 40C. The selection of this temperature is explained in Appendix B. To ensure temperature control throughout the duration of the experiment, a (insert model of oven) was used.2.2 Safety Hazards Wear gloves, lab coat and goggles Hydrogen and oxygen gas are explosive and self-igniting Pressurized gas canisters must be secured and handled properly2.3 ProcedureFollowing is a description of the procedure used for each trial of the experiment. The gas flows were adjusted to investigate cell performance in multiple flow ratios, both varying the amount of gas as well as the hydrogen:oxygen ratio. The flowrates tested may be found in Table 1.Table 1: Gas flowrates used in the performance investigation.Flowrates of Hydrogen:Oxygen (mL/min)

100 : 50

150 : 75

200 : 100

50 : 100

100 : 200

100 : 100

1. It was ensured that the gas lines were attached to the large 3-cell PEM fuel cell.2. The hydrogen and oxygen gas tanks were turned on with one full turn and then the pressures were adjusted with the regulator.3. The mass flow controllers were adjusted to produce the desired gas flowrates.4. The oven was turned on to 50C, and it was ensured that it remained near this operating temperature throughout the experiment. If the temperature fluctuated while measurements were taken it was noted.5. The humidifier was set to an operating temperature of 40C.6. The ammeter and voltmeter were turned on. It was ensured that the ammeter was connected in series and the voltmeter connected in a parallel circuit.7. The temperature was noted. The resistance was applied to the cell in increments of 0.1 , slowly moving up in larger increments. All 0.1 increments were recorded followed by 5 measurements in the 1 range, 3 or 4 in the 10 range and 2 at a time in the 100 range.8. With each resistance level the voltage and current were recorded.9. Steps 1-8 were repeated with various flow rates of hydrogen and oxygen.10. After completion of the experiment the main valve on the hydrogen and oxygen tanks were turned off first followed by the pressure gauge.

Figure 4: Schematic of experimental setup.3.0 Results and Discussion3.1 Cell Performance CurvesFigure 5 summarizes the cell performance at each flowrate and stoichiometry analyzed. It is clear from the plot that the cell shows highly non-ideal performance characteristics. The figure also shows that the cell operates most predictably when the hydrogen:oxygen ratio is 2:1. As can be seen in Appendix B, this is what would be expected. Each mole of oxygen reduced requires 4 electrons, while each mole of hydrogen oxidized only provides 2. It is clear from Figure 5 that operating in a reverse stoichiometric ratio of 1:2 hydrogen:oxygen highly impacts the cells functionality. Almost no voltage is provided when current is drawn from the cell. This is likely because this gas mixture leaves the cell starved for fuel.

Figure 5: Cell Performance at various flowrates and stoichiometries.

3.2 Cell Power OutputWhile previously discussed results make it clear that the fuel cell functions best when gas is fed in stoichiometric ratios, further analysis is needed to examine how varying flowrate impacts the overall power output of the cell. Cell power at the 100:50, 150:75, and 200:100 flowrates was calculated, and is summarized in Figure 6.

Figure 6: Power Output for the 2:1 ratio flowrates.

3.3 Linear fit of the region of Ohmic Polarization (Resistance Loss).

Table 2 Summary of the lines of best fit applied to the data recorded. All the plots can be found in the appendix in figures XXX to XXX.H2 FlowrateO2 flowrateSlope of linear sectionY-InterceptR2 Value

ml/minml/minmV/AmV-

10050-3878.21904.60.9953

15075-3383.01884.10.9994

200100-3275.01591.90.9920

250125-3151.8140309788

100100-5024.32342.30.9687

100200-9404.91195.20.7888

50100-607.1821.50.0833

As it can be observed from the table above, a linear trend line was found to be a very good fit for the region of Ohmic polarization for stoichiometric ratio of 2(H2):1(O2) as the R2 values are above 99% for the first 3 data sets and nearly 98% for the fourth data set. Furthermore, it was also found that the higher the total flow rate at this stoichiometric ratio, the better the cell performs in this region of the graph as the slope of the linear portion becomes less steep. This indicates that the difference between the ideal and actual voltage decreased meaning that it would be recommended to operate the cell at higher flow rates. Furthermore, the higher the current is the greater the performance difference is.In the case of the data resulting from the 1:1 H2 to O2 ratio experiment, it was possible to fit a straight line that resulted in a R2 value of 96% by avoiding a couple of point which would have caused the deviation to increase greatly. Consequently, a linear relationship could be established in a considerable portion of the graph (see fig. XX). As expected, the cell does not perform as well as with a 2:1 ratio. The slope was nearly 33% higher than the one of corresponding to the 100 (H2): 50 (O2) data set which suggests that an oversupply of oxygen causes a significant drop in cell performance.For the data sets corresponding to the experiments performed with a reverse stoichiometric ratio (1(H2): 2 (O2)) it was impossible to find a good linear fit. As it can be seen on table XX. the R2 value corresponding to the trend line was 8.33% which is too low and implies that there is practically no linear relationship between the voltage and the current.Overall, this table not only confirms that the stoichiometric ratio of 2:1 results in the best performance but also suggests that a higher flowrate results in improved performance. However, it is important to note that increasing the flowrate increases the costs as the exhaust gases are not recycled.

3.4 Error propagation and main sources of error.The major source of error came from the voltmeter readings. In general, the greater the resistance was, the greater the error. For the 2:1 stoichiometric readings, the voltmeters values oscillated around +-1 mV at the beginning with a total resistance of less than 1 ohm but ended varying in almost 20mV when the total resistancce was increased to 1000 ohms. The ampeter vas fairly constant and only varied very slightly giving a maximum absolute error of +-0.01A. These errors are relatively small as the maximum relative error for the voltage readings is

The flow meters seemed to be very constant throughout the experiment and an absolute error of +-0.5 ml/min was assumed. This translates into a maxmum relative error of 1% for our lowest flow rate (50 mL/min) and 0.25% for our highest flow rate (200 mL/min).The humidifier settings were not touched and it was assumed that it was correctly calibrated and maintained a humidity level of 40%.In our pre-lab calculations, it was determined that temperature was not a major factor in the performance of the cell if the cell was to operate ideally. This is because the EOCV was nearly identical at 50 C than at 25 C. To avoid increasing our total error, the oven was opened when the temperature surpassed 51.0 C and was closed as soon as the temperature dropped to 50 C.

4.0 Conclusions

5.0 ReferencesCalay, R. K., & Mustafa, M. Y. (2013). Challenges facing hydrogen fuel cell technology to replace combustion. Advanced Materials Research.Department of Chemical Engineering. (2015). Factors Affecting The Performance of a Polymer Electrolyte Fuel Cell. Queen's University.H2 Economy. (2008). PEM Fuel Cell Stacks ProFC-XU. Armenia.Larminie, J., & Dicks, A. (2003). Fuel Cell Systems Explained. England: John Wiley & Sons Ltd.PerkinElmer Inc. (2010). (PerkinElmer) Retrieved January 18, 2015, from Quality Control of Polymer Electrolyte Membrane Fuel Cells by Thermogravimetric Analysis: www.perkinelemer.comProietti, E., Jaouen, F., Lefevre, M., Larouche, N., Tian, J., Herranz, J., & Dodelet, J.-P. (2011). Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells. Nature Communications. doi:10.1038/ncomms1427Smitha, B., Sridhar, S., & Khan, A. A. (2005). Solid polymer electrolyte membranes for fuel cell applications-a review. Journal of Membrane Science, 12-15.U.S. Department of Energy. (2000). Fuel Cell Handbook (Fifth Edition). West Virginia: Natinal Energy Technology Laboratory.

Appendix A: Raw Data

I) Data for Calculating Equilibrium PotentialThe following data from Fuel Cell Systems Explained is used to determine Gibbs free energy for the calculation of the equilibrium potential, EOCV (Larminie & Dicks, 2003). Table 3: Gibb's free energy of liquid water at different temperaturesForm of Water ProductTemperature [C]Gibbs Free Energy, [kJ/mol]

Liquid25-237.2

Liquid80-228.2

II) Physical Properties of Anode and Cathode ReagentsThe table below summarizes some of the physical properties of oxygen and hydrogen, used in deriving the expressions for oxygen and hydrogen usage (Larminie & Dicks, 2003): Table 4: Physical properties of gasesType of GasMolecular Weight [g/mol]Density [g/cm3]

O2320.0013 @ 25C and 1 atm

H220.0001 @ 0C and 1 atm

Appendix B: Sample CalculationsI) Calculating the Energy of Open Circuit VoltageThe equilibrium potential is given by the following expression:

Where:EOCV is the energy of the open circuit voltage [V] = [J/C] G is the Gibbs free energy released [kJ/mol]F is Faradays constant [96485 C/mol]n is the number of transferred electrons [mol]An approximate value for Gibbs free energy for the reaction at 50C is calculated assuming a linear relationship between Gibbs free energy and temperature. The linear interpolation was performed based on tabulated values (see Appendix A, Section I) and resulted in the following:

Now, the energy of the open circuit voltage can be determined:

II) Finding the Humidifier Operating TemperatureFor a cell operating at 50C with a relative humidity of 60%, this corresponds to a partial pressure of water in the gas of 7.54 kPa. This is found using the definition of relative humidity and data from steam tables.

Rearranging gives a required Pw of 7.41 kPa. Referring to the steam tables, the saturation temperature at this pressure is 40C. This is the desired temperature for the humidifier.III) Derivation of Oxygen and Hydrogen FlowratesThe usage of anode and cathode reactants can be calculated based on Faraday's Law (Larminie & Dicks, 2003).

WhereF is Faradays constant [96485 C/mol]I is current [C/s] is the electron stoichiometric coefficient [mol]

Equation (1) depicts that four electrons are transferred for every mole of oxygen. Equation (2) depicts that 2 moles of electrons are transferred for every mole of hydrogen. Therefore, the flow-rate of oxygen and hydrogen can be expressed as:

For a stack of n fuel cells, the reactant usages become:

Typically, the units of mol/s is not practical. In order to write the usages in mL/min, one must use the molecular weight and density of the gas at the given temperature. These values are located in Appendix A, section II. Assuming that the density of the gases remain relatively constant with temperature, the usage expressions can be written in terms of mL/min as follows: