The rig developed was used to characterise a MEA. As is normal with unused MEAs, they must be used in a specific way to bring the MEA up to its optimum performance before use. This process is the initial bedding in of the key layers of the MEA such as the CL. Common practise among the fuel cell community is to run the new MEA at a steady state for around one to two hours (as to let the membrane fully hydrate), then run groups of polarisation curves at
Table 6.1: Table of parameters logged
Parameter Unit
Time Seconds
Relative Humidity @ anode inlet % Relative Humidity @ cathode inlet % Anode pressure before humidifier bar Anode pressure before cell bar Cathode pressure before humidifier bar Cathode pressure before cell bar Ambient temperature oC Cell temp 1 - 7 oC Anode mass flow rate slpm Cathode mass flow rate slpm Cathode gas temperature (inlet) oC Anode gas temperature (inlet) oC
Coolant temperature oC
Heater tank temperature oC
Set Point Anode mass flow rate slpm Set Point Cathode mass flow rate slpm Set Point heater tank temperature oC
regular intervals as to cycle the MEA through its full current range. Once the cell has reached an equilibrium at its highest current range, the MEA is considered conditioned.
After a full conditioning run (required before each use), an MEA was characterised using polarisation curve and EIS techniques to form a comparison baseline for degradation testing. EIS is an experimental technique where the different loss mechanisms observed in a fuel cell can be quantified. It is a non-intrusive technique that doesn’t degrade the cell whereas techniques such as Cyclic Voltammetry (CV) would. During operation of the cell, AC potential is applied through the fuel cell and the current through the cell is measured. The resulting data is often displayed via a Nyquist plot similar to Figure 6.11.
Figure 6.11: EIS Nyquist plot interpretation [71]
the use of EIS techniques. This tool was used to increase the confidence in certain failure mode’s affects on certain regions of the potential losses. EIS techniques are ran in either galvanostatic or potentiostatic conditions (current held, or voltage/potential held respectively).
For the conditioning runs, the MEA was run for one hour, then at 30 minute intervals after that point, polarisation curves were run in sets of 5 in order to ascertain the health of the cell. Galvanostatic EIS was also undertaken at low, medium and high current densities (0.2, 0.4 and 0.6 A/cm2 respectively). This technique was repeated until the MEA showed no more
signs of improvement. At this stage, the MEA was considered to be characterised in its highest performance.
The test was run with feed gas pressures of 0.55 bar, the temperatures of the gas feeds at around 30◦C, and the cell temperature kept at 39◦C as in Figures 6.12 & 6.13. The cell temperature was kept higher than the gas feeds as to avoid the flooding of the cell due to condensation occurring in the cell when a higher temperature gas meets a lower temperature environment. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 x 104 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Time [S] Pressure [bar] Air Hydrogen
Figure 6.12: Gas feed pressures
The variation in H2gas pressure in Figure 6.12 is due to other lab users demanding H2from the same bottle being used during the experiment. The variation is negligible at around 0.05 bar, and therefore won’t affect the results of the validation. The final variations in pressures are experienced during the shut-down of the rig, and are after the FC was operated.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 x 104 20 25 30 35 40 45 50 Time [S] Temperature [°C] Cell Temp Air Hydrogen
Figure 6.13: Cell and gas feed temperatures
The initial spike in temperature seen in Figure 6.13 is due to the transient response delay of the system control, where the heating element overshoots their target, then returned to the set-point. This will not have an impact upon results as the testing is only commenced when all operating parameters has reached their desired values.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 x 104 −5 0 5 10 15 20 Time [S]
Mass Flow Rate [SLPM]
Air Hydrogen
Figure 6.14: Feed gas flow rates
Gas feed flow rates were kept at a hydrogen stoichiometry of 1.2 and air stoichiometry of around 2.5. The flow rates for the experimentation run can be seen in Figure 6.14. The first
spike in flow rate is due to the N2 purge at the beginning of the cell operation, and the final spike at around 1.9x104seconds is due to the shut-down N2 purge as is expected.
Relative humidities were set at 70%, however due to the nature of the bubbler type humidifier system, an error of ± 5% was found in the air supply, and an error of ± 10% was found for the H2humidifier during operation. As can be seen at 1.4x104seconds in Figure 6.15, the hydrogen
feed RH sensor experienced a flooding event, and became unreliable. However, the values from this point are assumed to be the same as what was experienced previous to the flooding event at around 70%. This assumption can be made when analysing the data from 0.7x104 seconds
to 1.3x104 seconds. Here it can be assumed that (as with the Air feed) the RH measurements
were levelling out after the initial fluctuations of transient response delay experienced from 0 seconds to 0.7x104. Therefore, from a visual analysis of the Figure, it is assumed that the RH
levels would carry on to repeat what was seen from 0.7x104 seconds to 1.3x104 seconds.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 x 104 0 10 20 30 40 50 60 70 80 90 100 Time [S] Temperature [°C] Air Hydrogen
Figure 6.15: Feed gas relative humidities
Once steady state had been reached with the operational variables, the characterisation commenced. Cell potential was maintained at 0.65V in order to avoid degradation as much as was practicable (see Figure 6.16).
Figure 6.16 shows the voltage and current profile over time for the conditioning run of the MEA. Once all of the ancillary equipment was up to pressure and temperature, the H2 supply was passed through the cell. As soon as the chemical reactions began, the voltage of the cell was set to 0.65V as is common practice with MEA conditioning. After 3600 seconds the cell was temporarily cycled to its maximum current capability to ascertain the performance of the cell. It was assumed at this stage that the cell needed a little longer to hydrate before the full polarisation runs were to be started. At 0.5x1004seconds the first set of polarisation runs were
started. The green line shows how the voltage of the cell starts at its highest (OCV), then as the current is increased per time-step, it drops to its lowest value. The blue line shows how the current starts at 0, and is increased per time-step to its highest value before the cell voltage gets to its saturation limit. The five peaks in the green line indicates that the polarisation runs were completed five times per polarisation run.
This process was repeated three more times as can be seen by the profiles in Figure 6.16, until equilibrium was reached and the MEA was deemed conditioned. The process of closing down the test rig saw the slow drop off of the current as to not shock the newly conditioned MEA until the cell was no longer producing current, then the rig was purged and shut down.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 x 104 0 10 20 30 40 50 60 70 80 90 100 Current [A] Time [S] 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 x 104 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Voltage [V]
Figure 6.16: Current and Voltage profile throughout test
The tail off of the green voltage profile at the very end of the test is due to the N2 purge replacing all of the fuel H2in the cell during the purge, and therefore the OCV dropping to low levels until the operator is sure that there is no more H2 in the system.
The overall operating parameters throughout the characterisation are listed in Table 6.2:
Table 6.2: Table of fuel cell operational parameters
Parameter Value Unit Relative Humidity 70 ±5 %
Pressure 0.55 bar
Flow Rate H2 1.5 slpm
Flow Rate Air 8.5 slpm
H2Temp 31 Celsius
Air Temp 30.5 Celsius Cell Temp 39 Celsius
The RH, pressure and flow rates were all desired values that are commonly used in the FC community. However, the temperatures were the maximum values available from the FC rig. All three temperatures would ideally need to be at 70-80oC.
The averages of each of the sets of five polarisation curves were taken and plotted on the same figure in order to ascertain the health of the cell, see Figure 6.17.
0 10 20 30 40 50 60 70 80 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Current [A] Voltage [V] 1st cycle 2nd cycle 3rd cycle 4th cycle
Figure 6.17: Polarisation curve averages for each set of tests
It can be seen that after the 3rd set of polarisation curves, an equilibrium state was achieved, and the health of the cell could be assumed to be optimum. We can see that the polarisation
curves start to correlate quite positively after 3 cycles. A visual indication of an equilibrium of performance is all that is needed to signify an activated MEA.