FCH JU Grant Agreement number: 325262 Project acronym: CISTEM Project title: Construction of Improved HT-PEM MEAs and Stacks for Long Term Stable Modular CHP Units Work package: 2 - Materials beyond State of the Art Deliverable: 2.1 Optimized PBI based membranes Period covered: 01.06.2013 31.05.2014 Name, title and organization of the scientific representative of the project's coordinator: Peter Wagner NEXT ENERGY EWE Forschungszentrum für Energietechnologie e.v. Carl-von-Ossietzky-Str. 15 26129 Oldenburg / Germany Tel: + 49 441 999 06 316 Fax: + 49 441 999 06 109 E-mail: peter.wagner@next-energy.de Project website address: www.project-cistem.eu 1
Contents 1. Introduction... 3 2. Long-term testing with State of the Art Membrane-Electrode-Assemblies.. 3 3. Long-term testing with Optimized PBI membranes in Membrane-Electrode Assemblies... 5 3.1. Composite PBI membranes in Membrane-Electrode-Assemblies... 5 3.2. Thermally cured PBI membranes in Membrane-Electrode-Assemblies.. 9 3.2.1. Materials beyond SoA: Long-term test... 14 4. Conclusion... 16 2
1. Introduction Deliverable 2.1 states that long term test results with new PBI based HT-PEM membranes from DPS and UCLM with a durability of 1,000 hours will be provided (Month 12). Two different new PBI based membranes have been tested to reach the 1,000 h long test, one thermally cured membrane from DPS and Titanium based composite PBI membrane (2 % TiO2) from UCLM. DPS manufactured two MEAs from composite PBI membrane material from UCLM with standard electrodes and performed the first long term tests. UCLM received a MEA with the thermally cured membrane from DPS to carry out the 1,000 hour test. 2. Long-term testing with State of the Art Membrane- Electrode-Assemblies Long-term testing at constant current density has been carried out at NEXT ENERGY facilities with improved SoA MEAs. The specifications of the MEA used for the long-term test are summarized in Table 1. Table 1 MEA specifications MEA ID no. / Type MEA-13-466 Dapozol Batch no. G55-13- 013 Active cell area [ cm² ] Catalyst loading [ mg/cm² ] 21.1 Cathode: 1.6 Anode: 1.6 Membrane / MEA thickness [ µm ] 40 / 649 (at NE) Edge Reinforcem ent Kapton Figure 1 shows the 1,000 h test of the improved SoA Dapozol MEA at constant load without including break-in procedure time span. The constant load conditions have been interrupted for MEA characterization once per week (black points in Figure 1). Moreover, the MEA has been also electrochemically characterized right after the break-in procedure (day 0). A slight voltage improvement during the first days of the test can be observed. After MEA characterization in day 7, fuel cell voltage starts to decline. In addition, the fuel cell voltage shows a stepwise decrease after each one of the MEA characterization days, emphasizing that the characterization process causes additional degradation. Thus, the fuel cell voltage loss rate for the whole longterm test is 56.5 μv/h. The MEA thickness is nearly constant during the first 27 3
j (A/cm 2 ) E (V) d ( m) Deliverable 2.1: Optimized PBI based membranes days of the test. Few hours before the fifth MEA characterization (day 28), the MEA thickness suddenly decreased in one step from -1 to -6 μm and afterwards remained constant until the end of the test. 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1-56.5 V/h at 0.3 A/cm 2 0-20 -40-60 -80-100 -120 0.0-140 0 5 10 15 20 25 30 35 t (day) Figure 1 Long-term test for the fuel cell operating with H 2 /Air (λ H2 /λ Air =1.5/2). Constant load operation was interrupted for MEA characterization on day: 0, 7, 14, 18, 28 and 35 (black circles). T=160 C, p=1 atm, 5-fold serpentine flow field, P c =0.75 MPa The polarization curves recorded during the interruptions for MEA characterization are shown in Figure 2. The results from the first day of the test and day 7 show similar fuel cell behavior at the intermediate current density range, although the fuel cell performance in the first day of characterization is the lowest one at the low current density range. Thus, the fuel cell behavior observed in both Figures 1 and 2 during the first 7 days of the test suggests catalyst activity enhancement to ORR as impedance analysis also reveals (not included in this report). Moreover, the fuel cell shows 5 % voltage loss during the whole test and electrical efficiency loss from 33.0 % (day 0) to 31.4 % (day 35) at 0.3 A/cm 2. 4
E (V) P (W/cm 2 ) Deliverable 2.1: Optimized PBI based membranes 1.0 0.9 0.8 0 day 7 day 14 day 18 day 28 day 35 day 0.4 0.3 0.7 0.6 0.2 0.5 0.1 0.4 0.3 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 j (A/cm 2 ) -5 % V loss at 0.3 A/cm 2 Figure 2 Polarization curves for the fuel cell operating with H 2 /Air (λ H2 /λ Air =1.5/2) at different long-term test times. T=160 C, p=1 atm, 5-fold serpentine flow field, P c =0.75 MPa 3. Long-term testing with Optimized PBI membranes in Membrane-Electrode Assemblies 3.1. Composite PBI membranes in Membrane-Electrode- Assemblies Figure 3 shows images of the membrane developed by UCLM and the Fuel cell station and manufactured MEA for the 1,000 h essay at DPS facilities. The different MEAs were manufactured with the following materials, which are listed in Table 2. It must be pointed out that DPS has settled to a new building and some equipment has been left at the original location. Thus, all required protocol test routines can at this point of time not be performed. 5
(a) (b) Figure 3.(a) PBI + TiO 2 membrane sample developed by UCLM and (b) Fuel cell device and manufactured MEA with PBI + TiO 2 membrane for 1,000 h durability test at DPS facilities. Table 2 Materials and operation conditions for MEA testing for the Deliverable 2.1 Fuel cell No. Electrodes Membrane MEA-RD-14-077 E-14-002 (A/C): 1.4 /1.4 mg Pt/cm 2 M-13-578; Thermally cured D.L 10.3 MEA-RD-14-078 MEA-RD-14-089 E-14-002 (A/C): 1.4 /1.4 mg Pt/cm 2 Improved PBI+TiO 2 D.L 8 E-14-002 (A/C): 1.4 /1.4 mg Pt/cm 2 Improved PBI+TiO 2 D.L. 8.4 *DL=Acid Doping level. Operation conditions: λ H2 /λ air =1.5/2.5, 160 C, PBI as binder. The MEA-RD-14-078 was tested for the 1000 h durability test, which is presented in Figure 4. Continuous increase on MEA performance was observed at the beginning of life (BoL). The increased voltage rate was 116 µv h -1. Failure of the hydrogen supply took place after 238 hours. 6
Figure 4 Durability test with materials beyond state of art. Improved uniformity PBI membrane + 2 w% TiO 2 & Standard DPS electrodes. MEA ID : MEA-RD-14-078, 160 C, λ H2 /λ air =1.5/2.5 Due to the unexpected failure on the hydrogen supply, MEA-RD-14-089 was manufactured which is a replicate of MEA-RD-14-078 (See Table 2). Figure 5 shows the cell voltage with time for the second MEA and unfortunately can be observed that again a failure in the control system of the equipment led to the finish of the experiment. Figure 5 Durability test with materials beyond state of art. Improved uniformity PBI membrane + 2 w% TiO 2 & Standard DPS electrodes. MEA ID : MEA-RD-14-089, 160 C, λ H2 /λ air =1.5/2.5 7
In both cases, Figures 4 and 5, it can be observed that during the long term experiment the cell voltage drop was nearly Zero which means that the probabilities of success were very high if the unexpected failures in the system would not occur. Figure 6 shows the polarization curves at two different times and it can be observed that they performed very similar which confirms the above statement. (b) Figure 6 (a) Polarization curve after 168 h running at 200 ma/cm 2, (b) polarization curves after 336 h running at 200 ma/cm 2. 160 C, λ H2 /λ air =1.5/2.5 8
3.2. Thermally cured PBI membranes in Membrane-Electrode- Assemblies Figure 7 shows the break-in process for the MEA with a thermally cured PBI membrane from DPS carried out at 120 C with hydrogen as fuel and air as oxidant during 48 hours. It is observed that the voltage value is stable along the break in process. The cell voltage was around 0.625 V. Figure 7 Break in process for MEA (MEA-RD-14-077) Pc = 0.75 MPa, T=120 C, *λ H2 =1.2; λ Air =2, I = 0.1 A/cm 2 Figure 8 shows the stability test carried out. The stars in the time line mark the protocol test routines carried out during this experiment. According to the established protocol to evaluate the long term stability, at least once a week electrochemical characterization was necessary. This protocol test routine consisted of polarization curves with air and oxygen, impedance analysis at different current densities with air and with oxygen, LSV and cyclic voltammetries. 9
Voltage (V) Deliverable 2.1: Optimized PBI based membranes 0.66 0.64 0.62 0.6 0.58 0.56 0.54 1 3 EoL 0 2 1 0 100 200 300 400 500 600 700 800 900 1 Figure 8: Stability test MEA (MEA-RD-14-077) P c = 0.75 MPa, T=160 C, *λ H2 =1.2; λ Air =2, I= 0.2 A/cm 2 10 Time (h) In the Figure 8 is observed that once the break-in process finished a protocol test routine was performed and the performance increased to 0.65 V from 0.63 V, the initial value. A high plateau was reached during the following 200 hours and not protocol test routine was performed because it was holidays (Easter week). The day after holidays the test routine was carried out and a high decrease in voltage was obtained, around 30 mv with respect to the maximum value of cell voltage. Moreover, at the following weekend a decrease of 8 mv was observed and not explanation could be found. It will be discussed in the next project meeting. After this period, a slight decrease of cell voltage (about 10 mv) after each protocol test routine applied was reached, not 30 mv as in the second one. The end of life was reached after 766 hours, at a cell voltage value of 0.56 V (a 10 % of the initial value of voltage = 0.63 V). The strange decrease observed after the second protocol test routine (time 200 h approx.) currently cannot be explained. No failure of the electricity in UCLM facilities occurred, no lack of fuel and no lack of air supply has been registered. The phosphate and Pt in the cathode and anode side was measured during all the experiment but no phosphate either Pt leaching were observed. Therefore, we cannot explain why this high performance decay occurred. Figure 9 shows the variation of the OCV registered during the different protocol test routine performed during this long term study and the maximum power densities calculated from the polarization curves. After the 200 hours of running, decay in the value of the OCV, when air was the oxidant, can be observed, whereas in the case of the oxygen, this decay is very low. After that, it seems that the OCV values try to recover the initial values and an increase of the OCV values is observed till the end of the experiment. Nevertheless, it must be pointed out that during the whole long test the OCV remained almost stable
Voltage (V) Deliverable 2.1: Optimized PBI based membranes which means that the membrane was stable, no big pin holes where formed and hydrogen neither oxygen pass through the membrane. On the other hand, the maximum power density went down after the 200 h, from 350 mwcm -2 at the beginning to 250 mwcm -2 at the end of the experiment, for the case of oxygen. Whereas for the case of air, the loss of power density was not very appreciable. (a) 1.03 1.02 1.01 1 0.99 0.98 0.97 0.96 0.95 OCV(V) air OCV (V) O2 0 100 200 300 400 500 600 700 800 900 Time (h) (b) Figure 9 a) OCV variation for oxygen and air during the durability test. b) Evolution of Power density values measured at 0.4 A/cm 2 for Air (circles) and at 0.6 A/cm 2 for Oxygen (triangles) Figure 10 shows the five polarization curves (CP) carried out using air and oxygen. It can be observed that during the first 200 h (the two first polarization curves (CP1 and CP2) the performance was very similar, overall, for the case of air. At 0.4 V and working with air, 20 A can be reached during the first 200 h while at the end of the experiment, 12 A was the maximum current. In the case 11
of oxygen values higher than 20 A are not shown because of the limitation of the equipment. The booster had a maximum current of 20 A. (a) (b) Figure 10 Evolution of polarization curves during the durability test. A) with air; B) with Oxygen: CP1 at t = 0 h; CP2 at t= 200 h approx. CP3 at t= 400 h approx; CP4 at t =600 h and CP5 at t = 766 h Table 3 shows the values of the Electrochemical Surface Area (ECSA) obtained during the stability test calculated from the cyclic voltammetries. It is observed a high decrease of the value between the initial ECSA and the ECSA obtained in the second protocol test routine. This could explain the strange decrease obtained after second protocol test routine at 200 h. The values are almost the half of the 12
beginning which means that the catalyst layer has suffered a high degradation after this test routine (30 mv approx at 200 h). After that, the decrease is slower which explains the lower degradation obtained after the protocol test routine (10 mv approx.). Table 3 Electrochemical active surface obtained from each protocol routine during the durability test Protocol test routine ECSA (m 2 /g) 1 22.25 2 14.24 3 12.75 4 10.54 EoL 9.81 Table 4 shows the values of the ohmic resistance at the beginning, at 200 h and at the end of the experiment 766 h) values obtained from the different impedances, carried out with oxygen and air at different intensities in each protocol test routine, during the stability test. It is observed the degradation of the MEA, traduced in an increase of Ohmic resistance values. But if the two first values are compared it can be seen that the degradation is not so high which means that the loss of performance is more due to a degradation of the catalyst than a degradation of the membrane. The degradation rate (slope of the curve) is very similar in both cases with air and oxygen. Only the value for the lowest current density shows a very big value but it could be due to the impedance spectra showed very noise. Table 4 Ohmic resistance variations obtained for each impedance spectra Current density (A/cm 2 ) R ohm t= 0h mω cm 2 R ohm t= 200h mω cm 2 Air R ohm t= 766h mω cm 2 Rate mωcm 2 h -1 0.03 9.4 9.4 14.5 0.0064 (R 2 =0.889) 0.1 8.2 8.4 13 0.0068 (R 2 =0.930) 0.2 8.2 8.7 13 0.0065 (R 2 =0.957) 13 R ohm t=0h mωc m 2 R ohm t= 200h mω cm 2 Oxygen R ohm t= 766h mω cm 2 Rate mωcm 2 h -1 8.5 9.5 16.3 0.0097 (R 2 =0.895) 8.5 9.5 12.8 0.0059 (R 2 =0.984) 8.4 9.4 12.3 0.0055 (R 2 =0.963)
Current (A) Deliverable 2.1: Optimized PBI based membranes 0.3 8.2 8.5 12.5 0.006 (R 2 =0.946) 0.4 8.1 8.4 12.2 0.0056 (R 2 =0.930) 8.4 9.2 12.3 0.0055 (R 2 =0.970) 8.4 9.2 12.3 0.0055 (R 2 =0.960) Figure 11 shows the linear sweep voltammetries. A slight increase of the registered intensities is observed, which means a little crossover through the membrane. This is a signal of damage on the membrane, but the slope of the curves is similar in all cases, so the membrane did not experiment a high degradation during the experiment as it has been concluded from other characterization techniques. As conclusion, it must be said that the proposed 1,000 h have not been reached, but from evaluating the electrochemical characterization techniques any cause which could explain the dramatically cell voltage drop at 200 h cannot be deduced at this point of time. So a repetition of the experiment and further investigation is necessary. One MEA test alone does not allow the partners to make any further assumptions. 3.00E-02 2.50E-02 Linear Sweep Voltametric 2.00E-02 1.50E-02 1.00E-02 5.00E-03 0.00E+00 0.3-5.00E-03 0.35 0.4 0.45 0.5 0.55 Routine (0) Routine (I) Routine (II) Routine (III) Routine (Final) -1.00E-02 Pontential (V) Figure 11 Linear sweep voltammetries from different protocol routines 3.2.1.Materials beyond SoA: Long-term test Figure 12 shows the results from 1000 hours long-term test. MEA-RD-14-135 was based on the thermally cured membrane. After 48 hours of break in procedure the current density was increased up to 0.3 A/cm 2. The constant load conditions were interrupted in order to perform electrochemical characterisation. 14
j (A/cm 2 ) E (V) Deliverable 2.1: Optimized PBI based membranes Table 5. Materials for MEA manufacturing. Fuel cell No. RD-14-135 *D.L. Doping Level Electrodes E-12-041(A/C):1 mg Pt/cm 2 Membrane M-13-384; D.L.* 10,5 1.0 0.8 Performance drop 0.6 0.4 0.2 0.0 0 200 400 600 800 1000 Time (hours) Figure 122. Long-term test for the fuel cell operating with λ H2 /λ air =1.5/2, 160 C. Current density was interrupted for MEA characterization at time: 168 h, 336 h, 504 h, 672 h and 840 h (red circles). As observed in Figure 12, the performance drop was quite significant after the first chemical characterization that was performed after 168 h of long term testing. The MEA performance increased slightly over time for the rest of the durability test. Polarization curves with air and oxygen as oxidant was obtained at 168, 336, 504, 672 and 840 hours, until 1000 hours long term test goal was achieved. 15
Voltage (V) Voltage (V) Deliverable 2.1: Optimized PBI based membranes 1.0 0.9 0.8 (a) t = 168 h t = 336 h t = 504 h t = 672 h t = 840 h 1.0 0.9 0.8 (b) t = 168 h t = 336 h t = 504 h t = 672 h t = 840 h 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Current density (A/cm 2 ) 0.4 0.0 0.2 0.4 0.6 0.8 1.0 Current density (A/cm 2 ) Figure 13. (a) polarization curves during long term test, oxygen as comburent λ H2 /λ O2 =1.5/9.5, 160 C. (b) Polarization curves during long-term test using air as comburent λ H2 /λ air =1.5/2, 160 C. In Figure 13.a, the obtained polarization curve at 168 h reached a maximum current density of 1.5 A/cm 2, (using O 2 as oxidant), the voltage at the mentioned current density was 0.407 V. The performance loss after the high current exposure was found to be 4 % at 0.3 A/cm 2. A performance improvement was observed for the latest test (840 hours of long-term test), with respect to the previous data obtained at 504 and 672 h. In Figure 13.b, the obtained polarization curve at 168 h reached a maximum current density of approximately 0.81 A/cm 2 at 0.4 V (using air as oxidant). The performance loss was 15% at 0.3 A/cm 2. In this case, the performance was consistently decreasing over time until the 1,000 hours durability was reached. The dramatic drop performance at 168 h (See Figure 12) was believed to be due to the high current density during the first polarization curves, using hydrogen and oxygen as oxidant at 168 h (1.5 A/cm 2 ). Similar results were observed by UCLM when thermally cured membranes were tested into an MEA. In order to avoid further degradation in the MEA, the maximum current density was set to approx. 0.53 A/cm 2 for the rest of the test. 4. Conclusion All objectives of the Deliverable 2.1 have been achieved on time. MEAs with optimized PBI based membranes have been assembled. MEAs with thermally cured membranes, provided by DPS, fulfill the purpose of 1000h of durability. On an overall basis, WP 2 has a general conformity with the timescale. 16