500 h Continuous aging life test on PBI/H 3 PO 4

Transcription

1 International Journal of Hydrogen Energy 31 (2006) h Continuous aging life test on PBI/H 3 PO 4 high-temperature PEMFC Jingwei Hu a,b, Huamin Zhang a,, Yunfeng Zhai a,b, Gang Liu a,b, Baolian Yi a a Dalian Institute of Chemical Physics, Chinese Academy of Sciences, PEMFC Key Materials and Technology Lab, Dalian, Liaoning Province, , China b Graduate School of the Chinese Academy of Science, Chinese Academy of Sciences, Beijing , China Received 17 January 2006; received in revised form 10 April 2006; accepted 19 May 2006 Available online 27 July 2006 Abstract 500 h continuous aging test on high-temperature PEMFC (phosphoric acid-doped PBI system) was conducted, steady-state performance (current density = 640 ma cm 2 ) was recorded, polarization curves were tested every 24 h, and electrochemical techniques cyclic voltametry (CV) and electrochemical impedance spectroscopy (EIS) were used to study the relationship between electrochemical surface area (ESA), internal resistance (R in ) and charge transfer resistance (R ct ) with time. Our research results showed that after about 100 h activation, the cell performance began to degrade, and the cell voltage degradation rate is about 150 μvh 1 at 640 ma cm 2. AC impedance measurements showed that there was no obvious membrane degradation for internal resistance remains almost stable. The main reason for performance degradation is the decrease of ESA and the increase of R ct, which implied that obvious degradation occurs at catalyst International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: High-temperature PEMFC; PBI/H 3 PO 4 ; Voltage degradation; Internal resistance; Performance degradation 1. Introduction Proton exchange membrane fuel cells (PEMFCs) are very promising power generators for electrically powered vehicles and distributed power generation for they possess many advantages, such as near zero pollutant emission, high efficiency and low noise, etc. The common operation temperature for PEMFC is below 80 C, but with research goes further, researchers come to realize that the mid-low PEMFCs (operation temperature lower than 80 C) meet with many challenges. These challenges can be concluded as below: the catalytic activity is not satisfactory when Corresponding author. Tel.: ; fax: address: zhanghm@dicp.ac.cn (H. Zhang). operation temperature is below 80 C; catalyst tends to be poisoned by impurities (CO, nitrides or sulfides) in fuel stream; major part of water product is liquid and this will result in flooding in electrode; in addition, if operated below 80 C, the small difference between operation temperature and ambient temperature requires intensive cooling, especially when the ambient temperature is high or the cell is under high load. Recently, many researchers proposed different ways to solve above problems, their efforts include developing high activity electro-catalyst to increase cell performance [1,2], improving electrode structure for higher catalyst utilization [3], developing CO tolerant catalyst to alleviate catalyst poisonous effect [4] and proposing novel gas diffusion layers to reduce flooding [5]. However, there is still long way to go before solving above /$ International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi: /j.ijhydene

2 1856 J. Hu et al. / International Journal of Hydrogen Energy 31 (2006) problems satisfactorily. At the same time, other researchers developed PEMFCs operated at higher temperature (above 120 C) to solve the problems. For the common used perfluorosulfonic acid (PFSA) membranes cannot be used at higher temperature, different high-temperature PEMFC systems were proposed [6 9], as reviewed recently [10]. Among them one of the most successful system is PBI/H 3 PO 4 (phosphoric acid-doped polybenzimidazole) hightemperature PEMFC for PBI membrane possesses many advantages, such as high glass transition temperature ( C), good chemical resistance, excellent textile fiber properties [11] and good proton conductivity after being doped with H 3 PO 4 [12]. As to PBI/H 3 PO 4 high-temperature PEMFC, researchers focused their research work on its proton conductivity mechanism [12,13], catalyst activity [14] and stack development [15]. It is generally accepted that durability and life are the most considerations for the real application and commercialization of fuel cells, however, the degradation studies are not well documented and the degradation causes and mechanisms are not fully understood as to PBI/H 3 PO 4 high-temperature PEMFC. In this study, 500 h continuous aging test was conducted on PBI/H 3 PO 4 hightemperature PEMFC unit cell, steady-state performance curves and polarization curves were recorded during the life test, electrochemical methods cyclic voltametry (CV) and electrochemical impedance spectroscopy (EIS) were applied here to study the performance degradation on catalyst layer and membrane, respectively. Meanwhile, a simple electrochemical model was used here to interpret the linear relationship between steady-state voltage and time observed during aging test. 2. Experimental 2.1. Electrode preparation The gas diffusion layer (GDL) was from SGL company with 300 μm thickness, the catalyst ink was prepared by adding PBI solution with NMP to catalyst powder (commercial 46.5% Pt/C from TKK, Japan), after dispersed in ultrasonic bath for 30 min, the catalyst ink was sprayed on the GDL evenly, the GDE was prepared after evaporation at suitable temperatures (2 h at 60 C, 2 h at 80 C and 10 min at 120 C). The mean catalyst loading is 1 mg cm 2, the active area of unit cell is 7.8cm 2. In this work, the cathode side and the anode side use the same electrode Unit cell test After electrode was prepared, the electrode and H 3 PO 4 -doped PBI membrane (the 35 μm PBI membrane was supplied by our international research partner). Before use, the PBI membrane was immersed in 85% H 3 PO 4 for 20 min at 80 C to obtain proton conduction, then the electrodes and H 3 PO 4 -doped membrane were assembled in a home-made unit cell, the assembling force was 3 N m. The unit cell was installed in our home-made fuel cell test platform. The cell was operated at 150 C, 0.1 MPa pressure was applied at both anode side and cathode side, and the flow rate is 100 ml s 1 at both sides Steady-state performance test The unit cell was operated at constant load (640 ma cm 2 ) using our home-made test platform. Every 24 h the voltage at this current was recorded Polarization test The polarization curves were recorded every 24 h on our home-made platform, the voltage was recorded when it reaches its steady state at different currents EIS measurement Every 24 h, the EIS measurement was conducted on unit cell operated at constant load (640 ma cm 2 ). The test frequency scope was from 1 mhz to 1000 Hz with logarithmic spacing, the stimulate AC amplitude was 10 mv. The electrochemical working station is EG&G model 1025 FRA and model 263A Unit cell CV test Every 24 h, the cathode side was purged by nitrogen and hydrogen flowed though anode side. In a threeelectrode configuration working electrode was cathode side, anode side acted as reference electrode (DHE) and counter electrode simultaneously the CV curves were recorded from 0V (vs. DHE) to 0.6V (vs. DHE) with the scan rate of 20 mv s 1. The electrochemical working station is EG&G model 263A Characterization by transmission electron microscopy (TEM) The catalyst morphology before and after aging test was examined by TEM (JEOL, JEM-1200EX).

3 J. Hu et al. / International Journal of Hydrogen Energy 31 (2006) Characterization by scanning electron micrograph (SEM) The cross section morphology of MEA before and after aging test was examined by SEM (JEOL, JSM- 6360LV SEM). 3. Results and discussion Steady-state degradation curves at constant load (640 ma cm 2 ) were listed in Fig. 1. Two regions were obvious in Fig. 1, before 100 h operation, the cell was in activation phase, the voltage increased from 0.5 to 0.58 V, in the activation phase, the three-phase zone (TPZ) was expanded and cell performance was increasing; the cell was in degradation phase after 100 h, cell voltage decreased from 0.58 to 0.52V in the following 400 h, the fitted line of Fig. 1 degradation part showed that the mean degradation rate was about 150 μvh 1.It should be noted that the little voltage increases in Fig. 1 indicate nitrogen purging and electrochemical measurements, and the sharp voltage increase at 400 h indicates gas replacement, but these fluctuations do not affect the overall degradation tendency. The overall degradation tendency can be concluded as linear degradation for approximate linear relationship exits between cell voltage and time during degradation phase in Fig. 1. The typical polarization curves are listed in Fig. 2, the results was coincident with steady-state degradation test, at about 100 h, the cell reaches its best performance. From Figs. 1 and 2, it can be concluded that at initial 500 h aging test, the performance degradation was obvious. The next problem that should be answered is that what caused the degradation. As to unit cell, two factors determined the degradation procedure, one is the performance degradation of membrane, the other is the performance degradation of catalyst that caused by high-temperature operation. In addition, delamination of electrode and membrane (the stability of MEA construction) was also found be to a common reason for performance degradation [17]. This work was focused on the evaluation of the effects of different factors on performance degradation. Two electrochemical methods were used to determine the performance degradation on membrane and electrocatalyst layer. One of the methods is EIS. In the equivalent circuit as shown in Fig. 3(a), L represents the inductance of wires; R hf represents high frequency resistance that reflects the internal resistance of the unit cell; R ct represents charge transfer resistance that reflects the resistance of chemical reaction. Since the anodic reaction resistance is negligible, R ct stands for the Fig. 1. Steady-state voltage plot with test time during constant load (640 ma cm 2 ). Fig. 2. Typical polarization curves during steady-state aging test. charge transfer resistance of cathodic reaction; CPE is the constant phase element that used to simulate the porous surface of the electrode. From Fig. 3(b) the simulated EIS result compare very well with the experimental data, so the proposed equivalent circuit in Fig. 3(a) is reasonable to interpret the AC impedance behavior of unit cell. Fig. 4 listed the typical AC impedance diagram tested at different times during the aging test. As to the performance degradation of membrane, the changes of internal resistance (R hf ) can be used to evaluate the performance degradation of membrane; the reason why the evolution of R hf can be used to evaluate the performance degradation of PBI membrane is that main part of the changes of internal resistance is due to the changes of membrane resistance for other components of internal resistance (resistance of end-plates, flow field plates and contact resistance between them,

4 1858 J. Hu et al. / International Journal of Hydrogen Energy 31 (2006) Fig. 3. (a) Equivalent circuit and (b) typical impedance and simulation results; circle, experimental data; line, simulation data. Fig. 4. Typical AC impedance diagram test at different time during aging test. etc.) remain almost unchanged. The evolution of internal resistance was given in Fig. 5. From Fig. 5, no obvious increase of internal resistance was observed in our test, which proved that no obvious membrane degradation occurred. In a word, no obvious performance degradation was found on membrane during 500 h continuous aging test. As to the performance degradation of catalyst, two parameters can be used to evaluate the catalyst degradation. One parameter is R ct which can be derived from equivalent circuit analysis from EIS. The evolution of R ct was shown in Fig. 6. From Fig. 6, it is obvious that the R ct is very sensitive to performance changes: at about 100 h, R ct reaches its minimum value, indicating lowest cathode reaction resistance; after 100 h activation, R ct began to increase from it lowest value (86 mω cm 2 ) to 150 mω cm 2 at 500 h. R ct is widely used as an index of catalyst activity, the increase of R ct reflects obvious loss of catalyst activity in degradation phase. The other parameter used here to evaluate

5 J. Hu et al. / International Journal of Hydrogen Energy 31 (2006) Fig. 5. The evolution of internal resistance of unit cell with time. Fig. 7. Typical hydrogen desorption curves at different times during aging test. Fig. 6. The evolution of R ct with time. Fig. 8. The relationship between In(ESA) and time during aging test. performance degradation of catalyst is electrochemical surface area (ESA) which can be calculated from hydrogen desorption oxidation curves in CV curves [16]. Fig. 7 listed the typical hydrogen desorption oxidation peaks and the evolution of natural logarithm of ESA (ln(esa)) was shown in Fig. 8. From Fig. 8, the ESA is also very sensitive to cell performance degradation, after about 100 h activation phase, the ESA reached its maximum and began to decrease at degradation phase, this phenomenon indicated that the main reason that causes performance degradation is the decrease of ESA. It is widely accepted that higher-temperature operation favors the catalyst efficiency, it is true because the initial ESA after fully activation was very high; ESA reached 700 cm 2 (equivalent to 89 cm 2 mg 1 Pt). Unfortunately, the degradation was relatively fast in initial aging test, ESA decreased to 132 cm 2 (or 16.9cm 2 mg 1 Pt) after about 500 h continuous operation. It is also interesting that a good liner relationship existed between ln(esa) and time (Fig. 8) in the degradation phase. From basic electrochemical law if the anodic overpotential is omitted, cell voltage can be calculated as the following: E = E r K ln i i 0 ir ( in this test i = 0.64 A ), where E represents cell voltage, E r represents reverse voltage, i and i 0 represent current density and apparent exchange current density of ORR reaction, r represents internal resistance and A represents ESA. According to our experimental results, internal resistance can be treated as a constant (Fig. 5), so from above equation cell voltage will has a linear relationship with

6 1860 J. Hu et al. / International Journal of Hydrogen Energy 31 (2006) Fig. 9. TEM images of the cathode catalyst: (a) before and (b) after 500 h aging test. InA. Further, InA have linear relationship with time according to Fig. 8, for this reason it is natural that the cell voltage will have a linear relationship with time, please refer to Fig. 1. Fig. 9 is the TEM image of cathode catalyst before and after 500 h continuous aging test, Fig. 10 is the particle size distribution of catalyst before and after aging test derived from Fig. 9. From Figs. 9 and 10 it is obvious that serious sintering occurred in catalyst, the mean particle size increased from 3.8 to 6.9 nm. The TEM results agreed with electrochemical measurement, it proved the catalyst tended to agglomerate during high-temperature operation, the agglomerate of catalyst caused the decrease of ESA and increase of R ct, and as a result it caused the performance degradation. Delamination of the membrane and electrode is a common means of failure in fuel cell membranes [17]. SEM characterizations of new MEA and MEA after 500 h continuous aging test were conducted to investigate the delamination of MEA in aging test, as shown in Fig. 11. In this study, the electrodes (anode and cathode) and PBI membrane were not hot-pressed to make MEA as in usual low-temperature PEMFCs, the electrode and MEA were just put together by the assembling force. Since the electrodes and membrane are not hot-pressured, delamination was found even on new MEA-refer to Fig. 11(a1) and (a2); but there was aqueous H 3 PO 4 existed between the gap of electrode and membrane (delamination site), the aqueous H 3 PO 4 served as electrolyte to conduct proton between electrode and membrane phase, that is the reason why the internal resistance was very low even there existed delamination between electrodes and membrane, that is also the reason why the electrodes and membrane were not needed to be hot-pressed to make MEA for high-temperature PBI system. And from our SEM observations, compared with new MEA, no more serious delamination was found (Fig. 11). It was reported that differences in thermal and hydrated expansion properties of the different materials may also be responsible for delamination over time [17]. In this study, the cell was operated at a constant temperature (150 C) so the difference of electrodes and membrane in thermal expansion properties was not significant; more importantly, the gases do not need to be humidified and the proton conduction of H 3 PO 4 -doped PBI membrane is less dependent on (almost independent on) water, so the difference of electrodes and membrane in hydrated expansion properties can be wholly neglected. For these reasons, no more serious delamination was found. As a conclusion, the existence of liquid H 3 PO 4 at the delamination sites and no further obvious delamination during aging test make the delamination not Fig. 10. Particle size distribution for cathode catalyst: (a) before and (b) after 500 h aging test.

7 J. Hu et al. / International Journal of Hydrogen Energy 31 (2006) Fig. 11. SEM photographs of MEA cross section before (a1 and a2) and after (a1 and a2) 500 h continuous aging test. a significant reason for performance degradation during 500 h continuous aging test in this study. In conclusion, the linear degradation of voltage at constant load is determined by the catalyst performance degradation the increase of reaction resistance and the decrease of ESA, and no obvious degradation on proton conduction of membrane was observed in 500 h aging test. In addition, the delamination of membrane and electrode is not a significant factor. 4. Conclusions After about 100 h activation, the PBI/H 3 PO 4 hightemperature PEMFC showed obvious degradation during continuous operation at 150 C, the mean degradation rate is about 150 μvh 1. Electrochemical experiments showed that the initial continuous degradation behavior is caused by catalyst degradation, and no significant loss of proton conduction was found on H 3 PO 4 -doped PBI membrane. And the delamination of membrane and electrode is not a significant factor. The catalyst degradation at higher operation temperature is critical to high-temperature PEMFC performance; further researches on high-stable catalyst for high-temperature PEMFC is needed. Acknowledgment This work is partly supported by the National Natural Science Foundation of China (Grant No ) and the Innovation Foundation of Dalian Institute of Chemical Physics. References [1] Lebedeva NP, Janssen GJM. Electrochim Acta 2005;51:29. [2] Wang BJ. J Power Sources 2005;152:1. [3] Fernández R, Aparicio PF, Daza L. J Power Sources 2005;151:18. [4] Lebedeva NPG, Janssen JM. Electrochim Acta 2005;51:29. [5] Bultel Y, Wiezell K, Aouen JF. Electrochim Acta 2005;151:474. [6] Hogarth WHJ, Costa JC, Lu GQ. J Power Sources 2005;142:223. [7] Kim YT, K Song M, Kim KH. et al. Electrochim Acta 2004;50:645. [8] Kwak SH, Yang TH, Kim CS. et al. Electrochim Acta 2004;50:653. [9] Kim YM, Choi SH, Lee HC. et al. Electrochim Acta 2004;49:4787. [10] Li QF, He RH, Jensen OJ, Bjerrum NJ. Chem Mater 2003;15:4896. [11] Chung TS. Macromol J, Sci, Part C, Rev Macromol Chem 1997;37:277. [12] Li QF, He RH, Berg RW, Hjuler HA, Bjerrum NJ. Solid State Ionics 2004;168:177.

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