- Email: [email protected]
thaw cycles
international journal of hydrogen energy 35 (2010) 2986–2993
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Degradation behaviors of polymer electrolyte membrane fuel cell under freeze/thaw cycles Maji Luoa,*, Chengyong Huangb, Wei Liua, Zhiping Luob, Mu Panb a
School of Automobile Engineering, Wuhan University of Technology, Wuhan 430070, PR China State Key Laboratory of Advanced Technology for Materials Synthesis & Processing, Wuhan University of Technology, Wuhan 430070, PR China
b
article info
abstract
Article history:
Degradation behaviors of polymer electrolyte membrane fuel cell (PEMFC) in high current
Received 14 April 2009
density region were investigated under Freeze/Thaw cycles. Different dehumidification
Received in revised form
scenarios namely hot purge, cold purge and no purge were adopted for comparison.
21 May 2009
Micrographs from scanning electron microscopy proved little change in catalyst-coated
Accepted 15 June 2009
membrane (CCM) integrity, no delamination or segregation occurred after many freeze/
Available online 7 July 2009
thaw cycles. Cyclic Voltammetry (CV) measurement revealed reduction in electrochemical active surface area of CCM. The observed performance decay in the high current density
Keywords:
region was mainly attributed to the increased interface contact resistance and degraded
Freezing/thaw
electric and gas coupling characteristics at interfaces between CCM and GDL in this paper.
Performance decay
Meanwhile, the performance degradation under low current densities (for example
High current density
400 mA cm2 or even lower) was mainly ascribed to the degraded characteristics of catalyst
Performance degradation
layers referring to CCM as cyclic voltammetry indicated. Proper dehumidification through
Polymer electrolyte membrane fuel
gas purging is effective to maintain stable preference under subzero temperature.
cell (PEMFC)
1.
ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Introduction
There are several technical barriers to be overcome before commercializing polymer electrolyte membrane fuel cell (PEMFC) as power sources in automobiles. One of the emerging problems is their operation in cold climates. Significant activity has been taken in subzero temperature characteristics of PEMFCs. However, few literature specially addresses degradation phenomenon in high current density region (namely 0.8 A cm2 or above, if not indicated in this paper) after freeze/thaw operations. According to the United States Department of Energy’s technical targets, freeze durability is vital importance for fuel cell [1]. To ascertain whether performance degradation occurred, it is necessary to
investigate integral performance over the whole discharge region after subzero operation. As St-Pierre and co-workers validated [2], there is a close relationship between water management and freeze/thaw related performance decay. They carried out gas purging before repetitive freeze/thaw cycles to 20 C at the moment stack was shut down and at room temperature led to discrepant performance decay. The former scenario indicated deeper performance decay at high current densities, which was attributed to mass transport limitation (the difference between oxygen and air at high current densities increased with each cycle). However, the latter scenario revealed no performance decay over the entire current density range after 3 freeze/thaw cycles and performance loss amounted to as
* Corresponding author. Tel.: þ86 27 8785 9136; fax: þ86 27 8785 9247. E-mail address: [email protected] (M. Luo). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.06.036
international journal of hydrogen energy 35 (2010) 2986–2993
low as approximately 0.1 mV cycle1 at 0.5 A cm2 after 55 freeze/thaw cycles. It was also discovered to operate fuel cell at elevated working temperature (100 C) for a period during initial phase would be an effective way to recover cell performance after freeze/thaw events. Performance degradation at high current densities was also found in Hou’s [3] work. Gas purging with a certain relative humidity performed before repetitive freeze/thaw operations revealed no deeper performance loss in the mass transport limitation region. However, the GDL was easily flooded in high current density region. His group speculated that volume expansion of water freezing most likely slightly changed the GDL structure, so the loss of hydrophobicity in the GDL might exist some other reasons, as Oszcipok [4] proposed before. However, Thompson [5] recently indicated that large scale cracking was observed on the MEA electrode of water-clearing fuel cell following repeated cold start events, but no damage was observed in fine pore structure of electrode or micro-porous layers even in the wettest region. Obviously, there have been conflicting reports on whether microstructure of electrode and micro-porous layers changes or not regarding cycling through sub-freezing temperatures. Different gas purge scenarios were adopted to find out the main influencing factor for performance degradation under high current density. The effect of resided water on PEM fuel cell after cold start (from 5 C) was also experimentally investigated by Hou et al. [6]. They found that there was no change in the electrochemical active surface area (ECA) and charge transfer resistance at low current density while the mass-transport process slightly changed with the residual water amount in the cell at high current density. In this study, degradation behaviors of PEMFCs in high current density region were investigated under freeze/thaw cycles. Different dehumidification scenarios namely hot purge, cold purge and no purge were adopted for comparison. Polarization curves and power density curves were utilized to compare the cell performance evolution for one and the same. The degradation behavior was characterized by maximum power density, upper limit current density along with voltage degradation rate at 0.8 A cm2 and 1.2 A cm2. Scanning electron microscopy (SEM) and Cyclic Voltammetry (CV) were used to investigate the performance change of the cell after freeze/thaw cycles.
2.
Experimental
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ink was sprayed onto Teflon membrane. Then desiccation treatment was performed at 60 C for 10 min and at 90 C in nitrogen atmosphere for 3 min, respectively. The catalyst layer was then transferred onto the proton exchange membrane (Nafion 211 from DuPont without treatment) at 125 C and 1.5 MPa by the decal method to form the catalystcoated membrane (CCM). The Pt loading was controlled by weight at 0.2 mgPt cm2 for both the anode and cathode catalyst layers. After the Teflon support was peeled away, CCM was sandwiched between GDLs to form the MEA finally. It has been reported that MEA constructed from CCM and GDLs provides better power density due to an extended catalyst/ionomer interface and improvement of catalyst utilization [7].
2.2.
Single cell tests
Single cell was constructed from the prepared MEA, Teflon gasket, graphite plate hardware with a single serpentine flow field of 25 cm2 or 50 cm2 active areas and gold plated stainless steel plates on both sides of the MEA. Performance of the single cell was evaluated by measuring the I–V curves using a fuel cell test station (G50, GreenLight). Pure hydrogen and air were utilized as the reactants at the anode and cathode, respectively. The anode and cathode gas flows depending on cell active area were controlled at 0.5 Normal Litres per Minute (NLPM) and 2.0 NLPM for 25 cm2 active areas, 0.7 NLPM and 2.3 NLPM for 50 cm2 active areas, respectively. All cells were tested at temperature 65 C, the reactant gas humidification temperatures were 65 C with zero back pressure. The dew point temperature was controlled by dew point humidifier at 65 C. Prior to recording the polarization curves, cells were activated by changing current load till stable performance was achieved. After operation, different water management scenarios were adopted for comparison. The total experiment was divided into three groups. For the 1st group experiment, 0.3 MPa (meter pressure) dry nitrogen wasn’t adopted to purge extra water for 0.5 h until cell temperature was brought down to below 30 C, namely ‘‘cold purge’’. For the 2nd group experiment, 0.3 MPa (meter pressure) dry nitrogen was adopted to purge extra water for 5 min the moment cell was shut down, namely ‘‘hot purge’’. For the 3rd group experiment, no gas purge scenario was performed. The cell was hermetically sealed with dead-end vessels without purging and directly transferred to environmental chamber the moment cell temperature was cooled down to 30 C or below.
2.1. Preparation of membrane electrode assemblies (MEAs)
2.3.
The fabrication process of the electrodes was divided into two steps. Firstly, a homogeneous suspension composed of PTFE and carbon powder was sprayed on the wet-proofed carbon paper (TGP-060, Toray Inc.) to form the gas diffusion layer (GDL) with a 50 mm thick micro-porous layer. Secondly, the catalyst ink for the electrodes was prepared by mixing the catalyst powder (60 wt.% Pt/C, Johnson Matthey), Nafion solution (DE 520, 5 wt.%, EW 1000, DuPont), and isopropyl alcohol. After ultrasonic treatment for 30 min and high-speed homogenization for 1.5 h, respectively, the prepared catalyst
All freeze/thaw operations were conducted in an environmental chamber (CTP401F, Chongqing Sida Experiment Instrument Co., Ltd). The temperature was cycled from 10 C or 20 C to 40 C. It is of vital importance to ensure that cells investigated in freeze/thaw operations were frozen through thoroughly, so as to achieve reliable results and meet the practical application. Because according to one-dimensional transient model for frost heave [8], the freezing process in a PEMFC is mostly an unsaturated freezing process, ice lens growth is caused by the unfrozen water flow into the ice lens.
Freeze/thaw operations
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international journal of hydrogen energy 35 (2010) 2986–2993
Whether the frost front will propagate depends on the balance between the rate of heat loss and unfrozen water flow. So temperature held at 10 C or 20 C for 10 h during each cycle was necessary and adopted. Simultaneously, a plastic basin with water of half the amount of accommodation was employed as a reference to determine whether the water inside the cell had been thawed thoroughly as expected. It was found that even the chamber temperature approached over 30 C with a calefactive rate of 1.7 C min1, the water inside the plastic basin remained considerable ice blocks. This phenomenon indicated that ice suffered less meltingness from heat transfer resistance of cell components. Thus 2 h duration at 40 C was adopted here in order to achieve thorough thawing of cell from frozen state. The internal cell temperature was monitored by Pt100 thermocouple temperature meter.
2.4.
Characterization of catalyst-coated membranes
Cell performance evolution was revealed by polarization curves and power density curves. Performance degradation in high current density region was characterized by the degradation rates of maximum power density, upper limit current density and voltage under certain current densities. According to St-Pierre [2], degradation rate can be calculated by formula as follows: 1 degradation rate mV cycle ¼
final performanceðmVÞ original performanceðmVÞ cycle number
(1)
The morphologies of the catalyst-coated membranes were observed by scanning electron microscopy (SEM, JEOL JSM5610LV, Japan). Cross-section specimens of the catalystcoated membranes were prepared by breaking the membrane under liquid nitrogen (77 K). The samples were Au-sputtered under vacuum before the SEM examination. Cyclic voltammetry was implemented to determine the electrochemical surface area (ECSA) of the cathode electrode available for hydrogen adsorption or desorption. Cyclic voltammetry was performed on Autolab test branch (PGSTAT30/ GPES, Eco Chemie, Netherlands). Single cells with 4 cm2 active areas were used for test. Cyclic voltammetry was measured by feeding humidified nitrogen and hydrogen to the working and the counter electrode, respectively. Hydrogen flow rate was 40 mL min1, Nitrogen flow rate was 40 mL min1. The hydrogen electrode also served as the reference electrode. For the CV, three cycles were performed at a sweep rate of 50 mV s1 and cell potential was scanned from 0.05 to 1.2 V. All the measurements were carried out at 65 C with a temperature controller. The electrochemical active surface area was calculated by formula as follows [9]: ECSA m2 g1 ¼
Qarea ðmC cm2 Þ 10 QH ðmC cm2 Þ MPt ðmg cm2 Þ
3.
Results and discussion
3.1. Fuel cell performances under different purging scenarios As for the 1st group experiment, the cell performance characterized by polarization curves and power density curves after different freeze/thaw cycles between 10 C and 40 C were both plotted as Fig. 1 indicated. Freeze/thaw operations were performed under cold purge scenario between the 1st and 10th cycle. For comparison, freeze/thaw operations after hot purge without cooling were cycled between the 11th and 25th cycle. It showed excellent integrity in the whole discharge region unless thorough gas purge operation was performed on membrane electrode assembly (MEA) after cell temperature was lowered down to around room temperature. However, once dry gas purge was performed the moment the cell was shut down without cooling, performance in high current density region would result in irreversible loss. Two polarization curves regarding the 15th and 25th operations proved repetitive fluctuation phenomenon in the high current density region. When gas purge scenario was reverted to cold purge after the 25th operation (as Curve 50th indicated), it was found that performance in high current density region achieved steady recovering. However, it was also found that once the maximum power density at about 1.7 A cm2 was excelled and higher current density was reached, irreversible performance loss was discovered obviously in Curve 50th. The upper limit current density of cell was reduced from 2.20 A cm2 to 2.08 A cm2 simultaneously. It is estimated that diffusion resistance was increased, thus resulting in original current load drawn failure. Maximum power density reduced from 0.856 W cm2 to 0.854 W cm2 after the 50th freeze/thaw operation at a degradation rate of 0.005% cycle1. The voltage degradation rate was approximately 0.18 mV cycle1 at 0.8 A cm2 and 0.20 mV cycle1 at 1.2 A cm2, respectively. Performance was unstable and the recovering phenomenon was obvious as indicated in Fig. 2 which describes the performance evolution of single cell during the activation phase after the 25th freeze/thaw operation. The repetitive
(2)
where QH is the charge area of the H-desorption on the smooth Pt for 210 mC cm2. Qarea is integral area under the H-desorption peak of cyclic voltammetry curves. MPt is Pt loading of CCM.
Fig. 1 – Performance evolution after different freeze/thaw cycles between L10 8C and 40 8C for a 25 cm2 single cell under cold purge and hot purge.
international journal of hydrogen energy 35 (2010) 2986–2993
Fig. 2 – Performance evolutions during the activation course after the 25th freeze/thaw cycle for a 25 cm2 single cell under hot purge.
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Fig. 4 – Performance evolution after different freeze/thaw cycles between L20 8C and 40 8C for a 50 cm2 single cell under hot purge.
fluctuation of voltage and power density at about 1.92 A cm2 suggested some uncertainty and minor change in material or microstructure. The voltage and power density below 1.2 A cm2 retained excellent stability. According to the results from St-Pierre [2] and Hou [3], both humidity of gas for dehumidification and purging time are important. The 2nd group experiment was designed to seek optimum duration for dry gas ‘‘hot purge’’ to furthest reduce the performance decay. The experiment was carried out as explained in Section 2.3. Performance evolution during the activation phase after 50 freeze/thaw cycles between 20 C and 40 C for fuel cell with 50 cm2 active areas was shown in Fig. 3. It was found that the single cell integral performance was stable as seen from performance evolution during eight activation operations. As activation operation increased, cell performance decreased stably in low current density region while increased placidly in high current density region. Fig. 4 describes the cell performance evolution after different freeze/thaw cycles by polarization curves and power density
curves. It was most obvious that there was little difference in cell integral performance after 50 freeze/thaw cycles under this scenario. The maximum power density was 0.711 W cm2 initially and increased to 0.737 W cm2 after the 50th freeze/ thaw operation at a degradation rate of 0.07% cycle1. The voltage degradation rate was approximately 0.32 mV cycle1 at 0.8 A cm2 and 0.14 mV cycle1 at 1.2 A cm2, respectively. Thus, a conclusion can be drawn here is that proper hot purge by dry gas can effectively avoid performance decay from subzero operation and storage. Cell performance evolution was also investigated without any dehumidification operation as Fig. 5 indicated. Performance evolution of fuel cell with 25 cm2 active areas during activation phase after the 20th freeze/thaw operation revealed that full performance couldn’t recover without activation effort under up to 8 operations. Further, voltage in the high current density region changed acutely. Comparing with the former two scenarios (cold purge and hot purge), it is most likely that the residual water inside GDL or between CCM and GDL has worsened the hydrosphere transport and migratory
Fig. 3 – Performance evolution during the activation course after 50 freeze/thaw cycles between L20 8C and 40 8C for a 50 cm2 single cell under hot purge.
Fig. 5 – Performance evolution during the activation course after 20 freeze/thaw cycles between L20 8C and 40 8C for a 25 cm2 single cell without dehumidification.
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international journal of hydrogen energy 35 (2010) 2986–2993
diffusion characteristics. There is a great discrepancy between Curve zero and Curve 7th as Fig. 5 shown. Integral optimum performance was achieved after the 7th activation operation and was stable enough. This phenomenon might be explained in this way. As activation operation increased, membrane electrode assembly was adequately swelled, the interfacial contact resistance between CCM and GDL was reduced sufficiently, and thus coupling characteristics became a little better. So a successive voltage recovering phenomenon was observed. However, there was some minor change in hydrophobicity or microstructure in gas diffusion layer after freeze/ thaw operation. The hydrosphere transport and migratory diffusion characteristics there were no longer coupled so well. The hindrance for water and gas transport would result in water flooding or reactant starvation, thus sharp voltage dropped to nearly zero and recovered to only 0.1–0.2 V repetitively occurred. Fig. 6 indicated the performance evolution during activation phase after the 30th freeze/thaw operation. There is also a great discrepancy before and after activation operation as can be seen from this figure. The upper limit current density was reduced from 2.00 A cm2 to 1.84 A cm2 this time. When current load was applied, the voltage sharply dropped down to zero and slowly returned to 0.1–0.2 V repeatedly, but never recovered again. It is proposed here that the root for this voltage fluctuation phenomenon was mass diffusion backup. It was also validated in Fig. 7 that not only the maximum power density and upper limit current density were both reduced, but also performance degradation was observed through the entire discharge region. The maximum power density reduced from 0.798 W cm2 to 0.774 W cm2 after the 30th freeze/thaw operation at a degradation rate of 0.1% cycle1. The voltage degradation rate was approximately 0.17 mV cycle1 at 0.8 A cm2 and 0.47 mV cycle1 at 1.2 A cm2, respectively. By comparing the three scenarios, the degradation rate of the maximum power density was only 0.005% cycle1 and 0.07% cycle1 for ‘‘cold purge’’ and ‘‘hot purge’’, respectively. The voltage degradation rate was approximately 0.18 mV cycle1 and 0.32 mV cycle1 at 0.8 A cm2, 0.20 mV cycle1 and 0.14 mV cycle1 at 1.2 A cm2, respectively. However, the degradation rate of the maximum power density was 0.1%
Fig. 7 – Performance evolution after different freeze/thaw cycles between L20 8C and 40 8C for a 25 cm2 single cell without dehumidification. cycle1 in situation without any dehumidification operation. The voltage degradation rate for cell without purge was approximately 0.17 mV cycle1 at 0.8 A cm2 and 0.47 mV cycle1 at 1.2 A cm2, respectively. That’s to say the voltage degradation rate was minus at 1.2 A cm2 only in the 3rd group experiment which indicated real performance decay. The severely reduced upper limit current density and sharp voltage fluctuation phenomenon also suggested potential change in hydrophobicity and microstructure of GDL or CCM/ GDL interfacial coupling characteristics. According to Garzon’s report from Los Alamos National Laboratory (LANL) [10], the degradation in GDL might be a potential failure mechanism under subzero condition. By this token, the result obtained here tallied with their speculation. It is rather obvious that both cold purge and hot purge are suitable for subzero storage. It is necessary to perform proper dehumidification scenario before undergoing sub-freezing temperature.
3.2.
SEM analysis
The cross-section morphologies of catalyst-coated membranes (CCMs) were characterized by scanning electron microscopy (SEM). As from Fig. 8a–d indicated, CCM retained good integrity and no delamination or segregation occurred in the catalyst layers. Obviously, catalyst layers on the CCM bear excellent frozen invulnerability. There is not any visible physical damage and destruction. The succeeding work was concentrated on electrochemical performance of CCM after sub-freezing operations.
3.3. Cylic voltammograms under different purging scenarios
Fig. 6 – Performance evolution during activation phase after 30 freeze/thaw cycles between L20 8C and 40 8C for a 25 cm2 single cell without dehumidification.
Fig. 9 shows cyclic voltammograms of single cell before and after freeze/thaw operations under different operation scenarios. As Fig. 9a indicated, under cold purge scenario, there is little difference between cell hydrogen oxidation peak and sorption peak before and after 20 freeze/thaw operations, but oxygen deoxidization peak and desorption peak ratherish decreased. This phenomenon tallies with the voltage
international journal of hydrogen energy 35 (2010) 2986–2993
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Fig. 8 – SEM Images of CCM: a) fresh CCM without freeze/thaw operation; b) for a 25 cm2 single cell after 50 freeze/thaw cycles between L10 8C and 40 8C under cold purge; c) for a 50 cm2 single cell after 50 freeze/thaw cycles between L20 8C and 40 8C under hot purge; d) for a 25 cm2 single cell after 30 freeze/thaw cycles between L20 8C and 40 8C without dehumidification operation.
fluctuation phenomenon in Fig. 1. It indicated that oxygen deoxidization dynamics and diffusion mass transport dynamics play main role. Calculation revealed that electrochemical surface areas before and after freeze/thaw operations were 44.5 m2 g1 and 42.8 m2 g1, respectively, with a reduction rate of 3.8%. The ECSA degradation rate was 0.085 m2 g1 per freeze/thaw cycle. The evolution course tallies with the phenomenon in Fig. 4 regarding polarization curves and cell power density curves as seen from Fig. 9b. This picture revealed that performance degradation occurred in low current density region and performance improvement in high current density region. The area of hydrogen oxidation peak and sorption peak appreciably decreased while area of oxygen deoxidization peak and desorption peak increased somewhat. It suggested that electrochemical surface area has much matter with cell performance under low current density. The area of oxygen deoxidization peak and desorption peak has much matter with diffusion mass transport and electric and gas coupling characteristics at interfaces between CCM and GDL, which play a main role in cell performance in high current density region. Calculation revealed that electrochemical surface areas before and after freeze/thaw operations were 42.7 m2 g1 and 40.5 m2 g1, respectively, with a reduction rate of 5.2%. The ECSA degradation rate was 0.11 m2 g1 per freeze/thaw cycle. Fig. 9c indicated the cell performance evolution under no purge. Hydrogen oxidation peak and sorption peak
appreciably decreased, however it was more remarkable as for reduction of the area of oxygen deoxidization peak and desorption peak. This phenomenon corresponds to performance evolution in Fig. 7. It suggested that electrochemical active surface area has much matter with cell performance in low current density region. The area of oxygen deoxidization peak and desorption peak has much matter with oxygen deoxidization dynamics and diffusion mass transport dynamics. Mass transport in GDL and electric and gas coupling characteristics at interfaces between CCM and GDL have much influence on cell performance in high current density region. Calculation revealed that electrochemical surface areas before and after freeze/thaw operations were 45.2 m2 g1 and 39.4 m2 g1, respectively, with a reduction rate of 12.8%. The ECSA degradation rate was 0.29 m2 g1 per freeze/thaw cycle. Combining views from Oszcipok [4] and Thompson [5] with the analysis of UI, CV, SEM results in this paper, it is proposed here that the performance decay in the high current density region could be ascribed to variational contact resistance at CCM/GDL interfaces as freeze/thaw cycles increased. Once there is some change in hydrophobicity or microstructure of GDLs, the contact state would change accordingly, thus resulting in worsening of hydrosphere transport and mass diffusion. Hereunto, the fluctuation phenomenon of voltage in the high current density region and the reduced upper limit current density could be easily explained as above.
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international journal of hydrogen energy 35 (2010) 2986–2993
4.
Conclusions
Degradation behaviors of polymer electrolyte membrane fuel cell (PEMFC) in high current density region were compared by cold purge, hot purge and no purge. The degradation rate of the maximum power density was only 0.005% cycle1 and 0.07% cycle1 for ‘‘cold purge’’ and ‘‘hot purge’’, respectively. However, the degradation rate of the maximum power density was 0.1% cycle1 under no purge. The voltage degradation rates were approximately 0.18 mV cycle1 and 0.32 mV cycle1 at 0.8 A cm2, 0.20 mV cycle1 and 0.14 mV cycle1at 1.2 A cm2, under ‘‘cold purge’’ and ‘‘hot purge’’ respectively. Correspondingly, the voltage degradation rate under no purge was 0.17 mV cycle1 at 0.8 A cm2 and 0.47 mV cycle1 at 1.2 A cm2, respectively. By comparing the three scenarios, integral long-term performance could be maintained unless proper dehumidification scenario is adopted. No visible physical damage and destruction was observed by crosssection SEM. The result validated that homemade CCM bore all-right freeze invulnerability. By means of single cell performance test and cyclic voltammetry, it was proposed that reduction of electrochemical active surface area was main cause of cell performance degradation in low current density region; mass transport in GDL and electric and gas coupling characteristics at interfaces between CCM and GDLs were main causes of cell performance degradation in high current density region.
Acknowledgements The work was financially supported by the Ministry of Science and Technology 863 Hi-Technology Research and Development Program of China (no. 2007AA05Z126 and 2008AA11A106) and the National Natural Science Foundation of China (no. 50632050).
references
Fig. 9 – Cyclic voltammograms of single cell before and after freeze/thaw operations under different operation scenarios: a) under cold purge; b) under hot purge; c) without dehumidification.
[1] Garland N. DOE program/targets and workshop objectives[EB/OL], http://www1.eere.energy.gov/ hydrogenandfuelcells/pdfs/01_garland_freeze.pdf [accessed 01.02.05]. [2] St-Pierre J, Roberts J, Colbow K, Campbell S, Nelson A. PEMFC operational and design strategies for sub zero environments. Journal of New Materials for Electrochemical Systems 2005; 8(3):163–76. [3] Hou JB, Yu HM, Zhang SS, Sun SC, Wang HW, Yi BL, et al. Analysis of PEMFC freeze degradation at 20 C after gas purging. Journal of Power Sources 2006;162(1):513–20. [4] Oszcipok M, Riemann D, Kronenwett U, Kreideweis M, Zedda M. Statistic analysis of operational influences on the cold start behaviour of PEM fuel cells. Journal of Power Sources 2005;145(2):407–15. [5] Thompson EL. Electron microscope analysis of low temperature damage in PEM fuel cells, http://www.optics.rochester.edu/ workgroups/cml/opt307/spr05/eric/ [accessed 30.05.05]. [6] Hou JB, Yi BL, Yu HM, Hao LX, Song W, Fu Y, et al. Investigation of resided water effects on PEM fuel cell after cold start. International Journal of Hydrogen Energy 2007; 32(17):4503–9.
international journal of hydrogen energy 35 (2010) 2986–2993
[7] Prasanna M, Ha HY, Cho EA, Hong SA, Oh IH, et al. Investigation of oxygen gain in polymer electrolyte membrane fuel cells. Journal of Power Sources 2004;137(1): 1–8. [8] He SH, Mench MM. One-dimensional transient model for frost heave in polymer electrolyte fuel cells. Journal of the Electrochemical Society 2006;153(9):A1724–31.
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[9] Ralph TR, Hards GA, Keating JE, Campbell SA, Wilkinson DP, Davis M, et al. Low cost electrodes for proton exchange membrane fuel cells. Journal of the Electrochemical Society 1997;144(11):3845–57. [10] Garzon F, Kim YS, Mukundan R, Pivovar B. Sub freezing fuel cell effects, http://www.hydrogen.energy.gov/pdfs/review06/ fc_30_mukundan.pdf [accessed 18.05.06].
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Degradation behaviors of polymer electrolyte membrane fuel cell under freeze/thaw cycles Maji Luoa,*, Chengyong Huangb, Wei Liua, Zhiping Luob, Mu Panb a
School of Automobile Engineering, Wuhan University of Technology, Wuhan 430070, PR China State Key Laboratory of Advanced Technology for Materials Synthesis & Processing, Wuhan University of Technology, Wuhan 430070, PR China
b
article info
abstract
Article history:
Degradation behaviors of polymer electrolyte membrane fuel cell (PEMFC) in high current
Received 14 April 2009
density region were investigated under Freeze/Thaw cycles. Different dehumidification
Received in revised form
scenarios namely hot purge, cold purge and no purge were adopted for comparison.
21 May 2009
Micrographs from scanning electron microscopy proved little change in catalyst-coated
Accepted 15 June 2009
membrane (CCM) integrity, no delamination or segregation occurred after many freeze/
Available online 7 July 2009
thaw cycles. Cyclic Voltammetry (CV) measurement revealed reduction in electrochemical active surface area of CCM. The observed performance decay in the high current density
Keywords:
region was mainly attributed to the increased interface contact resistance and degraded
Freezing/thaw
electric and gas coupling characteristics at interfaces between CCM and GDL in this paper.
Performance decay
Meanwhile, the performance degradation under low current densities (for example
High current density
400 mA cm2 or even lower) was mainly ascribed to the degraded characteristics of catalyst
Performance degradation
layers referring to CCM as cyclic voltammetry indicated. Proper dehumidification through
Polymer electrolyte membrane fuel
gas purging is effective to maintain stable preference under subzero temperature.
cell (PEMFC)
1.
ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Introduction
There are several technical barriers to be overcome before commercializing polymer electrolyte membrane fuel cell (PEMFC) as power sources in automobiles. One of the emerging problems is their operation in cold climates. Significant activity has been taken in subzero temperature characteristics of PEMFCs. However, few literature specially addresses degradation phenomenon in high current density region (namely 0.8 A cm2 or above, if not indicated in this paper) after freeze/thaw operations. According to the United States Department of Energy’s technical targets, freeze durability is vital importance for fuel cell [1]. To ascertain whether performance degradation occurred, it is necessary to
investigate integral performance over the whole discharge region after subzero operation. As St-Pierre and co-workers validated [2], there is a close relationship between water management and freeze/thaw related performance decay. They carried out gas purging before repetitive freeze/thaw cycles to 20 C at the moment stack was shut down and at room temperature led to discrepant performance decay. The former scenario indicated deeper performance decay at high current densities, which was attributed to mass transport limitation (the difference between oxygen and air at high current densities increased with each cycle). However, the latter scenario revealed no performance decay over the entire current density range after 3 freeze/thaw cycles and performance loss amounted to as
* Corresponding author. Tel.: þ86 27 8785 9136; fax: þ86 27 8785 9247. E-mail address: [email protected] (M. Luo). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.06.036
international journal of hydrogen energy 35 (2010) 2986–2993
low as approximately 0.1 mV cycle1 at 0.5 A cm2 after 55 freeze/thaw cycles. It was also discovered to operate fuel cell at elevated working temperature (100 C) for a period during initial phase would be an effective way to recover cell performance after freeze/thaw events. Performance degradation at high current densities was also found in Hou’s [3] work. Gas purging with a certain relative humidity performed before repetitive freeze/thaw operations revealed no deeper performance loss in the mass transport limitation region. However, the GDL was easily flooded in high current density region. His group speculated that volume expansion of water freezing most likely slightly changed the GDL structure, so the loss of hydrophobicity in the GDL might exist some other reasons, as Oszcipok [4] proposed before. However, Thompson [5] recently indicated that large scale cracking was observed on the MEA electrode of water-clearing fuel cell following repeated cold start events, but no damage was observed in fine pore structure of electrode or micro-porous layers even in the wettest region. Obviously, there have been conflicting reports on whether microstructure of electrode and micro-porous layers changes or not regarding cycling through sub-freezing temperatures. Different gas purge scenarios were adopted to find out the main influencing factor for performance degradation under high current density. The effect of resided water on PEM fuel cell after cold start (from 5 C) was also experimentally investigated by Hou et al. [6]. They found that there was no change in the electrochemical active surface area (ECA) and charge transfer resistance at low current density while the mass-transport process slightly changed with the residual water amount in the cell at high current density. In this study, degradation behaviors of PEMFCs in high current density region were investigated under freeze/thaw cycles. Different dehumidification scenarios namely hot purge, cold purge and no purge were adopted for comparison. Polarization curves and power density curves were utilized to compare the cell performance evolution for one and the same. The degradation behavior was characterized by maximum power density, upper limit current density along with voltage degradation rate at 0.8 A cm2 and 1.2 A cm2. Scanning electron microscopy (SEM) and Cyclic Voltammetry (CV) were used to investigate the performance change of the cell after freeze/thaw cycles.
2.
Experimental
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ink was sprayed onto Teflon membrane. Then desiccation treatment was performed at 60 C for 10 min and at 90 C in nitrogen atmosphere for 3 min, respectively. The catalyst layer was then transferred onto the proton exchange membrane (Nafion 211 from DuPont without treatment) at 125 C and 1.5 MPa by the decal method to form the catalystcoated membrane (CCM). The Pt loading was controlled by weight at 0.2 mgPt cm2 for both the anode and cathode catalyst layers. After the Teflon support was peeled away, CCM was sandwiched between GDLs to form the MEA finally. It has been reported that MEA constructed from CCM and GDLs provides better power density due to an extended catalyst/ionomer interface and improvement of catalyst utilization [7].
2.2.
Single cell tests
Single cell was constructed from the prepared MEA, Teflon gasket, graphite plate hardware with a single serpentine flow field of 25 cm2 or 50 cm2 active areas and gold plated stainless steel plates on both sides of the MEA. Performance of the single cell was evaluated by measuring the I–V curves using a fuel cell test station (G50, GreenLight). Pure hydrogen and air were utilized as the reactants at the anode and cathode, respectively. The anode and cathode gas flows depending on cell active area were controlled at 0.5 Normal Litres per Minute (NLPM) and 2.0 NLPM for 25 cm2 active areas, 0.7 NLPM and 2.3 NLPM for 50 cm2 active areas, respectively. All cells were tested at temperature 65 C, the reactant gas humidification temperatures were 65 C with zero back pressure. The dew point temperature was controlled by dew point humidifier at 65 C. Prior to recording the polarization curves, cells were activated by changing current load till stable performance was achieved. After operation, different water management scenarios were adopted for comparison. The total experiment was divided into three groups. For the 1st group experiment, 0.3 MPa (meter pressure) dry nitrogen wasn’t adopted to purge extra water for 0.5 h until cell temperature was brought down to below 30 C, namely ‘‘cold purge’’. For the 2nd group experiment, 0.3 MPa (meter pressure) dry nitrogen was adopted to purge extra water for 5 min the moment cell was shut down, namely ‘‘hot purge’’. For the 3rd group experiment, no gas purge scenario was performed. The cell was hermetically sealed with dead-end vessels without purging and directly transferred to environmental chamber the moment cell temperature was cooled down to 30 C or below.
2.1. Preparation of membrane electrode assemblies (MEAs)
2.3.
The fabrication process of the electrodes was divided into two steps. Firstly, a homogeneous suspension composed of PTFE and carbon powder was sprayed on the wet-proofed carbon paper (TGP-060, Toray Inc.) to form the gas diffusion layer (GDL) with a 50 mm thick micro-porous layer. Secondly, the catalyst ink for the electrodes was prepared by mixing the catalyst powder (60 wt.% Pt/C, Johnson Matthey), Nafion solution (DE 520, 5 wt.%, EW 1000, DuPont), and isopropyl alcohol. After ultrasonic treatment for 30 min and high-speed homogenization for 1.5 h, respectively, the prepared catalyst
All freeze/thaw operations were conducted in an environmental chamber (CTP401F, Chongqing Sida Experiment Instrument Co., Ltd). The temperature was cycled from 10 C or 20 C to 40 C. It is of vital importance to ensure that cells investigated in freeze/thaw operations were frozen through thoroughly, so as to achieve reliable results and meet the practical application. Because according to one-dimensional transient model for frost heave [8], the freezing process in a PEMFC is mostly an unsaturated freezing process, ice lens growth is caused by the unfrozen water flow into the ice lens.
Freeze/thaw operations
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Whether the frost front will propagate depends on the balance between the rate of heat loss and unfrozen water flow. So temperature held at 10 C or 20 C for 10 h during each cycle was necessary and adopted. Simultaneously, a plastic basin with water of half the amount of accommodation was employed as a reference to determine whether the water inside the cell had been thawed thoroughly as expected. It was found that even the chamber temperature approached over 30 C with a calefactive rate of 1.7 C min1, the water inside the plastic basin remained considerable ice blocks. This phenomenon indicated that ice suffered less meltingness from heat transfer resistance of cell components. Thus 2 h duration at 40 C was adopted here in order to achieve thorough thawing of cell from frozen state. The internal cell temperature was monitored by Pt100 thermocouple temperature meter.
2.4.
Characterization of catalyst-coated membranes
Cell performance evolution was revealed by polarization curves and power density curves. Performance degradation in high current density region was characterized by the degradation rates of maximum power density, upper limit current density and voltage under certain current densities. According to St-Pierre [2], degradation rate can be calculated by formula as follows: 1 degradation rate mV cycle ¼
final performanceðmVÞ original performanceðmVÞ cycle number
(1)
The morphologies of the catalyst-coated membranes were observed by scanning electron microscopy (SEM, JEOL JSM5610LV, Japan). Cross-section specimens of the catalystcoated membranes were prepared by breaking the membrane under liquid nitrogen (77 K). The samples were Au-sputtered under vacuum before the SEM examination. Cyclic voltammetry was implemented to determine the electrochemical surface area (ECSA) of the cathode electrode available for hydrogen adsorption or desorption. Cyclic voltammetry was performed on Autolab test branch (PGSTAT30/ GPES, Eco Chemie, Netherlands). Single cells with 4 cm2 active areas were used for test. Cyclic voltammetry was measured by feeding humidified nitrogen and hydrogen to the working and the counter electrode, respectively. Hydrogen flow rate was 40 mL min1, Nitrogen flow rate was 40 mL min1. The hydrogen electrode also served as the reference electrode. For the CV, three cycles were performed at a sweep rate of 50 mV s1 and cell potential was scanned from 0.05 to 1.2 V. All the measurements were carried out at 65 C with a temperature controller. The electrochemical active surface area was calculated by formula as follows [9]: ECSA m2 g1 ¼
Qarea ðmC cm2 Þ 10 QH ðmC cm2 Þ MPt ðmg cm2 Þ
3.
Results and discussion
3.1. Fuel cell performances under different purging scenarios As for the 1st group experiment, the cell performance characterized by polarization curves and power density curves after different freeze/thaw cycles between 10 C and 40 C were both plotted as Fig. 1 indicated. Freeze/thaw operations were performed under cold purge scenario between the 1st and 10th cycle. For comparison, freeze/thaw operations after hot purge without cooling were cycled between the 11th and 25th cycle. It showed excellent integrity in the whole discharge region unless thorough gas purge operation was performed on membrane electrode assembly (MEA) after cell temperature was lowered down to around room temperature. However, once dry gas purge was performed the moment the cell was shut down without cooling, performance in high current density region would result in irreversible loss. Two polarization curves regarding the 15th and 25th operations proved repetitive fluctuation phenomenon in the high current density region. When gas purge scenario was reverted to cold purge after the 25th operation (as Curve 50th indicated), it was found that performance in high current density region achieved steady recovering. However, it was also found that once the maximum power density at about 1.7 A cm2 was excelled and higher current density was reached, irreversible performance loss was discovered obviously in Curve 50th. The upper limit current density of cell was reduced from 2.20 A cm2 to 2.08 A cm2 simultaneously. It is estimated that diffusion resistance was increased, thus resulting in original current load drawn failure. Maximum power density reduced from 0.856 W cm2 to 0.854 W cm2 after the 50th freeze/thaw operation at a degradation rate of 0.005% cycle1. The voltage degradation rate was approximately 0.18 mV cycle1 at 0.8 A cm2 and 0.20 mV cycle1 at 1.2 A cm2, respectively. Performance was unstable and the recovering phenomenon was obvious as indicated in Fig. 2 which describes the performance evolution of single cell during the activation phase after the 25th freeze/thaw operation. The repetitive
(2)
where QH is the charge area of the H-desorption on the smooth Pt for 210 mC cm2. Qarea is integral area under the H-desorption peak of cyclic voltammetry curves. MPt is Pt loading of CCM.
Fig. 1 – Performance evolution after different freeze/thaw cycles between L10 8C and 40 8C for a 25 cm2 single cell under cold purge and hot purge.
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Fig. 2 – Performance evolutions during the activation course after the 25th freeze/thaw cycle for a 25 cm2 single cell under hot purge.
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Fig. 4 – Performance evolution after different freeze/thaw cycles between L20 8C and 40 8C for a 50 cm2 single cell under hot purge.
fluctuation of voltage and power density at about 1.92 A cm2 suggested some uncertainty and minor change in material or microstructure. The voltage and power density below 1.2 A cm2 retained excellent stability. According to the results from St-Pierre [2] and Hou [3], both humidity of gas for dehumidification and purging time are important. The 2nd group experiment was designed to seek optimum duration for dry gas ‘‘hot purge’’ to furthest reduce the performance decay. The experiment was carried out as explained in Section 2.3. Performance evolution during the activation phase after 50 freeze/thaw cycles between 20 C and 40 C for fuel cell with 50 cm2 active areas was shown in Fig. 3. It was found that the single cell integral performance was stable as seen from performance evolution during eight activation operations. As activation operation increased, cell performance decreased stably in low current density region while increased placidly in high current density region. Fig. 4 describes the cell performance evolution after different freeze/thaw cycles by polarization curves and power density
curves. It was most obvious that there was little difference in cell integral performance after 50 freeze/thaw cycles under this scenario. The maximum power density was 0.711 W cm2 initially and increased to 0.737 W cm2 after the 50th freeze/ thaw operation at a degradation rate of 0.07% cycle1. The voltage degradation rate was approximately 0.32 mV cycle1 at 0.8 A cm2 and 0.14 mV cycle1 at 1.2 A cm2, respectively. Thus, a conclusion can be drawn here is that proper hot purge by dry gas can effectively avoid performance decay from subzero operation and storage. Cell performance evolution was also investigated without any dehumidification operation as Fig. 5 indicated. Performance evolution of fuel cell with 25 cm2 active areas during activation phase after the 20th freeze/thaw operation revealed that full performance couldn’t recover without activation effort under up to 8 operations. Further, voltage in the high current density region changed acutely. Comparing with the former two scenarios (cold purge and hot purge), it is most likely that the residual water inside GDL or between CCM and GDL has worsened the hydrosphere transport and migratory
Fig. 3 – Performance evolution during the activation course after 50 freeze/thaw cycles between L20 8C and 40 8C for a 50 cm2 single cell under hot purge.
Fig. 5 – Performance evolution during the activation course after 20 freeze/thaw cycles between L20 8C and 40 8C for a 25 cm2 single cell without dehumidification.
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diffusion characteristics. There is a great discrepancy between Curve zero and Curve 7th as Fig. 5 shown. Integral optimum performance was achieved after the 7th activation operation and was stable enough. This phenomenon might be explained in this way. As activation operation increased, membrane electrode assembly was adequately swelled, the interfacial contact resistance between CCM and GDL was reduced sufficiently, and thus coupling characteristics became a little better. So a successive voltage recovering phenomenon was observed. However, there was some minor change in hydrophobicity or microstructure in gas diffusion layer after freeze/ thaw operation. The hydrosphere transport and migratory diffusion characteristics there were no longer coupled so well. The hindrance for water and gas transport would result in water flooding or reactant starvation, thus sharp voltage dropped to nearly zero and recovered to only 0.1–0.2 V repetitively occurred. Fig. 6 indicated the performance evolution during activation phase after the 30th freeze/thaw operation. There is also a great discrepancy before and after activation operation as can be seen from this figure. The upper limit current density was reduced from 2.00 A cm2 to 1.84 A cm2 this time. When current load was applied, the voltage sharply dropped down to zero and slowly returned to 0.1–0.2 V repeatedly, but never recovered again. It is proposed here that the root for this voltage fluctuation phenomenon was mass diffusion backup. It was also validated in Fig. 7 that not only the maximum power density and upper limit current density were both reduced, but also performance degradation was observed through the entire discharge region. The maximum power density reduced from 0.798 W cm2 to 0.774 W cm2 after the 30th freeze/thaw operation at a degradation rate of 0.1% cycle1. The voltage degradation rate was approximately 0.17 mV cycle1 at 0.8 A cm2 and 0.47 mV cycle1 at 1.2 A cm2, respectively. By comparing the three scenarios, the degradation rate of the maximum power density was only 0.005% cycle1 and 0.07% cycle1 for ‘‘cold purge’’ and ‘‘hot purge’’, respectively. The voltage degradation rate was approximately 0.18 mV cycle1 and 0.32 mV cycle1 at 0.8 A cm2, 0.20 mV cycle1 and 0.14 mV cycle1 at 1.2 A cm2, respectively. However, the degradation rate of the maximum power density was 0.1%
Fig. 7 – Performance evolution after different freeze/thaw cycles between L20 8C and 40 8C for a 25 cm2 single cell without dehumidification. cycle1 in situation without any dehumidification operation. The voltage degradation rate for cell without purge was approximately 0.17 mV cycle1 at 0.8 A cm2 and 0.47 mV cycle1 at 1.2 A cm2, respectively. That’s to say the voltage degradation rate was minus at 1.2 A cm2 only in the 3rd group experiment which indicated real performance decay. The severely reduced upper limit current density and sharp voltage fluctuation phenomenon also suggested potential change in hydrophobicity and microstructure of GDL or CCM/ GDL interfacial coupling characteristics. According to Garzon’s report from Los Alamos National Laboratory (LANL) [10], the degradation in GDL might be a potential failure mechanism under subzero condition. By this token, the result obtained here tallied with their speculation. It is rather obvious that both cold purge and hot purge are suitable for subzero storage. It is necessary to perform proper dehumidification scenario before undergoing sub-freezing temperature.
3.2.
SEM analysis
The cross-section morphologies of catalyst-coated membranes (CCMs) were characterized by scanning electron microscopy (SEM). As from Fig. 8a–d indicated, CCM retained good integrity and no delamination or segregation occurred in the catalyst layers. Obviously, catalyst layers on the CCM bear excellent frozen invulnerability. There is not any visible physical damage and destruction. The succeeding work was concentrated on electrochemical performance of CCM after sub-freezing operations.
3.3. Cylic voltammograms under different purging scenarios
Fig. 6 – Performance evolution during activation phase after 30 freeze/thaw cycles between L20 8C and 40 8C for a 25 cm2 single cell without dehumidification.
Fig. 9 shows cyclic voltammograms of single cell before and after freeze/thaw operations under different operation scenarios. As Fig. 9a indicated, under cold purge scenario, there is little difference between cell hydrogen oxidation peak and sorption peak before and after 20 freeze/thaw operations, but oxygen deoxidization peak and desorption peak ratherish decreased. This phenomenon tallies with the voltage
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Fig. 8 – SEM Images of CCM: a) fresh CCM without freeze/thaw operation; b) for a 25 cm2 single cell after 50 freeze/thaw cycles between L10 8C and 40 8C under cold purge; c) for a 50 cm2 single cell after 50 freeze/thaw cycles between L20 8C and 40 8C under hot purge; d) for a 25 cm2 single cell after 30 freeze/thaw cycles between L20 8C and 40 8C without dehumidification operation.
fluctuation phenomenon in Fig. 1. It indicated that oxygen deoxidization dynamics and diffusion mass transport dynamics play main role. Calculation revealed that electrochemical surface areas before and after freeze/thaw operations were 44.5 m2 g1 and 42.8 m2 g1, respectively, with a reduction rate of 3.8%. The ECSA degradation rate was 0.085 m2 g1 per freeze/thaw cycle. The evolution course tallies with the phenomenon in Fig. 4 regarding polarization curves and cell power density curves as seen from Fig. 9b. This picture revealed that performance degradation occurred in low current density region and performance improvement in high current density region. The area of hydrogen oxidation peak and sorption peak appreciably decreased while area of oxygen deoxidization peak and desorption peak increased somewhat. It suggested that electrochemical surface area has much matter with cell performance under low current density. The area of oxygen deoxidization peak and desorption peak has much matter with diffusion mass transport and electric and gas coupling characteristics at interfaces between CCM and GDL, which play a main role in cell performance in high current density region. Calculation revealed that electrochemical surface areas before and after freeze/thaw operations were 42.7 m2 g1 and 40.5 m2 g1, respectively, with a reduction rate of 5.2%. The ECSA degradation rate was 0.11 m2 g1 per freeze/thaw cycle. Fig. 9c indicated the cell performance evolution under no purge. Hydrogen oxidation peak and sorption peak
appreciably decreased, however it was more remarkable as for reduction of the area of oxygen deoxidization peak and desorption peak. This phenomenon corresponds to performance evolution in Fig. 7. It suggested that electrochemical active surface area has much matter with cell performance in low current density region. The area of oxygen deoxidization peak and desorption peak has much matter with oxygen deoxidization dynamics and diffusion mass transport dynamics. Mass transport in GDL and electric and gas coupling characteristics at interfaces between CCM and GDL have much influence on cell performance in high current density region. Calculation revealed that electrochemical surface areas before and after freeze/thaw operations were 45.2 m2 g1 and 39.4 m2 g1, respectively, with a reduction rate of 12.8%. The ECSA degradation rate was 0.29 m2 g1 per freeze/thaw cycle. Combining views from Oszcipok [4] and Thompson [5] with the analysis of UI, CV, SEM results in this paper, it is proposed here that the performance decay in the high current density region could be ascribed to variational contact resistance at CCM/GDL interfaces as freeze/thaw cycles increased. Once there is some change in hydrophobicity or microstructure of GDLs, the contact state would change accordingly, thus resulting in worsening of hydrosphere transport and mass diffusion. Hereunto, the fluctuation phenomenon of voltage in the high current density region and the reduced upper limit current density could be easily explained as above.
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4.
Conclusions
Degradation behaviors of polymer electrolyte membrane fuel cell (PEMFC) in high current density region were compared by cold purge, hot purge and no purge. The degradation rate of the maximum power density was only 0.005% cycle1 and 0.07% cycle1 for ‘‘cold purge’’ and ‘‘hot purge’’, respectively. However, the degradation rate of the maximum power density was 0.1% cycle1 under no purge. The voltage degradation rates were approximately 0.18 mV cycle1 and 0.32 mV cycle1 at 0.8 A cm2, 0.20 mV cycle1 and 0.14 mV cycle1at 1.2 A cm2, under ‘‘cold purge’’ and ‘‘hot purge’’ respectively. Correspondingly, the voltage degradation rate under no purge was 0.17 mV cycle1 at 0.8 A cm2 and 0.47 mV cycle1 at 1.2 A cm2, respectively. By comparing the three scenarios, integral long-term performance could be maintained unless proper dehumidification scenario is adopted. No visible physical damage and destruction was observed by crosssection SEM. The result validated that homemade CCM bore all-right freeze invulnerability. By means of single cell performance test and cyclic voltammetry, it was proposed that reduction of electrochemical active surface area was main cause of cell performance degradation in low current density region; mass transport in GDL and electric and gas coupling characteristics at interfaces between CCM and GDLs were main causes of cell performance degradation in high current density region.
Acknowledgements The work was financially supported by the Ministry of Science and Technology 863 Hi-Technology Research and Development Program of China (no. 2007AA05Z126 and 2008AA11A106) and the National Natural Science Foundation of China (no. 50632050).
references
Fig. 9 – Cyclic voltammograms of single cell before and after freeze/thaw operations under different operation scenarios: a) under cold purge; b) under hot purge; c) without dehumidification.
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