1 Introduction
Proton exchange membrane fuel cells (PEMFCs), combined with the hydrogen of high pressure (i.e., 35 MPa and 70 MPa) storage, have distinct technical advantages on energy density and power density over other electrochemical power sources, and therefore, have longer cruise range and stronger driven power. Moreover, its hydrogen refueling time, which usually lasts for only several minutes (much shorter than the recharge time of battery), provides much more convenience for end users. Hence, it is believed that the PEMFC should be the most ideal power source for vehicles, especially for in heavy duty and long range cases. As shown in Fig. 1, fuel cell vehicles (FCVs) are coming to the commercial stage after the performance validation of the PEMFC in phase I, the durability improvement in phase II, and the cost reduction as well as the H2 infrastructure extension in phase III by the vehicle industry.
The durability of PEMFC remains to be one of the most crucial technical problems during the development of FCV. It is hoped that the ideal durability of PEMFC should roughly be over 5000 h and10000 h for cars and buses, respectively [
1,
2]. A lot of research on PEMFC performance degradation under vehicle duty have been conducted [
3–
5], and degradation mechanisms of key components have been disclosed, such as the membrane crossover, catalyst activity loss by Pt particle agglomeration and carbon corrosion, gas diffusion layer (GDL) and bipolar plate degradations. The challenges to improve the durability of the fuel cell system (FCS) have actually been multilateral because of its either strong or weak correlation to multi-scale and cross-discipline technical problems, such as the transfer regulation of water and heat at scales of system, stack, and electrode, the material stability under interaction of electric potential, chemical environment and mechanical stress [
6]. In terms of the feasibilities of performance evaluation and degradation mechanism investigation, many of the durability research have been performed by investigating the performance evolution under simulating vehicle working conditions in single cell scale and by characterizing the material status before and after the durability test. However, the impact of the power system design and vehicle operation mode on fuel cell durability are likely to be filtered roughly by the single cell scale simulation experiment. Therefore, it is necessary to investigate directly the performance evolution of the FCS and disclose the failure mode of fuel cell by analyzing the running data combined with electrode material characterization [
7,
8].
The durability investigations at system scale of three types of FCS have been reported in this paper. According to the analysis of the running data, the durability status has been evaluated. The electrode samples from the FCSs have been characterized to investigate the mechanism of the performance degradation. Based on the knowledge of classic degradation mechanism [
3–
5], the failure modes of fuel cells of dry operation, start/stop operation and GDL hydrophilicity have been disclosed and found to be related to the system water management design and control strategy, including the gas humidification status, the thermal management strategy, and the way of gas emission etc. According to the optimization of the design against the failure modes, the durability of FCS has been improved step by step.
2 Experimental
The specification of FCSs are listed in Table 1. The 2008FCS was developed by Sunrise Power Co., Ltd. in Dalian, Liaoning Province in 2008 funded by the “Energy saving and new energy vehicle project” in the 11th Five-Year Plan. The 2008FCS was applied by fuel cell car demonstrated in “2008 Beijing Olympic Games” and “2010 Shanghai EXPO,” and characterized with low back pressure operation and gas to gas humidification by enthalpy wheel. The 2013FCS was developed by Sunrise Power Co., Ltd. in 2013 funded by the “Electric vehicle project” in the 12th Five-Year Plan, which was innovated with the novel system water management strategy and start/stop tolerant system design. The 2014FCS was developed jointly by Sunrise Power Co., Ltd. and Shanghai Automotive Industry Corporation (SAIC). Sunrise Power Co., Ltd. improved the robustness and performance of the fuel cell stack and the stack management strategy, while SAIC developed the balance of plant (BOP) of the FCS, which was characterized with hydrogen recirculation design and start/stop tolerant. The 2008FCS and 2013FCS were of similar layout design since they were applied by the same series of powertrain [
9], while 2014FCS was different for its layout to fit for the powertrain integration requirement of SAIC [
10].
The fuel cell stack in 2008FCS was based on the XY100 type of stack by Sunrise Power Co., Ltd., which is operated at low air back pressure and the performance was depended heavily on the humidification. The fuel cell stack in 2013FCS and 2014FCS were based on the stack of XY200a type by Sunrise Power Co., Ltd., which was characterized by median air back pressure operation, lower humidification dependence, higher power density, and lower Pt loading compared with those of XY100.
The durability research of 2008FCS was performed by road running (RR). To investigate the main failure mode, some of the low cell samples from the stack after RR were performed several kinds of characterizations, including the cyclic voltammetry (CV) for both anode and cathode to judge the active area of the electrode, the scan electron spectroscopy (SEM) to observe electrode structure, the transmission electron microscope (TEM) and the X-ray diffraction detection (XRD) to acquire information of catalyst particle size. The chosen low cell samples were stripped into three pieces, and performed with the above characterizations piece by piece, the so-called partition investigation method.
The durability investigation of 2013FCS was performed by test bench running for over 1500 h on the system test bench. The duty cycle was shown in Fig. 2, and every cycle was combined with cycle 1 and 2. To investigate the failure mode, two low cell samples were chosen to perform the status characterization. Except for the CV and TEM of the electrodes, more than 40 positions of the membrane electrode assembly (MEA) for both anode and cathode sides were tested of the contact angle.
The durability investigation of 2014FCS was performed by TBR for over 3000 h on the system test bench under duty cycles, as depicted in Fig. 3.
The experimental information of the durability evaluation and the failure mode investigation was summarized in Table 2.
3 Results and discussion
3.1 FCS durability investigation of 2008FCS
The 2008FCS was applied in the FCV demonstration in 2010 Shanghai EXPO. The FCV fleets of 30 fuel cell cars served as VIP service car for several months, and some of the cars kept running after the EXPO. Figure 4 shows the fuel cell performance evolution of car No. 3 of 14785 km total mileage from May, 2010 to March, 2011, and the detailed information can be referred to in Ref. [
8]. Figure 5 demonstrates the average single cell voltage evolution at 80 A correlated to the running time at a supposed average speed of 20 km/h. Generally, the durability of the 2008FCS was believed to be only hundreds of hours under the RR condition regarding 10% performance loss as the end of life performance.
The analysis of the running data of the 2008FCSs and the experimental investigation of the degradation routes indicate that two kinds of failure modes are the most remarkable.
3.1.1 Dry operation
The aging MEAs from the FCS after the demonstration running were characterized, and one fresh MEA was also characterized to be the benchmark. The characterization results of one of the aging MEAs and fresh MEA are listed in Table 3. The performance of the aging MEA degrades drastically compared with that of the fresh MEA. The active area suggested by CV results for both anode and cathode are much lower than those of the fresh MEA, indicating a remarkable loss of the electrode activity. However, it should be noticed that the catalyst particle size of the aging MEA does not grow according to the results of TEM and XRD. Hence, the activity loss of the electrodes of the aging MEA is not caused by the catalyst particle growth.
Figure 6 illustrates the cross section by SEM of the aging MEA (a) and fresh MEA (b). It can be seen that both the membrane and the catalyst layer become thinner after the RR. The layer-by-layer structures of the aging MEA shrinks and is separated from each other, unlike the close combination structure of the fresh MEA. Although the catalyst particle size remains stable for the aging MEA, the ionomer bonding of the catalyst in the catalyst layer and the membrane shrink during the operation, causing the drastic active area degradation of the electrodes. On the other hand, the humidification design of 2008FCS is almost invalid (30% relative humidity of air inlet) at low loading operation area, as listed in Table 1, and the catalyst layer would experience stringent dry/wet cycle during the RR operation. Therefore, the FC dry operation caused by the poor design of water management, i.e., the humidification design of BOP, is believed to be one of the main failure modes of 2008FCS.
3.1.2 Start/stop operation
As shown in Fig. 7, another low cell sample from 2008FCS was stripped into three pieces along the hydrogen flow, and marked with positions 1#, 2#, and 3# for the pieces at the hydrogen inlet, the middle, and the outlet respectively. The samples presented a performance gradient of 1#>2#>3# as displayed in Fig. 8. To explain this phenomenon, the CV and TEM characterizations for both anode and cathode of the stripped MEA were performed. The catalyst TEM images of the stripped MEA are shown in Fig. 8, and the gathering results are listed in Table 4. It can be seen that the performance gradient is consistent with the order of cathode catalyst particle size, and the performance degradation is, therefore, attributed to the cathode catalyst activity loss. Another phenomenon which should be noticed is that the carbon support of cathode side catalyst seems to disappear along the reverse direction of the air flow, indicating that the carbon corrosion becomes more severe as it gets closer to the air inlet, i.e., the H
2 exit at another MEA side. The carbon corrosion is dominated by two kinds of mechanism [
4], i.e., the high potential at the cathode side generated by the start/stop operation [
11] and the cell reversal caused by fuel starvation [
12]. Based on the “start/stop” mechanism, although the carbon corrosion occurs at the cathode side according to Fig. 7, the cause should be the air invasion of the anode side at the outlet during start/stop operation. The hydrogen/oxygen interface is, therefore, formed and caused high cathode potential once current generation, and the carbon support of cathode catalyst corrodes under such a high potential.
3.2 Durability investigation of 2013FCS
The 2013FCS was developed after optimization against the failure mode of FCS2008, i.e., the humidification was enhanced and the system was start/stop operation tolerant according to BOP design as listed in Table 1. The 2013FCS was tested on the FCS test bench under the duty cycle as shown in Fig. 2. The average single cell at 150 A against the running time was exhibited in Fig. 9, and the durability was considered to be about 1400 h at a 10% performance loss.
The optimization of 2013FCS over 2008FCS against dry operation and start/stop cycle was confirmed to be effective by the elimination of performance fast drop within hundreds of hours. However, the degradation presented acceleration after 1000 h according to Fig. 9. Among the characterizations of the low cell from the 2013FCS, the contact angle of the GDL reduced a lot compared with the value at the beginning, as indicated in Fig. 10. The hydrophobic structure of the GDL was constructed by the three dimension porous skeleton of carbon black and PTFE mixture, and promises the gas reactant transferring in and water by-product draining out of the electrode. Due to the corruption of the hydrophobic structure, two areas in the electrode showed hydrophilic trend, corresponding to two kinds of failure modes.
3.2.1 GDL degradation by flooding operation
The area, marked with rectangle frame in Fig. 10, is the corner of the air outlet at both sides of the MEA. It was believed that the concentrated water at the air outlet damaged the hydrophobic structure of the GDL by long-terms of flooding water impregnation under effect of electro-potential [
13]. It was believed that the operating strategy between the stack thermal management and the humidification design of the 2013FCS could not guarantee that the water by-product in cathode and the liquid accumulated at the cathode outlet could drain out effectively, causing the GDL degradation.
3.2.2 GDL degradation by fuel starvation
The area marked with oval frame in Fig. 10, is the corner of the hydrogen outlet at both sides of the MEA. It was believed that the carbon corrosion of the electrode caused the hydrophilic trend of the area near the hydrogen outlet. Since the hydrogen emission of the 2013FCS was by way of purge according to Table 1, it is the purge operation of hydrogen that cause fuel starvation during the loading variation because of the possible accumulation of droplets and inert gas at the end of anode. As mentioned above, the 2008FCS was proven to degrade following the start/stop carbon corrosion mechanism, while the carbon corrosion following the fuel starvation mechanism would cause the loss of hydrophobic structure of the electrode of 2013FCS, as well as the performance degradation [
12].
3.3 Durability investigation of 2014FCS
Against the failure mode disclosed by durability investigation of 2013FCS, the 2014FCS modified the system water management strategy by deploying innovatory control combination of air humidification, stack thermal management and stoichiometry and hydrogen recirculation. Besides, the GDL degradation caused by “flooding operation” was proven to be overcome by the durability test, as presented in Fig. 11. The fuel cell performance degraded less than 10% after 3000 h running, which was three times as long as 2013FCS.
The failure modes and the durability status of the 2008FCS, 2013FCS, and 2014FCS were summarized in Fig. 12. The failure modes are marked with the durability curves of each generation of FCSs. The durability was improved step by step according to the optimization of the FCS design against the failure modes.
4 Conclusions
It is very crucial to improve the durability of the FCSs by distinguishing the failure modes of the stack and enhancing the design and operating strategy of the FCSs. The failure modes of dry operation and the start/stop cycle, which can destroy the electrode structure in hundreds of hours, are destructive to the FCS durability as shown by 2008FCS. The failure modes of flooding operation and the fuel starvation would lead to GDL degradation after roughly 1000 h of operation as shown by 2013FCS. According to enhancement against the failure modes of 2008FCS and 2013FCS, the durability of 2014FCS is improved to over 3000 h by proper water and thermal management design.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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