Cold-start icing characteristics of proton-exchange membrane fuel cells

Cold-start icing characteristics of proton-exchange membrane fuel cells

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Cold-start icing characteristics of proton-exchange membrane fuel cells Linjun Li a, Shixue Wang a,b,*, Like Yue a, Guozhuo Wang a a

School of Mechanical Engineering, Tianjin University, Tianjin 300350, China Key Laboratory of Efficient Utilization of Low and Medium Grade Energy (Tianjin University), Ministry of Education, Tianjin 300350, China

b

article info

abstract

Article history:

Understanding the icing characteristics of proton-exchange membrane fuel cells (PEMFCs)

Received 6 October 2018

is essential for optimizing their cold-start performance. This study examined the effects of

Received in revised form

start-up temperature, current density, and microporous layer (MPL) hydrophobicity on the

12 March 2019

cold-start performance and icing characteristics of PEMFCs. Further, the cold-start icing

Accepted 15 March 2019

characteristics of PEMFCs were studied by testing the PEMFC output voltage, impedance,

Available online 5 April 2019

and temperature changes at different positions of the cathode gas diffusion layer. Observation of the MPL surface after cold-start failure allowed determination of the distribution

Keywords:

of ice formation at the catalytic layer/MPL interface. At fuel cell temperatures below 0  C,

Proton exchange membrane fuel cell

supercooled water in the cell was more likely to undergo concentrated instantaneous

Cold start

freezing at higher temperatures ( 4 and

5  C), whereas the cathode tended to freeze in



Impedance test

sequence at lower temperatures ( 8 C). In addition, a more hydrophobic MPL resulted in

Icing characteristics

two successive instantaneous icing phenomena in the fuel cell and improved the cold-start

Hydrophobicity

performance. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The cold starting of proton-exchange membrane fuel cells (PEMFCs) is the biggest challenge facing the use of fuel cell vehicles in cold environments. When a PEMFC is started at a temperature below 0  C, not only is the energy conversion efficiency low but also the water generated by the cathode catalytic layer (CL) in the fuel cell is likely to freeze, thus preventing air flow into the CL and causing a cold-start failure and damage to the cell components [1,2]. To realize improved performance, many factors that affect the cold starting of fuel

cells have been investigated in recent years. Ishikawa et al. [3] found that water does not exist in a supercooled state and instead freezes directly in PEMFCs that are not purged before cold starting, resulting in poor cold-start performance. Purging a fuel cell with nitrogen gas after each run until the moisture content of the membrane electrode assembly (MEA) drops below 3% allows the performance degradation with each cold start to be controlled to within 0.06%, thereby improving its durability [4,5]. Jiang et al. [6] used numerical simulations to study the effect of current density on the coldstart performance of fuel cells. It is believed that increasing the starting current density of a fuel cell can accelerate the

Abbreviations: PEMFC, proton-exchange membrane fuel cell; CL, catalytic layer; MEA, membrane electrode assembly; GDL, gas diffusion layer; MPL, microporous layer; PEM, proton-exchange membrane. * Corresponding author. School of Mechanical Engineering,Tianjin University,Tianjin 300350, China. E-mail address: [email protected] (S. Wang). https://doi.org/10.1016/j.ijhydene.2019.03.115 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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temperature increase of the fuel cell, but it will also accelerate the accumulation of water in the cathode, which is likely to cause a cold-start failure. Reducing the starting current density can enhance the cold-start capability of a fuel cell [7]. However, Li et al. [8] found that a larger starting current density increases the icing probability in a fuel cell during cold starting. Tabe et al. [9] used scanning electron microscopy to study the distribution of ice in fuel cells after cold-start failure. They found that the addition of a microporous layer (MPL) to the gas diffusion layer (GDL) of the cathode enhances the surface contact between the CL and the GDL compared with that in the absence of the MPL. The GDL with an MPL inhibits the accumulation of water on the CL surface, improving the cold-start capability of the fuel cell and somewhat reducing the damage of the fuel cell after a cold-start failure. Ko et al. found that the presence of an MPL in the cathode increases the storage space of ice, thus extending the runtime of fuel cells during cold starting [10,11]. Jiang et al. [12] studied the nonisothermal cold starting of fuel cells and concluded that a proton-exchange membrane (PEM) greatly influences the cold-start performance. Icing on the PEM surface is not only related to the initial water content in the fuel cell but also to the thickness of the PEM [13e15]. In addition, Kagami et al. [16] and Du et al. [17] reported that although the saturated vapor pressure of water below 0  C is very low, a higher cathode gas flow rate quickly removes the water generated in the fuel cell. However, the reaction heat is also quickly removed, resulted in an extended cold-start process. The above factors that affect the cold-start performance of fuel cells may actually influence the movement of generated water and the formation and distribution of ice in the fuel cell during the cold-start process. Kagami et al. [18] found that icing in the fuel cell during the cold-start process mainly occurs in the CL, MPL, and GDL of the cathode and does not extend into the flow channel in most cases. Jiao et al. [19] also found that icing in cathode CL is a common cause of cold-start failure. Ichikawa et al. [20] showed that ice is first formed on the CL side of the cathode when the fuel cell is started from 20  C and then continues to expand toward the GDL side of the cathode until the CL is completely covered. Tabe et al. [9] also found that when a fuel cell is started from 10  C, icing mainly occurs at the interface between the CL and GDL in the cathode. Further, Hou et al. [21] reported that the ice is mainly concentrated in the GDL and does not reach the flow channels when starting from 5  C. Santamaria et al. [22] used neutron imaging technology to investigate icing in fuel cells started from 5  C, and found that icing occurs mainly in areas under flow channels. The location of ice formation is related to the movement of water before icing, and Ge et al. [23] used optical microscopy to show that the water produced by the cathode CL is in a supercooled state. Ishikawa et al. [24], who also confirmed the existence of supercooled water, suggested that the cold starting of PEMFCs would be successful if the water remained in the supercooled phase during the cold-start process. By dividing the fuel cell reaction zone into several sub-areas, Jiao et al. [25] found that the current density is concentrated at the inlet and the middle of the fuel cell cathode and that these areas produce more water. Experimental investigations of the fuel cell cold-start process by Lin et al. [26] revealed that the highest current density is

concentrated in the center of the cell reaction region. Oberholzer et al. [27] used neutron imaging technology to observe the distribution of water in the fuel cell during the cold-start process and found that ice forms in the GDL and the channels in the cathode owing to water transmission [28], with the rapid freezing of supercooled water causing a sudden drop in the output voltage. Dursch et al. [29] found that lower cell temperatures during the cold starting of fuel cells decreased the time for which supercooled water is maintained in the fuel cell. Based on kinetic and thermodynamic studies, Dursch et al. [30] concluded that the induction time of supercooled water in fuel cells is mainly related to the type of carbonsupport material and the ionomer fraction. Yang et al. [31] also indicated that the movement of water droplets and the distortion of the contact line at the three-phase interface promote the formation of an ice core when studying supercooled water droplets. Additionally, Khandelwal et al. [32] found that the temperature distribution within the PEMFC is greatly affected by the heat capacity and thermal conductivity of each component during the cold-start process. These previous studies suggest that many factors affect the cold-start performance of PEMFCs, with each factor influencing the movement of water and the formation of ice in the fuel cell during the cold-start process. Although previous studies have clearly revealed the distribution of ice in the cathode layers (CL, GDL, flow channels) of the fuel cell during cold starting, there has been insufficient research on the distribution of supercooled water and ice formation in the direction of the reaction surface. Therefore, in this study, the voltage, impedance, and GDL surface temperature changes of the fuel cell were tested under different experimental conditions. Further, the effects of the starting temperature, current density, and MPL hydrophobicity on the icing characteristics and the fuel cell cold-start performance were analyzed.

Materials and methods Fuel cell module and experimental system Fig. 1 shows a schematic of the experimental set-up used for the fuel cell GDL temperature tests. To investigate the temperature changes of the fuel cell cathode GDL during cold starting, the temperature was measured at four points (Fig. 1a). To reduce the influence of the thermocouples on the cathode gas flow field, the four thermocouples (diameter: 0.25 mm) were inserted into the fuel cell from the back of the cathode bipolar plate and the surface temperature of the GDL was measured (Fig. 1b). One of the thermocouples was located at the central axis of the reaction zone, whereas the other three were located at distances of d1, d2, and d3 from the central axis of the reaction zone, as summarized in Table 1. A GDL with an MPL has a strong moisturizing ability, which is beneficial for improving flooding and the cold-start performance [9,33]. Therefore, an MPL was added to the GDL used in this study. The membrane electrode was obtained from Kunshan Sunlaite New Energy Technology Corp.. To reduce the influence of the end plate heat capacity on the fuel cell during cold starting, a rigid silica gel plate (thickness: 10 mm), used for heat insulation, was placed between the end plate

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Fig. 1 e Schematic diagram of the experimental set-up used for fuel cell temperature measurements: (a) cathode GDL thermocouple distribution and (b) thermocouple assembly.

and the current collecting plate. The other fuel cell parameters are listed in Table 1. Fig. 2 shows the PEMFC cold-start experimental system. The intake gas temperature affects the temperature of the fuel cell during cold starting, which in turn affects the fuel cell cold-start performance [34]. To reduce the influence of the intake gas temperature on the fuel cell during cold starting, a cooling coil was used to cool dry hydrogen (5 N) and air (5 N) to the required temperature before entering the fuel cell. During the cold-start process, the temperatures at the test points of the fuel cell cathode GDL and the environment temperature were measured using K-type thermocouples (accuracy: 0.1  C) and recorded using an MV1000 data acquisition instrument. To reduce the impact of the mechanical vibrations generated during the operation of the cryogenic box on the icing of the fuel cell, the cryogenic box was shut down during the coldstart process. The fuel cell impedance was measured using an electrochemical workstation (Interface 5000E) with a minimum response time of 10 ms. At high frequencies (>1000 Hz), an electrochemical workstation primarily measures the ohmic resistance, Rohm [35], which is composed of the

Table 1 e Main parameters of the fuel cell. Parameter PEM type Anode platinum loading Cathode platinum loading Reaction area Clamping pressure Bipolar plate thickness h1 Hole diameter F1 CL thickness GDL thickness

Value Nafion212 0.4 mg cm 0.4 mg cm 25 cm2 3 MPa 4 mm 0.4 mm 10 mm 160 mm

2 2

Parameter

Value

PEM thickness MPL thickness Ridge width dr Channel width dc Channel depth h2 Channel length d1 d2 d3

50 mm 50 mm 1 mm 1 mm 1 mm 50 mm 4 mm 6 mm 24 mm

resistance of ions passing through the MEA, the electrode resistance, and the contact resistance between the electrodes [36]. When icing occurs during cold starting of a fuel cell, the ice layer at the interface between the MPL and the CL causes an increase in the contact resistance [37].

Cold-start experimental conditions and methods To ensure that the performance of the fuel cell was consistent before and after cold starting, the fuel cell was pretreated using the conditions shown in Table 2 to restore its performance before each cold-start experiment [38]. First, air and hydrogen, both at a temperature of 40  C and a relative humidity of 100%, were introduced into the fuel cell at 40  C, which was then continuously operated at a current density of 0.8 A cm 2 for 20 min. After shutdown, the fuel cell was purged with dry nitrogen for 1 h at room temperature (RT, 25  C). Finally, the fuel cell was cooled to the cold-start temperature and frozen for 3 h. After completion of this preparation process, air and hydrogen cooled by the coils were passed into the frozen fuel cell at the required start-up temperature, and cold-start experiments were conducted using the conditions shown in Table 3. In experiments examining the effects of temperature and current density on the cold-start performance, the fuel cells were started from 3, 4, 5, or 8  C at a constant current density of 0.04 A cm 2, and then started from 4 or 5  C at 0.10 A cm 2. The voltage, impedance, and temperature changes in the fuel cells were monitored simultaneously. During the cold-start process (> 10  C), icing in the fuel cell exists to a certain degree in the cathode CL/MPL interface [9]. Therefore, it was assumed that the effect of the MPL on the cold starting of the fuel cell was mainly due to its surface hydrophobicity. Before investigating the effect of the cathode

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Fig. 2 e PEMFC cold-start experimental system.

Table 2 e Pretreatment conditions before cold starting. Parameter Current density Hydrogen flow Air flow Hydrogen relative humidity Air relative humidity

Value

Parameter

Value

0.8 A cm 2 760 sccm 1810 sccm 100% 100%

Fuel cell temperature Operation time Purge time after shutdown Fuel cell freezing time after purging

40  C 20 min 1h 3h

Table 3 e Cold-start experimental conditions. Parameter Hydrogen flow Air flow Impedance test frequency

Value

Parameter

Value

58 sccm 226 sccm 2500 Hz

Hydrogen relative humidity Air relative humidity Disturbing current

0% 0% 0.01 A rms

MPL hydrophobicity on the fuel cell cold-start performance, the MPL surface contact angles were determined using a contact angle measuring instrument (accuracy: 0.1 ). For two hydrophobic MPL samples (hereafter referred to as sample 1 and 2, respectively, Supplier: Kunshan Sunlaite New Energy Technology Corp.) used in present experiment, the average contact angles of sample 1 and 2 were found to be 139.2 and 147.8 , respectively, thereby indicating that the latter sample was more hydrophobic. Subsequently, the fuel cells with these different MPLs were started from 5 or 8  C.

Results and discussion Effect of temperature on PEMFC cold-start performance To test the effect of temperature on the PEMFC cold-start performance at a current density of 0.04 A cm 2, sample 1 was used as the MPL. Fig. 3 shows the changes in the voltage and impedance for the fuel cells started from different temperatures, and Fig. 4 shows the corresponding temperature

changes of the cathode GDL. The surface ice distributions on the MPL after cold-start failure from 5 and 8  C are shown in Fig. 5. As shown in Fig. 3, the cell voltage did not change significantly when the startup temperature was 3  C. However, when the fuel cell was started from 4, 5, and 8  C, the voltage was greatly reduced, beginning at times of 462, 297, and 200 s, respectively. This behavior occurs because lowering the fuel cell temperature shortens the time before the water produced by the CL begins to freeze. As shown in Figs. 3 and 4a, when the fuel cell was started from 3  C, the voltage remained stable and the cell impedance did not increase, indicating that no ice was formed in the cell. In addition, the fuel cell impedance dropped at the beginning of the start-up process and then remained stable. During the early stage of the cold-start process, water is continuously absorbed by the MEA, resulting in a continuous decrease in the impedance of the MEA. However, the fuel cell impedance remains relatively constant after the MEA is sufficiently wetted in the later stages of the cold-start process. When the fuel cell was started from 4  C, the fuel cell impedance started to increase at 462 s (Fig. 3), coinciding with

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Fig. 3 e Changes in voltage and Rohm for fuel cells cold-started at 0.04 A cm¡2 (sample 1).

Fig. 4 e Changes in cathode GDL temperatures for fuel cells at a current density of 0.04 A cm¡2 (sample 1): (a) ¡3  C, (b) ¡4  C, (c) ¡5  C, and (d) ¡8  C.

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Fig. 5 e Ice distribution on the surface of the cathode MPL at (a) ¡5  C and (b) ¡8  C. the time at which the voltage began to drop. An increase in the Rohm of the fuel cell indicates an increase in the contact resistance between the CL and the MPL, meaning that ice is formed at the interface between these layers. As shown in Fig. 4b, the temperatures at measuring points P1, P2, and P3 appear to increase instantaneously owing to the latent heat released by icing in the vicinity of these points. The largest temperature increase (3  C) was observed at P1. Similarly, as shown in Fig. 4c, when the fuel cell was started from 5  C, the temperature in the central region of the GDL increased instantaneously at 297 s, but the temperature at the edge (measuring point P4) did not increase significantly. Thus, it can be speculated that icing occurs mainly in the center of the reaction zone. As shown in Fig. 4d, when the fuel cell started from 8  C began to freeze, no phenomenon associated with an instantaneous temperature increase was observed at any of the GDL measurement points. Moreover, the impedance growth of the fuel cells starting to freeze from 4  C and 5  C was more obvious than that of the fuel cell starting to freeze from 8  C (Fig. 3). Thus, it is inferred that the icing process in fuel cells started from 4 and 5  C differs from that in cells started from 8  C. That is, when the icing started, the former fuel cells were prone to concentrated instantaneous icing, whereas the latter was prone to icing in sequence. As shown in Fig. 5, when the fuel cell failed to start from 5  C, the ice layer on the surface of the MPL was mainly concentrated under the flow channels, and the ice under adjacent flow channels was connected together to form a larger structure. In contrast, after the fuel cell failed to start from 8  C, although the ice on the surface of the MPL mainly accumulated under the flow channels, the distribution was relatively scattered and the area of the ice layer was small. This analysis suggests that the water at the interface between the CL and the MPL remains in a supercooled state for a longer time and aggregates over a larger area before freezing when the fuel cell is started from a higher temperature (e.g., 5  C). When induced by the ice core, the ice quickly spread to the entire area of connected supercooled water to form concentrated ice, and the temperature of the central area where the latent heat was released increased instantaneously. Moreover, the formation of a large icing area at the interface between the CL and the MPL in the cathode severely reduced the contact area between the two layers (CL and MPL), resulting in a large increase in the

contact resistance between the two layers. When the fuel cell start-up temperature was lowered to 8  C, as the water was kept in a supercooled state for a shorter period, it was unable to concentrate into a large area, resulting in the formation of dispersed ice. Consequently, it was impossible to concentrate the release of latent heat. Simultaneously, as the icing area was small, the contact resistance between the CL and the MPL increased slowly.

Effect of current density on PEMFC cold-start performance Fig. 6 shows the changes in voltage and impedance for the fuel cells using sample 1 as the MPL started from 4 and 5  C at 0.04 and 0.10 A cm 2. Fig. 7 shows the changes in the surface temperature of the cathode GDL when the fuel cells with a current density of 0.10 A cm 2 were started from 4 and 5  C. As shown in Figs. 6a and 7, when the fuel cell at 0.10 A cm 2 was started from 4  C, the output voltage was stable and the fuel cell temperature was increased to above 0  C by the reaction heat at approximately 850 s. In contrast, the temperature of the fuel cell at 0.04 A cm 2 increased to approximately 2  C at 850 s (Fig. 4b). At a current density of 0.10 A cm 2, the reaction exothermic power of the fuel cell was approximately 2.5 times greater than that at 0.04 A cm 2, so the temperature of the former fuel cell increased at a greater rate. Moreover, when the fuel cell was started from 4  C at a current density of 0.10 A cm 2, there was no sudden increase in the GDL temperature and the fuel cell impedance because no ice was formed in the fuel cell. When the overall temperature of the fuel cell increased to above 0  C, the voltage remained stable, indicating that the cold start was successful. As the amounts of heat and water produced by the fuel cell at a current density of 0.10 A cm 2 were large, the fuel cell impedance under this condition also decreased at a higher rate in the early stage of the cold-start process. As shown in Fig. 6b, when the fuel cell was started from 5  C at 0.10 A cm 2, the voltage started to drop at approximately 100 s, decreasing to 0 V in approximately 100 s. When the current density was 0.04 A cm 2, the time between the beginning of the voltage drop and 0 V was approximately 350 s (Fig. 6b). When the current density was 0.10 A cm 2, the impedance of the fuel cell started from 5  C began to rise continuously at approximately 100 s, whereas at 0.04 A cm 2,

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Fig. 6 e Effect of current density on fuel cell voltage and impedance (sample 1): (a) ¡4  C and (b) ¡5  C.

Fig. 7 e Changes in cathode GDL temperatures for fuel cells at a current density of 0.10 A cm¡2 (sample 1): (a) ¡4  C and (b) ¡5  C. this phenomenon occurred approximately 200 s later. Increasing the starting current density of the fuel cell started from 5  C shortened the time required to progress from startup to icing and the duration of the continuous icing process. In Fig. 6b, the impedance growth rate observed for the fuel cell at 0.10 A cm 2 is approximately 40% of that at 0.04 A cm 2. Considering that the amounts of water generated by the cathode CL were similar when the fuel cells at the two different starting current densities began to freeze and that the temperatures at the measuring points did not increase suddenly (Fig. 7b), it can be inferred that the former fuel cell undergoes gradual icing. It is suggested that increasing the water generation rate increases the disturbance caused by the

movement of water, thus making it is easier to induce icing. In summary, when the fuel cell was cold-started at a large current density, although the temperature of the cell increased at a higher rate, the amount of time that water in the fuel cell was maintained in the supercooled state was greatly reduced. Therefore, using a smaller current density is advantageous for improving the fuel cell cold-start performance.

Effect of MPL surface hydrophobicity on PEMFC cold-start performance Fig. 8a and b shows the changes in voltage and impedance for the fuel cells with sample 1 and sample 2 as the MPL started

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Fig. 8 e Effect of MPL surface hydrophobicity on the voltage and impedance of fuel cells cold-started at (a) ¡5  C and (b) ¡8  C. from 5 and 8  C, respectively. Fig. 9 shows the GDL temperature changes of the fuel cell with sample 2 as the MPL. As shown in Fig. 8, when the fuel cell was started from 5  C, the fuel cell using sample 2 ran for 25 s longer than the fuel cell using sample 1; when started from 8  C, the fuel cell using sample 2 ran approximately 40 s longer. During the start-up of the fuel cell using sample 2 at 5 and 8  C, the impedance exhibited two large increases (Fig. 8), indicating that two concentrated icing processes occurred at the interface between the CL and the MPL in the cathode. The fuel cell using sample 1 only showed a significant increase in impedance when it was started from 5  C. Based on these results, it can

be speculated that the hydrophobicity of the MPL surface affects the way that ice forms at the interface between the CL and the MPL in the cathode. As the MPL surface of sample 1 is less hydrophobic than that of sample 2, it is inferred that water penetrates the MPL more easily, which may cause the water on the surface of the MPL to be more dispersed. Therefore, when the first concentrated icing process occurs, the ice formed at the interface between the CL and the MPL in the cathode is relatively dispersed. Freezing of the water in the cathode is easily induced by this dispersed ice, making reaggregation and concentrated icing again difficult. Because the MPL surface of sample 2 is more hydrophobic, the water does

Fig. 9 e Changes in temperature for fuel cells at a current density of 0.04 A cm¡2 (sample 2): (a) ¡5  C and (b) ¡8  C.

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not easily penetrate the MPL and is more likely to accumulate at the interface between the CL and the MPL. After the first concentrated icing process, it is inferred that the distribution of ice on the surface of the MPL is relatively concentrated. Thus, there may be more space at the CL and MPL interface for the water generated by the cathode CL to reaggregate and concentrate icing. When the fuel cell using sample 2 was started from 5  C, the temperatures at the central measuring points (P1, P2, and P3) of the reaction zone increased rapidly when the first concentrated icing process occurred at 260 s (Fig. 9a). Among them, the temperature rise at P1 was still the highest (approximately 2  C), but no rapid increase in temperature was detected at the edge measurement point (P4). However, when the second concentrated icing process occurred at approximately 400 s, the temperature increase at P4 was the largest (approximately 1  C), whereas insignificant temperature increases were observed in the central zone. It can be inferred that the concentrated icing area was transferred from the center to the edge of the reaction zone. Because the current density of the central reaction zone was higher than that at the edge at the beginning of the cold-start process, icing is first concentrated at the center of the fuel cell reaction zone. After the first concentrated icing process, most of the central zone in the reaction area was covered by ice, and the high current density was transferred from the center to the edge of the reaction zone. Therefore, when the second concentrated icing process occurred, the largest temperature increase was observed at the edge measuring point (P4). As shown in Fig. 9b, concentrated icing also occurred when the fuel cell was started from 8  C, and the temperature of P1 increased slightly at 160 s. This behavior is different from that of the fuel cell using sample 1. Thus, it can be considered that icing in sequence and concentrated icing coexist at this time under this starting condition. In summary, a fuel cell with a more hydrophobic MPL has a better cold-start performance.

Conclusion To study the effects of start-up temperature, current density, and MPL surface hydrophobicity on the cold-start performance and icing characteristics of PEMFCs, the voltage, impedance, and GDL temperature of the fuel cells during cold starting were measured experimentally. The following conclusions were drawn from the experimental results: 1) When the fuel cell was cold-started at 0.04 A cm 2, if the starting temperature was 3  C, the water generated in the fuel cell was not frozen; if the starting temperature was 4 or 5  C, the generated water accumulated at the interface between the MPL and the CL, resulting in concentrated instantaneous icing; and if the starting temperature was 8  C, the generated water was dispersed and sequentially frozen at the interface between the MPL and the CL. 2) Increasing the start-up current density slightly reduced the fuel cell temperature for a successful cold-start process, but it greatly shortened the time from start-up to the beginning of icing and the duration of the icing process, which was not conducive to improving the cold-start performance of fuel cells.

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3) Enhancing the hydrophobicity of the MPL improved the cold-start performance of fuel cells. When the MPL was highly hydrophobic, two concentrated instantaneous icing phenomena occurred in the fuel cell cathode, with the first icing process occurring mainly in the central reaction zone and the second mainly at the edge of the reaction zone. Finally, to improve the cold-start performance of the fuel cell, it is better to appropriately reduce the thermal conductivity and heat capacity of the bipolar plate and to use more hydrophobic MPL.

Acknowledgements This research is supported by Ministry of Science And Technology program: project of international cooperation (2016YFE0118600).

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