Experimental study on spatiotemporal distribution and variation characteristics of temperature in an open cathode proton exchange membrane fuel cell stack

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Experimental study on spatiotemporal distribution and variation characteristics of temperature in an open cathode proton exchange membrane fuel cell stack Qifei Jian*, Jing Zhao School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, Guangdong, China

highlights
Temperature distribution and evolution during dynamic process are analyzed.
Air flow rate significantly affect thermal response and temperature uniformity.
There is temperature drift during load stepwise increase and decrease.
There is a time lag between temperature and voltage when the load changes.

article info

abstract

Article history:

This paper experimentally explores the spatiotemporal distribution and variation charac-

Received 17 June 2019

teristics of temperature in an open cathode proton exchange membrane fuel cell stack

Received in revised form

based on thermal imager and thermocouples inserted in the cathode flow channels. The

16 August 2019

temperature distribution and evolution during the dynamic process are analyzed in detail.

Accepted 21 August 2019

Besides, the effects of air flow rate and load current on the thermal characteristics of the

Available online 17 September 2019

stack are also investigated. The results show that during the start-up, the hot spot first sprouts in the central area and then spreads rapidly to the surrounding area. During the

Keywords:

shutdown, the central and lower regions are first cooled, followed by the hydrogen inlet

Open cathode proton exchange

region, and finally the endplates. The temperature during the load stepwise increase is

membrane fuel cell

inconsistent with that during the load stepwise decrease, showing a temperature drift

Spatiotemporal distribution

phenomenon. Moreover, there is a time lag in the response of temperature and voltage to

Temperature variation

changes in current.

characteristics

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

Parameter study

Introduction The development and research of new energy technologies are urgent due to the global energy crisis and intensifying environmental problems [1]. Proton exchange membrane fuel cells (PEMFCs) have attracted increasing attention over the

last decades as their high efficiency, pollution-free and low operating temperature [2e4]. However, there are still some thorny problems to be solved before large-scale commercialization. It is well known that PEMFCs will generate a large amount of heat during operation, which will increase the cell temperature. On the one hand, proper temperature rise can accelerate

* Corresponding author. E-mail address: [email protected] (Q. Jian). https://doi.org/10.1016/j.ijhydene.2019.08.177 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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electrochemical reaction kinetics and mitigate flooding, thereby improving output performance [5]. However, on the other hand, the excessive temperature in turn can dehydrate proton exchange membranes, degrade output performance, and may even cause irreversible damage to the cells [6]. Therefore, when it comes to the thermal engineering of PEMFCs, the temperature issue, known as thermal management, needs to be given special attention [7]. Studying the temperature distribution of PEMFCs is one of the important contents of thermal management. It is important to master the useful information of temperature distribution for the verification of numerical models, the structural design and optimization of fuel cells, and the improvement of performance [8]. Recently, a large number of studies on the temperature distribution of PEMFCs have been reported, which are mainly carried out from two aspects: numerical simulation and experimental testing. In terms of the numerical simulation: Jung et al. [9] proposed a two-dimensional non-isothermal model based on the water and heat balance of PEMFC. The model can well predict the temperature distribution on the surface of the membrane electrode assembly (MEA). Cao et al. [10] developed a three-dimensional non-isothermal numerical model to investigate the effect of thermal contact resistance on the temperature distribution of a fuel cell. Robin et al. [11] presented a mathematical analysis model to study the homogeneity of the temperature distribution inside a PEMFC. The simulation results showed that the hot spots occurred in a place where the airflow velocity was relatively small. Rahgoshay et al. [12] simulated the temperature profile of PEMFCs with parallel and serpentine flow fields through a typical three-dimensional non-isothermal model. The study demonstrated that the surface temperature distribution on the serpentine flow field was more uniform due to its better cooling performance. Akira et al. [13] built a validated 3D model and revealed that the relative humidity of the reaction gas was important for the temperature distribution of a fuel cell. Xing et al. [14] introduced a two-phase flow numerical model that can better characterize the effects of phase change and transport of water on temperature distribution in PEMFCs. Salva et al. [15] compared the polarization curves and temperature distributions of a PEMFC stack under different stoichiometry, relative humidity and pressure through an experimentally validated model. Amirfazli et al. [16] developed an experimentally validated model to investigate the effects of geometrical parameters on the temperature distribution of a PEMFC stack. The results showed that reducing the cooling channel size can effectively improve the temperature distribution uniformity of the stack. Numerical simulation can enhance the understanding of the heat distribution inside the PEMFCs to some extent. However, due to some assumptions and simplifications of the model as well as the complexity and uncertainty of the actual working conditions, it is very likely that there will be a significant inconsistency between the simulation results and the actual results, thus causing unreasonable judgment and analysis. Therefore, experimental testing is a fundamental solution to this problem. So far, several methods have been developed to experimentally measure the temperature of PEMFCs. Thermal imaging is a useful experimental technique that can visualize

temperature distribution and capture regional hot spots [7,17]. Shimoi et al. [18] replaced a part of the cathode endplate with an optical window to observe the temperature field on the membrane surface. The temperature distribution under different operating conditions was investigated by thermography technology and proved to be closely related to overpotential. Hakenjos et al. [19] self-designed a PEMFC and revealed the effect of water flooding on the temperature distribution by the thermal imager. Wang et al. [20] investigated the temperature distribution on the MEA surface with the help of the thermal imager. The results showed that the temperature of the middle channels was the highest, and the uniformity of temperature was gradually deteriorated with the increase of the current density. A significant advantage of this technique is its high temporal and spatial resolution. Moreover, due to the non-invasive measurement, the method can also effectively eliminate the effects of the temperature measuring device on fluid flow and electrochemical reactions [21]. The main limitation of this method is that it can only obtain the temperature distribution on the outer surface of the tested object, and there is nothing it can do about the internal temperature [22]. In addition, some scholars have also proposed the use of micro-sensors based on micro-electromechanical system technology (MEMS) [23e25] for internal temperature measurement. Although the method has the advantages of small size, high sensitivity and good robustness [7,8], the complex configuration modification to a cell or stack and the high use cost limit its application and popularization. For most researchers, thermocouples are still the main way to acquire the internal temperature of the PEMFCs because they are low-cost and easily available on the market. Typically, they are inserted at different locations within the fuel cells, for example between different layers (ribs/gas diffusion layer (GDL)/catalytic layer/membrane) [6,17,26], in the flow channels [27,28], or behind the flow field plate [7]. Wen and Huang [27] embedded 11 thermocouples on the cathode flow channel plate to measure the temperature distribution of a PEMFC with an active area of 25 cm2. The experimental results showed that local temperature was closely related to the distribution of liquid water in the flow channel. Atan and Mohamed [29] investigated the temperature distribution on a 3-cell air-cooled PEMFC stack. They inserted 8 thermocouples into the cooling channels on the cathode side to plot the temperature change curve in different positions and analyze the thermal behavior of fuel cells in different situations. Lin et al. [7] inserted thermocouples by drilling to monitor the temperature change on the backside of the anodic flow field plate. They pointed out that the temperature distribution on the cathode side of the flow field plate was worse than that on the anode side, but the difference between the average temperatures on both sides was small. The main problem with temperature measurement using thermocouples is that for fuel cells with closed cathode and anode, the insertion of thermocouples into the specified position still requires the design modification (such as drilling, grooving), which can easily destroy the original closed operating conditions and may affect the operating characteristics of the fuel cells. In contrast, the open cathode PEMFCs do not have this problem because their cathode flow channels and cooling flow channels are combined and directly open to the outside

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environment. In this case, the thermocouples can be inserted directly into the cathode channels from outside the stack to minimize the impact on the fuel cells. Most of the current work focuses on the steady-state temperature distribution, but there is still insufficient discussion on the evolution process of temperature over time during the dynamic operation such as start-up and shutdown. In addition, few scholars have reported the thermal response characteristics of open cathode PEMFCs, since most of the previous studies are on fully enclosed fuel cells. Therefore, this paper will investigate the temporal and spatial distribution and dynamic characteristics of temperature under different operating conditions by means of thermal imager and thermocouples to deepen the understanding of the thermal characteristics of the open cathode PEMFCs.

Thermal analysis Heat generation In addition to generating electrical energy during operation, PEMFCs also release a large amount of waste heat, which typically accounts for about 40%e60% of the total energy. These heat mainly includes the entropic heat of the reaction, the irreversible heat of the electrochemical reaction, the ohmic heat and the condensation heat [30,31]. The energy balance of the PEMFCs can be represented by Equation (1) [32]: iA Hfuel ncell ¼ Qgen þ iAVcell ncell 2F

(1)

where i is the current density; A is the effective area; F is the Faraday constant; Hfuel is the heat value of the fuel; ncell is the number of cells, Qgen is the heat production, and Vcell is the single cell voltage. In general, heat production can be estimated by the following formula: Qgen ¼ iAncell ðEr  Vcell Þ

(2)

where Er is the theoretical reversible voltage; Vcell is a function of current density, which can be further expressed as [32]: Vcell ¼ Er 



 RT i RT iL ln ln  iRi  aF i0 nF iL  i

(3)

where R is the universal gas constant; T is the stack temperature; a is the transfer coefficient; F is the Faraday constant; i0 is the exchange current density; Ri is the ohmic resistance; n is the number of transferred electrons; iL isthe limiting current density. Substituting formula (3) into formula (2) yields: Qgen ¼ iAncell



  RT i RT iL ln ln þ iRi þ aF i0 nF iL  i

(4)

Temperature uniformity index In this study, the temperature uniformity index (TUI) is used to quantitatively analyze the uniformity of temperature distribution in different cells. Referring to Ref. [33], TUI is defined as follows:

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N P

jTn  Tj TUI ¼ n¼1 N

(5)

where Tn is the local temperature of the measuring point; T is the average temperature; N is the number of measuring points; A smaller TUI value is desirable because it represents a smaller deviation between the local temperature and the average temperature and more uniform temperature distribution.

Experimental setup Experimental system Fig. 1 shows the experimental schematic of the study, which mainly includes a PEMFC stack, a data acquisition system, an electronic load, and a reactant supply subsystem. A 10-cell open cathode PEMFC stack is tested in this study. The thickness of the membrane (DMR100, Dongyue Group Co., Ltd., China) is 15 mm and the effective active area is 62 cm2. The material of the GDL is carbon paper with a thickness of 300 mm. The catalysts for both anode and cathode are Pt, and the loading capacity is 0.16 mg/cm2 and 0.64 mg/cm2, respectively. The bipolar plates are made of graphite material, and parallel rectangular flow channels are engraved on both sides to provide fuel and oxide for the stack. The width and depth of the cathode channel are 2.1 mm and 1.3 mm, respectively, while the corresponding dimensions at the anode side are 0.4 mm and 1.2 mm. In the reactant supply subsystem, dry and unheated hydrogen with a purity of 99.999% is supplied to the anode side. The pressure of hydrogen can be regulated by a pressure reducing valve (LEVAN-WR57, Ningbo Weirui Fluid Technology Co., Ltd., China), and hydrogen flow rate can be read by a rotameter (LZM-8M, Guangzhou Chengxin Flow Meter Co., Ltd., China). Due to the open cathode configuration, ambient air, as an oxidant and coolant, is supplied directly into the cathode flow channels by two identical fans (TFB0412SHN, Delta Electronics Public Co., Ltd, China). In this study, the fans work in the blowing mode with positive pressure. Compared with suction mode, although this fan configuration usually results in a slightly larger temperature gradient along the direction of airflow. However, in this mode, the airflow has a higher turbulivity, resulting in a better cooling effect [34]. During the experiments, an electronic load (JT6331A, Jartul Electronics Co., Ltd, China) with a resolution of 0.1 mV and an accuracy of 0.03% is used to provide different load currents. The data acquisition instrument (Smart DAC þ GM, Yokogawa Electric Corp., Japan) collects the voltage and temperature signals of single cells at a frequency of 2 HZ to monitor the working state of the stack. Finally, all the data is stored on a computer for subsequent analysis.

Thermal imaging A thermal imager (Ti25, Fluke, USA) is used to capture the temperature distribution of the outer surface of the stack. The focal plane array of the instrument is 160  120, allowing

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Fig. 1 e PEMFC stack experimental system: (a) schematic diagram; (b) tested single cells; (c) thermocouple layout and regional division.

infrared light with wavelengths between 7.5 mm and 14 mm to be detected. The instrument uses a 20 mm lens to ensure a minimum focusing distance of 15 cm. In addition, its noise equivalent temperature difference is 100 mK, which shows high thermal sensitivity.

Local temperature measurement Due to the open cathode configuration, thermocouples can be easily inserted into the cathode flow channels for monitoring of local temperature changes near the cathode GDL surface. As shown in Fig. 1b above, 5 of 10 cells are selected as survey objects to investigate the difference in temperature variation at different locations within the stack. According to their relative positions, these five cells are named Cell 1, Cell 3, Cell 5, Cell 7, Cell 9, respectively. For each tested cell, 6

thermocouples are symmetrically inserted into the open cathode channels as shown in Fig. 1c. Taking Cell 1 as an example, the depth of the six measuring points inserted into the flow channel is approximately 15 mm, and they are also numbered sequentially, from 1 to 30, for the distinction. These 30 measuring points are arranged along the airflow direction. Therefore, the plane, where the 15 measuring points are located, near the air inlet side is named the upstream plane, while the plane where the remaining measuring points are located is named the downstream plane (see Fig. 1b). In addition, in order to facilitate differentiation and comparison, the bipolar plates are divided into different regions, as shown in Fig. 1c. The thermocouple probe is sufficiently thin compared to the cross-sectional dimensions of the cathode flow channels. So the effects of the thermocouples on the heat and mass

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transfer on the cathode side are considered to be negligible. In addition, we compare the stack performance before and after thermocouple insertion, as shown in Fig. 2 below. It can be seen from the figure that the difference in the performance of the stack in both cases is sufficiently small. In fact, some scholars also used the same method to study the temperature characteristics of this type of stack and obtained satisfactory results [29,34].

Experimental conditions Prior to the experiments, thermocouples and a high-precision mercury thermometer are placed in a constant temperature water tank to calibrate the thermocouples. Their maximum difference does not exceed 0.4  C in the temperature range of 25e60  C. The stack is placed vertically and runs in deadended anode mode to improve fuel efficiency. Hydrogen flows from the inlet at the upper end into the stack at a constant pressure of 0.05 MPa. The solenoid valve (Fa0520 series, Dongguan Chengyi Electronics Co., Ltd., China) mounted at the anode outlet line is purged for 0.5 s every 18 s to regularly flush out water and impurities accumulated in the anode compartment. The temperature and humidity of the air are approximately 29  C and 70%, respectively. The air flow rate is controlled by adjusting the fan voltage and is measured by an anemometer. Due to the existence of measurement errors, the accuracy of the relevant instruments and the uncertainty of the key parameters are listed in Table 1. The research work is mainly carried out from the following three aspects. In the first part, the thermal imager is used to capture the temperature distribution of the outer surface of the stack during the start-up and shutdown. The main purpose is to obtain more details of the heat generation and evolution process in the stack by comparing the temperature profiles at different times. The second part will mainly discuss the effects of the air flow rate on the thermal characteristics of the stack. The third part will investigate the effects of load current on the temperature variation characteristics of the stack. It is worth pointing out that the following experimental results have been proven to be repeatable.

Fig. 2 e Comparison of polarization curves of PEMFC stack with and without thermocouples.

Table 1 e The accuracy of the instrument and the uncertainty of the parameters. Parameters Voltage (electronic load) Current (electronic load) Velocity (anemometer) Temperature (thermocouple) TUI Thermal power Electric power

Accuracy/uncertainty ±0.03% ±0.05% ±5% ±0.4  C ±0.067  C ±0.73% ±0.61%

Results and discussion Temperature distribution during start-up and shutdown Start-up and shutdown are two common operating conditions for fuel cells. However, few studies have reported on the temperature distribution and evolution during these dynamic processes. In this section, the stack is initially in open circuit and then the load is suddenly stepped to the set current (20 A) to simulate the start-up phase of the stack. After the stack reaches the quasi-steady state, the load drops again to the open circuit to investigate the temperature variation characteristics during the shutdown. During the whole experiment, the air flow rate is maintained at 1.1  102 m3s1, which ensures adequate cooling of the stack. Fig. 3 shows the temperature distribution of the outer surface of the stack at different times during the start-up. It should be pointed out that the temperature scales of all subgraphs have been unified by the proprietary post-processing software of thermal imager. Therefore, the temperature distribution at different times can be directly compared. It can be seen from the diagram that the stack is almost integrated with the environment in the open circuit, showing a very high consistency of temperature distribution. In the following tens of seconds, the contrast between the color of the outer surface of the stack and that of the environment is becoming more intense. This is because the temperature of the stack is rapidly increasing, thereby expanding the temperature difference between the stack and the environment. Combined with Fig. 3bef, it can be found that the hot spot (high-temperature region) sprouts from the central region of the stack and then spreads around. During this time, although the temperature of the stack is still relatively low, it can be observed that the obvious temperature gradient has been formed across the stack (see Fig. 3f). As time progress, the temperature gradient in the stack increases further. It can be clearly seen from Fig. 3gei that the temperature of the central and lower regions of the stack is significantly higher than that of the inlet region.  pez-Sabiro  n et al. [35] and Guo et al. [36] also reported Lo similar results. It should be noted that the uniformity of airflow has little effect on temperature distribution, because the results of airflow velocity measurement show that on the whole, the airflow distribution over the entire air inlet crosssection is relatively uniform, although the velocity in the middle region is slightly larger. However, the effect of this slight difference in airflow distribution (velocity difference is less than 0.1 m/s) on the temperature distribution is considered to be negligible and not enough to lead to such a

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Fig. 3 e Temperature distribution at different times during startup.

significant uneven temperature distribution. This phenomenon is considered to be related to the uneven water distribution within the stack. In this study, dry hydrogen is supplied directly to the stack. So the membrane near the anode inlet region (see Fig. 1c) is relatively dry, resulting in lower performance. Because the stack is placed vertically and has parallel straight channels at the anode side, the water in the membrane and channels tends to flow downward under the action of gravity and hydrogen flow [37]. As time progress, the water content of the membrane would gradually increase along the direction of hydrogen flow. Consequently, better hydration can be achieved in the central and lower regions of the membrane, resulting in more active electrochemical reactions and more heat production there. Another interesting phenomenon is that, after 90 s, the temperature changes in the central and lower regions are no longer obvious, while the temperature in the inlet region and inlet distributor (marked by the dashed box) is gradually increasing. One possible reason is that although the membrane of the anode inlet region has a lower activity at the initial stage, the water produced at the cathode is transported to the anode by back diffusion over time so that the membrane of the inlet region is gradually wetted. Since the region

is close to the anode inlet, where the hydrogen concentration is higher, the electrochemical reaction there will become more active with the gradual hydration of the membrane. Accordingly, heat generation will also increase and most of which will be transferred to the inlet distributor through heat conduction. Because there are no cooling channels at the outermost ends of the bipolar plates (see Fig. 1c), the hydrogen inlet distributor mainly relies on natural convection for heat dissipation. Insufficient cooling may lead to the accumulation of heat and a local high-temperature region eventually forms there. The results of Fig. 4 are obtained by processing the thermal image data at different times in Fig. 3. The ordinate in the figure represents the number of pixels corresponding to a certain temperature in the thermographic image. The larger the value, the more extensive the corresponding temperature distribution is. As can be seen from the figure, at the initial stage (at 5 s), the temperature is an approximately normal distribution. At this time, the intensity of the electrochemical reaction is relatively weak, so most of the region in the picture is still close to the ambient temperature, with an overall temperature difference of no more than 2.5  C. However, in only 10 s, the temperature in the thermographic image has

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Fig. 4 e Statistics of the pixel points in the thermographic images at different times.

spread rapidly to approximately 38  C, and the local temperature has even exceeded 40  C. Over time, on the one hand, the high-temperature front continues to extend to the right along the abscissa. On the other hand, the percentage of the hightemperature area gradually increases. Eventually, the overall temperature distribution is shifted to the right. Fig. 5 shows the temperature distribution and evolution of the outer surface of the stack during the shutdown. It can be clearly seen that the temperature change in the stack is not synchronous during the shutdown. Once the load is switched to open circuit, the temperature in the central region will drop immediately, and the hot spot will quickly shrink toward the center. As can be seen from Fig. 5aei, the temperature of most areas can be uniform in around 20 s, except for the top region. Especially for inlet distributor, where it takes longer to fall to a lower temperature. As mentioned earlier, the outermost ends of the bipolar plates are not provided with the cooling channels. Therefore, initially, this phenomenon is considered to be the result of insufficient cooling. However, a question arises from this: why the outlet distributor can be cooled as quickly as the central region since the cooling conditions there are also poor? This indicates that cooling conditions may not the main cause of the above phenomenon. In addition, it should be noted that the hydrogen supplied to the anode inlet is unheated. The hydrogen temperature at the anode inlet is measured and found to be very close to the ambient. This fact eliminates the possibility of hydrogen stream heating the inlet distributor. A similar phenomenon can also be found in the study of Abdullah et al. [38]. Combined with their experimental results and analysis, the phenomenon that there is still a small temperature difference within the stack at the open circuit is thought to be related to hydrogen crossover or internal current at the inlet region. Although the proton exchange

membranes are selectively permeable, there are still a small amount of hydrogen or electrons that can pass through the membranes. Hydrogen, which penetrates from the anode to the cathode, reacts chemically or electrochemically with oxygen under the action of Pt catalyst, releasing the reaction heat to increase the stack temperature [39]. This phenomenon is usually negligible during fuel cells operation. However, its negative effect will become significant when the fuel cells are at the open circuit [32]. To confirm the above possibility, we conduct a test as shown in Fig. 6 below. In this test, the stack is always at the open circuit and the local temperature is monitored through 30 thermocouples. It should be noted that the numbers in the legend represent different temperature measuring points. The numbering rule has been explained in section Local temperature measurement. In the initial stage, hydrogen is not supplied into the stack. It can be seen that the readings of the thermocouples are relatively stable, and the temperatures between the measuring points are very close (<0.3  C). Once hydrogen is introduced into the anode, the temperature of all measuring points is climbed rapidly, indicating that hydrogen does have a significant effect on the stack at the open circuit. It can be clearly seen from the diagram that the temperature rise of the measuring points at the inlet region is more significant. This is easy to understand: the main driving force of hydrogen crossover is the local pressure difference. The hydrogen pressure in the inlet region is higher, so the hydrogen permeability may be higher there. The analysis is confirmed by Ref. [40] where the authors concluded that the hydrogen penetration at the inlet was the largest and believed that the partial pressure gradient of hydrogen was one of the reasons for the local variation of hydrogen crossover in the fuel cell. In addition, in the tens of seconds after the introduction of hydrogen, the temperature of the measuring points decreases

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Fig. 5 e Temperature distribution at different times during shutdown.

gradually due to the combined action mechanism of heat conduction and convection cooling. When the thermal balance is built again, the temperature of all measuring points tends to converge. This process corresponds well to the temperature evolution pattern in Fig. 5. Therefore, a reasonable explanation for the temperature evolution of the inlet region and inlet distributor in Fig. 5 may be that: when the load shuts down, the inlet region still maintains relatively strong fuel crossover due to the higher hydrogen concentration, which causes additional heat generation. A small part of the heat generated there is transferred downwards and carried away by the forced airflow in the cathode flow channels, so the influence of which on the temperature distribution in the central and lower regions is not obvious. However, the rest is transferred upwards to the hydrogen inlet distributor. Since there are no cooling channels in the edge region (see Fig. 1c), the heat transfer conditions there are worse than that at the region with the cooling channels. As a result, the heat transferred to there can offset part of the heat dissipation, which slows down the cooling

Fig. 6 e Temperature changes before and after the hydrogen is introduced into the stack.

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Fig. 7 e Temperature variation under different air flow rates: (a) 5.3 £ 10¡3 m3s¡1; (b) 1.1 £ 10¡2 m3s¡1; (c) 1.5 £ 10¡2 m3s¡1; (d) Comparison of average temperature and temperature difference.

rate. As time progress, on the one hand, the hydrogen concentration tends to be uniform within the stack, so that the difference of hydrogen crossover over the active area is no longer significant. On the other hand, continuous cooling also accelerates heat dissipation in local high-temperature areas. Eventually, the local high temperature gradually faded away and the temperature of the entire stack tends to be uniform. It can also be observed in Fig. 5 that it takes a longer time to cool down the endplates. This is because the material of the endplates is bakelite, which has a very low thermal conductivity, so it relies mainly on natural convective to heat dissipation.

The effects of air flow rate In the previous section, the thermal imager provided high spatial resolution to help better understand the temperature distribution and evolution of the stack. In the next two sections, thermocouples will be applied to explore the local thermal characteristics inside the stack under different operating conditions. The experimental procedure is as follows: The load is first increased from the open circuit to 10 A in increments of 5 A, and each step stays for 60 s. Then, the current steps to 15 A and lasts for 15 min to make the stack temperature reach a quasi-stable state. Next, the load is further increased to 20 A and maintained for 20 min. This section will analyze the thermal characteristics of the stack when the load changes from 15 A to 20 A under different air flow rates. The step-bystep change of current from 0 A to 15A is to avoid

irreversible damage to the stack caused by the sharp increase of the load. Fig. 7aec shows the temperature variation curves of all measuring points at different air flow rates. It can be seen from these figures that when the load is stepped from 15 A to 20 A, the local temperature can respond quickly and rise sharply. Over time, the temperature tends to be gentle. This asymptotic behavior of temperature is the result of the balance of heat production and heat dissipation in the fuel cell system. It can be seen from Fig. 7d that as the air flow rate drops, the cooling conditions deteriorate rapidly, resulting in a significant increase in the average stack temperature and the temperature difference. Fig. 8 shows the temperature variation curves of the five tested cells and the upstream and downstream sections. The average value of the 6 temperature measuring points on the tested cell is taken as the overall temperature of the cell. The temperature variation curves of upstream and downstream sections are also obtained by this method. The figure reveals some interesting points: (i) There is a significant temperature difference within the stack, especially at high currents (20 A); (ii) The temperature profile of the cells shows a parabola distribution pattern, that is, the temperature of the intermediate cell is the highest, while the temperature of the cells on both sides is relatively lower; (iii) The temperature of the downstream section is significantly higher than that of the upstream section. Fig. 9 quantitatively compares the uniformity of the temperature distribution of PEMFCs under different air flow rates. It can be clearly seen from the figure that the relationship

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Fig. 8 e Temperature variation curves of different cells and sections.

between the air flow rate and temperature uniformity index (TUI) is not monotonous. More specifically, the TUI first increases and then decreases with the increase in air flow rate. This phenomenon occurs because when the air flow rate is small (e.g. 5.3  103 m3s1), the heat taken away from the stack through forced convection is very limited. In this case, the thermal conduction mechanism will dominate, which will promote thermal diffusion and lead to a more uniform temperature distribution. As the air flow rate increases, the convection cooling will be gradually enhanced and more heat will be removed from the stack through the airflow. This will increase the temperature gradient within the in-plane (from the air inlet to outlet), resulting in a larger TUI. However, when the air flow rate increases further (e.g. 1.5  102 m3s1), the forced convection effect will be further enhanced, so that the downstream airflow will have a greater cooling capacity. As a result, the uniformity of temperature distribution will be improved to a certain extent. In addition, it can also be found from Fig. 9 that the TUI of the intermediate cell and the downstream is the highest regardless of the change in the air flow rate, which is believed to be related to the worse heat transfer conditions where they are located.

In order to better study the effects of the air flow rate on the thermal response characteristics of the stack, an indicator, thermal response time t, is introduced in this study. It is defined as the time it takes for the transient temperature to reach a 98% steady-state temperature value during the load changes. Fig. 10 compares the thermal response time at different air flow rates. Fig. 10a shows that the thermal response time of the intermediate cell is shorter than that of the cells on both sides. This is because the heat of the intermediate cell is mainly taken away by the forced airflow in the cathode channels, while the cells on both sides can dissipate heat directly to the environment through edge cooling in addition to the forced convection of air. Therefore, the intermediate cell is more vulnerable to overheating [41]. As a result, when the load changes, the temperature of the intermediate cell will rise faster, which shortens the time it takes to reach a steady state. In addition, when the air flow rate is reduced from 1.5  102 m3s1 to 5.3  103 m3s1, the effects of overheating on the cells are more significant, so the thermal response time of all cells increases dramatically from tens of seconds to hundreds of seconds. This result is slightly different from the

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Fig. 9 e Comparison of temperature uniformity index under different air flow rates.

data reported in Refs. [26,42] due to the difference in cell configuration (such as size, structure) and operating conditions. But they are consistent in the order of magnitude. In other words, the thermal response times in these documents are also at the level of hundreds of seconds. Fig. 10b shows a comparison of the thermal response times for the upstream and downstream sections. It can be found that the thermal response time of the downstream section is slightly larger than that of the upstream section. This can be attributed to the fact that the coolant (air) with a lower temperature first contacts the upstream and carries the heat downstream, so the cooling effect of the upstream is better than that of the downstream, and the thermal response time is also shorter.

The effects of load current The load current is another important factor affecting temperature. This section will investigate the dynamic characteristics of the temperature at different currents. The experimental procedure is as follows: the current is gradually increased from 0A to 24A in an increment of 8 A, and then

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Fig. 10 e Effects of air flow rate on thermal response time: (a) comparison between different cells; (b) comparison of upstream and downstream sections.

returns to the open circuit again in the same way. Each stage runs for 15 min. It should be noted that because the time span of the experiment is long, a larger air flow rate (1.5  102 m3s1) is selected in order to reduce the thermal response time. Fig. 11a shows the instantaneous temperature curves under different currents. It can be clearly seen that the temperatures of the measuring points are positively correlated with the current. The higher the current, the more the heat production and the higher the temperature. The local temperature and TUI at different currents are compared in Fig. 11b. It can be clearly seen that the temperature difference within the stack is gradually expanded with the increase of the current. At 24 A, the maximum temperature difference between the measuring points can reach 9  C, which is very close to the results reported by Wu et al. [43]. By fitting the data points, it can be found that there is a very good second-order relationship between the average temperature and the current. Similar results have been reported by Mench et al. [44]. This is because the heat production is proportional to the square of the current density, that is,

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Fig. 11 e Effects of load current on temperature: (a) temperature variation curves; (b) comparison of characteristic temperature and TUI.

Qgen fi2 , as shown in Equation (4) in Section Heat generation. So the second-order change in heat production will cause the temperature to show the same change pattern. Fig. 11b also reveals the effect of current on the TUI. As expected,

Fig. 12 e Variation curves of cell temperature and the output voltage at different currents.

the heat load increases with the increase of the current, which will lead to a larger temperature gradient within the stack, thus worsening the temperature homogeneity. Therefore, special attention should be paid to thermal management problems at high currents to avoid irreversible damage to the PEMFCs. In addition, Fig. 11a also shows an interesting variation feature: the temperature during the current stepwise increase seems to be lower than that during the current stepwise decrease. This temperature drift phenomenon is more pronounced in Fig. 12. It should be pointed out that this phenomenon is repeatable and not caused by experimental errors. This behavior may be related to the operating temperature of the stack and the hydration of the membrane. At the stage where the current is stepped up, the initial operating temperature of the stack is lower, so catalyst activity is inhibited. Moreover, since the dry hydrogen is introduced into the anode, the membrane is relatively dry at this time. As a result, the electrochemical reaction rate is at a low level, resulting in relatively less heat production. Conversely, during the load descent stage, the initial temperature of the stack is higher and the membrane has also been well wetted as the reaction proceeds. Therefore, the electrochemical reaction is strengthened, and the heat production increase. The above hypothesis can be confirmed, because, overall, the average output voltage in the latter stage is higher than that in the previous stage as shown in Fig. 12. Another possibility is that during the load stepwise decrease, the latent heat released by water vapor condensation may also increase the stack temperature to a certain extent [19]. Compared with the process of load stepwise increase, the time required for cells to reach quasi-stable is increased during the load stepwise decrease. Especially when the current steps from 16 A to 8 A, the temperature continues to decline and shows no sign of stability. The output voltage also shows the same change law. This may be because a large amount of water vapor condenses rapidly during the load stepwise decrease, causing liquid water to accumulate on the anode side, covering the active sites, blocking the GDL and the flow channels, and hindering the hydrogen transport. As a result, the concentration polarization is significantly enhanced, and the output voltage shows a slow downward trend. Furthermore, the lower the working temperature stack is, the faster the condensation rate of water vapor is, and the more significant the negative effect is, which coincides well with the temperature variation feature in Fig. 12. In summary, the temperature variation characteristics of the PEMFCs may be affected by several factors, and these factors may be interrelated and interacting with each other. Another interesting phenomenon in the transient analysis is the time lag between temperature response and voltage response. As shown in Fig. 13, the temperature always lags the voltage by approximately 0.5e1s at different loads. This is easy to understand because the electrochemical response is much faster than the physical response (heat generation, heat transfer, heat convection). Therefore, once the load changes, the voltage can respond almost at the same time, while the heat flow generated by the cathode catalytic layer needs to be first conducted to the GDL and then transferred to the air by

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Fig. 13 e Comparison of the dynamic response of temperature and output voltage at different loads: (a) 8e16A; (b) 16e24A; (c) 24e16A; (d) 16e8A.

convective heat transfer before being detected by the probes. Therefore, there is an inconspicuous time lag between the two.

Conclusion The spatial-temporal temperature distribution and variation characteristics of an open cathode PEMFC stack under different operating conditions are investigated experimentally by thermal imager and thermocouples. Through the above research work, the following conclusions can be drawn: (1) During the start-up, the hot spot begins to sprout from the central region of the stack and then spreads to the surrounding area. After the quasi-stable state is reached, the temperature distribution inside the stack is not uniform. During the shutdown, the central and lower regions are first cooled down, followed by the anode inlet region, and finally the endplates. (2) The intermediate cell has the shortest thermal response time. The farther away from the middle, the longer the thermal response time of the cell. Besides, the thermal response time of the upstream section is shorter than that of the downstream. (3) The air flow rate significantly affects the thermal response characteristics and temperature homogeneity of the stack. The thermal response time decreases significantly with the increase of air flow rate, while the temperature uniformity index shows the change pattern of increasing first and then decreasing. (4) The relationship between the average temperature of the stack and current can be well characterized by a

quadratic polynomial. The temperature during the load stepwise increase is asymmetric with that during the load stepwise decrease. Overall, the temperature in the load stepwise decrease stage is higher than that in the load stepwise increase stage. Moreover, there is a time lag in the response of temperature and voltage to changes in current.

Acknowledgments This research was supported by the National Natural Science Foundation of China (21776095), and the Guangzhou Science and Technology Program (201804020048).

Nomenclature GDL MEA MEMS PEMFC TUI A Er F i i0 iL n ncell N

Gas diffusion layer Membrane electrode assembly Micro-electro-mechanical system Proton exchange membrane fuel cell Temperature uniformity index,  C Effective active area, cm2 Theoretical reversible voltage, V Faraday constant Current density, A cm2 Exchange current density, A cm2 Limiting current density, A cm2 Number of transferred electrons Number of cells Number of measuring points

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Qgen R Ri T T Vcell a

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Heat production, W Universal gas constant ohmic resistance, U cm2 Stack temperature,  C Average temperature,  C Single cell voltage, V Transfer coefficient

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