Keywords

1 Introduction

The PEMFC is regarded as one of the most promising power sources for vehicles and various portable electronic applications due to its high energy conversion rate, high reliability and low pollution emission in operation [1]. Currently, commercial fuel cells usually need hundreds of cells and large active area for each single cell to realize high power applications [2]. Scaling up the active area from no more than 50 cm2 in laboratory level to over 150 cm2 for commercial use encounters challenges in reaction gas flow distribution, water-heat management, and reaction uniformity [3]. In order to optimize the design of commercial PEMFC, it is necessary to carefully consider the coupling relationship of various uneven parameters such as temperature, reaction gas concentration, stress distribution and current density distribution [4].

Conventional fuel cell diagnostic methods such as polarization curve can give an overall performance but lack of spatial information at different positions for the large reaction active area. As current density is directly linked to electrochemical reaction of fuel cells, its distribution over the active area can give the local performance of the cells [5]. Thus, relevant diagnostic methods need to be developed to obtain the current density distribution in the reaction process of commercial PEMFCs with large active area. So far, various researches have been conducted to investigate the current density distribution inside PEMFCs with large active area. In order to study the influence of relative humidity (RH) on the performance and durability of reversal-tolerant-anodes (RTAs) during hydrogen starvation, Wang et al. [6] employed an advanced segmented technique to examine the coupling reactions by simultaneously measuring current density, RH and temperature in a PEMFC with large active area. It is found that the inlet of RTAs undergoes degradation earlier than the outlet and the membrane electrode assembly (MEA) with a RTA has an optimal humidity during cell reversal. Lin et al. [7] studied the changes of the overall performance and current density distribution of commercial PEMFCs under dynamic gas operating parameters especially transient process through fast segmented cell technology. The results showed that the cathode stoichiometric ratio has a more significant effect on the cell performance than the back pressure. The work leads to suggestions on the control strategy during dynamic load demands and lays a foundation for further study into the durability under variable gas operation parameters of the fuel cell operation.

Moreover, some researches have been conducted to study the relationship between stress distribution and current density distribution as well as the cell overall performance. Vijayakumar et al. [8] measured the current distribution profile along the cathode flow field channel using a segmented current measurement plate to understand the influence of uneven clamping pressure distribution on the MEA. Although scholars have done some research on the relationship between the stress distribution and the current density distribution affected by the clamping force, few studies on the stress distribution and its relationship with the current density distribution caused by the cell structure and clamping strategy have been conducted so far inside commercial PEMFCs with large active area.

In this work, two different clamping strategies are applied to a commercial PEMFC with the active area of 360 cm2. The current density distribution is tested by segmented cell technology (from Yuanzhu technology, www.szfcet.com) and the stress distribution is obtained. The results of both strategies are compared not only to analyze the influence of different structures on stress distribution but also to examine the relationship between stress distribution and current density distribution as well as the cell overall performance.

2 Experimental Part

The research is carried out on a commercial single PEMFC with the active area of 360 cm2 with two different clamping strategies (strategy 1 and strategy 2) under the standard operating conditions. The conditions contain: coolant temperature of 75℃, anode/cathode back pressure of 100/95 kPa, anode/cathode stoichiometry of 1.5/2.2, anode/cathode RH of 80%/80%. The segmented cell technology is from Yuanzhu technology (www.szfcet.com), which is applied to test the current density distribution of the fuel cell and the stress distribution is obtained by pressure measurement film.

3 Results and Discussion

3.1 Influence of Different Structures on Stress Distribution

Fig. 1.
figure 1

The stress distribution (a) and the flow field structure (c) of the contact interface of the cell with strategy 1; the stress distribution (b) of the cell with strategy 2; cross section of the parallel areas (d) and the vertical areas (e, f) of the flow field structure.

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Figure 1 analyzes the influence of different structures on stress distribution. It can be found that the stress distribution of the cell with strategy 1 (Fig. 1(a)) is less uniform than the stress distribution of the cell with strategy 2 (Fig. 1(b)). As shown in Fig. 1(c), the contact interface of the cell with strategy 1 can be divided into two parts. One is the area where the ribs on the contact interface are vertical to each other (the triangle areas of Fig. 1(c)), and the other is the area where the ribs on the contact interface are parallel to each other (the oval areas of Fig. 1 (c)). Comparing the stress distribution and the flow field structure on the contact interface, we can find that the contact where ribs are vertical to each other is better than that where ribs are parallel to each other. In vertical areas, the ribs and channels on both sides of the interface are interlaced. The contact forms of the interface can be divided into three types: rib to rib, rib to channel, channel to channel (Fig. 1(e, f)). From Fig. 1(a) we can find that both the rib-to-rib form and the rib-to-channel form can achieve good contact stress, while the channel-to-channel form gains poor contact. As most of the vertical areas are rib-to-rib and rib-to-channel forms, these areas generally result in good contact. On the other hand, the parallel areas only have rib-to-rib and channel-to-channel forms. And as the proportion of the channel-to-channel form which has poor contact is considerably large (near 50%), the parallel areas generally result in poor contact. As shown in Fig. 1(b), the interface of the cell with strategy 2 only has rib-to-channel and rib-to-rib forms (Fig. 1(e)). Thus, the stress distribution on the entire contact interface is more uniform and generally better.

3.2 Relationship Between Stress Distribution and Current Density Distribution

Fig. 2.
figure 2

The current density distribution of the cell with strategy 1 (a) and the cell with strategy 2 (b) at 1000 mA/cm2.

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Figure 2 shows the current density distribution of the cell with two clamping strategies at 1000 mA/cm2 respectively, which investigates the relationship between stress distribution and current density distribution. As shown in Fig. 2(a), it is found that the local current density in some areas (the oval areas of Fig. 2(a)) is lower than the normal value. It can be found from the stress distribution and flow field structure in Fig. 1(c) that these low current density areas are consistent with the poor contact areas (parallel areas). Therefore, the local poor contact may affect the reaction in these areas and lead to low local current density. On the other hand, by comparing the triangle areas of the stress distribution (Fig. 1 (c)) and the current density distribution (Fig. 2(a)), the results show that local good contact contributes to the reaction in these areas, which leads to the increase of local current density. Moreover, the current density distribution of the cell with strategy 2 (Fig. 2(b)) is much more uniform than that of the cell with strategy 1 (Fig. 2(a)). By combining the phenomenon with the stress distribution of the cells with two clamping strategies respectively (Fig. 1(a) and (b)), we can find that a more uniform stress distribution can lead to a more uniform reaction current density distribution.

3.3 Relationship Between Current Density Distribution and the Cell Overall Performance

Fig. 3.
figure 3

Polarization curves of the cell with both clamping strategies.

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As shown in Fig. 3, the red line is the polarization curve of strategy 2, and the black line is the polarization curve of strategy 1. It can be found that the overall performance of the cell with strategy 2 is better than that of the cell with strategy 1, especially at medium and low current density region. Since the stress distribution and current density distribution of the cell with strategy 2 is more uniform than that of strategy 1 (Figs. 1 and 2), it can be deduced that the well uniformity of the stress distribution and current density distribution has a positive impact on the improvement of the cell overall performance.

4 Conclusions

In this study, two different clamping strategies are applied to a commercial PEMFC with the active area of 360 cm2. The stress distribution and current density distribution as well as the overall performance of the cell in both cases are tested under the same operating conditions. The results show that the vertical areas where rib-to-rib and rib-to-channel forms occupy the majority have good contact while the parallel areas where the proportion of the channel-to-channel form which has poor contact is considerably large have poor contact, which leads to the uneven stress distribution of the cell with strategy 1. The cell with strategy 2 of which the contact interface only has rib-to-channel and rib-to-rib forms has more uniform and generally better stress distribution on the entire contact interface. By comparing the relationship between the stress distribution and the current density distribution of both strategies, it is found that a more uniform stress distribution can lead to a more uniform reaction current density distribution. From the results of the overall performance of the cells in both strategies, it can be considered that the good uniformity of the stress distribution and current density distribution has a positive impact on the improvement of the cell overall performance. Thus, it is significant to improve the clamping strategy and the contact form of the flow field in order to improve the uniformity of the stress distribution and reaction current density distribution, and ultimately improve the cell overall performance.