Investigation of water transport and its effect on performance of high-temperature PEM fuel cells

Electrochimica Acta 149 (2014) 271–277

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Investigation of water transport and its effect on performance of hightemperature PEM fuel cells Caizhi Zhang a,b, *, Lan Zhang b , Weijiang Zhou b , Youyi Wang c, Siew Hwa Chan a,b, * a

School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore Energy Research Institute @ Nanyang Technological University, 50 Nanyang Avenue, 637553, Singapore c School of Electrical & Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 25 August 2014 Received in revised form 2 October 2014 Accepted 13 October 2014 Available online 18 October 2014

The phosphoric acid doped polybenzimidazole (PBI) membrane is the most commonly used electrolyte in high-temperature PEM fuel cells (HT-PEMFCs). The aims of this study are to investigate the transport of water vapour from the cathode to the anode and the effect of water vapour on the performance of a single cell HT-PEMFC using a commercial PBI-based membrane-electrode-assembly (MEA). The amount of water vapour transportation is determined by weighing the water vapour condensed from the fuel offgas stream located at the anodic outlet of HT-PEMFC under a flow-through mode. Experimental results show that up to 31.7% of the generated water vapour is transported from the cathode to the anode. To study the effect of water on the cell performance, the polarization curves, transient voltage curve and transient ohmic resistance (due to on/off of the outlet solenoid valve) are measured under anodic deadend and flow-through modes. The study shows that excessive water vapour in the anode can significantly deteriorate the performance of a cell operated in an anodic dead-end mode, but negligible in an anodic flow-through mode. The performance can rapidly recover after switching the cell from a dead-end mode to a flow-through mode. The study allows fundamental understanding of the operation in anodic deadend and flow-through modes, which offers guidelines for operating a HT-PEMFC with improved fuel utilization. ã 2014 Elsevier Ltd. All rights reserved.

Keywords: HT-PEMFC Water vapour transport Transient performance Transient resistance

1. Introduction Among all the different types of fuel cells, the high-temperature PEM fuel cell (HT-PEMFC), which operates in a temperature range of 120
C to 200
C, has attracted significant attention in recent years[1–3]. In a HT-PEMFC, the proton conduction relies on the doped phosphoric acid (PA, H3PO4) in polybenzimidazole (PBI) membrane even at an anhydrous state (hop mechanisms or Grotthuss mechanism), which is different from the LT-PEMFC relying on liquid water to assist the proton migration from the anode to the cathode (vehicle mechanism) [4–6]. Because of high temperature operation, HT-PEMFC has several advantages such as faster reaction kinetics, higher CO tolerance, simpler system design and improved overall efficiency [1,7,8]. It is perhaps the most promising candidate for combined heat and power (CHP) and combined cooling, heat, and power(CCHP) applications [9–11]. Generally, a fuel cell operates at a stoichiometric ratio of anodic

* Corresponding authors. E-mail addresses: [email protected] (C. Zhang), [email protected] (S.H. Chan). http://dx.doi.org/10.1016/j.electacta.2014.10.059 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.

fuel higher than 1[12–14] and the non-reacted H2 molecules are discharged to the atmosphere. To improve the fuel utilization efficiency, it is advisable to run the fuel cell under anodic dead-end mode other than flow-through mode, because by doing so one can achieve hydrogen utilization of close to 100%. Essentially, the anodic dead-end mode is to block the anode outlet with marginally increase in fuel pressure, while flow-through mode is to operate the cell normally under the ambient pressure. Water management in a typical low-temperature PEM fuel cell (LT-PEMFC) operating on anodic dead-end mode is nothing new. In a LT-PEMFC, blocking the anode outlet may lead to serious flooding problems due to accumulation of liquid water in the anodic compartment, arising from water back diffusion from the cathode [15–20]. Bussayajarn et al. [15] studied planar air breathing LTPEMFC and concluded that the cell performance could deteriorate quickly when the cell is operated at anodic dead-end mode. Dumercy et al. [16], Choi et al. [17] and Belvedere et al. [18] experimentally studied the flooding phenomenon and voltage variations versus time of a fuel cell operated in a dead-end mode. Meyer et al. [19] investigated the effect of anode dead-end and purge processes on the cell performance by adopting a combinational approach, which involves voltage transients, EIS, off-gas

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analysis and thermal imaging of the cell. In HT-PEMFC, numerical models were developed by Chippar et al. [21,22] to study the effect of reactants crossover on performance. They concluded that reactants crossover in a fresh PBI membrane exhibited a negligible impact on performance and appeared more impact in a more severely degraded membrane or with pinholes in membrane. Daletou et al. [23] studied the gases permeability in a PBI/ polysulfone copolymer blends and observed water was transported. There is only a handful studies on the phenomena of water vapour transport from the cathode to the anode in HT-PEMFC [5,24] and a few literatures reporting the performance characteristics of HT-PEMFC operating on anodic dead-end mode, especially the transient behaviour of HT-PEMFC. In this study, the water transport in HT-PEMFC in anodic flowthrough mode and the effect of water vapour on the cell performance of a HT-PEMFC in anodic dead-end mode were investigated. The study includes quantitative analysis of water transport, polarization measurement, understanding transient behaviour of the cell performance and determination of transient resistance of the electrolyte membrane. The detailed information of water transport and the effect of water on the cell performance are presented, which provide useful insights for one to design a control strategy for water management in a HT-PEMFC.

was regulated at 0.1 bar (gauge pressure). The regulators (2, 9) were used to step down the pressure of the compressed gases. A microcomputer controller was used to control the on/off of the solenoid valves located at the inlet and outlet of the anode chambers. 2.2. Water transport measurement

2. Experimental

When HT-PEMFC was fed with dry H2 and dry air, water vapour molecules were produced by the electrochemical reaction at the cathode side. If the generated water molecules are transported from the cathode to the anode through the PBI electrolyte, the migrated water should be found at anode exit [24]. To quantify the amount of water vapour transported from the cathode to the anode, two dryers (4, 6) with desiccant in them have been used to absorb the water vapour as shown in Fig. 1. Dryer (4) located at the anodic inlet of HT-PEMFC was used to remove the water molecules in the fuel stream. Dryer (6) was used to absorb the water vapour at the anodic exit when the cell was operated in flow-through mode (Valve 5 open). The cold water bath (7) acts as a cooler to condense the water vapour in the anodic exhaust when it passes through the dryer. The mass of the dryer (6) before and after the adsorption of water at the anodic exhaust was weighed to determine the amount of water transported from the cathode to the anode.

2.1. Experimental setup

2.3. Characterization of water vapour effect on performance

The schematic of HT-PEMFC test rig used in this study is shown in Fig. 1. The test rig includes a single cell HT-PEMFC, an oven for temperature control, a load bank, a data acquisition system and a gas supply system. The membrane-electrode-assembly (MEA, Celtec1 P1000) was purchased from BASF Fuel Cell GmbH (Frankfurt, Germany) with an active area of 45 cm2. The properties of the MEA has been reported elsewhere [25,26]. The flow-field in graphite plates of anode and cathode are 5-stepserpentine and the geometrical specifications of flow-field are presented in Table 1. The cell was housed in a programmable oven (Memmert GmbH) to control the operating temperature at 160
C. The PLZ-4WA electronic load (KIKUSUI, Japan), fuel cell impedance meter KFM2150 (KIKUSUI, Japan) and the application software were employed for fuel cell measurement. To simplify the study, pure hydrogen was used as the fuel for the cell with fuel and air flow arranged in co-flow configuration. Reasonably dry H2 and air were supplied to the cell from compressed gas cylinders (1, 10). The air flow rate at the cathode was controlled at 300 sccm by a mass flow controller (8) at atmospheric pressure. At the anode side, the cell operated at dead-end mode or flow-through mode can be switched desirably by a solenoid valve (5). The pressure at the anode inlet

To study the effect of water on the performance of HT-PEMFC, the following experiments were conducted. A 30 min stabilising operation under flow-through mode was conducted at the beginning. Subsequently, polarization curves were measured underflow-through mode and dead-end mode. Then, transient behaviour of voltage (due to on/off of the outlet solenoid valve) at constant current of 9 A (0.2 A/cm2) under flow-through mode and (valve (5) open) and dead-end mode (valve (5) closed) were carried out alternately. Finally, the transient resistance of the membrane (due to on/off of the outlet solenoid valve) was measured by Electrochemical Impedance Spectroscopy (EIS) technique under both dead-end mode and flow through mode. 3. Results 3.1. Phosphoric acid distribution in MEA Scanning electron microscopy (SEM) image of Celtec1 P1000 MEA in cross section is shown in Fig. 2(a), while the corresponding SEM element map of phosphorus reflected as red dots is shown in Fig. 2(b). It was observed that phosphoric acid (PA) appears everywhere in the electrodes, but mainly in the electrolyte membrane. Kwon et al. [27] explained that the distribution of the PA is the result of squeezing it out of the membrane during assembly and diffusing into the electrodes mostly by capillary forces. The distribution of PA would affect the water transport and balance in different layers of MEA since PA has strong affinity to water absorption. It is noted that PA is filled in the microstructure of PBI electrolyte in HT-PEMFC, whereas water filled in the Nafion1 electrolyte of LT-PEMFC. Hence, the mechanism of water transport

Table 1 Geometrical specifications (mm). Channel

Fig. 1. Schematic of HT-PEMFC test rig.

Rib

Plate

Gasket

width

depth

width

thickness

dimension

thickness

1

1

1

2.5

100  100

0.33

C. Zhang et al. / Electrochimica Acta 149 (2014) 271–277

273

Fig. 2. (a) SEM images of cross-sectional MEA; (b) The corresponding SEM element map of phosphorus (red dots).

from the cathode to the anode in HT-PEMFC is different from that of Nafion1 membrane based PEMFC. 3.2. Water transport in HT-PEMFC The mass of water vapour at the anodic exit as a function of current and testing duration is presented in Fig. 3. Results evidently show that there was significant amount of water transported from the cathode to the anode. Under a fixed amount of current drawn (9 A, 4.5 A or 1 A), the mass of accumulated water changed linearly with testing duration, which implicitly means that the rate of water transport from the cathode to the anode is constant under a fixed current or the rate of water generated due to electrochemical reactions under the flow-through operational mode. The water flux calculated from the slopes of the accumulated mass of water against time in Fig. 3 is shown in Fig. 4(a). The water flux appears to be linearly varying with increased current drawn, indicating that the water transport from the cathode to the anode is proportional to the water production at the cathode. A comparison of water transport flux between present study and that reported by Galbiati et al. [24] under the same conditions is shown in Fig. 4(b). A higher water transport rate of present study is observed. It is attributed to the higher anodic flow rate (H2 stoichiometry: lH2 ¼ 1:9) in this study as against that reported by Galbiati et al. with lH2 ¼ 1:75 [24]. According to the data reported by Galbiati et al. [24] as shown in Fig 4(b), the water transported speed increased 102.3% with the flow rate increasing 34.6% (lH2 ¼ 1:3 to lH2 ¼ 1:75). 5 1A 4.5 A 9A

Water vapour weight (g)

4

The percentage of water transported in this study is defined as the molar flow rate of water transported from the cathode to the anode per unit membrane area over the molar flow rate of water generated per unit membrane area as follows: The electrochemical reaction at the cathode is shown as: 1 2Hþ þ 2e þ O2 ! H2 O þ Heat 2

(1)

Based on conservation of atoms, 1 mole of H2O produced is accompanied with 2 moles of electrons generated. One coulomb of charges is approximately equal to 1.04105 mol of electrons. Thus, when the current density (I) is 0.2 A/cm2, the molar flow rate of water generated per unit area (QWater) is: Q Water ¼ ð1:04  105 molC1 =2Þ  0:2C: s1 cm2 ¼ 1:04106 mols1 cm2 The molar flow rate of water transported at 0.2 A/cm2 taken from the results in Fig. 4(b) is 3.30  107 mols1cm2. Thus, the percentage of water transported from the cathode to the anode at 0.2 A/cm2 is: Transported water flux ratio ¼ ð3:30  107 =1:04  106 Þ  100% ¼ 31:7% In other words, 31.7% of water generated in present study were transported from the cathode to the anode when HT-PEMFC was fed with dry hydrogen (H2 stoichiometry: lH2 ¼ 1:9) and dry air (air stoichiometry: lair = 2) at 0.2 A/cm2, while only 18% was reported by Galbiati et al. [24] under the same conditions except that H2 stoichiometry of the latter was lH2 ¼ 1:75. 3.3. Water effect on cell performance

3

2

1

0 0

50

100

150

200

250

300

350

Time (min) Fig. 3. Accumulated mass of water vapour at the anodic exit as a function of current and testing duration.

The polarization curves recorded under flow-through and deadend modes are shown in Fig. 5, which also includes the powercurrent curves. The polarization curve of flow-through mode was recorded after 30 min of cell operation at 9 A. Another polarization curve was recorded after switching from flow-through mode to dead-end mode. It can be seen that the polarization behaviours under the two testing modes are the same in a voltage range of above 0.6 V. The deviation shave become more profound for the voltages below 0.6 V. It was observed that the performance degradation is more seriously in the dead-end mode. Fig. 6 shows the voltage transient behaviour of HT-PEMFC under alternate flow-through and dead-end modes of operation. The flow-through mode period is marked with “F”, while the dead-end mode period is marked with “D”. The flow-through mode and

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Fig. 4. (a) Water flux from cathode to anode versus current with dry hydrogen, (b) Water flux at different stoichiometric ratios of H2 under a current density of 0.2 A cm2.

dead-end mode testing were alternately conducted for four times. It can be seen from this results that the transient voltage behaviour was highly repeatable. Under the flow-through mode of operation, the output voltage of the single cell dropped 16 mV initially, which is marked with a dash square box in Fig. 6. The voltage then stabilises before switching to the dead-end mode. Such transient voltage behaviour can be the cause of changing membrane conductivity, which is linked to the water content in the membrane. Further discussion can be found in Section 3.3. Under the dead-ended mode, a slight increase in voltage of about 3 mV was observed immediately after switching from flowthrough to dead-end modes, which is marked with a dash circle in Fig. 6. The voltage then dropped slightly, followed by a rapid drop before switching to the flow-through mode. This behaviour can be explained by the Nernst equation in Eq. (2) [28]. The initial increase of voltage in dead-end mode may be related to the increase of reversible voltage, due to the momentary increase in H2 pressure from atmospheric pressure to 0.1 bar gauge pressure in the anode [19]. The subsequent rapid drop in performance can be attributed to the decreased hydrogen concentration due to large amount of water transported from the cathode to the anode and trapped in the anode compartment with concomitantly hydrogen consumption during that period [15]. According to Eq. 2, both hydrogen consumption and dilution effect would lead to performance drop, hence reduced equilibrium voltage (E).

1=2

E ¼ E0 þ

RT PH2 PO2 lnð Þ 2F P H2 O

(2)

After switching from the dead-end mode to the flow-through mode, the cell performance recovers rapidly. This is due to effective removal of the accumulated water vapour trapped in the anode compartment and the replenishing of fresh hydrogen. 3.4. Transient resistance of membrane The membrane resistance may be affected by dead-end mode of operation as there may be transported water vapour accumulated in the anode compartment, thus improving the humidity level of the electrolyte. The additional water in the electrolyte increases the charge carriers through following reaction [23,29,30]. H3 PO4 þ H2 O! H3 Oþ þ H2 PO4 

(3)

Though EIS is a powerful tool which can provide insightful information [31], the technique is mainly used to study an electrochemical system under the steady-state or pseudo-steadystate, because some artefacts can add in the EIS result, especially at low frequency measurement, if a measurement were to be conducted in a wide range of frequency and repeatedly [19]. To investigate the transient resistance of a membrane in flow-through mode and dead-end mode, the Cole-Cole measurements were

0.60 1.0

Constant current @ 9A

8 Flow through Dead end

0.55

0.8

4 0.4 2 0.2

0.0 0

5

10

15

20

25

30

0 35

Current (A) Fig. 5. I-V and I-P curves under flow-through and dead-end modes.

Voltage (V)

0.6

Power (W)

Voltage (V)

6

0.50

D

F

0.45

0.40

0.35 0

10

20

30

40

50

60

Time (min) Fig. 6. Voltage transient behaviour under through-flow and dead-end modes.

C. Zhang et al. / Electrochimica Acta 149 (2014) 271–277

275

Fig. 7. Schematic of transient resistance measurement signal.

Fig. 9. Chemical structure of (a) PBI and (b) H3PO4 doped PBI.

conducted at 9 A. The schematic of the testing cycle is shown in Fig. 7. Each of the Cole-Cole measurement took 2 s within every 1 min interval and each of the measurement is conducted only under high frequency range from 2000 to 500 Hz to save the scanning time. The ohmic resistance extracted was taken as the membrane resistance because it is the major contributor to the overall ohmic resistance [29].The transient resistance of membrane is shown in Fig. 8(b), while the corresponding cell voltage is illustrated in Fig. 8(a). The results show that the membrane resistances and the voltage curve are quite repetitive. Since there were current perturbations during the measurement, the corresponding voltage was found to be harmonic. In the flow-through mode of operation, the membrane resistance increased rapidly in the beginning from low value and then slowed down when approaching the maximum resistance value. This phenomenon may be attributed to the decrease of hydration level in the anode, because water vapour in anodic compartment was not accumulated and the humidity level of the membrane was reduced by releasing water into the fuel stream. During this period, the corresponding cell voltage (performance) curve in Fig. 8(a) (and Fig. 6) shows a sharp increase at the beginning, then drops gradually and finally stabilises at a certain value. The membrane resistance decreased linearly after switching from the flow-through mode to the dead-end mode due to increase in hydration level of the membrane. This is not fully agreeable to

with the resistance change in LT-PEMFC reported by Meyer et al. [19]. They observed that the resistance was stable after the sharp drop in the initial 80 s but the resistance increased in the next 100 s. The different behaviours of membrane resistance between Nafion1 and PBI can be explained that the PBI membrane with PA has stronger affinity to absorb water and higher solubility of water than Nafion1 membrane. Thus, the resistance of PBI can be reduced continuously with increased humidity.

(a)

Transient voltage

Voltage (V)

0.56

0.54

4.1. Mechanism of water transport In LT-PEMFC, free water molecules is filled in the microstructures of Nafion1 membrane, the water transportation between the anode and the cathode is mainly governed by two processes: electro-osmotic drag and back diffusion [32–35]. The electro-osmotic drag represents that water molecules are transported with protons from the anode to the cathode. The back diffusion is associated with the water transported from the cathode to the anode due to concentration difference [36]. In phosphoric acid doped PBI membrane used in HT-PEMFC, the nearzero coefficient of electro-osmotic drag was reported by Weng et al. [37]. PBI polymer contains a lot of repeated units with two polar imidazole groups as shown in Fig. 9(a) [30,38,39]. When the polymer is immersed in PA solution, the maximum degree of protonation of polymer is that two molecules of PA reacted with the imidazole groups of each PBI repeat unit to form stable Hbonding as shown in Fig. 9(b) through reaction (4). This process is termed PA doping. The doping level is defined as the mole number of PAper repeat unit of the PBI polymer [6]. H3 PO4 þ½C ¼ N ¼ H2 PO4  þ ½C ¼ NHþ

Flow through

Dead end

0.52 0

10

20

30

(b) 190

Flow through

40 50 Time (min)

60

70

80

90

60

70

80

90

Dead end

2

Resistance (mΩ cm )

4. Discussion

Transient R

185 180 175 170 0

10

20

30

40 50 Time (min)

Fig. 8. (a) Transient voltage and (b) transient resistance for flow-through and deadend modes at 0.2 A cm2.

(4)

In practice, the conductivity of the membrane is too low to be used as electrolyte in fuel cell if the doped level of PA is less than or even equal to the maximum degree of protonation per repeat unit [30]. Normally, higher doping levels of PA are obtained to improve the conductivity of PBI membrane. For example, a doping level at 3-9 (in 40-80 wt.%) of PA was tested by Li et al. [40], and the membrane with higher doping level of PA (more than 95 wt% per PBI repeat unit) was used in this study which is the same product as in reference [25]. The excess PA within the matrix of the polymer blend forms the proton carriers, H2PO4, through reaction (5) by self-ionization and self-dehydration which will significantly increase the conductivity of membrane even at high temperature and low humidity [6]. The protons migrate along the mixed H2PO4/HPO42 anionic chain. That are why the HT-PEMFC can operate at dry condition and high temperature.

276

5H3 PO4 Ð

C. Zhang et al. / Electrochimica Acta 149 (2014) 271–277

2H4 POþ 4

þ

þ H3 O þ

H2 PO 4

þ

H2 P2 O2 7

(5)

The PA in MEA which is shown in Fig. 2 starts to release water and form pyrophosphoric acid (H4P2O7) by acid dimerization via reaction (6) at around 130-140
C under dry conditions [29,30]. In this study, H4P2O7 may exist in the membrane, anode and cathode when the cell was heated to 160
C under dry condition. However, reaction (6) is a highly reversible process. The H4P2O7 was considered sufficiently small amount to be ignored in RH range of 5-30% [30]. During the operation, H4P2O7 in the cathode can be hydrolysed and converted to H3PO4 by the adsorption of water produced at the cathode. The excess adsorbed water can form chemical bonds with PA. The water concentration in cathode side is higher than the anode side leading to migration of the water from the cathode to the anode via the bond or chemical interaction with PA [23], namely by reaction or chemical transport phenomena. The transported water at anode side is then released to the fuel stream because the anode fuel is at dry condition and is far from saturation (the water in saturated hydrogen is reduced to 0.65%, 0.3% and 0.15% at 140
C, 170
C and 200
C respectively [6]). It means that the anode flow stream can accommodate significant amount of water vapour. 2H3 PO4 ¼ H4 P2 O7 þH2 O "

(6)

In summary, the transportation of water is continuously carried out via adsorbing water vapour at the cathode side, transporting via the chemical interaction or bonding of PA and releasing water vapour at the anode side. The dominant driving force to push the water vapour from the cathode to the anode is the water vapour concentration gradient. The water concentration gradient is proportional to water production in the cathode or the amount of current generation, which is the reason why the amount of collected water was linearly increased with time elapsing (Fig. 3) and proportional to the amount of current generated (Fig. 4(a)). 4.2. Water effect on performance It clearly indicates that the performance of HT-PEMFC is affected more seriously in dead-end mode of operation according to the study of polarization and transient voltage. The deviation of the two polarization curves shown in Fig. 5 can be explained by the following: sufficient hydrogen concentration in the channel guaranteed a negligible performance drop at the beginning of dead-end mode of operation (at voltage above 0.6 V). However, subsequent drop in hydrogen concentration due to electrochemical reactions and dilution effect caused by the water transported from the cathode to the anode (at voltage below 0.6 V) [29]. According to the transient study, the cell performance was quite stable under the flow-through mode, while it exhibits fast performance dropping under the dead-end mode. The transient voltage patterns are similar to the result of LT-PEMFC reported by Meyer et al. [19]. Like LT-PEMFC, HT-PEMFC also has the “flooding problem”, though the water molecules inside the fuel cell are in gaseous phase. It is not advisable to fully block the outlet of HTPEMFC all the time to improve the fuel utilization, because the accumulation of water vapour in the anode compartment can cause significant cell performance degradation. In other words, anodic gas purging by periodically opening the outlet valve is inevitable to minimize the “flooding problem” and to stabilize the cell performance. Another observation we have is that the performance (voltage curve) decreased in the dead-end operation even though the resistance decreased during this period as shown in Fig. 9. The

increased humidity level of PBI membrane in the dead-end anode brings about the advantage of reduced membrane resistance, and therefore a positive effect on the cell performance. On the other hand, excessive water vapour in the anode and flow channel would lead to hydrogen dilution and ineffective diffusion, causing adverse effect on the cell performance. The results show that the negative effect of water is more dominant in HT-PEMFC.

5. Conclusion The water transport in HT-PEMFC has been investigated in this study. Water transport appeared to have linear relationship with the operating current. It was found that up to 31.7% of generated water is transported to the anode with dry hydrogen at 0.2 A/cm2. The driving force is related to the water gradient across the membrane and its chemical interaction with PA in the electrolyte. The water vapour effect on cell performance was studied by measuring the polarization, transient voltage, and transient ohmic resistance. The results showed that water vapour exhibits a little effect on the cell performance under the flow-through mode of operation, but more significant on performance and stability under the dead-end mode of operation. The crossover water vapour can hinder the hydrogen diffusion and reduce the hydrogen concentration in the anode, and consequently led to reduced performance of the HT-PEMFC, though the membrane resistance was reduced by elevated humidity. It is unlikely that one can fully block the anode outlet to improve the fuel utilization, because the water vapour can degrade the cell performance and stability of HT-PEMFC significantly. Understanding of the water vapour effect on HT-PEMFC performance and the transient resistance under flow-through mode and dead-end mode provides rational operational parameters for designing a control system to improve the fuel utilization and enhance the performance stability.

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