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Performance degradation of a proton exchange membrane fuel cell with dead-ended cathode and anode
Accepted Manuscript Performance Degradation of a Proton Exchange Membrane Fuel Cell with Deadended Cathode and Anode Ben Chen, Yonghua Cai, Jun Shen, Zhengkai Tu, Siew Hwa Chan PII: DOI: Reference:
S1359-4311(17)36409-8 https://doi.org/10.1016/j.applthermaleng.2017.12.078 ATE 11599
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
6 October 2017 30 November 2017 21 December 2017
Please cite this article as: B. Chen, Y. Cai, J. Shen, Z. Tu, S. Hwa Chan, Performance Degradation of a Proton Exchange Membrane Fuel Cell with Dead-ended Cathode and Anode, Applied Thermal Engineering (2017), doi: https://doi.org/10.1016/j.applthermaleng.2017.12.078
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Performance Degradation of a Proton Exchange Membrane Fuel Cell with Dead-ended Cathode and Anode Ben Chen1, Yonghua Cai1,*, Jun Shen2, Zhengkai Tu2,3,*, Siew Hwa Chan3 1
Hubei Key Laboratory of Advanced Technology for Automotive Components, Wuhan University of Technology, Wuhan, 430070, China 2
School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China
3
Energy Research Institute @ Nanyang Technological University, 50 Nanyang Avenue, 637553, Singapore
*
Corresponding author. Tel.: +86 (0) 15102756731; fax: +86 27 87540724.
E-mail:[email protected](Y. Cai), [email protected] (Z.K. Tu)
1
Abstract: Proton exchange membrane fuel cells with dead-ended cathodes and anodes can simplify the fuel cell system and reduce costs. An experiment was performed to determine the performance degradation characteristics of Proton exchange membrane fuel cells with dead-ended cathodes and anodes. The effects of operating temperature and pressure differences between the cathode and the anode on the purging period were investigated in detail. The performance and cyclic voltammetry before and after the dead-ended operation were analyzed and compared. After the experiment, the membrane electrode assembly was cut to analyze the catalytic layer cross-section membrane morphology by scanning electron microscopy. The results showed that during operation, the fuel cell performance gradually decreases until the setting value, and then quickly recovers when the cathode outlet solenoid valve is triggered during a purging cycle. The dead-ended operating period decreases with an increase in operating temperature but increases with an increase in the pressure difference between the cathode and the anode. Flooding occurs easily in a Proton exchange membrane fuel cell with a dead-ended cathode and anode, causing performance degradation. Moreover, it may cause a decrease in the electrochemical surface area of the catalyst layer. The scanning electron microscopy images showed that both the upper and middle regions of the catalyst layers remained unchanged, whereas the downstream region corroded and become thinner in the dead-ended mode after 60 hours. Keywords: Proton exchange membrane fuel cell; dead-ended; purging period; performance degradation 2
1. Introduction Fuel cells directly transfer chemical energy stored in hydrogen and oxygen into electrical energy. The conversion process is not constrained by the Carnot cycle, and thus, the cells can achieve high efficiency. Fuel cells are currently a hot topic in energy research because they are environment-friendly, produce little noise, show high reliability, and have a wide range of applications [1-3]. The proton exchange membrane fuel cell (PEMFC) are considered one of the best power technologies, as it has additional advantages of high specific power, low operating temperature, fast startup, and long lifespan [4, 5]. Water management is a crucial issue for PEMFCs [6-10] because they often operate under high current density for high power density. Which results in the generation of a large amount of water inside the catalyst layer (CL), gas diffusion layer (GDL), or flow channel. In addition, when using pure oxygen as the oxidant, the reactant stoichiometric ratio is almost equal to 1 so that gas utilization can be improved. Under these circumstances, water flooding can easily occur if the generated water cannot be drained promptly. Water gathered in the flow channel can block the transportation of the reactant in GDLs and can cause reactant starvation in the flooding area, eventually leading to an uneven distribution of current density and poor performance [11, 12]. Furthermore, flooding can result in voltage reversal and thus cause serious corrosion of the carbon support in the catalyst layer and irreversible PEMFC performance degradation [13-15]. During operation, the membrane must be properly wetted to reduce proton conduction resistance. On one hand, if the water content in the 3
membrane is insufficient, the membrane will be dehydrated; this will increase the proton conduction resistance, leading to degradation of the PEMFC performance. Furthermore, small pinholes and brittle ruptures will occur in the membrane when the PEMFC is operated under dry conditions for a long time [16]. On the other hand, if the membrane is over hydrated, water will begin to gather from the catalyst layer to the gas diffusion layer, eventually resulting in flooding. PEMFCs operating in dead-ended mode are an effective option to simplify the system and improve reactant utilization, and have thus attracted widespread attention in recent years. However, water management has become a troublesome issue for PEMFCs operating in dead-ended mode. During operation, the generated water and impure gas accumulated inside the PEMFC dilute the reactant and thus cause water flooding, leading to performance degradation [17-21]. The excess water and impure gas must be removed periodically to ensure stable operation of the PEMFC. The PEMFC performance can recover rapidly once the solenoid valve is triggered to discharge the excess water and impure gas. Periodic degradation and recovery form the whole purging cycle during dead-ended operation. Most current research focuses on PEMFCs with a dead-ended anode for improving reactant utilization and reducing performance degradation. Nikiforow et al. [22] optimized the purging period, so that hydrogen utilization was increased to 99.9%. Our previous study found that the anode outlet solenoid valve can close for approximately 26 h due to the design based on the self-water detachment theory during operation; thus, hydrogen utilization was increased to 100 % [23]. Gomez et al. 4
[24] studied a dead-ended anode PEMFC stack with an active area of 300 cm2 under different operating conditions, and the results showed that liquid water and nitrogen accumulating in the anode directly lead to voltage degradation. Furthermore, current density has an effect on the purging period: PEMFCs with dead-ended anodes are suitable for operation at low current densities and air stoichiometry because of the small amount of water generated and the low nitrogen diffusion rates. Jang et al. [25] studied the purging cycle and duration of a kilowatt stack with dead-ended anode, with results showing that the purging duration increases with an increase in current density for draining out water and impure gas from the anode. Matsuura et al. [26] investigated the effect of operating temperature and cathode relative humidity on the lifespan of PEMFCs with dead-ended anodes. They found that the cathode catalyst layer corroded easily under dead-ended anode mode operation. High cathode relative humidity and low operating temperatures were beneficial to improve the PEMFC lifespan, while small pinholes would occur in the membrane under low cathode relative humidity, accelerating the decay of the PEMFC lifetime. Chen et al. [27] found that water and nitrogen diffused from the cathode accumulated at the end of the anode flow channel, where the hydrogen content was low. As a result, severe corrosion occurred at the end of the anode. Patterson et al. [28] studied the influence of partial hydrogen starvation on catalyst layer corrosion by placing a blockage in the anode channels. Severe corrosion occurred in the blocked region, while other parts of the cathode catalyst layer did not suffer corrosion even after 100 h. However, there is only limited research on PEMFCs with a dead-ended cathode. In 5
certain cases, for example, submarine and aerospace applications, dead-ended cathodes and anodes are required to improve reactant utilization and to prevent the discharge of unreacted gas for safety purposes. Choi et al. [29] studied the operating characteristics of PEMFCs with dead-ended cathodes for submarine and aerospace applications to establish the relationship between purging period and purging duration. However, the effect of operating time on the PEMFC was not taken into account. Moçotéguy et al. [30] experimentally and numerically studied five single hydrogen-oxygen PEMFC stacks with dead-ended cathodes and anodes. They found that water flooding occurred easily in the cathode, blocking the transport channel of oxygen and resulting in oxygen starvation and performance degradation. As a consequence, water management is of great importance to PEMFCs with dead-ended cathodes and anodes. More research is needed to understand the mechanism underlying the degradation of PEMFCs with dead-ended cathodes and anodes. In this study, we aim to investigate the impact of various operating conditions on the performance degradation of a single PEMFC with dead-ended cathode and anode, particularly the effect of operating temperature and pressure difference between the cathode and the anode on the purging period. The effect of dead-ended cathode and anode operation on the catalyst layer corrosion is analyzed using electrochemical technique and the morphology changes in different part of the membrane electrode assembly. The results of this study can provide reference for engineering application. 2 Experimental 2.1 Experimental system 6
The single fuel cell configuration with a straight flow field at both the cathode and the anode for self-water removal ability has been described in our previous work [7, 8] and used in this experiment. Under gravity, water generated in the cathode and anode can overcome viscous forces and move to the buffer region at the outlet, as shown in Fig.1 (a). The cathode and anode bipolar plates made of commercial graphite materials were hydrophobic with a contact angle of 145o and resistance of 100 mS·cm-1. The geometric parameters of the single cell are given in Table 1. The membrane electrode assembly (MEA) provided by Wuhan WUT New Energy Co., Ltd was fabricated with the catalyst-coated membrane (CCM) technique with 0.4 mg·cm-2 Pt loading (HiSPECTM 9100, Johnson Matthey) at both the anode and the cathode. The Nafion® XL membrane from Dupont™ was used as an electrolyte. Carbon paper (TGP-060, Toray), hydrophobically treated in PTFE with a contact angle of 145o was used as the gas diffusion layer (GDL), and a microporous layer (MPL) was brushed directly on the carbon paper. Fig.1 (b) shows a schematic of the testing system. Hydrogen and oxygen without humidification were controlled by two regulators with set operating pressures for both the anode and cathode. Two solenoid valves were set at the outlet to control gas purging. Electronic load, cell temperature, high frequency resistance (HFR), and voltage were recorded using the HTS Fuel Cell Station produced by Hephas Energy Corporation of Taiwan. This equipment is specially designed for a single cell test, and can accurately control the operating parameters including gas flow, gas temperature, relative humidity, dew point temperature, operation temperature, etc. In addition, 7
cyclic voltammetry (CV) and linear scanning (LSV) analyses of the PEMFC can be carried out using this equipment. 2.2 Experimental scheme The single fuel cell was placed vertically to enable liquid water removal under the effect of gravity. To investigate the characteristics of a cell with a dead-ended cathode and anode under different operating conditions, the initial operating temperature, loading current, and inlet pressure of both the cathode and anode were set to 50 ℃, 20 A, and 150 kPa, respectively. The cathode outlet solenoid valve was opened once the voltage decreased by 15%, while the solenoid valve at the anode was kept closed to improve hydrogen utilization. Next, the operating temperature was set to 65 ℃, and the cathode and anode inlet pressures were set separately to 200 kPa to study the effect of operating conditions on the operating characteristics and purging cycles of PEMFCs. To evaluate the initial PEMFC performance prior to the experiment, the single cell was pre-activated by increasing the current with fully humidified reactants for approximately 3 h under ambient pressure and open-ended before the polarization curves were recorded. The fuel cell polarization curve after operation in the dead-ended cathode and anode mode represents the final cell performance. Before and after the experiment, the MEA was analyzed with respect to performance decay, and the electrochemical surface area (ECSA) loss was compared to that for the fresh MEA using cyclic voltammetry (CV, Autolab, PGSTAT 302N). Finally, the degraded MEA was cut into nine segments, as shown in Figure 2, and the cross-sectional morphology 8
was observed by scanning electron microscopy (SEM, Hitachi S-4800) to understand the catalyst degradation after long-term operation in the dead-ended cathode and anode mode. We previously found that increasing the outlet opening size could effectively alleviate water flooding in the cathode, mitigate performance degradation, and prolong the PEMFC lifespan [21]. In this paper, both the cathode and anode outlet of cell have double outlets. Prior to the single cell performance evaluation, the internal and external reaction gas leakage was tested, with an acceptable result. This was done to meet international standard requirements and to ensure that the PEMFC operated effectively with the dead-ended cathode and anode. The hydrogen and oxygen purities were 99.99 % and 99.999 %, respectively, and the ambient temperature was 25 ℃. 3 Results and discussion 3.1 Effect of operating temperature on performance To improve hydrogen utilization, the anode outlet solenoid valve was kept closed at all times during the experiment, while the cathode outlet solenoid was only triggered when the voltage decreased by 15%, to drain out accumulated water or impure gas. Fig. 3 shows the voltage evolution at different temperatures, while the absolute operating pressures for the cathode and anode were both 150 kPa. During a single purging cycle, the voltage first decreased gradually and then remained stable at about 0.65 V for several seconds. Finally, the voltage declined rapidly until the cathode outlet solenoid valve was triggered with a purging period of about 950 s. The voltage
9
recovered instantly when the solenoid valve was opened during the next purging process. The voltage declined gradually due to the accumulation of water inside the PEMFC in a single purging cycle. Excess water could block the pores in the gas diffusion layer, preventing the reactants from reaching the catalysts, thus leading to gas starvation in the water flooding area. As a result, the voltage dropped quickly [17, 31]. At 50 ℃, the purging period lasted about 950 s at a 20 A current. As the operating temperature increased to 65 ℃, the voltage declined more quickly than that at 50 ℃. This was because the corresponding water vapor saturation pressure increased from 12.33 kPa to 25.01 kPa with the increase in operating temperature. Thus, the hydrogen partial pressure decreased at elevated temperatures. According to the Nerst Equation [32] as follow, Ecell E0,cell
RT ln( p H 2 pO0.25 ) 2F
(1)
where E0,cell is the reference potential, p H 2 and p O2 is the hydrogen and oxygen pressure, respectively. The voltage would decline rapidly and the purging cycle duration would shorten. 3.2 Effect of pressure difference between the cathode and the anode on the purging period Fig. 4 shows the voltage evolution in the dead-ended cathode and anode operation at 50 ℃, with different cathode and anode operating pressures at 20 A. The purging period could not be maintained at a stable value, and it decreased from 1000 s to 800 s when the cathode and anode absolute pressures were 150 kPa and 200 kPa, respectively. Further, the voltage could not fully restore, as it was slightly lower than 10
that in the previous cycle after purging. On the contrary, the performance recovered completely and the purging period was maintained at 1300 s when the cathode and anode pressures were 200 kPa and 150 kPa, respectively. As mentioned previously, operating temperature impacts not only the performance but also the purging period. When the operating temperature was increased to 65 ℃, the purging period decreased to about 240 s under the former pressure conditions and decreased to 400 s under the latter conditions, as seen in Fig. 5. This was because water transportation by back diffusion from the cathode to the anode was suppressed due to the pressure difference [33]. As a result, water accumulation in the cathode accelerated, increasing the cathode purging burden and accelerating flooding. This led to a shorter purging period, especially under high current density. In the situation of cathode and anode with the pressure of 200kPa and 150kPa, respectively, water transportation by back diffusion from the cathode to the anode was increased due to the positive pressure difference between the electrodes, which relieved the accumulated water in the cathode, and thus alleviated water flooding in the cathode. As a result, performance degradation was suppressed and the purging period was prolonged. Generally, the anode may also show water flooding due to back diffusion. However, water in the anode transferred from the cathode was far less than that generated in the cathode. In our design, water could easily detach from the flow channel to the buffer because of the influence of gravity. As a consequence, the anode showed good flood resistance in the experiment. What’s more, the increase of back diffusion water can make better humidification of dry hydrogen in the anode, which promoting mass transfer, reducing internal 11
resistance and getting better performance [11]. As mentioned above, the purging period was closely related to the operating temperature and pressure difference (Fig. 6). The purging period decreased with an increase in the operating temperature because elevated temperatures could lower the reactant concentrations and accelerate performance degradation, as mentioned in Section 3.1. The positive pressure difference between the cathode and the anode can increase the purging period because it could accelerate water transfer from the cathode to the anode, reducing the possibility of cathode flooding. 3.3 Performance evaluation degradation 3.3.1 Performance degradation After 60 h of operation in the dead-ended cathode and anode mode, the polarization curve was recorded to investigate the performance decay of the PEMFC, as shown in Figure. 7. The performance degradation rate increased with an increase in the current density. At 0.6 A·cm-2, the output voltage decreased by 13 mV and the degradation rate reached 1.6%, whereas the corresponding values were about 25 mV and 4.4% at 1.2 A·cm-2. CV measurements were performed to investigate the reason for the performance decay of the assembled PEMFC. Figure 8 shows the recorded CV curves and the calculated ECSA values of the MEA before and after 60 h of operation. the results showed that the adsorption peak and desorption peak in the cathode clearly dropped after the experiment. The ECSA can be calculated according to
ECSA
QH [ Pt ] 0.21
(2)
Where QH represents the charge area of the hydrogen desorption with unit of mC·cm-2, 12
[Pt] is the mass of platinum per unit area with unit of mg·cm-2, and 0.21 is the required charge to oxidized hydrogen located at the surface of platinum with the unit of mC·cm-2. The cathode ECSA decreased from 61.15 m2·g-1 to 52.76 m2·g-1, indicating a decay of 13.7%. The decrease in the MEA ECSA was likely the major reason for the performance decay. 3.3.2 SEM analysis of MEA cross section To further understand the cell performance decay and the decrease in the ECSA, after 60 h of dead-ended cathode and anode operation, the MEA was cut into 9 pieces according to Figure 2, and the cross-sectional morphology of each piece was investigated in detail. Fig. 9 shows the cross-sectional SEM images of each MEA segment after 60 h of dead-ended operation. Both the cathode and the anode exhibited serious degradation in the lower region of the single cell (numbered as 7, 8, and 9) and only a slight decrease in electrode thickness in the other regions (numbered as 1 through 6). Furthermore, the decrease in thickness at the cathode was more serious than that at the anode after the experiment. Unlike a PEMFC with flow-through mode operation, a greater amount of water accumulated inside the PEMFC with dead-ended cathode and anode operation, and thus, flooding was more likely. Since water was generated at the cathode, flooding occurred more frequently at this electrode. Although hydrogen was not humidified in the experiment, water could also be transported from the cathode to the anode due to the effects of back diffusion, osmotic drag, pressure, and heat [34-36]. In our experiment, water flooding still could occur at the anode after long-term operation 13
because the anode outlet solenoid valve was kept closed at all times during the experiment. The generated water at the upstream could drop from the GDL surface due to gravity in our design. However, water would accumulate at the bottom of the flow channel. Water adhered to the surface of the flow channel and GDL, blocking oxygen transport. The induced high potential resulted in corrosion of the carbon support and loss of the platinum electrocatalyst. Accordingly, the thickness in this region decreased significantly [37]. Moreover, anode flooding led to hydrogen starvation in the corresponding region, which could cause hydrolysis of water. The oxygen generated in the anode could form an oxygen-hydrogen interface, resulting in high potential (~1.6 VRHE) in the corresponding region in cathode, leading to corrosion of the carbon support. Thus, in addition to carbon corrosion by cathode flooding, corrosion of carbon and the consequent loss of platinum were more serious in the cathode than in the anode [38-40]. 4 Conclusion In this study, the performance degradation of a hydrogen-oxygen PEMFC with a dead-ended cathode and anode was investigated experimentally. The effects of operating temperature and pressure differences between the cathode and the anode on the purging cycle were investigated in detail. The following conclusions were drawn: (1) The purging period increased with an increase in the positive pressure difference between the cathode and the anode. More water was transferred from the cathode to the anode at a large pressure difference. Thus, flooding in the cathode was suppressed, and the purging period was prolonged. Moreover, the water vapor 14
saturation pressure increased with an increase in the operating temperature; thus, both the hydrogen and oxygen partial pressures decreased at elevated temperatures, leading to decreased performance and a shorter purging period. Therefore, to obtain high performance and a long purging period, PEMFCs with dead-ended anodes and cathodes should operate at a suitable temperature, with a higher operating pressure in the cathode. (2) The performance decay of the PEMFC under dead-ended cathode and anode operation mainly resulted from carbon corrosion induced by water flooding in on the anode and cathode sides. By segmental analysis of the MEA after 60 h of operation, the degradation of the catalyst layer was much more serious in the downstream region than elsewhere. Consequently, when operating PEMFCs in the dead-ended mode, special attention should be paid to water management downstream of the flow channel, especially at the outlet. Acknowledgements This work was supported by the Natural Science Foundation of China (Nos. 51476119 and 51776144) and the Natural Science Foundation of Hubei Province (No. 2016CFA041). References [1] L.B. Braga, J.L. Silveira, M.E.D. Silva, et al. Comparative analysis between a PEM fuel cell and an internal combustion engine driving an electricity generator: Technical, economical and ecological aspects. Applied Thermal Engineering, 2014, 63(1):354-361. 15
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Figure Captions Fig.1 Schematic diagram of designed dead-ended cathode and anode PEMFC Fig.2 Segment design for the investigation of electrode degradation Fig.3 Voltage variation of a PEMFC with dead-ended cathode and anode operating under 20 A loading current at different temperatures Fig.4 Voltage variation of a PEMFC with dead-ended cathode and anode operating at 50℃ and at different pressures Fig.5 Voltage variation of a PEMFC with dead-ended cathode and anode operating at 65℃ and at different pressures Fig.6 Purging period curves of a PEMFC with dead-ended cathode and anode under different operating conditions Fig.7 Performance degradation Fig.8 Cyclic voltammetry degradation Fig.9 SEM images of segments of MEA according to Figure 2 after dead-ended cathode and anode operation (the upper region is the cathode and the lower region is the anode)
List of tables Table 1 Geometric parameters of a PEMFC
22
Parameter
Value
Electrochemical active area (m2)
50×10-3
Depth of channels (m)
1.0×10-3
Width of channels (m)
2.0×10-3
Width of ribs (m)
1.0×10-3
Thickness of GDLs (m)
2.5×10-4
Thickness of membrane (m)
2.5×10-5
Thickness of catalyst layer (m)
1.0×10--5
Tables Table 1 Geometric parameters of a PEMFC
23
Fig.1 Schematic diagram of designed dead-ended cathode and anode PEMFC
(a) The designed flow field of cathode and anode
(b) Schematic diagram of the operation PEMFC with dead-ended cathode and anode.
24
Fig.2 Segments design for the investigation of electrode degradation
25
Fig.3 Voltage variation of a PEMFC with dead-ended cathode and anode operating under 20 A loading current at different temperatures
(a) at 50℃
(b) at 65℃
26
Fig.4 Voltage variation of a PEMFC with dead-ended cathode and anode operating at 50℃ and at different pressures
(a) cathode pressure at 150kPa, anode pressure at 200kPa
(b) cathode pressure at 200kPa, anode pressure at 150kPa
27
Fig.5 Voltage variation of a PEMFC with dead-ended cathode and anode operating at 65℃ and at different pressures
(a) cathode pressure at 150kPa, anode pressure at 200kPa
(b) cathode pressure at 200kPa, anode pressure at 150kPa
28
Fig.6 Purging period curves of a PEMFC with dead-ended cathode and anode under different operating conditions
29
Fig.7 Performance degradation
30
Fig.8 Cyclic voltammetry degradation
31
Fig.9 SEM images of segments of MEA according to Figure 2 after dead-ended cathode and anode operation (the upper region is the cathode and the lower region is the anode)
32
Performance Degradation of a Proton Exchange Membrane Fuel Cell with Dead-ended Cathode and Anode
> Water flooding was the main reason for performance degradation during dead-ended operation
> Purging period was determined by the operating temperature and pressure difference
> Water flooding induced carbon corrosion in the downstream both of anode and cathode
33
S1359-4311(17)36409-8 https://doi.org/10.1016/j.applthermaleng.2017.12.078 ATE 11599
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
6 October 2017 30 November 2017 21 December 2017
Please cite this article as: B. Chen, Y. Cai, J. Shen, Z. Tu, S. Hwa Chan, Performance Degradation of a Proton Exchange Membrane Fuel Cell with Dead-ended Cathode and Anode, Applied Thermal Engineering (2017), doi: https://doi.org/10.1016/j.applthermaleng.2017.12.078
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Performance Degradation of a Proton Exchange Membrane Fuel Cell with Dead-ended Cathode and Anode Ben Chen1, Yonghua Cai1,*, Jun Shen2, Zhengkai Tu2,3,*, Siew Hwa Chan3 1
Hubei Key Laboratory of Advanced Technology for Automotive Components, Wuhan University of Technology, Wuhan, 430070, China 2
School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China
3
Energy Research Institute @ Nanyang Technological University, 50 Nanyang Avenue, 637553, Singapore
*
Corresponding author. Tel.: +86 (0) 15102756731; fax: +86 27 87540724.
E-mail:[email protected](Y. Cai), [email protected] (Z.K. Tu)
1
Abstract: Proton exchange membrane fuel cells with dead-ended cathodes and anodes can simplify the fuel cell system and reduce costs. An experiment was performed to determine the performance degradation characteristics of Proton exchange membrane fuel cells with dead-ended cathodes and anodes. The effects of operating temperature and pressure differences between the cathode and the anode on the purging period were investigated in detail. The performance and cyclic voltammetry before and after the dead-ended operation were analyzed and compared. After the experiment, the membrane electrode assembly was cut to analyze the catalytic layer cross-section membrane morphology by scanning electron microscopy. The results showed that during operation, the fuel cell performance gradually decreases until the setting value, and then quickly recovers when the cathode outlet solenoid valve is triggered during a purging cycle. The dead-ended operating period decreases with an increase in operating temperature but increases with an increase in the pressure difference between the cathode and the anode. Flooding occurs easily in a Proton exchange membrane fuel cell with a dead-ended cathode and anode, causing performance degradation. Moreover, it may cause a decrease in the electrochemical surface area of the catalyst layer. The scanning electron microscopy images showed that both the upper and middle regions of the catalyst layers remained unchanged, whereas the downstream region corroded and become thinner in the dead-ended mode after 60 hours. Keywords: Proton exchange membrane fuel cell; dead-ended; purging period; performance degradation 2
1. Introduction Fuel cells directly transfer chemical energy stored in hydrogen and oxygen into electrical energy. The conversion process is not constrained by the Carnot cycle, and thus, the cells can achieve high efficiency. Fuel cells are currently a hot topic in energy research because they are environment-friendly, produce little noise, show high reliability, and have a wide range of applications [1-3]. The proton exchange membrane fuel cell (PEMFC) are considered one of the best power technologies, as it has additional advantages of high specific power, low operating temperature, fast startup, and long lifespan [4, 5]. Water management is a crucial issue for PEMFCs [6-10] because they often operate under high current density for high power density. Which results in the generation of a large amount of water inside the catalyst layer (CL), gas diffusion layer (GDL), or flow channel. In addition, when using pure oxygen as the oxidant, the reactant stoichiometric ratio is almost equal to 1 so that gas utilization can be improved. Under these circumstances, water flooding can easily occur if the generated water cannot be drained promptly. Water gathered in the flow channel can block the transportation of the reactant in GDLs and can cause reactant starvation in the flooding area, eventually leading to an uneven distribution of current density and poor performance [11, 12]. Furthermore, flooding can result in voltage reversal and thus cause serious corrosion of the carbon support in the catalyst layer and irreversible PEMFC performance degradation [13-15]. During operation, the membrane must be properly wetted to reduce proton conduction resistance. On one hand, if the water content in the 3
membrane is insufficient, the membrane will be dehydrated; this will increase the proton conduction resistance, leading to degradation of the PEMFC performance. Furthermore, small pinholes and brittle ruptures will occur in the membrane when the PEMFC is operated under dry conditions for a long time [16]. On the other hand, if the membrane is over hydrated, water will begin to gather from the catalyst layer to the gas diffusion layer, eventually resulting in flooding. PEMFCs operating in dead-ended mode are an effective option to simplify the system and improve reactant utilization, and have thus attracted widespread attention in recent years. However, water management has become a troublesome issue for PEMFCs operating in dead-ended mode. During operation, the generated water and impure gas accumulated inside the PEMFC dilute the reactant and thus cause water flooding, leading to performance degradation [17-21]. The excess water and impure gas must be removed periodically to ensure stable operation of the PEMFC. The PEMFC performance can recover rapidly once the solenoid valve is triggered to discharge the excess water and impure gas. Periodic degradation and recovery form the whole purging cycle during dead-ended operation. Most current research focuses on PEMFCs with a dead-ended anode for improving reactant utilization and reducing performance degradation. Nikiforow et al. [22] optimized the purging period, so that hydrogen utilization was increased to 99.9%. Our previous study found that the anode outlet solenoid valve can close for approximately 26 h due to the design based on the self-water detachment theory during operation; thus, hydrogen utilization was increased to 100 % [23]. Gomez et al. 4
[24] studied a dead-ended anode PEMFC stack with an active area of 300 cm2 under different operating conditions, and the results showed that liquid water and nitrogen accumulating in the anode directly lead to voltage degradation. Furthermore, current density has an effect on the purging period: PEMFCs with dead-ended anodes are suitable for operation at low current densities and air stoichiometry because of the small amount of water generated and the low nitrogen diffusion rates. Jang et al. [25] studied the purging cycle and duration of a kilowatt stack with dead-ended anode, with results showing that the purging duration increases with an increase in current density for draining out water and impure gas from the anode. Matsuura et al. [26] investigated the effect of operating temperature and cathode relative humidity on the lifespan of PEMFCs with dead-ended anodes. They found that the cathode catalyst layer corroded easily under dead-ended anode mode operation. High cathode relative humidity and low operating temperatures were beneficial to improve the PEMFC lifespan, while small pinholes would occur in the membrane under low cathode relative humidity, accelerating the decay of the PEMFC lifetime. Chen et al. [27] found that water and nitrogen diffused from the cathode accumulated at the end of the anode flow channel, where the hydrogen content was low. As a result, severe corrosion occurred at the end of the anode. Patterson et al. [28] studied the influence of partial hydrogen starvation on catalyst layer corrosion by placing a blockage in the anode channels. Severe corrosion occurred in the blocked region, while other parts of the cathode catalyst layer did not suffer corrosion even after 100 h. However, there is only limited research on PEMFCs with a dead-ended cathode. In 5
certain cases, for example, submarine and aerospace applications, dead-ended cathodes and anodes are required to improve reactant utilization and to prevent the discharge of unreacted gas for safety purposes. Choi et al. [29] studied the operating characteristics of PEMFCs with dead-ended cathodes for submarine and aerospace applications to establish the relationship between purging period and purging duration. However, the effect of operating time on the PEMFC was not taken into account. Moçotéguy et al. [30] experimentally and numerically studied five single hydrogen-oxygen PEMFC stacks with dead-ended cathodes and anodes. They found that water flooding occurred easily in the cathode, blocking the transport channel of oxygen and resulting in oxygen starvation and performance degradation. As a consequence, water management is of great importance to PEMFCs with dead-ended cathodes and anodes. More research is needed to understand the mechanism underlying the degradation of PEMFCs with dead-ended cathodes and anodes. In this study, we aim to investigate the impact of various operating conditions on the performance degradation of a single PEMFC with dead-ended cathode and anode, particularly the effect of operating temperature and pressure difference between the cathode and the anode on the purging period. The effect of dead-ended cathode and anode operation on the catalyst layer corrosion is analyzed using electrochemical technique and the morphology changes in different part of the membrane electrode assembly. The results of this study can provide reference for engineering application. 2 Experimental 2.1 Experimental system 6
The single fuel cell configuration with a straight flow field at both the cathode and the anode for self-water removal ability has been described in our previous work [7, 8] and used in this experiment. Under gravity, water generated in the cathode and anode can overcome viscous forces and move to the buffer region at the outlet, as shown in Fig.1 (a). The cathode and anode bipolar plates made of commercial graphite materials were hydrophobic with a contact angle of 145o and resistance of 100 mS·cm-1. The geometric parameters of the single cell are given in Table 1. The membrane electrode assembly (MEA) provided by Wuhan WUT New Energy Co., Ltd was fabricated with the catalyst-coated membrane (CCM) technique with 0.4 mg·cm-2 Pt loading (HiSPECTM 9100, Johnson Matthey) at both the anode and the cathode. The Nafion® XL membrane from Dupont™ was used as an electrolyte. Carbon paper (TGP-060, Toray), hydrophobically treated in PTFE with a contact angle of 145o was used as the gas diffusion layer (GDL), and a microporous layer (MPL) was brushed directly on the carbon paper. Fig.1 (b) shows a schematic of the testing system. Hydrogen and oxygen without humidification were controlled by two regulators with set operating pressures for both the anode and cathode. Two solenoid valves were set at the outlet to control gas purging. Electronic load, cell temperature, high frequency resistance (HFR), and voltage were recorded using the HTS Fuel Cell Station produced by Hephas Energy Corporation of Taiwan. This equipment is specially designed for a single cell test, and can accurately control the operating parameters including gas flow, gas temperature, relative humidity, dew point temperature, operation temperature, etc. In addition, 7
cyclic voltammetry (CV) and linear scanning (LSV) analyses of the PEMFC can be carried out using this equipment. 2.2 Experimental scheme The single fuel cell was placed vertically to enable liquid water removal under the effect of gravity. To investigate the characteristics of a cell with a dead-ended cathode and anode under different operating conditions, the initial operating temperature, loading current, and inlet pressure of both the cathode and anode were set to 50 ℃, 20 A, and 150 kPa, respectively. The cathode outlet solenoid valve was opened once the voltage decreased by 15%, while the solenoid valve at the anode was kept closed to improve hydrogen utilization. Next, the operating temperature was set to 65 ℃, and the cathode and anode inlet pressures were set separately to 200 kPa to study the effect of operating conditions on the operating characteristics and purging cycles of PEMFCs. To evaluate the initial PEMFC performance prior to the experiment, the single cell was pre-activated by increasing the current with fully humidified reactants for approximately 3 h under ambient pressure and open-ended before the polarization curves were recorded. The fuel cell polarization curve after operation in the dead-ended cathode and anode mode represents the final cell performance. Before and after the experiment, the MEA was analyzed with respect to performance decay, and the electrochemical surface area (ECSA) loss was compared to that for the fresh MEA using cyclic voltammetry (CV, Autolab, PGSTAT 302N). Finally, the degraded MEA was cut into nine segments, as shown in Figure 2, and the cross-sectional morphology 8
was observed by scanning electron microscopy (SEM, Hitachi S-4800) to understand the catalyst degradation after long-term operation in the dead-ended cathode and anode mode. We previously found that increasing the outlet opening size could effectively alleviate water flooding in the cathode, mitigate performance degradation, and prolong the PEMFC lifespan [21]. In this paper, both the cathode and anode outlet of cell have double outlets. Prior to the single cell performance evaluation, the internal and external reaction gas leakage was tested, with an acceptable result. This was done to meet international standard requirements and to ensure that the PEMFC operated effectively with the dead-ended cathode and anode. The hydrogen and oxygen purities were 99.99 % and 99.999 %, respectively, and the ambient temperature was 25 ℃. 3 Results and discussion 3.1 Effect of operating temperature on performance To improve hydrogen utilization, the anode outlet solenoid valve was kept closed at all times during the experiment, while the cathode outlet solenoid was only triggered when the voltage decreased by 15%, to drain out accumulated water or impure gas. Fig. 3 shows the voltage evolution at different temperatures, while the absolute operating pressures for the cathode and anode were both 150 kPa. During a single purging cycle, the voltage first decreased gradually and then remained stable at about 0.65 V for several seconds. Finally, the voltage declined rapidly until the cathode outlet solenoid valve was triggered with a purging period of about 950 s. The voltage
9
recovered instantly when the solenoid valve was opened during the next purging process. The voltage declined gradually due to the accumulation of water inside the PEMFC in a single purging cycle. Excess water could block the pores in the gas diffusion layer, preventing the reactants from reaching the catalysts, thus leading to gas starvation in the water flooding area. As a result, the voltage dropped quickly [17, 31]. At 50 ℃, the purging period lasted about 950 s at a 20 A current. As the operating temperature increased to 65 ℃, the voltage declined more quickly than that at 50 ℃. This was because the corresponding water vapor saturation pressure increased from 12.33 kPa to 25.01 kPa with the increase in operating temperature. Thus, the hydrogen partial pressure decreased at elevated temperatures. According to the Nerst Equation [32] as follow, Ecell E0,cell
RT ln( p H 2 pO0.25 ) 2F
(1)
where E0,cell is the reference potential, p H 2 and p O2 is the hydrogen and oxygen pressure, respectively. The voltage would decline rapidly and the purging cycle duration would shorten. 3.2 Effect of pressure difference between the cathode and the anode on the purging period Fig. 4 shows the voltage evolution in the dead-ended cathode and anode operation at 50 ℃, with different cathode and anode operating pressures at 20 A. The purging period could not be maintained at a stable value, and it decreased from 1000 s to 800 s when the cathode and anode absolute pressures were 150 kPa and 200 kPa, respectively. Further, the voltage could not fully restore, as it was slightly lower than 10
that in the previous cycle after purging. On the contrary, the performance recovered completely and the purging period was maintained at 1300 s when the cathode and anode pressures were 200 kPa and 150 kPa, respectively. As mentioned previously, operating temperature impacts not only the performance but also the purging period. When the operating temperature was increased to 65 ℃, the purging period decreased to about 240 s under the former pressure conditions and decreased to 400 s under the latter conditions, as seen in Fig. 5. This was because water transportation by back diffusion from the cathode to the anode was suppressed due to the pressure difference [33]. As a result, water accumulation in the cathode accelerated, increasing the cathode purging burden and accelerating flooding. This led to a shorter purging period, especially under high current density. In the situation of cathode and anode with the pressure of 200kPa and 150kPa, respectively, water transportation by back diffusion from the cathode to the anode was increased due to the positive pressure difference between the electrodes, which relieved the accumulated water in the cathode, and thus alleviated water flooding in the cathode. As a result, performance degradation was suppressed and the purging period was prolonged. Generally, the anode may also show water flooding due to back diffusion. However, water in the anode transferred from the cathode was far less than that generated in the cathode. In our design, water could easily detach from the flow channel to the buffer because of the influence of gravity. As a consequence, the anode showed good flood resistance in the experiment. What’s more, the increase of back diffusion water can make better humidification of dry hydrogen in the anode, which promoting mass transfer, reducing internal 11
resistance and getting better performance [11]. As mentioned above, the purging period was closely related to the operating temperature and pressure difference (Fig. 6). The purging period decreased with an increase in the operating temperature because elevated temperatures could lower the reactant concentrations and accelerate performance degradation, as mentioned in Section 3.1. The positive pressure difference between the cathode and the anode can increase the purging period because it could accelerate water transfer from the cathode to the anode, reducing the possibility of cathode flooding. 3.3 Performance evaluation degradation 3.3.1 Performance degradation After 60 h of operation in the dead-ended cathode and anode mode, the polarization curve was recorded to investigate the performance decay of the PEMFC, as shown in Figure. 7. The performance degradation rate increased with an increase in the current density. At 0.6 A·cm-2, the output voltage decreased by 13 mV and the degradation rate reached 1.6%, whereas the corresponding values were about 25 mV and 4.4% at 1.2 A·cm-2. CV measurements were performed to investigate the reason for the performance decay of the assembled PEMFC. Figure 8 shows the recorded CV curves and the calculated ECSA values of the MEA before and after 60 h of operation. the results showed that the adsorption peak and desorption peak in the cathode clearly dropped after the experiment. The ECSA can be calculated according to
ECSA
QH [ Pt ] 0.21
(2)
Where QH represents the charge area of the hydrogen desorption with unit of mC·cm-2, 12
[Pt] is the mass of platinum per unit area with unit of mg·cm-2, and 0.21 is the required charge to oxidized hydrogen located at the surface of platinum with the unit of mC·cm-2. The cathode ECSA decreased from 61.15 m2·g-1 to 52.76 m2·g-1, indicating a decay of 13.7%. The decrease in the MEA ECSA was likely the major reason for the performance decay. 3.3.2 SEM analysis of MEA cross section To further understand the cell performance decay and the decrease in the ECSA, after 60 h of dead-ended cathode and anode operation, the MEA was cut into 9 pieces according to Figure 2, and the cross-sectional morphology of each piece was investigated in detail. Fig. 9 shows the cross-sectional SEM images of each MEA segment after 60 h of dead-ended operation. Both the cathode and the anode exhibited serious degradation in the lower region of the single cell (numbered as 7, 8, and 9) and only a slight decrease in electrode thickness in the other regions (numbered as 1 through 6). Furthermore, the decrease in thickness at the cathode was more serious than that at the anode after the experiment. Unlike a PEMFC with flow-through mode operation, a greater amount of water accumulated inside the PEMFC with dead-ended cathode and anode operation, and thus, flooding was more likely. Since water was generated at the cathode, flooding occurred more frequently at this electrode. Although hydrogen was not humidified in the experiment, water could also be transported from the cathode to the anode due to the effects of back diffusion, osmotic drag, pressure, and heat [34-36]. In our experiment, water flooding still could occur at the anode after long-term operation 13
because the anode outlet solenoid valve was kept closed at all times during the experiment. The generated water at the upstream could drop from the GDL surface due to gravity in our design. However, water would accumulate at the bottom of the flow channel. Water adhered to the surface of the flow channel and GDL, blocking oxygen transport. The induced high potential resulted in corrosion of the carbon support and loss of the platinum electrocatalyst. Accordingly, the thickness in this region decreased significantly [37]. Moreover, anode flooding led to hydrogen starvation in the corresponding region, which could cause hydrolysis of water. The oxygen generated in the anode could form an oxygen-hydrogen interface, resulting in high potential (~1.6 VRHE) in the corresponding region in cathode, leading to corrosion of the carbon support. Thus, in addition to carbon corrosion by cathode flooding, corrosion of carbon and the consequent loss of platinum were more serious in the cathode than in the anode [38-40]. 4 Conclusion In this study, the performance degradation of a hydrogen-oxygen PEMFC with a dead-ended cathode and anode was investigated experimentally. The effects of operating temperature and pressure differences between the cathode and the anode on the purging cycle were investigated in detail. The following conclusions were drawn: (1) The purging period increased with an increase in the positive pressure difference between the cathode and the anode. More water was transferred from the cathode to the anode at a large pressure difference. Thus, flooding in the cathode was suppressed, and the purging period was prolonged. Moreover, the water vapor 14
saturation pressure increased with an increase in the operating temperature; thus, both the hydrogen and oxygen partial pressures decreased at elevated temperatures, leading to decreased performance and a shorter purging period. Therefore, to obtain high performance and a long purging period, PEMFCs with dead-ended anodes and cathodes should operate at a suitable temperature, with a higher operating pressure in the cathode. (2) The performance decay of the PEMFC under dead-ended cathode and anode operation mainly resulted from carbon corrosion induced by water flooding in on the anode and cathode sides. By segmental analysis of the MEA after 60 h of operation, the degradation of the catalyst layer was much more serious in the downstream region than elsewhere. Consequently, when operating PEMFCs in the dead-ended mode, special attention should be paid to water management downstream of the flow channel, especially at the outlet. Acknowledgements This work was supported by the Natural Science Foundation of China (Nos. 51476119 and 51776144) and the Natural Science Foundation of Hubei Province (No. 2016CFA041). References [1] L.B. Braga, J.L. Silveira, M.E.D. Silva, et al. Comparative analysis between a PEM fuel cell and an internal combustion engine driving an electricity generator: Technical, economical and ecological aspects. Applied Thermal Engineering, 2014, 63(1):354-361. 15
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Figure Captions Fig.1 Schematic diagram of designed dead-ended cathode and anode PEMFC Fig.2 Segment design for the investigation of electrode degradation Fig.3 Voltage variation of a PEMFC with dead-ended cathode and anode operating under 20 A loading current at different temperatures Fig.4 Voltage variation of a PEMFC with dead-ended cathode and anode operating at 50℃ and at different pressures Fig.5 Voltage variation of a PEMFC with dead-ended cathode and anode operating at 65℃ and at different pressures Fig.6 Purging period curves of a PEMFC with dead-ended cathode and anode under different operating conditions Fig.7 Performance degradation Fig.8 Cyclic voltammetry degradation Fig.9 SEM images of segments of MEA according to Figure 2 after dead-ended cathode and anode operation (the upper region is the cathode and the lower region is the anode)
List of tables Table 1 Geometric parameters of a PEMFC
22
Parameter
Value
Electrochemical active area (m2)
50×10-3
Depth of channels (m)
1.0×10-3
Width of channels (m)
2.0×10-3
Width of ribs (m)
1.0×10-3
Thickness of GDLs (m)
2.5×10-4
Thickness of membrane (m)
2.5×10-5
Thickness of catalyst layer (m)
1.0×10--5
Tables Table 1 Geometric parameters of a PEMFC
23
Fig.1 Schematic diagram of designed dead-ended cathode and anode PEMFC
(a) The designed flow field of cathode and anode
(b) Schematic diagram of the operation PEMFC with dead-ended cathode and anode.
24
Fig.2 Segments design for the investigation of electrode degradation
25
Fig.3 Voltage variation of a PEMFC with dead-ended cathode and anode operating under 20 A loading current at different temperatures
(a) at 50℃
(b) at 65℃
26
Fig.4 Voltage variation of a PEMFC with dead-ended cathode and anode operating at 50℃ and at different pressures
(a) cathode pressure at 150kPa, anode pressure at 200kPa
(b) cathode pressure at 200kPa, anode pressure at 150kPa
27
Fig.5 Voltage variation of a PEMFC with dead-ended cathode and anode operating at 65℃ and at different pressures
(a) cathode pressure at 150kPa, anode pressure at 200kPa
(b) cathode pressure at 200kPa, anode pressure at 150kPa
28
Fig.6 Purging period curves of a PEMFC with dead-ended cathode and anode under different operating conditions
29
Fig.7 Performance degradation
30
Fig.8 Cyclic voltammetry degradation
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Fig.9 SEM images of segments of MEA according to Figure 2 after dead-ended cathode and anode operation (the upper region is the cathode and the lower region is the anode)
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Performance Degradation of a Proton Exchange Membrane Fuel Cell with Dead-ended Cathode and Anode
> Water flooding was the main reason for performance degradation during dead-ended operation
> Purging period was determined by the operating temperature and pressure difference
> Water flooding induced carbon corrosion in the downstream both of anode and cathode
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