Performance evaluation of an air-breathing high-temperature proton exchange membrane fuel cell

Applied Energy 160 (2015) 146–152

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Performance evaluation of an air-breathing high-temperature proton exchange membrane fuel cell Qixing Wu ⇑, Haiyang Li, Wenxiang Yuan, Zhongkuan Luo, Fang Wang ⇑, Hongyuan Sun, Xuxin Zhao, Huide Fu College of Chemistry and Environmental Engineering, Shenzhen University, Nanhai Ave 3688, Shenzhen 518060, Guangdong, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 An air-breathing HT-PEMFC was

designed and evaluated experimentally.  The peak power density of the airbreathing HT-PEMFC was 220.5 mW cm 2 at 200 °C.  Break-in behavior and effects of temperature and anodic stoichiometry were studied.  The effect of cell orientations on the performance was investigated.  The degradation rate of the airbreathing HT-PEMFC was around 58.32 lV h 1.

a r t i c l e

i n f o

Article history: Received 27 March 2015 Received in revised form 4 September 2015 Accepted 8 September 2015

Keywords: PEMFC High temperature Air breathing Polybenzimidazole Phosphoric acid Durability

a b s t r a c t The air-breathing proton exchange membrane fuel cell (PEMFC) is of great interest in mobile power sources because of its simple system design and low parasitic power consumption. Different from previous low-temperature air-breathing PEMFCs, a high-temperature PEMFC with a phosphoric acid doped polybenzimidazole (PBI) membrane as the polymer electrolyte is designed and investigated under airbreathing conditions. The preliminary results show that a peak power density of 220.5 mW cm 2 at 200 °C can be achieved without employing any water managements, which is comparable to those with conventional NafionÒ membranes operated at low temperatures. In addition, it is found that with the present cell design, the limiting current density arising from the oxygen transfer limitation is around 700 mA cm 2 even at 200 °C. The short-term durability test at 200 mA cm 2 and 180 °C reveals that all the cells exhibit a gradual decrease in the voltage along with a rise in the internal resistance. The degradation rate of continuous operation is around 58.32 lV h 1, which is much smaller than those of start/ stop cycling operations. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction ⇑ Corresponding authors. Tel.: +86 755 26557393; fax: +86 755 26536141. E-mail addresses: [email protected] (Q. Wu), [email protected] (F. Wang). http://dx.doi.org/10.1016/j.apenergy.2015.09.042 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.

Since the last decade, considerable attentions [1–3] have been paid to proton exchange membrane fuel cells (PEMFC) because of

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their appealing advantages, such as high energy-conversion efficiency (typically higher than 50%), large power density, zero pollution and low noise. These favorable features make this kind of fuel cells suitable for automobiles, backup power sources and off-grid residential power systems. In an operating PEMFC, sufficient oxygen must be fed to the electrode to sustain oxygen reduction reaction (ORR) while excess water produced by ORR needs to be removed. For a large power system, forced convection generated by blowers/compressors is usually employed to ensure the efficient supply of reactants and removal of products. Nevertheless, such auxiliary devices not only consume additional power but also are difficult to miniaturize for small power applications. Hence, air-breathing PEMFCs that make use of natural convection and molecular diffusion have been proposed [4–14] as portable power sources for smart phones, laptops and mobile chargers. Generally, the power density of air-breathing PEMFCs is 80– 350 mW cm 2 [4–14], around 1/3 as that of active PEMFCs. The poor performance of air-breathing PEMFCs mainly arises from two issues: (1) slow mass-transfer rate of oxygen by diffusion and natural convection; (2) difficulty in maintaining a delicate balance between drying out of membranes and water flooding without employing active water management techniques. In order to improve the performance, a number of interesting efforts have been devoted to addressing such issues in recent years. Krumbholz et al. [11] investigated the current collector design parameters of an air-breathing PEMFC and found that when the open ratio was less than 0.1, the cell impedance was substantially low as water predominately accumulated inside the electrodes. In addition to open ratios, Bussayajarn et al. [13] experimentally studied the geometry of cathode current collector and its influences on the oxygen transport and membrane resistance and found that circular holes yielded the best performance. To retain the water inside the catalyst layers (CL), Poh et al. [14] improved the hydrophilicity of Pt/C catalysts by citric acid treatment and demonstrated an increase of 23.4% in power density with such self-humidifying catalysts. Although the performance can be increased to some extent by above mentioned methods, previous air-breathing PEMFCs still suffer from slow oxygen supply and water flooding/starvation problems. Obviously, one simple way to enhance the mass transport and avoid water flooding is elevating the cell temperature. For water starvation or drying out of polymer electrolyte membranes, employing a membrane with its conductivity independent or slightly dependent on water may be an ultimate solution. Wainright et al. [15] combined such two methods and firstly introduced acid-doped polybenzimidazole (PBI) membranes for high-temperature proton exchange membrane fuel cells (HT-PEMFC). Since this pioneering work [15], extensive attentions were paid to the proton conduction in acid-doped membranes [16–18], development of alternative acid-doped membranes [19–22], durability of HT-PEMFCs [23–25], electrode designs [26–28] and demonstration of stacks [29,30]. In addition to experimental works, numerical modeling was also of great interest to understand heat/mass transport phenomena occurring in a HT-PEMFC, including the gas crossover [31], flow field designs [32], effect of gas compositions [33] and the thermal effects [34]. However, a close look at the literature reveals that previous HT-PEMFCs rely on active supply of oxygen and their operating characteristics under air-breathing conditions remain unclear. In this work, an air-breathing HT-PEMFC based on a phosphoric acid (PA) doped PBI polymer electrolyte is designed and investigated under various operating conditions, with an aim to gain a general understanding on its performance characteristics.

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2. Experimental 2.1. Fabrication of PBI based membrane electrode assemblies (MEA) The preparation procedures of MEAs were similar to our previous work [35]. A PBI membrane with a thickness of 40 lm and a doping level of approximately 6.5 was applied as a protonconducting membrane. A 356 lm thick AvCarbÒ 1071-HCB carbon cloth with a microporous layer (MPL) was employed as the gas diffusion layer (GDL); the polytetrafluoroethylene (PTFE) contents in the carbon cloth and the MPL were 5 wt.% and 15 wt.%, respectively. The CLs were fabricated by spraying catalytic inks, consisting of HispecÒ 9100 Pt/C, PTFE and ethanol, onto the surface the MPLs; the metal loading and PTFE content in CLs were maintained to be about 1 mg cm 2 and 10 wt.%, respectively. To increase the active surfaces in the CL, all the electrodes were sprayed with additional 5 mg cm 2 PA before hot pressing. Finally, the electrodes and the membrane were hot pressed at 180 °C for 3 min to form a compact MEA, the active area of which was 3.0  3.0 cm2.

2.2. Single cell design of an air-breathing HT-PEMFC Fig. 1 shows the single cell design of the air-breathing HTPEMFC. The present single cell consisted of an aluminum cathode end plate, a cathode flow field, a MEA, an anode flow field and an aluminum anode end plate with a hydrogen inlet and outlet. Both the anode and cathode end plates were square shape of 75 mm  75 mm and have a thickness of 20 mm. For the cathode end plate, there was a square hole of 30 mm  30 mm at the center for the transport of oxygen. The cathode flow field was made of 316L stainless steel with a thickness of 2 mm and an open ratio of 46.94% formed by 64 £ 2.9 mm drilled circular holes, while the anode flow field was a 2 mm-thick graphite plate grooved with double-pass serpentine channels; the channel depth, channel width and the rib width were 1 mm, 1.5 mm and 1.1 mm, respectively. To control the fuel cell temperature, an electrical heater and a thermal couple were inserted to the holes (£ 6.5 mm) of the anode end plate. It should be noted that electrical heating is unnecessary in practice as high temperatures can be achieved by supplying reformates to the anode directly or by self-heating from reactions with appropriate thermal insulation.

2.3. Electrochemical characterizations and test conditions The electrochemical impedance spectra (EIS) of the airbreathing HT-PEMFC were attained by the ZahnerÒ Zennium workstation. To conduct the measurement, the counter and reference electrode of the workstation were connected to the anode whereas the working electrode was connected to the cathode. The measurement was performed under galvanostatic mode at 1.8 A with a perturbation amplitude of 0.1 A and the impedance data were recorded from 10 2 to 105 Hz with 8 steps per decade. For fuel cell performance test, an ArbinÒ BT-5HC testing system was used to control the external load and record the corresponding measured data. To attain the stable polarization curves, pure hydrogen (99.999%) with various anodic stoichiometry (kA) was fed to the anode and the HT-PEMFCs were discharged at a series of predefined current for 10 min until reproducible data was achieved. During the short-term durability test, the internal resistances were monitored and measured at every 24 h by the Arbin test station with the current interruption method. The environmental temperature and humidity during tests were, respectively, in the ranges of 23–25 °C and 45–55%.

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Fig. 1. Single cell design of the air-breathing HT-PEMFC used in this work.

3. Results and discussion 0.5

3.1. Activation behavior

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In general, a HT-PEMFC based on an acid-doped polymer electrolyte will undergo an activation or break-in process when a fresh MEA is used. To investigate the activation behaviors of the inhouse air-breathing HT-PEMFCs, the evolution of cell voltage and electrochemical impedance spectra before and after activation were recorded at 200 mA cm 2 and 180 °C with an anode stoichiometry of 1.2; the results were presented in Figs. 2 and 3. It is found in Fig. 2 that the cell voltage increases gradually from 604.6 to 623.9 mV (an increase of 19.3 mV) at the initial 16 h, and it remains nearly unchanged for the following 5 h. This behavior is similar to those of HT-PEMFCs operated with active supply of fuel and oxidant [36–38]. The improvement in the performance can be understood by comparing the EIS results before and after activation. As shown in Fig. 3, the cell after activation shows a slightly smaller internal resistance than does the one before activation; the internal resistances, calculated from the intersections with real axis at high frequencies, are 0.3082 X cm2 and 0.3154 X cm2, respectively, for the cell after and before activation. The reduction in the internal resistance can be attributed to the increased proton

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conductivity of the PA doped PBI membrane due to a higher water content in the electrolyte after discharge [16]. It should be noted that, however, the decrease in the internal resistance is not a major factor contributing to the improved performance after activation as the variation in the internal resistance is rather small. In contrast to the impedances at high frequencies, the cell after activation exhibits a much smaller semi-circle than does the one before activation at low frequencies. The low frequencies semi-circle is usually interpreted as the resistance of charge transfer [36]. The decrease in this resistance is mainly attributed to the PA redistribution in the MEA [27,36–38]: during discharge, the viscosity of PA can be lowered by the dilution effect of water produced from ORR, facilitating the migration of PA to the voids of CLs and hence increasing the triple phase boundaries (TPB). In summary, the above EIS results imply that the improved performance of the air-breathing HT-PEMFC after activation should be resulted primarily from the reduction in the resistance of charge transfer rather than the internal resistance. 3.2. Influences of temperature and anodic stoichiometry

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The effects of temperature and kA were investigated by changing the operating temperature from 120 to 200 °C with various kA from 1.2 to 8; the polarization curves obtained after 24 h activa-

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somehow weakens the natural convection effect as the flow of air has to overcome additional hydraulic resistance and shear stress [40], and hence a larger limiting current density may be attained by decreasing the aspect ratio even under a smaller temperature difference [12,13]. The above performance results indicate that under the air-breathing mode, oxygen transport is still a critical issue even for PEMFCs operated at elevated temperatures. Hence, future study on the detailed transport phenomena of oxygen through the PA doped MEA is crucial to improve the fuel cell performance. 3.3. Effect of cell orientations As natural convection can be greatly influenced by the cell orientation, it is interesting to investigate its effect on the oxygen supply and fuel cell performance. In this work, three different cathode orientations are considered: (1) vertical; (2) horizontal with the cathode facing upward; (3) horizontal with the cathode facing downward. The polarization curves with various cell orientations are shown in Fig. 6. It can be seen in Fig. 6a that the performance of the cell orientated vertically is slightly higher than that orientated horizontally with the cathode facing upward and the poorest performance is observed in the cell orientated horizontally with the cathode facing downward. To exclude the influence of hydrogen supply when changing orientations, another performance test is performed with a large anodic stoichiometry of 4, and as shown in Fig. 6b, the polarization curves remain almost the same, indicating the variation in the performance is indeed caused by oxygen supply. The difference in the performance of the cell with various orientations can be explained by the fact that the rate of heat/mass transfer reaches a maximum on a vertical heated plate and a minimum on a horizontal downward-facing plate under the same conditions [41]. Additionally, the above results are in qualitative agreement with both the numerical [42] and experimental [12] study on the effect of orientations in a low-temperature airbreathing PEMFC, though the limiting current density (around 300 mA cm 2) with the cathode facing downward is somehow smaller than expected, lower than a half of those with the other two orientations. The reason accounting for this phenomenon might be that the aspect ratio of the cathode plate is relatively high as compared to those used in previous works [4–14], such that the cathode seems more like a heated plate with an open cavity on which the natural convection is expected to be weaker than that on a flat plate [40]. Therefore, in terms of enhancing oxygen transfer, the cathode plate should not be faced downward and its aspect ratio should be minimized. 3.4. Short-term durability test

Power density (mW cm )

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tion were shown in Figs. 4 and 5. It can be seen from Fig. 4 that the peak power density increases substantially with temperatures; the peak power density boosts from 117.3 to 220.5 mW cm 2 when increasing the cell temperature from 120 to 200 °C. Key contributions to the improved performance include the acceleration in reaction kinetics, increase in conductivity of PA and enhancement of mass transfer at elevated temperatures. Meanwhile, it is worth mentioning that the preliminary performance of the in-house airbreathing HT-PEMFC is rather encouraging as it is comparable to those of conventional air-breathing PEMFCs operated at low temperatures [4–14]. In addition, it is found in Fig. 5a–e that at the same temperature, the polarization curves almost coincide when kA P 2, suggesting hydrogen is enough for all the preset currents. Furthermore, Fig. 5 also reveals that the cell with kA = 1.2 yields the same performance as the ones with bigger kA in the low current density region (<300 mA cm 2), whereas it exhibits a lower voltage than do the ones with bigger kA at high current densities, especially under low temperatures (<160 °C). Such a decrease in voltage arises from insufficient hydrogen supply under a high load. It should be noted that, however, as shown in Fig. 5d and e, the difference in voltage becomes smaller at higher temperatures due to increased mass-transfer rates. Another interesting phenomenon can be observed in Fig. 5d and e is that for all kA, the polarization curves at the temperatures of 180 °C and 200 °C exhibit much larger slopes in the range of 600–700 mA cm 2. For the lowtemperature air-breathing PEMFCs, such a rapid drop of voltage may be induced by an insufficient supply of hydrogen or oxygen [6,7,11–14] as well as dryout of the NafionÒ-type membrane at elevated temperatures [39]. Since the conductivity of the PA doped PBI membrane only slightly depends on water content and increasing kA from 2 to 8 results in no improvement in the performance, it can be concluded that the large voltage drop is caused by oxygen transport limitation. Owing to the enhanced natural convection created by the large temperature difference, this limiting current density (around 700 mA cm 2) is much higher than most of those observed in low-temperature air-breathing PEMFCs [4–11] (<500 mA cm 2), except for the works reported by Bussayajarn et al. [13] (680 mA cm 2) and Kim et al. [12] (800 mA cm 2). To understand this discrepancy, the aspect ratios of cavities for the cathode end plates, defined as length (or thickness)/height, are calculated. It turns out that the aspect ratio in the present work is substantially higher than those used in the works by Bussayajarn et al. [13] and Kim et al. [12]; the aspect ratios are 0.67 (20 mm/30 mm), 0 (without an end plate) and 0.19 (2.7 mm/14 mm), respectively. The high aspect ratio in our work

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600

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Current density (mA cm ) Fig. 4. Effect of cell temperature on the performance of the air-breathing HT-PEMFC with kA = 2.

To gain insights on the short-term durability of the airbreathing HT-PEMFC, three different operations were employed: (1) continuous operation at 200 mA cm 2 and 180 °C with kA = 1.2; (2) start/stop cycling with a 12 h test and a 12 h shutdown under temperature control at 180 °C; (3) start/stop cycling with a 12 h test and a 12 h shutdown under natural cooling. Note that when the cell was shutdown, no hydrogen was supplied to the anode while ambient oxygen can still diffuse to the cathode. The transient cell voltage and the internal resistance during durability test were shown in Fig. 7a. The degradation rates, the slopes calculated from linear fitting of voltage–time curves (see Fig. 7b), are 58.32 lV h 1, 237.9 lV h 1 and 297.5 lV h 1, respectively, for the continuous operation, cycling with temperature control and cycling with natural cooling. Such degradation rates are somewhat higher than those of active HT-PEMFCs, the rates of which are 5–150 lV h 1 [23–25,43,44]. The larger degradation of the air-breathing HT-PEMFC may result from two possible reasons:

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(a) faster evaporative loss of PA through natural convection induced by large temperature variation between the cathode and environment; (b) undesirable migration of PA. Both the loss and maldistribution of PA will lead to an increase in the internal resistance as confirmed in Fig. 7a and a decrease in TPB, thereby lowering the cell voltage. Moreover, Fig. 7b shows that the degradation rate under continuous operation is much smaller than those under cycling operations. This might because no water is produced dur-

ing shutdown period in cycling operations and hence PA could become dehydrated to form less conductive pyrophosphoric acid [45], leading to the undesirable redistribution of electrolyte phase in the electrode. Hence, as shown in Fig. 7a, the internal resistances of cycling operations increase more rapidly than does the one of continuous operation. The redistribution of PA may become more significant with natural cooling, because PA is expected to experience volume expansion and contraction induced by the changes of

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Fig. 6. Effect of cell orientations on the performance of the air-breathing HT-PEMFC: (a) kA = 2; (b) kA = 4.

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temperature and concentration [24] and thus the cycling operation without temperature control shows the poorest durability. 4. Conclusions The performance characteristics of an air-breathing HT-PEMFC, based on PA doped PBI membrane, were investigated under various conditions with special focuses on its activation behavior, operating temperature, anodic stoichiometry, cell orientation and durability. The preliminary results demonstrated that the unoptimized air-breathing HT-PEMFC can produce a comparable performance to that of a traditional air-breathing PEMFC. In addition, it is found that oxygen transport remains a problem in the airbreathing HT-PEMFC under a high load. The durability test indicates that the loss and redistribution of PA play significant roles in performance degradation. Hence, future modeling and experimental works on the migration of PA in air-breathing HT-PEMFCs are of great importance to the improvement of performance and durability. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51306125), Natural Science Foundation of

Guangdong Province (No. S2013040016860), Shenzhen Science and Technology Foundation (No. JCYJ20130329102823481, No. KQCX20140519105122378 and No. NYSW20140331010052), Natural Science Foundation of SZU (No. 827-000015) and Shenzhen Key Laboratory of New Lithium-ion Batteries and Mesoporous Materials.

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