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A durable polyvinyl butyral-CsH2PO4 composite electrolyte for solid acid fuel cells
Journal of Power Sources 359 (2017) 1e6
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
A durable polyvinyl butyral-CsH2PO4 composite electrolyte for solid acid fuel cells Dai Dang a, b, 1, Bote Zhao a, 1, Dongchang Chen a, Seonyoung Yoo a, Samson Y. Lai a, Brian Doyle a, Shuge Dai a, Yu Chen a, Chong Qu a, Lei Zhang a, Shijun Liao b, **, Meilin Liu a, * a
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, 30332-0245, USA School of Chemistry and Chemical Engineering, South China University of Technology, New Energy Research Institute, The Key Laboratory of Fuel Cell Technology of Guangdong Province & The Key Laboratory of New Energy Technology of Guangdong Universities, Guangzhou, 510641, China
b
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
A PVB-CsH2PO4 composite electrolyte has been prepared via a facile method. 3 wt% PVB/CsH2PO4 electrolyte has a high conductivity of ~28 mS cm1 at 260 C. The MEA with composite electrolyte achieves a peak power density of 108 mW cm2. The MEA shows no degradation after three thermal-cycling test.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 December 2016 Received in revised form 6 May 2017 Accepted 9 May 2017
A composite electrolyte membrane composed of polyvinyl butyral (PVB) and CsH2PO4 has been prepared via a facile and cost-effective method for solid acid fuel cells. The effect of PVB content on conductivity, mechanical integrity, and fuel cell performance is investigated. A minimum amount of 3 wt% PVB in the CsH2PO4-based composite electrolyte not only offers the required mechanical integrity but also allows high conductivity (~28 mS cm1 at 260 C). Single cells based on the composite electrolytes demonstrate a peak power density of 108 mW cm2 at 260 C. Almost no degradation in electrochemical performance could be observed during the stability test for 10 h and three thermal-cycling test in H2/O2 fuel cell, indicating the promising application of the composite electrolyte in solid acid fuel cells. © 2017 Elsevier B.V. All rights reserved.
Keywords: Electrolytes Intermediate-temperature fuel cells Polymers Composites Membrane electrode assembly
1. Introduction
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (M. Liu). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2017.05.023 0378-7753/© 2017 Elsevier B.V. All rights reserved.
With ever-increasing energy demands and environmental concerns, the search for new energy technologies is a grand challenge facing the world today [1e4]. Fuel cells are one of the most promising options to replace the conventional energy technologies (e.g., internal combustion engines) because of their high energy
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efficiency, and low emission [3,5,6]. Of all types of fuel cells, the intermediate-temperature fuel cells (ITFC) operated at 200e500 C have attracted much attention in the past few years because of its potential to overcome the technical difficulties associated with low-temperature fuel cells [7e14]. For example, ITFCs would have simplified water management system with enhanced electrochemical kinetics and high carbon monoxide-tolerance compared to the low temperature fuel cells [9e11,15,16]. Cesium dihydrogen phosphate (CsH2PO4), one of the most promising solid acid electrolytes, has been investigated due to its high proton conductivity (>10 mS cm1) with the phase transition from monoclinic to superprotonic phase over 230e260 C [9,17e22]. However, CsH2PO4 usually suffers from poor mechanical strength and can be easily destroyed by a small physical force, leading to a severe gas crossover [23e30]. Therefore, an electrolyte thicker than 250 mm is usually required to ensure mechanical integrity and prevent the electrolyte from voids or pinholes [18e20,31]. The cell performance is highly dependent on the thickness of the electrolyte [10,31]. The extremely thin electrolyte with an acceptable mechanical integrity is eagerly pursued to obtain high performance with high ionic conductivity and long term durability. Introduction of the polymers in the CsH2PO4 electrolyte is one of the effective approaches to reduce the thickness in addition to enhancing the mechanical integrity. In recent years, numerous efforts have been devoted to developing polymer-CsH2PO4 composite electrolytes. Qing et al. [28] reported a CsH2PO4-epoxy composite electrolyte with 20 wt% epoxy, which showed a good mechanical strength and stable conductivity at 259 C. However, the peak power density of the corresponding cell was only 38 mW cm2 due mainly to the relatively high content of epoxy in the electrolyte that reduces the ionic conductivity. Xie et al. [24] prepared a composite electrolyte composed of CsH2PO4 and sulfonated poly(ether ether ketone) (SPEEK) with phosphosilicate sol, which exhibited excellent mechanical property; but its proton conductivity (1 mS cm1) was relatively low. Moreover, the complicated and time-consuming preparation procedures of these reported composite electrolytes would still hamper their applications. To date, it is usually suggested that the high content of polymer (10e30 wt%) is required to compensate the poor mechanical integrity of CsH2PO4-based electrolyte [29]. However, the excessive polymer would block the proton pathway, resulting in decreased ionic conductivity of the electrolyte and thus poor cell performance. Thus, it is of great importance to develop new composite materials with the aim of reducing the polymer content, simplifying fabrication approach, and maintaining the physical property and ionic conductivity. In this communication, we report our findings in the preparation and electrochemical study of a CsH2PO4-polyvinyl butyral (PVB) composite electrolyte via a facile and cost-effective process. Chemically sturdy and mechanically flexible, PVB is widely used as an adhesive agent and a matrix former in the safety glass for automobiles windshields, where strong binding, toughness, flexibility, and thermal stability are required. In this work, we have systematically investigated the effect of the PVB content on the electrochemical properties of CsH2PO4-PVB composite electrolytes and on the fuel cell performance. Under optimal conditions, the PVB content can be as low as ~3 wt% in a composite electrolyte (~120 mm in thickness) while maintaining high proton conductivity (28 mS cm1 at 260 C). When tested in a single cell, it demonstrated a peak power density of ~108 mW cm2 at 260 C. More importantly, after three thermal-cycling test (between room temperature and 260 C), the cell performance was still maintained, demonstrating the robustness and durability of the composite electrolyte for practical applications.
2. Experimental 2.1. Synthesis of CsH2PO4-PVB composite electrolyte CsH2PO4 was synthesized by methanol-induced precipitation method [10] (see details in Supplementary Materials). PVB (Richard E. Mistler, Inc.) solution was prepared by dissolving 2 g of PVB in 81 mL of ethanol, followed by sonication for 30 min to obtain a homogenous 3 wt% PVB solution. Then, 3 wt% PVB was mixed with the CsH2PO4 powders and the mixture was ground to ensure good mixing as ethanol was volatilized. The CsH2PO4-PVB composite powder was collected and pressed into disc for further use and analysis. The composite electrolytes with different contents of PVB (3e10 wt%) (denoted as CsH2PO4-3%PVB, CsH2PO4-6%PVB and CsH2PO4-10%PVB) were prepared using the same procedure. The detailed characterization methods could be found in Supplementary Materials. 2.2. Preparation and electrochemical measurement of membrane electrode assembly (MEA) See preparation procedure in Supplementary Materials. The cells were tested at temperature from 240 to 260 C. The flow rate of both H2 and O2 was 45 sccm and both gases were passing through a water bubbler at ~80 C to keep water partial pressure at 0.47 atm. The electrochemical performance of the MEAs was evaluated under atmospheric pressure using a fuel cell test system (Scribner Associates Model 890CL). 3. Results and discussion For the pure CsH2PO4 electrolyte, an electrolyte disc could be obtained by pressing when the thickness was above 200 mm. However, it was difficult to press an intact CsH2PO4 electrolyte membrane with thickness <200 mm that would have sufficient mechanical strength for subsequent handling. As seen in Fig. 1a, some pinholes can be observed. In contrast, a CsH2PO4-PVB composite electrolyte disc, even with as little as 3 wt% PVB, can maintain its shape and reasonable mechanical strength for handling. The surface morphology of CsH2PO4-PVB composite electrolyte is shown in Fig. 1b. It is clear that the micrometer-sized CsH2PO4 particles were well stacked together to form a uniform and dense composite electrolyte with addition of the PVB. There were no obvious pinholes or voids observed for the composite electrolyte with only 3 wt% PVB. The pinholes or cracks in the electrolyte will result in degradation or destruction of the electrodes. The PVB plays an important role in maintaining the mechanical integrity of the electrolyte membrane, in preventing leakage through the electrolyte, and in minimizing the ohmic loss of the electrolyte since thinner electrolyte membrane of good conductivity becomes possible. The cross-sections of the composite electrolytes were also examined using SEM. The thickness of the composite electrolyte with 3 wt% PVB was around 120 mm. As shown in the Fig. 1(c and d), the CsH2PO4 particles and PVB were uniformly distributed across the region to form the dense structure of electrolyte disc. The distribution of the CsH2PO4 and PVB binder in the composite electrolyte was investigated by elemental mapping (Fig. S1). With increasing the PVB content from 3% to 10%, the C signal becomes stronger. The high content of PVB in the composite electrolyte is beneficial to the mechanical property of the electrolyte. However, excessive amount of PVB may block the active site of the CsH2PO4 particles for proton transport. As expected, the conductivity of the
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Fig. 1. (a, b) SEM images of the surface of (a) CsH2PO4 electrolyte and (b) CsH2PO4-3%PVB composite electrolyte; (c, d) SEM images of the cross-section of CsH2PO4-3%PVB composite electrolyte.
composite electrolyte decreases as the PVB content increases. Thus, we attempt to keep the PVB content as low as possible while obtaining good physical property. X-ray diffraction patterns are shown in Fig. 2a to reveal the structural difference of pure CsH2PO4, pure PVB, and CsH2PO4-PVB composites. The pure PVB was amorphous polymer with a broad diffraction peak at ~20 , in accordance with the literature [32]. The XRD pattern of pure CsH2PO4 was consistent with that in the reports [28]. After adding different amounts of PVB in the CsH2PO4PVB composite electrolytes, the composites showed quite similar XRD patterns compared to that of the pure CsH2PO4 electrolyte, implying that the crystalline structure of CsH2PO4 remains the same with the presence of PVB. Meanwhile, the peak associated with PVB was not visible for the composite electrolytes and this might be explained by the relatively low content of PVB (<10 wt%) in the composite electrolytes. Fig. 2b displays the TGA profiles of the composite electrolytes with different PVB contents, together with the profiles of pure CsH2PO4 and PVB for comparison. It is seen that the weight loss of pure CsH2PO4 begins at 225 C, and the initial losses are ascribed to dehydration expressed by the following reaction. CsH2PO4 (s) / CsH2
2xPO4 x
(s) þ xH2O(g), (0 x 1)
When CsH2PO4 is completely transformed to CsPO3 at 500 C, the cumulative weight loss is 8% (losing 1 molecule of H2O per CsH2PO4 molecule). As shown in our TGA result of pure PVB (Fig. 2b), the weight loss of PVB at 260 C was only 1.4%, and it became to be 2.4% at 270 C. The major weight loss of PVB happened at the temperature range of 270e340 C. The small weight loss of 1.4% for PVB suggests that it is still relatively stable at the fuel cell operating temperature (260 C). In the DTA profile of the CsH2PO4 sample, there were three endothermic peaks from room temperature to 350 C, corresponding to specific steps during dehydration processes. According
to a previous study [33], CsH2PO4 undergoes a phase transition from a low conductivity phase (monoclinic) to a high conductivity phase (cubic) at 234 C. Thus, in the DTA profile of the CsH2PO4, the first endothermic peak at around 234 C overlaps with the peaks of phase transition and dehydration. The DTA profiles of the CsH2PO4PVB composite samples displayed the characteristics of both the CsH2PO4 and the PVB samples. As shown in the Fig. 3a and b, the DTA and TGA profiles at temperatures below 280 C reflect mainly the thermal behavior of CsH2PO4. However, when the temperature was increased to above 280 (to 500 C), the behaviors of the composite electrolytes were dominated by the exothermic peaks, which can be attributed to the thermal degradation of PVB. For the composite electrolyte especially with higher PVB content of 6 and 10 wt%, an obviously exothermic peak at 340 C was observed, which may be attributed to the decomposition of PVB. The thermal behavior difference between pure PVB and composite electrolyte may be attributed to the water released from the CsH2PO4, which can influence the decomposition behavior of PVB. To better study the electrochemical properties of the composite electrolyte, the proton conductivity of the CsH2PO4-3%PVB composite electrolyte under N2 atmosphere (0.47 atm H2O) at different temperature was determined from electrochemical impedance spectroscopy (EIS). Shown in Fig. 3a are the conductivities, calculated from the resistances of the electrolyte determined from EIS, of a CsH2PO4-3%PVB composite electrolyte in the temperature range from 118 to 272 C. Clearly, a dramatic increase in conductivity was observed as the temperature was increased from 210 to 234 C, due mainly to the phase transition of CsH2PO4 from monoclinic to cubic phase. This is in good agreement with the results published elsewhere [10,29]. It should be mentioned that the composite electrolyte with 3 wt% PVB shows high conductivity at 260 C of 28 mS cm1, which is comparable to that of the pure CsH2PO4 reported in the literature [29], but higher than the conductivities reported for the CsH2PO4-polymer composite electrolytes [28]. Also, the stability of the composite electrolyte is crucial to
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Fig. 2. (a) XRD patterns, (b) TGA and (c) DTA profiles of pure CsH2PO4, pure PVB, and CsH2PO4-PVB composites with different PVB contents.
Fig. 3. (a) Temperature dependence of the conductivity of CsH2PO4-3%PVB composite electrolytes and (b) its stability test under N2 atmosphere (0.47 atm H2O) at 260 C.
practical applications. Fig. 3b shows some preliminary stability data of the CsH2PO4-3%PVB composite electrolyte at 260 C. It can be seen that a steady-state conductivity of 27e29 mS cm1 was achieved during the durability test. The composite electrolyte showed a good stability during the 45 h test under the conditions. Fig. 4a shows some typical I-V and I-P curves of the MEAs based
on CsH2PO4-PVB composite electrolytes with different PVB contents at 260 C. Generally, the polarization curves of solid acid fuel cell could be divided into two parts based on different electrochemical processes. At very beginning, the voltage drop in low current density zone can be ascribed to the sluggish kinetics of oxygen reduction at the cathode side, which is mostly related to the intrinsic ORR activity of the cathode. As shown in the Fig. 4a, the three MEAs with different PVB contents in the composite electrolyte showed a small difference in the region with cell voltage above 0.8 V, due largely to the fact that the same electrodes were used in all MEAs. In the region with cell voltage below 0.8 V, the polarization loss is dominated by the ohmic loss of the cell, including contributions from ionic conduction through the composite electrolyte and current collection between electrode and current collector. It can be seen in the Fig. 4a that the three MEAs exhibited distinctively different polarization curves in the high current density region, indicating that they have different ohmic resistance. Since all the MEAs were prepared using the same procedures and the same apparatus, the difference in the voltage drop in this region could be attributed to the different content of PVB in the composite electrolytes used in the MEAs. Clearly, the cell based on the composite electrolyte with the lowest PVB content (3 wt%) showed the highest power output and the lowest ohmic resistance. This is reasonable because the PVB in the composite electrolytes will block the continuous proton pathway, which would definitely increase the electrolyte resistance and decrease cell performance. As shown in the Fig. 4b, the peak power density was enhanced with decreasing PVB content from 10 to 3 wt% in the composite electrolyte at each given cell temperature. The peak power density of MEA using CsH2PO4-3%PVB composite electrolyte was as high as 108 mW cm2 whereas the MEA using CsH2PO4-10%PVB electrolyte had the poorest cell performance (58 mW cm2) at 260 C. The results suggest that maintaining sufficient amount of CsH2PO4 and creating continuous proton pathways in the composite electrolyte is crucial to achieving high performance MEA. However, when we further reduce the PVB content in the composite electrolyte, it will cause mechanical strength deterioration. Thus the optimized PVB content was fixed to 3 wt% in the composite electrolyte. On the other hand, single cell performance could be strongly influenced by cell temperature for the composite electrolytes MEAs. Fig. S2 shows the polarization curves for the MEA using CsH2PO4-3%PVB composite electrolyte at different cell temperature. When the cell temperature was increased from 240 C to 260 C, cell performance of three types of MEAs were all improved and the peak power density increased gradually. For comparison, we fabricated a MEA with CsH2PO4 electrolyte membrane in ~120 mm thickness and tested at 260 C. It should be mentioned that an intact CsH2PO4
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Fig. 4. (a) Typical I-V and I-P curves of the cells based on CsH2PO4-PVB composite electrolytes (with different PVB contents) at 260 C; (b) peak power density bar graph for the cells with different electrolytes at different cell temperatures, where the 3%PVB indicates the cell using CsH2PO4-3%PVB composite electrolyte; (c) I-V and I-P curves of the cells using CsH2PO4-3%PVB composite electrolytes with three thermal-cycling (room temperature - 260 C - room temperature) at 260 C; (d) chronoamperometry curve of the cell using CsH2PO4-3%PVB composite electrolyte (Tcell ¼ 260 C).
electrolyte membrane with a thickness of 120 mm was difficult to obtain without the addition of PVB by press method, which can be destroyed easily during the MEA fabrication process by a small force. We made our best efforts and eventually fabricated a MEA with CsH2PO4 electrolyte membrane in ~120 mm thickness for electrochemical measurement for comparison. The initial polarization curve of the MEA with pure CsH2PO4 electrolyte membrane was similar with that of CsH2PO4-3%PVB composite electrolyte (Fig. S3). However, the performance was not stable with the current irreversibly decreased after 3 h operation (Fig. S4). Fig. 4c shows the electrochemical performances of a cell based on a CsH2PO4-3%PVB composite electrolyte underwent three thermal-cycling tests (room temperature - 260 C - room temperature). The single cell performance decreased slightly after the first thermal-cycling test and remained stable during the second and third cycles. It is important and interesting to note that, even after three thermal-cycling tests, there was no observable decrease in cell performance and the peak power density was still as high as 102 mW cm2, demonstrating good reproducibility of the CsH2PO43%PVB composite electrolytes in solid acid fuel cell practical applications. Fig. 4d shows a preliminary stability testing of the fuel cell with CsH2PO4-3%PVB composite electrolyte for 10 h under constant discharge operation at 0.5 V. The output current density maintained at 160 mA cm2 during the test and no obvious decay was observed for the fuel cell performance, which demonstrated reliability of the CsH2PO4-3%PVB composite electrolyte. In Fig. S5, before and after fuel cell test for 10 h, Raman spectroscopy of
CsH2PO4-10%PVB shows the similar peaks which are characteristic of C-H in PVB, indicating that PVB still exists in the composite electrolyte. 4. Conclusion In summary, we have successfully prepared a CsH2PO4-PVB composite electrolyte. The distribution of both the CsH2PO4 and the PVB phases are relatively uniform and the composite electrolyte membranes have excellent mechanical integrity and good ionic conductivity. When tested in single cells, the MEA based on the CsH2PO4-3%PVB electrolyte membranes exhibited excellent performance, demonstrating peak power density as high as 108 mW cm2 and superior conductivity of 28 mS cm1 at 260 C (using humidified gases for both anode and cathode). Moreover, the MEA showed good stability during a short-term constant current discharge process and thermal-cycling test. The proposed composite electrolyte is believed to have good potential for applications in solid acid fuel cells. Acknowledgements This work was supported by the US Department of Energy ARPA-E REBELS Program (under award number DE-AR0000501) and Guangdong Innovative and Entrepreneurial Research Team Program (No. 2014ZT05N200). D.D. acknowledges the financial support of a scholarship from Guangzhou Elite Program.
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Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.05.023. References [1] D. Larcher, J.M. Tarascon, Nat. Chem. 7 (2015) 19e29. [2] S. Dai, W. Xu, Y. Xi, M. Wang, X. Gu, D. Guo, C. Hu, Nano Energy 19 (2016) 363e372. [3] M. Winter, R.J. Brodd, Chem. Rev. 104 (2004) 4245e4270. [4] B. Zhao, R. Ran, M. Liu, Z. Shao, Mater. Sci. Eng. R Rep. 98 (2015) 1e71. [5] D. Dang, H. Zou, Z.A. Xiong, S. Hou, T. Shu, H. Nan, X. Zeng, J. Zeng, S. Liao, ACS Catal. 5 (2015) 4318e4324. [6] A. Rabis, P. Rodriguez, T.J. Schmidt, ACS Catal. 2 (2012) 864e890. [7] K. Scott, C. Xu, X. Wu, Wiley Interdiscip. Rev. Energy Environ. 3 (2014) 24e41. [8] N. Mohammad, A.B. Mohamad, A.A.H. Kadhum, K.S. Loh, J. Power Sources 322 (2016) 77e92. [9] S.M. Haile, C.R.I. Chisholm, K. Sasaki, D.A. Boysen, T. Uda, Faraday Discuss. 134 (2007) 17e39. [10] D.A. Boysen, T. Uda, C.R. Chisholm, S.M. Haile, Science 303 (2004) 68e70. [11] A.B. Papandrew, S.S. John, R.A. Elgammal, D.L. Wilson, R.W. Atkinson, J.S. Lawton, T.M. Arruda, T.A. Zawodzinski, J. Electrochem. Soc. 163 (2016) F464eF469. [12] G. Kim, J.M. Griffin, F. Blanc, S.M. Haile, C.P. Grey, J. Am. Chem. Soc. 137 (2015) 3867e3876. [13] G. Qing, R. Kikuchi, A. Takagaki, T. Sugawara, S.T. Oyama, J. Power Sources 306 (2016) 578e586. [14] G. Qing, R. Kikuchi, A. Takagaki, T. Sugawara, S.T. Oyama, J. Power Sources 272 (2014) 1018e1029.
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
A durable polyvinyl butyral-CsH2PO4 composite electrolyte for solid acid fuel cells Dai Dang a, b, 1, Bote Zhao a, 1, Dongchang Chen a, Seonyoung Yoo a, Samson Y. Lai a, Brian Doyle a, Shuge Dai a, Yu Chen a, Chong Qu a, Lei Zhang a, Shijun Liao b, **, Meilin Liu a, * a
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, 30332-0245, USA School of Chemistry and Chemical Engineering, South China University of Technology, New Energy Research Institute, The Key Laboratory of Fuel Cell Technology of Guangdong Province & The Key Laboratory of New Energy Technology of Guangdong Universities, Guangzhou, 510641, China
b
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
A PVB-CsH2PO4 composite electrolyte has been prepared via a facile method. 3 wt% PVB/CsH2PO4 electrolyte has a high conductivity of ~28 mS cm1 at 260 C. The MEA with composite electrolyte achieves a peak power density of 108 mW cm2. The MEA shows no degradation after three thermal-cycling test.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 December 2016 Received in revised form 6 May 2017 Accepted 9 May 2017
A composite electrolyte membrane composed of polyvinyl butyral (PVB) and CsH2PO4 has been prepared via a facile and cost-effective method for solid acid fuel cells. The effect of PVB content on conductivity, mechanical integrity, and fuel cell performance is investigated. A minimum amount of 3 wt% PVB in the CsH2PO4-based composite electrolyte not only offers the required mechanical integrity but also allows high conductivity (~28 mS cm1 at 260 C). Single cells based on the composite electrolytes demonstrate a peak power density of 108 mW cm2 at 260 C. Almost no degradation in electrochemical performance could be observed during the stability test for 10 h and three thermal-cycling test in H2/O2 fuel cell, indicating the promising application of the composite electrolyte in solid acid fuel cells. © 2017 Elsevier B.V. All rights reserved.
Keywords: Electrolytes Intermediate-temperature fuel cells Polymers Composites Membrane electrode assembly
1. Introduction
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (M. Liu). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2017.05.023 0378-7753/© 2017 Elsevier B.V. All rights reserved.
With ever-increasing energy demands and environmental concerns, the search for new energy technologies is a grand challenge facing the world today [1e4]. Fuel cells are one of the most promising options to replace the conventional energy technologies (e.g., internal combustion engines) because of their high energy
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efficiency, and low emission [3,5,6]. Of all types of fuel cells, the intermediate-temperature fuel cells (ITFC) operated at 200e500 C have attracted much attention in the past few years because of its potential to overcome the technical difficulties associated with low-temperature fuel cells [7e14]. For example, ITFCs would have simplified water management system with enhanced electrochemical kinetics and high carbon monoxide-tolerance compared to the low temperature fuel cells [9e11,15,16]. Cesium dihydrogen phosphate (CsH2PO4), one of the most promising solid acid electrolytes, has been investigated due to its high proton conductivity (>10 mS cm1) with the phase transition from monoclinic to superprotonic phase over 230e260 C [9,17e22]. However, CsH2PO4 usually suffers from poor mechanical strength and can be easily destroyed by a small physical force, leading to a severe gas crossover [23e30]. Therefore, an electrolyte thicker than 250 mm is usually required to ensure mechanical integrity and prevent the electrolyte from voids or pinholes [18e20,31]. The cell performance is highly dependent on the thickness of the electrolyte [10,31]. The extremely thin electrolyte with an acceptable mechanical integrity is eagerly pursued to obtain high performance with high ionic conductivity and long term durability. Introduction of the polymers in the CsH2PO4 electrolyte is one of the effective approaches to reduce the thickness in addition to enhancing the mechanical integrity. In recent years, numerous efforts have been devoted to developing polymer-CsH2PO4 composite electrolytes. Qing et al. [28] reported a CsH2PO4-epoxy composite electrolyte with 20 wt% epoxy, which showed a good mechanical strength and stable conductivity at 259 C. However, the peak power density of the corresponding cell was only 38 mW cm2 due mainly to the relatively high content of epoxy in the electrolyte that reduces the ionic conductivity. Xie et al. [24] prepared a composite electrolyte composed of CsH2PO4 and sulfonated poly(ether ether ketone) (SPEEK) with phosphosilicate sol, which exhibited excellent mechanical property; but its proton conductivity (1 mS cm1) was relatively low. Moreover, the complicated and time-consuming preparation procedures of these reported composite electrolytes would still hamper their applications. To date, it is usually suggested that the high content of polymer (10e30 wt%) is required to compensate the poor mechanical integrity of CsH2PO4-based electrolyte [29]. However, the excessive polymer would block the proton pathway, resulting in decreased ionic conductivity of the electrolyte and thus poor cell performance. Thus, it is of great importance to develop new composite materials with the aim of reducing the polymer content, simplifying fabrication approach, and maintaining the physical property and ionic conductivity. In this communication, we report our findings in the preparation and electrochemical study of a CsH2PO4-polyvinyl butyral (PVB) composite electrolyte via a facile and cost-effective process. Chemically sturdy and mechanically flexible, PVB is widely used as an adhesive agent and a matrix former in the safety glass for automobiles windshields, where strong binding, toughness, flexibility, and thermal stability are required. In this work, we have systematically investigated the effect of the PVB content on the electrochemical properties of CsH2PO4-PVB composite electrolytes and on the fuel cell performance. Under optimal conditions, the PVB content can be as low as ~3 wt% in a composite electrolyte (~120 mm in thickness) while maintaining high proton conductivity (28 mS cm1 at 260 C). When tested in a single cell, it demonstrated a peak power density of ~108 mW cm2 at 260 C. More importantly, after three thermal-cycling test (between room temperature and 260 C), the cell performance was still maintained, demonstrating the robustness and durability of the composite electrolyte for practical applications.
2. Experimental 2.1. Synthesis of CsH2PO4-PVB composite electrolyte CsH2PO4 was synthesized by methanol-induced precipitation method [10] (see details in Supplementary Materials). PVB (Richard E. Mistler, Inc.) solution was prepared by dissolving 2 g of PVB in 81 mL of ethanol, followed by sonication for 30 min to obtain a homogenous 3 wt% PVB solution. Then, 3 wt% PVB was mixed with the CsH2PO4 powders and the mixture was ground to ensure good mixing as ethanol was volatilized. The CsH2PO4-PVB composite powder was collected and pressed into disc for further use and analysis. The composite electrolytes with different contents of PVB (3e10 wt%) (denoted as CsH2PO4-3%PVB, CsH2PO4-6%PVB and CsH2PO4-10%PVB) were prepared using the same procedure. The detailed characterization methods could be found in Supplementary Materials. 2.2. Preparation and electrochemical measurement of membrane electrode assembly (MEA) See preparation procedure in Supplementary Materials. The cells were tested at temperature from 240 to 260 C. The flow rate of both H2 and O2 was 45 sccm and both gases were passing through a water bubbler at ~80 C to keep water partial pressure at 0.47 atm. The electrochemical performance of the MEAs was evaluated under atmospheric pressure using a fuel cell test system (Scribner Associates Model 890CL). 3. Results and discussion For the pure CsH2PO4 electrolyte, an electrolyte disc could be obtained by pressing when the thickness was above 200 mm. However, it was difficult to press an intact CsH2PO4 electrolyte membrane with thickness <200 mm that would have sufficient mechanical strength for subsequent handling. As seen in Fig. 1a, some pinholes can be observed. In contrast, a CsH2PO4-PVB composite electrolyte disc, even with as little as 3 wt% PVB, can maintain its shape and reasonable mechanical strength for handling. The surface morphology of CsH2PO4-PVB composite electrolyte is shown in Fig. 1b. It is clear that the micrometer-sized CsH2PO4 particles were well stacked together to form a uniform and dense composite electrolyte with addition of the PVB. There were no obvious pinholes or voids observed for the composite electrolyte with only 3 wt% PVB. The pinholes or cracks in the electrolyte will result in degradation or destruction of the electrodes. The PVB plays an important role in maintaining the mechanical integrity of the electrolyte membrane, in preventing leakage through the electrolyte, and in minimizing the ohmic loss of the electrolyte since thinner electrolyte membrane of good conductivity becomes possible. The cross-sections of the composite electrolytes were also examined using SEM. The thickness of the composite electrolyte with 3 wt% PVB was around 120 mm. As shown in the Fig. 1(c and d), the CsH2PO4 particles and PVB were uniformly distributed across the region to form the dense structure of electrolyte disc. The distribution of the CsH2PO4 and PVB binder in the composite electrolyte was investigated by elemental mapping (Fig. S1). With increasing the PVB content from 3% to 10%, the C signal becomes stronger. The high content of PVB in the composite electrolyte is beneficial to the mechanical property of the electrolyte. However, excessive amount of PVB may block the active site of the CsH2PO4 particles for proton transport. As expected, the conductivity of the
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Fig. 1. (a, b) SEM images of the surface of (a) CsH2PO4 electrolyte and (b) CsH2PO4-3%PVB composite electrolyte; (c, d) SEM images of the cross-section of CsH2PO4-3%PVB composite electrolyte.
composite electrolyte decreases as the PVB content increases. Thus, we attempt to keep the PVB content as low as possible while obtaining good physical property. X-ray diffraction patterns are shown in Fig. 2a to reveal the structural difference of pure CsH2PO4, pure PVB, and CsH2PO4-PVB composites. The pure PVB was amorphous polymer with a broad diffraction peak at ~20 , in accordance with the literature [32]. The XRD pattern of pure CsH2PO4 was consistent with that in the reports [28]. After adding different amounts of PVB in the CsH2PO4PVB composite electrolytes, the composites showed quite similar XRD patterns compared to that of the pure CsH2PO4 electrolyte, implying that the crystalline structure of CsH2PO4 remains the same with the presence of PVB. Meanwhile, the peak associated with PVB was not visible for the composite electrolytes and this might be explained by the relatively low content of PVB (<10 wt%) in the composite electrolytes. Fig. 2b displays the TGA profiles of the composite electrolytes with different PVB contents, together with the profiles of pure CsH2PO4 and PVB for comparison. It is seen that the weight loss of pure CsH2PO4 begins at 225 C, and the initial losses are ascribed to dehydration expressed by the following reaction. CsH2PO4 (s) / CsH2
2xPO4 x
(s) þ xH2O(g), (0 x 1)
When CsH2PO4 is completely transformed to CsPO3 at 500 C, the cumulative weight loss is 8% (losing 1 molecule of H2O per CsH2PO4 molecule). As shown in our TGA result of pure PVB (Fig. 2b), the weight loss of PVB at 260 C was only 1.4%, and it became to be 2.4% at 270 C. The major weight loss of PVB happened at the temperature range of 270e340 C. The small weight loss of 1.4% for PVB suggests that it is still relatively stable at the fuel cell operating temperature (260 C). In the DTA profile of the CsH2PO4 sample, there were three endothermic peaks from room temperature to 350 C, corresponding to specific steps during dehydration processes. According
to a previous study [33], CsH2PO4 undergoes a phase transition from a low conductivity phase (monoclinic) to a high conductivity phase (cubic) at 234 C. Thus, in the DTA profile of the CsH2PO4, the first endothermic peak at around 234 C overlaps with the peaks of phase transition and dehydration. The DTA profiles of the CsH2PO4PVB composite samples displayed the characteristics of both the CsH2PO4 and the PVB samples. As shown in the Fig. 3a and b, the DTA and TGA profiles at temperatures below 280 C reflect mainly the thermal behavior of CsH2PO4. However, when the temperature was increased to above 280 (to 500 C), the behaviors of the composite electrolytes were dominated by the exothermic peaks, which can be attributed to the thermal degradation of PVB. For the composite electrolyte especially with higher PVB content of 6 and 10 wt%, an obviously exothermic peak at 340 C was observed, which may be attributed to the decomposition of PVB. The thermal behavior difference between pure PVB and composite electrolyte may be attributed to the water released from the CsH2PO4, which can influence the decomposition behavior of PVB. To better study the electrochemical properties of the composite electrolyte, the proton conductivity of the CsH2PO4-3%PVB composite electrolyte under N2 atmosphere (0.47 atm H2O) at different temperature was determined from electrochemical impedance spectroscopy (EIS). Shown in Fig. 3a are the conductivities, calculated from the resistances of the electrolyte determined from EIS, of a CsH2PO4-3%PVB composite electrolyte in the temperature range from 118 to 272 C. Clearly, a dramatic increase in conductivity was observed as the temperature was increased from 210 to 234 C, due mainly to the phase transition of CsH2PO4 from monoclinic to cubic phase. This is in good agreement with the results published elsewhere [10,29]. It should be mentioned that the composite electrolyte with 3 wt% PVB shows high conductivity at 260 C of 28 mS cm1, which is comparable to that of the pure CsH2PO4 reported in the literature [29], but higher than the conductivities reported for the CsH2PO4-polymer composite electrolytes [28]. Also, the stability of the composite electrolyte is crucial to
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Fig. 2. (a) XRD patterns, (b) TGA and (c) DTA profiles of pure CsH2PO4, pure PVB, and CsH2PO4-PVB composites with different PVB contents.
Fig. 3. (a) Temperature dependence of the conductivity of CsH2PO4-3%PVB composite electrolytes and (b) its stability test under N2 atmosphere (0.47 atm H2O) at 260 C.
practical applications. Fig. 3b shows some preliminary stability data of the CsH2PO4-3%PVB composite electrolyte at 260 C. It can be seen that a steady-state conductivity of 27e29 mS cm1 was achieved during the durability test. The composite electrolyte showed a good stability during the 45 h test under the conditions. Fig. 4a shows some typical I-V and I-P curves of the MEAs based
on CsH2PO4-PVB composite electrolytes with different PVB contents at 260 C. Generally, the polarization curves of solid acid fuel cell could be divided into two parts based on different electrochemical processes. At very beginning, the voltage drop in low current density zone can be ascribed to the sluggish kinetics of oxygen reduction at the cathode side, which is mostly related to the intrinsic ORR activity of the cathode. As shown in the Fig. 4a, the three MEAs with different PVB contents in the composite electrolyte showed a small difference in the region with cell voltage above 0.8 V, due largely to the fact that the same electrodes were used in all MEAs. In the region with cell voltage below 0.8 V, the polarization loss is dominated by the ohmic loss of the cell, including contributions from ionic conduction through the composite electrolyte and current collection between electrode and current collector. It can be seen in the Fig. 4a that the three MEAs exhibited distinctively different polarization curves in the high current density region, indicating that they have different ohmic resistance. Since all the MEAs were prepared using the same procedures and the same apparatus, the difference in the voltage drop in this region could be attributed to the different content of PVB in the composite electrolytes used in the MEAs. Clearly, the cell based on the composite electrolyte with the lowest PVB content (3 wt%) showed the highest power output and the lowest ohmic resistance. This is reasonable because the PVB in the composite electrolytes will block the continuous proton pathway, which would definitely increase the electrolyte resistance and decrease cell performance. As shown in the Fig. 4b, the peak power density was enhanced with decreasing PVB content from 10 to 3 wt% in the composite electrolyte at each given cell temperature. The peak power density of MEA using CsH2PO4-3%PVB composite electrolyte was as high as 108 mW cm2 whereas the MEA using CsH2PO4-10%PVB electrolyte had the poorest cell performance (58 mW cm2) at 260 C. The results suggest that maintaining sufficient amount of CsH2PO4 and creating continuous proton pathways in the composite electrolyte is crucial to achieving high performance MEA. However, when we further reduce the PVB content in the composite electrolyte, it will cause mechanical strength deterioration. Thus the optimized PVB content was fixed to 3 wt% in the composite electrolyte. On the other hand, single cell performance could be strongly influenced by cell temperature for the composite electrolytes MEAs. Fig. S2 shows the polarization curves for the MEA using CsH2PO4-3%PVB composite electrolyte at different cell temperature. When the cell temperature was increased from 240 C to 260 C, cell performance of three types of MEAs were all improved and the peak power density increased gradually. For comparison, we fabricated a MEA with CsH2PO4 electrolyte membrane in ~120 mm thickness and tested at 260 C. It should be mentioned that an intact CsH2PO4
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Fig. 4. (a) Typical I-V and I-P curves of the cells based on CsH2PO4-PVB composite electrolytes (with different PVB contents) at 260 C; (b) peak power density bar graph for the cells with different electrolytes at different cell temperatures, where the 3%PVB indicates the cell using CsH2PO4-3%PVB composite electrolyte; (c) I-V and I-P curves of the cells using CsH2PO4-3%PVB composite electrolytes with three thermal-cycling (room temperature - 260 C - room temperature) at 260 C; (d) chronoamperometry curve of the cell using CsH2PO4-3%PVB composite electrolyte (Tcell ¼ 260 C).
electrolyte membrane with a thickness of 120 mm was difficult to obtain without the addition of PVB by press method, which can be destroyed easily during the MEA fabrication process by a small force. We made our best efforts and eventually fabricated a MEA with CsH2PO4 electrolyte membrane in ~120 mm thickness for electrochemical measurement for comparison. The initial polarization curve of the MEA with pure CsH2PO4 electrolyte membrane was similar with that of CsH2PO4-3%PVB composite electrolyte (Fig. S3). However, the performance was not stable with the current irreversibly decreased after 3 h operation (Fig. S4). Fig. 4c shows the electrochemical performances of a cell based on a CsH2PO4-3%PVB composite electrolyte underwent three thermal-cycling tests (room temperature - 260 C - room temperature). The single cell performance decreased slightly after the first thermal-cycling test and remained stable during the second and third cycles. It is important and interesting to note that, even after three thermal-cycling tests, there was no observable decrease in cell performance and the peak power density was still as high as 102 mW cm2, demonstrating good reproducibility of the CsH2PO43%PVB composite electrolytes in solid acid fuel cell practical applications. Fig. 4d shows a preliminary stability testing of the fuel cell with CsH2PO4-3%PVB composite electrolyte for 10 h under constant discharge operation at 0.5 V. The output current density maintained at 160 mA cm2 during the test and no obvious decay was observed for the fuel cell performance, which demonstrated reliability of the CsH2PO4-3%PVB composite electrolyte. In Fig. S5, before and after fuel cell test for 10 h, Raman spectroscopy of
CsH2PO4-10%PVB shows the similar peaks which are characteristic of C-H in PVB, indicating that PVB still exists in the composite electrolyte. 4. Conclusion In summary, we have successfully prepared a CsH2PO4-PVB composite electrolyte. The distribution of both the CsH2PO4 and the PVB phases are relatively uniform and the composite electrolyte membranes have excellent mechanical integrity and good ionic conductivity. When tested in single cells, the MEA based on the CsH2PO4-3%PVB electrolyte membranes exhibited excellent performance, demonstrating peak power density as high as 108 mW cm2 and superior conductivity of 28 mS cm1 at 260 C (using humidified gases for both anode and cathode). Moreover, the MEA showed good stability during a short-term constant current discharge process and thermal-cycling test. The proposed composite electrolyte is believed to have good potential for applications in solid acid fuel cells. Acknowledgements This work was supported by the US Department of Energy ARPA-E REBELS Program (under award number DE-AR0000501) and Guangdong Innovative and Entrepreneurial Research Team Program (No. 2014ZT05N200). D.D. acknowledges the financial support of a scholarship from Guangzhou Elite Program.
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