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Journal of Power Sources 294 (2015) 232e238 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour A hydrophilic-hydrophobic dual-layer microporous layer enabling the improved water management of direct methanol fuel cells operating with neat methanol X.H. Yan, T.S. Zhao*, G. Zhao, L. An, X.L. Zhou Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China h i g h l i g h t s A hydrophilic-hydrophobic dual-layer MPL for passive DMFCs is proposed. The novel MPL enables a significant improvement of water management. The use of this MPL to either electrode leads to a significant performance boost. a r t i c l e i n f o a b s t r a c t Article history: Received 1 February 2015 Received in revised form 15 May 2015 Accepted 8 June 2015 Available online xxx Passive direct methanol fuel cells (DMFCs) operating with neat methanol can achieve the maximum system energy density. However, the anodic methanol oxidation reaction requires reactant water, which is completely supplied by water generated at the cathode, causing the system to experience a critical issue known as water starvation. A solution to this problem involves increasing the water recovery flux to meet the rate of water consumption of the anodic reaction, and increase the local water concentration as high as possible at the anode catalyst layer (CL) to improve the anodic kinetics. In the present work, a new microporous layer (MPL) consisting of a hydrophilic layer and a hydrophobic layer is proposed. The purposes of these two layers are to, respectively, trap and retain water and to create capillary pressure to prevent water loss. Our experiments have shown that the use of this novel MPL at the anode and cathode can increase the rate of water recovery and water retention, resulting in an increase in the local water concentration. As a result, the use of this dual-layer MPL to either electrode of a passive DMFC operating with neat methanol leads to a significant performance boost. © 2015 Elsevier B.V. All rights reserved. Keywords: Fuel cells Direct methanol fuel cells Neat methanol operation Water starvation 1. Introduction Passive DMFCs have garnered increasing attention as the next generation power source for portable electronic devices [1e4], owing to its unique features, such as compact structure, convenience in fuel storage, low operating temperature and high theoretical energy density. Conventional passive DMFCs operate with diluted methanol (i.e., 2.0 M4.0 M) to minimize the hindrances caused by methanol crossover, but the most striking feature of high energy density output (~4800 Wh L1) is sacrificed as a result. Thus, it is ideal to operate passive DMFCs with highly concentrated fuel, * Corresponding author. E-mail address: [email protected] (T.S. Zhao). http://dx.doi.org/10.1016/j.jpowsour.2015.06.058 0378-7753/© 2015 Elsevier B.V. All rights reserved. and retard the rate of methanol crossover to maintain high performance. Past efforts to achieve this objective have been extensively made. The approaches attempted can be divided into three categories: developing new electrolyte membranes with low methanol permeability [5,6]; modifying the anode diffusion layer to increase the transport resistance of methanol [7,8]; and introducing a barrier layer to control the rate of methanol transport before it reaches the electrode [9]. Among them, the vapor-feed supply mode is one of the most effective approaches [10e14], which enables passive DMFCs to operate with neat methanol. Under this setup, however, water starvation is a major issue as water is one of the reactants of the methanol oxidation reaction (MOR) at the anode, and relies completely on the supply of production from the cathode. To address this issue, a sufficiently large water recovery flux must be X.H. Yan et al. / Journal of Power Sources 294 (2015) 232e238 ensured. Masdar et al. [15] adopted a hydrophobic air filter at the cathode to retard water loss and thus enhance water recovery. Zhang and Feng [16] treated the cathode backing layer by increasing the PTFE content to 40 wt. %, to enhance the water backflow effects. Li and Faghri [17] attempted to improve the water recovery by optimizing the cathode open ratio. Xu et al. [18] proposed various approaches to increase the rate of water back transport, including thinning the membrane thickness, placing an air filter, and adopting a water management layer at the cathode. Previous efforts have indeed achieved varying degrees of success in water management, but have altered the original structure of DMFCs, resulting in increased system complexity. In the present work, we propose a new microporous layer (MPL) consisting of a hydrophilic layer and a hydrophobic layer. Highly hydrophobic material, polytetrafluoroethylene (PTFE), is used as the binder in the hydrophobic layer, and cross-linked polyvinyl alcohol (PVA), both hygroscopic and highly hydrophilic, is used in the hydrophilic layer. Taking advantage of the disparate features of these two binder materials, the hydrophilic layer traps and retains water, while the hydrophobic layer creates capillary pressure to prevent the water loss, resulting in an overall increase in the local water concentration. The schematic description of the hydrophilichydrophobic dual-layer MPL is shown in Fig. 1. A water gradient is created throughout the passive DMFC membrane, due to the higher concentration of water generated at the cathode in contrast with the anode, which produces no water. This gradient drives the water to diffuse from the cathode toward the anode. The corresponding flux can be expressed as: Cc Ca dm Km rs cos q ε 1=2 Jc ¼ JðsÞ mMH2 O dm K (2) where Km is the permeability of the membrane, r is water density, m is viscosity of liquid water, MH2O is molecular weight of water, and scosq(ε/K)1/2 J(s) is the capillary pressure created by the MPL [19]. Another contributor to the water crossover is electro-osmotic drag by proton transport from the anode to the cathode, which can be expressed as: Jeo ¼ nd i F (3) where nd, i and F denote, respectively, the electro-osmotic drag coefficient in the membrane, current density and Faraday's constant. Therefore, the net water recovery flux can be obtained from Eqs. (1)e(3) as: Jw ¼ Jd þ Jc Jeo ¼ Deff Clc Cla Km rs cos q ε 1=2 i JðsÞ nd mMH2 O dm K F dm (4) Meanwhile, the rate of water consumption by the anodic MOR is obtained as: 2. Theoretical Jd ¼ Deff 233 (1) where Deff is the effective diffusivity of water through the membrane, dm is the thickness of membrane, Cc and Ca represent the water concentrations at the cathode and anode surfaces facing the membrane, respectively. The generation of water at the cathode creates a gradual pressure difference throughout the membrane, resulting in a convection flux directed from the cathode to the anode: JMOR ¼ i 6F (5) From these equations, it is apparent that the basic requirement for a complete 6-electron transfer reaction to be true is to ensure Jw JMOR > 0. As shown in Eq. (4), a larger water back flux can be achieved by upgrading the water concentration at the cathode CL and increasing the capillary pressure. For this reason, a hydrophilichydrophobic dual-layer MPL is proposed, in which the hydrophilic layer located between the CL and hydrophobic layer elevates the water concentration at the cathode and enhances the effect of back diffusion. In addition, the hydrophobic layer is used to maintain the convection flux. It is noteworthy to mention that Wu et al. [20] found that there exists a mole ratio of water to methanol at the anode CL under neat methanol operation. When the mole ratio is larger than the critical value, the anode potential stabilizes to a value close to that of the Fig. 1. Schematic description of the hydrophilic-hydrophobic dual-layer MPL. 234 X.H. Yan et al. / Journal of Power Sources 294 (2015) 232e238 diluted-methanol operation and typically this value is larger than 2. Therefore, it is inadequate to focus entirely on ensuring high water recovery flux of Jw, but that water concentration at the anode CL must also be monitored to improve the anodic kinetics. Since water concentration at the anode CL is higher than that of the fuel reservoir, further loss of water toward the fuel reservoir should be observed. Based on this knowledge, the dual-layer MPL is also proposed for the anode, in which the hydrophobic layer is used to retard water loss and the hydrophilic layer is employed to trap and retain the recovered water. 3. Experimental 3.1. Membrane electrode assembly 3.1.1. Standard MEA A pretreated Nafion® 212 membrane of thickness 50 mm was employed in this work. Toary-060 carbon papers with 15 wt.% PTFE wet-proofing treatment worked as the backing layers. At the anode and cathode, the conventional hydrophobic MPL was fabricated using XC-72 carbon powder with a loading of 3.0 mg cm2 and adopted PTFE as the binder, with mass ratio of 30 wt.%. The catalyst layers of the anode and cathode consisted of PtRu black (Johnson Matthey®) with a loading of 4.0 mg cm2 and carbon supported Pt (60% Pt, Johnson Matthey®) with a loading of 2.0 mg cm2, respectively; both the mass ratios of Nafion ionomer were 20 wt.%. The catalyst layers were fabricated by a catalyst-coated membrane (CCM) method to provide direct coating on the membrane, ensuring the effect of different MPL structures to be investigated under a constant condition. The active area of the MEA was 4.0 cm2. 3.2. Single cell and measurement instrumentation The MEA was placed between an anode and a cathode perforated current collector. This setup was sandwiched by the anode and the cathode fixture and a 5 mL fuel tank was constructed at the anode. In contrast with conventional passive DMFC structure, a gas gap with a thickness of 10 mm, a perforated plate with an open ratio of 15% and a 28 mm pervaporation membrane were employed at the anode to realize the neat methanol operation as shown in Fig. 3. An Arbin BT2000 electrical load was used to control the discharge condition and record the data. Prior to the performance test, the MEA was installed in an active cell fixture and activated for 12 h at 60 C. During the activation process, 1.0 M methanol was fed at 1.0 mL min1, while dry air was supplied under the atmospheric pressure at a flow rate of 100 mL min1, and discharge tests were performed for several times until the performance reached a plateau without a further change. To ensure that the DMFC was running with neat methanol, the MEA was dried at ambient temperature for 12 h before the performance test to remove the water that initially existed inside the membrane. During the performance test, the voltage stabilized at about 60s for each discharge current point. To measure the water recovery flux, the cathode fixture was changed to an active mode and a water trap filled with Drierite (anhydrous CaSO4) was connected. The water was gathered at the open circuit voltage for 8.0 h. The details about this test can be found from Ref. [21]. The various MPLs were characterized by a contact angle measurement system, scanning electron microscope (JEOL-6300) and energy dispersive X-ray spectrum. 4. Results and discussion 3.1.2. MEA with dual-layer MPL To form the dual-layer MPL, an inner hydrophilic layer was fabricated between the CL and the outer hydrophobic layer with polyvinyl alcohol as the binder. The loading of the XC-72 carbon powder changed from 0.3 mg cm2 to 1.5 mg cm2, which determined the thickness of the hydrophilic layer; the mass ratio of the PVA binder changed from 5 wt. % to 20 wt. %. Due to the watersoluble nature of PVA, hydrogel must be formed through a crosslinking reaction, after the fabrication of the hydrophilic layer. This was formed by immersing the GDL coated hydrophilic layer into 5 wt.% glutaraldehyde (GA) (which worked as the crosslinker), and adding several drops of 5wt.% HCl to work as the catalyst as shown in Fig. 2. The entire process lasted for 2 h. The MPL was then washed with DI water. Fig. 2. The cross-linking reaction between PVA and glutaraldehyde. 4.1. Effect of the dual-layer MPL at the cathode Ensuring a sufficient water recovery flux to supply the water required by the MOR is imperative. To increase the water back Fig. 3. Schematic of a passive DMFC operating with neat methanol: 1-Fuel reservoir; 2-Pervaporation membrane; 3-Perforated plate; 4-Gas gap; 5-CO2 venting hole; 6Current collectors; 7-MEA. X.H. Yan et al. / Journal of Power Sources 294 (2015) 232e238 transport rate, the dual-layer MPL was used at the cathode. Fig. 4 gives a cross-sectional view of the dual-layer MPL structure (hydrophilic layer: 0.3 mg cm2 carbon, 10 wt.% PVA). From the EDX mappings, the hydrophilic part does not consist of F element, indicating that a very thin hydrophilic layer was coated on the conventional hydrophobic layer. Fig. 5 shows the effect of the dual-layer MPL on the cell performance of the DMFC operating with neat methanol. It demonstrates regardless of changes in the carbon loading of the hydrophilic layer, the MEA with dual-layer MPL always exhibits a better performance than does the conventional MEA. For instance, the power density of the MEA with hydrophilic layer of 0.5 mg cm2 carbon is 29.2 mW cm2, 29% larger than that of a conventional MEA (22.6 mW cm2). The cross-linked PVA is a highly hygroscopic and hydrophilic material, as a result the hydrophilic layer adopting the PVA binder can help to trap and retain produced water of the cathode ORR to enhance the water concentration at the cathode catalyst layer, the higher water concentration increase the water diffusion to the anode and further improve the kinetics of the anodic MOR, thereby boosting the cell performance. The increase in water recovery flux by the dual-layer MPL is proven in Fig. 6. Improvement in water back transport can be further confirmed by the limiting current density. In our previous work, we have proven that the limiting current density of the passive DMFCs under neat methanol operation is due to the lack of water for MOR [14]. Fig. 5 indicates that the current density increases significantly after the application of dual-layer MPL, thereby removing this limitation. These results suggest that the cathode dual-layer MPL has excellent water trapping and retention abilities and is able to improve water recovery for passive DMFCs operating with neat methanol. The effect of carbon loading in the hydrophilic layer (the mass ration of PVA is 10 wt.%) is shown in Fig. 5. It shows that there is a slight improvement in cell performance as the carbon loading increases from 0.3 mg cm2 to 0.5 mg cm2. A further increase in the carbon loading, however, leads to a performance decline. This can be attributed to the fact that a larger carbon loading indicates a thicker hydrophilic layer which exhibits better water retention ability. Therefore, enhancing the water concentration at the cathode CL and increasing the water back transport helps to boost the anodic MOR. It should be noted, however, that the hydrophilic layer has a negative effect on oxygen transport to the cathode CL. When water accumulates in the hydrophilic layer, oxygen transport 235 Fig. 5. Effect of the dual-layer MPL at the cathode with different carbon loadings on cell performance (the PVA content is 10 wt.%). Fig. 6. Effect of the dual-layer MPL at the cathode on the water recovery flux with different carbon loading. Fig. 4. Cross-sectional view of the hydrophilic-hydrophobic dual-layer MPL and the EDX mappings of fluorine and carbon. 236 X.H. Yan et al. / Journal of Power Sources 294 (2015) 232e238 through the water-rich layer will be hindered which will lower the cathode ORR, since the order of magnitude for oxygen diffusivity in air is 105, while the value in water is just 109 [22,23]. This phenomenon suggests a thin hydrophilic layer is preferred at the cathode. Fig. 7 shows the contact angles of various MPLs adopting different binders. The contact angle of the conventional hydrophobic MPL is as high as 140.3 . For MPLs adopting the PVA binder, the contact angles are smaller than 50 , meaning that the PVA can introduce the highly hydrophilic property. An increase in PVA content from 5 wt.% to 20 wt.% decreases the contact angle from 42 to 23 , thus improving the ability to trap water. Fig. 8 exhibits the effect of the dual cathode MPL, with different PVA contents, on the performance of passive DMFCs under neat methanol operation. It is demonstrated that increasing the PVA content from 5 wt.% to 15 wt.% improves cell performance up to a point, but any further increase in PVA content results in a performance decrease. The discussion and reasoning behind this is similar to that of the above analysis. As shown in Fig. 7, larger PVA content introduces better water trapping and retention abilities, which enhances water recovery. Though similarly, an accumulation of too much water hinders oxygen transport and lowers ORR. Nevertheless, even with PVA content above the higher boundary of Fig. 8. Effect of the dual-layer MPL at the cathode with different PVA content on cell performance (the carbon loading is 0.5 mg cm2). 20 wt.%, the peak power density can still reach 26 mW cm2, a result that is 15% higher than the cell performance of a conventional hydrophobic structure. An optimum PVA content within the Fig. 7. Contact angles of various MPLs: (a) hydrophilic layer with 5 wt.% PVA; (b) hydrophilic layer with 10 wt.% PVA; (c) hydrophilic layer with 15 wt.% PVA; (d) hydrophilic layer with 20 wt.% PVA; (e) conventional hydrophobic MPL with 30 wt.% PTFE. X.H. Yan et al. / Journal of Power Sources 294 (2015) 232e238 237 boundaries of 5 wt.% and 15 wt.% should be found to maximize the cell performance. These results suggest that the hydrophilic-hydrophobic duallayer MPL at the cathode can help to trap and retain the produced water at the cathode to enhance the water back transport. Moreover, since the water-rich layer shows the negative effects on the oxygen transport, both the PVA content and the thickness of the hydrophilic layer show significant influence on the cell performance. 4.2. Effect of the dual-layer MPL at the anode It is crucial to recognize that the MOR at the anode requires not only a sufficiently large water recovery flux, but also a high local water concentration at the anode CL. Hence it is useful to retain the recovered water at the anode CL by the hydrophilic-hydrophobic dual-layer MPL. The influence of the anode dual-layer MPL on the water management and cell performance was also investigated. The effect of the dual-layer MPL at the anode only on the cell performance is shown in Fig. 9. It is apparent that the DMFC with dual-layer MPL yields a much higher power density than does the conventional DMFC, demonstrating the favorable effect of the duallayer MPL at the anode. For instance, when the carbon loading of the hydrophilic layer is at 1.5 mg cm2, the peak power density is measured to be 32.6 mW cm2, which is 44% higher than the yields of conventional MEA. Meanwhile, it can be seen that performance improves with an increase in carbon loading. This is due to the thicker hydrophilic layer, which has a better ability to retain water, elevating the water concentration at the anode CL and improving the kinetics of anodic MOR. It should be noted, however, that although the highest power density is achieved at the carbon loading of 1.5 mg cm2, the voltage is lower at the high current density region, indicating mass transport is hindered by the thickness of the layer. Therefore the optimal carbon loading for the hydrophilic layer is 1.0 mg cm2, where the peak power density is still as high as 31.5 mW cm2. These results indicate that a relative thick hydrophilic layer is preferred at the anode. Fig. 10 shows the influence of the dual-layer MPL with varying PVA content in hydrophilic part. When the PVA content is at the lower end of 5 wt.%, water conservation at the anode CL is insufficient. However, when the PVA content is increased to 15 wt.%, the cell performance begins to decline. It can be understood similarly to the situation at the cathode, whereby excessive enhancements of the hydrophilic property leads to an increase in accumulated water, Fig. 9. Effect of the dual-layer MPL at the anode with different carbon loadings on cell performance (the PVA content is 10 wt.%). Fig. 10. Effect of the dual-layer MPL at the anode with different PVA content on cell performance (the carbon loading is 1.0 mg cm2). slowing down the removal of CO2 gas. The long-term discharge behaviors of the DMFCs with the optimized dual-layer MPL and the conventional one were tested at 40 mA cm2 and are shown in Fig. 11. It is evident that the use of this dual-layer MPL to either electrode enabled a significant increase in the cell voltage. The gradual decrease in the cell voltage with time is attributed to the decrease in the contact area between the liquid methanol and the pervaporation membrane (the cell is placed vertically) and the condensation of liquid water on the surface of the pervaporation membrane, which leads to a decrease in the methanol delivery rate. In addition, a sudden drop in the cell voltage is seen when the contact area between the liquid methanol and the pervaporation membrane becomes zero, meaning that there is no more methanol supply. The fuel efficiency is obtained from the equation: h ¼ Dmrea;MeOH DmMeOH ; (6) where Dmrea,MeOH is the methanol consumed by the anode reaction and DmMeOH is the total methanol supplied [24,25]. The fuel efficiency increases from 41.8% to 44.6% and 48.6% when the dual-layer MPL is used at the cathode and the anode, respectively. As Fig. 11. Long-term discharge behavior at the current density of 40 mA cm2. 238 X.H. Yan et al. / Journal of Power Sources 294 (2015) 232e238 discussed above, the dual-layer MPL at either side can enhance the local water concentration at the anode CL, which leads to a lower methanol crossover rate and hence a larger fuel efficiency. 5. Concluding remarks In the present work, we have demonstrated that the hydrophilic-hydrophobic dual-layer MPL is effective for trapping and retaining water by taking the advantage of the disparate features of PVA and PTFE. The dual-layer MPL at the cathode can increase the cathode water concentration to facilitate the water back diffusion which supplies the required reactant for anodic MOR. Meanwhile, the dual layer MPL at the anode conserves water from the cathode at the anode catalyst layer. The effects of both result in a higher water concentration, improve the kinetics of the anodic reaction and enhance the overall cell performance. The parameters of the dual-layer MPL structure have been optimized and the modification at the anode shows more significant improvement. Acknowledgements The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. HKUST9/CRF/11G). References [1] T. Shimizu, T. Momma, M. Mohamedi, T. Osaka, S. Sarangapani, J.Power Sources 137 (2004) 277e283. [2] Z. Guo, A. Faghri, J.Power Sources 160 (2006) 1183e1194. [3] X.H. Yan, T.S. Zhao, L. An, G. Zhao, L. Zeng, Electrochim. Acta 139 (2014) 7e12. [4] J. Zheng, Q. He, N. Gao, T. Yuan, S. Zhang, H. Yang, J.Power Sources 261 (2014) 38e45. [5] B. Yang, A. Manthiram, Electrochem. Commun. 6 (2004) 231e236. [6] T. Yamaguchi, H. Zhou, S. Nakazawa, N. Hara, Adv. Mater. 19 (2007) 592e596. [7] G.Q. Lu, C.Y. Wang, T.J. Yen, X. Zhang, Electrochim. Acta 49 (2004) 821e828. [8] H. Zhang, I.- Hsing, J.Power Sources 167 (2007) 450e454. [9] N. Nakagawa, K. 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