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Triple-layer proton exchange membranes based on chitosan biopolymer with reduced methanol crossover for high-performance direct methanol fuel cells application
Polymer 53 (2012) 2643e2651
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Triple-layer proton exchange membranes based on chitosan biopolymer with reduced methanol crossover for high-performance direct methanol fuel cells application Mohammad Mahdi Hasani-Sadrabadi a, b, Erfan Dashtimoghadam a, Nassir Mokarram d, Fatemeh S. Majedi b, c, Karl I. Jacob d, e, * a
Polymer Engineering and Color Technology Department, Amirkabir University of Technology, Tehran, Iran Institute of Bioengineering, Swiss Federal Institutes of Technology, Lausanne (EPFL), Lausanne, Switzerland Biomedical Engineering Department, Amirkabir University of Technology, Tehran, Iran d School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, USA e G. W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, USA b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 January 2012 Received in revised form 22 March 2012 Accepted 26 March 2012 Available online 2 April 2012
A novel triple-layer proton exchange membrane comprising two thin layers of structurally modified chitosan, as methanol barrier layers, both sides coated with NafionÒ105 is prepared and tested for highperformance direct methanol fuel cell applications. A tight adherence is detected between layers from SEM and EDX data for the cross-sectional area of the newly designed membrane, which are attributed to high affinity of opposite charged polyelectrolyte layers. Proton conductivity and methanol permeability measurements show improved transport properties for the multi-layer membrane compared to NafionÒ117 with approximately the same thickness. Moreover, direct methanol fuel cell tests reveal higher open circuit voltage, power density output, and overall fuel cell efficiency for the triple-layer membrane than NafionÒ117, especially at concentrated methanol solutions. A power output of 68.10 mW cm2 at 5 M methanol feed is supplied using multi-layer membrane, which is found to be about 72% more than that of for NafionÒ117. In addition, fuel cell efficiency for multi-layer membrane is measured about 19.55% and 18.45% at 1 and 5 M methanol concentrations, respectively. Owing to the ability to provide high power output, significantly reduced methanol crossover, ease of preparation and low cost, the triple-layer membrane under study could be considered as a promising polyelectrolyte for high-performance direct methanol fuel cell applications. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Structurally modified chitosan Proton exchange membrane Direct methanol fuel cell
1. Introduction Over the past years, fuel cells have attracted growing attention in both industry and academia. Fuel cells, as eco-friendly electrical energy generation systems, are able to directly convert chemical energy to electricity via redox reactions with reduced adverse environmental impacts. Among different types of fuel cells, direct methanol fuel cells (DMFCs), owing to their capability of providing high energy density, simplified system design and convenient fuel storage have been proposed as a means of energy supply for future portable electronic devices and replacement for lithium-ion batteries [1]. * Corresponding author. School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0295, USA. Tel.: þ1 404 894 2541; fax: þ1 404 894 8780. E-mail address: [email protected] (K.I. Jacob). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.03.052
The interior part of DMFCs is a proton exchange membrane (PEM), which is usually considered as the most important and often performance-limiting component [2]. The role of PEMs, which are composed of electron nonconductive materials, is to transfer newly formed protons from anode to cathode side of the cell. For such an application, an ideal PEM should fulfill a number of requirements including high proton conductivity, long-term chemical and hydrolytic durability under heated and humidified conditions, and low methanol crossover [3]. Perfluorinated ionomers such as NafionÒ, with fluoroalkyl ether side chains and sulphonic acid end groups on TeflonÔ-like backbones, have been the most commonly used PEMs so far. Although NafionÒ-based membranes are the dominant PEMs for hydrogen fueled fuel cells, but relatively high methanol permeability has hindered their wide-spread utilization in DMFCs [4]. The permeation of unreacted methanol across PEMs hinders oxygen reduction at cathode and reduces the efficiency of the
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DMFC significantly [5,6]. To alleviate such a problem, methanol is usually delivered to the anode as a dilute solution and a relatively thick NafionÒ membranes (NafionÒ117, 1100 EW and 178 mm thickness) are utilized. However, low energy density is provided at dilute methanol feeds and thick NafionÒ membranes increase the resistive losses of the cell [7]. In this regard, numerous researches have devoted considerable time to develop alternative materials for NafionÒ membranes [7e17]. Using the existing NafionÒ membranes, another approach is to reduce methanol permeability is to coat them with methanol barrier layers [18e20] or with layer-by-layer self-assembly thin polyelectrolyte films [21], and incorporation of inorganic particles into NafionÒ matrices [22e24]. Until now, a variety of NafionÒ-based multi-layer membranes have been evaluated to achieve suppressed methanol crossover, such as a triple-layer membrane composed of poly(vinylalcohol)/ poly(styrene sulfonic acid) blend coated on both sides of thin NafionÒ layers [25], casting a mixture of polyvinyl alcohol and NafionÒ on the NafionÒ [26], polybenzimidazol and NafionÒ117 multi-layer membrane [27], NafionÒ-sulfonated polyimide-NafionÒ [28] and NafionÒ-sulfonated poly(ether ether ketone)-NafionÒ [29]. Our group also recently developed new class of nanofiber based multi-layer membranes for fuel cells applications [30,31]. Recently, chitosan biopolymer has attracted considerable attention as a PEM material [17e19,32e35]. Such an interest in chitosan have arisen from its unique features of high hydrophilicity, facile chemical modification, good chemical and thermal resistance, convenient film forming properties and low price [17]. Our objective in the current research is to design and investigate a triple-layer membrane composed of NafionÒ105 sandwiched between two thin structurally modified chitosan barrier layers as a novel proton exchange membrane for high-performance DMFC applications. In fact, the choice of chitosan as the barrier layer was inspired due to the presence of several amino and hydroxyl groups
on its chains, which can interact with sulfonate groups in NafionÒ structure and provide firm interfaces. 2. Experimental 2.1. Materials High molecular weight chitosan was acquired from Aldrich. Methanol, acetic acid, glutaraldehyde (GA, 25 wt% solution in water), sulfosuccinic acid (SSA, 70 wt% solution in water) were all obtained from Merck and used as received. NafionÒ105, NafionÒ115, and NafionÒ117 membranes were purchased from E.I. DuPont de Nemours Company and were used as core layer and DMFC reference membrane, respectively. Deionized water (purified with Millipore) was used in the current work. 2.2. Membrane preparation NafionÒ membranes were treated as follows to be H-formed: Firstly, boiled in 3 vol% hydrogen peroxide for an hour, washed exhaustively with deionized water and boiled for another hour in deionized water. Then, the membranes were boiled in dilute sulfuric acid solution for an hour and finally, washed several times with deionized water. A 2 wt % aqueous solution of chitosan was prepared through dissolving 2 g of polymer in 100 ml of deionized water containing 2 ml of acetic acid. Aqueous GA/SSA (2/12 wt% of chitosan) solution, as crosslinking agent, was mixed with chitosan solution and stirred for at least 12 h at 25 C to provide a homogeneous solution. The resultant homogeneous mixture was then cast onto surface of the treated Nafion membranes and dried in an oven at 50 C for 12 h. The same procedure carried out to coat another side of the NafionÒ membranes. Finally, prepared triple-layer membranes were washed repeatedly with deionized water until neutral.
Scheme 1. Presumptive representation of proton and methanol migration through CGS-12/NafionÒ/CGS-12 triple-layer membrane.
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2.3. Preparation of membrane electrode assembly (MEA)
3. Results and discussion
The electrodes were prepared via catalyst painting technique [11e19]. Pt-black and Pt/Ru black were used as catalysts for anode cathode, respectively. Catalysts were first mixed with NafionÒ solution and several drops of glycerol (as suspension/painting agent). Then, the suspension was brushed directly (4 mg cm2) onto dry membranes, and hot-pressed at 100 C for 90 s to increase the contact area between the catalyst layer and membranes. Finally, the newly prepared MEAs were boiled in a dilute acidic solution and washed several times with deionized water.
Chitosan, owing to its distinct advantages of excellent water to alcohol selectivity and ion-conductivity properties, has been studied for application in the field of ion exchange membranes. In this regard, a series of proton exchange membranes was prepared based on crosslinked chitosan and evaluated as polyelectrolytes for DMFC applications [17,18]. It has been shown that proposed binary crosslinking agent comprising sulfosuccinic acid (SSA) and glutaraldehyde (GA) for structural modification of chitosan results in promising ionomers. Based on consistent results obtained from membrane selectivity parameter, water/methanol swelling behavior, proton conductivity and activation energy measurements,
2.4. Characterization The cross-sectional area of multi-layer membrane was studied via scanning electron microscopy (Vega Tescan SEM operating typically at 20 kV). Dried samples were then dipped and fractured in liquid nitrogen and coated with gold using a Polaron sputtercoater. In addition, the distribution of various elements was studied by using of an energy-dispersive X-ray spectrophotometer (EDX) (Oxford, INCA 200). Proton conductivity measurements were performed via BT-112 Conductivity Cell, BekkTech LLC, which has been designed for an in-plane, 4-point probe measurement to obtain bulk conductivity of membranes. The cell was set into a humidity chamber at 95% RH and 25 C, for at least 3 h before conductivity measurements of fully hydrated membranes were made. To get an insight into temperature dependency of proton conductivity, conductivity measurements were also performed on conductivity at higher temperatures up to 90 C. Methanol permeability measurements were carried out using a two-compartment glass diffusion cell. One side of the diffusion cell (cell A) contained methanol solution and the other side (cell B) was filled with pure water. Both solutions were continuously stirred to ensure homogeneity. Then, the concentration of the methanol in cell B was measured by gas chromatography method, and methanol permeability was calculated using following equation:
CBðtÞ ¼
A DK Cðt t0 Þ VB L
(1)
where, CB(t), DK (¼P), C, VB, A and L are methanol concentration in cell B (in mol L1), methanol permeability (in cm2 s1), methanol concentration in cell A (in mol L1), diffusion reservoir volume (in cm3), membrane area (in cm2) and membrane thickness (in cm), respectively. Methanol permeability of triple-layer membrane was also evaluated at elevated temperatures and compared with NafionÒ117. DMFC performance of fabricated membranes was investigated using a laboratory made DMFC single cell. The cell was composed of four 316 stainless steel (for end plates and flow fields), two carbon papers (gas diffusion layers, GDL, TGP-H-120 Toray) and the membrane electrode assembly. Silicon rubber sheets were used to seal the internal sections. Fuel cell performance evaluations were performed at two methanol concentrations of 1 and 5 M. Methanol was fed to anode side at 20 psi back pressure for 1 h, and air was introduced to cathode side with a gradual pressure increase up to 20 psi. Cell was allowed to run for half an hour before collecting polarization data. All single cell tests were conducted three times, and the results were presented as mean values. For methanol crossover measurements, humidified nitrogen was fed to cathode side at 70 C, and the fuel cell was performed until a limiting current occurs. In fact, the limiting current obtained at the open circuit condition indicates oxidation current of methanol crossover from anode to cathode.
Fig. 1. (a) SEM micrograph of cross-sectional area of the CGS-12/NafionÒ105/CGS-12 multi-layer membrane (b) and its energy dispersive spectrum along thickness.
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optimum composition of crosslinking agent turned out to be 12 and 2 wt% for SSA and GA, respectively and corresponding membrane was assigned as CGS-12 [18]. A proton conductivity of 0.0452 S cm1 and relatively low methanol permeability of 9.6 107 cm2 s1 was measured for CGS-12 membrane, which resulted in an improved membrane selectivity of about 47,100 compared to 40,500 for NafionÒ117. In addition, other favorable properties such as convenient preparation, thermal stability, low cost and recycling potential, were reported for CGS-12 membranes. Nonetheless, relatively low proton conductivity of CGS-12 propelled us to design a novel high-performance multi-layer PEM comprising CGS-12 as fuel barrier layers. Hence, NafionÒ105 owing to its higher proton conductivity compared to other types of available NafionÒ-based membranes was selected as the core layer. A presumptive representation of proton and methanol migration through the new triplelayer membrane has been schematically shown in Scheme 1. SEM micrograph and energy dispersive spectrum of crosssectional area of the multi-layer membrane has been shown in Fig. 1. A triple-layer structure including a core NafionÒ105 layer with thickness of 100 mm sandwiched between two crosslinked
chitosan barrier layers of 25 mm on its top and bottom is clearly seen in Fig. 1(a). No visible crack or voids indicates tight interfacial bonding between crosslinked chitosan layers and NafionÒ substrate. In other words, because of strong adherence between layers, no real delamination is observed at interfaces even after two weeks immersion of the membrane sample in aqueous environment. Indeed, such firm interfaces originates from possible hydrogen bondings as well as electrostatic interactions between sulfonate groups of NafionÒ and amino/hydroxyl groups on chitosan chains. Fig. 1(b) shows energy dispersive spectrum of the membrane along its thickness. As seen, gold, sulfur, fluorine, and carbon elements were detected through freeze-fractured cross-section of the triple-layer structure. EDX spectrum is again confirms good affinity between layers. The presence of sulfur at chitosan layers is attributed to SSA molecules, which have been incorporated into chitosan matrices as proton conducting and physical crosslinking agents [17]. EDX mapping of various elements across multi-layer membrane has been shown in Fig. 2. The intensity of the white dots
Fig. 2. Energy dispersive spectrum of CGS-12/NafionÒ105/CGS-12 triple-layer membrane (a) EDX mapping of (b) C (carbon) (c) S (sulfur) and (d) F (fluorine). The intensity of the white dots qualitatively indicates the amount of the specified elements.
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qualitatively indicates the amount of the specified elements. As seen in Fig. 2(b) and (d), carbon abound in chitosan layer as a carbohydrate biopolymer whereas fluorine is just detected in NafionÒ layer. Fig. 2(c) exhibit scattering of sulfur element through the triple-layer structure, which qualitatively propose a homogenous ionomeric structure. However, sulfur containing SSA molecules are not the only proton conducting groups in structurally modified chitosan layers. Proton conductivity of the multi-layer membranes has been compared with CGS-12 and NafionÒ117 membranes at different temperatures in Fig. 3a. A higher proton conductivity value of 0.088 S cm1 was measured for CGS-12/NafionÒ105/CGS-12 compared to 0.0452 S cm1 and 0.081 S cm1 for CGS-12 and NafionÒ117 at 25 C, respectively. It was found that proton conductivity for all membranes is increased with increasing temperatures and reaches to 0.1 S cm1, 0.143 S cm1, and 0.1635 S cm1 at 90 C for CGS-12, NafionÒ117, and multi-layer membranes respectively. Moreover, conductivity plots indicate superiority of multi-layer membrane over NafionÒ117 at higher temperatures, which might be associated with capability of triplelayer design as well as intrinsic resistance of chitosan-based CGS network against dehydration to retain water in core NafionÒ ionomeric matrix.
Fig. 3. (a) Proton conductivity of CGS-12/NafionÒ105/CGS-12 and CGS-12/NafionÒ115/ CGS-12 membranes compared to CGS-12 and NafionÒ105, NafionÒ117 membranes at different temperatures, and (b) their Arrhenius plot of conductivity.
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To get an insight into conductivity-temperature relationship of membranes, conductivity results at different temperatures were also investigated based on Arrhenius equation (s¼s0 exp(Ea/RT)), where s, s0, Ea, R and T are the proton conductivity, pre-exponential parameter, activation energy of proton migration through the sample, universal gas constant and the absolute temperature, respectively [17]. From Arrhenius plots of conductivity displayed in Fig. 3b, it can be found that all membranes exhibit positive temperature-conductivity dependency, which implies a thermally activated process for proton migration. Furthermore, activation energy for proton conduction were calculated based on Arrhenius equation as 9.0 kJ mol1, 7.0 kJ mol1, 8.8 kJ mol1, and 10.5 kJ mol1 for multi-layer, NafionÒ105, NafionÒ117, and CGS-12 membranes, respectively. As known, both proton conduction mechanisms (proton hopping and vehicle mechanism) have been accepted to exist for NafionÒ-based membranes [35], close activation energies of the triple-layer membrane and NafionÒ could be considered as another sign for suitable affinity of layers which has resulted in homogenous proton migration pathways. Interestingly, it was found that activation energy of multi-layer membrane we studied is related to the activation energy and thickness of isolated layers. It has been well understood that methanol permeation across proton exchange membranes in DMFC applications results in reduced overall cell performance in terms of fuel loss and catalyst poisoning at cathode [20]. So, to obtain permeability properties of designed triple-layer membrane at different temperatures, methanol permeation measurements were carried out in the temperature range of 25 Ce80 C. Fig. 4(a) provides a comparison between methanol permeability of multi-layer and NafionÒ117 membranes. As seen, with increasing temperature from 25 C to 80 C, methanol permeability is increased from 2.52 107 cm2 s1 to 7.74 107 cm2 s1 for CGS-12/NafionÒ105/CGS-12. Although methanol permeability of both membranes is increased with increasing temperature, but the destructive influence of temperature on methanol permeation rate is more considerable for NafionÒ117. Actually, permeability of triple-layer membrane was found to be remarkably 87e89% lower than NafionÒ117 in the whole temperature range. It should be noticed that methanol permeability of NafionÒ105 membrane is about 3.01 106 cm2 s1 at 25 C and higher than NafionÒ117 membrane. Methanol permeability of CGS-12/NafionÒ115/CGS-12 is also characterized and shown in Fig. 4. The permeability of membranes based on NafionÒ115 is in the same order compared with multi-layer membranes based on NafionÒ105. Interestingly, it was found that permeability as a function of temperature is consistent with the Arrhenius equation. So, methanol permeability results at different temperatures were investigated based on P¼P0 exp(Ea,MeOH/RT) equation, wherein P, P0 and Ea,MeOH are methanol permeability, pre-exponential factors and corresponding activation energy, respectively (Fig. 4(b)) [13]. Accordingly, the activation energy of methanol permeability for CGS-12/NafionÒ105/CGS-12 and NafionÒ117 membranes were calculated as 19.0 and 17.4 kJ mol1, respectively. Higher methanol permeation activation energy for laminated membrane again indicates their lower permeability owing to crosslinked chitosan barrier layers. It has been shown that overall electrochemical performance of a PEM at DMFC operational conditions is concurrently influenced by both conductivity and permeability properties. Usually, the higher membrane selectivity (ratio of proton conductivity to methanol permeability) provides a higher cell performance, especially at elevated methanol concentrations [11e19]. Hence, as both transport properties of CGS-12/NafionÒ105/CGS-12 membrane were found to have increased with increasing temperature,
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Fig. 5. Temperature dependency of selectivity parameter for multi-layer membrane compared to NafionÒ117 in temperature range of 25e80 C.
Fig. 4. (a) Methanol permeability of CGS-12/NafionÒ105/CGS-12 and CGS-12/ NafionÒ115/CGS-12 membranes compared to NafionÒ117 at different temperatures, and (b) corresponding Arrhenius plots.
selectivity parameter was used here to evaluate the performance of fabricated membrane at different temperatures. Temperature dependency of membrane selectivity parameter for multi-layer as well as NafionÒ117 membranes has been shown in Fig. 5. As seen, multi-layer membrane is shown to be significantly more selective than NafionÒ117 in the whole temperature range. The maximum selectivity was similarly achieved at room temperature. This behavior originates from the greater effect of increasing temperature on methanol permeability compared to its influence on proton conductivity as previously reported in the same trends for different membranes [13,19]. Due to lower selectivity of NafionÒ105 membrane which is about 34,400 S s cm3 in comparison with 40,400 S s cm3 for NafionÒ117 all of electrochemical characterizations were performed using NafionÒ117 as a reference membrane. Moreover, it should be noticed that due to dependency of electrochemical properties on resistance and thus on membrane thickness, it is more reliable to perform this type of tests for membranes with the same thickness. So for electrochemical
evaluations, we select CGS-12/NafionÒ105/CGS-12, NafionÒ117 as well as CGS-12 membranes with approximately the same thickness in the range of 150e170 mm. To investigate the effect of methanol crossover on performance of the multi-layer membrane at operational conditions, methanol crossover current density, IC, was calculated according to the relation IC ¼ IC,OC (1I/IL), where IC,OC, I and IL are methanol crossover current density at open circuit, operating current density and anode mass transport limiting current density, respectively all in mA cm2 [32]. The results of methanol crossover current density measurements at open circuit condition and limiting current density for fabricated MEAs based on multi-layer membrane, CGS-12 and NafionÒ117 have been summarized in Table 1. As shown, limiting current density results is consistent with methanol permeability properties observed for these membranes. The results of methanol crossover measurements for multi-layer membrane in comparison with NafionÒ117 and CGS-12 at 1 and 5M methanol concentrations have been exhibited in Fig. 6. Overall, it could be found that introducing crosslinked chitosan barrier layers onto NafionÒ membrane has significantly reduced its methanol permeation rate, especially at higher methanol concentration. Direct methanol-air single fuel cell performance tests were performed at two different methanol concentrations of 1 and 5 M, and obtained polarization curves of power density versus current density at 70 C for the multi-layer membrane, which are shown and compared with NafionÒ117 and CGS-12 membranes in Fig. 7. Open circuit voltage (OCV) was measured as 0.705, 0.663, and 0.686
Table 1 Methanol crossover current density at open circuit condition and limiting current density for MEAs comprising NafionÒ117, CGS-12 and CGS-12/NafionÒ105/CGS-12 membranes at 1 and 5 M methanol concentrations and 70 C. Sample
Methanol concentration (M)
Crossover current (mA/cm2)a
Limiting current (mA/cm2)b
NafionÒ117 NafionÒ117 CGS-12 CGS-12 Multi-layer Multi-layer
1.0 5.0 1.0 5.0 1.0 5.0
156 518 142 460 136 420
530 260 445 285 575 385
a Crossover current at open circuit condition obtained from methanol/nitrogen polarization curves. b Limiting current obtained from methanol/air polarization curves.
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at 1M methanol feed, and 0.651, 0.581, and 0.572 at 5 M methanol concentration for multi-layer, CGS-12 and NafionÒ117 membranes, respectively. Actually, providing high OCV in the case of triple-layer membrane, especially at higher methanol concentration clearly shows the effectiveness of the newly designed laminated structure in suppressing methanol crossover, which again supports ex situ methanol permeability and methanol crossover results discussed before. As shown in Fig. 7, a maximum power density output of 68.10 mW cm2 was measured for fabricated multi-layer membrane in comparison with 39.45 mW cm2 for NafionÒ117 and 36.54 mW cm2 for CGS-12 at methanol concentration of 5M. Actually, 72% more power output supplied by CGS-12/NafionÒ105/ CGS-12 membrane compared to commercial NafionÒ117 reveals electrochemical performance superiority of newly tested triplelayer membrane. In other words, drastically reduced methanol crossover through multi-layer membrane is clearly reflected in supplying higher power output at concentrated 5 M methanol feed, which is preferred from an application point of view. The newly designed membranes were characterized based on their electro-osmotic drag coefficient for methanol transport based on polarization curves and according to a reported method by Moll and Compa [36]. Essentially we have V (I, C) ¼ EA1 ln(I/I0) A2CA3I, where, A1 the sum of the slopes of the polarization curves
Fig. 7. Polarization curves of methanol-air single fuel cells consisted of CGS-12/ NafionÒ105/CGS-12, NafionÒ117 and CGS-12 at 70 C using (a) 1 M, and (b) 5 M methanol solutions.
for anode and cathode, A2 is a term relating the overvoltage to the methanol crossover by diffusion, A3 is a term relating the overvoltage influenced by the sum of the protonic resistance and the methanol electro-osmotic effects. Based on curve fitting of this equation (R2 > 0.996) on VeI curves A3 can be calculated. By knowing the membrane thickness clamped between the anode and cathode electrode layers (L) and conductivity of the membrane (s) and according to A3 ¼ L/sþcJeos, cJeos can be determined as an electro-osmotic drag of methanol across the membrane.
Fig. 6. Methanol crossover results of CGS-12/NafionÒ105/CGS-12 compared to NafionÒ117 as well as CGS-12 membrane at (a) 1 M (a) and (b) 5 M methanol concentrations.
Fig. 8. _cJeos (Vcm2 mA1) the electro-osmotic diffusion of methanol across of CGS-12/ NafionÒ105/CGS-12, NafionÒ117 and CGS-12 membranes at two methanol concentrations of 1 and 5 M.
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As shown in Fig. 8, and as expected from polarization curves triple-layer membrane exhibit the same methanol crossover by electro-osmosis at low methanol concentration as Nafion117 but this membrane shows the lower electro-osmotic drag of methanol at elevated methanol concentration in comparison with Nafion117. The pure CGS-12 membrane has a lowest coefficient due to its impermeable structure; but due to its low proton conductivity (high protonic resistance) and as a result of its selectivity, the overall performance of this type of pure membrane is poor and it is not suitable in a practical point of view. To confirm strong interface of Nafion-CGS-12 indirectly, the durability test is performed to check the stability of this triple-layer membrane in operational conditions. Presence or creation of any crack at the interfacial region can affect in performanceetime behavior and result in falling output voltage which is not happening in our case as shown in Fig. 9. In order to evaluate the long-term stability and performance at fuel cell operational conditions, the endurance testing of the fabricated MEAs comprising CGS-12/NafionÒ105/CGS-12 membranes was performed using 1 M and 5 M methanol solution at 70 C under fuel circulation for 100 h. The output voltage at 200 mA cm2 of the single DMFCs was measured and the results obtained are displayed in Fig. 9. As shown, during the first 10 h, voltage is commonly increased. Such an observation could be attributed to the electro-catalyst activation and decreasing internal resistance of the cell. Fuel efficiency of cells, hFuel, were also calculated using methanol crossover results using the following equation: hFuel ¼ I/(IþIC). Thermodynamic efficiency, hTherm, is defined as the ratio of Gibbs free energy change per mole of methanol to enthalpy change per mole of methanol. Voltage efficiency of fuel cell, hVolt, is determined as ratio of the real operating voltage to theoretical maximum voltage of fuel cell. Accordingly, real efficiency of fuel cell, hReal, is obtained the combination of fuel, thermodynamic, and voltage efficiency values: hReal ¼ hFuel. hTherm. hVolt [22]. The results of real efficiency assessment of fuel cell at two different methanol concentrations for CGS-12/NafionÒ105/CGS-12, CGS-12, and NafionÒ117 membranes have been displayed in Fig. 10. As shown, the maximum cell efficiency was decreased from 17.23% to 10.9% for NafionÒ117 membranes when the methanol concentration was increased from 1 to 5 M. Corresponding increase in the methanol content for the CGS-12 membranes showed a decrease in fuel cell
Fig. 10. A comparison between real fuel cell efficiency of CGS-12/NafionÒ105/CGS-12, NafionÒ117 and CGS-12 membranes at two methanol concentrations of (a) 1 and (b) 5 M.
efficiency from 10.82% to 8.88%, while the current density at the maximum efficiency is also decreased. However, lower overall efficiency reduction from 19.55% to 18.45% is observed with increasing methanol concentration from 1 to 5 M for triple-layer membrane, which again emphasize its electrochemical superiority for high-performance DMFC applications. Thus, a higher cell efficiency and a larger current density range were obtained for the triple-layer membrane compared to other membranes we studied.
4. Conclusion
Fig. 9. Endurance testing (cell potential at the constant current condition) of CGS-12/ NafionÒ105/CGS-12, NafionÒ117 and CGS-12 membranes using two methanol concentrations of 1 and 5 M at 70 C.
A novel high-performance triple-layer proton exchange membrane comprising two structurally modified chitosan layers coated onto NafionÒ105 sides was designed and prepared in the laboratory. Investigating the cross-sectional area of fabricated multi-layer membrane using SEM and EDX techniques showed high affinity and firm interfaces between layers. The results of proton conductivity and methanol permeability measurements revealed a remarkably reduced methanol crossover and significantly higher membrane selectivity parameter for fabricated multi-layer membrane compared to NafionÒ117. From electrochemical characterization of CGS-12/NafionÒ105/CGS-12 at DMFC operational condition, its ability to provide over 72% higher power output than
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NafionÒ117 was found at 5M methanol concentration, which is promising from an application point of view. Moreover, fuel cell efficiency for multi-layer membrane was measured as 19.55% and 18.45% at 1M and 5 M methanol concentration, respectively. Owing to the favorable properties resulting from suppressed methanol crossover, low activation energy, ease of preparation and low cost, the triple-layer proton exchange membrane we studied could be considered as a promising polyelectrolyte for high-performance DMFC applications.
References [1] Winter M, Brodd RJ. What are batteries, fuel cells, and supercapacitors? Chem Rev 2004;104:4245e70. [2] Haile SM. Fuel cell materials and components. Acta Mater 2003;51: 5981e6000. [3] Kerres JA. Development of ionomer membranes for fuel cells. J Membr Sci 2001;185:3e27. [4] Neburchilov V, Martin J, Wang H, Zhang J. A review of polymer electrolyte membranes for direct methanol fuel cells. J Power Sources 2007;169:221e38. [5] Argun AA, Ashcraft JN, Hammond PT. Highly conductive, methanol resistant polyelectrolyte multilayers. Adv Mater 2008;20:1539e43. [6] Kerres JA. Blended and cross-linked ionomer membranes for application in membrane fuel cells. Fuel Cells 2005;5:230e47. [7] Hickner MA, Ghassemi H, Seung Kim Y, Einsla BR, McGrath JE. Alternative polymer systems for proton exchange membranes (PEMs). Chem Rev 2004; 104:4587e612. [8] Kreuer KD. On the development of proton conducting polymer membranes for hydrogen and methanol fuel cells. J Membr Sci 2001;185:29e39. [9] Smitha B, Sridhar S, Khan AA. Solid polymer electrolyte membranes for fuel cell applicationsda review. J Membr Sci 2005;259:10e26. [10] Ahmad H, Kamarudin SK, Hasran UA, Daud WRW. Overview of hybrid membranes for direct-methanol fuel-cell applications. Int J Hydrogen Energy 2010;35:2160e75. [11] Hasani-Sadrabadi MM, Emami SH, Ghaffarian R, Moaddel H. Nanocomposite membranes made from sulfonated poly (ether ether ketone) and montmorillonite clay for fuel cells application. Energy Fuels 2008;22:2539e42. [12] Hasani-Sadrabadi MM, Emami SH, Moaddel H. Preparation and characterization of nanocomposite membranes made of poly (2,6-dimethyl-1,4-phenylene oxide) and montmorillonite for direct methanol fuel cells. J Power Sources 2008;183:551e6. [13] Hasani-Sadrabadi MM, Dashtimoghadam E, Sarikhani K, Majedi FS, Khanbabaei G. Electrochemical investigation of sulfonated poly (ether ether ketone)/clay nanocomposite membranes for moderate temperature fuel cell applications. J Power Sources 2010;195:2450e6. [14] Hasani-Sadrabadi MM, Ghaffarian SR, Mokarram N, Dashtimoghadam E, Majedi FS. Characterization of nanohybrid membranes for direct methanol fuel cell. Solid State Ion 2009;180:1497e504. [15] Hasani-Sadrabadi MM, Mokarram N, Ghaffarian SR, Dashtimoghadam E, Sarikhani K, Majedi FS. Investigation the effects of nanoclay addition on the transport and electrochemical performances of proton conducting membranes. J Appl Polym Sci 2010;117:1227e33. [16] Hasani-Sadrabadi MM, Dashtimoghadam E, Ghaffarian SR, Sadrabadi MHH, Heidari M, Moaddel H. High-performance proton exchange nanocomposite membranes based on poly (ether sulfone). Renewable Energy 2010;35:226e31.
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[17] Dashtimoghadam E, Hasani-Sadrabadi MM, Moaddel H. Modification of chitosan biopolymer as a novel polyelectrolyte membrane for green power generation. Polym Adv Tech 2010;21:726e34. [18] Hasani-Sadrabadi MM, Dashtimoghadam E, Majedi FS. Investigation of a double layer membrane for direct methanol fuel cell applications. ECS Trans 2009;25:1709e17. [19] Hasani-Sadrabadi MM, Dashtimoghadam E, Majedi FS, Emami SH, Moaddel H. A high-performance chitosan-based double layer proton exchange membrane with reduced methanol crossover. Int J Hydrogen Energy 2011;36:6105e11. [20] Yang B, Manthiram A. Multilayered membranes with suppressed fuel crossover for direct methanol fuel cells. Electrochem Commun 2004;6:231e6. [21] Jiang SP, Liu Z, Tian ZQ. Layer-by-layer self-assembly of composite polyelectrolyte/Nafion membrane for direct methanol fuel cells. Adv Mater 2006; 18:1068e72. [22] Hasani-Sadrabadi MM, Mokarram N, Azami M, Dashtimoghadam E, Majedi FS, Jacob KI. Preparation and characterization of polyelectrolyte nanocomposite membranes based on NafionÒ ionomer and nanocrystalline hydroxyapatite. Polymer 2011;52:1286e96. [23] Hasani-Sadrabadi MM, Dashtimoghadam E, Majedi FS, Kabiri K. Nafion/ biofunctionalized montmorillonite nanohybrids as novel polyelectrolyte membranes for direct methanol fuel cells. J Power Sources 2009;190:318e21. [24] Hasani-Sadrabadi MM, Dashtimoghadam E, Majedi FS, Kabiri K, Mokarram N, Solati-Hashjin M, et al. Novel high-performance nanohybrid polyelectrolyte membranes based on bio-functionalized montmoriloniote for fuel cells applications. Chem Commun 2010;46:6500e2 [F. De Martino, N. Vatistas, V. Tricoli, Concept to design membranes for pemfc: Triple-layer ion-conducting membrane, J Electrochem Soc 156(2009) B59eB65]. [25] Shao ZG, Wang X, Hsing IM. Composite Nafion/polyvinyl alcohol membranes for the direct methanol fuel cell. J Membr Sci 2002;210:147e53. [26] Hobson LJ, Nakano Y, Ozu H, Hayase S. Targeting improved DMFC performance. J Power Sources 2002;104:79e84. [27] Wang L, Yi BL, Zhang HM, Liu YH, Xing DM, Shao ZG, et al. Novel multilayer Nafion/SPI/Nafion composite membrane for PEMFCs. J Power Sources 2007; 164:80e5. [28] Ren SZ, Li CN, Zhao XS, Wu ZM, Wang SL, Sun GQ, et al. Surface modification of sulfonated poly(ether ether ketone) membranes using Nafion solution for direct methanol fuel cells. J Membr Sci 2005;247:59e63. [29] Hasani-Sadrabadi MM, Shabani I, Soleimani M. Novel nanofiber-based triple layer proton exchange membranes for fuel cell applications. J Power Sources 2011;196:4599e603. [30] Shabani I, Hasani-Sadrabadi MM, Haddadi V, Soleimani M. Nanofiber-based polyelectrolytes as novel membranes for fuel cell applications. J Membr Sci 2011;368:233e40. [31] Mukoma P, Jooste BR, Vosloo HCM. A comparison of methanol permeability in Chitosan and Nafion 117 membranes at high to medium methanol concentrations. J Membr Sci 2004;243:293e9. [32] Soontarapa K, Intra U. Chitosan-based fuel cell membranes. Chem Eng Commun 2006;193:855e68. [33] Cui Z, Liu C, Lu T, Xing W. Polyelectrolyte complexes of chitosan and phosphotungstic acid as proton-conducting membranes for direct methanol fuel cells. J Power Sources 2007;167:94e9. [34] Smitha B, Sridhar S, Khan AA. Polyelectrolyte complexes of chitosan and poly(acrylic acid) as proton exchange membranes for fuel cells. Macromolecules 2004;37:2233e9. [35] Hasani-Sadrabadi MM, Dashtimoghadam E, Majedi FS, Kabiri K, SolatiHashjin M, Moaddel H. Novel nanocomposite proton exchange membranes based on NafionÒ and AMPS-modified montmorillonite for fuel cell applications. J Membr Sci 2010;365:286e93. [36] Mollá S, Compan V. Performance of composite Nafion/PVA membranes for direct methanol fuel cells. Power Sources 2011;196:2699e708.
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Polymer journal homepage: www.elsevier.com/locate/polymer
Triple-layer proton exchange membranes based on chitosan biopolymer with reduced methanol crossover for high-performance direct methanol fuel cells application Mohammad Mahdi Hasani-Sadrabadi a, b, Erfan Dashtimoghadam a, Nassir Mokarram d, Fatemeh S. Majedi b, c, Karl I. Jacob d, e, * a
Polymer Engineering and Color Technology Department, Amirkabir University of Technology, Tehran, Iran Institute of Bioengineering, Swiss Federal Institutes of Technology, Lausanne (EPFL), Lausanne, Switzerland Biomedical Engineering Department, Amirkabir University of Technology, Tehran, Iran d School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, USA e G. W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, USA b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 January 2012 Received in revised form 22 March 2012 Accepted 26 March 2012 Available online 2 April 2012
A novel triple-layer proton exchange membrane comprising two thin layers of structurally modified chitosan, as methanol barrier layers, both sides coated with NafionÒ105 is prepared and tested for highperformance direct methanol fuel cell applications. A tight adherence is detected between layers from SEM and EDX data for the cross-sectional area of the newly designed membrane, which are attributed to high affinity of opposite charged polyelectrolyte layers. Proton conductivity and methanol permeability measurements show improved transport properties for the multi-layer membrane compared to NafionÒ117 with approximately the same thickness. Moreover, direct methanol fuel cell tests reveal higher open circuit voltage, power density output, and overall fuel cell efficiency for the triple-layer membrane than NafionÒ117, especially at concentrated methanol solutions. A power output of 68.10 mW cm2 at 5 M methanol feed is supplied using multi-layer membrane, which is found to be about 72% more than that of for NafionÒ117. In addition, fuel cell efficiency for multi-layer membrane is measured about 19.55% and 18.45% at 1 and 5 M methanol concentrations, respectively. Owing to the ability to provide high power output, significantly reduced methanol crossover, ease of preparation and low cost, the triple-layer membrane under study could be considered as a promising polyelectrolyte for high-performance direct methanol fuel cell applications. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Structurally modified chitosan Proton exchange membrane Direct methanol fuel cell
1. Introduction Over the past years, fuel cells have attracted growing attention in both industry and academia. Fuel cells, as eco-friendly electrical energy generation systems, are able to directly convert chemical energy to electricity via redox reactions with reduced adverse environmental impacts. Among different types of fuel cells, direct methanol fuel cells (DMFCs), owing to their capability of providing high energy density, simplified system design and convenient fuel storage have been proposed as a means of energy supply for future portable electronic devices and replacement for lithium-ion batteries [1]. * Corresponding author. School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0295, USA. Tel.: þ1 404 894 2541; fax: þ1 404 894 8780. E-mail address: [email protected] (K.I. Jacob). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.03.052
The interior part of DMFCs is a proton exchange membrane (PEM), which is usually considered as the most important and often performance-limiting component [2]. The role of PEMs, which are composed of electron nonconductive materials, is to transfer newly formed protons from anode to cathode side of the cell. For such an application, an ideal PEM should fulfill a number of requirements including high proton conductivity, long-term chemical and hydrolytic durability under heated and humidified conditions, and low methanol crossover [3]. Perfluorinated ionomers such as NafionÒ, with fluoroalkyl ether side chains and sulphonic acid end groups on TeflonÔ-like backbones, have been the most commonly used PEMs so far. Although NafionÒ-based membranes are the dominant PEMs for hydrogen fueled fuel cells, but relatively high methanol permeability has hindered their wide-spread utilization in DMFCs [4]. The permeation of unreacted methanol across PEMs hinders oxygen reduction at cathode and reduces the efficiency of the
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DMFC significantly [5,6]. To alleviate such a problem, methanol is usually delivered to the anode as a dilute solution and a relatively thick NafionÒ membranes (NafionÒ117, 1100 EW and 178 mm thickness) are utilized. However, low energy density is provided at dilute methanol feeds and thick NafionÒ membranes increase the resistive losses of the cell [7]. In this regard, numerous researches have devoted considerable time to develop alternative materials for NafionÒ membranes [7e17]. Using the existing NafionÒ membranes, another approach is to reduce methanol permeability is to coat them with methanol barrier layers [18e20] or with layer-by-layer self-assembly thin polyelectrolyte films [21], and incorporation of inorganic particles into NafionÒ matrices [22e24]. Until now, a variety of NafionÒ-based multi-layer membranes have been evaluated to achieve suppressed methanol crossover, such as a triple-layer membrane composed of poly(vinylalcohol)/ poly(styrene sulfonic acid) blend coated on both sides of thin NafionÒ layers [25], casting a mixture of polyvinyl alcohol and NafionÒ on the NafionÒ [26], polybenzimidazol and NafionÒ117 multi-layer membrane [27], NafionÒ-sulfonated polyimide-NafionÒ [28] and NafionÒ-sulfonated poly(ether ether ketone)-NafionÒ [29]. Our group also recently developed new class of nanofiber based multi-layer membranes for fuel cells applications [30,31]. Recently, chitosan biopolymer has attracted considerable attention as a PEM material [17e19,32e35]. Such an interest in chitosan have arisen from its unique features of high hydrophilicity, facile chemical modification, good chemical and thermal resistance, convenient film forming properties and low price [17]. Our objective in the current research is to design and investigate a triple-layer membrane composed of NafionÒ105 sandwiched between two thin structurally modified chitosan barrier layers as a novel proton exchange membrane for high-performance DMFC applications. In fact, the choice of chitosan as the barrier layer was inspired due to the presence of several amino and hydroxyl groups
on its chains, which can interact with sulfonate groups in NafionÒ structure and provide firm interfaces. 2. Experimental 2.1. Materials High molecular weight chitosan was acquired from Aldrich. Methanol, acetic acid, glutaraldehyde (GA, 25 wt% solution in water), sulfosuccinic acid (SSA, 70 wt% solution in water) were all obtained from Merck and used as received. NafionÒ105, NafionÒ115, and NafionÒ117 membranes were purchased from E.I. DuPont de Nemours Company and were used as core layer and DMFC reference membrane, respectively. Deionized water (purified with Millipore) was used in the current work. 2.2. Membrane preparation NafionÒ membranes were treated as follows to be H-formed: Firstly, boiled in 3 vol% hydrogen peroxide for an hour, washed exhaustively with deionized water and boiled for another hour in deionized water. Then, the membranes were boiled in dilute sulfuric acid solution for an hour and finally, washed several times with deionized water. A 2 wt % aqueous solution of chitosan was prepared through dissolving 2 g of polymer in 100 ml of deionized water containing 2 ml of acetic acid. Aqueous GA/SSA (2/12 wt% of chitosan) solution, as crosslinking agent, was mixed with chitosan solution and stirred for at least 12 h at 25 C to provide a homogeneous solution. The resultant homogeneous mixture was then cast onto surface of the treated Nafion membranes and dried in an oven at 50 C for 12 h. The same procedure carried out to coat another side of the NafionÒ membranes. Finally, prepared triple-layer membranes were washed repeatedly with deionized water until neutral.
Scheme 1. Presumptive representation of proton and methanol migration through CGS-12/NafionÒ/CGS-12 triple-layer membrane.
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2.3. Preparation of membrane electrode assembly (MEA)
3. Results and discussion
The electrodes were prepared via catalyst painting technique [11e19]. Pt-black and Pt/Ru black were used as catalysts for anode cathode, respectively. Catalysts were first mixed with NafionÒ solution and several drops of glycerol (as suspension/painting agent). Then, the suspension was brushed directly (4 mg cm2) onto dry membranes, and hot-pressed at 100 C for 90 s to increase the contact area between the catalyst layer and membranes. Finally, the newly prepared MEAs were boiled in a dilute acidic solution and washed several times with deionized water.
Chitosan, owing to its distinct advantages of excellent water to alcohol selectivity and ion-conductivity properties, has been studied for application in the field of ion exchange membranes. In this regard, a series of proton exchange membranes was prepared based on crosslinked chitosan and evaluated as polyelectrolytes for DMFC applications [17,18]. It has been shown that proposed binary crosslinking agent comprising sulfosuccinic acid (SSA) and glutaraldehyde (GA) for structural modification of chitosan results in promising ionomers. Based on consistent results obtained from membrane selectivity parameter, water/methanol swelling behavior, proton conductivity and activation energy measurements,
2.4. Characterization The cross-sectional area of multi-layer membrane was studied via scanning electron microscopy (Vega Tescan SEM operating typically at 20 kV). Dried samples were then dipped and fractured in liquid nitrogen and coated with gold using a Polaron sputtercoater. In addition, the distribution of various elements was studied by using of an energy-dispersive X-ray spectrophotometer (EDX) (Oxford, INCA 200). Proton conductivity measurements were performed via BT-112 Conductivity Cell, BekkTech LLC, which has been designed for an in-plane, 4-point probe measurement to obtain bulk conductivity of membranes. The cell was set into a humidity chamber at 95% RH and 25 C, for at least 3 h before conductivity measurements of fully hydrated membranes were made. To get an insight into temperature dependency of proton conductivity, conductivity measurements were also performed on conductivity at higher temperatures up to 90 C. Methanol permeability measurements were carried out using a two-compartment glass diffusion cell. One side of the diffusion cell (cell A) contained methanol solution and the other side (cell B) was filled with pure water. Both solutions were continuously stirred to ensure homogeneity. Then, the concentration of the methanol in cell B was measured by gas chromatography method, and methanol permeability was calculated using following equation:
CBðtÞ ¼
A DK Cðt t0 Þ VB L
(1)
where, CB(t), DK (¼P), C, VB, A and L are methanol concentration in cell B (in mol L1), methanol permeability (in cm2 s1), methanol concentration in cell A (in mol L1), diffusion reservoir volume (in cm3), membrane area (in cm2) and membrane thickness (in cm), respectively. Methanol permeability of triple-layer membrane was also evaluated at elevated temperatures and compared with NafionÒ117. DMFC performance of fabricated membranes was investigated using a laboratory made DMFC single cell. The cell was composed of four 316 stainless steel (for end plates and flow fields), two carbon papers (gas diffusion layers, GDL, TGP-H-120 Toray) and the membrane electrode assembly. Silicon rubber sheets were used to seal the internal sections. Fuel cell performance evaluations were performed at two methanol concentrations of 1 and 5 M. Methanol was fed to anode side at 20 psi back pressure for 1 h, and air was introduced to cathode side with a gradual pressure increase up to 20 psi. Cell was allowed to run for half an hour before collecting polarization data. All single cell tests were conducted three times, and the results were presented as mean values. For methanol crossover measurements, humidified nitrogen was fed to cathode side at 70 C, and the fuel cell was performed until a limiting current occurs. In fact, the limiting current obtained at the open circuit condition indicates oxidation current of methanol crossover from anode to cathode.
Fig. 1. (a) SEM micrograph of cross-sectional area of the CGS-12/NafionÒ105/CGS-12 multi-layer membrane (b) and its energy dispersive spectrum along thickness.
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optimum composition of crosslinking agent turned out to be 12 and 2 wt% for SSA and GA, respectively and corresponding membrane was assigned as CGS-12 [18]. A proton conductivity of 0.0452 S cm1 and relatively low methanol permeability of 9.6 107 cm2 s1 was measured for CGS-12 membrane, which resulted in an improved membrane selectivity of about 47,100 compared to 40,500 for NafionÒ117. In addition, other favorable properties such as convenient preparation, thermal stability, low cost and recycling potential, were reported for CGS-12 membranes. Nonetheless, relatively low proton conductivity of CGS-12 propelled us to design a novel high-performance multi-layer PEM comprising CGS-12 as fuel barrier layers. Hence, NafionÒ105 owing to its higher proton conductivity compared to other types of available NafionÒ-based membranes was selected as the core layer. A presumptive representation of proton and methanol migration through the new triplelayer membrane has been schematically shown in Scheme 1. SEM micrograph and energy dispersive spectrum of crosssectional area of the multi-layer membrane has been shown in Fig. 1. A triple-layer structure including a core NafionÒ105 layer with thickness of 100 mm sandwiched between two crosslinked
chitosan barrier layers of 25 mm on its top and bottom is clearly seen in Fig. 1(a). No visible crack or voids indicates tight interfacial bonding between crosslinked chitosan layers and NafionÒ substrate. In other words, because of strong adherence between layers, no real delamination is observed at interfaces even after two weeks immersion of the membrane sample in aqueous environment. Indeed, such firm interfaces originates from possible hydrogen bondings as well as electrostatic interactions between sulfonate groups of NafionÒ and amino/hydroxyl groups on chitosan chains. Fig. 1(b) shows energy dispersive spectrum of the membrane along its thickness. As seen, gold, sulfur, fluorine, and carbon elements were detected through freeze-fractured cross-section of the triple-layer structure. EDX spectrum is again confirms good affinity between layers. The presence of sulfur at chitosan layers is attributed to SSA molecules, which have been incorporated into chitosan matrices as proton conducting and physical crosslinking agents [17]. EDX mapping of various elements across multi-layer membrane has been shown in Fig. 2. The intensity of the white dots
Fig. 2. Energy dispersive spectrum of CGS-12/NafionÒ105/CGS-12 triple-layer membrane (a) EDX mapping of (b) C (carbon) (c) S (sulfur) and (d) F (fluorine). The intensity of the white dots qualitatively indicates the amount of the specified elements.
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qualitatively indicates the amount of the specified elements. As seen in Fig. 2(b) and (d), carbon abound in chitosan layer as a carbohydrate biopolymer whereas fluorine is just detected in NafionÒ layer. Fig. 2(c) exhibit scattering of sulfur element through the triple-layer structure, which qualitatively propose a homogenous ionomeric structure. However, sulfur containing SSA molecules are not the only proton conducting groups in structurally modified chitosan layers. Proton conductivity of the multi-layer membranes has been compared with CGS-12 and NafionÒ117 membranes at different temperatures in Fig. 3a. A higher proton conductivity value of 0.088 S cm1 was measured for CGS-12/NafionÒ105/CGS-12 compared to 0.0452 S cm1 and 0.081 S cm1 for CGS-12 and NafionÒ117 at 25 C, respectively. It was found that proton conductivity for all membranes is increased with increasing temperatures and reaches to 0.1 S cm1, 0.143 S cm1, and 0.1635 S cm1 at 90 C for CGS-12, NafionÒ117, and multi-layer membranes respectively. Moreover, conductivity plots indicate superiority of multi-layer membrane over NafionÒ117 at higher temperatures, which might be associated with capability of triplelayer design as well as intrinsic resistance of chitosan-based CGS network against dehydration to retain water in core NafionÒ ionomeric matrix.
Fig. 3. (a) Proton conductivity of CGS-12/NafionÒ105/CGS-12 and CGS-12/NafionÒ115/ CGS-12 membranes compared to CGS-12 and NafionÒ105, NafionÒ117 membranes at different temperatures, and (b) their Arrhenius plot of conductivity.
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To get an insight into conductivity-temperature relationship of membranes, conductivity results at different temperatures were also investigated based on Arrhenius equation (s¼s0 exp(Ea/RT)), where s, s0, Ea, R and T are the proton conductivity, pre-exponential parameter, activation energy of proton migration through the sample, universal gas constant and the absolute temperature, respectively [17]. From Arrhenius plots of conductivity displayed in Fig. 3b, it can be found that all membranes exhibit positive temperature-conductivity dependency, which implies a thermally activated process for proton migration. Furthermore, activation energy for proton conduction were calculated based on Arrhenius equation as 9.0 kJ mol1, 7.0 kJ mol1, 8.8 kJ mol1, and 10.5 kJ mol1 for multi-layer, NafionÒ105, NafionÒ117, and CGS-12 membranes, respectively. As known, both proton conduction mechanisms (proton hopping and vehicle mechanism) have been accepted to exist for NafionÒ-based membranes [35], close activation energies of the triple-layer membrane and NafionÒ could be considered as another sign for suitable affinity of layers which has resulted in homogenous proton migration pathways. Interestingly, it was found that activation energy of multi-layer membrane we studied is related to the activation energy and thickness of isolated layers. It has been well understood that methanol permeation across proton exchange membranes in DMFC applications results in reduced overall cell performance in terms of fuel loss and catalyst poisoning at cathode [20]. So, to obtain permeability properties of designed triple-layer membrane at different temperatures, methanol permeation measurements were carried out in the temperature range of 25 Ce80 C. Fig. 4(a) provides a comparison between methanol permeability of multi-layer and NafionÒ117 membranes. As seen, with increasing temperature from 25 C to 80 C, methanol permeability is increased from 2.52 107 cm2 s1 to 7.74 107 cm2 s1 for CGS-12/NafionÒ105/CGS-12. Although methanol permeability of both membranes is increased with increasing temperature, but the destructive influence of temperature on methanol permeation rate is more considerable for NafionÒ117. Actually, permeability of triple-layer membrane was found to be remarkably 87e89% lower than NafionÒ117 in the whole temperature range. It should be noticed that methanol permeability of NafionÒ105 membrane is about 3.01 106 cm2 s1 at 25 C and higher than NafionÒ117 membrane. Methanol permeability of CGS-12/NafionÒ115/CGS-12 is also characterized and shown in Fig. 4. The permeability of membranes based on NafionÒ115 is in the same order compared with multi-layer membranes based on NafionÒ105. Interestingly, it was found that permeability as a function of temperature is consistent with the Arrhenius equation. So, methanol permeability results at different temperatures were investigated based on P¼P0 exp(Ea,MeOH/RT) equation, wherein P, P0 and Ea,MeOH are methanol permeability, pre-exponential factors and corresponding activation energy, respectively (Fig. 4(b)) [13]. Accordingly, the activation energy of methanol permeability for CGS-12/NafionÒ105/CGS-12 and NafionÒ117 membranes were calculated as 19.0 and 17.4 kJ mol1, respectively. Higher methanol permeation activation energy for laminated membrane again indicates their lower permeability owing to crosslinked chitosan barrier layers. It has been shown that overall electrochemical performance of a PEM at DMFC operational conditions is concurrently influenced by both conductivity and permeability properties. Usually, the higher membrane selectivity (ratio of proton conductivity to methanol permeability) provides a higher cell performance, especially at elevated methanol concentrations [11e19]. Hence, as both transport properties of CGS-12/NafionÒ105/CGS-12 membrane were found to have increased with increasing temperature,
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Fig. 5. Temperature dependency of selectivity parameter for multi-layer membrane compared to NafionÒ117 in temperature range of 25e80 C.
Fig. 4. (a) Methanol permeability of CGS-12/NafionÒ105/CGS-12 and CGS-12/ NafionÒ115/CGS-12 membranes compared to NafionÒ117 at different temperatures, and (b) corresponding Arrhenius plots.
selectivity parameter was used here to evaluate the performance of fabricated membrane at different temperatures. Temperature dependency of membrane selectivity parameter for multi-layer as well as NafionÒ117 membranes has been shown in Fig. 5. As seen, multi-layer membrane is shown to be significantly more selective than NafionÒ117 in the whole temperature range. The maximum selectivity was similarly achieved at room temperature. This behavior originates from the greater effect of increasing temperature on methanol permeability compared to its influence on proton conductivity as previously reported in the same trends for different membranes [13,19]. Due to lower selectivity of NafionÒ105 membrane which is about 34,400 S s cm3 in comparison with 40,400 S s cm3 for NafionÒ117 all of electrochemical characterizations were performed using NafionÒ117 as a reference membrane. Moreover, it should be noticed that due to dependency of electrochemical properties on resistance and thus on membrane thickness, it is more reliable to perform this type of tests for membranes with the same thickness. So for electrochemical
evaluations, we select CGS-12/NafionÒ105/CGS-12, NafionÒ117 as well as CGS-12 membranes with approximately the same thickness in the range of 150e170 mm. To investigate the effect of methanol crossover on performance of the multi-layer membrane at operational conditions, methanol crossover current density, IC, was calculated according to the relation IC ¼ IC,OC (1I/IL), where IC,OC, I and IL are methanol crossover current density at open circuit, operating current density and anode mass transport limiting current density, respectively all in mA cm2 [32]. The results of methanol crossover current density measurements at open circuit condition and limiting current density for fabricated MEAs based on multi-layer membrane, CGS-12 and NafionÒ117 have been summarized in Table 1. As shown, limiting current density results is consistent with methanol permeability properties observed for these membranes. The results of methanol crossover measurements for multi-layer membrane in comparison with NafionÒ117 and CGS-12 at 1 and 5M methanol concentrations have been exhibited in Fig. 6. Overall, it could be found that introducing crosslinked chitosan barrier layers onto NafionÒ membrane has significantly reduced its methanol permeation rate, especially at higher methanol concentration. Direct methanol-air single fuel cell performance tests were performed at two different methanol concentrations of 1 and 5 M, and obtained polarization curves of power density versus current density at 70 C for the multi-layer membrane, which are shown and compared with NafionÒ117 and CGS-12 membranes in Fig. 7. Open circuit voltage (OCV) was measured as 0.705, 0.663, and 0.686
Table 1 Methanol crossover current density at open circuit condition and limiting current density for MEAs comprising NafionÒ117, CGS-12 and CGS-12/NafionÒ105/CGS-12 membranes at 1 and 5 M methanol concentrations and 70 C. Sample
Methanol concentration (M)
Crossover current (mA/cm2)a
Limiting current (mA/cm2)b
NafionÒ117 NafionÒ117 CGS-12 CGS-12 Multi-layer Multi-layer
1.0 5.0 1.0 5.0 1.0 5.0
156 518 142 460 136 420
530 260 445 285 575 385
a Crossover current at open circuit condition obtained from methanol/nitrogen polarization curves. b Limiting current obtained from methanol/air polarization curves.
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at 1M methanol feed, and 0.651, 0.581, and 0.572 at 5 M methanol concentration for multi-layer, CGS-12 and NafionÒ117 membranes, respectively. Actually, providing high OCV in the case of triple-layer membrane, especially at higher methanol concentration clearly shows the effectiveness of the newly designed laminated structure in suppressing methanol crossover, which again supports ex situ methanol permeability and methanol crossover results discussed before. As shown in Fig. 7, a maximum power density output of 68.10 mW cm2 was measured for fabricated multi-layer membrane in comparison with 39.45 mW cm2 for NafionÒ117 and 36.54 mW cm2 for CGS-12 at methanol concentration of 5M. Actually, 72% more power output supplied by CGS-12/NafionÒ105/ CGS-12 membrane compared to commercial NafionÒ117 reveals electrochemical performance superiority of newly tested triplelayer membrane. In other words, drastically reduced methanol crossover through multi-layer membrane is clearly reflected in supplying higher power output at concentrated 5 M methanol feed, which is preferred from an application point of view. The newly designed membranes were characterized based on their electro-osmotic drag coefficient for methanol transport based on polarization curves and according to a reported method by Moll and Compa [36]. Essentially we have V (I, C) ¼ EA1 ln(I/I0) A2CA3I, where, A1 the sum of the slopes of the polarization curves
Fig. 7. Polarization curves of methanol-air single fuel cells consisted of CGS-12/ NafionÒ105/CGS-12, NafionÒ117 and CGS-12 at 70 C using (a) 1 M, and (b) 5 M methanol solutions.
for anode and cathode, A2 is a term relating the overvoltage to the methanol crossover by diffusion, A3 is a term relating the overvoltage influenced by the sum of the protonic resistance and the methanol electro-osmotic effects. Based on curve fitting of this equation (R2 > 0.996) on VeI curves A3 can be calculated. By knowing the membrane thickness clamped between the anode and cathode electrode layers (L) and conductivity of the membrane (s) and according to A3 ¼ L/sþcJeos, cJeos can be determined as an electro-osmotic drag of methanol across the membrane.
Fig. 6. Methanol crossover results of CGS-12/NafionÒ105/CGS-12 compared to NafionÒ117 as well as CGS-12 membrane at (a) 1 M (a) and (b) 5 M methanol concentrations.
Fig. 8. _cJeos (Vcm2 mA1) the electro-osmotic diffusion of methanol across of CGS-12/ NafionÒ105/CGS-12, NafionÒ117 and CGS-12 membranes at two methanol concentrations of 1 and 5 M.
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As shown in Fig. 8, and as expected from polarization curves triple-layer membrane exhibit the same methanol crossover by electro-osmosis at low methanol concentration as Nafion117 but this membrane shows the lower electro-osmotic drag of methanol at elevated methanol concentration in comparison with Nafion117. The pure CGS-12 membrane has a lowest coefficient due to its impermeable structure; but due to its low proton conductivity (high protonic resistance) and as a result of its selectivity, the overall performance of this type of pure membrane is poor and it is not suitable in a practical point of view. To confirm strong interface of Nafion-CGS-12 indirectly, the durability test is performed to check the stability of this triple-layer membrane in operational conditions. Presence or creation of any crack at the interfacial region can affect in performanceetime behavior and result in falling output voltage which is not happening in our case as shown in Fig. 9. In order to evaluate the long-term stability and performance at fuel cell operational conditions, the endurance testing of the fabricated MEAs comprising CGS-12/NafionÒ105/CGS-12 membranes was performed using 1 M and 5 M methanol solution at 70 C under fuel circulation for 100 h. The output voltage at 200 mA cm2 of the single DMFCs was measured and the results obtained are displayed in Fig. 9. As shown, during the first 10 h, voltage is commonly increased. Such an observation could be attributed to the electro-catalyst activation and decreasing internal resistance of the cell. Fuel efficiency of cells, hFuel, were also calculated using methanol crossover results using the following equation: hFuel ¼ I/(IþIC). Thermodynamic efficiency, hTherm, is defined as the ratio of Gibbs free energy change per mole of methanol to enthalpy change per mole of methanol. Voltage efficiency of fuel cell, hVolt, is determined as ratio of the real operating voltage to theoretical maximum voltage of fuel cell. Accordingly, real efficiency of fuel cell, hReal, is obtained the combination of fuel, thermodynamic, and voltage efficiency values: hReal ¼ hFuel. hTherm. hVolt [22]. The results of real efficiency assessment of fuel cell at two different methanol concentrations for CGS-12/NafionÒ105/CGS-12, CGS-12, and NafionÒ117 membranes have been displayed in Fig. 10. As shown, the maximum cell efficiency was decreased from 17.23% to 10.9% for NafionÒ117 membranes when the methanol concentration was increased from 1 to 5 M. Corresponding increase in the methanol content for the CGS-12 membranes showed a decrease in fuel cell
Fig. 10. A comparison between real fuel cell efficiency of CGS-12/NafionÒ105/CGS-12, NafionÒ117 and CGS-12 membranes at two methanol concentrations of (a) 1 and (b) 5 M.
efficiency from 10.82% to 8.88%, while the current density at the maximum efficiency is also decreased. However, lower overall efficiency reduction from 19.55% to 18.45% is observed with increasing methanol concentration from 1 to 5 M for triple-layer membrane, which again emphasize its electrochemical superiority for high-performance DMFC applications. Thus, a higher cell efficiency and a larger current density range were obtained for the triple-layer membrane compared to other membranes we studied.
4. Conclusion
Fig. 9. Endurance testing (cell potential at the constant current condition) of CGS-12/ NafionÒ105/CGS-12, NafionÒ117 and CGS-12 membranes using two methanol concentrations of 1 and 5 M at 70 C.
A novel high-performance triple-layer proton exchange membrane comprising two structurally modified chitosan layers coated onto NafionÒ105 sides was designed and prepared in the laboratory. Investigating the cross-sectional area of fabricated multi-layer membrane using SEM and EDX techniques showed high affinity and firm interfaces between layers. The results of proton conductivity and methanol permeability measurements revealed a remarkably reduced methanol crossover and significantly higher membrane selectivity parameter for fabricated multi-layer membrane compared to NafionÒ117. From electrochemical characterization of CGS-12/NafionÒ105/CGS-12 at DMFC operational condition, its ability to provide over 72% higher power output than
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NafionÒ117 was found at 5M methanol concentration, which is promising from an application point of view. Moreover, fuel cell efficiency for multi-layer membrane was measured as 19.55% and 18.45% at 1M and 5 M methanol concentration, respectively. Owing to the favorable properties resulting from suppressed methanol crossover, low activation energy, ease of preparation and low cost, the triple-layer proton exchange membrane we studied could be considered as a promising polyelectrolyte for high-performance DMFC applications.
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