i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 9 1 7 e2 9 2 8 Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he A novel approach on ionic liquid-based poly(vinyl alcohol) proton conductive polymer electrolytes for fuel cell applications Chiam-Wen Liew, S. Ramesh*, A.K. Arof Centre for Ionics of University of Malaya, Department of Physics, Faculty of Science, University of Malaya, Lembah Pantai, 50603 Kuala Lumpur, Malaysia article info abstract Article history: The preparation of proton conducting-polymer electrolytes based on poly(vinyl alcohol) Received 8 December 2012 (PVA)/ammonium acetate (CH3COONH4)/1-butyl-3-methylimidazolium chloride (BmImCl) Received in revised form was done by solution casting technique. The ionic conductivity increased with ionic liquid 4 July 2013 mass loadings. The highest ionic conductivity of (5.74 0.01) mS cm1 was achieved upon Accepted 23 July 2013 addition of 50 wt% of BmImCl. The thermal characteristic of proton conducting-polymer Available online 17 August 2013 electrolytes is enhanced with doping of ionic liquid by showing higher initial decomposition temperature. The most conducting polymer electrolyte is stable up to 250 C. Attenuated total reflectance-Fourier Transform Infrared (ATR-FTIR) confirmed the complexation Keywords: PVA Ionic liquid Proton conductivity between PVA, CH3COONH4 and BmImCl. Polymer electrolyte membrane fuel cell (PEMFC) was fabricated. This electrochemical cell achieved the maximum power density of 18 mW cm2 at room temperature. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights Fuel cell reserved. Power density 1. Introduction Depletion of non-renewable fossil fuel catalyzes the development of environmentally friendly alternative energy sources. Fuel cells are promising candidates as power sources for a wide range of applications, ranging from portable devices to electric vehicles [1,2]. Fuel cells are of great interest in the recent research due to their attractive properties. They offer many advantages, such as environmentally benign, high energy efficiency and lower emission of pollutants [3,4]. Fuel cells also do not require the need for rapid recharging process in the presence of electricity compared to the lithium rechargeable batteries . Apart from that, fuel cells exhibit higher energy density than the batteries . The main component in PEMFCs is the proton exchange membrane (PEM) which is used as proton (or charge carriers) provider from anode to cathode. The basic requirement of PEM is the membrane must be proton conductive with some satisfactory properties like excellent mechanical strength and chemically stable. Perfluorosulfonated membrane, Nafion produced by Dupont has widely been used as host polymer in PEMFCs. However, high methanol transfer is the main shortcoming of using Nafion as primary polymer membrane in fuel cells [6,7]. High methanol crossover may cause the loss of fuel, slow methanol oxidation kinetics and formation of carbon monoxide (CO) intermediate on the platinum catalyst layer at the cathode as well as cell polarization. As a consequence, the performance of this electrochemical cell could be decreased * Corresponding author. Tel.: þ60 3 7967 4391; fax: þ60 3 7967 4146. E-mail addresses: [email protected] (C.-W. Liew), [email protected] (S. Ramesh), [email protected] (A.K. Arof). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.07.092 2918 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 9 1 7 e2 9 2 8 [6e8]. It also possesses some drawbacks such as expensive, high water swelling characteristic which reduces the lifespan of the membrane and decrease in conductivity at elevated temperature . So, the utilization of Nafion membrane in fuel cell is limited. Therefore, many attempts have been done to overcome the weakness of Nafion. Non-perfluorosulfonated polymer membrane was employed to replace Nafion in DMFC. Poly(vinyl alcohol) (PVA) is a potential material because of its cheaper price . PVA is a biodegradable semi-crystalline synthetic polymer bearing with hydroxyl functional group [7,10,11]. Superior chemical stability, non- toxic, high hydrophilicity, ease of preparation and biocompatibility are advantages of PVA [10e13]. Furthermore, PVA exhibits excellent thermal and chemical stabilities and high abrasion resistance as well as high flexibility [13,14]. PVA also possesses high dielectric permittivity and excellent capacity for charge storage which is an important feature for a supercapacitor, as reported by Hirankumar et al. . In order to prepare proton-conducting polymer electrolytes, ammonium salt must be embedded to provide the charge carriers as it is well-known as proton donor. Ammonium acetate (CH3COONH4) was chosen in this project due to its plasticizing effect. However, the ionic conductivity is relatively low. Therefore, much effort has been devoted to enhance the ionic conductivity, for instance polymer blending, addition of plasticizer, incorporation of filler, mixed salt system and doping of ionic liquid. Among all these approaches, impregnation of ionic liquid can be considered as the most feasible method because it is well known as environmentally friendly compound. Ionic liquid brings multiple advantages such as wider electrochemical stability, relatively high conductivity, negligible vapor pressure, high resistance to chemical substances and excellent thermo-stability, as reported in our previous works [16e18]. Several PVA-based fuel cells had been fabricated and analyzed by researchers. Higa and his co-workers fabricated a direct methanol fuel cell (DMFC) with a polymer electrolyte membrane where PVA is cross-linked 2 mol% of 2-methyl-1propanesulfonic acid (AMPS). The maximum power density of 2.4 mW cm2 was delivered by this DMFC . In addition, PVAbased nanocomposite polymer electrolytes were frequently used as separator in direct methanol alkaline fuel cell (DMAFC) and studied by Yang CC research group. The maximum power density of 15.3 mW cm2 was achieved using PVA/potassium hydroxide (KOH) with doping of 20% of fumed silica . However, lower maximum power density of 7.54 mW cm2 was obtained in PVA/titania (TiO2)-based DMFC . Sekhon et al. synthesized polymer electrolyte containing poly(vinylidene fluoride-co-hexafluoropropylene (PVdF-coHFP), triflic acid (HCF3SO3) and 2,3-dimethyl-1-octylimidazolium triflate (DmOImTf) for a PEMFC. Power density of 0.5 mW cm2 was obtained. This result is quite low compared to the Nafion membrane. However, it is comparable with the literature . The objective of the current project is to prepare environmentally friendly polymer electrolytes with high electrical properties and excellent thermal stability. Polymer electrolyte membrane fuel cells (PEMFCs) are thus fabricated using prepared polymer films. The electrochemical performances of PEMFCs such as power density are eventually studied in the present work. Another aim of this current project is to explore the effect of ionic liquid onto the electrochemical properties of polymer electrolytes and fuel cells. 2. Experimental 2.1. Materials Proton conducting polymer electrolytes comprising of PVA, CH3COONH4 and BmImCl were prepared in this present work. PVA (Sigma-Aldrich, USA, 99% hydrolyzed with molecular weight of 130000 g mol1), CH3COONH4 (Sigma, Japan) and BmImCl (Acros organic, USA) were used as polymer, salt and ionic liquid, respectively. All the materials were used as received. 2.2. Preparation of ionic liquid based-poly(vinyl alcohol) proton conducting polymer electrolytes Ionic liquid based-poly(vinyl alcohol) proton conducting polymer electrolytes were prepared by solution casting technique. PVA was initially dissolved in distilled water. Appropriate amount of CH3COONH4 was subsequently mixed in PVA solution. The weight ratio of PVA:CH3COONH4 was kept at 70:30. Different mass fraction of BmImCl was thus embedded into the PVA-CH3COONH4 mixture to prepare ionic liquidbased proton conducting polymer electrolyte. CL 2, CL 5 and CL 6 were the designations of polymer electrolytes with respect to addition of 20 wt%, 50 wt% and 60 wt% of BmImCl, whereas CL 0 was for the polymer electrolyte without incorporation of BmImCl. The resulting solution was stirred thoroughly and heated at 70 C for few hours until a homogenous colorless solution is obtained. The solution was eventually casted on glass Petri dish and dried in an oven at 60 C to obtain a free-standing proton conducting polymer electrolyte film. 2.3. Characterization of ionic liquid based-PVA proton conducting polymer electrolytes 2.3.1. Ambient temperature-ionic conductivity studies Freshly prepared samples were subjected to ac-impedance spectroscopy for ionic conductivity determination. The thickness of the film was measured by digital micrometer screw gauge. The ionic conductivity of the polymer electrolytes was measured by HIOKI 3532-50 LCR HiTESTER impedance analyzer over a frequency regime between 50 Hz and 5 MHz at ambient temperature. The data acquisition was recorded at a signal level of 10 mV. Proton conducting polymer electrolytes were sandwiched on the sample holder under spring pressure in the configuration of stainless steel (SS) blocking electrode/polymer electrolyte/SS electrode. 2.3.2. Thermogravimetric analysis (TGA) TGA was accomplished by thermogravimetric analyzer, TA Instrument Universal Analyzer 2000 with Universal V4.7A software. Sample weighing 2e3 mg placed into 150 ml of silica crucible. The samples were then heated from 25 C to 600 C at a heating rate of 50 C min1 in a nitrogen atmosphere with a flow rate of 60 ml min1. 2919 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 9 1 7 e2 9 2 8 2.3.3. Attenuated total reflectance-Fourier Transform Infrared (ATR-FTIR) Themoscientific Nicolet iS10 FTIR Spectrometer (from USA) was employed to perform ATR-FTIR study. This spectrometer is equipped with an ATR internal reflection system. The FTIR spectra were recorded with a resolution of 4 cm1 in transmittance mode over the wavenumber range from 4000 to 650 cm1. The FTIR spectra and peak deconvolution were scrutinized by OMNIC 8 software which is provided by Thermo Fischer Scientific Inc. The transmittance mode of FTIR spectra was initially converted into absorbance mode for peak deconvolution process. In order to deconvolute the FTIR spectra, baseline correction and curve fitting must be implemented. The FTIR curve was fitted with gaussian-lorentzian mixed mode. 2.4. Fuel cell fabrication The most conducting polymer electrolyte was used for PEMFC fabrication. The polymer membrane was soaked in distilled water for 5 min prior the fabrication. The surface area of electrodes is 16 cm2. The polymer electrolyte was then placed in the PEMFC kit which is purchased from H-tec, Germany. The assembled kit was eventually subjected to the Electrolyzer 10 (from H-tec, Germany) for electrochemical analysis with hydrogen flow rate of 10 cm3 min1 and oxygen flow rate of 5 cm3 min1. 2.4.1. Cell performances The performance of PEMFC was done by fuel cell measurement setup which is comprised of Programmable DC power supply (Model no: 3644A, Array Electronics Co. Ltd. from Taiwan), rheostat (Model no: ZX97E, Tinsley from United Kingdom) and Digital Bench Type True RMS multimeter (UT803, Uni-Trend from Guangdong, Zhina). DC power supply is employed to provide the current flow through the cell, whereas the rheostat is used to adjust the current flow. The measurements of voltage and current flow were probed using multimeter. The cell potential in every 10 mA of current was noted down, starting from 10 mA. The current density was determined by dividing the operational current into surface area of electrodes (16 cm2). On the other hand, the power density was calculated using the fundamental equation below: P ¼ IV (1) where P is power (W), I is operational current (A) and V is cell potential (V). Again, the power density was eventually computed by dividing the power into surface area of electrodes. where l is the thickness of polymer electrolytes (cm); Rb is bulk resistance (U) of polymer electrolytes and A is the known surface area (cm2) of blocking electrodes. Fig. 1 shows a typical impedance plot of CL 0 at ambient temperature. The Rb is determined as shown in Fig. 1. Since the impedance plot is not starting from origin, so the series resistance (Rs) is appeared in the figure. The total resistance (combination of Rb and Rs) is calculated from the intercept of the spike with the extrapolation of the semicircular region on Z real axis. The Rb is computed from the deduction of the total resistance with the initial Rs. On the other hand, the spike of the plot indicates the charge accumulation at the electrolyteeelectrode interface. Fig. 2 depicts the ionic conductivity of proton-conducting polymer electrolytes at different mass fraction of BmImCl. Upon inclusion of ionic liquid, the ionic conductivity is increased. As expected, the ionic conductivity increases with mass loadings of ionic liquid, up to an optimum level. The ionic conductivity increases gradually when 10 wt% of BmImCl is added into the polymer electrolyte. However, the ionic conductivity is found to be increased rapidly with doping of 20 wt% of BmImCl. The highest ionic conductivity of (5.74 0.01) mS cm1 corresponding to addition of 50 wt% of BmImCl is observed. This conductivity has been increased by two orders of magnitude in comparison to the polymer electrolyte without impregnation of ionic liquid. The abrupt increase in ionic conductivity is due to strong plasticizing effect of ionic liquid [16,18]. Strong plasticizing effect of ionic liquid aids to soften the polymer backbone and hence increases the flexibility of polymer chain. The proton can be transported easily within the polymer matrix with highly flexible polymer 400 350 300 250 200 150 100 Rs 50 Rs+Rb 0 0 3. Results and discussion 3.1. Ambient temperature-ionic conductivity studies 50 100 150 200 250 300 350 The ionic conductivity of polymer electrolytes is determined using the following equation: s¼ l Rb A (2) Fig. 1 e ColeeCole impedance plot of CL 0 at room temperature. 400 2920 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 9 1 7 e2 9 2 8 -2.00 H -2.50 .. Deprotonation + N N N + +H N -3.00 -3.50 -4.00 This carbene could then interact with the hydrogen in hydroxyl group of PVA and result carbocation in the imidazolium ring. -4.50 -5.00 -5.50 0 10 20 30 40 50 60 70 Weight percent of BmImCl (wt %) 80 90 CH CH2 Fig. 2 e The ionic conductivity of proton conducting polymer electrolytes at different mass ratio of BmImCl. N chains. Besides, higher flexibility of polymer chains improves the mobility of polymer segments and assists the ionic transportation within polymer complexes. Consequently, the ionic conductivity is higher compared to the polymer electrolyte without doping of ionic liquid. Moreover, ionic liquid increases the amorphousness of the polymer matrix. Ionic liquid could weaken the transient coordinative bonds among the molecules in the crystalline region and thus turn the polymer chains into flexible characteristic leading to higher amorphous degree of polymer complexes. Ionic liquid also promotes the ionic conduction process by providing more conducting sites. This can be done by interacting BmImþ with hydrogen of the side chain in the polymer backbone. This interaction not only provides more conducting sites, but it also provides more mobile proton upon the deprotonation. In order to understand the principle in the proton-conducting polymer electrolytes, the proton hopping mechanism must be well studied. We propose a mechanism pertaining to transport of conducting ions for proton-conducting polymer electrolytes illustrated as follows. The ionic liquid: imidazolium cation (BmImþ) and chloride (Cl) can be easily detached from the transient partial bonding due to the bulky size of the cation. Thus, the hydrogen at C2-position of the mobile BmImþ is initially de-protonated to form a stabilized carbene [21,22]. CH H C CH H CH2 N N N CH O OH OH n C N This interaction leads to the partial hydrogen bonding between oxygen and hydrogen. In addition, the de-protonated hydrogen in this step can help in accelerating the proton conduction as it is in mobile state. When the hydrogen bonding is partially bonded, the dissociated chloride and acetate anions from ionic liquid and salt will prefer to interact with the carbocation and thus break the hydrogen bond between the imidazolium cations and side chain of PVA. After the disintegration of hydrogen bonding, the oxygen atom in the hydroxyl group of PVA turns into negatively charged or forms anions. Therefore, these electron deficient oxygen anions would accept the electron from proton from the loosely bounded ammonium cations or from imidazolium cations. Two possible side products could be formed in this segment, viz. 3-butyl-2-chloro-1-methyl-4,5-dihydro-1Himidazole and 3-butyl-1-methyl-4,5-dihydro-1H-imidazole-2carboxylic acid methyl ester, as shown in the mechanism below. 3-butyl-2-chloro-1-methyl-4,5-dihydro-1H-imidazole CH2 CH + Cl Cl or / and CH3COO + CH H n OH O N CH2 n OH O .. CH2 CH CH2 CH :O ..: CH2 CH n + N OH N or / and OOCCH3 N N 3-butyl-1-methyl-4,5-dihydro-1H-imidazole-2-carboxylic acid methyl ester 2921 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 9 1 7 e2 9 2 8 cations into immobile state. As a consequence, the conducting sites could be lesser and ultimately reduce the ionic conductivity. These neutral ion pairs and ion aggregates could impede the conducting pathway and reduce the flexibility of polymer chains. This phenomenon can be proven by determining the activation energy of the proton transportation. The activation energies of polymer electrolytes containing 20 wt%, 50 wt% and 60 wt% of BmImCl are 7.54, 7.11 and 7.74 meV, respectively (not shown in this manuscript). Higher activation energy of the polymer electrolytes infers higher energy that the proton requires to overcome the barriers for the transportation. Therefore, we can conclude that the lower ionic conductivity of the polymer electrolytes containing high mass fraction of BmImCl is attributed to the formation of ion pairs Beyond this step, there are two possibilities of proton conduction process. The proton transportation can be generated from the hydrogen in ammonium cations or the hydrogen in BmIm cations. The dissociated proton from loosely bounded ammonium cations of doping salt would then interact with this negatively charged oxygen and result the formation of hydroxyl group through hydrogen bonding. This partial hydrogen bonding would result the CONH coordination bond. On the other hand, the detached chloride or/ and acetate anions will abstract an electron from hydrogen. This hydrogen will thus leave the hydroxyl group and hence turn into anions. The adjacent hydrogen could interact with this oxygen and hence generate the proton transportation from site to another adjacent site. CH CH2 H CH CH2 :.. O: + O CH CH2 n O H CH2 CH n :O ..: H Cl or / and CH3COO and ion aggregates as the ion pairs can block the transport pathway within the polymer chains. Apart from that, we suggest that proton conduction can also take place at the BmIm cations. For this proton transport, BmIm cations could interact with the oxygen in PVA and form NeO bond temporarily. Similarly, the detached chloride or/ and acetate anions will abstract an electron from hydrogen of another branched site and produce oxygen which is appeared as anions. As next step, the electron from the hydrogen from C2-position in BmIm cations could be donated to this electrondeficient oxygen. In order to form a stable compound, the NeO bond must be broken down. Hence, this bond detachment could lead to the formation of carbene, which is an important chemical compound in weakening the transient coordination bond within the macromolecules. Eventually, the proton conduction mechanism is repeated again with the formation of carbene. These two possible mechanisms will cause continuous ionic hopping mechanism in the polymer matrix. 3.2. Thermogravimetric analysis (TGA) Fig. 3 portrays thermogravimetric analysis for pure PVA, CL 0 and ionic liquid-based polymer electrolytes. Pure PVA shows three major weight losses, as described in Yang et al. . However, four thermal degradation steps are observed for salt and/or ionic liquid-doped polymer electrolytes. The initial thermal degradation in the region of 25 Ce125 C is due to the dehydration of water or moisture as polymer is a hydroscopic compound . The insignificant weight loss in this stage is also due to the elimination of impurities in the polymer electrolytes. Small weight losses of 5%, 5% and 6% are observed for pure PVA, CL 0 and CL 6, respectively in this re.. CH2 CH :O ..: N + CH2 CH O n H N CH2 CH CH2 O H N CH n + N N :O ..: N Cl or / and CH3COO Beyond adulteration of 50 wt% of BmImCl, the ionic conductivity is found to be lower in value. This is ascribed to the agglomeration of ions. Excessive ions in the polymer electrolyte could form ion pairs and ion aggregates. These ion pairs and ion aggregates might block the conducting pathway and hinder the proton from transportation, leading to lower ionic conductivity. The amount of proton for conduction is also decreased in the presence of ion pairs and ion aggregates. At high concentration of ionic liquid, NHþ 4 cations could rather form ion pairs with other anions than provide proton for transportation. Similar observation goes to BmImþ cations. These cations help in providing the conducting site as aforementioned above. However, at high mass loadings, the cations could form ion neutral pairs with anions and thus turn the gion. However, higher mass losses of 9% have been observed for CL 2 and CL 5. Beyond this stage, the weight of all the samples remains the same until 200 C. Upon the dehydration, ammonium acetate could be decomposed easily into acetamide, CH3C(O)NH2. On the contrary, two PVA polymer chains might be crosslinked as a result of water removal. This elimination could lead to the formation of ether cross-linkages . Above 200 C, an abrupt decrease in weight is observed for all the samples. Among all the samples, pristine PVA portrays the highest mass loss from 210 C to 235 C in this segment. Elimination of side chain in polymer backbone of PVA is the main attributor to this thermal degradation for pure PVA . Apart from formation of ether cross-linkages, the polymer 2922 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 9 1 7 e2 9 2 8 backbone of PVA could start a rapid chain stripping on the polymer backbone upon heating and hence turn into conjugated double bonds, giving rise to formation of polyene, as illustrated as below : electrolytes containing ionic liquid. Moreover, BmImCl starts to be degraded at 237 C as reported in Lee et al. . As a result, decomposition of BmImCl also causes the mass loss in this region. Hence, the weight loss of ionic liquid-based polymer electrolytes is found to be higher than CL 0 with this additional attributor. This is in good agreement with the result obtained where higher weight loss is obtained for those ionic liquid-plasticized polymer electrolytes. As can be seen, CL 5 depicts the highest decomposition temperature. This implies that the most conducting polymer electrolytes exhibit better thermal properties than other polymer complexes. A consecutive enormous loss in weight is observed thereafter for all polymer electrolytes. However, pure PVA exemplifies insignificant mass loss of 8% from 235 C until 410 C. The weight loss of all polymer electrolytes remains in the regime of 20e30%. CL 0 has weight loss of 20%, starting from 245 C until 375 C, whereas CL 2 exhibits mass loss of 21%, from 285 C to 365 C. Similarly, CL 6 also shows around 21% of weight loss in the temperature range of 245 C and 350 C. On the contrary, the weight loss of CL 5 is increased by 5%, which is around 26% whereby the degradation temperature starts from 305 C to 355 C. The thermal degradation in this phase arises from the chemical degradation processes. These chemical decomposition processes include the breakdown of ether cross-linkages and random chain scissoring between carbonecarbon bonds . Random bond Upon addition of salt and ionic liquid, the mass loss in this stage is significantly decreased. Besides, the starting decomposition temperature in this stage is improved. These observations deduct the contribution from the complexation scission yields two intermediate products and thus forms methyl-terminated allylic polyene in cis and trans isomerisms via elimination process, as demonstrated as below . between salt and/or ionic liquid and PVA. This is because higher energy would be required to break the transient coordination bond in the complexation process. Weight losses of 32%, 43%, 33% and 44% corresponding to the temperature range of 240 Ce245 C, 245 Ce285 C, 250 Ce305 C, 235 Ce245 C are attained for CL 0, CL 2, CL 5 and CL 6, respectively. The contributor of this thermal decomposition stage is suggestive of the degradation of acetamide where its decomposition temperature is around 222 C. The chain stripping mechanism is less favorable in CL 0 when the salt is complexed with PVA. In contrast, the carbene produced from ionic liquid interacts with the hydrogen from hydroxyl group in PVA and forms the coordination bond. Therefore, the chain stripping process might also be absent in the polymer Final weight loss is attained at elevated temperature. Pure PVA and CL 0 show 20% and 26.8% of weight losses, along with 12% and 11% of mass residues at 575 C, respectively. Ionic liquid-based polymer electrolytes display lower mass loss in comparison to CL 0. CL 2 exhibits 21% of weight loss from 365 C to 570 C with residual mass of 3%, whereas CL 5 reveals higher mass loss, which is around 25% with 6% of residue in the temperature range of 355 C-565 C. CL 6 starts to loss its weight around 22% with 9% of residue from 350 C to 575 C. This final weight loss is connected to the breakdown of the polymer backbone as reported in Yang et al. . Upon further heating, the double bond of polyene would be broken down into single bond and eventually converted into aliphatic polymer chains at this stage . Beyond this stage, the weight 120 100 Weight (%) 80 60 40 20 0 0 100 200 Pure PVA 300 CL 0 400 CL 2 CL 5 500 600 CL 6 Fig. 3 e Thermogravimetric analysis of pure PVA, CL 0, CL 2, CL 5 and CL 6. i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 9 1 7 e2 9 2 8 of samples remains unchanged. This observation infers the fully decomposition of the polymer matrix. Although the residual mass of ionic liquid-based polymer electrolytes is lesser, however, CL 5 is still a promising candidate as polymer electrolyte due to its higher first decomposition temperature. 3.3. Attenuated total reflectance-Fourier Transform Infrared (ATR-FTIR) The PVAeCH3COONH4 polymer electrolyte systems have been prepared and investigated. The FTIR results and the complexation between PVA and CH3COONH4 have been elucidated in the literature [15,20,26e32]. Therefore, the interaction between PVA and CH3COONH4 is not discussed in this present work. The interactions between ionic liquid and ammonium acetate-doped polymer electrolyte are also analyzed and discussed in details. Fig. 4 depicts the ATR-FTIR spectra of CL 0, BmImCl, CL 2, CL 5 and CL 6. On the other hand, the band assignment of the spectra is summarized in Table 1. The ATR-FTIR spectrum of CL 5 is investigated as it achieves the maximum ionic conducting as aforementioned in section 3.1. Upon adulteration of BmImCl, sixteen new peaks have been formed by comparing Fig. 4(a) with Fig. 3(d). These peaks are CeH vibrational mode for cyclic BmImþ at 655 cm1, 693 cm1, 3109 cm1 and 3159 cm1 , CeH bending mode for cyclic BmImþ at 752 cm1 , in-plane CeH bending mode of imidazolium ring at 890 cm1 , in-plane CeNeC bending mode at 951 cm1 , out-of-plane CeH wagging mode in alkyl chain at 1019 cm1 , CH3eN stretching mode at 1168 cm1 , CH2 symmetric bending mode at 1335 cm1 , CH3 asymmetric stretching mode at 1378 cm1 , CH3 asymmetric bending mode at 1428 cm1 and 1456 cm1 , C]N stretching mode at 1540 cm1  and CeC and CeN bending modes of imidazolium ring at 1696 cm1 . Some characteristic peaks of CL 0 are disappeared with doping of BmImCl. Examples of these characteristic peaks are the combinations of CHOH bending mode, CH3 in-plane deformation and CeH wagging mode at Fig. 4 e ATReFTIR spectra of (a) CL 0, (b) BmImCl, (c) Cl 2, (d) CL 5 and (e) CL 6. 2923 1329 cm1 cm1, CeH deformation mode at 1414 cm1 and C] O stretching mode at 1701 cm1. Apart from that, the CeCl stretching mode of BmImCl at 791 cm1 is disappeared. This entails the chloride dissociation from BmIm cations. Therefore, the dissociated BmIm cations could form the carbene compound after the deprotonation process, as suggested in section 3.1. This idea is supported by the absences of out-of-plane CeH bending mode of imidazolium ring at 730 cm1 , CeH bending mode in methyl group at 1132 cm1 , CeN stretching mode of imidazolium ring at 1289 cm1  and ¼ CeH stretching mode at 3091 cm1 . The proton conduction in ammonium salt-doped polymer electrolyte is based on the formation of CONH bond, as explained in section 3.1. However, the proton is migrated in a different way for polymer electrolytes containing BmImCl, as suggested in section 3.1. The CONH bonding mode of CL 0 is absent in all the ionic liquid-based polymer electrolytes and further verifies the proposed mechanism. The CeH bending mode of CL 0 is shifted to lower wavenumber from 662 cm1 to 655 cm1 due to the overlapping of CeH vibration mode for cyclic BmImþ. The CeH vibration mode for cyclic BmImþ at 698 cm1 also exhibit downward shift to 693 cm1 by comparing Fig. 4(a) with Fig. 3(d). The sharp peak at 844 cm1 in Fig. 4(a) is designated as skeletal CeH rocking mode of PVA and shifted to 846 cm1 with doping of ionic liquid. This peak not only exhibits the change in shift, but it also undergoes the change in peak intensity. The intensity of peak is gradually reduced from 11.28% to 8.66%, in transmittance mode, as demonstrated in Fig. 5. This phenomenon indicates the interaction between ionic liquid and polymer matrix. Another apparent proof to show the interaction between CeH bonding in imidazolium ring and CL 0 is also attained at the wavenumber of 890 cm1. A sharp peak with its intensity of 19.12% is observed in ionic liquid spectra; however it has been turned into a weak peak with its intensity of 0.14% in transmittance mode. Fig. 4(b) depicts a sharp peak at 900 cm1 with a shoulder peak at 940 cm1. Upon addition of PVA and ammonium acetate, this band becomes two new peaks with an extremely low intensity at 918 cm1 and 951 cm1. These two weak peaks are assigned as OeH bending mode and in-plane CeNeC bending mode at 918 cm1 and 951 cm1, respectively, reflecting the formation of NO bond as aforementioned in section 3.1 . The intensity of first peak is also reduced from 3.35% to 1.06% in transmittance mode. A sharp peak is observed at 1089 cm1 in the wavenumber range of 1100 cm1e1000 cm1 for CL 0 and assigned as CeO stretching mode. But, it has been changed to a medium sharp peak at 1092 cm1 with a shoulder peak at 1021 cm1 when BmImCl is embedded into the polymer complex. This indicates the overlapping of out-of-plane CeH wagging mode in alkyl chain with polymer matrix and further deduces the interaction between CeO bond from polymer matrix and CeH functional group from ionic liquid. This result agrees well with our proposed mechanism in section 3.1 where the hydrogen from ionic liquid will be abstracted to the adjacent electrondeficient oxygen for the proton hopping mechanism. Even though the CeO stretching mode still remains unchanged at 1236 cm1, however, the intensity of the peak is relatively reduced upon impregnation of ionic liquid. The peak reduction entails that the proton conduction could be taken place at 2924 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 9 1 7 e2 9 2 8 Table 1 e The assignments of the vibration modes of BmImCl, CL 0, CL 2, CL 5 and CL 6. Wavenumber (cm1) Descriptions of vibration modes BmImCl CeH bending mode CeH vibration mode for cyclic BmImþ Outeofeplane CeH bending mode of imidazolium ring CeH bending mode for cyclic BmImþ CeCl stretching mode Skeletal CeH rocking mode Ineplane CeH bending mode of imidazolium ring OeH bending mode Ineplane CeNeC bending mode Outeofeplane CeH wagging mode in alkyl chain CeO stretching mode CeH bending mode in methyl group CeC and CeO stretching modes of doubly H ebonded OH in crystalline region CH3eN stretching mode CeO stretching mode CeN stretching mode of imidazolium ring CHOH bending mode, CH3 ineplane deformation and CeH wagging mode CH2 symmetric bending mode CH3 asymmetric stretching mode CeH deformation mode CH3 asymmetric bending mode CH3 symmetric bending mode C¼C stretching mode C¼N stretching mode NeH bending mode CeH stretching mode eCONHe bonding mode CeC and CeN bending mode of imidazolium ring C¼O stretching mode CeH symmetric stretching mode in methyl group of alkyl chain OeH stretching mode of OH group ¼CeH stretching mode CL 0 CL 2 CL 5 CL 6 655 655, 693, 3109, 3159 e 657 657, 703, e, e e e 662, 698, 3113, 3152 730 e 655 655, 698, 3103, 3159 e 746 791 e 900 e e 844 e 753 e 846 885 752 e 846 890 751 e 848 e e 940 1018 918 e e 918 951 1019 918 951 1021 918 e 1019 e 1132 e 1089 e 1140 1092 e 1143 1092 e 1140 1089 e 1142 1163 e 1289 e e 1236 e 1329 1169 1236 e e 1168 1236 e e e 1236 e e 1337 1388 1413 1434 1465 1517 1560 e e e 1675 e e 1414 e e e e 1561 1648 1669 e 1335 1378 1409 e 1453 1504 e 1567 1648 e 1696 1335 1378 e 1428 1456 e 1540 1571 1647 e 1696 1332 1375 1410 e e 1504 e 1559 1652 e 1699 e 2872, 2950, 2966 e 3091 1701 2850, 2906, 2937 3259 e e 2876, 2911, 2940 3332 e e 2870, 2939 e 2850, 2911, 2938 3260 e the CeO coordination bond when the ionic liquid is complexed with polymer backbone. The sharp peak at 1163 cm1 is designated as CH3eN stretching mode, whereas the weak peak at 1388 cm1 is associated with CH3 asymmetric stretching mode. The first peak exhibits upward shift to 1168 cm1 with reduced peak intensity of 31.11%, from 44.11% to 13% in transmittance mode. On the contrary, the latter peak shifts to lower wavenumber to 1378 where its intensity is gradually reduced from 3.17% to 0.73% in transmittance mode. The peak at 1414 cm1 is designated as CeH deformation mode. However, the inclusion of 50 wt% of ionic liquid induces to the formation double peaks at 1456 cm1 and 1428 cm1, as shown in Fig. 6. The change in shape is suggestive of the overlap of CeH deformation mode at 1413 cm1, CH3 asymmetric bending mode at 1434 cm1 and CH3 symmetric bending mode at 1465 cm1. This can be proven using the deconvolution method. The peaks are fitted and deconvoluted as illustrated 662 e 3347 e in Fig. 7. Three deconvoluted peaks are obtained in this wavenumber range. This result is in good agreement with our suggestion. Again, this signifies the interaction of hydrogen from CeH bond with polymer membrane where the proton conduction can take place at. A medium sharp peak at 1561 cm1 corresponding to NeH bending mode is attained in CL 0 spectra. Nevertheless, the peak has been developed into a sharp peak at 1571 with a shoulder peak at 1540 cm1. The intensity of the sharp peak is increased from 6.56% to 10.01% in transmittance mode. The overlapping of C]N stretching mode at 1560 cm1 and C]C stretching mode from ionic liquid at 1517 cm1 contributes to this changes of peak intensity, peak location and shape. This can be a sign of interaction between ionic liquid and CL 0. The weak peak analogous to CeH stretching mode at 1648 cm1 is changed to a medium sharp peak at 1647 cm1. The intensity of this peak is increased from 0.43% to 5.63% in transmittance mode as observed in Fig. 4(a and d). This finding reveals the i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 9 1 7 e2 9 2 8 Fig. 5 e The change in peak intensity of skeletal CeH rocking mode of (a) CL 0 and (b) CL 5 in the wavenumber region of 850 cmL1e800 cmL1. complexation between ionic liquid and macromolecules and verifies the proton conduction. Another proof to show the proton conduction is also detected in the range of 3000 cm1e2800 cm1. Double peaks with a shoulder peak are appeared as a broad band within this region. This band is designated as CeH symmetric stretching mode in methyl group of alkyl chain. However, only a peak with a shoulder peak is observed at their respective wavenumber of 2939 cm1 and 2870 cm1 upon inclusion of BmImCl. Similar phenomenon is attained for the band within the range of 3400 cm1e3000 cm1. CL 0 illustrates a broad peak at 3259 which is related to OeH stretching vibration mode of OH group in PVA. Two additional shoulder peaks are scrutinized at 3109 cm1 and 3159 cm1 in which the OeH 0.05 0.04 0.03 1465 cm-1 Absorbance 1434 cm-1 0.02 0.01 1413 cm-1 0 -0.01 1380 1400 1420 Original curve 1460 1440 Wavenumber (cm-1) Fitted curve 1480 1500 Deconvoluted peaks Fig. 6 e The change in shape of vibration modes of (a) CL 0 and (b) CL 5 in the wavenumber region of 1500 cmL1e1400 cmL1. 2925 Fig. 7 e The original and fitted curves with the deconvoluted peaks in the wavenumber region of 1495 cmL1e1390 cmL1. stretching mode is shifted to 3347 cm1 with doping of BmImCl. These newly formed shoulder peaks are known as CeH vibration mode for cyclic BmImþ of ionic liquid. Therefore, these changes in shift and peak position can be indicative of interaction between CeH vibration mode of ionic liquid and OeH stretching mode of PVA which aids in the proton conduction. Among all the samples, we found out that the highest ionic conductivity is achieved by adding 50 wt% of BmImCl as shown in Fig. 2. One of the attributors in enhancing the ionic conductivity is correlated to lower crystalline region (or higher amorphous degree) in the polymer electrolytes. The vital feature can be proven in the ATR-FTIR spectra. CeC and CeO stretching modes of doubly H-bonded OH in crystalline region is located at 1140 cm1 with its intensity of 5.57% in Fig. 4(a). The intensity of this peak is gradually reduced to 3.97% corresponding to CL 2 at 1143 cm1 and 1.61% at 1140 cm1 for CL 5. Therefore, it can be concluded that CL 5 has the lowest crystalline region compared to CL 0 and CL 2. It is in good agreement with the suggestion that we proposed in section 3.1. The intensity of this characteristic peak of CL 6 is increased to 6.42% in comparison to CL 0, CL 2 and CL 5, as shown in Fig. 4(e). When the ionic liquid is added further, the crystalline phase of CL 6 becomes higher due to the ion pairing and ion aggregation as a result of excessive ion. The formation of ion pairs and ion aggregates can be further proven by the disappearance of some characteristic peaks in Fig. 4(e). These peaks are: in-plane CeH bending mode of imidazolium ring at 900 cm1, in-plane CeNeC bending mode at 940 cm1, CH3eN stretching mode at 1163 cm1, CH3 asymmetric bending mode at 1434 cm1, CH3 symmetric bending mode at 1465 cm1, C] N stretching mode 1560 cm1, and CeH vibration modes for cyclic BmImþ at 3113 cm1 and 3152 cm1. The changes in peak intensity, peak position and shape establish the complexation between ionic liquid and polymer matrix. 2926 3.4. i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 9 1 7 e2 9 2 8 Cell performances In order to study the effect of ionic liquid onto the electrochemical performances of fuel cell, four fuel cells are assembled by sandwiching CL0, CL 2, CL 5 or CL 6 between anode and cathode and further analyzed. Electrolyzer is used to generate hydrogen and oxygen gases. The hydrogen gas is connected to anode, whereas the oxygen is channeled to cathode. Below are the general reactions in both electrodes. Anode : 2H2 /4Hþ þ 4e Cathode : 4Hþ þ O2 /2H2 O Complete reaction : 2H2 þ O2 /2H2 O Hydrogen gas which acts as fuel will be split into positively charged hydrogen (or known as proton) and negatively charged electrons in the presence of catalyst layer at anode. For next step, the polymer electrolyte membrane allows the protons to travel from anode to cathode. In contrast, the electrons are transported along the external circuit to cathode and hence create electrical current. Eventually, the combinations between proton and oxygen with the help of electrons will form water as by-product at cathode side. This by-product will be flowed out from the system. The open-circuit voltage (Voc) is determined when no current flows into the fuel cell. This is known as the initial voltage prior to the cell voltage measurement. Figs. 8 and 9 depict the cell potential and power density of the electrochemical cells with different current density at room temperature, respectively. The Voc of CL 0, CL 2, CL 5 and CL 6 are 0.27 V, 0.37 V, 0.66 V and 0.56 V, respectively as illustrated in Fig. 8. As can been seen in current density-potential curves, the cell potential of ionic liquid-based polymer electrolytes are higher than that of polymer electrolytes without inclusion of ionic liquid. This is mainly attributed to the strong plasticizing effect of ionic liquid. The plasticizing effect of ionic liquid not only helps in promoting the proton conduction by weakening 0.7 0.6 Cell potential (V) 0.5 0.4 0.3 0.2 0.1 0 0 10 20 30 40 50 Current density (mAcm-2) CL 0 CL 2 CL 5 60 70 CL 6 Fig. 8 e The cell potential of the fabricated PEMFC containing CL0, CL 2, CL 5 and CL 6 polymer electrolytes at different current density. Fig. 9 e The power density of the fabricated PEMFC containing CL0, CL 2, CL 5 and CL 6 polymer electrolytes at different current density with inset of figure. the transient bonds, but it also produces sticky samples. The sticky feature of the sample can improve the contact between the cathode and anode. Therefore, the proton can be traveled easily. All the curves manifest the same pattern with respect to current density. The cell potential of fuel cell decreases with increasing the current density. The result is same as the findings that other researchers have reported previously [3e6]. Ionic liquid-based polymer electrolytes possess better electrochemical performance than CL 0 by showing higher current density at the end of the curve. This occurrence indicates that fuel cell comprising of ionic liquid-based polymer electrolytes can withstand higher current before the complete degradation process. CL 0 has been fully decomposed when 620 mA of current (or equivalent with current density of 38.75 mA cm2) is applied into the cell. Upon inclusion of 20 wt % of ionic liquid into the polymer electrolyte, the membrane is degraded at the current of 660 mA (or corresponding to 41.25 mA cm2). On the other hand, the fuel cell consisting CL 5 stops the function at the current 950 mA or current density of 59.34 mA cm2. However, the electrochemical properties of fuel cell are decreased when BmImCl is further added into the polymer matrix. Comparing CL 5 and CL 6, CL 6 is decomposed at lower current that is 700 mA with current density of 43.75 mA cm2. Power density of fuel cell is also determined. The power density of fuel cell increases with current density, up to a maximum level, as depicted in Fig. 9. The inset of figure portrays the peak obtained for fuel cell based on CL 0, CL 2 and CL 5 in a smaller scale. Beyond the highest value, the power density of fuel cell is found to be decreased gradually. This decrement of power density is correlated to the degradation of polymer electrolytes at higher current supply. The doping of ionic liquid can enhance the electrochemical properties of fuel cell as explained above. The same conclusion can also be drawn using the power density result. Fuel cell based on CL 2 i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 9 1 7 e2 9 2 8 polymer membrane exemplifies higher power density than CL 0. The maximum power density of 0.73 mW cm2 is observed at 0.05 V with a peak current density of 15.63 mA cm2. This peak is shifted to higher current with a higher power density with addition of 20 wt% of BmImCl. Fuel cell comprising of CL 2 displays a maximum power density of 1.13 mW cm2 at a maximum potential of 0.06 V with a peak current density of 18.13 mA cm2. Among all the ionic liquid-based polymer electrolytes, the fabricated fuel cell using CL 5 offers excellent electrochemical characteristics, where the power density of 18 mW cm2 at a peak current density of 46.88 mA cm2 with a maximum operational potential of 0.38 V is observed in Fig. 9. The increase in power density of fuel cell as increases the mass loading of ionic liquid into the polymer matrix is strongly due to the higher ionic conductivity of ionic liquidbased polymer electrolytes. Numerous protons with higher mobility are allowed to be transported from anode to cathode if the polymer electrolyte is conductive. Therefore, higher conductive characteristic of polymer electrolyte promotes the conversion of the fuel into electricity through a redox process in a fuel cell system and thereby increases the power density of the electrochemical cell. Nevertheless, the further inclusion of ionic liquid alters the electrochemical properties of fuel cell. Fuel cell based on CL 6 exhibits the maximum power density of 2.96 mW cm2 at a maximum potential of 0.25 V with a peak current density of 11.88 mA cm2 is observed in Fig. 9. Upon addition of 60 wt% of BmImCl not only decreases the power density of cell, but it also reduces the peak current density, as depicted in the inset of Fig. 9. In other words, excessive ionic liquid concentration in the polymer electrolytes could produce the ion aggregation which inhibits the conducting routes within the polymer chains. As a result, this proton conducting-polymer electrolyte possesses lower ionic conductivity and hence leads to lower electrochemical performance of fuel cell. Composite polymer electrolytes-based direct methanol fuel cells (DMFCs) were also prepared by Yang group. The maximum power density of 6.77 mW cm2 was reported in the literature. Another type of DMAFC using crosslinked PVA/SSA was also prepared by Yang. This DMAFC generates the highest power density of 4.13 mW cm2 at 60 C and 1 atm [39,40]. The power density of PEMFC obtained in this present work is higher than these two literature and the literature that we mentioned in the introduction. So, this implies that the prepared ionic liquid-based proton conducting polymer electrolytes exhibit excellent electrochemical performance in fuel cell. It can be concluded that CL 5 is the promising candidate as polymer electrolyte in a fuel cell as it can achieves the highest cell power density of 18 mW cm2 with improved electrochemical properties. 4. Conclusion Ionic liquid-based proton conducting polymer electrolytes were prepared by solution casting method. The ionic conductivity of these polymer electrolytes is greatly increased with doping of ionic liquid due to the strong plasticizing effect. The ionic conductivity also increased with mass fraction of ionic liquid, up to a maximum level, which corresponds to 2927 inclusion of 50 wt% of BmImCl. Upon addition of 50 wt% of BmImCl, the highest ionic conductivity of (5.74 0.01) mS cm1 is achieved at ambient temperature. This most conductive polymer membrane exhibited better thermal properties than other polymer electrolytes as it is stable up to 250 C. The temperature of first thermal degradation stage is shifted to higher temperature with doping of salt and ionic liquid due to the complexation. ATR-FTIR showed the interactions between PVA, ammonium acetate and BmImCl and further proved the complexation within the polymer electrolytes. Fuel cells were eventually fabricated using the electrolyzer and fuel cell membrane kit. Ionic liquid also helped in enhancing the electrochemical properties of fuel cells. The open-circuit voltage and power density of fuel cell have been greatly improved with doping of ionic liquid. The maximum power density of 18 mW cm2 was achieved in the fuel cell containing the most conducting polymer electrolyte at room temperature, along with operational current of 750 mA. Acknowledgment This work was supported by the High Impact Research Grant (J-21002-73851) from University of Malaya and eScienceFund (16e02e03e6031) from Ministry of Science, Technology and Innovation (MOSTI), Malaysia. Gratitude also goes to “Peruntukan Penyelidikan Pascasiswazah (PPP), Universiti Malaysia”. One of the authors, ChiameWen Liew gratefully acknowledges the “Skim Bright Sparks Universiti Malaya” (SBSUM) for scholarship award. references  Kim DS, Park HB, Rhim JW, Lee YM. Preparation and characterization of crosslinked PVA/SiO2 hybrid membranes containing sulfonic acid groups for direct methanol fuel cell applications. J Membrane Sci 2004;240:37e48.  Qiao J, Okada T, Ono H. 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