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Cellulose microfibers surface treated with imidazole as new proton conductors
Materials Chemistry and Physics 239 (2020) 122056
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Cellulose microfibers surface treated with imidazole as new proton conductors �ski a, Iga Jankowska a, *, Paweł Ławniczak a, Katarzyna Pogorzelec-Glaser a, Andrzej Łapin b a Radosław Pankiewicz , Jadwiga Tritt-Goc a b
Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Pozna� n, Poland Faculty of Chemistry, Adam Mickiewicz University in Pozna� n, Umultowska 89b, 61-614 Pozna� n, Poland
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Cellulose microfibers (CMF) are func tionalized with imidazole molecules. � Structural, thermal and electrical prop erties of the new material are determined. � Imidazole cations and anions are present in the material at room temperature. � The highest proton conductivity is measured at 150 � C. � The composite shows four orders of magnitude higher conductivity value than CMF.
A R T I C L E I N F O
A B S T R A C T
Keywords: Polymer-matrix composites Cellulose microfibers Imidazole dissociation Thermal properties Proton conductivity
A newly synthesized polymeric proton-conductive composite (3.5CMF-Im), based on pure cellulose microfibers (CMF) functionalized with imidazole molecules (Im) on their surface, was comprehensively studied in terms of structural, thermal, and electrical properties. According to elemental analysis of 3.5CMF-Im composite contains on average one imidazole molecule per 3.5 glucose units. Fourier transform infrared spectroscopy (FTIR) was used to identify the crystalline structure and hydrogen bond network. Thermogravimetric analysis (TGA þ DTG) and differential scanning calorimetry (DSC) tests were carried out to examine the stability and thermal decomposition of studied materials. In order to determine temperature dependences of electrical conductivity, the impedance spectroscopy was used. For the first time, evidence of imidazole dissociative mechanism in this type of material using FTIR was obtained. Imidazole may dissociate into an imidazolium cation and an imidazole anion, and this mechanism may contribute to the proton conductivity of cellulose-imidazole composites. The new material exhibits a maximum conductivity of 2.7 � 10 4 S m 1 at 150 � C, which is four orders of magnitude higher than that of pure cellulose microfibers. The composite is environmentally friendly solid polymer elec trolyte operating in the temperature range above the water boiling point.
Abbreviations: CMF, cellulose microfibers; Im, imidazole; CMF-Im, cellulose microfibers doped with imidazole composite. * Corresponding author. Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Pozna� n, Poland. E-mail addresses: [email protected] (I. Jankowska), [email protected] (P. Ławniczak), [email protected] (K. Pogorzelec-Glaser), [email protected] (A. Łapi� nski), [email protected] (R. Pankiewicz), [email protected] (J. Tritt-Goc). https://doi.org/10.1016/j.matchemphys.2019.122056 Received 10 May 2019; Received in revised form 20 August 2019; Accepted 22 August 2019 Available online 23 August 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Materials Chemistry and Physics 239 (2020) 122056
1. Introduction
electrical impedance spectroscopy (EIS).
In the last several decades, the development of modern civilization has led to a continuous increase in energy consumption, which entails the need to search for new sources of green energy. Electrochemical devices, such as fuel cells that convert chemical energy into electricity without generating pollution, can be such a source. A very important component of the hydrogen fuel cell is the proton exchange membrane (PEM), which separates the two electrodes from each other. Currently, the most commonly used material for the production of PEM is Nafion, which exhibits high proton conductivity (>10 S m 1), but only in hy dration conditions, besides it is relatively expensive (about 1700 $ m 2) and complicated in production. Hence, the search for new proton conductive materials has become an urgency. The new proton conduc tors should be liquid-free, work under anhydrous conditions at a tem perature higher than 100 � C, thermally stable, easy to manufacture, low cost, and moreover environmentally friendly [1–11]. Therefore, the attention of researchers is focused on the study of conductive materials based on natural polymers [6,7,12–17], among them on various cellu lose modifications [18–23]. Cellulose is a polysaccharide and is the most common natural poly mer in nature. The repeating polymer unit consists of two glucose resi dues connected by a β-1,4 glycosidic linkage. The structure is stabilized by a network of hydrogen bonds between numerous hydroxyl OH groups – each glucose residue contains three hydroxyl groups. Cellulose is environmentally friendly, low cost has high hydrophilicity and resis tance to hydrolysis, and low solubility [24–29]. Cellulose materials can be divided in terms of their structure on the dimensional scale, e.g., cellulose microcrystals (CMC), cellulose microfibers (CMF), cellulose nanocrystals (CNC), and cellulose nanofibrils (CNF). They also differ in the degree of crystallinity. A comparison of the structural, thermal and electrical properties of these types of cellulose can be found in a previous article [30]. The electrical conductivity is very low for pure cellulose, and the material technically is not conductive. The hydroxyl groups on the cellulose surface form a stable structure of the hydrogen bonding network that can support the immobilization of the heterocycles in the polymer matrix. The heterocyclic compounds, such as imidazole (Im), have essential properties to be used as fillers in proton conducting liquid-free composites; they can form the hydrogen bonding network similar to that of water molecules, have amphoteric character, a higher boiling point than water, and show a high degree of self-dissociation [8–10]. Previously reported results showed that it is possible to replace water molecules with heterocycles as conducting filler to obtain proton conducting liquid-free cellulose composites above 100 � C [31–34]. However, the content of imidazole in composites is still unsatisfactory. One of the ways to increase the imidazole concentration in the composite is to use more amorphous cellulose materials, such as cellu lose microfibers, which have a higher number of available OH groups on their surface. In addition to increasing the conductivity value, the higher concentration of imidazole in the composite enables spectroscopic studies of subtle phenomena occurring in the material, such as dissoci ation of imidazole. As a result, evidence for the existence of imidazolium cations and imidazolate anions in the cellulose composite by the FTIR method was obtained for the first time. Tautomerization of imidazole molecules in composite based on microcrystalline cellulose was previ ously observed by high-resolution 15N solid-state NMR spectroscopy method [34]. Mechanism of proton tautomerism and proton exchange of imidazole with the hydroxyl groups of cellulose is a suggested mecha nism of conductivity in cellulose composites. In this paper, the newly synthesized 3.5CMF-Im composite was tested to determine its structural, thermal, and electrical properties. The powdered CMF and 3.5CMF-Im samples were examined by elemental analysis, scanning electron microscopy (SEM), X-Ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA þ DTG), differential scanning calorimetry (DSC), and
2. Experimental 2.1. Synthesis The aim of the synthesis was to obtain a composite with a maximum concentration of imidazole. The high purity imidazole (0.952 g, 14.0 mmol) was dissolved in dichloromethane (25 mL). Then, commer cially available highly purified cellulose microfibers, Sigmall cellulose type 101 provided by Sigma-Aldrich Company (1 g, 5.6 mmol) was added to this solution upon vigorous stirring. The stirring was continued at 30 � C for 1.5 h. Afterward, the suspension was ultrasonicated for 0.5 h at 30 � C. After that, suspension of cellulose was filtered off under reduced pressure. The residue at the glass frit was well rinsed by a large amount of dichloromethane and dried at 40 � C for 2 h. Finally, about 1 g white powder was obtained. Cellulose is not soluble in dichloromethane and forms only suspension. The interaction with the molecules of imidazole can only take place at the surface of the fibers. 2.2. Elemental analysis The chemical composition of the newly synthesized composite was determined using a Vario EL III Elemental Analyzer, GmbH Germany, equipped with a standard CHN detector. The elemental analysis of 3.5CMF-Im samples of 20 mg was carried out three times and the dif ferences were smaller than 0.2%. 2.3. Scanning electron microscopy (SEM) Scanning electron microscopy images of CMF sample were taken at room temperature with a Fei NovaSEM 650 microscope. The vacuum was better than 6 � 10 4 Pa. 2.4. X-Ray diffraction (XRD) XRD patterns for CMF and 3.5CMF samples were collected at room temperature in Bragg-Brentano geometry with an X’Pert PAN analytical diffractometer using CuKα radiation (λ ¼ 1.5418 Å). The structure was refined using a High Score Plus package and Rietveld method. 2.5. Fourier transform infrared spectroscopy (FTIR) The infrared absorption spectra in KBr pellet (c ¼ 1:2000) were recorded using a Bruker Equinox 55 FT-IR spectrometer for the powdered materials of Im, CMF, and 3.5CMF-Im. The spectral range was from 650 cm 1 to 4000 cm 1; spectral resolution was 2 cm 1. In order to analyze the obtained experimental data, theoretical calculations with the Gaussian03 sets of codes were performed [35]. The geometries of the investigated species were fully optimized using MP2(Full)/6-311þþG (d,p) method. Calculations of normal modes were performed at the same level as that one applied for optimizations. The results of optimization corresponded to energy minima since no imaginary frequencies were detected. The computed frequencies were multiplied by the factor of 0.984 to obtain a reasonable estimate of the experimental results and to eliminate known systematic errors related to anharmonicity. 2.6. Thermogravimetric analysis (TGA þ DTG) TGA tests of pure CMF powder and 3.5CMF-Im composite were performed on a Perkin-Elmer TGA4000 instrument from room temper ature to 600 � C. The measurements were carried out under a nitrogen atmosphere at a heating rate of 10 � C min 1, and appropriate ther mogravimetric derivatization (DTG) data were collected at the same time. This method allows for observing even small changes in the TGA curve. 2
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2.7. Differential scanning calorimetry (DSC)
per 3.5 glucose units, which was calculated on the basis of the nitrogen and carbon data. Cellulose is not soluble in most solvents, also in dichloromethane used in the synthesis of the composite, and forms suspension. Hydrogen bonds are most likely formed between molecules of imidazole and hydroxyl groups of the cellulose molecules located on the surface of the fibers. This fact limits the cellulose-glucose residue/ imidazole molecule ratio in the resulting product; in CMC-based com posites, the highest concentration is 5.4 [31,32]. X-ray diffraction patterns of CMF and 3.5CMF-Im powders (Fig. 2) confirm that the tested samples belong to Iβ type of cellulose [36,37]. The first peak at approximately 15.0� containing two components at 2θ ¼ 14.8� , 16.4� , and the second peak at 22.2� are assigned to the planes (11 0), (1 1 0), and (2 0 0), respectively. The crystallinity index (CI) of CMF and 3.5CMF-Im powders was calculated and were about 19% and 16%, respectively. The fact that the CI of composite samples does not differ much from that of pure CMF shows that the cellulose was not dissolved during the synthesis and that imidazole was introduced into the CMF matrix by functionalizing the surface of the cellulose fibers. A similar situation was observed for microcrystalline and nanocrystal line cellulose composites. Compared to microcrystalline cellulose (CI ¼ 66%), CMF is more amorphous [30–33]. This result confirms that for more amorphous material it is possible to increase the concentration of heterocyclic molecules compared to that of crystalline cellulose (in CMC-based composites the highest concentration equals 5.4).
DSC measurements for CMF and 3.5CMF-Im materials were carried out using a NETZSCH DSC 200F3 calorimeter in a helium atmosphere. Thermograms were obtained for two heating cycles at a scanning rate of 10 � C min 1. The temperature range for the first heating cycles was from 5 � C to 110 � C, and then the samples were annealed at 110 � C for 0.5 h and cooled to 5 � C. Subsequently, the samples were subjected to the second heating from 5 � C to 250 � C. Powdered samples with a weight of about 5 mg were placed in closed pans with a small hole at the top. 2.8. Electrical impedance spectroscopy (EIS) The electrical conductivity of powdered CMF and 3.5CMF-Im sam ples was examined using impedance spectroscopy. The real and imagi nary parts of the complex impedance of the samples were obtained using an Alpha-A high-performance analyzer (Novocontrol GmbH) in combi nation with Quatro Cryosystem, to the accuracy better than � 0.01 � C. Impedance measurements were carried out in the frequency range from 0.1 Hz to 10 MHz with �1 V voltage oscillation. Pellet samples were prepared at room temperature by pressing at 10 MPa and then covered with electrodes made from Wolbring GmbH Hans silver paste. The size of the pellets was 1.0 mm in thickness and 6.0 mm in diameter. The measurements were carried out during two heating cycles and one cooling cycle: the first heating from 0 � C to 110 � C; annealing at this temperature for 0.5 h followed by a cooling cycle to 0 � C; the second heating cycle from 0 � C to 250 � C. In order to check the thermal stability of 3.5CMF-Im conductivity, the sample was annealed at every 10 � C as a function of time in temperatures from 80 � C to about 150 � C.
3.2. FTIR analysis FTIR spectroscopy is a powerful tool for obtaining information about the chemical structure of celluloses samples [37–40]. Fig. 3 shows the
3. Results and discussion 3.1. Structure of CMF and 3.5CMF-Im Surface morphology and CMF grains size were examined by SEM and are illustrated in Fig. 1. The average CMF particle size was 20 μm. The content of weakly bonded water in CMF and 3.5CMF-Im was 8 and 3%, respectively. Anhydrous cellulose cannot be functionalized with imid azole – the synthesis tests failed. Previous NMR studies of microcrys talline cellulose composite have confirmed that Im molecules form a hydrogen bonds network with the hydroxyl groups of cellulose and the water molecules [34]. For the new 3.5CMF-Im composite, NMR results will be published in a separate paper. The chemical composition of the new composite based on CMF was determined by elemental analysis. The calculated average percentage of carbon, hydrogen, and nitrogen atoms is 42.57, 6.13, and 4.12%, respectively. Sulfur and chlorine atoms were not found in the samples. The content of imidazole molecules in the composite is one Im molecule
Fig. 2. X-ray diffraction patterns of powders: CMF polymer matrix and 3.5CMF-Im composite.
Fig. 1. SEM images of CMF matrix. The scale bar in the first image corresponds to 200 μm and in the second one to 50 μm. 3
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spectra of (a) 3.5CMF-Im, (b) CMF, and (d) Im. The spectrum (c) was obtained by subtracting the spectrum (b) of cellulose microfibers (CMF) from the spectrum (a) of 3.5CMF-Im. In the first region, one can find the OH and CH stretching vibrations, whereas in the second one, named the “fingerprint” region, different stretching and bending vibrations of different groups are present. The strong, broad band appearing at about 3330 cm 1 is due to OH stretching intermolecular and intramolecular hydrogen bonds. This band is more prominent for CMF than for 3.5CMF-Im, which can be due to a larger number of hydrogen bonds in pure cellulose. In the spectral region of 3000-2250 cm 1, the CH stretching of asymmetric and symmetric methyl and methylene groups can be found [38]. The obtained spectra confirm that the structure of the studied cellulose belongs to the Iβ crystalline type [41,42]. The broad band at 1635 cm 1 is associated with adsorbed water in cellulose [38, 39,41]. The bands at 1429, 1371, 1320, and 1254 cm 1 are assigned to CH2 symmetric bending, CH bending, CH2 wagging vibration, respec tively [41]. The bands observed at 1157, 1111, 1058, 1030, 896 cm 1 are due to the asymmetric C-O-C bridge stretching, the anhydroglucose ring asymmetric stretching, C-O stretching, C-H in-plane deformation, C-H deformation of cellulose, respectively [39,41,43]. The infrared technique is commonly used to assess the degree of crystallinity of cellulose samples [39–47]. In order to analyze the degree of crystallinity, Nelson and O’Connor [47] proposed the total crystalline index (TCI) defined as the ratio between the heights of the bands at 1372 and 2900 cm 1; it is proportional to the crystallinity degree of cellulose [40]. The ratio of the areas of the bands observed at 1429 and 897 cm 1 is used as a lateral order index (LOI) [41]. The bands at 1429 cm 1 and 898 cm 1 are associated with the amount of crystalline structure of cellulose and with the amorphous region in cellulose, respectively [45]. LOI depends on the degree of order in cellulose; it increases with the crystallinity degree decrease [41,43]. Another ratio of the absorbance bands at 3400 and 1320 cm 1, the hydrogen bond intensity (HBI), is used to evaluate the degree of intermolecular regularity of cellulose samples [46]; HBI is proportional to the crystallinity degree of cellulose. For investigated CMF and 3.5CMF-Im compounds, the TCI, LOI, and HBI values were determined (see Table 1). The HBI value is higher for CMF than for 3.5CMF-Im sample. It indicates that CMF contains much more cellulose chains in a highly organized form, which can lead to higher hydrogen bond intensity between neighbor cellulose chains and result in more packed cellulose structure and higher crystallinity than in 3.5CMF-Im. Also, TCI and LOI values indicate that a higher degree of crystallinity and a more ordered cellulose structure characterize CMF rather than 3.5CMF-Im.
Fig. 3 reveals that in the spectrum of 3.5CMF-Im composite (a), in addition to the bands associated with cellulose (b), one can find the features that are associated with the imidazole molecules (c). In order to find out whether in 3.5CMF-Im we have neutral or ionized imidazole molecule, the DFT calculations of normal modes were performed. For the spectral analysis, we have taken into account the imidazolium ion and neutral imidazole of the C2v and Cs symmetry, respectively. The “free” ion contains ten atomic nuclei, so 24 internal vibrational modes (3n-6, where n is the number of atoms) should be observed. It has one plane of symmetry in the plane of the molecule, and another, perpen dicular to it. Assuming C2v symmetry, four groups of normal vibrations should be observed: 9 vibrations of class A1 (symmetrical to both planes of symmetry; active in IR and Raman), 3 of class A2 (antisymmetric to both planes; Raman), 4 of class B1 (symmetric to the plane of the molecule; IR, Raman), and 8 of class B2 (antisymmetric to the plane of the molecules; IR, Raman). The neutral imidazole contains nine atomic nuclei, and therefore 21 internal vibration modes must be present. One can find two groups of normal vibrations: 15 vibrations of class A’ (symmetric to the plane of the molecule; IR, Raman) and 6 vibrations of class A" (antisymmetric to the plane of the molecules; IR, Raman). Fig. 4 shows the experimental and theoretical IR spectra of the imi dazolium ion and neutral imidazole. The results of ab initio calculations for the vibrational spectra of the molecules under study were compared with experimental data in order to check if the proton transfer to imidazole molecule takes place. One can find that the most significant and intensive absorption bands that appear at 1532, 1492, 1324, 1257, 1091, 1063, and 933 cm 1 are related to the neutral molecule. This conclusion seems to be reasonable if confronted with the spectrum recorded for neutral imidazole (see Fig. 4, plot a), in which the corre sponding bands are observed at 1540, 1497, 1328, 1266, 1100, 1061, – C, C-N and 937 cm 1. These bands are mainly due to the presence of C– vibrations, C-H, N-H bending, and ring deformation. However, in addition to the bands associated with the dynamics of the neutral molecule, some weak bands that are also found can be related to the imidazolium ion. The bands related to the cation are present at 1597, 1454, 1214, and 1125 cm 1. The first mode is due to the stretching of – C bonds in the pentagonal ring and is located at the same position C– – C bond in the proton (1597 cm 1) as the line related to the stretching C– conducting system (ImH2)2SeO4⋅2H2O [48]. Zieba et al. have shown the presence of imidazole cation in this salt using X-Ray data, Raman spectroscopy, and DFT investigations. The other bands observed at 1454, 1214, and 1125 cm 1 are mainly due to the carbon-nitrogen stretching vibrations, ring deformations and carbon-hydrogen bending, respectively. For (ImH2)2SeO4⋅2H2O these bands appear at 1464, 1217 and 1124 cm 1 [48]. On the basis of the above spectral analysis, it can be concluded that for the investigated composite at room temperature the neutral and also ionized imidazole molecules occur in the system. This is the first evi dence of the existence of imidazole cations and anions in the cellulose composite obtained by means of infrared spectroscopy. Previously, the dissociation mechanism of imidazole in the microcrystalline cellulose composite was shown in the solid state high-resolution NMR experi ment. In this method, 15N-enriched imidazole composite samples were used because low sensitivity at 15N natural abundance is the main obstacle in its application [34]. Imidazole-15N is commercially avail able, but most other 15N-enriched heterocycles are not available. For this reason, it is very important to be able to study proton-conducting
Table 1 Cellulose infrared crystallinity ratios and hydrogen bond intensity. Cellulose samples
Fig. 3. Absorption spectra of 3.5CMF-Im (a), CMF (b), and Im (d) recorded at room temperature in KBr pellet. The spectrum (c) was obtained by subtracting the spectrum of cellulose microfibers (b) from the spectrum of 3.5CMF-Im (a).
CMF 3.5CMF-Im
4
IR crystallinity ratio
HBI
H1371/H2880 (TCI)
A1429/A896 (LOI)
A3330/A1320
0.79 0.56
1.81 2.72
22.33 11.49
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Fig. 4. The absorption spectrum of imidazole (a) and subtracted spectrum (b) (upper panel); Calculated IR spectra of the imidazolium ion (ImH)þ and neutral imidazole (Im)0 with C2v and Cs symmetry, respectively. Note: The spectrum (b in upper panel) was obtained by subtracting the spectrum of cellulose micro fibers (b) from the spectrum of 3.5CMF-Im (b) (see Fig. 3); at theory level of MP2(Full)/6-311þþG(d,p); the computed frequencies were multiplied by a scaling factor of 0.984.
composites containing heterocycles by FTIR method. 3.3. Thermal properties The thermal stability and degradation profiles of the tested materials were characterized by the thermal analysis (TGA, DSC). Fig. 5a shows TGA thermograms and the corresponding DTG curves (Fig. 5b) for pure CMF and 3.5CMF-Im composite samples. Detailed information about the onset and endset temperature, maximum degradation temperature (T0) of processes and weight loss (ΔY) for both samples are presented in Table 2. During the temperature increase to about 100 � C, the initial weight loss of 8% for CMF sample and 3% for 3.5CMF-Im is associated with evaporation of weakly bonded water and adsorbed humidity. In CMF sample, the water content is higher compared with that in micro crystalline cellulose, 5% [31], because CMF is a more amorphous ma terial. Water molecules are mostly adsorbed in cellulose by hydroxyl groups in the amorphous region [29]. The weakly bonded water content in cellulose fibers composites is similar to that in the composite based on microcrystalline cellulose [31,32]. In the temperature range from 100 � C up to about 250 � C, the cel lulose microfibers are thermally stable (Fig. 5a). For 3.5CMF-Im com posite above approximately 120 � C, one can observe the thermal degradation process taking place at two stages. The first stage is asso ciated with the loss of imidazole from the material as a result of breaking of the hydrogen bonds the Im molecules and CMF matrix. The second one includes cellulose pyrolysis and is observed for both tested samples with a maximum at about 350 � C. The loss of Im molecules from 3.5CMF takes place at a lower temperature than the boiling point of imidazole (of 256 � C), as shown for other cellulose composites [31–33]. In these composites, imidazole molecules can form hydrogen bonds with cellu lose in two different ways: the first is the direct attachment to cellulose hydroxyl groups, and the other is the attachment via strongly bonded water molecules [34]. The latter way can explain the lower temperature of Im loss because the strongly bonded water evaporates from the cel lulose at a lower temperature than the boiling point of imidazole. Fig. 6 shows the DSC thermograms obtained for CMF and 3.5CMF-Im samples. During the first heating cycle, a broad endothermic peak is observed for both samples: CMF and 3.5CMF-Im composite. This peak is
Fig. 5. Thermogravimetric (a) and the corresponding derivative TG curves (b) of CMF and 3.5CMF-Im powders at a heating rate of 10 � C min 1. Table 2 Characteristic temperatures of the decomposition reactions of CMF and 3.5CMFIm materials: the onset and endset temperatures, maximum degradation tem perature (T0), and weight loss (ΔY). CMF ΔY [%] Onset [� C] Endset [� C] T0 [� C] 3.5CMF-Im ΔY [%] Onset [� C] Endset [� C] T0 [� C]
I
II
III
7.068 28.36 78.66 52.80
54.624 325.54 362.08 350.48
7.637 462.18 545.31 498.90
I
II
III
IV
1.403 42.86 68.73 51.83
7.330 127.21 194.34 155.29
47.680 325.92 360.86 349.02
8.001 467.26 560.53 522.90
associated with the evaporation of weakly bonded water and adsorbed humidity. Differences in the shape of these peaks indicate that in the composite material the water content is lower compared to that in pure CMF and the hydrogen-bonded network has also changed. Then, after annealing for 0.5 h at 110 � C and cooling to 5 � C, the second heating cycle of the sample was carried out as shown in Fig. 6, from 5 � C to 250 � C. For the composite sample, an additional endothermic peak with a maximum at 205 � C is observed, which is related to 3.5CMF-Im decomposition and the loss of imidazole molecules, which is 5
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current resistance of the sample, τ ¼ 2πRC is the characteristic time constant (C is the capacity), α is an empirical exponent (0 < α � 1) indicating the deviation from the Debye model. The solid lines in Fig. 7 represent the best fit of Equation (1) to the experimental points. The bulk dc conductivity can be calculated from the simple formula: σ DC ¼ 1/ RDC (d/A), where d is the thickness, and A is the surface area of the sample. Fig. 8 shows the results obtained for the annealed sample of CMF (the second heating; black circles) in comparison with the results obtained for the composite material, 3.5CMF-Im, as a function of reciprocal temperature. The increase in the conductivity value for CMF is associ ated with the increase in the dynamics of cellulose OH groups and polymer chains performing segmental movements. The segmental mo tions are dependent on the structure of the hydrogen bonds network, and for cellulose, they become less limited at temperatures above 150 � C. The conductivity values for pure CMF are very low, which indicates that in pure cellulose the conductivity is mostly due to the mobility of water molecules [31]. The measurements for the composite include the first heating cycle (red up triangles), the cooling cycle (blue down triangles) and the sec ond heating cycle (open red triangles). There is a notable difference in the total conductivity values of the composite in the first heating cycle and the next cycles. This difference results from the presence of moisture in the material during the first heating cycle. After evaporation of water, the points corresponding to the conductivity values of the sample studied in the cooling cycle and the second heating coincides, which means that the electrical conductivity is thermally stable in this tem perature range. During the second heating cycle, the maximum con ductivity value at 150 � C is observed, followed by a rapid drop. This is related to the thermal degradation of 3.5CMF-Im composite observed by other experimental methods. As the temperature increases, the hydrogen bonds between the Im molecules and the cellulose chains start to break, and an increasing number of imidazole molecules detach from the polymer matrix. Above 210 � C, the conductivity values of a 3.5CMFIm compound are the same as those of the pure CMF matrix; which means that the composite proton conductor has been decomposed. The obtained maximum value of conductivity at 150 � C is equal to 2.7 � 10 4 S m 1 and is four orders of magnitude higher than that of CMF at the same temperature, of 1.0� 10 8 S m 1. The newly synthe sized proton conductor shows a much higher conductivity value at temperatures higher than the boiling point of water, and the proton transport is based not on water molecules, but on imidazole molecules and depends on their dynamics. The conductivity values of the studied samples increased linearly in
Fig. 6. DSC thermograms of 3.5CMF-Im powder in comparison to pure CMF powder obtained during the first and the second heating.
consistent with the TGA þ DTG results. A very broad endothermic peak of a smaller amplitude can be observed for both materials with a maximum at about 130 � C which is related to the loss of strongly bonded water. The obtained results confirm that even heating up to 110 � C does not result in evaporation of all water from the cellulose. This observation is consistent with the previously reported data [30–34]. At higher temperatures, the CMF sample is thermally stable up to 250 � C to which measurements were carried out. The results of thermal measurements confirm that imidazole molecules were successfully introduced into the cellulose matrix, although the most important proof of that comes from the study of electrical properties of the newly synthesized material. 3.4. Proton conductivity Electrical impedance spectroscopy measurements were carried out to investigate the temperature behavior of the conductivity of the studied samples. Fig. 7 shows the exemplary impedance spectra for selected temperatures in the form of Nyquist plots for the samples of CMF (the upper panel) and 3.5CMF-Im (the lower panel) in the frequency window from 1 Hz to 10 MHz. The response of the materials can be estimated as a single semicircle, and the experimental data were analyzed on the basis of the Cole equation: Z * ðωÞ ¼ Z ’ ðωÞ þ iZ } ðωÞ ¼
RDC ; 1 þ ðiωτÞα
(1)
where Z*(ω) is the complex impedance at angular frequency ω (ω ¼ 2πf; f is the linear frequency of the probing electric field), Z0 (ω) and Z”(ω) is the real and imagine part of complex impedance, RDC is the direct
Fig. 8. Arrhenius plots of the dc conductivity with activation energies of CMF and 3.5CMF-Im materials measured as a function of temperature. The solid lines are the best fits to the Arrhenius equation (Equation (2)).
Fig. 7. Nyquist plots for CMF and 3.5CMF-Im at 130, 150 and 160 � C. Solid lines represent the best fit of Equation (1) to the experimental points. 6
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the specific ranges. Therefore these dependencies can be described by the Arrhenius law:
σ(T) ¼ σ0 exp(-Ea/kT),
concentration in the composite can be increased up to 1 Im molecule per 3.5 glucose units, compared to 5.4 glucose units in microcrystalline cellulose-based composites. The higher imidazole concentration for the first time allowed to obtain FTIR evidence for the dissociation mecha nism of imidazole, and imidazole cations and anions are present in the cellulose composite material at room temperature. The dissociation mechanism coupled to proton exchange with hydroxyl groups of cellu lose together with the hydrogen bond reorganization is responsible for the transport of protons which contribute to proton conductivity in this class of cellulose composites. The newly synthesized 3.5CMF-Im poly meric composite is inexpensive to obtain, non-hazardous, environmen tally friendly. The next stage of our research will be the synthesis and characterization of the properties of nanocomposites based on cellulose nanofibers.
(2)
where σ0 is the pre-exponential factor, Ea is the activation Energy, k denotes the Boltzmann constant, and T is the temperature. The experi mental data were fitted with Equation (2) to determine the activation energy of proton transport. The Ea values for the pure polymer matrix and the composite were calculated and compared. The best-fit curves are shown in Fig. 8 (solid lines), and the estimated error is �0.01 eV. The activation energy value for the pure CMF is equal to 1.20 eV, and for 3.5CMF-Im composite, it is 0.95 eV. The decrease the activation energy for the composite compared to that for the pure cellulose matrix is related to the presence of imidazole molecules in the material, whose tautomerization and dynamics allow the transport of protons in the system. The value of activation energy is similar to those obtained for other cellulose-based composites [30,33,34]. It is higher than for the fully swollen Nafion membrane, Ea ¼ 0.16 eV, and hydrated membranes of cellulose nanocrystals and nanofibers, Ea ¼ 0.24 eV and Ea ¼ 0.21 eV, respectively [4,7]. In order to verify the thermal stability of 3.5CMF-Im, the conduc tivity EIS measurements as a function of time were carried out. Fig. 9 shows the time dependencies of the conductivity in the temperature ranges from 80 to 130 � C. 3.5CMF-Im sample was annealed for 1 h at every 10 � C. At 80 � C, the observed decrease in the conductivity with the annealing time is associated with the evaporation of the weakly bonded water from the material. From 90 � C, the conductivity of 3.5CMF-Im is almost constant up to about 120 � C at individual temperatures. At 130 � C, a visible decrease in the conductivity with the annealing time is observed. It is related to the beginning of thermal decomposition of the composite and the breaking of hydrogen bonds between the imidazole molecules and the cellulose matrix. These results lead to the conclusion that the new composite based on CMF doped with imidazole is thermally stable to about 120 � C.
Acknowledgments This work was supported by the National Science Centre, Poland [grant number 2017/24/C/ST5/00156]. The authors wish to thank Prof. A. Pietraszko, from the Institute of Low Temperature and Structure Research, Polish Academy of Sciences in Wrocław, for help in X-ray study and Dr. Michał Matczak, from the Institute of Molecular Physics, Polish Academy of Sciences in Pozna� n, for preparing SEM images. References [1] O.Z. Sharaf, M.F. Orhan, An overview of fuel cell technology: fundamentals and applications, Renew. Sustain. Energy Rev. 32 (2014) 810–853. https://doi.org/10 .1016/j.rser.2014.01.012. [2] C.Y. Wong, W.Y. Wong, K. Ramya, M. Khalid, K.S. Loh, W.R.W. Daud, K.L. Lim, R. Walvekar, A.A.H. Kadhum, Additives in proton exchange membranes for lowand high-temperature fuel cell applications: a review, Int. J. Hydrogen Energy 44 (2019) 6116–6135. https://doi.org/10.1016/j.ijhydene.2019.01.084. [3] B.R. Tiwari, Md.T. Noori, M.M. Ghangrekar, A novel low cost polyvinyl alcoholNafion-borosilicate membrane separator for microbial fuel cell, Mater. Chem. Phys. 182 (2016) 86–93. https://doi.org/10.1016/j.matchemphys.2016.07.008. [4] L. Liu, W. Chen, Y. Li, A statistical study of proton conduction in Nafion®-based composite membranes: prediction, filler selection and fabrication methods, J. Membr. Sci. 549 (2018) 393–402. https://doi.org/10.1016/j.memsci.2017.12.0 25. [5] Q. Liu, N. Ni, Q. Sun, X. Wu, X. Bao, Z. Fan, R. Zhang, S. Hu, F. Zhao, X. Li, Poly (2,5-benzimidazole)/trisilanolphenyl POSS composite membranes for intermediate temperature PEM fuel cells, J. Wuhan Univ. Technol.-Materials Sci. Ed. 33 (2018) 212–220. https://doi.org/10.1007/s11595-018-1808-x. [6] G. Hern� andez-Flores, H.M. Poggi-Varaldo, O. Solorza-Feria, Comparison of alternative membranes to replace high cost Nafion ones in microbial fuel cells, Int. J. Hydrogen Energy 41 (2016) 23354–23362. https://doi.org/10.1016/j.ijh ydene.2016.08.206. [7] T. Bayer, B.V. Cunning, R. Selyanchyn, M. Nishihara, S. Fujikawa, K. Sasaki, S. M. Lyth, High temperature proton conduction in nanocellulose membranes: paper fuel cells, Chem. Mater. 28 (2016) 4805–4814. https://doi.org/10.1021/acs.che mmater.6b01990. [8] S.Ü. Celik, A. Bozkurt, S.S. Hosseini, Alternatives toward proton conductive anhydrous membranes for fuel cells: heterocyclic protogenic solvents comprising polymer electrolytes, Prog. Polym. Sci. 37 (2012) 1265–1291. https://doi.org/10 .1016/j.progpolymsci.2011.11.006. [9] M.F.H. Schuster, W.H. Meyer, M. Schuster, K.D. Kreuer, Toward a new type of anhydrous organic proton conductor based on immobilized imidazole, Chem. Mater. 16 (2004) 329–337. https://doi.org/10.1021/cm021298q. [10] J. Li, Z. Wu, H. Li, H. Liang, S. Li, Layered-structure microporous poly (benzimidazole)-loaded imidazole for non-aqueous proton conduction, New J. Chem. 42 (2018) 1604–1607. https://doi.org/10.1039/C7NJ04239F. [11] S.B. Aziz, T.J. Woo, M.F.Z. Kadir, H.M. Ahmed, A conceptual review on polymer electrolytes and ion transport models, J. Sci. Adv. Mater. Dev. 3 (2018) 1–17. http s://doi.org/10.1016/j.jsamd.2018.01.002. [12] J. Kalaiselvimary, K. Selvakumar, S. Rajendran, G. Sowmya, M.R. Prabhu, Effect of surface-modified montmorillonite incorporated biopolymer membranes for PEM fuel cell applications, Polym. Compos. 40 (2019). E301–E311, https://doi. org/10.1002/pc.24655. [13] K. Jesuraj, R.P. Manimuthu, Preparation and characterization of hybrid chitosan/ PEO–silica membrane doped with phosphotungstic acid for PEM fuel cell application, Polym. Plast. Technol. 58 (2018) 14–30. https://doi.org/10.1080/03 602559.2018.1455862. [14] M. Yamada, A. Goto, Proton conduction of DNA–imidazole composite material under anhydrous condition, Polym. J. 44 (2012) 415–420. https://doi.org/ 10.1038/pj.2012.5.
4. Conclusions The obtained results have shown the differences between the prop erties of the polymer matrix – pure cellulose fibers and 3.5CMF-Im composite. The resulting composite at 150 � C reveals a close to four orders of magnitude higher value of conductivity than that of CMF, 2.7� 10 4 S m 1 and 1.0 � 10 8 S m 1, respectively. The conductivity of 3.5CMF-Im under anhydrous conditions is related to the dynamics of imidazole molecules arranged in the polymer matrix. Thanks to using a more amorphous material, i.e., cellulose fibers, the imidazole
Fig. 9. The thermal stability of the proton conductivity of 3.5CMF-Im com posite during annealing at selected temperatures. 7
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Cellulose microfibers surface treated with imidazole as new proton conductors �ski a, Iga Jankowska a, *, Paweł Ławniczak a, Katarzyna Pogorzelec-Glaser a, Andrzej Łapin b a Radosław Pankiewicz , Jadwiga Tritt-Goc a b
Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Pozna� n, Poland Faculty of Chemistry, Adam Mickiewicz University in Pozna� n, Umultowska 89b, 61-614 Pozna� n, Poland
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Cellulose microfibers (CMF) are func tionalized with imidazole molecules. � Structural, thermal and electrical prop erties of the new material are determined. � Imidazole cations and anions are present in the material at room temperature. � The highest proton conductivity is measured at 150 � C. � The composite shows four orders of magnitude higher conductivity value than CMF.
A R T I C L E I N F O
A B S T R A C T
Keywords: Polymer-matrix composites Cellulose microfibers Imidazole dissociation Thermal properties Proton conductivity
A newly synthesized polymeric proton-conductive composite (3.5CMF-Im), based on pure cellulose microfibers (CMF) functionalized with imidazole molecules (Im) on their surface, was comprehensively studied in terms of structural, thermal, and electrical properties. According to elemental analysis of 3.5CMF-Im composite contains on average one imidazole molecule per 3.5 glucose units. Fourier transform infrared spectroscopy (FTIR) was used to identify the crystalline structure and hydrogen bond network. Thermogravimetric analysis (TGA þ DTG) and differential scanning calorimetry (DSC) tests were carried out to examine the stability and thermal decomposition of studied materials. In order to determine temperature dependences of electrical conductivity, the impedance spectroscopy was used. For the first time, evidence of imidazole dissociative mechanism in this type of material using FTIR was obtained. Imidazole may dissociate into an imidazolium cation and an imidazole anion, and this mechanism may contribute to the proton conductivity of cellulose-imidazole composites. The new material exhibits a maximum conductivity of 2.7 � 10 4 S m 1 at 150 � C, which is four orders of magnitude higher than that of pure cellulose microfibers. The composite is environmentally friendly solid polymer elec trolyte operating in the temperature range above the water boiling point.
Abbreviations: CMF, cellulose microfibers; Im, imidazole; CMF-Im, cellulose microfibers doped with imidazole composite. * Corresponding author. Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Pozna� n, Poland. E-mail addresses: [email protected] (I. Jankowska), [email protected] (P. Ławniczak), [email protected] (K. Pogorzelec-Glaser), [email protected] (A. Łapi� nski), [email protected] (R. Pankiewicz), [email protected] (J. Tritt-Goc). https://doi.org/10.1016/j.matchemphys.2019.122056 Received 10 May 2019; Received in revised form 20 August 2019; Accepted 22 August 2019 Available online 23 August 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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1. Introduction
electrical impedance spectroscopy (EIS).
In the last several decades, the development of modern civilization has led to a continuous increase in energy consumption, which entails the need to search for new sources of green energy. Electrochemical devices, such as fuel cells that convert chemical energy into electricity without generating pollution, can be such a source. A very important component of the hydrogen fuel cell is the proton exchange membrane (PEM), which separates the two electrodes from each other. Currently, the most commonly used material for the production of PEM is Nafion, which exhibits high proton conductivity (>10 S m 1), but only in hy dration conditions, besides it is relatively expensive (about 1700 $ m 2) and complicated in production. Hence, the search for new proton conductive materials has become an urgency. The new proton conduc tors should be liquid-free, work under anhydrous conditions at a tem perature higher than 100 � C, thermally stable, easy to manufacture, low cost, and moreover environmentally friendly [1–11]. Therefore, the attention of researchers is focused on the study of conductive materials based on natural polymers [6,7,12–17], among them on various cellu lose modifications [18–23]. Cellulose is a polysaccharide and is the most common natural poly mer in nature. The repeating polymer unit consists of two glucose resi dues connected by a β-1,4 glycosidic linkage. The structure is stabilized by a network of hydrogen bonds between numerous hydroxyl OH groups – each glucose residue contains three hydroxyl groups. Cellulose is environmentally friendly, low cost has high hydrophilicity and resis tance to hydrolysis, and low solubility [24–29]. Cellulose materials can be divided in terms of their structure on the dimensional scale, e.g., cellulose microcrystals (CMC), cellulose microfibers (CMF), cellulose nanocrystals (CNC), and cellulose nanofibrils (CNF). They also differ in the degree of crystallinity. A comparison of the structural, thermal and electrical properties of these types of cellulose can be found in a previous article [30]. The electrical conductivity is very low for pure cellulose, and the material technically is not conductive. The hydroxyl groups on the cellulose surface form a stable structure of the hydrogen bonding network that can support the immobilization of the heterocycles in the polymer matrix. The heterocyclic compounds, such as imidazole (Im), have essential properties to be used as fillers in proton conducting liquid-free composites; they can form the hydrogen bonding network similar to that of water molecules, have amphoteric character, a higher boiling point than water, and show a high degree of self-dissociation [8–10]. Previously reported results showed that it is possible to replace water molecules with heterocycles as conducting filler to obtain proton conducting liquid-free cellulose composites above 100 � C [31–34]. However, the content of imidazole in composites is still unsatisfactory. One of the ways to increase the imidazole concentration in the composite is to use more amorphous cellulose materials, such as cellu lose microfibers, which have a higher number of available OH groups on their surface. In addition to increasing the conductivity value, the higher concentration of imidazole in the composite enables spectroscopic studies of subtle phenomena occurring in the material, such as dissoci ation of imidazole. As a result, evidence for the existence of imidazolium cations and imidazolate anions in the cellulose composite by the FTIR method was obtained for the first time. Tautomerization of imidazole molecules in composite based on microcrystalline cellulose was previ ously observed by high-resolution 15N solid-state NMR spectroscopy method [34]. Mechanism of proton tautomerism and proton exchange of imidazole with the hydroxyl groups of cellulose is a suggested mecha nism of conductivity in cellulose composites. In this paper, the newly synthesized 3.5CMF-Im composite was tested to determine its structural, thermal, and electrical properties. The powdered CMF and 3.5CMF-Im samples were examined by elemental analysis, scanning electron microscopy (SEM), X-Ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA þ DTG), differential scanning calorimetry (DSC), and
2. Experimental 2.1. Synthesis The aim of the synthesis was to obtain a composite with a maximum concentration of imidazole. The high purity imidazole (0.952 g, 14.0 mmol) was dissolved in dichloromethane (25 mL). Then, commer cially available highly purified cellulose microfibers, Sigmall cellulose type 101 provided by Sigma-Aldrich Company (1 g, 5.6 mmol) was added to this solution upon vigorous stirring. The stirring was continued at 30 � C for 1.5 h. Afterward, the suspension was ultrasonicated for 0.5 h at 30 � C. After that, suspension of cellulose was filtered off under reduced pressure. The residue at the glass frit was well rinsed by a large amount of dichloromethane and dried at 40 � C for 2 h. Finally, about 1 g white powder was obtained. Cellulose is not soluble in dichloromethane and forms only suspension. The interaction with the molecules of imidazole can only take place at the surface of the fibers. 2.2. Elemental analysis The chemical composition of the newly synthesized composite was determined using a Vario EL III Elemental Analyzer, GmbH Germany, equipped with a standard CHN detector. The elemental analysis of 3.5CMF-Im samples of 20 mg was carried out three times and the dif ferences were smaller than 0.2%. 2.3. Scanning electron microscopy (SEM) Scanning electron microscopy images of CMF sample were taken at room temperature with a Fei NovaSEM 650 microscope. The vacuum was better than 6 � 10 4 Pa. 2.4. X-Ray diffraction (XRD) XRD patterns for CMF and 3.5CMF samples were collected at room temperature in Bragg-Brentano geometry with an X’Pert PAN analytical diffractometer using CuKα radiation (λ ¼ 1.5418 Å). The structure was refined using a High Score Plus package and Rietveld method. 2.5. Fourier transform infrared spectroscopy (FTIR) The infrared absorption spectra in KBr pellet (c ¼ 1:2000) were recorded using a Bruker Equinox 55 FT-IR spectrometer for the powdered materials of Im, CMF, and 3.5CMF-Im. The spectral range was from 650 cm 1 to 4000 cm 1; spectral resolution was 2 cm 1. In order to analyze the obtained experimental data, theoretical calculations with the Gaussian03 sets of codes were performed [35]. The geometries of the investigated species were fully optimized using MP2(Full)/6-311þþG (d,p) method. Calculations of normal modes were performed at the same level as that one applied for optimizations. The results of optimization corresponded to energy minima since no imaginary frequencies were detected. The computed frequencies were multiplied by the factor of 0.984 to obtain a reasonable estimate of the experimental results and to eliminate known systematic errors related to anharmonicity. 2.6. Thermogravimetric analysis (TGA þ DTG) TGA tests of pure CMF powder and 3.5CMF-Im composite were performed on a Perkin-Elmer TGA4000 instrument from room temper ature to 600 � C. The measurements were carried out under a nitrogen atmosphere at a heating rate of 10 � C min 1, and appropriate ther mogravimetric derivatization (DTG) data were collected at the same time. This method allows for observing even small changes in the TGA curve. 2
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2.7. Differential scanning calorimetry (DSC)
per 3.5 glucose units, which was calculated on the basis of the nitrogen and carbon data. Cellulose is not soluble in most solvents, also in dichloromethane used in the synthesis of the composite, and forms suspension. Hydrogen bonds are most likely formed between molecules of imidazole and hydroxyl groups of the cellulose molecules located on the surface of the fibers. This fact limits the cellulose-glucose residue/ imidazole molecule ratio in the resulting product; in CMC-based com posites, the highest concentration is 5.4 [31,32]. X-ray diffraction patterns of CMF and 3.5CMF-Im powders (Fig. 2) confirm that the tested samples belong to Iβ type of cellulose [36,37]. The first peak at approximately 15.0� containing two components at 2θ ¼ 14.8� , 16.4� , and the second peak at 22.2� are assigned to the planes (11 0), (1 1 0), and (2 0 0), respectively. The crystallinity index (CI) of CMF and 3.5CMF-Im powders was calculated and were about 19% and 16%, respectively. The fact that the CI of composite samples does not differ much from that of pure CMF shows that the cellulose was not dissolved during the synthesis and that imidazole was introduced into the CMF matrix by functionalizing the surface of the cellulose fibers. A similar situation was observed for microcrystalline and nanocrystal line cellulose composites. Compared to microcrystalline cellulose (CI ¼ 66%), CMF is more amorphous [30–33]. This result confirms that for more amorphous material it is possible to increase the concentration of heterocyclic molecules compared to that of crystalline cellulose (in CMC-based composites the highest concentration equals 5.4).
DSC measurements for CMF and 3.5CMF-Im materials were carried out using a NETZSCH DSC 200F3 calorimeter in a helium atmosphere. Thermograms were obtained for two heating cycles at a scanning rate of 10 � C min 1. The temperature range for the first heating cycles was from 5 � C to 110 � C, and then the samples were annealed at 110 � C for 0.5 h and cooled to 5 � C. Subsequently, the samples were subjected to the second heating from 5 � C to 250 � C. Powdered samples with a weight of about 5 mg were placed in closed pans with a small hole at the top. 2.8. Electrical impedance spectroscopy (EIS) The electrical conductivity of powdered CMF and 3.5CMF-Im sam ples was examined using impedance spectroscopy. The real and imagi nary parts of the complex impedance of the samples were obtained using an Alpha-A high-performance analyzer (Novocontrol GmbH) in combi nation with Quatro Cryosystem, to the accuracy better than � 0.01 � C. Impedance measurements were carried out in the frequency range from 0.1 Hz to 10 MHz with �1 V voltage oscillation. Pellet samples were prepared at room temperature by pressing at 10 MPa and then covered with electrodes made from Wolbring GmbH Hans silver paste. The size of the pellets was 1.0 mm in thickness and 6.0 mm in diameter. The measurements were carried out during two heating cycles and one cooling cycle: the first heating from 0 � C to 110 � C; annealing at this temperature for 0.5 h followed by a cooling cycle to 0 � C; the second heating cycle from 0 � C to 250 � C. In order to check the thermal stability of 3.5CMF-Im conductivity, the sample was annealed at every 10 � C as a function of time in temperatures from 80 � C to about 150 � C.
3.2. FTIR analysis FTIR spectroscopy is a powerful tool for obtaining information about the chemical structure of celluloses samples [37–40]. Fig. 3 shows the
3. Results and discussion 3.1. Structure of CMF and 3.5CMF-Im Surface morphology and CMF grains size were examined by SEM and are illustrated in Fig. 1. The average CMF particle size was 20 μm. The content of weakly bonded water in CMF and 3.5CMF-Im was 8 and 3%, respectively. Anhydrous cellulose cannot be functionalized with imid azole – the synthesis tests failed. Previous NMR studies of microcrys talline cellulose composite have confirmed that Im molecules form a hydrogen bonds network with the hydroxyl groups of cellulose and the water molecules [34]. For the new 3.5CMF-Im composite, NMR results will be published in a separate paper. The chemical composition of the new composite based on CMF was determined by elemental analysis. The calculated average percentage of carbon, hydrogen, and nitrogen atoms is 42.57, 6.13, and 4.12%, respectively. Sulfur and chlorine atoms were not found in the samples. The content of imidazole molecules in the composite is one Im molecule
Fig. 2. X-ray diffraction patterns of powders: CMF polymer matrix and 3.5CMF-Im composite.
Fig. 1. SEM images of CMF matrix. The scale bar in the first image corresponds to 200 μm and in the second one to 50 μm. 3
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spectra of (a) 3.5CMF-Im, (b) CMF, and (d) Im. The spectrum (c) was obtained by subtracting the spectrum (b) of cellulose microfibers (CMF) from the spectrum (a) of 3.5CMF-Im. In the first region, one can find the OH and CH stretching vibrations, whereas in the second one, named the “fingerprint” region, different stretching and bending vibrations of different groups are present. The strong, broad band appearing at about 3330 cm 1 is due to OH stretching intermolecular and intramolecular hydrogen bonds. This band is more prominent for CMF than for 3.5CMF-Im, which can be due to a larger number of hydrogen bonds in pure cellulose. In the spectral region of 3000-2250 cm 1, the CH stretching of asymmetric and symmetric methyl and methylene groups can be found [38]. The obtained spectra confirm that the structure of the studied cellulose belongs to the Iβ crystalline type [41,42]. The broad band at 1635 cm 1 is associated with adsorbed water in cellulose [38, 39,41]. The bands at 1429, 1371, 1320, and 1254 cm 1 are assigned to CH2 symmetric bending, CH bending, CH2 wagging vibration, respec tively [41]. The bands observed at 1157, 1111, 1058, 1030, 896 cm 1 are due to the asymmetric C-O-C bridge stretching, the anhydroglucose ring asymmetric stretching, C-O stretching, C-H in-plane deformation, C-H deformation of cellulose, respectively [39,41,43]. The infrared technique is commonly used to assess the degree of crystallinity of cellulose samples [39–47]. In order to analyze the degree of crystallinity, Nelson and O’Connor [47] proposed the total crystalline index (TCI) defined as the ratio between the heights of the bands at 1372 and 2900 cm 1; it is proportional to the crystallinity degree of cellulose [40]. The ratio of the areas of the bands observed at 1429 and 897 cm 1 is used as a lateral order index (LOI) [41]. The bands at 1429 cm 1 and 898 cm 1 are associated with the amount of crystalline structure of cellulose and with the amorphous region in cellulose, respectively [45]. LOI depends on the degree of order in cellulose; it increases with the crystallinity degree decrease [41,43]. Another ratio of the absorbance bands at 3400 and 1320 cm 1, the hydrogen bond intensity (HBI), is used to evaluate the degree of intermolecular regularity of cellulose samples [46]; HBI is proportional to the crystallinity degree of cellulose. For investigated CMF and 3.5CMF-Im compounds, the TCI, LOI, and HBI values were determined (see Table 1). The HBI value is higher for CMF than for 3.5CMF-Im sample. It indicates that CMF contains much more cellulose chains in a highly organized form, which can lead to higher hydrogen bond intensity between neighbor cellulose chains and result in more packed cellulose structure and higher crystallinity than in 3.5CMF-Im. Also, TCI and LOI values indicate that a higher degree of crystallinity and a more ordered cellulose structure characterize CMF rather than 3.5CMF-Im.
Fig. 3 reveals that in the spectrum of 3.5CMF-Im composite (a), in addition to the bands associated with cellulose (b), one can find the features that are associated with the imidazole molecules (c). In order to find out whether in 3.5CMF-Im we have neutral or ionized imidazole molecule, the DFT calculations of normal modes were performed. For the spectral analysis, we have taken into account the imidazolium ion and neutral imidazole of the C2v and Cs symmetry, respectively. The “free” ion contains ten atomic nuclei, so 24 internal vibrational modes (3n-6, where n is the number of atoms) should be observed. It has one plane of symmetry in the plane of the molecule, and another, perpen dicular to it. Assuming C2v symmetry, four groups of normal vibrations should be observed: 9 vibrations of class A1 (symmetrical to both planes of symmetry; active in IR and Raman), 3 of class A2 (antisymmetric to both planes; Raman), 4 of class B1 (symmetric to the plane of the molecule; IR, Raman), and 8 of class B2 (antisymmetric to the plane of the molecules; IR, Raman). The neutral imidazole contains nine atomic nuclei, and therefore 21 internal vibration modes must be present. One can find two groups of normal vibrations: 15 vibrations of class A’ (symmetric to the plane of the molecule; IR, Raman) and 6 vibrations of class A" (antisymmetric to the plane of the molecules; IR, Raman). Fig. 4 shows the experimental and theoretical IR spectra of the imi dazolium ion and neutral imidazole. The results of ab initio calculations for the vibrational spectra of the molecules under study were compared with experimental data in order to check if the proton transfer to imidazole molecule takes place. One can find that the most significant and intensive absorption bands that appear at 1532, 1492, 1324, 1257, 1091, 1063, and 933 cm 1 are related to the neutral molecule. This conclusion seems to be reasonable if confronted with the spectrum recorded for neutral imidazole (see Fig. 4, plot a), in which the corre sponding bands are observed at 1540, 1497, 1328, 1266, 1100, 1061, – C, C-N and 937 cm 1. These bands are mainly due to the presence of C– vibrations, C-H, N-H bending, and ring deformation. However, in addition to the bands associated with the dynamics of the neutral molecule, some weak bands that are also found can be related to the imidazolium ion. The bands related to the cation are present at 1597, 1454, 1214, and 1125 cm 1. The first mode is due to the stretching of – C bonds in the pentagonal ring and is located at the same position C– – C bond in the proton (1597 cm 1) as the line related to the stretching C– conducting system (ImH2)2SeO4⋅2H2O [48]. Zieba et al. have shown the presence of imidazole cation in this salt using X-Ray data, Raman spectroscopy, and DFT investigations. The other bands observed at 1454, 1214, and 1125 cm 1 are mainly due to the carbon-nitrogen stretching vibrations, ring deformations and carbon-hydrogen bending, respectively. For (ImH2)2SeO4⋅2H2O these bands appear at 1464, 1217 and 1124 cm 1 [48]. On the basis of the above spectral analysis, it can be concluded that for the investigated composite at room temperature the neutral and also ionized imidazole molecules occur in the system. This is the first evi dence of the existence of imidazole cations and anions in the cellulose composite obtained by means of infrared spectroscopy. Previously, the dissociation mechanism of imidazole in the microcrystalline cellulose composite was shown in the solid state high-resolution NMR experi ment. In this method, 15N-enriched imidazole composite samples were used because low sensitivity at 15N natural abundance is the main obstacle in its application [34]. Imidazole-15N is commercially avail able, but most other 15N-enriched heterocycles are not available. For this reason, it is very important to be able to study proton-conducting
Table 1 Cellulose infrared crystallinity ratios and hydrogen bond intensity. Cellulose samples
Fig. 3. Absorption spectra of 3.5CMF-Im (a), CMF (b), and Im (d) recorded at room temperature in KBr pellet. The spectrum (c) was obtained by subtracting the spectrum of cellulose microfibers (b) from the spectrum of 3.5CMF-Im (a).
CMF 3.5CMF-Im
4
IR crystallinity ratio
HBI
H1371/H2880 (TCI)
A1429/A896 (LOI)
A3330/A1320
0.79 0.56
1.81 2.72
22.33 11.49
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Fig. 4. The absorption spectrum of imidazole (a) and subtracted spectrum (b) (upper panel); Calculated IR spectra of the imidazolium ion (ImH)þ and neutral imidazole (Im)0 with C2v and Cs symmetry, respectively. Note: The spectrum (b in upper panel) was obtained by subtracting the spectrum of cellulose micro fibers (b) from the spectrum of 3.5CMF-Im (b) (see Fig. 3); at theory level of MP2(Full)/6-311þþG(d,p); the computed frequencies were multiplied by a scaling factor of 0.984.
composites containing heterocycles by FTIR method. 3.3. Thermal properties The thermal stability and degradation profiles of the tested materials were characterized by the thermal analysis (TGA, DSC). Fig. 5a shows TGA thermograms and the corresponding DTG curves (Fig. 5b) for pure CMF and 3.5CMF-Im composite samples. Detailed information about the onset and endset temperature, maximum degradation temperature (T0) of processes and weight loss (ΔY) for both samples are presented in Table 2. During the temperature increase to about 100 � C, the initial weight loss of 8% for CMF sample and 3% for 3.5CMF-Im is associated with evaporation of weakly bonded water and adsorbed humidity. In CMF sample, the water content is higher compared with that in micro crystalline cellulose, 5% [31], because CMF is a more amorphous ma terial. Water molecules are mostly adsorbed in cellulose by hydroxyl groups in the amorphous region [29]. The weakly bonded water content in cellulose fibers composites is similar to that in the composite based on microcrystalline cellulose [31,32]. In the temperature range from 100 � C up to about 250 � C, the cel lulose microfibers are thermally stable (Fig. 5a). For 3.5CMF-Im com posite above approximately 120 � C, one can observe the thermal degradation process taking place at two stages. The first stage is asso ciated with the loss of imidazole from the material as a result of breaking of the hydrogen bonds the Im molecules and CMF matrix. The second one includes cellulose pyrolysis and is observed for both tested samples with a maximum at about 350 � C. The loss of Im molecules from 3.5CMF takes place at a lower temperature than the boiling point of imidazole (of 256 � C), as shown for other cellulose composites [31–33]. In these composites, imidazole molecules can form hydrogen bonds with cellu lose in two different ways: the first is the direct attachment to cellulose hydroxyl groups, and the other is the attachment via strongly bonded water molecules [34]. The latter way can explain the lower temperature of Im loss because the strongly bonded water evaporates from the cel lulose at a lower temperature than the boiling point of imidazole. Fig. 6 shows the DSC thermograms obtained for CMF and 3.5CMF-Im samples. During the first heating cycle, a broad endothermic peak is observed for both samples: CMF and 3.5CMF-Im composite. This peak is
Fig. 5. Thermogravimetric (a) and the corresponding derivative TG curves (b) of CMF and 3.5CMF-Im powders at a heating rate of 10 � C min 1. Table 2 Characteristic temperatures of the decomposition reactions of CMF and 3.5CMFIm materials: the onset and endset temperatures, maximum degradation tem perature (T0), and weight loss (ΔY). CMF ΔY [%] Onset [� C] Endset [� C] T0 [� C] 3.5CMF-Im ΔY [%] Onset [� C] Endset [� C] T0 [� C]
I
II
III
7.068 28.36 78.66 52.80
54.624 325.54 362.08 350.48
7.637 462.18 545.31 498.90
I
II
III
IV
1.403 42.86 68.73 51.83
7.330 127.21 194.34 155.29
47.680 325.92 360.86 349.02
8.001 467.26 560.53 522.90
associated with the evaporation of weakly bonded water and adsorbed humidity. Differences in the shape of these peaks indicate that in the composite material the water content is lower compared to that in pure CMF and the hydrogen-bonded network has also changed. Then, after annealing for 0.5 h at 110 � C and cooling to 5 � C, the second heating cycle of the sample was carried out as shown in Fig. 6, from 5 � C to 250 � C. For the composite sample, an additional endothermic peak with a maximum at 205 � C is observed, which is related to 3.5CMF-Im decomposition and the loss of imidazole molecules, which is 5
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current resistance of the sample, τ ¼ 2πRC is the characteristic time constant (C is the capacity), α is an empirical exponent (0 < α � 1) indicating the deviation from the Debye model. The solid lines in Fig. 7 represent the best fit of Equation (1) to the experimental points. The bulk dc conductivity can be calculated from the simple formula: σ DC ¼ 1/ RDC (d/A), where d is the thickness, and A is the surface area of the sample. Fig. 8 shows the results obtained for the annealed sample of CMF (the second heating; black circles) in comparison with the results obtained for the composite material, 3.5CMF-Im, as a function of reciprocal temperature. The increase in the conductivity value for CMF is associ ated with the increase in the dynamics of cellulose OH groups and polymer chains performing segmental movements. The segmental mo tions are dependent on the structure of the hydrogen bonds network, and for cellulose, they become less limited at temperatures above 150 � C. The conductivity values for pure CMF are very low, which indicates that in pure cellulose the conductivity is mostly due to the mobility of water molecules [31]. The measurements for the composite include the first heating cycle (red up triangles), the cooling cycle (blue down triangles) and the sec ond heating cycle (open red triangles). There is a notable difference in the total conductivity values of the composite in the first heating cycle and the next cycles. This difference results from the presence of moisture in the material during the first heating cycle. After evaporation of water, the points corresponding to the conductivity values of the sample studied in the cooling cycle and the second heating coincides, which means that the electrical conductivity is thermally stable in this tem perature range. During the second heating cycle, the maximum con ductivity value at 150 � C is observed, followed by a rapid drop. This is related to the thermal degradation of 3.5CMF-Im composite observed by other experimental methods. As the temperature increases, the hydrogen bonds between the Im molecules and the cellulose chains start to break, and an increasing number of imidazole molecules detach from the polymer matrix. Above 210 � C, the conductivity values of a 3.5CMFIm compound are the same as those of the pure CMF matrix; which means that the composite proton conductor has been decomposed. The obtained maximum value of conductivity at 150 � C is equal to 2.7 � 10 4 S m 1 and is four orders of magnitude higher than that of CMF at the same temperature, of 1.0� 10 8 S m 1. The newly synthe sized proton conductor shows a much higher conductivity value at temperatures higher than the boiling point of water, and the proton transport is based not on water molecules, but on imidazole molecules and depends on their dynamics. The conductivity values of the studied samples increased linearly in
Fig. 6. DSC thermograms of 3.5CMF-Im powder in comparison to pure CMF powder obtained during the first and the second heating.
consistent with the TGA þ DTG results. A very broad endothermic peak of a smaller amplitude can be observed for both materials with a maximum at about 130 � C which is related to the loss of strongly bonded water. The obtained results confirm that even heating up to 110 � C does not result in evaporation of all water from the cellulose. This observation is consistent with the previously reported data [30–34]. At higher temperatures, the CMF sample is thermally stable up to 250 � C to which measurements were carried out. The results of thermal measurements confirm that imidazole molecules were successfully introduced into the cellulose matrix, although the most important proof of that comes from the study of electrical properties of the newly synthesized material. 3.4. Proton conductivity Electrical impedance spectroscopy measurements were carried out to investigate the temperature behavior of the conductivity of the studied samples. Fig. 7 shows the exemplary impedance spectra for selected temperatures in the form of Nyquist plots for the samples of CMF (the upper panel) and 3.5CMF-Im (the lower panel) in the frequency window from 1 Hz to 10 MHz. The response of the materials can be estimated as a single semicircle, and the experimental data were analyzed on the basis of the Cole equation: Z * ðωÞ ¼ Z ’ ðωÞ þ iZ } ðωÞ ¼
RDC ; 1 þ ðiωτÞα
(1)
where Z*(ω) is the complex impedance at angular frequency ω (ω ¼ 2πf; f is the linear frequency of the probing electric field), Z0 (ω) and Z”(ω) is the real and imagine part of complex impedance, RDC is the direct
Fig. 8. Arrhenius plots of the dc conductivity with activation energies of CMF and 3.5CMF-Im materials measured as a function of temperature. The solid lines are the best fits to the Arrhenius equation (Equation (2)).
Fig. 7. Nyquist plots for CMF and 3.5CMF-Im at 130, 150 and 160 � C. Solid lines represent the best fit of Equation (1) to the experimental points. 6
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the specific ranges. Therefore these dependencies can be described by the Arrhenius law:
σ(T) ¼ σ0 exp(-Ea/kT),
concentration in the composite can be increased up to 1 Im molecule per 3.5 glucose units, compared to 5.4 glucose units in microcrystalline cellulose-based composites. The higher imidazole concentration for the first time allowed to obtain FTIR evidence for the dissociation mecha nism of imidazole, and imidazole cations and anions are present in the cellulose composite material at room temperature. The dissociation mechanism coupled to proton exchange with hydroxyl groups of cellu lose together with the hydrogen bond reorganization is responsible for the transport of protons which contribute to proton conductivity in this class of cellulose composites. The newly synthesized 3.5CMF-Im poly meric composite is inexpensive to obtain, non-hazardous, environmen tally friendly. The next stage of our research will be the synthesis and characterization of the properties of nanocomposites based on cellulose nanofibers.
(2)
where σ0 is the pre-exponential factor, Ea is the activation Energy, k denotes the Boltzmann constant, and T is the temperature. The experi mental data were fitted with Equation (2) to determine the activation energy of proton transport. The Ea values for the pure polymer matrix and the composite were calculated and compared. The best-fit curves are shown in Fig. 8 (solid lines), and the estimated error is �0.01 eV. The activation energy value for the pure CMF is equal to 1.20 eV, and for 3.5CMF-Im composite, it is 0.95 eV. The decrease the activation energy for the composite compared to that for the pure cellulose matrix is related to the presence of imidazole molecules in the material, whose tautomerization and dynamics allow the transport of protons in the system. The value of activation energy is similar to those obtained for other cellulose-based composites [30,33,34]. It is higher than for the fully swollen Nafion membrane, Ea ¼ 0.16 eV, and hydrated membranes of cellulose nanocrystals and nanofibers, Ea ¼ 0.24 eV and Ea ¼ 0.21 eV, respectively [4,7]. In order to verify the thermal stability of 3.5CMF-Im, the conduc tivity EIS measurements as a function of time were carried out. Fig. 9 shows the time dependencies of the conductivity in the temperature ranges from 80 to 130 � C. 3.5CMF-Im sample was annealed for 1 h at every 10 � C. At 80 � C, the observed decrease in the conductivity with the annealing time is associated with the evaporation of the weakly bonded water from the material. From 90 � C, the conductivity of 3.5CMF-Im is almost constant up to about 120 � C at individual temperatures. At 130 � C, a visible decrease in the conductivity with the annealing time is observed. It is related to the beginning of thermal decomposition of the composite and the breaking of hydrogen bonds between the imidazole molecules and the cellulose matrix. These results lead to the conclusion that the new composite based on CMF doped with imidazole is thermally stable to about 120 � C.
Acknowledgments This work was supported by the National Science Centre, Poland [grant number 2017/24/C/ST5/00156]. The authors wish to thank Prof. A. Pietraszko, from the Institute of Low Temperature and Structure Research, Polish Academy of Sciences in Wrocław, for help in X-ray study and Dr. Michał Matczak, from the Institute of Molecular Physics, Polish Academy of Sciences in Pozna� n, for preparing SEM images. References [1] O.Z. Sharaf, M.F. Orhan, An overview of fuel cell technology: fundamentals and applications, Renew. Sustain. Energy Rev. 32 (2014) 810–853. https://doi.org/10 .1016/j.rser.2014.01.012. [2] C.Y. Wong, W.Y. Wong, K. Ramya, M. Khalid, K.S. Loh, W.R.W. Daud, K.L. Lim, R. Walvekar, A.A.H. Kadhum, Additives in proton exchange membranes for lowand high-temperature fuel cell applications: a review, Int. J. Hydrogen Energy 44 (2019) 6116–6135. https://doi.org/10.1016/j.ijhydene.2019.01.084. [3] B.R. Tiwari, Md.T. Noori, M.M. Ghangrekar, A novel low cost polyvinyl alcoholNafion-borosilicate membrane separator for microbial fuel cell, Mater. Chem. Phys. 182 (2016) 86–93. https://doi.org/10.1016/j.matchemphys.2016.07.008. [4] L. Liu, W. Chen, Y. Li, A statistical study of proton conduction in Nafion®-based composite membranes: prediction, filler selection and fabrication methods, J. Membr. Sci. 549 (2018) 393–402. https://doi.org/10.1016/j.memsci.2017.12.0 25. [5] Q. Liu, N. Ni, Q. Sun, X. Wu, X. Bao, Z. Fan, R. Zhang, S. Hu, F. Zhao, X. Li, Poly (2,5-benzimidazole)/trisilanolphenyl POSS composite membranes for intermediate temperature PEM fuel cells, J. Wuhan Univ. Technol.-Materials Sci. Ed. 33 (2018) 212–220. https://doi.org/10.1007/s11595-018-1808-x. [6] G. Hern� andez-Flores, H.M. Poggi-Varaldo, O. Solorza-Feria, Comparison of alternative membranes to replace high cost Nafion ones in microbial fuel cells, Int. J. Hydrogen Energy 41 (2016) 23354–23362. https://doi.org/10.1016/j.ijh ydene.2016.08.206. [7] T. Bayer, B.V. Cunning, R. Selyanchyn, M. Nishihara, S. Fujikawa, K. Sasaki, S. M. Lyth, High temperature proton conduction in nanocellulose membranes: paper fuel cells, Chem. Mater. 28 (2016) 4805–4814. https://doi.org/10.1021/acs.che mmater.6b01990. [8] S.Ü. Celik, A. Bozkurt, S.S. Hosseini, Alternatives toward proton conductive anhydrous membranes for fuel cells: heterocyclic protogenic solvents comprising polymer electrolytes, Prog. Polym. Sci. 37 (2012) 1265–1291. https://doi.org/10 .1016/j.progpolymsci.2011.11.006. [9] M.F.H. Schuster, W.H. Meyer, M. Schuster, K.D. Kreuer, Toward a new type of anhydrous organic proton conductor based on immobilized imidazole, Chem. Mater. 16 (2004) 329–337. https://doi.org/10.1021/cm021298q. [10] J. Li, Z. Wu, H. Li, H. Liang, S. Li, Layered-structure microporous poly (benzimidazole)-loaded imidazole for non-aqueous proton conduction, New J. Chem. 42 (2018) 1604–1607. https://doi.org/10.1039/C7NJ04239F. [11] S.B. Aziz, T.J. Woo, M.F.Z. Kadir, H.M. Ahmed, A conceptual review on polymer electrolytes and ion transport models, J. Sci. Adv. Mater. Dev. 3 (2018) 1–17. http s://doi.org/10.1016/j.jsamd.2018.01.002. [12] J. Kalaiselvimary, K. Selvakumar, S. Rajendran, G. Sowmya, M.R. Prabhu, Effect of surface-modified montmorillonite incorporated biopolymer membranes for PEM fuel cell applications, Polym. Compos. 40 (2019). E301–E311, https://doi. org/10.1002/pc.24655. [13] K. Jesuraj, R.P. Manimuthu, Preparation and characterization of hybrid chitosan/ PEO–silica membrane doped with phosphotungstic acid for PEM fuel cell application, Polym. Plast. Technol. 58 (2018) 14–30. https://doi.org/10.1080/03 602559.2018.1455862. [14] M. Yamada, A. Goto, Proton conduction of DNA–imidazole composite material under anhydrous condition, Polym. J. 44 (2012) 415–420. https://doi.org/ 10.1038/pj.2012.5.
4. Conclusions The obtained results have shown the differences between the prop erties of the polymer matrix – pure cellulose fibers and 3.5CMF-Im composite. The resulting composite at 150 � C reveals a close to four orders of magnitude higher value of conductivity than that of CMF, 2.7� 10 4 S m 1 and 1.0 � 10 8 S m 1, respectively. The conductivity of 3.5CMF-Im under anhydrous conditions is related to the dynamics of imidazole molecules arranged in the polymer matrix. Thanks to using a more amorphous material, i.e., cellulose fibers, the imidazole
Fig. 9. The thermal stability of the proton conductivity of 3.5CMF-Im com posite during annealing at selected temperatures. 7
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