Chitosan–ammonium acetate–ethylene carbonate membrane for proton batteries

2.1 Preparation and characterization of chitosan–NH₄CH₃COO–EC membranes

The membranes were prepared using the solution-casting technique. One gram chitosan powder (CP; Chito-Chem, Malaysia) was dissolved in 100 ml of 1% acetic acid solution (CH₃COOH, Wako Pure Chemical Industries). The mixture was continuously stirred with a magnetic stirrer at room temperature (25 °C). NH₄CH₃COO (RDH) and EC (Aldrich) were added accordingly at different concentrations. Once completely dissolved for 24 h, the solution was placed onto a Petri dish and left to dry at 25 °C to obtain membranes of CA, CA–NH₄CH₃COO, and CA–NH₄CH₃COO–EC systems as coded in Table 1. The membranes were maintained in desiccators with silica gel desiccants for further drying. The membranes were cut into suitable sizes and mounted on the conductivity holder with stainless steel (SS) electrodes under spring pressure. The conductivity of the dry membranes was determined using the Frequency Response Analyzer of Autolab PGSTAT 30 (Eco Chemie B.V.) in a frequency range between 1 Hz and 1 MHz. The measurements were carried out at 25 °C.

To determine the chemical functional group of the CA, CA–NH₄CH₃COO, and CA–NH₄CH₃COO–EC membrane systems, Fourier transform infrared (FTIR, Perkin–Elmer®) analysis was performed for all samples between 4000 and 550 cm⁻¹ wave number. The membranes were selected based on the conductivity results.

The mechanical properties of the selected CA, CA–NH₄CH₃COO, and CA–NH₄CH₃COO–EC membrane systems based on conductivity results were measured using a universal testing machine (Lloyd Instrument, 9911) at room temperature. All samples were cut into rectangular membranes (1.5 cm × 3 cm) before testing. A strain rate of 25 mm min⁻¹ and gauge length of 50 mm were applied to the membranes.

2.2 Fabrication and characterization of proton batteries

The batteries were fabricated using the membrane with the highest conductivity. Zn powder (4.50 g; Merck) and ZnSO₄·7H₂O powder (1.50 g; Univar) were mixed with 0.15 g acetylene black (AB, Gunbai) and 0.05 g polytetrafluoroethylene (PTFE, Fluka) to form an anode pellet. Two different cathodes, namely, V₂O₅ (RDH) and MnO₂ (Aldrich) (4.50 g), were prepared by mixing with 0.40 g AB and 0.05 g PTFE to form cathode pellets. The current collector, SS mesh, was placed in the middle of both pellets. The batteries were designed as Zn + ZnSO₄·7H₂O ‖ chitosan membrane ‖ MnO₂ and Zn + ZnSO₄·7H₂O ‖ chitosan membrane ‖ V₂O₅ proton batteries.

The Arbin BT 2000 system was used to characterize the battery properties. The OCV of the batteries was measured for 48 h. The batteries were discharged using a constant current of 1.0 mA. The I–V and J–P curves were plotted using a discharge current ranging from 20 μA to 100 mA. The average voltage of both batteries was monitored for each current drain after a 10 s operation. All of the battery characteristics were measured using the galvanostat of Autolab PGSTAT 30 GPES (Eco Chemie B.V.).

3.1 Conductivity of chitosan–NH₄CH₃COO–EC membranes

Fig. 1 depicts the variation of ionic conductivity of CA membrane as a function of NH₄CH₃COO and EC plasticizer contents at room temperature. The conductivity of CA increased proportionally with NH₄CH₃COO amounts (Fig. 1a). The CA conductivity in different NH₄CH₃COO amounts was between 10⁻¹⁰ and 10⁻⁴ S cm⁻¹. The highest conductivity of (1.47 ± 1.17) × 10⁻⁴ S cm⁻¹ was obtained by CA with 50 wt.% of NH₄CH₃COO (CA50N). However, when the amounts of NH₄CH₃COO increased to 60–70 wt.%, the conductivity steadily decreased.

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Figure 1 Conductivity of (a) CA with different concentrations of NH₄CH₃COO and (b) CA50N with different concentrations of EC.

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Given that CA50N membrane had the highest conductivity value for CA–salt system, EC was added as a plasticizer to enhance its conductivity. Fig. 1b shows that the same trend also occurred in CA50N in different EC plasticizer contents. The conductivity progressively increased proportionally with EC contents. In addition, the conductivity was much higher compared with the CA–salt system at a range between 10^–4 and 10^–3 S cm⁻¹ until 70 wt.% of EC. The highest conductivity of (3.83 ± 0.72) × 10^–3 S cm^–1 was achieved in the CA50N70E membrane.

The sharp initial increase continued by gradually increasing the conductivity of CA in various NH₄CH₃COO concentrations. This phenomenon was attributed to the increase of free charge carrier movement in the membranes. When the optimum conductivity was reached, it started to decrease beyond the addition of 60–70 wt.% of NH₄CH₃COO. During the increase in conductivity, the charge carriers became more densely packed as the salt concentrations increased. Thus, the attractive interactions between these free charge carriers also increased (Lewandowski et al., 2001). However, when the conductivity decreased, the host matrix became more packed with dopant ions. This overcrowding reduced the number of charge carriers because of the limitation of charge carrier mobility (Ng and Mohamad, 2006).

The steady conductivity increase for CA50 as a function of EC plasticizer can be related to the negative dissociation of salt. In this case, EC did not increase the charge carrier numbers similar to its effect on the function of salt. However, EC assisted the dissociation of salt by increasing the number of mobile charge carriers. This increase in numbers led to conductivity enhancement compared to the unplasticized membrane (Ng and Mohamad, 2006).

3.2 Mechanical properties

Fig. 2 shows the tensile strength of CA, CA50N, CA70N, and CA50N70E membranes. Based on the results, the mechanical properties of SPEs were expected to decrease beyond 70 wt.% of EC because of the changing crystalline structure arrangement of the samples. The growth of larger crystals occurred beyond 70 wt.% of EC by expanding the smaller crystals. This growth will affect the tensile properties of the membrane. Low mechanical properties were not appropriate in applications for proton battery membranes. Table 2 presents the tensile values of the membranes.

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Figure 2 Tensile strength of CA, CA50N, CA70N, and CA50N70E.

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To ensure adequate mechanical properties, the amount of EC was maintained at 70 wt.%, which was similar to the study of Ng and Mohamad (2008). Moreover, the tensile strength values were almost similar to the hydrated non-porous polypyrrole/chitosan between ∼0.07 and 0.53 M Pa (Wan et al., 2004). Wan et al. (2004) stated that chitosan is a great hydrophilic and semi-crystalline copolymer because it contains polar hydroxyl and amino groups. The hydrogen bonding of inter- and intra-molecules, which formerly appeared in dry state, will no longer exist in the hydrated state of chitosan. Thus, the tensile strength indeed reduced.

3.3 FTIR analysis of chitosan–NH₄CH₃COO–EC membranes

Fig. 3 presents the FTIR spectra of CP, CH₃COOH solution, and CA membrane at 4000–2500 cm⁻¹ and at 2000–500 cm⁻¹. Some of the peaks shifted after the CP was dissolved in CH₃COOH solution. The new peaks of 3202 and 3025 cm⁻¹ (CA membrane) were observed because of the shifting of 3352 cm⁻¹ of CP (Tan et al., 2005) and 3306 cm⁻¹ of CH₃COOH solution (stretching vibration of N–H and O–H bands). These peaks could be assigned to the axial stretching vibration of O–H superposed to the N–H stretching band and chitosan inter-hydrogen bonds (Enescu et al., 2009). The C–H band of CP at 2969 and 2829 cm⁻¹ also stretched to 2948 and 2830 cm⁻¹ (Fig. 3a).

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Figure 3 FTIR spectra for CP, CH₃COOH, and CA at (a) 4000–2500 cm⁻¹ and at (b) 2000–500 cm⁻¹.

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In Fig. 3b, the symmetry of C⚌O stretch peak shifted from 1640 cm⁻¹ (CH₃COOH solution) to 1633 cm⁻¹ (CA membrane). In this study, the cation of CH₃COOH solution interacted with the nitrogen atom of NH₂ in chitosan. Consequently, NH₂ and other bands shifted. The existence of 1534 cm⁻¹ confirmed the NH₂ deformation when peaks 1591 and 1562 cm⁻¹ of CP shifted after dissolving in CH₃COOH solution (Yahya and Arof, 2003). The O–H band of CH₃COOH solution at 1440 cm⁻¹ also shifted to 1400 cm⁻¹ in CA membrane. Moreover, the C–O stretch of CH₃COOH solution shifted from 1383 and 1269 cm⁻¹ to 1147 cm⁻¹ of CA.

In addition, the C–O bond stretching vibration of CP at 1079 and 1036 cm⁻¹ (Tan et al., 2005) shifted to 1066 and 1016 cm⁻¹ in the CA membrane. The two new peaks exhibited were 992 and 880 cm⁻¹ (C–O–H deformation). Meanwhile, the 647 and 615 cm⁻¹ peaks of the CA membrane were denoted as a O–C⚌O bond shift of CH₃COOH solution (609 cm⁻¹). The shifting of some FTIR peaks proved that the CP dissolved well in CH₃COOH solution.

Fig. 4 shows the effects of various NH₄CH₃COO salt concentrations based on the FTIR spectra. The broad 3184 cm⁻¹ peak (Fig. 4a) showed the NH₄⁺ band, whereas the two broad bands of 2934 and 2820 represent the shift of the C–H stretch of chitosan acetate after the addition of NH₄CH₃COO (Fig. 4b). Moreover, the 1399 cm⁻¹ peak (Fig. 4c) was referred to the NH₂ deformation of NH₄⁺. This result proved that complexation occurred between the salt and the nitrogen atom of the amine group (Yahya and Arof, 2003). Other peaks exhibited in Fig. 4d and e were almost similar (overlaps) to those in Fig. 3a and b. However, the difference between CA, CA50N, and CA70N can still be observed based on the peak intensity.

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Figure 4 FTIR spectra for CA, CA50N, and CA70N at (a) 4000–3000 cm⁻¹, (b) 3000–2000 cm⁻¹, (c) 2070–1170 cm⁻¹, (d) 1170–720 cm⁻¹, and (e) 700–550 cm⁻¹.

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All of the FTIR peaks of CA50N were higher compared with the CA and CA70N peaks. This result was attributed to the existence of the highest amount of charge carrier compared with the CA (without salt). Therefore, CA can easily complex with 50 wt.% of NH₄CH₃COO salt. On the other hand, CA70N had lower intensity of peaks compared with CA50N. The charge carrier of 70 wt.% of NH₄CH₃COO in CA70N reached the maximum dissociation to free charge carriers, and then associated again. The number of charge carrier that can supply mobile protons reduced. Consequently, decreasing the complexation between the salt and the CA resulted in a decreased sample.

Fig. 5 depicts the effect of additional 70 wt.% EC to CA50N. This amount was selected for the analysis because it produced the highest membrane conductivity in this study. All the peaks of CA, CA50N, and CA50N70E existed as in Fig. 5a and b, which were almost similar to those in Figs. 3 and 4. The plasticizer (EC) only enhanced the interaction between the salt and the chitosan acid to form complexation. This enhancement implied that the plasticizer has the same function as the filler (Majid and Arof, 2009).

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Figure 5 FTIR spectra for CA, CA50N, EC, and CA50N70E at (a) 4000–3000 cm⁻¹, (b) 3000–2000 cm⁻¹, (c) 2100–1120 cm⁻¹, (d) 1170–650 cm⁻¹, and (e) 700–550 cm⁻¹.

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Nevertheless, six new peaks of EC were observed, as shown in Fig. 5c and d. These peaks were referred to as C⚌O stretch (1750 cm⁻¹) (Osman and Arof, 2003), C–C(O)–C stretch (1210 and 1160 cm⁻¹), symmetrical C–O–C stretch (1100 cm⁻¹), and O–C–O bond (764 and 709 cm⁻¹), which shifted to some new peaks for CA50N70E. Thus, this result demonstrated that complexation between CA50N and 70 wt.% of EC was good. Moreover, no significant peaks shifted in Fig. 5e. Table 3 summarizes all the functional groups in Figs. 3–5.

Fig. 6 illustrates the chemical interaction during the conduction mechanism between CP, CH₃COOH solution, CA, NH₄CH₃COO, and EC. The formation of hydrogen bond between CP and diluted CH₃COOH solution possibly occurred because chitosan contains a hydroxyl group (OH) along the chain (Fig. 6a). This formation was due to the existence of H---O–H in diluted CH₃COOH solution that can allow the formation of hydrogen bond, similar to the interaction between glycerol and water (Dashnau et al., 2006). In the same manner, hydrogen bonding occurred through the lone pair of the amine group, that is, H₂N---H₂O. The formation of hydrogen bond confirmed that CP can be dissolved in diluted CH₃COOH solution, which was supported by the FTIR analysis, thus contributing to the conduction mechanism.

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Figure 6 Interaction of (a) chitosan power and diluted CH₃COOH solution, (b) CA–NH₄CH₃COO, and (c) CA50N-70 wt.% EC.

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Fig. 6b shows the interaction between the CA membrane and the NH₄CH₃COO at different concentrations. When different concentrations of NH₄CH₃COO were added to CA (in acidic media), the free amino group (–NH₂) of chitosan was protonated with H⁺ ion of NH₄CH₃COO by hopping mechanism. Two of the four hydrogen atoms of NH₄⁺ ions were identically bound. The third H⁺ was bound more rigidly, whereas the fourth H⁺ was bound more weakly. The weakly bound H⁺ of NH₄⁺ can easily be dissociated under the influence of an electric field. The movement of H⁺ charge transportation in the bulk between the CA and the NH₄CH₃COO was similar to the previous study of ion movement in PEO electrolyte (Maurya et al., 1992). Nonetheless, beyond the addition of 60–70 wt.% of NH₄CH₃COO, the re-association of H⁺ charge carriers (ion pairing) occurred when the salt reached its maximum dissociation to free H⁺ charge carriers.

Fig. 6c shows the interaction between the CA50N membrane and the 70 wt.% EC. EC only moved along the CA50N70E chain because the EC plasticizer only contributed to the dissociation of salt enhancement and did not increase the H⁺ charge carrier numbers. All of the conduction mechanisms were also supported by FTIR analysis in terms of chemical interactions between CP, CH₃COOH solution, NH₄CH₃COO, and EC plasticizer.

3.4 Surface morphology and structural properties of chitosan–NH₄CH₃COO–EC membrane

Fig. 7 shows the FESEM images and XRD diffractogram patterns of CP, CA, CA50N, CA70N, and CA50N70E membranes. Fig. 7a shows the scattered and bulky CP surface. The CP was partially crystalline, with a broad peak of 2θ between 16° and 24°. Upon mixing the CH₃COOH solution, the CA membrane surface became clear and uniform in appearance (Fig. 7b). The broad XRD peak of CA became less intense with the existence of some new peaks of 2θ between 12° and 40°. Meanwhile, the uniform morphology with a few small particles appeared when 50 wt.% of NH₄CH₃COO was added to CA50N membrane, as shown in Fig. 7c. The three slightly broadened humps of XRD peaks appeared at 2θ = 16°, 30°, and 40°.

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Figure 7 FESEM and XRD images of (a) chitosan powder, (b) CA, (c) CA50N, (d) CA70N, and (e) CA50N70E.

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After the addition of 70 wt.% of NH₄CH₃COO, numerous small particles with rough surfaces can be observed on the CA70N membrane surface (Fig. 7d). Furthermore, the addition of NH₄CH₃COO resulted in a decreased intensity of three XRD humps in the CA70N membrane. Only the broad peak of 2θ that shifted between 16° and 40° can be observed because the peak of 2θ = 30° diminished. However, in Fig. 7e, the CA50N70E membrane surface transformed into a slightly uniform surface, which was influenced by the EC plasticizer. The XRD broad peak was slightly increased and shifted to the left of 2θ = 24°.

The agglomerated and bulky surface was attributed to the partially crystalline CP, which agreed with the XRD results. Meanwhile, after mixing CH₃COOH solution with CP, the CA membrane surface turned clear and uniform because of the reaction between solid (CP) and liquid (CH₃COOH). The addition of 50 wt.% NH₄CH₃COO salt contributed to the smooth and uniform CA50N membrane surface. However, when the amount of NH₄CH₃COO reached 70 wt.%, the surface of the CA70N membrane became rough. This finding was attributed to the excess amount of salt reacting with CA solution. Compared with the CA70N membrane, the appearance of the CA50N70E membrane was jelly-like with a smooth surface. This surface was due to the plasticizer, which improved the structural properties of the membranes by steady enhancement in the polymer crystal and amorphous phase.

In general, the crystallinity of CP decreased upon mixing CH₃COOH solution with CA. Thus, the intensity of the broadened peak also decreased. When 50 wt.% NH₄CH₃COO was added to the samples, the original CP peaks (2θ = 20°) shifted to the right (2θ = 30°). This finding indicated that the crystallinity of the sample also further decreased and became an amorphous film. Meanwhile, further destruction of the CA70N membrane crystalline at 2θ = 16–40° was attributed to the reaction between the chitosan and the cation of salt (Majid and Arof, 2009). This destruction made the membrane too pliable, and thus, unsuitable for SPE application.

Nonetheless, in the CA50N70E membrane, the intensity of the broad peak slightly shifted to the left, implying the reaction between the chitosan membrane and the plasticizer. In this study, the plasticizer altered the membrane structure by moving along the membrane chain. The movement of this plasticizer directly opened and widened the path along the membrane chain and improved the structural properties. Thus, the CA50N70E membrane structure was better compared with the CA70N membrane surface. In addition, as shown in FESEM, the CA50N70E membrane surface transformed to a more uniform surface compared with the CA70N membrane surface.

The XRD patterns and FESEM also supported the result of the conductivity study. The change in polymer–salt conductivity with the addition of different concentrations of salt and plasticizer indicated the structural changes within the samples (Koh et al., 2012). Moreover, the transformation of the membrane surface was observed as a result of the addition of NH₄CH₃COO and EC plasticizer.

Fig. 8 presents the EDX analysis of CP, CA, CA50N, CA70N, and CA50N70E membranes, which supported the FESEM images. The increasing amount of nitrogen wt.% in CA50N was proportional to the salt concentrations in CA70N. Nevertheless, the decrease of nitrogen wt.% in CA50N70E proved that the electron donor atoms in the polymer interacted with the salt cation and EC plasticizer.

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Figure 8 EDX of (a) chitosan powder, (b) CA, (c) CA50N, (d) CA70N, and (e) CA50N70E.

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3.5 Proton battery performance analysis

Fig. 9 shows the OCV of Zn + ZnSO₄·7H₂O ‖ CA50N70E ‖ MnO₂ and Zn + ZnSO₄·7H₂O ‖ CA50N70E ‖ V₂O₅ proton batteries. The proton battery using MnO₂ cathode was much more stable (1.60 V) compared with the proton battery using V₂O₅ (1.27 V) for 48 h. The stable potential provided better function and more durable proton battery. The chemical reactions that possibly occurred in the proton battery using MnO₂ cathode are as follows (Weast, 1977):

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Figure 9 Open circuit voltage of proton batteries using CA50N70E membranes.

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At the negative (anode) electrode, Zn was oxidized with the release of two electrons, and ZnSO₄·7H₂O provided the H⁺ ions:

• At the positive (cathode) electrode, MnO₂ was reduced with the acceptance of electrons:

• The overall proton battery reaction was calculated based on the standard electrode potential (the oxidation potential was the negative value of the reduction potential) (Linden, 2002):

• At the negative (anode) electrode, Zn was oxidized with the release of two electrons, and ZnSO₄·7H₂O provided the H⁺ ions:

• At the positive (cathode) electrode, V₂O₅ was reduced with the acceptance of electrons:

• The overall proton battery reaction is as follows:

In this study, the overall reaction provided the cell with E°_cell of 1.28 and 1.02 V for proton battery using MnO₂ and V₂O₅. However, the E°_cells of both batteries were 1.60 and 1.27 V, respectively. Eqns. (2)–(8) are possible because the fabrication of both batteries achieved OCV values higher than the theoretical calculation. The OCV values obtained in this study were more or less the same with the values obtained by previous works on proton battery using the same cathode materials (MnO₂ and V₂O₅), which were 1.56 (Ng and Mohamad, 2006), 1.57 (Pratap et al., 2006), and 1.48 V (Ng and Mohamad, 2008).

Fig. 10 indicates the discharge profile of Zn + ZnSO₄·7H₂O ‖ CA50N70E ‖ MnO₂ and Zn + ZnSO₄·7H₂O ‖ CA50N70E ‖ V₂O₅ proton batteries at 1.0 mA. The initial voltages of the proton batteries that used MnO₂ and V₂O₅ cathodes were 1.59 and 1.39 V, respectively. The results showed that both voltages were reduced to 0.50 V after sustaining for 52 min (MnO₂) and 49 min (V₂O₅). The discharge capacities of the proton batteries using MnO₂ and V₂O₅ cathode were 45.0 and 34.7 mA h, respectively. Based on the discharge profile, the proton batteries using MnO₂ cathode achieved higher discharge capacity compared with those using V₂O₅ cathode. In general, cathode materials are reactive with well-known electrolyte solutions, thus achieving rich surface chemistry (Aurbach et al., 2007).

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Figure 10 Discharge profile at 1.0 mA of proton batteries using CA50N70E membranes.

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The maximum discharge current was controlled by the three processes that occurred during discharge: (i) hydrogen was inserted into the cathode material, (ii) electrons from the anode reduced Mn²⁺ and VO²⁺ ions in the cathode to a lower valence, and (iii) admittance of the H⁺ ions in the electrolyte to the electrode surface. Exchange of H⁺ ions with the electrolyte occurred at the electrode–electrolyte interface. Besides the intrinsic electrochemical properties of the material, cathode performance depended significantly on the electrode microstructure and morphology. These reactions were almost similar to the cathode materials for Li⁺ batteries (Fergus, 2010).

The discharge capacity of the battery with V₂O₅ cathode was lower compared with the battery with MnO₂ cathode. This finding was attributed to the properties of V₂O₅. Generally, V₂O₅ has a high specific capacity of crystallinity. However, V₂O₅ undergoes structural conversion produced by the mechanical stress during deep charge–discharge cycles. Therefore, this conversion directly reduces the battery specific capacity besides decreasing the operating voltages (Fergus, 2010; Feng et al., 2008).

Fig. 11 shows the characteristics of I–V and J–P of Zn + ZnSO₄·7H₂O ‖ CA50N70E ‖ MnO₂ and Zn + ZnSO₄·7H₂O ‖ CA50N70E ‖ V₂O₅ proton batteries using current drains ranging from 2 μA to 100 mA. The voltage dropped from 1.70 to 0.34 V for the proton battery using MnO₂. Meanwhile, for the proton battery with V₂O₅, the voltage dropped from 1.56 V to 0.40 V. The maximum power densities were 1.83 mW cm⁻² for the battery with MnO₂ and 1.36 mW cm⁻² for the battery with V₂O₅ cathode based on the J–P curves.

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Figure 11 Plot of I–V and J–P using CA50N70E membranes for proton batteries.

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The I–V curves for both batteries were linear, which denoted that the ohmic contribution was mainly controlled for the polarization of the electrode. However, the maximum power densities obtained for both batteries were lower compared with those from prior studies (Kadir et al., 2010; Ng and Mohamad, 2006, 2008). These lower densities were attributed to the adhesion between the electrolyte and the MnO₂ surface, which was contributed by the battery properties. On the other hand, in the study done by Pratap et al., (2006), V₂O₅ has been mixed with other intercalating oxides such as PbO₂ in order to give a better battery performance.