Recently, biomaterials have been attracting attention as clean next-generation materials [1] [2] [3]. As well-known, tissue-derived biomaterials are biodegradable and environmentally friendly, and therefore it is expected that tissue-derived materials become new functional materials toward the next-generation. Chitin is one of the tissue-derived biomaterials and is a polymer of N-acetylglucosamine. In addition, chitin is well-known as the main component of shell such as crabs and shrimps and is abundantly present in nature. Recently, chitin’s excellent biocompatibility and strength are widely used not only in the medical field but also in the industrial field. Allan et al. reported that chitosan gives the wet strength of paper [4]. Nair and Madhavan have investigated the method for the elimination of Hg in solution using chitosan [5]. Schwarz et al. reported the adsorption effect of iron and sulfate ions on chitosan membrane [6]. Leslie et al. showed that chitin and chitin derivatives have an effect of promoting wound healing [7]. Nunthanid et al. reported the results of using spray-dried chitosan acetate and ethylcellulose as a new compressed coat of 5-aminosalicylic acid tablets [8]. Tripathi et al. reported on the preparation and antibacterial activity of chitosan-based films for food packaging [9]. Kim et al. reported on the potential of chitosan bio-devices [10]. Kenawi et al. Reported that chitosan and its derivatives could be used for medicinal purposes [11]. However, the applications of chitin are few in the energy field. As well known, in recent years, fuel cells have attracted attention as a next-generation energy. The problem with fuel cells is that the cost of the electrolyte is high, and environmental impact is imposed during manufacturing and disposal. Recently, we have fabricated the fuel cell using the electrolyte of chitin or chitosan which is known as the deacetylated chitin. Chitin and chitosan are available at low cost and are environmentally friendly. Figure 1(a) and Figure 1(b) respectively show the current-cell voltage characteristics of fuel cell using the chitin electrolyte and chitosan electrolyte obtained by the injections of hydrogen and oxygen fuel gas. It is evident that these current-cell voltage characteristics show typical characteristics of the fuel cell. These results indicate that the chitin and chitosan become the electrolyte of fuel cells. Further, as shown in Figure 1(a), the fuel cell-based on chitin electrolyte operates at maximum power density 1.35 mW/cm², while the fuel cell using the chitosan electrolyte shows the maximum power density of 0.032 mW/cm² [12] [13]. The prepared fuel cell used platinum electrodes and a 70 µm thick chitin or chitosan sheet as the electrolyte. In this way, the power density of the fuel cell using chitin electrolyte is considerably larger than that using chitosan electrolyte. These results indicate that proton conductivity of chitin is different from

that of chitosan. More recently, we have revealed that chitin becomes superior proton conductor among the natural bio-electrolyte and that the chitin in the humidified condition shows approximately 1.0 × 10⁻¹ S/m, which is approximately 10³ times higher than that in chitosan [12] [13].

From these facts, it is evident that the acetyl group plays an important role in proton conductivity in chitin system. In addition, it is noted that the open circuit voltage of the fuel cell using the chitosan electrolyte is slightly higher than that using the chitin electrolyte, as shown in Figure 1(a) and Figure 1(b). These results indicate that proton conductivity may change by changing the chitin-chitosan mixing ratio. Very recently, we have measured the characteristic feature of proton conductivity in the chitin-chitosan mixed compounds (chitin)_x(chitosan)_1−x and obtained the result that the mixing ratio x dependence of proton conductivity shows anomalous behavior. In the present work, in order to clarify the main factor determining proton conductivity in (chitin)_x(chitosan)_1−x, we have investigated the cause of anomalous behavior of proton conductivity observed in the chitin-chitosan mixed compounds, which is significantly important for fabricating chitin based fuel cell or other electrical devices. The obtained results include the useful information that the properties of biopolymers can be easily controlled by the mixing ratio of polymers. Therefore, the obtained results will be helpful not only to use the fabrication of electrical devices such as field effect transistor or fuel cell based on chitin-chitosan mixed compound but also to fully understand the mechanism of proton conductivity on biopolymers.

Figure 2 shows the photograph of the chitin-chitosan mixed membrane of (chitin)_0.5(chitosan)_0.5. Films with other mixing ratios (chitin)_x(chitosan)_{1 − x} have the same color appearance, moisture resistance and will not break or melt under humid conditions.

Figure 3 shows the Nyquist plot obtained by the real and imaginary parts of the impedance measurement data for the several mixing ratios around the 80% relative humidity. As shown in Figure 3, in the high frequency region, the relation between the real and imaginary parts of impedance becomes semi-circle arc. Therefore, the obtained results can be described by a parallel equivalent circuit of the components of capacitance C and of resistance R_DC. These results indicate that R_DC is obtained from intersects of the horizontal axis of semi-circle arc as described above. As seen in Figure 3, R_DC of chitin is considerably smaller than

that of chitosan. This result means that proton conductivity in chitin is considerably higher than that in chitosan and therefore is consistent with previous study [10]. Furthermore, we note that the semi-circle arc size doesn’t monotonously reduce by the reduction of the mixing ratio x. That is, the mixing ratio x dependence of R_DC isn’t monotonous change but shows the anomalous behavior.

The DC conductivity σ DC is obtained from the DC resistance R_DC. Figure 4 shows the relative humidity dependence of σ DC obtained from the analysis of Nyquist plot in Figure 3. Here, x in Figure 4 denotes the mixing ratio x. The humidity dependence of proton conductivity in each mixing membrane shows a monotonous increase with the increase in relative humidity. In this way, the behavior of the increase in conductivity by increasing R_H is the same in all specimens.

On the other hand, it is noted that the conductivity changes approximately 10 to 100 times by the change of x. Moreover, as shown in Figure 4, it is evident that proton conductivity shows anomalous behavior with changes of x. For example, with increasing x, proton conductivity at the 80% relative humidity increases from x = 0 to x ~ 0.4, but decreases at x ~ 0.6 and increases again with

the increase in x. Thus, considering that the mixing ratio x dependence of σ₀ shows the anomalous behavior, x contains an important factor in determining proton conductivity.

In order to examine the mixing ratio x and proton conductivity in detail, the relation between x and σ₀ at the 80% relative humidity is plotted in Figure 5. As shown in Figure 5, it was found that there is an anomalous correlation between x and σ₀ and that a large peak around x = 0.75 and a small peak around x = 0.4 are observed. It has been already clarified that the difference of proton conductivity between chitin and chitosan is mainly caused by the presence of the acetyl group. In the case of chitin, high-proton conductivity is realized by the water network formed between the fibers. On the other hand, chitosan, which is a deacetylated form of chitin, forms strong hydrogen bonds between chitosan-main chains due to the amino group exposed by the deacetylation, and the hydration number decreases and therefore proton conductivity decreases. Considering these facts, it seems that the increase in chitosan content, that is, the decrease in x, leads to a monotonous decrease in proton conductivity. However, the measured result shows the two peaks around x = 0.4 and 0.75. This result is very interesting, because we can control proton conductivity by adjusting the amount of acetyl-group (that is, the amount of x) and will be helpful to understand the main factor for the appearance of proton conductivity in chitin-type polymer.

In order to understand this anomalous behavior of proton conductivity, we have investigated the change of hydration for the mixing ratio using IR spectra. Figure 6 shows IR spectra in the samples with various mixing ratios of chitin and chitosan. Here, IR spectra of dry and wet conditions are shown in Figure 6(a) and Figure 6(b), respectively. In these figures, IR spectra are normalized by

the absorbance of C-O stretching (1070 cm⁻¹) at which no change between chitin and chitosan is observed in the IR spectrum. As shown in Figure 6, the IR spectra remarkably changes by water content and the drastic spectrum changes due to the existence of water are observed in the wet sample. As shown in Figure 6(a), it can be seen that the peak appearing in the vicinity of 1600 cm⁻¹ increases as the mixing ratio x = 0 increases to x = 1. It is known that the absorption band appearing near 1600 cm⁻¹ is the absorption of Amide I derived from the amino acetyl group of chitin. Therefore, the results in Figure 6(a) show that the mixing ratio and the absorbance peak at 1600 cm⁻¹ in the dried sample are consistent. Comparing Figure 6(a) and Figure 6(b), the wet sample shows large change in peak at around 600 cm⁻¹ and 1600 cm⁻¹, and above 3000 cm⁻¹. It was also found that there was a peak that appeared only in the wet sample even at 2100 to 2200 cm⁻¹. These results indicate that the difference between wet and dry samples is due to the presence or absence of water molecule. In order to make clear the effect of hydration, it is necessary to extract only the absorbance of water molecule without the overlapping with other spectra. It is known that there are several absorption bands of chitin and chitosan around 650 - 690 cm⁻¹ (OH out of plane), 1630 - 1660 cm⁻¹ (Amide I band) and 3100 - 3450 cm⁻¹ (NH and OH stretching), and therefore it is difficult to extract only the effect of water molecule using the IR spectra around these bands [14] [15] [16]. It is also known that IR spectrum around 2110 cm⁻¹ remarkably changes by hydration and that doesn’t overlap with other peaks [17] [18] [19] [20]. Therefore, in the present work, the effect of hydration is extracted using the difference spectra at the peak around 2110 cm⁻¹.

Figure 7 shows relationship between absorption intensity and the mixing ratio x. Here, the absorption intensity is calculated by the integration of the difference between dry and wet spectra around 2110 cm⁻¹. As shown in Figure 7, it is evident that the change in the intensity of the peak is not monotonous change but the abnormal behavior with the change in the mixing ratio x. The absorption intensity of chitin (x = 1) is the strongest, and the peaks of absorption intensity are observed around x = 0.75 and 0.4. It is noted that as shown in Figure 5 proton conductivity also shows the peaks around x = 0.75 and 0.4. In this way, the anomalous behavior of proton conductivity in Figure 5 is closely related to that of the absorption intensity corresponding to the effect of water. Considering that the absorption intensity is determined by the number of water content, the anomalous behavior of proton conductivity in Figure 5 is caused by the amount

of water molecule included in chitin-chitosan mixed compounds. That is, the main factor determining proton conductivity in chitin-chitosan mixed compounds is the amount of water content. Therefore, these results indicate that we can control proton conductivity in the chitin-type polymer by adjusting the amount of water content. These results can be used to produce the conducting film with various proton conductivities and can be applied to the fabrication of fuel-cell electrolyte or electrical devices. The challenges to these applications will appear in other articles. In addition, it is known that the number of hydrogen bonds between water molecules and the side chain in chitin is many in comparison with that in chitosan. On the other hand, the space, where water molecule is bonded with the side chain of chitin or chitosan will be expanded by deacetylation. Considering these facts, the anomalous behavior of water contents in Figure 7 may be caused by the water contents determined by the balances between the amount of the side chain-water molecule hydrogen bond and the number of the space where waters bind with the chitin or chitosan molecule. We are now planning to measure the structure of hydrated chitin-chitosan mixed compounds using the X-ray and neutron diffraction measurements, in order to understand the hydration process in the chitin-chitosan mixed compounds. These results will appear in next issue.

In order to investigate the main factor of proton conductivity in chitin-chitosan mixed compounds, the mixed membranes of (chitin)_x(chitosan)_1−x were prepared and proton conductivity was investigated. Proton conductivity in the chitin-chitosan mixed compounds monotonously increases by increasing relative humidity. These results indicate that the humidified condition is important for the realization of proton conductivity in chitin-type polymers. Moreover, proton conductivity in chitin is approximately 10³ times higher than that in chitosan. From these results, it seems that proton conductivity monotonously decreases by decreasing x in (chitin)_x(chitosan)_1−x. However, we can obtain that the mixing ratio x dependence of proton conductivity has two peaks around x = 0.75 and x = 0.4. That is, proton conductivity has optimal conditions for the realization of high-proton conductivity in (chitin)_x(chitosan)_1−x. This result is very interesting and allows us to control proton conductivity by adjusting the number of acetyl-group. Furthermore, in order to investigate the cause of the anomalous behavior for the x dependence of proton conductivity, we have measured FT-IR spectra in dry and wet (chitin)_x(chitosan)_1−x specimens. From these results, it was clarified that the absorption intensity around 2110 cm⁻¹, where the intensity remarkably changes by the amount of water content, has peaks around x = 0.75 and 0.4. These values are in good agreement with the values of x, where proton conductivity has peaks as shown in Figure 5. From these results, it was found that proton conductivity in (chitin)_x(chitosan)_1−x is determined by the amount of water content changed by x. In addition, using these results, we can control proton conductivity in the chitin-type polymer by adjusting the amount of the mixing ratio x, and will be able to apply to the fabrication of the conducting film such as fuel-cell electrolytes or thin film transistors.

We would like to appreciate Prof. Hiroshi Matsui in Tohoku University for useful discussion on FT-IR measurements. This work was partially supported by JSPS KAKENHI Grant Number 18K11741.

The authors declare no conflict of interest.

Kawabata, T., Takahashi, Y. and Matsuo, Y. (2020) Anomalous Proton Conductivity in Chitin-Chitosan Mixed Compounds. Materials Sciences and Applications, 11, 1-11. https://doi.org/10.4236/msa.2020.111001