As well-known, fuel cells attract attention as a clean next-generation energy. Among them, solid fuel cells are particularly attracting attention in terms of portability and stability. Therefore, these are required to find new electrolytes of the fuel cell, which is low cost and has no environmental load in the production process of the fuel cell. Recently, we focus on the tissue-derived biomaterials as the electrolyte of a fuel cell, because the tissue-derived biomaterials are low cost and abundant in nature. Chitin, which is one of tissue-derived biomaterials, is obtained from the shells and tendon, such as crabs, shrimps, etc., and is biomass of huge amount in nature. Furthermore, the chitin and the deacetylate chitin (chitosan) have been recently used in the several fields such as the industrial and medical care field, because chitin and chitosan have superior strength, thermal stability and biocompatibility as the biomaterial. Malette et al. have investigated the curative effect of chitosan on the vulnerary [1] . Sandford et al. have suggested the useful substituent effect of chitosan to skin [2] [3] . Nair and Madhavan have investigated the method for the elimination of Hg in solution using chitosan [4] . Peniche-Covas et al. have investigated the efficiency of adsorption of Hg [5] . Jha et al. have also shown that chitosan powder has the effect on the adsorption of Cd ions [6] . Allan et al. reported that chitosan gives the wet strength of paper [7] . In the field of electrical device, Schulte et al. have shown that chitin can be used as an electrochemical sensor component [8] . In this way, a lot of investigations on chitin and chitosan have been carried out. This fact is realized by the characteristic features of chitin and chitosan which are easy to handle.

Recently, we have found that the fuel cell based on the biopolymer “collagen” electrolyte shows the maximum power density 0.86 mW/cm² with the humidified condition and that operates LED lighting as shown in Figure 1 [9] . This result indicates that tissue-derived biomaterials become the electrolyte of a fuel cell. Very recently, we have discovered the new bio-electrolyte of the fuel cell. This electrolyte consists of chitin, and the fuel cell based on chitin electrolyte exhibits higher power density compared with that based on other biomaterials, which are DNA and collagen and so on [9] [10] . Moreover, we have investigated the key factor in the appearance of proton conductivity in chitin. In the present paper, these results are shown. These results will be helpful for the development of further new fuel cells based on the biomaterials.

Figure 5 shows the current density―cell voltage (i-V_cell) curve of the fuel cell based on chitin electrolyte with the 100% relative humidity. As shown in Figure 5, the cell voltage steeply decreases due to the anode over-potential at the very lower current region and then linearly decreases with increasing current density. In this way, the i-V_cell curve shows the typical i-V_cell characteristics of a fuel cell. This result indicates that the chitin becomes the electrolyte of the fuel cell and the chitin exhibits proton conductivity with the humidified condition. Further, it is also noted that the fuel cell using chitin electrolyte exhibits the maximum power density 1.35 mW/cm². This value is large compared with the fuel cell based on other biomaterials of collagen and DNA [10] .

Figure 6 shows the relation between the relative humidity R_H and n in the humidified chitin. Here, n is the number of water molecules per one molecule of the chitin at 300 K. This result indicates that n monotonically increases with the increase of humidity. This result indicates that there is no hydration-induced transition in the humidity region between 50% and 100% in chitin, and chitin has 2 water molecules per 1 molecule at the 100% relative humidity. Therefore, for the analysis of proton conductivity, we can use not the relative humidity but the number “n” as an index of the hydration.

Figure 7 shows the frequency dependence of AC conductivity. In Figure 7, AC conductivity is calculated with the equation σ = ωε₀ε” obtained from the simple Capacitance-Resistance (C-R) parallel equivalent circuit for specimen. Here, ω is angular frequency, ε₀ is the dielectric constant in vacuum, and ε” is the imaginary part of dielectric constant. We can clearly see that all AC conductivity increases with the increase of the hydration number n. These results indicate that AC conductivity increases by the formation of the bonding between the chitin and water molecules. Moreover, it is noted that the conductivity calculated by σ = ωε₀ε” strongly depends on the frequency. This result means that the chitin cannot be described by simple C-R equivalent parallel circuit. In order to clear the origin of frequency dependence of AC electrical conductivity, we show the relation between the real and imaginary parts obtained from the measured impedance data in Figure 8. As shown in Figure 8, the observed impedance data Z shows the semicircle part and linear parts in the impedance plane. This result indicates that two components are contained in the measured impedance. The almost linear behavior in impedance plane is well known as the contribution of Warburg impedance caused by the double layer capacitance and charge transfer resistance at the interface between the electrode and the specimen [14] . On the other hand, the part of semicircle behavior in impedance plane is also observed. When the specimen is considered by the simple C-R parallel equivalent circuit, the impedance Z can be described by the following equation,

Z = R 1 + ( ω C R ) 2 − j ω C R 2 1 + ( ω C R ) 2 (1)

Here, R and C are resistance and capacitance in the simple C-R parallel equivalent circuit, respectively, and j is imaginary unit.

For Z ' = R 1 + ( ω C R ) 2 and Z ' ' = j ω C R 2 1 + ( ω C R ) 2 , the relation between Z’ and Z” becomes the following equation,

( Z ' − R / 2 ) 2 + Z ' ' 2 = ( R / 2 ) 2 . (2)

In this way, the semicircle part in the impedance plane is caused by the simple C-R parallel equivalent circuit in the specimen. That is, considering these results, the observed semicircle part of Z represents the conductivity and polarization property in bulk of chitin. In addition, we note that DC proton conductivity in the bulk of chitin can be derived by extrapolating the circular arc to the real axis in the impedance plane.

Figure 9 shows the relationship between DC proton conductivity σ_DC and n in the bulk of chitin. The DC proton conductivity is obtained from the extrapolation of the circular arc using the method shown in Figure 8. As shown in Figure 9, DC proton conductivity increases with increasing n and tends to saturate in the region of n > 1. This result indicates that the formation of proton conduction path changes at around n = 1. That is, the bonding of water molecules to chitin molecule at around n = 1 changes from that below n < 1. Considering that the bonding between chitin and water molecule is formed by hydrogen bond, the formation of hydrogen bond between chitin and water molecule plays an important role in proton conductivity. In addition, as shown in Figure 9, it is noted that there is the anisotropy for proton conductivity in chitin. It is evident that the water number n dependences of σ_p and σ_N show the similar behavior. Moreover, it is also noted that σ_p becomes approximately 1 × 10⁻¹ S/m at n = 2. On the other hand, σ_N becomes about 2 × 10⁻² S/m. The conductivity normal to plane is 5 times smaller than that in plane. These results indicate that the direction of bonding between the chitin and water molecules is important for the determination of the value of proton conductivity. It is well known that the chitin has fiber structure. Therefore, in order to investigate the orientational dependence of proton conductivity in detail, we have measured the difference between proton conductivities along the fiber σ_PF and perpendicular to the fiber σ_NF.

Figure 10 shows the humidity dependences of σ_PF and σ_NF. Here, σ_PF and σ_NF are proton conductivity parallel and perpendicular to chitin fiber, respectively. It is evident that σ_PF has approximately 6 times higher than σ_NF. In the sheet sample in Figure 9, it is deduced that chitin fiber is lied in plane of chitin sheet and that therefore the conductivity in plane is mainly related to proton conductivity along the fiber. On the other hand, proton conductivity normal to the chitin sheet is mainly contributed by that perpendicular fiber direction. Therefore, the anisotropy for proton conductivity of chitin fiber in Figure 10 is consistent with that in sheet sample (Figure 9). Considering the results for anisotropic proton conductivity, it appears that the relation between chitin hydrated structure and proton conduction path formed by the hydrogen bond with water molecule is significantly important.

Figure 11 shows the crystal structures in the a-c and b-c planes in the hydrated chitin reported by many researchers [15] - [20] . As shown in Figure 11(a), it is evident that the hydrated chitin has water network which is formed along the chitin fiber direction. On the contrary, as shown in Figure 11(b), the formation of water network perpendicular to the chitin fiber direction is difficult compared with that along the fiber direction, because water molecules are bonded between chitin fibers. Therefore, proton transport in perpendicular to the fiber direction is prevented by difficulty of formation of the water network. These results indicate that the direction of water network is closely related to the value of proton conductivity. It is also noted that these water molecules form hydrogen bonds between hydroxyl group and amino-acetyl group in chitin, or between water molecule and next water molecule. Considering that proton conductivity increases with the increase in the number of water molecule and that proton conductivity along the direction of water network becomes high, these results indicate that the realization of proton conductivity is achieved by proton transfer via the water network formed between chitin and water molecules with the breaking and rearrangement of hydrogen bond. In addition, it seems that the hydroxyl group and amino-acetyl group in the side chain of chitin, which form the hydrogen bonds with water molecules, facilitate the generation of hydronium ions. It is deduced that the generated hydronium ions in the water network move with the breaking and rearrangement of hydrogen bond, and consequently proton conductivity is realized in the water network. It is necessary to investigate the quantitative analyses between the water network and proton conductivity, in

order to understand the anisotropic conductivity in chitin in detail. These results will appear future issues.

We have fabricated the chitin fuel cell and investigated whether the chitin becomes the electrolyte or not. As a result, chitin becomes proton conductor and operates as the electrolyte of the fuel cell. The proton conductivity increases with the increase in the number of water molecule. Moreover, chitin has the anisotropic conductivity from the measurement of proton conductivity in the sheet and highly-orientated specimens. It is found that proton conductivity along the chitin fiber direction becomes higher than that perpendicular to the chitin fiber direction. Considering that water molecules is bonded with the side chain of chitin along the chitin fiber direction, the appearance of proton conductivity in chitin is caused by the proton transfer via the water network formed between the chitin and water molecules.

The authors declare no conflicts of interest regarding the publication of this paper.

Kawabata, T. and Matsuo, Y. (2018) Chitin Based Fuel Cell and Its Proton Conductivity. Materials Sciences and Applications, 9, 779-789. https://doi.org/10.4236/msa.2018.910056