**3.3 Percolation conductivity in chitin**

In order to clarify the role of water molecules in chitin, we obtained the volume fraction of water molecules with respect to chitin molecules from the results of the relative humidity dependence of the hydration number, and investigated the relationship with proton conductivity. Yamada *et al*. measure the resistance value that changes by gradually increasing the volume fraction of the conductor material in the non-conductor material [12]. As a result, Yamada *et al*. report that the resistance

**Figure 5.** *FT-IR spectra of chitin sample [10].*

#### **Figure 6.**

*Relationship between volume fraction of water molecules and percolation conduction in chitin [11].*

value becomes constant at a volume fraction of about 30%. It is well known that the crystal structures of chitin and chitosan have been clarified for a long time and many reports have been published [13–19]. Sawada *et al.* report on the crystal structure of β-chitin dihydrate. We have found from water content experiments of chitin that each constituent unit has two water molecules, and that the proton conductivity is saturated during this dihydrate formation [8, 9]. Moreover, it has already confirmed that XRD diffraction pattern of our chitin sample is consistent with pattern of general chitin. Based on these results, the volume fraction of water molecules contained in the crystal lattice was estimated with reference to the report by Sawada *et al* [14–16]. **Figure 6** shows the relationship between the volume fraction of water molecules in chitin hydrate and the proton conductivity. The relationship between the volume fraction and the conductivity of the conductor in the non-conductor of **Figure 6** is a value calculated from the resistance value reported by Yamada *et al*. As can be seen in **Figure 6**, water molecules in chitin behave very much like conductors in non-conductors. In other words, this result indicates that water molecules in chitin function as a proton transport pathway.

#### **3.4 Activation energy and proton pathway**

Investigation of proton transport pathways in chitin is important for the future development and development of polysaccharide electrolyte membranes. So far, it has been shown that chitin has proton conductivity, and it is important that the introduction of water molecules behaves like a conductor for the proton conductivity. In addition, since the structure of chitin hydrate has been clarified, we approached the proton conduction pathway of chitin by measuring impedance using an oriented sample. As a result, it was confirmed that chitin and chitosan have orientation dependence of proton conductivity [8, 9]. From this result, the temperature dependence of the proton conductivity of the chitin system was investigated. **Figure 7** shows the proton conductivity in the chitin fiber direction when only the temperature factor is changed while maintaining a constant wet weight. As shown in **Figure 7**, the relationship between the reciprocal of temperature and the proton conductivity of chitin shows an Arrhenius-like linear change. This result indicates that the proton conductivity of chitin has thermal activity. **Table 1** shows the activation energy of proton conductivity derived from the Arrhenius equation

**171**

µ

following equation:

Here, σ and σ

**Figure 7.**

**Table 1.**

tion energy.

*Proton Conductivity in Chitin System*

*DOI: http://dx.doi.org/10.5772/intechopen.96799*

*Arrhenius plot of proton conductivity in chitin [11].*

*Activation energy of proton conduction in the chitin system.*

in each fiber direction of chitin and chitosan. Arrhenius equation is represented by

= − **0exp**

As shown in **Table 1**, the activation energy of chitin-based proton conduction changed depending on the fiber direction in the case of chitin, and no change was observed in the case of chitosan. In general, a decrease in activation energy indicates a decrease in energy required for proton transport, and is therefore expected to contribute to the realization of high proton conductivity. Considering this, it is considered that the decrease in activation energy of chitin in the vertical direction of the fiber is appropriate. However, in the oriented sample, the activation energy of chitin in each fiber direction seems to be inconsistent, considering that the proton conduction of chitin in the fiber direction is the highest. Since the activation energy is not the only element required for proton conduction, the following equation:

quency is 0, *k* and *T* are boltzmann constant and temperature, ∆*E* is activa-

σ

the charge concentration *e* is that the amount of water of crystallization is *e*,

 µ

Here, *z* and *n* are ion valence and number of proton transport pathway, *e* and

are charge density and mobility. Since the charge of the proton is +1 and *z* = 1,

σ σ

Chitin 0.55 0.30 Chitosan 0.57 0.58

> *E kT*

**Fiber direction (eV) Fiber vertical direction (eV)**

**<sup>0</sup>** are proton conductivity and proton conductivity when fre-

<sup>∆</sup> (2)

= *zne* (3)

*Proton Conductivity in Chitin System DOI: http://dx.doi.org/10.5772/intechopen.96799*

#### **Figure 7.**

*Chitin and Chitosan - Physicochemical Properties and Industrial Applications*

value becomes constant at a volume fraction of about 30%. It is well known that the crystal structures of chitin and chitosan have been clarified for a long time and many reports have been published [13–19]. Sawada *et al.* report on the crystal structure of β-chitin dihydrate. We have found from water content experiments of chitin that each constituent unit has two water molecules, and that the proton conductivity is saturated during this dihydrate formation [8, 9]. Moreover, it has already confirmed that XRD diffraction pattern of our chitin sample is consistent with pattern of general chitin. Based on these results, the volume fraction of water molecules contained in the crystal lattice was estimated with reference to the report by Sawada *et al* [14–16]. **Figure 6** shows the relationship between the volume fraction of water molecules in chitin hydrate and the proton conductivity. The relationship between the volume fraction and the conductivity of the conductor in the non-conductor of **Figure 6** is a value calculated from the resistance value reported by Yamada *et al*. As can be seen in **Figure 6**, water molecules in chitin behave very much like conductors in non-conductors. In other words, this result indicates that

*Relationship between volume fraction of water molecules and percolation conduction in chitin [11].*

water molecules in chitin function as a proton transport pathway.

Investigation of proton transport pathways in chitin is important for the future development and development of polysaccharide electrolyte membranes. So far, it has been shown that chitin has proton conductivity, and it is important that the introduction of water molecules behaves like a conductor for the proton conductivity. In addition, since the structure of chitin hydrate has been clarified, we approached the proton conduction pathway of chitin by measuring impedance using an oriented sample. As a result, it was confirmed that chitin and chitosan have orientation dependence of proton conductivity [8, 9]. From this result, the temperature dependence of the proton conductivity of the chitin system was investigated. **Figure 7** shows the proton conductivity in the chitin fiber direction when only the temperature factor is changed while maintaining a constant wet weight. As shown in **Figure 7**, the relationship between the reciprocal of temperature and the proton conductivity of chitin shows an Arrhenius-like linear change. This result indicates that the proton conductivity of chitin has thermal activity. **Table 1** shows the activation energy of proton conductivity derived from the Arrhenius equation

**3.4 Activation energy and proton pathway**

**170**

**Figure 6.**

*Arrhenius plot of proton conductivity in chitin [11].*


#### **Table 1.**

*Activation energy of proton conduction in the chitin system.*

in each fiber direction of chitin and chitosan. Arrhenius equation is represented by following equation:

$$
\sigma = \sigma\_o \exp\left(-\frac{\Delta E}{kT}\right) \tag{2}
$$

Here, σ and σ **<sup>0</sup>** are proton conductivity and proton conductivity when frequency is 0, *k* and *T* are boltzmann constant and temperature, ∆*E* is activation energy.

As shown in **Table 1**, the activation energy of chitin-based proton conduction changed depending on the fiber direction in the case of chitin, and no change was observed in the case of chitosan. In general, a decrease in activation energy indicates a decrease in energy required for proton transport, and is therefore expected to contribute to the realization of high proton conductivity. Considering this, it is considered that the decrease in activation energy of chitin in the vertical direction of the fiber is appropriate. However, in the oriented sample, the activation energy of chitin in each fiber direction seems to be inconsistent, considering that the proton conduction of chitin in the fiber direction is the highest. Since the activation energy is not the only element required for proton conduction, the following equation:

$$
\sigma = \pi n e \,\mu\tag{3}
$$

Here, *z* and *n* are ion valence and number of proton transport pathway, *e* and µ are charge density and mobility. Since the charge of the proton is +1 and *z* = 1, the charge concentration *e* is that the amount of water of crystallization is *e*,

assuming that the proton conduction is through the water of crystallization of the chitin crystal. Furthermore, from Eq. 3, considering that the mobility *μ* is related to the activation energy, the value of the proton conductivity *σ* on the left side is determined, so that the high proton conductivity in the fiber direction of the oriented chitin is determined. It is suggested that it is brought about by the number *n* of proton transport pathways. Taking these things into consideration, we gained insight into the relationship between the crystal structure of chitin and chitosan hydrates and the proton transport pathway from the hydrogen bond distance.

**Figure 8** shows the hydrogen bonds formed in chitin hydrate. In **Figure 8**, each color line shows hydrogen bonds, pink is about 2.6 Å of water-chitin molecule, yellow is 2.98 Å, and light blue is between chitin and chitin. Further, broken line shows the hydrogen bond formed along the fiber direction of chitin, and the rigid line shows the hydrogen bond formed in the direction perpendicular to the fiber. As shown in **Figure 8**, among the hydrogen bonds formed in chitin hydrate, the hydrogen bonds between the water molecule and chitin are formed at a distance of about 2.6 Å in the fiber vertical direction (*a*-axis direction). On the other hand, hydrogen bonds with a distance of 2.6 Å and 2.98 Å are alternately formed in the fiber direction. **Figure 9** shows the results in the case of chitosan. This result suggests that the proton transport pathway of chitosan is mediated by the hydrogen bond of 3.0 Å, which is the yellow dotted line in **Figure 9**, which is common in both the fiber direction and the fiber vertical direction. From these results, it is considered that the relationship between the proton conductivity of chitin and chitosan and the activation energy is due to the hydrogen bond distance of approximately 3.0 Å, which is common to both samples, as a bottleneck. It is expected that the high proton conductivity generated in the fiber direction of oriented chitin will be realized by increasing the number of pathways.

From the above results, the proton conduction of chitin having high proton conductivity is expected as shown in **Figure 10**. Proton conduction in chitin is considered to be realized by the Grosus mechanism in consideration of the relationship between the result of percolation conduction and the hydration structure. In the proton conduction, the crystal water becomes an oxonium ion by the proton, and the proton is passed to the adjacent water molecule by repeating the breaking and rearrangement of the hydrogen bond. In addition, as shown in **Figure 10**, it is considered that the high proton conductivity of chitin in the fiber direction was caused by the increase in the number of pathways due to the hydrogen bond of the bottleneck and the vertical path with low activation energy. On the other hand,

#### **Figure 8.**

*a-b plane of the crystal structure of chitin hydrate [11, 14–16]. White, gray, blue and red balls show hydrogen, carbon, nitrogen and oxygen.*

**173**

of 33 mW/cm2

reduced.

**Figure 10.**

**Figure 9.**

*Proton Conductivity in Chitin System*

*DOI: http://dx.doi.org/10.5772/intechopen.96799*

the decrease in the proton conductivity in the vertical direction of the fibers of the oriented chitin can pass through the low activation energy pathway in the vertical direction, but the bottleneck is difficult to utilize, so that the number of pathways is

*a-c plane of the crystal structure of chitosan hydrate [11, 17–19]. White, gray, blue and red balls show hydrogen, carbon, nitrogen and oxygen. Yellow ball shows oxygen derived from water molecule.*

The high proton conductivity of chitin appeared in the oriented sample and in the fiber direction. From this result, fuel cell using an oriented chitin sample was prepared. **Figure 11** shows schematic diagram of the fuel cell when oriented chitin is used and its *i-V* characteristics. As shown in **Figure 11**, the fuel cell using oriented chitin showed a much higher power density than the sheet sample, and a maximum

was obtained. In other words, chitin indicates that the output can be

**3.5 Creation of fuel cells using oriented chitin**

*Schematic diagram of proton transport pathway in chitin [11, 14–16].*

#### **Figure 9.**

*Chitin and Chitosan - Physicochemical Properties and Industrial Applications*

realized by increasing the number of pathways.

assuming that the proton conduction is through the water of crystallization of the chitin crystal. Furthermore, from Eq. 3, considering that the mobility *μ* is related to the activation energy, the value of the proton conductivity *σ* on the left side is determined, so that the high proton conductivity in the fiber direction of the oriented chitin is determined. It is suggested that it is brought about by the number *n* of proton transport pathways. Taking these things into consideration, we gained insight into the relationship between the crystal structure of chitin and chitosan hydrates and the proton transport pathway from the hydrogen bond distance.

**Figure 8** shows the hydrogen bonds formed in chitin hydrate. In **Figure 8**, each color line shows hydrogen bonds, pink is about 2.6 Å of water-chitin molecule, yellow is 2.98 Å, and light blue is between chitin and chitin. Further, broken line shows the hydrogen bond formed along the fiber direction of chitin, and the rigid line shows the hydrogen bond formed in the direction perpendicular to the fiber. As shown in **Figure 8**, among the hydrogen bonds formed in chitin hydrate, the hydrogen bonds between the water molecule and chitin are formed at a distance of about 2.6 Å in the fiber vertical direction (*a*-axis direction). On the other hand, hydrogen bonds with a distance of 2.6 Å and 2.98 Å are alternately formed in the fiber direction. **Figure 9** shows the results in the case of chitosan. This result suggests that the proton transport pathway of chitosan is mediated by the hydrogen bond of 3.0 Å, which is the yellow dotted line in **Figure 9**, which is common in both the fiber direction and the fiber vertical direction. From these results, it is considered that the relationship between the proton conductivity of chitin and chitosan and the activation energy is due to the hydrogen bond distance of approximately 3.0 Å, which is common to both samples, as a bottleneck. It is expected that the high proton conductivity generated in the fiber direction of oriented chitin will be

From the above results, the proton conduction of chitin having high proton conductivity is expected as shown in **Figure 10**. Proton conduction in chitin is considered to be realized by the Grosus mechanism in consideration of the relationship between the result of percolation conduction and the hydration structure. In the proton conduction, the crystal water becomes an oxonium ion by the proton, and the proton is passed to the adjacent water molecule by repeating the breaking and rearrangement of the hydrogen bond. In addition, as shown in **Figure 10**, it is considered that the high proton conductivity of chitin in the fiber direction was caused by the increase in the number of pathways due to the hydrogen bond of the bottleneck and the vertical path with low activation energy. On the other hand,

*a-b plane of the crystal structure of chitin hydrate [11, 14–16]. White, gray, blue and red balls show hydrogen,* 

**172**

**Figure 8.**

*carbon, nitrogen and oxygen.*

*a-c plane of the crystal structure of chitosan hydrate [11, 17–19]. White, gray, blue and red balls show hydrogen, carbon, nitrogen and oxygen. Yellow ball shows oxygen derived from water molecule.*

**Figure 10.** *Schematic diagram of proton transport pathway in chitin [11, 14–16].*

the decrease in the proton conductivity in the vertical direction of the fibers of the oriented chitin can pass through the low activation energy pathway in the vertical direction, but the bottleneck is difficult to utilize, so that the number of pathways is reduced.
