We have fabricated a fuel cell based on the tissue-derived biomaterial “chitin”, and investigated its proton conductivity. It was found that chitin becomes the electrolyte of the fuel cell in the humidified condition, and power density of the fuel cell using chitin electrolyte becomes typically 1.35 mW/cm 2 at the 100% relative humidity. This result is the first result showing that the polysaccharide obtained from nature becomes the fuel cell electrolyte. Moreover, this result indicates that chitin is proton conductor in the humidified condition. In the chitin sheet plane, proton conductivity in chitin is observed approximately 0.1 S/m. Further, it was also found that chitin has the anisotropic proton conductivity. The proton conductivity along the chitin fiber direction is higher than that perpendicular to the chitin fiber direction. From these results, it is deduced that the formation of water bridges accompanied by hydration plays an important role in the appearance of proton conductivity in chitin.
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 [
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
Chitin films were prepared with the purified chitin obtained from crab shells (Sugino Machine Limited). This purified chitin slurry was well dispersed in distilled water and the chitin sheets were prepared by suction filtration.
The chitin fiber specimens were prepared by purifying chitin obtained from the tendon of crab’s legs. Based on the article of Prosky et al., the purification method was performed using a group of enzymes obtained from Streptomyces griseus (Wako Pure Chemical Industries, Ltd.) [
The water contents were measured from the relative humidity dependence of the weight of chitin using the electronic analytical balance (OHAUS Inc.) and the number of water molecules per a chitin molecule was calculated from the obtained water contents and molecular weights of water and mono-chitin.
The measurement of electrical conductivity was carried out using precision LCR meter (E4980A, Agilent Technologies Inc.). The relative humidity and temperature were controlled by the humidified gas-flow control system (Auto PEM). In the impedance measurement, the electrical conductivities perpendicular to the surface and parallel to the surface in chitin sheet were measured. In the case of chitin fiber specimens, impedance measurements were performed for specimens along the fiber direction and normal to the fiber direction, respectively.
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.
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