We have fabricated the fuel cell based on the tissue derived biomaterial “collagen” and investigated its proton transfer. It was found that “collagen” becomes the electrolyte of fuel cell in the humidified condition. The power density of the fuel cell becomes typically 8.6 W/m 2 in the 80% humidity. Further, these results indicate that collagen exhibits proton conductivity in the humidified condition. Both of proton conductivity and dielectric constant increase by the increase of humidity. From the analyses of the frequency dependence of AC conductivity, it was found that proton conductivity and the dielectric dispersion observed in the humidified condition are caused by the formation of the water bridge, which is bonded with the collagen peptide chain. Considering that hydration induces the formation of the water bridge and that increases proton conductivity and dielectric constant, it is deduced that proton transfer in the fuel cell based on collagen is caused by the breaking and rearrangement of hydrogen bond in the water bridge.
It is well-known that tissue-derived biomaterials are abundant in nature and are widely used in biomedical field. Especially, it is known that these materials are used to the formation of artificial blood vessel, artificial bone filling material and so on. Recently, even in the field of electrical conductor, the application of tissue-derived biomaterials to devices begins to investigate, because the tissue-derived biomaterials have the features of charge transfer such as proton pump and ion channel, and further have a potential to achieve a low-carbon society. For example, the electrical properties of deoxyribo-nucleic acid (DNA) have been investigated by many researchers, and there are many reports [
The films of collagen were prepared from the decalcified scales of Tilapia fish (Nitta Gelatin Inc).
For the impedance measurement, the admittance mode which consists of the parallel circuit of capacitance and conductance was used as the equivalent circuit of collagen film. In this case, the electrical conductivity σAC is calculated from the simple equation of σAC = ωε0ε”, where ω is angular frequency, ε” and ε0 are the imaginary part of complex dielectric constant and the dielectric constant in vacuum, respectively. In the case that σAC is proportional to ω, the equivalent circuit corresponds to the simplest equivalent circuit which consists of the parallel circuit of capacitor and resistor.
On the other hand, when the measured AC conductivity is not proportional to ω and shows the complex frequency dependence, we cannot use the simple equation σAC = ωε0ε”, because the AC electrical conductivity includes the components of dielectric dispersion. The AC conductivity including dielectric dispersion is described by the following equation:
σ AC = σ DC − Im [ ω ε 0 ε ∞ + ω ε 0 ( ε s − ε ∞ ) 1 + ( j ω τ ) β ] = σ DC + ω ε 0 ( ε s − ε ∞ ) ( ω τ ) β sin ( π 2 β ) ( 1 + ( ω τ ) β cos ( π 2 β ) ) 2 + ( ( ω τ ) β sin ( π 2 β ) ) 2 (1)
where σDC is DC electrical conductivity, εs and ε∞ are the static and unrelaxed dielectric constants, respectively, τ and β are the relaxation time for dielectric
dispersion and the degree of mono-dispersion, respectively. The second term of Equation (1) shows the component of dielectric dispersion and the step-like anomaly appears in the frequency dependence of AC electrical conductivity. In other words, we can obtain the motion of molecules causing the dielectric dispersion from the frequency dependence of AC conductivity using Equation (1). The AC electrical conductivity measurement was carried out using precision LCR meter (E4980A, Agilent Technologies Inc.).
fuel cell. In addition, these results in
In order to investigate the key factor for the appearance of proton conductivity, we have carried out the impedance measurements.
As described above, the AC conductivity including the dielectric dispersion is described by Equation (1). The AC conductivity σAC calculated from Equation (1) is shown in
As shown in
Relative humidity (%) | σDC | εs − ε∞ | τ | β |
---|---|---|---|---|
100 | 4.0 × 10−3 | 2.0 × 103 | 1.0 × 10−6 | 0.40 |
81 | 5.0 × 10−5 | 9.1 × 102 | 1.9 × 10−5 | 0.60 |
53 | 1.0 × 10−5 | 8.7 × 102 | 8.1 × 10−5 | 0.63 |
humidity.
acid) are easily able to give and take protons of water molecule in the water bridge. The generated hydronium ions moves with the proton transfer in the water bridge, as shown in
Considering these results, it is deduced that mobile protons appear by the breaking and rearrangement of hydrogen bond in the water bridges and then proton conductivity and large dielectric polarization are realized. In the present work, it is also found that the slope of the humidity dependences of σDC and εs − ε∞ changes at the 80% relative humidity. This anomaly may be caused by the hydration-induced phase transition. In order to clarify the anomalous behavior of σDC and εs − ε∞ at the 80% relative humidity, we are now planning the investigation of the structure change at the 80% relative humidity by neutron diffraction measurement. These results will appear future issues.
In the present paper, it was found that collagen becomes the electrolyte of fuel cell and that the collagen based fuel cell shows the power density of typically 8.6 W/m2 in the 80% humidity. These results also indicate that collagen exhibits proton conductivity in the humidified condition. From the humidity dependences of proton conductivity and dielectric constant, proton conductivity but also dielectric increase by the increase in humidity. From the analyses of the frequency dependence of AC conductivity, it was found that the dielectric dispersion observed in the humidified condition is caused by the formation of the water bridge, which is bonded with the collagen peptide chain. Considering that hydration induces the formation of the water bridge and that increases proton conductivity and dielectric constant, it is deduced that proton transfer in collagen is caused by the formation of the water bridge and that proton transfer is realized by the breaking and rearrangement of proton in water bridge.
This study was supported by the Adaptable and Seamless Technology transfer Program through target-driven R&D from Japan Science and Technology Agency.
Matsuo, Y., Ikeda, H., Kawabata, T., Hatori, J. and Oyama, H. (2017) Collagen-Based Fuel Cell and Its Proton Transfer. Materials Sciences and Applications, 8, 747-756. https://doi.org/10.4236/msa.2017.811054