Fuel cells using biomaterials have the potential for environmentally friendly clean energy and have attracted a lot of interest. Moreover, biomaterials are expected to develop into in vivo electrical devices such as pacemakers with no side effects. Ion channels, which are membrane proteins, are known to have a fast ion transport capacity. Therefore, by using ion channels, the realization of fuel cell electrolytes with high-proton conductivity can be expected. In this study, we have fabricated a fuel cell using an ion channel electrolyte for the first time and investigated the electrical properties of the ion channel electrolyte. It was found that the fuel cell using the ion channel membrane shows a power density of 0.78 W/cm2 in the humidified condition. On the other hand, the power density of the fuel cell blocking the ion channel with the channel blocker drastically decreased. These results indicate that the fuel cell using the ion channel electrolyte operates through the existence of the ion channel and that the ion channel membrane can be used as the electrolyte of the fuel cell in humidified conditions. Furthermore, the proton conductivity of the ion channel electrolyte drastically increases above 85% relative humidity (RH) and becomes 2 × 10−2 S/m at 96% RH. This result indicates that the ion channel becomes active above 96%RH. In addition, it was deduced from the impedance analysis that the high proton conductivity of the ion channel electrolyte above 96% RH is caused by the activation of ion channels, which are closely related to the fractionalization of water molecule clusters. From these results, it was found that a fuel cell using the squid axon becomes a new fuel cell using the function of the ion channel above 96% RH.

It is well known that a proton conductor is needed as an electrolyte of hydrogen fuel cells, which are attracting attention as an environmentally friendly next-generation device. In particular, anhydrous proton-conducting electrolytes are highly desired because of their advantages, such as high catalytic efficiency and the ability to operate at high temperatures, which will lead to the further development of fuel cells. In this study, we have investigated the proton-conducting properties of the hydroxyapatite (HAp)-collagen composite without external humidification conditions. It was found that, by injecting HAp into collagen, the electrical conductivity becomes higher than that of the HAp or the collagen. Moreover, the motional narrowing of the proton NMR line is observed above 130 °C. These results indicate that the electrical conductivity observed in the HAp-collagen composite is caused by mobile protons. Furthermore, we measured the proton conduction of HAp-collagen composite films with different HAp contents and investigated the necessity of the appearance of proton conductivity in HAp-collagen composites. HAp content (n = 0–0.38) is the number of HAp per collagen peptide representing Gly-Pro-Hyp. These results indicate that injection of HAp into collagen decreases the activation energy of proton conduction which becomes almost constant above a HAp content n of 0.3. It is deduced that the proton-conduction pathway in the HAp-collagen composite is fully formed above n = 0.3. Furthermore, these results indicate that the value of the activation energy of proton conductivity was lowered, accompanied by the formation of the HAp-collagen composite, and saturated at n > 0.3. From these results, the HAp-collagen composite forms the proton-conduction pathway n > 0.3 and becomes the proton conductor with no external humidification in the condition of n > 0.3 above 130 °C.

Recently, hydrogen-fuel cells have attracted attention as an environmentally friendly next-generation energy device. Very recently, biomaterials such as collagen and chitin have realized proton conductivity via water bridges under humidity condition, and the fabrication of fuel cells using biomaterials is possible. However, the fuel cell electrolyte via water has demerits, such as the complication of fuel cell instruments and the operating temperature limit. Therefore, fuel cell electrolytes without humidified conditions are desired. In the present work, we have synthesized an anhydrous proton conductor using imidazole and collagen, which are biomaterials, and investigated the anhydrous proton conductivity in imidazole–collagen composites. It was found that an imidazole–collagen composite is a high-proton conductor above 10−3 S/m and above 200 °C without the humidified condition compared with other anhydrous bio-proton conductors such as the hydroxyapatite–collagen composite. Moreover, the motional narrowing of the 1H-NMR line width reveals that the proton conductivity is realized in the temperature region from 120 to 200 °C. In addition, the DTA measurement and the impedance analyses reveal that the imidazole–collagen composite film undergoes the phase transition at 120 °C. Furthermore, the proton conductivity in the imidazole–collagen composite strongly depends on n, which is the imidazole concentration per collagen molecule and takes a maximum at n = 2.0. In addition, the proton conductivity perpendicular to the collagen fiber is approximately ten times higher than that parallel to the collagen fiber. From these results, it can be deduced that the proton conductivity in the imidazole–collagen composite is caused by breaking and rearranging the hydrogen bonds of the collagen side chain with the imidazole molecule formed between the collagen fibers.