Role of pheophytin in power generation of photosynthetic fuel cells

Photosynthetic fuel cells, which yield hydrogen energy from photosynthesis, are attracting attention as a next-generation clean energy source. However, the relationship between the power density of photosynthetic fuel cells and the wavelength of incident light has not been made clear despite being an important factor concerning light absorption. In this study, we have measured the dependence of the power generation in photosynthetic fuel cells for the wavelength of incident light and investigated the key molecules that lead to the power generation. It was found that the power density peaks around the wavelength of the light absorption of chlorophyll. These results indicate that the electron generated by the light absorption in chlorophyll becomes the trigger of power generation, the same as the electrons created by the light absorption of chlorophyll become the trigger of the photosynthetic reaction. In addition, we observed the power density enhancement around the light’s wavelength of 520 nm. Considering that the absorption of chlorophyll cannot be observed at around 520 nm, this result indicates that in the power generation of photosynthetic fuel cells, another molecule besides chlorophyll also leads to power generation. Furthermore, we found that Photosynthetic fuel cells using the pheophytin degraded by the desorption of the Mg ion from chlorophyll can also generate power density by light irradiation. From the fact that pheophytin exhibits light absorption at 520 nm, where power density becomes enhanced, the power generation of photosynthetic fuel cells is caused by not only the light absorption of chlorophyll but also the light absorption of pheophytin is also important for the power generation.