A Novel Photoelectrochemical Secondary Battery Featuring Phthalocyanine Photocathode Toward the Application to Ubiquitous Sensors

The conventional photovoltaic cell can be one of the most suitable energy sources for driving ubiquitous sensors when it also has the function as a secondary battery in the dark. However, such a system tends to require high cost and is also of difficulty in the viewpoint of miniaturization. Alternatively, a photoelectrochemical cell can offer a simple and promising battery originating in chemical reaction under solar irradiation. In this study, we proposed a novel photoelectrochemical battery (denoted as PEB) with a photoelectrode of metal-free phthalocyanine (denoted as H₂Pc, a p-type semiconductor) deposited on ITO. H₂Pc has a great potential in terms of an efficient absorption over the visible-light energy range, low cost, harmlessness, and ease of deposition, which can be expected to be an ideal photoelectrode for versatile PEB. When the photoelectrode was irradiated using the visible-light energy of λ＞500 nm, Fe^III(CN)₆^3– (electron acceptor) was reduced at the H₂Pc surface, whereas Fe^II(CN)₆^4– (electron donor) was concurrently oxidized at the counter electrode. This result indicates that H₂Pc worked as the photocathode (i.e., H₂Pc induced the generation of photocathodic currents for charging the PEB under the visible-light irradiation). The photocurrent density during charging was ca. 30 μA/cm². Furthermore, when turning off the light, discharge occurred between the counter electrode and the discharge electrode. Such charge-discharge phenomena were confirmed to occur repeatedly. These results showed that the PEB featuring H₂Pc photoelectrode can be an attractive electric power source toward ubiquitous sensors. In addition to the details of the aforementioned results, the origin of the present photocurrent will be discussed in terms of the Fe^III(CN)₆^3– reduction at the H₂Pc photoelectrode.