AsiaChem | Chemistry in Japan | December 2021 Volume 2 Issue 1

86 | December 2021 www.facs.website charged into the CB in the cathode followed by their delayed consumption in the PtPpy-catalyzed HER. This behavior is clearly attributable to the uncontrollable nature of the driving force for the HER using the CB of TiO2. We also confirmed that both PtP-py and CoP-py are intact for at least an hour under the above water electrolysis conditions. Thus, our study, for the first time, demonstrated that a molecular-catalyst-based water electrolyzer is achievable by employing the mesoporous TiO2 films as the electrode materials on both anode and cathode. The band engineering enabling the fine tuning of the CB levels in both electrodes is considered as one of the important strategies towards the development of advanced technology for the solar hydrogen generation. Summary and Outlook We have emphasized the importance of keep tackling to innovate the solar-driven hydrogen generation technology by appreciating rather highly advanced technology in the hydrogen fuel cells which are likely to offer a significant contribution to our future society because of their sufficiently high fuel-to-electricity conversion efficiency together with the high capacity enabling the large scale energy supply based on the storable high energy density fuels. In sharp contrast with the artificial fuel generation methods recently explored by other researchers, our fuel generation method is designed to avoid the gas separation facility together with the double photon pumping route to transfer one electron. These strategies intend to make our target photosynthetic devices amenable to produce the highest achievable energy on the basis of the solar light energy absorbed. Nature has somehow achieved such photosynthetic systems after repeating a few billion years of evolution processes. Nature does not evolve flammable fuels mixed with dioxygen by smartly converting their fuels into water-soluble as well as recognizable forms. The side product, that is, dioxygen is simply ejected from the organism. The chemical engineering features are thus well advanced in natural organisms. We notify that such chemical engineering part of research has not been well advanced in the field of artificial photosynthesis. The advanced studies on such issues are thus likely to open up a new avenue of research. In this review, we also discussed some of our successful advancement in getting deeper insights into the mechanisms of molecular catalysis related to energy conversion processes. Development of molecular-anchored photosynthetic systems largely relies on the knowledge gained from the basics studies on the small molecular systems. Since our ability to control the molecular catalytic properties is still quite limited, we should further advance our knowledge and artificial skills to finely control all the photochemical and electrochemical actions of the molecular systems in our hand. ◆ Acknowledgment Thi s work was suppor ted by JSPS KAKENHI Grant Numbers JP16K05726, JP18H01996, JP18H05171, JP19K05502 and JP21H01952. Figure 7. (a) Linear sweep voltammograms (LSV) for our PECs under intermittent irradiation (λ > 400 nm). Photographs of the FTO/TiO2 and the FTO/TiO2/PtP-py cathodes before (b,d) and after (c,e) the LSV measurements under irradiation condition. (f) Spectral changes during the visible light irradiation to the FTO/TiO2/Ru-dpqpy electrode submerged into an acetate buffer solution containing 30 mM EDTA under Ar. (g) Time course of the EMF and the amount of H2 evolved under intermittent irradiation. 44 Figure 8. (a) Schematic representation of molecular-catalyst-anchored water electrolyzer consisting of the FTO/TiO2/CoP-py anode and the FTO/TiO2/PtP-py cathode (coverage of catalyst, 0.10 μmol/cm2 for each). (b) The pH dependences in the LSVs measured for either the anode or cathode (100 mV/s). (c) LSVs with the cathode short connected to the reference terminal (0.1 M borate buffer, pH=9.0). (d) The H2 and O2 evolved over time during 1 h of electrolysis with the anode potential held at 2.2 V vs. cathode (pH=9.0), where the dashed line corresponds to the amount of each gas expected for the 100% Faradaic efficiency.48

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