196th ECS Meeting | Honolulu, HI | Oct. 18, 1999
Recent Progress in Photoelectrochemistry
The technical program officially began with a plenary lecture by Professor Kenichi Honda of the Tokyo Institute of Polytechnics. Professor Honda can truly be regarded as one of the pioneers of modern photoelectrochemistry. He is perhaps best known, outside of this specialized community, for his landmark study of the solar-assisted electrolysis of water with irradiated titanium dioxide photoanodes. This work was carried out when Honda was at the University of Tokyo. There, in the 1970s, he established an active group in the field of photoelectrochemistry comprised of Fujishima, Watanabe, and other researchers—many of whom have had illustrious careers of their own since then. Indeed the 1970s can be regarded as the halcyon days of photoelectrochemistry, with intense activity in all parts of the world, including Japan.
Professor Honda began his lecture with an historical perspective of how the field evolved, as early as in 1839, with Becquerel’s experiments of the influence of light on metal electrodes. His excellent review of the progress made in photoelectrochemistry was organized into photoelectrode preparation aspects, photoelectrochemical reactions, and applications.
Beginning with the first topic, Honda discussed tailored photoelectrode assemblies comprised of multiple layers and semiconductor-metal composites. Chemical modification of semiconductor surfaces (e.g., with Langmuir-Blodgett films) is a tactic for precise control of the interfacial chemistry and electrostatics. This area continues to be actively investigated by researchers in many countries. Size quantization—where the optical properties of the semiconductor (for example) exhibit interesting changes (from the bulk counterpart) when the particle size approaches the exciton dimension—is no longer a topic confined to the solidstate physics community. This lecture contained examples of similar studies in semiconductor-liquid interfacial environments, which have provided interesting data in recent years.
Under the second topic of photoelectrochemical reactions, water splitting (a reaction undoubtedly close to Professor Honda’s heart!), CO2 reduction and oxidative decomposition of organic compounds were discussed. The individual HER and OER water splitting components can be accomplished with sacrificial electron donors and acceptors respectively. Photoelectrodes, with layered and tunnel chemical structures, have been demonstrated to be capable of driving these reactions in a sustained fashion without photocorrosion. Materials such as inorganic niobates, titanates, and tantalates have been added to the library of candidate photoelectrodes alongside the TiO2 workhorse. Honda’s review of this topic was primarily confined to a discussion of the progress made by Japanese researchers. Finally, he also described two-photon water-splitting systems and efforts to split water with visible light.
In these times of increasing concerns with CO2 emissions from fossil fuel combustion and implications for global climate change, photoelectrochemical methods for CO2 reduction have assumed particular relevance. A crucial factor, as pointed out by Honda, rests with the fact that electrolytic approaches for driving this difficult reaction would make little sense if the electricity needed is derived from fossil sources. Thus, even if the solar-assisted approaches are much less efficient (than their “dark” counterparts), the former would win out on environmental considerations alone. Both gas phase and solution phase photoelectrochemical CO2 reduction pathways are being studied in many laboratories, especially in Japan.
Professor Honda concluded his review with a short discussion of possible technological applications emerging from the field of photoelectrochemistry. These include solar energy conversion using photovoltaic and photoelectrolytic approaches, and environmental remediation. At least to this writer, one of the most impressive aspects of Honda’s lecture (other than the technical ones) was the total lack of reference to his own singular contributions to the field in the areas of energy conversion and dye sensitization—a feature of this talk underlining his humility and graciousness.