Introduction Hydrogen gas (H2 ) is an important chemical feedstock; it is used as a reductant in petroleum refining, ammonia synthesis, the manufacture of stainless steel, and the production of core chemicals (e.g., methanol) and food (e.g., margarine). H2 is also regarded to be a critical and indispensable clean fuel for sustainable energy systems because of its extremely high energy density of 142 kJ g−1 (vs. 44 kJ g−1 for gasoline) and low pollution emissions.1) However, H2 is presently produced by conversion of hydrocarbons or fossil fuels and is accompanied by enormous energy consumption and greenhouse gas emissions. To realize carbon neutral and sustainable circulation technology, carbon-free and green H2 production methods, such as water-splitting, are required. 2) Results and discussion This presentation describes organic π-conjugated polymers as a novel photoelectrocatalyst towards water-splitting.2-4) π-Conjugated polymers synthesized without residual metals were developed to function in "water", which defied common sense, leading to this novel catalytic ability. In detail, (1) we succeeded in synthesizing polythiophene, which is a type of π-conjugated polymer, in a film form without residual metal by our unique iodine-vapor-phase polymerization method (2) We found that this polythiophene film worked as a novel photoelectrocatalyst that absorbs visible-light and highly efficiently reduces water to generate H2 . (3) In addition, based on a rigid π-conjugated polymer structure, this polythiophene film catalyzed water-splitting/ H2 production with surprisingly high durability (more than 1 month without no degradation). The development of environmentally-friendly organic catalysts that do not use precious metals will lead to the production of truly green H2 .We will also describe developments to other catalytic functions (O2 reduction/ H2O2 production etc.) of organic π-conjugated polymers. References 1) Schlapbach, L. et al., Nature, 2001, 414, 353-358.2) Oka, K. et al., Adv. Energy Mater., 2021, 11, 2003724.3) Oka, K. et al., Energy Environ. Sci., 2018, 11, 1335-1342.4) Oka, K. et al., Adv. Energy Mater. 2019, 9, 1803286. Acknowledgment This work was partially supported by Grants-in-Aids for Scientific Research (No. JP23K17945, JP23H03827, JP24K01552) from MEXT, Japan. K. Oka. also acknowledges the support from Shorai Foundation for Science and Technology, TEPCO Memorial Foundation, Amano Industry Technology Laboratory, Sugiyama Houkoukai, The Yamada Science Foundation, Kenjiro Takayanagi Foundation, Kansai Research Foundation for Technology Promotion, and JACI Prize for Encouraging Young Researcher.
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