The supply of clean and sustainable energy is regarded as one of the most important scientific and technological challenges in the 21st century. The most probable alternatives to fossil fuels are renewable energies such as solar, geothermal, and so on. Energy harvested from sunlight offers a desirable approach for minimal environmental impact, and what is even better, solar energy is a decentralized and inexhaustible natural resource. In our laboratory, we are investigating innovative photocatalytic materials for the conversion of solar energy to hydrogen through a water-splitting reaction and to valuable chemicals through carbon dioxide reduction. In addition, thermoelectric materials and systems are being investigated because of their potential for utilizing geothermal energy, ambient thermal energy (energy harvesting), and recycling exhaust heat, and are therefore expected to generate environmentally clean electric power.

1. Solar energy conversion to hydrogen or valuable chemicals

The hydrogen generated through photocatalytic water-splitting represents a clean and renewable fuel source. We have attempted to fabricate a photocatalytic material or construct a photocatalytic system that can accomplish water splitting to produce hydrogen and oxygen at a ratio of 2:1 (overall water-splitting) under visible light. One approach is that we prepared a composite photocatalyst, composed of hydrogen-evolution and oxygen-evolution photocatalysts with a conducting layer in between, and the composite was able to split pure water irradiated with visible light. Recently, we succeeded in preparing an overall water-splitting composite that could utilize visible light up to wavelengths of 740 nm, which represents nearly the entire visible light spectrum and, to our knowledge, is the longest wavelength reported to date.

The photocatalytic conversion of carbon dioxide using solar energy, so-called artificial photosynthesis, is also currently underway in our laboratory. At present, carbon monoxide and methane were simultaneously detected over our photocatalyst, so that we now have the challenge of forming these useful organic molecules more selectively. In addition, we are trying to realize a photocatalyst to form the carbon-carbon bond by artificial carbon dioxide conversion.

2. Thermal energy conversion to electric power

Thermoelectricity is a technology for the direct conversion of heat to electric power. Recently, energy harvesting has been attracting attention to realize the IoT (Internet of things) society. Unused, ambient thermal energy is one of the most important energy sources for energy harvesting. Solar power, including both photon and thermal energies, is also regarded as one of the most important ambient, renewable energies. We have already demonstrated an enhancement of the thermal-electric conversion efficiency by utilizing the photon energy, accompanied by a photochromic phenomenon. This demonstration can lead to a multifunctional material as a component of photochromic smart windows, which are expected to provide shade from sunlight while simultaneously generating electric power.