Photocatalysts are materials that can use sunlight to produce hydrogen from water or break down harmful substances, and they are widely regarded as a promising technology for tackling energy and environmental challenges. Many researchers have refined the composition and structure of photocatalysts in pursuit of higher performance, yet it remains far from straightforward to identify where each modification actually takes effect—whether it improves light absorption, facilitates electron flow, or accelerates surface reactions. As a result, even after layers of improvements, it has often been unclear what to try next. With years of experience in photoelectrochemical measurement techniques, our group dissects how each modification influences each step of the reaction. Building on the insights obtained, we aim to develop high-performance photocatalyst materials and, through them, contribute to solving global energy challenges.

Converting methane—an abundant resource—and carbon dioxide—the main driver of global warming—into industrially valuable substances is a critically important challenge from both energy and environmental perspectives. Our group has previously demonstrated that these gases can be converted into hydrogen and carbon monoxide using light energy. As a further challenge, we are working to uncover the true bottlenecks that emerge only when reactions are attempted under weaker light and lower temperatures, with the goal of realizing such reactions under milder conditions.

Photocatalyst research to date has focused much of its effort on refining a small set of materials such as titanium dioxide. However, many other materials possess promising properties such as strong light absorption, and once their combinations are taken into account, the number of candidates worth exploring becomes vast. Our group makes use of high-throughput experimentation—a method that allows many materials to be tested efficiently in parallel—to uncover yet-unknown high-performance photocatalysts from this vast materials space. Our challenge extends beyond materials to the discovery of new reactions themselves, where we seek the overlap between reactions at which photocatalysts truly excel and those that society needs today, working to open up entirely new chemical reactions that bring out the full potential of photocatalysis.

Toward a carbon-neutral society, the widespread use of hydrogen energy requires the development of hydrogen carriers that can safely store and transport hydrogen and release it with low energy input. High-pressure hydrogen gas cylinders, which are commonly used as hydrogen carriers, have low hydrogen storage density and pose risks of explosion. Liquid hydrogen carriers such as ammonia, formic acid, and organic hydrides also present challenges, including toxicity and corrosiveness, as well as the need for high-temperature heating to release hydrogen. In addition, solid hydrogen carriers such as metal alloys have low gravimetric hydrogen density, making lightweight storage and transportation difficult. In our laboratory, we are studying nanosheet-based two-dimensional materials that contain large amounts of hydrogen.

As hydrogen-containing two-dimensional materials, we are studying hydrogen boride sheets (HB sheets), and layered hydrogenated silicene (L-HSi). These materials exhibit extremely high gravimetric hydrogen density (8.5% in the case of HB sheets), are lightweight, and do not pose a risk of explosion. In addition, they can be synthesized by a facile solution-based method, making large-scale production feasible. In our laboratory, we aim to achieve energy-efficient hydrogen release by applying various external stimuli (such as heat, photon, and electricity) to these materials, and we are also working on re-storage of hydrogen after release. Our goal is to realize lightweight, safe, and energy-efficient hydrogen carriers.

Electrochemistry is one of the promising approaches for realizing strategies to reduce carbon dioxide (CO₂) emissions, which are a cause of global warming. We are developing electrode materials and catalysts for “electrochemical CO₂ reduction,” which converts electricity generated from renewable energy sources into useful chemical raw materials. As for electrocatalysts, we are mainly focusing on metal sulfides, inspired by the hypothesis in earth science that “chemical transformations occurred on metal sulfide minerals at deep sea hydrothermal vents, leading to the origin of the first life.” Recently, we have also been working on reactions to synthesize ammonia (NH₃), which is used as fertilizer, from nitrogen (N₂) and nitrate (HNO₃).

To design highly active catalysts, it is necessary to identify which physical properties contribute to their activity. Information technology, exemplified by machine learning, is a powerful tool for achieving this. We synthesize and evaluate the activity of catalysts ourselves, and then use machine learning such as multi-regression analysis, on the obtained activity and the catalyst’s physicochemical parameters to investigate the correlation between activity (and/or selectivity) and those parameters. In this way, we identify the parameters that mainly contribute to activity and feed this information back into catalyst design, leading to the development of higher-performance catalysts.

Conventional research on electrochemical reactions in aqueous solution, particularly electrochemical CO₂ reduction reactions, has been almost entirely limited to room temperature and atmospheric pressure. Our laboratory focuses on “hydrothermal electrochemistry,” which involves reactions at high temperatures and high pressures, aiming for synthesis of highly efficient catalyst, increased chemical conversion efficiency. This equipment was developed in our laboratory and allows for experiments under high temperatures and high pressures that are impossible with conventional electrochemical reactors. Catalysts synthesized by electrodeposition in such a special condition exhibit different performance than catalysts synthesized under normal conditions. Furthermore, we have found that when electrochemical reactions are carried out directly under hydrothermal conditions, the activity and selectivity differ depending on the temperature. In addition to catalyst synthesis and chemical conversion, we are also challenging ourselves with the creation of new scientific theories such as “electron transfer and proton transfer under hydrothermal conditions.”