HIGH-EFFICIENCY ENERGY CONVERSION
TOWARD A ZERO-EMISSION SOCIETY
――The future of clean energy production lies in perfecting the nanoscale details of fuel cell components to create an innovative carbon-neutral energy system.
Research Keywords: Fuel Cell, Photocatalyst for Water Splitting, Organic Nanoarchtecture
The race is on to find ways to replace our reliance on fossil fuels with clean, sustainable methods of generating power. Current solar and hydrogen technologies are not yet efficient enough to produce energy on a large scale. With this in mind, researchers from the High-Efficiency Energy Conversion program aim to improve current solar and hydrogen fuel cells, while finding ways to reduce the pollutants already in the environment.
“We hope to develop efficient energy conversion systems that don’t emit carbon dioxide and pollutants,” says Nagahiro Hoshi, lead researcher on the program. “We will create a system that generates power fueled by sunlight and water. To do this, we must perfect the structures of the nanomaterials used to make solar and hydrogen fuel cells to optimize their ability to harness energy.”
Hoshi’s team works with various nanostructures, including nanoparticles, single-crystal surfaces and organic nanoassemblies, to build devices that can capture as much energy as possible without wastage.
Enhancing energy conversion
The surface structures of catalysts used in solar cells and hydrogen fuel cells can have a remarkable impact on conversion efficiency. For example, the orientation of atoms on a solar cell’s surface significantly affects the distribution of light across the cell. One way of enhancing a solar cell is to coat its surface with a dye. The atomic make-up of the dye contributes to the cell’s ability to generate electric current when exposed to light, providing it is adsorbed onto an appropriate surface.
The researchers recently explored the effects of using a zinc porphyrin (ZnP) dye on single-crystal titanium dioxide (TiO2) substrate surfaces. They trialed three different TiO2 substrates and found that their surface structures significantly altered the ability of the ZnP dye to generate an electric current efficiently. In fact, one of the ZnP TiO2 substrates had an incident photon-to-current conversion efficiency 13 times higher than one of the others.
Hoshi attributes the high conversion efficiency on this substrate to the bent configuration of the adsorbed porphyrin ring, which results in a shorter distance between the ring and surface. Such attention to detail in terms of nanoscale surface structures will generate the breakthroughs needed to create highly efficient, scalable and cost-effective solar power.
At present, hydrogen is mainly manufactured by using heat and pressure to separate it from natural gases. But since this also creates large amounts of carbon dioxide, scientists are exploring ways to generate hydrogen from water instead. When burned, hydrogen releases only water and steam, and it has the potential to fuel all kinds of systems. Hoshi’s team has developed a hydrogen-based method that could transform excess carbon dioxide into usable fuel.
“We are researching photocatalysts capable of decomposing pollutants such as carbon oxides, ammonia and waste oil,” says Hoshi. “We have had recent success in investigating the photocatalytic conversion of carbon dioxide into methane — the reaction pressure needed to reduce the carbon dioxide was created using hydrogen gas.”
To help realize this vision of a zero-emission society, Hoshi and his team welcome collaborations with researchers and private companies interested in this burgeoning field of nanotechnology.
Members
Principal Investigator
| Name | Title, Affiliation | Research Themes |
|---|---|---|
| HOSHI Nagahiro | Professor, Graduate School of Engineering | Surface Electrochemictry |
Co-Investigatior
| Name | Title, Affiliation | Research Themes |
|---|---|---|
| NAKAMURA Masashi | Associate Professor, Graduate School of Engineering | Surface Electrochemictry |
| IZUMI Yasuo | Professor, Graduate School of Science | Surface Chemistry, X-ray spectrometry |
| YAGAI Shiki | Professor, IGPR/Graduate School of Engineering | Organic Functional Materials Chemistry |
| KOJIMA Takashi | Associate Professor, Graduate School of Engineering | Inorganic Synthesis Chemistry |
Research report（2016〜2018）
Four groups have collaborated on the basic studies of fuel cells, photocatalysts for water splitting and solar cell using well-defined surfaces and shape-controlled nano materials. The achievements are summarized below.
Fuel cell and photocatalysts for water splitting (Hoshi & Nakamura group)
They enhance the catalytic activities for the oxygen reduction reaction (ORR) of a fuel cell and photocatalysts for hydrogen generation by water splitting using single crystal electrodes of Pt and TiO2. In the study on the ORR, modification of Pt(111) with tetrahexyl ammonium cation (THA+) activates the ORR 8 times higher than that on bare Pt(111). This result was published in Nat. Commun. 9, 4378 (2018). Modification of Pt(111) with alkyl amines (OA/PA) also enhances the ORR activity remarkably. Infrared spectroscopy shows that the enhancement of the activity on Pt(111) by OA/PA is attributed to the formation of ice-like water with smaller cluster size. Modification by melamine activates the ORR on Pt(331) of which activity is the highest in bare Pt single crystal electrodes, giving the highest activity in Pt electrodes.In the study on photocatalysts for water splitting, hydrogen evolution rate on TiO2(100) modified with eosin Y and cuboctahedral Pt nanoparticles is 11 times as high as that on TiO2(110) modified in the same way.
Photofuel cell (Izumi group and Kojma group)
By the structural control of photoelectrodes in a photofuel cell, higher voltage (> 2 V per cell) and greater power (>0.1 mW cm−2) were enabled. Homogeneously size-distributed TiO2 crystallines were well aligned on anode and the TiO2 film was further doped with delphinidin dye to utilize visible light. As the result, open-circuit voltage of 2.11 V was enabled for the first time in the world (ACS Sustainable Chem. Eng. 6, 11892 (2018)). The cell design is unique to use photocatalysts on both electrodes and a report to obtain high voltage of 2.11 V is only one in comparison to polymer electrolyte fuel cells, perovskite solar cells, and the other solar cells. It is especially specific to investigate photoelectrode by the control of particle morphology, crystalline face, size distribution, and its orientation for the photofuel cell. Furthermore, as a photocatalyst for cathode, time course of 13CO2 photoconversion and the reaction route were reported for the first time (J. Am. Chem. Soc. 141, 6292 (2019)).
Organic solar cell (Yagai group)
By collaborating with Hoshi&Nakamura group, we have extensively worked on the construction of dimension-controlled organic-inorganic composites, and the application of these materials to various optoelectronic devices. As prominent achievements, we have developed hydrogen-bond-oriented molecular design for organic dyes that can spontaneously organize into highly-ordered rod-shaped nanostructures. These nanorods have been shown to exhibit power conversion efficiency (PCE) around 3% in organic thin film solar cells (Chem. Commun. 52, 7874 (2016)). Furthermore, by collaborating with Hoshi, we have revealed self-assembled structures of functional dyes by means of scanning tunneling microscope. These self-assembled dyes were further found to hierarchical organize into superhelical structures, and exhibited PCE of 4.5% which is remarkably high among hydrogen-bonding organic dye materials (Chem. Sci. 9, 3638 (2018); selected as a cover graphic).