In this blog post, we’ll explore photocatalytic technology—which uses the sun and water to produce hydrogen—and examine why hydrogen energy is gaining attention as a future eco-friendly energy source.
Eco-Friendly Energy to Replace Fossil Fuels
For over 100 years, thanks to fossil fuels such as oil and coal, humanity has been able to travel conveniently by car and enjoy warm living conditions even during cold winters. The development of modern civilization, including the Industrial Revolution, was also made possible by fossil fuels. However, these gifts of nature are not infinite. While there were once predictions that fossil fuels would be completely depleted within a few decades, forecasts regarding the timing of depletion continue to change due to the discovery of new reserves and advances in extraction technology. Nevertheless, the fact that fossil fuels are a finite resource remains unchanged, and in the long term, the development of alternative energy sources is absolutely necessary.
The problem with fossil fuels is not limited to resource constraints. Air pollution and greenhouse gas emissions generated during the use of fossil fuels also have a significant impact on the global environment. Pollutants emitted by countless vehicles and industrial facilities degrade air quality, and carbon dioxide produced during combustion is considered one of the primary causes of global warming. For these reasons, humanity is continuously striving to develop eco-friendly energy sources to replace fossil fuels.
How is hydrogen energy produced?
Hydrogen energy is one of the most promising next-generation eco-friendly energy sources among these alternatives. Hydrogen does not directly emit carbon dioxide during use and can produce electrical energy with high efficiency through fuel cells and other means. However, since hydrogen rarely exists in a pure form in nature, it must undergo a production process.
There are three main methods for producing hydrogen. These include electrolysis, which uses electricity to split water; reforming, which involves the conversion of fossil fuels such as natural gas; and the use of photocatalysts. Electrolysis requires a power supply, and while fossil fuel reforming is currently the most widely used method, it produces carbon dioxide as a byproduct. In contrast, the photocatalyst method directly utilizes solar energy to split water, making it an eco-friendly technology that minimizes the use of external energy and is attracting significant attention.
For these reasons, water splitting technology using photocatalysts is being actively researched around the world as an eco-friendly method for producing hydrogen. Hydrogen production using photocatalysts requires three main elements: sunlight, water, and a photocatalyst.
The term “photocatalyst” combines the Korean characters for “light” (光) and “catalyst” (觸媒); it refers to a substance that, when exposed to sunlight, facilitates the decomposition of water into hydrogen and oxygen. Light consists of particles called photons, and photocatalysts absorb the energy of these photons to generate electrons and holes. The generated electrons react with water to produce hydrogen, while the holes participate in oxidation reactions, enabling the water splitting process to continue.
Why are photocatalysts important?
Just as you must score above the cutoff to pass an exam, a certain minimum level of solar energy is required to produce hydrogen. The minimum energy required for this is called the bandgap energy. In other words, to generate the electrons and holes needed for hydrogen production, light with energy greater than the photocatalyst’s bandgap energy must be absorbed.
Bandgap energy is an inherent property of a photocatalyst, and its value varies from material to material. Generally, photocatalysts with smaller bandgap energies can generate electrons and holes more easily, so they are more likely to have higher hydrogen production efficiency. This can be understood using the same principle as how it is easier to pass an exam when the cutoff score is lower. Therefore, research is steadily underway to develop photocatalysts that operate stably while having a lower bandgap energy, thereby increasing hydrogen production efficiency.
How far has photocatalyst research advanced?
Research on photocatalysts has continued steadily since 1972, when Professors Akira Fujishima and Kenichi Honda of the University of Tokyo published their findings on photoelectrochemical water splitting using titanium dioxide (TiO₂). Since then, various types of photocatalysts have been developed, and research to discover new materials is still actively underway.
Titanium dioxide (TiO₂) is a representative photocatalyst, and various other materials—including oxides and nitride-oxide compounds such as TaON and BaTaO₂N—are also being studied. Since photocatalysts can only absorb light with energy equal to or greater than their bandgap, developing materials capable of utilizing a broader range of solar wavelengths is a key research challenge.
Currently, many photocatalysts—both commercial and in the research stage—primarily utilize light in the ultraviolet (UV) spectrum. However, UV light accounts for only a small portion of the solar energy reaching the Earth’s surface. Visible light, on the other hand, constitutes a much larger proportion. Therefore, if photocatalysts capable of effectively absorbing not only UV light but also visible light are developed, solar energy utilization efficiency could be significantly improved, and this is expected to mark a major turning point in enhancing hydrogen production.
How will the future of hydrogen energy change?
Hydrogen energy is expected to continue playing a vital role as one of the key energy sources for realizing a carbon-neutral society. If hydrogen production technologies utilizing sunlight and water continue to advance, they can significantly contribute to reducing dependence on fossil fuels and cutting greenhouse gas emissions. Furthermore, water is abundant on Earth, and photocatalysts have the advantage of being reusable without being consumed during the reaction process.
Of course, challenges remain that must be addressed for hydrogen energy to become widely adopted, including reducing production costs, improving storage and transportation technologies, and building the necessary infrastructure. Nevertheless, if water splitting technology using photocatalysts continues to advance, the economic viability of eco-friendly hydrogen production will increase, and the potential for hydrogen to be utilized more widely across various industries and transportation sectors will also grow.
If, one day, hydrogen produced solely from sunlight and water becomes the primary energy source powering cars, ships, aircraft, and industrial facilities, we will be able to usher in a more sustainable society with significantly reduced dependence on fossil fuels. The sooner that day arrives, the more we will be able to pass on a cleaner sky and a healthier Earth to future generations.
