In this blog post, we’ll explore the significance of hydrogen energy as a solution to the limitations of fossil fuels, the principle behind producing hydrogen by using solar power to split water, and the challenges that must be overcome for this technology to evolve into a sustainable energy source.
Recently, the world has been seeking various solutions to energy problems. Fossil fuels, which have been the primary energy source for the past several decades, have caused not only price volatility—as exemplified by oil crises—but also various environmental problems, such as greenhouse gas emissions and air pollution generated during their use. Furthermore, fossil fuels are geologically finite resources, which means they cannot serve as a sustainable energy source in the long term. However, it is difficult to conclusively state that fossil fuel reserves will be completely depleted in the near future, rendering them unusable. What is important is not the exact timing of reserve depletion itself, but rather reducing dependence on fossil fuels and expanding sustainable energy sources to address issues such as climate change, energy security, and price volatility. To solve these problems, the world is developing various alternative energy sources and low-carbon energy technologies, one of which is hydrogen energy. While hydrogen differs in nature from solar or wind power—as it is an energy carrier that can be produced and stored using other forms of energy rather than being an energy source in itself—it is gaining attention as a highly promising energy carrier in sectors where decarbonization through electricity alone is difficult, such as industry and long-distance transportation. As of 2026, global hydrogen production still relies heavily on fossil fuels, but low-emission hydrogen production technologies are gradually expanding. According to the International Energy Agency (IEA), low-emission hydrogen production reached approximately 1 million metric tons in 2025, and the global installed capacity of electrolysers—devices that use electrolysis to split water—exceeded 4 GW in 2025.
Existing methods for producing hydrogen include reforming or cracking fossil fuels and electrolysis of water. While producing hydrogen from fossil fuels such as natural gas is currently the most widely used method, it generates carbon dioxide during the production process; therefore, producing hydrogen alone cannot solve the environmental problems associated with fossil fuels. The method of obtaining hydrogen by electrolytically decomposing water requires additional energy because it relies on electricity, and the environmental impact of the hydrogen production process varies depending on how the electricity used is generated. Therefore, methods that use renewable energy sources such as solar or wind power to generate electricity, which is then used to electrolyze water to produce hydrogen, are being researched as important low-emission hydrogen production methods. In addition, photocatalytic and photoelectrochemical water splitting technologies that directly utilize solar energy to split water are also being studied. Water splitting using solar energy has a significant advantage in that it can replace some or all of the externally supplied electricity with direct solar energy. However, hydrogen is not automatically produced simply by shining sunlight on water; appropriate photocatalysts, semiconductor materials, electrodes, and reaction conditions are required to trigger the water splitting reaction. Furthermore, when using seawater, additional technical issues—such as corrosion and side reactions caused by salinity and impurities—may arise. Therefore, it cannot be simply stated that the problem of drinking water scarcity can be completely solved merely because seawater is abundant.
The key elements in the process of producing hydrogen from water are water, sunlight, and a photocatalyst or suitable semiconductor material that absorbs light to catalyze the water-splitting reaction. A certain amount of energy is required to split water. The photocatalyst plays a crucial role in utilizing the energy from sunlight to trigger the oxidation-reduction reactions necessary for water splitting. Simply shining sunlight directly on water does not cause it to easily decompose into hydrogen and oxygen; however, when an appropriate photocatalyst absorbs light, it can use that energy to generate charges and promote the water splitting reaction. Key concepts in photocatalysis are the valence band (V.B.) and the conduction band (C.B.).
In semiconductors, electrons generally reside in the valence band; when they absorb sufficient energy from an external source, they can move to the conduction band, a higher energy state. The energy difference between the conduction band and the valence band is called the band gap. When an electron absorbs energy equal to or greater than the band gap, it moves from the valence band to the conduction band, leaving a hole in its place. To put it simply, this is similar to climbing stairs. Just as you need enough strength to lift your leg to the height of a single step, an electron requires a certain minimum amount of energy to move to a higher energy state. When an electron receives sufficient energy from sunlight, it moves from the valence band to the conduction band, resulting in the creation of two different charge carriers: electrons and holes. The electron that has moved to the conduction band and the hole remaining in the valence band can each participate in reduction and oxidation reactions, respectively; when the appropriate energy levels and reaction conditions are met, hydrogen and oxygen can be produced through water splitting. This is the basic principle of water splitting using sunlight.
This explanation may be somewhat difficult for non-specialists to understand. To put it simply, just as it takes force to cut an apple in half, energy is also required to split water. To decompose water using electricity or physical force, a separate device and additional costs are required to supply that energy. Therefore, research is underway to supply the energy needed for water splitting using solar energy, a renewable energy source. When sunlight is shone on a photocatalyst or semiconductor material, the material absorbs the light’s energy and converts it into a form that can be used for the oxidation and reduction reactions of water. If the electrons and holes generated in this way move and react appropriately, water can be split into hydrogen and oxygen. This method holds significant importance as one of the future low-emission hydrogen production technologies, as it reduces dependence on fossil fuels and utilizes renewable energy and water. However, current technologies that directly use sunlight to split water must overcome challenges such as efficiency, durability, material stability, production costs, and the construction of large-scale systems. Across the hydrogen industry as a whole, low-emission hydrogen is often still more expensive than fossil-fuel-based hydrogen, and the International Energy Agency reports that, as of 2026, fossil-fuel-based hydrogen will remain cheaper in most regions in the short term. Nevertheless, efforts to scale up electrolyzer technology, expand renewable energy, and advance materials and catalyst technologies are ongoing, and low-emission hydrogen production is expanding, centered on actual projects. Therefore, rather than concluding that technology for producing hydrogen using water and solar power is a fully mature commercial technology, it is more appropriate to view it as an important area of research where efficiency, cost-effectiveness, and durability must be improved in the future. If this technology becomes sufficiently competitive, hydrogen derived from water could become a key energy carrier that reduces dependence on fossil fuels and supports the decarbonization of industry and energy systems.
