Thermochemical cycle

A thermochemical cycle is a special process scientists are studying to make hydrogen in a clean and sustainable way. Instead of using electricity, like in electrolysis, these cycles use very high heat from sources such as solar concentrators or nuclear reactors. The heat drives a chain of chemical reactions that split water into hydrogen and oxygen. The key idea is that the chemicals used in the process are recycled at the end of each cycle, so the system can keep running without needing new materials every time.^[1]

One of the best-known examples is the sulfur–iodine cycle. This cycle has three main steps. First, sulfur dioxide, iodine, and water react to form two different acids. Then, sulfuric acid is broken apart at extremely high heat (around 850 °C) to release oxygen and recycle sulfur dioxide. Finally, hydrogen iodide is broken apart at about 450 °C to release hydrogen and recycle iodine.^[2] Because everything is reused, the cycle can continue, and it can reach an efficiency of more than 50%, which is very high for such processes.^[3]

Another example is the copper–chlorine cycle, which works at lower temperatures, around 500 °C. This makes it easier to pair with nuclear power plants, which can provide that level of heat. In this cycle, compounds of copper and chlorine go through several steps, including reacting with water, releasing oxygen, and a special kind of electrolysis combined with heat. Together, these steps produce hydrogen while reusing the copper and chlorine.^[4]

There are also metal oxide cycles, such as those that use cerium oxide (CeO₂). In these cycles, concentrated solar heat reduces cerium oxide into a lower form. When this lower form reacts with water, it releases hydrogen and turns back into regular cerium oxide. This makes the process repeatable and powered directly by solar energy.^[5]

Thermochemical cycles are exciting because they do not need large amounts of electricity, which can be expensive. Instead, they can use heat from the sun or nuclear plants.^[1] However, they face challenges, like dealing with corrosive chemicals, building materials that can survive extreme heat, and making the reactions stable on a large scale. Most of these cycles are still in the research stage, but scientists are testing improvements such as new catalysts, combining heat with small amounts of electricity, and connecting the systems with solar power plants. If these cycles become practical, they could make massive amounts of hydrogen with very high efficiency, enough to support industries like steelmaking, fertilizer production, and heavy transport. This could be an important step toward a cleaner, carbon-free future.^[6]