# Direct Solar Thermal Water-Splitting Using Iron and Iron Oxides at High Temperatures: A Review
## 1 Introduction  
The highest levels of solar radiation are typically found in arid and hyperarid climates. The Atacama Desert in northern Chile offers ideal solar radiation, making it widely recognised for its potential usefulness in producing solar hydrogen (H\({}_{2}\)). Additionally, the region boasts a stable and predictable climate, with minimal rainfall and a lack of extreme temperature variations. This climatic consistency is crucial for the long-term performance and reliability of solar energy systems. Moreover, the desert's vast expanses of land provide ample space for large-scale solar installations, facilitating the deployment of cutting-edge solar technologies and enabling the generation of significant amounts of clean energy. The knowledge gained from studying solar resources in this unique environment not only contributes to advancements in solar technology but also serves as a valuable blueprint for harnessing solar power in other regions with similar radiation characteristics. There are numerous reasons that explain why the thermochemical process [1, 2] plays an important role in producing hydrogen (H\({}_{2}\)) in arid and hyperarid zones. Zones with high solar radiation have the following advantage: the high levels of solar radiation present in arid regions, such as the Atacama Desert, make them ideal for harnessing solar power. Water electrolysis is a promising method for hydrogen production due to its ability to produce high-purity hydrogen, its integration with renewable energy sources, and its adaptability to different scales of operation. However, its high energy consumption and the challenge of reducing the use of exotic materials remain significant challenges for this type of technology, affecting energy-conversion efficiency and posing a major barrier to its large-scale adoption. Although thermochemical solar cycles using ceria have 25% annual efficiency compared to 14% for the electrolysis process [3,4], the cycles are still impeded by a low level of technological preparation for the production of low-emission hydrogen [5, 6, 7, 8]. The utilization of abundant solar energy in thermochemical processes can result in the generation of hydrogen and has the following advantages. (i) The efficient splitting of water molecules into hydrogen and oxygen is facilitated using concentrated solar energy to achieve high temperatures in thermochemical water splitting. When high solar irradiance is available, this method can achieve higher efficiency than is achieved with other hydrogen-production methods. (ii) Hydrogen production is made sustainable and environmentally friendly by utilizing renewable solar energy in the process. This approach contributes to cleaner energy production by reducing dependence on fossil fuels and decreasing greenhouse gas emissions. (iii) Iron or iron oxide can be used as abundant and low-cost materials for the thermochemical process, which makes it economically viable. Cost-effective production is guaranteed in regions with high solar potential thanks to the high availability of these materials globally. (iv) Arid zones tend to have predictable and stable weather patterns with little rainfall and small variations in temperature. Solar thermochemical systems' performance and reliability are enhanced by the consistent climatic conditions, which ensure reliable and continuous operation, and (v) technological advancements can be achieved by researching and developing solar thermochemical hydrogen production in high-irradiation zones with high solar potential. Through the application of this knowledge, global hydrogen-production efforts can benefit from improved efficiency, cost reduction, and the development of new materials. The thermochemical solar process (TSP) for recovering solar hydrogen is considered environmentally favourable and inexpensive. The electrodes used in this process are generally made from iron or iron oxide, materials that are abundant worldwide.