Thermochemical Water Splitting

Thermo-chemical water splitting is a new advancement in hydrogen production technologies, leveraging high temperature chemical reactions to facilitate splitting of water into hydrogen and oxygen. Particularly focused on large-scale applications, thermo-chemical water splitting processes may utilise solar energy or nuclear energy to drive the reaction at high efficiencies and provides an alternative to using electrical energy for production (electrolysis). The process involves a series of cyclic chemical reactions, depending on the intermediate compounds used. The intermediate compounds are reduced and oxidised in the process.

The first step of the process is achieving the high temperatures required for thermo-chemical water splitting, usually achievable through solar concentrators or nuclear reactors. Once the activation energy is achieved, the chemical reaction of the intermediatory compounds takes place. Here, an example of sulphur-iodine (S-I) is explained in Figure X, where iodine and sulphur react with water to produce hydrogen iodide and sulphuric acid. The sulphuric acid decomposes into sulphur, water, and oxygen, and the hydrogen iodide decomposes into iodine and hydrogen. The decomposition reactions are heat-induced and reversible, hence intermediate chemicals are recycled.

The main advantage of thermo-chemical water splitting lies in its large-scale hydrogen production capacity and use of renewable solar energy as the heating source. Although in early stages of development, thermo-chemical water splitting may prove to compete with electrolysis for hydrogen production and contribute extensively towards the future hydrogen economy. Yet, critical challenges hinder its widespread adoption. Thermo-chemical water splitting plants have a very high investment cost, corrosive intermediate compounds that lead to reduction in plant lifetime, and slow rates of reactions are some critical challenges that need to be addressed before pilot projects and commercialisation may occur.