Production of Hydrogen with Solar Energy

Abstract

Abstract One of the main requirements for a future Hydrogen Economy is a clean and efficient process for producing hydrogen using renewable energy sources. Hydrogen is a promising energy carrier because of its high energy content and clean combustion. In particular, the production of hydrogen from water and solar energy, i.e., photocatalysis and photoelectrolysis are methods for both renewable and sustainable energy production. Systems based on a photocatalyst powder suspended in an aqueous solution comprise single particles, layered materials, composite particles, or particle mixtures. The oxidation and reduction reactions take place at the same surface of a single particle. In order to avoid recombination processes, co‐catalysts are deposited on the surface of the photocatalyst to ensure that oxygen and hydrogen evolve at spatially separate surface sites. While water splitting can be carried out with coupled solar cells‐commercial water electrolyzers, the direct photoelectrolysis with a semiconductor photoelectrode is a more elegant and cheaper approach and the direct photoelectrolysis is the Holy Grail of electrochemistry, using the power of light. Here, the photoactive semiconductor is immersed in water and the photogenerated electrons and holes are directly used to reduce and oxidize water, respectively, in a PhotoElectroChemical cell (PEC cell). This article focuses on the materials science and engineering of photocatalysts, co‐catalysts, and photoelectrodes and will illustrate that the activities in the field of solar energy conversion into hydrogen are numerous. The principle of the PEC cell will be presented along with materials requirements for application in a PEC cell. Characterizations of the physical and chemical properties of state‐of‐the‐art and novel materials guide materials scientists to optimal photocatalysts and photoelectrodes. Engineering on an atomic scale is achieved by cation and anion doping effects, deviations from stoichiometry, and, in case of ternary and more complex materials, deviations from molecularity. Defect chemical aspects will be presented. To date, the decreasing length scale to the nanoscale of the functional materials for photocatalytical and photoelectrochemical applications attracts widespread attention. Besides water, also aqueous solutions of hydrobromic acid and hydrogen sulfide have been explored for hydrogen production using solar radiation. The large‐scale conversion of hydrogen sulfide from the Black Sea would be beneficial from an environmental and energy point of view.