OLÍVIA R. FURTADO
LNEG – Laboratório Nacional de Energia e Geologia, Lisboa, Portugal

Abstract

Metal oxides are used in concentrated solar energy plants (CSP) to store heat, which can be used in the absence of the sun to generate electricity or produce solar fuels. Energy can be stored in this form for long periods, being released when necessary. Two step redox thermochemical cycles using metal oxides are a valuable solar energy storage option to produce H₂ and CO. 

This brief review paper summarizes the role of metal oxide on the sustainable production of alternative fuels. It contemplates the formation of oxygen vacancies, combination of metals and the progress made to date on the utilization of metal oxides as promising materials with potential application in water splitting for production of clean hydrogen and solar fuels.

THE THERMOCHEMICAL STORAGE OF SOLAR ENERGY

The sun is an unlimited source of energy. This energy can be converted and stored in the form of chemical fuels, such as hydrogen or syngas (synthesis gas), being an ecological alternative to fossil fuels (1). Solar energy stored in the form of solar fuels can be transportable and stored for long periods. 

Hydrogen is a promising fuel option as it allows high storage capacity and energy density. Hydrogen can be generated from solar energy by electrochemical, photochemical or thermochemical processes (Figure 1) (2).

Recently, Steinfeld and coworkers reviewed important aspects concerning thermochemical energy storage using concentrated solar radiation (3). Through thermochemical cycles, there is no need to separate oxygen from hydrogen, produced in different stages, at temperatures below the thermolysis process and there is no need to consume electrical energy as through electrolysis, where it is necessary to consume large amounts of electricity.

Metal oxides are used in concentrated solar energy plants (CSP) to store heat and in the absence ...