Metal Oxides Applied to Thermochemical Water-Splitting for Hydrogen Production Using Concentrated Solar Energy

Abanades Abanades

doi:10.3390/chemengineering3030063

Abstract

Solar thermochemical processes have the potential to efficiently convert high-temperature solar heat into storable and transportable chemical fuels such as hydrogen. In such processes, the thermal energy required for the endothermic reaction is supplied by concentrated solar energy and the hydrogen production routes differ as a function of the feedstock resource. While hydrogen production should still rely on carbonaceous feedstocks in a transition period, thermochemical water-splitting using metal oxide redox reactions is considered to date as one of the most attractive methods in the long-term to produce renewable H2 for direct use in fuel cells or further conversion to synthetic liquid hydrocarbon fuels. The two-step redox cycles generally consist of the endothermic solar thermal reduction of a metal oxide releasing oxygen with concentrated solar energy used as the high-temperature heat source for providing reaction enthalpy; and the exothermic oxidation of the reduced oxide with H2O to generate H2. This approach requires the development of redox-active and thermally-stable oxide materials able to split water with both high fuel productivities and chemical conversion rates. The main relevant two-step metal oxide systems are commonly based on volatile (ZnO/Zn, SnO2/SnO) and non-volatile redox pairs (Fe3O4/FeO, ferrites, CeO2/CeO2−, perovskites). These promising hydrogen production cycles are described by providing an overview of the best performing redox systems, with special focus on their capabilities to produce solar hydrogen with high yields, rapid reaction rates, and thermochemical performance stability, and on the solar reactor technologies developed to operate the solid–gas reaction systems.

Highlights

Solar thermochemical processes are efficient routes for converting high temperature solar heat into valuable and sustainable chemical energy carriers
Thermochemical water-splitting cycles consist of the thermal conversion of water into separate streams of hydrogen and oxygen via a series of endothermic and exothermic chemical reactions
The interest in thermochemical water-splitting cycles boomed in the late 70s and early 80s with the oil crisis [5,6,7,8,9], and most of the cycles were proposed for being combined with a primary nuclear energy source, thereby imposing constraints on the operating temperature that should remain below 900 ◦C

Summary

Introduction

Solar thermochemical processes are efficient routes for converting high temperature solar heat into valuable and sustainable chemical energy carriers (solar fuels). A large research effort has mainly been focused on UT-3 and Iodine-Sulphur (I-S) cycles [10,11,12,13,14,15,16], in which the primary energy input was the high-temperature heat released from an advanced nuclear reactor (4th generation power station) These cycles have been proposed in combination with a solar energy source [17,18,19,20,21,22]. This electrolysis step requires only about one tenth of the electrical power needed for conventional water electrolysis, thereby reducing the global process energy demand for hydrogen production, which is crucial for future industrial technology commercialization Such complex cycles operating below 1000 ◦C usually involve incomplete reactions or electrochemical steps (hybrid cycles), additional separation steps, hazardous or corrosive reactants and/or products, which results in materials issues for reactor construction and may compromise viable commercial process implementation. The products of the high-temperature reduction reaction are in the gaseous state for the volatile oxide cycle category, whereas redox reactions proceed in the condensed state for the non-volatile oxides

Volatile Metal Oxide Cycles

Findings

Fe2O 3 - Fe3O4 2