Additive manufacturing and two-step redox cycling of ordered porous ceria structures for solar-driven thermochemical fuel production

Abstract

This study focuses on thermochemical H2O and CO2-splitting processes using non-stoichiometric metal oxides and concentrated solar energy to produce solar fuels. The redox process involves two distinct reactions: (i) a thermal reduction at high temperature of the oxide with creation of oxygen vacancies in its crystallographic structure, resulting in released O2; (ii) a re-oxidation of the metal oxide by H2O and/or CO2, yielding H2 and/or CO. Hierarchically-ordered porous ceria materials offer high potential for solar-driven thermochemical fuel production based on two-step redox cycles for H2O and CO2-splitting. The emergence of additive manufacturing processes allows to develop architected reactive materials with 3D-ordered geometry and hierarchical structure (porosity gradient), able to enhance the volumetric solar absorptivity and provide homogeneous heating of the oxygen carrier. The investigation of additive-manufactured ordered porous ceria monoliths made from 3D-printed polymer scaffolds was performed in a solar reactor.In comparison with reticulated ceria foams, an improvement of the oxygen yields was achieved with 3D-ordered porous structures. A low total pressure during the reduction step (inducing low pO2) favored the reduction extent and associated fuel yields. The fuel production rate during the exothermal oxidation step was enhanced by decreasing the temperature and by increasing the CO2 partial pressure. In addition, since the oxidation step is a surface-controlled reaction, a natural pore former (woody biomass) was used to create µm-size pores within the struts of ceria scaffolds. This enhanced the oxidation kinetics with a maximal CO production rate of 4.9 mL/min/g and fuel yield up to 333 µmol/g (with a reduction step at 1400 °C under ~0.10 bar of total pressure). Thus, using a 3D-ordered open-cell geometry with addition of micro-scale porosity in the struts enhanced the thermochemical performance.