Solar thermochemical fuel production from H2O and CO2 splitting via two-step redox cycling of reticulated porous ceria structures integrated in a monolithic cavity-type reactor

Abstract

Solar thermochemical H2O and CO2 splitting cycles represent an efficient route for converting high temperature concentrated solar heat into valuable chemical energy carriers (solar fuels). A new monolithic solar reactor compatible with ceria redox reactions was designed, constructed and tested under concentrated solar radiation. The ceria redox material was shaped and integrated as reticulated porous structures with controlled cell sizes and gradient (10–60 ppi, pores per inch) enabling efficient volumetric solar radiation absorption and micro-scale interconnected porosity favouring the solid-gas reactions. Temperature-swing redox cycling experiments were performed to demonstrate solar reactor reliability during continuous operation. The foams were first thermally activated by increasing the reactor temperature (1400–1450 °C) for O2 release and then exposed to H2O or CO2 stream to produce pure H2 or CO (700–1100 °C), allowing cyclic operation in the same reactor. The influence of operating conditions (including reduction and oxidation temperatures, pressure and type of oxidizing gas) on reactor performance was investigated. An increase of the reduction temperature or a decrease of the operating pressure improved both the ceria reduction extent and fuel production yields (up to 341 μmol/g), while a decrease of the CO:CO2 ratio (by increasing total inlet gas flow-rate) or an increase of the inlet CO2 concentration enhanced oxidation rates (up to 9.3 mL/g/min). The obtained fuel production rates outperformed the maximum previously reported values by up to 8 times using the highly-reactive manufactured ceria porous foams cycled between 1400 °C and 900 °C with oxidation performed in 100% CO2 upon dynamic cooling. An average H2/CO production of ∼280 Ncm3/cycle (64 cycles performed) was achieved with solar-to-fuel efficiency up to ∼7.5% and remarkable material performance stability.