Materiais para a Energia - ME
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Browsing Materiais para a Energia - ME by Author "Abanades, Stéphane"
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- High performance cork-templated ceria for solar thermochemical hydrogen production via two-step water-splitting cyclesPublication . Oliveira, Fernando Almeida Costa; Barreiros, M. Alexandra; Haeussler, Anita; Caetano, Ana P. F.; Mouquinho, Ana; Silva, P. Oliveira e; Novais, Rui M.; Pullar, Robert C.; Abanades, StéphaneABSTRACT: Water splitting by solar energy-driven two-step thermochemical cycles is a promising approach for large-scale production of renewable fuels (e.g. hydrogen). The key challenge is developing materials capable of withstanding the harsh environmental conditions and to ensure high reliability in use, particularly in terms of redox kinetics and better activity at low operation temperatures. In this work, we demonstrate that cork-templated ceria can significantly enhance the hydrogen production performance under solar irradiation heating. Three types of ceria morphologies were synthesised and investigated in two-step thermochemical redox cycles, namely ceria granules (ecoceramics) prepared from cork templates based on either a green water-based or an acetone solvent-based approach, as well as ceria foams replicated from polyurethane templates. These materials were cycled in a high-temperature indirectly-irradiated solar tubular reactor, heated via concentrated solar light, using a temperature-swing process. Samples were typically thermally reduced at 1400-1450 degrees C and subsequently re-oxidised with H2O between 950-1150 degrees C. The green synthesis ceria granules had up to 25% and 32% higher average H-2 production yields than the acetone-based ecoceramics and replicated ceria foams, respectively. On average, H-2 production rates for cork-templated ceria granules (1.3 +/- 0.2 mL min(-1) g(-1)) were up to similar to 60% higher than for ceria foams (0.8 +/- 0.3 mL min(-1) g(-1)), indicating that the morphology of this three-dimensionally ordered macroporous (3-DOM) CeO2 improves the reaction kinetics. This is attributed to the smaller mean cell size of the cork-derived ecoceramic (25 mu m) compared to that of the replicated ceria foam (575 mu m), suggesting that their semi-closed wall cells enhanced reaction rates. The increase in reduction temperature from 1400 to 1450 degrees C resulted in the highest H-2 production rate (1.6 mL min(-1) g(-1)) reported so far for 3-DOM ceria. Neither loss in redox performance nor change in grain morphology was observed from the first to the last cycle. These findings show that cork-like structural features are key to engineering efficient materials for enhanced solar thermochemical fuel production.
- Solar redox cycling of ceria structures based on fiber boards, foams, and biomimetic cork-derived ecoceramics for two-step thermochemical H2O and CO2 splittingPublication . Haeussler, Anita; Abanades, Stéphane; Oliveira, Fernando Almeida Costa; Barreiros, M. Alexandra; Caetano, Ana P. F.; Novais, Rui M.; Pullar, Robert C.ABSTRACT: Solar thermochemical conversion of H2O and captured CO2 is considered for the production of high-value solar fuels and CO2 valorization, using nonstoichiometric oxygen-exchange redox materials. This work aims to compare the thermochemical cycle performance of different ceria structures, including biomimetic cork-templated ceria (CTCe), ceria foams (CeF), and ceria bulk fiber boards (CeFB), to study the effect of the morphology on fuel production from two-step H2O and CO2 splitting via solar redox cycling. The considered materials underwent thermochemical cycles in a directly irradiated solar reactor under various operating conditions. Typically, a thermal reduction at 1400 degrees C under Ar at atmospheric pressure, using concentrated solar energy, was carried out followed by an oxidation step with H2O or CO2 between 800 and 1050 degrees C. The comparison of the fuel production rate and yield from the reactive materials highlighted the importance of the material thermal stability during cycling. CTCe and CeF showed good O-2 and fuel production stability over repeated cycles, while CeFB exhibited a decrease of the production because of sintering and thermal gradient due to its low thermal conductivity. Biomimetic CTCe showed a higher fuel production rate compared to the other investigated materials, explained by the favorable microstructure of the cork-based ceramic. The morphology obtained from the cork structure led to the improvement of the redox activity, demonstrating the relevance of studying this material for thermochemical H2O and CO2 splitting cycles. In addition, the impact of the operating conditions was investigated. A decrease of the starting oxidation temperature, an increase of the CO2 molar fraction (lower CO/CO2 ratio), or a high total gas flow rate favoring gas product dilution had a beneficial impact on the CO (or H-2) production rate.