Reduction enthalpy and charge distribution of substituted ferrites and doped ceria for thermochemical water and carbon dioxide splitting with DFT+U

Abstract

The thermal reduction step of substituted ferrites (MFe2O4 where M = Fe, Ni, Co, Gd) and doped ceria (MxCe1-xO2, where M = Ce, Zr, Hf and x = 0.25) in two-step thermochemical cycles for H2O and CO2 splitting is investigated within the DFT+U framework. This thermal reduction step is described as the oxygen vacancy formation energy (reduction enthalpy), i.e. the energy required to create an oxygen vacancy in the crystal lattice. Oxides with a lower oxygen vacancy creation energy are easier to reduce. A Bader charge analysis of the reduction mechanism is carried out providing the charge distribution of the bulk and reduced ions, enabling interrelations of the substitute ions and the resulting reduction energies. Based on the approach presented here, interesting solar fuels producing materials are CoFe2O4, NiFe2O4 and Hf0.25Ce0.75O2.

Thermochemical H2O and CO2 splitting redox cycles in a NiFe2O4 structured redox reactor: Design, development and experiments in a high flux solar simulator

“…The third term is calculated from the reduction enthalpy of NiFe 2 O 4 (DH redNiFe2O4 = 416 kJ/0.5 mol O 2 (Dimitrakis et al, 2016) and a simulation run of the NiFe 2 O 4 monolith kinetics (Kostoglou et al, 2014) and is negligible compared to the other terms.…”

Thermochemical cycles over redox structured reactors

“…The redox material selected for the development of porous ceramic structures was the NiFe 2 O 4 , which is a well-known and widely investigated composition [12,14,21,27,28]. NiFe 2 O 4 has also been investigated via computational chemistry (DFT calculations, [29]) where it was shown that it is one of the state-of-the-art formulations in the family of ferrite materials. The nickel ferrite powder was synthesized via the Selfpropagating High-temperature Synthesis (SHS) technique from a mixture of Fe, NiO and Fe 2 O 3 , according to the following reaction scheme:…”

Solar Redox Cycling of Ceria Structures Based on Fiber Boards, Foams, and Biomimetic Cork-Derived Ecoceramics for Two-Step Thermochemical H2O and CO2 Splitting

“…5 Ceria is currently designated as the benchmark material to perform thermochemical cycles, given its ability to maintain its crystallographic structure over a large range of non-stoichiometry, together with its thermodynamically favorable oxidation and high oxygen storage capacity, which makes it suitable to perform thermochemical cycles with high performance stability. Numerous studies have been devoted to ceria [6][7][8][9][10][11][12][13][14] , and the addition of dopants [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] to enhance its redox performance. However, ceria redox activity depends on the reduction temperature and oxygen partial pressure applied during the high-temperature step, which is generally limited to 1500°C due to undesired oversintering and ceria sublimation.…”