Design Principles for Metal Oxide Redox Materials for Solar‐Driven Isothermal Fuel Production

Abstract

The performance of metal oxides as redox materials is limited by their oxygen conductivity and thermochemical stability. Predicting these properties from the electronic structure can support the screening of advanced metal oxides and accelerate their development for clean energy applications. Specifically, reducible metal oxide catalysts and potential redox materials for the solar-thermochemical splitting of CO2 and H2O via an isothermal redox cycle are examined. A volcano-type correlation is developed from available experimental data and density functional theory. It is found that the energy of the oxygen-vacancy formation at the most stable surfaces of TiO2, Ti2O3, Cu2O, ZnO, ZrO2, MoO3, Ag2O, CeO2, yttria-stabilized zirconia, and three perovskites scales with the Gibbs free energy of formation of the bulk oxides. Analogously, the experimental oxygen self-diffusion constants correlate with the transition-state energy of oxygen conduction. A simple descriptor is derived for rapid screening of oxygen-diffusion trends across a large set of metal oxide compositions. These general trends are rationalized with the electronic charge localized at the lattice oxygen and can be utilized to predict the surface activity, the free energy of complex bulk metal oxides, and their oxygen conductivity.

Rational design of metal nitride redox materials for solar-driven ammonia synthesis

“…Such novel theory-assisted design is analogous to the rational design of metal oxide redox materials [39][40][41], where reactivity is controlled by the oxygen vacancy formation energy [39,40]. To verify the concept, we experimentally demonstrate controlling nitrogen vacancy formation in manganese based redox materials.…”

Carbon Dioxide Reforming of Methane using an Isothermal Redox Membrane Reactor

“…(2)] for binary metal oxides vs.the enthalpy of the oxide reduction. Thea nalysis displays at rade-off inherent to redox materials: [10,39,40] materials that facilitate high CO 2 conversion bind lattice oxygens trongly,w hereas materials that facilitate high CH 4 conversion bind lattice oxygen weakly. Thei deal materials compositions are wheret hese effects balance, located at the intersectiono fb oth curves.I deally,b oth reac- tions are exergonic, which can be achieved by increasing the reaction temperature,a ss hown with Figure 4B,C.A nalogous to the Sabatier principle in catalysis, [41] the redox cycle is either limited by too strongly or too weaklyb ound lattice oxygen, as described by the most endergonic reactione nergies markedw ith the volcano-shapedl ine in Figure 4C.F or DRM, the location of the volcanot op is determined mainly by the temperature of the metal oxide reduction, which is due to the large entropy change of the gas participating in the reaction.…”

“…[25,31] We employ electronic structure theory to quantify the thermodynamic limitations of LSCF redox membranes and to identify advanced perovskite compositionsf or solar-drivenD RM. Thet hermochemical stability and the reactione nergetics for perovskites are calculated from the scaling relation [10,39] [39] shown with Figure 4D.P lotting the data for perovskites togetherw ith those for binary metal oxides shows,i nF igure 4A,t hat perovskites can reproduce the redox energetics of expensive or toxic materials [15] -such as LaCuO 3 ,a nd CO from CH 4 at about the same rate as CO from CO 2 .C H 4 conversion reaches 17 %, whereas the CO 2 conversion is at maximum 8.0% due to CH 4 decomposition. Electronic structure calculations show how CO 2 reduction limits the dryr eforming of methane with LSCF redox membranes.C O 2 conversion may be increased by using La 0.5 Sr 0.5 MnO 3Àd and La 0.5 Sr 0.5 Mn 0.5 Co 0.5 O 3Àd with lower oxygenv acancy stabilities.T he developed principles may be useful for the rational designo fa dvanced redox materials for solar-drivenprocesses.…”

“…Solid-state analysis: Membranes were characterized by using XRD (Cu K a radiation, 20-808 2q,0 .068 min À1 scan rate,4 5kV/ 20 mA output, PA Nalytical/XPert MPD/DY636, Philips) and SEM using aZ eiss Supra 50VP microscope. Thermodynamic equilibrium calculations:T or ationally design metal-oxide redox materials for DRM, we computed av olcanotype plot [10,39,40] from the thermochemical equilibrium of 38 redox pairs as af unction of temperature and pressure by employing tabulated free energy data. [26] Electronic structure computations:C omputational details are given in-depth elsewhere [10,39] and in the Supporting Information.…”

“…Thermodynamic equilibrium calculations:T or ationally design metal-oxide redox materials for DRM, we computed av olcanotype plot [10,39,40] from the thermochemical equilibrium of 38 redox pairs as af unction of temperature and pressure by employing tabulated free energy data. [26] Electronic structure computations:C omputational details are given in-depth elsewhere [10,39] and in the Supporting Information. Briefly,t he thermochemical stability of 10 La 0.5 Sr 0.5 B 0.5 B' 0.5 O 3Àd (B,B' = Mn, Fe,Co, Cu) perovskites and the free energy of forming oxygen vacancies at the (010) facet were computed by using DFT with the Grid-based projector-augmented wave (GPAW ) code [42,43] in the atomic simulation environment (ASE).…”