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

The Case for a Defect Genome Initiative

The Materials Genome Initiative (MGI) has streamlined the materials discovery effort by leveraging generic traits of materials, with focus largely on perfect solids. Defects such as impurities and perturbations, however, drive many attractive functional properties of materials. The rich tapestry of charge, spin, and bonding states hosted by defects are not accessible to elements and perfect crystals, and defects can thus be viewed as another class of "elements" that lie beyond the periodic table. Accordingly, a Defect Genome Initiative (DGI) to accelerate functional defect discovery for energy, quantum information, and other applications is proposed. First, major advances made under the MGI are highlighted, followed by a delineation of pathways for accelerating the discovery and design of functional defects under the DGI. Near-term goals for the DGI are suggested. The construction of open defect platforms and design of data-driven functional defects, along with approaches for fabrication and characterization of defects, are discussed. The associated challenges and opportunities are considered and recent advances towards controlled introduction of functional defects at the atomic scale are reviewed. It is hoped this perspective will spur a community-wide interest in undertaking a DGI effort in recognition of the importance of defects in enabling unique functionalities in materials.

Fabrication of optimal oxygen vacancy amount in P/Ag/Ag2O/Ag3PO4/TiO2 through a green photoreduction process for sustainable H2 evolution under solar light

Defects engineering on photocatalysts such as oxygen vacancies (OVs) is an effective approach for improving photocatalytic hydrogen (H₂) evolution efficiency. In this study, OVs modified P/Ag/Ag₂O/Ag₃PO₄/TiO₂ (PAgT) composite was successfully fabricated via a photoreduction process by controlling the ratio of PAgT to ethanol (16, 12, 8, 6 and 4 g·L^-1) under simulated solar light irradiation for the first time. Characterization methods confirmed the presence of OVs in the modified catalysts. Meanwhile, the OVs amount and their effects on the light absorption ability, charge transfer rate, conduction band and H₂ evolution efficiency of the catalysts were also investigated. The results indicated that the optimal OVs amount endowed OVs-PAgT-12 with the strongest light absorption, the fastest electron transfer rate and suitable band gap for H₂ evolution, leading to the highest H₂ yield (863 μmol·h^-1·g^-1) under solar light irradiation. Moreover, OVs-PAgT-12 exhibited a superior stability during cyclic experiment, indicating its great potential for practical application. Furthermore, a sustainable H₂ evolution process was proposed based on a combination of sustainable bio-ethanol resource, stable OVs-PAgT, abundant solar energy and recyclable methanol. This study would provide new insights into the design of defects modified composite photocatalyst for enhanced solar-to-hydrogen conversion.

Recyclable, Self-Healing Solid Polymer Electrolytes by Soy Protein-Based Dynamic Network

Compared to traditional organic liquid electrolytes, which often present leakage, flammability, and chemical stability problems, solid polymer electrolytes (SPEs) are widely regarded as one of the most promising candidates for the development of safer lithium-ion batteries. Vitrimers are a new class of polymer materials consisting of dynamic covalent networks that can change their topology by thermally activated bond-exchange reactions. Herein, the recyclable and self-healing solid polymer electrolytes (SPEs) with a soy protein isolate (SPI)-based imine bond dynamic network are reported. This malleable covalent cross-linked network polymer can be reshaped and recycled at high temperature (100 °C) or only with water at ambient temperature (25 °C), which may realize the green processing of energy materials. The introduction of bis(trifluoromethane) sulfonimide lithium (LiTFSI) significantly reinforces the conductivity of the dynamic network to a maximum of 3.3 × 10-4 S cm-1 . This simple and applicable method establishes new principles for designing scalable and flexible strategies for fabricating polymer electrolytes.

Unraveling CoNiP-CoP2 3D-on-1D Hybrid Nanoarchitecture for Long-Lasting Electrochemical Hybrid Cells and Oxygen Evolution Reaction

Evolving cost-effective transition metal phosphides (TMPs) using general approaches for energy storage is pivotal but challenging. Besides, the absence of noble metals and high electrocatalytic activity of TMPs allow their applicability as catalysts in oxygen evolution reaction (OER). Herein, CoNiP-CoP2 (CNP-CP) composite is in situ deposited on carbon fabric by a one-step hydrothermal technique. The CNP-CP reveals hybrid nanoarchitecture (3D-on-1D HNA), i.e., cashew fruit-like nanostructures and nanocones. The CNP-CP HNA electrode delivers higher areal capacity (82.8 μAh cm-2 ) than the other electrodes. Furthermore, a hybrid cell assembled with CNP-CP HNA shows maximum energy and power densities of 31 μWh cm-2 and 10.9 mW cm-2 , respectively. Exclusively, the hybrid cell demonstrates remarkable durability over 30 000 cycles. In situ/operando X-ray absorption near-edge structure analysis confirms the reversible changes in valency of Co and Ni elements in CNP-CP material during real-time electrochemical reactions.  Besides, a quasi-solid-state device unveils its practicability by powering electronic components. Meanwhile, the CNP-CP HNA verifies its higher OER activity than the other catalysts by revealing lower overpotential (230 mV). Also, it exhibits relatively small Tafel slope (38 mV dec-1 ) and stable OER activity over 24 h. This preparation strategy may initiate the design of advanced TMP-based materials for multifunctional applications.

CaCo0.05Mn0.95O3-δ: A Promising Perovskite Solid Solution for Solar Thermochemical Energy Storage

The redox cycle of doped CaMnO_3-δ has emerged as an attractive way for cost-effective thermochemical energy storage (TCES) at high temperatures in concentrating solar power. The role of dopants is mainly to improve the thermal stability of CaMnO_3-δ at high temperatures and the overall TCES density of the material. Herein, Co-doped CaMnO_3-δ (CaCo_xMn_1-xO_3-δ, x = 0-0.5) perovskites have been proposed as a promising candidate for TCES materials for the first time. The phase compositions, redox capacities, TCES densities, reaction rates, and redox chemistry of the samples have been explored via experimental analysis and theoretical calculations. The results demonstrate that CaCo_0.05Mn_0.95O_3-δ showed an enhanced redox capacity (1000 °C at pO₂ = 10^-5 bar) without decomposition and provided the highest TCES density of ∼571 kJ kg^-1 reported so far. The effective Co doping tended to increase the valence states of B-site cations in perovskite and facilitate the diffusion of the lattice oxygen atoms into the surface-active oxygen sites. Furthermore, the high cooling rates deteriorated the microstructure of CaCo_0.05Mn_0.95O_3-δ particles and resulted in incomplete heat release, which is instructive to the design and operation of the TCES systems.

Thermochemical CO2 splitting performance of perovskite coated porous ceramics

In this paper, we investigate the redox performance of perovskite coated porous ceramics with various architectures. For this purpose, reticulated porous ceramics (RPCs) in three different pore sizes (5, 12, 75 ppi) were fabricated to represent a broad range of structures and pore sizes. The perovskite material is based on lanthanum manganite and was synthesized and doped with Ca and Al through the Pechini method. Using a deep coating method, the surface of RPC substrates was modified by a thin-film coating with a thickness of ∼15 μm. We evaluated the CO₂ conversion performance of the developed materials in a gold-image IR furnace. X-ray micro-computed tomography along with SEM/EDX were utilized in different steps of the work for a thorough study of the bulk and surface features. Results reveal that the intermediate pore size of 12 ppi delivers the maximum perovskite loading with a high degree of coating homogeneity and connectivity while CO₂ conversion tests showed the highest CO yield for 75 ppi. Our results show that the extreme conditions inside the furnace combined with the flow of gaseous phases cause the RPCs to shrink in length up to 23% resulting in the alteration of the pore phase and elimination of small pores reducing the total specific surface area. Further our results reveal an important mechanism resulting in the inhibition of CO₂ conversion where the perovskite coating layer migrates into the matrix of the RPC frame.

A representative volume of LCMA coated porous SiC showing a maximum of 23% shrinkage when subject to high-temperature CO₂ conversion redox reactions. This results in significant structural changes including a reduction in specific surface area.

Endogenous Nanoparticles Strain Perovskite Host Lattice Providing Oxygen Capacity and Driving Oxygen Exchange and CH4 Conversion to Syngas

Particles dispersed on the surface of oxide supports have enabled a wealth of applications in electrocatalysis, photocatalysis, and heterogeneous catalysis. Dispersing nanoparticles within the bulk of oxides is, however, synthetically much more challenging and therefore less explored, but could open new dimensions to control material properties analogous to substitutional doping of ions in crystal lattices. Here we demonstrate such a concept allowing extensive, controlled growth of metallic nanoparticles, at nanoscale proximity, within a perovskite oxide lattice as well as on its surface. By employing operando techniques, we show that in the emergent nanostructure, the endogenous nanoparticles and the perovskite lattice become reciprocally strained and seamlessly connected, enabling enhanced oxygen exchange. Additionally, even deeply embedded nanoparticles can reversibly exchange oxygen with a methane stream, driving its redox conversion to syngas with remarkable selectivity and long term cyclability while surface particles are present. These results not only exemplify the means to create extensive, self-strained nanoarchitectures with enhanced oxygen transport and storage capabilities, but also demonstrate that deeply submerged, redox-active nanoparticles could be entirely accessible to reaction environments, driving redox transformations and thus offering intriguing new alternatives to design materials underpinning several energy conversion technologies.

Mechanism of oxygen vacancy assisted water-splitting of LaMnO3: inorganic perovskite prediction for fast solar thermochemical H2 production

The mechanism of water-splitting and H₂ production around the oxygen vacancy site of the LaMnO₃ defective surface is explored for the purpose of quick identification of kinetically favorable dopants such as Mo.

Solar Energy on Demand: A Review on High Temperature Thermochemical Heat Storage Systems and Materials

Among renewable energies, wind and solar are inherently intermittent and therefore both require efficient energy storage systems to facilitate a round-the-clock electricity production at a global scale. In this context, concentrated solar power (CSP) stands out among other sustainable technologies because it offers the interesting possibility of storing energy collected from the sun as heat by sensible, latent, or thermochemical means. Accordingly, continuous electricity generation in the power block is possible even during off-sun periods, providing CSP plants with a remarkable dispatchability. Sensible heat storage has been already incorporated to commercial CSP plants. However, because of its potentially higher energy storage density, thermochemical heat storage (TCS) systems emerge as an attractive alternative for the design of next-generation power plants, which are expected to operate at higher temperatures. Through these systems, thermal energy is used to drive endothermic chemical reactions, which can subsequently release the stored energy when needed through a reversible exothermic step. This review analyzes the status of this prominent energy storage technology, its major challenges, and future perspectives, covering in detail the numerous strategies proposed for the improvement of materials and thermochemical reactors. Thermodynamic calculations allow selecting high energy density systems, but experimental findings indicate that sufficiently rapid kinetics and long-term stability trough continuous cycles of chemical transformation are also necessary for practical implementation. In addition, selecting easy-to-handle materials with reduced cost and limited toxicity is crucial for large-scale deployment of this technology. In this work, the possible utilization of materials as diverse as metal hydrides, hydroxides, or carbonates for thermochemical storage is discussed. Furthermore, special attention is paid to the development of redox metal oxides, such as Co₃O₄/CoO, Mn₂O₃/Mn₃O₄, and perovskites of different compositions, as an auspicious new class of TCS materials due to the advantage of working with atmospheric air as reactant, avoiding the need of gas storage tanks. Current knowledge about the structural, morphological, and chemical modifications of these solids, either caused during redox transformations or induced wittingly as a way to improve their properties, is revised in detail. In addition, the design of new reactor concepts proposed for the most efficient use of TCS in concentrated solar facilities is also critically considered. Finally, strategies for the harmonic integration of these units in functioning solar power plants as well as the economic aspects are also briefly assessed.

Functional Materials and Systems for Rewritable Paper

"Paper" has greatly contributed to the development and spread of civilization. Even in today's "digitalized" world, paper continues to play a key role in socioeconomic growth, as is evidenced by the growth in global paper consumption. Unfortunately, the use of paper has its cost in terms of the exhaustion of world's natural resources. Consequently, new, cost-effective technologies that preserve natural resources are required for this purpose. Functional materials have revolutionized the way people think about developing new technologies. Especially important in this regard are "smart reactive materials," which are capable of actively responding to external stimuli such as heat, light, mechanical stress, and specific molecular orientations. Moreover, functionalized chromogenic materials, which undergo reversible color switching upon external stimulation, have attracted great attention in the context of developing rewritable paper. Here, investigations of various materials and systems that are devised for use as rewritable paper are reviewed with the hope that the coverage will stimulate and guide future studies in this area.

Solar-Driven Thermochemical Splitting of CO2 and In Situ Separation of CO and O2 across a Ceria Redox Membrane Reactor

Splitting CO2 with a thermochemical redox cycle utilizes the entire solar spectrum and provides a favorable path to the synthesis of solar fuels at high rates and efficiencies. However, the temperature/pressure swing commonly applied between reduction and oxidation steps incurs irreversible energy losses and severe material stresses. Here, we experimentally demonstrate for the first time the single-step continuous splitting of CO2 into separate streams of CO and O2 under steady-state isothermal/isobaric conditions. This is accomplished using a solar-driven ceria membrane reactor conducting oxygen ions, electrons, and vacancies induced by the oxygen chemical potential gradient. Guided by the limitations imposed by thermodynamic equilibrium of CO2 thermolysis, we operated the solar reactor at 1,600°C, 3·10-6 bar [Formula: see text] and 3,500 suns radiation, yielding total selectivity of CO2 to CO + ½O2 with a conversion rate of 0.024 μmol·s-1 per cm2 membrane. The dynamics of the oxygen vacancy exchange, tracked by GC and XPS, further validated stable fuel production.

Design principles of perovskites for solar-driven thermochemical splitting of CO2

Perovskites are attractive redox materials for thermo/electrochemical fuel synthesis. To design perovskites with balanced redox energetics for thermochemically splitting CO2, the activity of lattice oxygen vacancies and stability against crystal phase changes and detrimental carbonate formation are predicted for a representative range of perovskites by electronic structure computations. Systematic trends in these materials properties when doping with selected metal cations are described in the free energy range defined for isothermal and temperature-swing redox cycles. To confirm that the predicted materials properties root in the bulk chemical composition, selected perovskites are synthesized and characterized by X-ray diffraction, transmission electron microscopy, and thermogravimetric analysis. On one hand, due to the oxidation equilibrium, none of the investigated compositions outperforms non-stoichiometric ceria - the benchmark redox material for CO2 splitting with temperature-swings in the range of 800-1500 °C. On the other hand, certain promising perovskites remain redox-active at relatively low oxide reduction temperatures at which ceria is redox-inactive. This trade-off in the redox energetics is established for YFeO3, YCo0.5Fe0.5O3 and LaFe0.5Ni0.5O3, identified as stable against phase changes and capable to convert CO2 to CO at 600 °C and 10 mbar CO in CO2, and to being decomposed at 1400 °C and 0.1 mbar O2 with an enthalpy change of 440-630 kJ mol-1 O2.

High Redox Capacity of Al-Doped La1-x Srx MnO3-δ Perovskites for Splitting CO2 and H2 O at Mn-Enriched Surfaces

Perovskites are attractive candidates for the solar-driven thermochemical redox splitting of CO₂ and H₂ O into CO and H₂ (syngas) and O₂ . This work investigates the surface activity of La_1-x Sr_x Mn_1-y Al_y O_3-δ (0≤x≤1, 0≤y≤1) and La_0.6 Ca_0.4 Mn_0.6 Al_0.4 O_3-δ . At 1623 K and 15 mbar O₂ , the oxygen non-stoichiometry of La_0.2 Sr_0.8 Mn_0.8 Al_0.2 O_3-δ increases with the strontium content and reaches a maximum of δ=0.351. X-ray photoelectron spectroscopy analysis indicates that manganese is the only redox-active metal at the surface. All La_1-x Sr_x Mn_1-y Al_y O_3-δ compositions exhibit surfaces enriched in manganese and depleted in strontium. We discuss how these compositional differences of the surface from the bulk lead to the beneficially higher reduction extents and lower strontium carbonate concentrations at the aluminum-doped surfaces. Using first principles calculations, we validate the experimental reduction trends and elucidate the mechanism of the partial electronic charge redistribution upon perovskite reduction.

Tunable thermodynamic activity of La x Sr1-x Mn y Al1-y O3-δ (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) perovskites for solar thermochemical fuel synthesis

Nonstoichiometric metal oxides with variable valence are attractive redox materials for thermochemical and electrochemical fuel processing. To guide the design of advanced redox materials for solar-driven splitting of CO₂ and/or H₂O to produce CO and/or H₂ (syngas), we investigate the equilibrium thermodynamics of the La _x Sr_1-x Mn _y Al_1-y O_3-δ perovskite family (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) and La_0.6Ca_0.4Mn_0.8Al_0.2O_3-δ , and compare them to those of CeO₂ as the baseline. Oxygen nonstoichiometry measurements from 1573 to 1773 K and from 0.206 to 180 mbar O₂ show a tunable reduction extent, increasing with increasing Sr content. Maximal nonstoichiometry of 0.32 is established with La_0.2Sr_0.8Mn_0.8Al_0.2O_3-δ at 1773 K and 2.37 mbar O₂. As a trend, we find that oxygen capacities are most sensitive to the A-cation composition. Partial molar enthalpy, entropy and Gibbs free energy changes for oxide reduction are extracted from the experimental data using defect models for Mn⁴⁺/Mn³⁺ and Mn³⁺/Mn²⁺ redox couples. We find that perovskites exhibit typically decreasing enthalpy changes with increasing nonstoichiometries. This desirable characteristic is most pronounced by La_0.6Sr_0.4Mn_0.4Al_0.6O_3-δ , rendering it attractive for CO₂ and H₂O splitting. Generally, perovskites show lower enthalpy and entropy changes than ceria, resulting in more favorable reduction but less favorable oxidation equilibria. The energy penalties due to larger temperature swings and excess oxidants are discussed in particular. Using electronic structure theory, we conclude with a practical methodology estimating thermodynamic activity to rationally design perovskites with variable stoichiometry and valence.

3D Cathodes of Cupric Oxide Nanosheets Coated onto Macroporous Antimony-Doped Tin Oxide for Photoelectrochemical Water Splitting

Cupric oxide (CuO), a narrow-bandgap semiconductor, has a band alignment that makes it an ideal photocathode for the renewable production of solar fuels. However, the photoelectrochemical performance of CuO is limited by its poor conductivity and short electron diffusion lengths. Herein, a three-dimensional (3D) architecture consisting of CuO nanosheets supported onto transparent conducting macroporous antimony-doped tin oxide (mpATO@CuONSs) is designed as an excellent photocathode for promoting the hydrogen evolution reaction (HER). Owing to the 3D structure affording superior light-harvesting characteristics, large contact areas with the electrolyte, and highly conductive pathways for separation and transport of charge carriers, the mpATO@CuONSs photocathode produces an impressively high photocurrent density of -4.6 mA cm^-2 at 0 V versus the reversible hydrogen electrode (RHE), which is much higher than that of the CuONSs array onto planar FTO glass (-1.9 mA cm^-2 ).

Chemical looping of metal nitride catalysts: low-pressure ammonia synthesis for energy storage

The activity of many heterogeneous catalysts is limited by strong correlations between activation energies and adsorption energies of reaction intermediates. Although the reaction is thermodynamically favourable at ambient temperature and pressure, the catalytic synthesis of ammonia (NH3), a fertilizer and chemical fuel, from N2 and H2 requires some of the most extreme conditions of the chemical industry. We demonstrate how ammonia can be produced at ambient pressure from air, water, and concentrated sunlight as renewable source of process heat via nitrogen reduction with a looped metal nitride, followed by separate hydrogenation of the lattice nitrogen into ammonia. Separating ammonia synthesis into two reaction steps introduces an additional degree of freedom when designing catalysts with desirable activation and adsorption energies. We discuss the hydrogenation of alkali and alkaline earth metal nitrides and the reduction of transition metal nitrides to outline a promoting role of lattice hydrogen in ammonia evolution. This is rationalized via electronic structure calculations with the activity of nitrogen vacancies controlling the redox-intercalation of hydrogen and the formation and hydrogenation of adsorbed nitrogen species. The predicted trends are confirmed experimentally with evolution of 56.3, 80.7, and 128 μmol NH3 per mol metal per min at 1 bar and above 550 °C via reduction of Mn6N2.58 to Mn4N and hydrogenation of Ca3N2 and Sr2N to Ca2NH and SrH2, respectively.

Carbon Dioxide Reforming of Methane using an Isothermal Redox Membrane Reactor

The continuous production of carbon monoxide (CO) and hydrogen (H₂) by dry reforming of methane (CH₄) is demonstrated isothermally using a ceramic redox membrane in absence of additional catalysts. The reactor technology realizes the continuous splitting of CO₂ to CO on the inner side of a tubular membrane and the partial oxidation of CH₄ with the lattice oxygen to form syngas on the outer side. La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) membranes evaluated at 840-1030 °C yielded up to 1.27 μmol _CO   s^-1 from CO₂, 3.77 μmol_H₂ g^-1 s^-1 from CH₄ , and CO from CH₄ at approximately the same rate as CO from CO₂. We compute the free energy of the oxygen vacancy formation for La_0.5Sr_0.5B_0.5B'_0.5O_3-δ (B, B'=Mn, Fe, Co, Cu) using electronic structure theory to understand how CO₂ reduction limits dry reforming of methane using LSCF and to show how the CO₂ conversion can be increased by using advanced redox materials such as La_0.5Sr_0.5MnO_3-δ and La_0.5Sr_0.5Mn_0.5Co_0.5O_3-δ .

Design Principles of Perovskites for Thermochemical Oxygen Separation

Separation and concentration of O2 from gas mixtures is central to several sustainable energy technologies, such as solar-driven synthesis of liquid hydrocarbon fuels from CO2 , H2 O, and concentrated sunlight. We introduce a rationale for designing metal oxide redox materials for oxygen separation through "thermochemical pumping" of O2 against a pO2 gradient with low-grade process heat. Electronic structure calculations show that the activity of O vacancies in metal oxides pinpoints the ideal oxygen exchange capacity of perovskites. Thermogravimetric analysis and high-temperature X-ray diffraction for SrCoO3-δ , BaCoO3-δ and BaMnO3-δ perovskites and Ag2 O and Cu2 O references confirm the predicted performance of SrCoO3-δ , which surpasses the performance of state-of-the-art Cu2 O at these conditions with an oxygen exchange capacity of 44 mmol O 2 mol SrCoO 3-δ(-1) exchanged at 12.1 μmol O 2 min(-1)  g(-1) at 600-900 K. The redox trends are understood due to lattice expansion and electronic charge transfer.

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

Fixed nitrogen is an essential chemical building block for plant and animal protein, which makes ammonia (NH3) a central component of synthetic fertilizer for the global production of food and biofuels. A global project on artificial photosynthesis may foster the development of production technologies for renewable NH3 fertilizer, hydrogen carrier and combustion fuel. This article presents an alternative path for the production of NH3 from nitrogen, water and solar energy. The process is based on a thermochemical redox cycle driven by concentrated solar process heat at 700-1200°C that yields NH3 via the oxidation of a metal nitride with water. The metal nitride is recycled via solar-driven reduction of the oxidized redox material with nitrogen at atmospheric pressure. We employ electronic structure theory for the rational high-throughput design of novel metal nitride redox materials and to show how transition-metal doping controls the formation and consumption of nitrogen vacancies in metal nitrides. We confirm experimentally that iron doping of manganese nitride increases the concentration of nitrogen vacancies compared with no doping. The experiments are rationalized through the average energy of the dopant d-states, a descriptor for the theory-based design of advanced metal nitride redox materials to produce sustainable solar thermochemical ammonia.