Solar thermochemical water splitting enables hydrogen production by cycling metal oxides between reduced and oxidized states, typically through an oxygen vacancy mechanism. However, recent experimental work suggests that cation vacancies have a greater influence on the redox behavior of iron aluminate spinels used in water splitting. This remains debated, as calculations predict that such cation vacancies are thermodynamically unfavorable. In the current work, we show that Fe vacancies in (Fe zeta Al1-zeta)3O4 become accessible only when facilitated by inversion between Fe and Al. This antisite disorder lowers the formation energy of octahedral Fe vacancies in Al-rich spinels (zeta = 1/3) from over 3 eV to just 0.62 eV when one-third of the cation sites are inverted, allowing high Fe vacancy concentrations under oxidizing conditions. This mechanism supports high H2 yields up to 361 mu mol/g, consistent with experimental observations. Our findings support the notion that solar thermochemical water splitting can occur through a cation vacancy mechanism. They also clarify how site inversion, vacancy energetics, and defect interactions each contribute to redox performance, offering general design principles for identifying and optimizing materials that operate through cation vacancy cycling.

The ability to control metal nanoparticle size and morphology on supported catalysts is crucial for optimizing catalytic performance in targeted applications. This work presents a systematic approach for tuning Ni particle and crystallite size on an unconventional, low-porosity silica fume support through select thermal treatments. The catalyst was synthesized via the deposition of nickelocene onto silica fume, resulting in well-dispersed Ni nanoparticles. A face-centered central composite design was employed to systematically assess the effects of time, temperature, and sintering gas environment on metal particle growth. The results demonstrate that the sintering gas environment is the primary factor governing particle and crystallite evolution, with temperature as the next most significant influence. Nickel nanoparticles sintered at temperatures of 650 degrees C and above under inert conditions exhibited substantial growth and polycrystalline structures, whereas samples treated in oxidative environments formed NiO, restricting particle mobility. Minimally oxidative (500 ppm O2) environments facilitated rapid sintering while effectively removing residual ligands from the one-step nickelocene deposition process. Extensive structural characterization via a combination of scanning transmission electron microscopy, Xray diffraction, hydrogen temperature programmed reduction, and small-angle X-ray scattering revealed that oxidative treatments enhanced metal-support interactions, as evidenced by increased reduction temperatures and narrower particle size distributions. These findings establish quantitative relationships between sintering parameters and Ni nanoparticle characteristics, providing a framework for rational catalyst design through controlled thermal treatments. This methodology is broadly applicable to other catalytic systems and provides a quantitative foundation for catalyst design.

This paper examines feasibility of a solar indirect heating plant for CO2 splitting with enhanced performance. The conceptual plant comprises a solar receiver, a reactor and a heat storage unit connected thermally with a heat transfer fluid. Such a configuration of the system enables a larger amount of metal oxide as reactive material than direct irradiation technology, and is suitably applicable to regenerating exhaust heat incurred by a temperature swing between two levels of reduction and oxidation temperatures. A ceria foam device is tested experimentally in an externally heated tube reactor, similar to the indirect heating plant. The experiment indicated that the externally heated reactor could produce a comparable volume of product gas to the direct irradiation method with various amounts of reactive material mass and the temperatures for reduction and oxidation. A numerical simulation was used to solve the one-dimensional unsteady process for solar heating and reduction. The simulation demonstrated that the solar to fuel efficiency for the proposed system increased with an increasing load of the material and could exceed that of the direct irradiation plant when recovering the exhaust heat.

The widely used trimethylaluminum (TMA)/water atomic layer deposition (ALD) chemistry has been shown to improve the electrochemical cycling stability of LiMO2 materials, including nickel-rich LiNixMnyCo1-x-yO2 (x > 0.5). However, there are many process options users must select when choosing to perform ALD, which makes comparison of ALD-on-LiMO2 papers difficult. This work studies the significance of three ALD process parameters (temperature, number of ALD cycles, and termination step) on the polycrystalline LiNi0.8Mn0.1Co0.1O2 (NMC811) electrochemical performance using a 2(3) full factorial design. We observe that termination on a half-step (TMA) leads to inferior electrochemical performance. This work also evaluates the role of the TMA/water ALD process on doped NMC811 powder. When fabricated into full cells and assessed for discharge capacity and energy, overall cell resistances at low and high states of delithiation, and pulse power, the ALD-coated sample did not perform significantly better or worse compared to the uncoated, doped NMC811. Our findings suggest that rather than serving as a barrier film from the electrolyte, low cycle counts of TMA/H2O ALD fulfill a role similar to that of the dopants added during the NMC synthesis steps.

Tungsten‑rhenium (WRe) solid solution alloys provide a unique combination of mechanical and thermal properties that are utilized in many high-temperature applications. A homogeneous rhenium concentration is crucial for mechanical performance, but this is difficult to achieve in near-net-shape manufacturing without employing specialized powder preparation and consolidation techniques. Here we show that coating tungsten powder with a perrhenic acid solution substantially improves alloy homogeneity by reducing diffusion distances and promoting pre-alloying relative to metal powder mixtures. This strengthens the rhenium softening effect to produce superior WRe alloys with conventional, commercially relevant powder metallurgy techniques.

Tungsten-rhenium (W-Re) solid solution alloys provide a unique combination of mechanical and thermal properties that are utilized in many high-temperature applications. A homogeneous rhenium concentration is crucial for mechanical performance, but this is difficult to achieve in near-net-shape manufacturing without employing specialized powder preparation and consolidation techniques. Here we show that coating tungsten powder with a perrhenic acid solution substantially improves alloy homogeneity by reducing diffusion distances and promoting pre-alloying relative to metal powder mixtures. This strengthens the rhenium softening effect to produce superior W-Re alloys with conventional, commercially relevant powder metallurgy techniques.

Solar thermal water splitting (STWS) produces renewable (or green) hydrogen from water using concentrated sunlight. Because STWS utilizes energy from the entire solar spectrum to drive the reduction–oxidation (redox) reactions that split water, it can achieve high theoretical solar-to-hydrogen efficiencies. In a two-step STWS process, a metal oxide that serves as a redox mediator is first heated with concentrated sunlight to high temperatures ( T  >1000°C) to reduce it and evolve oxygen. In the second step, the reduced material is exposed to steam to reoxidize it to its original oxidation state and produce hydrogen. Various aspects of this process are comprehensively reviewed in this work, including the reduction and oxidation chemistries of active materials considered to date, the solar reactors developed to facilitate the STWS reactions, and the effects of operating conditions—including the recent innovation of elevated oxidant pressure—on efficiency. To conclude the review, a perspective on the future optimization of STWS is provided. This article is categorized under:

Sintering of tungsten nominally requires several hours at ultrahigh temperatures. We show this refractory metal can be sintered quickly by direct injection of current into dog bone shaped specimens. The current rate was varied from 10 A s-1 (fast) to 0.1 A s-1 (slow), leading to sintering in 2-200 s, respectively. Sintering occurred at the same current density, regardless of the current rate. In all instances, the samples sintered when they reached 1000 degrees C. The phenomenological behavior of flash sintering of metals is described by three stages: an incubation time followed by electroluminescence, and finally by abrupt sintering to full density. It is conjectured that rapid sintering is instigated by the formation of Frenkel pairs (vacancies and interstitials), as well as electrons and holes. The point defects accelerate mass transport, whereas electrons and holes recombine to form photons. Calorimetric measurements show an endothermic reaction attributed to the creation of defects. Estimates suggest an unusually large concentration of Frenkel pairs. PS: Flash sintering is different than electro-discharge-sintering where a capacitor is discharged in a few milliseconds to sinter a metal. Here, instead of dumping large amount of energy at once, a power supply is programed to control the rate of current injection.

An international research project has been undertaken to integrate a unique solar thermal processing reactor system with ceria and iron aluminate as active redox materials for CO2 splitting. Experimental investigations for CO2 splitting were conducted using a solar simulator and tube furnace at Niigata University, followed by demonstrations using a high-flux solar furnace (HFSF) at the National Renewable Energy Laboratory (NREL) in Golden, CO. Each experimental setup consisted of foam devices composed of reticulated porous ceramic (RPC). The RPC has a full ceria or iron aluminate body. It fabricated using the replica method and subjected to a two-step redox reaction, which iteratively separated a stream of CO2 into O-2 and CO. Reactivity was evaluated using CO production per mass of the reactive material. The tubular furnace yielded a CO production of 6.41 mL/g at a reduction temperature of 1600 degrees C, showcasing a higher CO production rate and total amount than those obtained from experiments conducted with solar simulators and solar furnace setups. For iron aluminate RPC, the productivity was measured as 3.57 mL/g using HFSF at a reduction temperature of 1450 degrees C. These results are somewhat higher than those of the previous experiment at lower reduction temperatures of 1400.-1500 degrees C. Additionally, the production of CO in the case of ceria RPC was compared with the steady flow model simulation, which assumed chemical equilibrium at various levels of oxygen partial pressure during the reduction process. On the basis of these results, this study proposes a solar fuel system with an open receiver that uses a high-temperature heat transfer fluid.

We show that rhenium can be sintered from powders to nearly full density (99.96 pct) by directly injecting electrical current into dogbone shaped specimens. The current was increased at a rate of 1 A s−1. The specimen sintered abruptly after about 30 seconds when its temperature had risen to 900 °C. The experiments were carried out without furnace heating, within a glove box in Ar atmosphere. The following in-operando measurements are reported, (i) shrinkage strain with a rapid rate camera, (ii) resistivity measured by voltage and current, (iii) temperature measured with a pyrometer, and (iv) electroluminescence spectra measured with a spectrometer. The sintering cycle, the first, during which the sample sintered to full density, was followed by two more flash cycles with the same specimen. In the first cycle, the change in resistance exhibited a peak arising from abatement of interparticle interface resistance; the peak was absent in the second and third cycles. The rapid sintering is attributed to the generation of defects in the form of vacancy-interstitial (Frenkel) pairs. The concentration of the Frenkels was estimated from in-situ calorimetry, where the difference between the electrical input energy, and the energy lost to radiation, convection and specific heat, was attributed to an endothermic reaction for defect generation. In this way we calculated a concentration of ∼ 10 mol pct of Frenkel pairs. The resistance of the flash sintered specimens was higher than literature values, presumably due to these defects. The very low sintering temperature and the anonymously high defect concentrations mean that flash sintering of metals is a far-from-equilibrium phenomenon.

We identified the perovskite oxides LaMn0.5Ni0.5O3 (L2MN), Gd0.5La0.5Mn0.5Ni0.5O3 (GLMN), and GdMn0.5Ni0.5O3 (G2MN) as candidate solar thermal chemical hydrogen (STCH) redox mediators from their density functional theory (DFT)-computed electronic and oxygen vacancy properties following a high-throughput computational screening of AA ' BB ' O-6 compositions that are likely to form as perovskites and split water. At a thermal reduction temperature of 1350 degrees C and a water splitting temperature of 850 degrees C, the L2MN and GLMN perovskites produced similar to 65 mu mol g(-1) of hydrogen per cycle with no phase degradation over three redox cycles at 40 mol % steam, while the G2MN perovskite did not produce STCH under these conditions. When reoxidized by exposure to a gas flow with a H2O:H-2 molar ratio of 1333:1, which represents operating conditions where the thermodynamic driving force of water splitting is lowered by orders of magnitude relative to 40 mol % steam, the L2MN and GLMN perovskites each produced similar to 35 mu mol g(-1) of hydrogen per cycle. Guided by DFT, we propose that L2MN and GLMN's STCH activities arise from B-site cation antisite defects that facilitate oxygen vacancy formation and thus redox cycling, whereas the synthesized G2MN has few antisite defects and is therefore inactive for STCH.

Efficient solar thermochemical fuel production has been hindered by either solid-solid heat recuperation and material stability challenges associated with conventional temperature swing operation or low reactant conversion associated with isothermal operation. Increasing the oxidation pressure has emerged as a method for achieving efficient and practical solar-to-fuel conversion with certain redox mediators. When coupled with isothermal operation, pressure-swing redox cycling with iron aluminates eliminates irreversible heating penalties associated with large temperature swings while simultaneously enabling greater reactant conversion. However, remaining questions persist regarding 1) kinetics, 2) co-splitting capabilities, and 3) continuous processing prior to implementation beyond the lab scale. Here, we provide mechanistic insight on how pressure impacts the oxidation kinetics of iron aluminates, demonstrate syngas production with H2:CO ratios ranging from 1.3 +/- 0.05 to 3.2 +/- 0.25, and produce fuel continuously over an eight-hour period using dual fluidized bed reactors to evaluate the key considerations for large scale implementation. This work demonstrates the feasibility of a continuous solar thermochemical fuel production process via pressure-swing, isothermal redox cycling.

We report first-time results for in-operando flash sintering synchrotron experiments carried out in current rate mode where the specimen, held at a constant temperature, is fed current that is increased at a constant rate. These experiments are unique because the time dependence of the sintering behavior can be stretched out over a longer period (by changing the current rate) than in voltage-to-current experiments in which sintering occurs in a burst at the onset of the flash. Two results are presented: (i) A comparison of temperatures measured with the platinum standard to those predicted by the black body radiation model leading to estimates of the emissivity as a function of porosity whereby emissivity increases from 0.65 to 0.9 as the sample sinters from its green state to full density, and (ii) measurements of the excess lattice expansion as a function of density as the sample sinters continuously while the current is increased. The present work highlights the promise of current rate experiments to obtain results while the sample sinters gradually from its green density to full density (somewhat akin to conventional sintering) for gaining further insights into the mechanisms of flash sintering.

A moving porous media (MPM) modeling methodology was developed for reactor-scale CFD simulations of continuous spatial particle atomic layer deposition (ALD). The continuous vibrating reactor process for particle atomic layer deposition (CVR-ALD) was modeled by treating the powder bed as a porous media which conveys as a sliding and layering dynamic mesh zone inside the vibrating reactor zone. Candidate porous reactor baseplates were experimentally characterized using x-ray computed tomography (XRCT), porometry, porosimetry, and atomic force microscopy (AFM) before permeabilities from flow tests were used as inputs to the MPM model. Parameter sweeps over vibration magnitude, powder bed convection speed, and precursor mass fraction revealed the dependence of surface titration uniformity and residual gas breakthrough on operating conditions and powder properties. Parasitic chemical vapor deposition (CVD) reactions were observed in the first precursor zone when dose start times and inlet gas compositions were not optimized.

The commercial path is described for two new powder processing technologies that resulted in two new businesses. Both developments started out as laboratory curiosities and had to overcome significant skepticism and technical and financial challenges along the way. One was an industrial endeavor with a major chemical company to start a new business for the synthesis of advanced non-oxide powder materials using a newly discovered processing route. The other was an academic endeavor resulting in a spinoff company from the university lab, bootstrapping, merging, and finally making technology believers out of skeptics for the novel coating of particles to functionalize surfaces. Both represented significant advances in the two respective project areas. Key to both successes was an understanding of scientific and engineering fundamentals, intellectual property to protect the risk takers, and viable market opportunities for products providing for cost/performance advantages over the competition and lower cost products for the end-user. An inside perspective is provided for these two success stories with the hope that others with similar opportunities can plod forward in the face of similar inevitable headwinds. It's easy to quit and the road from a laboratory curiosity to a commercial process/product is not an easy one.

The thermochemical dissociation of water and/or carbon dioxide over a reduced metal oxide has long been thought to be independent of total pressure, as the number of moles of gaseous reactants (i.e., H2O and/or CO2) and gaseous products (i.e., H2 and/or CO) is equal. In this study, however, through careful experimentation, we conclusively demonstrate that in an open system—where product gases are swept away from the reaction site—operating at elevated pressures improves both the equilibrium extent and rate of the aforementioned equimolar oxidation reaction. These findings have important implications for the viability of commercial systems, as the discovery of a temperature-independent technique for increasing reactant conversion not only enables the use of more earth-abundant materials but may also finally facilitate the development of a process for the production of green hydrogen (or syngas) that is both practical and efficient.

Alumina surface coatings are commonly applied to layered oxide cathode particles for lithium-ion battery applications. Atomic layer deposition (ALD) is one such surface coating technique, and ultrathin alumina ALD films (<2 nm) are shown to improve the electrochemical performance of LiNixMnyCo1-x-yO2 materials, with groups hypothesizing that a beneficial Li-Al-O product is being formed during the alumina ALD process. However, the atomic structure of these films is still not well understood, and quantifying the interface of ultrathin (∼1 nm) ALD films is an arduous experimental task. Here, we perform molecular dynamics simulations of amorphous alumina films of varying thickness in contact with the (0001) LiCoO2 (LCO) surface to quantify the film nanostructure. We calculate elemental mass density profiles through the films and observe that the Li-Al-O interphase extends ∼2 nm from the LCO surface. Additionally, we observe layering of Al and O atoms at the LCO-film interface that extends for ∼1.5 nm. To access the short-range order of the amorphous film, we calculated the Al coordination numbers through the film. We find that while [4]Al is the prevailing coordination environment, significant amounts of [6]Al exist at the interface between the LiCoO2 surface and the film. Taken together, these principal findings point to a pseudomorphic Li-Al-O overlayer that approximates the underlying layered LiCoO2 lattice but does not exactly replicate it. Additionally, with sufficient thickness, the Li-Al-O film transitions to an amorphous alumina structure. We anticipate that our findings on the ALD-like, Li-Al-O film nanostructure can be applied to other layered LiNixMnyCo1-x-yO2 materials because of their shared crystal structure with LiCoO2. This work provides insight into the nanostructure of amorphous ALD alumina films to help inform their use as protective coatings for Li-ion battery cathode active materials.

The production of syngas by simultaneous splitting of direct-air-captured CO2 and H2O via a solar thermochemical redox cycle is a competitive alternative to electrolysis-based pathways. Isothermal or near-isothermal operation using high-entropy oxides that are readily available, robust, and flowable is recommended on the basis of practical considerations and improved performance, both mechanical and thermodynamic. Ongoing research efforts should direct attention toward devising compatible thermal energy storage technologies and/or incorporating hybrid solar-electric heating to (1) mitigate the effects of solar intermittency and (2) provide a continuous feed for downstream gas-to-liquid processing. The path forward involves funding the development and characterization of fully integrated laboratory systems.