Symposium EN05—Materials Challenges and Opportunities in Concentrated Solar Power Technologies

The efficiency of concentrated solar power (CSP) plants for generating electricity may be appreciably increased by operating at higher temperatures. By increasing turbine inlet temperatures to >750^oC and by using high-pressure, supercritical carbon dioxide (sCO₂) power cycles, instead of conventional cycles at <550^oC, the relative heat-to-electricity conversion efficiency may be increased by >20% (a key step towards direct competition with fossil fuel power plants and a large decrease in greenhouse gas emissions). However, inlet sCO₂ temperatures for CSP turbines are severely limited by current printed-circuit-type heat exchangers (PCHEXs) that are used to transfer heat to the sCO₂; that is, conventional metal alloys used in such PCHEXs exhibit a significant decrease in mechanical performance at >600^oC.

Here we present a robust composite of zirconium carbide (ZrC) and tungsten (W) for use in PCHEXs at >750^oC. Co-continuous ZrC/W composites possess an attractive combination of properties for this application. ZrC and W are chemically compatible; that is, these solids exhibit limited mutual solid solubility, do not react to form other compounds, and exhibit a solidus temperature of 2800^oC. Measurements of the thermal diffusivity and heat capacity of co-continuous ZrC/W composites have yielded an average thermal conductivity value of 66 W/m-K at 800^oC, which is appreciably higher than values reported for advanced Ni-based alloys at this temperature (e.g., 22.1 W/m-K for IN740H). Polycrystalline ZrC and W exhibit similar modest values of thermal expansion from 25^oC to 750^oC, and have been found to be resistant to thermal cycling and extreme thermal shock (e.g., >1000^oC/sec). The interconnected carbide phase can provide ZrC/W composites with high-temperature stiffness and creep resistance, whereas W exhibits high-temperature ductility for enhanced resistance to fracture relative to monolithic ZrC. Indeed, co-continuous ZrC/W composites have been found to possess average failure strengths at 800^oC of 600 MPa (well above the maximum allowed stresses of Ni-based superalloys at this temperature at elevated pressures). Although not inherently resistant to high-temperature oxidation, ZrC/W composites have been endowed with such resistance via modest CO additions to sCO₂ (to generate a supercritical reducing fluid) and by placing a layer of Cu (a relatively noble metal) on the ZrC/W surface. Such Cu-bearing ZrC/W composites have exhibited negligible oxidation after 1000 h at 750^oC in mixtures of sCO₂ with 50 ppm CO at 20 MPa.

The fabrication of PCHEXs comprised of dense co-continuous ZrC/W-based HEX plates with tailorable channel patterns will also be discussed. Porous WC preform plates with desired channel patterns may be generated by low-cost forming (e.g., casting, pressing) of WC/organic mixtures, followed by organic pyrolysis and light (initial stage) sintering. The resulting rigid, porous WC preform plates can then be converted into dense ZrC/W plates via reactive pressureless infiltration with a Zr-bearing liquid. The Zr in the liquid undergoes a displacement reaction with the WC in the preform to yield ZrC and W as solid reaction products. These products possess a combined volume that is larger than the volume of WC consumed, so that the prior pores in the preform become filled with these new solids with little change in external preform shape, surface features, and size (typically <1% dimension changes). Because the channels present in the easily-formed porous WC preforms are retained in the dense ZrC/W plates, prolonged chemical etching (required for current metal-based PCHEXs) is not needed to produce such channels, and unique channel morphologies (higher aspect ratios, non-circular cross-sections) can be generated for tailored fluid flow and heat transfer. Techno-economic analyses indicate that ZrC/W-based PCHEXs can strongly outperform Ni-superalloy PCHEXs at lower cost.

Robust and accurate thermophysical properties, namely thermal diffusivity, thermal conductivity, and specific heat of high-temperature ( >1000 K) containment materials (CMs) and heat transfer materials (HTMs) are lacking, (i) because few measurement techniques exists, and (ii) unbeknown measurement inaccuracies exist in many commercially available techniques that are not well document in literature. This has resulted in surprisingly large spread in reported thermophysical properties at high temperatures of both CMs and HTMs, begging the question what are the true thermophysical properties of many CMs and HTMs. This presents a unique challenge for designing high-temperature thermal systems when there is such large uncertainty in physical properties of materials. It has been challenging to calculate (or model) the thermophysical properties due to the composite (or multi-component alloy) nature of these materials often resulting in microstructural phase transitions and spatially heterogenous composites. At elevated temperatures chemical stability (both oxidation and corrosion) and radiation effects make accurate measurements exceedingly difficult. To address this challenge, numerous repeated measurements on multiple samples spanning both spatial (e.g., heterogeneity) and temporal (e.g., aging) variations and subsequent statistical analysis (e.g., 95% confidence-interval) are required to confirm or refute the often single-measurements reported on material specification sheets or literature. This invited talk will highlight some “problematic materials” whose properties have been mis-reported previously, commenting on “likely” sources of error. This talk will also highlight new measurement techniques that are trying to address some of these identified short comings. And finally, this talk will present “lessons learned” from a more robust statistical approach to reporting thermophysical properties.

The use of solar energy for power generation provides an efficient sustainable energy solution. Among a number of technologies developed for power generation using solar energy, concentrating solar power (CSP) is encouraging because of the capability of thermal energy storage that makes it possible for the generation beyond sunset and in times of cloud cover. Although the use of molten salts as heat transfer fluid and thermal storage in CSP has various advantages, they have a major disadvantage as they make the component systems highly susceptible to corrosion. Different approaches to prolong life by suppressing corrosion have been adapted including the use of high alloy steels and the use of high purity molten salts, coating is a promising option because coatings are believed to provide shield to suppress corrosion.
In this study, we investigate the use of protective coatings to enhance the corrosion resistance of the component systems against molten salts at an elevated temperature. Nickel aluminide coatings are deposited using plasma spray system, as they hold high-temperature mechanical strength and it has a remarkable oxidation resistance performance as substrate component. Moreover, nickel aluminide has low solubility in the molten nitrate salt. Ni3Al coatings are much preferred to be used as corrosion resistant coatings as they possess strength at high temperature, oxidation protection and creep properties.
Corrosion behaviour of stainless steel 347 (SS347) and Ni3Al coated SS347 was investigated in molten nitrate salt (60wt% NaNO₃ + 40wt% KNO₃) immersion at 565^oC for 500 hours intervals up to 3000 hours. A growth of stratified oxide layers was observed on SS347 sample surface comprising of NaFeO₂, Fe₂O₃ and Fe₃O₄. The Ni₃Al coated SS347 samples were observed to undergo rapid oxidation within first 500 hours. The results presented in this study suggest that Ni3Al coating supresses the formation of oxide layers on the surface of stainless steel substrates and can be used to suppress corrosion in presence of molten nitrate salts. The fact, that Ni3Al coated SS347 gives mass change of one order of magnitude lower than the bare SS347, it means that these coatings can be used to prolong the lifetime of bare SS347 in molten nitrate salt at 565oC, which is of relevance to thermal energy storage applications. In my presentation, I will show the corrosion test results and discuss the potential of nickel aluminide coatings to suppress hot corrosion by molten salts in an environment that simulates that of concentrating solar power plants.

High temperature particle-based receivers offer distinct advantages over conventional molten salt receivers due to their ability to achieve temperatures above 700 C, direct absorption of solar energy as they fall through a beam of concentrated sunlight, and the relative ease of storage (and retrieval using a secondary working fluid) in insulated storage tanks. The use of particles, however, also raises concerns with material degradation from the flow of hot or cold particles through discharge hoppers or along the inner receiver surfaces and other system components (e.g. tubes, valves etc.), depending on operating mode of the receiver. The flow of particles over surfaces may result in loss of material from abrasive wear, impact erosion from impingement under gravitational fall and particle attrition as particles fall and move on top of each other.
In the present study, the performance of candidate materials and particles were evaluated through a series of abrasion erosion and particle attrition experiments at room temperature as well as at 800 C. Candidate materials were subject to abrasive wear from particles at low particle to material velocities inside a resistance heated kiln, and analyzed for changes in mass and surface morphology using cross-sectional scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) tests. The wear rate for different specimens was noted to be largely driven by the strength of chromia scales built on the specimens from exposure to high temperature.
Particle attrition measurements explored the susceptibility of particles to breakdown from particle to particle interaction and the generation of fines from this process. At the low velocities expected in particle based Concentrated Solar Power (CSP) plants, the particles tested exhibited near negligible breakdown. However, changes in the particle hardness at 800 C resulted in a significantly higher particle breakdown to sizes <40 microns raising potential environmental concerns. In addition to particle breakdown, it was also noted that the sample had oxides from the stainless steel test setup mixed in with the particles. Similar oxides can be expected to turn up in the utility scale particle based CSP plants as well. The presence of oxide was also noted to affect the solar absorptivity of the mixture compared to a clean initial specimen, potentially resulting in a change in the overall efficiency of the CSP plant.

The thermal conductivity of liquids has traditionally been determined by measuring either the steady-state temperature gradient or time-dependent temperature rise in a sample due to an applied heat flux. The transient hot-wire method has become the standard for measuring liquid thermal conductivity; in this method, direct current Joule heats a long, thin wire immersed in a liquid sample, and the measured temperature rise over a particular time interval is fit to a heat transfer model to extract the thermal conductivity of the liquid. However, in the high-temperature environments of molten salts, long metal wires are vulnerable to corrosion, and convection errors are magnified in larger volumes of molten salts. Here, we utilize a new frequency-domain, corrosion-resistant, short hot-wire technique (sensor and model) to measure the thermal conductivity of molten NaNO₃ (2.78% standard uncertainty), KNO₃ (2.38% standard uncertainty), and Solar Salt mixture (2.66% standard uncertainty). Operating in the frequency domain and utilizing a full 3D model allows the use of short sensor wires, reducing the total contact area to minimize corrosion, and the use of small sample volumes, mitigating convection errors. An alternating current at varying frequencies Joule heats a short platinum wire sensor (6.5 mm), which is protected by a 1.6 mm thick Al₂O₃ coating, submerged in molten salt. The frequency-dependent temperature response of the sensor surrounded by salt is measured and fit to a 3D thermal model to obtain the thermal conductivity of the molten salt. Together, the new sensor design and 3D model keep the probed molten salt volume below 1 μL. Our frequency-domain measurements show an ~11-15% higher thermal conductivity of molten nitrate salts than the current reference correlations made from steady-state and time-domain measurements.

The efficiency of concentrated solar power (CSP) plants for generating electricity may be appreciably increased by operating at higher temperatures. By increasing turbine inlet temperatures to >750^oC and by using high-pressure, supercritical carbon dioxide (sCO₂) power cycles, instead of conventional cycles at <550^oC, the relative heat-to-electricity conversion efficiency may be increased by >20% (a key step towards direct competition with fossil fuel power plants and a large decrease in greenhouse gas emissions). However, inlet sCO₂ temperatures for CSP turbines are severely limited by current printed-circuit-type heat exchangers (PCHEXs) that are used to transfer heat to the sCO₂; that is, conventional metal alloys used in such PCHEXs exhibit a significant decrease in mechanical performance at >600^oC.

Here we present a robust composite of zirconium carbide (ZrC) and tungsten (W) for use in PCHEXs at >750^oC. Co-continuous ZrC/W composites possess an attractive combination of properties for this application. ZrC and W are chemically compatible; that is, these solids exhibit limited mutual solid solubility, do not react to form other compounds, and exhibit a solidus temperature of 2800^oC. Measurements of the thermal diffusivity and heat capacity of co-continuous ZrC/W composites have yielded an average thermal conductivity value of 66 W/m-K at 800^oC, which is appreciably higher than values reported for advanced Ni-based alloys at this temperature (e.g., 22.1 W/m-K for IN740H). Polycrystalline ZrC and W exhibit similar modest values of thermal expansion from 25^oC to 750^oC, and have been found to be resistant to thermal cycling and extreme thermal shock (e.g., >1000^oC/sec). The interconnected carbide phase can provide ZrC/W composites with high-temperature stiffness and creep resistance, whereas W exhibits high-temperature ductility for enhanced resistance to fracture relative to monolithic ZrC. Indeed, co-continuous ZrC/W composites have been found to possess average failure strengths at 800^oC of 600 MPa (well above the maximum allowed stresses of Ni-based superalloys at this temperature at elevated pressures). Although not inherently resistant to high-temperature oxidation, ZrC/W composites have been endowed with such resistance via modest CO additions to sCO₂ (to generate a supercritical reducing fluid) and by placing a layer of Cu (a relatively noble metal) on the ZrC/W surface. Such Cu-bearing ZrC/W composites have exhibited negligible oxidation after 1000 h at 750^oC in mixtures of sCO₂ with 50 ppm CO at 20 MPa.

The fabrication of PCHEXs comprised of dense co-continuous ZrC/W-based HEX plates with tailorable channel patterns will also be discussed. Porous WC preform plates with desired channel patterns may be generated by low-cost forming (e.g., casting, pressing) of WC/organic mixtures, followed by organic pyrolysis and light (initial stage) sintering. The resulting rigid, porous WC preform plates can then be converted into dense ZrC/W plates via reactive pressureless infiltration with a Zr-bearing liquid. The Zr in the liquid undergoes a displacement reaction with the WC in the preform to yield ZrC and W as solid reaction products. These products possess a combined volume that is larger than the volume of WC consumed, so that the prior pores in the preform become filled with these new solids with little change in external preform shape, surface features, and size (typically <1% dimension changes). Because the channels present in the easily-formed porous WC preforms are retained in the dense ZrC/W plates, prolonged chemical etching (required for current metal-based PCHEXs) is not needed to produce such channels, and unique channel morphologies (higher aspect ratios, non-circular cross-sections) can be generated for tailored fluid flow and heat transfer. Techno-economic analyses indicate that ZrC/W-based PCHEXs can strongly outperform Ni-superalloy PCHEXs at lower cost.

Thermal energy storage within high-temperature molten salts is critical to overcome solar illumination intermittency and ensure stable operation of concentrated solar-thermal systems. Current molten salt phase change materials, however, generally suffer from a low thermal conductivity, which seriously limits both the charging and discharging processes. It is challenging to effectively increase the thermal conductivity of high-temperature molten salts without affecting their heat storage capacity and other inherent thermophysical properties. In this work, we reported that magnetically moving mesh-structured solar absorbers within solar salts along the solar illumination path could significantly accelerate the solar-thermal energy storage rates. The mesh absorber continuously advances the melting solid/liquid interface thus achieves fast latent heat storage of solar-thermal energy within solar salts. Compared to conventional static charging processes, such magnetically-accelerated movable charging strategy increases the latent heat solar-thermal energy harvesting rate by 107% and maintains 100% heat storage capacity of solar salts. We also demonstrated that the movable charging supports large-area charging of solar salts with small-sized solar-thermal converters and continuous batch-to-batch solar-thermal storage. The movable charging system can be readily integrated with heat exchanging systems to serve as energy sources for water and space heating by using abundant clean solar-thermal energy. Alternatively, the high-temperature heat stored within melted solar salts can be direct converted to electricity through thermoelectric conversion, and the converted electricity can simultaneously light up multiple LEDs.

Many chemical processes depend on having an environment that is low in oxygen partial pressure (PO2 < 100 Pa); sorption pumps are a promising route to establishing that environment by either oxygen pumping or oxygen separation from an inert gas. Near ambient sorption-based processes rely on either pressure or thermal swings, requiring no moving parts, no electricity, and neither very high nor very low temperatures. In this work, we use ab initio calculations to explore zeolites as a class of materials for sorption based oxygen pumping/separation. Our calculations indicate that while the neat ALPO-5 zeolite does not adsorb O2, O2 does adsorb on zeolites selectively doped with transition metals and metalloids and hence, can enable separation and pumping. ALPO-5 doped with Si, Ge, Sn, Pd, Pt, Ti, V, Cr, Mn, Zr, Mo, Hf, W, Ce, and Pr provides ad-sorption energies ranging from -0.19 to -3.92 eV, (-) indicates exothermic process. Additionally, we provide a comprehensive understanding of what controls the adsorption energy: 1) the dopant must be able to adopt an oxidation state that is more positive than the cation it replaces, and 2) the size of the pore into which the O2 adsorbs to the wall. Lastly, we conduct a thermodynamic analysis of a thermal swing cycle to approximate the optimal O2 binding energy for low energy O2 pumping/separation. We find that the minimum energy cost likely occurs when the adsorption energy is in the range of 0.75 – 1.00 eV (72 – 97 kJ*mol-1), which corresponds to Ge, V, Pt, or Ce doped ALPO-5.

Materials for solar thermochemical production of hydrogen (STCH) and other fuels often relies on the use of highly thermally stable multinary oxides. Often these oxides possess three or more cations and represent an alloy or line compound within a complex multinary phase space. These complex phase spaces often possess one or more other refractory phases which, if formed during synthesis, remain as secondary phases through repeated sintering. These secondary phases are typically inactive thermochemically, create a convoluted signal in measurements which can complicate isolating the performance and behaviors of candidate materials. This work discusses a number of best practices and potential routes for synthesizing single phase multinary oxides for thermochemical applications. Common synthetic routes discussed include mixed carbonate and oxide sintering, unary oxide blends, and modified Pechini sol-gel processing. The formation of different secondary phases based on the synthesis route is demonstrated for example systems such as BaCe_0.25Mn_0.75O₃. Results of sintering model systems at different peak temperatures demonstrate the need for furnace temperature in excess of 1200°C in many systems. Additionally, the effect of isothermal reduction and re-oxidation cycling on removing secondary phases, especially containing softer cations such as Mn, is demonstrated. These reduction and oxidation cycling results partially explain the commonly observed drift in performance during the initial few cycles of STCH materials. This work provides an important roadmap to synthesizing single phase thermochemical materials.

High temperature endothermic reduction of metal oxides, which can liberate oxygen gas at achievable conditions, converts thermal energy to stored chemical energy. A subsequent re-oxidation step either recovers the stored energy as heat or utilizes it to drive other chemical reactions. If the re-oxidation step restores the material to its original state so that the two-steps can repeat indefinitely, the sequence of reactions constitutes a thermochemical cycle. Two related processes (reversibly re-oxidizing with oxygen or affecting bond breaking by re-oxidizing with CO₂ and/or water) are somewhat analogous; the fuel production option is significantly more constrained by thermodynamics and hence subject to greater challenges to efficiency and cost. Nonetheless, both chemistries have been of increasing interest due to their potential for high efficiency utilization of the sun and economic competitiveness for clean energy and most recently as a role in achieving negative emissions and closing the carbon cycle.

This presentation will discuss thermodynamic requirements and opportunities presented by the redox-active metal oxide thermochemistry and show recent progress towards developing the materials and cycles for thermochemical energy storage and CO₂/H₂O splitting.

The layered perovskite BaCe_0.25Mn_0.75O₃ (BCM) is a proven solar thermochemical (STCH) water splitting material demonstrating higher hydrogen generation and improved redox kinetics relative to current state of the art materials. Recent results have revealed that BCM belongs to a larger family of STCH materials, BXM (X = Ce, Nb, Pr). Despite the three materials having identical structures, and differing only in 1/8 of their cation species, their water splitting performances are markedly different. Improved understanding of the mechanisms and material properties that determine performance of STCH materials is necessary for designing materials that better synergize with the operating temperatures of concentrated solar power (CSP). In this work, we present a comparative study of the electronic response of BXM materials to varying degrees of reduction, and use modeling to reveal the mechanisms driving these changes. Probing the oxygen with X-ray absorption spectroscopy (XAS) reveals substantial differences in the electronic structure between the BXM materials in both oxidized and reduced states. These results demonstrate that the oxygen K-edge is sensitive to even small concentrations of oxygen vacancies in large part due to the finding that the effect of vacancies on the electronic structure of the material extends far beyond second nearest neighbors. Comparisons of the oxygen K-edge measured by XAS and the complementary technique electron energy loss spectroscopy (EELS) confirm the veracity of the observed changes in electronic structure. Local density of states (LDOS) calculations carried out on oxidized and defected structures enable specific peaks in the XAS spectra to be correlated to hybridized molecular orbitals, providing valuable insights into the underlying electronic changes resulting in shifts in energy and intensity. These LDOS calculations correctly predict changes in XAS due to reduction. These results are a key component of elucidating structure-property relationships for this class of STCH materials.

Ammonia (NH₃) is an energy-dense chemical and a vital component of fertilizer. In addition, it is a carbon-neutral liquid fuel and a potential candidate for thermochemical energy storage for high-temperature concentrating solar power (CSP). NH₃ is currently synthesized via the Haber-Bosch process, which requires pressures of 15- 25 MPa and temperatures of 400-500 °C. Nitrogen (N₂) and hydrogen (H₂) are essential feedstocks for producing NH₃: H₂ is generally derived by steam reforming of methane, and N₂ is sourced from air, after O₂ removal, via combustion of methane. Burning hydrocarbons produces the heat and mechanical energy required to drive the NH₃ reaction. All three processes (sourcing the energy, producing H₂ and N₂) produce CO₂ emissions. The development of a renewable pathway to NH₃ synthesis that utilizes concentrated solar irradiation for the process heat instead of hydrocarbon combustion and operates under relatively low pressure, will result in both a decrease (or elimination) in greenhouse gas emissions and avoid the cost, complexity, and safety issues inherent in high-pressure processes.

The aim of the Solar-Thermal Ammonia Production (STAP) project is to develop a solar thermochemical looping technology to produce and shuttle N₂ from air for the subsequent production of NH₃ in an advanced two-stage process. The endothermic thermal reduction of redox-active metal oxide particles is driven by concentrated solar irradiation; subsequent exposure to air re-oxidizes the particles, removing O₂ and producing relatively pure N₂ gas. The N₂ serves as a feedstock for a renewable NH₃ production process in an advanced two-step, low pressure, looping process. In the first step, H₂ reacts with metal nitride particles to produce NH₃ , resulting in a reduced (nitrogen deficient) metal nitride. In the second step, the nitride is regenerated utilizing the nitrogen produced in the air separation step. The net result is NH₃ produced from sunlight, air, and (green) H₂, while the metal oxide and nitride particles are recycled.

The STAP project consists of four thrusts: (1) Systematic synthesis, characterization, and thermodynamic analysis of oxides for N₂ recovery from air; (2) materials investigation and development of nitrides for NH₃ production and re-nitridation; (3) process and reactor design, modeling, fabrication, and small-scale demonstration; and (4) full system and techno-economic analyses. The system concept will be summarized in this presentation, and the synthesis and characterization of the working materials will be discussed, including x-ray diffraction, thermogravimetric analysis, and reaction performance.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Phosphatic Pebble or Dolomite, calcium magnesium carbonate (CaMg(CO₃)₂), is considered an undesirable mined resource for phosphoric acid production in Florida; as such, large quantities of the mineral are available. This study is aimed to characterize the phosphatic high concentration dolomite pebbles received from the Florida Industrial and Phosphate Research Institute (FIPR) and investigated their feasibility for thermochemical energy storage (TCES). The chemical composition of these dolomite minerals was studied using different techniques such as X-ray Fluorescence (XRF), Fourier-transform infrared spectroscopy (FTIR), and Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM, EDS), which confirmed the phosphatic pebbles received from FIPR contained high percentages of dolomite. Thermogravimetric analysis (TGA) was used to calculate the activation energy for calcination and carbonation of the dolomite pebbles in Nitrogen (N₂) ambient and CO₂ ambient conditions respectively, and with temperatures up to 800^°C. Values important for thermochemical energy storage such as heat capacity and enthalpy of reaction were also calculated using this TGA data. Changes in crystal structure after calcination, carbonation, and wet ball milling of the dolomite minerals were observed using X-ray diffraction (XRD). The effects of wet ball milling to reduce particle size were observed on the cyclic stability and carbonation of the dolomite pebbles. Particle size and surface area were measured using XRD and N₂ adsorption with the BET method. The structural characteristics of dolomite as explored via metrological techniques (QEMSCAN) thus demonstrating the presence of magnesium and calcium in combinations of carbonates, Calcium Oxide, Magnesium Oxides, and residual Carbon dioxide.

Thermochemical cycles of the sulphur family recycle sulphur as the central element that appears in different compounds at changing oxidation state. Such cycles like the Hybrid Sulphur (HyS) and the Sulphur-Iodine (SI) cycle were originally conceived to produce hydrogen via water-splitting, but can be modified to produce solid sulphur that can be combusted in air to produce high-temperature heat and hence can be used both as a renewable fuel as well as a seasonal solar energy storage means. The HyS, SI and solid sulphur-based thermochemical cycles share the common step of the decomposition reaction of sulphuric acid: first thermally to steam and SO₃ and subsequently the catalytic SO₃ dissociation (splitting) to SO₂ and oxygen. The latter is their highest-temperature (700-900^oC), endothermic reaction step that can be aided by heat supplied by concentrated solar irradiation, either directly on porous ceramic volumetric receivers/sulphuric acid decomposition reactors or indirectly via solar-receiver-heated gaseous or solid particle streams operating as heat transfer fluids and transferring their enthalpy to the sulphuric acid decomposer downstream of the solar receiver. A common issue in all cycles has to do with the particular characteristics of sulphuric acid chemistry: highly corrosive environment combined with high temperatures. These peculiarities impose further necessities and research needs with respect to the structural materials and components to be employed, in addition to research on catalyst compositions and reactor designs. The work will present recent progress in this field in the context of designing, building and operating a novel, sulphuric acid decomposer/sulphur trioxide splitting reactor indirectly heated via hot moving particle streams. Aspects to be covered include materials solutions for the manifolds for the injection of the sulphuric acid solution, evaporator tubes, sealants, special ceramic adhesives to join anti-corrosive metallic-to-ceramic tubing e.g. (stainless steel to SiC) and materials and reactor solutions to compensate for thermal expansion/contraction. These results will be complemented with analogous results from long-term testing (up to 1,000 operation hours) of sulphur trioxide catalyst compositions shaped in the form of either catalytic particles or structured porous ceramic assemblies - and relevant post-operation characterization studies. The final technical solutions are articulated into an integrated sulphuric acid decomposer/sulphur trioxide splitting reactor from qualified materials and concepts, capable of concentrated solar irradiation-driven, long-term operation under the required demanding conditions.

Concentrated solar energy provides a clean source of process heat in excess of 600 °C and thus enables the decarbonization of several key energy-intensive industrial processes, such as mineral and metallurgical extraction, lime and cement manufacturing, biomass pyrolysis and gasification, as well as thermal production of power and fuels. These processes can be performed round-the-clock with solar process heat by making use of the endothermic-exothermic effect associated with the reduction-oxidation (redox) of metal oxides to store and release the high-temperature heat. This category of thermochemical energy storage (TCS) materials typically features high values of energy storage density and the advantage of employing ambient air as gaseous reagent. Of particular interest are the redox pairs BaO₂/BaO, Co₃O₄/CoO, CuO/Cu₂O, Fe₂O₃/Fe₃O₄ and Mn₂O₃/Mn₃O₄, which have their equilibrium temperature above 600 °C in atmospheric air. A common problem for these metal oxides when approaching their melting point is thermal sintering, which progressively reduces the gas-solid contact area and, hence, their redox rate and redox cyclic performance. In this work, we described the manufacturing of mm-size granules with tailored morphological properties that exhibit stable reaction kinetics and cyclic performance over multiple redox cycles. The material synthesis process involved the mixing of the reactive metal oxide with pore formers and sintering inhibitors - e.g., Al₃O₄, CaO, CeO₂, Fe₂O₃, MgO, Mn₃O₄, SrO, Yr₂O₃-stabilized ZrO₂ (YSZ) - which were added in different concentrations and combinations to form binary and ternary mixtures. The synthetized granules were tested by thermogravimetry over up to 100 consecutive redox cycles at conditions relevant for the intended TCS applications. Thermodynamic and kinetics analyses were carried out for each binary and ternary system. In terms of cyclic stability, the best performing materials were Fe₂O₃ in pure form, the binary mixture CuO-YSZ (65-35wt%), and the ternary mixtures Co₃O₄-YSZ-MgO (80-15-5wt%), CuO-Al₂O₃-YSZ (70-22.5-7.5wt%), and CuO-Al₂O₃-MgO (60-30-10wt%), yielding gravimetric heat storage capacities of 421, 559, 609, 565, 439 kJ for kg of oxidized mixture, respectively. In these materials, the additives served as inert supports. The binary mixtures Mn₂O₃-Al₂O₃, Mn₂O₃-Fe₂O₃, Mn₂O₃-MgO, where the additives participated in the redox reactions, exhibited stable cyclic performance as well.

An alternative concept to achieve solar selectivity for solar thermal materials and applications consists in the use of spectrally selective transmitter coatings.[1] These are characterized by a high transmittance in the solar range and a high reflectance in the thermal range of the electromagnetic spectrum. Suitable materials for selective transmitters are dielectric/metal/dielectric multilayers and transparent conductive oxides (TCOs).[2] The concept has a series of advantages compared to multilayer- or cermet-based solar-selective coatings (SSCs) like the easiness of manufacturing, the possibility to use standard materials as transmitter (e.g., indium tin oxide (ITO)) and absorber (e.g. Pyromark or black chrome), and the adaptability to specific requirements with respect to receiver temperature and solar concentration factor.

After a conceptual introduction, the analysis of solar plant parameters, i.e., operation temperature and solar concentration, for which this concept provides a better solar efficiency than state-of-the-art bare black body absorber, will be given.[3] We will then review the recent developments in the field, which include an excellent high-temperature in-air stability of such type of solar coatings.[4] In the second part of the talk, we will report own results toward a new TCO on black body absorber coating. Vacuum and in-air stability of the TCO SnO2:Ta at 800 °C and its structural properties before and after heat exposure are demonstrated. As potential absorber, the formation, structure, and optical properties of dense, PVD-grown CuCr2O4 thin films are studied. They are obtained in high purity from as- deposited samples by a simple in-air annealing step at 800 °C and absorb light in the whole solar range from 300 nm to 2500 nm.

[1] C.E. Kennedy, Review of Mid- to High-Temperature Solar Selective Absorber Materials, NREL Technical Reports, NREL - National Renewable Energy Laboratory, Golden, Colorado, USA, 2002.
[2] J.C.C. Fan, F.J. Bachner, Transparent heat mirrors for solar-energy applications, Applied Optics 15(4) (1976) 1012-1017.
[3] F. Lungwitz, R. Escobar-Galindo, D. Janke, E. Schumann, R. Wenisch, S. Gemming, M. Krause, Transparent conductive tantalum doped tin oxide as selectively solar-transmitting coating for high temperature solar thermal applications, Solar Energy Materials and Solar Cells 196 (2019) 84-93.
[4] H. Wang, I. Haechler, S. Kaur, J. Freedman, R. Prasher, Spectrally selective solar absorber stable up to 900 degrees C for 120 h under ambient conditions, Solar Energy 174 (2018) 305- 311.

Infrared emissivity measurements at working temperatures are of great importance for solar absorber coatings in central tower plants, as they allow proper estimations of the conversion efficiencies of these materials, as well as heat transfer management. This is especially important in situations where the concentration flux of the plant is reduced (e.g., the start-up phase). To that end, the high-temperature directional spectral emissivities of new copper-alloyed spinel coatings with different structures have been researched for concentrated solar power applications.
Four new coatings based on black spinel nanoparticles [1], as well as two nanoneedle-based ones [2], were measured up to their desired temperature operation range (800 °C and 600 °C, respectively). The nanoparticle-based coatings feature improved thermal stability that allows them to withstand higher temperatures than the industry standard Pyromark® 2500. The emissivities of these coatings are highly dependent on the morphology of the nanoparticle agglomerates, even more than on sample composition. In particular, a porous Cu_0.5Cr_1.1Mn_1.4O₄ coating showed a promising combination of an enhanced solar absorptance and emissivity in the normal direction, and a reduced total hemispherical emissivity at all temperatures. Both effects can be linked to the porosity of the sample, by means of enhanced light trapping in the normal direction and increased semitransparency at high polar angles of emission. Thus, this coating shows signs of directional selectivity. Besides, using a porous coating would be more cost-effective, since less mass of the material would be required for the coating. When combined with absorptance measurements, these emissivity measurements allow estimating the real efficiencies of the coatings, which can even reach values around 0.93 [3].
CuCo₂O₄ nanoneedle-based coatings, on the other hand, possess an extremely high 99% solar absorptance, which is also coupled with very high hemispherical emissivity values, with values between 0.82 and 0.92 at working temperatures. It was observed that coating these nanostructures with aluminum-doped zinc oxide (AZO) is highly effective in increasing their emissivities at all angles and temperatures. Thus, outside of concentrated solar power applications, these thermally stable thin coatings can also find use as highly efficient black absorbers in the visible and IR ranges.

[1] E.B. Rubin, Y. Chen, R. Chen, Optical properties and thermal stability of Cu spinel oxide nanoparticle solar absorber coatings, Sol. Energy Mater. Sol. Cells 195 (2019) 81–88.
[2] E.B. Rubin, S. Shin, Y. Chen, R. Chen, High-temperature stable refractory nanoneedles with over 99% solar absorptance, APL Mater. 7 (2019) 031101.
[3] I. González de Arrieta, T. Echániz, R. Fuente, E. Rubin, R. Chen, J.M. Igartua, M.J. Tello, G.A. López, Infrared emissivity of copper-alloyed spinel black coatings for concentrated solar power systems, Sol. Energy Mater. Sol. Cells 200 (2019) 109961.

Two multilayer solar selective absorber coatings [Al/CrN_0.95/Cr_0.96Al_0.04N_1.08/Cr_0.53Al_0.47N_1.12/Al₂O₃ (stack #1) and Cr_0.96Al_0.04N_0.89/Cr_0.62Al_0.38N_1.00/Cr_0.53Al_0.47N_1.12/Al₂O₃ (stack #2)] were deposited on 316L steel by combining direct current (DC) and high power impulse magnetron sputtering (HiPIMS) technologies with the aim of increasing the working limit temperature. The composition and thickness of the constituent layers were optimized using CODE software to achieve a high solar absorptance (α) and low values of thermal emittance (ε) in the infrared region. The deposited multilayered stacks were heated for 2 hours in air at 600, 700 and 800 ^oC to study their thermal stability and optical performance. Compositional, structural and optical characterization of the stacks (as-prepared and after thermal treatment) was performed. Both stacks presented a good solar selectivity with α > 95% and ε_{25 ^oC} < 15%, were stable up to 600 ^oC and fulfilled the performance criterion PC < 5% after 600 and 700 ^oC treatments. Despite the stacks suffered chemical transformations above 600 ^oC, partial oxidation (stack #1) and Cr₂N formation (stack #1 and #2), the optical properties were optimum up to 700 ^oC for stack #1 (α = 94%, ε_{25 ^oC} =12%) and 600 ^oC for stack #2 (α = 93%, ε_{25 ^oC} =13%).The solar-to-mechanical energy conversion efficiencies (η) of the as-deposited and annealed (600 and 700 ^oC) samples were up to 20 percentage points higher than the absorber paint commercially used (Pyromark). At 800 ^oC, they underwent a further structural transformation, provoked by the oxidation of the inner layers, and they consequently lost their solar selectivity.

In this work we present the microstructural and optical characterization of CrN_y and Cr_1-xAl_xN_y films to be used in multilayered solar selective coatings (SSC) for high temperature application. The films were prepared by direct current (DC) and high-power impulse magnetron sputtering (HiPIMS) technology. The deposition parameters: N₂ flux, HiPIMS frequency and the aluminum power target, were modified to get a wide variety of stoichiometries. The composition, morphology, phases and electronic structure of the films were characterized in depth. The optical behavior was determined by UV-Vis-NIR and FTIR spectroscopies, and the optical constants were obtained from the measured transmittance and reflectance spectra based on appropriate dielectric function models. Small changes in the layer composition influence the optical constants. Our results indicate that a metallic-like behavior was obtained for CrN_y layers with N vacancies (y=0.67 and y=0.95) while a semiconductor-like behavior was observed for y=1.08. The optical properties of Cr_1-xAl_xN_y layers can also be tuned from metallic to semiconductor-like behavior depending on the chemical composition. In particular, we demonstrated that CrN_y films could be used as effective absorber layer in multilayer-based SSC, and namely, the CrN_0.67 film also as IR reflector/absorber layer. Indeed, the absorption coefficients of Cr_1-xAl_xN_y films with optimized Al content and N-vacancies are comparable to those reported for state-of-the-art materials such as TiAlN or TiAlCrN. In addition, a Cr_0.96Al_0.04N_0.89 film was found to be a suitable IR reflector/absorber layer. The ensemble of results highlights the potential of these materials as efficient components in multilayer-based SSC for high temperature applications. Three different multilayered- SSC were designed by applying optical simulations, obtaining excellent optical selective properties (a > 95% and e < 15%).

High power impulse magnetron sputtering (HiPIMS) technology has emerged as a recent promising physical vapor deposition technique for coating with improved properties. The application of short high power pulses to the magnetron sources enables higher plasma densities and ionization rates, leading to improved film density and functionality. In this paper we explore the possibilities of this technique to design multilayered systems based on Cr(Al)N with controlled chemical composition and microstructure for solar selective absorption. By changing the pulse length (20 to 100 ms) and frequencies (200 to 1000 Hz) differences in the film stoichiometry, crystallinity, lattice parameters and morphology can be obtained, influencing the properties of interest for the foreseen application. In particular, we will present how to tailor the optical properties and the oxidation resistance thanks to the variation of pulse parameters in the single individual layers. The benefits obtained by combining the high power pulse delivered to the target with a negative voltage applied to the susbtrate (bias) will be shown. A full design based on a multilayered CrAlN systems capped with an antireflective Al₂O₃ layer is prepared on 316L steel with and without the application of bias. The results demonstrated that the bias helped to densify the structure, decreasing the intercolumnar spaces, although affected the targeted film stoichiometry. The thermal stability and oxidation resistance properties of both stacks will be compared and discussed to establish a correlationship between synthesis conditions and practical performance at different temperatures (room temperature, 600, 700 and 800°C).

Nickel-based superalloys are promising materials for next generation heat-exchangers in concentrated solar power technologies, including the supercritical CO₂ Brayton cycle, due to their superior corrosion resistance at high-temperatures of 1000 °C. However, many of these alloys lack sufficient yield and creep rupture stress required in power plants as strengthening from gamma prime precipitates is no longer effective at these elevated temperatures. One of the most promising methods of increasing yield stress at high temperatures is through the introduction of oxide dispersion strengthening (ODS) particles. At these high temperatures grain boundary sliding is activated; the ODS particles pin the grain boundaries reducing sliding improving creep and yield stress. Using the existing Reppich particle pinning model, our results indicate that by simply mixing in 0.3 wt% of 40 nm diameter ODS (Y₂O₃) particles, the yield stress of a typical Ni alloy, Haynes 214, can more than double at 1000 °C showing the promise of the ODS approach. Increasing the amount of ODS particles can result in improved strengthening of the base alloy, but only up to a certain mass-fraction. As the weight percent of ODS powders is increased clusters of ODS particles form causing stress concentrations, which leads to alloy cracking and to changes in grain recrystallization. Traditionally, the maximum mass-fraction of ODS powders is determined through trial and error, based on observing its effects on cracking during solidification. This process is time consuming and offers limited insight about the fundamental mechanisms at play. In this work, we apply phase-field modelling to inform how the microstructural evolution is influenced by the mass-fraction and initial distribution of the ODS particles. The model will be applied to explore the effect of ODS particle clustering on grain growth, which can influence creep and strength. Processing parameters, such as cooling rate, will be linked to initial ODS particle distribution offering insight into the driving forces that lead to particle clustering. These findings will shed light onto the mechanisms that determine the effectiveness of oxide dispersion particle strengthening on improving mechanical properties for high temperature applications to help enable next generation concentrated solar power technologies.

We propose a high performance, compact and durable ceramic heat exchanger design capable of operating over 1200 °C and 250 bar for aerospace and CSP applications. Our checkered pattern design leverages porosity at the centimeter-scale that serves as macrochannels for the flow of working fluids. Meanwhile, a micrometer-scale porous core is embedded into these channels to significantly improve heat transfer and structural strength. Compared to traditional porous media approach, this multiscale porous configuration with straight ducts enables high heat transfer and a low pressure drop penalty. Furthermore, these structures can be fabricated using a ceramic co-extrusion process capable of creating both the channels and porous internal structure in one step. Overall, the heat exchanger is able to operate with hot flows over 1200°C and supercritical CO2 at 300°C and 250 bar, providing high volumetric power density and less than 1% pressure drop.

Due to the large difference in the feature size of the heat exchanger, it is computationally prohibitive to include all the detailed feature in one simulation. Therefore, a hierarchical model is proposed to optimize the design and predict the mechanical strength, pressure drop and thermal performance. An analytical thermal resistance model is also developed to optimize the thermal performance of the heat exchanger. Thermal performance is evaluated and optimized as volume power density, weight power density and effectiveness.

The proposed high-performance counterflow heat exchanger can enable efficient heat exchange at extreme conditions in a wide range of power generation application, due to its high thermal performance, low volume, light weight and scalable fabrication process.

Advanced Iron (Fe) and Nickel (Ni) based alloys are potential candidates to serve as reactor components in high temperature, super-critical carbon dioxide (s-CO₂) heat exchangers. These high temperature (up to 1100°C) and high pressure (up to 20 atm s-CO₂) conditions pose the critical challenge of carburization induced corrosion, in addition to enhanced surface oxidation. In this work, we evaluate the relative performance of 3 such alloys: Fe-based Inconel MA 956, Ni-based Haynes 214 and Ni-based Amdry 386. Thermodynamic calculations and kinetic simulations are implemented using a CALPHAD (CALculation of PHAse Diagrams) based approach to understand the propensity of oxide and carbide formation. Thermodynamic analysis supports reported experimental findings that that carbide precipitates (M₂₃C₆, M₇C₃ etc.) are stable at the interface between the surface oxide (Corundum, rutile, spinel etc.) and the underlying metallic alloy. We propose a diffusion-driven model that combines oxide growth rates and gas transport through the oxide, to estimate the carbon activity at this interface. The reduction of carbon dioxide (CO₂) to carbon monoxide (CO) results in the formation of elemental Carbon (C), enhancing the thermodynamic driving force for carburization. Under similar boundary conditions for the interfacial carbon activity, Ni-based alloys exhibit better carburization resistance as compared to Fe-based alloys. Further, we also implement isothermal and non-isothermal heat treatment simulations to study the microstructural changes resulting from exposure to high temperatures and pressures (upto 1100°C and 20 atm). Thermo-kinetic nucleation and growth models employed in this study will be used to inform experiments about the design of particular heat treatments that promote/arrest the growth of desired high strength precipitate phases. The results from our work will guide the selection and processing of structural materials used as high temperature components in the next generation of concentrated solar power technologies.

Aerogels can be made transparent in the solar spectrum but opaque in the infrared spectrum. Such optically transparent and thermally insulating aerogels can be used in solar collectors, replacing the need for selective surfaces and vacuum in solar thermal systems. This talk will summarize several aspects of aerogel solar collectors. Theoretical modeling suggests that aerogel collectors can have attractive performance. Optimization of the synthesis process for silica aerogel has led to highly transparent aerogels across the entire solar spectrum. Several experimental implementations of the aerogel collectors will be discussed. One configuration is to combine solar thermal with solar PV in one optical path to increase dispatchability of PV, enabled by the transparent aerogel. In another configuration, transparent aerogel will be used in linear Fresnel concentrators to achieve high temperatures. Experimental results on the system performance will be discussed. The transparent aerogel also enables nonconcentrating system to achieve high temperatures for other thermal applications such as sterilization.

Population growth and continued industrialization of developing countries may double global energy consumption by 2035. If such energy demands are largely met by fossil fuels, the deleterious effects of anthropogenic carbon loading of the earth’s ecosystems will greatly intensify. As such, developing technologies to create renewable and sustainable fuels from sunlight is widely seen as a global imperative.

While technologies for converting solar irradiance into hydrogen or hydrocarbons
predicated on low-temperature pathways such as electrolysis and natural or artificial photosynthesis offer potential solutions, there is significant recent interest in utilizing high-temperature pathways powered by concentrated solar energy. Here we offer a perspective on a process that utilizes highly concentrated thermal energy from the sun to drive chemical reactions that split water into hydrogen or carbon dioxide into carbon monoxide, thereby creating simple fuels or precursors to more complex energy carriers like fungible liquids. Our implementation for converting sunlight to fuel is based upon a deceptively simple two-step redox cycle. In step one, a concentrated solar resource heats and reduces a non-stoichiometric oxide, spontaneously creating oxygen defects and depositing the solar energy directly into the reduced oxide, much like charging a battery. Once “charged,” the reduced oxide is exposed to water or carbon dioxide in a second step under conditions that result in spontaneous gas splitting, yielding molecular hydrogen or carbon monoxide (i.e., solar fuel or solar syngas).

Effective and efficient redox-active materials are key to the commercial success of solar thermochemical water splitting technologies, and material discovery efforts at laboratories around the globe are currently underway. This talk will discuss the use of complex, multi-cation oxides to accomplish the redox cycle chemistry and describe our efforts to explore the fundamental underpinnings of important relationships between oxide composition, electronic structure, and functionality. Furthermore, we will review progress made by the HydroGEN Advanced Water Splitting Materials Consortium (h2awsm.org), which is a research community brought together and funded by the US DOE as an Energy Materials Network initiative in late 2017 and tasked with using high throughput computational material science approaches to discovering thermochemically active oxides. We will also review challenges and opportunities for discovering new redox cycle chemistries within the context of achieving high solar-to-fuel conversion efficiency in engineerable and appropriately designed solar reactors.

Lowering the peak temperature required by solar thermochemical hydrogen (STCH) production cycles might be key to overcoming a range of technology readiness challenges. For oxide perovskite based STCH, the peak cycling temperature occurs during the oxide reduction step where reactive oxygen vacancies are generated, typically at temperatures >1300°C. Generally, oxygen vacancies are charge compensated by reduction of a single cation species on either the A or B site. However, recent modeling suggests a strategy for reducing the temperature would be to tune the enthalpy through doping while maximizing the entropy of reduction with dual reduction on both A and B site cations. We will report on experimentally validating these models and demonstrating dual cation reduction of Ce⁴⁺ to Ce³⁺ and Mn³⁺ to Mn²⁺ in (Ca,Ce)(Mn,Ti)O₃ perovskite. We designed techniques to measure samples with varying degrees of reduction in ex situ experiments through complementary use of in situ measurements. In situ techniques including TGA and XRD were used to quantify the reduction of ex situ samples, where reduced states are quenched into samples cooled under reducing environments. The oxidation state of cations in samples, both oxidized and reduced, were then determined through a combination of ex situ X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) measurements. We report the performance of this material during temperature and pO₂ cycling. These experiments both validate predicted routes to design STCH materials and demonstrate techniques to quantifiably quench reduction into samples for ex situ measurements.

Solar thermochemical hydrogen production (STCH) is a two-step process that utilizes concentrated solar heat and metal oxide materials to produce hydrogen gas by splitting water molecules through oxide redox chemistry. Among the many different materials investigated, perovskite (ABO₃) materials, including Sr_xLa_1-xMn_yAl_1-yO₃ (x, y = 0.4, 0.6)¹ and ATi_0.5Mn_0.5O₃ (A = Ca and Sr),^2,3 have been reported that are competitive with CeO₂, the current benchmark material. Multiple combinations of perovskite-like materials are possible with diverse crystal structures due to partial cation substitution and vacancy formation on the metal and oxygen sites, which influence the electronic structure and consequent STCH performance. Recently, BaCe_0.25Mn_0.75O₃ (BCM), a layered perovskite, was shown to have faster water oxidation kinetics than Sr_xLa_1-xMn_yAl_1-yO₃ (x, y = 0.4, 0.6) and better hydrogen production capability than ceria at a lower thermal reduction temperature (T_R=1350 °C).⁴ In the course of determining the chemical underpinnings of BCM’s performance, three new water-splitting and isostructural materials, BaNb_0.25Mn_0.75O₃ (BNM), BaPr_0.25Mn_0.75O₃ (BPM), and BaTi_0.5Mn_0.5O₃ (BTM), were synthesized and characterized. Comparing the performance and structural changes of these materials in operando aids in the understanding of the relationship between hydrogen production capability and the different local environments of the metal-oxygen bonds resulting from the chemical modification. From this, a better fundamental understanding can be developed to aid in identifying new materials for solar thermochemical fuel production.
1. McDaniel, A. H.; Miller, E. C.; Arifin, D.; Ambrosini, A.; Coker, E. N.; O'Hayre, R.; Chueh, W. C.; Tong, J. Energy Environ. Sci. 2013, 6, 2424.
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Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. The views expressed in this article do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

The two-step metal oxide water-splitting cycle is one of the most viable approach for Solar Thermochemical Hydrogen (STCH) production. Challenges exist in finding suitable oxides that can satisfy thermodynamics of the STCH redox cycle under viable range of temperatures and partial pressures. Recently, BXM family members, i.e., Ba₄NbMn₃O₁₂(BNM), Ba₄CeMn₃O₁₂ (BCM), Ba₄PrMn₃O₁₂ (BPM) are discovered to exhibit promising STCH performance. However, the magnetic degrees of freedom of their experimental crystal structures is not characterized, and only hypothetical models exist for their electronic structure. We performed Monte-Carlo sampling for the magnetic spin arrangement of Mn atoms to provide explicit atomic structure models for their ground state 12R polytype and determined the most stable spin configuration for each member after structure relaxation using density functional theory (DFT) calculations. We also elucidate the most stable charge configuration (Mn³⁺/Mn⁴⁺ arrangement) for BNM among the existing hypothesis in literature. We investigate these strongly correlated oxides using different levels of theory (DFT+U, Hybrid-DFT) to develop better understanding of their electronic structure. These calculations allowed us to develop a model based on molecular orbital interactions that help explain the observed changes in their experimental x-ray spectroscopy measurements. Thereby, help link differences in electronic structure of BXM family members to their varying water-splitting behavior. These structure models will further facilitate future in-depth defect studies to model STCH redox cycle of these oxides.

Renewable and sustainable sources of energy are playing a much larger role in powering modern society driven by the need to limit global CO2 emissions and mitigate the anthropogenic influences on climate change. Hydrogen, a clean energy source with a high gravimetric energy density (≈140 MJ kg-1) and zero carbon emission, is a possible clean route to lowering emissions from transportation and as an alternate source of energy storage. Currently, the majority of hydrogen is produced from reforming of fossil fuels which is carbon intensive and non-renewable. Renewable production of hydrogen, such as electrolysis and solar thermochemical, are less developed and more expensive than hydrocarbon reforming. The primary challenge of renewable hydrogen production is reducing the cost of production technologies to make the resulting hydrogen cost competitive with conventional transportation fuels. We propose a novel route to hydrogen production via a hybrid thermochemical-electrochemical process utilizing liquid metal solutions (LMS) to effect a water splitting reaction. The process can be driven by heat and electricity from renewable concentrating solar power (CSP). In the first step, steam is thermochemically reduced by the LMS producing hydrogen and a metal oxide by product, while in the second step, an endothermic process, the metal oxide is electrochemically reduced back to metal, and oxygen is evolved. The LMS process has the potential to produce hydrogen via water splitting at temperatures much lower than current CSP-driven thermochemical processes (>> 1000 °C) and at applied potentials lower than conventional high-temperature electrolysis. A number of possible binary LMS compositions that reduce at reasonably low overpotentials have been identified using machine learning. Here we test several alloy candidates for their ability to produce hydrogen in a flow reactor set-up. The basic proof of concept will be elucidated. X-ray diffraction and scanning electron microscopy of the LMS before and after the reaction to analyze byproducts and changes in microstructure, grain sizes of tin oxide, coalescence will be presented.

Many classes of materials for production of solar thermochemical hydrogen (STCH) and other fuels depend on reversible formation of oxygen vacancies which react with oxygen in steam or other gases. The change in oxygen vacancy stoichiometry during the gas reduction step (Δδ) is directly proportional to the hydrogen produced each cycle. Despite being a key property of STCH materials, the oxygen stoichiometry is rarely quantitatively measured with sufficient accuracy to capture changes in vacancy concentration. In many cases, thermal gravimetric analysis (TGA) is used to observe mass loss during cycling, which is then attributed to oxygen loss. However, the dynamic nature of TGA measurements prevent its use to directly measure vacancy content of ex-situ and postmortem samples used in other measurements. In this work, we introduce the use of Rutherford Backscattering Spectrometry (RBS) to directly measure the oxygen and cation stoichiometry of multinary STCH oxide samples, both powders and thin films, at sufficient accuracies to determine the vacancy stoichiometry by comparing oxidized and reduced samples. The operating principles of RBS are described, and the capabilities of the technique are compared to complimentary elemental analysis techniques including X-ray fluorescence spectroscopy (XRF), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The non-destructive depth profiling capabilities of RBS are demonstrated on representative STCH materials. Additionally, the sensitivity of RBS to light elements allows the determination of whether secondary phases are forming at surfaces, including the formation of carbonates which is a concern for STCH materials featuring alkaline rare earth elements. Together, this work demonstrates an underutilized technique for quantitative analysis of oxygen vacancy stoichiometry in STCH materials; a key material property to STCH performance which has been under-reported in the field.

Solar thermochemical (STC) processes have to potential to be an efficient way to generate renewable fuels or fuel precursors using concentrated sunlight. Specifically, an STC process can use redox-active, transition-metal oxide substrates to split water and CO₂, generating syngas, H₂ and CO. Importantly, STC processes benefit from oxide substrates that can i) withstand large temperature swings, ii) sustain high degrees of oxygen off-stoichiometry and iii) be resistant to adverse phase transformations. Currently, the state-of-the-art material used in STC applications is pure CeO₂; identifying materials that can operate at lower temperatures and achieve higher efficiency is an active area of research. Here, we explore the chemical space of Ca-Ce-M-O (M=3d transition metal) oxide perovskites, with Ca and/or Ce occupying the A site and M occupying the B site within an ABO₃ framework, as potential STC candidates, using density functional theory (DFT) based calculations. To account properly for electron exchange and correlation, we use the strongly constrained and appropriately normed functional, with an appropriately determined Hubbard U correction to reduce the inherently large self-interaction error DFT engenders when describing 3d (M) and 4f (Ce) electrons. While we consider only Ca and/or Ce on the A-site because of their similar size and the potential redox-activity of Ce, we evaluate all 3d transition metals except Zn on the B-site (Zn⁺³ is uncommon in solids). We evaluate the oxygen vacancy formation energy (~enthalpy of reduction in an STC process), electronic properties, and thermodynamic stability of ternary Ca-M-O, Ce-M-O, and quaternary Ca-Ce-M-O perovskites, identify promising candidates, and extract metrics that govern the enthalpy and entropy of reduction that are key to improving STC efficiencies.