Chemical looping reforming of methane (CLRM) is a promising route for low-emission synthesis gas production. Current CLRM technology remains primarily at the laboratory scale where the most promising redox intermediates have been tested under relatively benign conditions, such as ambient pressure, low (<10%) CH4 concentrations and with limited cycling stability. This work investigates the stability and kinetic behavior during dry CLRM using the candidate redox intermediate Ni-ceria (Ni-CeO2), under industrially relevant conditions-i.e., 90% CH4, 90% CO2, and 5 bar pressure. Specifically, parametric experimental studies were conducted to understand the effects of residence time (t(res) = 6.6-26.38 s), temperature (T = 700, 800, 900 degrees C), and controlled bed-average oxygen nonstoichiometry ranges (delta = 0-0.14) via nonsteady cycling on syngas yields and CH4 conversions. It was observed that under conditions which enabled the highest conversions (i.e., high t(res)) selectivity to H-2 and CO were negatively impacted. We find that in intermediate conditions (t(res) = 13.19 s) high conversions (>98%) and selectivities (H-2 > 90% and CO > 80%) can be achieved, specifically at (T = 900 degrees C, t(res) = 13.19 s, and 0.1 < delta < 0.14). These identified intermediate conditions were used to demonstrate redox stability over 1000 redox cycles. Results demonstrated consistent redox cycling behavior, high methane conversion (similar to 0.99), and effective syngas selectivities (S-H2 similar to 0.91, S-CO similar to 0.83) at 5 bar and 90% vol of reactants. These results establish Ni-ceria as a stable, durable, and kinetically favorable redox intermediate for pressurized CLRM, combining rapid reforming rates with long-term redox stability, supporting its potential for industrial-scale in situ syngas production for applications such as Fischer-Tropsch synthesis, methanol production, and other syngas-based processes amid the ongoing energy transition.

Hydrogen production via multi-step thermochemical cycles may be an affordable, low-carbon alternative to steam methane reforming or water electrolysis. Conventional thermochemical water splitting generates hydrogen while coproducing an oxidized product, oxygen, that has little value. Instead, we use elemental sulfur as an input and coproduce sulfur dioxide. This process, which we call Steam Sulfur Reforming (SSR), could function as a value-added substitute for the sulfur burning step of sulfuric acid manufacturing. Steam Sulfur Reforming is demonstrated with experiments using ceria and iron oxide, thermodynamic modeling, and technoeconomic analysis. Sulfur oxidation is favorable at temperatures near 1200 °C, comparable to conventional sulfur burning and lower than many thermochemical water splitting cycles. Experiments with ceria demonstrate stability over dozens of cycles, with sulfur conversions of up to 90% at 1200 °C and steam conversions of up to 50% between 700 and 900 °C. Experiments with iron oxide show the intended cycle but are challenged by sulfidation side reactions and low steam conversion, highlighting opportunities for metal oxide modification. SSR may achieve costs below 2 $/kg H2 with CeO2 or Fe3O4, making the process cost-competitive with fossil hydrogen and a potential drop-in hydrogen replacement for the fertilizer industry.

This work reports kinetics and improved long-term stability of A-site deficient Ga-doped La–Sr–Mn (LSM) perovskite for two-step thermochemical H 2 O splitting over 100 redox cycles (163 h).

Thermodynamic properties of a Ga-doped La–Sr–Mn perovskite are experimentally extracted and used to compare its water splitting behavior to ceria.

In this paper, we present a comprehensive thermodynamic model for efficiency analysis of a solar thermochemical water splitting system utilizing indirect-contact solid-gas heat exchangers, where heat flows between the metal oxide and steam/or sweep gas (i.e., nitrogen). Three different counterflow reduction reactor configurations are considered within the model, including a conventional one inlet and exit reactor, a two-exit reactor, and a two-inlet-two-exit reactor. The last two better approximate the ideal reduction process compared to the conventional. The impact of several process parameters, such as reduction and oxidation temperatures, steam and sweep gas flowrates, and purity of sweep gas, on the solar-to-fuel efficiency and solar power distribution is assessed, and optimal operating conditions are found. We show that while operating under a practical reduction temperature of 1500 degrees C and specific optimal operating parameters, it is possible to achieve a solar-to-fuel efficiency of 10.2% with 70% solid-gas heat recovery. Based on our modeling assumptions, the typical system with solid-solid heat exchangers achieves the same efficiency under its optimal operating parameters when assuming 95% gas heat recovery, and 35% solid heat recovery.

Chemical-looping reforming of methane (CLRM) presents a promising pathway toward renewable syngas production. CLRM involves two reaction steps using a metal oxide redox intermediate: 1) oxide reduction via oxidation of CH4 to produce syngas, followed by 2) oxide oxidation via CO2 and/or H2O dissociation to selectively produce CO and/or H2. Ni-CeO2 (or Ni-Ceria) has been demonstrated to facilitate high reactant conversions and product selectivity toward syngas, but has yet to be studied at high pressures and CH4 concentrations, which are relevant for industrial adoption. To study the behavior under more relevant industrial conditions, we have developed a packed bed reactor that affords total pressure control up to 5 bar and supplies up to 100 vol% CH4, and present CLRM results herein for the first time. Using response surface methodology, CLRM experiments were performed with 5 wt% Ni-Ceria, between 1 and 5 bar, 10-95 vol% CH4, 700-900 degrees C, and 0.15-6.6 s residence time. During this campaign, total cycle selectivity to CO approached 0.99 at 5 bar, indicating nearly 100 % carbon utilization. Repeatable and stable cycling behavior was observed across 32 cycles conducted at the experimental design midpoint. Increasing pressure resulted in improved syngas yields and process performance, while elevated methane concentration was found to increase yields but at the expense of conversion and syngas selectivity. The insights derived from this study clarify the performance and viability of CLRM with Ni-Ceria for large-scale syngas production.

Solar thermochemical hydrogen production using redox-active metal oxides is a promising pathway for the production of green hydrogen and synthetic fuel precursors. Herein, the perovskite material (La0.6Sr0.4)(0.95)Mn0.8Ga0.2O3-delta (LSMG6482) is identified as a promising metal oxide for thermochemical water splitting. LSMG6482, along with more-established water splitters ceria and (La0.6Sr0.4)(0.95)MnxAl1-x O3-delta (LSMA) perovskites, is experimentally characterized via thermogravimetric (TGA) analysis and high-temperature water splitting in a reactor simulating solar concentrating conditions. TGA analysis demonstrated that LSMG6482 has high and stable oxygen exchange capacity under controlled pO(2 )redox cycling, demonstrated by large changes in oxygen nonstoichiometry (delta) relative to ceria. Water splitting experiments using laser heating (T red = 1400 degrees C, Tox = 1200 degrees C) resulted in H-2 yields of 165.1 mu mol g(-1) for the candidate LSMG6482 composition, exceeding that of all benchmark materials tested. Under high conversion oxidation conditions, where H2 is cointroduced with H2O (150 <= nH2O/nH2 <= 500), H2 yields were greatest for LSMG6482 and LSMA6482, up to four times that of ceria at the highest nH(2)O/nH(2) conditions. Crystallographic analysis showed that over the course of experimentation, there is some secondary phase growth for all perovskite compositions, except for LSMA6482, but there was no observable degradation in H2 yields.

Solar thermochemical hydrogen (STCH) production from water splitting typically requires performing redox cycles at temperatures above 1200 °C to reduce and re-oxidize the bulk of a reversible material. Bulk processes such as oxygen vacancy formation and oxygen diffusion energies dictate the viability of a material for STCH. The surface plays an important role in the formation and destruction of vacancies and interacts with gas phase water and surface adsorbed species. These surface processes can lead to surface reconfigurations and even the formation of surface phases with stoichiometry and oxygen content very different from the bulk composition. Understanding in-situ the surface chemical state and its evolution under water splitting is important to design nonstoichiometric oxides capable of longer-lasting STCH generation at lower temperatures. In this work, we describe the water splitting active defect sites in LSM ((La0.65Sr0.35)0.95MnO3−δ) and Ga-doped LSM ((La0.6Sr0.4)0.95(Mn0.8Ga0.2)O3−δ) perovskites during Operando thermochemical water splitting conditions using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) experiments at 800 °C under steam. We show that sub-stoichiometric La+3 in the oxygen-vacancy rich surface at operating conditions can be used to correlate surface water splitting activity and the creation of surface hydroxide intermediates. The addition of Ga in LSM is shown to drastically stabilize the surface chemical composition by preventing Sr segregation and stabilizing catalytically active surface defects that promote the binding of adsorbed hydroxides. We use Operando AP-XPS quantification of metastable surface hydroxide intermediates (La(OH)3) to determine the amount of catalytically active surface sites in LSM (2.9

Recently, thermodynamic modeling has demonstrated that reduction of nonstoichiometric oxides in a counterflow gas current with a single inlet and exit must result in an equilibrium temperature gradient that deviates from the typically assumed isothermal operation at every single point but one, under assumed mass balance constraints. This necessity for a temperature gradient results in real processes that are near isothermal to deviate from the ideal reversible process, except for the single point where the Gibbs free energy change is zero. In this paper, new configurations with additional inlets and exits that can better approximate reversible and isothermal countercurrent flow reactors are considered and thermodynamically modeled. We show that it is possible to decrease irreversibilities while operating under the same maximum temperature, gas flowrates and initial nonstoichiometry simply by employing two or more exits and/or inlets. Under the most optimal conditions where the normalized irreversibilities are lowest, utilizing either an extra exit or extra inlet and exit show improved results in terms of minimizing the Gibbs free energy and lowering the needed separation work for the gases. Generally, as more irreversibilities are generated within the reactor, the more significant the usage of extra inlets/exits becomes.

Synthesis of solar thermochemical hydrogen (STCH) production redox materials with engineered structures, for example, replica foams, can enable efficient heat and mass transport and are critical for scaled-up systems. Prior work has motivated the use of lanthanum strontium manganese (LSM)-type perovskites as foamed STCH materials, but the effect of their morphology on bulk and kinetic behavior has not been reported. In this work, replica and direct foamed samples of La0.65Sr0.35MnO3-delta (LSM35) were fabricated and compared to synthesized powders and dense monoliths, and similarly synthesized CeO2-delta (ceria) foams, regarding their specific reaction rates and bulk oxygen capacity/H-2 yields. Changes in oxygen capacity (Delta delta) and reaction rates were measured between 1200 degrees C and 1400 degrees C by using fixed ratios of steam and hydrogen during both reduction and oxidation steps, allowing for analysis under practical high conversion conditions. Results suggest bulk behavior and reaction rates of the foamed LSM materials are comparable to their powder analogues. Differences in reaction rates were observed only when replica foamed samples were subjected to rapid laser heating (emulating conditions expected in solar furnaces), which is expected but has not been demonstrated at such a small scale. Foamed samples were further subjected to 50 redox cycles at 1400 degrees C to evaluate their stability. Results show no statistically significant decrease in hydrogen production for any of the foamed samples, but the direct foamed samples became brittle with time. Together, these results demonstrate the viability of replica foamed LSM perovskites for integration in scaled-up STCH systems.

Chemical looping reforming of methane utilizing the oxygen exchange material CeO 2 is catalytically enhanced with metallic Ni to improve reaction rates at low operating temperatures. A kinetic study and comparison to noncatalyzed CeO 2 is presented.

Professor Aldo Steinfeld’s contributions to the fields of solar thermochemistry and energy conversion are extensive and impressive. His work has greatly contributed to the ongoing transition from fossil to renewable fuels. We, his former doctoral students and postdoctoral researchers, take a look back at his life and honor his contributions. His work has redefined the field and created a legacy that reverberates throughout the world. His impact is being realized through his tireless efforts towards developing cutting edge solar technologies, writing seminal papers, and his undying commitment to and leadership in the solar energy and renewable energy technology communities and beyond. This legacy has been recognized by numerous accolades and will continue after his retirement through his mentorship and guidance of the next generations of researchers dedicated to continuing his solar research.

Synthesis of solar thermochemical H2 (STCH) production redox materials with engineered structures, for example replica foams, can enable efficient heat and mass transport and are critical for scaled-up systems. Prior work has motivated the use of LSM-type perovskites as foamed STCH materials, but the effect of their morphology on bulk and kinetic behavior has not been reported. In this work, replica and direct foamed samples of La0.65Sr0.35MnO3-δ (LSM35) were fabricated and compared to synthesized powders and dense monoliths, and similarly synthesized CeO2-δ (ceria) foams, regarding their specific reaction rates and bulk oxygen capacity/H2 yields. Changes in oxygen capacity (Δδ) and reaction rates were measured between 1200 °C −1400 °C by using fixed ratios of steam and hydrogen during both reduction and oxidation steps, allowing for analysis under practical high conversion conditions. Results suggest bulk behavior and reaction rates of the foamed LSM materials are comparable to their powder analogues. Differences in reaction rates were observed only when replica foamed samples were subjected to rapid laser heating (emulating conditions expected in solar furnaces), which is expected but has not been demonstrated at such a small scale. Foamed samples were further subjected to 50 redox cycles at 1400 °C to evaluate their stability. Results show no statistically significant decrease in hydrogen production for any of the foamed samples, but the direct foamed samples became brittle with time. Together, these results demonstrate the viability of replica foamed LSM perovskites for integration in scaled up STCH systems.

Methane reforming and gasification processes are valuable pathways toward hydrogen production, and currently they represent nearly 79% of the worldwide hydrogen production market share. Chemical looping-water gas shift (CL-WGS) is a promising downstream process that can be integrated with reforming and gasification reactors and utilizes redox active metal oxides as oxygen exchange materials to produce separate streams of H2 and CO2. Thus, this technology can enable the production of high-purity hydrogen coupled with carbon capture without the need for an additional CO2 separation system. Although a number of redox-active metal oxides have been proposed in the literature as viable candidates for CL-WGS, the majority are based on stoichiometric metal oxides. However, non-stoichiometric oxides, which have the benefit of stability, rapid kinetics, and thermodynamic tunability, have only been proposed a handful of times. And of the prior proposed non-stoichiometric materials, none of the so called "water splitting" redox materials of the solar thermochemical H2O splitting (STCH) fields have been discussed for CL-WGS applications, despite their potentially favorable thermodynamic properties. In this research, we have identified 13 potential oxygen carrier materials and screened these for CL-WGS viability based on a simplified closed-system thermodynamic analysis. Ceria (CeO2-$), CZO20 (Ce0.8Zr0.2O2-$), and LSMA4060 (La0.6Sr0.4Mn0.4Al0.6O3-$) are identified to have the most suitable properties in terms of operating temperatures, specific H2 yields, and conversions of syngas, during reduction, and steam, during oxidation. Results show that CZO20 and LSMA4060 have especially excellent reduction and oxidation extents at relatively low operating temperatures. For example, CZO20 and LSMA4060 have 85% syngas and steam conversions when cycled between 485-912 and 100-635 degrees C, respectively. Results show that only CZO20 is viable for isothermal operation, but higher temperatures are required; at 1023 degrees C, syngas and steam conversions are 80 and 12%.(c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

While ceria is the standard material for two-step water splitting, perovskites are emerging as viable alternatives. In this work, based on the orthorhombic LaMnO3 supercell, we substitute Li Na K Rb Mg Ca Sr Ba on the A-sites (La sites) and Al Ga In Mg Zn on the B-sites (Mn sites) at a concentration of 37.5%. The range of temperature and oxygen partial pressure at which each composition is stable is predicted. For compositions that are stable in relevant temperature and pressure ranges, the oxygen vacancy formation energies are determined for all of the oxygen vacancy site positions available in the computational supercell. Mg, Ca, Sr, and Ba A-site-substituted LaMnO3 and Al and In B-site-substituted LaMnO3 meet these two criteria for candidates in solar-thermal water splitting applications. Oxygen vacancy formation energy can also be controlled by adjusting the doping strategy.

A cycling strategy is presented for simultaneous improvement of conversion and syngas selectivity during chemical looping reforming. Periodic recharging is used to re-establish a favorable nonstoichiometry profile during non-steady cycling.

This Special Issue on solar hydrogen production focuses on innovative approaches and emerging technologies to transform solar energy into H2 or derivative energy carriers via water splitting pathways; those discussed include photoelectrochemical, photocatalytic, and thermochemical processes. The articles published in this edition range from fundamental materials research to integration in operational components and photoreactors, and experimentation. Sustainable and, particularly, solar-driven hydrogen production is an important topic of global interest because it can enable a shift from fossil fuels towards sustainable (solar) fuels. Because of the inherent variability of solar energy (and other renewables), cost-effective conversion and storage solutions are necessary in order to realize a truly sustainable energy future. Hydrogen is attractive as an energy vector because of its high mass-specific energy density, its inconsequential emissions upon combustion, limited reliance on precious metals and raw material mining (e.g. lithium),[1] and its capability to be utilized in a variety of applications. For example, it can be used as (seasonal) energy storage solution at the utility and residential scale and as fuel in fuel cell electric vehicles and heavy duty transport such as rail and shipping. Hydrogen can even be flexibly blended with natural gas for cleaner power production, or act as feedstock for chemical process industry. In 2017, Japan adopted a “Basic Hydrogen Strategy” aimed at decreasing the cost of hydrogen production and competing with gasoline and liquified natural gas.[2] In March 2020, the European Union launched the European Clean Hydrogen Alliance to install at least 40 GW of renewable hydrogen electrolysers by 2030.[3] Hydrogen has also been promoted in June 2021 by the U.S. Department of Energy's H2 Earthshot Program with the aim of producing $1/kg H2 in 1 decade.[4] To achieve this goal, large technological advancements must be made. Currently, about 75% of the world's H2 is produced from natural gas[5] (followed by coal) via steam methane reforming and the water gas shift reaction (CH4 + 2H2O → CO2 + 4H2). This is not a long-term, sustainable strategy because of the reliance on fossil fuels, but it can be coupled with CO2 sequestration to produce carbon free hydrogen, or so called “blue hydrogen”.[6] Storage of renewable energy as “green hydrogen” can be achieved via a variety of technologies and other renewable feedstocks such as biomass and water.[6] The focus of this Special Issue is specific to water splitting pathways that focus on driving the reversible chemical reaction 2H2O ↔ 2H2 + O2 using solar energy. This endothermic reaction may be driven directly by photons (photoelectrochemical[7, 8] and photocatalytic[8, 9]), renewable electrons (high and low temperature electrolysis),[10] renewable thermal energy (thermochemical processes)[11] or a combination of these. This Special Issue has research articles focused on photocatalytic, photoelectrochemical and thermochemical, as well as hybrid processes aimed at utilizing solar energy to drive the reforming of biomass derivatives or methane, which can offset a large percentage of the CO2 emissions because of the reduced heating demands. Articles ente.202100525, ente.202100469, ente.202100356, ente.202100302, ente.202100265, ente.202100259, ente.202100188, ente.202100161, and ente.202000950 focus on photocatalytic, ente.202100570, ente.202100461, ente.202100457, and ente.202100181 on photocatalytic and photoelectrochemical, and ente.202100515, ente.202100491, ente.202100473, ente.202100222, ente.202100220, and ente.202000925 focus on thermochemical. We hope you enjoy this Special Issue on Solar Hydrogen Production and are grateful to all the authors and editorial staff at Energy Technology that made this possible. Sincerely, Jonathan R. Scheffe, Sophia Haussener, Greta R. Patzke Jonathan Scheffe is an Associate Professor in the Department of Mechanical and Aerospace Engineering at the University of Florida. Prof. Scheffe is Principle Investigator of the Renewable Energy Conversion Laboratory that is focused on research in the area of conversion and storage of solar energy. Applications include the production of renewable fuels/electricity, H2 production and fuel reforming. He has co-authored more than 40 peer received publications in the field of solar thermal energy conversion. Sophia Haussener is an Associate Professor heading the Laboratory of Renewable Energy Science and Engineering at the Ecole Polytechnique Fédérale de Lausanne (EPFL). Her research is focused on providing multi-scale design guidelines for thermal, thermochemical, electrochemical and photoelectrochemical energy conversion reactors and processes through multi-physics modeling and demonstration. She is a co-founder of the startup SoHHytec, the former chair of ASME's Solar Energy Division, and member of multiple scientific advisory boards promoting energy conversion and solar fuels. Greta R. Patzke is a Full Professor in the Department of Chemistry at the University of Zurich. Her research includes the synthesis and monitoring of nanomaterials and composites for energy and environmental applications. This encompasses a wide range of molecular, nanostructured and solid transition metal-based catalysts for artificial photosynthesis. She is a board member of the UZH Research Priority Program “Light to Chemical Energy Conversion”, and she serves on various other panels for sustainable energy research.

Zr‐doped ceria and Sr‐doped lanthanum manganese perovskites are investigated as candidate redox materials for the chemical‐looping reforming of methane to produce syngas. Herein, the effect that these compositions have on syngas selectivity, partial oxidation of methane (POM) rate, and yield in the temperature range of 800−1000 °C is probed. POM reaction rates increase when using 10% Zr‐doped ceria (CZO10) compared with pure ceria at all conditions, but the yields are generally less except at 800 °C. In the case of (La 0.65 Sr 0.35 ) 0.95 MnO 3 (LSM35), POM yields are equal to or greater than ceria under all conditions investigated. For all materials, selectivity to syngas during POM is relatively high and typically above 85%. However, selectivity of CZO10 and LSM35 is lower than ceria, likely because of their lower reduction enthalpies and more accessible surface oxygen. This is validated by demonstrating improvements in POM selectivity by suppressing the oxidation extent; in this case, there is no observable H 2 O or CO 2 formation. POM rates increase and activation energy ( E a ) decreases for all materials following oxidation with O 2 versus CO 2 . Ceria and CZO10 E a decrease from about 225 to <50 kJ mol −1 and LSM35 from 150 to 100 kJ mol −1 .

Porous ceramic foams are utilized in various fields and applications due to their advantageous properties such as high porosity, high specific surface area, and controlled permeability. Non-stoichiometric oxide foams are of particular interest for the solar thermochemical production of hydrogen in directly irradiated solar receivers due to their enhanced heat transfer capabilities and structural integrity. We report the fabrication of lanthanum strontium manganite replica foams and their solar thermochemical water splitting performance. Highly porous (>80%) foams were produced with controlled structure and tested in a high temperature water splitting furnace to demonstrate stability and overall performance. The temperature profile of the burnout and sintering of the foam structure were altered based on simultaneous differential scanning calorimetry and thermal gravimetry analysis. Two-step sintering was used both to retain micron-sized pores (D-50 = 0.4 mu m) on the foam's struts and to limit grain growth to a grain size of similar to 1 mu m. The structural characteristics were confirmed by porosimetry and scanning electron microscopy. The foams were repeatedly cycled, producing approximately 200 mu mol/g of H-2 per cycle. Water splitting performance was tested for 50 cycles and approximately 35 h at 1400 degrees C. No statistical difference in the effective hydrogen production per cycle was observed, which is promising for long-term redox cycle stability. (C) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.