Thermochemical Splitting Research Articles

Characterisation and thermochemical stability analysis of 3D printed porous ceria structures fabricated via composite extrusion Modelling

Composite Extrusion Modelling (CEM) is an advanced additive manufacturing technique that enables rapid, cost-effective production of complex, customisable designs. In this study, CEM 3D printing of porous ceria structures is reported for the first time. First, the ceramic injection molding (CIM) feedstock with thermoplastic properties was prepared and optimised in terms of its rheological properties to ensure good workability for the printing process at 140–150 °C. Subsequently, the thermoplastic feedstock was used to print porous ceria structures for thermochemical splitting. The feasibility of printing various porous structures with different dimensions, geometries, and macropore sizes was investigated, and the printing parameters: extrusion multiplier (EM), extrusion temperature (ET), nozzle velocity (NV), and layer thickness (LT), were optimised to prevent clogging of the printing nozzle and to achieve homogeneous overlap of the printed layers. The optimised printing parameters for ceria structures are EM 1.3, ET 150 °C, LT ∼ 0.13 mm, and NV 50 mm/s, which were determined by multiple response optimisation process. The sintered 3D-printed bars yielded relative densities of ≥ 98 % and a total microporosity of 0.29 %, measured with a Hg porosimeter. Thermogravimetric analysis (TGA) and cyclic stability experiments were performed on the printed porous ceria structures and showed stability over 100-redox cycles.

Selection of iron-based oxygen carriers for two-step solar thermochemical splitting of carbon dioxide

The two-step solar thermochemical cycle is a prospective clean energy technology that enables the direct splitting of water and carbon dioxide for solar fuel production. However, immature oxygen carriers fundamentally restrict the solar-to-fuel conversion efficiency and thus prevent this technology from immediate industrial uptake. Focusing on the conventional iron-based oxygen carriers, this study intends to find a modified oxygen carrier with low reaction temperature requirements and excellent reaction performance, so as to attain dual optimization in terms of thermal efficiency and chemical efficiency. The thermochemical reaction characteristics of six common ferrites and four metal dopants are compared via thermogravimetric analysis under the same programmed temperature control. For further practical applications of the selected oxygen carrier, a foam-structured material with SiC as support is prepared and subsequently experimented in a self-designed 18-kW thermochemical system. The results indicate that the modified oxygen carriers obtained by direct doping of CoFe2O4 with 50wt% NiO exhibit optimal reaction performance, giving the highest CO yield of 439μmol/g. Additionally, the foam-structured material enable a peak CO yield of 7.0 mL min−1 g−1 and CO2 conversion of 45.5% at a reduction temperature of only 1100°C, which might be attributed to the micron-scale pore structure as well as the generation of aluminosilicate crystal structures.

Cost effective decarbonisation of blast furnace – basic oxygen furnace steel production through thermochemical sector coupling

We present here a first-principles study of the sector coupling between a thermochemical carbon dioxide (CO2) splitting cycle and existing blast furnace – basic oxygen furnace (BF-BOF) steel making for cost-effective decarbonisation. A double perovskite, Ba2Ca0.66Nb0.34FeO6, is proposed for the thermochemical splitting of CO2, a viable candidate due to its low reaction temperatures, high carbon monoxide (CO) yields, and 100% selectivity towards CO. The CO produced by the TC cycle replaces expensive metallurgical coke for the reduction of iron ore to metallic iron in the blast furnace (BF). The CO2 produced from the BF is used in the TC cycle to produce more CO, therefore creating a closed carbon loop, allowing for the decoupling of steel production from greenhouse gas emissions. Techno-economic analysis of the implementation of this system in UK BF-BOFs could reduce steel sector emissions by 88% while increasing the cost-competitiveness of UK steel on the global market through cost reduction. After five years, this system would save the UK steel industry £1.28 billion while reducing UK-wide emissions by 2.9%. Implementation of this system in the world's BF-BOFs could allow the steel sector to decarbonise in line with the Paris Climate Agreement to limit warming to 1.5 °C.

Investigation of CO2 Splitting on Ceria-Based Redox Materials for Low-Temperature Solar Thermochemical Cycling with Oxygen Isotope Exchange Experiments

The surface exchange and bulk transport of oxygen are highly relevant to ceria-based redox materials, which are envisaged for the solar thermochemical splitting of carbon dioxide in the future. Experimental investigations of oxygen isotope exchange on CeO2-δ, Ce0.9M3+0.1O1.95-δ (with M3+ = Y, Sm) and Ce0.9M4+0.1O2-δ (with M4+ = Zr) samples were carried out for the first time utilizing oxygen-isotope-enriched C18O2 gas atmospheres as the tracer source, followed by Secondary Ion Mass Spectrometry (SIMS), at the temperature range 300 ≤ T ≤ 800 °C. The experimental K˜O and D˜O data reveal promising results in terms of CO2 splitting when trivalent (especially Sm)-doped ceria is employed. The reaction temperatures are lower than previously proposed/reported due to the weak temperature dependency of the parameters K˜O and D˜O. The majority of isotope exchange experiments show higher values of K˜O and D˜O for Sm-doped cerium dioxide in comparison to Y-doped and Zr-doped ceria, as well as nominally undoped ceria. The apparent activation energies for both K˜O and D˜O are lowest for Sm-doped ceria. Using Zr-doped cerium oxide exhibits various negative aspects. The Zr-doping of ceria enhances the reducibility, but the possible Zr-based surface alteration effects and dopant-induced migration barrier enhancement in Zr-doped ceria are detrimental to surface exchange and oxygen diffusion at lower temperatures of T ≤ 800 °C.

Thermochemical activation of CO2 into syngas over ceria-supported niobium oxide catalyst: An integrated experimental-DFT study

Thermochemical splitting of a mixture of CO2 and water into syngas (and methane) remains a viable approach toward an industrial-scale treatment of CO2 emission. However, most deployed catalysts encompass expensive metallic ingredients such as Pd or Pt. In an integrated experimental-modelling approach, this work reports a high conversion of CO2 over alternative and cost-effective configurations. Ceria-based catalysts are proven to be effective in numerous catalytic processes owing to the facile switch in the Ce redox cycle. Evolution of H2 and CO from thermochemical splitting of water and carbon dioxide is an important process in the production of syngas and in fuel cell applications. In this study, NbOx ceria catalysts are prepared, characterized, and applied for production of syngas via the thermochemical splitting of CO2 and gaseous water. It was observed that presence of Nb in the ceria matrix up to 12 wt%, increases the reaction yield at higher temperature (up to 79% CO2 conversion with 80% selectivity to syngas at 600 ºC). Moreover, the catalytic reaction was found to display a higher selectivity towards syngas over the ceria supported catalysts. The attained conversion is higher than other ceria-supported catalysts such as Rh-CeO2, CeO2-ZrO2, and V2O5-CeO2. Mapped-out reaction pathways by DFT calculations portray accessible routes into syngas. Results provided herein should be useful to optimize a continuous process for the valorization of CO2 into syngas over relatively affordable a catalytic formulation.

Recent advances in the solar thermochemical splitting of carbon dioxide into synthetic fuels

Recent years have seen a sharp rise in CO2 emissions into the atmosphere, which has contributed to the issue of global warming. In response to this several technologies have been developed to convert CO2 into fuel. It is discovered that the employment of a solar-driven thermochemical process (S-DTCP) that transforms CO2 into fuels can increase the efficiency of the production of sustainable fuels. The process involves the reduction of metal oxide (MO) and oxidizing it with CO2 in a two-step process using concentrated solar power (CSP) at higher and lower temperatures, respectively. This study summarizes current advancements in CO2 conversion methods based on MO thermochemical cycles (ThCy), including their operating parameters, types of cycles, and working principles. It was revealed that the efficiency of the solar conversion of CO2 to fuel is not only influenced by the composition of the MO, but also by its morphology as well as the available surface area for solid/gas reactions and the diffusion length. The conversion mechanism is governed by surface reaction, which is influenced by these two parameters (diffusion length and specific surface area). Solar energy contributes to the reduction and oxidation steps by promoting reaction kinetics and heat and mass transport in the material. The information on recent advances in metal oxide-based carbon dioxide conversion into fuels will be beneficial to both the industrial and academic sectors of the economy.

Redox behavior of potassium doped and transition metal co-doped Ce0.75Zr0.25O2 for thermochemical H2O/CO2 splitting.

CeO2 slow redox kinetics as well as low oxygen exchange ability limit its application as a catalyst in solar thermochemical two-step cycles. In this study, Ce0.75Zr0.25O2 catalysts doped with potassium or transition metals (Cu, Mn, Fe), as well as co-doped materials were synthesized. Samples were investigated by X-ray diffraction (XRD), N2 sorption (BET), as well as by electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) to gain insight into surface and bulk features, which were connected to redox properties assessed both in a thermogravimetric (TG) balance and in a fixed bed reactor. Obtained results revealed that doping as well as co-doping with non-reducible K cations promoted the increase of both surface and bulk oxygen vacancies. Accordingly, K-doped and Fe-K co-doped materials show the best redox performances evidencing the highest reduction degree, the largest H2 amounts and the fastest kinetics, thus emerging as very interesting materials for solar thermochemical splitting cycles.

A solar concentrator/receiver/storage/reactor system for thermochemical splitting cycles based on perovskites

A solar concentrator/receiver/storage/reactor system for thermochemical splitting cycles based on perovskites

A perspective on the production of hydrogen from solar-driven thermal decomposition of methane

In addition to “green” hydrogen from electrolysis of the water molecule with solar-photovoltaic or wind electricity, and “white” hydrogen, based on solar-thermal driven thermochemical splitting of the water molecule, there is another emerging opportunity to produce CO2 free hydrogen at a reduced cost. The perspective advocates in favor of “aquamarine” hydrogen, based on the solar-thermal driven thermal decomposition of methane. This pathway has an energy requirement that is much less than white and green hydrogen, and even if based on hydrocarbon fuel, has no direct production of CO2 as a by-product, but rather carbon particles of commercial interest. Catalytic methane decomposition can be based on self-standing/supported metal-based catalysts such as Fe, Ni, Co, and Cu, metal oxide supports such as SiO2, Al2O3, and TiO2, and carbon-based catalysts such as carbon blacks, carbon nanotubes, and activated carbons, the pathway of higher technology readiness level (TRL). Thus, catalytic methane decomposition appears to be a highly promising approach, with undoubtedly many challenges, but also huge opportunities following pathways to be further refined through research and development (R&D).

Synergies between Direct Air Capture Technologies and Solar Thermochemical Cycles in the Production of Methanol

Methanol is an example of a valuable chemical that can be produced from water and carbon dioxide through a chemical process that is fully powered by concentrated solar thermal energy and involves three steps: direct air capture (DAC), thermochemical splitting and methanol synthesis. In the present work, we consider the whole value chain from the harvesting of raw materials to the final product. We also identify synergies between the aforementioned steps and collect them in five possible scenarios aimed to reduce the specific energy consumption. To assess the scenarios, we combined data from low and high temperature DAC with an Aspen Plus® model of a plant that includes water and carbon dioxide splitting units via thermochemical cycles (TCC), CO/CO2 separation, storage and methanol synthesis. We paid special attention to the energy required for the generation of low oxygen partial pressures in the reduction step of the TCC, as well as the overall water consumption. Results show that suggested synergies, in particular, co-generation, are effective and can lead to solar-to-fuel efficiencies up to 10.2% (compared to the 8.8% baseline). In addition, we appoint vacuum as the most adequate strategy for obtaining low oxygen partial pressures.

There are hydrogen production pathways with better than green hydrogen economic and environmental costs

While the present hydrogen (H2) is mostly grey, from steam reforming of methane (CH4) with direct carbon dioxide (CO2) emission, different pathways have been proposed to produce H2 more environmentally friendly. Blue H2, adopting carbon capture and storage (CCS), has no direct CO2 emission, but the additional cost to capture and store the CO2. Thus, green H2, from the electrolysis of the water (H2O) molecule, is becoming the preferential pathway for future H2 production. Two additional pathways are proposed for reduced economic and environmental costs, both free of direct CO2 emissions, based on the use of concentrated solar energy (CSE). White H2 is produced from the catalytic solar thermochemical splitting of the H2O molecule. Aquamarine H2 is produced from solar thermochemical CH4 pyrolysis with a carbon catalyst. Here we provide an estimation of costs by 2030 of white and aquamarine H2 lower than the cost of not only green H2 but also grey H2 disregarding the CO2 emission.

Perspectives of Perovskites for Solar Thermochemical Splitting of CO2 or H2O Molecules

Solar thermochemical splitting cycles (TSCs) are a promising technology for producing renewable hydrogen, or hydrocarbon fuels, from a feedstock of H2O, or H2O and CO2, and solar energy. At present, perovskites materials have been only investigated in isolation for potential use in high‐temperature two‐step TSC. Their ability in isolation to gain or lose oxygen at different temperatures and their heating duties for CO2 or H2O are not satisfactory arguments in favor of or against the specific use. The design, prototyping, and testing of a specific solar receiver/reactor for the production of CO or H2 are necessary to prove the technology.

Isothermal redox cycling of A‐ and B‐site substituted manganite‐based perovskites for CO2 conversion

AbstractConversion of CO2 into fuels and valuable chemicals can simultaneously address two major global issues: climate change and the increasing energy demand. Most of the previous studies regarding the thermochemical splitting of CO2 into CO have been carried out at elevated temperatures of up to 1400°C. In this study, under isothermal reaction conditions, manganite‐based perovskites are used to convert CO2 to CO in a thermogravimetric analyzer (TGA). In addition, the use of CH4 as a reducing agent has lowered the reaction temperatures (≤900°C). This study primarily focuses on the influence of A‐ and B‐site substitution on the redox performances of the manganite‐based perovskites. Based on the redox performances of four samples (CaMnO3, La0.5Ca0.5MnO3, La0.5Sr0.5MnO3, and Y0.5Sr0.5MnO3) with different A‐site compositions, La0.5Sr0.5MnO3 displays the best stability. Similarly, in the La1‐xSrxMnO3 perovskite family (x = 0.5, 0.25, and 0.10), the performance of La0.5Sr0.5MnO3 outshines the other two compositions. It is observed that as the Sr content increases, the activation energy for reduction decreases. Substituting Mn with Al and Fe in the B‐site of the La1‐xSrxMnO3 perovskite, did not enhance the performance. The perovskite materials investigated have shown decrease in activity after the first cycle, possibly due to sintering and powder densification. However, after the first cycle, the perovskites illustrated good stability for 10 redox cycles. In this paper, size variance and metal‐oxygen bond strength provide the best explanation for the trends observed during the reduction of the perovskites.

K-doped CeO2-ZrO2 for CO2 thermochemical catalytic splitting.

Green syngas production is a sustainable energy-development goal. Thermochemical H2O/CO2 splitting is a very promising sustainable technology allowing the production of H2 and CO with only oxygen as the by-product. CeO2–ZrO2 systems are well known thermochemical splitting catalysts, since they combine stability at high temperature with rapid kinetics and redox cyclability. However, redox performances of these materials must be improved to allow their use in large scale plants. K-doped systems show good redox properties and repeatable performances. In this work, we studied the effect of potassium content on the performances of ceria–zirconia for CO2 splitting. A kinetic model was developed to get insight into the nature of the catalytic sites. Fitting results confirmed the hypothesis about the existence of two types of redox sites in the investigated catalytic systems and their role at different K contents. Moreover, the model was used to predict the influence of key parameters, such as the process conditions.

Favorable Redox Thermodynamics of SrTi0.5Mn0.5O3−δ in Solar Thermochemical Water Splitting

Two-step, solar thermochemical splitting of water using nonstoichiometric redox-active metal oxides has emerged as an intriguing approach for large-scale hydrogen production. Perovskites have been ...

A kinetic model for evolution of H2 and CO over Zr-doped ceria

Among notable catalytic assisted processes by ceria-abased materials are the production reactions of H2 and CO from solar thermochemical splitting of water and carbon dioxide, respectively, for applications pertinent to hydrogen fuel cell and synthesis of syngas. Doping ceria with isovalent cations, most notably Zr, was shown to enhance the yield of O2, H2, and CO. Via sophisticated measurements techniques, literature provided several global-like rates for the overall evolutions of H2 and CO. However, these values remain without a mechanistic verification by means of first-principles calculations. Herein, we map out mechanisms and construct a detail chemistry kinetic model for the dissociative adsorption of H2O and CO2 over pure and Zr-decorated ceria surfaces. The overarching objective of the kinetic model is to deduce temperature-dependent profiles that account for the experimentally observed evolution of O2, H2, and CO during the redox cycles of ceria and to reveal the effect of Zr dopants. Ce surface atoms in the latter incur higher positive charges, and hence more affinity to capture gas phase molecules. Splitting of H2O into a gas phase H2 over the Zr-substituted surface ensues in two consecutive steps through activation energies of 77.7 kJ/mol and 191.2 kJ/mol. Our computed rate constant for evolution of H2 over ceria amounts to 2.67 mL/g. min, within the narrow range of the experimental values scattered between 1.5–5.4 mL/g. min. Formation of H2 and CO arises at a ∼ 100 K lower temperature over the Zr-doped surface in reference to pure ceria. The developed kinetic model is expected to describe - on a precise atomic basis - the H2O/CO2 redox cycles of ceria-based materials and the promoting effects of dopants.

Anti-sintering non-stoichiometric nickel ferrite for highly efficient and thermal-stable thermochemical CO2 splitting

The thermochemical splitting of CO2 into CO based on redox reaction demonstrates great promising for clean energy production. However, the redox material inevitably suffers from high-temperature sintering, leading to an increased ion diffusion barrier that deactivates the bulk phase. In this study, a distinct strategy is demonstrated to neutralize the adverse effect of sintering by engineering a non-stoichiometric nickel ferrite for the CO2 splitting. It maintains a very high CO yield over multiple consecutive cycles, performing better than state-of-the-art redox materials. Extensive characterizations and first principles calculations reveal that the bulk cation diffusion boosts the splitting reaction. Such cation diffusion mode in the non-stoichiometric ferrite provides a more accessible diffusion path for the bulk reaction, leading to accelerated CO2 splitting kinetics. Meanwhile, high stability is demonstrated for the non-stoichiometric nickel ferrite with 100% of the initial reactivity maintaining often 20 redox cycles. Our findings guide to design newly anti-sintering redox materials toward highly efficient energy conversion and production.

Application of Li-, Mg-, Ba-, Sr-, Ca-, and Sn-doped ceria for solar-driven thermochemical conversion of carbon dioxide

The redox reactivity of the Li-, Mg-, Ca-, Sr-, Ba-, and Sn-doped ceria (Ce0.9A0.1O2−δ) toward thermochemical CO2 splitting is investigated. Proposed Ce0.9A0.1O2−δ materials are prepared via co-precipitation of the hydroxide technique. The composition, morphology, and the average particle size of the Ce0.9A0.1O2−δ materials are determined by using suitable characterization methods. By utilizing a thermogravimetric analyzer setup, the long-term redox performance of each Ce0.9A0.1O2−δ material is estimated. The results obtained indicate that all the Ce0.9A0.1O2−δ materials are able to produce steady amounts of O2 and CO from cycle 4 to cycle 10. Based on the average n_{{{text{O}}_{2} }} released and n_{text{CO}} produced, the Ce0.899Sn0.102O2.002 and Ce0.895Ca0.099O1.889 are observed to be the top and bottom-most choices. When compared with the CeO2 material, all Ce0.9A0.1O2−δ materials showed elevated levels of O2 release and CO production.

Application of chromium oxide-based redox reactions for hydrogen production via solar thermochemical splitting of water

Thermodynamic equilibrium, as well as efficiency analysis of the Cr2O3/Cr water splitting (Cr-WS) cycle, was conducted in this study. The thermodynamic properties required for the computations were obtained from an HSC Chemistry 9.9 software. An increase in thermal reduction (TR) temperature (TH) from 1800 K to 2230 K was responsible for the rise in the percentage TR of Cr2O3 (Tr-Cr) from 0% to 100%. The equilibrium analysis additionally indicates that the re-oxidation of Cr into Cr2O3 via WS reaction is feasible at any temperature from 300 to 3000 K (we have selected 1300 K for this study). The efficiency analysis indicates that the Q̇solar-reactor-Cr-WS and Q̇solar-heater-Cr-WS were enhanced by 3636.8 kW and 260.0 kW due to the increment in the TH from 1800 K to 2230 K. The increase in the Q̇solar-reactor-Cr-WS and Q̇solar-heater-Cr-WS resulted into a rise in the Q̇solar-cycle-Cr-WS by 3896.8 kW. The ηsolar-to-fuel-Cr-WS increased from 9.5% to 26.4% when the TH was augmented from 1800 K to 2000 K. A further rise in the TH from 2000 K to 2230 K resulted in a reduction in the ηsolar-to-fuel-Cr-WS from 26.4% to 21.3%. After employing the 100% heat recuperation, theηsolar-to-fuel-HR-Cr-WS of the Cr-WS cycle was improved up to 48.3% at TH = 2000 K.

Solar thermochemical splitting of H2O using Ca-Ferrite based redox reactions: Effect of partial pressure of O2

The influence of the partial pressure of O2(PO2) on the viability of the Ca-ferrite based solar thermochemical H2O splitting cycle was thermodynamically scrutinized. The thermodynamic parameters were estimated by maintaining a steady thermal reduction (TR) and water splitting (WS) temperature equal to 1800 K and 1200 K, respectively. The reduction in the PO2 from 10−1 atm to 10−5 atm resulted in an improvement in the degree of non-stoichiometry (δ) associated with the Ca-ferrite from 0.172 to 0.720. Besides, a similar drop in the PO2increased the Q˙CaFe2O4−red−CFWS and Q˙H2O−heating−CFWS by 222.4 kW and 43.1 kW, separately. An upsurge in both Q˙CaFe2O4−red−CFWS and Q˙H2O−heating−CFWS significantly affected the Q˙solar−cycle−CFWS, and it was enhanced from 382.5 kW to 704.8 kW when the PO2 increased from 10−1 atm to 10−5 atm. Even though Q˙solar−cycle−CFWS was enlarged, due to the augmentation in H2 production, ηsolar−to−fuel−CFWS was escalated from 12.8% to 29.2% when the PO2 was decreased from 10−1 atm to 10−5 atm. By employing heat recuperation, the maximumηsolar−to−fuel−HR−CFWS (61.1%) was recorded for PO2=10−5atm.