Solar Thermochemical H2O Splitting Research Articles

High-entropy spinel oxide (Fe0.2Mg0.2Mn0.1Al0.3Cr0.2)3O4 as a highly active and stable redox material for methane driven solar thermochemical water splitting

Solar thermochemical H2O splitting has attained wide attention as a promising solution for hydrogen production. However, it remains a daunting challenge to produce hydrogen with high kinetics and stability for the oxide materials at lower temperature because of insufficient active sites for H2O activation and splitting. Herein, we report a high-entropy spinel oxide that consists of five cations (HEO) and can highly-effectively thermochemically split H2O to H2 with ultrahigh production rate and productivity of 182.9 mL min-1 g-1 and 68.5 mL g-1, respectively, several times higher than state-of-the-art materials via a two-step cycle process with methane driven reduction. High-entropy effect facilitated the preservation of single-phase structure even at large amount of oxygen converted (almost 70 mL g-1), which promoted the exsolution and stabilization of substantial Fe0 nanoparticles (20–30 nm vs. 100 nm) and their dissolution into spinel structure during redox process, resulting in extensive metal-oxide interfaces with metal-oxygen vacancy pairs responsible for the efficient and stable water splitting. Such findings provide a class of viable material with high configurational entropy for efficient hydrogen generation.

Thermodynamic screening of candidate ceria and perovskite systems as potential oxygen carrier materials for chemical looping water gas shift

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-δ), CZO20 (Ce0.8Zr0.2O2-δ), and LSMA4060 (La0.6Sr0.4Mn0.4Al0.6O3-δ) 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 °C, respectively. Results show that only CZO20 is viable for isothermal operation, but higher temperatures are required; at 1023 °C, syngas and steam conversions are 80 and 12%.

Reactivity and stability of Zr–doped CeO2 for solar thermochemical H2O splitting in combination with partial oxidation of methane via isothermal cycles

Reactivity and stability of Zr–doped CeO2 for solar thermochemical H2O splitting in combination with partial oxidation of methane via isothermal cycles

Thermodynamic study of the effect of partial thermal reduction of dysprosium oxide on solar-to-fuel energy conversion efficiency

A thermodynamic equilibrium and efficiency analysis of the dysprosium oxide-based solar thermochemical H2O splitting (Dy-WS) cycle is conducted. The objective of this study is to understand the effect of partial thermal reduction (TR) of Dy2O3 on the solar-to-fuel energy conversion efficiency (ηsolar-to-fuel-Dy-WS) of the Dy-WS cycle. The equilibrium analysis indicate a rise in the percentage TR of Dy2O3 (%TR-Dy) from 0.3% up to 100% when the TR temperature (TH) is increased from 2000 K to 2530 K. The upsurge in the TH yielded a considerable surge in the Q̇solar-reactor-Dy-WS and Q̇solar-heater-Dy-WS. Overall results of this study showed that the ηsolar-to-fuel-Dy-WS is amplified from 0.6% to 6.5% in three zones (slow zone, medium-fast zone, and fast zone) as the TH is amplified from 2000 K up to 2280 K. A further rise in the TH from 2280 K up to 2530 K resulted in a drop in the ηsolar-to-fuel-Dy-WS from 6.5% to 3.5%. By employing the heat recuperation, the ηsolar-to-fuel-HR-Dy-WS is improved further up to 11.4% (at TH = 2280 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.

Thermodynamic analysis of Ni-ferrite based solar thermochemical H2O splitting cycle for H2 production

Abstract In this paper, a comprehensive thermodynamic analysis of Ni-ferrite based H2O splitting cycle is performed using HSC Chemistry 7.1 software and its thermodynamic database. The influence of partial pressure of O2 ( P O 2 ) in the inert N2 and thermal reduction temperature ( T H ) on δ (degree of nonstiochiometry) introduced in the Ni-ferrite crystal structure is investigated. With the increase in the P O 2 in the inert N2 and T H the δ increases, which results into higher levels of H2 production via H2O splitting reaction. Variations in the other thermodynamic process parameters such as total amount of solar energy required to run the cycle ( Q ˙ s o l a r − c y c l e ) , total amount of heat energy re-radiated from the cycle ( Q ˙ r e − r a d − c y c l e ) , amount of heat energy released by water splitting reactor ( Q ˙ s p l i t t i n g − r e a c t o r ) , cycle ( η c y c l e ) and solar-to-fuel energy conversion efficiency ( η s o l a r − t o − f u e l ) , and others as a function of P O 2 in the inert N2, T H , and water splitting temperature ( T L ) are also researched. The η c y c l e and η s o l a r − t o − f u e l upsurges with the reduction in the P O 2 in the inert N2, T H , and T L . For instance, η c y c l e = 29.5 % and η s o l a r − t o − f u e l = 35.5 % can be achieved at P O 2 in the inert N2 = 10−5 atm, T H = 1600 K , and T L = 1000 K .

Thermodynamics of paired charge-compensating doped ceria with superior redox performance for solar thermochemical splitting of H2O and CO2

Paired charge-compensating doped ceria has great potential for solar thermochemical splitting of H2O and CO2 because of its balanced reduction and oxidation properties.

Generation of H2 and CO by solar thermochemical splitting of H2O and CO2 by employing metal oxides

Generation of H2 and CO by splitting H2O and CO2 respectively constitutes an important aspect of the present-day concerns with energy and environment. The solar thermochemical route making use of metal oxides is a viable means of accomplishing these reduction reactions. The method essentially involves reducing a metal oxide by heating and passing H2O or CO2 over the nonstoichiometric oxide to cause reverse oxidation by abstracting oxygen from H2O or CO2. While ceria, perovskites and other oxides have been investigated for this purpose, recent studies have demonstrated the superior performance of perovskites of the type Ln1−xAxMn1−yMyO3 (Ln=rare earth, A=alkaline earth, M=various +2 and +3 metal ions), in the thermochemical generation of H2 and CO. We present the important results obtained hitherto to point out how the alkaine earth and the Ln ions, specially the radius of the latter, determine the performance of the perovskites. The encouraging results obtained are exemplefied by Y0.5Sr0.5MnO3 which releases 483µmol/g of O2 at 1673K and produces 757µmol/g of CO from CO2 at 1173K. The production of H2 from H2O is also quite appreciable. Modification of the B site ion of the perovskite also affects the performance. In addition to perovskites, we present the generation of H2 based on the Mn3O4/NaMnO2 cycle briefly.

Oxygen nonstoichiometry, defect equilibria, and thermodynamic characterization of LaMnO3 perovskites with Ca/Sr A-site and Al B-site doping

This work encompasses the thermodynamic characterization of four doped lanthanum manganites, namely La0.6A0.4Mn1−yAlyO3 (A = Ca, Sr and y = 0, 0.4), all showed to be promising redox materials for the solar thermochemical splitting of H2O and CO2 to H2 and CO. We present oxygen nonstoichiometry measurements in the temperature range T = 1573 K–1773 K and oxygen partial pressure range pO2 = 4.5066 × 10−2 bar–9.9 × 10−5 bar. For a given T and pO2, oxygen nonstoichiometry is found to be higher when replacing the divalent dopant Sr in La0.6Sr0.4MnO3 by the divalent Ca but also increases significantly when additionally doping 40 mol-% Al to the Mn-site. La0.6Ca0.4Mn0.6Al0.4O3 revealed the highest mass specific oxygen release, 0.290 mol O2 per kg metal oxide at T = 1773 K and pO2 = 2.360 × 10−3 bar and 0.039 mol kg−1 at T = 1573 K and pO2 = 4.5066 × 10−2 bar. It is shown that the chemical defect equilibrium of all four perovskites can be accurately described by the two simultaneous redox couples Mn4+/Mn3+ and Mn3+/Mn2+. Thermodynamic properties, namely partial molar enthalpy, entropy and Gibbs free energy are consequently extracted from the defect models. Partial molar enthalpy decreases with increasing oxygen nonstoichiometry for the Al-doped perovskites whereas the opposite trend is observed for the others. The enthalpy falls within the range 260–300 kJ mol−1 for all the materials. Equilibrium hydrogen yields upon oxidation with H2O are determined as a function of redox conditions. Although reduction extents of the perovskites are greater compared to CeO2, oxidation with H2O and CO2 is thermodynamically less favorable. This leads to lower mass specific fuel productivity compared to CeO2 under most conditions relevant for solar thermochemical cycles.

Lanthanum Manganite Perovskites with Ca/Sr A‐site and Al B‐site Doping as Effective Oxygen Exchange Materials for Solar Thermochemical Fuel Production

AbstractPerovskite oxides have recently been proposed as promising redox intermediates for solar thermochemical splitting of H2O and CO2, offering the benefit of significantly reduced operating temperatures. We present a systematic experimental screening of doped lanthanum manganites within the composition space La1−x(Ca,Sr)xMn1−yAlyO3 and identify several promising redox materials. In particular, La0.6Sr0.4Mn0.6Al0.4O3 and La0.6Ca0.4Mn0.6Al0.4O3 boast a five‐ to thirteen‐fold improvement in the reduction extent compared to the state‐of‐the‐art material CeO2 in the temperature range 1200–1400 °C. The materials are shown to be capable of splitting CO2 into CO fuel when isothermally cycled between low‐pO2 and high‐pCO2 environments at 1240 °C and to approach full reoxidation in CO2 with temperature swings as low as 200 °C, with mass‐specific fuel yields up to ten times that of CeO2. The underlying material thermodynamics are investigated and used to explain the favorable redox behavior.