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Two-step thermochemical water splitting (II-TWS), involving concentrating sunlight, has become prominent for green hydrogen production that does not require H2/O2 separation steps at high temperatures. The kinetics and thermodynamics of redox reactions are important factors that determine hydrogen production efficiency. This efficiency is strongly influenced by the structural properties of active materials used in II-TWS reactions. Perovskite oxides are one of the promising active materials for II-TWS due to their superior oxygen exchange abilities. In this study, La1-xCaxMn0.8Co0.2O3 (LCMC) type perovskites with a wide range of calcium substitution (x = 0, 0.2, 0.4, 0.6, 0.8) were examined for hydrogen production in terms of their structural properties, kinetics, O2/H2 production capacities, and cyclabilities. According to our test results, La0.8Ca0.2Mn0.8Co0.2O3 (LCMC8282) and La0.6Ca0.4Mn0.8Co0.2O3 (LCMC6482) displayed higher H2 production capacity with 256 imol g-1 and 88 imol g-1 as compared to the other selected perovskites. After three consecutive cycles, La0.8Ca0.2Mn0.8Co0.2O3 lost 83% of its H2 production capacity whereas La0.6Ca0.4Mn0.8Co0.2O3 preserved 61% of its H2 production capacity achieved in the first cycle.
Two-step thermochemical water splitting (II-TWS), involving concentrating sunlight, has become prominent for green hydrogen production that does not require H2/O2 separation steps at high temperatures. The kinetics and thermodynamics of redox reactions are important factors that determine hydrogen production efficiency. This efficiency is strongly influenced by the structural properties of active materials used in II-TWS reactions. Perovskite oxides are one of the promising active materials for II-TWS due to their superior oxygen exchange abilities. In this study, La1-xCaxMn0.8Co0.2O3 (LCMC) type perovskites with a wide range of calcium substitution (x = 0, 0.2, 0.4, 0.6, 0.8) were examined for hydrogen production in terms of their structural properties, kinetics, O2/H2 production capacities, and cyclabilities. According to our test results, La0.8Ca0.2Mn0.8Co0.2O3 (LCMC8282) and La0.6Ca0.4Mn0.8Co0.2O3 (LCMC6482) displayed higher H2 production capacity with 256 imol g-1 and 88 imol g-1 as compared to the other selected perovskites. After three consecutive cycles, La0.8Ca0.2Mn0.8Co0.2O3 lost 83% of its H2 production capacity whereas La0.6Ca0.4Mn0.8Co0.2O3 preserved 61% of its H2 production capacity achieved in the first cycle.
(ProQuest: ... denotes formulae omitted.)
INTRODUCTION
The results of the burning of fossil fuels, industrialization, and deforestation have led to an increase in the concentration of greenhouse gases, particularly carbon dioxide, in the world's atmosphere. Greenhouse gas emissions cause climate change that negatively affects the environment. In this regard, the use of hydrogen gas (H2) in transportation and stationary applications may offer a renewable and carbon-free solution to reduce greenhouse gas emissions.1,2 H2 is an energy carrier that has a much higher specific energy, 143 MJ kg-1, than any fossil fuel.2 H2 can be produced either from fossil fuels (i.e., oil, natural gas, etc.) or renewable energy sources (i.e., solar, wind/PV, etc.). Unfortunately, the major issue in the production of H2 from fossil fuels is the simultaneous generation of carbon dioxide as a by-product.3,4 Because of this, H2 must be obtained using renewable energy sources, especially solar energy, to overcome this problem. Recently, two-step thermochemical water splitting (II-TWS) has become one of the most prominent hydrogen production methods. Unlike single-step thermochemical water splitting (thermolysis), IITWS requires relatively lower reaction temperatures (< 2000 °C) without additional steps to separate O2 and H2. In this process, an active metal oxide (MOx) is thermally reduced under low partial oxygen pressure at high temperatures (much lower than 2000 °C) to drive the formation of oxygen vacancies, resulting in the production of O2, as shown in Eq. 1. Water vapor is then introduced to re-oxidize the reduced metal oxides (MOx-d) at slightly lower temperatures (typically 5001000 °C) followed by the liberation of H2 as a result of breaking the bond between hydrogen and oxygen.5-7 The oxidation reaction is shown in Eq. 2.
... (1)
... (2)
The expectations for an ideal redox-active metal oxide would include higher H2 production capacity, faster kinetics, lower reduction temperatures, higher cycle stability, being thermodynamically favorable, being non-hazardous, and having low cost.5,6
Redox-active materials for two-step thermochemical water splitting can be classified as volatile and non-volatile. Although volatile metal oxides exhibit high H2 productivity, they are not promising for two-step thermochemical water splitting because of the need for high temperatures for reduction, slow reduction-oxidation rates, and challenges in the separation process to prohibit the re-oxidation of reduced metal with the produced O2.5,6,8 Undoped magnetite (Fe3O4), as an attractive non-volatile material, has a high theoretical specific H2 production capacity. However, undoped magnetite suffers from the sintering of active material during the thermal reduction due to having a low melting temperature. This sintering leads to a decrease in the surface area, limiting oxygen production capacity.9 Even though the sintering problem appears to have been solved by metals such as Ni or Co-doped Fe3O4, thermochemical behaviors of Fe3O4 have not reached the desired level in terms of hydrogen production capacity and oxidation kinetics.10-12 In addition to this, ceria (CeO2) is the benchmark material that uses the oxygen vacancy mechanism for II-TWS. Ceria displays fast reduction kinetics and high cycle stability, even after hundreds of cycles. However, high reduction temperatures (« 1500 °C) to produce a reasonable amount of H2 preclude ceria becoming an ideal II-TWS material.13-16
In this context, perovskite-type oxides with the chemical formula of ABO3 can be proposed as a catalyst for solar-to-fuel applications because perovskites can accommodate relatively high amounts of oxygen vacancies and preserve their crystal structures under reducing and oxidizing environments.17 Moreover, oxygen sites, about 2%, are activated in cerium, while 16.7% of the total oxygen sites in a perovskite structure can be activated by two-step thermochemical water splitting.18 Also, perovskite-type oxides have the potential to reduce the temperature of the reduction (1300-1400 °C) and oxidation reaction (800-1000 °C) compared with ceria counterparts. Partial or total substitutions on the A- and/or B-sites govern the catalytic activity of perovskites by oxidation state modification, oxygen lattice mobility, oxygen vacancy generation, and the formation of the other structural defects. Herein, A-site cations serve to stabilize the unusual oxidation state of B-site cations by creating oxygen vacancies, whereas B-site cations designate the catalytic activity of the perovskite.19 In this regard, Sr-substituted LaMnO3 is the most studied perovskite for thermochemical H2O and CO2 splitting.20-22 A substituted lower charge metal cation (i.e., Sr2+, Ca2+, Ba2+) into the A-site of LaMnO3 causes the transformation of some of Mn3+ into Mn4+ in order to balance charge differences, which results in a significant increase in the extent of oxygen vacancy.16,20,23-28 Moreover, LaCoO3-based perovskite oxides can be proposed as suitable redoxactive materials for II-TWS due to their superior oxygen exchange performance. Further, it can be also noted that a perovskite material that includes cobalt promotes the generation of oxygen vacancies.29 Demont et al. carried out a range of investigations related to Sr-doped LaCoO3 and Sr, Fe doped LaCoO3, and found that the presence of cobalt provided a larger reduction extent at 1200 °C. However, La0.6Sr0.4CoO3 decomposed into Ruddlesden-Popper (RP) phase and cobalt oxides.20 Orfila et al. suggested that La0.8Sr0.2CoO3 was a promising material for II-TWS due to its redox stable performance and having relatively low reduction temperatures compared with Sr-doped LaMnO3.30 Furthermore, the Ca2+ ion can be substituted into the A-site of the lanthanum-cobaltite perovskite or the lanthanum-manganese perovskite. The ionic radius of Ca2+(coordination XII, 1.34 A) is cantly smaller Sr2+ (coordination XII, 1.44 A).31 An increment in the Ca doping content shrinks the lattice parameter, resulting in a change in the oxidation state of manganese.17 Wang et al. studied Ca2+ dopant as an A-site cation in LaMnO3 and LaCoO3. They observed that there is a linear relationship between the amount of Ca additive and the degree of reduction. However, the re-oxidation yield decreased when the Ca amount was higher than 40% for both LaMnO3 and LaCoO3.32,33 Nair et al. studied Mn-doped LaCoO3 for thermochemical CO production. Their findings showed that Mndoped LaCoO3 exhibited significantly lower oxygen production, ~ 83 imol/g than undoped LaCoO3 during the reduction step at 1400 °C while having a higher re-oxidation yield. Besides, it was observed that the manganese enhanced resistance to sintering and increased cycling performance stability.29,34 Recent studies have focused on perovskite oxides, and have shown that cobalt offers improved redox performance.10,30,33,35,36 Typically, a small amount of cobalt is preferred to avoid conspicuous grain growth during thermochemical cycles, since it is utilized as sintering agent.37
In this study, we investigated the redox behaviors of La1-xCaxMn0.8Co0.2O3 (LCMC) for H2O splitting to investigate the effect of A-site substitution by various amounts of Ca where x = 0, 0.2, 0.4, 0.6, and 0.8. The thermochemical cycles were performed between 1400 and 800 °C for 2 h to evaluate the oxygen and hydrogen production capabilities of the perovskite oxides. We discuss the influence of calcium to evaluate the structural stability in terms of the effects on O2 and H2 production and the thermochemical cycling efficiency of LCMC perovskite oxides.
EXPERIMENTAL PROCEDURE
The Lai_xCaxMn0.8Co0.2O3_d (LCMC) was synthesized by a sol-gel-based Pechini method.38 The compositions and their abbreviations for the perovskite oxides studied and discussed in this study are summarized in Supplementary Table S1, online.
Stoichiometric amounts of La(NO3)3·6H2O (99.99%, Alfa Aesar), Ca(NO3?·4?O (> 99.98% Alfa Aesar), Mn(NO3)2·4H2O (> 98.5%, Merck) and, Co(NO3)2·6H2O (> 97.7% Alfa Aesar) were dissolved in distilled water to synthesize the perovskite oxides. C6H8O7 (citric acid) (> 99.5%, Carlo Erba) was then added into the solution to form a metalcitrate complex at the ratio of 1.5 mol citric acid to 1 mol metal cations concentration. The solution was kept at 70 °C with continuous stirring until a gel formed. The ǧel was left overniǧht in a dryinǧ oven at 70 °C and pre-calcined at 250 °C for 2 h to remove residual organic substances and nitrates. Finally, the obtained resin was successfully calcined at 900 °C for 6 h to achieve the desired crystal structure. After the calcination, the structural and crystallographic analyses of the perovskites were performed by x-ray diffraction (XRD, Rigaku Smartlab) using Cu-Ka radiation. Diffraction angles (20) were measured in the range of 10°-80° with a scan rate of 2°· min-1. Rietveld refinement was utilized to calculate the phase fractions, lattice parameters, and unit cell volumes using MAUD.39 Scanning electron microscopy (SEM, JEOL JSM6400) was performed to characterize the particle morphology of perovskites synthesized. The Brunauer-EmmettTeller (BET) method was used to determine the specific surface area.
The surface composition was investigated by xray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe) for the as-synthesized perovskite oxides and after thermochemical cycles. La3d, Ca2p, Mn2p and Mn3s, Co2p, and O1s were examined within the range of 827 to 867, 342 to 361, 633 to 672 and 53 to 93, 765 to 815, and 520 to 540 in terms of binding energy, respectively. The spot area of the samples scanned was set at 100 im. Excitation of the samples was done using monochromatic Al Ka radiation with an analyzer that was at a 45° angle to the surface of the samples. Pass energy was 58.70 eV when examining the spectral scans of the elements.
Thermochemical cycling was performed to determine the O2 and H2 production capacities in a hightemperature furnace which was attached to a gas analyzer. Approximately 0.5 g of the perovskite powder was placed into the center of an alumina tube in an alumina crucible. High purity Ar gas (~ 99.999%) was used as a carrier gas with a flow rate of 100 SCCM. The reduction of the perovskite powders was carried out under an Ar atmosphere at 1400 °C. Then, 800 °C was set and the cooling rate was 5 °C min-1 to give the start for the second step. A syringe pump (New Era NE-300) was used to introduce H2O at a rate of 150 il/min into a vaporizer reactor to oxidize the reduced perovskite. During the thermal reduction and water splitting steps, the oxygen and hydrogen generated were measured by mass spectrometer (HIDEN QGA) and gas chromatography (Shimadzu). The perovskites were held for 2 h for both reduction and oxidation steps, and a schematic of the set-up for the thermochemical cycles is given in Fig. 1.
RESULTS AND DISCUSSION
Structural Characterization
Figure 2a depicts x-ray diffraction patterns of all LCMC perovskites. Among all the as-synthesized perovskites, LCMC4682 completely adopts the typical cubic ABO3 structure with a Pm3m space group (COD-4124852) while LCMC8282, LCMC6482, and LCMC2882 have a partial cubic structure. Likewise, LMC1082 was composed of a cubic and a rhombohedral perovskite structure (COD-1531294) without any secondary phases. The refined parameters and calculated phase fractions are summarized in Supplementary Table S2. The results showed that a relatively high amount, ~ 27.23% and ~ 13.75%, of ABO3 orthorhombic structure with a Pnma space group (COD-1520947), was observed for LCMC8282 and LCMC6482, respectively, whereas LCMC4682 had complete cubic symmetry. Interestingly, a remarkable amount of secondary phase Ca3MnCoO6 having a hexagonal symmetry with a space group (R3C) was observed for LCMC2882. The diffraction peak belonging to the (100) plane at ca. 20 = 23°, indicating that calcium stabilizes the cubic structure, disappeared with the increase in the amount of Ca.40 As the calcium content increased, there was a shift in the main reflections (ca. 20 = 32.5°), with the slight shift toward a greater diffraction angle due to the smaller ionic radius of Ca2+ (1.34 A) compared with La3+ (1.36 A), Fig. 1b. The lattice constant and unit cell volume decreased with increasing calcium dopant due to the smaller ionic radius of Ca, as mentioned above.
The surface morphology of the samples was characterized by SEM. After the calcination, it was observed that all perovskites synthesized had similar particle morphology which was independent of the choice of B-site doping element. As seen in Fig. 3, all LCMC perovskites have some spherical aggregates consisting of equiaxed primary particles which were ~ 100 nm. BET results, see Supplementary Table S2, showed that the specific surface area of LCMC perovskites significantly decreased when the calcium amount was equal to or bigger than x = 0.4. However, the specific surface area was lower at lower calcium amounts. The highest specific surface areas of the relative perovskites were 11.87 m2/g for LCMC6482, 8.72 m2/g for LCMC4682, 5. m2/g for LCMC8282, 4.41 m2/g for LCMC2882, and 1. m2/g for LMC1082.
Thermochemical Water Splitting
The oxygen and hydrogen production capacities of the perovskites investigated are demonstrated in Fig. 4. As seen in Fig. 4a, all O2 gas data were collected starting at 200 °C in order not to miss any important reduction events. The O2 release began at approximately 850 °C for LCMC6482, LCMC4682, and LCMC2882. On the other hand, LMC1082 displayed a different reduction behavior with two reduction regions. The first O2 release initiated at 600 °C and ended at ~ 1000 °C. Then, when the temperature reached 1350 °C, the second O2 release occurred. Contrarily, the reduction of LCMC8282 began above 1350 °C. The oxygen release rate was increased as the calcium dopant amount was increased from 0.2 to 0.8 for LCMC perovskites, apart from LMC1082. Total O2 production capacities were calculated as 186 imol g-1, 161 imol g-1, 275 imol g-1, 722 imol g-1, and 924 imol g-1 for LMC1082, LCMC8282, LCMC6482, LCMC4682, and LCMC2882, respectively. On the other hand, the hydrogen release rate started to decrease with an increase in Ca content, as shown in Fig. 4b. Just like in O2 production, LMC1082 broke this trend in terms of hydrogen production with 21 imol g-1. LCMC8282 displayed fairly high H2 production of 256 ımol g-1, whereas LCMC6482 exhibited much lower H2 production capacity with 69 imol g-1. Unfortunately, LCMC4682 and LCMC2882 were not sufficient in terms of hydrogen production during thermochemical water-splitting cycles. LCMC4682 and LCMC2882, havinǧ a hiǧher Ca content of more than 50%, showed rapid sintering and coarsening which limits the re-oxidation of the perovskite compared with the others. This may be the reason why it also has a high O2 production capacity during thermochemical water splitting. The O2 produced during the cycling was related to the instability of LCMC4682 and LCMC2882 perovskite oxides at the reaction temperature, and can be evaluated as not related to thermochemical water splitting reactions. The low H2 production capacity (< 30 imol g-1) also proved this behavior for LCMC4682 and LCMC2882.
The amount of oxygen released is summarized for the first thermochemical cycle in Supplementary Table S3. Out of the samples, LCMC8282 and LCMC6482 were considered worth calculating the re-oxidation efficiency for, due to their relatively higher H2 production capacities. In addition to that, higher O2 production yield shows that LCMC4682 and LCMC2882 decompose under the studied conditions durinǧ thermochemical cyclinǧ.
LCMC8282 and LCMC6482 perovskite oxides were subjected to three thermochemical cycles to reveal their O2 and H2 yield, re-oxidation capacity, and structural stability during cycling (Fig. 5). The temperature of the reduction reaction and water splitting reaction was 1400 °C and 800 °C for 2 h, respectively. The preserved amount of hydrogen yield by the end of the cycling was compared with the first cycle to evaluate the thermochemical cycling behavior of perovskite oxides.
It was observed that LCMC8282 produced 161 imol g-1, 183 imol g-1, and 159 imol g-1 of O2 for three consecutive cycles, respectively. Although LCMC8282 has comparable hydrogen production, a significant decrease was observed for the further cycles, see Supplementary Table S4. Also, the hydrogen production rate decreases as the number of cycles increase for both perovskites (LCMC8282 and LMC6482). Even though LCMC8282 has reasonable hydrogen production, it was not able to preserve its capacity for further cycles. The LMC6482, on the other hand, provided better conservation over the course of cycles, despite the lower hydrogen production amounts.
The highest O2 production rate of 0.046 imol s-1 g-1 was achieved by LCMC6482 at 1400 °C during the first cycle. In contrast, the kinetic behavior of LCMC8282 was much slower with 0.021 imol s-1 g-1. In contrast, LCMC8282 reached the maximum H2 production rate of 0.088 imol s-1 g-1 approximately 50 min later, and then decreased slowly. Nevertheless, the H2 continued to be produced at a rate of ~ 0.025 imol s-1 g-1 even after 2 h. In contrast to LCMC8282, LCMC6482 showed rather slow kinetic behavior with an H2 production rate of ~ 0.026 imol s-1 g-1. All O2 and H2 production rates obtained are summarized in Supplementary Table S5.
Figure 6 demonstrates the comparison of the reoxidation performance between LCMC8282 and LCMC6482. The re-oxidation efficiency of LCMC8282 dramatically decreased from 80 to 22% after the first thermochemical cycle, whereas LCMC6482 exhibited more stable H2 production behavior despite its lower H2 production. The H2 production capacity of LCMC8282 in the second and third cycles was 82 imol g-1 and 45 imol g-1, respectively. On the other hand, LCMC6482 has more stable hydrogen production which may be related to this composition having a stable structure after thermochemical cycles and preserving its cubic structure compared with LCMC8282, given Fig. 7.
Figure 7 depicts XRD patterns of cycled LCMC6482 and LCMC8282 perovskites. It was found that both LCMC8282 and LCMC6482 have a partially cubic structure. Refined XRD patterns are given in Fig. 8. and the calculated phase fractions are summarized in Supplementary Table S6. The results showed that a relatively high amount, - 27.23% and - 13.75%, of ABO3 orthorhombic structure with a Pnma space group (COD-1520947), was observed for LCMC8282 and LCMC6482, respectively. However, after the first cycle of LCMC8282, the orthorhombic structure becomes predominant, rather than the cubic structure, and a similar trend can be seen for the third cycle. On the other hand, although LCMC6482 mostly preserved its structure, the CaO phase was detected as a secondary phase which is less than 5 wt.% after the first cycle. Moreover, the amount of orthorhombic structure in LCMC6482 increased by the third cycle, whereas CaO remained at the same amount. It was observed that the higher number of cycles caused an expansion of the lattice parameter and unit cell volume for LCMC6482. In contrast to LCMC6482, the unit cell volume of LCMC8282 tends to increase with increasing cycle numbers.
Figure 9 shows SEM micrographs of LCMC6482 and LCMC8282 before and after thermochemical cycles. It was observed that particles coalesce with further cycles for LCMC perovskites. Although the initial particle size was smaller than 1 im, later particle sizes ranged between 5 and 10 im. Larger particles resulted in lower hydrogen production capacities for LCMC perovskite oxides.
In addition, the surface composition was examined by XPS. C1s was calibrated at 284.8 eV to be able to use it as a reference to examine the oxidation states of elements. The corresponding XPS spectra of La3d, Ca2p, Mn2p, Mn3s, Co2p, and O1s of LCMC6482 and LCMC8282 are given in Fig. 10 and Fig. 11, respectively. La3d spectra of LCMC6482 and LCMC8282 before and after relative thermochemical cycles exhibit two doublet peaks, La3d3/2 and La3d5/2 with two splitting peaks indicating lanthanum has 3 + for all samples. The energy difference between La3d3/2 and La3d5 was measured at - 17 eV, corresponding to La3+ ions.41,42 The energy differences between the La 3d5/2 doublets were around 3.9 eV, corresponding to La(OH)3, except for the XPS spectrum of LCMC6482 after the first cycle, which was around 4.6 eV, indicating La2O3. The measured La 3d5/2 splitting energy, corresponding to the carbonates, was the result of the tendency of the sample surface to the oxygen during oxidation step.43,44
The Ca2p spectra had spaced spin-orbit components with energy differences which were around 3.5 eV, indicating CaCO3 for all samples.45,46 The Ca2p3/2 component centered at around 345 eV represents Ca2 + valance states. On the other hand, the Ca2p3/2 component at around 346.9 eV can be attributed to the formation of CaO on the surface of perovskite,47,48 as confirmed by XRD refinement results.
Mn2p spectra of LCMC6482 and LCMC8282 before and after relative thermochemical cycles are given in Figs. 10 and 11, respectively. The main Mn2p3/2 was observed between 640 and 642 eV for all samples. Although MnO2 has a much narrower and distinguishable profile at the top of the Mn2p3/ 2 peak when compared with other possibilities (MnO or Mn2O3), it was hard to be sure that the valence state of manganese was 4 + . Thus, Mn3s spectra were collected in order to measure the magnitude of peak splitting to diagnose oxidation states for all samples. It was concluded that manganese has a 4 + oxidation state since the energy differences between the Mn3s splitting were measured at around 4.7 eV for all samples.49,50
Positions of Co2p3/2 and Co2p1/2 components were measured at around 778-779 eV and 795-796 eV, respectively. This indicated the presence of Co3+ for all samples except the samples corresponding to before thermochemical cycles. Since LCMC6482 and LCMC8282 oxides had an observable satellite feature at - 786 eV,51,52 one can say that both samples had a 2 + valence state before thermochemical cycling.
There are two split components for O1s spectra, as given in Fig. 10e. The first component appeared at around 528.5 eV which corresponds to lattice oxygen, whereas the second one at around 531 eV corresponds to adsorbed oxygen.53 Lattice oxygen is the oxygen which bonds with the other elements in the perovskite structure. On the other hand, adsorbed oxygen is typically related to the adsorbed humidity from the air, having a hydroxyl group.54-57 The ratio of lattice oxygen to adsorbed oxygen can be utilized to determine the performance of the oxygen carriers during thermochemical cycling.41 This ratio is 1.628 before thermochemical cycling and decreases to 1.050 and 1.005 during further cycles for LCMC6482. Similarly, this ratio has a higher value of 1.944 before cycling, followed by 1.098 and 1.291 for LCMC8282. This relationship is coherent with the results obtained in the thermochemical water splitting cycles.
CONCLUSION
The series of La1-xCaxMn0.8Co0.2O3 perovskites (LCMC) (where x = 0, 0.2, 0.4, 0.6, 0.8) were synthesized by the sol-gel-based Pechini method for two-step thermochemical cycles. The effect of Ca dopant amount on the structural, morphological, and redox properties of synthesized perovskites was investigated.
The structural study revealed that all as-synthesized perovskites, except LCMC2882, have a mixture of perovskite structures composed of cubic, orthorhombic, and rhombohedral space groups: Pm3 3 Pnma, and R-3c, respectively. As a secondary phase in LCMC2882, quasi-one-dimensional Ca3CoMnO6, which is rhombohedral, space group R-3c, exists at 14.25 wt.%. The perovskite samples investigated had similar particle morphologies, with some spherical aggregates which consist of equiaxed primary particles of ~ 100 nm. The highest specific surface area, 11.87 m2/g, belongs to LCMC6482.
In the thermal reduction step, it was observed that the O2 release rate increased as the calcium amount increased. However, a Ca amount higher than 50% led to a decomposition reaction that occurred at 1400 °C in the Ar atmosphere. In the re-oxidation step, LCMC8282 and LCMC6482 exhibited relatively much higher H2 production capacities with 256 imol g- and 88 imol g-1, respectively, and both perovskites were deemed worthy of running through further thermochemical cycles. The hydrogen production rate declined as the number of cycles increased for both perovskites. The highest O2 production capacity of 275 imol g-1 was obtained with an O2 production rate of 0.046 imol s-1 g-1 in LCMC6482. We observed that the H2 production rate belonging to LCMC8282 reached a maximum point at ~ 0.088 imol s-1 g-1 after 50 min. After three consecutive cycles, LCMC8282 lost 83% of the H2 production capacity it achieved in the first cycle, whereas only 29% of the H2 production capacity of LCMC6482 could not be preserved. After two-step thermochemical water splitting, LCMC8282 possessed a completely perovskite structure without any secondary phases. Even though LCMC6482 substantially adopts a perovskite structure, the CaO phase was detected as an impurity that was less than 5 wt.% after the first cycle.
The present work makes it clear that increasing the Ca additive up to a certain amount of x = 0.4 results in a reasonable improvement in the amount of hydrogen production. However, the cycling behaviors need to be further investigated under longer consecutive cycles focusing on the improvement method for the structural stability of LCMC perovskite oxides. Although our main results call for further studies correlating the structure-stability of LCMC perovskite oxides, the perovskite oxides examined in this study may shed more light on newer compositions for thermochemical water splitting studies.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge TUBITAK (The Scientific and Technological Research Council of Turkey) for their support and funding (project number 119M420). The authors also wish to acknowledge Prof. Dr. Mehmet Oztürk and Prof. Dr. Mehmet Emin Duru for support with mass spectrometer analysis.
CONFLICT OF INTEREST
The authors declare that they have no conflict of interest.
SUPPLEMENTARY INFORMATION
The online version contains supplementary material available at https://doi.org/10.1007/s11837022-05493-9.
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