Compositionally Complex Perovskite Oxides for Solar Thermochemical Water Splitting

Solar thermochemical hydrogen generation (STCH) is a promising approach for eco-friendly H2 production, but conventional STCH redox compounds often suffer from thermodynamic and kinetic limitations with limited tunability. Expanding from the nascent high-entropy ceramics field, this study explores a new class of compositionally complex perovskite oxides (La0.8Sr0.2)(Mn(1-x)/3Fe(1-x)/3CoxAl(1-x)/3)O3 for STCH. In situ X-ray diffraction demonstrates the phase stability during redox cycling and in situ X-ray photoelectron spectroscopy shows preferential redox of Co. The extent of reduction increases, but the intrinsic kinetics decreases, with increased Co content. Consequently, (La0.8Sr0.2)(Mn0.2Fe0.2Co0.4Al0.2)O3-{\delta} achieves an optimal balance between the thermodynamics and kinetics properties. The combination of a moderate enthalpy of reduction, high entropy of reduction, and preferable surface oxygen exchange kinetics enables a maximum H2 yield of 395 +- 11 {\mu}mol g-1 in a short 1-hour redox duration. Entropy stabilization expectedly contributes to the structure stability during redox without phase transformation, which enables an exceptional STCH stability for>50 cycles under harsh interrupted conditions. The underlying redox mechanism is further elucidated by the density functional theory based parallel Monte Carlo computation, which represents a new computation paradigm first established here. This study suggests a new class of non-equimolar compositionally complex ceramics for STCH and chemical looping.


Keywords:
Solar thermochemical hydrogen production Water splitting Perovskite Compositionally complex ceramics High-entropy ceramics A new class of compositionally complex perovskite oxides (La0.8Sr0.2)(Mn(1-x)/3Fe(1-x)/3CoxAl(1-x)/3)O3 were discovered for two-step redox solar thermochemical hydrogen production by expanding the emerging field of high-entropy ceramics to non-equimolar compositional complex ceramics. The in-situ characterization and density functional theory based parallel Monte Carlo computation elucidated the redox sequence and underlying mechanism.

Introduction
Hydrogen has critical applications in modern industries and energy systems for future hydrogen economy. [1][2][3][4][5] To realize sustainable sunlight-driven water splitting and store the intermittent solar energy, two-step solar thermochemical hydrogen production (STCH) has attracted considerable attention due to its capability of utilizing the full solar spectrum energy and potential to generate industrial-scale quantities of solar fuels. [6][7][8][9][10][11] The two-step STCH process involves an endothermic reduction step at a high temperature (≥1200 C) under low P O 2 with a suitable non-stoichiometric redox metal oxide to release oxygen and a subsequent oxidation step via flowing steam to the reduced oxide at a relatively lower temperature (~800-1100 C). The most investigated non-stoichiometric metal oxide materials for STCH are undoped and doped ceria materials due to their structural stability and fast redox kinetics. [12][13][14][15] However, ceria suffers from the requirement of extremely high reduction temperature (>1500 C) to reach a small extent of reduction (Δδ ≈ 0.03-0.06), challenging the design of suitable reactors. [14,16] The perovskite oxides (ABO3-δ) are considered as a promising alternative candidate to ceria for STCH, as they have favorable merits including structural stability under large non-stoichiometry redox swing, tunable defect chemistry with A/B site doping, and a large compositional space. [17,18] For instance, Ca(Ti0.5Mn0.5)O3 [19] and (La1-xSrx)(Mn1-yAly)O3 [20] perovskites have been investigated for STCH. Even though perovskites can have much larger redox capacities (Δδ > 0.15), longer reaction time (total dwell time for reduction and oxidation steps > 2 h) is usually required to reach a fair H2 yield in the reports. [21][22][23] Understanding of the tradeoff between thermodynamic and kinetics properties is needed to optimize the perovskite composition to achieve high H2 yield within a short time ( 1h).
High-entropy ceramics (HECs) have been synthesized in several material families including rocksalt, [24,25] perovskite, [26] and fluorite [27][28][29] oxides, as well as other non-oxides such as borides, [30] carbides [31] and silicides. [32] Notably, an equimolar poly-cation (Fe0.25Mg0.25Co0.25Ni0.25)Ox was reported for STCH undergoing reversible phase transformation between the spinel and rocksalt structure for 10 cycles. [33] However, the repetitive phase transformation is not desirable for long-term cycling stability, reversibility, and oxygen exchange kinetics due to its larger energy barrier. Fe was proposed as the redox active element, although the ex-situ characterization in that work may not precisely track the instantaneous redox change and the redox center valence may subject to change in the quenching/cooling process. It remains unclear why a particular cation dominates the redox chemistry and what factors govern the priority of redox behavior in the HECs during the STCH process. Recently, compositionally complex ceramics (CCCs) have been introduced to broaden the field of HECs by including non-equimolar compositions that reduce the configurational entropy but allow more engineering space to improve the properties. [34,35] CCCs can provide additional tunability of the physical properties (e.g. thermomechanical properties in compositionally complex fluorite-based oxides). [36,37] Here, we hypothesize that a vast compositional space in compositionally complex pervoskite oxides (CCPOs) will enable us to create a new class of stable STCH materials, where non-equimolar compositional designs can be leveraged to balance the thermodynamic and kinetic properties.
Unlike (Fe0.25Mg0.25Co0.25Ni0.25)Ox [33] and BaCexMn1-xO3, [38] this LS_MFCxA series of CCPOs did not show any phase transformation during the STCH redox cycling for x < 0.61. In this series, the thermodynamic and intrinsic kinetics properties showed different correlations with the Co content.
LS_MFC0.4A or (La0.8Sr0.2)(Mn0.2Fe0.2Co0.4Al0.2)O3 (x = 0.4), showed the best balance between the thermodynamics and kinetics properties, rendering a high hydrogen yield of 395 ± 11 μmol g -1 within a short 1 h duration at an optimized STCH condition. In situ characterization indicated that Co dominated the redox. Furthermore, Monte Carlo (MC) sampling based on density functional theory (DFT) was applied in this field, for the first time to our knowledge, to demonstrate that oxygen vacancies prefer to form on the Co octahedron position among all B-site metal octahedron positions and the valence change of Co is the most obvious. Furthermore, we found that the Co-O bond in the CCPO is substantially weakened in comparison to both Mn-O, Al-O and Fe-O bonds and their respective simple perovskites, rationalizing the experimental observations. Moreover, LS_MFC0.4A exhibited exceptional structural stability and cycling durability even after 51 cycles under very harsh interrupted cycling conditions involving startup heating and shutdown cooling, which simulates the real-world day-night cycle conditions for STCH. This work opens up a new direction to explore novel CCCs for STCH and provides a new computation paradigm for mechanistic understanding of redox chemistries of STCH materials.   Table 1 shows the sample abbreviations and nominal compositions for this LS_MFCxA series of CCPOs. The X-ray diffraction (XRD) pattern ( Figure 1a) reveals that LS_MFCxA adopted a rhombohedral structure (R3 c), with the peak splitting at 2θ of 32.8 o (Figure 1b). Rietveld refinements confirm the R3 c phase for all compositions by assuming random B site occupation of Mn, Fe, Co, and Al ( Figure S1 and Table S1, Supporting Information). As the Co content increases in LS_MFCxA, the peak splitting becomes larger due to more severe lattice distortion and the diffraction peaks gradually shift to higher 2θ angle indicating the decrease of lattice parameters. To probe the structural stability of LS_MFCxA under thermally reducing environments, LS_MFC0.16A was selected to perform hightemperature XRD. Figure 1c displays that LS_MFC0.16A showed structural stability at 1350 C and maintained the R3 c rhombohedral structure without phase transition under the reducing conditions, except for the slight peak shifts (lattice expansion) due to the thermal expansion and oxygen loss. After the O2 uptake at 800 C, the lattice was recovered to the initial state. Typical HT-XRD patterns were refined ( Figure S2) showing the crystallographic structure was maintained during the thermal reduction and oxidation conditions. The atomic structure and elemental distribution of LS_MFC0.25A were characterized by the scanning transmission electron microscopy (STEM) high-angle annular dark field (HAADF) imaging with the energy dispersive spectroscopy (EDS) (Figure 1d), which suggested a homogenous elemental distribution of cation elements. The XRD and EDS mapping results indicate a long-range random B site occupation. To quantify the redox capability, LS_MFCxA samples were tested by temperature-programmed reduction (TPR) in thermogravimetric analysis (TGA). [39] Figure 2a shows slightly increased indicating pyrolysis of the surface absorbed La(OH)3 species at 800 C. Sr also exhibited a side peak evolution, which was assigned to slight Sr segregation that is widely found in solid oxide fuel cells. [40][41][42] For the B-site elements, Mn, Fe, and Co can be potentially redox active. Al 2p showed negligible shift due to the stable Al 3+ valence state. Co 2p showed an increased intensity and broadening of the side peak located at 787 eV, which is a Co 2+ 2p3/2 satellite peak. This satellite peak evolution suggests that Co 3+ is reduced to Co 2+ during the in-situ heating process, in agreement with previous reports. [43][44][45] In contrast, Fe 2p and Mn 2p had negligible changes upon heating. Meanwhile, the ex-situ STEM electron energy loss spectroscopy (EELS) of LS_MFC0.4A demonstrates that only Co has obvious valence change ( Figure S6, Supporting Information). Therefore, Co is likely the primary redox element, consistent with the correlation between Δδ and Co content in the TPR. The dependences of Δδ (measured by TPR) and oxygen surface exchange coefficient kex (measured by electrical conductivity relaxation) on the Co content in LS_MFCxA. The thermodynamic (Δδ) and kinetic (kex) properties demonstrate opposite correlation with Co molar ratio in LS_MFCxA. c) H2 production stability of LS_MFC0.4A under different flow rate during the uninterrupted cycling (TRe = 1350 C for 30 min in N2 and TOx = 1100 C for 30 min in 40 vol% steam). d) Long-term STCH cycling of LS_MFC0.4A under harsh interrupted cycling conditions involving startup heating and shutdown cooling, at a STCH condition producing significant (but not maximized) H2 yield. Specifically, the sample was heated in the morning and cooled down to room temperature at night and four cycles were performed per day (using the STCH condition: TRe = 1350 C for 45 min in N2 and TOx = 1100 C for 15 min in 40 vol% steam with a gas flow rate of 100 sccm). This simulates the real day-night cycle conditions for STCH than uninterrupted cycling. Background H2 production from catalytic water thermolysis was subtracted from the total H2 yield and only the H2 production from the redox reaction was illustrated.
The STCH performances of LS_MFCxA materials were investigated using a homemade flow reactor ( Figure S7, Supporting Information). It is worth noting that the water thermolysis catalyzed by the alumina tube and samples could take place at 1100 C in 40 vol% steam flow producing a small quantity of H2, which has been recently reported. [46][47][48] Thus, the H2 production from direct catalytic water splitting was measured ( Figure S8, Supporting Information) and subtracted from the total H2 yield to extract the true H2 yield from the redox reaction. Figure 3a shows H2 yields of various LS_MFCxA under the given conditions. The hydrogen yield increased from LS_MFC0.16A to LS_MFC0.4A and then rapidly fell after x = 0.61. The relationship of H2 yield with the Co content has a volcano shape with the peak H2 yield achieved by LS_MFC0.4A, similar to the observed trend in La(Ga1-yCoy)O3. [21] Under this condition, LS_MFC0.4A produced the highest H2 yield (219 μmol/g) although it had a medium Δδ. In contrast, the simple perovskite LSC showed the minimal H2 yield (24 μmol/g) despite its largest Δδ. The molar ratio of H2 to O2 produced in the oxidation and reduction steps, respectively, is 2:1 ( Figure S9), suggesting the water splitting is realized by the redox reaction.
The volcano shaped plot of hydrogen production vs. Co molar ratio may relate to the trade-off between the thermodynamic and kinetic properties as shown in Figure 3b. The intrinsic kinetic properties of the LS_MFCxA series were examined by using the electrical conductivity relaxation (ECR) technique, which can quantify the oxygen surface reaction coefficient kex (i.e., the proportionality between the rate of oxygen incorporation and deviation of surface oxygen concentration) by monitoring the response of electrical conductivity to the change of P O 2 . [49,50] It is found that the thermodynamic and kinetic properties can be readily tailored by tuning the Co content in LS_MFCxA. Figure 3b illustrates an opposite correlation of the thermodynamic (Δδ) and kinetics (kex) properties with the Co content, where more Co decreases the kinetics but increases Δδ. In this series, LS_MFC0.4A likely achieved a balance between the thermodynamic and kinetics properties, delivering the highest H2 yield within the given duration. The oxygen uptake and release tests further confirm the superior oxygen incorporating kinetics of LS_MFCxA with a lower Co content ( Figure S10, Supporting information). Additionally, LS_MFCxA with a high Co content (x > 0.52) showed inferior structural stability, where the formation of secondary RP phase ( Figure   2c and d) is unfavorable for the H2 production. Therefore, we focused on the optimal composition LS_MFC0.4A to further investigate its STCH performance. The hydrogen production is influenced by the testing conditions, such as the gas flow rate, reaction temperature (TRe and TOx) and reaction time (tRe and tOx). Therefore, the effects of these factors on the H2 yield of LS_MFC0.4A were systematically investigated. The reduction temperature was fixed at 1350 C. Table 2 summarizes the cumulative H2 production in the stabilized cycle under different conditions. A larger temperature swing by reducing the oxidation temperature (C1, C2 and C3) reduced the H2 yield in the 1-hour cycle, although the larger temperature swing is supposed to thermodynamically favor the H2 production. We believe that the oxidative water splitting step is primarily limited by the kinetics. The elevated oxidation temperature of (C3) rendered H2 production four times higher than the lower oxidation temperature (C1), as the former promoted the reaction kinetics. Furthermore, when TOx was higher than 1200 C, the alumina tube produced a substantial H2 background level arising from its catalytic water thermolysis; hence, we did not further investigate higher oxidation temperatures. The extended oxidation time enhanced the H2 yield, confirming the kinetically limited oxidative water splitting step. Increasing the flow rate of steam also boosted the H2 yield from 270 μmol g -1 at 100 sccm to 395 μmol g -1 at 200 sccm within 30-min oxidation time likely due to the improved mass transfer of steam and rapid removal of H2 product. The highest H2 yield of 395 μmol g -1 in a short duration (30 min oxidation and 30 min reduction) for LS_MFC0.4A is comparable to those of best reported reported STCH materials (Table S2, Supporting Information), indicating the great potential of CCPOs for STCH.
We investigated the cyclability of LS_MFC0.4A under both uninterrupted and harsher interrupted conditions. Figure 3c exhibits the uninterrupted cycling test results (similar to most prior reports). After the initial two activation cycles, LS_MFC0.4A maintained a stable H2 yield of 270 ± 9 and 395 ± 11 μmol g -1 at a steam flow rate of 100 and 200 sccm for the following 10 consecutive cycles, respectively. The particle morphology and EDS mapping of pristine and cycled LS_MFC0.4A are shown in Figure S11. The LS_MFC0.4A after 12 cycles did not show significant element segregation despite apparent sintered particles.
Note that the seemingly inhomogeneity in EDS maps of some elements such as Sr and Al compared to other elements is owing to the different element interaction volume rather than their inhomogeneous distribution. For the practical STCH applications, the diurnal cycle of sunlight irradiation should be considered without the viable efficient heat storage technology, as the STCH materials may subject to drastic temperature swing between elevated temperatures under 8-hour strong sunlight irradiation in the day and room temperature of long-time cooling stage in the night. In stark contrast to the ideal uninterrupted cycling condition, such interrupted cycling involving startup heating and shutdown cooling is a harsh condition to the STCH application, which can induce thermal fatigue, impair the structural integrity and dramatically deteriorate the H2 production cycle. [51,52] A similar challenge exists in other solar-driven hydrogen production technologies such as photovoltaic driven water electrolysis, where the startup and shutdown cycling due to the day-night cycle causes degradation of electrocatalysts. [53][54][55][56] Few research on development of STCH materials has paid attention to the cyclability under this harsh condition. [51] Therefore, the interrupted cycling stability of LS_MFC0.4A for thermochemical hydrogen production was evaluated under the daily startup and shutdown cycling for 13 days (51 cycles) to simulate the real diurnal cycle (we note the more cycles per day can be carried out in a real reactor with faster ramping rates, in comparison with our home-made testing reactor).

Figure 3d
demonstrates that LS_MFC0.4A had outstanding stability against the thermal fatigue and redox degradation without significant decay in H2 production under this harsh condition. This is possibly attributed to the no phase transformation of the CCPO during redox cycling and a possible contribution from entropy stabilization, which make our CCPO stand out among other state-of-the-art STCH materials (Table S2) Figure 4b shows the respective magnetic moment magnitude changes of Mn, Fe, and Co in bulk and vacancy configurations. Before the vacancy is formed (i.e., bulk), the median magnetic moment magnitudes for Mn, Fe, and Co are 3.0, 4.0, and 2.9, corresponding to oxidation states of Mn 4+ , Fe 3+ , and Co 3+ , respectively. After introducing a single oxygen vacancy, Co is the element that has the greatest magnetic moment magnitude change (Co 3+  Co 2+ ) and broadest distribution.
The magnetic moment distribution of Mn is slightly broadened. The magnetic moment distribution of Fe is almost unchanged. Therefore, the Co 3+ /Co 2+ redox pair is the most active in response to the formation of oxygen vacancies in the LS_MFCxA systems, possibly followed by Mn 4+ /Mn 3+ . A possible reason for the Co preference in vacancy configurations is the Co-O bond stretching effect.
Since neutral vacancy formation causes volume expansion in many oxides, and a tensile stress (bond stretching) in the bulk will lower the vacancy formation energy. [57][58][59] In the CCPOs, local bond stretching may create more space to favor oxygen vacancy formation. To capture the local bond strength, the bond valence sum (BVS) descriptor was analyzed (see details in Supporting Information). [60] The BVS for each B-site cation in a perfect structure is close to its oxidation number. A lower (higher) value is considered as an elongated (compressed) bond with weaker (stronger) bond strength. For example, using the tabulated parameters (developed by fitting experiments [61] and listed in Table S3), the BVS for Mn 4+ in SrMnO3, Fe 3+ in LaFeO3, and Co 3+ in LaCoO3, and Al 3+ in LaAlO3 (dash lines in Figure 4c) are very close to 4.0, 3.0, 2.9, and 3.0 for their DFT computed relaxed structures, respectively. The DFT calculated BVS of ternary perovskites (e.g., SrMnO3, LaFeO3, LaCoO3, and LaAlO3) serves as the references to compare with that of the bulk (La0.75Sr0.25)(Mn0.25Fe0.25Co0.25Al0.25)O3 (solid lines and box plots in Figure 4c). In addition to the proven excellent thermal stability and intrinsic surface oxygen exchange kinetics, two thermodynamic properties, enthalpy and entropy of reduction, are important factors determining the performance of STCH materials. [67][68][69] Therefore, we carried out the measurements of the oxygen nonstoichiometry over a range of temperatures and P O 2 values for LS_MFC0.4A using a widely reported TGA protocol (see details in Figure S19, Supporting Information). [19,66,[70][71][72] Figure 5a shows the oxygen nonstoichiometry of LS_MFC0.4A as a function of temperature measured by continuous (P O 2 ≥ 0.028 atm) and stepwise (P O 2 ≤ 1.7×10 -3 atm) heating under the given P O 2 values. Following the van't Hoff method, [19,63,73] the enthalpy and entropy of reduction can be correlated with temperature and P O 2 according to the following equation: where R is the universal gas constant, and P O 2 is equal to the oxygen partial pressure referenced to the standard gas pressure (1 atm). T is the temperature in Kelvin. ∆ ( ) and ∆ ( ) are the standard enthalpy and entropy of reduction, respectively, which are dependent on δ and defined on a per mole of oxygen basis but have negligible dependence on temperature. 1000/T for extraction of thermodynamic properties (e.g., enthalpy and entropy of reduction) by the van't Hoff method at typical fixed δ values as indicated. Symbols and dash lines in the profile represent the measured and linearly fitted results, respectively. c) Standard enthalpy and d) entropy of reduction calculated for LS_MFC0.4A and comparison to those of some reported STCH and redox oxide materials. The error bars of LS_MFC0.4A came from the error of its linear fitting. The literature data for CeO2, [63] La0.6Sr0.4Mn0.4Al0.6O3 (LSMA6446), [64] cubic CaMn0.5Ti0.5O3 (CTM55), [19] La0.6Sr0.4MnO3 (LSM64), [65] and La0.8Sr0.2CoO3 (LSC) [66] is presented for comparison.
An Arrhenius plot of ln(P O 2 ) vs. 1000/T at each specified δ value with linear fitting can be obtained (Figure 5b), from which ∆ ( ) and ∆ ( ) can be extracted from the slope and intercept, respectively. In this way, the enthalpy and entropy of LS_MFC0.4A as a function of δ are obtained ( Figure   5c and 5d) and compared with those of some reported STCH and redox oxide materials such as CeO2, [63] cubic Ca(Mn0.5Ti0.5)O3-δ (CTM55), [19] La0.6Sr0.4MnO3-δ (LSM64), [65] La0.6Sr0.4Mn0.4Al0.6O3-δ (LSMA6446), [64] and La0.8Sr0.2CoO3 (LSC). [66] Previous experimental and theoretical studies suggested that a combination of intermediate enthalpy and large entropy is favorable for STCH from water splitting. [19,64,72,74] For perovskite materials, the enthalpy of reduction can be tuned by the dopants, [75] and is desirably constrained to a certain moderate range (neither too high nor too low), which may follow a Sabatier principle. A low enthalpy of reduction usually accompanies a low entropy of reduction, rendering favorable reduction extents (large Δδ) but unfavorable oxidative water splitting. [74,76] In contrast, a high enthalpy of reduction enables a high energy penalty for thermal reduction. LS_MFC0.4A shows enthalpy and entropy values comparable to LSMA6446 and CTM55 which have been identified to have prominent hydrogen production with an appropriate combination of moderate enthalpy and relatively large entropy. [19,64] Compared with ceria, LS_MFC0.4A possess a much lower enthalpy but similar entropy of reduction when Δδ is more than 0.1, suggesting its potential for high H2 yield under a lower reduction temperature. As shown in the TPR results ( Figure 2), the Δδ value of LS_MFCxA increases with x, possibly due to the decreased enthalpy of reduction upon the growing content of Co. For example, LSC material shows a small enthalpy (< 200 kJ mol -1 ) and entropy simultaneously, suggesting that it tends to have a large Δδ but poor oxidative water splitting capability. This is in good agreement with our TGA and STCH results for LSC. LS_MFC0.4A displays the highest hydrogen production in this LS_MFCxA series. This finding suggests that while maintaining a moderate enthalpy of reduction, multiple cations in LS_MFCxA may introduce extra configurational entropy to fulfill the needs of oxygen vacancy formation and electronic and ionic configurational entropy change. [77,78]

Conclusions
In this study, we demonstrated a new class of medium-to high-entropy CCPOs, (La0.8Sr0.2)(Mn(1- In general, we showed the importance to achieve a trade-off of thermodynamic and kinetic properties for optimized STCH performance. This study further exemplifies that non-equimolar compositions outperform their higher-entropy equimolar counterparts in this two-step STCH application. Thus, while an entropy effect with the multiple cations may stabilize oxygen vacancy formation without phase transformation, maximizing configurational entropy is not needed. Instead, we should use non-equimolar compositional design to balance the thermodynamic and kinetic properties to achieve better STCH performance, which represent a new design strategy in general.
This work provides a new pathway to explore CCPOs with tunable redox, thermodynamic and kinetics properties for STCH and chemical looping and a new computation paradigm to predict the redox behavior.