Solar thermochemical CO₂ splitting with doped perovskite LaCo_0.7Zr_0.3O₃: thermodynamic performance and solar-to-fuel efficiency

Abstract

The research of thermochemical CO₂ splitting based on perovskites is a promising approach to green energy development. Performance evaluation was performed towards the doped perovskite LaCo_0.7Zr_0.3O₃ (LCZ-73) based two-step thermochemical CO₂ splitting process thermodynamically based on the experimentally derived parameters for the first time. The impacts of vacuum pump and inert gas purge to reduce oxygen partial pressure and CO₂ heating on the performance parameter η_{solar-to-fuel} have been analyzed. The results showed that at the P_O2 of 10⁻⁵ bar, non-stoichiometric oxygen δ increased by more than 3 times as the reduction temperature varied from 1000 °C to 1300 °C, however, no significant deviation of δ was observed between 1300 °C and 1400 °C. The reaction enthalpy ranged from 60 to 130 kJ mol⁻¹ corresponding to δ = 0.05–0.40. Comparing the abovementioned two ways to reduce the oxygen partial pressure, the η_{solar-to-fuel} of 0.39% and 0.1% can be achieved with 75% and without heat recovery with the CO₂ flow rate of 40 sccm under experimental conditions, respectively. The energy cost for CO₂ heating during the thermodynamic process as the n_CO2/n_LCZ-73 increases was obtained from the perspective of energy analysis. The ratio of n_CO2/n_LCZ-73 at lower temperature required more demanding conditions for the aim of commercialization. Finally, the ability of perovskite to split CO₂ and thermochemical performance were tested under different CO₂ flow rates. The results showed that high CO₂ flow rate was conducive to the production of CO, but at the cost of low η_{solar-to-fuel}. The maximum solar-to-fuel efficiency of 1.36% was achieved experimentally at a CO₂ flow rate of 10 sccm in the oxidation step and 75% heat recovery.

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1. Introduction

CO₂ is the main greenhouse gas of the Earth because of its stable thermodynamic properties and large amounts of emission in a short period of time.¹ Thermochemical CO₂ splitting is an effective method to mitigate CO₂ emissions and provide a sustainable energy development path.² As the most abundant energy source on the Earth, solar energy is clean and pollution-free, and has received the attention of many countries in the world. Using solar energy as a source of energy for thermochemical fuel production can effectively alleviate the global energy crisis and global warming effect.^3,4 Solar thermochemical CO₂ splitting takes sufficient solar energy as the heat source and greenhouse gas CO₂ as the carbon source, which can realize the sustainable production of carbonaceous fuel,^5,6 and can be converted into liquid fuels through existing industrial facilities for long-distance transportation and storage.⁷ At the same time, in this process, discontinuous and low energy density solar energy is stored in the form of fuel chemical energy with high energy density.

The solar two-step thermochemical fuel production with metal oxide perovskite, ABO₃, as the oxygen carrier has received more and more attention from the researchers in recent years.^8,9 Compared to the more studied binary oxide cerium oxide, perovskites have the advantage of being able to be reduced at lower operating temperatures, while having a greater amount of oxygen released so that more fuel can be produced in the oxidation step.^10,11 In recent years, it has been reported that perovskite is used as oxygen carrier to split CO₂ for CO production.^12,13 Dey et al. studied the substituted La_0.5Sr_0.5MnO₃ with trivalent metal ion at B site, and the results showed that the performance of thermochemical CO₂ splitting was significantly improved by 5% Sc substitution at B position, and the yield of O₂ and CO increased by 2 times and 1.7 times, respectively.¹⁴ Muhich et al. compared the performance of perovskite and ceria in thermochemical splitting of CO₂, and found that perovskite has a high CO production capacity under a large CO₂ flow rate, but at the same time, the solar-to-fuel efficiency is lower due to the high heating cost.¹⁵ Ezbiri et al. extensively screened the perovskite redox activity and thermochemical stability used for solar thermochemical splitting of CO₂ by density functional theory, and verified the calculation results through experiments and applied them to fine-tune the oxygen exchange capacity of selected perovskite components.¹⁶ Galvez and coworkers studied the splitting of CO₂ by Ca and Sr-doped LaMnO₃ perovskites and found that the formation of carbonates resulted in a decrease in thermochemical efficiency due to incomplete conversion of CO₂.¹⁷ Demont et al. studied the solar CO₂ splitting via two-step thermochemical cycle with A- and B-site substituted Mn-perovskites, the results showed the effectiveness of the incorporation of Y in A-site and Mg in B-site in terms of CO production when compared with ceria.¹⁸ Cooper et al. studied LaMnO₃ perovskite with Ca/Sr A-site and Al B-site doping for CO₂ splitting, the results show that these materials could split CO₂ into CO at 1240 °C and could approach complete oxidation at 1040 °C. The fuel production per unit mass was 10 times that of CeO₂ under the same conditions.¹⁹ Bork and coworkers found a material, La_0.6Sr_0.4Cr_0.8Co_0.2O_3−δ with efficient solar-to-fuel conversion at lower temperatures. Thermogravimetric experiments showed that the ability to decompose CO₂ is more than 25 times that of CeO₂ in the two-step thermochemical cycle of 800–1200 °C.²⁰ Demont et al. studied the splitting of CO₂ by two-step solar thermochemical of LaMnO₃ perovskite doped with Sr, and characterized the redox thermodynamics properties. The results showed that a certain amount of Sr can tune the redox thermodynamics of the series of materials, which has a high activity for the splitting of CO₂.²¹ Takalkar et al. studied the thermochemical CO₂ splitting by Pr_xSr_1−xMnO₃ perovskites by thermogravimetry. It was found that lower Pr and higher Sr can effectively increase CO production, and two optimal perovskites were identified as promising materials for thermochemical CO₂ splitting.²² Jiang et al. studied the loaded LaFeO₃ perovskite with A- and B-sites doping. The CO production increased by 2–3 times, and the controlling mechanism and kinetic model of CO₂ splitting process were analyzed.²³

Currently, in the two-step thermochemical CO₂ splitting with perovskite as the oxygen carrier, most of the researches focus on one or the perovskite series through the different contents of elemental doping. The main goal of the studies is to find a high-performance perovskite that increases fuel production and thus achieves high solar-to-fuel efficiency (η_{solar-to-fuel}). The direction of efforts is mainly focused on the improvement of redox thermodynamics, kinetic properties and thermal stability of the material thermochemical cycle. Although researchers have made a lot of efforts in these areas and made great progress in some areas, the current researches in the field of thermochemical fuel research have low efficiency and cannot be commercialized. At present, the world's largest thermochemical fuel system is in pilot scale,²⁴ and how to achieve efficient fuel production remains a major challenge. It is beneficial to explore materials that can balance the advantages of reduction and oxidation reactions, and thermodynamic analysis should be performed when conducting primary materials screening.²⁵ However, there are few studies on the thermodynamics of perovskite thermochemical fuel production. Thermodynamic analysis was only performed theoretically in literatures. Carrillo and Scheffe established a thermodynamic model to calculate the solar-to-fuel efficiency of La_1−x(Sr,Ca)_xMn_1−yAl_yO₃ perovskites, showing the potential to improve the efficiency during isothermal or near-isothermal thermochemical cycles.²⁶ Muhich et al. implemented a thermodynamic system model of solar thermochemical fuel production based on mass and energy balances, the results showed the promising of perovskites due to the high fuel production capacities under large H₂O/CO₂ flow rates, but the poor solar-to-fuel efficiency due to the high heating load.¹⁵ Bork et al. performed a Calphad defect model to optimize and calculate the redox thermodynamics of Cr-doped La_0.6Sr_0.4MnO₃ perovskite for solar-to-fuel conversion, the results revealed that both the thermochemical fuel production and the solar-to-fuel efficiency of the Cr-doped materials were improved. These predictive perspectives are expected to provide prospects for the thermodynamic engineering of perovskite thermochemical reactions towards high solar-to-fuel efficiency.²⁷ Overall, the existing studies on thermodynamic analysis and solar-to-fuel efficiency mainly focus on theoretical analysis, but the extent to which level of this efficiency, especially the solar-to-fuel efficiency still needs more experimental proof.

To our knowledge, there are no studies addressing the thermodynamic analysis of CO₂ conversion into fuels using Zr doped LaCoO₃. This paper focuses on the report of a detailed thermodynamic analysis of a two-step thermochemical CO₂ splitting process with perovskite LaCo_0.7Zr_0.3O₃ (LCZ-73) as the oxygen carrier based on the obtained thermodynamic parameters experimentally. Material synthesis, characterization and thermochemical performance of CO₂ splitting have been reported in the previous experimental work,²⁸ in which studies of elements doping, doping concentration, the redox temperatures, heating rates on the performance of the doped LaCoO₃ perovskites based two-step thermochemical CO₂ splitting and reduction kinetics were illustrated. The results demonstrated the stability of the material before and after the thermochemical redox cycles, the activity of thermochemical CO₂ splitting and better reduction kinetics than other perovskite metal oxides and cerium oxides reported in the literature. These studies focused on discussing the performance of thermochemical CO₂ splitting from the perspective of experiments, but did not involve the thermodynamics of this process. Therefore, this paper discussed the thermodynamics of thermochemical CO₂ splitting from the perspective of solar to fuel conversion. The thermochemical reduction enthalpy was obtained and the thermodynamic efficiency was analyzed with the consideration of gas–gas, gas–solid phase heat recuperation as well as the use of vacuum pump and vacuum pump combined with inert gas purge to reduce the oxygen partial pressure. Based on the experimental conditions, the solar-to-fuel efficiency of the solar thermochemical fuel production process was analyzed by combining the vacuum pump with the inert gas purge and directly the vacuum pump to reduce the oxygen partial pressure, respectively. The energy consumed for CO₂ heating took up the bulk of energy consumption of the thermodynamic process as the n_CO2/n_LCZ-73 increases based on the energy analysis. Finally, the thermodynamic model was verified by experimentally testing the CO₂ splitting performance under different CO₂ flow rates and analyzing its solar-to-fuel efficiency. This paper revealed that one of the main problems in the field of solar thermochemical fuel production was the huge energy input caused by excessive oxidant in the oxidation step, which resulted in low efficiency.

2. Experiment and mathematical model

2.1. Experimental procedure and apparatus

LaCo_0.7Zr_0.3O₃ perovskite powder was prepared by citric acid sol–gel method, which was described in detail in previous studies.²⁸ Metal nitrate precursors were obtained commercially (Aladdin and Macklin) and were specified to be >99.5% pure. XRD measurements confirmed the existence of perovskite structure LaCo_0.7Zr_0.3O₃, SEM image showed that the sample was irregularly shaped particles with a size in the a few microns and EDS analysis verified that the synthesized sample compositions were in line with expectations. Similarly, XRD of the powder sample after reduction showed that it still maintained the perovskite phase structure. The measurements of the redox reactions were carried out with a high temperature thermogravimetric analyzer (TGA, LINSEIS STA PT 1600). To avoid the effects of thermal inertia during testing, fresh material of LaCo_0.7Zr_0.3O₃ (LCZ-73) oxide of the same batch of synthetic sample powder was used in each test. Granular powders with an average weight of approximately 10 mg were evenly placed in an alumina crucible with a volume of about 100 μl throughout all the measurement series. Before the formal start of the redox experiments, a vacuum pump was used to vacuum the furnace chamber to 5.5 × 10⁻⁵ atm for a low oxygen environment, and then purged with high purity argon (99.999%) for one hour to provide an inert environment and further reduce the oxygen partial pressure. The inert atmosphere was always maintained throughout the reduction process and during the process of reducing from reduction temperature to the oxidation temperature to blow the released oxygen out of the furnace in time. High purity argon was used as the purge and protective gas, the same high purity carbon dioxide (99.999%) was used as the oxidant during the oxidation process. The overall gas flow rate entering the reaction chamber was maintained at 80 sccm, with the balance being protected with argon flow of 40 sccm at all times. In the reduction process, the sample was heated to 1000, 1100, 1200, 1300, 1400 °C, respectively, at a heating rate of 30 °C min⁻¹ and maintained for 60 min with continuous and constant Ar flow. Then the temperature was lowered to 800 °C at the same rate for oxidation. In the oxidation step, the high purity Ar (40 sccm) and high purity CO₂ with a flow rate of 40 sccm (P_CO2 = 0.5 atm) were mixed and introduced into the furnace chamber to react with the oxygen-deficient perovskite and produce CO. The trace oxygen was blown out and diluted to further reduce the impact on CO₂ splitting. This oxidation process proceeded for 60 min. The specific experimental platform was shown in the Fig. 1.

Fig. 1 Schematic diagram of the experimental platform for thermochemical CO₂ splitting.

Since the redox behavior analysis of perovskite in this work was mainly based on thermogravimetric analysis, the mass change in sample was induced by the release and uptake of oxygen under high temperatures and specific oxygen partial conditions, then the non-stoichiometry of the perovskite can be determined by measuring the mass change. The relationship between the mass change Δm and the nonstoichiometric oxygen δ is as follow:²⁹

m_perovskite is the initial mass of the perovskite sample, and M̄_perovskite and M̄_O are the molar mass of the perovskite and the atomic oxygen, respectively.

Thermodynamic parameters are extracted from the Van't Hoff approach. The standard molar Gibbs free energy of the reduction reaction can be expressed as follow:^8,30

ΔG^Θ(T) = −R̄T ln K_red = ΔH^Θ_red − TΔS^Θ_red (2)

where K_red is the equilibrium constant, and K_red = P_O2/P^Θ. P_O2, P^Θ, R̄ and T are the oxygen partial pressure, the standard pressure and P^Θ = 1 bar, the universal gas constant and the reduction temperature, respectively. Inserting the relationship of K_red into eqn (1), for constant oxygen non-stoichiometry, δ, the above equation can be rearranged asΔH̄^Θ_red is the standard molar enthalpy and can be determined through an Arrhenius plot for sets of temperature T and ln(P_O2).

2.2. Mathematical model

A typical thermochemical redox cycle of CO₂ splitting for a two-step process based on perovskite can be exemplified in the following reactions:where δ_i and δ_f are the oxygen non-stoichiometry of perovskite after reduction and oxidation reactions, respectively. Δδ = δ_i − δ_f is the non-stoichiometry net change in oxygen, which is also a numerical equivalent of the amount of CO produced by 1 molar of perovskite in a single thermochemical cycle. In the first step, the perovskite oxide is partly reduced at a high reduction temperature (T_red), releasing lattice oxygen and generating oxygen vacancies, eqn (4). In the second oxidation step, the reduced perovskite is reoxidized by CO₂ at a lower temperature (T_oxi), eqn (5), CO₂ is reduced to CO. For a non-isothermal thermochemical cycle, T_red > T_oxi.

2.2.1. Reduction process. Here we consider a dynamic thermochemical cycle rather than a process of heating the perovskite from room temperature to reduction temperature, and a quasi-steady state is assumed for the thermochemical cycle. Thus, the energy required to heat the perovskite from oxidation temperature to reduction temperature is:

The energy required to drive the reduction reaction of one molar perovskite is:

where C̄_p,LCZ-73 is the molar specific heat capacity of perovskite LCZ-73 used in the test, which can be obtained by differential scanning calorimeter. According to the thermal effect obtained by differential scanning calorimetry and the formula ΔQ = (m/M̄)C̄_pΔT, the value of C̄_p can be obtained. In fact, the specific heat capacity is a function of temperature, but for convenience, the average value of C̄_p,LCZ-73 = 157.5 J (mol⁻¹ K⁻¹) is considered.

Researches have shown that decreasing oxygen partial pressure in the thermal reduction process is beneficial to the increase of δ and the efficiency of thermochemical fuel production.³¹ There are two common ways to reduce oxygen partial pressure of the reduction process in thermochemical fuel research: vacuum pump and inert gas purge.

2.2.1.1. Pumping. A vacuum pump is needed to eliminate the residual air in the reactor to provide a lower oxygen partial pressure environment during the thermal reduction process. Q_pump is the energy consumed to obtain a lower oxygen partial pressure with a vacuum pump. In this paper, the ideal pump work is used to calculate the performance of solar-to-fuel efficiency.³²

Q_pump = n_O2R̄T_pump ln(P₀/P_O2)/η_S→W (8)

where n_O2 = n_CO/2 follows from the reaction stoichiometry, R̄ is the universal gas constant, T_pump is the operating temperature of the pump. In this study, the temperature is the ambient temperature. P₀ is the experimental pressure at atmospheric pressure. P_O2 is the oxygen partial pressure inside the instrument after vacuuming, η_S→W is the solar energy to pump work conversion efficiency and its value is assumed to be 0.1 in this paper.³²
2.2.1.2. Sweep gas. Argon was used as the sweep gas in the reduction step and as the shielding gas for the balance chamber throughout the thermochemical process. In our experiment, high purity Ar (99.999%) with an oxygen impurity concentration less than 10 ppm was used. Considering that the high-temperature exhaust gas in the reduction process carries a large amount of heat, it can be used to preheat Ar required for the reduction reaction. Therefore, the energy required to heat sweep gas by considering the heat recovery of exhaust gas is calculated as:where ε_{heat exchanger} is the heat exchange coefficient of the heat exchanger.³³ T_amb is the ambient temperature and T_red is the reduction temperature. The molar specific heat capacity of Ar is considered constant at a certain temperature (T ≤ 2000 K), and C̄_p,Ar = 20.786 J (mol⁻¹ K⁻¹).³⁴

2.2.2. Oxidation process. Energy cost during the oxidation process is the cost of heating CO₂. A heat exchanger can be used to heat CO₂ using the outflowing oxidizer and fuel from the oxidation reactor. The same heat exchanger coefficient is used for simplicity.

C̄_p,CO2(T) = 20.99 + 6.768 × 10⁻²T − 4.960 × 10⁻⁵T² + 1.779 × 10⁻⁸T³ − 2.495 × 10⁻¹²T⁴ (11)

where n_CO2 is the molars of CO₂ introduced in the oxidation process, T_oxi is the oxidation temperature, C̄_p,CO2 is the molar specific heat capacity of CO₂ under a standard atmospheric pressure and is a function of temperature T, which can be given as a polynomial fits in J (mol⁻¹ K⁻¹).³³
2.2.2.1. Heat recovery. It is difficult to realize the heat utilization of solid reactants by heating low temperature solid with high temperature solid³⁵ and may present some other problems. In many previous studies, the heat recovery of solid reactants was not considered.³³ The idea of using a heat exchanger to convert solid heat into process heat to heat the oxidant to improve efficiency³³ is also used in the analysis herein.

Under the experimental conditions, in order to maintain the oxidation reaction at a constant temperature, continuous additional heating is necessary. The heat transfer for one molar LCZ-73 during the oxidation reaction is calculated:

Q_oxi = −Q_red + (δ_i − δ_f)HHV_CO (12)

The available amount of the recovered heat from the solid reactants is limited by the heat required for CO₂ heating in the oxidation process,

where ε_rec is the effectiveness of heat recovery. This assumes that ε_rec = 0.6 (ref. 33) and HHV_CO is the high calorific value of CO.

2.2.3. Solar-to-fuel efficiency. An ideal blackbody reactor is considered in the thermochemical cycling reactions, therefore, the absorption efficiency of the reactor which is a function of T_red and concentration ratio C.^36,37 Considering the radiative heat loss of the solar reactor, the heat required to power all the processes of the thermodynamic system can be supplied with the absorption efficiency η_abs. It can be defined asHere σ is the Stefan–Boltzmann constant, 5.67 × 10⁻⁸ W (m⁻² K⁻⁴). I = 1000 W m⁻², C represents the solar concentration ratio in suns, here, C = 3000 (ref. 38 and 39) is employed in this paper. Here, we assume that there is no heat loss of convection and heat conduction from the solar reactor, thus there is only the heat loss rooted in radiation to the ambient.

An energy balance equation for the entire thermochemical processes yields the following equation:^33,38

Here we assume that the required electrical heating energy, pumping power, energy for gas purging, CO₂ heating and energy required to maintain a constant temperature for oxidation and other energy-consuming processes of the thermochemical cycle are all provided by solar energy. The available heat recovery of the system is also considered.

According to the first law of thermodynamic, η_{solar-to-fuel} of the two-step thermochemical redox processes is chosen as the merit of the thermochemical cycling performance,^40,41 which is defined as the ratio of the calorific value of the fuel to the total solar energy input in the system:^33,38,42

where HHV_CO is the higher heating value of CO.

3. Results and discussion

3.1. Thermodynamic characteristics of LCZ-73

Here, the relationship between oxygen partial pressure and non-stoichiometric oxygen δ during the reduction process after reaching the target temperatures is revealed. As shown in Fig. 2, the non-stoichiometric oxygen δ in LaCo_0.7Zr_0.3O_3−δ is highly dependent on reduction temperature and the oxygen partial pressure (P_O2). As the oxygen partial pressure (P_O2) decreases, the non-stoichiometric oxygen δ gradually increases, but the increase rate slows down as the reduction reaction proceeds. Under the same oxygen partial pressure environment, the high temperature is more favorable for the generation of non-stoichiometric oxygen in the reduction process. But it is not that the higher the temperature is, the better it will be. At 1300 °C and 1400 °C, the ability to produce non-stoichiometric oxygen under both conditions is nearly the same. At the oxygen partial pressure of 10⁻⁵ bar, the non-stoichiometric oxygen at the reduction temperature of 1000–1400 °C is 0.18, 0.34, 0.44, 0.60 and 0.60, respectively. The non-stoichiometric oxygen δ increases by more than 3 times when the temperature varies from 1000 °C to 1300 °C. At 1000 °C, the non-stoichiometric oxygen δ is doubled when P_O2 changes from 10⁻⁴ to 10⁻⁵ bar, while, at 1300 °C, this value increases by a third. Lower oxygen partial pressure is required when the same non-stoichiometric oxygen is reached at low temperature. At δ = 0.2, the oxygen partial pressure P_O2 during 1000–1400 °C varies from 10^−5.75 to 10^−3.55 bar.

Fig. 2 Oxygen non-stoichiometry δ of ∼10 mg LCZ-73 perovskite powder reduced at 1000–1400 °C for 60 min with a heating rate of 30 °C min⁻¹ (in this analysis, only the reduction under a constant temperature process is considered).

Based on the above analysis, the derived ln(P_O2) versus inverse temperature for LCZ-73 is shown in Fig. 3. Considering that there is a certain error in the oxygen partial pressure at some points, the linearity is not ideal for all when performing linear fitting on the data points, especially for the δ = 0.20 and δ = 0.25. Similarly, it can be found that the non-stoichiometric oxygen increases with decreasing the oxygen partial pressure at the same temperature. In order to achieve the same δ, a lower oxygen partial pressure is required at low temperature. Fig. 3 also shows two ways to obtain larger non-stoichiometric oxygen δ: increasing temperature and decreasing oxygen partial pressure. According to the existing data with better linearity, it can be seen that ΔH^Θ_red in eqn (3) is likely to be temperature-independent, which is confirmed in other studies.³⁰ Simultaneously, this also shows the reliability of our experimental data. Based on this, we can get the relationship between the reduction enthalpy ΔH^Θ_red and the different oxygen stoichiometry δ during the thermochemical reduction process, which will be discussed next.

Fig. 3 Arrhenius plots of oxygen partial pressure versus inverse temperature for LaCo_0.7Zr_0.3O₃ (LCZ-73) corresponding to fixed δ between 0.05 and 0.40.

The energy required to generate oxygen vacancies in the perovskite LCZ-73 can be obtained via the change of reduction enthalpy, ΔH^Θ_red. Similar studies have shown that reduction enthalpy is only related to stoichiometry δ.^8,43 As shown in Fig. 4, the curve for the change in standard molar reduction enthalpy is fit with a polynomial. The value of ΔH^Θ_red is 60–130 kJ mol⁻¹ when δ in the range of 0.05–0.40. The following thermodynamic analysis will be based on these results for discussion.

ΔH^Θ_red(δ) = 58.529 + 71.905δ + 905.361δ² − 1724.966δ³ (17)

Fig. 4 Variation of reduction enthalpy of LaCo_0.7Zr_0.3O₃ (LCZ-73) as a function of oxygen non-stoichiometry δ.

3.2. Solar-to-fuel efficiency analysis for LCZ-73 based solar CO production system

3.2.1. Decreasing P_O2 via vacuum pump. In the case of decreasing the oxygen partial pressure by vacuum pump, then no sweep gas is needed and Q_{Ar heating} = 0. Solar-to-fuel efficiency of the two-step solar thermochemical CO₂ splitting based on LCZ-73 perovskite as functions of reduction temperature and the ratio of n_CO2/n_LCZ-73 with different heat exchange coefficients ε_{heat exchanger} is analyzed in the range of reduction temperature of 1000–1400 °C and oxidation at 800 °C and is shown in Fig. 5(a)–(d). Meanwhile, the relationship between the ratio of n_CO2/n_LCZ-73 corresponding to η_{solar-to-fuel} = 20% and reduction temperatures is shown in Fig. 5(e).

Fig. 5 Solar-to-fuel efficiency (η_{solar-to-fuel}) as functions of reduction temperature and molar ratio of CO₂ to perovskite and the molar ratio of reactants required to achieve η_{solar-to-fuel} = 20% under different heat recovery conditions when using a vacuum pump to reduce oxygen partial pressure. (a)–(d) η_{solar-to-fuel} as functions of reduction temperature and molar ratio of CO₂ to LCZ-73 under different heat exchange coefficients ε_{heat exchanger}. (e) Molar ratio of CO₂ to perovskite corresponding to η_{solar-to-fuel} = 20% with the reduction at 1000–1400 °C and oxidation at 800 °C.

As shown in Fig. 5(a)–(d), at the molar ratio of n_CO2/n_LCZ-73 ≤ 500, η_{solar-to-fuel} decreases rapidly with the increase of the ratio, and decreases slowly when the molar ratio is greater than 500. In Fig. 5(a)–(c), η_{solar-to-fuel} ≤ 1% at n_CO2/n_LCZ-73 ≥ 500, the molar ratio of n_CO2/n_LCZ-73 is enlarged to 1000 in Fig. 5(d). This also means that higher efficiency can be achieved with higher heat recovery. This can also be found from Fig. 5(a)–(d) that with the increase of ε_{heat exchanger}, η_{solar-to-fuel} increases gradually. In the current experimental situation, the molar ratio of n_CO2/n_LCZ-73 is 2737 : 1, η_{solar-to-fuel} = 0.39% is obtained at the reduction temperature of 1300 °C with 75% heat recovery. While, when no heat recovery is considered, this efficiency is just 0.1%.

The insets are the partial enlargements with the molar ratio of less than 50. Overall, under the same conditions, the highest η_{solar-to-fuel} is achieved at 1300 °C and n_CO2/n_LCZ-73 ≥ 5 in Fig. 5(a)–(c) and n_CO2/n_LCZ-73 ≥ 10 in Fig. 5(d). When the ratio of n_CO2/n_LCZ-73 < 10, η_{solar-to-fuel} will reach 20% or more in Fig. 5(a)–(c) and for higher heat recovery in Fig. 5(d), η_{solar-to-fuel} of 20% can be achieved at n_CO2/n_LCZ-73 < 50. This efficiency value is also proposed based on estimated qualitatively in reference and is considered as the benchmark for commercially achievable system-level annual average solar-to-fuel efficiency for a viable alternative to conventional electrolysis.⁴⁴ For this competitive efficiency reference value, further analysis is shown in Fig. 5(e). When the efficiency reaches 20%, n_CO2/n_LCZ-73 = 47.59 at 1300 °C, and n_CO2/n_LCZ-73 = 35.12 at 1000 °C with 75% heat recovery. The molar ratios of n_CO2/n_LCZ-73 are 12.4 and 6.74 with no heat recovery, respectively. Compared with higher temperature conditions (1300 and 1400 °C), the molar ratio of n_CO2/n_LCZ-73 at low temperature requires more demanding conditions with the prospects of commercialization. This is still a big challenge in the current research background of thermochemical fuels. However, under the same conditions, the larger molar ratio of n_CO2/n_LCZ-73 is more accessible under current technical conditions.

3.2.2. Decreasing P_O2 via vacuum pump combined with Ar sweeping. When inert gas purging is used to reduce oxygen partial pressure, then no vacuum pump is needed and Q_pump = 0. A strategy of combined vacuum pump with Ar purging during the reduction process was adopted to reduce the partial pressure of oxygen in the reaction chamber rapidly and reduce the amount of Ar required for purging. It has also been shown that the combination of reducing the oxygen partial pressure via vacuum pump and inert sweeping helps to increase the efficiency of the inert-swept reactor.³¹ The evolution of η_{solar-to-fuel} as functions of reduction temperature and molar ratio of CO₂ to LCZ-73 with different heat exchange coefficients ε_{heat exchanger} is depicted in Fig. 6(a)–(d). As shown, in the reduction temperature range of 1000–1400 °C, η_{solar-to-fuel} decreases rapidly as the molar ratio of CO₂ to perovskite increases. When n_CO2/n_LCZ-73 ≥ 1000, η_{solar-to-fuel} < 1%, regardless of the temperature is in this range, especially in Fig. 6(a)–(c), the range of this ratio expanded to n_CO2/n_LCZ-73 ≥ 500. In Fig. 6(d), ε_{heat exchanger} = 0.75, η_{solar-to-fuel} > 1% at n_CO2/n_LCZ-73 ≤ 500. When n_CO2/n_LCZ-73 < 500, the efficiency slowly increases as the ratio decreases. For n_CO2/n_LCZ-73 < 200, the efficiency increases rapidly as the ratio decreases. When it comes to the ratio of n_CO2/n_LCZ-73 ≤ 200, η_{solar-to-fuel} > 1% except in the case of ε_{heat exchanger} = 0 and reduction temperatures of 1000–1100 °C. With the increase of ε_{heat exchanger}, η_{solar-to-fuel} at different temperatures are improved and reached the highest at 1300 °C. Under the experimental conditions of this work, the flow rate of CO₂ is 40 sccm, the molar ratio of n_CO2/n_LCZ-73 = 2737, η_{solar-to-fuel} = 0.1% without considering heat recovery at the reduction temperature of 1300 °C, η_{solar-to-fuel} = 0.39% with 75% heat recovery. For a theoretical deduction, if the molar ratio of n_CO2/n_LCZ-73 can be further reduced to 0.5, the theoretical maximum value of η_{solar-to-fuel} of 52.5% and 59.6% can be achieved with no heat recovery and 75% heat recovery were considered, respectively. Despite this considerable efficiency value, it is necessary to know that such a low ratio poses great challenges for experimental verification. The inset in each figure of Fig. 6(a)–(d) is for partial amplification in the range of molar ratio less than 50. It can be seen from the insets that the highest efficiency value is obtained over the entire molar ratio when the reduction temperature is 1300 °C, and the lowest value at 1000 °C. At this point we consider a considerable solar-to-fuel efficiency value of 20% which is a viable alternative with that more traditional and reliable technology based on electrolysis.⁴⁴

Fig. 6 Solar-to-fuel efficiency (η_{solar-to-fuel}) as functions of reduction temperature and molar ratio of CO₂ to perovskite and the molar ratio of reactants required to achieve η_{solar-to-fuel} = 20% under different heat recovery conditions when using vacuum pump combined with gas purging to reduce oxygen partial pressure. (a)–(d) η_{solar-to-fuel} as functions of heat exchange coefficient ε_{heat exchanger} and molar ratio of CO₂ to LCZ-73. (e) Variation of molar ratio of CO₂ to LCZ-73 corresponding to η_{solar-to-fuel} = 20% with the reduction at 1000–1400 °C and oxidation at 800 °C.

Variation of molar ratio of n_CO2/n_LCZ-73 corresponding to the solar-to-fuel efficiency of 20% with heat exchange coefficient ε_{heat exchanger} at different reduction temperatures was shown in Fig. 6(e). Similarly, we find that when the efficiency reaches 20% and as the benchmark, the ratio of n_CO2/n_LCZ-73 is maximum at 1300 °C, reaching about 50, which is the easiest to reach in the temperature range in question. At the same time, this means that if there is a more efficient way to reduce the ratio of n_CO2/n_LCZ-73, then at 1300 °C, solar-to-fuel efficiency will be higher than 20%. This is a great encouragement for commercialization. However, in most of the current experimental studies of two-step solar thermochemical fuel production cycles, the far excessive amount of oxidant in oxidation step is used for a high fuel production. How to effectively reduce the amount of oxidant introduced into the oxidation step becomes the key to improve the solar-to-fuel efficiency. Another finding is that as the heat exchange coefficient increases, n_CO2/n_LCZ-73 at the same temperature changes monotonously, indicating that as the heat recovery increases, the conditions for commercialization are more moderate, and the solar-to-fuel efficiency is higher.

3.2.3. Analysis of the energy for CO₂ heating. Fig. 7 shows the relationship between the energy of each part and reduction temperature during the solar two-step thermochemical CO₂ splitting process via Ar purging. The variations of energy with reduction temperature and the relative magnitude are shown in Fig. 7(a). It can be seen that Q_red, Q_LCZ-73, Q_oxi, Q_rec and the fuel energy generated increase with the increase of reduction temperature. The energy required for the reduction reaction is the minimum, while the energy required for the sample heating and the oxidation reaction is the maximum of the energy consumption items mentioned above. The energy required for heating Ar as functions of reduction temperature and heat exchange coefficient of heat exchanger is shown in Fig. 7(b). We can find that the energy consumed for heating Ar increases with the increase of reduction temperature, and this value at 1400 °C is about twice of that at 1000 °C. At the same temperature, the energy consumed decreases with the increase of heat exchange coefficient ε_{heat exchanger}.

Fig. 7 Relationship between the energy and reduction temperature during the solar two-step thermochemical CO₂ splitting process via Ar purging. (a) Energy required as a function of reduction temperature, (b) energy required for Ar heating as functions of reduction temperature and heat transfer coefficient ε_{heat exchanger}.

Similarly, the energy consumed for CO₂ heating with heat recovery as functions of reduction temperature and molar ratio of n_CO2/n_LCZ-73 is given in Fig. 8(a). The molar ratio of n_CO2/n_LCZ-73 increases as the amount of CO₂ increases, and the energy consumed for CO₂ heating increases significantly. When n_CO2/n_LCZ-73 varies from 100 to 1000, the energy consumed increases by a corresponding multiple. Compared with the molar ratio of n_CO2/n_LCZ-73, the reduction temperature has much smaller impact on the energy consumed for CO₂ heating. The energy increases slightly with the increase of reduction temperature, but the trend is gentle. The energy for CO₂ heating is two or three orders of magnitude higher than those in Fig. 7(a) and (b).

Fig. 8 Energy for CO₂ heating as functions of reduction temperature and molar ratio of n_CO2/n_LCZ-73 under different heat recovery conditions. (a) Energy required for CO₂ heating and (b) the proportion of energy for CO₂ heating in total input energy as functions of reduction temperature and molar ratio of n_CO2/n_LCZ-73 with different heat transfer coefficients ε_{heat exchanger}.

Finally, the proportion of energy required for CO₂ heating in total input energy is summarized in Fig. 8(b). It can be seen that the proportion rapidly increases with the increase of n_CO2/n_LCZ-73, but decreases slightly with the increase of the reduction temperature, which is mainly because the reduction temperature has little influence on the energy of CO₂ heating, but has great impact on the total input energy. With the increase of heat exchanger coefficient, the proportion of energy consumption for CO₂ heating gradually decreases. As the molar ratio of n_CO2/n_LCZ-73 increases, the proportion of energy for CO₂ heating in total input energy increases rapidly, resulting in lower solar-to-fuel efficiency. At 1300 °C and ε_{heat exchanger} = 0.5, when n_CO2/n_LCZ-73 = 0.5, the proportion of CO₂ heating is only 5.35%, while, when n_CO2/n_LCZ-73 = 50, the proportion reaches nearly 85.0%. When the amount of CO₂ is further increased, this proportion reaches 95.8% as the ratio of n_CO2/n_LCZ-73 increased to 200. This also reveals the reason why the current thermochemical fuel research is less efficient.

3.2.4. The effect of CO₂ flow rates on solar-to-fuel efficiency. In order to explore the thermodynamic properties of perovskite LCZ-73 based thermochemical CO₂ splitting under different CO₂ content atmospheres, experimental tests are carried out with different CO₂ content in Ar. Approximately 10 mg LCZ-73 powder is used for each test and the sample is heated to 1300 °C in Ar (80 sccm) with the ramp of 10 °C min⁻¹ and maintained for 60 min. Then the sample is cooled down to 800 °C for oxidation with the ramp of 20 °C min⁻¹ and maintained for 60 min in the atmosphere of 10–70 sccm CO₂ balanced with Ar, the corresponding ratio of n_CO2/n_LCZ-73 varies from 684 to 4788, the total gas flow is maintained at 80 sccm. The CO yield and solar-to-fuel efficiency as functions of CO₂ flow rate are obtained and calculated based on the thermodynamic model mentioned above. The details are shown in Fig. 9.

Fig. 9 CO yield and solar-to-fuel efficiency as functions of CO₂ flow rate.

As shown, the effects of CO₂ flow rate on CO yield and solar-to-fuel efficiency at various heat exchanger coefficient ε_{heat exchanger} are analyzed. It can be clearly seen from Fig. 9(a) that the yield of CO increases gradually with the increase of CO₂ flow rate, and the high CO₂ concentration is favorable for the oxidation reaction. At the same time, the ratio of n_CO2/n_CO changes monotonically with the increase of CO₂ flow rate. When the CO₂ flow rate in the oxidation step is 10 sccm, n_CO2/n_LCZ-73 = 684, n_CO2/n_CO = 1926, and the same value is 10 648 when the CO₂ flow rate is 70 sccm at n_CO2/n_LCZ-73 = 4788, the latter represents an increase of nearly 27% in CO yield over the former. However, it is also noted that solar-to-fuel efficiency is negatively correlated with the increase of CO₂ flow rate in Fig. 9(b). Although CO yield is low at low CO₂ flow rate, solar-to-fuel efficiency is high. In order to prove that this material can still split CO₂ under the condition of lower molar ratios, further experiments would be needed for verification, such as the CO₂ flow rate is less than 10 sccm (n_CO2/n_LCZ-73 < 684). It should be noted that an excessively low CO₂ flow rate may impose higher requirements on the experimental devices in actual operation. Considering the effect of heat transfer coefficient, solar-to-fuel efficiency increases with the increase of heat transfer coefficient at the same CO₂ flow rate. The maximum solar-to-fuel efficiencies of 1.36% and 0.35% are achieved with 75% heat recovery and without heat recovery with the CO₂ flow rate of 10 sccm (n_CO2/n_LCZ-73 = 684). However, the solar-to-fuel efficiencies under the same heat exchange conditions are only 0.25% and 0.06% with the CO₂ flow rate of 70 sccm.

4. Conclusions

This work performs the performance evaluation towards the doped perovskite LaCo_0.7Zr_0.3O₃ (LCZ-73) based thermochemical CO₂ splitting process thermodynamically via the experimentally derived parameters for the first time. The impacts of vacuum pump and vacuum pump combined with inert gas purge to reduce oxygen partial pressure and CO₂ heating on the performance parameter η_{solar-to-fuel} have been analyzed with the consideration of gas–gas, gas–solid phase heat recuperation. The energy cost of CO₂ heating during the redox process is underscored. At the oxygen partial pressure of 10⁻⁵ bar, non-stoichiometric oxygen δ increases by more than 3 times as the reduction temperature increases from 1000 °C to 1300 °C. However, the deviation of δ is negligible between 1300 °C and 1400 °C. The reaction enthalpy ranges from 60 to 130 kJ mol⁻¹ corresponding to δ = 0.05–0.40. Comparing η_{solar-to-fuel} via vacuum pump with the case of vacuum pump combined with inert gas purging, the efficiencies of 0.39% and 0.1% can be achieved with 75% and without heat recovery under experimental conditions, respectively. Based on the energy analysis, the energy cost of CO₂ heating took up the bulk of energy consumption of the thermodynamic process as the n_CO2/n_LCZ-73 increases, which is two or three orders of magnitude higher than other heating items. Compared with higher temperature conditions, n_CO2/n_LCZ-73 at lower temperature requires more demanding conditions for the aim of commercialization. Finally, the ability of the perovskite LCZ-73 to split CO₂ and thermochemical performance were tested under different CO₂ flow rates. The results show that high CO₂ flow rate is conductive to the production of CO, but at the cost of low η_{solar-to-fuel}. The maximum solar-to-fuel efficiencies of 1.36% and 0.35% are achieved with 75% heat recovery and without heat recovery with the CO₂ flow rate of 10 sccm. However, values under the same heat exchange conditions are only 0.25% and 0.06% with the CO₂ flow rate of 70 sccm. On the premise of meeting the requirements of the experimental device, further experiments are needed to verify the ability of the perovskite to split CO₂ at lower CO₂ flow rate (e.g., less than 10 sccm, n_CO2/n_LCZ-73 < 684).

Nomenclature

M	The molar mass of perovskite, g mol^−1
m	Mass of the matter, g
R̄	Universal molar gas constant, 8.314 J mol^−1 K^−1
T	Temperature, K
K	The equilibrium constant
P	Pressure, atm
Q	The energy required or recovered for each process, J mol^−1
C̄p	The specific molar heat capacity, J mol^−1 K^−1
n	Amount of substance, mol
HHV	Higher heating value, J mol^−1
C	Solar concentration ratio, suns
I	1000 W m^−2
nCO2/nLCZ-73	Molar ratio of CO2 to perovskite LaCo0.7Zr0.3O3

Greek symbols

Δm	Mass change, g
δ	Nonstoichiometric oxygen
ΔG^Θ	Standard molar Gibbs free energy, J mol^−1
ΔH^Θred	Standard molar enthalpy for reduction, J mol^−1
ΔS^Θred	Standard molar entropy for reduction, J mol^−1
Δδ	Non-stoichiometry change
ΔQ	The energy change in a process, J mol^−1
ΔT	Temperature swing, K
ηS→W	Solar energy to vacuum pumping conversion efficiency
εheat exchanger	The heat exchange coefficient of the heat exchanger
εrec	The effectiveness of heat recovery, 0.6
ηabs	Heat absorption efficiency of solar reactor
σ	Stefan–Boltzmann constant, 5.67 × 10^−8 W m^−2 K^−4
ηsolar-tio-fuel	Solar to fuel conversion efficiency

Subscripts

O	Oxygen atom
O2	Oxygen gas
red	Reduction
oxi	Oxidation
i	The state of non-stoichiometric oxygen after reduction
f	The state of non-stoichiometric oxygen after oxidation
pump	Vacuum pump
0	The atmospheric pressure, 1 atm
Ar heating	Preheat the sweeping gas Ar
amb	Ambient condition
CO2	Carbon dioxide
CO	Carbon monoxide
rec	Recovery
CO2 heating	Preheat the CO2 gas
abs	Absorption

Superscripts

Θ	Standard condition at T and Patm

Abbreviations

LCZ-73

LaCo0.7Zr0.3O3

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China (Grant No. 2018YFB1502005), the National Natural Science Foundation of China (Grant No. 51806209) and Institute of Electrical Engineering, Chinese Academy of Sciences (Grant No. Y770111CSC).

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