Recent Developments in Ceria-Driven Solar Thermochemical Water and Carbon Dioxide Splitting Redox Cycle

Abstract

Metal oxide (MO) based solar thermochemical H₂O (WS) and CO₂ splitting (CDS) is one of the most promising and potential-containing processes that can be used to produce H₂ and syngas (liquid fuel precursor). Several non-volatile and volatile MOs were considered redox materials for the solar-driven WS and CDS operation. Among all the examined redox materials, based on their high O² storage capacity, faster oxidation kinetics, and good stability, ceria and doped ceria materials are deemed to be one of the best alternatives for the operation of the thermochemical redox reactions associated with the WS and CDS. Pure ceria was used for solar fuel production for the first time in 2006. A review paper highlighting the work done on the ceria-based solar thermochemical redox WS and CDS cycle from 2006 until 2016 is already published elsewhere by the author. This review paper presents all the significant findings reported in applying pure ceria and doped ceria materials for the WS and CDS by research teams worldwide.

1. Introduction

The continuous rise in the concentration of atmospheric CO₂ is deemed one of the prime reasons for the greenhouse effect and subsequent global warming [1]. One of the significant sources of CO₂ emissions is the incessant utilization of fossil fuels in power plants and transportation fuels [2]. To overcome the issues mentioned above, the production of alternative fuels such as H₂ or precursors for the synthesis of transportation fuel, i.e., syngas via solar-driven water splitting (WS) and carbon dioxide splitting (CDS) has been investigated considerably by using metal oxide (MO)-driven thermochemical redox reactions [3]. The MO-based redox cycle is divided into two steps: (1) thermal reduction (TR) of the MO and (2) re-oxidation (RO) of the MO [4]. The TR of the MO is an endothermic solar-driven step. In contrast, the RO of MO is a slightly exothermic non-solar-driven step [5]. The TR of MO results in O₂-deficient MO (called reduced MO) and the release of O₂ [6]. The RO of MO is responsible for the regeneration of the MO via the splitting of H₂O and/or CO_2, producing H₂ and/or CO [7].

Two kinds of MOs were examined for the solar thermochemical production of fuel via WS and CDS: volatile and non-volatile MOs. In the case of the volatile MOs, most studies used ZnO/Zn [8,9,10] and SnO₂/SnO cycles [11,12,13]. In contrast, multiple numbers of non-volatile MOs were investigated, which mainly belong to the ferrites [14,15,16,17,18,19], ceria [20,21,22], and perovskites [6,23,24,25] categories. Among all the MOs investigated until now, the ceria-based redox materials are considered as most suitable because of their better thermal stability and lower sintering during high-temperature operation, elevated O₂ storage capacity, and rapid re-oxidation kinetics during the WS or CDS steps.

In 2006, Abanades and Flammant [26] reported the application of ceria for the thermochemical conversion of H₂O into H₂ for the first time. Since then, plenty of investigations have been carried out to examine the suitability of pure ceria (CeO₂) or doped ceria for solar thermochemical fuel production applications. The redox reactions associated with using pure ceria (Equations (1)–(3) and doped ceria (Equations (4)–(6)) are presented below.

Application of pure ceria:

$${\mathrm{CeO}}_2\to {\mathrm{CeO}}_{2-\mathsf{\delta }}+\raisebox{1ex}{$\mathsf{\delta }$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\mathrm{O}}_2$$

(1)

$${\mathrm{CeO}}_{2-\mathsf{\delta }}+{ \mathsf{\delta }\mathrm{H}}_2\mathrm{O}\to {\mathrm{CeO}}_2+{\mathsf{\delta }\mathrm{H}}_2$$

(2)

$${\mathrm{CeO}}_{2-\mathsf{\delta }}+{ \mathsf{\delta }\mathrm{CO}}_2\to {\mathrm{CeO}}_2+\mathsf{\delta }\mathrm{CO}$$

(3)

Application of doped ceria:

$${\mathrm{Ce}}_{\mathrm{x}}{\mathrm{A}}_{1-\mathrm{x}}{\mathrm{O}}_2\to {\mathrm{Ce}}_{\mathrm{x}}{\mathrm{A}}_{1-\mathrm{x}}{\mathrm{O}}_{2-\mathsf{\delta }}+\raisebox{1ex}{$\mathsf{\delta }$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\mathrm{O}}_2$$

(4)

$${\mathrm{Ce}}_{\mathrm{x}}{\mathrm{A}}_{1-\mathrm{x}}{\mathrm{O}}_{2-\mathsf{\delta }}+{ \mathsf{\delta }\mathrm{H}}_2\mathrm{O}\to {\mathrm{Ce}}_{\mathrm{x}}{\mathrm{A}}_{1-\mathrm{x}}{\mathrm{O}}_2+{\mathsf{\delta }\mathrm{H}}_2$$

(5)

$${\mathrm{Ce}}_{\mathrm{x}}{\mathrm{A}}_{1-\mathrm{x}}{\mathrm{O}}_{2-\mathsf{\delta }}+{ \mathsf{\delta }\mathrm{CO}}_2\to {\mathrm{Ce}}_{\mathrm{x}}{\mathrm{A}}_{1-\mathrm{x}}{\mathrm{O}}_2+\mathsf{\delta }\mathrm{CO}$$

(6)

A = dopants (transition, alkali, alkaline earth, post-transition, or lanthanides).

In recent years, multiple review papers on solar thermochemical fuel production via WS and CDS reactions have been published by Haeussler et al. [27], Yadav and Banerjee [28], Carrillo and Scheffe [29], Budama et al. [30], Abanades [31], Warren and Weimer [32] and others. The published review articles cover the application of various metal oxides, including ceria materials, to solar fuel production. However, as the focus of the previously published review articles is very wide, the information reported about the application of the ceria materials is not very detailed. In the past, I have published a review paper covering detailed information about each study associated with ceria and doped ceria for solar thermochemical fuel production from 2006 to 2016 [33]. Since 2017, as new developments have been registered in ceria-based solar thermochemical WS and CDS research, this review paper is prepared to enlighten the solar thermochemical community.

2. Advances in Ceria-Based Solar Thermochemical Fuel Production since 2017

2.1. Year 2017

To achieve continuous and maximum production of solar fuel, it is crucial to understand the role of dopants in the performance of doped ceria toward the thermochemical WS and CDS cycle. Muhich and Steinfeld [34] conducted Density functional theory (DFT) computations to investigate how the dopants help the ceria materials attain a better solar fuel production rate. The authors reported that the tetravalent Zr- and Hf-doped ceria performed well in solar thermochemical applications due to the tendency of the Zr- or Hf–O bonds to store the energy released during the O₂ vacancy formation. Furthermore, they have also notified that the di- and trivalent dopants are unsuitable as they cannot promote the two electrons to the high energy Ce f-band during reduction, which is required to store sufficient energy released during the O₂ vacancy formation.

Hoes et al. [35] investigated the application of trivalent and pentavalent dopants via synthesizing and testing the paired charge-compensating doped ceria (PCCD) materials for the thermochemical WS and CDS by using a thermogravimetric analyzer (TGA) for the temperature and partial pressure of O₂ in the ranges of 900 °C to 1500 °C and 10⁻¹⁵ to 10⁻¹ atm, respectively. Several combinations of the PCCD were synthesized, which included Ce_0.9A_0.05Nb_0.05O₂ (A = Y, La, Sc) and Ce_xLa_(1−x)/2Nb_(1−x)/2O₂ (x = 0.75, 0.95). Among the various PCCD materials examined, the greatest reduction extent was reported for Ce_0.9Sc_0.05Nb_0.05O₂. The authors have also developed a model according to which PCCD materials can attain a higher solar-to-fuel energy conversion efficiency compared to the other ceria-based redox materials.

To avoid the heat losses and material stresses that occurred due to the temperature and pressure swing TR and RO steps, Tou et al. [36] developed a ceria membrane reactor (Figure 1) capable of working under constant temperature and pressure conditions. Solar experiments conducted at temperature = 1600 °C, pressure = 3 × 10⁻⁶ bar, and solar concentration = 3500 suns resulted in (a) 100% selectivity of CO₂ into CO and 0.5O₂ and (b) conversion rate equal to 0.024 μmol/s·m².

Ackermann et al. [37] simulated ceria foam structure (reticulated porous ceramic, RPC) containing mm-range and μm-range dual scale porosity to understand the heat and mass transfer associated with the thermochemical WS and CDS reactions (Figure 2). The accurate 3D digital geometry of the ceria foam structure (obtained from tomography scans) was used for the pore-level simulations and Monte Carlo ray tracing. The maximum O₂ yield per mass of ceria was recorded at porosity = 0.75 and pore size = 2.2 mm (due to the elevated mass transfer, moderate optical thickness, and good permeability). To reach the highest solar-to-fuel energy conversion efficiency, the authors advised using the ceria foam structure with the front side consisting of large pores (2.2 mm) and the rear side containing tiny pores (0.6 mm).

Jacot et al. [38] screened M_0.1Ce_0.9O_2−δ (M = Si, Ti, V, Cr, Zr, Nb, Rh, Hf, Ta, Nb, V, Pr, and Tb) redox materials for solar thermochemical WS and CDS application. A modified Pechini-type sol–gel method was applied to synthesize M_0.1Ce_0.9O_2−δ. The O₂ exchange capacity of each M_0.1Ce_0.9O_2−δ redox material was examined by performing TGA experiments. Moreover, scanning electron microscopy (SEM) and powder X-ray diffraction (PXRD) were utilized to assess the solubility and incorporation of each dopant in the ceria lattice structure. Because of the higher flexibility in O₂ vacancy formation, Hf-, Zr-, and Ta-doped ceria showed the maximum O₂ exchange capacity.

To improve the mass conversion, selectivity, and solar-to-fuel energy conversion efficiency of the solar thermochemical conversion of CO₂, Marxer et al. [39] utilized a 4 kW solar reactor containing a ceria RPC structure. This reactor was operated in a temperature and pressure swing operation mode under the radiative flux equal to 3000 suns. The authors reported that using ceria RPC possessing mm and μm-sized porosity helped improve the heat and mass transfer, further reflected in improved reaction kinetics. The results obtained during 500 consecutive thermochemical cycles indicate that application of ceria RPC supported achieving selectivity = 100%, mass conversion = 83%, and solar-to-fuel energy conversion efficiency = 5.25%.

Rothensteiner et al. [40] utilized a ceria-hafnia pyrochlore phase (Ce₂Hf₂O₇) for the solar thermochemical WS and CDS application. The Ce₂Hf₂O₇ was prepared by reducing the ceria-hafnia powder by employing a chemical reduction (in the presence of H₂/He) and then the inert gas reduction at 1552 °C. The thermogravimetric experimental results indicate the 1005 conversion of Ce⁴⁺ to Ce³⁺.

Takacs et al. [41] tested (a) RPCs with interconnected single-scale and dual-scale porosity made up of ceria and 10% and 20% Zr doped ceria and (b) commercially obtained ceria fibers for the solar thermochemical application using a solar thermogravimetric analyzer. The solar-TGA was operated in the temperature range of 927–1677 °C for the TR step and 677–1127 °C for the RO step. An efficient heat transfer rate was recorded in the case of ceria RPC with 8 ppi. In contrast, ceria RPC with 10 ppi showed the highest O₂ release and CO production. The O₂ release was improved by utilizing Zr-doped ceria at the expense of lower CO production. Compared to the RPC structures, the ceria fiber sample performed disappointingly.

A study of the role of onsite electronic configurational entropy on the solar thermochemical WS and CDS application was conducted by Naghavi et al. [42]. The authors reported that the onsite electronic configurational entropy was higher in the case of ceria (Ce⁴⁺/Ce³⁺ reduction). Hence, the ceria effectively converted the H₂O and CO₂ into solar fuels via thermochemical redox reactions. The authors also highlighted that terbium dioxide could be a suitable candidate for WS and CDS application due to its high electronic entropy.

Pappacena et al. [43] reported that the modifications in the surface defect structure/configuration of ceria-based redox materials could improve their catalytic ability toward WS and CDS. The authors have investigated the structural changes occurring on Ce_0.85Zr_0.15O₂ during its solar thermochemical fuel production application. They have reported that the formation of a zirconia–oxynitride phase via thermal aging of the Ce_0.85Zr_0.15O₂ in the presence of N₂ was beneficial for the formation of large surface vacancy clusters with an appropriate defect configuration, which further resulted in the improvement in the H₂ production via WS.

Ruan et al. [44] identified Ce₂Sn₂O₇ pyrochlore as a suitable candidate for the solar thermochemical WS application, superior to ceria. During the TR step at 1400 °C, the CeO₂ and SnO₂ reacted together to ultimately reduce Ce^IV to Ce^III via producing stable Ce₂Sn₂O₇ pyrochlore rather than metastable CeO_2−δ. During the RO step at 800 °C, the Ce₂Sn₂O₇ pyrochlore dissociated (Ce^III to Ce^IV) into CeO₂ and SnO₂, producing 3.8 times higher H₂ than the phase pure ceria.

Grobbel et al. [45] studied the heat transfer in the ceria particle bed experimentally (Figure 3). The obtained results were compared with the numerical simulations performed by using the Zehner–Bauer–Schlünder (ZBS) model. A solar simulator was used to irradiate the ceria particle bed. The variation in the temperatures across the bed was recorded at ambient as well as various vacuum pressures. At the end of the study, the authors concluded that the experimentally obtained results matched the numerical results generated by the model.

Using three approaches: flame spray pyrolysis, dry impregnation, and polymerized complex method, Mostrou et al. [46] synthesized Cr-doped ceria materials for the solar thermochemical WS and CDS application. The dry impregnation and polymerized complex methods produced mixed phase composition containing ceria and chromia. Phase pure Cr-doped ceria production was accomplished in the case of flame-spray pyrolysis. The presence of chromia was beneficial as it exhibited improved O₂ exchange and WS and CDS capacity of the Cr-doped ceria materials. Moreover, the H₂ production via WS was doubled with a significant reduction in the cycle time by 38 min. The H₂ production rate for Cr-doped ceria materials derived by the polymerized complex and flame-spray-pyrolysis methods is shown in Figure 4. Compared with the phase pure ceria, the Cr-doped ceria showed 20 and 500 times more H₂ and CO production at similar operating conditions.

2.2. Year 2018

Takalkar et al. [47] derived Ce_0.9M_0.1O_2−δ materials (where, M = Ni, Zn, Mn, Fe, Cu, Cr, Co, Zr) via co-precipitation method (Figure 5) and tested them in 10 consecutive thermochemical CDS cycles using TGA. Based on the TR and CDS ability at 1400 °C and 1000 °C, respectively, Ce_0.9Zn_0.1O_2−δ and Ce_0.9Fe_0.1O_2−δ were reported to be the best candidates. The authors further confirmed that during the 10 thermochemical CDS cycles, the phase composition of the Ce_0.9M_0.1O_2−δ materials remained unchanged. In contrast, the crystallize size increased considerably due to high-temperature sintering.

Oliveira et al. [48] examined ceria granules and ceria prepared from cork templates and polyurethane templates for thermochemical CDS in a solar reactor. At TR and CDS, temperatures equal to 1400 °C and 1000–1200 °C, the CO production by ceria granules was recorded to be twice that of the ceria foam and three times higher than the ceria RPC. The authors further noticed that the rise in the TR temperature helped improve the CO production yield. In contrast, a drop in the CDS temperature enhanced the CO production rate.

Muhich et al. [49] thermodynamically examined six different ceria and perovskite-based candidates for solar syngas production. The authors reported that the ceria and doped ceria materials are better candidates for the solar thermochemical cycles than the perovskites as they favor the RO step. Further, a higher solid-to-solid heat recovery was essential for ceria materials to attain a higher solar-to-fuel energy conversion efficiency. Contradictory to this, perovskites needed better gas-to-gas heat recovery. The maximum solar-to-fuel energy conversion efficiency was reported in the case of Zr-doped ceria, whereas the lowest was observed for La_0.6Sr_0.4Mn_0.6Al_0.4O₃.

Sol–gel-derived Ce_1−xM_xO_2−δ (M = Zr, Ni, Cr; x = 0, 0.05, 0.10, 0.15, 0.20) were utilized for the thermochemical CDS by Zhu et al. [50] The experimental setup shown in Figure 6 was employed to perform the TR and RO step at 1500 °C and 1000 °C, respectively. Ni-doped and Cr-doped ceria performed inadequately when compared to the Zr-doped ceria. Although 15% Zr-doped ceria produced a higher amount of CO (315.40 μmol/g), the oxidation rate was considerably lower when compared to the phase pure ceria material.

By using the same experimental setup (Figure 6), Zhu et al. [51] investigated the redox capacity and kinetics-based thermodynamic efficiency of the pure and doped ceria materials (doped with La, Ca, Pr, and Zr) by following the isothermal (1300, 1400, and 1500 °C) and near isothermal (TR at 1500 °C and CDS at 1300 °C) operating modes. Zr-doped ceria showed the maximum fuel production capacity compared to La-, Ca-, and Pr-doped ceria materials. Furthermore, La, Ca, and Pr doping in the Zr-doped ceria (forming ternary oxides) did not significantly improve fuel production aptitude. Although the doping of Zr in the ceria crystal structure helped improve fuel production, the oxidation step’s kinetics was considerably poor compared to the pure ceria sample. Pure ceria behaved better than the Zr-doped ceria regarding kinetics and thermodynamic efficiency.

Arifin and Weimer [52] investigated the rate-controlling mechanism and kinetic parameters of the ceria-driven WS and CDS. The experiments were carried out in a stagnation flow reactor. The authors reported that the WS reaction (at 750–950 °C and 20–40 vol% H₂O) followed the first-order reaction kinetics with an activation energy of 29 kJ/mol. However, the CDS reaction (at 650–875 °C and 10–40 vol% CO₂) was recorded to follow a complex kinetic model that heavily depends on the surface of the redox material.

Muhich et al. [53] conducted Density Functional Theory (DFT) calculations and TGA experiments to examine the application of doping of the paired charge compensating dopants in the ceria crystal structure for the solar thermochemical CDS. Using the modified Pechini method, Ce_0.9A_0.05B_0.05O₂ (where A = La, Y, Sc, and B = Nb) and Ce_0.9Hf_0.1O₂ combinations were prepared and examined. The DFT calculations and TGA experiments confirmed that the combined doping of trivalent and pentavalent dopant made the trivalent dopant behave like a tetravalent dopant. During the combined doping, the trivalent dopant had a more pronounced effect on reducing the doped ceria than the pentavalent dopant.

Falter and Pitz-Paal [54] analyzed the influence of vacuum pumping and inert gas sweeping on the thermodynamic efficiency of the ceria-driven thermochemical WS and CDS cycle. By using the published data, the authors have conducted a detailed energy balance analysis and reported that a thermochemical cycle operated at 1627 °C under vacuum pumping (pump efficiency = 50%) by using radiative flux = 5000 suns and 70% heat recuperation resulted in energy conversion efficiency ˃20%.

To identify a benchmark ceria-doped redox material for thermochemical WS and CDS, Jacot et al. [55] synthesized and tested ceria doped with (a) 10% Zr, (b) 10% Hf, (c) 7% Ta, and (d) 5%Nb in 50 thermochemical cycles by using a TGA (TR at 1500 °C for 90 min and CDS at 1000 °C for 40 min). Zr-, Hf-, and Nb-doped ceria showed promising results regarding high O₂ exchange capacity and long-term stability. The authors concluded that ceria co-doped with Hf, Zr, and Nb dopants (optimal average dopant radius of 0.8 Å) could be the most promising doped ceria material to produce solar thermochemical fuel.

Farooqui et al. [56] studied the kinetics of the ceria re-oxidation in the presence of CO₂ (temperature range = 700–1000 °C, and CO₂ feed concentration = 20 to 40 vol%). The authors reported that the CO production rate was considerably influenced by oxidation temperature and CO₂ concentration; however, the CO production yield was mainly affected by the oxidation temperature. Among the various kinetic models attempted, the Sestak–Berggren (SB) model, which belongs to the nucleation and grain growth reaction mechanism, provided a good fit. As shown in Figure 7, the activation energy for the ceria reoxidation via CDS was evaluated to be equal to 79.1 ± 6.5 kJ/mol for CO₂ feed concentration = 40%.

Budama et al. [57] proposed the utilization of ceria-based redox reactions for the co-production of H₂ and electricity (Figure 8). A thermodynamic model comprised of 23 components and 45 states was developed by assuming a quasi-steady state. Authors reported that a total of 10.96 MW total output (5.55 MW of H₂ and 5.41 MW of electricity) could be achieved by using a single tower driven by a solar radiation input = 27.74 MW (DNI = 900 W/m² and aperture area = 9.424 m²). The authors further stated that the H₂ production could be improved by enhancing the particle loop recuperator effectiveness.

Arribas et al. [58] employed a directly irradiated solar reactor (using a 7 kWth high flux solar simulator) to produce H₂ via a commercially purchased ceria-driven redox thermochemical WS cycle. Authors identified that a higher irradiance helped to attain a higher TR temperature; however, it was also responsible for the degradation of the redox sample. The reported results indicated that irradiating the ceria sample with maximum power for 9.5 min helped achieve a higher conversion but at the expense of the degradation of the redox material. In contrast, a similar conversion level was attained by exposing the ceria sample for 2 min only (irradiated at maximum power) without degrading the sample.

Li et al. [59,60] developed and utilized a thermodynamic model by considering the conservation of mass and energy and Gibbs criteria to investigate the effects of various operating parameters on the solar-to-fuel energy conversion efficiency of the ceria-based redox materials. The authors reported that applying 10% and 15% Zr-doped ceria resulted in higher efficiency than the pure ceria (solid heat recovery = 0%, gas heat recovery = 75%, and TR temperature = 1427–1577 °C). WS’s maximum efficiency (7.8%) was recorded at 1500 °C, gas heat recovery = 95%, and solid heat recovery = 0%. Applying gas heat recovery = 95% combined with a solid heat recovery = 90% resulted in a rise in the solar-to-fuel energy conversion efficiency up to 26.4% (for inert gas sweeping) and 25.2% (for vacuum pumping) at 1627 °C.

Roberts et al. [61] explored the influence of adding ceria to the iron oxide/zirconia solid solution on thermochemical WS application. At a constant TR temperature = 1450 °C, adding ceria to the iron oxide/zirconia solid solution significantly improved H₂ production at a WS temperature of 1200 °C. In contrast, there was no enhancement in the H₂ production at a WS temperature equal to 1000 °C. Dähler et al. [62] designed and fabricated a solar dish concentrating system for solar thermochemical fuel production (Figure 9). To conduct both TR and RO reactions simultaneously by alternating the solar input, the solar dish concentrating system contained (a) sun-tracking 4.4 m-dia. Solar dish concentrator, and (b) planar rotating reflector. The solar experiments using ceria redox material indicated achieving a peak solar concentration ratio of 5010 suns.

Bhosale and Takalkar [63] synthesized Ce_0.9Ln_0.1O₂; Ln = La, Pr, Nd, Sm, Gd, Tb, Dy, or Er to study the effect of doping of lanthanides in the ceria crystal structure on thermochemical fuel production. A TGA setup was used to test the reactivity of the derived Ce_0.9Ln_0.1O₂ materials by performing 10 thermochemical CDS cycles (TR at 1400 °C and CDS at 1000 °C) and out of 8 dopants, Sm, Er, Tb, Dy, and La helped ceria to improve its TR ability. On the other hand, only La could improve the CDS ability of the ceria.

2.3. Year 2019

Kong et al. [64] proposed a modification in the heat recovery system to improve the efficiency of the ceria-driven solar thermochemical fuel production process (Figure 10). Instead of the conventional approaches, the authors suggested that steam can cool the ceria after the TR step. As the finding reported, the steam injection can help with (a) in situ heat recovery and (b) quick cooling of the reduced ceria. Authors further claimed that the steam injection helped increase the solar-to-fuel energy conversion efficiency up to 24.36% (no gas and solid heat recovery needed) at 1327 °C.

By using additive manufacturing and the Schwartzwald replication method for coating, Hoes et al. [65] developed hierarchically ordered porous ceria structures for the thermochemical WS and CDS application. The results obtained via Monte-Carlo ray-tracing simulations and experiments indicate that the ordered structures possess a better radiation penetration depth and more uniform temperature distribution than the ceria RPC structure. It was also reported that the ordered structures could lead to a superior redox performance as they reached the peak temperature with the volume due to the higher heating rates and improved volumetric radiative absorption.

By using a co-precipitation of hydroxides method, Takalkar et al. [66] synthesized Ce_0.9M_0.05Ag_0.05O_2−δ materials (M = Ni, Zn, Mn, Fe, Cu, Cr, Co, Zr) for thermochemical conversion of CO₂. Multiple TGA-based CDS runs were conducted for each ceria-doped material, and the results obtained were analyzed using the Calisto Software (https://setaramsolutions.com/software/calisto, 1 July 2023). The redox reactivity of all the Ce_0.9M_0.05Ag_0.05O_2−δ materials was improved due to the inclusion of Ag in the ceria crystal structure. In terms of O₂ released, Ce_0.9Zn_0.05Ag_0.05O_2−δ was reported to be the best combination, whereas Ce_0.9Cr_0.05Ag_0.05O_2−δ was capable of producing higher levels of CO as compared to other Ce_0.9M_0.05Ag_0.05O_2−δ materials.

Tou et al. [67] developed a solar-driven membrane reactor. They tested it for syngas production via simultaneous WS and CDS. The solar reactor was operated using a solar simulator (4200 suns) and fed by a CO₂/H₂O mixture at molar ratios from 3:4 to 2:1. The reactor was operated isothermally (1500 to 1600 °C) and isobarically (0.2 to 1.7 Pa of O₂ pressure). The experimental results indicated a maximum reactant conversion of 0.7% and a syngas production rate equal to 2.3 μmol/cm²·min at 1600 °C and 0.2 Pa O₂ pressure.

Farooqui et al. [68] developed and tested a model for the WS and CDS driven by the ceria redox cycle. As shown in Figure 11, the oxidation and reduction reactor associated with the model was simulated by using ASPEN Plus^® (a series of CSTRs). The TR temperature and operating pressure have a considerable influence on the efficiency of the process. The maximum efficiency was recorded in the case of CDS only (35.41%), followed by a combined WS and CDS (35.26%) and WS only (30.84%). The techno-economic analysis indicated that the H₂ compressor and solar field + tower require 19% and 39% of the total equipment cost.

Li et al. [69] developed and tested a thermodynamic model for an inert sweep gas-driven isothermally operated ceria membrane reactor for the simultaneous WS and CDS. The thermodynamic computations performed at 1627 °C and gas heat recovery effectiveness of equal to 95% indicated solar-to-fuel energy conversion efficiency of 1.3% and 0.73% for WS under counter- and parallel-flow configurations, respectively. The CDS reactions acquired 3.2% and 2.0% solar-to-fuel energy conversion efficiency at similar operating conditions when the model was operated under counter- and parallel-flow configurations, respectively. The authors concluded that the efficiency values reported in the published literature were considerably higher than those reported because the thermodynamic models reported elsewhere may have violated the Gibbs criterion.

Carrillo et al. [70] reported developing a high-temperature tubular reactor that can assess the thermodynamic and kinetic behavior of a thermochemical WS and CDS cycle. Authors reported that this reactor arrangement could heat up to 1600 °C, pressure can be varied from the ambient to vacuum, the O₂ partial pressure can be decreased to 10⁻²⁹ atm, can handle a variety of redox material morphologies, maintain well-defined flow, and easily integrate with the steam generator. Constant temperature relaxation experiments were carried out using pure ceria to validate the reactor setup and the experimental procedure. The equilibrium data obtained for the ceria-based redox reactions in the temperature range of 900 to 1200 °C by maintaining the O₂ partial pressure from 4.54 × 10⁻¹⁸ to 1.02 × 10⁻⁹ atm well agreed with the published data. Furthermore, the kinetic and thermodynamic parameters, such as activation energy and defect formation enthalpies and entropies for the redox reactions, were also estimated and recorded to agree with the published literature.

Zoller et al. [71] estimated the variation in the solar-to-fuel energy conversion efficiency of a scaled-up ceria RPC solar reactor using a transient heat transfer model, which was experimentally validated. As per the numerical simulations, solar-to-fuel energy conversion efficiency can be increased from 6.12% to 12.75% by applying solar radiative input power of 50 kW and complete sensible heat recovery. It was also reported that if the macropore diameter of the ceria RPC was increased from 2 mm to 7 mm, the solar-to-fuel energy conversion efficiency was enhanced from 7.34% to 10.2%. The authors also indicated that increasing the solar radiative input power can decrease the time required for the TR of ceria.

Lee and Scheffe [72] developed a 200 W CO₂ laser-based heating system coupled with operando Raman spectroscopy, which can be applied to conduct thermochemical redox reactions. The system was characterized by reducing 10 mol% Gd-doped and pure ceria pellets in the presence of H₂. Samples were heated, and the full width at half maximum (FWHM) was monitored as a function of the reduction extent. In the case of Gd-doped ceria, as the reduction extent was increased (maximum up to 0.056), the FWHM was enhanced. However, a further rise in the reduction extent (greater than 0.056) exhibited a drop in the FWHM. The authors concluded that the laser-based heating system coupled with operando Raman spectroscopy is a valuable tool for studying redox materials for thermochemical fuel production applications.

De la Calle and Bayon [73] developed a model to simulate a dynamic 1 MWth ceria-driven solar thermochemical discontinuous and continuous syngas production plants (Figure 12). The authors reported that the efficiency values acceptable for large-scale implementations could be attained if ceria and inert gas heat exchangers are operated carefully. When no solid and gas heat recovery was applied, the system attained lower efficiency values (4 to 9%). The mass flow of ceria was also reported to be a crucial factor that affected efficiency considerably. At the optimized mass flow of ceria, the maximum efficiency of the plant was in the range of 9 to 9.3%. In addition to the mass flow of ceria, the location at which the plant is operated also plays an important role. Alice Springs was reported to be the best location choice as it helped to attain the highest efficiency in the range of 9.5 to 9.7%.

Takalkar et al. [74] investigated the co-precipitation synthesized Ce_0.75Zr_0.2M_0.05O_2−δ (M = Cr, Mn, Fe, Ni, Co, Zn) materials for the solar thermochemical CDS. The phase composition analysis (XRD) confirmed the formation of targeted metal oxide with no metal impurities (Figure 13). In contrast, the morphology analysis (SEM) showed agglomeration of roundish particles. The synthesized materials were tested for 10 thermochemical cycles. The TR and CDS steps were carried out at 1400 °C and 1000 °C, respectively. Except for the Ce_0.75Zr_0.2Mn_0.05O_2−δ, all the other Ce_0.75Zr_0.2M_0.05O_2−δ materials showed enhanced levels of O₂ released and CO production compared to the Ce_0.75Zr_0.25O_2−δ. The maximum O₂ release was recorded in the case of Ce_0.75Zr_0.2Zn_0.05O_2−δ (105.1 μmol/g·cycle), whereas Ce_0.75Zr_0.2Ni_0.05O_2−δ produced the highest amount of CO (170.5 μmol/g·cycle).

Luciani et al. [75] reported the importance of the hydrothermal method in preparing the ceria-based redox materials for improved O₂ exchange and redox reactivity. Authors have synthesized Ce_0.75Zr_0.25O₂ using the hydrothermal method with urea as the chelating agent. They have compared its redox activity toward the WS and CDS to the previously investigated Zr-doped ceria materials. The physicochemical characterization confirmed the formation of Ce_0.75Zr_0.25O₂ with high surface area and O₂ mobility. The TGA results further corroborated that the Ce_0.75Zr_0.25O₂ produced a higher quantity of CO and H₂ than the Zr-doped ceria investigated in the past.

Shi et al. [76] conducted a study in which three different approaches, namely, one-pot evaporation-induced self-assembly (EISA), co-precipitation, and hydrothermal methods, were scrutinized for the synthesis of Ce_0.5Zr_0.5O₂ materials, which was further utilized for the thermochemical CDS. The TGA-based thermochemical CDS experiments were carried out in the temperature range of 1100 °C to 1400 °C. The Ce_0.5Zr_0.5O₂ synthesized via EISA produced 0.54 and 0.69 mL of O₂/g·cycle and 0.98 and 1.56 mL of CO/g·cycle more than the Ce_0.5Zr_0.5O₂ synthesized via co-precipitation and hydrothermal methods, respectively. Authors concluded that the higher lattice defect and O₂ vacancies and greater O₂ exchange capacity of the EISA-derived Ce_0.5Zr_0.5O₂ were responsible for the better redox reactivity than the co-precipitation and hydrothermal synthesized Ce_0.5Zr_0.5O₂.

2.4. Year 2020

Haeussler et al. [77] developed ceria RPC (10–60 ppi) and tested it in a solar reactor (Figure 14) in which the TR and WS/CDS reactions were conducted in the temperature ranges of 1400–1450 °C and 700–1100 °C, respectively. Authors reported that ceria’s TR and fuel production ability increased with a rise in the TR temperature and reduced pressure. Moreover, the oxidation rate was improved as the CO:CO₂ ratio decreased, or the inlet CO₂ concentration increased. In total, 64 cycles were conducted in which the ceria RPC produced an average of ∼280 Ncm³/cycle (TR = 1400 °C and WS/CDS = 900 °C), eight times higher than the previously investigated ceria porous foams.

In a similar work, Haeussler et al. [78] investigated the effect of operating parameters on the CO production ability of the ceria RPC (open-cell foam structure shown in Figure 15). The TR yield of the ceria was improved by increasing the TR temperature by reducing the pressure. Moreover, the CO-produced rate was enhanced due to (a) the drop in the RO temperature and (b) the surge in the concentration of CO₂. The authors concluded that due to the stable granular microstructure, the ceria RPC could produce 10 mL/min·g of CO, which was much higher than the data reported in the published literature.

Oliveira et al. [79] tested ceria granules developed by using cork templates via (a) green water and (b) an acetone solvent method and ceria RPC (developed from the polyurethane templates) for the solar thermochemical WS application (TR at 1400–1450 °C and WS at 950–1150 °C). The authors reported that using the ceria granules improved the fuel yield and kinetics of the thermochemical WS cycle. A 25% and 32% higher average H₂ generation was recorded in the case of ceria granules prepared via the green water method compared to the ceria granules developed via the acetone solvent method and ceria RPC, respectively. Moreover, the H₂ production rate reported in the case of ceria RPC was 40% less than the rate observed in the case of ceria granules. The authors concluded that the lower mean cell size of the cork-derived ceria granules was responsible for the higher WS rate.

The influence of alkali (Li), alkaline earth (Mg, Ca, Sr, Ba), and post-transition (Sn) metal dopants on the redox reactivity ceria toward thermochemical CDS was examined by Takalkar et al. [80]. The doped ceria materials were derived via co-precipitation methods and characterized via X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The XRD analysis confirmed the pure phase composition. The SEM images indicated an average particle size of 150 to 200 nm for each doped ceria material. Ten TGA-based thermochemical CDS cycles were carried out in the temperature range of 1400 and 1000 °C to test the TR and CDS ability of the synthesized doped ceria materials. The experimental results confirmed that the post-transition Sn-doped ceria was the most redox-active composition, resulting in the release of 107.6 μmol of O₂/g·cycle and the production of 180.5 μmol of CO/g·cycle.

The effect of the different morphologies of ceria on the thermochemical WS and CDS was experimentally investigated by Haeussler et al. [81]. For the first cycle, the highest redox reactivity was recorded in the case of ceria fibers. However, for the subsequent cycles, the cork-templated ceria and the ceria foam represented better redox reactivity than ceria fiber due to the lower thermal sintering. The redox reactivity was improved in the case of CDS by increasing the inlet CO₂ concentration. In contrast, the rise in the inlet steam concentration did not affect the WS reactivity. The investigation was concluded by reporting the cork-based ceria as the top choice, with a CO production rate of 3.1 mL/min/g higher than the structured ceria reactors examined in the past.

Takalkar and Bhosale [82] examined various combinations of co-precipitation-synthesized Zr-doped ceria for thermochemical CDS by using a TGA setup. XRD analysis indicated that the crystallite size of the Zr-doped ceria was increased with a rise in the dopant concentration. SEM images confirmed an average particle size of 40 to 100 nm for all Zr-doped ceria samples. The TGA experiments further exhibited that the 15% and 30% doping of Zr in the ceria crystal structure was beneficial to attain maximum O₂ release and CO production, respectively.

Naghavi et al. [83] screened several Ce⁺⁴ redox oxides to find a suitable combination with a high entropy of reduction and low reduction enthalpy using DFT calculations. The authors scrutinized a variety of Ce⁺⁴ redox oxides by considering the Inorganic Crystal Structure Database (ICSD) and published literature. CeTi₂O₆ with the brannerite structure was reported to be the best candidate for a smaller reduction enthalpy, a large entropy of reduction, and better thermal stability than the other Ce⁺⁴ redox oxides.

For a better understanding of the ceria-based thermochemical WS and CDS reactions, Rawadieh et al. [84] investigated the mechanism. They developed a kinetic model using the experimentally obtained temperature-dependent O₂ and fuel production profiles. The authors stated that the TR of Zr-doped ceria and pure ceria initiated at 777 °C and 877 °C, respectively. For both samples, the release of O₂ reached maxima at 1.6 μmol/g, and the O anions were removed from the surface at 1047 °C and 1127 °C for Zr-doped ceria and pure ceria, respectively. The authors added that in the case of WS, the production of H₂ from H₂O happened in two steps with activations energies equal to 77.7 kJ/mol and 191.2 kJ/mol.

Arifin et al. [85] tested co-precipitation synthesized Zr-, Gd-Zr-, and Pr-Zr-doped ceria materials in a stagnation flow reactor to examine their WS performance. A 10% to 25% doping of Zr into the ceria structure was responsible for improving both TR and WS ability. The rate of O₂ evolution for Zr-, Gd-Zr-, and Pr-Zr-doped ceria materials is shown in Figure 16. On the other hand, the Gd-Zr-, and Pr-Zr-doped ceria produced less amount of H₂ as compared to the 10–25% Zr-doped ceria. For all doped ceria materials, the kinetic analysis indicated that the WS conducted at 1000 °C and with 30 vol% H₂O was a surface-controlled reaction and followed a deceleratory power law F-model (like pure ceria).

Hao and Jin [86] thermodynamically studied the isothermal operation of the ceria-based solar reactor for (a) individual and (b) combined WS and CDS in the temperature range of 1500 to 1600 °C. Regarding the solar-to-fuel energy conversion efficiency, rate of conversion, and fuel production aptitude per cycle, the isothermal CDS was observed to be better than the isothermal WS. As per the authors, the CDS was more favorable than WS because of (a) the use of CO₂, which provided a higher partial pressure of O₂ during oxidation; (b) no need for a phase change process in CDS (no liquid-to-gas conversion required); and (c) the required solar radiation concentration and temperatures can be easily obtained with contemporary technologies. While investigating a combined WS and CDS, authors recorded that syngas production heavily depends upon the CO₂:H₂O feed ratio. Furthermore, the syngas composition can be adjusted by varying (a) the partial pressure of O₂ and (b) the isothermal temperature.

Luciani et al. [87] explored the application of K⁺ and Cu²⁺/Fe³⁺ co-doped ceria-zirconia for the combined splitting of H₂O and CO₂. In the case of K-Cu-CeZr oxide, the WS reaction was more favorable with no CO₂ reduction. K-Fe-CeZr oxide helped produce H₂ at low and CO at high temperatures. The kinetic analysis indicated that WS needed more neighboring reduced sites than CDS. The authors concluded that the utilization of the K⁺ and Cu²⁺/Fe³⁺ co-doped ceria-zirconia worked as per the following three schemes: scheme-1: WS is promising below 650 °C, scheme-2: WS and CDS both are favorable in the temperature range of 560 to 700 °C and scheme-3: CDS is promising above 700 °C.

Takalkar et al. [88] developed Ce_0.9Ln_0.05Ag_0.05O_2−δ materials (Ln = La, Pr, Nd, Sm, Gd, Tb, Dy, Er) via co-precipitation. They examined their redox reactivity in 10 thermochemical cycles. The TGA-based TR and CDS were conducted at 1400 °C and 1000 °C, respectively. The XRD and SEM characterizations confirmed targeted composition formation, crystallite size in the 32 to 64 nm range, and spherical nanostructured particle morphology. As per the TGA results, Ce_0.911La_0.053Ag_0.047O_1.925 was the best choice in terms of a higher average TR (72.2 μmol/g cycle) and CDS (136.6 μmol/g cycle) ability over 10 thermochemical cycles (Figure 16).

By using FORTRAN and ASPEN Plus^®, Farooqui et al. [89] simulated TR and combined WS and CDS steps of the ceria-driven redox cycles by developing and using a moving-bed reactor model. Simulation results indicated that 0.198 of non-stoichiometry can be achieved in ceria during the TR step at 1600 °C and 10⁻⁷ bar vacuum pressure. During the RO step, the WS yield (97%) was recorded to be more favorable than the CDS (91%). Moreover, in the case of both WS and CDS steps, the selectivity and the ceria conversion were recorded to be higher than 90%.

2.5. Year 2021

Abanades and Haeussler [90] used indirectly heated tubular packed-bed and directly irradiated cavity-type solar reactors to examine the redox performance of the fibrous ceria pellets obtained from Zircar Zirconia Inc. Florida, NY, USA, toward thermochemical WS and CDS. In the case of the packed tubular bed, 2.3 mL/g·min of H₂ production rate was recorded when the TR and WS steps were conducted at 1400 °C and 950 °C. By operating the cavity-type solar reactor under reduced pressure for the TR step at 1400 °C and using pure CO₂ for the CDS step at 950 °C, 9.5 mL/g·min of CO production was realized with a solar-to-fuel energy efficiency = 9.4%. The authors reported that a further improvement in the fuel production rate and oxidation kinetics could be achieved by (a) applying low O₂ partial pressure, (b) decrease in the RO temperature, and (c) increasing the CO₂ partial pressure.

Yang et al. [91] projected and numerically examined a solar-driven fixed-bed cavity receiver–reactor array by considering the ceria-based thermochemical WS cycle. The numerical analysis used a linear matrix model (for heat recuperation) and a lumped parameter model (for TR and RO steps). For the concentration ratio = 3000 suns, the effectiveness of the gas-phase heat recovery = 0.95, TR temperature = 1627 °C, relative partial pressure of O₂ = 0.01, the effectiveness of the solid-phase heat recovery and solar-to-fuel energy conversion efficiency was recorded to be 80% and 27%, respectively. The authors reported that the solar-to-fuel energy conversion efficiency could be improved by employing more receiver–reactors. Moreover, the upsurge in the concentration ratio (5000 suns) and the effectiveness of the gas-phase heat recovery (100%) resulted in a high solar-to-fuel energy conversion efficiency (42%).

Haeussler and Abanades [92] developed and tested ordered ceria porous structures (Figure 17) for thermochemical fuel production using a solar reactor system. Employing a low partial pressure of O₂ was favorable for attaining a higher reduction extent and fuel yield. On the other hand, the reduction in the temperature or rise in the partial pressure of CO₂ fed to the reactor increased the RO of the ceria structure. Compared to the non-ordered ceria RPC foams, the ordered ceria porous structures with axial porosity gradient showed a better TR extent with no improvement in the RO extent. Wood particles were added while synthesizing the ordered ceria porous structures to improve the micro-scale interconnected porosity within the struts, enhancing fuel production.

A process consisting of a ceria membrane solar reactor (for CDS) integrated with the mixed ionic-electronic conducting ceria membrane (for constant temperature O₂ removal) was designed, developed, and used by Abanades et al. [93]. This system produced 10 times higher CO than the previously used solar membrane reactors at 1550 °C and partial inert gas pressure equal to 10⁻⁵ bar (Figure 18). The authors reported that the CO production yield could be improved by increasing the TR temperature from 1450 to 1550 °C and the CO₂ feed flow rate. Authors concluded that this reactor system could be improved by (a) using perovskite-based mixed ionic-electronic conducting, (b) increasing the stability of the membrane, and (c) enhancing the active surface area of the redox material by coating a thin dense MIEC membrane on porous redox support.

Abanades et al. [88] employed ceria porous microspheres for thermochemical WS and CDS to utilize porous microspheres’ excellent flowability and large surface area. A directly and indirectly irradiated packed bed solar reactor was filled with the ∼150–350 μm size ceria porous microspheres. Authors reported that the investigated ceria porous microspheres produced 1.8 mL of fuel/min·g when TR and RO were carried out at 1400 °C and <1050 °C, respectively. The authors confirmed that the shape and internal porosity of the ceria microspheres were maintained even after exposure to the high flux solar irradiation for 32 h (19 redox cycles).

Based on their usefulness in fluidized-bed-type solar thermochemical reactors, porous Zr-doped ceria (Ce_0.8Zr_0.2O₂ and Ce_0.9Zr_0.1O₂) microspheres were synthesized and tested for the CDS by Chen et al. [94] The porous Zr-doped ceria microspheres were developed by using the acrylamide-assisted sol–gel method followed by sintering at 1500 °C for 1 h. Thermochemical CDS cycles conducted in the 800 to 1400 °C temperature range confirmed that the Zr-doped ceria microspheres produced more CO than the Zr-doped ceria powder samples. After undergoing 12 cycles, more thermal degradation was recorded in the case of Ce_0.9Zr_0.1O₂ microspheres as compared to the Ce_0.8Zr_0.2O₂ microspheres.

Sediva et al. [95] used in situ Raman spectroscopy integrated with a packed bed reactor for the investigation of the defect chemistry and redox kinetics of the trivalent (La) and tetravalent (Hf) cation doped ceria-based solar thermochemical WS and CDS cycles. The results indicated that the Hf-doped ceria produced higher fuel than the La-doped ceria. The in situ Raman spectroscopy further helped to understand that the doping of ceria with a tetravalent dopant (due to the intrinsic O₂ vacancies) supported the higher amount of fuel production, whereas trivalent dopant (due to the extrinsic O₂ vacancies) resulted in a rise in the rate of fuel production.

Orfila et al. [96] synthesized Ce_0.9Me_0.1O_y (Me = Fe, Co, Mn, and Zr) powder samples via the co-precipitation method and tested them for the WS application. A high-temperature tubular reactor coupled with the gas analyzer was used. The TR and WS steps were performed at 1300 °C and 800 °C, respectively. Ce_0.9Fe_0.1O_y exhibited better redox reactivity and thermal cyclability among the investigated doped ceria powder samples. The authors have further developed the Ce_0.9Fe_0.1O_y RPC structure and tested them for the WS. The reported results confirmed that the Ce_0.9Fe_0.1O_y RPC resulted in a higher fuel production rate than the Ce_0.9Fe_0.1O_y powder sample.

2.6. Year 2022

Li et al. [97] developed a thermodynamic model (based on the first and second laws of thermodynamics) to estimate the maximum possible solar-to-fuel energy conversion efficiency of a metal oxide-based solar thermochemical WS and CDS cycle by considering more realistic operating conditions. Although La-Co-Mn-Al perovskite was better in the perovskite category, pure ceria, and Zr-doped ceria seemed the best choices for the thermochemical WS and CDS. The authors concluded that ceria materials still have issues to be addressed, such as the unfavorable reduction (needed high temperatures, and the rate was slow) and the requirement of the pre-heating, which affected the efficiency of the process.

Ben-Arfa et al. [98] used robocasting to fabricate the 50 mm diameter ceria scaffold discs for the thermochemical WS and CDS application. The ceria scaffold discs obtained after sintering possessed (a) surface area = 1.58 m²/g, (b) ceria struts contained mesopores <75 Å, and had a micropore (<20 Å) surface area of 0.29 m²/g, (c) size of the voids = 500 μm, and (d) diameter of the struts = 450 μm. The CDS ability of the ceria scaffold disc was evaluated by performing TGA experiments (TR in the temperature range of 1050–1400 °C under Ar and CDS at 1050 °C under 50% CO₂). To attain 90% TR and RO, the ceria scaffold disc needed 40 and 10 min, respectively. The CO production rates of the ceria scaffold disc were comparable to the ceria RPC structures and higher than the ceria powder samples.

To study the ceria-based solar thermochemical WS, Wang et al. [99] developed a mathematical reactor model by considering solar irradiation, the surface interface reaction, and macro- and micro-scale heat and mass transport processes. The results obtained after employing this model indicated that (a) the temperature distribution axially and the kinetics of the redox reaction were affected significantly by the thermal non-equilibrium, (b) The RO step required half of the time needed for the TR step, (c) a higher bed temperature and rate of reaction can be attained by applying an elevated irradiant heat flux and (d) more defects formation was realized in the small size particles.

Thanda et al. [100] designed, developed, and tested a conjoined receiver/reactor system (Figure 19) containing ceria-coated zirconia RPC for the thermochemical WS to produce solar H₂. The thermochemical redox reactions were carried out in the 1000 °C to 1400 °C temperature range using 150 kW of radiative power. After a successful demonstration, this reactor system produced 8.8 g of H₂/cycle, lower than the theoretical maximum. The authors reported that the lower H₂ production happened because of non-homogeneous temperature distribution inside the receiver. The authors further stated that the heat losses via thermal radiation from the receiver surface were relatively high. They hence advised to use either a shutter that covers the aperture or a small window to reduce the thermal losses.

Wang et al. [101] analyzed the ceria-driven thermochemical WS using an epitrochoidal rotary reactor (Figure 20), which can comfortably achieve solid-phase heat recovery. The thermodynamic analysis indicated a solar-to-fuel energy conversion efficiency of 13.2% when TR and WS were carried out at 1500 °C and 800 °C, respectively (by keeping solar concentration ratio = 3000 suns, the effectiveness of the gas phase heat recovery = 0.95, effectiveness of the mechanical energy recovery = 0.85, and molar ratio of inert sweep gas and ceria = 10).

To improve the rate of CO production, which directly enhanced the solar-to-fuel energy conversion efficiency, Chen et al. [102] doped the Ce_0.85Zr_0.15O₂ with noble metal catalysts such as RuO_x, PtO_x, and IrO_x. Among the three noble metal catalysts, doping/loading of IrO_x helped the Ce_0.85Zr_0.15O₂ the most to improve the CO production rate. A 0.4 atom% doping of IrO_x improved the CO production rate of Ce_0.85Zr_0.15O₂ by 35%. A further increase in the atom% doping (1%) of IrO_x helped Ce_0.85Zr_0.15O₂ enhance its CO production rate by three times. Multiple thermochemical CDS cycles were conducted, and the obtained results indicated a stable catalytic performance of the IrO_x doped Ce_0.85Zr_0.15O₂.

As the solar-to-fuel energy conversion efficiency heavily depends upon the TR of the metal oxide, Bai et al. [103] examined the application of a high-temperature electrochemical O₂ pump (EOP) capable of supporting the O₂ removal during the TR of ceria-based redox thermochemical cycle. EOP was considered a replacement for inert gas sweeping as the later procedure requires extra heating and gas separation energy. The transient model developed by the authors indicated that using EOP instead of NS helped improve the solar-to-fuel energy conversion efficiency of the ceria redox cycle by a factor of 1.64.

Onigbajumo et al. [104] used an indirectly irradiated fluidized bed reactor containing ceria particles for thermochemical H₂ production via WS (Simulation study using Aspen Plus). This setup was also used to produce electricity via integrating with the O₂ co-production and heat recovery units. The simulation results confirmed that this reactor arrangement helped achieve H₂ production with a minimum selling price of 3.92 USD/kg H₂. The authors indicated the following factors responsible for the determination of the minimum selling price of H₂: (a) rate of discount, (b) steam Rankine cycle, (c) power block, (d) redox material cost, (e) storage of H₂, and (f) O₂ price.

Zoller et al. [105] demonstrated the production of kerosene via WS and CDS by using a solar-driven 50 kW ceria-based thermochemical reactor system shown in Figure 21 (operated by using IMDEA energy, Spain, with a mean solar concentration ratio = 2500 suns). The performance of the solar reactor system was evaluated by considering the following metrics, (a) selectivity of the WS and CDS reactions, (b) quality of the syngas produced, (c) purity of fuel produced, (d) energy efficiency, and (e) stability of the materials used during high-temperature operation. A total of 62 thermochemical cycles were performed (6–8 cycles per day), and solar-to-syngas energy conversion efficiency equal to 4.1% was attained.

Lee et al. [106] investigated the oxidation kinetics of Ce_0.9La_xYb_yZr_0.1−x−yO_2−δ (x = 0, 0.05, 0.1, y = 0, 0.05, 0.1) using synthetic air as the oxidant. TGA experiments were conducted by performing an isothermal TR step (at 1400 °C). The RO step was carried out by varying the temperature from 400 °C to 1200 °C. The results indicated that the oxidation kinetics of the Ce_0.9La_xYb_yZr_0.1−x−yO_2−δ was quicker than Zr-doped ceria (10%) but slower than the pure ceria. RO conducted below 700 °C showed better oxidation kinetics and lower activation energy than the RO carried out above 700 °C. The authors concluded the study by stating that the valence of the dopant dictates the oxidation kinetics rather than the ionic radius.

Lampe et al. [107] attempted to optimize the H₂-producing ceria-based solar reactor operation to achieve the highest solar-to-fuel energy conversion efficiency. The authors have identified and optimized the most critical process parameters associated with the temperature of the reactions, times required for the cycle, and mass flows used during the operation. The authors reported that after optimizing most of the processing conditions, a maximum solar-to-fuel energy conversion efficiency of 1.1% was attained.

2.7. Year 2023

Using the Pechini sol–gel method, Lee et al. [108] synthesized La- and Yb-doped zirconia-ceria redox materials and tested their redox reactivity as a function of ionic radius and valence for thermochemical WS and CDS. Regarding the TR ability, La- and Yb-doped zirconia-ceria showed promising results compared to pure and Zr-doped ceria. Regarding the CDS step, La-doped zirconia-ceria showed quicker RO kinetics when compared to the Yb-doped zirconia-ceria and Zr-doped ceria. The detailed analysis of the CDS kinetics confirmed that the oxidation rate was controlled mainly by the first layer of the oxidation and surface exchange process. It was also concluded that instead of extrinsic O₂ vacancies, the ionic radius of the dopants had a significant influence on the CDS kinetics.

For the very first time, Uxa et al. [109] investigated the O₂ isotope exchangeability of the CeO_2−δ, Ce_0.9M³⁺_0.1O_1.95−δ (with M³⁺ = Y, Sm), and Ce_0.9M⁴⁺_0.1O_2−δ (with M⁴⁺ = Zr) samples by performing experiments in the presence of C¹⁸O₂ gas atmospheres. The reactions were carried out in the 300 to 800 °C temperature range using an IR radiation-heated experimental setup integrated with a Secondary Ion Mass Spectrometry (SIMS). The Sm-doped ceria showed promising results regarding the improved surface exchange coefficients and diffusion coefficients of O₂ compared to the 10% Y-doped and Zr-doped ceria and pure ceria materials. The authors also reported that the lowest possible activation energy was reported in Sm-doped ceria.

Wang et al. [110] proposed an epitrochoidal rotary reactor for the thermochemical WS application. They optimized its geometry to attain the most favorable solar-to-fuel energy conversion efficiency. A hit-or-miss Monte Carlo method was used to guesstimate the geometric compression ratio. Various geometrical parameters of the epitrochoidal rotary reactor were evaluated. Among all, the cam-to-rotor size ratio and the number of rotor sides were found to have the utmost noteworthy influence on the solar-to-fuel energy conversion efficiency. The authors concluded the study by reporting the optimal geometrical compression ratio = 13 and solar-to-fuel energy conversion efficiency = 17% at a solar concentration ratio = 3000 suns.

Ma et al. [111] fabricated porous ceria microspheres using a sol–gel process assisted by the acrylamide as a solidification and pore-generation agent. After sintering at 1500 °C, ceria microspheres of 800 μm diameter and 2.9 m²/g surface area were obtained. The thermochemical CDS ability of the porous ceria microspheres was evaluated by performing TGA experiments in the temperature range of 1000 to 1400 °C. These porous ceria microspheres released 49 μmol of O₂/g and produced 88 μmol of CO/g. The porous ceria microspheres showed better redox reactivity and thermal stability than the porous ceria granules examined by the same research team.

Razmgar et al. [112] synthesized the NbO_x ceria catalyst using the incipient wetness impregnation method. They tested it for syngas production via thermochemical WS and CDS. At 600 °C, CO₂ conversion = 79% with syngas selectivity = 80% was recorded for the 12 wt% inclusion of Nb in the ceria structure. Regarding the conversion rates, NbO_x ceria catalysts performed better than the Rh-CeO₂, CeO₂-ZrO₂, and V₂O₅-CeO₂ supported catalysts.

3. Summary

Since 2017, much progress has been made in applying ceria-based solar thermochemical WS and CDS process. Plenty of dopants (Ni, Zn, Mn, Fe, Cu, Cr, Co, Zr, Hf, Si, Ti, V, Nb, Rh, Ta, La, Pr, Nd, Sm, Gd, Tb, Dy, Er, Mg, Ca, Sr, Ba, Sn) were used for the preparation of binary, ternary, as well quaternary ceria oxide powders by using various synthesis approaches such as sol–gel, Pechini, and co-precipitation methods. These ceria oxide powders were tested for multiple WS or CDS cycles using laboratory-scale high-temperature TGA, packed bed, and fluidized bed reactors. The influence of various process parameters, such as TR and RO temperatures and times, material loading, flow rates of the gases, etc., were investigated. The selected ceria oxide powders were transformed into RPCs, porous granules, microspheres, and ordered porous structures. These structures were tested in direct and indirect solar-irradiated reactor arrangements. Various new reactor arrangements were proposed and developed, including membrane reactor, cavity-based reactor, in situ Raman spectroscopy integrated with a packed bed reactor, solar dish concentrating reactor system, receiver–reactor arrays, and Epitrochoidal rotary reactor. Small-scale (up to 10 kW) and large-scale (up to 250 kW) solar-driven ceria-based systems were tested for the pilot-scale application of WS and CDS. The kinetics and thermodynamics associated with the ceria-driven thermochemical cycle were also scrutinized using the experimental results. In addition to the experimental work, a lot of theoretical studies were also conducted to estimate the solar-to-fuel energy conversion efficiency, to understand the mechanism of O₂ vacancy formation and the role of the dopants in improving TR and RO abilities, to estimate the electronic configuration entropy, to study the surface defects and structural changes, to investigate the role of heat recovery and vacuum pumping, and to explore the heat and mass transfer associated with the solar reactors. In some of the studies, to improve the process’s economics and reduce the cost, a co-production of H₂ and electricity was also considered. The research directions the solar thermochemical community can explore in the case of solar thermochemical fuel production using ceria materials are as follows:

• Couple the ceria materials with perovskites (chemically and mechanically) to utilize the advantage of both redox materials.
• Use CH₄ instead of an inert gas as the reducing agent to decrease the cycle temperatures and achieve fuel production in thermal reduction and oxidation steps.
• Invest more into developing the pilot scale reactor system under concentrated solar power.
• Convert the powdered samples into porous structures, which can attain the highest possible mass and heat transfer rates with minimum resistance.