In photocatalytic CO₂ conversion, the regulation of structure and the binding site location plays an important role in improving conversion efficiency. Oxides of rare Earth metals not only exhibit an excellent capability during the CO₂ adsorption process, but also present a high charge separation efficiency via the addition of surface oxygen vacancies (Muhammad et al., 2020). Hence, rare Earth metals have been considered as the highly viable options for photocatalytic CO₂ conversion. Ceria was widely studied due to its high chemical stability and outstanding oxygen storage-and-release capability. Besides above mentioned advantages, Fiorenza and co-workers have further emphasized the photocatalytic capability of ceria. They pointed out that the combination of light and temperature can greatly enhance the performance of ceria photocatalysis (Fiorenza et al., 2022). Despite the intrinsic ability of pristine CeO₂, a higher CO₂ conversion performance is desired by the optimization of the conduction band (CB) position and quick electron-hole recombination. To achieve high activity and product selectivity in photocatalytic CO₂ conversion, numerous catalysts modified techniques, such as metal or non-metal doping (Wang M. et al., 2022), building heterojunction (Hamidah Abdullah et al., 2017), and oxygen vacancy engineering (Hezam et al., 2020), have been reported, which benefit to promote CO₂ molecule activation and adsorption. We mainly summarized the improved CeO₂-based photocatalysts through element doping, heterojunction building and vacancy engineering in this section.

Elements doping has impacts on the electronic structures of the semiconductor photocatalyst, particularly the bandgap. Transition metal are the common dopant elements for optical and photoelectrochemical semiconductor modification, among which the most widely used include Fe, Ni, Cr, Ag, and so on (Wang Y. et al., 2019; Prajapati et al., 2022). Through the dispersion of Ag particles on CeO₂ surfaces, Cai and co-authors have investigated the impact of surface plasmon effect on photocatalytic CO₂ conversion efficiency. It is noteworthy that the Ag-CeO₂ photocatalyst, which was created via a straightforward solvent-based method, can simultaneously produce CH₄ and CH₃OH after 6 h of visible light irradiation, giving 100 and 35 mol·g⁻¹ h⁻¹ of CH₄ and CH₃OH, respectively (Cai et al., 2018). Furthermore, in numerous studies, non-metal doping has been mentioned as a viable method to improve photocatalytic activity. Non-metal-loaded elements that could widen the absorption zone to absorb visible light include nitrogen (N) (Shen et al., 2020), sulfur (S) (Wang et al., 2021), and phosphate (P) (Li W. et al., 2021). Excellent CO₂ reduction was demonstrated by nitrogen-doped CeO₂, with CO and CH₄ yields of 1.83 and 1.25 μmol·g⁻¹, respectively, being 2.5 times higher than those of nanocasted ordered mesoporous CeO₂ (OMCe) (Figures 7A–C) (Shen et al., 2020).

The photocatalytic efficiency of pure CeO₂ as a photocatalyst is insufficient. Heterojunction of CeO₂ and other nanomaterials is able to overcome the limitations of single component and lead to the synergistic effect, which has been considered as one of the most promising structures to improve its photocatalytic activity. Heterojunction possesses a great benefit in separating photoinduced (e)—(h+) pairs and takes full advantage of the individual functional properties of each component (Zhang W. et al., 2020). Dai and co-workers prepared the CeO₂/Bi₂MoO₆ heterostructured microspheres with different CeO₂ contents via a facile solvothermal route. The heterojunction of CeO₂/Bi₂MoO₆ nanocomposite showed a high specific surface area and a significantly enhanced response to visible light, which is conductive to improve the charge carrier separation and transfer efficiency. As a result, 5% CeO₂-Bi₂MoO₆ with the best activity in photocatalytic CO₂ reduction toward the generation of CH₃OH and C₂H₅OH realized the yields of 32.5 and 25.9 μmol·g_cat ⁻¹ for CH₃OH and C₂H₅OH, respectively (Dai W. et al., 2018). Layered graphitic carbon compound (g-C₃N₄) exhibits wonderful photocatalytic activity due to its high reducibility and visual lightweight absorption, but the low specific area and speedy charge recombination in pristine g-C₃N₄ have prevented its practical use (Bian et al., 2018). Liang et al. (2019) designed a hollow g-C₃N₄@CeO₂ heterostructure photocatalyst with abundant oxygen vacancies to enhance light utilization and catalytic activities. Because of the unique structure, the synergetic effect and oxygen vacancies, g-C₃N₄@CeO₂ makes multiple reflections of light in the cavity and contributes greatly to the enhanced CO₂ adsorption capability, thus achieving a much earlier CH₄ generating and a higher CH₄ concentration in comparison to that of the pristine g-C₃N₄ and CeO₂. The g-C₃N₄@CeO₂ with a CeO₂ loading of 49.7 wt% showed the best CO₂ photoreduction performance with the yields of CH₃OH (5.2 μmol·g⁻¹), CO (16.8 μmol·g⁻¹) and CH₄ (3.5 μmol·g⁻¹) (Figure 7D). Further mechanism analysis deduced that the photogenerated electrons in the CB of the g-C₃N₄ are able to migrate to the CB of the CeO₂ while the holes generated in the CeO₂ will transfer to the valence band (VB) of the g-C₃N₄ for the g-C₃N₄@CeO₂ composites (Figure 7E). Such interfacial electron transfer contributes greatly to the separation efficiency of electron-hole pairs resulting and thus the accumulated electron-hole pairs on the CeO₂ surface promote the generation of CH₄, which was demonstrated by the obvious surface photovoltage spectroscopy (SPS) signal in visible light region of g-C₃N₄@CeO₂ (Figure 7F). This work provides a novel approach to produce g-C₃N₄-based photocatalysts in the absence of noble metal for the high efficiency CO₂ photocatalytic reduction and clear the charge transfer and catalytic mechanism from the perspectives of structural design and atomic-level regulation.

The poor photocatalytic CO₂ performances on pure CeO₂ are mainly due to its wide band gap and low light absorption (Xie et al., 2017). Therefore, oxygen vacancy introduction in the CeO₂ nanocrystal structure also has attracted strong interest, which can enhance visible-light absorption ability. Introducing oxygen vacancies can make CO₂ molecules more easily adsorbed and activated on the photocatalyst surface due to the abilities of providing active sites and increasing the CO₂ adsorption energy. Furthermore, the defect energy level generated by oxygen vacancies is supposed to promote the separation and suppress the recombination of electron-hole and change the transfer path of carriers (Wang et al., 2020). Lai et al. (2022) have introduced surface oxygen vacancies by preparing porous iron-doped ceria, which significantly improved the light absorption performance of ceria in the application of photocatalytic CO₂ conversion. In spite of introducing oxygen vacancies is able to significantly enhance the CO₂ photoreduction performance of CeO₂, its activity would decrease because the stability of the generated oxygen vacancies is insufficient with gradually filling or losing during the photoreduction process. In order to achieve and maintain a high CO₂ reductive activity of CeO₂, Wang and co-workers introduce Cu into CeO₂ to generate and stabilize oxygen vacancies (CeO_2–x ) (Wang M. et al., 2019). They found that the Cu-introduced sample show a lower photoluminescence intensity, indicating the extremely enhanced charge transfer between CeO₂ and Cu species and the effectively inhibited electron-hole recombination, thus leading to a prolonged carriers’ lifetime and stabilized oxygen vacancies. By further catalytic mechanism analysis, they proposed the possible reaction mechanism of photocatalytic CO₂ conversion on the Cu/CeO_2-x catalysts and confirmed that the introduction of Cu alters the configurations of the adsorbed CO₂ on CeO_2–x . Therein, Cu/CeO_2-x -0.1 presented the best photocatalytic performance with a CO yield of 8.25 μmol·g⁻¹ during 5 h of Xe-light irradiation and excellent chemical stability (Figures 7G–I).

Because of the concerns on the sustainability of fossil fuels, photocatalysis is always research focus to create effective, sustainable, and varied energy storage technologies. H₂ is considered as a promising renewable energy source due to its high energy density of 143 kJ·g⁻¹ and the advantages of low emission and no pollution (Xu and Xu, 2015). For a long time, most of the H₂ is produced by hydrocarbon steam reforming or coal gasification, which are high energy-consuming (Zhao et al., 2020). Currently, the photocatalytic hydrogen evolution reaction (HER) has been considered as a prospective approach to produce H₂ by an environment-friendly way and attracted lots of research. However, HER is a multielectron, endothermic uphill reaction that requires a high positive Gibb’s free energy (Sultana et al., 2021). Generally, 2.458 eV energy is required to split one water molecule to generate one hydrogen molecule, thus a highly active photocatalyst that possesses capability of decreasing the energy barrier is necessary. Numerous semiconductor photocatalysts with a narrow bandgap and imperative photoredox behavior have been applied in the photolytic HER. Recently, on account of the easy conversion between Ce³⁺ and Ce⁴⁺ and abundant oxygen vacancies, CeO₂ has been used in the photocatalytic HER. Dong et al. have synthesized the CeO₂ nanorods and found that the pure CeO₂ presented a favorable photocatalytic activity with a high H₂ production rate of ∼25.10 μmol·g⁻¹ (after solar light irradiation for 5 h) (Dong et al., 2018). Moreover, a variety of CeO₂-based nanostructures, such as Au/CeO₂ (Primo et al., 2012), ZnO/CeO₂ (Zeng et al., 2014), CeO₂/g-C₃N₄ (Zou et al., 2017), etc., were reported to strengthen the photocatalytic activity of HER. In this section, we overviewed the mechanism of photocatalytic water splitting for H₂ production on CeO₂-based catalysts. Meanwhile, applications of various CeO₂-based nanostructures in photocatalytic HER were summarized from pristine CeO₂ to element-doped CeO₂ and CeO₂-based heterostructure.

As introduced by previous reports, water splitting over ceria mainly includes water hydroxylation and H₂ formation. As shown in Figure 8A, water molecule first adsorbs by the oxygen atom on the top of the cerium atom of CeO₂ (111) and occurs dissociation near the oxygen vacancy of defect enriched CeO₂ (111) accompanied by the bonding between one hydrogen atom of water and the surface oxygen atom of CeO₂. Water dissociation into hydroxyl occurs, followed by hydroxyl decomposition and H₂ liberation through an asymmetric process. Therein, the surface vacancies facilitate the water dissociation step and the process is accompanied by the oxidation of Ce³⁺ to Ce⁴⁺ (Lu et al., 2011; Chen et al., 2013; Hansen and Wolverton, 2014; Wu et al., 2019). Because different CeO₂ facets show different oxygen and cerium coordination, many studies have developed CeO₂ nanostructures with highly active exposed crystal planes in photocatalytic HER. Tong and co-authors prepared the oriented hexagonal CeO₂ nanorods with (110) as the predominantly exposed planes by a facile electrochemical method (Figures 8B,C) (Lu et al., 2011). Hexagonal CeO₂ nanorods present an excellent redox capability thus a super photocatalytic activity for HER. The H₂ evolution rate on CeO₂ nanorods was about 741 μmol g⁻¹ h⁻¹ with Na₂S-Na₂SO₃ sacrificial agents, which was much higher than that of the commercial CeO₂ or CdS. However, the photocatalytic efficiency of pure CeO₂ is still not desirable on account of the rapid electron-hole recombination and low proportion of surface active sites. Therefore, non-metallic element doped CeO₂ and CeO₂-based heterojunction structures have been extensively studied. Hao et al. synthesized a N,S-doped C-encapsulated CeO₂ hinge-like nanostructure (CeO₂@N,S-C HN) to improve the separation efficiency of photoinduced electrons-hole pairs and expose more accessible active sites in the photocatalytic reactions (Figures 8D,E) (Hao et al., 2020). The CeO₂@N,S-C HN showed a mass-normalized rate of H₂ production of 555 μmol h⁻¹·g⁻¹, which is remarkably higher than that of CeO₂@C HN (405 μmol h⁻¹·g⁻¹), CeO₂ HN (325 μmol h⁻¹·g⁻¹), and commercial CeO₂ (195 μmol h⁻¹·g⁻¹) (Figure 8F). Moreover, the CeO₂@N,S-C HN had a long-term stability without visible activity degeneration after four cycles. Through the spin-polarized DFT calculations, they found that the CeO₂@N,S-C HN with a lower Gibbs free energy for the formation of the intermediate state (H*) of 0.08 eV exhibited the best HER performance in comparison to pristine CeO₂ (0.30 eV) and N,S-C (0.93 eV) (Figure 8G). They further deduced that the strong interaction between N,S-C HN and CeO₂ (111) was beneficial to the photogenerated carriers transfer and separation, thereby enhancing the catalytic performance for HER. In order to reduce the bandgap energy and enhance plasmonic characteristics toward the visible-light range, introducing nitrogen dopants (such as substitutional and interstitial N sites) to ceria is often used, which can increase oxygen vacancies and Ce³⁺ active defects. Van Dao et al. (2021) synthesized a nitrogen-doped ceria coupled with nitrogen-doped graphene (3.9% N-CeO₂/N-Gr), which exhibited better photocatalytic properties than N-CeO₂ and CeO₂. In this case, a super HER rate of 3.7 μmol·mg_cat ⁻¹·h⁻¹ under visible-light irradiation and remarkable durability were realized by the obtained N-CeO₂/N-Gr photocatalyst. The reduced bandgap energy confirmed by the DFT calculations can be ascribed to the synergistically electronic effects between 3.9% N-CeO₂ and N-Gr. Besides, the unique structure with a N-Gr shell, N-Gr network and 3.9% N-CeO₂ core benefits to play the best role for every component. For the N-CeO₂/N-Gr photocatalyst, the N-CeO₂ possesses more oxygen vacancies and Ce³⁺ active defects for enhancing plasmonic properties in the visible-light range; and the N-Gr perfectly performs as an electron reservoir to accumulate plasmon-induced electrons traveling from 3.9% N-CeO₂ and to suppress the recombination of photoinduced electron-hole pairs. The photocatalytic performance of the heterojunctions is strongly influenced by the interfacial contact between the CeO₂ and the other semiconductors (Xu et al., 2014). Therefore, CeO₂ is often combined with other photocatalysts/co-catalysts. Shen and co-authors composited CeO₂ with W₁₈O₄₉ to prepare the novel Z-scheme heterojunction photocatalyst W₁₈O₄₉/CeO₂. The W₁₈O₄₉/CeO₂ composite showed a high hydrogen production efficiency of ∼0.2061 mmol·g⁻¹·h⁻¹, which was about 1.93 times higher than that of the pure CeO₂. They proved that the Z-scheme heterojunction structure at the contact interface of W₁₈O₄₉ and CeO₂ greatly increased the accumulation of photo-generated electrons and the separation efficiency of the charge carriers, thus enhancing the photocatalytic performance for hydrogen evolution.

Splitting water electrochemically and its reversed process form hydrogen and oxygen cycle for energy storage and energy conversion, which involve four critical half-cell reactions, i.e., the HER and oxygen evolution reaction (OER) for energy storage by water electrolysis, and the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) for energy conversion in fuel cells. Photocatalytic HER is a promising approach to producing H₂ in an environment-friendly way, CeO₂-based photocatalysts for HER have been overviewed in the previous section of photocatalytic applications. In this section, we mainly focus on the OER in the water-splitting process. Because of the slow kinetics of the four-electron process, it is generally known that the OER is the bottleneck in the water-splitting processes. In this regard, it is of a great desire to design active OER catalysts that can accelerate O-H bond breaking and O-O bond formation (Gao et al., 2018; Zhang et al., 2021). On account of its superior ionic conductivity and large oxygen storage capacity, CeO₂ is frequently utilized as a cocatalyst to enhance the charge transfer and energy conversion efficiency of OER catalysts, as well as the OER kinetics (He et al., 2019). The synergistic effect, high surface area, and unique structure of the catalyst all contribute to the enhanced activities (Zhao et al., 2018). Cobalt-based spinel oxides, as one of the promising OER electrocatalysts in alkaline medium, show limited catalytic activity due to the abundant existence of relatively inactive Co³⁺ octahedral coordination (Wang et al., 2016). Qiu and co-workers have built the CeO₂-induced interfacial Co²⁺ octahedral sites and oxygen vacancies to improve the OER performance of Co₃O₄ (Figure 9A) (Qiu et al., 2019). As shown in Figure 9B, the ratio of Co³⁺/Co²⁺ in CeO₂/Co₃O₄ was lower than that of the pristine Co₃O₄, accompanied by the increased ratio of Ce⁴⁺/Ce³⁺ and oxygen vacancies, which was evidenced by X-ray photoelectron spectroscopy (XPS) and Co L-edge X-ray absorption near-edge structure (XANES). As expected, CeO₂/Co₃O₄ interfacial nanotubes show excellent OER performance with an obviously decreased overvoltage of 265 mV at a current density of 10 mA·cm⁻², which is much lower than those of Co₃O₄ (340 mV) and commercially available RuO₂ catalysts (360 mV) (Figure 9C). Meanwhile, CeO₂/Co₃O₄ interface nanotubes with a Ce/Co ration of 2:20 can achieve an ultrahigh mass activity with a current density of 128.6 A·g⁻¹ at a given overpotential of 340 mV and enable an OER Faradaic efficiency of ∼99%. Li et al. (2022) have reported a very different conclusion on the valence of active Co ions by using CeO₂ nanoparticles anchored Co layered double hydroxide (LDH) as a catalyst. They deemed that the Co³⁺ with strong Lewis acidity helps the binding of OH⁻ and thus benefiting the formation and transformation of oxygen-containing intermediates by forming CoOOH active species (Li Z. et al., 2021). Besides the electrochemical water-splitting in alkaline electrolyzers, the proton exchange membrane water electrolyzers show more promise for practical applications and have benefits for overall water-splitting. However, OER electrocatalysts in acidic conditions are facing great challenges in their longevity on account of the highly oxidizing and corrosively acidic operating environments. Gou and co-workers have prepared the amorphous IrO _x /CeO₂ nanowire electrocatalysts, featured by nanoscale intimacy and amorphous structure, for water oxidation in 0.5 M H₂SO₄, which are providing abundant binary interfaces and favorable kinetics for acidic OER (Gou et al., 2022). They pointed out that CeO₂ as an electron buffer can not only regulate the adsorption of oxygen intermediates and lowers the activation barrier of OER, but also suppress the over-oxidation and dissolution of iridium (Figure 9D). As a result, IrO _x /CeO₂ significantly enhanced the OER activity and stability. Iridium/CeO₂ ratios of 0.6 M (IrO_x/CeO₂-0.6) delivered the best electrocatalytic OER performances with a high mass activity of 167 A g_Ir ⁻¹ at 1.51 V, a low overpotential of 220 mV at 10 mA·cm⁻², and a stable performance for 300 h of continuous operation in acid (Figures 9E,F). This work provides a reasonable strategy for constructing acid-resistant OER electrocatalysts.

The proton exchange membrane fuel cell (PEMFC) with considerable power density and energy efficiency is one of the most promising candidates for renewable and sustainable energy conversion devices because of its zero CO₂ emissions, which is widely used as clean energy conversion devices, especially in vehicles and some mobile systems powering. The configuration of PEMFC can be seen in Figure 10A. During the PEMFC working, H₂ gas at the anode is oxidized to release protons and electrons, then the released electrons generate electricity at the external circuit. The protons, i.e., hydrogen ions, migrate through the polymer electrolyte (proton exchange membrane) to recombine with electrons and oxygen and produce water at the cathode (Majlan et al., 2018). Therein, the catalyst is necessary for accelerating the oxidation of hydrogen gas and reducing oxygen gas to water. The high cost owing to the use of noble metals as catalysts slows the development and commercialization of PEMFCs (Tasic et al., 2009). Therefore, reducing the use of noble metal catalysts and gradually replacing noble metal with a non-noble metal in the anode, is an important direction for the development of PEMFCs.

Fiala and co-workers have reported a novel carbon supported anode catalysts consisting of thin films of ceria with low Pt loadings (Pt²⁺-CeO₂) (Fiala et al., 2016). They pointed out that the presence of Pt²⁺ is important for enhancing electrocatalytic activity and the stability of Pt²⁺ on ceria enabled the resistance to hydrogenation reduction. Meanwhile, the formation of such stable surface complexes benefits to prevent the degradation of the composite catalysts and improve the durability. To clear the reasonable amount of Pt in the Pt-doped CeO₂ thin film, the chemical state of Pt was related to the Pt content in the ceria film via XPS spectra analysis (Figure 10B). They found that low Pt loadings would give such thin film with Pt species in the +2 state only. However, the excess Pt content would give rise to the appearance of metallic Pt, indicating that the sites for the accommodation of Pt²⁺ in square-planar coordination of O atoms were limited. Remarkably, only small amounts of surface Pt²⁺ can realize a high power density value of 0.41W cm⁻², which is comparable to the reference commercial catalysts (Figure 10C). When calculated into specific power, this value was 205 kW·g⁻¹ _(Pt), much higher than that of reference commercial catalysts (0.22 kW·g_(Pt) ⁻¹).

Solid oxide fuel cells (SOFCs) are a form of fuel cell that consists of a porous anode and cathode separated by a highly dense electrolyte [such as yttria-stabilized zirconia (YSZ) or gadolinium doped ceria (GDC)]. Because of their considerable electrical efficiency, the possibility of using a variety of fuels, and the benign environmental impact, SOFCs have attracted wide attention (Menzler et al., 2010). Although many key materials for SOFCs have been developed in the last decades, there are still great challenges in improving durability and decreasing cost (Suntivich et al., 2011). Currently, the researches on SOFCs materials mainly focus on the optimization of anode, cathode and electrolyte. As CeO₂ can be not only used as catalysts or catalyst supports in electrodes, but also explored as electrolytes with improved ionic conductivity, it was widely used in SOFCs (Mogensen, 1994). For example, non-doping CeO₂ nanocubes were used as an electrolyte in advanced fuel cells and exhibited outstanding performance (Li et al., 2018). In the case of the CeO₂-coated NaFeO₂ proton-conducting electrolyte, the addition of the CeO₂ shell layer not only increased the number of oxygen vacancies for proton transport but also introduced heterointerface for enhancing ionic boundary conductivity (Xing et al., 2021). This section summarizes the applications of CeO₂ used as an electrolyte and electrode component in SOFCs.

When used as an electrolyte, ceria is generally doped with other trivalent element (or less commonly bivalent) to realize a significant improvement of the ionic conductivity. Common dopants include calcium (Sudarsan and Moorthy, 2019), yttrium, samarium (Bhabu et al., 2016), and gadolinium (Khan et al., 2019). In comparison to the single-phase electrolytes, doping increases the oxygen vacancy concentration, which dictates ionic conduction. Li and co-workers have synthesized CeO₂ nanocube with exposed the (100) and (110) active crystal facets and obtained an excellent power density of 406 mW·cm⁻² at 600 C by using the un-doped CeO₂ nanocubes as an electrolyte for advanced fuel cell (Figures 11A,B) (Li et al., 2018). To reveal the origin of the excellent fuel cell performance, they calculated the number of oxygen vacancies of the H₂-treated CeO₂ (CeO_2-δ) and CeO₂ nanocubes from the XPS spectra, which showed an increased amount of oxygen vacancies and a higher Ce³⁺ percentage for the H₂-treated CeO₂ (Figures 11C,D). Moreover, a lower activation energy of 0.63 eV was found for CeO₂ nanocubes-introduced fuel cells, which is superior to that of YSZ (0.91 eV) (Strlckler and Carlson, 1965) and other oxide ion conductors (Rupp and Gauckler, 2006). As confirmed by previous studies, major proton conducting mechanism in oxides is conducted by oxygen vacancies, thus proton conduction is highly related to the concentration of oxygen vacancies. Therefore, they inferred that the surface of the defective CeO₂ nanocube can enhance proton transport through its abundant oxygen vacancies and its conductivity is governed by interfacial ionic transportation. As shown in Figure 11E, the semiconducting energy band differences between the reduced CeO₂ (CeO_2-δ) at the anode and oxidized CeO₂ at the cathode side form the double layer device, which can achieve effective charge separation and avoid the device short-circuiting. This work demonstrated the CeO₂/CeO_2-δ heterogeneous interfaces with a high ionic conductive path conductor and provided a well electrolyte choice for advanced SOFCs, which widen the selecting range of various electrolyte candidates. Besides, ceria is often combined with other components, for example, transition metal layered oxides (TMLOs), carbonates, oxides (Raza et al., 2011), hydroxides (Hu et al., 2006), sulfates (Liu et al., 2005), and halides (Zhu et al., 2001), etc., to create a better low-temperature SOFCs (LT-SOFCs) (Wang et al., 2008; Benamira et al., 2011; Fan et al., 2013). Xing and co-workers reported the CeO₂ coated NaFeO₂ as an electrolyte material for LT-SOFC (Xing et al., 2021). They found that the CeO₂ shell layer introduces more oxygen vacancies for proton transfer in the obtained electrolyte material. Meanwhile, the heterointerface is in favor of the O²⁻ grain boundary conductivity. As a result, both the open-circuit voltage (OCV) and the power output of the fuel cells were greatly improved. The fuel cell delivered an admirable power output of 727 mW·cm⁻² at 550°C by using the core cell NaFeO₂-CeO₂ composites as an electrolyte.

Another widely explored field in SOFCs developments is ceria-based composite cathodes and anodes, such as Pd@CeO₂ (Adijanto et al., 2013), Ru-CeO₂ (Zhan and Barnett, 2005), Ni-CeO₂ (Lee et al., 2013), and so on. As a high cost of the noble-metal catalysts, Thieu and co-workers prepared the catalyst-modified cells with Cu and CeO₂ (Cu-Ce-cell), in which Cu was inserted directly near the electrolyte-anode interface and CeO₂ was incorporated into the anode support to effectively facilitate thermochemical reforming reactions (Figure 11F) (Thieu et al., 2022). As shown in Figure 11G, Cu-Ce-cell exhibited a record high performance with a peak power density of 1,120 mW·cm⁻² at 600°C. The peak power densities of Cu-Ce-cell did not significantly change with the different steam-to-carbon ratios (SCRs). Furthermore, Cu-Ce-cell showed long-term performances with only slight voltage degradation of ∼2.76% over 250 h under a constant current load of 0.15 A·cm⁻² by using butane fuel with an SCR of 3 at 600°C.

Lithium-sulfur (Li-S) batteries are regarded as a promising energy storage system for new generation portable electronic devices and electric vehicles due to their high theoretical energy density (2,600 Wh·kg⁻¹) and specific capacity (1,675 mAh·g⁻¹) as well as the low cost, natural abundance, and environmentally friendly nature of sulfur (Urbonaite et al., 2015). However, the insulating property of sulfur and its discharge products leads to limited reaction kinetics during the redox processes, which results in low utilization of sulfur and insufficient practical specific capacity. Furthermore, in the multistep sulfur reduction reaction, the conversion of the soluble lithium polysulfide intermediates (LiPSs) into insoluble Li₂S₂/Li₂S has a much higher apparent activation energy, which will lead to the accumulation of polysulfides in the liquid electrolyte, a continuous loss of active sulfur from the cathode and the final battery failure. Therefore, introducing electrocatalysts in Li-S cells towards fast sulfur conversions is of great significance for decreasing the activation energy of the precipitation of Li₂S₂/Li₂S solids and improving Li-S battery performances (Liu et al., 2021). Metal oxides, sulfides, nitrides, phosphides, and their heterostructures have been screened in a large number of studies (Yuan et al., 2016; Sun et al., 2017; Zhang M. et al., 2020; Hua et al., 2021; Xia et al., 2021). Liang et al. (2016) have pointed out that only materials with redox potentials in a targeted window can react with polysulfides to form active surface-bound polythionate species. And they deemed that the formed active surface-bound polythionate species have direct correlation with the superior Li-S cell performance (Figure 12A). These metal oxides with redox potentials between 2.4 and 3.05 V (such as VO₂ and CuO) possess a window that lies just above the redox voltage of soluble polysulfides and thus promotes polythionate formation. A higher redox potential will lead to excessive oxidization of polysulfides, and a lower one shows no redox reaction with polysulfides. CeO₂ is a polar metal oxide that can chemically adsorb the sulfur species and boost redox reaction through catalysis. Meanwhile, CeO₂ nanocrystals possess a proper redox potential of 2.72 V vs. Li/Li⁺, which is higher than that of polysulfides (2.10 V vs. Li/Li⁺). Accordingly, the CeO₂ nanocrystals might oxidize the intermediate polysulfides to thiosulfates and polythionates via the surface redox chemistry (Figures 12B,C) (Ma et al., 2017). Therefore, many CeO₂-based nanostructures have been reported to be used as electrocatalysts to promote Li-S cell performance.

Ma and co-workers demonstrate an advanced sulfur host material prepared by implanting CeO₂ nanocrystals homogeneously into bimodal micromesoporous nitrogen-rich carbon nanospheres (CeO₂/MMNC) (Ma et al., 2017). This hybrid with high conductivity and abundant hierarchical pore structures can effectively store and capture sulfur species. Meanwhile, the CeO₂ can promote the chemical redox reactions of polysulfides, thus significantly enhancing their retentions upon cycling. As a result, CeO₂/MMNC-S cathodes showed the excellent capacity and rate performance, as well as an ultralong cycle life. Specifically, the cathode with a sulfur mass loading of 1.4 mg·cm⁻² realized the reversible capacities of 1,066 mAh·g⁻¹ at 0.2 C after 200 cycles and 836 mAh· g⁻¹ at 1.0 C after 500 cycles, and a high cycle stability of 721 mAh·g⁻¹ at 2.0 C after 1,000 cycles with a low capacity decay of 0.024% per cycle. Besides carbon nanospheres, other carbon materials, such as carbon nanotubes (CNTs) and graphene, are often used as supports to disperse the active CeO₂ nanocrystal (Ma et al., 2017). Yuan et al. have designed a three-dimension porous conductive network of CeO₂-webbed carbon nanotubes (CeO₂@CNT) to provide fast electron paths and achieve a good rate capability in the Li-S batteries (Xiao et al., 2018). In order to increase the sulfur utilization, CNT particles with high tap density were applied to realize a uniform melt-diffusion of sulfur by Gueon and co-authors. They further demonstrated microdomain sulfur by coating the CeO₂ nanoparticles in a CNT with an open pore structure. The open pore structure of CeO₂/CNTP and the microdomain sulfur enabled fast kinetics in the redox reaction of sulfur, and therefore achieving excellent cycling stability of only 0.044% per cycle for 300 cycles at 2 C and a high capacity of 5.6 mAh·cm⁻² even at high sulfur loading (Gueon et al., 2020). In addition, CeO₂ was used as cathode materials alone by constructing phosphorus-modulated porous CeO₂ (P-CeO₂) as reported by Tao et al. (2022). The P-CeO₂ cathode showed a better oxidation-reduction kinetics of LiPSs and a faster Li⁺ diffusion rate in comparison to that of bare CeO₂. Meanwhile, they have confirmed that the P-CeO₂ cathode presented stronger adsorption of Li₂S₆, higher redox peak current, and earlier precipitation of Li₂S in comparison to the bare CeO₂. Therefore, introducing P resulted in an improved initial capacity of 1,027 mA·h·g⁻¹ (bare CeO₂: 895.7 mA·h·g⁻¹) at 0.2 C.

Apart from the sulfur host, CeO₂ has also been used in the separator modification. Generally, the soluble polysulfides can be immobilized in the cathode side by the multifunctional modified interlayer. Cheng et al. have designed a multifunctional separator modified by CeO₂ decorated graphene (CeO₂@G) to accelerate polysulfide redox reaction and immobilize polysulfides by strong chemisorption (Cheng et al., 2021). Zhang and co-authors fabricated a functional bilayer separator based on 0D (CeO₂ nanocrystals)/1D (carbon nanofibers) composite mats (PAN/CNF-CeO₂) (Zhang J. et al., 2020). By integrating the advantages of highly conductive carbon nanofibers and electrocatalytically active CeO₂ nanocrystals, the Li-S batteries with the obtained PAN/CNF-CeO₂ separators showed high S utilization, excellent thermal stability, superior rate performance and enhanced cycling stability (Figure 12D). Specifically, the Li-S cell exhibited an initial reversible capacity of 1,359 mAh·g⁻¹ at 0.2 C and a low capacity decay rate of 0.04% per cycle at 0.5 C over 300 cycles (Figure 12E).