1. Introduction

In recent times, there has been a growing interest in lanthanides, particularly Cerium (Ce) [1]. This surge is attributed to their economic competitiveness [2], with Cerium, in particular, being as abundant as copper [3]. Moreover, the Rare Earth (RE) elements are recognized for their low toxicity, posing no harm to health through ingestion or inhalation [4, 5]. Consequently, various Cerium compounds are now being explored for prospective applications in diverse sectors, such as electrocatalytic hydrogen generation [6, 7, 8], thermochemical fuel production [9, 10, 11], water-splitting devices [12, 13, 14, 15, 16, 17, 18, 19, 20], biodiesel production catalysts [21, 22], sensors [23, 24, 25, 26, 27, 28], biosensors [29, 30, 31, 32], and more. Recent investigations into the use of ceria nanoparticles for medical applications [33, 34, 35, 36, 37, 38, 39, 40, 41] underscore the potential of cerium oxide-based nanomaterials.

2. Application of Cerium oxide and soluble salts-based materials for corrosion protection

Highly doped metal alloys serve as essential construction materials for various purposes [42, 43]. Like any solid-state industrial product, their exceptional mechanical strength is derived from both their chemical composition and structure (Figure 1).

Figure 1.
Correlations between the chemical composition, structure, form, and properties that predetermine the application of any solid-state product.

The strength enhancement of these materials is primarily influenced by intermetallic inclusions [44] and the granulometry of the basic metallic lattice [45, 46]. However, the presence of doping elements leads to galvanic corrosion [47, 48, 49], which can progress into other localized corrosion forms [50], such as crystallographic pitting, intergranular corrosion [51], and crevice corrosion [52, 53]. These corrosion types can lead to stress corrosion cracking [47, 50, 54]. Another factor contributing to the increased vulnerability of highly doped alloys compared to low-doped ones is the disruption of the native oxide layer on their surface [55]. Additionally, the native oxide layers on Al-based alloys demonstrate amphoteric behavior, rendering them susceptible to dissolution in both alkaline and acidic environments [50, 56, 57].

The foundational principles governing the use of cerium compounds for efficient metal corrosion mitigation through inhibition and the formation of conversion coatings were first proposed by the groundbreaking works of Hinton et al. [58, 59] in the mid-1980s, followed by Davenport et al. [60, 61] and Mansfeld et al. [62, 63] in the early 1990s. Since then, the interest in cerium conversion coatings has steadily grown, particularly in optimizing the conditions for their deposition.

The initial research on lanthanides focused on their cathodic inhibition ability, arising from the reaction between lanthanide ions and hydroxyl ions produced by the oxygen reduction reaction (ORR), as depicted in Eqs. (1)–(5) [64, 65, 66]:

O2+2H2O+4e−→4OH−E°=0.401VSHEE1

O2+2H2O+4e−→H2O2+2OH−E°=0.682VSHEE2

H2O2+2e−→2OH−E°=0.710VSHEE3

H2O2+2e−→2H2OE°=1.776VSHEE4

H2O2+2H3O++2e−→2H2OE°=1.776VSHEE5

The hydroxyl anions/OH⁻ anions generated by these reactions react with dissolved Ce³⁺ and Ce⁴⁺ ions, forming insoluble cerium hydroxide precipitates according to Eqs. (6) and (7):

Ce3++3OH−→CeOH3E6

Ce4++4OH−→CeOH4E7

These reactions lead to the sealing of cathodic areas on the AA2024 alloy through the deposition of insoluble precipitates, as illustrated in Figure 2.

Figure 2.
Visual model of Al-Cu alloy with Ce-inhibitor deposits on its surface: (1) Inhibitor deposits; (2) Surface oxide layer; (3) Aluminum matrix; (4) Cathodic copper intermetallic inclusions [67].

In this context, Bethencourt et al. conducted a comparative study on the corrosion inhibition efficiencies of chlorides from various lanthanide elements [68]. As a primary outcome, the authors determined that CeCl₃ exhibited superior efficiency compared to other chlorides, including elements such as Pr, Nd, Ni, Co, La, Y, and Fe.

Recent elucidations on the inhibitor efficiency of various cerium salts, including inorganic ones such as CeCl₃ [69, 70, 71, 72, 73, 74], Ce(NO₃)₃ [74, 75, 76, 77, 78, 79, 80], Ce(SO₃)₂ [81], Ce₂(MoO₄)₃ [82], as well as some organic Ce-salts like (CH₃COO)₃Ce [83, 84], (HOC₂H₅COO)₃Ce [85], and other Ce-carboxylates [86], have been reported. Furthermore, the substantial potential of Ce-compounds as corrosion inhibitors is highlighted in various review papers [87, 88, 89, 90, 91, 92, 93].

Subsequently, the apparent cathodic inhibition capability of cerium-based compounds found successful application in the deposition of cerium conversion coatings (CeCC). The possibility of forming continuous coating layers through CeCC deposition was realized by Conde et al. [94], which was confirmed by other authors [95]. These researchers proposed additional chemisorption processes on the superficial oxide layer of aluminum, enabling the complete coverage by a uniform Ce-O-Al layer. Consequently, CeCC formation initiates from nobler intermetallic inclusions, sealing them completely, and subsequently spreading over the remaining surface. Thus, CeCC growth undergoes a transformation in the formation mechanism from island-like nucleation to subsequent layer formation [96], as illustrated in Figure 3.

Figure 3.
Illustration of CeCC layer on Cu-doped Al alloy: (1) Grain boundaries; (2) Grains of the basic Aluminum matrix; (3) Cathodic copper intermetallic inclusions; (4) Native oxide layer; (5) CeCC layer; (6) Initial Ce-oxide/hydroxide deposits [96].

However, as established by Rodriguez et al. [97], the practical deposition of CeCC faces some challenges related to undesirable precipitation rather than the formation of uniform, dense, and adhesive layers. Recent evidence suggests that the optimal concentration of the Ce-precursor should range between 0.03 and 0.05 mol dm⁻³ [98, 99], and the maximum amount of added 30% H₂O₂ as a deposition activator should be around 10 ml for up to 1000 ml of precursor solution. Experiences thus far indicate that the deposition of these coating layers can be accomplished through dip-coating [96, 97], electrodeposition under galvanostatic [46] or potentiostatic [100, 101] regimes. In all cases, CeCC formation requires activation energy in the form of elevated temperatures (up to 60°C to avoid considerable thermal H₂O₂ decomposition), applied potential (up to −1 V) vs. Ag/AgCl reference electrode, or defined current density (from −2 to −5 mA cm⁻²). Superior values of these parameters result in Ce(OH)₃/Ce(OH)₄ precipitation rather than thin, uniform CeCC formation.

X-ray photoelectron spectroscopy (XPS) studies [102, 103] have revealed that CeCC layers, deposited from Ce(III) soluble salts, convert to Ce(IV) oxides and hydroxides, primarily CeO₂. This conversion occurs due to the more complex mechanism of cerium oxide and hydroxide formation, proceeding under the given regime via the following reactions:

4Ce3+aq+O2aq+4OH−+2H2O→4CeOH22+aqE8

2Ce3+aq+H2O2aq+2OH−aq→2CeOH22+aqE9

The Ce(IV) ions subsequently precipitate as hydroxides (reaction (10)) and oxides (reaction (11)).

2CeOH22+aq+2OH−aq→CeOH4sE10

CeOH4s→CeO2s+2H2OE11

Additionally, Covelo et al. [104] propose direct oxidation, as indicated by the reaction below (Eq. (12)):

4Ce3++12OH−+O2→2CeO2+6H2OE12

Furthermore, the oxidation of Ce³⁺ ions to Ce(IV)-oxides and hydroxides could be further enhanced under conditions of electrochemical deposition, following reaction (13):

Ce4+→Ce3++e−E°=1.61VSHEE13

The deposition of uniform, dense, and adherent CeCC films, instead of obtaining discrete Ce-precipitates, poses a significant challenge, primarily due to the complicated chemical mechanism. Beyond the influence of Ce-compound type and concentration, and, of course, the H₂O₂ activator content, the deposition mechanism and rate can be influenced by regulating various factors, such as pH, temperature, and the presence of additives, among others. Various additives have been shown to have a beneficial effect, including black cuprous oxide “smut” [105], Al³⁺ and Cl⁻ ions [106], pH buffers [107], etc. Similar studies are also dedicated to the formation of CeCC layers on other metallic substrates, such as galvanized [108] and stainless [109] steel, and zinc substrates [110]. The works of Mansfeld et al. on Ce-Mo [62], Jiang et al. [111] on Ce-V, as well as the recent works of Song et al. [112] on Ce-Ti, Zhan et al. [113] on Ce-Ti-Zr, and Zhou et al. [114] on Ce-La-Y combined conversion layers for the protection of aluminum and magnesium alloys open up an entirely new direction for the development of combined conversion coatings.

Especially in the context of aluminum-based alloys, the CeCC formation was considered as an approach for the elaboration of “stainless” aluminum by Mansfeld et al. [63] decades ago. However, the current interest in corrosion protection through CeCC coating primer deposition persists. Numerous research works on this topic attempt to be summarized in several extended reviews. In these reviews, the authors focus on the beneficial properties of CeO₂-based anticorrosion layers [115, 116, 117], as environmental chromate substitutions [118, 119, 120]. Other authors highlight the technological aspects of CeCC layer depositions [121], considering these types of layers as efficient sealants for anodic oxide layers [122, 123].

In this context, experimental activities follow the main streams, aiming to optimize CeCC deposition conditions [124, 125, 126], explore applications of Ce-oxide sealants for anodic oxide layers [104, 127, 128, 129, 130, 131], and utilize cerium compounds as corrosion inhibitors for the active protection of epoxy [132] and sol-gel-derived [133, 134, 135, 136, 137, 138, 139] protective coatings.

Regardless of the method of CeCC deposition, the sequence of related operations includes: (i) preliminary treatment; (ii) layer deposition procedure, and (iii) final CeCC sealing. In this context, the treatment of already deposited CeCC layers with hot phosphate solutions appears to be an attractive CeCC sealing procedure, as proposed by Heller et al. [140, 141].

Anodization and plasma electrochemical oxidation (PEO) are considered the most suitable approaches for preliminary surface treatment. Some authors define anodization as a process of electrochemical oxidation of metals and/or alloys under conditions of anodic polarization [142]. This method enables the formation of highly textured surfaces, composed of arrays of regularly ordered tubular pores with equal size dimensions, as illustrated in Figure 4 [142].

Figure 4.
Schematic illustration of a typical anodic aluminum oxide layer [142].

The topology of the anodic aluminum oxide (AAO) layers depends on various factors during layer formation, including: (i) nature and concentration of the contact electrolyte [143]; (ii) applied current density or formation voltage [128, 144]; (iii) composition of the to-be-anodized aluminum alloy [145, 146, 147, 148, 149] and its structure [150]; (iv) temperature [151]; (v) duration [55, 152]; (vi) presence of additives [153], including Ce(IV) species [154], and of course, (vii) preliminary surface treatment [155, 156, 157]. Additionally, it should be mentioned that anodization at rather high voltages alters the process mechanism, converting it to plasma electrolytic oxidation (PEO). The above-listed parameters impact the PEO process, and the resulting surface modifications are investigated in detail by Egorkin et al. [158], Pezzato et al. [159], Cerchier et al. [160], and Kostelac et al. [161]. In this context, Arunnellaiappan et al. [162] propose the formation of combined AAO/CeCC layers by applying PEO, combined with simultaneous electrophoresis of CeO₂ nanoparticles. Furthermore, the synergism between the CeCC layer and AAO sublayer, obtained under conventional conditions, was established by Andreeva et al. [163], Kozhukharov et al. [164], and confirmed by L. Selegård et al. [165] and Ramirez et al. [166].

Finally, the term “conversion coating” encompasses any metal oxide layer deposited on a metal surface through chemical or electrochemical processes that partially or completely replace the native surface metal oxide, according to Arenas and Damborenea [167]. Consequently, the formation of sufficiently thick anodic aluminum oxide (AAO) layers enables their partial substitution by Ce-oxides. This substitution is possible due to the definitively acidic properties of the Ce-precursor solutions [168, 169] and the previously mentioned amphoteric character of the aluminum oxide layers, including AAO.

Although CeO₂ is a rather stable metallic oxide, commonly used as a high-performance ceramic material, it is soluble through electrochemical polarization in strongly acidic media under aeration conditions. According to the encyclopedia of Sukhotina [170], it can undergo dissolution through the following reaction:

CeO2+4H++O2+e−→Ce3++2H2OE0=1.4VSHEE14

The solubility of CeO₂ in acidic media (i.e., 0.01 M NaClO₄) has been thoroughly investigated by Plakhova et al. [171], comparing theoretical thermodynamic calculations with practical experiments. Additionally, Joshi et al. [172] conducted an extended experimental study on Ce-leaching from CeCC deposited on AA7075-T6 aircraft alloy in a 0.1 M NaCl model corrosive medium. In both cases, the authors claim that CeO₂ is less soluble than Ce(III) oxides and hydroxides, which constitute the CeCC layers. However, CeO₂ does possess, albeit insignificant, solubility that could be enhanced in strongly acidic media. Furthermore, the solubility of CeO₂ could be additionally enhanced when this oxide is in the form of nanoparticles [171, 172], particularly as in the CeCC layers obtained by Selegård et al. [165], composed of nanoglobules. Therefore, the post-treatment procedure is necessary for improvement of the CeCC performance in corrosive media.

In this context, phosphatation appears to be among the most attractive approaches for the final sealing of CeCC layers due to the remarkable phosphate anion adsorption aptitude of CeO₂ [173]. One of the pioneering works in this field is authored by Heller et al. [140]. The authors conducted a comparative research on the effect of NH₄H₂PO₄, Na₃PO₄, K₄P₂O₇, Na₂H₂P₂O₇, and Na₅P₃O₁₀ as phosphate sources. Furthermore, the temperature and duration of the post-treatment have been the subject of research [140, 141]. The authors immersed CeCC-coated AA2024-T3 plates in a 2.5 wt.% NH₄H₂PO₄ solution for different times (10, 30, 120, and 600 s) and temperatures (55, 70, and 85°C), respectively. It was concluded that the best coating is obtained for at least 10 minutes at a minimum of 70°C and pH = 4.5. Additionally, at room temperature, no changes have been observed. The authors explained the beneficial effect of the post-treatment with the formation of supplemental hydrated CePO₄, which fills the cracks of the CeCC coating, making it much denser. Furthermore, similar methods are proposed for the corrosion protection of zinc [174] and low carbon steel [175]. Recently, Tsanev et al. [176] proposed phosphatation of the CeCC layer deposited on AA1050 alloy by exposure for 15 min to 0.5 M NaH₂PO₄ with the addition of 0.1 M Ca(NO₃)₂ at 85°C. Considering the amphoteric properties of the AAO layers and the CeO₂ solubility in definitely acidic media, Portolesi et al. [177] propose two buffering solution compositions enabling the final sealing of the AAO/CeCC layers at nearly neutral pH, namely: Na₂HPO₄/KH₂PO₄ and Na₂B4O₇ / HCl. In this sense, other systems, like Na₂B₄O₇/H₃BO₃, are of potential interest for further neutral solutions for final thermal treatment.

Direct Ce-phosphate deposition on AA2024-T3 alloy has also been proposed [178, 179]. Recently, Wang et al. [180] introduced chemically bonded phosphate ceramic coatings modified with organic-inorganic hybrid nano alumina. Other authors suggest hydrothermal sealing of AAO layers after CeCC deposition [165] or incorporation of copper oxide [181].

Steels, as critical metallic materials in civil engineering, are under scrutiny for corrosion protection through Ce-compounds via inhibition [182, 183, 184], CeCC deposition [185, 186, 187, 188], and the utilization of Ce-compounds as active ingredients in protective coatings [183].

Similar to aluminum alloys, the properties of the CeCC layer on steel substrates are contingent upon preliminary treatment and phosphatation. Therefore, steel substrates should undergo preliminary galvanization, as iron is susceptible to severe uniform corrosion attack in CeCC deposition solutions [189]. This susceptibility is due to the inherent acidic nature of CeCC deposition solutions and the oxidant properties of the Ce(IV) species, discussed below (Figure 5).

Figure 5.
Illustration of CeCC deposition on steel after preliminary Zn –galvanization according to [189].

In this context, Zn-based coatings for steels can also be applied through electrochemical methods. The quality of the resulting galvanic layers depends on both the electrolyte composition [190, 191, 192], the applied galvanizing regime [191], and the post-treatment applied. This procedure can be executed through anodization [192, 193, 194, 195, 196], similar to the case of aluminum discussed earlier, or by the deposition of Ce-modified hydrotalcites, as proposed by Pham et al. [197, 198]. In summary, among the most promising approaches for corrosion protection could be achieved by subsequent performance of: (i) galvanization; (ii) galvanic layer anodization; (iii) CeCC layer deposition; (iv) phosphate sealing; and (v) hydrotalcite finishing, combined with (vi) advanced polymer deposition.

All electrochemical layer formation processes should be conducted after at least a brief analysis of the system to be elaborated, considering the interactions between the metal substrate and the metallic ions in the deposition electrolyte. For this reason, the standard electrode potentials, defined against the Standard Hydrogen Electrode (SHE), of the redox couples of ceric to cerous ions and some of the metals recently subjected to CeCC, are summarized in Table 1 [170].

Table 1.

Standard electrode potentials of Ce^4+/Ce³⁺ and some metals.

Moreover, theoretically, the Ce(IV) and Ce(III) compounds from the CeCC deposition electrolytes could also be reduced to metallic cerium:

Ce3++3e−→Ce0E0=−2.483VSHEE15

Ce4++4e−→Ce0E0=1.68VSHEE16

However, no data are found for electrochemical or chemical metallic cerium layer deposition, probably because the conditions necessary for the formation of metallic Ce layers are extremely severe for the metals to be coated. The potentials shown in Table 1 should be recalculated to be adopted for real conditions. For instance, the potentials of the saturated calomel (SCE) and the silver/silver chloride (Ag/AgCl 3 M KCl) reference electrodes are as follows: E⁰_SCE = 0.268 V_SHE and E⁰_Ag/AgCl = 0.222 V_SHE.

Additionally, the real standard potential values should be recalculated using the Nernst equation and considering the Pourbaix E/pH diagrams, as proposed elsewhere [50]. Particularly, the CeCC formation solutions are definitely acidic, commonly with pH ≈ 2.5 [108, 109].

Finally, the remarkable oxidant properties of the Ce⁴⁺ ions find application in various fields, including analytical, environmental, and nanomedicine applications [199, 200, 201]. For example, enzymes, being biocatalysts with protein-based composition and various specific activities, can be substituted with CeO₂ nanoparticles, as proposed by Xiao et al. [202]. CeO₂ nanoparticles have potential applications in oxidative nanozymes, environmental [203], medical [204], and bactericide [205] applications.

Advanced corrosion-protective coatings should be composed of various layers, each performing its own function. Coating primers, like the aforementioned CeCC, support adhesion between metallic surfaces and intermediate layers. These intermediate layers, composed of organic or hybrid polymers, are susceptible to ultraviolet (UV)-assisted chemical bond disruption or oxidation. Hence, they should be protected by outer UV-absorptive coating finishes. Both the intermediate layers and the coating finishes should possess distinguishable hydrophobicity.

Efficient advanced coating systems should respond to a variety of requirements to provide passive and active corrosion protection. The former pertains to the barrier properties of the coating layer between the protected metal and the corrosive medium. The latter relates to the residual barrier effects after the disruption of coating integrity, as summarized in a recent review [206]. Recent research activities are focused on acquiring superhydrophobicity and self-healing ability [207, 208, 209, 210, 211, 212]. The illustration of recent and upcoming trends in the design of advanced corrosion-protective layers for aluminum alloys is provided below (Figure 6).

Figure 6.
Recent trends regarding the elaboration of advanced corrosion-protective coatings for aluminum alloys.

In this context, the sol-gel method emerges as the most suitable approach for the development of intermediate and finishing layers in advanced corrosion-protective coating systems. The obtained gels represent an intermediate stage in the coating formation process, followed by appropriate drying and thermal treatment to achieve dense and adherent solid layers. Generally, gels are considered micro-heterogeneous colloidal systems, composed of a liquid medium equally distributed in a polymeric matrix [213]. The application of this approach requires monitoring various factors, including: (i) the composition and nature of the precursors, (ii) the used solvents, (iii) the medium pH, (iv) the addition of modifiers such as detergents and complexing agents, (v) temperature, and (vi) the surrounding gaseous medium composition and pressure [214, 215].

The sol-gel method allows for the direct incorporation of CeO₂ nanoparticles into the initial liquid sol. This approach leads to significantly higher scratch resistance in the resulting sol/gel-derived intermediate layers. Moreover, these particles can be pre-loaded with water-soluble Ce-compounds, such as Ce(NO₃)₃ [80, 215, 216], CeCl₃ [215], and (CH₃COO)₃Ce [215], to ensure their gradual release as efficient corrosion inhibitors, as illustrated below (Figure 7).

Figure 7.
3D schematic view for the role of nanocontainers for decrease of the undesirable leaching of the inhibitor; (a) – inhibitor involved directly in GMT-matrix and (b) – inhibitor-loaded nanoparticles [217].

This concept enables the use of a variety of corrosion inhibitors and nanocontainers [217, 218, 219, 220, 221, 222, 223, 224, 225, 226], allowing for the combination of diverse polymer matrix materials, solid nanoparticle phases and inhibitors.

Specifically, the sol-gel method proves to be a versatile approach for the synthesis and deposition of functional thin films. Sol-gel-derived layers can serve various functions, being effective corrosion protectors, as previously mentioned, as well as active sensor components and electrodes for alternative energy sources.

3. Cerium compounds for sensor elements

The sensors are indispensable components essential for the operation of every monitoring and automatic control system. In classical automation, automatic systems are generally classified into Systems for Automatic Regulation (SAR) and Systems for Automatic Control (SAC). The key distinction lies in the fact that SAR controls a single parameter, while SAC manages multiple parameters simultaneously. Contemporary automation trends emphasize the use of sensor arrays for precise and reliable control. This stands in contrast to earlier generations of automatic regulation systems that heavily relied on the technical functionality of a single sensor, often without the ability to verify its status.

However, new challenges arise in modern automatic systems as the use of multiple parallel sensors introduces the potential for deviations among the data provided by each sensor. In cases where significant data discrepancies emerge, it indicates a malfunction in at least one of these elements.

The term “sensor element” encompasses all electronic components that generate electric signals or modify their parameters as a response to their environmental conditions [227]. Sensor elements generating electric signals are termed “primary elements,” while those relying on an external source of electric energy for their function are considered “secondary elements.” Chemical sensors play a crucial role as interfaces between chemical industrial installations and automatic control systems. These sensors convert variables in the surrounding medium (inside tubes, chemical reactors, reservoirs, etc., where the sensor is installed) into electric output signal parameters. Examples of environmental variables and resulting output signal parameters are illustrated in Figure 8.

Figure 8.
Schematic illustration of sensor element characteristics [227].

The sensors, depending on their intended use, play a crucial role in detecting and monitoring various environmental parameters, such as temperature (°t), pressure (P), concentration of a chemical compound [C], acidity or alkalinity of liquids (pH), relative humidity of gases (RH%), light intensity (candela) or wavelength (meter), etc.

The construction of these sensors determines the electric output signal parameters that include electric current (I), potential (U), ohmic resistance (R), capacitance (C), or inductance (L). The development of sensor elements involves analyzing the correlations between environmental variables and output signal parameters. In addition to sensitivity (tg(α)), these elements have inherent detection and output signal ranges, critical for detectable values of the environmental variable and the output signal. Outside these ranges, the sensor does not alter the output signal characteristics and may even be damaged.

Beyond the parameters mentioned, sensors in development undergo testing for durability during storage under various conditions and reliability after extended usage. These characteristics are determined through long-term test procedures, considering data repeatability and reproducibility. Repeatability pertains to a sensor’s ability to consistently perform under multiple repetitions of environmental variable alterations, while reproducibility is concerned with the similarity of performance among a relatively large number of identical sensor elements. Statistical data acquisition is employed through multiple repetitions of sensor characteristics’ determination under the same conditions. This approach allows for statistical analysis, resulting in the final result (FR), which includes the respective standard deviation. Thus, the final results of multiple measurements are defined by the following formula:

FR=∑i=1i=nARin±∑i=1i=nARi−ARav2nn−1;E17

Where: FR is the final result. AR_i is the acquired result of each measurement with i ranging from at least 2 to n. n is the total number of measurements. AR_av is the average value obtained by dividing the sum of all measurement results by their number.

Nonetheless, sensors undergo changes in behavior over time due to aging and/or corrosion of their components. The former (i.e., aging) are result of internal non desirable structural and compositional alterations, caused by internal factors, such as structural relaxation, residual crystallization, solid-state diffusion, etc. The latter is (i.e., corrosion) result of interactions with the surrounding medium, as was already mentioned. Additional important characteristics of sensors include response time delay and recovery time. Response time delay refers to the time between the moment of notable alteration of the controlled parameter (i.e., environmental variable) and the moment of its registration. Recovery time predetermines the hysteresis area. Both these parameters should have as lower values as possible, achievable by decreasing the size dimensions of the sensor elements, for instance, by using nanosized materials like nanocrystalline CeO₂, as proposed by Knecht et al. [228], and by employing precise equipment for sample assembly.

Chemical sensors also should exhibit selectivity regarding the substance (i.e., chemical element or compound) whose concentration should be controlled.

Nanocrystalline CeO₂-based materials hold great potential for use in sensor elements due to their recently described semiconductor properties arising from the composition of CeO₂ lattice [229, 230, 231, 232, 233], which has a highly defective and nonstoichiometric structure [234, 235, 236]. Utilizing the sol-gel method, humidity sensor elements based on Ce-doped titanium dioxide (TiO₂) layers were developed [237], even though obtaining Ce-Ti-O proved challenging due to the large atomic radius of Ce preventing its involvement in the TiO₂ crystal lattice. Nevertheless, the coexistence of both oxides could result in conjunction with interesting optical and electric properties. This approach was further enhanced, leading to the development of sol-gel-derived CeO₂-TiO₂-SiO₂ humidity sensors with remarkable properties [238, 239]. The introduction of CeO₂ nanoparticles into a polymer matrix is also employed for the development of advanced biosensors [240, 241].

Another approach to enhance the sensing ability of CeO₂-based materials involves modifying their lattices by doping with other elements [25, 240, 241, 242]. The zinc oxide (ZnO) modification by Ce is particularly attractive, as the treatment of ZnO/TiO₂ sol-gel-derived film at 400°C results in the formation of the simonkolleite phase, which possesses exceptional hygroscopicity [243]. The beneficial effect of ZnO-TiO₂-SiO₂ thin films is confirmed by other authors [244, 245, 246, 247], with the procedures proposed allowing for the involvement of Ce-compounds aiming at further improvement of the resulting film sensitivity.

4. Cerium compounds for alternative energy sources

The aforementioned specific semiconductor properties of CeO₂-based materials and the possibilities for their doping make them suitable for use in alternative energy sources, such as solid oxide fuel cells (SOFCs). SOFCs are solid-state multilayer ceramic electrochemical devices, composed of at least two porous electrodes separated by a dense ion-conductive electrolyte. The SOFCs enable simultaneous thermal and electric energy generation (Figure 9a) and efficient electrocatalytic oxidation of volatile organic compounds (VOCs), as electrocatalytic oxidation precursors or environmental contaminants (Figure 9b).

Figure 9.
Schematic illustration of SOFC, working as energy source (a) and electrocatalyst (b) 1, 5-contact elements (platinum (Pt) or stainless steel); 2-porous anode; 3-dense solid electrolyte (usually yttria-stabilized zirconia); 4-porous cathode.

The characteristics of the output signals generated by SOFCs depend on the gas flows and concentrations. Consequently, SOFCs could function as gas sensors in the transport and industrial sectors. In transportation, SOFC sensors could detect incompletely combusted exhaust gases. In industry, they could be used for various gaseous flows of oxidants (O₂, O₃, etc.) or reductants (H₂ vapors of volatile organic compounds, such as the light fraction combustibles in the petrol refineries, etc.), as well as for detecting toxic gases (especially carbon monoxide) [248], applicable in metallurgical plants and thermal electric power stations.

In this regard, cerium oxide (CeO₂) and doped ceria-based materials are well suited for SOFC applications due to their previously mentioned electrical conductivity, oxygen adsorption ability, and extended mechanical properties [249]. Additionally, their conductivity and other properties can be easily modified by varying their structure and porosity [250, 251]. Further improvement can be achieved when CeO₂ is doped with other elements, such as La [252], Gd [253, 254, 255], In [256], Y, Zr [257], Fe, Cu [258, 259], etc.

The hydrogen required for feeding SOFCs can be provided by photoelectrochemical water-splitting electrolysers [260]. These devices compensate for the low efficiency of classical electrolysers by utilizing sunlight energy captured by photosensitive electrodes. In this context, attention is focused on CeO₂ materials doped with Cu [261], Fe [262, 263], Ni [263, 264, 265], Se [264, 265], Co [266, 267], etc.

In this context, CeO₂ phosphatation appears to be a rather attractive approach, not only for efficient corrosion protection, as discussed in the previous paragraph, but also for the development of photoactive electrodes for water-splitting [268, 269, 270].

Finally, as CeO₂-based materials belong to the class of metal oxides, their synthesis is possible by drop spreading of solutions and/or colloids containing Ce-salts across high-temperature workspaces or on heated surfaces. This approach is well known as spray pyrolysis. In the former case, this method enables the spray pyrolysis synthesis (SPS) of nanosized powders [271]. In the latter case, spray pyrolysis deposition (SPD) allows the formation of thin CeO₂ layers [272], facilitating the development of advanced devices such as sensor elements [273]. The CeO₂-based nanomaterials have recently encountered other applications for energy storage, like supercapacitors [274, 275, 276, 277, 278], environmental applications [279], Zn-air batteries [280, 281], etc.

Moreover, the unique properties of CeO₂-based materials are under investigation for potential applications as superconductors [282], magnetic materials [283], and proton ceramic membrane reactors [284]. Additionally, spray freezing methods have been developed as an alternative to the spray pyrolysis approach [285]. Other alternative methods for CeO₂-based nanomaterials synthesis are the hydrothermal method, proposed by Hunpratub et al. [277], and gel combustion method, proposed recently by various authors [286, 287, 288, 289].

5. Conclusions

Recently, interest in the application of Ce-based materials has dramatically increased due to their unique properties and relatively reasonable economic accessibility. Cerium compounds can be employed in various forms for efficient corrosion protection of various metallic substrates, as modern protective coatings comprise multiple layers with distinct functions.

For advanced coating primers on Al-based alloys, a process involving Al substrate anodization, CeCC deposition, and phosphate sealing can be employed.

Similarly, coating primers for steels can be developed through initial galvanizing, followed by CeCC deposition and final phosphatation.

Cerium compounds can also be utilized for intermediate and finishing coating layers, providing both passive and active corrosion protection. In the former case, CeO₂ can serve as a reinforcing phase to enhance the mechanical properties of the coating. In the latter case, various water-soluble Ce-compounds (mainly salts) can function as corrosion inhibitors, acting even after coating integrity is compromised.

The specific semiconductive properties exhibited by CeO₂ make this oxide particularly intriguing for the development of advanced sensor elements.

This oxide is extensively studied as part of alternative energy sources, such as solid oxide fuel cells (SOFCs) and photoelectrochemical water splitters. SOFCs generate electricity and thermal energy while consuming hydrogen, and photoelectrochemical water splitters produce hydrogen essential for SOFC operation. Additionally, Ce-based SOFCs can be applied for electrocatalysis of reactions for various chemical and environmental purposes. Besides, the nanosized ceria can also find applications in biochemical fields, serving as a nanozyme with potential medical applications. Furthermore, ceria has demonstrated utility in superconductors and magnetic materials.

Several promising technological methods, including the sol-gel method, spray pyrolysis, spray freezing, hydrothermal and gel combustion methods, are considered as prospective approaches for ceria-based nanosized materials synthesis.

Acknowledgments

This work is developed as part of contract №: BG-RRP-2.004-0002-C01, project name: BiOrgaMCT, Procedure BG-RRP-2.004 “Establishing of a network of research higher education institutions in Bulgaria,” funded by Bulgarian National Recovery and Resilience Plan. Also, the authors are grateful for the funding of this research to the Bulgarian National Science Fund contract No. КП-06-ПН79/1 “Protective and functional layers on aluminum and its alloys.”

Takahiro Maruyama

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