Sol-Gel Materials for Electrochemical Applications: Comparison
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Modified electrodes for sensors and supercapacitors as well as anti-corrosion are described. Sol-gel synthesis expands the capabilities of technologists to obtain highly porous, homogeneous, and hybrid thin-film materials for supercapacitor electrode application. The widespread materials are transition metal oxides, but due to their low conductivity, they greatly impede the rate capability of electrochemical supercapacitors. The way to optimize their properties is the production of complex oxides or different composites. Among the new materials, a special place is occupied by perovskites and materials with an olivine-type structure, which can be easily obtained by the sol-gel method. The sol-gel coating process has demonstrated excellent chemical stability to advance the corrosion resistance of the various metal alloy substrates. 

  • sol-gel
  • corrosion protection
  • coatings
  • supercapacitors
  • electrochemical sensors

1. Introduction

Sol-gel thin film technology has been around for more than six decades. The sol-gel process has been proposed as an alternative to traditional methods, such as sputtering, CVD, and plasma spray, for applying thin ceramic coatings. Sol-gel thin films are technically visible alternatives to these methods in several known as well as commercially viable alternatives [1][2][3]. The inorganic thin films obtained by the sol-gel method are successfully used in various fields [4][5]. For example, gas sensors based on sol-gel coatings have been investigated in the last 20 years. Several advantages can be expected in the development of sensitive gas sensors. In the context, of the sensors controlling the impurities via stoichiometry, low-temperature fabrication should provide better control over the structural morphology [6][7][8][9][10][11][12]. These factors contribute to low conductivity and high purity for excellent sensitivity. Finally, the high surface area materials and their porosity composed by sol-gel methods should increase sensitivity in mechanisms dominated by surface phenomena.Sol-gel is progressively enticing the consideration of the electrochemical community as a versatile way to produce coatings for supercapacitors [13][14], electrochemical sensors [15], solid electrolytes [16], electrochromic devices [17], anti-corrosion protection [18][19], etc.
The hybrid sol-gel coating has attracted great attention due to its promising properties and insisting to explores a wide range of applications in a couple of decades. The combination of organic and inorganic materials in a single phase provides exceptional opportunities for electrochemical applications, anti-corrosion, opticals, and sensors. This no-holds-barred design concept has led to the development of hybrid coatings for diverse applications, such as highly optically transparent materials such as glass and glasses to protect the metal substrates from the penetrations of abrasion and corrosion.
In recent years, xerogels, aerogels, and even conductive hydrous gel applications have been produced and investigated. Many scientists have concluded that amorphous materials hold more promise than their crystalline nature, i.e., those that require rapid diffusions, such as Li batteries, electrolytes, and electrodes for supercapacitors.
Sol-gel chemistry is the production of inorganic or ceramics polymeric materials from solution by converting liquid precursors into sol and forming the gel network structure [20]. Traditionally, sol formation obtains via hydrolysis and is followed by condensation in the bottom-up approach. In this practice, the ultimate sol-gel products are made by the performance of several irreversible chemical reactions. There are several several advantages of the sol-gel method due to its unique characteristics. [21]. Another important advantage is the possibility of joint deposition of several hydroxides or carbonates. Further heat treatment allows forming of complex oxides with particles of various shapes and sizes [22][23]. In addition, the sol-gel process makes it possible to obtain a highly dispersed homogenous composite with a high degree of purity [24]. The very important advantage of this method compared to conventional methods is the lower process temperature, which is the creation of metallic and ceramic nanomaterial in this process at various temperature ranges (70–320 °C) [25][26].
The last review on sol-gel materials for electrochemistry was published in 1997 [15]. Two new trends were noted: (1) the increased attention to amorphous gels; (2) the rapid spread of organic-inorganic hybrids in all areas of electrochemistry. A book [27] focused on the way to synthesize, assemble, and modify material that find use in the system designed for energy conversion and energy storage was published in 2012 and continues to be of great significant interest. The sol-gel process is used for various systems on the energy market.

2. Sol-Gel Materials for Supercapacitors Application

A large specific surface area with high purity is required for the production of electrode materials for supercapacitors. Considering the advantages of the sol-gel method, it can be observed as a great potential process to prepare these materials. In general, tailor-made metal oxides and metal hydroxide-based electrodes follow a pseudocapacitance (redox) mechanism, whereas carbon-based electrodes store the charge via electric double-layer capacitance (EDLC). The materials to integrate both types of charge-storing mechanisms can be made using the universal process, which is a soft chemical approach. This is practiced to produce high porous metal oxides at adequate temperatures. The properties of supercapacitors depend on the electrochemical stability, conductivity, porosity, and structure features of the electrode materials for high specific capacitance at low resistance enables.

3. Sol-Gel Technologies in Electrochemical Sensors

Electrochemical sensors are a class of chemical sensors in which an electrode is used as a transducer element in the presence of the analyte. Modern electrochemical sensors use several properties to determine various parameters in our daily life, whether physical, chemical, or biological parameters. In this regard, they are very widely applicable. Some examples are sensors for monitoring the environment, health, and appliances, as well as sensors related to machines such as cars, airplanes, mobile phones, and technological media. Currently, new materials, production methods, and strategies are actively used in the development of electrochemical sensors to increase selectivity and detection limits. Sol-gel matrices are useful in the production of electrochemical sensors due to their chemical inertia, mechanical stability, biocompatibility, and high specific surface area provided by porous grids with an open frame. All these features allow sol-gel nanocomposite sensors to demonstrate increased sensitivity and low detection limits about a wide range of target analytes [28]. The sol-gel method is most applicable in the formation of sensitive materials based on oxides, complex oxides, and their composites.

3.1. Oxide Materials and Materials Based on Them

Materials based on oxides of various metals obtained by the sol-gel method are most actively used in electrochemical sensors. Zinc oxide-based materials are very popular. Golli, A.E et al. [29] reported the preparation of ZnO nanoparticles by the sol-gel method. Zinc acetate was dissolved in methanol at room temperature with constant stirring for 2 h, then copper chloride was added into the mixture, and the resultant product dried to get aerogel nanomaterial and the final product was annealed. The SEM image of fabricated Cu/ZnO showed spheroid-like spherical nanoparticles (Figure 1).
Figure 1. (a) FE-SEM image of a ZnO on Cu NPs. (b) High-resolution image of aggregated ZnO [29].
The resulting material was used in an amperometric glucose sensor and showed a high reaction and selectivity. Haque, M et al. [30] reported that copper-doped zinc oxide was prepared via the implementation method of the sol-gel process. At first, zinc acetate was dissolved in 0.1 M sodium hydroxide and stirred at 60 °C for 30 min; then, copper acetate was added dropwise into the solution, and the resultant mixture was stirred at 80 °C. SEM images of the ZnO and Cu-doped ZnO nanoparticles showed ~20 nm spherical nanoparticles. Electrochemical sensing of myoglobulin was studied in blood using Cu-ZnO nanoparticles, myoglobulin was detected in the range of 3–15 mM with 0.46 nM myoglobulin detection limit at high sensitive 10.14 μAnM−1cm−2. Zhan, B et al. [31] prepared ZnO with the sol-gel method, Zn(CH₃CO2)2 was dissolved in ethanol, then a stabilizer of ethanolamine was added in the above solution, and finally, the mixture was stirred to form transparent homogenous at 70 °C for one hour, after which it was left undisturbed for 48 h. Similarly, ZnO was prepared with different solvents, i.e., isopropyl alcohol, ethylene glycol, and methyl ether, respectively. SEM images of the ZnO (ZnOeth, ZnOme, and ZnOgme) showed that the crystal growth depends on solvents, which controlled the shape and size of ZnO particles. Electrochemical sensing of clenbuterol was studied using ZnO nanoparticles; clenbuterol was detected in the range 0.3–1000 ng/mL, with a low detection limit of 0.12 ng. There is research on the production of electrochemical sensors based on tin oxide [32][33][34][35][36][37]. In these works, electrochemical sensors are used to detect various harmful substances. They have different principles of operation. These works are united by the fact that the sensitive material is obtained by various variations of the sol-gel method. Lete, C et al. [33] prepared tin oxide in the sol-gel method by dissolving Sn(II) 2-ethyl hexanoate in absolute alcohol and triethanolamine. The resulting mixture solution was deposited a multilayer film at 450 °C and thermally treated at 600 °C for 1 h.AFM topographical images of SnO2 film display a very smooth surface with RMS > 3 nm, with 30 nm deep, and a diameter of 100 nm (see Figure 2).
Figure 2. AFM image of SnO2 films [38][115].
Tin oxide films were used in the sensor platform to detect nitrites in milk, mineral water, and beer by impedance and conductometric methods. There is also research on the production of electrochemical sensors based on oxides and titanium and tungsten [38][39][40][41][42]. The preparation of tungsten trioxide [4039] and its morphology is shown in Figure 3. Using fabricated WO3 were studied for dopamine sensing vby cyclic voltammetry analysis, and the material showed better sensing of dopamine with excellent stability.
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Figure 3. Schematic representation of the synthesis of WO3 rods. (i) SEM image of WO3 rods at different magnifications (a–d) (ii) [4039].
Composite materials based on oxides are also obtained by the sol-gel method for subsequent use in electrochemical sensors. Abdullah M. M et al. [4342] have prepared α-Fe2O3 doped CdSe in ultra-sonication method and examined the material for methanol detection. The SEM image of the α-Fe2O3-doped CdSe nanoparticles showed ~15 nm rod-like morphology. As prepared α-Fe2O3-doped CdSe nanoparticles were studied for methanol sensing, showing good methanol sensing performance with greater sensitivity. Tareen, A.K et al. prepared [4443] MnO-CrN composite via the ammonolysis process, and performed chromium and manganese nitrate ammonolysis at 800 °C for 8 h. SEM and TEM images of MnO-CrN show nanoparticles with aggregation with an average particle size of about ~10 nm. Naikoo, G.A et al. [4544] prepared monoliths based on NiO using the sol-gel method, and fabricated NiO@@Si-NPs monolith on silica nanoparticles calcinated at 650 °C. The SEM image of the NiO@@Si-NPs showed nanocube-like morphology and the resulting material was studied with glucose sensing in the range of 10–100 mV/s. Paramparambath, S et al. [4645] have prepared CuO-MgO composition by the sol-gel method in the aqueous medium, which was then calcined at 500 °C for 4 h. SEM images of CuO-MgO NC showed highly aggregated morphology of nanoparticles in the range of 200-500 nm. As-prepared CuO-MgO NC was studied dopamine sensing, and the material shows good sensitivity to dopamine with a sensitivity of 69 μAcm−2mM−1. Lu, Q et al. [4746] have prepared FeVO4 and modified FeVO4 with a different mole ratio of metal oxides (NiO, SnO2, WO3) by the sol-gel method in the aqueous medium and obtained the material calcined at 800 °C for 2 h. Finally, FeVO4 and modified FeVO4 materials were studied using ammonia sensors in the YSZ electrolyte. SEM images of FeVO4 nanoparticles showed rod-like morphology with an irregular structural arrangement. FeVO4 modified with NiO nanoparticles detected ammonia in the range of 53–83 mV.

3.2. Materials Based on Complex Oxides

Double and triple oxides of semiconductor metals are a popular object of research in electrochemical sensors. This is due to their high conductivity and stability of chemical properties. Oftentimes, such materials are obtained using the sol-gel method. Peng S et al. [4847] prepared cadmium indium oxides using the sol-gel method in the aqueous medium and studied glucose biosensors. SEM images of nickel-coated CdIn2O4 showed multilayer stacked nanoplates, and it shows good selectivity of glucose. Similarly, Shu, H et al. [4948] prepared CdIn2O4 with iron modification by sol-gel method in the aqueous medium and examined glucose biosensors. SEM images of iron-modified CdIn2O4 showed aggregated nanoparticles (Figure 4) and it provides good electrochemical conductivity to the detection of glucose.
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Figure 4. SEM images of Fe-CdIn2O4 on nickel foam. (a–c) 25%; (d–f) 30%; and (g–i) 35% [4948].
Petruleviciene, M et al. [5049] prepared BiVO4 thin films on fluoride-doped tin oxide substrates by the sol-gel method in the aqueous medium. Fabrication of BiVO4 thin film, urea, and polyvinyl alcohol was used as a stabilizer and the layer deposition was annealed at 450 °C for 2 h. SEM images of BiVO4 thin films revealed that morphology, the influence of PVA additive on the urea-PVA treated coating, and nanoparticles are interconnected to BiVO4 thin films (Figure 15). PVA-modified coating BiVO4 thin films have a greater affinity to glucose adsorption.
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Figure 5. SEM images of Bi_Urea and Bi_Urea_PVA coatings [5049].
The MnFe2O4 nanoparticles and the MnFe2O4 nanoparticles modified carbon electrode paste (CPE) were prepared via the sol-gel process [5150]. The AuNPs were formed onto the MnFe2O4/CPE surface by electrodeposition of HAuCl4.3H2O. The SEM image indicates a nanostructure layer of the AuNPs onto the modified CPE/MnFe2O4NPs surface. This sensor showed good sensitivity to flunitrazepam. MWCNTs/CuFe2O4 (carbon nanotubes in combination with copper ferrite) nanocomposite was successfully fabricated by the sol-gel method [5251]. After the preparation of CuFe2O4, citric acid was added to the mixture and stirred at 80 °C for 3 h. Then, MWCNTs were added and ultra-sonicated at 80 °C for 1 h. The resultant material was calcined at 400 °C for 4 h. The SEM images of MWCNTs/CuFe2O4 nanoparticles showed tubular-like spherical morphology with a diameter of 32 nm (Figure 6). As a result, CuFe2O4 loading on MWCNTs reduces the particle size, which exposes more active centers in the nanocomposite. This feature of the morphology of the material, according to the authors of the work, contributes to a better sensitivity to bisphenol A.
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Figure 6. SEM (A,B) and TEM (C,D) images of MWCNTs and MWCNTs/CuFe2O4 nanocomposite [5251].
Zhang, Y et al. [5352] prepared La2NiFeO6 by the sol-gel method, and citric acid was used as a complexing agent. The resulting material was annealed at 400 °C for 4 h and the final materials were sintered at 1000 °C, 1100 °C, and 1200 °C respectively. The SEM images of La2NiFeO6 materials exhibit particle average size of 300 nm (sintered at 1000 °C) and 500 nm (sintered at 1100 °C), whereas materials sintered at 1000 °C show a slightly agglomerated average size of 1 micron (Figure 7).
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Figure 7. SEM image of La2NiFeO6 (sintered at three different temperatures; (a) 1000, (b) 1100, and (c) 1200 °C) [5352].
Usually, the structures of complex oxides are used in the creation of electrochemical biosensors. In most of the works presented, they are detectors of complex organic substances in standard solutions of electrolytes: red blood salt and potassium chloride. However, there are also works in which the obtained materials are used to detect inorganic substances and organic gases. Similarly, Hao X et al. [5453] have prepared La2NiO4 by sol-gel method, and citric acid is used as a complexing agent. The resulting material was thermally treated at 80 °C for 2 h and followed by sintering at 1000 °C for 2 h. The fabricating La2NiO4 material has potential electrochemical sensors for sub-ppm H2S detection. Wang, J et al. [5554] prepared CdTiO3 by sol-gel method. Subsequentiallycitric acid was injected and stirred at room temperature for 3 h. The resulting final material was was calcined at 1000 °C for 3 h. The SEM images of CdTiO3 nanoparticles showed irregularly arranged multilayered structures with a porous inner channel (Figure 8). The resultant CdTiO3 nanoparticles quickly detect the acetylene gas.
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Figure 8. (a,b) SEM and (c,d) TEM images of CdTiO3 [5554].
La2CuO4 nanocrystals were synthesized by the sol-gel process and utilized for hydrogen peroxide detection [5655]. The components of the gel-forming solution were mixed in deionized water. Then, the temperature treatment of the resulting gel took place. The sensitivity of the resulting gel for hydrogen peroxide reached 100%. The functional features of the materials considered are presented in Table 1.
Table 1. Characteristics of some electrochemical sensors based on sol-gel materials.
Sol-Gel Material Detectable

Substance		 Maximum

Gas Sensitivity		 Linear Range Ref.
Cu/ZnO nanocomposite glucose 36.641 μAmM−1cm−2 0.01–1, 1–7 mM [29]
Cu-doped ZnO nanoparticles myoglobin 2.13–10.14 µAnM−1cm−2 3–15 nM [30]
ZnO nanoparticles clenbuterol - 0.3–1000 ng/mL [31]
SiO2/Al2O3/C nitrite 410 μAμM−1 0.2–280 μM [32]
SnO2 coatings nitrite 22.56 μAμM−1 10–400 μM [33]
Pt-SnO2 nanoparticles hydrogen - 0.08–500 ppm [34]
Au–SnO2 nanoparticles vitamin B12 110.843 μApM−1 0–1500 pM [35]
Cu-doped SnO2 nanoparticles ethyl acetate 4.8 µA/ppb 1–20 ppb [36]
CuNPs on the C/SiO2 working electrode glucose - 53–670 mg/L O2 [37]
p-TiO2 nanoparticles ethanol ~50% 1–100 ppm [3938]
WO3 rods dopamin 3.66 μAμM−1cm−2 1–250 μM [4039]
Nitrogen-doped carbon sheets wrapped in SnO2 nanoparticles glucose 215 nAμM−1cm−2 0.05–10 μM [4140]
α-Fe2O3 doped CdSe aqueous methanol 0.2744 μAmM−1cm−2 0.2–48 mM [4241]
MnO–CrN nanocomposite hydrogen peroxide 2156.25 μAmM−1cm−2 0.33–15 000 μM [4443]
NiO nanoporous materials glucose 445 μAm−1cm−2 - [4544]
CuO-MgO composite dopamine 69 μAmM−1cm−2 10–100 μM [4645]
WO3@SnO2-20 ammonia - 5–50 ppm [4746]
CdIn2O4 nanoparticles glucose 3292.5 μAmM−1cm−2 1.0 μM–1.0 mM [4847]
Fe-CdIn2O4 nanoparticles glucose 8992 μAmM−1cm−2 0.01–1 mM [4948]
BiVO4 glucose - 1–35 mM [5049]
MnFe2O4 flunitrazepam - 0.1–100 μM [5150]
Multiwalled carbon nanotubes CuFe2O4 bisphenol 0.355 μAμM−1 0.01–120 μM [5251]
La2NiFeO6 triethylamine - 0.5–200 ppm [5352]
K2NiF4-type oxides La2NiO4 hydrogen sulfide −70 mV 0.05–2 ppm [5453]
CdTiO3 acetylene −91 mV 5–100 ppm [5554]
Ln2CuO4 nanocrystals hydrogen peroxide - 0.50–15.87 μM [5655]
As can be seen from the table, the materials considered are used to detect a wide range of substances. However, they are most applicable as biomarkers. When they are used in this capacity, the boundaries of the definition of the detected substance are at the level of mM or μM. When detecting gases, the lower detection limit is below 1 ppm. All the materials considered are characterized by a relatively high sensitivity to the substance being determined. Glucose sensors are characterized by the highest sensitivity. Thus, when obtaining materials for electrochemical sensors, three varieties of sol-gel methods are mainly used: anhydrous, aqueous, and citrate. As a result of using these methods, it is possible to obtain sensors that are more highly sensitive than their analogs. These electrochemical sensors are most often used to detect complex organic substances in liquid media. The most popular detectable substance is glucose.

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