An Electrochemical Characterisation of Silica–Zirconia Oxide Nanostructured Materials for Fuel Cells

Abstract

Silica–zirconia nanoparticles were successfully synthesised using the precipitation process. The surface area and shape of the Si-ZrO₂ nanoparticles were investigated using BET, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (HRTEM). The HRTEM results demonstrate that silica was successfully integrated into ZrO₂ nanoparticles with a mixture of nanorod and nanosphere shapes. The element analysis (EDX) reveals the presence of silica (14.61%) and zirconia (1.18%) nanoparticles, as well as oxygen (83.65) on the surface. The BET results demonstrate a larger surface area of 185 m²/g and pore volume (0.14 cm³/g). The XRD measurements confirmed the transition of amorphous silica into the monoclinic phase of the zirconia nanoparticles. The electrochemical characteristics of the silica–zirconia nanoparticles were tested in a potassium chloride solution. With a large specific surface area and an appropriate pore size distribution, a pair of broad and symmetric redox peaks were centred at −0.15 V and 0.6 V.

1. Introduction

Nanotechnology has become an important field of study due to the unique chemical, biological, electrical, optical, and magnetic properties of nanomaterials [1]. Because the field of nanotechnology has so many uses, such as dental care, biological sensors, medical imaging, assays, diagnostic kits, athletic gear, sunscreens, cosmetics, environmental cleanup, textiles, and gene inactivation, many researchers focus on this field of study [1]. The growth of nanotechnology has enabled researchers to look for new materials and modify already-existing materials to create new procedures. One such approach is the levitation technique, which is a cheap, efficient, reasonably easy, and adaptable method of making materials [2]. Materials of sizes between 1 and 100 nanometres are known as nanoparticles and have special characteristics that distinguish them from bulk materials [3,4]. Their improved surface-to-volume ratio, quantum effects, and altered chemical and physical properties on a nanoscale are the sources of these properties [5,6]. Oxide compounds are not conductive to electricity; however, certain perovskite-structured oxides are electronically conductive, finding application in the cathodes of solid oxide fuel cells and oxygen generation systems. They are insoluble in aqueous solutions and extremely stable, making them useful in ceramic structures and as lightweight structural components in aerospace and electrochemical applications such as fuel cells in which they exhibit ionic conductivity. Zirconia oxide (ZrO₂) finds extensive applications in a variety of fields, including biological fields (e.g., biosensors, cancer treatment, and hip replacement), optical coatings, solar cells, fuel cells, dentistry, oxidation selectivity, and catalysis [5,7,8,9,10]. Zirconia is an appealing material for numerous applications due to its exceptional qualities, including its redox, acid–basic, and chemical stability, as well as its mechanical resistance [5,7], high surface-to-volume ratio, and small size [8,9,10]. With a high melting temperature of 2750 °C, which makes it valuable as a refractory material, ZrO₂ has a special combination of features that set it apart from all other ceramic materials [11]. Because of the low thermal conductivity of zirconia, it is a suitable choice for thermal protection coatings and insulation materials at high temperatures [12]. Zirconia is characterised by a high hardness and strength and is a durable material for applications that require mechanical stress resistance. ZrO₂ can also be used as an additive to enhance the performance of materials, drug delivery, and medical imaging due to its biocompatibility [13]. This property makes it useful in applications related to oxygen sensing and in high-temperature fuel cells [14].

Zirconia is a special high-temperature solid electrolyte because it forms structural defects with oxygen ion vacancies upon doping with specific aliovalent oxides. This significantly increases oxygen ion conductivity [11]. Moreover, zirconia can provide good ionic conductivity and thermochemical stability when doped with different transition metal cations [15] and has shown the possibility of electrochemical measurement using solid-state galvanic cells based on zirconium solid electrolytes [16]. However, ZrO₂ has limited applications in industry due to its low acidity, basicity, and porosity, as well as its low BET surface area of 50 m²/g compared to other oxide materials such as SiO₂, Al₂O₃, and TiO₂ [17]. Mesoporous silica has a high surface area and well-defined pore structure, ensuring sufficient active sites for catalytic reactions, adsorption processes, and drug delivery applications [18]. The combination of mesoporous silicon and zirconium oxide nanoparticles can achieve an optimal balance between surface area, conductivity, stability, pore size distribution, and mechanical stability, making it very useful for various applications such as electrical equipment, sensors, catalyst supports, and solar cells [7]. The zirconium oxide phase is shown to be more stable when combined with silicon dioxide (SiO₂) due to the creation of Si-O-Zr bonds [19]. Furthermore, the benefits of ZrO₂/SiO₂ binary oxide include the synthesis of Si-OH (silanol) and Zr-OH groups, which operate as sites for increasing the formation tendency of an apatite layer. Silanol groups can connect to biomolecules, resulting in a multifunctional coating for drug delivery, cell targeting, bioimaging, and biosensing [19].

2. Experiment

2.1. Materials

Sodium hydroxide pellets (NaOH) (Sigma-Aldrich (PTY) Ltd., Address: No.4 Aviation Pk 17 Pomona Rd, Pomona AH, Gauteng, 1619, South Africa), ethanol (Sigma-Aldrich), zirconium oxychloride hydrate (ZrOCl₂·8H₂O) (Sigma-Aldrich), Tetraethyl orthosilicate (TEOS) (Sigma-Aldrich, 98% purity), potassium chloride (KCl) (Sigma-Aldrich), Nafion^® solution, D521-Alcohol based 1100 EW at 5 wt.% (Ion Power, New Castle, DE, USA), and silver nitrate (AgNO₃) (Sigma-Aldrich).

2.2. Preparation of Si-ZrO₂ Nanoparticles

The silica–zirconia oxide (Si-ZrO₂) was synthesised by the precipitation method. In total, 0.2 M of ZrOCl₂·8H₂O and 2 N of NaOH were mixed in a 250 mL glass beaker and stirred with a magnetic stirrer for 2 h [18,19]. A silica precursor, tetraethyl orthosilicate (TEOS), was added to the obtained precipitate. TEOS was added dropwise, and the mixture was set to stir for 5 h. The resultant precipitate was placed in the oven at 150 °C for 24 h [9], then centrifuged and washed with deionised water to remove all traces of chlorine ions (Cl⁻). The sample was then dried at 100 °C for 24 h, calcined at 600 °C for 2 h [18,19], and named Si-ZrO₂.

2.3. Characterisation

Brunauer–Emmett–Teller (BET), X-ray diffractometer (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), thermogravimetric analysis (TGA), and Fourier transform infrared (FTIR) spectroscopy were used to characterise silica–zirconia nanoparticles.

2.4. Electrochemical Measurements

Three electrodes were used for the electrochemical experiments: silver–silver chloride (Ag/AgCl) as the reference electrode, Si-ZrO₂ nanoparticles coated on glassy carbon as the working electrode, and Pt wire as the counter electrode. The working electrodes were prepared by ultrasonic mixing of 0.03 g Si-ZrO₂ and Nafion^® solution (0.5 mL) and absolute ethanol for 30 min. The obtained solution was piped to the working electrode and dried [20]. For electrochemical measurements, 2 M of KCl solution was prepared. To observe the reaction behaviour of Si-ZrO₂ nanoparticles, cyclic voltammetry (CV) was conducted at various scanning rates of 100, 50, 30, 20, and 10 mV/s. Electrochemical impedance spectroscopy (EIS) measured the charge transfer properties of the nanoparticles at frequencies from 100 kHz to 0.01 Hz.

3. Results and Discussion

3.1. The High-Resolution Transmission Electron Microscopy

The HRTEM images for Si-ZrO₂ nanoparticles are presented in Figure 1. The results obtained for Si-ZrO₂ nanoparticles show a mixture of spherical and rod-like shapes as shown in Figure 1a,c, with a nanometre size ranging between 2 and 5 nm. Figure 1a,c reveal that ageing the Si-ZrO₂ nanoparticles at a higher temperature resulted in smaller crystal size nanoparticles that are less than 2 nm [19,20,21,22], compared to the 27 nm obtained by Kocjan et al. [23]; this may be due to the synthesis temperature. Figure 1b shows the selected area electron diffraction (SAED) pattern for Si-ZrO₂, which clearly displays (1 1 0), (0 1 5), (0 0 6), and (2 2 0) lattice planes for ZrO₂ and (1 1 1), (1 1 3), (1 3 3), and (2 2 4) lattice planes for Silica. The lattice planes were detected using the CrysTBox software server 1.10 [24]. The SAED pattern reflects the higher crystallisation for the monoclinic plane [18]. The concentric circles and clear scattering spots in Figure 1 may be due to the sample crystallisation. The HRTEM images and lattice spacing of Si-ZrO₂ nanoparticles are shown in Figure 1c. Figure 1c reveals that very crystallised nanoparticles with well-oriented and -defined lattice fringes. The spacing between the lattice fringes was found to be 0.5343 nm, which was confirmed by the XRD monoclinic structure of the (111) plane, indicating that silicon doping can also affect the lattice properties of zirconium nanoparticles. In contrast, Lestari et al. obtained a distance between the fringes of 0.329 nm on zircon nanopowder [25]. The lattice spacing of highly crystalline nanoparticles is consistent with the predicted value of typical monoclinic ZrO₂ structures as shown in Figure 1c [26]. The highly crystallised Si-ZrO₂ nanoparticles obtained will be a good inorganic nanofiller for the Nafion^® membrane and could enhance its performance in fuel cell applications. The EDAX results in Figure 2 demonstrate that the main elemental composition of Si-ZrO₂ nanoparticles is silicon (S) at 14.6%, zirconium (Zr) at 1.2%, and oxygen (O) at 83.7%. The EDAX results in Figure 2 (insert) confirmed that the nanoparticles were more than 99.5% pure, and that the purity of the nanoparticles was consistent with the synthesis method.

3.2. Structural Analysis

Figure 3 and the insert show the XRD results of Si-ZrO₂ and Si nanoparticles. No amorphous silica was found, owing to the crystallisation state of zirconia oxide when calcined over 600 °C, as shown in Figure 3. The insert in Figure 3 shows a broad amorphous peak at 2θ = 20.7–26.2° for the silica nanoparticles; this may be due to their incomplete inner structure and small crystalline size. The diffraction patterns of the Si-ZrO₂ nanoparticles changed significantly, with the amorphous phase of silica transforming into a baddeleyite (monoclinic) structure, as seen in Figure 3. Other studies have confirmed that the monoclinic phase is more thermodynamically stable than the tetragonal and cubic phases [20,25,27]. The primary peaks at 2θ were located at 30.4°, 50.6°, 60.1°, and 74.0°, which correspond to the planes (1 1 1), (3 0 0), (1 3 1), and (4 1 1) in Figure 3 [21]. The diffraction pattern exhibits strong and well-defined peaks, indicating a crystalline structure. This shows the successful synthesis of silica–zirconia nanoparticles in one pot, as amorphous silica was transformed into monoclinic zirconia nanoparticles, and that the silica core is well blended with zirconia [27].

3.3. Scanning Electron Microscopy (SEM) Analysis

Figure 4a,b show the SEM morphology of the Si-ZrO₂ nanoparticles in 100 nm and 1 µm scale bars. The Si-ZrO₂ nanoparticles show a mixture of rod-like and spherical shapes as shown in Figure 4a., which is confirmed by Lei Xu et al., who obtained a rod-shaped morphology with a diameter ranging between 150 and 200 nm [28].

3.4. TGA and Derivative Thermo-Gravimetric (DTG)

Figure 5 shows the TGA and DTG curves of the Si-ZrO₂ nanoparticles. As seen in Figure 5, thermal degradation has two weight loss stages. Figure 5 shows an initial weight-loss of 4wt% between 40 °C and 100 °C, which is caused by the evaporation of adsorbed water. The second stage of weight loss occurs between 150 and 900 °C, as a result of unreacted alkoxy groups, organic component degradation, and hydroxyl group removal [20]. The TGA results show that the Si-ZrO₂ nanoparticles decompose less and remain stable at temperatures higher than 900 °C with the total weight loss of 12%. The DTG curves in Figure 5 confirm the two weight loss stages observed by TGA, with broad endothermic peaks between 100 °C and 300 °C due to desorbed water [17].

3.5. FTIR Spectrum of ZrO₂ Nanoparticles

The FTIR results of the Si-ZrO₂ nanoparticles are shown in Figure 6. The bands at 1627 cm⁻¹, 1062 cm⁻¹, and 797 cm⁻¹ are attributed to the symmetric stretching vibration Si-O-Si; this may be due to the addition of TEOS [29,30]. The absorption peaks observed at 1062 cm⁻¹ and 945 cm⁻¹ are assigned to Si–O–Zr bonds generated by silica and the zirconium raw materials [27,31]. The peak at 1452 cm⁻¹ is due to O-H bonding, and the peaks in the region of 1536 cm⁻¹ may be due to the adsorbed moisture. The peak at 2366 cm⁻¹ corresponds to the Zr–OH stretching vibration band, indicating the presence of zirconium nanoparticles. The peaks at 3411 cm⁻¹ are attributed to O-H stretching vibration bands [30,31].

3.6. Nitrogen Adsorption–Desorption

Figure 7 and

Table 1. BET of ZrO₂ nanoparticles: (a) Si-ZrO₂ and (b) ZrO₂.

Sample ID	Surface Area (m2/g)	Pore Volume (cm3/g)
Si-ZrO2	185	0.14
ZrO2	162	0.33

show the N₂ adsorption–desorption isotherms and pore size distribution curves and BET-specific surface area of the Si-ZrO₂ nanoparticles. Figure 7 shows that the effective diameter of the Si-ZrO₂ pores was reduced due to the incorporation of silica into the pores, which also increased the surface area [32]. The BET results in Figure 7a,b and Table 1 show that Si-ZrO₂ has a microporous structure with a higher surface area of 185 m²/g. When the samples are aged at a higher temperature of 150 °C, the adsorption–desorption isotherm curves are type IV, with an H3 hysteresis loop, showing that the material has a mesoporous structure [19]. Figure 7b shows that the Si-ZrO₂ pore size distribution is narrower, with only one pore size distribution detected, this could be owing to the higher synthesis temperature. The pore distribution data indicate that ageing temperature has an impact on the porosity of zirconia nanoparticles. Si-ZrO₂ had a pore volume of 0.14 cm³/g as shown in Table 1. Bumajdad et al. obtained values greater than 144 m²/g and 83 m²/g [33,34]. This is due to the incorporation of tetraethyl orthosilicate into the zirconia solution.

3.7. Electrochemical Results

The cyclic voltammetry of Si-ZrO₂ is presented in Figure 8. The Si-ZrO₂ nanoparticle electrodes demonstrated characteristic electric double-layer capacitive behaviour with pseudo-rectangular voltametric diagrams in Figure 8. Figure 8 shows that increasing the scan rate from 10 mV s⁻¹ to 100 mV s⁻¹ increases the current due to resistance effects in the pores [20]. The volumetric cycle is higher and faster to complete with an increased electrode capacitance at 50 mV s⁻¹ and 100 mV s⁻¹ than at lower scan rates, demonstrating that the current reactivity of the Si-ZrO₂ electrode increases as the scan rate increases, as shown in Figure 8. Furthermore, at an increased scan rate of 100 mV s⁻¹, Si-ZrO₂ had a greater area under the voltametric curve and a stronger charge transfer resistance [20], as shown in Figure 8. This may be due to a high BET surface area and wide pores, which contribute to its equivalent performance in electrochemical processes measured with cyclic voltammetry. However, the quasi-rectangular shape, which is indicative of capacitive behaviour, increases dramatically between 50 and 100 mV s^–1 which makes it suitable for use in higher charge–discharge devices [35].

The fundamental electrochemical behaviour of Si-ZrO₂ in 2 M KCl electrolytes was investigated using electrochemical impendence spectroscopy, as shown in Figure 9. Figure 9 and (insert) show the Nyquist plots for these cells over the frequency range of 100 kHz to 0.01 Hz. Si-ZrO₂ has vertical Nyquist plots without a curve zone, as seen in Figure 9 and (insert), due to electron transmission through its porous nanoparticle structure, which generally indicates excellent capacitive behaviour [36]. Figure 9 (insert) shows that, at low frequencies, the curve is virtually vertical, showing optimal capacitive behaviour. The Si-ZrO₂ nanoparticles have the same low equivalent series resistance of only 6.25 Ohm, indicating better electrochemical performance in asymmetric devices due to increased electrolyte ion diffusion and mobility during charging and discharging cycles [34]. Furthermore, the huge surface area-to-volume ratio allows for a lot of charge accumulation, which leads to efficient charging. Such properties contribute to increased conductivity, making Si-ZrO₂ ideal for use in fuel cell applications.

4. Conclusions

The incorporation of amorphous SiO₂ into ZrO₂ nanoparticles enhances charge transmission resistance while also providing outstanding capacitance performance, as seen from the electro-chemical studies. Si-ZrO₂ nanoparticles exhibit thermal stability under TGA, with a total weight loss of less than 12%. Based on EDX measurements, the major elemental composition of Si-ZrO₂ nanoparticles is 14.6% silicon (S), 1.2% zirconium (Zr), and 83.7% oxygen. Both Si and Zr structures are formed in the SAED pattern, which is consistent with the XRD findings. Furthermore, the BET results show that Si-ZrO₂ has a higher surface area of 185 m²/g with the adsorption–desorption of type IV isotherm curves. In addition, FTIR reveals Si-O-Zr structures, showing the presence of silica in the zirconia nanoparticles. TEM data show that the composited nanoparticles are crystallised with a particle size of around 5 nm.