Development of a Au/TiO₂/Ti Electrocatalyst for the Oxygen Reduction Reaction in a Bicarbonate Medium

Abstract

The oxygen reduction reaction (ORR) is a pivotal electrochemical process in energy technologies and in the generation of hydrogen peroxide (H₂O₂), which serves as both an effective agent for dye degradation and a fuel in H₂O₂-based fuel cells. In this regard, a titanium (Ti) sheet was anodized to generate a TiO₂ layer, and then the oxide layer was modified with gold (presented as Au/TiO₂/Ti) via electrodeposition. The developed electrocatalyst was confirmed by X-ray photoelectron spectroscopy (XPS), which showed characteristic binding energies for Ti⁴⁺ in TiO₂ and metallic Au. In addition, the Nyquist plot verified the electrode modification process, since the diameter of the semicircular arc, corresponding to charge transfer resistance, significantly decreased due to Au deposition. Voltametric studies revealed that the TiO₂ layer with a Ti surface exhibited a good synergistic effect on Au and the ORR in a bicarbonate medium (0.1 M KHCO₃) by lowering the overpotential, enhancing current density, and boosting durability. The scan rate-dependent study of the ORR produced by the developed electrocatalyst showed a Tafel slope of 180 ± 2 mV dec⁻¹ over a scan rate range of 0.05–0.4 V s⁻¹, thereby indicating a 2e⁻ transfer process in which the initial electron transfer process was the rate-limiting step. The study also revealed that the Au/TiO₂/Ti electrode caused oxygen electro-reduction with a heterogenous rate constant (k⁰) of $4.40\times {10}^{-3}$ cm s⁻¹ at a formal potential (E⁰′) of 0.54 V vs. RHE.

1. Introduction

The global energy landscape faces a critical and escalating paradox: an infrastructure under unprecedented strain from the rapid depletion of finite fossil fuels and the continuously growing demand for power [1]. This unsustainable trajectory is driven by the fact that the world’s insatiable energy appetite, propelled by rampant industrialization and demographic expansion, is exhausting the planet’s finite reserves of coal, oil, and natural gas at a catastrophic pace [2]. In addition, this reliance on carbon-intensive fuels is the primary contributor to severe environmental pollution, manifesting as atmospheric greenhouse gas accumulation, climate change, ocean acidification, and public health crises [3]. It is precisely this convergence of pressing challenges like resource depletion, soaring demand, environmental degradation, and the current limitations of alternatives that has catalyzed an urgent global imperative [4]. Consequently, the focus has decisively shifted from mere consideration to the active and strategic development of a diversified green energy portfolio recently termed as green bond financing [5,6]. The mission is no longer to simply utilize renewables but to advance their efficiency, scalability, and economic viability through innovation, infrastructure investment, and policy support, thereby securing a sustainable and resilient energy future [7].

This urgent challenge has, in turn, galvanized research into sustainable energy alternatives. Among these, fuel cells are recognized as highly efficient, low-emission energy conversion technologies pivotal for mitigating climate impacts. Concomitantly, the development of highly efficient electrocatalysts constitutes a central thrust in modern materials science research [8]. This intense focus is driven by the pivotal role that these catalysts play in enhancing kinetic rates and minimizing overpotential, thereby directly determining the energy conversion efficiency and practical viability of next-generation technologies such as fuel cells, electrolyzers, and metal–air batteries [9]. Within the pursuit of sustainable energy, the oxygen reduction reaction (ORR) is the crucial electrochemical process at the heart of next-generation energy technologies like fuel cells and metal–air batteries [10,11]. This reaction is also highly promising for the generation of hydrogen peroxide (H₂O₂), which serves as both an effective agent for dye degradation and a fuel for H₂O₂-based fuel cells. It is worth mentioning that H₂O₂ has garnered significant interest as an alternative fuel in fuel cell applications due to its abundance, low cost, high volumetric energy density, and carbon-free emissions [12]. It is already adopted as a promising oxidant due to the following advantages: (i) being a carbon-free liquid [13]; (ii) its operation in an oxygen-free environment [14]; (iii) its ability to enhance overall cell performance by increasing cell voltage [15]; (iv) its ability to reduce activation loss [16]; (v) its ability to alleviate the water flooding problem in fuel cells [17]. Generally, hydrogen peroxide is used as oxidant in conventional two-compartment fuel cells where ethanol [15], sodium borohydrides [14], hydrazine [18], or metals [19] are used as fuel. In this type of configuration, H₂O₂ undergoes a reduction reaction and produces water molecules. The efficacy of H₂O₂ as an oxidant in these systems is governed by its reduction mechanism.

However, it is well established that this cathodic process can proceed via two primary pathways in a basic aqueous medium: efficient four-electron (4e⁻) transfer to hydroxide (OH⁻) or two-electron (2e⁻) transfer to peroxide (HO₂⁻) as per Equations (1) and (2) [20].

$$\mathbf{4}{\mathbf{e}}^{-} \mathbf{pathway}:{\mathrm{O}}_2 + 2{\mathrm{H}}_2\mathrm{O} + 4{\mathrm{e}}^{-} \to 4{\mathrm{OH}}^{-}$$

(1)

$$\mathbf{2}{\mathbf{e}}^{-} \mathbf{pathway}:{\mathrm{O}}_2 + {\mathrm{H}}_2\mathrm{O} + 2{\mathrm{e}}^{-} \to \mathrm{H}{\mathrm{O}}_{\mathrm{2}}^{-} +{\mathrm{OH}}^{-}$$

(2)

The 4e⁻ pathway is preferred for fuel cells due to its high current density and energy efficiency, while the 2e⁻ pathway is industrially significant for the sustainable production of hydrogen peroxide [21,22]. The electrochemical generation of H₂O₂ from oxygen via the 2e⁻ pathway has drawn significant attention, often preferred to conventional techniques such as direct synthesis from H₂ and O₂ gases and the anthraquinone method, as this method is eco-friendly and sustainable, avoids explosive reagents, provides on-site production, consumes relatively low energy, and shows high product selectivity [23,24,25,26,27]. So, the strategic development of catalysts is essential to steer this reaction toward the production of H₂O₂ through the 2e⁻ pathway which has further utilization in fuel cells or dye degradation. Consequently, the quest to develop high-performance ORR catalysts is a major frontier in materials science and electrochemistry, especially for proton exchange membranes (PEMs) and H₂O₂-based fuel cells. To address the inherent inefficiencies of the currently developed catalysts, researchers are employing sophisticated strategies such as nano-structuring, doping, and defect engineering to tailor the electronic properties and surface chemistry of materials [28,29,30]. Moreover, catalysts such as N-doped carbon, metal–organic frameworks (MOFs), and single-atom catalysts are promising materials available to improve ORR efficiency [31,32,33,34]. The aim is to optimize the binding energy of oxygen intermediates, thereby steering the reaction toward the production of H₂O₂ via a more efficient pathway and dramatically boosting the performance of electrochemical devices.

In parallel, it is established that the incorporation of semiconducting metal oxides, such as TiO₂, SnO₂, or WO₃, significantly enhances the durability of metal–carbon electrocatalysts [35]. These oxides function as robust, corrosion-resistant supports, mitigating the carbon corrosion that typically occurs under harsh electrochemical operating conditions [36]. One of the most commonly used metal oxides is TiO₂, and of interest are TiO₂ nanotube (TNT) arrays, which have been extensively studied and used for a wide range of electrochemical applications [37,38,39,40]. However, conventionally, TiO₂ is a poor electrocatalyst for the oxygen reduction reaction (ORR) due to its inherently low electrical conductivity and lack of electroactive sites. Moreover, TiO₂ is highly resistant to oxidation and corrosion under fuel cell operating conditions [41]. In anodically prepared TiO₂ electrodes, such as the ones in our study, it has been confirmed that a four-electron process is the dominant reduction process in alkaline media [42]. The foundational TiO₂ layer, grown on a Ti substrate, provides a stable, high-surface-area scaffold but remains electrochemically inert [43]. Within this context, gold nanoparticles (Au NPs) have emerged as a promising, highly tunable platform due to their unique catalytic properties, which are known to be profoundly influenced by their size and morphology. Notably, it is also found that Au has superior catalytic ability regarding the ORR. A study by Wang et al. [44] provided a seminal investigation into the precise structure–property relationships that govern ORR activity on shaped-controlled Au nanoparticles. In our recent study, we incorporated Au nanoparticles on a GCE electrode for dye degradation via the ORR [45]. With this in mind, in our current study, the addition of gold (Au) nanoparticles over anodized TiO₂ primarily addresses the critical issue of conductivity, creating electron pathways and providing initial active sites for oxygen adsorption.

Guided by established design principles for high-performance electrocatalytic systems, we engineered a composite electrode architecture (Au/TiO₂/Ti) aimed at enhancing both stability and activity for the ORR in neutral media. Specifically, the foundation consists of an anodically grown TiO₂ layer on a titanium sheet, providing a robust, high-surface-area support conducive to efficient mass transport. The incorporation of Au is intended to electronically modify the surface. This strategic combination is designed to optimize the adsorption behavior of oxygenated intermediates on catalytic sites, thereby favorably enhancing ORR kinetics. Following fabrication, to validate the surface morphology and chemical composition of the Au/TiO₂/Ti electrode, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were employed. These techniques confirmed the successful formation and distribution of the Au particles on the TiO₂ layer. To complement these structural analyses, for electrochemical characterization, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) were employed to evaluate the catalytic activity, reaction kinetics, and charge transfer behavior of the electrode toward the oxygen reduction reaction (ORR) in a slightly basic medium, namely a bicarbonate medium.

2. Results and Discussion

2.1. Surface Characterization

X-ray photoelectron spectroscopy (XPS) was employed to obtain information about the elemental composition and chemical/electronic state of the developed electrode material. Figure 1A represents the spectrum of the anodized Ti electrode, which illustrates two peaks at 458.26 and 463.86 eV, with a peak difference of 5.6 eV. As per previous studies [46,47,48], these two peaks can be assigned to Ti 2p_3/2 at 458.26 eV and to Ti 2p_1/2 at 463.86 eV; such observation indicates the Ti⁴⁺ of TiO₂ in anatase form.

After depositing the Au metal, XPS analysis of the modified electrode was performed. The peak position of Ti in Au-modified TiO₂ was again assessed to find any changes in the chemical states. It is observed in Figure 1B that the peaks corresponding to Ti 2p_3/2 and Ti 2p_1/2 retained approximately the same position after Au deposition. The O 1 s spectrum of the TiO₂ surface (see Figure 1C) exhibits a prominent peak centered at 531.50 eV, which is characteristic of lattice oxygen (O²⁻) in the Ti–O–Ti framework of TiO₂. In addition, a secondary peak is also observed at higher binding energy (533.25 eV), which corresponds to surface hydroxyl groups (–OH) or adsorbed oxygen species (O₂²⁻/O⁻) on the TiO₂ surface. Such surface oxygen species and hydroxyl groups can enhance surface reactivity and play a crucial role in catalytic and electrochemical processes by facilitating charge transfer and the adsorption of reactant molecules (e.g., O₂ or intermediates during ORR). Figure 1D shows two peaks at 84.03 and 87.71 eV, which can be assigned to Au 4f_7/2 and Au 4f_5/2, respectively, and such observation indicates Au in its metallic state, thereby demonstrating the successful deposition of Au on the TiO₂ layer.

In addition to XPS characterization, X-ray diffraction (XRD) analysis was performed to examine the crystalline structure and phase composition of the synthesized electrocatalyst. Figure 2 depicts the comparative XRD patterns of pristine Ti, TiO₂/Ti, and Au/TiO₂/Ti samples, in which no detectable new diffraction peaks appeared after the anodization process. This observation from these XRD signals indicates that the anodized Ti surface does not contain any crystalline TiO₂ phase detectable by XRD, meaning the oxide layer formed is either amorphous or too thin to generate measurable diffraction patterns [49,50,51,52,53,54]. In line with previous studies, the absence of distinct TiO₂ peaks is often observed for anodically produced TiO₂ films, which typically remain amorphous prior to thermal treatment [49,50,51,52,53]. It is observed from the figure that the Ti diffraction peaks of (101) and (200) planes at 2θ = 36.26° and 42.11° weakened or vanished after the anodization process. This finding can be attributed to the coating of the Ti surface by a new amorphous oxide film and alterations in the Ti’s underlying crystal structure during anodization [2,3]. The relative intensity changes among the remaining Ti reflections further suggest a texture evolution in the underlying substrate. Moreover, the changes in intensities of the Ti diffraction peaks suggest a textural evolution underlying the Ti substrate. Following Au deposition, new peaks appeared in the diffraction pattern, confirming the presence of a face-centered cubic (fcc) Au structure. The strong intensity of the (111) reflection relative to other Au peaks reveals that the deposited Au illustrates a pronounced (111) preferred orientation [55].

At this point, the surface of the developed electrode was assessed with a scanning electron microscope (SEM). Figure 3A,B show the SEM micrograph of the Au/TiO₂/Ti composite electrode at different magnifications. The surface appears to be composed of aggregated clusters, which enhance the effective surface area of the electrode. The distribution of bright regions across the surface corresponds to the presence of Au particles, which exhibit higher electron density and thus appear brighter in the SEM image. The underlying porous TiO₂ network serves as a framework, providing anchoring sites for Au particles. The overall morphology demonstrates that anodization followed by Au deposition produced a heterogeneous yet well-integrated composite surface, which is expected to promote enhanced catalytic activity due to the synergistic interaction between Au and TiO₂.

The EDX spectrum of the Au/TiO₂/Ti electrode (see Figure 4) confirms the elemental composition of the surface. The detected elements are Ti, O, Au, and C, consistent with the expected structure of the anodized TiO₂ layer decorated with Au nanoparticles. The elemental composition of the Au/TiO₂/Ti electrode is summarized in

Table 1. Elemental composition of Au/TiO₂/Ti electrode resulting from EDX analysis.

Elements	Atomic %	Atomic % Error	Weight %	Weight % Error	Net Counts
O	14.8	4.0	4.7	1.3	86
C *	7.9	0.8	1.9	0.2	131
Ti	70.8	0.9	67.7	0.9	9 825
Au	6.5	0.7	25.7	2.9	440

* C is observed due to carbon tap.

.

A strong Ti signal (70.8 at%) is observed due to the Ti substrate and the anodically formed TiO₂ layer. The O peak (14.8 at%) arises from the oxide layer on the Ti surface, confirming the presence of TiO₂. The Au signal (6.5 at%) indicates the successful deposition of Au particles on the anodized TiO₂ surface. The relatively low atomic percentage of Au compared to Ti is expected since Au is present as a dispersed particulate layer. The corresponding weight percentages (Ti: 67.7 wt.%, Au: 25.7 wt.%, O: 4.7 wt.%) further emphasize the dominance of the Ti substrate and the significant presence of the deposited Au. These findings confirm that the electrode surface comprises mainly Ti and Au, with a TiO₂ layer, validating the successful fabrication of the Au/TiO₂/Ti composite structure.

2.2. Electrochemical Characterization

When there is no current flowing through an electrode, its internal potential can be expressed by measuring the open circuit potential (OCP). To distinguish the interfacial electrochemical properties that emerged at the surface of the Au/TiO₂/Ti electrode in the presence and absence of O₂, the OCP was examined in this study in an O₂ and N₂-saturated 0.1 M KHCO₃ electrolyte.

The polarization curves recorded for the Au/TiO₂/Ti electrode in O₂-saturated and N₂-saturated 0.1 M KHCO₃ medium, as portrayed in Figure 5, show that the OCP value for O₂-saturated alkaline medium is +0.81 V, but when the electrolyte becomes saturated with N₂, the value shifts to +0.84 V. The less positive charge developed in the oxygen-saturated medium indicates that applying a minimal amount of negative potential would catalyze the reduction process in the presence of oxygen species.

Electrochemical impedance spectroscopy (EIS) is a powerful tool for examining an electrode surface’s electrical characteristics. To study the charge transfer resistance (R_ct), EIS spectra were recorded at same potential for bare Ti, TiO₂/Ti, and Au/TiO₂/Ti electrodes in O₂-saturated 0.1 M KHCO₃ to differentiate between the electrical properties and efficiency of the electrodes.

Figure 6 illustrates three Nyquist plots: a bigger semicircular EIS spectrum of a pristine Ti electrode and two progressively smaller semicircular spectra for the TiO₂/Ti and Au/TiO₂/Ti electrodes. The values of the corresponding circuit elements are summarized in

Table 2. EIS properties recorded for bare Ti, TiO₂/Ti, and Au/TiO₂/Ti for ORR at −0.15 V vs. RHE potential in experimental conditions.

Electrodes	Rs/Ω	Rct/kΩ	CPE/µMho
Ti	571	36.6	27
TiO2	515	7.55	145
Au/TiO2/Ti	507	3.80	25.2

. The value of the R_ct corresponding to bare Ti is 36.6 kΩ, which decreases significantly to 7.55 kΩ after formation of the TiO₂ layer over Ti via anodization, demonstrating enhanced charge transfer kinetics due to the formation of the TiO₂ layer. Further attenuation of R_ct to 3.80 kΩ can be observed upon Au deposition on the TiO₂ surface, indicating that the particles of Au over the layer facilitate faster electron transfer and improve the overall interfacial conductivity. The markedly lower R_ct of the Au/TiO₂/Ti electrode signifies minimal barrier to charge transfer during the electrochemical reaction, while the higher resistance of bare Ti reflects sluggish reaction kinetics. Thus, the comparison clearly demonstrates that Au/TiO₂/Ti exhibits superior electrochemical activity and is more effective for the ORR than the unmodified Ti electrode.

2.3. Catalytic Activity

In order to elucidate the viability of the reduction process, the activity of bare Ti and the modified Ti electrodes for the ORR was examined by utilizing the cyclic voltametric technique, in which the range of the applied potential was from 0.3 to −0.8 V vs. RHE at a scan rate of 0.1 V s⁻¹.

Figure 7A depicts the cyclic voltammograms (CVs) of the oxygen reduction reaction on the Ti electrode in N₂ and O₂-saturated 0.1 M KHCO₃ electrolytic solutions. t is evident from the figure that the peak current density of 1.64 mA cm⁻² at −0.78 V vs. RHE with an onset potential of −0.38 V vs. RHE on bare Ti in O₂-saturated 0.1 M KHCO₃ medium compared to in the N₂-saturated medium, indicating the reduction of O₂ on the electrode. The formation of the TiO₂ layer on the Ti electrode via anodization diminished the activity of the ORR rather than enhancing the ORR, as shown in Figure 7B. Furthermore, Au deposition on the Ti electrode enhanced ORR performance by lowering the peak potential and onset potential by 480 and 650 mV, respectively. However, the combination of Au and Ti was still insufficient since the electrode reduced the current density by 0.3 mA cm⁻², as shown in in Figure 7C. On the contrary, the deposition of Au on the TiO₂ layer modifying the Ti electrode considerably improved ORR performance, as illustrated in Figure 7D, since it reduced oxygen molecules with a current density of −1.81 mA cm⁻² at −0.18 V vs. RHE, with a set potential of 0.26 V vs. RHE. The reduction performance was improved in terms of peak current density, peak potential, and onset potential, as presented in

Table 3. Voltametric properties obtained by using Ti, TiO₂/Ti, Au/Ti, and Au/TiO₂/Ti electrodes in O₂-saturated 0.1 M KHCO₃ at 0.1 V s⁻¹.

Electrode	Ei/V vs. RHE	Ep/V vs. RHE	jp/mA cm−2
Bare Ti	−0.38	−0.78	$-$1.63
TiO2/Ti	-	-	-
Au/Ti	0.27	−0.32	$-$1.35
Au/TiO2/Ti	0.27	−0.18	$-$1.81

; therefore, the combination of Au and TiO₂ with a Ti surface has a good synergistic effect.

2.4. Scan Rate Effect

A scan rate-dependent study helps to elucidate the important information about an electrochemical process in terms of mechanisms and kinetics; therefore, the CVs of the Au/TiO₂/Ti electrode in related to the ORR were recorded at variable scan rates (0.05–0.4 V s⁻¹). The voltammograms, as illustrated in Figure 8A, show two fundamental features of an irreversible electrode process, i.e., a gradual increase in the peak current density (j_p) and the negative shifting of the peak potential (E_p) with scan rate (see Figure S1A,B) [45,56,57,58].

In connection to the irreversible electrode process, the potential separation between the value of the peak (E_p) and half peak (E_p/2), i.e., ΔE_p/2, was evaluated for each scan rate and was found to be around 0.51 V over the investigated scan rate range, as illustrated in Figure S1C. The value was almost invariant with respect to scan rate, thereby indicating that the electrode process followed a conventional kinetic framework given by the Butler–Volmer equation [45,56,57,59]. Regarding this adherence, the Tafel slope was evaluated by plotting E vs. log (−j) and was found to be 180 ± 2 mV dec⁻¹ as per Equation (3) over the scan rate range of 0.05–0.4 V s⁻¹ (see Figure 8B). The observed Tafel slope implies that the electron-transfer kinetics play an important role in controlling the overall two-electron reduction of O₂ to H₂O₂ on the Au/TiO₂/Ti surface [60].

$$E=\left(E^{0\prime }+b\times \log \left(j_0\right)\right)-b\times \log \left(j\right)$$

(3)

where $b={\displaystyle \frac{2.303RT}{\alpha F}}$ is the Tafel slope and $j_0$ is the exchange current density.

In order to evaluate the formal potential from the intercept of the Tafel slope in this particular case, the value of $j_0$ must be known. The value of the exchange current density can be determined indirectly from the relation $j_0=nFC{k}^0$, which connects j₀ with a heterogeneous rate constant. The value of k⁰ is only applicable to the case in which the diffusional current density shows deviation from linearity with further scan rate increment, which can be evaluated using Equation (4).

$$k^0={\displaystyle \frac{1.11 \sqrt{D}\sqrt{\vartheta }}{\sqrt{E_p-E_{p/2}} }}$$

(4)

where D is the diffusion coefficient of O₂ (2.0 × 10⁻⁵ cm² s⁻¹), υ is the scan rate (V s⁻¹) after which the linearity deviates, and E_p−E_p/2 is the difference between the peak potential and the half-peak potential (0.51 V).

The developed electrode showed deviation from linearity after reaching a 0.4 V s⁻¹ scan rate, and the value of the k⁰ was calculated to be $4.40\times {10}^{-3}$ cm s⁻¹ by evaluating the peak width at this scan rate. By using the values of these parameters, the formal electrode potential was calculated to be 0.54 V vs. RHE.

2.5. Stability Evaluation

A catalyst can be considered efficient and practically applicable if it exhibits sufficient stability. This is the key factor in choosing an electrode catalyst with confirmed durability during the reaction process. The modified Au/TiO₂/Ti electrode catalyst was assessed in terms of stability using the voltametric technique for the ORR in O₂-saturated 0.1 M KHCO₃. As illustrated in Figure 9, both the potential and peak current density demonstrated negligible changes after 500 voltametric cycles, indicating excellent stability. Furthermore, only a slight decrease in current density—approximately 5% after 1250 cycles and 12% after 2000 cycles—was observed. These results indicate that the modified electrode possesses outstanding durability and stability for the ORR under the tested conditions.

3. Experimental

3.1. Chemicals and Instruments

All chemicals employed in this study were of analytical grade and used as received, without any additional purification. A sheet of titanium (Ti) metal, tetrachloroauric (III) acid (HAuCl₄), 98% sulfuric acid (H₂SO₄), 37% hydrochloric acid (HCl), sodium hydroxide (NaOH) pellets, potassium bicarbonate (KHCO₃), absolute ethanol (C₂H₅OH), and acetone (CH₃COCH₃) were purchased from Sigma Aldrich (St. Louis, MO, USA).

The electrochemical experiments were carried out in a three-electrode Pyrex glass cell utilizing an Autolab PGSTAT 128N or a CHI 602E (CH Instruments) potentiostat. The electrochemical cell consists of a Ti sheet working electrode, an Ag/AgCl (sat. KCl) reference electrode, and a Pt counter electrode. A Milli-Q water system provided the high-purity water for all solutions. The experimental setup also employed a sonicator for electrode preparation, a hot plate with temperature control, and N₂/O₂ cylinders for solution purging. All measured potentials versus the Ag/AgCl reference electrode were converted to the reversible hydrogen electrode (RHE) scale according to Equation (5) (note: pH of 0.1 M KHCO₃ is 8.3) [61]:

ERHE = EAg/AgCl + 0.059 pH + 0.1971

(5)

3.2. Electrode Fabrication

3.2.1. Synthesis of TiO₂ Layer on Ti Sheet

The formation of the TiO₂ layer was achieved through the anodization process. In order to accomplish this, a Ti sheet with a geometric area of 0.196 cm² was taken and cleaned by sonicating it in a mixture of acetone and deionized water for 10 min to remove adsorbed substances from the Ti surface. The sonicated sheet was then placed in a beaker containing 6.0 M HCl and heated at 85 °C on a hot plate for another 10 min. Subsequently, the sheet was sonicated in deionized water for an additional 10 min. Once thoroughly cleaned, the sheet was placed in an electrochemical cell with a three-electrode configuration containing 1.0 M H₂SO₄. A potential of 12.0 V was applied using a DC power adapter (12 V, 2 A) [62]. After 2 h of treatment, the color of the Ti sheet changed from gray to brown, indicating the formation of TiO₂.

3.2.2. Au/Ti Electrode Fabrication

Metallic gold was electrodeposited onto the titanium sheet substrate from a 0.05 M chloroauric acid (HAuCl₄) solution. The electrolyte was prepared by dissolving 0.0759 g of HAuCl₄ in 5 mL of Milli-Q water. Deposition was performed using a three-electrode configuration via cyclic voltammetry by scanning the potential from 0 to −1.0 V (vs. Ag/AgCl (sat. KCl)) at a rate of 0.1 V s⁻¹. This process was repeated for three consecutive cycles to ensure complete coverage.

3.2.3. Au/TiO₂/Ti Electrode Fabrication

Metallic Au was deposited on the anodized Ti sheet (that is, TiO₂/Ti) via electrochemical deposition. Briefly, the TiO₂/Ti electrode was subjected to the cyclic voltametric technique, with electrochemical scanning from 0 V to −1.0 V at a scan rate of 0.1 V s⁻¹ for three cycles in an electrochemical cell configured with a three-electrode system, containing 10 mL of a 0.05 M chloroauric acid solution.

3.3. Surface Properties Analysis

Surface morphological characterization was performed via scanning electron microscopy (SEM) using a Hitachi TM3030 Plus system (Hitachi, Tokyo, Japan), coupled with energy-dispersive X-ray spectroscopy (EDX) for elemental analysis. To evaluate the surface chemical states and elemental composition, X-ray photoelectron spectroscopy (XPS) was conducted on a Kratos Axis-Ultra instrument (Shimazu-Kratos, Tokyo, Japan) equipped with a monochromatic Al Kα source (1486.68 eV). Furthermore, the crystalline structure of the synthesized electrocatalyst was analyzed using an X-ray diffractometer (SmartLab, Rigaku, Japan) equipped with Cu Kα radiation (λ = 1.54 Å). The diffraction patterns were recorded over an appropriate 2θ range at room temperature to identify the crystalline phases present.

4. Conclusions

This study successfully developed and characterized a novel Au/TiO₂/Ti composite electrode for the electrocatalytic oxygen reduction reaction; the electrode was fabricated via the anodization of a Ti sheet, followed by the electrochemical deposition of Au particles. Extensive surface characterization including XPS and SEM confirmed the successful formation of metallic Au nanoparticles on the anodized TiO₂ surface. Subsequently, electrochemical evaluation demonstrated enhanced ORR activity, as evidenced by a higher cathodic current density (−1.81 mA cm⁻²) and a drastically lower charge transfer resistance (3.80 kΩ) compared to the pure Ti sheet (36.6 kΩ). Additionally, the kinetic evaluation revealed a Tafel slope of 180 mV dec⁻¹, implying that the rate-limiting step involves the initial electron transfer. Furthermore, the electrode exhibited exceptional durability, maintaining its catalytic performance over 2000 cycles without significant degradation. In summary, the synergistic combination of Au particles and the TiO₂/Ti substrate creates a highly active, stable, and selective electrocatalyst for the ORR, thereby presenting a promising candidate for applications in sustainable energy conversion and environmental remediation.