Bifunctional Pt/TiO₂-O_v catalysts for enhanced electron transfer and CO tolerance in acidic HOR and ORR

Abstract

The development of anti-corrosion and anti-poison electrocatalysts for both the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) is of great importance for effective applications of proton exchange membrane fuel cells (PEMFCs). In this study, a non-carbon supported catalyst, Pt/TiO₂-O_v, enriched with oxygen vacancies (O_v), is successfully synthesized using a microwave-assisted method. This catalyst is developed as a bifunctional electrocatalyst with superior contamination tolerance, enabling efficient HOR and ORR performance. The electronic metal-support interaction (EMSI) is leveraged to facilitate electron transfer between Pt and Ti atoms, induced by the formation of oxygen vacancy channels in the small-sized, high surface area TiO₂-O_v support. Notably, TiO₂-O_v has a lower bandgap than commercial TiO₂, enhancing its catalytic properties. In a 0.1 mol/L HClO₄ electrolyte, the normalized Pt mass activity (j_k,m) and specific activity (j_0,s) of Pt/TiO₂-O_v are 1.24 times higher than those of commercial Pt/C. Furthermore, Pt/TiO₂-O_v catalyst exhibits minimal current density decay after a prolonged durability testing under hydrogen and oxygen atmospheres. Remarkably, under a H₂/(1000×10^–6) CO atmosphere, the relative retention rate of Pt/TiO₂-O_v significantly exceeds that of Pt/C catalyst, demonstrating its superior CO tolerance and promising potential for practical applications in PEMFCs. This study highlights the critical role of the strong metal-support interaction between the reducible oxide support and the noble metal Pt in improving long-term performance and CO poisoning resistance.

Graphical abstract

Keywords

CO poisoning / hydrogen oxidation reaction / oxygen reduction reaction / proton exchange membrane fuel cells (PEMFCs) / Pt/TiO₂ catalyst

1 Introduction

Hydrogen fuel cells, as zero emission devices converting hydrogen to electricity, have been extensively developed and recognized as a viable technology for automotive power units [1–4]. Specifically, proton exchange membrane fuel cells (PEMFCs), which use H₂/O₂ as reactants and operate under acidic conditions, have successfully been demonstrated to power electric vehicles for various transportation applications. However, practical PEMFCs require substantial amounts of carbon-supported platinum (Pt) catalysts at both the cathode and anode. The long-term stability of conventional carbon supports for Pt-based catalysts under high cathode potential and acidic conditions, along with contamination from impurities, have been identified as two challenges [5].

To address the issue of corrosion of carbon supports under practical PEMFC operating conditions, recent studies have shown that modifying or replacing carbon supports with non-carbon supports holds promise. For addressing contamination caused by hydrogen fuel impurities such as CO and H₂S, the development of non-carbon supported Pt-based catalysts has also proven effective [6,7].

Contamination or deactivation of the catalyst due to poisoning from hydrogen fuel impurities such as CO and H₂S, is a major challenge. These impurities can competitively bind to the surface reaction sites on Pt nanoparticles (NPs), significantly decreasing the availability of active catalytic sites and hindering overall catalytic performance. In the case of CO contamination, the catalysts may fail to mitigate the strong CO/Pt binding caused by electron donation from the CO σ-orbital to the Pt d-orbital, exacerbating CO poisoning and impairing catalytic performance [8]. This strong adsorption of CO, driven by the electronic interactions between Pt and CO, remains a major challenge in the design of CO-resistant metallic catalysts for the anode of PEMFCs [9].

Confining metallic Pt onto metal oxide supports, especially transition metal oxides, has been found to stabilize Pt NPs for both HOR and ORR [10]. The intense electronic transfer between Pt and the metal oxides, facilitated by electronic metal-support interaction (EMSI), markedly reduces Pt leaching and inhibits Oswald ripening, thereby enhancing catalyst activity and long-term stability. Furthermore, metal oxide supports can facilitate additional electron transfer pathways, enriching the electron environment on the Pt NPs. This shift in the Pt d-band center to a lower energetic state creates an electron-deficient environment, weakening the interaction between Pt’s d-orbitals and the antibonding orbitals of CO [8]. Consequently, CO adsorption on Pt is markedly diminished, mitigating CO poisoning and enhancing catalyst performance. However, under severe environments, the inherently low conductivity and limited durability of oxide supports pose significant challenges to their practical application in PEMFCs.

To address these challenges, Peng et al. [11] ingeniously embedded CoO_x into noble metal Pt NPs, where a perfectly encapsulating Pt layer effectively prevented transition metal dissolution. This design allowed the beneficial electronic modification of Pt by the oxide to be fully realized, and the resulting CoO_x@Pt/C catalyst achieved an excellent balance of stability and activity in acidic electrolytes. Similarly, Lin et al. [12] introduced a novel reactive metal-support interaction (RMSI) strategy, synthesizing a PtNi/ZrO₂ catalyst through the simultaneous reduction of Pt and a transition metal-doped oxide support. During calcination, the migration of Ni within the support increased the oxygen vacancy concentration in Ni-ZrO₂, significantly enhancing its electronic conductivity.

Inspired by these studies, transition metal oxides, particularly TiO₂, have emerged as promising alternatives to carbon-based supports due to their excellent stability, cost-effectiveness, and intrinsic catalytic properties [13,14]. In this context, a high-surface-area TiO₂ material enriched with oxygen vacancies (TiO₂-O_v) was successfully synthesized. By utilizing microwave irradiation, which offers a shorter reaction time and precise heating control, Pt NPs were rapidly immobilized onto the TiO₂-O_v support. This approach effectively addresses the sintering of Pt NPs, a common limitation of high-temperature calcination, and demonstrates scalability, making it a promising pathway for large-scale synthesis of catalysts [15]

The Pt/TiO₂-O_v catalyst exhibits excellent catalytic performance for both HOR and ORR in PEMFCs. The intrinsic ORR performance exhibits an exceptionally low half-wave potential (E_1/2) loss of only 7 mV after 5000 cycles of accelerated durability tests. Additionally, the current density shows a minimal decay of only 13.44% after a prolonged 24-h test under HOR conditions. The oxygen vacancies-enriched TiO₂ support accelerates electron transfer [16], providing an excellent model support for elucidating the cooperative effects of Pt and TiO₂-O_v in mitigating poisoning and improving CO tolerance. This results in enhanced intrinsic activity and CO tolerance for the Pt/TiO₂-O_v catalyst.

2 Experimental section

2.1 Synthesis of TiO₂/SiO₂

The TiO₂ nanoparticles embedded in SiO₂ were prepared using a sol-gel technique. In brief, 3.8 g of Ti(SO₄)₂ and 1.9 g of SiO₂ were separately dissolved in 33 mL of H₂O, followed by 60 min of ice bath sonication until complete dissolution. The two solutions were then combined and subjected to ice bath sonication for an additional 2 h. NH₃·H₂O was then added to adjust the pH to 6, resulting in a milky white precipitate. This precipitate was centrifuged and filtered with pure H₂O for three times.

Afterward, 33 mL of H₂O was mixed with the precipitate, and 2.5 mL of H₂O₂ was added. The mixture was sonicated for 1 h in a cooling bath, forming a homogeneous light-yellow solution. The solution was then heated in an oil bath at 110 °C for 40 h, after which it was transferred to a Teflon-coated autoclave and heated at 180 °C for 15 h. The final compound was washed thoroughly three times with ultrapure water and subjected to vacuum drying at 60 °C.

After milling, the powder was thermally treated in air at 500 °C for 3 h (at a rate of 2 °C/min), resulting in the final TiO₂/SiO₂ sample.

2.2 Synthesis of TiO₂-O_v

To optimize both yield and specific surface area, TiO₂/SiO₂ samples were etched with varying concentrations of HF (0.1%, 0.3%, and 0.5%) for 12 h. The TiO₂ samples etched with different HF concentrations were labeled as TiO₂-0.1%, TiO₂-0.3%, and TiO₂-0.5%, respectively. Given the significant impact of higher HF concentrations on the yield of TiO₂, no further increase in HF concentration was explored in this study. After etching, the TiO₂ samples underwent high-temperature hydrogen annealing at 800 °C. The resulting support material was designated as TiO₂-O_v.

2.3 Synthesis of Pt/ TiO₂-O_v

A total of 120 mg of the above TiO₂-O_v was dispersed in 20 mL of ethylene glycol using magnetic stirring. Once a uniform dispersion was achieved, 0.25 mmol of NaOH solution was introduced, and the mixture was stirred at 55 °C for 4 h. Next, the calculated H₂PtCl₆·6H₂O soluble salt and potassium bromide particles were added to 80 mL of a blended medium of ethylene glycol, 0.25 mmol of NaOH, and 0.1 g of KBr. The mixture was subjected to ultrasonic treatment for 2 h in a cooling bath under an inert argon atmosphere. It was then gradually added to the dispersed TiO₂-O_v solution while maintaining constant mixing. The solution was sonicated for an additional hour, followed by microwave synthesis at 400 W for 4 min. The final products were washed with deionized water and ethanol to remove residual impurities and then dried at 40 °C for 24 h.

3 Results and discussion

3.1 Structure characterizations

In this study, TiO₂ is successfully synthesized using a straightforward Stöber method, which involves sol-gel and gelation stages. Figure 1(a) illustrates a schematic of the Pt/TiO₂-O_v design. The synthesized TiO₂/SiO₂ is first subjected to air calcination, followed by etching and high-temperature reduction under H₂, resulting in a TiO₂-O_v support with a high specific surface area and oxygen vacancies. Then, Pt NPs are firmly anchored onto the TiO₂-O_v support through the reducing action of ethylene glycol (EG) and rapid microwave heating reaction, forming the final Pt/TiO₂-O_v catalyst. This entire procedure is user-friendly, scalable, and expected to be feasible for the mass production of Pt/TiO₂-O_v for practical applications.

X-ray diffraction (XRD) measurements were performed to investigate the structural characteristics. As depicted in Fig. 1(b), all characteristic diffraction peaks of Pt/TiO₂-O_v align well with those of pure Pt (JCPDS Card No. 01-087-0646) and anatase TiO₂ (JCPDS Card No. 98-000-0081), confirming the successful incorporation of Pt NPs onto a pure TiO₂ substrate. It should be noted that, regions previously assigned to SiO₂, which were removed through hydrofluoric acid (HF) etching, contribute to the formation of high-surface-area TiO₂ supports by creating new support sites.

This conclusion is supported by a series of control experiments in which varying concentrations of HF is tested to achieve an optimal balance between the surface area and TiO₂ yield. The BET surface areas of TiO₂ etched with HF concentrations of 0.1%, 0.3%, and 0.5% were found to be 123.9, 141.4, and 152.7 m²/g, respectively (Figs. 1(d) and S1). The corresponding pore size distribution data (Fig. S2) show that TiO₂-0.5% exhibits the smallest pore size due to the more extensive etching of SiO₂.

The ORR activity of various Pt/TiO₂ catalysts was evaluated in a 0.1 mol/L HClO₄ electrolyte, as illustrated in Fig. S3, to determine the most suitable support for Pt loading. Among the tested samples, Pt/TiO₂-0.5% exhibited the highest ORR activity, which was attributed to its superior specific surface area. Therefore, TiO₂-0.5% was selected as the optimal support for further investigation. According to the XRD patterns in Fig. S4, no SiO₂ peaks are observed in the TiO₂-0.5% samples after 12 h of HF etching, indicating the near-complete removal of SiO₂ from the TiO₂ surface. Following H₂ annealing, this material is designated as TiO₂-O_v.

As shown in the scanning electron microscopy (SEM) images in Fig. 1(c), the embedding of SiO₂ into TiO₂ via the hard template method followed by HF etching results in a porous architecture after the SiO₂ is removed. However, a small residual content of SiO₂ remains detectable in the final sample. Energy Dispersive Spectroscopy (EDS) mapping (Fig. S5 and Table S1) of SEM images reveals a residual Si content of 5.08% in the TiO₂ substrate, compared to 10.29% and 14.78% for TiO₂ etched with 0.3% and 0.1% HF, respectively. This suggests the inherent challenges of achieving complete SiO₂ removal during the HF etching process. The HF concentration positively correlates with the final surface area but negatively correlates with the yield of the product. Additionally, high HF concentrations and longer etching times lead to corrosion of the TiO₂, degrading the performance of the TiO₂ substrate.

The morphology of the Pt/TiO₂-O_v catalyst was further examined using transmission electron microscopy (TEM). As illustrated in Figs. 1(e)–1(g), the TEM images reveal a uniform distribution of abundant Pt NPs, approximately 2.75 nm in size, evenly dispersed across the TiO₂ support (inset in Fig. 1(f)). The corresponding high-resolution transmission electron microscopy (HRTEM) image reveals lattice fringes with spacings of 0.232 and 0.208 nm, corresponding to the Pt (111) and (200) planes of standard Pt, which is consistent with the XRD pattern. This confirms the successful loading of Pt NPs onto the TiO₂-O_v support. Isolated Pt NPs, highlighted by yellow circles, are clearly distinguishable from the surrounding TiO₂-O_v support. The lattice fringe spacings of TiO₂ are measured to be 0.239, 0.245, and 0.350 nm, corresponding to the (004), (103), and (101) crystal planes of standard anatase TiO₂, respectively.

The metallic composition of the catalyst was further analyzed using High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM), as shown in Fig. 1(h). The higher atomic number of Pt (Z = 78) results in a stronger contrast in HAADF-STEM imaging, which facilitates better visualization and characterization of Pt NPs on the TiO₂-O_v support. The EDX mappings (Figs. 1(i)–1(m)) confirm that the Pt elements, represented by blue luminescent spots, are homogeneously distributed across the TiO₂-O_v support. The green, yellow, and red regions correspond to the distribution of Ti, O, and Si elements, respectively. This highly uniform Pt loading method is expected to significantly enhance the utilization and MA of the noble metal, creating an efficient catalyst structure.

Since the discovery of oxygen vacancies by Tompkins and colleagues in 1960, extensive research has demonstrated that the dissociation of oxygen within the lattice can lead to the formation of O_v [17]. These vacancies play a crucial role in modulating the electronic state of metal oxides, thereby influencing their electrical conductivity. Annealing in a reducing atmosphere is known to induce defects sites within the TiO₂ lattice by reducing the transition metal cations to a lower valence state [18]. The interaction between the TiO₂ support and hydrogen molecules at elevated temperatures facilitates the formation of O_v, which effectively lowers the activation energy barrier for TiO₂ lattice rearrangement, accelerating the process [19,20].

In this study, to characterize the oxygen vacancies on the oxide surface, several well-established techniques were employed, including X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and UV-vis-NIR diffuse reflectance spectra (UV-vis-NIR DRS). The structural properties of TiO₂, TiO₂-O_v, and Pt/TiO₂-O_v samples were examined using Raman spectroscopy (Fig. S6). The peaks at 394.5, 506.7, and 622.4 are characteristic of anatase TiO₂ bands [21]. Notably, as the catalyst synthesis progresses, these peaks broaden and attenuate, indicating structural changes induced by the introduced oxygen vacancies, which disrupt the symmetry of the TiO₂ lattice during H₂ annealing [22,23].

A key advantage of using metal oxides as supports, compared with traditional carbon supports, is their ability to facilitate additional electron transport pathways. This characteristic, known as the electronic metal-support interaction (EMSI) effect, enhances electron transfer between Pt and the support. To investigate the chemical environment of surface elements, X-ray photoelectron spectroscopy (XPS) measurements were conducted. Figures S7 and S8 present the survey spectra for TiO₂-O_v and Pt/ TiO₂-O_v. The spectrum for TiO₂-O_v reveals only the presence of Ti, O, and Si, whereas the spectrum for Pt/TiO₂-O_v displays an additional signal for Pt, confirming the successful deposition of Pt NPs onto the TiO₂-O_v support via microwave reduction.

Figure 2(a) illustrates the high-resolution XPS spectra of Ti 2p, with the primary peaks for Ti⁴⁺(459.08, 464.96 eV) corresponding to the spin-orbit splitting of Ti 2p_3/2 and Ti 2p_1/2. These peaks shift to higher binding energies (459.18 and 465.02 eV) after the loading of Pt NPs. Additionally, Ti contains low-valence Ti states, generated by hydrogen reduction in TiO₂-O_v. The main peak for Ti³⁺ exhibits a similar shifting, suggesting electron transfer from Ti to Pt across the entire catalytic system.

The oxygen vacancies in TiO₂ are believed to induce the formation of Ti³⁺ states by creating defect sites within the crystal lattice where electrons can be localized. In these vacancies, Ti⁴⁺ ions capture electrons and undergo reduction to form Ti³⁺ states. This electron transfer, facilitated by oxygen vacancies, influences the electronic structure and properties of TiO₂. Specifically, Ti³⁺ creates a new donor level within the electronic band structure of TiO₂ support, facilitating the transfer of d-orbital electrons from TiO₂ support to Pt [24]. This electronic modification promotes the interaction between Pt and the support [25,26].

The high-resolution XPS spectrum of O 1s (Fig. 2(b)) displays a positive binding energy shift compared with pristine TiO₂. The peaks at 533.13 and 533.03 eV are associated defected oxygen in TiO₂-O_v and TiO₂. The significantly increased peak area ratio of defected oxygen to lattice oxygen in TiO₂-O_v confirms a greater concentration of defected oxygen, which enhances the conductivity of the reducible oxide carrier and facilitates the catalytic process.

Figure 2(c) presents the high-resolution XPS spectrum of Pt 4f spectra in Pt/ TiO₂-O_v, with peaks at binding energies of 70.52 and 73.88 eV, corresponding to the 4f_7/2 peaks of Pt⁰ sites. Additionally, the negative shift of the Pt 4f spectrum compared with commercial Pt/C shown (Fig. 2(d)) provides clear evidence that Pt can accept electrons from Ti. This electron transfer is attributed to charge redistribution during Pt loading, which induces the EMSI effect, causing electron disturbance and surface electron enrichment on Pt.

As established in prior research, an increase in oxygen vacancies in anatase TiO₂ results in a reduced bandgap [27]. UV-vis spectra were deployed to assess the charge transfer capabilities of TiO₂-O_v (Fig. S9(a)). The TiO₂-O_v sample shows significant border absorption across the entire UV-vis wavelength range, with the absorption edge around 400 nm for both TiO₂-O_v and TiO₂. However, TiO₂-O_v demonstrates a marked increase in absorbance, attributed to changes in the bandgap associated with the anatase phase and the introduction of oxygen vacancies at the interfaces [28]. As illustrated in Fig. S9(b), the Tauc plot reveals a reduced bandgap of 2.97 eV for TiO₂-O_v after hydrogen annealing. This reduction in bandgap enhances electron transfer efficiency, thereby increasing conductivity and facilitating rapid charge transfer between surface-adsorbed oxygen species and the oxide support [28,29]. However, the interaction between oxygen vacancies and Pt ions is insufficient to ensure the complete adsorption of all Pt ions on the support surface [30]. Based on the findings from ICP characterization (Table S2), the actual Pt loading in Pt/TiO₂-O_v sample is 15.99 wt%.

3.2 Electrocatalytic performances

The electrocatalytic HOR and ORR activities of Pt/TiO₂-O_v and Pt/C (20 wt%, TKK) were evaluated in a 0.1 mol/L HClO₄ solution through rotating disk electrode (RDE) measurements. To assess performance differences between the catalyst and the support, both Pt/TiO₂-O_v and TiO₂-O_v were tested under acidic conditions. As shown in Fig. S10, the comparison of their cyclic voltammetry (CV) curves and ORR linear sweep voltammetry (LSV) curves clearly reveals the extremely low activity of the TiO₂-O_v support, which exhibits negligible catalytic performance. Upon Pt loading, electronic transfer between Pt and the TiO₂-O_v support modifies the electronic structure of the material, significantly enhancing the catalytic activity of Pt/TiO₂-O_v compared to TiO₂-O_v. This interaction underscores the critical role of the interplay between TiO₂-O_v and Pt NPs in influencing the reaction rate.

For assessing the HOR performance of Pt/TiO₂-O_v and Pt/C, LSV experiments were performed to demonstrate that the acidic HOR activity of Pt/TiO₂-O_v exceeds that of Pt/C. As illustrated in Fig. 3(a), with increasing applied potential, the LSV curve of Pt/TiO₂-O_v catalyst reaches a limiting current density of 3.23 mA/cm², which is achieved more rapidly than that of Pt/C, which reaches a limiting current density of 3.20 mA/cm². This is further corroborated by the observation that the half-wave potential of Pt/TiO₂-O_v is 5 mV lower than that of Pt/C, indicating a faster kinetic rate for the HOR. As depicted in Fig. 3(b), an increase in electrode rotation speed leads to a continuous rise in the limiting current density. The Koutechy-Levich plots (Fig. S11) can be constructed from the reciprocals of current densities observed at various rotation rates at 45 mV. The slope obtained from linear fitting of the points corresponding to rotation speeds of 400, 900, 1600, and 2500 r/min is found to be 4.54 cm²/(mA²∙s)^−1/2, which is in close agreement with the postulated value of 4.87 cm²/(mA²∙s)^−1/2 for a two-electron transfer mechanism in the HOR [31]. Moreover, to quantitatively assess the reaction rates and catalytic activity, the kinetic current densities (j_k) were calculated using K-L equation. Corresponding Tafel plots, as depicted in Fig. 3(c), indicate that at 50 mV versus RHE, the j_k for Pt/TiO₂-O_v is nearly twice that of Pt/C, specifically 639.11 mA/cm² compared to 292.33 mA/cm². As presented in Fig. S12, even when normalizing the j_k to Pt loading, Pt/TiO₂-O_v catalyst outperforms Pt/C.

Additionally, Pt/TiO₂-O_v demonstrates outstanding durability even after a prolonged 24-h test at 0.1 V. As illustrated in Fig. 3(g), the current density exhibits only a 13.44% decay after the durability test, highlighting the remarkable stability of the catalyst. Given the similarity in particle size and uniform distribution of metal nanoparticles on both the commercially available Pt/C catalyst and the Pt/TiO₂-O_v catalyst prepared in the experiment, the main difference lies in the choice of support. The EMSI effect in Pt/TiO₂-O_v catalyst system likely explains this enhanced performance, as electron transfer from the oxide to Pt generates a conduction pathway, modulating the hydrogen binding energy (HBE) [14,32]. The optimized electronic properties of Pt refine the adsorption behavior at active sites, ultimately enhancing the kinetics of HOR in acidic environments.

After the demonstration of the remarkable performance of Pt/TiO₂-O_v catalyst in HOR, attention is directed toward the proficiency in the ORR, emphasizing its dual functionality. The CV peaks in Fig. 3(d) between 0.6 and 1.0 V clearly shows the Pt-OH then Pt-O formation and Pt-O reduction, confirming the active ORR reaction [32]. These peaks provide insights into the HBE and oxygen-hydrogen binding energy (OHBE), which are critical for evaluating electrochemical performance [33]. The hydrogen desorption peak for Pt/TiO₂-O_v is observed at 0.144 V, reflecting a reduced binding affinity for dissociated H_ads or an increased stability of OH_ads, compared to the peak for Pt/C at 0.153 V.

Hydrogen adsorption has a “sweet point” —excessive or insufficient binding between the catalysts and the hydrogen can lead to an unfavorable state for HOR [34]. When Pt NPs are loaded on TiO₂-O_v, in contrast to commercial carbon support, the EMSI effect arises between TiO₂-O_v and Pt NPs, resulting in altered electronic states that reduce hydrogen adsorption at the catalyst interface. Regarding the Pt/C catalyst, the lack of electron transfer confines the proton transfer and hinders the desorption of intermediate due to accumulation of hydrogen deteriorate the stable state on the surface of catalyst [34,35]. Additionally, the hydrogen underpotential deposition (H_upd) experiments reveals that the electrochemically active surface area (ECSA) of Pt/TiO₂-O_v measures 62.34 m²/g, outperforming Pt/C (57.39 m²/g).

This enhancement in ECSA can be ascribed not only to the O_v within TiO₂, which influences surface properties, but also to the synergistic effect of EMSI and ligand interactions, as demonstrated by XPS results.

These interactions help better disperse and stabilize Pt NPs, optimizing their catalytic efficiency. Moreover, the onset of platinum oxidation is shifted to higher potential for Pt/TiO₂-O_v catalyst, as the potential increases. This same trend is observed during the potential decrease, corresponding to the reduction of platinum oxide. The CV curves reveal a robust interaction between the non-carbon substrate and the Pt NPs, proved by XPS results, resulting in a weakened oxophilicity. This diminished oxophilicity of Pt, attributed to the ligand effect, is crucial for enhancing ORR activity by facilitating the capture of intermediates [36,37].

Consequently, Pt/TiO₂-O_v catalyst exhibits superior E_1/2 and MA compared to commercial Pt/C catalyst. As illustrated in Fig. 3(e), LSV was employed to measure the ORR performance. The potential was systematically swept from 0.05 to 1.0 V while the electrode was spun at 1600 r/min. The limiting current densities for Pt/TiO₂-O_v and Pt/C are 5.80 and 5.60 mA/cm², respectively. Additionally, the E_1/2 for Pt/TiO₂-O_v is 0.88 V, surpassing Pt/C at 0.87 V. Various rotation speeds (400, 900, 1600, and 2500 r/min) were also used to probe ORR dynamics. Figure S13 illustrates that as the rotation speed increases, the current density rises, which can be attributed to enhanced mass transport dynamics and reduced concentration polarization facilitated by hybrid kinetic diffusion. Accelerated deterioration tests (ADTs) were performed to evaluate the durability of Pt/TiO₂-O_v and Pt/C under 5000 CV cycles. Pt/TiO₂-O_v catalyst manifests only a 7 mV decrease in E_1/2 throughout degradation tests, contrasting sharply to the significant drop noted in the commercial Pt/C catalyst, which endures extensive Pt agglomeration under identical harsh potential sweep conditions.

As shown in Figs. 3(f) and S14, the ECSA of Pt/TiO₂-O_v maintains exceptional stability, consistently surpassing Pt/C throughout electrochemical cycles, underscoring its remarkable durability during ORR performance. After ADTs, as shown in Fig. S15, the ECSA retention rates for Pt/TiO₂-O_v and Pt/C are 65.5% and 43.5%, respectively.

Additionally, Figs. S16 (a) and S16(b) highlight the MA and specific activity (SA) retention rates of Pt/TiO₂-O_v, which significantly outperform Pt/C. Specifically, after durability testing, the MA retention rates for Pt/TiO₂-O_v and its counterpart are 62.6% and 25%, respectively, reflecting a difference of more than twofold. Similarly, SA retention rates are 95.6% for Pt/TiO₂-O_v compared to 59.6% for Pt/C, underscoring the superior stability and resilience of Pt/TiO₂-O_v catalyst under harsh operational conditions. The structure integrity of the catalysts after ADTs was also assessed via TEM. As shown in Fig. S17 and its inset, Pt/TiO₂-O_v catalyst maintains excellent structural stability even after 5000 cycles, attributed to the EMSI effect. This interaction effectively anchors Pt NPs, suppresses agglomeration, and ensures minimal change in particle size distribution, with the average Pt particle size increasing only slightly from 2.75 to 3.67 nm, confirming the uniform dispersion of nanoparticles.

In summary, oxygen vacancies facilitate enhanced electron transfer, enabling the support to form stronger bonds with Pt, thereby suppressing the migration and aggregation of Pt NPs, ultimately improving catalyst stability [38].

3.3 CO tolerance measurements

Impurities such as CO, H₂S, and NH₃, which are inevitably generated during industrial hydrogen production, can strongly adsorb onto Pt-group metal catalysts, poisoning the active sites and significantly diminishing catalytic performance [1,6,31,39–42]. For example, when CO interacts with the Pt surface, the 2π* and 5σ orbitals of CO molecule undergo a reduction in energy and an expansion in width. The 5σ orbital splits into bonding and antibonding states [42]. At the Pt/CO interface, two distinct electron transfer pathways emerge: one involves electron transfer from the antibonding 5σ orbital of CO to the metal d-band, while the other describes electron flow from the metal d-band to the 2π* orbitals of CO [43]. The coexistence of these two electron transfer pathways strengthens the interaction between Pt and CO, resulting in CO poisoning.

Inspired by this mechanism, the Pt/TiO₂-O_v catalyst developed in this study is designed to reduce the interaction between Pt and CO by limiting the electron migration from CO to Pt. As shown in the XPS spectra in Figs. 2(a) and 2(b), the positive shift in Ti 3p orbitals and the negative shift in Pt 4f orbitals relative to standard Pt/C confirm the electron transfer from the titanium oxide support to Pt in the Pt/TiO₂-O_v system. This electron-enriched Pt NPs in the catalyst system not only prevent electron transfer from CO to Pt but also modulate both the HBE and carbon monoxide binding energy (COBE), ultimately contributing to improved catalytic performance [14,32].

The introduction of CO during the HOR process has a profound impact on catalyst performance. CO can obstruct hydrogen uptake and its subsequent cleavage at the catalyst surface, leading to a significant reduction in HOR efficiency. The polarization curves of both Pt/TiO₂-O_v and Pt/C were also examined under a H₂/(1000×10^–6) CO atmosphere. After injecting 1000×10^–6 CO into the HOR process, the LSV comparison between Pt/TiO₂-O_v and Pt/C (Figs. 4(a) and 4(b)) reveal a notable reduction in limiting current densities. Pt/TiO₂-O_v exhibits only a 3.67% decline in current density, while Pt/C suffers nearly three times the attenuation, underscoring the superior CO tolerance of Pt/TiO₂-O_v.

To better quantify the impact of CO poisoning, the exchange current density (j₀) was evaluated by fitting the data from the polarization curves in the micro-polarization region (−0.005 to 0.005 V), facilitating a more nuanced comparison and offering clearer insights into performance degradation. As shown in Fig. 4(c), the relative j₀ retention rates for Pt/TiO₂-O_v and Pt/C are 90.52% and 65.27%, respectively, indicating that the non-carbon support facilitates additional electron transfer pathways, limiting excessive CO adsorption and enhancing CO tolerance.

Extending from the effect of CO impurities on the HOR process, even trace levels of CO in the oxygen supply can critically affect the ORR process [44]. When transitioning to the ORR process with 1000×10^–6 CO injected into O₂, the LSV results for both samples (Figs. 4(d) and 4(e)) show a significant reduction in limiting current densities by 0.673 and 0.888 mA/cm², respectively, highlighting the occupation of active sites by CO as it competes with O₂ on the catalyst surface. Additionally, CO adsorption at the catalyst interface hampers the accessibility of active sites for oxygen reduction, causing a negative shift in the E_1/2. For the as-prepared Pt/TiO₂-O_v, the E_1/2 presents a negligible change, while the commercial Pt/C experiences an 11.5 mV loss after exposure to 1000×10^–6 CO, manifesting a stronger interaction between Pt NPs and the oxide support, which enhances their kinetic reactivity.

To further explore the CO poisoning effect on the catalysts, chronoamperometric tests at E_1/2 were also conducted for both HOR and ORR in the presence of 1000×10^–6 CO to evaluate long-term durability. The experimental results in Figs. 4(g) and 4(h), show significant differences in CO resistance between Pt/TiO₂-O_v and Pt/C catalysts under H₂/(1000×10^–6) CO and O₂/(1000×10^–6) CO atmospheres. The activity decline for Pt/TiO₂-O_v, shown by the red dotted line in Fig. 4(g), is comparatively smaller than that observed for Pt/C, which exhibits a nearly threefold greater decline in relative current. When the atmosphere changes from H₂/(1000×10^–6) CO to O₂/(1000×10^–6) CO, both Pt/TiO₂-O_v and Pt/C exhibit a similar trend of decay during the 2000-s chronoamperometric test, with current densities decreasing by 66.36% and 72.53%, respectively. The performance degradation stems from CO aggravating the inherently slow kinetics of the acidic ORR.

The adsorption energy of CO was further investigated using CO stripping voltammetry. The CO adsorption procedure was conducted by polarizing the electrode at 0.1 V and introducing a CO atmosphere into the HClO₄ solution for 25 min, followed by an Ar flow for 10 min to remove dissolved CO from the electrolyte. As shown in Fig. 4(f), the first forward scan of CV represents the stripping of a single layer of CO. Both Pt/C and Pt/TiO₂-O_v catalysts exhibit a prominent CO stripping peak between 0.65 and 0.75 V. For Pt/C, the CO stripping peak is recorded at 0.74 V, while Pt/TiO₂-O_v shows a peak at 0.68 V, indicating a negative shift of 60 mV compared to the commercial catalyst. This shift indicates an enhanced OHBE on Pt/TiO₂-O_v, attributed to the abundant oxygen vacancies, which effectively weaken the Pt–CO interaction and promote the rapid oxidation of CO [45,46]. The faster kinetics of HOR make it more susceptible to CO poisoning, particularly in the absence of self-regeneration mechanisms in the acidic environment. However, by leveraging the additional electron transfer between Pt and non-carbon support (TiO₂-O_v), the diminished OHBE facilitates OH species adherence on the catalyst interface, accelerating CO oxidation [43].

The above findings suggest that the Pt/TiO₂-O_v catalyst demonstrates enhanced resistance to poisoning in both the HOR and ORR processes by leveraging the EMSI effect, which is introduced by substituting oxide for carbon support. This capability effectively mitigates performance degradation arising from unforeseen CO introduction during the actual operational conditions in PEMFC [43].

4 Conclusions

In summary, the Stöber method is employed to synthesize small-sized TiO₂ embedded with SiO₂, achieving a higher surface area through controlled HF etching. High-temperature hydrogen calcination further enriches the TiO₂ with oxygen vacancies. The combination of ethylene glycol impregnation reduction and microwave loading methods proves to be both efficient and scalable for large-scale synthesis. XPS characterization of the resulting Pt/TiO₂-O_v catalyst reveals electron transfer between TiO₂-O_v and Pt NPs, indicative of the EMSI effect. This electron transfer forms a strong foundation for long-term stability, enhancing electrocatalytic performance in both HOR and ORR reactions in 0.1 mol/L HClO₄, and provides significant resistance to noble metal poisoning, particularly in the presence of 1000×10^–6 CO.

This study provides new insights into noble the development of metal-loaded electrocatalysts with oxygen-rich reducible oxide supports, improving both their catalytic efficiency in energy conversion reactions and their resilience against CO poisoning. The enhanced stability of the Pt/TiO₂-O_v catalyst in both HOR and ORR processes under acidic conditions is attributed to the electron transfer from TiO₂-O_v support to Pt NPs. This electron transfer, facilitated by substituting the support with TiO₂, results in the EMSI effect, which strengthens the interaction between the noble metal and the support. This robust interaction ensures that the catalyst remains securely anchored during prolonged stability tests, effectively preventing significant detachment and agglomeration. As a result, these factors collectively contribute to the enhanced long-term stability of the catalyst.

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