Highly Dispersed and Crystalline Ta2O5 Anchored Pt Electrocatalyst

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Highly Dispersed and Crystalline Ta2O5 Anchored Pt Electrocatalyst with Improved Activity and Durability Towards Oxygen Reduction: Promotion by Atomic-Scale Pt–Ta2O5 Interactions wenbin gao, zhengping zhang, Meiling Dou, and Feng Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04505 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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Highly Dispersed and Crystalline Ta2O5 Anchored Pt Electrocatalyst with Improved Activity and Durability Towards Oxygen Reduction: Promotion by Atomic-Scale Pt–Ta2O5 Interactions Wenbin Gao,a,b Zhengping Zhang,a,b Meiling Dou,a,b,* Feng Wang a,b,*

a

State Key Laboratory of Chemical Resource Engineering, Laboratory of Electrochemical Process

and Technology for materials, Beijing University of Chemical Technology, Beijing 100029, China b Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University

of Chemical Technology, Beijing 100029, P. R. China

ABSTRACT: Developing highly active and durable Pt-based electrocatalysts for the oxygen reduction reaction (ORR) is a crucial target if actual commercial application of proton exchange membrane fuel cells (PEMFCs) is to be realized. Herein we show that utilizing highly dispersed and crystalline Ta2O5-modified carbon nanotubes (CNTs) as a support can stabilize Pt nanoparticles (NPs) by strengthening the metal–support interactions at the atomic scale, and that

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this offers an efficient strategy to improve the ORR catalytic activity and durability of the Pt NPs. These were found to be selectively anchored on the interface of well-dispersed Ta2O5 NPs and CNTs, showing an atomic-coupled interfacial structure between Pt and Ta2O5 with lattice overlap of Pt (200) and Ta2O5 (001). X-ray absorption near edge structure (XANES) analysis shows that the electronic structure of Pt is perturbed by Ta2O5 by virtue of the formation of strong Pt–O–Ta bonds. The presence of highly crystalline Ta2O5 also induces the growth of polyhedral-structured Pt NPs with the exposure of abundant (111) and (100) facets, leading to an improved ORR activity for Pt–Ta2O5/CNT. As a result, our Pt–Ta2O5/CNT electrocatalyst exhibits high ORR activity with a large electrochemical surface area of 78.4 m2 g–1 and a mass activity of 0.23 A mg–1Pt at 0.9 V (this represents a 3.4- and 2.2-fold improvement over the corresponding activities of commercial Pt/C and Pt/CNT catalysts, respectively). Most importantly, Pt–Ta2O5/CNT possesses superior long-term durability without any obvious degradation after 10,000 cycles and thus outperforms both of the commercial catalysts. Our strategy of using highly dispersed and crystalline Ta2O5 to stabilize Pt NPs, resulting in strengthened metal–support interactions, should facilitate the development of high-performance Pt-based ORR electrocatalysts for use in fuel cells.

KEYWORDS: Ta2O5 nanoparticles, Pt–Ta2O5 interfaces, strong metal-support interactions, electrocatalysts, oxygen reduction reaction

INTRODUCTION


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Polymer electrolyte membrane fuel cells (PEMFCs) are one of the most promising energy conversion devices for automobile propulsion due to their high efﬁciency and zero emission.1-3 Carbon-supported Pt nanoparticles (NPs) are most frequently used as electrocatalysts to accelerate the kinetics of the oxygen reduction reaction (ORR) at the cathode of PEMFCs.4,5 However, the high cost and poor durability of Pt-based electrocatalysts hinder actual practical application of PEMFCs.6,7 Especially under the harsh working conditions of fuel cells (e.g., frequent load cycles, high potential and strongly acidic and oxidizing conditions8-10), Pt NPs tend to suffer from dissolution, migration, and Ostwald ripening/coalescence due to the relatively weak interactions between Pt and the carbon support,11,12 leading to a significant degradation of performance. Furthermore, the supports tend to undergo carbon corrosion especially under the high potentials occurred during the start-up/shut-down or fuel starvation conditions; this results in the detachment of Pt NPs from the carbon support and thus a decrease in performance.13,14 To address these issues, a number of approaches have been developed, such as introducing a second transition metal (TM) to alloy with Pt15 or fabricate core–shell (TM@Pt) structures,16 modifying the carbon support or Pt NPs with metal oxides,13 and developing durable carbon-free supports17). Of these, modification of pristine carbon by metal oxides (e.g., MnOx, TiOx, SnOx18-20) has been shown to be an effective strategy to not only enhance the corrosion resistance of the support and reduce the degradation rate of Pt-based electrocatalysts, but also improve the ORR activity by virtue of the strong metal– support interactions (SMSI) generated in these hybrid structures.11,12 The SMSI effects between Pt NPs and metal oxides mainly arise from the Pt–metal oxide interface, and offer several advantages: (i) tailoring of O2 adsorption and O–O bond cleavage by modifying the electronic state of Pt,21,22 (ii) reducing the OH coverage on the Pt surface by virtue of the spillover of OH onto the metal oxide,23,24 and (iii) preventing the dissolution and aggregation of Pt NPs by suppressing the


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formation of Pt–oxide species.21,25 Therefore, creating abundant Pt–metal oxide interfaces to promote SMSI effects is a promising way to improve the activity and durability of Pt-based electrocatalysts for ORR. Tantalum oxide (TaOx) is very stable in acidic media due to the formation of a passive layer of Ta2O5,26,27 and has been shown to have potential application as a component of ORR electrocatalysts (as a support or catalyst modifier) in PEMFCs.28,29 To the best of our knowledge, there have been few studies of the effect of TaOx on the ORR performance of Pt-based electrocatalysts.23,30 Baturina et al. prepared a Pt/Ta2O5/Vulcan carbon (VC) electrocatalyst by supporting Ta2O5 on VC followed by the deposition of Pt colloids and showed that the resulting material had a higher area-specific ORR activity at 0.9 V than the unmodified Pt/VC.28 However, the Pt/Ta2O5/VC electrocatalyst had a rather lower electrochemical surface area (ECSA) (24 m2 g– 1

Pt)

than the unmodified Pt/VC (63 m2 g–1Pt). Wesselmark et al. also reported that the introduction

of TaOx decreased the ECSA of Pt/TaOx electrode prepared by depositing TaOx followed by a Pt layer on a gas diffusion layer via thermal evaporation in a vacuum.31 More promisingly, Ohsaka et al. showed that using a glassy carbon (GC) as a support afforded Pt/TaOx/GC electrocatalysts with ORR activity and durability substantially better than the unmodified Pt/GC catalysts; they attributed the high ORR performance to the strong interaction between Pt and TaOx.23,32 However, these Pt/TaOx/GC electrocatalysts still exhibited low ECSAs (31.7–47 m2 g–1)23,32 and these result in low Pt utilization and limit further improvement of the ORR activity. Although the enhanced ORR activity is attributed to the strong SMSI effect, it is still a challenge to further strengthen the Pt–TaOx interaction due to the difficulty in controlling the Pt–TaOx interface. Furthermore, previous reports of the synthesis of Pt–TaOx/C electrocatalysts have generally adopted the electrodeposition approach,23,31 which is not suitable for scale-up due to its poor controllability.


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Therefore, developing an efficient and scalable strategy to synthesize highly active and durable Pt–TaOx/C electrocatalyst with enhanced SMSI effect is urgently needed. In this work, we demonstrate an effective strategy to synthesize a robust Pt–Ta2O5/CNT electrocatalyst by utilizing highly dispersed and crystalline Ta2O5-modified carbon nanotubes (CNTs) as a support to stabilize Pt NPs and strengthen the metal–support interactions at the atomic scale. The as-prepared Ta2O5 NPs with small average particle size of 8.7 nm were uniformly dispersed on the surface of CNTs, and the supported Pt NPs were found to be selectively anchored on the interface of Ta2O5 and CNTs, forming an atomic-coupled interfacial structure between Pt and Ta2O5 in the form of Pt–O–Ta bonds. The introduction of highly crystalline Ta2O5 with good dispersion not only ensures the formation of a ternary Pt–Ta2O5–CNT interface, but also induces the growth of polyhedral-structured Pt NPs with the exposure of abundant (111) and (100) facets, leading to an improved ORR activity for Pt–Ta2O5/CNT. The ORR electrocatalytic properties of the resulting Pt–Ta2O5/CNT were shown to compare favorably with commercial Pt/C and Pt/CNT electrocatalysts, making them attractive potential ORR electrocatalysts for application in fuel cells. EXPERIMENTAL METHODS Synthesis of Ta2O5/CNT composites. Prior to modification with Ta2O5, CNTs were treated with concentrated H2SO4–HNO3 in a volume ratio of 3:1 at 60 °C for 5 h to functionalize their surface. In a typical synthesis, 100 mg of functionalized CNTs were dispersed in ethanol (50 mL) by ultrasonication for 0.5 h. Meanwhile, TaCl5 (100 mg) was dispersed in a mixture of benzyl alcohol (140 μL) and anhydrous ethanol (10 mL) that had been pre-purged with nitrogen for 0.5 h to remove the dissolved oxygen. The resulting solution was added dropwise to the CNTs suspension and the mixture stirred for 12 h, before being transferred into a microwave reactor and kept at 150


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°C for 1 h. After cooling, the precipitate was collected by filtration, washed with ultrapure water, and dried at 80 °C. Finally, the Ta2O5/CNT composites were obtained by heating at different temperatures (600–1000 °C) for 3 h in argon; the products are denoted as Ta2O5/CNT-T (where T represents the temperature). Ta2O5/CNT composites with different mass ratios of Ta2O5/CNT were also prepared by varying the mass ratio of TaCl5 and CNTs in the precursor mixtures from 0.25 to 4. Synthesis of Pt–Ta2O5/CNT electrocatalysts. Typically, Ta2O5/CNT (100 mg) was dispersed in ethylene glycol (100 ml) followed by addition of 0.01 mol L–1 H2PtCl6 solution (10.25 mL) and ultrasonication for 0.5 h. The pH of the solution was adjusted to 10 by addition of 1 M NaOH solution and the resulting solution was stirred at 130 °C for 3 h. After cooling to room temperature, the mixture was washed and dried to obtain the Pt–Ta2O5/CNT electrocatalyst. For comparison, Pt/CNT electrocatalyst with the same Pt loading was also prepared following the same synthetic procedure but omitting the addition of TaCl5. Material Characterization. The crystal phase of the products was identified using a D/max-2500 X-ray diffraction diffractometer (XRD) equipped with Cu Kα radiation. The morphologies were examined using a scanning electron microscope (SEM, JEOL FE-JSM-6701F), a high-resolution transmission electron microscope (HR-TEM, JSM-2100) and a high-angle annular dark-field scanning transmission electron microscope (HADDF-STEM, JEM-ARM200F). Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 6700 FTIR spectrophotometer in the range 400–4000 cm−1. The Ta2O5 loading was determined using a STA7300 thermal gravimetric analyzer by heating in air with a ramp rate of 10 °C min−1. Surface properties were characterized by X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific ESCALAB 250) using an 6 ACS Paragon Plus Environment

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Al Kα source. The Pt loading was determined by an inductively coupled plasma mass spectrometry (ICP-MS) (7700, Agilent Technologies Inc., USA). Electrical conductivity was measured using a 𝐼

four-point probe working station (SB118) according to the equation (σ = 2𝜋𝑑𝑉 =

𝑙𝑛2 𝜋𝑑

𝐼

× 𝑉 , where σ

is the electrical conductivity, d is the thickness of the sample, I is the current, and V is the voltage drop). X-ray absorption fine structure (XAFS) spectra were recorded on the beamline 1W1B station of the Beijing Synchrotron Radiation Facility. The catalysts were characterized in fluorescence mode and the energy of Pt and Ta L3-edges was calibrated using standard Pt foil and Ta2O5 powder (transmission mode), respectively. The standard procedures were used to extract and process to acquire the extended X-ray absorption ﬁne structure (EXAFS) data using the ATHENA module implemented in the IFEFFIT software packages. Electrochemical measurements. The electrochemical measurements were performed on a CHI760e electrochemical workstation (Shanghai Chenhua Instrument Corporation, China) in a conventional three-electrode system. The working electrode was a catalyst-coated glassy-carbon (GC) rotating disk electrode (RDE) (S = 0.247 cm–2) from Pine Instrument Company. A saturated calomel electrode (SCE) and Pt wire were used as the reference and counter electrode, respectively. Catalyst ink was prepared by ultrasonically blending the catalyst (2.5 mg) with ethanol (1 mL) and 5 wt.% Nafion solution (10 μL). The volume of catalyst ink required to give a Pt loading of 20 μg cm−2 was then pipetted onto the polished GC electrode to act as the working electrode. All potentials were calibrated versus the reversible hydrogen electrode (RHE) according to the formula (E

vs. RHE

=E

vs. SCE

+ 0.241 V + 0.059 × pH). The ECSAs of all samples were calculated by

evaluating the areas in the hydrogen adsorption/desorption region assuming a factor of 210 μC cm−2 for adsorption of a hydrogen monolayer in N2-saturated 0.1 M HClO4 at 50 mV s−1. The kinetic current density (Jk) was calculated from the ORR polarization curves using the Koutecky– 7 ACS Paragon Plus Environment

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Levich equation: 1/J = 1/Jk + 1/Jd (where J is the current density at 0.9 V and Jd is the diffusionlimiting current density). RESULTS AND DISCUSSION Growth of Ta2O5 NPs on CNTs. As shown in Scheme 1, the synthetic process involved the growth of Ta2O5 NPs on CNTs followed by supporting Pt NPs on the resulting Ta2O5/CNT composite. The CNTs support was first functionalized by treatment with a mixture of concentrated sulfuric and nitric acids to introduce an abundance of oxygen-containing groups (e.g., carboxyl and hydroxyl groups, Figure S1) on the surface of the CNTs; these serve as nucleation and anchoring sites for the subsequent growth of Ta2O5. The formation of Ta2O5 NPs on the CNT surface probably involves three reaction steps:33-35 alcoholysis, hydrolysis and polycondensation (Equations (1)–(3)). TaCl5 reacted with –OC2H5 groups (from anhydrous ethanol) to form TaCl5– x(OC2H5)x

species and then formed Ta(OH)m(OC2H5)n through a hydrolysis process caused by the

very small amount of water (< 0.3%) in the ethanol via a microwave-assisted hydrothermal reaction. Finally, the Ta(OH)m(OC2H5)n species underwent a complex polycondensation process to form highly dispersed TaOx NPs on the CNT surface. TaCl5 + C2H5OH

―𝑥Cl ―

TaCl5 ― 𝑥(OC2H5)𝑥 + H2O

Ta(OH)𝑚(OC2H5)𝑛

(1)

TaCl5 ― 𝑥(OC2H5)𝑥 ―𝑥Cl ― /OC2H5

― H2O OC2H5

Ta(OH)𝑚(OC2H5)𝑛

Ta2O5

(2)

(3)

The resulting product was heat-treated at different temperatures in an argon atmosphere to generate the final Ta2O5/CNT composites. It should be noted that due to the high sensitivity of Ta8 ACS Paragon Plus Environment

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containing compounds such as TaCl5 and Ta(OC2H5)5 to moisture,36 control of the water content plays an important role in optimizing the dispersion of TaOx NPs on CNTs through varying the hydrolysis rate of TaCl5. For example, when 98% ethanol was used in place of anhydrous ethanol as the solvent, TEM images (Figure S2) show that only aggregated Ta2O5 NPs with large particle size were formed on the surface of the CNTs. The effect of varying the Ta2O5 loading on the physical and chemical properties of Ta2O5/CNT was first investigated. Samples with different Ta2O5 loadings of 12.1, 17.3, 21.8 and 57.8 wt.% (as determined by TG analysis (Figure S3)) were prepared. As the Ta2O5 loading was increased, Ta2O5 NPs tend to aggregate on the CNTs surface, leading to increased particle size (Figure S4), and the electrical conductivity of the Ta2O5/CNT gradually decreases from 58.2 to 24.2 μS cm–1 (Table S1), respectively. As a compromise between increasing Ta2O5 loading and decreasing conductivity, Ta2O5/CNT with a Ta2O5 loading of 17.3%, which has a conductivity of 50.9 μS cm– 1,

was selected as the support for anchoring Pt NPs. The effect of varying the calcination

temperature during the synthesis of Ta2O5/CNT was also investigated over the range 600–1000 °C. XRD patterns of Ta2O5/CNT-600 and Ta2O5/CNT-700 exhibit only the characteristic C (002) diffraction peak at 26.2° (Figure S5). However, when the temperature is increased to 800 °C, intense diffraction peaks appeare at 22.90°, 28.29° and 36.67°, which can be respectively indexed as the (0 0 1), (1 11 0) and (1 11 1) planes of orthorhombic Ta2O5 nanocrystals (JCPDS No. 250922) (Figure 1a and Figure S5), indicating the formation of highly crystalline Ta2O5. TEM images show that the average particle size of Ta2O5 NPs increases from 1.9 to 14.6 nm with increasing calcination temperature from 600–1000 °C, with the particle size distribution becoming broader and asymmetric (Figure S6 and Table S2). Therefore, Ta2O5/CNT prepared by calcination at 800


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°C was selected as the optimum support for anchoring Pt NPs, since the material has highly crystalline and relatively uniform Ta2O5 NPs on the surface of the CNTs. The average Ta2O5 crystallite size in the optimal Ta2O5/CNT material was calculated to be approximately 8.8 nm according to the Scherrer equation. TEM images (Figure 1b) show that Ta2O5 NPs are uniformly dispersed on the surface of CNTs without any obvious aggregation. From TEM images, the average particle size of Ta2O5 NPs is approximately 8.7 nm (Figure 1b), consistent with the average crystallite size estimated by XRD analysis, indicating that an individual NP is a single crystal of Ta2O5. HR-TEM images and fast Fourier transform (FFT) patterns display lattice fringes of 0.38 and 0.24 nm, which can be assigned to the (0 0 1) and (1 11 1) planes of Ta2O5 nanocrystals (Figure 1c), respectively. The selected area electron diffraction (SAED) pattern shows a series of diffraction rings which can be ascribed to (0 0 2), (1 11 1), (1 11 0) and (0 0 1) planes of Ta2O5, which confirms the formation of highly crystalline Ta2O5 (Figure 1d). The XPS survey spectrum of Ta2O5/CNT (Figure 2a) shows the presence of Ta (1.29 at.%), O (4.64 at.%) and C (94.08 at.%). The high-resolution Ta 4f XPS spectrum exhibits a pair of peaks at binding energies of 28.6 eV and 26.7 eV corresponding to Ta5+ species (Figure 2b),27,29 consistent with the formation of stoichiometric Ta2O5. In the deconvoluted C 1s XPS spectrum (Figure 2c), the peaks centered at 285.0 eV and 286.3 eV can be assigned to sp2 hybridized C=C and C–O,37,38 respectively. The higher relative content of C–O specie in Ta2O5/CNT (28.3%) compared with pristine CNTs (7.5%) is indicative of the formation of C–O–Ta bonds and thus a strong interaction between Ta2O5 and CNTs. The O 1s XPS spectrum of Ta2O5/CNT can be deconvoluted into four peaks which can be assigned to Ta–O (530.7 eV), Ta–OH (531.6 eV), C=O (532.4 eV) and O– C=O (533.9 eV) (Figure 2d),39,40 respectively.


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Anchoring Pt NPs on Ta2O5/CNT. The Ta2O5/CNT composite and acid CNTs were used as supports to anchor Pt NPs by means of the chemical reduction of H2PtCl6 with ethylene glycol at 130 °C (Scheme 1). The Pt loading in Pt–Ta2O5/CNT and Pt/CNT was determined to be 9.06% and 8.89%, respectively, by ICP-MS. The XRD pattern of Pt–Ta2O5/CNT shows diffraction peaks at 39.89°, 46.37° and 67.59° corresponding to the (111), (200) and (220) planes, respectively, of face-centered cubic (fcc) structured Pt (JCPDS No. 04-0802) (Figure 3a). The Pt peaks are broader than those of Ta2O5, indicating the smaller crystalline size of Pt. This was confirmed by TEM image—Pt NPs were observed to be highly dispersed on the Ta2O5/CNT composite with a small average particle size of approximately 3 nm without any obvious aggregation, similar to the uniform distribution of Ta2O5 NPs (Figure 3b). The presence of a crystalline Pt phase was also verified by the SAED pattern with two obvious diffraction rings corresponding to Pt (111) and (200) planes (Figure 3c). Further investigation shows that most of Pt NPs are anchored adjacent to Ta2O5 NPs on the CNTs support, and form ternary Pt–Ta2O5–CNT moieties (Figure 3d). This is consistent with previous reports in the literature that anchoring of Pt NPs at metal oxide–carbon junctions is thermodynamically favorable.13 HADDF-STEM images show that several Pt NPs are anchored at the junctions of CNTs and a single Ta2O5 nanocrystal (the inset FFT pattern shows the (001), (200) and (201) oriented-directions of Ta2O5) rather than on the surface of Ta2O5 or CNTs (Figure 3e), generating a rather clear interfacial structure between Pt and Ta2O5. Pt NPs anchored at the Ta2O5–CNTs junctions show a polyhedral structure with the exposure of abundant Pt (111) and (100) planes (Figure 3e), different from the near-spherical Pt NPs with polyoriented surface for Pt/C.41,42 The Pt-Ta2O5 interfacial region shown in Figure 3f shows an obvious overlap of lattice fringes in the vertical direction with the lattice spacing of 0.19 nm and 0.38 nm attributed to the Pt (200) and Ta2O5 (001) planes, respectively. Further investigation, combined with FFT analysis,


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reveals the existence of parallel Pt (200) (0.19 nm) and Ta2O5 (200) (0.31 nm) planes at the Pt– Ta2O5 interface. Since the lattice spacing of Ta2O5 (001) is very close to twice that of Pt (200), Pt NPs probably grow epitaxially on Ta2O5 NPs through Pt–O–Ta bonds at the Pt–Ta2O5 interface. This is consistent with Ohsaka’s work on the Pt/TaOx/GC electrocatalyst,32 and other studies which have reported the presence of Pt–O–TM bonds between Pt and metal oxides (e.g., Pt–O–Ta bonds in Pt–TaOx,23 Pt–O–Sn bonds in Pt–SnO2,43 and Pt–O–Ti bonds in Pt–TiO244). HADDF-STEM and EDX elemental mapping images show several small Pt NPs (~3 nm) anchored to a large Ta2O5 NP (~8.7 nm) (Figure 3g). Since the average size of Ta2O5 NPs is roughly three times that of Pt NPs, the presence of several Pt NPs anchored on a single Ta2O5 NP structure will provide more Pt–Ta2O5 interfaces and stronger interactions between them than if the Pt and Ta2O5 NPs have a similar particle size. To explore the mechanism of formation of the Pt–Ta2O5 interfacial structure, the Ta2O5/CNT600 sample was also used to support Pt NPs. Although the XRD pattern of Ta2O5/CNT-600 shows no diffraction peaks characteristic of a Ta2O5 phase, the high-resolution Ta 4f spectrum can be deconvoluted into only two Ta5+ peaks at 28.6 eV and 26.7 eV,27,29 verifying the formation of Ta2O5 phase (Figure S7). When Pt NPs were supported on Ta2O5/CNT-600, which contains poorly crystalline and rather small (approximately 1.9 nm, as shown in Figure S6) Ta2O5 NPs, a Ta2O5capped Pt NPs structure was formed (Figure S8d-h). The formation of this structure can best be ascribed to the diffusion of Ta and O atoms to the surface of Pt NPs in order to minimize total energy,45,46 as has often been observed in metal and reducible metal oxide systems (e.g., Pd/TiO2,47 Pt/Fe3O4,48 Au/TiO249). In contrast, when using Ta2O5/CNT-800 containing highly crystalline Ta2O5 as Pt support, a ternary Pt–Ta2O5–CNT structure with Ta2O5-anchored Pt NPs was formed. Thus the high crystallinity of Ta2O5 in Ta2O5/CNT-800 clearly favors the formation of an 12 ACS Paragon Plus Environment

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atomically-coupled large-area Pt–Ta2O5 interface. Unlike the structure of Ta2O5-capped Pt NPs, in which most of Pt sites are covered by Ta2O5, the Pt sites in the highly crystalline Ta2O5-anchored Pt NPs in Pt–Ta2O5/CNT are highly exposed. Further investigations show that most of Pt NPs in Pt–Ta2O5/CNT-600 exhibit near-spherical structure when using low crystalline Ta2O5 as support (Figure S8e), completely different from the polyhedral structure of Pt NPs in Pt–Ta2O5/CNT observed in HAADF-STEM image. This result reveals that the introduction of high crystalline Ta2O5 in Pt–Ta2O5/CNT tends to promote the formation of polyhedral structured Pt NPs, as compared with that of using low crystalline Ta2O5. The formed polyhedral structure of Pt NPs in Pt–Ta2O5/CNT could expose more abundant (111) and (100) planes that are active towards ORR, than that of near-spherical structured Pt NPs,42 thereby leading to an enhancement of ORR activity. Based on above analysis, the Pt–Ta2O5 interfacial structure by introducing high crystalline Ta2O5 is believed to not only enhance the ORR activity through the SMSI effects induced by Ta2O5, but also improve the durability of Pt–Ta2O5/CNT electrocatalyst by preventing the dissolution, migration and aggregation of Pt NPs due to the strong anchoring of Pt by Ta2O5. XPS measurements were performed to analyze near-surface composition and bonding structure of Pt and Ta. Four elements, Pt (8.48 at.%), Ta (4.32 at.%), O (9.7 at.%), and C (77.5 at.%) were detected in Pt–Ta2O5/CNT (Figure 4a). The high-resolution O 1s spectrum of Pt–Ta2O5/CNT can be deconvoluted into four peaks assigned to Ta–O (530.6 eV), Pt–O (531.3 eV), Ta–OH (531.6 eV), C=O (532.4 eV) and O–C=O bonds (533.9 eV)39,40 (Figure S9 and Table S3). It should be noted that the Pt 4f5/2 peak centered at 71.95 eV shifts to higher binding energy compared with the corresponding peak in Pt/CNT (71.75 eV) (Figure 4b), indicating a strong electronic coupling effect between Pt and Ta2O5, with electron transfer from Pt to Ta2O5. Such electron donation from 13 ACS Paragon Plus Environment

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Pt to Ta2O5 is consistent with the reported literatures for the Pt–TaOx/C or Pt/TaOx electrocatalysts.23,32 The deconvoluted Pt 4f spectrum of Pt–Ta2O5/CNT shows that Pt presents primarily in the form of Pt0 (peaks at 71.66 eV and 75 eV), Pt2+ (peaks at 72.5 eV and 75.9 eV) and Pt4+ (peaks at 74.9 eV and 78.1 eV) (Figure 4b)50,51. The percentage of Pt0 in Pt–Ta2O5/CNT was calculated to be 38.7%, lower than that of Pt/CNT (43.3%) (Table S4), suggesting an increased content of oxidized Pt. In the Ta 4f spectrum, the Ta 4f5/2 peak also shifts to lower binding energy (by 0.33 eV) compared to Ta2O5/CNT (Figure 4c). This is consistent with an increased electron density of Ta due to the electron transfer from Pt to Ta2O5, which is in agreement with the Pt 4f spectrum. By comparing the deconvoluted Ta 4f spectra of Ta2O5/CNT and Pt–Ta2O5/CNT (Figure 4c), in addition to Ta5+ peaks at 28.6 eV and 26.7 eV (as observed for Ta2O5/CNT), two other peaks at 27.8 and 25.9 eV which can be assigned to Ta

Highly Dispersed and Crystalline Ta2O5 Anchored Pt Electrocatalyst

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