CeO₂ nano-nail-induced interface electronic effects boost ethanol oxidation

Abstract

Developing ethanol oxidation reaction (EOR) electrocatalysts for direct ethanol fuel cells (DEFCs) with high catalytic activity, durability, and resistance to CO poisoning remains a major challenge. Herein, we synthesize a Pt/nCeO₂@NPC catalyst, consisting of Pt nanoparticles supported on a hybrid support of CeO₂ nano-nails (nCeO₂) and N-doped porous carbon (NPC). The unique nCeO₂ efficiently forms and adjusts the Pt–CeO₂ interface, modulating the electronic structure of the Pt nanoparticles. This electronic modulation yields an efficient and poison-resistant electrocatalyst for the ethanol oxidation reaction (EOR), thereby significantly enhancing the EOR catalytic activity of the catalyst. As a result, the optimized catalyst (Pt/nCeO₂@NPC) demonstrates a mass activity of 1374 mA mg_Pt⁻¹ in ethanol oxidation, which is 3.87 times higher than that of commercial Pt/C (355 mA mg_Pt⁻¹). Additionally, it maintains 72% and 48% of its initial activity after 500 and 2000 CV cycles, respectively, and shows a 20 mV negative shift in onset potential compared to that of Pt/NPC, indicating excellent durability and enhanced tolerance to CO poisoning. Meanwhile, combined experimental and theoretical analyses reveal that the strong electronic interaction between Pt-based nanoparticles and nCeO₂ modulates CO adsorption strength and supplies OH species to facilitate CO removal, thereby significantly enhancing the CO tolerance of the catalyst. This study thus proposes an innovative strategy for the design of highly efficient catalysts for DEFCs and advances the development of clean and sustainable energy technologies.

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Introduction

Compared with internal combustion engines, fuel cells have garnered significant attention due to their superior energy conversion efficiency. Among various fuel cells, direct ethanol fuel cells (DEFCs) exhibit considerable application potential owing to their high volumetric energy density, renewability, and the ease of handling of ethanol.^1–3 Nevertheless, the ethanol oxidation reaction (EOR) at the anode of DEFCs encompasses a complex 12-electron transfer process, leading to extremely sluggish reaction kinetics.^4–6 Furthermore, achieving complete ethanol oxidation via C–C bond cleavage (the C1 pathway) remains highly challenging, even with optimal Pt-based catalysts (Pt/C).^7–11 Instead, incomplete oxidation to acetaldehyde (2e⁻) and/or acetic acid (4e⁻) is more commonly observed (the C2 pathway).^12–17 Additionally, on Pt/C catalysts, the strongly adsorbed CO species generated during the ethanol oxidation process tend to continuously accumulate, thereby blocking the active sites and poisoning the catalyst, further undermining its catalytic performance.^18–22 Hence, the development of Pt-based catalysts featuring high activity, good durability, and excellent anti-poisoning capability is important for advancing the development of DEFC.

The introduction of transition metal oxides (TMOs), such as CeO₂, SnO₂, TiO₂, and WO₃, as cocatalysts constitutes an effective strategy for enhancing the activity and anti-poisoning capacity of Pt-based catalysts in the EOR.^23–28 These TMOs not only have the capability to accelerate the oxidation and desorption of CO but can also modulate the electronic structure of Pt via strong metal–support interactions (SMSIs) to reduce the adsorption strength of CO on the active sites.^29–31 Among them, CeO₂ has garnered significant attention owing to its reversible redox capability between Ce⁴⁺ and Ce³⁺, which promotes the formation of oxygen vacancies.^32–34 These oxygen vacancies can promote the transport of OH, thereby effectively eliminating CO intermediates, significantly enhancing the reaction kinetics.^35–37 Nevertheless, the low conductivity of CeO₂ restricts its application in DEFCs.^38,39 To address this issue, the construction of a metal–TMO–carbon triple interaction has been demonstrated to be an efficacious strategy.^40,41 The integration of CeO₂ with a carbon support effectively combines the advantages of metal oxides with the high conductivity of carbon materials. Furthermore, the interaction between CeO₂ and Pt modulates the electronic structures of both Pt and Ce, thereby enhancing the intrinsic activity and selectivity of the catalyst. However, in composite structures of metal–TMO–carbon, the TMOs usually cover the active sites, resulting in a decline in Pt utilization and thereby attenuating the catalytic performance. Hence, the design of metal–TMO–carbon catalyst structures that exhibit enhanced intrinsic activity and anti-poisoning performance and ensure the full exposure of the catalytically active sites is of paramount importance for the design of EOR catalysts.

Herein, we accurately tailor the Pt–CeO₂ interface to modulate the electronic structure of Pt nanoparticles by constructing CeO₂ nano-nails on the surface of N-doped porous carbon (NPC), thereby realizing an efficient and poison-resistant electrocatalyst for the EOR. The unique CeO₂ nano-nails form the Pt–CeO₂ interface efficiently and ensure full exposure of the active sites, significantly enhancing the EOR catalytic activity of the catalyst. Meanwhile, the strong electronic interactions between the CeO₂ nano-nails and the Pt-based nanoparticles can modulate the adsorption strength for CO and provide OH species to accelerate the removal of CO, thereby enhancing the anti-CO poisoning performance of the catalyst. The optimized catalyst (Pt/nCeO₂@NPC) exhibits superior catalytic performance in ethanol oxidation, achieving a mass activity (1374 mA mg_Pt⁻¹) 3.87 times greater than that of commercial Pt/C (355 mA mg_Pt⁻¹). The catalyst retains 72% and 48% of its initial activity after 500 and 2000 CV cycles, respectively, demonstrating superior long-term stability compared to commercial Pt/C. Furthermore, it shows a 20 mV negative shift in onset potential compared to that of Pt/NPC, confirming its enhanced tolerance to CO.

Experimental

Materials and methods

Materials. Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O; >99.0%), cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O; >99.0%), chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O; >37.5%), 2-methyleimidazole (C₄H₆N₂; 99.0%), ethylene glycol (99.5%), sulfuric acid (H₂SO₄; 95%–98%), methanol (99.5%), ethanol (99.7%), sodium hydroxide (NaOH; 96%), commercial Pt/C (30%, Johnson Matthey), and deionized water (18.25 MΩ cm⁻¹) were used as received.

Synthesis of ZIF-8 and NPC. ZIF-8 was prepared according to the reported method.⁴² Briefly, 6 g of zinc nitrate hexahydrate and 7 g of 2-methylimidazole were separately dissolved in 250 mL of methanol. The two solutions were then mixed and stirred for 8 h, followed by aging for 12 h. The resulting precipitate was collected, washed with methanol, and dried to obtain ZIF-8. Thereafter, the as-prepared ZIF-8 underwent thermal treatment at 1000 °C under a N₂ atmosphere for 2 hours to obtain the NPC.

Synthesis of nCeO₂@NPC. Ce(NO₃)₃·6H₂O (20 mg) was dispersed in 20 mL of ethanol and ultrasonicated for 30 minutes to prepare a homogeneous Ce³⁺ precursor solution with a concentration of 1 mg mL⁻¹. Afterwards, 50 mg of NPC (pre-treated under vacuum for 30 minutes) was incorporated into the above solution, and thorough dispersion was achieved by stirring and ultrasonication. The resultant mixture was then washed three times with ethanol to remove impurities and dried in an oven. Finally, the nCeO₂@NPC composite support was successfully synthesized by heat-treating the dried powder under a N₂ atmosphere at 300 °C for 1 hour. In addition to the lower concentration of the Ce³⁺ precursor solution, the identical preparation method was employed to successfully fabricate the CeO₂@NPC composite support, wherein CeO₂ was effectively confined within the porous structure of NPC. Pure CeO₂ was synthesized by dissolving 2 g of Ce(NO₃)₃·6H₂O in 100 mL of deionized water, followed by dropwise addition of 2.0 mL of 15 wt% aqueous ammonia solution under vigorous stirring. The mixture was thoroughly washed with deionized water, dried overnight at 60 °C in an oven, and subsequently calcined at 500 °C for 2 h to obtain CeO₂.

Synthesis of Pt/nCeO₂@NPC. The Pt nanoparticles were supported on the nCeO₂@NPC composite support via the polyol reduction method. NaOH, nCeO₂@NPC, and H₂PtCl₆·6H₂O were uniformly dispersed in ethylene glycol via stirring and ultrasonication. The mixed solution exhibited a pH of around 9–10. Subsequently, the mixture was heated under continuous stirring at 230 °C for 2 h in a N₂ atmosphere. After cooling to room temperature, the solution was adjusted to pH 2–3 with H₂SO₄. The resultant mixture was then washed with deionized water, followed by drying at 60 °C in an oven. Finally, the obtained powder was calcined at 300 °C for 1 hour under a 10% H₂/N₂ mixed atmosphere to synthesize Pt/nCeO₂@NPC. Using the preparation method described above, Pt/NPC catalysts with pure NPC as the support and Pt/CeO₂@NPC catalysts were also successfully synthesized. By employing the same preparation procedure described above, Pt/CeO₂@NPC catalysts with CeO₂@NPC as the support, as well as Pt/NPC catalysts with pure NPC as the support, were successfully obtained. By employing the same preparation procedure described above, Pt/CeO₂@NPC catalysts with CeO₂@NPC as the support, Pt/CeO₂ catalysts with CeO₂ as the support, as well as Pt/NPC catalysts with pure NPC as the support, were also successfully obtained.

Material characterization

The structure and morphology of all the catalysts were comprehensively characterized using X-ray powder diffraction (XRD; ULTIMA III), scanning electron microscopy (SEM; Supra 55), and transmission electron microscopy (TEM; FEI Tecnai G2F20). The chemical states and elemental compositions were analyzed by X-ray photoelectron spectroscopy (XPS; AXIS Ultra DLD), with Al Kα radiation employed as the excitation source. X-ray absorption fine structure (XAFS) measurements were collected at the RapidXAFS 2M. The metal loading was determined by inductively coupled plasma mass spectrometry (ICP-MS; iCAP 7400).

Electrochemical measurements

The electrochemical performance tests were carried out with a CHI760E electrochemical workstation at room temperature using a three-electrode system. All potentials were normalized with respect to the reversible hydrogen electrode (RHE). Before the tests, all electrolytes were purged with N₂ to reach saturation, and all catalysts were activated through cyclic voltammetry (CV) within 0.05–1.20 V vs. RHE. The working electrode was a glassy carbon electrode (0.196 cm⁻²), the reference electrode was a saturated Ag/AgCl electrode, and the counter electrode was a graphite rod electrode. The uniform catalyst ink was prepared by mixing 2 mg of the catalyst with 500 μL of deionized water, 500 μL of isopropanol, and 10 μL of 5 wt% Nafion solution, followed by ultrasonic dispersion. Then, the working electrode was prepared by uniformly depositing 10 μL of the catalyst ink on the surface of the glassy carbon electrode. The Pt loading on the working electrode was 10 μg cm⁻² for all catalysts. The CV curves for the EOR were recorded in N₂-saturated 0.5 M H₂SO₄ + 1 M CH₃CH₂OH electrolyte within 0.05–1.20 V vs. RHE at 50 mV s⁻¹. Linear sweep voltammetry (LSV) curves were recorded in an N₂-saturated 0.5 M H₂SO₄ + 1 M CH₃CH₂OH solution at 5 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) was conducted in N₂-saturated 0.5 M H₂SO₄ + 1 M CH₃CH₂OH at 0.7 V vs. RHE in the frequency range of 10⁻²–10⁵ Hz. We conducted the accelerated durability tests (ADT) at 50 mV s⁻¹ for 2000 cycles (0.05–1.20 V vs. RHE). The I–t curves were obtained during a chronoamperometry test for 3 hours at 0.7 V vs. RHE in 0.5 M H₂SO₄ + 1 M CH₃CH₂OH electrolyte. Furthermore, to evaluate the CO poisoning resistance of the catalysts, CO-stripping voltammetry was performed at 50 mV s⁻¹ in N₂-saturated 0.5 M H₂SO₄ after CO adsorption onto the working electrode surface.

DFT calculations

DFT computations were performed using the Vienna Ab initio Simulation Package (VASP). The projector-augmented wave (PAW) was employed to model the electron exchange–correlation interaction. The exchange–correlation functional was optimized using the Perdew–Burke–Ernzerhof (PBE) and generalized gradient approximation (GGA). A kinetic energy cut-off of 500 eV was applied for all calculations. Convergence thresholds were set to 0.02 eV for forces and 1 × 10⁻⁵ eV for energy.

Results and discussion

Synthesis and structure characterization

Fig. 1a illustrates the preparation strategy for the Pt/nCeO₂@NPC catalyst. Firstly, NPC was synthesized via the high-temperature pyrolysis of ZIF-8 nanocrystals. Subsequently, CeO₂ nano-nails were obtained using the wet-impregnation method, followed by high-temperature treatment in N₂. Finally, the Pt NPs were anchored on the nCeO₂@NPC by the polyol reduction method and then annealed under a reducing atmosphere to obtain Pt/nCeO₂@NPC. The metal contents were analyzed using inductively coupled plasma mass spectrometry (ICP-MS) (Table S1).

Fig. 1 (a) Schematic of the synthesis of Pt/nCeO₂@NPC. (b) XRD patterns of Pt/nCeO₂@NPC, Pt/NPC, Pt/C, and nCeO₂@NPC. (c) HRTEM image of Pt/nCeO₂@NPC and the enlarged images of blue and purple regions. (d) HAADF-STEM image of Pt/nCeO₂@NPC and the corresponding particle size distribution. (e) High-resolution N 1s XPS spectra of Pt/CeO₂@NPC-300 and Pt/NPC. (f) High-resolution Pt 4f XPS spectra of Pt/nCeO₂@NPC and Pt/NPC.

Fig. S1 presents the X-ray powder diffraction (XRD) spectra of ZIF-8, the characteristic peaks of which align well with literature reports.⁴¹ After annealing in a N₂ atmosphere at 1000 °C for 2 hours, ZIF-8 was successfully transformed into NPC. The resulting XRD spectra (Fig. S2) exhibited only two peaks located at 25° and 43°, which were attributed to the (002) and (101) crystalline planes of graphitic carbon, respectively.⁴³ For the nCeO₂@NPC composites, the XRD spectra closely match the CeO₂ standard card (JCPDS No. 34-0394), confirming a face-centered cubic (fcc) structure (Fig. S3). As the Ce content increased, the characteristic diffraction peaks of CeO₂ became more pronounced at 28.6°, 33.1°, 47.5°, and 56.3°, corresponding to the (111), (200), (220), and (311) crystalline planes of CeO₂, respectively. This trend indicates the gradual growth of CeO₂ nanoparticles with increasing Ce content and confirms the successful synthesis of CeO₂. Scanning electron microscopy (SEM) images (Fig. S4 and S5) reveal that ZIF-8 retained its original morphology, with a particle size of approximately 40–50 nm, even after high-temperature carbonization at 1000 °C. As depicted in Fig. S6, the thermal decomposition of Ce(NO₃)₃·6H₂O loaded onto the NPC resulted in the gradual growth of CeO₂ nanoparticles as the Ce content increased.

To further explore the morphology and microstructure of nCeO₂@NPC, we conducted transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), as well as high-resolution transmission electron microscopy (HRTEM). As shown in Fig. S7, no distinct CeO₂ particles were observed in NPC at low Ce content, suggesting that small CeO₂ particles were primarily confined within the NPC's pore structure. However, with increasing Ce content, the CeO₂ nanoparticles gradually grew and became more uniformly distributed, appearing as distinct black particles (Fig. S8). In the HRTEM images, lattice fringes corresponding to CeO₂ were clearly visible, with interplanar spacings of approximately 0.32 nm and 0.27 nm, which are consistent with the (111) and (200) crystal planes of fcc-structured CeO₂, respectively (Fig. S9). These findings are in agreement with the results obtained from XRD analysis. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) revealed bright spots associated with small CeO₂ particles within the NPC at low Ce content (Fig. S10), indicating that CeO₂ was primarily confined within the pores. With increasing Ce content, the nano-nail structures of CeO₂ became more pronounced and uniformly distributed across both the surface and pores of the NPC, with statistical analysis revealing a particle size range of 1–3 nm and an average size of 2.05 nm (Fig. S11). Additionally, energy-dispersive X-ray spectroscopy (EDS) elemental mapping (Fig. S12) demonstrated that C, N, Ce, and O were homogeneously distributed, corresponding well with the observed morphological features. The formation of ultra-small CeO₂ nanoclusters and nano-nails can be attributed to the spatial confinement effect of NPC, which effectively adsorbs and immobilizes Ce elements, preventing their agglomeration into larger particles.⁴² These results indicate that, as the Ce content increases, CeO₂ confined within the NPC nanopores gradually migrates to the surface, forming unique CeO₂ nails (nano-nail CeO₂, nCeO₂).

As presented in Fig. 1b, the XRD patterns of the Pt/nCeO₂@NPC catalyst display less pronounced characteristic peaks of Pt, which may be attributed to the confinement effect or anchoring sites provided by nCeO₂@NPC and NPC, which effectively reduce the nanoparticle size of Pt and result in weaker crystallinity. Nonetheless, a distinctly broadened diffraction peak corresponding to the Pt (111) plane remains observable, and no obvious shift is observed in the Pt (111) peak positions among the Pt/nCeO₂@NPC, Pt/NPC, and Pt/C catalysts, indicating successful Pt deposition without significant lattice strain. Moreover, the absence of carbon-related peaks is likely due to the small particle size of NPC (≈50 nm), leading to an increased amorphous phase content. Notably, the characteristic peaks of CeO₂ were absent in the Pt/nCeO₂@NPC, likely due to the low relative content of CeO₂, the diffraction intensities of which were masked after Pt loading. Compared to the commercial Pt/C catalysts, the full width at half maximum (FWHM) of Pt/NPC and Pt/nCeO₂@NPC were significantly broader, indicating smaller Pt nanoparticle sizes. Furthermore, particle sizes of Pt/nCeO₂@NPC, Pt/NPC, and commercial Pt/C were analyzed according to the Debye–Scherrer equation, with the Pt/nCeO₂@NPC having smaller NPs (1.93 nm) than Pt/NPC (1.98 nm) and commercial Pt/C (2.47 nm). This may be due to the stronger interaction between the nCeO₂/NPC support and Pt, leading to smaller Pt nanoparticles in Pt/nCeO₂@NPC compared to Pt/NPC and Pt/C. These findings highlight the pivotal role of nCeO₂ in regulating the size of Pt nanoparticles and demonstrate that the synergistic effect of N-doped carbon and nCeO₂ effectively restricts Pt nanoparticle growth.

Further analysis through TEM and HRTEM (Fig. 1c and S13) reveals that Pt nanoparticles are uniformly distributed on the supports, with lattice spacings of 0.192 nm and 0.226 nm ascribed to the CeO₂ (110) and Pt (111) crystal planes, respectively. Notably, one or more CeO₂ nanoparticles are located adjacent to the Pt nanoparticles, forming a Pt/nCeO₂ heterointerface. This heterogeneous interface effectively inhibits Pt nanoparticle growth while enhancing the catalyst's stability and durability. Additionally, HAADF-STEM and corresponding particle size images (Fig. 1d) indicate that numerous bright nanoparticles with a particle size of about 2 nm are evenly dispersed on the CeO₂@NPC support without agglomeration. The elemental mapping further confirms the uniform distribution of O, Ce, and Pt elements throughout the catalyst (Fig. S14).

The chemical valence states of the elements and their interactions in the Pt/nCeO₂@NPC catalyst were elucidated through XPS characterization. Fig. 1e presents the high-resolution XPS spectra of N 1s, revealing four characteristic peaks at 398.27 eV, 399.04 eV, 400.59 eV, and 404.43 eV, which are attributed to pyridine nitrogen, pyrrole nitrogen, graphitic nitrogen, and nitrogen oxide, respectively.⁴⁴ These nitrogen species provide abundant electrons and effectively regulate the charge distribution of the catalyst, thereby optimizing its catalytic performance. In Fig. 1f, the high-resolution Pt 4f XPS spectrum reveals that, compared to Pt/NPC, the binding energy of Pt⁰ in Pt/nCeO₂@NPC decreases to 71.55 eV (Pt 4f_7/2) and 74.86 eV (Pt 4f_5/2), while the binding energies of Pt²⁺ are reduced to 72.52 eV (Pt 4f_7/2) and 75.78 eV (Pt 4f_5/2). This negative shift in binding energy arises from the strong electronic interaction between Pt and the nCeO₂@NPC support, which leaves Pt in an electron-rich state. Such an electronic configuration not only stabilizes the Pt nanoparticles and inhibits their agglomeration but also shifts the d-band center away from the Fermi level, thereby potentially optimizing the intrinsic activity and durability of the catalyst. Moreover, this facilitates the weak adsorption of CO* during the oxidation process and thus accelerates the EOR.⁴⁵ This phenomenon is ascribed to the strong charge-transfer effect between Pt and nCeO₂@NPC. Collectively, these results demonstrate that the synergistic effect of NPC and nCeO₂ can effectively modulate the surface electronic state of Pt, thereby enhancing the activity and stability of the catalyst.

The interaction between Pt and nCeO₂ was further confirmed by the high-resolution Ce 3d XPS spectra. Fig. S15 shows the Ce 3d XPS spectra, indicating the coexistence of Ce³⁺ and Ce⁴⁺ valence states in the catalyst. The characteristic peaks of Ce³⁺ are observed at 880.00 eV, 885.5 eV, 899.29 eV, and 904.29 eV, while the characteristic peaks of Ce⁴⁺ are at 882.40 eV, 889.13 eV, 898.47 eV, 901.20 eV, 908.31 eV, and 916.50 eV. What is noteworthy is that the content of Ce³⁺ in Pt/nCeO₂@NPC increases significantly compared to that in nCeO₂@NPC (Table S2); this can be ascribed to the interaction between nCeO₂ and Pt, which induces the release of lattice oxygen from the CeO₂ lattice and the formation of oxygen vacancies, thereby promoting the generation of more Ce³⁺.⁴⁶ The increased Ce³⁺ content suggests the creation of more oxygen vacancies, which not only promotes the transport of OH intermediates during the electrocatalytic process but also effectively facilitates the oxidation and desorption of CO intermediates in the EOR process, thereby remarkably enhancing the resistance to CO poisoning of the catalysts.

X-ray absorption spectroscopy (XAS) was employed to investigate the electronic and coordination structures of Pt in both Pt/nCeO₂@NPC and Pt/NPC. The X-ray absorption near-edge structure (XANES) spectra (Fig. 2a and b) reveal that the near-edge absorption position for Pt/nCeO₂@NPC is lower than that for Pt/NPC, implying a reduced Pt valence by the incorporation of nCeO₂. This finding further confirms the presence of a strong electronic interaction between Pt and nCeO₂, along with electron transfer from nCeO₂ to Pt, which is in excellent agreement with the XPS results. The Fourier transform extended X-ray absorption fine structures (FT-EXAFS) spectra presented in Fig. 2c reveal the coordination structures for Pt/nCeO₂@NPC, Pt/NPC, and Pt foil. The Pt–Pt bond length in both Pt/NPC and Pt/nCeO₂@NPC is shorter than that in metallic Pt foil, suggesting strong interfacial interactions between Pt and the support.⁴⁷ Moreover, two prominent peaks are observed for Pt/NPC, corresponding to the first-shell Pt–N/C and second-shell Pt–Pt scattering paths, respectively. Notably, the distance for first-shell peak in Pt/nCeO₂@NPC is slightly reduced relative to that for Pt/NPC, indicating contributions from additional coordination pathways beyond Pt–N/C. In conjunction with the XPS data, this shift is likely associated with the formation of Pt–O bonding interactions facilitated by nCeO₂. Furthermore, the fitted curves in R space (Fig. 2d–f and Table S3) indicate coordination numbers (CN) of 1.1 for Pt–N and 7.3 for Pt–Pt in Pt/NPC. In contrast, Pt/nCeO₂@NPC exhibits a lower CN of 0.9 for Pt–N and 6.8 for Pt–Pt, which may be ascribed to the formation of the Pt–CeO₂ interface, which alters the local atomic arrangement. Additional insights into the coordination environment are provided by wavelet transform analysis (Fig. 2g–i), which further confirms the existence of a distinct scattering path in Pt/nCeO₂@NPC, supporting the conclusion that the Pt coordination is modified due to the interfacial engineering.

Fig. 2 (a) Normalized Pt L₃-edge XANES of Pt/nCeO₂@NPC, Pt/NPC, and the Pt foil and (b) enlarged images of the purple regions. (c) Pt L₃-edge FT-EXAFS of Pt/nCeO₂@NPC, Pt/NPC, and the Pt foil. EXAFS fitting result of (d) Pt/nCeO₂@NPC, (e) Pt/NPC, and (f) the Pt foil. Wavelet transform (WT) EXAFS spectra of (g) Pt/nCeO₂@NPC, (h) Pt/NPC, and (i) the Pt foil.

Electrocatalytic performance

To thoroughly investigate the effects of CeO₂ nano-nails and NPC on the catalytic performance of Pt nanoparticles, the Pt/CeO₂ catalyst was supplemented (Fig. S16) and electrochemical tests for all the catalysts were conducted. Fig. 3a and Fig. S17 display the cyclic voltammetry (CV) curves of all the synthesized catalysts with a scan rate of 50 mV s⁻¹ and a potential range of 0.05 to 1.2 V (relative to the reversible hydrogen electrode, RHE) in 0.5 M H₂SO₄ + 1 M CH₃CH₂OH electrolyte. For comparison, the EOR currents of all the electrodes were normalized to the mass of Pt. It is worth noting that the CV curves reveal distinct oxidation peaks during both the forward and reverse scans. Typically, the current density of the forward scan peak is used to assess the EOR performance of the catalysts. The peak current densities follow the trend: Pt/nCeO₂@NPC > Pt/NPC > Pt/CeO₂ > Pt/C. Specifically, the peak current density of Pt/nCeO₂@NPC reached 1374 mA mg_Pt⁻¹, which is 2.15 times higher than that of Pt/NPC (639 mA mg_Pt⁻¹), 3.49 times higher than that of Pt/CeO₂ (394 mA mg_Pt⁻¹), and 3.87 times higher than that of commercial Pt/C (355 mA mg_Pt⁻¹) (Fig. 3b). Furthermore, the electrochemical active surface area (ECSA)-normalized specific activity of Pt/nCeO₂@NPC attains 2.04 mA cm⁻²—surpassing those of Pt/NPC (1.13 mA cm⁻²), Pt/CeO₂ (0.90 mA cm⁻²), and commercial Pt/C (0.52 mA cm⁻²) (Fig. S18 and 19). Furthermore, Pt/nCeO₂@NPC delivers a mass activity of 1004 mA mg_Metal⁻¹ and a specific activity of 1.5 mA cm⁻²—both normalized to the mass of all metals—exceeding the corresponding values for Pt/NPC, Pt/CeO₂, and Pt/C (Table S4). The result demonstrates that both CeO₂ nano-nails and NPC significantly improve the catalytic activity of Pt, while their synergistic effect further boosts catalytic activity. Moreover, the onset potential of Pt/nCeO₂@NPC is lower than those of Pt/NPC, Pt/CeO₂ and Pt/C, as determined by linear sweep voltammetry (LSV) measurements (Fig. 3c and S20). The 140 mV negative shift in the onset potential of Pt/NPC relative to Pt/C suggests that the NPC support substantially improves the EOR performance of Pt. Furthermore, the onset potential of Pt/nCeO₂@NPC is further reduced due to the synergistic effect between nCeO₂ and NPC, which optimizes the reaction kinetics of Pt. Additionally, Fig. 3d illustrates that Pt/nCeO₂@NPC exhibits exceptional catalytic activity across various potentials, thereby further highlighting its improved electrocatalytic performance. The Nyquist plots demonstrate that Pt/nCeO₂@NPC displays the smallest charge-transfer resistance, indicative of accelerated electron-transfer kinetics during the EOR, as shown in Fig. S21 and Table S5.

Fig. 3 (a) CV curves for the EOR in a 0.5 M H₂SO₄ + 1 M CH₃CH₂OH solution. (b) Peak current density for the EOR with the catalysts. (c) LSV curves of the EOR in a 0.5 M H₂SO₄ + 1 M CH₃CH₂OH solution. (d) Mass activity toward the EOR at different potentials. (e) Chronoamperometric curves of the catalysts in an N₂-saturated 0.5 M H₂SO₄ + 1 M CH₃CH₂OH solution at 0.7 V vs. RHE for 3 h. (f) Current density at 0.7 V vs. RHE after 3 h for the catalysts. (g) CV curves of Pt/nCeO₂@NPC in an N₂-saturated 0.5 M H₂SO₄ + 1 M CH₃CH₂OH solution at a scan rate of 50 mV s⁻¹ during the durability tests. (h) Changes in peak current densities of the EOR during potential cycling of Pt/nCeO₂@NPC and Pt/C. (i) CO stripping curves recorded with Pt/C, Pt/NPC and Pt/nCeO₂@NPC in 0.5 M H₂SO₄.

To assess the stability and resistance to poisoning of the catalysts, chronoamperometry tests were performed at 0.70 V vs. RHE. As depicted in the I–t curves for the Pt/nCeO₂@NPC, Pt/NPC, and commercial Pt/C, the current density exhibits a rapid decline in the initial stage, which can be attributed to the formation of intermediate products (e.g., CO and CH_x) that block the active sites on the Pt surface (Fig. 3e). However, as the adsorption and oxidation of these intermediates reach equilibrium, the current gradually stabilizes. Throughout the testing period, Pt/nCeO₂@NPC exhibited the lowest decay rate and sustained a significantly higher current density compared to Pt/NPC and Pt/C catalysts, thereby underscoring its exceptional stability. After the 3 h chronoamperometry test (Fig. 3f), the current density of Pt/nCeO₂@NPC (79 mA mg_Pt⁻¹) was 2.72 times and 26.33 times higher than Pt/NPC (29 mA mg_Pt⁻¹) and Pt/C (3 mA mg_Pt⁻¹), respectively, demonstrating the superior long-term stability of Pt/nCeO₂@NPC. Moreover, a 2000-cycle accelerated durability test (ADT) was performed on Pt/nCeO₂@NPC as shown in Fig. 3g and h and S22. The Pt/nCeO₂@NPC catalyst retained 72% and 48% of its initial activity after 500 and 2000 CV cycles, respectively—demonstrating markedly superior stability compared to the commercial Pt/C, which retained only 45% of its initial activity after 500 CV cycles. Moreover, structural and morphological characterization of Pt/nCeO₂@NPC after ADT also confirms its exceptional stability during the EOR (Fig. S23 and 24). This enhanced durability is primarily attributed to the strong interaction between the CeO₂ nano-nails and Pt nanoparticles, which effectively prevents the migration and agglomeration of the Pt nanoparticles.

The resistance to CO poisoning is a critical indicator in assessing catalysts for DEFCs. Fig. 3i presents the CV curves for CO electrooxidation on different catalysts. The CO oxidation onset potential for Pt/NPC (0.84 V) is significantly lower than for commercial Pt/C (0.88 V), which may be attributed to NPC regulation of CO binding energy on the Pt surface. Moreover, the CO oxidation onset potential for Pt/nCeO₂@NPC (0.82 V) was further negatively shifted by 20 mV in comparison to that of Pt/NPC. The superior performance of Pt/nCeO₂@NPC can be ascribed to the synergistic effect between the CeO₂ nano-nails and Pt, which regulates the charge distribution of Pt and supplies more OH intermediates to facilitate the oxidative desorption of CO. The analysis of peak potentials for Pt/nCeO₂@NPC, Pt/NPC, and Pt/C catalysts further confirms that the synergistic interaction between CeO₂ nano-nails and Pt substantially enhances CO oxidation efficiency. The aforementioned experimental results indicate that the Pt/nCeO₂@NPC catalyst has outstanding performance in the EOR and remarkable resistance to CO poisoning.

Enhancement mechanism

To further verify the promoting effect of CeO₂ nano-nails on the electrooxidation of ethanol performance, we incorporated a comparative Pt/CeO₂@NPC catalyst, wherein Pt nanoparticles are uniformly anchored onto CeO₂ nanoclusters confined within NPC. The catalytic performance of Pt/CeO₂@NPC and Pt/nCeO₂@NPC is compared in Fig. 4a. The peak current density of Pt/nCeO₂@NPC was superior to that of Pt/CeO₂@NPC, with peak current densities of 1374 mA mg_Pt⁻¹ and 1000 mA mg_Pt⁻¹, respectively (Fig. 4b). The specific activity of Pt/nCeO₂@NPC is 1.24 times that of Pt/CeO₂@NPC (1.64 mA cm⁻²), as presented in Fig. S25 and S26. Moreover, Pt/nCeO₂@NPC exhibits a mass activity of 1004 mA mg_Metal⁻¹ and a specific activity of 1.5 mA cm⁻² (both normalized to the total metal content), outperforming Pt/CeO₂@NPC in both metrics (Table S4). Furthermore, EIS studies further revealed the exceptional charge-transfer characteristics of Pt/nCeO₂@NPC (Fig. S27 and Table S5). The Pt/nCeO₂@NPC catalyst exhibits a superior EOR performance, outperforming recently reported state-of-the-art Pt-based catalysts, as summarized in Table S6. The influence of scanning rate on the EOR activity was investigated to evaluate the mass transfer effect of the catalysts. The CV curves of all the catalysts with different scan rates (10–100 mV s⁻¹) revealed an increase in mass activity with increasing scan rate (Fig. S28). A linear dependence of mass activity on the square root of the scan rate confirms the diffusion-controlled process and the rapid mass transfer rate of Pt/nCeO₂@NPC (Fig. S29).^48,49 In addition, Pt/nCeO₂@NPC showed higher stability and durability in both the 3-hour chronoamperometry test and the 2000-cycle ADT. Pt/nCeO₂@NPC can maintain a higher current density than Pt/CeO₂@NPC during the I–t testing period, which suggests its superior resistance to CO poisoning may originate from unique nCeO₂ (Fig. 4c). The mass activities (MA) of Pt/nCeO₂@NPC reached 79 mA mg_Pt⁻¹ after the 3-hour chronoamperometry test (Fig. 4d), which is about 3.43 times higher than that of Pt/CeO₂@NPC (23 mA mg_Pt⁻¹). This further confirms the enhanced long-term stability of Pt/nCeO₂@NPC. Furthermore, after the 2000-cycle ADT (Fig. S30 and Fig. 3g), Pt/nCeO₂@NPC retained 48% of its initial activity, which was significantly higher than Pt/CeO₂@NPC (35%) (Fig. 4e and f). These results demonstrate that the introduction of uniquely nailed CeO₂ significantly improved the comprehensive performance of the catalyst.

Fig. 4 (a) CV curves of the EOR in a 0.5 M H₂SO₄ + 1 M CH₃CH₂OH solution. (b) Peak current density of the EOR for the catalysts. (c) Chronoamperometric curves of the catalysts in an N₂-saturated 0.5 M H₂SO₄ + 1 M CH₃CH₂OH solution at 0.7 V vs. RHE for 3 h. (d) Current density at 0.7 V vs. RHE after 3 h for the catalysts. (e) Changes in the peak current densities of Pt/nCeO₂@NPC and Pt/CeO₂@NPC during 2000 cycles ADT. (f) Residual performance after ADT. (g) HRTEM image of Pt/CeO₂@NPC. The blue and purple regions correspond to Pt and CeO₂, respectively. (h) High-resolution Pt 4f XPS spectra of Pt/nCeO₂@NPC and Pt/CeO₂@NPC. (i) High-resolution Ce 3d XPS spectra of Pt/nCeO₂@NPC and Pt/CeO₂@NPC.

To gain a deeper understanding of the mechanism by which the CeO₂ nano-nails promote catalysis, we characterized the morphology of Pt/CeO₂@NPC and Pt/nCeO₂@NPC catalysts by HRTEM. As illustrated in Fig. 4g and 1c, within the Pt/nCeO₂@NPC structure, Pt nanoparticles are in close contact with the CeO₂ nano-nails, thereby constructing the Pt–CeO₂ interaction interface. The fully exposed Pt–nCeO₂ interface in Pt/nCeO₂@NPC ensures greater electrochemical accessibility of active sites than that in Pt/CeO₂@NPC, significantly enhancing the EOR catalytic activity of the catalyst. Moreover, this structure not only inhibited the migration and agglomeration of Pt nanoparticles, but also enhanced the stability and durability of the catalyst by the synergistic effect of CeO₂ on the surface and in the interior from different dimensions.

In addition, we analyzed the electronic states and metal–support interactions of Pt/nCeO₂@NPC and Pt/CeO₂@NPC, making use of XPS. Fig. 4h shows that the binding energy of Pt 4f in Pt/nCeO₂@NPC is shifted by 0.15 eV towards lower binding energy compared with that in Pt/CeO₂@NPC, indicating stronger electronic interactions between the CeO₂ nano-nails and Pt. Notably, as depicted in Fig. 4i and Table S7, the content of Ce³⁺ in Pt/nCeO₂@NPC (45.86%) was significantly higher than that in Pt/CeO₂@NPC (38.80%), demonstrating that it maintained a strong oxygen vacancy activity, which is mainly attributed to the heterogeneous interfacial effect between the CeO₂ nano-nails and Pt.

Density functional theory (DFT) calculations were implemented to investigate the promoting mechanism of CeO₂ nano-nails on Pt/nCeO₂@NPC. As illustrated in Fig. S31–33, models of Pt/nCeO₂@NPC, Pt/CeO₂@NPC, and Pt (111) were constructed. The charge density difference distribution of Pt/CeO₂@NPC and Pt/nCeO₂@NPC reveals significant electron interactions between CeO₂@NPC or nCeO₂@NPC and Pt, along with obvious electron transfer from CeO₂ or nCeO₂ to Pt (Fig. S34 and 35).

Moreover, the projected density of states (PDOS) for Pt/nCeO₂@NPC, Pt/CeO₂@NPC, and Pt (111) are presented in Fig. 5a. The DOS of Pt/CeO₂@NPC and Pt/nCeO₂@NPC exhibit significant overlap, indicating a strong coupling between the Ce 4f orbitals and the Pt 5d orbitals. Notably, in comparison to Pt/CeO₂@NPC, the coupling between the Pt 5d orbitals and the Ce 4f orbitals in Pt/nCeO₂@NPC is higher, suggesting that a more robust interaction has formed between nCeO₂@NPC and Pt. As shown in Fig. 5b, the d-band center of Pt in Pt/CeO₂@NPC (−2.16 eV) has shifted away from the Fermi level compared to Pt (111) (−1.96 eV). Notably, when contrasted with Pt/CeO₂@NPC, the d-band center of Pt in Pt/nCeO₂@NPC (−2.34 eV) exhibits a more pronounced negative shift. This is consistent with the previous analysis results regarding the electronic structure and indicates that the Pt sites in the Pt/nCeO₂@NPC exhibit a reduced adsorption for CO* compared with Pt/CeO₂@NPC and Pt (111).⁵⁰ These findings are consistent with the results derived from XPS and HRTEM analysis.

Fig. 5 (a) PDOS of Pt (111), Pt/CeO₂@NPC, and Pt/nCeO₂@NPC. (b) d-band centers of Pt (111), Pt/CeO₂@NPC, and Pt/nCeO₂@NPC. (c) The adsorption model of CO* and OH* on Pt (111), Pt/CeO₂@NPC, and Pt/nCeO₂@NPC. Gray, purple, blue, and red spheres represent Pt, C, Ce, and O atoms, respectively. (d) Adsorption comparison of CO* on Pt (111), Pt/CeO₂@NPC, and Pt/nCeO₂@NPC. (e) Adsorption comparison of OH* on Pt (111), Pt/CeO₂@NPC, and Pt/nCeO₂@NPC. (f) Schematic of the Pt/nCeO₂@NPC enhancement mechanism for the EOR.

To study the thermodynamic tendencies of the C1 and C2 pathways, we found that, for Pt/nCeO₂@NPC, the conversion step of CH₃CO* → CH₃* + CO* is exothermic, while the step of CH₃CO* + OH* → CH₃COOH* is endothermic. This indicates that the C1 pathway of ethanol complete oxidation is thermodynamically favorable for Pt/nCeO₂@NPC (Fig. S36). To examine the improved resistance to CO poisoning in Pt/nCeO₂@NPC, a comparative analysis of the adsorption energies of CO* on Pt/nCeO₂@NPC, Pt/CeO₂@NPC, and Pt (111) was conducted; the corresponding adsorption models are established in Fig. 5c and S37. Fig. 5d presents the adsorption energy of CO* on Pt/nCeO₂@NPC, Pt/CeO₂@NPC, and Pt (111), suggesting weaker binding of CO* on the active Pt sites of Pt/nCeO₂@NPC (−2.15 eV) than of Pt/CeO₂@NPC (−2.75 eV) and Pt (111) (−3.69 eV). This demonstrates the exceptional CO poisoning resistance for Pt/nCeO₂@NPC during the ethanol oxidation process, consistent with the above DOS results. Additionally, the adsorption models of OH* on Ce sites in Pt/nCeO₂@NPC, as well as on Pt sites in Pt/CeO₂@NPC and Pt (111), were established, as shown in Fig. 5c and S38. Notably, the results demonstrate a stronger bonding strength for the formation of Ce–OH compared to the formation of Pt–OH (Fig. 5e). This further confirms that CeO₂ nano-nails play a critical role in promoting the removal of carbonaceous poisons and accelerating the kinetics of ethanol oxidation by supplying abundant OH* species due to the strong interaction between nCeO₂@NPC and Pt (Fig. 5f).

Conclusions

In this work, we constructed CeO₂ nano-nails on N-doped porous carbon (NPC) to modulate the electronic structure of Pt nanoparticles by accurately tailoring the Pt–CeO₂ interface, thereby realizing an efficient and poison-resistant electrocatalyst for the EOR. The unique CeO₂ nano-nails can construct the Pt–CeO₂ interface efficiently and ensure full exposure of the active sites, significantly enhancing the EOR catalytic activity of the catalyst. Meanwhile, the strong electronic interactions between Pt-based nanoparticles and the CeO₂ nano-nails can modulate the adsorption strength for CO and provide OH species to accelerate the removal of CO, thereby enhancing the anti-CO poisoning performance of the catalyst. The optimized catalyst (Pt/nCeO₂@NPC) demonstrates a mass activity of 1374 mA mg_Pt⁻¹ in ethanol oxidation, which is 3.87 times higher than that of commercial Pt/C (355 mA mg_Pt⁻¹). Additionally, it maintains 72% and 48% of its initial activity after 500 and 2000 CV cycles, respectively, and shows a 20 mV negative shift in onset potential compared to that of Pt/NPC, indicating excellent durability and enhanced tolerance to CO poisoning. This study has proposed an innovative strategy for the design of highly efficient catalysts for DEFCs, thereby advancing the development of clean and sustainable energy technologies.

Author contributions

Haoran Jiang: writing – original draft, writing – review & editing, data curation, formal analysis, conceptualization, methodology. Min Ouyang: writing – review & editing, data curation, formal analysis, methodology. Zichen Wang: visualization, data curation, formal analysis. Yinghui Jiang: formal analysis, investigation. Wangbin Zhu: software, data curation. Qiliang Wei: writing – review & editing, methodology. Niancai Cheng: writing – review & editing, funding acquisition, supervision.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available: Physical characterization (XRD, TEM, XPS) before and after ADT, electrochemical evaluation with durability test results, and DFT models for Pt/nCeO₂@NPC, Pt/CeO₂@NPC, and Pt(111). See DOI: https://doi.org/10.1039/d6qi00024j.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (grant no. 21875039 and 22478205).

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