Immobilization of Platinum Nanoparticles on Covalent Organic Framework‐Derived Carbon for Oxygen Reduction Catalysis

Platinum (Pt)‐based catalysts are considered as the most active catalysts for the oxygen reduction reaction (ORR). However, their applications have remained limited because of the high cost of Pt, and developing catalysts with low Pt contents is a challenge. Herein, a highly active catalyst (Pt–COF800) is constructed for the ORR by immobilizing hierarchical Pt subnano‐ and nanoparticles on covalent organic framework (COF)‐derived carbon. The catalyst shows excellent activity in alkaline conditions. The physical characterization demonstrates low nuclear Pt atoms and nanoparticles and confirms the role of heterogeneous active sites. This work paves the way for the construction of functional porous carbon materials with dual‐scale Pt clusters and may be applied to industrial catalytic reactions.


Introduction
The cathodic oxygen reduction reaction (ORR) plays a key role in energy conversion devices, such as fuel cells, and metal-air batteries because the inferior activity of the cathode hinders device performance and efficiency. Platinum (Pt)-based noble catalysts have been suggested as the most active catalysts for the ORR. [1] However, the high cost of Pt hindered their widespread use. [2] Various strategies have been adopted to develop highly active catalysts for the ORR. [3] The low-nuclearity Pt or non-noble catalysts supported on carbon have increasingly attracted interest because of their high atomic utilization efficiency and chemical stability. [4] Mostly, their activity and selectivity in 4e À pathway are lower than those of Pt/C catalysts. [5] For example, Li, and co-workers developed Pt-CuSx with a high density of isolated Pt atom sites, which displayed high H 2 O 2 selectivity (92-96%) over a wide potential range (0.05-0.7 V vs RHE) in an HClO 4 electrolyte. [6a] Meanwhile, Shi et al. reported Pt-modified AuCu nanostructures, exhibiting mainly a 2e À pathway with a H 2 O 2 selectivity of 91.8% at 0.35 V (vs RHE). [6b] More recently, Bian and co-workers comparatively investigated H 2 O 2 selectivity over a series of graphene-supported Pt and showed that the catalyst comprising isolated Pt atoms exhibited a H 2 O 2 selectivity of up to 95% in 0.1 M KOH in the potential range of 0.3-0.8 V (vs RHE). [6c] Catalysts assembled with hierarchical Pt structures can integrate activity, selectivity, and stability for the ORR while balancing the cost of Pt.
Covalent organic frameworks (COFs) are a class of crystalline porous polymers comprising designable building blocks linked by covalent bonds. [7] As they can comprise plenty of active sites, they offer a promising way to design ORR catalysts with high performance. [8] For example, Li et al. developed JUC-528, functionalized by bipentacyclic thiophene sulfur, which showed a more positive half-wave potential (0.70 V) than that functionalized with one thiophene sulfur (JUC-527). [9] However, the most pristine COFs with poor electrical conductivity and restrictive active site types. [10] Due to their low density, high porosity, and structural periodicity, COFs have been commonly used as precursors to fabricate functional carbons with high surface areas, abundant defect sites, and standing carbon layers. [11] In this work, we demonstrate a highly active and stable ORR catalyst using COF-derived functional carbon to support Pt subnano-and nanoparticles. The COF-derived carbon exhibits a high surface area and abundant N atoms, which immobilize Pt nanoparticles in the ORR process. The high porosity of the catalysts facilitates mass transport to the active sites. Owing to these advantages, the catalyst catalyzed the ORR with high activity and DOI: 10.1002/sstr.202200320 Platinum (Pt)-based catalysts are considered as the most active catalysts for the oxygen reduction reaction (ORR). However, their applications have remained limited because of the high cost of Pt, and developing catalysts with low Pt contents is a challenge. Herein, a highly active catalyst (Pt-COF 800 ) is constructed for the ORR by immobilizing hierarchical Pt subnano-and nanoparticles on covalent organic framework (COF)-derived carbon. The catalyst shows excellent activity in alkaline conditions. The physical characterization demonstrates low nuclear Pt atoms and nanoparticles and confirms the role of heterogeneous active sites. This work paves the way for the construction of functional porous carbon materials with dual-scale Pt clusters and may be applied to industrial catalytic reactions.
stability, and a half-wave potential of 0.88 V was achieved, which was 30 mV more positive than that of Pt/C in 0.1 M KOH.
The crystal structures of TP-BPY-COF and Pt-COF were revealed by powder X-ray diffraction (PXRD). The PXRD patterns of TP-BPY-COF showed peaks at 3.0°, 8.7°, and 26.3°, which were attributed to the (100), (200), and (001) facets ( Figure 2A, blue curve). [13] The peaks in Pt-COF were also observed. The metal peaks were absent among the high 2θ range, indicating the low aggregation of Pt atoms in the frameworks ( Figure 2A, green curve). The porosities of TP-BPY-COF and Pt-COF were investigated by N 2 adsorption isotherms at 77 K. TP-BPY-COF and Pt-COF displayed type-IV isotherms, indicating microporous and mesoporous structures. The Brunauer-Emmett-Teller (BET)-specific surface areas of TP-BPY-COF and Pt-COF were 575 and 383 m 2 g À1 , respectively. The pore size distribution curves of TP-BPY-COF and Pt-COF showed pore volumes of 1.14 and 0.78 cm 3 g À1 , respectively ( Figure 2B and S1, Supporting Information). The decreased surface areas and pore volumes were attributed to the Pt species dispersed in the pores.
Fourier-transform infrared (FT-IR) spectroscopy was used to investigate the chemical structures of TP-BPY-COF and Pt-COF. The formation of C═N bonds was verified for TP-BPY-COF at 1580 cm À1 . In addition, the C─N bond band broadened at 1270 cm À1 for Pt-COF, indicating that Pt ions were coordinated to the bipyridine structure of TP-BPY-COF ( Figure S2, Supporting Information). [14] The morphologies of TP-BPY-COF and Pt-COF were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM images of the TP-BPY-COF crystal showed regular aggregated bulks ( Figure 3A). In addition, the TEM images confirmed their morphologies, indicating an ordered lattice structure for TP-BPY-COF ( Figure 3B). As shown in the elemental mapping results obtained by energy-dispersive spectroscopy (EDS) ( Figure     Pt-COF showed that the morphology was well maintained ( Figure 3D). Upon the introduction of Pt ions, the layered structure was not destroyed ( Figure 3E). As for the EDS images, the catalyst particles demonstrated uniformly dispersed Pt, N, O, and C species ( Figure 3F). Thermogravimetric analysis (TGA) was conducted to study the thermal stability of TP-BPY-COF and Pt-COF from 40 to 800°C under N 2 . The corresponding TGA curves showed that TP-BPY-COF and Pt-COF did not decompose below 350 and 360°C, respectively, revealing that TP-BPY-COF and Pt-COF exhibited similar thermal stability in N 2 ( Figure S3, Supporting Information). The loss of water for Pt-COF decreased compared with that of TP-BPY-COF because of the occupancy of pores by Pt ions, leading to a decrease in the ratio of water in TP-BPY-COF.
The TP-BPY-COF and Pt-COF were then pyrolyzed at 800°C to yield TP-BPY-COF 800 and Pt-COF 800 , respectively. The PXRD patterns of the Pt-COF 800 catalysts showed typical Pt face-centered cubic (fcc) features. The broad peak located at 24.1°was assigned to the C (002) plane. Meanwhile, the Pt (111), Pt (200), Pt (220), Pt (311), and Pt (222) peaks appeared at 40.0°, 46.5°, 67.8°, 81.9°, and 86.5°, respectively, consistent with the fcc Pt lattice of the PDF#04-0802 support ( Figure 4A). Figure S4A, Supporting Information, shows the representative shapes, which are best represented by the top slice of hollow-shell ellipsoidal shape. To determine the type of the top facet of the above shape, the fast Fourier transform (FFT) of the selected area was further carried out, where the obvious lattice fringes (0.22 nm) indexed as the typical (111) plane of Pt nanoparticles were clearly observed from the HRTEM image. [15] In contrast, the PXRD pattern of TP-BPY-COF 800 showed the (002) and (100) peaks of carbon at 22.9 and 43.8°( Figure S4B, Supporting Information).
N 2 adsorption isotherms were obtained to evaluate the porosities of COFs after pyrolysis ( Figure 4B). Pt-COF 800 exhibited a type-IV isotherm, suggesting the coexistence of micropores and mesopores, with a BET surface area of 517 m 2 g À1 . The pore size distribution curves showed that the pore volume for micropores and mesopores was 1.07 cm 3 g À1 ( Figure S5, Supporting Information). TP-BPY-COF 800 displayed IV sorption behavior,  www.advancedsciencenews.com www.small-structures.com and the corresponding BET surface area and pore volume were 317 m 2 g À1 and 0.69 cm 3 g À1 , respectively ( Figure S6, Supporting Information). The Pt in the pores of TP-BPY-COF hindered the collapse of pores in the pyrolysis process, resulting in the high BET surface area and pore volume. The SEM images of TP-BPY-COF 800 and Pt-COF 800 indicated that the morphologies were well maintained with no visible collapse ( Figure 5A,D). [16] The TEM images showed that TP-BPY-COF 800 comprised carbon sheets ( Figure 5B), and the TEM images of Pt-COF 800 showed that Pt nanoclusters or nanoparticles were uniformly distributed on the resulting TP-BPY-COF-derived carbon ( Figure 5E). The average particle size for Pt in Pt-COF 800 was 9.3 nm with a narrow size distribution. According to the inductively coupled plasma-optical emission spectroscopy (ICP-OES) results, the Pt content in Pt-COF 800 was 8.12 wt%. The EDS images confirmed that C, N, and O elements were well dispersed ( Figure 5C). As shown in Figure 5F, the Pt element was distributed uniformly across the TP-BPY-COF-derived carbon, further indicating the excellent dispersity of Pt nanoclusters or nanoparticles.
The Raman spectra showed that the characteristic peaks of TP-BPY-COF 800 and Pt-COF 800 at 1350 and 1580 cm À1 belonged to defective carbon (D band) and graphitized carbon (G band), respectively, where the I D /I G ratios were 1.00 ( Figure S7, Supporting Information), indicating that the nanosized 2D graphitic structure was retained with the addition of Pt ions. Notably, the difference in the performance of the ORR was not due to the carbon defect. [17] 2.2. Chemical State and Atomic Structure Analysis X-ray photoelectron spectroscopy (XPS) was conducted to investigate the electronic states of the catalysts. The XPS spectrum of Pt-COF 800 showed C 1s, N 1s, O 1s, and Pt 4f peaks, and the contents of C, N, O, and Pt were 68.55, 2.74, 4.47, and 3.95 wt%, respectively ( Figure S8, Supporting Information). From the high-resolution XPS spectrum of N 1s for Pt-COF 800 , the peaks at 398.5, 399.5, 401.1, and 404.4 eV were assigned to pyridine N (N1), pyrrole N (N2), graphitic N (N3), and oxidic N (N4), and the relative contents of N1, N2, N3, and N4 were 0.11%, 0.07%, 0.34%, and 0.12% ( Figure S9, Supporting Information). [18] As well known, the dissolution of Pt atoms via the formation of Pt─O bonds would also be hindered. The XPS spectrum of O 1s is depicted in Figure S10, Supporting Information, the peaks at 532.5 eV belonged to the Pt─O bond (66.2%). The peak at 533.9 eV was attributed to the adsorbed H 2 O with a content of 33.8%. [19] To better understand the differences before and after the carbonization process in Pt-COF, the XPS spectrum of Pt-COF was also conducted. In detail, the spectrum of the Pt 4f region for Pt-COF 800 displayed doublet peaks including a lower energy (Pt 4f 7/2 ) band and a higher energy (Pt 4f 5/2 ) band ( Figure 6A). Two pairs of doublets were found from the deconvolution of the spectrum: 1) 71.5 eV for Pt 4f 7/2 and 74.9 eV for Pt 4f 5/2 , attributed to Pt (0); and 2) 72.7 eV for Pt 4f 7/2 and 77.3 eV for Pt 4f 5/2 , which can be assigned to Pt (IV) species. [20a] In contrast, the binding energies of the Pt 4f peaks for Pt-COF showed no Pt (0) when compared with those of the Pt-COF catalyst, indicating that Pt particles were formed after the pyrolysis of Pt-COF. Previous studies have demonstrated that any change to the electronic structure may influence the binding of oxygen species and thereby the ORR activity. [20b] Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) measurements were performed to further determine the local atomic and electronic structures of Pt-COF 800 . Figure 6B shows the XANES curves at the Pt L 3 -edge of Pt-COF 800 (red curve), the Pt foil (blue curve), and PtO 2 (black curve). The white line intensity directly reflects the unoccupied density of states of Pt 5d orbitals. Compared to the Pt foil, the same intensity of the white line peak for Pt-COF 800 indicates the valence state of Pt (0), which is consistent with the XPS analyses.
The Fourier-transformed (FT) k 2 -weighted χ(k)-function of the EXAFS spectra was employed to study the local coordination environment of Pt atoms. The 1.53 and 2.44 Å scatterings on  Pt L 3 -edge FT-EXAFS could be assigned to the main coordination shell of Pt-N and Pt-Pt (without phase shift) ( Figure 6C). The least-squares EXAFS fitting of Pt-COF 800 in the R space was further performed to acquire quantitative structural parameters. As the K-edge FT-EXAFS was present ( Figure S11 and Table S1, Supporting Information), the first peak at (%1.95 Å) in Pt-COF 800 was assigned to the Pt─N bond, and a secondary peak at a high R value (%2.72 Å) found in Pt-COF 800 demonstrated the Pt-Pt path. According to the fitting results in Table S1, Supporting Information, the coordination numbers of Pt-N and Pt-Pt were evaluated to be 0.5 and 9.7, respectively. The average size of Pt NPs was found to be 3.3 nm based on the coordination number of Pt-Pt. [21] The larger particle size of Pt-COF 800 observed by TEM was caused by the lack of close packing in large NPs. Moreover, the fitted Pt-N coordination number was too large, which may be caused by the coexistence of Pt SACs and nanoparticles. The wavelet-transform (WT) EXAFS analysis showed the WT contour plots of the three k 2 -weighted χ(k) signals based on Morlet wavelets with optimum resolution ( Figure 6D). Pt-COF 800 showed a WT maximum with a k value of 4.8 Å À1 for the PtÀN path, which can be matched to the Pt-N/C (PtO 2 ) path and a k value of 8.4 Å À1 for the PtÀPt path, which can be matched to the Pt-Pt (Pt foil) path.

Oxygen Reduction Catalysis
The electrocatalytic performance of TP-BPY-COF 800 and Pt-COF 800 toward the ORR was evaluated in alkaline conditions (0.1 M KOH aqueous solution). For comparison, the catalytic behavior for benchmark Pt/C (20%) was also investigated. Cyclic voltammetry (CV) measurements were conducted at scan rates of 1600 mV s À1 from 0.2 to 1.0 V versus RHE. The CV curve of Pt-COF 800 showed an obvious reduction peak at 0.75 V in an O 2 -saturated solution, while such a peak was absent in a N 2 -saturated solution ( Figure S12, Supporting Information). In addition, linear sweep voltammetry (LSV) measurements ( Figure 7A) revealed that Pt-COF 800 was superior to Pt/C in terms of the half-wave potential (E 1/2 = 0.88 and 0.85 V for Pt-COF 800 and Pt/C, respectively) and the onset potential (E onset = 1.00 and 0.99 V for Pt-COF 800 and Pt/C, respectively). For the TP-BPY-COF 800 catalyst without Pt species, the halfwave potential and the onset potential showed negative shifts to 0.70 and 0.78 V, respectively, indicating its weak activity. The diffusion-limiting current density of Pt-COF 800 (6.79 mA cm À2 ) was much higher than that of Pt/C and TP-BPY-COF 800 (5.40 and 5.50 mA cm À2 , respectively). Such a high limiting current density offers high power density. As for reaction kinetics (Figure 7B), the superior performance of Pt-COF 800 was confirmed by the lower Tafel slope (93 mV dec À1 ) compared to those of Pt/C (96 mV dec À1 ) and TP-BPY-COF 800 (126 mV dec À1 ). LSV measurements under different rotating rates and the corresponding Koutecky-Levich plots were used to study the electron transfer number during the ORR process ( Figure S13, Supporting Information). The results indicated that Pt-COF 800 follows a four-electron transfer pathway in the ORR process. To further investigate the H 2 O 2 yield, rotating-ring disk electrode (RRDE) measurements were performed ( Figure 7C). The H 2 O 2 yield of Pt-COF 800 remained below 5%, close to that of Pt/C within the potential range of 0.2-1.0 V versus RHE. The ORR process was dominated by the four-electron transfer pathway, similar to that in the case of Pt/C in the corresponding potential range. The results demonstrated that the nanometer-sized Pt cluster-incorporated Pt-COF 800 was a more efficient ORR electrocatalyst in an alkaline medium. The stability measurements showed that the ORR current for the Pt-COF 800 catalyst remained at 94% after 18 h of testing ( Figure S14, Supporting Information). After the stability test, SEM, TEM, EDS mapping, and XRD analyses were performed to reveal the morphology and phase changes of Pt-COF 800 and TP-BPY-COF 800 , as shown in Figure S15 and S16, Supporting Information. Compared to initial samples, the SEM images showed that regular aggregated polyhedral structures remained intact after 18 h ORR test ( Figure S15A,D, Supporting Information). TEM analyses showed the particles of Pt-COF 800 were found to have no obvious agglomeration, which proved the excellent stability of Pt-COF 800 catalyst and verified that  the derived COF could effectively hinder the migration of Pt nanoparticles on the carbon carrier ( Figure S15B,E, Supporting Information). In addition, the EDS mapping also showed a uniform distribution of C, N, O, and Pt ( Figure S15C,F, Supporting Information). The XRD images showed that the sizes of metallic Pt nanoparticles reduced and verified that some Pt nanoparticles were dissolved during the ORR ( Figure S16, Supporting Information).
To further explore the active sites of Pt-COF 800 , we added potassium thiocyanate (KSCN) to the electrolyte. The strong affinity between SCN À and Pt ions can poison Pt-associated active sites. The ORR current decreased after adding a trace amount of KSCN to the electrolyte ( Figure S17, Supporting Information). For example, after adding KSCN, the current only retained about 0.80 V of the previous current (0.88 V). This result demonstrated that the highly active Pt single-atom sites in the Pt-COF 800 catalyst were deactivated and the retained activity originated from Pt NPs.
To gain insight into the kinetic process of ORR, the electrochemical impedance spectroscopy (EIS) experiments were performed. The diameter of the semicircle corresponds to R ct , which represents the activation resistance toward the ORR. [22] Nyquist plots are shown in Figure S18, Supporting Information, and the R ct of Pt-COF 800 was 36.15 Ω cm À2 , which is much lower than that of TP-BPY-COF 800 (47.45 Ω cm À2 ), indicating that Pt-COF 800 has demonstrated fastest electron transfer process. Apart from the intrinsic activity, the electrochemical activity is also positively correlated with the number of available active sites, i.e., with the electrochemical active surface area (ECSA). [23] As shown in Figure S19, Supporting Information, the Cdl of Pt-COF 800 and TP-BPY-COF 800 was 3.5 and 3.9 μF cm À2 , respectively, which indicated that some of the pore channels were filled with Pt ions, resulting in a decrease in the ECSA.
Although the derived functional carbon showed the ORR activity, the current contribution at the high potential range mostly originated from the metallic Pt sites. As shown in Figure 7D, the constructed hierarchical Pt atom clusters in Pt-COF 800 contained low-nuclearity Pt atoms and larger Pt NPs. The highly dispersed low-nuclearity sites induced the 2e À pathway for OOH À production via end-on adsorption. [24] Further, the most exposed facet on Pt NPs was (111), which was the high-activity facet in the Pt crystal, reducing oxygen to H 2 O via the 4e À pathway. The Pt (111) facets followed the O═O dissociation and OOH dissociation mechanism. [25] In addition, a large number of the products of 2e À ORR were migrated to Pt crystal surfaces and reduced to H 2 O in the microenvironment of carbon layers. [26] Therefore, the indirect 4e À and direct 4e À processes conduced to efficient energy conversion via the COF-derived catalytic action.

Conclusion
We developed TP-BPY-COF coordinated with highly dispersed Pt atoms on functional carbon materials. The controllable thermal metal aggregation in COFs induced the formation of low nuclear sites and Pt NPs with potential catalytic activities. Compared with the commercial 20 wt% Pt/C, Pt-COF 800 with a lower noble metal content showed excellent performance with a half-wave potential of 0.88 V (vs RHE) and a diffusion-limiting current density of 6.79 mA cm À2 . These heterogeneous active sites demonstrated a high potential for energy conversion via the electrocatalytic process. This work offers new insights into advanced carbon material design and industrial catalytic system development through the manipulation of suitable active sites for various applications.
Preparation of Pt-COF Catalysts: The Pt nitrate solution (18.02 wt% Pt) (20 mg) was added into TP-BPY-COF (100 mg) in MeOH (100 mL), which was stirred for 24 h at room temperature. The obtained black powder was filtered, washed with MeOH 3 times, and dried at 80°C in a vacuum oven for 12 h to obtain Pt-COF (yield: 83%).
Preparation of Pt-COF 800 and TP-BPY-COF 800 Catalysts: The Pt-COF samples were added to a crucible. The crucible was then placed in a tube furnace and heated to 800°C for 2 h at a heating rate of 5°C min À1 under a N 2 atmosphere. After cooling to room temperature, Pt-COF 800 (black powder) was obtained. The product was used directly without further treatment. For comparison, TP-BPY-COF 800 was synthesized by the pyrolysis of COFs using the same process.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.