Photochemical Solid-Phase Synthesis of Platinum Single Atoms on

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Photochemical solid-phase synthesis of platinum single atoms on nitrogen-doped carbon with high loading as bifunctional catalysts for hydrogen evolution and oxygen reduction reactions Tuanfeng Li, Jingjun Liu, Ye Song, and Feng Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02288 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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isolated Pt atoms to the coordinated N atoms in this catalyst. The Pt1/NPC can be employed as bifunctional catalysts for fuel cells and hydrogen production.

KEYWORDS: platinum single-atoms, porous carbon, photochemical solid-phase reduction, hydrogen evolution reaction, oxygen reduction reaction

1. Introduction The precious metal platinum has been currently regarded as one of the best electrocatalyst for several important chemical reactions, such as hydrogen evolution reaction (HER), oxygen reduction reaction (ORR) and methanol electrooxidation that are applied in various energy storage and conversion devices.1-3 However, the high cost and rare reserve of the metal seriously restrict its large-scale commercialized application in these above fields. To address this issue, reducing the size of active Pt nanoparticles to clusters or even single atoms may offer a smart way to achieve the maximum utilization and minimum consumption of the metal. More importantly, as platinum is atomically divided into isolated single atoms anchored on supports like carbon materials and transition metal oxides, the Pt single-atoms would exhibit surprising catalytic properties.4-8 It is attributed to the atomic-level Pt displaying unique chemical and physical properties, relative to its bulk counterpart. To date, there are several synthesis strategies, such as solution chemical method,9, 10 atomic layer deposition (ALD),11-13 and photochemical synthesis,14-16 proposed for the promising Pt-based single atom catalysts (SACs). Among these above synthesis strategies, the traditional solution chemical route has been considered as one of the mainstreams to fabricate the so-called SACs by now.17 For example, Zhang et al.17 prepared atomically dispersed Pt on iron oxide nanoparticles (Pt1/FeOx) by coprecipitation of chloroplatinic acid and ferric nitrate in reductive solutions. As a result, although

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the loading of the Pt is very low (0.17 wt.%), the synthesized Pt1/FeOx catalyst still shows a significantly improved activity for CO oxidation in comparison with that of the commonly used Au/Fe2O3 catalyst. The enhanced activity may come from the intrinsic activity of the isolated Pt atoms with the partially unoccupied 5d orbitals and high-valence, which can contribute to reduce the adsorption energy and lower the activation barriers of CO. It is believed that loading of the Pt is usually low due to many nanocrystals formed by diffusion of the ions and atoms via the solution chemical method. To overcome this issue, Sun et al.12,

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prepared atomic-level Pt

supported on nitrogen-doped graphene nanosheets (Pt1/NGNs) using the well-known ALD method. The Pt loading is up to about 2.1 wt.%,13 but the obtained Pt1/NGNs are always mixed with more or less nanoclusters and even nanoparticles of Pt, which would decline the electrocatalytic performance. Recently, Wu and coworkers15 proposed an iced-photochemical reduction to fabricate atomic Pt catalysts via ultraviolet irradiation reduction of frozen chloroplatinic acid aqueous solution. And then, the melting frozen solution containing Pt atoms is physically mixed with various supports like graphene or TiO2 nanoparticles to synthesize Ptbased SACs. This work suggests that photochemical reduction may be suitable to facilely fabricate Pt single-atoms on matrixes. In this work, we developed well-defined Pt single-atoms on a nitrogen-doped porous carbon (NPC) via a modified photochemical solid-phase reduction method. Using this synthesis strategy, PtCl62- ions are directly reduced by ultraviolet light irradiation and then deposit on the carbon surface, without post physical or chemical treatment process. Surprisingly, not only is the loading of homogeneously Pt single atoms very high (3.8 wt.%), but is the aggregation of Pt atoms prevented, as evidenced by aberration-corrected scanning transmission electron microscopy (STEM) and X-ray absorption fine structure (XAFS) results. In an acidic or alkaline


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solution, the synthesized Pt-based catalyst shows an outstanding electrocatalytic activity and a good stability for HER and ORR, respectively. It may open a door to simply synthesize precious metal-based single atom catalysts for various energy storage and conversion devices.

2. Experimental Section 2.1 Materials and Instrumentations Potassium hydroxide (82%), sodium hydroxide (96%) and concentrated hydrochloric acid (37%) were purchased from Beijing Chemical Works. Chloroplatinic acid (99.9%) was purchased from J&K Scientific Ltd.. Commercial Pt/C (20 wt%, Johnson Matthey Company, Hispec 3000) and nafion solution (5 wt%, Dupont Company, D520) were used in this work. The mercury lamp (GPH212T5L/4P, 10 W of power, 254 nm of ultraviolet wavelength) was employed in this work. Preparation of nitrogen-doped porous carbon. Prior to the synthesis of the atomic-level Pt on a nitrogen-doped porous carbon (NPC), a nitrogen-doped porous carbon was initially fabricated through the pyrolysis of a cattle bone. The cattle bone used in this work was purchased from a food market in Beijing. Firstly, the fresh cattle bone was boiled in water for 1 h to remove grease, and then washed with ultrapure water for several times. After drying at 80 °C for more than 12 h, the cattle bone was shattered into powder as raw material for the further pyrolysis process. The obtained cattle bone powder was pre-carbonized in a tubular furnace at 400 ℃ for 3 h under argon atmosphere. Subsequently, the pre-carbonized product was grinded and mixed with KOH as active agent at a mass ratio of 1:1, then activated at 800 ℃for 1 h under argon atmosphere. Next, the obtained powder was washed with 2.4 M HCL aqueous solution, followed by rinsing with deionized water several times and drying at 70 ℃. The final product


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was gained by heating the sample up to 1000 ℃ for 3 h under ammonia atmosphere in a tubular furnace. Repeat experiment results show that there is a reproducibility for the NPC derived from different batches of cattle. The detail synthesis procedure of the NPC is shown in Reference.18 Preparation of platinum single-atoms on carbon. Based on the as-synthesized NPC, we proposed a facile and fast route to fabricate the isolated Pt atoms on the NPC (Pt1/NPC) with a high loading using a photochemical solid-phase reduction method. In a typical synthesis of Pt1/NPC catalyst, 100 mg of the obtained NPC was added into 250 mL of 0.4 mM H2PtCl6 aqueous solution. After dispersed by sonicating for 30 min, the mixed solution was kept stirring for 24 h at room temperature. The resulting mixed solution was filtrated and washed with dilute NaOH solution and then DI water several times, and was then natural drying. The obtained dried precursor power was spread to form a thin film on a glass plate for allowing light to pass through, as shown in Figure S1. After that, the thin film was irradiated by ultraviolet light from a mercury lamp under the distance of 10 cm for 1 h at room temperature, where the adsorbed PtCl62- ions on the NPC are directly reduced into Pt atoms by ultraviolet light irradiation. The obtained samples don’t suffer post physical or chemical treatment process after the photochemical solid-phase reduction. 2.2 Characterization of the Pt1/NPC catalysts Physical Characterization. Scanning electron microscope (SEM, FE-JMS-6701F) and highresolution transmission electron microscopy (HRTEM, JEM-3010) were used to determine the morphologies of the as-synthesized Pt1/NPC catalysts. The dispersion of single Pt atoms was explored by an atomic-resolution aberration-corrected scanning transmission electron microscope (STEM, JEM-ARM200F). The nitrogen adsorption/desorption isotherm was


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performed for the samples by using an automatic Quantachrome II analyzer at 77 K. The specific surface area and the pore size distribution of the samples were calculated according to the Brunauer-Emmett-Teller (BET) and the density functional theory (DFT) model, respectively. The powder X-ray diffraction (XRD) was performed on a Rigaku D/max-2500 diffractometer, using Cu Kα radiation (γ=1.54056 Å) as the X-ray source at 30 KV and 40 mA. To determine the loadings of Pt on the carbon, the thermalgravimetric-differential thermal analysis (TG-DTA) was carried out on a Rigaku TG-8120 with a heating rate of 5 K min-1 under air atmosphere to determine the Pt loadings. Moreover, an inductively coupled plasma optical emission spectroscopy (ICP-OES, IRIS INTREPID II, Thermo Fisher) was used to further determine the loadings of Pt. The surface electronic structure of the Pt1/NPC catalysts were characterized by using X-ray photoelectron spectrometer (XPS, Thermo ESCALAB 250). All the XPS spectra were calibrated relative to the C 1s peak at 285 eV. Including X-ray absorption near edge structure (XANES) region and extended X-ray absorption fine structure (EXAFS) region, X-ray absorption spectra (XAS) at both the Pt L3- and L2-edge were recorded at the beamline 1W1B of the Beijing Synchrotron Radiation Facility (BSRF, China). The typical energy of the storage ring was 2.5 GeV in BSRF. Pt foil was used to calibrate the Si (111) double crystal monochromator and then collected the XAFS data measured in the transmission mode for the reference spectrum. The XAS data at both the Pt L3- and L2-edge for the Pt1/NPC and Pt/C were recorded in the fluorescence mode. The acquired XAS data were extracted and processed according to the standard procedures using the ATHENA module implemented in the IFEFFIT software packages. Electrochemical Tests. All the electrochemical measurements were performed by a threeelectrode system with a glassy carbon electrode (GCE) as the working electrode, a saturated


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calomel electrode (SCE) as the reference electrode, and a graphite rod as the counter electrode. The iR-compensation was used to correct the solution resistant during all the electrochemical measurements. Moreover, the measured potentials were normalized by reversible hydrogen electrode (RHE) by the following equation: E (RHE) = E (SCE) + 0.241 + 0.0591 × pH. For the HER measurements, the loading mass of the catalysts on the working electrode is 3.8 µg Pt cm-2 for the Pt1/NPC catalyst and 20 µg Pt cm-2 for the commercial Pt/C catalyst. The HER activity of these catalysts was measured by linear sweep voltammetry (LSV) in N2-saturated 0.5 M H2SO4 at room temperature, with a scan rate of 10 mV s-1. The catalytic durability was evaluated by accelerated durability test (ADT) recorded at a potential range from 0.65 to 1.05 V and a scan rate of 100 mV s-1 in N2-saturated 0.5 M H2SO4 for 3000 CV cycles and the chronopotentiometric measurements recorded under the constant current density of 20 mA cm-2 in N2-saturated 0.5 M H2SO4 for 10000 s. For the ORR measurements on the catalysts, LSV was carried out on a rotating ring-disk electrode device (AFCBP1 type, PINE Company, working electrode area of 0.247 cm2), recorded in O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s-1. The loading mass for the Pt1/NPC and Pt/C on the working electrode were 15.2 µg Pt cm2

and 20 µg

Pt

cm-2, respectively. The accelerated durability test (ADT) was carried out by

continuous CV method in N2-saturated 0.1 M KOH solution at a scan rate of 200 mV s-1 for 10000 cycles at a polarization potential range from 0.65 to 1.05 V, versus RHE. The chronoamperometry measurement in O2-saturated 0.1 M KOH solution at a given potential of 0.82 V (vs RHE) and a rotation rate of 1600 rpm. 2.3 DFT modeling For the Pt1/NPC catalyst, density functional theory (DFT) calculations were performed using DMol3 module in Material Studio 5.5 software package.19, 20 Based on the literature,21 a periodic


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5 × 5 graphene supercell containing 50 carbon atoms was introduced to model the Pt–N4 species like pyridine-, and pyrrole-type structures. The geometrical configurations of the catalyst are fully optimized by using the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) method. The core electrons for the metal Pt are treated by effective core potentials (ECP). The double numerical plus polarization (DNP) basis sets with a real-space global orbital cutoff radius of 4.5 Å are adopted. The convergence criteria for structure optimizations are set as energy tolerance of 2.72×10-4 eV per atom, maximum force tolerance of 0.0544 eV/Å and maximum displacement tolerance of 5.0× 10-3 Å. The converged criterion of self-consistent field (SCF) is within 2.72×10-5 eV per atom. The pyridine-type Pt–N4 system is built by removing two carbon atoms, followed by the substitution of four carbon atoms around the divacancy site with four nitrogen atoms to provide the anchoring site for single Pt atom. For comparison, the pyrroletype Pt–N4 system was built by removing six carbon atoms and subsequently replacing the corresponding carbon atoms in five-membered rings with four nitrogen atoms to anchor Pt atom. A vacuum space between the graphene sheets in the z-direction was set to 15 Å, which is sufficiently large to avoid their interactions. After fully relaxed with DMol3, all of the tructures are subjected to population analysis with the Hirshfeld scheme and the densities of states (DOS) are obtained on a Monkhorst-Pack grid with the 7×7×1 k points. In this work, the binding energy (Eb) was chosen to calculate the stabilities for the pyridine-type Pt–N4 and pyrrole-type Pt–N4 systems and the formula is as following: Eb = EPt1/NPC – (EPt1 + ENPC),

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where EPt1/NPC is the

total energy of pyridine-type Pt–N4 or pyrrole-type Pt–N4 system (For the sake of simplification, pyridine-type Pt–N4 and pyrrole-type Pt–N4 system are donated as “1” and “2”, respectively); EPt1 is the energy of single Pt atom; ENPC is the energy of defective graphene substrate without Pt atom (ENPC, 1 = Egraphene – 6×EC atom + 4×EN atom; ENPC, 2 = Egraphene – 10×EC atom + 4×EN atom).


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According to the definition of binding energy, the Eb is always a negative number, and the more negative the Eb is, the more stable the chemical structure. To compare the stabilities of pyridinetype Pt–N4 and pyrrole-type Pt–N4 structures, the binding energy difference (∆Eb) between them is calculated as ∆Eb = Eb, 2 – Eb, 1 = (EPt1/NPC, 2 – (EPt1 + ENPC, 2)) – (EPt1/NPC, 1 – (EPt1 + ENPC, 1)) = (EPt1/NPC, 2 – (EPt1 + (Egraphene – 10×EC atom + 4×EN atom)) – (EPt1/NPC, 1 – (EPt1 + (Egraphene – 6×EC atom + 4×EN atom)) = EPt1/NPC, 2 –EPt1/NPC, 1 + 4×EC atom = 0.13 eV, where EPt1/NPC, 2, EPt1/NPC, 1, and 4×EC atom

are calculated as listed: -54409.74 eV, -50297.81 eV and -1027.95 eV, respectively.

3. Results and discussion 3.1 Formation of isolated Pt atoms on nitrogen-doped carbon Figure 1 shows the typical photochemical solid-phase synthesis route for the isolated Pt atoms supported on the home-made nitrogen-doped porous carbon (NPC) with a high specific surface area (2816.4 m2 g-1). As the NPC is physically mixed with H2PtCl6 solution, it can adsorb many PtCl62- ions from the solution. First, the carbon has rich heteroatoms like nitrogen and oxygen shown in Table S1 derived from survey XPS spectra of the NPC. These heteroatoms provide active sites for chemical adsorption of PtCl62- ions.23 Second, the carbon displays a large number of crumpled honeycomb-shaped macro-pores with about 100 nm diameter, as shown in Figure 2A-B. Also, the NPC exhibits abundant micropores (0.8-2 nm) and mesopores (2.7-5.2 nm), as evidenced by the nitrogen adsorption-desorption measurement (Figure S2). These formed micro-, meso-, and macro-pores are especially propitious to the chemical adsorption and anchorage of PtCl62- ions. Third, we can clearly observe the presence of many lattice fringes with short-range ordered structure spreading all over the carbon sheets and these fringe as defects also contribute the adsorption of PtCl62- ions on the NPC.13 After adsorbing and drying, ultraviolet light was employed to irradiate the mixture of PtCl62- and the carbon, leading to the reduction of PtCl62-


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ions. Figure 2C-D show the typical morphology of the final products after irradiation, which is almost similar to that of the pure carbon. No Pt nanoclusters or nanoparticles have been found on the carbon surface. It implies that the reduced Pt may be at the atomic level, forming the isolated Pt atoms anchored on the N-doped porous carbon (Pt1/NPC).

Figure 1. Schematic illustration of the formation of Pt1/NPC catalyst. (A) The resultant NPC substrate; (B) PtCl62- ions adsorbed on the NPC; (C) Pt single-atoms anchored on the NPC. To confirm this conjecture, atomic-resolution aberration-corrected scanning transmission electron microscopy (STEM) were performed for the synthesized Pt1/NPC sample. From HAADF image shown in Figure 2E, a lot of the isolated Pt atoms (white spots) are uniformly dispersed on the carbon at atomic-level. These Pt atoms have a homogeneous distribution with a narrow size range (less than 0.4 nm), as shown in Figure 2F. No Pt clusters or nanoparticles have been observed from the STEM bright field image shown in Figure 2G, further proving that the all the Pt are in form of the isolated single atoms on the carbon. This outcome is in accordance with the XRD results shown in Figure 2H. From Figure 2H, there are only two characteristic


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diffraction peaks assigned to the carbon support have been observed. No peaks associated with metallic Pt have been found, but the TG results indeed reveal the presence of a certain amount of platinum in the Pt1/NPC catalyst, as displayed in Figure 2I. According to inductively coupled plasma optical emission spectrometry (ICP-OES) measurements, the loading of the atomically platinum is up to 3.8 wt.%. For the as-fabricated Pt1/NPC, the recorded loading of the Pt is much higher than that of most published Pt single-atom catalysts.12, 13, 15, 17-20, 24-29


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Figure 2. (A) SEM and (B) HRTEM images of the pure NPC. The insets show the enlarged images; (C) SEM and (D) HRTEM images of the Pt1/NPC catalyst. The insets showing the enlarged images; (E), (F) HADDF-STEM image and enlarged image of Pt1/NPC catalyst; (G) The STEM bright field image of Pt1/NPC catalyst; (H) XRD patterns of Pt1/NPC and NPC; (I) TG cures of Pt1/NPC and NPC with a heating rate of 10 ℃ min-1 at air. To further investigate the chemical structure of the isolated Pt atoms, X-ray absorption spectra were recorded for the synthesized Pt1/NPC sample. Figure 3A shows the normalized XANES spectra for the sample at the Pt L3-edge. As observed, the obtained whiteline (WL) intensity of the sample is significantly higher than that obtained by a Pt foil (inset of Figure 3A), revealing more unoccupied 5d orbitals (5d5/2 or 5d3/2) of the isolated Pt relative to the metallic Pt foil.13 Moreover, the fourier transforms (FT) curves of the EXAFS oscillations were plotted to analyze the local atomic structure of the single-atom Pt. As shown in Figure 3B, the Pt foil displays a distinct Pt–Pt bonds at approximately 2.59 Å, corresponded to the metallic Pt. For comparison, the Pt1/NPC presents no such characteristic peaks derived from the Pt–Pt bonds but exhibits a dominant peak at about 1.93 Å, assigned to Pt–N bonds.30 It reveals the isolated Pt atoms should bond with N atoms. Moreover, the additional Pt L2-edge spectra can give more information about the formation of the Pt–N bonds at electronic-orbit level. As depicted in Figure 3C-D, the Pt single-atoms also present a much higher WL intensity compared with the Pt foil at the Pt L2edge. And also, there are clear Pt–N bonds and disappeared Pt–Pt bonds for the Pt1/NPC. These results are almost consistent with the L3-edge results (Figure 3A-B). However, the recorded WL intensity of Pt foil at L2-edge is very weak while the intensity of the isolated Pt in the Pt1/NPC is extremely strong, as shown in Figure 3C. It illustrates that the isolated Pt have much more vacant 5d3/2 orbits, compared with that of the metallic Pt with filled Pt 5d3/2 orbits by accepting electron


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transition from Pt 2p1/2 orbits.13 So, the increased unfilled Pt 5d3/2 orbits of the Pt single-atoms may be associated with the formation of Pt–N coordinations in the Pt1/NPC.

Figure 3. (A) The normalized XANES spectra at Pt L3-edge for the Pt1/NPC catalyst and Pt foil; (B) Fourier transform of EXAFS spectra at Pt L3-edge and corresponding R-space fitting curves for the Pt1/NPC catalyst and Pt foil. The insets show the atomic structure models for the Pt1/NPC and Pt foil; (C) The normalized XANES spectra and (D) Fourier transform (FT) of EXAFS spectra at Pt L2-edge for the Pt1/NPC catalyst and Pt foil; (E) High-resolution XPS Pt 4f patterns of the Pt1/NPC and a commercial Pt/C catalyst; (F) The nitrogen content of the Pt1/NPC and the NPC,

respectively.

Moreover, XPS spectra can be used to identify the valence of the single-atom Pt in the Pt1/NPC. As depicted in Figure 3E, the recorded Pt 4f XPS spectrum presents a strong peak of Pt2+ and a weak peak of Pt4+. No peaks assigned to metallic Pt0 has been observed for the sample. In addition, the relative content of the Pt2+ is much higher than that of the Pt4+. As evidenced by


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Figure S3, the content of the pyridinic and pyrrolic N is dominant, which is much higher than the other N species like oxidized N or graphitic N for the Pt1/NPC and the NPC samples, as shown in Figure 3F and Table S2. These above Pt 4f and N 1s XPS results imply that the predominant Pt2+ ions with a typical d8 electron configuration are possibly coordinated with pyrrolic or pyridinic N atoms to form a stable square-planar-type Pt–N4 configuration in this Pt1/NPC catalyst.31 To provide a strong evidence for the formation of the Pt-N4 bonds in the Pt1/NPC, the XAS data were fitted by using Pt-N4 structure model shown in Figure 3B inset. The obtained Rspace fitting curves and parameters are shown in Figure 3B and Table S3, respectively. The fitted curves for the first shell of Pt atoms, obtained at the R-space range from 1.5 to 3.5 Å, are in accordance with the experimental data shown in Figure 3B, revealing the formation of the Pt-N4 bonds in the Pt1/NPC. To further indentify the Pt–N4 coordinations, density functional theory (DFT) calculations were performed by us for the Pt1/NPC catalyst. Figure 4A and B present the optimized geometrical structures of the pyridine- and pyrrole-type Pt–N4 coordinations in this catalyst. Figure 4C and D give the partial density of states (PDOS) plots for the two Pt–N4 structures, respectively. Compared with the pyrrole-type Pt–N4, the pyridine-type structure shows more overlap areas between the d orbitals of Pt and p orbitals of N, as shown in Figure 4C and D. It reveals that the pyridine-type is more stable than the pyrrole-type Pt–N4 structure. This finding can be further confirmed by their binding energy difference (∆Eb = 0.13 eV, the detailed calculation in experimental section). The positive ∆Eb means that the pyridine-type Pt–N4 structure is more stable than the pyrrole-type. Besides, the calculated bond length of the Pt–N bonds in the pyridine-type Pt–N4 structure is 1.963 Å, which is closed to the result of FTNEXAFS (1.92 Å, see Figure 3B). Therefore, these DFT and FT-NEXAFS results give the clear


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evidences for the formation of the pyridine-type Pt–N4 coordination in the catalyst. Thus, the pyridine-type Pt–N4 may sever as the active sites for HER or ORR electrocatalysis.

Figure 4. Optimized geometrical structures for (A) pyridine-type Pt–N4 and (B) pyrrole-type Pt– N4. The unit of bond length is Å. Partial density of states for (C) pyridine-type Pt–N4 and (D) pyrrole-type Pt–N4. The dashed line at zero energy represents the Fermi level. 2.2 The activity for hydrogen evolution reaction The electrocatalytic activity of the synthesized Pt1/NPC catalyst for hydrogen evolution reaction (HER) was performed in N2-saturated 0.5 M H2SO4 aqueous solution. As depicted in Figure 5A, the catalyst exhibits an outstanding catalytic activity for the HER, which is substantially higher than a commercial 20% Pt/C or the pure NPC. At a polarization current density of -10 mA cm-2, the recorded potential for Pt1/NPC catalyst is -0.025 V (vs. RHE), which is more positive than that obtained by the Pt/C and other catalysts reported in literature.13, 15, 32


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Figure 5B gives the mass activities of the Pt1/NPC catalyst at an overpotential range from 0 V to 0.05 V. As observed, at the overpotential of 0.025 V, the recorded mass activity of the catalyst is 2.86 A mg-1Pt, which is 24 times greater than that of the Pt/C (0.12 A mg-1Pt). To study the intrinsic kinetics of the single-atom Pt catalyst, the electrochemical surface areas (ECSAs) are determined for the Pt-based samples, based on CO stripping voltammetry in Figure S4. The obtained ECSAs of the Pt1/NPC and the Pt/C are 121.1 m2 g,Pt-1 and 79.8 m2 g,Pt-1, respectively. Accordingly, the recorded the specific activity of Pt1/NPC catalyst is 2.4 mA cm-2, which is much higher than that of 20 wt% Pt/C catalyst (0.15 mA cm-2), at the overpotential of 0.025 V vs RHE (Figure 5C). It reveals that the intrinsic ORR kinetics over the single-atom Pt catalyst is faster than that of the Pt/C.

Figure 5. (A) The HER polarization curves of the Pt1/NPC, NPC, and 20% Pt/C catalysts recorded at a scan rate of 5 mV s-1 in N2-saturated 0.5 M H2SO4 solution. The inset shows the enlarged curves at the onset potential region of the HER; (B) mass activity. The inset shows the


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mass activity at 0.025 V vs RHE; (C) Specific activity. The inset shows the specific activity at 0.025 V vs RHE; (D) The Tafel plots of these catalysts; (E) The H2 turnover frequencies (TOFs); (F) The polarization curves recorded in N2-saturated 0.5 M H2SO4 before and after 3000 CV cycles. To explore the intrinsic kinetics process of the HER, Tafel slopes of these Pt-based catalysts were calculated and the obtained results are shown in Figure 5D. As observed, the Tafel slope of the Pt1/NPC catalyst is 28 mV dec-1, which is slightly lower than that of the Pt/C (31 mV dec-1). The low Tafel slop suggests that the HER process over this catalyst follows the Volmer–Tafel mechanism and the Tafel step is the rate-limiting step at low overpotentials.13, 33 From catalytic efficiency view, the H2 turnover frequency (TOF) was estimated to study the quantify of H2 product per unit time on per Pt site. At an overpotential of 0.025 V vs RHE, the TOF of the Pt1/NPC (per active site) is up to 2.93 s-1, which is almost 6 times higher than that of the state-ofthe-art Pt/C (0.55 s-1), as shown in Figure 5E. The remarkably enhanced HER catalytic efficiency can be attributed to the formation of the Pt–N4 coordination sites in this Pt1/NPC catalyst. In addition, the durability of the Pt1/NPC was evaluated by the accelerated durability test (ADT) for 3000 CV cycles in N2-saturated 0.5 M H2SO4 solution. As shown in Figure 5F, the catalyst shows a negligible negative shift (only 1 mV) at the current density of 100 mA cm-2, which is much lower than that for the Pt/C. The chronopotentiometric tests were also recorded under a constant current density of 20 mA cm-2 in N2-saturated 0.5 M H2SO4 for 10000 s, as shown in Figure S5. As a result, the catalyst displays an improved stability with respect to the Pt/C. The outstanding durability of the Pt1/NPC catalyst is related to the stable Pt–N4 structures that prevent the agglomeration and abscission of Pt active sites.13 Moreover, the Pt1/NPC catalyst


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also exhibits an enhanced HER activity, compared with the commercial Pt/C in an alkaline condition (0.1 M KOH), as shown in Figure S6. 2.3 The activity for oxygen reduction reaction More importantly, the ORR activity of the synthesized Pt1/NPC catalyst has been benchmarked in this work. As illustrated in Figure 6A, the catalyst shows surprising superior ORR activity to the commercial 20% Pt/C as well as the pure NPC catalyst in 0.1 M KOH solution. Among these catalysts, the Pt1/NPC displays the most positive half-wave potential (inset of Figure 6A), illustrating a high electrocatalytic activity for the ORR. This finding is evidenced by the largest Jk of 3.23 mA cm-2 for the Pt1/NPC catalyst shown in Figure S7, which is about 4.3 times larger than that of the Pt/C (0.77 mA cm-2), at a given potential of 0.9 V vs RHE. At this polarization potential, the determined mass activity (MA) of the Pt1/NPC catalyst is 0.17 A mg-1Pt, which is over 4 times higher than that of the Pt/C (0.04 A mg-1Pt), as shown in Figure 6B. Its specific activity is 0.14 mA cm-2, which is much higher than that of the Pt/C (0.05 mA cm-2), as shown in Figure 6C. Moreover, the recorded Tafel slop of the Pt1/NPC catalyst is 55 mV dec-1, which is smaller than that of the Pt/C (69 mV dec-1) shown in Figure 6D, revealing a faster ORR kinetics catalyzed by the atomic Pt atoms. The remarkably enhanced ORR activity is also verified by the cyclic voltammetry results shown in Figure S8. Although it is still in debate on whether Pt single-atoms are active for the ORR, this work confirms that Pt singleatoms can boost the ORR in an alkaline solution.


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Figure 6. (A) The ORR polarization curves of the Pt1/NPC, NPC and 20% Pt/C catalysts recorded at a rotation rate of 1600 rpm and a scan rate of 5 mV s-1 in O2-saturated 0.1 M KOH solution. The inset shows the half-wave potentials for the different catalysts; (B) ORR mass activity at different potentials of the Pt1/NPC and Pt/C. The inset shows the mass activity at 0.9 V vs RHE; (C) ORR specific activity. The inset shows the specific activity at 0.9 V vs RHE; (D) The Tafel plots for the above catalysts; (E) The electronic transfer numbers (n) and hydrogen peroxide yield (HO2- %); (F) The polarization curves recorded in O2-saturated 0.1 M KOH before and after 10K CV cycles. Moreover, the ORR pathway on the Pt1/NPC catalyst has been studied and the obtained electronic transfer numbers (n) and hydrogen peroxide yield (HO2- %) results are shown in Figure 6E. For the catalyst, the electronic transfer numbers are about 3.95, revealing a nearly 4electron pathway during the ORR.34 This conclusion can be proved by the negligible hydrogen peroxide yield (~2%) shown in Figure 6E. To give insight into catalytic durability of the catalyst,


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an accelerated durability test was performed in 0.1 M KOH solution. As a result, the assynthesized Pt1/NPC catalyst shows a negligible negative shift (only 7 mV) in linear sweep voltammetry after 10K cycles, revealing that the single-atom catalyst still shows an excellent stability during ORR shown in Figure 6F. For comparison, the commercial Pt/C shifts negatively more than 20 mV after 10K cycles under the same test condition, showing a poor stability relative to the Pt1/NPC catalyst. This conclusion is further confirmed by the results obtained by chronoamperometric measurement shown in Figure S9. Therefore, the good stability of the Pt1/NPC catalyst may be related to the presence of the stable Pt–N4 structure shown in Figure 4A. However, in 0.1 M HClO4, the Pt1/NPC catalyst displays a poor ORR activity, compared with the commercial Pt/C, as shown in Figure S10. The reason may be attributed to the singleatom Pt selectively catalyzing a two-electron ORR pathway producing H2O2 in acidic solutions, as reported by published papers.20, 21, 35-38 2.4 The origin of electrocatalytic activity of the Pt1/NPC In this work, based on the unique electronic structure of the isolated Pt atoms in form of the Pt–N4 coordinations (Figure 4A), we explored the origin of the enhanced HER and ORR activity of the as-synthesized Pt1/NPC in acidic or alkaline solution, as illustrated in Figure 7A and B. The HER proceeds by a Volmer–Tafel mechanism and the Tafel stage is the rate-limiting step at low overpotentials for the Pt1/NPC catalyst, according to the calculated Tafel slop from Figure 5D. In an acidic solution, the proposed overall HER process catalyzed by the catalyst is illustrated in Figure 7A. Two H+ ions in solution are reduced into a H2 gas molecule, through two initial Volmer steps (H+ + e- → H*) and a subsequent Tafel step (2H* → H2).39 During the initial Volmer steps, absorbed H+ ions are chemically bonded to the Pt in the form of Pt–H bonds. Afterward, during the Tafel reaction, the two protons bonded with Pt would combine and


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generate H2 dimer, leaving for the solution. For the Pt1/NPC, a strong electron transfer from the single Pt atoms to adjacent coordinated N atoms can contribute the H coverage on the Pt surface. Such electron transfer is evidenced by the increased binding energy of Pt 4f in the Pt1/NPC with respect to the metallic Pt as reference, as shown in Figure 7C. In this way, as depicted by Figure 3A and C, the Pt single-atoms with rich unoccupied 5d orbitals can contribute to the H coverage on the Pt atoms through the interaction between the Pt 5d and H 1s orbitals to form electron pairing and hydride.13 The higher H coverage on a Pt atom is more favorable to the Tafel reaction, because the increase in H coverage can efficiently decrease the adsorption strength of H2 species, as reported in literature.13, 39, 40 Therefore, the improved HER electrocatalytic activity of the Pt1/NPC should be ascribed to more unoccupied Pt 5d orbitals shown in Figure 3A and C, which can not only increase the number of chemically bonded H atoms but also decrease the adsorption energy of H2 product for HER, boosting the HER kinetics.


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ACS Catalysis

Figure 7. The proposed mechanism for (A) HER and (B) ORR catalyzed by Pt1/NPC; (C) Highresolution XPS Pt 4f patterns of the Pt1/NPC and Pt black; (D) the calculated Hirshfeld charges for the Pt1/NPC and Pt bulk. Moreover, for the Pt1/NPC catalyst, the increased unoccupied Pt 5d orbitals can also improve the ORR process. Figure 7B shows the overall ORR process catalyzed by the catalyst in an alkaline solution. Since the Pt is in form of the isolated state, the adsorption of active O2 usually follows side-on or end-on adsorption. For the end-on adsorption, the adsorbed O2 tends to be reduced to HO2- intermediate, which is disagree with the result of little hydrogen peroxide yield in Figure 6E. So, in this case, the active O2 must follows side-on adsorption on the isolated Pt atom.41 For this adsorption model, the isolated Pt atom with rich vacant 5d-orbitals can significantly facilitate the adsorption of O2, due to the back-donation from the antibonding π* orbital of active O2 to the vacant 5d-orbitals of Pt.42 However, the desorption of the OH- product from Pt-based catalysts to release the active sites is difficult, which has been identified as the rate-determining step in the ORR over Pt-based catalysts.42, 43 For this Pt1/NPC catalyst, the four N atoms in form of the Pt-N4 bonds show strongly negative charge state, caused by the strong electron transition from Pt and C atoms to these adjacent N species shown in Figure 7C. Such electron transition can be triggered by the higher electronegativity of N (3.04) than that of Pt (2.28) and C (2.55). DFT calculations can provide a direct evidence to the strong electron transfer from the Pt atoms to the coordinated N atoms, caused by the stable Pt–N4 coordinations formed in this Pt1/NPC catalyst. According to the Hirshfeld charge analysis, the calculated Pt atoms in the Pt1/NPC catalyst have a positive charge of 0.2494 |e|, while Pt atoms in bulk show a neglectable charge of 0.0062 |e|, as shown in Figure 7D. This outcome clearly indicates that there is a strong 22 ACS Paragon Plus Environment

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electron transfer from the isolated Pt atoms to the coordinated N atoms in this catalyst. As a result, these negatively charged N atoms around Pt atom can effectively repel the adsorbed OHspecies away from the Pt site through a lateral repulsion, caused by the increased negatively charged N atoms. It contributes to generate more exposed Pt sites for the catalyst, which would boost the ORR kinetics. Therefore, the synthesized Pt1/NPC (3.8 wt.%) shows a superior ORR activity to the commercial Pt/C (20 wt.%).

4. Conclusion In summary, well-defined Pt single-atoms on a nitrogen-doped porous carbon (Pt1/NPC) were successfully fabricated by a simple photochemical solid-phase reduction method. The proposed photo-chemical route can not only avoid agglomeration of precious metals that is typical of conditional wet chemistry methods but also is easier to access and potentially more scalable than the current ALD. Besides, the loading of the as-synthesized Pt single atoms on the carbon is very high (3.8 wt.%). These Pt single atoms can be used as bifunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) in acidic or alkaline solutions. It exhibits ultrahigh electrocatalytic activity and stability for HER in an acidic solution, which much higher those obtained by a commercial Pt/C (20 wt.% Pt). The catalyst also displays superior ORR activity and stability to the Pt/C in an alkaline solution. These remarkably improved electrocatalytic activities are attributed to the unique electronic structure of the Pt–N4 coordination sites in the Pt1/NPC. The formation of the stable Pt–N4 structure in this catalyst has been confirm by DFT calculation. This work provides a novel photochemical solid-phase reduction to design precious metal based (such as Pt, Ag, Au, Ir, Ru and Pd) single-atom catalyst as next generation electrocatalytic materials for fuel cells and hydrogen production.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional figures: Digital photograph; Supported characterizations by BET, XPS and electrochemical tests. Additional tables: Elemental compositions; Relative contents of N functionalities; Parameters of EXAFS fits. Additional notes: The turnover frequency; The electronic transfer numbers and hydrogen peroxide yield. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (Jingjun Liu); [email protected] (Feng Wang) Tel/Fax: +86-10-64411301 Author Contributions All authors have given approval to the final version of the manuscript. ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Funds of China (Grant No. 51572013, 51432003). The authors thank beam line 1W1B of Beijing Synchrotron Radiation Facility (BSRF) for providing the beam time. REFERENCES


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Photochemical Solid-Phase Synthesis of Platinum Single Atoms on

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