Ni2P Entwined by Graphite Layers as Low-Pt Electrocatalyst in Acidic

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Ni2P Entwined by Graphite Layers as Low-Pt Electrocatalyst in Acidic Media for Oxygen Reduction Ruihong Wang, Lei Wang, Wei Zhou, Yajie Chen, Haijing Yan, Zhiyu Ren, Chungui Tian, Keying Shi, and Honggang Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16167 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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ACS Applied Materials & Interfaces

Ni2P Entwined by Graphite Layers as Low-Pt Electrocatalyst in Acidic Media for Oxygen Reduction Ruihong Wang,ab Lei Wang,a Wei Zhou,a Yajie Chen,a Haijing Yan,a Zhiyu Ren,a Chungui Tian,a Keying Shi*a and Honggang Fu*a a

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the

People’s Republic of China, Heilongjiang University,150080 Harbin P. R. China. b State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, 130012, P. R. China. KEYWORDS:

Ni2P; graphite layers; low-Pt electrocatalyst; oxygen reduction; synergistic

effect; acidic media ABSTRACT: A simple and feasible strategy was reported to intentionally construct the Ni2P nanostructures entwined by graphite layers (Ni2P/GC). In this process, a commercial amino phosphonic acid chelating resin was adopted as both the phosphorus and carbon resources. Then, Ni2+ was introduced into the resin framework via the ionic exchange and chelation to form a resin-Ni2+precursor. After carbonization, the highly dispersed Ni2P particles, coupled with thin graphite layers were simultaneously synthesized in situ. A ternary 7.5%Pt-Ni2P/GC catalyst was further obtained by loading 7.5 wt% Pt on Ni2P/GC. For the oxygen reduction reaction in acidic

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media, the 7.5%Pt-Ni2P/GC catalyst exhibited even more positive onset (1.03 V) and half-wave (0.93 V) potentials, as well as a rather higher mass activity of 565.3 mA/mgPt and a better longterm stability than the commercial 20%Pt/C (JM) electrocatalyst. The improved reaction kinetics are mainly attributed to the synergistic effect between Pt and Ni2P/GC. This work not only provides a method for the synthesis of phosphides, but also gives insight into the synergy between Pt and Ni2P, which is helpful for the development of more low-Pt catalysts in acidic media. 1. INTRODUCTION The oxygen reduction reaction (ORR, O2 + 4H+ + 4e = 2H2O in acid) is a crucial process occurring at the cathode of polymer electrolyte membrane fuel cells (PEMFCs).1,2 Platinum (Pt) has long been considered as the most effective electrocatalyst for the ORR. However, prohibitive costs and insufficient durability make the widespread commercialization of fuel cells unrealistic.3 Considerable research efforts have been concentrated on developing alternative non-Pt or lowPt, which are both important strategies for the economic viability of fuel cells.4 Desirably, Pt could be replaced entirely, and use the non-precious metals catalysts made from abundant and inexpensive elements, such as metal-nitrogen/carbon compounds5-7, metal oxide/oxysalt8,9, metal-organic-frameworks10 or carbons could be used.11,12Although these non-precious metal alternatives display activities close to or even better than that of Pt in alkaline electrolyte, most have achieved neither an activity nor a stability as good as the Pt-based catalyst in acidic environments. In fact, the current commercially available applications of fuel cells technology (such as vehicles, portable electronics etc.) are mainly based on proton exchange membranes in which an acidic electrolyte is a key component, and the commercial implementation of an alkaline membrane remains insufficient. 13-15


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Given the above challenge, many research efforts have been focused on improving the catalytic activity of Pt-based catalysts in acidic media with less Pt loading. Alloying Pt with transition metals has been attempted, and the electronic structure of the Pt surface is modified by the underlying alloy, leading to improved ORR activity. 16,17 Another powerful solution is to use certain transition metal interstitial compounds as co-catalysts, such as transition metal carbides (TMCs) or transition metal nitrides (TMNs) due to their Pt-like catalytic properties and excellent corrosion resistance under fuel cell operating conditions.18-20Catalysts composed of Pt/TMC or Pt/TMN are reported to show better ORR activity and stability than conventional Pt/C catalysts. The intense synergistic effect between Pt and interstitial compounds was proposed as the essential reason for the promoted catalytic performance.21-25 Additionally, we explored various synthesis strategies to prepare interstitial compounds, such as WC,26,27 WN,28-30 Mo2C,31 Mo2N,32 and VN,33 in our previous work and we investigated their interactions with the Pt catalyst. These results reliably indicated that the co-catalysts can effectively improve the utilization efficiency of Pt and substantially reduce the use of the noble metal. Transition metal phosphides (TMPs) are important metal interstitial compounds and are also candidates to replace Pt. They possess similar structures to those of carbides and nitrides.34,35 In electrocatalysis, TMPs predominantly emerge as the high-performance catalysts for the hydrogen evolution reaction (HER).

36-38

Among TMPs, nickel phosphide (Ni2P) exhibits attractive

electrocatalytic activity and long-term stability towards the HER in acidic medium, and it is even competitive with Pt.

39,40

Recently, Prof. Xing and co-workers discovered the effective co-

catalytic ability of Ni2P as an anode catalysts of fuel cells. 41,42 This finding is a significant step towards developing TMPs as co-catalysts in electrochemistry. However, the current attention placed on TMPs as co-catalysts is less than that for TMCs and TMNs, and the attention is


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primarily focused on the anode catalysts.43-48 To date, the effect of TMPs on promoting the oxygen reduction reaction at the cathode has not been well established. To study the TMP effect on ORR at the cathode, we designed and prepared Ni2P nanoparticles (NPs) entwined by graphite layers (Ni2P/GC) to investigate the co-catalytic ability of Ni2P/GC towards ORR in acid solution and its synergistic effect with Pt. The purpose of hybridizing Ni2P with graphitic carbon (GC) is to enhance the conductivity and corrosion resistance of the carrier. The fabrication procedure reported here is scalable, simple and inexpensive, and it involves only two reagents: nickel chloride hexahydrate (NiCl2·6H2O) as the nickel resource, and a commercial amino phosphonic acid macro porous chelating resin as both the phosphorus and carbon resources. Other organic reagents, such as trioctylphosphane, trioctylphosphane oxide, or any other surfactant, are unneeded.

Scheme 1. The synthesis route of Ni2P/GC The synthetic scheme is given in Scheme 1. Originally, Ni2+ was introduced into a commercial amino phosphonic acid chelating resin by electrostatic force and group coordination. Subsequently, Ni2+ was strongly coordinated by charged oxygen-containing or/and uncharged


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nitrogen-containing ligands to form the stable resin-Ni2+ composite. Owing to the plentiful and uniform pore tunnels in the resin, Ni2+ could be well distributed and anchored inside of the resin. In the following pyrolysis, the resin provided a phosphorus resource to form Ni2P in situ. Meanwhile, the resin carbon around the Ni2P was catalyzed into graphitic carbon. The synthesis was facile and highly reproducible in a production batch. A Ni2P/GC-supported Pt catalyst (7.5%Pt-Ni2P/GC) was fabricated with a Pt loading of 7.5 wt%. The electrocatalytic performance of Ni2P/GC, 7.5%Pt-Ni2P/GC, 7.5%Pt/C and commercially available 20%Pt/C (JM) towards ORR in acid was investigated, respectively. The results demonstrated that the electrocatalytic activity of Pt was evidently promoted by the introduction of Ni2P/GC. The as-synthesized 7.5%Pt-Ni2P/GC electrocatalyst exhibited a more positive onset potential (1.03 V) and half-wave potential (0.93 V) as well as a higher limiting current and stability than the commercial 20%Pt/C catalyst. 2. EXPERIMENTAL METHODS 2.1.Synthesis of Ni2P/GC and 7.5%Pt-Ni2P/GC For the synthesis of the resin-Ni2+ precursor, 10 g of amino phosphonic acid chelating resin and 250 mL of 0.15 mol L-1 NiCl2 solution were adopted without additional purification. The Ni2P/GC composite was finally obtained by carbonizing the resin-Ni2+ precursor at 1000 °C. The detailed preparation procedure was described in our previous work.27,31 For comparison, the bare resin carbon was also prepared via directly carbonizing the amino phosphonic acid chelating resin at 1000 °C. To fabricate the 7.5%Pt-Ni2P/GC catalyst, the conventional NaBH4 reduction method was adopted.27, 31, 49 The overall weight percentage of Pt was confirmed by inductively coupled plasma (ICP) analysis. The resin carbon-supported 7.5 wt% Pt catalyst (7.5%Pt/GC) was


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also synthesized in the same way. 2.2 Material Characterization X-ray diffraction (XRD) patterns were performed on a RigakuD/max-IIIB diffractometer with an accelerating voltage of 40 KV. Transmission electron microscopy (TEM) experiments were performed by a JEOL JEM-2100 electron microscope with a 200-kV acceleration voltage. The Raman spectrum was measured on a Jobin Yvon HR80000. N2 adsorption-desorption isotherms were taken on a Tristar 3000 adsorption instrument. X-ray photoelectron spectroscopy (XPS) was tested on a VG ESCALAB MK II with an achromatic Mg Kα X-ray source. X-ray absorption fine structure (XAFS) at the Pt L3-edge and Ni K-edge were operated at beam line BL14W1 of the Shanghai Synchrotron Radiation Facility (SSRF), China. The data processing and fitting were performed via the ATHENA and ARTEMIS programs, respectively.50 2.3 Electrochemical study The electrochemical study was conducted at PINE electrochemical workstation by a standard three-electrode system, in which a rotating ring-disk electrode (RRDE) was used as the working electrode, and a Pt flake and reversible hydrogen electrode (RHE) were adopted as the counter and reference electrodes, respectively. The catalyst ink was made as follows: 5 mg of 7.5%PtNi2P/GC catalyst was mixed with a Nafion solution (0.5 mL, 0.5 wt%) and then dispersed in 1.5 mL of isopropanol. After ultrasonicating for 30 min, a 30-µL volume of the mixture was loaded onto a polished RRDE substrate. All the electrochemical measurements were obtained in O2saturated 0.1 M HClO4, and the potentials are given versus a reversible hydrogen electrode (vs. RHE). The temperature was controlled at 25°C in an electro-thermostatic water bath.


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The electron transfer number (n) and percentage of peroxide (H2O2%) are calculated via Equations (1, 2). The jd is the disk current, jr is the ring current, and N is the current collection efficiency of the ring-risk electrode (0.37). (1)

(2)

For the kinetic current (jk) calculation, the Koutecky–Levich equation is described as follows (Equation 3). (3)

The j represents the experimentally measured current and jlim represents diffusion-limiting current. Also the number of electron transfer can be calculated based on Equation 3 and Equation 4. (4)

The F is Faraday constant (96,485 C mol−1), C0 is oxygen solubility (1.6 × 10−3 mol L−1), D0 is oxygen diffusivity (1.1 × 10−5 cm2 s−1), ν is electrolyte kinematic viscosity (1.00 × 10–2 cm2s–1).


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3. RESULTS AND DISCUSSION 3.1.Characterization of Ni2P/GC and 7.5%Pt-Ni2P/GC

Figure 1. (a, b) The low-magnification TEM and (c, d) HRTEM images of Ni2P/GC, and the inset is the FTIR image of the Ni2P/GC sample Figure 1a and b illustrate the low-magnification TEM images. Ni2P nanoparticles (NPs) are found to be well-dispersed on the carbon carrier owing to the spatially confined effect of the resin. Figure S1 provides the particle size distribution of Ni2P. The average grain diameter is calculated to be 51.7 nm. In fact, the representative amino phosphonic acid chelating resin is composed of macropores and a homogeneous cross-linked structure modified by the ligands and exchanged ions.51 This macroporous structure could provide convenient channels for Ni2+ ions to move into the interior of the resin. Meanwhile, the ligands and exchanged ions enable Ni2+ to


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become strongly embedded and uniformly distributed in the resin framework because of group coordination and coulomb forces, respectively. Subsequently, being subjected to carbonization, the resin acted not only as the in situ phosphorus source and carbon source but also as the bonding medium, limiting the growth of the Ni2P NPs and preventing their aggregation. A closer look at Ni2P/GC is provided from the HRTEM images (Figure 1c and d). For a typical nanoparticle, the neat lattices with d=0.17 nm and 0.22 nm are consistent with the (300) and (111) planes of the Ni2P phase, respectively. A single Ni2P particle was entwined around by six to eleven graphitic layers with a typical crystalline graphite (002) lattice (d=0.34 nm). Furthermore, FTIR image of Ni2P/GC in the inset further confirms the multiphase co-existence. Notably, these Ni2P particles are entwined rather than fully covered by graphite layers. This structure is likely to offer the following advantages: First, in an acidic medium, the graphite layers could stabilize the Ni2P particles, preventing their aggregation and migration. Second, graphitic carbon could improve the conductivity of the catalyst, strengthening the electron transport between Ni2P and Pt, which is beneficial for the charge-transfer process in the ORR. More critically, the exposed Ni2P NPs may provide more active sites to form the Ni2P-Pt contacts to help realize the highest potential co-catalytic ability of Ni2P.


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Figure 2. (a) The XRD pattern and (b) Raman spectra for Ni2P/GC Figure 2 shows the XRD pattern of Ni2P/GC. A set of prominent diffraction peaks at 2θ = 30.5°, 31.9°, 40.8°, 44.6°, 47.4°, 54.2°, 55.1°, 66.4°, 69.9°, 72.6° and 74.7° were consistent with the (110), (011), (111), (021), (210), (300), (211), (310), (221), (311) and (212) crystal planes of hexagonal Ni2P, demonstrating the highly crystalline character of Ni2P. The diffraction peak of graphite (002) at 2θ = 26.2° was also detected when the intensity was increased 10-fold. Raman spectroscopy provides the degree of graphitization of carbon. As displayed in Figure 2b, the intensity ratio between the G-band (1575 cm-1) and D-band (1360 cm-1) peaks (IG/ID) is 1.38 for Ni2P/GC, implying the good graphitization of carbon in the Ni2P/GC composite.52 In addition, the symmetric 2D-band at 2722 cm-1 is in accord with graphitic carbon in the literature.53 These results further indicate the successful preparation of Ni2P and graphitic carbon. The contents of different elements in the Ni2P/GC composite were determined by energy dispersive X-ray spectroscopy (EDX) analysis, in which the relatively content of Ni, P, O, N, and C were 63.50 wt%, 25.24 wt%, 2.95 wt%, 2.94 wt% and 5.36 wt%, respectively (Figure S2). To further investigate the evolution of Ni2P/GC, Figure S3 shows the XRD patterns and Raman spectra of


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the products by carbonizing resin-Ni2+ precursor at different temperatures. Originally, with carbonization temperature rising to 650°C, the Ni12P5 phase was detected as the major product together with the minor Ni2P phase. As the temperature increased to 700°C, the intensity of Ni2P increased, and the diffraction peak of Ni12P5 notably declined. The peak intensity of Ni2P was further improved as the temperature was increased further. The XRD results show that at a lower temperature, Ni12P5 primarily formed due to the lack of P and the possible loss of P during pyrolytic process.54 At higher temperatures, more P species were available originating from breaking of the P-O bond, thus, forming the Ni2P phase. The Raman spectra in Figure S3b present the intensity ratio of IG/ID rises with the increasing carbonizing temperature, suggesting gradually enhanced graphitization. To study the effect of the carbonizing temperature on the specific surface area and pore volume, the N2 adsorption/desorption isotherms and pore size distributions of the samples are displayed in Figure S4 and Table S1. All the samples exhibit similar H3-type hysteresis loops with a slit-shaped mesoporous structure and broad pore size distribution (insets). From the data in Table S1, when the temperature rose from 600 to 900°C, the specific surface area and pore volume obviously improved due to the decomposition of the resin matrix and organic functional groups. However, as the temperature rose to 1000°C, the specific surface area and pore volume began to decrease since the crystallization of both Ni2P and carbon tends to be more defect-free. These results reveal that the in situ and simultaneous synthesis route could not only realize a facile synthesis for highly dispersed and well-crystallized Ni2P particles entwined with graphite layers but also construct large accessible interfaces, which are especially beneficial for the mass transfer process in the ORR. Among all the samples, Ni2P/GC formed by carbonizing the resin-Ni2+ precursor at 1000°C displays the large BET


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specific surface area (396.81 m2/g) and total pore volume (0.50 cm3g-1) as well as the high graphitization.

Figure 3. (a) The XPS spectra of Ni 2p and (b) P 2p in Ni2P/GC Next, XPS was measured to further study the chemical composition and valence states of the Ni2P/GC. Focusing initially on the XPS spectrum of Ni 2p (Figure 3a), two peaks at 853.4 eV and 871.1 eV are respectively identified as the Ni 2p3/2 and Ni 2p1/2 for Niδ+ in Ni2P. Meanwhile, the peaks at 856.5 eV and 874.5 eV are assigned to Ni 2p3/2 and Ni 2p1/2 for Ni2+ in the nickel oxides due to the superficial oxidation of Ni2P.55 There is no significant signal for nickel oxides in XRD, suggesting a low amount of nickel oxides or amorphous characteristics in the surface of Ni2P/GC. The satellite peaks of Ni at 861.1 eV and 879.6 eV are also detected. Two peaks for P 2p at 130.2 eV and 134.7 eV are exhibited, corresponding to Pδ- in Ni2P phase and P5+ in surface phosphorus oxides or phosphate.56 The XPS spectra of C1s, N1s and O1s are also given in Figure S5. Peak with bond energy of 285.7 eV at C1s XPS spectra is ascribed to the carbon in C-N bond.57 The formation of C–N bonds should be attributed to the reaction of the amino groups (-


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NH-) with the carbon in resin. The fact indicates that during the pyrolysis process, nitrogen could be simultaneously doped in graphitic carbon. The XPS N1s spectrum in the supporting information further confirms the formation of doped-N in Ni2P/GC composite. Therefore, combining the results of XRD, Raman, TEM and XPS, we can conclude that the Ni2P/GC hybrid was successfully fabricated. Originally, in the pyrolysis procedure, the resin acted as the in situ phosphorus source in the formation of Ni2P. Simultaneously, the resin carbon around the Ni2P NPs was catalyzed into graphite, and nitrogen was incorporated into the carbon phase. Hence, the as-obtained Ni2P/GC composite exhibit several desirable features, including the combined functionalities of Ni2P, GC and doped-N, a high BET surface area and porous structures. Therefore, Ni2P/GC is expected to play an important role in enhancing the catalytic activity of Pt catalysts toward ORR.

Figure 4．．(a)TEM (b, c) HRTEM images of 7.5%Pt-Ni2P/GC (d) XPS spectra of Pt 4f


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After loading 7.5 wt% Pt on the Ni2P/GC hybrid, a ternary catalyst 7.5%Pt-Ni2P/GC was obtained. The weight percentage of Pt was measured by ICP. Figure S6a shows the lowmagnification TEM image. It was found that there were densely dotted Pt NPs deposited on the Ni2P/GC surface on a large scale. The distribution of Pt particles was rather uniform, implying the Ni2P/GC hybrid was helpful for supporting the Pt NPs, providing a high density of available active sites for the electrocatalytic reaction. Figure S6b and 6c exhibit that the particle size of Pt is 2-3 nm. Energy dispersive X-ray measurements (EDX) were performed to determine the spatial distribution of elements in 7.5%Pt-Ni2P/GC. Figure S7 exhibits scanning transmission electron microscopy (STEM) in bright-field and EDX mapping. All elements, including Pt, Ni, P, C, N and O, were recorded and are shown to be uniformly dispersed. For detailed structural information, we further characterized 7.5%Pt-Ni2P/GC by using high-magnification TEM (Figure 4a) and HRTEM (Figure 4b and 4c). In Figure 4a, the larger particles are the typical Ni2P while the relatively small particles with sizes of 2-3 nm are Pt NPs. Figure 4b and c show the Pt particles supported on Ni2P/GC are high crystalline with clear lattice fringes of Pt (111). They are located on either the Ni2P particles or the graphite layers. It should be noted that the lattice width of Pt (111) (d= 0.23 nm) is very close to that of Ni2P (111) (d=0.22 nm), and the obvious interface connection between Ni2P (111) and Pt (111) is identified, implying their direct and strong interaction. 3.2 Further insights into the interaction between Pt and Ni2P/GC Following, all the X-ray spectra including XRD, XPS and XAFS were adopted to investigate the crystalline phase, valence states and coordination environment of the ternary 7.5%PtNi2P/GC catalyst. As shown in Figure S8a, for the 7.5%Pt/GC sample, the XRD peaks at 39.8°, 46.2° and 67.2° are assigned as the (111), (200) and (220) diffraction peaks for face-centered


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cubic Pt, respectively. However, for 7.5%Pt-Ni2P/GC, no distinct property of the fcc Pt can be identified. The strong peaks of Ni2P (111) and Ni2P (210) may cover or integrate the peaks of Pt, forming the broadened peaks at approximately 40° and 46°. This result may be ascribed to the low amount of Pt loading and small size as well as the much stronger diffraction peak for Ni2P. Figure S8b shows the XPS wide scan spectrum of 7.5%Pt-Ni2P/GC, further confirming the presence of Pt in the composite. The XPS high-resolution spectrum of Pt 4f is exhibited in Figure 4d. Evidently, the Pt 4f peaks for 7.5%Pt-Ni2P/GC are shifted to the lower binding energies of approximately 0.33 eV compared with 7.5%Pt/GC sample. The negative shift should be due to the partial electron transfer from Ni2P to Pt. Correspondingly, from Figure S9, an obvious enhancement of the Ni2+ XPS peak was detected in the 7.5%Pt-Ni2P/GC sample, suggesting more highly charged nickel ions were formed on the Ni2P surface after loading Pt NPs. This results from the electron donation from Ni2P to Pt and finally induces a strong electronic interaction between Pt and Ni2P.

Figure 5．．(a) Pt L3-edge XANES spectra of Pt foil, 7.5%Pt-Ni2P/GC and 7.5% Pt/C. (b) Pt L3edge EXAFS spectra after Fourier transform for all samples The interaction between Pt and Ni2P/GC is further investigated by XANES and EXAFS. Figure


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5a presents the normalized XANES spectra of 7.5%Pt-Ni2P/GC, 7.5%Pt/C and Pt foil at Pt L3 edges. It is found that both nano-Pt and bulk-Pt possess the analogous resonance patterns above L3 edge, which provides additional proof of the successful loading of Pt particles. It is well known that the first resonance at the edge is commonly recognized as a white line arising from the 2p → 5d dipole transition.58,59 For 7.5%Pt-Ni2P/GC, the white line is broadened to a higher energy than those of Pt foil and 7.5%Pt/C, demonstrating that the Pt coordination environment was evidently affected in the presence of Ni2P/GC. The Fourier transforms (FT) of the EXAFS spectra at Pt L3-edge for 7.5%Pt-Ni2P/GC, 7.5%Pt/C and Pt foil are displayed in Figure 5b. The two peaks in the region of 2.0~3.2 Å are corresponding to Pt-Pt coordination shells. And the unique peak at 1.7 Å in 7.5%Pt-Ni2P/GC can be assigned to the Pt–P coordination. Table1. Fitting parameters of Pt EXAFS spectra Sample

Shell

N[a]

R(Å)[b]

σ2(×10-3 Å2)[c]

∆E0(eV)[d]

Pt foil

Pt-Pt

11.0±0.6

2.76±0.01

3.8±0.6

7.7±0.4

7.5% Pt/C

Pt-Pt

9.7 ± 0.8

2.75 ± 0.01

5.8 ± 0.5

6.4

7.5% PtNi2P/GC

Pt-Pt

5.0 ± 0.4

2.76 ± 0.02

4.5 ± 1.6

7

Pt-P

2.0 ± 0.5

2.30 ± 0.02

3.4 ± 2.2

4.6

[a] coordination number; [b] bond distance; [c] Debye–Waller factor; [d] inner potential correction.

Accordingly, the structural parameters such as the coordination number (N), bond distance (R), Debye-Waller factor and inner potential correction were derived from the EXAFS curve-fitting of Pt L3-edge as summarized in Table 1. The Pt-P coordination number (NPt-P) is calculated to be 2.0 ± 0.5. These observations may result from the direct interaction between Pt and Ni2P, which agrees with the TEM and XPS results. Additionally, it was found that the NPt-Pt in 7.5%PtNi2P/GC is 5.0 ± 0.4, which is smaller than that of 7.5%Pt/C (NPt-Pt =9.7 ± 0.8). This result indicates that the Pt NPs supported on Ni2P/GC possess much smaller sizes, which would lead to


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more Pt active sites available for the ORR. In addition to the Pt L3-edge, XANES, EXAFS data of Ni K-edges and fitted structural parameters are plotted in Figure S10 and Table S2. As shown in Figure S10a, the pre-edge peaks at 8334 eV corresponding to the 1s → 3d dipole transition are similar for both Ni2P/GC and 7.5%Pt-Ni2P/GC. However, in the case of 7.5%Pt-Ni2P/GC, an obvious absorption peak at approximately 8351 eV was detected, possibly resulting from the presence of more highly charged nickel, which agrees with the result of XPS. This viewpoint could be further emphasized by the EXAFS analysis of the Ni K-edges. In Figure S10b, the two main peaks at 1.8 Ǻ and 2.3 Ǻ belong to the Ni-P and Ni-Ni contributions, respectively. There is no apparently change in the Ni-P coordination number in 7.5%Pt-Ni2P/GC (NNi-P=3.5 ± 0.9) and Ni2P/GC (NNi-P=3.4 ± 0.3). However, the Ni-Ni contributions in 7.5%Pt-Ni2P/GC (NNi-Ni= 3.5 ± 0.7) were much lower than that in Ni2P/GC (NNi-Ni=5.4 ± 0.4) due to the change in the Ni coordination environment on the surface. In short, the above X-ray spectra, including the XAFS and XPS investigations, provide evidence that the dominant local structure of Ni2P/GC remained unaltered after supporting Pt particles; nevertheless, the coordination environment of both Pt and Ni on 7.5%Pt-Ni2P/GC surface was significantly changed due to the intensive interaction between Pt and Ni2P/GC. The unique composition and structure profile make 7.5%Pt-Ni2P/GC more appropriate for ORR. 3.3. Electrochemical Tests To investigate the catalytic activity of 7.5%Pt-Ni2P/GC towards ORR, ring-disk electrode (RRDE) experiments were tested in an O2-saturated HClO4 solution. The pure Ni2P/GC, 7.5%Pt/GC and commercial 20%Pt/C catalysts were also measured for comparison. As shown in Figure S11, Ni2P/GC alone exhibited some activity in acidic electrolyte with an onset potential of nearly 0.5 V, which is not competitive with Pt-based catalysts. Likewise, neither the onset


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potential (Eonset=0.97 V) nor the half-wave potential (E1/2=0.85 V) of the 7.5%Pt/GC catalyst is advantageous because of the low amount of Pt catalyst. In Figure 6a, a conventional 20%Pt/C (JM) catalyst shows an outstanding catalytic activity towards ORR, including the positive Eonset (1.00 V) and E1/2 (0.91 V). Still, it is noteworthy that the as-synthesized 7.5%Pt-Ni2P/GC catalyst

Figure 6 (a) RRDE voltammograms on 7.5%Pt-Ni2P/GC and 20% Pt/C(JM) in an O2-saturated 0.1 M HClO4 electrolyte with scan rate of 5 mV/s and rotating speed of 1600 rpm. (b), (c) Electron transfer number (n) and percentage of peroxide (H2O2%) at various potentials. (d), (e)


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ORR polarization curves on 7.5%Pt-Ni2P/GC and 20%Pt/C (JM) with rotation rate from 400 to 2025 rpm; Insets are Koutecky–Levich plots (j–1 vs. ω–1/2) analyzed at 0.8 V and 0.9 V. (f) Corresponding Tafel plots for 7.5%Pt-Ni2P/GC and 20% Pt/C(JM).

displays a more excellent electrocatalytic performance than the 20%Pt/C (JM) catalyst, taking into account the more positive Eonset (1.03 V) and E1/2 (0.93 V). The parallel measurements based on the 7.5%Pt-Ni2P/GC catalyst have been taken in order to obtain the stable ORR activity with good reproducibility. The ORR polarization curves and the statistical analysis on experimental data were summarized in supporting information（Figure S12 and Table S3 ). The average values of Eonset, E1/2 and jlim were 1.035 V, 0.924 V and 5.783 mA/cm2 respectively via twentyfour parallel tests. The enhanced ORR onset potential and half-wave potential for 7.5%PtNi2P/GC is a straightforward evidence revealing the important co-catalytic effect of Ni2P/GC. In addition, the limiting current for 7.5%Pt-Ni2P/GC (5.70 mA cm-2) is very near to that of the 20% Pt/C catalyst (5.28 mA cm-2). Therefore, a much lower amount of Pt is needed for the 7.5%PtNi2P/GC catalyst to reach the equivalent current density of the tradition Pt/C catalyst, thus substantially reducing the noble metal use. Following, the number of electron transfer (n) and the percentage of peroxide (H2O2%) in the full potential range are calculated according to the experimental ring and disk current based on Equations 1 and 2.60 As shown in Figure 6b, the average n values for 7.5%Pt-Ni2P/GC and 20%Pt/C(JM) are 3.95 and 3.84, respectively. Figure 6c shows the H2O2 yield on 7.5%Pt-Ni2P/GC is below 6.1%, which is lower than the yield (9.6%) on 20%Pt/C (JM). The facts indicate the ORR mechanism on both 7.5%Pt-Ni2P/GC and 20%Pt/C follows a direct 4e- pathway along with the efficient reduction of O2 to H2O. Subsequently, Figure 6d and 6e recorded the polarization curves of 7.5%Pt-Ni2P/GC and


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20%Pt/C (JM) at different rotation rates. Apparently, the current density increased with the accelerated rotation rate, demonstrating a higher catalyst turnover rate for ORR at catalysts. Insets present the corresponding Koutecky-Levich plots at 0.8 V and 0.9 V, which display the linear relation between j–1 and ω–1/2. The transferred electron number (n) can also be calculated by Equations 3 and 4. Finally, the calculated n values for 7.5%Pt-Ni2P/GC and 20%Pt/C are 3.92 and 3.89 respectively, which agree with the values obtained from RRDE measurements. As far as the kinetic current densities (jk) is concerned, it can be calculated by Koutecky-Levich equation (Equations 3).61 After normalization of kinetic currents to the loading amount of Pt, the mass activity of 7.5%Pt-Ni2P/GC at 0.9 V was found to be 565.3 mAmg–1Pt, which is 5.51 times greater than that of 20%Pt/C (102.6 mAmg–1Pt). The fact indicates the greatly improved electrocatalytic activity of ORR on 7.5%Pt-Ni2P/GC. Figure 6f displays the Tafel plots extracted from the ORR polarization curves of the 7.5%Pt-Ni2P/GC and 20% Pt/C(JM) in the overpotential range of 0.80-1.05 V. Two Tafel linear regions (i) low current density (potential range 0.85-1.05 V, near onset of the ORR) and (ii) high current density (potential range 0.80-0.85 V) were assigned, where the values of Tafel slopes were estimated. As a result, the Tafel slopes for 7.5%Pt-Ni2P/GC are calculated to be 62.5 mV/dec at the low current density and 101.3 mV/dec at the high current density. The values for 20%Pt/C are 67.9 mV/dec and 92.1 mV/dec, respectively. These results are consistent with the representative values of Tafel slopes for Pt catalysts in reported work17, 29, 62, 63 and confirm the similar ORR mechanization on 7.5%PtNi2P/GC and 20%Pt/C. A certain difference of Tafel slopes was found. We suppose this fact is associated with an influence of the porous structure of Ni2P/GC support. To further understand the improved ORR activity on 7.5%Pt-Ni2P/GC, the oxygen temperature-programmed desorption (O2-TPD) was adopted to examine the adsorption ability of


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O2 on different catalysts, since the ORR kinetics are largely decided by the free sites available for O2 adsorption.64 The experiment was conducted in a homemade flow apparatus,65 and the detailed operation procedure is described in the supporting information. As shown in Figure S13, the TPD profiles of all samples exhibit the distinct O2 desorption peaks under 550 °C, which corresponds to the adsorbed oxygen on catalyst surface.66 In comparison, the desorption peak on Ni2P/GC occurred at nearly 345 °C that is higher than that on 20%Pt/C (250 °C), indicating a stronger O2 adsorption ability on Ni2P/GC surface. More importantly, the O2-TPD profile of 7.5%Pt-Ni2P/GC features two obvious peaks located at 265 °C and 345 °C, respectively, along with more higher desorption intensity. This result presents the fairly better O2 capture ability on 7.5%Pt-Ni2P/GC that finally contributes to the remarkable ORR activity. Although the nature and mode of oxygen adsorption remained unsolved due to the limitation of TPD technique, it provided a qualitative comparison for the O2 adsorption−activity relationship and further revealed the important role of Ni2P/GC in enhancing ORR activity and reducing the usage of Pt. In addition to the free sites available for O2 adsorption, the electronic state of the Pt catalyst prominently influences the catalyst activity. As investigated by XPS, the Pt 4f peak shifted to a lower binding energy in the 7.5%Pt-Ni2P/GC catalyst, meaning that a partial electron transfer may occur from Ni2P to Pt. This electron transfer would modify the surface electronic character of the Pt, which facilitates the oxygen capture on the Pt active sites, meanwhile, weakening the adsorption energies of intermediates on Pt surface. Finally, the accelerated ORR kinetics of the 7.5%Pt-Ni2P/GC catalyst are associated with the excellent electrical conductivity of the graphite layers, the high BET and the appropriate porous structures, which provided a favorable path for electron transfer and mass transfer, and thus affected the overall activity of the catalyst.


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Figure 7 (a), (c) RDE voltammograms of 20%Pt/C(JM) and 7.5%Pt-Ni2P/GC in an O2-saturated 0.1 M HClO4 electrolyte with a rotating speed of 1600 rpm initial (solid line) and after (dotted line) a 10000-cycle durability test; (b), (d) Cyclic voltammograms of the 20%Pt/C(JM) and 7.5%Pt-Ni2P/GC catalysts for the initial (solid line) and last (dotted line) cycle in an O2-saturated 0.1 M HClO4 electrolyte at 50 mV/s. In addition to the catalytic activity, the durability of the catalyst is another crucial factor. Although Pt is recognized as being stable over a wide potential range, corrosion of the electrocatalyst occurs when the cycling is accelerated at the electrode potential in an acidic electrolyte. As shown in Figure 7, on oxidative cycling, the definite descending of the intensities and shift of the half-wave potential are observed for both 20%Pt/C and 7.5%Pt-Ni2P/GC, which is associated with a decrease of the ECSA of catalysts. For 20%Pt/C catalyst, the E1/2 degrades by 39 mV and the loss of ECSA was approximately 37%. Contrastively, the E1/2 degrades by 22 mV and the loss of ECSA was 21% on 7.5%Pt-Ni2P/GC. The huge decline of electrocatalytic


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activity and ECSA for 20%Pt/C is mainly caused by the agglomeration of Pt particles and the deterioration of carbon support. The improved stability for 7.5%Pt-Ni2P/GC catalysts is attributed not only to the stability of the components (Ni2P, graphitic carbon, Pt) individually but also to the interactions between Ni2P/GC and Pt that effectively reduces the passivation of the Pt particles. 4. CONCLUSIONS In summary, we designed and synthesized a Ni2P/GC composite by an in situ and simultaneous route. As a co-catalyst, Ni2P/GC provided the active sites for O2 adsorption, the appropriate BET, high conductivity and mesoporous structure for mass and charge transport. Moreover, the strong interaction between the Ni2P and Pt enables their synergistic effect to be played fully. Based on these unique electronic and structural properties, the 7.5%Pt-Ni2P/GC composite exhibits improved catalytic activity and stability superior compared to a commercial 20%Pt/C catalyst and substantially reduces the required loading amount of Pt. Finally, this work offers a strategy for the rational design and development of a novel low-Pt ORR catalyst using low-cost starting materials and a facile synthesis procedure.

ASSOCIATED CONTENT Supporting Information：： The supporting information is available free of charge on the ACS Publications website, including EDX, XRD, Raman, XPS, TEM, XAFS, LSV, TPD and N2 adsorption-desorption isotherms. Corresponding Author：：


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Honggang Fu* E-mail: [email protected],[email protected] Shikeying* E-mail: [email protected]. ACKNOWLEDGMENT We acknowledge the support of this research by the National Natural Science Foundation of China (21631004, 21573062, 21371053, 51672073, 21771059), the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (UNPYSCT-2016076), the Science and technology innovation talents research project in Harbin (2016RQQXJ102) and the Basic research expenditure of universities and colleges in Heilongjiang Province, special fund of Heilongjiang University (HDJCCX-201605). REFERENCES (1) Steele, B. C.; Heinzel, A. Materials for Fuel-Cell Technologies. Nature. 2001, 414, 345-352. (2) Shao, M.; Chang, Q.; Dodelet, J. P.; Chenitz, R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116, 3594-3657. (3) Nie, Y.; Li, L.; Wei, Z. Recent Advancements in Pt and Pt-free Catalysts for Oxygen Reduction Reaction. Chem. Soc. Rev. 2015, 44, 2168-2201. (4) Morozan, A.; Jousselme, B.; Palacin, S. Low-Platinum and Platinum-Free Catalysts for the Oxygen Reduction Reaction at Fuel Cell Cathodes. Energy Environ. Sci. 2011, 4, 1238-1254. (5) Kramm, U. I.; Herrmann-Geppert, I.; Behrends, J.; Lips, K.; Fiechter, S.; Bogdanoff, P. On an Easy Way to Prepare Metal-Nitrogen Doped Carbon with Exclusive Presence of MeN4‑type Sites Active for the ORR. J. Am. Chem. Soc. 2016, 138, 635-640. (6) Hu, Y.; Jensen, J. O.; Zhang, W.; Cleemann, L. N.; Xing, W.; Bjerrum, N. J.; Li, Q. Hollow Spheres of Iron Carbide Nanoparticles Encased in Graphitic Layers as Oxygen Reduction Catalysts. Angew. Chem. Int. Ed. 2014, 53, 3675-3679. (7) Zhang, J.; Li, Q.; Wu, H.; Zhang, C.; Cheng, K.; Zhou, H.; Pan, M.; Mu, S. Nitrogen-SelfDoped Carbon with a Porous Graphene-Like Structure as a Highly Efficient Catalyst for Oxygen Reduction. J. Mater. Chem. A 2015, 3, 10851-10857. (8) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 Nanocrystals on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Materials. 2011, 10,


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