Enhanced Electrocatalytic Oxygen Reduction on NiWOx Solid Solution

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Enhanced Electrocatalytic Oxygen Reduction on NiWOx Solid solution with induced oxygen defects Meiqin Shi, Xue Tong, Wang Li, Jun Fang, Li-Tao Chen, and Chun-An Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10891 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 22, 2017

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ACS Applied Materials & Interfaces

Enhanced Electrocatalytic Oxygen Reduction on NiWOx Solid Solution with induced oxygen defects Meiqin Shi, Xue Tong, Wang Li，Jun Fang，Litao Chen, Chun-an Ma∗ State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032,Zhejiang, China KEYWORDS: Electrocatalysis, Oxygen Reduction Reaction, Transition Metal, NiWOx Solid Solution, Oxygen Defects, Alkaline ABSTRACT: The continuous solid solution NiWOx is successfully prepared by using precursor W18O49 with plenty of oxygen defects. The NiWOx nanoparticles are characterized by X-ray diffraction, High resolution transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and X-ray absorption spectroscopy. The crystallographic phase of NiWOx is stable and characterized by the same feature of the parent lattice W18O49 even with various concentration of dopant Ni which indicates the existence of oxygen defects. The NiWOx nanoparticles could be processed as the appropriate promoter after loading 10 wt% Pt. The Pt/NiWOx displays remarkable response for oxygen reduction reaction in alkaline medium compared with the commercial Pt/C. The analysis of the electrochemistry data shows that the existence of abundant oxygen defects in the solid solution NiWOx is the key factor for the improved ORR catalyst performance. Ni is effective in the catalysts because of its compatibility with W in the solid solution and its active participation in oxygen reduction reaction.

1. Introduction The study of oxygen reduction reaction (ORR) has been focused on because it is one of the most important reactions in various electrochemical energy storage/conversion systems such as fuel cells, metal−air batteries, electrocatalytic and photocatalytic water splitting and so forth.1-4 One of the active research in this study is the development of efficient ORR electrocatalysts because the commercialization of fuel cells has been severely hampered by the intrinsic slowness of the ORR, catalyst instability and high cost of a noble-metal catalyst.5-8 It was found that the formation of Oads and hydroxide anion derived from the electrosplitting of O2,ads species is the rate-determining step in ORR.9 The alkaline medium is an effective system because the inherently faster kinetics of the ORR in alkaline media than that in acid media and an alkaline media also provides a less corrosive environment to the catalysts and electrodes.10-11 In alkaline medium fast adsorption/desorption involving oxygen-containing species such as O, OH-, O2−, HO2- and H2O2 from the surfaces of catalysts were required.12 In another word, a channel where oxide species can move fluently may accumulate the kinetic of reaction. The aim of this work is to fabricate a cheap catalyst with oxygen defects and multiple active sites for facilitating the removal of intermediate HO2-. As a potential Pt-alternative catalyst, metal oxides such as spinels AB2O413 and perovskite ABO314 have received significant attention. The mechanism of ORR at transition metal oxide surfaces is different to those at Pt-based catalysts since this type of oxides possesses flexible oxygen

coordination. This changeable valence of cations leads to the distribution of H2O over the catalyst surface by coordinating with the oxygen of H2O in order to fulfill their full oxygen coordination. Then M−OH− species are formed via the reduction of surface cation M such as Mn4+, Co3+, Fe3+, Ni3+, and soforth. The M−OH−species further interact with O2 which adsorb on oxide surfaces.11, 13, 15-16 Another explanation can be oxygen vacancies existing in the catalyst which act as active sites and catalyze the O−O bond cleavage, consequently, favoring the direct 4e- reaction path way from O2 to OH-.17 In order to endow much of oxygen vacancies, another type of metal oxides ABOx has been designed and fabricated in this work in which A is Ni and B is W. As a matter of fact, solid solution of NiWOx which has mixed valence oxides may exhibit electrical conductivity or semi-conductivity enabling their direct use as electrode materials. The same as spinels AB2O4, cations in NiWOx can change their oxidation state， and "electroconductivity chains" are thus formed providing movement of either electrons or holes. Ni-W is a good sample to fabricate and analysis since it is a stable solid solution with tunable interdiffusion coefficients.18-19 Another reason for us to consider the Ni-W-O system is both nickel oxide and tungsten oxide are effective catalysts for the ORR. While in early research, only one ternary oxide, nickel tungstate (NiWO4) was mainly investigated in the Ni-W-O system by heating the component oxides NiO and WO3 inevacuatedquartz capsules at 1050 K for 20 h.20 Therefore, we intend to synthesize NiWOx nanomaterials by using the precursor W18O49 with plenty of oxygen vacancies. Besides being a precursor, tung-

∗

Corresponding author.Tel/fax. +86 571 88320830. E-mail address: [email protected] (C. Ma).

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sten oxide can modify the electronic structure and shift the local d-band center of Pt relative to its Fermi level by the synergetic effect between them.21-23 It is expected that the solid solution NiWOx formed by doping Ni on W18O49 can work as the electrocatalyst support and active components for ORR. In this paper, we developed an efficient strategy for the synthesis of the solid solution NiWOx system by using W18O49 as precursors. Then Pt/NiWOx was formed after loading 10 wt% Pt. The electrochemical studies showed that, compared to the Pt/W18O49 and commercial 20 wt% Pt/C, the obtained Pt/NiWOx exhibited not only enhanced ORR electrocatalytic activity but also superior stability in alkaline media, which render them a class of high performance catalyst for ORR.

2. Experimental section 2.1 Synthesis of tungsten oxide (W18O49) electrocatalyst. All materials or chemicals were used as received and were of analytical grade. In a typical synthesis, WCl6（0.2 g, 0.5 mmol）was dissolved in 20 ml n-butyl alcohol. After vigorous stirring for 10 min, the resulting homogeneous solution was transferred into a sealed teflon autoclave and hydrothermally treated at 200 °C for 12 h before it was cooled to room temperature. The blue precipitate was collected by centrifuging, then was thoroughly washed with ethanol and deionized water to remove the n-butyl alcohol. Finally, the precipitate was dried at 60 °C for 5 h under the vacuum condition to form a fine, blue W18O49 powder for further use. 2.2 Synthesis of nickel–doped tungsten oxides system. For preparation of nickel–doped tungsten oxides materials, appropriate amount of WCl6 and Ni(acac)2 were dissolved in 10 ml n-butyl alcohol respectively. After vigorous stirring for 10 min, the solution were mixed together and continually stirred for another 2 min. The resulting homogeneous mixture was transferred into a sealed teflon autoclave and hydrothermally treated at 200 °C for 12 h before it was cooled to room temperature. The light green precipitate was collected by centrifuging, then was thoroughly washed with ethanol and deionized water to remove the n-butyl alcohol. Finally, the precipitate was dried at 60 °C for 5 h under the vacuum condition to form a fine, green powder for further use. In order to optimize the ratio of nickel to tungsten in the hybrid oxides, we prepared NiWOx solid solution by adjusting WCl6/ Ni(acac)2 reactant ratio, the different samples are marked asW18O49 (without doping), W/Ni=2:1, W/Ni=1:1 (NiWOx) and W/Ni=1:2. 2.3 Synthesis of Pt/W18O49 and Pt/NiWOx electrocatalysts. Typically, Pt/W18O49 and Pt/NiWOx electrocatalysts with a Pt loading of 10 wt% were prepared by a microwave assisted polyol method. The prepared W18O49 and NiWOx as the supporting materials were added 10ml H2PtCl6 solution (5 mM), After 30 min ultrasonic dispersion, appropriate amount of ethylene glycol was mixed in the above suspension. After the pH value of the above well-mixed slurry was adjusted to 8-10 with moderate KOH solution, the mixture transferred into microwave system (Biotage, Initiator EX), where it was rapidly heated to 180 °C for 30 min. The resultant was washed by water and ethanol several times and dried at 60 ℃ for 5 h under the vacuum circumstance. And the commercial Pt/C catalyst (20 wt% Pt on Vulcan XC-72R, Johnson Matthey Corp.) was also tested for comparison.

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2.4 Material Characterization. The crystal structures of W18O49 and NiWOx solid solution with different W/Ni ratio were characterizedby the X-ray diffraction (XRD), using an X’Pert Pro patterns (PANalytical, Netherlands) and Cu Kα source. The scan range is from 10° to 80°. Scanning electron microscope (SEM) (Hitachi S-4700II, Japan) was used to observed the morphologies of the materials, using Cu Kα radiation (λ = 0.154 nm)..Transmission electron microscopy (TEM) was performed with a Tecnai G2 F30 STwin microscope (FEI, Netherlands), along with energy dispersive X-Ray spectrometer (EDX, Thermo NORAN VANSTAGE ESI), also using Cu Kα radiation. The element distribution of the powders was observed by EDX. X-ray photoelectron spectroscopy (XPS, KRATOS AXIS ULTRA (DLD)) was used to prove the elements type and valence of the elements on the surface of materials. XAFS measurements at Pt LIII-edge, W LIII-edge and Ni K-edge in both transmission (for Pt foil) and fluorescence (for samples) mode were performed at the BL14W124 in Shanghai Synchrotron Radiation Facility(SSRF). The electron beam energy was 3.5 GeV and the stored current was 230 mA (topup). A 38-pole wiggler with the maximum magnetic field of 1.2 T inserted in the straight section of the storage ring was used. XAFS data were collected using a fixed-exit doublecrystal Si(111) monochromator. A Lytle detector was used to collect the fluorescence signal, and the energy was calibrated using Pt foil. The photon flux at the sample position was 2.6×1012 photons per second. The raw data analysis was performed using IFEFFIT software package according to the standard data analysis procedures.25 The spectra were calibrated, averaged, pre-edge background subtracted, and post-edge normalized using Athena program in IFEFFIT software package. 2.5 Electrochemical Measurements. All measurements were carried out with a CHI 660C electrochemical workstation(Chen-Hua, Shanghai, China) in a standard three-electrode cell. Working electrodes were prepared by mixing the catalyst with absolute ethanol and Nafion (5 wt%) (v/v = 9/1) by sonication at least 30 min to form a homogeneous ink. 10 µl catalyst ink was deposited onto the polished surface of a 5 mm RDE made of glassy carbon (geometric area 0.196 cm2), and dried in the air. A Pt foil (2 cm2) and a Hg/HgO electrode were used as the counter and reference electrode, respectively. All potentials referred to in this paper are converted to the pH-independent reversible hydrogen electrode (RHE) to compare with the value in literatures according to the equation (1),26

Evs.RHE =Evs.Hg/HgO +0.059*pH+E°Hg/HgO (1) In the electrochemical measurements, all the electrodes were pretreated by cycling the potential between -0.7 and 0.3 V (vs. Hg/HgO) at a sweep rate of 50 mVs-1 for 20 cycles to activate electrodes and remove the dissolved oxygen before the ORR activity tests in N2-saturated 0.1 M KOH solution until a stable cyclic voltammogram was recorded. Then, the electrolyte was saturated with oxygen by bubbling O2 for 30 min prior to the ORR activity tests. A flow of O2 was maintained over the electrolyte to ensure O2 saturation during the recording of CV. Linear-sweep voltammetry (LSV) measurements were conducted under O2 saturated circumstance by sweeping the potential negatively from 0.3 V (vs. Hg/HgO) to-


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0.7 V at a scan rate of 5 mVs-1 with varying rotating speed from 1600 to 100 rpm. Koutecky−Levich plots (i−1 vs.ω−1/2) were analyzed at various electrode potentials. The slopes of their best linear fit lines were used to calculate the number of electrons transferred (n) on the basis of the Koutecky−Levich equation: 1 J

1

1

Jk

JL

= +

=

1 Bω1/2

+

1

(2)

Jk =JL J/JL -J

(3)

B=0.62nFAD2/3 v-1/6 CO2

(4)

Where J is the experimentally obtained current, JL refers to the measured diffusion-limited current, and Jk is the masstransport free kinetic current. n is the number of electrons transferred, Frefers to the Faraday’s constant (96485 Cmol-1), A is corresponding to the area of the electrode (0.196 cm2), D represents the diffusion coefficient of O2 in 0.1 M KOH solution (1.9 × 10−5 cm2s-1), v reflects the kinematic viscosity of the electrolyte (0.01 cm2s-1), ω is the angular frequency of rotation, ω=2πf/60, f is the RDE rotation rate in rpm, and CO2 is the concentration of molecular oxygen in 0.1 M KOH solution (1.2 × 10−3 molL-1). Specific activities were determined via calculation of Jk and normalization with the electrochemically active surface area (ECSA) and the noble metal (Pt) loading. In order to calculate ECSA, the as-prepared electrode was first tested with cyclic voltammogram (CV) technology between -0.7 and 0.3 V (vs. Hg/HgO) at 50 mVs-1 in N2-saturated 0.1M KOH solution until a steady hydrogen adsorption–desorption curve was obtained. The stability tests of the catalysts were conducted by applying a cyclic potential sweep between -0.6 V and 0 V (vs. Hg/HgO) in O2-saturated 0.1 M KOH solution, with a scan rate of 50 mVs-1.

3. Results and discussion Figure 1 shows XRD profiles of the prepared samples and XRD standards of Ni(OH)2 (JCPDS No. 00-003-0177), NiO (JCPDS No. 00-047-1049), NiWO4 (JCPDS No. 01-0720480) and W18O49 (JCPDS No. 01-071-2450). The top line (8) is the pattern of tungsten oxide without Ni doping which fits well the diffraction characterization of W18O49 published in literatures.27-28 The two intense diffraction peaks at 23.33°and 47.87° are assigned to the (010) and (020) crystal faces of W18O49 with monoclinic structure (JCPDS NO: 01-071-2450). It is worth noticing that the rest of patterns recorded from nickel–doped tungsten oxides (NiWOx) preserve the feature of its parent W18O49 except for the decrease of intensity and widening of diffraction peaks because the diffusion of Ni weakened the crystallinity of the prepared NiWOx. Standard W18O49 (JCPDS No. 01-071-2450), pattern No.4 shows multiple diffraction peaks, in which 23.479° (010) is the main peak. Peaks around 27, 34-35, 53-54, 56 are the other diffraction peaks belong to pure W18O49 which have weak intensities.28 These peaks become more ambiguous in our prepared NiWOx samples due to the substitution of Ni atoms. The XRD patterns of NiWOx samples still display the profile of host lattice W18O49. However, compared with the pure W18O49, the slightly left shift of the (010) peak can be observed indicates that Ni2+ is introduced in W6+ sites where the effective ionic radius of Ni2+ (0.069 nm) is superior to that of W6+ (0.060 nm). More important, no peaks associated with NiO and Ni(OH)2 could be observed in any doped NiWOx which indicates that there are

no crystalline phase of NiO or Ni(OH)2 in these samples. The XRD results prove the success synthesis of NiWOx which has the typical lattice structure of W18O49 with substitution of Ni cations in host lattice sites. This also can be confirmed by comparing the diffraction intensity of (010) crystal plane in NiWOx samples with different ratio of Ni/W. Obviously, the crystallinity of NiWOx samples decrease with the increase of the Ni concentrations while the parent lattice structure of W18O49 has not been destroyed. The pattern of sample with the ratio of W/Ni=1:1(pattern No. 6) has been analyzed carefully because the three peaks located around 15.5°, 35.7° and 54.0° are observed while those have not been found for other NiWOx samples. We think the appearance of these peaks result from the introduction of Ni atom. In previous researches, the typical nickel tungsten oxide is NiWO4 and its structure is found to be complicated which is related to the grain size. For small grains, nickel and tungsten ions have plane square-like [NiO4]O2 and nearly tetrahedral-like [WO4]O2 coordinations. For polycrystalline NiWO4, the [NiO6] octahedral possess small tetragonal distortion, whereas [WO6] octahedral are strongly distorted with tungsten ions being off-center.20, 29-31 However, the pattern can not match pure NiWO4 very well because the clear peak left-shift can be noticed, and the preferential plane (111) in pure NiWO4which appears at 30.96° (JCPDS No. 01-072-0480) can not be identified. The structure of NiWOx systems are complex and then its characterization was further investigated using EXAFS, see in Figure 2 and Table1. Figure 2 displayed the W L3-edge and Ni K-edge k3weighted Fourier transform spectra from EXAFS for samples NiWOx (W:Ni=1:1) and pure W18O49. The fitting results of NiWOx are shown in Table 1. According to analysis the [NiO6] octahedral-likes structure described in previous researches is strongly distorted in our prepared NiWOx samples. The number of oxygen atoms around Ni(co-ordination number) in NiWOx is found to be 4～6, the oxygen atoms are divided into two groups of 3.1～3.7 at RNi–O ≈ 2.05 Å and 1.1～1.5 at RNi– O≈ 2.25 Å. The tungsten atom in NiWOx is surrounded by four oxygen atoms which coordinated with ～2.2 O at RW–O ≈ 1.78 Å and ～2.4 O at RW–O≈ 2.06 Å, while for pure W18O49, the tungsten atom is surrounded by six oxygen atoms which coordinated with ～3.6 O at RW–O ≈ 1.78 Å and ～3.4 O at RW–O≈ 2.05 Å. In the case of NiWO4, the WO6 octahedra is strongly distorted: the six oxygens are divided into two groups of four (at ～ 1:83Å) and two (at ～ 2:15Å) atoms.32 Different from the NiWO4, the NiWOx series samples in this paper are continuous solid solutions which the components in our case are the Ni and W atoms are miscible in all proportions. Furthermore the fit result shows that NiWOx series samples have the distorted structure andthe coordination number of W–O and Ni-O atoms in prepared NiWOx both decreased leading to the formation of oxygen vacancies. Figure 3a-d show the SEM images of the pristine W18O49 and the nickel-doped tungsten oxide with different molar ratios of W/Ni. It’s worth noting that the morphologies of the four samples with different doping amount are similar. For all the four samples, the relatively thin and curved nanosheets with thickness of 50–200 nm seem stack randomly. The similar morphologies for the different molar ratios of W/N indicates that few effects can be caused from the different doping of Nickel. To further understand the structure of NiWOx solid solution, the sample with W/Ni = 1:1 (NiWOx) is chosen for TEM characterization since it displays more active as the ORR electrocatalyst support than the pristine W18O49 and other NiWOx


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(see Electrochemical Measurements for details). TEM investigations were conducted tofurther investigate the crystalline

Figure 1. XRD patterns for pure W18O49 and Nickel-doped tungsten oxide NiWOx solid solution with different ratio of W/Ni: (1) Ni(OH)2 (JCPDS No. 00-003-0177) (2) NiO (JCPDS No. 00047-1049) (3) NiWO4 (JCPDS No. 01-072-0480) (4) W18O49 (JCPDS No. 01-071-2450) (5) NiWOx,W:Ni=1:2 (6) NiWOx, W:Ni=1:1 (7) NiWOx,W:Ni=2:1 (8) pure W18O49

Figure 2: (a,b) The W L3-edge k3-weighted Fourier transform spectra from EXAFS for samplesNiWOx (W:Ni=1:1) and pure W18O49. The black solid line are experimental results, and the red dotted line are best-fit curves for R = 1.0–2.3 Å, using corresponding k3 χ(k) functions in k = 3.0–13.6 Å-1. (c,d) The Ni K-edge k3-weighted Fourier transform spectra from EXAFS for sampleNiWOx (W:Ni=1:1) . The black solid line are experimental results, and the red dotted line are best-fit curves for R = 1.0–2.3 Å, using corresponding k3 χ(k) functions in k = 3.0–14.0 Å-1 Table 1. Fit results for the first two shells, Ni-O and W-O in NiWOx, W-O in pure W18O49 Sample Ni W Pure-W18O49

Shell

N

Ni-O1 Ni-O2 W-O1 W-O2 W-O1 W-O2

3.4±0.3 1.3±0.2 2.1±0.1 2.0±0.4 3.1±0.5 3.1±0.3

σ2(10-3 Å2)

R(Å) 2.05±0.01 2.25±0.02 1.78±0.01 2.06±0.01 1.78±0.01 2.05±0.01

4.1±1.0 3.0 3.0 8.0±3.0 4.8±1.2 10.0

N, coordination number; R, distance between absorber and backscatter; σ2, Debye–Waller factor

Figure 3.

SEM images of a) pure W18O49b) W/Ni=2:1

c) W/Ni=1:1 d) W/Ni=1:2;

structure of the NiWOx (Figure 4a) and Pt/NiWOx (Figure 4c) with a Pt loading of 10 wt%. As indicated in Figure 4b, the measured lattice distance of NiWOx samples is 0.380 nm, which is assigned to the (010) lattice plane of NiWOx and close to that of tungsten oxide, and some local disorders can be seen. It suggests that the introduction of nickel doesn’t change the lattice structure of host tungsten oxide, but only decreases the whole crystallinity, which is consistent with the XRD results. Figure 4c is TEM image of Pt/NiWOx, the Figure 4d and 4e are the corresponding HRTEM image and the particle size distribution image of Pt nanoparticles. In addition, the Pt size distribution and TEM images about NiWOx, Pt/C and Pt/W18O49 (shown as Figure S1 and S2) can be compared as supporting evidence. It can be seen that Pt size distributions in all the three samples are uniform and less than 5 nm. Particularly, for the sample of Pt/NiWOx, the Pt nanoparticles (Pt NPs) with the size of about 3 nm are dispersed homogeneously on the surfaces of the NiWOx support, which is attributed to the interaction between Pt NPs and NiWOx support. What’s more, the incorporation of nickel cations can supply more anchor points for the deposition of Pt NPs, subsequently forming the local Pt-Ni adjacent structure on the surface of the support due to the highly effective of nickel in balancing the Pt surface energetic.33 And then this unique structure can improve the process of oxygen reduction reaction (ORR).The EDX test from the selected yellow square region of Pt/NiWOx is shown in Figure 4f, confirming the existence of W, O, Ni and Pt elements. The W/Ni molar ratio is ~1 and the weight ratio of Pt is 10%, which is close to the designed ratio. The EDX elemental mapping for each element indicates that W, Ni, O and Pt elements uniformly distribute in the as-produced sample and form the continuous phase, which prove that the nickel is doped successfully into the tungsten oxide lattice and the solid solution is obtained. The influence of Ni induced defects on the electronic structure of surface/near-surface of NiWOx solid solution was examined by X-ray photoelectron spectroscopy (XPS). The full-range XPS spectrum of NiWOx solid solution show the elements W, Ni, O and also some carbon due to the surface contamination (Figure 5a). More detailed information on chemical state of these elements can be obtained from the high-resolution XPS spectra of W 4f and Ni 2p, as shown in


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Figure 4 TEM images of NiWOx(a) and Pt/NiWOx (c) and the corresponding HRTEM image (b.d) e) the size distribution image of Pt particles. f) the EDX spectrum of Pt/NiWOx from the selected yellow square region and its corresponding mapping images of this region

Figure 5b and c. The W4f core-level spectrum can be deconvoluted into three doublets which are associated with three different oxidation states of surface W atoms, and the fitted results are also shown in the Figure 5b (red line). The main doublet peaks, W 4f5/2 at37.6 eV and W 4f7/2 at35.5 eV are attributed to the W6+ oxidation state (Figure 5b, blue line). The second couple, with lower binding energy at 37.1 eV and 35.0 eV, is derived from the emission of W 4f5/2 and W 4f7/2 core levels from the five coordination tungsten atoms (Figure 5b green line).In addition, the third couple, observed at 36.3 eV and 34.3 eV, corresponding to the unsaturated coordination W4+ oxidation state (Figure 5b,orange line). The whole three typical oxidation states show a clearly shift to higher binding energy (BE) compared with that of pure W18O49 reported in previous researches, which should be related to the presence of Ni atoms in the surrounding environment of W6+ (W5+ or W4+) species.27-28, 34 In addition, the semi-quantitative analysis of XPS spectra shows that the ratio of W5+/W6+ is 0.81, this component suggesting a higher concentration of oxygen vacancies as a result of the introduction of nickel and being the most crucial reason for the superior ORR electrocatalytic performance. The XPS Ni 2p3/2 spectrum has been analyzed into three components at 855.7 eV, 856.5 eV and 857.9 eV, corresponding to NiO, Ni(OH)2 and NiOOH, respectively. The fitted results are marked in red lines (Figure 5c). Similar to the shift of W6+( W5+ or W4+), NiO, Ni(OH)2 and NiOOH significantly shift to higher binding energe position compared with that of pure NiO, Ni(OH)2 and NiOOH reported in literature, because of the effect of tungsten atoms around the nickel atoms.35-36 The abundant hydroxyl groups on the solid solution surface may be caused by the relatively low synthesis temperature without a thermal-treating process.15 Ni element in the NiWOx solid solution play a role in its catalytic activity via a sequential mechanism which are speculated to be the reason for enhancing electrocatalytic performance for ORR. First, Ni can supply abundant sites for catalysts to adsorb O2.Then Ni2+

itself can promote the reduction of oxygen and is expected to stabilize the valence of W by effectively absorbing intermediate products produced by oxygen reduction and generating more Ni3+ (NiOOH) species.11, 16, 37 Figure 5d shows the measured Raman scattering spectra of pristine W18O49 and NiWOx solid solution samples. The Raman spectra of NiWOx are sensitive mainly to the tungsten sublattice, since the nickel environment shows relatively weak Raman activity.18 It can be clearly seen that the original W-O stretching vibration mode ofthe pristineW18O49 located at 680 and 800 cm−1 shifts to higher wave number 797 and 904 cm-1. The Raman spectra in Figure 5d show W18O49 and NiWOx have different surface structures which indicate the different molecular structures, especially the W atoms and its surrounding state, exist in the two samples. The broader peaks compared with the pristine W18O49 can be attributed to the substitutional disorder and lead to the decrease of material crystallinity. Meanwhile, the original peak located at 283 cm−1 associates to the W-O bonding vibration modes disappears and a new peak at 958 cm-1 appears. After comparing the Raman spectrum of NiWOx with those of reference compounds including ideal WO4 tetrahedral and WO6 octahedral structure of tungsten oxides such as Na2WO4, CaWO4 , CuWO4 and Na2W2O7 etc.38 The peak at 958cm-1 can be attributed to the distorted octahedral-like W=O, which indicated that the coordination of local W atoms was in the transition state from octahedron structure to tetrahedron structure in view of the abundant oxygen vacancies.38-40 The ratio of the integrated Raman intensity of W=O to W-O bands in the NiWOx solid solution indicates an increase in the number of defects (oxygen vacancy) compared with the pristine W18O49.41-44 The weak peak located at around 531 cm-1 can be attributed to the shaking peaks of Ni-O bonding which in good agreement with the reported values of Ni-O bond for nanosized NiO particles.45-47 However, the crystal phase of NiO can not be found in the XRD pattern. It can be deduced that the appearance of Ni-O bonds in the NiWOx sample was due to the relatively high Ni doping content to form amorphous Ni


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oxides. The more important reason is the oxygen vacancies in the W18O49 lattice become the channel for Ni atoms diffusion and lead to altering the symmetry of NiWOx sample. The other reason for the observed peak of Ni-O bonding at around 531 cm-1 may be that the excess nickel oxide species presented on the surface of NiWOx solid solution, where the doping of Ni exceeded its solubility in W18O49.The analyses results from the Raman scattering spectra are in accordance with those of XRD, EXAFS and XPS. All the characterization proves the structure of the NiWOx solid solution. In our prepared samples, the valence of W is +6 （+5 or +4）, for the W6+, one W ion is balanced with three O ions (-2 charge). The main valence of the Ni impurity in the lattice is +2, then each Ni impurity requires one O atoms. Consequently the addition of one Ni atom would lead to the removal of two O atom from the lattice, in other words, the formation of two O vacancies.17

Figure 5. XPS spectra of the as-prepared NiWOx solid solution electrocatalyst for ORR a) Full spectra b) W 4f and c) Ni 2p d) the Raman scattering spectra of pristine W18O49 and NiWOx

To study the effect of tungsten/nickel ratio on its catalytic activity, the ORR polarization curves of the pure W18O49 and NiWOx system with different W/Ni molar ratio in an O2saturated 0.1 M KOH solution are shown in Figure 6a. It indicates that the sample of NiWOx shows the best performance for ORR and the onset potentials are about 35 mV, 65 mV and 75 mV positively shifting, compared to that of the NiWOx solid solution with W/Ni =2:1, pure W18O49 and the NiWOx system with W/Ni=1:2, respectively. The enhanced activity of the nickel-doped tungsten oxide samples can be attributed to the incorporation of the Ni element which generates more oxygen vacancies. Because of the charge mismatch between W and Ni, the addition of Ni2+ atoms would lead to the removal of two O atoms from the lattice and create the oxygen vacancies which have been verified to facilitate the oxygen reduction reaction (ORR) significantly. It can be deduced that with increasing of the doping amount of Ni, the concentration of the oxygen vacancies can be increased so as to improve the catalyst activity. On the other side, as discussed in the literatures, formation of solid solutions with di- or trivalent dopant cations can cause the migration of oxygen ions via a vacancy mechanism so that the ionic conductivity is enhanced by the increased concentration of the oxygen-ion vacancies. However, maximum conductivity is achieved by inducing dopants which cause very little expansion or contraction of the host crystal lattice.48-50 In our experiments, with the increasing of Ni dop-

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ing, more Ni atoms can be introduced into the tungsten oxide host lattice and replace the W atoms, which results in distortion and expansion in monoclinic tungsten oxide lattice. When the doping amount exceeds the critical value (solubility limit), distortion and expansion of tungsten oxide lattice will become more severely, which may induce the large amount of disorder and interfere the electron transfer and thus lower the electric conductivity. This could be the reason that NiWOx sample with W/Ni=1:2 can not give a better performance. NiWOxis then used as a promoter to load a small amount of noble metal Pt (10 wt%), denoted as Pt/NiWOx and its electrocatalytic cactivity for ORR is shown in Figure 6b and 6c. For comparison, the ORR polarization curves obtained from the Pt/W18O49 and commercial Pt/C are also shown in Figure 6b and 6c. It can be seen that the limit current density of the Pt/NiWOx normalized by electrode geometric area is nearly same as that of the other two electrocatalysts, ~(-5.6~6.0) mA·cm-2, which is close to the theoretical diffusion limiting current of 5.8 mAcm-2.51 After normalized against the mass of noble metal Pt, the limit current density of the Pt/NiWOx is almost two times higher compared to that of the commercial Pt/C (Figure 6c). In addition, the ORR onset potential and half-wave potential (E1/2) are closely combined with the catalytic activity of electrocatalysts,52 thereby, which can be used to quickly evaluate the activity of a catalyst. The Pt/NiWOx exhibits a more positive onset potential (about 0.986 V) and half-wave potential (about 0.905 V) than that of Pt/W18O49 (0.898 V, 0.826 V) and commercial Pt/C (0.948 V, 0.850 V).The results mentioned above indicate that the electrocatalyst of Ni-substituted tungsten oxide as a promoter and active site has a relative higher catalytic activity towards the ORR in an alkaline medium. The Koutecky–Levich (K–L) plots of the Pt/NiWOx electrocatalyst, and the inset shows the corresponding rotating-disk voltammograms at different rotation speeds are shown in Figure 6d. It can be observed that all of the four lines have the relatively good linearity and the similar slopes over the designated potential range (0.75 to 0.9 V), which indicates the first-order dependence of O2 and similar electron transfer numbers for ORR at different electrode potentials.10, 53 The electron-transfer number is then calculated to be 3.94 on the basis of the slopes of the Koutecky−Levich plots, which demonstrates that the four-electron process is the dominating pathway for the ORR on the Pt/NiWOx electrocatalyst. In order to investigate the intrinsic activity of Pt/NiWOx electrocatalysts for the ORR, the kinetic currents normalized to the mass activity at 0.85 V and 0.9 V (based on the mass of noble metal Pt) and specific activity (based on the electrochemically active surface area (ECSA) ) are shown in Figure 6e and 6f. The ECSA of electrocatalysts can be calculated according to following equation:

ECSA

charge QH , µC cm-2 cm2 = g 210 µC cm-2 *electrode loading g cm-2

210 µC•cm-2 is employed for the monolayer adsorption of hydrogen species on Pt surface.54 As shown in Figure 6e, not only at 0.85 V, the Pt/NiWOx exhibits the excellent mass activity at 0.90 V, which is considered as an activity benchmarks for different ORR catalysts.55 The tafel plots of studied electrocatalysts are shown in Figure 6f. Typical tafel slopes can be seen for the Pt/NiWOx, which are close to -60 mVdec-1 at the low overpotential region and -120 mVdec-1 at the high overpotential region. It means different reaction mechanism at the corresponding potential ranges for ORR. In


6

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Figure 6. a) ORR polarization curves on the pure W18O49 and NiWOx solid solution with different W/Ni molar ratio in O2-saturated 0.1 M KOH solution, with rotation rate of 1600 rpm and a potential sweep rate of 5 mV·s-1. b, c) ORR polarization curves based on b) the per unit electrode geometric area and c) per unit Pt mass on the10 wt% Pt/NiWOx, 10 wt% Pt/W18O49, 20 wt% commercial Pt/C in O2saturated 0.1 M KOH solution, with rotation rate of 1600 rpm and a potential sweep rate of 5 mVs-1. d) Koutecky−Levichplots on Pt/NiWOx at different potentials. Symbols are experimental data from the corresponding rotating-disk voltammograms shown in the inset. e) Mass activities for the Pt/NiWOx, Pt/W18O49, commercial Pt/C at 0.85 and 0.90 V. f) Tafel plots from the different electrocatalysts and the comparison of their specific activities.

general, it indicates that the first-electron transfer reaction is the rate-determining step for the tafel slope of -60 mVdec-1, and the two-electron transfer reaction is the rate determinin step for the tafel slope of -120 mVdec-1.56 On the contrary, the quite different behaviors of the tafel slope of the other catalysts can be noted. For the commercial Pt/C, a single tafel slope of nearly -120 mVdec-1 appears in the whole potential range, while for the Pt/W18O49, the surprising tafel slope of 120 mVdec-1 at the low overpotential region and -60 mVdec1 at the high overpotential region can beobtained, which suggests that the reaction pathway and the rate-determining step are different on these catalysts. The ORR results from the three electrocatalysts are summarized in Table2. It can be seen from the comparison that the prepared Pt/NiWOx has a higher electrocatalytic activity relative to the others towards the ORR in an alkaline medium and making it a promising alternative material. Table 2.Electrocatalytic Performance of Pt/NiWOx, Pt/W18O49, Pt/C for ORR Sample

Pt/ NiWOx Pt/W18O49 Pt/C

onset potential (V)

E1/2 (V)

0.986 0.898 0.948

0.905 0.826 0.850

mass activities (at 0.9 V, A/mg Pt) 0.14 0.02 0.04

specific activities (at 0.9 V mA /cm2Pt ) 0.31 0.06 0.07

There are several possible reasons for the superior ORR catalyst performance of our synthesized NiWOx solid solutionand its related Pt/NiWOx electrocatalyst. The existence of abundant oxygen defects on the material surface is the most important reason for the enhanced ORR catalyst performance. The ratio of W5+/W6+ (0.81) calculated from the results of XPS indicates a large number of oxygen

vacancies (defects) can be produced along with the introduction of nickel into the tungsten oxide crystal lattice to keep the whole electric neutrality. The specific catalytic mechanism of surface oxygen vacancies (SOVs) for ORR is shown in scheme 1. First of all, O2 molecule tends to absorb onto the vacancies, which can be considered as the active sites. The tungsten and nickel cations located on the subsurface can adjust the charge distribution and make electrontransfer to the chemisorbed O2 molecule frommolecule can be further activated to transition state, such as superoxo or peroxo. As a result, the bond length of O-O would be significantly increased by the positively charged oxygen vacancies and ultimately lead to the O-O bond cleavage.17 In this work, more abundant oxygen vacancies can be induced because of the difference valence caused by nickel atoms taking the place of tungsten in the host tungsten oxide lattice. The above described circulatory reaction speed can be improved greatly, which ultimately lead a onset potential 65 mV positive shift of NiWOx system compared with that of pure W18O49 before Pt loading (Figure 6a). In prepared Pt/NiWOx, NiO (Ni(OH)2)/NiOOH redox species may function as the catalyst for ORR similar to NiMnOx/C electrocatalyst.9 This multi-valence of Ni cations leads to the distribution of H2O over the catalyst surface by coordinating with the oxygen of H2O in order to fulfill their full oxygen coordination. The generated Ni−OH− species further interact with O2 which adsorb on oxide surfaces. Then Ni−OH−species are formed again via the reduction of a surface Ni3+. The reaction mechanism is shown as follows:

NiO+H2 O→Ni OH2

(1)

-

(2)

O2 +e- →O2,ads


7


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Scheme 1. Schematic Mechanism of surface oxygen vacancy (SOV) as an important catalytic site for ORR -

2Ni(OH)2 +O2,ads +e- →2NiOOH+2OH-

(3)

NiOOH+H2 O+e- →Ni(OH)2 +OH-

(4)

Where the reaction (4) is the rate-determining step (RDS). Interestingly, according to the previous research, the divalent cations doping, such as Ni2+ and Mg2+, especially Ni2+, can be effective for the reducing of peroxide ions intermediate （HO2-）because of the decreasing of activation energy of the hydrogen peroxide decomposition. Thus, we can reasonably speculate that the nickel oxide (hydroxide) has the ability to promote the residual peroxide ions intermediate（HO2-） to transfer timely from Pt surface to nickel oxide (hydroxide) surface to eliminate it and sequentially prevent from its accumulation. The probable reaction mechanism as follow:

Pt+O2 →Pt-O2,ads

(5) -

Pt-O2,ads +H2 O+e- →Pt-HO2,ads +OH-

(6)

Ni OH2 + HO2 +e →NiOOH+2OH-

(7)

NiOOH+H2 O+e- →Ni OH2 +OH-

(8)

-

The stability of Pt/NiWOx electrocatalysts for the ORR was evaluated by the chronoamperometry measurement. The data was recorded at -0.1 V (vs. MMO.) for 10000 s in the O2saturated 0.1 M KOH and the rotating speed was 1600 rpm. As shown Figure 7a, a rapid decrease within about 2000 s polarization time can be seen for the commercial Pt/C catalyst, which results from the charge and discharge in the electric double layer. The computed average degradation rate of the ORR current on the commercial Pt/C is 0.89 mA•cm-2•h-1.The obvious decrease of catalyst performance may be the reason of the occupation of active sites on the surface of Pt by the abundant oxygen-containing intermediate （HO2- ）produced during the reduction of oxygen. The noble metal (Pt) falls off from the supported carbon along with the reduction reaction of oxygen is another possible reason for the sharp degradation of ORR current. In contrast, Pt/NiWOx electrocatalysts displays the integral stability with a much lower degradation rate of 0.43 mA•cm-2•h-1 because the NiWOx solid solution works as both a good support and promoter. Simultaneously, it can be noted that the onset potential and the half wave potential (E1/2)

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of the Pt/NiWOx hardly change after the 10000 s chronoamperometry test, which only negatively shift 5 mV and 8 mV, respectively, compared with 20 mV and 25 mVof the commercial Pt/C (Figure 7b and Figure 7c). To further distinguish the variation of electron transfer number before and after the chronoamperometry test, the K-L plots are again delineated as shown Figure 7d. The displayed curves are both linear and nearly parallel. The computed electron-transfer number (n) only alter from 3.94 to 3.88, which indicates that the direct 4e- reaction pathway of ORR is same after the long time chronoamperometry measurement. The possible factor for the superior stability of ORR current is that nickel oxide (hydroxide) can promote the remainder oxygen-containing intermediate （HO2- ）to transfer timely from Pt surface to nickel oxide (hydroxide) surface and sequentially prevent from its accumulation. On the other hand, according to the XAFS experiment performed on the 10 wt % Pt/NiWOx nanocrystals and 20 wt % Pt/C, the .results are shown in Figure 8, which reveals that the intensities of the absorption peaks, i.e., the white line, of heterostructural 10 wt % Pt/NiWOx nanocrystals at the Pt LIII edge were lower than the corresponding peaks for the Pt/C and Pt foil. The Pt LIII edge white line is caused by transitions from 2p3/2 to unoccupied 5d3/2 and 5d5/2 states. Its intensity is determined by density of unoccupied final states, i.e. for NPs, two factors could contribute to the observed differences: Pt particle size effect or unequal charge transfer. In the present study, Pt particles sizes are nearly comparative for the 10 wt % Pt/NiWOx and 20 wt % Pt/C from the above Pt particles size distribution images. Hence, the particle size effect can be ruled out. Thus, the unequal charge transfer is the only possible reason for the differences in the white line intensity.57 The lower white line intensity for the 10 wt % Pt/NiWOx indicates the decreased dband vacancy which is attributed to the strong metal-support interaction (SMSI) between the Pt atoms and the NiWOxnanosupport, which generated by the electronic transfer from the NiWOx nanosupport to the Pt surface.58-61 The results indicatesthat the strong interaction between tungsten oxide and Pt through the electron transferring from the NiWOx to Pt and causing the redistribution of Pt d-band electron can anchor tightly Pt nanoparticles on the support surface and therefore avoid its drop. The analyses for the two possible factors are consistent with the above discussion results.

Figure 7. catalysts stability tests a) the K-L plots comparison of the Pt/NiWOx before and after the 10000 s chronoamperometry test at 0.9 V b, c) is the comparison of polarization curves of the


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Pt/NiWOx (b) and commercial Pt/C (c) before and after the 10000 s chronoamperometry test at a potential range of 0.4-1.2 V (vs. RHE.) in the O2-saturated 0.1M KOH and the rotating speed is 1600 rpm. d) 10000 s chronoamperometry test of Pt/NiWOx and commercial Pt/C at a potential of −0.1 V (vs. MMO.) in the O2saturated 0.1M KOH and the rotating speed is 1600 rpm;

LQ15B030004).We also thank beamline BL14W1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.

References

Figure 8 . Normalized XANES spectra at the Pt LIII edge of 10 wt % Pt/NiWOx and 20 wt % Pt/C nanocrystals with Pt foil as the reference

4. Conclusions NiWOx solid solutionwas designed by the displacement of Ni in the W18O49 lattice to fabricate a continuous solid solution with abundant oxygen defects. NiWOxwas processed as the support for loading small amount of Pt and the Pt/NiWOx electrocatalysts displayed improved activity towards ORR. The excellent catalytic activity could be attributed to theabundant oxygen vacancies induced by the introduction of Ni, which can greatly accelerate the breakage of O-O bond. In addition,the multi-valence of Ni cations also participate into the process of oxygen reduction and contribute to the enhanced performance.The durability of the Pt/NiWOx hybrid nanomaterials was also enhanced because the interfaces between Pt and the NiWOx solid solution provide more active sites for O2 adsorption and activation.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional figures, including TEM images of NiWOx and the corresponding HRTEM image, TEM micrographs and the size distribution image of the Pt particles for different samples, ORR polarization curves on the Ni-W-O solid solution with different W/Ni molar ratio and commercial Pt/C, ORR polarization curves on the 10 wt.% Pt/NiWOx, 20 wt.% Pt/NiWOx and 20 wt.% Pt/C (PDF)

Acknowledgments This work was supported by National Natural Science Foundation of China (2137622）, Natural Science Foundation of Zhejiang Province (LY16B060009, LY12B03008,

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Enhanced electrocatalytic oxygen reduction on NiWOx solid solution with induced oxygen defects

Table of Contents/Abstract Graphic


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Schematic Mechanism of surface oxygen vacancy (SOV) as an important catalytic site for ORR 337x134mm (300 x 300 DPI)


Enhanced Electrocatalytic Oxygen Reduction on NiWOx Solid Solution

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