Template-Free Preparation of 3D Porous Co-doped VN Nanosheet

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Energy, Environmental, and Catalysis Applications

Template-Free Preparation of 3D Porous Co-doped VN Nanosheetassembled Microflowers with Enhanced Oxygen Reduction Activity Haibo Tang, Junming Luo, Xin Long Tian, Yuanyuan Dong, Jing Li, Mingrui Liu, Lina Liu, Huiyu Song, and Shijun Liao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18504 • Publication Date (Web): 21 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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ACS Applied Materials & Interfaces

Template-Free Preparation of 3D Porous Co-doped VN Nanosheet-assembled Microflowers with Enhanced Oxygen Reduction Activity Haibo Tang, Junming Luo, Xin Long Tian, Yuanyuan Dong, Jing Li, Mingrui Liu, Lina Liu, Huiyu

Song*, Shijun Liao* The Key Laboratory of Fuel Cell Technology of Guangdong Province & The Key Laboratory of New Energy Technology of Guangdong Universities, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China

Abstract Developing cheap and stable electrocatalysts is considered the key factor to achieve the largescale application of fuel cells. In this paper, 3D porous Co-doped vanadium nitride (VN) nanosheet-assembled microflowers are prepared with a facile solvothermal approach followed by nitridation at 500 °C in NH3. It is found that the microflower morphology and the Co doping both significantly enhance the oxygen reduction reaction (ORR) performance of the materials. Since the unique 3D porous structure provides higher specific surface area and more active sites as well as enriching the d electrons of V via doping, Co also improves the intrinsic activity of VN. Our optimal V0.95Co0.05N microflowers achieve a half-wave potential for the ORR of up to 0.80 V in 0.1 M KOH solution, which is almost comparable to that of commercial 20% Pt/C. More importantly, the catalysts show superior durability with little current decline (less than 12%) during chronoamperometric evaluation for over 25,000 s. These features make the V0.95Co0.05N microflowers attractive for fuel cell applications. *

Corresponding authors, e-mail: [email protected], [email protected], fax +86 20 87113586 1

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Keywords: vanadium nitride; microflowers; oxygen reduction reaction; Co-doped; d electrons; fuel cells

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INTRODUCTION

In the quest to minimize fossil fuel consumption, fuel cells, supercapacitors, lithium-ion battery, and solar cell technologies have garnered significant attention as the most promising energy alternatives.1-8 The development of new electrode materials for energy conversion technologies, particularly fuel cell cathode electrodes for the oxygen reduction reaction, is particularly important.9-11 To date, Pt-based nanomaterials have been the dominant and best electrocatalysts for promoting the ORR.12-16 However, Pt’s scarcity, high cost, and susceptibility to MeOH crossover and CO poisoning effects hinder its practical application in fuel cell devices.17-18 Thus, low-cost, efficient, and durable catalysts for the ORR are prerequisites for the commercialization of fuel cell technology but have remained challenging to develop. To achieve this goal, various studies have reported attempts to rationally design non-precious metal catalysts (NPMCs) with activity and stability comparable or even superior to Pt-based materials, including chalcogenides,19 transition metal oxides,20 carbides,21 phosphides22 and nitrides,23 as well as carbon/graphene-based materials.24 Among them, transition metal nitrides (TMNs), such as NbN,25 TiN,26-27 Ti0.7Mo0.3N,28 and Co0.5Mo0.5Ny,29 have evolved as possible alternatives for the ORR or as supports for precious metals, due to their excellent conductivity, high corrosion resistance, and moderate ORR activity. Although TMN-based NPMCs show superb stability, their activity is still not satisfactory.30 Therefore, new strategies to develop TMN-based efficient ORR catalysts remain a major challenge. Our recent study found that for VN nanoparticles (NPs), enriching its d electrons by doping with Co is an effective way to promote its ORR activity.31 However, the prepared VCoN NPs lacked the desired size and morphology controls and did not readily agglomerate, leading to low surface area, so their performance is still unattractive. The designed synthesis of nanostructures has always been a key method for developing high-performance 3



catalysts.32-33 Furthermore, recent studies show that highly porous structures could provide excellent structural rigidity to prevent aggregation; consequently, their durability is significantly enhanced without compromising their intrinsic high electrocatalytic activity.34 Nevertheless, the usual synthetic approach for porous VN has depended on the use of templates as components of a preliminary composite,35 and the removal of templates is also tedious. To our knowledge, the synthesis of 3D binary TMN composite nanostructures, especially hierarchically porous microflowers (MFs), without templates has not previously been reported. Based on the above considerations, our goal was to develop a catalyst that is not only highly active but also highly stable, robust, facile to create, and easily scalable. In this article, we describe a facile method for synthesizing novel 3D porous VCoN hierarchical MFs based on a template-free solvothermal method and subsequent nitridation process. The porous 3D VCoN are characterized by ultrathin nanosheet-assembled MFs, a porous nanostructure, and anisotropic qualities, which impart advantages such as high electrocatalyst utilization, high transport rate of electroactive species, and large surface area. These multiple advantages have the potential to bring noticeable improvement in electrocatalytic activity for TMNs. EXPERIMENTAL SECTION 1. Preparation of VCoN and VN Microflowers The V0.95Co0.05N MFs were prepared by solvothermal method and then nitridation in an NH3 atmosphere. In a typical synthesis, 400 ul vanadium oxytriisopropoxide (VOT, 96%) was dropwise added into 60 mL of isopropyl alcohol (IPA, 99.7%) under stirring for 60 min. Then 31.6 mg of cobalt (III) 2, 4-pentanedionate (Co(C5H7O2)3, 99%) were dispersed into the above solution with continuous stirring for 0.5 h. The mixed solution was then transferred into a Teflon-lined autoclave (100 mL), which was sealed and heated at 200 °C for 12 h. 4


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After natural cooling, the precipitate was washed and collected by centrifugation. Finally, the obtained samples were nitrided at ultra-low nitridation temperatures (450–550 °C) for 2 h under NH3 to yield hybrid 3D porous Co-doped VN. VN MFs were obtained with a similar routine without added cobalt (III) 2, 4-pentanedionate.36 For comparison, V0.95Co0.05N NPs and VN NPs were synthesized with the same method which has been reported previously by our group.31 2. Catalyst Characterization Crystallographic data of our catalysts were collected on a TD-3500 powder diffractometer (Tongda, China) with monochromatized Cu Kα radiation (λ = 1.5418 Å) operated at 20 mA and 30 kV. Transmission electron microscopy (TEM) analysis was accomplished by dispersing 0.5 mg of the powdered sample in 2 mL ethyl alcohol. From this suspension, 5 µL were collected and then dropped on a 200 mesh Cu grid. The sample-coated Cu grid was dried and analyzed using a JEM-2100 transmission electron microscope (JEOL, Japan). Scanning electron microscopy (SEM) images were generated on a Merlin field emission SEM (Carl Zeiss). XPS was performed on an X-ray photoelectron spectrometer using monochromated Al Kα X-ray sources (hν = 1486.6 eV, Axis Ultra DLD). The surface area and pore distribution date were collected using a TriStar II 3020 gas adsorption analyzer from nitrogen adsorption–desorption. ICP-AES (Leema PROFILE, America) was used to determine the actual content of V and Co. 3. Catalyst Evaluation All electrochemical experiments were carried out on an electrochemical workstation (Ivium), with a three-electrode cell. The system was composed by a glassy carbon-based working electrode (GC, diameter 5mm), a Hg/HgO reference electrode, and a Pt wire counter electrode. All working electrodes were made as follows. Firstly, the catalyst inks were prepared by adding 5 mg catalyst to 1 mL Nafion ethanol solution (0.25 wt%) with 30 min 5



ultrasonic. Then the electrode was covered with 5 µL catalyst ink and dried under an infrared lamp. RESULTS AND DISCUSSION Figure 1a shows the XRD patterns of the synthesized VN MFs nitrided at different temperatures. It can be seen that the precursor was completely transformed into nitride in the ammonia flow at a very low temperature of 500 ºC, which is much lower than the typical nitridation temperatures previously reported.37-38 The diffraction peaks of the synthesized VN MFs, centered at 37.7 °, 44.0 °, 63.9 °, 76.8 °, and 80.9 °, correspond to the face-centered cubic structure of VN (JCPDS no. 35-0768). Once the nitriding temperature was below 500ºC, the peak for VN could not be observed, indicating that 500 ºC was the minimum temperature for the nitridation of the vanadium oxide precursors. We found that a higher nitriding temperature resulted in better nitride crystallization but also led to larger particle size in the VN MFs. As shown in Figure 1b, the Co-doped V0.95Co0.05N MFs had almost the same XRD pattern as VN, suggesting that the doped Co may have been incorporated into the VN lattice. Interestingly, an obvious shift in peak position could be observed after Co doping, verifying the successful doping of Co in the nitride. Furthermore, the variations in the XRD patterns of V0.95Co0.05N MFs with different nitridation temperatures were almost the same as for the VN MFs sample without Co doping. The higher the calcination temperature, the larger the crystallite size. The XRD patterns of V0.95Co0.05N NPs annealed at 500 °C are shown in figure S1. It can be seen that the precursor of V0.95Co0.05N NPs is completely transformed into nitride in the ammonia flow at 500 ºC. Furthermore, no peaks belonging to cobalt nitride or cobalt metal can be observed, indicating that the doped Co completely entered the lattice of the vanadium nitride nanoparticles. Importantly, V0.95Co0.05N NPs exhibits the XRD pattern 6


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of VN almost same as V0.95Co0.05N MFs, however, its diffraction peaks are much narrower than those of the latter, which should be caused by the thinner flower structure of the V0.95Co0.05N MFs. In other words, the structure differences could be observed through the XRD patterns. Figure 1c shows the XRD patterns of samples with various Co content. No peaks belonging to cobalt nitride were observable, even when the Co content was as high as 7 at%, indicating that the doped Co completely entered the lattice of the vanadium nitride. Nevertheless, it is interesting that as the Co content increased, the main peak (200) shifted negatively (Figure 1d), reflecting the increase in the lattice parameters of the material doped with Co. This could be explained by the Co2+ ion having a larger ionic radius (0.74 Å) than the V3+ ion (0.64 Å).

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Figure 1. (a) XRD patterns of VN MFs and (b) V0.95Co0.05N MFs annealed at various temperatures. (c) XRD patterns of V1-xCoxN MFs annealed at 500 °C. (d) XRD patterns of V1-xCoxN MFs annealed at 500 °C obtained using a slow scanning rate. The strategy for synthesizing VN MFs and V0.95Co0.05N MFs is schematically depicted in Figure 2. First, the 3D VOx MFs are prepared from the solution of VOT in IPA via a facile solvothermal approach at 200 °C for 12 h. Next, the precursor structure is nitrided in NH3 at 500 °C for 2 h to obtain VN porous MFs. The V0.95Co0.05N MFs are prepared using the same method except for the addition of a small amount of Co(C5H7O2)3. Interestingly, the structure and morphology of the catalysts is significantly affected by Co doping. We suspect that this is because the cobalt ions promote the growth of the VN MFs.

Figure 2. Schematic illustration of the procedure used to fabricate VN MFs and V0.95Co0.05N MFs. The morphology and structure of the products before and after nitriding were examined via TEM and SEM. As shown in Figure 3a, the VOx precursor MFs were highly uniform with relatively smooth surfaces, and the diameter was around 810 nm. A close examination 8


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(Figure 3b) reveals the ultrathin nanoflake subunits of the as-synthesized MFs. After the nitridation process, the VOx MFs precursor was converted into VN MFs, but the MF arrangements of the nanoflakes (Figure 3c) were essentially retained, suggesting good structural stability. The enlarged TEM image (Figure 3d) reveals that the primary nanoflakes became porous structures. Compared with VOx MFs, the nanoflake subunits of the as-synthesized VCoOx MFs (Figure 3e and 3f) were more denser and had a larger diameter (~1.2 µm). This result shows that the addition of cobalt promoted the growth of the MFs. After the nitridation process, the V0.95Co0.05Ox MFs precursor was converted into V0.95Co0.05N MFs, but the morphology was retained. Unlike the V0.95Co0.05Ox MFs precursor, the nanoflakes of the V0.95Co0.05N MFs were porous (Figure 3g, 3h). This 3D porous structure has proven extremely useful for the ORR. To further investigate the elemental distribution in the V0.95Co0.05N MFs composite, elemental mapping was carried out, as shown in Figure 3i–l. V, Co, and N were well dispersed and well distributed throughout the MFs. We also analyzed the catalyst by inductively coupled plasma mass spectrometry (ICP-MS), according to which the molar ratio of V to Co was 47.45:2.51, agreeing well with its recipe composition of 19:1.

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Figure 3. (a, b) TEM images of VOx MFs. (c, d) TEM images of VN MFs. (e, f) TEM images of V0.95Co0.05Ox MFs. (g, h) TEM images of V0.95Co0.05N MFs. (i) ADF-TEM image of V0.95Co0.05N MFs and the corresponding elemental mappings: (j) V, (k) Co, and (l) N. A panoramic SEM image shows that MF structures of the prepared VOx MFs precursor and V0.95Co0.05Ox MFs precursor had a uniform size and were formed on a large scale (Figure 4). The morphology of the VN MFs and V1-xCoxN MFs has also been characterized by SEM (Figure S2). V0.97Co0.03N and V0.93Co0.07N also exhibit micro flower like morphology similar as V0.95Co0.05N, however, obvious differences could be observed for three samples, clearly revealing that the morphology and structure of the catalysts could be significantly affected by the doping of cobalt and doping amount.

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Figure 4. SEM images of (a) VOx MFs and (b) V0.95Co0.05Ox MFs. Figure 5a shows the nitrogen adsorption and desorption isotherms. The samples prepared by the solvothermal method clearly exhibited type IV isotherms with distinct hysteretic loops, implying the presence of mesoporous structures in the VN MFs and V0.95Co0.05N MFs samples, which might have arisen from the decomposition of ammonium during the calcination process. This result agrees very well with the pore-size distribution curve (Figure 5b) and TEM results. The BET specific surface areas of the VN MFs and V0.95Co0.05N MFs were estimated to be 59.6 and 47.2 m2 g–1, respectively. Hence, the surface area decreased slightly with Co doping, but compared to the V0.95Co0.05N NPs (7.9 m2 g–1) without a flowery structure, the V0.95Co0.05N MFs possessed a higher specific surface area and more mesopores. We expected that the high surface area and unique porous structure of our microflowers sample would lead to better ORR performance.

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Figure 5. (a) N2 adsorption–desorption isotherms of VN MFs and V0.95Co0.05N MFs for various annealing temperatures. (b) Pore-size distribution of VN MFs and V0.95Co0.05N MFs. Figure 6a shows the polarization curves of VN MFs and V0.95Co0.05N MFs in O2saturated 0.1 M KOH solution at a scanning rate of 10 mV s–1. For comparison, VN NPs, V0.95Co0.05N NPs, and commercial Pt/C were also measured. Our optimal V0.95Co0.05N MFs exhibit excellecnt performance, which is one of the best catalysts among TMNs for ORR (Table S1).31 The ORR onset potential and activity for the five catalysts in an alkaline medium followed the order VN NPs < VN MFs < V0.95Co0.05N NPs < V0.95Co0.05N MFs < JM 20 wt% Pt/C. Compared with VN MFs,

V0.95Co0.05N MFs exhibited much better

performance, with an onset potential of 0.903 V (vs. RHE) and a half-wave potential of 0.802 V (vs. RHE), which are about 80 and 180 mV more positive than those of VN MFs — indicating that Co doping can dramatically improve the ORR performance of VN. Furthermore, the onset potential and half-wave potential of V0.95Co0.05N MFs are about 32 and 35 mV more positive than those of V0.95Co0.05N NPs, showing that transforming the morphology from nanoparticles to microflowers also leads to better ORR performance. These results are consistent with our original intention. Figure 6b shows the effects of nitriding temperature on the performance of the V0.95Co0.05N MFs catalyst and indicates that 500 ºC seems to be the optimal preparation 12


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temperature. Notably, an annealing temperature over 500 ºC was needed to completely form V0.95Co0.05N; below that, the final product could not be completely nitrated (Figure 1b). Above 500 ºC, though, the ORR activity of the V0.95Co0.05N MFs began to fall, which might have been because the structure collapsed and the surface area decreased as the temperature rose. It is very interesting that the flowery precursor could be nitrided at such a low temperature (500 ºC), which is much lower than reported previously by our group or other groups. Generally, low temperature results in high surface area, good porous structure, and high catalytic performance, which may be one of the reasons that our catalysts exhibited excellent ORR performance. We also investigated the effects of Co amount on the ORR performance of the Co-doped catalysts (Figure 6c) and found that doping with only a small amount of Co could significantly enhance the ORR performance. The catalyst performed optimally at 5% dopant concentration, while both lower and higher dopant concentrations led to decreased ORR activity. To investigate the durability of V0.95Co0.05N MFs in an alkaline medium, the current– time (i–t) chronoamperometric response was measured in 0.1 M KOH solution (Figure 6d). The stability test showed that V0.95Co0.05N MFs could maintain ∼88% ORR activity for 25,000 s, compared to 84% and 94% for the VN NPs and JM Pt/C under the same testing conditions and for the same time.

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Figure 6. LSV curves of (a) VN NPs, VN MFs, V0.95Co0.05N NPs, V0.95Co0.05N MFs, and 20% Pt/C; (b) V0.95Co0.05N MFs annealed at various temperatures; (c) V1-xCoxN MFs annealed at 500 °C, all calculated by subtracting N2-saturated solution from O2-saturated solution at a rotation speed of 1600 rpm in 0.1 M KOH solution. (d) Catalytic stability of V0.95Co0.05N MFs, V0.95Co0.05N NPs, and JM 20% Pt/C polarized at 0.57 V (vs. RHE) during 25,000 s in O2-saturated 0.1 M KOH solution at a rotation rate of 900 rpm. Figure 7 shows the electrochemical impedance spectra of the VN NPs, VN MFs, V0.93Co0.07N MFs, V0.97Co0.03N MFs, V0.95Co0.05N NPs, and V0.95Co0.05N MFs in O2-saturated 0.1M KOH solution. Clearly, the doping of cobalt results in the significantly decrease of the charge transfer resistance, furthermore, the flower like samples exhibited obvious lower charge transfer resistance than the nanoparticles samples, and the V0.95Co0.05N MFs exhibited the lowest charge transfer resistance among the three flower samples doped with various 14


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amount of Co. We can expect that V0.95Co0.05N with microflower-like morphology will exhibit the best ORR performance, because the lower charge transfer resistance generally results in the high catalytic performance of the materials. 600 500

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Figure 7. Nyquist plots of the synthesized catalysts at 0.7 V. To understand the kinetics of the ORR on our catalysts, we recorded the LSV curves of VN MFs and V0.95Co0.05N MFs at different rotation rates (Figure 8) and further analyzed using the Koutecky–Levich (K–L) equations. The average electron transfer numbers were calculated to be 3.9 and 4.0 for VN MFs and V0.95Co0.05N MFs, respectively, which reveals that the ORR process of the two catalysts follows a one-step, four-electron pathway; it also seems that the Co-doped flower catalyst performed better than the one without Co doping.

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Figure 8. ORR polarization plots of (a) VN MFs and (b) V0.95Co0.05N MFs in O2-saturated 0.1 M KOH electrolyte at different rotation rates, and (inset) the Koutecky–Levich plots. To obtain a deeper understanding of the excellent ORR performance of the V0.95Co0.05N MFs, we performed XPS analysis of our prepared VN MFs, V0.95Co0.05N MFs, and V0.95Co0.05N NPs. Figure 9a shows the survey scanning spectra of the three samples, wherein V, N, Co, O, and C are observable. The presence of O implies that the catalysts were slightly oxidized.39 All catalysts show three doublets for V 2p in their high-resolution XPS spectra. The higher BE doublet is associated with V5+–O (V1),40-41 which might have been generated by exposure of the catalyst to air, while the lower-energy one is clearly identifiable as the nitride V3+–N (V3).42-43 The addition of another doublet is necessary to fit the experimental spectrum. The peak located at 515.5 eV can be indexed to V3+ in V2O3 (V2).44-45 Since an N atom has a lower electronegativity than an O atom, the oxidation state of V3 was below three. Comparing the XPS peaks of VN MFs and V0.95Co0.05N MFs shows that the binding energies for V1 and V2 in the V0.95Co0.05N MFs shifted negatively by 0.2 and 0.5 eV, respectively, relative to VN MFs (Figure 9b). This result indicates that the coordination environment of V had significantly changed due to Co doping—that is to say, charge transfer from Co to V occurred with Co doping. To better compare the proportional variations induced by Co doping, we calculated the proportions of V1, V2, and V3 on the basis of their corresponding peak areas in the V 3p3/2 spectra. Compared with VN MFs, the proportion of V3 dramatically increased with Co doping (Figure 9c). In other words, Co doping increased the proportion of V in a low-valence state, a result ascribable to charge transfer from Co to V, which has been shown to improve the ORR activity of catalysts.25 As indicated in Figure S3 (b), the proportion of V3 obviously increased as the Co content increased, revealing that the coordination environment of V in VCoN could be changed by the doping amount of Co. 16


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Unlike the survey scan spectrum of VN MFs, the peaks observed at 781.0 and 796.9 eV are ascribed to Co 2p3/2 and Co 2p1/2,46-47 respectively, which confirms the existence of Co2+-like species, further indicating successful Co doping; this is in agreement with the EDS mapping results.

N 1s

Intensity/ a.u.

VN MFs

800 600 400 Binding Energy/ eV

V3

V0.95Co0.05N MFs

V0.95Co0.05N NPs

200

0.7

0

V1 V2 V3

0.6

528

dV

524

520 516 Binding Energy/ eV

Co0.05N MFs

512

Co 2p

0.95

0.5

Intensity/ a.u.

c

1000

V1 V2

C 1s

V0.95Co0.05N-NPs

O 1s V 2p

V0.95Co0.05N MFs

1200

V 2p

b

VN MFs

Co 2p

Intensity/ a.u.

a

Proportion of V 2p3/2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4 0.3

V0.95Co0.05N-NPs

0.2 0.1 0.0

VN MFs

V0.95Co0.05N MFs V0.95Co0.05N NPs 810

805

800

795 790 785 Binding Energy/ eV

780

775

Figure 9. (a) XPS spectra of VN MFs, V0.95Co0.05N MFs, and V0.95Co0.05N NPs. (b) Highresolution V 2p of VN MFs, V0.95Co0.05N MFs, and V0.95Co0.05N NPs. (c) Proportion of V 2p3/2 in VN MFs, V0.95Co0.05N MFs, and V0.95Co0.05N NPs. (d) High-resolution Co 2p of V0.95Co0.05N MFs and V0.95Co 0.05N NPs. The surface compositions of the VN MFs, V0.97Co0.03N MFs, V0.95Co0.05N MFs, V0.93Co0.07N MFs, and V0.95Co0.05N NPs measured by XPS are given in Table 1. It reveals that the samples doped with various amount of Co exhibit quite different surface 17



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compositions, in term of the contents of N, Co, V, and the molar ratio of Co/V, indicating that the doping amount of Co significantly affected the surface composition and structure of the samples, and these results provide a reasonable explanation for the performance differences of the three samples. The V0.95Co0.05N MFs showed minor differences between the bulk and surface Co:V ratios (5.30% versus 6.95%, as determined by ICP and XPS, respectively), but the V0.95Co0.05N NPs showed a remarkable difference between the bulk and surface Co:V ratios (5.30% versus 14.95%, respectively). This surface ratio enhancement could have occurred due to cobalt segregation during calcination and the cobalt ion distribution not being uniform in the V0.95Co0.05N NPs. In addition, the result indicates that V0.95Co0.05N MFs synthesized by this solvothermal approach possess better uniformity, which is consistent with the observations from elemental mapping. This might be an important reason for the enhanced performance of the V0.95Co0.05N MFs. As a result of higher cobalt content on the surface of the V0.95Co0.05N NPs, significant displacement occurred in the high-resolution spectra of V2p and Co 2p compared with the V0.95Co0.05N MFs (Figure 9b, d). Table 1 The atomic compositions of VN MFs, V0.97Co0.03N MFs, V0.95Co0.05N MFs, V0.93Co0.07N MFs, and V0.95Co0.05N NPs measured by XPS VN MFs

V0.97Co0.03N MFs

V0.95Co0.05N MFs

V0.93Co0.07N MFs

V0.95Co0.05N NPs

O

21.27

25.62

26.11

26.26

35.70

N

53.24

42.73

47.42

41.06

30.42

V

25.48

31.11

24.72

29.85

29.46

Co

1.44

1.72

2.81

4.40

Co/V

4.63%

6.95%

9.41%

14.95%

In conclusion, the significantly enhanced ORR performance of our Co-doped VN materials can be attributed to two factors: a better, well-defined porous structure, which leads 18


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to high surface area and high active site exposure; and charge transfer from the doped Co to V (Figure 10), which may play a crucial role in performance enhancement.

Figure 10. Electron-transfer model for the enhanced ORR activity of V0.95Co0.05N MFs. CONCLUSIONS In summary, we have combined morphology control with doping effects and successfully developed a template-free approach for the produce of 3D porous Co-doped vanadium nitride microflowers. The optimal nitriding temperature was 500 °C, and doping with 5 at% Co significantly enhanced the performance of the catalyst. Our optimal catalyst, V0.95Co0.05N MFs, in an alkaline medium exhibited an ORR performance comparable to that of commercial 20% Pt/C. In addition to its good activity, this catalyst also exhibited excellent stability. Based on our kinetic analysis and characterization results, we ascribe the catalyst’s high performance to a unique porous structure, thin sheets, high surface area, and charge donation from doped Co to V atoms. All of these characteristics may make the material a new alternative to replace traditional Pt-based ORR catalysts for PEMFCs, and our approach may provide a novel pathway for developing NPMC catalysts for fuel cell applications. 19



ASSOCIATED CONTENT Supporting Information XRD patterns of V0.95Co0.05N NPs; SEM and XPS of V1-xCoxN MFs; a comparison table of the ORR activity of TMNs. AUTHOR INFORMATION Corresponding Author * Email: [email protected], [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (Project Nos. 2017YFB0102900, 2016YFB0101201), the Guangdong Provincial Department of Science and Technology (Project No. 2015B010106012), and the Guangzhou Science Technology and Innovation Committee (Project Nos. 201504281614372, 2016GJ006), the National Natural Science Foundation of China (NSFC Project Nos. 21476088, 21776105), National Natural Science Foundation of China (21706080), Fundamental Research Funds for the Central Universities (2017BQ066). References

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Template-Free Preparation of 3D Porous Co-doped VN Nanosheet

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