Titanium nitride hollow spheres consisted by TiN nanosheets and its

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Kinetics, Catalysis, and Reaction Engineering

Titanium nitride hollow spheres consisted by TiN nanosheets and its controllable carbon-nitrogen active sites as efficient electrocatalyst for oxygen reduction reaction Jinwei Chen, Xiaoyang Wei, Jie Zhang, Yan Luo, Yihan Chen, Gang Wang, and Ruilin Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05719 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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Industrial & Engineering Chemistry Research

Titanium nitride hollow spheres consisted by TiN nanosheets and its controllable carbon-nitrogen active sites as efficient electrocatalyst for oxygen reduction reaction Jinwei Chen*, Xiaoyang Wei, Jie Zhang, Yan Luo, Yihan Chen, Gang Wang, and Ruilin Wang* College of Materials Science and Engineering, Sichuan University, Chengdu 610065, China. Corresponding Authors * Tel.: 86 028 8541-8786. E-mail: [email protected] (J. C.) * Tel.: 86 028 8541-8786. E-mail: [email protected] (R. W.)

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Abstract: Titanium nitride hollow spheres (TiN HSs) consisted by TiN nanosheets have been developed with a carbon-template-assisted strategy. The residual carbon is activated during the removal of the carbon template by calcining at high temperature in the air, and is beneficial for the formation of more carbon-nitrogen active site. The TiN HSs prepared exhibits porous and uniform shell with the thickness of ~30 nm with superior catalytic activity for the oxygen reduction reaction (ORR). The TiN HSs calcined at 325 oC (TiN HSs-325) show the more positive onset potential (0.85 V vs. RHE) for ORR and the enhanced limiting current density of up to 4.5 mA cm-2 due to its advantages of unique nanostructure and the controllable active sites. This work provides a promising approach for the design and synthesis of hollow transition metal nitrides with significantly enhanced performances for ORR and other electrochemical energy conversion/storage devices. Keywords: Oxygen reduction reaction, Titanium nitride, Hollow structure consisted by nanosheet, Nitrogen doped carbon

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INTRODUCTION In the past few decades, fuel cells (FCs) have attracted tremendous attention of many researchers as a kind of promising energy conversion device. With the emergence of two major issues that the environment pollution and energy scarcity, the FCs with low pollution and high energy conversion efficiency are becoming more attractive. The fuel cell electrode reaction includes an oxygen reduction reaction (ORR) occurring at cathode and a hydrogen oxidation reaction (HOR) occurring at the anode. Now, one of the biggest problems of fuel cell is how to improve the sluggish kinetics of the ORR.1-5 At present, as the most common ORR catalyst, platinum (Pt) is rare and expensive. On the one hand it cannot meet the needs of industry, on the other hand, platinum is not stable enough under current operating conditions.6 Therefore, the ORR catalyst greatly restricts the development of fuel cells. Many researchers have been searching for a kind of inexpensive, durable and high-performance oxygen reduction catalyst. Carbon doped with heteroatomic atoms, such as nitrogen (N), phosphorus(P) and sulfur (S) will create more active sites for ORR.7-9 Recently, many attempts have been made to develop nitrogen-doped carbon (N-C) material which are required to be active and stable enough to replace the Pt-group noble metals catalyst.10 However, its performance is still dependence on many factors, such as precursor material, synthesis conditions, the type of nitrogen and microstructure, etc.11 In order to achieve a better performance, especially activity and stability in practical application, N-C modified with other electro catalytic materials (transition metal nitrides/oxides/carbides, etc.) 3



should be developed. Due to the excellent physical property, electrical conductivity, thermal and chemical stability, transition metal nitrides (TMNs) have attracted much attention of researchers.12-14 TMNs have been used in hydrogen storage, lithium-batteries and fuel cells.15-18 The introduction of nitrogen atoms in the lattice gap of transition metals will increase its d-electron density, so the transition metal nitrides have some similar electronic structure to those of precious metals.19 Therefore, TMNs are considered as potential replacements for noble metal platinum for ORR catalyst. Many methods have been proposed to prepare TMNs, including in situ carbothermal, magnetron sputtering, sol-gel/ammonia reduction, chemical vapor deposition (CVD), hydrothermal/ammonia reduction.20-22 However, many TMN nanoparticles mentioned above are agglomerated. The morphology and microstructure of TMNs greatly affect its performance for ORR. For example, by changing the morphology of TiN catalysts from nanoparticles to nanotubes, the shape-specific activity toward the ORR has an unexpectedly improvement and becomes comparable to that of Pt/C.23 In addition, the structure and morphology of the TMN nanostructures were also slightly affected by the doping of various transition metals.24 However, the most problem is how to effectively control the morphology and microstructure of TMNs. TMNs with different morphology and structure have been prepared. Among them, the hollow structure has aroused great interest of researchers due to that the hollow structure has a high specific surface area, which can expose more active sites and improve the ability of oxygen molecules or ions to contact with active sites, at the same time shorting the diffusion distance of 4


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electrolyte.25 At present, several hollow transition metal nitrides have been reported. One of the most effective methods is the hard template method and researchers usually use metal oxide as the hard template, such as MgO.26 It is also possible to use active carbon as a template. Due to the relative advantages of active carbon such as the size of the particle is easy to control, the removal of the template will not introduce impurities and so on, we choose carbon sphere as template. Furthermore, as one of the metal nitride family, titanium nitride (TiN) possesses high electronic conductivity and good electrochemical activity, which enable it to be applied widely in electrochemistry. TiN nanostructures were recently shown to be active for ion adsorption and desorption in alkaline based electrolyte in a super capacitor and oxygen reduction reaction.27,

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However, TiN nanoparticles mentioned above are

likely to be agglomerated and show insufficient active sites for electrochemical reaction. The discovery of TiN combined with N-doped carbon materials as well as their ORR performance is of great interest and significance. Thus, controlling the reservation of carbon template to form more N-C active sites may further promote the performance of hollow TiN catalysts. In this paper, carbon was used as hard template to synthesize TiN hollow spheres (TiN HSs) assembled by two dimensional nanosheets. By controlling the calcining temperature in air during the process of removing the template, the role of carbon content in the formation process of the TiN HSs morphology and effect on the oxygen reduction performance were investigated. It has been found that the different calcination temperature leads to obtain the as-prepared carbon with different content 5



in the samples. In addition, the performance of prepared TiN HSs catalysts for ORR was evaluated and the possible mechanisms for enhanced activity also were discussed. EXPERIMENTAL SECTION Synthesis of TiN HSs All chemical reagents were of analytical grade and used without further purification. First, carbon spheres are synthesized according to the method in the literature.29 14.4 g glucose was dissolved in 80 ml of deionized water to form glucose solution, then the solution was hydrothermally treated at 180 oC for 8 h. The brown black product was filtered three times with deionized water and ethanol aqueous solution respectively. Then it was dried in vacuum at 60 oC for overnight, grinded and collected. To prepare TiN HSs, the carbon spheres prepared by the above process were used as hard templates, and a homogeneous titanium dioxide shell was deposited by sol-gel processing of the carbon spheres in ethanol and deionized water.30 First to prepare the uniform dispersed carbon spheres ethanol solution, appropriate carbon spheres were dispersed in ethanol by stirring (0.5h) and ultrasonic treatment (2.5h). Tetrabutyl titanate was first added slowly to this carbon spheres solution, followed by stirring for 2h to make the titanium precursor dispersed in carbon spheres colloidal solution. Then, it was allowed to use a separating funnel to add dropwise deionized water into the solution with continuous stirring. After that, the colloidal solution was aged overnight. Subsequently, hydrothermal treatment was conducted at 180 oC for 6 h to obtain the carbon@titanium dioxide (C@TiO2) spheres. The product was filtered with deionized water and ethanol aqueous solution. The product was dried in vacuum for 6


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60 oC overnight, grinded and collected. To remove carbon spheres, the C@TiO2 was calcined in the air atmosphere at 250, 325 and 400 oC, respectively. During the calcination process, the calcination ramping rate is 3 oC min-1 and then kept it for 1 hour. The products were denoted as TiO2 HSs-250, TiO2 HSs-325 and TiO2 HSs-400, respectively. Finally, the as-prepared C@TiO2, TiO2 HSs-250, TiO2 HSs-325 and TiO2HSs-400 were calcined in a furnace under NH3 atmosphere at 900°C for 2h to accomplishing nitriding process. Before heating, the furnace was purged with Ar for 30min to remove air and water. The sample was heated to the desired reaction temperature with Ar (50 sccm) and NH3 (50 sccm). The heating rate is 5 oC min-1 and then kept for 2 h with gas flow ofNH3 and Ar (50 sccm /50 sccm). After cooling the furnace by purging Ar, the TiN HSs were collected. The products were denoted as C@TiN, TiN HSs-250, TiN HSs-325 and TiN HSs-400, respectively. For comparison, bulk TiN was obtained by directly nitriding commercial TiO2 powder in the same conditions. In addition, all the as-prepared catalysts were compared with that of the commercial Pt/C (20 wt. %, Johnson Matthey Co.). Materials characterization The XRD patterns were carried out by a DX-2000 x-ray diffractometer(Dandong Ltd, China, Cu-Kα，λ=1.54178 Å) at a scanning rate of 3 °min-1 with 40 kV voltage and 50 mA current with Cu-Kα radiation. The thermogravimetry (TG) analysis was performed from 20 to 500 oC in O2 with a heating of 5oC min-1. Specific surface areas were recorded by Brunauer-Emmett-Teller (BET) nitrogen adsorption–desorption on 7



a gas adsorption analyser (Quantachrome, USA). XPS spectra were obtained using an ESCALAB 250Xi (Thermo Fisher Scientific, USA) spectrometer with an Al target as a Kα x-ray source, and TEM images were obtained using a field emission transmission electron micro-scope (FEI Tecnai G2 F20, USA). The morphology of particle was observed with scanning electron microscope (SEM, S-4800, HitachiJapan) with operating voltage of 5 kV. Electrochemical measurements Evaluation of prepared catalysts performance for ORR was carried out on Autolab electrochemical workstation using rotating disc electrode technique. A standard three electrode system was used for electrochemical performance test, the Hg/HgO (1 M KOH) electrode as the reference electrode, Carbon rod as the counter electrode and the working electrode is glassy carbon with catalyst layer and the electrolyte with 0.1M KOH solution. The preparation process of the catalyst layer are as follows: 10 mg of the catalyst was dispersed into a mixed solvent containing 1 mL of isopropyl alcohol and 1 ml deionized water, then the 5% wt Nafion solution was added to it. Next, a certain amount of the dispersion was uniformly dropped to the surface of freshly polished glassy carbon electrode (5 mm diameter, 0.196 cm2 geometrical surface area), which was dried under ambient conditions. The ORR tests were performed in O2-saturated 0.1 M KOH electrolyte. The potential range is cyclically scanned between -0.8 and 0.2 V with a scan rate of 10 mVs-1. The potential dependence of the current density (mA cm-2 of geometrical surface area) is recorded in cyclic voltammetry (CV) and linear-sweep voltammetry (LSV) characterization. 8


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The CV and LSV were obtained at room temperature after purging O2 or N2 gas for 30 min. The potential cycling was repeated until stable voltammogram curves were obtained. Rotating disk electrode (RDE) measurements were carried out at various rotating speeds from 400 to 2025 rpm at a scan rate of 10 mVs-1. The exact kinetic parameters were investigated on the basis of Koutecky–Levich (K-L) equations. RESULT AND DISCUSSION Structural analyses A facile template-assisted method was used to synthesize TiN HSs. In this process, carbon spheres were fabricated via the polymerization of glucose and the subsequent hydrolysis of tetrabutyl titanate on the surface of carbon spheres, which leads to the formation of C@Ti(OH)4. Then, C@TiO2 was obtained by hydrothermal reaction. Finally, C@TiO2 was calcined in air with different temperature (250, 325 and 400 oC) and further nitriding in NH3 to obtain TiN HSs. According to the TG curve analysis in Figure S1 (the Supporting Information), the first step (100–250 °C) is correlated with related to the loss of adsorbed water. The main weight loss occurs at approximately 250–450 °C, which is attributed to the decomposition of carbon in air. It can be concluded that the reserved carbon content canberetainedaround~45% (TiO2 HSs-400), 70% (TiO2 HSs-325), and 85 % (TiO2 HSs-250). The results indicate that the different calcination temperature leads to obtain the as-prepared carbon with different content in the TiN HSs samples. To verify the crystal structure of these prepared catalysts, the powder XRD of C@TiO2 are presented in Figure 1a. It is clearly identified at about 25°, 37°and 48°corresponding to the TiO2(101), (004) and 9



(200) plane. This indicates that C@TiO2 has been successfully transformed from C@Ti(OH)4 in the hydrothermal process. All the XRD patterns of TiN HSs nanostructure calcined with different temperatures were shown in Figure 1b, denoted as TiN HSs-400, TiN HSs-325, TiN HSs-250 and C@TiN, respectively. It reveals distinct peaks at about 36.8°, 42.7°, 62°, 74.4°, 78.3° corresponding to the TiN (111), (200), (220), (311) and (222) plane(PDF 87-0632). In addition, diffraction peaks of all the TiN samples display different intensities. For comparison, C@TiN HSs also was prepared by directly nitriding C@TiO2 in NH3 under the same conditions. It shows the similar diffraction peaks and crystallinity as that of TiN HSs. The calcination temperature did not give an effect on the synthesis of TiN phase, but it could affect the content of carbon and the crystallization of TiN. In addition, it can be found that the intensity of the peaks displays variation with the different calcination temperature. TiN HSs-400 exhibits the highest diffraction peak intensity and the degree of crystallinity, because calcination at 400 °C is beneficial to crystallization. The BET surface areas derived from C@TiO2,TiO2 HSs-325, and TiNHSs-325 are shown in Figure S2 and the relevant data are listed in Table S1.The specific surface area and average pore size of TiO2 HSs-325 was calculated to be 316.7 m2 g-1, higher than that of C@TiO2(107.9 m2 g-1). The hierarchical mesoporous structure and larger specific surface area of TiO2 HSs-325 would be favorable for the formation of hollow TiN during the subsequent nitridation process. The average pore size is about 3.74 nm and the BET surface areas of TiNHSs-325 are 358.3 m2 g-1, which is higher than that of TiO2 HSs-325. It indicates that the high surface areas and pores of the carbon 10


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template can be well maintained after the nitridation process, which is the key for enhancing the transport of oxygen and the electrolyte onto the catalyst surface. Figure 2 shows the SEM images of C@TiN and TiN HSs-325, respectively. The Figure 2a and 2b present the SEM image of C@TiN under different magnification. A slight agglomeration was observed in Figure 2b due to the high temperature and unevenly dispersed carbon sphere. As shown in Figure 2a, TiN is almost uniformly coated on the surface of carbon spheres. The phenomenon also implies that the core-shell structure has been prepared successfully. Figure 2c and 2d show the morphologies of TiN HSs-325, it is hard to observe the bare carbon sphere from the TiN HSs. Especially, the surface morphology of TiN has been changed after the calcination and nitriding process. The surface of TiO2 is relatively smooth and compact as shown in Figure S3a, but that of TiN HSs-325 consists of numerous irregular two-dimensional nanosheets through larger multiples of the SEM image, which may provide a higher specific surface area. During the nitriding process, the porous structure can be formed on the surface of TiN due to gas production in the pyrolysis. At the same time, high temperature nitridation causes TiN to grow toward a specific crystal, thus assembly forming the two-dimensional sheets. The microstructures of the as-obtained TiN HSs are characterized in detail by TEM. Figure 3 shows the TEM images of TiN HSs-250, TiN HSs-325, and TiN HSs-400. In all samples, TiN HSs nanostructures exhibited an obvious hollow structure assembled by the two-dimensional TiN nanosheets, which revealed the TiN HSs were well dispersed with a shell thickness of ~30 nm. As shown in Figure 3a, 3b and 3c, no 11



significant morphological change was observed in TiN HSs samples prepared with different calcination temperature. For TiN HSs-250, Figure 3d and 3g show that undecomposed carbon can be found in the core part. On the contrary, there is no carbon observed in TiN HSs-400, as displayed in Figure 3f and 3i. However, moderate carbon content retained in TiN HSs-325, accompanied by a homogeneous distribution of TiN shell and hollow carbon sphere (Figure 3b, 3e and 3h). Thus, we speculate that the novel structure favors the enhancement of ORR activity due to its ideal porosity and more exposed active sites. It is demonstrated that moderate carbon not only played the main catalytic role in the formation of two-dimensional TiN nanosheets but also promoted the crystallization of TiN. In addition, HRTEM (inset of Figure 3i) shows that the distance between lattice fringes was measured to be 0.21 nm, which is in accordance with the (002) plane of cubic TiN. To explore the surface electronic state and constituent of TiN HSs-325, XPS spectra were collected and are shown in Figure 4. The survey spectrum (Figure 4a) shows the presence of Ti and N, along with C and O elements. The high-resolution Ti2p spectrum (Figure 4b) can be fitted into four peaks at 464.1, 461.7, 458.6, and 456.3 eV. The two peaks at 464.1 and 461.7 eV are assigned to the N-Ti-N groups in the surface region of TiN HSs-325. The peak at 458.6 eV is assigned to the O-Ti-O species originated from TiO2 and the peak at 456.3 eV is connected with the spectrum of N-Ti-O groups, indicating the presence of oxide layer on the surface of TiN HSs-325.31-33 Figure 4c displays the fitted results of the high-resolution N 1s spectra, where the peak at 398.7 eV is correspond to N-O-H originated from hydrated oxygen 12


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species. The peaks at 397 and 396.2 eV are attributed to N bonding to Ti (from TiN species) and pyridinic-N, respectively. Pyridinic-N contains a lone electron pair, which can enhance the conductivity and facilitate four-electron process during the reduction reaction.34 The C 1s spectrum can be deconvoluted into three peaks at 288.7, 286.1 and 284.7 eV (Figure 4d), corresponding to C-N, C-O and graphitic C, indicating the successful doping of heteroatoms on the carbon lattice.35 According to areas of the fitted peaks, graphitic C occupies a large proportion in TiN HSs-325, leading to the improvement of high conductivity and limiting current density.32,

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Based on the results from TEM and XPS spectra, we can infer that an enhancement caused by alterative electronegativity between the metal titanium and nitrogen atoms, accelerating the charges transfer process in the nitride.37, 38 Furthermore, the improved charge transfer could facilitate the formation of active site, which could promote the dissociation and adsorption of oxygen. In addition, the hollow structure is beneficial to shorten mass transfer distance and carbon as an excellent conductor also shows a certain ORR catalytic activity. Therefore, an excellent electrocatalytic activity towards ORR can be achieved on TiN HSs, which is attributed to the structure and electronegativity. Catalytic activity and durability towards the ORR The effects of their served carbon content on ORR performance of the TiN HSs catalysts have been intensively investigated. Figure 5a shows the CV curves of TiN HSs-325 and bulk TiN in N2- and O2-saturated 0.1 M KOH at a sweep rate of 10 mV s-1. It can be seen that the onset potential and peak potential of ORR on the bulk TiN 13



catalyst are about 0.6 V and 0.5 V, respectively. It suggests the poor ORR activity of the bulk TiN catalyst. However, the TiN HSs-325 catalyst shows more positive potential. The ORR takes place from 0.83 V, and reaches a peak at around 0.68 V. It suggests that TiN HSs-325 shows a smaller over potential and transfer limitations for ORR. Figure 5b shows the RDE voltammograms of TiN HSs-325, bulk TiN, N-C with TiN and commercial Pt/C (20 wt.%) in O2-saturated 0.1 M KOH with a scan rate of 10 mV s-1 at 1600 rpm. The ORR activity of TiN HSs-325 exhibit much higher onset potential (0.85 V) as well as the limited current density than the N-C without TiN-introduced. It also can be clearly seen that the onset potential of TiN HSs-325 is more positive than that of bulk TiN, indicating a significantly improved electrocatalytic activity of TiN HSs. The limiting current density of TiN HSs-325 is about 4.5 mA cm-2, which is much higher than that of bulk TiN (3.2 mA cm-2). The bulk TiN shows an inferior ORR activity, which is attributed to the severely agglomerate to form massive particle without adequate active sites. In addition, the onset potential and limiting current density of TiN HSs-325 also is close to that of Pt/C. We speculate the excellent ORR catalytic activity of TiN HSs-325 is related to the increased exposed active sites and better access for the electrolyte to the active sites, promoting the mass transfer of the reactant gas and conducting ions on the catalyst surface. In order to investigate the effects of the reserved carbon content for the ORR, LSV voltammograms of TiN HSs-400, TiN HSs-325, TiN HSs-250 and C@TiN were recorded in O2-saturated 0.1 M KOH with a scan rate of 10 mV s-1 at 1600rpm, as 14


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shown in Figure 5c. The carbon content varies with the calcination temperature; the corresponding oxygen reduction properties of the samples are quite different. C@TiN was prepared without the calcination process in air, we can consider that it presents a solid sphere consisting of TiN and carbon. This structure may cause an inferior electron transfer efficiency due to the prolonged electron-transfer path between TiN nanosheets and N–C core. The onset potential changes with a further increase in calcination temperature, in which TiN HSs-325 exhibits the more positive onset potential compared with others. Additionally, the current density for the TiN HSs-400, TiN HSs-325, TiN HSs-250, and TiN@C at 0.58 V are 0.1, 2.8, 1.5, and 0.4 mA cm-2, respectively. The combined effect of the onset potential and current density induced by TiN HSs-325 facilitates the ORR process. To gain further insight into the ORR process of TiN HSs-325, Figure 5d shows LSV polarization curves of TiN HSs-325 at different rotation rates. The limiting current density gradually increased with the rotation speed, which is attributed to the enhanced diffusion rate of the electrolyte and easier to the electrode surface for oxygen.39 Figure 5e shows the corresponding Koutecky–Levic(K-L) plots obtained from TiN HSs-325. It reveals first-order reaction with respect to the dissolved oxygen concentration and similar electron transfer number for the ORR process at different potentials. According to K-L plots, the electron transfer number per oxygen molecule (as displayed in Figure 5f) in the ORR on the TiN HSs-325 electrode was calculated between 3.5 and 3.9 at potentials ranging from 0.28 to 0.58 V, suggesting the oxygen reduction is mainly through the four-electron pathway. It can be concluded that the 15



unique microstructure of TiN HSs is beneficial to improve the electrochemical kinetics for the ORR and the suitable carbon content helps to form more active sites, which is beneficial to the ORR. We speculate that the reasons for high catalytic performance on TiN HSs-325 are the following. First, hollow structure has a high specific surface area and a porous surface, which facilitates the exposure of the active sites and shorts mass transfer distance.25 Secondly, the surface consisted by nanosheets provide higher specific surface. Finally, by adjusting the calcination temperature, the carbon content is controlled, which increases the conductivity of the prepared catalyst and form more N-C active sites.40 In short, the presence of TiN HSs and N-C in the composite strengthened the electronic structure and increased the contents of active sites, which resulted in improved ORR performance. Besides the catalytic activity, the methanol tolerance and stability are also crucial factors for fuel cell catalysts and need to be considered. Figure 6a shows when the 3 M methanol is added into the electrolyte, the ORR current density of Pt/C quickly decreased due to the methanol oxidation intermediate products. For the TiN HSs catalyst, the addition of methanol had no significant influence on ORR current density, revealing indicating that TiN HSs has better methanol resistance than Pt/C. As depicted in Figure 6b, the current–time (i–t) chronoamperometric responses were performed at -0.35 V (vs. Hg/HgO) in 0.1 M KOH solution at a rotation rate of 1600 rpm. After 15000 s, the Pt/C catalyst suffers a loss of 25% in the current density, whereas TiN HSs-325 only shows a loss of 17 %, suggesting a superior long-term durability of the TiN HSs-325 for the ORR. 16


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CONCLUSION In this study, a hollow structure TiN HSs was fabricated using hard template method and followed with a nitridation process with controllable structure and carbon content. The obtained TiN HSs consist of two-dimensional TiN nanosheets and porous structure produced during nitridation, which can provide high surface area, superior electroconductivity, shortened diffusion pathway, and exposed active sites. Most importantly, TiN HSs-325 displays excellent ORR activity and stability as well as good ORR selectivity in alkaline solution, which is close to that of Pt/C. We believe that the reserved carbon could increase the active sites and improve the conductivity, which is also directly related to the formation of two dimensional nanosheet structures. This work not only develops a new strategy of building novel nanostructure from TMNs but also provides an idea of controllable constructing materials which satisfy both energy conversion and storage fields.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Details on TG curve of carbon spheres, BET data and SEM images of samples (Figure S1 to S3 and Table S1)

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AUTHOR INFORMATION Corresponding Author Corresponding Authors * Tel.: 86 028 8541-8786. E-mail: [email protected] (J. C.) * Tel.: 86 028 8541-8786. E-mail:[email protected] (R. W.)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51602209), the Provincial Nature Science Foundation of Sichuan (2017CC0017, 2018FZ0105) and the Fundamental Research Funds supported by Ministry of Education of the People’s Republic of China (No. YJ201746, 2018SCUH0025).

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Figure captions: Figure 1. XRD patterns of (a) C@TiO2, (b) XRD patterns of TiN HSs-400, TiN HSs-325, TiN HSs-250, and C@TiN. Figure 2. SEM images of C@TiN (a and b) and TiN HSs-325(c and d). Figure 3. TEM images of (a, d and g) TiN HSs-250, (b, e and h) TiN HSs-325, and (c, f and i) TiN HSs-400 (The inset of Fig. 3i is the corresponding HRTEM image). Figure 4. XPS spectra of TiN HSs-325: (a) a survey spectrum, (b) high-resolution Ti 2p, (c) N 1s, and (d) C 1s. Figure 5. (a) CV curves of TiN HSs-325 and bulk TiN in N2- and O2-saturated 0.1 M KOH at a sweep rate of 50 mV s-1, (b) RDE voltammograms of TiN HSs, bulk TiN, N-C and Pt/C in O2-saturated 0.1 M KOH with a scan rate of 10 mV s-1 at 1600 rpm, (c) RDE voltammograms of TiN HSs-250, TiN HSs-325, TiN HSs-400 and TiN@C in O2-saturated 0.1 M KOH with a scan rate of 10 mV s-1 at 1600 rpm, (d) LSV of TiN HSs-325 at different rotation rates, and (e) Koutecky–Levich plots at different potentials for TiN HSs-325, and (f) Electron-transfer number n as a function of the electrode potential. Figure 6. (a) Chronoamperometric response of commercial Pt/C and TiN HSs at 0.2 V in O2-saturated 0.1 M KOH solution. 3M Methanol was introduced into the electrolyte at 3000 s. (b) Current–time (i–t) chronoamperometric responses of TiN HSs and Pt/C electrodes at -0.35 V (vs. Hg/HgO) in O2-saturated 0.1 M KOH at a rotation rate of 1600 rpm.

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TOC 85x47mm (300 x 300 DPI)



Figure 1. XRD patterns of (a) C@TiO2, (b) XRD patterns of TiN HSs-400, TiN HSs-325, TiN HSs-250, and C@TiN. 540x209mm (300 x 300 DPI)


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Figure 2. SEM images of C@TiN (a and b) and TiN HSs-325(c and d).



Figure 3. TEM images of (a, d and g) TiN HSs-250, (b, e and h) TiN HSs-325, and (c, f and i) TiN HSs-400 (The inset of Fig. 3i is the corresponding HRTEM image).


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Figure 4. XPS spectra of TiN HSs-325: (a) a survey spectrum, (b) high-resolution Ti 2p, (c) N 1s, and (d) C 1s. 119x90mm (300 x 300 DPI)



Figure 5. (a) CV curves of TiN HSs-325 and bulk TiN in N2- and O2-saturated 0.1 M KOH at a sweep rate of 50 mV s-1, (b) RDE voltammograms of TiN HSs, bulk TiN, N-C and Pt/C in O2-saturated 0.1 M KOH with a scan rate of 10 mV s-1 at 1600 rpm, (c) RDE voltammograms of TiN HSs-250, TiN HSs-325, TiN HSs-400

and C@TiN in O2-saturated 0.1 M KOH with a scan rate of 10 mV s-1at 1600 rpm, (d) LSV of TiN HSs-325 at different rotation rates, and (e) Koutecky–Levich plots at different potentials for TiN HSs-325, and (f) Electron-transfer number n as a function of the electrode potential.


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Figure 6. (a) Chronoamperometric response of commercial Pt/C and TiN HSs at 0.2 V in O2-saturated 0.1 M KOH solution. 3M Methanol was introduced into the electrolyte at 3000 s. (b) Current–time (i–t) chronoamperometric responses of TiN HSs and Pt/C electrodes at -0.35 V (vs. Hg/HgO) in O2-saturated 0.1 M KOH at a rotation rate of 1600 rpm.


Titanium nitride hollow spheres consisted by TiN nanosheets and its

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