Scaffold-Like Titanium Nitride Nanotubes with a Highly Conductive

May 19, 2016 - The accelerated durability test (ADT) revealed that this nanotubular supporting material dramatically enhanced the durability of the ca...
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Research Article pubs.acs.org/acscatalysis

Scaffold-Like Titanium Nitride Nanotubes with a Highly Conductive Porous Architecture as a Nanoparticle Catalyst Support for Oxygen Reduction Heejong Shin,†,‡,∥ Hyoung-il Kim,§,∥ Dong Young Chung,†,‡ Ji Mun Yoo,†,‡ Seunghyun Weon,§ Wonyong Choi,*,§ and Yung-Eun Sung*,†,‡ †

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 151-742, Korea School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742, Korea § School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea ‡

S Supporting Information *

ABSTRACT: We designed a scaffold-like porous titanium nitride (TiN) nanotube (NT) as a catalyst support for Pt to facilitate the oxygen reduction reaction. Bulk titanium nitride, which is known as an electrically conductive material, is compatible with other metals. As the size of TiN particles decreases, however, they lose their intrinsic high electrical conductivity, due to a series of nanoparticle grain boundaries acting as electron reservoirs and traps. A designed grainboundary-free scaffold-like porous TiN NT which is analogous to the shape of the one-dimensional porous human spine exhibits high electrical conductivity in spite of having a surface area similar to that of TiN nanoparticle (NPs). The electrical conductivity of TiN NTs is ca. 30-fold higher than that of spherical TiN NPs. The electrochemical oxygen reduction measurements between porous TiN NT and TiN NPs after Pt loading clearly exhibit the superiority of TiN NT as a catalyst support. The results from various electrochemical measurements suggest that the electrocatalytic activity per site did not change from a kinetic viewpoint, but the utilization (the amount of triggered catalytic active sites) in the catalyst layer on the electrode decreased. The Pt/TiN NT composite catalyst exhibited higher activity in comparison to TiN NPs as well as conventional Pt/C catalysts. The accelerated durability test (ADT) revealed that this nanotubular supporting material dramatically enhanced the durability of the catalyst and maintained the electrochemically active surface area (ECSA) of Pt nanoparticles, thus exhibiting performance higher than that of the commercial Pt/C catalyst. X-ray spectroscopy results verified the strong metal−support interaction between Pt nanoparticles and the TiN NT support. This approach opens a reliable path for designing innovative transition-metal oxides, nitrides, or carbides as catalyst supports for use in a wide range of energy conversion applications. KEYWORDS: titanium nitride, electrocatalysis, oxygen reduction reaction, strong metal−support interaction, accelerated durability test, Pt/TiN

1. INTRODUCTION

the harsh conditions of PEMFCs. To solve these issues, several studies have concentrated on adapting materials that are corrosion resistant,8 including some with graphitic character: for example, nanofibers, carbon nanotubes, and graphene.9−12 Although these nanostructured carbons have corrosion damage lower than that of carbon black, they cannot still avoid long-term corrosion damage because they are basically composed of carbon materials, like carbon black. Therefore, it is vital to develop new strategies and improve catalyst material design to increase both catalyst durability and the efficiency of the ORR.

Polymer electrolyte membrane fuel cells (PEMFCs) have emerged as promising candidates for use in the automobile industry and portable electronics because they have high power density and are portable.1,2 However, despite recent advances, oxidation of carbon catalyst supports is a great concern for the long-term stability of PEMFCs. Sudden changes in operating conditions such as partial fuel starvation and start up/shutdown cycles can result in cathode potentials of up to 1.4 V, at which the cathode carbon support material rapidly oxidizes.2−4 That is, the carbon materials are highly susceptible to corrosion, thus leading to detachment of the metal nanoparticles from carbon supports and a consequent decrease in the catalytic activity over long-term operation.5−7 It is challenging to catalyze the sluggish oxygen reduction reaction (ORR) at the cathode under © XXXX American Chemical Society

Received: February 5, 2016 Revised: April 14, 2016

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10 min. The well-mixed suspension was placed into a sealed Teflon container (ca. 120 mL) and then hydrothermally reacted at either 120 °C for 24 h or 200 °C for 48 h. After the reactor was cooled to room temperature, the precipitates were filtered and washed with distilled water until a neutral filtrate was obtained. The product was dried in an oven at 80 °C for 12 h. The dried titanate nanofibers were placed into a flow furnace and were annealed at 800 °C for 2 h under NH3 flow (150 mL/min) with a progressive heating ramp (room temperature to 300 °C (5 °C/min); ∼700 °C (2 °C/min); ∼800 °C (1 °C/min)) to minimize the structural change. The obtained dark powder was carefully ground with a mortar and pestle. Spherical TiN NPs were obtained by the same heat treatment with porous NTs (at 800 °C for 2 h under NH3 flow) from the titanate fibers treated at 120 °C. 2.2. Growth of Pt Nanoparticles. To coat the TiN nanostructures with Pt nanoparticles,34 ethylene glycol (20 mL, EG, Sigma-Aldrich) was initially injected into a three-neck flask and heated in air at 110 °C for 30 min. TiN nanostructures (38.74 mg) were then added to the EG and heated for an additional 30 min. PVP (225 mg, 400 mM, Mw ≈ 55000, SigmaAldrich) and H2PtCl6·6H2O (25.71 mg, 20 wt % of Pt to TiN nanostructures, Sigma-Aldrich) were prepared separately in EG (10 mL) at room temperature. These two solutions, each 5 mL in volume, were then placed simultaneously into the flask over a period of 1.5 min. The reaction mixture was heated at 110 °C for 3 h. The final product was washed thoroughly with ethanol and water to remove EG and excess PVP. The dried samples were placed into a flow furnace, followed by heat treatment by annealing at 400 °C for 3 h under an H2 5%/Ar 95% flow (150 mL/min) with a heating ramp (room temperature to 400 °C (5 °C/min)). 2.3. Materials Characterization. High-resolution transmission electron microscopy (HR-TEM) images were measured using an F20 FEG Technai instrument at 200 kV (Research Institute of Advanced Materials, SNU). Field emission scanning electron microscopy (FE-SEM) images (JSM-6700F) were measured to characterize the material morphology. Powder X-ray diffraction (XRD) was measured using a Rigaku D/MAX 2500 instrument with Cu Kα radiation (λ = 0.15418 nm), ranging from 20 to 80° in 2θ (generator settings were 40 kV and 200 mA). The atomic concentration of materials was determined by means of inductively coupled plasma mass spectrometry (ICP-MS, NexION 350D, PerkinElmer SCIEX). Specific surface areas and pore distributions were measured using a Micromeritics TriStar II 3020 instrument. A four-point probe device (CMT-SP 2000N) was used to estimate the conductivity of the compressed powders. X-ray photoelectron spectroscopy (XPS) was conducted to characterize the core level and using a synchrotron X-ray source at the Pohang Light Source-II (PLS-II beamline 8A1, 3 GeV). X-ray absorption near-edge structure (XANES) of the Pt L3 edge was performed using synchrotron X-rays at PLS-II beamline 8C. 2.4. Electrochemical Measurements. The prepared Pt/TiN nanostructure composites (10 mg) were mixed with 60 μL of Nafion resin (Nafion perfluorinated resin solution (5 wt %) in a mixture of lower aliphatic alcohols and water (45% water), Sigma-Aldrich, 15 wt %) as the binder and dissolved in 700 μL of 2-propanol (Sigma-Aldrich). The solution (7 μL) was deposited on a rotating ring−disk electrode (RRDE, Pine, E7R9 Series tips). The loading density of Pt catalyst on the electrode was about 0.744 mgPt cm−2. All electrochemical experiments were evaluated using an Autolab potentiostat (PGSTAT302N, Metrohm) in a standard three-electrode cell

Recently, much attention has centered on the development of noncarbon support materials such as highly stable, conducting transition-metal oxides that can prevent the corrosion of the catalyst support.13,14 However, carbon-based support materials are still more conductive than metal oxides by several orders of magnitude. Many investigators have resolved the conduction problem by doping support materials with other metals or adding a carbon conducting agent: for example, as in lithium ion batteries.12,15−17 However, carbon conducting agents are not a practical substitute because they also corrode under PEMFC conditions. Over the past few years, transition-metal nitrides and carbides have attracted increasing research interest for their potential applications as electrocatalysts, catalyst supports, lithium ion batteries, and solar cell materials.18−20 In particular, titanium nitride (TiN) as a high lyconducting material (ca. 40 kS cm−1 in the bulk phase) has been recently studied as a catalyst support and showed improved resistance to corrosion damage.21−28 To maximize the catalytic activity by increasing the number of active sites on the catalyst surface, nanoscale catalyst particles and support materials with high surface area are favored.29 However, as the size of TiN particles decreases, they lose their intrinsic high electrical conductivity. This occurs because the number of interparticle grain boundaries (ohmic contacts) increases and these grain boundaries act as resistance during electron transfer. To overcome this problem, researchers have been seeking to increase the surface area of TiN while maintaining the conductivity. One resent approach is to add carbon materials as conducting agents in the TiN nanoparticle (NP) network,24,25 but this approach has a carbon corrosion problem. Another recent approach is to control the TiN morphology as one-dimensional fiber or tubular structures, but previously applied one-demensional TiN structures by electrospinning and solvothermal methods were consisted of closely packed TiN NPs, still containing a large number of grain boundaries.30,31 Herein, we report a new type of TiN support for Pt to facilitate the oxygen reduction reaction. The distinctive boundary-free TiN support from a single-crystalline TiO2 nanofiber, with a scaffold-like, wrinkled, nanotube structure, exhibits a superior conductivity (118.73 S cm−1) and stability in comparison to conventional carbon supports (ca. 5 S cm−1).31 In order to investigate the structural benefit of the present TiN NTs as a catalyst support, we prepared an agglomerate of spherical TiN NPs which has a similar surface area but a great number of grain boundaries and compared various electrochemical properties between NPs and NTs. In comparison to TiN NPs, the TiN NTs have shown a much higher conductivity (ca. 30-fold), supercapacitor performance, and ORR activity. In addition, we first demonstrated that, although the electrocatalytic activity per site does not change from a kinetic viewpoint, the catalytic current density can be improved by an increase in active catalyst area, utilization, on the electrode. Thus, this new approach allowed the production of a superior support material for Pt that showed much improved activity and durability over commercial Pt/C catalysts for the ORR for potential application in PEMFCs.

2. EXPERIMENTAL SECTION 2.1. Preparation of Titanium Nitride Nanoparticles and Porous Nanotubes. Titanate nanofibers were prepared by alkali-assisted hydrothermal treatment.32,33 For example, P25 (1 g) was used as the source of TiO2 and was added to a concentrated NaOH solution (60 mL, 10 M). The suspension was then vigorously stirred with a magnetic bar, followed by sonication for 3915

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(JCPDS file 21-1272) observed, which means anatase titania was completely converted to TiN during the thermal nitridation treatment. SEM and TEM images were used to observe the surface morphology of the TiN materials (Figure 1a−d). The scaffold-like TiN NTs had very distinctive structural features. The length of TiN NT is approximately 5−10 μm with a diameter of 250 nm, while the TiN NPs form agglomerates consisting of NPs with an average size between 30 and 40 nm. These different morphologies were attributed to the contraction of TiO2 after nitridation processes. The density difference between the TiN and TiO2 phases led to a porous and wrinkled scaffold-like TiN structure. In brief, the structures were maintained after nitridation in the case of TiO2 fibers synthesized at 200 °C by the hydrothermal method.35 However, the TiO2 fibers made at 120 °C were broken and rearranged during nitridation, leading to spherical pieces of these TiN nanostructures. Images of the original TiO2 nanofibers are shown in Figure S2 in the Supporting Information. The Brunauer−Emmett−Teller (BET) nitrogen adsorption and desorption isotherms (Figure S3 in the Supporting Information) of the TiN materials are typical type IV isotherms with a distinct hysteresis loop, attributable to the presence of mesoporous structures in the TiN nanostructures.36 This is supported by the pore size distribution curve, as shown in the inset of Figure S3 in the Supporting Information. TiN NTs and NPs had comparable pore distributions, which indicates that these TiN support materials have similar surface pore structures. The BET surface areas of the TiN-based support materials are ca. 25.19 m2 g−1 for TiN NTs and ca. 39.10 m2 g−1 for spherical TiN NPs, respectively. Although the specific surface area of NTs was even smaller than that of spherical NPs, the electronic conductivity of TiN NTs was much higher than that of spherical TiN NPs, with a value of 118.73 S cm−1 (28 times higher than that of NPs (4.17 S cm−1)), as confirmed by four-point probe measurements conducted at a compressive pressure of 5 tons cm−2. 3.2. Characterization and Electrocatalytic Performances of Pt/TiN Composites. The formation of crystalline Pt nanoparticles in the Pt/TiN NTs and Pt/TiN NPs was verified by XRD measurements (Figure S1 in the Supporting Information). Figure 1 shows the TEM images of Pt/TiN composites: Figure 1e for Pt/TiN NPs and Figure 1f−h for

with a Pt wire as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. All of the potentials shown are relative to the reversible hydrogen electrode (RHE), which was calibrated by the determining the potential at which hydrogen oxidation and evolution occurred. Before the electrocatalytic performance was tested, 100 cycles of cyclic voltammetry were conducted from 0.05 to 1.2 V in an Ar-saturated 0.1 M HClO4 (Sigma-Aldrich, 70%) solution to obtain stable signals. The ORR performance was measured in an O2-saturated 0.1 M HClO4 solution at a rotating rate of 1600 rpm and a scan rate of 10 mV s−1 at 293 K in a water bath. The nonFaradaic capacitance term was calibrated by reduction of CV under Ar at the same scan rate. The electron transfer number was calculated from the disk current and ring current of the RRDE system. For addition of carbon black (Vulcan XC 72R, Cabot Corporation) as a conducting agent to the catalyst ink, Pt/TiN composites (5 mg) and carbon black (5 mg) were mixed with the same amount of Nafion binder and 2-propanol mentioned earlier. The accelerating durability test (ADT) was conducted in the following manner. The potential was scanned for 10000 cycles sequentially from 0.6 to 1.2 V vs RHE at a scan rate of 50 mV s−1 in Ar. Supercapacitor performance was measured by using the same three-electrode system, and the procedure is described in section 2.3. The specific capacitances of the TiN nanostructures were measured using a 6 M KOH solution purged with Ar. Electrochemical impedance spectroscopy (EIS) was carried out under open circuit voltage from 50 to 100 kHz with a 5 mV amplitude.

3. RESULTS AND DISCUSSION 3.1. Characterization of TiN Support Nanostructures. We synthesized two different TiN nanostructures as ORR catalyst supports by hydrothermal treatment and subsequent thermal nitridation (under NH3 flow, up to 800 °C). The structures of the TiN materials were confirmed by XRD measurements (Figure S1 in the Supporting Information). The diffraction pattern of TiN NTs and TiN NPs showed five dominant face-centered cubic (fcc) TiN phase peaks (JCPDS file 38-1420). There were no diffraction patterns characteristic of anatase-TiO2

Figure 1. HR-TEM images: (a, b) TiN NPs; (c) scaffold-like TiN NTs; (e) Pt/TiN NP catalysts; (f−h) Pt/TiN NT catalysts. FE-SEM image: (d) TiN NTs. 3916

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were deposited (Figure S6b in the Supporting Information). This indicates that the Ti in the support materials transfers electrons to Pt. Therefore, unlike carbon supports, the TiN supports changed the electronic structure of Pt nanoparticles. Intriguingly, the Pt nanoparticles deposited on TiN NP had lower ORR catalytic performance; specifically, the limiting cathodic current was lower than that of the Pt/TiN NT catalysts. As the ORR is kinetically a very fast reaction at high overpotential regions, we can consider that the liming current is mass-transfer limited. In this case, the cathodic limiting current is expressible by the Levich equation:

Pt/TiN NTs. The Pt nanoparticles were highly dispersed and showed good contact with the TiN supports. The average size of Pt nanoparticles on TiN NT is 3.65(±0.52) nm, and that of TiN NP is 3.80(±0.41) nm (Figure S4 in the Supporting Information). The Pt nanoparticles had a hemispherical rather than spherical shape on the TiN support surface (Figure 1h and Figure S4 in the Supporting Information). These wetted hemispherical Pt nanoparticles may indicate a strong metal−support interaction (SMSI).37 The ORR catalytic performance of the designed Pt/TiN composites was measured. For comparison, we controlled the quantity of platinum nanoparticles loaded on the TiN supports to be 20 wt % by regulating the amount of Pt precursor (more details are given in the Experimental Section). We also confirmed the quantity of Pt loading by ICP-MS analyses (Table S1 in the Supporting Information), which indicated the amounts of Pt loading in Pt/TiN NP and Pt/TiN NT are 19.7 and 19.2 wt %, respectively. Figure 2a shows the ORR polarization curves of

il,c =

0.62nFADO2/3ω1/2CO* ν1/6

If some experimental conditions contained in the above equation were fixed, such as the diffusivity (DO) and the concentration (CO*) of oxygen in saturated electrolyte solution at 293 K, the rotating speed (ω) of the RRDE, the voltage scan rate (v), and the electrode area of the catalyst layer (A), we could simplify this equation. That is, the limiting current becomes proportional to “the total electron-transfer number (n)” only; therefore, it is generally accepted as an indicator of ORR kinetics. However, the electron transfer term alone was not enough to elucidate the relation between the poor conductivity of the sample and the ORR activities. We considered that the electrode area (A) was not constant, since poor conductivity could affect the utilization of the catalyst layer. Thus, we asked how the poor electrical conductivity of the catalyst support could affect the catalytic performance: that is, is it due to the reduced electron-transfer number (n) per catalytically active site or less utilization of the catalyst layer (A), i.e., electrode area? To improve the poor electric conductivity of the TiN NPs, we mixed the Pt/TiN NP catalysts with an auxiliary carbon conducting agent. The added carbon as an assisting conducting agent caused an improvement in its limiting current density, whose value was even equal to that of both the commercial Pt/C catalyst and the Pt/TiN NT catalyst, as shown in Figure 2c. This can be explained by the fact that the added auxiliary carbon allowed use of the entire Pt in the electrode, by simply increasing the electrical conductivity of the supporter. Using Iring and Idisk of the RRDE system, the total electron-transfer number (n) was calculated in this case.40 Surprisingly, it was found not to have changed when carbon was added (Figure 2d). This indicates that the improved cathodic limiting current was caused by an increase in the electrochemically active electrode area (A), since there was nothing to be changed apart from it. There was another possibility that the low limiting current would occur because the Pt/TiN NP catalysts did not cover the entire area of the electrode. It is well-known that TiN materials are denser in comparison with other carbon-based materials. In that case, the addition of carbon black would fill the blank area of the electrode, increasing the catalyst area on the electrode. To testify to the existence of a blank area on the electrode, we increased loading of only the Pt/TiN NP catalyst (without additional carbon black) on the glassy-carbon electrode. Therefore, we loaded 2 (14 μL) or 3 times (21 μL) the amount of the Pt/TiN NP catalyst onto the disk electrode. A comparison of their limiting currents, as shown in Figure 2c, revealed that there was no change with increasing loading amount, which indicated that the original catalyst loading (7 μL) was sufficient to fill the electrode area. That is, the catalyst coverage of the electrode was not the cause of the lower limiting current.

Figure 2. Oxygen reduction reaction activity: (a) ORR curves with 1600 rpm rotation; (b) mass specific activity at 0.9 and 0.95 V (vs RHE); (c) ORR activity of Pt/TiN NP with carbon and without carbon; (d) total electron transfer number (n) of each case at 0.7 V.

the Pt/TiN NT catalysts in comparison to commercial Pt/C (Johnson Matthey, 20 wt %) and also Pt/TiN NP catalysts. The Pt/TiN NT catalysts had better catalytic activities in comparison to the commercial Pt/C. The higher activity of the Pt/TiN NT catalysts for ORR may be attributed to the SMSI between Pt nanoparticles as well as the excellent conductivity of the scaffold TiN NTs. The superior mass activities of Pt/TiN NT catalysts at 0.90 and 0.95 V (vs RHE) are shown in Figure 2b. The specific activities for the ORR normalized by the electrochemical active surface area (ECSA) of Pt are also presented in Figure S5 and Table S2 in the Supporting Information. SMSI can modify the electronic structure of the metal catalyst.38,39 A modified electronic structure results in different catalytic properties. To elucidate the origin of the enhanced catalytic performance and the stabilization of the Pt/TiN NT catalyst, the Pt 4f XPS spectra of the commercial Pt/C and the Pt/TiN NTs (Figure S6a in the Supporting Information) were used to verify the strong interaction between the Pt nanoparticles and TiN supports. The binding energy of the Pt 4f signal for platinum deposited on the TiN NT was significantly lower (0.195 eV) in comparison with the samples of Pt deposited on carbon. On the other hand, the binding energy of the Ti 2p signal from the TiN NTs increased (0.274 eV) after Pt nanoparticles 3917

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easily transported in one-dimensional TiN NTs than in the zero-dimensional spherical powder systems. As the two TiN nanostructures have similar pore size distributions and very small BET surface areas in comparison to typical carbon nanomaterials (Figure S3 in the Supporting Information), their different rate performances did not result from differences in the mobility of the electrolyte ions. Instead, this suggests that TiN NP lost electrical conductivity, due to a series of nanoparticle boundaries acting as electron reservoirs and traps. That is, the unique porous, scaffold-like, cylindrical structure allowed facilitated carrier diffusion in TiN materials; thus improving electrical transport. In addition, we conducted a frequency response analysis using EIS measurements to confirm the electrical and ionic conductivities. The Nyquist plots of both scaffold TiN NTs and TiN NPs are shown in Figure 3c. The magnified Nyquist plots in the inset clearly display a single capacitive arc in both NT and NP samples. These arcs, which are assigned as a complex electronic resistance of the intergranular electronic resistance between the TiN particles and the interface between TiN particles and the electrode, mainly were determined by the intrinsic electronic properties of TiN particles.41,42 The smaller arc in the TiN NT sample indicates a much superior electronic property in comparison to TiN NPs, being derived by the morphological advance of the one-dimensional tubular structure as an ORR catalyst support. This is consistent with our previous CV results. We can also estimate the rate capability of materials from EIS measurements by calculating the complex capacitance (Figure 3d). Complex capacitance analysis is a useful tool for extracting frequency-related information. Complex capacitance is defined as43 1 Z(ω) = jωC(ω)

The increased onset potential in this case was just attributed to the larger quantity of loaded catalyst on electrode for testing. If the thickness of catalyst layer was a problem in the case of Pt/TiN NP, Pt/TiN NT should be under the same conditions. TiN NTs and TiN NPs have similar volume densities, as they are the same material only with different morphologies. Thus, the thickness of catalyst loading on the GC electrode would be almost identical in both cases, as we loaded the same quantity of each of them. In comparison, almost all of the mass of Pt in Pt/TiN NT was utilized without any auxiliary carbon; thus its limiting current was similar to that of the commercial Pt/C, shown in Figure 2a. Therefore, we concluded that the inactive and unutilized area is directly related to the use of a poorly conductive support. After carbon was added as an auxiliary conducting agent, the total active area of the catalyst increased, allowing the use of the totality of the Pt mass present in the electrode. This caused the Pt/TiN NP materials to overcome their poor electrical conductivity. In summary, the poor conductivity of the electrocatalyst support may cause low utilization of the catalyst layer. In this sense, our scaffold-like morphology could help to overcome these limitations without any auxiliary carbons. 3.3. Supercapacitor and Transport Characterizations of the Designed TiN Supports. To confirm the role of the conductivity of catalytic supports, the capacitive measurements were carried out in the three-electrode aqueous electrolyte system. The cyclic voltammograms (CV) were measured at different scan rates to investigate the properties of the supercapacitors, as shown in Figure 3a,b. The samples were tested over

C(ω) = C′(ω) − jC″(ω) C′(ω) =

−Z″(ω) ω |Z(ω)|2

C″(ω) =

Z″(ω) ω |Z(ω)|2

The value of C′(ω), detected at low frequency, corresponds to the static capacitance. C″(ω) is also correlated to the energy dissipation of the capacitor by an irreversible process: for example, irreversible Faradaic charge transfer or iR drop. From the peak frequency of C″(ω), the relaxation time constant can be calculated, which reflects the kinetic performance of the materials.44−46 The C″(ω) plot of TiN NTs (red squares), as shown in Figure 3d, has a peak at 17.78 Hz in the measured frequency range. In contrast, the C″(ω) plot of the TiN NPs (red upper triangle) did not have an absolute peak in this frequency region; however, it is possible that the peak frequency of the TiN NPs occurs at a lower frequency. The higher peak frequency of TiN NTs indicates a shorter relaxation time constant, as well as faster kinetics. That is, scaffold-like TiN could offer direct electrical pathways for electrons and increase the electron transport rate, which may improve the electrocatalytic performance because it could minimize the interparticle ohmic contacts with the one-dimensional structure.47 Of course, the surface areas of one-dimensional materials become smaller than spherical nanostructured particles. However, the scaffold-like structure can also overcome these demerits, as it has a very porous and hollow morphology. Therefore, we used these highly conductive TiN nanostructures as electrocatalyst supports for the ORR (Figure 4).

Figure 3. CV curves in KOH solution with various scan rates of (a) TiN NT and (b) TiN NP catalysts. (c) Nyquist plots of EIS results. (d) Real and imaginary capacitance plots for the complex capacitance analysis.

a potential range of 0.05−1.0 V (vs RHE) in a 6 M KOH solution for 50 scans. No special Faradaic redox currents were observed in their CV curves, which indicates the electrochemical stability of the TiN nanostructures. Due to the excellent conductivity and porous structure of the TiN NTs, the rectangular CV curves observed at scanning rates from 20 to 500 mV s−1 were retained, even at the high scan rate of 500 mV s−1, indicating excellent capacitive behavior. However, the specific capacitance of the TiN NP decreased with increasing scan sweep rates. The total specific capacitance of TiN NP was also smaller than that of the TiN NTs at low scan rates, even though the BET surface area is larger. This indicates that the electrons and electrolyte ions are more 3918

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suggesting that carbon corrosion at high potentials was responsible for this effect. The TEM image of the Pt/TiN NT after ADT is strikingly different from that of the commercial Pt/C (Figure S8 in the Supporting Information). TEM images of Pt/TiN NT catalysts show that the morphology of the scaffold-like nanotube and the similar sizes of the Pt nanoparticles were preserved, indicating the electrochemical stability of the Pt particles. We also made an effort to verify the stability of TiN NTs as well as that of Pt NPs. To verify the stability of TiN materials in our case, we conducted XPS characterizations (the core level of Ti 2p) and XANES analyses (the Ti K edge) in comparison between initial and 10000-cycle ADT tested samples of TiN NTs (Figure S7 in the Supporting Information). As shown by the data, there are no significant differences after the ADT test, which means that the TiN surface barely changed. This suggests that the surface is not predominated by the oxide layer after the electrochemical treatment. However, TEM results after the ADT test (Figure S8b,c in the Supporting Information) showed that surface corrosion of the support material was not that severe in contrast with carbon support materials. The commercial Pt catalyst showed a significant decrease in the number of Pt nanoparticles on the carbon support, and many Pt nanoparticles also agglomerated together (Figure S8 in the Supporting Information). TiN and other metal oxide materials are also widely known as they have strong interaction with metal particles. The experimental data verified the SMSI effects between Pt nanoparticles and the TiN NTs support. The intensity of the whiteline of the XANES Pt L3 edge in the Pt/TiN NT samples became lower than that for Pt/C (Figure S6c in the Supporting Information). The Pt L3 edge peak indicates an electronic transition from 2p3/2 to the unoccupied 5d states,23 and the smaller whiteline peak indicates that the 5d band of Pt nanoparticles was more populated. That is, the electrons moved from the TiN support to Pt 5d band. Both XPS and XANES results revealed that the electron density of Pt metal increased for the Pt/TiN NT catalysts, and then these modifications might improve both the catalytic activity for ORR performance and the electrochemical stability.48

Figure 4. Scheme of Pt/TiN composite catalysts, spherical TiN NPs, and scaffold-like TiN NTs. We used highly conductive scaffold-like TiN nanostructures as electrocatalyst supports for the ORR. The wetted hemispherical Pt nanoparticles may indicate a strong metal−support interaction (SMSI).

3.4. Durability of the Scaffold-Like Pt/TiN NTs. The stability of the electrocatalyst is an important issue that must be addressed before PEMFCs can be widely applied. The accelerated durability test (ADT) was conducted to compare the longterm electrochemical stabilities of Pt/TiN NT catalysts and the commercial Pt/C catalysts (Figure 5). The ADTs of the catalysts

4. CONCLUSION We have designed noncarbon, scaffold-like, titanium nitride nanotube supports that show a high supercapacitor performance and have a high activity and durability for the ORR in comparison with commercial Pt/C catalysts. This catalytic enhancement and high stability are the result of the electronic structure modification of Pt caused by its stronger interaction with the TiN NT support than with carbons. We also investigated the effect on the electrochemical process with spherical TiN NP as catalyst supports which have a much reduced conductivity. Our result suggests that, although the electrocatalytic activity per site did not change from a kinetic viewpoint, the improved current density was caused by an increase in active catalyst area on the electrode. The insufficient conductivity of a supporter material derived by interparticle ohmic contacts (or grain boundaries) led to a less utilizable catalyst. In contrast with the spherical TiN nanoparticles, the scaffold-like TiN NT support as a grainboundary-free structure which has a high electrical performance does not require the addition of carbon-based conducting agents, resulting in an outstanding stability. This study provides a model strategy for the design of new transition metal oxides, nitrides, or carbides, which are widely known to be poorly conductive materials, for use as catalyst supports. The scaffold-like TiN NT

Figure 5. CV curves from initial to 10000 cycles of (a) Pt/TiN NTs and (b) commercial Pt/C catalyst. (c) Long-term durability test by ADT protocol. (d) Mass activity losses at 0.90 and 0.95 V after ADT.

were carried out by applying potential steps between 0.6 and 1.2 V (vs RHE) in an Ar-saturated 0.1 M HClO4 solution at 20 °C. Notably the Pt/TiN NTs showed no degradation in their electrochemical surface area (ECSA) after 10000 cycles (Figure 5a). The ADT result revealed that this supporting material can dramatically enhance the durability of the catalyst and maintain the ECSA of Pt. In fact, the loss of ECSA is almost insignificant (97% of the initial ECSA remained after 10000 ADT cycles; Figure S8 in the Supporting Information). In contrast, of the ECSA of the commercial Pt/C catalyst, only 55% remained after 10000 cycles. In the mixed kinetic−diffusion-controlled region, it is clear that the ORR polarization curves of the commercial Pt/C catalyst showed a more noticeable current drop after the ADT, while there was a slight change for the Pt/TiN NT catalysts. The mass activity at 0.9 V (vs RHE) of the commercial Pt/C showed activity degradation of approximately 59.3%, while that of Pt/TiN NT was reduced by approximately 16.1% (Figure 5d), 3919

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Research Article

ACS Catalysis

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support not only can replace the traditional catalytic structures but also can be extended to applications in electrical energy conversion.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b00384. Experimental details, XRD patterns of Pt/TiN composites and TiN, N2 adsorption−desorption isotherm of TiN, ICP-MS result of Pt/TiN composites, additional TEM images, and specific activities of catalysts (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for W.C.: [email protected]. *E-mail for Y.-E.S.: [email protected]. Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by IBS-R006-G1. REFERENCES

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DOI: 10.1021/acscatal.6b00384 ACS Catal. 2016, 6, 3914−3920