Mesoporous Ti0.5Nb0.5N Ternary Nitride as


Sep 23, 2013 - interest for potential use in a variety of power applications. (i.e., automotive ... Ti:Nb ratio was approximately 1:1 with about 4% K ...
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Mesoporous Ti0.5Nb0.5N Ternary Nitride as a Novel Noncarbon Support for Oxygen Reduction Reaction in Acid and Alkaline Electrolytes Zhiming Cui,† Raymond G. Burns,† and Francis J. DiSalvo* Baker Laboratory, Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853-1301, USA S Supporting Information *

KEYWORDS: fuel cell, titanium nitride, formic acid, methanol, electro-oxidation

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synthesis produced nitrides that were highly sintered, in a random network morphology with a crystallite size of 50−70 nm and a pore size of 30−50 nm. Energy dispersive X-ray spectra (Supporting Information Figure S1b) showed that the Ti:Nb ratio was approximately 1:1 with about 4% K remaining postnitridation despite washing the nanopowder oxide prior to calcination. The residual K in the sample likely exists as a crystalline or amorphous K−M−O compound (M = Ti and/or Nb). The mole fraction of this species is too low to detect by Xray diffraction. Residual K in the surface passivation layer significantly improves the oxidative stability of Ti0.5Nb0.5N as evidenced by TGA data (shown in Supporting Information Figure S2). The exact role of the residual K is unknown at present and is not discussed further in this report. Nitrogen physisorption measurements showed that the material had a BET surface area of 45 m2 g−1. The conductivity of the Ti0.5Nb0.5N and Vulcan XC-72 carbon black was measured using a custom built 4-point probe (Supporting Information Figure S3) at 200 PSI (to mimic the internal mechanical pressure in a fuel cell). The conductivity of the pressed Ti0.5Nb0.5N powder was 3.9 S cm−1, which is about 2.5 times higher than that of Vulcan XC-72 carbon black (1.5 S cm−1). The phase purity of the product is confirmed by powder Xray diffraction (PXRD) in Figure 1a. For as-prepared Ti0.5Nb0.5N, the diffraction peaks at 36.2°, 42.2°, 61.3°, 73.4°, and 77.3° can be assigned to (111), (200), (220), (311), and (222) lattice planes of face-centered cubic Ti0.5Nb0.5N with (PDF no. 04-008-5126). To investigate the chemical stability of nitrides, Ti0.5Nb0.5N, TiN, and NbN were soaked in 0.1 M HClO4 solution and 0.1 M KOH solution for two months. Figure 1b,c shows XRD patterns of Ti0.5Nb0.5N after stability tests. There are no observable changes in the pXRD pattern before and after stability testing, and the Ti0.5Nb0.5N remains black (Supporting Information Figure S4) indicating that the material is still conducting. Stability tests on the single-metal nitrides (TiN and NbN) are shown in Supporting Information Figure S4 as well as pXRD patterns in Supporting Information Figure S5. These results indicate that TiN is chemically stable in alkaline solutions but not in acidic solutions, while NbN is

roton exchange membrane fuel cells (PEMFC) are of interest for potential use in a variety of power applications (i.e., automotive, portable electronics, etc.) due to their potential to efficiently convert chemical energy (i.e., hydrogen) directly to electrical energy with useful power density and low to zero emissions.1,2 Despite recent advances, several challenges continue to inhibit the full deployment of fuel cell technologies, including electrochemical stability for both catalysts and catalyst support materials and the slow kinetics of the oxygen reduction reaction (ORR) at the fuel cell cathode.2,3 Carbon black is still the most practical catalyst support for both the anode and cathode. Unfortunately, carbon is only kinetically stable under fuel cell operating conditions and has been shown to degrade after only a few hundred to a few thousand hours depending upon operating conditions.2,4,5 Therefore, it is necessary to explore alternatives to replace carbon support materials to improve the durability of PEMFC. Recently, much attention has been centered on the development of robust noncarbon support materials, such as carbides,6,7 nitrides,8 and conducting metal oxides.9 Nitrides of the early transition metals are more electrically conducting than doped or partially reduced oxides of the same metals, due to the more covalent nature of the metal−nitrogen bond compared to metal−oxygen. Various nitrides including CrN and TiN have been explored for catalyst and catalyst support applications in fuel cells.10−12 Most literature reports for nitrides as PEMFC catalyst supports focus on simple binary nitrides, with the large emphasis on TiN. TiN has a number of desirable properties such as excellent conductivity, high hardness, and high melting points.13 However, TiN suffers from wear and corrosion in highly acidic environments.5,10,11 Therefore, TiN alone may not possess the necessary stability to replace carbon based supports. Here we describe the synthesis and characterization of mesoporous Ti0.5Nb0.5N supported Pt catalyst and report its enhanced chemical stability and electrocatalytic activity for ORR in acid and alkaline media. Mesoporous Ti0.5Nb0.5N was prepared by the coprecipitation of metal precursors to form mixed-metal oxides followed by thermal treatment under flowing ammonia. The experimental details are shown in Supporting Information. The morphology and structure of the mesoporous Ti0.5Nb0.5N were characterized using scanning electron microscopy as shown in Figure S1a (Supporting Information). The coprecipitation/ammonolysis © 2013 American Chemical Society

Received: August 15, 2013 Revised: September 13, 2013 Published: September 23, 2013 3782

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Chemistry of Materials

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Ti0.5Nb0.5N support are similar in size to those on carbon, which indicates there is little effect on particle size using different supports. The electrochemical stability of Ti0.5Nb0.5N powder is examined under applied potential conditions comparable to those of a fuel cell. The sample was tested over a potential range from −0.5 V to +1.5 V (vs RHE) in 0.1 M HClO4 and 0.1 M KOH solutions at room temperature. As shown in Supporting Information Figure S6a,b, the nitride is not electrochemically active in the potential range of fuel cell operation (0 to +1.2 V vs RHE), suggesting a stable surface. Moreover, when cycling repeatedly between those potentials for 10 cycles, there is no sign of redox peaks associated with Ti0.5Nb0.5N, indicating that Ti0.5Nb0.5N has at least short-term electrochemically stability. The cyclic voltammograms of the catalysts measured in 0.1 M HClO4 at a scan rate of 10 mV s−1 are shown in Supporting Information Figure S6c,d. The electrochemically active area (ECSA) of the Pt catalyst was calculated by measuring the charge collected in the Hupd adsorption/desorption region after double-layer correction with a value of 210 mC·cm−2 assumed for the adsorption of a hydrogen monolayer.3 The specific ECSA of Pt/Ti0.5Nb0.5N (48 m2·g−1) is lower than that of Pt/C (56 m2·g−1), which may be attributed to the larger particle size of the former. Figure 3a shows polarization curves for Pt/Ti0.5Nb0.5N catalyst in 0.1 M HClO4 saturated with oxygen, using a

Figure 1. XRD patterns of (a) original Ti0.5Nb0.5N; (b) Ti0.5Nb0.5N after soaking in 0.1 M HClO4; (c) Ti0.5Nb0.5N after soaking in 0.1 M KOH; (d) Pt/Ti0.5Nb0.5N; and (e) Pt/C.

stable in acidic solutions but not in alkaline. However, it appears that the mixed metal Ti0.5Nb0.5N is stable in both acid and alkaline solutions, marrying the properties of the parent single-metal nitrides. Pt nanoparticles were deposited on Ti0.5Nb0.5N using a polyol process with ethylene glycol (EG) as reducing agent.14 For comparison, Pt/C was also prepared through the same polyol process. The XRD patterns for mesoporous Pt/ Ti0.5Nb0.5N and Pt/C are shown in Figure 1e,f. All the diffraction peaks of Ti0.5Nb0.5N are observed in Pt/Ti0.5Nb0.5N catalyst. The Pt particles crystallize in space group Fm3̅m with refined lattice parameter a = 3.9301(1) Å. For Pt/C, the diffraction peaks of Pt are almost identical to those of Pt on mesoporous Ti0.5Nb0.5N. The actual Pt loadings in Pt/ Ti0.5Nb0.5N and Pt/C are 19.8 wt %, 20.1 wt %, respectively, which are analyzed by EDX. Figure 2a,c shows the TEM images

Figure 3. (a) ORR polarization curves in O2-saturated 0.1 M HClO4. Rotation rate: 1600 rmp; sweep rate: 5 mV s−1. The inset is kinetic current density for ORR. (b) Bar plot of the kinetic current density at 0.9 V in 0.1 M HClO4 before and after stability tests of 5000 cycles. (c) ORR polarization curves in O2-saturated 0.1 M KOH. Rotation rate: 1600 rmp; sweep rate: 5 mV s−1. The inset is kinetic current density for ORR. (d) Bar plot of the kinetic current density at 0.9 V in 0.1 M KOH before and after stability tests of 5000 cycles. Figure 2. (a) TEM image of Pt/Ti0.5Nb0.5N catalyst; (b) the histograms of Pt particle sizes for Pt/Ti0.5Nb0.5N catalyst; (c) TEM image of Pt/C catalyst; (d) the histograms of Pt particle sizes for Pt/C catalyst.

rotating disk electrode at 1600 rpm. The current density was normalized to the geometric surface area of the electrodes. The polarization curves for the Pt/C sample are included for comparison. The Pt/Ti0.5Nb0.5N catalyst exhibited excellent catalytic activity toward ORR compared to Pt/C. For instance, the onset potential of ORR on Pt/Ti0.5Nb0.5N is about 1.04 V, which is more positive than that on Pt/C (0.99 V), indicating that oxygen is more easily reduced on Pt/Ti0.5Nb0.5N. The halfwave potentials for the Pt/Ti0.5Nb0.5N and Pt/C catalysts were 0.94 and 0.91 V, respectively. The Pt/Ti0.5Nb0.5N catalyst showed a significant 30 mV shift to more positive potentials

of sonicated Pt/Ti0.5Nb0.5N and Pt/C catalysts. The average sizes of the Pt particles in the Pt/Ti0.5Nb0.5N and Pt/C catalysts as estimated from their histograms are 4.1 ± 0.6 nm and 4.0 ± 0.5 nm, respectively. Histograms of Pt particle diameters were obtained by measuring the size of 50 particles in random regions as shown in Figure 2b,d. The Pt particles on the Pt/ 3783

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Chemistry of Materials

Communication

studies of mixed metal nitride support materials are clearly warranted.

relative to the Pt/C catalysts. The half-wave potential for the Pt/Ti0.5Nb0.5N is also higher than those of other noncarbon supported Pt catalysts.3,9,15 For example, Hwang et al. reported Ti0.7Mo0.3O2 supported Pt catalyst which exhibited a halfpotential of 0.9 V. The inset in Figure 3a shows the kinetic current density (Jk) for the ORR obtained from RDE data. Jk was normalized to the Pt metal loading. The Pt/Ti0.5Nb0.5N produces currents as high as 256 mA·mgPt−1 at 0.9 V, which is ∼1.8 times higher than those of the Pt/C sample (142 mA· mgPt−1 at 0.9 V). The catalytic activity of Pt/Ti0.5Nb0.5N is also higher than those of the state-of-the-art Pt/C.16−18 For example, Nesselberger et al. achieved 103 mA·mgPt−1 at 0.9 V in 0.05 M H2SO4 on carbon supported 2−3 nm Pt catalysts;16 Garsany et al. obtained 160−210 mA·mgPt−1 at 0.9 V in 0.1 M HClO4 on 20% Pt/Vulcan carbon.17 To further investigate the role of supports, the specific surface activities of these catalysts, in terms of specific current normalized to the electrochemically active specific surface area, are also evaluated. The calculated specific surface activities at 0.9 V for the Pt/Ti0.5Nb0.5N and Pt/ C catalysts are 5.3 A·m−2 and 2.5 A·m−2, respectively. The much higher specific surface activities of the Pt/Ti0.5Nb0.5N catalyst toward ORR may be due to the excellent conductivity of Ti0.5Nb0.5N. The stability of the catalysts was assessed by applying potential steps between 0.6 and 1.0 V in O2-saturated 0.1 M HClO4 electrolytes at 50 mV s−1. The activity of the catalysts before and after the stability test is depicted in Figure 3b. The Pt/Ti0.5Nb0.5N catalyst was more stable under ORR conditions than the Pt/C. After 5000 cycles, the current is 207 mA·mgPt−1 at 0.9 V, which is reduced by 19.2% from the start of the cycling. In contrast, a larger activity loss was observed in the carbon supported Pt catalysts (29.4% for Pt/C) decreasing the initial lower Pt/C activity even further. Currently there is renewed interest in the oxygen reduction reaction (ORR) in alkaline media for alkaline fuel cells and metal−air batteries. The catalytic activity of Pt/Ti0.5Nb0.5N for ORR was also investigated in alkaline media. Figure 3c shows polarization curves for Pt/Ti0.5Nb0.5N and Pt/C catalysts in 0.1 M KOH. The onset potential and half-wave potential of Pt/ Ti0.5Nb0.5N are 1.01 V and 0.90, which are more positive than those on Pt/C (0.98 V and 0.86 V), indicating that oxygen is more easily reduced on Pt/Ti0.5Nb0.5N. The kinetic current density of Pt/Ti0.5Nb0.5N (74.5 mA·mgPt−1 at 0.9 V) is ∼2.2 times higher than that of Pt/C (34.1 mA·mgPt−1 at 0.9 V). The activity of the catalysts before and after the stability test is depicted in Figure 3d. The stability of the catalysts was assessed by applying potential steps between 0.6 and 1.0 V (NHE) in O2-saturated 0.1 M KOH electrolytes at 50 mV s−1. The Pt/ Ti0.5Nb0.5N is also more stable in alkaline media than Pt/C. After 5000 cycles, Pt/Ti0.5Nb0.5N shows an activity degradation of ∼23%, while the activity of Pt/C is reduced by ∼34%. In conclusion, we have developed a robust noncarbon Ti0.5Nb0.5N support with excellent electronic conductivity and high surface area by a coprecipitation method. This ternary nitride appears kinetically stable in both acid media and alkaline media, combining the stability of TiN in base with the stability of NbN in acid. Electrochemical studies in the potential range of fuel cell cathode operation show lower rates of loss of activity than with Pt/C. Ti0.5Nb0.5N supported Pt catalyst exhibits a higher activity and stability than carbon supported Pt catalyst. The coprecipitation approach could be potentially applied to rationally design other conducting, porous, and chemically stable materials like ternary nitrides for technologies including but not limited to fuel cells, solar cells, and batteries. Further



ASSOCIATED CONTENT

* Supporting Information S

Synthesis of Ti 0.5 Nb 0.5 N and Pt/Ti 0.5 Nb 0.5 N, TGA of Ti0.5Nb0.5N, XRD patterns of TiN and NbN, electrode preparation, and electrochemical measurements.This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Author

*Tel.: +1 607 255 7238. Fax: +1 607 255 4137. E-mail: [email protected] cornell.edu (Francis J. DiSalvo). Author Contributions †

These authors contributed equally (Z.C. and R.G.B.).

Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by the Energy Materials Center at Cornell, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award Number DE-SC0001086. We thank Dr. Chinmayee V. Subban for designing and building conductivity apparatus.



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