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Partially oxidized titanium carbonitride as a non-noble catalyst for oxygen reduction reactions
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 5 1 3 5 e1 5 1 3 9
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
Partially oxidized titanium carbonitride as a non-noble catalyst for oxygen reduction reactions Duc Tai Dam, Kyung-Don Nam, Hao Song, Xin Wang, Jong-Min Lee* School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore
article info
abstract
Article history:
Partially oxidized titanium carbonitride (TiCNO), newly synthesized from partial oxidation
Received 20 December 2011
of TiC0.5N0.5 powder under a flow rate of 500 cm3 min1 of nitrogen containing 4% H2 and
Received in revised form
0.5% O2 by volume at 800 C for 20 h, is examined as a non-platinum cathode for polymer
8 July 2012
electrolyte membrane fuel cells (PEMFCs). TiCNO shows significantly enhanced electro-
Accepted 30 July 2012
catalytic activity for the oxygen reduction reaction (ORR) compared with as-prepared
Available online 21 August 2012
TiC0.5N0.5 and fully oxidized TiO2. In addition, TiCNO has a high chemical stability in
Keywords:
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
0.1 mol dm3 sulfuric acid at 30 C. Titanium carbonitride
reserved.
Cathode Polymer electrolyte membrane fuel cells
1.
Introduction
Recently polymer electrolyte membrane fuel cells (PEMFCs) utilizing the oxygen reduction reaction (ORR) have been extensively studied as alternative electricity source for electric vehicles and transportable applications due to their low temperature operation, high theoretical energy efficiency and less emission of pollutants. However, in order for FEMFCs to be widely commercialized, several problems need to be overcome. Firstly, the platinum used as electrocatalysts at both anode and cathode happens to be the most serious problem due to its high cost and limited resource of 37 ktons [1e3]. On the other hand, the slow kinetic rate of the oxygen reduction reaction (ORR) not only leads to higher reduction overpotential in the performance of cathode [4e7] but also limits energy conversion efficiency of the state-of-the-art
PEMFCs. Therefore, development of an alternative material that possesses high stability and high catalytic activity toward the ORR is essential. Transition metal carbides and nitrides are well-known to possess good electrocatalytic activity and corrosion resistance. In the past several decades, extensive works have been performed on these materials as electrocatalysts for oxygen reduction reactions in acid solutions. Kunchan Lee et al. proved that addition of tantalum into tungsten carbide increased the stability in acidic solution and the electrocatalytic activity toward ORR significantly [8]. Furthermore, Ken-ichiro Ota’s group reported that tantalum oxynitride [9,10], zirconium oxynitride [11,12], chromium carbonitride [13], partially oxidized tantalum carbonitride [14] and niobium carbonitride [2] were stable in an acid solution and had a good catalytic activity for the ORR. Titanium is the ninth most abundant element in the Earth’s crust (0.56% by
* Corresponding author. Tel.: þ65 651 381 29. E-mail address: [email protected] (J.-M. Lee). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.07.129
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 5 1 3 5 e1 5 1 3 9
mass) [15]. More importantly, heat-treated titanium oxide demonstrated a significant increase in onset potential to about 0.65 V vs. RHE [16]. Hence, titanium-based materials stand out as an alternative to replace Pt in PEMFC. In this study, titanium carbonitride (denoted as TiCN) was used as a starting material. TiCN was partially oxidized to obtain titanium oxide-based compound. Physical and electrochemical properties of this compound were evaluated to explore the feasibility of using it as non-platinum cathode for PEMFCs.
2.
Experimental
Titanium carbonitride powder (TiCN) was synthesized from a mixture of titanium dioxide (TiO2, <5 mm, 99.9%, SigmaeAldrich) and carbon black (4 mm, Sinopharm Chemical Reagent Co. Ltd, China). TiO2 and carbon black powders were uniformly mixed with the molar ratio of TiO2:C ¼ 1:2.5. The above mixture was heated at 1800 C for 5 h in tube furnace under nitrogen atmosphere to synthesize TiCN as follows TiO2 þ 2:5C þ 0:25N2 ðgÞ/TiC0:5 N0:5 þ 2COðgÞ
voltammetry reached a steady state, slow scan voltammetry (SSV) was performed from 0 to 1 V vs. RHE in saturated N2 and O2 atmospheres separately at a scan rate of 5 mV s1 to obtain the potentialecurrent density curves for the ORR. The difference between the current densities obtained under O2 and N2 (iO2 and iN2 , respectively) was defined as the oxygen reduction current density (i.e. iORR ¼ iO2 iN2 ). The onset potential for the ORR (EORR) was defined as the electrode potential at the iORR ¼ 0.2 mA cm2. The onset potential and the iORR at 0.6 V vs. RHE obtained from SSV were utilized to evaluate the catalytic activity for the ORR in this study. The crystalline structures of the specimens were characterized by using powder X-ray diffraction measurements (Bruker D8 Advance X-Ray Diffractometer) with Cu Ka radiation (l ¼ 0.15406 nm) in the range from 10 to 80 . In order to analyze chemical bonding states of the surface, Raman measurements were performed using a Renishaw inVia Raman Microscope equipped with charge couple device (CCD) detector. Arþ ion laser with the wavelength of 514 nm was used as an excitation source. The laser intensity at the sample surface was maintained at around 1.5 mW for all measurements.
(1)
The carbon and nitrogen contents were determined by using a CHNS elemental analyzer. The overall chemical composition of as-prepared TiCN was confirmed to be TiC0.5N0.5. TiCN powder was then partially oxidized so as to enhance its catalytic activity toward ORR. As-synthesized TiCN powder was evenly spread in alumina boat and placed in the isothermal zone of quartz tube furnace. Heat treatment was conducted in a quartz tube with low pressure of oxygen to investigate the effect of the partial oxidation on the catalytic activity for the ORR. Nitrogen containing 4% H2 and 0.5% O2 by volume was introduced to tube inlet at a flow rate of 500 sccm. The addition of H2 in heat treatment atmosphere was to ensure homogenous oxidation of TiCN. TiCN was heat-treated at 800 C with the holding time of 20 h to prepare partially oxidized TiCN powder, denoted as TiCNO in this paper. The color of powder changed from black to dimgray due to the partial oxidation. The original TiCN and heat-treated TiCNO powders were separately mixed with Ketjen Black (5 wt%). The mixture was suspended in a mixture of ethanol and deionized water and sonicated to ensure good dispersion. The suspension was then evenly dropped onto the glassy carbon rod (6 mmØ). About 5 mg of the catalyst was loaded on the glassy carbon rod. Diluted Nafion 117 solution (0.5 wt%, SigmaeAldrich) was dropped onto the surface. The coated glassy carbon rod was then dried in inert atmosphere at 120 C for at least 1 h in a drying oven to prepare a working electrode. Electrochemical measurements were performed in 0.1 mol dm3 sulfuric acid at 30 C in N2 and O2 atmospheres separately using a threeelectrode cell equipped with a carbon plate counter electrode and a Ag/AgCl (3 mol L1 KCl) reference electrode. All measured potentials in this work were converted to the reversible hydrogen electrode (RHE) scale. Fast scan voltammetry (FSV) was carried out from 0.2 V to 1.0 V vs. Ag/AgCl (i.e. 0e1.2 V vs. RHE) in saturated O2 atmosphere at a scan rate of 50 mV s1 to stabilize working electrode and measure electrochemical stability of specimens. When cyclic
3.
Results and discussion
Fig. 1 demonstrates the XRD patterns of TiCN, partially oxidized TiCNO at 800 C and fully oxidized powder. Based on Vegard’s law [17], TiC and TiN have the same crystalline structure as sodium chloride and are soluble in each other. The XRD peak patterns of TiC (JCPDS 32-1383) and TiN (JCPDS 38-1420) are similar to each other and each peak shifts to a higher angle with increasing nitrogen content. As-prepared TiCN, possibly considered as a binary system formed by carbide and nitride, possesses the same crystalline structure as NaCl with XRD peaks existing between TiC and TiN. The XRD pattern in Fig. 1(a) indicated that as-synthesized TiCN forms a complete solid solution without any other secondary
Fig. 1 e XRD patterns of the specimens with and without heat-treatment: (a) as-prepared TiCN, (b) TiCNO heattreated at 800 C for 20 h and (c) completely oxidized TiO2 (rutile).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 5 1 3 5 e1 5 1 3 9
phase such as metallic Ti, Ti2C, and Ti2N. The lattice constant and the chemical composition of the solid solution TiCN have a linear relation between TiC and TiN according to Vegard’s law. The lattice constant and the chemical composition of the TiCN were estimated to be 4.284 A and TiC0.5N0.5, respectively. In Fig. 1c, TiCN was fully oxidized in oxygen-rich atmosphere and its product was confirmed to be rutile TiO2. In addition, no structures associated with other titania phases (e.g. anatase and brookite) were detected due to the high stability of rutile structure at elevated temperature. The peaks of TiC0.5N0.5 and TiO2 are observed for TiCNO in Fig. 1b, due to the partial surface oxidation, similar to the previous results [2,15]. The slow cyclic voltammogram of TiCNO mixed with Ketjen Black in 0.1 mol dm3 sulfuric acid at 30 C in saturated N2 atmosphere is shown in Fig. 2. After the second cycle, the electrode reached the steady state and the quasi-rectangular shape of CV hardly changed, indicating high electrochemical stability in acidic solution in the potential range from 0 to 1 V. Moreover, there was no specific peak due to anode dissolution observed in the CV, indicating that it has a high electrochemical stability in acidic solution in the potential range from 0 to 1 V. In addition, electrochemical stability can be evaluated by the difference between anodic and cathodic charges in the CV of powder with heat treatment. The anodic and cathodic charges were determined using the following equation [14]: Z Q¼
Z idt ¼
i
dE 1 ¼ v v
Z idE
where Q, i, t, v, and E were charge, current, time, scan rate, and electrode potential, respectively. The anodic and cathodic charges calculated from the CV at steady state were 26.5 mC and 26.4 mC, respectively. These charge values were in good agreement with each other, which implied that charges were likely due to charge/discharge of a double layer and no onesided reaction occurred. This result clearly suggests that heat-treated specimens have a high electrochemical stability in the potential range of 0e1 V.
Fig. 2 e Cyclic voltammogram of TiCNO mixed with Ketjen black in 0.1 mol dmL3 sulfuric acid at 30 C in saturated N2 with the scan rate of 5 mV sL1.
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The potentialecurrent density curves for the ORR of TiCN, TiCNO partially oxidized at 800 C and fully oxidized TiO2 are shown in Fig. 3. A potentialecurrent density curve of 46.3 wt % Pt/C (Tanaka Kikinzoku Kogyo, Japan) was also plotted for comparison: the Pt loading was 4 mg/GC rod (6 mmØ) or 14 mg cm2. The ORR currents of TiCN and TiO2 were observed below 0.55 V vs. RHE, showing low electrocatalytic activity toward ORR. However, the ORR current of TiCNO prepared at 800 C and 20 h was observed at 0.88 V vs. RHE. The partial surface oxidation of TiCN clearly enhanced the electrocatalytic activity for the ORR, possibly due to an increase of the active surface area or the oxygen adsorption area on the particle, resulting in a significant increase in the ORR current (iORR ¼ 0:795 mA cm2 at 0.6 V vs. RHE) and the onset potential (0.88 V vs. RHE). This ORR current is comparable to that of partially oxidized niobium carbonitride (ca. 0.8 mA cm2) [2] and much higher than reported jiORR j of heattreated zirconium carbonitride (ca. 0.42 mA cm2) [18]. The onset potential of Pt/C was 1.05 V, which was about 0.17 V higher than that of as-prepared TiCNO, and the catalyst loading of TiCNO was much greater than that of Pt/C. However, since Ti has abundant resource and partially oxidized TiCNO reached onset potential of 0.88 V vs. RHE. On top of that, more experimental heat treatments can be done and catalyst activity can be further improved by tuning degree of surface oxidation. Thus, TiCNO may be a promising candidate of non-noble cathode material for PEMFCs. Fig. 4 presents the Raman spectra of TiC, TiN, TiCN, TiCNO, and TiO2. Raman spectra of titanium carbide (TiC, 2 mm, 99.5%, Alfa Aesar) and titanium nitride (TiN, <10 mm, 99.7%, Alfa Aesar) were included for comparison. TiCN has a very similar Raman spectrum to those of TiC and TiN in the range from 150 to 800 cm1 because TiC, TiN and as-prepared TiCN possess a face-centered cubic (FCC) crystal structure, showing the peaks at 260, 405 and 607 cm1 arising from the longitudinal acoustic (LA), second-order acoustic (2A) and transverse optical (TO) modes, respectively. The surface of TiC, TiN and
Fig. 3 e PotentialeORR current density curves of TiCN, TiCNO partially oxidized at 800 C, and fully oxidized TiO2 in 0.1 mol dmL3 sulfuric acid at 30 C with the scan rate of 5 mV sL1.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 5 1 3 5 e1 5 1 3 9
preferentially excite P states, visible Raman spectra directly depend on the sp2 sites and configuration of the sp2 phase [20]. Therefore, the results show sp2 sites in amorphous carbon matrix were formed due to the breakage of TieC bond. Raman spectra of as-prepared TiCN and fully oxidized TiO2 display broad graphite peaks whereas sharp D and G band were observed from Raman spectrum of TiCNO, clearly illustrating the formation of free carbon during the heat treatment process. Carbonitride materials have low electrical conductivity (<102 S cm1) [21], which is a major obstacle for their application in PEMFCs. The formation of free carbon in TiCNO may play an important role in its catalytic activity, enhancing electrical conductivity and electron transport during the oxygen reduction process.
4.
Fig. 4 e Raman spectra of TiC, TiN, TiCN, TiCNO, and TiO2 in Raman shift range of (a) 150e800 cmL1 and (b) 1000e2000 cmL1.
TiCN powders might have been slightly oxidized in air and/or oxygen molecules adsorbed on the surface as reported in Ref. [2]. When titanium was fully oxidized, the 607 cm1 mode did not show any frequency shift but the 405 cm1 mode was considerably shifted to the 439 cm1, due to the planar OeO interaction influenced by the oxygen deficiency rather than the TieO stretching interaction [19], a similar behavior shown for TiCNO. The presence of free carbon was indicated by two bands centered at 1350 cm1 and 1595 cm1, labeled D and G, respectively, which are characteristic of CeC bonding. In carbon materials, the D (defect) band is a breathing mode of A1g symmetry associated with optical phonons close to the K zone boundary in graphite. The D band is usually absent in the perfect graphite and only becomes active in the presence of disorder due to the breathing modes of sp2 atoms in ring and the limitation in graphite domain size induced by imperfections. On the other hand, the G (graphite) band, due to the bond stretching modes (optical phonon modes of E2g) of sp2 pairs in both ring and chain, indicates high disorder and dispersion in amorphous network. Since visible photons
Conclusions
The partially oxidized titanium carbonitride (TiCNO) was evaluated as a non-Pt cathode for PEMFCs. TiC0.5N0.5 powder was synthesized from the uniform mixture of TiO2 powder and carbon black at 1800 C in a tube furnace under nitrogen atmosphere. As-prepared TiC0.5N0.5 was heat-treated under a flow rate of 500 cm3 min1 of nitrogen containing 4% H2 and 0.5% O2 by volume at 800 C for 20 h to prepare TiCNO. The influence of the heat treatment on the catalytic activity toward oxygen reduction reaction and the properties of TiCNO were investigated. The onset potential for the ORR of the TiCNO was 0.88 V vs. RHE, which was much higher than those of as-prepared TiC0.5N0.5 and fully oxidized TiO2. ORR current (jiORR j at 0.6 V vs. RHE) was substantially enhanced to around 0.795 mA. More importantly, TiCNO has a high chemical stability in 0.1 mol dm3 sulfuric acid at 30 C. Raman measurements suggest that existence of sp2 carbon matrix in crystal structure may play an important role in its catalytic activity, enhancing electrical conductivity and electron transport during the oxygen reduction process.
Acknowledgments This work is supported by Startup Grant of Nanyang Technological University and Academic Research Fund (RG21/09) of Ministry of Education in Singapore. The authors thank Lion Corporation (Japan) and Ultrachem (S) Pte Ltd (Singapore) for providing Ketjen Black.
references
[1] Sammes N. Fuel cell technology reaching towards commercialization. In: Engineering Materials and Processes. London: Springer; 2006. [2] Nam K-D, Ishihara A, Matsuzawa K, Mitsushima S, Ota K-I, Matsumoto M, et al. Partially oxidized niobium carbonitride as a non-platinum catalyst for the reduction of oxygen in acidic medium. Electrochimica Acta 2010;55:7290e7. [3] Bashyam R, Zelenay P. A class of non-precious metal composite catalysts for fuel cells. Nature 2006;443:63e6.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 5 1 3 5 e1 5 1 3 9
[4] Nørskov JK, Rossmeisl J, Logadottir A, Lindqvist L, Kitchin JR, Bligaard T, et al. Origin of the overpotential for oxygen reduction at a fuel-cell cathode. The Journal of Physical Chemistry B 2004;108(46):17886e92. [5] Lee J-M. Numerical analysis of galvanic corrosion of Zn/Fe interface beneath a thin electrolyte. Electrochimica Acta 2006;51(16):3256e60. [6] Lee J-M, Dyar H, Taylor JE, Carpio R. Current distribution for the metallization of resistive wafer substrates under controlled geometric variations. Journal of the Electrochemical Society 2006;153:C265e71. [7] Cao Y, Lee J-M, West AC. Designs for minimizing resistive substrate effects on wafer metallization. Plating and Surface Finishing 2003;90:40e5. [8] Lee K, Ishihara A, Mitsushima S, Kamiya N, Ota K-I. Stability and electrocatalytic activity for oxygen reduction in WC þ Ta catalyst. Electrochimica Acta 2004;49:3479e85. [9] Ishihara A, Lee K, Doi S, Mitsushima S, Kamiya N, Hara M, et al. Tantalum oxynitride for a novel cathode of PEFC. Electrochemical and Solid-State Letters 2005;8:A201e3. [10] Shibata Y, Ishihara A, Mitsushima S, Kamiya N, Ota K-I. Effect of heat treatment on catalytic activity for oxygen reduction reaction of TaOxNy/Ti prepared by electrophoretic deposition. Electrochemical and Solid-State Letters 2007;10:B43e6. [11] Doi S, Ishihara A, Mitsushim S, Kamiya N, Ota K-I. Zirconium-based compounds for cathode of polymer electrolyte fuel cell. Journal of the Electrochemical Society 2007;154:B362e9. [12] Maekawa Y, Ishihara A, Kim J-H, Mitsushima S, Ota K-I. Catalytic activity of zirconium oxynitride prepared by reactive sputtering for ORR in sulfuric acid. Electrochemical and Solid-State Letters 2008;11:B109e12.
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[13] Kim J-H, Ishihara A, Mitsushima S, Kamiya N, Ota K-I. New non-platinum cathode based on chromium for PEFC. Chemistry Letters 2007;36:514e5. [14] Ishihara A, Tamura M, Matsuzawa K, Mitsushima S, Ota K-I. Tantalum oxide-based compounds as new non-noble cathodes for polymer electrolyte fuel cell. Electrochimica Acta 2010;55:7581e9. [15] Enghag P. Encyclopedia of the elements: technical data, history, processing, applications. Weinheim: Wiley-VCH; 2004. [16] Kim J-H, Ishihara A, Mitsushima S, Kamiya N, Ota K-I. Catalytic activity of titanium oxide for oxygen reduction reaction as a non-platinum catalyst for PEFC. Electrochimica Acta 2007;52:2492e7. [17] Duwez P, Odell F. Phase relationships in the binary systems of nitrides and carbides of zirconium, columbium, titanium, and vanadium. Journal of the Electrochemical Society 1950; 97:299e304. [18] Ohgi Y, Ishihara A, Matsuzawa K, Mitsushima S, Ota K. Zirconium oxide-based compound as new cathode without platinum group metals for PEFC. Journal of the Electrochemical Society 2010;157(6):B885e91. [19] Wu X, Zhang M-S, Yin Z, Ji X, Chen Q. Temperature characteristics of Raman spectra in nanometer material titanium dioxide. Chinese Physics Letters 1994;11:685e8. [20] Ferrari AC, Robertson J. Interpretation of Raman spectra of disordered and amorphous carbon. Physical Review B 2000; 61:14095e107. [21] Yang S, Feng X, Wang X, Mullen K. Graphene-based carbon nitride nanosheets as efficient metal-free electrocatalysts for oxygen reduction reactions. Angewandte Chemie International Edition 2011;50:5339e43.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
Partially oxidized titanium carbonitride as a non-noble catalyst for oxygen reduction reactions Duc Tai Dam, Kyung-Don Nam, Hao Song, Xin Wang, Jong-Min Lee* School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore
article info
abstract
Article history:
Partially oxidized titanium carbonitride (TiCNO), newly synthesized from partial oxidation
Received 20 December 2011
of TiC0.5N0.5 powder under a flow rate of 500 cm3 min1 of nitrogen containing 4% H2 and
Received in revised form
0.5% O2 by volume at 800 C for 20 h, is examined as a non-platinum cathode for polymer
8 July 2012
electrolyte membrane fuel cells (PEMFCs). TiCNO shows significantly enhanced electro-
Accepted 30 July 2012
catalytic activity for the oxygen reduction reaction (ORR) compared with as-prepared
Available online 21 August 2012
TiC0.5N0.5 and fully oxidized TiO2. In addition, TiCNO has a high chemical stability in
Keywords:
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
0.1 mol dm3 sulfuric acid at 30 C. Titanium carbonitride
reserved.
Cathode Polymer electrolyte membrane fuel cells
1.
Introduction
Recently polymer electrolyte membrane fuel cells (PEMFCs) utilizing the oxygen reduction reaction (ORR) have been extensively studied as alternative electricity source for electric vehicles and transportable applications due to their low temperature operation, high theoretical energy efficiency and less emission of pollutants. However, in order for FEMFCs to be widely commercialized, several problems need to be overcome. Firstly, the platinum used as electrocatalysts at both anode and cathode happens to be the most serious problem due to its high cost and limited resource of 37 ktons [1e3]. On the other hand, the slow kinetic rate of the oxygen reduction reaction (ORR) not only leads to higher reduction overpotential in the performance of cathode [4e7] but also limits energy conversion efficiency of the state-of-the-art
PEMFCs. Therefore, development of an alternative material that possesses high stability and high catalytic activity toward the ORR is essential. Transition metal carbides and nitrides are well-known to possess good electrocatalytic activity and corrosion resistance. In the past several decades, extensive works have been performed on these materials as electrocatalysts for oxygen reduction reactions in acid solutions. Kunchan Lee et al. proved that addition of tantalum into tungsten carbide increased the stability in acidic solution and the electrocatalytic activity toward ORR significantly [8]. Furthermore, Ken-ichiro Ota’s group reported that tantalum oxynitride [9,10], zirconium oxynitride [11,12], chromium carbonitride [13], partially oxidized tantalum carbonitride [14] and niobium carbonitride [2] were stable in an acid solution and had a good catalytic activity for the ORR. Titanium is the ninth most abundant element in the Earth’s crust (0.56% by
* Corresponding author. Tel.: þ65 651 381 29. E-mail address: [email protected] (J.-M. Lee). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.07.129
15136
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 5 1 3 5 e1 5 1 3 9
mass) [15]. More importantly, heat-treated titanium oxide demonstrated a significant increase in onset potential to about 0.65 V vs. RHE [16]. Hence, titanium-based materials stand out as an alternative to replace Pt in PEMFC. In this study, titanium carbonitride (denoted as TiCN) was used as a starting material. TiCN was partially oxidized to obtain titanium oxide-based compound. Physical and electrochemical properties of this compound were evaluated to explore the feasibility of using it as non-platinum cathode for PEMFCs.
2.
Experimental
Titanium carbonitride powder (TiCN) was synthesized from a mixture of titanium dioxide (TiO2, <5 mm, 99.9%, SigmaeAldrich) and carbon black (4 mm, Sinopharm Chemical Reagent Co. Ltd, China). TiO2 and carbon black powders were uniformly mixed with the molar ratio of TiO2:C ¼ 1:2.5. The above mixture was heated at 1800 C for 5 h in tube furnace under nitrogen atmosphere to synthesize TiCN as follows TiO2 þ 2:5C þ 0:25N2 ðgÞ/TiC0:5 N0:5 þ 2COðgÞ
voltammetry reached a steady state, slow scan voltammetry (SSV) was performed from 0 to 1 V vs. RHE in saturated N2 and O2 atmospheres separately at a scan rate of 5 mV s1 to obtain the potentialecurrent density curves for the ORR. The difference between the current densities obtained under O2 and N2 (iO2 and iN2 , respectively) was defined as the oxygen reduction current density (i.e. iORR ¼ iO2 iN2 ). The onset potential for the ORR (EORR) was defined as the electrode potential at the iORR ¼ 0.2 mA cm2. The onset potential and the iORR at 0.6 V vs. RHE obtained from SSV were utilized to evaluate the catalytic activity for the ORR in this study. The crystalline structures of the specimens were characterized by using powder X-ray diffraction measurements (Bruker D8 Advance X-Ray Diffractometer) with Cu Ka radiation (l ¼ 0.15406 nm) in the range from 10 to 80 . In order to analyze chemical bonding states of the surface, Raman measurements were performed using a Renishaw inVia Raman Microscope equipped with charge couple device (CCD) detector. Arþ ion laser with the wavelength of 514 nm was used as an excitation source. The laser intensity at the sample surface was maintained at around 1.5 mW for all measurements.
(1)
The carbon and nitrogen contents were determined by using a CHNS elemental analyzer. The overall chemical composition of as-prepared TiCN was confirmed to be TiC0.5N0.5. TiCN powder was then partially oxidized so as to enhance its catalytic activity toward ORR. As-synthesized TiCN powder was evenly spread in alumina boat and placed in the isothermal zone of quartz tube furnace. Heat treatment was conducted in a quartz tube with low pressure of oxygen to investigate the effect of the partial oxidation on the catalytic activity for the ORR. Nitrogen containing 4% H2 and 0.5% O2 by volume was introduced to tube inlet at a flow rate of 500 sccm. The addition of H2 in heat treatment atmosphere was to ensure homogenous oxidation of TiCN. TiCN was heat-treated at 800 C with the holding time of 20 h to prepare partially oxidized TiCN powder, denoted as TiCNO in this paper. The color of powder changed from black to dimgray due to the partial oxidation. The original TiCN and heat-treated TiCNO powders were separately mixed with Ketjen Black (5 wt%). The mixture was suspended in a mixture of ethanol and deionized water and sonicated to ensure good dispersion. The suspension was then evenly dropped onto the glassy carbon rod (6 mmØ). About 5 mg of the catalyst was loaded on the glassy carbon rod. Diluted Nafion 117 solution (0.5 wt%, SigmaeAldrich) was dropped onto the surface. The coated glassy carbon rod was then dried in inert atmosphere at 120 C for at least 1 h in a drying oven to prepare a working electrode. Electrochemical measurements were performed in 0.1 mol dm3 sulfuric acid at 30 C in N2 and O2 atmospheres separately using a threeelectrode cell equipped with a carbon plate counter electrode and a Ag/AgCl (3 mol L1 KCl) reference electrode. All measured potentials in this work were converted to the reversible hydrogen electrode (RHE) scale. Fast scan voltammetry (FSV) was carried out from 0.2 V to 1.0 V vs. Ag/AgCl (i.e. 0e1.2 V vs. RHE) in saturated O2 atmosphere at a scan rate of 50 mV s1 to stabilize working electrode and measure electrochemical stability of specimens. When cyclic
3.
Results and discussion
Fig. 1 demonstrates the XRD patterns of TiCN, partially oxidized TiCNO at 800 C and fully oxidized powder. Based on Vegard’s law [17], TiC and TiN have the same crystalline structure as sodium chloride and are soluble in each other. The XRD peak patterns of TiC (JCPDS 32-1383) and TiN (JCPDS 38-1420) are similar to each other and each peak shifts to a higher angle with increasing nitrogen content. As-prepared TiCN, possibly considered as a binary system formed by carbide and nitride, possesses the same crystalline structure as NaCl with XRD peaks existing between TiC and TiN. The XRD pattern in Fig. 1(a) indicated that as-synthesized TiCN forms a complete solid solution without any other secondary
Fig. 1 e XRD patterns of the specimens with and without heat-treatment: (a) as-prepared TiCN, (b) TiCNO heattreated at 800 C for 20 h and (c) completely oxidized TiO2 (rutile).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 5 1 3 5 e1 5 1 3 9
phase such as metallic Ti, Ti2C, and Ti2N. The lattice constant and the chemical composition of the solid solution TiCN have a linear relation between TiC and TiN according to Vegard’s law. The lattice constant and the chemical composition of the TiCN were estimated to be 4.284 A and TiC0.5N0.5, respectively. In Fig. 1c, TiCN was fully oxidized in oxygen-rich atmosphere and its product was confirmed to be rutile TiO2. In addition, no structures associated with other titania phases (e.g. anatase and brookite) were detected due to the high stability of rutile structure at elevated temperature. The peaks of TiC0.5N0.5 and TiO2 are observed for TiCNO in Fig. 1b, due to the partial surface oxidation, similar to the previous results [2,15]. The slow cyclic voltammogram of TiCNO mixed with Ketjen Black in 0.1 mol dm3 sulfuric acid at 30 C in saturated N2 atmosphere is shown in Fig. 2. After the second cycle, the electrode reached the steady state and the quasi-rectangular shape of CV hardly changed, indicating high electrochemical stability in acidic solution in the potential range from 0 to 1 V. Moreover, there was no specific peak due to anode dissolution observed in the CV, indicating that it has a high electrochemical stability in acidic solution in the potential range from 0 to 1 V. In addition, electrochemical stability can be evaluated by the difference between anodic and cathodic charges in the CV of powder with heat treatment. The anodic and cathodic charges were determined using the following equation [14]: Z Q¼
Z idt ¼
i
dE 1 ¼ v v
Z idE
where Q, i, t, v, and E were charge, current, time, scan rate, and electrode potential, respectively. The anodic and cathodic charges calculated from the CV at steady state were 26.5 mC and 26.4 mC, respectively. These charge values were in good agreement with each other, which implied that charges were likely due to charge/discharge of a double layer and no onesided reaction occurred. This result clearly suggests that heat-treated specimens have a high electrochemical stability in the potential range of 0e1 V.
Fig. 2 e Cyclic voltammogram of TiCNO mixed with Ketjen black in 0.1 mol dmL3 sulfuric acid at 30 C in saturated N2 with the scan rate of 5 mV sL1.
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The potentialecurrent density curves for the ORR of TiCN, TiCNO partially oxidized at 800 C and fully oxidized TiO2 are shown in Fig. 3. A potentialecurrent density curve of 46.3 wt % Pt/C (Tanaka Kikinzoku Kogyo, Japan) was also plotted for comparison: the Pt loading was 4 mg/GC rod (6 mmØ) or 14 mg cm2. The ORR currents of TiCN and TiO2 were observed below 0.55 V vs. RHE, showing low electrocatalytic activity toward ORR. However, the ORR current of TiCNO prepared at 800 C and 20 h was observed at 0.88 V vs. RHE. The partial surface oxidation of TiCN clearly enhanced the electrocatalytic activity for the ORR, possibly due to an increase of the active surface area or the oxygen adsorption area on the particle, resulting in a significant increase in the ORR current (iORR ¼ 0:795 mA cm2 at 0.6 V vs. RHE) and the onset potential (0.88 V vs. RHE). This ORR current is comparable to that of partially oxidized niobium carbonitride (ca. 0.8 mA cm2) [2] and much higher than reported jiORR j of heattreated zirconium carbonitride (ca. 0.42 mA cm2) [18]. The onset potential of Pt/C was 1.05 V, which was about 0.17 V higher than that of as-prepared TiCNO, and the catalyst loading of TiCNO was much greater than that of Pt/C. However, since Ti has abundant resource and partially oxidized TiCNO reached onset potential of 0.88 V vs. RHE. On top of that, more experimental heat treatments can be done and catalyst activity can be further improved by tuning degree of surface oxidation. Thus, TiCNO may be a promising candidate of non-noble cathode material for PEMFCs. Fig. 4 presents the Raman spectra of TiC, TiN, TiCN, TiCNO, and TiO2. Raman spectra of titanium carbide (TiC, 2 mm, 99.5%, Alfa Aesar) and titanium nitride (TiN, <10 mm, 99.7%, Alfa Aesar) were included for comparison. TiCN has a very similar Raman spectrum to those of TiC and TiN in the range from 150 to 800 cm1 because TiC, TiN and as-prepared TiCN possess a face-centered cubic (FCC) crystal structure, showing the peaks at 260, 405 and 607 cm1 arising from the longitudinal acoustic (LA), second-order acoustic (2A) and transverse optical (TO) modes, respectively. The surface of TiC, TiN and
Fig. 3 e PotentialeORR current density curves of TiCN, TiCNO partially oxidized at 800 C, and fully oxidized TiO2 in 0.1 mol dmL3 sulfuric acid at 30 C with the scan rate of 5 mV sL1.
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preferentially excite P states, visible Raman spectra directly depend on the sp2 sites and configuration of the sp2 phase [20]. Therefore, the results show sp2 sites in amorphous carbon matrix were formed due to the breakage of TieC bond. Raman spectra of as-prepared TiCN and fully oxidized TiO2 display broad graphite peaks whereas sharp D and G band were observed from Raman spectrum of TiCNO, clearly illustrating the formation of free carbon during the heat treatment process. Carbonitride materials have low electrical conductivity (<102 S cm1) [21], which is a major obstacle for their application in PEMFCs. The formation of free carbon in TiCNO may play an important role in its catalytic activity, enhancing electrical conductivity and electron transport during the oxygen reduction process.
4.
Fig. 4 e Raman spectra of TiC, TiN, TiCN, TiCNO, and TiO2 in Raman shift range of (a) 150e800 cmL1 and (b) 1000e2000 cmL1.
TiCN powders might have been slightly oxidized in air and/or oxygen molecules adsorbed on the surface as reported in Ref. [2]. When titanium was fully oxidized, the 607 cm1 mode did not show any frequency shift but the 405 cm1 mode was considerably shifted to the 439 cm1, due to the planar OeO interaction influenced by the oxygen deficiency rather than the TieO stretching interaction [19], a similar behavior shown for TiCNO. The presence of free carbon was indicated by two bands centered at 1350 cm1 and 1595 cm1, labeled D and G, respectively, which are characteristic of CeC bonding. In carbon materials, the D (defect) band is a breathing mode of A1g symmetry associated with optical phonons close to the K zone boundary in graphite. The D band is usually absent in the perfect graphite and only becomes active in the presence of disorder due to the breathing modes of sp2 atoms in ring and the limitation in graphite domain size induced by imperfections. On the other hand, the G (graphite) band, due to the bond stretching modes (optical phonon modes of E2g) of sp2 pairs in both ring and chain, indicates high disorder and dispersion in amorphous network. Since visible photons
Conclusions
The partially oxidized titanium carbonitride (TiCNO) was evaluated as a non-Pt cathode for PEMFCs. TiC0.5N0.5 powder was synthesized from the uniform mixture of TiO2 powder and carbon black at 1800 C in a tube furnace under nitrogen atmosphere. As-prepared TiC0.5N0.5 was heat-treated under a flow rate of 500 cm3 min1 of nitrogen containing 4% H2 and 0.5% O2 by volume at 800 C for 20 h to prepare TiCNO. The influence of the heat treatment on the catalytic activity toward oxygen reduction reaction and the properties of TiCNO were investigated. The onset potential for the ORR of the TiCNO was 0.88 V vs. RHE, which was much higher than those of as-prepared TiC0.5N0.5 and fully oxidized TiO2. ORR current (jiORR j at 0.6 V vs. RHE) was substantially enhanced to around 0.795 mA. More importantly, TiCNO has a high chemical stability in 0.1 mol dm3 sulfuric acid at 30 C. Raman measurements suggest that existence of sp2 carbon matrix in crystal structure may play an important role in its catalytic activity, enhancing electrical conductivity and electron transport during the oxygen reduction process.
Acknowledgments This work is supported by Startup Grant of Nanyang Technological University and Academic Research Fund (RG21/09) of Ministry of Education in Singapore. The authors thank Lion Corporation (Japan) and Ultrachem (S) Pte Ltd (Singapore) for providing Ketjen Black.
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