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In-situ synthesized TiC@CNT as high-performance catalysts for oxygen reduction reaction
Carbon 126 (2018) 566e573
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Carbon journal homepage: www.elsevier.com/locate/carbon
In-situ synthesized TiC@CNT as high-performance catalysts for oxygen reduction reaction Taizhong Huang a, *, Hengyi Fang a, Shun Mao b, **, Jiemei Yu a, Lei Qi a a Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, School of Chemistry and Chemical Engineering, University of Jinan, 336 West Nanxinzhuang Road, Jinan, Shandong 250022, China b State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 July 2017 Received in revised form 2 October 2017 Accepted 22 October 2017 Available online 26 October 2017
Developing low cost and high performance catalyst for oxygen reduction reaction (ORR) is a critical factor for novel energy sources. In this paper, we adopt carbon nanotube (CNT) as carbon source to synthesize titanium carbide (TiC). With the catalysis of iodine and sodium, the CNT react with titanium particles and in-situ synthesizes co-axial TiC@carbon nanotube (TiC@CNT). The co-axial structure is confirmed by Xray diffraction, scanning electron microscope, X-ray photo-electron spectroscopy and transmission electron microscopy tests. The electrocatalytic performances for ORR are investigated by cyclic voltammetry, Tafel, linear sweeping voltammetry, rotating disc electrode and rotating ring disc electrode methods. Results show that the TiC@CNT has high catalytic performance for ORR in both alkaline and acidic electrolytes. The ORR mainly happens through 4-electron pathway. The CNT plays the role of electron conductor and the TiC plays the role of active center for ORR. The high catalytic activity should be attributed to the synergistic effect of CNT and TiC. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction The accelerating development of fuel cells has a great demand for low cost and high performance catalysts to catalyze the reactions at the anodic and cathodic electrode. The high cost and rare sources of traditional precious metal-based catalysts inhibit the large-scale applications of fuel cells [1,2]. Developing low-cost and non-precious metal-based catalysts is the best way to overcome this limitation [3]. The catalytic characteristics of transition metalbased materials for cathode have been widely reported [4,5]. For instance, transition metal oxide, carbide, and nitride-based catalysts have been demonstrated to have great potential to be alternate for Pt-based catalysts for oxygen reduction reaction (ORR) [6e8]. To improve the conductivity of transition metal-based catalysts, carbon and other conductive materials are usually adopted as a support. The carbon-based supports also have a great impact on the catalytic performance of the catalysts, especially for transition metal carbides [9,10]. Among the carbon supports, carbon
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T. Huang), [email protected] (S. Mao). https://doi.org/10.1016/j.carbon.2017.10.074 0008-6223/© 2017 Elsevier Ltd. All rights reserved.
nanotube (CNT) is normally adopted as support for precious metalbased catalysts [11]. Functionalized-CNT can provide active sites for metal ions, which eventually enlarged the application fields of CNT [12]. For instance, functionalized-CNT supported Pt showed high catalytic activity for ORR in proton exchange membrane fuel cell [5,13], and it is proposed that the advanced carbon nanomaterials brings a lot of opportunities to high-efficiency electrocatalysts for ORR [14]. Great attention has been given to transition metal carbides for ORR in recent years due to their high catalytic activity and stability. Co-based carbide showed high catalytic performance for ORR in alkaline electrolyte [15]. Nitrogen-enriched core-shell structured Fe/Fe3C-C nanorods exhibited promising electrocatalytic performance for ORR [16]. On the other hand, the graphene supported core-shell structured Fe/Fe3C boxes that made from metaleorganic frameworks (MOFs) was reported to have high catalytic performance for ORR [17]. Moreover, vanadium-based carbides have also been investigated and demonstrated good catalytic activities for ORR [18]. In comparison, graphene-supported VC showed better catalytic performance than that of the industrialized VC [19], and the catalytic activity enhancement was attributed to the specific VC morphology and graphene support. So far, some of the reported current densities of transition metal carbides for ORR even
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surpassed that of the commercial Pt/C catalyst, although the onset potential was still not as good as that of Pt/C. It is reported that hybrid catalysts composed of transition metal carbides and carbon nanomaterials have a great potential to be the next generation electrocatalysts for ORR [20]. Up to now, investigations on metal carbides for ORR were mainly conducted in the alkaline electrolyte [21]. This is because many metal carbides are soluble in acidic electrolyte. The solubility of catalyst in the acidic electrolyte greatly limits the applications of metal carbides for ORR in acidic fuel cells. Therefore, developing metal carbides or hybrid catalysts of metal carbide/nanocarbon with high endurance to acidic electrolyte is needed for the future development of fuel cells. In this study, we adopted CNT as carbon source to synthesize titanium carbide (TiC) as catalyst for ORR by a facile one-pot method. The titanium reacted with CNT and formed TiC on the outer layer of CNT, producing a co-axial structure. The dentritic TiC@CNT catalyst combines the advantages of CNT and TiC, which leads to a high catalytic activity and good long-time running stability both in alkaline and acidic electrolytes. The outer layer of TiC played the role of active center and the inner CNT core assured the good conductivity and high stability for ORR. 2. Experimental methods 2.1. Material synthesis The fabrication process of dentritic TiC is schematically showed in Fig. 1a. 2.4 g titanium powder, 0.6 g multiwall CNT, 25.4 g iodine and 4.6 g sodium were thoroughly mixed. Then the mixture was sealed in a stainless reactor and heated up to 573 K and kept for 5 h. All the above-mentioned operations were conducted in an argon atmosphere. At last, the reactor was naturally cooling down to ambient temperature. A black powder was obtained from the reactor and the powder was boiled in 6 M HCl for 15 min. The residue iodine, titanium and other by-product were dissolved and removed from the powder. At last, the obtained precipitate (TiC@CNT) was washed with DI water several times and fully dried. 2.2. Structure characterizations X-ray diffraction (XRD) tests were performed on a Bruker D8-
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advance X-ray powder diffractometer with Cu Ka radiation (l ¼ 1.5418 Å). The morphology and the elemental mapping of the catalysts were obtained by using a Hitachi (S-4800) scan electron microscope (SEM) equipped with an energy-dispersive spectroscopy analyzer. Transmission electron microscopy (TEM) images were obtained by a Tecnai 20 U-TWIN instrument at accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) tests were conducted by using a Perkin Elmer PHI5300 spectrometer with monochromatized Mg Ka radiation. The BrunnerEmmetTeller (BET) measurements of TiC@CNT were conducted by using a Quantachrome Autosorb gas-sorption system. The N2 adsorptionedesorption measurements were carried out at 77 K. 2.3. Electrochemical tests The electrochemical tests were conducted using a CHI 760E electrochemical workstation. In the electrochemical testing system, carbon electrode with the diameter of 5 mm was the counterelectrode and glassy carbon electrode with catalysts was the working electrode. 0.1 M KOH or 0.5 M H2SO4 were used as the electrolyte solution. The working electrode was made by follows: 5.0 mg catalyst was mixed into 50 ml Nafion solution (Aldrich, 5% in aliphatic alcohols) and 450 ml DI water and the mixture was sonicated for 40 min. Then 5.0 ml of the suspension was dropped onto a glassy-carbon electrode (3 mm in diameter) and fully dried. In the alkaline electrolyte, the Hg/Hg2Cl2 electrode was used as reference electrode. Before electrochemical tests, the cyclic voltammetry tests of working electrode was conducted at a sweeping rate of 0.05 V/s between 0.2 and 0.8 V versus Hg/Hg2Cl2 until reproducible results were obtained. Thereafter, the electrolyte was saturated with oxygen and cyclic voltammogram tests were conducted for oxygen reduction from 0.2 to 0.8 V versus Hg/Hg2Cl2 at sweeping rates of 5, 10, 20, 50 and 100 mV/s, respectively. The linearly sweeping voltammetry (LSV) and Tafel tests were conducted at the sweeping rate of 5 mV/s. The electrochemical impedance spectroscopy (EIS) tests were carried out at a half wave potential from 0.01 to 105 Hz. The long-time running stability test was recorded for 5000 s at the potential of 0.2 V versus Hg/Hg2Cl2. The Ag/AgCl reference electrode was used for the acidic 0.5 M H2SO4 electrolyte. The CV tests were conducted in the range of 0.2 Ve1.2 V. The other electrochemical tests are similar to that of
Fig. 1. (a) Schematic illustration of the synthesis process of dentritic TiC@CNT. (b) SEM image, elemental mapping, and EDS results of the dentritic TiC@CNT. (A colour version of this figure can be viewed online.)
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alkaline electrolyte. The rotating disc electrode (RDE) and rotating ring disc electrode (RRDE) tests were carried out on a RRDE-3A rotating ring disc electrode combined with CHI 760E electrochemical work station. The RDE tests were conducted at the sweeping rate of 5 mV/s with different rotating speeds. RRDE tests were carried out on a rotating speed of 1600 rpm with the sweeping rate of 5 mV/s. The potential of the ring electrode of RRDE tests was 0.5 V (v.s. Hg/Hg2Cl2) for the 0.1 M KOH and 1.0 V (v.s. Ag/ AgCl) for the 0.5 M H2SO4 electrolyte. 3. Results and discussion The synthesis process of dentritic TiC@CNT is schematically shown in Fig. 1a. With the CNT as the carbon source and back bone, hierarchical dentritic TiC are grown on the CNT. This hybrid structure enables the fully exposure of TiC surface to electrolyte during ORR and leads to a high conductive catalyst structure for charge transfer between the catalyst and electrode. Fig. 1b showed the SEM, elemental mapping and energy dispersive spectra (EDS) of TiC@CNT. It was clearly displayed that the TiC was in a dentritic structure. This should be attributed to the reaction of Ti particles and CNT, which was catalyzed by iodine and sodium. The titanium atoms of titanium particles diffused along the axis of CNT and reacted with the carbon atoms of outer layer CNT. On the other hand, the carbon atoms also diffused into titanium particles, simultaneously. The dentritic structure was eventually formed with the mutual diffusion of both elements. The elemental mapping of Fig. 1b showed that the distribution of titanium and carbon atoms were consistent with each other. The signal of silicon in Fig. 1b came from the substrate of the test. The EDS proved the co-existence of titanium and carbon atoms. The SEM tests certified the successive synthesis of dentritic TiC. It has been reported that the TiC nanoparticles can form plate film structure [22]. However, dentritic structure in this case is more
favoured for catalyst application since this open structure allows the free diffusion of ions to the catalyst surface and presents high density of reaction cites for the ORR. The XRD patterns of TiC@CNT and CNT are showed in Fig. 2a. It was indexed that the diffraction peaks were corresponding to the JCPDF No. 32-1383 of TiC. The corresponding XRD patterns of TiC@CNT confirmed the successful synthesis of TiC. TiC had a cubic structure and the corresponding space group was Fm-3m(225). The calculated cell parameter a of TiC was 0.433 nm, which was consistent with the reported results [23]. Fig. 2b showed the Raman spectra of TiC@CNT. The D peak and G peak that centered at 1333 and 1582 cm1 are found in the spectra, respectively. The two peaks are typical Raman spectra of carbon containing catalysts [24,25]. This should be attributed to the co-existed CNT of TiC@CNT. The structure of TiC@CNT was also characterized by TEM images. The TEM and high resolution TEM (HRTEM) images are showed in Fig. 2c and d. Based on TEM images, TiC dentrites are found overlapped with each other and the CNT is the “axis” and backbone for the TiC dentrites. The arrow pointed area clearly showed that CNT was enclosed by the TiC dentritic structure. Fig. 2d showed the HRTEM image of the dentritic TiC. Both the dentritic TiC and CNT were detected. The (111) facet of TiC was observed, which confirmed the successful synthesis of TiC. On the other hand, as shown in Fig. 2d, the (002) facet of CNT was also detected, confirming the intimate contact between the TiC and CNT. The synthesized TiC was formed on the surface of CNT. The interlayer area, which was labelled in the dotted region in the Fig. 2d, proved the synthesis of TiC on the outer layer of CNT. The inset of Fig. 2c is the selected area electron diffraction (SAED) of TiC@CNT. The SAED patterns of CNT showed a bright ring style [26], and the cubic (111) facet of TiC showed hexagonal point pattern. The surface chemical compositions of TiC@CNT were characterized by XPS tests. Fig. 3a showed the XPS of TiC, which clearly proved the existence of Ti and C elements. On the other hand, the signal of oxygen
Fig. 2. (a) XRD patterns and (b) Raman spectra of CNT and TiC@CNT. (c) TEM image and (d) HRTEM images of TiC@CNT. (A colour version of this figure can be viewed online.)
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Fig. 3. XPS spectra of TiC@CNT. (a) Survey XPS spectrum of TiC@CNT; High resolution XPS of (b) C 1s, (c) Ti 2p, and (d) O 1s. (A colour version of this figure can be viewed online.)
was also detected that was come from the oxygen in the synthesis process. Micro amount of oxygen react with titanium and formed TiO2. The amount of TiO2 was so scarce that it could not be detected by XRD and TEM tests. And the existence of oxygen cannot be found in EDS spectrum, either. The XPS of pure CNT are supplied in Fig. S1 (Supplementary Materials) showed that only the signal of carbon was detected. Fig. 3b showed the high resolution XPS spectrum of carbon. The signal of C 1s should be attributed to the co-existed CNT and TiC. The binding energy of the C 1s peak was centered at 281.9 eV. Researches on the XPS of winged CNT also proved the C-C bond with the similar binding energy [27]. The bond of C-Ti was also detected in the spectrum, which centered at 281.3 eV [28]. Similar C-Ti bond was also found in the study of carbon-doped TiO2 [29]. Fig. 3c showed the XPS spectra of Ti2p. The peaks centered at 452.2 eV, 455.9 eV, 453.2 eV, and 458.5 eV, which were attributed to the Ti 2p3/2, Ti 2p1/2, Ti-C and Ti-O bonds, respectively. The Ti-C bond was corresponding to the C-Ti bond of carbon elements. The results clearly proved the successful synthesis of TiC catalyst. The bond of Ti-O should be attributed to the impurity of TiO2. Fig. 3d showed the high resolution XPS spectra of O 1s. The peaks centered at 527.5 eV and 529.4 eV were attributed to O 1s and O-Ti bond, separately [30]. The O-Ti bond was also detected in the research of TiO2 based photocatalysts [31]. The surface area was critical factor to the performance of the electrocatalyst for ORR. To study the specific surface area, BrunnerEmmetTeller (BET) measurements were conducted on the TiC@CNT. Fig. S2a showed the result of nitrogen absorptiondesorption tests. Based on the BET tests, the obtained specific surface area of TiC@CNT was 31.8 m2/g. Fig. S2b showed that the average pore size of TiC@CNT was 8.43 nm. The electrocatalytic characteristics of the co-axis dentritic TiC@CNT was examined by cyclic voltammetry (CV) tests both in
nitrogen- and oxygen-saturated electrolyte at the sweeping rate of 0.05 V/s and the results are shown in Fig. 4a. It was clearly observed that the CV test of nitrogen saturated electrolyte was a smooth line and there was no peak detected. As comparison, an obvious peak of oxygen reduction was clearly observed, which proved the good catalytic performance of the catalyst. The peak current intensity of the catalyst even surpassed that of the reported Pt/C catalyst [32]. The catalytic performance of TiC was influenced by its morphology [33]. The reported TiC nanowire had a higher catalytic activity than bulk TiC. However, the catalytic activity of co-axial TiC@CNT greatly surpassed that of the reported TiC nanowire, indicating the dentritic morphology is more favoured in ORR than nanowire morphology. As comparison, the CV tests of pure CNT was also conducted in the same electrolyte with the same sweeping rate and the results are showed in Fig. S3 (Supplementary materials). The CV tests of TiC@CNT with different sweeping rates are shown in Fig. S4a. It was showed that the current intensity increased with increasing sweeping rates. Fig. S4b showed that the peak current intensity linearly increased with the square roots of sweeping rates. This meant that the ORR on the electrode was a diffusion controlled process [34]. Fig. 4b shows the Tafel test of TiC@CNT for oxygen reduction. The Tafel slope of the catalyst was 0.163 V/decade. Based on the Tafel test, the exchange current intensity (i0) and electron transfer coefficient (a) were calculated according to the Tafel equation. The calculated i0 and a were 8.32 107 A/cm2 and 0.08, respectively. Fig. 4c showed the electrochemical impedance spectroscopy (EIS) tests of the TiC@CNT catalyst. It was shown that the modulated results matched well with the tested results. R0 is the Ohmic resistance that is originated from the contact resistance of the catalyst with the electrode. R1 and R2 were oxygen reduction reaction resistances. The values of R1 and R2 were 70.78 U and 2628 U,
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Fig. 4. Electrochemical tests of TiC@CNT for oxygen reduction in 0.1 M KOH electrolyte. CV in nitrogen- and oxygen-saturated electrolyte (a), Tafel (b), EIS (c), i-t tests (d), RDE tests (e) and corresponding fitted K-L calculations (f). (A colour version of this figure can be viewed online.)
respectively. It is usually believed that the oxygen reduction involves 4-electron and 2-electron reactions. The oxygen molecular was directly transformed into hydroxide ion in 4-electron reaction style; while the intermediate of H2O2 was produced in the 2-electron reaction process and then transformed into hydroxide ion in the following reaction. The R1 corresponds to the mixed reaction of 4-electron and the first step of 2-elelectron ORR. Thus, the R2 corresponds to the second step of 2-electron ORR. The value of R1 was much lower than that of R2, based on which it could be deduced that the intermediate H2O2 should be in a low concentration. The 4-electron and 2-electron reactions will be discussed with the rotating disc electrode tests. Fig. 4d shows the i-t test results of TiC@CNT, which confirms that the current intensity decreased with time going. This should be attributed to both the decreased oxygen content in the electrolyte and the slow diffusion of oxygen on the electrode. The inset in Fig. 4d shows the CV test results at the time of t ¼ 0 and t ¼ 5000 s with a sweeping rate of 0.05 V/s. The inset proved that the current
intensity and the onset potential of oxygen reduction at t ¼ 5000 s was much higher than that at t ¼ 0 s. This should be attributed to the continuous activation of the catalyst. To investigate the catalytic mechanism of TiC@CNT for ORR, the RDE tests were conducted and the results are shown in Fig. 4e. It is observed that the current intensity increased with increasing rotating speeds. Based on the RDE tests, the electron transfer number was calculated according to the Koutecky-Levich equation [35]. The calculation methods and equation were provided in the Supplementary Materials. The calculated n at different potentials are displayed in Fig. 4f. The calculated n is distributed in the range of 3 and 4, which should be attributed to different ORR happening pathways existed on the electrode surface. The suggested ORR process include the following reactions [36]:
2H2 O þ O2 þ 4e /4OH
(1)
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2H2 O þ O2 þ 2e /2OH þ H2 O2 ;
(2)
H2 O2 þ 2e /2OH
(3)
The value of n was very close to 4, which meant that the 4electron style was the major reaction pathway on the electrode. The values of n also indicated that both the 4-electron and 2electron reactions coexisted on the catalyst surface. The R1 of modulated EIS is corresponding to the mixed 4-electron reaction (reaction 1) and the first step of 2-electron reaction (reaction 2). The R2 of the EIS modulated reaction is corresponding to the second step of the 2-electron reaction (reaction 3). The obtained values n (3.74) is quite similar to that of the N, B-doped graphene foams (n ¼ 3.5) [37] and mesoporous Pt-C catalyst (n ¼ 3.7) [38]. The catalytic mechanism of the TiC@CNT was also investigated by RRDE tests and the results are shown in Fig. 5. Fig. 5a showed that the disc current intensity was much higher than that of the ring. The electron transfer number and hydrogen peroxide content on the electrode at different potentials were calculated according to the following equations (4) and (5), respectively [13],
n¼
4 Id ; ðId þ Ir =NÞ
H2 O2 % ¼ 200
(4)
Ir =N ; Id þ Ir =N
(5)
where Id is the current of disk, Ir is current of ring and N is the geometrical current collection coefficient of Pt ring in RRDE as 0.4. The calculated electron transfer number and percentage of H2O2 at different potentials are shown in Fig. 5b. It is clearly showed that the electron transfer number was between 3 and 4. This result is consistent with the RDE test results. Correspondingly, the percentage of H2O2 decreased with the increase of electron transfer number n. Reviews on the high performance catalysts for ORR drew similar conclusions [14]. It is deduced that the 4-electron pathway dominated the ORR reaction. Researches on the RRDE tests of nitrogen doped electrocatalysts for ORR observed similar results [15]. Compared with Pt/C catalyst, metal carbides are usually dissolved in acidic electrolyte, which limit the application of the metal carbide [14]. TiC is stable in acidic electrolyte that ensures the usage of TiC in acidic working condition. In this study, the catalytic characteristics of TiC@CNT for oxygen reduction in 0.5 M H2SO4 were investigated and the results are showed in Fig. 6. Fig. 6a showed the CV tests of TiC@CNT in nitrogen- and oxygen-
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saturated 0.5 M H2SO4 electrolyte. It was shown that there was no peak observed in the nitrogen-saturated electrolyte. This result certified that the TiC@CNT was stable in the acidic electrolyte. In contrast, one peak was observed in the oxygen-saturated 0.5 M H2SO4 electrolyte. The CV curves of TiC@CNT proved that the TiC@CNT can catalyze the ORR in an acidic electrolyte. Similarly, the catalytic performance of pure CNT was also investigated by CV tests in both nitrogen- and oxygen-saturated 0.5 M H2SO4 and the results are shown in Fig. S5 (Supplementary materials). The CV tests of TiC@CNT for ORR at different sweeping rates are shown in Fig. S6. The results show that the current intensity increases with increasing sweeping rates. The peak current intensity is also linearly increased with the increase of square roots of sweeping rates, indicating that the TiC@CNT catalyzed ORR in acidic electrolyte is a diffusion controlled reaction. Fig. 6b shows the i-t test results of TiC@CNT for ORR in 0.5 M H2SO4. It is shown that the current intensity decreased with the running time. The decrease should be originated from both the decreased oxygen intensity of the electrolyte and the sluggish diffusion of oxygen on the electrode surface. The inset showed the CV results at t ¼ 0 and t ¼ 4500 s. It was confirmed that, after 4500 s running, both the peak current intensity and the onset potential of oxygen reduction were increased. This result was similar to the test result in alkaline electrolyte. Fig. 6c shows the Tafel slopes of ORR in H2SO4 electrolyte. The Tafel slope was 0.14 V/decade. The value of the Tafel slope was quite approach to that of Fe-N-C and Pt-C catalysts, which was ascribed to the transfer of the first electron as a rate-determining step [39]. Based on the Tafel tests, the calculated electron transfer coefficient (a) and the exchange current intensity (i0) was 0.1 and 2.28 106 A/cm2, respectively. Fig. 6d showed the measured and modulated EIS results of ORR in 0.5 M H2SO4 electrolyte. The inset was the modulated equivalent circuit. It was found that the modulated data were in consistence with the measured data. The R0 of the circuit (contact resistance) was 18.8 U. The values of R1 and R2 were 68.75 U and 1227 U, respectively, which was corresponding to the two reaction styles [40]. The R1 is corresponding to the mixture of the following reactions (6) and (7):
O2 þ 4Hþ þ 4e /2H2 O
(6)
O2 þ 2Hþ þ 2e /H2 O2
(7)
The R2 is corresponding to the reduction reaction of H2O2:
H2 O2 þ 2Hþ þ 2e /2H2 O
(8)
The low value of R1 indicates that the oxygen could be easily
Fig. 5. RRDE tests of TiC@CNT in 0.1 M KOH at 1600 rpm. (A colour version of this figure can be viewed online.)
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Fig. 6. Cyclic voltammetry (a), current (i)-time (t) (b), Tafel (c), EIS (d), RDE (e) and RRDE (f) test results of TiC@CNT for ORR in 0.5 M H2SO4 electrolyte. (A colour version of this figure can be viewed online.)
reduced through the pathway of reaction (6) and (7). The high value of R2 is resulted from the reduction of H2O2, which is corresponding to reaction (8). The reaction mechanism of TiC@CNT in an acidic electrolyte was studied by RDE and RRDE tests and the results are shown in Fig. 6e and f, respectively. Fig. 6e showed that the current intensity slightly increased with the increase of rotating speeds. However, the currents under different rotating speeds were quite similar at the lower potential (0.45e0.6 V), which made the electron transfer number (n) was difficult to be accurately calculated. Fig. 6f showed the RRDE tests of the TiC@CNT in the acidic electrolyte. Based on the RRDE test, it was deduced that the electron transfer number (n) of the acidic electrolyte was 3.6, which was close to the calculated electron transfer number of the alkaline electrolyte. The RRDE study proved that the TiC@CNT catalyzed ORR mainly happened through 4-electron pathway. The catalytic activity study indicated that the dentritic TiC@CNT had an excellent catalytic performance for ORR in acidic electrolyte. Comparison of the electron transfer number showed that the n of acidic electrolyte was slighter lower than that of alkaline electrolyte, which could be attributed to the preferred formation of peroxide species in the alkaline medium which desorbs more readily than that in the acid counterpart [41]. Density functional theory study on the catalytic activity of CoNX
showed that the strong interaction between peroxide and Conitride defect supports a 2 2e single site ORR mechanism in alkaline and acidic media [42]. The study on nitrogen-doped CNT also showed that the current of ORR in alkaline electrolyte was higher than that of acidic electrolyte [43]. The reason for the difference was attributed to the doped nitrogen. The different performance of TiC@CNT in alkaline and acidic electrolytes should also be attributed to the structure defects in the co-axial structure. The onset potential of ORR in 0.1 M KOH and 0.5 M H2SO4 were 0.2 V (vs. Hg/Hg2Cl2) and 0.78 V (vs. Ag/AgCl), respectively, which was only 0.15 V and 0.1 V lower than those of Pt/C catalyst [44]. This indicates that the TiC@CNT catalyst has good catalytic performance for ORR both in alkaline and acidic electrolytes. 4. Conclusion In this paper, the CNT was used as carbon source to synthesize TiC. The TiC and CNT formed a co-axial structure. The TiC@CNT catalyst shows high electrocatalytic performance for ORR both in alkaline and acidic electrolytes. The electron transfer number of TiC@CNT catalyzed ORR is close to 4 in both alkaline and acidic electrolytes, which indicates that the ORR mainly happens through 4-electron pathway. In addition to the high catalytic activity, the
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TiC@CNT catalyst also has a good long-term running stability in both alkaline and acidic electrolytes. The outer layer TiC plays the role of catalytic active center for ORR, thus, the inner CNT plays the role of electron conductor. The synergistic effect between TiC and CNT assures the high catalytic activity of TiC@CNT. The TiC@CNT catalyst has great potential to be high performance catalysts for ORR. Acknowledgements This work was financial supported by the Science Development Project of Shandong Provincial (No. 2017GGX40115, 2016GGX102038) and the National Natural Science Foundation of China (No. 21407060). S. M. thanks 1000 Talents Plan of China for financial support. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.carbon.2017.10.074. References [1] Z. Chen, D. Higgins, A. Yu, L. Zhang, J. Zhang, A review on non-precious metal electrocatalysts for PEM fuel cells, Energy Environ. Sci. 4 (9) (2011) 3167e3192. [2] T. Fujita, P. Guan, K. McKenna, X. Lang, A. Hirata, L. Zhang, et al., Atomic origins of the high catalytic activity of nanoporous gold, Nat. Mater. 11 (9) (2012) 775e780. [3] D. He, L. Zhang, D. He, G. Zhou, Y. Lin, Z. Deng, et al., Amorphous nickel boride membrane on a platinumenickel alloy surface for enhanced oxygen reduction reaction, Nat. Commun. 7 (2016) 12362e12369. [4] W. He, Y. Wang, C. Jiang, L. Lu, Structural effects of a carbon matrix in nonprecious metal O2-reduction electrocatalysts, Chem. Soc. Rev. 45 (9) (2016) 2396e2409. [5] D. Higgins, P. Zamani, A. Yu, Z. Chen, The application of graphene and its composites in oxygen reduction electrocatalysis: a perspective and review of recent progress, Energy Environ. Sci. 9 (2) (2016) 357e390. [6] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, et al., Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction, Nat. Mater. 10 (10) (2011) 780e786. [7] T. Huang, S. Mao, H. Pu, Z. Wen, X. Huang, S. Ci, et al., Nitrogen-doped graphene-vanadium carbide hybrids as a high-performance oxygen reduction reaction electrocatalyst support in alkaline media, J. Mater. Chem. A 1 (2013) 13404e13410. [8] T. Huang, S. Mao, G. Zhou, Z. Wen, X. Huang, S. Ci, et al., Hydrothermal synthesis of vanadium nitride and modulation of its catalytic performance for oxygen reduction reaction, Nanoscale 6 (16) (2014) 9608e9613. [9] S. Guo, S. Zhang, S. Sun, Tuning nanoparticle catalysis for the oxygen reduction reaction, Angew. Chem. Int. Ed. 52 (33) (2013) 8526e8544. [10] D.C. Higgins, M.A. Hoque, F. Hassan, J.-Y. Choi, B. Kim, et al., Oxygen reduction on grapheneecarbon nanotube composites doped sequentially with nitrogen and sulfur, Acs Catal. 4 (8) (2014) 2734e2740. [11] Y. Liang, H. Wang, P. Diao, W. Chang, G. Hong, Y. Li, et al., Oxygen reduction electrocatalyst based on strongly coupled cobalt oxide nanocrystals and carbon nanotubes, J. Am. Chem. Soc. 134 (38) (2012) 15849e15857. [12] B. Kim, W.M. Sigmund, Functionalized multiwall carbon nanotube/gold nanoparticle composites, Langmuir 20 (19) (2004) 8239e8242. [13] Z. Liu, X. Lin, J.Y. Lee, W. Zhang, M. Han, L.M. Gan, Preparation and characterization of platinum-based electrocatalysts on multiwalled carbon nanotubes for proton exchange membrane fuel cells, Langmuir 18 (10) (2002) 4054e4060. [14] M. Zhou, H.-L. Wang, S. Guo, Towards high-efficiency nanoelectrocatalysts for oxygen reduction through engineering advanced carbon nanomaterials, Chem. Soc. Rev. 45 (5) (2016) 1273e1307. [15] M.D. Meganathan, S. Mao, T. Huang, G. Sun, Reduced graphene oxide intercalated Co2C or Co4N nanoparticles as an efficient and durable fuel cell catalyst for oxygen reduction, J. Mater. Chem. A 5 (2017) 2972e2980. [16] Z. Wen, S. Ci, F. Zhang, X. Feng, S. Cui, S. Mao, et al., Nitrogen-enriched coreshell structured Fe/Fe3C-C nanorods as advanced electrocatalysts for oxygen reduction reaction, Adv. Mater. 24 (11) (2012) 1399e1404. [17] Y. Hou, T. Huang, Z. Wen, S. Mao, S. Cui, J. Chen, MetalOrganic frameworkderived nitrogen-doped core-shell-structured porous Fe/Fe3C@C nanoboxes Supported on Graphene Sheets for Efficient Oxygen Reduction Reactions, Adv.
573
Energy Mater. 1400337 (2014) 1e8. [18] T. Huang, S. Mao, G. Zhou, Z. Zhang, Z. Wen, X. Huang, et al., A high-performance catalyst support for methanol oxidation with graphene and vanadium carbonitride, Nanoscale 7 (4) (2015) 1301e1307. [19] T. Huang, J. Yu, J. Han, Z. Zhang, Y. Xing, C. Wen, et al., Oxygen reduction catalytic characteristics of vanadium carbide and nitrogen doped vanadium carbide, J. Power Sources 300 (2015) 483e490. [20] C.R. Raj, A. Samanta, S.H. Noh, S. Mondal, T. Okajima, T. Ohsaka, Emerging new generation electrocatalysts for the oxygen reduction reaction, J. Mater. Chem. A 4 (29) (2016) 11156e11178. [21] J. Xu, R. Zhang, P. Chen, S. Ge, Effects of adding ethanol to KOH electrolyte on electrochemical performance of titanium carbide-derived carbon, J. Power Sources 246 (2014) 132e140. [22] Y. Ou, X. Cui, X. Zhang, Z. Jiang, Titanium carbide nanoparticles supported Pt catalysts for methanol electrooxidation in acidic media, J. Power Sources 195 (5) (2010) 1365e1369. [23] N.J. Welham, P.E. Willis, Formation of TiN/TiC-Fe composites from ilmenite (FeTiO3) concentrate, Metall. Mater. Trans. B 29 (5) (1998) 1077e1083. [24] R. Zhang, Y. Zhang, Q. Zhang, H. Xie, W. Qian, F. Wei, Growth of half-meter long carbon nanotubes based on schulzeflory distribution, ACS Nano 7 (7) (2013) 6156e6161. [25] F. Tuinstra, J.L. Koenig, Raman spectrum of graphite, J. Chem. Phys. 53 (3) (1970) 1126e1130. [26] T. Huang, S. Mao, M. Qiu, O. Mao, C. Yuan, J. Chen, Nitrogen-boron dipolardoped nanocarbon as a high-efficiency electrocatalyst for oxygen reduction reaction, Electrochim. Acta 222 (2016) 481e487. [27] Y. Cheng, H. Zhang, C.V. Varanasi, J. Liu, Highly efficient oxygen reduction electrocatalysts based on winged carbon nanotubes, Sci. Rep. 3 (2013) 3195e3201. [28] O. Rist, P.T. Murray, Growth of TiC films by pulsed laser evaporation (PLE) and characterization by XPS and AES, Anal. Bioanal. Chem. 341 (5) (1991) 360e364. [29] H. He, P. Xiao, M. Zhou, F. Liu, S. Yu, L. Qiao, et al., PtNi alloy nanoparticles supported on carbon-doped TiO2 nanotube arrays for photo-assisted methanol oxidation, Electrochim. Acta 88 (2013) 782e789. [30] D.O. Scanlon, C.W. Dunnill, J. Buckeridge, S.A. Shevlin, A.J. Logsdail, S.M. Woodley, et al., Band alignment of rutile and anatase TiO2, Nat. Mater. 12 (9) (2013) 798e801. [31] T. Huang, S. Mao, J. Yu, Z. Wen, G. Lu, J. Chen, Effects of N and F doping on structure and photocatalytic properties of anatase TiO2 nanoparticles, Rsc Adv. 3 (37) (2013) 16657e16661. [32] G. Fu, K. Wu, J. Lin, Y. Tang, Y. Chen, Y. Zhou, et al., One-pot water-based synthesis of PtePd alloy nanoflowers and their superior electrocatalytic activity for the oxygen reduction reaction and remarkable methanol-tolerant ability in acid media, J. Phys. Chem. C 117 (19) (2013) 9826e9834. [33] V. Kiran, K. Srinivasu, S. Sampath, Morphology dependent oxygen reduction activity of titanium carbide: bulk vs. nanowires, Phys. Chem. Chem. Phys. 15 (22) (2013) 8744e8751. [34] Y. Wang, D. Zhang, M. Tang, S. Xu, M. Li, Electrocatalysis of gold nanoparticles/ layered double hydroxides nanocomposites toward methanol electrooxidation in alkaline medium, Electrochim. Acta 55 (12) (2010) 4045e4049. [35] J.W. Hong, S.W. Kang, B.-S. Choi, D. Kim, S.B. Lee, S.W. Han, Controlled synthesis of PdePt alloy hollow nanostructures with enhanced catalytic activities for oxygen reduction, ACS Nano 6 (3) (2012) 2410e2419. [36] J. Xu, G. Fu, Y. Tang, Y. Zhou, Y. Chen, T. Lu, Non-precious Co3O4 nano-rod electrocatalyst for oxygen reduction reaction in anion-exchange membrane fuel cells, Energy Environ. Sci. 5 (1) (2012) 5333e5339. [37] Y. Xue, D. Yu, L. Dai, R. Wang, D. Li, A. Roy, et al., Three-dimensional B,N-doped graphene foam as a metal-free catalyst for oxygen reduction reaction, Phys. Chem. Chem. Phys. 15 (29) (2013) 12220e12226. [38] Z. Wu, Y. Lv, Y. Xia, P.A. Webley, D. Zhao, Ordered mesoporous Platinum@ Graphitic carbon embedded nanophase as a highly active, stable, and methanol-tolerant oxygen reduction electrocatalyst, J. Am. Chem. Soc. 134 (4) (2011) 2236e2245. [39] H. Xiao, Z.-G. Shao, G. Zhang, Y. Gao, W. Lu, B. Yi, FeeNecarbon black for the oxygen reduction reaction in sulfuric acid, Carbon 57 (1) (2013) 443e451. [40] E. Biddinger, D. Deak, U. Ozkan, Nitrogen-containing carbon nanostructures as oxygen-reduction catalysts, Top. Catal. 52 (11) (2009) 1566e1574. [41] F. Bidault, D.J.L. Brett, P.H. Middleton, N.P. Brandon, Review of gas diffusion cathodes for alkaline fuel cells, J. Power Sources 187 (1) (2009) 39e48. [42] S. Kattel, P. Atanassov, B. Kiefer, Catalytic activity of Co-Nx/C electrocatalysts for oxygen reduction reaction: a density functional theory study, Phys. Chem. Chem. Phys. 15 (1) (2013) 148e153. [43] Z. Mo, S. Liao, Y. Zheng, Z. Fu, Preparation of nitrogen-doped carbon nanotube arrays and their catalysis towards cathodic oxygen reduction in acidic and alkaline media, Carbon 50 (7) (2012) 2620e2627. [44] Y. Bing, H. Liu, L. Zhang, D. Ghosh, J. Zhang, Nanostructured Pt-alloy electrocatalysts for PEM fuel cell oxygen reduction reaction, Chem. Soc. Rev. 39 (6) (2010) 2184e2202.
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In-situ synthesized TiC@CNT as high-performance catalysts for oxygen reduction reaction Taizhong Huang a, *, Hengyi Fang a, Shun Mao b, **, Jiemei Yu a, Lei Qi a a Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, School of Chemistry and Chemical Engineering, University of Jinan, 336 West Nanxinzhuang Road, Jinan, Shandong 250022, China b State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 July 2017 Received in revised form 2 October 2017 Accepted 22 October 2017 Available online 26 October 2017
Developing low cost and high performance catalyst for oxygen reduction reaction (ORR) is a critical factor for novel energy sources. In this paper, we adopt carbon nanotube (CNT) as carbon source to synthesize titanium carbide (TiC). With the catalysis of iodine and sodium, the CNT react with titanium particles and in-situ synthesizes co-axial TiC@carbon nanotube (TiC@CNT). The co-axial structure is confirmed by Xray diffraction, scanning electron microscope, X-ray photo-electron spectroscopy and transmission electron microscopy tests. The electrocatalytic performances for ORR are investigated by cyclic voltammetry, Tafel, linear sweeping voltammetry, rotating disc electrode and rotating ring disc electrode methods. Results show that the TiC@CNT has high catalytic performance for ORR in both alkaline and acidic electrolytes. The ORR mainly happens through 4-electron pathway. The CNT plays the role of electron conductor and the TiC plays the role of active center for ORR. The high catalytic activity should be attributed to the synergistic effect of CNT and TiC. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction The accelerating development of fuel cells has a great demand for low cost and high performance catalysts to catalyze the reactions at the anodic and cathodic electrode. The high cost and rare sources of traditional precious metal-based catalysts inhibit the large-scale applications of fuel cells [1,2]. Developing low-cost and non-precious metal-based catalysts is the best way to overcome this limitation [3]. The catalytic characteristics of transition metalbased materials for cathode have been widely reported [4,5]. For instance, transition metal oxide, carbide, and nitride-based catalysts have been demonstrated to have great potential to be alternate for Pt-based catalysts for oxygen reduction reaction (ORR) [6e8]. To improve the conductivity of transition metal-based catalysts, carbon and other conductive materials are usually adopted as a support. The carbon-based supports also have a great impact on the catalytic performance of the catalysts, especially for transition metal carbides [9,10]. Among the carbon supports, carbon
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T. Huang), [email protected] (S. Mao). https://doi.org/10.1016/j.carbon.2017.10.074 0008-6223/© 2017 Elsevier Ltd. All rights reserved.
nanotube (CNT) is normally adopted as support for precious metalbased catalysts [11]. Functionalized-CNT can provide active sites for metal ions, which eventually enlarged the application fields of CNT [12]. For instance, functionalized-CNT supported Pt showed high catalytic activity for ORR in proton exchange membrane fuel cell [5,13], and it is proposed that the advanced carbon nanomaterials brings a lot of opportunities to high-efficiency electrocatalysts for ORR [14]. Great attention has been given to transition metal carbides for ORR in recent years due to their high catalytic activity and stability. Co-based carbide showed high catalytic performance for ORR in alkaline electrolyte [15]. Nitrogen-enriched core-shell structured Fe/Fe3C-C nanorods exhibited promising electrocatalytic performance for ORR [16]. On the other hand, the graphene supported core-shell structured Fe/Fe3C boxes that made from metaleorganic frameworks (MOFs) was reported to have high catalytic performance for ORR [17]. Moreover, vanadium-based carbides have also been investigated and demonstrated good catalytic activities for ORR [18]. In comparison, graphene-supported VC showed better catalytic performance than that of the industrialized VC [19], and the catalytic activity enhancement was attributed to the specific VC morphology and graphene support. So far, some of the reported current densities of transition metal carbides for ORR even
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surpassed that of the commercial Pt/C catalyst, although the onset potential was still not as good as that of Pt/C. It is reported that hybrid catalysts composed of transition metal carbides and carbon nanomaterials have a great potential to be the next generation electrocatalysts for ORR [20]. Up to now, investigations on metal carbides for ORR were mainly conducted in the alkaline electrolyte [21]. This is because many metal carbides are soluble in acidic electrolyte. The solubility of catalyst in the acidic electrolyte greatly limits the applications of metal carbides for ORR in acidic fuel cells. Therefore, developing metal carbides or hybrid catalysts of metal carbide/nanocarbon with high endurance to acidic electrolyte is needed for the future development of fuel cells. In this study, we adopted CNT as carbon source to synthesize titanium carbide (TiC) as catalyst for ORR by a facile one-pot method. The titanium reacted with CNT and formed TiC on the outer layer of CNT, producing a co-axial structure. The dentritic TiC@CNT catalyst combines the advantages of CNT and TiC, which leads to a high catalytic activity and good long-time running stability both in alkaline and acidic electrolytes. The outer layer of TiC played the role of active center and the inner CNT core assured the good conductivity and high stability for ORR. 2. Experimental methods 2.1. Material synthesis The fabrication process of dentritic TiC is schematically showed in Fig. 1a. 2.4 g titanium powder, 0.6 g multiwall CNT, 25.4 g iodine and 4.6 g sodium were thoroughly mixed. Then the mixture was sealed in a stainless reactor and heated up to 573 K and kept for 5 h. All the above-mentioned operations were conducted in an argon atmosphere. At last, the reactor was naturally cooling down to ambient temperature. A black powder was obtained from the reactor and the powder was boiled in 6 M HCl for 15 min. The residue iodine, titanium and other by-product were dissolved and removed from the powder. At last, the obtained precipitate (TiC@CNT) was washed with DI water several times and fully dried. 2.2. Structure characterizations X-ray diffraction (XRD) tests were performed on a Bruker D8-
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advance X-ray powder diffractometer with Cu Ka radiation (l ¼ 1.5418 Å). The morphology and the elemental mapping of the catalysts were obtained by using a Hitachi (S-4800) scan electron microscope (SEM) equipped with an energy-dispersive spectroscopy analyzer. Transmission electron microscopy (TEM) images were obtained by a Tecnai 20 U-TWIN instrument at accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) tests were conducted by using a Perkin Elmer PHI5300 spectrometer with monochromatized Mg Ka radiation. The BrunnerEmmetTeller (BET) measurements of TiC@CNT were conducted by using a Quantachrome Autosorb gas-sorption system. The N2 adsorptionedesorption measurements were carried out at 77 K. 2.3. Electrochemical tests The electrochemical tests were conducted using a CHI 760E electrochemical workstation. In the electrochemical testing system, carbon electrode with the diameter of 5 mm was the counterelectrode and glassy carbon electrode with catalysts was the working electrode. 0.1 M KOH or 0.5 M H2SO4 were used as the electrolyte solution. The working electrode was made by follows: 5.0 mg catalyst was mixed into 50 ml Nafion solution (Aldrich, 5% in aliphatic alcohols) and 450 ml DI water and the mixture was sonicated for 40 min. Then 5.0 ml of the suspension was dropped onto a glassy-carbon electrode (3 mm in diameter) and fully dried. In the alkaline electrolyte, the Hg/Hg2Cl2 electrode was used as reference electrode. Before electrochemical tests, the cyclic voltammetry tests of working electrode was conducted at a sweeping rate of 0.05 V/s between 0.2 and 0.8 V versus Hg/Hg2Cl2 until reproducible results were obtained. Thereafter, the electrolyte was saturated with oxygen and cyclic voltammogram tests were conducted for oxygen reduction from 0.2 to 0.8 V versus Hg/Hg2Cl2 at sweeping rates of 5, 10, 20, 50 and 100 mV/s, respectively. The linearly sweeping voltammetry (LSV) and Tafel tests were conducted at the sweeping rate of 5 mV/s. The electrochemical impedance spectroscopy (EIS) tests were carried out at a half wave potential from 0.01 to 105 Hz. The long-time running stability test was recorded for 5000 s at the potential of 0.2 V versus Hg/Hg2Cl2. The Ag/AgCl reference electrode was used for the acidic 0.5 M H2SO4 electrolyte. The CV tests were conducted in the range of 0.2 Ve1.2 V. The other electrochemical tests are similar to that of
Fig. 1. (a) Schematic illustration of the synthesis process of dentritic TiC@CNT. (b) SEM image, elemental mapping, and EDS results of the dentritic TiC@CNT. (A colour version of this figure can be viewed online.)
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alkaline electrolyte. The rotating disc electrode (RDE) and rotating ring disc electrode (RRDE) tests were carried out on a RRDE-3A rotating ring disc electrode combined with CHI 760E electrochemical work station. The RDE tests were conducted at the sweeping rate of 5 mV/s with different rotating speeds. RRDE tests were carried out on a rotating speed of 1600 rpm with the sweeping rate of 5 mV/s. The potential of the ring electrode of RRDE tests was 0.5 V (v.s. Hg/Hg2Cl2) for the 0.1 M KOH and 1.0 V (v.s. Ag/ AgCl) for the 0.5 M H2SO4 electrolyte. 3. Results and discussion The synthesis process of dentritic TiC@CNT is schematically shown in Fig. 1a. With the CNT as the carbon source and back bone, hierarchical dentritic TiC are grown on the CNT. This hybrid structure enables the fully exposure of TiC surface to electrolyte during ORR and leads to a high conductive catalyst structure for charge transfer between the catalyst and electrode. Fig. 1b showed the SEM, elemental mapping and energy dispersive spectra (EDS) of TiC@CNT. It was clearly displayed that the TiC was in a dentritic structure. This should be attributed to the reaction of Ti particles and CNT, which was catalyzed by iodine and sodium. The titanium atoms of titanium particles diffused along the axis of CNT and reacted with the carbon atoms of outer layer CNT. On the other hand, the carbon atoms also diffused into titanium particles, simultaneously. The dentritic structure was eventually formed with the mutual diffusion of both elements. The elemental mapping of Fig. 1b showed that the distribution of titanium and carbon atoms were consistent with each other. The signal of silicon in Fig. 1b came from the substrate of the test. The EDS proved the co-existence of titanium and carbon atoms. The SEM tests certified the successive synthesis of dentritic TiC. It has been reported that the TiC nanoparticles can form plate film structure [22]. However, dentritic structure in this case is more
favoured for catalyst application since this open structure allows the free diffusion of ions to the catalyst surface and presents high density of reaction cites for the ORR. The XRD patterns of TiC@CNT and CNT are showed in Fig. 2a. It was indexed that the diffraction peaks were corresponding to the JCPDF No. 32-1383 of TiC. The corresponding XRD patterns of TiC@CNT confirmed the successful synthesis of TiC. TiC had a cubic structure and the corresponding space group was Fm-3m(225). The calculated cell parameter a of TiC was 0.433 nm, which was consistent with the reported results [23]. Fig. 2b showed the Raman spectra of TiC@CNT. The D peak and G peak that centered at 1333 and 1582 cm1 are found in the spectra, respectively. The two peaks are typical Raman spectra of carbon containing catalysts [24,25]. This should be attributed to the co-existed CNT of TiC@CNT. The structure of TiC@CNT was also characterized by TEM images. The TEM and high resolution TEM (HRTEM) images are showed in Fig. 2c and d. Based on TEM images, TiC dentrites are found overlapped with each other and the CNT is the “axis” and backbone for the TiC dentrites. The arrow pointed area clearly showed that CNT was enclosed by the TiC dentritic structure. Fig. 2d showed the HRTEM image of the dentritic TiC. Both the dentritic TiC and CNT were detected. The (111) facet of TiC was observed, which confirmed the successful synthesis of TiC. On the other hand, as shown in Fig. 2d, the (002) facet of CNT was also detected, confirming the intimate contact between the TiC and CNT. The synthesized TiC was formed on the surface of CNT. The interlayer area, which was labelled in the dotted region in the Fig. 2d, proved the synthesis of TiC on the outer layer of CNT. The inset of Fig. 2c is the selected area electron diffraction (SAED) of TiC@CNT. The SAED patterns of CNT showed a bright ring style [26], and the cubic (111) facet of TiC showed hexagonal point pattern. The surface chemical compositions of TiC@CNT were characterized by XPS tests. Fig. 3a showed the XPS of TiC, which clearly proved the existence of Ti and C elements. On the other hand, the signal of oxygen
Fig. 2. (a) XRD patterns and (b) Raman spectra of CNT and TiC@CNT. (c) TEM image and (d) HRTEM images of TiC@CNT. (A colour version of this figure can be viewed online.)
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Fig. 3. XPS spectra of TiC@CNT. (a) Survey XPS spectrum of TiC@CNT; High resolution XPS of (b) C 1s, (c) Ti 2p, and (d) O 1s. (A colour version of this figure can be viewed online.)
was also detected that was come from the oxygen in the synthesis process. Micro amount of oxygen react with titanium and formed TiO2. The amount of TiO2 was so scarce that it could not be detected by XRD and TEM tests. And the existence of oxygen cannot be found in EDS spectrum, either. The XPS of pure CNT are supplied in Fig. S1 (Supplementary Materials) showed that only the signal of carbon was detected. Fig. 3b showed the high resolution XPS spectrum of carbon. The signal of C 1s should be attributed to the co-existed CNT and TiC. The binding energy of the C 1s peak was centered at 281.9 eV. Researches on the XPS of winged CNT also proved the C-C bond with the similar binding energy [27]. The bond of C-Ti was also detected in the spectrum, which centered at 281.3 eV [28]. Similar C-Ti bond was also found in the study of carbon-doped TiO2 [29]. Fig. 3c showed the XPS spectra of Ti2p. The peaks centered at 452.2 eV, 455.9 eV, 453.2 eV, and 458.5 eV, which were attributed to the Ti 2p3/2, Ti 2p1/2, Ti-C and Ti-O bonds, respectively. The Ti-C bond was corresponding to the C-Ti bond of carbon elements. The results clearly proved the successful synthesis of TiC catalyst. The bond of Ti-O should be attributed to the impurity of TiO2. Fig. 3d showed the high resolution XPS spectra of O 1s. The peaks centered at 527.5 eV and 529.4 eV were attributed to O 1s and O-Ti bond, separately [30]. The O-Ti bond was also detected in the research of TiO2 based photocatalysts [31]. The surface area was critical factor to the performance of the electrocatalyst for ORR. To study the specific surface area, BrunnerEmmetTeller (BET) measurements were conducted on the TiC@CNT. Fig. S2a showed the result of nitrogen absorptiondesorption tests. Based on the BET tests, the obtained specific surface area of TiC@CNT was 31.8 m2/g. Fig. S2b showed that the average pore size of TiC@CNT was 8.43 nm. The electrocatalytic characteristics of the co-axis dentritic TiC@CNT was examined by cyclic voltammetry (CV) tests both in
nitrogen- and oxygen-saturated electrolyte at the sweeping rate of 0.05 V/s and the results are shown in Fig. 4a. It was clearly observed that the CV test of nitrogen saturated electrolyte was a smooth line and there was no peak detected. As comparison, an obvious peak of oxygen reduction was clearly observed, which proved the good catalytic performance of the catalyst. The peak current intensity of the catalyst even surpassed that of the reported Pt/C catalyst [32]. The catalytic performance of TiC was influenced by its morphology [33]. The reported TiC nanowire had a higher catalytic activity than bulk TiC. However, the catalytic activity of co-axial TiC@CNT greatly surpassed that of the reported TiC nanowire, indicating the dentritic morphology is more favoured in ORR than nanowire morphology. As comparison, the CV tests of pure CNT was also conducted in the same electrolyte with the same sweeping rate and the results are showed in Fig. S3 (Supplementary materials). The CV tests of TiC@CNT with different sweeping rates are shown in Fig. S4a. It was showed that the current intensity increased with increasing sweeping rates. Fig. S4b showed that the peak current intensity linearly increased with the square roots of sweeping rates. This meant that the ORR on the electrode was a diffusion controlled process [34]. Fig. 4b shows the Tafel test of TiC@CNT for oxygen reduction. The Tafel slope of the catalyst was 0.163 V/decade. Based on the Tafel test, the exchange current intensity (i0) and electron transfer coefficient (a) were calculated according to the Tafel equation. The calculated i0 and a were 8.32 107 A/cm2 and 0.08, respectively. Fig. 4c showed the electrochemical impedance spectroscopy (EIS) tests of the TiC@CNT catalyst. It was shown that the modulated results matched well with the tested results. R0 is the Ohmic resistance that is originated from the contact resistance of the catalyst with the electrode. R1 and R2 were oxygen reduction reaction resistances. The values of R1 and R2 were 70.78 U and 2628 U,
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Fig. 4. Electrochemical tests of TiC@CNT for oxygen reduction in 0.1 M KOH electrolyte. CV in nitrogen- and oxygen-saturated electrolyte (a), Tafel (b), EIS (c), i-t tests (d), RDE tests (e) and corresponding fitted K-L calculations (f). (A colour version of this figure can be viewed online.)
respectively. It is usually believed that the oxygen reduction involves 4-electron and 2-electron reactions. The oxygen molecular was directly transformed into hydroxide ion in 4-electron reaction style; while the intermediate of H2O2 was produced in the 2-electron reaction process and then transformed into hydroxide ion in the following reaction. The R1 corresponds to the mixed reaction of 4-electron and the first step of 2-elelectron ORR. Thus, the R2 corresponds to the second step of 2-electron ORR. The value of R1 was much lower than that of R2, based on which it could be deduced that the intermediate H2O2 should be in a low concentration. The 4-electron and 2-electron reactions will be discussed with the rotating disc electrode tests. Fig. 4d shows the i-t test results of TiC@CNT, which confirms that the current intensity decreased with time going. This should be attributed to both the decreased oxygen content in the electrolyte and the slow diffusion of oxygen on the electrode. The inset in Fig. 4d shows the CV test results at the time of t ¼ 0 and t ¼ 5000 s with a sweeping rate of 0.05 V/s. The inset proved that the current
intensity and the onset potential of oxygen reduction at t ¼ 5000 s was much higher than that at t ¼ 0 s. This should be attributed to the continuous activation of the catalyst. To investigate the catalytic mechanism of TiC@CNT for ORR, the RDE tests were conducted and the results are shown in Fig. 4e. It is observed that the current intensity increased with increasing rotating speeds. Based on the RDE tests, the electron transfer number was calculated according to the Koutecky-Levich equation [35]. The calculation methods and equation were provided in the Supplementary Materials. The calculated n at different potentials are displayed in Fig. 4f. The calculated n is distributed in the range of 3 and 4, which should be attributed to different ORR happening pathways existed on the electrode surface. The suggested ORR process include the following reactions [36]:
2H2 O þ O2 þ 4e /4OH
(1)
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2H2 O þ O2 þ 2e /2OH þ H2 O2 ;
(2)
H2 O2 þ 2e /2OH
(3)
The value of n was very close to 4, which meant that the 4electron style was the major reaction pathway on the electrode. The values of n also indicated that both the 4-electron and 2electron reactions coexisted on the catalyst surface. The R1 of modulated EIS is corresponding to the mixed 4-electron reaction (reaction 1) and the first step of 2-electron reaction (reaction 2). The R2 of the EIS modulated reaction is corresponding to the second step of the 2-electron reaction (reaction 3). The obtained values n (3.74) is quite similar to that of the N, B-doped graphene foams (n ¼ 3.5) [37] and mesoporous Pt-C catalyst (n ¼ 3.7) [38]. The catalytic mechanism of the TiC@CNT was also investigated by RRDE tests and the results are shown in Fig. 5. Fig. 5a showed that the disc current intensity was much higher than that of the ring. The electron transfer number and hydrogen peroxide content on the electrode at different potentials were calculated according to the following equations (4) and (5), respectively [13],
n¼
4 Id ; ðId þ Ir =NÞ
H2 O2 % ¼ 200
(4)
Ir =N ; Id þ Ir =N
(5)
where Id is the current of disk, Ir is current of ring and N is the geometrical current collection coefficient of Pt ring in RRDE as 0.4. The calculated electron transfer number and percentage of H2O2 at different potentials are shown in Fig. 5b. It is clearly showed that the electron transfer number was between 3 and 4. This result is consistent with the RDE test results. Correspondingly, the percentage of H2O2 decreased with the increase of electron transfer number n. Reviews on the high performance catalysts for ORR drew similar conclusions [14]. It is deduced that the 4-electron pathway dominated the ORR reaction. Researches on the RRDE tests of nitrogen doped electrocatalysts for ORR observed similar results [15]. Compared with Pt/C catalyst, metal carbides are usually dissolved in acidic electrolyte, which limit the application of the metal carbide [14]. TiC is stable in acidic electrolyte that ensures the usage of TiC in acidic working condition. In this study, the catalytic characteristics of TiC@CNT for oxygen reduction in 0.5 M H2SO4 were investigated and the results are showed in Fig. 6. Fig. 6a showed the CV tests of TiC@CNT in nitrogen- and oxygen-
571
saturated 0.5 M H2SO4 electrolyte. It was shown that there was no peak observed in the nitrogen-saturated electrolyte. This result certified that the TiC@CNT was stable in the acidic electrolyte. In contrast, one peak was observed in the oxygen-saturated 0.5 M H2SO4 electrolyte. The CV curves of TiC@CNT proved that the TiC@CNT can catalyze the ORR in an acidic electrolyte. Similarly, the catalytic performance of pure CNT was also investigated by CV tests in both nitrogen- and oxygen-saturated 0.5 M H2SO4 and the results are shown in Fig. S5 (Supplementary materials). The CV tests of TiC@CNT for ORR at different sweeping rates are shown in Fig. S6. The results show that the current intensity increases with increasing sweeping rates. The peak current intensity is also linearly increased with the increase of square roots of sweeping rates, indicating that the TiC@CNT catalyzed ORR in acidic electrolyte is a diffusion controlled reaction. Fig. 6b shows the i-t test results of TiC@CNT for ORR in 0.5 M H2SO4. It is shown that the current intensity decreased with the running time. The decrease should be originated from both the decreased oxygen intensity of the electrolyte and the sluggish diffusion of oxygen on the electrode surface. The inset showed the CV results at t ¼ 0 and t ¼ 4500 s. It was confirmed that, after 4500 s running, both the peak current intensity and the onset potential of oxygen reduction were increased. This result was similar to the test result in alkaline electrolyte. Fig. 6c shows the Tafel slopes of ORR in H2SO4 electrolyte. The Tafel slope was 0.14 V/decade. The value of the Tafel slope was quite approach to that of Fe-N-C and Pt-C catalysts, which was ascribed to the transfer of the first electron as a rate-determining step [39]. Based on the Tafel tests, the calculated electron transfer coefficient (a) and the exchange current intensity (i0) was 0.1 and 2.28 106 A/cm2, respectively. Fig. 6d showed the measured and modulated EIS results of ORR in 0.5 M H2SO4 electrolyte. The inset was the modulated equivalent circuit. It was found that the modulated data were in consistence with the measured data. The R0 of the circuit (contact resistance) was 18.8 U. The values of R1 and R2 were 68.75 U and 1227 U, respectively, which was corresponding to the two reaction styles [40]. The R1 is corresponding to the mixture of the following reactions (6) and (7):
O2 þ 4Hþ þ 4e /2H2 O
(6)
O2 þ 2Hþ þ 2e /H2 O2
(7)
The R2 is corresponding to the reduction reaction of H2O2:
H2 O2 þ 2Hþ þ 2e /2H2 O
(8)
The low value of R1 indicates that the oxygen could be easily
Fig. 5. RRDE tests of TiC@CNT in 0.1 M KOH at 1600 rpm. (A colour version of this figure can be viewed online.)
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Fig. 6. Cyclic voltammetry (a), current (i)-time (t) (b), Tafel (c), EIS (d), RDE (e) and RRDE (f) test results of TiC@CNT for ORR in 0.5 M H2SO4 electrolyte. (A colour version of this figure can be viewed online.)
reduced through the pathway of reaction (6) and (7). The high value of R2 is resulted from the reduction of H2O2, which is corresponding to reaction (8). The reaction mechanism of TiC@CNT in an acidic electrolyte was studied by RDE and RRDE tests and the results are shown in Fig. 6e and f, respectively. Fig. 6e showed that the current intensity slightly increased with the increase of rotating speeds. However, the currents under different rotating speeds were quite similar at the lower potential (0.45e0.6 V), which made the electron transfer number (n) was difficult to be accurately calculated. Fig. 6f showed the RRDE tests of the TiC@CNT in the acidic electrolyte. Based on the RRDE test, it was deduced that the electron transfer number (n) of the acidic electrolyte was 3.6, which was close to the calculated electron transfer number of the alkaline electrolyte. The RRDE study proved that the TiC@CNT catalyzed ORR mainly happened through 4-electron pathway. The catalytic activity study indicated that the dentritic TiC@CNT had an excellent catalytic performance for ORR in acidic electrolyte. Comparison of the electron transfer number showed that the n of acidic electrolyte was slighter lower than that of alkaline electrolyte, which could be attributed to the preferred formation of peroxide species in the alkaline medium which desorbs more readily than that in the acid counterpart [41]. Density functional theory study on the catalytic activity of CoNX
showed that the strong interaction between peroxide and Conitride defect supports a 2 2e single site ORR mechanism in alkaline and acidic media [42]. The study on nitrogen-doped CNT also showed that the current of ORR in alkaline electrolyte was higher than that of acidic electrolyte [43]. The reason for the difference was attributed to the doped nitrogen. The different performance of TiC@CNT in alkaline and acidic electrolytes should also be attributed to the structure defects in the co-axial structure. The onset potential of ORR in 0.1 M KOH and 0.5 M H2SO4 were 0.2 V (vs. Hg/Hg2Cl2) and 0.78 V (vs. Ag/AgCl), respectively, which was only 0.15 V and 0.1 V lower than those of Pt/C catalyst [44]. This indicates that the TiC@CNT catalyst has good catalytic performance for ORR both in alkaline and acidic electrolytes. 4. Conclusion In this paper, the CNT was used as carbon source to synthesize TiC. The TiC and CNT formed a co-axial structure. The TiC@CNT catalyst shows high electrocatalytic performance for ORR both in alkaline and acidic electrolytes. The electron transfer number of TiC@CNT catalyzed ORR is close to 4 in both alkaline and acidic electrolytes, which indicates that the ORR mainly happens through 4-electron pathway. In addition to the high catalytic activity, the
T. Huang et al. / Carbon 126 (2018) 566e573
TiC@CNT catalyst also has a good long-term running stability in both alkaline and acidic electrolytes. The outer layer TiC plays the role of catalytic active center for ORR, thus, the inner CNT plays the role of electron conductor. The synergistic effect between TiC and CNT assures the high catalytic activity of TiC@CNT. The TiC@CNT catalyst has great potential to be high performance catalysts for ORR. Acknowledgements This work was financial supported by the Science Development Project of Shandong Provincial (No. 2017GGX40115, 2016GGX102038) and the National Natural Science Foundation of China (No. 21407060). S. M. thanks 1000 Talents Plan of China for financial support. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.carbon.2017.10.074. References [1] Z. Chen, D. Higgins, A. Yu, L. Zhang, J. Zhang, A review on non-precious metal electrocatalysts for PEM fuel cells, Energy Environ. Sci. 4 (9) (2011) 3167e3192. [2] T. Fujita, P. Guan, K. McKenna, X. Lang, A. Hirata, L. Zhang, et al., Atomic origins of the high catalytic activity of nanoporous gold, Nat. Mater. 11 (9) (2012) 775e780. [3] D. He, L. Zhang, D. He, G. Zhou, Y. Lin, Z. Deng, et al., Amorphous nickel boride membrane on a platinumenickel alloy surface for enhanced oxygen reduction reaction, Nat. Commun. 7 (2016) 12362e12369. [4] W. He, Y. Wang, C. Jiang, L. Lu, Structural effects of a carbon matrix in nonprecious metal O2-reduction electrocatalysts, Chem. Soc. Rev. 45 (9) (2016) 2396e2409. [5] D. Higgins, P. Zamani, A. Yu, Z. Chen, The application of graphene and its composites in oxygen reduction electrocatalysis: a perspective and review of recent progress, Energy Environ. Sci. 9 (2) (2016) 357e390. [6] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, et al., Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction, Nat. Mater. 10 (10) (2011) 780e786. [7] T. Huang, S. Mao, H. Pu, Z. Wen, X. Huang, S. Ci, et al., Nitrogen-doped graphene-vanadium carbide hybrids as a high-performance oxygen reduction reaction electrocatalyst support in alkaline media, J. Mater. Chem. A 1 (2013) 13404e13410. [8] T. Huang, S. Mao, G. Zhou, Z. Wen, X. Huang, S. Ci, et al., Hydrothermal synthesis of vanadium nitride and modulation of its catalytic performance for oxygen reduction reaction, Nanoscale 6 (16) (2014) 9608e9613. [9] S. Guo, S. Zhang, S. Sun, Tuning nanoparticle catalysis for the oxygen reduction reaction, Angew. Chem. Int. Ed. 52 (33) (2013) 8526e8544. [10] D.C. Higgins, M.A. Hoque, F. Hassan, J.-Y. Choi, B. Kim, et al., Oxygen reduction on grapheneecarbon nanotube composites doped sequentially with nitrogen and sulfur, Acs Catal. 4 (8) (2014) 2734e2740. [11] Y. Liang, H. Wang, P. Diao, W. Chang, G. Hong, Y. Li, et al., Oxygen reduction electrocatalyst based on strongly coupled cobalt oxide nanocrystals and carbon nanotubes, J. Am. Chem. Soc. 134 (38) (2012) 15849e15857. [12] B. Kim, W.M. Sigmund, Functionalized multiwall carbon nanotube/gold nanoparticle composites, Langmuir 20 (19) (2004) 8239e8242. [13] Z. Liu, X. Lin, J.Y. Lee, W. Zhang, M. Han, L.M. Gan, Preparation and characterization of platinum-based electrocatalysts on multiwalled carbon nanotubes for proton exchange membrane fuel cells, Langmuir 18 (10) (2002) 4054e4060. [14] M. Zhou, H.-L. Wang, S. Guo, Towards high-efficiency nanoelectrocatalysts for oxygen reduction through engineering advanced carbon nanomaterials, Chem. Soc. Rev. 45 (5) (2016) 1273e1307. [15] M.D. Meganathan, S. Mao, T. Huang, G. Sun, Reduced graphene oxide intercalated Co2C or Co4N nanoparticles as an efficient and durable fuel cell catalyst for oxygen reduction, J. Mater. Chem. A 5 (2017) 2972e2980. [16] Z. Wen, S. Ci, F. Zhang, X. Feng, S. Cui, S. Mao, et al., Nitrogen-enriched coreshell structured Fe/Fe3C-C nanorods as advanced electrocatalysts for oxygen reduction reaction, Adv. Mater. 24 (11) (2012) 1399e1404. [17] Y. Hou, T. Huang, Z. Wen, S. Mao, S. Cui, J. Chen, MetalOrganic frameworkderived nitrogen-doped core-shell-structured porous Fe/Fe3C@C nanoboxes Supported on Graphene Sheets for Efficient Oxygen Reduction Reactions, Adv.
573
Energy Mater. 1400337 (2014) 1e8. [18] T. Huang, S. Mao, G. Zhou, Z. Zhang, Z. Wen, X. Huang, et al., A high-performance catalyst support for methanol oxidation with graphene and vanadium carbonitride, Nanoscale 7 (4) (2015) 1301e1307. [19] T. Huang, J. Yu, J. Han, Z. Zhang, Y. Xing, C. Wen, et al., Oxygen reduction catalytic characteristics of vanadium carbide and nitrogen doped vanadium carbide, J. Power Sources 300 (2015) 483e490. [20] C.R. Raj, A. Samanta, S.H. Noh, S. Mondal, T. Okajima, T. Ohsaka, Emerging new generation electrocatalysts for the oxygen reduction reaction, J. Mater. Chem. A 4 (29) (2016) 11156e11178. [21] J. Xu, R. Zhang, P. Chen, S. Ge, Effects of adding ethanol to KOH electrolyte on electrochemical performance of titanium carbide-derived carbon, J. Power Sources 246 (2014) 132e140. [22] Y. Ou, X. Cui, X. Zhang, Z. Jiang, Titanium carbide nanoparticles supported Pt catalysts for methanol electrooxidation in acidic media, J. Power Sources 195 (5) (2010) 1365e1369. [23] N.J. Welham, P.E. Willis, Formation of TiN/TiC-Fe composites from ilmenite (FeTiO3) concentrate, Metall. Mater. Trans. B 29 (5) (1998) 1077e1083. [24] R. Zhang, Y. Zhang, Q. Zhang, H. Xie, W. Qian, F. Wei, Growth of half-meter long carbon nanotubes based on schulzeflory distribution, ACS Nano 7 (7) (2013) 6156e6161. [25] F. Tuinstra, J.L. Koenig, Raman spectrum of graphite, J. Chem. Phys. 53 (3) (1970) 1126e1130. [26] T. Huang, S. Mao, M. Qiu, O. Mao, C. Yuan, J. Chen, Nitrogen-boron dipolardoped nanocarbon as a high-efficiency electrocatalyst for oxygen reduction reaction, Electrochim. Acta 222 (2016) 481e487. [27] Y. Cheng, H. Zhang, C.V. Varanasi, J. Liu, Highly efficient oxygen reduction electrocatalysts based on winged carbon nanotubes, Sci. Rep. 3 (2013) 3195e3201. [28] O. Rist, P.T. Murray, Growth of TiC films by pulsed laser evaporation (PLE) and characterization by XPS and AES, Anal. Bioanal. Chem. 341 (5) (1991) 360e364. [29] H. He, P. Xiao, M. Zhou, F. Liu, S. Yu, L. Qiao, et al., PtNi alloy nanoparticles supported on carbon-doped TiO2 nanotube arrays for photo-assisted methanol oxidation, Electrochim. Acta 88 (2013) 782e789. [30] D.O. Scanlon, C.W. Dunnill, J. Buckeridge, S.A. Shevlin, A.J. Logsdail, S.M. Woodley, et al., Band alignment of rutile and anatase TiO2, Nat. Mater. 12 (9) (2013) 798e801. [31] T. Huang, S. Mao, J. Yu, Z. Wen, G. Lu, J. Chen, Effects of N and F doping on structure and photocatalytic properties of anatase TiO2 nanoparticles, Rsc Adv. 3 (37) (2013) 16657e16661. [32] G. Fu, K. Wu, J. Lin, Y. Tang, Y. Chen, Y. Zhou, et al., One-pot water-based synthesis of PtePd alloy nanoflowers and their superior electrocatalytic activity for the oxygen reduction reaction and remarkable methanol-tolerant ability in acid media, J. Phys. Chem. C 117 (19) (2013) 9826e9834. [33] V. Kiran, K. Srinivasu, S. Sampath, Morphology dependent oxygen reduction activity of titanium carbide: bulk vs. nanowires, Phys. Chem. Chem. Phys. 15 (22) (2013) 8744e8751. [34] Y. Wang, D. Zhang, M. Tang, S. Xu, M. Li, Electrocatalysis of gold nanoparticles/ layered double hydroxides nanocomposites toward methanol electrooxidation in alkaline medium, Electrochim. Acta 55 (12) (2010) 4045e4049. [35] J.W. Hong, S.W. Kang, B.-S. Choi, D. Kim, S.B. Lee, S.W. Han, Controlled synthesis of PdePt alloy hollow nanostructures with enhanced catalytic activities for oxygen reduction, ACS Nano 6 (3) (2012) 2410e2419. [36] J. Xu, G. Fu, Y. Tang, Y. Zhou, Y. Chen, T. Lu, Non-precious Co3O4 nano-rod electrocatalyst for oxygen reduction reaction in anion-exchange membrane fuel cells, Energy Environ. Sci. 5 (1) (2012) 5333e5339. [37] Y. Xue, D. Yu, L. Dai, R. Wang, D. Li, A. Roy, et al., Three-dimensional B,N-doped graphene foam as a metal-free catalyst for oxygen reduction reaction, Phys. Chem. Chem. Phys. 15 (29) (2013) 12220e12226. [38] Z. Wu, Y. Lv, Y. Xia, P.A. Webley, D. Zhao, Ordered mesoporous Platinum@ Graphitic carbon embedded nanophase as a highly active, stable, and methanol-tolerant oxygen reduction electrocatalyst, J. Am. Chem. Soc. 134 (4) (2011) 2236e2245. [39] H. Xiao, Z.-G. Shao, G. Zhang, Y. Gao, W. Lu, B. Yi, FeeNecarbon black for the oxygen reduction reaction in sulfuric acid, Carbon 57 (1) (2013) 443e451. [40] E. Biddinger, D. Deak, U. Ozkan, Nitrogen-containing carbon nanostructures as oxygen-reduction catalysts, Top. Catal. 52 (11) (2009) 1566e1574. [41] F. Bidault, D.J.L. Brett, P.H. Middleton, N.P. Brandon, Review of gas diffusion cathodes for alkaline fuel cells, J. Power Sources 187 (1) (2009) 39e48. [42] S. Kattel, P. Atanassov, B. Kiefer, Catalytic activity of Co-Nx/C electrocatalysts for oxygen reduction reaction: a density functional theory study, Phys. Chem. Chem. Phys. 15 (1) (2013) 148e153. [43] Z. Mo, S. Liao, Y. Zheng, Z. Fu, Preparation of nitrogen-doped carbon nanotube arrays and their catalysis towards cathodic oxygen reduction in acidic and alkaline media, Carbon 50 (7) (2012) 2620e2627. [44] Y. Bing, H. Liu, L. Zhang, D. Ghosh, J. Zhang, Nanostructured Pt-alloy electrocatalysts for PEM fuel cell oxygen reduction reaction, Chem. Soc. Rev. 39 (6) (2010) 2184e2202.