Journal ofMaterials Chemistry A

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aSchool of Nano-Bioscience and Chemical

Science & Technology (UNIST), Ulsan 689-7bDepartment of Chemical Engineering, Poha

(POSTECH), Pohang, 790-784 KoreacBeamline Research Division, Pohang Accele

Korea

† Electronic supplementary informationmethod, XRD patterns, SEM images, TEMLSV results. See DOI: 10.1039/c3ta11135k

Cite this: J. Mater. Chem. A, 2013, 1,8007

Received 20th March 2013Accepted 9th May 2013

DOI: 10.1039/c3ta11135k

www.rsc.org/MaterialsA

This journal is ª The Royal Society of

A highly efficient transition metal nitride-basedelectrocatalyst for oxygen reduction reaction: TiN on aCNT–graphene hybrid support†

Duck Hyun Youn,b Ganghong Bae,b Suenghoon Han,b Jae Young Kim,b

Ji-Wook Jang,b Hunmin Park,b Sun Hee Choic and Jae Sung Lee*ab

Transition metal nitrides of group 4–6 (Mo2N, W2N, NbN, Ta3N5, and TiN) were synthesized by the urea-

glass route and screened for oxygen reduction reaction (ORR) electrodes in PEMFCs. In terms of

electrochemical stability and activity, TiN was selected as the most promising candidate as a catalyst for

ORR. To further enhance the activity for ORR, TiN was modified with nanostructured carbon supports

including CNTs, graphene (GR), and CNT–GR hybrid. The obtained nanocarbon-supported TiN catalysts

exhibited small particle sizes of TiN (<7 nm) and a good TiN–support interaction with reduced

aggregation and no free-standing TiN particles away from the supports compared to bare TiN. In

particular, TiN supported on the CNT–GR hybrid (TiN/CNT–GR) showed greatly enhanced ORR activity

than bare TiN and other supported TiN catalysts. It exhibited a high onset potential (0.83 V) and the

highest current density among the reported nitride-based electrocatalysts. The enhancement was

ascribed to a synergistic effect between TiN nanoparticles (NPs) and CNT–GR hybird support, roles of

which were to provide active sites for ORR and a facile electron pathway to NPs, respectively. Besides,

TiN/CNT–GR exhibited large mesopores that could allow easy access of the electrolyte due to the

formation of a 3-D CNT–GR structure assembled between 2-D graphene and 1-D CNTs. Further, it

showed an excellent methanol tolerance compared to the commercial Pt/C catalyst. Thus, our TiN/CNT–

GR could be a promising ORR electrocatalyst for PEMFCs and DMFCs.

Introduction

Polymer electrolyte membrane fuel cells (PEMFCs) are attractinggreat attention as a promising power source for transportationand residential applications due to their high energy conversionefficiency, noise-free, pollutant-free operation, and long-lastingsystems due to a lack ofmoving parts.1However, the high price ofPt, the most effective electrocatalyst for both anode and cathode,restricts wide-spread dissemination of PEMFCs. In particular,sluggish and corrosive oxygen reduction reaction (ORR) in cath-odes of PEMFCs requires a large amount of Pt and causes dura-bility problems.2 Thus, development of noble metal-free andstable alternative materials for ORR is of great importance tomake PEMFCs commercially viable. So far, many noble metal-

Engineering, Ulsan National Institute of

98, Korea. E-mail: jlee1234@unist.ac.kr

ng University of Science and Technology

rator Laboratory (PAL), Pohang, 790-784

(ESI) available: A detailed syntheticimages, XANES, EXAFS spectra, and

Chemistry 2013

free catalyst candidates have been reported for ORR, includingmacrocycles,3 chalcogenides,4 metal oxides,5 carbides,6 nitrides,7

and nitrogen-doped carbons.8 Unfortunately, these materials donot satisfy the activity and stability requirements at the sametime. Here, we investigate the electrochemical properties of puretransition metal nitrides (TMNs) of group 4–6 metals as a newclass of ORR catalysts. Among the tested TMNs, the TiN catalystexhibited the most promising performance in activity andstability, and it was modied with nanocarbon supports of CNTs,graphene (GR), and CNT–GR hybrid. The obtained catalyst,i.e. TiN nanoparticles (NPs) on a CNT–GR hybrid support (TiN/CNT–GR), represents a low cost, noble metal-free ORR catalystexhibiting a high onset potential of 0.83 V comparable to Pd-based catalysts9–11 and the best ORR current density among thereported nitride-based materials.7,12–15

TMNs have unique physical properties, such as hardness,wear resistance, and superconductivity, responsible for theirvarious applications like coating agents for cutting tools andrefractorymaterials. They also have been considered as a possiblereplacement for Pt-group metal catalysts in various heteroge-neous catalyses, including isomerization, hydrodesulfurization,and hydrogenation reactions16 due to their similar electronicstructures to those of noble metals.17 Recently, many nitride

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materials have received considerable attention in energy appli-cations including fuel cells,12,18 photocatalysts,19,20 dye-sensitizedsolar cells (DSSCs),21,22 and hydrogen evolution reactions(HERs).23 Particularly in fuel cell applications, many nitride-based materials, oxynitrides,24 carbonitrides,25 and partiallyoxidized carbonitrides (CNO)13 have been reported as potentialORR catalysts, pioneered by Ota and coworkers. Their Zr, Nb, andTa-based CNO catalysts showed very high onset potentials above0.85 V with high stability in acid media. However, due to theirhigh synthetic temperature (>1500 �C), these catalysts did notprovide high surface area, small particle size, or high density ofactive sites, resulting in low current densities. To increase theactivity of nitrides for ORR, TiN NPs were loaded on carbon black(CB) using a mesoporous graphitic carbon nitride (mpg-C3N4)template by Domen et al.12 Here, mpg-C3N4 acts as a nitrogensource as well as a template for growth of TiN NPs. Due to the aidof the template, uniform-sized TiN NPs were synthesized on CBand the resultant TiN/CB exhibited better activity for ORR thanbare TiN NPs, and high onset potential (0.8 V). However, varioussteps in their synthetic method (mpg-C3N4 synthesis on CB, silicaremoval, Ti precursor impregnation, and heat treatment) requirelong time and tedious efforts. Thus, to use nitride materials asORR catalysts, general electrochemical information about theactivity and stability of pure TMNs is necessary for proper selec-tion of candidate materials, and a simple but effective syntheticmethod is also needed to enhance their performance.

In the present work, to broaden the scope of TMNs as elec-trocatalysts for ORR, electrochemical screening of group 4–6TMNs for ORR was carried out. The simple synthetic method ofurea-glass route26,27 was employed to fabricate the TMNs. Next,to provide a highly conductive electron pathway and largesurface area, nanostructured carbons (CNTs, GR, and CNT–GR)were introduced to form TMN–carbon composites by themodied urea-glass route proposed here. The resultant TiN/CNT–GR exhibited small TiN particle sizes of less than 7 nm anda good metal–support interaction featuring less aggregationand no free-standing TiN particles away from the support.Furthermore, it showed increased large mesopores that allowedeasy access of the electrolyte to active TiN sites due to theformation of a 3-D-like structure between CNTs and GR. Thus,our TiN/CNT–GR exhibited high activity and stability for ORR inacidmedia with a high onset potential and an enhanced currentdensity due to a synergetic effect between TiN NPs and CNT–GRhybrid support. In addition, it also showed a strong methanoltolerance for direct methanol fuel cells (DMFCs). We havedemonstrated a useful general strategy to develop an efficient,Pt-free electrocatalyst for ORR, i.e. search of an active and stablephase for ORR (TiN in our case) and its modication by usingthe CNT–GR hybrid support to provide a highly conductiveelectron pathway and large mesopores for facile access of theelectrolyte to the active phase.

ExperimentalSynthesis of TMNs by the urea-glass route

Metal chlorides (MoCl5, WCl4, NbCl5, TaCl5, and TiCl4) wereused as metal precursors for synthesis of TMNs. In a typical

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synthesis, 1 g of metal chloride was dispersed in 2.53 ml ofethanol, and an appropriate amount of urea, as a nitrogensource, was added to the solution. The molar ratios of urea tometal precursors were 1–9. Aer 1 h stirring, the viscous metal–urea complex formed was transferred to a tubular furnace andcalcined at 750 �C for 3 h under a N2 atmosphere. More detaileddescriptions for the synthesis of TMNs including the exactamount of reagents and post-treatment conditions (for NbN asan example) were given in the ESI.†

Synthesis of TiN NPs supported on GR, CNTs, and CNT–GR bythe modied urea-glass route

Nanocarbon-supported TiN catalysts were synthesized by asimple modication of the urea-glass route named themodiedurea-glass route. Graphene oxide (GO) was synthesized byHummer's method,28 and CNTs were purchased from HanwhaNanotech (CMP-310F). GO, CNTs, and a mixture of GO andCNTs with a 1 : 1 weight ratio were used as startingmaterials forsupports. The amount of Ti metal was xed to 60 wt% in sup-ported TiN catalysts. Thus, 2.53 ml of ethanol solution con-taining 1 g of TiCl4 was dispersed ultrasonically in 15 mlethanol solution containing 95 mg of each support (GO, CNTs,and CNT–GO). Aer vigorous stirring for 1 h with 1583 mg ofurea, the resulting solution was dried in a 100 �C oven toevaporate the excess amount of ethanol. Aer heat treatmentunder the same conditions as for TiN synthesis, supported TiNnanoparticles were obtained.

Catalysts' characterization

The crystalline structure of synthesized catalysts was examined byX-ray diffraction (XRD) using a PANalytical pw 3040/60 X'pertdiffractometer, and structural information was elucidated with ascanning electron microscope (SEM, JEOL JSM-7410F) and a highresolution transmission electron microscope (TEM, JEOL JEM-2100F). Surface areas and pore structures of the catalysts wereobtained from N2 sorption isotherms measured at 77 K on aconstant volume adsorption apparatus (Nanoporosity-XQ, MiraeScientic Instruments, Korea). X-ray absorption ne structure(XAFS) was applied to investigate the local structure of TiN NPs,from data collected on 7D beamline of the Pohang AcceleratorLaboratory (PLS-II, 3.0 GeV), Korea. The incident beam wasmonochromatized using a Si (111) double crystal monochromator.At room temperature, the spectra were taken for the K-edge of Ti(E0¼ 4966 eV) in a transmission mode with He-lled and N2-lledIC Spec ionization chambers for incident and transmitted beams,respectively. The obtained data were analyzed with ATHENA andARTEMIS in the IFEFIT suite of soware programs.29 The refer-ence material used as a standard for tting experimentally derivedRadial structural functions (RSF) was generated with Feff 8.2code30 by using a known TiN crystal structure.31

Electrochemical measurements

Electrochemical measurements including cyclic voltammetry(CV) and linear sweep voltammetry (LSV) were carried out in aconventional three electrode cell with N2 or O2 saturatedaqueous solution of 0.5 M H2SO4 using an Ivium potentiostat

This journal is ª The Royal Society of Chemistry 2013

Fig. 1 Cyclic voltammetry (CV) results of (a) Mo2N, (b) W2N, (c) NbN, (d) Ta3N5,and (e) TiN in 0.5 M H2SO4. Dotted lines represent the trace of the first scan.

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(Ivium technologies). The Ag/AgCl (3 M NaCl) electrode and a Ptplate were used as the reference and counter electrodes, respec-tively. In this paper, all the potentials were referred to thereversible hydrogen electrode (RHE) without specication. Forelectrochemical screening tests of TMNs, working electrodeswere prepared by dispersing 20 mg of catalyst in 2 ml of deion-ized water and 40 ml of 5% Naon solution and pipetting out 5 mlof slurry onto a glassy carbon electrode (0.0707 cm2). 3 ml ofNaon solution was added on top to x the electrocatalyst. TheCV tests were performed in a range of 0 V–1.2 V at a scan rate of50 mV s�1. The LSV measurements of TMNs were carried out atthe potential range of 1.1 V to 0.2 V with a scan rate of 5 mV s�1.The ORR on the TiN-based catalysts was studied using a rotatingdisk electrode (RDE, PAR Model 636 RDE system). Fabrication ofcatalyst slurry with TiN-based catalysts is the same as that ofTMNs. 15 ml of slurry and 9 ml of Naon solution were transferredonto a glassy carbon electrode (0.19635 cm2) to prepare a workingelectrode. During CV and LSV measurements, the working elec-trode was rotated at 1600 rpm with the same applied potentialsand scan rates for TMNs. In the identical cell setup, electro-chemical impedance spectroscopy (EIS) was carried out. Thefrequency range was from 100 kHz to 1 mHz with a modulationamplitude of 10 mV at 0.6 V bias voltage. The EIS spectra weretted by the Z-view soware.

Fig. 2 Linear sweep voltammetry (LSV) results of (a) Mo2N, (b) W2N, (c) NbN, (d)Ta3N5, and (e) TiN in 0.5 M H2SO4. (Dotted lines under O2; solid lines under N2.)

Results and discussionScreening of TMNs for ORR

For electrochemical screening for ORR, pure phases of TMNswere fabricated by a simple synthetic method of urea-glassroute.26,27 Thus, nitrides of group 4–6 (Mo2N, W2N, NbN, Ta3N5,and TiN) were synthesized. Fig. S1a† displays the XRD patternsof synthesized TMNs. All the TMNs exhibited cubic structuresexcept for Ta3N5, which showed a hexagonal structure. TMNswere well-matched to their reference XRD patterns of Mo2N(JCPDS no. 00-25-1366), W2N (00-025-1257), NbN (00-038-1155),Ta3N5 (01-079-1533), and TiN (00-038-1420). No peaks ofimpurities, including oxides or metals, were observed. Usingthe Scherrer equation, the average particle sizes of TMNs wereestimated to be 6.8 nm, 2.1 nm, 6.4 nm, 26 nm, and 7.5 nm forMo2N, W2N, NbN, Ta3N5, and TiN, respectively. SEM images ofsynthesized TMNs are presented in Fig. S1b–f.† In all cases,particles of TMNs exhibited a spherical morphology and noimpurity phases were observed, which revealed the large scalehomogeneity of synthesized TMNs.

Electrochemical stabilities of TMNs in an acid medium (0.5 MH2SO4) were investigated using cyclic voltammetry (CV). SomeTMNs are known to be weak under corrosive conditions ofORR.32–34 Thus, general potentials with 20 scans were applied tosort out unstable catalysts from the synthesized TMNs. The CVresults are presented in Fig. 1 with the dotted lines representingthe rst scan.Mo2N showed a very large oxidation current even inthe rst scan of CV measurements from 0.6 V, which was thecorrosion current originated from the transformation of Mo2Ninto molybdenum oxide. Thus, Mo2N is not suitable as an ORRcatalyst for PEMFCs due to its instability in the acid medium.W2N exhibited relatively better stability without the large

This journal is ª The Royal Society of Chemistry 2013

oxidation current compared to Mo2N. However, reduction ofpeak intensity in the hydrogen adsorption/desorption region wasobserved with increasing number of scans due to dissolution ofW2N in the acid medium.32 Further, there exists a small oxidationcurrent from 0.6 V. And also, W2N is not a good catalyst for ORRconsidering the electrochemical instability. On the other hand,NbN, Ta3N5, and TiN showed similar good stability in CVmeasurements. Their currents increased with increasing numberof scans and were stabilized before the 20th scan. No specicoxidation currents were observed in their CV traces. The resultsveried the electrochemical stability of NbN, Ta3N5, and TiN.Therefore, the electrochemical stability of TMNs can be alignedin the following order: Mo2N � W2N < NbN, Ta3N5, TiN.

Fig. 2 shows the linear sweep voltammetry (LSV) results ofTMNs for ORR to reveal the electrochemical activities for ORR.Dotted and solid lines exhibit the measured currents in O2– andN2–saturated sulfuric acid solutions, respectively. All the LSV tests(except for Mo2N) were performed aer CV measurements toclean and stabilize the catalyst surface. In the case of Mo2N, wewere not able to observe any ORR activity aer CV due to theformation of inactive molybdenum oxide. Hence, fresh Mo2Ncatalysts without prior CV tests were used for LSV measurements.The ORR current density ( JORR) could be determined by thedifference of current densities between O2 and N2 saturatedsolutions. Mo2N exhibited a fairly large ORR current density from0.6 V as shown in Fig. 2a. However, due to the instability in thepotential range of ORR, its activity has no practical meaning.W2Ndid not generate a signicant ORR current density in Fig. 2b. Atiny ORR current density was observed from 0.3 V, but the onset

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Fig. 3 SEM images of (a) TiN, (b) TiN/GR, (c) TiN/CNT, and (d) TiN/CNT–GR. Insetimages describe the schematic diagram for each sample.

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potential and the scale of ORR current density were negligiblysmall. Considering its activity and stability, W2N is not a suitablecatalyst for ORR either. These results on Mo2N and W2N are notconsistent with previous reports. Zhong et al. showed goodactivities and stabilities of carbon supported Mo2N and W2N.14,15

However, we suspect that a large portion of their activity andstability may have originated from nitrogen-doped carbon. Forthe synthesis of nitride materials, they used the typical ammo-nolysis method using NH3 gas. Heat treatment of carbon in NH3

gas would form nitrogen-doped carbonwithMo2N orW2N, whichis known as an active catalyst for ORR. The effect of nitrogen-doped carbon on ORR will be discussed in the next section.

NbN exhibited ORR activity from 0.6 V with a current densityof �1 mA. The onset potential of Ta3N5 was 0.5 V, which wassimilar to the one reported in a previous study.7 Still, its ORRcurrent density was rather low. In the case of TiN, its onsetpotential for ORR was 0.6 V. However, TiN exhibited nearly 10times higher ORR current densities compared to NbN andTa3N5. As a result, electrochemical activities of TMNs can bealigned in the following order: W2N < NbN, Ta3N5 � TiN <Mo2N. The results lead us to conclude that TiN is the mostpromising ORR catalyst considering the electrochemicalstability as well as the activity in the potential range of ORR. Yet,its activity is still rather low compared to noble metal-basedORR catalysts including Pt or Pd-based catalysts. As an effort toimprove the performance of TiN catalysts, nanocarbon supportswere introduced to these TiN NPs.

TiN on a CNT–graphene hybrid support

Three nanostructured carbon materials, CNTs, GR, and CNT–GR hybrid, were used as supports for TiN NPs. X-ray diffraction(XRD) patterns of synthesized TiN supported on various nano-structured carbons are presented in Fig. S2† with those of bareTiN for comparison. Their XRD patterns were well-matched toreference XRD patterns of TiN (JCPDS no. 38-1420) without anyundesired product peaks such as titanium dioxide or titaniummetal, indicating the synthesis of TiN with a pure phase. Andsupported TiN catalysts showed small peaks around 24–26� inFig. S2b†, which originated from CNTs or GR.36 By the Scherrerequation, the average particle sizes of TiN NPs were estimated tobe 7.5 nm, 7.0 nm, 6.8 nm, and 6.9 nm for TiN, TiN/GR, TiN/CNT, and TiN/CNT–GR, respectively. Estimated particle sizeswere generally consistent with TEM results shown below. Thesupported TiN, in general, exhibited a little smaller particle sizealthough the difference was marginal. Thus, the signicantdifference in physicochemical properties and electrochemicalperformances between bare and supported TiN catalysts (asdiscussed below) could not be attributed to particle size effects.However, synthesized TiN catalysts exhibited small particlesizes less than 7 nm with high crystallinity and high purity,which are general requirements for good catalysts or electro-catalysts. Furthermore, considering the synthesis of smallparticles of TiN of less than 10 nm is rare35 due to hightemperatures involved in the synthesis (>750 �C), these resultsdemonstrate the effectiveness of our modied urea-glass routemethod to fabricate TiN NPs on nanocarbon supports.

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SEM images of synthesized catalysts are exhibited in Fig. 3together with the schematic representation of the morphology ofeach sample in the inset. Spherical-shaped TiN NPs were heavilyagglomerated as shown in Fig. 3a. In the case of TiN/GR inFig. 3b, the morphology of GR was clearly found and TiN NPswere loaded on the GR sheets, which were horizontally stackedeach other. The stacking of GR sheets is commonly observed forthis 2-D carbon and decreases potential sites for active compo-nent loading present between GR sheets.36 Even for the activecomponent placed between the two GR sheets, it is hard to beaccessible by the electrolyte. In Fig. 3c, bundled CNTs wereobserved with randomly located TiN NPs. The TiN NPs generallylocated on the outer surface of bundled CNTs (inset of Fig. 3c),which reduces the actual surface area for TiN particles to settledown on the support. Thus, we could not utilize the high surfacearea of CNTs effectively.37 Furthermore, like stacked GR sheets inTiN/GR, TiN NPs somehow intruded into the CNT bundles andare not easily accessible by the electrolyte, and thus the numberof TiN NPs that can join ORR decreases. However, in CNT–GRhybrid layers shown in Fig. 3d, monotubes of CNTs were locatedbetween GR monosheets. Apparently, CNTs act as a spacer toblock the stacking of GR sheets. At the same time, GR sheets alsoblock the bundling of CNTs. As shown in Fig. S4,† highly bundledand entangled CNTs were observed in the SEM image of TiN/CNT(Fig. S4a†). In contrast, in the presence of GR, the CNTs weremuch more disentangled compared to CNTs alone (Fig. S4b†).These observations are consistent with previous reports on thecomposite of graphene oxide and CNTs.38,39 Thus, our CNT–GRhybrid could alleviate the stacking of graphene and bundling ofCNTs by acting as a spacer for each other.

Fig. 4 displays the TEM images to analyze the structuraldetails of synthesized catalysts. Spherical TiN NPs with anaverage particle size of 7.7 nm were observed in Fig. 4a. Thelattice spacings of 0.242 nm and 0.207 nm shown in the inset

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Fig. 4 TEM images of (a) TiN, (b) TiN/GR, (c) TiN/CNT, and (d) TiN/CNT–GR. Insetsshow high resolution TEM images of each sample. (See Fig. S3† for enlarged insetimages.)

Fig. 5 (a) N2 sorption isotherms and (b) pore size distribution (PSD) plots of thesynthesized TiN-based catalysts.

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image of Fig. 4a are consistent with the known d values of (111)and (200) planes of TiN. However, heavy aggregation of theprimary TiN NPs was observed. In the TEM images of TiN/GR inFig. 4b, formation of GR from reduction of graphene oxidecould be conrmed by a wrinkled paper-like morphology of GRsheets and an interlayer distance of 0.345 nm corresponding tothe d(002) value of GR (0.34 nm).40 Thermal reduction of gra-phene oxide is a method to fabricate graphene from grapheneoxide, and the high synthetic temperature (750 �C) is enough toreduce graphene oxide.41,42 In a stark contrast to bare TiN, theTiN NPs with an average particle size of 7.3 nm were highlydispersed over the GR sheets in TiN/GR without severe aggre-gation. Furthermore, there were no free-standing TiN particleswhich were located away from the GR sheets (Fig. 4b andFig. S5a†).

Less aggregation and no free-standing particles of TiN werecommon observation for TiN/CNT (Fig. 4c and Fig. S5b†) andTiN/CNT–GR (Fig. 4d and Fig. S5c†). These results lead us toconclude that there exists a good TiN–support interaction,43,44

which could prevent TiN NPs from self-agglomeration andseparation from the supports. The interaction could also facil-itate the electron transfer from the nanocarbon supports to TiNNPs when applied as an ORR catalyst. Fig. 4c shows the TEMimages of TiN/CNT with a clear multi-walled CNT (MWCNT)structure (inset image). The TiN particles, with a size of 7.1 nm,adhered well onto CNTs. In Fig. 4d, the TEM images of TiN/CNT–GR exhibit that CNTs were randomly distributed on GRsheets, and TiN particles with an average size of 7.1 nm wereattached well on both CNTs and GR. By XRD and TEM analyses,we could conrm that our preparation method (the modiedurea-glass route) is an effective way to synthesize hybrid mate-rials of TiN NPs on nanocarbon supports featuring a goodmetal–support interaction, which has been revealed by reduced

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aggregation and no free-standing particles away from thenanocarbon support. We believe this preparation methodproposed here can be easily applied to the synthesis of variousnitride materials on nanocarbon supports for other electro-chemical reactions.

The local structures around Ti in bare and supported TiN wereexamined with X-ray absorption ne structure (XAFS). Noparticular changes were observed in the Ti K-edge X-ray absorp-tion near edge structure (XANES) of TiN NPs including the pre-edge peak due to 1s / 3d transition (Fig. S6a†). It is due to anoctahedral symmetry of Ti having one 3d electron. However, theFourier-transformed EXAFS (extended X-ray absorption nestructure) in Fig. S6b† showed that the peak intensities for sup-ported TiN catalysts were all similar but clearly lower than that ofbare TiN catalysts. The decrease in the peak intensity indicates alower coordination number or a larger Debye–Waller factor.Therefore, we calculated structural parameters through anonlinear least squares tting and the results are given in TableS2.† The Ti–Ti coordination number of bare TiN was slightlylarger than those of carbon-supported catalysts, in agreementwith its slightly larger particle size. In addition, the Debye–Wallerfactors of the supported TiN, an indicator of the structuraldisorder, were larger than those of bare TiN. The distances ofTi–N and Ti–Ti for supported TiN were shorter by 0.01 A thanthose for bare TiN catalysts. These results suggest that carbonsupports interfere with the crystallization process of TiN NPs tosome degree and provide additional evidence of TiN–carboninteraction in supported TiN catalysts. But these structuraldifferences are not large enough to account for the markeddifferences in electrochemical properties between bare andsupported TiN NPs as discussed below.

Textural properties of synthesized catalysts were analyzedusing N2 sorption isotherms. The surface area and pore sizedistribution (PSD) of the catalysts were determined by Bru-nauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH)methods, respectively. TiN-based catalysts exhibited type IVisotherms as in Fig. 5a, which was characteristic of mesoporousmaterials.45 The BET surface area of bare TiN (156 m2 g�1) wasrelatively high compared to other TiN catalysts synthesized bythe traditional ammonolysis method or commercial TiN NPs(<60 m2 g�1).18,46 TiN/CNT showed the highest BET surface area(300 m2 g�1) because of the high surface area of CNTs.37

However, the BET surface area of TiN/GR (168 m2 g�1) increasedonly slightly higher due to the stacking of GR sheets. Byadopting the CNT–GR hybrid as a support for TiN, the BET

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surface area increased markedly (270 m2 g�1), only slightly lessthan that of TiN/CNT. This trend of BET surface area wasconsistent with that of nanocarbon supports without TiN.47

Fig. 5b shows the BJH pore size distribution plot. Sharp peaksshown at 3.5 nm in every catalyst may be originated from thepores in TiN NPs. In addition, all carbon supported catalystsdeveloped larger pores of 7–20 nm in the following order: GR�CNTs < CNT–GR. Thus the total pore volume of TiN/CNT–GR(0.5460 cm3 g�1) was the highest, followed by TiN/CNT (0.4744cm3 g�1), TiN/GR (0.3456 cm3 g�1), and bare TiN (0.2404 cm3

g�1). Large pores could play an important role in electro-chemical reactions, because they could facilitate the contactbetween catalysts and electrolyte. The creation of large poresmay come from the formation of 3-D structure between 1-DCNTs and 2-D GR, which avoids the stacking of GR andbundling of CNTs as shown in the schematic diagram in Fig. 3d.

Electrochemical properties and oxygen reduction reaction

Electrochemical properties of synthesized catalysts were inves-tigated by CV and the results are shown in Fig. 6a. As conrmedin CV results of TMNs (Fig. 1), bare TiN was highly stable in thepotential range of ORR. Nanocarbon supported TiN catalystswere also stable, showing saturated current densities before the20th scan without any sign of corrosion. Most interesting in theCV results was the huge double layer-induced current exhibitedby TiN/CNT–GR in spite of its lower BET surface area than TiN/CNT. The large pores in TiN/CNT–GR (see Fig. 5b) created by theformation of a 3-D structure between CNTs and GR couldpromote the contact with the electrolyte, thus resulted in thelargest double layer current.

Fig. 6b displays the LSV results of synthesized catalysts usinga rotating disk electrode (RDE) at 1600 rpm. Nanocarbon sup-ported TiN catalysts showed enhanced onset potentials of 0.8 Vfor TiN/GR and TiN/CNT, and 0.83 V for TiN/CNT–GR comparedto 0.6 V for bare TiN. The good conductivity of carbon supportmaterials may serve to increase the activity of TiN NPs for ORR.The onset potential of TiN/CNT–GR was comparable to those ofPd-based catalysts synthesized by simple chemical reduction orthermal decomposition methods, which are frequently studiedfor ORR as a substitute for Pt. Reported onset potentials of Pd-based catalysts for ORR were between 0.6 and 0.9 V.For example, 0.68 V for Pd/C,10,11 0.83 V for PdFe/C,10 0.82 V forPdV/C,11 and 0.80 V for PdCo/C.9 In general, onset potentials ofPd-based catalysts are highly affected by synthetic methods.Morphology controlled Pd-based catalysts, such as Pd or PdFe

Fig. 6 (a) CV results of synthesized TiN-based catalysts; (b) LSV results.

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nanorods, exhibited high onset potentials of 0.9 V.48,49 However,Pd and Pd alloys have a durability problem due to dissolution ofPd or leaching of 2nd metals in acid media,50 and Pd is a rela-tively expensive noble metal itself.

In addition to enhancement in onset potential, TiN/CNT–GRexhibited higher ORR current density than bare and othersupported TiN catalysts in the whole potential range. Particu-larly, it showed enhanced activity in the high potential region(i.e. in the low overpotential region). The ORR current density( JORR) of our TiN/CNT–GR at 0.8 V (�0.4 mA cm�2) and 0.7 V(�1.4 mA cm�2) represents the best values among the reportednitride-based catalysts, including carbon supported Mo and Wnitrides (not active at 0.7 V),14,15 TaON (�5 mA cm�2 at 0.7 V),7 Zr,Nb, and Ta-based CNO (�0.1 mA cm�2 at 0.7 V),13 and TiN/CB(�0.6 mA cm�2 at 0.7 V).12 The activities of nitride-basedmaterials for ORR are summarized in Table S3.†High activity inthe high potential region is of importance considering theoperation of single cells. This activity enhancement in TiN/CNT–GR is ascribed to the increased number of active sites (TiNNPs) accessible by the electrolyte due to the unique morphologyof the CNT–GR hybrid with markedly increased large meso-pores, as the structural properties of TiN NPs (includingprimary particle sizes) were similar both in supported and bareTiN catalysts as conrmed by XRD and XAFS analyses. This alsoleads us to attribute the difference in ORR performance amongdifferent nanocarbon supports to their different morphologies.For TiN NPs loaded on the CNT–GR hybrid, the chances tocontact with the electrolyte increase owing to the abundantlarge mesopores, thus more TiN NPs could participate in theORR process and contribute to the increased current density.The ORR activity of TiN/CNT–GR is not that much inferior to thestate-of-the-art Pt/C catalysts, which exhibit high onset poten-tials of 0.95–1.0 V with high current density (�1 mA cm�2 at0.9 V).44,50 The activities of TiN-based catalysts were comparedwith that of Pt/C and are shown in Fig. S7a.† Besides highactivity, the ORR performance of the TiN/CNT–GR catalyst didnot decrease signicantly aer 500 cycles of potential sweepbetween 0.6 and 1.0 V, indicating a good stability for ORR, asshown in Fig. S7b.†

As discussed in the previous section, nitrogen-doped carboncould be formed inadvertently in supported TiN catalysts underthe synthetic conditions involving carbon, urea as a nitrogensource, and high temperature. To clarify the inuence ofnitrogen-doped carbon on ORR activity in our catalysts, wecarried out various sets of LSV measurements for a referencenitrogen-doped carbon sample (urea/CNT–GR) prepared by thesame synthetic method of TiN/CNT–GR but in the absence of Tiprecursor. In addition, to identify the effect of nanocarbonsupport on ORR, TiN powders were physically mixed with theCNT–GR hybrid support (TiN–CNT–GR). As shown in Fig. S8,†relative to bare TiN, urea/CNT–GR exhibited the similar onsetpotential (0.6 V) and a slightly increased activity for ORR. Theonset potential of 0.6 V was reported in nitrogen-doped carbonshowing a moderate activity for ORR.51,52 The performance is instark contrast to TiN/CNT–GR that exhibited an enhanced onsetpotential of 0.83 V and a marked increase in ORR activity. Thus,the formation of nitrogen-doped carbon had an insignicant

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effect on the active phase of ORR in our TiN/CNT–GR. Physicallymixed TiN–CNT–GR showed an onset potential of 0.8 V, andORR activity was slightly better than urea/CNT–GR, but muchlower than TiN/CNT–GR. The result indicates that the strongTiN–support interaction is essential for the high ORR activity.Essentially the same observation was made for TiN/CNT(Fig. S9†) and TiN/GR (Fig. S10†). Therefore, the effect ofnanocarbon supports, not that of nitrogen-doped carbon, wasthe dominant reason for the enhanced ORR activity of TiN/CNT–GR. Nitrogen-doped carbon may have merely contributedto the increased current density below 0.6 V.

In addition to the improved textural properties of carbon-supported TiN, the nanostructured carbons could also provide ahighly conductive electron pathway to enhance the performanceof the electrocatalyst. Thus, the electron transport processoccurring in bare and carbon-supported TiN was investigatedusing electrochemical impedance spectroscopy (EIS). Fig. 7ashows the Nyquist plots of EIS measurements for the synthe-sized catalysts. The Nyquist plots were also tted with a simpleequivalent RC circuit model shown in Fig. 7b, which consistedof charge-transfer resistances (Rct) across the interfaces presentin the electrocatalyst and constant phase elements (CPE) inparallel, and the resulting tting parameters are summarized inFig. 7b. A semicircle shown in the Nyquist plot is closely relatedto the interfacial charge transfer process at the catalyst–elec-trolyte interface.

Compared to the bare TiN, supported TiN catalysts showedincreased simulated capacitances indicating increased surfaceareas of the catalyst layer, which were consistent with PSD(Fig. 5b) and CV (Fig. 6a) results. In particular, TiN/CNT–GRexhibited 20 times higher capacitance value than that of bare

Fig. 7 (a) Nyquist plots of the synthesized catalysts at 0.6 V bias voltage.Measured spectra and fitting results were exhibited by circles and lines, respec-tively. The high frequency region was magnified and shown in the inset. (b) Theequivalent circuit model and relevant fitting values to fit the measured EISspectra.

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TiN, which revealed the effectiveness of the CNT–GR hybrid as acatalyst support providing a high surface area. Introduction ofcarbon supports also greatly decreased the charge transferresistance (Rct), which varied inversely with the activity of theelectrocatalysts. The lowest value of Rct in TiN/CNT–GR(2.25 kU) was nearly 15 times lower than that of bare TiN (33.5kU), which is consistent with its highest electrocatalytic activityfor ORR.

Thus, the EIS results lead us to conclude that the enhancedactivity of TiN/CNT–GR for ORR mainly originated from thesynergistic effect between TiN NPs and CNT–GR hybrid support,roles of which were to act as active sites for ORR, and to providea large surface area and a facile electron pathway to TiN NPs,respectively. Introduction of the CNT–GR hybrid support doesnot affect much of the properties of TiN itself except hinderingthe aggregation of NPs, but it dramatically improves its ORRactivity by two major roles. First, it provides large mesoporesdue to the formation of a 3-D like morphology assembled from1-D CNTs and 2-D GR, which allows easy access of the electrolyteto active TiN sites. Second, it provides a highly conductiveelectron pathway that shuttles electrons from an external circuitto TiN NPs. For the second role, electric connectivity betweenTiN and the support should be critical, and such a close inter-action seems realized in the present system because the supporthinders the aggregation of TiN NPs and there are no free-standing NPs away from the support.

Methanol tolerance of supported TiN electrocatalysts

An efficient ORR catalyst is needed for both PEMFCs and directmethanol fuel cells (DMFCs). For application in DMFCs, ORRcatalysts should satisfy an additional requirement, i.e. satis-factory tolerance to methanol that crossed from the anode sidethrough the membrane. Methanol tolerance properties of theTiN/CNT–GR catalyst were investigated in comparison with acommercial Pt/C (E-TEK) catalyst using CV and LSV measure-ments in the presence of 0.1 M methanol. During the CVmeasurement, the Pt/C catalyst showed typical large oxidation

Fig. 8 CV and LSV results of TiN/CNT–GR and commercial Pt/C (E-TEK) to testmethanol tolerance; (a and b) for TiN/CNT–GR and (c and d) for Pt/C.

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peaks in a methanol-containing sulfuric acid solution (Fig. 8c).However, regardless of the presence of methanol, TiN/CNT–GRexhibited no oxidation peaks in its CV traces as shown inFig. 8a. Like CV results, Pt/C showed a large oxidation peakagain in the LSV trace, indicating a drastic decrease in the ORRactivity (Fig. 8d). In contrast, TiN/CNT–GR showed little activityloss (Fig. 8b) indicating an excellent tolerance to methanol.Thus, CV and LSV results in the presence of methanol demon-strate the potential use of TiN/CNT–GR as a promising meth-anol tolerant cathode catalyst for DMFCs.

Conclusions

Five TMNs (Mo2N, W2N, NbN, Ta3N5, and TiN) were synthesizedby the urea-glass route and screened for their electrochemicalstability and activity for ORR by CV and LSV. The electro-chemical stability could be aligned as Mo2N � W2N < NbN,Ta3N5, TiN, whereas the activity was aligned as W2N < NbN,Ta3N5 � TiN < Mo2N. The selected TiN was modied withnanocarbon supports for further improved activities for ORR.Nanocarbon-supported TiN NPs exhibited small particle sizes ofTiN (<7 nm) and a good TiN–support interaction with reducedaggregation and no free-standing TiN particles away from thesupports compared to bare TiN. In particular, TiN/CNT–GRshowed much higher ORR activity than other reported nitride-based catalysts as well as bare TiN, TiN/GR, and TiN/CNT. Thisdramatic enhancement was ascribed to a synergistic effectbetween TiN NPs and CNT–GR hybrid, roles of which were toprovide active sites for ORR and an electron pathway to TiN NPs,respectively. In addition, TiN/CNT–GR exhibited the formationof large mesopores due to the formation of 3-D like CNT–GRstructure assembled between 2-D GR and 1-D CNTs, whichcould allow easy access of the electrolyte. Furthermore, TiN/CNT–GR also showed a good methanol tolerance for DMFCs,indicating that our TiN/CNT–GR could be a promising ORRelectrocatalyst for DMFCs as well as PEMFCs.

Acknowledgements

This work has been supported by BK 21, Basic Science ResearchProgram (no. 2012-017247), the Hydrogen Energy R & D Center,Korean Centre for Articial Photosynthesis (NRF-2011-C1AAA0001-2011-0030278), and WCU (R31-30005) funded by theMinistry of Education, Science, and Technology of Republic ofKorea, and special professor program funded by POSCO/RIST.

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