self‐sacrificial template synthesis of a nitrogen‐doped ... · probably because the metal...

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Self-Sacrificial Template Synthesis of a Nitrogen-DopedMicrostructured Carbon Tube as Electrocatalyst for OxygenReductionLimeng Yang,[a, b] Yachao Zeng,[c] Xuejun Tang,[a, b] Dongyan Xu,*[d] Dahui Fang,[a, b]

He Huang,[a, b] Zhigang Shao,*[a] and Baolian Yi[a]

The development of low-cost, efficient and stable electro-catalysts for the oxygen reduction reaction (ORR) is desirablebut remains a great challenge. We report a self-sacrificialtemplate synthesis approach for nitrogen-doped hollow carbonmicrotubes as electrocatalyst for ORR. Typically, Fe-MIL (Materi-aux de l’Institute Lavosier) nanocrystals cast as self-sacrificialtemplate. Polyaniline (PANI) was in situ-synthesized and depos-ited on the surface of Fe-MIL nanocrystals. By a two-steppyrolysis of the MIL-101 (Fe)@PANI hybrids, unique micro-structured carbon tubes with Fe3O4 nanoparticles encapsulatedinside the wall were fabricated after the self-sacrificial templatedecomposed. The optimized C-PANI-MIL-2 catalyst exhibits a

high onset potential of 1.0 V in alkaline media. Out of thenegative effect of the disruption of metal-organic frameworks(MOF) morphology, the ingenious catalytic activities may beascribed to the well-defined microtube architecture with a highaccessible surface area, the existence of FeN2+2/FeN4 and richactive nitrogen atoms. Moreover, C-PANI-MIL-2 exhibited ex-cellent methanol tolerance and durability compared to acommercial Pt/C catalyst in both electrolytes. This work mayprovide a new strategy for the design and preparation ofmicrostructured non-precious metal based catalysts supportedon hollow carbon tubes for fuel cells.

1. Introduction

Because of its environmental benignity and high energyefficiency, fuel cells (FCs) are considered as one of the mostpromising power source candidates. Among various fuel celltypes, proton exchange membrane fuel cells (PEMFCs), whichemploy proton-conducting electrolytes are most encouragingfor transportation and portable applications. In addition,alkaline anion exchange membrane fuel cell (AAEMFC), whichemploy a solid alkaline electrolyte membrane and mitigate theelectrolyte leakage issue, also draw much attention. Never-theless, the high consumption of noble metals (i. e., Pt and Pd)in cathode catalysts has significantly increased the cost of fuelcell stacks.[1] In light of that, it’s fundamental to develop non-precious metal catalysts (NPMCs) with excellent ORR activity

and durability in both acidic and alkaline electrolytes. Thereported NPMCs varies a lot,[2] in the midst of them, transitionmetal doped carbon-supported nitrogen materials (M� Nx/C) aremost promising.[3]

Among the M� Nx/C catalysts, porous hollow carbon micro-spheres are generally fabricated.[4] Even with merits ofenhanced surface area, shortened distance for mass/chargetransfer, the common adaption of polystyrene or silica as hardtemplates complicates the synthesis routine and influences theaccording ORR performance. On the other hand, with excellentelectric conductivity, both large external and internal surfacearea and the flexibility to be functionalized[5] for ORR activesites, we set our sights on hollow carbon tubes. Except forcommercial carbon nanotubes (CNTs), there are reports aboutnitrogen-doped CNTs derived from polyaniline.[6] Due to thesimilar aromatic structures of the CNTs and PANI[7], carbon tubescan easily be acquired in metal-based catalysts derived fromPANI. Non-precious metal, such as Fe, appears to catalyze thedecomposition of PANI and formation of C-Nx species which arebeneficial to rearranging and coalescing to harvest highlygraphitic structures.[1]

Additionally, MOFs are viewed as a gorgeous template orprecursor to synthesize new formulas of M–Nx/C materials,[8]

taking the following advantages in: (1) metal ions can act ascatalytic centers, (2) ultrahigh surface areas can host moreactive sites and (3) high porosity can provide more channels.Ma et al. reported that nitrogen doped carbon materialscontaining iron showed better ORR catalytic activities than Coand Zn.[9] Therefore, Fe-MIL, a kind of MOF, was chosen as ironsource to synthesize M� Nx/C catalysts. However, there are stilldefects obstructing the performance of MOF derived cathode

[a] L. Yang, X. Tang, D. Fang, H. Huang, Prof. Dr. Z. Shao, B. YiDalian National Laboratories for Clean Energy, Dalian Institute of ChemicalPhysics, Chinese Academy of Sciences457 Zhongshan Road, Dalian 116023, ChinaE-mail: [email protected]

[b] L. Yang, X. Tang, D. Fang, H. HuangUniversity of Chinese Academy of Sciences19A Yuquan Road, Beijing 100049, China

[c] Y. ZengUniversity of New South Wales2052, Kensington, Sydney, Australia

[d] D. XuState Key Laboratory Base of Eco-chemical EngineeringCollege of Chemical EngineeringQingdao University of Science and Technology99 Songling Road, Qingdao 266042, ChinaE-mail: [email protected] information for this article is available on the WWW underhttps://doi.org/10.1002/celc.201801050

ArticlesDOI: 10.1002/celc.201801050

3731ChemElectroChem 2018, 5, 3731–3740 © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Wiley VCH Dienstag, 20.11.2018

1823 / 121153 [S. 3731/3740] 1

http://orcid.org/0000-0003-2777-6362

https://doi.org/10.1002/celc.201801050

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electrocatalysts. One of the defects is the devastation ofmorphology during carbonization.[10] Specifically, the collapse ofMOF and the agglomeration of conductive carbon caused ahuge surface area decline of MOF precursor.[11] In addition, themetal species inevitably agglomerated during calcination whichresulted in the lessening of active sites density. Although it ischallenging to control the morphology of MOF duringpyrolysis,[12] we could still harness the loss control of the crystalmorphology by utilizing MOF as self-sacrificial template. Due tothe lower decomposition temperature of Fe-MIL compared withPANI, hollow carbon tubes can be constructed by applying thedecomposition of Fe-MIL to form hollow pores. The Fe-MIL mayact as both iron source and self-sacrificial template forpreparing M� Nx/C catalysts.

Herein, we report a synthetic approach via a two-steppyrolysis of synthesizing PANI on self-sacrificial template Fe-MILnanocrystals, which leads to an unprecedented hollow carbonmicrotubes with giant inner diameter structured NPMC for ORRin both acidic and alkaline media. Macroscopically, with thepresence of hollow cavity in catalysts, the reactants andproducts could transfer directly to and from the active sitesacross macropores with slight transport resistance.[13] It mani-fests effective utilization of reagent due to high accessibility ofthe inner pores to the surrounding substance and then the ORRactivity could be enhanced. Different from traditional one-steppyrolysis or hard template method,[3b,14] this work innovativelycombined acid etching, two-step pyrolysis with self-sacrificialtemplate and constructed a brand new-type NPMC. Theobtained NPMC shows a high onset potential of 1.0 V in alkalineelectrolyte. It exhibits satisfying ORR activity in acidic mediumwith a 0.86 V onset potential. The high ORR activity of thehollow carbon microstructured catalyst may originate from alarge accessible surface area of the hollow cavity of themicrotube, the abundant pyridinic-/graphitic-N and the FeN4/FeN2+2 species. Such an inventive combination of well-arrangedmicroscale morphology and highly active sites leads to theunique performance in both acidic and alkaline medium.Moreover, the C-PANI-MIL-2 exhibited excellent methanoltolerance than commercial Pt/C and a satisfying durability of10000 and 5000 cycles in alkaline and acidic electrolytes,respectively.

2. Results and Discussion

2.1 Structural Characterization

Scheme 1 illustrates the synthesis route of PANI-MIL derivednitrogen-doped hollow carbon microtubes. In brief, we synthe-sized MIL-101(Fe) firstly, and then soaked the MOF powder inaniline for several days, the MIL-101(Fe)@PANI composites wereobtained through an in situ polymerization of aniline by(NH4)2S2O8. Next, MIL-101(Fe)@PANI composites were trans-formed to nitrogen-doped hollow carbon microtubes aftercarbonization at 900 °C in N2. The resultant sample weredenoted as C-PANI-MIL-FP (first pyrolysis), C-PANI-MIL-AT (acidtreatment) and C-PANI-MIL-SP (second pyrolysis), respectively. If

there isn’t denoted particularly in this paper, C-PANI-MIL-FP, C-PANI-MIL-AT and C-PANI-MIL-SP catalysts are all derived from 2days’ aniline soaking. The structure and electrochemical activityof the PANI-MIL derived hollow carbon microtubes have beencharacterized.

Figure 1 shows the electron microscopy images of corre-sponding MIL-101(Fe) precursor, MIL-101(Fe)@PANI composite,C-PANI-MIL-AT, C-PANI-MIL-FP and C-PANI-MIL-SP catalysts. Fig-ure 1a and Figure 1b exhibit a rigid zeotype octahedralstructure with a length of 200–300 nm which is correspondingwith the reference reported.[15] Moreover, the phase of obtainedMIL-101(Fe) precursor was characterized by XRD in Figure S1. Itcan be found that the major Bragg positions of synthesizedMIL-101(Fe)’s PXRD pattern meets those of the simulated well.Thus, we can tell that the precursor is exactly MIL-101(Fe).[16]

Scheme 1. Synthesis of the C-PANI-MIL oxygen reduction catalyst.

Figure 1. SEM images of as-prepared a) MIL-101(Fe), c) MIL-101(Fe)@PANImircorods, d) C-PANI-MIL-FP, and e) C-PANI-MIL-AT; TEM and STEM imagesof corresponding b) MIL-101(Fe) and f) C-PANI-MIL-SP.

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3732ChemElectroChem 2018, 5, 3731–3740 www.chemelectrochem.org © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim


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After coated PANI, the MIL-101(Fe)@PANI composites turnedinto dark green microrods with a diameter of 1 μm and a lengthof 4–6 μm. The morphology of microrods may take shape dueto the aggregating of more than several MIL-101(Fe) crystals atfirst and then the coating of PANI. In Figure 1d, there are hollowcarbon microtubes with an inner cavity of 500 nm diameter anda length of approximately 3 μm. The wall of the carbonmicrotube is composed of N-doped graphene-like layers with athickness of ca.100 nm. Obvious bright metal particles can beseen wrapped in the walls of carbon microtubes. The brightmetal particles are inhomogeneous from tens of nanometers tohundreds of nanometers. While in Figure 1e, there aren’tobvious metal particles on the acid treated carbon microtubes,and after second pyrolysis, there exists some tiny metal particlesagain in the walls of carbon microtubes. However, the size ofmetal particles is much smaller and more uniform at ca. 50 nm.The disappeared metal particles by acid etched reappearedprobably because the metal particles weren’t totally etched byacid. Remained tiny metal particles may be wrapped under thewall of the carbon tubes in Figure 1e and they aggregatedagain due to the intermetallic force during second pyrolysis.Thus, it seems the disappeared metal particles have reappearedagain in C-PANI-MIL-SP catalyst.[17]

We characterized the C-PANI-MIL-AT, C-PANI-MIL-FP and C-PANI-MIL-SP catalysts by the back-electron detector (BED) ofscanning electron microscope in Figure S2, which can distin-guish heavy metal elements from light elements. From Fig-ure S2a, S2d and S2 g, there are many noticeable metal particlesafter the first pyrolysis, while after the acid treatment, there arerare of them on the surface of C-PANI-MIL-AT in Figure S2b,S2e, S2 h. After second pyrolysis, there exists some tiny metalparticles again but less obvious than the catalyst after firstpyrolysis in Figure S2c, S2 f and S2i. From ICP-AES, the ironamounts (wt.%) in C-PANI-MIL-FP is 9.39%, which is more than1.13% of C-PANI-MIL-AT and 1.29% C-PANI-MIL-SP. The resultsare in good accordance with SEM-BED images above.

To investigate the instinct of iron particles encased in thewall and the detailed morphology of carbon microtubes, wecharacterized the C-PANI-MIL-SP by high magnification TEM.The HRTEM image of a metal nanoparticle (Figure 2b, Inset)presents lattice fringes of Fe3O4 along (111) panel, manifestingthat the nanoparticle is composed of Fe3O4.

[4c,18] It is found thatabout 14 graphene layers wrapped metal particles on the wallof carbon microtubes (Figure 2d). The lattice spacing of theshells is about 0.34 nm along (002) facet and the graphenelayers seems inhomogeneous. It recommends that graphiticlayers formed during the calcination are not ideal but slightlydisordered, such as turbostratic.[1] There are reports declaringthat the graphite layers wrapping out of metal particles and thenumber of the graphitic layers are closely associated with theORR activity and durability of catalysts.[19] The elementalcomposition analysis in Figure 2f clearly show the ferruginousparticles uniformly wrapped in the wall of the carbon micro-tube. The doped N displaying in Figure 2g seems barelydetectable probably because its content in the catalyst is low,which is proved by the 2.21 at% nitrogen contents from theresults of XPS in Tab. S2.

In order to examine the porous features of the catalysts,nitrogen isothermal adsorption-desorption measurements wereexecuted. As shown in Figure 3a, all the N2 adsorption-desorption plots of four C-PANI-MIL catalysts exhibit a type-IVisotherm with a tiny hysteresis loop and a vertical tail inrelatively high-pressure region, demonstrating the presence ofmeso- and macro-pores. Besides, the hasty rise at low pressureindicates the existence of micropores. By means of these plots,it was calculated that C-PANI-MIL-2 catalyst owned the highestBET surface area of 884 m2g� 1 in Table S1. All the pore sizedistribution plots in Figure 3b show micropores (2–50 nm)under 5 nm. Particularly, C-PANI-MIL-2 catalyst exhibits anobvious single peak at the range of 0–2 nm, demonstrating themost microporous structure of it. From the results of t-plotmicropore analysis method (Table S1), a highest micropore areaof 746 m2g� 1 in C-PANI-MIL-2 is also confirmed. Since themicroporous structure may hold active sites for O2 adsorptionand reduction,[20] C-PANI-MIL-2 catalyst, with a superior micro-pore structure and BET surface area, may show a greater ORRbehavior than other C-PANI-MIL catalysts accordingly. On theother hand, the meso- and macro-pores of C-PANI-MIL-2, whichalso appeared in SEM and TEM images of Figure 2, not onlyguarantee rapid diffusion of the reactant across the surface ofcatalyst, but also afford extra electrode-O2-electrolyte triple-phase interface to accelerate ORR.

Furthermore, XRD was employed to analyze the phasespecies of several precursors and C-PANI-MIL-SP catalysts afterdifferent ANI soaking time. A series of peaks centered at 35.4°,30.1° and 62.5° of Fe3O4 (JCPDS card No. 01-019-0629) areobserved in most of the C-PANI-MIL-SP catalysts patterns

Figure 2. a) SEM image, b), d) TEM images of C-PANI-MIL-SP, c) HAADF-TEMimage and e)-g) typical TEM images of C-PANI-MIL-SP and the correspondingC, Fe and N elemental mappings.

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(Figure 3d). It is in good agreement with the TEM image result.Besides, metallic iron exists in C-PANI-MIL-1 and C-PANI-MIL-3catalysts with peaks of 44.6°, 65.1° and 82.5° (JCPDS card No.01-087-0722). The broad and strong peaks centered at 26.1°and 43.3° can be separately attributed to (002) and (101) facetof the disordered carbon in both C-PANI-MIL-1 and C-PANI-MIL-3 catalysts (JCPDS card No. 00–034-0567). Compared with thediffraction patterns of C-MIL-900 and C-PANI-900, it can bespeculated that the metallic iron species and disordered carbonin C-PANI-MIL-SP catalysts may derived from the calcination ofMIL-101 and PANI precursor, respectively.

To probe the substance interactions during the synthesisand to find the shape tuning mechanism of the hollow carbonmicrotubes, the thermal stability of MIL-101(Fe) precursor, PANIprecursor and PANI-MIL-BH (before heat treatment) wasevaluated by thermogravimetric analysis (TGA) (Figure 4). TheTGA curve firstly shows a slight weight loss at 40–200 °C inFigure 4a, which is owing to the evaporation of guest solventDMF in the pores of MIL-101 precursor. The next weight loss at300–600 °C is ascribed to the partial decomposition of the Fe-MIL-101 precursor[15] while the decomposition of PANI starts atca. 300 °C in Figure 4b. At final 1000 °C, there are still 40% ofPANI residues remained confirming a better thermal stability,while only 5% and 20% of the MIL-101(Fe) and PANI-MIL-BH

residues remain, respectively. At 500 °C where the most weightloss of PANI appears, there aren’t great weight loss of PANI-MIL-BH. Nevertheless, at 320 °C where the most weight loss of MIL-101(Fe) occurs, PANI-MIL-BH also loses the most weight. Thus,the hollow cavity of carbon microtubes of C-PANI-MIL catalystsmay originate from the decomposition of MIL-101(Fe) precursor.Furthermore, SEM and EDS characterization of intermediatebetween PANI-MIL-BH and C-PANI-MIL-FP, i. e. the 0.5 h-pyro-lyzed PANI-MIL-BH (denoted as C-PANI-MIL-0.5 h), have beenproceeded to study the transitional state of the catalyst. Fromthe SEM image of the C-PANI-MIL-0.5 h in Figure S4, it can beseen that there are analogous polyhedrons existing in the wallof carbontubes. Then the metal-based particles around thecarbontube are selected for EDS characterization and ironelement is detected, which demonstrated that the particles arederived from MIL-101(Fe) indirectly (Figure S4b and S4c). Yet,none of iron element is detected on the wall of the carbontube(Figure S4d). Therefore, together with the TGA results, the shapetuning mechanism of the hollow carbon microtubes can beillustrated by the decomposition of MIL-101(Fe) precursor.

Figure 3. Porous feature of catalysts: a) N2 sorption isotherms (closed, adsorption; open, desorption); b) QSDFT pore size distribution, and c)–d) XRD patternsof precursors and C-PANI-MIL-SP with different ANI soaking time. (δ Fe3O4; π Fe3C; ν disordered carbon; χ C3N4; ϕ Fe)

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2.2 Electrochemical Characterization

To evaluate the influence of different ANI soaking time, weinvestigated electrocatalysts with different ANI soaking timevaried from 0.5 day to 2 days. Figure 5a and Figure 5b show theRDE polarization curves of the electrocatalysts with differentANI soaking time in alkaline and acidic media. It can be seenthat the C-PANI-MIL-2 catalyst performs best in both media. Thesecond-best catalyst is C-PANI-MIL-0.5. In O2 saturated 0.1 MKOH, the C-PANI-MIL-2 catalyst shows the same ORR catalyticactivity with 20 wt.% Pt/C, with a onset potential of 1.0 V and ahalf-wave potential of 0.87 V. In acidic condition, it also shows a0.86 V onset potential which is only slightly negative shift(0.1 V) compared with 20 wt.% Pt/C. The half-wave potential ofC-PANI-MIL-2 is also 0.1 V negative shift from 20 wt.% Pt/C. It ishigher than other NPMCs with equivalent catalyst loading(Table S5). The onset potential and half-wave potential of all

non-precious metal electrocatalysts in this work are listed inTable S7 and Table S8 for ORR property comparing. It can beconcluded that C-PANI-MIL-1 and C-PANI-MIL-3 catalyst can’trival C-PANI-MIL-2 in both alkaline and acidic media. However,the C-PANI-MIL catalysts generally behave inferior in HClO4 thanin KOH electrolyte. The fact may reveal the different ORRmechanism or different active sites in different kinds ofmedium.[21] With respect to the better catalytic activity of C-PANI-MIL-2 and C-PANI-MIL-0.5, it may be attributed to thelarger surface area and the rich pyridinic- and graphitic-Ncontents in Table S1 and Table S3. The role of second pyrolysisstep and the synergistic effect of PANI and MIL can beelucidated from the poor electroactivity of contrast sample inFigure S5. It can be concluded that the ORR activity of C-PANI-

Figure 4. TGA-DTG plot of a) MIL-101(Fe), b) PANI, c) PANI-MIL-BH (beforeheat treatment).

Figure 5. RDE polarization curves of C-PANI-MIL electrocatalysts of withdifferent ANI soaking time: a) in O2 saturated 0.1 M KOH, and b) in O2

saturated 0.1 M HClO4. c) Stability evaluation of C-PANI-MIL-2 and Pt/C after10000 cycles in 0.1 M KOH. d) Chronoamperometric (0.6 V) response at1600 rpm in KOH, the arrow indicates the introduction of 10 vol% methanol.e) C-PANI-MIL-2 catalyst at different rotating speed in 0.1 M KOH; f) K� L plotsat different potentials. % H2O2 and transferred electron number (n) duringORR process g) in O2 saturated 0.1 M KOH and h) in O2 saturated 0.1 MHClO4.

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MIL-SP catalyst enhanced a lot after acid treatment and thesecond pyrolysis, which may result from the reconstruction ofactive sites.

Figure 5c and Figure S6a display the electroactivity degrada-tion of C-PANI-MIL-2 catalyst and Pt/C after being cycled in arange of 0.6–1.0 V with a scan speed of 0.1 V s� 1 for 10000cycles in KOH and 5000 cycles in HClO4, respectively. It isobvious in Figure S6c and S6d that 82% and 70% of the initialcurrent density of C-PANI-MIL-2 have been retained in alkalineand acidic media, which is superior to those of Pt/C (23% and28%).

The anti-methanol test was carried out by injecting10 vol.% methanol into electrolyte under chronoamperometrictechnique. From Figure 5d and Figure S6b, it can be obviouslydrawn that C-PANI-MIL-2 exhibited methanol tolerance thancommercial Pt/C in both acidic and alkaline media. A harshgrowth of the current density for Pt/C was perceived after theaddition of 10 vol.% methanol into the electrolyte (Figure 5dand Figure S6b, the arrow represents the introduction), whilethe C-PANI-MIL-2 was only faintly disturbed. By virtue of this, itwill be valuable for further blossom of nitrogen-doped NPMCsfor fuel cells, such as direct methanol fuel cells (DMFCs) andPEMFCs.

To investigate ORR activity and selectivity of the optimizedC-PANI-MIL-2 catalyst, we chose K� L plots and RRDE test torecognize the transferred electron number (n) of the ORRprocess. The linear sweep voltammograms gained at differentrotation rates from RDE experiments are shown in Figure 5eand Figure S6c for C-PANI-MIL-2 catalyst. As shown in Figure 5fand Figure S6f, the resultant K� L plots show a good linearity atdifferent potentials. The transferred electrons numbers werealso estimated by K� L equation via K� L plots. The linear plot inFigure 5f approve the first order ORR kinetics above 0.5 V inalkaline medium. According to the K� L equation, the n for C-PANI-MIL-2 is calculated to be about 3.7 and 3.2 separatelyagainst 0.5 to 0.8 V in alkaline and 0.2 to 0.5 V acidic media.Additionally, RRDE tests were proceeded for C-PANI-MIL-2 andcommercial Pt/C to identify the transferred electron number.Figure 5g and 5 h show the H2O2 yield and transferred electronnumber (n) during ORR process in O2 saturated 0.1 M KOH andHClO4 separately. The upper part of Figure S6g and S6 h plots isthe ring current (Ir) over applied potential when the lower partplots is the disk current density (jd). The platinum ring cancollect H2O2 produced during ORR, thus Ir is closely related withthe hydrogen peroxide yield (% H2O2) which can be calculatedfrom Equation (1). In Figure S6g, both the Id curve and the Ircurve of C-PANI-MIL-2 are close to the curves of Pt/C. It predictsa similar low H2O2 yield below 4% and a near 4 electron-transfernumber for C-PANI-MIL-2 and Pt/C. The n derived from RRDEtest for C-PANI-MIL-2 is in line with the value 3.7 calculated byK� L plots, which demonstrates that the ORR on C-PANI-MIL-2 inKOH is governed by a primarily direct 4e� process same withcommercial Pt/C. While the situations in acidic medium (Fig-ure S6h) are not optimistic compared to those in alkalinemedium (Figure S6g). The Ir even reaches 0.04 mA at 0.2 Vwhich is four-fold higher than that of Pt/C. It leads to a higherH2O2 yield ca. 15% and a relatively lower n value about 3.5. The

electron-transfer number calculated by RRDE in acidic mediumis larger than that of K� L plots perhaps because the theoreticalcollection efficiency of the ring electrode is larger than thepractical. Overall, they demonstrate that the ORR on C-PANI-MIL-2 catalyst in HClO4 is governed by both direct and indirect4e� reduction process.

2.3 The Origin of the High Electrocatalytic Performance

Nowadays, by various probing method and advanced exper-imental techniques, such as in situ X-ray absorption spectro-scopy (XAS), 57Fe Mössbauer spectra and quantum chemistrycalculations, four kinds of active sites were mainly promoted:Fe� Nx, Fe@NxCy, NxCy and Fe single atom sites for Fe� N/Ccatalysts[20,22]. Fe-Nx configuration is comprised of Fe� N2/C,Fe� N4/C

[23] and Fe� N2+2/C[24] primarily. Fe� N2/C is established

on the chelation of a Fe atom and two pyridinic-N atomsencapsulated in a graphene matrix, while Fe� N4/C or Fe� N2+2/Care based on a graphene-type matrix embedding the chelationof a Fe atom with four pyridinic-N or in-plane pyrrolic-N atoms.Fe@NxCy consists of Fe particles encapsulated by single ormultiple graphitic layers. As to metal-free NxCy sites, thecontroversy primarily focus on the chemical types of the Nelement, especially the pyridinic-N or graphitic-N.

In order to probe the origin of the high electrocatalyticperformance in this work, we applied XPS and 57Fe Mössbauerspectra to explore the atomic active site structures.

The chemical states of N in the composites were deter-mined using XPS. Four states of N deconvoluted from N 1s XPSspectra are shown in Figure 6[25]. Pyridinic and graphitic nitro-gen are considered to deserve the largest positive effect[5b,26] toORR catalytic sites through the ages. However, there are stilldebates above active sites basing on whether created bypyridinic-N or graphitic-N currently. Demonstrated promotingthe 4e� process in ORR, pyridinic-N has been deemed as activesite.[27] Lately, the carbon atoms adjacent to pyridinic-N withLewis basicity were pointed as active sites by Nakamura et al.[28]

However, because of their reduced adsorption energy, graph-itic-N have been reported to enhance ORR activity bothexperimentally and theoretically.[29] It has been suggested thatthe graphitic-N introducing to the graphene edge not onlyshowed better stability and preference for the 4e� reductionpathway, but also enhanced the first electron transfer rateparticularly.[30]

The relative elements content of different C-PANI-MILcatalysts are summarized in Figure 7 (The related data presentsin Table S2). Different atomic concentration of N-configurationswas obtained from the deconvoluted N 1s HR-XPS spectra. InFe� N/C catalysts, N can be seemed as an n-type carbon dopantwhich results in the formation of disordered carbon (Fig-ure 3d).[1] The nitrogen also can donate electrons to theconjugated π orbital in carbon matrix, which may arise itsfeedback electrons to the π* orbital of O2 molecule, thusenabling the O� O bond splitting.[31] Thus, relatively high nitro-gen doping amount may raise the ORR catalytic activity tosome extent. Coincidentally, the C-PANI-MIL-2 catalyst, which

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preserves the highest amount of N, owns a better performancethan other C-PANI-MIL catalysts.

It can be seen in Figure 7 that the N types content ofdifferent C-PANI-MIL catalysts varies. The C-PANI-MIL-2 catalystowns both the highest quantity of pyridinic- and graphitic-N,and C-PANI-MIL-3 catalyst preserves second total quantity ofgraphitic- and oxidized-N. The active N species amount of C-PANI-MIL-3 is higher than that of C-PANI-MIL-0.5, while the ORRactivity of C-PANI-MIL-3 is inferior to C-PANI-MIL-0.5. It’spossibly influenced by the nearly half-lower surface area of C-PANI-MIL-3 than C-PANI-MIL-0.5 (Table S1). The poor surface

area may prohibit mass transfer seriously. In a word, C-PANI-MIL-2 catalyst which holds the highest total amount ofpyridinic- and graphitic-N deserves the best ORR performanceamong C-PANI-MIL series catalysts.

The 57Fe-Mössbauer spectra were recorded at 298 K.Mössbauer spectra for the optimized sample C-PANI-MIL-2 areshown in Figure 8. The spectra were deconvoluted to onesinglet, one doublet and three sextets. Owing to the toughoverlap of the lines, it was required to partly constrain theparameters. The sextet-1 of a 20.7 Tesla magnetic field aresupposed to represent a carbide. As the magnetic field of 21Tesla is a typical signature of a carbide and the 0.19 mms� 1

isomer shift (IS) of sextet-1 is also a typical IS of a carbide. Whileaccording to the literature on pure carbides,[32] the hyperfine

Figure 6. High-resolution XPS spectra of N 1s of a) C-PANI-MIL-0.5, b) C-PANI-MIL-1, c) C-PANI-MIL-2 and d) C-PANI-MIL-3 catalysts.

Figure 7. Different atomic concentration of nitrogen configurations and thecorresponding binding energy range

Figure 8. Experimental 57Fe Mössbauer transmission spectra measured for C-PANI-MIL-2 and their fittings.

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field of 26 T is expected in pure Fe3C. Another possibility for thesextet-1 could be nitride, since a smaller field of 17.6 T isexpected in a nitride.[33] The observed value 21 T is between thevalues expected for pure carbide and for the nitride. It may bethat the carbide is doped with N in C-PANI-MIL-2 which leadsthe field reducing from 26 T to 21 T. Herein, together with theabsence of Fe3C phase in XRD patterns (Figure 3d), the pureFe3C coated by graphitic layers as active sites doesn’t seem tofit catalysts in this work.[20] Concerning with sextet-2, it mayrepresent γ-Fe2O3, because γ-Fe2O3 is represented by the broadsextet with IS of 0.32 mms� 1 and hyperfine magnetic field of487 kOe. The observed hyperfine magnetic field of sextet-2 is485 kOe which is close to 487 kOe. The γ-Fe2O3 is probablyderived from the atmospheric oxidation after calcination, sinceFe nanoparticles are prone to form a passivation layer ofmaghemite after being exposed in air[34]. Sextet-3 may assign tomagnetic domains, such as magnetite (Fe3O4) which is inaccordance with the results of TEM and XRD (Figure 2b,Figure 3d). Unfortunately, Fe-oxides, such as magnetite (Fe3O4)and hematite (Fe2O3), are sometimes supposed inactive toORR.[35] The singlet is assigned to γ-Fe (Figure 3d) and thedoublet is assigned to FeN4 or FeN2+2, with a 0.38 mms� 1 ISvalue and a 0.75 mms� 1 quadrupole splitting value. Even if FeN4

is considered one category of the early active sites in Fe� N/Ccatalysts, they are demonstrated not required to ORR lately.[35]

Meanwhile, the XPS outcome in Figure 6 and Figure 7 displaythat the amount of pyridinic-N in C-PANI-MIL-2 is much morethan that of pyrrolic-N. Since Fe is chelated with four pyridinic-N in FeN2+2, the doublet may be assigned to FeN2+2 morepossibly. However, the turn over frequence (TOF) of the Fe-N2+

2/C site was proved similar in alkaline and acidic electrolytes.[20]

When the C-PANI-MIL-2 shows different ORR performance inthe alkaline and acidic electrolytes, more characterization suchas XAFS should be conducted to recognize whether it’s FeN2+2

or FeN4.It’s possibly ascribed to the stable graphitic-N[36] and the

multiple layered graphene structure (Figure 3d), the durabilityof C-PANI-MIL-2 seems to be enhanced compared to Pt/C owingto the exposure of inner ORR sites until all the outmost layersbeing consumed.

3. Conclusions

In summary, a novel nitrogen-doped microstructured catalyst C-PANI-MIL was constructed through a self-sacrificial templateapproach for oxygen reduction. Typically, home-made Fe-MILnanocrystals, which are cost effective and environmentalfriendly, cast as self-sacrificial template. Polyaniline (PANI) wasin situ-synthesized and deposited on the surface of Fe-MILnanocrystals. After the template decomposition during calcina-tion, unique microtubes architecture with Fe3O4 nanoparticlesencapsulated in the wall appeared. The obtained NPMC exhibitsa high onset potential of 1.0 V in alkaline electrolyte. In acidicmedium, it only shows slightly negative shift (0.1 V) comparedwith 20 wt.% Pt/C, in terms of both onset potential and half-wave potential. It performs better than other NPMCs with

equivalent catalyst loading. The excellent surface area, existenceof FeN2+2/ FeN4 and rich active nitrogen atoms of C-PANI-MIL-2catalyst might be contributed to the outstanding catalyticactivities in both alkaline and acidic media. Moreover, C-PANI-MIL-2 exhibited excellent methanol tolerance and durabilitythan commercial Pt/C catalyst in both kinds of electrolytes. Thiswork may provide a new perspective for the preparation ofnon-precious metal derived hollow microstructured catalysts forPEMFCs and DMFCs.

Experimental Section

Synthesis of Fe-MIL

The Fe-MIL nanoparticles were synthesized according to theprevious Ref [20]. Briefly, N, N-dimethylformamide (DMF, 197 mL),Iron (III) chloride-hexahydrate (1.084 mg) and 1,4-benzenedicarbox-ylic acid (H2BDC, 0.664 g) were mixed in a flask by magnetic stirringat 25 °C to a DMF/Fe3+/H2BDC molar ratio of 635 :1 :1. Then themixture was heated to 150 °C under stirring for 24 h. Next, theprecipitated products were collected by centrifugation. The lightbrown precipitate was washed twice with DMF and activated invacuum oven at 150 °C overnight, the precursor was denoted asMIL-101(Fe).

Synthesis of PANI-MIL Composites

An amount of MIL-101 (0.20 g, 0.28 mmol) was soaked in aniline(1.7 mL, 0.02 mol) for different days. The brown tar-like ink wasfiltered off and washed twice with a small amount of diethyl ether(1 ml). The composite was obtained after drying at 60 °C undervacuum. The received product was denoted as PANI-MIL.

Synthesis of Nitrogen-Doped Hollow Carbon Microtubes

The PANI-MIL compound (0.2 g, 0.17 mmol) were added to 5 ml HClsolution (1 M, aqueous) below 4 °C. Then a mixture of (NH4)2S2O8

(0.3 g, 1.32 mmol) and 1.7 mL HCl solution (1 M, aqueous) wasadded dropwise for half an hour. The subsequent mixture wasstirred below 4 °C for 4 h vigorously. The dark green precipitate wasfiltered off, washed with deionized (DI) water and then dried at60 °C under vacuum. The subsequent heat treatment was per-formed at 900 °C in N2 for 1 h. After the first pyrolysis in nitrogen at900 °C, the product was ground in a mortar, leached in 0.5 M H2SO4

at 80 °C for 8 h, and washed with DI water to neutral. After dryingat 80 °C oven overnight in vacuum, the powder was calcined againat 900 °C in N2 atmosphere for 3 h to obtain the final product. Theprepared catalysts are denoted as C-PANI-MIL� T, where T (T=0.5,1, 2 and 3) was the MIL-101 soaking days duration in aniline.

Materials Characterization

The morphology of MIL-101(Fe) precursor was investigated byscanning electron microscope (JSM-7800, JEOL) operating at anacceleration voltage of 1.50 kV. The sample was sputtered withplatinum atoms to increase its conductivity. Transmission electronmicroscopy (TEM) was carried out on JEM-2100 at 120 kV.

Powder x-ray diffraction (XRD) were conducted on a D/MAX2500VB2/PC with a 2θ range from 5° to 90° at a scan rate of5 °min� 1. The electronic structure and surface species were identi-

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fied by X-ray photoelectron spectroscopy (XPS) (ESCA Lab250 Xi,Thermo Scientific).

Through inductively coupled plasma atomic emission spectrometry(ICP-AES, Perkin Elmer 7300DV), the metal content in the electro-catalysts was determined. Thermogravimetric analysis (TGA) wereconducted on a Netzsch-STA 449 F3 system at a heating speed of10 °Cmin� 1 at a range of 40–1000 °C in N2.

Aiming at exploring the porous features of the catalysts, nitrogensorption measurements were carried out on QuantachromeAutosorb-IQ gas adsorption analyzer at 77 K. The catalysts weredegassed before tests at 150 °C for 5 h under dynamic vacuum. Thepore size distributions of samples were fitted from Quenched SolidDensity Functional Theory (QSDFT) method and the BET surfaceareas were calculated from the adsorption branches.57Fe Mössbauer spectra were measured at room temperature(295 K) using a 57Co source encapsulated in a rhodium matrix and aconstant accelerator spectrometer. The isomer shifts were refer-enced at test temperature relative to that of α-Fe. At least 100 mgcatalyst powder was pressed into pill-shape of ca. 15 mm-diameterand 2 mm-thick.

Electrochemical Characterization

Electrochemical measurements were performed using a GamryElectrochemical Station in a three-electrode system at constant25 °C. Platinum foil (2 cm×1.5 cm) and a saturated calomelelectrode (SCE) electrode or Ag/AgCl electrode were separatelyused as the counter and reference electrode. The catalyst layer wasprepared as follows: the nitrogen-doped hollow carbon microtubescatalyst powder was dissolved in a mixture of Nafion (5.0 wt.%,DuPont Corp.), water and isopropanol with a ratio of 0.1 : 1 : 9 (v/v/v)to get 2 mgmL� 1 ink under sonication. Then the homogenouslydispersed ink was syringed onto the disk of the working electrode(0.19625 cm2), which was then left to dry in air under an infraredheat lamp. The procedure was repeated to obtain a catalyst loadingof ca. 400 μgcm� 2 finally. In contrast, the commercial Pt/C (20 wt.%,Johnson Matthey) loading was set to 20 μgPt cm

� 2. The electro-chemical potential in this work was referenced to reversiblehydrogen electrode (RHE). The calibrated ΔE was � 0.308 V (vs. SCE)and � 0.962 V (vs. Ag/AgCl) in Figure S1. The cyclic voltammetry(CV) curves were attained after purging N2 or O2 for at least 30 min.The loaded electrode was subjected to cycling scan between 0.05and 1.2 V (vs. RHE) at 0.1 Vs� 1 until steady voltammograms weregained. If there isn’t denoted particularly in this paper, all potentialsare vs. RHE.

The background capacitive currents were test in N2-saturatedelectrolyte at a scan rate of 10 mV·s� 1 from 0.2 to 1.2 V in rotatingdisk electrode (RDE) tests. Then linear sweep voltammograms (LSV)were tested at an electrode rotation rate of 1600 rpm in O2-saturated electrolyte. The final O2 reduction current was calibratedby the background capacitive current.

The ring potential was controlled at 1.466 V in KOH or 1.150 V inHClO4 to oxidize the H2O2 produced on the disk electrode duringORR in rotating ring-disk electrode (RRDE, PINE AFE7R9GCPT) tests.The hydrogen peroxide yield (%H2O2) and the number of trans-ferred electron (n) were calculated using Equations (1) and (2):

%H2O2 ¼200Ir

N Idj j þ Ir� 100% ð1Þ

n ¼ 4 � 2�%H2O2

100ð2Þ

Herein N represents the collection efficiency, taking value0.37, the Id and Ir are the disk current and ring current.

Koutecky-Levich (K� L) plots were obtained basing on RDEtests at different rotating speeds. The transferred electronnumber (n) of per O2 molecule in the course of ORR can beregulated from the slopes of K� L plots via Equation (3):

jlim ¼ 0:62nFD2=30 n� 1=6C0w1=2 ð3Þ

Jlim is the limiting current density here; F is the Faradayconstant (96485.309 Cmol� 1); D0 is the diffusion coefficient ofO2 (2.20×10� 5 cm2 · s� 1 for 0.1 M KOH solution and 1.93×10� 5 cm2 · s� 1 for 0.1 M HClO4 solution), ν is the kinetic viscosityof both the alkaline and acidic media for (0.01009 cm2 · s� 1);[25] C0

is the bulk concentration of O2 (1.274×10� 3 molcm� 3 for 0.1 MKOH and 1.260×10� 3 molcm� 3 for 0.1 M HClO4) and ω is theelectrode rotation speed (rads� 1).

During the accelerated durability test (ADT), the electrodeswere cycled with a scan speed of 0.1 Vs� 1 between 0.6 and1.0 V for 10000 cycles in O2 saturated KOH or 5000 cycles in O2

saturated HClO4.

Acknowledgements

This work is financially supported by the National Key Researchand Development Program of China (No.2016YFB0101208), theKey Program of National Natural Science Foundations of China(No. 21436003, No. 21306190, No. U1508202).

Conflict of Interest

The authors declare no conflict of interest.

Keywords: fuel cells · materials science · microstructuredcarbon tubes · non-precious metal catalysts · oxygen reductionreaction

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Manuscript received: July 31, 2018Accepted Article published: September 14, 2018Version of record online: October 1, 2018

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