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Carbon 139 (2018) 234e240

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Carbon

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Pomegranate-like molybdenum phosphide@phosphorus-dopedcarbon nanospheres coupled with carbon nanotubes for efficienthydrogen evolution reaction

Ji-Sen Li a, b, *, Xiao-Rong Wang a, Jia-Yi Li a, Shuai Zhang a, Jing-Quan Sha a,Guo-Dong Liu a, Bo Tang b, *

a Key Laboratory of Inorganic Chemistry in Universities of Shandong, Department of Chemistry and Chemical Engineering, Jining University, Qufu, Shandong273155, PR Chinab College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging inUniversities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Provincial Key Laboratory of Clean Production ofFine Chemicals, Shandong Normal University, Jinan, Shandong 250014, PR China

a r t i c l e i n f o

Article history:Received 8 May 2018Received in revised form11 June 2018Accepted 25 June 2018Available online 26 June 2018

Keywords:Carbon nanotubesHydrogen evolution reactionMetal-organic frameworksMolybdenum phosphidePorous carbon

* Corresponding authors.E-mail address: tangb@sdnu.edu.cn (B. Tang).

https://doi.org/10.1016/j.carbon.2018.06.0580008-6223/© 2018 Elsevier Ltd. All rights reserved.

a b s t r a c t

Rational design and synthesis of low-cost electrocatalysts with high-performance is of urgency forhydrogen evolution reaction (HER). Herein, we report a facile synthetic approach to construct apomegranate-like MoP@P-doped porous carbon nanospheres coupled with carbon nanotubes (CNTs)composite using metal-organic frameworks-CNTs hybrid as precursor, which was successfully employedas a robust electrocatalyst for the HER. Ascribed to the distinct nanostructure together with the syner-gistic coupling, the resultant composite exhibits highly efficient electrocatalytic performance, featured bya low onset overpotential of 75mV, small Tafel slope of 55.9mV dec�1, as well as long-term stability for10 h, which is comparable or even superior to the best MoP-based catalysts toward HER up to now. Moreimportantly, this strategy can provide a new route in the construction of highly efficient Pt-free elec-trocatalysts for energy conversion fields.

© 2018 Elsevier Ltd. All rights reserved.

1. Introduction

To overcome environmental crisis and energy shortage issues,hydrogen (H2), as clean, sustainable, and renewable energy, hasstimulated extensive research. In terms of large-scale H2 produc-tion, electrochemical water splitting is widely regarded as aneffective approach to produce high-purity H2 [1,2]. Currently, thehighest performing electrocatalysts toward hydrogen evolutionreaction (HER) are platinum-based nanomaterials owing to theirextraordinary catalytic activity. Unfortunately, the low-abundanceand high-cost critically limited their widespread applications[3,4]. As a consequence, seeking and exploring earth-abundanceand inexpensive non-precious metal HER catalysts with high effi-ciency become a hot subject in the catalytic field.

In the past few years, 3d transition metal (TM)-based materialssuch as sulfides [5e9], carbides [10e14], nitrides [15e17], and

phosphide [18e22], have attracted great interest as potential al-ternatives for HER, due to their abundance and favorable activity.Especially, molybdenum phosphide (MoP)-based composites asoutstanding HER electrocatalysts have been a focus of researchowing to their high activity and electrical conductivity [23e29].Regretfully, their electrocatalytic performance is still far fromsatisfactory in comparison with noble metal-based nanomaterials,which is associated with aggregation and surface oxidation of MoPnanoparticles during preparation and long-term storage. To furtherboost their catalytic activity, coupling TM-based catalysts withcarbon materials (graphene, carbon nanotubes (CNTs), etc.) isfrequently identified as a hopeful strategy to increase the conduc-tivity and facilitate the electron transfer [30e36]. Although sub-stantial progress have been achieved in enhancing HER catalyticperformance, rational design and development of high-efficiencycatalysts toward HER is still at its infancy stage.

Metal-organic frameworks (MOFs)-derived functional materialshave received much attention due to their fascinating physical andchemical properties [37e43]. Noteworthily, polyoxometalate-basedMOFs (POMOFs), as a new class of MOFs, can be considered as ideal

J.-S. Li et al. / Carbon 139 (2018) 234e240 235

candidates to synthesize Mo, W, and V-contained materials, whichare almost impossible to implement using sole MOFs as precursors[44]. Despite a considerable development in the field of POMOFs,until now there have been few reports of the exploration of Mo-based electrocatalysts derived from POMOFs for HER [45e48]. Inparticular, to the best of our knowledge, little attention has beenpaid to the investigation of MoP-based materials utilizing POMOFs-CNTs composite as precursor toward HER.

Inspired by the above-mentioned, we herein have ingeniouslyconceived and fabricated pomegranate-like MoP@P-doped porouscarbon nanospheres accompanied with CNTs nanocomposite(denoted as MoP@PC-CNTs) through a POMOFs-assisted method.Impressively, the resultant MoP@PC-CNTs is endowed with threeprominent features: (1) small-sized MoP nanocrystal seeds can beconfined in P-doped carbon shell as peel; (2) porous structures; (3)the introduction of CNTs can not only boost the interfacial contactbetween nanospheres, but offer more sufficient catalytic sites.Benefiting from these merits, the composite exhibits superiorelectrocatalytic properties and prominent long-term stability forthe HER, which can even compete the recently reported MoP-basedelectrocatalysts for hydrogen production.

2. Experimental

2.1. Materials

Copper acetate monohydrate (Cu(OAc)2$H2O), phosphomolyb-dic acid (H3PMo12O40$nH2O), phosphotungstic acid(H3PW12O40$nH2O), polyvinylpyrrolidone (PVP, K30), L-glutamicacid (C5H9NO4), 1,3,5-benzenetricarboxylic acid (H3BTC, 98%), iron(Ⅲ) chloride hexahydrate (FeCl3$6H2O) and diammoniumhydrogen phosphate ((NH4)2HPO4) were purchased from Sino-pharm Chemical Reagent Co. Ltd. Multiwall CNTs (>97%, diameter:60e100 nm, length: 5e15 mm) were obtained from ShenzhenNanotech Port Co., Ltd. Pt/C catalyst (20wt% Pt/C) was purchasedfrom Johnson Matthey. Nafion (5.0wt%) was purchased fromSigma-Aldrich. All chemicals were used without further purifica-tion. The water used in the experiments was ultra-purified water(18.25MU).

2.2. Synthesis of pretreatment of CNTs

2 g of pristine CNTs were refluxed in a 300mL mixture of sul-furic acid and nitric acid (1:3) in a water bath at 80 �C for 3 h withstirring. And then, the resulting sample were collected by filtrationand rinsed with ultra-purified water until the pH value was 7.0,then dried at 80 �C for 12 h.

2.3. Synthesis of NENU-5-CNTs and NENU-5

In a typical procedure, 10mg of pretreated CNTs and 200mg ofPVP were first added in 40mL of ultra-purified water. After soni-cation for 2 h, 0.2 g of Cu(OAc)2$H2O, 0.3 g of H3PMo12O40$nH2Oand 0.07 g of C5H9NO4 were dissolved into form solution A. H3BTC(0.14 g) was dispersed in an equal amount of water, and then addeddropwise into A. After vigorous stirring for 16 h, the product wascleansed with alcohol/water for several times and dried at 70 �C invacuum (defined as NENU-5-CNTs).

For comparison, NENU-5 was synthesized under the samecondition without adding CNTs.

2.4. Synthesis of MoO2@PC-CNTs and MoO2@PC

The NENU-5-CNTs was calcined at 600 �C for 6 h under Ar gasflow; and then, the sample was dispersed in 0.1M FeCl3 solution

and stirred for 6 h to remove metallic Cu. The resulting sample wascollected by filtration, washed with ultra-purified water and driedunder vacuum at 70 �C (termed MoO2@PC-CNTs).

In control experiment, MoO2@PC was gained by the identicalmethod as that for MoO2@PC-CNTs using NENU-5 as precursors.

2.5. Synthesis of MoP@PC-CNTs and MoP@PC

The prepared MoO2@PC-CNTs (100mg) was placed in a tubefurnace, and (NH4)2HPO4 (500mg) was put at the upstream side.Under H2/Ar, the heating temperature was elevated to 850 �C for2 h, and the final product was named as MoP@PC-CNTs. In controlexperiments, MoP@PC-CNTs (5mg) and MoP@PC-CNTs (15mg)were also synthesized by identical condition except that the con-tent of CNTs is 5 and 15mg, respectively.

In addition, the MoP@PC was prepared by a similar approach,but utilizing MoO2@PC as raw materials, respectively.

2.6. Instruments

The surface morphologies of these materials were characterizedby transmission electron microscopy (TEM, JEOL-2100F), scanningelectron microscope (SEM, JSM-7600F). The powder X-Raydiffraction (PXRD) patterns were carried out on a D/max 2500VL/PCdiffractometer equipped with Cu Ka line (l¼ 1.54060 Å). X-rayphoton spectroscopy (XPS) analysis was performed on PHI 5000Verasa using Al ka X-ray source. Nitrogen sorption isotherms weremeasured on a Micromeritics ASAP 2050 system. Surface area wasdetermined according to Barret-Joyner-Halenda (BJH) model.

2.7. Electrochemical study

The electrochemical evaluations were conducted by an elec-trochemistry workstation (CHI 760D). A saturated calomel elec-trode (SCE) is utilized as a reference electrode. A graphite rod isused as a counter electrode. The sample-coated glassy carbonelectrode (GCE, 0.0706 cm2) is adopted as a working electrode. Thesample ink is composed of 4mg sample dispersed in 2mL of ultra-purified water/ethanol solution (3:1 v/v %) and sonicated for 12 h.Subsequently, 5 mL sample ink was dropped on the GCE and dried atambient condition. Finally, the working electrode was achieved bydropping 5 mL Nafion solution (0.1wt%) on the surface of GCE. Thesample loading is ~0.14mg cm�2. The measured potentials wereconverted to a reversible hydrogen electrode (RHE) without IRcorrection. In 0.5MH2SO4 (pH¼ 0), ERHE¼ ESCE þ 0.241 V.

Linear sweep voltammetry (LSV) at a scan rate of 2mV s�1 wasperformed in 0.5MH2SO4 (purge with high-purity N2). Stabilitywas tested by cyclic voltammograms (CVs) at 100mV s�1. Mean-while, the accelerated durability was also characterized by chro-noamperometry (CA) at 10mA cm�2 current density. Theelectrochemical active surface areas of these catalysts weremeasured by using CV within -59 - 41mV with various scan ratesranging from 20 to 200mV s�1.

3. Results and discussion

As exhibited in Fig. 1, a multi-step method was performed tofabricate well-definedMoP@PC-CNTs composite. Firstly, the NENU-5-CNTs achieved by a facile solution method at ambient tempera-ture were carbonized at 600 �C for 6 h, where PMo12 werecompletely converted into MoO2; at the same time, Cu ions werereduced to metallic Cu nanoparticles. In order to remove metallicCu, the resultant sample was subsequently etched with FeCl3 so-lution. Finally, the MoP@PC-CNTs was obtained after phosphida-tion, where (NH4)2HPO4 was used as a phosphorus source.

Fig. 1. Schematic illustration of the fabrication of MoP@PC-CNTs nanocomposite. (Acolour version of this figure can be viewed online.)

J.-S. Li et al. / Carbon 139 (2018) 234e240236

Interestingly, the in situ generated MoP nanocrystals as seeds wereencapsulated into P-doped porous carbon matrix as peel, furtherforming pomegranate-like MoP@PC nanospheres.

The representative scanning electron microscopy (SEM) imagesof NENU-5-CNTs in Fig. S1a, b show that the NENU-5 nanosphereswith a size of about 300 nm and smooth surface were in situ coatedon the CNTs backbone. From Fig. S1c, the transmission electronmicroscopy (TEM) image reveals that the uniformly distributedNENU-5 nanospheres are closely intertwined with CNTs. Simulta-neously, it is clearly observed that CNTs stretch out from the surfaceof a NENU-5 crystal, further stringing other NENU-5 crystalstogether, which is beneficial to improve the connection amongcrystals. As displayed in Fig. S1d, the powder X-ray diffraction(PXRD) spectra of NENU-5-CNTs matches well with the simulatedNENU-5 [46]. While, the broad peak at approximate 26� can beattributable to the (002) diffraction peak of graphitic carbon [31].

Fig. 2a shows the nanospherical morphology of the NENU-5crystal with a shrunken size of about 200 nm is still well pre-served. However, the surfaces became much rougher and rich tinyholes are clearly visible, which may be associated with the libera-tion of H2O and CO2 during heating treatment [49]. Additionally, itshould be noted that theMoP@PC is intimately interlinked by CNTs.As exhibited in Fig. 2b and c, the interpenetration is also confirmedby the clear observation of CNTs penetrating throughout thenanospheres and connecting with the adjacent ones. This uniquestructure would be expected to form a connected conductivenetwork and provide abundant active interfaces and robustconjugation between MoP@PC and CNTs [32,50]. Plentiful ofnanoparticles (dark spots) encapsulated in a carbon shell wereprobed. However, it was found that a small part of MoP nano-particles is outside the carbon shell after carbonization and phos-phidation processes. This may be attributed to the followingreason: in the synthetic process, a very small amount ofH3PMo12O40 may be adsorbed on the surface of CNTs, which werenot encapsulated by NENU crystals. The high-resolution (HR) TEMin the inset of Fig. 2b demonstrates well-defined lattice fringes of0.21 and 0.34 nm, which match with the (101) facet of hexagonalMoP and (002) plane of graphitic carbon, respectively, reconfirmingthat these dark nanoparticles should be MoP nanocrystals. Theelemental mapping images in Fig. 2deg shows the existence of C,Mo, and P elements in the MoP@PC-CNTs composite and the CNTsbackbone is decorated with MoP@PC. Also, it is found that theelemental distribution profiles of P and Mo are very similar,implying that MoP nanoparticles was successfully fabricated andpartial P atoms were also doped into carbon matrix. Therefore, the

above results unambiguously prove that a pomegranate-likenanospherical morhphology with MoP nanocrystals as seeds andP-doped carbon layers as shell was formed, and coated on the CNTsbackbone. From Fig. 2h, the PXRD shows the characteristic peaks at27.91, 32.03, 43.01, 57.19, 64.74, 67.54, and 74.08�, indexed to (001),(100), (101), (110), (111), (102), and (201) planes of hexagonal MoP(JCPDS, No. 65e6487), respectively. No other peaks are detectedexcept for the weak (002) diffraction peak of graphitic carbonlocated at 26�, which may stem from the loading of MoP with highdensity. The surface area of MoP@PC-CNTs is 355m2 g�1 calculatedby Brunauer-Emmett-Teller analysis, which favors the exposure ofmore active sites. The corresponding pore size distribution ismainly centered at 3.7 nm (Fig. 2i), which can provide effectivechannels to promote mass transfer and numerous accessible cata-lytic sites toward HER [23,24,27].

As a comparison, MoO2@PC-CNTs and MoP@PC were achievedand characterized in detail. As seen from Fig. S2, the morphology ofMoO2@PC-CNTs is similar to that of MoP@PC-CNTs; whereas theMoP@PC hybrid exhibits an octahedral or slightly truncated octa-hedral morphology. In this regard, it can verify that the existence ofCNTs has a significant influence on the morphologies of the resul-tant samples, further affecting their electrocatalytic properties forHER. In Fig. S3, the PXRD pattern also validates the successfulsynthesis of MoO2@PC-CNTs and MoP@PC.

X-ray photoelectron spectroscopy (XPS) was further employedto identify the chemical state and composition in the MoP@PC-CNTs hybrid. From Fig. 3a, the survey spectrum demonstrates theco-existence of C, P, Mo, and O elements. As observed from Fig. 3b,the high-resolution C1s XPS can be fitted to four peaks at 284.7,285.2, 286.5 and 290.4 eV, assignable to C-C/C¼C, C-P, C-O, and O-C¼O, respectively [45]. In terms of P 2p (Fig. 3c), the binding en-ergies are related to 129.6 eV for P 2p3/2 and 130.5 eV for P 2p1/2,signifying the formation of MoP [23,27]. Additionally, the rest oftwo peaks are presented at 132.8 eV accompanied with P-C and133.9 eV, assigned to P-O, respectively [10]. Fig. 3d shows the high-resolution scan Mo 3d electrons, which could be fitted into threedoublets. Among them, the doublet (228.4/231.5 eV) can beassigned to MoP species in MoP@PC-CNTs, which are regarded asthe active sites for HER [24,28]. The additional two doublets locatedat 229.2/232.2 eV (Mo4þ) and 232.25/236.1 eV (Mo6þ) are ascribedto MoO2 and MoO3, which is related to the high surface energy ofMoP, as previously reported [10,23,48]. For comparison, MoO2@PC-CNTs and MoP@PC were also investigated by XPS, demonstrated inFig. S4-5, respectively. It can be seen that these results are similar tothose for MoP@PC-CNTs. The corresponding atomic percent ofvarious catalysts are described in Table S1.

To assess the electrocatalytic performance of these materialstowards the HER, a standard three-electrode system was carriedout in 0.5MH2SO4 solution. As a comparison, CNTs, MoO2@PC-CNTs, MoP@PC, and 20% Pt-C were also examined. Fig. 4a displaysthe polarization curves of different samples without IR compen-sation. As observed, commercial 20% Pt-C shows outstanding HERactivity; however, CNTs and MoO2@PC-CNTs indicate relativelypoor catalytic performance for the HER. In sharp contrast, MoP@PC-CNTs possesses a low onset potential (�75mV) as compared to thatof MoP@PC (�104mV). Moreover, the current density rises rapidlyas applied potential becomes more negative. Impressively, theMoP@PC-CNTs requires an overpotential of approximate 220mV toderive the current density of 10mA cm�2, which outperforms thatof MoP@PC (282mV).

To further understand the HER mechanism, the Tafel slopes ofas-prepared electrocatalysts were analyzed by extrapolating thecorresponding polarization curves based on Tafel equation (h¼ blog (j) þ a, where b is the Tafel slope), demonstrated in Fig. 4b.Strikingly, the MoP@PC-CNTs has the smallest Tafel slope of

Fig. 2. Morphological and structural characteristics of MoP@PC-CNTs. (a) SEM, (b, c) TEM images. Inset of (b): HRTEM image. (deg) C, Mo, P, and overlap elemental mapping images.Scale bar: 50 nm. (h) PXRD pattern and (i) N2 sorption isotherm. Inset of (i): pore size distribution. (A colour version of this figure can be viewed online.)

J.-S. Li et al. / Carbon 139 (2018) 234e240 237

55.9mV dec�1 than those of MoP@PC (61.1mV dec�1), MoO2@PC-CNTs (260.7mV dec�1), and CNTs (238.2mV dec�1); but it is stillinferior to that of commercial 20% Pt-C (30mV dec�1) [27,45,46].This results imply the favorable HER kinetics via a Volmer-Heyrovsky mechanism with the rate-limiting step of the electro-chemical desorption [31,45]. It is worth mentioning that the valueof Tafel slope forMoP@PC-CNTs is comparable to or obviously lowerthan those of recently reported MoP-based nanomaterials for HER(Table S2) [23,24,27,28]. As per our best knowledge, such a smallTafel slope accompanying with a remarkably low onset over-potential has rarely been reported for MoP-based electrocatalysts,strongly supporting the prominent HER activity of MoP@PC-CNTs.

Cyclic voltammetry (CV) was adopted to further determine theeffective surface area, which is expected to be proportional to thecapacitance of double-layer (Cdl). As shown in Fig. 4c, the measured

Cdl value of MoP@PC-CNTs is about 13.7mF cm�2, which is over 2.7times, 10.4 times, and 12.1 times larger than those of MoP@PC(5.03mF cm�2), CNTs (1.31mF cm�2), and MoO2@PC-CNTs(1.13mF cm�2), respectively (Fig. S6). This illustrates that the con-ductivity is improved and abundant electrochemical active sites aregenerated after the introduction of CNTs and phosphorization,which are beneficial to boosting the HER performance.

Stability is a critical parameter for the practical application ofelectrocatalysts for HER. From Fig. 4d, the current-time chro-noamperometric response of MoP@PC-CNTs presents a muchslower decay rate and 90.1% of the initial current could be stillmaintained for at least 10 h. Meanwhile, continuous CV sweepswere tested from �200 to 200mV at a scan rate of 100mV s�1. Theinset of Fig. 4d exhibits negligible change after 1000 cycles. All theresults highlight that the as-synthesized MoP@PC-CNTs composite

Fig. 3. XPS of MoP@PC-CNTs: (a) survey spectrum, (b) C 1s, (c) P 2p, and (d) Mo 3d spectrum. (A colour version of this figure can be viewed online.)

Fig. 4. HER performance. (a) LSV curves of different catalysts. (b) Corresponding Tafel plots. (c) Plot of capacitive current at �9mV against the scan rate. Inset: CV curves ofMoP@PC-CNTs under various rates from 20 to 200mV s�1. (d) Time-dependent durability by chronoamperometric response for MoP@PC-CNTs at 10mA cm�2. Inset: Polarizationplots initially and after 1000 CV cycles. (A colour version of this figure can be viewed online.)

J.-S. Li et al. / Carbon 139 (2018) 234e240238

exhibits not only high-efficiency HER activity but also good dura-bility in acidic solution.

In consideration of the mentioned results, the excellent elec-trochemical performance of MoP@PC-CNTs for the HER can beassigned to the special constitutions and structural superiority. (i)

As is well known, MoP nanoparticles possess excellent electro-catalytic activity for HER due to its Pt-like electronic structure[28,29]. Equally important, the dopant of P could regulate theelectronic structure of carbon, which contribute to the improve-ment of catalytic performance for HER [10,45]. (ii) The

Fig. 5. (a) Polarization curves for MoP@PC-CNTs with different loadings of CNTs (5, 10, and 15mg). (b) Tafel plots of the corresponding polarization curves. (A colour version of thisfigure can be viewed online.)

J.-S. Li et al. / Carbon 139 (2018) 234e240 239

pomegranate-like structure affords accessible and rich active sites[14]. Likewise, the carbon layer can provide powerful protection forhighly active MoP nanoparticles; the mesopores are beneficial tothe significant mass transfer and the effective exposure of catalyticsites toward the HER [23,24,48]. (iii) The interconnected networkformed by outstretched CNTs enhances the electron transportationand provides effective long-range conductivity. Just as importantly,the functional integration of P-doped carbon nanospheres andCNTs can reduce electrochemical impedance and improve the long-term stability [31,32,50].

To examine the effect of CNTs content on the HER activity,MoP@PC-CNTs with different loadings of CNTs (5 and 15mg) wereas-synthesized through the same method. From the aspect of theonset overpotential and Tafel slope toward the HER (Fig. 5), theMoP@PC-CNTs (10mg) composite manifests the optimum combi-nation of MoP@PC and CNTs, which can be further proven by theSEM and TEM images (Fig. S7). Namely, due to the lower amount ofCNTs in MoP@PC-CNTs (5mg), the corresponding electrical con-ductivity was inferior to that of MoP@PC-CNTs (10mg). Whereas, inthe case of MoP@PC-CNTs (15mg), the content of MoP as the cat-alytic sites was dramatically decreased with the increase of theamount of CNTs. Consequently, the result reveals that, although thecontent of CNTs has significant influence on the electrocatalyticactivity toward the HER, they are not the only determinant forenhanced catalytic performance. In other words, it can be reason-ably presumed that a perfect balance between conductivity andactive sites is desired for achieving the optimal electrochemicalperformance for the HER.

4. Conclusions

In summary, utilizing POMOFs-CNTs composite as precursor, thepomegranate-like MoP@PC coupled with carbon nanotubes hybridhas been synthesized through a facile carbonization followed byphosphidation process for the first time. Amazedly, owing to thepore confinement effect of MOFs, MoP nanoparticles can be effec-tively encapsulated in the porous carbon matrix as seeds. As well,the carbon shell stemming from the organic ligands as peel canboth protect from surface oxidation and agglomeration of MoPnanoparticles and function as an electron highway. Moreover, theincorporation of CNTs contributes to the enhancement of long-range conductivity. Taking advantage of the rationally designedcomponents and unique nanostructural advantage, the compositeexhibits remarkably enhanced catalytic activity and excellent sta-bility toward the HER. As a result, this work can provide moreopportunities to develop competitive alternatives to Pt-basednanomaterials for various applications such as hydrogen evolu-tion, oxygen evolution, and battery applications.

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (No. 21535004, 21390411, and91753111), the China Postdoctoral Science Foundation (No.2016M602178), the Natural Science Foundation of ShandongProvince (No. ZR2014BQ037), the Project of Shandong ProvinceHigher Educational Science and Technology Program (No. J16LC02),the Foundation of Basic Discipline Construction and TheoreticalResearch Center of Jining University (No. 2017JCXK005), and theTalent Team Culturing Plan for Leading Disciplines of University inShandong Province.

Appendix A. Supplementary data

Supplementary data related to this article can be found athttps://doi.org/10.1016/j.carbon.2018.06.058.

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