Electrochimica Acta 180 (2015) 155–163

Edge-rich MoS2 Naonosheets Rooting into Polyaniline Nanofibers asEffective Catalyst for Electrochemical Hydrogen Evolution

Nan Zhanga,b, Weiguang Maa,b, Tongshun Wua,*, Haoyu Wanga,*, Dongxue Hana, Li Niua

a State Key Laboratory of Electroanalytical Chemistry, c/o Engineering Laboratory for Modern Analytical Techniques, Changchun Institute of Applied Chemistry,Chinese Academy of Sciences, Changchun 130022, Jilin, ChinabUniversity of the Chinese Academy of Sciences, Beijing 100039, China

A R T I C L E I N F O

Article history:Received 21 May 2015Received in revised form 17 August 2015Accepted 19 August 2015Available online 24 August 2015

Keywords:polyanilineintegrative hybridedge-richMoS2 nanosheetshydrogen evolution reaction

A B S T R A C T

Conductive polymer polyaniline (PANI) with abundant protonated sites which are beneficial to hydrogenevolution reaction (HER), was applied as the support of MoS2 for enhanced HER performance for the firsttime. The novel three dimensional (3D) HER catalyst (MoS2/PANI) was constructed with two dimensional(2D) MoS2 building blocks rooting into the integrative nanowires. PANI nanofibers acted as excellentsubstrates for the uniform, dense and approximate vertical growth of MoS2 nanosheets exposingabundant active edges. Consequently, excellent HER performance has been achieved with a low onsetoverpotential of 100 mV and a small Tafel slope of 45 mV dec�1. Most importantly, it only needed 200 and247 mV overpotential to reach the current density of 30 and 100 mA/cm2 respectively. Additionally,MoS2/PANI has achieved superior stability over other MoS2-polymer-based HER electrocatalyst. Ingeneral, for the first time, employing PANI for the construction of the edge-rich integrative hybrid hassuccessfully achieved an outstanding HER performance.

ã 2015 Elsevier Ltd. All rights reserved.

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1. Introduction

Hydrogen is a promising clean and sustainable energy carrier tosubstitute the traditional fossil fuels, then alleviate the energycrisis and environmental pollution [1–3]. Electrochemical splittingwater is an efficient and clean technology [4]. Although the mostefficient electrocatalysts for hydrogen evolution reaction (HER) arePt-group metals, their high cost and resource scarcity hinder thepractical application [5,6].

The hydrogen binding energy of MoS2 has been identified to beclose to that of Pt-group metals by density functional theorycalculations [7–9], and MoS2 has been confirmed to be the talentedalternative for Pt as active, acid-stable and non-noble HERelectrocatalyst [10,11]. By experimental together with computa-tional studies [12,13] and direct experimental comparison reportedrecently [14], the catalytic activity of MoS2 has been verified tostem mainly from the edges rather than the basal surface. Inconsequence, many investigations with the aim of exposing moreactive edges have been reported [15–18]. Unfortunately, MoS2appears to be stacking due to the unstable thermos-dynamicallyand van der Waals interactions therefore the number of active sites

* Corresponding authors. Tel.: +86-0431-85262533.E-mail address: tswu@ciac.ac.cn (T. Wu).

http://dx.doi.org/10.1016/j.electacta.2015.08.1080013-4686/ã 2015 Elsevier Ltd. All rights reserved.

should be decreased [19,20]. To obstacle the agglomeration anddisperse MoS2 nanosheets for maximum edges exposure, kinds ofsubstrates have been utilized, such as (carbon nanotubes) CNTs[21], graphene [22,23], amorphous carbon [24], carbon cloth[25,26] etc.

Conducting polymers have been widely used as matrix inrelative electrochemical fields [27–33] due to their low cost, easypreparation, large electrochemical surface area as well as goodstability [34,35]. Furthermore, unlike most materials with fixedmorphology such as graphene, CNT and carbon fiber, conductingpolymers have kinds of variable morphologies, which impliesfurther regulation for enhanced performances for differentapplications [36–40]. In this work, we employed polyaniline(PANI) with the morphology of nanowire to construct thehierarchical integrative hybrid with MoS2 (MoS2/PANI). A novelthree dimensional (3D) HER catalyst with MoS2 naonosheetsrooting into PANI has been fabricated. And PANI nanofibers asexcellent substrates have successfully realized the dense, uniformand approximate vertical growth of MoS2 nanosheets with greatexposure of active edges. On the other hand, effective protonadsorption facilitates the overall HER kinetics [41,42]. Herein PANIwas chosen for its abundant protonated sites [43,44] and excellentadsorption ability [45,46]. The presence of numerous protons inclose vicinity of active sites contribute significantly to the superiorHER performance. Finally, a low onset overpotential of 100 mV and

156 N. Zhang et al. / Electrochimica Acta 180 (2015) 155–163

a small Tafel slope of 45 mV dec�1 have been achieved. Mostimportantly, it only needed 200 and 247 mV overpotential to reachthe current density of 30 and 100 mA/cm2 respectively. Addition-ally, excellent stability has been acquired by this 3D HER catalyst.

2. Experimental Section

2.1. Reagent

Commercial Pt/C (20 wt%), (NH4)6Mo7O24�4H2O and Nafion(5 wt%) were purchased from Alfa-Aesar. Aniline was obtainedfrom Xilong Chemicals (Guangdong, China) and distillated beforeuse. HCl, (NH4)2S2O8, H2SO4 and thiourea were purchased fromBeijing Chemicals, China. Unless otherwise stated, reagents were of

Fig. 1. (a) SEM image of MoS2/PANI, inset: high-magnification SEM image of MoS2/PANI;HRTEM image); (d) TEM image of pure MoS2; (e) STEM image of MoS2/PANI and the c

analytical grade and used as received. All aqueous solutions wereprepared with doubly distilled water from a Millipore system(>18 MV cm).

2.2. Synthesis of MoS2/PANI

1.3 g (NH4)6Mo7O24�4H2O and 3.34 g aniline were dissolved in40 mL water, and aqueous HCl (1 M) was dropwise added to adjustpH to 4-5 until white precipitate appeared. After stirring at 50 �Cfor 2 hours, the white product of Mo3O10(C6H8N2)2�2H2O wascollected by centrifugation and dried at 50 �C subsequently. Then0.34 g Mo3O10(C6H8N2)2�2H2O and 0.50 g (NH4)6Mo7O24�4H2Owere mixed with 0.57 g (NH4)2S2O8 in 40 mL water and 1 M HClwas added to adjust pH to about 2. After stirring vigorously for

(b) SEM image of pure MoS2; (c) TEM image of MoS2/PANI (inset: the correspondingorresponding elemental mapping images of C, N, Mo, and S.

N. Zhang et al. / Electrochimica Acta 180 (2015) 155–163 157

6 hours, dark green MoOx/PANI was collected by centrifugation andthen dried at 50 �C. After this, 0.40 g MoOx/PANI nanowires weredispersed in 20 mL water containing 0.30 g thiourea, thentransferred to a 40 mL Teflon-lined stainless-steel autoclave andtreated at 200 �C for 48 h. The product hierarchical MoS2/PANInanowires were collected by centrifugation then thoroughlywashed with water and ethanol several times and finally driedat 50 �C overnight.

Pure MoS2 for contrast experiments was synthesized asfollowing. 0.40 g (NH4)6Mo7O24�4H2O was introduced in 20 mLwater containing 0.50 g thiourea, then transferred to a 40 mLTeflon-lined stainless-steel autoclave and treated at 200 �C for 48 h.Pure MoS2 was collected by centrifugation, thoroughly wash andfinally drying at 50 �C overnight.

Pure PANI for contrast experiments was synthesized asfollowing. 0.3 mL aniline and 0.18 g of (NH4)2S2O8 were added in10 mL aqueous HCl (1 M). After stirring vigorously for 6 hours, purePANI was collected by centrifugation, thoroughly wash and finallydrying at 50 �C overnight.

2.3. Apparatus

Scanning electron microscope (SEM) images were taken on anXL30 ESEM FEG field emission scanning electron microscope.Transmission electron microscopy (TEM) images, high-resolutionTEM (HRTEM), scanning transmission electron microscopy (STEM)and elemental mapping images were all carried out on a Fei Tecnai

Fig. 2. (a) XRD pattern of MoS2/PANI; (b) FTIR spectrum of MoS2/PANI; (c) XPS survdeconvolution.

G2 F20 S-TWIN transmission electron microscope operating at200 kV. X-Ray diffraction pattern (XRD) was obtained with aSiemens D5005 diffractometer with Cu Ka radiation, and wasapplied to investigate the crystallographic structure of theas-fabricated products. X-Ray photoelectron spectroscopy (XPS)analysis was carried out on an ESCALAB MKII X-ray photoelectronspectrometer with Al Ka X-ray radiation as the X-ray source forexcitation. Fourier-transform infrared (FTIR) spectra of KBrpowder-pressed pellets were recorded on a Bruker Vector22 Spectrometer. The average percentage of MoS2 in hybrid wasdetermined by measuring the concentration of Mo using aninductive coupled plasma emission spectrometer (ICP, Perki-nElmer OPTIMA 3300DV). The average percentages of C, H, Nwere measured by the combustion elemental analysis (Vario ELcube, Elementar, Germany).

2.4. Electrochemical Measurements

Catalyst inks for electrochemical testing were respectivelyprepared by adding 5 mg prepared sample and 20 mL Nafionsolution (5 wt%) into 490 mL ethanol and 500 mL water, thentreated by sonication until a homogeneous ink was obtained. Afterthat, 5 mL catalyst ink was deposited onto a well-polished glassycarbon electrode (GCE, 3 mm in diameter) as the working electrodeand dried at room temperature. All the electrochemical measure-ments were performed in electrolyte of 0.5 M H2SO4 with a threeelectrodes system. Apart from the decorated GCE as working

ey spectrum of MoS2/PANI (inset: S 2p spectrum); (d) Mo 3d spectrum and its

158 N. Zhang et al. / Electrochimica Acta 180 (2015) 155–163

electrode, a saturated calomel electrode (SCE) and a carbon rodwere employed as the reference electrode and counter electrode,respectively.

Linear Sweep Voltammetry (LSV) was operated with a scan rateof 5 mV/s. While cyclic voltammetry (CV) was conducted with ascan rate of 100 mV/s in potential ranging from -0.2 V to 0.2 V andperformed from 1000 cycles to 3000 cycles. Amperometric i-tcurve was obtained at a static overpotential of 220 mV for 50000 s.The measurements above mentioned were all conducted by anelectrochemical CHI 920c workstation (CH Instruments, Inc.,Shanghai). All data were iR corrected determined by CHI 920cvia the resistance test.

Electrochemical impedance spectroscopy (EIS) measurementswere carried out with Solartron 1255B Frequency ResponseAnalyzer (Solartron Inc., UK) with frequencies ranging from100 kHz to 0.1 Hz at various overpotentials from 165 mV to 200 mV.

All the experiments were conducted after bubbling nitrogen for10 min at room temperature and without activation progress. Allthe potentials are given with respect to RHE.

3. Results and Discussions

3.1. Characterization of MoS2/PANI

Firstly, Mo3O10 (C6H8N2)2�2H2O integrative hybrid nanowireswere fabricated (Fig. S1y). Then the 1D precursor converted toMoOx/PANI nanowires with a tougher surface and larger diameterafter polymerization (Fig. S2y). Finally, hierarchical MoS2/PANInanowires were achieved by the treatment of thiourea to MoOx/

Fig. 3. (a) Polarization curves of Pt/C, MoS2/PANI, MoS2-PANI, MoS2 and PANI (b) the poPANI, MoS2; (d) Polarization curves of MoS2/PANI with different percentages of MoS2 i

PANI in hydrothermal process. SEM image of MoS2/PANI nanowireswith uniform length and diameter is shown in Fig. 1a. Moreover,the uniform and dense growth of MoS2 nanosheets exhibits anoutstanding edge-rich morphology. Approximately, MoS2 nano-sheets root into PANI with a vertical direction (inset of Fig. 1a),which further increases the active edges exposure as reported[23–26]. Compared with the pure MoS2 nanoclusters foldingthemselves with lots of active edges buried inside (Fig. 1b), moreactive edges exposure has been achieved by PANI as excellentsubstrate to enfold MoS2 nanoclusters against their trend of curlingup. In addition, pure PANI nanofibers synthesized withouthybridization with Mo precursor have much smaller length anddiameter compared with MoS2/PANI (Fig. S3ay). And pure MoS2physically mixed with pure PANI (MoS2-PANI) with the samepercentage of MoS2/PANI maintains their separate nanocluster andnanofiber morphologies (Fig. S3by). Compared with the physicalmix, the chemical synthesis for integrative hybridization of MoS2/PANI is essential for edge-rich MoS2 naonosheets rooting into PANInanofibers. Furthermore, in contrast with pure MoS2 (Fig. 1d),MoS2/PANI (Fig. 1c) is confirmed to have a superior configurationaccumulating thin MoS2 nanosheets into the edge-rich morpholo-gy. It's worth noticing that two dimensional (2D) MoS2 buildingblocks are assembled into the nanowires forming 3D integrativehybrid. The interlayer distance of 0.62 nm between parallel lines inHRTEM image of MoS2/PANI (inset of Fig. 1c) is attributed to the(0 0 2) lattice of MoS2. The elemental mapping images of C, N, Mo, Scorresponding to scanning transmission electron microscopy(STEM) image (Fig. 1e) substantiate that Mo and S elements areuniformly and intensively distributed on each PANI nanofiber.

larization curves in the magnified version; (c) Tafel plots of Pt/C, MoS2/PANI, MoS2-n the hybrid.

N. Zhang et al. / Electrochimica Acta 180 (2015) 155–163 159

To further characterize the identity and structure of MoS2/PANI,XRD was measured. As shown in Fig. 2a, the characteristic peaks at2u = 13.62, 33.10, 57.48� correspond to the (0 0 2), (10 0) and (110)planes of MoS2, which confirms the well-defined and hexagonallysymmetric structured MoS2. Moreover, MoS2/PANI compositionwas confirmed by FTIR spectrum in Fig. 2b. The bands at 1585 and1498 cm�1 are associated with the stretching vibration ofquinonoid (Q) and benzenoid (B) rings, respectively [47,48]. Thepeaks at 1293 and 1220 cm�1 are assigned to nC-N in Q-B-Q and Bunits, and those at 1128 and 815 cm�1 are attributed to dC-H in Qand B rings. To further confirm the chemical states of PANI andMoS2 in MoS2/PANI, XPS was conducted. The peaks at about395 and 285 eV in survey spectrum (Fig. 2c) are related to N 1s andC 1s, which are the characteristic peaks of PANI as reported [49–51]. Peaks in S 2p spectrum (inset of Fig. 2c) at 161.8 and 162.9 eVattributed to S 2p3/2 and S 2p1/2 indicate the domain oxidation stateof S2� [52]. As shown in Mo 3d spectrum (Fig. 2d), peaks at228.9 and 232.1 eV are assigned to Mo 3d5/2 and Mo 3d3/2

respectively [53–56]. Furthermore, according to the deconvolu-tion, Mo 3d spectrum exclusively consists of Mo4+ peaks with theco-existence of 2H and a small portion of 1T phase [57]. Asreported, the presence of 1T phase plays a crucial rule for thecatalytic activity [58,59], therefore the coexistence of 1T phaseprovides a prediction of the better HER performance.

3.2. HER electrocatalytic activities of MoS2/PANI

The HER catalytic activity of MoS2/PANI has been evaluated byLSV measurements. Meanwhile, pure MoS2, pure PANI and MoS2-PANI have also been measured as comparisons. As Fig. 3a shown,the polarization curve of Pt/C exhibits the highest HER catalyticperformance with a near zero onset overpotential as expected,while PANI hardly shows any HER activity. A pretty large onsetoverpotential of pure MoS2 (190 mV) as reported has beenachieved [22,24,54,60]. Compared with the onset overpotentialof pure MoS2 and MoS2-PANI (160 mV), MoS2/PANI shows a muchsmaller one of 100 mV, which is clearly shown by the polarizationcurves in the magnified version (Fig. 3b). Moreover, furthernegative potential leads to rapidly increased cathodic currentdensity. To afford current density of 30 and 100 mA/cm2, MoS2/PANI only needs overpotential of 200 and 247 mV, respectively.

Table 1Comparison of HER performance of MoS2/PANI with other MoS2-based HER electrocata

catalyst Onset h (mV) Tafe(mV

MoO3-MoS2 core-shell nanowires 150-200 50-6MoS2 nanoparticles/mesoporous Graphene 100 42

MoS2 nanosheets/CNTs composite 90 44.6MoS2 nanoflowers/graphene 190 95

layer confined MoS2 naonosheets/graphene �140 41

MoSx nanoparticles/graphene 130 43

MoS2 nanosheets/3D-Graphene hierarchical framework 121 46.3MoS2 nanoparticles/ CNT-graphene hybrid 140 100S-MoS2 nanoplates/N-doped carbon nanofibers 30 38

MoS2 nanoplates within carbon nanofibers – 42

Amorphous MoS2/nanoporous gold 125 41

Edge oriented nanoporous MoS2 films on Mo substrate 150-200 50-7MoS2 nanosheets perpendicular to graphene – 43

Vertically oriented MoS2 nanosheets on carbon cloth 100 50

few layer MoS2 nanoroses/3D graphene 115 60

MoS2 nanoparticles on carbon nanofiber foam 120 44

MoS2 nanosheets on S-doped carbon 60 72

MoS2 nanosheets rooting into PANI 100 45

Much higher catalytic activity of MoS2/PANI has been achieved incontrast with that of MoS2 and other transition metal dichalco-genides (TMDs) [61–63], which can be ascribed to the great activeedges exposure configuration with the participation of PANI. Theonset overpotential of MoS2/PANI as well as the overpotentials toreach the particular current densities are quite low compared withthose of other MoS2-based HER electrocatalysts (MoS2 on variedconductive substrates) as shown in Table 1. It substantiates thatPANI as support can rival high conductive substrates applied inHER, such as graphene [23,56,64,65], CNTs [18,21], carbon nano-fiber [60,66], carbon cloth [26], even metals like nanoporous gold[55] and molybdenum [67]. In addition, the efficient hydrogengeneration is presented by the photograph of Fig. S4y.

The Tafel plots, which are recorded with the linear regions fittedinto the Tafel equation, yielding Tafel slopes of 30 mV dec�1 for Pt/C, 45 mV dec�1 for MoS2/PANI, 82 mV dec�1 for MoS2-PANI and121 mV dec�1 for pure MoS2 as previously reported [22,24,54,60](Fig. 3c). The drastically reduced Tafel slope of the integrativehybrid indicates the significant improvement of the electro-catalytic activity. As reported, Tafel slope value can also imply themechanism of hydrogen reduction on cathodes [68]. In acidsolutions, three principal reaction steps as following, Eq. (1) to (3)are involved with Tafel slopes of 120, 40, 30 mV dec�1 respectively[56,69]. Based on this point, Tafel slope of 45 mV dec�1 obtained byMoS2/PANI corresponds to the Volmer-Heyrovsky mechanism withHeyrovsky reaction as the rate limiting step [68]. Besides, thenumerous protonated sites [43,44] and excellent proton adsorp-tion ability [45,46] of PANI not only indicate the fast formation ofcatalyst-H therefore the rate limiting role of Heyrovsky step, butalso accelerate the Heyrovsky reaction which greatly contributedto the small Tafel slope of MoS2/PANI.

Volmer reaction: H3O+ + e + catalyst ! catalyst-H + H2O (1)

Heyrovsky reaction: H3O+ + e + catalyst-H ! catalyst+ H2+ H2O (2)

Tafel reaction: catalyst-H + catalyst-H ! 2catalyst + H2 (3)

lysts (MoS2 on varied conductive substrates) in acid media.

l slope dec�1)

Current density(j, mA cm�2)

h at the corresponding j (mV) reference

0 20 272 [77]30-40 200 [54]

15-20 200 [21]�30 300 [64]23 200 [22]17 200 [56]

10 220 [20] 10 255 [78]

1070

120300

[60]

80.3 300 [66]5.7 200 [55]

2 18.6 300 [67]10 172 [23]86 250 [26]10 210 [79]16 230 [80]1040

160�230

[81]

30100

200247

This work

160 N. Zhang et al. / Electrochimica Acta 180 (2015) 155–163

Moreover, the effect of the percentage of MoS2 in MoS2/PANI onthe HERperformancewas explored.The percentage of MoS2 in MoS2/PANI was determined to be 79.6% percentage by ICP with the C, H, Npercentage of 15.1%, 1.5%, and 3.4% respectively measured by thecombustion elemental analysis. MoS2 in hybrid with otherpercentages of 62.6%, 72.3% and 82.9% present inferior HERperformances as shown in Fig. 3d, suggesting 79.6% is the optimalpercentage inourexperiment. Increased percentagewithmoreMoS2nanosheets brought more exposed active edges, which can enhancethe HER performance. However, too many MoS2 nanosheets mayhinder the edge exposure therefore draw back the HER activity.

The electrode kinetics of HER catalyzed by the MoS2/PANI wasinvestigated by the electrochemical impedance spectroscopy (EIS).Experimental data were fitted using an equivalent circuit modelshown in Fig. 4a. It consists of a resistor (Rs) in series with twoparallel units of resistor and capacitor [70–72], in which Rs

represents the uncompensated solution resistance while Rct

corresponds to charge transfer resistance arisen from HER. Theconstant unit, R1 and CPE1, is probably related to the contactbetween the GCE and the catalyst [73]. In Nyquist plots shown inFig. 4b and c, empty patterns represent the experiment data andsolid line for fitted curve. As the Nyquist plots in Fig. 4b shown, Rct

of MoS2, MoS2-PANI and MoS2/PANI under 200 mV overpotentialare 421.65, 228.71 and 70.92 V, respectively. Under the protonreduction potential, lower Rct value reflects more efficient electrontransfer at the interface between MoS2/PANI and electrolyte,therefore more efficient hydrogen generation. The effect of PANI ispresented by the lower Rct of MoS2-PANI compared with that ofMoS2. As is well known, PANI acted as protonated emeraldine salts

Fig. 4. (a) Equivalent circuit model used for fitting the Nyquist plots. (b) Nyquist plots of MPANI cathode at various overpotentials from 165 to 200 mV; (d) Dependence of the charge

with numerous protonated sites [43,44]. Moreover, nitrogen siteswith electron donating ability as basic sites [74,75] has excellentadsorption capacity [45,46]. Nitrogen has been involved tofacilitate the proton adsorption for enhanced HER performancerecently [76]. Herein, numerous protonated basic sites and protonseffectively and continuously adsorbed by nitrogen sites in the non-catalytic PANI contributed to the effect electron transfer and theefficient hydrogen generation reaction. Furthermore, the muchlower Rct of MoS2/PANI compared with that of MoS2-PANI and pureMoS2 should result from the hybrid’s morphology exposingabundant active edges which fully contacted with the electrolyteand make the electron transfer between the catalyst andelectrolyte more efficiently. Besides, the Nyquist plots of MoS2/PANI at different overpotentials are shown in Fig. 4b. The Rct are404.48, 323.19, 193.90, 109.64 and 70.92 V corresponding tooverpotential of 165, 170, 180, 190 and 200 mV, respectively. Itindicates that at increased overpotential, more rapid electrontransfer and more efficient hydrogen generation took place, asillustrated by the polarization curve of MoS2/PANI (Fig. 3a).Moreover, the Tafel slope can be derived from the plot ofoverpotantial versus log (Rct

�1) (Fig. 4d, inset) [70–72], anddetermined to be 45.28 mV dec�1 which agrees well with thatobtained from polarization curve. It worth noticing that the Tafelslope obtained this way reflects the electrode kinetics better thanthose obtained from the polarization curve.

Additionally, stability is an essential issue for evaluating anadvanced electrocatalyst considering the practical application.First, we employed CV measurement for this investigation. Fig. 5ashows the polarization curves of MoS2/PANI initial and after 1000,

oS2, MoS2-PANI and MoS2/PANI at 200 mV overpotential; (c) Nyquist plots of MoS2/-transfer resistance (Rct) on the overpotential. Inset: overpotential versus log(Rct

�1).

Fig. 5. (a) Polarization curves for MoS2/PANI initially and after 1000, 2000 and 3000 cycles (inset: the polarization curves in the magnified version); (b) Time-dependence ofcathodic current density curve for MoS2/PANI under 220 mV static overpotential for 50000 s.

N. Zhang et al. / Electrochimica Acta 180 (2015) 155–163 161

2000 and 3000 CV cycles. The slight decays and the graduallyreduced deterioration (inset of Fig. 5a) in current density suggestthe high stability of MoS2/PANI. In addition, the time-dependentcurrent density curve of MoS2/PANI (Fig. 5b) at a cathodic currentdensity of 50 mA cm�2 shows slight degradation during this50000 s. It confirms the strong binding between MoS2 nanosheetsand PANI under a long term bubbles-generation-release condition.Herein, MoS2/PANI exhibits superior stability over the previouslyreported polypyrrole/MoSx [33]. It mainly ascribes to the strongmechanical interaction between MoS2 nanosheets and PANI whichwere assembled together integrally.

4. Conclusions

PANI with numerous protonated sites which are beneficial toHER has been first applied as the support of MoS2 for enhancedHER performance in this work. Novel 3D HER catalyst MoS2/PANI isconstructed with MoS2 nanosheets rooting into the PANI nano-wires integrally. With the participation of PANI as substrate, thehierarchical integrative hybrid has realized the edge-rich mor-phology of uniform, dense and approximate vertical alignment ofMoS2 nanosheets. Consequently, MoS2/PANI has achieved anexcellent HER activity with a low onset overpotential of 100 mVand a small Tafel slope of 45 mV dec�1. It’s worth to note that it onlyneeded 200 and 247 mV overpotential to reach the current densityof 30 and 100 mA/cm2 respectively. The outstanding HER catalyticactivity of MoS2/PANI confirmed the successful application of PANIin HER electrocatalyst, which can rival other high conductivesubstrates like graphene, CNTs, carbon cloth, carbon nanofiber,even metals like gold or molybdenum etc. Furthermore, MoS2/PANIhas achieved superior stability over other MoS2-polymer-basedHER electrocatalyst.

Acknowledgment

The authors are most grateful to the NSFC, China (No. 21205112,No. 21225524 and No. 21475122 and No. 21127006), the Depart-ment of Science and Techniques of Jilin Province (No. 20120308,No. 201215091 and SYHZ0006) and Chinese Academy of Sciences(YZ201354, YZ201355) for their financial support.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.electacta.2015.08.108.

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