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carbon nanotube film decorated with NiSe nanoparticles for electrocatalytic hydrogen evolution reactions
Electrochimica Acta 243 (2017) 291–298
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Reduced graphene oxide-polyimide/carbon nanotube film decorated with NiSe nanoparticles for electrocatalytic hydrogen evolution reactions Tingxia Wanga , Xin Lia , Yimin Jiangb , Yaxin Zhoua , Lingpu Jiaa , Chunming Wanga,* a State Key Laboratory of Applied Organic Chemistry, , Lanzhou University, Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization, Gansu Province, Lanzhou University and College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China b College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China
A R T I C L E I N F O
Article history: Received 1 March 2017 Received in revised form 21 April 2017 Accepted 14 May 2017 Available online 15 May 2017 Keywords: Hydrogen evolution reaction Green electrodeposition method NiSe nanoparticle Polyimide/carbon nanotube film Reduced graphene oxide
A B S T R A C T
An efficient electrocatalyst based earth-abundant materials for hydrogen evolution reaction (HER) is a prerequisite for improving the efficiency of producing hydrogen. However, the synthesis of such electrocatalysts by a simple and environmentally friendly method is still a tremendous challenge. Herein, NiSe nanoparticles were in situ deposited on the reduced graphene oxide-polyimide/carbon nanotube (RGO-PI/CNT) film by a green electrodeposition method. The NiSe nanoparticles on the RGO-PI/CNT (NiSe-RGO-PI/CNT) film was employed directly as a highly active HER electrocatalyst. The RGO-PI/CNT film allowed NiSe nanoparticles to grow rapidly on its surface. NiSe nanoparticles with a small size of 35– 45 nm were distributed uniformly on the surface of the RGO-PI/CNT film. The good dispersity of NiSe nanoparticles permitted the exposure of more active sites, which enhanced the activity of HER on the NiSe-RGO-PI/CNT film. The NiSe-RGO-PI/CNT film performed well HER behaviors featured by a low overpotential (270 mV), a small Tafel slope (61 mV dec1) and a considerable stability. The efficient HER performance of the NiSe-RGO-PI/CNT film was originated from the synergistic effect between the RGOPI/CNT film and NiSe nanoparticles. This exploration provided a straightforward and green method to prepare a film electrocatalyst for HER. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction The development of sustainable and environmentally friendly energy is an inevitable challenge in the contemporary world because of dramatic depletion of fossil fuels and its deleterious impacts on the environment and human health [1]. Currently, hydrogen is considered as an ideal candidate to replace fossil fuels [2–4]. The electrocatalytic HER by water splitting is a feasible approach of producing hydrogen on a large scale and a remarkable electrocatalyst should be employed for HER to obtain a high current density at a minimal overpotential [5]. As is well-know, Pt and Pt-alloys served as the most active catalysts for HER in acidic media, whereas the widespread application has been impede greatly by their high cost and scarcity. Therefore, the exploration of efficient, acid-stable, low-cost and earth-abundant catalysts for HER remains an ongoing and important topic [6,7].
* Corresponding author. E-mail address: [email protected] (C. Wang). http://dx.doi.org/10.1016/j.electacta.2017.05.084 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
Recently, transition metal chalcogenides [8,9], carbides [10,11], borides [12], phosphides [13,14] and heteroatom-doped nanocarbons [15] have been applied successfully for HER to substitute noble metals. Among these alternatives, Ni-based chalcogenides have been utilized intensively in water splitting due to their good electrocatalytic properties and extremely cheap cost [16,17]. Selenium (Se) owns more obviously metallic property than sulfur (S), which indicates the better electrical conductivity of Se. The intrinsically electrical conductivity of Se enhances electrocatalytic performance of transition metal selenides [6]. The Ni and Se can form different phases such as NiSe, NiSe2 and Ni3Se2 [18,19], which have been applied in water splitting [20–22]. Particularly, NiSe and doped NiSe have been proven to be efficient electrocatalysts for HER and OER [23,24]. Although Ni-based selenides were active for HER, the HER performances were not perfect enough owing to their poor conductivity [21]. Moreover, when stacked with each other, the electrocatalysts have poor catalytic activities arisen from a low density of exposed active sites [25]. The RGO has been proposed as a conductive support to intensify the conductivity and dispersity of electrocatalysts [26]. For example, Dai and his co-
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workers have synthesized MoS2 nanoparticles grown on RGO as a highly efficient electrocatalyst for HER [8]. Hu and his groups have obtained superior HER performances by preparing WSe2/Co0.85Se/ RGO hybrids materials [27]. These findings have indicated that RGO can enhance exchange current density and the overall conductivity of catalysts by modulating electron density and the distribution of electronic potential in the hybrid materials. The synthesis of NiSe and doped NiSe has been reported by using a hydrothermal method [5,23]. However, few reports have presented about preparing NiSe by an electrodeposition method. Sun and his groups have demonstrated that NiSe2 electrodeposited on the Ti foam can catalyze efficiently the process of water splitting [35]. The electrodeposition method is time-saving and frees from the toxic H2Se gas compared with the hydrothermal method [29]. In this work, NiSe-RGO-PI/CNT film was prepared by a green electrodeposition method. The PI/CNT film was employed as the growing membrane of NiSe nanoparticles because this film possessed superior properties such as excellent electrical conductivity, good resistances to strong acidic and alkaline solutions, outstanding thermal stability and so on [28]. To further improve the conductivity and dispersity of NiSe nanoparticles, RGO has been fabricated on the PI/CNT film. The prepared NiSe-RGO-PI/CNT film was used directly as HER electrocatalyst without further treatments, which exhibited well HER catalytic activity. This work is expected to open up new possibilities in exploring electrocatalysts in the form of film by using the RGO-PI/CNT film as substrate. 2. Experimental sections
were prepared with deionized water (DI water, 18 MV cm1), which was obtained from a Milli-Q water purification system. 2.2. Preparation of the NiSe-RGO-PI/CNT film All electrochemical experiments were performed at room temperature with a CHI 660 electrochemical workstation (CH Instrument, USA) using a conventional three electrode system consisted of a RGO-PI/CNT film as the working electrode, a Pt wire counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The PI-CNT film was prepared according to a method described in the previous work [28]. (Graphene oxide) GO was synthesized from natural graphite powder through a modified Hummers method [30] and 10 mL GO aqueous solution (1 mg mL1) was added on the surface of a PI/CNT film. Subsequently, GO aqueous solution was dried at room temperature. The RGO-PI/CNT film was obtained by electrochemical reduction of GO in 0.1 M Na2SO4 aqueous solution at an applied potential of 1.25 V vs. RHE. The NiSe nanoparticles was electrodeposited cathodically on the RGO-PI/CNT film at a deposition potential of 0.25 V vs. RHE for different time (30, 45, 60, 90, 120 and 300 s) in a solution of 0.1 M Na2SO4, 5.0 mM NiSO4 and 10 mM SeO2 aqueous solution. The electrodeposition process of NiSe nanoparticles was indicated in Scheme 1. In detail, at the initial stage, SeO2 was hydrolyzed to H2SeO3, which was further reduced to Se. The Se continued to reduce to Se2 [29]. Subsequently, the Ni2+ in the electrolyte moved rapidly to the surface of the working electrode, in which the Ni2+ bonded with Se2 to form the NiSe. The corresponding reactions as follows: SeO2 þ H2 O ! H2 SeO3
ð1Þ
H2 SeO3 þ 4Hþ þ 4e ! SeðsÞ þ 3H2 O
ð2Þ
SeðsÞ þ 2e ! Se2
ð3Þ
2.1. Reagents Nickel sulfate hexahydrate (NiSO46 H2O) was purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Selenium dioxide (SeO2) was obtained from Aladdin Ltd. (Shanghai, China). Sulfuric acid (H2SO4) was obtained from Chengdu Kelong Chemical Reagent Factory (Chengdu, China). Sodium sulfate anhydrous (Na2SO4) were purchased from Tianjin Chemical Co. Ltd. (Tianjin, China). All reagents used in this study were of analytical grade and without further purifications. All aqueous solutions
Ni
2þ
þ Se2 ! NiSeðsÞ
Scheme 1. The illustration for electrodeposition process of the NiSe-RGO-PI/CNT film.
ð4Þ
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Then, the prepared NiSe-RGO-PI/CNT film was used directly as HER catalysts in acidic solution. 2.3. Structural Characterizations The morphology of the as-prepared products was characterized by Scanning electron microscopy (SEM Hitachi S-4800, Japan) with an accelerating voltage of 5 kV and Transmission electron microscopy (TEM TecnaiTM G2 F30, FEI, USA) operated at 300 kV. The chemical composition of the products was measured by energy dispersive spectrum (EDS) recorded on an EDAX-4800. The crystal structure of the samples was characterized by X-ray powder diffraction (XRD RigakuD/max-2400) equipped with Cu Ka radiation (l = 0.154178 nm) in the 2theta range from 20 to 65 (Scan step size = 0.03 , Time per step = 0.2 s). The X-Ray photoelectron spectroscopy (XPS) data was collected by X-Ray photoelectron spectroscopy (XPS AXIS ULTRA DLD).
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2.4. Electrochemical Characterizations Electrochemical measurements were implemented at room temperature with a CHI 660 electrochemical workstation in 0.5 M H2SO4 solution. A traditional three electrodes system was used, including a NiSe-RGO-PI/CNT film as the working electrode, a Pt wire as the counter electrode and a SCE (with saturated KCl aqueous solution) as the reference electrode. The geometric area of the working electrode is 0.1256 cm2. All the potential in this work was converted totally to a reversible hydrogen electrode (RHE) according to the Nernst equation in 0.5 M H2SO4. The Nernst equation as follows: EðRHEÞ ¼ EðSCEÞ þ 0:059pH þ 0:251
ð5Þ
0.251 is standard potential value of SCE at 25 C (The standard potential value of SCE depends on the concentration of KCl and experimental temperature.). Linear sweep voltammetry (LSV) was
Fig. 1. (a) XRD pattern and (b) EDS spectrum of the NiSe-RGO-PI/CNT film..
Fig. 2. XPS of (a) the NiSe-RGO-PI/CNT film survey spectrum, (b) C spectrum, (c) Ni spectrum and (d) Se spectrum.
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used to evaluate electrocatalytic behaviors of the NiSe-RGO-PI/CNT film in 0.5 M H2SO4 solution with a potential scanning from 0 to 0.5 V vs. RHE and scan rate of 5 mV s1. 3. Results and discussions 3.1. Structure and morphology The crystal structure of the NiSe-RGO-PI/CNT film was characterized by XRD. As shown in Fig. 1a, the XRD pattern of the NiSe-RGO-PI/CNT film revealed coexistence of NiSe nanoparticles and RGO nanosheets in this sample. Three obviously typical peaks located at 33.15 , 44.69 and 61.70 can be identified as the (101), (102) and (201) planes of the NiSe nanoparticles. All the diffraction peaks matched with the JPCDS 02-0892. The (101) plane was further confirmed by the finding of HRTEM (Fig. 3c). The peaks at 44.69 and 61.70 were agreement with the result of SAED (Fig. 3d). Besides, the broad diffraction peak at 25.48 was assigned to be the (002) plane of RGO nanosheets [31]. Fig. 1b shows the EDS spectrum of the NiSe-RGO-PI/CNT film, which reveals the distinct peaks from Ni, Se, O and C and suggests the presences of Ni, Se, O and C in the NiSe-RGO-PI/CNT film. The chemical composition and surface electronic states of the as-synthesized NiSe-RGO-PI/CNT film were analyzed by XPS. The typical survey of XPS spectrums (Fig. 2a) reveals that the sample is consisted of Ni, Se, C and O, which is consistent with the result of EDS. Fig. 2b–d show XPS spectra of the C 1s, Ni 2p and Se 3d regions, respectively. As depicted in Fig. 2b, the binding energy of C
1s states was located at 284.5 eV belonging to the value of C-C (sp2) group [32]. The binding energies of the Ni 2p1/2 and Ni 2p3/2 states were confirmed at 873.9 and 856.5 eV in Fig. 2c, indicating the Ni (II) species. In addition, the two peaks at 877.73 and 860.1 eV were assigned to the shakeup satellites of Ni (II) [33]. The Se 3d XPS spectrum (Fig. 2d) shows two peaks for Se 3d3/2 at 55.2 eV and 3d5/ 2 at 54.5 eV, suggesting divalent of Se [34]. The broad peak near 59.7 eV in Fig. 2d indicated the surface oxidation state of Se species due to exposure to atmosphere [35]. The morphology and structure of NiSe nanoparticles was observed by TEM. Fig. 3a shows TEM image of the NiSe-RGO-PI/ CNT film with a low magnification, which illustrates that dense NiSe nanoparticles are deposited uniformly on the RGO-PI/CNT film. Fig. 3b shows clearly that the diameter of NiSe nanoparticles is mainly 35–45 nm. Further insight into the structure of NiSe nanoparticles was performed by using high resolution TEM (HRTEM) and selected-area electron diffraction (SAED). The HRTEM image of a single NiSe nanoparticles (Fig. 3c) demonstrates a clear lattice fringe with the inter-planar distance of 0.27 nm, corresponding to the (101) plane of NiSe nanoparticles. The SAED image (Fig. 3d) discloses three distinct diffraction rings, which is indexed to the (100), (102) and (201) planes of NiSe nanoparticles and suggests the poly-crystalline structure of the as-prepared samples. The surface morphology of electrocatalysts affects directly the number of exposed active sites, which is greatly significant for HER. The RGO-PI/CNT film was used as a good substrate for the growth of NiSe nanoparticles. The electrocatalytic activity of the NiSe-RGO-
Fig. 3. (a,b) TEM images, (c) HRTEM image and (d) SAED pattern of the NiSe-RGO-PI/CNT film.
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Fig. 4. SEM images of NiSe-RGO-PI/CNT film with different electrodeposition time: (a) 30, (b) 45, (c) 60, (d) 90, (e) 120 and (f) 300 s.
PI/CNT film was related to the structure of NiSe, thus the research on the morphology evolution of NiSe was indispensable. SEM was used to observe the growing process of the NiSe-RGO-PI/CNT film. Fig. 4 shows the SEM images of the NiSe-RGO-PI/CNT film at different stages, which indicates that the morphologies of asprepared products are all nanoparticles. At the early stage for 30 s, a small quantity of NiSe nanoparticles with a small size of 20– 30 nm were deposited on the RGO-PI/CNT film and the drapes of RGO nanosheets were apparent (Fig. 4a). When the electrodeposition time was increased to 45 s, more nanoparticles with the diameter of 30–35 nm were formed on the RGO-PI/CNT film (Fig. 4b). After 60 s, a considerable number of NiSe nanoparticles with a size of 35–45 nm appeared densely on the surface of the RGO-PI/CNT film (Fig. 4c). With the electrodeposition time was changed from 30 to 60 s (Fig. 4a–c), the size of NiSe nanoparticles increased slightly. However, the quantity of NiSe nanoparticles were increased dramatically, which would enhance the HER performance of the NiSe-RGO-PI/CNT film. When the reaction continued to 90 s (Fig. 4d), NiSe nanoparticles tended to gather with each other. Further prolonging time to 120 and 300 s (Fig. 4e and f), NiSe nanoparticles almost agglomerated completely. As the electrodeposition proceeding from 60 to 300 s, NiSe nanoparticles connected with each other, decreasing the quantity of exposed
active sites. In order to explore the role of RGO in this electrodeposition process, the morphologies of the RGO-PI/CNT film and NiSe nanoparticles on the PI/CNT (NiSe-PI/CNT) film was also assessed. As shown in Fig. S1 a, the morphology of bare RGOPI/CNT film was nanosheets with obvious drapes. The SEM image of the NiSe-PI/CNT film was shown in Fig. S1 b, which revealed that the morphology of the NiSe on the PI/CNT film were irregular because of the agglomeration of small NiSe nanoparticles. The growing mechanism of NiSe nanoparticles was that the RGO-PI/ CNT or PI/CNT film provided the nucleation sites for the subsequent growth of NiSe nanoparticles on its surface. The RGO-PI/CNT film was decorated with more dispersive NiSe nanoparticles compared with the PI/CNT film, which indicated that RGO can reduce efficiently the agglomeration and improve the dispersity of NiSe nanoparticles. 3.2. HER electrocatalytic properties of the NiSe-RGO-PI/CNT film Electrochemical measurements were performed at room temperature using a three electrodes system. The RGO-PI/CNT film was loaded with below 0.04 mg cm2 NiSe nanoparticles. The polarization curves of the NiSe-RGO-PI/CNT film with different electrodeposition time from 30 to 120 s were shown in Fig. 5a.
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Fig. 5. Polarization curves of (a) the NiSe-RGO-PI/CNT film with different electrodeposition time and (b) the PI/CNT, RGO-PI/CNT, NiSe-PI/CNT, and NiSe-RGO-PI/CNT film in 0.5 M H2SO4 with scan rates of 5 mV s1.
Table 1 Comparison of catalytic parameters of different HER catalysts in 0.5 M H2SO4. Materials
Substrate
Mass loading (mg cm2)
h5 (mV)
h10 (mV)
Tafel slope (mV dec1)
Ref.
RGO-MoSSe Co-WSe2/rGO PhLi MoSe2 MoSe2 CoSe2 NiSe NiSe
GCD GCE GCE rGO-PI CF GCE RGO-PI/CNT
0.198 0.57 – – – 0.28 <0.04
135 190 330 540 127 252 240
158 217 351 – 141 272 270
53 64 54 82 68 64 61
[36] [27] [37] [38] [39] [23] This work
h5: Overpotential of the catalysts at the current density of 5 mA cm2. GCD: glassy carbon disk. GCE: glassy carbon electrode. CF: carbon fiber. rGO-PI: reduced graphene oxide/polyimide.
According to the polarization curves, electrodeposition time was increased from 30 to 60 s, causing a rapid rise of the cathodic current density. The enhanced HER performances were attributed to a sharp increase in the quantity of exposed active sites and small-size NiSe nanoparticles. However, the current density decreased gradually when the electrodeposition process continued. The reduction was caused by the formation of the aggregated NiSe on the RGO-PI/CNT film. It should be noted that the NiSe nanoparticles with electrodeposition time 60 s on the RGO-PI/CNT film exhibited more apparent HER electrocatalytic performance. In principle, controlling the duration of electrodeposition could fabricate NiSe nanoparticles with different morphology on the RGO-PI/CNT film. The electrocatalytic activity of NiSe-RGO-PI/CNT film was influenced by the number and reactivity of active sites. The greatly dense distribution and small size of the NiSe nanoparticles provided more active sites, which enhanced electrocatalytic activity of the NiSe-RGO-PI/CNT film. Moreover, the HER performance of NiSe nanoparticles at different electrodeposition time were consistent with the number of exposed active sites at the corresponding stages. For comparison, the electrochemical HER performances of the PI/CNT film, RGO-PI/CNT film and NiSePI/CNT film were also tested in the same conditions. The bare PI/ CNT film, and the RGO-PI/CNT film showed negligible or poor HER catalytic activity in the measurement voltage range, which indicated that efficient HER performance was originated from the NiSe nanoparticles, in spite the RGO-PI/CNT film performed a cathodic current density of 1.67 mA cm2 at 0.50 mV. The NiSePI/CNT showed a poor HER performance, which was arisen from a small number of NiSe with a low density of active sites. In strong contrast, the NiSe-RGO-PI/CNT exhibited apparently electrocatalytic activity with a current density of 47.60 mA cm2 at 0.50 mV. Therefore, the high HER performance of the NiSe-RGO-PI/CNT film was attributed to the coupling effect of the RGO-PI/CNT film and
NiSe nanoparticles. Taken together, the highly electrocatalytic activity of NiSe-RGO-PI/CNT film was rationalized as follows: (a) The conductive RGO-PI/CNT film allowed the intimate growth of small-size NiSe nanoparticles on its surface. (b) The RGO improved the dispersity and electrical conductivity of the NiSe nanoparticles and prevented the agglomeration of NiSe nanoparticles on RGO-PI/ CNT film. (c) Although the mass loading of NiSe nanoparticles was small, the high density of small size NiSe nanoparticles provided a great quantity of exposed active sites, which enhanced the HER performance. In order to further assess the HER electrocatalytic activity of the NiSe-RGO-PI/CNT film, the HER performance of other transition metal selenides were compared in Table 1.
Fig. 6. The Tafel slope of the NiSe-RGO-PI/CNT film.
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Fig. 7. (a) Chronoamperometric responses (i-t) at a constant applied potential of 0.35 V and (b) polarization curves of NiSe-RGO-PI/CNT film before and after 500 cycles.
3.3. The mechanism of HER on the NiSe-RGO-PI/CNT film Three reaction steps and two mechanisms (Volmer-Heyrovsky and Tafel-Heyrovsky mechanism) have been demonstrated for HER in acidic solution [40]. In detail, H+ converting to H2 involves three reactions [41,42]: Volmer reaction :
H3 Oþ þ e ! Hads þ H2 O b ¼ 120mVdec1
Heyrovsky reaction :
Tafel reaction :
Hads þ Hþ þ e ! H2 ðgÞ b ¼ 39mVdec1
Hads þ Hads ! H2 ðgÞ b ¼ 29mVdec1
ð6Þ
ð7Þ
curves under constant applied overpotential of 0.3 V in 0.5 M H2SO4 was shown in Fig. 7a, which suggested that the current density were decreased slightly after 12 h HER. This decrease was ascribed to the consumption of H+ and the production of hydrogen bubbles. As the HER continued, a large quantity of hydrogen released rapidly and aggregated subsequently on the surface of the electrode, which impeded the H+ moving to the surface of the electrode. As shown in Fig. 7b, the polarization curves of NiSeRGO-PI/CNT film before and after 500 cycles displayed negligible current loss. The morphology of the NiSe-RGO-PI/CNT film after 12 h HER was investigated by TEM (Fig. S2), which disclosed that the nanostructure of NiSe nanoparticles changed negligibly after a long time HER. The electrocatalytic and structural stabilities of NiSe-RGO-PI/CNT film confirmed a promise of the catalyst for practical application.
ð8Þ
The first step is a primary discharge process (Volmer reaction), the following step either an electrochemical desorption (Heyrovsky reaction) step or chemical desorption (Tafel reaction) step. The Tafel slope is an inherent property of catalysts for HER and can been obtained by measuring Tafel plot of the as-prepared catalyst. The equation of the Tafel plot is h = a + blog|j|, in which a is the Tafel constant, b is the Tafel slope, j is the current density and h is the overpotential [43]. Tafel slope has been used to investigate the dominate reaction mechanism and the rate-limiting step in the HER process promoted by an electrocatalyst. A Tafel slope of 120 mV dec1 indicates that the Volmer reaction is rate-limiting step. When the Tafel slope is 29 or 39 mV dec1, the Tafel reaction or Heyrovsky reaction would be rate-limiting step respectively. In this work, the measured Tafel slope of the NiSe-RGO-PI/CNT film (Fig. 6) is 61 mV dec1, which suggests the mechanism of this HER is Volmer-Heyrovsky mechanism and the rate-limiting step is Volmer reaction. Moreover, the Tafel slope of this film was consistent with the values of the non-precious metal HER catalysts (ranging from 40 to 120 mV dec1). This Tafel slope was comparable or even smaller than other efficient HER electrocatalysts (Table 1), implying that a high current density can be required at a lower applied overpotential.
4. Conclusion The NiSe-RGO-PI/CNT film was synthesized by a facile and green method of electrodeposition and used directly as an efficient HER electrocatalyst in acidic solution. The characterizations of structure and morphology demonstrated the conductive RGO-PI/ CNT film held a great potential for the rapid growth of a large quantity of a small-size NiSe nanoparticles on its surface. The electrocatalytic investigation verified that the good HER performance of NiSe-RGO-PI/CNT film was attributed to the synergistic effect between NiSe nanoparticles and RGO-PI/CNT film. The NiSeRGO-PI/CNT film can obtain the current density of 10 mA cm2 at a small overpotential of 270 mV and perform a low Tafel slope (61 mV dec1). The facile synthetic method can be extended to employ the RGO-PI/CNT film as a superior substrate for preparing other nanomaterials modified RGO-PI/CNT film for HER electrocatalysts. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant No. 51372106).
3.4. The durability of NiSe-RGO-PI/CNT film for HER
Appendix A. Supplementary data
Good structural and electrocatalytic stabilities are important criterions to evaluate electrocatalysts in practical HER. In an electrocatalytic process, electrocatalysts may suffer from destruction of the structure or the decrease of the HER performance, especially considering the HER electrocatalyst usually works in a strong acid condition. In order to examine the durability of the NiSe-RGO-PI/CNT film, i-t and LSV were performed for a long time in 0.5 M H2SO4 solution. The time-dependent current density
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta.2017. 05.084. References [1] J.A. Turner, A Realizable Renewable Energy Future, Science 285 (1999) 687– 689.
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Reduced graphene oxide-polyimide/carbon nanotube film decorated with NiSe nanoparticles for electrocatalytic hydrogen evolution reactions Tingxia Wanga , Xin Lia , Yimin Jiangb , Yaxin Zhoua , Lingpu Jiaa , Chunming Wanga,* a State Key Laboratory of Applied Organic Chemistry, , Lanzhou University, Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization, Gansu Province, Lanzhou University and College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China b College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China
A R T I C L E I N F O
Article history: Received 1 March 2017 Received in revised form 21 April 2017 Accepted 14 May 2017 Available online 15 May 2017 Keywords: Hydrogen evolution reaction Green electrodeposition method NiSe nanoparticle Polyimide/carbon nanotube film Reduced graphene oxide
A B S T R A C T
An efficient electrocatalyst based earth-abundant materials for hydrogen evolution reaction (HER) is a prerequisite for improving the efficiency of producing hydrogen. However, the synthesis of such electrocatalysts by a simple and environmentally friendly method is still a tremendous challenge. Herein, NiSe nanoparticles were in situ deposited on the reduced graphene oxide-polyimide/carbon nanotube (RGO-PI/CNT) film by a green electrodeposition method. The NiSe nanoparticles on the RGO-PI/CNT (NiSe-RGO-PI/CNT) film was employed directly as a highly active HER electrocatalyst. The RGO-PI/CNT film allowed NiSe nanoparticles to grow rapidly on its surface. NiSe nanoparticles with a small size of 35– 45 nm were distributed uniformly on the surface of the RGO-PI/CNT film. The good dispersity of NiSe nanoparticles permitted the exposure of more active sites, which enhanced the activity of HER on the NiSe-RGO-PI/CNT film. The NiSe-RGO-PI/CNT film performed well HER behaviors featured by a low overpotential (270 mV), a small Tafel slope (61 mV dec1) and a considerable stability. The efficient HER performance of the NiSe-RGO-PI/CNT film was originated from the synergistic effect between the RGOPI/CNT film and NiSe nanoparticles. This exploration provided a straightforward and green method to prepare a film electrocatalyst for HER. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction The development of sustainable and environmentally friendly energy is an inevitable challenge in the contemporary world because of dramatic depletion of fossil fuels and its deleterious impacts on the environment and human health [1]. Currently, hydrogen is considered as an ideal candidate to replace fossil fuels [2–4]. The electrocatalytic HER by water splitting is a feasible approach of producing hydrogen on a large scale and a remarkable electrocatalyst should be employed for HER to obtain a high current density at a minimal overpotential [5]. As is well-know, Pt and Pt-alloys served as the most active catalysts for HER in acidic media, whereas the widespread application has been impede greatly by their high cost and scarcity. Therefore, the exploration of efficient, acid-stable, low-cost and earth-abundant catalysts for HER remains an ongoing and important topic [6,7].
* Corresponding author. E-mail address: [email protected] (C. Wang). http://dx.doi.org/10.1016/j.electacta.2017.05.084 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
Recently, transition metal chalcogenides [8,9], carbides [10,11], borides [12], phosphides [13,14] and heteroatom-doped nanocarbons [15] have been applied successfully for HER to substitute noble metals. Among these alternatives, Ni-based chalcogenides have been utilized intensively in water splitting due to their good electrocatalytic properties and extremely cheap cost [16,17]. Selenium (Se) owns more obviously metallic property than sulfur (S), which indicates the better electrical conductivity of Se. The intrinsically electrical conductivity of Se enhances electrocatalytic performance of transition metal selenides [6]. The Ni and Se can form different phases such as NiSe, NiSe2 and Ni3Se2 [18,19], which have been applied in water splitting [20–22]. Particularly, NiSe and doped NiSe have been proven to be efficient electrocatalysts for HER and OER [23,24]. Although Ni-based selenides were active for HER, the HER performances were not perfect enough owing to their poor conductivity [21]. Moreover, when stacked with each other, the electrocatalysts have poor catalytic activities arisen from a low density of exposed active sites [25]. The RGO has been proposed as a conductive support to intensify the conductivity and dispersity of electrocatalysts [26]. For example, Dai and his co-
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workers have synthesized MoS2 nanoparticles grown on RGO as a highly efficient electrocatalyst for HER [8]. Hu and his groups have obtained superior HER performances by preparing WSe2/Co0.85Se/ RGO hybrids materials [27]. These findings have indicated that RGO can enhance exchange current density and the overall conductivity of catalysts by modulating electron density and the distribution of electronic potential in the hybrid materials. The synthesis of NiSe and doped NiSe has been reported by using a hydrothermal method [5,23]. However, few reports have presented about preparing NiSe by an electrodeposition method. Sun and his groups have demonstrated that NiSe2 electrodeposited on the Ti foam can catalyze efficiently the process of water splitting [35]. The electrodeposition method is time-saving and frees from the toxic H2Se gas compared with the hydrothermal method [29]. In this work, NiSe-RGO-PI/CNT film was prepared by a green electrodeposition method. The PI/CNT film was employed as the growing membrane of NiSe nanoparticles because this film possessed superior properties such as excellent electrical conductivity, good resistances to strong acidic and alkaline solutions, outstanding thermal stability and so on [28]. To further improve the conductivity and dispersity of NiSe nanoparticles, RGO has been fabricated on the PI/CNT film. The prepared NiSe-RGO-PI/CNT film was used directly as HER electrocatalyst without further treatments, which exhibited well HER catalytic activity. This work is expected to open up new possibilities in exploring electrocatalysts in the form of film by using the RGO-PI/CNT film as substrate. 2. Experimental sections
were prepared with deionized water (DI water, 18 MV cm1), which was obtained from a Milli-Q water purification system. 2.2. Preparation of the NiSe-RGO-PI/CNT film All electrochemical experiments were performed at room temperature with a CHI 660 electrochemical workstation (CH Instrument, USA) using a conventional three electrode system consisted of a RGO-PI/CNT film as the working electrode, a Pt wire counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The PI-CNT film was prepared according to a method described in the previous work [28]. (Graphene oxide) GO was synthesized from natural graphite powder through a modified Hummers method [30] and 10 mL GO aqueous solution (1 mg mL1) was added on the surface of a PI/CNT film. Subsequently, GO aqueous solution was dried at room temperature. The RGO-PI/CNT film was obtained by electrochemical reduction of GO in 0.1 M Na2SO4 aqueous solution at an applied potential of 1.25 V vs. RHE. The NiSe nanoparticles was electrodeposited cathodically on the RGO-PI/CNT film at a deposition potential of 0.25 V vs. RHE for different time (30, 45, 60, 90, 120 and 300 s) in a solution of 0.1 M Na2SO4, 5.0 mM NiSO4 and 10 mM SeO2 aqueous solution. The electrodeposition process of NiSe nanoparticles was indicated in Scheme 1. In detail, at the initial stage, SeO2 was hydrolyzed to H2SeO3, which was further reduced to Se. The Se continued to reduce to Se2 [29]. Subsequently, the Ni2+ in the electrolyte moved rapidly to the surface of the working electrode, in which the Ni2+ bonded with Se2 to form the NiSe. The corresponding reactions as follows: SeO2 þ H2 O ! H2 SeO3
ð1Þ
H2 SeO3 þ 4Hþ þ 4e ! SeðsÞ þ 3H2 O
ð2Þ
SeðsÞ þ 2e ! Se2
ð3Þ
2.1. Reagents Nickel sulfate hexahydrate (NiSO46 H2O) was purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Selenium dioxide (SeO2) was obtained from Aladdin Ltd. (Shanghai, China). Sulfuric acid (H2SO4) was obtained from Chengdu Kelong Chemical Reagent Factory (Chengdu, China). Sodium sulfate anhydrous (Na2SO4) were purchased from Tianjin Chemical Co. Ltd. (Tianjin, China). All reagents used in this study were of analytical grade and without further purifications. All aqueous solutions
Ni
2þ
þ Se2 ! NiSeðsÞ
Scheme 1. The illustration for electrodeposition process of the NiSe-RGO-PI/CNT film.
ð4Þ
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Then, the prepared NiSe-RGO-PI/CNT film was used directly as HER catalysts in acidic solution. 2.3. Structural Characterizations The morphology of the as-prepared products was characterized by Scanning electron microscopy (SEM Hitachi S-4800, Japan) with an accelerating voltage of 5 kV and Transmission electron microscopy (TEM TecnaiTM G2 F30, FEI, USA) operated at 300 kV. The chemical composition of the products was measured by energy dispersive spectrum (EDS) recorded on an EDAX-4800. The crystal structure of the samples was characterized by X-ray powder diffraction (XRD RigakuD/max-2400) equipped with Cu Ka radiation (l = 0.154178 nm) in the 2theta range from 20 to 65 (Scan step size = 0.03 , Time per step = 0.2 s). The X-Ray photoelectron spectroscopy (XPS) data was collected by X-Ray photoelectron spectroscopy (XPS AXIS ULTRA DLD).
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2.4. Electrochemical Characterizations Electrochemical measurements were implemented at room temperature with a CHI 660 electrochemical workstation in 0.5 M H2SO4 solution. A traditional three electrodes system was used, including a NiSe-RGO-PI/CNT film as the working electrode, a Pt wire as the counter electrode and a SCE (with saturated KCl aqueous solution) as the reference electrode. The geometric area of the working electrode is 0.1256 cm2. All the potential in this work was converted totally to a reversible hydrogen electrode (RHE) according to the Nernst equation in 0.5 M H2SO4. The Nernst equation as follows: EðRHEÞ ¼ EðSCEÞ þ 0:059pH þ 0:251
ð5Þ
0.251 is standard potential value of SCE at 25 C (The standard potential value of SCE depends on the concentration of KCl and experimental temperature.). Linear sweep voltammetry (LSV) was
Fig. 1. (a) XRD pattern and (b) EDS spectrum of the NiSe-RGO-PI/CNT film..
Fig. 2. XPS of (a) the NiSe-RGO-PI/CNT film survey spectrum, (b) C spectrum, (c) Ni spectrum and (d) Se spectrum.
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used to evaluate electrocatalytic behaviors of the NiSe-RGO-PI/CNT film in 0.5 M H2SO4 solution with a potential scanning from 0 to 0.5 V vs. RHE and scan rate of 5 mV s1. 3. Results and discussions 3.1. Structure and morphology The crystal structure of the NiSe-RGO-PI/CNT film was characterized by XRD. As shown in Fig. 1a, the XRD pattern of the NiSe-RGO-PI/CNT film revealed coexistence of NiSe nanoparticles and RGO nanosheets in this sample. Three obviously typical peaks located at 33.15 , 44.69 and 61.70 can be identified as the (101), (102) and (201) planes of the NiSe nanoparticles. All the diffraction peaks matched with the JPCDS 02-0892. The (101) plane was further confirmed by the finding of HRTEM (Fig. 3c). The peaks at 44.69 and 61.70 were agreement with the result of SAED (Fig. 3d). Besides, the broad diffraction peak at 25.48 was assigned to be the (002) plane of RGO nanosheets [31]. Fig. 1b shows the EDS spectrum of the NiSe-RGO-PI/CNT film, which reveals the distinct peaks from Ni, Se, O and C and suggests the presences of Ni, Se, O and C in the NiSe-RGO-PI/CNT film. The chemical composition and surface electronic states of the as-synthesized NiSe-RGO-PI/CNT film were analyzed by XPS. The typical survey of XPS spectrums (Fig. 2a) reveals that the sample is consisted of Ni, Se, C and O, which is consistent with the result of EDS. Fig. 2b–d show XPS spectra of the C 1s, Ni 2p and Se 3d regions, respectively. As depicted in Fig. 2b, the binding energy of C
1s states was located at 284.5 eV belonging to the value of C-C (sp2) group [32]. The binding energies of the Ni 2p1/2 and Ni 2p3/2 states were confirmed at 873.9 and 856.5 eV in Fig. 2c, indicating the Ni (II) species. In addition, the two peaks at 877.73 and 860.1 eV were assigned to the shakeup satellites of Ni (II) [33]. The Se 3d XPS spectrum (Fig. 2d) shows two peaks for Se 3d3/2 at 55.2 eV and 3d5/ 2 at 54.5 eV, suggesting divalent of Se [34]. The broad peak near 59.7 eV in Fig. 2d indicated the surface oxidation state of Se species due to exposure to atmosphere [35]. The morphology and structure of NiSe nanoparticles was observed by TEM. Fig. 3a shows TEM image of the NiSe-RGO-PI/ CNT film with a low magnification, which illustrates that dense NiSe nanoparticles are deposited uniformly on the RGO-PI/CNT film. Fig. 3b shows clearly that the diameter of NiSe nanoparticles is mainly 35–45 nm. Further insight into the structure of NiSe nanoparticles was performed by using high resolution TEM (HRTEM) and selected-area electron diffraction (SAED). The HRTEM image of a single NiSe nanoparticles (Fig. 3c) demonstrates a clear lattice fringe with the inter-planar distance of 0.27 nm, corresponding to the (101) plane of NiSe nanoparticles. The SAED image (Fig. 3d) discloses three distinct diffraction rings, which is indexed to the (100), (102) and (201) planes of NiSe nanoparticles and suggests the poly-crystalline structure of the as-prepared samples. The surface morphology of electrocatalysts affects directly the number of exposed active sites, which is greatly significant for HER. The RGO-PI/CNT film was used as a good substrate for the growth of NiSe nanoparticles. The electrocatalytic activity of the NiSe-RGO-
Fig. 3. (a,b) TEM images, (c) HRTEM image and (d) SAED pattern of the NiSe-RGO-PI/CNT film.
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Fig. 4. SEM images of NiSe-RGO-PI/CNT film with different electrodeposition time: (a) 30, (b) 45, (c) 60, (d) 90, (e) 120 and (f) 300 s.
PI/CNT film was related to the structure of NiSe, thus the research on the morphology evolution of NiSe was indispensable. SEM was used to observe the growing process of the NiSe-RGO-PI/CNT film. Fig. 4 shows the SEM images of the NiSe-RGO-PI/CNT film at different stages, which indicates that the morphologies of asprepared products are all nanoparticles. At the early stage for 30 s, a small quantity of NiSe nanoparticles with a small size of 20– 30 nm were deposited on the RGO-PI/CNT film and the drapes of RGO nanosheets were apparent (Fig. 4a). When the electrodeposition time was increased to 45 s, more nanoparticles with the diameter of 30–35 nm were formed on the RGO-PI/CNT film (Fig. 4b). After 60 s, a considerable number of NiSe nanoparticles with a size of 35–45 nm appeared densely on the surface of the RGO-PI/CNT film (Fig. 4c). With the electrodeposition time was changed from 30 to 60 s (Fig. 4a–c), the size of NiSe nanoparticles increased slightly. However, the quantity of NiSe nanoparticles were increased dramatically, which would enhance the HER performance of the NiSe-RGO-PI/CNT film. When the reaction continued to 90 s (Fig. 4d), NiSe nanoparticles tended to gather with each other. Further prolonging time to 120 and 300 s (Fig. 4e and f), NiSe nanoparticles almost agglomerated completely. As the electrodeposition proceeding from 60 to 300 s, NiSe nanoparticles connected with each other, decreasing the quantity of exposed
active sites. In order to explore the role of RGO in this electrodeposition process, the morphologies of the RGO-PI/CNT film and NiSe nanoparticles on the PI/CNT (NiSe-PI/CNT) film was also assessed. As shown in Fig. S1 a, the morphology of bare RGOPI/CNT film was nanosheets with obvious drapes. The SEM image of the NiSe-PI/CNT film was shown in Fig. S1 b, which revealed that the morphology of the NiSe on the PI/CNT film were irregular because of the agglomeration of small NiSe nanoparticles. The growing mechanism of NiSe nanoparticles was that the RGO-PI/ CNT or PI/CNT film provided the nucleation sites for the subsequent growth of NiSe nanoparticles on its surface. The RGO-PI/CNT film was decorated with more dispersive NiSe nanoparticles compared with the PI/CNT film, which indicated that RGO can reduce efficiently the agglomeration and improve the dispersity of NiSe nanoparticles. 3.2. HER electrocatalytic properties of the NiSe-RGO-PI/CNT film Electrochemical measurements were performed at room temperature using a three electrodes system. The RGO-PI/CNT film was loaded with below 0.04 mg cm2 NiSe nanoparticles. The polarization curves of the NiSe-RGO-PI/CNT film with different electrodeposition time from 30 to 120 s were shown in Fig. 5a.
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Fig. 5. Polarization curves of (a) the NiSe-RGO-PI/CNT film with different electrodeposition time and (b) the PI/CNT, RGO-PI/CNT, NiSe-PI/CNT, and NiSe-RGO-PI/CNT film in 0.5 M H2SO4 with scan rates of 5 mV s1.
Table 1 Comparison of catalytic parameters of different HER catalysts in 0.5 M H2SO4. Materials
Substrate
Mass loading (mg cm2)
h5 (mV)
h10 (mV)
Tafel slope (mV dec1)
Ref.
RGO-MoSSe Co-WSe2/rGO PhLi MoSe2 MoSe2 CoSe2 NiSe NiSe
GCD GCE GCE rGO-PI CF GCE RGO-PI/CNT
0.198 0.57 – – – 0.28 <0.04
135 190 330 540 127 252 240
158 217 351 – 141 272 270
53 64 54 82 68 64 61
[36] [27] [37] [38] [39] [23] This work
h5: Overpotential of the catalysts at the current density of 5 mA cm2. GCD: glassy carbon disk. GCE: glassy carbon electrode. CF: carbon fiber. rGO-PI: reduced graphene oxide/polyimide.
According to the polarization curves, electrodeposition time was increased from 30 to 60 s, causing a rapid rise of the cathodic current density. The enhanced HER performances were attributed to a sharp increase in the quantity of exposed active sites and small-size NiSe nanoparticles. However, the current density decreased gradually when the electrodeposition process continued. The reduction was caused by the formation of the aggregated NiSe on the RGO-PI/CNT film. It should be noted that the NiSe nanoparticles with electrodeposition time 60 s on the RGO-PI/CNT film exhibited more apparent HER electrocatalytic performance. In principle, controlling the duration of electrodeposition could fabricate NiSe nanoparticles with different morphology on the RGO-PI/CNT film. The electrocatalytic activity of NiSe-RGO-PI/CNT film was influenced by the number and reactivity of active sites. The greatly dense distribution and small size of the NiSe nanoparticles provided more active sites, which enhanced electrocatalytic activity of the NiSe-RGO-PI/CNT film. Moreover, the HER performance of NiSe nanoparticles at different electrodeposition time were consistent with the number of exposed active sites at the corresponding stages. For comparison, the electrochemical HER performances of the PI/CNT film, RGO-PI/CNT film and NiSePI/CNT film were also tested in the same conditions. The bare PI/ CNT film, and the RGO-PI/CNT film showed negligible or poor HER catalytic activity in the measurement voltage range, which indicated that efficient HER performance was originated from the NiSe nanoparticles, in spite the RGO-PI/CNT film performed a cathodic current density of 1.67 mA cm2 at 0.50 mV. The NiSePI/CNT showed a poor HER performance, which was arisen from a small number of NiSe with a low density of active sites. In strong contrast, the NiSe-RGO-PI/CNT exhibited apparently electrocatalytic activity with a current density of 47.60 mA cm2 at 0.50 mV. Therefore, the high HER performance of the NiSe-RGO-PI/CNT film was attributed to the coupling effect of the RGO-PI/CNT film and
NiSe nanoparticles. Taken together, the highly electrocatalytic activity of NiSe-RGO-PI/CNT film was rationalized as follows: (a) The conductive RGO-PI/CNT film allowed the intimate growth of small-size NiSe nanoparticles on its surface. (b) The RGO improved the dispersity and electrical conductivity of the NiSe nanoparticles and prevented the agglomeration of NiSe nanoparticles on RGO-PI/ CNT film. (c) Although the mass loading of NiSe nanoparticles was small, the high density of small size NiSe nanoparticles provided a great quantity of exposed active sites, which enhanced the HER performance. In order to further assess the HER electrocatalytic activity of the NiSe-RGO-PI/CNT film, the HER performance of other transition metal selenides were compared in Table 1.
Fig. 6. The Tafel slope of the NiSe-RGO-PI/CNT film.
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Fig. 7. (a) Chronoamperometric responses (i-t) at a constant applied potential of 0.35 V and (b) polarization curves of NiSe-RGO-PI/CNT film before and after 500 cycles.
3.3. The mechanism of HER on the NiSe-RGO-PI/CNT film Three reaction steps and two mechanisms (Volmer-Heyrovsky and Tafel-Heyrovsky mechanism) have been demonstrated for HER in acidic solution [40]. In detail, H+ converting to H2 involves three reactions [41,42]: Volmer reaction :
H3 Oþ þ e ! Hads þ H2 O b ¼ 120mVdec1
Heyrovsky reaction :
Tafel reaction :
Hads þ Hþ þ e ! H2 ðgÞ b ¼ 39mVdec1
Hads þ Hads ! H2 ðgÞ b ¼ 29mVdec1
ð6Þ
ð7Þ
curves under constant applied overpotential of 0.3 V in 0.5 M H2SO4 was shown in Fig. 7a, which suggested that the current density were decreased slightly after 12 h HER. This decrease was ascribed to the consumption of H+ and the production of hydrogen bubbles. As the HER continued, a large quantity of hydrogen released rapidly and aggregated subsequently on the surface of the electrode, which impeded the H+ moving to the surface of the electrode. As shown in Fig. 7b, the polarization curves of NiSeRGO-PI/CNT film before and after 500 cycles displayed negligible current loss. The morphology of the NiSe-RGO-PI/CNT film after 12 h HER was investigated by TEM (Fig. S2), which disclosed that the nanostructure of NiSe nanoparticles changed negligibly after a long time HER. The electrocatalytic and structural stabilities of NiSe-RGO-PI/CNT film confirmed a promise of the catalyst for practical application.
ð8Þ
The first step is a primary discharge process (Volmer reaction), the following step either an electrochemical desorption (Heyrovsky reaction) step or chemical desorption (Tafel reaction) step. The Tafel slope is an inherent property of catalysts for HER and can been obtained by measuring Tafel plot of the as-prepared catalyst. The equation of the Tafel plot is h = a + blog|j|, in which a is the Tafel constant, b is the Tafel slope, j is the current density and h is the overpotential [43]. Tafel slope has been used to investigate the dominate reaction mechanism and the rate-limiting step in the HER process promoted by an electrocatalyst. A Tafel slope of 120 mV dec1 indicates that the Volmer reaction is rate-limiting step. When the Tafel slope is 29 or 39 mV dec1, the Tafel reaction or Heyrovsky reaction would be rate-limiting step respectively. In this work, the measured Tafel slope of the NiSe-RGO-PI/CNT film (Fig. 6) is 61 mV dec1, which suggests the mechanism of this HER is Volmer-Heyrovsky mechanism and the rate-limiting step is Volmer reaction. Moreover, the Tafel slope of this film was consistent with the values of the non-precious metal HER catalysts (ranging from 40 to 120 mV dec1). This Tafel slope was comparable or even smaller than other efficient HER electrocatalysts (Table 1), implying that a high current density can be required at a lower applied overpotential.
4. Conclusion The NiSe-RGO-PI/CNT film was synthesized by a facile and green method of electrodeposition and used directly as an efficient HER electrocatalyst in acidic solution. The characterizations of structure and morphology demonstrated the conductive RGO-PI/ CNT film held a great potential for the rapid growth of a large quantity of a small-size NiSe nanoparticles on its surface. The electrocatalytic investigation verified that the good HER performance of NiSe-RGO-PI/CNT film was attributed to the synergistic effect between NiSe nanoparticles and RGO-PI/CNT film. The NiSeRGO-PI/CNT film can obtain the current density of 10 mA cm2 at a small overpotential of 270 mV and perform a low Tafel slope (61 mV dec1). The facile synthetic method can be extended to employ the RGO-PI/CNT film as a superior substrate for preparing other nanomaterials modified RGO-PI/CNT film for HER electrocatalysts. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant No. 51372106).
3.4. The durability of NiSe-RGO-PI/CNT film for HER
Appendix A. Supplementary data
Good structural and electrocatalytic stabilities are important criterions to evaluate electrocatalysts in practical HER. In an electrocatalytic process, electrocatalysts may suffer from destruction of the structure or the decrease of the HER performance, especially considering the HER electrocatalyst usually works in a strong acid condition. In order to examine the durability of the NiSe-RGO-PI/CNT film, i-t and LSV were performed for a long time in 0.5 M H2SO4 solution. The time-dependent current density
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