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The Dominating Role of Ni0 on the Interface of Ni/ NiO for Enhanced Hydrogen Evolution Reaction Jing Wang, Shanjun Mao, Zeyan Liu, Zhongzhe Wei, Haiyan Wang, Yiqing Chen, and Yong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15377 • Publication Date (Web): 06 Feb 2017 Downloaded from http://pubs.acs.org on February 11, 2017

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ACS Applied Materials & Interfaces

The Dominating Role of Ni0 on the Interface of Ni/NiO for Enhanced Hydrogen Evolution Reaction Jing Wang, Shanjun Mao, Zeyan Liu, Zhongzhe Wei, Haiyan Wang, Yiqing Chen, Yong Wang* Advanced Materials and Catalysis Group, Center for Chemistry of High-performance and Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou 310028, P. R. China. E-mail: [email protected] KEYWORDS: NiOx-based hybrids, electrocatalysis, interface, hydrogen evolution reaction, DFT

ABSTRACT: The research of a robust catalytic system based on single NiOx electrocatalyst for hydrogen evolution reaction (HER) remains a huge challenge. Particularly, the factors that dominate the catalytic properties of NiOx-based hybrids for HER have not been clearly demonstrated. Herein, a convenient protocol for the fabrication of NiOx@bamboo-like carbon nanotubes hybrids (NiOx@BCNTs) is designed. The hybrids exhibit superb catalytic ability and considerable durability in alkaline solution. A benchmark HER current density of 10 mA cm -2 has been achieved at an overpotential of ~79 mV. Combined with the experimental results and density functional theory (DFT) calculations, it for the first time definitely validates that the

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inherent high Ni0 ratio and the Ni0 on the interface of Ni/NiO play a vital role in the outstanding catalytic performance. Especially, the Ni0 on the interface of Ni/NiO performs superior activity for water splitting compared with bulk Ni0. These conclusions provide guidance for the rational design of the future non-noble metallic catalysts.

1. Introduction

The growing energy crises and environmental problems have stimulated the exploration of renewable alternative energies. As a clean, efficient and sustainable energy, H2 is regarded as an ideal energy carrier. HER in electrocatalytic water splitting is one of the most efficient strategies to produce high-purity H2.1-3 Especially, the alkaline HER plays a crucial role in current wateralkali and chlor-alkali electrolyzers.4 Nevertheless, for low-temperature electrochemical devices under alkaline condition, the kinetic limitation of the hydrogen evolution is a major obstacle.5 Platinum (Pt) has hitherto been the element of choice for catalyzing HER due to their outstanding electrocatalytic performance. Unfortunately, the exorbitant cost and low reserve of Pt have compromised the large-scale deployment. Since the first report of the use of MoS2 nanoparticles (NPs) for HER,6 the exploration of earth–abundant candidates to substitute Pt has undergone rapid development.7-12 Such candidates typically contain Mo-based materials,13-18 CoS2,19-20 FeP,21 VS217 and CoP22-24. Although they have presented encouraging catalytic activities in acid medium, the HER performance of these catalysts in alkaline medium have been rarely reported. Developing cost-effective and highly active electrocatalyst in alkaline condition is highly desirable. Nickel is typically used in industry for water reduction in basic medium. However, Ni metal is not an ideal HER catalyst due to its high overpotential and large Tafel slope.25 Normally, the


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enhanced activity of Ni-based catalysts is realized by the introduction of foreign active components.26-29 For example, metallic Ni2P,30 Ni3S2,31 Ni alloys,32-33 and Ni/NiO/CoSe234 have been demonstrated as promising catalysts.35 However, the complicated process, the high metal consumption, low specific surface area and the poor electrical conductivity are such limitations. Nanocarbon materials have been widely employed in energy conversion field because of their large specific surface area and unique electronic properties.36-41 The combination of active metal Ni phases and nanocarbon materials has been proved as an impressive strategy. Ni/Mo-C,33 NiAl2O3@GO,42 FeNi-Graphene,43 and Ni3C-GNRs44 have attracted great attention recently. Despite tremendous efforts in this direction, the amelioration of catalytic activity is still mainly depended on the foreign active components. The development of a robust catalytic system based on single NiOx electrocatalyst for the HER still remains a challenge.45 Herein, we design a convenient protocol for the fabrication of NiOx@bamboo-like carbon nanotubes hybrids (NiOx@BCNTs) through calcining melamine/Ni(NO3)2·6H2O. Excellent HER activity is obtained by adjusting the mass ratio of the precursors and the pyrolytic program. NiOx@BCNTs possesses an overpotential of ~79 mV at a current density of 10 mA cm-2 and exhibits considerable durability. By designing catalysts with different Ni0 ratios via H2temperature programmed reduction (H2-TPR) technique, we concluded that both bulk Ni0 and the Ni0 on the interface of Ni/NiO contributed to the enhanced performance. Especially, the Ni0 on the interface of Ni/NiO showed stronger ability for water splitting compared with bulk Ni0. Besides, the in situ generated Ni0 from NiO during HER process also accelerated the hydrogen generation kinetics. These conclusions reveal the essence of the catalytic active sites, and provide guidance for the design of highly active non-noble metallic catalysts for the conversion and storage of renewable energy.



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2. EXPERIMENTAL SECTION 2.1 Materials. Melamine, Ni(NO3)2·6H2O and commercial Ni were received from Aladdin. All the chemical reagents were commercially available analytical grade. 2.2 Synthesis of bamboo-like carbon nanotubes composites (NiOx@BCNTs). The materials in this work were synthesized by annealing the solid mixture of melamine/Ni(NO3)2·6H2O. In the typical synthesis of NiOx@BCNTs, 1.11 g Ni(NO3)2·6H2O was first dissolved in an appropriate amount of deionized water. Then, 20 g melamine was added into the above solution. The water completely evaporated in 80 oC oil bath with magnetic stirring. Afterwards, the mixture was ground into powder, and transferred into a 30 ml-crucible. The solid mixture experienced two sections of temperature program: first heated to 600 oC for 1 h and further rose to 700 oC for 2 h in a Muffle furnace in N2 flow (400 mL min-1). Finally, the products were cooled down to room temperature, and NiOx@BCNTs was obtained. 2.3 Synthesis of the contrastive samples The synthesis of the contrastive samples was similar to that of NiOx@BCNTs. The contrastive samples can be obtained by varying the mass ratio of the precursors, pyrolysis temperature or the holding time. The corresponding synthetic parameters of the contrastive samples were provided in Table S1. All the products, which were used for exploring the growth process of BCNTs, were treated by 2 M HCl. In order to explore the role of nanocarbon, NiO was prepared by direct calcinating Ni(NO3)2·6H2O at 800 oC for 1 h.


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2.4 Characterization The microstructures of the samples were observed by SEM, which performed on a SU-70 microscope. HRTEM was conducted on Tecnai G2 F30 S-Twin to further detect the crystalline structure. Powder X-ray diffraction (XRD) patterns were obtained on a D/tex-Ultima TV wide angle X-ray diffractometer (Cu Kα radiation, λ = 1.54 Å). The chemical states of C and Ni species were demonstrated by X-ray photoelectron spectra (XPS). An aluminum anode (Al 1486.6 eV) served as X-ray source. The Raman spectra were carried out on a Raman spectrometer (JY, HR 800) using 514-nm laser. The surface physical structures of the products were studied by N2 adsorption analysis, which operated at 77 K using a Micromeritics ASAP 2020. The samples were first outgassed at 423 K for 8 h. The conventional Brunauere-EmmetteTeller (BET) method was employed to calculate the speciﬁc surface area. The pore size distribution (PSD) plot was recorded by the Barrett-Joyner-Halenda (BJH) algorithm. The Ni content was determined by ICP-AES (PerkinElmer Optima OES 8000) and aqua regia was used to dissolve the sample. As the ICP-AES result revealed, the nickel content of NiOx@BCNTs was 39.74 wt%. 2.5 H2-Temperature programmed reduction (H2-TPR) experiments H2-TPR experiments of NiOx@BCNTs were carried out in a quartz reactor. Before a TPR run, catalysts were pretreated in He at 600 oC for 1 h, and then cooled down to room temperature. After that, the TPR was operated in the stream of H2/Ar at a heating rate of 10 oC min-1 up to 850 o

C. A series of contrast samples were treated under similar operating conditions at 400 oC, 550

o

C, 650 oC, 750 oC and 850 oC, respectively.



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Take the sample treated at 650 oC as an example. 50 mg NiOx@BCNTs was loaded in the quartz reactor. Before a TPR run, catalysts were pretreated in He at 600 oC for 1 h, and then cooled down to room temperature. Next, the TPR experiment was carried out in the stream of H2/Ar at a heating rate of 10 oC min-1. Upon the temperature rise to 650 oC, the gas immediately switched to He, and cool down to the room temperature in He atmosphere. 2.6 Electrochemical measurements The electrocatalytic properties of the products for HER were evaluated with a three electrode system using a CHI electrochemical workstation (model 750E). The as-prepared composites were employed as working electrode. The auxiliary and the reference electrodes were a graphite rod electrode and a saturated calomel electrode (SCE), respectively. The preparation process of the electrode is shown as follows: First, 3 mg sample was dispersed into a mixed solution containing 10 μl ~5 wt% Nafion and 300 μl alcohol. The mixture was under ultrasonic dispersion for 20 min. Then, 20 μl of the homogeneous mixture was dropped on the surface of the glassy carbon electrode (5 mm). Before measurement, the electrode was allowed to dry at room temperature. The reported potentials were all referenced to the reversible hydrogen electrode (RHE) according to Evs RHE = Evs SCE + EoSCE + 0.059 pH. Linear sweep voltammetry (LSV) was conducted in 1 M KOH solution and 1 M phosphate buffer solution (PBS) with scan rate of 5 mV s-1. Electrochemical impedance spectroscopy (EIS) was carried out in potentiostatic mode from 105-0.01 Hz. All the polarization curves are the steady-state ones after several cycles. All polarization curves were IR corrected. In order to explore the changes in morphology, composition and structure for NiOx@BCNTs after stability test, we fabricated the electrode by loading NiOx@BCNTs on commercial carbon cloth (1×1 cm, mass loading: 3 mg cm-2).


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2.7 DFT Calculations DFT calculations were performed by using periodic, spin-polarized DFT as implemented in Vienna ab initio program package (VASP). Detailed computational process was shown in Supporting Information. 3. RESULTS AND DISCUSSION 3.1 Synthesis and characterization of NiOx@BCNTs

Figure 1. a) TEM image of NiOx@BCNTs. Inset is metal particle size distribution histogram. b) TEM image of NiOx@BCNTs treated with acid. c, d, e) HRTEM images of NiOx@BCNTs. Inset in d is the Fast Fourier transform (FFT) images of Ni0. The synthesis process of products was illustrated in Scheme S1. The morphology can be easily controlled by tailoring different parameters (Figure S1-3). The as-prepared optimal product is denoted as NiOx@BCNTs (700 oC, 2 h). The microstructure of NiOx@BCNTs was



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investigated by transmission electron microscopy (TEM). Well-defined bamboo-like carbon nanotubes were readily fabricated by "solid precipitation" method (Figure 1). The as-synthesized CNTs had diameters ranging from 10 to 20 nm (Figure 1b). The histogram in Figure 1a evidences the Ni-based NPs with a mean size of 12.5 nm. High-resolution transmission electron microscopy (HRTEM) images (Figure 1c) further disclosed the high crystallinity features of NiOx@BCNTs. As Figure 1d revealed, an encapsulated Ni NP with well-defined lattice fringes were consistent with the (111) and (200) facet of the Ni0 crystal. The external sidewalls of NPs are consisted of discontinuous graphitic layers. The obvious defects of the carbon shell can allow reactant molecules to reach the NPs. It is worth mentioning that the unique interface of Ni/NiO indwelled in the hybrids (Figure 1e). Such interfacial structure may play an important part on the electrocatalytic property. Further, energy-dispersive X-ray (EDX) element mapping gives more structural details about NiOx@BCNTs. Just as we expected, the C, N species are distributed homogeneously throughout the composites (Figure S4). And relatively uniform and discrete Nibased NPs have been formed.


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Figure 2. a) XRD patterns, b) Ni2p XPS spectrum, c) Raman spectrum, d) Adsorption/desorption isotherms of NiOx@BCNTs. X-ray diffraction (XRD) was performed to investigate the crystallographic structure of NiOx@BCNTs (Figure 2a). Both the characteristic peaks of the graphite-type lattice (C: PDF 41-1487) and Ni/NiO (Ni: PDF 65-2865, NiO: PDF 65-5745) were recorded. X-ray photoelectron spectroscopy (XPS) further investigated the chemical valence of Ni. As shown in Figure 2b, the peaks centering at 870.2 and 853.0 eV are corresponded to the position of Ni0 and the rest of the peaks are assigned to the form of NiO.4, 46-47 Besides, relying on the result of high resolution C 1s spectra (Figure S5), NiOx@BCNTs showed three types of carbon coordination, assigning to C=C (284.5 eV), C-C (285.4 eV), and C-N-C (~287.0 eV).48 The existence of CN-C bonding indicated that the N atoms were successfully incorporated into the graphitic carbon texture. The doped N in the carbon skeleton would hinder the agglomeration of the metal NPs through the interaction between metal and nitrogen atoms,49 thus improving the dispersions of metal NPs, and reducing the particle size of the metals. The graphitic structure of NiOx@BCNTs was also detected by Raman spectrum. Not only typical D- and G-bands of carbon but also a well-defined 2D peak (Figure 2c) was observed. The high ID/IG value indicated more structural defects, which might be caused by N dopants.50 N2 adsorption–desorption analysis was employed to study the textural properties of NiOx@BCNTs (Table S2). The N2 adsorption isotherm measured for NiOx@BCNTs resembled type IV with a large specific surface area (314 m2g-1) and mesoporous feature (Figure 2d), which is conducive to the mass transfer. 3.2 Electrochemical HER activity of products Motivated by the above-mentioned merits, the as-synthesized composites were evaluated as cathode materials for HER. First, we evaluated the role of nanocarbon through comparing the



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activities of composites (NiOx@NC-800), NiO (obtained by direct calcinating Ni(NO3)2·6H2O at 800 oC for 1 h, Figure S6) and commercial Ni (Figure S7). The NiO displayed almost no electrocatalytic activity. Despite the high mass loading, metal Ni showed poor catalytic activity. Pleasingly, the composites delivered higher catalytic activity after introducing the carbon supports. Based on the electrochemical impedance spectroscopy (EIS) analysis (Figure S8), the charge-transfer resistance of the samples significantly increased from NiOx@NC-800, metal Ni to NiO, which manifested that the intrinsic high conductive carbon facilitated the fast electron transfer.51 We then explored the HER activity under different calcination temperature in the range of 700-1000 oC (holding time was 1 h). The as-prepared products are named as NiOx@NC-700, NiOx@NC-800, NiOx@NC-900 and NiOx@NC-1000, respectively. The results shown in Figure 3 indicated that the catalytic performance enhanced with increasing pyrolysis temperature from 700 to 800 oC, and then declined from 900 to 1000 oC. To figure out the reason for different HER activity, the EIS measurements were carried out. For NiOx@NC-700, a large number of g-C3N4 did not completely decompose, which led to the poor conductivity (Figure S9a). Whereas, NiOx@NC-800, NiOx@NC-900 and NiOx@NC-1000 displayed the similar semicircle diameters, which indicates that the conductivity is not the reason for the activity difference of the three samples. Based on the analysis of high resolution C 1s spectra (Figure S5), NiOx@NC-700 had relatively low C=C content among the samples, suggesting the poor graphitization degree. While, NiOx@NC-800, NiOx@NC-900 and NiOx@NC-1000 have equivalent distributions of carbon species (C=C, C-C and C-N-C) according to Figure S5, which excluded the influence of carbon species on the activity. Additionally, NiOx@NC-800, NiOx@NC-900 and NiOx@NC-1000 have the similar ID/IG values on the basis of Raman results (Figure S10). And all of the three samples featured the characteristic peaks of the graphite-type


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lattice and Ni/NiO (Figure S11). It is generally known that textural properties play an important role in the catalytic performance. As summarized in Table S2, NiOx@NC-800 possessed higher specific surface area (SSA) than that of NiOx@NC-900 and NiOx@NC-1000. The larger SSA is beneficial for exposing more active sites, thus resulting in superior activity.52 More importantly, the calcination temperature can obviously affect the Ni bonding configurations in the samples. Figure 3c and 3d summarized the percentage of Ni functionality based on the deconvolution. An apparent decline in Ni0 content has been observed (Figure 3d, Table S3) along with the increasing pyrolysis temperature, which is consistent with the tendency of activity. Based on the XPS analysis, we speculate that the better HER performance is derived from the higher Ni0 content.

Figure 3. a) Polarization curves for samples calcined at different temperature. b) HER polarization curves for NiOx@NC-700, NiOx@BCNTs and NiOx@BCNTs-Ni foam. c)



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High-resolution Ni2p XPS spectra of samples. d) The content of the Ni/NiO species in products and the corresponding overpotential under 10 mA cm -2. All the HER tests were measured in 1 M KOH. To further elucidate the above conjecture, we take the sample calcinated at 700 oC as an example. By extending calcination time to 2 h, g-C3N4 underwent complete pyrolysis process to form NiOx@BCNTs. Pleasingly, NiOx@BCNTs shows superb activity with an overpotential of ~183 mV at 10 mA cm-2 (Figure 3b), which is comparative to the reported works (Table S4). Next, we studied the effect of catalyst loadings on the catalytic performance. As exhibited in Figure S12, a higher mass loading enables a better the HER activity. Additionally, loading the NiOx@BCNTs on nickel foam (NiOx@BCNTs-Ni foam) not only makes for exposing more active sites, but also benefits the mass transfer process. After a long time HER test (Figure S13), the stabilized NiOx@BCNTs-Ni foam presented superb activity, that the overpotential can be so small as ~79 mV at 10 mA cm-2 with a loading of ~3.18 mg cm-2 based on the mass of nickel (Figure 3b). The current density of NiOx@BCNTs-Ni foam at -0.2 V is about 8 times higher than that of pure commercially used Ni foam (Figure S14). Interestingly, the performance of NiOx@BCNTs is similar to that of NiOx@NC-800. It has been reported that the N dopants activated the adjacent C atoms to further enhance the catalytic activity.53-54 Both NiOx@BCNTs and NiOx@NC-800 have the similar N content on the basis of XPS data (Table S3), thus excluding the influence of N heteroatoms. Also, NiOx@BCNTs and NiOx@NC-800 possess the comparative conductivity and almost the same SSA (Table S2). Moreover, the content of Ni0 in NiOx@BCNTs (36.2%) was similar to NiOx@NC-800 (32.8%), indicating the relative higher Ni0 content resulted in the decent performance. To sum up, we have reason to believe that the inherent high Ni0 level is the key factor for the excellent catalytic activity.


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Figure 4. a) Polarization curves of NiOx@BCNTs tested for 13 times. b) The content of Ni/NiO in NiOx@BCNTs and NiOx@BCNTs-HER from XPS. Additionally, an interesting phenomenon has been found during the process of test, the overpotential is reduced gradually after potential cycling (Figure 4a), which captivated our attention. We speculated that the phenomenon was the result of the change of metal valence state. The chemical configuration evolution of Ni0 and NiO in the fresh and after HER tested (NiOx@BCNTs-HER) catalyst was determined by semiquantitative analysis of XPS. Interestingly, the Ni0 content increased substantially from 36.2% to 55.4% after tested (Figure 4b and Figure S15). The NiO was in situ reduced to Ni0 under the outer circuit during the HER process. The in situ produced Ni0 combined with the intrinsic Ni0 greatly promoted the catalytic activity. 3.3 The real active sites analysis



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Figure 5. a) H2-TPR plot of NiOx@BCNTs. b) The corresponding LSV curves of NiOx@BCNTs and samples treated by H2-TPR process. c) Volcano curve of exchange current density (log(i0)) and HBE values,5 the green and red lines represent the HBE values of Ni and Ni/NiO. d) The corresponding Tafel values of samples according to b. To deeply reveal the veil of the catalytic activity, NiOx@BCNTs was further treated by H2TPR technique. The delicate experiment layout shed light on the real active sites via the purposive reduction samples at different temperatures. According to the TPR results (Figure 5a), NiO started to be reduced at 400 oC, and could be thoroughly reduced at 850 oC. Furthermore, the defined curve contained no carbon decomposition peak, suggesting the superior stability of carbon skeleton in our system. On the basis of TPR data, we designed a sequence of samples that treated at 400, 550, 650, 750 and 850 oC in H2/Ar, respectively, to obtain the different levels of Ni0 in the samples. The corresponding TPR plots were shown in Figure S16. According to the


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HRTEM images of TPR-treated samples (Figure S17-21), Ni, NiO and bamboo-like carbon nanotubes were retained after TPR process. Furthermore, the TPR-treated samples exhibited the characteristic peaks of metallic Ni depending on XRD results (Figure S22). For TPR-treated samples from 400 to 750 oC, the partial reduction of the NiO led to the decreased crystalline degree, which is characterized by the decrease in the diffraction peak intensity, and thus the characteristics peaks of NiO in the XRD pattern is not obvious. Besides, all the TPR-treated samples possessed similar carbon species distributions (Figure S23). Then we believed that the differences of catalytic activity among the samples derived from the distributions of nickel species. Just as we expected, the proportion of Ni0 improved with the increase of the final temperature (Figure S24, Table S3). After being treated with TPR method, all the samples exhibited enhanced catalytic activity compared with the fresh catalyst (NiOx@BCNTs). This result indicates that the increased Ni0 concentration is conducive to the improvement of the catalytic activity, which is well consistent with the above conclusion. Specifically, with the rising of the treated temperature from 400 to 650 oC, the overpotential declined sharply from about 161 to 97 mV. Then, the overpotential increases gradually to 135 mV for the totally reduced product (treated at 850 oC). That is, the higher content of Ni0 leads to a better catalytic activity only within a certain range. During the TPR treatment, part of the NiO was in situ reduced to Ni0, thus resulting in newly formed Ni0 on the interface of Ni/NiO. The different activity of samples treated at 650 and 850 oC might be attributed to the metal phase change. As for the former sample, a moderate newly Ni0 formed on the Ni/NiO interface due to the partial reduction. While, it was almost totally bulk Ni0 for the sample treated at 850 oC. Although TPR-850 oC possesses relative higher Ni0 content, the Ni0 mainly exists in the form of bulk phase. So, the absence of Ni/NiO interfaces may cause the



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decrease of activity. Therefore, we propose that the newly in situ formed Ni0 on the Ni/NiO interface possess a better activity towards the HER. The hydrogen binding energy (HBE) has been verified as a useful screening tool to detect the HER activity.55-56 To confirm the deduction, DFT calculation was performed to value the hydrogen binding energies for bulk Ni0 and Ni0 at the Ni/NiO interface. According to the calculation models with optimized structures for hydrogen binding in Figure S32, the Ni/NiO interface lowers the HBE of the newly in situ formed Ni0 (-0.47 eV) compared with that of the bulk one (-0.54 eV). Based on the volcano curve (Figure 5c),5 we can predict the activity trend. We found that the HBE values of the in situ formed Ni0 on the Ni/NiO interface were closer to the best predicted HBE value (~ -0.35 eV), suggesting that it deserved to show better activity than the bulk one. And not surprisingly, the big HBE values on NiO (-1.18 and -0.76 eV at Ni and O sites, respectively) implied that NiO was not active. To further confirm the result, the Tafel slopes of the samples with TPR treatment were plotted with corresponding temperatures (Figure 5d). The concave-downward curve clearly testifies the good HER activity for the interface Ni0. The result can then be explained as follows: the ratio of Ni0 of both the bulk and the interface is rather low at 400 oC, which results in high Tafel slope. The ratios of them both increase along with the increase of the temperature from 400 to 650 oC, leading to a decrease of the Tafel slope. When the temperature goes beyond 650 oC, the ratio of bulk Ni0 becomes dominant and the in situ formed interface Ni0 deceases accordingly. Then the Tafel slope mainly displays the activity of the bulk Ni0, which corresponds to an increase of Tafel slope. It is thus reasonable to conclude that the Ni0 on the interface of Ni/NiO showed higher activity than the bulk Ni0.


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It is also a key concern to clarify the HER pathway. We deduced the mechanism from Tafel slopes values. As shown in Figure 6, the NiOx@BCNTs exhibited the lowest Tafel slope of b ≈ 119 mV dec-1 among the products. Such a small Tafel slope will lead to a faster increment of the HER rate. As we all know, the HER proceeds in the sequence of three principal reaction steps, as shown in the eqs 1−3 (Scheme S2), which are involved with Tafel slopes of 120, 40, and 30 mV dec-1, respectively.57-59 The Tafel slope is close to 120 mV dec-1, suggesting that the rate determining step for NiOx@BCNTs is the Volmer reaction in basic medium.57 Besides, as shown in Figure S25, NiOx@BCNTs displayed a large double-layer capacitance (Cdl) of 38.8 mF cm-2. The calculated higher exchange current density (Table S5) for the NiOx@BCNTs implies a remarkable improvement in the charge carrier mobility as well. Based on the above results, such

Figure 6. a) Tafel plots of NiOx@NC-800, NiOx@NC-900, NiOx@NC-1000 and NiOx@BCNTs. b) Chronoamperometric response of NiOx@BCNTs. superior HER performance of NiOx@BCNTs can be explained as follows: (1) the high concentration of Ni0 (both bulk Ni0 and the interface of Ni/NiO) played a vital role in the HER process; (2) The Ni0 on the interface of Ni/NiO showed distinguished ability for water splitting



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compared with bulk Ni0 through modifying the HBE; (3) the in situ produced Ni0 during the electrocatalytic process further facilitated the activity. For practical use, the durability of catalysts should be taken into consideration. As displayed in Figure 6b and Figure S26, ~90% of the current density of NiOx@BCNTs can be maintained after continuous reaction for 10 h, indicating the good stability of NiOx@BCNTs. XRD data (Figure S27) manifests the tested sample still remains the characteristic peaks of NiO and Ni. According to the analysis of XPS, the content of Ni0 increases after long-term HER electrolysis (Figure S28). The enhanced Ni0 ratio is caused by the electrochemical activation. The HRTEM studies further reveal the bamboo-like structure of the tested electrode. Besides, NiO and Ni0 are also confirmed by HRTEM images. While, the particle size of NiOx@BCNTs after the durability test is about 20.36 nm (Figure S29), which is larger than the fresh one. The aggregation of the active metal NPs leads to a decline in catalytic activity. Besides, NiOx@BCNTs showed superior catalytic activity even in higher concentration of electrolyte (2 M KOH and 6 M KOH, Figure S30), which provides a possibility for its application in high concentration alkaline electrolyte. We further evaluated the catalytic ability of NiOx@BCNTs in neutral media. From Figure S31, the overpotential of NiOx@BCNTs at 2 mA cm-2 is 287 mV. As was expected, H2-TPR-650 delivered small overpotential of 132 mV at 2 mA cm-2, indicating the potential application in HER. 4. Conclusions In summary, NiOx@BCNTs hybrids were prepared and used as highly active and robust HER electrocatalysts in alkaline solution. The experimental and theoretical results provide important insights into the understanding of the catalytic activity nature. Both the bulk Ni0 and the Ni0 on Ni/NiO interface promote the HER performance. Especially, the Ni0 on Ni/NiO interface shows


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better ability for hydrogen generation. Besides, the in situ produced Ni0 from NiO during the HER process further facilitated the splitting of water. This work provides guidance for the design of efficient catalysts in HER. Moreover, the convenient synthetic methodology and good catalytic ability towards alkaline HER promise its utilization in industrial applications. Supporting Information. Detailed analysis of NiOx@BCNTs and the control samples, electrochemical parameters, and supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Tel.: 86-571-88273551. Fax: +86-571-87951895. E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT Financial support from the National Natural Science Foundation of China (21622308, 91534114, 21376208), the MOST (2016YFA0202900), the Fundamental Research Funds for the Central Universities, and the computing time supported by Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) are greatly appreciated. We would like to thank Prof. J. G. Chen (Columbia University) for the assistance in TPR experimental design section. REFERENCES



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