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Encasing Copper Boosts the Electrocatalytic Activity of N-Doped Carbon Nanotubes for Hydrogen Evolution Yun Zhang, Yuling Ma, Yu-Yun Chen, Lu Zhao, Lin-Bo Huang, Hao Luo, Wen-Jie Jiang, Xing Zhang, Shuai Niu, Daojiang Gao, Jian Bi, Guangyin Fan, and Jin-Song Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11748 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 3, 2017

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ACS Applied Materials & Interfaces

Encasing Copper Boosts the Electrocatalytic Activity of N-Doped Carbon Nanotubes for Hydrogen Evolution Yun Zhang, a,b Yuling Ma, a,b Yu-Yun Chen,b Lu Zhao,b,c Lin-Bo Huang,b Hao Luo,b Wen-Jie Jiang,b Xing Zhang,b,c Shuai Niu,b Daojiang Gao,a Jian Bi,a Guangyin Fan,a,* Jin-Song Hub,c,* a

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610068,

China. b

CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, CAS Research

/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, 2 North first Street, Zhongguancun, Beijing 100190, China. c

School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences,

Beijing 100049, China. KEYWORDS: HER, electrocatalysis, electronic modulation, nanostructures, hydrogen production

ACS Paragon Plus Environment

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ABSTRACT

Nitrogen-doped carbons combined with transition metal nanoparticles are attractive as alternatives to the state-of-the-art precious metal catalysts for hydrogen evolution reaction (HER). Herein, we demonstrate a strategy for fabricating three-dimensional Cu encased N-doped carbon nanotube arrays which are directly grown on Cu foam (Cu@NC NT/CF) as a new efficient HER electrocatalyst. Cu nanoparticles are encased here instead of common transition metals (Fe, Co or Ni) for pursuing a well-controllable morphology and an excellent activity by taking advantage of its more stable nature at high temperature and in acidic or alkaline electrolyte. It is discovered that metallic Cu exhibits the strong electronic modulation on N-doped carbon to boost its electrocatalytic activity for HER. Such nanostructure not only offers plenty of accessible highly active sites, but also provides a 3D conductive open network for fast electron/mass transfer and facilitates gas escaping for prompt mass exchange. As a result, Cu@NC NT/CF electrode exhibits superior HER performance and durability, outperforming most of reported M@NC materials. Furthermore, the etching experiments together with XPS analysis reveal that the electronic modulation from encased Cu significantly enhances the HER activity of N-doped carbon. These findings open up opportunities for exploring other Cu-based nanomaterials as efficient electrocatalysts and understanding their catalytic processes.


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INTRODUCTION Hydrogen as a renewable and clean energy carrier is a sustainable alternative to fossil fuels, contributing to the massive global energy supply without environmental concerns.[1-3] Unlike the dominant steam reforming route with massive CO2 emission, advanced electrocatalytically water splitting with high energy conversion efficiency, especially driven by renewable energy (such as wind, solar and tidal energy), is a promising and clean route for hydrogen production.[4-5] Pt-based materials are the state-of-the-art catalysts for hydrogen evolution reaction (HER) in electrolysis. With the objective of reducing the cost of hydrogen production, it is imperative to develop the cost-effective alternatives with satisfying activities and stabilities for lowering the electricity consumption and materials cost.[6-7] During the past decade, various non-noble metal catalysts based on carbon materials and/or transition metals have been extensively explored, including heteroatom-doped carbon materials, metal alloys, hydroxides, carbides, phosphides, chalcogenides, and their composites.[8-17] In particular, nitrogen-doped carbon materials encapsulated with transition metal nanoparticles (M@NC, M=Co, Ni, Fe or their oxides) attracted much attention due to the special core-shell structure for excellent tolerance to acidic/alkaline electrolyte and high activity for HER.[18-21] It is reported that the N-doped carbon shell in M@NC is the active part while the transition metalbased core does not directly participate in catalyzing HER but can modify the electronic structure and decrease the local work function of N-doped carbon shell, thus boosting its catalytic activity.[2226]

Generally, M@NC was prepared by the pyrolysis of nanostructured transition metal sources and

nitrogen source or pre-prepared metal organic framework (MOF) precursors.[27-30] Since the commonly used transition metals (Fe, Co, or Ni ) are very active at high temperature, the pyrolysis process usually causes the outmigration and aggregation of metal nanostructure and thus the


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disordered microstructure, which is unfavorable to the performance of the catalysts and makes the catalytic mechanism ambiguous. Besides, such M@NC was usually prepared in form of powder which requires polymer binder as film-forming agent to immobilize catalyst onto a conductive substrate, causing the loss of active sites, the suffering in electron and mass transport, thus the activity decline.[31-33] Such glued catalyst was also suffered from the peel off electrode, seriously threatening the electrode durability. Therefore, it is highly desirable to explore well-defined new metal encapsulated doped carbon materials for efficient HER, especially directly grown on threedimensional (3D) electrodes in view of the practical requirements of vast gas evolution and effective mass transfer at large current output. Taking into account these facts, we developed herein a strategy for fabricating new 3D copper encased N-doped carbon nanotube arrays directly grown on copper foam (CF) (Cu@NC NT/CF). Copper was selected instead of commonly used Fe, Co or Ni in consideration of its more manageable characteristic at high temperature, stable nature in acidic/alkaline electrolyte, and similar electron donating effect to potentially modulate the electronic structure of N-doped carbon. Copper foam was used as Cu source to firstly construct vertically aligned Cu2O nanowire arrays on CF, followed by uniformly coating of polydopamine (PDA) shells. After thermal annealing, well-defined N-doped carbon nanotube arrays encasing Cu nanoparticles with controllable shell thicknesses were easily constructed on copper foam to achieve a 3D nanostructured electrode. The electrochemical experiments demonstrated that such Cu@NC NT/CF electrode exhibited superior HER performance with a low overpotential of 123 mV at 10 mA cm-2 as well as high stability. The control experiments and detailed investigation disclosed that the electronic tuning from encased Cu significantly enhanced the HER activity of N-doped carbon. Together with the structural merits from 3D nanotube arrays on open porous Cu foam for exposing sufficient active sites and


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providing efficient electron/mass transfer pathways, boosting the intrinsic activity of active sites with such electronic modulation achieved a high-performance Cu@NC electrode for HER. These new findings may open up opportunities for the development of new Cu-based electrocatalysts for diverse energy applications. EXPERIMENTAL SECTION Synthesis of Cu2O NW/CF. A copper foam (CF, 3 × 2 cm2) was firstly washed by acetone and subsequently with ethanol to remove impurities. The cleaned CF was immediately immersed into 15 mL aqueous solution containing 1.6 g NaOH and 0.456 g (NH4)2S2O8 in ice bath for 15 min. Then, it was washed with water and ethanol several times and dried in air. After that, such precursor was annealed at 500 oC in argon atmosphere for 2 h to achieve Cu2O NW/CF. Synthesis of Cu2O@PDA NW/CF. The Cu2O NW/CF was immersed into a mixed solution of 320 mL water and 40 mL ethanol. Then, 0.4 g tris-base was added into above solution under magnetic stirring for 10 min. After the addition of 0.64 g dopamine, the resultant solution was gently stirred for several hours to coat polydopamine on Cu2O NW. The product was thoroughly washed with water and ethanol and dried in air to obtain Cu2O@PDA NW/CF. The thickness of PDA shell increased with the polymerization time. Synthesis of Cu@NC NT/CF. The as-obtained Cu2O@PDA NW/CF with 30 nm PDA shell was heated to different temperature under Ar atmosphere and then kept for 2 h to achieve Cu@NC NT/CF. The samples annealed at the temperature of 300, 500 and 600 oC were noted as Cu@NC NT/CF-300, -500 and -600, respectively. The samples with 15, 30 and 60 nm PDA shell annealed at 500 oC were noted as PDA-15, -30 and -60, respectively.


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Synthesis of control samples. The acid-leached samples were prepared as follows. The Cu2O@PDA NW/CF with 30 nm thick PDA shell was firstly immersed into 0.5 M H2SO4 for different time to etch away Cu2O. The etched samples were then annealed at 500 oC for 2 h, similar to that for Cu@NC NT/CF. The samples obtained with the leaching time of 1 and 2 h were noted as acid-leached-1 and acid-leached-2, respectively. Characterization. Field-emission scanning electron microscope (Hitachi, S-4800, Japan) worked at 15 kV and transmission electron microscope (JEOL, JEM-2100F, Japan) operated at an accelerating voltage of 200 kV were carried out to investigate the morphology of each matrial. Xray photoelectron spectroscopy (XPS) spectra were collected on a Thermo Scientific ESCALab 250Xi with a monochromic Al Kα radiation (VG, USA). X-ray diffraction (XRD) measurements experiments were performed on a Regaku D/Max-2500 diffractometer equipped with a Cu Kα1 radiation (λ = 1.54056 Å, Rigaku Corporation, Tokyo, Japan). The Raman experiments were carried out using a LabRAM HR Evolution spectroscope (HORIBA, France). Electrochemical measurements. Beside the electrochemical impedance spectroscopy measurements on an Autolab workstation (PGSTAT 302N, Metrohm, Switzerland), other electrochemical tests were carried out on an CHI electrochemical workstation (CHI 660E, CH Instruments, China) in a three-electrode configuration at room temperature. Hg/HgO (1 M NaOH) and graphite rod were utilized as reference and counter electrode, respectively. The above-obtained samples were directly used as working electrode (0.5 × 0.5 cm2). Linear sweep voltammetry (LSV) tests were performed between 0.05 to -1.0 V vs. RHE at a scan rate of 5 mV s-1 in 1 M KOH or 0.5 M H2SO4. Cyclic voltammetry tests were applied at different scan rates in 1 M KOH to probe the electrochemical double layer capacitances at non-faradaic potentials for estimating the effective


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electrochemical surface areas (ECSA). Stability test was conducted under a constant current density of 10 mA cm-2 for 10 h. All the potentials were calibrated with respect to a reversible hydrogen electrode unless specified. All the polarization curve measurements were carried out with iR-correction unless specified. RESULTS AND DISCUSSION

Figure 1. Schematic illustration of the preparation of Cu@NC NT/CF. Well-defined Cu@NC NT/CF was prepared using copper foam as Cu source and dopamine as C and N sources, as illustrated in Figure 1 (details seen in Experimental Section). In brief, Cu (OH)2 nanowire arrays were first grown via simply treating CF in alkaline solution, followed by annealing in argon to form Cu2O nanowire arrays (Cu2O NW/CF). The PDA shells with varying thickness were then coated on Cu2O nanowires (Cu2O@PDA NW/CF). The representative thickness of 30


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nm was used hereinafter unless specifically illustrated. After thermal annealing at a set temperature in argon, PDA shells transformed into N-doped carbon nanotubes and Cu2O nanowire cores were reduced into Cu nanoparticles which were well encased inside the nanotubes, resulting in a Cu@NC NT/CF electrode. The representative annealing temperature of 500 oC was applied hereinafter unless specifically illustrated. Typical scanning electron microscopy (SEM) image of Cu2O NW/CF showed that the entire surface of CF was covered by plenty of Cu2O NWs in a diameter of about 100-300 nm and length of several micrometers (Figure S1a and S2). X-ray diffraction (XRD) experiment discovered the characteristic diffraction peaks of cubic Cu2O at 36.4, 42.3, 61.4, and 73.6o according to JCPDS card No. 78-2076 (Figure S1b). The three strong diffraction peaks (43.3, 50.4, and 74.1o) should originate from CF substrate. This result indicated that Cu2O NW arrays were successfully achieved on CF. After PDA coating, low-magnification SEM image displayed that the diameter of nanowire became larger (Figure S1c, d and S3). A zoom-in SEM image clearly evidenced that every Cu2O NW in brighter contrast were entirely coated with a shell (inset in Figure S1d). Furthermore, TEM image in Figure S1e clearly discovered the core-shell structure with a darker core in a diameter of about 120 nm and a brighter shell in a thickness of about 30 nm, consistent with the SEM result in Figure S1d. The Cu2O NW/CF before and after PDA coating was further monitored by Raman technique. As shown in Figure S1f, Cu2O NW/CF presented a set of characteristic Raman peaks of Cu2O at 146, 219, 295, 411, and 628 cm-1. After PDA coating, these Cu2O peaks remained at their inherent positions but their intensity turned weaker. In addition, two broad peaks from PDA were distinguished at about 1350 and 1600 cm-1, coming from the stretching and deformation of catechols which was the key component of dopamine molecule.

[34-36]

The Raman results

corroborated that the Cu2O NW/CF was successfully coated by PDA shell.


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Figure 2. Typical (a) SEM, (b) TEM, and (c) HRTEM images of Cu@NC NT/CF. (d) XRD pattern of Cu@NC NT/CF. After thermal annealing at 500 oC, SEM images depicted that the one-dimensional core-shell nanowire structure was well preserved. The nanowire cores turned to be discrete nanoparticles in a size of several hundred nanometers and the shell turned thinner (Figure 2a and S4). TEM image demonstrated these darker nanoparticles encased in a brighter shell and that the thickness of carbon layer was several to 10 nanometers (Figure 2b). High-resolution TEM (HRTEM) image in Figure 2c taken on a particle displayed the continuous lattice fringes with a distance of 0.21 nm, corresponding to the inter-distance of (111) plane of cubic Cu which suggested that these encased nanoparticles were Cu nanocrystals. The graphitized carbon layers were also observed in the shell which was the characteristic feature of pyrolyzed carbon materials. The crystal structure of Cu nanoparticles and the graphitization of PDA shell were further evidenced by XRD and Raman


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analysis, respectively. The XRD pattern (Figure 2d) showed only three peaks at 43.3, 50.4, and 74.1o, which were characteristic diffraction of cubic Cu according to JCPDS card No. 04-0836. All diffractions from Cu2O disappeared. Therefore, it could be inferred that Cu2O nanowire were completely reduced into Cu after thermal annealing. The Raman features of D band (1370 cm-1) and G band (1580 cm-1) verified the existence of disordered defects and sp2 carbon in the carbon shell, respectively (Figure S5). Such defects could supply abundant active sites for electrocatalytic HER while sp2 carbon structure was the bedrock for an efficient electron transport. As expected from the pyrolysis of PDA, X-ray photoelectron spectroscopy (XPS) measurements clearly evidenced the N 1s signal, indicating the shell part should be N-doped carbon layer (Figure S6). In short, the above results revealed that the thermal annealing of Cu2O NW/CF at 500 oC in argon atmosphere achieved the N-doped carbon nanotubes encased with metallic Cu nanocrystals on copper foam, i.e. Cu@NC NT/CF. It should be noted that during the synthesis we first pre-treated the as-grown Cu(OH)2 nanowires to obtain Cu2O nanowire, followed by PDA coating and thermal annealing, to prepare Cu@NC NT. If we directly use Cu(OH)2 nanowires to coat PDA it cannot remain the nanowire morphology after the coating (Figure S7). If we firstly reduce Cu2O to Cu, the nanowire morphology cannot be reserved due to the melting of Cu (Figure S8). It was expected that such unique core-shell structured Cu@NC NT/CF would hold the following merits for electrocatalytic process: 1) Encased Cu nanocrystals may improve the electronic structure of Ndoped carbon for catalyzing HER and abridge the electron diffusion length; 2) The interconnected nanotube arrays provided plenty of accessible active sites and enable efficient electron and mass transfer; 3) The structure of nanotube arrays on 3D open CF substrate facilitated the gas release.


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Figure 3. (a) Polarization curves of Pt/C, Cu foam, and Cu@NC NT/CFs prepared at different temperature (PDA thickness in precursor is 30 nm). (b) Corresponding Tafel plots of these Cu@NC NT/CFs. (c) V-t curve of Cu@NC NT/CF-500. (d, f) SEM images of Cu2O@PDA NW/CF with 15 nm PDA shell (d) and the resulting Cu@NC NT/CFs (f). (e, g) SEM images of Cu2O@PDA NW/CF with 60 nm PDA shell (e) and the resulting Cu@NC NT/CF (g). (h) Polarization curves of Cu@NC NT/CFs prepared by tuning PDA shells. The sample PDA-30 here is same as the above-mentioned Cu@NC NT/CF-500. In order to investigate the influence of annealing temperature on the structure and electrocatalytic HER performance of Cu@NC NT/CF, three catalysts were prepared at different temperature (300, 500, and 600 oC) and denoted as Cu@NC NT/CF-300, -500, and -600, respectively. As shown in the SEM images (Figure S9), Cu@NC NT/CF-300 had the similar morphology to Cu@NC NT/CF-500 but the nanowire-like morphology for Cu@NC NT/CF-600 showed a bit collapse. All


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three catalysts showed the similar XRD patterns (Figure 2d and S10). The electrocatalytic HER performance for three Cu@NC NT/CFs were explored in 1M KOH solution. For comparison, the pure Cu foam and the-state-of-the-art commercial Pt/C catalyst were also investigated in parallel. The HER polarization curves in Figure 3a showed that Pt/C exhibited excellent electrocatalytic activity for HER with the smallest overpotential. The Cu foam displayed a large overpotential of 300 mV at 10 mA cm-2, indicating that Cu itself had a very poor HER activity. Among three Cu@NC NT/CF samples, Cu@NC NT/CF-500 achieved the best HER activity. The overpotential required to drive the cathodic current density of 10 mA cm-2 was 123 mV, smaller than those of Cu@NC NT/CF-300 (149 mV) and Cu@NC NT/CF-600 (183 mV). It was appreciably better than those reported analogues in alkaline medium, such as Ni@NC (190 mV),[27] Co@BCN (183 mV),[37] and Co-P/NC (191 mV).[38] Moreover, Cu@NC NT/CF-500 exhibited the smallest onset overpotential (40 mV). Interestingly, the corresponding Tafel slope of Cu@NC NT/CF-500 was 63 mV dec-1, smaller than that of Cu@NC NT/CF-300 (83 mV dec-1), and -600 (101 mV dec-1), indicating that Volmer-Heyrovsky mechanism dominated the HER, where the recombination with an additional proton was the rate-limiting step (Figure 3b).[39-40] Extrapolating the Tafel plot to η at 0 V, Cu@NC NT/CF-500 achieved an exchange current density (j0) of 0.36 mA cm-2 (Figure S11). Furthermore, the cyclic voltammetry (CV) measurement was employed for researching the capacitance of the double layer, thus the ECSA of electrode (Figure S12a, c and e). Based on the plots of the current densities vs. scan rates (Figure S12b, d and f), the capacitance values were calculated to be 91, 101 and 66 mF cm-2 for Cu@NC NT/CF-300, -500, and -600, respectively. The difference in capacitance for the three catalysts agreed well with the difference in their HER activity. It should be noted that the higher temperature usually delivered a higher ECSA during the pyrolysis of carbonaceous materials. In our case, the smallest ECSA for Cu@NC NT/CF-600


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should be ascribed to the partial collapse of the nanowire structures probably due to the low melting point for Cu nanowires. Additionally, the capacitance of Cu@NC NT/CF-500 was larger than that of N-Co@G (10 mF cm-2),[28] Co@BCN (83 mF cm-2),[37] and Co@NC/NG (37 mF cm-2),[24] suggesting its higher ECSA and abundant active sites which contributed to the enhanced HER performance. The durability was further examined using chronoamperometric measurement at a constant current density of 10 mA cm-2 for 10 h in 1M KOH solution (Figure 3c). It can be seen that the V-t curve shifts negligibly degraded during the test, suggesting the excellent durability of Cu@NC NT/CF-500 for HER in alkaline solution. The catalytic performance of Cu@NC NT/CF in 0.5 M H2SO4 was also evaluated and the results were presented in Figure S13. Among three samples, the Cu@NC NT/CF-500 showed the best activity with an overpotential of 290 mV at 10 mA cm-2, which was comparable to the Co-NRCNTs (260 mV),[41] FeCo@NCNTs-NH (~280 mV)[42] and Co@NC/NG (~280 mV). [24] Additionally, the Cu@NC NT/CF-500 showed negligible overpotential loss after 10 h chronoamperometry test, implying its excellent durability for HER in acid medium. Therefore, 500 oC was used for all following experiments. Furthermore, the thickness of PDA shell was adjusted through controlling the polymerization time of dopamine to investigate its influence on the thickness of NC nanotube and the morphology of Cu@NC NT/CF, thus on the electrocatalytic performance. It was found that when PDA shell was about 15 nm, the prepared Cu@NC NT/CF displayed a disordered morphology, where Cu nanoparticles suffered serious aggregation probably because such a thin shell was not strong enough to confine the reduced Cu nanoparticles during the thermal annealing process (Figure 3d and f). As discussed above, when the thickness of PDA shell increased to about 30 nm, the carbonized shell was able to effectively encapsulate Cu nanoparticles without outmigration. When the thickness further increased to 60 nm, the similar intact nanotube arrays were obtained (Figure


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3e and g). The electrochemical measurements for HER showed that such difference in the thickness of NC nanotube and morphology significantly affected their electrocatalytic performance. The Cu@NC NT/CF prepared with 15 nm PDA shell exhibited the poor HER activity worse than that prepared with 30 nm PDA shell, indicating the aggregation of Cu nanoparticles appreciably deteriorated the electrocatalytic activity. Moreover, the Cu@NC NT/CF prepared with 60 nm PDA shell demonstrated the worst HER activity although it had the similar ordered 3D nanotube morphology, which implied that thicker NC nanotubes were not favorable for HER and sharply degraded the activity of Cu@NC NT/CF. These results implicated that the state of encased Cu nanoparticles in Cu@NC NT/CF significantly affected HER activity.

Figure 4. (a) Polarization curves of Cu@NC NT/CF before and after acid leaching. Typical SEM images of the sample (b) acid-leached-1 and (c) acid-leached-2. (d) Typical TEM image of the sample acid-leached-2. (e) N content in terms of N/C ratio from XPS measurements and (f) N1s XPS spectra of Cu@NC NT/CF before and after acid leaching.


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Inspired by the above discussion, to further understand the role of encased Cu nanoparticles in Cu@NC NT/CF, the control experiments were performed by deliberately removing the encased Cu nanoparticles without appreciably changing the morphology. Because Cu was very stable in normal acidic solution, the acid leaching experiments were carried out by pre-etching Cu2O nanowire cores in Cu2O@PDA NW/CF instead of directly etching Cu, followed by the exactly same thermal annealing process as that for preparing Cu@NC NT/CF (details seen in experimental section). For clearly understanding the effect of encased Cu, two experiments with different leaching time (1 and 2 h) in 0.5 M H2SO4 were carried out and the obtained products were noted as acid-leached-1 and -2, respectively. The electrocatalytic activity of Cu@NC NT/CF before and after acid leaching for HER were evaluated and the polarization curves were shown in Figure 4a. It could be seen that after the removal of Cu nanoparticles the HER activity of catalyst (acidleached-1) apparently declined in comparison with unleached Cu@NC NT/CF. Increasing acid leaching time to completely remove Cu, the catalyst (acid-leached-2) exhibited the lowest HER activity. This result revealed that acid leaching to remove Cu significantly degraded the HER activity of catalysts. The morphology of Cu@NC NT/CF before and after acid leaching were monitored by SEM and TEM techniques (details seen in supplementary information). After acid leaching for 1h, most of Cu nanoparticles encased in NC nanotube were removed and a small fraction of Cu remained as shown in SEM image (Figure 4b). Increasing acid leaching time to 2 h, Cu nanoparticles were entirely removed as evidenced by SEM and TEM images (Figure 4c, d and S14). The N content and solution resistance (Rs) of the electrodes were further assessed by XPS and electrical impedance spectroscopy (EIS) experiments, respectively. The results in Figure 4e and S15 displayed that the N content in terms of the ratio of N/C in the samples and Rs in both acid-leached-


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1 and acid-leached-2 were very close to unleached Cu@NC NT/CF. The unchanged N content and similar Rs indicated that the number of catalytically active sites and the conductivity should not be changed by acid leaching. Given that the N-doped carbon should be responsible for catalyzing HER in view of all Cu nanoparticles were encased in the nanotubes and Cu itself did not show the effective HER activity, this result implied that the intrinsic activity of N-doped carbon may be tuned by the encased Cu nanoparticles. The high-resolution XPS N1s spectra were therefore analyzed, which clearly showed that the binding energy down shifted by 0.2 eV from 398.8 eV for Cu@NC NT/CF to 398.6 eV for acid-leached-1 (Figure 4f). The down shifted value was further increased to 0.6 eV for acid-leached-2 (398.2 eV), which suggested that the electronic structure of N-doped carbon in Cu@NC NT/CF was modulated by the encased Cu. CONCLUSION In summary, 3D well-defined Cu nanoparticles encased N-doped carbon nanotube arrays on Cu foam was developed as a highly active non-precious metal HER electrode by using Cu foam as Cu source to grow Cu2O nanowire arrays, then coating polydopamine as N-containing carbonaceous shells, followed by an annealing process. The thickness and morphology of N-doped carbon nanotubes can be controlled by the thickness of the polydopamine shells and encased Cu nanoparticles were achieved by in-situ reducing Cu2O nanowire cores during the carbonization of PDA shells. Such Cu@NC NT/CF electrode hold several merits for HER application: 1) Compared with the commonly-used transition metals (Fe, Co, or Ni), encased Cu nanoparticles are more stable and manageable at high temperature but offer similar electron donating effect to potentially modulate the electronic structure of N-doped carbon for improving HER activity, which enables the preservation of well-defined catalyst morphology for the fast electron transportation and effective mass transfer; 2) The one-dimensional nanotube arrays vertically grown on Cu foam with


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3D open structure benefit the prompt gas escaping for efficient mass exchange and exposure of plenty of highly accessible active sites for catalyzing HER; 3) the electronic structure of N-doped carbon was appreciably modulated by the encased Cu, thus significantly enhancing HER activity. Consequently, the Cu@NC NT/CF electrode exhibited superior HER performance with a low overpotential of 123 mV at 10 mA cm-2 and a high stability, outperforming most of reported M@NC materials. Furthermore, the etching experiments and detailed XPS analyses corroborated that the electronic modulation from encased Cu significantly boosted the HER activity of N-doped carbon. These findings would inspire the further exploration of other Cu-based nanomaterials as electrocatalysts for efficient water electrolysis. ASSOCIATED CONTENT Supporting Information Supporting Information is available free of charge via the Internet at http://pubs.acs.org: Additional SEM images, TEM images, Raman spectra, XPS spectrum, XRD patterns, and electrochemical data. AUTHOR INFORMATION Corresponding Author *Email: [email protected]; *Email: [email protected]. ACKNOWLEDGMENT This work was financially supported by the National Key Research and Development Program of China (2016YFB0101200), the National Natural Science Foundation of China (21773263, 91645123 and 21573249), the Strategic Priority Research Program of the Chinese Academy of


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Sciences (XDB12020100), the Sichuan Youth Science and Technology Foundation (2016JQ0052), as well as by the Beijing National Laboratory for Molecular Sciences (BNLMS, 20160106). The authors also thank Dr. Yang Sun at the Center for Analysis and Testing, ICCAS for their help for the XRD analysis. REFERENCES (1)

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Graphic Abstract

Well-defined Cu@NC NT/CF (Cu nanoparticles encased N-doped carbon nanotube arrays on Cu foam) is explored as a new efficient HER electrode with superior electrocatalytic performance due to the electronic modulation of Cu nanoparticle core on outer N-doped carbon shell.


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Encased Copper Boosts the Electrocatalytic ... - ACS Publications

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