Efficient electrocatalysis of hydrogen evolution by ultralow-Pt-loading bamboo-like nitrogen-doped carbon nanotubes

Materials Today Energy 6 (2017) 173e180

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Efficient electrocatalysis of hydrogen evolution by ultralow-Pt-loading bamboo-like nitrogen-doped carbon nanotubes Ruguang Ma a, Yao Zhou a, Fangfang Wang a, Kang Yan b, ***, Qian Liu a, c, **, Jiacheng Wang a, c, * a

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China State Key Laboratory of Mechanics and Control of Mechanical Structures, College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, PR China c Shanghai Institute of Materials Genome, 99 Shangda Road, Shanghai, 200444, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 May 2017 Received in revised form 14 September 2017 Accepted 2 October 2017

The highly effective catalysts for the electrochemical hydrogen generation with substantially reduced cost are strongly desirable, but difficult to achieve. The reduction of Pt-loading by downsizing the Pt nanoparticles is an efficient strategy for obtaining low-cost and high-activity HER electrocatalysts. Herein, we describe a facile strategy to the formation of ultrafine Pt nanoparticles (NPs) bonding to Ndoped bamboo-like carbon nanotubes, serving as a highly active and durable catalyst with ultra-low Pt loading for the electrochemical hydrogen generation. With the addition of mesoporous silica to change the wettability of the electrode from hydrophobicity to hydrophilicity of the electrode, the optimized nanocomposite catalyst with ultra-low Pt loading (0.74 wt%) shows a near-zero onset potential (Uonset), an extremely low overpotential of 40 mV to reach 10 mA cm
2 (h10), a small Tafel slope of 33 mV dec
1, and excellent long-term stability, which are comparable to those of 20 wt% Pt/C catalyst. The outstanding properties ensure this promising nanocomposite with significantly reduced Pt loading to become one of the most active catalysts towards the electrochemical hydrogen generation in acid medium. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Carbon nanostructures Nitrogen doping Defects Pt nanoparticles Electrochemical hydrogen generation

1. Introduction The depletion of fossil fuels and environmental challenges (e.g. global warming) promote the development of clean energy systems to meet the needs of present and future generations [16]. Thus the renewable clean energy source has become one of the world's foremost challenges. Hydrogen (H2) is the cleanest fuel available for replacing fossil fuels in the future [4]. Therefore, the novel and clean hydrogen evolution technology could solve the problems of both energy shortage and environmental pollution caused by fossil

* Corresponding author. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China. ** Corresponding author. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China. *** Corresponding author. E-mail addresses: [email protected] (K. Yan), [email protected] (Q. Liu), [email protected] (J. Wang). https://doi.org/10.1016/j.mtener.2017.10.003 2468-6069/© 2017 Elsevier Ltd. All rights reserved.

fuels, beneficial for the sustainable development of society. At present, H2 is mainly produced from steam-reformed methane and coal, and this process still consumes large amount of fossil reserves and leads to the emission of a substantial amount of CO2 [27]. In sharp comparison, H2 from water splitting powered by renewable resources-derived electricity (e.g., sunlight and wind) is especially attractive as a clean energy carrier for replacing the current fossil fuel-based energy systems [15]. The water covers 75% area of the Earth, and it is much plentiful as the source for H2 production by water electrolysis. Pt is the most active for electrocatalytic hydrogen evolution from water, but the high cost hinders its wide applications in electrocatalysis [7,18,44]. It remains challenging to develop highly active hydrogen evolution catalysts with more abundance at lower costs. In order to overcome this barrier, various strategies have been employed to eliminate or reduce the utilization of Pt. For example, many earth abundant transition metal (e. g. Co, Ni, Fe, Mo, W, etc.) sulfides, carbides, phosphides, and borides have been investigated to catalyze the hydrogen evolution reaction (HER) [9,20,21,24,25,29,33,36,42,43,48,50], but these attempts seldom

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succeeded because of large overpotentials of over 100 mV (vs. reversible hydrogen electrode (RHE) for all potentials reported in this study) to reach 10 mA cm
2 (h10) [7,50]. Another strategy to overcome the above challenges is to increase the surface to bulk atomic ratio of Pt, thus significantly reducing the amount of Pt loading while maintaining the high HER activity. Recently, various methods have been developed for decreasing the Pt loading in the Pt-based HER electrocatalysts. Monolayer (ML) Pt could be prepared on the WC and TiC films by atomic layer deposition (ALD) technique [1,10,11,19]. The resultant ML Pt-WC and ML Pt-TiC showed the comparable HER activity to commercial 20 wt% Pt/C. The combined experimental and theoretical results revealed the important insights into the understanding of the Pt-like properties of transition carbides, playing the important role in supporting ML Pt and promoting the HER synergistically. Recently, unique Pt-Pd-graphene stack structures with controllable Pt shell thickness were synthesized, the HER activity of which increases with a decrease in the Pt thickness. It can be explained by the surface polarization mechanism as suggested by the first-principle theoretical calculation [3]. In 2016, ultrafine Pt nanoclusters (NCs) of less than ~18 Pt atoms were successfully confined with a calixarenebased [Ni24] coordination cage with the sulfur atoms on the edges [38]. The as-synthesized Pt NCs exhibited higher electrocatalytic activity than commercial Pt/C toward the HER. However, these strategies of reducing Pt loading for the HER involved the usage of expensive equipment and/or complicated steps/reagents. Thus, it is urgent to search for a more facile strategy to prepare low-Pt loading HER electrocatalysts by downsizing the Pt NPs. Carbon nanotubes (CNTs) are promising supports for Pt-based NPs in various electrocatalytic applications [40]. However, the additional treatments (e.g., chemical/thermal oxidation/activation, heteroatom doping, etc.) are necessary to implement defects and heteroatoms before loading active NPs [26,31]. The N doping can activate p electrons of adjacent carbon atoms by conjugating with long-pair electrons from N dopants [14]. It is believed that the structural defects as well as N dopants not only beneficially stabilize active metallic NPs, but also improve the proton adsorption and reduction kinetics during the HER process [6]. Herein, we describe a facile strategy of preparing ultrafine Pt NPs bonding onto the bamboo-like, CNTs with congenital structural defects and N doping, functioning as a highly active and stable HER electrocatalyst in acidic electrolyte (Fig. 1). The Co2þ/g-C3N4derived NCNTs were used as the support for ultrafine Pt NPs. The optimized Pt/NCNTs nanocomposite catalyst with only 0.74 wt% Pt loading obtained an excellent HER activity with a near-zero Uonset, a small h10 of 40 mV, and a small Tafel slope of 33 mV dec
1, almost the same as those for commercial 20 wt% Pt/C. Furthermore, this

Fig. 1. Schematic illustration for the preparation of Pt NPs bonding to the NCNTs (Pt/ NCNTs) as a highly active catalyst for electrochemical hydrogen generation. (a) pyrolysis of Co2þ/g-C3N4 to form Co/NCNTs in Ar at 700  C, (b) reduction of chloroplatinic acid by Co/NCNTs, and (c) removal of abundant Co species by acid washing to obtain Pt/ NCNTs.

activity could be retained with only a small loss of ~10 mV at h10 after 2000 cyclic voltammograms (CV) scans. 2. Experimental section 2.1. Reagents Nafion solution (5 wt%), chloroplatinic acid and dicyanamide (C8H11N5, DCA) were purchased from Aldrich. Anhydrous ethanol, nitric acid (HNO3, 69%), concentrated sulfuric acid (H2SO4, 98%), sodium borohydride (NaBH4), and cobalt nitrate hexahydrate (Co(NO3)2$6H2O)) were purchased from Sinopharm Chemical Reagent Co., Ltd. The reference catalyst 20 wt% Pt/C was purchased from Johnson Matthey (United Kingdom). The commercial carbon nanotubes (CNTs) were purchased from Chengdu Organic Chemicals Co., LTD. All chemical reagents were used as received without further purification. 2.2. Materials synthesis Synthesis of Co/NCNTs by pyrolysis of the mixture of DCA and cobalt nitrate hexahydrate. Typically, DCA (47.6 mmol) was dissolved in water at 100  C under magnetic stirring until a transparent solution was formed. Then, a calculated amount of cobalt nitrate hexahydrate was added to the above solution and the resulting solution was kept at 100  C until a dry powder was obtained. Afterward, this powder was placed on an alumina boat and heated at 400  C for 30 min with a ramp of 2  C min
1 in nitrogen atmosphere and further 700  C for 6 h at a ramp of 5  C min
1 to produce a black powder. It was further ground to obtain a fine powder composed of metallic Co nanoparticles and bamboo-like N-doped carbon nanotubes (NCNTs), thus named as Co/NCNTs [24,49]. Synthesis of Pt NPs supported on the NCNTs (Pt/NCNTs). The asprepared Co/NCNTs (1 g) was dispersed into deionized water (200 mL) by ultrasonication. Then, the calculated amounts (3.5, 8, or 15 mL) of chloroplatinic acid solution (5 mM) were slowly added into the above solution under the continuously magnetic stirring at room temperature. After stirring for 2 h, an excess amount of 0.5 M H2SO4 solution (50 mL) was introduced into the solution to etch off the redundant metallic Co nanoparticles. After the separation by centrifugation, washing and drying, the obtained black powder is denoted as Pt/NCNT-x (x ¼ I, II, or III) depending on the different amount of chloroplatinic acid solution used above. The inductively coupled plasma atomic emission spectroscopy (ICPAES) analysis showed that Pt/NCNT-I, II and III possessed 0.36, 0.74, and 1.35 wt% Pt loading, respectively. Unless stated specifically, the Pt/NCNTs stands for the sample with 0.74 wt% Pt loading. Synthesis of NCNTs. The resulting Co/NCNTs, prepared by the pyrolysis of the mixture of DCA and cobalt nitrate hexahydrate, was treated by mixing with 0.5 M H2SO4 solution overnight to etch off exposed metallic Co NPs. And the obtainable material was designated as NCNTs which still contain large amount of Co nanoparticles inside the NCNTs due to the protective graphitic layers. Synthesis of Pt NPs supported on commercially available CNTs (Pt/ comm-CNTs) for comparison. The commercially available CNTs (comm-CNTs) were refluxed with nitric acid for several hours to remove any metallic impurities, followed by centrifugation, washing and drying. Then, metallic Co NPs were supported on comm-CNTs (1 g) by adding NaBH4 solution (4 mmol in 5 mL water) into the aqueous mixture (20 mL) of comm-CNTs and Co(NO3)2$6H2O (0.4 mmol). This solution was allowed for further stirring for 2 h at room temperature to form metallic Co nanoparticles on the comm-CNTs (Co/comm-CNTs). After filtering, washing, and drying, the Co/comm-CNTs were obtained, which were further used as the substrate to prepare Pt/comm-CNTs following the procedure of

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preparing Pt/NCNTs. The calculated amount (8 mL) of chloroplatinic acid solution (5 mM) was slowly added into the dispersion of Co/ comm-CNTs under the continuously magnetic stirring. After 2 h, an excess amount of 0.5 M H2SO4 solution (50 mL) was introduced into the solution to etch off the redundant metallic Co nanoparticles. After the separation by centrifugation, washing and drying, the obtained black powder is denoted as Pt/comm-CNTs. 2.3. Structural characterization The morphology of the catalysts was observed by scanning electron microscopy (SEM) on an FEI field emission Magellan 400 SEM equipped with an Oxford Instruments XEDS system. The sample was dispersed in ultra-high purity ethanol, and then a drop of the suspension was dropped onto a Al coil. Samples were prepared for transition electron microscopy (TEM) examination by dispersing the catalyst powder in ultra-high purity ethanol. A drop of the suspension was then allowed to evaporate on a carbon microgrid supported by a 300 mesh copper grid. Samples were examined in a JEOL JEM 2100F transmission electron microscope operating at an accelerating voltage of 200 kV. This instrument was also fitted with an Oxford Instruments energy-dispersive X-ray spectrometer (XEDS) for compositional analysis. Nitrogen sorption isotherms were measured using a Micromeritics ASAP 2010 surface area and pore size analyzer at liquid nitrogen temperature (
196  C). Prior to measurement, the hierarchically porous materials were dehydrated under vacuum at 200  C overnight. The specific surface areas were calculated by the Brunauer-Emmett-Teller (BET) method. The total pore volume was calculated from the amount of nitrogen adsorbed at a relative pressure of 0.99. The pore size distribution curves were calculated from the analysis of the desorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) model. X-ray powder diffraction (XRD) measurements were carried out at a step-size of 5 min
1 over the 2theta angle range of 20e80 on a Rigaku D/MAX-2250 V diffractometer with Cu Ka radiation. X-ray photoelectron spectroscopy (XPS) was recorded with an ESCALAB 250 X-ray photoelectron spectrometer with Al Ka

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(hy ¼ 1486.6 eV) radiation. The carbon monoliths were ground to powder and then glued onto indium (In) metal particles by pressing for measurements. All spectra were calibrated using 284.5 eV as the line position of adventitious carbon. For each sample, the XPS measurements were performed three times, and the average data of the N contents and the ratio of graphitic to pyridinic N were reported. Raman spectra were recorded on a DXR Raman Microscope (Thermal Scientific Co., USA) with 532 nm excitation length. The Pt and Co contents in the composites were analyzed using an Agilent 4100 microwave plasma-atomic emission spectrometer. 2.4. Electrode preparation and electrochemical measurements The active material (5 mg) and rod-like mesoporous silica nanoparticles (2 mg) were dispersed in the mixture of water (0.25 mL) and ethanol (0.25 mL) containing 5% Nafion solution (25 mL) under ultrasonic irradiation for ca. 20 min until a homogeneous ink was formed. Then, 5 mL ink containing 50 mg catalyst was transferred onto a glassy carbon electrode (GCE) with 5 mm diameter, yielding a catalyst level of 0.25 mg cm
2. And the calculated Pt loading level of the commercial Pt/C electrode is 0.05 mgPt cm2. The electrode with the catalyst was dried at 50  C, which was used as the working electrode for further electrochemical measurements. For electrode preparation, mesoporous silica nanoparticles are added into the inks of the tested active materials including Pt/NCNTs, Pt/comm-CNTs, and NCNTs. In the case of commercial 20 wt% Pt/C, no mesoporous silica was used. For comparison, a Pt/NCNTs modified GCE was also prepared without the addition of mesoporous silica nanoparticles. Electrochemical activity of the working electrode was using a standard three-electrode cell with a Pt plate as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. An aqueous solution of 0.5 M H2SO4 was used as the electrolyte for the electrochemical studies. The measured potentials vs. SCE were converted to a reversible hydrogen electrode (RHE) scale. In 0.5 M H2SO4, ERHE ¼ ESCE þ 0.2412 þ 0.059  pH. The electrochemical cell was connected to an electrochemical

Fig. 2. a) SEM, b-c) high-magnification SEM images showing b) Co NPs with about 50e100 nm in diameter encased in the NCNTs and c) other small-sized NPs on the surface, and d) the cross-sectional line scans (EDS spectrum) of a single Pt NP from a small particle marked with a yellow dashed circle in Fig. 2c. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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workstation (CHI760) coupled with a rotating disk electrode (RDE) system (AFMSRCE3529, Pine Research Instrumentation, USA). Prior to the measurements, the electrolyte was saturated by N2 for 15 min. The working electrode was cycled for fifty times at a scan rate of 100 mV s
1 before the data were recorded with a scan rate of 2 mV s
1 at 1500 rpm. Current density was normalized to the geometrical area of the working electrode. The applied voltages were corrected for the ohmic resistance of the cells (the iR compensation), to give an effective voltage (Veffective) for the potential-current polarization curves according to the formula [34]: Veffective ¼ Vapplied
iR, where i is the current flowing through the cell and R is the resistance of the cell. The cell resistances were automatically measured by the iR test function available on the potentiostats (CHI760). The ESCA of the catalysts were calculated using the double-layer capacitor based on the reported electrochemical method. The cyclic voltammograms (CVs) were obtained

at different rates from 20 to 180 mV s
1 in the potential range of 0.3e0.42 V (vs. RHE). The stability of the catalysts was studied at a scan rate of 5 mV s
1 for 2000 cycles. The long-term (18000 s) stability was also tested at the controlled overpotential (40 mV). The electrochemical impedance spectroscopy (EIS) measurements were carried out from 100 KHz to 0.1 Hz in the same configuration at different h values of from 0 to 300 mV. 3. Results and discussion The Pt/NCNTs were prepared via galvanic reduction of chloroplatinic acid by Co2þ/g-C3N4-derived NCNTs containing Co NPs (Co/NCNTs, Fig. 1 and S1) [49], following the reaction equation: 2þ 2Co þ PtCl2
þ 6Cl
. A large positive redox potential 6 / Pt þ 2Co (0.458 V) of E(PtCl2
/Pt, 0.735 V) minus E(Co2þ/Co, 0.277 V) could 6 effectively drive this redox reaction, leading to the nucleation and

Fig. 3. a) TEM, b) HAADF-STEM micrographs, c) EDS spectrum for a region marked with a green dashed circle in Fig. 3b, d) particle-size distribution for Pt-based NPs on the surface, e) HRTEM image of Co NPs encased within the NCNTs, and f) HRTEM images of a Pt NP supported on the surface of the NCNTs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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growth of Pt around Co NPs [35], followed by etching off abundant Co species by acid washing. The Pt contents for various Pt/NCNTs, determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES), could be controlled by adjusting volume of chloroplatinic acid solution (Table S1). When performing the HER experiments, it is observed that silica modification on the Pt/NCNTs electrode significantly improves the activity in terms of h10 (Fig. S2), because the addition of hydrophilic mesoporous silica changes the wettability of the electrode from hydrophobicity to hydrophilicity of the working electrode (Fig. S3eS5). This is advantageous for fast proton adsorption onto the hydrophilic electrode operated in aqueous medium. The optimal performance was obtained for Pt/NCNTs with 0.74 and 1.35 wt% Pt loading, extremely close to that for 20 wt% Pt/C (Fig. S6). Taking the cost into consideration, the sample with 0.74 wt% Pt (named as Pt/NCNTs unless stated otherwise) was selected as a model catalyst for further analysis. Scanning electron microscopy (SEM) images show that Pt/ NCNTs consists of tens-of-mm-long bamboo-like nanotubes with diameters ranging from 50 to 300 nm (Fig. 2a). Co NPs with varied sizes are found at the tip or inside of nanotubes (Fig. 2a-b, S7), which correlated with the reported reports [24,49]. Thus, these Co NPs cannot be removed by acid washing due to the outside protective graphitic layers. The pyrolysis of pure g-C3N4 without Co2þ led to complete decomposition (Fig. S8), implicating the key role of Co in the catalytic growth of NCNTs. It is clearly observed that there are some very small NPs bonding to the surface of the NCNTs (Fig. 2c), and the line scan EDS of SEM confirmed they are Pt NPs (Fig. 2d). Transmission electron microscopy (TEM) image also indicates the existence of large black particles in the Pt/NCNTs (Fig. 3), and the high-resolution (HR) TEM image confirms that they are metallic Co NPs a d(111) spacing of 0.21 nm encapsulated within the NCNTs

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(Fig. 3e). Ascribed to the encapsulation of these Co particles by graphitic layers, they cannot be removed off by treating in acidic solution. Also high-angle annular dark-field scanning TEM (HAADFSTEM) image indicates small NPs are present on the surface of the NCNTs (Fig. 3b). EDS analysis verifies that they are Pt NPs (Fig. 3c). No peaks attributed to Pt are found in the XRD pattern, suggesting lower Pt content than the detection limit (Fig. S1). The HR-TEM images clearly demonstrate that these Pt NPs have an average diameter of ~2.3 nm (Fig. 3d) and they are crystalline with the d(111) spacing of 0.22 nm (Fig. 3f). The X-ray photoelectron spectroscopy (XPS) survey confirms that Pt/NCNTs are composed of Pt (1.02 wt%), Co (0.72 wt%), C (88.08 wt%), N (5.42 wt%), and O (4.76 wt%) (Fig. S9, Table S1). The higher Pt content (1.02 wt%) from XPS analysis than 0.74 wt% determined by ICP-AES implies that Pt-based NPs are rich on the surface. Such a unique nanostructure is advantageous for fast electron transport from Co-encased NCNTs to Pt-based active sites, thus improving the activity. The high-resolution XPS spectra further provide the valuable information about the chemical state of Pt/NCNTs. Fig. 4a shows the Pt 4f spectrum, where two peaks at 75.1 and 71.7 eV can be ascribed to Pt (0) 4f5/2 and 4f7/2, respectively. The doped N atoms mainly exist as the forms of graphitic and pyridinic N (Fig. 4b, S10d) [17,32]. The long-pair electrons of N dopants can enrich the electron density of the adjacent carbon atoms, thus enhancing the proton adsorption [49]. The weak peaks are found at 778.9 and 794.4 eV corresponding to the Co 2p3/2 and 2p1/2 states, respectively, which are consistent with zero-valent Co element (Fig. 4c, S10e). The Co content of 6.7 wt% determined by ICP-AES is significantly larger than 0.72 wt% from XPS, suggesting the encapsulation of major Co species inside the NCNTs (Table S1). This unique core-shell nanostructure is thus favourable for promoting electron penetration from encased Co NPs to NCNTs [9].

Fig. 4. High-resolution a) Pt 4f, b) N1s, and c) Co 2p XPS spectra with deconvoluted peaks, and d) N2 sorption isotherms for Pt/NCNTs (inset: the corresponding pore size distribution curve) for Pt/NCNTs.

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The structural information of the NCNTs was further investigated by the Raman spectroscopy technique through the determination of the G (related to pristine sp2 graphitic layer) bands at ~1580 cm
1, and D (related to defect in sp2 lattice) bands at ~1340 cm
1 [2,5,13]. The intensity of the D to G band (ID/IG) is used to provide the qualitative information on the graphitic degree in carbon materials. The Pt/NCNTs shows a ID/IG value of 1.16 (Fig. S11), evidently larger than 0.2e0.4 for CNTs and graphene prepared by chemical vapour deposition [12,30]. The band broadness as well as large ID/IG value implies that a large number of defects are present in NCNTs, possibly resulting from the N dopants and concomitant microstructural rearrangement of carbon atoms [23]. Accordingly, NCNTs shows a larger lattice spacing (0.35 nm) than that (0.335 nm) of graphite, determined by HRTEM (Fig. 3e). These structural defects can both anchor/stabilize active NPs and adjust the proton adsorption equilibrium. Moreover, Pt/NCNTs has bimodal mesopores of 3.9 and 24.2 nm and a moderate specific surface area of 148.5 m2 g
1 (Fig. 4d), facilitating rapid masstransfer and reaction kinetics and thus improving the reactivity [8,28]. The HER activity of Pt/NCNTs as well as the control samples was investigated in 0.5 M H2SO4 medium. The active materials were mixed with 30% mesoporous silica rods and Nafion solution in

water-ethanol solution by ultrasonication for electrode preparation. Blank glassy carbon electrode (GCE) had a negligible activity (Fig. 5a), and commercial Pt/C showed superior activity with a nearzero Uonset. The pure NCNTs possessed a moderate activity with a Uonset of ~150 mV, while only 0.74 wt% Pt loading (Pt/NCNTs) revealed a significantly enhanced activity with a small Uonset of ~0 mV, the same as that for Pt/C. The more negative potential results in the fast increase of the cathodic current. Its h10 value (40 mV) is only 6 mV more than that (34 mV) of Pt/C, implying the exceptional HER activity of Pt/NCNTs. To highlight the key role of defective NCNTs, Pt NPs were supported on commercial CNTs, designated as Pt/comm-CNTs for comparison. Comm-CNTs with less defects resulted in the aggregation of Pt NPs in spite of low Pt loading of 0.79 wt% (Fig. S12-S13). Pt/comm-CNTs has an inferior activity to Pt/ NCNTs (Fig. 5a, Table S1), although comm-CNTs show a surface area (124 m2 g
1) comparable to the NCNTs (Fig. S14). This reveals a significantly positive effect of innate defects in the NCNTs on the dispersion and catalytic activity of Pt NPs (Fig. S13). The defects could improve the reduction kinetics and thus achieve large current density at low overpotential. Moreover, encased Co NPs could also promote fast electron transfer, thus improving the activity [9,39]. The HER mechanism was further studied by the Tafel slopes (Fig. 5b). The Tafel slope of Pt/NCNTs is 33 mV dec
1, much smaller

Fig. 5. a) The polarization curves after iR compensation for a blank GCE, NCNTs, Pt/NCNTs, Pt/comm-CNTs, and Pt/C in 0.5 M H2SO4 at a scan rate of 2 mV s
1. b) Tafel plots of NCNTs, Pt/comm-CNTs, Pt/NCNTs and Pt/C. c) CV curves for Pt/NCNTs with different rates from 10 to 200 mV s
1 in the potential range of 0.30e0.42 V. d) Capacitive current at 0.36 V as a function of scan rate for Pt/NCNTs and Pt/comm-CNTs (Dj0 ¼ ja e jc). e) Electrochemical impedance spectroscopy (EIS) Nyquist plots collected at a bias voltage of
0.1 V. f) HER polarization curves for Pt/NCNTs before and after 2000 cycles (Inset: current-time response at 40 mV over 18000 s).

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than 78 mV dec
1 of NCNTs, but very close to 31 mV dec
1 of Pt/C [21]. It implies a Volmer-Tafel mechanism for Pt/NCNTs and the recombination of two adsorbed protons to form a hydrogen molecule is the rate-limiting step. In sharp contrast, the replacement of NCNTs by comm-CNTs yielded a large Tafel slope (66 mV dec
1) for Pt/comm-CNTs, implying a Volmer-Heyrovsky HER mechanism. This result discloses that the defective NCNT matrix is beneficial for H2 desorption via a Tafel pathway, thus leading to the significantly enhanced HER activity of Pt/NCNTs. In terms of overpotential (h10) and the Tafel slope, Pt/NCNTs with ultra-low Pt loading is comparable to Pt/C, and strongly superior to the latest metal-based electrocatalysts, such as Au@NC (125 mV, 77 mV dec
1) [47], Rh2S3 (88 mV, 44 mV dec
1) [46], Mo2C@NC (125 mV, 60 mV dec
1) [22], P-WN/rGO (85 mV, 54 mV dec
1) [45], Se-rich NiSe2 (117 mV, 32 mV dec
1) [37], etc (Fig. 6 and Table S2). The electrochemical surface area (ECSA) was evaluated to gain further insights into the electrocatalysts by calculating the electrochemical double layer capacitances (Cdl) (Fig. 5c, S15). The Cdl (13.1 mF cm
2) of Pt/NCNTs is much larger than 0.8 mF cm
2 of Pt/ comm-CNTs (Fig. 5d), possibly resulted from the existence of large amount of the defective, N-doped structural sites in Pt/NCNTs. A larger Cdl value implies a higher effective active surface area and amount of adsorbed protons, resulting in improved proton exchangeability between active sites and the electrolyte. Accordingly, more hydrogen was evolved at a low overpotential [21]. The activity enhancement could be further reflected by the electrochemical impedance spectroscopy (EIS). The Nyquist plots show the charge transfer resistance (RCT) significantly decreased from 18 to 5.2 U when Pt loading increased from 0.36 to 0.74 wt% in Pt/NCNTs (Fig. S16). In contrast, the replacement of NCNTs by comm-CNTs led to a larger RCT value (79.3 U), showing distinctly slow electrode kinetics towards HER on Pt/comm-CNTs (Fig. 5e, S17) [22], although a similar content of Pt was loaded on both samples. The fast electron transfer in Pt/NCNTs should result from the electronically cooperative functions of Pt NPs, N-doped dopants, and encased Co NPs [51,52]. Moreover, the activity of Pt/NCNTs could be retained with a small loss of ~10 mV at h10 after 2000 CV scans. And ~92% of the current density was maintained after 18000 s testing at a constant overpotential of ~40 mV, indicating its superior stability towards electrocatalytic HER (Fig. 5f). The above results imply the successful synthesis of highly active bimetallic HER electrocatalysts with ultra-low Pt content. Its exceptional performance can be ascribed to the synergistic effects of the following factors: i) ultra-small sizes for Pt NPs can efficiently enhance the density of active sites; ii) the inborn structural defects and N-dopants in the NCNTs not only can anchor and stabilize ultrafine Pt NPs, advantageous for the improved activity and stability; iii) the N dopants could lead to enhanced interaction with protons

Fig. 6. Tafel slope vs. overpotential at 10 mA cm
2 for Pt/NCNTs with ultra-low Pt loading compared with other catalysts.

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owing to the presence of long-pair electrons on the N atoms [49]; iv) the encased Co NPs could decrease the local work function on the NCNTs' surface, contributed by fast electron transfer from metal NPs to NCNTs [9,41]. 4. Conclusions In summary, we have successfully synthesized a new family of bimetallic Pt NPs bonding to Co2þ/g-C3N4-derived, bamboo-like NCNTs with congenital defects by galvanic replacement, functioning as a highly active and stable catalyst for electrochemical HER. The optimized catalyst with ultra-low Pt loading (0.74 wt%) showed a comparable activity to 20 wt% Pt/C in acidic electrolyte. The present research paves a new avenue for designing/preparing novel nanostructured electrocatalysts with significantly reduced loading of precious metals in water splitting, fuel cells, etc. Acknowledgements The authors thank the financial support from the National Key Research and Development Program of China (2016YFB0700204), NSFC (51602332, 51502327), Science and Technology Commission of Shanghai Municipality (15520720400, 15YF1413800, 14DZ2261203, 16DZ2260603), Key Project for Young Researcher of State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Computational Center for Shanghai Institute of Ceramics (Y61ZC8180G), and One Hundred Talent Plan of Chinese Academy of Sciences. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.mtener.2017.10.003. References [1] D.D.V. Anicijevic, V.M. Nikolic, M.P. Marcetakaninski, I.A. Pasti, Int. J. Hydrogen Energy 38 (2013) 16071e16079. [2] W.S. Bacsa, J.S. Lannin, D.L. Pappas, J.J. Cuomo, Phys. Rev. B 47 (1993) 10931e10934. [3] S. Bai, C. Wang, M. Deng, M. Gong, Y. Bai, J. Jiang, Y. Xiong, Angew. Chem. 53 (2014) 12120e12124. [4] M. Balat, Int. J. Hydrogen Energy 33 (2008) 4013e4029. [5] J. By A, R. Saito, G. Dresselhaus, M.S. Dresselhaus, Phil. Trans. R. Soc. Lond. A 362 (2004) 2311e2366. [6] J.C. Charlier, Defects in carbon nanotubes, Acc. Chem. Res. 35 (2002) 1063e1069. [7] H.M. Chen, C.K. Chen, R.S. Liu, L. Zhang, J.J. Zhang, D.P. Wilkinson, Chem. Soc. Rev. 41 (2012) 5654e5671. [8] Y. Chen, Q. Liu, J. Wang, Nano Adv. 1 (2016) 79e89. [9] J. Deng, P. Ren, D. Deng, X. Bao, Angew. Chem. 54 (2015) 2100e2104. [10] D.V. Esposito, J.G. Chen, Energy Environ. Sci. 4 (2011) 3900e3912. [11] D.V. Esposito, S.T. Hunt, A.L. Stottlemyer, K.D. Dobson, B.E. Mccless, R.W. Birkmire, J.G. Chen, Angew. Chem. Int. Ed. 49 (2010) 9859e9862. [12] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Phys. Rev. Lett. 97 (2006), 187401. [13] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095e14107. [14] K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, Science 323 (2009) 760e763. [15] J. Greeley, T.F. Jaramillo, J. Bonde, I. Chorkendorff, J.K. Norskov, Nat. Mater. 5 (2006) 909e913. [16] M. Hoel, S. Kverndokk, Resour. Energy Econ. 18 (1996) 115e136. [17] F.-X. Hu, L. Li, K. Lin, L. Cui, C.-J. Shi, A.S. Sayyar, S. Cui, J. Inorg. Mater. 31 (2016) 827e833. [18] J. Kibsgaard, Z.B. Chen, B.N. Reinecke, T.F. Jaramillo, Nat. Mater. 11 (2012) 963e969. [19] Y.C. Kimmel, L. Yang, T.G. Kelly, S.A. Rykov, J.G. Chen, J. Catal. 312 (2014) 216e220. [20] H. Li, C. Tsai, A.L. Koh, L. Cai, A.W. Contryman, A.H. Fragapane, J. Zhao, H.S. Han, H.C. Manoharan, F. Abildpedersen, Nat. Mater. 15 (2016) 48. [21] H. Lin, Z. Shi, S. He, X. Yu, S. Wang, Q. Gao, Y. Tang, Chem. Sci. 7 (2016) 3399e3405. [22] Y. Liu, G. Yu, G. Li, Y. Sun, T. Asefa, W. Chen, X. Zou, Angew. Chem. 127 (2015) 10902e10907.

180

R. Ma et al. / Materials Today Energy 6 (2017) 173e180

[23] R. Ma, Y. Ma, Y. Dong, J.M. Lee, Nano Adv. 1 (2016) 50e61. [24] R. Ma, E. Song, Y. Zhou, Z. Zhou, G. Liu, Q. Liu, J. Liu, Y. Zhu, J. Wang, Energy Storage Mater. 6 (2017) 104e111. [25] R. Ma, Y. Zhou, Y. Chen, P. Li, Q. Liu, J. Wang, Angew. Chem. Int. Ed. 54 (2015) 14723e14727. [26] L.F. Mabena, S. Sinha Ray, S.D. Mhlanga, N.J. Coville, Appl. Nanosci. 1 (2011) 67e77. [27] M. Mbodji, J.M. Commenge, L. Falk, D.D. Marco, F. Rossignol, L. Prost, S. Valentin, R. Joly, P. Delgallo, Chem. Eng. J. 207 (2012) 871e884. [28] C.M. Parlett, K. Wilson, A.F. Lee, Soc. Rev. 42 (2013) 3876e3893. [29] S. Peng, L. Li, X. Han, W. Sun, M. Srinivasan, S.G. Mhaisalkar, F. Cheng, Q. Yan, J. Chen, S. Ramakrishna, Angew. Chem. Int. Ed. 53 (2014) 12594e12599. [30] R. Rao, J. Reppert, R. Podila, X. Zhang, A.M. Rao, S. Talapatra, B. Maruyama, Carbon 49 (2011) 1318e1325. [31] E. Raymundo-Pinero, A. Cazorla-Amoros, S. Delpeux, E. Frackowiak, K. Szostak, F. Beguin, Carbon 40 (2002) 1614e1617. [32] Q. Shi, Y.-P. Lei, Y.-D. Wang, Z.-M. Wang, J. Inorg. Mater. 31 (2016) 351e357. [33] Y. Shi, B. Zhang, Chem. Soc. Rev. 45 (2016) 1529e1541. [34] M.D. Symes, L. Cronin, Nat. Chem. 5 (2013) 403e409. [35] Y. Vasquez, A.K. Sra, R.E. Schaak, J. Am. Chem. Soc. 127 (2005) 12504. [36] H. Vrubel, X. Hu, Angew. Chem. Int. Ed. 124 (2012) 12703e12706. [37] F. Wang, Y. Li, T.A. Shifa, K. Liu, F. Wang, Z. Wang, P. Xu, Q. Wang, J. He, Angew. Chem. 55 (24) (2016) 6919e6924. [38] S. Wang, X. Gao, X. Hang, X. Zhu, H. Han, W. Liao, W. Chen, J. Am. Chem. Soc. 138 (2016) 16236e16239.

[39] S. Wang, J. Wang, M. Zhu, X. Bao, B. Xiao, D. Su, H. Li, Y. Wang, J. Am. Chem. Soc. 137 (2015) 15753e15759. [40] Y.-J. Wang, B. Fang, H. Li, X.T. Bi, H. Wang, Prog. Mater. Sci. 82 (2016) 445e498. [41] G. Wu, K.L. More, C.M. Johnston, P. Zelenay, Science 332 (2011) 443e447. [42] H.B. Wu, B.Y. Xia, L. Yu, X.-Y. Yu, X.W. Lou, Nat. Commun. 6 (2015) 6512. [43] Z. Xing, Q. Liu, A.M. Asiri, X. Sun, Adv. Mater. 26 (2014) 5702e5707. [44] M.L. Xu, B. Duan, Y.-J. Zhang, G.-T. Yang, P. Dong, S.-B. Xia, X.-W. Yang, J. Inorg. Mater. 30 (2015) 931e936. [45] H. Yan, C. Tian, L. Wang, A. Wu, M. Meng, L. Zhao, H. Fu, Angew. Chem. Int. Ed. 54 (2015) 6325e6329. [46] D. Yoon, B. Seo, J. Lee, K.S. Nam, B. Kim, S. Park, H. Baik, S.H. Joo, K. Lee, Energy Environ. Sci. 9 (2016) 850e856. [47] W. Zhou, T. Xiong, C. Shi, J. Zhou, K. Zhou, N. Zhu, L. Li, Z. Tang, S. Chen, Angew. Chem. 128 (2016) 8556e8560. [48] Y. Zhou, R. Ma, P. Li, Y. Chen, Q. Liu, G. Cao, J. Wang, J. Mater. Chem. A 4 (2016) 8204e8210. [49] X. Zou, X. Huang, A. Goswami, R. Silva, B.R. Sathe, E.k. Mikmekov a, T. Asefa, Angew. Chem. 126 (2014) 4461e4465. [50] X. Zou, Y. Zhang, Chem. Soc. Rev. 44 (2015) 5148e5180. [51] C.Q. Shang, M.C. Li, Z.Y. Wang, S.F. Wu, Z.G. Lu, ChemElectroChem 3 (9) (2016) 1437e1445. [52] Y. Liu, Y. Liu, S.H.S. Cheng, S.C. Yu, B. Nan, H.D. Bian, K. Md, M. Wang, C.Y. Chung, Z.G. Lu, Electrochim. Acta 219 (2016) 560e567.