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Jun 22, 2017 - Promotion of Electrocatalytic Hydrogen. Evolution Reaction on Nitrogen-Doped Carbon. Nanosheets with Secondary Heteroatoms. Konggang ...
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Promotion of Electrocatalytic Hydrogen Evolution Reaction on Nitrogen-Doped Carbon Nanosheets with Secondary Heteroatoms Konggang Qu,†,‡ Yao Zheng,‡ Xianxi Zhang,† Ken Davey,‡ Sheng Dai,*,‡,§ and Shi Zhang Qiao*,‡ †

Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, China ‡ School of Chemical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia § School of Chemical Engineering and Advanced Materials, Newcastle University, Newcastle upon Tyne NE1 7RU, United Kingdom S Supporting Information *

ABSTRACT: Dual heteroatom-doped carbon materials are efficient electrocatalysts via a synergistic effect. With nitrogen as the primary dopant, boron, sulfur, and phosphorus can be used as secondary elements for co-doped carbons. However, evaluation and analysis of the promotional effect of B, P, and S to N-doped carbons has not been widely researched. Here we report a robust platform that is constructed through polydopamine to prepare N,B-, N,P-, and N,Sco-doped carbon nanosheets, characterized by similar N species content and efficient B, P, and S doping. Systematic investigation reveals S to have the greatest promotional effect in hydrogen evolution reactions (HER) followed by P and that B decreases the activity of N-doped carbons. Experimental and theoretical analyses show the secondary heteroatom promotional effect is impacted by the intrinsic structures and extrinsic surface areas of both materials, i.e., electronic structures exclusively determine the catalytic activity of active sites, while large surface areas optimize apparent HER performance. KEYWORDS: dual-doped carbons, hydrogen evolution reaction, polydopamine, graphene, water splitting

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Single-doped carbons have been manipulated through codoping with various secondary heteroatoms. This can increase the number of active sites and bring about so-called “synergistic coupling” and, as a result, significantly boost the electrocatlytical activities of carbons.19,20 Current dual-doping approaches are the introduction of N to sp2-carbon frames, combined with another element, such as B, S, or P.10,19−22 So far, however, research has focused on attempting diverse strategies for dual-heteroatom doping,17,23 and there is little research on comparative analyses of the promotion effect of secondary B, P, and S doping to N-doped carbons. This is actually an important to guide future good design of electrocatalytic materials. In summary, research has shown two current problems: (1) catalytic activity of N-doping carbons varies with content of N, especially different N structures of pyridinic N (p-N) and graphitic N (g-N);11,24,25 and (2) poor doping efficiency of B,

o meet the challenge of dwindling fossil fuels, electrochemically renewable energy conversion and storage have been widely researched.1−5 However, a replacement for scarce and nondurable noble-metal electrocatalysts is urgently required.6,7 Newly emerging heteroatomdoped carbon nanomaterials have attracted significant interest, principally because of their abundance, excellent electrical conductivities, tunable molecular structures, and tolerance to acidic/alkaline environments.8−10 Doping of carbon with nitrogen is important to tailor electrocatalytic property. This is achieved with high doping efficiency together with consequent electron-donor property and enhanced π bonding in carbon frameworks.8,9,11−14 Nitrogen-doped carbons exhibit excellent electrocatalytic activity and durability for oxygen electrode electrocatalysis, such as oxygen reduction reaction (ORR)8 and oxygen evolution reaction (OER),15,16 that are comparable with metal counterparts. However, previous research implies that single nitrogen doping is limited in improving catalytic performance for hydrogen evolution reactions (HER).9,17,18 © 2017 American Chemical Society

Received: May 11, 2017 Accepted: June 22, 2017 Published: June 22, 2017 7293

DOI: 10.1021/acsnano.7b03290 ACS Nano 2017, 11, 7293−7300

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ACS Nano Scheme 1. Fabrication of the Co-Doped Carbon Nanosheets of N,B-CN, N,P-CN, and N,S-CNa

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R-SH is 2-mercaptoethanol. In doped graphene models, green, pink, blue, red, yellow, and purple represent C, B, N, O, S, and P atoms, respectively.

Figure 1. (A) FTIR spectra of GP, GPB, GPP, and GPS. (B) Raman spectra of N-CN, N,B-CN, N,P-CN, and N,S-CN. (C) TEM and (D) the magnified TEM images of N,S-CN.

Here, we propose a strategy using PDA to synthesize in situ N,B-, N,P-, and N,S-co-doped carbon nanosheets for systematic investigation of the promotional effect of B, P, and S to Ndoped carbons in electrocatalytic HER. Theoretical and experimental results confirm S with greater promotion in HER activity to N-doped carbon compared with P, while codoping with B decreases activity of N-doped carbon. Elctrocatalytic analyses reveal electrocatalytic HER performance is dominated by the intrinsic electronic structures of carbon electrocatalysts. Performance can be improved by optimizing surface area with a benign intrinsic structure.

P, or S because of oxidation susceptibility at high temperature of these,20,23 despite using large amounts of doping precursors. Therefore, new doping strategies with well-controlled components and high doping efficiency are needed. Based on our previous research,26−28 homogeneous doping and identical N contents of N-doped carbons can be achieved via polydopamine (PDA)-derived doped carbons. In addition to low toxicity, easy dispersibility, and high carbon yield, PDA can undergo multiple postmodification reactions. For example, the catechol groups of PDA can conjugate boric acid.29,30 PDA is highly reactive to thiol functional groups via Schiff base or Michael addition reaction.29,31,32 The positive-charged amino groups of PDA can bind to phosphate groups through electrostatic attraction.21 Significantly these reactions proceed efficiently at ambient temperature and do not require harsh reaction conditions. Crucially, these reactions take place on both surface and interior of PDA films to provide the possibility of highly efficient doping, due to the intrinsic swellability and permeability of PDA. 33−35 PDA therefore appears to simultaneously address both current problems.

RESULTS AND DISCUSSION As illustrated in Scheme 1, graphene-PDA (GP) hybrids were prepared by mixing dopamine (DA) and graphene oxide (GO) in a PBS buffer (pH ∼ 8.5).36 DA self-polymerized to deposit a PDA thin-film layer directly on the GO surface.26,27 Boric acid (BA), 2-mercaptoethanol (ME), and 1,4-butylenebisphosphonic acid (BPA) as the corresponding secondary-heteroatom 7294

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Figure 2. XPS surveys and the corresponding high-resolution spectra of (A) N 1s, B 1s for N,B-CN; (B) N 1s, P 2p for N,P-CN; and (C) N 1s, S 2p for N,S-CN. (D) Contents of total N, specific N species, and secondary dopants in three co-doped electrocatalysts.

precursors were introduced separately to PDA via either covalent interaction or electrostatic and/or hydrophobic interactions. Resulting GP hybrids with B, S, and P heteroatoms were denoted as GPB, GPS, and GPP.31,37 The obtained GPB, GPS, and GPP hybrids were pyrolyzed to derive corresponding N,B-, N,S-, and N,P-co-doped carbon nanosheets, represented as N,B-CN, N,S-CN, and N,P-CN. A single N-doping carbon nanosheet (N-CN) was produced by the pyrolysis of GP as a control. Fourier transform infrared (FTIR) spectra were employed to verify formation, and subsequent modification, of PDA thinfilms (Figure 1A). For GP, the characteristic peaks at 1504 and 1610, cm−1 are consistent with indole or indoline structures of PDA,36 confirming successful deposition of PDA film onto the GO surface. Atomic force microscopy (AFM) analysis (Figure S1) shows that PDA forms a uniform layer coating on the surface of GO, with a thickness of ∼2.5 nm.26 The spectrum of GPB displays the absorption band from asymmetric B−O stretching at 1415 cm−1 that characterizes formation of boric acid ester.38 For GPP, the strong band at 1150 cm−1 and the weak band at 1252 cm−1 originate from vibration of P−O and PO,39 while the weak peaks at 2846 and 2930 cm−1 are attributed to C−H streching of aliphatic −CH2−.30,40 For the GPS, the weak band at 634 cm−1 corresponds to the C−S, indicating the effective grafting of ME to PDA.26 Raman spectra of the four doped carbons (Figure 1B) contain typical D and G bands at 1349 and 1586 cm−1. However, the ID/IG ratios of N,B-CN (1.17), N,P-CN (1.18), and N,S-CN (1.15) are greater than that of N-CN (1.08). This indicates successful

introduction of boron, phosphorus, and sulfur atoms. According to the empirical Tuinstra−Koening relation,41,42 the average size of graphene sp2 domains (La), which is inversely proportional to the ID/IG, is about 3.76, 3.73, and 3.82 nm for the N,B-CN, N,P-CN, and N,S-CN, respectively. This suggests similar graphitization levels of the doped samples, excluding any influence of different graphitization levels in these samples.8,43 The transmission electron microscopy (TEM) images (Figures 1C,D and S2) showed three codoped samples retained good dispersibility together with separate sheet morphologies (several micrometre) with rough surfaces that are distinctly crumpled and wrinkled. This facilitates the formation of large interlayer pores that would be beneficial to electrocatalytical mass transfer and gas diffusion. The magnified TEM images clearly show the abundant lattice structures of graphene sheets (also referred to as graphene domains) in three co-doped samples. Noteworthy is that most of the graphene domains are disordered and tortuous as a result of defects. These highly interconnected graphene domains ensure in-plane charge transfer for electrocatalytic reaction. The compositions and chemical status of elements in the asprepared carbon materials were analyzed by X-ray photoelectron spectroscopy (XPS) (Figure 2). In the high-resolution B 1s spectrum of N,B-CN (Figure 2A), the peak at 189.0 eV is attributed to the BC3 structure, while the minor peak centered at 188.0 eV is assigned to B4C. The intense peak at 190.4 eV corresponds to the structure of boron atoms bonding to carbon and oxygen atoms (BC2O). This suggests a significant fraction 7295

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Figure 3. HER polarization curves and the corresponding Tafel plots of N-CN, N,B-CN, N,P-CN, and N,S-CN in 0.5 M H2SO4 (A, B) and 1 M KOH (C, D). (E) The corresponding difference in the current density at 1.055 V plotted against scan rate (see Figure S2), the calculated Cdl values are shown as the inset. (F) The relationship between the Cdl and ΔGH* values of four electrocatalysts and their overpotentials at the current density of 10 mA cm−2.

comparison of various B, P, or S-doping methods is summarized in Tables S1−S3. Because PDA film is highly permeable and swellable to small molecules,33−35 it makes possible postdecoration of PDA from surface to interior, which results in efficient doping of secondary heteroatoms. Meanwhile, typical elemental mapping images (Figure S3) indicate the uniform dispersion of different dopants via this versatile codoping method, also confirming the homogeneous PDA deposition and the following postmodification reactions. The electrocatalytic activities of the as-prepared doped carbon nanosheets toward HER were evaluated in both acidic (0.5 M H2SO4) and alkaline (1 M KOH) aqueous solutions. Polarization curves were obtained from linear sweep voltammetry (LSV) measurements with a sweep rate of 5 mV s−1. Figure 3A presents the HER polarization curves of various electrocatalysts in 0.5 M H2SO4. To deliver a 10.0 mA cm−2 current density (Ej=10), a critical metric in solar fuel production, the operating potential of N,S-CN is −0.29 V. This is lower than that for N,P-CN (−0.55 V), N-CN (−0.62 V), and N,B-

of boron exists in the form of epoxy. The signal at 191.9 eV reveals that boron atoms are surrounded by carbon and oxygen atoms (BCO2), thereby indicating presence of a boronic acid group in the sample.38,44 The P 2p spectrum of N,P-CN (Figure 2B) can be deconvoluted into two peaks centered at 133.2 and 134.3 eV that can be ascribed to P−C and P−O.10 The peak area of P−C is approximately twice that of the P−O. This indicates that most P atoms have been incorporated into the carbon framework.45 For the N,S-CN (Figure 2C), the high-resolution S 2p peaks are deconvoluted into three peaks associated with C−S−C (162.8 eV for S 2p3/2, 164.1 eV for S 2p 1/2 ) and C−SO x −C (168.1 eV) species. 19,26 Most importantly, three dual-doped carbon nanosheets contain similar content of total N (3.2−3.7 at. %) and specific N species as shown in Figure 2D. Specifically, g-N moiety (2.2− 2.6 at. %) dominates in all samples over p-N (0.9−1.2 at. %). The amount of secondary dopants are 4.2 at. % B for N,B-CN, 5.4 at. % P for N,P-CN, and 5.8 at. % S for N,S-CN. This is much greater than similar counterparts.19,20,23 A detailed 7296

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Figure 4. (A) Comparison of the HER activities in acidic and alkaline solutions of the as-prepared four electrocatalysts and other reported carbon-based electrocatalysts. (B) Polarization curves of N,S-CN recorded before and after 1000 potential sweeps (0.0 to −0.8 V vs RHE) in acidic and alkaline solutions. (C) HER current−time chronoamperometric response of N,S-CN in acidic solution.

eV). Similarly, N,P-G exhibits a reduced ΔGH* (0.53 eV), while conversely, N,B-G gives an increased ΔGH* (1.10 eV). The underlying reason for this is that N,S-G showed a closer-density of state peak to the Fermi level than that of the models for N,PG and N,B-G. It is concluded therefore that greater H* adsorption can be obtained. Importantly, the experimentally determined catalytic activities agree well with the theoretically ΔGH*. The apparent catalytic activity for an electrocatalyst is governed by the unit activity on each active site and the number of these active sites.26 Unit activity is related to the adsorption property of active sites, while the number of these is reflected through a fine nanostructure of the materials. Therefore, increasing the extrinsic surface area by increased exposure of active sites is a robust strategy to improve catalytic activity. As is seen in Figures 3E and S5, the active surface area of these carbon materials can be estimated from values of the electrochemical double-layer capacitance (Cdl) because of a proportional relationship. Figure 3F illustrates this relationship of overpotential (η) at a current density of 10 mA cm−2 in acidic solution with the Cdl and ΔGH* values. For N,B-CN, despite a high Cdl value, ∼3-fold that of N-CN, it also shows a degraded activity (90 mV increase in η) due to its large ΔGH* value, caused by the introduction of B. This confirms ΔGH* plays a controlling role in catalytic activity. Although N,P-CN has a smaller ΔGH*, its low Cdl results in a small increase in HER performance (70 mV decrease in η). This suggests there is room for improvement by adjusting extrinsic surface area. For N,S-CN, the smallest value of ΔGH* and the largest Cdl together determine optimized HER activity (330 mV decrease in η). This is a significant enhancement compared with N-CN and N,P-CN. A similar trend was observed in alkaline solution (Figure S6). Here, the influence of secondary dopant concentration to catalyst apparent activity is beyond the

CN (−0.71 V). Additionally, the N,S-CN exhibited a Tafel slope of 76.9 mV decade−1 (Figure 3B), which was smaller than that for N,P-CN (139.3 mV decade−1), N-CN (159.3 mV decade−1), and N,B-CN (198.2 mV decade−1). In 1 M KOH solution (Figure 3C,D), the operating potential of N,S-CN to deliver a cathodic current density of 10 mA cm−2 is −0.38 V with a low Tafel slope of 103 mV decade−1. This is greater than for N,P-CN (−0.49 V, 118 mV decade−1), N-CN (−0.57 V, 97 mV decade−1), and N,B-CN (−0.73 V, 87 mV decade−1). In both acidic and alkaline electrolyte, the apparent HER performance of three dual-doped elecrocatalysts shared a similar trend, namely, N,S-CNT showed significantly improved HER activity and N,P-CN a moderate increase, compared with that of N-CN. N,B-CN however showed a reverse-trend. Given the nearly identical contents of total N and specific N species in these dual-doped carbons, this HER performance is reasonably attributable to selective doping of secondary heteroatoms (S, P, or B). This selective doping is significant to the intrinsic electronic structure of resultant carbons through the synergistially coupling effect. The electronic structure has a direct correlation to surface adsorption ability of electrocatalysts. It is widely accepted that HER involves Volmer−Heyrovsky or Volmer−Tafel pathways.46 Based on Tafel slope, the Volmer step (hydrogen adsorption on the catalyst surface) is the ratedetermining step for these four catalysts because of weak hydrogen adsorption on graphene surface. The strength of hydrogen adsorption can be expressed as the free energy change for this step (ΔGH*). A low value of |ΔGH*| indicates increased activity for HER (H* designates an H atom adsorbed at the active sites of the catalyst). Based on theoretical computations (Figure S4) of the atomic configurations of the three co-doped graphene models,47 the most active dual-doped graphene model is N,S-G. This is represented by a low ΔGH* value of 0.23 eV and is significantly less than that for N-G (0.81 7297

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ACS Nano consideration, because the large ΔGH* of N,B-CN determines its degraded performance regardless of B doping concentration, and N,P-CN and N,S-CN have similar levels of secondary dopants, about 5.4% P and 5.8% S. Additionally, electrochemical impedance spectrum (EIS), reflecting the contact and charge-transfer impedance during the HER, conforms a better electrochemical activity with a smaller impedance (Figure S7). It is concluded that to enhance electrocatalysts design, an improved intrinsic electronic structure is a first priority, and the second is to increase surface area. The significantly improved HER catalytic activity of N,S-CN can be clearly illustrated by comparison with other reported carbon materials. As is shown in Figure 4A, in 0.5 M H2SO4, the operating potential of N,S-CN is −0.29 V to achieve a current density of 10 mA cm−2. This is meaningfully lower than that of N,P-co-doped graphene (N,P-G, −0.42 V),23 singledoped graphene23,47 including N-doped graphene (N-G, −0.48 V), B-doped graphene (B-G, −0.52 V), P-doped graphene (PG, −0.54 V), and S-doped graphene (S-G, −0.56 V) and comparable with that of N,S-co-doped porous graphene (N,SPG, −0.28 V)17 and carbon nitride@N-doped graphene (C3N4@NG, −0.25 V).9 N,S-CN in 1 M KOH however requires an overpotential of −0.35 V at a current density of 5 mA cm−2. This is much lower than that of C3N4@NG (−0.57 V),9 N,P-G (−0.58 V),23 N-G (−0.63 V), P-G (−0.69 V), S-G (−0.72 V), and B-G (−0.74 V). N,S-CN has a potential reduction of 50 mV to deliver 5 mA cm−2 in comparison with that of N,S-co-doped carbon nanotube (N,S-CNT, −0.40 V),36 the present best carbon counterpart in alkaline solutions. Given that water splitting is more readily conducted in alkaline electrolytes, the significantly greater activity of N,S-CN for HER therefore is seen to hold promise for carbon-based water splitting electrocatalysts. Additionally, durability tests were conducted in both acidic and alkaline aqueous solutions for 1000 cycles using potential sweeps at a scan rate of 50 mV s−1 between 0.0 V and −0.8, V (vs RHE). The resulting polarization curves, Figures 4B and S8, compare HER activities before and after potential cycling. No obvious degradation is observed for both N,S-CN and N,P-CN catalysts, thereby revealing stability for sustainable hydrogen production. Long-term durability was also evaluated for 4 h in 0.5 M H2SO4, Figure 4C. As is seen in the figure, the initial current shows a slow attenuation, probably due to N,S-CN peeling from electrodes during evolution of H2 gas. There the current is seen to remain almost unchanged. This result confirms the excellent stability of the N,S-CNT in catalyzing H2 evolution. This stability of dual-doped carbon nanosheets can be attributed to strong tolerance of metal-free active sites to both acidic and alkaline environments as well as the structurally two-component graphene-PDA integration.

performance is dominated by the intrinsic electronic structure of co-doped carbons and can be optimized by increasing surface area of a benign intrinsic structure. Our finding would advance knowledge for the rational design of high-performance carbon electrocatalysts in the future.

EXPERIMENTAL SECTION Chemicals and Materials. Graphite flakes, sulfuric acid (H2SO4, 95−98%), potassium permanganate (KMnO4, 99%), phosphorous acid (H3PO4, 85%), hydrogen peroxide (H2O2, 30%), dopamine hydrochloride, 2-mercaptoethanol, boric acid, 1,4-butylenebisphosphonic acid, and disodium hydrogen phosphate (Na2HPO4) were purchased from Sigma-Aldrich. All chemicals were used as received without further purification. Milli-Q water (18.2 MΩ) was used throughout. Characterizations. FTIR spectra were collected on the transmission module of a Thermo Nicolet 6700 FTIR spectrometer at 2 cm−1 resolution and 64 scans. TEM images were determined on a JEM-2100 microscopy. AFM images were obtained under ambient conditions with Ntegra Solaris AFM (NT-MDT) operated in a tapping mode. The Raman spectra were collected on iHR550 from HORIBA Scientific with a 532 nm solid laser as the excitation source. XPS analysis was conducted on an Axis Ultra spectrometer (Kratos Analytical Ltd.) with monochromated Al Kα radiation at ca. 5 × 10−9 Pa. Electrochemical Characterizations. Electrochemical measurements were performed with a CHI 760C electrochemical analyzer (CH Instruments, USA) in a standard three-electrode system that included an Ag/AgCl/KCl (4 M) reference electrode (a graphite rod as the counter electrode for the HER test) in a glass cell containing 100 mL 0.5 M H2SO4 and 1 M aqueous KOH as an acidic and alkaline electrolyte, respectively. All potentials measured were calibrated to RHE using the following equation: E(RHE) = E(Ag/AgCl) + 0.205 V + 0.059 × pH. Cyclic voltammograms (CVs) were performed with the scan rate of 10 mV s−1, and working electrodes were scanned several times until stabilization before CV data were collected. The HER polarization curves were obtained using the LSVs with a scan rate of 5 mVs −1 . The long-term durability test was performed using chronopotentiometric measurements. All currents presented are corrected against ohmic potential drop. For the electrochemical tests, 2 mg of the fabricated catalysts was dispersed in 1 mL of Milli-Q water. The mixture was ultrasonicated to give a homogeneous catalyst ink. To prepare the working electrode for electrochemical measurements, 20 μL of the ink was dripped onto a mirror-polished glass carbon electrode. After 5 μL of 0.5 wt % Nafion aqueous solution was dripped on the electrode and dried at ambient temperature as a binder. Preparation of GO. GO was synthesized from natural graphite flake by an improved Hummers’ method with detailed procedures described in Supporting Information.48 Preparation of GPB, GPS, and GPP Hybrids. In a typical experiment, 85 mL GO dispersion (2 mg mL−1) was mixed with 125 mg DA dissolved in 10 mL Milli-Q water. The mixture was mixed with 130 mL Milli-Q water and sonicated for 5 min. Twenty-five mL PBS buffer (0.4 M, pH = 8.5) was added. The mixture was continuously stirred at ambient temperature for 24 h to obtain the GP hybrids. After that, 125 mg ME, 100 mg BA, and 250 mg BPA were added to the above-prepared GP hybrids, respectively, and stirred continually for 12 h. The GDB, GDP, and GDS hybrids were obtained after centrifugation and washed three times with water. Preparation of Dual-Doped Carbon Nanosheets. The N,BCN, N,P-CN, and N,S-CN were prepared through the carbonization of the GDB, GDP, and GDS hybrids, in a temperature programmable tube furnace under N2 atmosphere at 400 °C for 2 h with a heating rate of 1 °C min−1. This was followed by further treatment at 800 °C for 3 h with a heating rate of 5 °C min−1. For the control, the GD hybrids were pyrolyzed under the same conditions to obtain N-CN.

CONCLUSION In summary, a robust strategy is first constructed by means of facile PDA platform to fabricate three dual-doped carbon nanosheets. On the basis of achievably similar N doping and high-level secondary-heteroatoms introduction, the comparative investigations were accordingly made on the promotional effect of secondary heteroatoms to N-doped carbons in HER, and it reveals that S dopant owns the strongest promotion effect followed by P dopant, while the import of B dopant reduces the activity of N-doped carbons. The origin of this promotion effect was elucidated through theoretical computations together with electrocatalytical evidence. Electrocatalytic 7298

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03290. Preparation of GO; electrochemical characterization; AFM image of GP hybrid; TEM images of N,B-CN and N,P-CN; Cdl calculation based on CV curves at different scan rates; difference of the overpotentials in alkaline solution; electrochemical impedance spectra of N-CN, N,B-CN, N,P-CN, and N,S-CN; polarization curves of N,P-CN recorded before and after 1000 potential sweeps in acidic and alkaline solution; Tables S1−S3 for comparison on the recently reported methods for S, B, or P-doping (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Shi Zhang Qiao: 0000-0002-4568-8422 Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acsnano.7b03290 ACS Nano 2017, 11, 7293−7300