Enhanced electrocatalytic hydrogen evolution in graphene via defect engineering and heteroatoms co-doping

Accepted Manuscript Title: Enhanced electrocatalytic hydrogen evolution in graphene via defect engineering and heteroatoms co-doping Author: Ye Tian Rui Mei Da-zhong Xue Xiao Zhang Wei Peng PII: DOI: Reference:

S0013-4686(16)32142-9 http://dx.doi.org/doi:10.1016/j.electacta.2016.10.055 EA 28143

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

1-8-2016 30-9-2016 9-10-2016

Please cite this article as: Ye Tian, Rui Mei, Da-zhong Xue, Xiao Zhang, Wei Peng, Enhanced electrocatalytic hydrogen evolution in graphene via defect engineering and heteroatoms co-doping, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.10.055 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enhanced electrocatalytic hydrogen evolution in graphene via defect engineering and heteroatoms co-doping

Ye Tian1*, Rui Mei1, Da-zhong Xue2, Xiao Zhang3*, Wei Peng3

1 College

of Science, Hebei North University, Zhangjiakou 075000, Hebei, China

2College

of Basic Medicine, Hebei North University, Zhangjiakou 075000, Hebei, China

3

College of Information Science and Engineering, Hebei North University, Zhangjiakou 075000, Hebei, China

*Corresponding author. E-mail address: [email protected] (Y. Tian)；[email protected] (Xiao Zhang)

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Abstract Development of efficient and metal-free electrocatalysts toward hydrogen evolution reaction (HER) is of great significance for the renewable energy conversion and storage technologies. Heteroatom doping, especially two heteroatoms co-doping, is one of the most effective strategies to improve the catalytic activity of carbon-based electrocatalysts. Here, we reported that the plasma-etching could produce additional structural defects on N, S co-doped graphene (NSG), which further improved the HER activity of NSG. It was demonstrated that the synergetic coupling effect of N, S co-doping and plasma-induced structural defects could maximize the number of exposed active sites and significantly enhance the HER activity of graphene in both acidic and alkaline media. This study thus indicates that combining defect engineering with heteroatoms co-doping is an efficient strategy for dramatically improving the HER activity of graphene.

Keywords: Hydrogen evolution reaction; Heteroatoms co-doping; Plasma-etching; Defect engineering.

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1 Introduction Electrochemical hydrogen evolution reaction (HER) from the splitting of water molecules has been deemed as the most economical and environmentally friendly method for large-scale hydrogen production[1]. Although Pt-based electrocatalysts have been considered as the most active catalysts and remain the benchmark for HER, their scarcity and high cost limit the practical applications[2]. To this end, earth-abundant non-noble transition-metal materials including chalcogenides[3], phosphides[4], carbides[5], nitrides[5] and their complexes have been extensively studied as alternatives to Pt-based electrocatalysts toward HER owing to their low cost and high catalytic activity. However, most of these transition-metal based electrocatalysts have poor electrical conductivity and suffer from chemical corrosion/passivation under acidic conditions, which therefore compromise their catalytic efficiency[6]. Therefore, it is desirable to develop efficient and metal-free electrocatalysts with comparable electrocatalytic activity toward HER. Heteroatom doped carbon materials have been emerging as promising metal-free electrocatalysts due to their low cost, high electrical conductivity, robust stability in acid/alkaline environments, and potentially high electrocatalytic activities[7-9]. Especially, co-doping of two different heteroatoms (i.e., N/S[10], N/P[11], N/B[12]) can effectively modulate the electronic structures and physicochemical properties of carbon-based catalysts to further increase the electrocatalytic activity due to their synergetic effect[6, 7]. Although heteroatoms co-doped carbon materials have been extensively studied for oxygen reduction reaction (ORR) as excellent and metal-free

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electrocatalysts [13-15], their catalytic activity toward HER has rarely been investigated. Recently, several heteroatoms co-doped carbon materials, including N, P co-doped graphene[16], N, S co-doped nanoporous graphene[17], N, S co-doped carbon nanosheets[18] and N, P co-doped porous graphitic carbons[11] have been reported to exhibit favorable catalytic activity for HER. Qiao’s group[17] conducted a pioneering work in this context and reported that the N, P co-doped graphene showed improved HER activity compared with single doped counterparts. Density functional theory (DFT) calculations revealed that the valence orbital energy levels of the C atoms next to the N, P dopants could be modified and thus coactivated because of N, P co-doping, resulting in a synergistically enhanced HER activity. Chen’s group[17] synthesized the N, S co-doped nanoporous graphene by chemical vapor deposition, which exhibited a high HER activity with an onset potential of 130 mV and a Tafel slope of 80.5 mV for their best sample. Based on their theoretical predictions, the improved HER activity originated from the interplay between the chemical dopants and geometric lattice defects of the nanoporous graphene. Zhou et al.[18] reported a green method to prepare N, S co-doped carbon nanosheets by thermal treatment of peanut root nodules. The obtained electrocatalysts displayed a very small onset potential of 27 mV and a Tafel slope of 67.8 mV. They ascribed the increased HER activity to the important role of S doping that could effectively modulate the electronic energy structures and improve the adsorption ability of H atoms. Very recently, Dai’s group[11] synthesized three-dimensional (3D) N, P co-doped porous graphitic carbons by pyrolysis of self-assembled melamine and phytic acid in the

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presence of graphene oxide (GO). The synthesized electrocatalysts exhibited a pronounced HER activity with an overpotential of 163 mV at 10 mA·cm-2 and a Tafel slope of 89 mV. They proved that both N, P co-doping and 3D porous graphitic network were important factors for enhanced HER activity. Despite these efforts, the HER performance of heteroatoms co-doped carbon materials is still markedly lower in

comparison

with

that

of

Pt-based

and

most

transition-metal-based

electrocatalysts[1]. Therefore, the rational design of carbon-based metal-free HER electrocatalysts with competitive HER activity is critical and remains challenging. In this study, an efficient strategy was proposed to explore the potentially high HER activity of graphene by combining defect engineering with heteroatoms co-doping. The newly developed HER electrocatalyst was prepared through a two-step method involving the microwave-assisted solvethermal synthesis of N, S co-doped graphene (NSG), followed by plasma-etching in an Ar atmosphere (denoted as P-NSG for the final electrocatalyst), as illustrated in Scheme 1. Plasma technique has already been demonstrated as an effective route to introduce geometric defects and improve the electrocatalytic activity of a series of materials, such as carbon materials[19], MoS2[20], ZnO2[21], Co3O4[22], and NiMoN[23]. Previously, plasma treatment was employed to create structural defects on carbon materials without doping, which showed enhanced ORR activity attributed to its enriched defects[19]. Here, we applied the plasma technique to introduce additional structure defects in NSG to further enhance the HER activity of NSG. As we proved that this kind of design could induce the synergetic coupling effect of N, S co-doping and

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plasma-induced structural defects that was able to maximize the number of active sites for achieving the best HER performance of graphene.

2 Experimental 2.1. Synthesis Graphene oxide (GO) was prepared from graphite powder (200 μm, 99.9% purity, Qingdao Dongkai Graphite Co., Ltd.) by a modified Hummers method[24]. NSG was synthesized by a microwave-assisted solvethermal method using thiourea as both N and S sources, as reported in our previous paper[25]. In a typical procedure, 0.5 g of thiourea was dispersed in 200 mL GO solution (1.0 mg·mL-1) under ultrasonication for 1 h. The mixture was then sealed in a quartz tube and transferred to a commercial microwave oven (G80F23YSL-X1, 2450 MHz, 800 W) and subjected to microwave irradiation for 10 min. After cooling to room temperature naturally, the products were collected by centrifugation, washed several times with deionized water and dried at 60 oC for 24 h under vacuum. For comparison, single doped graphene, including N doped graphene (urea as the N source) and S doped graphene (NaS2 as the S source), and undoped reduced graphene oxide (RGO, without addition of thiourea) were also synthesized under the similar conditions. 2.2. Plasma treatment Plasma treatment was carried out in the experimental setup consisting of AX-1000 plasma system equipped with a radiofrequency (RF, 13.56 MHz) power generator, as illustrated in Scheme S1 (Supporting Information). Prior to plasma treatment, the quartz tube was evacuated to a base pressure of 0.01 Pa, and then Ar

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gas was fed into the quartz tube at a gas flow rate of 20 sccm. After that, the as-synthesized samples were treated with Ar plasma discharged at a RF power level of 200 W for 0-40 min. Unless otherwise specified, P-NSG means the NSG sample treated by plasma for 30 min. 2.3. Characterizations and electrochemical measurements Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) elemental mapping were carried out on a JEM-2100 instrument. Raman spectra were recorded using a Raman spectroscope (Alpha 300R, WITEC). X-ray diffraction (XRD) pattern was recorded on a powder diffractometer (Bruker D8 Advanced Diffractometer System). XPS analysis was conducted on a PHI 5000C ESCA system. Electrochemical

measurements

were

conducted

on

a

PGSTAT-302N

electrochemical workstation using a conventional three-electrode system at room temperature. A modified glassy carbon electrode (GCE, 3 mm in diameter) served as the working electrode, a Ag/AgCl (in 3 M KCl solution) electrode and a graphite rod were used as reference and counter electrodes, respectively. For the preparation of catalyst ink as working electrode, 5 mg of the catalysts was dispersed into 1 mL of water/ethanol (v/v=24:25) solvent containing 5% Nafion solution (20 μL) under ultrasonication for 1 h to form a homogeneous ink. Then, 4 μL aliquot of the as-prepared catalyst ink was transferred onto the surface of the GCE substrate and then dried in an ambient environment, yielding an catalyst loading of approximate 0.28 mg·cm-2. Commercial 20% Pt/C catalyst was also used as a reference sample.

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HER test was performed by linear sweep voltammetry (LSV) scans at room temperature in both acidic (0.5 M H2SO4) and alkaline (1.0 M KOH) media. The reference electrode was calibrated with respect to a reversible hydrogen electrode (RHE). In 0.5 M H2SO4, ERHE = EAg/AgCl + 0.196 V, In 1.0 M KOH, ERHE = EAg/AgCl + 1.038 V. All polarization curves were iR-corrected using the CHI software. The onset potential was determined based on the beginning of the linear part of the Tafel plot. Electrochemical impedance spectroscopy (EIS) spectra were recorded at AC voltage amplitude of 10 mV within the frequency range from 100 kHz to 0.01 Hz.

3 Results and discussion 3.1. Characterizations of NSG and P-NSG The fabrication procedure of P-NSG is illustrated in Scheme 1. First, the NSG was synthesized through a microwave-assisted solvethermal method, which enables the simultaneous removal of oxygen-containing groups on GO and incorporation of N, S atoms into the graphene frameworks[26]. Subsequently, the as-prepared NSG is subjected to Ar plasma etching to engrave the graphene structure and produce more structural defects on NSG, and P-NSG is obtained. The structural morphologies of NSG before and after plasma-etching were initially examined by TEM. From the low magnification TEM images (Figs. 1a, b), it is seen that the two samples have a similar graphene morphology showing a crumpled and wrinkled sheet structure, arising mainly from the structural distortion caused by the heteroatoms co-doping[27]. However, a closer examination from the high magnification TEM images show that the surface of P-NSG is coved with many

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in-plane nanosized holes (Fig. 1d), which do not exist in NSG without plasma-etching (Fig. 1c). The presence of nanosized holes in P-NSG stems from the etching effect of plasma to expose new structural defects[22]. The defect-rich feature of P-NSG is further proved by selected area electron diffraction (SAED). The SAED pattern of NSG (Fig. 1e) shows a two-set of ring-like diffraction pattern with symmetric hexagonal dispersed spots indicative of few-layered graphene with relatively high level of graphitic crystallinity[28]. In contrast, the SAED pattern of P-NSG displays an intense diffusive ring feature (Fig. 1f), which reveals the high degree of disorder or the existence of rich defects[29]. Thus, the plasma-etching is able to noticeably increase the defect sites in NSG by generating amounts of in-plane nanosized holes, which are believed to be favorable for the electrocatalytic hydrogen evolution. Raman spectroscopy was used to directly estimate the defect level of obtained NSG before and after plasma-etching, as shown in Fig. 2a. Raman spectra show G-band at ~1590 cm-1 and D-band at ~1350 cm-1. The G-band corresponds to graphitic carbons and the D-band corresponds to the disorder or defects present in the graphitic domains[30, 31]. The ID/IG intensity ratio is a qualitative measure of the quantity of defects in carbon materials. The ID/IG ratio increases from NSG (1.15) to P-NSG (1.38), suggesting that more defects are generated within NSG by plasma-etching, which is consistent with the results obtained from the TEM/SAED observations (Fig. 1). XPS was performed to gain further insight into the chemical states and doping structures of NSG and P-NSG. Figs. 2 b-d displays the XPS survey spectra (Fig. 2b)

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and high-resolution N 1s (Fig. 2c) and S 2p (Fig. 2d) spectra for both NSG and P-NSG. The corresponding information related to doping level and deconvoluted N, S species and their contents are summarized in Table S1. It is seen from Fig. 2b that the XPS survey scan of both NSG and P-NSG reveals four characteristic peaks of C 1s, O 1s, N 1s and S 2p/S 2s, while no N 1s and S 2p/S 2s peaks are detected for GO, revealing that N and S heteroatoms are successfully co-doped into the graphene frameworks of NSG and P-NSG. In addition, after plasma-etching, there are only negligible changes in the N, S doping level. As seen in Table S1, P-NSG contains 5.7 at.% N and 1.1 at.% S, similar to the N (6.1 at.%) and S (0.8 at.%) levels of NSG. This indicates the plasma-etching has little impact on the N, S concentrations of NSG. Moreover, the changes in high-resolution C1s, N 1s and S 2p spectra of NSG before and after plasma-etching are investigated. High-resolution C 1s spectra of NSG can be deconvoluted into four peaks, which are assigned to sp2 C=C (284.5 eV), sp3 C-C (285.5 eV), C-N/C-S (286.7 eV), and C-O/C=O (288.3 eV). It is clear that the content of defective sp3 C-C (22.8 at.%) in P-NSG is markedly higher than that of untreated NSG (15.1 at.%). This further corroborates the evidence that plasma-etching can lead to the exposure of more structural defects, consistent with the aforementioned TEM and Raman results. High-resolution N 1s spectra (Fig. 2c) can be deconvoluted into two peaks corresponding to pyridinic-N (399.7 eV) and graphitic-N (400.8 eV)[32]. High-resolution S 2p spectra (Fig. 2d) can be deconvoluted into three peaks, in which the two intense peaks at 164.6 and 165.9 eV corresponds to thiophene-S, and the

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weak and broad peak at 169.5 eV corresponds to oxidized-S[33]. Together with the Table S1, it seems that the percentages of pyridinic-N and thiophene-S species are slightly increased after plasma-etching accompanied by the reduced percentages of graphitic-N and oxidized-S species. Actually, the N, S doping species are closely associated with the plasma treatment time. It is shown in Fig. 2f with increasing plasma time from 10 min to 40 min, the pyridinic-N is firstly decreased at 10 min and then increased beyond 10 min, while the content of thiophene-S is in direct proportion to the plasma time (Fig. 2g). It is well documented that the plasma treatment is coupled with the surface heating that can lead to the elevated temperature (> 600 oC) at the local surface region of the treated materials due to the plasma bombardment[34, 35]. Accordingly, the decreased content of pyridinic-N at 10 min is attributed probably to the plasma-induced surface heating that can convert some thermally unstable pyridinic-N to graphitic-N because graphitic-N generally tends to form at relatively high temperature compared to pyridinic-N[36, 37]. With further prolonging the plasma time (20-40 min), the plasma-etching effect is more significant and it can noticeably damage the basal surfaces of graphene and break central graphitic-N structures into edged pyridinic-N. The increased thiophene-S with increasing plasma time is also relevant to the plasma-induced defects since the disruption of carboatomic ring is an important factor to incorporate thiophene-S into graphene lattice[38], whereas this is not critical for oxidized-S. Therefore, the N, S dopants configurations are highly dependent on the plasma-induced defects and the creation of more graphene defects is beneficial to gain more thiophene-S and pyridinic-N species.

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From the above results, it is concluded that plasma-etching is able to produce more structure defects present as the form of in-plane nanosized holes while making the preferable formation of more thiophene-S and pyridinic-N species. Such doping-derived active sites and structure defects are expected to induce the synergetic coupling effect, which can strongly boost the HER performance of graphene. 3.2 HER activities To verify our hypothesis, the HER catalytic activity of P-NSG was first evaluated in acid media (0.5 M H2SO4) purged with N2 and the polarization curves based on LSV test is shown in Fig. 3a. As references, pristine RGO, NSG, and commercial 20% Pt/C were also tested. As expected, the Pt/C catalyst exhibits the highest HER activity with negligible onset potential, while RGO is nearly catalytically inactive for HER with onset potential exceeding 500 mV. In contrast, the NSG has much improved activity compared with RGO with a small onset potential of 189 mV, indicating the positive role of N, S co-doping for enhancing the HER of graphene, consistent with the literature results[16-18]. Impressively, the HER performance of NSG can be further improved by plasma-etching, and the P-NSG exhibits a much smaller onset potential of 58 mV, which is very close to that of commercial Pt/C, suggesting its superior HER activity. The overpotential that is required to achieve a current density of 10 mA·cm-2 (η10) is an important parameter for HER activity evaluation[1]. Accordingly, the P-NSG shows a η10 value of 149 mV, which is much smaller than that of NSG (η10 = 322 mV). These results strongly indicate the importance of plasma-etching to further enhance the HER activity of

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graphene in addition to heteroatoms co-doping, since plasma-etching produces a larger number of catalytically defective/active sites corresponding to large electrochemical surface area (ECSA) available for HER[39]. To verify this, the ECSA values of both NSG and P-NSG were measured (Fig. S1, Supporting Information). Generally, the ECSA is proportional to the electrochemical double-layer capacitance (EDLC, Cdl)[40], and therefore, the Cdl can be measured to indirectly investigate the ECSA behavior. As shown in Fig. S1, the value of Cdl measured for P-NSG is 19.6 mF·cm-2, which is higher than that of NSG (Cdl = 10.1 mF·cm-2). This thus provides direct evidence for the enlarged ECSA of P-NSG compared to NSG that provides more catalytically active sites, leading to the enhanced HER activity. To further investigate the interplay of N, S co-doping and plasma-induced structure defects on the HER activity, controlled experiments were performed for the plasma-etched un-doped RGO (P-G), plasma-etched N doped graphene (P-NG), and plasma-etched S doped graphene (P-SG) under the identical plasma treatment and HER test conditions (Fig. S2a, Supporting Information). Obviously, the P-NSG exhibits the highest HER activity among all the tested samples, which can be possibly explained by its more plasma-induced structural defects (Fig. S2b, Supporting Information) and relatively higher doping level (Fig. S2c, Supporting Information). Nevertheless, the P-NG with a comparable defect level but a higher doping concentration shows a much worse HER activity than P-NSG, which may be related to their differences in dopants bonding configurations. The pyridinic-N and thiophene-S have been recently demonstrated as the most active sites for

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electrocatalysis[41, 42]. On the basis of XPS analysis shown in Fig. S2d (Supporting Information), it is reasonable to believe that the higher concentrations of highly-active pyridinic-N and thiophene-S in P-NSG relative to P-NG accounts for its higher HER activity. This also implies the amount of active doping species may be the dominant factor for the enhanced catalytic activity rather than the element doping level. It is worth noting that, without doping, the P-G with only plasma-etching shows the poorest HER performance, indicating the structural defects alone do not result in a noticeably improved HER activity. Based on the above results, it can be concluded that the synergistic coupling effect of plasma-induced structural defects and active pyridinic-N/thiophene-S species are responsible for the significantly improved HER activity in P-NSG. Furthermore, the plasma time has an important effect on the HER performance, and the HER activity of P-NSG at different plasma time (10-40 min) was investigated (Fig. S3a, Supporting Information). It is seen that, with increasing plasma time, the HER activity is initially increased (10-30min) and then decreased (40 min), reaching the maximum at an optimal plasma time of 30 min. According to the aforementioned results, high defect level and large amount of active doping species are both beneficial for the HER activity. Therefore, the short plasma time (10-20 min) can not provide enough defective sites (Fig. S3b, Supporting Information) and active doping species (Figs. 2f, g), resulting in an improved HER activity far from satisfactory. It is noteworthy that, although P-NSG treated for 10 min possesses a much higher defect level than untreated NSG, their HER activities are comparable, which can be ascribed

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to the lower amount of active pyridinic-N/thiophene-S species in the former (Fig. 2f). This, once again, confirms that the active pyridinic-N/thiophene-S species are possibly the most HER favorable doping sites. Moreover, the reduced HER activity of P-NSG treated for 40 min can mostly be attributed to its large charge-transfer resistance caused by the presence of too many defects due to excessive plasma treatment, as confirmed in EIS analysis (Fig. S3c, Supporting Information). Too many defects are detrimental to electrical transport properties and thus hinder the electron transfer rate of the HER kinetics[43]. Therefore, it is crucial to select a rational plasma-etching duration to make the balance between the creation of rich defects/active doping species and retaining the effective electron transport. The P-NSG with 30 min plasma treatment possesses enriched defects and active doping species while maintaining a relatively high electrical conductivity of NSG, thus exhibiting the best HER activity. To obtain further insight into the reaction kinetics during the HER, Tafel slopes of various catalysts were investigated (Fig. 3b). Tafel slope can be obtained by linear fitting to the Tafel equation (η = b log j + a , where η is the overpotencial, b is the Tafel slope, j is the current density and a is the constant)[1]. The Tafel slope of Pt/C is 34 mV, consistent with the reported values[2]. The P-NSG exhibits a much smaller Tafel slope (78 mV) than NSG (118 mV) indicating a smaller activation energy needed for HER. This once again demonstrates the enhanced HER activity of P-NSG due to plasma-etching. Additionally, the Tafel slope of 78 mV for P-NSG reveals that the HER proceeds through a Volmer-Heyrovsky mechanism with electrochemical

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desorption of hydrogen as the rate-limiting step[44, 45]. Furthermore, the exchange current density (j0) is also an important parameter to reflect the HER catalytic efficiency[1], which can be calculated by the extrapolation of Tafel plots (Fig. S4, Supporting Information). The obtained j0 of P-NSG is 5.5×10-2 mA·cm-2, which is 4.2 times higher than that of NSG (1.3×10-2 mA·cm-2), further confirming the most favorable HER kinetics of P-NSG. Moreover, the turnover frequency (TOF) of P-NSG is estimated to be 0.025 s−1 at a HER overpotential of 0.2 V vs. RHE by assuming all the pyridinic-N/thiophene-S species are next to the structural defects (see Note S1 for details, Supporting Information). Such TOF value is fairly comparable to that of most transition-metal-based catalysts[2, 46]. Overall, the P-NSG displays excellent HER activities with a low onset potential of 58 mV (η10 = 149 mV), a small Tafel slope of 78 mV, and a large j0 of 5.5×10-2 mA·cm-2. These values are superior or comparable to those of previously reported carbon-based metal-free catalysts and even the state-of-the-art transition-metal-based catalysts in acidic media (Tables S2, S3), suggesting the impressively synergetic effect of N, S co-doping and plasma-etching to significantly enhance the HER activity of graphene. Stability is another important aspect in the development of advanced electrocatalysts for the practical applications[1]. The electrochemical stability of P-NSG was evaluated by the long-term LSV test. As shown in Fig. 3c, negligible difference can be observed between the polarization curves measured at the initial cycle and after 3000 cycles. The durability of P-NSG as HER catalysts was also examined using chronoamperometry (j-t) test at a constant potential of 0.2 V vs. RHE,

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and the current density remains stable over 20 h (Fig. 3d). These results indicate that P-NSG has the good long-term durability as HER catalyst, which can be attributed to the stable defective structure and strong covalent bonding of N and S heteroatoms with C atoms of graphene[10]. Furthermore, the H2 generated from the chronoamperometry (i-t) test over P-NSG was collected and examined by gas chromatography (GC) analysis (Fig. S5, Supporting Information). It is seen that measured values for the amount of H2 is very close to the theoretically calculated values, suggesting the nearly 100% Faradaic efficiency of P-NSG for HER. On the other hand, the design of HER catalysts capable of operating high efficiency over a wide pH range is greatly desired[47]. Nevertheless, the carbon-based metal-free electrocatalysts that can drive HER in both acidic and alkaline systems have rarely been reported in the literature. Next, we evaluate the HER performances of both NSG and P-NSG in alkaline media, as shown in Fig. 4. According to the polarization curves (Fig. 4a) and corresponding Tafel plots (Fig. 4b), the η10 of P-NSG is determined to be 197 mV and the Tafel slope is calculated to be 101 mV. Following the same trend in acidic media, the P-NSG exhibits improved HER activity when compared to NSG (η10 = 377 mV, 153 mV in Tafel slope) in alkaline media under the same test conditions. It is noteworthy that in alkaline media, the HER activities of both NSG and P-NSG are obviously lower than those in acidic media (Fig. 3a, b). This can be explained that there is an additional water dissociation step in the alkaline solution that leads to a relative high kinetic energy barrier toward HER[6, 16]. Furthermore, the P-NSG also exhibits excellent durability in alkaline media. The

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polarization curves before and after operating 3000 cycles show negligible difference (Fig. 4c), and the change of current density is less than 5% at a constant potential of 0.2 V vs. RHE for 20 h test (Fig. 4d). Therefore, the P-NSG shows superb HER activity and stability in both acid and alkaline solution. Strikingly, the HER activities of P-NSG are also competitive with those of recently developed representative HER catalysts in alkaline media (Table S3), demonstrating the great potential of P-NSG as

a high-performance HER electrocatalyst in both acidic and alkaline media.

4 Conclusions In summary, we have disclosed an efficient strategy to explore the potentially high HER activity of graphene by using plasma-etching method to produce additional structural defects on NSG. TEM/SAED, Raman, XRD, and XPS analysis confirmed that the plasma-etching produced additional structure defects present as the form of in-plane nanosized holes while making the preferable formation of more thiophene-S and pyridinic-N species. The P-NSG showed a much improved HER activities in both acidic and alkaline media compared to NSG without plasma-etching. Particularly in acidic medium, the P-NSG displayed a low overpotential of 149 mV at 10 mA·cm-2, a small Tafel slope of 58 mV, a large exchange current density of 5.5×10-2 mA·cm-2, as well as the good stability with a nearly 100% Faradaic efficiency. Such HER performances were superior or comparable to those of previously reported metal-free carbon-based catalysts and even the state-of-the-art transition-metal-based catalysts. The excellent HER activity of P-NSG was ascribed to the synergistic coupling effect of plasma-induced structural defects and active pyridinic-N/thiophene-S species. We

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anticipate that the combination of heteroatoms co-doping and defect engineering may open a new avenue to develop highly efficient metal-free carbon-based HER catalysts. The current strategy might be extended to tailor and enhance the electrocatalytic performances of carbon-based materials for other electrocatalytic applications beyond HER.

Acknowledgement This work is supported by "Engineering Technology Research Center of Population Health Informatization in Hebei Province", "Application Technology Research and Development Center of Medical Informatics in Hebei Universities", "Institute of New Energy Science and Technology of Hebei North University",

Science and Technology Department of Hebei Province Project (No.16961301D), Hebei Provincial Education Department Youth Fund Project (No.QN2015148), Zhangjiakou Science and Technology Research and Development Project (No.1511075B ).

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Fig. 1. Low magnification TEM images of NSG (a) and P-NSG (b). High magnification TEM images of NSG (c) and P-NSG (d). SAED patterns of NSG (e) and P-NSG(f).

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Fig. 2. (a) Raman spectra of NSG and P-NSG. (b) XPS survey scan of GO, NSG and

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P-NSG. High-resolution C 1s (c), N 1s (d), and S 2p (e) spectra of NSG and P-NSG. Effect of plasma treatment time on the percentages of N species (f) and S species (g) of P-NSG.

Fig. 3. Polarization curves (a) and corresponding Tafel plots (b) of RGO, NSG, P-NSG and commercial 20% Pt/C on GCE in 0.5 M H2SO4 solution at a scan rate of 2 mV·s-1. (c) Polarization curves for P-NSG initially and after 3000 cycles. (d) Chronoamperometry (j-t) test for P-NSG under a constant overpotential of -0.2 V vs. RHE for 20 h.

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Fig. 4. Polarization curves (a) and corresponding Tafel plots (b) of NSG and P-NSG on GCE in 1.0 M KOH solution at a scan rate of 2 mV·s-1. (c) Polarization curves for P-NSG initially and after 3000 cycles. (d) Chronoamperometry (j-t) test for P-NSG under a static overpotential of -0.2 V vs. RHE for 20 h.

1

Scheme 1. Schematic illustration of the synthesis process of P-NSG

1