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Understanding the Roles of Nitrogen Configurations for Hydrogen Evolution: Trace Atomic Cobalt Boost the Activity of Planar Nitrogen doped Graphene Huaixin Wang, Na Yang, Wei Li, Wei Ding, Ke Chen, Jing Li, Li Li, Jianchuan Wang, Jinxia Jiang, Fengqiu Jia, and Zidong Wei ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Energy Letters

Understanding the Roles of Nitrogen Configurations for Hydrogen Evolution: Trace Atomic Cobalt Boost the Activity of Planar Nitrogen doped Graphene Huaixin Wang,#,1,2 Na Yang,#,1 Wei Li,#,1 Wei Ding,*,1 Ke Chen,*,2 Jing Li,1 Li Li,1 Jianchuan Wang,1 Jinxia Jiang,1 Fengqiu Jia,2 Zidong Wei *,1 #

These authors contributed equally to this work

1

Chongqing Key Laboratory of Chemical Process for Clean Energy and Resource Utilization,

College of Chemistry and Chemical Engineering, Chongqing University, 400044, Chongqing, China. 2

College of Material Science and Engineering, Chongqing University, 400044, Chongqing,

China. Corresponding Author E-mails:

[email protected]

(Wei

Ding),

[email protected]

(Ke

Chen),

[email protected] (Zidong Wei).

ACS Paragon Plus Environment

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ABSTRACT: Revealing the exact roles of nitrogen configurations and precise controlling the nitrogen configuration of nitrogen doped graphene (NG) are extremely important for realizing its advance functions in the clean energy technologies. Herein, for the first time, we established that the hydrogen evolution reaction (HER) activities of NG display definite trends upon its nitrogen configurations which was selectively generated by using the layer-structured montmorillonite (MMT) with different layer distance and function modulated by Co2+, Ni2+, Na+, H+ ions. We found that, among the three types of N, i.e., pyridine, pyrrolic and quaternary N, quaternary N is the most active one for HER in a metal-free NG catalyst. While with an introduction of trace atomic cobalt, the planar (pyridine and pyrrolic) N becomes the better one. In contrast, when trace atomic Ni is involved to replace the Co, the former results in heavily depressed HER activities for NG catalyst. Density functional theory calculations further revealed that (i) the carbon atoms are highly activated for HER by the nearby quaternary N but not by planar N, and (ii) nickel blocks but cobalt promotes the hydrogen adsorption after coordinated with planar N, leading to an excellent HER performance.

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ACS Energy Letters

With the ever increasing demand for clean energy at the global scale, the hydrogen technologies such as water splitting,1,2 hydrogen storage3 and fuel cells,4,5 have been attracting intense attentions. The water electrolysis (WE) driven by renewable energy is a promising technology to produce hydrogen at industrial scale.6-8 However, practical implementation has been impeded by the high energy consumptions and the high costs of electrocatalysts for hydrogen and oxygen evolution reactions (HER and OER).9-11 Recently, N-doped graphene (NG) shows an acceptable activity for the catalysis of HER as a low-cost electrocatalyst.12-14 To meet the requirement of WE applications in an efficient and cost-saving way, further promotion of NG activity in catalysis of HER is still necessary. It has been confirmed that the nitrogen configuration plays a critical role in affecting the properties of NG for oxygen reduction reaction (ORR).15-17 For example, pyridine N and pyrrolic N doped graphene with a planar configuration (planar NG), where the N atoms with a sp2 hybrid orbit located in a π-π conjugated system, is conducive for electron transfer on their planar structures. Whereas the quaternary N doped graphene (quaternary NG), where the N atoms with a sp3 hybrid orbit interrupt the π-π conjugation, is not conducive for electron transfer on their three-dimension structure.18 Meanwhile, planar NG and quaternary NG also show different activity for O2 protonization, that is, from OO becoming to OOH.19 It was also reported that the planar NG exhibits a much higher ORR activity than quaternary NG in acid solution.18,20,21 However, in the catalysis of HER , the nitrogen configuration-activity relationship has not been set up yet, which it is crucial to obtain catalytic active sites with an optimal hydrogen binding energy and then to catalyze HER efficiently.21,22 Herein, for the first time, we try to establish the relationship between the HER activities and NG configurations including planar N, quaternary N, atomic cobalt modified planar N, and


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atomic nickel modified planar N. We found that the HER catalytic activity is positively correlated with quaternary N content in a metal-free NG catalyst, while with an assistance of trace atomic Co, the planar N doped NG exhibits a much better HER activity than that of quaternary N doped NG. Density functional theory (DFT) calculations revealed that the carbon atoms in the NG catalyst can be highly activated by the nearby quaternary N but not by the nearby planar N. Therefore, the quaternary N doped NG shows much lower adsorption energy for hydrogen (EHads) than that on the planar N doped NG. After incorporating the trace atomic Co into the planar N doped NG, a highly active Co site forms, and the nearby nitrogen and carbon atoms are also activated. As a result, the cobalt modified planar N doped NG shows the smallest free energy for HER among all investigated NG catalysts. In contrast, the incorporation of the trace Ni into NG heavily blocks the adsorption of hydrogen on the nearby nitrogen atom, leading to a depressed HER activity.

Scheme 1. Schematic representation of the synthesis of NG catalyst with different content of quaternary N content by using the nanoreactor MMT with different interspace width (δ).


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ACS Energy Letters

We here extend our previously invented space-confinement-induced method not only to precisely control the nitrogen configurations,22 but also to exactly introduce trace level active metal into nitrogen configurations of the NG catalyst by using the layered montmorillonite (MMT) as a quasi-closed flat nanoreactor (see the supporting information for details and Scheme S1). Scheme 1 presents the relationship between the interspace widths of MMT and the nitrogen content of produced NG catalysts. With this concept, we are able to control the nitrogen configuration by tuning the MMT nanoreactors with different layer distance and functions. As shown in scheme 1, because of the torsion angle of the C-N bond is 0° for planar sp2 nitrogen and 60° for tetrahedral sp3 nitrogen,23 quaternary N doped NG would form a 3D structure, whereas pyridinic N and pyrrolic N doped NG forms a 2D structure flat platelets. The NG with planar N is massively produced when the MMT with a small interspace width (δ) was used; otherwise, in the cases with larger-δ MMT utilized, the N atoms preferentially occupy the positions of quaternary-N or oxidized-N for stabilizing purpose (Scheme 1). The δ of the nanoreactor can be exactly adjusted by introducing different cations into the layer interspaces of MMT via cation exchange. When the active metal cations are intercalated into the layer, the trace atomic active metal can be in-situ introduced into nitrogen configuration during the NG formation. As shown in figure S1, the value of δ is well controlled within the sub-nano scale from 0.46 nm in H-MMT, to 0.53 nm in Na-MMT, to 0.59 nm in Ni-MMT, and finally to 0.60 nm in Co-MMT. The resultant NG samples produced by using these nanoreactors are accordingly denoted as NG@H-MMT, NG@Na-MMT, NG@Ni-MMT, and NG@Co-MMT, respectively. For comparison, the contrast sample NCΩMMT was prepared without MMT participation of polymerization and pyrolysis. While a physical mixture of Na-MMT and AN


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during polymerization and pyrolysis is denoted as NC＆Na-MMT, which simulates the case of NC formed between Na-MMT particles.24


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Figure 1. Representative SEM images of (a) NG@Na-MMT, (b) NG@H-MMT, (c) NG@NiMMT, (d, e) NG@Co-MMT; and (f-h) are EDS elemental maps of C and N for the NG@CoMMT. Figure 1a-e show distinct ultrathin and wrinkled graphene structures for all the produced NG. The uniform distribution of flower-like nanosheets after removal of MMT indicates the high efficiency of the space-confinement method in producing graphene. The powder X-ray diffraction (XRD) pattern of the as-synthesized NG shows a broad diffraction peak at 26° emerges which attributed to the graphene (002) (Figure S2), indicating a graphite structure with a dislocated layer by layer stack. Besides, the presences of G band and weak 2D band in Raman spectra further indicate a graphene structure for all studied NG (Figure S3). And a relatively high peak intensity ratio of D band to G band (ID/IG) of 1.06 tells that the produced NG catalysts have a relatively high concentration of defects including heteroatom-defects and ring-defects. The elemental mapping analysis of a single NG@Co-MMT graphene sheet in figure 1f shows that C and N atoms are distributed homogeneously throughout the entire NG nanosheet. The N content is about 0.4 wt.% according to the EDS spectrum (Figure 1h). Strangely, only a tiny signal of Co (0.012 wt.%) is detected in the NG@Co-MMT by the Inductively Coupled Plasma (ICP) although 10 wt.% Co ion was used in the synthesis. Similarly, Ni content in the NG@Ni-MMT is also at the trace level of 0.019 wt.%.


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ACS Energy Letters

Figure 2. N1s XPS spectra of (a) NG@Na-MMT and NG@H-MMT; (b) NG@Ni-MMT and NG@Co-MMT; (c) NCΩMMT and NC＆Na-MMT; (d) the absolute contents of each N species. The X-ray photoelectron spectroscopy (XPS) measurements were carried out to investigate the nitrogen configurations in NG. As shown in figure S4, the produced NG consists of carbon, nitrogen, oxygen and a small ratio of MMT residue. The nitrogen contents for all the NG catalysts are around 4 at.%, indicating a successful nitrogen doping. The existence of C-N and CC (graphite-type) bonds in the matrix further confirms the N-doped graphite structure of the prepared samples (Figure S5). Almost no S was detected in XPS on most of the produced catalyst surface, and only the NCΩMMT shows a tiny single of S with an S/N ratio as low as


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0.07 (Figure S6). This may be due to the limit contact with APS in our synthesis where only the edges of interspace in the MMT open to reaction. As shown in figure 2, the N 1s XPS spectra of NG could be deconvoluted into four different signals, i.e., pyridinic N (398.6 eV), pyrrolic N (400.5 eV), quaternary N (401.3 eV) and oxidized N (402~405 eV).8,12,18 Particularly, the content of quaternary N in NG@Na-MMT and NG@H-MMT is as low as 23.1 % and 15.2 %, respectively, while planar N dominates a ratio of 76.9 % and 84.8 % in both materials (Figure 2a). In contrast, 75.7 % and 44.6 % of nitrogen presents in the quaternary N in NCΩMMT and NC＆Na-MMT, respectively (Figure 2c). The lowest yield of quaternary N doped NG occurred where H-MMT with the smallest interspace δ of 0.46 nm was used as a nanoreactor, followed by gradually increased quaternary N content when the reactors with larger δ were utilized, i.e. from δ of 0.53 nm, to about 1 µm, to unlimited space, that is no MMT involved (Figure 2d). These results indicate that the MMT nanoreactor could effectively inhibit the production of quaternary N doped NG but accelerate the planar N doped NG. As shown in figure 2b and 2d, the planar N doped NG is dominantly formed in the cases where the Co-MMT with the δ of 0.60 nm and the Ni-MMT with the δ of 0.59 nm were used as nanoreactors. The planar N content is 60.8 % and 69.6 % in NG@Co-MMT and NG@Ni-MMT, respectively, which are slightly lower than that in NG@Na-MMT (76.9 %) due to the use of slightly larger space δ of the used nanoreactors. The quaternary N doped NG, however, is only 13.1 % and 28.6 % in NG@Ni-MMT and NG@CoMMT, respectively, indicating a planar N doped NG dominates configuration of NG@H-MMT and NG@Na-MMT.


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0

j / mA .cm-2

-10

(b)

NG@Na-MMT NG@H-MMT NG@Ni-MMT NG@Co-MMT NC&Na-MMT NCΩMMT Carbon Pt/C

-20

(c)

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NG@Na-MMT NG@H-MMT NG@Ni-MMT NG@Co-MMT NC&Na-MMT NCΩMMT

-15

-20 -0.6

-0.5

-0.6

-0.4

-0.2

0

0

-0.2

-0.1

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NG@H-MMT NG@Co-MMT

-10

-20

-30 -1.0

0.0

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E / V (vs .RHE)

(e)

-0.3

204 mV

60 mV

-0.8

-0.4

E / V (vs. RHE)

(d)

-20

-30 -1.0

0

E / V (vs .RHE) NCΩMMT 4.50 wt.% Co modified NCΩMMT

-10

j / mA .cm-2

-0.2

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Specific Activity normalized by BET surface area (mA m-2)

(a)

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NG@Co-MMT 2.58 wt.% Co modified NG@Co-MMT

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Co-MMT NG@Co-MMT without removal of nanoreactor

NG@Co-MMT

13 mV

j / mA .cm-2

-10

j / mA .cm-2

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ACS Energy Letters

-20

-30 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1

0.0

-10 with removal of all the nanoreactor

NG@Co-MMT

with removal of 60% the nanoreactor

-20

-30 -1.0

-0.8

-0.6

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E / V (vs .RHE)

E / V (vs .RHE)

Figure 3. HER polarization plots (a, c-f) measured with different catalysts in N2-saturated 0.5 M H2SO4 at a potential scan rate of 10 mV s-1; (b) The specific activities of catalysts normalized by the relative value of BET surface area. The HER activities of the electrodes made from various NG catalysts were tested in N2 saturated 0.5 M H2SO4 at a scan rate of 10 mV s-1. As shown in figure 3a and S7, among the


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metal free NG catalysts, the highest activity is achieved on the NCΩMMT catalysed electrode with a smallest overpotential of -316 mV at 10 mA cm-2, and decreased in the order of NC＆NaMMT > NG@Na-MMT > NG@H-MMT with the overpotential of -343 mV, -405 mV, and -421 mV at 10 mA cm-2, respectively. Considered together with the nitrogen configurations shown in figure 2, it is found that the HER catalytic activity is positively correlated with the absolute quaternary N content in these metal-free NG catalysts. The NG@H-MMT synthesized in HMMT with a δ of 0.46 nm contains the highest absolute content of planar N (3.65 at.%), but exhibits the lowest HER activity. Oppositely, NCΩMMT, the most active metal-free catalyst in this work, synthesized without the confinement of MMT (δ→∞) contains the highest absolute content of quaternary N (2.65 at.%). Interestingly, when Ni/Co metals are introduced into NG catalysts, the quaternary N no longer plays a key role in catalysis of HER. The NG@Co-MMT with planar nitrogen dominated configuration as NG@Na-MMT shows significantly better HER activity than all other catalysts (Figure 3a). A HER current density of 10 mA cm-2 is achieved at a much smaller overpotential of -210 mV on a NG@Co-MMT catalyzed electrode, which is even comparable to that of the state of the art Pt/C catalyst and is among the best carbon based catalysts for HER reported so far (see Table S1).1,6,8,25-28 In contrast, NG@Ni-MMT shows the lowest activity among the produced NG catalysts, even though both configurations and contents of nitrogen are almost same with those of NG@Co-MMT (see Figure 3a and 2d). These results indicate that the Co ions, after coordinated with planar-N doped NG, well promote the HER catalytic activity, but the Ni ions depress it. The Brunauer-Emmet-Teller (BET) surface areas of the produced catalyst were measured by nitrogen adsorption-desorption at 77K (Figure S8), and the activities of these NG catalysts were normalized by their specific surface areas to investigate the intrinsic activity. As shown figure 3b, the same increasing trend of specific activities as that


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ACS Energy Letters

of apparent HER activities was observed, which tell that the specific activities deeply depend on the type of the catalysts rather than porosity. We further modified the quaternary-N dominated NCΩMMT catalyst with Co species to investigate the enhancement effect of Co. As shown in figure 3c, although the Co species reached 4.5 wt.% in the NCΩMMT catalyst after modification, only a tiny positive shift of about 60 mV in the overpotential was observed. In contrast, the HER activity of the planar-N dominated NG catalyst after introducing Co (NG@Co-MMT) exceeds that of NG@H-MMT catalyst by 204 mV at a trace Co content level of 0.012 wt.% (Figure 3d). It indicates that the enhancement effect of Co is much more efficient on planar N than on quaternary-N. When further increasing the Co content to 2.58 wt.% in NG@Co-MMT, however, almost no change can be observed (Figure 3e). This may be attributed to the limited planar N content in the catalyst leading to a relatively low saturated concentration for the coordination. Besides, we evaluated the HER activities for the catalysts with nanoreactors removed at different degrees to investigate supports effect on activity. As shown in figure 3f, the extremely low catalytic current at a quite negative potential of -0.6 V vs RHE on the Co-MMT modified electrode tells that the nanoreactors are indeed inert for the catalysis of HER. After the in-situ formation of NG inside the Co-MMT nanoreactors, the HER activity is remarkably enhanced probably due to the formation of a large number of active sites. The same phenomenon also occurred on all the other nanoreactors (Figure S9). Further enhanced HER activity is observed on the NG catalyst with the nanoreactor totally removed, which owns well-exposed active sites and indicates that most active sites are formed inside rather than outside of the nanoreactor. There is a quite large interface between the MMT and produced NG catalyst after NG formation between the layers of MMT. Thus, the original sample before removing MMT nanoreactor would be the best HER catalyst in


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principle if the support interaction plays a dominate function. However, the results indicate that the best performance is achieved on the catalyst with 60 wt.% MMT removed, rather than the one with the largest NG-MMT interface (Figure 3f and S9c). Therefore, the support interaction shows little electronic influence but only hinder the restacking of NG sheets. Our N1s XPS data of NG catalysts which contain 40 wt.% MMT show no shift compared with the standard N1s data, further indicating no influence of support interaction on the N-electronic environment (Figure 2). But other effects such as wettability of MMT cannot be excluded at this time.

Figure 4. The adsorption energy of hydrogen adsorbed on quaternary-NG, pyridine-NG, Ni modified pyridine-NG and Co modified pyridine-NG (a); the free energy of HER on different


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ACS Energy Letters

catalysts (b); the relationship among the interspace width, free energy and HER activity of these NG catalysts (c). The tafel slope has often been used to identify the rate-determining step of HER (see in supporting information for details). As shown in figure S10, the tafel slopes for the NG catalysts are detected to be around 120 mV dec-1, while that for the Pt/C catalysts is 53 mV dec-1. The results indicate that the rate-determining step of HER on the NG catalysts is the volmer reaction, i.e. the adsorption of hydrogen, rather than the desorption of hydrogen (Heyrovsky reaction) as that on Pt.26,29-31 Based on these observations, we investigated the hydrogen adsorption on these nitrogen configurations by DFT calculation (see in Figure S11-13 for details). Several possible HER active sites on the proposed models were investigated, including the nitrogen itself, the carbon sites around it, and the metal site. As shown in figure 4a, after doping of the quaternary-N atom in the graphene structure, the EHads shows a remarkable decrease on both in-plane carbon sites from 1.538 to 0.424 or 0.554 eV, and edge carbon sites from 0.662 to -0.144 eV. It indicates that the carbon atoms are highly activated by the quaternary N for HER. The EHads on quaternary N itself is 1.599 eV which is so high that the quaternary N cannot adsorb hydrogen for HER. As for the pyridine-N doped graphene, the values of EHads on the carbon sites are almost same with those of pristine graphene, while the pyridine-N site shows a relatively low EHads of -0.027 eV. These results indicate that, differing from the quaternary N, the pyridine-N participates in the HER but cannot activate the nearby carbons at all. It is noted that one quaternary N atom can activate three carbon sites for HER but one doped pyridine-N atom only represents a single active site. In addition, the most active site on quaternary N doped NG shows a much lower EHads of -0.144 eV than that of pyridine-N (-0.027 eV). Consequently, the quaternary-N doped NG displays a much better HER performance than that of the pyridine-N doped NG catalyst. When


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introducing the metals into the NG catalyst, the hydrogen adsorption properties are totally different. As shown in figure 4a, a highly active Co site for HER was formed with the lowest EHads of -0.186 eV after introducing Co species into pyridine-N doped NG. Its hydrogen adsorption capacity is very close to the ideal level for HER, i.e. EHads of -0.24 eV (∆G = 0), and comparable to that on Pt (-0.33 eV). More importantly, the EHads on carbon atoms obviously decrease from 1.167 to 0.618 eV. This indicates that the incorporation of Co into the planar N doped NG not only introduces a highly active Co site for HER but also activates the nearby carbon atoms leading to the highest activity for HER among the NG catalysts. Although the incorporation of Co increases EHads of pyridine-N site to 0.606 eV, it is still active enough for HER. In contrast, the incorporation of Ni averages the EHads among the pyridine-N site (0.744 eV), carbon site (0.856 eV) and Ni site (0.729 eV) leading to the worse adsorption property. The strength of hydrogen adsorption on the most active sites for each model was computed to obtain the free energy barrier for this step. As shown in figure 4b, the free energy change values of rate-determining step are maximized for Ni-planar-NG and decrease successively for planar-NG, quaternary-NG and Co-planar-NG. Thus, as illustrated in figure 4c, an enlarged interspace width increases the quaternary N content, leading to a lower free energy change and higher activity of HER. Besides, the Co species greatly enhanced HER activity of the planar N doped NG, owing to both the formation of highly active Co sites and their activation effects on the nearby carbon and nitrogen atom. The Ni ions, however, homogenize hydrogen adsorption on their active sites and depressed them for catalysis of HER. The d-band structure of Co/Ni may be responsible for this phenomenon. As shown in figure S14, the resonant states around -3 eV caused by the hybridizations of p-orbital of N and d-orbital of Co is stronger than that of N and Ni, indicating a stronger electronic interaction between Co and NG than that between Ni and


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ACS Energy Letters

NG. The relatively higher peak in Co d-orbital at the Fermi level, compared with that in Ni, indicates a much better mobility of electrons in Co d-orbital. Considering together with the wider d-orbital distribution of Co than that of Ni in the range from 0 to 3 eV, we can conclude that the electrons is relatively delocalized in Co d-orbital but it is relatively localized in Ni d-orbital; the former facilities the electron transfer along the Co-N-C binding leading to an activation effect on the carbon an nitrogen sites during the catalysis, while the later hampers the electrons transfer leading to a depressed HER activity. In summary, we have adopted the space-confinement-induced method to prepare NG catalyst with different nitrogen configuration by using layered MMT as a quasi-closed flat nanoreactor to investigate the effects of nitrogen configurations of NG on the catalysis of HER. We found that, among the metal-free NG catalysts, the HER catalytic activity increased with the increase of quaternary N content. With an assistance of trace atomic Co, the planar N doped NG exhibits a much higher HER activity than that of quaternary N doped NG. The NG@Co-MMT displays an excellent HER activity with a very small onset potential of -49 mV (vs. RHE) and a much lower overpotential of -210 mV at 10 mA cm-2, which is among the best carbon based HER catalysts reported so far. DFT calculations revealed that the carbon atoms in the NG catalyst are highly activated by the nearby quaternary N but are nonfunctional with the doping of planar N, leading to higher adsorption energy for hydrogen on the quaternary N doped NG than that on the planar N doped NG. The activity enhancement of Co incorporation is due to the formation of active Co sites and their activation effects on the nearby carbon and nitrogen atoms leading to a lowest free energy barrier for HER. The Ni, however, depress HER activity due to the homogenization effect on hydrogen adsorption property. This work provides a new insight


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for constructing a new type of nanostructured carbon materials as a low-cost and highly efficient HER catalyst.

Acknowledgements This work was financially supported by the National Key Research and Development Program of China (2016YFB0101202), and by the National Natural Science Foundation of China (Grant Nos. 91534205, 21436003, 21573029 and 21776024), and by the Fundamental Research

Funds

for

the

Central

Universities

(106112017CDJXY220002,

106112017CDJXSYY0001 and 106112015CDJXY130008), and by sharing fund of Chongqing university’s large-scale equipment (201612150016 and 201612150015), by Chongqing’s postgraduate research innovation projects (CYS17003). Author contributions H. Wang and W. Li performed the experimental work and analyzed the results; N. Yang provided density functional theory calculation; L. Li, J. Wang, J. Jiang and F. Jia participated in the planning and analysis of the experiments; J, Li polished languages of manuscript. W. Ding, K. Chen and Z. Wei conceived and designed the experiments. Competing interests The authors declare no competing financial interests. Additional information Supporting Information. Experiment details, additional structural characterization and supporting electrochemical characterization are presented in Supporting Information.


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