Graphitic Carbon Composites as

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In-situ Formation of Cobalt Nitrides/Graphitic Carbon Composites as Efficient Bifunctional Electrocatalyst for Overall Water Splitting Ziliang Chen, Yuan Ha, Yang Liu, Hao Wang, Hongyuan Yang, Hongbin Xu, Yanjun Li, and Renbing Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18858 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018

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ACS Applied Materials & Interfaces

In-situ

Formation

of

Cobalt

Nitrides/Graphitic

Carbon

Composites as Efficient Bifunctional Electrocatalyst for Overall Water Splitting

Ziliang Chen,†,‡ Yuan Ha,†,‡ Yang Liu,§ Hao Wang,† Hongyuan Yang,† Hongbin Xu,† Yanjun Li,⊥and Renbing Wu*,†

†

Department of Materials Science, Fudan University, Shanghai 200433, China

§

Department of Chemistry, Fudan University, Shanghai 200433, China

⊥

Shanghai Institute of Measurement and Testing Technology, Shanghai 200233, China

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT:

Developing

cost-effective

and

highly

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efficient

bifunctional

electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is of great interest for overall water splitting, but still remains challenging issue. Herein, a self-template route is employed to fabricate a unique hybrid composite constructed by encapsulating cobalt nitrides (Co5.47N) nanoparticles within three-dimensional (3D) N-doped porous carbon polyhedra (Co5.47N NP@N-PC), which can be served as a highly active bifunctional electrocatalyst. To afford a current density of 10 mA cm−2, the as-fabricated Co5.47N NP@N-PC only requires overpotentials as low as 149 mV and 248 mV for HER and OER, respectively. Moreover, an electrolyzer with Co5.47N NP@N-PC electrodes as both the cathode and anode catalyst in alkaline solutions can drive a current density of 10 mA cm−2 at a cell voltage of only 1.62 V, superior to that of the Pt/IrO2 couple.The excellent electrocatalytic activity of Co5.47N NP@N-PC can be mainly ascribed to the high inherent conductivity and rich nitrogen vacancies of Co5.47N lattice, the electronic modulation of N-doped carbon towards Co5.47N as well as the hierarchically porous structure design.

KEYWORDS: Co5.47N, electrocatalyst, hydrogen evolution reaction, oxygen evolution reaction, overall water splitting


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INTRODUCTION Electrochemical water splitting is considered as the most promising technology for clean hydrogen production.1−3 However, as two important half-reaction involved in water splitting, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are kinetically sluggish and thermodynamically uphill, thus imposing a high driving overpotential in the practical electrolysis process.4−6 To address this issue, considerable efforts have been devoted to developing electrocatalysts which are often applied to reduce overpotential and realize less energy-intensive processes.7−11 To date, precious Pt is the most efficient HER electrocatalyst with a near-zero overpotential, while noble IrO2 and RuO2 are known as the state-of-the-art OER electrocatalysts.12,13 Unfortunately, the scarcity and electrochemical instability of these precious metal-based catalysts hinder their large-utilization in commercial electrolyzes.14 As an alternative to precious-metal catalysts, a large number of materials comprised of earth-abundant metals have been explored as HER and/or OER electrocatalysts.15−18 In particular, owing to the tunable electronic structure,19 3d transition metal-based catalysts such as carbides,20,21 chalcogenides,22,23 and phosphides24,25 have shown promising HER catalytic performance in acidic media, whereas transition metal oxides/hydroxides have been extensively studied for OER,26−28 and exhibited reasonable activity and stability in alkaline solution. Nevertheless, most electrocatalysts mentioned above are only active toward either the HER or the OER, and thus cannot be effectively used as bifunctional electrocatalysts for overall water splitting in the same electrolyte, leading to the low-efficiency of water electrolysis and



complexity of the system.29,30 Although a few recent works reported that bimetallic layered double hydroxide (LDH) or heterogeneous metal oxide/chalcogenides such as Ni-Fe LDH,31 MoS2/Ni3S2,32 and CoS/MoS233 could be used as the bifunctional electrocatalysts, a high operating voltage is still required to drive the overall water splitting, possibly be due to their poor intrinsic conductivity and slow charge transfer rate at the interface. It is therefore highly imperative to develop non-precious transition metal-based catalysts to achieve efficient overall water splitting. Compared to transition metal hydroxides and chalcogenides, their corresponding nitrides, in which the N atoms incorporating into the interstitials of the metal host lattice are bonded covalently to the metal atoms, generally exhibit metal-like feature and much better conductivities and stabilities.34−36 In addition, the formation of metal-nitrogen bond in the density of states of the metal d-band would endow the metal nitrides an electron donating character, and this could contribute to the high electrocatalytic activity of metal nitrides towards HER and OER for the overall water splitting.37−42 For instance, Zheng et al. reported that myriophyllum-like hierarchical TiN@Ni3N nanowire only required the overpotentials of 21 mV for the HER and 350 mV for the OER to afford a current density of 10 mA cm–2.41 Chen et al. found that bimetallic Ni-Mo nitride nanotubes could drive a current density of 10 mA cm–2 with the overpotentials of 89 mV for the HER and 295 mV for the OER, respectively.42 Despite these impressive advances made recently, the available synthesis procedures for above heterostructures usually involve multinary nitrides and complicated synthetic conditions (e.g. hydrothermal reaction combined with pyrolysis), which may not be suitable for the


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large-scale application. Hence, using a convenient approach to fabricate the simple metal nitrides as high efficiency catalyst towards both HER and OER is highly desirable, but remains a great challenge to date. On the other hand, previous studies have demonstrated that the fabrication of porous structures composed of nano-sized transition metal-based compounds and graphitic carbon is beneficial for enhancing the catalytic activity since such structures not only possess the high electrical conductivity and guarantee the intimate contact of the electrolyte with electrocatalysts, but offer numerous exposed active sites and allow fast diffusion of the generated gases.43−48 In addition, compared with the stoichiometric metal-based compounds, the non-stoichiometric ones are favorable to the generation of rich active sites induced by the vacancies and thus could exhibit higher catalytic activity towards HER and OER.49−51 For instance, Ding et al. reported that the overpotential for non-stoichiometric CoS2 (CoS1.84) with abundant sulfur vacancies to drive a current density of 10 mA cm−2 was as low as 167 mV, much smaller than that of pure CoS2 (~240 mV).49 Zhu et al reported that non-stoichiometric iron-cobalt oxide with abundant oxygen vacancies delivered a much smaller overpotential (308 mV) at a current density of 10 mA cm−2 than that of corresponding stoichiometric one (400 mV).50 Inspired

by

these

ideas,

herein,

we

in-situ

encapsulate

binary

and

non-stoichiometric cobalt nitrides nanocrystals, i.e., Co5.47N, into N-doped porous carbon polyhedra (Co5.47N NP@N-PC) via a simultaneous nitridation and carbonization procedure by using ZIF-67 as the self-template. As expected, the fabricated three-dimensional (3D) hybrid architectures can be served as highly active bifunctional



electrocatalyst. They exhibit not only small overpotential of ~149 mV at a current density of 10 mA cm–2 and Tafel slope as low as ~86 mV decade–1 for the HER, but also small overpotential of 248 mV at a current density of 10 mA cm–2 and Tafel slope as low as ~54 mV decade–1 for the OER, showing comparable performance to state-of-the-art Pt and IrO2 catalyst, respectively. Moreover, when Co5.47N NP@N-PC is used as electrocatalyst for overall water splitting, it is even better than the Pt/IrO2 counterpart. RESULTS AND DISCUSSION The synthesis strategy for Co5.47N NP@N-PC is illustrated in Figure 1, from which Co-based zeolitic imidazolate framework (ZIF-67) precursors were directly nitridated in the presence of flowing gas of NH3. During the nitridation, the Co ions liberated from ZIF-67 reacted with NH3 and formed Co5.47N nanoparticles, while the coordinated organic ligands were in-situ decomposed and carbonized to N-doped porous carbon, resulting in the formation of 3D hybrid composites constructed by encapsulating ultrafine Co5.47N nanoparticles within N-doped porous carbon.

Figure 1. Schematic illustration of the synthesis procedure for the Co5.47N NP@N-PC. ACS Paragon Plus Environment

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The as-obtained precursors and their nitridated products were first analyzed by X-ray diffraction (XRD). The XRD pattern of the precursors in the 2θ range of 5–45° was shown in Figure S1, from which the precursors were ZIF-67 pure phase with high crystallinity. For comparison, Figure 2a shows the XRD pattern of the nitridated products. The diffraction peaks positioned at around 43.7, 50.8, and 74.9° corresponded to (111), (200), and (220) crystal planes of Co5.47N, respectively, while the diffraction peaks positioned at around 26.3 and 44.4° corresponded to (002) and (101) crystal planes of carbon, respectively. Obviously, in addition to Co5.47N phase and graphitic carbon, no diffraction peaks of precursors and impurities were observed, suggesting a complete phase conversion from precursors to Co5.47N NP@N-PC. To elucidate the crystal structure of Co5.47N phase, Figure 2b illustrates the structure model of Co5.47N. As previously reported,52 the crystal structure of Co5.47N is quite similar to that of the Co4N but only difference in the amount of N atoms. For Co4N, the N atoms only and fully occupied in the octahedral interstitials of the Co metal lattice, while some N atoms were absent in the octahedral interstitials and formed corresponding vacancies in Co5.47N. Such an occupation of N atoms in Co5.47N not only endows the cobalt nitride metallic property, but also strengthens the tendency of electron donation via the formation of Co-N bond as well as provides more active sites via the generation of vacancies. To further clarify the formation of the N vacancies in the Co5.47N crystal structure, the Rietveld refinement has been performed for the XRD pattern of the as-prepared Co5.47N NP@N-PC composite. During the refinement, the initial structure model was firstly taken from the Co4N. Nevertheless, no satisfactory results could be



obtained. In contrast, if we used the structure model derived from the Co5.47N, in which some N vacancies are introduced into the Co4N, a well-consistent result could be achieved (Figure S2, Supporting Information). Furthermore, from the refined result (Table S1, Supporting Information), it can be seen that the lattice parameters (a = 3.589 Å) of the Co5.47N are smaller than those (a = 3.738 Å) of Co4N, which may be due to the presence of N vacancies. Figure 2c presents the Raman spectrum of the composite in the range of 1000–2000 cm‒1. Two broad peaks were found at 1342 cm‒1 (D-band) and 1588 cm‒1 (G-band), respectively, in which the D- and G-band represent the feature of amorphization and graphitization, respectively. The relative intensity ratio of D-band and G-band (ID/IG) for the Co5.47N NP@N-PC was calculated as 0.99. This value was smaller than that (1.05) for the Co NP@N-PC composite prepared by the calcination of ZIF-67 under Ar atmosphere at 700 °C, indicating the higher graphitization degree of carbon. Furthermore, as an indication of the pore features of the Co5.47N NP@N-PC, nitrogen gas (N2) sorption experiment was performed. As shown in the N2 sorption isotherm analysis (Figure 2d), it could be seen the existence of mesopores in the composites, which is also verified by the pore size distribution (inset in Figure 2d). The specific surface area evaluated by the Brunauer–Emmett–Teller (BET) method was approximately 191 m2 g−1. Such high surface area and rich porous channel are believed to be beneficial for improving the electrocatalytic performance.


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Figure 2. (a) XRD pattern of the Co5.47N NP@N-PC composites, (b) crystal structure model of the Co5.47N, (c) Raman spectra of the Co5.47N NP@N-PC and Co NP@N-PC composites, (d) N2 sorption isotherms measured at 77 K for Co5.47N NP@N-PC. Inset in (d): the pore-size distribution calculated using the Barrett–Joyner–Halenda (BJH) method. The morphology of ZIF-67 precursors and Co5.47N NP@N-PC were characterized by field-emission scanning electron microscopy (FESEM). As shown in Figure 3a-b, the ZIF-67 crystals exhibited polyhedral morphology with sizes ranging from 500 to 700 nm and their surfaces are very smooth. After nitridation treatment under flowing gas of NH3 at 700 °C, the particles still inherited well the dodecahedron-like morphology (Figure 3c), whereas its surface was shrunk and presented much rougher (Figure 3d), suggesting its porous feature. Moreover, a broken Co5.47N NP@N-PC particle can be found in Figure S3, indicating that some of them may have hollow structure.



Figure 3. FESEM images of the (a, b) ZIF-67 precursors and (c, d) Co5.47N NP@N-PC composite. To investigate the microstructure of Co5.47N NP@N-PC composites, transmission electron microscope (TEM) was further conducted. The low-magnification TEM image (Figure 4a and 4b) clearly exhibited that the polyhedron-like composites were uniformly dispersed and their surfaces were very rough, which are well consistent with the FESEM observations. Moreover, the magnified TEM images shown in Figure 4c suggested that the polyhedron was composed of numerous nanocrystals embedded within carbon matrix. The high-resolution TEM (HRTEM) image (in Figure 4d) further revealed that the carbon matrix is mainly composed of the graphitic carbon with porous structure. The embedded nanocrystals exhibit an obvious lattice fringe with a d-spacing of 0.208 nm, corresponding to the (111) lattice of the Co5.47N. On the outside, they are encapsulated by a thickness of about 3–5 graphitic carbon layers with an interlayer


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distance of 0.340 nm (Figure 4e), strongly indicating an intimate contact between Co5.47N and carbon matrix, which is favourable to the stability of catalysts. Furthermore, a large number of defects including twin boundaries (marked as “TB”) and stacking faults (marked as “SF”) can be observed in the exterior surfaces of the ultrafine Co5.47N particles. These defects are believed to promote the molecular adsorption and give rise to enhanced catalytic activity. A typical high-angle annular dark-field scanning TEM (HAADF-STEM) image of a Co5.47N NP@N-PC polyhedra showed a uniform dispersion of bright subunits, indicating that the Co5.47N nanocrystals were highly confined and homogenously dispersed (Figure 4f). The corresponding elemental mapping analysis revealed the coexistence and the homogenous dispersion of Co, N, and C elements within the polyhedra framework (Figure 4g−i). Meanwhile, the EDS spectra recorded from a representative Co5.47N NP@N-PC composite showed that the atomic ratio of Co and N (5.03:1) is smaller than 5.47:1 (Figure S4), which could be due to the additional amount of N element doped in the carbon matrix derived from the ZIF–67.



Figure 4. (a-c) TEM images and (d, e) HRTEM images of Co5.47N NP@N-PC; (f) HAADF-STEM image and corresponding (g-i) elemental mapping images showing the homogenous distribution of all three elements of Co, N, and C in the architecture. On the basis of the above results, we have confirmed the in-situ formation of a strong coupling between Co5.47N and N-doped carbon by nitridating the ZIF-67 template. To further check the electron interaction between Co5.47N and N-doped carbon, the X-ray photoelectron spectroscopy (XPS) measurement was employed to identify the element composition and chemical state in Co5.47N NP@N-PC composites. For comparison, XPS measurement was also conducted for the Co NP@N-PC sample. The XPS survey spectra of Co5.47N NP@N-PC and Co NP@N-PC samples were shown in Figure 5a, from which the peaks of C 1s, N 1s, and Co 2p could be clearly observed.


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The presence of Co-N bond in Co5.47N NP@N-PC was firstly verified by high-resolution Co 2p and N 1s spectra. As shown in the high resolution Co 2p spectrum of Co5.47N NP@N-PC (Figure 5b), Co atoms are in the oxidation state, implying the presence of the Co-N bond in the Co5.47N. In contrast, most Co atoms in Co NP@N-PC are in the metal state. In addition, the high-resolution N 1s spectra of Co5.47N NP@N-PC (Figure 5c) indicates that except the existence of pyridinic N, pyrrolic N, and graphitic N, N-Co bond assigned to the peak of 398.9 eV was obviously observed, further demonstrating the presence of Co-N bond in the Co5.47N.53 The chemical coupling between the N-doped carbon and Co-N bond in Co5.47N was further demonstrated by comparing the high-resolution C 1s spectra of Co NP@N-PC and Co5.47N NP@N-PC. As shown in Figure 5d, in the high-resolution C 1s spectrum of Co NP@N-PC, three main peaks with binding energies of 284.3 and 285.8 eV were assigned to C-C/C=C and C=N bonds, respectively.54 The presence of C=N bonds strongly confirms the successful doping of N element in the carbon matrix. In the high-resolution C 1s spectrum of Co5.47N NP@N-PC, the relative peak intensity of C=N was strikingly augmented as compared to that in Co NP@N-PC, which should be due to more N amount in the Co5.47N NP@N-PC. Since the Co NP@N-PC was prepared by thermal-annealing ZIF-67 under Ar, the N in Co NP@N-PC is therefore derived from the decomposition of organic group in ZIF-67. Upon thermal annealing ZIF-67 precursors under NH3 at the same temperature, the molar ratio of N to Co in the composites increased from 0.98 to 1.15 (based on the XPS results), indicating that the excessive N may originate from NH3. Theoretically, the formation of Co5.47N can make



the molar ratio of N to Co increase to 1.12, lower than the measured value (1.15), implying that NH3 would also contribute to the high N amount in the porous carbon, as previous report demonstrated.55 The relative peak intensity of C=N was strikingly augmented as compared to that in Co NP@N-PC, indicating that the carbon matrix not only interacted with the doped nitrogen, but also had a strong electronic interaction with Co-N bond in the Co5.47N host lattice. The formation of Co-N-C bond is believed to effectively enhance the catalytic activity for OER and HER by lowering the energy barriers of intermediates.56

Figure 5. (a) the survey XPS spectra for Co5.47N NP@N-PC (up) and Co NP@N-PC (down), and their corresponding high resolution (b) Co 2p spectra, (c) N 1s spectra, and (d) C 1s spectra. To evaluate the HER and OER catalytic activities, a three-electrode cell was


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employed by using the Co5.47N NP@N-PC, saturated calomel electrode, and carbon rod as the working electrode, reference electrode, and counter electrode, respectively. Linear sweep voltammetry (LSV) was performed at a scan rate of 10 mV s–1 in 1 M KOH aqueous electrolyte. For comparison, the Co NP@N-PC electrodes were also measured using the same method. As presented in Figure 6a, the overpotential to drive the current density of 10 mA cm–2 for Co5.47N NP@N-PC is as low as 149 mV, which is much smaller than that of Co NP@N-PC (561 mV). In addition, the Tafel slope (86 mV dec–1) of the Co5.47N NP@N-PC is also much lower than that (124 mV dec–1) of Co NP@N-PC. The smaller overpotential at a current density of 10 mA cm–2 as well as lower Tafel slope reveal the higher HER catalytic performance of Co5.47N NP@N-PC as compared to Co NP@N-PC electrocatalysts in alkaline electrolytes. It should also be remarked that the HER catalytic performance of the Co5.47N NP@N-PC is close to the commercial Pt@C (Figure 6a and 6b) and superior to most recently reported Co-based electrocatalysts (Table S2), demonstrating the potential in the electrochemical application. To unravel the possible origin for the high HER catalytic performance of Co5.47N NP@N-PC, the electrochemical surface area (ECSA) and the electrochemical impedance spectra (EIS) of the catalysts were evaluated. To estimate the ECSA, the double-layer capacitance (Cdl), which is in proportion to ECSA, is obtained by deriving from the cyclic voltammetry (CV) curves versus the scan rate (Figure 6c, 6d and Figure S5). The Co5.47N NP@N-PC has much larger Cdl value (24.8 mF cm–2) than the Co NP@N-PC (5.5 mF cm–2), suggesting its enriched and more exposed catalytically active sites for electrocatalysis during HER. Nyquist plots of Co5.47N NP@N-PC and Co5.47N



NP@N-PC are presented in Figure 6e. The semicircular diameter (corresponding to charge transfer resistance Rct) in the EIS of Co5.47N NP@N-PC is much smaller than that of Co NP@N-PC, revealing the importance of strong interactions between Co5.47N and N-PC in enhancing the electron transport rates. In addition to the high activity, the long-term stability is another crucial criterion to evaluate an advanced electrocatalyst. To assess the durability of the Co5.47N NP@N-PC catalysts, controlled potential electrolysis was performed in 1.0 M KOH. After 10 h, the current density was relatively stable and 85% of the initial value was retained for the Co5.47N NP@N-PC, much better than those of Co NP@N-PC (29%) and state-of-the-art Pt@C (56%) at an overpotential of 561 mV (Figure 6f). The morphology of the Co5.47N NP@N-PC after the cycle durability test was also characterized. As shown in Figure S6, the characteristic of polyhedral morphology is still retained.


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Figure 6. (a) IR-corrected LSV curves of Co5.47N NP@N-PC, Co5.47N NP@N-PC and Pt@C samples for hydrogen evolution, and (b) the corresponding Tafel curves, (c) CVs curves at different scan rates of Co5.47N NP@N-PC, (d) plots of the capacitive currents as a function of scan rate, (e) EIS of Co5.47N NP@N-PC and Co NP@N-PC, and (f) the current–time curves of Co5.47N NP@N-PC, Co NP@N-PC, and Pt@C. Inset in (e): the equivalent circuit of the Co5.47N NP@N-PC and Co5.47N NP@N-PC electrodes. To explore the possibility of our Co5.47N NP@N-PC catalysts for overall water splitting, we further evaluated the OER activity of the hybrid electrode in 1 M KOH. As expected, the Co5.47N NP@N-PC catalyst also showed excellent OER activity in the alkaline medium, which is much better than the Co NP@N-PC (Figure 7a). Specifically,



it only exhibits an overpotential as low as 248 mV to achieve a current density of 10 mA cm–2, much lower than that of the Co NP@N-PC (390 mV), and even lower than that of the commercial IrO2@C (281 mV) and most recently reported Co-based electrocatalysts (Table S3). Moreover, the Tafel slope of the Co5.47N NP@N-PC catalyst is calculated to be 54 mV dec–1, significantly smaller than those of the Co NP@N-PC and commerical IrO2@C. In addition, as shown in Figure 7c, the Co5.47N NP@N-PC composite keeps a nearly 82% retention of current density at an overpotential of 248 mV even after 10 h, showing higher stability than that of Co NP@N-PC and the state-of-the-art IrO2@C (56%). Noted that the curves for the durability test of Co5.47N NP@N-PC gently decline firstly, and then increase, and decline again. The fluctuation during the stability test can be commonly observed in other porous electrocatalysts.57,58 Such phenomenon is believed to be mainly ascribed to the alternate processes of the bubble accumulation and release. To understand the influence of nitridation temperature on the phase composition, morphology, and electrocatalytic performance, the ZIF-67 precursors were also nitridated at 600 and 800 °C, respectively. Figure S7 showed the XRD patterns of the nitridated products at 600 and 800 °C. Co phase instead of cobalt nitrides phase could be observed in the products obtained at 600 °C, while Co5.47N phase and carbon phase were formed in the products obtained at 800 °C. This result indicates that the formation of Co5.47N phase requires an appropriate nitridation temperature. The products obtained at 600 and and 800 °C were also characterized by FESEM (Figure S8). It can be found that the surface of products become much rougher and shrunk with increasing


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temperature, indicating the particle growth at higher temperature. Figure S9 showed the LSV curves of the products towards HER and OER in the 1.0 KOH solution. Obviously, all of them show the inferior HER and OER activity to those of Co5.47N NP@N-PC. The excellent HER/OER activity of Co5.47N NP@N-PC might be due to the balance of phase composition, graphitization degree and particle size. On the other hand, the overpotentials at the current density of 10 mA cm–2 towards HER and OER for bare cobalt nitride and N-doped carbon are also much higher than those of Co5.47N NP@N-PC (Figure S10), demonstrating the advantage of in-situ encapsulation of cobalt nitride by the N-doped porous carbon polyhedra. To get an insight into the origin of the superior OER performance, the turnover frequencies (TOF) were evaluated, which is believed to be directly related to the intrinsic activities of catalytic sites. The TOF value for Co5.47N NP@N-PC samples at an overpotential of 280 mV is 0.054 s–1, whereas the value for Co NP@N-PC at the same overpotential is almost negligible. This strongly suggests the rich active sites for catalytic reactions in Co5.47N NP@N-PC electrode, leading to the enhanced OER activity. Meanwhile, EIS measurement results shown in Figure 7d indicate that the Co5.47N NP@N-PC electrode possesses a much smaller Rct as compared to that of Co NP@N-PC, meaning the desirable electron transport and favorable catalytic kinetics. In addition, the morphology of the Co5.47N NP@N-PC electrode after the cycle durability still maintains well (Figure S11), which should be contributed to the high OER stability.



Figure 7. (a) IR-compensated LSV curves of Co5.47N NP@N-PC, Co NP@N-PC and IrO2@C samples for oxygen evolution, (b) the corresponding Tafel curves, (c) the current–time curves of Co5.47N NP@N-PC, Co NP@N-PC and Pt@C, (d) EIS of Co5.47N NP@N-PC and Co NP@N-PC; (e) high resolution XPS spectra of O 1s in Co5.47N NP@N-PC after current–time measurement; (f) the overall water splitting performance of the Co5.47N NP@N-PC and (Pt@C)||(IrO2@C). Inset in (d): the equivalent circuit of the Co5.47N NP@N-PC and Co5.47N NP@N-PC electrodes; Inset in (f): the schematic diagram for overall water splitting reaction in a two-electrode configuration. The XPS characterizations of Co5.47N NP@N-PC after the OER cycle durability


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test were conducted to further understand the origin of the superior OER activity. As compared to the high resolution Co 2p spectra achieved before OER test, except the peak assigned to Con+, a new peak positioned at higher binding energy was found in the high resolution Co 2p spectra achieved after OER test, which should be corresponded to the Co with higher valence (Figure S12). This indicates the partial oxidation of Co5.47N during the OER process. Moreover, the deconvolution peaks of the O 1 s spectrum are centered at 530, 531.8, and 533.2 eV (Figure 7e), which are usually ascribed to metal-oxygen bonds, low-coordinated oxygen ions at the surface and defect sites, and adsorbed

water,

respectively.59 The

presence

of

metal-oxygen

bonds

and

low-coordinated oxygen ions further suggests the formation of the cobalt oxides/hydroxides on the surface of the Co5.47N NP@N-PC electrode during the OER process. To further confirm the above results, the Co5.47N NP@N-PC electrode after the OER process were characterized by TEM. As shown in Figure S13a-c, the lattice distances of 0.237 nm and 0.207 nm in the outside part and the core part corresponded to the (011) and (111) lattice planes of the Co(OH)2 and Co5.47N phases, respectively. Furthermore, the EDS result (Figure S13d) showed the significant enhancement in the peak intensity of oxygen as compared to that of the Co5.47N NP@N-PC before OER process. These results further demonstrate the formation of the cobalt hydroxides on the surface of the Co5.47N NP@N-PC during the OER process. Noted that a weak oxidation peak positioned at about 1.39 mV vs. RHE can be found in the LSV curve (Figure S14). With prolonged cycle test, the weak oxidation peak gradually disappeared, possibly be due to the gradual steady of the oxidation process.60,61 Furthermore, with the prolonged



OER tests, the surface oxidation layer generated on the non-oxide Co-based catalysts can gradually achieve a steady state and prevent the Co-based core from further oxidation, resulting in the formation of interface between non-oxide and oxides Co-based compounds.60,61 Such interfaces are beneficial to enhance the OH− adsorption ability and reduce the Gibbs free energy of the reaction intermediate, which improves the OER catalytic activity.62,63 Inspired by the superior HER and OER activities, an alkaline electrolyzer with two-electrode system using Co5.47N NP@N-PC electrodes as both anode and cathode was designed in 1.0 M KOH electrolyte solution. As shown in Figure 7f, the polarization curve achieves a water-splitting current density of 10 mA cm−2 at a cell voltage of only 1.62 V, which is lower than those of (Pt@C)||(IrO2@C) (Figure 7f) (1.65 V) and many other recently reported cobalt-based catalysts (Table S4), showing its potential in practical applications. The outstanding catalytic performance of the Co5.47N NP@N-PC toward both HER and OER could be attributed to the following factors: (1) the intrinsic high conductivity of Co5.47N can accelerate the electron transfer during the OER and HER process; (2) the rich N vacancies in Co5.47N possibly serve as new active sites during electrocatalysis; (3) the construction of Co5.47N nanoparticles embedded in porous N-doped carbon, not only ensures more active sites and mechanical stability, but also is favorable for the mass transportation; (4) the chemical coupling between Co5.47N and N-doped carbon effectively lower the energy barriers of intermediates and thus promote the catalytic reactions. All these factors synergistically facilitate the catalytic activity of Co5.47N NP@N-PC for overall water splitting.


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CONCLUSIONS In summary, we have developed a facile and scalable self-template approach to fabricate a 3D Co5.47N NP@N-PC electrocatalyst for highly efficient overall water splitting. Benefiting from the intrinsic high conductivity to accelerate the electron transfer, larger surface area and rich defects to expose more active sites as well as the unique 3D configuration, the catalyst exhibits outstanding catalytic performance for both HER and OER, achieving a current density of 10 mA cm−2 at low overpotentials of 149 mV for HER and 248 mV for OER, respectively. Moreover, the alkaline electrolyzer using Co5.47N NP@N-PC as both anodes and cathodes also exhibit superior water splitting efficiency. The findings in the present work may open up a new avenue for the development of other high-performance bifunctional electrocatalyst for water splitting system. EXPERIMENTAL SECTION Preparation of ZIF-67 precursor: The preparation of ZIF-67 is similar to previous reports.64,65 In a typical preparation, 1.233 g of cobalt nitrate hexahydrate were dissolved in 25 ml methanol. The solutions were thoroughly mixed by stirring 1 h. Then, the as-obtained solutions were slowly injected into 25 ml methanol solution with 1.466 g of 2-methylimidazole. After about 5 min vigorously stirred, the mixture was placed at room temperature for 18 h. The resulting purple precipitates (ZIF-67) were centrifuged and washed with ethanol for at least three times, and finally dried in a vacuum at 60 °C.



Synthesis of Co5.47N NP@N-PC composite: The as-obtained Co-ZIF were placed in a corundum crucible, which was then loaded within a tube furnace. Then, the samples were heated to 700 °C at a ramping rate of 2 °C min−1 and kept at this target temperature for 2 h under a flow NH3 gas. After naturally cooling down to 25 °C, the Co5.47N NP@N-PC powders with black color were obtained. For comparison, Co NP@N-PC composites were synthesized by a process similar to that for making Co5.47N NP@N-PC but in the presence of Ar. Materials Characterization: To evaluate phase structures, powder X-ray diffraction (XRD) measurements were carried out on a D8 ADVANCE X-ray diffractometer with Cu Kα radiation, and the obtained XRD pattern were then further analyzed by the Rietveld method on the RIETAN-2000 program.66 Field-emission scanning electron microscope (FESEM, JEOL JSM-6700F) and transmission electron microscope (TEM, JEOL JEM-2100F) were used to examine the morphology and microstructure of the obtained products, respectively. The specific surface area (SSA) of samples was measured on ASAP 2020 Plus HD88 instrument. Raman spectroscopy was recorded to acquire the carbon structure by using Renishaw via plus laser Raman spectrometer. X-ray spectra (XPS) measurements were conducted on a Kratos XSAM-800 spectrometer with an Mg Kα radiation source. A monochromatic Al Kα1 X-ray (hγ= 1486.6 eV) implemented with a SPECS PHOIBOS 150 electron energy analyzer (total energy resolution: 0.50 eV) was employed to acquire high-resolution XPS. A polycrystalline Ag foil was used to calibrate the binding energy.


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Preparation of Working Electrodes: To prepare the working electrodes, powder samples (4 mg) were firstly dispersed in ethanol (976 µL), followed by the addition of Dupont Nafion 117 solution (24 µL, 10 wt.%). After that, the mixed solution was sonicated for 0.5 h to acquire a homogeneous catalyst suspension. Finally, 8 µL of catalyst suspension was cast onto the surface of glass carbon (GC) electrode with a 3 mm diameter, and the working electrode was naturally dried at 25 °C before electrochemical measurements. For comparison, commerical noble electrocatalysts were also evaulated. The commerical Pt@C and IrO2@C powders were directly purchased, and the weight percentage of Pt in Pt@C and IrO2 in IrO2@C electrode are 10% and 75%, respectively. Electrochemical

Measurements:

All

measurements

were

carried

out

in

a

three-electrode cell with an electrochemical workstation (AutoLab PGSTAT302N) at 25 °C in the O2 saturated 1 M KOH solution for both HER and OER. A carbon rod and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. Linear sweep voltammetry (LSV) was measured at a scan rate of 10 mV s–1. All potentials were calibrated with respect to reversible hydrogen electrode (RHE) based on following equation: Evs.RHE = Evs.SCE + 0.2412 + 0.05916pH – iRs. The method for iR-compensation is described as follows: the value of uncompensated solution resistance (Rs) based on the potentiostatic electrochemical impedance spectroscopy (EIS) was first extracted, then the i·80% Rs (80% of Rs value) drops were manually performed through the NOVA 2.1 software on the Autolab equipment. On the basis of the LSV, TOF was calculated as the following equation:



TOF = j/(4·F·m/M), where j, F, m, and M represent the current density (mA cm–2) at a certain overpotential, the faraday constant (~96485 C mol–1), the mass loading of the catalyst (mg cm–2), and the molecular weight of the catalyst, respectively. In order to evaluate the electrochemical surface area (ECSA) of the materials, the electrochemical double-layer capacitance (Cdl) was calculated as it was directly proportional to the ECSA. Briefly, cyclic voltammetry (CV) was conducted within non-Faradaic region at the scan rates from 2 to 10 mV s–1. The difference (Δjc-d) between charging and discharging current density was plotted linearly against the scan rates, and the fitted slope was the double of Cdl value. EIS for HER and OER were tested in a frequency range of 100 kHz-0.01 Hz at the potential 0.15 V vs. RHE and 0.3 V vs. RHE, respectively. The cycle durability were tested by the chronoamperometric response in the O2 saturated 1.0 M KOH solution.

ASSOCIATED CONTENT Supporting Information XRD patterns of the ZIF-67 precursor, nitridated products at 600 and 800 °C; Rietveld refinement for the XRD pattern of the Co5.47N NP@N-PC composites; FESEM images of the broken Co5.47N NP@N-PC polyhedron, the Co5.47N NP@N-PC composites after HER/OER processes and nitridated products at 600 and 800 °C; EDS results recorded from the representative Co5.47N NP@N-PC composite before and after OER process; comparison of the high-resolution Co XPS spectrum from the Co5.47N NP@N-PC composites before and after OER process; TEM image of the Co5.47N NP@N-PC composites after OER process; other electrochemical data including CV curves and


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LSV curves. This material is available free of charge via the Internet at http://pubs.acs.org. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions ‡

Z. Chen and Y. Ha contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (51672049), the Recruitment Program of Global Youth Experts (National Thousand Young Talents Program), the Research Grant for Talent Introduction of Fudan University, China (JJH2021103) and Fudan's Undergraduate Research Opportunities Program (FDUROP).

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Graphitic Carbon Composites as

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