Metallic Cobalt@Nitrogen-Doped Carbon Nanocomposites: Carbon

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Metallic Cobalt@Nitrogen-Doped Carbon Nanocomposites: CarbonShell Regulation toward Efficient Bi-Functional Electrocatalysis Qijie Mo, Nana Chen, Mengdie Deng, Lichun Yang, and Qingsheng Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10853 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 15, 2017

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ACS Applied Materials & Interfaces

Metallic

Cobalt@Nitrogen-Doped

Carbon

Nanocomposites: Carbon-Shell Regulation toward Efficient Bi-Functional Electrocatalysis Qijie Mo,a Nana Chen,a Mengdie Deng,a,b Lichun Yang,*,b and Qingsheng Gao*,a a

Department of Chemistry, Jinan University, No. 601 Huangpu Avenue West, 510632

Guangzhou, P. R. China b

School of Materials Science and Engineering, South China University of Technology, No. 381

Wushan Road, 510641 Guangzhou, P. R. China KEYWORDS:

noble-metal-free

electrocatalysts,

metal-organic-frameworks,

controlled

pyrolysis, cobalt@N-doped carbon nanostructures, carbon shells, bi-functional electrocatalysis

ABSTRACT: To advance hydrogen economy, noble-metal-free electrocatalysts with good efficiency are urgently demanded. They can be developed from metal-organic frameworks (MOFs) with abundant structure-variety, in which a controlled pyrolysis is desired to rationalize nanostructure and maximize catalytic activity. Herein, the efficient regulation is proposed for the first time on the carbon-shell of MOFs-derived Co@NC nanocomposites via varying temperature and flow-rate during pyrolysis, enabling the good accessibility and the electronic optimization of active Co cores. With moderated temperature and flow-rate, the resulting ultrathin carbon-shell on the one hand renders Co cores easily accessible to electrolytes, and on the other hand

1 ACS Paragon Plus Environment


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promotes the electronic penetration to optimize metallic Co active-sites. As expected, the optimal Co@NC affords the benchmarking performance of noble-metal-free electrocatalysts in hydrogen evolution and oxygen reduction reactions, featured by the low overpotentials, the striking kinetic metrics, and the outstanding long-term stability. Elucidating the feasibility to design efficient electrocatalysts via controlled MOFs pyrolysis, this work will open up new opportunities for the development of cost-effective materials in energy field.

■ INTRODUCTION The blueprint of hydrogen economy critically depends on two key technologies: water electrolyzers and fuel cells.1-2 Water electrolysis store energy generated from renewable sources (e.g. wind and solar) into hydrogen bonds via the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode.3 Then, fuel cells provide ondemand power to end-users by the hydrogen oxidation reaction (HOR) at the anode and the oxygen reduction reaction (ORR) at the cathode. During the above inter-conversions between chemical energy and electricity, efficient electrocatalysts are desired to decrease the dynamic overpotentials that mainly spawn excessive energy consumption.4-6 So far, noble metals are the benchmark electrocatalysts,7-8 i.e., platinum for HER, HOR and ORR, and iridium and ruthenium oxides for OER. However, their high cost and scarcity seriously impede the widespread application. It’s urgently demanded to explore cost-efficient electrocatalysts based on Earthabundant elements. 8-10 Transition metals (e.g., Fe, Co and Ni) and their oxides, phosphides, sulfides and carbides, are recently discovered as promising noble-metal-free electrocatalysts,11-15 which are highlighted by the low cost and the high Earth-abundance. In particular, metal@carbon (M@C) core-shell nanostructures have demonstrated the promising performance in electrocatalytic HER and


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ORR.16-20 The carbon matrix serves as important components to improve metal dispersity, resulting in rich active sites for electrocatalysis.21-23 More importantly, the strong interactions associated with carbon-shell endow composites with engineered electronic configuration and promoted electrocatalysis,24-25 which is in contrast to the traditional cognition that carbon-shell in M@C structures will render the metal cores inert. Bao and co-workers proposed that the electronic interactions between metallic cores and carbon-shell might change the work function of the shell, and thereby afford it with unexpectedly high activity.26 Li et al further proved that the carbon-shell was not seamless, which enabled good accessibility to metal cores, but unfortunately caused leaching or poisoning during electrocatalysis.27 The ultrathin shell is rationally desired to promote the accessibility and electronic modulation of active sites, while, the thick one is demanded to prevent active species from corrosive electrolytes or poisons. Obviously, the control on the thickness and chemical states of carbon-shell is of great significance in electrocatalyst design.28 As highlighted in the recent work by Han et al,29-31 a quantitatively modulation on the ORR activity of M@C was accomplished via controlling the thickness of the carbon-shell and the nitrogen-doping level. However, the facile and precise control over shell structures still remains as great challenges during exploiting cost-efficient M@C electrocatalysts. Metal-organic-frameworks (MOFs) are attractive precursors toward M@C, because their abundant organic ligands and atomic periodic structure can enable the homogeneous generation of ultrafine nanostructures.32-34 The highly excessive organic molecules beyond metals will easily result in thick carbon-shell during pyrolysis, and thus the precise control is highly desired.35 For example, Co-based zeolitic imidazolate frameworks-67 (ZIF-67), assembled by cobalt ions and N rich imidazole ligands, is feasible to achieve various Co@C nanostructures.36-



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However, the modulation on carbon-shell is usually ignored, which unfortunately prohibits the

electrocatalysis on the encapsulated Co cores.21,39 Further improvement requires extra N and B doping into carbon-shell21 or introducing carbon-based supports,39-40 indicating the poor transportation of electrolytes or electrons through the heavy carbon-shell. Thus, the controlled pyrolysis of the ZIF-67 precursors is highly desired for the rational catalyst design based on MOFs. Herein, we attempt to engineer the carbon-shell in Co@N-doped carbon (Co@NC) by simply and directly varying temperature and flow rate during ZIF-67 pyrolysis (Scheme 1). The above two factors are crucial to modulate the chemical composition and morphology of activesites in [email protected],41-42 For example, the pyrolysis at relatively low temperature and slow flow-rate results in thick carbon-shell on Co due to slow graphitization,, which obstructs Co active surface and impedes electrocatalysis. And the increasing temperature and flow-rate cause the formation of incomplete carbon-shell with rich seams, which cannot prevent the Co cores from etching or poison. Therefore, a controlled pyrolysis at moderated temperature and flow-rate is employed to accomplish the balance between the exposure and protection of Co nanoparticles (NPs), and more importantly enable effective electronic interactions between Co and carbon-shell. As expected, the optimal Co@NC affords a robust and superior activity in both HER and ORR, performing among the best of current noble-metal-free electrocatalysts.


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Scheme1. Schematic illustration for the fabrication of Co@NC nanocomposites. T and vAr denote for the temperature and argon flow-rate adopted for pyrolysis, respectively. ■ RESULTS AND DISCUSSION As illustrated in Scheme 1, Co-based ZIF-67 was fabricated via reacting Co2+ with 2methylimidazole (2-MI) in methanol, and identified by X-ray diffraction (XRD) and N2 sorption isotherm analysis (Figure S1 in Supporting Information). After pyrolysis under argon flow at different temperature, the Co/N-doped carbon (Co/NC) composites were received. The increasing temperature and Ar flow rate lead to the reduced thickness of carbon-shell and the enrich seams, and the moderated condition (T = 775 oC, vAr = 150 mL min-1) is employed to optimize the carbon-shell on Co cores. The Co/NC composites were further treated with 1 M HCl (aq.) to remove the residual Co species out of carbon-shell, resulting in Co@NC core-shell nanostructures. The samples before and after HCl treatment were denoted as Co/NC-T and Co@NC-T, respectively. To investigate the texture of carbon species, Raman spectra was conducted on a series of Co/NC obtained with varied T (700 ~ 850 oC) and fixed vAr (150 mL min-1). The as-received samples clearly present the characteristic bands of carbon materials after normalization (Figure 1a). The two peaks located at 1385 and 1580 cm-1 are assigned to the typical D- and G-bands, respectively, and the board one around 2786 cm-1 corresponds to the 2D band.43-44 The D-band is associated with the inter-valley double-resonance process that is often activated by grain boundaries or defects. And the G-band is a first-order scattering process that refers to optical phonons at the Brillouin zone center, corresponding to in-plane carbon atom stretching vibrations.45 It is well known that 2D-band is intrinsic resonant modes of few-layered graphene and can be designated as inter-valley process involving two iTO phonons near the K point.45-46



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Thus, the intensity ratio of D-band and G-band (ID/IG), and of 2D-band and G-band (I2D/IG), can be an indicator for the defect induced by N-doping, and the stacking mode of graphene layers, respectively.47 It is obvious that the value of ID/IG remains at the same level (0.97 ± 0.02) in the three samples (Figure 1b), indicating the similar defects of carbon. By contrast, the I2D/IG visibly decreases with increasing temperature, which suggests the reduced thickness of graphitized carbon-shell in Co/NC. This evolution can be further verified by transition electronic microscopy (TEM) investigation (Figures 1c ~ 1e). The Co/NC-700 clearly presents the thick and highlygraphitized carbon-shell on Co NP (Figure 1c). With the increasing temperature for pyrolysis, such shell turns to be thin and amorphous in Co/NC-775 and Co/NC-850 (Figures 1d and 1e).


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Figure 1. (a) Raman spectra of Co/NC (I: Co/NC-700, II: Co/NC-775, III: Co/NC-850) obtained with the vAr of 150 mL min-1, (b) their corresponding ID/IG and I2D/IG. HR-TEM images of Co/NC (c) -700, (d) -775, and (e) -850 received with the vAr of 150 mL min-1, and Co/NC obtained at 775 oC with the varied vAr of (f) 100 and (g) 200 mL min-1. (h) The evolving Co loading in Co/NC (vAr = 150 mL min-1) before and after 1 M HCl (aq.) etching. Because Co is the good catalyst for graphitization,48 the carbon-rich species (e.g., 2-MI and its derivatives) will deposit to carbon-shell on its surface. At low temperature (700 oC), the ZIF67 decomposes at a relatively slow rate, which benefits the formation of well-graphitized and intimate carbon-shell. With the accelerated decomposition at higher temperature (775 and 850 o

C), the ligand and its fragments cannot stay on Co surface for sufficient time, resulting in the

thin and amorphous carbon-shell. In this regard, the varied flow rate of carrier gas will also make the similar influences because it alters the contacting time between carbon-rich species and Co surface. As evidenced, the slower vAr of 100 mL min-1 at 775 oC causes the thicker carbon-shell on Co (Figure 1f). And the fastened flow (e.g., 200 min-1) leads to the visibly decreasing thickness because of the shortened time of organic molecules staying on Co (Figure 1g). As the Co/NC samples are treated by 1 M HCl, the different Co leaching is observed relying on the varied carbon-shell (Figure 1h). The inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was employed to determine the Co content in Co/NC and Co@NC. Only 3.8% of Co in the Co/NC-700 leaches after acid etching, while, a drastically increasing leaching (35.4%) is observed on the Co/NC-850, consistent with the reduced thickness of carbon-shell after increasing pyrolysis temperature. Because of the regulated pyrolysis at moderated temperature, the Co/NC-775 presents a moderated decrease in Co content after H+ etching, retaining the sufficient Co in the Co@NC-775 for electrocatalysis.



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After removing Co species out of carbon-shell by 1 M HCl, Co@NC composites were harvested. Figure 2a displays the XRD patterns of Co@NC fabricated with the fixed vAr of 150 mL min-1. The diffraction peaks at 2θ = 44.2, 51.5 and 75.8° correspond to the (111), (200) and (220) of metallic Co (JCPDS No.: 15-0806), respectively, and that at 26.0° is ascribed to the (002) of graphitized carbon. Interestingly, such peaks become broad with increasing pyrolysis temperature, indicating the reduced size of Co NPs. This should be ascribed to the quick generation of carbon-shell that remarkably prohibits the aggregation and growth of Co cores. The Rietveld analysis was further conducted to assess the varied composition and particle size associated with pyrolysis temperature (Figure S2 and Table S1 in Supporting Information). The calculated Co content is slightly higher than that by ICP-AES, because of the ignored amorphous carbon in XRD Rietveld analysis. Meanwhile, the textural structure of Co@NC was studied by N2 adsorption-desorption isothermals (Figure 2b). The Co@NC-775 and -850 present the specific surface area (360 ~ 390 m2 g-1) higher than that of Co@NC-700 (154.6 m2 g-1). The lower surface area of Co@NC-700 is possibly associated with its thick carbon-shell and high Co content. Raman spectra further reveal the characteristic bands of carbon at 1350 cm-1 (D-band), 1580 cm-1 (G-band), and 2700 cm-1 (2D-band) in the three samples (Figure 2c). The value of I2D/IG in Co@NC-700, as an indicator for the thickness of carbon-shell, is greater than that for Co@NC-775 and -850 (Figure 2d), consistent with the Co/NC precursors before HCl treatment.


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Figure 2. (a) XRD patterns, (b) N2 sorption isothermals, and (c) Raman spectra of Co@NC (I: Co@NC-700, II: Co@NC-775, III: Co@NC-850). (d) ID/IG and I2D/IG obtained from Raman spectra. XPS spectra of (e) Co and (f) N in Co@NC. These samples were further analyzed by X-ray photoelectron spectroscopy (XPS). Figure 2e displays the Co 2p3/2 profiles that can be deconvoluted into four species. In the Co@NC-775, the peaks at 777.9, 781.0 and 783.6 eV correspond to metallic Co, Co-N-C and Co nitrate, respectively, and that at 788.0 eV is associated with the shake-up satellites.49 The similar profiles of Co 2p3/2 are also detected on the Co@NC-700 and -850. However, the peak of metallic Co in the Co@NC-775 is visibly red-shifted (~ 0.3 eV) in comparison with the Co@NC-700 and -850, indicating the enriched electrons around Co. This could be ascribed to the strong interactions with ultrathin N-doped carbon-shell.50 Meanwhile, the relative content of different Co species was determined by XPS analysis (Table S2 in Supporting Information). The Co@NC-775 shows the higher content of metallic Co (~ 34.4%) than that in the Co@NC-700 (~21.2%) and -850



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(~15.3%), suggesting more metallic Co remaining in the Co@NC-775. Moreover, the highresolution profile of N 1s in the Co@NC-775 is deconvoluted into five peaks (Figure 2f), corresponding to pyridinic-N (397.8 eV), Co-N (399.0 eV), pyrrolic-N (399.6 eV), graphitic-N (400.6 eV), and oxidized-N (403.0 eV), respectively.51 The pyridinic-N and pyrrolic-N will promote the electrical conductivity of the composites due to their planar structure in carbon matrix.52 The presence of Co–N species suggests the interactions between metallic Co and the Ndoped carbon shell. Such peak is dominant and blue-shifted (~ 0.2 eV) in the Co@NC-775, indicating the possible electronic penetration from N to Co. The contents of various N are summarized in Table S3 of Supporting Information. Scanning electronic microscopy (SEM) and TEM investigations were further employed to study the morphology of Co@NC composites, taking the Co@NC-775 as the model sample. As shown in SEM (Figure 3a), semi-sphere shells with obvious pores are obtained, which should result from gas emission during ZIF-67 pyrolysis. The TEM displays the uniform Co NPs dispersed in carbon matrix, with a size of 5~9 nm (Figure 3b and 3c). This is consistent with the particle size derived from the Rietveld analysis of XRD pattern (Table S1 in Supporting Information). In comparison with the Co/NC before HCl treatment, the Co NPs are uniform and small, indicating that the large Co free from carbon-shell protection has been removed by HCl. The obvious lattice fringes with a d-spacing of 0.21 and 0.17 nm are ascribed to the Co (111) and (200), respectively. Remarkably, thin carbon-shell is observed on Co, which is responsible for the good maintenance of Co free from H+ etching, as well as the enhanced electronic interactions with Co. In addition, the energy dispersive spectrum (EDS) identifies the presence of C, N, and Co (Figure S4 in Supporting Information). Their corresponding elemental mapping further confirms the uniform distribution in the samples (Figure 3d).


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Figure 3. (a) SEM, (b) TEM and (c) HRTEM images of Co@NC-775, and (inset of a) SEM image of ZIF-67 precursor. Inset of b is the distribution of Co particle size. The magnified TEM image and the corresponding particle-size distribution are displayed in the Figure S5 of Supporting Information. (d) HAADF TEM image of Co@NC-775 and elemental mapping of C, N, and Co. Cobalt-carbon composites are bi-functional electrocatalysts for the HER and ORR,27 serving as a promising substitution for noble-metals. The controlled carbon-shell on Co surface has been demonstrated feasible to promote the accessibility and electronic configuration of Co cores, which will benefit the electrocatalysis. To this end, the above samples were loaded onto glassy carbon electrodes (GCEs), and tested as HER electrocatalysts in an acidic electrolyte (0.5 M H2SO4). Figure 4a displays their polarization curves after iR-drop corrections, along with that of the benchmark Pt/C catalyst (40 wt% Pt on carbon black from Johnson Matthey) for reference. Obviously, the Co@NC-775 presents the best activity among Co@NC samples. It delivers an ultra-low overpotential (η10) of 82 mV to reach a current density (j) of -10 mA cm-2, which is obviously lower than that of Co@NC-700 (233 mV) and Co@NC-850 (173 mV). Accordingly,



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the Tafel analysis identifies the fast kinetic of the Co@NC-775, feature by the low Tafel slope of 38.0 mV dec-1 and the low ηonset (overpotential referring to the beginning of the linear regime in the Tafel plot) of 50 mV (Figure 4b). Such low slope suggests the possible Volmer-Tafel reaction mechanism, in which the Tafel step is rate-determined. By contrast, the Co@NC-700 and -850 present obviously higher slopes, indicating the relatively slower kinetics. In comparison with current noble metal-free catalysts, our Co@NC-775 performs among the best (Table S4 in Supporting Information). In the case of Co-carbon nanocomposites derived from ZIF-67, the η10 of 82 mV on the Co@NC-775 is lower than that of Co-doped carbon polyhedron/N-doped graphene (229 mV),22 Co-NC (181 mV),53 Co-NC/CNT (201 mV),47 and Co@B,N-doped carbon (96 mV).21 Moreover, the activity outperforms that of carbon supported Co fabricated via other strategies (Co-C-N: 138 mV,54 CoNS-C: 180 mV,55 Co@N-rich carbonitride nanotubes: 150 mV,56 Co/N-doped porous carbon: 230 mV57). In the further comparison with metal carbides, sulfides, nitrides and phosphides, the Co@NC-775 is still featured by its high activity.


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Figure 4. (a) Polarization curves and (b) corresponding Tafel plots of Co@NC modified GCEs in 0.5 M H2SO4, along with that of commercial Pt/C (I: Co@NC-700, II: Co@NC-775, III: Co@NC-850, IV: Pt/C). (c) Estimation of Cdl through plotting the current density variation at 150 mV vs. RHE; data was obtained from the CV in Figure S5 in Supporting Information. (d) Polarization curves (I, II and III) before and (I’, II’ and III’) after introducing 5.0 mM KSCN. (e) Nyquist plots (at η = 200 mV) of the above Co@NC (f) Long term stability of Co@NC in 0.5 M H2SO4. We further analyzed the electrochemical surface area (ECSA) of the Co@NC. Because of the unclear capacitive behavior, the accurate measurement of ECSA cannot be determined directly. An alternative calculation of double-layer capacitances (Cdl) is herein employed, which is proportional to ECSA and can provide a relative comparison. As shown in Figure 4c, the Co@NC-775 reveals a Cdl of 88.7 mF·cm−2, higher than that of the Co@NC-700 (22.0 mF·cm−2)



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and Co@NC-850 (25.2 mF·cm−2). This trend is slightly different from their BET surface areas that represent all the surface of materials. Due to the large amount of carbon species in Co@NC, most of the BET surface area is contributed by the carbon. By contrast, the Cdl is associated with the electrochemical active surface of Co. Thus, the difference between the BET surface and Cdl is acceptable. In this regard, the Co@NC-775 with sufficient and accessible Co species presents the high value of Cdl. And the obviously lower Co content (c.f. Figure 1h) in the Co@NC-850 is responsible for the lower Cdl, in comparison with the Co@NC-775. Although the Co@NC-700 possesses higher Co content (c.f. Figure 1h), the thick carbon-shell has severely prohibited its exposure, resulting in the lower Cdl. The active-sites and activity of Co@NC should be qualitatively discussed in details. Ndoped carbon-shell was considered as the active-sites for the HER in some previous work.58 However, in this work the HER test clearly identifies the remarkable variation of activity on Co@NC, despite their similar N-doping. And this varied HER activity depends on different carbon-shell, which makes influences on the exposure of Co surface and the electronic interactions with Co. These evidences support the main contribution of Co sites for the HER. To further identify the Co NPs with carbon capsulation as the active species, we introduced KSCN to poison the Co@NC electrocatalysts (Figure 4d), in the regard that Co will be deactivated by the SCN− due to the strong coordination with surface Co atoms.21 The Co@NC-850 drastically deactivates after introducing KSCN, suggesting the negligible protection by the loose and amorphous carbon-shell. By contrast, the Co@NC-700 with the thicker and highly-graphitized carbon-shell delivers a small decrease in HER activity. And the loss of activity on Co@NC-775 is moderate, in line with the carbon-shell showing low graphitization and reduced thickness. The carbon-shell on Co NPs is double-edged, which on one hand protect the Co cores from KSCN


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poison, and on the other hand block the diffusion of reactants (H+ and H2O) toward active Co. Therefore, the Co@NC-775 with a well-controlled carbon-shell presents the high HER activity. Although the Co@NC-850 has loose and amorphous carbon-shell, its lower Co content (20.2%) than that of the Co@NC-775 (25.5%) is responsible to the reduced activity. Furthermore, the carbon-shell has been already demonstrated influenced by the flow rate of carrier gas during pyrolysis, which will be illustrated by their electrocatalytic performance. The Co@NC-775 received with a varied vAr affords the different HER activity in 0.5 M H2SO4 (Figure S6 in Supporting Information). With a moderated flow rate of 150 mL min-1, the composites possessing optimal carbon-shell deliver the higher current density than that of others received with slower (100 mL min-1) or faster (200 mL min-1) Ar flow. Obviously, the Co species in Co@NC is active for the HER, and its carbon-shell makes influences on the quantity of active-sites and the activity. Thus, the intrinsic activity of various Co@NC can be accessed via normalizing the catalytic current by ECSA (Figure S7 in Supporting Information). The remarkably high value on Co@NC-775 confirms its intrinsically improved catalytic activity due to the electronic interactions between Co and N-doped carbonshell. As indicated by the XPS profile of Co (Figure 2e), the binding energy for metallic Co species in Co@NC-775 presents a red-shift (~ 0.3 eV) in comparison with that in the Co@NC700 and -850. It’s indicated that the strong electronic interactions drive the electron penetration from N-doped carbon-shell to Co cores, which will bring about the down-shift of d-band center to weaken the hydrogen binding on Co surface.59-61 As a result, the HER kinetic will be intrinsically promoted owing to the facilitated Hads desorption. In addition, the activity of Pt/C reference was also normalized by its ECSA (Figure S7 in Supporting Information), showing the



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higher activity nature of Pt/C than our proposed Co@NC. However, due to the low-cost and high earth-abundance of Co, the Co@NC is still promising as the cost-efficient HER electrocatalysts. Furthermore, the role of N can be ascribed to the improved conductivity and the enhanced interactions between carbon-shell and Co cores. According to the previous reports,52,62 the pyridinic and pyrrolic N atoms retaining the planar structure in carbon matrix will promote the conductivity due to its excessive electrons. In contrast, quaternary and oxidized N atoms, possessing a 3D structure, lead to the poor conductivity because of the interruption of their π-π conjugation. The quantitative XPS analysis (Figure 2f and Table S3 in Supporting Information) shows the higher content of pyridinic and pyrrolic N species than that of quaternary and oxidized N in Co@NC. This feature enables the fast electron transfer during the electrocatalytic HER. Meanwhile, the N-doping in carbon matrix will enhance the interactions with Co species, which on one hand benefits the good dispersion of Co, and on the other hand penetrates electrons into Co to optimize its electrocatalysis. Ion and charge transports are also crucial for efficient electrocatalysis. We measured the electrochemical impedance spectroscopy (EIS) at η of 150 mV from 0.01 Hz to 100 MHz (Figure 4e). There are two semi-circles with different diameters in the EIS profile (Figure S8 in Supporting Information), suggesting an equivalent circuit characterized by two time constants (inset of Figure 4e).63 In this simulation, Rs and R1 represent the system resistance and the resistance for the adsorption of reactants on electrode surface, respectively.64 And Rct indicates the charge transfer resistance related to the interface charge-transfer process, and the lower Rct value identifies a faster reaction rate. The fitting results clear present the lowest Rct on the Co@NC-775 (88 Ω) among a series of Co@NC, confirming its best activity for the HER.


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Moreover, to assess the long-term durability of the Co@NC-775 in acid solution, 5000 continuous cycles and a chronoamperometric test at η = 200 mV are carried out. After 5000 cycles, the polarization curve shows a small decline (Figure 4f). As been further evaluated by prolonged electrolysis at η = 160 mV, the Co@NC-775 exhibits a high catalytic current around 20 mA cm−2 over 20 hours (inset of Figure 4f), suggesting the outstanding stability. Such Co@NC composites also show the high efficiency for HER in basic electrolytes (Figure 5a). The Co@NC-775 affords a remarkable activity with a η10 of 95 mV in 1.0 M KOH, superior to that of the Co@NC-700 (165 mV) and Co@NC-850 (201 mV). This confirms the excellent HER activity of the Co@NC-775 associated with its suitable carbon-shell. Accordingly, the consistent order in the analysis of Tafel plots (inset of Figure 5a), Cdl (Figure 5b) and Rct (Figure 5c) further identifies the enhanced kinetics and active surface on the Co@NC-775. It’s noticed that the Tafel analysis shows the lower kinetics of these Co@NC in 1.0 M KOH as compared with those in 0.5 M H2SO4 (c.f. Figure 4b), indicating the limitation of Volmer step via the dissociation of H2O, rather than the direct reduction of H+ in acidic electrolytes.65 Moreover, its long-term stability is satisfied in basic electrolytes. For example, after 5000 continuous cycles or a chronoamperometric test for 20 hours, it delivers a negligible loss in activity (Figure 5d).



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Figure 5. (a) Polarization curves and (inset of a) corresponding Tafel plots of Co@NC modified GCEs in 1.0 M KOH, along with that of commercial Pt/C (I: Co@NC-700, II: Co@NC-775, III: Co@NC-850, IV: Pt/C). (b) Estimation of Cdl through plotting the current density variation at 150 mV vs. RHE; data was obtained from the CV in Figure S9 in Supporting Information. (c) Nyquist plots (at η = 200 mV) of the above Co@NC. (d) Long term stability of Co@NC in 1.0 M KOH. The ORR performance on Co@NC was evaluated using a rotating disk electrode technique. The CV test on a series of Co@NC presents a clear oxygen reduction peak in O2-saturated 1.0 M KOH and 0.5 M H2SO4 (Figures S10 and S11 in Supporting Information), confirming the activity for ORR. Figures 6a and 6c display their polarization curves at a rotating speed of 1600 rpm in O2-saturated 1.0 M KOH and 0.5 M H2SO4, respectively, along with commercial Pt/C. In 1.0 M KOH solution, the Co@NC-775 exhibits a half-wave potential (E1/2) of 0.90 V vs. RHE (Figure 6a), higher than that on the Co@NC-700 (0.81 V) and Co@NC-850 (0.85 V). The E1/2 on Co@NC-775 is quite close to that commercial Pt/C (0.91 V), indicating the outstanding ORR activity. At 0.3 V vs. RHE, the Co@NC-775 delivers a much higher limiting current density of


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−4.5 mA·cm−2 than the Co@NC-700 (−3.1 mA·cm−2) and Co@NC-850 (−3.4 mA·cm−2). Furthermore, the Koutechy–Levich (K-L) plots of the Co@NC calculated from LSV curves at different rotating speeds exhibit a good linearity with the different slop (Figure 6b and Figure S12 in Supporting Information). The derived number of electron transfer on the Co@NC-775 is 3.8, suggesting the four-electron reduction, while the numbers of 3.1 and 3.3 for the Co@NC700 and -850, respectively, indicate the mixed four- and two-electron pathway. The K-L plot of Pt/C reference presents an even higher (4.0) number of electron transfer, corresponding with its superior ORR activity to Co@NC. The activity order of Co@NC is similar in 0.5 M H2SO4 (Figure 6c), and their polarization curves clearly present the E1/2 of 0.60 V, 0.78 V and 0.68 V on the Co@NC-700, -775 and -850, respectively. Particularly, the value on the Co@NC-775 (0.78 V) is closed to that of the commercial Pt/C (0.83 V). The K-L plots further illustrate the efficient ORR via four-electron pathway on the Co@NC-775 (Figure 6d). The CV measurement at the same scan rate in N2-saturated electrolytes confirms the higher ECSA on the Co@NC-775, featured by the higher current and thus the larger integral area as compared with the other two (Figure S13 in Supporting Information). In this regard, the ORR currents in kinetic region normalized by the integral areas of CVs can provide relative comparison for their specific activity (Table S5 in Supporting Information). As expected, the highest value on the Co@NC775 confirms the promoted ORR kinetics associated with the engineered carbon-shell on Co surface. However, this value is still lower than that of Pt/C reference. The role of N-doping was further taken into account to interpret the efficient ORR. The Ndoped carbon materials are active for the ORR in alkaline electrolytes,66-68 in which the pyridinic and pyrrolic N species are considered as active species. In our work, a series of Co@NC with the consistent ratio of pyridinic and pyrrolic N (37 ~ 39%, Table S3 in Supporting Information)



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however deliver the remarkable difference in the ORR performance. And such difference is evidently associated with the thickness of carbon-shell, which hinders the exposure of active Co. In these regard, the N-doping in carbon-shell promotes the ORR via the enhancement in electrical conductivity and interactions with Co active species. In particular, the electronic interactions between N-doped carbon-shell and Co cores in the Co@NC-775 can enrich the electrons around Co Fermi level, which will promote O2 activation due to the electron donation to π*O-O, as indicated by the high specific activity on the Co@NC-775 (Table S5 in Supporting Information).

Figure 6. Polarization curves of Co@NC and Pt/C in O2-saturated (a) 1.0 M KOH and (c) 0.5 M H2SO4 at 1600 rpm with a scan rate 10 mV s-1 (I: Co@NC-700, II: Co@NC-775, III: Co@NC850, IV: Pt/C), and (b and d) the corresponding plots for the K-L equations. The


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chronoamperometry curves of Co@NC-775 and Pt/C for ORR in (e) 1.0 M KOH and (f) 0.5 M H2SO4. The long-term stability of our Co@NC-775 is further investigated by chronoamperometry. In both 1.0 M KOH and 0.5 M H2SO4, the Co@NC-775 presents a steady current density for ORR (Figure 6e and 6f). By contrast, the Pt/C suffers obvious deactivation. This highlights the outstanding long-term stability of our Co@NC. The stability is also confirmed by the continuous 1000 cycles, in which negligible variation is observed in CV curve (Figure S14 in Supporting Information). ■ CONCLUSION In summary, we demonstrated a facile control over carbon-shell in MOFs-derived Co@NC electrocatalysts,

accomplishing

the

benchmarking

performance

of

noble-metal-free

electrocatalysts for the HER and ORR. The temperature and flow-rate during pyrolysis are the key factors to tailor the shell thickness and other textural features, making obvious influence on the accessibility and electronic configuration of Co cores. With moderated temperature and flowrate, the optimal Co@NC affords superior activity for HER and ORR, featured by the low overpotentials, the striking kinetic metrics, and the outstanding long-term stability. Elucidating the feasibility toward prominent electrocatalysts via controlled MOFs pyrolysis, this work will boost the exploration of cost-effective materials in energy field. ■ EXPERIMENTAL SECTION Materials. All reagents were purchased from commercial sources and in analytical or reagent grade when possible. 2-methylimidazole (2-MI), Co(NO3)2·6H2O, sulphuric acid and hydrochloric acid were purchased from Aladdin. Methanol, ethanol and KSCN were purchased



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from Mackin. Nafion solution (5 wt% in lower aliphatic alcohols and water) was bought from Sigma-Aldrich. All aqueous solutions were prepared using ultrapure water (> 18 MΩ cm). Synthesis of Co/NC and Co@NC nanocomposites. The precursor of Co-based ZIF-67 was synthesized according to the previous report:69 0.89 g of Co(NO3)2·6H2O was dissolved in 30 mL of methanol to form a solution, which was subsequently tranfered into 20 mL of methanol containing 2-MI (1.90 g). The solution was further stirred at room temperature for 24 hours. The resulting dark-blue precipitates were collected by centrifuging, washed with water and methanol in sequence for at least three times, and finally dried in vacuum at 50 °C overnight. The Co/NC composites were harvested via the pyrolysis of ZIF-67 at varied temperature (700 ~ 850 °C) for 5 hours under Ar flow (vAr = 100 ~ 200 mL min-1). To identify the effect of pyrolysis temperature, the vAr was fixed at 150 mL min-1. And the temperature was fixed as 775 o

C to study the influence by Ar flow rate. The final Co@NC were received via treating the above

Co/NC by 1 M HCl (aq.) for 24 hours, which successfully removed the residual Co species out of carbon shells. Physical measurements. XRD was carried out with a Bruker D8 Focus operating at 40 kV and 40 mA equipped with a nickel-filtered Cu Kα radiation (λ = 1.54056 Å). SEM and TEM images were obtained using a ZEISS ULTRA55 and JOEL JEM-2100F, respectively. The EDS, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and the corresponding elemental mapping were conducted on JEM-2100F. BET surface area was determined using N2 adsorption/desorption isotherm measurements at −196 °C on an automatic gas adsorption analyzer (Quantachrome Autosorb iQ-MP). The CHN elemental analysis was carried out on a Vario EL Elementar. Raman spectra were recorded on a laser confocal Raman micro-spectrometer (HR-800, Horiba Jobin Yvon, Ltd.), with an excitation laser wavelength of


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532 nm. XPS data were obtained on a PHI-tools equipped with Al Kα radiation, using C 1s (284.6 eV) as a reference. The Co content of Co@NC and Co/NC was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Electrochemical HER test: The working electrode was prepared on a glassy carbon electrode (GCE). Typically, 4 mg of catalyst and 40 µL of 5 wt% Nafion solution were dispersed in 1 ml of 4:1 (v/v) water/ethanol by at least 30 min sonication to form a homogeneous ink. Then 5 µL of the catalyst ink was loaded onto a GCE of 3 mm in diameter. Linear sweep voltammetry was conducted with the scan rate of 10 mV s-1 in 0.5 M H2SO4 or 1 M KOH on a potentiostat of CHI760 (CH Instruments), using a saturated calomel electrode (SCE) as the reference electrode, a graphite electrode as the counter electrode. All of the potentials reported in this work were referenced to a reversible hydrogen electrode (RHE) by adding a value of (0.241 + 0.059 pH) V. AC impedance measurements were carried out in the same configuration from 0.01 to 1 000 000 Hz and an amplitude of 5 mV. Electrochemical ORR test: CV and LSV voltammetry were performed by using a CHI650e electrochemical analyzer in a conventional three-electrode electrochemical cell. A graphite electrode, a saturated Ag/AgCl (saturated with 3 M KCl) reference electrode, and rotating disk working electrode (Pine Research Instrumentation) were used. To prepare the working electrode, 4 mg of catalysts were dispersed in 1 mL of ethanol (containing 40 µL of 5.0 wt% Nafion) solution under ultrasonic agitation to form an electrocatalysts ink. Then 20 μL of the ink was dropped on the surface of the pre-cleaned rotating disk electrode (5 mm diameter, 0.196 cm2) and dried at room temperature. ASSOCIATED CONTENT



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Supporting Information. XRD pattern and N2 sorption isothermals the ZIF-67 precursor, Co/NC and Co@NC, composition of Co@NC, EDS of Co@NC-775, CV and specific activity of Co@NC, LSV of Co@NC received with various vAr, and the comparison with recently report noble-metal-free HER electrocatalysts. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * L. C. Yang. Email: [email protected] * Q. S. Gao. Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We appreciate the financial support from National Natural Science Foundation of China (21373102, 21433002 and 51671089), Guangdong Natural Science Funds for Distinguished Young Scholar (2015A030306014 and 2017B030306004), Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2017), and Science and Technology Program of Guangzhou (201707010268). REFERENCES (1) Crabtree, G. W.; Dresselhaus, M. S.; Buchanan, M. V., The Hydrogen Economy. Phys. Today 2004, 57, 39-44. (2) Lewis, N. S.; Nocera, D. G., Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. USA 2006, 103, 15729-15735.


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Facile regulation on the carbon-shell in MOFs-derived Co@NC is achieved via varying temperature and flow-rate in controlled pyrolysis, resulting in the ultrathin carbon-shell to optimize the accessibility and electronic configuration of the Co cores toward efficient electrocatalysis.


Metallic Cobalt@Nitrogen-Doped Carbon Nanocomposites: Carbon

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