Carbon-Based Nanostructures Vertically Arrayed on Layered

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Carbon-based nanostructures vertically arrayed on layered lanthanum oxycarbonate as highly efficient catalysts for oxygen reduction reaction Lu Bai, Jingjun Liu, Weiwei Gu, Ye Song, and Feng Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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ACS Applied Materials & Interfaces

Carbon-based Nanostructures Vertically Arrayed on Layered

Lanthanum

Oxycarbonate

as

Highly

Efficient Catalysts for Oxygen Reduction Reaction Lu Bai, Jingjun Liu *, Weiwei Gu, Ye Song and Feng Wang *

Static Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, P. R. China. KEYWORDS Zeolitic imidazolate framework-67, La2O2CO3, carbon-based nanostructures, oxygen reduction reaction, electrocatalytic activity.

ABSTRACT Controllable pyrolysis of collapsible metal-organic frameworks (MOFs) into carbon-based nanostructures without obvious collapse and aggregation is of importance for the fabrication of well catalytic active and durable carbon-based catalysts for the oxygen reduction reaction (ORR). Herein, we fabricate morphology-controlled carbon-based nanostructures derived from Co-based zeolitic imidazolate framework (ZIF-67) that epitaxially grows on layered lanthanum oxycarbonate (La2O2CO3) as a structure-oriented template, followed by a pyrolysis at 800 °C. These synthesized carbon-based nanostructures show a well-defined dodecahedron morphology and vertically array on the template surface. In 0.1M KOH solution, the ORR activity and

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durability of the carbon-based nanostructures are not only much higher than those obtained by pyrolytic carbons derived from pure ZIF-67 but also exceed a commercial Pt/C (20 wt. %, Pt). The significantly improved ORR performance can be ascribed the increased Co-Nx level, high specific surface area and graphitization of the pyrolytic carbon, caused by the introducing of the La2O2CO3 phase into the composite catalyst. Therefore, using La2O2CO3 as template may be a smart synthetic strategy for MOF-derived nanocarbons with controlled morphology and composition for energy storages and conversions. Introduction Various carbon nanomaterials have been emerged as potential non-noble-metal-free ORR electrocatalysts, such as heteroatom-doped porous carbon,1, 2 and transitional-metal-nitrides, 3, 4, 5, 6 etc.

Metal organic frameworks (MOFs) derived carbon materials have been regarded currently as

a kind of efficient catalysts for H2 and O2 generation or ORR.7, 8 Nowadays, MOFs-based carbon materials have received increasing attentions owing to their richened nanopores with tailorable structure,9, 10 abundant nitrogen, variable transition metal centers and extreme surface-to-volume ratio11 that are suitable for the ORR in energy storages and conversions.12,13,14,15 For example, Xia et al.16 reported N-doped carbon nanotube frameworks (NCNTFs) derived from zeolitic imidazolate framework-67 (ZIF-67) at 700 ℃ in an Ar/H2 atmosphere, followed by acid treatment and the NCNTFs exhibits high ORR activities, raised by the hierarchical hollow structure, high graphitic degree and N-doped level compared with other nitrogen-doped carbon material. What’s more, Hu and co-workers17 constructed a hierarchically porous carbon, through directly pyrolyzing zeolitic imidazolate framework materials. The resulting porous carbon exhibits high ORR performance but it still lower than that of the-art-of-state Pt/C in alkaline electrolyte. The poor activity relative to the Pt/C is attributed to the serious collapse or aggregation of original


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pores of these ZIF materials during pyrolysis at elevated temperatures (> 700 ℃). As a result, the destructive pore structure can not only decrease the density of exposed active sites like N-bonded with central metal atoms18, 19, 20 but also impede the mass transport of the reactant species like active O2 and then seriously decay the ORR kinetics. 21, 22, 23, 24 Controllable pyrolysis of collapsible metal-organic frameworks (MOFs) into carbon-based nanostructures without obvious collapse and aggregation is of importance for the fabrication of remarkably active and durable carbon-based catalysts for performing the oxygen reduction reaction (ORR). Therefore, how to efficiently preserve innate pore structures of the above pristine materials is one of the key factors for further improving the ORR activity over MOFs-based nanocarbons. Using templates is one of the rational strategies to solve this problem above,25 For example, Wei et al.26 used a 2D inorganic matrix, Co and Al-based layered double hydroxides (CoAl-LDHs) with the edge-sharing metal-O6 octahedra as a hard template, to epitaxially grow ZIF-67 on the surface of CoAl-LDHs nanoplatelets. After subsequent pyrolysis process, the obtained 2D carbonbased network derived from ZIF-67 displays excellent activity for the ORR compared with that of a commercial Pt/C catalyst. The excellent ORR performance can be associated with the hierarchical micro-/mesoporous structure of the pyrolytic carbon coated on the CoAl-LDHs. In addition, Cai et al.27 and Young et al.28 also provided a template-assisted synthesis strategy for well-aligned ZIF-67 arrays on transition metal-based oxides like CoO， NiO and ZnO served as self-sacrificing templates. These templates contain the reagent ions (Co2+, Ni2+) that can promote the in situ nucleation and directed epitaxial growth of ZIF-67 or ZIF-8. The obtained pyrolytic carbons with aligned hierarchical morphology shows wonderful electrocatalytic activity, which surpass the pristine ZIF-based counterparts.


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Inspired by the above mentioned LDHs and transition metal-based oxides, the layered lanthanum oxycarbonate (La2O2CO3) may be an ideal structure-oriented template to synthesized the ZIFbased porous carbons, since the lanthanum-based compound has a well-defined separated layer structure, where the CO32- layer is parallel with [La2O2]2+ layer.29 On the one hand, the layered and separated carbonate groups in this compound can serve as active sites for direct growth of ZIF-67, through a strongly chemical interaction between the CO32- species in the template and Co ions in the ZIF-67. Such directed epitaxial growth can not only facilitate the in situ nucleation but also favour to controlling the morphology of ZIF-67. On the other hand, the abundant exposed CO32species can also stabilize the central Co ions of the ZIF-67, which would contribute to keeping the chemical composition of the pyrolytic carbon derived from ZIF-67, significantly improving the density of Co-N active sites for the ORR. Some published papers confirmed that the increasing Co-N moieties included other transfer metal-nitrogen structure in the porous carbons can remarkably improve their ORR activity.19,

30

Different from LDHs, CoO or NiO templates,

La2O2CO3 isn’t required to be cleared away from the hybrid, since it has ORR activity.31 The hybrid composed of La2O2CO3 avoids the recognized problem of removing hard templates like LDHs, CoO or NiO, which would decay the structure of the pyrolytic carbons in their hybrid.32 Herein, we fabricate well-defined Co-based zeolitic imidazolate framework (ZIF-67), through in situ nucleation and epitaxial growth on layered lanthanum oxycarbonate (La2O2CO3) as a structure-oriented template. After subsequent pyrolysis, the ZIF-67 directly converts into morphology-controlled carbon-based nanostructures that vertically arrays on the La2O2CO3 surface to form a hybrid. In an alkaline solution, the ORR activity of the above hybrid is not only much higher than that obtained by pure pyrolytic carbon derived from ZIF-67 or La2O2CO3 but also exceeds a commercial Pt/C (20 wt. %, Pt). The introducing of La2O2CO3 can not only avoid


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obvious depression and aggregation but also promote the formation of active Co-N coordination for the carbon-based nanostructures. These should be account for the ORR activity of the hybrid. Experimental Section 2.1 Preparation of carbon-based nanostructures on lanthanum oxycarbonate. First, lanthanum oxycarbonate (La2O2CO3) with a main size of 5 μm was firstly prepared by the calcination of lanthanum carbonate under a growing temperature rate of 5 °C min-1 from room temperature to 600 °C in air rate, following a keeping warm for another 2 h. Second, the synthesized La2O2CO3 serves as a hard template to induce ZIF-67 nanocrystals grown on it. The typical synthesis procedures are as following. 1.455 mg cobaltous nitrate hexahydrate were added into 100 mL methanol solution containing 0.1 g lanthanum oxycarbonate, and then 50 mL 2methylimidazole methanol solution (0.8 M) were poured quickly into the former solution at room temperature under magnetic stirring for 15 min. The obtained ZIF-67 nanostructures supported on La2O2CO3 (marked as ZIF-67@La2O2CO3) were washed several times by methanol and dried at 80 °C for several hours. The dried ZIF-67@La2O2CO3 hybrid was then calcinated though the same growing temperature rate above up to 800 °C under argon (Ar) atmosphere, following 2 hours of heat preservation to obtain ZIF-67-derived carbon on the compound (noted ZIF-67800@La2O2CO3). For comparison, the pure ZIF-67-derived carbon (noted ZIF-67-800) was obtained by operated the same pyrolytic procedures as well without added the La2O2CO3 template. 2.2 Physical characterizations. X-ray diffraction (XRD) patterns of these ZIF-67-derived carbons were operated with a scan rate of 5 per minute in the range from 10° to 90° (Rigaku RINT 2200 V/PC). In addition, the plot


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of XRD can obtain the average particle size of hybrid calculated by the Scherrer equation: D= Kγ / Bcosθ (1) (D is the average thickness of the crystal grains perpendicular to the crystal plane direction, K is the Scherrer constant, γ is the X-ray wavelength, which is 0.154056 nm, B is the half-height width of the diffraction peak of the measured sample and θ is the diffraction angle.) The morphologies of the samples were examined through scanning electron microscopy (SEM) (FE-SEM, JEOL, JSM-6701F) and high resolution transmission electron microscopy (TEM) (JEOL TEM 2010 microscope). The nitrogen-doping characteristics of the above samples were carried out by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250), where the Lorentzian-Gaussian parameter was fixed at 20 %. In addition, the Brunauer−Emmett−Teller (BET) specific surface area and pore structure were obtained by Quadrasorb SI (Quantachrome Instruments) with calculated method of Quenched solid density functional theory (QSDFT). The graphitization degree of the synthesized samples was performed by Raman spectra (Horiba Jobin Yvon LabRam HR800). 2.3 Electrochemical activity characterizations. The oxygen reaction reduction electrocatalytic activity of ZIF-67-derived carbon@La2O2CO3 hybrids were measured through a rotating disk machine (AFCBP1 type, PINE, USA) with typical three electrodes system. The counter electrode in that system is graphite, while the saturated calomel electrode (SCE) is used as reference electrode in our work. All electrochemical tests are managed in 0.1 M KOH oxygen saturated solution. The carbon catalyst ink of the sample is prepared by dispersing 5 mg catalyst in a mixture of 0.7 ml alcohol and 0.3 ml deionized water, following with 50 µl Nafion solution (5 wt %) for 30 minutes by ultrasonically dispersing. 20 µl of the uniformly dispersing electrocatalyst suspension has been pipetted to evenly lay over the


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rotating disk electrode (0.247 mg cm-2) for all the carbon catalysts, while for the commercial Pt/C (20 wt. %) is 5µl. the final work electrode will form after drying at room temperature. For the cyclic voltammetry (CV) test, the catalysts were tested under a scanning rate of 20 mV/s, while 5 mV/s is used for obtaining the polarization curve in the rotating disk electrode (RDE) experiments. Chronoamperometric curves for the catalysts are conducted by RDE connected with CHI660E electrochemical work station system in 0.1 M KOH oxygen saturated solution for 10000 s. During the electrochemical tests of our work, the potentials were normalized with respect to the reversible hydrogen electrode (RHE). The potential vs. RHE (Evs.RHE) was calculated by the following equation: Evs.RHE=Evs.SCE+0.241+0.059pH (2) (Evs.SCE stands for the potential vs. SCE). The electron transfer number (n) and the kinetic current (jk) performed in oxygen reaction reduction can both be carried out by the Koutecky-Levich (K-L) plots. In addition, the hydrogen peroxide yield (H2O2 %) was obtained by equation below from RRDE measurement. The H2O2 % computational formula as well as K-L equations for calculating n and jk are shown below: H2O2 %= (200IR/n0)/(IR/n0+ID)

(3)

1/j = 1/jK +1/jL = 1/Bω1/2 + 1/jK

(4)

B = 0.62nFC0D02/3ν-1/6

(5)

jK = nFkC0

(6)

From the equation above, the IR represented ring current while ID denoted as disk current, n0 (0.37) is a constant value, which is defined as the disk current collection efficiency of the machine. What’s more, j represents current density, the following jK and jL serve as kinetic current densities and the limiting diffusion current densities, respectively. D0 is the oxygen diffusion coefficient, ω


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is the electron rotating rate, and k is defined as the electron transfer rate constant, while ν is kinematic viscosity. n, C0 and F in the equations of B as well as jK are termed as the electrons transferred number, bulk concentration of O2 and the Faraday constant (96485 C mol-1), respectively. In addition, the electrochemical impedance spectroscopy (EIS) played a necessary role in reflecting the ability to impede charge transfer of the hybrid catalyst. The frequency range of operation is from 10-5 to 0.1 Hz in the 0.1 M KOH oxygen saturated electrolyte at a certain potential. The electrochemical fitting software (Zsimdemo) was used to process the data and simulate the corresponding equivalent circle to obtain the reaction kinetic parameters of the sample electrodes. Results and discussion 3.1 Fabrication of carbon-based nanostructures on lanthanum oxycarbonate


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Figure 1. (a) Schematic diagram of ZIF-67-derived carbon-based nanostructures on lanthanum oxycarbonate; (b) SEM images of ZIF-67@La2O2CO3 hybrid; (c) SEM images of the ZIF-67800@La2O2CO3 and (d) the ZIF-67-800; (e) TEM image of the ZIF-67-800@La2O2CO3; (f) HRTEM image of Co and C with the interlayer spacing of (111) and (002) plane indicated, respectively. (g) The XRD patterns of ZIF-67-800@La2O2CO3, ZIF-67-800 and lanthanum oxycarbonate, respectively.

Figure 1(a) illustrates the overall preparation procedure of the Co-based zeolitic imidazolate framework (ZIF-67) on layered La2O2CO3 as a structure-oriented template (denoted as ZIF67@La2O2CO3), the final carbon-based nanostructures derived from ZIF-67 is obtained under a


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subsequent pyrolysis. Using this above synthetic strategy, the zeolitic imidazolate framework can directly and evenly grow on the template surface through an epitaxial growth, caused by a strongly chemical interaction between the CO32- species in La2O2CO3 and Co ions in ZIF-67, as shown in Figure 1(b) and Figure S1. As evidenced by additional XRD results shown in Figure S2, the synthesized ZIF-67@La2O2CO3 corresponds to a superimposition of the pure ZIF-67 indicating the formation of ZIF-67 on the template.26Moreover, the directly grown ZIF-67 shows a regular polygon morphology with well-defined rhombus faces and straight edges, which are almost similar to that of the pure ZIF-67 shown in Figure S3. After pyrolysis at 800 °C, the epitaxial grown zeolitic imidazolate framework converts into a carbon-based nanostructures located on the La2O2CO3 surface (denoted as ZIF-67-800@La2O2CO3 hybrid), as shown in Figure 1(c). The obtained carbon-based nanostructures are separated and vertically arrayed on the template surface to form a carbon-based nanoarray. Besides, the vertically grown carbon nanostructures still retain the well-defined dodecahedron nanostructure with an average particle size about 300 nm, as shown in Figure 1(e). However, as shown in Figure 1(d), the pyrolytic carbons derived from pristine ZIF67 at 800 °C (marked as ZIF-67-800) just give agglomerated nanoparticles.33 It reveals that the introducing of the La2O2CO3 can contribute to hindering collapse and aggregation of ZIF-67, even under elevated temperatures in this case. For the hybrid, the mass ratio of the carbon-based nanostructures to La2O2CO3 is 3:7, based on TG/DTA result shown in Figure S4. For the obtained carbons, there exist some uniformly distributed metallic Co nanocrystals derived from ZIF-67, as shown in Figure 1(e). This finding can be further confirmed by Figure 1(f), where a well-resolved lattice fringe of 0.203 nm is assigned to Co (111) plane while the inter-planar distance of 0.332 nm corresponds to the C (002) plane. It implies the presence of the metallic Co in the carbons. As illustrated in Figure 1(g), X-


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ray diffraction (XRD) patterns for the ZIF-67-800@La2O2CO3 hybrid show obvious diffraction peaks that are attributed to the La2O2CO3 phase, revealing the presence of the lanthanum-based compound in this hybrid. Besides, the recorded diffraction peak at about 26°corresponds to the (002) plane of carbon while the other reflections at about 44° and 51°are assigned to the (111) and (200) crystal planes of face-centered cubic (fcc) Co phase.26 These results are in agreement with the pure ZIF-67-800 sample shown in Figure 1(g). However, based on the above XRD results, the calculated size of the Co nanoparticles is 12.4 nm for the hybrid, which is smaller than of the ZIF67-800 (15.2 nm) calculated by Scherrer Formula.34 It implies that the addition of La2O2CO3 may impact pyrolytic process of ZIF-67, leading more atomic level Co atoms entering into the carbon skeleton in the hybrid. To further explore the effect of La2O2CO3 on the pyrolysis of ZIF-67 into carbon, X-ray photoelectron spectroscope (XPS) analysis were performed for the ZIF-67-800@La2O2CO3 hybrid. As displayed in Figure S5, the obtained survey spectrum reveals signal of C, N, O, Co, and La respectively and the relative contents of these above elements is displayed in Table S1. We found that the C content slightly decreases but the N content increases for the hybrid relative to the pure ZIF-67-800 sample, which result from the addition of the La2O2CO3 phase. As observed in Figure 2(a), the C 1s spectrum of the hybrid can be fitted into the C-C (285 eV), C-O (286.1 eV) and CO32- (289 eV) groups, respectively.35 Compared with the ZIF-67-800, the high intensity of the CO32- for the hybrid can be attributed to the CO32- ions existed in La2O2CO3 phase.


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Figure 2. The high resolution XPS spectra of (a) C 1s, (b) N 1s and (c) Co 2p for the ZIF-67800@La2O2CO3 and ZIF-67-800; (d) The relative contents of Co-Nx moieties of the above two samples, determined from the above N 1s and Co 2p spectra.

As shown in Figure 2(b), to obtain more accurate signal for the bonding type of nitrogen, the recorded N 1s spectrum for the hybrid can be fitted into pridinic N (398.7 eV), Co-Nx (399.8 eV), pyrrolic N (400.8 eV), graphitic N (401.9 eV) and oxidized N (403.0 eV) species, respectively.36 The relative contents of these N-coordinated species are illustrated in Table S2. Compared with the ZIF-67-800, the higher level Co-Nx species of the hybrid reveals the introducing of the template can contribute to the formation of these moieties. This finding can be evidenced by high-resolution


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Co 2p spectra for the hybrid. For the Co 2p3/2 spectra in Figure 2(c), the fitted peaks at about 778.3, 779.8, 781.1 and 783.5 eV correspond to the Co-Co, Co-Ox, Co-Nx bonds and the shake-up (satellites) respectively.37, 38 The contents of these above Co-based moieties are shown in Table S3. The intensity of Co-Nx bonds is higher than that of the ZIF-67-800, which is consistent with results from the N 1s spectra, as shown in Figure 2(d). This gives a direct evidence for that La2O2CO3 facilitate more Co atoms entering into the carbon skeletons to form more Co-Nx moieties at atomic level. This result may be associated with a strong chemical interaction between the La-based compound and the carbon, which should be responsible for the increased Co-Nx level for this hybrid. 3.2 Interfacial interaction in this hybrid To probe into the chemical interaction relationship between the La-based compound and the carbon-based nanostructures, which leads to the increase in the Co-Nx content shown in Figure 2(d), we further subdivided and fitted the O 1s spectra for the hybrid, ZIF-67-800, and La2O2CO3, respectively. The results are displayed in Figure 3(a). According to the analysis, the recorded O 1s curves for these samples could be fitted four peaks at about 530.1, 531.4, 532.3 and 533.5 eV, respectively. The peak at about 532.3 eV is explained to the C=O bonds while the three peaks at around 530.1, 533.5 and 531.4 eV are assigned to La-O, C-O and CO32- bonds derived from CO32in lanthanum oxycarbonate.35 As expected, the position of the recorded CO32- peak for the hybrid has an obvious negative shift, relative to the pure La2O2CO3. After hydrochloric acid-treatment for the synthesized hybrid, the negative shift of the corresponding peak disappears. It reveals a chemical interaction between the CO32- component in the La-based compound and the Co atoms in the carbon-based nanostructures. As proved from Figure 3(b) and Figure S6, the recorded Co2+ peak shows a positive shift for the hybrid compared with the pure ZIF-67-800. After the acid


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etching, the shifted Co2+ peak also move back to the similar position of the ZIF-67-800. This offers another evidence for the chemical interaction at the interface for the hybrid.

Figure 3. XPS spectra for the synthesized ZIF-67-800@La2O2CO3 hybrid, HCl-treated hybrid, La2O2CO3, and ZIF-67-800, respectively; (a) O 1s spectra; (b) Co 2pspectra; (c) The interfacial interaction between ZIF-67 and La2O2CO3 components in their hybrid.

Together O1s with Co2+ XPS results, we can see that the chemical interaction results from the CO32- component in the La-based compound and the Co2+ ions from the pyrolytic carbon derived form ZIF-67. Since the carbon nanostructures are directly grown onto the layered lanthanum oxycarbonate surface, the chemically intimate contact between the two phases


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contributes to the formation of the covalent Co-[CO2]2- bonds at the interface, as shown in Figure 3(c). The formed the Co-[CO2]2- bonds can lead to Co2+ ions are located out of the planar Co(II)N4 coordination to form a out-of-plane structure. 39, 40, 41This out-of-plane structure can promote the ability of the axial adsorption for active O2 on the exposed Co (II) atoms, which may boost the ORR rate.42 3.3 Enhanced ORR activity of the hybrid To demonstrate ORR electrocatalytic activity of the covalent ZIF-67-800@La2O2CO3 hybrid, the plot of cyclic voltammograms (CVs) were firstly carried out at room condition during the 0.1 M KOH oxygen and nitrogen saturated solutions. As shown in Figure 4(a). The ZIF-67-800 and a commercial Pt/C (20wt. %, Pt) were also characterized for comparison. The CVs curves have been compared under O2 and N2 saturated atmosphere for electrolytes, the oxygen reduction peak of the hybrid shows clearly around 0.85 V with largest current peak compared with the other catalysts under O2-saturated condition, revealing an excellent ORR activity. To confirm the outcomes above, rotating disk electrode (RDE) measurements were performed in 0.1 M KOH oxygen saturated solutions. As shown in Figure 4(b), the hybrid catalyst exhibits the highest ORR activity, which is more excellent than that of pure ZIF-67-800 or the Pt/C catalyst. As illustrated in Figure 4(c), the recorded half-wave potentials (E1/2) for the oxygen reduction over the hybrid, ZIF-67800, Pt/C are 0.85, 0.83, and 0.84 V vs. RHE, respectively. The relative E1/2 of the hybrid is 20 mV over that obtained by the ZIF-67-derived carbon, and 10 mV surpass that of the Pt/C. For comparison, the LSV curve of the Lanthanum oxycarbonate template has been performed and shown in Figure S7. The recorded half-wave potential of the template is 0.5 V vs. RHE, which is much lower than that of the hybrid catalyst. Moreover, the mass specific activity (MA) is another important indicator to determine the ORR activity of catalysts. Figure S8 gives the recorded mass


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specific activity (MA) for the tested catalyst, normalizing the kinetic currents ([email protected] V vs. RHE) by using catalyst mass on electrode. 43, 44 For the hybrid catalyst, the obtained MA is up to 1.07 A g-1, which is higher than that of the ZIF-67-800 (0.84 A g-1). It illustrates the remarkably improved ORR activity of the hybrid exhibits with respect to the ZIF-67-800.

Figure 4. (a) CV curves of ZIF-67-800@La2O2CO3, ZIF-67-800, and a commercial Pt/C (20 wt%, Pt) in 0.1 M KOH. The dashed line is tested in N2-saturated solution and solid line is tested in O2saturated solution. (b) LSV curves for the above three catalysts in 0.1 M KOH oxygen saturated solution; (c) The half-wave potentials of the samples and (d) the fitted Tafel slopes from the LSV curves for the catalysts.


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To further investigate the remarkably improved ORR activity, Tafel curves were plotted for these above catalysts shown in Figure 4(d). The hybrid catalyst as observed exhibits a lower Tafel slope (63 mV dec-1), with respect to the ZIF-67-derived carbon (67 mV dec-1) and the Pt/C (66 mV dec-1). The lower Tafel slope indicates a faster ORR kinetics over the hybrid catalyst with respect to the ZIF-67-derived carbon or Pt/C catalyst. It implies that this hybrid catalyst can facilitate the superficial adsorption of active O2, that is, a sluggish electron transfer process (O2 (ad) + e → O2- (ad))

17, 45, 46.

For the hybrid catalyst, the enhanced ORR activity can be proven by the

remarkably improved kinetic current density (JK) for the hybrid, which surpasses the sum of both La2O2CO3 and ZIF-67-800 catalysts, as evidenced by Figure S8(b). The remarkably enhanced ORR activity possibly connected with the formation of the well-defined carbon-based nanostructures covalently located on the La2O2CO3 surface. The comparison of the ORR performance between the synthesized ZIF-67-derived carbon@La2O2CO3 hybrid catalyst and previously reported carbon-based catalysts derived from MOFs is given in Table S7. We found that the ORR performance of the hybrid catalyst exceed most of the previously reported MOFbased catalysts.


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Figure 5. (a) LSV curves of the ZIF-67-800@La2O2CO3 hybrid in 0.1 M KOH solution under different rotating speed. The K-L curves at selected potentials derived from the LSV curves of (b) the hybrid sample and (c) Pt/C (20 wt. %, Pt). (d) The chronopotentiometry curves in 0.1M KOH oxygen-saturated solution for the hybrid, ZIF-67-800 and Pt/C, respectively. To explore the ORR pathway, the LSV curves of the hybrid were measured at various rotation rates (400-1600 rpm), as shown in Figure 5(a). Based on the data in Figure 5(a), at different polarization potentials, the extracted slopes of the linear K-L correlations are shown in Figure 5(b). As a result, the determined electron transfer number for the ORR over the ZIF-67800@La2O2CO3 hybrid is closed to 4, indicating the ORR over the hybrid undergoing an approximately four-electron process.47 This is in agreement with that tested by the commercial Pt/C catalyst, as observed in Figure 5(c). The rotating ring-disk electrode (RRDE) measurements


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were carried out to calculate hydrogen peroxide yield (% HO2-) of the above catalysts, which has been shown in the Figure S9. As result, the HO2- yield of the hybrid catalyst is 2.8 % in 0.1 M KOH solution, which is very similar to that acquired by the Pt/C (1.91 %). The very low hydrogen peroxide yield on the hybrid catalyst can prove the four-electron pathway during the ORR. The electrochemical impedance spectroscopy (EIS) has been performed for the catalysts shown in Figure S10. The EIS data were fitted and the resulting outcomes are shown in Table S4. As a result, the ZIF-67-800@La2O2CO3 displays a lower charge transfer resistance of 2.164 Ω.cm2 than the commercial Pt/C (5.286 Ω.cm2) at 0.84 V vs. RHE. It intuitively demonstrates that the fast ORR rate may result from the low charge transfer resistance of the hybrid catalyst. In addition, the long-term durability serve as a key parameter is especially for judging the stability of the hybrid catalyst under reaction condition.48 Therefore, the durability for the ZIF-67800@La2O2CO3 was measured via the chronoamperometric (i-t) response at 0.85 V (vs. RHE) in 0.1 M KOH solution for 10000 s. As observed in Figure 5(d), the hybrid catalyst exhibits an excellent electrochemical stability, compared with ZIF-67-800, even surpass the Pt/C. After 10000s, there is still 95 % current remaining for the hybrid, whereas only 90 % for pure ZIF-67derived carbon and 76 % for Pt/C catalyst. This superior stability has also surpassed other previously MOF-based catalysts reported by Li et.al49 or Chen’s group50. It suggests a superiority in long-term operation for the hybrid catalyst with respect to the ZIF-67-800 and Pt/C. The notably improved durability may derived from the vertically grown carbon-based nanostructures chemically coupled with the La2O2CO3 in this hybrid shown in Figure 3(c). The stable nanocarbonbased array structure with uncollapsed ZIF-67-derived dodecahedron structure arrangement provide a feasible way to improve the stability of MOF-based catalysts. 3.4. Origin of electro-catalytic activity of the hybrid.


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For the vertically arrayed carbon-based nanostructures covalently coupled with La2O2CO3 in their hybrid, the significant enhancement of the ORR performance may connect with three possibilities: (1) the increased content of Co-Nx bonds in the carbons from ZIF-67 shown Figure 2(d); (2) the large surface area, which is also contribute to expose more catalytically active sites like Co-Nx moieties; (3) the high graphitization of the carbons, serve as another feature for ORR performance. To further confirm the above hypothesis, we fabricated a physical mixture of the pure ZIF-67-derived carbon with La2O2CO3 (marked as ZIF-67-800/La2O2CO3 blend). The morphology and composition of the blend are shown in Figure 6(a) and (b). For the blend sample, the ratio of the ZIF-67-derived carbon to the La2O2CO3 is similar to that of the as-synthesized hybrid. However, in contrast with the hybrid, the blend exhibits very weak ORR activity, as indicated in Figure 6(c). Based on the N1s XPS results shown in Figure 11, the content of the CoNx moieties in the blend (0.91 at. %) is far lower that of the hybrid (1.27 at. %), as shown in Figure 6(d). This finding is also confirmed by the content of the Co-Nx species determined from Co 2p spectra shown in Table S5. These outcomes reveal the increased Co-Nx coordinate serve as a crucial point in deciding the ORR activity of the hybrid. It is reported that Co ions coordinated with N atoms can absorb oxygen and transfer electrons to N thereby lowing the over-potential, further boosting ORR activity.30,

51

Besides the increased active Co-Nx species, the enriched

porosity and large surface area for active site exposure give an another vital factor to facilitate the ORR activity of porous carbon-based catalysts derived from ZIFs.25,52,53


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Figure 6. (a) SEM image of the ZIF-67-800/La2O2CO3 blend; (b) XRD patterns of ZIF-67800@La2O2CO3 hybrid and ZIF-67-800/La2O2CO3 blend; (c) Line-sweep voltammorgrams in 0.1M KOH under saturated oxygen condition with a rotated speed of 1600 rpm cooperated with a 5 mV s-1 scan rate for the above catalysts; (d) The content of Co-Nx for the above samples determined by N 1s spectra.

To explore the pore structure of the tested catalysts, adsorption-desorption isotherms of N2 were performed for the hybrid and ZIF-67-800/La2O2CO3 blend as reference. Figure 7(a) exhibits a type-IV isotherm curve with a pronounced hysteresis loop, suggesting that both micropores and mesopores are existed for the hybrid. It is almost similar to that obtained by the blend. Different


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from the blend, however, the hybrid shows well-defined hierarchical pores with micro- (1.2 nm) and meso-pores (2.3 nm), based on the pore size distribution (PSD) in Figure S12, determined by using density functional theory.54 Furthermore, the determined BET surface area of the hybrid is up to 855.593 m2 g-1 relative to the carbon and total pore volume for the hybrid is 0.165 cm3 g-1, both of which are much more than those obtained by the blend (the relative number 594.17 m2 g-1 and 0.143 cm3 g-1 respectively), as shown in Figure 7(b) and Table S6. The high BET surface area and pore volume can contribute to the exposure of the active sites like Co-Nx species for the hybrid, which can boost the ORR kinetics through promoting mass transport.55, 56, 57

Figure 7. (a) N2 sorption isotherms of the ZIF-67-800@La2O2CO3 hybrid and the ZIF-67800/La2O2CO3 blend; (b) The surface area and total pore volume for the hybrid and the blend; (c)


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Raman spectra and (d) the value of intensity ratio for fitted D-band to G-band for the above samples, respectively.

Moreover, the addition of the La2O2CO3 can also promote the graphitization of the carbonbased nanostructures, as evidenced by Raman spectra shown in Figure 7(c). There are two characteristic peaks at about 1345 cm-1 and 1585 cm-1, respectively. The D band is attributed to structural disorder, while the G band is associated with structural order of carbons.58 The intensity ratio of these two bands (remarked as ID/IG) can be used to evaluate the graphitization of carbons.59 So, we calculated the intensity ratios of ID/IG for these above carbon samples and the relative results are displayed on Figure 7(d). As observed, the ID/IG value of the hybrid (1.16) is lower than that of the blend (1.43), revealing a relatively high graphitization degree of the carbon in this hybrid. It reveals that La2O2CO3 can facilitate the graphitization of the carbon-based nanostructures derived from ZIF-67 epitaxially grown the La-based compound shown in Figure 1(b)-(c). This can be explained by the smaller metallic Co nanoparticles highly dispersed into the carbon-based nanostructures in the hybrid with respect to the ZIF-67-derived carbons shown in Figure 1(g). In general, the metallic Co can efficiently promote the graphitization of the pyrolytic carbons at elevated temperatures. Therefore, the high graphitization can further enhance the ORR rate, through reducing the charge transfer resistance of the hybrid catalysts. Conclusion In conclusion, we have devised a smart synthesis pathway for morphology-controlled carbonbased nanostructures derived from ZIF-67, using layered lanthanum oxycarbonate (La2O2CO3) as a structure-oriented template. The obtained carbon-based nanostructures vertically grow on the


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template surface and show a well-defined dodecahedron morphology. The comparison have been made with the pure pyrolytic carbon derived from ZIF-67 or La2O2CO3 alone, the ZIF-67800@La2O2CO3 hybrid exhibits distinctly improved ORR activity and durability in 0.1M KOH solution, surpassing those displayed by a commercial Pt/C (20 wt. % Pt). The introducing of La2O2CO3 can not only avoid obvious depression and aggregation but also promote the formation of active Co-N coordination for the carbon-based nanostructures. In addition, the carbon-based nanostructures show the increased graphitization degree and richened porous structure, caused by the added La2O2CO3. All of these could serve as a reliable evidence for the substantially improved ORR activity of the ZIF-67-800@La2O2CO3 hybrid. Since La2O2CO3 having ORR activity, no additional tedious post-treatments like acid leaching is required to remove the compound in this work. Therefore, using the layered La2O2CO3 as a novel template may be a smart route to precisely and effectively manipulate the performances of MOF-derived carbons with controllable morphology and composition for energy storages and conversions. ASSOCIATED CONTENT Supporting Information. The representative SEM image of ZIF-67 on the La2O2CO3 surface through an epitaxial growth; the XRD patterns of ZIF-67@La2O2CO3 and ZIF-67; the SEM image of ZIF-67; the TG and DTA for the ZIF-67-800@La2O2CO3; the obtained survey spectrum of ZIF-67-800@La2O2CO3; the high-resolution XPS Co 2p spectra of ZIF-67-800@La2O2CO3, ZIF-67-800 and ZIF-67800@La2O2CO3-acid; the LSV curves of La2O2CO3 and ZIF-67-800@ La2O2CO3 were operated under 1600 rpm electrode rotating rate following 5 mV s-1 scan speed in 0.1 M KOH oxygen saturated electrolyte; the mass specific activity of these obtained ZIF-67-derived carbon


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and ZIF-67-800@La2O2CO3 from the polarization curves of ORR depended on a 0.1 M KOH oxygen saturated electrolyte at a fixed rotation rate of 1600 rpm, normalized by catalyst mass coated on electrode; the kinetic current densities (Jk) of ZIF-67-800@La2O2CO3, ZIF-67-800, and La2O2CO3; the high-resolution XPS N 1s spectra of ZIF-67-800@La2O2CO3, ZIF-67-800 and ZIF67-800/La2O2CO3; the pore size distribution of the ZIF-67-800@La2O2CO3 and ZIF-67800/La2O2CO3; the atomic percentage (%) results for the ZIF-67-800@La2O2CO3 and ZIF-67800; the atomic percentage (%) results and configuration of N for the ZIF-67-800@La2O2CO3 and ZIF-67-800 determined by the XPS of N 1s spectra; the atomic percentage (%) results and configuration of Co for the ZIF-67-800@La2O2CO3 and ZIF-67-800 derived from the XPS of Co 2p spectra; the atomic percentage (%) results and configuration of N and Co for the ZIF-67800@La2O2CO3 and ZIF-67-800/La2O2CO3 determined by N 1s and Co 2p spectra; analysis of special surface relative to carbon and the distribution of pore structural parameters for the ZIF67-800@La2O2CO3 and ZIF-67-800/La2O2CO3, respectively. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (J. Liu). *E-mail: [email protected] (F. Wang) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Funds of China (Grant Nos. 51572013).


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Carbon-Based Nanostructures Vertically Arrayed on Layered

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