Anchoring Cobalt Nanocrystals through the Plane of Graphene: Highly

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Anchoring Cobalt Nanocrystals through the Plane of Graphene: Highly Integrated Electrocatalyst for Oxygen Reduction Reaction Xin-Hao Li Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm503988n • Publication Date (Web): 23 Dec 2014 Downloaded from http://pubs.acs.org on January 1, 2015

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Anchoring Cobalt Nanocrystals through the Plane of Graphene: Highly Integrated Electrocatalyst for Oxygen Reduction Reaction

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

Chemistry of Materials cm-2014-03988n.R2 Article 22-Dec-2014 Lv, Li-Bing; Shanghai Jiao Tong University, Department of Polymer Science and Engineering Ye, Tian-Nan; Shanghai Jiao Tong University, School of Chemistry and Chemical Engineering Gong, Ling-Hong; Shanghai Jiao Tong University, School of Chemistry and Chemical Engineering Wang, Kai-Xue; Shanghai Jiao Tong University, School of Chemistry and Chemical Engineering Su, Juan; Shanghai Jiao Tong University, School of Chemistry and Chemical Engineering Li, Xin-Hao; Shanghai Jiao Tong University, School of Chemistry and Chemical Engineering Chen, Jie-Sheng; Shanghai Jiao Tong University, School of Chemistry and Chemical Engineering

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Anchoring Cobalt Nanocrystals through the Plane of Graphene: Highly Integrated Electrocatalyst for Oxygen Reduction Reaction Li-Bing Lv, Tian-Nan Ye, Ling-Hong Gong, Kai-Xue Wang, Juan Su, Xin-Hao Li* and Jie-Sheng Chen* School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China. ABSTRACT: Due to the high cost of Pt-based materials for electrocatalysis, substitute catalysts composed of non-precious metals have been in high demand. Herein, an ultra-stable electrocatalyst with cobalt nanocrystals grown through the plane of graphene subunits of nitrogen-doped graphenes was synthesised via a one-step route. The catalyst has more positive onset and half-wave potential than Pt/C, high methanol crossover tolerance and superior stability. It is the introduction of strongly bonded cobalt nanocrystals into the network of graphenes that modulate the electronic properties of the latter, resulting in the superb electrocatalytic performance.

INTRODUCTION Efficient and sustainable electrocatalysts are highly desirable for practical applications of proton-exchange membrane (PEM) fuel cells.1,2 Much effort has been focused on developing non-precious metal or metal-free electrocatalysts3,4 due to the low stability of expensive Ptbased catalysts, which are however the best electrocatalysts for comercial fuel cells in the present.5,6 The extraction of graphene by Geim group led to great endeavors to exploit the unique properties of such atomic layers for vairous applications, including catalysis and electrocatalysis.7,8 The high surface area and charge mobility of graphene-based materials are rather suitable to overcome the sluggish kinetics and high Pt loadings for commercialization of the relevant electrocatalysts.9-11 Pristine graphene could only offer limited activity for various electro-catalytic reactions, including oxygen reduction reactions (ORR). Further functionalization of graphenes by introducing guest components was essential to trigger the catalytic activity of graphene-based materials.12-18 Within this respect, non-precious metals and their oxides, hydroxides, sulphides, nitrides or even phosphates were rationally anchored onto the surface of graphene sheets or assemblies. The final activity of these composite electrocatalysts is significantly affected by the dispersion and stability of the guest components. Strong interactions between the graphene sheets and supported nanoparticles play the key role in dominating the final ORR activity and stability. Highly integrated nanoparticle-graphene complexes are thus highly preferred for practical uses.

Scheme 1. Schematic structures of the nanoparticles graphene hybrids. The functional nanoparticles (exemplified with Co here) can be grown on the surface (a) of nitrogen-doped graphenes (NG) as described in the literature and through the plane (b) of NG in this work. .

In principle, direct insertion of nanoparticles through the plane of graphene, as depicted in Scheme 1b, can result in highly strong contact between these two components and thus better dispersion and stability of nanoparticles, which are beneficial for both catalysis and electrocatalysis. Indeed, recent theoretical studies showed that small metal clusters growing on both sides of the graphene sheet by penetrating the plane have stronger binding between them and thus higher thermodynamic stability as compared with graphene-supported metal clusters grown on one side.19 Better catalytic performance can also be expected as the in-situ formed defects through the graphene plane effectively introduce a band gap and thus supply us a possible means of modulating the electronic



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properties of graphene. A Mott Schottky effect at the metal-doped carbon interface was also possible, further benefiting its potential applications in catalysis and electrocatalysis.20 Although approaches for generating through-plane holes in the graphene sheets were reported,21-23 developing effective methods to anchor guest nanoparticles inside these holes to form graphene-based nanocomposites in large quantities for catalysis still remains a great challenge.

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Ultra DLD spectrometer using a monochromated Al Kα radiation. Scanning electron microscopy (SEM) measurements were operated on a FEI Nova NanoSEM 2300. The TEM and HRTEM images were taken with a JEM-2100F microscope operated at an acceleration voltage of 200 kV.

RESULTS AND DISCUSSION

Herein, we report a single-step approach for directly fabricating nitrogen-doped graphene (NG) with nonprecious metal nanoparticles grown through the plane of graphene layers in the graphenes simply by thermal condensation of biomass and corresponding metal salts.24,25 The as-obtained metal nanoparticle-graphene monolith complexes, here exemplified with Co (CoNPs@NG), show superb electrocatalytic activity and stability for ORR, outperforming the state-of-the-art catalyst Pt/C. The cost of CoNPs@NG catalyst is, however, much lower than that of Pt/C for practical applications.

EXPERIMENTAL SECTION Material synthesis. A homogeneous mixture of CoCl2, glucose and dicyandiamide (DCDA) with a mass ratio of 40:1 (DCDA : glucose) and 0.05:1 (Co : C content in glucose) was heated at 1000 °C (3.3 °C min-1), held at that temperature for 1h under the protection of N2 flow (20 mL min-1), and cooled down to room temperature. The asobtained monolith was directly used for characterizations and electrochemical tests. Electrochemical testing. 5 mg of the sample or Pt/C (20 wt.%) was dispersed in a solution consisting of 240 µL of Nafion (used as binder here) and 800 µL of deionized (DI) water. The mixture was ultrasonicated for 30 minutes to get a homogenous catalyst ink. 10 μL of the ink was dropped on a mirror polished glass carbon electrode and dried at room temperature. The as-obtained electrode with catalyst loading of 0.245 mg cm-2 was used as working electrode. We used platinum wire as a counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The electrochemical test was performed on a CHI 660C electrochemical workstation. Before electrochemical tests, the electrolyte (0.1 M KOH or 0.5 M H2SO4) was saturated with N2 or O2. Cyclic voltammetry (CV) was carried out from 1.2 V to -0.2 V versus reversible hydrogen electrode (RHE) with a sweep rate of 50 mV s-1. Linear sweep voltammetry (LSV) was performed from 1.2 to 0 V at 1600 rpm with a sweep rate of 10 mV s-1. For the RRDE measurements, the disk electrode was scanned cathodically at a rate of 5 mV s-1 and the ring potential was constant at 1.5 V vs RHE. Characterization. Powder X-ray diffraction patterns were recorded on a Bruker D8 Advance X-ray diffractometer with Cu-Kα radiation (λ = 1.5418 Å) with a scan rate of 20° min-1. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos Axis

Figure 1. Characterization of CoNPs@NG. (a) The synthetic progress of CoNPs@NG. XRD pattern (b), XPS sweep scan (c), TEM image (d) and HRTEM (e) image of CoNPs@NG. Insets: (e) the selected area electron diffraction pattern.

The overall fabrication process of CoNPs@NG is depicted in Figure 1a. A homogeneous mixture of dicyandiamide (DCDA), glucose and metal salts (CoCl2) were directly transformed into CoNPs@NG at 1000 °C under the protection of N2 flow. Generally, inert gas (N2 flow) was used to remove oxygen gas from the synthetic system. CoCl2 was then reduced to metallic Co NPs by highly reductive nitrogen-rich carbon species formed from the decomposition of DCDA and glucose at high temperature. The formation of layered g-C3N4 from DCDA at a temperature between 300 and 750 °C (Figure S1 and Figure S2) was believed to be key step for confining both the Co species and carbon intermediates into the interlayer gaps of g-C3N4. Such a confinement effect obviously depressed the formation of carbon nanotubes and capsules, which were main products in pyrolysis of transition metal-C-N complexes at a temperature between 350 to 500 °C in the literature.26 Both the graphene layers and the Co NPs were released after the complete thermolysis of g-C3N4 at 1000 °C (Figure S3), resulting in the final hybrid CoNPs@NG. The metal loadings can be facilely controlled by varying the weight ratios of metal salts and glucose in


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the precursor, resulting in CoNPs@NG-x % (x % = weight

percentages of metal species in the precursor).

Figure 2. The electrocatalytic activity of various samples in alkaline (a-c) and acid (d-f) electrolyte. Rotating ring-disk electrode voltammograms (a,d), the electron transfer number (b,e) and corresponding tafel slopes (c,f) of Pt/C, bare NG and CoNPs@NG. All -1 measurements were conducted on glassy carbon electrodes at 1600 rpm in O2-saturated electrolytes with a sweep rate of 10 mV s .

The layered structure is the major morphology of subunits comprising the as-formed CoNPs@NG as revealed by the representative scanning electron microscopic (SEM) images (Figure S4b). Similar to bare NG sample (Figure S4a), the wrinkled planes of CoNPs@NG suggest the thin and flexible feature of the graphene sheets. Under optimized ratios of Co and C (1-10%), large area graphene sheets are the major. This was further confirmed by the transmission electron microscopy (TEM) observation (Figure 1d and Figure S5). High resolution transmission electron microscopic image (HRTEM) of the graphene sheets (Figure S6) reveals the presence of both monolayer and few-layer graphenes (layer number < 10) in as-formed samples. Moreover, free-standing particles in micrometer or hundreds of nanometer scale are hardly seen via SEM observation, rather speaking for the highly integrated structure of the Co NPs and NG. The introduction of Co species did not disturb the formation of graphene sheets. The presence of Co NPs in the as-formed monolith can be confirmed by the X-ray diffraction (XRD) analysis and TEM observation. Figure 1b and Figure S7 shows the typical XRD patterns of CoNPs@NG samples with three peaks matching well with the (111), (200) and (220) peaks of metallic Co (standard PDF card 15-0806). In contrary to graphene oxide with broad peaks around ten degrees, no obvious XRD peaks were observed in the XRD patterns of our samples, rather speaking for the absence of oxygenrich functional groups in the graphene lattices of our samples. No peaks corresponding to the phases of cobalt oxides, carbides or nitrides were detected, indicating that

the nanoparticles are pure metallic cobalt crystals. According to the X-ray photoelectron spectroscopy (XPS) result (Figure 1c), the atomic concentration of N was estimated to be 4.6 %, comparable to that of bare NG (4.3%) fabricated under the same conditions. Additional nitrogen atoms in the CoNPs@NG sample was mainly in the form of N-Co bonds on the surface of Co NPs, which will be discussed in the following section. The TEM (Figure 1d and Figure S5) and HRTEM images unambiguously revealed the formation of metallic Co NPs with a mean size of 15 nm (Figure 1e and Figure S8) well dispersed through all the planes. Note that Figure 1e demonstrates the welldefined crystalline lattice spacing of 0.2 nm, which matches well with the d spacing value of Co (111) plane. The selected area electron diffraction (SAED) in Figure 1e further confirms the crystalline structure of Co NPs. These observations also suggest the very high thermal stability of the as-formed Co NPs without obvious aggregation even at a temperature up to 1000 °C. Such a high thermal stability of Co NPs can be attributed to their strong interaction with the support NG. We firstly investigated the electrocatalytic activities of the CoNPs@NG catalysts by cyclic voltammetry (CV) (Figure S10a) in O2- or N2-saturated alkaline electrolyte. At the same time, the NG and the benchmark catalyst Pt/C (20 wt.%) were also tested for comparison (Figure S10 b-c). There are no obvious redox peaks in the CV curve of CoNPs@NG sample in N2 saturated electrolyte, suggesting



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Figure 3. Real structure of CoNPs@NG. HRTEM images of CoNPs@NG with the Co NPs uncovered (a), partly covered (b)

and completely covered (c) by graphene layers. (d) High resolution XPS N 1s spectra of NG and CoNPs@NG. (e) High resolution XPS O 1s spectra of NG and CoNPs@NG. (f) High resolution XPS Co 2p spectrum of CoNPs@NG with both metallic Co and Co-X bonds (X might be C, N or O). their good electrochemical stability. The ORR onset potential and peak potential of CoNPs@NG sample in O2saturated electrolyte were 1.06 and 0.88 V, respectively, vs the reversible hydrogen electrode (RHE), more positive than those (0.98V and 0.81 V vs RHE) of NG (Figure S10a and Figure S10c), surpassing those nanoparticle/graphene based electrocatalysts described in the literature (Table S1). Linear sweep voltammetry (LSV) curves measured on rotating ring-disk electrode (RRDE) (Figure 2a,d) showed similar trends. The CoNPs@NG sample has the most positive half-wave potential at 1.01 V vs RHE among the samples tested including Pt/C (0.87 V vs RHE) and bare NG (0.81 V vs RHE) in 0.1 M KOH. The Co content of CoNPs@NG was optimized to be 5% to achieve the best ORR activity (Figure S12) and was thus used for all the following electrochemical characterization except specially noted. It should be noted that the half-wave potential of CoNPs@NG was 140 mV more positive than that of Pt/C. Moreover, the CoNPs@NG sample also shows much better methanol tolerance as compared with Pt/C (Figure S11). Only moderate disturbance was observed after the addition of a certain amount of methanol (final concentration: 1 mol L-1). As shown in Figure 2b, the electron transfer numbers of CoNPs@NG through all potential range (0.2-1.0 V vs RHE) varied from 3.80 to 3.99 (in 0.1 M KOH), rather speaking for a four-electron process of the

ORR over CoNPs@NG. Moreover, the Tafel slope of 85 mV (in 0.1 M KOH) per decade for CoNPs@NG is much smaller than that of bare NG and close to that of Pt/C, again revealing a more favourable ORR kinetics of CoNPs@NG (Figure 2c). All these results reveal the excellent electrocatalytic activity of CoNPs@NG for ORR in alkaline electrolyte, promising its great potential as a cheap but efficient electrocatalyst for practical applications in alkaline fuel cells or Li/Zn air batteries.27-29 The promoting effect of Co components on the ORR activity of graphene based nanocatalyst was also obvious in acid electrolyte (0.5 M H2SO4) for their potential application in proton-exchange membrane fuel cells. The positive half-wave potential (Figure 2d), high selectivity to four-electron process (Figure 2e) and much lowered (61 mV per decade, Figure 2f) in 0.5 M H2SO4 make CoNPs@NG among one of the best noble-metal-free electrocatalysts for ORR in acid electrolyte.30-32 Besides the excellent ORR activity, the real role that Co components acted in the CoNPs@NG complex for activating oxygen here is also of great importance for further improving ORR performance of graphene-based hybrid nanocatalysts. Considering the significant promoting effect of Co NPs on the ORR activity, we firstly investigated the real structure of Co NPs in our samples. The textural details of the


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CoNPs@NG complexes can be clearly observed in their HRTEM images (Figure 3a-c and Figure S13 a-c). Three types of metallic Co NPs were observed, including uncovered (Figure 3a), partly covered (Figure 3b) and completely covered (Figure 3c) nanoparticles by graphene layers. All the XPS O 1s peaks of bare NG and our sample (Figure 3e and Figure S9) centered at 532.3 eV revealed the presence of oxygen in the form of absorbed H2O, O2 and/or CO2. The amount of Co-Ox species with the signal centered at 529.2 eV is thus negligible in our sample.33 Further XPS Co 2p spectra (Figure 3f) with broad Co 2p peaks revealed the presence of a certain amount of Co-N bonds in our sample. This is to say that the metallic Co NPs were anchored through the plane of NG via Co-N bonds, which could be confirmed by the up shift in the XPS N 1s peaks of CoNPs@NG (Figure 3d). More importantly, the involvement of Co in the formation process of NG led to an obvious increase in the percentage of graphitic nitrogen atoms (Figure 3d), which have been proved to result in highly active electrocatalytic sites and better conductivity in the carbon networks.

Figure 4. Effect of the structure on the ORR activity. (a) Scheme of the active cite at the interface of Co NPs and NG. Effect of SCN ̄ ions on ORR activity of CoNPs@NG (b) catalyst was tested in 0.5 M H2SO4. The final concentration of SCN ̄ was 5 mM. (c) Cartoon schemes and TEM images of + typical nanostructures in CoNPs@NG-H (obtained by etching CoNPs@NG sample in 7.5 M HNO3 for 24 h at room tem+ perature). (b) LSV curves of CoNPs@NG and CoNPs@NG-H tested in alkaline electrolyte.

To determine if the electrocatalytic activity of CoNPs@NG derives exclusively from Co species, we employed a series of control tests. For pyrolyzed transition metal/C/N electrocatalysts, there is a consensus that metal-Nx complexes function as active centers for ORR in acid solution.16,34-39 Recent studies demonstrated that CN- or SCN- ions are generally poisonous for the metal-Nx/C catalyst in acid electrolyte. Indeed, a moderate negative shift (about 90 mV) on the half-wave potential was observed in the LSV curves of CoNPs@NG in oxygensaturated 0.5 M H2SO4 electrolyte after the addition of KSCN with a final concentration of 5 mM (Figure 4b).

Such an observation demonstrated that Co-N-C macrocyclic compounds, which located mainly at the interface of Co NPs and NG according to the TEM observation (Figure 3a-c) and XPS analysis (Figure 3d-f), could activate oxygen molecules for reduction reactions. It should be noted that the synthetic temperature for metal-N-C macrocyclic compounds described in the literature were usually optimized to be around 900 °C. Even higher temperature (e.g. 1000 °C) could destroy these active.16 On the contrary, the best sample in our system was the one obtained at 1000 °C. There seems to be additional active centers that were not sensitive to the SCN ̄. The fact that the poisoned CoNPs@NG sample (Figure 4b) with SCN ̄ still offered better ORR activity as compared with bare NG (Figure 2d) revealed the presence of additional active centers rather than Co-Nx bonds. We thus further removed the possible Co-centers by using acid solution (7.5 M HNO3) under relative mild conditions, resulting in the sample CoNPs@NG-H+. As expected, three types of nanostructures (Type I-Type III) were observed in the TEM images of the etched sample as depicted in Figure 4c and Figure S16, revealing the unique through-plane hybrid manner of Co NPs and NG. Moreover, our etching process was mild enough to keep the graphene sheets from oxidation according to the XPS (Figure S14) and XRD analysis (Figure S15). After the acid etching process, even 44 at.% of the Co component was left in the CoNPs@NG-H+, presumably in the form of completely coated Co NPs (Type III in Figure 4c). Surprisingly, the ORR onset potential and half-wave potential were not varied too much with only a slight decrease in the half-wave potential (Figure 4d). It is obvious that completely coated Co NPs could not directly act as the active cites here. As both Co NPs and NG are structurally tightly bonded (Figure 4a), electrons will flow at their interface to equilibrate their Fermi levels. Such an electron-redistribution between Co, C and N atoms could introduce a band gap into graphene, making graphene domains adjacent to Co NPs more “noble” and active for ORR. As a result, the limited current intensity decreased (Figure 4d) when part of Co NPs was removed via acid etching. Generally speaking, a dual-active-site mechanism may be responsible for excellent ORR activity of CoNPs@NG. The nitrogen-doped graphene domains adjacent to the Co NPs should be the main active cites in the catalysts, whilst the Co-N-C macrocyclic compounds at the Co NPs and NG interface help to activate oxygen for better ORR activity. The metallic Co NPs exposed to the electrolyte and oxygen can improve the charge transport of electrodes for significantly elevating the current intensity (Figure 4d) due to their high conductivity. An ideal electrocatalyst should also have an excellent electrochemical stability for practical applications besides low cost and high activity. We thus further tested the electrochemical stability and chemical stability of the samples. The chronoamperometric responses of the samples were thus measured to evaluate their ORR stability.



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As shown in Figure 5, only a slight attenuation of the activity of the CoNPs@NG sample was observed after 30,000 seconds with 91 % of its initial activity retained in alkaline electrolyte and 96 of that in acid one. Note that such an attenuation rate of the CoNPs@NG sample is the same with that of bare NG and other graphene-based metalfree electrocatalysts, rather speaking for its high ORR stability. Pt/C lost 50 % of its initial current under the same conditions. As a comparison, a control sample obtained by depositing Co NPs on the surface of NG (Co+NG) was also tested. Although the attenuation rate of the Co+NG sample is lower than that of Pt/C at the earlier stage of the chronoamperometric test, a constant loss in activity was observed and reached to 50 % after 30,000 seconds.

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half-wave potentials, high limited current density and ultra-high stability, surpassing the benchmarked catalyst Pt/C in alkaline electrolyte. More importantly, low cost of the synthetic method combined with their promising catalytic activity makes NPMNPs@NG (NPM: non-precious metals not limited to Co) an excellent class of electrocatalysts for the next generation of fuel cells and Li/Zn air batteries. More applications of MNPs@NG nanodyads, such as in catalysis, oxygen sensors, and water treatment, can also be expected and will be tested in our following research.

ASSOCIATED CONTENT Supporting Information. Experimental details, more characterization results, and detailed discussions. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (XHL) *E-mail: [email protected] (JSC)

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was financially supported by the National Basic Research Program of China (2013CB934102, 2011CB808703) and the National Natural Science Foundation of China (21331004, 21301116). Figure 5. Chronoamperometric responses (percentage of current retained versus operation time) of CoNPs@NG, Pt/C and Co+NG (obtained by post deposition of Co NPs on the surface of NG).

Chemical stability of the CoNPs@NG was also high due to the unique Co-N-graphene hybrid structure, where Ngraphene acted as stabilizer like the manner of organic ligands. As demonstrated simply by XRD analysis, the sample remained stable after a storage of five months under ambient conditions without obvious change in the XRD peaks (Figure S17a). The well reservation of the metallic Co NPs after long-term storage in air can also be facilely demonstrated by its respondence to a magnet bar (Figure S17b), whilst oxidized Co NPs showed no obvious response to external magnetic field. More importantly, the CoNPs@NG could also remain stable in corresponding ink for several months. All these results indicate the superior recycling stability of CoNPs@NG, matching well with the prediction of theoretical studies.19

CONCLUSIONS In summary, we developed a single-step approach for constructing highly integrated non-precious metal Colayered carbon complexes with the Co NPs penetrating through the plane of the graphene sheets. The bonded Co-graphene complexes afforded the material excellent electrocatalytic activities with highly positive onset and

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Chemistry of Materials .

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SYNOPSIS TOC


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