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Graphdiyne: a Promising Catalyst-Support to Stabilize Cobalt Nanoparticles for Oxygen Evolution Jian Li, Xin Gao, Xin Jiang, Xu-Bing Li, Zhongfan Liu, Jin Zhang, Chen-Ho Tung, and Li-Zhu Wu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01781 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 6, 2017

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Graphdiyne: A Promising CatalystCatalyst-Support to Stabilize Cobalt Nanoparticles for Oxygen Evolution Jian Li,1,2 Xin Gao,3 Xin Jiang,1,2 Xu-Bing Li,1,2 Zhongfan Liu,3 Jin Zhang,3* Chen-Ho Tung,1,2 Li-Zhu Wu1,2* 1 Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry & University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, P. R. China 2 University of Chinese Academy of Sciences, Beijing 100049, P. R. China 3

Center for Nanochemistry, Beijing Science and Engineering Center for Nanocarbons, Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P.R. China ABSTRACT: Graphdiyne (GDY), with highly π-conjugated structure of sp2- and sp-hybridized carbons, has recently appeared as an allotropic form of carbon nanomaterials. However, the application of this material is far behind its sister graphene. Herein, we attempt to use GDY as catalyst-support to stabilize cobalt nanoparticles for oxygen evolution, which is considered as the bottleneck for water splitting. In terms of close interaction between metal ions and alkyne π-conjugated networks, the self-supported electrode is made in situ by a facile chemical reduction of Co2+ salt precursor in aqueous solution. The prepared 3D Cu@GDY/Co electrode shows high OER electrocatalytic activity with a small overpotential of nearly 0.3 V and a large unit mass activity of 413 A g−1 at 1.60 V vs RHE. In the course of 4 h electrolysis, the electrode maintains the relatively constant current density. Our results indicate that the GDY is a promising catalyst-support to stabilize metal NPs for oxygen evolution.

KEYWORDS: graphdiyne, catalyst-support, cobalt nanoparticles, water oxidation, catalysis

To deal with energy crisis and environmental issue, water splitting into molecular oxygen and hydrogen provides one of the most promising solutions.1-7 Because of energetically and mechanistically high demands, the water oxidation reaction possesses the sluggish kinetics and large overpotential.8-17 Over the past decades, a large number of oxygen-evolving catalysts have been designed and Iridium and Ruthenium complexes/oxides have been demonstrated promising in oxygen evolution reaction (OER).18-23 However, their appealing features are outweighted by their low abundance and relatively high price, thereby making their large-scale application impractical. Recently, cobalt based catalysts in relation to molecular complexes,24-27 oxides,28-34 nanoparticles (NPs)35-36 and layered structure37-40 have appeared at the forefront of OER. By tuning the size of the catalysts into nanoscale or introducing conductive catalyst-support, the exposed active sites and electronic conductivity of the OER catalysts could be greatly improved.22,31,35 Unfortunately, these high-surface-energy, small-sized nanoparticles usually suffer from serious aggregation, which results in dramatic decrease of their catalytic performance. Thus, seeking for appropriate and efficient catalyst-support material, to improve the dispersity of catalysts and simultaneously to facilitate the mass-transport and electron-transfer kinetics, represents a major target from the scientific and applicable points of view. Graphdiyne (GDY), a new metal-free nanomaterials, is constructed by inserting the acetylenic linkage (−C≡C−)

between two bonded carbons in graphene.41-43 Owing to its highly π-conjugated structure, large surface area, uniform-distributed pores, good chemical stability and excellent electronic conductivity, GDY has been demonstrated promising applications in Li ion battery,44 photoelectric device,45-46 environmental remediation47 and catalysis.48-49 However, the attempt, to the best of our knowledge, to use GDY for oxygen evolution is unknown. With this in mind, we initiated the integration of the flat sp- and sp2 hybridized GDY and catalysts for OER. It is anticipated that (1) the interaction between metal catalysts, such as Co2+/Co, and the alkyne and aryl π-conjugated networks would help stabilize nanoparticles from aggregation;50-51 (2) the unique porous structure of GDY could embed the metal nanoparticles with a strong adsorption energy;52-53 (3) the excellent electrical conductivity and chemical stability allow for GDY to serve as support materials for the catalysts. In this contribution, we report the design and fabrication of self-supported 3D Cu@GDY/Co electrode for OER. As shown in Scheme 1, the Co NPs on GDY substrate is simply made by a facile chemical reduction of Co2+ salt precursor in aqueous solution. The as-prepared 3D Cu@GDY/Co electrode shows superior OER activity with a small overpotential of nearly 0.3 V and a large unit mass activity of 413 A g−1 at 1.60 V vs RHE, which is comparable to the best value of Co NPs based hybrid catalysts for OER.35-36 In the course of 4 h electrolysis, the assembled electrode maintains the relatively constant current density. These results render

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GDY to be a promising catalyst-support to stabilize metal NPs for OER.

Scheme 1. Schematic illustration of the preparation of Cu@GDY/Co composites. Scheme 1 illustrated the fabrication procedure of self-supported 3D Cu@GDY/Co electrode. First, Cu foam was chosen as both the catalyst to grow GDY and the substrate to support the nanostructures. Compared to the electrocatalysts fabricated on glassy carbon electrode (GCE), the 3D porous and highly conductive scaffolds were expected to exhibit great increase in OER performance, profitting from the sufficient contaction with electrolyte and abundant catalysts loading. After an acetylenic coupling reaction catalyzed by Cu foam, GDY nanowalls were in situ synthesized successfully, covering the entire surface of 3D Cu skeleton. The as-prepared Cu@GDY foam was immersed into the Co2+ solution subsequently to induce the adsorption of Co2+ onto the negatively charged surface of GDY. The absorbed Co2+ ions were then reduced to Co NPs by addition of NaBH4 solution. The formation of the Co NP on the surface of GDY was visually observed by the color changes of GDY, from light-yellow to light-black after the reduction of Co2+ by NaBH4. The morphology of Cu@GDY/Co electrode was investigated by scanning electron microscope (SEM). Individual Cu foam and GDY-based Cu foam were also prepared for comparision. As shown in Figure S1, the Cu foam possesses a 3D porous structure and maintains its basic skeleton in the entire fabrication process. After the growth of GDY, the smooth surface of Cu foam was covered uniformly by vertically regular cross-linked nanowalls with plenty of submicrometer voids (Figure 1a), which is beneficial for the sufficient adsorption of Co ions and the contaction with electrolyte. Furthermore, we observed that densely and uniformly packed Co NPs had been deposited on the surface of GDY with average diameters below 10 nm (Figure 1b). SEM-EDX mapping suggested that element Co was distributed uniformly over the entire surface of GDY nanowalls (Figure 1c, 1d). Meanwhile, a series of identical operations were performed for bare Cu foam without GDY. No significant changes emerged before and after the adsorption of Co NPs and the element Co signal was not detected by the EDX analysis at all (Figure S2), indicating that the GDY is the base for the adsorption of metal NPs.

Figure 1. SEM images of (a) Cu@GDY, (b) the prepared Cu@GDY/Co electrode (scale bars: 200 nm); the SEM elemental mapping of (c) cobalt and (d) carbon; TEM (e) and HRTEM (f) images of GDY/Co composites. The morphology of the formed Co NP/GDY nanocomposite was further characterized by TEM. Figure 1e displayed the low-resolution TEM image of Co-loaded GDY nanocomposite. No bulky agglomeration was seen and the average size of Co NPs is nearly 4 nm (Figure S3), suggesting the fair dispersion of Co NPs on the surface of GDY. From the HRTEM image, we identified that the Co NP was well-crystallized with the lattice fringe spacing of ~0.200 nm, close to the (111) lattice spacing of the face-centered cubic structure of Co (Figure 1f). The results revealed that GDY could interact with Co NPs and served as an excellent substrate to stabilize Co NPs, as predicted theoretically.52-53 The XRD patterns of the prepared samples were also investigated (Figure S4). The two peaks at 2θ values of 43.64°, 50.80° were consistent with (111), (200) diffractions of face-centered cubic (fcc) metallic Cu (JCPDS No. 03–1005). The absence of well-defined XRD pattern for GDY was probably due to the low amount and relatively low crystallization of GDY. The signals of Co were not detected either for XRD test, possibly due to low amount. The surface chemical composition of the prepared electrode was also characterized by X-ray photoelectron spectrocopy (XPS) and raman spectroscopy, which would provide insightful chemical bond information. Unlike the Cu foam/GDY, only composing of elemental Cu and C, the Cu@GDY/Co electrode showed additional characteristic binding energy of Co element (Figure 2a), in agreement with the EDX results. Figure 2b displayed the C 1s orbital of the Cu@GDY/Co electrode, which consisted of four sub-peaks at 284.6, 285.3, 286.8 and 288.4 eV, corresponding to sp2 (C=C), sp (C≡C), C−O and C=O. The

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existence of oxygen containing carbon species is ascribed to oxidation of some terminal alkyne.42-43 The presence of cobalt in this composite was confirmed by Co 2p peaks, and the high resolution XPS spectrum of Co 2p showed a characteristic peak centred at 777.9 eV, corresponding to the metallic Co 2p3/2 state. Additional shoulder peak at 780.2 indicated that part of metal Co NPs with high surface energy were oxidized to CoO. Moreover, Raman spectra of Cu@GDY electrode displayed four prominent peaks around 1408.7, 1578.2, 1923.3, 2171.4 cm-1, respectively. The peak located at 1408.7 cm-1 corresponds to the breathing vibration of sp2 carbon domains in aromatic rings (D band). The peak at 1578.2 cm-1 could be assigned to the first-order scattering of the E2g mode observed for in-phase stretching vibration of sp2 carbon lattice in aromatic rings (G band). The peak at 2171.1 represents the vibration of conjugated diyne links (-C ≡ C-). After anchoring Co NPs, the G band peak exhibited a hypsochromic shift indicating the chemical interactions between GDY and Co NPs.54-55 Similar phenomenon was also observed in the sp carbon peak.

electrode to generate a current density of 10 mA/cm2 dropped to nearly 0.35 V (Figure S5). After the adsorption of Co NPs, however, the Cu foam without GDY had almost no enhancement in OER activity compared to the bare Cu foam before the adsorption (Figure S6). The amount of Co bound to the Cu@GDY electrode was determined using ICP-MS to be ~8.2 μg cm-2, which was much higher than that of bare Cu foam (0.6 μg cm-2), implying the interaction between GDY and Co NPs. Further, we compared the unit mass activity of the Cu@GDY/Co and Cu/Co electrode at 1.6 V vs RHE. After deducting the background current, the mass activity of Co deposited on Cu/GDY was calculated to be ~413 A g-1, higher than that of Co deposited on Cu foam directly (~317 A g-1), indicating that GDY could not only increase the loading of Co, but also have some effect on the activity. In addition, the GDY supported on Cu foil was evaluated by LSV. Both GDY/Cu foil and Co NPs/GDY/Cu foil displayed lower OER catalytic activity than their corresponding 3D foam substrates (Figure S7), highlighting the exceptional advantanges of the unique 3D configuration. Therefore, the superior electrocatalytic OER performance may benefit from the synergistic effect between Co NPs, GDY and 3D Cu foam, where the Co NPs provide catalytically active sites for OER and GDY layer supported on 3D Cu foam serves as channels for mass and generated electrons transfer during rapid OER. Although it is hard to compare the catalytic performance with other carbon based support such as graphene-Co NPs at this stage due to the varied growth conditions used, the above obtained values are comparable to the best values of Co NPs based catalysts reported in the literatures.35-36

Figure 2. (a) Full XPS spectra of Cu@GDY/Co electrode; (b) high resolution XPS spectra of C 1s and (c) Co 2p for Cu@GDY/Co electrode; (d) Raman spectra of Cu@GDY and Cu@GDY/Co electrodes. The electrocatalytic OER activity of Cu@GDY/Co electrode was measured by linear sweep voltammetry (LSV) in 0.1 M KOH solution. For comparison, the electrocatalytic activity of bare Cu foam and GDY supported on Cu foam was also measured. As shown in Figure 3a, the bare Cu foam displayed no apparent anodic current within 1.6 V vs RHE and there were no significant difference in OER activity after the growth of GDY, revealing that GDY itself has almost no catalytic activity for OER. However, the Cu@GDY/Co foam exhibited a much higher catalytic current than that of the above two electrodes with an OER onset potential of ~1.53 V vs RHE, corresponding to an overpotential of nearly 0.3 V. At an applied potential of nearly 1.65 V vs RHE, the current density could achieve 10 mA cm-2. It should be pointed out that no iR-correction was performed in all the LSV curves. In consideration of compensated resistance of the electrochemical cell, the overpotential of Cu@GDY/Co

Figure 3. (a) LSV curves for the Cu foam, Cu@GDY and Cu@GDY/Co electrodes in 0.1 M KOH; (b) Tafel plots of the corresponding electrodes; (c) the corresponding electrochemical impedance plots at the potential of 1.60 V vs RHE; (d) the chronoamperometric test of the Cu@GDY/Co electrode at 1.6 V vs RHE. Tafel plot and electrochemical impedance spectroscopy were employed to probe the OER catalytic activity of the prepared electrodes. As known to us, a smaller Tafel slope would be more conducive to OER, because it provides a larger OER rate with a decreasing overpotential. The much smaller tafel slope of Cu@GDY/Co electrode (148 mV/decade) than that of bare Cu foam and GDY/Cu

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foam(Figure 3b) suggested the high conductivity and OER rate of Cu@GDY/Co electrode. In addition, the EIS Nyquist plots showed that Cu@GDY/Co electrode exhibited the smallest semicircle among the three electrodes (Figure 3c), indicating the most efficient interfacial electron transfer for OER. The stability of Cu@GDY/Co electrode was tested by using a potentiostatic method at 1.60 V vs RHE. As shown in Figure 3d, the assembled electrode maintained the relatively constant current density during 4 h bulk water electrolysis (only nearly 5% decline). A faradaic efficiency of 85% for oxygen was obtained by using gas chromatography (GC) (Figure S8). SEM and TEM of the electrode after electrolysis were examined. Partial GDY nanowalls were destroyed under a high current density for a long time, and the small particles were aggregated into large particles in a few to tens of nanometers range (Figure S9). TEM images showed that the particles after electrolysis were still on the surface of GDY and from HRTEM image, we identified (200) lattice spacing of cubic structure of CoO with the interplanar spacing of 0.229 nm (Figure S10), indicating that most of Co nanoparticles were oxidized to CoO after electrolysis. The result was in line with the observation of high resolution XPS spectra of Co 2p after electrolysis (Figure S11). Moreover, the high resolution XPS spectra of C 1s could be deconvoluted into four types of peaks consisting of C-C (sp2) at 284.5 eV, C-C (sp) at 285.2 eV, C−O at 286.9 eV, and C=O at 288.5 eV, respectively, with slight increase of C−O and C=O (Figure S12), which could be ascribed to the partial oxidation of diacetylenic linkages between carbon hexagons. After electrolysis, Raman spectra of Cu@GDY/Co electrode also showed four typical peaks of GDY (Figure S13). However, these peaks became weaker than prior to electrolysis possibly arising from the dissociation and partial oxidation of some GDY from the Cu foam during electrolysis. In summary, we have demonstrated that GDY can be used as a novel catalyst-support to stablize metal NPs for OER. This hybrid electrocatalyst is successfully fabricated by an in situ chemical reduction strategy and the prepared electrode shows a superior OER activity with a low overpotential of nearly 0.3 V and a large unit mass activity of 413 A g−1 at 1.60 V vs RHE in 0.1 M KOH aqueous solution. Though the catalyst support is partially destroyed after electrolysis, the results presented profit from the exposure of sufficient catalytically active sites and improved conductivity, which not only affords a simple and facile method to prepare nanocomposite, but also enriches the family of catalyst support to stabilize NPs. The concept of catalyst-support with GDY to enhance dispersity and conductivity of catalyst is certainly not confined to Co NPs, but should be extended to other catalysts for catalytic applications and related works are in progress in our laboratory. ASSOCIATED CONTENT Supporting Information. Experimental procedures, methods, and product characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author [email protected] [email protected]

Author contributions J.L. and X.G. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technology of China (2014CB239402, 2016YFA0200104, and 2013CB834505), the National Science Foundation of China (91427303, 21390404, 51432002 and 50973125) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17030300), and the Chinese Academy of Sciences..

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