Deep-Eutectic Solvents Derived Nitrogen-Doped Graphitic Carbon as

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Deep-eutectic solvents derived nitrogen-doped graphitic carbon as a superior electrocatalyst for oxygen reduction Rui Luo, Chao Liu, Jiansheng Li, Chaohai Wang, Xiuyun Sun, Jinyou Shen, Weiqing Han, and Lianjun Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09707 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 14, 2017

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ACS Applied Materials & Interfaces

Deep-eutectic solvents derived nitrogen-doped graphitic carbon as a superior electrocatalyst for oxygen reduction

Rui Luo, Chao Liu, Jiansheng Li*, Chaohai Wang, Xiuyun Sun, Jinyou Shen, Weiqing Han, Lianjun Wang*

Key Laboratory of Jiangsu Province for Chemical Pollution Control and Resources Reuse School of Environment and Biological Engineering Nanjing University of Science and Technology, Nanjing 210094, China. Tel: +(86) 025-84315351 E-mail: [email protected]; [email protected]

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Abstract: The activity and stability of electrocatalyst for oxygen reduction reaction (ORR) essentially depends on its structural and compositional properties. Herein, we report the facile preparation of nitrogen-doped graphitic carbon (NGC) via pyrolysis of deep-eutectic solvents (DESs) as a superior electrocatalyst for ORR. The resulting NGCs possess high surface areas, rich nitrogen content and favorable graphitization degree, which are highly desired for ORR catalysts. The effects of the pyrolysis temperature on the ORR performance of the final products are explored. The results implied that the material fabricated at 900 oC (NGC900) is identified as the best ORR catalyst in the series of samples. Specifically, NGC900 shows efficient performance toward ORR with an onset potential of 0.97 V and a half potential of 0.84 V, which bears comparison with commercial Pt/C catalyst with enhanced stability in alkaline media. The superior ORR performance of NGC900 may be ascribed to the balance between surface area, pyridine-nitrogen and defect of NGCs. The rational design of NGCs with efficient ORR activity and stability based on the low-cost DESs implies adequate support for the development of energy devices in practical application. Keywords: N-doped graphitic carbon; deep-eutectic solvents; oxygen reduction; electrocatalyst; activity and stability.

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Introduction Fuel cell is widely deemed to be a promising technology for sustainable development, where the oxygen reduction reaction (ORR) is the crucial process.1 Pt-based electrocatalysts demonstrated the state-of-the-art performance for ORR, while the low reserves and high cost of noble metal hindered the implementation for commercialization of fuel cell.2,3 In this context, tremendous effort has been expended for developing cost-efficient and outstanding electrocatalysts.4-7 Currently, significant advances have been achieved in studying noble-metal-free (M-N-C) and heteroatom-doped carbon catalysts.8-11 Among the various catalysts, nitrogen-doped carbon materials received much attention due to the high stability and excellent activity.8 Very recently, the surface charge distribution of nanocarbon induced by pyridinic nitrogen was proved to be the heart of the high performance of nitrogen-doped carbon for ORR.12 Based on the discovery, extensive research was concentrated on the synthesis of nanoporous carbon enriched with pyridinic nitrogen. For achieving the objective, carbonization of unique precursors containing nitrogen element is considered as an effective pathway. Zhao et al. recently reported the synthesis of porous carbon with high nitrogen content derived from metal-organic frameworks.13 Using a brilliant strategy, Shi et al. have successfully designed molecule precursor with the function of self-polymerization based on dopamine to improve the nitrogen content of carbon.14 The derived nitrogen-doped carbon with higher nitrogen content displayed superior catalytic activity for ORR. In spite of that, the facile and scale-up synthesis of nitrogen-rich carbon with outstanding catalytic efficiency derived from cost-efficient precursors is highly desired towards the practical applications of ORR. Deep-eutectic solvents (DESs) are molecular complexes typically formed with two components, one hydrogen-bond acceptor and one hydrogen-bond donor.15 As an analogue of ionic liquids, DESs have received special attention because of the diversity and multifunctional property.16-18 Leveraging the features, carbonization of DESs has been regarded as an appealing way to prepare heteroatom-doped carbon, especially nitrogen-doped carbon.16,19 For instance, Francisco and co-authors have reported a series of nitrogen-doped porous carbon derived from DESs containing choline chloride.20,21 Recently, a detail study of tailoring the textural properties of porous carbon by controlling the components and ratio was reported.22 Very lately, a new class of DESs composed of phenols/ketones and urea with hydrogen-bond interaction was presented for preparing 3



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nitrogen-doped carbon with high pyridinic nitrogen content.23 For this DES system, dopant and graphitization of carbon were integrated tactfully. Thus, DESs based on the phenols/ketones and urea is promising pathway to achieve the goal: the convenient synthesis of prominent electrocatalyst based on cost-efficient precursors for practical application of ORR. In this work, we first report a novel and convenient synthesis of nitrogen-doped graphitic carbon (NGC) as a superior electrocatalyst for ORR, where tannin acid, urea and zinc chloride were employed as carbon precursor, nitrogen precursor and template, respectively. Taking advantage of hydrogen-bond interaction, NGC was prepared by precondensation and precarbonization of tannin and urea induced by evaporation, followed by carbonization at high temperatures. Zinc chloride not only serves as the function of template but also plays an important role to promote the precondensation process. The resulting NGCs possess high surface areas, rich nitrogen content and obvious graphitization degree. Furthermore, the results of electrochemical tests demonstrated approximate catalytic performance of commercial Pt/C for ORR with improved stability. Hence, the rational design of nitrogen-doped carbon based on the low-cost DESs provides adequate support for the development of ORR in practical application.

Experimental Section Material preparation NGCs were synthesized by precondensation and precarbonization of tannin acid (AR) and urea (99.5%) induced by evaporation using ZnCl2 (AR) as template, followed by carbonization at high temperatures. In brief, 0.5 g tannin and 0.5 g urea were dissolved in 10 mL of deionized water in ceramic crucible at room temperature, and then 10 g of ZnCl2 was added (none for NGC0) and stirred for 10 min to form a uniform solution. The solution was further transferred to the oven and hold 30 h at 105oC for volatilization. Then, the resulting mixture was annealed for 1 h at different temperatures (700-1000 oC) with a ramp rate of 5 oC/min under 50 sccm high-purity N2 atmosphere. After carbonization, the solid was washed in 3 M HCl solution for 12 h to remove Zn impurity. Finally, the NGCs were obtained after washing with deionized water several times and drying at 80 o

C for 12 h.

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Characterazition The morphology and structure of NGCs were determined by scanning electron microscopy (SEM, FEI Quanta 250F system) and transmission electron microscopy (TEM, FEI Tecnai 20 electron microscope). N2 adsorption-desorption isotherms of NGCs were analyzed by Micromeritics ASAP-2020 at 77 K. Based on the Brunauer-Emmett-Teller and nonlocal density functional theory method, the specific surface area and the pore size distribution of samples were calculated. The X-ray diffraction (XRD, Bruker D8) was used to analyze the graphitization of NGCs at 40 kV and 40 mA. Raman spectroscopy was conducted at Renishaw in Via reflex spectrometer system. The measurements of X-ray photoelectron spectroscopy (XPS) were conducted on a photoelectron spectrometer (PHI Quantera II ESCA System). Inductive Coupled Plasma Emission Spectrometer (Optima 7000DV) was used to detect the content of Zn in NGCs. Electrochemical measurement Electrochemical measurements of NGCs for ORR were performed in a three-electrode cell (CHI 760E, CH Instrument, Shanghai) at 25 oC, where NGCs-modified glassy carbon, Ag/AgCl (3M) electrode and platinum wire were the working electrode, reference and counter electrode, respectively. The preparation of working electrode is as following: 5 mg of catalyst was first dispersed in 1mL mixed solution of water/isopropanol (80:20). Then, 50 µL of the suspension was dripped onto a RDE electrode with diameter of 5 mm. After drying, 5 µL of nafion solution (5% in isopropanol) as the binder was dripped onto the RDE electrode. Finally, the electrode was dried at 70 o

C for 3 h. Cyclic voltammetry (CV) tests of catalysts were measured from 0.1 to 1.1 V vs. RHE

(ERHE=EAg/AgCl + 0.965) in N2/O2-saturated 0.1 M KOH aqueous solution (the scan rate: 10 mV/s). Linear sweep voltammogram (LSV) measurements of NGCs and Pt/C (the loading content of Pt is 20%) were performed under various rotation in O2-saturated 0.1 M KOH aqueous solution and the background values in N2-saturated 0.1 M KOH solution were subtracted (scan rate: 10 mV/s). Current-time (I-t) tests of NGC900 and Pt/C (20%) were executed at rotation rate of 1600 rpm in O2-saturated 0.1 M KOH aqueous solution. The electrochemical impedance spectroscopy (EIS) measurements were operated in the frequency range of 0.01-100000 Hz with 5 mV AC amplitude. The electron transfer numbers (n) were calculated according to Koutecky-Levich (K-L) equation as following: 5



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1 1 1 = + 1/2 j jk Bw n=

B 0.2 F ( Do )

2/3

(V )

−1/6

CO 2

Where w is the rotation rate (rpm), F is the Faraday constant (96485 C mol-1), Do is the diffusion coefficient of oxygen (1.9×10-5 cm/s, 0.1 M KOH), V is the kinetic viscosity (0.01 cm/s, 0.1 M KOH), and Co2 is the concentration of oxygen (1.2×10-6 mol cm-3).

Result and discussion

Scheme 1 Illustration of the preparation of the NGCs based on DESs of tannin and urea

Scheme 1 reveals the simple protocol for the preparation of the NGCs based on DESs. During the process of fabrication, ZnCl2 serves as the function of template and an auxiliary to promote the precondensation process. As shown in Figure S1, darkgrey solid formed after precondensation only when ZnCl2 was added, but another two samples transferred to melt. This verifies the important role of ZnCl2 in the preparation process. Besides ZnCl2, the precondensation of precursors also could be achieved by other salts including AlCl3, NaCl, KCl, FeCl3 and CoCl2 (Figure S2), which might be due to the hypersaline condition that plays a role in the precondensation process.24 The samples obtained after carbonization at different temperature were denoted as NGC700, NGC800, NGC900 and NGC1000, respectively. The morphology and structure of NGCs were determined by SEM and TEM. NGC900 consists of assembled clusters with rough surface and 6


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shows the interconnected spherical shape with abundant interval space like aerogel structure (Figure 1a), which is a typical morphology of the carbon based on DESs.25,26 This sort of morphology should be explained by the formation of a polymer-rich phase during the polycondensation stage.25 Meanwhile, abundant mesopores and micropores were observed in TEM image (Figure 1b). Moreover, NGC700, NGC800 and NGC1000 demonstrate the similar morphologies and structures (Figure S3). The high resolution TEM (HRTEM) images show obvious interplanar crystalline space of 0.349, 0.368, 0.386 and 0.403 nm for NGC700, NGC800, NGC900 and NGC1000 (Figure 1c and S3), respectively, which shows an increasing trend with the higher carbonization temperatures and indicates the graphitic structure of NGCs.27,28 EDS elemental mapping clearly reveals the N and C elements homogenously distribute throughout the material (Figure 1d), indicating the successful doping of N.

Figure 1 Structural characterizations of NGC900. (a) SEM, (b) TEM, (c) HRTEM images, (d) EDS elemental mapping of NGC900

Figure 2a shows the adsorption-desorption N2 isotherms of NGCs. According to the N2 adsorption curves, NGCs exhibit conspicuous type IV isotherms and type IV hysteresis loops, suggesting the hierarchical structure with micropores and mesopores.29 The NGCs show high BET 7



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surface areas, which reach to 1180, 1387, 1192 and 801 m2/g for NGC700, NGC800, NGC900 and NGC1000, respectively. However, the NGC obtained without adding ZnCl2 (NGC0) shows low BET surface areas of 35 m2/g (Figure S4), indicating the significant contribution of ZnCl2 to pore structures of NGCs.30-32 The surface areas of NGCs show a little increase from pyrolysis temperature of 700 to 800oC, while NGC900 and NGC1000 exhibit a slight decrease due to the destruction of pores in the graphitization process at elevated temperatures.33 The total pore volumes of NGCs are in the range of 0.48-0.79 cm3/g (Table S1). As seen from Figure 2b, similar pore distributions of the samples were observed, which demonstrates the pore size of NGCs mainly distribute in micropores (0.6-2 nm) and mesopores (2-7 nm). The hierarchical pores benefit the mass transfer of reactants and reaction products in ORR.

Figure 2 (a) N2 adsorption-desorption isotherms of NGCs, (b) the distributions of pores of NGCs derived from the N2 adsorption isotherms

Raman spectroscopy and X-ray diffraction (XRD) are efficient methods for discerning the graphitization and disorder of carbon-based materials. Analysis of NGCs by XRD exhibits obvious diffraction peaks and weak peaks around 25 and 43o, corresponding to the d-spacing and in-plane structure, respectively (Figure 3a).34 This is indicative of the graphitic structure of NGCs.35,36 Moreover, according to the change of peaks around 43o, pyrolysis of the precursor under higher temperature makes pronounced aromatic stacking peaks.23 However, the peaks around 25o show slight left shift and the intensity displays a little decrease, suggesting disorder and larger interlayer distance of NGCs that coincides exactly with the HRTEM results (Figure 1c and S3).37 In addition, there is no peak of Zn species in the samples that means complete removal of impurity. The Zn 8


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contents of NGCs were further determined by ICP, and almost no Zn species (

Deep-Eutectic Solvents Derived Nitrogen-Doped Graphitic Carbon as

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