CdS-Polydopamine-Derived N,S-Codoped Hierarchically Porous

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CdS-polydopamine derived N, S co-doped hierarchically porous carbons as highly active electrocatalyst for oxygen reduction Hui Zhao, Chen-Chen Weng, Zhong-Pan Hu, Li Ge, and Zhong-Yong Yuan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01875 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 22, 2017

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ACS Sustainable Chemistry & Engineering

CdS-polydopamine derived N, S co-doped hierarchically porous carbons as highly active electrocatalyst for oxygen reduction Hui Zhao,a,b Chen-Chen Weng,a,b Zhong-Pan Hu,a,b Li Ge,a,b Zhong-Yong Yuan a,b,* a

National Institute for Advanced Materials, School of Materials Science and Engineering, Nankai University, Tianjin 300350, China. E-mail: [email protected] b

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education). Collaborative

Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China

* Corresponding author: Professor Zhong-Yong Yuan National Institute for Advanced Materials, School of Materials Science and Engineering, Nankai University, Tongyan Road 38, Haihe Educational Park, Tianjin 300353, China E-mail: [email protected]

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Abstract Rational design of highly active electrocatalyst for oxygen reduction reaction (ORR) is critical for several advanced energy conversion and storage technologies such as fuel cells and rechargeable metal-air batteries. Engineering carbonaceous materials with heteroatoms can achieve optimal electronic and porous structures and show considerable electrocatalytic performance. In this work, a facile and highly efficient method for nitrogen and sulfur incorporation into carbon skeleton has been developed based on CdS-polydopamine composites to derive the N, S-co-doped hierarchical porous carbons. CdS plays an important role in the formation of this unique structure and the sulfur-doping. Through pyrolyzing under inert atmosphere, the CdS-polydopamine can be easily transformed into N and S co-doped porous carbons. The resultant N, S-co-doped carbons possess hierarchically porous structures with high specific surface area, demonstrating superior ORR performance which is higher than that of commercial Pt/C catalyst in alkaline media in terms of onset potential, half-wave potential and diffuse limiting current density. The high ORR performance is also shown in both neutral and acidic media. In addition, the much higher stability and better methanol tolerance than Pt/C allow them to be a potential candidate for large-scale practical applications. Keywords: CdS, polydopamine, hierarchically porous carbon, oxygen reduction

INTRODUCTION Tremendous researches concerning novel metal-free materials with highly efficient catalytic performance have been conducted over a long period. Carbon

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materials have gained great attention due to their abundant resources, high electronic conductivity, tailored physicochemical property, good thermal stability and environmental acceptability.1-4 Although the great progress has been made in exploring carbon materials as high-efficiency metal-free catalysts, the large-scale preparation and commercialization is still not satisfactory. It is crucial that the properties of carbon materials should satisfy the need of future applications. Recently, the application of carbon materials in the field of energy conversion and storage has been a hot research topic of high priority.5-8 With the continuous depletion of fossil fuels, it is urgent to find clean and high-efficiency energy resources. Many advanced energy conversion and storage technologies such as fuel cells and rechargeable metal-air batteries have been attracted much attention. Nevertheless, their development is limited by the slow kinetics of electrocatalytic reactions such as oxygen reduction (ORR) that occurs on the cathode of fuel cells and anode of rechargeable metal–air batteries.9-11 Noble metal Pt-based catalysts are the conventional materials used for catalytic ORR process, but the high cost, poor methanol resistance and stability hinder their further application.12 Many researchers have endeavored to develop the carbon nanomaterials with highly efficient catalytic performance. Generally, the properties of carbon can be controlled by two strategies, the design of pore structure and the introduction of functional groups within the carbon framework or onto the carbon surface.13-15 Pore structure of carbon materials, as a critical parameter that affects the catalytic performance, has been extensively studied.16,17 Microporous carbons possess a high surface area and favor the exposure of active sites. However, microporous structure often leads to the poor reactant accessibility which greatly

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limits its utilization and mass transportation. On the contrary, macroporous carbons can provide good reactant accessibility to allow the mass diffusion, yet the low surface area hinders the exposure of active sites, further declining the catalytic performance. Mesoporous carbons inherit both advantages that allowing the exposure of active sites and the good mass transportation. Notably, designing hierarchically porous carbons which combine micro-, meso- and macroporosity should be a more promising way to achieve the optimal catalytic activity. Besides pore structure, the introduction of functional groups is also an important strategy governing the surface properties of carbons. For example, carbon nanotube

(CNT)

and

graphene

can

be

functionalized

by

poly(diallyldimethylammonium chloride) (PDDA) to create net positive charge for facilitating the ORR reaction.18,19 Additionally, doping heteroatom into the carbon framework can not only modify the electronic structure but also form additional functional groups on the carbon surface.20 Among various heteroatoms, nitrogen is the most popular one. N-doped carbons have been used in many fields such as CO2 adsorption, fuel cells, supercapacitors and batteries.21-26 Nitrogen can generate catalytic/basic sites in the carbon, increasing the chemical reactivity for redox or acid-base chemistry.5 Through the pyrolysis of N-containing precursors or post-treatments, nitrogen can be facilely introduced. Also, porous carbons co-doped with different heteroatoms,

such as N, P-co-doped and N, P,

B-co-doped carbons, have been reported as metal-free electrocatalysts for ORR.27-30 According to the previous reports, there exhibits synergistic effect between nitrogen and sulfur which can remarkably promote the catalytic performance. N, S-co-doped carbons, such as N, S-co-doped graphene, N, S-codoped microporous carbon nanobelts and N, S-codoped carbon nanosheets,

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have been employed in the fields of fuel cells and batteries.

31-33

However, the

synthetic routes for N, S-co-doped carbons often involve complex experimental conditions and toxic precursors.34,35 Hence, it is still a great challenge to explore a facile and efficient method to achieve N, S-co-doped porous carbons. In addition, it is of great significance to integrate the hierarchically porous structure with optimal heteroatom doping. In this work, we synthesized N, S-co-doped carbons with hierarchically porous structure and high surface area by utilizing CdS as the template, dopamine as the nitrogen and carbon source. Dopamine can self-polymerize into polydopamine on the surface of CdS.36 During the high-temperature calcination under inert atmosphere, polydopamine carbonized into N-doped carbons and reduced Cd2+ into Cd, further occurred the evaporation of Cd due to its low boiling point (765 °C).37 The evaporation of Cd leads to the presence of abundant pores in carbon skeleton. Finally, N, S-co-doped carbons with hierarchically porous structure and high surface area were achieved. The resulting carbons exhibit much higher ORR performance, better methanol tolerance and stability than the commercial Pt/C catalyst in 0.1 M KOH solution. Besides, the ORR performance is excellent in neutral media as well. It is found that the pyrolysis temperature has a great influence on the electrocatalytic performance.

EXPERIMENTAL Preparation of CdS nanomaterials. CdS nanoparticles were prepared by using our previously reported method.38 Briefly, 30 mmol L-1 of Na2S ethanol solution was added dropwise into 30 mmol L-1 of Cd(NO3)2 ethanol solution. After stirring for 15 min at room temperature, the 5



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mixture was immersed in ultrasonic water bath with mechanical stirring for 5 min, followed by stirring for several hours. The product was collected and washed by water and ethanol, and dried at 80 °C for 6h. Preparation of N, S-co-doped hierarchically porous carbons. For the preparation of N, S-co-doped hierarchically porous carbons, 0.2 g of CdS was dispersed in 25 mL tris-HCl solution (10 mmol, pH 8.5). After stirring for 1h, 0.2 g of dopamine hydrochloride was slowly dropwise introduced into the above mixture, followed by stirring for 6h at room temperature (25 °C). The obtained precipitates were collected by centrifugation, washed by water and ethanol, and dried at 80 °C for 6h in a vacuum oven. Then the resultant powder was calcined at a certain temperature for 2h under N2 atmosphere with a ramp rate of 5 °C min-1. The final product was denoted as NSC-T, where T represents the calcination temperature. NSC-T-acid was used as the code name of NSC-T dealt with acid. Preparation of N-doped porous carbon. For comparison, N-doped porous carbon was prepared without the addition of CdS. The resulting product was calcined in N2 at 1000 °C for 2h and named as NC-1000. Characterizations The scanning electron microscopy (SEM) images were obtained on a Jeol JSF-7500L microscope at 5 kV. X-ray diffraction (XRD) study was conducted on a Bruker D8 Focus Diffractometer with Cu-Kα radiation (λ=0.15418 nm), with an operation voltage at 40 kV and current at 40 mA. X-ray photoelectron spectroscopy (XPS) were collected from a Kratos Axis Ultra DLD (delay line detector) spectrometer with Al-Kα as the X-ray source (1486.6 eV). A Quantachrome Autosorb-1MP sorption analyzer was used to measure N2 adsorption−desorption 6


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isotherms at -196 °C. Prior to measurement, all the samples were outgassed at 200 °C for 12h. Surface areas were calculated using adsorption data in a relative pressure range 0.05-0.3 by the multipoint Brunauer-Emmett-Teller (BET) method. Total pore volume was calculated from the volume adsorbed at a relative pressure of 0.99. Pore size distribution curves were computed from the adsorption branches using the Barrett, Joyner, and Halenda (BJH) method. Raman spectroscopic analysis was carried out on a Thermo-Fisher Scientific DXR spectrometer with a laser radiation of 514 nm. Electrochemical measurements 2 mg of the catalyst was ultrasonically dispersed in 1 mL of distilled water (Milli-Q) to obtain a catalyst ink. 20 µL of the ink was drop casted onto a polished glassy carbon electrode with an active diameter of 5 mm. After drying under ambient conditions overnight, 3 µL of 0.5 wt% Nafion aqueous solution was coated on the electrode and dried in air. The resulting electrode was used as the working electrode. All electrochemical tests were conducted using a three-electrode system. Platinum wire was used as the counter electrode, and the reference electrode was Ag/AgCl. The electrochemical data were collected using an electrochemical analysis station (Pine, USA). All the potentials initially measured versus Ag/AgCl (3 M KOH) were converted to the ones versus the reversible hydrogen electrode (RHE) by adding a value of (0.205+0.059×pH) V. For the ORR, the electrolyte (0.1 M KOH or 0.1 M PBS) was saturated with O2 during the recording of electrochemical measurements. Cyclic voltammograms (CV) was first performed at a scan rate of 50 mV s-1 in a potential range of 1.2 - 0 V (vs RHE) for 100 cycles. Then, linear sweep voltammograms (LSV) and CV were carried out in a potential range of 1.2 - 0 V (vs RHE) at a scanning rate of 5 and 20 7



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mV s-1, respectively. Rotating ring-disk electrode (RRDE) measurements were performed by using a glassy carbon disk with a Pt ring. The electron number (n) and peroxide percentage were calculated by following equations: H O =

=

⁄

⁄

(1)

× 100%

(2)

⁄

where iR, ID and N refer to the ring current, disk current and collection efficiency, respectively.

RESULTS AND DISCUSSION The synthesis process of N, S-co-doped hierarchically porous carbons is shown in Scheme 1. CdS nanomaterials were first obtained by a simple sonochemistry– assisted synthesis method. As shown in Figure S1, there exhibits a large number of CdS particles of several nanometers in size. The form of CdS-polydopamine composite involved the self-polymerization of dopamine on the surface of CdS in weak alkaline condition at room temperature. Accordingly, N, S-co-doped carbons could be obtained by direct pyrolysis of CdS-polydopamine composite in the nitrogen atmosphere, wherein CdS and polydopamine perform as heteroatom sources for S and N doping, respectively. To be specific, polydopamine can be pyrolyzed to derive the N-doped carbons and further reduce Cd2+ to metal Cd under inert atmosphere. Instantaneously, the evaporation of Cd occurred owing to its low boiling point (765 °C). Cd and S vapors could transfer freely through the carbon framework. The abundant pores were formed during the evaporation of Cd. Herein, the synthesis of N, S-co-doped porous carbons with homogenously distributed N and S could be achieved. The Cd metal vapor can be collected by the 8


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stove plug in tube furnace during the pyrolysis procedure, so this synthesis method is environmentally friendly.

Scheme 1 Schematic illustration of N, S-co-doped porous carbon catalyst preparation process.

Figure 1 XRD patterns of (a) CdS and the as-synthesized carbon catalysts, (b) NSC-800-acid and NSC-900-acid.

The generation of carbon can be verified by wide-angle powder X-ray diffraction (XRD). Figure 1a shows that the CdS nanomaterial presents diffraction peaks of hexagonal cadmium sulfide phase (PDF#65-3414). For NC-1000 and NSC-1000, a broad diffraction peak centered at 2θ = 24° is ascribed to the carbon (002) peak, suggesting a low graphitization degree.39 NSC-800 and NSC-900 possess obvious diffraction peaks of CdS. This reveals that CdS phase still exists after calcination at 800 and 900 °C. Therefore, it is necessary to treat NSC-800 and NSC-900 with 9



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acid to remove CdS species. As shown in Figure 1b, the characteristic diffraction peaks of CdS can no longer be detected after acid treatment, instead a broad weak peak situated at 2θ = 24° is clearly observed, indicating the transformation to N,S-co-doped carbons. SEM was conducted to assess the morphologies and microstructure of the carbon materials. The NC-1000 without using CdS exhibits a plate-like morphology (Figure 2d). The NSC-800 possesses a compact surface with several cracks resulting from the evaporation of Cd (Figure 2a). The NSC-900 exhibits both compact and unconsolidated surfaces with the increase of the calcination temperature (Figure 2b). For NSC-1000, the entire carbon surface becomes unconsolidated and emerges abundant large pores (Figure 2c). It is observed that the NSC-1000 is composed of a great deal of small nanoparticles. These large pores and unconsolidated surfaces may be favorable for the mass/charge transfer during the electrocatalytic process. SEM-EDS mapping confirmed that the uniform distribution of C, N, S, O elements in the NSC-1000 and C, N, O elements in the NC-1000 (Figure S2 and S3). The results above suggest that CdS-polydopamine composite can serve as a reliable precursor to synthesize N, S-co-doped porous carbons. The dopamine can adsorb on the surface of CdS, and subsequent high-temperature carbonization results in the evaporation of Cd metal from the thermal reduction of Cd2+ by the polydopamine-derived carbonaceous materials. Cd and S can freely diffuse through the carbon skeleton to form the N and S homogeneously distributed porous carbons.

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Figure 2 SEM images of (a) NSC-800-acid, (b) NSC-900-acid, (c) NSC-1000, (d) NC-1000.

XPS measurements were carried out to examine the surface chemistry of the synthesized carbon materials. As depicted in Figure 3a, the XPS survey scans of NSC-1000 verify the existence of C, O, N and S elements. Deconvolution of C 1s spectra reveals the presence of C-C（284.6 eV），C-N (286.0 eV), C=O (287.1 eV), O-C=O (288.3 eV) (Figure 3b).40 The high-resolution N 1s spectra can be resolved into three contributions, ascribed to pyridinic N (398.6 eV), graphitic N (401.2 eV), and pyridinic-Noxides (403.3 eV) (Figure 3c).41 According to the reports, pyridinic N and graphitic N are both favorable for the formation of electrocatalytic active sites.42 The atom percentage (atom%) of the nitrogen increases with the increase of pyrolysis temperature (Table S1). The pyridinic nitrogen content in the carbons decreases while the graphitic nitrogen content increases with the increase of the pyrolysis temperature (Table S2), indicating there exhibits more graphitic nitrogen on the surface of NSC-1000. With respect to the S 2p spectra, two peaks situated

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at 164.1 and 165.3 eV correspond to S 2p3/2 (-C-S-) and S 2p1/2 (-C-S-), respectively, and a weak peak at 168.5 eV corresponds to oxygen-containing sulfur groups (such as –SOx-) (Figure 3d).43 Notably, the atom% of the sulfur is found to be relatively low and it decreases with the increase of the pyrolysis temperature. On the basis of the XPS results, it can be confirmed that N and S have been successfully introduced into the carbon materials. The XPS data for NSC-800-acid, NSC-900-acid and NC-1000 are displayed in Figures S4, S5 and S6, respectively.

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Figure 3 (a) XPS survey, (b) C 1s, (c) N 1s and (d) S 2p spectra of NSC-1000. (e) Raman spectra and (f) N2 adsorption-desorption isotherms of the synthesized carbons.

Raman analysis is an effective method to investigate the structural properties of carbon materials. As shown in Figure 3e, all the carbon materials show typical D and G bands at 1360 and 1590 cm-1, respectively. The D band corresponding to the 13



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defects, edges and disordered carbon sites and the G band deriving from E2g vibration of sp2–hybridized graphitic carbon. The higher peak appeared at 2700 cm-1 could be ascribed to 2D bands.16 The intensity ratio of D and G bands (ID/IG) indicates the degree of defects in the carbon. It is found that NSC-1000 exhibits higher ratio value (1.48) than NC-1000 (1.20), signifying that NSC-1000 possesses more disordered structures, which can be ascribed to the introduction of defects by the S-doping. As the pyrolysis temperature decreased from 1000 to 900°C and 800 °C, the ratio of ID/IG shrinks from 1.48 to 1.26 and 1.13, respectively, indicating the high degree of disordered carbons in NSC-1000 (Table 1).44 Figure 3f and Figure S7 show the N2 adsorption-desorption isotherms and pore size distribution curves of the synthesized carbon materials, and the corresponding porous characteristics are listed in Table 1. For N,S-co-doped carbon materials, the adsorption below P/P0

CdS-Polydopamine-Derived N,S-Codoped Hierarchically Porous

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