Research Article pubs.acs.org/journal/ascecg
Synthesis of Nitrogen-Doped Porous Carbon Spheres with Improved Porosity toward the Electrocatalytic Oxygen Reduction Daguo Gu,† Ruguang Ma,*,‡ Yao Zhou,‡ Fangfang Wang,‡ Kang Yan,*,§ Qian Liu,*,‡,∥ and Jiacheng Wang*,‡,∥ †
School of Materials Engineering, Yancheng Institute of Technology, 1 Xiwang Avenue, Yancheng 224051, Jiangsu Province P. R. China ‡ State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, P. R. China § State Key Laboratory of Mechanics and Control of Mechanical Structures, College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, P. R. China ∥ Shanghai Institute of Materials Genome, 99 Shangda Road, Shanghai, 200444, P. R. China S Supporting Information *
ABSTRACT: In this study, a series of activated N-doped porous carbon spheres (ANCSs) have been prepared from biomass as the carbon source to be used as highly active and stable electrocatalysts toward the electrocatalytic oxygen reduction reaction (ORR). Hydrothermal carbonization of biomass glucose, which obtains uniform carbon nanopsheres, is followed by doping N atoms by treatment in ammonia and subsequent activation treatment to form ANCSs. The resultant ANCSs possess a large specific surface area of up to 2813 m2/g and pore volume of up to 1.384 cm3/g, and adjustable N contents (2.38−4.53 atom %) with increasing activation temperature. The graphitic and pyridinic-N groups dominate in various N functional groups in the ANCSs. Remarkably, the 1000 °C-activated sample demonstrates competitive activity and outstanding stability and methanol crossover toward the ORR with a four-electron transfer pathway in alkaline media compared to commercial Pt/C catalyst. This excellent performance should be mainly due to effective N-doping and high porosity which can boost the mass transfer and charge transfer and provide a larger number of active sites for the ORR. The unique spherical morphologies with improved porosity as well as excellent stability and recyclability make these ANCSs among the most promising ORR electrocatalysts in practical applications. KEYWORDS: Carbon nanospheres, N-Doping, Porosity, Electrocatalysis, Oxygen reduction reaction
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INTRODUCTION The rapid increase in the use of traditional fossil fuels results in a serious global energy crisis and environmental pollution, significantly imperilling human health and survival.1,2 Therefore, it is necessary to search for economic and sustainable clean energy to replace traditional fossil fuels. The fuel cells invented in the 1830s can directly convert the chemical energy of fuels to electricity through electrochemical processes.3−5 The energy conversion efficiencies of fuel cells are about two times greater than those of the conversion fire-powered plants. Oxygen electrochemistry plays an important role in the development of such advanced energy conversion systems.6 Unfortunately, the sluggish oxygen reduction reaction (ORR) kinetics at the cathode significantly limits their performance and efficiency,7−9 which becomes an obstacle on the way to practical applications.10 Thus, various electrocatalysts could be explored to reduce the ORR overpotentials.11,12 The noble metal platinum (Pt) and its alloy have been considered as the most © XXXX American Chemical Society
active ORR electrocatalysts in fuel cells. However, these catalysts suffer from various drawbacks such as high cost, poor stability, methanol crossover, and low natural abundance.4,13 During recent years, much effort has been made to develop a wide range of efficient alternative catalysts,12,14−19 such as metal-N doped carbons,20,21 transition metal oxides,22,23 carbides and nitrides,24 carbon-based materials,16,25,26 and so on.27,28 Among various non-noble metal-based ORR electrocatalysts, carbon-based materials have received much attention due their essential properties including excellent electronic conductivity, low cost, lightweight, adjustable porosity, controllable heteroatom doping and high resistance to chemical corrosiveness.29−34 However, the electrochemical activity and stability of Received: September 1, 2017 Revised: September 19, 2017
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DOI: 10.1021/acssuschemeng.7b03046 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
lined stainless-steel autoclave and heated at 180 °C for 8 h. After cooling down to room temperature, the as-formed CSs were collected by centrifugation, washed three times with deionized water, and finally dried at 50 °C under vacuum for 8 h. The NCSs were prepared by subsequent calcination of CSs in NH3 flow to introduce nitrogen atoms and remove abundant oxygen atoms in the carbon framework.61,62 Preparation of Activated NCSs (ANCSs) by Treatment with CO2. Further activation was performed by heating NCSs at the expected temperature (800−1050 °C) with a ramp rate of 10 °C/min, and then holding for 0−90 min under a CO2 flow of 50 mL/min. After cooling to room temperature, the activated samples were collected and weighed, and were labeled as ANCS-X-Y, where ANCS means ‘activated nitrogen-doped carbon sphere”, X stands for the activation temperature in °C, and Y is the activation time in min. At the same time, the NCSs were treated at 1000 °C for 45 min in Ar flow as a control sample (named as NCS-1000-45 (Ar)). For comparison, pure N-free CSs were activated at 1000 °C for 45 min in CO2 flow, and the resulting material was named as CS-1000-45. Structural Characterization. Nitrogen sorption isotherms were measured at liquid nitrogen temperature (−196 °C) with an ASAP 2010 accelerated surface area and pore size analyzer system (Micrometitics, Norcross, GA). Prior to the measurement, the samples were treated at 300 °C overnight under vacuum. The specific surface area was calculated using the Brunauer−Emmett−Teller (BET) method. The total pore volume was calculated from the amount of nitrogen adsorbed at a relative pressure of 0.99. The pore size distribution curves were calculated from an analysis of the desorption branch of the isotherm using the Barrett−Joyner−Halenda (BJH) model. The morphology of the samples was examined by field emission scanning electron microscope (FESEM, JEOL JSM-5610) and transmission electron microscopy (TEM, JEM-2100F). The surface nitrogen content of the samples was determined by X-ray photoelectron spectroscopy (XPS) measurement recorded with an ESCALAB 250 X-ray photoelectron spectrometer using Al Kα (hv = 1486.6 eV) radiation to analyze the surface of the obtained samples. Electrode Preparation and Electrochemical Experiments. The carbon sample (5 mg) was dispersed in a mixture of 0.5 mL water and 0.5 mL ethanol under ultrasonic irradiation for ca. 2 h. Then 5% Nafion solution (25 μL) was added into the above suspension, and further ultrasonic treatment was performed until a homogeneous ink was formed. A 20 μL aliquot of ink containing 62.5 μg of catalyst was transferred onto the glassy rotating disk electrode with 5 mm diameter, yielding a catalyst level of 0.32 mg/cm2. For 40% Pt/C commercial catalyst (JM company), 31.2 μg catalyst was deposited on the glassy rotating disk electrode. The electrode with the catalyst was dried at 50 °C which was used as the working electrode for further electrochemical measurements. Electrochemical activity of the working electrode was studied by cyclic voltammetry (CV) and rotating disk electrode (RDE) using a standard three-electrode cell with a Pt plate as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. An aqueous solution of 0.1 M KOH was used as the electrolyte for the electrochemical studies. The electrochemical cell was controlled by a bipotentiostat (Pine Instrument Co.) equipped with a RDE. Prior to the measurements, the electrolyte was saturated by bubbling O2 or N2 for 15 min. The working electrode was cycled at least 30 times before the CV data were recorded at a scan rate of 50 mV/s. The RDE measurements were performed at different rotating rates varying from 400 to 1600 rpm with the scan rate of 5 mV/s. For the RDE tests, the CV measurement was performed from 0.1 to −1 V (vs SCE) in O2-saturated in 0.1 M KOH solution with a sweep rate of 50 mV/s for at least 30 cycles for the activating electrode and 10 mV/s for three cycles for data recording. Linear sweep voltammetry (LSV) measurements were conducted from 0 to −1.0 V (vs SCE) in O2-saturated with a sweep rate of 10 mV s−1 at different rotating speeds of 400, 625, 900, 1225, and 1600 rpm. The Koutechy−Levich (K−L) analyses were based on the RDE data at different electrode potentials. The slopes of their linear fitting lines are used to calculate
these reported carbon materials are still evidently worse than those of Pt-based materials.35,36 To further enhance the ORR activity of the carbon-based catalysts, various strategies including heteroatom doping,16,36−39 forming hierarchical porosity,40−42 and making the defects have been widely explored.43,44 The doping of different heteroatoms (e.g., N, P, etc.) within the carbon framework could evidently modify their electronic structures and chemical activities due to different electronegativity, thus facilitating the electron transfer and improving the ORR activity.45 The enhanced porosity for carbon materials could expose many active sites, beneficial for the effective transfer of reactants to the active sites, and thus improve the ORR activity greatly. As expected, the combination of doping heteroatoms and increasing porosity could be highly effective to obtain highly active carbon-based ORR electrocatalysts with respect to dual effects. Various organic precursors are widely used for synthesizing functional carbon materials for a variety of applications including gas adsorption and storage,31,46−51 electrochemical energy conversion and storage,51−54 catalysis,55 adsorption for waste oil and heavy metal ions,56 etc. The structural properties (e.g., elemental composition, porosity, and morphologies) of the as-prepared carbons could be controlled by adjusting the composition of starting materials, pyrolysis parameters, etc. Biomass materials are promising carbon sources for synthesizing carbon-based catalysts because of low-cost and easy accessibility. 57,58 However, direct pyrolysis of biomass precursors under high temperatures led to low-content Ndoping and low porosity of carbon materials,48,59 and thus the resultant carbon materials showed poor activity due to lack of both high density of active sites and hierarchical pores for mass transfer. In this study, we show the successful preparation of highly porous nitrogen-doped carbon nanospheres from biomass glucose and their use as efficient and stable ORR electrocatalysts with superior activity comparable to commercial Pt/C in basic solution. Glucose was hydrothermally treated to form colloidal carbon spheres (CSs), followed by heat-treatment in ammonia atmosphere to increase N-doping levels and subsequent activation by CO2 at high temperature to enhanced the porosity. The as-prepared activated N-doped carbons spheres (ANCSs) showed very high specific surface areas (up to 2813 m2/g) and pore volumes (up to 1.384 cm3/g), as well as adjustable nitrogen contents (2.38−6.55 atom %) depending on the activation temperatures and periods. Ascribed to their unique properties, the resultant ANCSs showed not only excellent ORR activity, but also superior stability and tolerance to MeOH crossover to commercial Pt/C electrocatalyst.
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EXPERIMENT
Chemicals. Potassium hydroxide (KOH) and anhydrous ethanol (EtOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. Carbon dioxide (CO2), ammonia (NH3), and argon (Ar) were obtained from Shanghai Minxing Gas Supplier. The Nafion solution (5 wt %) was purchased from Aldrich. The commercial Pt/C (20 wt %) catalyst was purchased from Johnson Matthey (United Kingdom). All solvents and chemicals mentioned were analytical grade and were utilized without further purification. Deionized water was used throughout all experiments. Materials Synthesis. Synthesis of Carbon Spheres (CSs) and Nitrogen-Doped Carbon Spheres (NCSs). The CSs were synthesized using glucose as the carbon source by the hydrothermal method according to a previous report.60 Typically, a solution of 8 g of glucose dissolved in 80 mL of water was transferred into a 100 mL TeflonB
DOI: 10.1021/acssuschemeng.7b03046 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering
Figure 1. Schematic of highly porous activated N-doped carbon spheres (ANCSs) as efficient and stable ORR electrocatalysts via a multistep strategy: (I) hydrothermal treatment of glucose solution at 180 °C to form CSs; (II) N-doping by thermal treatment with NH3 at 650 °C; and (III) CO2 activation to produce large amount of pores on the surface of nanospheres.
Figure 2. Effects of (a) activation periods (activation temperature: 800 °C) and (b) temperatures (activation period: 45 min) on the weight loss for the activated samples (ANCSs). Weight loss (%) = (WNCSs − WANCSs)/WNCSs × 100. the electron transfer number (n) based on the following K−L eqs 1 and 2: 1 1 1 = + J Jk Bω1/2
(1)
B = 0.2nFCo2Do2 2/3υ−1/6
(2)
n=4
Id Id + Ir /N
%H 2O2 = 200
Ir / N Id + Ir /N
(3)
(4)
where Id is the disk current, Ir is the ring current, and N (N = 0.37) is the collection efficiency of the Pt ring electrode.
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RESULTS AND DISCUSSION As shown in Figure 1, the activated nitrogen-doped carbon spheres (ANCSs) were prepared via a combined multistep procedure of hydrothermal treatment, N doping, and CO2 activation. The uniform carbon spheres (CSs) were simply obtained by the hydrothermal treatment of glucose in water at 180 °C. The SEM observation confirmed the spherical morphology of the hydrothermal carbon products derived from glucose (Figure S1).14,64 Many chemical reactions could take place under hydrothermal treatment of glucose in the sealed autoclave, thus resulting in the formation of complex organic compounds. It has been supposed that the CSs were produced according to the LaMer model including the polymerization step and subsequent carbonization and growth step.65 The N doping was performed by further heating CSs in ammonia flow via the reaction of ammonia and various oxygen-
in which J is the measured current density, JK is the kinetic limiting current density, ω is the electrode rotating rate, n is the electron transfer number, F is the Faraday constant (F = 96485 C mol−1), Co2 is the bulk concentration of O2 (Co2 = 1.2 × 10−6 mol cm−3),63 Do2 is the diffusion coefficient of O2 in 0.1 M KOH (Do2 = 1.9 × 10−5 cm2 s−1),63 and υ is the kinetic viscosity (υ = 0.01 cm2 s−1). The constant 0.2 is adopted when the rotation rate is expressed in rpm. For the rotating ring disk electrode (RRDE) tests, a CG disk (0.2475 cm2) was used as a working electrode which was surrounded by a Pt ring (0.1866 cm2). The ring potential was held at 0.2 V (vs SCE) with a rotating speed of 1600 rpm while the scanning rate was 10 mV s−1. The ring current and disk current were collected in O2saturated 0.1 M KOH from GC disc and Pt ring, respectively. The apparent number of electrons transferred (n) and the percentage of H2O2 released (%H2O2) during the ORR process are calculated as follows: C
DOI: 10.1021/acssuschemeng.7b03046 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering
Figure 3. SEM (a) and TEM (b) images of ANCS-1000-45, and the corresponding (c) carbon, (d) nitrogen, and (e) oxygen elemental mapping images.
Figure 4. (a) The N2 adsorption−desorption isotherms and (b) pore size distribution (PSD) curves of various samples (inset: the enlarged part of the PSD), and (c) the relationship of specific surface area (SBET) and the ratio of micropore volume (Vmicro) to total pore volume (Vtotal) with the activation temperatures.
increase in the holding time could not result in the larger carbon loss, implying the activation main happens at the surface of the spheres. The resulting micropores on the surface could capture CO2 molecules, thus preventing the activation extending into the inner of the spheres. As shown in Figure 2b, the increase in the activation temperature could greatly lead to the combustion of more carbon. The carbon loss for 800 °C sample is 15.3%, and it significantly increased to 72.1% for 1000 °C-activated sample. At 1050 °C, the carbon loss is as high as ∼98%, implying the carbon combusted completely and no carbon was almost left. The plentiful pores were produced by the reaction of carbon and CO2, and thus the porosity of the activated samples could be highly correlated with the amount of carbon loss. The activation at high temperature caused the evident carbon loss, but the spherical morphology of the activated samples was still retained, as shown in Figure 3a,b and Supporting Information Figure S3. Figure 3 panels a and b show the SEM and TEM images of ANCS-1000-45 prepared by CO2 activation of NCSs at 1000 °C for 45 min. Despite a 72.1%
containing groups in the CSs at high temperature to form Ndoped carbon spheres (NCSs). And the resulting NCSs still retained the spherical morphology (Figure S2). Therefore, the additional pores could be formed due to the etching effect of ammonia at high temperature.66 The porosity of NCSs could be greatly improved by etching the surface via the reaction of carbon with CO2 at high temperatures.67,68 Thus, the activation temperatures and periods have a significant influence on the carbon yields and textural properties of the activated carbons,69,70 as discussed in the latter. The CO2 activation proceeds as a stoichoimetric gas−solid reaction, according to the equation C + CO2 → 2CO.70 Thus, this process could result in the significant carbon loss depending on the activation periods and temperatures. As shown in Figure 2, the effects of the activation periods and temperatures on the weight loss for the activated samples were investigated in detail. At 800 °C, only 3.0% carbon combusted when the holding time is zero min. This carbon loss significantly increased to 11.5% for 15 min and 15.2% for 30 min activated samples (Figure 2a). However, the further D
DOI: 10.1021/acssuschemeng.7b03046 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
activation temperature (Figure 4c and Table 1), which is positively related to the mass loss during the activation treatment. With an increase of the activation temperature from 800 to 950 °C, the specific surface areas of ANCSs increased to 551, 630, 813, and 1228 m2/g, respectively. At the same time, the total pore volume (Vtotal), micropore surface area (Smicro), and micropore volume (Vmicro) of these samples also improve as the increased activation temperature, matching well with the trend of the surface area values (Table 1). These activated samples have the Vmicro/Vtotal ratios of 81.0−85.4%, showing their microporous properties. It is notable that the surface area significantly increased to 2813 m2/g for the sample activated at 1000 °C (Table 1), associated with a maximum mass loss among these activated samples. In sharp contrast, the treatment of NCSs in Ar at 1000 °C only obtains a moderate surface area of 778 m2/g for NCS-1000-45 (Ar) (Table 1). Meanwhile, the percent of micropore volume for ANCS-100045 greatly decreases to 20.5%, significantly lower than those of other activated samples, ascribed to the pore broadening after overactivation. Indeed, it is observed that the pore sizes evidently broaden when the activation temperature increases to 950−1000 °C (Figure 4b). For comparison, ACS-1000-45 prepared by CO2 activation of CSs without N-doping has a lower specific surface area of 2483 m2/g and larger micropore volume percent (50.8%) than those of NACS-1000-45 (2813 m2/g, 50.8%). It clearly implies that the doped nitrogen groups could activate the carbon framework, beneficial for CO2 activation at high temperatures and thus the CO2 treatment on the NCSs results in the larger surface area and pore widening. Thus, the CO2 activation treatment should have a significant influence on the contents and types of N functional groups for the activated samples, as discussed in the latter. It has been reported that the micropores and small mesopores in these activated samples could contribute to the adsorption of the electrolyte ions.68 The unique spherical morphology of these N-doped carbon spheres with large surface areas could be advantageous for the enhanced exposure of N-related active sites and fast diffusion of the electrolyte ions to the active sites, thus increasing the ORR activity of the activated samples significantly.40 The treatment of NCSs with CO2 resulted in not only the significant increase in the porosity, but also an evident change in the elemental compositions of the ANCSs determined by Xray photoelectron spectroscopy (XPS). The XPS spectra of various ANCSs demonstrate three peaks at ca. 284.2, 400.6, and 533.4 eV, which are attributed to C 1s, N 1s and O 1s, respectively (Figure 5a),47,48 implying the existence of N-doped groups in the activated samples in spite of evident mass loss due to CO2 activation at high temperatures. Table 2 shows the
carbon loss, the resulting sample is still composed of spherical particles with the diameter of approximately hundreds of nanometers which interconnected with each other, implying the CO2 activation treatment has no great influence on the spherical morphology of NCSs (Figure 3a and Figure S3). Moreover, the EDS mapping images show that the N and O elements are uniformly distributed throughout the sample (Figure 3c−e). It suggests that a large amount of N dopants are still doped within the carbon framework of the activated samples, although the large carbon loss happened accompanying the introduction of some oxygen elements. The CO2 activation treatment has no great effect on the morphology of the carbon spheres, but it significantly improves the porosity of the activated samples as evidenced by the N2 adsorption measurement performed at −196 °C. The N2 adsorption−desorption isotherms and pore size distribution curves of a series of samples are shown in Figure 4a,b, and the textural properties are shown in Table 1. The as-prepared Table 1. Physical Properties of Pure CSs, NCSs, Various CO2-Activated Samples (ANCSs), and a NCS Sample without CO2 Activation (NCS-1000-45 (Ar)) samples CSs NCSs ANCS-800−45 ANCS-850-45 ANCS-900-45 ANCS-950-45 ANCS-1000-45 ACS-1000-45 NCS-1000-45 (Ar)
SBETa (m2/g)
Vporeb (cm3/g)
Smicroc (m2/g)
Vmicrod (cm3/g)
Vmicro/ Vpore (%)
13 468 551 630 813 1228 2813 2483 778
0.021 0.235 0.270 0.308 0.394 0.595 1.384 1.187 0.382
0 410 499 575 736 1065 794 1433 728
0 0.188 0.228 0.263 0.336 0.482 0.284 0.603 0.334
0 80.0 84.4 85.4 85.3 81.0 20.5 50.8 87.4
a
BET specific surface area. bSingle point adsorption total pore volume of pores less than 189.5 nm diameter at P/P0 = 0.99. ct-Plot micropore area. dt-Plot micropore area.
sample (CSs) almost has no adsorption for nitrogen, implying it is nonporous (Figure 4a). Indeed, its specific surface area is only 13 m2/g, determined by the Brunauer−Emmett−Teller (BET) method.71 The isotherms for other samples except for ANCS-1000-45 present the type I behavior with the absence of a hysteresis loop, typical for microporous materials.72−75 In the isotherm of ANCS-1000-45, a small hysteresis loop could be observed at the relative pressure of 0.2−0.3, implying an increased porosity in the mesopore range.76−78 The quantity of adsorbed nitrogen significantly increases as the activation temperature increases (Figure 4a), meaning greatly improved specific surface areas. Moreover, in the isotherms of the activated samples, there exists an obtuse curvature at low relative pressures, implying a widening of micropores that even extends to the mesopore range. As shown in Figure 4b, the pore sizes of the activated samples increase as the activation temperature, and the micropore sizes of the samples activated at 950 and 1000 °C evidently extend to the range of small mesopores (
