Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9709-9717
Rupturing Cotton Microfibers into Mesoporous Nitrogen-Doped Carbon Nanosheets as Metal-Free Catalysts for Efficient Oxygen Electroreduction Xiuxia Lin,†,⊥ Xiufang Wang,†,⊥ Ligui Li,*,‡,§ Mingfang Yan,*,∥ and Yong Tian*,† †
School of Pharmacy, Guangdong Pharmaceutical University, 280 Waihuan Dong Road, University Town, Guangzhou 510006, China Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy Research Institute, School of Environment and Energy, and §Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, College of Environment and Energy, South China University of Technology, 382 Waihuan Dong Road, University Town, Guangzhou 510006, China ∥ School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical University, 9-13 Changmingshui Road, Wuguishan, Zhongshan City 528458, China ‡
S Supporting Information *
ABSTRACT: Mechanical grinding is exploited to effectively rupture biomass cotton microfibers into metal-free, nitrogendoped carbon nanosheets with a large number of mesoporous textures. Experimentally, raw microfibers of absorbent cotton are presoaked with fuming sulfuric acid to generate plenty of hierarchical pores/cavities, which sufficiently expose the inner parts of cotton microfibers to nitrogen source for efficient incorporation of nitrogen dopants onto carbon skeletons in subsequent thermal annealing process. Mechanical grinding of these thermally annealed carbon microfibers leads to exfoliated nitrogen-doped thin carbon nanosheets with a high surface area of 912.1 m2/g as well as abundant mesopores and a considerable nitrogen content of 8.5 at. %. These characteristics contribute to an excellent electrocatalyst with marked catalytic activities toward oxygen reduction reaction in an alkaline electrolyte solution, including a more positive half-wave potential, much higher diffusionlimiting current, remarkably enhanced operation stability, and stronger immunity against fuel-crossover effects, as compared to commercial Pt/C catalysts. The present results provide a novel facile method to the scalable preparation of biomass-derived highly porous two-dimensional carbons for efficient electrochemical energy devices. KEYWORDS: Oxygen reduction reaction, Cotton microfiber, Mesoporous carbon nanosheet, Nitrogen doping, Mechanical grinding
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INTRODUCTION Oxygen reduction reaction (ORR) represents a fundamental process at the cathodes of several important electrochemical energy devices, including the promising metal−air batteries, microbial fuel cells, and proton-exchange membrane fuel cells (PEMFCs). When compared to the oxidation process of fuel molecules at anodes, ORR is more complicated and usually exhibits rather sluggish charge-transfer kinetics. Therefore, a large quantity of electrocatalysts are generally required to generate a sufficiently high output current for practical applications.1,2 Currently, nanomaterials of platinum-based noble metals and their alloys with various transition metals are the primary catalysts of choice due to their outstanding ORR activity.1−3 Yet, the apparent drawbacks of Pt-based noble metals, such as scarcity, high cost, low tolerance/resistance against fuel crossover and CO poisoning, as well as poor operation stability, have been the main barriers that heavily impeded the massive commercialization of the aforementioned green electrochemical energy conversion technologies. Re© 2017 American Chemical Society
cently, considerable research efforts have been devoted to the exploration of noble-metal-free catalysts,4−6 in particular the various heteroatom-doped carbon-based ORR catalysts.7−14 Of these heteroatom-doped carbon materials, nitrogen-doped (Ndoped) carbons have emerged as a class of viable alternatives to Pt-based conventional ORR electrocatalysts due to multiple advantages: (i) the introduction of nitrogen dopants onto carbon skeletons causes fluctuation of electron density on the surface of graphitized carbon due to the stronger electron affinity of nitrogen as compared with carbon, which is beneficial to the adsorption and dissociation of oxygen molecules and finally leads to remarkable ORR activity; (ii) the inherent high electrical conductivity of graphitized carbons; (iii) wide electrochemical stability window; (iv) abundantly available source materials; (v) facile synthesis; and so on.9,15−19 Received: May 5, 2017 Revised: August 21, 2017 Published: September 29, 2017 9709
DOI: 10.1021/acssuschemeng.7b01398 ACS Sustainable Chem. Eng. 2017, 5, 9709−9717
Research Article
ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Image Showing the Synthesis of Mesoporous N-Doped Carbon Nanosheets
as catalyst supports39 in catalysis. Herein, we report a facile route to the readily scalable synthesis of N-doped carbon nanosheets with abundant mesopores via mechanical grinding of the thermally annealed cotton microfibers presoaked with fuming sulfuric acid. Detailed electrochemical studies confirm that the catalyst synthesized at an annealing temperature of 800 °C shows the best ORR activity among all of the investigated carbon catalysts, which is highly comparable to the benchmark Pt/C catalyst. The results in the present work enrich the approaches to the preparation of highly porous 2D carbons for electrochemical energy conversion and storage devices.
Besides the nature of active species, the catalytic activity of N-doped carbon nanomaterials toward ORR is significantly influenced by the amounts of accessible active species that closely correlate to specific surface area and porous texture.20−22 Among the huge number of carbon-based ORR catalysts reported in the literature, graphene-like two-dimensional (2D) carbon nanosheets are of special interest from the viewpoint of fundamental research because they can provide a high surface area for efficient exposure of active sites and continuous 2D channels for fast electron transfer.11,23−26 However, carbon nanosheets usually suffer from restacking/ aggregation problems because of the strong π−π interactions between carbon nanosheets, which drastically decrease the effective electrochemical surface area that is highly important in catalysis, leading to a decreased ORR activity. Yet, generating a large number of porous structures in carbon nanosheets represents a judicious strategy to effectively mitigate or even exclude the negative effects of restacking/aggregation on catalytic activity.16−18,23,27 Within this context, rigid inorganic templates such as structured SiO2, Al2O3, MgO, etc., have been widely used to form nanopores/nanocavities in/on 2D carbon catalysts.21,23,28−31 However, the conventional template-assisted methods inevitably entail multiple complicated and timeconsuming processes, including the preparation of uniform nanostructured templates, homogeneously mixing the preformed nanotemplates with carbonaceous precursors, chemical etching of the sacrificial templates after calcination, catalyst extraction/purification, etc., which undoubtedly complicate the catalyst preparation and make it difficult for mass production. Meanwhile, the ORR activity of catalysts is inevitably compromised because partial active components are concomitantly removed during harsh acidic/basic etching. Moreover, most of the carbonaceous precursors reported in the literature for the preparation of porous carbon nanosheets are actually high-value products/derivates of nonrenewable petroleum resources,16,18,32−35 which is beneficial to neither cost reduction nor environment protection. Therefore, is it possible to prepare highly porous 2D-carbon catalysts toward ORR from the renewable natural resources without using any templates? This is the primary motivation of the present work. Cotton is a widely available natural resource, which largely consists of cellulose microfibers. Carbonization of cotton affords carbon microfibers with remarkable surface areas that are extensively used in sample separation/purification36−38 and
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EXPERIMENTAL SECTION
Sample Preparation. The synthesis of N-doped mesoporous carbon nanosheets was schematically illustrated in Scheme 1. Briefly, commercially available absorbent cotton was dipped into fuming H2SO4 for about 30 s, and then washed thoroughly with copious deionized water before drying at 70 °C for 3 h. Subsequently, the dried black solid was blended with nitrogen source NH4Cl (H2SO4-treated cotton:NH4Cl = 1:30, w/w) and thermally annealed at controlled temperatures (i.e., 700, 800, and 850 °C) for 2 h under an inert atmosphere of N2. The resultants were further mechanically grinded with an agate mortar for about 5 min, affording N-doped mesoporous carbon nanosheets N/MCNS-T, where T is the thermal annealing temperature. The control sample N/UTCF-800 was synthesized in a similar fashion but without soaking with fuming H2SO4 before thermal annealing. Characterization. Transmission electron microscopic (TEM) images were obtained on a Tecnai G2-F20 instrument operated at an acceleration voltage of 100 kV. For TEM measurement, catalyst ink was dropcast from the corresponding dispersion directly onto a copper grid precoated with a thin holey carbon film as supporting layer. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Phi X-tool XPS instrument with Al Kα X-ray as excitation beam source. The Brunauer−Emmet−Teller (BET) surface area of catalysts was determined on a Micromeritics ASAP 2010 instrument by nitrogen adsorption and desorption at 77 K using a Barrett−Joyner− Halenda (BJH) method. Electrochemistry. A CHI 750E electrochemical workstation (CH Instruments, Chenhua Co., China) was used for electrochemical measurements in a conventional three-electrode cell equipped with a gas flow system. Platinum wire was used as counter electrode, while an Ag/AgCl electrode and a glassy carbon electrode (GCE) loaded with catalysts were used as reference electrode and working electrode, respectively. Catalysts were dispersed in a solution containing water, isopropanol, and Nafion (5%) at a volume ratio of 4:1:0.025 under ultrasonic agitation. Subsequently, the catalyst suspension was evenly 9710
DOI: 10.1021/acssuschemeng.7b01398 ACS Sustainable Chem. Eng. 2017, 5, 9709−9717
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ACS Sustainable Chemistry & Engineering
Figure 1. TEM images of (a,b) pristine cotton fibers and (c,d) cotton fibers soaked with fuming H2SO4 for about 30 s.
Figure 2. SEM images with different magnifications showing the morphology of (a−c) N/UTCF-800 and (d,e) carbonized cotton microfibers after thermal annealing of the H2SO4-treated absorbent cotton. (f) The enlarged SEM image of the area indicated by a dashed square in panel (e). cast on the clean surface of GCE with a micropipette and then dried in air. The loading of catalyst was calculated to be 0.4 mg cm−2 except for 0.2 mg cm−2 for commercial Pt/C. Linear sweep voltammograms (LSV) were recorded in a commonly used 0.1 M KOH aqueous solution that was saturated with oxygen. Electrode rotation speed was steadily increased from 400 to 2025 rpm. The number of electron transfer (n) for ORR was calculated using eq 1: n=
4IDisk IRing /N + IDisk
In a series of measurements, the reference electrode Ag/AgCl was precalibrated with a reversible hydrogen electrode (RHE). Briefly, the calibration was conducted in ultrahigh pure H2 (99.999%) saturated electrolyte, using two Pt wires as the corresponding working and counter electrode, respectively. Cyclic voltammogram (CV) measurements were performed at a potential sweep rate of 1 mV/s. The thermodynamic potential of the RHE was then determined by the average of the two potential values at which the current crossed zero, and in KOH aqueous solution with a concentration of 0.1 mol/L, EAg/AgCl = ERHE + 0.964 V.
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(1)
where N is the collection efficiency correlated with electrode geometry (37% in our case),40−42 while IDisk and IRing correspond to the voltammetric currents detected at disk and ring electrode, respectively. The commercially available Pt/C (20% w/w) from Alfa Aesar was used as a reference catalyst.
RESULTS AND DISCUSSION Pristine cotton fibers show a dominant diameter of about 20 μm (Figure S1) and display a smooth surface without apparent pores or cavities (Figure 1a,b). In sharp contrast, a large 9711
DOI: 10.1021/acssuschemeng.7b01398 ACS Sustainable Chem. Eng. 2017, 5, 9709−9717
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ACS Sustainable Chemistry & Engineering
Figure 3. (a,b) SEM and (c−e) TEM images of N/MCNS-800. The inset to panel (e) is the corresponding SAED pattern.
number of pores/cavities are observed on cotton fibers, which have been soaked in fuming sulfuric acid (Figure 1c). In the TEM image with a higher magnification, one can find that the width of pores/cavities is within the range of about 3−20 nm. These observations clearly demonstrate that soaking of absorbent cotton with fuming H2SO4 generates plenty of nanopores/nanocavities on cotton microfibers likely due to dehydration of cellulose.43 As for Raman spectrum measurements, H2SO4-treated cotton shows a distinct Raman peak at about 1600 cm−1 (Figure S2a) that is ascribed to the G band of ordered graphite-like carbon. In contrast, no such Raman peak is observed for pristine cotton (Figure S2b). These observations clearly confirm that cotton is dehydrated by fuming H2SO4 in present work. Correspondingly, a marked difference in morphology is also observed for the thermally annealed cotton fibers. From Figure 2a, fibrillar structures with a diameter ranging from ca. 10 to 20 μm are found to retain after mechanical grinding. In the SEM image with a higher magnification (Figure 2b), no apparent pore/cavity structure is observed for N/UTCF-800. This is further supported by the result of TEM measurement shown in Figure 2c, where a nanofiber (ca. 80 nm in diameter) is observed to show a very smooth surface without any identified pores or cavities, whereas the thermally annealed cotton sample that was presoaked with fuming sulfuric acid comprised broken microfibers and large fragments with a lateral size of about 20− 100 μm that contain a large number of micropores (Figure 2d). The SEM image of the broken microfibers revealed a rough surface likely consisting of numerous nanosheets (Figure 2e,f). Surprisingly, mechanical grinding of the obtained broken carbon microfibers with an agate mortar leads to thin carbon nanosheets (Figure 3a) with a thickness of ca. 20 nm. A closer look at the SEM image reveals that the surface of these 2D carbon nanosheets consists of many porous textures (Figure 3b). In the corresponding TEM image shown in Figure 3c, a sheet-like morphology is also observed. Interestingly, from the TEM image with a higher magnification (Figure 3d), plenty of
pores/cavities with a diameter of about 10 nm are observed for N/MCNS-800 (see also Figure S3), which indicates that the large number of pores/cavities generated by fuming H2SO4 are largely reserved during the subsequent high-temperature annealing process. Therefore, the roles of fuming H2SO4 treatment are two aspects: (i) to generate a large number of pores/cavities in precursor; and (ii) to carbonize precursor to avoid the shrink or collapse of porous precursor matrix during the subsequent high-temperature thermal annealing, which is also further supported by the pore diameter of N/MCNS-800 being highly comparable to that of its parent precursor, that is, H2SO4-treated cotton (Figure S4). In the high-resolution TEM image shown in Figure 3e, wavy strips with a d-spacing of ca. 3.5 Å are clearly identified, signifying the formation of thin graphite nanosheets. In the corresponding SAED image (inset to Figure 3e), two pronounced diffraction rings are resolved. The d-spacings of the inner and outer diffraction ring are calculated to be 2.10 and 1.21 Å, corresponding to the (100) and (110) planes of graphite,44 respectively, again indicating the formation of graphite-like ordered structures. Taken together, the above results demonstrate that the N/MCNS-800 sample is actually comprised of mesoporous graphitized carbon nanosheets. BET measurements are then conducted to probe the specific surface area of the aforementioned cotton-derived carbon samples. As shown in Figure 4, the adsorption pore volume of all of the investigated samples is found to increase from 237.8 m3/g for N/UTCF-800 to 350.7 m3/g for both N/MCNS-700 and N/MCNS-850 and then 556.0 m3/g for N/MCNS-800. Note that the N/MCNS-800 sample shows a mixed type-I/IV adsorption/desorption isotherm of N2 at 77 K with a hysteresis loop within the relative pressure range of P/P0 = 0.4−1.0, suggesting the coexistence of micropores and mesopores. Indeed, from the corresponding pore size distribution depicted in Figure S4, one can find that the pore diameter of N/MCNS800 is ranging from about 0.8 to 10 nm. Accordingly, a specific surface area of 750.7 m2/g is determined for N/MCNS-700, 9712
DOI: 10.1021/acssuschemeng.7b01398 ACS Sustainable Chem. Eng. 2017, 5, 9709−9717
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then probed by high-resolution XPS measurement. As depicted in Figure 5b, deconvolution of the XPS spectrum of N 1s electrons (black line) leads to three peaks at 400.8, 400.0, and 398.2 eV, coinciding with the graphitic N, pyrrolic N, and pyridinic N (Figure 5c) documented in the literature,16,45−47 respectively. Similar results are also observed for N/MCNS700, N/MCNS-850, and N/UTCF-800 samples (Figures S5− 7). The appearance of these three deconvoluted peaks of N 1s electrons signifies that nitrogen from NH4Cl has been successfully doped into the honeycomb-like carbon skeleton of aforementioned cotton-derived samples. It is also worthy to note that the contents of these three nitrogen doping configurations are highly comparable in both N/MCNS-700 and N/MCNS-800 samples; yet at a higher annealing temperature of 850 °C, the contents of graphitic N and pyridinic N suddenly decrease from 3.9 and 4.0 at. % for N/ MCNS-800 to 2.6 and 2.4 at. % for N/MCNS-850 (Table 1), respectively. Consequently, the total nitrogen content shows an abrupt decrease from 8.5 at. % for N/MCNS-800 to 5.6 at. % for N/MCNS-850 (Figure 5d), likely due to the exclusion of nitrogen dopants during graphitization/crystallization of carbon matrix. Interestingly, the control sample N/UTCF-800 has a much lower content of nitrogen dopants as compared to N/ MCNS-800, which suggests that the abundant porous structures generated by fuming H2SO4 soaking help better expose the inner part of cotton microfibers to NH4Cl for more efficient incorporation of nitrogen dopants onto carbon skeletons during thermal annealing. Although the detailed active sites/species for ORR on N-doped carbons are still under active debate,48−52 carbon atoms adjacent to N dopants are generally reported to be responsible for ORR.7,49,53 Therefore, on the basis of the high content of nitrogen dopants along with a high surface area consisting of a large number of mesopores, N/MCNS-800 is expected to show a high ORR activity. The catalytic activity of the above N/MCNS catalysts toward ORR is screened by cyclic voltammetric (CV) studies in 0.1 M KOH aqueous electrolyte solution. As depicted in Figure 6, when CV measurements are conducted in a N2-saturated solution, only a rectangle-like double-layer charging profile (black dashed-dotted lines) is observed for all of the N/MCNS samples in the potential range from −0.04 to +1.16 V (vs RHE) . On basis of these rectangle-like charging profiles, one can find that the electrochemical surface area of N/MCNS-800 is apparently much larger than that of N/MCNS-700 and N/ MCNS-850. This conclusion coincides with the observations in BET surface area measurements (Figure 4 and Table 1). When CV scans are conducted in an O2-saturated solution (red solid curves), a broad cathodic peak resulting from oxygen electroreduction can be well resolved at +0.778 V for N/ MCNS-700, while it is identified at a much more positive potential of +0.846 V for both N/MCNS-800 and N/MCNS850, which demonstrate that, although these three samples possess an apparent catalytic activity for ORR, the ORR activity of N/MCNS-800 and N/MCNS-850 is superior to that of N/ MCNS-700. One may note that the cathodic peak of N/ MCNS-800 locates at a potential that is nearly identical to that of benchmark Pt/C (20 wt %) catalyst, which suggests that the ORR activity of N/MCNS-800 is highly comparable to that of commercial Pt/C catalyst. The detailed ORR activity of the N/MCNS catalysts is then investigated by a rotating ring-disk electrode (RRDE) voltammetric technique. As depicted in Figure 7a, when the potential of disk electrode is continuously increased from
Figure 4. N2 adsorption and desorption isothermals for different carbon samples.
and it steadily increases to 912.1 m2/g for N/MCNS-800. However, at a higher annealing temperature of 850 °C, an abrupt decrease of the surface area to only 672.3 m2/g is observed for N/MCNS-850, which is attributed to the collapse of nanopores at high temperature.45 Significantly, the specific surface area of N/MCNS-800 is much higher than the 324.5 m2/g of N/UTCF-800, which unambiguously demonstrates that soaking of cotton microfibers with fuming H2SO4 helps generate abundant pore structures on their surface. Concurrently forming a mesoporous structure and remarkable surface area in N/MCNS-800 may substantially facilitate the mass transfer process and hence finally promote its ORR activity. The component details of the above cotton-derived carbon samples are quantitatively analyzed by XPS measurements. For the XPS survey spectrum of N/MCNS-800 shown in Figure 5a, five peaks corresponding to the elements of S, Cl, C, N, and O are clearly resolved. Note that S and Cl elements are likely mainly resulting from the trace amount of −SO3− and NH4Cl. On the basis of the results of XPS measurements, a total nitrogen content of 8.5 at. % is determined for N/MCNS-800. The doping configurations of nitrogen in carbon skeleton are
Figure 5. (a) XPS survey spectrum and (b) high-resolution N 1s scanning curve of the N/MCNS-800 sample. (c) Schematic illustration of the three primary nitrogen doping configurations on graphene. (d) Histograms showing the contents of the three primary nitrogen doping configurations in different carbon samples. 9713
DOI: 10.1021/acssuschemeng.7b01398 ACS Sustainable Chem. Eng. 2017, 5, 9709−9717
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ACS Sustainable Chemistry & Engineering Table 1. Specific Surface Area and Component Details for Different Carbon Samples N (at. %) sample name
surface area (m2 g−1)
C (at. %)
O (at. %)
S (at. %)
pyridinic N
pyrrolic N
graphitic N
N/MCNS-700 N/MCNS-800 N/MCNS-850 N/UTCF-800
750.7 912.1 672.3 324.5
80.6 83.0 86.7 86.0
10.1 7.7 7.0 7.8
0.4 0.8 0.5 0.2
3.9 4.0 2.4 2.8
0.9 0.6 0.6 1.1
4.1 3.9 2.6 2.1
and N/MCNS-850 are observed at the same potential position, the half-wave potentials for these two samples appear at different potentials. For instance, a half-wave potential is observed at +0.812 V for N/MCNS-850; in contrast, a higher value of +0.820 V is observed for N/MCNS-800. Meanwhile, the diffusion-limiting currents of the N/MCNS catalysts are also significantly influenced by annealing temperature, increasing from 3.756 mA/cm2 for N/MCNS-700 at +0.200 V to 5.286 mA/cm2 for N/MCNS-800, and then decreasing to 4.844 mA/cm2 for N/MCNS-850. Taking these results together, one can conclude that the N/MCNS-800 sample outperforms the other two samples in the series. Note that N/MCNS-800 shows a high content of pyridinic N (4.0 at. %) and graphitic N (3.9 at. %), which indicates that both pyridinic N and graphitic N are likely responsible for the ORR activity for N/MCNS-T samples. Actually, the ORR activity of N/MCNS-800 is found to be highly comparable to that of commercial Pt/C catalyst. As shown in Figure 7b, the onset potential of N/MCNS-800 appears at the same potential as that of commercial Pt/C catalyst. Note that under the same testing condition, the Pt/C catalyst demonstrates a half-wave potential of +0.818 V, which is slightly lower than the +0.820 V for N/MCNS-800. In addition, a higher diffusion-limiting current is observed for N/ MCNS-800 at +0.200 V as compared to that of benchmark Pt/
Figure 6. CV curves of different cotton-derived carbon samples and commercial Pt/C catalyst conducted in an aqueous solution of 0.1 M KOH saturated with nitrogen (black dashed line) and oxygen (red solid line). The potential sweeping rate is set at 50 mV/s.
+1.100 to −0.320 V (vs RHE), the corresponding current detected on disk electrode is observed to show an abrupt increase for all samples due to the emergence of oxygen electroreduction. Specifically, an onset potential for such an abrupt increase of current is identified at +0.920 V (vs RHE) for N/MCNS-700, while a much more positive one of +0.981 V is observed for both N/MCNS-800 and N/MCNS-850. Interestingly, although the onset potentials for N/MCNS-800
Figure 7. (a) LSVs of the series N/MCNS samples. (b) Comparison of the linear polarized curves of N/MCNS-800 and Pt/C. (c) The plots of number of electron transfer n against sweeping potential. (d) A series of linear polarized curves of N/MCNS-800 at an electrode rotation speed between 225 and 2025 rpm, and (e) the correlated K−L plots. (f) The corresponding Tafel plots of N/MCNS-800 and Pt/C. All of the electrochemical measurements here were conducted on a glassy-carbon electrode modified with 0.4 mg cm−2 of N/MCNS catalysts and 0.2 mg cm−2 of commercial Pt/C in 0.1 M KOH aqueous solution saturated with O2 at an electrode rotating speed of 1600 rpm unless otherwise indicated. The potential sweeping rate was set at 10 mV s−1. 9714
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Figure 8. (a) Plot of chronoamperometric current against operation time for electrodes modified with N/MCNS-800 and Pt/C catalyst. (b) Chronoamperometric current evolution of N/MCNS-800 and Pt/C in the presence of 1 M methanol solution. The chronoamperometric current was measured at +0.70 V (vs RHE) in a 0.1 M KOH aqueous solution saturated with O2. The electrode rotating speed was set at 900 rpm.
C catalyst (4.933 mA/cm2), again indicative of a high catalytic activity for N/MCNS-800. Figure 7c displays the plots of electron-transfer number n against sweeping potential, where one can find that, although the n values of N/MCNS-800 at high overpotentials are slightly lower than that of Pt/C catalyst, N/MCNS-800 shows an electron-transfer number of almost 4 over a wide range of potential from +0.800 to +0.000 V. Specifically, the average n value of N/MCNS-800 is calculated to be 3.92, very close to the 3.96 of Pt/C, which suggests a high selectivity toward the preferred 4e− pathway for N/MCNS-800 in ORR catalysis. This conclusion is also supported by the low hydrogen peroxide yield observed for N/MCNS-800 over a wide range of potentials (Figure S8). From the series of LSVs for N/MCNS-800 catalyst (Figure 7d), one can find that the diffusion-limiting currents are concurrently increased with increasing electrode rotation speed. Figure 7e shows the correlated Koutecky−Levich (K−L) plots, where a series of linear fitting curves between +0.45 and +0.73 V are found to show highly comparable slopes, signifying that the reaction rate of ORR on N/MCNS-800 is proportional to the concentration of dissolved oxygen in electrolyte solution, therefore corresponding to a first-order kinetics. Similarly, from the linear fittings in the Tafel plots (Figure 7f), equivalent slopes are identified for N/MCNS-800 (60 mV/dec) and Pt/C (59 mV/dec), which suggest that the overall 4e− ORR on both catalysts is largely dominated by the first electron reduction process.5,16,54 Operation stability is of paramount importance in practical catalysis. Figure 8a depicts the comparison studies on the stability of both commercial Pt/C and N/MCNS-800 catalysts by chronoamperometric measurements, where one can find that after continuously catalyzing ORR for 15 000 s, the voltammetric current of benchmark Pt/C-modified electrode decreases to about 87.1% of that at the beginning, while it is observed to slightly decrease to 95.0% of its initial value for the N/MCNS-800-modified electrode. These observations clearly demonstrate that N/MCNS-800 has higher operation stability than commercial Pt/C in ORR electrocatalysis. Moreover, the tolerance to fuel crossover is also an important issue for ORR catalysts in real devices. Herein, we use methanol, a common fuel in fuel cells, as an illustrating example to conduct comparison studies. As shown in Figure 8b, when 1 M methanol solution is injected into the electrolyte solution, the voltammetric current detected on Pt/C-modified electrode drastically decreases with only about 50% retention. This is
because of the presence of methanol oxidation reaction on this Pt/C-modified electrode, indicating a poor selectivity of Pt/C catalyst toward ORR. After continuous operation for 1000 s, the voltammetric current only recovers to about 80% of that before the injection of methanol. In sharp contrast, injection of 1 M methanol solution causes no apparent decrease but a slight increase in the voltammetric current detected on N/MCNS800-modified electrode due to the agitation of methanol injection, which unambiguously signifies that N/MCNS-800 is totally immune to fuel crossover effect in ORR catalysis, a desired ability of excellent ORR catalyst. The aforementioned results signify that N-doped carbon nanosheets comprising abundant mesopores with a diameter of ca. 10 nm are successfully prepared by rupturing the thermally annealed cotton microfibers pretreated with fuming H2SO4. Those N-doped 2D carbons show a remarkable ORR activity, which is mainly ascribed to the synergetic effects of a relatively high content of nitrogen dopants, a high surface area, and the numerous mesoporous textures. Further advancing the ORR activity of N/MCNS catalysts might rely on optimizing the conditions for H2SO4 soaking and thermal annealing, and codoping with other heteroatoms such as S,55 P,11 B,10 F,56 Fe,14 and Co.14,57
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CONCLUSIONS In summary, metal-free thin carbon nanosheets can be facilely prepared by mechanical grinding of the thermally annealed cotton microfibers, which have been subjected to soaking of fuming H2SO4. This soaking treatment not only helps generate abundant pores in the carbonized cotton microfibers via dehydration of cellulose, but also effectively exposes the inner parts of cotton microfibers to nitrogen-containing dopants so that sufficient N dopants can be incorporated into the carbon matrix during subsequent high-temperature thermal annealing. BET measurements reveal that thus-synthesized N/MCNS catalysts show a remarkable specific surface area together with plenty of homogeneously distributed mesoporous structures. Apparent ORR activity is identified for the resulting N/MCNS catalysts in alkaline electrolyte, and the one prepared at a thermal temperature of 800 °C (N/MCNS-800) is found to demonstrate dramatically higher ORR activity than the others in the series, along with a higher half-wave potential, substantially improved operation stability, and excellent immunity to fuel-crossover effect as compared to Pt/C catalyst. Such a superb ORR activity of N/MCNS-800 is primarily 9715
DOI: 10.1021/acssuschemeng.7b01398 ACS Sustainable Chem. Eng. 2017, 5, 9709−9717
Research Article
ACS Sustainable Chemistry & Engineering
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ascribed to the considerable content of nitrogen dopants and remarkable surface area consisting of abundant mesopores. The results in the present work pave the way for a novel avenue to the scalable fabrication of highly porous 2D carbons from the low-cost, renewable bioresources for electrochemical energy devices.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01398. Table showing component details, SEM images, TEM images, Raman spectra, pore size distribution plots, additional figures of XPS spectra, and plots of hydrogen peroxide yield (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*Tel.: +86-20 39380520. E-mail:
[email protected]. *E-mail:
[email protected]. *Tel./fax: +86-20 39352129. E-mail: tian_yong_tian@163. com. ORCID
Ligui Li: 0000-0002-1636-9342 Author Contributions ⊥
X.L. and X.W. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC 51402111 and NSFC 21373061), the Guangdong Innovative and Entrepreneurial Research Team Program (2014ZT05N200), the Key Project for Natural Science Foundation of Guangdong Province (2014A030311038, 2016A030313733), and the Innovation and Strengthen University project of Guangdong Pharmaceutical University (2015KQNCX073).
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REFERENCES
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DOI: 10.1021/acssuschemeng.7b01398 ACS Sustainable Chem. Eng. 2017, 5, 9709−9717
