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Tea-leaf-residual derived electrocatalyst: Hierarchical pore structure and self nitrogen and fluorine co-doping for efficient oxygen reduction reaction
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Tea-leaf-residual derived electrocatalyst: Hierarchical pore structure and self nitrogen and fluorine co-doping for efficient oxygen reduction reaction Danyang Wu, Yantao Shi*, Hongyu Jing, Xun Wang, Xuedan Song, Duanhui Si, Suxia Liang, Ce Hao State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology, Dalian, 116024, Liaoning, China
article info
abstract
Article history:
The heteroatom-doped carbon materials as metal-free catalysts show great potential for
Received 11 April 2018
oxygen reduction reaction (ORR) due to the high electrocatalytic activity, low cost, long-
Received in revised form
term stability, and environmental friendliness. Utilizing biomass as precursors has
29 August 2018
offered facile and extremely low-cost strategy to large-scale fabrication of highly efficient
Accepted 30 August 2018
carbon materials with abundant pore structures and elements. In this work, tea residue
Available online xxx
was used as precursor to synthesize nitrogen (N) and fluorine (F) co-doped porous carbon materials via one-step annealing process without any activation or post-treatment. The
Keywords:
morphology, pore structure, elemental composition of our sample as well as its electro-
Tea-leaf
catalytic performance in ORR were studied. This biomass-based carbon materials has high
Oxygen reduction reaction
specific surface area (855.6 m2 g1) and hierarchical pore structure, which can offer
Nitrogen and fluorine co-doping
abundant active sites and meanwhile is favorable to ion diffusion of liquid electrolyte.
Electrocatalytic activity
Secondly, the heteroatoms (especially F) doping can induce charge redistribution thanks to
Biomass-derived carbon material
the electronegativity difference, which can facilitate electron transfer in ORR. When compared to commercial Pt/C, our catalyst shows a higher limited current density and a high electron transfer number of about 3.8. Finally, the catalyst also demonstrates excellent methanol tolerance in the alkaline medium. Our work provides an economical and facile strategy to synthesize efficient electrocatalyst for ORR. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cell known as a new energy device with environmentfriendly characteristics has high energy conversion efficiency and power density, which favors to address the environmental and energy crisis nowadays [1e4]. However, the
sluggish kinetic of the oxygen reduction reaction (ORR) at the cathode is the main obstacle to the practical applications [5e7]. The heteroatom-doped carbon materials as Pt-free catalysts show great potential for ORR due to high electrocatalytic activity, low cost, long-term stability, and environmental friendliness. The introduction of heteroatoms (e.g, N, B and S) into carbon-based materials can lead to the charge
* Corresponding author. E-mail address: [email protected] (Y. Shi). https://doi.org/10.1016/j.ijhydene.2018.08.201 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Wu D, et al., Tea-leaf-residual derived electrocatalyst: Hierarchical pore structure and self nitrogen and fluorine co-doping for efficient oxygen reduction reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.08.201
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density or spin density redistribution, playing a positive role on oxygen absorption and subsequent reduction process [8e15]. Previous studies revealed that, due to synergetic effect of different atoms, the binary or ternary heteroatom-doped carbon materials had higher catalytic activity than those with single doping [16e20]. However, the synthesis of carbon materials with co-doped heteroatoms requires complex procedures or hazardous precursors [10,21,22]. Recent years, researchers are turning their attention to explore an alternative facile and green synthesis method for commercial production of heteroatom-doped carbon materials with high performance for ORR [23e26]. Alternatively, Earth's biodiversity endows our human beings abundant biomass resources that can be easily converted into various function carbon materials through carbonization, offering us an extremely low-cost, facile and environmental friendly strategy. More importantly, biomass is an ideal template so as to obtain hierarchical pore structures to facilitate mass transport and increase surface area simultaneously. In addition, biomass has multi-elements that may produce highly efficient active sites through self-doping. Utilizing biomass as precursors provides us a broad horizon to tune the properties of carbon materials in various catalytic reactions, such as ORR [27e30]. To date, some studies have confirmed the N-doped carbon materials derived from biomass are efficient electrocatalysts for ORR, e.g. coconut shells [31], nori [32], catkin [33], typha orientalis [34], ginkgo leaf [35], etc. Tea is known as a favorite drink all over the world, widely grown in the Asian region especially in China. Each year an enormous number of brewed tea residue has been discarded. To date, preparation of tea leaf into carbon materials for ORR has been merely reported. Recently, Dai and co-worker has prepared un-brewed tea leaf into 3D porous carbon with good performance in ORR, especially after doping with Fe [36]. As mentioned above, so far there have been some reports on heteroatoms-doped carbon materials successfully synthesized by pyrolysis of biomass [37e41]. However, tedious synthetic processes were usually needed to get access to materials with good electrocatalytic performance, for example adding additional organic compound into precursor, annealing in NH3 atmosphere, using NaOH or ZnCl2 for activation and employing MgO or silica as template. In this work,
tea residue rather than un-brewed tea leaf is used as both carbon and heteroatoms sources for preparation of nitrogen and fluorine co-doped carbon material via one-step thermal treatment at 1000 C without any activation or posttreatment, as shown in Fig. 1. The carbon material synthesized in this work demonstrates good structure characteristics and excellent electrocatalytic performance. We believe that our synthetic strategy have great potential for the large-scale commercial production in future.
Experimental section Preparation of N, F-doped porous carbon from tea residue Tea was purchased from a local market in Dalian, brewed several times with boiling water until the tea water changed from dark to colorless. Then the tea residue was collected and washed with deionized water. The tea residue was completely dried at 70 C for 12 h. Subsequently, the tea residue was pyrolyzed at 1000 C for 2 h in a tubular furnace under N2 atmosphere with a heating rate of 10 C min1. After the tubular furnace cooled down to room temperature, the sample was washed with HCl (2.0 M) and deionized water several times to remove metal deposits and followed by drying at 70 C for 12 h. The final product was labeled as T-NFC.
Characterizations The surface morphology of T-NFC was observed via scanning electron microscopy (SEM, S-4800) and transmission electron microscopy (TEM, Titan 80-300). X-ray diffraction (XRD) patterns were obtained using a Bruker-D8 diffractometer with Cu K〈alpha〉 radiation at room temperature. Raman spectra were measured using a Renishaw in via unit with 532 nm laser excitation. X-ray photoelectron spectroscopy (XPS) analysis was carried out with a Thermo Escalab 250Xi unit to research the chemical elements and sample states. Nitrogen adsorption-desorption isotherms were conducted at 77 K using ASAP 2020-Physisorption Analyzer to obtain the Brunauer-Emmett-Teller (BET) surface area and pore size distribution.
Fig. 1 e Fabrication process for the T-NFC catalyst. Please cite this article in press as: Wu D, et al., Tea-leaf-residual derived electrocatalyst: Hierarchical pore structure and self nitrogen and fluorine co-doping for efficient oxygen reduction reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.08.201
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Electrochemical measurements The electrochemical measurements of the sample were carried out on the CHI 700E electrochemical workstation with a three electrodes system. A glassy carbon rotating disk electrode (RDE) (5 mm in diameter, Pine Research) and rotating ring-disk electrode (RRDE) (5 mm in diameter, Pine Research) were used as working electrodes. The counter and reference electrodes were Pt wire and Ag/AgCl filled with saturated KCl aqueous solution, respectively. All the potentials were given with respect to the reversible hydrogen electrode (RHE). The potential values converted to the RHE scale according to the following equation [42]. EðRHEÞ ¼ EðAg=AgClÞ þ EqðAg=AgClÞ þ 0:059PH
(1)
In a typical preparation, 4.0 mg sample was dispersed ultrasonically in 960 mL ethanol and 40 mL Nafion solution (5 wt%) for 60 min. Then, 10 mL catalyst ink was dipped onto the GC surface and then dried thoroughly at room temperature. Catalyst loading on testing electrode was calculated to be 0.2 mg cm2. For comparison, a commercial Pt/C catalyst (20 wt % Pt) was prepared in the same way. The loading of Pt/C was also 0.2 mg cm2. The electrochemical measurements were carried out in O2-saturated 0.1 M KOH solution at room temperature.
Results and discussion Physico-chemical characterisations The morphology and structure of the as-prepared sample was firstly investigated by SEM measurements. As shown in
3
Fig. 2a, the T-NFC displays an irregular 3-dimensional interconnected framework structure with a rather rough surface and a large number of pores with different diameters, which can be attributed to the multistage structure of the raw tea residue. This structure is desired to improve the electrocatalytic property, such as a large surface area and abundant active sites. We performed TEM measurements to further explore the morphology, as indicated in Fig. 2b and d. It can be easily confirmed that the product is composed of wrinkled and stacked graphite-like sheet planes. Moreover, in Fig. 2d we can clearly note that some poorly crystallized particles exist in the edge of the as-synthesized product result from the doping of heteroatoms into carbon substrate. So we can predict that the hierarchically porous structure with active open-edge sites will benefit ORR catalytic activity. Then we carried out XRD measurements to evaluate the formation of graphitic phase. The X-ray diffraction patterns of the sample, as seen in Fig. 3a, exhibit two major diffraction peaks centered at 2q ¼ 23.6 and 43.8 that could be assigned to the (002) and (100) graphite plane. The XRD results confirm the existence of graphitic carbon, endowing our sample with good electrical conductivity [43,44]. Fig. 3b shows Raman spectra of the prepared product, exhibiting the characteristic “D” and “G” peaks at 1350 and 1580 cm1, respectively. Generally, the D band is associated with the A1g vibration mode of the disordered carbon and the G band is attributed to the E2g vibration mode of the ordered graphitic carbon. Normally, we evaluate the structural defects by analyzing the intensity ratio of the D and G bands (ID/IG). Based on the spectra, the ratio ID/IG is about 0.95 for the T-NFC sample, indicating a considerable degree of graphitization. This characterization further demonstrates that the T-NFC sample has a good electrical
Fig. 2 e (a) SEM and (bed) TEM images of the T-NFC sample. Please cite this article in press as: Wu D, et al., Tea-leaf-residual derived electrocatalyst: Hierarchical pore structure and self nitrogen and fluorine co-doping for efficient oxygen reduction reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.08.201
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Fig. 3 e (a) XRD patterns, (b) Raman spectra, (c) nitrogen adsorption-desorption isotherms, (d) pore size distribution of T-NFC.
conductivity as confirmed by XRD results. In addition, we also observe a broader 2D peak at about 2800 cm1, which is due to N defects in the graphene layers as predicted in several previous studies [45,46]. Fig. 3c and d show the nitrogen adsorptionedesorption isotherms and the curve pore size distribution, respectively. The isotherms shape is classified as the type IV isotherm with a typical hysteresis loop (appearing in the range from P/ P0 ¼ 0.45 to 1.0), which usually correspond to materials with mesoporous structure. The pore size distribution curve in Fig. 3d clearly indicates that the diameter of mesopores is centered around 3 nm. The BET specific surface area and the total pore volume of the T-NFC sample are determined to be 855.6 m2 g1 and 0.65 cm3 g1, respectively. The high specific surface area and hierarchically porous structure can not only provide more accessible active sites, but also facilitate the diffusion of liquid electrolyte. XPS measurements were performed to confirm the elemental composition and their valence states. As demonstrated in Fig. 4a, we not only observe the presence of carbon and oxygen, the characteristic peaks of nitrogen and fluorine are also found at 400.5 eV and 687.5 eV respectively. Surface content of each element including C, O, N and F are depicted in Fig. 4b. The atomic percentages of N and F were calculated to be 2.8% and 2.2%, respectively. The high-resolution N 1s XPS spectrum is shown in Fig. 4c. Three peaks could be deconvoluted at 398.1 eV, 399.8 eV, 401.0 eV, which correspond to pyridinic N, pyrrolic N and graphitic N, respectively. The percentages of pyridinic nitrogen and graphitic nitrogen are obviously higher than that of pyrrolic nitrogen. Generally, graphitic and pyridinic nitrogen have been proved to play a crucial role in improving the electrocatalytic activity for ORR [47e55]. Fig. 4d shows the high-resolution F 1s XPS spectrum,
in which one dominant peak located at 684.6 eV is clearly found and can be ascribed to CeF bond. As mentioned in previous literature, charge redistribution induced by electronegativity can facilitate electrocatalytic reaction. Therefore, the evident difference of electronegativity between C and F atoms can facilitate electron transfer in ORR. According to above analysis, we draw a conclusion that N and F are successfully doped into our carbon materials.
ORR studies of the T-NFC catalyst We evaluated the electrocatalytic activity for ORR through various electrochemical characterizations. Fig. 5a shows the cyclic voltammetry (CV) of T-NFC in N2e or O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s1. First, no obvious redox peak can be observed in N2-saturated 0.1 M KOH solution. When the electrolyte is saturated with O2, a visible oxygen reduction peak with the potential of 0.75 V (vs. RHE) appears, which qualitatively suggests electrocatalytic properties of the prepared T-NFC for ORR in an alkaline electrolyte. To further assess the ORR catalytic performance of sample, we conducted the linear sweep voltammetry (LSV) measurements in O2-saturated 0.1 M KOH solution. For comparison, we also performed the LSV using commercial Pt/C (the content of Pt is about 20 wt%). As shown in Fig. 5b, the onset and halfwave potentials of the T-NFC sample are located at 0.81 and 0.66 V vs. RHE, respectively, which are lower than those of commercial Pt/C (0.96 and 0.81 V vs. RHE). And the diffusionlimited current density value of the T-NFC sample (5.1 mA cm2) at 0.4 V vs RHE is comparable to that of Pt/C (5.0 mA cm2). The limited current density is closely related to the electrocatalytic activity which is determined by number of active sites and mass transfer process. The T-NFC is a nitrogen
Please cite this article in press as: Wu D, et al., Tea-leaf-residual derived electrocatalyst: Hierarchical pore structure and self nitrogen and fluorine co-doping for efficient oxygen reduction reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.08.201
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Fig. 4 e (a) XPS survey of the T-NFC; (b) the surface contents of carbon, nitrogen, oxygen and fluorine; (c) high-resolution N1s and (d) high-resolution F1s XPS spectra.
Fig. 5 e (a) Cyclic voltammetry (CV) of T-NFC in N2e or O2-saturated 0.1 M KOH solution; (b) linear sweep voltammetry (LSV) of T-NFC and Pt/C RDE electrodes at 1600 rpm with 10 mV s¡1; (c) polarization curves for ORR in O2-saturated 0.1 M KOH solution on T-NFC electrode at various rotation rates; (d) K-L plots derived from Fig. (c) (insert: the dependence of n on potential).
and fluorine co-doped functional carbon material with high specific surface area and hierarchically porous structure, which can offer abundant active sites and in the meantime facilitate mass transfer. Fig. 5c shows the RDE measurements
of the T-NFC sample for ORR at various rotating speeds from 625 to 2500 rpm. The analysis of Koutecky-Levich (K-L) plots is an effective method to calculate electron transfer numbers for ORR. The K-L equations is shown as follows:
Please cite this article in press as: Wu D, et al., Tea-leaf-residual derived electrocatalyst: Hierarchical pore structure and self nitrogen and fluorine co-doping for efficient oxygen reduction reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.08.201
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Fig. 6 e (a) RRDE voltammograms for T-NFC in an O2-saturated 0.1 M KOH solution at 1600 rpm with 10 mV s¡1; (b) n and H2O2 yield calculated from the RRDE voltammograms for T-NFC. (c) durability test for T-NFC in an O2-saturated 0.1 M KOH solution; (d) methanol selectivity test for T-NFC in an O2-saturated 0.1 M KOH solution (rotation speed 1600 rpm).
1 1 1 ¼ þ j jk Bu12
(2)
B ¼ 0:62nFCO2 DO2 2=3 g1=6
(3)
=
where jk is the kinetic limiting current density, u is the electrode rotating rate (rad s1), F is the Faradaic constant (96485 C mol1), CO2 is the oxygen concentration in 0.1 M KOH (1.2 106 mol cm3), DO2 is the oxygen diffusion coefficient in 0.1 M KOH (1.90 105 cm2s1) [56], g is the kinematic viscosity of the electrolyte solution (0.01 cm2 s1). In Fig. 5d, the datapoints obtained from different rotating speeds exhibit good linearity and the fitting lines at different potentials also show similar slope, suggesting first order reaction for ORR. According to slopes of the K-L plots, the average number of electron transfer (n) is estimated to be ~3.8 per O2 molecule. Hence, we predict the ORR for T-NFC is mainly dominated by an efficient four-electron pathway. The RRDE tests can simultaneously obtain the disk current (ID) and the ring current (IR), and then through equations (4) and (5) to determine the electron transfer number (n) and peroxide yield (H2O2). ID n¼4 ID þ IR =N %ðH2 O2 Þ ¼ 200
(4) IR =N ID þ IR =N
(5)
As presented in Fig. 6a, the IR of the T-NFC is extremely low (approaching zero), indicating the formation of very small amount of H2O2 during ORR. The calculated yield of H2O2 is
less than 2%, and the measured electron transfer number (n) is around 3.98, revealing a dominated four-electron pathway in ORR for our T-NFC. In addition to excellent electrocatalytic activity, long-term durability of ORR electrocatalysts is crucial for practical application in fuel cell. In order to evaluate durability, we recorded the initial LSV curve and final LSV data after 5000 continuous potential cycles, as shown in Fig. 6c. Compared with the initial curve, the final curve exhibits negligible performance loss, suggesting that the as-synthesized catalyst has long-term stability for ORR in O2-saturated 0.1 M KOH solution. The tolerance to the methanol poisoning effect is another essential factor to evaluate the performance of catalysts. To examine the tolerance of the T-NFC product, 3 M methanol was introduced into the electrolyte. Fig. 6d shows that the LSV curves remain nearly unchanged with and without methanol, suggesting the sample is insensitive to methanol and has superior tolerance to the methanol poisoning effect. It can be concluded that the T-NFC catalyst is a good candidate for ORR in the alkaline electrolytes.
Conclusion In summary, N and F co-doped carbon material was successfully produced by the one-step pyrolysis of tea residue at controlled temperature under nitrogen atmosphere without any post treatment (such as the addition of organic compound in the precursor, annealing in NH3 atmosphere, activation process using NaOH or ZnCl2, and employing MgO or silica as template, etc). In our experiment tea residue served as the single precursor for heteroatoms and carbon source, avoiding
Please cite this article in press as: Wu D, et al., Tea-leaf-residual derived electrocatalyst: Hierarchical pore structure and self nitrogen and fluorine co-doping for efficient oxygen reduction reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.08.201
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the employment of hazardous chemicals in the synthesis process. Due to the synergetic effect of N and F co-doping, the high specific surface area, hierarchically porous structure and the high contents of pyridinic and graphitic nitrogen, the obtained sample was proved to be an alternative catalyst for ORR with high catalytic activity, excellent stability and high selectivity. Our work offers a novel strategy to develop heteroatom-doped functionalized carbons based on discarded biomass waste.
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Acknowledgments
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The authors appreciate the financial support from the National Natural Science Foundation of China (Grant No. 51402036 and 51773025), and the International Science & Technology Cooperation Program of China (Grant No. 2013DFA51000).
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Please cite this article in press as: Wu D, et al., Tea-leaf-residual derived electrocatalyst: Hierarchical pore structure and self nitrogen and fluorine co-doping for efficient oxygen reduction reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.08.201
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Tea-leaf-residual derived electrocatalyst: Hierarchical pore structure and self nitrogen and fluorine co-doping for efficient oxygen reduction reaction Danyang Wu, Yantao Shi*, Hongyu Jing, Xun Wang, Xuedan Song, Duanhui Si, Suxia Liang, Ce Hao State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology, Dalian, 116024, Liaoning, China
article info
abstract
Article history:
The heteroatom-doped carbon materials as metal-free catalysts show great potential for
Received 11 April 2018
oxygen reduction reaction (ORR) due to the high electrocatalytic activity, low cost, long-
Received in revised form
term stability, and environmental friendliness. Utilizing biomass as precursors has
29 August 2018
offered facile and extremely low-cost strategy to large-scale fabrication of highly efficient
Accepted 30 August 2018
carbon materials with abundant pore structures and elements. In this work, tea residue
Available online xxx
was used as precursor to synthesize nitrogen (N) and fluorine (F) co-doped porous carbon materials via one-step annealing process without any activation or post-treatment. The
Keywords:
morphology, pore structure, elemental composition of our sample as well as its electro-
Tea-leaf
catalytic performance in ORR were studied. This biomass-based carbon materials has high
Oxygen reduction reaction
specific surface area (855.6 m2 g1) and hierarchical pore structure, which can offer
Nitrogen and fluorine co-doping
abundant active sites and meanwhile is favorable to ion diffusion of liquid electrolyte.
Electrocatalytic activity
Secondly, the heteroatoms (especially F) doping can induce charge redistribution thanks to
Biomass-derived carbon material
the electronegativity difference, which can facilitate electron transfer in ORR. When compared to commercial Pt/C, our catalyst shows a higher limited current density and a high electron transfer number of about 3.8. Finally, the catalyst also demonstrates excellent methanol tolerance in the alkaline medium. Our work provides an economical and facile strategy to synthesize efficient electrocatalyst for ORR. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cell known as a new energy device with environmentfriendly characteristics has high energy conversion efficiency and power density, which favors to address the environmental and energy crisis nowadays [1e4]. However, the
sluggish kinetic of the oxygen reduction reaction (ORR) at the cathode is the main obstacle to the practical applications [5e7]. The heteroatom-doped carbon materials as Pt-free catalysts show great potential for ORR due to high electrocatalytic activity, low cost, long-term stability, and environmental friendliness. The introduction of heteroatoms (e.g, N, B and S) into carbon-based materials can lead to the charge
* Corresponding author. E-mail address: [email protected] (Y. Shi). https://doi.org/10.1016/j.ijhydene.2018.08.201 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Wu D, et al., Tea-leaf-residual derived electrocatalyst: Hierarchical pore structure and self nitrogen and fluorine co-doping for efficient oxygen reduction reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.08.201
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density or spin density redistribution, playing a positive role on oxygen absorption and subsequent reduction process [8e15]. Previous studies revealed that, due to synergetic effect of different atoms, the binary or ternary heteroatom-doped carbon materials had higher catalytic activity than those with single doping [16e20]. However, the synthesis of carbon materials with co-doped heteroatoms requires complex procedures or hazardous precursors [10,21,22]. Recent years, researchers are turning their attention to explore an alternative facile and green synthesis method for commercial production of heteroatom-doped carbon materials with high performance for ORR [23e26]. Alternatively, Earth's biodiversity endows our human beings abundant biomass resources that can be easily converted into various function carbon materials through carbonization, offering us an extremely low-cost, facile and environmental friendly strategy. More importantly, biomass is an ideal template so as to obtain hierarchical pore structures to facilitate mass transport and increase surface area simultaneously. In addition, biomass has multi-elements that may produce highly efficient active sites through self-doping. Utilizing biomass as precursors provides us a broad horizon to tune the properties of carbon materials in various catalytic reactions, such as ORR [27e30]. To date, some studies have confirmed the N-doped carbon materials derived from biomass are efficient electrocatalysts for ORR, e.g. coconut shells [31], nori [32], catkin [33], typha orientalis [34], ginkgo leaf [35], etc. Tea is known as a favorite drink all over the world, widely grown in the Asian region especially in China. Each year an enormous number of brewed tea residue has been discarded. To date, preparation of tea leaf into carbon materials for ORR has been merely reported. Recently, Dai and co-worker has prepared un-brewed tea leaf into 3D porous carbon with good performance in ORR, especially after doping with Fe [36]. As mentioned above, so far there have been some reports on heteroatoms-doped carbon materials successfully synthesized by pyrolysis of biomass [37e41]. However, tedious synthetic processes were usually needed to get access to materials with good electrocatalytic performance, for example adding additional organic compound into precursor, annealing in NH3 atmosphere, using NaOH or ZnCl2 for activation and employing MgO or silica as template. In this work,
tea residue rather than un-brewed tea leaf is used as both carbon and heteroatoms sources for preparation of nitrogen and fluorine co-doped carbon material via one-step thermal treatment at 1000 C without any activation or posttreatment, as shown in Fig. 1. The carbon material synthesized in this work demonstrates good structure characteristics and excellent electrocatalytic performance. We believe that our synthetic strategy have great potential for the large-scale commercial production in future.
Experimental section Preparation of N, F-doped porous carbon from tea residue Tea was purchased from a local market in Dalian, brewed several times with boiling water until the tea water changed from dark to colorless. Then the tea residue was collected and washed with deionized water. The tea residue was completely dried at 70 C for 12 h. Subsequently, the tea residue was pyrolyzed at 1000 C for 2 h in a tubular furnace under N2 atmosphere with a heating rate of 10 C min1. After the tubular furnace cooled down to room temperature, the sample was washed with HCl (2.0 M) and deionized water several times to remove metal deposits and followed by drying at 70 C for 12 h. The final product was labeled as T-NFC.
Characterizations The surface morphology of T-NFC was observed via scanning electron microscopy (SEM, S-4800) and transmission electron microscopy (TEM, Titan 80-300). X-ray diffraction (XRD) patterns were obtained using a Bruker-D8 diffractometer with Cu K〈alpha〉 radiation at room temperature. Raman spectra were measured using a Renishaw in via unit with 532 nm laser excitation. X-ray photoelectron spectroscopy (XPS) analysis was carried out with a Thermo Escalab 250Xi unit to research the chemical elements and sample states. Nitrogen adsorption-desorption isotherms were conducted at 77 K using ASAP 2020-Physisorption Analyzer to obtain the Brunauer-Emmett-Teller (BET) surface area and pore size distribution.
Fig. 1 e Fabrication process for the T-NFC catalyst. Please cite this article in press as: Wu D, et al., Tea-leaf-residual derived electrocatalyst: Hierarchical pore structure and self nitrogen and fluorine co-doping for efficient oxygen reduction reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.08.201
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Electrochemical measurements The electrochemical measurements of the sample were carried out on the CHI 700E electrochemical workstation with a three electrodes system. A glassy carbon rotating disk electrode (RDE) (5 mm in diameter, Pine Research) and rotating ring-disk electrode (RRDE) (5 mm in diameter, Pine Research) were used as working electrodes. The counter and reference electrodes were Pt wire and Ag/AgCl filled with saturated KCl aqueous solution, respectively. All the potentials were given with respect to the reversible hydrogen electrode (RHE). The potential values converted to the RHE scale according to the following equation [42]. EðRHEÞ ¼ EðAg=AgClÞ þ EqðAg=AgClÞ þ 0:059PH
(1)
In a typical preparation, 4.0 mg sample was dispersed ultrasonically in 960 mL ethanol and 40 mL Nafion solution (5 wt%) for 60 min. Then, 10 mL catalyst ink was dipped onto the GC surface and then dried thoroughly at room temperature. Catalyst loading on testing electrode was calculated to be 0.2 mg cm2. For comparison, a commercial Pt/C catalyst (20 wt % Pt) was prepared in the same way. The loading of Pt/C was also 0.2 mg cm2. The electrochemical measurements were carried out in O2-saturated 0.1 M KOH solution at room temperature.
Results and discussion Physico-chemical characterisations The morphology and structure of the as-prepared sample was firstly investigated by SEM measurements. As shown in
3
Fig. 2a, the T-NFC displays an irregular 3-dimensional interconnected framework structure with a rather rough surface and a large number of pores with different diameters, which can be attributed to the multistage structure of the raw tea residue. This structure is desired to improve the electrocatalytic property, such as a large surface area and abundant active sites. We performed TEM measurements to further explore the morphology, as indicated in Fig. 2b and d. It can be easily confirmed that the product is composed of wrinkled and stacked graphite-like sheet planes. Moreover, in Fig. 2d we can clearly note that some poorly crystallized particles exist in the edge of the as-synthesized product result from the doping of heteroatoms into carbon substrate. So we can predict that the hierarchically porous structure with active open-edge sites will benefit ORR catalytic activity. Then we carried out XRD measurements to evaluate the formation of graphitic phase. The X-ray diffraction patterns of the sample, as seen in Fig. 3a, exhibit two major diffraction peaks centered at 2q ¼ 23.6 and 43.8 that could be assigned to the (002) and (100) graphite plane. The XRD results confirm the existence of graphitic carbon, endowing our sample with good electrical conductivity [43,44]. Fig. 3b shows Raman spectra of the prepared product, exhibiting the characteristic “D” and “G” peaks at 1350 and 1580 cm1, respectively. Generally, the D band is associated with the A1g vibration mode of the disordered carbon and the G band is attributed to the E2g vibration mode of the ordered graphitic carbon. Normally, we evaluate the structural defects by analyzing the intensity ratio of the D and G bands (ID/IG). Based on the spectra, the ratio ID/IG is about 0.95 for the T-NFC sample, indicating a considerable degree of graphitization. This characterization further demonstrates that the T-NFC sample has a good electrical
Fig. 2 e (a) SEM and (bed) TEM images of the T-NFC sample. Please cite this article in press as: Wu D, et al., Tea-leaf-residual derived electrocatalyst: Hierarchical pore structure and self nitrogen and fluorine co-doping for efficient oxygen reduction reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.08.201
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Fig. 3 e (a) XRD patterns, (b) Raman spectra, (c) nitrogen adsorption-desorption isotherms, (d) pore size distribution of T-NFC.
conductivity as confirmed by XRD results. In addition, we also observe a broader 2D peak at about 2800 cm1, which is due to N defects in the graphene layers as predicted in several previous studies [45,46]. Fig. 3c and d show the nitrogen adsorptionedesorption isotherms and the curve pore size distribution, respectively. The isotherms shape is classified as the type IV isotherm with a typical hysteresis loop (appearing in the range from P/ P0 ¼ 0.45 to 1.0), which usually correspond to materials with mesoporous structure. The pore size distribution curve in Fig. 3d clearly indicates that the diameter of mesopores is centered around 3 nm. The BET specific surface area and the total pore volume of the T-NFC sample are determined to be 855.6 m2 g1 and 0.65 cm3 g1, respectively. The high specific surface area and hierarchically porous structure can not only provide more accessible active sites, but also facilitate the diffusion of liquid electrolyte. XPS measurements were performed to confirm the elemental composition and their valence states. As demonstrated in Fig. 4a, we not only observe the presence of carbon and oxygen, the characteristic peaks of nitrogen and fluorine are also found at 400.5 eV and 687.5 eV respectively. Surface content of each element including C, O, N and F are depicted in Fig. 4b. The atomic percentages of N and F were calculated to be 2.8% and 2.2%, respectively. The high-resolution N 1s XPS spectrum is shown in Fig. 4c. Three peaks could be deconvoluted at 398.1 eV, 399.8 eV, 401.0 eV, which correspond to pyridinic N, pyrrolic N and graphitic N, respectively. The percentages of pyridinic nitrogen and graphitic nitrogen are obviously higher than that of pyrrolic nitrogen. Generally, graphitic and pyridinic nitrogen have been proved to play a crucial role in improving the electrocatalytic activity for ORR [47e55]. Fig. 4d shows the high-resolution F 1s XPS spectrum,
in which one dominant peak located at 684.6 eV is clearly found and can be ascribed to CeF bond. As mentioned in previous literature, charge redistribution induced by electronegativity can facilitate electrocatalytic reaction. Therefore, the evident difference of electronegativity between C and F atoms can facilitate electron transfer in ORR. According to above analysis, we draw a conclusion that N and F are successfully doped into our carbon materials.
ORR studies of the T-NFC catalyst We evaluated the electrocatalytic activity for ORR through various electrochemical characterizations. Fig. 5a shows the cyclic voltammetry (CV) of T-NFC in N2e or O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s1. First, no obvious redox peak can be observed in N2-saturated 0.1 M KOH solution. When the electrolyte is saturated with O2, a visible oxygen reduction peak with the potential of 0.75 V (vs. RHE) appears, which qualitatively suggests electrocatalytic properties of the prepared T-NFC for ORR in an alkaline electrolyte. To further assess the ORR catalytic performance of sample, we conducted the linear sweep voltammetry (LSV) measurements in O2-saturated 0.1 M KOH solution. For comparison, we also performed the LSV using commercial Pt/C (the content of Pt is about 20 wt%). As shown in Fig. 5b, the onset and halfwave potentials of the T-NFC sample are located at 0.81 and 0.66 V vs. RHE, respectively, which are lower than those of commercial Pt/C (0.96 and 0.81 V vs. RHE). And the diffusionlimited current density value of the T-NFC sample (5.1 mA cm2) at 0.4 V vs RHE is comparable to that of Pt/C (5.0 mA cm2). The limited current density is closely related to the electrocatalytic activity which is determined by number of active sites and mass transfer process. The T-NFC is a nitrogen
Please cite this article in press as: Wu D, et al., Tea-leaf-residual derived electrocatalyst: Hierarchical pore structure and self nitrogen and fluorine co-doping for efficient oxygen reduction reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.08.201
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Fig. 4 e (a) XPS survey of the T-NFC; (b) the surface contents of carbon, nitrogen, oxygen and fluorine; (c) high-resolution N1s and (d) high-resolution F1s XPS spectra.
Fig. 5 e (a) Cyclic voltammetry (CV) of T-NFC in N2e or O2-saturated 0.1 M KOH solution; (b) linear sweep voltammetry (LSV) of T-NFC and Pt/C RDE electrodes at 1600 rpm with 10 mV s¡1; (c) polarization curves for ORR in O2-saturated 0.1 M KOH solution on T-NFC electrode at various rotation rates; (d) K-L plots derived from Fig. (c) (insert: the dependence of n on potential).
and fluorine co-doped functional carbon material with high specific surface area and hierarchically porous structure, which can offer abundant active sites and in the meantime facilitate mass transfer. Fig. 5c shows the RDE measurements
of the T-NFC sample for ORR at various rotating speeds from 625 to 2500 rpm. The analysis of Koutecky-Levich (K-L) plots is an effective method to calculate electron transfer numbers for ORR. The K-L equations is shown as follows:
Please cite this article in press as: Wu D, et al., Tea-leaf-residual derived electrocatalyst: Hierarchical pore structure and self nitrogen and fluorine co-doping for efficient oxygen reduction reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.08.201
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Fig. 6 e (a) RRDE voltammograms for T-NFC in an O2-saturated 0.1 M KOH solution at 1600 rpm with 10 mV s¡1; (b) n and H2O2 yield calculated from the RRDE voltammograms for T-NFC. (c) durability test for T-NFC in an O2-saturated 0.1 M KOH solution; (d) methanol selectivity test for T-NFC in an O2-saturated 0.1 M KOH solution (rotation speed 1600 rpm).
1 1 1 ¼ þ j jk Bu12
(2)
B ¼ 0:62nFCO2 DO2 2=3 g1=6
(3)
=
where jk is the kinetic limiting current density, u is the electrode rotating rate (rad s1), F is the Faradaic constant (96485 C mol1), CO2 is the oxygen concentration in 0.1 M KOH (1.2 106 mol cm3), DO2 is the oxygen diffusion coefficient in 0.1 M KOH (1.90 105 cm2s1) [56], g is the kinematic viscosity of the electrolyte solution (0.01 cm2 s1). In Fig. 5d, the datapoints obtained from different rotating speeds exhibit good linearity and the fitting lines at different potentials also show similar slope, suggesting first order reaction for ORR. According to slopes of the K-L plots, the average number of electron transfer (n) is estimated to be ~3.8 per O2 molecule. Hence, we predict the ORR for T-NFC is mainly dominated by an efficient four-electron pathway. The RRDE tests can simultaneously obtain the disk current (ID) and the ring current (IR), and then through equations (4) and (5) to determine the electron transfer number (n) and peroxide yield (H2O2). ID n¼4 ID þ IR =N %ðH2 O2 Þ ¼ 200
(4) IR =N ID þ IR =N
(5)
As presented in Fig. 6a, the IR of the T-NFC is extremely low (approaching zero), indicating the formation of very small amount of H2O2 during ORR. The calculated yield of H2O2 is
less than 2%, and the measured electron transfer number (n) is around 3.98, revealing a dominated four-electron pathway in ORR for our T-NFC. In addition to excellent electrocatalytic activity, long-term durability of ORR electrocatalysts is crucial for practical application in fuel cell. In order to evaluate durability, we recorded the initial LSV curve and final LSV data after 5000 continuous potential cycles, as shown in Fig. 6c. Compared with the initial curve, the final curve exhibits negligible performance loss, suggesting that the as-synthesized catalyst has long-term stability for ORR in O2-saturated 0.1 M KOH solution. The tolerance to the methanol poisoning effect is another essential factor to evaluate the performance of catalysts. To examine the tolerance of the T-NFC product, 3 M methanol was introduced into the electrolyte. Fig. 6d shows that the LSV curves remain nearly unchanged with and without methanol, suggesting the sample is insensitive to methanol and has superior tolerance to the methanol poisoning effect. It can be concluded that the T-NFC catalyst is a good candidate for ORR in the alkaline electrolytes.
Conclusion In summary, N and F co-doped carbon material was successfully produced by the one-step pyrolysis of tea residue at controlled temperature under nitrogen atmosphere without any post treatment (such as the addition of organic compound in the precursor, annealing in NH3 atmosphere, activation process using NaOH or ZnCl2, and employing MgO or silica as template, etc). In our experiment tea residue served as the single precursor for heteroatoms and carbon source, avoiding
Please cite this article in press as: Wu D, et al., Tea-leaf-residual derived electrocatalyst: Hierarchical pore structure and self nitrogen and fluorine co-doping for efficient oxygen reduction reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.08.201
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the employment of hazardous chemicals in the synthesis process. Due to the synergetic effect of N and F co-doping, the high specific surface area, hierarchically porous structure and the high contents of pyridinic and graphitic nitrogen, the obtained sample was proved to be an alternative catalyst for ORR with high catalytic activity, excellent stability and high selectivity. Our work offers a novel strategy to develop heteroatom-doped functionalized carbons based on discarded biomass waste.
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Acknowledgments
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The authors appreciate the financial support from the National Natural Science Foundation of China (Grant No. 51402036 and 51773025), and the International Science & Technology Cooperation Program of China (Grant No. 2013DFA51000).
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Please cite this article in press as: Wu D, et al., Tea-leaf-residual derived electrocatalyst: Hierarchical pore structure and self nitrogen and fluorine co-doping for efficient oxygen reduction reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.08.201