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Confined polyaniline derived mesoporous carbon for oxygen reduction reaction
European Polymer Journal 88 (2017) 1–8
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Confined polyaniline derived mesoporous carbon for oxygen reduction reaction Shuangxi Xing a,⇑, Xiaodan Yu a, Guibao Wang b, Yue Yu a, Yuanhong Wang a, Yan Xing a,⇑ a b
Faculty of Chemistry, Northeast Normal University, Changchun 130024, PR China Changchun Institute of Engineering Technology, Changchun 130024, PR China
a r t i c l e
i n f o
Article history: Received 13 December 2016 Received in revised form 30 December 2016 Accepted 10 January 2017 Available online 11 January 2017 Keywords: Polyaniline SiO2 Carbon Oxygen reduction reaction
a b s t r a c t Based on the previous generation of yolk-shell nanostructured polyaniline@SiO2 particles, a confined carbonization process followed by etching the SiO2 layer is carried out to achieve carbon particles. The existence of the SiO2 shell helps to preserve the original well-dispersed morphology of the polyaniline that retains amounts of active sites for oxygen reduction reaction. Furthermore, the CeO2 oxidant and the SiO2 shell lead to the increased amount of oxygen existing in the product and the oxygen reduction reaction results show the well-maintained nanostructure with mesopores and the effective O doping benefit for achieving enhanced catalytic activity. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction In order to solve the energy problems, fuel cells and metal-air batteries are constructed and developed quickly in recent years [1–4]. However, the sluggish reaction in the cathode (oxygen reduction reaction, ORR) often inhibits the effective utilization of the devices. Pt/C has been considered as the most effective one to catalyze the oxygen reduction reaction in cathode for its low overpotential and high current density. Nevertheless, the Pt/C catalysts often suffer from CO poisoning and unsatisfied cycling stability. To solve these problems, kinds of substitutes have been adopted, including alloyed metals, metal oxides/sulphides/nitrides/carbides and heteroatom doped carbon materials [5–12]. Carbon has been widely used in the field of energy storage and conversion [13–17], and the carbon materials doped with different non-metal elements, such as N, P, S and P, received much attention for the ORR application. The doping of these atoms into the carbon can alter the charge density of the centred C owing the different electron negativity or spin density, which facilitates the oxygen reduction process. To achieve such doped carbon, various strategies have been desired, for example, thermally annealing graphic oxides in the presence of ammonia and boric acid and direct carbonization precursors doped with heteroatoms [11,12]. The carbonization of the conducting polymers, such as polyaniline (PANI) and polypyrrole, has been considered as a facile route to directly obtain N-doped carbon and the catalytic measurement has confirmed their excellent performance in oxygen reduction reaction and even oxygen evolution reaction [18–20]. Introducing proper elements can lead to the formation of multiple heteroatom-doped carbon catalysts. For instance, N- and O-doped mesoporous carbons were generated via pyrolysis of the PANI confining in pores of SBA-15 that could efficiently catalyze ORR with high current density and low overpotential [18]; Poly(o-phenylenediamine) doped with glycine can also be carbonized to produce N- and O-doped carbon for supercapacitors [21]. It should be noted there are seldom researches reporting the generation of doped C mainly/ ⇑ Corresponding authors. E-mail addresses: [email protected] (S. Xing), [email protected] (Y. Xing). http://dx.doi.org/10.1016/j.eurpolymj.2017.01.011 0014-3057/Ó 2017 Elsevier Ltd. All rights reserved.
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dominatingly doped with O, which is considered to increase the ORR activity upon increasing the O dopant [19]. Apart from that, in order to attain satisfied catalytic performance, the well-constructed nanostructure should be realized to keep the high specific surface area and attainable mesopores. On the other hand, as mentioned above, the confined PANI has been pyrolized into mesoporous carbon and presented excellent electrochemical performance [18]. In this issue, the limited space for loading the carbon-precursor plays an important role to maintain the morphology of the materials during the carbonization process and ensures the achievement of high specific surface area and mesoporous nanostructure [19,20,22]. Many kinds of materials were selected to construct the proper space, including SiO2, SBA-15, montmorillonite and polyvinyl pyrrolidone. In this work, the confined PANI nanoparticles in SiO2 shell that were generated using the CeO2 cores as sacrificing oxidants [23] were pyrolized at high temperature to produce carbon materials. The existence of the SiO2 as protecting layer and the utilization of CeO2 as oxidants led to the effective O doping and the N exhausting/losing occurred at the same time. The confined of the SiO2 allowed the generation of mesoporous nanostructure for the derived carbon. As a result, the product displayed excellent ORR catalytic activity owing to the effective O doping and proper mesoporous nanostructure. 2. Experiments 2.1. Materials All solutions were prepared using ultrapure water with resistance of 18.2 MX cm1. Cerium(III) nitrate hexahydrate (99%), Nafion (5 wt%, Aldrich), acetic acid, ethyl silicate ethylene glycol, isopropanol, ammonia solution, hydrochloric acid, sodium hydroxide and potassium hydroxide (Beijing Chemical), polyvinylpyrrolidone (K15, TCI) were used as received. Aniline (98%, TCI) was distilled before use. 2.2. Synthesis of PANI@SiO2 The synthesis of PANI@SiO2 followed our previously reported procedure. Briefly, CeO2 nanoparticles (NPs) obtained from a typical hydrothermal route were coated by a layer of SiO2 via a Stöber method. After that, the CeO2@SiO2 NPs were incubated in aniline solution followed by addition of HCl aqueous solution to induce the polymerization of aniline in the SiO2 shell. 2.3. Synthesis of carbonized PANI The obtained PANI@SiO2 NPs were pyrolized under an N2 atmosphere in a tube furnace. The temperature was raised to a predetermined value (700, 800, 900, 1000 °C) at a rate of 5 °C min1 and was kept stable for 2 h. After that, the sample was cooled to room temperature and the so-obtained product was dispersed in NaOH aqueous solution (0.5 mg mL1). The mixture was then transferred into a Teflon-line stainless-steel autoclave and maintained at 150 °C for 24 h. Finally, the product was cooled to room temperature and was marked as C-NCNP-700 (800, 900 and 1000). 2.4. Characterization Transmission electron microscopic (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were obtained on a JEM-2100F microscope with an accelerating voltage of 200 kV. Scanning electron microscope (SEM) images were collected on a JEOL SM-6360LV field-emission-gun scanning electron microscope. The Brunauer–Emmett–Tel ler (BET) surface area was calculated by nitrogen adsorption and desorption. A Thermo ESCALAB 250X-ray photoelectron spectroscope (XPS) equipped with a standard and monochromatic source (Al Ka hm = 1486.6 eV) was employed for surface analysis. Raman spectra were conducted on a LabRAM XploRA laser Raman spectrometer (HORIBA Jobin Yvon CO. Ltd) using a 532 nm laser with incident power of 1 mW. 2.5. Oxygen reduction reaction measurements The ORR activity of the catalysts was investigated via the rotating disk electrode (RDE) technique accompanied by an electrochemical workstation (CHI660E) with a typical three-electrode system. The glassy carbon (GC) rotating ring-disk electrode (RDE, PINE, 5 mm in diameter and 0.196 cm2 in area) was used as working electrode. The platinum sheet and Ag/AgCl electrode were used as counter and reference electrode, respectively. KOH (0.1 M) aqueous solution was used as electrolyte and high-purity O2 was purged into the above solution for 30 min to give oxygen saturated one. To prepare the working electrode, a certain amount of catalyst was dispersed in a mixture containing Nafion (5 wt%), isopropanol and water (V:V:V = 5:19:76) and give a ink with concentration of 4 mg mL1. The catalyst ink was then dipped onto the surface of the pre-polished glassy carbon electrode and a final mass loading of 0.20 mg cm2 was achieved. Linear sweep voltammetry (LSV) curves were collected at various rotating speeds ranging from 400 to 1600 rpm with a sweeping rate of
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5 mV s1. All potentials in this work were applied against the reversible hydrogen electrode (RHE) using the following equation:
ERHE ¼ EAg=AgCl þ 0:1976V þ 0:059 pH Chronoamperometric responses were conducted at 0.65 V vs. RHE for 10,000 s under the rotating rate of 1600 rpm. The Koutechy-Levich (K-L) plots were analyzed at different electrode potentials. The electron transfer number (n) could be calculated from the slope of the K-L plot based on the K-L equation:
1 1 1 1 1 ¼ þ ¼ þ j jL jK Bx1=2 jK B ¼ 0:62nFC 0 ðD0 Þ2=3 ðtÞ1=6 where j represents the measured current density, jK is the kinetic limiting current density, x is the electrode rotating rate, jL is the diffusion-limiting current density, F is the Faraday constant (F = 96,485 C mol1), C0 is the bulk concentration of O2 (C0 = 1.2 106 mol cm3), D0 is the diffusion coefficient of O2, and m is the kinematic viscosity of the electrolyte. 3. Results and discussion The confined polymerization of aniline is realized based on our previously reported method [23], where the CeO2 particles encapsulated in SiO2 layer are utilized as a sacrificed reactive template. After addition of aniline and acid, the Ce4+ ions slowly release to oxidize the aniline molecules that penetrate the SiO2 shell. The obtained yolk-shell nanostructured PANI@SiO2 particles are heated at high temperature to carbonize the PANI. Finally, the SiO2 shell is etched by NaOH aqueous solution to achieve the doped carbon particles. In order to set the optimal carbonization condition, the PANI@SiO2 particles were thermally treated at different temperature. After removing the SiO2 shell, the products were evaluated on their ORR performance in 0.1 M KOH aqueous
Fig. 1. (a) LSV curves for the samples pyrolized at different temperatures in O2-saturated 0.1 M KOH with rotating speed of 1600 rpm; (b) CV curves of C-NCNP-900 recorded in N2 and O2 saturated 0.1 M KOH; (c) LSV curves of C-NCNP-900 measured at different rotating rates; (d) K-L plots of C-NCNP-900 in various potential range (vs. RHE).
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electrolytes. The polarization curves are presented in Fig. 1. The sample C-NCNP-900 displays the most positive onset potentials with the lowest overpotential for ORR (0.85 V). Furthermore, C-NCNP-900 gives the highest kinetic current density among these samples (Table 1). All these data demonstrate the carbonized PANI obtained from 900 °C heat treatment behaves the best electrocatalytic performance for ORR application. Therefore, C-NCNP-900 was selected as a typical example for further investigation. The cyclic voltammeter (CV) curves of C-NCNP-900 in N2 or O2-saturated aqueous KOH solution are shown in Fig. 1b, where no observable reduction peak is detected in the former system, while a well-defined cathodic peak centred at 0.85 V is found in the later one, indicating its ORR activity. The ORR polarization curves at different rotation rates along with the corresponding K-L plots are presented in Fig. 1c and d. The current density increases steadily when the rotation rate is changed from 400 to 1600 rpm. The slopes maintain nearly constant within the range from 0.2 to 0.4 V, indicating the similar electron transfer numbers, which is calculated from the slopes as ca. 4. In comparison, C-NCNP-700 and C-NCNP-1000 both present an average electron transfer number of 2 between the same potential ranges. Besides, C-NCNP-800 illustrates an unstable electron transfer number from 4 to 2.5 upon increasing the potential from 0.2 to 0.5 V. This further reveals the best ORR performance of the sample obtained at 900 °C. As a control experiment, another sample by using CeO2 particles without SiO2 shell were used as reactive templates to oxidize aniline to generate PANI, which was further carbonized at 900 °C and labelled as NCNP-900. The LSV curves of the two samples (C-NCNP-900 and NCNP-900) along with the commercial Pt/C catalyst are shown in Fig. 2a. NCNP-900 gives a most negative onset potential at 0.75 V and lowest current density (Table 1). Besides, the electron transfer number originated from this sample is about 2.4 in the range of 0.2–0.5 V, all of which demonstrate the worse electrocatalytic performance for NCNP-900 than C-NCNP-900 (Fig. 2b). In order to make clear the difference of the ORR activity between these two samples, the morphology was first examined by TEM and SEM. After pyrolyzed at 900 °C, the yolk-shell PANI@SiO2 maintains well its original shape with a moving core in the shell (Fig. 3a). However, the size of the core turns smaller (64 nm vs. 57 nm) because of the contraction effect during the heating process [24]. After etching process, the cores are well separated and give a raspberry-like morphology with rough surface (Fig. 3b). The particles are swollen by the etching solution and give an increasing diameter of about 86 nm. The SEM image in Fig. 3d reveals these cores are assembled by many smaller nanoparticles with the average size of 14 nm. The HRTEM image demonstrates no ordered structure exists in the sample, indicating C-NCNP-900 consists of amorphous carbon (Fig. 3c). In comparison, NCNP-900 that was prepared from the sample directly via using CeO2 as oxidant in the presence of acid (Fig. 4a) gives an apparent aggregated morphology (Fig. 4b). This confirms the advantage of the confined effect that a separated space for carbonization of the precursor is beneficial for achieving products with uniform nanostructure. The corresponding SEM image of NCNP-900 (Fig. 4c) displays the accumulation of layered sheets/particles. This different morphological result seems inducing great difference in the specific surface area, which directly influence the ORR performance, as concluded in many reports. However, it is found the two samples present almost same specific surface area (688.1 m2 g1 for C-NCNP-900 vs. 686.6 m2 g1 for NCNP-900). This might be related to the presence of a great deal of micropores in both of the samples, as shown in Fig. 5a. Further analysis reveals larger amounts of mesopores exist in C-NCNP-900 than in NCNP-900 (Fig. 5b), which is beneficial to the kinetic mass transport during the ORR process. In this case, the intermediates are easier to penetrate the inside of the mesopores to undergo a complete 4 electron reduction way; on the contrary, a 2 electron reduction process dominates the whole reaction because the intermediates are difficult to enter the micropores and leave the surface quickly. The Raman spectra of the two kinds of samples were collected to further seek the reason for the difference of the ORR activities between them. Two prominent peaks located at ca. 1350 and 1596 cm1 are observed (Fig. 5c), where the former one is induced by the topological defects in the carbonized PANI and the later one originates from the crystalline graphitic domains. The two peaks are commonly ascribed to D and G bands of distorted graphitic carbon materials and a higher relative intensity (ID/IG) with more defects in the sample indicates a better electrocatalytic performance owing to the presence of larger amounts of catalytic active sites. C-NCNP-900 illustrates a higher value of ID/IG (0.982 vs. 0.928), consistent with its better ORR performance [25]. The N content in the doped carbon materials is another important factor to affect the electrocatalytic performance of the samples. A higher N content commonly means a better ORR activity because more positive charged C atoms are formed via
Table 1 Comparison of the ORR catalytic performances. Sample
C-NCNP-700
C-NCNP-800
C-NCNP-900
Eonset (V) j-0.4V (mA cm2) Sample Eonset (V) j-0.4V (mA cm2)
0.65 0.53 C-NCNP-1000 0.68 0.60
0.74 1.41 NCNP-900 0.75 1.47
0.85 2.76 Pt/C 0.96 5.38
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Fig. 2. (a) LSV curves of C-NCNP-900, NCNP-900 and Pt/C with rotating speed of 1600 rpm; (b) the electron transfer numbers of different samples in the potential range of 0.2 V and 0.5 V on the basis of K-L equations.
Fig. 3. (a) TEM image of carbonized PANI@SiO2 achieved at 900 °C before etching; TEM (b), HRTEM (c) and SEM (d) images of C-NCNP-900; the inset in (a) presents the TEM image of PANI@SiO2 before carbonization.
the neighbouring N atoms with larger electronegativity (3.04 for N vs. 2.55 for C). However, there are only tiny amount of N (0.21%) in C-NCNP-900, much less than that of NCNP-900 (2.22%). On the contrary, C-NCNP-900 presents a greater O content than NCNP-900 with the value of 19.12% vs. 6.41%, respectively, as shown in Fig. 5d. The higher O amount should be related to the combination of original CeO2 cores and the SiO2 protection. Meanwhile, the O doping is considered to be beneficial to improve the ORR performance since the co-doping of N and O may generate asymmetric spins and better charged structure [18,19].
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Fig. 4. (a) TEM image PANI obtained without SiO2 shell; TEM (b) and SEM (c) images of NCNP-900.
Fig. 5. N2 adsorption-desorption isotherms (a), pore size distribution (b), Raman (c) and XPS (d) spectra of C-NCNP-900 and NCNP-900.
In order to further evaluate the stability of the samples for electrocatalytic ORR, chronoamperometry test was carried out on C-NCNP-900, NCNP-900 and commercial Pt/C. After 10,000 s, the later two shows a quick degradation in the first 4000 and 2000 s, respectively, and maintains ca. 60% of original value. In comparison, C-NCNP-900 presents a much slower decrease rate and only 10% current density fading is observed, demonstrating its much better stability than the other two. Furthermore, methanol crossover often occurs in the ORR catalytic systems. As revealed in Fig. 6, a remarkable current decreasing is observed upon addition of 11.03 mL of methanol (Cfinal = 3 M) for the Pt/C-containing systems. On the contrary, little current change is found for C-NCNP-900, demonstrating its strong tolerance to methanol crossover, which is also much better than that of NCNP-900.
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Fig. 6. (a) Chronoamperometric curves of different samples at 0.65 V; (b) chronoamperometric curves of different samples at 0.65 V before and after addition of methanol. All measurements were conducted in an O2-saturated 0.1 M KOH aqueous solution with an electrode rotating rate of 1600 rpm.
4. Conclusions Polyaniline nanospheres confined in SiO2 shell that were synthesized using CeO2 cores as reactive sacrificed oxidant were carbonized to generate O-dominatingly doped carbon particles after high temperature pyrolysis and etching processes. The SiO2 layer helped maintaining the nanostructure of the original polyaniline, which endowed the product large specific surface area and mesoporous structure. Meanwhile, the usage of CeO2 as oxidant and SiO2 as protecting layer benefited for the effective O doping for the carbonized polyaniline. As a result, the sample presented higher catalytic activity towards oxygen reduction reaction than that obtained without SiO2 protection, although the later one showed higher N doping amount. Furthermore, the sample illustrated high stability and tolerance to CH3OH, indicating its proper application in energy conversion. Acknowledgements The authors thank Jilin Provincial Science and Technology Development Foundation (Grant No. 20140101109JC), 12th Five-Year Plan in Science and Technology of the Education Department of Jilin Province (2015-546) and Jilin Provincial Key Laboratory of Advanced Energy Materials (Northeast Normal University) for financial support. References [1] J. Wang, B. Li, T. Yersak, D. Yang, Q. Xiao, J. Zhang, C. Zhang, Recent advances in Pt-based octahedral nanocrystals as high performance fuel cell catalysts, J. Mater. Chem. A 4 (2016) 11559–11581. [2] D.U. Lee, P. Xu, Z.P. Cano, A.G. Kashkooli, M.G. Park, Z. Chen, Recent progress and perspectives on bi-functional oxygen electrocatalysts for advanced rechargeable metal-air batteries, J. Mater. Chem. A 4 (2016) 7107–7134. [3] G. Li, X. Wang, J. Fu, J. Li, M.G. Park, Y. Zhang, G. Lui, Z. Chen, Pomegranate-inspired design of highly active and durable bifunctional electrocatalysts for rechargeable metal-air batteries, Angew. Chem. Int. Ed. 55 (2016) 4977–4982. [4] K.-N. Jung, J. Kim, Y. Yamauchi, M.-S. Park, J.-W. Lee, J.H. Kim, Rechargeable lithium-air batteries: a perspective on the development of oxygen electrodes, J. Mater. Chem. A 4 (2016) 14050–14068. [5] T.W. Chen, J.X. Kang, D.F. Zhang, L. Guo, Ultralong PtNi alloy nanowires enabled by the coordination effect with superior ORR durability, RSC Adv. 6 (2016) 71501–71506. [6] L. Gan, S. Rudi, C. Cui, M. Heggen, P. Strasser, Size-controlled synthesis of sub-10 nm PtNi3 Alloy nanoparticles and their unusual volcano-shaped size effect on ORR electrocatalysis, Small (2016) 3189–3196. [7] G. Gnana Kumar, M. Christy, H. Jang, K.S. Nahm, Cobaltite oxide nanosheets anchored graphene nanocomposite as an efficient oxygen reduction reaction (ORR) catalyst for the application of lithium-air batteries, J. Power Sources 288 (2015) 451–460. [8] H. Huang, X. Feng, C. Du, W. Song, High-quality phosphorus-doped MoS2 ultrathin nanosheets with amenable ORR catalytic activity, Chem. Commun. 51 (2015) 7903–7906. [9] X. Pan, X. Song, S. Lin, K. Bi, Y. Hao, Y. Du, J. Liu, D. Fan, Y. Wang, M. Lei, A facile route to graphite-tungsten nitride and graphite-molybdenum nitride nanocomposites and their ORR performances, Ceram. Int. (2016). [10] W. Chu, D. Higgins, Z. Chen, R. Cai, Non-precious metal oxides and metal carbides for ORR in alkaline-based fuel cells, Non-Noble Met. Fuel Cell Catal. (2014) 357–388. [11] T. Asefa, Metal-free and noble metal-free heteroatom-doped nanostructured carbons as prospective sustainable electrocatalysts, Acc. Chem. Res. 49 (2016) 1873–1883. [12] L. Dai, Y. Xue, L. Qu, H.J. Choi, J.B. Baek, Metal-free catalysts for oxygen reduction reaction, Chem. Rev. 115 (2015) 4823–4892. [13] S.J. Zhu, J. Zhang, J.J. Ma, Y.X. Zhang, K.X. Yao, Rational design of coaxial mesoporous birnessite manganese dioxide/amorphous-carbon nanotubes arrays for advanced asymmetric supercapacitors, J. Power Sources 278 (2015) 555–561. [14] M. Huang, R. Mi, H. Liu, F. Li, X.L. Zhao, W. Zhang, S.X. He, Y.X. Zhang, Layered manganese oxides-decorated and nickel foam-supported carbon nanotubes as advanced binder-free supercapacitor electrodes, J. Power Sources 269 (2014) 760–767. [15] Y. Zhang, M. Dong, S. Zhu, C. Liu, Z. Wen, MnO2@colloid carbon spheres nanocomposites with tunable interior architecture for supercapacitors, Mater. Res. Bull. 49 (2014) 448–453.
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Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Confined polyaniline derived mesoporous carbon for oxygen reduction reaction Shuangxi Xing a,⇑, Xiaodan Yu a, Guibao Wang b, Yue Yu a, Yuanhong Wang a, Yan Xing a,⇑ a b
Faculty of Chemistry, Northeast Normal University, Changchun 130024, PR China Changchun Institute of Engineering Technology, Changchun 130024, PR China
a r t i c l e
i n f o
Article history: Received 13 December 2016 Received in revised form 30 December 2016 Accepted 10 January 2017 Available online 11 January 2017 Keywords: Polyaniline SiO2 Carbon Oxygen reduction reaction
a b s t r a c t Based on the previous generation of yolk-shell nanostructured polyaniline@SiO2 particles, a confined carbonization process followed by etching the SiO2 layer is carried out to achieve carbon particles. The existence of the SiO2 shell helps to preserve the original well-dispersed morphology of the polyaniline that retains amounts of active sites for oxygen reduction reaction. Furthermore, the CeO2 oxidant and the SiO2 shell lead to the increased amount of oxygen existing in the product and the oxygen reduction reaction results show the well-maintained nanostructure with mesopores and the effective O doping benefit for achieving enhanced catalytic activity. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction In order to solve the energy problems, fuel cells and metal-air batteries are constructed and developed quickly in recent years [1–4]. However, the sluggish reaction in the cathode (oxygen reduction reaction, ORR) often inhibits the effective utilization of the devices. Pt/C has been considered as the most effective one to catalyze the oxygen reduction reaction in cathode for its low overpotential and high current density. Nevertheless, the Pt/C catalysts often suffer from CO poisoning and unsatisfied cycling stability. To solve these problems, kinds of substitutes have been adopted, including alloyed metals, metal oxides/sulphides/nitrides/carbides and heteroatom doped carbon materials [5–12]. Carbon has been widely used in the field of energy storage and conversion [13–17], and the carbon materials doped with different non-metal elements, such as N, P, S and P, received much attention for the ORR application. The doping of these atoms into the carbon can alter the charge density of the centred C owing the different electron negativity or spin density, which facilitates the oxygen reduction process. To achieve such doped carbon, various strategies have been desired, for example, thermally annealing graphic oxides in the presence of ammonia and boric acid and direct carbonization precursors doped with heteroatoms [11,12]. The carbonization of the conducting polymers, such as polyaniline (PANI) and polypyrrole, has been considered as a facile route to directly obtain N-doped carbon and the catalytic measurement has confirmed their excellent performance in oxygen reduction reaction and even oxygen evolution reaction [18–20]. Introducing proper elements can lead to the formation of multiple heteroatom-doped carbon catalysts. For instance, N- and O-doped mesoporous carbons were generated via pyrolysis of the PANI confining in pores of SBA-15 that could efficiently catalyze ORR with high current density and low overpotential [18]; Poly(o-phenylenediamine) doped with glycine can also be carbonized to produce N- and O-doped carbon for supercapacitors [21]. It should be noted there are seldom researches reporting the generation of doped C mainly/ ⇑ Corresponding authors. E-mail addresses: [email protected] (S. Xing), [email protected] (Y. Xing). http://dx.doi.org/10.1016/j.eurpolymj.2017.01.011 0014-3057/Ó 2017 Elsevier Ltd. All rights reserved.
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dominatingly doped with O, which is considered to increase the ORR activity upon increasing the O dopant [19]. Apart from that, in order to attain satisfied catalytic performance, the well-constructed nanostructure should be realized to keep the high specific surface area and attainable mesopores. On the other hand, as mentioned above, the confined PANI has been pyrolized into mesoporous carbon and presented excellent electrochemical performance [18]. In this issue, the limited space for loading the carbon-precursor plays an important role to maintain the morphology of the materials during the carbonization process and ensures the achievement of high specific surface area and mesoporous nanostructure [19,20,22]. Many kinds of materials were selected to construct the proper space, including SiO2, SBA-15, montmorillonite and polyvinyl pyrrolidone. In this work, the confined PANI nanoparticles in SiO2 shell that were generated using the CeO2 cores as sacrificing oxidants [23] were pyrolized at high temperature to produce carbon materials. The existence of the SiO2 as protecting layer and the utilization of CeO2 as oxidants led to the effective O doping and the N exhausting/losing occurred at the same time. The confined of the SiO2 allowed the generation of mesoporous nanostructure for the derived carbon. As a result, the product displayed excellent ORR catalytic activity owing to the effective O doping and proper mesoporous nanostructure. 2. Experiments 2.1. Materials All solutions were prepared using ultrapure water with resistance of 18.2 MX cm1. Cerium(III) nitrate hexahydrate (99%), Nafion (5 wt%, Aldrich), acetic acid, ethyl silicate ethylene glycol, isopropanol, ammonia solution, hydrochloric acid, sodium hydroxide and potassium hydroxide (Beijing Chemical), polyvinylpyrrolidone (K15, TCI) were used as received. Aniline (98%, TCI) was distilled before use. 2.2. Synthesis of PANI@SiO2 The synthesis of PANI@SiO2 followed our previously reported procedure. Briefly, CeO2 nanoparticles (NPs) obtained from a typical hydrothermal route were coated by a layer of SiO2 via a Stöber method. After that, the CeO2@SiO2 NPs were incubated in aniline solution followed by addition of HCl aqueous solution to induce the polymerization of aniline in the SiO2 shell. 2.3. Synthesis of carbonized PANI The obtained PANI@SiO2 NPs were pyrolized under an N2 atmosphere in a tube furnace. The temperature was raised to a predetermined value (700, 800, 900, 1000 °C) at a rate of 5 °C min1 and was kept stable for 2 h. After that, the sample was cooled to room temperature and the so-obtained product was dispersed in NaOH aqueous solution (0.5 mg mL1). The mixture was then transferred into a Teflon-line stainless-steel autoclave and maintained at 150 °C for 24 h. Finally, the product was cooled to room temperature and was marked as C-NCNP-700 (800, 900 and 1000). 2.4. Characterization Transmission electron microscopic (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were obtained on a JEM-2100F microscope with an accelerating voltage of 200 kV. Scanning electron microscope (SEM) images were collected on a JEOL SM-6360LV field-emission-gun scanning electron microscope. The Brunauer–Emmett–Tel ler (BET) surface area was calculated by nitrogen adsorption and desorption. A Thermo ESCALAB 250X-ray photoelectron spectroscope (XPS) equipped with a standard and monochromatic source (Al Ka hm = 1486.6 eV) was employed for surface analysis. Raman spectra were conducted on a LabRAM XploRA laser Raman spectrometer (HORIBA Jobin Yvon CO. Ltd) using a 532 nm laser with incident power of 1 mW. 2.5. Oxygen reduction reaction measurements The ORR activity of the catalysts was investigated via the rotating disk electrode (RDE) technique accompanied by an electrochemical workstation (CHI660E) with a typical three-electrode system. The glassy carbon (GC) rotating ring-disk electrode (RDE, PINE, 5 mm in diameter and 0.196 cm2 in area) was used as working electrode. The platinum sheet and Ag/AgCl electrode were used as counter and reference electrode, respectively. KOH (0.1 M) aqueous solution was used as electrolyte and high-purity O2 was purged into the above solution for 30 min to give oxygen saturated one. To prepare the working electrode, a certain amount of catalyst was dispersed in a mixture containing Nafion (5 wt%), isopropanol and water (V:V:V = 5:19:76) and give a ink with concentration of 4 mg mL1. The catalyst ink was then dipped onto the surface of the pre-polished glassy carbon electrode and a final mass loading of 0.20 mg cm2 was achieved. Linear sweep voltammetry (LSV) curves were collected at various rotating speeds ranging from 400 to 1600 rpm with a sweeping rate of
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5 mV s1. All potentials in this work were applied against the reversible hydrogen electrode (RHE) using the following equation:
ERHE ¼ EAg=AgCl þ 0:1976V þ 0:059 pH Chronoamperometric responses were conducted at 0.65 V vs. RHE for 10,000 s under the rotating rate of 1600 rpm. The Koutechy-Levich (K-L) plots were analyzed at different electrode potentials. The electron transfer number (n) could be calculated from the slope of the K-L plot based on the K-L equation:
1 1 1 1 1 ¼ þ ¼ þ j jL jK Bx1=2 jK B ¼ 0:62nFC 0 ðD0 Þ2=3 ðtÞ1=6 where j represents the measured current density, jK is the kinetic limiting current density, x is the electrode rotating rate, jL is the diffusion-limiting current density, F is the Faraday constant (F = 96,485 C mol1), C0 is the bulk concentration of O2 (C0 = 1.2 106 mol cm3), D0 is the diffusion coefficient of O2, and m is the kinematic viscosity of the electrolyte. 3. Results and discussion The confined polymerization of aniline is realized based on our previously reported method [23], where the CeO2 particles encapsulated in SiO2 layer are utilized as a sacrificed reactive template. After addition of aniline and acid, the Ce4+ ions slowly release to oxidize the aniline molecules that penetrate the SiO2 shell. The obtained yolk-shell nanostructured PANI@SiO2 particles are heated at high temperature to carbonize the PANI. Finally, the SiO2 shell is etched by NaOH aqueous solution to achieve the doped carbon particles. In order to set the optimal carbonization condition, the PANI@SiO2 particles were thermally treated at different temperature. After removing the SiO2 shell, the products were evaluated on their ORR performance in 0.1 M KOH aqueous
Fig. 1. (a) LSV curves for the samples pyrolized at different temperatures in O2-saturated 0.1 M KOH with rotating speed of 1600 rpm; (b) CV curves of C-NCNP-900 recorded in N2 and O2 saturated 0.1 M KOH; (c) LSV curves of C-NCNP-900 measured at different rotating rates; (d) K-L plots of C-NCNP-900 in various potential range (vs. RHE).
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electrolytes. The polarization curves are presented in Fig. 1. The sample C-NCNP-900 displays the most positive onset potentials with the lowest overpotential for ORR (0.85 V). Furthermore, C-NCNP-900 gives the highest kinetic current density among these samples (Table 1). All these data demonstrate the carbonized PANI obtained from 900 °C heat treatment behaves the best electrocatalytic performance for ORR application. Therefore, C-NCNP-900 was selected as a typical example for further investigation. The cyclic voltammeter (CV) curves of C-NCNP-900 in N2 or O2-saturated aqueous KOH solution are shown in Fig. 1b, where no observable reduction peak is detected in the former system, while a well-defined cathodic peak centred at 0.85 V is found in the later one, indicating its ORR activity. The ORR polarization curves at different rotation rates along with the corresponding K-L plots are presented in Fig. 1c and d. The current density increases steadily when the rotation rate is changed from 400 to 1600 rpm. The slopes maintain nearly constant within the range from 0.2 to 0.4 V, indicating the similar electron transfer numbers, which is calculated from the slopes as ca. 4. In comparison, C-NCNP-700 and C-NCNP-1000 both present an average electron transfer number of 2 between the same potential ranges. Besides, C-NCNP-800 illustrates an unstable electron transfer number from 4 to 2.5 upon increasing the potential from 0.2 to 0.5 V. This further reveals the best ORR performance of the sample obtained at 900 °C. As a control experiment, another sample by using CeO2 particles without SiO2 shell were used as reactive templates to oxidize aniline to generate PANI, which was further carbonized at 900 °C and labelled as NCNP-900. The LSV curves of the two samples (C-NCNP-900 and NCNP-900) along with the commercial Pt/C catalyst are shown in Fig. 2a. NCNP-900 gives a most negative onset potential at 0.75 V and lowest current density (Table 1). Besides, the electron transfer number originated from this sample is about 2.4 in the range of 0.2–0.5 V, all of which demonstrate the worse electrocatalytic performance for NCNP-900 than C-NCNP-900 (Fig. 2b). In order to make clear the difference of the ORR activity between these two samples, the morphology was first examined by TEM and SEM. After pyrolyzed at 900 °C, the yolk-shell PANI@SiO2 maintains well its original shape with a moving core in the shell (Fig. 3a). However, the size of the core turns smaller (64 nm vs. 57 nm) because of the contraction effect during the heating process [24]. After etching process, the cores are well separated and give a raspberry-like morphology with rough surface (Fig. 3b). The particles are swollen by the etching solution and give an increasing diameter of about 86 nm. The SEM image in Fig. 3d reveals these cores are assembled by many smaller nanoparticles with the average size of 14 nm. The HRTEM image demonstrates no ordered structure exists in the sample, indicating C-NCNP-900 consists of amorphous carbon (Fig. 3c). In comparison, NCNP-900 that was prepared from the sample directly via using CeO2 as oxidant in the presence of acid (Fig. 4a) gives an apparent aggregated morphology (Fig. 4b). This confirms the advantage of the confined effect that a separated space for carbonization of the precursor is beneficial for achieving products with uniform nanostructure. The corresponding SEM image of NCNP-900 (Fig. 4c) displays the accumulation of layered sheets/particles. This different morphological result seems inducing great difference in the specific surface area, which directly influence the ORR performance, as concluded in many reports. However, it is found the two samples present almost same specific surface area (688.1 m2 g1 for C-NCNP-900 vs. 686.6 m2 g1 for NCNP-900). This might be related to the presence of a great deal of micropores in both of the samples, as shown in Fig. 5a. Further analysis reveals larger amounts of mesopores exist in C-NCNP-900 than in NCNP-900 (Fig. 5b), which is beneficial to the kinetic mass transport during the ORR process. In this case, the intermediates are easier to penetrate the inside of the mesopores to undergo a complete 4 electron reduction way; on the contrary, a 2 electron reduction process dominates the whole reaction because the intermediates are difficult to enter the micropores and leave the surface quickly. The Raman spectra of the two kinds of samples were collected to further seek the reason for the difference of the ORR activities between them. Two prominent peaks located at ca. 1350 and 1596 cm1 are observed (Fig. 5c), where the former one is induced by the topological defects in the carbonized PANI and the later one originates from the crystalline graphitic domains. The two peaks are commonly ascribed to D and G bands of distorted graphitic carbon materials and a higher relative intensity (ID/IG) with more defects in the sample indicates a better electrocatalytic performance owing to the presence of larger amounts of catalytic active sites. C-NCNP-900 illustrates a higher value of ID/IG (0.982 vs. 0.928), consistent with its better ORR performance [25]. The N content in the doped carbon materials is another important factor to affect the electrocatalytic performance of the samples. A higher N content commonly means a better ORR activity because more positive charged C atoms are formed via
Table 1 Comparison of the ORR catalytic performances. Sample
C-NCNP-700
C-NCNP-800
C-NCNP-900
Eonset (V) j-0.4V (mA cm2) Sample Eonset (V) j-0.4V (mA cm2)
0.65 0.53 C-NCNP-1000 0.68 0.60
0.74 1.41 NCNP-900 0.75 1.47
0.85 2.76 Pt/C 0.96 5.38
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Fig. 2. (a) LSV curves of C-NCNP-900, NCNP-900 and Pt/C with rotating speed of 1600 rpm; (b) the electron transfer numbers of different samples in the potential range of 0.2 V and 0.5 V on the basis of K-L equations.
Fig. 3. (a) TEM image of carbonized PANI@SiO2 achieved at 900 °C before etching; TEM (b), HRTEM (c) and SEM (d) images of C-NCNP-900; the inset in (a) presents the TEM image of PANI@SiO2 before carbonization.
the neighbouring N atoms with larger electronegativity (3.04 for N vs. 2.55 for C). However, there are only tiny amount of N (0.21%) in C-NCNP-900, much less than that of NCNP-900 (2.22%). On the contrary, C-NCNP-900 presents a greater O content than NCNP-900 with the value of 19.12% vs. 6.41%, respectively, as shown in Fig. 5d. The higher O amount should be related to the combination of original CeO2 cores and the SiO2 protection. Meanwhile, the O doping is considered to be beneficial to improve the ORR performance since the co-doping of N and O may generate asymmetric spins and better charged structure [18,19].
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Fig. 4. (a) TEM image PANI obtained without SiO2 shell; TEM (b) and SEM (c) images of NCNP-900.
Fig. 5. N2 adsorption-desorption isotherms (a), pore size distribution (b), Raman (c) and XPS (d) spectra of C-NCNP-900 and NCNP-900.
In order to further evaluate the stability of the samples for electrocatalytic ORR, chronoamperometry test was carried out on C-NCNP-900, NCNP-900 and commercial Pt/C. After 10,000 s, the later two shows a quick degradation in the first 4000 and 2000 s, respectively, and maintains ca. 60% of original value. In comparison, C-NCNP-900 presents a much slower decrease rate and only 10% current density fading is observed, demonstrating its much better stability than the other two. Furthermore, methanol crossover often occurs in the ORR catalytic systems. As revealed in Fig. 6, a remarkable current decreasing is observed upon addition of 11.03 mL of methanol (Cfinal = 3 M) for the Pt/C-containing systems. On the contrary, little current change is found for C-NCNP-900, demonstrating its strong tolerance to methanol crossover, which is also much better than that of NCNP-900.
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Fig. 6. (a) Chronoamperometric curves of different samples at 0.65 V; (b) chronoamperometric curves of different samples at 0.65 V before and after addition of methanol. All measurements were conducted in an O2-saturated 0.1 M KOH aqueous solution with an electrode rotating rate of 1600 rpm.
4. Conclusions Polyaniline nanospheres confined in SiO2 shell that were synthesized using CeO2 cores as reactive sacrificed oxidant were carbonized to generate O-dominatingly doped carbon particles after high temperature pyrolysis and etching processes. The SiO2 layer helped maintaining the nanostructure of the original polyaniline, which endowed the product large specific surface area and mesoporous structure. Meanwhile, the usage of CeO2 as oxidant and SiO2 as protecting layer benefited for the effective O doping for the carbonized polyaniline. As a result, the sample presented higher catalytic activity towards oxygen reduction reaction than that obtained without SiO2 protection, although the later one showed higher N doping amount. Furthermore, the sample illustrated high stability and tolerance to CH3OH, indicating its proper application in energy conversion. Acknowledgements The authors thank Jilin Provincial Science and Technology Development Foundation (Grant No. 20140101109JC), 12th Five-Year Plan in Science and Technology of the Education Department of Jilin Province (2015-546) and Jilin Provincial Key Laboratory of Advanced Energy Materials (Northeast Normal University) for financial support. References [1] J. Wang, B. Li, T. Yersak, D. Yang, Q. Xiao, J. Zhang, C. Zhang, Recent advances in Pt-based octahedral nanocrystals as high performance fuel cell catalysts, J. Mater. Chem. A 4 (2016) 11559–11581. [2] D.U. Lee, P. Xu, Z.P. Cano, A.G. Kashkooli, M.G. Park, Z. Chen, Recent progress and perspectives on bi-functional oxygen electrocatalysts for advanced rechargeable metal-air batteries, J. Mater. Chem. A 4 (2016) 7107–7134. [3] G. Li, X. Wang, J. Fu, J. Li, M.G. Park, Y. Zhang, G. Lui, Z. Chen, Pomegranate-inspired design of highly active and durable bifunctional electrocatalysts for rechargeable metal-air batteries, Angew. Chem. Int. Ed. 55 (2016) 4977–4982. [4] K.-N. Jung, J. Kim, Y. Yamauchi, M.-S. Park, J.-W. Lee, J.H. Kim, Rechargeable lithium-air batteries: a perspective on the development of oxygen electrodes, J. Mater. Chem. A 4 (2016) 14050–14068. [5] T.W. Chen, J.X. Kang, D.F. Zhang, L. Guo, Ultralong PtNi alloy nanowires enabled by the coordination effect with superior ORR durability, RSC Adv. 6 (2016) 71501–71506. [6] L. Gan, S. Rudi, C. Cui, M. Heggen, P. Strasser, Size-controlled synthesis of sub-10 nm PtNi3 Alloy nanoparticles and their unusual volcano-shaped size effect on ORR electrocatalysis, Small (2016) 3189–3196. [7] G. Gnana Kumar, M. Christy, H. Jang, K.S. Nahm, Cobaltite oxide nanosheets anchored graphene nanocomposite as an efficient oxygen reduction reaction (ORR) catalyst for the application of lithium-air batteries, J. Power Sources 288 (2015) 451–460. [8] H. Huang, X. Feng, C. Du, W. Song, High-quality phosphorus-doped MoS2 ultrathin nanosheets with amenable ORR catalytic activity, Chem. Commun. 51 (2015) 7903–7906. [9] X. Pan, X. Song, S. Lin, K. Bi, Y. Hao, Y. Du, J. Liu, D. Fan, Y. Wang, M. Lei, A facile route to graphite-tungsten nitride and graphite-molybdenum nitride nanocomposites and their ORR performances, Ceram. Int. (2016). [10] W. Chu, D. Higgins, Z. Chen, R. Cai, Non-precious metal oxides and metal carbides for ORR in alkaline-based fuel cells, Non-Noble Met. Fuel Cell Catal. (2014) 357–388. [11] T. Asefa, Metal-free and noble metal-free heteroatom-doped nanostructured carbons as prospective sustainable electrocatalysts, Acc. Chem. Res. 49 (2016) 1873–1883. [12] L. Dai, Y. Xue, L. Qu, H.J. Choi, J.B. Baek, Metal-free catalysts for oxygen reduction reaction, Chem. Rev. 115 (2015) 4823–4892. [13] S.J. Zhu, J. Zhang, J.J. Ma, Y.X. Zhang, K.X. Yao, Rational design of coaxial mesoporous birnessite manganese dioxide/amorphous-carbon nanotubes arrays for advanced asymmetric supercapacitors, J. Power Sources 278 (2015) 555–561. [14] M. Huang, R. Mi, H. Liu, F. Li, X.L. Zhao, W. Zhang, S.X. He, Y.X. Zhang, Layered manganese oxides-decorated and nickel foam-supported carbon nanotubes as advanced binder-free supercapacitor electrodes, J. Power Sources 269 (2014) 760–767. [15] Y. Zhang, M. Dong, S. Zhu, C. Liu, Z. Wen, MnO2@colloid carbon spheres nanocomposites with tunable interior architecture for supercapacitors, Mater. Res. Bull. 49 (2014) 448–453.
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