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N,B-codoped defect-rich graphitic carbon nanocages as high performance multifunctional electrocatalysts
Author’s Accepted Manuscript N,B-codoped Defect-rich Graphitic Carbon Nanocages as High Performance Multifunctional Electrocatalysts Ziyang Lu, Jing Wang, Shifei Huang, Yanglong Hou, Yanguang Li, Yueping Zhao, Shichun Mu, Jiujun Zhang, Yufeng Zhao www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(17)30681-X https://doi.org/10.1016/j.nanoen.2017.11.004 NANOEN2305
To appear in: Nano Energy Received date: 14 August 2017 Revised date: 5 October 2017 Accepted date: 2 November 2017 Cite this article as: Ziyang Lu, Jing Wang, Shifei Huang, Yanglong Hou, Yanguang Li, Yueping Zhao, Shichun Mu, Jiujun Zhang and Yufeng Zhao, N,Bcodoped Defect-rich Graphitic Carbon Nanocages as High Performance Multifunctional Electrocatalysts, Nano Energy, https://doi.org/10.1016/j.nanoen.2017.11.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
N,B-codoped Defect-rich Graphitic Carbon Nanocages as High Performance Multifunctional Electrocatalysts Ziyang Lu11, Jing Wang11, Shifei Huang1, Yanglong Hou2*, Yanguang Li3*,Yueping Zhao1, Shichun Mu4, Jiujun Zhang5, Yufeng Zhao1* 1 Key Laboratory of Applied Chemistry, Yanshan University, Qinhuangdao 066004, China 2 Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing Key Laboratory for Magnetoelectric Materials and Devices (BKLMMD), Beijing 100871, China 3 Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123 , China 4 State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China 5 Institute of Sustainable Energy, Shanghai University, Shanghai University, Shanghai, 200444, P. R. China [email protected] [email protected] [email protected] *Corresponding authors. Abstract Nanocarbon materials recognized as effective and inexpensive catalysts for independent electrochemical reactions, are anticipated to possess a broader spectrum of multifunctionality toward oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). A rational design of trifunctional nanocarbon catalyst requires balancing the heteroatoms-doping and defect-engineering to afford desired active centers and satisfied electric conductivity, which however is conceptually challenging while desires in-depth research both experimentally and theoretically. This work reports a N,B-codoped graphitic carbon nanocage (NB-CN) with graphitic yet defect-rich characteristic as a promising
1
These authors contribute equally to this work.
trifunctional electrocatalyst through a facile thermal pyrolysis assisted in-situ catalytic graphitization (TPCG) process. Density functional theory (DFT) calculations are conducted, for the first time, to demonstrate that the best performance for ORR/OER and HER can be originated from the configuration with B meta to a pyridinic-N, which presents a minimum theoretical overpotential of 0.34 V for ORR, 0.39 V for OER, and a lowest Gibbs free-energy (ΔGads) of 0.013 eV for HER. A primary zinc-air battery is assembled presenting a maximum power density of 320 mW cm-2 along with excellent operation durability, evidencing great potential in practical applications. Graphical abstract The N,B-codoped defect-rich graphitic carbon nanocages (NB-CN) are prepared through a technique of in situ vapor deposition. The NB-CN exhibits efficient multifunctional catalytic activity for ORR, OER and HER, shows potential practical performances in the fields of metal-air battery, fuel cell and water splitting.
Keywords: N,B-codoping, carbon nanocage, multifunctional catalysis, Zinc–air battery
Introduction The sluggish kinetics in oxygen reduction reaction (ORR), oxygen evolution reaction (OER) or hydrogen evolution reaction (HER) demonstrates critical challenges for high efficiency renewable energy storage and conversion devices, such as metal-air batteries, water splitting cells or fuel cells.[1-4] Precious metals (e.g. Pt) or transition metal-oxides are considered as efficient electrocatalysts, which however suffer from high price, inferior durability or poor electric conductivity.[5-11] Nanocarbon based catalysts with good conductivity and modifiable surface chemistry have gained paramount interest in recent years.[12-19] The catalytic activity of nanocarbon materials can be tailored through heteroatom doping (e.g., B, N, P, S, F etc.) by modifying the electronegativity, charge distribution as well as the electron transfer behavior.[14,20-27] Codoping of heteroatoms with different electronegativity can further boost the catalytic performance by introducing more active centers.[15,28-30] For example, nanocarbons codoped with electron-giving atoms (e.g. S or P) and electron-accepting atoms (e.g. N) can serve as effective bifunctional catalyst for ORR-HER, ORR-OER or OER-HER with significantly enhanced electrocatalytic activity, in which the positively charged electron-giving atoms (S or P) would function as active centers together with the positively charged C induced by electron-accepting atoms (N).[31-36] Nevertheless, the design and development of multifunctional electrocatalyst are still
challenging however attractive for the rapid development of renewable energy technologies. Recently, Dai et al.[37] reported a N, P, F-codoped graphene as a trifunctional electrocatalyst for ORR, OER and HER. Thus it becomes possible to construct an integrated self-powered water-splitting units which can supply green gases (H2 and O2) in sustainable energy storage and conversion. As neighboring atoms of C (electronegativity χ=2.55) in the periodic table, B (χ=2.04) and N (χ=3.04) with different electronegativities are also anticipated as promising co-dopants for trifunctional catalyst. However the existing studies regarding N,B-codoped carbons are mostly about independent ORR catalysis,[38-42] their multifunctional catalytic activity for three key electrochemical reactions hasn’t been investigated either theoretically or experimentally. Meanwhile, significant amount of studies have declared that the electrocatalytic activity of nanocarbons can also be enhanced by introducing more surface functional groups, edge dangling bonds, or defective sites. [43,44]
Yao et al.[44] on the other hand, testified the multifunctionality of dopant-free
defective graphene through experiment and corresponding DFT calculation, and proposed a defect-dominated catalytic mechanism. Therefore, a more comprehensive carbon architecture inclusive both heteroatom doping and defective effect, should be considered for an effective design of trifunctional electrocatalyst. In this work, we report a unique N,B-codoped graphitic carbon nanocage (NB-CN) with the shell thickness around 5~14 graphitic layers and defect-rich characteristic, synthesized through a facile thermal pyrolysis assisted in-situ catalytic graphitization
(TPCG) process. Compared with other literatures,[45-49] the carbon nanocage prepared by this method possesses a smaller radius and wall thickness. The as-prepared material exhibits splendid trifunctional catalytic activities for ORR, OER and HER along with superior stability. Density functional theory (DFT) calculations are conducted to uncover the origin of this multifunctional catalytic activity, which demonstrates that the N,B-codoped carbon can present a minimum theoretical overpotential of 0.34 V for ORR, 0.39 V for OER, and a lowest Gibbs free-energy (ΔGads) of 0.013 eV for HER, evidencing great potential as a trifunctional electrocatalyst. A primary Zn-air battery was assembled in order to demonstrate the practical application of NB-CN. The maximum power density reached up 320 mW cm-2, and no obvious activity decay was observed after galvanostatic discharge at current density of 10 mA cm-2 for 14 h or 50 mA cm-2 for 8 h.
2. Experiment 2.1 Materials preparation In a traditional way, 1.2 g chitosan, 1.2 g cobaltous acetate hexahydrate and 2.4 g boric acid were mixed in 100ml deionized water and kept stirring for an hour. Then 0.8 g red phosphorus powders were added into the mixture respectively and continued stirring for 2 hours. Subsequently, 3.6g KOH was dissolved in 10ml deionized water, added into the mixture and kept stirring for 6 hours. Obtained mixture was heated at 100 °C and kept stirring constantly until it turned into a paste. Next, the paste was transferred into a quartz tube and heated to 750 °C kept for 4 hours at a heating rate of
2°C /min in flowing argon atmosphere. The obtained samples were washed with 8M HNO3 and deionized water, then dried at 85℃. Lastly, the samples were heated to 600 ℃ for 3 hours at a heating rate of 10℃ /min in a flowing H2/Ar (20% H2) combination gas for annealing. The final product was obtained and denoted as NB-CN. For comparison, the sample without boric acid and red phosphorus was prepared through the same method above, and referred as N-C. 2.2 Characterization Raman spectra were recorded on a Renishaw in Via Raman microscope with an Arion laser at the excitation wavelength of 514.5 nm. Hitachi-X-ray photoelectron spectrum (XPS) was measured by a VG ESCALAB MKIIX-ray photoelectron spectrometer using Mg−Kα as the exciting source (1253.6 eV). Carl Zeiss SUPRA 55 SAPPHIRE field emission scanning electron microscope (FESEM, Germany, 15 kV) Hitachi-7650 transmission electron microscopy (TEM, Japan, 80 kV), and high resolution transmission electron microscopy (HRTEM, JEOL JEM-3000F) were used to investigate the morphology and microstructure. The specific surface area (SSA) and pore size of the materials were measured using a micromeritics (ASAP 2020 V4.00 (V4.00 H)). Fourier-transform infrared (FTIR) spectra were collected by EQUINOX55 FTIR spectrometer operated from 400 to 4000 cm−1. 2.3 Electrochemical measurement Electrochemical experiments were carried out using the PARSTAT 4000 electrochemical workstation (America) in a three-electrode configuration. A glassy carbon disk electrode (0.196 cm2) coated with the catalyst layer as the working
electrode, saturated calomel electrode (SCE) and Hg/HgO electrode used as reference electrode in 0.1M KOH and 0.5M H2SO4, respectively. A Pt foil (1 cm2) was used as the counter electrode for ORR and OER tests and a graphite rod was used as the counter electrode for HER tests. RHE was used in this article, E(RHE)= E(SCE) +0.2412 + 0.059×pH, E(RHE) = E( Hg/HgO) + 0.098 V + 0.059 × pH. The rotating disk electrode (RDE) was performed on a MSRX electrode rotator (Pine Instrument) and the PARSTAT 4000 electrochemical workstation. For ORR and OER measurements, 2.5 mg sample powders were mixed with 1000 μL isopropyl alcohol and 15 μL 5 wt% Nafion. Then the mixture was ultrasonically suspended for 30 min. After that 20 μL of this homogenous mixture was dropped on the glassy carbon disk and dried at room temperature to obtain a catalyst loading of 255.1 μg cm-2. For HER measurements, the catalyst loading was 382.7 μg cm-2. For comparison, commercial Pt/C (20 wt%) and IrO2 (20 wt%) were also measured. The loading of Pt/C and IrO2 catalysts on glassy carbon disk electrode was 127.6 μg cm-2 and 255.1 μg cm-2 respectively. The catalytic activity of the electrocatalyst toward ORR and OER were all performed in 0.1 M KOH aqueous solution. Before the experiment, the electrolyte was bubbled with O2 or N2 for >20 min. ORR tests under acidic condition in a similar way. In order to investigate the ORR procedure, the oxygen reduction test was performed at various rotating speeds from 400 to 2000 rpm in O2-saturated electrolyte at a sweep rate of 5 mV s-1. The electron transfer number (n) was evaluated according to Koutecky–Levich (K-L) equation.
1
J
1
JL
1
JK
1
B
1/2
1
JK
(1)
Where J is the measured current density, JK is the kinetic-limiting current density, ω is the electrode rotation rate.
B 0.62nFCo(Do )2 / 3 1 / 6
(2)
B could be determined from the slope of the Koutecky–Levich plots based on the Koutecky–Levich equation, here n is the number of transferred electrons, F is the faraday constant (96485 C mol-1), Co (1.2×10−6 mol cm-3) and Do (1.9×10−5 cm2 s−1) are the bulk concentration and diffusion coeffcient of O2 in electrolyte, υ (0.01 cm2 s−1) is the kinematic viscosity of the electrolyte. The oxygen evolution text was performed in O2-saturated electrolyte at a sweep rate of 5 mV s-1. Results and Discussion The N,B-codoped carbon nanocage is prepared through a thermal pyrolysis assisted in-situ catalytic graphitization (TPCG) method at 750 oC in argon atmosphere, as illustrated in Figure 1a. In a typical synthesis, chitosan was used as C and N resources, while boracic acid as B resource. Certain amount of KOH, red phosphorus (RP), and cobaltous acetate are added for multiple purposes: on the one hand, these chemicals will react with each other through a series of reactions forming Co 2P nanoparticles, which not only serve as catalysts in the following graphitization process, but also regulate the morphology by depositing the C atoms on the surface, forming a C-Co2P core-shell structure.[50] This can be confirmed by XRD patterns and SEM images of the intermediate products (Figure S1-S2). On the other hand KOH and RP will facilitate the in-situ exfoliation of carbon layers, and guarantee the
few-layer characteristic of the cage wall.[51,52] The N,B-codoped graphitic carbon nanocage will be finally obtained by leaching out the Co2P core and post-annealing treatment (see supporting information for the detailed information). For comparison, single N-doped carbon (N-C) is also prepared and characterized (Figure S3). The scanning electron microscope (SEM) and transmission electron microscope (TEM) images (Figure 1b-c, Figure S4) demonstrate uniform nanocage structure with a diameter size of 30-80 nm. Figure 1d corresponds to the C-Co2P core-shell structure before acid etching. The high resolution (HR) TEM image (Figure 1e,f) indicates the hollow structure of the carbon nanocage, revealing good crystalline few-layer (5~14 layers) nanostructure of the carbon nanocage with a interlayer distance of ~0.35 nm. Prominently, despite the relatively ordered graphitic feature, the as-prepared NB-CN demonstrates defect-rich characteristic (Figure 1e), including the topological disclinations at the corner, the surficial broken fringes, and the micropores,
[53,54]
which might be attributed to the removal of inorganic ingredients, B,N-codoping or partial subtraction of N element from chitosan precursors.[43,44] Although the impact of the type of defects on the catalytic is not clear, there is no doubt that defects have a enhancement on the catalytic and the diversity of defects is also associated with the diversity of catalytic performance. XRD patterns in Figure 2a show the (002) diffraction peaks at 25.6° demonstrating an apparently left shift with increased d spacing of 0.348 nm，which is in consistent with the HR-TEM result. Raman spectrum of NB-CN (Figure 2b) shows characteristic peak of D-band and G-band at ~1598 cm-1 and 1338 cm-1
respectively.[55] An ID/IG ratio of 1.05 indicates the defect rich characteristic caused by the B, N doping in the sp2 carbon structure, which is consistent with the HRTEM result. The BET (Brunauer-Emmet-Teller) specific surface area was obtained as 796 m2/g through the nitrogen adsorption measurement (Figure S5). The electron energy loss spectrum (EELS) (Figure 2c) and X-ray photoelectron spectroscopy (XPS) (Figure 2d) confirmed the coexistence of B and N in the NB-CN. The characteristic peaks of C1s, O1s, N1s and B1s can be clearly identified from the XPS survey spectrum (Figure 2d) with an atom ratio of 82.18, 10.68, 4.13 and 3.0 at%, respectively. The high-resolution N1s spectrum (Figure 2e) can be assigned into three N species corresponding to pyridinic-N (398.5 eV), pyrrolic-N (399.7 eV), and graphitic-N (401.1 eV), of which the percentages are 68.5%, 18.3% and 13.2% respectively. The large percentage of pyridinic-N and pyrrolic-N would generate activation for neighboring atoms leading high electrocatalytic activity for ORR, OER and HER.[56-60] B1s spectrum (Figure 2f) can be deconvoluted into three peaks, corresponding to BCO2 (192.3 eV), O-B-C (191.2 eV) and BC2O (190.6 eV).[61,62] The high-resolution XPS spectra of P 2p and Co 2p were shown in Figure S6. No significant signal for P and Co was distinguished. The high-resolution C1s and O1s were shown in Figure S7. Furthermore, the Fourier transform infrared (FTIR) (Figure S8) also confirm the co-existence of B and N. As contrast, single N-doped carbon (N-C) was also prepared using the similar method and related characterizations are provided in the supporting information (Figure S9). The possible residual cobalt and phosphorus cannot be detected by XPS (Figure S6).
However, the ICP-MS (inductively coupled plasma mass spectrometry) result indicates the presence of trace amount (0.1% atomic ratio) of cobalt element. First-principles calculations using density functional theory (DFT) were carried out to provide microscopic scenario of the catalytic reactions for the NB-CN. Four co-doped structures with N/B in different sites are constructed (Figure S10, see detailed description in the supporting information). For convenience, only pyridinic-N is modeled which is dominant (68.5% from XPS) and evaluates the ORR/OER activities. According to the XPS results, oxygen atom is inclusive in the sample, which is also included in our models and it is found to be positive to the reactions. To study the ORR (OER) catalytic activities, four steps are considered in the alkaline media, i.e., O2 dissociation into *OOH, *OOH decomposition into *O, *O alkalization into *OH, *OH detachment into OH- (the reverse progress started with OH- adsorption is for OER). Our results show that although the structure with B para to a pyridinic-N (Figure S10d) demonstrates the lowest energy, the best performance for ORR/OER and HER is from the configuration with B meta to a pyridinic-N (labeled with number 4 in Figure 2g). This is in consistent with the model constructed by Zhang et al [30], in which a B atom meta to a pyridinic N atom, shows the highest Ead value (the energy of HO2 adsorption) and we also notice that this relative position is also adopted in N,P-codoped graphene. Therefore, we focus on the B meta to a pyridinic-N case in the following studies. The overpotential η as a critical parameter of catalytic activities is identified to be as low as 0.34V for the ORR, with the active site located at boron atom. This value is much lower than that of commercial Pt (~0.45V).[30] The
minimum overpotential for OER can be obtained as 0.39V considering the active site transport to the neighboring carbon atom (labeled with number 3 in Figure 2g) which is also reported in literature.[63] The effect of different electrode potentials U is also investigated and the free energy diagrams are plotted in Figure 2h and 2i for ORR and OER, respectively. It is discovered that at zero potential, all the reactions are exothermic for both ORR and OER in view of the fact that all the elementary reaction steps are downhill (uphill). The catalyst performs better with negative values of electrode potentials for both ORR and OER in basic media. In general, carbon materials exhibit superior hydrogen evolution activity under acidic conditions than alkaline conditions.[64-65] Therefore, this work only focuses on the HER in acid solution, in which Gibbs free-energy (ΔGads) of the adsorption of atomic hydrogen is a main descriptor to determine the HER activity of a catalyst.[66-67] The lowest ΔGads value is calculated to be 0.013 eV with the reaction site of carbon (labeled with number 11 in Figure 2g). It is well known that the closer to zero (for the ΔGads value), the better the catalyst is as the positive (negative) ΔGads cannot efficiently absorb on (desorb from) carbon. Compared with the Pt (ΔGads =~0.90 eV) and other N- and/or S-doped graphene (0.12 eV for N/S co-doped graphene),[68] the minimum ΔGads value ensures the NB-CN to be the best HER catalyst for now. The optimized adsorption structures (top view and side view) for minimum ORR, OER overpotential and minimum HER free energy calculations are shown in Figure S11-13 respectively. The evaluation of the electrocatalytic activity of NB-CN was initially carried out by examining the cathodic ORR performance in O2-saturated 0.1M KOH aqueous
solution. A series of linear sweep voltammograms (LSVs) were collected on a rotating disk electrode (RDE) at 1600 rpm for different samples. The LSV curves in Figure 3a confirms the outstanding electrocatalytic activity of NB-CN, with an onset potential of 0.92 V versus reversible hydrogen electrode (RHE) and a half-wave potential of 0.835 V, comparable to that of commercial Pt/C (0.95V and 0.85V for onset and half-wave potentials respectively), outperforming the single N doped sample (N-C) and most of the electrocatalysts literatures.[15,28,32] And the limiting current of the NB-CN is even larger than commercial Pt/C. NB-CN exhibits a Tafel slope of 65 mV dec-1 (Figure S14), much lower than that of Pt/C (69 mV dec-1) suggesting the faultless ORR kinetics of the NB-CN catalyst. In order to evaluate the electron transferred number, the LSV curves of NB-CN at various rotating speeds from 400 to 2000 rpm were measured (Figure 3b). The electron transfer number for ORR can be calculated according to the Koutechy-Levich (K-L) equation. Over the potential range from of 0.45-0.60 V Koutechy-Levich (K-L) plots obtained from LSV curves and show linear relationships between j-1 and ω-1/2 (Figure 3c). The electron transfer numbers of NB-CN was evaluated to be 3.9, exhibiting a four-electron pathway for ORR. Furthermore, the NB-CN displays excellent stability and methanol tolerance (Figure S15). Specifically, after 6 h tests in 0.1 M KOH at a constant potential of 0.60 V vs RHE current retains 98.7% of the initial current, while the commercial Pt/C catalyst declines to about 81.7% under the same conditions. The ORR activity of NB-CN in acidic media was also tested, which demonstrates convincible catalytic performance in 0.5 M H2SO4 solution (Figure S16), with 0.79 V
and 0.58 V and for onset and half-wave potential respectively. For comparison, the electron transfer numbers of N-C was also tested (Figure S17). To demonstrate the multifunctional catalytic activities of the NB-CN, the electrocatalytic performance in OER and HER of different carbon catalysts were also examined. For OER, a potential of 1.65 V for NB-CN is needed to reach a 10 mA cm-2 current density in O2-saturated 0.1 M KOH solution (Figure 3d), much lower than that of commercial IrO2. The stability test through V–t response (Figure S18) shows that, after reaction for 10000 s, NB-CN exhibits better stability than that of IrO2. This performance is far superior to N-C whether in onset potential or in current density. For HER, NB-CN exhibits excellent catalytic performance under acidic conditions, the overpotential to drive a -10 mA cm−2 current density for NB-CN is 175.3 mV in 0.5 M H2SO4 (Figure 3e) which is superior to most of carbon based catalysts (Table S1). According to the Tafel equation (h=blogj+ a, where j is the current density and b is the Tafel slope), Tafel slope can be obtained by linear fitting the polarization curves. The NB-CN exhibits the lowest Tafel slope of 82.1 mV/decade (Pt/C: 40 mV/decade) (Figure S19), demonstrating advanced reaction kinetics for HER. Figure 3f shows excellent bifunctional activities for NB-CN evaluated by potential difference ΔE,[69] (ΔE=Ej=10-E1/2 Ej=10, the operating potentials to deliver a 10 mA cm-2 current density for OER; E1/2 is the ORR half-wave potential). The ΔE value is 0.815 V for NB-CN, this value is smaller than most of carbon based catalysts, even lower than metal oxides catalysts (Table S2). The outstanding multifunctional activities should be attributed to the unique structure
of NB-CN. Based on the theoretical calculation results, the high doping amount of 4.13 at% N and 3.0 at% B in NB-CN can provide highly concentrated active sites and favorable overpotentials for multiple catalytic reactions of ORR, OER and HER. Meanwhile, the unique semi-open graphitic carbon nanocage structure can provide abundant edge-rich structure, while the codoing of N and B would also lead to a large amount of defects, which is consistent with the Raman analysis results (ID/IG = 1.05). This defect-rich characteristic together with the heretoatoms doping would faliciate enhanced catalytic activity with more active sites.[43,44] while the relatively ordered graphitic feature with favorable small thickness of NB-CN would afford an expedite transmission speed for charge-transfer. In addition, the presence of trace Co may also contribute to the catalytic activity by forming new complex active sites in-plane Co-Nx centers or CoOx species.[70-75] Co-Nx centers can activate the ORR process by significantly decreasing the oxygen adsorption energy and extending the O–O bond,[70-72] while the CoOx species are effective catalysts for water electrolysis[73-75]. Therefore, combining the above factors, the as prepared NB-CN with trace Co is capable of multifunctionality as elecrocatalyst for ORR, OER and HER. Primary Zn-air batteries were assembled to demonstrate the practical application of the as prepared NB-CN as air cathode catalyst. In a typical configuration, both the NB-CN powder and commercial Pt/C were uniformly loaded onto hydrophobic carbon fiber paper electrode with the same loading of 1 mg cm-2 as the cathode, assessed in 6 M KOH in a three-electrode system and a Zn foil was paired as anode. Diagram and Digital picture of the Zn-air battery of the customized electrochemical
cell were shown in Figure 4a and 4b. The as assembled zinc-air battery based on NB-CN possesses an open circuit voltage of 1.4 V, with a peak power density of about 320 mW cm-2 (Figure 4c). Remarkable current densities of 68 mA cm-2 and 220 mA cm-2 can be reached at the voltage of 1.20 and 1.0 V respectively, which is comparable to Pt/C electrode and superior than previous works.[76-78] Besides, the as constructed primary zinc-air batteries are also very durable. Typical galvanostatic discharge curves in Figure 4d illustrate the voltage of 1.31V at current density of 10 mA cm-2 for 14 h, and 1.21 V at 50 mA cm-2 for 8 h, showing little voltage decay duing the test. Such impressive performances and durability of our primary zinc-air batteries should be afforded by the outstanding catalytic activity and stability of NB-CN.
Conclusions In conclusion, this work reported an facile fabrication of N,B-codoped graphitic carbon nanocage with unique defective nanocage structures. The N,B-codoping demonstrates impressive effect on the multifunctional catalytic activities toward ORR, OER and HER, which was confirmed both theoretically and experimentally for the first time. Meanwhile, the unique semi-open nanocage structures, and the defect-rich yet relatively ordered graphitic feature of the as-prepared NB-CN would also favor the splendid catalytic activity by providing more active sites and good charge transport. This multifunctional catalyst can be further used as ORR-OER, OER-HER bifunctional catalyst in metal–air battery and electrochemical water-splitting unit or
beyond. Coupled with the low-priced and easy preparation, the NB-CN would be a promising catalyst in the next generation of fuel cells, metal air batteries and water splitting etc. Acknowledgement We thank the financial supports from National Natural Sciecne Foundation of China (51774251, 21403185, 51590882, 51631001), the Hebei Science Foundation for Distinguished Young Scholars (B2017203313), Hebei
Province
(SLRC2017057),
the
Hundreds of Innovative Talents in
National
R&D
Program of China
(2016YFA0200102), NSFC-RGC Joint Research Scheme (51361165201), Scientific Research Foundation for the Returned Overseas Chinese Scholars (CG2014003002) and the open funding from State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology, 2017-KF-14). References [1] M. R. Gao, Y. F. Xu, J. Jiang, S. H. Yu, Chem. Soc. Rev. 42 (2013) 2986-3017. [2] M. K. Debe, Nature 486（2012）43-51. [3] Z. Peng, S. A. Freunberger, Y. Chen, P. G. Bruce, Science 337（2012）563-566. [4] W. F. Chen, C. H. Wang, K. Sasaki, N. Marinkovic, W. Xu, J. T. Muckerman, Y. Zhu, R. R. Adzic, Energy Environ. Sci. 6 (2013) 943-951. [5] T. Y. Ma, J. Ran, S. Dai, M. Jaroniec, S. Z. Qiao, Angew. Chem. Int. Ed. 54（2015） 4646-4650. [6] M. Tavakkoli, T. Kallio, O. Reynaud, A. G. Nasibulin, C. Johans, J. Sainio, H. Jiang, E. I. Kauppinen, K. Laasonen, Angew. Chem. Int. Ed. 54（2015）
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Figure 2. Element characterization and related theoretical calculations (a) The XRD pattern, (b) the Raman spectrum and (c) EELS spectrum of NB-CN. XPS survey spectra of NB-CN (d) with the high resolution N1s (e) and B1s (f) spectra. (g) Relative position of N, B and O for the N,B-codoped graphene considered in the calculations, number 1 to 13 refer to reaction sites. (h, i) Schematic energy profiles for the ORR pathway (h) and the OER pathway (i) on N,B-codoped graphitic carbon nanocage in alkaline media. Figure 3. Electrocatalytic tests (a) ORR Linear scan voltammogram (LSV) curves for N-C, NB-CN and commercial Pt/C in O2 saturated 0.1 M KOH solution. (b) LSV curves of NB-CN in oxygen saturated 0.1 M KOH at various rotating speeds. (c) K-L plots for NB-CN at various potentials and the electron transfer number in the corresponding voltages. (d) OER LSV curves of different catalysts for N-C, NB-CN and IrO2 at 1600 rpm in O2-saturated aqueous solution of 0.1 M KOH at a scan rate of 5 mV s-1. (e) HER LSV curves for N-C, NB-CN and commercial Pt/C in N2 saturated 0.5 M H2SO4 solution at a sweep rate of 5 mV s-1. (f) LSV curves of N-C, NB-CN, Pt/C and IrO2 catalyst on an RDE (1600 rpm) in 0.1 M KOH (scan rate, 5 mV s-1), showing the electrocatalytic activities towards ORR and OER. Figure 4. Applications of NB-CN as anode of primary Zn-air batteries (a) Diagram of Zn-air battery. (b) Digital picture of the customized electrochemical cell. (c) Polarization and power density curves of primary Zn-air batteries using NB-CN and Pt/C as catalyst. (d) Typical discharge curves of the primary Zn-air batteries using NB-CN and Pt/C as the air catalyst at 10 mA cm-2 and 50 mA cm-2.
Highlights
A N,B-codoped carbon nanocage is reoported for high efficiency multifunctional electrocatalyst.
DFT calculations reveal the catalytic compatibility of the N,B-codoping for ORR/OER/HER.
A theoretical overpotential of 0.34 V for ORR, 0.39 V for OER, and ΔGads of 0.013 eV for HER is obtained.
A primary zinc-air battery is assembled presenting a maximum power density of 320 mW cm-2.
Figure 1
Figure 2
Figure 3
Figure 4
PII: DOI: Reference:
S2211-2855(17)30681-X https://doi.org/10.1016/j.nanoen.2017.11.004 NANOEN2305
To appear in: Nano Energy Received date: 14 August 2017 Revised date: 5 October 2017 Accepted date: 2 November 2017 Cite this article as: Ziyang Lu, Jing Wang, Shifei Huang, Yanglong Hou, Yanguang Li, Yueping Zhao, Shichun Mu, Jiujun Zhang and Yufeng Zhao, N,Bcodoped Defect-rich Graphitic Carbon Nanocages as High Performance Multifunctional Electrocatalysts, Nano Energy, https://doi.org/10.1016/j.nanoen.2017.11.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
N,B-codoped Defect-rich Graphitic Carbon Nanocages as High Performance Multifunctional Electrocatalysts Ziyang Lu11, Jing Wang11, Shifei Huang1, Yanglong Hou2*, Yanguang Li3*,Yueping Zhao1, Shichun Mu4, Jiujun Zhang5, Yufeng Zhao1* 1 Key Laboratory of Applied Chemistry, Yanshan University, Qinhuangdao 066004, China 2 Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing Key Laboratory for Magnetoelectric Materials and Devices (BKLMMD), Beijing 100871, China 3 Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123 , China 4 State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China 5 Institute of Sustainable Energy, Shanghai University, Shanghai University, Shanghai, 200444, P. R. China [email protected] [email protected] [email protected] *Corresponding authors. Abstract Nanocarbon materials recognized as effective and inexpensive catalysts for independent electrochemical reactions, are anticipated to possess a broader spectrum of multifunctionality toward oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). A rational design of trifunctional nanocarbon catalyst requires balancing the heteroatoms-doping and defect-engineering to afford desired active centers and satisfied electric conductivity, which however is conceptually challenging while desires in-depth research both experimentally and theoretically. This work reports a N,B-codoped graphitic carbon nanocage (NB-CN) with graphitic yet defect-rich characteristic as a promising
1
These authors contribute equally to this work.
trifunctional electrocatalyst through a facile thermal pyrolysis assisted in-situ catalytic graphitization (TPCG) process. Density functional theory (DFT) calculations are conducted, for the first time, to demonstrate that the best performance for ORR/OER and HER can be originated from the configuration with B meta to a pyridinic-N, which presents a minimum theoretical overpotential of 0.34 V for ORR, 0.39 V for OER, and a lowest Gibbs free-energy (ΔGads) of 0.013 eV for HER. A primary zinc-air battery is assembled presenting a maximum power density of 320 mW cm-2 along with excellent operation durability, evidencing great potential in practical applications. Graphical abstract The N,B-codoped defect-rich graphitic carbon nanocages (NB-CN) are prepared through a technique of in situ vapor deposition. The NB-CN exhibits efficient multifunctional catalytic activity for ORR, OER and HER, shows potential practical performances in the fields of metal-air battery, fuel cell and water splitting.
Keywords: N,B-codoping, carbon nanocage, multifunctional catalysis, Zinc–air battery
Introduction The sluggish kinetics in oxygen reduction reaction (ORR), oxygen evolution reaction (OER) or hydrogen evolution reaction (HER) demonstrates critical challenges for high efficiency renewable energy storage and conversion devices, such as metal-air batteries, water splitting cells or fuel cells.[1-4] Precious metals (e.g. Pt) or transition metal-oxides are considered as efficient electrocatalysts, which however suffer from high price, inferior durability or poor electric conductivity.[5-11] Nanocarbon based catalysts with good conductivity and modifiable surface chemistry have gained paramount interest in recent years.[12-19] The catalytic activity of nanocarbon materials can be tailored through heteroatom doping (e.g., B, N, P, S, F etc.) by modifying the electronegativity, charge distribution as well as the electron transfer behavior.[14,20-27] Codoping of heteroatoms with different electronegativity can further boost the catalytic performance by introducing more active centers.[15,28-30] For example, nanocarbons codoped with electron-giving atoms (e.g. S or P) and electron-accepting atoms (e.g. N) can serve as effective bifunctional catalyst for ORR-HER, ORR-OER or OER-HER with significantly enhanced electrocatalytic activity, in which the positively charged electron-giving atoms (S or P) would function as active centers together with the positively charged C induced by electron-accepting atoms (N).[31-36] Nevertheless, the design and development of multifunctional electrocatalyst are still
challenging however attractive for the rapid development of renewable energy technologies. Recently, Dai et al.[37] reported a N, P, F-codoped graphene as a trifunctional electrocatalyst for ORR, OER and HER. Thus it becomes possible to construct an integrated self-powered water-splitting units which can supply green gases (H2 and O2) in sustainable energy storage and conversion. As neighboring atoms of C (electronegativity χ=2.55) in the periodic table, B (χ=2.04) and N (χ=3.04) with different electronegativities are also anticipated as promising co-dopants for trifunctional catalyst. However the existing studies regarding N,B-codoped carbons are mostly about independent ORR catalysis,[38-42] their multifunctional catalytic activity for three key electrochemical reactions hasn’t been investigated either theoretically or experimentally. Meanwhile, significant amount of studies have declared that the electrocatalytic activity of nanocarbons can also be enhanced by introducing more surface functional groups, edge dangling bonds, or defective sites. [43,44]
Yao et al.[44] on the other hand, testified the multifunctionality of dopant-free
defective graphene through experiment and corresponding DFT calculation, and proposed a defect-dominated catalytic mechanism. Therefore, a more comprehensive carbon architecture inclusive both heteroatom doping and defective effect, should be considered for an effective design of trifunctional electrocatalyst. In this work, we report a unique N,B-codoped graphitic carbon nanocage (NB-CN) with the shell thickness around 5~14 graphitic layers and defect-rich characteristic, synthesized through a facile thermal pyrolysis assisted in-situ catalytic graphitization
(TPCG) process. Compared with other literatures,[45-49] the carbon nanocage prepared by this method possesses a smaller radius and wall thickness. The as-prepared material exhibits splendid trifunctional catalytic activities for ORR, OER and HER along with superior stability. Density functional theory (DFT) calculations are conducted to uncover the origin of this multifunctional catalytic activity, which demonstrates that the N,B-codoped carbon can present a minimum theoretical overpotential of 0.34 V for ORR, 0.39 V for OER, and a lowest Gibbs free-energy (ΔGads) of 0.013 eV for HER, evidencing great potential as a trifunctional electrocatalyst. A primary Zn-air battery was assembled in order to demonstrate the practical application of NB-CN. The maximum power density reached up 320 mW cm-2, and no obvious activity decay was observed after galvanostatic discharge at current density of 10 mA cm-2 for 14 h or 50 mA cm-2 for 8 h.
2. Experiment 2.1 Materials preparation In a traditional way, 1.2 g chitosan, 1.2 g cobaltous acetate hexahydrate and 2.4 g boric acid were mixed in 100ml deionized water and kept stirring for an hour. Then 0.8 g red phosphorus powders were added into the mixture respectively and continued stirring for 2 hours. Subsequently, 3.6g KOH was dissolved in 10ml deionized water, added into the mixture and kept stirring for 6 hours. Obtained mixture was heated at 100 °C and kept stirring constantly until it turned into a paste. Next, the paste was transferred into a quartz tube and heated to 750 °C kept for 4 hours at a heating rate of
2°C /min in flowing argon atmosphere. The obtained samples were washed with 8M HNO3 and deionized water, then dried at 85℃. Lastly, the samples were heated to 600 ℃ for 3 hours at a heating rate of 10℃ /min in a flowing H2/Ar (20% H2) combination gas for annealing. The final product was obtained and denoted as NB-CN. For comparison, the sample without boric acid and red phosphorus was prepared through the same method above, and referred as N-C. 2.2 Characterization Raman spectra were recorded on a Renishaw in Via Raman microscope with an Arion laser at the excitation wavelength of 514.5 nm. Hitachi-X-ray photoelectron spectrum (XPS) was measured by a VG ESCALAB MKIIX-ray photoelectron spectrometer using Mg−Kα as the exciting source (1253.6 eV). Carl Zeiss SUPRA 55 SAPPHIRE field emission scanning electron microscope (FESEM, Germany, 15 kV) Hitachi-7650 transmission electron microscopy (TEM, Japan, 80 kV), and high resolution transmission electron microscopy (HRTEM, JEOL JEM-3000F) were used to investigate the morphology and microstructure. The specific surface area (SSA) and pore size of the materials were measured using a micromeritics (ASAP 2020 V4.00 (V4.00 H)). Fourier-transform infrared (FTIR) spectra were collected by EQUINOX55 FTIR spectrometer operated from 400 to 4000 cm−1. 2.3 Electrochemical measurement Electrochemical experiments were carried out using the PARSTAT 4000 electrochemical workstation (America) in a three-electrode configuration. A glassy carbon disk electrode (0.196 cm2) coated with the catalyst layer as the working
electrode, saturated calomel electrode (SCE) and Hg/HgO electrode used as reference electrode in 0.1M KOH and 0.5M H2SO4, respectively. A Pt foil (1 cm2) was used as the counter electrode for ORR and OER tests and a graphite rod was used as the counter electrode for HER tests. RHE was used in this article, E(RHE)= E(SCE) +0.2412 + 0.059×pH, E(RHE) = E( Hg/HgO) + 0.098 V + 0.059 × pH. The rotating disk electrode (RDE) was performed on a MSRX electrode rotator (Pine Instrument) and the PARSTAT 4000 electrochemical workstation. For ORR and OER measurements, 2.5 mg sample powders were mixed with 1000 μL isopropyl alcohol and 15 μL 5 wt% Nafion. Then the mixture was ultrasonically suspended for 30 min. After that 20 μL of this homogenous mixture was dropped on the glassy carbon disk and dried at room temperature to obtain a catalyst loading of 255.1 μg cm-2. For HER measurements, the catalyst loading was 382.7 μg cm-2. For comparison, commercial Pt/C (20 wt%) and IrO2 (20 wt%) were also measured. The loading of Pt/C and IrO2 catalysts on glassy carbon disk electrode was 127.6 μg cm-2 and 255.1 μg cm-2 respectively. The catalytic activity of the electrocatalyst toward ORR and OER were all performed in 0.1 M KOH aqueous solution. Before the experiment, the electrolyte was bubbled with O2 or N2 for >20 min. ORR tests under acidic condition in a similar way. In order to investigate the ORR procedure, the oxygen reduction test was performed at various rotating speeds from 400 to 2000 rpm in O2-saturated electrolyte at a sweep rate of 5 mV s-1. The electron transfer number (n) was evaluated according to Koutecky–Levich (K-L) equation.
1
J
1
JL
1
JK
1
B
1/2
1
JK
(1)
Where J is the measured current density, JK is the kinetic-limiting current density, ω is the electrode rotation rate.
B 0.62nFCo(Do )2 / 3 1 / 6
(2)
B could be determined from the slope of the Koutecky–Levich plots based on the Koutecky–Levich equation, here n is the number of transferred electrons, F is the faraday constant (96485 C mol-1), Co (1.2×10−6 mol cm-3) and Do (1.9×10−5 cm2 s−1) are the bulk concentration and diffusion coeffcient of O2 in electrolyte, υ (0.01 cm2 s−1) is the kinematic viscosity of the electrolyte. The oxygen evolution text was performed in O2-saturated electrolyte at a sweep rate of 5 mV s-1. Results and Discussion The N,B-codoped carbon nanocage is prepared through a thermal pyrolysis assisted in-situ catalytic graphitization (TPCG) method at 750 oC in argon atmosphere, as illustrated in Figure 1a. In a typical synthesis, chitosan was used as C and N resources, while boracic acid as B resource. Certain amount of KOH, red phosphorus (RP), and cobaltous acetate are added for multiple purposes: on the one hand, these chemicals will react with each other through a series of reactions forming Co 2P nanoparticles, which not only serve as catalysts in the following graphitization process, but also regulate the morphology by depositing the C atoms on the surface, forming a C-Co2P core-shell structure.[50] This can be confirmed by XRD patterns and SEM images of the intermediate products (Figure S1-S2). On the other hand KOH and RP will facilitate the in-situ exfoliation of carbon layers, and guarantee the
few-layer characteristic of the cage wall.[51,52] The N,B-codoped graphitic carbon nanocage will be finally obtained by leaching out the Co2P core and post-annealing treatment (see supporting information for the detailed information). For comparison, single N-doped carbon (N-C) is also prepared and characterized (Figure S3). The scanning electron microscope (SEM) and transmission electron microscope (TEM) images (Figure 1b-c, Figure S4) demonstrate uniform nanocage structure with a diameter size of 30-80 nm. Figure 1d corresponds to the C-Co2P core-shell structure before acid etching. The high resolution (HR) TEM image (Figure 1e,f) indicates the hollow structure of the carbon nanocage, revealing good crystalline few-layer (5~14 layers) nanostructure of the carbon nanocage with a interlayer distance of ~0.35 nm. Prominently, despite the relatively ordered graphitic feature, the as-prepared NB-CN demonstrates defect-rich characteristic (Figure 1e), including the topological disclinations at the corner, the surficial broken fringes, and the micropores,
[53,54]
which might be attributed to the removal of inorganic ingredients, B,N-codoping or partial subtraction of N element from chitosan precursors.[43,44] Although the impact of the type of defects on the catalytic is not clear, there is no doubt that defects have a enhancement on the catalytic and the diversity of defects is also associated with the diversity of catalytic performance. XRD patterns in Figure 2a show the (002) diffraction peaks at 25.6° demonstrating an apparently left shift with increased d spacing of 0.348 nm，which is in consistent with the HR-TEM result. Raman spectrum of NB-CN (Figure 2b) shows characteristic peak of D-band and G-band at ~1598 cm-1 and 1338 cm-1
respectively.[55] An ID/IG ratio of 1.05 indicates the defect rich characteristic caused by the B, N doping in the sp2 carbon structure, which is consistent with the HRTEM result. The BET (Brunauer-Emmet-Teller) specific surface area was obtained as 796 m2/g through the nitrogen adsorption measurement (Figure S5). The electron energy loss spectrum (EELS) (Figure 2c) and X-ray photoelectron spectroscopy (XPS) (Figure 2d) confirmed the coexistence of B and N in the NB-CN. The characteristic peaks of C1s, O1s, N1s and B1s can be clearly identified from the XPS survey spectrum (Figure 2d) with an atom ratio of 82.18, 10.68, 4.13 and 3.0 at%, respectively. The high-resolution N1s spectrum (Figure 2e) can be assigned into three N species corresponding to pyridinic-N (398.5 eV), pyrrolic-N (399.7 eV), and graphitic-N (401.1 eV), of which the percentages are 68.5%, 18.3% and 13.2% respectively. The large percentage of pyridinic-N and pyrrolic-N would generate activation for neighboring atoms leading high electrocatalytic activity for ORR, OER and HER.[56-60] B1s spectrum (Figure 2f) can be deconvoluted into three peaks, corresponding to BCO2 (192.3 eV), O-B-C (191.2 eV) and BC2O (190.6 eV).[61,62] The high-resolution XPS spectra of P 2p and Co 2p were shown in Figure S6. No significant signal for P and Co was distinguished. The high-resolution C1s and O1s were shown in Figure S7. Furthermore, the Fourier transform infrared (FTIR) (Figure S8) also confirm the co-existence of B and N. As contrast, single N-doped carbon (N-C) was also prepared using the similar method and related characterizations are provided in the supporting information (Figure S9). The possible residual cobalt and phosphorus cannot be detected by XPS (Figure S6).
However, the ICP-MS (inductively coupled plasma mass spectrometry) result indicates the presence of trace amount (0.1% atomic ratio) of cobalt element. First-principles calculations using density functional theory (DFT) were carried out to provide microscopic scenario of the catalytic reactions for the NB-CN. Four co-doped structures with N/B in different sites are constructed (Figure S10, see detailed description in the supporting information). For convenience, only pyridinic-N is modeled which is dominant (68.5% from XPS) and evaluates the ORR/OER activities. According to the XPS results, oxygen atom is inclusive in the sample, which is also included in our models and it is found to be positive to the reactions. To study the ORR (OER) catalytic activities, four steps are considered in the alkaline media, i.e., O2 dissociation into *OOH, *OOH decomposition into *O, *O alkalization into *OH, *OH detachment into OH- (the reverse progress started with OH- adsorption is for OER). Our results show that although the structure with B para to a pyridinic-N (Figure S10d) demonstrates the lowest energy, the best performance for ORR/OER and HER is from the configuration with B meta to a pyridinic-N (labeled with number 4 in Figure 2g). This is in consistent with the model constructed by Zhang et al [30], in which a B atom meta to a pyridinic N atom, shows the highest Ead value (the energy of HO2 adsorption) and we also notice that this relative position is also adopted in N,P-codoped graphene. Therefore, we focus on the B meta to a pyridinic-N case in the following studies. The overpotential η as a critical parameter of catalytic activities is identified to be as low as 0.34V for the ORR, with the active site located at boron atom. This value is much lower than that of commercial Pt (~0.45V).[30] The
minimum overpotential for OER can be obtained as 0.39V considering the active site transport to the neighboring carbon atom (labeled with number 3 in Figure 2g) which is also reported in literature.[63] The effect of different electrode potentials U is also investigated and the free energy diagrams are plotted in Figure 2h and 2i for ORR and OER, respectively. It is discovered that at zero potential, all the reactions are exothermic for both ORR and OER in view of the fact that all the elementary reaction steps are downhill (uphill). The catalyst performs better with negative values of electrode potentials for both ORR and OER in basic media. In general, carbon materials exhibit superior hydrogen evolution activity under acidic conditions than alkaline conditions.[64-65] Therefore, this work only focuses on the HER in acid solution, in which Gibbs free-energy (ΔGads) of the adsorption of atomic hydrogen is a main descriptor to determine the HER activity of a catalyst.[66-67] The lowest ΔGads value is calculated to be 0.013 eV with the reaction site of carbon (labeled with number 11 in Figure 2g). It is well known that the closer to zero (for the ΔGads value), the better the catalyst is as the positive (negative) ΔGads cannot efficiently absorb on (desorb from) carbon. Compared with the Pt (ΔGads =~0.90 eV) and other N- and/or S-doped graphene (0.12 eV for N/S co-doped graphene),[68] the minimum ΔGads value ensures the NB-CN to be the best HER catalyst for now. The optimized adsorption structures (top view and side view) for minimum ORR, OER overpotential and minimum HER free energy calculations are shown in Figure S11-13 respectively. The evaluation of the electrocatalytic activity of NB-CN was initially carried out by examining the cathodic ORR performance in O2-saturated 0.1M KOH aqueous
solution. A series of linear sweep voltammograms (LSVs) were collected on a rotating disk electrode (RDE) at 1600 rpm for different samples. The LSV curves in Figure 3a confirms the outstanding electrocatalytic activity of NB-CN, with an onset potential of 0.92 V versus reversible hydrogen electrode (RHE) and a half-wave potential of 0.835 V, comparable to that of commercial Pt/C (0.95V and 0.85V for onset and half-wave potentials respectively), outperforming the single N doped sample (N-C) and most of the electrocatalysts literatures.[15,28,32] And the limiting current of the NB-CN is even larger than commercial Pt/C. NB-CN exhibits a Tafel slope of 65 mV dec-1 (Figure S14), much lower than that of Pt/C (69 mV dec-1) suggesting the faultless ORR kinetics of the NB-CN catalyst. In order to evaluate the electron transferred number, the LSV curves of NB-CN at various rotating speeds from 400 to 2000 rpm were measured (Figure 3b). The electron transfer number for ORR can be calculated according to the Koutechy-Levich (K-L) equation. Over the potential range from of 0.45-0.60 V Koutechy-Levich (K-L) plots obtained from LSV curves and show linear relationships between j-1 and ω-1/2 (Figure 3c). The electron transfer numbers of NB-CN was evaluated to be 3.9, exhibiting a four-electron pathway for ORR. Furthermore, the NB-CN displays excellent stability and methanol tolerance (Figure S15). Specifically, after 6 h tests in 0.1 M KOH at a constant potential of 0.60 V vs RHE current retains 98.7% of the initial current, while the commercial Pt/C catalyst declines to about 81.7% under the same conditions. The ORR activity of NB-CN in acidic media was also tested, which demonstrates convincible catalytic performance in 0.5 M H2SO4 solution (Figure S16), with 0.79 V
and 0.58 V and for onset and half-wave potential respectively. For comparison, the electron transfer numbers of N-C was also tested (Figure S17). To demonstrate the multifunctional catalytic activities of the NB-CN, the electrocatalytic performance in OER and HER of different carbon catalysts were also examined. For OER, a potential of 1.65 V for NB-CN is needed to reach a 10 mA cm-2 current density in O2-saturated 0.1 M KOH solution (Figure 3d), much lower than that of commercial IrO2. The stability test through V–t response (Figure S18) shows that, after reaction for 10000 s, NB-CN exhibits better stability than that of IrO2. This performance is far superior to N-C whether in onset potential or in current density. For HER, NB-CN exhibits excellent catalytic performance under acidic conditions, the overpotential to drive a -10 mA cm−2 current density for NB-CN is 175.3 mV in 0.5 M H2SO4 (Figure 3e) which is superior to most of carbon based catalysts (Table S1). According to the Tafel equation (h=blogj+ a, where j is the current density and b is the Tafel slope), Tafel slope can be obtained by linear fitting the polarization curves. The NB-CN exhibits the lowest Tafel slope of 82.1 mV/decade (Pt/C: 40 mV/decade) (Figure S19), demonstrating advanced reaction kinetics for HER. Figure 3f shows excellent bifunctional activities for NB-CN evaluated by potential difference ΔE,[69] (ΔE=Ej=10-E1/2 Ej=10, the operating potentials to deliver a 10 mA cm-2 current density for OER; E1/2 is the ORR half-wave potential). The ΔE value is 0.815 V for NB-CN, this value is smaller than most of carbon based catalysts, even lower than metal oxides catalysts (Table S2). The outstanding multifunctional activities should be attributed to the unique structure
of NB-CN. Based on the theoretical calculation results, the high doping amount of 4.13 at% N and 3.0 at% B in NB-CN can provide highly concentrated active sites and favorable overpotentials for multiple catalytic reactions of ORR, OER and HER. Meanwhile, the unique semi-open graphitic carbon nanocage structure can provide abundant edge-rich structure, while the codoing of N and B would also lead to a large amount of defects, which is consistent with the Raman analysis results (ID/IG = 1.05). This defect-rich characteristic together with the heretoatoms doping would faliciate enhanced catalytic activity with more active sites.[43,44] while the relatively ordered graphitic feature with favorable small thickness of NB-CN would afford an expedite transmission speed for charge-transfer. In addition, the presence of trace Co may also contribute to the catalytic activity by forming new complex active sites in-plane Co-Nx centers or CoOx species.[70-75] Co-Nx centers can activate the ORR process by significantly decreasing the oxygen adsorption energy and extending the O–O bond,[70-72] while the CoOx species are effective catalysts for water electrolysis[73-75]. Therefore, combining the above factors, the as prepared NB-CN with trace Co is capable of multifunctionality as elecrocatalyst for ORR, OER and HER. Primary Zn-air batteries were assembled to demonstrate the practical application of the as prepared NB-CN as air cathode catalyst. In a typical configuration, both the NB-CN powder and commercial Pt/C were uniformly loaded onto hydrophobic carbon fiber paper electrode with the same loading of 1 mg cm-2 as the cathode, assessed in 6 M KOH in a three-electrode system and a Zn foil was paired as anode. Diagram and Digital picture of the Zn-air battery of the customized electrochemical
cell were shown in Figure 4a and 4b. The as assembled zinc-air battery based on NB-CN possesses an open circuit voltage of 1.4 V, with a peak power density of about 320 mW cm-2 (Figure 4c). Remarkable current densities of 68 mA cm-2 and 220 mA cm-2 can be reached at the voltage of 1.20 and 1.0 V respectively, which is comparable to Pt/C electrode and superior than previous works.[76-78] Besides, the as constructed primary zinc-air batteries are also very durable. Typical galvanostatic discharge curves in Figure 4d illustrate the voltage of 1.31V at current density of 10 mA cm-2 for 14 h, and 1.21 V at 50 mA cm-2 for 8 h, showing little voltage decay duing the test. Such impressive performances and durability of our primary zinc-air batteries should be afforded by the outstanding catalytic activity and stability of NB-CN.
Conclusions In conclusion, this work reported an facile fabrication of N,B-codoped graphitic carbon nanocage with unique defective nanocage structures. The N,B-codoping demonstrates impressive effect on the multifunctional catalytic activities toward ORR, OER and HER, which was confirmed both theoretically and experimentally for the first time. Meanwhile, the unique semi-open nanocage structures, and the defect-rich yet relatively ordered graphitic feature of the as-prepared NB-CN would also favor the splendid catalytic activity by providing more active sites and good charge transport. This multifunctional catalyst can be further used as ORR-OER, OER-HER bifunctional catalyst in metal–air battery and electrochemical water-splitting unit or
beyond. Coupled with the low-priced and easy preparation, the NB-CN would be a promising catalyst in the next generation of fuel cells, metal air batteries and water splitting etc. Acknowledgement We thank the financial supports from National Natural Sciecne Foundation of China (51774251, 21403185, 51590882, 51631001), the Hebei Science Foundation for Distinguished Young Scholars (B2017203313), Hebei
Province
(SLRC2017057),
the
Hundreds of Innovative Talents in
National
R&D
Program of China
(2016YFA0200102), NSFC-RGC Joint Research Scheme (51361165201), Scientific Research Foundation for the Returned Overseas Chinese Scholars (CG2014003002) and the open funding from State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology, 2017-KF-14). References [1] M. R. Gao, Y. F. Xu, J. Jiang, S. H. Yu, Chem. Soc. Rev. 42 (2013) 2986-3017. [2] M. K. Debe, Nature 486（2012）43-51. [3] Z. Peng, S. A. Freunberger, Y. Chen, P. G. Bruce, Science 337（2012）563-566. [4] W. F. Chen, C. H. Wang, K. Sasaki, N. Marinkovic, W. Xu, J. T. Muckerman, Y. Zhu, R. R. Adzic, Energy Environ. Sci. 6 (2013) 943-951. [5] T. Y. Ma, J. Ran, S. Dai, M. Jaroniec, S. Z. Qiao, Angew. Chem. Int. Ed. 54（2015） 4646-4650. [6] M. Tavakkoli, T. Kallio, O. Reynaud, A. G. Nasibulin, C. Johans, J. Sainio, H. Jiang, E. I. Kauppinen, K. Laasonen, Angew. Chem. Int. Ed. 54（2015）
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Figure 2. Element characterization and related theoretical calculations (a) The XRD pattern, (b) the Raman spectrum and (c) EELS spectrum of NB-CN. XPS survey spectra of NB-CN (d) with the high resolution N1s (e) and B1s (f) spectra. (g) Relative position of N, B and O for the N,B-codoped graphene considered in the calculations, number 1 to 13 refer to reaction sites. (h, i) Schematic energy profiles for the ORR pathway (h) and the OER pathway (i) on N,B-codoped graphitic carbon nanocage in alkaline media. Figure 3. Electrocatalytic tests (a) ORR Linear scan voltammogram (LSV) curves for N-C, NB-CN and commercial Pt/C in O2 saturated 0.1 M KOH solution. (b) LSV curves of NB-CN in oxygen saturated 0.1 M KOH at various rotating speeds. (c) K-L plots for NB-CN at various potentials and the electron transfer number in the corresponding voltages. (d) OER LSV curves of different catalysts for N-C, NB-CN and IrO2 at 1600 rpm in O2-saturated aqueous solution of 0.1 M KOH at a scan rate of 5 mV s-1. (e) HER LSV curves for N-C, NB-CN and commercial Pt/C in N2 saturated 0.5 M H2SO4 solution at a sweep rate of 5 mV s-1. (f) LSV curves of N-C, NB-CN, Pt/C and IrO2 catalyst on an RDE (1600 rpm) in 0.1 M KOH (scan rate, 5 mV s-1), showing the electrocatalytic activities towards ORR and OER. Figure 4. Applications of NB-CN as anode of primary Zn-air batteries (a) Diagram of Zn-air battery. (b) Digital picture of the customized electrochemical cell. (c) Polarization and power density curves of primary Zn-air batteries using NB-CN and Pt/C as catalyst. (d) Typical discharge curves of the primary Zn-air batteries using NB-CN and Pt/C as the air catalyst at 10 mA cm-2 and 50 mA cm-2.
Highlights
A N,B-codoped carbon nanocage is reoported for high efficiency multifunctional electrocatalyst.
DFT calculations reveal the catalytic compatibility of the N,B-codoping for ORR/OER/HER.
A theoretical overpotential of 0.34 V for ORR, 0.39 V for OER, and ΔGads of 0.013 eV for HER is obtained.
A primary zinc-air battery is assembled presenting a maximum power density of 320 mW cm-2.
Figure 1
Figure 2
Figure 3
Figure 4