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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Cobalt Single Atoms Immobilized N‑Doped Carbon Nanotubes for Enhanced Bifunctional Catalysis toward Oxygen Reduction and Oxygen Evolution Reactions Sobia Dilpazir,†,‡ Hongyan He,† Zehui Li,§ Meng Wang,†,‡ Peilong Lu,†,‡ Rongji Liu,† Zhujun Xie,∥ Denglei Gao,† and Guangjin Zhang*,†
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CAS Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China § State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, P. R. China ∥ School of Printing & Packaging, Beijing Institute of Graphic Communication, Beijing 102600, P. R. China S Supporting Information *
ABSTRACT: Novel Co atoms immobilized carbon nanotubes (CoSAs@CNTs) are synthesized by structural engineering of the zeolitic imidazolate framework (ZIF-67) upon treatment with dicyandiamide (DCD). A unique morphology and promising electrochemical performance are shown by the Co atoms immobilized CNTs. The electrocatalyst remarkably exhibits a highly positive onset potential of 0.99 V and half-wave potential of 0.86 V, both even more positive than the commercial Pt/C catalyst, and the current density is also greater than that of the Pt/C catalyst in alkaline media. A decent performance is observed in acidic media also. The electrocatalyst is extraordinarily stable to harsh environments. A promising performance for the oxygen evolution reaction (OER) is demonstrated by the electrocatalyst, while for bifunctional electrocatalysis a small overvoltage of 0.78 V is observed with onset potential at the lower overpotential of 300 mV announcing the advantage of its usage for practical energy conversion and storage systems. This novel study may provide a new road map for fuel cell technology. KEYWORDS: single atoms, oxygen reduction, oxygen evolution reactions, carbon nanotubes, bifunctional catalysis
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INTRODUCTION Fuel cell technology which provides clean and sustainable power1 is one of the promising solutions for the current energy crisis.2−7 These fuel cells directly generate electricity by electrochemical reduction of oxygen and oxidation of fuel.8−10 The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are integral to the renewable energy technologies, e.g., regenerative fuel cells, electrocatalytic water splitting, and metal−air batteries.11 These processes possess intrinsically slower kinetics.12−14 Therefore, to breed sustainable energy devices, speeding up these reactions by lowering their activation energies is vital, for which precious metals are employed traditionally.15 However, the use of precious metals trammels the industrial commercialization of these devices because of high cost, low tolerance to CO and methanol crossover effect, instability with time, and scarcity.16,17 Substantial efforts have been made in the development of efficient and cost-effective catalysts which reveal that the high density of active sites and abundant porosity lead to a faster kinetics.18−20 Carbon-based porous metal organic frameworks (MOFs) with some transition d metal serve as puissant candidates for the formation of the © XXXX American Chemical Society
electrocatalyst with propinquity in the efficiency as compared to that of precious noble metals and perhaps better CO and methanol tolerance and cost effectiveness.21−24 Careful engineering of the electronic structure and chemical composition could lead to an ultrafine structure more favorable for faster electron transfer.25 Because of their unique features, i.e., permanent porosity, high specific surface area (SSA), larger pore volume,26−29 good electrical conductivity, and high stability to harsh conditions, MOFs have attracted much attention.30−36 However, the key factor for the excellent performance is controlling the shape and morphology of central metals which have a strong connotation on performance of the electrocatalyst.37,38 Among these MOFs zeolite imidazolate frameworks (ZIFs) are a class of MOFs with a lucent pore structure, in which the transition metal is complexed to four nitrogens at the same time as with a carbon matrix.39 ZIF-67 with central cobalt metal is newly explored for bifunctional catalysis and has shown a good Received: March 28, 2018 Accepted: June 19, 2018
A
DOI: 10.1021/acsaem.8b00490 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials
electrode. The electrode was then placed in a preheated oven at 60 °C for 10 min. Cyclic Voltammetry (CV) and Linear Sweep Voltammetry (LSV). CV and LSV were performed by using a traditional three-electrode system with Ag/AgCl (saturated with 3 M KCl) as reference electrode and platinum wire as counter electrode. A 0.1 M KOH solution was used as electrolyte, while for electrochemical performance under acidic conditions, 0.1 M HClO4 solution was used as electrolyte, and a Hg calomel electrode was used as reference electrode. Calculation of Electron Transfer Number n. The electron transfer number was calculated from RDE data by using Koutechy−Levich (K−L) equation.
potential and current density as well, but is still not compatible with noble metals;40−44 however, at the active sites in these catalysts the metal is not fully exposed which subordinates the catalyst efficiency. Deeply embedded metal atoms if exposed fully and dispersed well could add to the performance of the bifunctional catalyst.45,46 This can be achieved by creating single atomic sites (SAs) in the material. SAs, because of their rational design, surface exposure, and other atomic level configurations, surpass the traditional nanoparticles in performance which have complex structures, geometry, and composition.47−49 The dispersed SAs increase the number of active sites and surface area for catalysis.50,51 Keeping in view the challenges of achieving highly concentrated and uniformly dispersed atomic metal sites, we have developed a novel and effective strategy for designing the best suited electrocatalyst. Herein we report a novel approach for the synthesis of nitrogen-doped porous carbon nanotubes with fully dispersed cobalt single atoms, i.e., CoSAs@NCNTs, using dicyandiamide-modified (DCD-modified) ZIF-67.
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1 1 1 1 1 = + = + j jk jl jk Bω1/2
(1) −2
Here, j is the current density measured (mA cm ), jk is the kinetic current density (mA cm−2), and jl is diffusion-limiting current density (mA cm−2). ω represents the rotating rate of the disk electrode. B is the slope of the Koutechy−Levich equation and can be directly determined from the slope of the K−L plot. B = 0.62nF(DO2)3/2 ν−1/6CO2
EXPERIMENTAL SECTION
(2)
Here, n represents the overall number of transferred electrons during the oxygen reduction reaction, F is the Faraday constant, CO2 denotes the bulk concentration of O2, DO2 is the diffusion coefficient of O2, ν represents the kinematic viscosity of the electrolyte, while k is the electron transfer rate constant. Overall transfer number of electrons (n) and the kinetic-limiting current jk can be obtained from the slope and intercept of the Koutecky−Levich plots (1/j versus ω−1/2) using eqs 1 and 2. The rotating ring-disk electrode (RRDE) percentage yield of H2O2 and the number of electrons transferred (n) were determined by RRDE measurements. The electrode as well as the catalyst ink were prepared by the same method as for the rotating disk electrode (RDE). The ring electrode was held constant to 1. 5 V versus reversible hydrogen electrode (RHE) to oxidize H2O2 diffusing from the disk electrode. The %H2O2 yield and transfer number (n) were calculated by following eqs 3 and 4.
Chemicals and Reagents. Cobalt nitrate hexahydrate Co(NO3)2, dicyandiamide (DCD), and 2-methylimidazole (2-MeIm) were purchased from Aladdin. Pt/C (Pt nominally 40% on highsurface-area carbon support) was obtained from Alfa Aesar. N,NDimethylformamide (DMF) and methanol (anhydrous) were bought from Xilong Scientific Corporation. Nafion solution (5 wt % in lower aliphatic alcohols and water) was obtained from Sigma-Aldrich. All the reagents were used as received without any further purification. Freshly prepared solutions in ultrapure water were used for all the electrochemical measurements. Synthesis of ZIF-67. ZIF-67 was synthesized by coprecipitation method. In the typical synthesis, 0.055 mmol of Co(NO3)2 was dissolved in 10 mL of anhydrous methanol. After stirring for 30 min, imidazole solution (1 mmol in 15 mL of methanol) was added slowly to the metal precursor solution with continuous stirring. A purple turbid solution was formed which was stirred for 4 h at room temperature. The final product was collected by centrifugation, washed twice with methanol, and dried under vacuum at 60 °C. Synthesis of Co@NC. Co@NC was obtained through pyrolysis by carbonization of ZIF-67 at 750 °C for 2 h under controlled Ar gas flow. The obtained black powder was washed with 2 M HCl with subsequent washing with water and methanol and then dried at 60 °C under vacuum. Synthesis of ZIF-67 @DCD. ZIF-67@DCD was synthesized by mixing 0.3 mmol of DCD with the Co(NO3)2 solution. Upon the formation of a clear homogeneous solution, imidazole solution was added dropwise. The solution was stirred for 4 h, then centrifuged followed by successive washing with anhydrous methanol, and finally dried at 60 °C for 4 h. A similar protocol was repeated for all the concentrations. Synthesis of CoSAs@CNT Nanotubes. Cobalt-atom-decorated nanotubes were obtained through pyrolysis by carbonization at 750 °C for 2 h under controlled Ar gas flow. The obtained black powder was washed with 2 M HCl with subsequent washing with water and methanol and then dried at 60 °C under vacuum. Evaluation of Electrocatalytic Performance. The electrocatalytic performance was tested by using the conventional three-electrode system with the CHI 760e (Shanghai, China) electrochemical workstation. Preparation of Electrocatalyst. The working electrode was prepared on a glassy carbon electrode. Catalyst ink was prepared typically by dispersing 4 mg of electrocatalyst in 1 μL of DMF. The catalyst was ultrasonicated with DMF for 30 min to form homogeneous ink. The catalyst was then loaded to the prepolished glassy carbon electrode with a loading amount of 0.3 mg cm−2. A 2.5 μL portion of 5 wt % Nafion was dropped at the surface of the
%H 2O2 =
n=
200Iring /N Iring /N + Idisk
4Idisk Iring /N + Idisk
(3)
(4)
Here, Iring is ring current, N is the collection efficiency of the Pt ring which was calibrated by the K3Fe(CN)6 redox reaction and was determined to be 0.40, while Idisk is the voltammetric current at the disk electrode. Structural Characterization. Powder X-ray diffraction (XRD) patterns were recorded by an X’Pert PRO PANalytical diffractometer operating at 40 kV and 30 mA with Cu Kα1 as radiation source (λ= 0.154 18 nm). Scanning electron microscopy (SEM) was carried out with a JEOL JSM-7610F microscope. The transmission electron microscopy (TEM) images were obtained by a field-emission JEM2100F microscope at an operating voltage of 200 kV. X-ray photoelectron spectroscopy was carried out for the elemental analysis of the samples (ESCALab220i-XL). Nitrogen adsorption−desorption isotherms were measured at 77 K (Autosorb-IQ, Quantachrome). The Brunauer−Emmett−Teller (BET) method was used to calculate specific surface area of the electrocatalyst and the total pore volume, and the surface was examined by V−t method. The discrete Fouriertransform (DFT) method was applied to measure the pore size distribution (PSD). The samples were degassed for 12 h at 200 °C under vacuum. For the monitoring of weight changes with temperature changes, thermogravimetric analysis (TG/DTA) was carried out by a TA Instruments SDT Q600 instrument under argon or air flow at a heating rate of 5 °C min−1 from room temperature to B
DOI: 10.1021/acsaem.8b00490 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials 900 °C. The Raman spectrum was recorded by a Raman spectrophotometer (Renishaw Raman system, inVia Reflex) with an excitation wavelength of 532 nm for the 20 mW air-cooled argon ion laser source. The power of the laser positioned at the sample was 4.0 mW with diameter 1 mm. Data acquisition time was set to 10 s.
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RESULTS AND DISCUSSION Scheme 1 illustrates the fabrication of CoSAs@CNTs which involved various steps and chemical transformations from DCD-modified ZIF-67 to the final catalyst. Scheme 1. Schematic Illustration of the Strategy for the Fabrication of CoSAs@CNTs
The structural and compositional transformation is manifested by phase analysis of the material. The presence of metallic cobalt is clearly made evident by the XRD pattern with major peaks at the position of (2θ) 44.3°, 51.6°, and 75.9° that is well-fitted to the face-centered growth of metallic cobalt along the 111, 200, and 220 crystal planes (JCPDS card 150806) while a broad peak at (2θ) 26° is a characteristic peak for graphitized carbon indexed well to JCPDS card 75-1621. Strong and sharp diffraction peaks indicate a well-defined crystalline structure (Figure 1).
Figure 2. (a) SEM image of uncarbonized ZIF-67/DCD. (b) SEM image of CoSAs@CNTs. (c) TEM image of CoSAs@CNTs. (d) HRTEM image of CoSAs@CNTs. (e, f) HAADF images of CoSAs@ CNTs. (g) HAADF image showing the atomic distribution of Cobalt atoms. (h−l) Elemental mapping of Co, C, N, and O.
(111) crystal plane of Co, while the interplanar distance of the graphitic carbon ordered in layers is 3.40 Å corresponding to the crystal plane (002). The results are in accordance with XRD results. Figure 2e shows the presence of bamboo-like carbon nanotubes bearing cobalt nanoparticles at the tip. The existence of Co nanoparticles at the tips of CNTs indicates catalyzing growth of the nanotubes. The diameter growth rate and alignment of nanotubes are strongly affected and controlled by Co nanoparticles. More importantly, the bright-field high-angle annular dark-field (HAADF) analysis shows that the single atoms of Co are homogeneously welldispersed throughout the network of nanotubes (Figure 2f). These single atomic sites are highlighted in Figure 2g. The presence of all the elements is manifested by EDS mapping (Figure 2h−l) which shows the uniform distribution of these elements. The orbital peaks of the XPS survey spectrum divulge the presence of Co, N, C, and O elements (Figure S3). Relative percentages of the elements C, N, O, and Co are 85%, 3.17%, 10.85%, and 0.14%, respectively. The high-resolution electron spectroscopy for chemical analysis (ESCA) of each element further avows the chemical states of the respective element. The ESCA spectrum of C 1s can be deconvoluted into three main peaks showing the hybridization of carbon atoms during the formation of the core−shell structures. The peaks centered at 284.5, 286.5, and 288.97 eV are ascribed as CC bonds (graphitic carbon), COC bonding, and OCO, respectively (Figure 3a).52 Four characteristics peaks are
Figure 1. Powder XRD spectrum of CoSAs@CNTs.
SEM images reveal that DCD-modified ZIF-67 (denoted as ZIF-67@DCD) showed a typical polyhedral morphology of ZIF crystals before carbonization (Figure 2a). It can be clearly envisioned from TEM that ZIF-67@DCD crystals are not hollow (Figure S1), indicating that DCD may be absorbed in the frameworks of ZIF-67. As shown in Figure 2b, upon subsequent pyrolysis, these crystals are converted to convoluted carbon nanotubes (denoted as CoSAs@CNTs). Whereas for simple carbonized ZIF-67 (Co@NC), the dodecahedral shape remains with a lot of Co nanoparticles embedded in the frameworks (Figure S2). The nanotubes are further enumerated by TEM images (Figure 2c), which showed that, at each tip of carbon nanotubes, there is a metallic nanoparticle. Details of the component unit and structure of nanotubes are characterized by high-resolution TEM (HRTEM) (Figure 2d). The metal coated by a carbon shell exhibits an interplanar spacing of 2.05 Å attributed to the C
DOI: 10.1021/acsaem.8b00490 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Figure 4. (a) N2 adsorption−desorption isotherm. (b) Pore size distribution. (c) Thermogravimetric analysis of ZIF-67@DCD. (d) Raman spectrum of CoSAs@CNTs.
Figure 3. High-resolution XPS spectrum of (a) C 1s, (b) Co 2p, (c) N 1s, and (d) O 1s.
transforming it into a dense carbon nanotubular structure, and the gradual weight loss during the third phase is due to further alignment of the structure and evaporation of gaseous materials such as CO2, CO, etc. from the material. Further heating may result in the deformation of the regular alignment of the channel structure which has a direct effect on its catalytic performance. Detailed analysis of TGA of DCD and ZIF-67 shows that both materials are highly vulnerable to the temperature changes emphasizing the selection of the carbonization temperature for the formation of a well-defined structure (Figures S4 and S5). The structural defects and transformation are shown by Raman spectrum. The band that arose due to structural deformation by sp3-hybridized carbon vibrations is positioned at 1357 cm−1 as the D band, while the G band is attributed by the sp2-hybridized carbon domain, arising because of the E2g scattering vibrational mode of first-ordered scattering in a hexagonal lattice. The judging criterion for degree of graphitization, i.e., the ratio of ID/IG, suggests that there is a decent degree of graphitization in the carbon domain.58 This value was determined as 0.86 (Figure 4d), indicating the high degree of graphitization of the prepared material. Growth Mechanism of Single Co Atoms Immobilized N-Doped Carbon Nanotubes. The synthesized DCDmodified ZIF-67 structure is treated thermally at elevated temperature under passive conditions which subsequently forms the CoSAs@CNT structure. The process is illustrated in Scheme 1. The possible mechanism of growth for the nanotubes might involve the transformation of DCD to a cyclic structure by increasing temperature. This involves the conversion of DCD to various intermediate cyclic azines which could lead to the tubular structures. Intermediate cyclic compounds like melamine and melem (2,5,8-triamino-tris-striazine) have low thermal stabilities. This might be responsible for significant weight loss in the region of structural transformations as it involves deamination. The final product of the self-reactions of DCD (melon) has unusual thermal stability up to 700 °C and is unreactive until 700 °C. This could be the deciding factor for selecting the temperature of carbonization. The self-reactions of DCD are disturbed because of the involvement of the carbon matrix and metal atoms. These cyclic compounds bond to the carbon matrix
shown by spatial resolution of the Co 2p3/2 XPS profile. The peaks corresponding to metallic Co, Co3O4, and CoNC are shown at 778.2, 779.7, and 781.0 eV, respectively, while that of shake up satellites appears at 798.2 eV (Figure 3b).53 The deconvolution of the N 1s high-resolution profile shows five peaks corresponding to CNC, CoN, tertiary nitrogen N(C3), amino functionality CNH, and oxidized N centered at 397.8, 398.9, 399.7, 400.5, and 402.7 eV, respectively (Figure 3c). The interaction of metallic cobalt with nitrogen is accredited to the presence of the CoN bond.54 Two characteristics peaks are shown in the ESCA of O 1s for CO at 532.4 eV and for CO at 533.5 eV (Figure 3d). The sample texture and porosity of the CoSAs@CNT catalyst are further analyzed by BET revealing the presence of a high density of pores in the material with a high specific surface area. A clear hysteresis loop of type IV is formed for BET analysis of the material with a specific surface area SBET of 512.8 m2 g−1, which is much higher than previously reported Co-containing MOFs,55,56 while the total pore volume of the material is 1.2 cm3 g−1 with average pore size of 9.5 nm calculated by density functional theory (DFT) method. The presence of DCD may be the reason for controlling the porosity and SSA of the material during pyrolysis (Figure 4a,b). The material is the best suited candidate for extraordinary catalytic performance by virtue of such a high pore density, their accessibility, and well-demarcated structure. Generally the high surface area is directly related to number of active sites. When the surface area is higher, the density of active sites is greater, which means that SSA directly affects the performance of the material.57 Three phases of loss are observed in the temperature profile of the CoSAs@CNT composite (Figure 4c). The presence of DCD has a direct effect on the thermal stability of the material. The mass loss during the very first phase is due to the vaporization of the solvent molecules from inside of the pores. Thermogravimetric analysis (TGA) results demonstrate the thermal stability of ZIF-67@DCD during the solvent evaporation process. When the temperature is increased above 200 °C there is a sudden weight loss indicating the phase decomposition of the ZIF-67@DCD structure and D
DOI: 10.1021/acsaem.8b00490 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials which serves as support for the formation of nanotubes while the growth rate is controlled by self-catalysis of cobalt nanoparticles (Scheme 2). In addition to the formation of Scheme 2. Schematic Illustration of the Growth Mechanism of CNTs
carbon nanotubes, DCD being rich in nitrogen is also responsible for creating stabilized Co single atomic sites. Nitrogen from DCD surrounds Co sites constraining the formation of aggregated nanoparticles. These atomic sites are dispersed well and uniformly over the entire carbon nanotube network as evident from SEM, TEM, and HAADF. Electrochemical Performance Evaluation. All the electrochemical measurements were performed versus reversible hydrogen electrode (RHE); for calibration, see Figure S6. The first intuition of ORR activity was made by performing cyclic voltammetry of the samples in the environment saturated with Ar and O2 gas. No prominent peak is observed in CV under the Ar-saturated environment. The CV is like the symmetrical loop without any oxidation or reduction peak, while a conspicuous cathodic peak at positive onset potential in propinquity to the Pt/C is observed in the CV with saturated oxygen (Figure 5a). Different ratios of Co/DCD (1:5, 1:6, and 1:7), carbonized ZIF-67(Co@NC), and Pt/C were tested under same conditions for comparison (Figure S7). The best results are shown by ratio 1:6, i.e., CoSAs@ CNTs. The performance was further evaluated by linear sweep voltammetry through rotating disk electrode (RDE) in an environment saturated with O2 as shown in Figure 5b. The LSV confirms the extraordinary electrocatalytic performance with onset potential (Eonset) of ∼0.99 V and half-wave potential (E1/2) 0.86 V both more positive than Eonset (0.96 V) and E1/2 (0.82 V) of Pt/C. The limiting current density is also remarkably higher than that of Pt/C. This performance surpasses many of the previously reported ZIF-67-based and carbon-based metal catalysts (Table S1). Such a high value of current density announces that the oxygen reduction proceeds directly through a four-electron process. There are two possible pathways for the ORR in alkaline media. The reduction can proceed through either a two-electron multistep process through the production of H2O2 or direct reduction of O2 to water via a four-electron process. The results imply the direct four-electron reduction process following the elementary steps of the Eley−Rideal mechanism (eqs S1−S6). To underpin the transfer number of electrons per oxygen molecule (n) linear sweep voltammetry was performed at different rotating speeds from 400 to 2500 rpm (Figure 5c), and the transfer number of electrons was calculated by corresponding K−L plots that were obtained at varying potentials (Figure 5d). A linear relationship is observed between J−1 and ω−1/2, and from the slope, n was calculated as 4.3 on average. These
Figure 5. Evaluation of ORR activity CoSAs@CNTs under alkaline medium. (a) Cyclic voltammogram for the oxygen-saturated and Arsaturated environment. (b) Polarization curves measured at 1600 rpm at a scan rate of 10 mV S−1. (c) Polarization curves obtained at different rotation speeds. (d) Corresponding K−L plots. (f) RRDE measurements for determination of average electron transfer number and percentage H2O2 yield. (e) Tafel slopes for the materials for kinetic evaluation of the catalyst.
results announce the complete selectivity of the catalyst for the four-electron oxygen reduction reaction. Rotating ring-disk electrode (RRDE) measurements were performed to figure out the other reaction pathways proceeding through H 2 O2 production. If there is an intermediate production of H2O2 during oxygen reduction taking place at the disk, it can be detected by the current produced at the ring. There was not any production of H2O2 as compared to the reduction reaction products. The electron transfer number was determined to be 4.3 (Figure 5e), which is in agreement with the results calculated through K−L plots. This envisioned that the progress of the ORR to water involves a complete fourelectron pathway. The kinetics of the oxygen reduction reaction was assessed by Tafel analysis of the polarization curves. The analysis of Tafel slopes epitomizes the faster kinetics for CoSAs@CNTs as compared to Co@NC and Pt/C with the value being calculated as 99 mV dec−1 which is much smaller than those of Pt/C (135 mV dec−1) and Co@NC (119 mV dec−1) (Figure 5f). Furthermore, the catalyst simultaneously exhibited far better tolerance to the methanol crossover effect and remarkable long-term stability which are depicted by the accelerated durability test (ADT) and methanol tolerance (MT) tests. For ADT, 3000 continuous cycles were run for the catalyst at an accelerated rate of 100 mV S−1. The catalyst showed a reduction in potential by only 8 mV, but no prominent negative shift was observed in Eonset (Figure 6a), while for methanol tolerance 3 M methanol was added during E
DOI: 10.1021/acsaem.8b00490 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Co@NC (Figure 7b). The general parameter used for the performance evaluation for a bifunctional catalyst is the potential gap (ΔE) between the half-wave potential of the ORR and OER potential at 10 mA cm−2. A similar catalyst loading is used for comparing the potential gap. The value of overvoltage 0.78 V is much smaller as compared to those of other catalysts, i.e., Pt/C and Co@NC (Figure 7c), and other precious metal catalysts such as IrO2 with the value of overvoltage 1.41 V and RuO2 possessing overvoltage of 1.27 V. The results suggest the promising bifunctional electrocatalytic activity as compared to other catalysts (Table S2).
Figure 6. Stability tests. (a) Accelerated durability test for CoSAs@ CNTs at 1600 rpm before and after 5000 continuous CV cycles. (b) Chronoamperometric responses of CoSAs@CNTs and Pt/C for methanol tolerance upon the addition of 3 M methanol.
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DISCUSSION The high catalytic activity is due to the presence of a welldefined geometry of the CNTs, high porosity of the CNTs, nitrogen doping, and uniformly distributed atomically dispersed cobalt atoms. The content of nitrogen is high in DCD which may lead to nitrogen doping of these CNTs. Studies show that nitrogen doping is responsible for the modification of charge distribution in the carbon network.59,60 This charge redistribution, along with the CNT network possessing large pore volume, high specific surface area, and appropriate pore size distribution, caters a synergistic effect for excellent catalytic activities and high stability both for the ORR and OER, outperforming the other previously reported bifunctional catalysts. According to the recent density-functional-theory-based (DFT-based) calculations, nitrogen incorporated in CNTs is not an active site itself, but it lowers the activation energy barriers of the adjacent carbon atoms by charge redistribution of the p-conjugated systems of C-rings in CNTs.61 The chemical environments and geometry of the active sites are responsible for the high efficiency of the catalyst. The d splitting of metal is effected by nitrogen as suggested by some theoretical calculations based on DFT.62 The calculations suggest that metals can effect the overall chemical environment of the active site M−N−C bridge. DCD could be responsible for creating single atomic sites. The calculations shown in Figure 8 indicate that metals can effect the overall chemical environment of the active site M−N−C bridge. It can be seen that the charge density of carbon is highly influenced by the presence of Co. The charge density of carbon in the structure without Co is only slightly positive (Figure 8a) while the charge density of carbon in the Co−N− C bridge is highly positive as compared to than N−C. This indicates an increase in electrophilicity of carbon in the bridge attracting more electrons and assisting the movement of these electrons toward the anode, hence enhancing the reduction reactions. As single atoms are the most reactive species, they need to be stabilized chemically. DCD being rich in nitrogen stabilizes the single Co atoms, preventing the formation of Co nanoparticles as shown in Figure 8b. The presence of a high loading of single atoms is responsible for creating multiple active sites in the catalyst leading to the full utilization of metal centers. Therefore, synergistic coupling of M−N−C and single atomic sites both add to the superior performance of the catalyst. Cobalt SAs allow the maximum utilization of metal atoms along with high efficiency and selectivity. Cobalt moieties bind too weakly to the oxygen molecules with longer bond lengths CoO−O as suggested by DFT-D calculations63 for efficient oxygen reduction. This adsorbed oxygen on single cobalt atoms with a longer bond is easily broken and reduced to water by a direct four-electron pathway.64 In addition to these active sites, accessibility of O2 to these highly active sites
chronoamperometry. A negligible current loss of less than 10% was observed for the CoSAs@CNTs in contrast to the relatively high current loss and instability of approximately 35% (Figure 6b). All results proclaim the auspicious operational stability of the prepared catalyst. The electrochemical performance of CoSAs@CNTs was also verified under acidic conditions by using 0.1 M HClO4. The respective cyclic voltammogram exhibits a cathodic peak indicating the oxygen reduction property which is further evaluated by RDE and RRDE measurements as described above for catalysis in basic conditions (Figure S9). The onset potential current density is not good as compared to Pt/C, but still it shows better stability for ADT, methanol crossover effect, and smaller value of Tafel slope compared to other catalysts, i.e., Pt/C and Co@NC. The OER performance of the bifunctional catalyst was investigated in basic electrolyte. The onset potential of CoSAs@CNTs appears at a potential of 1.53 V versus RHE (overpotential of 300 mV) and generates a current density of 10 mA cm−2 at a potential of 1.64 V versus RHE (overpotential of 410 mV) (Figure 7a), which is extraordinarily less than the benchmark catalyst IrO2, Pt/C, and other non-noble metal electrocatalysts. The value of the Tafel slope 85 mV dec−1 for CoSAs@CNTs is much smaller than those of IrO2, Pt/C, and
Figure 7. Evaluation of the OER activity of CoSAs@CNTs under basic medium. (a) Polarization curves recorded at 1600 rpm. (b) Corresponding Tafel plots. (c) Comparison of bifunctional performance of CoSAs@CNTs with Pt/C, ZIF-67, and Co@NC. (d) Histogram showing comparison of bifunctional activity. F
DOI: 10.1021/acsaem.8b00490 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00490.
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Additional data and figures including a TEM image, XPS survey spectrum, TGA/DT, reversible hydrogen electrode calibration, polarization curves, cyclic voltammograms, catalytic activity in acidic media, comparison of ORR and bifunctional activities, possible pathways for the oxygen reduction reaction in basic and acidic media, and scheme for the mechanism of growth of CNTs by cyclic transformations (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hongyan He: 0000-0003-1291-2771 Guangjin Zhang: 0000-0002-1234-4252 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (91545125 and U1662121), Youth Innovation Promotion Association of CAS, Beijing Municipal Commission Education (KM201710015009), and Cross training plan for high level talents in Beijing colleges and universities. S.D. is thankful to “CAS-TWAS President’s Fellowship for International PhD Students”.
Figure 8. DFT-calculated charge redistribution for CoSAs@CNTs. (a) Charge densities for N−C network. (b) Charge densities for M− N−C network.
is also a vital factor which is achieved by extensive porosity and high surface area allowing a faster mass transport. It has been reported that the catalytic performance of the material with porosity is much higher than the catalytic performance of the material without any porosity.65 The results of electrochemical performance are in complete agreement with these findings. Therefore, we propose that atomic sites and geometry of structures are crucial for efficient bifunctional catalysis.
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REFERENCES
(1) Pegis, M. L.; Roberts, J. A. S.; Wasylenko, D. J.; Mader, E. A.; Appel, A. M.; Mayer, J. M. Standard Reduction Potentials for Oxygen and Carbon Dioxide Couples in Acetonitrile and N,N-Dimethylformamide. Inorg. Chem. 2015, 54, 11883−11888. (2) Wang, H. F.; Tang, C.; Zhang, Q. Template Growth of Nitrogen-Doped Mesoporous Graphene on Metal Oxides and its use as a Metal-Free Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions. Catal. Today 2018, 301, 25−31. (3) Huang, Z. F.; Wang, J.; Peng, Y.; Jung, C. Y.; Fisher, A.; Wang, X. Design of Efficient Bifunctional Oxygen Reduction/Evolution Electrocatalyst: Recent Advances and Perspectives. Adv. Energy Mater. 2017, 7, 1−21. (4) Liu, Y.; Li, Y.; Kang, H.; Jin, T.; Jiao, L. Design, Synthesis, and Energy-Related Applications of Metal Sulfides. Mater. Horiz. 2016, 3, 402−421. (5) Lin, S.; Diercks, C. S.; Zhang, Y.-B.; Kornienko, N.; Nichols, E. M.; Zhao, Y.; Paris, A. R.; Kim, D.; Yang, P.; Yaghi, O. M.; Chang, C. J. Covalent Organic Frameworks Comprising Cobalt Porphyrins for Catalytic CO2 Reduction in Water. Science 2015, 349, 1208−1213. (6) Li, J.-S.; Wang, Y.; Liu, C.-H.; Li, S.-L.; Wang, Y.-G.; Dong, L.Z.; Dai, Z.-H.; Li, Y.-F.; Lan, Y.-Q. Coupled Molybdenum Carbide and Reduced Graphene Oxide Electrocatalysts for Efficient Hydrogen Evolution. Nat. Commun. 2016, 7, 11204−11211. (7) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446−6473.
CONCLUSION
In summary we have developed ZIF-67-derived new structures with excellent morphology. By a very simple and unique treatment with ZIF-67, we have successfully engineered metal atoms decorated highly organized nanotube arrays upon pyrolysis. To the best of our knowledge the strategy has not been reported previously. The new developed catalyst exhibits extraordinary ORR as well as decent OER performance. The catalyst also possesses remarkable long-term stability to harsh acidic and basic environments. The atomic dispersion of Co atoms is responsible for increasing the number of active sites available for catalysis in addition to the M−N−C active centers. The effective strategy was focused to increase the catalytic sites with well-defined morphology which is achieved successfully. This can open new floodgates to explore such competitive electrocatalysts for the ORR, OER, and other catalyst systems. G
DOI: 10.1021/acsaem.8b00490 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Energy Materials (8) Zhuang, G.-L.; Gao, Y.-F.; Zhou, X.; Tao, X.-Y.; Luo, J.-M.; Gao, Y.-J.; Yan, Y.-L.; Gao, P.-Y.; Zhong, X.; Wang, J.-G. ZIF-67/COFDerived Highly Dispersed Co3O4/N-Doped Porous Carbon with Excellent Performance for Oxygen Evolution Reaction and Li-ion Batteries. Chem. Eng. J. 2017, 330, 1255−1264. (9) Zhang, S.; Li, B. Heteroatom Doped Graphdiyne as Efficient Metal-Free Electrocatalyst for Oxygen Reduction Reaction in Alkaline Medium. J. Mater. Chem. A 2016, 4, 4738−4744. (10) Zhou, K.; Mousavi, B.; Luo, Z.; Phatanasri, S.; Chaemchuen, S.; Verpoort, F. Characterization and Properties of Zn/Co Zeolitic Imidazolate Frameworks vs. ZIF-8 and ZIF-67. J. Mater. Chem. A 2017, 5, 952−957. (11) Feng, Y.; Alonso-Vante, N. Nonprecious Metal Catalysts for the Molecular Oxygen-Reduction Reaction. Phys. Status Solidi B 2008, 245, 1792−1806. (12) Liang, Y.; Messinger, J. Improving BiVO4 Photoanodes for Solar Water Splitting Through Surface Passivation. Phys. Chem. Chem. Phys. 2014, 16, 12014−12020. (13) Yuan, H.; Hou, Y.; Abu-Reesh, I. M.; Chen, J.; He, Z. Oxygen Reduction Reaction Catalysts used in Microbial Fuel Cells for EnergyEfficient Wastewater Treatment: a Review. Mater. Horiz. 2016, 3, 382−401. (14) Gao, M.-R.; Xu, Y.-F.; Jiang, J.; Yu, S.-H. Nanostructured Metal Chalcogenides: Synthesis, Modification, and Applications in Energy Conversion and Storage Devices. Chem. Soc. Rev. 2013, 42, 2986− 3017. (15) Tang, J.; Yang, M.; Dong, W.; Yang, M.; Zhang, H.; Fan, S.; Wang, J.; Tan, L.; Wang, G. Highly Porous Carbons Derived from MOFs for Shape-Stabilized Phase Change Materials with High Storage Capacity and Thermal Conductivity. RSC Adv. 2016, 6, 40106−40114. (16) Zheng, Y.; Jiao, Y.; Chen, J.; Liu, J.; Liang, J.; Du, A.; Zhang, W.; Zhu, Z.; Smith, S. C.; Jaroniec, M.; Lu, G. Q.; Qiao, S. Z. Nanoporous Graphitic-C3N4@Carbon Metal-Free Electrocatalysts for Highly Efficient Oxygen Reduction. J. Am. Chem. Soc. 2011, 133, 20116−20119. (17) Alexeyeva, N.; Shulga, E.; Kisand, V.; Kink, I.; Tammeveski, K. Electroreduction of Oxygen on Nitrogen-Doped Carbon Nanotube Modified Glassy Carbon Electrodes in Acid and Alkaline Solutions. J. Electroanal. Chem. 2010, 648, 169−175. (18) Liu, Y.; Han, G.; Zhang, X.; Xing, C.; Du, C.; Cao, H.; Li, B. Co-Co3O4@Carbon Core−Shells Derived From Metal−Organic Framework Nanocrystals as Efficient Hydrogen Evolution Catalysts. Nano Res. 2017, 10, 3035−3048. (19) Lu, W.; Wei, Z.; Gu, Z.-Y.; Liu, T.-F.; Park, J.; Park, J.; Tian, J.; Zhang, M.; Zhang, Q.; Gentle, T., III; Bosch, M.; Zhou, H.-C. Tuning the Structure and Function of Metal-Organic Frameworks via Linker Design. Chem. Soc. Rev. 2014, 43, 5561−5593. (20) Qiu, B.; Guo, W.; Liang, Z.; Xia, W.; Gao, S.; Wang, Q.; Yu, X.; Zhao, R.; Zou, R. Fabrication of Co3O4 Nanoparticles in Thin Porous Carbon Shells from Metal−Organic Frameworks for Enhanced Electrochemical Performance. RSC Adv. 2017, 7, 13340−13346. (21) Li, W.; Liu, J.; Zhao, D. Mesoporous Materials for Energy Conversion and Storage Devices. Nat. Rev. Mater. 2016, 1, 1−17. (22) Liang, J.; Hassan, M.; Zhu, D.; Guo, L.; Bo, X. Cobalt Nanoparticles/Nitrogen-Doped Graphene with High Nitrogen Doping Efficiency as Noble Metal-Free Electrocatalysts for Oxygen Reduction Reaction. J. Colloid Interface Sci. 2017, 490, 576−586. (23) Yu, X.; Wang, D.; Liu, J.; Luo, Z.; Du, R.; Liu, L.-M.; Zhang, G.; Zhang, Y.; Cabot, A. Cu2ZnSnS4 Nanocrystals as Highly Active and Stable Electrocatalysts for the Oxygen Reduction Reaction. J. Phys. Chem. C 2016, 120, 24265−24270. (24) Zheng, Y.; Jiao, Y.; Li, L. H.; Xing, T.; Chen, Y.; Jaroniec, M.; Qiao, S. Z. Toward Design of Synergistically Active Carbon-Based Catalysts for Electrocatalytic Hydrogen Evolution. ACS Nano 2014, 8, 5290−5296. (25) Lori, O.; Elbaz, L. Advances in Ceramic Supports for Polymer Electrolyte Fuel Cells. Catalysts 2015, 5, 1445−1464.
(26) Zhang, W.; Lu, X. Morphology Control of Bimetallic Nanostructures for Electrochemical Catalysts. Nanotechnol. Rev. 2013, 2, 487−514. (27) Zhang, S.; Oms, O.; Hao, L.; Liu, R.; Wang, M.; Zhang, Y.; He, H. Y.; Dolbecq, A.; Marrot, J.; Keita, B.; Zhi, L.; Mialane, P.; Li, B.; Zhang, G. High Oxygen Reduction Reaction Performances of Cathode Materials Combining Polyoxometalates, Coordination Complexes, and Carboneous Supports. ACS Appl. Mater. Interfaces 2017, 9, 38486−38498. (28) Wang, Z.; Laddha, G.; Kanitkar, S.; Spivey, J. J. Metal Organic Framework-Mediated Synthesis of Potassium-Promoted Cobalt-Based Catalysts for Higher Oxygenates Synthesis. Catal. Today 2017, 298, 209. (29) Wang, C.; Wang, H.; Luo, R.; Liu, C.; Li, J.; Sun, X.; Shen, J.; Han, W.; Wang, L. Metal-Organic Framework One-Dimensional Fibers as Efficient Catalysts for Activating Peroxymonosulfate. Chem. Eng. J. 2017, 330, 262−271. (30) Zhang, G. R.; Wö llner, S. Hollowed Structured PtNi Bifunctional Electrocatalyst with Record Low Total Overpotential for Oxygen Reduction and Oxygen Evolution Reactions. Appl. Catal., B 2018, 222, 26−34. (31) Yu, D.; Ge, L.; Wu, B.; Wu, L.; Wang, H.; Xu, T. Precisely Tailoring ZIF-67 Nanostructures From Cobalt Carbonate Hydroxide Nanowire Arrays: Toward High-Performance Battery-Type Electrodes. J. Mater. Chem. A 2015, 3, 16688−16694. (32) Zhong, L.; Frandsen, C.; Mørup, S.; Hu, Y.; Pan, C.; Cleemann, L. N.; Jensen, J. O.; Li, Q. 57Fe-MÖ ssbauer Spectroscopy and Electrochemical Activities of Graphitic Layer Encapsulated Iron Electrocatalysts for the Oxygen Reduction Reaction. Appl. Catal., B 2018, 221, 406−412. (33) Guo, M.; Gao, T.; Ma, H.; Li, H. Weaving ZIF-67 by Employing Carbon Nanotubes to Constitute Hybrid Anode for Lithium Ions Battery. Mater. Lett. 2018, 212, 143−146. (34) Wang, Q.; Gao, F.; Xu, B.; Cai, F.; Zhan, F.; Gao, F.; Wang, Q. ZIF-67 Derived Amorphous CoNi2S4 Nanocages with Nanosheet Arrays on the Shell for a High-Performance Asymmetric Supercapacitor. Chem. Eng. J. 2017, 327, 387−396. (35) Wang, B.; Tan, W.; Fu, R.; Mao, H.; Kong, Y.; Qin, Y.; Tao, Y. Hierarchical Mesoporous Co3O4/C@MoS2 Core−Shell Structured Materials for Electrochemical Energy Storage with High Supercapacitive Performance. Synth. Met. 2017, 233, 101−110. (36) Guo, S.; Wang, E. Noble Metal Nanomaterials: Controllable Synthesis and Application in Fuel Cells and Analytical Sensors. Nano Today 2011, 6, 240−264. (37) Wang, F.; Shifa, T. A.; Zhan, X.; Huang, Y.; Liu, K.; Cheng, Z.; Jiang, C.; He, J. Recent Advances in Transition-Metal Dichalcogenide Based Nanomaterials for Water Splitting. Nanoscale 2015, 7, 19764− 19788. (38) Wang, J.; Cui, W.; Liu, Q.; Xing, Z.; Asiri, A. M.; Sun, X. Recent Progress in Cobalt-Based Heterogeneous Catalysts for Electrochemical Water Splitting. Adv. Mater. 2016, 28, 215−230. (39) Ruditskiy, A.; Choi, S.-I.; Peng, H.-C.; Xia, Y. Shape-Controlled Metal Nanocrystals for Catalytic Applications. MRS Bull. 2014, 39, 727−737. (40) Amarante, S. F.; Freire, M. A.; Mendes, D. T. S. L.; Freitas, L. S.; Ramos, A. L. D. ZIFs Metal Organic Frameworks in the Knoevenagel Condensation Reaction. Appl. Catal., A 2017, 548, 47−51. (41) Cho, D. W.; Jeong, K. H.; Kim, S.; Tsang, D. C. W.; Ok, Y. S.; Song, H. Synthesis of Cobalt-Impregnated Carbon Composite Derived from a Renewable Resource: Characterization and Catalytic Performance Evaluation. Sci. Total Environ. 2018, 612, 103−110. (42) Osgood, H.; Devaguptapu, S. V.; Xu, H.; Cho, J.; Wu, G. Transition Metal (Fe, Co, Ni, and Mn) Oxides for Oxygen Reduction and Evolution Bifunctional Catalysts in Alkaline Media. Nano Today 2016, 11, 601−625. (43) Singh, K.; Razmjooei, F.; Yu, J.-S. Active Sites and Factors Influencing them for Efficient Oxygen Reduction Reaction in Metal-N H
DOI: 10.1021/acsaem.8b00490 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Energy Materials Coordinated Pyrolyzed and Non-Pyrolyzed Catalysts: a Review. J. Mater. Chem. A 2017, 5, 20095−20119. (44) Wang, L.; Feng, X.; Ren, L.; Piao, Q.; Zhong, J.; Wang, Y.; Li, H.; Chen, Y.; Wang, B. Flexible Solid-State Supercapacitor Based on a Metal-Organic Framework Interwoven by Electrochemically-Deposited PANI. J. Am. Chem. Soc. 2015, 137, 4920−4923. (45) Xu, X.; Tang, J.; Qian, H.; Hou, S.; Bando, Y.; Hossain, M. S. A.; Pan, L.; Yamauchi, Y. Three-Dimensional Networked MetalOrganic Frameworks with Conductive Polypyrrole Tubes for Flexible Supercapacitors. ACS Appl. Mater. Interfaces 2017, 9, 38737−38744. (46) Zhu, C.; Fu, S.; Shi, Q.; Du, D.; Lin, Y. Single-Atom Electrocatalysts. Angew. Chem., Int. Ed. 2017, 56, 13944−13960. (47) Yin, P.; Yao, T.; Wu, Y.; Zheng, L.; Lin, Y.; Liu, W.; Ju, H.; Zhu, J.; Hong, X.; Deng, Z.; Zhou, G.; Wei, S.; Li, Y. Single Cobalt Atoms with Precise N-Coordination as Superior Oxygen Reduction Reaction Catalysts. Angew. Chem., Int. Ed. 2016, 55, 10800−10805. (48) Tang, C.; Wang, B.; Wang, H. F.; Zhang, Q. Defect Engineering toward Atomic Co−N x − C in Hierarchical Graphene for Rechargeable Flexible Solid Zn-Air Batteries. Adv. Mater. 2017, 29, 1−7. (49) Wan, G.; Yu, P.; Chen, H.; Wen, J.; Sun, C.; Zhou, H.; Zhang, N.; Li, Q.; Zhao, W.; Xie, B.; Li, T.; Shi, J. Engineering Single-Atom Cobalt Catalysts toward Improved Electrocatalysis. Small 2018, 14, 1704319-1−1704319-7. (50) Xiong, C.; Zhang, T.; Kong, W.; Zhang, Z.; Qu, H.; Chen, W.; Wang, Y.; Luo, L.; Zheng, L. ZIF-67 Derived Porous Co3O4 Hollow Nanopolyhedron Functionalized Solution-Gated Graphene Transistors for Simultaneous Detection of Glucose and Uric Acid in Tears. Biosens. Bioelectron. 2018, 101, 21−28. (51) Pal, J.; Pal, T. Faceted Metal and Metal Oxide Nanoparticles: Design, Fabrication and Catalysis. Nanoscale 2015, 7, 14159−14190. (52) Wen, Z.; Ci, S.; Zhang, F.; Feng, X.; Cui, S.; Mao, S.; Luo, S.; He, Z.; Chen, J. Nitrogen-Enriched Core-Shell Structured Fe/Fe3C-C Nanorods as Advanced Electrocatalysts for Oxygen Reduction Reaction. Adv. Mater. 2012, 24, 1399−1404. (53) Sui, Z.-Y.; Zhang, P.-Y.; Xu, M.-Y.; Liu, Y.; Wei, Z.; Han, B.-H. Metal−Organic Framework-Derived Metal Oxide Embedded in Nitrogen-Doped Graphene Network for High-Performance LithiumIon Batteries. ACS Appl. Mater. Interfaces 2017, 9, 43171−43178. (54) Li, H.; Gan, S.; Wang, H.; Han, D.; Niu, L. Intercorrelated Superhybrid of AgBr Supported on Graphitic-C3N4-Decorated Nitrogen-Doped Graphene: High Engineering Photocatalytic Activities for Water Purification and CO2 Reduction. Adv. Mater. 2015, 27, 6906−6913. (55) Wang, Y.; Laborda, E.; Ward, K. R.; Tschulik, K.; Compton, R. G. A Kinetic Study of Oxygen Reduction Reaction and Characterization on Electrodeposited Gold Nanoparticles of Diameter between 17 and 40 nm in 0.5 M Sulfuric Acid. Nanoscale 2013, 5, 9699−9708. (56) Yao, J.; He, M.; Wang, K.; Chen, R.; Zhong, Z.; Wang, H. High-Yield Synthesis of Zeolitic Imidazolate Frameworks from Stoichiometric Metal and Ligand Precursor Aqueous Solutions at Room Temperature. CrystEngComm 2013, 15, 3601−3606. (57) Mo, Q.; Chen, N.; Deng, M.; Yang, L.; Gao, Q. Metallic Cobalt@Nitrogen-Doped Carbon Nanocomposites: Carbon-Shell Regulation toward Efficient Bi-Functional Electrocatalysis. ACS Appl. Mater. Interfaces 2017, 9, 37721−37730. (58) Zhang, J.; Zhao, Z.; Xia, Z.; Dai, L. A Metal-Free Bifunctional Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions. Nat. Nanotechnol. 2015, 10, 444−452. (59) Tang, C.; Wang, H.-F.; Zhang, Q. Multiscale Principles To Boost Reactivity in Gas-Involving Energy Electrocatalysis. Acc. Chem. Res. 2018, 51, 881−889. (60) Wang, X. X.; Cullen, D. A.; Pan, Y.-T.; Hwang, S.; Wang, M.; Feng, Z.; Wang, J.; Engelhard, M. H.; Zhang, H.; He, Y.; Shao, Y.; Su, D.; More, K. L.; Spendelow, J. S.; Wu, G. Nitrogen-Coordinated Single Cobalt Atom Catalysts for Oxygen Reduction in Proton Exchange Membrane Fuel Cell. Adv. Mater. 2018, 30, 1706758-1− 1706758-11.
(61) Yang, H. B.; Miao, J.; Hung, S.-F.; Chen, J.; Tao, H. B.; Wang, X.; Zhang, L.; Chen, R.; Gao, J.; Chen, H. M.; Dai, L.; Liu, B. Identification of Catalytic Sites for Oxygen Reduction and Oxygen Evolution in N-Doped Graphene Materials: Development of Highly Efficient Metal-Free Bifunctional Electrocatalyst. 2016. Sci. Adv. 2016, 2, 1−11. (62) Calle-Vallejo, F.; Martínez, J. I.; Rossmeisl, J. Density Functional Studies of Functionalized Graphitic Materials with Late Transition Metals for Oxygen Reduction Reactions. Phys. Chem. Chem. Phys. 2011, 13, 15639−15643. (63) Zitolo, A.; Ranjbar-Sahraie, N.; Mineva, T.; Li, J.; Jia, Q.; Stamatin, S.; Harrington, G. F.; Lyth, S. M.; Krtil, P.; Mukerjee, S.; Fonda, E.; Jaouen, F. Identification of Catalytic Sites in CobaltNitrogen-Carbon Materials for the Oxygen Reduction Reaction. Nat. Commun. 2017, 8, 1−10. (64) Liu, J.; Jiao, M.; Lu, L.; Barkholtz, H. M.; Li, Y.; Jiang, L.; Wu, Z.; Liu, D. J.; Zhuang, L.; Ma, C.; Zeng, J.; Zhang, B.; Su, D.; Song, P.; Xing, W.; Xu, W.; Wang, Y.; Jiang, Z.; Sun, G. High Performance Platinum Single Atom Electrocatalyst for Oxygen Reduction Reaction. Nat. Commun. 2017, 8, 1−9. (65) Volosskiy, B.; Fei, H.; Zhao, Z.; Lee, S.; Li, M.; Lin, Z.; Papandrea, B.; Wang, C.; Huang, Y.; Duan, X. Tuning the Catalytic Activity of a Metal-Organic Framework Derived Copper and Nitrogen Co-Doped Carbon Composite for Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2016, 8, 26769−26774.
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DOI: 10.1021/acsaem.8b00490 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
