Abstract
A cobalt/nitrogen/carbon (Co/N/C) electrocatalyst has been successfully synthesized by the solvothermal method using cobalt acetate (Co(Ac)2), NH3·H2O and carbon quantum dots (CQDs) as the starting materials. The oxygen reduction reaction (ORR) activity of Co/N/C is better than single nitrogen-doped CQDs (NC) or cobalt-doped CQDs in terms of the onset potential and peak potential. The Co/N/C catalyst also exhibits better methanol durability and stability compared to the commercial Pt/C (20%). The electron-transferred number of Co/N/C for ORR is calculated to be about 3.51, indicating a four-electron pathway. Such a novel and high-efficient codoped carbon-based catalyst could be a promising substitute for commercial Pt/C in fuel cells as well as other energy storage and conversion systems.
1 Introduction
There is a growing interest in developing renewable energy including fuel cells and metal-air batteries. A catalyst for the oxygen reduction reaction (ORR) on the cathode is the key technology in energy storages and conversion systems [1–3]. Despite tremendous efforts, developing catalysts with high activity at low cost remains a great challenge. The main bottleneck resides in the sluggish kinetics, limited reserves and declined activities of the catalysts for ORR on the cathode side [4, 5]. Platinum or its alloys have been identified as the most effective catalysts for ORR; however, the high cost and poor durability of them hinder the large-scale commercialization of platinum-based catalysts [6–8]. To date, numerous studies have focused on the alternative catalysts based on doping materials, such as Co- or Fe-based catalysts, and metal-free materials, such as carbon-based materials (carbon nanotubes [CNTs], graphene, mesoporous graphitic arrays, mesoporous carbon) [9–14]. For example, Liang et al. [3] have investigated the catalyst properties of Co, N-codoped reduced graphene and proposed that the synergistic catalyst of Co and N exhibited high activity for ORR. Yang et al. [15] studied the effect of Co, N-codoped carbon material by pyrolyzing the cobalt-based macrocyclic compounds (VB12) and carbon quantum dots (CQDs). Although these Co, N-codoped materials showed good catalytic activity for ORR, there are still underlying problems to be dealt with urgently, such as the catalyst mechanism of Co and N on carbon material and construction of porous structures, which could greatly affect the catalytic activity.
CQDs are recently emerging carbon nanomaterials with a size <10 nm and have shown wide application advantages in bioimaging, drug delivery and catalysis due to their benign, biocompatible, accessible and distinctive catalytic properties [16, 17]. CQDs could be regarded as the desirable building blocks due to their tiny size and functionalized surface [18].
In this study, the Co/N/C catalyst was fabricated by a one-step process using Co(Ac)2, NH3·H2O and CQDs as the starting materials. The results show that Co3O4 nanocrystals, the material with little ORR activity by itself, when grown with CQDs and NH3·H2O exhibit high activity for ORR in alkaline solutions, indicating that more active sites were produced due to the synergetic effect of Co and N in the CQDs framework. According to the rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) results, the electron-transferred number of Co/N/C for the ORR is calculated to be about 3.51, indicating a four-electron pathway. Moreover, the Co/N/C catalyst also shows better methanol durability and stability compared to commercial Pt/C (20%).
2 Materials and methods
Cobalt acetate (Co(AC)2·6H2O) and ammonia (NH3·H2O, 30 mass%) were purchased from Beijing Chemical Reagent Co. Ltd (Beijing, China). Ultra-pure graphite rodes were purchased from Sigma-Aldrich (USA). All the chemical reagents used in this work were used as obtained without further purification.
2.1 Synthesis of N/C, Co/C, Co/N/C hybrids and free Co3O4 nanoparticles
The CQDs were synthesized directly from graphite rods by a facile electrochemical approach reported by our group previously [19]. CQDs were collected from the aqueous solution by high speed centrifugation (60,000 rpm) and then heated at 60°C to remove the residual water. The dried CQDs were dispersed in anhydrous ethanol, and the concentration of the final CQDs ethanol solution was about 0.3 mg/ml. To prepare Co/N/C hybrid with the addition of NH3·H2O, the first step reaction mixture was prepared by adding 1 ml of 0.2 m Co(Ac)2 aqueous solution to 20 ml of the above CQDs ethanol solution, followed by the addition of 0.5 ml of NH3·H2O (30% solution) and 0.5 ml of water at room temperature. After continuous stirring for 8 h, the reaction mixture was transferred into two 40 ml autoclaves for solvothermal reactions at 150°C for 8 h. The resulting product was collected by centrifugation and washed with ethanol and water. The same steps were applied for the preparation of N/C, Co/C and free Co3O4 nanopowders without the addition of Co(Ac)2, NH3·H2O, or CQDs to the reaction mixture. The schematic illustration of the preparation of Co/N/C catalysts is shown in Figure 1.
Schematic illustration of the preparation of Co/N/C catalysts and their catalytic ability for oxygen reduction reaction (ORR).
2.2 Morphology characterizations
Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained using an FEI Tecnai G2 F20 at an operation voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was carried out using a KRATOS Axis ultra-DLD X-ray photoelectron spectrometer with a monochromatized Mg Kα X-ray (hν=1283.3 eV).
2.3 Electrochemical measurement
The electrochemical characterization for ORR, including cyclic voltammetry (CV) and linear sweep voltammograms (LSV) measurements, was performed in a standard three-electrode electrochemical cell, which was connected to a computer-controlled CHI 760E workstation (CH Instruments, Shanghai, China) equipped with a rotating disk and ring-disk electrode apparatus (RRDE-3A, ALS, Co. Ltd., Japan). A platinum wire and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. In the RDE measurements, the working electrode was prepared by loading 6 μl of catalyst suspension (~4 mg/ml) on a glassy carbon electrode of 3 mm in diameter, followed by coating with 5 μl of Nafion solution (0.5 wt%) and dried at room temperature. The CV measurements were performed in Ar- and O2-saturated 0.1 m KOH solutions with a scanning rate of 50 mV/s. The RDE measurements were conducted in the O2-saturated 0.1 m KOH solution at rotation speeds ranging from 600 rpm to 2000 rpm with a scanning rate of 5 mV/s. The RRDE measurements were performed at a rotation speed of 1600 rpm with a scanning rate of 5 mV/s and the ring potential was constant at 0.5 V. It should be noted that all the measurements were performed at room temperature.
3 Results and discussion
In this study, nitrogen-doped CQDs (N/C), cobalt-doped CQDs (Co/C), and nitrogen and cobalt-codoped CQDs (Co/N/C) were synthesized to investigate the influence of heteroatom doping on carbon materials toward ORR. A TEM image of CQDs synthesized in the study is shown in Figure 2A, indicating a uniform distribution of CQDs with diameters in the range of 3–6 nm. Figure 2B shows the HRTEM image of CQDs. The inset in Figure 1B displays the lattice space of 0.321 nm, which is in good agreement with the lattice space of (002) plane of graphitic carbon [20].
(A) and (B) transmission electron microscopy (TEM) images of received carbon quantum dots (CQDs). The inset in (B) is the high-resolution TEM (HRTEM) image of CQDs.
For the doping system, the catalyst mechanism of the doping atoms of Co and N in the Co/N/C system is the key to investigate. To investigate the catalytic properties of the synthesized catalysts for ORR, CV experiments were performed in Ar and O2-saturated 0.1 m KOH solution at a scan rate of 50 mV/s. All potential values given here are in reference to the SCE. Figure 3A shows the CV curves of synthesized catalysts in O2or Ar-saturated 0.1 m KOH solution, in which all catalysts exhibit ORR peaks in O2-saturated solutions in contrast to the featureless peaks observed within the same potential range in Ar-saturated solutions (compare the dot lines and solid lines in Figure 3A), confirming the electrocatalytic activity of the synthesized catalysts for ORR in alkaline medium. It is noted that the nitrogen and cobalt-codoped CQDs (Co/N/C) show a more positive ORR peak potential (-0.168 V) than the potentials of free Co3O4, CQDs, N/C and Co/C, which are -0.41, -0.35, -0.34 and -0.32 V, respectively (the solid line), indicating more active sites produced owing to the ternary synergistic effect. The properties of the commercial Pt/C (20%) were evaluated and compared with those of the synthesized catalysts. Figure 3B compares the LSVs of Pt/C (20%), free Co3O4, Co/C, N/C and Co/N/C in O2-saturated 0.1 m KOH solution. The onset potential of Co/N/C in 0.1 m KOH is -0.08 V, which is comparable to that of Pt/C (20%) and more positive than those of free Co3O4, CQDs, N/C and Co/C.
(A) Cyclic voltammetry (CV) curves of free Co3O4, carbon quantum dots (CQDs), Co/C, N/C and Co/N/C in (solid line) O2-saturated or (dot line) Ar-saturated 0.1 m KOH solutions with a scan rate of 50 mV/s. (B) Linear sweep voltammograms (LSVs) of free Co3O4, CQDs, Co/C, N/C, Co/N/C and commercial Pt/C in an O2-saturated 0.1 m KOH solution at a rotation rate of 1600 rpm and a scanning rate of 5 mV/s.
As known, the ORR is a complex process involving two main possible reaction pathways: one direct reaction with four electrons transferred to produce H2O and the other reaction involving two electrons transferred to produce hydrogen peroxide (an intermediate that can be further reduced to H2O) [21]. The four-electron pathway is preferred to the two-electron pathway for the ORR because the intermediate H2O2 produced in the two-electron pathway may erode the membrane and electrocatalyst which frustrates the efficiency of fuel cells. To obtain an insight into the ORR process of the Co/N/C catalyst, RDEs were employed to evaluate the electron-transferred numbers and the rotating speeds ranged from 600 rpm to 2000 rpm at a scan rate of 5 mV/s in O2-saturated 0.1 m KOH solution (Figure 4A). As shown in Figure 4A, the oxygen reduction current density increased with increasing rotation rate. The transferred electron numbers per oxygen molecule could be calculated by the Koutechy-Levich equation:
(A) Rotating disk electrode (RDE) voltammograms of the Co/N/C in an O2-saturated 0.1 m KOH solution at 5 mV/s and various rotation speeds. The inset in (A) is the Koutecky-Levich plots of J-1 vs. ω-1/2 of the Co/N/C at potentials of -0.7 V, -0.8 V and -0.9 V. (B) Rotating ring-disk electrode (RRDE) linear sweep voltammogram (LSV) curves for Co/N/C with a rotation rate of 1600 rpm in an O2- saturated 0.1 m KOH solution at 5 mV/s; the upper part and the lower part are the ring current (Ir) and the disk current (Id), respectively. (C) Electron-transferred numbers (n) and (D) H2O2 yields in O2-saturated 0.1 m KOH solution based on the RRDE data.
in which j is the measured current density, ω is the rotation speed expressed with rad/s, and B is Levich slope given by:
where n is the electron-transferred numbers for ORR, F is the Faraday constant (96,485 C/mol), D is the diffusion coefficient of O2 in 0.1 m KOH solution, ν is the kinematic viscosity of 0.1 m KOH and C is the saturated concentration of O2. The values for D, ν and C in 0.1 m KOH solution are 1.9×10-5 cm2/s, 0.01 cm2/s and 1.2×10-6 mol/cm3, respectively [22]. The inset in Figure 4A shows the K-L plots obtained at different potentials of -0.7 V, -0.8 V and -0.9 V in O2-saturated 0.1 m KOH solution, in which the K-L plots displays good linearity, indicating the first-order kinetic reaction related to the dissolved O2. Based on the slopes of K-L plots in the inset of Figure 4A, the electron-transferred number of the Co/N/C catalyst is calculated to be about 3.51 at the potential range from -0.7 V to -0.9 V, indicating an effective four-electron process of ORR. In order to fully characterize the ORR efficiency of the Co/N/C catalyst, RRDE measurement was performed to calculate the electron-transferred number (n), a significant kinetic parameter to characterize the ORR efficiency of the Co/N/C catalyst (Figure 4B). The electron-transferred number (n) and percentage of H2O2 can be calculated according to the following equations:
where Id is the disk current, Ir is the ring current and N is the current collection efficiency (N=0.42). The electron-transferred number (n) of the Co/N/C catalyst is calculated to be about 3.49 at the potential range from -0.3 V to -0.9 V, almost in agreement with the result estimated from the slopes of the Koutecky-Levich plots (n=3.51), confirming a nearly four electron pathway for ORR (Figure 4C). Figure 4D shows the corresponding H2O2 yield, in which the percentage of H2O2 is about 26%. All the results obtained from RDE and RRDE measurements confirm that the ORR process catalyzed in 0.1 m KOH solution by Co/N/C is dominated by a 4e- process.
Figure 5A and B present typical TEM images of Co/N/C with a porous structure and homogeneous distribution with diameters of 5–8 nm. The inset in Figure 5B displays the diffraction rings of crystalline Co3O4 marked in the red dash rectangle in Figure 5B, confirming that the Co/N/C catalyst consists of crystalline Co3O4. A magnified TEM image of Co/N/C also shows the presence of the crystalline CQDs, which is identified according to the calculated lattice spacing of 0.321 nm shown in the inset of Figure 5C. The XPS was employed to characterize the chemical state of C, N and Co of the Co/N/C catalyst. The full XPS scan of Co/N/C is shown in Figure 5D, which displays the existence of C, O as well as limited N and Co without other impurities. The high-resolution XPS spectrums mainly exhibit the C1s, N1s and Co2p peaks of the synthesized Co/N/C catalyst (Figure 5E, F and G). The XPS spectrum of C1s can be fitted with three different peaks located at 284.6 eV, 285.8 eV and 288.5 eV, corresponding to C-C, C-O and C-N, respectively (Figure 5E) [23]. The existence of C-O bonding stemmed from the Co(Ac)2 precursor, and the C-N bonding was regarded as originating from C, which was bonded with three N neighbors. The N1s spectrum presented in Figure 5F is also fitted with three different peaks with binding energies of 401 eV, 399.8 eV and 398.8 eV, corresponding to N-N, N-(C)3 and C-N-C, respectively [9, 24–27]. Apart from the C1s and N1s spectrum, the high resolution XPS spectrum of Co2p is also displayed in Figure 5F, in which three peaks are fitted with bonding energies of 781.3 eV, 788.2 eV and 797.5 eV. These peaks are assigned to Co2p3/2, Co2p3/2 and Co2p1/2, corresponding to the doped Co coordinated with N (781.3 eV) [28] and undoped oxidized Co (Co3O4) (788.2 and 797.6 eV), respectively. The high resolution XPS of Co2p shows that a portion of Co was doped into the Co/N/C catalyst and the residual Co was in the form of Co3O4 nanoparticles, in agreement with the inset shown in Figure 5B. It can also be found in Figure 3A that the Co3O4 alone and N/C show weak catalytic activity toward ORR, indicating that the improvement in catalytic activity of Co/N/C is attributed to the synergetic effect of doped Co, N and CQDs in the Co/N/C catalyst.
(A), (B) and (C) transmission electron microscopy (TEM) images of the as-synthesized Co/N/C. The inset in (B) is the diffraction rings of Co3O4 marked in the red dash rectangle in (B), and the inset in (C) is the high-resolution TEM (HRTEM) image of crystalline C marked in the blue circle in (C). (D) The full X-ray photoelectron spectroscopy (XPS) scan of Co/N/C. (E) High resolution of C1s, (F) N1s and (G) Co2p spectra of Co/N/C.
To evaluate the durability of synthesized catalysts, chronoamperometric measurements for Co/N/C were performed in O2-saturated 0.1 m KOH solution compared with commercial Pt/C catalyst. As shown in Figure 6A, the continuous ORR causes tardy attenuation (9.1%) of current for the Co/N/C catalyst over 10 h in 0.1 m KOH solution, but the commercial Pt/C shows significant decline of stability within the same time, which merely kept 62.2% of the original current. The results demonstrate that Co/N/C processes better stability compared with commercial Pt/C toward ORR in an alkaline solution. In order to investigate the crossover effect, the catalysts of Co/N/C and Pt/C against the electrooxidation of methanol in O2-saturated 0.1 m KOH solution by dropping 5 ml of methanol (0.5 mol/l) into the solution at 2000 s was carried out. As shown in Figure 6B, the ORR current of Pt/C exhibited a sudden change after the methanol addition due to the electrooxidation of methanol at the present potential (-0.4 V), indicating the high sensitivity of commercial Pt/C toward methanol crossover. However, the Co/N/C remained almost unchanged current density under the same condition, confirming the superior durability of the obtained catalyst than commercial Pt/C toward methanol crossover effect.
![Figure 6: (A) The current-time (i-t) curves of Co/N/C and 20% Pt/C catalysts at -0.4 V [vs. saturated calomel electrode (SCE)] in an O2-saturated 0.1 m KOH solution for 10 h. (B) Chronoamperometric responses of Co/N/C and 20% Pt/C catalysts at -0.4 V in an O2-saturated 0.1 m KOH solution without methanol (0–2000 s) and with adding methanol (2000–6000 s). The arrow represents the addition of methanol.](/document/doi/10.1515/gps-2015-0030/asset/graphic/j_gps-2015-0030_fig_006.jpg)
(A) The current-time (i-t) curves of Co/N/C and 20% Pt/C catalysts at -0.4 V [vs. saturated calomel electrode (SCE)] in an O2-saturated 0.1 m KOH solution for 10 h. (B) Chronoamperometric responses of Co/N/C and 20% Pt/C catalysts at -0.4 V in an O2-saturated 0.1 m KOH solution without methanol (0–2000 s) and with adding methanol (2000–6000 s). The arrow represents the addition of methanol.
All of the above results indicate that Co/N/C possesses good catalytic ability compared to commercial Pt/C, superior stability and outperformed methanol durability than commercial Pt/C.
4 Conclusions
Co/N/C electrocatalyst has been successfully synthesized by the solvothermal process using Co(Ac)2, NH3·H2O and CQDs as the starting materials. The electrocatalytic activity of obtained catalysts was investigated in 0.1 m KOH solution. Remarkably, Co/N/C shows a surprising ORR onset potential (-0.08 V) and is comparable to that of commercial Pt/C (20%). The excellent catalyst ability of Co/N/C is attributed to the porous structure which results in the formation of a large interface and surface. The electron-transferred number of Co/N/C for ORR is calculated to be about 3.51, indicating a four-electron pathway. The Co/N/C also shows better methanol durability and stability compared to commercial Pt/C (20%). The synergetic effect of Co and N among the CQDs framework (Co/N/C) promoted more active sites toward ORR than those of the single nitrogen-doped CQDs (NC) or cobalt-doped CQDs, which dominate the whole ORR process. Such a novel and high-efficient Co, N codoped carbon-based catalyst could be a promising substitute for commercial Pt/C in fuel cells as well as other energy storage and conversion systems.
About the authors
Huihua Wang obtained her doctorate degree from Northeastern University, China. Currently, she works in the School of Iron and Steel, Soochow University, Suzhou, China. She is mainly interested in the development of novel materials used in environmentally friendly fields.
Yanmei Yang is pursuing her Master’s degree at the Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou, China. Currently, her primary research interest involves the development of catalysts applied in oxygen reduction reaction (ORR) and OER.
Tianpeng Qu is an Associate Professor in the School of Iron and Steel, Soochow University, Suzhou, China. He is mainly interested in the evaluation and multipurpose utilization of metal-based composite materials.
Zhenhui Kang works at the Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou, China. He is a known expert in the development and application of carbon-based catalyst materials.
Deyong Wang works at the School of Iron and Steel, Soochow University, Suzhou, China. He focuses on the comprehensive utilization of secondary resources, such as technological progress towards the friendly treatment of waste water and waste slag.
Acknowledgments
This work was supported by Jiangsu Key Laboratory for Carbon-based Functional Materials and Devices, and Collaborative Innovation Center of Suzhou Nano Science and Technology, the National Natural Science Foundation of China (No.51422207, 51444007, 51132006 and 21471106). Their support enabled us to perform the present investigation.
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