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C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction
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3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient Co-N/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction Shujun Chao*, Mingjiang Geng Key Laboratory of Medical Molecular Probes, School of Basic Medical Sciences, Xinxiang Medical University, Xinxiang 453003, PR China
article info
abstract
Article history:
Nowadays, the exploration of highly active and cost-effective bifunctional catalysts for
Received 15 January 2016
oxygen electrode reactions is crucial for various energy conversion applications. Herein, to
Received in revised form
obtain a highly effective Co-N/C non-precious metal bifunctional catalyst, 3,5-diamino-
8 May 2016
1,2,4-triazole (Hdatrz) was used for the first time as a Nitrogen precursor. Different
Accepted 30 May 2016
Co-Hdatrz complex weight percentages and thermal treatment temperatures had resulted
Available online xxx
in varying catalyst performance. Among the resulting catalysts, due to the highest active N content, the smallest charge transfer resistance and the synergy of CoO with carbon black,
Keywords:
the Co-N30%/C-600 catalyst (the Co-Hdatrz complex weight percentage was 30% and the
3,5-Diamino-1,2,4-triazole
thermal treatment temperature was 600 C) demonstrated the highest catalytic activity
Co-N/C
towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline
Non-precious metal bifunctional
medium, which was superior to Pt/C. Additionally, the Co-N30%/C-600 catalyst also showed
catalyst
excellent methanol tolerance and ORR and OER stabilities compared to Pt/C. Thus, the
Oxygen reduction reaction
Co-N30%/C-600 catalyst may serve as a promising bifunctional electrocatalyst in fuel cells,
Oxygen evolution reaction
metal-air batteries and water splitting cells. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The impending energy crisis and the increasing environment pollution problem have prompted intense research on the development of high capacity and environmentally-friendly energy conversion and storage systems, such as regenerative fuel cells, metal-air batteries and water splitting cells [1e8]. The efficiency of these renewable energy systems mainly depends on the cathodic oxygen reduction reaction (ORR) and the anodic oxygen evolution reaction (OER).
However, the high overpotentials and the slow kinetics of the ORR and OER lead to great energy efficiency lost. Until now, the most efficient catalysts in accelerating the ORR or OER rate are still noble-metal based catalysts, such as Pt, Ir and Ru [9e11]. But, the prohibitive costs and element scarcities hinder their large-scale applications. Furthermore, Pt and Pt-based alloys, the best known ORR catalysts, only show moderate activity for the OER, while Ir and Ru oxides, the most advanced OER catalysts, have poor activity for the ORR. Hence, it is highly concerned to develop efficient non-noble metal bifunctional catalysts for the ORR and OER.
* Corresponding author. Tel./fax: þ86 373 3029128. E-mail address: [email protected] (S. Chao). http://dx.doi.org/10.1016/j.ijhydene.2016.05.269 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269
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Among various non-noble metal bifunctional catalysts [12e28], pyrolyzed carbon supported transition metalnitrogen complexes (M-N/C, M ¼ Fe or Co) catalysts have received extensive attention due to their superior activity in oxygen electrocatalysis [12e17]. Transition metal-Nx, such as Co-Nx and Fe-Nx, pyridinic N and graphitic N have been regarded as the active sites for oxygen electrocatalysis [13,14,16,19,29,30]. Zhang et al. [29] reported that Co-Nx active sites have a close relationship with ORR and Wang et al. indicated that Fe-Nx is considered as the most active species in oxygen electro-catalysis [19]. Zhao et al. [30] also demonstrated that the high OER activity of N/C materials is from the pyridinic N or/and graphitic N related active sites. Therefore, it can be inferred that more transition metal-Nx, pyridinic N and graphitic N active sites can be more favorable to quickly accomplish the ORR and OER. Recent studies have suggested that the Nitrogen precursor has an important effect on the active sites, catalytic activity and stabilities of the M-N/C catalysts. Nallathambi et al. [31] demonstrated that the accessible active site density increased with increasing the N/C ratio of the Nitrogen precursor, which could lead to the active M-N/C catalysts. They found that the most active catalyst has a N/C ratio of 2 [31]. Therefore, in order to obtain more efficient bifunctional M-N/C catalysts, several different types of Nitrogen precursors have been used as Nitrogen sources, such as macrocycles [32,33], conductive polymers [13,16] and small molecules [12,14,17]. However, the N/C ratios of the reported Nitrogen precursors are generally not more than 2. And, to the best of our knowledge, 3,5-diamino-1,2,4-triazole (Hdatrz) with the N/C ratio of 2.5, which is larger than 2, is rarely used as the Nitrogen precursor to synthesize the highly efficient bifunctional M-N/C catalyst. Based on the above considerations, for the first time, we employed Hdatrz as the Nitrogen precursor and synthesized a novel effective Co-N/C catalyst. We showed that the assynthesized Co-N/C catalyst displayed a high ORR and OER activity in alkaline media. Furthermore, the Co-N/C catalyst outperformed Pt/C in methanol crossover resistance and stability in the half-cell test. These results suggest the potential of Co-N/C catalyst in large-scale applications in the nextgeneration energy conversion and storage devices.
Experimental section Materials CoCl2$6H2O and Hdatrz were obtained from J&K Scientific Ltd. Commercial 20 wt% Pt/C was purchased from Alfa Aesar. Vulcan XC-72 (carbon black) was bought from Cabot and employed as the support. All chemical reagents were used as received without further purification. Double distilled water was used throughout the experiments.
Synthesis of Co-Hdatrz complex A solution of CoCl2$6H2O (1 mmol) in absolute ethanol (10 mL) was added dropwise into a solution of Hdatrz (2 mmol) in absolute ethanol (30 mL). A light blue precipitate formed
immediately. The reaction mixture was stirred for 4 h at room temperature. The crude product was filtered, washed three times with absolute ethanol and dried at 40 C for 2 h.
Synthesis of Co-N30%/C-600 catalyst The Co-Hdatrz complex (60 mg) and carbon black (Vulcan XC72, 140 mg) were added into 40 mL absolute ethanol under ultrasonic dispersion for 60 min followed by constant stirring for 4 h at room temperature. The black mixture was obtained upon removal of the ethanol using a rotary evaporator. After drying, the mixture was thermal-treated at 600 C for 2 h under a N2 atmosphere at a heating rate of 3 C min1. The resulting catalyst was labeled as Co-N30%/C-600.
Apparatus X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (D8 Advance, Bruker, Germany) using Cu Ka radiation with a Ni filter (l ¼ 1.5406 A at 40 kV and 20 mA). Transmission electron microscopy (TEM) images were obtained using a Hitachi JEM-2100 transmission electron microscope operating at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG Scientific ESCALab220i-XL electron spectrometer with Al Ka radiation (1486.6 eV) operated at 300 W. The binding energies for the high resolution spectra were calibrated by setting C 1s peak to 284.8 eV.
Electrochemical measurements Electrochemical measurements were carried out on a computer-controlled CHI 660D workstation (CH Instruments, Chenhua, Shanghai, China) equipped with a model 616 electrode rotator (Princeton Applied Research) in a traditional three-electrode cell. A catalyst-coated glass carbon rotating disk electrode (RDE, 0.1256 cm2), Pt foil (1 cm2) and saturated calomel electrode (SCE) served as the working, counter and reference electrodes, respectively. Catalyst ink was prepared by mixing the catalyst powder (2 mg) with 1 mL ethanol and 40 mL 5 wt% Nafion and was then subjected to ultrasonication for 20 min. Next, 18 mL of catalyst ink was dropped onto the pre-polished RDE surface to yield a catalyst loading of 275.6 mg cm2. ORR steady-state polarization curve measurements were carried out in O2-saturated 0.1 M KOH solution between 0.2 and 1.0 V vs. SCE with a scan rate of 5 mV s1 at different rotating speeds. Polarization curve measurements for OER were carried out in N2-saturated 0.1 M KOH solution in the potential scan range of 0e1.0 V vs. SCE at a scan rate of 5 mV s1 at 900 rpm. AC impedance tests were conducted on a VMP3 multichannel potentiostats at potential of 0.62 V vs. SCE from 105 Hz to 102 Hz with an AC amplitude of 5 mV. The stability tests of the Co-N30%/C-600 and Pt/C catalysts towards ORR and OER based on cyclic voltammetry were performed from 0.2 to 1.0 V vs. SCE at 200 mV s1 for 10,000 cycles and from 0 to 0.8 V vs. SCE at 50 mV s1 for 300 cycles, respectively. After 10,000 and 300 cycles, ORR and OER steadystate polarization curves were recorded from 0.2 to 1.0 V vs.
Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269
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Scheme 1 e The synthesis process of the Co-N/C catalysts.
SCE at 5 mV s-1 at 1600 rpm and from 0 to 1.0 V at 5 mV s1 at 900 rpm, respectively. All electrochemical experiments were performed at 25 ± 1 C.
Results and discussion As shown in Scheme 1, the Co-N/C catalysts were synthesized via a simple three-step method, which included Co2þ coordinating with Hdatrz, mixing Co-Hdatrz complex with carbon black and thermal treatment of carbon supported Co-Hdatrz complex under N2 atmosphere. To find
the most active Co-N/C catalyst, the catalysts synthesized with different Co-Hdatrz complex weight percentages (10%, 20%, 30% and 40%) and under different thermal treatment temperatures (500 C, 600 C and 700 C) were investigated. Since the electrochemical tests indicated that the Co-N30%/C-600 catalyst exhibited the highest ORR and OER activity among the resulting catalysts, this catalyst was mainly studied by means of XRD, TEM, XPS and polarization curve measurements. XRD, TEM and XPS analysis confirmed that thermal treatment caused the carbon supported Co-Hdatrz complex to decompose into CoO, CoNx and graphitic N.
Fig. 1 e XRD patterns of (a) the Co-N30%/C catalysts after thermal treatment at different temperatures and (b) the Co-N/C-600 catalysts synthesized with different Co-Hdatrz complex weight percentages. Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269
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Fig. 2 e (a) TEM image, (b) the size frequency curve and (c) high-resolution TEM image of the Co-N30%/C-600 catalyst.
Fig. 3 e High-resolution Co 2p3/2 (a, b, c) and N 1s (d, e, f) spectra of the Co-N30%/C-500, Co-N30%/C-600 and Co-N30%/C-700 catalysts.
Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269
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Fig. 4 e (a) ORR polarization curves of the Co-N30%/C catalysts synthesized at different thermal treatment temperatures and Pt/C at 1600 rpm. (b) ORR polarization curves of the Co-N/C-600 catalysts with different Co-Hdatrz complex weight percentages at 1600 rpm. (c) and (e) ORR polarization curves of the Pt/C and Co-N30%/C-600 catalysts at different rotation speeds. (d) and (f) KeL plots of the Pt/C and Co-N30%/C-600 catalysts at different electrode potentials obtained from (c) and (e). (f) Tafel plots for the Co-N30%/C-600 and Pt/C catalysts obtained from (b). (h) The kinetic current densities (Jk) of the Co-N30%/ C-600 and Pt/C catalysts at different electrode potentials obtained from (d) and (f). Electrolyte: O2-saturated 0.1 M KOH solution; scan rate: 5 mV s¡1. XRD was used to characterize the crystallographic phases and sample purities. Fig. 1 shows XRD patterns of the Co-N30%/C catalysts after thermal treatment at different
temperatures and the Co-N/C-600 catalysts synthesized with different Co-Hdatrz weight percentages. In all XRD patterns (Fig. 1a and b), one broad diffraction peak exists at ~25 , which
Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269
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Fig. 5 e (a) OER activity of the Co-N30%/C catalysts synthesized at different thermal treatment temperatures and Pt/C at 5 mV s¡1 at 900 rpm. (b) OER activity of the Co-N/C-600 catalysts obtained with different Co-Hdatrz complex weight percentages at 5 mV s¡1 at 900 rpm. (c) Nyquist plots of the Co-N/C-600 catalysts measured at a potential of 0.62 V vs. SCE. The electrolyte is N2-saturated 0.1 M KOH solution.
can be attributed to (002) plane of graphitic carbon. Other diffraction peaks at 36.6 , 42.6 , 61.8 , 74.0 and 77.5 correspond to (111), (200), (220), (311) and (222) planes of CoO (JCPDS Card no. 01-1227), respectively. The results indicate that thermal treatment makes part of the Co-Hdatrz complex decompose into CoO. And, according to the intensity of these diffraction peaks of CoO, we can easily see that the Co-N30%/ C-600 catalyst possesses relatively higher CoO content than other catalysts. The particle size of CoO in the Co-N30%/C-600 catalyst is calculated to be 30.1 nm by Scherrer's equation. It has been reported that the hybridation of transition metal oxides and carbon-based materials can exhibit improved bifunctional electrocatalytic activity [5,16,20]. Therefore, the existence of CoO in these catalysts may help to enhance their ORR and OER performance. TEM was carried out to study the morphology and particle sizes of the prepared catalysts. Fig. 2a shows TEM image of the Co-N30%/C-600 catalyst. It can be observed that CoO nanoparticles are uniformly encapsulated in carbon black. The average particle size of CoO nanoparticles is about 28.2 nm (Fig. 2b). The high-resolution TEM image (Fig. 2c) further confirms the encapsulation of CoO nanoparticles in carbon black. These CoO nanoparticles have a crystalline lattice of 0.213 nm, matching well with the (200) crystal plane of CoO. The results agree with the XRD analysis. The chemical compositions of the Co-N30%/C-500, Co-N30%/ C-600 and Co-N30%/C-700 catalysts as determined by XPS analysis show the presence of C, N, O and Co. Among them, the Co-N30%/C-500 catalyst contains 96.86% C, 0.87% N, 1.74% O and 0.53% Co. The relative atomic percentages of C, N, O and Co in the Co-N30%/C-600 catalysts are 95.94%, 0.94%, 2.58% and 0.54%, respectively. 96.85% C, 0.63% N, 2% O and 0.52% Co exists in the Co-N30%/C-700 catalyst. Fig. 3a, b and c show the high-resolution Co 2p3/2 spectrum of the three catalysts. Peaks at binding energies of 780.0, 781.4 ± 0.1 and 786.2 eV are assigned to CoO, Co-Nx and Co2þ ions, respectively [16,34,35]. According to the fitting results, the content of CoO and Co-Nx in the three catalysts is 12.4% and 49.7%, 18.8% and 61.3%, and 11.1% and 45.9%, respectively. Co oxides and Co-Nx active sites are very important for the improvement of ORR and OER activity [5,8,16,20,29].
The XPS spectra of N 1s (Fig. 3def) indicate the presence of three dominating signals, including Co-Nx at 398.9 ± 0.1 eV, graphitic N at 401.1 ± 0.1 eV and pyridine-N-oxide at 402.6 or 404.3 eV [16,19,35,36]. The fitting results show that the Co-N30%/C-500 catalyst contains 15.4% Co-Nx and 71.2% graphitic N. The content of Co-Nx and graphitic N in the Co-N30%/C-600 catalyst is 54.6% and 45.4%, respectively. Similarly, 29.7% Co-Nx and 49.8% graphitic N exist in the Co-N30%/C-700 catalyst. The active N content in the three catalysts is 0.75%, 0.94% and 0.50%, respectively. The higher active N content contributes to improve ORR and OER activity [14,16,19,29,30,35]. Therefore, it is expected that the Co-N30%/ C-600 catalyst will show the highest catalytic activity towards ORR and OER among the three catalysts. Polarization curve measurements were carried out to evaluate the electrocatalytic activity towards ORR and OER. Fig. 4a shows the ORR polarization curves of the Co-N30%/C catalysts synthesized at different thermal treatment temperatures. The Co-N30%/C-500, Co-N30%/C-600 and Co-N30%/C-700 catalysts have no obvious difference in onset potential. But, the Co-N30%/C-600 catalyst shows higher limiting current density (5.73 mA cm2) than those of the Co-N30%/C-500 (5.12 mA cm2) and Co-N30%/C-700 catalysts (4.65 mA cm2). Therefore, 600 C may be the most appropriate thermal treatment temperature. Moreover, the limiting current density of the Co-N30%/C-600 catalyst is higher than that of Pt/C (5.28 mA cm2). The relatively higher active N content and the synergy of CoO with carbon black may be responsible for the high ORR activity of the Co-N30%/C-600 catalyst. Fig. 4b further evaluates the influence of different Co-Hdatrz complex weight percentages on ORR activity of the Co-N/C-600 catalysts. The onset potential of the CoN10%/C-600 catalyst is 0.14 V vs. SCE. When the Co-Hdatrz complex weight percentage is increased to 20% and 30%, the onset potential shifts to 0.13 V and 0.09 V vs. SCE, respectively, and the limiting current density increases from 4.76 to 5.16 and then to 5.73 mA cm2. With the continued increase in the Co-Hdatrz complex weight percentage to 40%, a negative shift in the onset potential (0.13 V vs. SCE) and a decrease in the limiting current density (5.07 mA cm2) are observed, which indicate that the
Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269
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Fig. 6 e ORR polarization curves of (a) the Co-N30%/C-600 and (b) Pt/C catalysts in O2-saturated 0.1 M KOH solution without and with 1 M methanol at 5 mV s¡1 at 1600 rpm. (c) ORR polarization curves of the Co-N30%/C-600 and Pt/C catalysts before and after 10,000 potential cycles in O2-saturated 0.1 M KOH solution at 5 mV s¡1 at 1600 rpm. (d) OER polarization curves for the Co-N30%/C-600 catalyst in N2-saturated 0.1 M KOH solution before cycling and after 300 cycles at 5 mV s¡1 at 900 rpm. (e) Chronoamperometric responses for ORR on the Co-N30%/C-600 and Pt/C catalysts in O2-saturated 0.1 M KOH solution at ¡0.3 V vs. SCE at 1600 rpm. (f) Chronoamperometric responses for OER on the Co-N30%/C-600 and Pt/C catalysts in N2saturated 0.1 M KOH solution at 0.7 V vs. SCE at 900 rpm.
optimum Co-Hdatrz content (at which the optimum ORR activity occurs) is 30%. The ORR processes on the Pt/C and Co-N30%/C-600 catalysts were further studied by polarization curve measurements. Fig. 4c and e display ORR polarization curves of the Pt/C and CoN30%/C-600 catalysts at different rotation speeds. For the both catalysts, the current density increases with an increase in
rotation speed, which can be explained by a shortened diffusion distance at higher speeds. Moreover, the Co-N30%/C-600 catalyst exhibits higher ORR limiting current densities than those for Pt/C at the same rotation speed. Using the KouteckyLevich equation [37], KeL plots (J1 versus u1/2) were drawn at different potentials (0.4 V, 0.5 V, 0.6 V, 0.7 V and 0.8 V vs. SCE) (Fig. 4d and f). All the KeL plots show good linearity.
Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269
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Fig. 7 e TEM images of the Co-N30%/C-600 (a and b) and Pt/C (c and d) catalysts before and after 10000 potential cycles. The corresponding size frequency curves of Pt/C (e and f) before and after 10,000 potential cycles.
According to the slopes of the KeL plots, the electron-transfer number (n) on Pt/C is calculated to be 3.9, indicating that the ORR on Pt/C is a complete four-electron process, which is consistent well with the previous reports [38,39]. In the same way, n is determined to be 3.7, 3.7, 3.7, 3.9 and 3.8 on Co-N30%/ C-600 in the potential of 0.4 V, 0.5 V, 0.6 V, 0.7 V and 0.8 V vs. SCE, respectively. So, the ORR on Co-N30%/C-600 is similar with Pt/C and follows a one-step four-electron process. To in-depth understand ORR performance of the Co-N30%/ C-600 and Pt/C catalysts, diffusion-corrected Tafel plots were also investigated. As shown in Fig. 4g, Tafel plots for the Co-N30%/C-600 and Pt/C catalysts exhibit two different Tafel slopes in the low and high overpotentials regions. The calculated Tafel slopes of the Co-N30%/C-600 and Pt/C catalysts are 62.8 and 80.1 mV dec1 in the low overpotentials regions, respectively. In the high overpotentials regions, the Tafel slope values are 127.0 and 136.8 mV dec1, respectively, which are close to theoretical value of 120 mV dec1, indicating that the rate-determining step is the first electron transfer [40]. Because of overpotential increasing rapidly with current density, the higher Tafel slopes can result in lower ORR activity [40]. On the basis of Tafel plots, the exchange current densities of the Co-N30%/C-600 and Pt/C catalysts are 5.5 104 and 2.1 104 mA cm2, respectively. Hence, the ORR limiting current densities of the Co-N30%/C-600 catalyst are higher than those of Pt/C. In addition, the Co-N30%/C-600 catalyst also outperforms Pt/C with respect to the kinetic current density (Jk) at different electrode potentials (Fig. 4h), which make it a promising cathodic catalyst candidate in fuel cells and metalair batteries. The OER polarization curves of the Co-N30%/C catalysts synthesized at different thermal treatment temperatures and Pt/C were shown in Fig. 5a. It can be seen that the Co-N30%/ C-600 catalyst exhibits the best OER activity among the
Co-N30%/C catalysts, which is mainly attributed to the relatively high active N content and the synergy of CoO with carbon black. Moreover, the OER activity of the Co-N30%/C-600 catalyst is better than Pt/C. A small value of the potential difference (△E) between ORR potential (3 mA cm2) and OER potential (10 mA cm2) represents a high ORR and OER activity. The △E of the Co-N30%/C-500, Co-N30%/C-600, Co-N30%/ C-700 and Pt/C catalysts is 1.08 V, 1.02 V, 1.1 V and 1.1 V, respectively. The △E of the Co-N30%/C-600 catalyst is lower than those of the Co-N30%/C-500, Co-N30%/C-700 and Pt/C catalysts. The results indicate that the Co-N30%/C-600 catalyst possesses a high bifunctionality for catalyzing ORR and OER. The OER activity of the Co-N/C-600 catalysts synthesized with different Co-Hdatrz complex weight percentages were further examined. As displayed by Fig. 5b, increasing the Co-Hdatrz complex weight percentage from 10% to 20% and then to 30%, the anodic current increases from 19.9 to 22.7 and then to 32.9 mA cm2. If the Co-Hdatrz complex weight percentage is increased higher, OER activity begins to decrease, suggesting that 30% is the optimized Co-Hdatrz complex weight percentage. The difference in OER activity of the Co-N/C-600 catalysts can be explained by the electrochemical impedance spectroscopy (EIS) analysis. As shown in Fig. 5c, the Nyquist plots indicate that the charge transfer resistance of the Co-N/C-600 catalysts decreases in the following order: Co-N10%/ C-600 > Co-N20%/C-600 > Co-N40%/C-600 > Co-N30%/C-600. The smaller charge transfer resistance can lead to more efficient charge transport in the OER process [41]. Thus, the OER activity of the Co-N/C-600 catalysts increases in the following sequence: Co-N10%/C-600 < Co-N20%/C-600 < Co-N40%/ C-600 < Co-N30%/C-600. Owing to the smallest charge transfer resistance, the Co-N30%/C-600 catalyst exhibits the highest OER activity.
Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269
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Catalyst selectivity against methanol electrooxidation was also investigated. Fig. 6a shows ORR polarization curves of the Co-N30%/C-600 catalyst in O2-saturated 0.1 M KOH in the absence or presence of 1 M methanol. The similarity between the ORR polarization curves without and with 1 M methanol indicates that the Co-N30%/C-600 catalyst has a good ORR selectivity. In contrast, when using Pt/C modified electrode, the disappearance of ORR peak and the appearance of obvious methanol oxidation peak suggest that the ORR process is largely retarded by methanol oxidation (Fig. 6b). The improved methanol resistance indicates that the Co-N30%/C-600 catalyst may function as a methanol-resistant ORR catalyst in fuel cells. The long-term stability of the catalyst in the ORR and OER is an important concern for fuel cells, metal-air batteries and water splitting cells. Accelerated stability tests were used to investigate the stabilities of the Co-N30%/C-600 and Pt/C catalysts for ORR. As shown in Fig. 6c, after 10,000 continuous potential cycles, ORR half-wave potential of the Co-N30%/C-600 catalyst exhibits a 20 mV negative shift, while a 36 mV negative shift in half-wave potential is observed for Pt/C. Fig. 6d shows the OER stability of the Co-N30%/C-600 catalyst. The Co-N30%/C-600 catalyst displays almost unchangeable polarization curves and anodic currents before and after 300 cycles. The ORR and OER stabilities of the Co-N30%/C-600 and Pt/C catalysts were further investigated using chronoamperometric method. Fig. 6e shows that the Co-N30%/C-600 catalyst possesses high stability and ORR current density decreases only 8% after 10,000 s at 0.3 V vs. SCE. However, Pt/C exhibits a poor stability with a 51.8% decrease in ORR current density under the same operation condition. The Co-N30%/C-600 catalyst also exhibits excellent OER stability. As displayed by Fig. 6f, the decay in OER current density for the Co-N30%/C-600 catalyst is just 4.2% over 10,000 s continuous operation at 0.7 V vs. SCE. But the decay of 67.3% in OER current density is observed for Pt/C, indicating a more serious degradation. The results demonstrate that the Co-N30%/C-600 catalyst has superior ORR and OER stabilities compared to Pt/C. The composition is responsible for the high ORR stability of the Co-N30%/C-600 catalyst. TEM images of the Co-N30%/C-600 catalyst before and after 10,000 potential cycles (Fig. 7a and b) evidence that CoO nanoparticles do not produce obvious aggregation. This may be because CoO nanoparticles are uniformly encapsulated in carbon black and protected by carbon layers, they are stable in alkaline medium. For Pt/C, before the stability testing, Pt nanoparitcles with the average particle size of 2.1 nm are uniformly dispersed on the surface of carbon black (Fig. 7c and e). However, after the stability testing, the aggregation of Pt nanoparticles can be clearly observed and the average particle size of Pt nanoparticles increases to 8.5 nm (Fig. 7d and f). The loss of ORR activity and stability on Pt/C could be ascribed to the migration or aggregation of Pt nanoparticles caused by successive potential cycling [42].
Conclusions In summary, a novel non-precious metal bifunctional catalyst Co-N/C has been synthesized using Hdatrz as the Nitrogen
9
precursor and CoCl2 as the metal precursor. The Co-Hdatrz complex weight percentage and thermal treatment temperature have a significant influence on the ORR and OER activity of the Co-N/C catalyst. Among the synthesized Co-N/C catalysts, the Co-N30%/C-600 catalyst shows the highest ORR activity with an onset potential of 0.09 V vs. SCE and the highest OER activity with the largest anodic current (32.9 mA cm2). More importantly, the potential difference (1.02 V) of the Co-N30%/C-600 catalyst is lower than Pt/C (1.10 V). The best ORR and OER activity of the Co-N30%/C-600 catalyst is mainly related with the highest active N content and the smallest charge transfer resistance as well as the synergy of CoO with carbon black. Additionally, the Co-N30%/ C-600 catalyst outperforms Pt/C in terms of anti-poisoning ability for methanol oxidation and long-term stability for ORR and OER. These properties indicate that the Co-N30%/ C-600 catalyst may be a promising non-precious metal bifunctional catalyst for energy related applications.
Acknowledgments This work was supported by the Doctoral Startup Fund of Xinxiang Medical University (505116).
references
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Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269
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3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient Co-N/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction Shujun Chao*, Mingjiang Geng Key Laboratory of Medical Molecular Probes, School of Basic Medical Sciences, Xinxiang Medical University, Xinxiang 453003, PR China
article info
abstract
Article history:
Nowadays, the exploration of highly active and cost-effective bifunctional catalysts for
Received 15 January 2016
oxygen electrode reactions is crucial for various energy conversion applications. Herein, to
Received in revised form
obtain a highly effective Co-N/C non-precious metal bifunctional catalyst, 3,5-diamino-
8 May 2016
1,2,4-triazole (Hdatrz) was used for the first time as a Nitrogen precursor. Different
Accepted 30 May 2016
Co-Hdatrz complex weight percentages and thermal treatment temperatures had resulted
Available online xxx
in varying catalyst performance. Among the resulting catalysts, due to the highest active N content, the smallest charge transfer resistance and the synergy of CoO with carbon black,
Keywords:
the Co-N30%/C-600 catalyst (the Co-Hdatrz complex weight percentage was 30% and the
3,5-Diamino-1,2,4-triazole
thermal treatment temperature was 600 C) demonstrated the highest catalytic activity
Co-N/C
towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline
Non-precious metal bifunctional
medium, which was superior to Pt/C. Additionally, the Co-N30%/C-600 catalyst also showed
catalyst
excellent methanol tolerance and ORR and OER stabilities compared to Pt/C. Thus, the
Oxygen reduction reaction
Co-N30%/C-600 catalyst may serve as a promising bifunctional electrocatalyst in fuel cells,
Oxygen evolution reaction
metal-air batteries and water splitting cells. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The impending energy crisis and the increasing environment pollution problem have prompted intense research on the development of high capacity and environmentally-friendly energy conversion and storage systems, such as regenerative fuel cells, metal-air batteries and water splitting cells [1e8]. The efficiency of these renewable energy systems mainly depends on the cathodic oxygen reduction reaction (ORR) and the anodic oxygen evolution reaction (OER).
However, the high overpotentials and the slow kinetics of the ORR and OER lead to great energy efficiency lost. Until now, the most efficient catalysts in accelerating the ORR or OER rate are still noble-metal based catalysts, such as Pt, Ir and Ru [9e11]. But, the prohibitive costs and element scarcities hinder their large-scale applications. Furthermore, Pt and Pt-based alloys, the best known ORR catalysts, only show moderate activity for the OER, while Ir and Ru oxides, the most advanced OER catalysts, have poor activity for the ORR. Hence, it is highly concerned to develop efficient non-noble metal bifunctional catalysts for the ORR and OER.
* Corresponding author. Tel./fax: þ86 373 3029128. E-mail address: [email protected] (S. Chao). http://dx.doi.org/10.1016/j.ijhydene.2016.05.269 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269
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Among various non-noble metal bifunctional catalysts [12e28], pyrolyzed carbon supported transition metalnitrogen complexes (M-N/C, M ¼ Fe or Co) catalysts have received extensive attention due to their superior activity in oxygen electrocatalysis [12e17]. Transition metal-Nx, such as Co-Nx and Fe-Nx, pyridinic N and graphitic N have been regarded as the active sites for oxygen electrocatalysis [13,14,16,19,29,30]. Zhang et al. [29] reported that Co-Nx active sites have a close relationship with ORR and Wang et al. indicated that Fe-Nx is considered as the most active species in oxygen electro-catalysis [19]. Zhao et al. [30] also demonstrated that the high OER activity of N/C materials is from the pyridinic N or/and graphitic N related active sites. Therefore, it can be inferred that more transition metal-Nx, pyridinic N and graphitic N active sites can be more favorable to quickly accomplish the ORR and OER. Recent studies have suggested that the Nitrogen precursor has an important effect on the active sites, catalytic activity and stabilities of the M-N/C catalysts. Nallathambi et al. [31] demonstrated that the accessible active site density increased with increasing the N/C ratio of the Nitrogen precursor, which could lead to the active M-N/C catalysts. They found that the most active catalyst has a N/C ratio of 2 [31]. Therefore, in order to obtain more efficient bifunctional M-N/C catalysts, several different types of Nitrogen precursors have been used as Nitrogen sources, such as macrocycles [32,33], conductive polymers [13,16] and small molecules [12,14,17]. However, the N/C ratios of the reported Nitrogen precursors are generally not more than 2. And, to the best of our knowledge, 3,5-diamino-1,2,4-triazole (Hdatrz) with the N/C ratio of 2.5, which is larger than 2, is rarely used as the Nitrogen precursor to synthesize the highly efficient bifunctional M-N/C catalyst. Based on the above considerations, for the first time, we employed Hdatrz as the Nitrogen precursor and synthesized a novel effective Co-N/C catalyst. We showed that the assynthesized Co-N/C catalyst displayed a high ORR and OER activity in alkaline media. Furthermore, the Co-N/C catalyst outperformed Pt/C in methanol crossover resistance and stability in the half-cell test. These results suggest the potential of Co-N/C catalyst in large-scale applications in the nextgeneration energy conversion and storage devices.
Experimental section Materials CoCl2$6H2O and Hdatrz were obtained from J&K Scientific Ltd. Commercial 20 wt% Pt/C was purchased from Alfa Aesar. Vulcan XC-72 (carbon black) was bought from Cabot and employed as the support. All chemical reagents were used as received without further purification. Double distilled water was used throughout the experiments.
Synthesis of Co-Hdatrz complex A solution of CoCl2$6H2O (1 mmol) in absolute ethanol (10 mL) was added dropwise into a solution of Hdatrz (2 mmol) in absolute ethanol (30 mL). A light blue precipitate formed
immediately. The reaction mixture was stirred for 4 h at room temperature. The crude product was filtered, washed three times with absolute ethanol and dried at 40 C for 2 h.
Synthesis of Co-N30%/C-600 catalyst The Co-Hdatrz complex (60 mg) and carbon black (Vulcan XC72, 140 mg) were added into 40 mL absolute ethanol under ultrasonic dispersion for 60 min followed by constant stirring for 4 h at room temperature. The black mixture was obtained upon removal of the ethanol using a rotary evaporator. After drying, the mixture was thermal-treated at 600 C for 2 h under a N2 atmosphere at a heating rate of 3 C min1. The resulting catalyst was labeled as Co-N30%/C-600.
Apparatus X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (D8 Advance, Bruker, Germany) using Cu Ka radiation with a Ni filter (l ¼ 1.5406 A at 40 kV and 20 mA). Transmission electron microscopy (TEM) images were obtained using a Hitachi JEM-2100 transmission electron microscope operating at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG Scientific ESCALab220i-XL electron spectrometer with Al Ka radiation (1486.6 eV) operated at 300 W. The binding energies for the high resolution spectra were calibrated by setting C 1s peak to 284.8 eV.
Electrochemical measurements Electrochemical measurements were carried out on a computer-controlled CHI 660D workstation (CH Instruments, Chenhua, Shanghai, China) equipped with a model 616 electrode rotator (Princeton Applied Research) in a traditional three-electrode cell. A catalyst-coated glass carbon rotating disk electrode (RDE, 0.1256 cm2), Pt foil (1 cm2) and saturated calomel electrode (SCE) served as the working, counter and reference electrodes, respectively. Catalyst ink was prepared by mixing the catalyst powder (2 mg) with 1 mL ethanol and 40 mL 5 wt% Nafion and was then subjected to ultrasonication for 20 min. Next, 18 mL of catalyst ink was dropped onto the pre-polished RDE surface to yield a catalyst loading of 275.6 mg cm2. ORR steady-state polarization curve measurements were carried out in O2-saturated 0.1 M KOH solution between 0.2 and 1.0 V vs. SCE with a scan rate of 5 mV s1 at different rotating speeds. Polarization curve measurements for OER were carried out in N2-saturated 0.1 M KOH solution in the potential scan range of 0e1.0 V vs. SCE at a scan rate of 5 mV s1 at 900 rpm. AC impedance tests were conducted on a VMP3 multichannel potentiostats at potential of 0.62 V vs. SCE from 105 Hz to 102 Hz with an AC amplitude of 5 mV. The stability tests of the Co-N30%/C-600 and Pt/C catalysts towards ORR and OER based on cyclic voltammetry were performed from 0.2 to 1.0 V vs. SCE at 200 mV s1 for 10,000 cycles and from 0 to 0.8 V vs. SCE at 50 mV s1 for 300 cycles, respectively. After 10,000 and 300 cycles, ORR and OER steadystate polarization curves were recorded from 0.2 to 1.0 V vs.
Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269
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Scheme 1 e The synthesis process of the Co-N/C catalysts.
SCE at 5 mV s-1 at 1600 rpm and from 0 to 1.0 V at 5 mV s1 at 900 rpm, respectively. All electrochemical experiments were performed at 25 ± 1 C.
Results and discussion As shown in Scheme 1, the Co-N/C catalysts were synthesized via a simple three-step method, which included Co2þ coordinating with Hdatrz, mixing Co-Hdatrz complex with carbon black and thermal treatment of carbon supported Co-Hdatrz complex under N2 atmosphere. To find
the most active Co-N/C catalyst, the catalysts synthesized with different Co-Hdatrz complex weight percentages (10%, 20%, 30% and 40%) and under different thermal treatment temperatures (500 C, 600 C and 700 C) were investigated. Since the electrochemical tests indicated that the Co-N30%/C-600 catalyst exhibited the highest ORR and OER activity among the resulting catalysts, this catalyst was mainly studied by means of XRD, TEM, XPS and polarization curve measurements. XRD, TEM and XPS analysis confirmed that thermal treatment caused the carbon supported Co-Hdatrz complex to decompose into CoO, CoNx and graphitic N.
Fig. 1 e XRD patterns of (a) the Co-N30%/C catalysts after thermal treatment at different temperatures and (b) the Co-N/C-600 catalysts synthesized with different Co-Hdatrz complex weight percentages. Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269
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Fig. 2 e (a) TEM image, (b) the size frequency curve and (c) high-resolution TEM image of the Co-N30%/C-600 catalyst.
Fig. 3 e High-resolution Co 2p3/2 (a, b, c) and N 1s (d, e, f) spectra of the Co-N30%/C-500, Co-N30%/C-600 and Co-N30%/C-700 catalysts.
Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269
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Fig. 4 e (a) ORR polarization curves of the Co-N30%/C catalysts synthesized at different thermal treatment temperatures and Pt/C at 1600 rpm. (b) ORR polarization curves of the Co-N/C-600 catalysts with different Co-Hdatrz complex weight percentages at 1600 rpm. (c) and (e) ORR polarization curves of the Pt/C and Co-N30%/C-600 catalysts at different rotation speeds. (d) and (f) KeL plots of the Pt/C and Co-N30%/C-600 catalysts at different electrode potentials obtained from (c) and (e). (f) Tafel plots for the Co-N30%/C-600 and Pt/C catalysts obtained from (b). (h) The kinetic current densities (Jk) of the Co-N30%/ C-600 and Pt/C catalysts at different electrode potentials obtained from (d) and (f). Electrolyte: O2-saturated 0.1 M KOH solution; scan rate: 5 mV s¡1. XRD was used to characterize the crystallographic phases and sample purities. Fig. 1 shows XRD patterns of the Co-N30%/C catalysts after thermal treatment at different
temperatures and the Co-N/C-600 catalysts synthesized with different Co-Hdatrz weight percentages. In all XRD patterns (Fig. 1a and b), one broad diffraction peak exists at ~25 , which
Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269
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Fig. 5 e (a) OER activity of the Co-N30%/C catalysts synthesized at different thermal treatment temperatures and Pt/C at 5 mV s¡1 at 900 rpm. (b) OER activity of the Co-N/C-600 catalysts obtained with different Co-Hdatrz complex weight percentages at 5 mV s¡1 at 900 rpm. (c) Nyquist plots of the Co-N/C-600 catalysts measured at a potential of 0.62 V vs. SCE. The electrolyte is N2-saturated 0.1 M KOH solution.
can be attributed to (002) plane of graphitic carbon. Other diffraction peaks at 36.6 , 42.6 , 61.8 , 74.0 and 77.5 correspond to (111), (200), (220), (311) and (222) planes of CoO (JCPDS Card no. 01-1227), respectively. The results indicate that thermal treatment makes part of the Co-Hdatrz complex decompose into CoO. And, according to the intensity of these diffraction peaks of CoO, we can easily see that the Co-N30%/ C-600 catalyst possesses relatively higher CoO content than other catalysts. The particle size of CoO in the Co-N30%/C-600 catalyst is calculated to be 30.1 nm by Scherrer's equation. It has been reported that the hybridation of transition metal oxides and carbon-based materials can exhibit improved bifunctional electrocatalytic activity [5,16,20]. Therefore, the existence of CoO in these catalysts may help to enhance their ORR and OER performance. TEM was carried out to study the morphology and particle sizes of the prepared catalysts. Fig. 2a shows TEM image of the Co-N30%/C-600 catalyst. It can be observed that CoO nanoparticles are uniformly encapsulated in carbon black. The average particle size of CoO nanoparticles is about 28.2 nm (Fig. 2b). The high-resolution TEM image (Fig. 2c) further confirms the encapsulation of CoO nanoparticles in carbon black. These CoO nanoparticles have a crystalline lattice of 0.213 nm, matching well with the (200) crystal plane of CoO. The results agree with the XRD analysis. The chemical compositions of the Co-N30%/C-500, Co-N30%/ C-600 and Co-N30%/C-700 catalysts as determined by XPS analysis show the presence of C, N, O and Co. Among them, the Co-N30%/C-500 catalyst contains 96.86% C, 0.87% N, 1.74% O and 0.53% Co. The relative atomic percentages of C, N, O and Co in the Co-N30%/C-600 catalysts are 95.94%, 0.94%, 2.58% and 0.54%, respectively. 96.85% C, 0.63% N, 2% O and 0.52% Co exists in the Co-N30%/C-700 catalyst. Fig. 3a, b and c show the high-resolution Co 2p3/2 spectrum of the three catalysts. Peaks at binding energies of 780.0, 781.4 ± 0.1 and 786.2 eV are assigned to CoO, Co-Nx and Co2þ ions, respectively [16,34,35]. According to the fitting results, the content of CoO and Co-Nx in the three catalysts is 12.4% and 49.7%, 18.8% and 61.3%, and 11.1% and 45.9%, respectively. Co oxides and Co-Nx active sites are very important for the improvement of ORR and OER activity [5,8,16,20,29].
The XPS spectra of N 1s (Fig. 3def) indicate the presence of three dominating signals, including Co-Nx at 398.9 ± 0.1 eV, graphitic N at 401.1 ± 0.1 eV and pyridine-N-oxide at 402.6 or 404.3 eV [16,19,35,36]. The fitting results show that the Co-N30%/C-500 catalyst contains 15.4% Co-Nx and 71.2% graphitic N. The content of Co-Nx and graphitic N in the Co-N30%/C-600 catalyst is 54.6% and 45.4%, respectively. Similarly, 29.7% Co-Nx and 49.8% graphitic N exist in the Co-N30%/C-700 catalyst. The active N content in the three catalysts is 0.75%, 0.94% and 0.50%, respectively. The higher active N content contributes to improve ORR and OER activity [14,16,19,29,30,35]. Therefore, it is expected that the Co-N30%/ C-600 catalyst will show the highest catalytic activity towards ORR and OER among the three catalysts. Polarization curve measurements were carried out to evaluate the electrocatalytic activity towards ORR and OER. Fig. 4a shows the ORR polarization curves of the Co-N30%/C catalysts synthesized at different thermal treatment temperatures. The Co-N30%/C-500, Co-N30%/C-600 and Co-N30%/C-700 catalysts have no obvious difference in onset potential. But, the Co-N30%/C-600 catalyst shows higher limiting current density (5.73 mA cm2) than those of the Co-N30%/C-500 (5.12 mA cm2) and Co-N30%/C-700 catalysts (4.65 mA cm2). Therefore, 600 C may be the most appropriate thermal treatment temperature. Moreover, the limiting current density of the Co-N30%/C-600 catalyst is higher than that of Pt/C (5.28 mA cm2). The relatively higher active N content and the synergy of CoO with carbon black may be responsible for the high ORR activity of the Co-N30%/C-600 catalyst. Fig. 4b further evaluates the influence of different Co-Hdatrz complex weight percentages on ORR activity of the Co-N/C-600 catalysts. The onset potential of the CoN10%/C-600 catalyst is 0.14 V vs. SCE. When the Co-Hdatrz complex weight percentage is increased to 20% and 30%, the onset potential shifts to 0.13 V and 0.09 V vs. SCE, respectively, and the limiting current density increases from 4.76 to 5.16 and then to 5.73 mA cm2. With the continued increase in the Co-Hdatrz complex weight percentage to 40%, a negative shift in the onset potential (0.13 V vs. SCE) and a decrease in the limiting current density (5.07 mA cm2) are observed, which indicate that the
Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269
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Fig. 6 e ORR polarization curves of (a) the Co-N30%/C-600 and (b) Pt/C catalysts in O2-saturated 0.1 M KOH solution without and with 1 M methanol at 5 mV s¡1 at 1600 rpm. (c) ORR polarization curves of the Co-N30%/C-600 and Pt/C catalysts before and after 10,000 potential cycles in O2-saturated 0.1 M KOH solution at 5 mV s¡1 at 1600 rpm. (d) OER polarization curves for the Co-N30%/C-600 catalyst in N2-saturated 0.1 M KOH solution before cycling and after 300 cycles at 5 mV s¡1 at 900 rpm. (e) Chronoamperometric responses for ORR on the Co-N30%/C-600 and Pt/C catalysts in O2-saturated 0.1 M KOH solution at ¡0.3 V vs. SCE at 1600 rpm. (f) Chronoamperometric responses for OER on the Co-N30%/C-600 and Pt/C catalysts in N2saturated 0.1 M KOH solution at 0.7 V vs. SCE at 900 rpm.
optimum Co-Hdatrz content (at which the optimum ORR activity occurs) is 30%. The ORR processes on the Pt/C and Co-N30%/C-600 catalysts were further studied by polarization curve measurements. Fig. 4c and e display ORR polarization curves of the Pt/C and CoN30%/C-600 catalysts at different rotation speeds. For the both catalysts, the current density increases with an increase in
rotation speed, which can be explained by a shortened diffusion distance at higher speeds. Moreover, the Co-N30%/C-600 catalyst exhibits higher ORR limiting current densities than those for Pt/C at the same rotation speed. Using the KouteckyLevich equation [37], KeL plots (J1 versus u1/2) were drawn at different potentials (0.4 V, 0.5 V, 0.6 V, 0.7 V and 0.8 V vs. SCE) (Fig. 4d and f). All the KeL plots show good linearity.
Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269
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Fig. 7 e TEM images of the Co-N30%/C-600 (a and b) and Pt/C (c and d) catalysts before and after 10000 potential cycles. The corresponding size frequency curves of Pt/C (e and f) before and after 10,000 potential cycles.
According to the slopes of the KeL plots, the electron-transfer number (n) on Pt/C is calculated to be 3.9, indicating that the ORR on Pt/C is a complete four-electron process, which is consistent well with the previous reports [38,39]. In the same way, n is determined to be 3.7, 3.7, 3.7, 3.9 and 3.8 on Co-N30%/ C-600 in the potential of 0.4 V, 0.5 V, 0.6 V, 0.7 V and 0.8 V vs. SCE, respectively. So, the ORR on Co-N30%/C-600 is similar with Pt/C and follows a one-step four-electron process. To in-depth understand ORR performance of the Co-N30%/ C-600 and Pt/C catalysts, diffusion-corrected Tafel plots were also investigated. As shown in Fig. 4g, Tafel plots for the Co-N30%/C-600 and Pt/C catalysts exhibit two different Tafel slopes in the low and high overpotentials regions. The calculated Tafel slopes of the Co-N30%/C-600 and Pt/C catalysts are 62.8 and 80.1 mV dec1 in the low overpotentials regions, respectively. In the high overpotentials regions, the Tafel slope values are 127.0 and 136.8 mV dec1, respectively, which are close to theoretical value of 120 mV dec1, indicating that the rate-determining step is the first electron transfer [40]. Because of overpotential increasing rapidly with current density, the higher Tafel slopes can result in lower ORR activity [40]. On the basis of Tafel plots, the exchange current densities of the Co-N30%/C-600 and Pt/C catalysts are 5.5 104 and 2.1 104 mA cm2, respectively. Hence, the ORR limiting current densities of the Co-N30%/C-600 catalyst are higher than those of Pt/C. In addition, the Co-N30%/C-600 catalyst also outperforms Pt/C with respect to the kinetic current density (Jk) at different electrode potentials (Fig. 4h), which make it a promising cathodic catalyst candidate in fuel cells and metalair batteries. The OER polarization curves of the Co-N30%/C catalysts synthesized at different thermal treatment temperatures and Pt/C were shown in Fig. 5a. It can be seen that the Co-N30%/ C-600 catalyst exhibits the best OER activity among the
Co-N30%/C catalysts, which is mainly attributed to the relatively high active N content and the synergy of CoO with carbon black. Moreover, the OER activity of the Co-N30%/C-600 catalyst is better than Pt/C. A small value of the potential difference (△E) between ORR potential (3 mA cm2) and OER potential (10 mA cm2) represents a high ORR and OER activity. The △E of the Co-N30%/C-500, Co-N30%/C-600, Co-N30%/ C-700 and Pt/C catalysts is 1.08 V, 1.02 V, 1.1 V and 1.1 V, respectively. The △E of the Co-N30%/C-600 catalyst is lower than those of the Co-N30%/C-500, Co-N30%/C-700 and Pt/C catalysts. The results indicate that the Co-N30%/C-600 catalyst possesses a high bifunctionality for catalyzing ORR and OER. The OER activity of the Co-N/C-600 catalysts synthesized with different Co-Hdatrz complex weight percentages were further examined. As displayed by Fig. 5b, increasing the Co-Hdatrz complex weight percentage from 10% to 20% and then to 30%, the anodic current increases from 19.9 to 22.7 and then to 32.9 mA cm2. If the Co-Hdatrz complex weight percentage is increased higher, OER activity begins to decrease, suggesting that 30% is the optimized Co-Hdatrz complex weight percentage. The difference in OER activity of the Co-N/C-600 catalysts can be explained by the electrochemical impedance spectroscopy (EIS) analysis. As shown in Fig. 5c, the Nyquist plots indicate that the charge transfer resistance of the Co-N/C-600 catalysts decreases in the following order: Co-N10%/ C-600 > Co-N20%/C-600 > Co-N40%/C-600 > Co-N30%/C-600. The smaller charge transfer resistance can lead to more efficient charge transport in the OER process [41]. Thus, the OER activity of the Co-N/C-600 catalysts increases in the following sequence: Co-N10%/C-600 < Co-N20%/C-600 < Co-N40%/ C-600 < Co-N30%/C-600. Owing to the smallest charge transfer resistance, the Co-N30%/C-600 catalyst exhibits the highest OER activity.
Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269
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Catalyst selectivity against methanol electrooxidation was also investigated. Fig. 6a shows ORR polarization curves of the Co-N30%/C-600 catalyst in O2-saturated 0.1 M KOH in the absence or presence of 1 M methanol. The similarity between the ORR polarization curves without and with 1 M methanol indicates that the Co-N30%/C-600 catalyst has a good ORR selectivity. In contrast, when using Pt/C modified electrode, the disappearance of ORR peak and the appearance of obvious methanol oxidation peak suggest that the ORR process is largely retarded by methanol oxidation (Fig. 6b). The improved methanol resistance indicates that the Co-N30%/C-600 catalyst may function as a methanol-resistant ORR catalyst in fuel cells. The long-term stability of the catalyst in the ORR and OER is an important concern for fuel cells, metal-air batteries and water splitting cells. Accelerated stability tests were used to investigate the stabilities of the Co-N30%/C-600 and Pt/C catalysts for ORR. As shown in Fig. 6c, after 10,000 continuous potential cycles, ORR half-wave potential of the Co-N30%/C-600 catalyst exhibits a 20 mV negative shift, while a 36 mV negative shift in half-wave potential is observed for Pt/C. Fig. 6d shows the OER stability of the Co-N30%/C-600 catalyst. The Co-N30%/C-600 catalyst displays almost unchangeable polarization curves and anodic currents before and after 300 cycles. The ORR and OER stabilities of the Co-N30%/C-600 and Pt/C catalysts were further investigated using chronoamperometric method. Fig. 6e shows that the Co-N30%/C-600 catalyst possesses high stability and ORR current density decreases only 8% after 10,000 s at 0.3 V vs. SCE. However, Pt/C exhibits a poor stability with a 51.8% decrease in ORR current density under the same operation condition. The Co-N30%/C-600 catalyst also exhibits excellent OER stability. As displayed by Fig. 6f, the decay in OER current density for the Co-N30%/C-600 catalyst is just 4.2% over 10,000 s continuous operation at 0.7 V vs. SCE. But the decay of 67.3% in OER current density is observed for Pt/C, indicating a more serious degradation. The results demonstrate that the Co-N30%/C-600 catalyst has superior ORR and OER stabilities compared to Pt/C. The composition is responsible for the high ORR stability of the Co-N30%/C-600 catalyst. TEM images of the Co-N30%/C-600 catalyst before and after 10,000 potential cycles (Fig. 7a and b) evidence that CoO nanoparticles do not produce obvious aggregation. This may be because CoO nanoparticles are uniformly encapsulated in carbon black and protected by carbon layers, they are stable in alkaline medium. For Pt/C, before the stability testing, Pt nanoparitcles with the average particle size of 2.1 nm are uniformly dispersed on the surface of carbon black (Fig. 7c and e). However, after the stability testing, the aggregation of Pt nanoparticles can be clearly observed and the average particle size of Pt nanoparticles increases to 8.5 nm (Fig. 7d and f). The loss of ORR activity and stability on Pt/C could be ascribed to the migration or aggregation of Pt nanoparticles caused by successive potential cycling [42].
Conclusions In summary, a novel non-precious metal bifunctional catalyst Co-N/C has been synthesized using Hdatrz as the Nitrogen
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precursor and CoCl2 as the metal precursor. The Co-Hdatrz complex weight percentage and thermal treatment temperature have a significant influence on the ORR and OER activity of the Co-N/C catalyst. Among the synthesized Co-N/C catalysts, the Co-N30%/C-600 catalyst shows the highest ORR activity with an onset potential of 0.09 V vs. SCE and the highest OER activity with the largest anodic current (32.9 mA cm2). More importantly, the potential difference (1.02 V) of the Co-N30%/C-600 catalyst is lower than Pt/C (1.10 V). The best ORR and OER activity of the Co-N30%/C-600 catalyst is mainly related with the highest active N content and the smallest charge transfer resistance as well as the synergy of CoO with carbon black. Additionally, the Co-N30%/ C-600 catalyst outperforms Pt/C in terms of anti-poisoning ability for methanol oxidation and long-term stability for ORR and OER. These properties indicate that the Co-N30%/ C-600 catalyst may be a promising non-precious metal bifunctional catalyst for energy related applications.
Acknowledgments This work was supported by the Doctoral Startup Fund of Xinxiang Medical University (505116).
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Please cite this article in press as: Chao S, Geng M, 3,5-Diamino-1,2,4-triazole as a Nitrogen precursor to synthesize highly efficient CoN/C non-precious metal bifunctional catalyst for oxygen reduction reaction and oxygen evolution reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.05.269