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The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation
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The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation Xiaomei Chang a,b, Fang Dong a, Fei Zha b, Zhicheng Tang a,* a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, and National Engineering Research Center for Fine Petrochemical Intermediates, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China b College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, 730070, China
highlights
graphical abstract
The 3D CNSs can be the excellent catalyst support for MOR. Ozone oxidation is an efficient method to introduce the oxygenated functional groups. The
enriched
carboxyl
groups
provide more deposition sites for PtCo on the 3D CNSs. Ozone oxidation is an efficient method to improve the catalytic efficiency of MOR.
article info
abstract
Article history:
In this paper, we perform three typical oxidation treatments, such as O3, HNO3 and H2O2, to
Received 11 November 2019
modify the surface structure of the novel three dimensional carbon nanosheets (3D CNSs).
Received in revised form
The different treatment generates kinds of oxygenated functional groups on the surface of
31 December 2019
support. One of them, the CNSs treated by ozone produces the greatest amount of carboxyl
Accepted 13 January 2020
groups and keeps the original morphology, which can be used as the excellent support for the
Available online xxx
electrochemical oxidation of methanol in acidic medium. Through various characterizations, it is discovered that the PtCo/CNSs-1 shows the excellent lamellar structure, good crystal-
Keywords:
linity, abundant pore structure and large interlayer spacing, which provide ultrahigh specific
PtCo alloy nanoparticle
surface area, superb electron and ionic conductivity. Furthermore, the superfine PtCo alloy
Methanol electro-oxidation reaction
nanoparticles evenly deposits on the 3D CNSs, which reveals superior electrocatalytic per-
Surface modification treatments
formance. More importantly, the capacity of anti-poisoning of which is enhanced obviously.
Oxygenated functional groups
© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Three-dimensional carbon nanosheets
* Corresponding author. E-mail addresses: [email protected], [email protected] (Z. Tang). https://doi.org/10.1016/j.ijhydene.2020.01.088 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Chang X et al., The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.088
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Introduction The environment pollution and energy shortage are the common topic of mankind to face. It is imperative to address these problems for the sustainable development of mankind. The efficient solution are exploiting the new energy and decreasing the emission of harmful substance[1]. It makes sense to do research and application for Direct Methanol Fuel Cell because it met the requirements very well[2]. It exhibits superior advantages, such as high power density, low pollution, higher security and low startup temperatures with respect to the conventional energy technologies[3]. At the same time, there exists some obstacles should be resolved in the extensive commercialization. The platinum mainly used as the active component in the noble metal catalysts, the high price and less reserve of Pt made the cost of catalysts prohibitively expensive. Pt is highly susceptible to CO poisoning which lowers the efficiency of catalyst[4]. Researchers have done extensively studies for augmenting the efficiency of catalyst. It was found that decreasing the size distribution of nanoparticles and modulating the morphology of catalysts are two effective ways to construct highly efficient catalysts. In addition, introducing the second noble metal or non-noble metal element into Pt-based catalysts to improve the activity and reduce the cost[5]. The bifunctional mechanism and electronic effects which resulted from the lattice contraction and downshift of the d-band center of Pt in the bimetallic structures will explain the improved activity[6]. While at present, Ru[7,8], Pd[9,10], Au [11], Ag [12], Cu [13] and the like are being studied mostly. PtCo alloy catalyst shows outstanding electrocatalytic performance and higher resistance to CO poisoning for methanol oxidation than pure Pt catalyst. In the meantime, Co is less expensive than noble metal Ru[14e16]. However, there is less work for Co catalyst than noble metal Ru in MOR, especially in the acid medium. So, it is necessary to do more detailed research on the PtCo alloy catalyst. The other important thing is to use the appropriate support or even perform the surface modification to produce the synergistic effect between support and metal nanoparticles. The nanoparticles anchored on the surface tightly and evenly will be benefit for the improvement of performance. The normally used carbon supports are carbon black[9], graphene [17], carbon fiber [18], carbon nanotubes [1], order mesoporous carbon [19] and so on. The researchers focus on onedimensional CNTs and two-dimensional graphene sheets for a long time because of their unique structures and properties. However, decreasing of surface area and deteriorating of electron transport are going to happen, because CNTs are apt to bundle and graphene sheets tend to stack irreversibly. As for the carbon fiber and order mesoporous carbon, they possessed the unsatisfactory conductivity because of low graphitization degree formed in the preparation process. The transition metal oxides are also used as catalyst support except the normally carbon support[20]. It is a pity that the conductivity is not as good as carbon material, though the ability of anti-poisoning is marvelous. So, it is significant to find and rational design novel catalyst support. The threedimensional carbon nanosheets with disordered, defective,
large interlayer spacing, high electron and ionic conductivity features is the promising support compared to traditional graphite. In this paper, the three-dimensional carbon nanosheets as raw support materials followed with different oxidation treatments, which further deposited PtCo alloy nanoparticles have been investigated in the MOR. The electrocatalytic activity and stability of catalysts were examined for getting a better insight into the relationship between the surface characteristic and catalytic performance of the CNSs and achieve efficient oxidation method of the CNSs in the MOR.
Experimental Synthesis of catalysts The 3D carbon nanosheets synthesized by the previous method reported in the paper[21]. Furthermore, we carried out the following treatments. (1) O3. The raw 3D carbon nanosheets were put into the tube furnace connected to an ozone generator and treated at 140 C for 30 min in the ozone atmosphere. We marked the sample as CNSs-1 (2) H2O2. The 50 mg pristine 3D carbon nanosheets were added into 50 ml 30% H2O2 solution at room temperature for 12 h under continuously magnetic stirring. The resulted solids were filtered, washed with deionized water, dried at 60 C in the vacuum for 3 h and crushed to obtain the CNSs-2. (3) HNO3. The 1 g untreated 3D carbon nanosheets were immersed into 100 ml HNO3 solution (67%) with sonicating for 30 min, and then refluxed at 100 C for 5 h. Finally, the product was dried for 12 h in vacuum at 60 C and denoted as CNSs-3. The pristine 3D carbon nanosheets and carbon black supported PtCo alloy as referenced samples to determine the performance of oxidized electrocatalysts. We marked the supports as pristine CNSs and pristine CB. Then the deposition of PtCo on support was performed. The oxidized and raw supports (20 mg) were dispersed into the ethylene glycol(90 ml) respectively with agitating in ultrasonic wave for 30 min. The pH of mixture was adjusted to 8 with NaOH solution. Additionally, 1 ml H2PtCl6 (38.6 mmol/L) and 10 mg cobalt acetate (C4H6CoO4) were added into the mixture, which was continued with magnetic stirring for 5 min at room temperature. Subsequently, this mixture had been heating reflux at 140 C for 3 h in the nitrogen (N2) atmosphere. While the reaction finished, the products were filtered and washed with ethanol and distilled water for several times, then dried in the vacuum oven for 3 h at 60 C. Finally, we obtained the five samples, PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/ CNSs-3 and PtCo/CB.
Physical characterizations of catalysts The structure and morphology of the catalysts were observed by the graphs got from Transmission electron microscope
Please cite this article as: Chang X et al., The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.088
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(JEOL JEM-2010 transmission electron microscope at 200 kV). The crystal structure of catalysts were explored by Xeray diffraction analysis, which was performed by a Rigaku D/ MAXeRB diffractometer with Cu Ka (l ¼ 0.154 nm; 60 kV and 55 mA) diffraction source. Raman spectra obtained by employed A RM 2000 microscope confocal Raman spectrometer with 532 nm laser. Fourier transform infrared spectroscopy (FT-IR) of samples was measured with KBr at room temperature on a Bruker IFS 66 spectrometer. X-ray photoelectron spectroscopy (XPS) was proceed on a VG ESCALAB 210 employing Mg Ka as X-ray source to analysis the surface composition and valence state. N2 adsorption-desorption isotherm was inspected by adsorbing N2 at 76.2 K on a Micromeritics ASAP 2020 determinator for knowing about the pore structure and specific surface area.
Electrochemical performance characterizations The CHI660E electrochemical workstation (Chenhua, Shanghai) with a typical three electrode system was used as assessment machine to test the electrochemical performance of catalysts. All the techniques were carried out in a N2saturated atmosphere at room temperature. A platinum wire and a saturated calomel electrode (SCE) were the counter and reference electrodes, respectively. A glassy carbon electrode (GCE, 3.0 mm diameter) polished sequentially with 0.3 and 0.05 mm alumina oxide powder before the modification by catalysts. Then it was cleaned with ethanol and deionized water by turns in the ultrasonic for 2 min and further coated with catalysts, which was used as the working electrode. Then the homogenous catalyst ink formed by dispersing 3.0 mg catalyst and 25 mL of 0.05% Nafion solution into 1 ml ethanol and agitated in ultrasonic wave for 30 min. The working electrode was readied by dropping 10 mL of the catalyst ink on the surface of GCE. Finally, made it dried in the room temperature naturally. The working electrode coated by catalyst was activated by cyclic voltammetry (CV) in 0.5 M H2SO4 solution. The test was performed in a potential range of 0.2 to 1.2 V (vs. SCE) with a scan rate of 50 mV s1. The electrocatalytic activities of catalysts for methanol oxidation were evaluated by CV in 0.5 M H2SO4 solution contained 1 M methanol. The potential range is 0e1.0 V (vs. SCE) with a scan rate of 50 mV s1, i-t curves was obtained at a potential of 0.65 V for 3000 s in the methanol solution. The electrochemical impedance spectroscopy (EIS) data was tested in form of Nyquist diagrams in a frequency range of 100 kHze0.1 Hz, in a constant potential mode at an amplitude of 5 mV. The recorded scanning rate of linear sweep voltammetry (LSV) measurement is 5 mV s1.
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on the intrinsic morphology by the O3 and H2O2 treatment. However, compared Fig. 1b with Fig. 1h, the carbon nanosheets treated by HNO3 could not maintain the primary morphology, but stacked irreversibly. The carbon black exhibited the morphology of nanospheres that non-uniform in size for comparing Fig. 1j with Fig. 1b. The ultrahigh specific surface area provided by the excellent morphology will be benefit for the efficient deposition of alloy nanoparticles. Just as we expected, Fig. 1a, c, e and g shows the ultrafine PtCo alloy nanoparticles anchored on the surface and edge of nanosheets without any agglomeration. Compared Fig. 1g with 1i, even if the HNO3 treatment made the nanosheeets stacked, the size of PtCo alloy nanoparticles on the CNSs-3 is still smaller than the PtCo alloy nanoparticles on the CB and so does the size of PtCo alloy nanoparticles on pristine CNSs. This is ascribed to the intrinsic superior structure of 3D nanosheets. From the size distribution graphs which exhibited in Fig. 1k-o, we know that the size of alloy nanoparticles supported on the CNSs is smaller than 2 nm. The increase of nanoparticles size of PtCo deposited on CNSs-3 verified the damage of structure. The size of PtCo alloy NPs deposited on the CB is much larger than on nanoparticles on the CNSs, which make a noticeable difference on the TEM images.
XRD analysis X-ray diffraction (XRD) analysis can be used to confirm the crystalline structure of materials[48,49]. Fig. 2 is the powder XRD patterns of catalysts. The all samples shows a broad peak at about 25 is the (002) crystal plane of graphite[22]. Except for the PtCo/CB, the else catalysts do not reveal the distinct peaks which can ascribe to the metal alloy nanoparticles. This may be due to the size of alloy nanoparticles is smaller than the 2 nm, which match well with the results of size distribution data obtained from TEM [23]. The PtCo/CB shows the characteristic peaks at 41.5 (111), 47.6 (200), 69.9 (220), 82.7 (311) [23,24]. The dark and red vertical lines represent the diffraction peaks of the pure Pt reflections (PDF card #65-2868) and pure Co reflections (PDF card #15-0806), respectively. The (111) facet of the PtCo nanoparticles is located between pure Pt and Co, indicating that Co is incorporated into the Pt fcc structure forming an alloy phase with a concomitant lattice contraction [25]. According to Bragg's Law, the interlayer spacing of PtCo/CNSs, PtCo/CNSs1 and PtCo/CNSs-2 are calculated to 0.354 nm[21]. The diffraction peak of PtCo/CNSs-3 shifts to a higher angle, which demonstrated the decrease of interlayer spacing resulted from stacking of carbon nanosheets.
Raman spectroscopy
Results and discussions Catalyst characterization TEM characterization In order to survey the morphology of prepared catalysts, TEM micrographs were obtained. From the TEM images shown in Fig. 1b, d, f, one intact and ultrathin lamellar structure is clearly observed. We see that there is no damage
Raman analysis was also performed to detect whether the degree of disorder of the carbon nanosheets were increased or not as an effect of the treatment conditions, which was showed in Fig. 3. There are two characteristic peaks at 1341 and 1595 cm1, corresponding to the D and G bands, which is associated with the disorder or sp3-hybridized carbon and the G band corresponds to the graphite in-plane vibration[26,27]. The degree of graphitization can be evaluated by the D-band to G-band intensity ratio in Raman spectra, where higher Dband intensity indicates a higher degree of defects or disorder
Please cite this article as: Chang X et al., The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.088
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Fig. 1 e Representative TEM images of catalysts with different magnifications and their size distributions. PtCo/CNSs(a, b, k), PtCo/CNSs-1 (c, d, l), PtCo/CNSs-2 (e, f, m), PtCo/CNSs-3 (g, h, n), PtCo/CB(i, j, o).
[28,29]. From Fig. 3, we know that after treating by three methods, all the degree of graphitization was reduced compared to the original CNSs. Especially, the ozone treatment produced the greatest degree of disorder and the HNO3 treatment followed. The carbon black has the most defective sites ascribed to the intrinsic structure.
FT-IR analysis Fig. 4 shows the FT-IR results of the catalysts, and all bonds are observed in the range of 500e4000 cm1. It does not appear the formation of new bond and the disappearance of old bond, it is demonstrated that the untreated carbon nanosheets and carbon black exists some oxygenated functional groups. The band at ca. 1059 cm1 is ascribed to the CeO stretching vibration of epoxy and alkoxy. The peak at ca. 1573 cm1 is the
CeC stretching vibration in the graphitized region and 1620 cm1 is the C¼C bending vibration. The peak at ca. 3430 cm1 and 1630 cm1 are the eOH stretching vibration and bending vibration (possibly from eCOOH and H2O). In addition, the peak at ca. 1728 cm1 is the C¼O stretching vibration and 2365 cm1 is the CO2 stretching vibration. Finally, the peaks at ca. 1454 and 2922, 2855 cm1are the eCH2e shear vibration and stretching vibration. Compared the sample of ozone oxidation with pristine carbon nanosheets, the intensity of peaks at ca. 3430 cm1 (-OH) and 1728 cm1 (C¼O) increased remarkably which shows an improvement in the quantity of carboxyl after the ozone treatment. The content of CeO (at ca. 1059 cm1) is also increased. However, the HNO3 oxidation mainly increase the content of C¼O (at ca. 1728 cm1) and the eCH2e (at ca. 2922 and 2855 cm1).
Please cite this article as: Chang X et al., The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.088
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BET measurement
Fig. 2 e The XRD patterns of PtCo/CNSs(a), PtCo/CNSs-1(b), PtCo/CNSs-2(c), PtCo/CNSs-3(d), PtCo/CB(e).
For the purpose of viewing the structure of pore and BET surface area, the N2 adsorption/desorption isotherms of catalysts are measured. The isotherms with H3 hysteresis loop observed in Fig. 5 are contributed to the layered structure and the loose assemblages of CNSs. The BET surface area of PtCo/ CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/CB was found to be 228.04, 229.57, 248.18, 16.45 and 125.58 m2 g-1, respectively. There is a pronounced diminution in the specific surface area for the CNSs treated by HNO3 due to the collapse of structure just like as exhibited in the TEM images. The specific surface area of untreated 3D CNSs is almost two times higher than the CB. At the same time, we know that the pore size of CNSs and CB is mainly distributed in 2e10 nm and about 10e50 nm from the pore size distribution graphs. The average pore diameter of PtCo/CNSs, PtCo/CNSs-1, PtCo/ CNSs-2, PtCo/CNSs-3, PtCo/CB was calculated to be 9.4, 11.6, 10.0, 11.1 and 17.6 nm, respectively. The small and uniform pore is conducive to restrain the alloy nanoparticles at the original position and prevent the agglomeration of them.
XPS analysis
Fig. 3 e The Raman spectra of PtCo/CNSs(a), PtCo/CNSs1(b), PtCo/CNSs-2(c), PtCo/CNSs-3(d), PtCo/CB(e).
So as to obtain more understanding on the surface chemical states for the effect of different treatment, XPS measurement is further performed[49,50]. Fig. 6a displays the C1s XPS peaks of catalysts with different pretreatment conditions. The peaks at 284.6, 285.9 and 289.4 eV belong to CeC bonds of the CNSs and CB, CeO, HOeC¼O bonding, individually[30,51]. It is found that the condition of oxidation treatment have an effect on the percentage of oxygen atom on the surface of the functionalized support. For example, O3 and HNO3 oxidation treatments produce higher amount of oxygenated functional groups on the CNSs, in comparison with pristine CNSs. The quantity of oxygen atoms in different chemical states on the functionalized supports are shown in Table 1. We found that the different oxidation treatment make difference on the amount of three types of oxygenated functional groups. In the study, O3 and HNO3 oxidation treatments introduce higher amount of oxygenated functional groups and especially carboxylic acid groups on the surface of the CNSs than that of those CNSs with another treatment methods. The XPS spectra of Pt4f, as show in Fig. 6b, could be deconvoluted into two pairs of doublets. The more intense doublet (71.2 and 74.2 eV) is due to metallic Pt (0). The less intense doublets (72.5 and 75.8eV) could be assigned to the Pt (II) species such as PtO or Pt (OH)2 [31]. With regard to the Co2p, the characteristic Co2p XPS spectra and their deconvolution to various cobalt oxidation states are displayed in Fig. 6c. The PtCo/ CNSs-1 and PtCo/CNSs-2 shows typical characteristic peaks, apart from the other three samples. It is demonstrated that the Co precursor reduced incompletely. It may have a connection with the performance of MOR. Initially cobalt Co3p (781.2 eV) and Co2p (786.8 eV) components, which can be attributed to the oxidation of metallic Co after the O2 exposure[16,32,33].
Electrochemical performance CV curve Fig. 4 e The FT-IR spectra of PtCo/CNSs(a), PtCo/CNSs-1(b), PtCo/CNSs-2(c), PtCo/CNSs-3(d), PtCo/CB(e).
The steady cyclic voltammetry curves of the catalysts in 0.5 M H2SO4 solution are presented in Fig. 7a, which exhibited
Please cite this article as: Chang X et al., The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.088
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Fig. 5 e Nitrogen adsorption-desorption isotherms of PtCo/CNSs(a), PtCo/CNSs-1(b), PtCo/CNSs-2(c), PtCo/CNSs-3(d), PtCo/ CB(e) and the inserted picture is the corresponding Pore-size distribution. The picture of magnification of pore-size distribution(f).
a pair of characteristic hydrogen adsorption/desorption peaks in the potential range of 0.2 to 0.1 V. The peak area can be obtained by integrating the charge in the hydrogen adsorption-desorption region of 0.2 to 0.1 V after subtracting the double layer contribution[8]. Then the electrochemical active surface area (ECSA)is calculated by the equation: ECSA ¼ QH/(MPt 0.21) where QH is the coulombic charge for hydrogen adsorption/desorption (mC cm2), MPt stands for the platinum loading (g cm2) in the electrode and 0.21 represents the charge required to oxidize a monolayer of hydrogen on bright Pt surface (mC cm2) [34e36]. We performed the CO stripping voltammograms to evaluate the electrochemical response of the catalysts for CO intermediates. The negative shift of onset potential means the easier oxidation of CO intermediates[37,38]. From Fig. 7b, we see that the catalysts of PtCo alloy nanoparticles supported on the CNSs with the similar onset potential and PtCo/CB with the much higher onset potential. It is demonstrated that the existence of favorable synergistic effect between PtCo alloy NPs and CNSs.
We obtained the electrochemical surface area (ECSA) of catalysts by integrating the electrical charge used in hydrogen adsorption and the stripping charge involved in CO oxidation. The ECSA calculated by two means are nearly with no difference. That also explained the accuracy of the two measuring methods. Our review of data, the PtCo/CNSs-1 with the highest peak current density possessed the largest ECSA. On one hand, the increased density of oxygenated functional groups provided more anchor sites for PtCo alloy nanoparticles, which lead to the uniform deposition. On the other hand, the smaller of particles size distribution contributed to the highest ECSA of PtCo/CNSs-1. The ECSA value of others catalysts are listed in Table 2. Fig. 7c shows the electrochemical responses of the catalysts in 0.5 M H2SO4 in the presence of methanol. The peak of methanol oxidation at about 0.65 V corresponds to forward anodic current peak (If), while the backward anodic current peak (Ib)at about 0.42 V can be responsible for the oxidation of the incompletely oxidized carbonaceous species generated in the forward sweep [39]. The PtCo/CNSs-1 shows the greatest
Please cite this article as: Chang X et al., The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.088
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Fig. 6 e C1s(a), Pt4f(b), Co2p(c) XPS spectra of PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/CB.
catalytic activity and the current density of which comes up to 85.30 mAcm2. The current density of PtCo/CNSs-2 is inferior to PtCo/CNSs-1. The activity of PtCo/CNSs-3 is not very satisfactory because of the damage of structure. However, the activity of PtCo/CB is better than the PtCo/CNSs, which attributed to the surface inert of pristine CNSs. As known, the If/Ib value is an necessary index to estimate the ability of antipoisoning toward CO-species for MOR, which is the ratio of the forward anodic peak current (If) to reverse one (Ib), where the higher ratio of If/Ib suggests a better poisoning resistance [40]. As shown in Table 2, the CNSs supported PtCo alloy nanoparticles have the similar resistance to poison due to the stronger adhesion of PtCo nanoparticles on the support. The PtCo/CB is not as good as the former. In addition, the CNSs supported PtCo alloy nanoparticles with the lower onset potential permit the reaction proceeding easily. This phenomenon will be ascribed to the synergistic effect between support and PtCo alloy. Fig. 7d presented the specific and mass activities of these five types of electrocatalysts, in which PtCo/ CNSs-1 exhibited the highest specific activity and mass activity, which is 6.16 times higher than that of the PtCo/CB catalyst respectively.
I-t curve The stability of catalysts is tested by the i-t curves. After 3000s tests, the PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3 and PtCo/CB shows a loss of 85%, 70%, 85%, 95% and 90%,
respectively (Fig. 7e). As seen from the data and graphs, the PtCo/CNSs-1 revealed the greatest retention rate and excellent long term stability. The catalysts contained Co species are unstable in the acidic surrounding, which restrict their application in MOR[32]. On the contrary, the PtCo/CNSs-1 shows extraordinary stability than the PtCo/CB in this work. Such a phenomenon is ascribed to the effective suppression of the Ostwald ripening effect when Co is incorporated into the crystal lattice of Pt[41]. In addition, the carbon nanosheets layers will protect the PtCo alloy nanoparticles from erosion by the acidic electrolyte and prevent PtCo alloy NPs from detaching and agglomerating because of the encapsulation of PtCo alloy NPs in the carbon layers, which just as we observed in the TEM images [42]. The highly stable electrochemical active surface is the one reason for the superior durability of the PtCo/CNSs-1. The high corrosion resistance of the support
Table 1 e Relative content of species in C1s peaks and O/C atomic ratio determined by XPS spectra of samples. Samples O/C
PtCo/CNSs PtCo/CNSs-1 PtCo/CNSs-2 PtCo/CNSs-3 PtCo/CB
Relative C1s peak area (%)
0.052 0.170 0.066 0.224 0.060
CeC
CeO
HOeC¼O
94.2 76.2 87.4 77.1 81.7
4.1 16.0 8.2 15.9 14.2
1.5 7.7 4.3 6.9 3.9
Please cite this article as: Chang X et al., The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.088
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Fig. 7 e (a) The Cyclic voltammograms of PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/CB catalyst-modified electrodes in 0.5 M H2SO4 at a scan rate of 50 mV s¡1. (b) CO-stripping of PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/CB in 0.5 M H2SO4 solution. (c) The Cyclic voltammograms of PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/ CB catalyst-modified electrodes in 0.5 M H2SO4 þ1 M methanol at a scan rate of 50 mV s¡1. (d) The corresponding specific and mass activities of PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/CB calculated from the peak currents in the positive scan. (e) The i-t curves of PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/CB catalyst-modified electrodes in 0.5 M H2SO4 þ1 M methanol.
Table 2 e Kinetic parameters for the MOR at selected potentials and electrode active surface area (ECSA) of the electrocatalysts. Catalyst PtCo/CNSs PtCo/CNSs-1 PtCo/CNSs-2 PtCo/CNSs-3 PtCo/CB
mAcm2 at 0.42 V
mAcm2 at 0.7 V
If/Ib
ECSACO m2g1
ECSAH m2g1
3.2 78.0 38.0 16.5 13.0
3.9 85.3 42.1 20.2 13.8
1.25 1.10 1.11 1.22 1.06
22.1 60.5 43.0 24.4 40.2
19.4 58.0 44.9 22.4 39.5
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is the other reason. The PtCo/CB reveals rapid decay, which means the deficiencies of the intrinsic structure of carbon black compare with 3D CNSs. The current density of PtCo/ CNSs-3 drop sharply with the worst stability.
Electrochemical impedance tests We carried out the Electrochemical Impedance Spectra (EIS) to explore the conductivity of catalysts, which shows in Fig. 8a [43,52]. The charge transfer resistance of the catalyst is evaluated by diameter of the primary semicircle. The smaller diameter demonstrates the faster transfer of charge in the methanol oxidation reaction[44]. We know that the catalysts of PtCo alloy nanoparticles deposited on the CNSs exhibit superior conductivity than the CB, which demonstrate the excellent structure of 3D CNSs. Such as large interlayer spacing and good crystallinity which both benefit for the transport of electron and ionic. Though the conductivity was reduced with producing more organic functional groups, it didn't make a huge difference. The conductivity of CNSs treated by HNO3 had the greatest reduction due to the decrease of crystallinity and interlayer spacing resulted from stacking. The equivalent circuit created by fitting the experimental data (inserted in Fig. 8a), where the RS represents the
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solution resistance, Cdl is the electrode double layer capacitance and RCT is the charge-transfer resistance[45]. According to the fitting results, which shows the electrolyte resistances (Rs) is 10.5, 8.6, 9.4, 0.9.9 and 8.0 U for the PtCo/CNSs, PtCo/ CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3 and PtCo/CB, respectively. The calculated charge transfer resistances (Rct) corresponded to them are about 641, 687, 656, 3510 and 800U. It can be easily noticed that the solution resistance can be ignored compared to the charge transfer resistance.
Tafel slope analysis Fig. 8b shows the Tafel plots based on polarization curves in the linear area. The exchange current density and Tafel slope for the cathodic process were evaluated using following equation:h ¼ a þ b log j. Where the a is exchange current density and the b is Tafel slope[17,46]. The fitted tafel slope of PtCo/CNSs-1 is smallest, which confirm the rapid oxidation kinetics for CO intermediates. The increased content of oxygenated functional groups promotes the consumption of CO intermediates. Just as the papers reported that the oxygenated functional groups act as the Ru[47]. The emptied Pt sites are helpful for the following adsorption of methanol molecules. This also increases the rate of methanol oxidation. Toward the PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, the tafel slope of them increased as the carboxyl content in these three treatment methods decreases. However, the PtCo/CNSs shows the relatively slow methanol oxidation kinetics because of the severe stacking of the CNSs layers. The PtCo/CB possessed the slowest methanol oxidation kinetics due to the unsatisfied morphology of support.
Conclusions In summary, we found a promising catalyst support for methanol electrooxidation with excellent structure characteristic. To overcome the problem of surface inert, we perform the three typical oxidation treatments to modify the surface structure. We found that O3 and HNO3 treatment produced more numerous carboxyl groups which act as the anchor sites for PtCo alloy nanoparticles. However, the HNO3 do irreversibly damage to the structure because of the strong acidity. What is more, the other two methods always accompanied with complicated post treatments. As a result, the ultrafine and homogenous PtCo alloy NPs supported on the 3D CNSs-1. Because of the confined space of the relative small cavities and the protection of carbon layer, the PtCo alloy nanoparticles avoid detachment and agglomeration. The PtCo/ CNSs-1 shows superb electrocatalytic activity and stability compared to PtCo/CB.
Acknowledgment Fig. 8 e (a) The EIS spectras of PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/CB catalyst-modified electrodes in 0.5 M H2SO4 þ1 M methanol. The inserted picture is the equivalent circuit diagram. (b) Tafel plots of PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/ CB electrode in 0.5 M H2SO4 þ1 M methanol.
This work was supported by the National Natural Science Foundation of China (51908535, 21707145, 51808529), Natural Science Foundation of Gansu Province (18JR3RA383), Key Science and Technology Program of Lanzhou City (2018-RC-65), and CAS “Light of West China” (Program 2016-B).
Please cite this article as: Chang X et al., The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.088
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Available online at www.sciencedirect.com
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The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation Xiaomei Chang a,b, Fang Dong a, Fei Zha b, Zhicheng Tang a,* a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, and National Engineering Research Center for Fine Petrochemical Intermediates, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China b College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, 730070, China
highlights
graphical abstract
The 3D CNSs can be the excellent catalyst support for MOR. Ozone oxidation is an efficient method to introduce the oxygenated functional groups. The
enriched
carboxyl
groups
provide more deposition sites for PtCo on the 3D CNSs. Ozone oxidation is an efficient method to improve the catalytic efficiency of MOR.
article info
abstract
Article history:
In this paper, we perform three typical oxidation treatments, such as O3, HNO3 and H2O2, to
Received 11 November 2019
modify the surface structure of the novel three dimensional carbon nanosheets (3D CNSs).
Received in revised form
The different treatment generates kinds of oxygenated functional groups on the surface of
31 December 2019
support. One of them, the CNSs treated by ozone produces the greatest amount of carboxyl
Accepted 13 January 2020
groups and keeps the original morphology, which can be used as the excellent support for the
Available online xxx
electrochemical oxidation of methanol in acidic medium. Through various characterizations, it is discovered that the PtCo/CNSs-1 shows the excellent lamellar structure, good crystal-
Keywords:
linity, abundant pore structure and large interlayer spacing, which provide ultrahigh specific
PtCo alloy nanoparticle
surface area, superb electron and ionic conductivity. Furthermore, the superfine PtCo alloy
Methanol electro-oxidation reaction
nanoparticles evenly deposits on the 3D CNSs, which reveals superior electrocatalytic per-
Surface modification treatments
formance. More importantly, the capacity of anti-poisoning of which is enhanced obviously.
Oxygenated functional groups
© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Three-dimensional carbon nanosheets
* Corresponding author. E-mail addresses: [email protected], [email protected] (Z. Tang). https://doi.org/10.1016/j.ijhydene.2020.01.088 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Chang X et al., The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.088
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Introduction The environment pollution and energy shortage are the common topic of mankind to face. It is imperative to address these problems for the sustainable development of mankind. The efficient solution are exploiting the new energy and decreasing the emission of harmful substance[1]. It makes sense to do research and application for Direct Methanol Fuel Cell because it met the requirements very well[2]. It exhibits superior advantages, such as high power density, low pollution, higher security and low startup temperatures with respect to the conventional energy technologies[3]. At the same time, there exists some obstacles should be resolved in the extensive commercialization. The platinum mainly used as the active component in the noble metal catalysts, the high price and less reserve of Pt made the cost of catalysts prohibitively expensive. Pt is highly susceptible to CO poisoning which lowers the efficiency of catalyst[4]. Researchers have done extensively studies for augmenting the efficiency of catalyst. It was found that decreasing the size distribution of nanoparticles and modulating the morphology of catalysts are two effective ways to construct highly efficient catalysts. In addition, introducing the second noble metal or non-noble metal element into Pt-based catalysts to improve the activity and reduce the cost[5]. The bifunctional mechanism and electronic effects which resulted from the lattice contraction and downshift of the d-band center of Pt in the bimetallic structures will explain the improved activity[6]. While at present, Ru[7,8], Pd[9,10], Au [11], Ag [12], Cu [13] and the like are being studied mostly. PtCo alloy catalyst shows outstanding electrocatalytic performance and higher resistance to CO poisoning for methanol oxidation than pure Pt catalyst. In the meantime, Co is less expensive than noble metal Ru[14e16]. However, there is less work for Co catalyst than noble metal Ru in MOR, especially in the acid medium. So, it is necessary to do more detailed research on the PtCo alloy catalyst. The other important thing is to use the appropriate support or even perform the surface modification to produce the synergistic effect between support and metal nanoparticles. The nanoparticles anchored on the surface tightly and evenly will be benefit for the improvement of performance. The normally used carbon supports are carbon black[9], graphene [17], carbon fiber [18], carbon nanotubes [1], order mesoporous carbon [19] and so on. The researchers focus on onedimensional CNTs and two-dimensional graphene sheets for a long time because of their unique structures and properties. However, decreasing of surface area and deteriorating of electron transport are going to happen, because CNTs are apt to bundle and graphene sheets tend to stack irreversibly. As for the carbon fiber and order mesoporous carbon, they possessed the unsatisfactory conductivity because of low graphitization degree formed in the preparation process. The transition metal oxides are also used as catalyst support except the normally carbon support[20]. It is a pity that the conductivity is not as good as carbon material, though the ability of anti-poisoning is marvelous. So, it is significant to find and rational design novel catalyst support. The threedimensional carbon nanosheets with disordered, defective,
large interlayer spacing, high electron and ionic conductivity features is the promising support compared to traditional graphite. In this paper, the three-dimensional carbon nanosheets as raw support materials followed with different oxidation treatments, which further deposited PtCo alloy nanoparticles have been investigated in the MOR. The electrocatalytic activity and stability of catalysts were examined for getting a better insight into the relationship between the surface characteristic and catalytic performance of the CNSs and achieve efficient oxidation method of the CNSs in the MOR.
Experimental Synthesis of catalysts The 3D carbon nanosheets synthesized by the previous method reported in the paper[21]. Furthermore, we carried out the following treatments. (1) O3. The raw 3D carbon nanosheets were put into the tube furnace connected to an ozone generator and treated at 140 C for 30 min in the ozone atmosphere. We marked the sample as CNSs-1 (2) H2O2. The 50 mg pristine 3D carbon nanosheets were added into 50 ml 30% H2O2 solution at room temperature for 12 h under continuously magnetic stirring. The resulted solids were filtered, washed with deionized water, dried at 60 C in the vacuum for 3 h and crushed to obtain the CNSs-2. (3) HNO3. The 1 g untreated 3D carbon nanosheets were immersed into 100 ml HNO3 solution (67%) with sonicating for 30 min, and then refluxed at 100 C for 5 h. Finally, the product was dried for 12 h in vacuum at 60 C and denoted as CNSs-3. The pristine 3D carbon nanosheets and carbon black supported PtCo alloy as referenced samples to determine the performance of oxidized electrocatalysts. We marked the supports as pristine CNSs and pristine CB. Then the deposition of PtCo on support was performed. The oxidized and raw supports (20 mg) were dispersed into the ethylene glycol(90 ml) respectively with agitating in ultrasonic wave for 30 min. The pH of mixture was adjusted to 8 with NaOH solution. Additionally, 1 ml H2PtCl6 (38.6 mmol/L) and 10 mg cobalt acetate (C4H6CoO4) were added into the mixture, which was continued with magnetic stirring for 5 min at room temperature. Subsequently, this mixture had been heating reflux at 140 C for 3 h in the nitrogen (N2) atmosphere. While the reaction finished, the products were filtered and washed with ethanol and distilled water for several times, then dried in the vacuum oven for 3 h at 60 C. Finally, we obtained the five samples, PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/ CNSs-3 and PtCo/CB.
Physical characterizations of catalysts The structure and morphology of the catalysts were observed by the graphs got from Transmission electron microscope
Please cite this article as: Chang X et al., The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.088
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(JEOL JEM-2010 transmission electron microscope at 200 kV). The crystal structure of catalysts were explored by Xeray diffraction analysis, which was performed by a Rigaku D/ MAXeRB diffractometer with Cu Ka (l ¼ 0.154 nm; 60 kV and 55 mA) diffraction source. Raman spectra obtained by employed A RM 2000 microscope confocal Raman spectrometer with 532 nm laser. Fourier transform infrared spectroscopy (FT-IR) of samples was measured with KBr at room temperature on a Bruker IFS 66 spectrometer. X-ray photoelectron spectroscopy (XPS) was proceed on a VG ESCALAB 210 employing Mg Ka as X-ray source to analysis the surface composition and valence state. N2 adsorption-desorption isotherm was inspected by adsorbing N2 at 76.2 K on a Micromeritics ASAP 2020 determinator for knowing about the pore structure and specific surface area.
Electrochemical performance characterizations The CHI660E electrochemical workstation (Chenhua, Shanghai) with a typical three electrode system was used as assessment machine to test the electrochemical performance of catalysts. All the techniques were carried out in a N2saturated atmosphere at room temperature. A platinum wire and a saturated calomel electrode (SCE) were the counter and reference electrodes, respectively. A glassy carbon electrode (GCE, 3.0 mm diameter) polished sequentially with 0.3 and 0.05 mm alumina oxide powder before the modification by catalysts. Then it was cleaned with ethanol and deionized water by turns in the ultrasonic for 2 min and further coated with catalysts, which was used as the working electrode. Then the homogenous catalyst ink formed by dispersing 3.0 mg catalyst and 25 mL of 0.05% Nafion solution into 1 ml ethanol and agitated in ultrasonic wave for 30 min. The working electrode was readied by dropping 10 mL of the catalyst ink on the surface of GCE. Finally, made it dried in the room temperature naturally. The working electrode coated by catalyst was activated by cyclic voltammetry (CV) in 0.5 M H2SO4 solution. The test was performed in a potential range of 0.2 to 1.2 V (vs. SCE) with a scan rate of 50 mV s1. The electrocatalytic activities of catalysts for methanol oxidation were evaluated by CV in 0.5 M H2SO4 solution contained 1 M methanol. The potential range is 0e1.0 V (vs. SCE) with a scan rate of 50 mV s1, i-t curves was obtained at a potential of 0.65 V for 3000 s in the methanol solution. The electrochemical impedance spectroscopy (EIS) data was tested in form of Nyquist diagrams in a frequency range of 100 kHze0.1 Hz, in a constant potential mode at an amplitude of 5 mV. The recorded scanning rate of linear sweep voltammetry (LSV) measurement is 5 mV s1.
3
on the intrinsic morphology by the O3 and H2O2 treatment. However, compared Fig. 1b with Fig. 1h, the carbon nanosheets treated by HNO3 could not maintain the primary morphology, but stacked irreversibly. The carbon black exhibited the morphology of nanospheres that non-uniform in size for comparing Fig. 1j with Fig. 1b. The ultrahigh specific surface area provided by the excellent morphology will be benefit for the efficient deposition of alloy nanoparticles. Just as we expected, Fig. 1a, c, e and g shows the ultrafine PtCo alloy nanoparticles anchored on the surface and edge of nanosheets without any agglomeration. Compared Fig. 1g with 1i, even if the HNO3 treatment made the nanosheeets stacked, the size of PtCo alloy nanoparticles on the CNSs-3 is still smaller than the PtCo alloy nanoparticles on the CB and so does the size of PtCo alloy nanoparticles on pristine CNSs. This is ascribed to the intrinsic superior structure of 3D nanosheets. From the size distribution graphs which exhibited in Fig. 1k-o, we know that the size of alloy nanoparticles supported on the CNSs is smaller than 2 nm. The increase of nanoparticles size of PtCo deposited on CNSs-3 verified the damage of structure. The size of PtCo alloy NPs deposited on the CB is much larger than on nanoparticles on the CNSs, which make a noticeable difference on the TEM images.
XRD analysis X-ray diffraction (XRD) analysis can be used to confirm the crystalline structure of materials[48,49]. Fig. 2 is the powder XRD patterns of catalysts. The all samples shows a broad peak at about 25 is the (002) crystal plane of graphite[22]. Except for the PtCo/CB, the else catalysts do not reveal the distinct peaks which can ascribe to the metal alloy nanoparticles. This may be due to the size of alloy nanoparticles is smaller than the 2 nm, which match well with the results of size distribution data obtained from TEM [23]. The PtCo/CB shows the characteristic peaks at 41.5 (111), 47.6 (200), 69.9 (220), 82.7 (311) [23,24]. The dark and red vertical lines represent the diffraction peaks of the pure Pt reflections (PDF card #65-2868) and pure Co reflections (PDF card #15-0806), respectively. The (111) facet of the PtCo nanoparticles is located between pure Pt and Co, indicating that Co is incorporated into the Pt fcc structure forming an alloy phase with a concomitant lattice contraction [25]. According to Bragg's Law, the interlayer spacing of PtCo/CNSs, PtCo/CNSs1 and PtCo/CNSs-2 are calculated to 0.354 nm[21]. The diffraction peak of PtCo/CNSs-3 shifts to a higher angle, which demonstrated the decrease of interlayer spacing resulted from stacking of carbon nanosheets.
Raman spectroscopy
Results and discussions Catalyst characterization TEM characterization In order to survey the morphology of prepared catalysts, TEM micrographs were obtained. From the TEM images shown in Fig. 1b, d, f, one intact and ultrathin lamellar structure is clearly observed. We see that there is no damage
Raman analysis was also performed to detect whether the degree of disorder of the carbon nanosheets were increased or not as an effect of the treatment conditions, which was showed in Fig. 3. There are two characteristic peaks at 1341 and 1595 cm1, corresponding to the D and G bands, which is associated with the disorder or sp3-hybridized carbon and the G band corresponds to the graphite in-plane vibration[26,27]. The degree of graphitization can be evaluated by the D-band to G-band intensity ratio in Raman spectra, where higher Dband intensity indicates a higher degree of defects or disorder
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Fig. 1 e Representative TEM images of catalysts with different magnifications and their size distributions. PtCo/CNSs(a, b, k), PtCo/CNSs-1 (c, d, l), PtCo/CNSs-2 (e, f, m), PtCo/CNSs-3 (g, h, n), PtCo/CB(i, j, o).
[28,29]. From Fig. 3, we know that after treating by three methods, all the degree of graphitization was reduced compared to the original CNSs. Especially, the ozone treatment produced the greatest degree of disorder and the HNO3 treatment followed. The carbon black has the most defective sites ascribed to the intrinsic structure.
FT-IR analysis Fig. 4 shows the FT-IR results of the catalysts, and all bonds are observed in the range of 500e4000 cm1. It does not appear the formation of new bond and the disappearance of old bond, it is demonstrated that the untreated carbon nanosheets and carbon black exists some oxygenated functional groups. The band at ca. 1059 cm1 is ascribed to the CeO stretching vibration of epoxy and alkoxy. The peak at ca. 1573 cm1 is the
CeC stretching vibration in the graphitized region and 1620 cm1 is the C¼C bending vibration. The peak at ca. 3430 cm1 and 1630 cm1 are the eOH stretching vibration and bending vibration (possibly from eCOOH and H2O). In addition, the peak at ca. 1728 cm1 is the C¼O stretching vibration and 2365 cm1 is the CO2 stretching vibration. Finally, the peaks at ca. 1454 and 2922, 2855 cm1are the eCH2e shear vibration and stretching vibration. Compared the sample of ozone oxidation with pristine carbon nanosheets, the intensity of peaks at ca. 3430 cm1 (-OH) and 1728 cm1 (C¼O) increased remarkably which shows an improvement in the quantity of carboxyl after the ozone treatment. The content of CeO (at ca. 1059 cm1) is also increased. However, the HNO3 oxidation mainly increase the content of C¼O (at ca. 1728 cm1) and the eCH2e (at ca. 2922 and 2855 cm1).
Please cite this article as: Chang X et al., The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.088
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BET measurement
Fig. 2 e The XRD patterns of PtCo/CNSs(a), PtCo/CNSs-1(b), PtCo/CNSs-2(c), PtCo/CNSs-3(d), PtCo/CB(e).
For the purpose of viewing the structure of pore and BET surface area, the N2 adsorption/desorption isotherms of catalysts are measured. The isotherms with H3 hysteresis loop observed in Fig. 5 are contributed to the layered structure and the loose assemblages of CNSs. The BET surface area of PtCo/ CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/CB was found to be 228.04, 229.57, 248.18, 16.45 and 125.58 m2 g-1, respectively. There is a pronounced diminution in the specific surface area for the CNSs treated by HNO3 due to the collapse of structure just like as exhibited in the TEM images. The specific surface area of untreated 3D CNSs is almost two times higher than the CB. At the same time, we know that the pore size of CNSs and CB is mainly distributed in 2e10 nm and about 10e50 nm from the pore size distribution graphs. The average pore diameter of PtCo/CNSs, PtCo/CNSs-1, PtCo/ CNSs-2, PtCo/CNSs-3, PtCo/CB was calculated to be 9.4, 11.6, 10.0, 11.1 and 17.6 nm, respectively. The small and uniform pore is conducive to restrain the alloy nanoparticles at the original position and prevent the agglomeration of them.
XPS analysis
Fig. 3 e The Raman spectra of PtCo/CNSs(a), PtCo/CNSs1(b), PtCo/CNSs-2(c), PtCo/CNSs-3(d), PtCo/CB(e).
So as to obtain more understanding on the surface chemical states for the effect of different treatment, XPS measurement is further performed[49,50]. Fig. 6a displays the C1s XPS peaks of catalysts with different pretreatment conditions. The peaks at 284.6, 285.9 and 289.4 eV belong to CeC bonds of the CNSs and CB, CeO, HOeC¼O bonding, individually[30,51]. It is found that the condition of oxidation treatment have an effect on the percentage of oxygen atom on the surface of the functionalized support. For example, O3 and HNO3 oxidation treatments produce higher amount of oxygenated functional groups on the CNSs, in comparison with pristine CNSs. The quantity of oxygen atoms in different chemical states on the functionalized supports are shown in Table 1. We found that the different oxidation treatment make difference on the amount of three types of oxygenated functional groups. In the study, O3 and HNO3 oxidation treatments introduce higher amount of oxygenated functional groups and especially carboxylic acid groups on the surface of the CNSs than that of those CNSs with another treatment methods. The XPS spectra of Pt4f, as show in Fig. 6b, could be deconvoluted into two pairs of doublets. The more intense doublet (71.2 and 74.2 eV) is due to metallic Pt (0). The less intense doublets (72.5 and 75.8eV) could be assigned to the Pt (II) species such as PtO or Pt (OH)2 [31]. With regard to the Co2p, the characteristic Co2p XPS spectra and their deconvolution to various cobalt oxidation states are displayed in Fig. 6c. The PtCo/ CNSs-1 and PtCo/CNSs-2 shows typical characteristic peaks, apart from the other three samples. It is demonstrated that the Co precursor reduced incompletely. It may have a connection with the performance of MOR. Initially cobalt Co3p (781.2 eV) and Co2p (786.8 eV) components, which can be attributed to the oxidation of metallic Co after the O2 exposure[16,32,33].
Electrochemical performance CV curve Fig. 4 e The FT-IR spectra of PtCo/CNSs(a), PtCo/CNSs-1(b), PtCo/CNSs-2(c), PtCo/CNSs-3(d), PtCo/CB(e).
The steady cyclic voltammetry curves of the catalysts in 0.5 M H2SO4 solution are presented in Fig. 7a, which exhibited
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Fig. 5 e Nitrogen adsorption-desorption isotherms of PtCo/CNSs(a), PtCo/CNSs-1(b), PtCo/CNSs-2(c), PtCo/CNSs-3(d), PtCo/ CB(e) and the inserted picture is the corresponding Pore-size distribution. The picture of magnification of pore-size distribution(f).
a pair of characteristic hydrogen adsorption/desorption peaks in the potential range of 0.2 to 0.1 V. The peak area can be obtained by integrating the charge in the hydrogen adsorption-desorption region of 0.2 to 0.1 V after subtracting the double layer contribution[8]. Then the electrochemical active surface area (ECSA)is calculated by the equation: ECSA ¼ QH/(MPt 0.21) where QH is the coulombic charge for hydrogen adsorption/desorption (mC cm2), MPt stands for the platinum loading (g cm2) in the electrode and 0.21 represents the charge required to oxidize a monolayer of hydrogen on bright Pt surface (mC cm2) [34e36]. We performed the CO stripping voltammograms to evaluate the electrochemical response of the catalysts for CO intermediates. The negative shift of onset potential means the easier oxidation of CO intermediates[37,38]. From Fig. 7b, we see that the catalysts of PtCo alloy nanoparticles supported on the CNSs with the similar onset potential and PtCo/CB with the much higher onset potential. It is demonstrated that the existence of favorable synergistic effect between PtCo alloy NPs and CNSs.
We obtained the electrochemical surface area (ECSA) of catalysts by integrating the electrical charge used in hydrogen adsorption and the stripping charge involved in CO oxidation. The ECSA calculated by two means are nearly with no difference. That also explained the accuracy of the two measuring methods. Our review of data, the PtCo/CNSs-1 with the highest peak current density possessed the largest ECSA. On one hand, the increased density of oxygenated functional groups provided more anchor sites for PtCo alloy nanoparticles, which lead to the uniform deposition. On the other hand, the smaller of particles size distribution contributed to the highest ECSA of PtCo/CNSs-1. The ECSA value of others catalysts are listed in Table 2. Fig. 7c shows the electrochemical responses of the catalysts in 0.5 M H2SO4 in the presence of methanol. The peak of methanol oxidation at about 0.65 V corresponds to forward anodic current peak (If), while the backward anodic current peak (Ib)at about 0.42 V can be responsible for the oxidation of the incompletely oxidized carbonaceous species generated in the forward sweep [39]. The PtCo/CNSs-1 shows the greatest
Please cite this article as: Chang X et al., The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.088
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Fig. 6 e C1s(a), Pt4f(b), Co2p(c) XPS spectra of PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/CB.
catalytic activity and the current density of which comes up to 85.30 mAcm2. The current density of PtCo/CNSs-2 is inferior to PtCo/CNSs-1. The activity of PtCo/CNSs-3 is not very satisfactory because of the damage of structure. However, the activity of PtCo/CB is better than the PtCo/CNSs, which attributed to the surface inert of pristine CNSs. As known, the If/Ib value is an necessary index to estimate the ability of antipoisoning toward CO-species for MOR, which is the ratio of the forward anodic peak current (If) to reverse one (Ib), where the higher ratio of If/Ib suggests a better poisoning resistance [40]. As shown in Table 2, the CNSs supported PtCo alloy nanoparticles have the similar resistance to poison due to the stronger adhesion of PtCo nanoparticles on the support. The PtCo/CB is not as good as the former. In addition, the CNSs supported PtCo alloy nanoparticles with the lower onset potential permit the reaction proceeding easily. This phenomenon will be ascribed to the synergistic effect between support and PtCo alloy. Fig. 7d presented the specific and mass activities of these five types of electrocatalysts, in which PtCo/ CNSs-1 exhibited the highest specific activity and mass activity, which is 6.16 times higher than that of the PtCo/CB catalyst respectively.
I-t curve The stability of catalysts is tested by the i-t curves. After 3000s tests, the PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3 and PtCo/CB shows a loss of 85%, 70%, 85%, 95% and 90%,
respectively (Fig. 7e). As seen from the data and graphs, the PtCo/CNSs-1 revealed the greatest retention rate and excellent long term stability. The catalysts contained Co species are unstable in the acidic surrounding, which restrict their application in MOR[32]. On the contrary, the PtCo/CNSs-1 shows extraordinary stability than the PtCo/CB in this work. Such a phenomenon is ascribed to the effective suppression of the Ostwald ripening effect when Co is incorporated into the crystal lattice of Pt[41]. In addition, the carbon nanosheets layers will protect the PtCo alloy nanoparticles from erosion by the acidic electrolyte and prevent PtCo alloy NPs from detaching and agglomerating because of the encapsulation of PtCo alloy NPs in the carbon layers, which just as we observed in the TEM images [42]. The highly stable electrochemical active surface is the one reason for the superior durability of the PtCo/CNSs-1. The high corrosion resistance of the support
Table 1 e Relative content of species in C1s peaks and O/C atomic ratio determined by XPS spectra of samples. Samples O/C
PtCo/CNSs PtCo/CNSs-1 PtCo/CNSs-2 PtCo/CNSs-3 PtCo/CB
Relative C1s peak area (%)
0.052 0.170 0.066 0.224 0.060
CeC
CeO
HOeC¼O
94.2 76.2 87.4 77.1 81.7
4.1 16.0 8.2 15.9 14.2
1.5 7.7 4.3 6.9 3.9
Please cite this article as: Chang X et al., The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.088
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Fig. 7 e (a) The Cyclic voltammograms of PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/CB catalyst-modified electrodes in 0.5 M H2SO4 at a scan rate of 50 mV s¡1. (b) CO-stripping of PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/CB in 0.5 M H2SO4 solution. (c) The Cyclic voltammograms of PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/ CB catalyst-modified electrodes in 0.5 M H2SO4 þ1 M methanol at a scan rate of 50 mV s¡1. (d) The corresponding specific and mass activities of PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/CB calculated from the peak currents in the positive scan. (e) The i-t curves of PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/CB catalyst-modified electrodes in 0.5 M H2SO4 þ1 M methanol.
Table 2 e Kinetic parameters for the MOR at selected potentials and electrode active surface area (ECSA) of the electrocatalysts. Catalyst PtCo/CNSs PtCo/CNSs-1 PtCo/CNSs-2 PtCo/CNSs-3 PtCo/CB
mAcm2 at 0.42 V
mAcm2 at 0.7 V
If/Ib
ECSACO m2g1
ECSAH m2g1
3.2 78.0 38.0 16.5 13.0
3.9 85.3 42.1 20.2 13.8
1.25 1.10 1.11 1.22 1.06
22.1 60.5 43.0 24.4 40.2
19.4 58.0 44.9 22.4 39.5
Please cite this article as: Chang X et al., The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.088
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is the other reason. The PtCo/CB reveals rapid decay, which means the deficiencies of the intrinsic structure of carbon black compare with 3D CNSs. The current density of PtCo/ CNSs-3 drop sharply with the worst stability.
Electrochemical impedance tests We carried out the Electrochemical Impedance Spectra (EIS) to explore the conductivity of catalysts, which shows in Fig. 8a [43,52]. The charge transfer resistance of the catalyst is evaluated by diameter of the primary semicircle. The smaller diameter demonstrates the faster transfer of charge in the methanol oxidation reaction[44]. We know that the catalysts of PtCo alloy nanoparticles deposited on the CNSs exhibit superior conductivity than the CB, which demonstrate the excellent structure of 3D CNSs. Such as large interlayer spacing and good crystallinity which both benefit for the transport of electron and ionic. Though the conductivity was reduced with producing more organic functional groups, it didn't make a huge difference. The conductivity of CNSs treated by HNO3 had the greatest reduction due to the decrease of crystallinity and interlayer spacing resulted from stacking. The equivalent circuit created by fitting the experimental data (inserted in Fig. 8a), where the RS represents the
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solution resistance, Cdl is the electrode double layer capacitance and RCT is the charge-transfer resistance[45]. According to the fitting results, which shows the electrolyte resistances (Rs) is 10.5, 8.6, 9.4, 0.9.9 and 8.0 U for the PtCo/CNSs, PtCo/ CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3 and PtCo/CB, respectively. The calculated charge transfer resistances (Rct) corresponded to them are about 641, 687, 656, 3510 and 800U. It can be easily noticed that the solution resistance can be ignored compared to the charge transfer resistance.
Tafel slope analysis Fig. 8b shows the Tafel plots based on polarization curves in the linear area. The exchange current density and Tafel slope for the cathodic process were evaluated using following equation:h ¼ a þ b log j. Where the a is exchange current density and the b is Tafel slope[17,46]. The fitted tafel slope of PtCo/CNSs-1 is smallest, which confirm the rapid oxidation kinetics for CO intermediates. The increased content of oxygenated functional groups promotes the consumption of CO intermediates. Just as the papers reported that the oxygenated functional groups act as the Ru[47]. The emptied Pt sites are helpful for the following adsorption of methanol molecules. This also increases the rate of methanol oxidation. Toward the PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, the tafel slope of them increased as the carboxyl content in these three treatment methods decreases. However, the PtCo/CNSs shows the relatively slow methanol oxidation kinetics because of the severe stacking of the CNSs layers. The PtCo/CB possessed the slowest methanol oxidation kinetics due to the unsatisfied morphology of support.
Conclusions In summary, we found a promising catalyst support for methanol electrooxidation with excellent structure characteristic. To overcome the problem of surface inert, we perform the three typical oxidation treatments to modify the surface structure. We found that O3 and HNO3 treatment produced more numerous carboxyl groups which act as the anchor sites for PtCo alloy nanoparticles. However, the HNO3 do irreversibly damage to the structure because of the strong acidity. What is more, the other two methods always accompanied with complicated post treatments. As a result, the ultrafine and homogenous PtCo alloy NPs supported on the 3D CNSs-1. Because of the confined space of the relative small cavities and the protection of carbon layer, the PtCo alloy nanoparticles avoid detachment and agglomeration. The PtCo/ CNSs-1 shows superb electrocatalytic activity and stability compared to PtCo/CB.
Acknowledgment Fig. 8 e (a) The EIS spectras of PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/CB catalyst-modified electrodes in 0.5 M H2SO4 þ1 M methanol. The inserted picture is the equivalent circuit diagram. (b) Tafel plots of PtCo/CNSs, PtCo/CNSs-1, PtCo/CNSs-2, PtCo/CNSs-3, PtCo/ CB electrode in 0.5 M H2SO4 þ1 M methanol.
This work was supported by the National Natural Science Foundation of China (51908535, 21707145, 51808529), Natural Science Foundation of Gansu Province (18JR3RA383), Key Science and Technology Program of Lanzhou City (2018-RC-65), and CAS “Light of West China” (Program 2016-B).
Please cite this article as: Chang X et al., The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.088
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Please cite this article as: Chang X et al., The three-dimensional carbon nanosheets as high performance catalyst support for methanol electrooxidation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.088