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CNT oxygen reduction catalysts by atomic layer deposition
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Active sites engineering of Pt/CNT oxygen reduction catalysts by atomic layer deposition Jie Gan a,1, Jiankang Zhang b,1, Baiyan Zhang b, Wenyao Chen a,∗, Dongfang Niu a, Yong Qin b, Xuezhi Duan a,∗, Xinggui Zhou a
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a b
State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, China
a r t i c l e
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Article history: Received 14 August 2019 Revised 23 September 2019 Accepted 26 September 2019 Available online xxx Keywords: Oxygen reduction Pt/CNT catalyst Atomic layer deposition Active sites
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a b s t r a c t Understanding carbon-supported Pt-catalyzed oxygen reduction reaction (ORR) from the perspective of the active sites is of fundamental and practical importance. In this study, three differently sized carbon nanotube-supported Pt nanoparticles (Pt/CNT) are prepared by both atomic layer deposition (ALD) and impregnation methods. The performances of the catalysts toward the ORR in acidic media are comparatively studied to probe the effects of the sizes of the Pt nanoparticles together with their distributions, electronic properties, and local environments. The ALD-Pt/CNT catalysts show much higher ORR activity and selectivity than the impregnation-Pt/CNT catalysts. This outstanding ORR performance is ascribed to the well-controlled Pt particle sizes and distributions, desirable Pt0 4f binding energy, and the Cl-free Pt surfaces based on the electrocatalytic measurements, catalyst characterizations, and model calculations. The insights reported here could guide the rational design and fine-tuning of carbon-supported Pt catalysts for the ORR. © 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
1. Introduction Carbon-supported Pt-based oxygen reduction reaction (ORR) electrocatalysts have long been regarded as state-of-the-art materials, particularly in acidic medium. The urgent necessity to minimize noble Pt usage has directed research efforts toward the development of highly efficient catalysts that can increase the Pt utilization efficiency [1–10]. This catalytic process is known to be a structure-sensitive reaction, i.e., the size-dependent ORR activity and selectivity, whose sensitivity stems from the chemistry occurring primarily on the Pt (111) active sites and is associated with the Pt electronic properties and local environments [11–18]. Therefore, from the perspective of the active sites, it is highly desirable to understand the effects of the sizes of Pt nanoparticles together with their distributions, electronic properties, and local environments on the ORR activity and selectivity. It is well known that the preparation method remarkably affects the surface structures and properties of the as-prepared
∗
Corresponding authors. E-mail addresses: [email protected] (W. Chen), [email protected] (X. Duan). 1 These authors contributed equally to this work.
carbon-supported Pt catalysts [19]. Impregnation with H2 PtCl6 as the Pt precursor, a simple yet widely studied method, inevitably results in a wide distribution of the Pt particle size and the presence of residual Cl− on the catalyst surfaces [19–24]. Conversely, the colloidal method is effective for the synthesis of well-controlled Pt particle sizes with the assistance of stabilizing agents or ligands [19,25]. In these studies, surface cleaning treatments were performed to remove a significant amount of residual ions and surfactants, and thus weaken the unfavorable effects on the ORR [25–27]. However, these treatments usually result in a change in the size and shape of the metal particles. Recently, the self-limiting gas-solid reaction of atomic layer deposition (ALD) has sparked tremendous attention, because it enables atomically precise control to achieve good uniformity in the sizes of Pt nanoparticles supported on carbon [28–31]. The outstanding advantages of ALD make it a promising technique with well-controlled particle sizes and desirable electronic properties as well as the local environment of carbon-supported Pt catalysts [19,28,31]. Some previous studies were devoted to the development of ALD-synthesized Pt electrocatalysts immobilized on various substrates [32–34]. Therefore, an attempt is made to employ such an ALD technique for the synthesis of the above-mentioned targeted carbon-supported Pt catalysts toward an enhanced ORR performance.
https://doi.org/10.1016/j.jechem.2019.09.024 2095-4956/© 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Please cite this article as: J. Gan, J. Zhang and B. Zhang et al., Active sites engineering of Pt/CNT oxygen reduction catalysts by atomic layer deposition, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.024
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In this work, ALD-synthesized Pt/CNT catalysts with three different sizes were prepared and tested for the ORR in acidic media. The performances of the three differently sized impregnationsynthesized ORR catalysts were compared for probing the effects of the Pt nanoparticles’ sizes along with their distributions, electronic properties, and local environments. The ALD-Pt/CNT catalysts showed significantly higher ORR activity and selectivity than the impregnation-Pt/CNT catalysts. Multiple electrocatalytic measurements, catalyst characterizations, and model calculations were conducted and discussed. Finally, a plausible relationship between the catalyst structures and the ORR performance was established.
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2. Experimental
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2.1. Catalyst preparation
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The Pt/CNT catalysts were synthesized by the ALD method: The multi-walled carbon nanotube (CNT) support was purchased from Shenzhen Nanotech. Port Co., and its textural properties are shown in Table S1. The pristine CNT was pretreated with HNO3 (68 wt%) for 6 h at 120 °C to remove the residual metals. A hot-wall closedchamber-type ALD reactor with the deposition temperature of 280 °C was used to deposit Pt nanoparticles onto the CNT [28,35]. Firstly, Pt ALD was performed by sequential exposure of the CNT to (methylcyclopentadienyl) trimethylplatinum (MeCpPtMe3 , Strem Chemicals, 99%) and ozone (O3 ), produced by an ozone generator. The pulse, exposure, and purge time for MeCpPtMe3 were 0.5, 12, and 25 s, respectively, and those for O3 were 1, 12, and 25 s, respectively. MeCpPtMe3 was contained in a stainless steel bubbler heated to 60 °C to provide sufficient vapor pressure, and the precursor inlet lines were maintained at a temperature of 150 °C to prevent condensation. Ultra-high purity nitrogen (99.999%) was used as the carrier and purge gas. The cycle numbers of Pt ALD were maintained at 10, 20, and 35. The as-obtained catalysts were denoted as ALD-Ptx /CNT (x = 10, 20, and 35). Synthesis of Pt/CNT by impregnation method: The pristine CNT was employed as the catalyst support without further pretreatment. Three Pt/CNT catalysts with different Pt contents (i.e., 5, 8, and 14 wt%) were synthesized by an incipient wetness impregnation method, where H2 PtCl6 (Sinopharm Chemical Reagent Co. Ltd) was used as the Pt precursor. The Pt/CNT catalyst precursors were dried in stagnant air at room temperature for 12 h, followed by 110 °C for 12 h. Subsequently, they were reduced by pure H2 at 250 °C for 2 h and passivated in 1% O2 /Ar at room temperature for 20 min. The as-obtained catalysts were denoted as impregnation5% Pt/CNT, impregnation-8% Pt/CNT, and impregnation-14% Pt/CNT, separately. The commercial 20 wt% Pt on Vulcan XC72 was purchased from Sigma-Aldrich, and it was denoted as commercial 20% Pt/C catalyst.
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2.2. Catalyst characterization
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The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM, Tecnai G2 F20 S-Twin) and high-resolution transmission electron microscopy (HRTEM, JEOL JSM-2100, Japan) were employed to characterize the Pt particle size and shape. The mean Pt particle sizes for the catalysts were calculated based on the sizes of at least 300 random particles. X-ray photoelectron spectrometry (XPS, Kratos AXIS SUPRA) was employed to analyze the surface compositions and electronic properties of the catalysts on an instrument equipped with Al Kα X-ray (1486.6 eV, excitation source working at 15 kV). The C 1 s peak at 284.6 eV was taken as an internal standard to correct the shift in the binding energy caused by sample charging. An inductively coupled plasma atomic emission spectrometer (Agilent 725-ES ICP-AES) was used to obtain the Pt contents of the ALD-Pt10 /CNT,
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ALD-Pt20 /CNT, and ALD-Pt35 /CNT catalysts, which are 5, 8, and 14 wt%, respectively. The Cl ion concentration was analyzed by ion chromatography (Dionex ICS-1100 IC system, ThermoFisher).
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2.3. Electrochemical measurements
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The measurements of the as-obtained Pt/CNT catalysts were performed on a computer-controlled AUTOLAB PGSTAT204 (Metrohm, Switzerland) using electrochemical measurements (Pine Instruments, USA). In a single-compartment conventional three-electrode cell, a working electrode was prepared using a 5-nm-diameter (0.196 cm2 ) rotating glassy carbon disk electrode coated by a catalyst film; a Pt wire was used as the counter electrode, and a saturated Ag/AgCl electrode as the reference electrode, with a 0.1 M HClO4 solution as the electrolyte. The catalyst ink for the ORR test was prepared by ultrasonic dispersion of the mixture, which consisted of 5 mg of the fine catalyst powder, 950 μL of absolute ethanol, and 50 μL of 5 wt% Nafion solution. Ten microliters of the catalyst ink were dropped onto the surface of the glassy carbon disk electrode, followed by air-drying to obtain a catalyst loading of 250 μg cm−2 . All the results based on the electrochemical measurements were calculated against the reversible hydrogen electrode (RHE) by
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E (vs. RHE ) = E (vs. Ag/ AgCl ) + E 0 (Ag/ AgCl ) + 0.0591 × pH, where E (vs. RHE) is the potential vs. RHE, E (vs. Ag/AgCl) is the potential vs. Ag/AgCl, and E0 (Ag/AgCl) is the potential of Ag/AgCl with respect to the standard hydrogen electrode. The Koutecky–Levich (K–L) equation was used to calculate the kinetic current by considering the mass-transport correction:
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1 1 1 1 1 = + = + j jk jl jk Bω 0.5 129
B = 0.2nF Co (Do )
2 3
1
v− 6
where j represents the measured current density; jk and jl , the kinetic current density and the limited diffusion current density, respectively; ω, the rotation rate of the rotating disk electrode (400, 625, 900, 1225, 1600, and 2025 rpm); n, the transferred electron number; F, the Faraday constant (F = 96,485 C mol−1 ); CO , the bulk concentration of O2 (CO = 1.26 × 10−3 mol/L); DO , the O2 diffusion coefficient (DO = 1.93 × 10−5 cm2 /s); and v, the kinematic viscosity of the electrolyte (v = 1.009 × 10−2 cm2 /s). The constant, 0.2, is adopted when the rotation speed is expressed in rpm. The kinetic current density at any potential was calculated with the following formula:
jk =
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j × jl . jl − j
The transferred electron number (n) and the H2 O2 yield were calculated based on the ring and disk currents from the rotating ring-disk electrodes (RRDE), and they were estimated by the following equations:
n=
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jring /N H2 O2 (% ) = 200 × jdisk + jring /N where jdisk and jring represent the disk and the ring current densities, respectively, and N is the current collection efficiency of the Pt ring determined by a solution of Fe(CN)3 − (N = 0.37).
Please cite this article as: J. Gan, J. Zhang and B. Zhang et al., Active sites engineering of Pt/CNT oxygen reduction catalysts by atomic layer deposition, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.024
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Fig. 1. (a) Schematic illustration of the synthesis process of ALD-Pt/CNT catalysts. The red, green, organic, and blue balls represent the Pt atoms, oxygen atoms, methyl, and ligands within the Pt precursor, respectively. (b–d) HAADF-STEM images and the corresponding particle-size distributions of the ALD-Pt/CNT catalysts. (e) LSV polarization curves of the catalysts at a scan rate of 5 mV s−1 at a rotation speed of 1600 rpm in an O2 -saturated 0.1 M HClO4 solution. (f) Mass activity of the catalysts at 0.90 V, as a function of the Pt particle size. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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3. Results and discussion
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3.1. Improved ORR activity of Pt/CNT catalysts synthesized by ALD
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The three differently sized Pt/CNT catalysts were precisely synthesized by the ALD technique, and the synthesis procedure is schematically shown in Fig. 1(a). The CNT support was pre-treated with harsh acid oxidation to introduce oxygen-containing groups as the anchoring sites of the Pt species to increase the Pt dispersion and distribution [28,29,36]. These as-synthesized catalysts were characterized by HAADF-STEM, and the results are shown in Figs. 1(b–d). The average Pt particle sizes of the ALD-Pt10 /CNT, ALD-Pt20 /CNT, and ALD-Pt35 /CNT catalysts are determined to be 1.5 ± 0.1, 1.9 ± 0.1, and 2.4 ± 0.3 nm, respectively. In contrast, the three differently sized Pt/CNT catalysts prepared by the impregnation method and the commercial 20% Pt/C catalyst in Fig. S1 show relatively wide size distributions of the Pt nanoparticles and even some agglomerated particles existing in the impregnation14% Pt/CNT and the commercial 20% Pt/C catalysts, as marked by red cycles. This indicates that the ALD is an effective technique for finely controlling the Pt particle size and distribution. Subsequently, the ORR performances of the ALD-Pt/CNT, impregnation-Pt/CNT, and commercial 20% Pt/C catalysts, as well as the CNT support, were comparatively studied. Evidently, from the ORR polarization curves in Fig. 1(e) and Fig. S2, the ALD-Pt/CNT catalysts show significantly higher ORR activity (higher onset potentials, half-wave potentials, and limited diffusion currents) than the impregnation-Pt/CNT catalysts, which is comparable to that of the commercial 20% Pt/C catalyst. Moreover, it can be seen that the CNT support shows very low onset potentials, indicating its inactivity toward the ORR. Notably, these catalysts have exhibited different Pt loadings and particle sizes, which have been reported to remarkably affect the ORR performance [11,37]. To make a fair comparison, the kinetic current was normalized in reference to the
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Pt loading, and the resulting mass activity was plotted with the Pt particle size. It can be clearly observed in Fig. 1(f) that with excluding the Pt size effects, the ALD-Pt/CNT catalysts result in about 2.5 times higher mass activity than the impregnation-Pt/CNT catalysts, and about 1.4 times higher mass activity than the commercial 20% Pt/C catalyst. These results demonstrate the outstanding ORR mass activity and consequently enhanced Pt utilization efficiency of the ALD-synthesized Pt/CNT catalysts.
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3.2. Model calculations and XPS measurements
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As noted above, there is a remarkable size and supportdependent ORR activity, which could be associated with the number of Pt active sites and their electronic properties [1,6,12]. By employing the model calculations developed in our previous work [38], the Pt (111) atoms were suggested as the dominant active sites for the ORR [11]. For such a method, HRTEM measurements and the corresponding fast Fourier transform (FFT) analysis of the ALD-Pt/CNT, impregnation-Pt/CNT, and commercial 20% Pt/C catalysts were carried out, and the results are shown in Figs. 2(a–b) and Fig. S3. In most cases, the Pt nanoparticles are prone to exist as a well-defined truncated cuboctahedron with a top surface of (100) based on the 70.5° angle between the (111) surfaces over these catalysts. Owing to the distribution of the Pt particle sizes, all the observed Pt nanoparticles were considered to calculate the number of Pt (111) atoms. As expected, the well-controlled ALDPt/CNT catalysts showed additional Pt (111) active sites in Fig. 2(c) due to the relatively narrow Pt particle size distribution mentioned above. Based on the determined number of the Pt (111) atoms, the TOF of the catalysts were obtained, and the results are shown in Fig 2(c). Interestingly, the ALD-Pt/CNT catalysts also exhibit higher TOF than the impregnation-Pt/CNT and commercial 20% Pt/C catalysts, strongly indicating their remarkable electronic effects.
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Please cite this article as: J. Gan, J. Zhang and B. Zhang et al., Active sites engineering of Pt/CNT oxygen reduction catalysts by atomic layer deposition, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.024
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Fig. 2. Typical HRTEM images and the corresponding FFT analysis of (a) the ALD-Pt/CNT and (b) the impregnation-Pt/CNT catalysts. The inset models present a truncated octahedron Pt shape. (c) Number of Pt (111) atoms per mole and the calculated turn over frequency (TOF) at 0.90 V for the ALD-Pt/CNT, impregnation-Pt/CNT, and the commercial 20% Pt/C catalysts.
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Additionally, XPS measurements were employed to determine the Pt electronic properties. Evidently, in Fig. 3, both the ALD-Pt/CNT and the impregnation-Pt/CNT catalysts show decreased Pt0 4f binding energies as the Pt particle size decreases. This is mainly because the electron-withdrawing capacity of the given CNT support leads to the decreased Pt0 4f binding energy of the supported Pt nanoparticles as the Pt loading increases. Previous studies indicated that the reduced Pt binding energy indicates a reduced adsorption strength of the reaction intermediates on the Pt surfaces and thus a relatively high ORR activity [11,15,39–41]. Therefore, these can provide a rational explanation for the increase in the TOF with the Pt particle size due to the decreased Pt0 4f binding energy. It can be also seen in Fig. 3 that with excluding the Pt size effects, the ALD-Pt/CNT catalysts show a higher Pt0 4f binding energy compared with the impregnation-Pt/CNT and commercial 20% Pt/C catalysts (Fig. S3). In principle, the relatively high Pt0 4f binding energy should correspond to the relatively low ORR activity [15], which could not explain why the ALD-Pt/CNT catalysts with relatively high binding energy exhibited enhanced activity. In other words, this strongly implies that another important issue (e.g., the catalyst local environment) could contribute to the increased TOF for the ALD-Pt/CNT catalysts.
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3.3. The crucial effects of the Pt local environments
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To test this idea, the Pt local environments, in terms of the surface properties, for these catalysts were further compared based on the XPS results. As shown in Fig. 4 and Fig. S3, both the impregnation-Pt/CNT and commercial 20% Pt/C catalysts show significant Cl signals, in comparison with the undetectable Cl signal for the ALD-Pt/CNT catalysts. This indicates the difficulty associated with the complete removal of the Cl species during the reduction process for the impregnation-Pt/CNT catalysts. Combining their significantly lower activities, it is reasonable to assume that the existence of Cl would inhibit the ORR activity [21,42,43]. To semi-quantify the information of the residual Cl over the Pt
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nanoparticles, ion chromatography was employed to determine the difference in the Cl concentrations over these catalysts. As shown in Figs. 4(c) and 4(d), the Cl− concentrations for the impregnation5% Pt/CNT, impregnation-8% Pt/CNT, and impregnation-14% Pt/CNT catalysts were determined to be 297, 99, and 149 μg g−1 , respectively. Moreover, the commercial 20% Pt/C catalyst shows the highest Cl− concentration of 706 μg g−1 , as shown in Table S2. It can be observed that the trend of the Cl− concentration for these catalysts is inconsistent with that of the activity, in which the commercial 20% Pt/C catalyst with more Cl− shows higher activity than the impregnation-Pt/CNT catalysts. Notably, the ion chromatography-detected Cl− could distribute either on the Pt surface or the support surface; thus, it is important to distinguish the influence of Cl− on the Pt surface from that on the support surface. Along this line, to separately investigate the Cl− influence on the Pt surface, CV measurements were conducted to evaluate the H-adsorption/desorption behaviors for these catalysts, which could further yield their under-potentially deposited hydrogen (Hupd ) [21,44]. As shown in Fig. 5 and Fig. S4, with the exclusion of the Pt size effects, the Hupd -region (noted as (2)) for the Cl-containing impregnation-Pt/CNT catalysts narrows with respect to the ALDPt/CNT catalysts, which could be due to an energetic modification of H-adsorption/desorption. Generally, Hupd corresponds to an ideal one-electron transfer assuming one H atom per Pt surface atom [44,45]. In other words, this suggests a one-to-one relationship between the Pt surface atom and the H atom. Thus, the narrow Hupd region for the impregnation-Pt/CNT catalysts indicates a relatively low amount of Pt surface sites, which could be blocked by Cl− . As a result, the impregnation-5% Pt/CNT catalyst in Fig. 5(a) exhibits the lowest amount of H-adsorption, which is consistent with the highest amount of Cl− concentration in Fig. 4(d). Moreover, the commercial 20% Pt/C catalyst in Fig. 5(c) shows a much higher H-adsorption than the other catalysts, which could be due to the preferential adsorption of Cl− on the support surface instead of the Pt surface. Therefore, it is evident that the Cl− ions prefer to adsorb on the Pt surface and inhibit the ORR activity for the impregnation-Pt/CNT catalysts. Conversely, the Cl− ions prefer
Please cite this article as: J. Gan, J. Zhang and B. Zhang et al., Active sites engineering of Pt/CNT oxygen reduction catalysts by atomic layer deposition, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.024
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Fig. 3. XPS Pt 4f spectra of the (a) ALD-Pt/CNT and (b) impregnation-Pt/CNT catalysts.
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to adsorb on the support surface and has limited influence for the commercial 20% Pt/C catalyst. These observations could account for the higher activity of the commercial 20% Pt/C catalyst compared to that of the impregnation-Pt/CNT catalysts. To further verify the negative effects of Cl− on the ORR, certain amounts of NaCl were added to the HClO4 solution to obtain different Cl− concentrations, which were used as electrolytes for the ORR test over the Cl-free ALD-Pt35 /CNT catalyst. It can be observed from the LSV curves in Fig. 5(d) that there is a gradual decrease in the ORR activity as the Cl− concentration increases, which is consistent with the above results. Specifically, the half-wave potential (Fig. 5(e)) exhibits a rapid decay from 0.86 to 0.65 V with the Cl− concentration increasing from 10−4 to 10−1 M. Additionally, based on the CV curves in Fig. 5(f), with the increase in the Cl− concentration, the peak of the oxygen-containing species adsorption downshifts (noted as (4)), and the oxygen reduction peak exhibits a left shift from 0.8 V to 0.6 V (noted as (3)). These results indicate that Cl− could compete with the reactants for the active sites over the Pt surfaces, and thus inhibit the ORR activity. Based on the above analyses, the well-controlled Pt particle size and narrow-sized distribution of the ALD-Pt/CNT catalysts result in additional Pt (111) active sites, and the relatively largesized Pt/CNT catalyst shows a relatively low Pt0 4f binding energy and thus a relatively high TOF. However, over the impregnationPt/CNT and commercial 20% Pt/C catalysts, the presence of Cl− leads to remarkable poisonous effects, which block the Pt surfaces
toward relatively few Pt active sites and thus decrease the ORR activity.
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3.4. Improved ORR selectivity of Pt/CNT catalysts synthesized by ALD
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In addition to the ORR activity, the ORR selectivity is another important issue, because the ORR proceeds preferably via a fourelectron pathway to form H2 O. This is much more efficient than the two-electron pathway, which forms H2 O2 [1,6,11,37]. Along this line, RRDE testing was carried out to determine the number of electron transfers (i.e., n) and the H2 O2 yield for these catalysts; the results are shown in Fig. 6. Based on the RRDE curves in Figs. 6(a) and 6(b), the average numbers of electron transfers (n) were calculated and summarized in Fig. 6(c). It was found that the ALD-Pt/CNT and commercial 20% Pt/C catalysts showed much higher electron transfer numbers and lower H2 O2 yield than the impregnation-Pt/CNT catalysts. This indicates that the ORR over the former two catalysts preferably proceeds through an efficient fourelectron pathway with a relatively high ORR selectivity, while the impregnation-Pt/CNT catalysts exhibit a relatively low selectivity. This could have resulted because the abundant Pt active sites are blocked by Cl− and thus suppress the breaking of the O–O bond for the impregnation-Pt/CNT catalysts. Conversely, for the Cl-free ALD-Pt/CNT catalysts, the ALDPt20 /CNT catalyst exhibits the highest ORR selectivity compared
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Please cite this article as: J. Gan, J. Zhang and B. Zhang et al., Active sites engineering of Pt/CNT oxygen reduction catalysts by atomic layer deposition, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.024
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Fig. 4. XPS Cl 2p spectra of the (a) ALD-Pt/CNT and (b) impregnation-Pt/CNT catalysts. The Cl− concentration of the impregnation-Pt/CNT catalysts determined by (c) XPS spectroscopy and (d) ion chromatography.
Fig. 5. (a–c) CV curves between 0.05 and 1.20 V in N2 -saturated 0.1 M HClO4 for the catalysts. (d) LSV polarization curves; (e) the E1/2 values and (f) CV curves of the ALD-Pt35 /CNT catalyst in O2 -saturated 0.1 M HClO4 with different Cl− concentrations. Notes (1) and (2) present the hydrogen desorption and adsorption peak, respectively. Meanwhile, notes (3) and (4) present the oxygen desorption and adsorption peak, respectively.
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with the ALD-Pt10 /CNT and ALD-Pt35 /CNT catalysts (Fig. 6), which could not be explained by the difference in the Cl− concentrations. Considering the medium Pt 4f binding energy of the ALD-Pt20 /CNT catalyst, we attribute this to the appropriate adsorption strength of the reaction intermediates on the Pt surfaces. Based on previous studies [46], if the metal binds with oxygen exceedingly strongly, it would result in a high degree of oxygenated species coverage, thus
blocking the Pt active sites and suppressing the cleavages of the O–O bond. Conversely, if the metal binds with oxygen too weakly, it would result in a low degree of oxygenated species coverage and thus insufficient O–O bond cleavage [46]. Therefore, the ALDPt20 /CNT catalyst with the optimal Pt binding energy shows the highest ORR selectivity. These results indicate that a cost-effective Pt/CNT catalyst should possess a desirable structure and electronic
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Fig. 6. (a, b) RRDE polarization curves for (a) the ALD-Pt/CNT and commercial 20% Pt/C as well as (b) the impregnation-Pt/CNT catalysts. (c) The corresponding average number of transferred electrons (n) and H2 O2 yield.
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properties as well as Cl-free local environment to achieve high ORR activity and selectivity.
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4. Conclusions
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In summary, three differently sized Pt nanocatalysts were prepared by both atomic layer deposition (ALD) and impregnation methods. Their performances toward the ORR in acidic media were comparatively studied to probe the effects of the Pt particle size together with the distributions, electronic properties, and local environments. The as-prepared ALD-Pt/CNT catalysts exhibited much higher ORR activity and selectivity than the impregnation-Pt/CNT catalysts. A combination of electrocatalytic measurements, catalyst characterizations, and model calculations was conducted to establish the relationship between the catalyst structure and ORR performance. It was found that the well-controlled Pt particle sizes and distributions, desirable Pt0 4f binding energy, and the Cl-free Pt surfaces of the ALD-Pt/CNT catalysts contributed to the outstanding ORR performance. The insights gained here could provide a new direction for the rational design and fine-tuning of carbonsupported Pt-based catalysts for the ORR.
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Acknowledgments
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This work was financially supported by the Natural Science Foundation of China (21922803 and 21776077), the Shanghai Natural Science Foundation (17ZR1407300 and 17ZR1407500), the Program for the Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the Shanghai RisingStar Program (17QA1401200), the State Key Laboratory of OrganicInorganic Composites (oic-201801007), and the Open Project of State Key Laboratory of Chemical Engineering (SKLChe-15C03).
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Supplementary materials
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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2019.09.024.
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References
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[1] Z.W. Seh, J. Kibsgaard, C.F. Dickens, I. Chorkendorff, J.K. Nørskov, T.F. Jaramillo, Science 355 (6321) (2017) eaad4998. [2] R. Chattot, O.L. Bacq, V. Beermann, S. Kühl, J. Herranz, S. Henning, L. Kühn, T. Asset, L. Guétaz, G. Renou, J. Drnec, P. Bordet, A. Pasturel, A. Eychmüller, T.J. Schmidt, P. Strasser, L. Dubau, F. Maillard, Nat. Mater. 17 (9) (2018) 827– 833. [3] M.F. Li, 1 Z.P. Zhao, T. Cheng, A. Fortunelli, C.Y. Chen, R. Yu, Q.H. Zhang, L. Gu, B. Merinov, Z.Y. Lin, E. Zhu, T. Yu, Q.Y. Jia, J.H. Guo, L. Zhang, W.A. Goddard III, Y. Huang, X.F. Duan, Science 354 (6318) (2016) 1414–1419. [4] A. Kulkarni, S. Siahrostami, A. Patel, J.K. Nørskov, Chem. Rev. 118 (5) (2018) 2302–2312. [5] C. Jackson, G.T. Smith, D.W. Inwood, A.S. Leach, P.S. Whalley, M. Callisti, T. Polcar, A.E. Russell, P. Levecque, D. Kramer, Nat. Commun. 8 (2017) 15802. [6] Y. Jiao, Y. Zheng, M. Jaroniec, S.Z. Qiao, Chem. Soc. Rev. 44 (8) (2015) 2060– 2086. [7] D.S. He, D.P. He, J. Wang, Y. Lin, P.Q. Yin, X. Hong, Y. Wu, Y.D. Li, J. Am. Chem. Soc. 138 (5) (2016) 1494–1497. [8] X. Liu, H.M. Wang, S.G. Chen, X.Q. Qi, H.L. Gao, Y. Hui, Y. Bai, L. Guo, W. Ding, Z.D. Wei, J. Energy Chem. 23 (3) (2014) 358–362. [9] C. Fan, Z.H. Huang, X.Y. Hu, Z.P. Shi, T.Y. Shen, Y.W. Tang, X.J. Wang, L. Xu, Green Energy Environ. 3 (2018) 310–317. [10] H.F. Zhang, J.B. Zhang, K.H. Liu, Y.Q. Zhu, X.Y. Qiu, D.M. Sun, Y.W. Tang, Green Energy Environ. 4 (2019) 245–253. [11] J. Gan, W. Luo, W.Y. Chen, J.N. Guo, Z.H. Xiang, B.X. Chen, F. Yang, Y.J. Cao, F. Song, X.Z. Duan, X.G. Zhou, Eur. J. Inorg. Chem. 27 (2019) 3210–3217. [12] Y. Nie, L. Li, Z.D. Wei, Chem. Soc. Rev. 44 (2015) 2168–2201. [13] A.M.G. Marín, R. Rizoa, J.M. Feliu, Catal. Sci. Technol. 4 (2014) 1685–1698. [14] I. Katsounaros, S. Cherevko, A.R. Zeradjanin, K.J.J. Mayrhofer, Angew. Chem. Int. Ed. 53 (1) (2014) 102–121. [15] Y. Peng, B.Z. Lu, N. Wang, L.G. Li, S.W. Chen, Phys. Chem. Chem. Phys. 19 (14) (2017) 9336–9348. [16] S. Sui, X.Y. Wang, X.T. Zhou, Y.H. Su, S. Riffatc, C.J. Liu, J. Mater. Chem. A 5 (2017) 1808–1825.
381Q3 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413
Please cite this article as: J. Gan, J. Zhang and B. Zhang et al., Active sites engineering of Pt/CNT oxygen reduction catalysts by atomic layer deposition, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.024
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[m5G;October 12, 2019;12:38]
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[17] M. Nesselberger, S. Ashton, J.C. Meier, I. Katsounaros, K.J.J. Mayrhofer, M. Arenz, J. Am. Chem. Soc. 133 (43) (2011) 17428–17433. [18] M.H. Shao, A. Peles, K. Shoemaker, Nano Lett. 11 (9) (2011) 3714–3719. [19] P. Munnik, P.E. de Jongh, K.P. de Jong, Chem. Rev. 115 (14) (2015) 6687–6718. [20] N. Job, M. Chatenet, S.B. Fabry, S. Hermans, F. Maillard, J. Power Sources 240 (2013) 294–305. [21] T.J. Schmidt, U.A. Paulus, H.A. Gasteiger, R.J. Behm, J. Electroanal. Chem. 508 (2001) 41–47. [22] X.W. Yu, S.Y. Ye, J. Power Sources 172 (1) (2007) 133–144. [23] T.M. Arruda, B. Shyam, J.M. Ziegelbauer, S. Mukerjee, D.E. Ramaker, J. Phys. Chem. C 112 (46) (2008) 18087–18097. [24] N.M. Markovic, P.N. Ross, J. Electroanal. Chem. 330 (1–2) (1992) 499–520. [25] Y.H. Chung, D.Y. Chung, N. Jung, Y.E. Sung, J. Phys. Chem. Lett. 4 (8) (2013) 1304–1309. [26] D.G. Li, C. Wang, D. Tripkovic, S.H. Sun, N.M. Markovic, V.R. Stamenkovic, ACS Catal. 2 (2012) 1358–1362. [27] F.J.V. Iglesias, J.S. Gullón, E. Herrero, V. Montiel, A. Aldaz, J.M. Feliu, Electrochem. Commun. 13 (2011) 502–505. [28] J.K. Zhang, W.Y. Chen, H.B. Ge, C.Q. Chen, W.J. Yan, Z. Gao, J. Gan, B.Y. Zhang, X.Z. Duan, Y. Qin, Appl. Catal. B: Environ. 235 (2018) 256–263. [29] Z. Gao, Y. Qin, Acc. Chem. Res. 50 (9) (2017) 2309–2316. [30] N. Cheng, S. Stambula, D. Wang, M.N. Banis, J. Liu, A. Riese, B.W. Xiao, R.Y. Li, T.K. Sham, L.M. Liu, G.A. Botton, X.L. Sun, Nat. Commun. 7 (2016) 13638. [31] C. Marichy, M. Bechelany, N. Pinna, Adv. Mater. 24 (2012) 1017–1032. [32] N.C. Cheng, Y.Y. Shao, J. Liu, X.L. Sun, Nano Energy 29 (2016) 220–242. [33] L. Zhang, Y. Zhao, M.N. Banis, K. Adair, Z.X. Song, L.J. Yang, M. Markiewicz, J.J. Li, S.Z. Wang, R.Y. Li, S.Y. Ye, X.L. Sun, Nano Energy 60 (2019) 111–118.
[34] S.C. Xu, Y.M. Kim, J. Park, D. Higgins, S.J. Shen, P. Schindler, D. Thian, J. Provine, J. Torgersen, T. Graf, T.D. Schladt, M. Orazov, B.H. Liu, T.F. Jaramillo, F.B. Prinz, Nat. Catal. 1 (2018) 624–630. [35] X.M. Wang, N. Li, J.A. Webb, L.D. Pfefferle, G.L. Haller, Appl. Catal. B: Environ. 101 (1–2) (2010) 21–30. [36] X.Y. Yan, X.L. Tong, Y.F. Zhang, X.D. Han, Y.Y. Wang, G.Q. Jin, Y. Qin, X.Y. Guo, Chem. Commun. 48 (13) (2012) 1892–1894. [37] L. Colmenares, E. Guerrini, Z. Jusys, K.S. Nagabhushana, E. Dinjus, S. Behrens, W. Habicht, H. Bo¨nnemann, R.J. Behm, J. Appl. Electrochem. 37 (2007) 1413– 1427. [38] W.Y. Chen, J. Ji, X. Feng, X.Z. Duan, G. Qian, P. Li, X.G. Zhou, D. Chen, W.K. Yuan, J. Am. Chem. Soc. 136 (2014) 16736–16739. [39] G.A. Tritsaris, J. Greeley, J. Rossmeisl, J.K. Nørskov, Catal. Lett. 141 (2011) 909– 913. [40] Z.Y. Zhou, X.W. Kang, Y. Song, S.W. Chen, J. Phys. Chem. C 116 (19) (2012) 10592–10598. [41] V. Stamenkovic, B.S. Mun, K.J.J. Mayrhofer, P.N. Ross, N.M. Markovic, J. Rossmeisl, J. Greeley, J.K. Nørskov, Angew. Chem. 118 (2006) 2963–2967. [42] K. Mamtani, Jain D, A.C. Co, U.S. Ozkan, Catal. Lett. 147 (2017) 2903–2909. [43] N.M. Markovic, T.J. Schmidt, V. Stamenkovic´, P.N. Ross, Fuel Cells 1 (2) (2001) 105–116. [44] D. Chen, Q. Tao, L.W. Liao, S.X. Liu, Y.X. Chen, S. Ye, Electrocatal. 2 (2011) 207– 219. [45] J.N. Tiwari, K. Nath, S. Kumar, R.N. Tiwari, K.C. Kemp, N.H. Le, D.H. Youn, J.S. Lee, K.S. Kim, Nat. Commun. 4 (2013) 2221. [46] M.G. Balcázar, F.M.C. Muniz, L. Á. Contreras, L.G. Arriaga, J.L. García, J. Power Sources 197 (2012) 121–124.
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Journal of Energy Chemistry journal homepage: www.elsevier.com/locate/jechem
Active sites engineering of Pt/CNT oxygen reduction catalysts by atomic layer deposition Jie Gan a,1, Jiankang Zhang b,1, Baiyan Zhang b, Wenyao Chen a,∗, Dongfang Niu a, Yong Qin b, Xuezhi Duan a,∗, Xinggui Zhou a
Q1
a b
State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, China
a r t i c l e
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Article history: Received 14 August 2019 Revised 23 September 2019 Accepted 26 September 2019 Available online xxx Keywords: Oxygen reduction Pt/CNT catalyst Atomic layer deposition Active sites
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a b s t r a c t Understanding carbon-supported Pt-catalyzed oxygen reduction reaction (ORR) from the perspective of the active sites is of fundamental and practical importance. In this study, three differently sized carbon nanotube-supported Pt nanoparticles (Pt/CNT) are prepared by both atomic layer deposition (ALD) and impregnation methods. The performances of the catalysts toward the ORR in acidic media are comparatively studied to probe the effects of the sizes of the Pt nanoparticles together with their distributions, electronic properties, and local environments. The ALD-Pt/CNT catalysts show much higher ORR activity and selectivity than the impregnation-Pt/CNT catalysts. This outstanding ORR performance is ascribed to the well-controlled Pt particle sizes and distributions, desirable Pt0 4f binding energy, and the Cl-free Pt surfaces based on the electrocatalytic measurements, catalyst characterizations, and model calculations. The insights reported here could guide the rational design and fine-tuning of carbon-supported Pt catalysts for the ORR. © 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
1. Introduction Carbon-supported Pt-based oxygen reduction reaction (ORR) electrocatalysts have long been regarded as state-of-the-art materials, particularly in acidic medium. The urgent necessity to minimize noble Pt usage has directed research efforts toward the development of highly efficient catalysts that can increase the Pt utilization efficiency [1–10]. This catalytic process is known to be a structure-sensitive reaction, i.e., the size-dependent ORR activity and selectivity, whose sensitivity stems from the chemistry occurring primarily on the Pt (111) active sites and is associated with the Pt electronic properties and local environments [11–18]. Therefore, from the perspective of the active sites, it is highly desirable to understand the effects of the sizes of Pt nanoparticles together with their distributions, electronic properties, and local environments on the ORR activity and selectivity. It is well known that the preparation method remarkably affects the surface structures and properties of the as-prepared
∗
Corresponding authors. E-mail addresses: [email protected] (W. Chen), [email protected] (X. Duan). 1 These authors contributed equally to this work.
carbon-supported Pt catalysts [19]. Impregnation with H2 PtCl6 as the Pt precursor, a simple yet widely studied method, inevitably results in a wide distribution of the Pt particle size and the presence of residual Cl− on the catalyst surfaces [19–24]. Conversely, the colloidal method is effective for the synthesis of well-controlled Pt particle sizes with the assistance of stabilizing agents or ligands [19,25]. In these studies, surface cleaning treatments were performed to remove a significant amount of residual ions and surfactants, and thus weaken the unfavorable effects on the ORR [25–27]. However, these treatments usually result in a change in the size and shape of the metal particles. Recently, the self-limiting gas-solid reaction of atomic layer deposition (ALD) has sparked tremendous attention, because it enables atomically precise control to achieve good uniformity in the sizes of Pt nanoparticles supported on carbon [28–31]. The outstanding advantages of ALD make it a promising technique with well-controlled particle sizes and desirable electronic properties as well as the local environment of carbon-supported Pt catalysts [19,28,31]. Some previous studies were devoted to the development of ALD-synthesized Pt electrocatalysts immobilized on various substrates [32–34]. Therefore, an attempt is made to employ such an ALD technique for the synthesis of the above-mentioned targeted carbon-supported Pt catalysts toward an enhanced ORR performance.
https://doi.org/10.1016/j.jechem.2019.09.024 2095-4956/© 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Please cite this article as: J. Gan, J. Zhang and B. Zhang et al., Active sites engineering of Pt/CNT oxygen reduction catalysts by atomic layer deposition, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.024
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In this work, ALD-synthesized Pt/CNT catalysts with three different sizes were prepared and tested for the ORR in acidic media. The performances of the three differently sized impregnationsynthesized ORR catalysts were compared for probing the effects of the Pt nanoparticles’ sizes along with their distributions, electronic properties, and local environments. The ALD-Pt/CNT catalysts showed significantly higher ORR activity and selectivity than the impregnation-Pt/CNT catalysts. Multiple electrocatalytic measurements, catalyst characterizations, and model calculations were conducted and discussed. Finally, a plausible relationship between the catalyst structures and the ORR performance was established.
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2. Experimental
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2.1. Catalyst preparation
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The Pt/CNT catalysts were synthesized by the ALD method: The multi-walled carbon nanotube (CNT) support was purchased from Shenzhen Nanotech. Port Co., and its textural properties are shown in Table S1. The pristine CNT was pretreated with HNO3 (68 wt%) for 6 h at 120 °C to remove the residual metals. A hot-wall closedchamber-type ALD reactor with the deposition temperature of 280 °C was used to deposit Pt nanoparticles onto the CNT [28,35]. Firstly, Pt ALD was performed by sequential exposure of the CNT to (methylcyclopentadienyl) trimethylplatinum (MeCpPtMe3 , Strem Chemicals, 99%) and ozone (O3 ), produced by an ozone generator. The pulse, exposure, and purge time for MeCpPtMe3 were 0.5, 12, and 25 s, respectively, and those for O3 were 1, 12, and 25 s, respectively. MeCpPtMe3 was contained in a stainless steel bubbler heated to 60 °C to provide sufficient vapor pressure, and the precursor inlet lines were maintained at a temperature of 150 °C to prevent condensation. Ultra-high purity nitrogen (99.999%) was used as the carrier and purge gas. The cycle numbers of Pt ALD were maintained at 10, 20, and 35. The as-obtained catalysts were denoted as ALD-Ptx /CNT (x = 10, 20, and 35). Synthesis of Pt/CNT by impregnation method: The pristine CNT was employed as the catalyst support without further pretreatment. Three Pt/CNT catalysts with different Pt contents (i.e., 5, 8, and 14 wt%) were synthesized by an incipient wetness impregnation method, where H2 PtCl6 (Sinopharm Chemical Reagent Co. Ltd) was used as the Pt precursor. The Pt/CNT catalyst precursors were dried in stagnant air at room temperature for 12 h, followed by 110 °C for 12 h. Subsequently, they were reduced by pure H2 at 250 °C for 2 h and passivated in 1% O2 /Ar at room temperature for 20 min. The as-obtained catalysts were denoted as impregnation5% Pt/CNT, impregnation-8% Pt/CNT, and impregnation-14% Pt/CNT, separately. The commercial 20 wt% Pt on Vulcan XC72 was purchased from Sigma-Aldrich, and it was denoted as commercial 20% Pt/C catalyst.
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2.2. Catalyst characterization
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The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM, Tecnai G2 F20 S-Twin) and high-resolution transmission electron microscopy (HRTEM, JEOL JSM-2100, Japan) were employed to characterize the Pt particle size and shape. The mean Pt particle sizes for the catalysts were calculated based on the sizes of at least 300 random particles. X-ray photoelectron spectrometry (XPS, Kratos AXIS SUPRA) was employed to analyze the surface compositions and electronic properties of the catalysts on an instrument equipped with Al Kα X-ray (1486.6 eV, excitation source working at 15 kV). The C 1 s peak at 284.6 eV was taken as an internal standard to correct the shift in the binding energy caused by sample charging. An inductively coupled plasma atomic emission spectrometer (Agilent 725-ES ICP-AES) was used to obtain the Pt contents of the ALD-Pt10 /CNT,
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ALD-Pt20 /CNT, and ALD-Pt35 /CNT catalysts, which are 5, 8, and 14 wt%, respectively. The Cl ion concentration was analyzed by ion chromatography (Dionex ICS-1100 IC system, ThermoFisher).
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2.3. Electrochemical measurements
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The measurements of the as-obtained Pt/CNT catalysts were performed on a computer-controlled AUTOLAB PGSTAT204 (Metrohm, Switzerland) using electrochemical measurements (Pine Instruments, USA). In a single-compartment conventional three-electrode cell, a working electrode was prepared using a 5-nm-diameter (0.196 cm2 ) rotating glassy carbon disk electrode coated by a catalyst film; a Pt wire was used as the counter electrode, and a saturated Ag/AgCl electrode as the reference electrode, with a 0.1 M HClO4 solution as the electrolyte. The catalyst ink for the ORR test was prepared by ultrasonic dispersion of the mixture, which consisted of 5 mg of the fine catalyst powder, 950 μL of absolute ethanol, and 50 μL of 5 wt% Nafion solution. Ten microliters of the catalyst ink were dropped onto the surface of the glassy carbon disk electrode, followed by air-drying to obtain a catalyst loading of 250 μg cm−2 . All the results based on the electrochemical measurements were calculated against the reversible hydrogen electrode (RHE) by
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E (vs. RHE ) = E (vs. Ag/ AgCl ) + E 0 (Ag/ AgCl ) + 0.0591 × pH, where E (vs. RHE) is the potential vs. RHE, E (vs. Ag/AgCl) is the potential vs. Ag/AgCl, and E0 (Ag/AgCl) is the potential of Ag/AgCl with respect to the standard hydrogen electrode. The Koutecky–Levich (K–L) equation was used to calculate the kinetic current by considering the mass-transport correction:
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1 1 1 1 1 = + = + j jk jl jk Bω 0.5 129
B = 0.2nF Co (Do )
2 3
1
v− 6
where j represents the measured current density; jk and jl , the kinetic current density and the limited diffusion current density, respectively; ω, the rotation rate of the rotating disk electrode (400, 625, 900, 1225, 1600, and 2025 rpm); n, the transferred electron number; F, the Faraday constant (F = 96,485 C mol−1 ); CO , the bulk concentration of O2 (CO = 1.26 × 10−3 mol/L); DO , the O2 diffusion coefficient (DO = 1.93 × 10−5 cm2 /s); and v, the kinematic viscosity of the electrolyte (v = 1.009 × 10−2 cm2 /s). The constant, 0.2, is adopted when the rotation speed is expressed in rpm. The kinetic current density at any potential was calculated with the following formula:
jk =
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j × jl . jl − j
The transferred electron number (n) and the H2 O2 yield were calculated based on the ring and disk currents from the rotating ring-disk electrodes (RRDE), and they were estimated by the following equations:
n=
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jring /N H2 O2 (% ) = 200 × jdisk + jring /N where jdisk and jring represent the disk and the ring current densities, respectively, and N is the current collection efficiency of the Pt ring determined by a solution of Fe(CN)3 − (N = 0.37).
Please cite this article as: J. Gan, J. Zhang and B. Zhang et al., Active sites engineering of Pt/CNT oxygen reduction catalysts by atomic layer deposition, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.024
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Fig. 1. (a) Schematic illustration of the synthesis process of ALD-Pt/CNT catalysts. The red, green, organic, and blue balls represent the Pt atoms, oxygen atoms, methyl, and ligands within the Pt precursor, respectively. (b–d) HAADF-STEM images and the corresponding particle-size distributions of the ALD-Pt/CNT catalysts. (e) LSV polarization curves of the catalysts at a scan rate of 5 mV s−1 at a rotation speed of 1600 rpm in an O2 -saturated 0.1 M HClO4 solution. (f) Mass activity of the catalysts at 0.90 V, as a function of the Pt particle size. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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3. Results and discussion
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3.1. Improved ORR activity of Pt/CNT catalysts synthesized by ALD
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The three differently sized Pt/CNT catalysts were precisely synthesized by the ALD technique, and the synthesis procedure is schematically shown in Fig. 1(a). The CNT support was pre-treated with harsh acid oxidation to introduce oxygen-containing groups as the anchoring sites of the Pt species to increase the Pt dispersion and distribution [28,29,36]. These as-synthesized catalysts were characterized by HAADF-STEM, and the results are shown in Figs. 1(b–d). The average Pt particle sizes of the ALD-Pt10 /CNT, ALD-Pt20 /CNT, and ALD-Pt35 /CNT catalysts are determined to be 1.5 ± 0.1, 1.9 ± 0.1, and 2.4 ± 0.3 nm, respectively. In contrast, the three differently sized Pt/CNT catalysts prepared by the impregnation method and the commercial 20% Pt/C catalyst in Fig. S1 show relatively wide size distributions of the Pt nanoparticles and even some agglomerated particles existing in the impregnation14% Pt/CNT and the commercial 20% Pt/C catalysts, as marked by red cycles. This indicates that the ALD is an effective technique for finely controlling the Pt particle size and distribution. Subsequently, the ORR performances of the ALD-Pt/CNT, impregnation-Pt/CNT, and commercial 20% Pt/C catalysts, as well as the CNT support, were comparatively studied. Evidently, from the ORR polarization curves in Fig. 1(e) and Fig. S2, the ALD-Pt/CNT catalysts show significantly higher ORR activity (higher onset potentials, half-wave potentials, and limited diffusion currents) than the impregnation-Pt/CNT catalysts, which is comparable to that of the commercial 20% Pt/C catalyst. Moreover, it can be seen that the CNT support shows very low onset potentials, indicating its inactivity toward the ORR. Notably, these catalysts have exhibited different Pt loadings and particle sizes, which have been reported to remarkably affect the ORR performance [11,37]. To make a fair comparison, the kinetic current was normalized in reference to the
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Q2
Pt loading, and the resulting mass activity was plotted with the Pt particle size. It can be clearly observed in Fig. 1(f) that with excluding the Pt size effects, the ALD-Pt/CNT catalysts result in about 2.5 times higher mass activity than the impregnation-Pt/CNT catalysts, and about 1.4 times higher mass activity than the commercial 20% Pt/C catalyst. These results demonstrate the outstanding ORR mass activity and consequently enhanced Pt utilization efficiency of the ALD-synthesized Pt/CNT catalysts.
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3.2. Model calculations and XPS measurements
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As noted above, there is a remarkable size and supportdependent ORR activity, which could be associated with the number of Pt active sites and their electronic properties [1,6,12]. By employing the model calculations developed in our previous work [38], the Pt (111) atoms were suggested as the dominant active sites for the ORR [11]. For such a method, HRTEM measurements and the corresponding fast Fourier transform (FFT) analysis of the ALD-Pt/CNT, impregnation-Pt/CNT, and commercial 20% Pt/C catalysts were carried out, and the results are shown in Figs. 2(a–b) and Fig. S3. In most cases, the Pt nanoparticles are prone to exist as a well-defined truncated cuboctahedron with a top surface of (100) based on the 70.5° angle between the (111) surfaces over these catalysts. Owing to the distribution of the Pt particle sizes, all the observed Pt nanoparticles were considered to calculate the number of Pt (111) atoms. As expected, the well-controlled ALDPt/CNT catalysts showed additional Pt (111) active sites in Fig. 2(c) due to the relatively narrow Pt particle size distribution mentioned above. Based on the determined number of the Pt (111) atoms, the TOF of the catalysts were obtained, and the results are shown in Fig 2(c). Interestingly, the ALD-Pt/CNT catalysts also exhibit higher TOF than the impregnation-Pt/CNT and commercial 20% Pt/C catalysts, strongly indicating their remarkable electronic effects.
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Fig. 2. Typical HRTEM images and the corresponding FFT analysis of (a) the ALD-Pt/CNT and (b) the impregnation-Pt/CNT catalysts. The inset models present a truncated octahedron Pt shape. (c) Number of Pt (111) atoms per mole and the calculated turn over frequency (TOF) at 0.90 V for the ALD-Pt/CNT, impregnation-Pt/CNT, and the commercial 20% Pt/C catalysts.
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Additionally, XPS measurements were employed to determine the Pt electronic properties. Evidently, in Fig. 3, both the ALD-Pt/CNT and the impregnation-Pt/CNT catalysts show decreased Pt0 4f binding energies as the Pt particle size decreases. This is mainly because the electron-withdrawing capacity of the given CNT support leads to the decreased Pt0 4f binding energy of the supported Pt nanoparticles as the Pt loading increases. Previous studies indicated that the reduced Pt binding energy indicates a reduced adsorption strength of the reaction intermediates on the Pt surfaces and thus a relatively high ORR activity [11,15,39–41]. Therefore, these can provide a rational explanation for the increase in the TOF with the Pt particle size due to the decreased Pt0 4f binding energy. It can be also seen in Fig. 3 that with excluding the Pt size effects, the ALD-Pt/CNT catalysts show a higher Pt0 4f binding energy compared with the impregnation-Pt/CNT and commercial 20% Pt/C catalysts (Fig. S3). In principle, the relatively high Pt0 4f binding energy should correspond to the relatively low ORR activity [15], which could not explain why the ALD-Pt/CNT catalysts with relatively high binding energy exhibited enhanced activity. In other words, this strongly implies that another important issue (e.g., the catalyst local environment) could contribute to the increased TOF for the ALD-Pt/CNT catalysts.
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3.3. The crucial effects of the Pt local environments
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To test this idea, the Pt local environments, in terms of the surface properties, for these catalysts were further compared based on the XPS results. As shown in Fig. 4 and Fig. S3, both the impregnation-Pt/CNT and commercial 20% Pt/C catalysts show significant Cl signals, in comparison with the undetectable Cl signal for the ALD-Pt/CNT catalysts. This indicates the difficulty associated with the complete removal of the Cl species during the reduction process for the impregnation-Pt/CNT catalysts. Combining their significantly lower activities, it is reasonable to assume that the existence of Cl would inhibit the ORR activity [21,42,43]. To semi-quantify the information of the residual Cl over the Pt
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nanoparticles, ion chromatography was employed to determine the difference in the Cl concentrations over these catalysts. As shown in Figs. 4(c) and 4(d), the Cl− concentrations for the impregnation5% Pt/CNT, impregnation-8% Pt/CNT, and impregnation-14% Pt/CNT catalysts were determined to be 297, 99, and 149 μg g−1 , respectively. Moreover, the commercial 20% Pt/C catalyst shows the highest Cl− concentration of 706 μg g−1 , as shown in Table S2. It can be observed that the trend of the Cl− concentration for these catalysts is inconsistent with that of the activity, in which the commercial 20% Pt/C catalyst with more Cl− shows higher activity than the impregnation-Pt/CNT catalysts. Notably, the ion chromatography-detected Cl− could distribute either on the Pt surface or the support surface; thus, it is important to distinguish the influence of Cl− on the Pt surface from that on the support surface. Along this line, to separately investigate the Cl− influence on the Pt surface, CV measurements were conducted to evaluate the H-adsorption/desorption behaviors for these catalysts, which could further yield their under-potentially deposited hydrogen (Hupd ) [21,44]. As shown in Fig. 5 and Fig. S4, with the exclusion of the Pt size effects, the Hupd -region (noted as (2)) for the Cl-containing impregnation-Pt/CNT catalysts narrows with respect to the ALDPt/CNT catalysts, which could be due to an energetic modification of H-adsorption/desorption. Generally, Hupd corresponds to an ideal one-electron transfer assuming one H atom per Pt surface atom [44,45]. In other words, this suggests a one-to-one relationship between the Pt surface atom and the H atom. Thus, the narrow Hupd region for the impregnation-Pt/CNT catalysts indicates a relatively low amount of Pt surface sites, which could be blocked by Cl− . As a result, the impregnation-5% Pt/CNT catalyst in Fig. 5(a) exhibits the lowest amount of H-adsorption, which is consistent with the highest amount of Cl− concentration in Fig. 4(d). Moreover, the commercial 20% Pt/C catalyst in Fig. 5(c) shows a much higher H-adsorption than the other catalysts, which could be due to the preferential adsorption of Cl− on the support surface instead of the Pt surface. Therefore, it is evident that the Cl− ions prefer to adsorb on the Pt surface and inhibit the ORR activity for the impregnation-Pt/CNT catalysts. Conversely, the Cl− ions prefer
Please cite this article as: J. Gan, J. Zhang and B. Zhang et al., Active sites engineering of Pt/CNT oxygen reduction catalysts by atomic layer deposition, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.024
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Fig. 3. XPS Pt 4f spectra of the (a) ALD-Pt/CNT and (b) impregnation-Pt/CNT catalysts.
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to adsorb on the support surface and has limited influence for the commercial 20% Pt/C catalyst. These observations could account for the higher activity of the commercial 20% Pt/C catalyst compared to that of the impregnation-Pt/CNT catalysts. To further verify the negative effects of Cl− on the ORR, certain amounts of NaCl were added to the HClO4 solution to obtain different Cl− concentrations, which were used as electrolytes for the ORR test over the Cl-free ALD-Pt35 /CNT catalyst. It can be observed from the LSV curves in Fig. 5(d) that there is a gradual decrease in the ORR activity as the Cl− concentration increases, which is consistent with the above results. Specifically, the half-wave potential (Fig. 5(e)) exhibits a rapid decay from 0.86 to 0.65 V with the Cl− concentration increasing from 10−4 to 10−1 M. Additionally, based on the CV curves in Fig. 5(f), with the increase in the Cl− concentration, the peak of the oxygen-containing species adsorption downshifts (noted as (4)), and the oxygen reduction peak exhibits a left shift from 0.8 V to 0.6 V (noted as (3)). These results indicate that Cl− could compete with the reactants for the active sites over the Pt surfaces, and thus inhibit the ORR activity. Based on the above analyses, the well-controlled Pt particle size and narrow-sized distribution of the ALD-Pt/CNT catalysts result in additional Pt (111) active sites, and the relatively largesized Pt/CNT catalyst shows a relatively low Pt0 4f binding energy and thus a relatively high TOF. However, over the impregnationPt/CNT and commercial 20% Pt/C catalysts, the presence of Cl− leads to remarkable poisonous effects, which block the Pt surfaces
toward relatively few Pt active sites and thus decrease the ORR activity.
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3.4. Improved ORR selectivity of Pt/CNT catalysts synthesized by ALD
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In addition to the ORR activity, the ORR selectivity is another important issue, because the ORR proceeds preferably via a fourelectron pathway to form H2 O. This is much more efficient than the two-electron pathway, which forms H2 O2 [1,6,11,37]. Along this line, RRDE testing was carried out to determine the number of electron transfers (i.e., n) and the H2 O2 yield for these catalysts; the results are shown in Fig. 6. Based on the RRDE curves in Figs. 6(a) and 6(b), the average numbers of electron transfers (n) were calculated and summarized in Fig. 6(c). It was found that the ALD-Pt/CNT and commercial 20% Pt/C catalysts showed much higher electron transfer numbers and lower H2 O2 yield than the impregnation-Pt/CNT catalysts. This indicates that the ORR over the former two catalysts preferably proceeds through an efficient fourelectron pathway with a relatively high ORR selectivity, while the impregnation-Pt/CNT catalysts exhibit a relatively low selectivity. This could have resulted because the abundant Pt active sites are blocked by Cl− and thus suppress the breaking of the O–O bond for the impregnation-Pt/CNT catalysts. Conversely, for the Cl-free ALD-Pt/CNT catalysts, the ALDPt20 /CNT catalyst exhibits the highest ORR selectivity compared
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Fig. 4. XPS Cl 2p spectra of the (a) ALD-Pt/CNT and (b) impregnation-Pt/CNT catalysts. The Cl− concentration of the impregnation-Pt/CNT catalysts determined by (c) XPS spectroscopy and (d) ion chromatography.
Fig. 5. (a–c) CV curves between 0.05 and 1.20 V in N2 -saturated 0.1 M HClO4 for the catalysts. (d) LSV polarization curves; (e) the E1/2 values and (f) CV curves of the ALD-Pt35 /CNT catalyst in O2 -saturated 0.1 M HClO4 with different Cl− concentrations. Notes (1) and (2) present the hydrogen desorption and adsorption peak, respectively. Meanwhile, notes (3) and (4) present the oxygen desorption and adsorption peak, respectively.
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with the ALD-Pt10 /CNT and ALD-Pt35 /CNT catalysts (Fig. 6), which could not be explained by the difference in the Cl− concentrations. Considering the medium Pt 4f binding energy of the ALD-Pt20 /CNT catalyst, we attribute this to the appropriate adsorption strength of the reaction intermediates on the Pt surfaces. Based on previous studies [46], if the metal binds with oxygen exceedingly strongly, it would result in a high degree of oxygenated species coverage, thus
blocking the Pt active sites and suppressing the cleavages of the O–O bond. Conversely, if the metal binds with oxygen too weakly, it would result in a low degree of oxygenated species coverage and thus insufficient O–O bond cleavage [46]. Therefore, the ALDPt20 /CNT catalyst with the optimal Pt binding energy shows the highest ORR selectivity. These results indicate that a cost-effective Pt/CNT catalyst should possess a desirable structure and electronic
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Fig. 6. (a, b) RRDE polarization curves for (a) the ALD-Pt/CNT and commercial 20% Pt/C as well as (b) the impregnation-Pt/CNT catalysts. (c) The corresponding average number of transferred electrons (n) and H2 O2 yield.
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properties as well as Cl-free local environment to achieve high ORR activity and selectivity.
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4. Conclusions
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In summary, three differently sized Pt nanocatalysts were prepared by both atomic layer deposition (ALD) and impregnation methods. Their performances toward the ORR in acidic media were comparatively studied to probe the effects of the Pt particle size together with the distributions, electronic properties, and local environments. The as-prepared ALD-Pt/CNT catalysts exhibited much higher ORR activity and selectivity than the impregnation-Pt/CNT catalysts. A combination of electrocatalytic measurements, catalyst characterizations, and model calculations was conducted to establish the relationship between the catalyst structure and ORR performance. It was found that the well-controlled Pt particle sizes and distributions, desirable Pt0 4f binding energy, and the Cl-free Pt surfaces of the ALD-Pt/CNT catalysts contributed to the outstanding ORR performance. The insights gained here could provide a new direction for the rational design and fine-tuning of carbonsupported Pt-based catalysts for the ORR.
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Acknowledgments
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This work was financially supported by the Natural Science Foundation of China (21922803 and 21776077), the Shanghai Natural Science Foundation (17ZR1407300 and 17ZR1407500), the Program for the Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the Shanghai RisingStar Program (17QA1401200), the State Key Laboratory of OrganicInorganic Composites (oic-201801007), and the Open Project of State Key Laboratory of Chemical Engineering (SKLChe-15C03).
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Supplementary materials
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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2019.09.024.
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References
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[1] Z.W. Seh, J. Kibsgaard, C.F. Dickens, I. Chorkendorff, J.K. Nørskov, T.F. Jaramillo, Science 355 (6321) (2017) eaad4998. [2] R. Chattot, O.L. Bacq, V. Beermann, S. Kühl, J. Herranz, S. Henning, L. Kühn, T. Asset, L. Guétaz, G. Renou, J. Drnec, P. Bordet, A. Pasturel, A. Eychmüller, T.J. Schmidt, P. Strasser, L. Dubau, F. Maillard, Nat. Mater. 17 (9) (2018) 827– 833. [3] M.F. Li, 1 Z.P. Zhao, T. Cheng, A. Fortunelli, C.Y. Chen, R. Yu, Q.H. Zhang, L. Gu, B. Merinov, Z.Y. Lin, E. Zhu, T. Yu, Q.Y. Jia, J.H. Guo, L. Zhang, W.A. Goddard III, Y. Huang, X.F. Duan, Science 354 (6318) (2016) 1414–1419. [4] A. Kulkarni, S. Siahrostami, A. Patel, J.K. Nørskov, Chem. Rev. 118 (5) (2018) 2302–2312. [5] C. Jackson, G.T. Smith, D.W. Inwood, A.S. Leach, P.S. Whalley, M. Callisti, T. Polcar, A.E. Russell, P. Levecque, D. Kramer, Nat. Commun. 8 (2017) 15802. [6] Y. Jiao, Y. Zheng, M. Jaroniec, S.Z. Qiao, Chem. Soc. Rev. 44 (8) (2015) 2060– 2086. [7] D.S. He, D.P. He, J. Wang, Y. Lin, P.Q. Yin, X. Hong, Y. Wu, Y.D. Li, J. Am. Chem. Soc. 138 (5) (2016) 1494–1497. [8] X. Liu, H.M. Wang, S.G. Chen, X.Q. Qi, H.L. Gao, Y. Hui, Y. Bai, L. Guo, W. Ding, Z.D. Wei, J. Energy Chem. 23 (3) (2014) 358–362. [9] C. Fan, Z.H. Huang, X.Y. Hu, Z.P. Shi, T.Y. Shen, Y.W. Tang, X.J. Wang, L. Xu, Green Energy Environ. 3 (2018) 310–317. [10] H.F. Zhang, J.B. Zhang, K.H. Liu, Y.Q. Zhu, X.Y. Qiu, D.M. Sun, Y.W. Tang, Green Energy Environ. 4 (2019) 245–253. [11] J. Gan, W. Luo, W.Y. Chen, J.N. Guo, Z.H. Xiang, B.X. Chen, F. Yang, Y.J. Cao, F. Song, X.Z. Duan, X.G. Zhou, Eur. J. Inorg. Chem. 27 (2019) 3210–3217. [12] Y. Nie, L. Li, Z.D. Wei, Chem. Soc. Rev. 44 (2015) 2168–2201. [13] A.M.G. Marín, R. Rizoa, J.M. Feliu, Catal. Sci. Technol. 4 (2014) 1685–1698. [14] I. Katsounaros, S. Cherevko, A.R. Zeradjanin, K.J.J. Mayrhofer, Angew. Chem. Int. Ed. 53 (1) (2014) 102–121. [15] Y. Peng, B.Z. Lu, N. Wang, L.G. Li, S.W. Chen, Phys. Chem. Chem. Phys. 19 (14) (2017) 9336–9348. [16] S. Sui, X.Y. Wang, X.T. Zhou, Y.H. Su, S. Riffatc, C.J. Liu, J. Mater. Chem. A 5 (2017) 1808–1825.
381Q3 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413
Please cite this article as: J. Gan, J. Zhang and B. Zhang et al., Active sites engineering of Pt/CNT oxygen reduction catalysts by atomic layer deposition, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.024
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414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440
ARTICLE IN PRESS
[m5G;October 12, 2019;12:38]
J. Gan, J. Zhang and B. Zhang et al. / Journal of Energy Chemistry xxx (xxxx) xxx
[17] M. Nesselberger, S. Ashton, J.C. Meier, I. Katsounaros, K.J.J. Mayrhofer, M. Arenz, J. Am. Chem. Soc. 133 (43) (2011) 17428–17433. [18] M.H. Shao, A. Peles, K. Shoemaker, Nano Lett. 11 (9) (2011) 3714–3719. [19] P. Munnik, P.E. de Jongh, K.P. de Jong, Chem. Rev. 115 (14) (2015) 6687–6718. [20] N. Job, M. Chatenet, S.B. Fabry, S. Hermans, F. Maillard, J. Power Sources 240 (2013) 294–305. [21] T.J. Schmidt, U.A. Paulus, H.A. Gasteiger, R.J. Behm, J. Electroanal. Chem. 508 (2001) 41–47. [22] X.W. Yu, S.Y. Ye, J. Power Sources 172 (1) (2007) 133–144. [23] T.M. Arruda, B. Shyam, J.M. Ziegelbauer, S. Mukerjee, D.E. Ramaker, J. Phys. Chem. C 112 (46) (2008) 18087–18097. [24] N.M. Markovic, P.N. Ross, J. Electroanal. Chem. 330 (1–2) (1992) 499–520. [25] Y.H. Chung, D.Y. Chung, N. Jung, Y.E. Sung, J. Phys. Chem. Lett. 4 (8) (2013) 1304–1309. [26] D.G. Li, C. Wang, D. Tripkovic, S.H. Sun, N.M. Markovic, V.R. Stamenkovic, ACS Catal. 2 (2012) 1358–1362. [27] F.J.V. Iglesias, J.S. Gullón, E. Herrero, V. Montiel, A. Aldaz, J.M. Feliu, Electrochem. Commun. 13 (2011) 502–505. [28] J.K. Zhang, W.Y. Chen, H.B. Ge, C.Q. Chen, W.J. Yan, Z. Gao, J. Gan, B.Y. Zhang, X.Z. Duan, Y. Qin, Appl. Catal. B: Environ. 235 (2018) 256–263. [29] Z. Gao, Y. Qin, Acc. Chem. Res. 50 (9) (2017) 2309–2316. [30] N. Cheng, S. Stambula, D. Wang, M.N. Banis, J. Liu, A. Riese, B.W. Xiao, R.Y. Li, T.K. Sham, L.M. Liu, G.A. Botton, X.L. Sun, Nat. Commun. 7 (2016) 13638. [31] C. Marichy, M. Bechelany, N. Pinna, Adv. Mater. 24 (2012) 1017–1032. [32] N.C. Cheng, Y.Y. Shao, J. Liu, X.L. Sun, Nano Energy 29 (2016) 220–242. [33] L. Zhang, Y. Zhao, M.N. Banis, K. Adair, Z.X. Song, L.J. Yang, M. Markiewicz, J.J. Li, S.Z. Wang, R.Y. Li, S.Y. Ye, X.L. Sun, Nano Energy 60 (2019) 111–118.
[34] S.C. Xu, Y.M. Kim, J. Park, D. Higgins, S.J. Shen, P. Schindler, D. Thian, J. Provine, J. Torgersen, T. Graf, T.D. Schladt, M. Orazov, B.H. Liu, T.F. Jaramillo, F.B. Prinz, Nat. Catal. 1 (2018) 624–630. [35] X.M. Wang, N. Li, J.A. Webb, L.D. Pfefferle, G.L. Haller, Appl. Catal. B: Environ. 101 (1–2) (2010) 21–30. [36] X.Y. Yan, X.L. Tong, Y.F. Zhang, X.D. Han, Y.Y. Wang, G.Q. Jin, Y. Qin, X.Y. Guo, Chem. Commun. 48 (13) (2012) 1892–1894. [37] L. Colmenares, E. Guerrini, Z. Jusys, K.S. Nagabhushana, E. Dinjus, S. Behrens, W. Habicht, H. Bo¨nnemann, R.J. Behm, J. Appl. Electrochem. 37 (2007) 1413– 1427. [38] W.Y. Chen, J. Ji, X. Feng, X.Z. Duan, G. Qian, P. Li, X.G. Zhou, D. Chen, W.K. Yuan, J. Am. Chem. Soc. 136 (2014) 16736–16739. [39] G.A. Tritsaris, J. Greeley, J. Rossmeisl, J.K. Nørskov, Catal. Lett. 141 (2011) 909– 913. [40] Z.Y. Zhou, X.W. Kang, Y. Song, S.W. Chen, J. Phys. Chem. C 116 (19) (2012) 10592–10598. [41] V. Stamenkovic, B.S. Mun, K.J.J. Mayrhofer, P.N. Ross, N.M. Markovic, J. Rossmeisl, J. Greeley, J.K. Nørskov, Angew. Chem. 118 (2006) 2963–2967. [42] K. Mamtani, Jain D, A.C. Co, U.S. Ozkan, Catal. Lett. 147 (2017) 2903–2909. [43] N.M. Markovic, T.J. Schmidt, V. Stamenkovic´, P.N. Ross, Fuel Cells 1 (2) (2001) 105–116. [44] D. Chen, Q. Tao, L.W. Liao, S.X. Liu, Y.X. Chen, S. Ye, Electrocatal. 2 (2011) 207– 219. [45] J.N. Tiwari, K. Nath, S. Kumar, R.N. Tiwari, K.C. Kemp, N.H. Le, D.H. Youn, J.S. Lee, K.S. Kim, Nat. Commun. 4 (2013) 2221. [46] M.G. Balcázar, F.M.C. Muniz, L. Á. Contreras, L.G. Arriaga, J.L. García, J. Power Sources 197 (2012) 121–124.
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