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Template-free synthesis of platinum hollow-opened structures in deep-eutectic solvents and their enhanced performance for methanol electrooxidation
Electrochimica Acta 337 (2020) 135742
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Template-free synthesis of platinum hollow-opened structures in deep-eutectic solvents and their enhanced performance for methanol electrooxidation Xiaoqu Wang a, Miaolan Sun a, Sheng Xiang a, Muhammad Waqas a, Youjun Fan a, **, Jingping Zhong a, Kexin Huang a, Wei Chen a, ***, Laijun Liu b, Jun Yang c, d, * a
Guangxi Key Laboratory of Low Carbon Energy Materials, College of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, Guilin 541004, China College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China c State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China d Zhongke Langfang Institute of Process Engineering, Fenghua Road No 1, Langfang Economic & Technical Development Zone, Hebei Province 065001, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 November 2019 Received in revised form 13 January 2020 Accepted 18 January 2020 Available online 20 January 2020
As a novel non-aqueous green solvent that can deem to be the optimal substitution of conventional solvents and ionic liquids, deep-eutectic solvents (DESs) have attracted significant attention from academia and industry in terms of chemical synthesis in recent years. Herein, we report the highly effective preparation of Pt nanoparticles with hollow-opened structures (Pt HOSs) as an efficient electrocatalyst for methanol oxidation reaction (MOR) via a facile template-free solvothermal synthesis in DESs. The as-obtained Pt HOSs are consisted of well assembled secondary and tertiary branches, which can remarkably improve the utilization efficiency and electrocatalytic activity of Pt via promoting the access of electroactive species to the full-extent Pt surface. In this regard, the as-prepared Pt HOSs possess higher electrochemically active surface areas (ECSAs) and show improved electrochemical properties for MOR. Furthermore, the Pt HOSs retain a higher reactivity after the long-term testing compared with commercial Pt black, suggesting that they have enhanced electrocatalytic durability. In specific, the rational design of Pt HOSs in DESs may enrich the synthetic strategies for producing more electrocatalysts with impressive activity and stability. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Deep eutectic solvents Hollow-opened structures Platinum nanoparticles Electrocatalyst Methanol oxidation reaction
1. Introduction Direct methanol fuel cells (DMFCs), which rely on the electrocatalytic oxidation of methanol at their anode, have been favored as the prospective power conversion devices for electric vehicles and portable power systems owing to the key characteristics, including high conversion efficiency, high power density, environmental friendliness, and structural simplicity [1e4]. Currently, the most effective electrocatalysts for MOR at the anode of DMFCs are
* Corresponding author. State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (Y. Fan), [email protected] (W. Chen), [email protected] (J. Yang). https://doi.org/10.1016/j.electacta.2020.135742 0013-4686/© 2020 Elsevier Ltd. All rights reserved.
arguably still dominated by platinum (Pt)-based catalysts, including the commercially representative non-supported Pt black [4e7]. However, achieving the large-scale deployment and widespread commercialization of DMFCs confronts with the following major challenges: 1) the low utilization and prohibitive cost of noble metal Pt catalyst [8]; 2) the sluggish reaction kinetics of MOR and the poisoning effect leading to a sharp decay of Pt activity during the catalytic process [9]. In principle, the enhancement of intrinsic activity and stability of an electrocatalyst highly depends on its exposed active sites, geometric features, compositions and atomic arrangements [10e14]. Shape-controlled synthesis undoubtedly has the advantages to equip the Pt-based electrocatalysts with desired characters favorable for MOR [15e17]. Unfortunately, the reported Pt-based catalysts with welcome topographies are basically solid particles, which are extremely unfavorable for noble metals that only catalyze chemical reactions through their surface active sites. The solid structure sacrifices a large proportion of the
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naturally cooling down to room temperature, the obtained black solids were collected by the centrifugation, and washed repeatedly with absolute ethanol to eliminate the residual impurities. The final products were sealed and stored in 2 mL of absolute ethanol.
metal active sites and therefore leads to severe decline in Pt utilization. In this sense, designing materials at nanoscale but with open hollow structures that maximize the decrease of Pt loading and increase Pt utilization has become one of the effective ways to develop highly efficient and stable anode electrocatalysts for promoting MOR [18e20]. So far, although a considerable number of strategies based on sacrificial templates [21e24] or electrochemical displacement method [25e28] have been developed for producing Pt nanoparticles with hollow structures, the synthetic processes are usually complicated, and the obtained catalysts are mostly closed capsulelike structures that are unfavorable for the use of their inner surfaces. Additionally, the template-mediated methods often involve the removal of internal templates after synthesis, and this would inevitably induce several inherent disadvantages ranging from the difficulty of obtaining high product yields from a multi-step synthetic process to the lack of structural robustness associated with the removal of supporting templates. Therefore, exploring a simple and effective strategy to synthesize Pt-based electrocatalysts with hollow-open structure and desired robustness is highly demanded. Deep eutectic solvents (DESs) are a novel type of anhydrous green solvents, have been considered to be a promising solvent candidate in synthesizing versatile nanomaterials due to their favorable characteristics compared to traditional solvents and ionic liquids, including negligible volatility, high thermal stability, good solubility and conductivity, environmental friendliness, economical affordability, and wide electrochemical window [29e33]. Herein, making use of an effective template-free method, we report the use of choline chloride/ethylene glycol DESs with the unique adaptability and flexibility as media for the preparation of Pt HOSs with well-defined boundaries and interior cavities. In comparison with the material synthesis conducted in conventional solvents, the proposed synthetic route with cost-effectiveness and energy efficiency. Simultaneously, the growth mechanism and technical conditions affecting the features of Pt HOSs are also systematically studied. In specific, as we will demonstrate, the as-prepared Pt HOSs exhibit better electrocatalytic performance for MOR compared with the commercial Pt black catalyst.
The electrochemical tests were performed on CHI 660D electrochemical workstation by using a conventional three-electrode system. A platinum foil and a saturated calomel electrode (SCE) were employed as the counter and reference electrodes, respectively. All potentials reported in the electrochemical measurements were versus the SCE. The working electrode was prepared by pipetting 5 mL of the prepared catalyst suspension on the pretreated glassy carbon electrode. After natural air-drying, the surface was exposed under a UV-ozone lamp (8 W, wavelength 245 nme354 nm) to irradiate for 12 h for removing the residual surfactants. Subsequently, 5 mL Nafion solution (0.2 wt%) was dropped and air-dried on the surface of the irradiated electrode at room temperature for the electrochemical studies. The activation step of the catalyst surface and CO stripping tests were performed by the CV technique in 0.5 M H2SO4 at a scan rate of 50 mV s1, and the electrocatalytic performance of catalysts for MOR was investigated in a 0.5 M H2SO4 solution containing 0.5 M CH3OH.
2. Experimental
3. Results and discussion
2.1. Materials
In the typical preparation of Pt HOSs, DESs serve as the reducing agent, while PVP and SDS act as capping agent or structuredirecting agent, respectively. The morphology and particle size of the as-obtained Pt HOSs from the template-free synthesis were firstly investigated by SEM. As observed from Fig. 1A and B, the feature of the achieved Pt products is dominated by well-defined hollow-opened structures (HOSs). Fig. 1C shows the SEM image of Pt HOSs at higher magnification, which reveals that the Pt HOSs are actually composed of relatively fine Pt nanoparticles, among which the gaps would establish channels for reactants to access the interiors of Pt HOSs. AFM was used to estimate the roughness of the Pt HOSs. As evinced by Fig. S1 of supplementary materials (SM) for the AFM image of an arbitrary single hollow-opened Pt particle, the root mean square roughness (Rrms) of the Pt HOSs is determined to be 11.2 nm, which means that the surface of the particles is very rough. Thus, this unique hollow-open structural feature as well as their rough surface endows the Pt products significant advantages in terms of improving their utilization and catalytic activity [36]. The Pt HOSs have an average size of 540 nm, as statistically shown by the inset of Fig. 1B for the histogram of particle size distribution. It is noteworthy that the distinctive hollow structure formed by particle aggregates and the gaps among fine particles may compensate for the defects caused by the overall larger particle size [37]. The crystal structures and phase composition of commercial Pt
Analytical grade choline chloride, ethylene glycol, methanol and ethanol from Sinopharm Chemical Reagent Co. Ltd., China, platinum acetylacetonate (Pt(acac)2, 99.95%), Commercial platinum (Pt) black (99.95%) and polyvinylpyrrolidone (PVP) from Alfa Aesar, Nafion solution (5 wt%) from Sigma-Aldrich, and sodium dodecyl sulfate (SDS) from Aladdin, were used as received. All aqueous solutions in the experiment were obtained by dissolving the corresponding precursors in tri-distilled water. 2.2. Preparation of Pt HOSs The synthesis of choline chloride/ethylene glycol DESs can refer to a previously reported procedure [34,35]. The eutectic mixture was obtained by mixing two components (1 Choline chloride: 2 ethylene glycol) at 80 C to form a uniform colorless liquid phase, sealed and stored for further synthesis process. The Pt HOSs were synthesized by a simple template-free chemical reduction method. Typically, 4.0 mg of Pt(acac)2, 40 mg of PVP and 25 mg of SDS were ultrasonically dissolved in 8 mL of DESs for 10 min at room temperature, followed by magnetic stirring for 5 min to disperse the precursor uniformly. Then, the above mixture was transferred into an oil bath (130 C) and kept there for 2 h under stirring. After
2.3. Physical characterization X-ray diffraction (XRD) were carried out on an X-ray diffractometer (Rigaku D/Max 2500 v/pc, Japan) using a Cu Ka radiation source. The surface morphologies, size and microstructures of the prepared samples were captured by scanning electron microscopy (SEM, FEI Quanta FEG 200, Holland) and high-resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F30, USA) operated at 200 kV. The roughness of the Pt HOSs was estimated by atomic force microscopy (AFM, BRUKE, DIMENSION FastSan Bio, Germany). X-ray photoelectron spectroscopy (XPS) analyses were performed by a Thermo Fisher Scientific EscaLab 250xi system with an Al Ka radiation as the excitation source. 2.4. Electrochemical measurements
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Fig. 1. (AeC) SEM images of the Pt HOSs with different magnifications. The inset in (B) is the corresponding histogram showing the size distribution of the as-prepared Pt HOSs. (D) XRD patterns of commercial Pt black and Pt HOSs.
black and Pt HOSs were disclosed by their XRD patterns (Fig. 1D). The diffraction peaks for Pt black and Pt HOSs at approximately 2q of 39.6 , 46.1, 67.5 , and 81.3 well match with the (111), (200), (220) and (311) planes of the face-centered cubic (fcc) Pt, respectively [38]. Additionally, the diffraction peaks of Pt HOSs are apparently broadened compared to Pt black because the particles that make up the Pt HOSs are smaller. The average size of fine particles in Pt HOSs is estimated to be ca. 3.15 nm in view of the Pt (220) peak. The microstructures of as-prepared Pt HOSs were also studied by TEM. As shown in Fig. 2A for the TEM image at low magnification, each of the Pt HOSs exhibits a hollow interior surrounded by a large number of tiny particles. HRTEM image (Fig. 2B) shows that the lattice fringes with distances of 0.23 nm and 0.198 nm are clearly visible, which perfectly matched with (111) faces and (200) faces of the fcc Pt, respectively [39]. Notably, the discontinuous lattice fringes and clearly visible grain boundaries indicate that the Pt HOSs are polycrystalline structures, which is in line with the reaction-limited aggregation (RLA) mechanism of spherical nanoparticles [40]. The polycrystalline nature of Pt HOSs could also be
verified by the selected area electron diffractions (SAEDs) of the same, which display ring-like patterns corresponding to the Pt (111), Pt (200), Pt (220) and Pt (311) faces, respectively (Fig. 2C). Further, a large quantity of crystalline defects such as interface dislocations, intragranular dislocations, and stacking faults are found, as indicated by the oval marks in Fig. 2B, on which the Pt atoms are generally considered to be highly active for catalytic reactions [41,42]. In addition, the intimate connections among fine Pt nanoparticles could guarantee the good conductivity of the asprepared Pt HOSs. The Pt HOSs were also examined by energy dispersive X-ray (EDX) spectrum (Fig. 2D), which proves that products are dominated by Pt elements. Other elements, e.g. Cu, C, O and N, are either from TEM cupper grid, or from the surface capping agent, i.e. PVP. For investigating the growth mechanism of the Pt HOSs, the evolution of the size and morphology of the Pt HOSs was recorded by TEM at different times, as shown in Fig. 3. It is noteworthy that almost no products are collected for the aliquot taken out from the reaction mixture at reaction time of 15 min since the precursors are yet to be reduced. After reaction for 30 min, microcrystalline
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Fig. 2. Representative TEM image (A), HRTEM image (B), SAED pattern (C) and EDX spectrum (D) of the as-prepared Pt HOSs.
dendrites with an overall size of ca. 10 nm are found (Fig. 3A). As the reaction time continues, the dendritic feature of the products becomes more pronounced, accompanied by an increase in the overall size, as evinced by Fig. 3B and C for the TEM images at 45 min and 60 min, respectively. After 90 min of reaction, the dendritic particles with an average overall size of ca. 29.4 nm are the dominant product, as display by Fig. 3D and E for their TEM images at different magnifications. The average size of the grains that make up the dendritic particles is ca. 2.92 nm, as shown by SM Fig. S2 for the histogram derived from Fig. 3D, which agrees basically with the result obtained from XRD calculation (ca. 3.15 nm). Eventually, the dendritic particles evolve into hollow-opened structures when the reaction time reaches 120 min (Fig. 3F), and this evolution may originate from the cross-linking phenomenon among the nanobranches of the dendritic particles under the synergistic effect of capping surfactants and structural directing agents. In order to understand the critical roles of SDS and PVP in the synthetic system, we have made some control experiments to research the effects of SDS and PVP on the final morphology of Pt HOSs. In the absence of PVP and SDS, the products are only aggregates of Pt particles with irregular shapes instead of hollow-
open structures, as illustrated by SM Fig. S3A. In contrast, the products obtained are isolated particles with an overage size of ca. 500 nm but still no hollow structures are observed in the presence of only PVP (SM Fig. S3B). Further, the existence of only SDS also results in aggregates consisting of relatively small Pt particles, as shown by SM Fig. S3C. Based on the controls, PVP makes an important impact on keeping the dispersibility of particles in the synthetic process, while SDS accounts for the assembly of the particles. The competition between particle assembly induced by SDS and PVP capping is essential for the formation of Pt dendrites with appropriate sizes before eventually evolution into Pt HOSs. In other words, the successful preparation of Pt HOSs is the result of a synergistic effect of PVP and SDS, which can be deeply understood as the precise regulation of optimal reaction kinetics through the introduction of capping surfactants and structure-directing agents [43]. Inevitably, the capping agent (PVP) in the synthetic system also causes the existence of some isolated Pt particles around the large Pt HOSs, as observed in Figs. 1C and 2A for the SEM and TEM images, which might be reduced by optimizing the reaction conditions, e.g. reaction time, temperature, and PVP/SDS concentrations.
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Fig. 3. TEM images of the intermediate products obtained at reaction time of 30 min (A), 45 min (B), 60 min (C), 90 min (D, E), and 120 min (F), respectively.
XPS analyses of the as-prepared Pt HOSs were performed to identify their surface composition and chemical states. Fig. 4A depicts the XPS full spectra of Pt HOSs, in which the Pt signals of Pt 4f (73.4 eV) and Pt 4d (315.2 eV) peaks are clearly observed. Notably, the signals of C 1s (284.6 eV), O 1s (531.7 eV) and N 1s (399.4 eV) peaks appear simultaneously, which are derived from the PVP absorbed on surface of the Pt HOSs. According to the highresolution spectra of Pt 4f shown in Fig. 4B, the chemical composition of the Pt was analyzed via peak fitting. The curve deconvolution fits two pairs of doublets. The more intense doublet (at binding energies of 71.8 eV and 75.3 eV) indicates the metallic Pt, while the relative weaker doublet at binding energies of 72.7 eV and 76.1 eV corresponds to the Pt(II) species, e.g. PtO or Pt(OH)2 [44,45]. The Pt HOSs need to be pre-treated to remove residual surfactants on their surfaces by UV-ozone lamps prior to conducting the electrochemical tests [43]. The electrocatalytic performance of the Pt HOSs were investigated via cyclic voltammograms (CVs) and benchmarked against commercial Pt black in N2-purged 0.5 M H2SO4 aqueous solution at 50 mV s1. As shown in Fig. 5A, both the Pt HOSs and Pt black have significant hydrogen adsorption/desorption peaks in the range of 0.26 V to 0.1 V. According to the hydrogen adsorption/desorption peak area integration, the electrochemically active surface areas (ECSAs) of Pt HOSs and Pt black are 24.7 m2 g1 and 6.7 m2 g1, respectively. The higher ECSAs of Pt HOSs can be attributed to their unique hollow-opened structure that is composed of finer but connected Pt grains, which improve the charge-transport capability despite their overall
larger sizes. The CVs of methanol electrooxidation for Pt HOSs and their Pt black counterparts were obtained in 0.5 M CH3OH þ 0.5 M H2SO4 solution. As illustrated in Fig. 5B and C, the two samples exhibit typical irreversible characteristic peaks in both forward and reverse scans, which could be assigned to the oxidation of methanol (0.63 V) and carbon-containing intermediate species (0.48 V), respectively [46]. As summarized in Fig. 5D, the Pt HOSs show much higher mass activity and specific activity toward MOR, which are about 5.4 and 1.5 times those of commercial Pt black catalyst, respectively. In particular, as revealed by the inset of Fig. 5B, the onset potential of MOR over the Pt HOSs is 0.26 V, much lower than that of commercial Pt black catalyst (0.34 V), indicating that the Pt HOSs can improve the inherent kinetics of MOR. Regarding the long-term activity and stability for the asprepared Pt HOSs, the chronoamperometry experiments were carried out for 7200 s in 0.5 M CH3OH þ 0.5 M H2SO4 aqueous solution at a constant potential of 0.6 V. As revealed by Fig. 6A, the Pt HOSs show higher initial and final current densities compared with commercial Pt black, indicating that the Pt HOSs have higher electrochemical activity and durability for MOR. The current density of Pt HOSs decay sharply at the initial time due to the poisoning effect from carbonaceous intermediates formed during MOR (e.g., COads, CH3OHads and CHOads), which have strong adsorption on the Pt surfaces. After a durability test for 7200 s, the Pt HOSs still retain a higher reactivity (29.3 mA mg1 Pt ) than that of Pt black (6.3 mA mg1 Pt ). In addition, the cyclic voltammetry tests were also performed to study the variation of peak current densities in 0.5 M CH3OH þ 0.5 M H2SO4 aqueous solution at a scan rate of 50 mV s1.
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As shown by SM Fig. S4, the Pt HOSs exhibit a lower attenuation rate (34%) than that of Pt black (42%) after 200 cycles, manifesting that the Pt HOSs have good tolerance to toxic intermediates [38,47]. The CO-tolerance properties of the as-prepared Pt HOSs and commercial Pt black catalyst were estimated by CO-stripping experiments in 0.5 M H2SO4 solution at a scan rate of 50 mV s1. As shown in Fig. 6B, because of the saturated adsorption of CO on the catalytic surface, the hydrogen adsorption/desorption peaks are completely suppressed in the first CV curves of both catalysts. After removal of pre-adsorbed CO by oxidation, the hydrogen adsorption/ desorption peaks appear again in the subsequent CV curves. In addition, the onset potential of CO oxidation on the Pt HOSs is ca. 0.24 V, which is more negative than that on the commercial Pt black (0.30 V), suggesting that the Pt HOSs have better CO-tolerance capability. In recent years, the pre-wave region of catalysts has attracted sustained research interest due to its close relationship with the CO oxidation process [48]. The understanding of this relationship is helpful to reduce the coverage of CO, thereby significantly reducing the impacts of Pt surface poisoning. As observed in Fig. 6B, the pre-wave region of Pt HOSs is broader relative to the Pt black, demonstrating a higher CO tolerance for the surface of the Pt HOSs. Moreover, the pre-oxidation peak of the Pt HOSs is sharper compared with Pt black, indicating a more favorable removal of absorbed CO from their surfaces.
4. Conclusion Fig. 4. XPS survey spectrum (A) and Pt 4f spectrum (B) of the Pt hollow-opened structures (Pt HOSs) as-prepared by a template-free synthesis.
In summary, we developed a facile template-free strategy to synthesize Pt HOSs by using choline chloride/ethylene glycol DESs as reaction medium. The systematic studies on the growth process of Pt HOSs as well as the controlling experiments demonstrate that
Fig. 5. (A) Cyclic voltammograms of Pt HOSs and commercial Pt black in nitrogen-saturated 0.5 M H2SO4 solution. (B) Mass activity, (C) specific activity and (D) performance comparison histogram of MOR on the Pt HOSs and commercial Pt black in 0.5 M CH3OH þ 0.5 M H2SO4 solution. The inset in (B) shows the magnified positive scan CV curves of the boxed area. The scan rate for all the CV tests is 50 mV s1.
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Foundation of Guangxi Province (2017GXNSFDA198031, 2016GXNSFAA380199, 2014GXNSFFA118003), the BAGUI Scholar Program (2014A001) and the Project of Talents Highland of Guangxi Province. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2020.135742. References
Fig. 6. (A) Current-time curves of methanol oxidation on Pt HOSs (a) and commercial Pt black (b) in 0.5 M CH3OH þ 0.5 M H2SO4 solution at a potential of 0.6 V. (B) CO stripping voltammograms of Pt HOSs (a) and commercial Pt black (b) in 0.5 M CH3OH þ 0.5 M H2SO4 with a scan rate of 50 mV s1.
PVP and SDS synergistically regulate the optimal reaction kinetics through balancing the competition between surface capping and particle assemblies for the formation of dendritic products with appropriate sizes, which are eventually evolved into hollowopened structures. In specific, the electrocatalytic evaluations disclose that the as-synthesized Pt HOSs have enhanced electrocatalytic activity, stability and anti-toxicity for MOR than those of commercial Pt black, which are mainly ascribed to their unique hollow-open structural feature. This study might have provided a prospective avenue for the design and fabrication of Pt-based nanostructures with desired catalytic properties. Declaration The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Xiaoqu Wang: Methodology, Investigation, Writing - original draft. Miaolan Sun: Methodology, Investigation. Sheng Xiang: Data curation, Investigation. Muhammad Waqas: Writing - review & editing. Youjun Fan: Conceptualization, Supervision. Jingping Zhong: Data curation, Investigation. Kexin Huang: Investigation. Wei Chen: Supervision. Laijun Liu: Data curation. Jun Yang: Supervision, Writing - review & editing. Acknowledgements This work was supported by the National Natural Science Foundation of China (21463007, 21573240), Natural Science
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Template-free synthesis of platinum hollow-opened structures in deep-eutectic solvents and their enhanced performance for methanol electrooxidation Xiaoqu Wang a, Miaolan Sun a, Sheng Xiang a, Muhammad Waqas a, Youjun Fan a, **, Jingping Zhong a, Kexin Huang a, Wei Chen a, ***, Laijun Liu b, Jun Yang c, d, * a
Guangxi Key Laboratory of Low Carbon Energy Materials, College of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, Guilin 541004, China College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China c State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China d Zhongke Langfang Institute of Process Engineering, Fenghua Road No 1, Langfang Economic & Technical Development Zone, Hebei Province 065001, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 November 2019 Received in revised form 13 January 2020 Accepted 18 January 2020 Available online 20 January 2020
As a novel non-aqueous green solvent that can deem to be the optimal substitution of conventional solvents and ionic liquids, deep-eutectic solvents (DESs) have attracted significant attention from academia and industry in terms of chemical synthesis in recent years. Herein, we report the highly effective preparation of Pt nanoparticles with hollow-opened structures (Pt HOSs) as an efficient electrocatalyst for methanol oxidation reaction (MOR) via a facile template-free solvothermal synthesis in DESs. The as-obtained Pt HOSs are consisted of well assembled secondary and tertiary branches, which can remarkably improve the utilization efficiency and electrocatalytic activity of Pt via promoting the access of electroactive species to the full-extent Pt surface. In this regard, the as-prepared Pt HOSs possess higher electrochemically active surface areas (ECSAs) and show improved electrochemical properties for MOR. Furthermore, the Pt HOSs retain a higher reactivity after the long-term testing compared with commercial Pt black, suggesting that they have enhanced electrocatalytic durability. In specific, the rational design of Pt HOSs in DESs may enrich the synthetic strategies for producing more electrocatalysts with impressive activity and stability. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Deep eutectic solvents Hollow-opened structures Platinum nanoparticles Electrocatalyst Methanol oxidation reaction
1. Introduction Direct methanol fuel cells (DMFCs), which rely on the electrocatalytic oxidation of methanol at their anode, have been favored as the prospective power conversion devices for electric vehicles and portable power systems owing to the key characteristics, including high conversion efficiency, high power density, environmental friendliness, and structural simplicity [1e4]. Currently, the most effective electrocatalysts for MOR at the anode of DMFCs are
* Corresponding author. State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (Y. Fan), [email protected] (W. Chen), [email protected] (J. Yang). https://doi.org/10.1016/j.electacta.2020.135742 0013-4686/© 2020 Elsevier Ltd. All rights reserved.
arguably still dominated by platinum (Pt)-based catalysts, including the commercially representative non-supported Pt black [4e7]. However, achieving the large-scale deployment and widespread commercialization of DMFCs confronts with the following major challenges: 1) the low utilization and prohibitive cost of noble metal Pt catalyst [8]; 2) the sluggish reaction kinetics of MOR and the poisoning effect leading to a sharp decay of Pt activity during the catalytic process [9]. In principle, the enhancement of intrinsic activity and stability of an electrocatalyst highly depends on its exposed active sites, geometric features, compositions and atomic arrangements [10e14]. Shape-controlled synthesis undoubtedly has the advantages to equip the Pt-based electrocatalysts with desired characters favorable for MOR [15e17]. Unfortunately, the reported Pt-based catalysts with welcome topographies are basically solid particles, which are extremely unfavorable for noble metals that only catalyze chemical reactions through their surface active sites. The solid structure sacrifices a large proportion of the
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naturally cooling down to room temperature, the obtained black solids were collected by the centrifugation, and washed repeatedly with absolute ethanol to eliminate the residual impurities. The final products were sealed and stored in 2 mL of absolute ethanol.
metal active sites and therefore leads to severe decline in Pt utilization. In this sense, designing materials at nanoscale but with open hollow structures that maximize the decrease of Pt loading and increase Pt utilization has become one of the effective ways to develop highly efficient and stable anode electrocatalysts for promoting MOR [18e20]. So far, although a considerable number of strategies based on sacrificial templates [21e24] or electrochemical displacement method [25e28] have been developed for producing Pt nanoparticles with hollow structures, the synthetic processes are usually complicated, and the obtained catalysts are mostly closed capsulelike structures that are unfavorable for the use of their inner surfaces. Additionally, the template-mediated methods often involve the removal of internal templates after synthesis, and this would inevitably induce several inherent disadvantages ranging from the difficulty of obtaining high product yields from a multi-step synthetic process to the lack of structural robustness associated with the removal of supporting templates. Therefore, exploring a simple and effective strategy to synthesize Pt-based electrocatalysts with hollow-open structure and desired robustness is highly demanded. Deep eutectic solvents (DESs) are a novel type of anhydrous green solvents, have been considered to be a promising solvent candidate in synthesizing versatile nanomaterials due to their favorable characteristics compared to traditional solvents and ionic liquids, including negligible volatility, high thermal stability, good solubility and conductivity, environmental friendliness, economical affordability, and wide electrochemical window [29e33]. Herein, making use of an effective template-free method, we report the use of choline chloride/ethylene glycol DESs with the unique adaptability and flexibility as media for the preparation of Pt HOSs with well-defined boundaries and interior cavities. In comparison with the material synthesis conducted in conventional solvents, the proposed synthetic route with cost-effectiveness and energy efficiency. Simultaneously, the growth mechanism and technical conditions affecting the features of Pt HOSs are also systematically studied. In specific, as we will demonstrate, the as-prepared Pt HOSs exhibit better electrocatalytic performance for MOR compared with the commercial Pt black catalyst.
The electrochemical tests were performed on CHI 660D electrochemical workstation by using a conventional three-electrode system. A platinum foil and a saturated calomel electrode (SCE) were employed as the counter and reference electrodes, respectively. All potentials reported in the electrochemical measurements were versus the SCE. The working electrode was prepared by pipetting 5 mL of the prepared catalyst suspension on the pretreated glassy carbon electrode. After natural air-drying, the surface was exposed under a UV-ozone lamp (8 W, wavelength 245 nme354 nm) to irradiate for 12 h for removing the residual surfactants. Subsequently, 5 mL Nafion solution (0.2 wt%) was dropped and air-dried on the surface of the irradiated electrode at room temperature for the electrochemical studies. The activation step of the catalyst surface and CO stripping tests were performed by the CV technique in 0.5 M H2SO4 at a scan rate of 50 mV s1, and the electrocatalytic performance of catalysts for MOR was investigated in a 0.5 M H2SO4 solution containing 0.5 M CH3OH.
2. Experimental
3. Results and discussion
2.1. Materials
In the typical preparation of Pt HOSs, DESs serve as the reducing agent, while PVP and SDS act as capping agent or structuredirecting agent, respectively. The morphology and particle size of the as-obtained Pt HOSs from the template-free synthesis were firstly investigated by SEM. As observed from Fig. 1A and B, the feature of the achieved Pt products is dominated by well-defined hollow-opened structures (HOSs). Fig. 1C shows the SEM image of Pt HOSs at higher magnification, which reveals that the Pt HOSs are actually composed of relatively fine Pt nanoparticles, among which the gaps would establish channels for reactants to access the interiors of Pt HOSs. AFM was used to estimate the roughness of the Pt HOSs. As evinced by Fig. S1 of supplementary materials (SM) for the AFM image of an arbitrary single hollow-opened Pt particle, the root mean square roughness (Rrms) of the Pt HOSs is determined to be 11.2 nm, which means that the surface of the particles is very rough. Thus, this unique hollow-open structural feature as well as their rough surface endows the Pt products significant advantages in terms of improving their utilization and catalytic activity [36]. The Pt HOSs have an average size of 540 nm, as statistically shown by the inset of Fig. 1B for the histogram of particle size distribution. It is noteworthy that the distinctive hollow structure formed by particle aggregates and the gaps among fine particles may compensate for the defects caused by the overall larger particle size [37]. The crystal structures and phase composition of commercial Pt
Analytical grade choline chloride, ethylene glycol, methanol and ethanol from Sinopharm Chemical Reagent Co. Ltd., China, platinum acetylacetonate (Pt(acac)2, 99.95%), Commercial platinum (Pt) black (99.95%) and polyvinylpyrrolidone (PVP) from Alfa Aesar, Nafion solution (5 wt%) from Sigma-Aldrich, and sodium dodecyl sulfate (SDS) from Aladdin, were used as received. All aqueous solutions in the experiment were obtained by dissolving the corresponding precursors in tri-distilled water. 2.2. Preparation of Pt HOSs The synthesis of choline chloride/ethylene glycol DESs can refer to a previously reported procedure [34,35]. The eutectic mixture was obtained by mixing two components (1 Choline chloride: 2 ethylene glycol) at 80 C to form a uniform colorless liquid phase, sealed and stored for further synthesis process. The Pt HOSs were synthesized by a simple template-free chemical reduction method. Typically, 4.0 mg of Pt(acac)2, 40 mg of PVP and 25 mg of SDS were ultrasonically dissolved in 8 mL of DESs for 10 min at room temperature, followed by magnetic stirring for 5 min to disperse the precursor uniformly. Then, the above mixture was transferred into an oil bath (130 C) and kept there for 2 h under stirring. After
2.3. Physical characterization X-ray diffraction (XRD) were carried out on an X-ray diffractometer (Rigaku D/Max 2500 v/pc, Japan) using a Cu Ka radiation source. The surface morphologies, size and microstructures of the prepared samples were captured by scanning electron microscopy (SEM, FEI Quanta FEG 200, Holland) and high-resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F30, USA) operated at 200 kV. The roughness of the Pt HOSs was estimated by atomic force microscopy (AFM, BRUKE, DIMENSION FastSan Bio, Germany). X-ray photoelectron spectroscopy (XPS) analyses were performed by a Thermo Fisher Scientific EscaLab 250xi system with an Al Ka radiation as the excitation source. 2.4. Electrochemical measurements
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Fig. 1. (AeC) SEM images of the Pt HOSs with different magnifications. The inset in (B) is the corresponding histogram showing the size distribution of the as-prepared Pt HOSs. (D) XRD patterns of commercial Pt black and Pt HOSs.
black and Pt HOSs were disclosed by their XRD patterns (Fig. 1D). The diffraction peaks for Pt black and Pt HOSs at approximately 2q of 39.6 , 46.1, 67.5 , and 81.3 well match with the (111), (200), (220) and (311) planes of the face-centered cubic (fcc) Pt, respectively [38]. Additionally, the diffraction peaks of Pt HOSs are apparently broadened compared to Pt black because the particles that make up the Pt HOSs are smaller. The average size of fine particles in Pt HOSs is estimated to be ca. 3.15 nm in view of the Pt (220) peak. The microstructures of as-prepared Pt HOSs were also studied by TEM. As shown in Fig. 2A for the TEM image at low magnification, each of the Pt HOSs exhibits a hollow interior surrounded by a large number of tiny particles. HRTEM image (Fig. 2B) shows that the lattice fringes with distances of 0.23 nm and 0.198 nm are clearly visible, which perfectly matched with (111) faces and (200) faces of the fcc Pt, respectively [39]. Notably, the discontinuous lattice fringes and clearly visible grain boundaries indicate that the Pt HOSs are polycrystalline structures, which is in line with the reaction-limited aggregation (RLA) mechanism of spherical nanoparticles [40]. The polycrystalline nature of Pt HOSs could also be
verified by the selected area electron diffractions (SAEDs) of the same, which display ring-like patterns corresponding to the Pt (111), Pt (200), Pt (220) and Pt (311) faces, respectively (Fig. 2C). Further, a large quantity of crystalline defects such as interface dislocations, intragranular dislocations, and stacking faults are found, as indicated by the oval marks in Fig. 2B, on which the Pt atoms are generally considered to be highly active for catalytic reactions [41,42]. In addition, the intimate connections among fine Pt nanoparticles could guarantee the good conductivity of the asprepared Pt HOSs. The Pt HOSs were also examined by energy dispersive X-ray (EDX) spectrum (Fig. 2D), which proves that products are dominated by Pt elements. Other elements, e.g. Cu, C, O and N, are either from TEM cupper grid, or from the surface capping agent, i.e. PVP. For investigating the growth mechanism of the Pt HOSs, the evolution of the size and morphology of the Pt HOSs was recorded by TEM at different times, as shown in Fig. 3. It is noteworthy that almost no products are collected for the aliquot taken out from the reaction mixture at reaction time of 15 min since the precursors are yet to be reduced. After reaction for 30 min, microcrystalline
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Fig. 2. Representative TEM image (A), HRTEM image (B), SAED pattern (C) and EDX spectrum (D) of the as-prepared Pt HOSs.
dendrites with an overall size of ca. 10 nm are found (Fig. 3A). As the reaction time continues, the dendritic feature of the products becomes more pronounced, accompanied by an increase in the overall size, as evinced by Fig. 3B and C for the TEM images at 45 min and 60 min, respectively. After 90 min of reaction, the dendritic particles with an average overall size of ca. 29.4 nm are the dominant product, as display by Fig. 3D and E for their TEM images at different magnifications. The average size of the grains that make up the dendritic particles is ca. 2.92 nm, as shown by SM Fig. S2 for the histogram derived from Fig. 3D, which agrees basically with the result obtained from XRD calculation (ca. 3.15 nm). Eventually, the dendritic particles evolve into hollow-opened structures when the reaction time reaches 120 min (Fig. 3F), and this evolution may originate from the cross-linking phenomenon among the nanobranches of the dendritic particles under the synergistic effect of capping surfactants and structural directing agents. In order to understand the critical roles of SDS and PVP in the synthetic system, we have made some control experiments to research the effects of SDS and PVP on the final morphology of Pt HOSs. In the absence of PVP and SDS, the products are only aggregates of Pt particles with irregular shapes instead of hollow-
open structures, as illustrated by SM Fig. S3A. In contrast, the products obtained are isolated particles with an overage size of ca. 500 nm but still no hollow structures are observed in the presence of only PVP (SM Fig. S3B). Further, the existence of only SDS also results in aggregates consisting of relatively small Pt particles, as shown by SM Fig. S3C. Based on the controls, PVP makes an important impact on keeping the dispersibility of particles in the synthetic process, while SDS accounts for the assembly of the particles. The competition between particle assembly induced by SDS and PVP capping is essential for the formation of Pt dendrites with appropriate sizes before eventually evolution into Pt HOSs. In other words, the successful preparation of Pt HOSs is the result of a synergistic effect of PVP and SDS, which can be deeply understood as the precise regulation of optimal reaction kinetics through the introduction of capping surfactants and structure-directing agents [43]. Inevitably, the capping agent (PVP) in the synthetic system also causes the existence of some isolated Pt particles around the large Pt HOSs, as observed in Figs. 1C and 2A for the SEM and TEM images, which might be reduced by optimizing the reaction conditions, e.g. reaction time, temperature, and PVP/SDS concentrations.
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Fig. 3. TEM images of the intermediate products obtained at reaction time of 30 min (A), 45 min (B), 60 min (C), 90 min (D, E), and 120 min (F), respectively.
XPS analyses of the as-prepared Pt HOSs were performed to identify their surface composition and chemical states. Fig. 4A depicts the XPS full spectra of Pt HOSs, in which the Pt signals of Pt 4f (73.4 eV) and Pt 4d (315.2 eV) peaks are clearly observed. Notably, the signals of C 1s (284.6 eV), O 1s (531.7 eV) and N 1s (399.4 eV) peaks appear simultaneously, which are derived from the PVP absorbed on surface of the Pt HOSs. According to the highresolution spectra of Pt 4f shown in Fig. 4B, the chemical composition of the Pt was analyzed via peak fitting. The curve deconvolution fits two pairs of doublets. The more intense doublet (at binding energies of 71.8 eV and 75.3 eV) indicates the metallic Pt, while the relative weaker doublet at binding energies of 72.7 eV and 76.1 eV corresponds to the Pt(II) species, e.g. PtO or Pt(OH)2 [44,45]. The Pt HOSs need to be pre-treated to remove residual surfactants on their surfaces by UV-ozone lamps prior to conducting the electrochemical tests [43]. The electrocatalytic performance of the Pt HOSs were investigated via cyclic voltammograms (CVs) and benchmarked against commercial Pt black in N2-purged 0.5 M H2SO4 aqueous solution at 50 mV s1. As shown in Fig. 5A, both the Pt HOSs and Pt black have significant hydrogen adsorption/desorption peaks in the range of 0.26 V to 0.1 V. According to the hydrogen adsorption/desorption peak area integration, the electrochemically active surface areas (ECSAs) of Pt HOSs and Pt black are 24.7 m2 g1 and 6.7 m2 g1, respectively. The higher ECSAs of Pt HOSs can be attributed to their unique hollow-opened structure that is composed of finer but connected Pt grains, which improve the charge-transport capability despite their overall
larger sizes. The CVs of methanol electrooxidation for Pt HOSs and their Pt black counterparts were obtained in 0.5 M CH3OH þ 0.5 M H2SO4 solution. As illustrated in Fig. 5B and C, the two samples exhibit typical irreversible characteristic peaks in both forward and reverse scans, which could be assigned to the oxidation of methanol (0.63 V) and carbon-containing intermediate species (0.48 V), respectively [46]. As summarized in Fig. 5D, the Pt HOSs show much higher mass activity and specific activity toward MOR, which are about 5.4 and 1.5 times those of commercial Pt black catalyst, respectively. In particular, as revealed by the inset of Fig. 5B, the onset potential of MOR over the Pt HOSs is 0.26 V, much lower than that of commercial Pt black catalyst (0.34 V), indicating that the Pt HOSs can improve the inherent kinetics of MOR. Regarding the long-term activity and stability for the asprepared Pt HOSs, the chronoamperometry experiments were carried out for 7200 s in 0.5 M CH3OH þ 0.5 M H2SO4 aqueous solution at a constant potential of 0.6 V. As revealed by Fig. 6A, the Pt HOSs show higher initial and final current densities compared with commercial Pt black, indicating that the Pt HOSs have higher electrochemical activity and durability for MOR. The current density of Pt HOSs decay sharply at the initial time due to the poisoning effect from carbonaceous intermediates formed during MOR (e.g., COads, CH3OHads and CHOads), which have strong adsorption on the Pt surfaces. After a durability test for 7200 s, the Pt HOSs still retain a higher reactivity (29.3 mA mg1 Pt ) than that of Pt black (6.3 mA mg1 Pt ). In addition, the cyclic voltammetry tests were also performed to study the variation of peak current densities in 0.5 M CH3OH þ 0.5 M H2SO4 aqueous solution at a scan rate of 50 mV s1.
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As shown by SM Fig. S4, the Pt HOSs exhibit a lower attenuation rate (34%) than that of Pt black (42%) after 200 cycles, manifesting that the Pt HOSs have good tolerance to toxic intermediates [38,47]. The CO-tolerance properties of the as-prepared Pt HOSs and commercial Pt black catalyst were estimated by CO-stripping experiments in 0.5 M H2SO4 solution at a scan rate of 50 mV s1. As shown in Fig. 6B, because of the saturated adsorption of CO on the catalytic surface, the hydrogen adsorption/desorption peaks are completely suppressed in the first CV curves of both catalysts. After removal of pre-adsorbed CO by oxidation, the hydrogen adsorption/ desorption peaks appear again in the subsequent CV curves. In addition, the onset potential of CO oxidation on the Pt HOSs is ca. 0.24 V, which is more negative than that on the commercial Pt black (0.30 V), suggesting that the Pt HOSs have better CO-tolerance capability. In recent years, the pre-wave region of catalysts has attracted sustained research interest due to its close relationship with the CO oxidation process [48]. The understanding of this relationship is helpful to reduce the coverage of CO, thereby significantly reducing the impacts of Pt surface poisoning. As observed in Fig. 6B, the pre-wave region of Pt HOSs is broader relative to the Pt black, demonstrating a higher CO tolerance for the surface of the Pt HOSs. Moreover, the pre-oxidation peak of the Pt HOSs is sharper compared with Pt black, indicating a more favorable removal of absorbed CO from their surfaces.
4. Conclusion Fig. 4. XPS survey spectrum (A) and Pt 4f spectrum (B) of the Pt hollow-opened structures (Pt HOSs) as-prepared by a template-free synthesis.
In summary, we developed a facile template-free strategy to synthesize Pt HOSs by using choline chloride/ethylene glycol DESs as reaction medium. The systematic studies on the growth process of Pt HOSs as well as the controlling experiments demonstrate that
Fig. 5. (A) Cyclic voltammograms of Pt HOSs and commercial Pt black in nitrogen-saturated 0.5 M H2SO4 solution. (B) Mass activity, (C) specific activity and (D) performance comparison histogram of MOR on the Pt HOSs and commercial Pt black in 0.5 M CH3OH þ 0.5 M H2SO4 solution. The inset in (B) shows the magnified positive scan CV curves of the boxed area. The scan rate for all the CV tests is 50 mV s1.
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Foundation of Guangxi Province (2017GXNSFDA198031, 2016GXNSFAA380199, 2014GXNSFFA118003), the BAGUI Scholar Program (2014A001) and the Project of Talents Highland of Guangxi Province. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2020.135742. References
Fig. 6. (A) Current-time curves of methanol oxidation on Pt HOSs (a) and commercial Pt black (b) in 0.5 M CH3OH þ 0.5 M H2SO4 solution at a potential of 0.6 V. (B) CO stripping voltammograms of Pt HOSs (a) and commercial Pt black (b) in 0.5 M CH3OH þ 0.5 M H2SO4 with a scan rate of 50 mV s1.
PVP and SDS synergistically regulate the optimal reaction kinetics through balancing the competition between surface capping and particle assemblies for the formation of dendritic products with appropriate sizes, which are eventually evolved into hollowopened structures. In specific, the electrocatalytic evaluations disclose that the as-synthesized Pt HOSs have enhanced electrocatalytic activity, stability and anti-toxicity for MOR than those of commercial Pt black, which are mainly ascribed to their unique hollow-open structural feature. This study might have provided a prospective avenue for the design and fabrication of Pt-based nanostructures with desired catalytic properties. Declaration The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Xiaoqu Wang: Methodology, Investigation, Writing - original draft. Miaolan Sun: Methodology, Investigation. Sheng Xiang: Data curation, Investigation. Muhammad Waqas: Writing - review & editing. Youjun Fan: Conceptualization, Supervision. Jingping Zhong: Data curation, Investigation. Kexin Huang: Investigation. Wei Chen: Supervision. Laijun Liu: Data curation. Jun Yang: Supervision, Writing - review & editing. Acknowledgements This work was supported by the National Natural Science Foundation of China (21463007, 21573240), Natural Science
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