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Pt-Ag nanostructured 3D architectures: A tunable catalyst for methanol oxidation reaction
Electrochimica Acta 298 (2019) 835e843
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Pt-Ag nanostructured 3D architectures: A tunable catalyst for methanol oxidation reaction Thulasi Radhakrishnan, N. Sandhyarani* Nanoscience Research Laboratory, School of Nano Science and Technology, National Institute of Technology Calicut, Calicut, Kerala, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 October 2018 Received in revised form 22 December 2018 Accepted 27 December 2018 Available online 28 December 2018
Platinum-noble metal binary nanoparticles with high surface energy and controlled size exposing high index planes represent an emerging class of nanomaterials for electro-catalytic applications. However, difficulty in the synthesis of a stable high index plane exposed nanoparticles pose a hurdle to their practical applications. Herein, we report a two-step method for the synthesis of large platinum-silver nanostructures with exposed high index facets formed by the self-assembly of small nanoparticles. In the first step, size and shape controlled platinum-silver nanoparticles are synthesized using formic acid as the reducing agent and polyvinylpyrrolidone as the surfactant. In the second step, square wave potential was applied which leads to preferential exposure of high index facets on these individual platinum-silver nanoparticles. The catalyst exhibits high electro-catalytic activity towards methanol oxidation reaction with a specific activity ~18 times higher than that of commercial platinum/carbon and 9 times higher than commercial platinum-ruthenium/carbon catalysts. This catalyst also proves to be efficient for ethanol and formic acid electro-oxidation, which shows the potential of this catalyst in practical fuel cell applications. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Methanol oxidation reaction MOR PtAg nanoparticle Electrocatalyst DMFC
1. Introduction The impending fossil fuel depletion and rising environmental pollution force the global energy sector to find alternative green energy technologies. On account of the high power density, low pollution and low operating temperatures over other conventional energy technologies, low temperature fuel cells are attracting momentous interest as an alternative clean energy source for transportation and electronics [1,2]. Fuel cells can also be an attractive substitute to batteries due to the high energy density on the system level [3,4]. Among different types of fuel cells, direct methanol fuel cell (DMFC) which is an extension of proton exchange membrane fuel cell (PEMFC), has attracted significant interest [5]. Precious metal catalysts, mainly platinum are largely used anode catalyst for methanol oxidation reaction (MOR) due to its efficient catalytic activity with remarkable potential for further improvement. However, platinum is highly susceptible to CO poisoning which reduces the efficiency of fuel cell [6]. The intermediates formed during the oxidation of methanol get strongly
* Corresponding author. E-mail address: [email protected] (N. Sandhyarani). https://doi.org/10.1016/j.electacta.2018.12.151 0013-4686/© 2018 Elsevier Ltd. All rights reserved.
bind over the platinum active site hindering the oxidation of fresh methanol [7]. Tremendous efforts have been made to achieve higher utilization efficiency of platinum catalysts. Structural tunability is a very important factor in the design of high performance electro-catalytic Pt nanoparticles. The activity of the catalyst can be tuned by either alloying thereby tuning its electronic properties [8e11] or by controlling the structure and exposing the most active planes [12e14]. Only the exposed outermost few layers of platinum are utilized in catalysis and hence restructuring of the catalyst with more exposed high index planes can lead to a poisoning free catalyst. Pt nanocrystal with exposed high index facets having high density of stepped surface atoms that exhibits excellent electro-catalytic activity [15,16]. Hence research is focused to control the morphology and to attain high index facets [17e19]. Catalytic performance can be augmented by reducing the nanoparticle size thereby increasing the surface area. However, the synthesis of small nanoparticles of <10 nm is challenging due to its poor stability. To increase the stability various surfactants such as polyvinylpyrrolidone (PVP), DNA, polydopamine, etc. were used for the synthesis of small sized nanoparticles [20e22]. However, the presence of surfactant can lead to a decrease in catalytic activity. Thus the synthesis of small nanoparticles with high surface area
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possessing high catalytic activity remains as a challenge. To meet this, branched Pt nanostructures such as nanodendrites were synthesized and reported to possess impressive catalytic activity [18]. Similar to this, an assembly of small nanoparticles also can lead to a high surface area than the individual large nanoparticles. At the same time, the synthesis of facet controlled large nanoparticles of micron size also is promising as these individual nanoparticles can act as new microelectrodes. In this context, the synthesis of smaller nanoparticles with large surface area and their assembly to facet controlled large nanostructures become very important in view of their high catalytic activity. Hetero elements modify the electronic structure and alter the binding energy of the adsorbed intermediates. Specifically, small amount of silver along with more noble metals such as Au or Pt, found to enhance the catalytic activity [23]. Thus on combining these three strategies, the high index faceted, high surface area, bi-metallic nanoparticle will have immense potential as an efficient electro-catalyst. In this work, we report an effective wet chemical synthetic approach for the synthesis of Pt-Ag nanostructures composed of individual nanoparticles of 5 nm. The small nanoparticles selfassemble to form a 3D architecture upon altering the ratios of the precursors and the surfactant PVP, which plays a major role in the self-assembly of nanoparticles. Though the self-assembly of the catalyst leads to a reduction in surface area of individual nanoparticles, this assembled Pt-Ag showed a good electro-catalytic activity for MOR. This catalyst possesses superior electro-catalytic activity towards methanol, ethanol and formic acid oxidation than commercially available Pt/C and PtRu/C catalysts. In the conventional method, the synthesis of high index faceted catalyst is difficult due to the high growth rate along the direction perpendicular to high index planes than those of low index planes which leads to the formation of predominant low index planes [17]. We demonstrate that with electrochemical treatment the Pt-Ag nanostructures synthesized here can easily be transformed into high index plane exposed large nanostructures. The resulting Pt-Ag HI exhibited enhanced catalytic activity towards MOR with their mass and specific activities significantly higher than commercial Pt/C and PtRu/C catalysts. The specific and mass activity of the synthesized catalyst was found to be higher than most of existing reports (see below). This catalyst also exhibited higher specific activities for ethanol and formic acid oxidation reactions. 2. Experimental 2.1. Chemicals Chloroplatinic acid (H2PtCl4$6H2O) was purchased from SigmaAldrich. Sulphuric acid, formic acid and methanol were purchased from Merck, India. Polyvinylpyrrolidone (PVP, MW-18000) used for the synthesis was purchased from SRL India. Carbon supported Pt (20% on carbon black) catalyst and Pt-Ru (20% and 10% on carbon black) were purchased from Alfa Aesar. All the experiments were done using ultrapure deionized (DI) water obtained using an ultrafiltration system (Milli-Q, Millipore) with a measured resistivity above 18MU cm. 2.2. Synthesis Briefly, 1 gm of PVP was dissolved in 10 mL of DI water by stirring it at 2000 rpm for 15 min. After the dissolution, 2 mL of formic acid was added followed by the addition of 60 mL of 10 mM AgNO3 and 200 mL of 40 mM H2PtCl4$6H2O. The solution was stirred for next 4 h at room temperature and the product was collected by centrifugation at 12000 rpm for 15 min followed by washing thrice with DI water. The use of formic acid as a reducing agent for the
preparation of Au-Pt alloy at room temperature is already reported.9 It was seen that no precipitate was obtained in the absence of either silver nitrate or chloroplatinic acid. The structure was highly dependent on each reaction conditions used (see supplementary information). By changing the concentration of chloroplatinic acid (20 mM instead of 40 mM, Fig. S1), amount of silver nitrate (2 mL instead of 60 mL, Fig. S2) and formic acid used (1 mL instead of 2 mL, Fig. S3) the morphology of the nanostructures obtained were different as shown in Figs. S1eS3. The detailed description regarding the synthesis is given in supporting information. On increasing/decreasing the amount of silver nitrate it was seen that the 3D architectures were not formed. The morphology obtained was entirely different while using cetyl trimethylammonium bromide (CTAB) as capping agent instead of PVP. The well arranged particles were not visible on using CTAB as clear from Fig. S4, which confirms that PVP plays a crucial role in the selfassembly of these nanostructures. From these observations it is clear that the optimum concentration of Pt, Ag precursors, formic acid and the stabilizing agent PVP are necessary for the selfassembly of the nanoparticles. The presence of PVP is evident from the IR spectrum given in Fig. S5. The Pt-Ag nanostructures synthesized at the optimum conditions were drop-casted on a glassy carbon electrode (GCE) and subjected to square wave potential (SWP) with an upper potential of 1.1 V and lower potential of 0.2 V at 1 Hz in 0.1 M H2SO4 and 30 mM ascorbic acid continuously for three cycles. The electrode after the post-synthesis electrochemical treatment was washed with DI water, collected the nanostructures and analyzed the morphology. These nanostructures showed preferential faceting and are referred in this report as Pt-Ag HI. It is seen that the assembled particles are stable even after the electrochemical treatment (Fig. S5). 2.3. Instrumentation The morphological analysis of the catalyst was carried out using field emission scanning electron microscope (FESEM) Hitachi S46600. Transmission Electron Microscopy (TEM) images were acquired using JEOL JEM-2100 at an accelerated voltage of 200 kV. Elemental composition was studied using Axis Ultra Shimadzu Xray photoelectron spectrometer (XPS). Grazing angle X-ray diffractograms (GXRD) were performed using PANalytical X'Pert PRO system. Fourier Transform Infrared Spectroscopy (FTIR) were performed using Perkin-Elmer Frontier FTIR spectrometer using ATRIR accessory. The samples were drop-casted on ITO for GXRD and XPS measurements. For morphological analysis the Pt-Ag HI was synthesized on an ITO plate. The samples were collected and dropcasted on carbon coated copper grid for TEM and on carbon tape for SEM measurements. The Pt-Ag HI on the ITO plates after electrochemical treatments were used for XRD, XPS and FTIR analysis. 2.4. Electrochemical measurements All electrochemical experiments were performed at room temperature in a standard three-electrode configuration in an electrochemical cell with Pt wire as the counter electrode, silver/silver chloride (Ag/AgCl) as reference electrode and GCE (~3 mm diameter) modified with catalyst as working electrode. Square wave voltammetry was performed using saturated calomel electrode (SCE) as reference electrode. All the electrochemical measurements were performed with CHI 760E electrochemical workstation (CH instruments, Texas, Austin). In this work, the methanol oxidation was performed in 0.1 M H2SO4. To investigate the chance of Pt contamination on using Pt-wire counter electrode, the methanol oxidation reaction was performed using stainless steel mesh as
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counter electrode. The MOR activity exhibited is given in Fig. S6 (details regarding the measurements are given in supplementary information). Both the catalysts exhibited more or less the same activity on using stainless steel or Pt wire as counter electrode and thus the conventional Pt wire was used as the auxiliary electrode for all the electrochemical measurements in this work.
3. Results and discussion 3.1. Physico-chemical characterization Morphology of the Pt-Ag and Pt-Ag HI were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM images shown in Fig. 1A and B reveals the formation of self-assembled 3D structures of Pt-Ag nanoparticles composed of smaller nanoparticle assembly. Majority of the 3D structures are around 2e3 mm in size. However, 3D assembled structures of size around 50 nm were also present. TEM images shown in Fig. 1C confirmed the formation of self-assembled
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microstructure. The regions where smaller nanoparticle assemblies formed of around 50 nm are shown in the inset of Fig. 1C and this region was analyzed in detail for EDS mapping and HRTEM (see below). The transformation of spherical structure composed of very small nanoparticles in Pt-Ag to planar structure with definite exposed planes for Pt-Ag HI, maintaining almost the same dimension is clear from low magnification SEM image shown in Fig. 1D. It is clear that the overall assembly and distribution of nanostructures in the specific area are not changed significantly after the electrochemical treatment; however the high magnification image in Fig. 1E indicates the change in morphology of individual assembled nanostructures to a more planar structure. The TEM image in Fig. 1F confirms the 3D nano/sub microstructure. Inset of Fig. 1F shows the magnified image of the Pt-Ag HI, where regions of smaller nanoparticle assemblies of around 50 nm are shown. To further understand the structure and to gain information regarding elemental composition and distribution of Pt and Ag in the catalyst, energy dispersive X-ray spectroscopy (EDS)
Fig. 1. (A and B) SEM images at two different magnifications, indicating the assembly of nanostructures, (C) TEM image of the 3D assembled structure with inset showing the regions of assembled structure with size of 50 nm, (D and E) low and high magnification SEM images of Pt-Ag HI, (F) TEM image of Pt-Ag HI with high magnification image showing the smaller assembled structures in the inset. (G, H and I) High magnification TEM image of Pt-Ag region used for EDS mapping and EDS mapping of Pt and Ag demonstrating the distribution of Pt and Ag in the nanostructure, respectively. (J, K and L) High magnification TEM image of Pt-Ag HI region used for EDS mapping and the distribution of Pt and Ag in the nanostructure, respectively as mapped by EDS.
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characterization was performed. The EDS spectrum given in Fig. S7 shows the presence of Pt and Ag, confirming the purity of the sample. The Ag to Pt ratio was found to be 0.08. In these catalysts Ag may remain alloyed with Pt or homogeneously distributed; however there is a possibility of local segregation of less noble metal Ag around the steps and kinks on the topmost layer [24]. The surface energy of Ag is less than that of Pt, which favors the surface segregation of Ag. These segregated Ag atoms may provide extra stability to the nanostructure assembly. Fig. S7B shows the EDS spectrum of Pt-Ag HI, which indicated a slightly lower ratio (0.044) of Ag to Pt suggesting that some of the Ag atoms on the surface have been removed during the electrochemical treatment. This was further confirmed from the ICP-OES analysis. Pt-Ag and Pt-Ag HI were dissolved in aqua regia and was diluted to 10 mL. The platinum and silver content in both the samples were verified and the ratio of Ag to Pt was compared for both the samples. The platinum and silver content in 100 mg of Pt-Ag were 92.8 mg and 7 mg respectively, whereas the Pt and Ag content in 100 mg of Pt-Ag HI were 95.96 mg and 4.03 mg respectively giving Ag to Pt ratio of 0.075 for Pt-Ag and 0.042 for Pt-Ag HI which was close to the results obtained from EDS. To ascertain the distribution and location of Pt and Ag in the sample EDS mapping of both the samples were done. Shown in Fig. 1G is the region of Pt-Ag where EDS mapping is done. Distribution of Pt and Ag are shown respectively in Fig. 1H and I. It appears that Ag is randomly distributed in Pt nanostructure with more segregation to the surface of the assembled nanostructure, which also is evident further in X-ray photoelectron spectroscopy (XPS) analysis (see below). EDS mapping of Pt-Ag HI (Fig. 1J) in the Pt and Ag regions are shown in Fig. 1K and L respectively, indicating the random distribution of Ag in the nanostructure. It may be noted here that the assembly of nanoparticle was slightly distorted due to the electrochemical etching induced reorientation of atoms in the nanostructure. TEM image shown in Fig. 2A revealed that the individual nanoparticles are of around 5 ± 2 nm in size and these nanoparticles are assembled to give the 3D architecture. To get a complete understanding on the nature and morphology of the individual nanoparticle high resolution transmission electron microscopy (HRTEM) images of the nanostructures were analyzed in detail (Fig. 2B) which confirmed that the nanoparticles are of spherical morphology. The marked region of the Fig. 2B is subjected
to fast Fourier transform (FFT) (see supplementary information Fig. S8A and S8B for details) and the FFT filtered image is shown in Fig. 2C. The lattice planes are clearly seen in the image. The selected area electron diffraction (SAED) pattern shown in Fig. 2D indicates the polycrystalline nature of the Pt-Ag nanoparticle. Planes are marked in the image. In Fig. 2E the high magnification TEM image of the Pt-Ag HI is shown where the transformation of spherical morphology in Pt-Ag to a polygonal structure in Pt-Ag HI for few particles is evident. To substantiate the formation of high index planes in Pt-Ag HI, HRTEM images of the catalyst was analyzed. HRTEM image shown in Fig. 2F evidently indicated the transformation of Pt-Ag shown in Fig. 2B to high index plane exposed Pt-Ag HI. The individual spherical Pt-Ag nanoparticles were converted to sharp edged particles with a slight increase in size. FFT filtered image shown in Fig. 2G ascertains the extent of conversion that happened on the application of SWP. The individual spherical nanoparticles were transformed to facet oriented square or polygon shaped nanoparticles and these are further assembled to give 3D architectures of facet oriented nano/sub-micron structures. Comprehensive analysis of these individual nanoparticles demonstrates that stepped surfaces are enclosed by continuously distributed high-index facets with (100) terraces and (110) steps, such as {410}, {310}, and {210} which usually serve as highly catalytically active sites [25e28]. The SAED pattern of Pt-Ag HI indicates the polycrystalline nature as given in Fig. 2H. Crystallographic details of Pt-Ag and Pt-Ag HI were examined using X-ray diffraction (XRD) (Fig. 3A). For XRD measurements, PtAg was drop casted on ITO and applied SWP for three cycles to obtain Pt-Ag HI. Grazing angle XRD measurements were performed at an angle of 0.5 . XRD pattern show the peak corresponding to platinum and silver. The peaks at 39.61, 45.62 , 67.4 , 81 and 85.3 correspond to FCC crystal structure of platinum, and the peak for Ag is observed at 37.6 which is marked in the figure with an arrow. As the reflections observed for platinum and silver are closer, reflections corresponding to each metal are not well resolved. The deconvoluted peaks for Pt (111) and Ag (111) are given in the Fig. 3B. Separate peaks for Pt and Ag suggest the segregation of components rather than the formation of alloy nanoparticles [29]. To ascertain the segregation of silver on the surface, Pt-Ag was subjected to an acid treatment to remove the silver present in the
Fig. 2. (A) High magnification TEM image of Pt-Ag (B) HRTEM (C) FFT filtered HRTEM image from the marked area in B indicating the lattice planes (D) SAED pattern (E) High magnification TEM image of Pt-Ag HI (F) HRTEM (G) FFT filtered HRTEM image from the marked area in F revealing the surface atomic steps and the high index facets (the edge atoms are marked in the figure with white dots) and (H) SAED pattern.
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Fig. 3. (A) XRD pattern of (a) Pt-Ag and (b) Pt-Ag HI, (B) the expanded and deconvoluted XRD pattern of Pt (111), Ag (111) and Pt (200) region of (a) Pt-Ag and (b) Pt-Ag HI. JCPDS value of Pt and Ag (Pt, JCPDS: 04-0802 and Ag, JCPDS file No. 04-0783) are shown (C) XPS spectra of Pt 4f region of (a) Pt-Ag and (b) Pt-Ag HI and (D) Ag 3d region of (a) Pt-Ag and (b) Pt-Ag HI.
catalyst. Pt-Ag was treated with concentrated nitric acid for one hour, washed thrice with DI water and collected the precipitate by centrifugation at 12000 rpm for 15 min. It is clear from the XRD measurement of the acid treated sample that the ratio of Ag(111) to Pt(111) is less compared to the Pt-Ag signifying the removal of segregated silver atoms (Fig. S9A). The destruction of the assembled 3D microstructure of Pt-Ag (Fig. S9B) is also observed in the SEM image shown in Fig. S9C. This shows that the presence of Ag provided extra stability to the assembled structure. The composition and oxidation state of the catalysts were examined using XPS (Fig. 3C). Pt 4f peaks were seen at 70.9 eV and 74.2 eV corresponding to Pt 4f7/2 and 4f5/2. Compared with the bulk Pt the peak values of Pt 4f7/2 and Pt 4f5/2 were down shifted which may be due to valency unsaturated atoms at edges and corners [30]. These peaks were deconvoluted and was seen that the Pt 4f7/2 and 4f5/2 are composed of two peaks corresponding to metallic platinum at high binding energy and a downward shifted peak with a significant intensity. The down shift of Pt 4f bands can also be due to the electronic effects, ie. the charge transfer from Ag to Pt. However, corresponding shifts are not observed in the Ag region, suggesting that the electronic effects are not the decisive factor in the down shift of platinum region. The two peaks at ca. 373.8 eV and 367.8 eV, which are ascribed to binding energies of metallic silver Ag 3d5/2 and Ag 3d3/2, respectively are shown in Fig. 3D. No alloy formation is evident from XPS. After electrochemical treatment both the Pt and Ag regions underwent a slight shift which is attributed to the reorientation of atoms within the nanoparticle during the SWP. These electronic effects can be a significant factor in the electro catalytic activity of Pt-Ag and Pt-Ag HI. Being a surface
sensitive technique (penetration depth ~3l), XPS will give more information regarding the segregation of silver nanoparticle over platinum nanoparticle. The ratio of Ag to Pt for Pt-Ag was found to be 0.143 which was almost two times higher than the ratio obtained from EDS (0.088) which gives elemental composition from the bulk of sample (of around 1 mm size). The higher ratio of Ag to Pt obtained from XPS for Pt-Ag than that obtained from EDS suggests the segregation of silver on the surface. Similar trend was seen for Pt-Ag HI exhibiting a ratio of 0.0765 from XPS and 0.044 from EDS, which indicate a surface segregation of silver atoms in Pt-Ag HI. 3.2. Electrochemical characterizations Electrochemical impedance spectroscopy (EIS) was carried out for both the catalysts at onset potential (0.4 V vs Ag/AgCl) of methanol oxidation in 1 M methanol and 0.1 M H2SO4 and was compared with commercial Pt/C and PtRu/C. Fig. 4A shows the Nyquist plot of the catalysts in the frequency ranging from 1 kHz to 0.01 Hz. The equivalent circuit to fit the EIS data is shown in the inset. Rs, Rct and Cdl represent solution resistance, charge transfer resistance and double layer capacitance, respectively. Nyquist plot of methanol oxidation on all the catalysts are constituted with single semicircle. The decreased diameter of semicircle reflects the decrease in charge transfer resistance (Rct) for MOR. The diameter of the arc decreases in the order Pt/C, PtRu/C, Pt-Ag and Pt-Ag HI. The enhanced charge transfer properties of Pt-Ag HI suggest faster reaction kinetics for electrocatalytic applications. Electrochemical voltammetric methods provide overall structural information on the exposed planes of catalysts compared to
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Fig. 4. (A) Nyquist plot of (a) Pt-Ag, (b) Pt-Ag HI, (c) Pt/C and (d) PtRu/C in 1 M methanol using 0.1 M H2SO4 as supporting electrolyte (B) Cyclic voltammogram of (a) Pt-Ag, (b) Pt-Ag HI, (c) Pt/C and (d) PtRu/C in 0.1 M H2SO4.
HRTEM where only limited number of particles are analyzed. Literature reports suggests that low-coordinate step atoms on Pt high index planes promotes oxygen adsorption and are susceptible to oxidation leading to a larger current at low potential in cyclic voltammograms when recorded in aqueous H2SO4 solution [15]. Thus the electric charges of oxygen adsorption can directly be correlated to the quantity of Pt atomic steps. Fig. 4B compares the cyclic voltammograms of Pt-Ag HI with Pt-Ag, Pt/C and PtRu/C recorded in 0.1 M H2SO4. As expected, a larger current of oxygen adsorption was observed at a lower potential (in the potential range 0.4 Ve0.95 V) on the Pt-Ag HI which confirms the presence of high index faceted platinum surface with low coordination numbers (CN). It is clear that the electric charge density of oxygen adsorption on the Pt-Ag HI catalyst is comparatively higher than the Pt-Ag and other commercially available catalyst Pt/C and PtRu/C which points to the high density of atomic steps of Pt-Ag HI. The higher oxygen adsorption peak on the Pt-Ag compared to Pt/C suggest that the PtAg also possess some high index facets compared to Pt/C but less than that of Pt-Ag HI. It was reported in literatures that adsorption/ desorption of oxygen generated by SWP plays a crucial role in the structural modifications. It is suggested that during the electrochemical treatment, H2O in solution undergoes dissociation and form oxides or hydroxides which react with platinum nanoparticle surface at upper potential. Oxygen atoms preferentially diffuse invade to the lattice structure replacing Pt atoms of (100) and (111) facets with surface atoms of high CN. These displaced Pt atoms cannot always return to their original position even after the desorption of oxygen atoms from the lattice site at lower potential, which leads to the destruction of ordered surface structure. On the contrary, high-index facets contain lots of step atoms with low CN, the oxygen atoms preferentially adsorb at those sites without replacing them, and hence the ordered surfaces are preserved during the electrochemical cycling. Thus, the adsorption/desorption of oxygen atoms plays the key role in the formation of high index facets [27,31]. To compare the catalyst surface area, the electrochemical active surface area (ECSA) was calculated by measuring the charge associated with hydrogen adsorption/desorption (QH) using the equation (1).
ECSA ¼
QH Q 0 mPt
(1)
Where, Q0 is the charge required for hydrogen adsorption on Pt surface (0.21 mC/cm2), and mPt is the mass of Pt nanoparticles (mg/
cm2) on the electrode 12. The ECSA values were found to be comparable for Pt-Ag and Pt-Ag HI. ECSA were calculated to be 27.88 m2/g, 25.11 m2/g, 91.1 m2/g and 74 m2/g for Pt-Ag, Pt-Ag HI, Pt/C and PtRu/C respectively. The slight decrease in the ECSA for PtAg HI after square wave potential treatment may be due to the small increase in the particle size as observed in Fig. 2E. It is reported that these high index faceted structure have limited surface area due to their large size [31,32]. 3.2.1. Electrochemical oxidation of fuels To test the electrocatalytic activity of the catalyst, methanol oxidation was performed in 1 M methanol using 0.1 M H2SO4 as supporting electrolyte. The catalyst (1 mg/mL) was dispersed in DI water and sonicated for 10 min 6 mL of the catalyst dispersion was dropped on a mechanically cleaned GCE, maintaining a loading of 6 mg for each measurement. Fig. 5A compares the cyclic voltammetric response of all the catalysts for methanol electro-oxidation at a scan rate of 50 mV/s. Lower onset potential of 0.4 V and higher oxidation peak current density of 400 mA/mg for the Pt-Ag HI are the highlight of the work. The methanol oxidation current obtained for Pt-Ag and Pt-Ag HI is higher than Pt/C and PtRu/C which is in agreement with the impedance analysis. In spite of the comparable ECSA values, the current obtained for MOR on Pt-Ag HI was two times higher than that of Pt-Ag. The comparable ECSA values of Pt-Ag and Pt-Ag HI together with the higher MOR activity of Pt-Ag HI confirm the high catalytic activity of the high index planes exposed on its surface which is one of the major contributing factors to the enhanced performance. The electrocatalytic activity of acid treated Pt-Ag was monitored and it was found that the activity exhibited by this catalyst is 12 times lower than Pt-Ag pointing to the role of Ag in the catalyst (Fig. S10B). This suggests that the enhanced activity of Pt-Ag and Pt-Ag HI is also due to the stability of the catalytically active site during electrochemical oxidation as a result of the segregated Ag atoms on the top most layer. The interesting feature observed in both Pt-Ag and Pt-Ag HI is the low intermediate oxidation current which is due to the oxidation of adsorbed intermediates [33e35]. These adsorbed intermediates can block the active sites of the catalyst inhibiting further oxidation of methanol. The ratio of forward current to backward current (If/Ib) indicates the electrocatalytic ability to remove the carbonaceous species that gets binds to the active site of the catalyst and also is a rough indicator of the efficiency of catalyst. The ratio, If/Ib greater than unity indicates enhanced tolerance to poisoning [36e39]. The catalysts Pt-Ag and Pt-Ag HI
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Fig. 5. (A) Cyclic voltammetry mass activity curves of (a) Pt-Ag, (b) Pt-Ag HI, (c) Pt/C and (d) PtRu/C (B) specific activity bar diagram of different electrocatalysts for MOR recorded at a scanning rate of 50 mV/s (C) Chronoamperometry analysis of (a) Pt-Ag, (b) Pt-Ag HI, (c) Pt/C and (d) PtRu/C at peak potential. All measurements were performed using 1 M methanol using 0.1 M H2SO4 as supporting electrolyte. (D) Schematic representation of methanol oxidation on Pt-Ag and Pt-Ag HI.
we reported here exhibited an If/Ib ratio of ca 1.4 which is at par with PtRu/C (1.4-which is the commercially used catalyst to enhance methanol oxidation during the forward bias) and higher than Pt/C (1.1) that point out to the high catalytic activity of these catalysts. It was also observed that the onset potential for methanol oxidation starts at 0.41 V on Pt-Ag HI and 0.44 V on Pt-Ag respectively, which is lower than Pt/C (0.47 V) indicating the superior activity of the catalysts compared to Pt/C. The specific activity of the catalyst is shown in Fig. 5B demonstrating that the specific activity of Pt-Ag HI is almost 18 times higher than that of Pt/C and 9 times higher than PtRu/C. The enhanced electrocatalytic activity observed in case of Pt-Ag HI is attributed to the synergetic effect of large surface area and high index facets on the bimetallic catalyst. Chronoamperometry measurements were performed to analyze the stability of the catalyst towards MOR. The presence of high index facets and the segregated Ag on these facets helps in improving the stability of the catalyst as indicated in Fig. 5C. Chronoamperometry measurements were done at 0.72 V vs Ag/ AgCl in 1 M methanol using 0.1 M H2SO4 as supporting electrolyte. The slow decay of the current in Pt-Ag HI compared to other catalysts shows the excellent stability and lower poisoning of the catalyst. Pt-Ag nanostructures exhibited higher catalytic activity than Pt/ C and PtRu/C owing to its higher surface area due to the very small size of the individual nanoparticles. However, the predominant
(111) plane are prone to poisoning and the intermediate species gets adsorbed on the plane as in the schematic representation shown in Fig. 5D. Upon applying SWP, the high index facets which are less prone to poisoning are formed, thereby the catalytically active sites are not blocked by the intermediates, leading to better electrochemical oxidation of organic molecules. The enhanced electrocatalytic property of the Pt-Ag HI can be attributed to the presence of Pt nanoparticles with preferentially exposed high indexed facets with surface segregation of Ag. The electronic effects ie., the charge transfer from Ag to Pt also may play a crucial role in enhancing the catalytic activity of both Pt-Ag and Pt-Ag HI. There is a possibility of PVP getting adsorbed on the active site affecting the catalytic activity by blocking the adsorption of methanol to the surface. However, presence of organic content on the Pt-Ag and PtAg HI did not significantly affect the electro catalytic activity, pointing that the active sites are not blocked by the PVP. It is also clear that the enhanced activity of Pt-Ag HI over Pt-Ag is not due to the presence of organic content as both the catalysts have the same organic content present as indicated by the FTIR spectra shown in Fig. S5. In Pt-Ag HI, the small nanoparticles with high indexed facets and small CN possessing abundant steps and kinks are assembled to give large structures with steps and terraces. These larger nanocrystals may act as new micro electrodes. Each individual nanoparticle is of approximately 5 nm in size contributing significantly to high surface area. During electro catalytic oxidations the
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less noble metal silver stabilizes the surface active sites and the 3D structure and there by enhances the catalytic activity and durability of the catalyst. The performance of both the catalysts toward methanol oxidation was compared with the previously reported work in literature and is shown in Fig. 6. Relatively higher mass and specific activity was exhibited by the Pt-Ag HI exposed with high index facets reported in this work. To see the electro catalytic activity of these catalyst towards
other organic molecules; electro-oxidation of ethanol and formic acid, the two fuels commonly used in fuel cell application are monitored. The mass activity and specific activity of the catalyst towards ethanol oxidation are shown in Fig. 7A and B. Pt-Ag HI exhibits the highest mass and specific activity. The Pt-Ag HI has the mass activity of 250.36 mA/mg, which is 1.5 times higher than PtAg, 1.2 times higher than PtRu/C and 2.8 times higher than Pt/C respectively. As shown in Fig. 7C and D, the Pt-Ag HI is highly active towards oxidation of formic acid also. The highest activity of the Pt-Ag HI towards methanol, ethanol and formic acid oxidation indicate that Pt-Ag HI can be used as a promising catalyst for the fuel cells.
4. Conclusions
Fig. 6. Comparison of literature reports on mass and specific activity of Pt based catalyst towards methanol oxidation. Reference numbers are indicated in brackets. PtAg and Pt-Ag HI are the catalysts reported in this work [40e43].
In summary, we have presented a robust wet chemical method to generate 3D architectures of Pt-Ag nano/sub microstructures with controllable high surface energy nanoparticles. This catalyst was then transformed to high index plane exposed structures upon electrochemical treatment. The nanostructures exhibited well defined high indexed exposed planes with stable kinks and surfaces. The catalyst showed a specific activity of 1.6 mA/cm2 which is almost 18 times higher than the commercial Pt/C and 9 times higher than commercial PtRu/C. High density of atomic steps with segregated silver atoms formed on the catalyst and the electronic effects contributed to the higher electrocatalytic activity towards fuel oxidation reactions. The Pt-Ag HI has higher stability than PtAg and commercial catalysts showing a novel Pt based bimetallic nanostructure with high catalytic performance and stability for forthcoming practical applications.
Fig. 7. (A) Cyclic voltammetric mass activity curves and (B) specific activity bar diagram of the electro-catalysts towards ethanol oxidation reaction in 1 M CH3CH2OH and 0.1 M H2SO4, (C) Cyclic voltammetric mass activity curves and (D) specific activity bar diagram of the electro-catalysts towards formic acid oxidation in 1 M HCOOH recorded at scanning rate of 50 mV/s of (a), (b), (c) and (d) represents Pt-Ag, Pt-Ag HI, Pt/C and PtRu/C respectively.
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Acknowledgements The authors would like to acknowledge Kerala State Council for Science, Environment and Technology for the financial support to the Fuel cell research (Grant No. 003/SRSPS/2014/CSTE) and Department of Science and Technology (DST) for the facilities provided for nanoscience research laboratory. We thank Dr. Subramanyan Namboodiri Varanakkottu for helping us with the FijiWin 64 software and the TEM analysis. We acknowledge the help received from Dr. M.K. Jayaraj, Nanophotonic & Optoelectronic Devices laboratory, Dept. of physics for XRD measurement under DST nanomission initiative program. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2018.12.151. References [1] A.A. Permyakova, B. Han, J.O. Jensen, N.J. Bjerrum, Y. Shao-Horn, PtdSi Bifunctional surfaces for CO and methanol electro-oxidation, J. Phys. Chem. C 119 (2015) 8023e8031. [2] T.-Z. Hong, Q. Xue, Z.-Y. Yang, Y.-P. Dong, Great-enhanced performance of Pt nanoparticles by the unique carbon quantum dot/reduced graphene oxide hybrid supports towards methanol electrochemical oxidation, J. Power Sources 303 (2016) 109e117. [3] Z. Zhu, T. Kin Tam, F. Sun, C. You, Y.-H. Percival Zhang, A high-energy-density sugar biobattery based on a synthetic enzymatic pathway, Nat. Commun. 5 (2014) 3026. [4] B. Kaur, R. Srivastava, B. Satpati, Highly efficient CeO2 decorated nano-ZSM-5 catalyst for electrochemical oxidation of methanol, ACS Catal. 6 (2016) 2654e2663. [5] Y.Y. Tong, C.D. Gu, J.L. Zhang, M.L. Huang, H. Tang, X.L. Wang, J.P. Tu, Threedimensional astrocyte-network NiePeO compound with superior electrocatalytic activity and stability for methanol oxidation in alkaline environments, J. Mater. Chem. A. 3 (2015) 4669e4678. [6] Y. Kang, J.B. Pyo, X. Ye, T.R. Gordon, C.B. Murray, Synthesis, shape control, and methanol electro-oxidation properties of PteZn alloy and Pt3Zn intermetallic nanocrystals, ACS Nano 6 (2012) 5642e5647. € Karatepe, Y. Yıldız, H. Pamuk, S. Eris, Z. Dasdelen, F. Sen, Enhanced elec[7] O. trocatalytic activity and durability of highly monodisperse Pt@PPyePANI nanocomposites as a novel catalyst for the electro-oxidation of methanol, RSC Adv. 6 (2016) 50851e50857. [8] H. Xu, A.-L. Wang, Y.-X. Tong, G.-R. Li, Enhanced catalytic activity and stability of Pt/CeO2/PANI hybrid hollow nanorod arrays for methanol electro-oxidation, ACS Catal. 6 (2016) 5198e5206. [9] L. Chen, L. Kuai, B. Geng, Shell structure-enhanced electrocatalytic performance of AuePt coreeshell catalyst, CrystEngComm 15 (2013) 2133e2136. [10] C.-C. Ting, C.-H. Liu, C.-Y. Tai, S.-C. Hsu, C.-S. Chao, F.-M. Pan, The size effect of titania-supported Pt nanoparticles on the electrocatalytic activity towards methanol oxidation reaction primarily via the bifunctional mechanism, J. Power Sources 280 (2015) 166e172. [11] D. Yu, E. Shamsaei, J. Yao, T. Xu, H. Wang, A hierarchically structured PtCo nanoflakesenanotube as an electrocatalyst for methanol oxidation, Inorg. Chem. Front. 4 (2017) 845e849. [12] T. Radhakrishnan, N. Sandhyarani, Three dimensional assembly of electrocatalytic platinum nanostructures on reduced graphene oxide e an electrochemical approach for high performance catalyst for methanol oxidation, Int. J. Hydrogen Energy 42 (2017) 7014e7022. [13] L. Wei, Z.-Y. Zhou, S.-P. Chen, C.-D. Xu, D. Su, M.E. Schuster, S.-G. Sun, Electrochemically shape-controlled synthesis in deep eutectic solvents: triambic icosahedral platinum nanocrystals with high-index facets and their enhanced catalytic activity, Chem. Commun. 49 (2013) 11152e11154. [14] T. Kwon, M. Jun, H.Y. Kim, A. Oh, J. Park, H. Baik, S.H. Joo, K. Lee, Vertexreinforced PtCuCo ternary nanoframes as efficient and stable electrocatalysts for the oxygen reduction reaction and the methanol oxidation reaction, Adv. Funct. Mater. 28 (2018) 1706440. [15] Z.-Y. Zhou, Z.-Z. Huang, D.-J. Chen, Q. Wang, N. Tian, S.-G. Sun, High-index faceted platinum nanocrystals supported on carbon black as highly efficient catalysts for ethanol electrooxidation, Angew. Chem. Int. Ed. 49 (2010) 411e414. [16] L. Wei, T. Sheng, J.-Y. Ye, B.-A. Lu, N. Tian, Z.-Y. Zhou, X.-S. Zhao, S.-G. Sun, Seeds and potentials mediated synthesis of high-index faceted gold nanocrystals with enhanced electrocatalytic activities, Langmuir 33 (2017) 6991e6998. [17] L. Shuiping, P.K. Shen, Concave platinumecopper octopod nanoframes bounded with multiple high-index facets for efficient electrooxidation catalysis, ACS Nano 11 (2016) 11946e11953.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Pt-Ag nanostructured 3D architectures: A tunable catalyst for methanol oxidation reaction Thulasi Radhakrishnan, N. Sandhyarani* Nanoscience Research Laboratory, School of Nano Science and Technology, National Institute of Technology Calicut, Calicut, Kerala, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 October 2018 Received in revised form 22 December 2018 Accepted 27 December 2018 Available online 28 December 2018
Platinum-noble metal binary nanoparticles with high surface energy and controlled size exposing high index planes represent an emerging class of nanomaterials for electro-catalytic applications. However, difficulty in the synthesis of a stable high index plane exposed nanoparticles pose a hurdle to their practical applications. Herein, we report a two-step method for the synthesis of large platinum-silver nanostructures with exposed high index facets formed by the self-assembly of small nanoparticles. In the first step, size and shape controlled platinum-silver nanoparticles are synthesized using formic acid as the reducing agent and polyvinylpyrrolidone as the surfactant. In the second step, square wave potential was applied which leads to preferential exposure of high index facets on these individual platinum-silver nanoparticles. The catalyst exhibits high electro-catalytic activity towards methanol oxidation reaction with a specific activity ~18 times higher than that of commercial platinum/carbon and 9 times higher than commercial platinum-ruthenium/carbon catalysts. This catalyst also proves to be efficient for ethanol and formic acid electro-oxidation, which shows the potential of this catalyst in practical fuel cell applications. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Methanol oxidation reaction MOR PtAg nanoparticle Electrocatalyst DMFC
1. Introduction The impending fossil fuel depletion and rising environmental pollution force the global energy sector to find alternative green energy technologies. On account of the high power density, low pollution and low operating temperatures over other conventional energy technologies, low temperature fuel cells are attracting momentous interest as an alternative clean energy source for transportation and electronics [1,2]. Fuel cells can also be an attractive substitute to batteries due to the high energy density on the system level [3,4]. Among different types of fuel cells, direct methanol fuel cell (DMFC) which is an extension of proton exchange membrane fuel cell (PEMFC), has attracted significant interest [5]. Precious metal catalysts, mainly platinum are largely used anode catalyst for methanol oxidation reaction (MOR) due to its efficient catalytic activity with remarkable potential for further improvement. However, platinum is highly susceptible to CO poisoning which reduces the efficiency of fuel cell [6]. The intermediates formed during the oxidation of methanol get strongly
* Corresponding author. E-mail address: [email protected] (N. Sandhyarani). https://doi.org/10.1016/j.electacta.2018.12.151 0013-4686/© 2018 Elsevier Ltd. All rights reserved.
bind over the platinum active site hindering the oxidation of fresh methanol [7]. Tremendous efforts have been made to achieve higher utilization efficiency of platinum catalysts. Structural tunability is a very important factor in the design of high performance electro-catalytic Pt nanoparticles. The activity of the catalyst can be tuned by either alloying thereby tuning its electronic properties [8e11] or by controlling the structure and exposing the most active planes [12e14]. Only the exposed outermost few layers of platinum are utilized in catalysis and hence restructuring of the catalyst with more exposed high index planes can lead to a poisoning free catalyst. Pt nanocrystal with exposed high index facets having high density of stepped surface atoms that exhibits excellent electro-catalytic activity [15,16]. Hence research is focused to control the morphology and to attain high index facets [17e19]. Catalytic performance can be augmented by reducing the nanoparticle size thereby increasing the surface area. However, the synthesis of small nanoparticles of <10 nm is challenging due to its poor stability. To increase the stability various surfactants such as polyvinylpyrrolidone (PVP), DNA, polydopamine, etc. were used for the synthesis of small sized nanoparticles [20e22]. However, the presence of surfactant can lead to a decrease in catalytic activity. Thus the synthesis of small nanoparticles with high surface area
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possessing high catalytic activity remains as a challenge. To meet this, branched Pt nanostructures such as nanodendrites were synthesized and reported to possess impressive catalytic activity [18]. Similar to this, an assembly of small nanoparticles also can lead to a high surface area than the individual large nanoparticles. At the same time, the synthesis of facet controlled large nanoparticles of micron size also is promising as these individual nanoparticles can act as new microelectrodes. In this context, the synthesis of smaller nanoparticles with large surface area and their assembly to facet controlled large nanostructures become very important in view of their high catalytic activity. Hetero elements modify the electronic structure and alter the binding energy of the adsorbed intermediates. Specifically, small amount of silver along with more noble metals such as Au or Pt, found to enhance the catalytic activity [23]. Thus on combining these three strategies, the high index faceted, high surface area, bi-metallic nanoparticle will have immense potential as an efficient electro-catalyst. In this work, we report an effective wet chemical synthetic approach for the synthesis of Pt-Ag nanostructures composed of individual nanoparticles of 5 nm. The small nanoparticles selfassemble to form a 3D architecture upon altering the ratios of the precursors and the surfactant PVP, which plays a major role in the self-assembly of nanoparticles. Though the self-assembly of the catalyst leads to a reduction in surface area of individual nanoparticles, this assembled Pt-Ag showed a good electro-catalytic activity for MOR. This catalyst possesses superior electro-catalytic activity towards methanol, ethanol and formic acid oxidation than commercially available Pt/C and PtRu/C catalysts. In the conventional method, the synthesis of high index faceted catalyst is difficult due to the high growth rate along the direction perpendicular to high index planes than those of low index planes which leads to the formation of predominant low index planes [17]. We demonstrate that with electrochemical treatment the Pt-Ag nanostructures synthesized here can easily be transformed into high index plane exposed large nanostructures. The resulting Pt-Ag HI exhibited enhanced catalytic activity towards MOR with their mass and specific activities significantly higher than commercial Pt/C and PtRu/C catalysts. The specific and mass activity of the synthesized catalyst was found to be higher than most of existing reports (see below). This catalyst also exhibited higher specific activities for ethanol and formic acid oxidation reactions. 2. Experimental 2.1. Chemicals Chloroplatinic acid (H2PtCl4$6H2O) was purchased from SigmaAldrich. Sulphuric acid, formic acid and methanol were purchased from Merck, India. Polyvinylpyrrolidone (PVP, MW-18000) used for the synthesis was purchased from SRL India. Carbon supported Pt (20% on carbon black) catalyst and Pt-Ru (20% and 10% on carbon black) were purchased from Alfa Aesar. All the experiments were done using ultrapure deionized (DI) water obtained using an ultrafiltration system (Milli-Q, Millipore) with a measured resistivity above 18MU cm. 2.2. Synthesis Briefly, 1 gm of PVP was dissolved in 10 mL of DI water by stirring it at 2000 rpm for 15 min. After the dissolution, 2 mL of formic acid was added followed by the addition of 60 mL of 10 mM AgNO3 and 200 mL of 40 mM H2PtCl4$6H2O. The solution was stirred for next 4 h at room temperature and the product was collected by centrifugation at 12000 rpm for 15 min followed by washing thrice with DI water. The use of formic acid as a reducing agent for the
preparation of Au-Pt alloy at room temperature is already reported.9 It was seen that no precipitate was obtained in the absence of either silver nitrate or chloroplatinic acid. The structure was highly dependent on each reaction conditions used (see supplementary information). By changing the concentration of chloroplatinic acid (20 mM instead of 40 mM, Fig. S1), amount of silver nitrate (2 mL instead of 60 mL, Fig. S2) and formic acid used (1 mL instead of 2 mL, Fig. S3) the morphology of the nanostructures obtained were different as shown in Figs. S1eS3. The detailed description regarding the synthesis is given in supporting information. On increasing/decreasing the amount of silver nitrate it was seen that the 3D architectures were not formed. The morphology obtained was entirely different while using cetyl trimethylammonium bromide (CTAB) as capping agent instead of PVP. The well arranged particles were not visible on using CTAB as clear from Fig. S4, which confirms that PVP plays a crucial role in the selfassembly of these nanostructures. From these observations it is clear that the optimum concentration of Pt, Ag precursors, formic acid and the stabilizing agent PVP are necessary for the selfassembly of the nanoparticles. The presence of PVP is evident from the IR spectrum given in Fig. S5. The Pt-Ag nanostructures synthesized at the optimum conditions were drop-casted on a glassy carbon electrode (GCE) and subjected to square wave potential (SWP) with an upper potential of 1.1 V and lower potential of 0.2 V at 1 Hz in 0.1 M H2SO4 and 30 mM ascorbic acid continuously for three cycles. The electrode after the post-synthesis electrochemical treatment was washed with DI water, collected the nanostructures and analyzed the morphology. These nanostructures showed preferential faceting and are referred in this report as Pt-Ag HI. It is seen that the assembled particles are stable even after the electrochemical treatment (Fig. S5). 2.3. Instrumentation The morphological analysis of the catalyst was carried out using field emission scanning electron microscope (FESEM) Hitachi S46600. Transmission Electron Microscopy (TEM) images were acquired using JEOL JEM-2100 at an accelerated voltage of 200 kV. Elemental composition was studied using Axis Ultra Shimadzu Xray photoelectron spectrometer (XPS). Grazing angle X-ray diffractograms (GXRD) were performed using PANalytical X'Pert PRO system. Fourier Transform Infrared Spectroscopy (FTIR) were performed using Perkin-Elmer Frontier FTIR spectrometer using ATRIR accessory. The samples were drop-casted on ITO for GXRD and XPS measurements. For morphological analysis the Pt-Ag HI was synthesized on an ITO plate. The samples were collected and dropcasted on carbon coated copper grid for TEM and on carbon tape for SEM measurements. The Pt-Ag HI on the ITO plates after electrochemical treatments were used for XRD, XPS and FTIR analysis. 2.4. Electrochemical measurements All electrochemical experiments were performed at room temperature in a standard three-electrode configuration in an electrochemical cell with Pt wire as the counter electrode, silver/silver chloride (Ag/AgCl) as reference electrode and GCE (~3 mm diameter) modified with catalyst as working electrode. Square wave voltammetry was performed using saturated calomel electrode (SCE) as reference electrode. All the electrochemical measurements were performed with CHI 760E electrochemical workstation (CH instruments, Texas, Austin). In this work, the methanol oxidation was performed in 0.1 M H2SO4. To investigate the chance of Pt contamination on using Pt-wire counter electrode, the methanol oxidation reaction was performed using stainless steel mesh as
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counter electrode. The MOR activity exhibited is given in Fig. S6 (details regarding the measurements are given in supplementary information). Both the catalysts exhibited more or less the same activity on using stainless steel or Pt wire as counter electrode and thus the conventional Pt wire was used as the auxiliary electrode for all the electrochemical measurements in this work.
3. Results and discussion 3.1. Physico-chemical characterization Morphology of the Pt-Ag and Pt-Ag HI were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM images shown in Fig. 1A and B reveals the formation of self-assembled 3D structures of Pt-Ag nanoparticles composed of smaller nanoparticle assembly. Majority of the 3D structures are around 2e3 mm in size. However, 3D assembled structures of size around 50 nm were also present. TEM images shown in Fig. 1C confirmed the formation of self-assembled
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microstructure. The regions where smaller nanoparticle assemblies formed of around 50 nm are shown in the inset of Fig. 1C and this region was analyzed in detail for EDS mapping and HRTEM (see below). The transformation of spherical structure composed of very small nanoparticles in Pt-Ag to planar structure with definite exposed planes for Pt-Ag HI, maintaining almost the same dimension is clear from low magnification SEM image shown in Fig. 1D. It is clear that the overall assembly and distribution of nanostructures in the specific area are not changed significantly after the electrochemical treatment; however the high magnification image in Fig. 1E indicates the change in morphology of individual assembled nanostructures to a more planar structure. The TEM image in Fig. 1F confirms the 3D nano/sub microstructure. Inset of Fig. 1F shows the magnified image of the Pt-Ag HI, where regions of smaller nanoparticle assemblies of around 50 nm are shown. To further understand the structure and to gain information regarding elemental composition and distribution of Pt and Ag in the catalyst, energy dispersive X-ray spectroscopy (EDS)
Fig. 1. (A and B) SEM images at two different magnifications, indicating the assembly of nanostructures, (C) TEM image of the 3D assembled structure with inset showing the regions of assembled structure with size of 50 nm, (D and E) low and high magnification SEM images of Pt-Ag HI, (F) TEM image of Pt-Ag HI with high magnification image showing the smaller assembled structures in the inset. (G, H and I) High magnification TEM image of Pt-Ag region used for EDS mapping and EDS mapping of Pt and Ag demonstrating the distribution of Pt and Ag in the nanostructure, respectively. (J, K and L) High magnification TEM image of Pt-Ag HI region used for EDS mapping and the distribution of Pt and Ag in the nanostructure, respectively as mapped by EDS.
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characterization was performed. The EDS spectrum given in Fig. S7 shows the presence of Pt and Ag, confirming the purity of the sample. The Ag to Pt ratio was found to be 0.08. In these catalysts Ag may remain alloyed with Pt or homogeneously distributed; however there is a possibility of local segregation of less noble metal Ag around the steps and kinks on the topmost layer [24]. The surface energy of Ag is less than that of Pt, which favors the surface segregation of Ag. These segregated Ag atoms may provide extra stability to the nanostructure assembly. Fig. S7B shows the EDS spectrum of Pt-Ag HI, which indicated a slightly lower ratio (0.044) of Ag to Pt suggesting that some of the Ag atoms on the surface have been removed during the electrochemical treatment. This was further confirmed from the ICP-OES analysis. Pt-Ag and Pt-Ag HI were dissolved in aqua regia and was diluted to 10 mL. The platinum and silver content in both the samples were verified and the ratio of Ag to Pt was compared for both the samples. The platinum and silver content in 100 mg of Pt-Ag were 92.8 mg and 7 mg respectively, whereas the Pt and Ag content in 100 mg of Pt-Ag HI were 95.96 mg and 4.03 mg respectively giving Ag to Pt ratio of 0.075 for Pt-Ag and 0.042 for Pt-Ag HI which was close to the results obtained from EDS. To ascertain the distribution and location of Pt and Ag in the sample EDS mapping of both the samples were done. Shown in Fig. 1G is the region of Pt-Ag where EDS mapping is done. Distribution of Pt and Ag are shown respectively in Fig. 1H and I. It appears that Ag is randomly distributed in Pt nanostructure with more segregation to the surface of the assembled nanostructure, which also is evident further in X-ray photoelectron spectroscopy (XPS) analysis (see below). EDS mapping of Pt-Ag HI (Fig. 1J) in the Pt and Ag regions are shown in Fig. 1K and L respectively, indicating the random distribution of Ag in the nanostructure. It may be noted here that the assembly of nanoparticle was slightly distorted due to the electrochemical etching induced reorientation of atoms in the nanostructure. TEM image shown in Fig. 2A revealed that the individual nanoparticles are of around 5 ± 2 nm in size and these nanoparticles are assembled to give the 3D architecture. To get a complete understanding on the nature and morphology of the individual nanoparticle high resolution transmission electron microscopy (HRTEM) images of the nanostructures were analyzed in detail (Fig. 2B) which confirmed that the nanoparticles are of spherical morphology. The marked region of the Fig. 2B is subjected
to fast Fourier transform (FFT) (see supplementary information Fig. S8A and S8B for details) and the FFT filtered image is shown in Fig. 2C. The lattice planes are clearly seen in the image. The selected area electron diffraction (SAED) pattern shown in Fig. 2D indicates the polycrystalline nature of the Pt-Ag nanoparticle. Planes are marked in the image. In Fig. 2E the high magnification TEM image of the Pt-Ag HI is shown where the transformation of spherical morphology in Pt-Ag to a polygonal structure in Pt-Ag HI for few particles is evident. To substantiate the formation of high index planes in Pt-Ag HI, HRTEM images of the catalyst was analyzed. HRTEM image shown in Fig. 2F evidently indicated the transformation of Pt-Ag shown in Fig. 2B to high index plane exposed Pt-Ag HI. The individual spherical Pt-Ag nanoparticles were converted to sharp edged particles with a slight increase in size. FFT filtered image shown in Fig. 2G ascertains the extent of conversion that happened on the application of SWP. The individual spherical nanoparticles were transformed to facet oriented square or polygon shaped nanoparticles and these are further assembled to give 3D architectures of facet oriented nano/sub-micron structures. Comprehensive analysis of these individual nanoparticles demonstrates that stepped surfaces are enclosed by continuously distributed high-index facets with (100) terraces and (110) steps, such as {410}, {310}, and {210} which usually serve as highly catalytically active sites [25e28]. The SAED pattern of Pt-Ag HI indicates the polycrystalline nature as given in Fig. 2H. Crystallographic details of Pt-Ag and Pt-Ag HI were examined using X-ray diffraction (XRD) (Fig. 3A). For XRD measurements, PtAg was drop casted on ITO and applied SWP for three cycles to obtain Pt-Ag HI. Grazing angle XRD measurements were performed at an angle of 0.5 . XRD pattern show the peak corresponding to platinum and silver. The peaks at 39.61, 45.62 , 67.4 , 81 and 85.3 correspond to FCC crystal structure of platinum, and the peak for Ag is observed at 37.6 which is marked in the figure with an arrow. As the reflections observed for platinum and silver are closer, reflections corresponding to each metal are not well resolved. The deconvoluted peaks for Pt (111) and Ag (111) are given in the Fig. 3B. Separate peaks for Pt and Ag suggest the segregation of components rather than the formation of alloy nanoparticles [29]. To ascertain the segregation of silver on the surface, Pt-Ag was subjected to an acid treatment to remove the silver present in the
Fig. 2. (A) High magnification TEM image of Pt-Ag (B) HRTEM (C) FFT filtered HRTEM image from the marked area in B indicating the lattice planes (D) SAED pattern (E) High magnification TEM image of Pt-Ag HI (F) HRTEM (G) FFT filtered HRTEM image from the marked area in F revealing the surface atomic steps and the high index facets (the edge atoms are marked in the figure with white dots) and (H) SAED pattern.
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Fig. 3. (A) XRD pattern of (a) Pt-Ag and (b) Pt-Ag HI, (B) the expanded and deconvoluted XRD pattern of Pt (111), Ag (111) and Pt (200) region of (a) Pt-Ag and (b) Pt-Ag HI. JCPDS value of Pt and Ag (Pt, JCPDS: 04-0802 and Ag, JCPDS file No. 04-0783) are shown (C) XPS spectra of Pt 4f region of (a) Pt-Ag and (b) Pt-Ag HI and (D) Ag 3d region of (a) Pt-Ag and (b) Pt-Ag HI.
catalyst. Pt-Ag was treated with concentrated nitric acid for one hour, washed thrice with DI water and collected the precipitate by centrifugation at 12000 rpm for 15 min. It is clear from the XRD measurement of the acid treated sample that the ratio of Ag(111) to Pt(111) is less compared to the Pt-Ag signifying the removal of segregated silver atoms (Fig. S9A). The destruction of the assembled 3D microstructure of Pt-Ag (Fig. S9B) is also observed in the SEM image shown in Fig. S9C. This shows that the presence of Ag provided extra stability to the assembled structure. The composition and oxidation state of the catalysts were examined using XPS (Fig. 3C). Pt 4f peaks were seen at 70.9 eV and 74.2 eV corresponding to Pt 4f7/2 and 4f5/2. Compared with the bulk Pt the peak values of Pt 4f7/2 and Pt 4f5/2 were down shifted which may be due to valency unsaturated atoms at edges and corners [30]. These peaks were deconvoluted and was seen that the Pt 4f7/2 and 4f5/2 are composed of two peaks corresponding to metallic platinum at high binding energy and a downward shifted peak with a significant intensity. The down shift of Pt 4f bands can also be due to the electronic effects, ie. the charge transfer from Ag to Pt. However, corresponding shifts are not observed in the Ag region, suggesting that the electronic effects are not the decisive factor in the down shift of platinum region. The two peaks at ca. 373.8 eV and 367.8 eV, which are ascribed to binding energies of metallic silver Ag 3d5/2 and Ag 3d3/2, respectively are shown in Fig. 3D. No alloy formation is evident from XPS. After electrochemical treatment both the Pt and Ag regions underwent a slight shift which is attributed to the reorientation of atoms within the nanoparticle during the SWP. These electronic effects can be a significant factor in the electro catalytic activity of Pt-Ag and Pt-Ag HI. Being a surface
sensitive technique (penetration depth ~3l), XPS will give more information regarding the segregation of silver nanoparticle over platinum nanoparticle. The ratio of Ag to Pt for Pt-Ag was found to be 0.143 which was almost two times higher than the ratio obtained from EDS (0.088) which gives elemental composition from the bulk of sample (of around 1 mm size). The higher ratio of Ag to Pt obtained from XPS for Pt-Ag than that obtained from EDS suggests the segregation of silver on the surface. Similar trend was seen for Pt-Ag HI exhibiting a ratio of 0.0765 from XPS and 0.044 from EDS, which indicate a surface segregation of silver atoms in Pt-Ag HI. 3.2. Electrochemical characterizations Electrochemical impedance spectroscopy (EIS) was carried out for both the catalysts at onset potential (0.4 V vs Ag/AgCl) of methanol oxidation in 1 M methanol and 0.1 M H2SO4 and was compared with commercial Pt/C and PtRu/C. Fig. 4A shows the Nyquist plot of the catalysts in the frequency ranging from 1 kHz to 0.01 Hz. The equivalent circuit to fit the EIS data is shown in the inset. Rs, Rct and Cdl represent solution resistance, charge transfer resistance and double layer capacitance, respectively. Nyquist plot of methanol oxidation on all the catalysts are constituted with single semicircle. The decreased diameter of semicircle reflects the decrease in charge transfer resistance (Rct) for MOR. The diameter of the arc decreases in the order Pt/C, PtRu/C, Pt-Ag and Pt-Ag HI. The enhanced charge transfer properties of Pt-Ag HI suggest faster reaction kinetics for electrocatalytic applications. Electrochemical voltammetric methods provide overall structural information on the exposed planes of catalysts compared to
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Fig. 4. (A) Nyquist plot of (a) Pt-Ag, (b) Pt-Ag HI, (c) Pt/C and (d) PtRu/C in 1 M methanol using 0.1 M H2SO4 as supporting electrolyte (B) Cyclic voltammogram of (a) Pt-Ag, (b) Pt-Ag HI, (c) Pt/C and (d) PtRu/C in 0.1 M H2SO4.
HRTEM where only limited number of particles are analyzed. Literature reports suggests that low-coordinate step atoms on Pt high index planes promotes oxygen adsorption and are susceptible to oxidation leading to a larger current at low potential in cyclic voltammograms when recorded in aqueous H2SO4 solution [15]. Thus the electric charges of oxygen adsorption can directly be correlated to the quantity of Pt atomic steps. Fig. 4B compares the cyclic voltammograms of Pt-Ag HI with Pt-Ag, Pt/C and PtRu/C recorded in 0.1 M H2SO4. As expected, a larger current of oxygen adsorption was observed at a lower potential (in the potential range 0.4 Ve0.95 V) on the Pt-Ag HI which confirms the presence of high index faceted platinum surface with low coordination numbers (CN). It is clear that the electric charge density of oxygen adsorption on the Pt-Ag HI catalyst is comparatively higher than the Pt-Ag and other commercially available catalyst Pt/C and PtRu/C which points to the high density of atomic steps of Pt-Ag HI. The higher oxygen adsorption peak on the Pt-Ag compared to Pt/C suggest that the PtAg also possess some high index facets compared to Pt/C but less than that of Pt-Ag HI. It was reported in literatures that adsorption/ desorption of oxygen generated by SWP plays a crucial role in the structural modifications. It is suggested that during the electrochemical treatment, H2O in solution undergoes dissociation and form oxides or hydroxides which react with platinum nanoparticle surface at upper potential. Oxygen atoms preferentially diffuse invade to the lattice structure replacing Pt atoms of (100) and (111) facets with surface atoms of high CN. These displaced Pt atoms cannot always return to their original position even after the desorption of oxygen atoms from the lattice site at lower potential, which leads to the destruction of ordered surface structure. On the contrary, high-index facets contain lots of step atoms with low CN, the oxygen atoms preferentially adsorb at those sites without replacing them, and hence the ordered surfaces are preserved during the electrochemical cycling. Thus, the adsorption/desorption of oxygen atoms plays the key role in the formation of high index facets [27,31]. To compare the catalyst surface area, the electrochemical active surface area (ECSA) was calculated by measuring the charge associated with hydrogen adsorption/desorption (QH) using the equation (1).
ECSA ¼
QH Q 0 mPt
(1)
Where, Q0 is the charge required for hydrogen adsorption on Pt surface (0.21 mC/cm2), and mPt is the mass of Pt nanoparticles (mg/
cm2) on the electrode 12. The ECSA values were found to be comparable for Pt-Ag and Pt-Ag HI. ECSA were calculated to be 27.88 m2/g, 25.11 m2/g, 91.1 m2/g and 74 m2/g for Pt-Ag, Pt-Ag HI, Pt/C and PtRu/C respectively. The slight decrease in the ECSA for PtAg HI after square wave potential treatment may be due to the small increase in the particle size as observed in Fig. 2E. It is reported that these high index faceted structure have limited surface area due to their large size [31,32]. 3.2.1. Electrochemical oxidation of fuels To test the electrocatalytic activity of the catalyst, methanol oxidation was performed in 1 M methanol using 0.1 M H2SO4 as supporting electrolyte. The catalyst (1 mg/mL) was dispersed in DI water and sonicated for 10 min 6 mL of the catalyst dispersion was dropped on a mechanically cleaned GCE, maintaining a loading of 6 mg for each measurement. Fig. 5A compares the cyclic voltammetric response of all the catalysts for methanol electro-oxidation at a scan rate of 50 mV/s. Lower onset potential of 0.4 V and higher oxidation peak current density of 400 mA/mg for the Pt-Ag HI are the highlight of the work. The methanol oxidation current obtained for Pt-Ag and Pt-Ag HI is higher than Pt/C and PtRu/C which is in agreement with the impedance analysis. In spite of the comparable ECSA values, the current obtained for MOR on Pt-Ag HI was two times higher than that of Pt-Ag. The comparable ECSA values of Pt-Ag and Pt-Ag HI together with the higher MOR activity of Pt-Ag HI confirm the high catalytic activity of the high index planes exposed on its surface which is one of the major contributing factors to the enhanced performance. The electrocatalytic activity of acid treated Pt-Ag was monitored and it was found that the activity exhibited by this catalyst is 12 times lower than Pt-Ag pointing to the role of Ag in the catalyst (Fig. S10B). This suggests that the enhanced activity of Pt-Ag and Pt-Ag HI is also due to the stability of the catalytically active site during electrochemical oxidation as a result of the segregated Ag atoms on the top most layer. The interesting feature observed in both Pt-Ag and Pt-Ag HI is the low intermediate oxidation current which is due to the oxidation of adsorbed intermediates [33e35]. These adsorbed intermediates can block the active sites of the catalyst inhibiting further oxidation of methanol. The ratio of forward current to backward current (If/Ib) indicates the electrocatalytic ability to remove the carbonaceous species that gets binds to the active site of the catalyst and also is a rough indicator of the efficiency of catalyst. The ratio, If/Ib greater than unity indicates enhanced tolerance to poisoning [36e39]. The catalysts Pt-Ag and Pt-Ag HI
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Fig. 5. (A) Cyclic voltammetry mass activity curves of (a) Pt-Ag, (b) Pt-Ag HI, (c) Pt/C and (d) PtRu/C (B) specific activity bar diagram of different electrocatalysts for MOR recorded at a scanning rate of 50 mV/s (C) Chronoamperometry analysis of (a) Pt-Ag, (b) Pt-Ag HI, (c) Pt/C and (d) PtRu/C at peak potential. All measurements were performed using 1 M methanol using 0.1 M H2SO4 as supporting electrolyte. (D) Schematic representation of methanol oxidation on Pt-Ag and Pt-Ag HI.
we reported here exhibited an If/Ib ratio of ca 1.4 which is at par with PtRu/C (1.4-which is the commercially used catalyst to enhance methanol oxidation during the forward bias) and higher than Pt/C (1.1) that point out to the high catalytic activity of these catalysts. It was also observed that the onset potential for methanol oxidation starts at 0.41 V on Pt-Ag HI and 0.44 V on Pt-Ag respectively, which is lower than Pt/C (0.47 V) indicating the superior activity of the catalysts compared to Pt/C. The specific activity of the catalyst is shown in Fig. 5B demonstrating that the specific activity of Pt-Ag HI is almost 18 times higher than that of Pt/C and 9 times higher than PtRu/C. The enhanced electrocatalytic activity observed in case of Pt-Ag HI is attributed to the synergetic effect of large surface area and high index facets on the bimetallic catalyst. Chronoamperometry measurements were performed to analyze the stability of the catalyst towards MOR. The presence of high index facets and the segregated Ag on these facets helps in improving the stability of the catalyst as indicated in Fig. 5C. Chronoamperometry measurements were done at 0.72 V vs Ag/ AgCl in 1 M methanol using 0.1 M H2SO4 as supporting electrolyte. The slow decay of the current in Pt-Ag HI compared to other catalysts shows the excellent stability and lower poisoning of the catalyst. Pt-Ag nanostructures exhibited higher catalytic activity than Pt/ C and PtRu/C owing to its higher surface area due to the very small size of the individual nanoparticles. However, the predominant
(111) plane are prone to poisoning and the intermediate species gets adsorbed on the plane as in the schematic representation shown in Fig. 5D. Upon applying SWP, the high index facets which are less prone to poisoning are formed, thereby the catalytically active sites are not blocked by the intermediates, leading to better electrochemical oxidation of organic molecules. The enhanced electrocatalytic property of the Pt-Ag HI can be attributed to the presence of Pt nanoparticles with preferentially exposed high indexed facets with surface segregation of Ag. The electronic effects ie., the charge transfer from Ag to Pt also may play a crucial role in enhancing the catalytic activity of both Pt-Ag and Pt-Ag HI. There is a possibility of PVP getting adsorbed on the active site affecting the catalytic activity by blocking the adsorption of methanol to the surface. However, presence of organic content on the Pt-Ag and PtAg HI did not significantly affect the electro catalytic activity, pointing that the active sites are not blocked by the PVP. It is also clear that the enhanced activity of Pt-Ag HI over Pt-Ag is not due to the presence of organic content as both the catalysts have the same organic content present as indicated by the FTIR spectra shown in Fig. S5. In Pt-Ag HI, the small nanoparticles with high indexed facets and small CN possessing abundant steps and kinks are assembled to give large structures with steps and terraces. These larger nanocrystals may act as new micro electrodes. Each individual nanoparticle is of approximately 5 nm in size contributing significantly to high surface area. During electro catalytic oxidations the
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less noble metal silver stabilizes the surface active sites and the 3D structure and there by enhances the catalytic activity and durability of the catalyst. The performance of both the catalysts toward methanol oxidation was compared with the previously reported work in literature and is shown in Fig. 6. Relatively higher mass and specific activity was exhibited by the Pt-Ag HI exposed with high index facets reported in this work. To see the electro catalytic activity of these catalyst towards
other organic molecules; electro-oxidation of ethanol and formic acid, the two fuels commonly used in fuel cell application are monitored. The mass activity and specific activity of the catalyst towards ethanol oxidation are shown in Fig. 7A and B. Pt-Ag HI exhibits the highest mass and specific activity. The Pt-Ag HI has the mass activity of 250.36 mA/mg, which is 1.5 times higher than PtAg, 1.2 times higher than PtRu/C and 2.8 times higher than Pt/C respectively. As shown in Fig. 7C and D, the Pt-Ag HI is highly active towards oxidation of formic acid also. The highest activity of the Pt-Ag HI towards methanol, ethanol and formic acid oxidation indicate that Pt-Ag HI can be used as a promising catalyst for the fuel cells.
4. Conclusions
Fig. 6. Comparison of literature reports on mass and specific activity of Pt based catalyst towards methanol oxidation. Reference numbers are indicated in brackets. PtAg and Pt-Ag HI are the catalysts reported in this work [40e43].
In summary, we have presented a robust wet chemical method to generate 3D architectures of Pt-Ag nano/sub microstructures with controllable high surface energy nanoparticles. This catalyst was then transformed to high index plane exposed structures upon electrochemical treatment. The nanostructures exhibited well defined high indexed exposed planes with stable kinks and surfaces. The catalyst showed a specific activity of 1.6 mA/cm2 which is almost 18 times higher than the commercial Pt/C and 9 times higher than commercial PtRu/C. High density of atomic steps with segregated silver atoms formed on the catalyst and the electronic effects contributed to the higher electrocatalytic activity towards fuel oxidation reactions. The Pt-Ag HI has higher stability than PtAg and commercial catalysts showing a novel Pt based bimetallic nanostructure with high catalytic performance and stability for forthcoming practical applications.
Fig. 7. (A) Cyclic voltammetric mass activity curves and (B) specific activity bar diagram of the electro-catalysts towards ethanol oxidation reaction in 1 M CH3CH2OH and 0.1 M H2SO4, (C) Cyclic voltammetric mass activity curves and (D) specific activity bar diagram of the electro-catalysts towards formic acid oxidation in 1 M HCOOH recorded at scanning rate of 50 mV/s of (a), (b), (c) and (d) represents Pt-Ag, Pt-Ag HI, Pt/C and PtRu/C respectively.
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Acknowledgements The authors would like to acknowledge Kerala State Council for Science, Environment and Technology for the financial support to the Fuel cell research (Grant No. 003/SRSPS/2014/CSTE) and Department of Science and Technology (DST) for the facilities provided for nanoscience research laboratory. We thank Dr. Subramanyan Namboodiri Varanakkottu for helping us with the FijiWin 64 software and the TEM analysis. We acknowledge the help received from Dr. M.K. Jayaraj, Nanophotonic & Optoelectronic Devices laboratory, Dept. of physics for XRD measurement under DST nanomission initiative program. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2018.12.151. References [1] A.A. Permyakova, B. Han, J.O. Jensen, N.J. Bjerrum, Y. Shao-Horn, PtdSi Bifunctional surfaces for CO and methanol electro-oxidation, J. Phys. Chem. C 119 (2015) 8023e8031. [2] T.-Z. Hong, Q. Xue, Z.-Y. Yang, Y.-P. Dong, Great-enhanced performance of Pt nanoparticles by the unique carbon quantum dot/reduced graphene oxide hybrid supports towards methanol electrochemical oxidation, J. Power Sources 303 (2016) 109e117. [3] Z. Zhu, T. Kin Tam, F. Sun, C. You, Y.-H. Percival Zhang, A high-energy-density sugar biobattery based on a synthetic enzymatic pathway, Nat. Commun. 5 (2014) 3026. [4] B. Kaur, R. Srivastava, B. Satpati, Highly efficient CeO2 decorated nano-ZSM-5 catalyst for electrochemical oxidation of methanol, ACS Catal. 6 (2016) 2654e2663. [5] Y.Y. Tong, C.D. Gu, J.L. Zhang, M.L. Huang, H. Tang, X.L. Wang, J.P. Tu, Threedimensional astrocyte-network NiePeO compound with superior electrocatalytic activity and stability for methanol oxidation in alkaline environments, J. Mater. Chem. A. 3 (2015) 4669e4678. [6] Y. Kang, J.B. Pyo, X. Ye, T.R. Gordon, C.B. Murray, Synthesis, shape control, and methanol electro-oxidation properties of PteZn alloy and Pt3Zn intermetallic nanocrystals, ACS Nano 6 (2012) 5642e5647. € Karatepe, Y. Yıldız, H. Pamuk, S. Eris, Z. Dasdelen, F. Sen, Enhanced elec[7] O. trocatalytic activity and durability of highly monodisperse Pt@PPyePANI nanocomposites as a novel catalyst for the electro-oxidation of methanol, RSC Adv. 6 (2016) 50851e50857. [8] H. Xu, A.-L. Wang, Y.-X. Tong, G.-R. Li, Enhanced catalytic activity and stability of Pt/CeO2/PANI hybrid hollow nanorod arrays for methanol electro-oxidation, ACS Catal. 6 (2016) 5198e5206. [9] L. Chen, L. Kuai, B. Geng, Shell structure-enhanced electrocatalytic performance of AuePt coreeshell catalyst, CrystEngComm 15 (2013) 2133e2136. [10] C.-C. Ting, C.-H. Liu, C.-Y. Tai, S.-C. Hsu, C.-S. Chao, F.-M. Pan, The size effect of titania-supported Pt nanoparticles on the electrocatalytic activity towards methanol oxidation reaction primarily via the bifunctional mechanism, J. Power Sources 280 (2015) 166e172. [11] D. Yu, E. Shamsaei, J. Yao, T. Xu, H. Wang, A hierarchically structured PtCo nanoflakesenanotube as an electrocatalyst for methanol oxidation, Inorg. Chem. Front. 4 (2017) 845e849. [12] T. Radhakrishnan, N. Sandhyarani, Three dimensional assembly of electrocatalytic platinum nanostructures on reduced graphene oxide e an electrochemical approach for high performance catalyst for methanol oxidation, Int. J. Hydrogen Energy 42 (2017) 7014e7022. [13] L. Wei, Z.-Y. Zhou, S.-P. Chen, C.-D. Xu, D. Su, M.E. Schuster, S.-G. Sun, Electrochemically shape-controlled synthesis in deep eutectic solvents: triambic icosahedral platinum nanocrystals with high-index facets and their enhanced catalytic activity, Chem. Commun. 49 (2013) 11152e11154. [14] T. Kwon, M. Jun, H.Y. Kim, A. Oh, J. Park, H. Baik, S.H. Joo, K. Lee, Vertexreinforced PtCuCo ternary nanoframes as efficient and stable electrocatalysts for the oxygen reduction reaction and the methanol oxidation reaction, Adv. Funct. Mater. 28 (2018) 1706440. [15] Z.-Y. Zhou, Z.-Z. Huang, D.-J. Chen, Q. Wang, N. Tian, S.-G. Sun, High-index faceted platinum nanocrystals supported on carbon black as highly efficient catalysts for ethanol electrooxidation, Angew. Chem. Int. Ed. 49 (2010) 411e414. [16] L. Wei, T. Sheng, J.-Y. Ye, B.-A. Lu, N. Tian, Z.-Y. Zhou, X.-S. Zhao, S.-G. Sun, Seeds and potentials mediated synthesis of high-index faceted gold nanocrystals with enhanced electrocatalytic activities, Langmuir 33 (2017) 6991e6998. [17] L. Shuiping, P.K. Shen, Concave platinumecopper octopod nanoframes bounded with multiple high-index facets for efficient electrooxidation catalysis, ACS Nano 11 (2016) 11946e11953.
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