Highly Active Multimetallic Palladium Nanoalloys Embedded in

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Highly active multimetallic palladium nanoalloys embedded in conducting polymer as anode catalyst for electrooxidation of ethanol Srabanti Ghosh, Susmita Bera, Sandip Bysakh, and Rajendra Nath Basu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08327 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 13, 2017

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ACS Applied Materials & Interfaces

Highly active multimetallic palladium nanoalloys embedded in conducting polymer as anode catalyst for electrooxidation of ethanol Srabanti Ghosha,* Susmita Beraa, Sandip Bysakhb, Rajendra Nath Basua* a b

Fuel Cell and Battery Division, CSIR - Central Glass and Ceramic Research Institute,

Materials Characterization Division, CSIR - Central Glass and Ceramic Research Institute 196, Raja S. C. Mullick Road, Kolkata-700032, India

*

Corresponding Authors Email:

[email protected] [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT: Fabrication of multimetallic nanocatalysts with controllable composition remains a challenge for the development of low-cost electrocatalysts and incorporating metal-based catalysts into active carbon nanoarchitecture represents an emerging strategy to improve the catalytic performance of electrocatalysts. Herein, a facile method developed for Pd nanoparticles (NPs) based multimetallic alloys incorporated on polypyrrole (Ppy) nanofibers by in situ nucleation and growth of NPs using colloidal radiolytic technique. Electrochemical measurement suggests that the as-prepared catalysts demonstrate dramatically enhanced electrocatalytic activity for the ethanol oxidation in alkaline medium. The ultrasmall Pd30Pt29Au41/Ppy nanohybrids (∼8 nm) exhibit an excellent electrocatalytic activity which is ∼5.5 times higher than monometallic counter parts (12 A per mg Pd, 5 times higher activity compared to Pd/C catalyst). Most importantly, ternary nanocatalyst shows no obvious change in chemical structure and the long-term stability reflected with 2% lost in forward current density during 1000 cycling. The superior catalytic activity and durability of nanohybrids have been achieved due to the formation of Pt−Pd−Au heterojunctions with cooperative action of the three metals in the alloy composition, and the strong interactions between Ppy nanofibers support with metal NPs. The facile synthetic approach provides a new generation polymer supported metal alloy hybrid nanostructures as potential electrocatalysts with superior catalytic activity for fuel cell applications.

KEYWORDS: Electrocatalysis, Pd based catalysts, alloys, Ppy nanofibers, ethanol electrooxidation


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1. INTRODUCTION Direct ethanol fuel cells (DEFCs) have been considered as a energy conversion devices using ethanol as abundant and inexpensive liquid fuel having high power density with facile storage, and easy refilling.1,2 The ethanol electrooxidation reaction (EOR) is kinetically slow process with overpotentials of 0.3−0.4 V and for the improvement of DEFC technology, highly efficient, durable and low cost electrocatalysts are required for ethanol oxidation with high power density and efficiency.3,4 The use of both noble and transition metal based electrocatalysts can improve kinetics at anode for alcohol oxidation. Particularly, platinum (Pt) has been recognized as bench marked catalysts for fuel cells applications; however, high cost and catalytic poisoning limit their large scale application.5,6 Alternatively, palladium nanostructures have been found to be the one of the best catalysts applied in DEFCs, due to their relatively high availability compared to Pt and improved CO resistance.7-9 However, catalyst poisoning and poor durability, limit the utility of Pd electrocatalysts for DEFCs. Moreover, palladium-based bimetallic or multi-metallic alloy or core-shell nanostructures such as PdPt,10 Pd-Ag,11 PdAu,12,13 Pd3Pb,14 PdGe,15 Pt−Pd/Graphene,16 Pd–M–P ternary phosphide NPs17 etc demonstrated enhanced electrocatalytic activity compared with pure Pd counterparts. Notably, the presence of second metal augmented the number of d-band vacancies with an ideal M–M inter-atomic distance and in parallel reduce the content of Pt or Pd in alloy composition.16, 18 The catalytic activity of multi metallic nanoalloys can be greatly enhanced by the availability of large number of active sites toward a specific reaction and accelerate mass transfer between different active sites ternary alloy composition have been explored as active catalysts for electrooxidation of alcohol such as ethanol, formic acid,



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ethylene glycol or glycerol etc. Thus, Pd based multimetallic nanoalloy would be useful to increase the EOR catalytic performance. To further improve the stability, and utilization efficiency of metals, various types of carbon based porous nano-architectures have been utilized as catalyst support which facilitates the mass diffusion to improve the utilization efficiency of metals, and prevent the aggregation and leaching of the metallic NPs.19, 20 For example, the Pt or Pd-based NPs supported on porous carbon are still the most commonly used electrocatalysts such as commercial Pt/C or Pd/C, however, corrosion of carbon supports in acid or alkaline electrolyte leads to the blocking of channels which lower the surface area and aggregation and leaching of Pd.21, 22 To avoid this fundamental issues, graphene, a two-dimensional (2-D) carbon having extremely porous structure and excellent electrical conductivity has been extensively used as a support for electrocatalysts.23-25 However, hydrophobic nature of graphene needs further funtionalization which may lower the interaction between carbon support with the metal catalysts and often, the presence of surfactant around NPs surface lower the available active sites, which consequently lower chemical activity. Recently, Ghosh et al. have shown that Au based multimetallic NPs deposited on reduced graphene oxides nanosheets without using surfactant molecule and found useful as active electrocatalysts for glucose oxidation.26 Other carbon based materials such as carbon nitride,27 carbon fibers,28 carbon nanotubes (CNTs),29 carbon aerogels and xerogels22 have been used for facile electron transfer and to avoid nanoparticle agglomeration. Moreover, metal oxides, metal nitrides and carbides also used as supports such as Pd/NiCoOx,30 Pd/TiN,31 and Pd/WC32 etc. Recently, other novel supports have been also explored to enhance catalytic activity and durability of electrocatalysts. Recently, Ye et al. developed a strongly coupled Pd nanoparticles/CoP nanosheets attached on the cloth of 4 ACS Paragon Plus Environment

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carbon fibers for significantly increase the catalytic activity and stability of Pd NPs for electrooxidation of ethanol.33 Nanostructured conducting polymers shows potential in various field including electronic devices, photovoltaic devices, sensors and catalysts owing to their excellent electrical, optical, and electrochemical properties.34-36 Particularly, one-dimensional (1D) conducting polymer nanostructures (CPN) are widely utilized for solar device application with unique phycochemical charactistics compared to the bulk materials.37, 38 Conducting polymer with πconjugated structures having good electronic conductivity and high corrosion resistance has been considered as a suitable template for the nanoparticles synthesis.39 Polymer supported Pt NPs demonstrated high electrocatalytic activity toward electrocatalytic oxidation of methanol (MOR).40 For example, Xu et al. also reported multilayered Pt/CeO2/PANI and ZnO/Pt/CeO2/PANI

hybrid

hollow

nanorod

arrays

which

demonstrate

elevated

electrocatalytic activity toward MOR due to having short diffusion paths for electroactive species.41 Pandey et al. developed a multistep process for the electrochemical deposition of Pd NPs polyaniline nanofibers or 3,4-polyethylenedioxythiophene film which displays electrochemical activity towards ethanol and methanol.42 Li and co-workers reported polypyrrole

functionalized

PtPd/PPy/PtPd

three-layered

nanotube

arrays

for

the

electrooxidation of liquid fuel (methanol, ethanol and formic acid).43 Recently, our group prepared Pd/Ppy nanocomposites that exhibit high catalytic performance for ethanol oxidation.44 However, conducting polymer supported trimetallic nanoalloys have not been utilized as efficient anode catalysts for ethanol electrooxidation so far. Herein, we have successfully synthesized crystalline, small multimetallic NPs on polymer nanofiber surface by a radiolysis technique without using conventional reducing agent at room temperature. Here, 5 ACS Paragon Plus Environment


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we have successfully synthesized composition tunable Pd based bimetallic and the trimetallic nanoalloys supported on conducting polymer nanostructures via radiolytic reduction method. Interestingly, each nanoparticle is uniformly deposited on polypyrrole nanofibers, showing a significantly high eletrocatalytic activity and durability compared with the Pd/C catalyst for electroxidation of ethanol under alkaline medium.

2. EXPERIMENTAL SECTION 2.1 Materials and Reagents. Cetyltrimethylammonium bromide (CTAB, 98% purity), ammonium

peroxydisulfate,

pyrrole,

palladium

(II)

acetylacetonate,

platinum

(II)

acetylacetonate, gold (III) acetate, 2-propanol, ethanol (≥ 99% for HPLC), KOH (97%), HCl, Nafion® suspension (5 wt.%), and acetone were purchased from Sigma-Aldrich. The entire reagent used in the present study as received without further purification. Ultrapure water (Millipore System, 18.2 MΩ cm) was used as solvents. 2.2. Synthetic Procedures. Conducting polymer nanostructure preparation. Surfactant mediated soft template has been employed for the fabrication of conducting polymer, following our previous method with some modification.

40, 44, 45

A white precipitate of

CTAB/APS complex formed by reacting 0.015 M cetrimonium bromide (CTAB, cationic surfactant) and 0.026 M ammonium persulfate (APS, anionic oxidant) in 1 M aqueous HCl, 1 M pyrrole was mixed at 5°C and polymerization was allowed to continue for 24 hours. The polypyrrole nanofiber powder was washed with distilled water, ethanol and finally acetone. Metal nanoparticles deposition on conducting polymer. Metal nanoparticles were deposited on polypyrrole nanostructures by using steady state gamma irradiation. In a typical experiment, 1 mg mL-1 of Ppy solution was mixed with 1 mM of palladium (II) 6 ACS Paragon Plus Environment

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acetylacetonate, platinum (II) acetylacetonate and gold (III) acetate solution and then deaerated under a N2 flow. This final solution was irradiated in Co-60 gamma irradiation chamber at a dose rate of 9.8 kGy/h (dose, 55 kGy/h). Finally, the mixture were centrifuged and washed with ethanol and then obtained precipitate dried at 50οC. Similarly, various compositions of bimetallic, Pd-Pt/Ppy, Pd-Au/Ppy and Pd-Pt-Au/Ppy have been prepared by selecting suitable mass ratio. 2.3. Characterization. High-resolution transmission electron microscopy (HRTEM) were obtained with Tecnai G2 30ST (FEI) high-resolution transmission electron microscope operating at 300 kV. X-ray diffraction (XRD) patterns of catalysts were recorded on a Philips X’Pert, The Netherlands, X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å). XPS measurement was performed on a PHI 5000 VersaProbe II spectrophotometer (Physical Electronics Inc., USA) with Al Kα (~1486.6 eV) X-ray. Charge correction has been considered by taking C 1s spectra as standard (284.5 eV). Thermogravimetric analysis (TGA) of the sample was performed on thermal analyzer (STA 449F, Netzsch, Germany). Samples were heated under argon flow upto 950°C at heating rate of 10 °C min−1. The Fourier transform infrared (FTIR) spectra of the samples were recorded using a JASCO FTIR-6300 spectrometer Raman spectra were collected using a JOBIN YVON HR800 Confocal Raman system employing a 632.8 nm laser beam. The spectra were recorded using 20X objective and accumulated for 30 s. The composition of metal/polymer nanohybrid was determined by Spectro Ciros Vision inductively coupled plasma atomic-emission spectroscopy (ICP-AES) instrument, Spectro GmbH, Germany. 2.4. Electrocatalytic experiment. All electrochemical measurements were carried out on a galvanostat−potentiostat (PGSTAT302N, Autolab, Netherlands). A conventional three7 ACS Paragon Plus Environment


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electrode system was used, with a platinum wire as the counter electrode, modified glassy carbon electrode as the working electrode, and Hg/HgO electrode as the reference. The electrolyte solution was purged with nitrogen gas in order to remove dissolved oxygen. Prior to surface coating, the surface of a glassy carbon electrode was polished with α-alumina powders. Then 5 µL of nanohybrid or commercial catalysts (20% Pd or Pt on Vulcan XC-72, JM) were deposited on the surface of GC electrode and dried before electrochemical measurement. Cyclic voltammetry (CV) was carried out in the 1 M KOH electrolyte and ethanol oxidation was followed in presence of 1 M ethanol at a scan rate 50 mV Sec-1. A two compartment, thermostated cell was used for ethanol oxidation measurements within the range of range of 15-45 °C (calibration was done prior to electrochemical measurement). Electrochemical impedance spectroscopy (EIS) measurements were recorded on a galvanostat−potentiostat. The frequency range was from 10 kHz to 10 mHz with an AC amplitude of 10 mV. 3. RESULTS AND DISCUSSION XRD patterns of the as-prepared materials are displayed in Figure 1a to illustrate the successful formation of bimetallic or trimetallic NPs and their crystalline structures. The diffraction peaks at 40.6°, 46.6°, 68.4°, 82° and 87° which can be indexed as the (111), (200), (220), (311) and (222) facets diffractions of face-centered cubic (fcc) Pd (JCPDS No 050681), respectively.46 For Pt/Ppy, the main peaks at 39.65, 46.1, 67.5, 81.1, 86.1 correspond to the (111), (200), (220), (311), and (222) faces of Pt crystal respectively. For bimetallic PdPt/Ppy composition, the major peaks at 39.89°, 46.5°, 67.85°, 81.8 and 86.5 correspond to (111), (200), (220), (311) and (222) plane, respectively, which confirm the face centered cubic (fcc) crystal structure having strong and prominent diffraction peaks of each metal. 8 ACS Paragon Plus Environment

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(111)

Intensity/Counts

(111)

(b)

Pt/Ppy

(200)

(220)

(311) (222)

(220)

(311)(222)

Pd89Pt11/Ppy Pd54Au46/Ppy Pd30Pt29Au41/Ppy (111)

30

60

75

2853 2924

1547 1624 2000

2500

3000

10000

Intensity /a.u.

1536 1639

1046 1159 962 1043 1160 1302

Ppy

1500

150

(d) Pd30Pt29Au41/Ppy

1000

40

0

90

2θ/°

(c)

Pd30Pt29Au41/Ppy

60

(311) (222)

(220)

45

Pd54Au46/Ppy

80

20

Au/Ppy

(200)

Ppy Pt/Ppy Pd/Ppy Pd89Pt11/Ppy

100

Pd/Ppy

(200)

Weight / wt.%

(a)

Transmittance /a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8000

300

450

600

750

900

Temperature /°C 1567

Ppy Pd/Ppy Pd54Au46/Ppy Pd30Pt29Au41/Ppy

6000 4000 962

1318 1375 1232 1054

2000 0

3500

800

-1

Wavenumber/cm

1200

1600 -1

Raman shift /cm

Figure 1. (a) XRD patterns for the synthesized Pd/Ppy, Pt/Ppy, Au/Ppy, Pd89Pt11/Ppy, Pd54Au46/Ppy, Pd30 Pt29Au41/Ppy nanohybrids. (b) TGA curves of Ppy nanofibers (black line) and Pd/Ppy (red line), Pt/Ppy (green line), Pd89Pt11/Ppy (blue line), Pd54Au46/Ppy (orange line), Pd30Pt29Au41/Ppy (pink line) nanohybrids. (c) FTIR spectra of Ppy and Pd30 Pt29Au41/Ppy nanohybrids. (d) Raman spectra of Ppy, Pd/Ppy, Pd54Au46/Ppy, Pd30 Pt29Au41/Ppy nanohybrids.

The shift in the diffraction peaks suggests the formation of alloy which is consistent with literature report.47 Moreover, compare to Au/Ppy (37.9°), the (111) peak positions for another bimetallic composition Pd64Au46/Ppy (38.26°) nanohybrids are shifted to higher angles and other strong peaks at 44.5°, 64.74°, 77.9 and 82.06 correspond to (200), (220), (311) and 9 ACS Paragon Plus Environment


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(222) plane also suggests formation of Pd-Au alloy phase. It is important to note that Pt, Pd and Au has the cubic structures and the main diffraction peaks from trimetallic Pd-Pt-Au/Ppy composition at 38.1° for (111) plane also corroborates the formation of alloys and other peaks are similar to the pure Pt, Pd, and Au. Hence, the XRD patterns suggest high purity and crystallinity of the as prepared Pd based nanoalloys. In general, polypyrrole nanofibers has broad feature of the X-ray peak with an amorphous polymeric structure. The loading of metal nanoparticle on polymer surface has been confirmed by thermal gravimetric analysis. Interestingly, the thermal stability of the nanohybrids have been improved in comparison to bare polymer structure as shown in Figure 1b which is consistent with our previous observation.44 The metal loading on Ppy nanofibers has been determined by thermo gravimetric analysis (TGA) using high residual mass of Ppy (∼15-51%) as summarized in Table 1. Additionally, composition of each nanohybrid has been determined by inductively coupled plasma atomic-emission spectroscopic data. Moreover, the chemical structures of nanohybrid and interaction of metal nanoparticles with polymer have been studied by FTIR analysis (Figure 1c and Figure S1 in the Supporting Information). An intense peak at 1624 cm-1 for Ppy (corresponds to –C=N stretching vibration) has been shifted to 1639 cm-1 in Pd30Pt29Au41/Ppy nanohybrid which suggests strong interaction of Pd30Pt29Au41 NPs with Ppy polymeric chain (Figure 1c).48,

49

Hence,

present methodology endow with a intimate contact between polymer and metallic nanoparticles due to radiolytic reduction of metallic precursors happened on polymer surface.


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Table 1. Quantification of metal loading on polymer nanofibers by using TGA and ICP-AES techniques.

Materials

TGA: total metal loading (wt.%)

ICP-AES Pd-Pt-Au Atomic content (at.%) Pd

Metal loading (wt.%)

Pt

Au

100

-

16±2

Pt/Ppy

15

Pd/Ppy

30

100

-

-

20±1

Pd89Pt11/Ppy

31

89

11

-

27±2

Pd54Au46/Ppy

40

54

-

46

26±3

Pd30 Pt29Au41/Ppy

49

30

29

41

30±2

On comparison of the two spectra, shifting of polypyrrole bands has been observed due to strong interaction between metal NPs and polymer matrix (Figure S1). Additionally, intense absorption bands at 2853, 2924 cm−1 for deformational vibrations of C–H and CH2 stretching modes of Ppy has been significantly lowered after formation of nanohybrid suggests templating effect of

the Ppy nanofibers during synthesis of nanoparticles at polymer

interface. The molecular structures of nanohybrids have been characterized by Raman spectra. Figure 1d shows the Raman spectra of Ppy and Pd/Ppy, Pd54Au46/Ppy, Pd30Pt29Au41/Ppy nanohybrids. From the Raman spectra of pure Ppy, the bands at approximately 932 and 989 cm−1 are assigned to the ring deformation associated with quinonoid bipolaron structure respectively, revealing the presence of doped Ppy structures.50 The peak at 1054 cm–1 is correspond to the quinonoid polaron structure. The C=C backbone stretching peak of polypyrrole shows a strong band located at ca. 1567 cm-1.51 The additional double peaks at about 1318 and 1375 cm–1 are associated with the ring-stretching mode of Ppy.52 The



prominent bands observed in the spectrum of Ppy can also be observed in the spectrum of nanohybrids. However, the intensity of polymer spectrum significantly lower after formation of nanohybrids also suggests enhanced interaction between metal NPs and polymer that would be favorable to attach the metal alloys onto the Ppy surface. X-Ray photoelectron spectroscopy is widely utilized to characterize the chemical composition of nanomaterials.

0

Ppy Nanofibers Pd30Pt29Au41/Ppy

(b) 3d5/2

Cl2p

200

400

600

800

Binding Energy/eV

(c)

4f7/2

3d3/2

334

1000

336

338

340

70

72 74 76 Binding Energy/ eV

Fitting Results

(d)

4f7/2

Pt 4f

344

346

Au 4f 4f5/2

82

78

342

Binding Energy/eV

Intensity/counts

4f5/2

Fitting Results

Pd 3d

Intensity/Counts

O 1s

C 1s Au 4d Pd 3d N 1s Pt 4f Au 4f

Intensity/counts

(a)

Intensity/Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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84

Fitting Results

86

88

90

Binding Energy/eV

Figure 2. (a) XPS pattern for the as-prepared Ppy, and Pd30Pt29Au41/Ppy nanohybrids, (b-d) Magnified XPS spectra for (b) Pd 3d, (c) Pt 4f and (d) Au 4f.

The presence of strong XPS signals at 196.5, 285.5, 401.5, and 532 eV in the survey spectrum of both Ppy and Pt24Pd26Au50/Ppy nanohybrid are corresponds to Cl 2p, C 1s, N 1s, and O 1s, respectively (Figure 2a). 44, 53 The signal corresponds to Cl, C, N, and O are contributed from 12 ACS Paragon Plus Environment

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Ppy. Other strong XPS signals at 70.0, 84.1 and 346.2, eV are corresponds to Pt 4f, Au 4f, and Pd 3d respectively in the survey spectrum Pd30Pt29Au41/Ppy supports the formation of trimetallic composition. Figure 2b-d illustrates the XPS spectra of Pd 3d, Pt 4f, and Au 4f regions. The core-level Pd 3d spectra display a doublet signal with binding energies of 335.3 and 340.4 eV for Pd 3d5/2 and Pd 3d3/2, respectively, corresponds to the Pd signal.54 Another small doublets around 336.7 and 342.1 can be assigned to the Pd 3d3/2 and Pd 3d5/2 peaks of PdO (Figure 2b).44 The Pt 4f signals represents intense doublet at 70.8 eV and 74.08 eV which could be assigned to the binding energies of Pt 4f7/2 and Pt 4f5/2 of metallic Pt0 and the second doublet (71.9 and 75.4 eV) can be ascribed to the +2 oxidation states of Pt respectively (Figure 2c).55, 56 The peak observed at ~83.4 and 87.1 eV from Au 4f7/2 and Au 4f5/2 spin-orbit doublets, respectively, correspond to a metallic-gold (Au0) which are well consistent with Au NPs (Figure 2d).57 Moreover, C1s core level spectra demonstrated a strong peak at 285.6 eV correspond to the conjugated carbon atoms from the Ppy (Figure S2 a-c in the Supporting Information). The signals located at 285.6, and 402.06 eV for C 1s and N 1s respectively in Ppy are slightly shifted to 284.5, and 402.25 eV for nanohybrids, suggesting that a strong interaction between the metal NPs and polymeric chain. For binary composition, presence of both Pt and Pd peak and Pd or Au peak suggests the formation of Pd89Pt11/Ppy, Pd54Au46/Ppy, Pd51Pt49/Ppy nanohybrids (Figure S2d in the Supporting Information). The morphology of the as-prepared nanohybrid was initially characterized by transmission electron microscopy (TEM). Figure 3a shows the TEM image of the as-prepared polymers. Ppy nanofibers with average size of 50-65 nm diameters and a few micrometers long are obtained using surfactant based oxidative template. Figure 3b displays TEM images of Pd/Ppy hybrid nanostructures at low and high magnification. The Pd/Ppy hybrid nanostructures are



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composed of Ppy nanofibers decorated with ultra-fine Pd NPs of about 2-3 nm in size. The nanoparticles are well dispersed on polymer nanofiber surface as shown in a magnified image (Figure 3c).

Figure 3. TEM images of (a) Ppy nanofibers, (b-c) Pd/Ppy nanohybrids at different magnifications. (d) HRTEM image, (e) SAED pattern, (f) EDX spectrum of Pd/Ppy nanohybrids. The high-resolution TEM (HRTEM) image shows the presence of many single crystalline nano-particles as shown in Figure 3d (see the white dashed circles with their crystallographic orientations). The inter-planar distance in the lattice fringes of one domain is measured to be 0.22 nm which corresponds to the (111) planes of metallic Pd. The characteristic selected area electron diffraction (SAED) pattern recorded from this region shows diffraction rings corresponding to the inter planar spacing of the various set of crystallographic planes of the cubic (fcc) crystal structure of palladium as shown in Figure 3e. The energy-disperse X-ray 14 ACS Paragon Plus Environment

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spectrum (EDX) illustrates the chemical composition of Pd/Ppy nanohybrid with the presence of Pd, carbon, chlorine (Figure 3f). Moreover, this methodology have been extended for the preparation of a series of bimetallic nanohybrids, Pd89Pt11/Ppy, Pd51Pt49/Ppy, Pd54Au46/Ppy and trimetallic, Pd30Pt29Au41/Ppy Pd26Pt24Au50/Ppy, Pd50Pt21Au29/Ppy and Pd19Pt49Au32/Ppy nanohybrids. Figure 4a, b demonstrated that the Ppy nanofibers with a high and a uniform coverage by Pd89Pt11 NPs and the average particle size of Pd89Pt11 NPs is found to about 5.7 nm.

Figure 4. TEM images of (a-b) Pd89Pt11/Ppy nanohybrids at different magnifications, (c) HRTEM of Pd89Pt11/Ppy, (d) HAADF-STEM image of Pd89Pt11/Ppy nanohybrids. (e-g) HAADF-STEM-EDS mapping images of Pd89Pt11/Ppy showing elemental distribution of Pd, Pt and Cl, respectively.

HRTEM image of Pd89Pt11/Ppy nanohybrids shows crystal lattice fringes confirming high crystallinity (Figure 4c). Figure 4d-g shows high-angle annular dark-field scanning TEM 15 ACS Paragon Plus Environment


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(HAADF-STEM) image of the Pd89Pt11/Ppy nanohybrids together with energy-dispersive Xray spectroscopy (EDS) elemental maps. Both the bright field and HAADF images suggest the formation of tiny NPs of Pd-Pt alloy on the Ppy nanofiber surface. The fine NPs are typically pure Pd or, Pd-rich, while the relatively coarse NPs tend to be rich in Pt. It is well accepted that formation of Pd-Pt bimetallic nanostructure favored due to similar crystal structure and insignificant lattice mismatch between Pd and Pt crystal.58 The presence of Pt and Pd in the nanoalloys has been determined using EDX analysis. Elemental mapping suggests that the nanofibers supported nanoparticles are made of a Pt−Pd alloys and both the Pd and Pt atoms have been well distributed (Figure 4e, f) among and within the nanoparticles. From the elemental mapping of Cl from doped Ppy nanofibers also suggests the PdPt nanoalloys well distributed on polymer surface. Pt NPs well distributed on Ppy nanofibers as observed from the TEM image and average particle size of Pt NPs is ca. 3.5 nm (Figure S3a, b in the Supporting Information). The HRTEM image illustrates the presence of continuous lattice fringes of Pt NPs and the inter-planar distance (0.234 nm) corresponds to the (111) planes of metallic Pt (Figure S3c in the Supporting Information). Moreover, TEM and HRTEM images of other bimetallic composite, Pd51Pt49/Ppy suggest homogeneous distribution and high crystallinity of Pd51Pt49 NPs on the polymer nanofiber (Figure S3d-f in the Supporting Information). Additionally, the formation of relatively larger Au nanoparticles has been observed with average size of about 20 nm in TEM image (Figure S4a and b in the Supporting Information). The HRTEM image demonstrates that the Au nanocrystals having a lattice fringe of 0.235 nm which associated to the (111) plane of Au NPs (Figure S4c in the Supporting Information). Another bimetallic composition, Pd54Au46/Ppy nanoalloys were also formed on the polymer nanofibers by radiolytic technique and homogeneous distribution and


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crystallinity shown in TEM and HRTEM images (Figure S4d-f in the Supporting Information). Further, SEM, HAADF-STEM and EDS structural characterization were utilized to obtain more insight into the morphology, and composition of trimetallic Pd-Pt-Au nanoalloys prepared with radiolytic technique. Figure 5 a, b shows the fine Pd30Pt29Au41NPs on the polymer surfaces with average particle sizes of ∼8 nm. Further, the HRTEM supports the crystallinity of the as prepared Pd30Pt29Au41 alloy NPs on Ppy surface (Figure 5c). The STEM-HAADF images of Pd30Pt29Au41 demonstrate the presence of fine NPs with different sizes (ranging from 6 nm to ~50 nm) attached to the polymer nanofibers (Figure 5d).

Figure 5. Typical TEM images of (a) Pd30Pt29Au41/Ppy nanohybrids, (b) HRTEM image of Pd30Pt29Au41/Ppy. (c) HAADF-STEM image of Pd30Pt29Au41/Ppy nanohybrids (d)-(g) HAADF-STEM-EDS mapping images of Pd30Pt29Au41/Ppy.



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The EDS elemental mapping images reveal the presence of Pd, Pt and Au in the trimetallic nanolloy (Figure 5e-g). The STEM-EDX mapping also showed that the large NPs (50 nm) are very rich in Au, with some small amounts of both Pd and Pt. The Pd:Pt:Au atomic ratio of the trimetallic nanoalloys was calculated to be 30:29:41 by ICP-AES which is matched well with EDS spectrum data in different region, respectively (Figure S4 in the Supporting Information and Table 1). EDS spectrum suggests the presence of major elements, Pd, Pt, Cl, C, and Cu and out of which Pd, Pt and Au came from the nanoalloys, while Cl, C, and Cu originated from the Ppy nanofiber and the Cu grid, respectively. These are in good agreement and further confirm the STEM-EDS results. The change in composition of each metal element and the variation of NPs size estimated by ICP-AES and TEM analyses, respectively.

3.2. Mechanism of formation of nanohybrid Polypyrrole nanofibers supported metal nanoalloys have been prepared via two step process (Scheme 1). Initially, polymer nanofibers are formed in the interface of a lamellar structure based soft template, (CTA)2S2O8 in presence of a chemical oxidant (ammonium persufate) and cetyltrimethylammonium bromide (CTAB).45 Then, bimetallic Pd–Pt and Pd–Au or trimetallic Pd-Pt-Au nanoparticles are successfully fabricated on polymer nanofibes by gamma irradiation without any chemical reducing agent. The mechanism of the formation of bimetallic or trimetallic nanoalloys by gamma irradiation was mainly attributed to the radiation induced formation of solvated electron (e-sol) via radiolysis of the propanol molecules as well as forming energetic radicals etc.59, 60


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Scheme 1. Proposed schematic diagram of the radiolytic synthesis of nanohybrids. (a) Synthesis of polymer nanofibers using surfactant based oxidative template. (b) Formation of metal nanoparticles on conducting polymer nanofibers by radiolysis.

These solvated electrons and 2-propanol radicals (CH3)2–C.OH can be trapped by metal ions, consequently, metal atoms formed during the irradiation process. (CH3)2CHOH → e-sol; solvated protons ((CH3)2CHOH2+), (CH3)2C.OH and other radiolytic products CH3)2CHOH + OH•(H•) → (CH3)2C•OH + H2O e-sol +

M+ → M0

(M= Pd, Pt, Au)

(CH3)2C•OH + M+→ (CH3)2CO + M0 19 ACS Paragon Plus Environment

(1) (2) (3)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PdII +e-sol→ PdI

(4)

PdII + (CH3)2C•OH → PdI + (CH3)2CO 2PdI n Pd0

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→ Pd0 + PdII → (Pd)n

(5) (6) (7)

The strong reduction potential of radicals is capable to reduce metal ions. Due to presence of enough propanol, which scavenge the oxidizing agent, OH• radicals generated during irradiation. Consequently reducing agents produced by radiolysis are sufficient to complete the reduction of metal ions. In general, radiolytic reduction of metal ions involves multistep process. Initially, atoms in unusual valence states are formed and further reduction and agglomeration happened to achieve a stable nanoparticle as proposed by Belloni and other coworkers.61 In presence of two or more metal ions, very fast reduction occurred among the metal depending on their relative abundance and rate constant of radiolytic species within the solution. During the radiation induced synthesis of nanoparticles, metal ions such as Pd, Pt and Au in the solution were reduced to zero-valance atoms forming ‘core’ of the particles. Subsequent growth of the nuclei and co-deposition of Pt and Au atoms on the nuclei could form large clusters of the bimetallic or trimetallic alloy. The possible formation of alloyed structure depends on the kinetic competition between the metal ions as well as radiolytic reduction of both metal ions. In presence of two or more metal ions, very fast reduction happened among the metal according to their relative abundance and rate constant of radiolytic species within the solution.


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3.3 Electrochemical activity The electrochemical behavior of the Pd/Ppy, Pt/Ppy, and binary, Pd89Pt11/Ppy, Pt49Pd51/Ppy, Pd54Au46/Ppy and ternary, Pd30Pt29Au41/Ppy modified glassy carbon electrodes were measured in argon-saturated 1M KOH as shown in Figure 6a. The cyclic voltammogram (CV) profiles for all the electrodes clearly shows two distinct regions associated with the electrochemical processes which is well consistent with literature for Pd electrode. 62, 63 M + OH− →M−OHads + e−

(M= Pd, Pt, Au)

M−OHads + OH− ⟷M−O + H2O + e− M−OHads + M−OHads ⟷M−(O) + H2O

(8) (9) (10)

The characteristics peak for hydrogen adsorption and desorption (Had/Hdes), oxide formation, and oxide reduction are also observed from the voltammograms of all the catalysts studied (marked with dotted spherical and dotted line in Figure 6a). The significant negative shift of hydrogen adsorption/desorption and oxide formation/reduction peak in comparison to monometallic Pd/Ppy suggests the formation of an alloys.15 The peak current density is higher for both bimetallic or trimetallic alloys in comparison to the pure Pt/Ppy and Pd/Ppy nanohybrids. The catalytic performance of the as-prepared Pd based catalysts for electrooxidation of ethanol has been investigated and the results have been compared with those of commercial Pd/C and Pt/C catalysts (Figure 6b and Figure S6a-d in the Supporting Information). The high peak current densities have been observed for bimetallic and trimetallic electrodes in comparison to monometallic counterpart. In order to choose the best trimetallic compositions, effect of different ratios of Pd, Pt and Au on the Ppy have been tested for electrooxidation of ethanol under similar reaction condition. The current density 21 ACS Paragon Plus Environment


obtained for four different compositions have been compared (Figure S6 in the Supporting Information). The Pd30Pt29Au41//Ppy catalyst demonstrates superior catalytic activity toward ethanol oxidation compared to other catalysts.

4 2

Pd/Ppy Pt/Ppy Pd89Pt11/Ppy

30 2

6

(b) 35

Pd/Ppy Pt/Ppy Pd89Pt11/Ppy Pd51Pt49/Ppy

j/mA/cm

Current density, j/mA/cm

2

(a) 8

Pd54Au46/Ppy Pd30Pt29Au41/Ppy

25

Pd51Pt49/Ppy Pd54Au46/Ppy

20

Pd30Pt29Au41/Ppy

15

0

10

-2

5 0 0.5

1.5

(d) 4

th

10000 7500

2

1

0.6

0.9

1.2

1.5

Potential/V vs RHE

6

1. Pd/Ppy 2. Pd51Pt49/Ppy 3. Pd89Pt11/Ppy

4

4. Pd30Pt29Au41/Ppy 5. Pd64Au46/Ppy 6. Pt/Ppy

2

0

py py py /P /P /P u 41 t 46 49 A u P A Pt 2 9 51 64 Pd d 30 Pd

/P py 11

Pt 89

Pd

0

6

Pd /C

2500

3

5

5000

Pt /P py Pd /P py

-1

100 cycle

-2

0.3

Pd loading interms of Mass Activity

(c)

1.0

Potential/ V vs RHE

12500

j mAcm mg

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1

2

3 4 Catalysts

5

6

P

Figure 6. (a) CVs of Pd/Ppy, Pt/Ppy, Pd89Pt11/Ppy, Pt49Pd51/Ppy, Pd54Au46/Ppy and Pd30Pt29Au41/Ppy in 1M KOH at a scan rate of 50 mVs-1. (b) CVs for the electrocatalytic oxidation of 1 M ethanol at 100th cycle by Pd/Ppy (red solid line), Pt/Ppy (green solid line), Pd89Pt11/Ppy (blue solid line), Pt49Pd51/Ppy (black solid line), Pd54Au46/Ppy (pink solid line), and Pd30Pt29Au41/Ppy (orange solid line) at a scan rate of 50 mV s-1 in 1 M KOH. The working electrode was a glassy carbon disk modified with the Pd based nanostructures. (c) Comparative values of mass activity of Pd/C, Pd/Ppy, Pt/Ppy, Pd89Pt11/Ppy, Pt49Pd51/Ppy,


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Pd54Au46/Ppy and Pd30 Pt29Au41/Ppy electrodes for ethanol oxidation in alkaline medium. (d) Volcano plot corresponding to mass activity of Pd/Ppy nanohybrids based electrocatalysts.

The different parameters like onset potential (Eonset), the anodic peak current densities in forward and backward scan (jf and jb), and the ratio of forward and backward peak current density (jf/jb) have been calculated from the CVs and are described in Table 2. The peak current densities of the CVs has been expressed as dividing current densities (mA cm−2) by the mass of total metal content of catalysts adsorbed per unit area of the surface (mg−1 of catalyst) to examine the effect of different composition on catalytic activity (Figure 6c). A volcano relationship has been observed for the Pd based nanohybrids having different metallic content (Figure 6d). The peak current density shows that Pd30Pt29Au41/Ppy exhibit significantly enhanced catalytic activity toward ethanol electrooxidation. The superior catalytic activity obtained for Pd89Pt11/Ppy and Pd30Pt29Au41/Ppy due to the formation of alloy which may be capable of controlling the poisoning effect while Au enhances the reactivity.15, 26

The catalytic efficiency increase in the order of the catalysts: Pd30Pt29Au41/Ppy (5.5-fold) >

Pd54Au46/Ppy (2.4-fold) > Pd89Pt11/Ppy (2.3-fold) > Pd51Pt49/Ppy (1.2-fold) >Pt/Ppy (1-fold) ≅ Pd/Ppy (choosen as reference). It is well accepted that when Pd combined with platinum or gold in the crystal structure, the d-band of Pd may be shifted up with larger number of active sites and as a result affects the overall electronic structure of the Pd.16,

18

In fact,

Pd30Pt29Au41/Ppy nanohybrids exhibit an excellent electrocatalytic activity which is ∼3 and 5 times higher than Pd/C and Pt/C respectively suggests a close contact between Pd NPs and polymer nanofibers which boost up the dispersity and availability of metal catalysts (Figure S7 in the Supporting Information) during catalysis. Mass activity value of Pd30Pt29Au41/Ppy 23 ACS Paragon Plus Environment


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for ethanol oxidation reaction are superior in comparison to various recently reported catalysts such as Pd/Ppy, Pd/PANI/Pd, Pd–PEDOT/GE, Pd–PEDOT, PtPd/PPy/PtPd nanotube, PdPt/C, Ni@PbPt/Graphene, PdPt/rGO, Pt–Cu/rGO, PdCo/CNF etc as shown in Table S.1. A negative shift of Eonset highlights the improvement in the kinetics of ethanol oxidation. For Pd/Ppy electrode, the Eonset shifts to a more negative potential from -610 mV at the 1st cycle to -640 mV after 100 cycles (Figure S8 in the Supporting Information). Moreover, the peak current density is also increased for both the forward and backward anodic peak with the ratio jf/jb of 1.6. This observation indicates that the formation of stable catalysts layer on glassy carbon and no loss of catalyst during the cycling of oxidation reaction. The peak current density of the commercial Pd/C and Pt/C both shows significant lowering of current density even after few cycles (Figure S7 b and d). Moreover, for Pd/C, the Eonset shifts to a more positive potential (from -640 mV at the 2nd cycle to -599 mV after 50th cycle) may suggests catalytic poisoning due to the formation of unoxidized intermediate species on the catalysts surface. Notably, the electro oxidation potential of EOR at Pd54Au46/Ppy electrode negatively shifts ca. 680 mV indicating more kinetically effective electrocatalysts for ethanol oxidation as the adsorption of OH− ions on gold surface may eliminate the poisonous CO in the form of CO2.64 The forward peak current of Pd30Pt29Au41/Ppy electrode is 17.4 and 5.2 times that of the Pd/Ppy or Pd/rGO (rGO, reduced graphene oxides nanosheets) electrodes, respectively, indicates the advantages of using conducting polymer supported multimetallic alloy as anode catalysts for EOR.46 The electrochemical performance of present catalysts have been compared with Pd-based catalysts from the literature in Table S1, Supporting Information. Here the activities of as prepared alloy NPs have been normalized to the total mass of the


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catalysts. It can be clearly observed from Table S1, the Pd30Pt29Au41electrode exhibits a relatively high mass activity for ethanol oxidation. Table 2. Comparison of the electrochemical performance of Pd/Ppy, Pt/Ppy, Pd89Pt11/Ppy, Pt49Pd51/Ppy, Pd54Au46/Ppy and Pd30Pt29Au41/Ppy nanohybrids for the oxidation of ethanol. The main characteristics measured from cyclic voltammograms associated with the electrocatalytic oxidation of 1 M ethanol in 1 M KOH at a scan rate of 50 mVs-1. The working electrode was a glassy carbon disc modified with the Pd nanostructures. The reference electrode was Hg/HgO electrode. The current density is referred to the geometric area of the glassy carbon support. Electrode (after 100 cycles)

jf (mA/cm²)

jb (mA/cm²)

jf/jb

Eonset (mV vs Hg/HgO)

Pd/Ppy

9.50

5.82

1.6

-640

Pt/Ppy

0.82

0.37

2.2

-625

Pd89Pt11/Ppy

15.8

6.7

2.3

-630

Pd51Pt49/Ppy

5.0

2.06

2.4

-606

Pd54Au46/Ppy

10.35

10.50

0.98

-680

32.45

12.86

2.4

-630

Pd30Pt29Au41/Ppy

The binary and ternary alloys possess higher catalytic activity in comparison to monometallic catalysts, due to synergistic electronic effects multimetallic components and good dispersion 25 ACS Paragon Plus Environment


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of Pd NPs on Ppy nanofibers, which make them suitable as anode catalysts for ethanol oxidation. The concept of using multimetallic alloys is to enhance the reactivity as well as to lower the usage of noble metal content. The contribution of individual element, particularly, Au showed the lowest level of catalytic performance for ethanol oxidation under alkaline condition (Figure S9 in the Supporting Information). Additionally, Ppy support has not contributed any catalytic activity for ethanol oxidation and catalytic activity of monometallic Pt/Ppy is not high as compared to Pd based electrode under similar reaction condition. Conducting polymers has been introduced to anchor the metal NPs. Indeed, metal NPs could strongly absorb on the polymer surface and furthermore, interaction between the metal NPs and π-conjugated carbon may possibly enhance the dispersity and availability of metal catalysts at catalytic site. Moreover, when NPs are deposited on bulk polymer, the catalytic efficiency is not significant higher. Ppy nanofibers supported Pd metal catalysts showed high current density, which is 5.1 times higher than that of Pd/Ppy (bulk polymer) (Figure S9, in the Supporting Information). The enhanced electrocatalytic activity may be associated with the facile charge transfer at conductive polymer interfaces and efficient diffusion paths for the electroactive species and high electronic conductivity. The stability of the nanohybrid electrode has been investigated by chronoamperometric (CA) measurement at constant potentials for ethanol electro-oxidation as shown in Figure 7a. For all the electrode, current density decay gradually upto 500 seconds due to the accumulation of poisonous intermediates and then attained a steady state.62 For Pd/Ppy or Pt/Ppy, the current rapidly decreasing, almost decrease to zero while other electrode materials reach a stable current.


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Figure 7. (a) Chronoamperometric curves for the ethanol oxidation at constant potential −0.25V vs Hg/HgO on Pd/Ppy, Pt/Ppy, Pd89Pt11/Ppy, Pt49Pd51/Ppy, Pd54Au46/Ppy and Pd30Pt29Au41/Ppy electrodes. (b) Long cycling study of Pd/C, Pd/Ppy, Pd54Au46/Ppy and Pd30Pt29Au41/Ppy electrodes in a solution of 1M KOH and 1M ethanol at scan rate of 50 mV Sec-1. (c) Stability of Pd/Ppy and Pd30Pt29Au41/Ppy catalysts in solution state after 240 days, compare current density at 100 cycles. (d) FESEM image of Pd30 Pt29Au41/Ppy electrodes after 1000 cycling of ethanol oxidation. However, Pd89Pt11/Ppy, Pd54Au46/Ppy and Pd30Pt29Au41/Ppy catalyst exhibited higher limiting as well as initial current, suggests superior stability than the Pd/Ppy catalysts and attained steady states after 1000 seconds. This clearly indicates that bimetallic and tri metallic alloy based nanohybrid performed as stable electrocatalyst compared to mono-metallic nanohybrid 27 ACS Paragon Plus Environment


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for EOR. The nonzero value of current density obtained after 3600 seconds for Pd89Pt11/Ppy, Pd54Au46/Ppy, Pd30Pt29Au41/Ppy electrodes is possibly due to the close contact between metal NPs and polymer nanofibers having strong interaction which may improve mass transfer rate. The Pd30Pt29Au41/Ppy illustrate much slower current decay over time Pd89Pt11/Ppy, Pd54Au46/Ppy and commercial Pd/C (Figure S11 in the Supporting Information), suggesting that the Pd30Pt29Au41/Ppy electrode have a much better durability for ethanol oxidation. Thus, this composition identified as best electrode for EOR. Furthermore, the stability of electrodes using Pd/C, Pd/Ppy, Pd54Au46/Ppy, and Pd30Pt29Au41/Ppy has been studied by cyclic voltammetry under similar condition up to 1000 cycles. Figure 7b illustrated that nearly no change in current density of the bi and trimetallic alloy based catalyst compared to Pd/Ppy for EOR. At the end of 1000 cycles, the current densities of Pd54Au46/Ppy and Pd30Pt29Au41/Ppy still remain about 93.4% and 97% of their highest values and 53% for Pd/Ppy. This suggests that Pd54Au46/Ppy and Pd30Pt29Au41/Ppy electrocatalysts could be efficiently recycled without noticeable loss of activity. Whereas, for commercial Pd/C, current density significantly decreased even in few cycles and 100 % decay observed after 1000 cycles. The stability of forward current density of Pd/Ppy and Pd30Pt29Au41/Ppy have been compared through monitoring the stability of catalysts ink. The decay of current density for the EOR on Pd30Pt29Au41/Ppy electrocatalysts is 4% which is significantly lower than Pd/Ppy (26%) after upto 240 days (Figure 7c). This indicate that presence of Ppy nanofibers significantly enhance the stability metal catalysts in solution. The morphology and chemical structure of the Pd30Pt29Au41/Ppy remained unchanged after 1000 cycles of oxidation reaction as observed in XRD pattern and FTIR spectrum (Figure S12 in the Supporting Information). FESEM image also reveals that after electrocatalysis, there is no significant change in the morphology of the


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hybrid nanostructures as shown in Figure7d. Hence, excellent catalytic activity and stability of Pd30Pt29Au41/Ppy electrocatalysts make them suitable for direct alcohol fuel cell application. The presence Ppy nanofibers not only increase the dispersion of metal catalysts, also enhance the stability through strong interaction between nanofibers with metal content which may facilitate the transportation of electroactive species. Further studies related to kinetics, catalytic poisoning and molecular transformation pathways are vital in order to utilize these novel catalysts for large scale applications. We further estimated the effects of ethanol concentration on the catalytic performance of hybrid catalysts. Figure 8a displays a typically the CVs of Pd/Ppy electrode in presence of 2 M mol L-1 (black solid line curve) and 0.1 mol L-1 (blue solid line curve) ethanol in 1 M KOH containing at a scan rate of 50 mVs-1. The increase in anodic current density suggests that the adsorption of ethoxy on the catalyst surface can be augmented with an increase in the ethanol concentration. Moreover, reaction orders has been calculated from the plot of log current vs log concentration of ethanol at a specific potential (Figure 8b). log I = log nFk + mlogc

(11)

where F is Faraday’s constant, k is the reaction constant, c is the ethanol concentration, and m is the reaction order with respect to ethanol. The overall reaction order is estimated as 0.12 which is consistent with other studies.30 Similarly, Pd30Pt29Au41/Ppy electrode follow the similar trend and Figure 8c demonstrated a significant increase of anodic forward and backward peak current density upto 42 mA/cm2 (2 mol/L, green solid line) from 1.04 mA/cm2 (0.1mol/L, black solid line) associated with the coverage of adsorbed methoxy at the catalyst surface.



(a)

Pd/Ppy Ethanol concentration 2M 0.1 M

10

(b)0.54 Y= 0.53 + 0.12X

0.51 0.48

Log j

j/mA/cm2

8 6 4

0.45 0.42 0.39

2

0.36

0 0.4

0.8

1.2

0.33

1.6

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2

Potential/V vs RHE

(c)

2

30

20

Y= 1.43 + 0.72X 1.5 1.2

Log j

Ethanol concentration 0.01M 0.1M 0.5M 1M 2M

0.0

LogCEtOH /M

(d)

Pd30Pt29Au41/Ppy

40

j /mA/cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.9 0.6

10 0.3 0 0.0 0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

-2.0

-1.5

-1.0

-0.5

0.0

LogCEtOH /M

Potential/V vs RHE

Figure 8. (a) CVs of ethanol oxidation using Pd/Ppy electrode at different ethanol concentration: 0.10 and 2.00 mol L-1. (b) The double logarithm graph between peak current and ethanol concentrations. (c) CVs of ethanol oxidation using Pd30Pt29Au41/Ppy electrode at different ethanol concentration. (d) The corresponding double logarithm graph between peak current and ethanol concentrations.

The effect of temperature on the electrocatalytic activity of Pd/Ppy and Pd30Pt29Au41/Ppy electrodes for EOR has been investigated at different temperature (from 20 to 50οC), as illustrated in Figure 9a. With increasing temperature, ethanol oxidation peak current densities in the both forward peak and reverse peak have been enhanced for Pd/Ppy which influences the ratio of jf/jb as shown in inset of Figure 9a. This suggests that the unoxidized ethanolic 30 ACS Paragon Plus Environment

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residues on the catalysts surface during the forward sweep can be oxidized at higher potentials and lowering catalytic poisoning. The activation energy has been calculated from the graph ∼22 kJmol-1 which is much lower than the Pd-PEDOT electrodes implying ethanol electrooxidation on Pd/Ppy nanohybrids kinetically favored.42

(a) 6

4 3

(b) 1640

o

Temperature (oC)

jf/jb

20

1.8

30

1.6

40

1.3

50

1.1

20 C o 30 C o 40 C o 50 C

Potential/mV vs RHE

5

j/mA/cm2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 1

1620

y = 1471 + 3.2x R2 = 0.989

1600 1580 1560 1540

0 0.2

0.3

0.4

0.5

20

25

30

35

40

45

50

Temperature/°C

Potential/V vs RHE

Figure 9. (a) Magnified portion of cyclic voltammetry curves for the electrooxidation of ethanol on Pd/Ppy catalysts with different temperatures. (b) Variation of onset potential at different temperature.

Figure 9b shows a negative shift of Eonset from −610 V to −720 V upon increasing temperature which suggests the improved kinetics of EOR. Pd30Pt29Au41/Ppy electrode also shows the similar trend and an increase of the anodic current density and highly negative Eonset has been observed upon increasing the temperature (Figure S13 in the Supporting Information). This indicates that the kinetics of the EOR has been enhanced by increasing the temperature up to 38 οC. However, a further increase in the temperature beyond 40οC leads to a decrease in the peak current. Moreover, the Tafel slopes offers constructive information on the reaction



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mechanism using potentiodynamic pseudo steady state polarization at low scan rate of 1 mV S-1 under similar condition using following the equation:

= + log =−

.

log 0 +

.

log

(12)

(13)

Where, η is the over potential, a is Tafel intercept and b is Tafel slope, 0 is called exchange current density, α is the charge transfer coefficient, F is the Faraday constant, R is the gas constant, and T is the absolute temperature in K. The exchange current density is calculated by extrapolating the linear fitted Tafel line where over potential equals to zero (Figure 10a).66 The calculated equilibrium exchange current density (j0) and charge transfer coeffcients (α) are presented in Table 3. Table 3. Comparative Tafel slope, exchange current density, charge transfer coefficient of Pd/C, Pd/Ppy, Pt/Ppy, Pd89Pt11/Ppy, Pd54Au46/Ppy, Pd30Pt29Au41/Ppy nanohybrids electrodes for ethanol oxidation at scan rate 1 mV S-1. Intercept/V

Slope/V dec-1

R2 value

j0 /mA cm-2

Pd/C

-0.46

0.255

0.974

0.015

0.23

Pd/Ppy

-0.42

0.340

0.999

0.058

0.167

Pd89Pt11/Ppy

-0.215

0.293

0.992

0.184

0.201

Pt49Pd51/Ppy

-0.528

0.402

0.993

0.048

0.148

Pd54Au46/Ppy

-0.315

0.305

0.996

0.092

0.165

Pd30Pt29Au41/Ppy

-0.27

0.348

0.989

0.167

0.171

Electrodes


α

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The higher exchange current density achieved on Pd89Pt11/Ppy of 18.4 ×10−2 mA cm−2 and Pd30Pt29Au41/Ppy of 16.7 ×10−2 mA cm−2 which is 12.2 and 11.1 times higher than that of Pd/C (1.5 ×10−2 mA cm−2).

0.66 0.63

Pd30Pt29Au41/Ppy Pd89Pt11/Ppy

Pd50Pt49/Ppy

Pd54Au46/Ppy

-2

0.69

(b) 30

Pd/C Pd/Ppy Pd89Pt11Ppy

Z''/Ohm cm

(a) 0.72 Potential/ V vs RHE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pd54Au46/Ppy Pd30Pt29Au41/Ppy

0.60

Pd/Ppy Ppy

20

10

0.57 0.54 0

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0

10

20

30

-2

Logj

Z'/Ohm cm

Figure 10. (a) Tafel plots of Pd/C, Pd/Ppy, Pt/Ppy, Pd89Pt11/Ppy, Pd54Au46/Ppy and Pd30 Pt29Au41/Ppy nanohybrids at a scan rate of 1 mV s−1. (b) Nuquist plots of Ppy, Pd/Ppy, Pd89Pt11/Ppy, Pd54Au46/Ppy and Pd30 Pt29Au41/Ppy nanohybrids. Electrochemical impedance spectroscopy (EIS) has been employed to investigate the electron transfer properties at the electrode/electrolyte interface for the different Ppy based nanohybrid electrodes. Figure 10b represents typical Nyquist plots for the Pd/Ppy, Pd89Pt11/Ppy, Pd54Au46/Ppy and Pd30Pt29Au41/Ppy. The impedance curve of all nanohybrid electrodes show a distorted semi-circle in the high-frequency region due to electrolytes diffusion to the catalysts

surface

and

also

represent

electron

transfer

limited

process

at

the

electrode/electrolyte interface.66 The diameter presents the charge-transfer resistance (Rct) over the interface between the electrode and electrolyte. A vertically linear spike in the low frequency region corresponds to diffusion process of the electrolytes. Figure 10b displays the values of charge transfer resistances for Ppy, Pd/Ppy, Pd89Pt11/Ppy, Pd54Au46/Ppy and 33 ACS Paragon Plus Environment


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Pd30Pt29Au41/Ppy are 20.67, 10.25, 8.04, 6.79 and 6.2 Ω cm−2, respectively. This suggests that the Pd30Pt29Au41/Ppy provide facile access (less resistance) for charge intercalation compared to the other nanohybrids. The electrocatalytic performance of Pd30Pt29Au41/Ppy may be described in terms of effective utilization of metal catalysts and strong chemical interaction between catalysts with support. First, Pd30Pt29Au41 alloy supported by Ppy nanofibers exhibits high catalytic activity due to the more uniform dispersion and smaller size of alloy NPs, as well as higher metal nanocatalyst utilization efficiency. Second, stable Ppy nanofibers support and synergistic coupling effects between Pd with other noble metals could effectively generate additional catalytically active sites as well as may slightly suppress the CO adsorption on the Pd surface as proposed by dband theory. We further carried out the end product analysis to understand the path of electrooxidation of ethanol. The electrooxidation of ethanol in alkaline medium can be plausibly described as follows:62, 67 M + CH3CH2OH ⟷ M−(CH3CH2OH)ads

(14)

M−(CH3CH2OH)ads + 3 OH− ⟷M−(CH3CO)ads + 3H2O + 3e−

(15)

M−(CH3CO)ads + M−OHads ⟷M−(CH3COOH) + H2O

(16)

M−(CH3COOH) + OH−⟷M+ CH3COO− + H2O

(17)

In general, the electrooxidation of ethanol on metal (M) surfaces in alkaline media proceeds via multistep process through the reactive intermediate or the poisoning-intermediate (CO) pathway. Where the M-OHads and M-(COCH3)ads intermediates are combined to generate acetate anions through reactive intermediate. On the other hand, *CH3CO may also completely decomposes into *CO and *CH3, which can block the active site of the catalysts.68


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To understand the catalytic pathway of anodic oxidation of ethanol, cyclic voltammograms of Pd/Ppy and Pd30Pt29Au41/Ppy electrodes in 1M KOH solutions containing ethanol, acetaldehyde and sodium acetate (100 mM) have been compared in the potential range of 0.8V to + 0.8V at a scan rate of 50 mVs-1. Figure 11 (a-c) depicts that the electro-oxidation of acetaldehyde has similar features compared to that of ethanol, but a higher current density and significant positively shifted peak potential obtained for Pd30Pt29Au41/Ppy electrodes. In contrast, no distinct oxidation current has been observed in the potassium acetate solution for all the electrodes, Pd/Ppy, Pd89Pt11/Ppy and Pd30Pt29Au41/Ppy electrodes.

(a) 20

2

5 0 -5

Pd89Pt11/Ppy Ethanol Acetaldehyde Acetate

10

j mA/cm

2

Ethanol Acetaldehyde Acetate

10

j mA/cm

(b) 15

Pd/Ppy

15

5

0

-10 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

-0.8

0.8

Potential /V vs Hg/HgO

(c) 10

(d)

Pd30Pt29Au41/Ppy Ethanol Acetaldehyde Acetate

-2

6

8

j/mA cm

2

8

j mA/cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Potential /V vs Hg/HgO Forward current density Ethanol (i) Pd/Ppy Acetaldehyde (ii) Pd89Pt11/Ppy

(iii) Pd30Pt29Au41/Ppy

6

4

2

2

0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Potential /V vs Hg/HgO

0

(i)

(ii)

(iii)

Figure 11. CVs of CH3CH2OH, CH3CHO, CH3COONa solutions of 100 mM concentration in 1 M aqueous KOH at scan rate 50mVs-1 using (a) Pd/Ppy, (b) Pd89Pt11/Ppy and (c) 35 ACS Paragon Plus Environment


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Pd30Pt29Au41/Ppy electrodes. (d) Comparative current densities for ethanol and acetaldehyde oxidation on Pd/Ppy, Pd89Pt11/Ppy and Pd30Pt29Au41/Ppy electrodes. Figure 11(a) indicates the respective peak current density in mAcm-2 of Pd/Ppy electrode follows the order: (1.9) CH3CHO < CH3CH2OH (6.1) where oxidation of acetate did not occur on the catalysts surface. In contrast, Pd30Pt29Au41/Ppy electrodes follow completely the reverse order, CH3CH2OH (0.846) < CH3CHO (2.3) which suggests the oxidation of ethanol is favorable more on Pd/Ppy but acetaldehyde oxidation preferred on Pd30Pt29Au41/Ppy electrode (Figure 11b, c). Hence, acetate salt might be the final product and acetaldehyde is an active intermediate for the EOR on the polymer supported metal catalysts. Liquid-phase oxidation products have been analyzed by a 1H NMR method and both acetaldehyde and acetic acid are observed in the solution after 1000 cycles of ethanol oxidation on Pd30Pt29Au41/Ppy electrode (Figure S14 in the Supporting Information). The singlet at 8.5 ppm indicates the formation of acetate, while those at 3.4 ppm are assigned to methyl formate which is consistent with literature report.68, 69 Figure 11 (d) depicts that the respective peak current density for acetaldehyde in mAcm-2 follows the order: Pd/Ppy (1.9) < Pd89Pt11/Ppy (2.1) < Pd30Pt29Au41/Ppy (2.3) representing the high oxidation capability of trimetallic composition in comparison to monometallic catalysts. In contrast, ethanol has low potential and easier pathway of oxidation on Pd/Ppy electrode. The presence Pd or Au in combination with Pt catalysts create facile *OH generation and high CO tolerance which may facilitate exceptional electrocatalytic performance of trimetallic alloy towards EOR.


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4. Conclusion In summary, we report a polymer-directed radiolytic route for the synthesis of new generation metal/polymer hybrid materials without any chemical reducing agent and investigate its electrocatalytic application for direct ethanol fuel cells (DEFC). Different kinds of metal nanoalloys (M= Pd-Pt, Pd-Au, Pd-Pt-Au) can be deposited on Ppy nanofibers with tunable compositions. The presence of polymer nanofibers significantly improved the catalytic activity of Pd alloy nanostructures for ethanol oxidation with high current density and longterm stability compared with commercial Pd/C catalysts. The mass activity has been successfully improved for Pd30Pt29Au41/Ppy electrodes and demonstrated better tolerance towards intermediate poisoning which may be attributed due to the smaller size, additional Pd active site on surface, and the good synergistic effects between Pd with Pt and Au atoms. The present strategy represents a general route for the synthesis of novel 1D hybrid nanostructures for various nanoalloys fabrication which may also find broad applications in clean, sustainable and renewable energy generation.

Supporting Information Additional supporting figures (Figure S1−S14) and tables (Table S1) for the FTIR spectra, magnified XPS spectra of Pd30Pt29Au41/Ppy nanohybrids, XPS spectra for the Pd/Ppy, Pd89Pt11/Ppy, Pt49Pd51/Ppy, and Pd54Au46/Ppy nanohybrids, TEM images of pure Pd, Pt, Au NPs, and EDX spectra of the Pd30Pt29Au41/Ppy nanohybrids, ethanol oxidation behaviors of various trimetallic alloys such as Pd26Pt24Au50/Ppy, Pd50Pt21Au29/Ppy, Pd19Pt49Au32/Ppy based electrocatalyst modified glassy carbon electrodes, cyclic voltammetry curves of commercial Pd/C and Pt/C catalysts, chronoamperometric curves for Pd/C and Pt/C catalysts, effect of 37 ACS Paragon Plus Environment


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temperature on ethanol oxidation by Pd30Pt29Au41/Ppy electrode, cycling behavior of Pd/Ppy electrode for ethanol oxidation, cyclic voltammetric profile of Ppy, Au/Ppy, Pt/Ppy, Pd/Ppy (bulk) for ethanol oxidation, XRD patterns and FTIR spectra of the catalysts after ethanol oxidation, 1H NMR spectra of ethanol after electrocatalysis as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements. The authors acknowledge Director, CSIR-CGCRI for his kind permission to publish the work. One of the authors (SG) is thankful to Council of Scientific & Industrial Research (CSIR), India, for providing CSIR-Senior Research Associate (Scientists’ Pool Scheme) award. SB is thankful to Department of Science and Technology (DST), Government of India for awarding INSPIRE junior research fellowship to carry out PhD. References (1) Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486, 43–51. (2) Ghosh, S.; Thandavarayan, M.; and Basu, R. N. Recent advances in nanostructured electrocatalysts for direct alcohol fuel cells in Electrocatalysts for Low Temperature Fuel Cells–Fundamentals and Recent trends, Wiley-VCH VerlagGmbh & Co. KGaA, Germany, 2017, Chapter 11, pp 347–372. (3) Vigier, F.; Coutanceau, C.; Perrard, A.; Belgsir, E. M.; Lamy, C. Development of Anode Catalysts for a Direct Ethanol Fuel Cell. J. Appl. Electrochem. 2004, 34, 439–446. (4) Badwal, S. P. S.; Giddey, S.; Kulkarni, A.; Goel, J.; Basu, S. Direct Ethanol Fuel Cells for Transport and Stationary Applications – A Comprehensive Review. Appl. Energy 2015, 145, 80–103.


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Highly Active Multimetallic Palladium Nanoalloys Embedded in

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