Highly Efficient SilverâCobalt Composite Nanotube Electrocatalysts for

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Highly Efficient Silver-Cobalt Composite Nanotube Electrocatalysts for Favorable Oxygen Reduction Reaction Areum Yu, Chongmok Lee, Nam-Suk Lee, Myung Hwa Kim, and Youngmi Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11073 • Publication Date (Web): 15 Nov 2016 Downloaded from http://pubs.acs.org on November 21, 2016

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ACS Applied Materials & Interfaces

Highly Efficient Silver-Cobalt Composite Nanotube Electrocatalysts for Favorable Oxygen Reduction Reaction Areum Yu,a Chongmok Lee,a Nam-Suk Lee,b Myung Hwa Kim,*a Youngmi Lee*a a

Department of Chemistry and Nano Science, Ewha Womans University, Seoul, 03760, Korea

b

National Institute for Nanomaterials Technology (NINT), Pohang University of Science and

Technology (POSTECH), Pohang, 37673, Korea *Co-corresponding authors: [email protected] (Y.L.); [email protected] (M.H.K.)

ACS Paragon Plus Environment

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ABSTRACT

This paper reports the synthesis and characterization of silver-cobalt (AgCo) bimetallic composite nanotubes. Cobalt oxide (Co3O4) nanotubes were fabricated by electrospinning and subsequent calcination in air; and then reduced to cobalt (Co) metal nanotubes via further calcination under H2/Ar atmosphere. As prepared Co nanotubes were then employed as templates for the following galvanic replacement reaction (GRR) with silver (Ag) precursor (AgNO3), which produced AgCo composite nanotubes. Various AgCo nanotubes were readily synthesized with applying different reaction times for the reduction of Co3O4 nanotubes and GRR. 1-h reduction was sufficiently long to convert Co3O4 to Co metal and 3-h GRR was enough to deposit Ag layer on Co nanotubes. The tube morphology and co-presence of Ag and Co in AgCo composite nanotubes were confirmed with SEM, HRTEM, XPS and XRD analyses. Electroactivity of as-prepared AgCo composite nanotubes was characterized for ORR with rotating disk electrode (RDE) voltammetry. Among differently synthesized AgCo composite nanotubes, the one synthesized via 1-h reduction and 3-h GRR showed the best ORR activity (the most positive onset potential, greatest limiting current density and highest number of electron transferred). Furthermore, the ORR performance of the optimized AgCo composite nanotubes was superior compared to pure Co nanotubes, pure Ag nanowires and bare platinum (Pt). High ethanol tolerance of AgCo composite nanotubes was also compared with the commercial Pt/C and then verified its excellent resistance to ethanol contamination.

KEYWORDS: Electrospinning; Bimetallic Nanotubes; Silver; Cobalt; Electrocatalyst; Oxygen Reduction Reaction


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1. Introduction Direct alkaline alcohol fuel cells (DAAFCs) have been researched over the past decade due to their faster kinetics of alcohol oxidation and oxygen reduction reaction (ORR) in alkaline solution, higher power generation efficiency, easy handling of the liquid fuels and the low emission of pollutants.1 ORR, a cathodic reaction of fuel cells, has been of great interest in electrocatalysis. ORR is known to be proceeding through several reaction steps including adsorption, desorption, and multiple electron transfer.2 The reaction complexity causes the high overpotential of ORR and therefore a relevant electrocatalyst is required in most cases. Currently, platinum (Pt) is the best catalyst material used for ORR at the cathode of a fuel cell.3-4 For a sustainable fuel cell commercialization, it is a necessary requirement to develop inexpensive non-precious metal catalyst materials having high ORR performance to replace expensive and rare Pt-based catalysts.5 Silver (Ag) is a good candidate for efficient ORR catalysts because it is relatively cheap, abundant, and also has a high ORR activity through 4-electron pathway; which is a desired one over 2-electron pathway due to the higher current generation and the absence of unwanted intermediates such as peroxide.6 However, Ag is less active than Pt because Ag binds oxygen atoms less strongly.7 The weaker interaction of Ag to oxygen makes the O-O bond cleavage less favorable at Ag, and eventually exhibiting lower ORR activity compared to Pt.8-10 It was reported that the ORR activity of Ag-based electrocatalysts could be improved via forming the alloys or bimetallic materials with cobalt (Co).7, 11-13 The materials based on mixed Ag and Co could bind oxygen-containing species more strongly than pure Ag, induced by a strong binding affinity of Co to oxygen. 14-17


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Recently, one-dimensional (1D) nanostructures have received growing interests as a result of their fascinating properties and applicability superior to bulk counterparts.18 One of the most promising methods for the mass production of 1D nanostructures is electrospinning. Electrospinning is a simple, cheap and versatile technique to fabricate polymer fibers containing diverse compositions of materials. In particular, variously modified electrospinning methods have been employed to synthesize 1D-nanostructured ORR electrocatalysts composed of two different metals or metal oxides: For examples, there are electrospinning at once with a mixed solution of different metal precursors;19-28 electrospinning based on a two-capillary coaxial spinneret;29 and the use of electrospun materials as a template on which secondary nanostructures were grown.30-34 In this study, we demonstrate a new type of synthetic strategy and characterization for silvercobalt bimetallic composite nanotubes (AgCo composite nanotubes) as an ORR electrocatalyst. First, cobalt oxide (Co3O4) nanotubes were synthesized by electrospinning and the following thermal annealing in air. The subsequent reduction of Co3O4 nanotubes via additional calcination under H2 flowing produced Co metal nanotubes which were used as templates for Ag layer deposition. In fact, galvanic replacement reaction (GRR) between Co nanotubes and Ag precursor formed AgCo composite nanotubes. Co nanostructures have been widely used as templates for GRR with various metals (Pd,35-36 Pt,37-39 Au40-41 and Ag42) due to its negatively low standard reduction potential. A few studies on electrospun Co3O4 nanotubes were reported previously only for simple physical characterization43 or lithium ion battery anode material.44-45 Within confinement of our knowledge, this is the first report of the use of electrospun 1Dnanostructured Co as a template for GRR. The synthetic conditions of AgCo composite


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nanotubes (reaction time spans for the reduction of Co3O4 nanotubes and the following GRR) were systematically varied and optimized for the most efficient ORR activity.

2. Experimental 2.1. Materials. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O), silver nitrate (AgNO3), sodium hydroxide (NaOH), poly(vinyl pyrrolidone) (PVP, MW ≈ 1,300,000) and Nafion (5 wt% solution) were purchased from Sigma-Aldrich (St.Louis, MO,USA). Ethanol (97 %) was supplied by Daejung (Korea). Commercial Pt/C (20 wt% metal loading on Vulcan XC-72) was purchased from E-TEK Company. All other chemicals used were of analytical grade, and all solutions were prepared with deionized water (resistivity ≥ 18 MΩ·cm) 2.2. Synthesis of AgCo composite Nanotubes. First, Co3O4 nanotubes were synthesized by electrospinning as follows. 110 mg of Co(NO3)2·6H2O was first dissolved in 4.5 mL of ethanol and sonicated for 20 min. 230 mg of PVP was then added to the solution and agitated overnight. This homogenously mixed solution was placed in a syringe and emitted through a needle connected to a voltage power supply (applied voltage = 12 kV) at a flow rate of 20 µL min−1 using an electrospinning system (NanoNC ESR200R2). An aluminum plate was placed at a distance of 11 cm apart from the needle tip to collect the nanofibers electrospun. The obtained electrospun nanofibers were transferred to a vacuum oven and dried for 1 h to eliminate the remaining solvent. The dried electrospun nanofibers were calcined at 400 °C for 1 h in air for the removal of PVP and the decomposition of remaining Co(NO3)2. Then, the resulting Co3O4 nanotubes after the calcination were reduced to metal Co at 250 °C with flowing of H2 at 10 sccm and Ar at 100 sccm. The reduction time of Co3O4 was varied from 15 min to 5 h.


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AgCo composite nanotubes were prepared via GRR between the reduced Co nanotubes and AgNO3 with diverse reaction times. 1.5 mg of Co nanotubes were dispersed in 3 mL of distilled water (0.5 mg mL−1) and 3 mL of 16 mM AgNO3 solution was added dropwise to a Co nanotube-dispersed solution using a syringe pump with the injection rate of 0.375 mL min−1; and the mixed solution was magnetically stirred for various time periods from 30 min to 24 h. The final products, AgCo composite nanotubes, were obtained by centrifugation and washed three times with water/ethanol. The morphologies and compositions of as prepared AgCo composite nanotubes were characterized by field-emission scanning electron microscopy (FE-SEM; JEOL JSM-6700F) equipped with an energy dispersive X–ray spectrometer (EDS), high-resolution transmission electron microscopy (HRTEM, Cs-corrected STEM, JEOL JEM-2100F), high resolution X-ray diffraction (XRD; Bruker D8 DISCOVER, Cu Kα radiation) and X-ray photoelectron spectroscopy (XPS; Theta Probe AR-XPS System. Al Kα radiation). For comparison, Ag nanowires were also synthesized as one-dimensional counterpart to AgCo composite nanotubes by polyol process previously reported.46-47 Briefly, 5 mL of ethylene glycol (EG) was preheated at 160 °C for 1 h. Then, 0.25 M AgNO3 (in 3.0 mL of EG) and 0.375 M poly(vinyl pyrrolidone) (in 3.0 mL of EG) were simultaneously added to the preheated solution (5.0 mL) using two-channel syringe pump at a rate of 0.375 mL min−1. The reaction mixture was maintained at 160 °C for 45 min. Ag nanowires were collected by centrifugation and several washing steps. 2.3. Electrodes and Electrochemical Measurements. AgCo composite nanotubes, Ag nanowires and Co nanotubes were loaded on a glassy carbon (GC) disk electrode (3 mm in diameter) independently for electrochemical characterization. 6 µL of each catalyst-dispersed solution (2.0 mg mL−1 in deionized water) was pipetted onto the cleaned GC electrode surface


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and then allowed to dry in oven at 60 °C for 10 min. These loading and drying steps were repeated five times to load 60 µg of each catalyst in total. Then, 10 µL of 0.05 wt% Nafion (in ethanol) was dropped onto the modified GC electrode and which was dried at room temperature. For the electrochemical characterization, a three-electrode cell was used with a GC electrode loaded with each catalyst or a bare Au disk electrode (3 mm in diameter) as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a coiled Pt wire as the counter electrode. Electrochemical activities of these nanomaterials for ORR were characterized with rotating disk electrode (RDE) voltammetry in 0.1 M NaOH aqueous solution. RDE voltammetry was carried out using a RDE-1 rotor/Epsilon electrochemical analyzer (BASi).

calcined

Figure 1. Synthetic scheme of AgCo composite nanotubes.

3. Results and Discussion


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3.1. Synthesis and Characterization of AgCo composite Nanotubes. Figure 1 shows a scheme of AgCo composite nanotube synthesis. Firstly, a solution of Co(NO3)2 mixed with PVP in ethanol was delivered through a needle tip at a flow rate of 20 µL min−1 and electrospun onto aluminum foil, leading to the formation of nanofibers comprising of PVP and Co(NO3)2. The electrospun nanofibers were thermally annealed at 400 °C in air for 1 h. This calcination process removed PVP via thermal decomposition and eventually produced Co3O4 nanotubes. The Co3O4 nanotubes were reduced to Co metal through heating at 250 °C under a mixed H2/Ar atmosphere for different time spans. Finally, Ag layers were spontaneously deposited onto the Co nanotubes by adding Ag precursor (AgNO3) into a Co nanotube-dispersed solution. This GRR between Ag+ ion and Co(0) metal, forming AgCo bimetallic nanotubes, was induced by the difference in their standard reduction potentials: Co2+/Co (−0.277 V vs. SHE), Ag+/Ag (0.7991V vs. SHE).

A

1 µm

B

1 µm

C

100 nm

Figure 2. SEM images of (A) electrospun Co(NO3)2/PVP and (B and C) Co3O4 nanotubes after the calcination at 400 °C for 1 h in air.

Figure 2A shows the representative SEM image of as-spun nanofibers, composed of Co(II) and PVP, before the calcination. The nanofibers had quite smooth surfaces with an average


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diameter of 590 ± 130 nm. After being calcined at 400 °C in air for 1 h, the as-spun nanofibers were transformed to Co3O4 having a porous nanotube-like morphology with the significant decrease of an average diameter to 150 ± 8 nm as seen in the SEM images (Figure 2B and 2C). No nitrogen was detected in EDS analysis, verifying the complete elimination of PVP and nitrate from Co precursor. In fact, the following processes are considered to be occurring during calcination: the decomposition of nitrate ions and oxidation of the PVP polymer component in as-spun nanofibers; the formation of metal oxide aggregates on the nanofiber surface and their agglomeration into a continuous shell, transforming from the nanofibers to the nanotubes; and the complete removal of remaining polymer.43 Generally, it is believed that the formation of the nanotubes could be originated from the relative kinetics of the gas evaporation of PVP as a polymer matrix between inner and outer of nanofibers during the thermal annealing process. The Co3O4 nanotubes of current study had a relatively large diameters, compared to previously reported ones (diameters ≤ 100 nm),43, 45 which is possibly advantageous for maintaining the tube morphology in AgCo formed after the following reduction and GRR. The Co3O4 nanotubes were simply reduced via additional calcination under a mixed H2/Ar atmosphere, and eventually converted to Co metal nanotubes. Figure S1 shows SEM images of the Co nanotubes produced with various reduction times from 15 min to 5 h. It confirmed that the morphology of Co3O4 nanotubes was maintained after the reduction for a time between 15 min and 3 h as seen in Figure S1A-D. However, the further increase in the reduction time (e.g., 5 h) caused a partial destruction of the tube structures and aggregation of the broken pieces as seen in Figure S1E. Figure 3 shows SEM images of the products generated after Co nanotubes underwent GRR with AgNO3: The Co nanotubes were from Co3O4 nanotubes with different reduction times. The


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time for GRR was fixed to 24 h to give sufficient time for GRR. No reaction is anticipated between Ag precursor and Co3O4 without proceeding the reduction to Co. As seen in Figure 3A, the morphology of Co3O4 nanotubes was not changed and Ag was not existent in the corresponding EDS data. In Figure 3B-F, their reduction times were varied between 15 min and 5 h at 250 °C. The nanotube-like morphology was comparatively well maintained in the AgCo nanocomposites formed after the galvanic replacement of Co with Ag. A closer inspection reveals that the porous rough surface of nanotubes becomes smoother as the reduction time increases up to 1 h and reaches to quite similar surface morphology with longer reduction. This suggests that a rather even and well-packed Ag layer is formed by the GRR. In addition to the nanotubes, some aggregated broken pieces were also observed in the products formed from Co nanomaterials reduced for 5 h due to the partially collapsed tube-shape of precursor Co nanotubes as shown in Figure S1E.


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A

100 nm

B

100 nm

C

100 nm

D

100 nm

E

100 nm

F

100 nm

Figure 3. SEM images of AgCo composite nanotubes prepared from Co nanotubes formed with different reduction times of (A) 0 min, (B) 15 min, (C) 30 min, (D) 1 h, (E) 3 h and (F) 5

Figure 4A presents atomic percent of Ag in AgCo composite nanotubes depending on the reduction time of precursor Co3O4 nanotubes. Because non-reduced Co3O4 did not react with Ag precursor, Ag was not present in the EDS data. As the reduction time became longer, the amount of replaced Ag also increased. This indicates that the longer reduction time, the more amount of Co metal the nanotubes have; and most of Co3O4 is converted to Co after ca. 1 h reduction.


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Atomic percent of Ag /%

60

A

50 40 30 20 10 0 0

1

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4

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40 30 20 10 0 2 10 4 12 6 14 2216 24

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Co(002)

Co(100)

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Co(101)

Co(110)

Intensity /a.u.

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B

50

Reduction time /h

Intensity /a.u.

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Atomic percent of Ag /%


Ag(111)

Ag(200) Ag(220) Ag(311)

30

40

50

60

Angle /2θ θ

70

80

30

40

50

60

70

80

Angle /2θ θ

Figure 4. Atomic percent of Ag (i.e., Ag amount out of total metal amount) in AgCo composite nanotubes synthesized with (A) different reduction times and (B) different GRR times. XRD spectra of (C) Co nanotubes prepared with the reduction time of 1 h and (D) AgCo composite nanotubes prepared from Co nanotubes formed with the reduction time of 1 h and the subsequent GRR for 3 h.

The GRR times were also varied. Figure 4A shows that the amount of replaced Ag does not increase substantially after the reduction for longer than 1 h. Therefore, the time of reduction was fixed to 1 h and GRR time was varied between 30 min and 24 h. Ag precursor solution was injected to Co nanotube-dispersed solution slowly using a syringe pump to conserve its morphology. Figure S2A-E show the SEM images of AgCo composite nanotubes, which were synthesized with different GRR times. GRR for a time ≥ 3 h resulted in a very similar Ag content


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in the produced AgCo composite nanotubes as shown in Figure 4B, supporting 3 h is sufficiently long for GRR completion between Co nanotubes and Ag precursor. Figure 4C-D shows XRD patterns of Co and AgCo composite nanotubes in a range of 25-80°. The XRD spectrum of Co nanotubes prepared with the reduction time of 1 h showed four distinct peaks at 41.6°, 44.4°, 47.3° and 75.9°, being assigned to (100), (002), (101) and (110) planes of Co metal hcp phase, respectively. This implies that all Co3O4 was reduced to Co metal. In contrast, the peaks of AgCo composite nanotubes prepared with 1-h reduction followed by 3-h GRR were observed at 38.1°, 44.2°, 64.4°and 77.2°, corresponding to (111), (200), (220) and (311) planes of Ag metal fcc phase, while any noticeable Co related peaks were not observed. This indicates that Ag layer formed by GRR covered the surface of Co nanotubes nearly completely. The inner hollow morphology of AgCo composite nanotubes was clearly identified as the difference in brightness in the low resolution and high resolution TEM images as shown in Figure 5A and 5B. Figure 5B represents that the inner diameter of a single AgCo composite nanotube is estimated by 93 nm (± 18 nm) and the thickness of wall in a tube is measured by 21 nm (± 2 nm), respectively. The texture of the surface of a AgCo composite nanotube is relatively rough as shown in SEM images. To determine the crystalline structure of AgCo composite nanotubes, the high resolution TEM (HRTEM) images were acquired in Figure 5C and 5D. HRTEM image and the fast Fourier transform (FFT) image confirm the existence of specific crystalline planes corresponding both Ag and Co3O4 crystal structures such as (200) for fcc Ag phase (Figure 5C inset) and (111) for cubic Co3O4 phase (Figure 5D inset), indicating of the polycrystalline nature in a nanotube, consistent with XRD measurement.


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A

C

200 nm

10 nm

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B

10 nm

D

10 nm

(111) 0.4503 nm

(200) 0.2196 nm

(11-1) 0.2358 nm

(002) 0.4022 nm

(1-11) 0.2476 nm

(11-1) 0.4671 nm

5 nm

Figure 5. (A and B) TEM images and (C and D) high-resolution TEM images of AgCo composite nanotubes prepared with the reduction time of 1 h and the GRR time of 3 h (insets: SAED patterns).

As survey XPS spectrum revealed that the co-presence of Ag and Co elements in AgCo composite nanotubes prepared via reduction for 1 h and GRR for 3 h (Figure 6A). Two peaks observed at 374.5, 368.5 eV in the high-resolution spectrum are corresponding to Ag 3d3/2 and 3d5/2 with a spin-orbit splitting of 6 eV, which indicates that Ag existed as Ag(0) metal in the composite (Figure 6B).48 In Figure 6 C, The high-resolution Co 2p spectrum exhibited two peaks at 796.3 and 780.4 eV, corresponding to the Co 2p1/2 and Co 2p3/2 spin-orbit peaks of Co3O4 rather than other forms of cobalt oxides or cobalt metal.49 Even with the reduction for 1 h, Co nanotubes were naturally converted to the stable oxide form, Co3O4, once they were exposed to


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air for 3 days, as confirmed with high resolution XPS spectrum for Co 2p (Figure S3). In other words, Co is present as Co(0) metal in the reduced nanotubes (Figure 4C) and the surface Co metal is replaced with Ag during GRR along with the natural oxidation of some remaining Co metal to Co3O4.

Ag 3d

A Intensity /a.u.

Co 2p O 1s

C 1s

0

200

400

600

800

Binding energy /eV

B

C

Ag 3d5/2

360

365

370

375

Co 2p3/2

Co 2p1/2

Intensity /a.u.

Ag 3d3/2

Intensity/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


380

Binding energy /eV

770

780

790

800

810

Binding Energy /eV

Figure 6. (A) Survey XPS spectrum, (B) high resolution Ag 3d XPS spectrum, and (C) high resolution Co 2p spectrum of AgCo composite nanotubes prepared with the reduction time of 1 h and the GRR time of 3 h.

The reduction of Co3O4 nanotubes (150 ± 8 nm) caused a slight decrease of the diameter of the resulting Co nanotubes (142 ± 13 nm); and the following GRR produced AgCo composite


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nanotubes with a slightly larger diameter (150 ± 11 nm). However, these size changes were very small in contrast to the significant decrease of an average diameter associated with the conversion of as-spun nanofibers (590 ± 130 nm) to Co3O4 nanotubes (150 ± 8 nm). 3.2. Electrocatalytic Activity of AgCo composite Nanotubes. Figure 7 represents the RDE voltammetric curves for ORR occurring at AgCo nanotubes synthesized under different conditions. For the preparation of AgCo composite nanotubes presented in Figure 7A, the reduction time of precursor Co3O4 nanotubes was varied between 15 min and 5 h while the following GRR time was fixed to 24 h. Each voltammetric curve was normalized to the corresponding geometric surface area (GSA) of an electrode. The GSA was determined with chronocoulometry

(CC)

carried

out

in

0.1

M

tetrabutylammonium

hexafluorophosphate/acetonitrile solution containing 5 mM ferrocene.50 The GSA was determined to be ca. 0.08 ~ 0.10 cm2 depending on the catalyst loading from the slope of the linear plot of the measured charge vs. time1/2. In this case, the ORR onset potential and limiting current became improved as the reduction time of Co3O4 increased upto 1 h, but the longer reduction than 1 h affected the ORR activity of AgCo composite nanotubes insignificantly. In fact, the further reduction induced a slight negative shift of the onset potential. This indicates that 1 h is sufficiently long to reduce Co3O4 to Co metal, well matched with XRD results (Figure 4C). In contrast, AgCo composite nanotubes in Figure 7B were prepared with the same reduction time of 1 h but their GRR times were changed from 30 min to 24 h. It was observed that the longer GRR time up to 3 h, the better ORR activity they had (i.e, more positive onset potential and higher limiting current). However, AgCo composite nanotubes prepared with 6-h GRR showed a similar ORR activity of 3-h GRR product; and the further increase in the GRR time caused to deteriorate the activity. Therefore, 1-h reduction of Co3O4 nanotubes to form Co nanotubes


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followed by 3-h GRR with Ag precursor is seemingly an optimized synthetic condition for AgCo composite nanotubes as a good ORR electrocatalyst.

0

A

j /mA cm

-2

-1

-2

-3

15 min 30 min 1h 3h 5h

-4

-5 -0.6

-0.4

-0.2

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Potential /V 0

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-2 -3 30 min 1h 3h 6h 24 h

-4 -5 -0.6

-0.4

-0.2

0.0

Potential /V

Figure 7. RDE voltammograms for the ORR on AgCo composite nanotubes prepared with various (A) reduction times and (B) GRR times in an O2-saturated 0.1 M NaOH solution at a −1 scan rate of 10 mV s with a rotation speed of 1600 rpm. Current values were normalized to the corresponding GSAs to obtain current densities (j).

The ORR activity of AgCo composite nanotubes prepared in the optimized condition was compared with that of bare Pt, Pt/C, Ag nanowires (i.e., one-dimensional counterpart composed


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of pure Ag) and Co nanotubes using RDE voltammetry. The Co nanotubes were the ones obtained after the reduction of electrospun Co3O4 nanotubes without proceeding the GRR with an Ag precursor; and the Ag nanowires were synthesized by the previously reported method called polyol method (See the Experimental section for details),46-47 and the diameters were ranged between 550 – 650 nm as seen in the SEM image (Figure S4). As seen in Figure 8A, AgCo composite nanotubes showed a higher ORR limiting current than both Co nanotubes and Ag nanowires and even better than bare Pt. The onset potential became more positive in the following order: Co nanotubes (−0.23 V) < Ag nanowires (−0.11 V) < Pt disk (−0.07 V) ≤ AgCo nanotubes (−0.067 V) < Pt/C (−0.006 V). Compared to Pt/C, the ORR onset potential of AgCo composite nanotubes was slightly less positive (approximately by 0.06 V), but the ORR limiting currents of both materials were nearly the same, indicating the fairly good catalytic efficiency of AgCo composite nanotubes. Figure 8B shows the Koutecky-Levich (K-L) plots for the tested various electrocatalysts based on K-L equation (equation 1) obtained from the RDE results showing limiting currents and the number of electrons transferred (n) during the ORR was calculated with the slopes of K-L plots.

= +

(1)

where j is the measured limiting current density, is the kinetic current density, and is the diffusion-limited current density expressed by the following equation.

/

= 0.62 / /

(2)

where n is the number of electrons transferred in the ORR, F is the Faraday constant, (1.2 × 10−6 mol cm−3) is the saturated concentration of oxygen, (1.9 × 10−5 cm2 s−1) is the diffusion


18

Page 19 of 30

coefficient of oxygen, (1.0 × 10−2 cm2 s−1) is the kinematic viscosity of the solution, and ω is the electrode rotation rate.50

0

A

j /mA cm

-2

-1

-2

-3 Pt/C AgCo nanotubes bare Pt Ag nanowires Co nanotubes

-4

-5 -0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

Potential /V

2

0.5

B

0.4

-1

j /mA cm

0.3

-1

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


Pt/C AgCo nanotubes bare Pt Ag nanowires

0.2

0.06

0.08

0.10 -1/2

ω

0.12

0.14

-1/2 1/2

/rad

s

Figure 8. (A) RDE voltammograms for the ORR in an O2-saturated 0.1 M NaOH solution at a −1 scan rate of 10 mV s with a rotation speed of 1600 rpm. Current values were normalized to the corresponding GSAs to obtain current densities (j). (B) Kotecky-Levich (K-L) plots for the ORR obtained from the RDE data at −0.7 V. Note that the data for Co nanotubes were not included because its RDE curve in (A) did not attain a steady-state current density.


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Page 20 of 30

The K-L plots ( / ) developed using j values of ORR measured at −0.7 V (vs. SCE) exhibited a fairly good linearity. The n values for AgCo composite nanotoubes, Ag nanowires, bare Pt and Pt/C were 3.80, 3.55, 3.40 and 3.89, respectively. This implies that the ORR at AgCo composite nanotubes occurs predominantly via a direct 4-electron transfer pathway rather than 2electron pathway, suggesting a nearly complete reduction of O2 to OH− in alkaline media. Tafel plots based on Tafel equation (equation 3) were constructed for AgCo composite nanotubes and Pt/C in the mixed kinetic-diffusion controlled regime (Figure S5).

= − "log& ) + "log& )

(3)

where E is the electrode potential, E0 and j0 are the standard electrode potential and the exchange current density for the ORR, respectively, b is 2.303 RT/αnF (α: transfer coefficient; R: gas constant; T: absolute temperature), and jk is the kinetic current density.

The obtained Tafel slopes were −63.7 and −64.3 mV per decade for AgCo nanotubes and Pt/C, respectively, and the slope value for Pt/C was similar to the one previously reported.51 The exchange current densities (j0) calculated from the Tafel plots were 1.32 × 10−6 mA cm−2 for AgCo and 8.49 × 10−6 mA−2 cm−2 for Pt/C. This suggests that the ORR at AgCo composite nanotubes and Pt/C proceeds in a similar pathway52 (with the fast initial electron transfer), while the electron transfer rate in the kinetic controlled region is slightly slower at AgCo than at Pt/C. Overall, AgCo composite nanotubes exhibited greater ORR limiting current, higher n value and more positive onset/half-wave potentials, clearly demonstrating the improved ORR activity compared to not only pure Ag nanowires but pure Co nanotubes. This is possibly attributed to the bimetallic synergic effect of Ag and Co, which could be originated from the interface sites


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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


between two distinct crystalline natures, in addition to the unique tubular morphology providing a high surface to volume ratio. AgCo composite nanotubes have structures of inner Co nanotubes coated with outer Ag layers. However, oxygen can access the Ag and Co interfacial places via the diffusion through the porous and rough AgCo nanotubes (See Figure 3). The interfaces of Ag and Co are reckoned to be important active places showing the synergic effect of co-existing Ag and Co. The ORR activity of AgCo composite nanotubes was compared with that of other catalysts based on Ag and Co which have been recently reported (Table 1). Current AgCo composite nanotubes exhibited the ORR activity better than or at least comparable to that of other catalysts composed of Ag and Co. In fact, AgCo composite nanotubes showed the most positive onset potential along with considerably high limiting current and large n value.

Table 1. Comparison of the ORR catalytic performance of AgCo composite nanotubes with other catalysts composed of Ag and Co. Catalysts

Onset Potential (vs. SCE)

Limiting Current Density (mA cm−2)

n Value

Solution

Reference No.

AgCo composite nanotubes

−0.067 V

−4.75b

3.80

0.1 M NaOH

This work

Carbon-supported Ag–Co nanoparticles

ca. −0.09 Va

−5.41c

3.92–4.03

0.1 M NaOH

6

Ag/Co3O4–C

−0.1 Va

−2.39b

3.8–4.0

1 M KOH

10

Ag-Co/C

−0.11 Va

−3.0b

3.0

1 M KOH

11

Ag/Co electrodeposition on GC

−0.07 Va

−3.18d

3.8–4.0

0.1 M KOH

12

Cobalt monolayer on Ag(111)

−0.14Va

−2.75d

2.9–3.0

0.1 M KOH

48

a

Potentials are converted to the values vs. SCE. b at 1600 rpm, c at 900 rpm, d at 500 rpm

Once a cathodic catalyst is utilized in direct ethanol fuel cells, the tolerance of this catalyst to ethanol crossover from the anodic compartment is an important requirement for the fuel cell performance. In order to confirm the ethanol tolerance of AgCo composite nanotubes, RDE


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Page 22 of 30

voltammetry was carried out for the ORR in the presence/absence of 10 mM ethanol (Figure 9). As known for the susceptibility to ethanol contamination, the ORR activity of Pt/C catalyst was significantly affected by the presence of ethanol. The RDE curve of Pt/C with ethanol had a drastic current deep in the ORR limiting current region, indicating partial blockage of the ORR active sites by the adsorption of alcohol or intermediates of alcohol oxidation.53 Contrastively, even in the presence of ethanol, AgCo composite nanotubes showed the RDE voltammetric curve similar to that without ethanol, confirming its excellent ethanol tolerance. To verify the stability of AgCo composite nanotubes as ORR catalysts, ORR current of AgCo at −0.7 V (vs. SCE) was measured continuously for a long time period. For 10000 s, the generated ORR current level of AgCo composite nanotubes was only reduced to 96.0% of the initial one, while a commercial Pt/C exhibited the current decrease down to 79.9% of the initial value (Figure S6). In addition, the SEM image of AgCo composite nanotubes after 10000-s ORR experiment confirmed that the morphology was not changed noticeably (Figure S7). This supports a decent stability of AgCo composite nanotubes. A problem of alkaline fuel cells is the poisoning by carbon dioxide (CO2). CO2 tolerance of AgCo composite nanotubes was assessed with a chronoamperometric measurement and compared with that of commercial Pt/C. Amperometric response of each electrode at −0.7 V (vs. SCE) was measured in 0.1 M NaOH solution, being purged with O2, CO2 and O2 in order. A quite stable current induced by ORR started to decrease with the initiation of CO2 purging because the dissolved O2 concentration decreased by CO2 purging. Then, the return to O2 purging made the current recover the initial steady-state ORR current value gradually. As shown in Figure S8, the ORR activity of AgCo composite nanotubes was not deteriorated with the exposure to CO2, which is better than Pt/C.


22

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0

w/o EtOH : dotted line w/ EtOH : solid line AgCo nanotubes

-100

Current /µ µA

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


-200

-300 Pt/C -400 -0.6

-0.4

-0.2

0.0

Potential /V

Figure 9. RDE voltammograms obtained with a GC electrode loaded with Pt/C (blue) or AgCo composite nanotubes (red) in an O2-saturated 0.1 M NaOH solution with (solid line) or without (dotted line) containing 10 mM ethanol. Rotation speed, 1600 rpm.

4. Conclusions AgCo composite nanotubes were readily synthesized as follows: (1) Electrospinning of a solution containing Co precursor and PVP to prepare nanofibers of PVP/Co(NO3)2; (2) calcination in air to form Co3O4 nanotubes; (3) calcination under H2/Ar atmosphere for the reduction of Co metal nanotubes; and (4) GRR of Co nanotubes with Ag precursor. Reaction times for the steps (3) and (4) were varied. 1-h reduction of Co3O4 nanotubes and 3-h GRR between Co nanotubes and Ag+ were confirmed as an optimized synthetic condition for AgCo composite nanotubes exhibiting best ORR activity (the most positive onset potential, greatest limiting current density and highest number of electron transferred). The ORR performance of


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Page 24 of 30

the optimized AgCo composite nanotubes was better compared to both pure Co nanotubes and pure Ag nanowires, suggesting the bimetallic synergic effect of Ag and Co in addition to the unique tubular morphology providing a high surface to volume ratio. In fact, the ORR activity of AgCo composite nanotubes was better than bare Pt and comparable to Pt/C while exhibiting high ethanol tolerance.

Acknowledgements This work was financially supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2014R1A2A2A05003769 for YL) and (2014R1A1A2059791 for MHK).

Supporting Information SEM images of Co nanotubes prepared after the reduction of Co3O4 nanotubes (Figure S1); SEM images of AgCo composite nanotubes prepared with different GRR times (Figure S2); high-resolution Co 2p XPS spectrum of Co nanotubes prepared with 1-h reduction which were exposed to air for 3 days (Figure S3); SEM image of an Ag nanowire (Figure S4); Tafel plots (Figure S5); stability test (Figure S6); SEM image of AgCo nanotubes after stability test (Figure S7); and CO2 tolerance test (Figure S8). This material is available free of charge via the Internet at http://pubs.acs.org.


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Toc Graphic


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Highly Efficient SilverâCobalt Composite Nanotube Electrocatalysts for

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