Pt nanocubes as oxygen reduction catalysts by co-surfactant synthesis method

Pt nanocubes as oxygen reduction catalysts by co-surfactant synthesis method

Applied Surface Science 467–468 (2019) 844–850 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/...

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Applied Surface Science 467–468 (2019) 844–850

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Full Length Article

Tuning saw-toothed morphology on Pd/Pt nanocubes as oxygen reduction catalysts by co-surfactant synthesis method Zheng-Wei Wu, Ming-Hung Chiang, Chien-Liang Lee

T



Department of Chemical and Materials Engineering, National Kaohsiung University of Science and Technology, Kaohsiung 807, Taiwan

A R T I C LE I N FO

A B S T R A C T

Keywords: High-index facets SAED Tafel plots RRDE CTAC

Herein, a facile co-surfactant method has been reported for the synthesis of core-shell Pd/Pt saw-toothed nanocubes (Pd/Pt STNCs). In this method, saw-toothed morphologies increase with decreasing ratio (R) of the concentration of cetyltrimethylammonium bromide to cetyltrimethylammonium chloride in the synthesis solution. High-resolution transmission electron microscopy (HR-TEM) images reveal that the Pd/Pt STNC prepared using R = 4 (Pd/Pt STNCR=4) has (3 1 0) and (4 1 0) facets on its edge, whereas the Pd/Pt STNCs prepared using R = 1 (Pd/Pt STNCR=1) and 0.25 (Pd/Pt STNCR=0.25) have (3 1 1) high-index facets. Furthermore, the comparison based on electrochemical surface area (ESA) shows that carbon-supported Pd/Pt STNCR=4 used for catalysing acidic oxygen reduction exhibits a kinetic current of 0.44 mA at 0.9 V (vs. RHE), which is 1.4-times greater than that obtained for commercial Pt/C (0.31 mA). The higher activity could be caused by enriched electrons on the Pt outershell with less adsorbed Cl− ions, as confirmed by the X-ray photoelectron analyses. Accelerated durability test results show that the Pd/Pt STNCR=4 catalyst is more stable than Pt/C.

1. Introduction Oxygen reduction reaction (ORR), the cathodic reaction for fuel cells, can directly limit the cell’s power output due to its slow kinetics. One possible approach to improve the ORR kinetics is to develop highly active catalysts. Defective metal catalysts with low-coordinated atoms (e.g. steps, edges, and corners) can provide more active sites, thus improving the efficiency of heterogeneous reactions due to the lower activation energy of the reactants [1]. It has been reported that concave nanoparticles are enclosed by high-index facets [2–4], which possess high density of low-coordinated atoms at the step and edge sites, which serve as highly active catalytic sites for the ORR [5,6] and for methanol or ethanol oxidation reactions [3,4,7,8]. Core-shell Pd/Pt nanoparticles also exhibit excellent activity due to the tuneable d-band centre of the outer Pt shell, arising from the electronic interaction between the Pt shell and the Pd core [9,10]. Halide ions can efficiently control the outer shell structure in the synthesis of Pd/Pt core-shell nanoparticles. Cl− ions have weaker adsorption power on Pt than Br− ions, leading to the island growth model and formation of dendritic Pd/Pt nanoparticles [11]. In contrast, the presence of Br− ions can form layered Pd/Pt core-shell nanoparticles in the layer growth mode [11]. In this study, a facile co-surfactant method, using the mix of cetyltrimethylammonium bromide (CTAB)



and cetyltrimethylammonium chloride (CTAC), was applied successfully to prepare saw-toothed Pt-shell-coated Pd nanocubes (Pt/Pd STNCs) with controlled concentration ratios (R). of cetyltrimethylammonium bromide (CTAB) to cetyltrimethylammonium chloride (CTAC) Various Pt/Pd STNCs (Pd/Pt STNCR=4, Pd/Pt STNCR=1, and Pd/Pt STNCR=0.25) were prepared in the solution with R = 4, 1, and 0.25, respectively. The carbon-supported Pd/Pt STNCR=4 (Pd/Pt STNCR=4/C), Pd/Pt STNCR=1 (Pd/Pt STNCR=1/C), and Pd/Pt STNCR=0.25 (Pd/Pt STNCR=0.25/C) are applied as acidic ORR catalysts. The electrochemical surface areas (ESA), ORR kinetics, and kinetic currents (at 0.9 V vs. RHE) of these saw-toothed catalysts and commercial Pt/C have been studied using rotating-ring disk electrode (RRDE) measurements. 2. Experimental 2.1. Synthesis of various Pd/Pt STNCs using co-surfactant method Various core-shell Pd/Pt STNCs (Pd/Pt STNCR=4, Pd/Pt STNCR=1, and Pd/Pt STNCR=0.25) were prepared using the co-surfactant method, in which R = 4, 1, and 0.25, respectively. Initially, 500 µL of a 0.01 M H2PdCl4 aqueous solution was added to a mixed solution containing 5 mL each of 2 × 10−2 M CTAB and 5 × 10−3 M CTAC aqueous

Corresponding author. E-mail addresses: [email protected], [email protected] (C.-L. Lee).

https://doi.org/10.1016/j.apsusc.2018.10.224 Received 26 July 2018; Received in revised form 30 September 2018; Accepted 28 October 2018 Available online 29 October 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.

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solutions at a fixed temperature of 95 °C and the mixture was stirred for 5 min. Subsequently, 0.96 mL of 0.4 M ascorbic acid (AA) was gradually added with continuous stirring to the mixed CTAB/CTAC and H2PdCl4 solution for 1 min. After aging for 30 min at 95 °C, a further 0.05 mL of 0.4 M AA aqueous solution was added and the solution was stirred for 1 min. H2PtCl6 aqueous solution (0.01 M, 83 µL) was then added to the solution and was stirred for 2 h, to obtain solution containing Pd/Pt STNCR=4. When the concentrations of CTAB and CTAC were reversed in the initial solution, Pd/Pt STNCR=0.25 was prepared in the same manner. When the concentrations of both CTAC and CTAB in the initial solution were 1.25 × 10−2 M, Pd/Pt STNCR=1 was prepared. Different concave Pd nanocubes (Pd CNCR=4, Pd CNCR=1, and Pd CNCR=0.25) were prepared using the same co-surfactant method, but without any H2PtCl6. The particle solutions were centrifuged at 23,360 ×g and dispersed in 10 mL deionised water to remove unreacted H2PdCl4, H2PtCl6, AA or free CTAC, and CTAB. The dispersions were centrifuged at 19,060 ×g and the precipitates were used for material analysis and preparation of ORR catalysts.

area: 0.196 cm2; Pt ring area: 0.11 cm2) was used as the working electrode. In addition, a Pt counter electrode and a saturated calomel reference electrode (SCE) were used for measurements in Ar- or O2-saturated 0.1 M HClO4(aq) solutions. For LSV measurements, the ring potential was maintained at 1.2 V to oxidize the H2O2 generated by O2 reduction at the disk electrode. All the voltammetry potentials were referenced to the reversible hydrogen electrode (RHE). Impurities in the catalysts were removed by sample pre-treatment prior to electrochemical analyses. The catalyst-loaded GCE electrode was rapidly scanned between 0.06 V and 1.06 V (vs. RHE) at a scanning rate of 150 mV s−1 in an Ar-saturated 0.1 M HClO4 (aq) solution over twenty cycles. The collection efficiency (N) of the ring electrode was ∼0.163, as determined by the electroreduction method for K3Fe(CN)6 on a disk electrode [13]. In the accelerated durability test (ADT), the catalyst-loaded GCE electrode was positively scanned from 0.08 V to 1.1 V (vs. RHE) by using LSV (scanning rate: 10 mV s−1) combined with an RRDE, at an electrode rotating speed of 1600 rpm. The loading metallic weight of Pd/Pt STNCR=4/C or Pt/C on GCE was fixed at 120 μg cm−2. Following this initial scan, 5000 cycles of repeated CV measurements were performed from 0.6 to 1 V (vs. RHE) at a scanning rate of 100 mV s−1. Thereafter, a final LSV curve was measured for comparison with the initial curve. All measurements were performed in an O2-saturated 0.1 M HClO4 solution.

2.2. Material analysis After purification, the Pd/Pt or Pd precipitates were dispersed in deionised water and drop-cast onto a copper grid (400 mesh) with supported carbon films. Their characteristic morphologies were determined by transmission electron microscopy (TEM; JEOL JEM-2100). The energy-dispersive X-ray (EDX) element mapping or line scanning data for a single core-shell particle was recorded on a scanning transmission electron microscope (JEOL JEM-2100F CS STEM). The crystal structures were determined using X-ray diffraction (XRD; Bruker D8, Cu anode, 1.54184 Å). The binding energies for the Pt shell and the atomic percentages (at%) of Pt and Pd in the three Pt/Pd STNCs were measured using X-ray photoelectron spectroscopy (XPS; Kratos Axis Ultra DLD) and inductively coupled plasma mass spectrometry (ICP-MS; ThermoElement XR), respectively.

3. Results and discussion 3.1. Characterisation and growth of various Pd/Pt STNCs The TEM images for Pd/Pt STNCR=4, Pd/Pt STNCR=1, and Pd/Pt STNCR=0.25 (Fig. 1A–C) show that all are composed of cubic particles with saw-toothed edges. The high yield of the three Pd/Pt STNCs can be observed in the low-magnified TEM images (Fig. S1A–C) and the statistics (Fig. S1D) using these TEM results shows 94.26%, 89.59%, and 91% for the yield percentage of Pd/Pt STNCR=4, Pd/Pt STNCR=1, and Pd/Pt STNCR=0.25, respectively. A detailed comparison with Pd/Pt STNCR=4 and Pd/Pt STNCR=1 reveals obvious concaves on the surface of Pd/Pt STNCR=0.25 (Fig. 1C). Based on the statistics (Fig. S2A–C) obtained using TEM results, the mean lengths of cross line (Lcross) on the Pd/Pt STNCR=4, Pd/Pt STNCR=1, and Pd/Pt STNCR=0.25 are 13.4 nm, 13.1 nm, and 12.8 nm, respectively. The particle sizes of these three Pd/ Pt STNC catalysts are nearly equal. Additionally, the mean Lcross values (Fig. S2D–F) are 12.6, 12.7, and 11.3 nm for Pd CNCR=4, Pd CNCR=1, and Pd CNCR=0.25, respectively, as shown in their corresponding TEM images and Fig. S3A–C. Obviously, the concavity of the concave Pd nanocubes is improved with decreasing R. As shown in the inset of Fig. S3D, the ESA obtained from the charge of the Pd oxide reduction peak around 0.62 V in CV curves (Fig. S3D) divided by 424 μC cm−2 was in the following order: Pd CNCR=4 < Pd CNCR=1 < Pd CNCR=0.25 [14,15]. The significant concave surface inevitably leads to small Lcross and larger ESA. Simultaneously, the extent of saw-toothed nature of the Pd/Pt STNC surface increases with decreasing R. In addition to size equalization, this produces an ESA of 5.18 cm2 for Pd/Pt STNCR=0.25, greater than 4.33 cm2 and 3.78 cm2 for Pd/Pt STNCR=1 and Pd/Pt STNCR=4, respectively. The ESA was estimated from the hydrogen adsorption/desorption area between 0.05 V and 0.3 V in the CV curve (Fig. S4) [12]. The structure and growth mode of these Pd/Pt STNCs were further determined by EDX, XRD, and HR-TEM analyses. Fig. 1D–F show the EDX element mapping results for Pd/Pt STNCR=4, Pd/Pt STNCR=1, and Pd/Pt STNCR=0.25, respectively. All the EDX data on a single particle in the marked squares reveal a Pd signal concentrated at the core, whereas Pt signals are spread over the entire particle. These results confirm the presence of core-shell structures and are consistent with the line scanning EDX results (Fig. S5), where Pt signals are stronger in the shell and intense Pd signals are found in the core. The ICP-MS results show that

2.3. ORR catalysis and electrochemical analysis Before the electrochemical analysis, 1 mg of carbon powder (Ketjen300 JD) was dispersed in 3 mL ethanol and mixed under ultrasonication for 40 min. Next, 1 mg of Pd/Pt powders was added to the carbon powder solution, followed by sequential ultrasonication and stirring for 10 min and 12 h, respectively. The solution was then heated to remove ethanol and dried at 30 °C in a vacuum oven for 40 min. One milligram of the resulting dry powder was added to a solution containing 0.24 mL deionised water, 0.56 mL isopropanol, and 10 µL Nafion (5%) for dispersal under ultrasonication for 30 min. The metallic weight percentages of the obtained carbon-powder-supported metal catalysts were measured using thermogravimetric analysis (TGA; DT/Q600). Thereafter, the ink solution containing the metal powder/C was dropcast onto a 0.196-cm2 glassy carbon electrode (GCE) disk, which was subsequently heated to 30 °C in a vacuum oven to evaporate the water and isopropanol. For a fair comparison of the kinetic currents toward ORRs, the ESA of the carbon-supported Pd/Pt STNCs (Pd/Pt STNC/C) or Pt/C was maintained at 3.8 cm2, estimated by cyclic voltammetry (CV) measurements in Ar-saturated 0.1 M HClO4 solution and the calculation of hydrogen adsorption/desorption area [12]. The The loading Pd/Pt weights on GCEs were 23.52 μg, 20.58 μg, and 17.2 μg for Pd/Pt STNCR=4, Pd/Pt STNCR=1, and Pd/Pt STNCR=0.25, respectively. For Pt/ C, the Pt weight on GCE was 11.37 μg. Activities of various Pd/Pt STNC/C catalysts, compared with commercial Pt/C (TKK; TEC10E50E), were evaluated by linear scan voltammetry (LSV) experiments using an RRDE. RRDE measurements were performed using a combination of an RRDE system (AFMSRCE, Pine Research Instrumentation) and a bipotentiostat (CHI 727D). A catalystmodified rotating glassy carbon disk/platinum ring electrode (disk 845

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(A)

(B)

(C)

Pd

(D)

Pt 50 nm

Pd

(E)

Pd

(F)

Pt

Pt 60 nm

60 nm

Fig. 1. TEM images (A–C) and EDX element mapping results (D–F) of Pd/Pt STNCR=4 (A, D), Pd/Pt STNCR=1 (B, E), and Pd/Pt STNCR=0.25 (C, F).

Pt particle is a single crystal with [1 0 0]-oriented with (2 0 0) facet. The HR-TEM images in Fig. 2G–I reveal lattice spaces of 1.99 Å, 1.94 Å, and 1.99 Å for Pd/Pt STNCR=4, Pd/Pt STNCR=1, and Pd/Pt STNCR=0.25, respectively. These values are close to the d-spacing of Pt(2 0 0) (1.98 Å) in the standard XRD spectrum of Pt (No. 88-2343). It has been proved that Pd CNCs are composed of Pd(1 0 0) planes [16]. The Pt (1 0 0) facets revealed by HR-TEM analyses indicate outer shell growth by layer-by-layer deposition, despite the island growth mode possibly induced by Cl− from CTAC [11]. This epitaxial growth is probably due to a small mismatch between the Pt and Pd lattices [17,18] and strong bonding of Br− from CTAB [11]. The facets on the saw-toothed edge of these Pd/Pt STNCs are high-indexed planes. The HR-TEM image (Fig. 2J) clearly displays (3 1 0) and (4 1 0) planes on the edge of single-

the at% of Pt atom in the Pd/Pt STNCR=4, Pd/Pt STNCR=1, and Pd/Pt STNCR=0.25 were 9.56%, 8.79%, and 8.09%, respectively. The corresponding Pd at% were 90.44%, 91.21%, and 91.91%. The crystalline structures of the Pd/Pt STNCs were initially determined using XRD analysis and compared with the standard spectra of Pd and Pt in Fig. S6A. Due to an undetectable signal from the thin Pt outer shell, the two larger peaks at approximately 40.1° and 46.6° were observed for all the Pd/Pt STNCs were close to the (1 1 1) and (2 0 0) diffraction planes of Pd (No. 05-0681), indicating that these core-shell Pd/Pt STNCs belong to face-centred cubic (fcc) structure. Their HR-TEM images and the corresponding selected area electron diffraction (SAED) patterns for single particles of different Pd/Pt STNCs are shown in Fig. 2A–C and Fig. 2D–F, respectively. All the SAED patterns show that the single Pd/ 846

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(A)

(B)

(C)

(D)

(E)

(F)

(G)

(H)

(I)

(J)

(K)

(L)

(M)

(310) (410)

(N)

(O)

(311) (311)

(410)

Fig. 2. HR-TEM images (A–C, G–I, and J–L), SAED (D–F), and schemes for the edge facets (M−O) of Pd/Pt STNCR=4 (A, D, G, J, and M), Pd/Pt STNCR=1 (B, E, H, K, and N), and Pd/Pt STNCR=0.25 (C, F, I, L, and O).

3.2. Pd/Pt STNCs used as ORR catalysts

crystalline Pd/Pt STNCR=4 when observing the marked square, as shown in Fig. 2A. The two facets belong to the [0 0 1] zone. Fig. 2K and L reveal the (3 1 1) planes ([001] zone) on the marked saw-toothed edge of single-crystalline Pd/Pt STNCR=1 and Pd/Pt STNCR=0.25 (Fig. 2B and C), respectively. The corresponding charts for these edge planes are shown in Fig. 2M–O.

The catalytic properties of the Pd/Pt STNC/C catalysts were studied by LSV and RRDE measurements. Fig. 3A shows the LSV curves for ORR catalysed by Pd/Pt STNCR=4/C, Pd/Pt STNCR=1/C, and Pd/Pt STNCR=0.25/C, as compared with Pt/C dispersed on the 0.196-cm2 disk 847

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j / mA cm -2

(A)

Pd/Pt NC R=1 /C Pd/Pt NC R=0.25 /C

0.03

0.5

Pt/C I / mA

jR jD

0.00 0

j / mA cm -2

(B) 1.0

Pd/Pt NC R=4 /C

0.06

0.0 -0.5

Pd/Pt STNCR=4 /C

-3

Pd/Pt STNCR=1/C

-1.0

Pd/Pt STNCR=0.25 /C

-6

Pt/C 0.2

0.4

0.6

0.8

0.0

1.0

E vs. RHE / V

At 0.9 V

0.6

(D) 1.00

i k / mA

0.31± 0.014 0.29±0.023

0.2

E vs. RHE / V

0.95

0.44± 0.014 0.36±0.021

0.4

0.6

0.8

1.0

1.2

E vs. RHE / V

(C)

0.4

0.2

Pd/Pt STNC R=4/C

slope = -60 mV

Pd/Pt STNC R=1/C Pd/Pt STNC R=0.25/C

0.90

Pt/C

0.85

slope = -120 mV

0.80 0.75

0.0

Pd/Pt STNCR=1/C

0.70

Pt/C

-2

Pd/Pt STNCR=0.25/C

Pd/Pt STNCR=4 /C

-1

0 -2

log (jk / mA cm )

Fig. 3. (A) Positive-scanned LSV curves obtained in RRDE experiments (1600 rpm) for Pd/Pt STNCR=4/C, Pd/Pt STNCR=1/C, Pd/Pt STNCR=0.25/C, and Pt/C in O2saturated 0.1 M HClO4 solutions. Scan rate: 10 mV s−1. (B) CV curves based on the same ESA (3.8 cm2) for Pd/Pt STNCR=4/C, Pd/Pt STNCR=1/C, Pd/Pt STNCR=0.25/ C, and Pt/C in Ar-saturated 0.1 M HClO4 aqueous solutions. Scanning rate: 50 mV s−1. (C) Bar plots of kinetic current at 0.9 V. (D) Tafel plots of jk.

Intensity / arbitral units

Raw Intensity Background 0 Pt (74.73 eV) 0 Pt (71.45 eV)

90

Raw Intensity Background 0 Pt (75.03 eV) 0 Pt (71.63 eV)

STNCR=0.25/C were 49.31%, 51.65%, and 49.97%, respectively, determined from the TGA curves (Fig. S6B). The ESA values of all the catalysts for the LSV analyses were fixed at ∼3.8 cm2, determined by the hydrogen adsorption/desorption area between 0.05 V and 0.32 V in the CV curves (Fig. 3B). The potential range of ESA is consistent with that by 1.95 nm Pt catalysts on reduced graphene oxide [19]. In the CV curves, the features for hydrogen adsorption and desorption by these Pd/Pt STNCs are dissimilar to reported sharp peaks by cubic and octahedral Pt nanoparticles [20] but similar to those for Pt/C (Fig. 3B), strongly indicating corners and edges on their surfaces. The disk current density (jD, current normalized to the geometric area of the electrode) induced by Pd/Pt STNCR=4/C starts before 0.97 V and via a mixed control of activation-diffusion, reduces to 0.75 V. A saturated diffusion control period was observed below 0.75 V. The jD of Pd/Pt STNCR=4/C is greater than those of Pt/C and other two Pd/Pt catalysts after 0.92 V. Compared with Pt/C, the three Pd/Pt catalysts show fast jD response after 0.85 V. Simultaneously, similar small current densities were detected by the ring electrode (jR) for all the catalysts. The Pd/Pt STNC/C catalysts showed fast catalysis for ORR, whereas the production of H2O2 as an intermediate was rare during the ORRs. To precisely compare the activities of the Pd/Pt STNC/C catalysts and Pt/C in ORR, an estimation of the mass-transfer-corrected kinetic current (ik) is necessary. According to a detailed study of various comparable Pt catalysts applied for ORRs involving a thin-film rotating disk experiment, ik at 0.9 V can be calculated by Eq. (1) [21].

4 f7/2 Pd/Pt STNCR=1

4 f5/2

Pd/Pt STNCR=0.25

Raw Intensity Background 0 Pt (75.12 eV) 0 Pt (71.72 eV)

85

4 f7/2 Pd/Pt STNCR=4

4 f5/2

4 f5/2

80

4 f7/2

75

70

65

Binding Energy / eV Fig. 4. XPS Pt4f spectra of Pd/Pt STNCR=4, Pd/Pt STNCR=1, and Pd/Pt STNCR=0.25.

electrode; H2O2 production was measured in situ by the Pt ring electrode in the 0.1 M HClO4 (aq) electrolyte. The weight percentages of Pd/Pt STNC in Pd/Pt STNCR=4/C, Pd/Pt STNCR=1/C, and Pd/Pt 848

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(A)

Pd/Pt STNCR=4/C 0

j / mA cm -2

-4

decay 31.82% -8

Pt/C

0

first cycle 5000 th cycle

-4

decay 59.65% -8 0.2

0.4

0.6

0.8

1.0

E vs. RHE / V

(B)

(C)

I / mA

(D)

1

Pd/Pt STNCR=4 /C

0

decay : 33%

-1 0.0

0.2

0.4

0.6

0.8

1.0

1.2

E vs. RHE / V Fig. 5. (A) LSV curves (1600 rpm; Scan rate: 10 mV s−1) of Pd/Pt STNCR=4/C before and after ADT experiments, as compared with Pt/C, (B-C) TEM images of Pd/Pt STNCR=4 LSV curves (1600 rpm; scan rate: 10 mV s−1) before (B) and after ADT (C) experiments, and (D) CV curves of Pd/Pt STNCR=4 in Ar-saturated 0.1 M HClO4 solutions (Scan rate: 10 mV s−1) before and after ADT experiments.

ik =

id × i id − i

(Fig. 3D) further reveals −60 mV at low overpotential and −120 mV at high overpotential in the mixed controlled region for these three Pd/Pt STNC/C catalysts and Pt/C. jk is kinetic current density, calculated from ik over ESA. The adsorbed O2 show Temkin isothermal behaviour, when the catalyst surfaces have high coverage of oxides at low overpotential. However, a Langmuir isotherm of O2 adsorption is observed with low coverage of oxides at higher overpotential [22]. In addition, based on

(1)

Here, id is the diffusion limiting current at 0.3 V (vs. RHE at 25 °C). The calculated ik values of Pd/Pt STNCR=4/C and Pd/Pt STNCR=1/C are 0.44 and 0.36 mA, respectively, greater than the value of 0.31 mA provided by Pt/C (Fig. 3C). The Tafel plots of potential vs. log jk 849

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References

the equation using N (0.163), disk current, and ring current at 0.9 V [23,24], the electron transfer number and efficiency for producing H2O by Pd/Pt STNCR=4/C-catalysed ORRs are calculated as 3.99 and 98.11%, respectively. Fig. 4 shows the XPS Pt 4f spectra for Pd/Pt STNCR=4, Pd/Pt STNCR=1, and Pd/Pt STNCR=0.25. The binding energy (BE) of Pt 4f7/2 for Pd/Pt STNCR=4 is 71.45 eV, smaller than the values of 71.63 V and 71.72 eV for Pd/Pt STNCR=1 and Pd/Pt STNCR=0.25, respectively. The order of BE is consistent with the order of ik. Additionally, the BE decreases with increasing R, indicating the effect of Cl− ions from CTAC on the activity of the Pd/Pt STNC catalysts. When less Cl− ions are adsorbed, the saw-toothed shell of Pd/Pt STNCR=4 can provide enriched electrons for effectively binding with O2 and processing ORRs, compared with the other two Pt/Pd STNCs. A comparison with typical jk (0.3–0.45 mA cm−2 at 0.9 V) [25–27] of Pt/C (TKK) shows that the suppressed jk (0.08 mA cm−2 at 0.9 V) of Pt/C is obtained here. In this study, the loading Pt weight and density on GCE for Pt/C were 11.37 μg and 58 μg cm−2, respectively. The loading density was greater than 8–18 μg cm−2 used in other studies [26]. In order to fix a greater amount of Pt/C powder on the GCE, a high concentration of Nafion was used. The lower jk could be resulted from the cover effect of Nafion on catalysts. Furthermore, the ORR stability of Pd/Pt STNCR=4/C compared with Pt/C was studied using ADT experiments, based on the comparison of the activities of the catalysts. Fig. 5A reveals comparable LSV curves for the jD values from these two catalysts before and after the ADT. The Pd/ Pt STNCR=4/C curves in the mixed-controlled region (0.7–0.95 V) before and after the ADT are closer together than the corresponding curves for Pt/C. The ik decay using Pd/Pt STNCR=4/C is 31.82%, smaller than the decay of 59.65% obtained for Pt/C, indicating that Pd/ Pt STNCR=4/C showed significant stability. The TEM images of Pd/Pt STNCR=4/C before and after ADT (Fig. 5B and C, respectively) reveal that hollow structures were formed after ADT, which can provide sufficient surface area and active sites for O2. This can be evidenced by the ESA decay of only 33%, as estimated from the CV curves (Fig. 5D) before and after ADT in Ar-saturated 0.1 M HClO4 electrolyte.

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4. Conclusions Saw-toothed Pd/Pt nanocubes with core-shell structures were successfully prepared by a co-surfactant method. The extent of saw-toothed surfaces increased with decreasing CTAB/CTAC concentration ratio. The (3 1 0) and (4 1 0) planes were on the saw-toothed edge of Pd/Pt STNCR=4, whereas the (3 1 1) planes were formed for Pd/Pt STNCR=1 and Pd/Pt STNCR=0.25. Comparison on the same ESA showed that the Pd/Pt STNCR=4/C catalyst had higher ORR activity than Pt/C. Additionally, the Pd/Pt STNCR=4/C catalyst showed promising stability, as supported by the ADT results. Acknowledgement The authors thank the Ministry of Science and Technology, Taiwan, for financially supporting this research under Contract No. MOST 1062221-E-151-039-MY3 as well as Mr. Shyne-Yen Yao at National ChenKung University and Mr. Hsien-Tsan Lin of Regional Instruments Center at National Sun Yat-Sen University for their help with TEM experiments. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2018.10.224.

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