One-pot solvothermal synthesis of reduced graphene oxide-supported uniform PtCo nanocrystals for efficient and robust electrocatalysis

Journal of Colloid and Interface Science 543 (2019) 17–24

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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

One-pot solvothermal synthesis of reduced graphene oxide-supported uniform PtCo nanocrystals for efficient and robust electrocatalysis Han-Bin Meng a, Xiao-Fang Zhang a, Yu-Lu Pu a, Xue-Lu Chen a, Jiu-Ju Feng a, De-Man Han b, Ai-Jun Wang a,⇑ a Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, College of Chemistry and Life Sciences, College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua 321004, China b Department of Chemistry, Taizhou University, Jiaojiang 318000, China

g r a p h i c a l a b s t r a c t H2

0

A

HER

-30

j / mA cm

-2

H+

Pt78Co22 NCs/rGO Pt83Co17 NPs/rGO

-60

Pt29Co71 NPs/rGO Pt52Co48 NPs

-90

Pt black Pt/C

-120 -0.60

-0.45 -0.30 -0.15 Potential / V vs. RHE

2

B -2

0

Pt83Co17 NPs/rGO Pt29Co71 NPs/rGO

-2

Pt52Co48 NPs Pt black Pt/C

-4

O2

a r t i c l e

ORR Pt78Co22 NCs/rGO

j / mA cm

OH-

0.00

0.4

0.6 0.8 1.0 Potential / V vs. RHE

i n f o

Article history: Received 6 December 2018 Revised 22 January 2019 Accepted 23 January 2019 Available online 28 January 2019 Keywords: Solvothermal method Bimentallic nanocrystals Reduced graphene oxide Oxygen reduction reaction Hydrogen evolution reaction

a b s t r a c t Pt-based nanocomposites with low Pt utilization and high-activity by incorporating with other transition metals have received significant interest in catalysis. Meanwhile, loading Pt-based catalysts on graphene has great research value for improved stability and dispersity of the catalysts. Herein, a facile L-prolinemediated solvothermal strategy was reported to construct reduced graphene oxide (rGO) supported sheet-like PtCo nanocrystals (Pt78Co22 NCs/rGO) in ethylene glycol (EG). The as-synthesized nanocomposite manifested remarkably improved catalytic properties and chemical stability for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER), surpassing home-made Pt29Co71 nanoparticles (NPs)/rGO, Pt83Co17 NPs/rGO, Pt52Co48 NPs, commercial Pt/C and Pt black catalysts. These scenarios demonstrated an improved catalytic performances by tailoring the feeding ratio of Pt:Co and introducing rGO as a support. This work provides some new insights to design rGO-supported Pt-based catalysts by engineering the shapes and compositions in practical fuel cells. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction Currently, energy crisis and environmental pollution have triggered a series of problems due to the overuse of fossil fuels [1]. Searching clean, renewable and secure alternatives is extremely

⇑ Corresponding author. E-mail address: [email protected] (A.-J. Wang). https://doi.org/10.1016/j.jcis.2019.01.110 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

necessary to solve these issues [2]. Fuel cells and hydrogen energy are found the best substitutes thanks to their advantages of pollution-free, abundance and reproducibility [3]. Among the new energy systems, hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) are vital half-cell reactions in water electrolysis and fuel cells [4,5], respectively. To improve the efficiency of electrocatalysis for the half-cell reactions, it is of great significance to seek appropriate electrocatalysts [6]. In general, Pt nanomaterials are considered to be

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highly efficient catalysts due to the lower overpotentials and superior catalytic properties for HER and ORR [7]. Regrettably, their limited availability, high cost and poor stability severely confine their broad applications [8]. Various attempts have been made on designing Pt-based bimetallic nanostructures with low Pt utilization and highactivity by doping Pt with other transition metals [9]. Among the bimetallic catalysts, PtCo alloy is of great concern due to its low cost and relatively high activity [10,11]. For example, Pt3Co nanocubes manifested increased catalytic activity for methanol oxidation reaction (MOR) [12]. In another example, PtCo nanosheets preformed well for HER, MOR and ORR [13]. The dramatic improvement in the catalysis is mainly ascribed to the ligand effects and ensemble effects resulted from the variation in Pt electronic structures. However, it is difficult yet attractive for researchers to find a catalyst with both excellent catalytic activity and good stability simultaneously until now, because of the serious aggregation and dissolution of the nanocatalysts [14]. Based on this, it is extremely vital to choose a suitable support to improve the durability of the catalysts [15]. Generally, porous carbon [16] and graphene [17] nanomaterials are normally adopted as the feasible supports. Particularly, graphene, as a superior carbon-based nanomaterial, is deemed as an effective support to enhance the durability and dispersivity of the immobilized catalysts on account of its unique features (i.e. large surface area, good conductivity and high mechanical strength) [18,19]. Hitherto, many graphenesupported PtCo nanocomposites have been prepared [20–23], all exhibiting the improved catalytic characters. As such, many strategies have been developed accordingly, such as dry plasma reduction (DPR) [20], microwave irradiation-assisted routine [21], galvanic replacement [22] and chemical reduction [23]. However, the complicated procedures seriously restricted their application. Thus, it is greatly favorable to develop a one-step strategy for facile preparation of graphene supported PtCo nanocatalysts. L-Proline (Fig. S1, Supporting Information, SI), as a natural amino-acid, contains carboxyl and amino groups that have strong chelating ability with metal precursors to decrease the reduction rate and modulate the metallic morphologies [24]. Furthermore, the interactions between L-proline molecules (including hydrogen bonds and electrostatic forces) facilitate the directional diffusion and nanocrystal growth [25]. Herein, a novel L-proline-assisted solvothermal method was developed for construction of Pt78Co22 nanocrystals loaded on reduced graphene oxide (Pt78Co22 NCs/ rGO), which was explored as a valid and stable nanocatalysts for HER and ORR.

2. Experimental 2.1. Synthesis of Pt78Co22 NCs/rGO Graphene oxide (GO) was synthesized by natural graphite powder via Hummer’s method with some modification [26]. After removing the remained impurities, the GO suspension was further ultrasonicated for 0.5 h before use. In a typical process, 45 mg of L-proline was soluble in 15.00 mL of ethylene glycol (EG) under magnetic stirring. Subsequently, the H2PtCl6 (0.78 mL, 38.62 mM) and CoCl2 (7 mg) solutions were sequentially added to the above mixture after the injection of 2.00 mL of the GO homogeneous suspension (0.5 mg mL 1). Then, the mixture was transferred into a Teflon-lined stainless-steel autoclave, heated up to 180 °C and kept for 12 h, accompanied by cooling down to ambient temperature naturally. The resulted precipitate was obtained by centrifugation and washed sufficiently with water and ethanol, and dried in vacuum. Contrast experiments were carried out by adjusting the Pt/Co ratios of the two precursors

or without GO, while keeping the other operation conditions constant. More details regard the Characterization and Electrochemical tests were displayed in SI.

3. Results and discussion 3.1. Characterization Transmission electron microscopy (TEM) characterization was utilized to evaluate the morphologies and structures of the representative product. As Fig. 1A–B exhibits, the product consists of numerous well-defined sheet-like crystals with an average diameter of 10.75 nm (Fig. 1E). Apparently, these crystals have the uniform dispersion on the rGO sheets (as shown by the red arrows in Fig. 1A–B). The high-resolution TEM (HRTEM) image reveals plenty of wellresolved lattice fringes from the marked squares (Fig. 1C–D). The lattice distances are calculated to be 2.13 Å, 2.12 Å and 1.91 Å (Fig. 1C–D), corresponding to the (1 1 1) and (2 0 0) planes of the face-centered cubic (fcc) CoPt alloy, respectively. This conclusion is further verified by the angle (55°) between the (2 0 0) and (1 1 1) planes, in good accordance with the theoretical value (54.7°) [27]. Beyond that, the selective area electron diffraction (SAED) pattern (Fig. 1F) displays the distinct diffraction rings, well indexed to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes, suggesting the high crystallinity feature [28]. The high-angle annular dark-field scanning TEM (HAADF-STEM) image was recorded to evaluate the elemental distribution. Fig. 2A–D and Fig. 2E reveal the homogeneous distribution of Pt and Co atoms in the architecture, indicating the alloy feature in this research. Also, the EDS spectrum (Fig. 2F) further strengthens the presence of Pt and Co, showing the Pt and Co atomic percentages of 78.49% and 21.51%, respectively. To this regard, the typical product is defined as Pt78Co22 NCs/rGO for simplicity. Also, the atom ratios of the other three PtCo samples (synthesized at the feeding Pt/Co ratios of 1:3, 3:1, and the lack of GO) are about 29:71, 83:17 and 52:48, referred as Pt29Co71 NPs/rGO, Pt83Co17 NPs/rGO and Pt52Co48 NPs for clarity, respectively (Figs. S2–S3, SI). The metal loading of the product is evaluated by thermogravimetric analysis (TGA, Fig. 3). As exhibited in Fig. 3, there are three degradation stages. The decreases at 100, 200 and 500 °C are assigned to the release of water, the pyrolyzation of oxygen containing groups, and the decomposition of carbon skeleton [19,29], respectively. Analogous degradation trends are also observed for Pt29Co71 NPs/rGO and Pt83Co17 NPs/rGO. Remarkably, the mass losses at 200 °C for the graphene-supported PtCo products with scalable Pt/Co ratios are clearly less than that of GO under the same operation conditions, showing the effective reduction of GO [29]. Furthermore, the metal loadings of Pt78Co22 NCs/ rGO, Pt83Co17 NPs/rGO and Pt29Co71 NPs/rGO are 85.79%, 92.00% and 97.05%, respectively. The crystal phase and bimetallic nature of PtCo alloy were analyzed by X-ray diffraction (XRD) analysis (Fig. 4), using pure Pt (JCPDS-04-0802) and Co (JCPDS-15-0806) as the references. For Pt78Co22 NCs/rGO, the diffraction peaks appear at 40.50°, 47.19°, 69.14°, 83.17° and 87.74°, coincidentally show up between those of bulk Pt and Co, which are attributed to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) facets of the fcc PtCo structures, demonstrating the formation of the PtCo alloy [30]. In addition, a broad peak emerged at 23.15° comes from the (0 0 2) planes of rGO, which differs from pure GO with a peak at 10.8° correlated with the (0 0 1) planes, reflecting the efficient reduction of GO [19]. The compositions and surface oxidation states were elucidated by X-ray photoelectron spectroscopy (XPS). As Fig. 5A exhibits, the

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A

B rGO

200 nm

rGO

100 nm

D

C d = 0.212 nm

d = 0.213 nm 1 nm

Frequency / %

E

55 º d = 0.191 nm

d = 0.212 nm

2 nm

F

28

(111)

(200)

21 14 (220)

7 0

(311)

6

9

12

15

18

Fig. 1. Low- (A), medium- (B) and high-resolution (C–D) TEM images of Pt78Co22 NCs/rGO. E and F show the particle-size distribution and SAED pattern of Pt78Co22 NCs anchored on rGO, respectively. The red arrows indicate the wrinkles of rGO. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

B

E Counts

A

20 nm

Pt-M

0

D

C

11

F

Element Pt Co

Weight 92.35 7.65

44

55

Atomic 78.49 21.51

Counts

C

22 33 Distance / nm

Cu Pt Pt Co

CuPt

Co-K

Overlap 0

4

8 12 Energy / KeV

16

Fig. 2. HAADF-STEM image (A), HAADF-STEM-EDS mappings (B–D), line scanning profiles (E), and EDS spectrum (F) of Pt78Co22 NCs decorated on rGO. Insets in E and F show the HAADF-STEM image, weight and atomic ratios of Pt to Co, respectively.

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100

Mass / %

80 60

Pt78Co22 NCs/rGO

40

Pt83Co17 NPs/rGO

Pt29Co71 NPs/rGO GO

20 0 0

200

400

600

800

o

Temperature / C Fig. 3. TGA curves of Pt78Co22 NCs/rGO, Pt83Co17 NPs/rGO, Pt29Co71 NPs/rGO and GO.

groups for Pt78Co22 NCs/rGO suggests the efficient reduction of GO [19]. Fourier transform infrared (FTIR) and Raman spectroscopy experiments were usually conducted to determine the degree of GO reduction and graphitizing [35]. Fig. S6 (SI) shows the FTIR spectra of GO and Pt78Co22 NCs/rGO. Normally, the vibrational peaks of GO at 3415, 1734, 1600, 1403, 1221, 1057 cm 1 are mapped well to the stretching of OAH of hydroxyl group, C@O of carboxyl and carbonyl, C@C, OAH deformation, and CAO stretching of carboxyl and alkoxy groups, respectively. However, most of them disappear after the reduction, while only C@O and C@C groups still retain, showing the effective reduction of oxygencontaining groups in GO [21]. Raman spectra of GO and Pt78Co22 NCs/rGO both display the D and G bands at around 1356 and 1612 cm 1 (Fig. S7, SI), respectively. The peak intensity ratio for Pt78Co22 NCs/rGO (1.01) is larger than that of GO (0.95), which indicates more defects easily available by anchoring PtCo nanocrystals on rGO [21]. 3.2. Formation mechanism

(220)

(002) 20

40

60

(311) (222)

Co (JCPDS-15-0806) Pt78Co22 NCs/rGO

(200)

Intensity / a.u.

(111)

Pt (JCPDS-04-0802)

80

2 / Degree Fig. 4. XRD pattern of Pt78Co22 NCs/rGO. Standard XRD spectra of bulk Pt (JCPDS04-0802) and Co (JCPDS-15-0806) were offered for comparison.

Pt, C, N, O and Co contents are about 15.78%, 54.52%, 5.25%, 19.52% and 4.93%, respectively. By fitting the peak areas of Pt 4f (Fig. 5B), metallic Pt (0) (appeared at 71.27 eV and 74.59 eV) is the main species in Pt78Co22 NCs on rGO, indicating the effective reduction of the Pt precursor. Similarly, the peak areas of Co0 at 780.90 and 797.08 eV are larger than those of Co2+ species (786.45 and 802.71 eV) [31] as illustrated in Fig. 5C, which also demonstrate the efficient reduction of the Co salt. To have deep insights on the variation in the electronic structures of Pt 4f, the PtCo alloys with different compositions were also analyzed by XPS similarly. The Pt 4f peak displays a distinctive blue shift as the content of Co enhances (Fig. S4, SI), illustrating the variation of Pt electronic structure [32]. Most notably, the Pt 4f binding energies of the rGO-supported a series of PtCo products show negative shifts relative to that of pure Pt52Co48 NPs, suggesting that the electron transfers from rGO to Pt [33]. This electron migration would generally increase the electron density of Pt, cause the down shift of the d-band center of Pt and weaken the adsorption with oxygen-containing species, ultimately resulting in the dramatic increase in catalysis [34]. The addition of Co would regulate the electronic structures of Pt for the Pt-Co alloy on carbon and improve the catalytic activity [11]. Meanwhile, the high-resolution C 1s XPS spectrum (Fig. 5D) can be divided into three peaks at 288.26, 285.79 and 284.59 eV corresponding to the C@O, CAO and CAC (sp2) species, respectively. Different from GO (Fig. S5, SI), the attenuation of oxygen-containing

In order to explore the growth mechanism of the as-synthesized catalysts, we carried out some controlled experiments accordingly. Firstly, the final morphologies of the products changed dramatically when the feeding ratios of the bimetal precursors (Pt/Co) altered from 1:3 to 3:1. More specially, well-dispersed PtCo NCs were hardly detected with the feeding ratio of 1:3 (Fig. S2A, SI) or 3:1 (Fig. S2C, SI). It means that the well-defined PtCo NCs only appear at the proper feeding ratio in this synthesis [7]. Secondly, the absence of GO just yields some agglomerated particles (Fig. S3A, SI) instead of the typical PtCo NCs emerged in the standard process, reflecting the crucial role of GO as a support to enhance the dispersibility and endurance of the anchored PtCo NCs for the strong affinity between the PtCo alloy and GO. As we know, the existence of carbonyl- and carboxyl-groups on the surface of GO promotes the adsorption of the metal salts onto GO and in-situ formation of the PtCo NCs, leading to the uniform dispersion of the PtCo architectures [36]. According to the above analysis, we outline the formation mechanism of Pt78Co22 NCs/rGO. Initially, the interactions between L-proline and the bimetal precursors promote the formation of the metal-proline complex, eventually decreasing the reduction kinetics [37]. Considering the fact that the standard redox potential of PtCl26 /Pt (0.74 V vs. RHE) [38] is much greater than that of Co2+/ Co ( 0.28 V vs. RHE) [39], PtCl26 is reduced firstly because of its higher reduction potential, accompanied by the simultaneous reduction of GO to rGO. The presence of Pt nuclei promote the reduction of the Co precursor autonomously [40]. As the bimetal atoms achieve the hyper-saturated states, they are quickly fused together to generate PtCo nuclei by nucleation [3], accompanied by fast capping with the adjacent L-proline. Ultimately, Pt78Co22 NCs are formed on rGO via crystal growth and anisotropicgrowth with the assistance of L-proline [41].

3.3. Electrochemical performances Generally, the electrochemically active surface area (ECSA) is a critical parameter to assess the electrocatalytic performances of nanocatalysts. Fig. 6 presents the cyclic voltammetry (CV) plots of Pt78Co22 NCs/rGO, Pt83Co17 NPs/rGO, Pt29Co71 NPs/rGO, Pt52Co48 NPs, Pt/C and Pt black catalysts. Evidently, several peaks appear on the CV curve of Pt78Co22 NCs/rGO, which are assigned to the oxidation and reduction of Pt hydr(oxide) (0.98 V and 0.76 V) and Co hydr(oxide) (0.71 V and 0.50 V) species [42] The ECSA was obtained by integrating the charges of hydrogen desorption/

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B

A N 1s

C 1s

1200

900 600 300 Binding Energy / eV

0

0

Pt II

Pt

81

0

78 75 72 Binding Energy / eV

Co

CoO

800 792 784 Binding Energy / eV

776

69

D C-C

Intensity / a.u

Intensity / a.u

Co 2p1/2

808

Pt 4f5/2

Co 2p3/2

C

Pt 4f7/2

Pt 4f

Intensity / a.u.

O 1s

Intensity / a.u.

Co 2p

C=O

294

C-O

291 288 285 Binding Energy / eV

282

Fig. 5. Survey (A), high-resolution Pt 4f (B), Co 2p (C), and C 1s (D) XPS spectra of Pt78Co22 NCs/rGO.

j / mA cm

-2

5 0 Pt78Co22 NCs/rGO

-5

Pt83Co17 NPs/rGO Pt29Co71 NPs/rGO

-10

Pt52Co48 NPs Pt black Pt/C

0.00

0.35

0.70

1.05

1.40

Potential / V vs. RHE Fig. 6. The CV plots of Pt78Co22 NCs/rGO, Pt83Co17 NPs/rGO, Pt29Co71 NPs/rGO, Pt52Co48 NPs, Pt/C and Pt black catalysts in 0.5 M KOH at 50 mV s 1.

adsorption sections in the CV curves (Fig. 6). The ECSA of Pt78Co22 NCs/rGO (22.87 m2 gPt1) is the highest in contrast with those of Pt83Co17 NPs/rGO (14.11 m2 gPt1), Pt29Co71 NPs/rGO (18.39 m2 gPt1) and Pt black (11.96 m2 gPt1), as well as other catalysts such as Pt3Co NPs (17.00 ± 4.00 m2 g 1) [42] and PtCu3 alloy NPs (8.00 ± 3.00 m2 g 1) [43]. The largest ECSA of Pt78Co22 NCs/rGO is ascribed to the smaller size of Pt78Co22 NCs relative to other catalysts, finally increasing the contact area between the catalysts and reactants. Remarkably, the ECSA values of the rGO-supported PtCo NPs with tunable Pt/Co ratios are also larger than that of Pt52Co48 NPs (10.61 m2 gPt1), mainly owing to the superior dispersion of the Pt78Co22 NCs on rGO surface [33]. However, this value of Pt78Co22 NCs/rGO is smaller than that of Pt/C (47.6 m2 gPt1), which is assigned to the small size of Pt particles in Pt/C [44].

Linear sweep voltammetry (LSV) tests were carried out to evaluate the HER property of Pt78Co22 NCs/rGO in N2-saturated electrolyte, using Pt29Co71 NPs/rGO, Pt83Co17 NPs/rGO, Pt52Co48 NPs, Pt black and Pt/C as the standards (Fig. 7A). Apparently, Pt78Co22 NCs/rGO ( 1.3 mV) displays the more positive onset potential (Eonset) than those of Pt83Co17 NPs/rGO ( 2.3 mV), Pt29Co71 NPs/ rGO ( 2.3 mV), Pt52Co48 NPs ( 10.3 mV), Pt black ( 14.3 mV) and Pt/C ( 8.8 mV). Furthermore, the Pt78Co22 NCs/rGO manifests an overpotential of 25.3 mV at 10 mA cm 2, much smaller when compared to those of Pt83Co17 NPs/rGO (31.3 mV), Pt29Co71 NPs/ rGO (30.3 mV), Pt/C (37 mV), Pt52Co48 NPs (46.3 mV) and Pt black (60.3 mV), confirming the enhanced HER performances of Pt78Co22 NCs/rGO. It reveals the vital importance of the incorporative effects between rGO and the bimetals [33]. Tafel slope is normally used to analyze the intrinsically electrocatalytic features and HER mechanism of nanocatalysts. As described in Fig. 7B, the Tafel slope of Pt78Co22 NCs/rGO (26 mV dec 1) is smaller than those of Pt/C (31 mV dec 1), Pt83Co17 NPs/rGO (35 mV dec 1), Pt29Co71 NPs/rGO (36 mV dec 1), Pt52Co48 NPs (38 mV dec 1) and Pt black (46 mV dec 1), indicating its faster HER kinetics and the effectiveness of rGO [45]. With the Tafel slope of 26 mV dec 1, the HER process at Pt78Co22 NCs/rGO follows Volmer-Tafel mechanism, and the combination of adsorptive H* species is the rate-limiting step [46]. Apart from the catalytic properties, the durability is also an important factor to estimate the properties of nanocatalysts [47]. Herein, the accelerated durability test (ADT) of the catalysts was carried out to investigate the stability. It is found that the current densities display negligible change after 1000 cycles (Fig. 7C). Besides, we conducted the chronopotentiometric test to assess the durability (Fig. 7D). The current density for Pt78Co22 NCs/rGO still maintains 98.9% of its original value, which is clearly bigger than those of Pt/C (90.7%), Pt83Co17 NPs/rGO (90.7%), Pt29Co71

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-2

-30

j / mA cm

0.064

A Overpotential / V

0

Pt78Co22 NCs/rGO Pt83Co17 NPs/rGO

-60

Pt29Co71 NPs/rGO Pt52Co48 NPs

-90

Pt black Pt/C

-120 -0.45 -0.30 -0.15 Potential / V vs. RHE

46

0.032 V 36 m

0.000

V dec

-1

Vd

ec

31

d mV

ec

-1

35 m

V dec

Pt78Co22 NCs/rGO Pt83Co17 NPs/rGO Pt29Co71 NPs/rGO Pt black Pt/C

0.0

0.00

0.3

0.6 0.9 -2 log (-j / mA cm )

1.2

Pt78Co22 NCs/rGO

D

C

Pt83Co17 NPs/rGO Pt29Co71 NPs/rGO Pt52Co48 NPs

-60 -90 Initial th 1000 cycle -0.3 -0.2 -0.1 0.0 Potential / V vs. RHE

j / mA cm-2

-2

j / mA cm

c

Pt52Co48 NPs

-30

-0.4

m

e Vd

38 m -1 dec -1

26 m

15

-120

-1

-1

-0.032

-0.60

0

B

Pt black Pt/C

0 -15 -30 0

2500

5000 7500 Time / s

10000

Fig. 7. HER polarization curves (A) of Pt78Co22 NCs/rGO, Pt83Co17 NPs/rGO, Pt29Co71 NPs/rGO, Pt52Co48 NPs, Pt/C and Pt black catalysts acquired at 5 mV s 1 with a rotation rate of 1600 rpm in N2-saturated 0.5 M H2SO4 solution. The HER Tafel plots (B). The polarization curves of Pt78Co22 NCs/rGO before and after 1000 cycles (C). The chronoamperometric curves at 0.03 V (D).

NPs/rGO (88.7%), Pt52Co48 NPs (23.3%), and Pt black (50.1%). These scenarios verify the remarkably improved stability of Pt78Co22 NCs/ rGO among the investigated catalysts. Beyond that, graphene as a support dramatically improves the stability and dispersivity of the attached nanocatalysts [18]. The ORR characters were further researched by LSV in O2saturated 0.5 M KOH electrolyte (Fig. 8). For comparison, Pt29Co71 NPs/rGO, Pt83Co17 NPs/rGO, Pt52Co48 NPs, Pt/C and Pt black catalysts were also assessed as the contrasts. As depicted in Fig. 8A, the Eonset (obtained at a current density of 2 lA cm 2) and halfwave potential (E1/2) [48] for Pt78Co22 NCs/rGO (1.117 V, 0.910 V) are much larger than those of Pt83Co17 NPs/rGO (1.111 V, 0.880 V), Pt29Co71 NPs/rGO (1.110 V, 0.886 V), Pt52Co48 NPs (1.100 V, 0.866 V), Pt black (1.091 V, 0.868 V) and Pt/C (1.092 V, 0.898 V). Moreover, the MA (Fig. 8B) at 0.85 V for Pt78Co22 NCs/ rGO is 83.64 mA mg 1, albeit with its lower than that of Pt/C (149.17 mA mg 1), but is 3.53-, 4.15-, 7.85- and 5.83-time enhancement alternative to those of Pt83Co17 NPs/rGO (23.67 mA mg 1), Pt29Co71 NPs/rGO (20.17 mA mg 1), Pt52Co48 NPs (10.66 mA mg 1) and Pt black (14.30 mA mg 1), respectively, verifying the remarkably improved ORR performance of Pt78Co22 NCs/rGO. These observations are explained by the specific Pt78Co22 NCs and the synergistic effects of the bimetals, as well as the introduction of graphene as a support [49]. The ORR polarization curves were acquired by LSV to estimate the ORR electron transfer kinetics with the rotation rates from 100 to 2500 rpm (Fig. S8, SI). Clearly, the enlargement of the rotation rates promotes the enhancement of the current densities, due to the fast diffusion of O2 [50]. Based on the K-L equation, the electron transfer numbers of Pt78Co22 NCs/rGO at 0.46, 0.47, 0.48 and 0.49 V are 3.91, 3.97, 4.02 and 4.07, respectively (Fig. 8C), confirming the efficient reduction of O2 to OH in the alkaline circumstance [51]. Rotating ring-disk electrode (RRDE) test was also carried out to illustrate the ORR mechanism by evaluating the H2O2 yield and

electron transfer number [52]. As described in Fig. S9 (SI), the H2O2 yield at Pt78Co22 NCs/rGO during ORR is less than 5% in the potential range of 0.3–0.8 V. The electron transfer numbers show up in the range of 4.01–4.08, which agree well with the results based on the K-L equation. It demonstrates that ORR undergoes a four electron pathway at Pt78Co22 NCs/rGO. The ORR mechanism is elucidated by the Tafel plots. The Tafel slopes (Fig. 8D) for Pt78Co22 NCs/rGO, Pt/C, Pt83Co17 NPs/rGO, Pt29Co71 NPs/rGO, Pt52Co48 NPs and Pt black are 75.61, 83.61, 80.04, 88.89, 93.35 and 89.37 mV dec 1, respectively. The smaller absolute value of the Tafel slope for Pt78Co22 NCs/rGO implies the significantly enhanced ORR efficiency [53]. Besides, the Tafel slope for Pt78Co22 NCs/rGO approaches to 59 mV dec 1, demonstrating that the cleavage of the OAO bond is the rate controlling step [54]. The tolerance of nanocatalysts is another critical parameter for ORR performance. The ADT is adopted to estimate the stability of Pt78Co22 NCs/rGO in 0.5 M KOH. There is a slight alteration of E1/2 (3 mV, Fig. S10A, SI) for Pt78Co22 NCs/rGO relative to those of Pt/ C (7 mV, Fig. S10B, SI), Pt83Co17 NPs/rGO (11 mV, Fig. S11A, SI), Pt29Co71 NPs/rGO (19 mV, Fig. S11B, SI), Pt52Co48 NPs (36 mV, Fig. S11C, SI) and Pt black (21 mV, Fig. S11D, SI), showing the excellent durability of the as-obtained Pt78Co22 NCs/rGO in this work. Besides, to assess the catalytic selectivity of Pt78Co22 NCs/rGO against methanol, the methanol-tolerance measurement was performed by chronopotentiometric test through injecting 0.5 M methanol to the alkaline solution. As compared to Pt/C (Fig. S12, SI), Pt78Co22 NCs/rGO exhibits little activity loss, mostly because the electron transfer between Co and Pt would decrease the bond energy of Pt-CO [32]. Briefly, the remarkably improved catalytic activity and tolerance of the as-synthesized catalyst are ascribed to the following aspects: (1) The smaller particle size of Pt78Co22 NCs provides the highly enlarged specific surface area for reactants; (2) The inducement of Co would regulate the electronic structures of Pt and make

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2

1.05

A

B

0

E / V vs.RHE

Pt83Co17 NPs/rGO

0.90

Pt29Co71 NPs/rGO Pt52Co48 NPs

-2

0.4

Pt29Co71 NPs/rGO Pt52Co48 NPs Pt black Pt/C

0

0.6 0.8 1.0 Potential / V vs. RHE

C

1.1

40 80 120 -1 jk / mA mg Pt

160

D

1.0

0.6

j-1 / cm2 mA

-1

Pt83Co17 NPs/rGO

0.60

-4

0.8

Pt78Co22 NCs/rGO

0.75

Pt black Pt/C

Potential / V

j / mA cm

-2

Pt78Co22 NCs/rGO

0.49 V 0.48 V 0.47 V 0.46 V

0.4

0.2 0.02

0.04

0.06 -1/2

0.08

0.10

0.9

Pt78Co22 NCs/rGO Pt83Co17 NPs/rGO

0.8

Pt29Co71 NPs/rGO

0.7

Pt black Pt/C

Pt52Co48 NPs

-1.8

-1/2

/ rpm

-1.2

-0.6 0.0 -2 0.6 log (-jk/mA cm )

Fig. 8. ORR polarization curves of Pt78Co22 NCs/rGO, Pt83Co17 NPs/rGO, Pt29Co71 NPs/rGO, Pt52Co48 NPs, Pt/C and Pt black catalysts in O2-saturated 0.5 M KOH at 5 mV s a rotation rate of 1600 rpm (A). The mass kinetic current densities taken at different potentials (B). The K-L plots at different potentials (C) and Tafel plots (D).

the downshift of the d-band center of Pt, finally improving the catalytic features [30]; (3) The introduction of graphene with large surface area as a superior support would improve the electroconductivity, dispersivity and stability of the attached catalysts, thanks to the strong electrostatic interactions resulted from the electron transport between graphene and the adhered nanocatalyst [18]. 4. Conclusions In this research, a facile L-proline-mediated solvothermal strategy was reported to construct reduced graphene oxide (rGO) supported PtCo nanocatalysts. Controlled experiments reveal that the feeding ratio of Pt:Co and rGO play the vital roles in the synthesis. Due to the smaller size of Pt78Co22 NCs and the cooperative effects of the bimetals, the as-obtained Pt78Co22 NCs/rGO manifested improved catalytic properties for ORR and HER. Furthermore, the introduction of graphene with large surface area as a support would improve the durable ability and the dispersivity of the nanocatalysts. Overall, this method provides a guideline for development of graphene-supported nanocomposites in water splitting and others. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 21475118), Zhejiang Province Basic Public Welfare Research Project (LGG18E010001) and Zhejiang Public Welfare Technology Application Research Project (LGG19B050001). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.01.110.

1

with

References [1] C. Bhowmik, S. Bhowmik, A. Ray, K.M. Pandey, Optimal green energy planning for sustainable development: a review, Renew. Sustain. Energy Rev. 71 (2017) 796–813. [2] I. Dincer, Renewable energy and sustainable development: a crucial review, Renew. Sustain. Energy Rev. 4 (2000) 157–175. [3] L.Y. Jiang, X.X. Lin, A.J. Wang, J.H. Yuan, J.J. Feng, X.S. Li, Facile solvothermal synthesis of monodisperse Pt2.6Co1 nanoflowers with enhanced electrocatalytic activity towards oxygen reduction and hydrogen evolution reactions, Electrochim. Acta 225 (2017) 525–532. [4] D.U. Lee, B.J. Kim, Z.W. Chen, One-pot synthesis of a mesoporous NiCo2O4 nanoplatelet and graphene hybrid and its oxygen reduction and evolution activities as an efficient bi-functional electrocatalyst, J. Mater. Chem. A 1 (2013) 4754–4762. [5] J. Yi, Z.L. Zhou, G.G. Ping, C. Hua, W. Bei, Z.J. Zhi, S.M. Teng, H. Min, Y.X. Cheng, Q.G. Ren, Z. Jin, D.A. Jun, Y.X. Dong, A heterostructure coupling of exfoliated NiFe hydroxide nanosheet and defective graphene as a bifunctional electrocatalyst for overall water splitting, Adv. Mater. 29 (2017) 1700017. [6] L.Z. Bu, S.J. Guo, X. Zhang, X. Shen, D. Su, G. Lu, X. Zhu, J.L. Yao, J. Guo, X.Q. Huang, Surface engineering of hierarchical platinum-cobalt nanowires for efficient electrocatalysis, Nat. Commun. 7 (2016) 11850. [7] L. Zhang, X.F. Zhang, X.L. Chen, A.J. Wang, D.M. Han, Z.G. Wang, J.J. Feng, Facile solvothermal synthesis of Pt71Co29 lamellar nanoflowers as an efficient catalyst for oxygen reduction and methanol oxidation reactions, J. Colloid Interface Sci. 536 (2019) 556–562. [8] J. Greeley, I.E.L. Stephens, A.S. Bondarenko, T.P. Johansson, H.A. Hansen, T.F. Jaramillo, J. Rossmeisl, I. Chorkendorff, J.K. Nørskov, Alloys of platinum and early transition metals as oxygen reduction electrocatalysts, Nat. Chem. 1 (2009) 552–556. [9] C. Wang, N.M. Markovic, V.R. Stamenkovic, Advanced platinum alloy electrocatalysts for the oxygen reduction reaction, ACS Catal. 2 (2012) 891– 898. [10] J.M. Zhang, S.N. Sun, Y. Li, X.J. Zhang, P.Y. Zhang, Y.J. Fan, A strategy in deep eutectic solvents for carbon nanotube-supported PtCo nanocatalysts with enhanced performance toward methanol electrooxidation, Int. J. Hydrogen Energy 42 (2017) 26744–26751. [11] X.L. Chen, L. Zhang, J.J. Feng, W.P. Wang, P.X. Yuan, D.M. Han, A.J. Wang, Facile solvothermal fabrication of polypyrrole sheets supported dendritic platinumcobalt nanoclusters for highly efficient oxygen reduction and ethylene glycol oxidation, J. Colloid Interface Sci. 530 (2018) 394–402. [12] H.Z. Yang, J. Zhang, K. Sun, S.Z. Zou, J.Y. Fang, Enhancing by weakening: electrooxidation of methanol on Pt3Co and Pt nanocubes, Angew. Chem. Int. Ed. 49 (2010) 6848–6851.

24

H.-B. Meng et al. / Journal of Colloid and Interface Science 543 (2019) 17–24

[13] Q.L. Chen, Z.M. Cao, G.F. Du, Q. Kuang, J. Huang, Z.X. Xie, L.S. Zheng, Excavated octahedral Pt-Co alloy nanocrystals built with ultrathin nanosheets as superior multifunctional electrocatalysts for energy conversion applications, Nano Energy 39 (2017) 582–589. [14] Y.G. Zhao, J.J. Liu, Y.H. Zhao, F. Wang, Y. Song, Pt-Co secondary solid solution nanocrystals supported on carbon as next-generation catalysts for the oxygen reduction reaction, J. Mater. Chem. A 3 (2015) 20086–20091. [15] S.H. Joo, J.Y. Park, C.K. Tsung, Y. Yamada, P. Yang, G.A. Somorjai, Thermally stable Pt/mesoporous silica core-shell nanocatalysts for high-temperature reactions, Nat. Mater. 8 (2008) 126–131. [16] Z.S. Li, C.Y. He, M. Cai, S. Kang, P.K. Shen, A strategy for easy synthesis of carbon supported Co@Pt core-shell configuration as highly active catalyst for oxygen reduction reaction, Int. J. Hydrogen Energy 37 (2012) 14152–14160. [17] M.Y. Wang, T. Shen, M. Wang, D.G. Zhang, J. Chen, One-pot green synthesis of Ag nanoparticles-decorated reduced graphene oxide for efficient nonenzymatic H2O2 biosensor, Mater. Lett. 107 (2013) 311–314. [18] X. Huang, X.Y. Qi, F. Boey, H. Zhang, Graphene-based composites, Chem. Soc. Rev. 41 (2012) 666–686. [19] Y.C. Shi, J.J. Feng, X.X. Lin, L. Zhang, J.H. Yuan, Q.L. Zhang, A.J. Wang, One-step hydrothermal synthesis of three-dimensional nitrogen-doped reduced graphene oxide hydrogels anchored PtPd alloyed nanoparticles for ethylene glycol oxidation and hydrogen evolution reactions, Electrochim. Acta 293 (2019) 504–513. [20] S.W. Yoon, V.D. Dao, L.L. Larina, J.K. Lee, H.S. Choi, Optimum strategy for designing PtCo alloy/reduced graphene oxide nanohybrid counter electrode for dye-sensitized solar cells, Carbon 96 (2016) 229–236. [21] M.Y. Zhang, J.J. Shi, W.S. Ning, Z.Y. Hou, Reduced graphene oxide decorated with PtCo bimetallic nanoparticles: facile fabrication and application for basefree oxidation of glycerol, Catal. Today 298 (2017) 234–240. [22] R. Ojani, J.-B. Raoof, M. Goli, R. Valiollahi, Pt-Co nanostructures electrodeposited on graphene nanosheets for methanol electrooxidation, J. Power Sources 264 (2014) 76–82. [23] R. Baronia, J. Goel, S. Tiwari, P. Singh, D. Singh, S.P. Singh, S.K. Singhal, Efficient electro-oxidation of methanol using PtCo nanocatalysts supported reduced graphene oxide matrix as anode for DMFC, Int. J. Hydrogen Energy 42 (2017) 10238–10247. [24] H. Aghahosseini, A. Ramazani, K. S´lepokura, T. Lis, The first protection-free synthesis of magnetic bifunctional L-proline as a highly active and versatile artificial enzyme: synthesis of imidazole derivatives, J. Colloid Interface Sci. 511 (2018) 222–232. [25] Y.F. Chen, G.T. Fu, Y.Y. Li, Q.S. Gu, L. Xu, D.M. Sun, Y.W. Tang, L-Glutamic acid derived PtPd@Pt core/satellite nanoassemblies as an effectively cathodic electrocatalyst, J. Mater. Chem. A 5 (2017) 3774–3779. [26] N.I. Kovtyukhova, P.J. Ollivier, B.R. Martin, T.E. Mallouk, S.A. Chizhik, E.V. Buzaneva, A.D. Gorchinskiy, Layer-by-layer assembly of ultrathin composite films from micron-sized graphite oxide sheets and polycations, Chem. Mater. 11 (1999) 771–778. [27] Y.N. Wang, X.P. Duan, J.W. Zheng, H.Q. Lin, Y.Z. Yuan, H. Ariga, S. Takakusagi, K. Asakura, Remarkable enhancement of Cu catalyst activity in hydrogenation of dimethyl oxalate to ethylene glycol using gold, Catal. Sci. Technol. 2 (2012) 1637–1639. [28] H.F. Liang, C. Xia, Q. Jiang, A.N. Gandi, U. Schwingenschlögl, H.N. Alshareef, Low temperature synthesis of ternary metal phosphides using plasma for asymmetric supercapacitors, Nano Energy 35 (2017) 331–340. [29] H.Y. Liu, T. Kuila, N.H. Kim, B.C. Ku, J.H. Lee, In situ synthesis of the reduced graphene oxide-polyethyleneimine composite and its gas barrier properties, J. Mater. Chem. A 1 (2013) 3739–3746. [30] M.J. Chen, B.Y. Lou, Z.J. Ni, B. Xu, PtCo nanoparticles supported on expanded graphite as electrocatalyst for direct methanol fuel cell, Electrochim. Acta 165 (2015) 105–109. [31] T.Y. Xia, J.L. Liu, S.G. Wang, C. Wang, Y. Sun, R.M. Wang, Nanomagnetic CoPt truncated octahedrons: facile synthesis, superior electrocatalytic activity and stability for methanol oxidation, Sci. Chin. Mater. 60 (2017) 57–67. [32] B.Y. Xia, H.B. Wu, N. Li, Y. Yan, X.W. Lou, X. Wang, One-pot synthesis of Pt-Co alloy nanowire assemblies with tunable composition and enhanced electrocatalytic properties, Angew. Chem. Int. Ed. 54 (2015) 3797–3801. [33] Y.P. Xiao, S. Wan, X. Zhang, J.S. Hu, Z.D. Wei, L.J. Wan, Hanging Pt hollow nanocrystal assemblies on graphene resulting in an enhanced electrocatalyst, Chem. Commun. 48 (2012) 10331–10333.

[34] S.Y. Wang, F. Yang, S.P. Jiang, S.L. Chen, X. Wang, Tuning the electrocatalytic activity of Pt nanoparticles on carbon nanotubes via surface functionalization, Electrochem. Commun. 12 (2010) 1646–1649. [35] A.A. Ensafi, M. Jafari-Asl, B. Rezaei, M.M. Abarghoui, H. Farrokhpour, Facile synthesis of Pt-Pd@Silicon nanostructure as an advanced electrocatalyst for direct methanol fuel cells, J. Power Sources 282 (2015) 452–461. [36] F.F. Ren, H.W. Wang, C.Y. Zhai, M.S. Zhu, R.R. Yue, Y.K. Du, P. Yang, J.K. Xu, W.S. Lu, Clean method for the synthesis of reduced graphene oxide-supported PtPd alloys with high electrocatalytic activity for ethanol oxidation in alkaline medium, ACS Appl. Mater. Inter. 6 (2014) 3607–3614. [37] G.T. Fu, X. Jiang, R. Wu, S.H. Wei, D.M. Sun, Y.W. Tang, T.H. Lu, Y. Chen, Arginine-assisted synthesis and catalytic properties of single-crystalline palladium tetrapods, ACS Appl. Mater. Inter. 6 (2014) 22790–22795. [38] Y.J. Hu, P. Wu, H. Zhang, C.X. Cai, Synthesis of graphene-supported hollow PtNi nanocatalysts for highly active electrocatalysis toward the methanol oxidation reaction, Electrochim. Acta 85 (2012) 314–321. [39] D.L. Wang, H.L. Xin, Y.C. Yu, H.S. Wang, E. Rus, D.A. Muller, H.D. Abruña, Ptdecorated PdCo@Pd/C core-shell nanoparticles with enhanced stability and electrocatalytic activity for the oxygen reduction reaction, J. Am. Chem. Soc. 132 (2010) 17664–17666. [40] S.L. Lu, K. Eid, D.H. Ge, J. Guo, L. Wang, H.J. Wang, H.W. Gu, One-pot synthesis of PtRu nanodendrites as efficient catalysts for methanol oxidation reaction, Nanoscale 9 (2017) 1033–1039. [41] S.L. Lu, K. Eid, Y.Y. Deng, J. Guo, L. Wang, H.J. Wang, H.W. Gu, One-pot synthesis of PtIr tripods with a dendritic surface as an efficient catalyst for the oxygen reduction reaction, J. Mater. Chem. A 5 (2017) 9107–9112. [42] M. Oezaslan, F. Hasché, P. Strasser, Oxygen electroreduction on PtCo3, PtCo and Pt3Co alloy nanoparticles for alkaline and acidic PEM fuel cells, J. Electrochem. Soc. 159 (2012) B394–B405. [43] M. Oezaslan, F. Hasché, P. Strasser, PtCu3, PtCu and Pt3Cu alloy nanoparticle electrocatalysts for oxygen reduction reaction in alkaline and acidic media, J. Electrochem. Soc. 159 (2012) B444–B454. [44] Z. Xu, H.M. Zhang, H.X. Zhong, Q.H. Lu, Y.F. Wang, D.S. Su, Effect of particle size on the activity and durability of the Pt/C electrocatalyst for proton exchange membrane fuel cells, Appl. Catal. B Environ. 111–112 (2012) 264–270. [45] F. Li, L. Zhang, J. Li, X.Q. Lin, X.Z. Li, Y.Y. Fang, J.W. Huang, W.Z. Li, M. Tian, J. Jin, R. Li, Synthesis of Cu-MoS2/rGO hybrid as non-noble metal electrocatalysts for the hydrogen evolution reaction, J. Power Sources 292 (2015) 15–22. [46] Y.G. Li, H.L. Wang, L.M. Xie, Y.Y. Liang, G.S. Hong, H.J. Dai, MoS2 nanoparticles grown on graphene: an advanced catalyst for the hydrogen evolution reaction, J. Am. Chem. Soc. 133 (2011) 7296–7299. [47] S.P. Luo, M. Tang, P.K. Shen, S.Y. Ye, Atomic-scale preparation of octopod nanoframes with high-index facets as highly active and stable catalysts, Adv. Mater. 29 (2017) 1601687. [48] M. Chokai, M. Taniguchi, S. Moriya, K. Matsubayashi, T. Shinoda, Y. Nabae, S. Kuroki, T. Hayakawa, M.-A. Kakimoto, J.-I. Ozaki, S. Miyata, Preparation of carbon alloy catalysts for polymer electrolyte fuel cells from nitrogencontaining rigid-rod polymers, J. Power Sources 195 (2010) 5947–5951. [49] G.R. Xu, B. Wang, J.Y. Zhu, F.Y. Liu, Y. Chen, J.H. Zeng, J.X. Jiang, Z.H. Liu, Y.W. Tang, J.M. Lee, Morphological and interfacial control of platinum nanostructures for electrocatalytic oxygen reduction, ACS Catal. 6 (2016) 5260–5267. [50] J. Gu, G.X. Lan, Y.Y. Jiang, Y.S. Xu, W. Zhu, C.H. Jin, Y.W. Zhang, Shaped Pt-Ni nanocrystals with an ultrathin Pt-enriched shell derived from one-pot hydrothermal synthesis as active electrocatalysts for oxygen reduction, Nano Res. 8 (2015) 1480–1496. [51] J.H. Shim, J. Kim, C. Lee, Y. Lee, Porous Pd layer-coated Au nanoparticles supported on carbon: synthesis and electrocatalytic activity for oxygen reduction in acid media, Chem. Mater. 23 (2011) 4694–4700. [52] Z.K. Yang, Z.W. Zhao, K. Liang, X. Zhou, C.C. Shen, Y.N. Liu, X. Wang, A.W. Xu, Synthesis of nanoporous structured iron carbide/Fe-N-carbon composites for efficient oxygen reduction reaction in Zn-air batteries, J. Mater. Chem. A 4 (2016) 19037–19044. [53] Y.H. Su, Y.H. Zhu, H.L. Jiang, J.H. Shen, X.L. Yang, W.J. Zou, J.D. Chen, C.Z. Li, Cobalt nanoparticles embedded in N-doped carbon as an efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions, Nanoscale 6 (2014) 15080–15089. [54] G. Zhang, W.T. Lu, F.F. Cao, Z.D. Xiao, X.S. Zheng, N-doped graphene coupled with Co nanoparticles as an efficient electrocatalyst for oxygen reduction in alkaline media, J. Power Sources 302 (2016) 114–125.