Properties and electrochemical behaviors of AuPt alloys prepared by direct-current electrodeposition for lithium air batteries

Electrochimica Acta 151 (2015) 415–422

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Properties and electrochemical behaviors of AuPt alloys prepared by direct-current electrodeposition for lithium air batteries Jinqiu Zhang a,b, *, Da Li a , Yiming Zhu a , Miaomiao Chen a , Maozhong An a,b , Peixia Yang a , Peng Wang b a

School of Chemical Engineering Technology, Harbin Institute of Technology, Harbin 150001, China School of Municipal and Environmental Engineering, State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 8 August 2014 Received in revised form 6 November 2014 Accepted 6 November 2014 Available online 11 November 2014

AuPt catalyst has a prospective application in a lithium air battery because of its bi-function on catalyzing Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER). Electrodeposition is an in-situ convenient technology for catalyst preparation without chemical residue. In an acid electrolyte, AuPt alloy catalysts were electrodeposited on carbon paper. The effect of main salt concentration, electrodeposition time and current density were studied by deposit micromorphology observation, structure analyses and composition testing. Catalytic abilities of AuPt alloys were measured by cyclic voltammetry (CV) in an ionic liquid of EMI-TFSI/Li-TFSI [1- Ethyl - 3- methylimidazolium–bis (trifluoromethanesulphonyl) imide/lithium–bis (trifluoromethanesulphonyl) imide]. The electrochemical behaviors of Au, Pt and AuPt deposits were also measured. An optimized direct-current electrodeposition process of getting high active AuPt catalyst is concluded, which is an aqueous solution containing 6.7
10 m mol  L1 HAuCl4, 10
13.3 mmol  L1 H2PtCl6 and 0.5 mol  L1 H2SO4 as the electrolyte, current density of 20mA  cm2 and electrodeposition time of 8
34 s. The co-deposition of AuPt alloy is an irregular co-deposition controlled by diffusion, while gold atoms enter the platinum’s crystal lattice in the structure of AuPt alloy. The increase of the concentration of H2PtCl6 in the electrolyte, the extension of the electrodeposition time or the raise of the current density can improve the content of Pt in the deposit. The clusters’ diameters of AuPt catalysts decrease to 150
250 nm by adjusting current densities during electrodeposition. ã 2014 Elsevier Ltd. All rights reserved.

Keywords: gold-platinum alloy lithium air battery oxygen electrode ionic liquid electrolyte linear sweep voltammetry

1. Introduction As electrical energy storage devices, lithium air batteries have received significant interest based on their higher theoretical specific energy density than that of all other battery types [1]. In lithium air batteries, there are many possible reactions involving lithium and air depending on the chemical environment. On the side of the oxygen electrode of non-aqueous lithium air batteries, Oxygen Reduction Reactions (ORR) and Oxygen Evolution Reactions (OER) take place during discharge process and charge process as followed equations shown [2]. O2 þ L iþ þ e ! Li O2 ð3V vs: L i=L iþ Þ

* Corresponding address. E-mail address: [email protected] (J. Zhang). http://dx.doi.org/10.1016/j.electacta.2014.11.038 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.

(1)

2LiO2 ! Li2O2 + O2

(2)

Li O2 þ Liþ þ e ! Li2 O2 ð3:1 V vs: Li=Liþ Þ

(3)

Li2O2 ! 2Li+ + 2e + O2

(4)

In the process of OER, the ORR product Li2O2 has a strong ionic bond that is hard to be reduced and decomposed if without using catalyst, and the product causes an actual charging voltage up to 4.2 V [3]. If byproducts, such as Li2CO3, are generated during the discharge, the charging voltage can be up to 4.5 V [4]. Therefore, catalysts are needed to accelerate the ORR and OER reactions in order to reduce the voltage gap between the charge and discharge, i.e., to increase the round-trip efficiency (the ratio of discharge to charge voltage) and improve the cycle performance [5]. Great effort has been devoted to study novel electrocatalysts. Metal oxides (such as MnO2 [6] and CaMnO3 [7]), carbon matrix

416

J. Zhang et al. / Electrochimica Acta 151 (2015) 415–422

0.4

0.2

S

0.2

-0.08V

b)

0.0

-0.4

M

N

0.66V

0.21V

-0.6 -0.8

H

a'

-0.6

N

b'

-0.8

a') 100mV·S-1 b') 250mV·S-1 c') 500mV·S-1 d') 750mV·S-1

c'

-1.0

d'

H

-1.2

a) Au b) Pt c) AuPt

-0.16V

-1.4

-1.2 -1.4 -0.4

M

-0.4

-0.2

-1.0

a)

-0.2

a)

Current / mA

Current / mA

0.0

c)

-1.6

-0.2

0.0

0.2

0.4

0.6

0.8

-1.8 -0.4

1.0

-0.2

0.0

Potential / V vs. SCE

0.4

0.6

0.8

1.2

Fig. 1. CV curves of Au, Pt and AuPt alloy electrodeposited from an electrolyte containing a) 10 m mol  L1 HAuCl4 and 0.5 mol  L1 H2SO4, b) 10 m mol  L1 H2PtCl6 and 0.5 mol  L1 H2SO4 and c) 10 m mol  L1 HAuCl4, 10 mmol  L1 H2PtCl6 and 0.5 mol  L1 H2SO4, respectively. The scan rate is 50mV  s1.

b) Peak M Peak N Peak H

1.0

0.8

Ip / mA

and graphene (such as mesoporous carbon [8] and diamond like carbon [9]), precious metals alloys (such as PtAu [10–12] and Pt-Ir [13]) and their composites (such as La0.5Ce0.5Fe0.5Mn0.5O3/ graphene [14] and a-MnO2/Pd[15]) have been used in lithium air batteries. It is reported that Pt0.5Au0.5 nanoparticles were reduced by chemical synthesis method and exhibited ORR at 2.7 V and OER at 3.6 V when the current density was 100mA  g 1, which had a prospective application in the lithium air battery with a round trip efficiency of 77% [10]. This chemical impregnation-reduction method is popularly used in the field of heterogeneous catalysis to prepare kinds of nanostructured catalysts in size of several nanometers with excellent catalytic properties [11,16]. However, it takes several to more than ten hours to complete the whole synthesis process including reduction reaction, particles powder drying, removal of surfactant contamination, etc [13,17]. Also, a binder, such as PVDF (Polyvinylidene Fluoride) or PTFE (Polytetrafluoroethylene), is needed to combine cathode material and catalyst’s powder with current collector [16,18]. Some binders can be broken by the large internal strain in electrodes during charge/ discharge cycling leading to electrode impedance growth and ultimately to cell failure [19]. Compared with chemical synthesis method, electrochemical method is an in-situ convenient and reproducible technology for catalyst preparation without chemical residue, complex operating conditions or expensive instruments, which is of interest for industrial development. Furthermore, atomic composition, loading mass, crystal surface and particle size of the catalysts can be controlled easily by adjusting the electrodeposition parameters [17]. For using in fuel cells, Pt-based catalysts, such as Pt, PtCo, PtNi, etc, have been successfully prepared by electrochemical method [20–22]. However, the preparation process and properties of electrodeposited AuPt particles using in lithium air batteries are not clear enough and need to be investigated [23]. Carbon powder materials, such as super P carbon, Vulcan carbon or ordered mesoporous carbon, are usually used as cathode materials of lithium air batteries, because they have high surface areas and large pore volumes for Li2O2 deposition during discharge process [24]. Carbon paper is a kind of commercial carbon fiber having uniform porosity and morphology, which can be used as a gas diffusion layer by coating slurry containing carbon powder and

0.2

Potential / V vs. SCE

0.6

0.4

0.2

0.0

10

15

20

25

30

υ1/2 / mV1/2 ·s-1/2

Fig. 2. a) CV curves of AuPt alloys electrodeposited in an electrolyte containing 10 m mol  L1 HAuCl4, 10 mmol  L1 H2PtCl6 and 0.5 mol  L1 H2SO4 at various scan rate of a0 ) 100 mV  s1, b0 ) 250mV  s1, c0 ) 500 mV  s1 and d0 ) 750 mV  s1. b) The linear relationship between Ip and v1/2, constructed from peak M(&), peak N(*) and peak H(~).

catalyst. However, in an actual experiment, the slurry is hard to be coated equably on the gas diffusion layer, which will affect the properties of the whole gas electrode. In order to compare the properties of catalyst without the influence factor of carbon powder material, AuPt alloy catalysts were electrodeposited directly on the carbon paper without carbon power coating on it in this paper. The carbon paper is not as good as carbon powder for providing reaction interfaces during catalytic process, but it is convenient for electrodeposition and comparison of catalysts. In the future work of assembling lithium air battery, carbon power will be coated on carbon paper at first, then AuPt catalysts will be deposited. In this paper, AuPt alloy catalysts were electrodeposited in acid electrolytes by direct current. The effect of electrolyte compositions and electrodeposition processes were studied. Electrochemical measurements were carried on to determine the electrodeposition behavior and to evaluate the catalytic abilities of the AuPt catalysts.

J. Zhang et al. / Electrochimica Acta 151 (2015) 415–422

417

Fig. 3. SEM images of a) Au catalyst electrodeposited in an electrolyte containing 20 mmol  L1 HAuCl4 and 0.5 mol  L1 H2SO4 and b) Pt catalyst electrodeposited in an electrolyte containing 20 mmol  L1 H2PtCl6 and 0.5 mol  L1 H2SO4 at 20 mA  cm2 for 17 s. c) CV curves of a0 ) Au/C and b0 ) Pt/C oxygen electrodes in oxygen-saturated electrolyte of EMI-TFSI mixing Li-TFSI at mass ratio of 1 to 0.4.

water and deionized water in sequence. After electrodeposition, the AuPt/C oxygen electrode was washed by cold water, deionized water and ultra-pure water orderly, then dried by hot-air.

2. Experimental 2.1. Electrodeposition of AuPt alloys Au, Pt or AuPt alloy were electrodeposited on carbon paper (15mm  15mm  0.3 mm, Toray Industries, Inc.) in electrolytes of 25  C containing HAuCl4 of 0
20 m mol  L1, H2PtCl6 of 0
20 mmol  L1 and H2SO4 of 0.5 mol  L1 to prepare Au/C, Pt/ C and AuPt/C oxygen electrodes. In the electrolytes, the total concentrations of HAuCl4 and H2PtCl6 were 20 mmol  L1, while their ratios were varied for studying the effect of main salts. The anode was a piece of graphite plate. Without special description, electrodeposition experiments were conducted at current densities of 10mA  cm2 and electrodeposition time of 17 s in different electrolytes. The loading mass of the AuPt catalyst on the carbon paper is around 0.2mg  cm2. Constant current density of 4mA  cm2 was carried on for 30 min to electrodeposit AuPt on a copper sheet for structure measurement. Before electrodeposition, the carbon paper was degreased in an aqueous solution containing 10g  L1 NaOH, 20g  L1 Na2CO3 and 20g  L1 Na2SiO3, alkalined washed in 0.2mol  L1 NaOH aqueous solution, and acid pickled in 0.2mol  L1 HCl aqueous solution step by step. After each step, the carbon paper was washed by cold

2.2. Property measurements of AuPt deposits A Scanning Electron Microscope (SEM, Quanta 200FEG, FEI Co.) was employed to observe the micromorphologies of Au, Pt and AuPt alloy. The compositions of the AuPt particles were measured by use of Energy Disperse Spectroscopy (EDS, Quanta 200FEG, FEI Co.). The structure of AuPt alloy was analyzed by X-ray diffraction (XRD, PANalytical B.V.) pattern. The activities of AuPt catalysts on ORR and OER were measured by cyclic voltammetry (CV) on a CHI750D electrochemical workstation at the scan rate of 10mV  s1 in an oxygen-saturated ionic liquid electrolyte composed by EMI-TFSI [1Ethyl - 3- methylimidazolium–bis (trifluoromethanesulphonyl) imide] and Li-TFSI [lithium–bis (trifluoromethanesulphonyl) imide] at mass ratio of 1 to 0.4. The work electrodes were Au/C, Pt/C and AuPt/C oxygen electrodes. The counter electrode was a platinum wire. The reference electrode was a Ag wire whose potential is 2.867 V vs. Li/Li+, which was measured experimentally by submerging the Ag wire into EMI-TFSI/LiTFSI ionic liquid electrolyte containing a clean lithium electrode and monitoring the potential between the Ag wire and the lithium foil. 2.3. Electrochemical evaluations

Cu

CV, linear sweep voltammetry (LSV) and constant potential electrodeposition experiments (for 1 min) in AuPt electrolytes of 25  1 C were carried out in a three-electrode glass cell on a CHI750D electrochemical workstation. A saturated calomel electrode (SCE) and a graphite rod were used as the reference electrode and the counter electrode, respectively. The working electrode was an L-shape glassy carbon electrode (GCE) with a working surface of 7.1 mm2 (f =3 mm) in CV and LSV measurements. Graphite sheets (10mm  5mm  0.1 mm) were used as

Intensity / a. u.

Cu

(111) Cu

(200)

AuPt 10

20

30

40

50

(220)

60

70

(311) 80

90

Pt 04-0802 Au 04-0784 2θ/° Fig. 4. XRD pattern of AuPt coating electrodeposited at 4 mA  cm2 for 30 min from the electrolyte containing 10mmol  L1 HAuCl4, 10mmol  L1 H2PtCl6 and 0.5mol  L1 H2SO4.

Table 1 The contents of Au and Pt in the AuPt alloy electrodeposited at different constant potentials in the electrolyte containing 10 m mol  L1 HAuCl4, 10 mmol  L1 H2PtCl6 and 0.5 mol  L1 H2SO4. Potential/V vs. SCE

0.7

0.6

0.4

0.2

0.1

0.2

Au/at% Pt/at%

100 0

100 0

100 0

90.49 9.51

68.37 31.63

57.36 42.64

418

J. Zhang et al. / Electrochimica Acta 151 (2015) 415–422

Fig. 5. SEM images (a
e) of AuPt catalysts electrodeposited at 10mA  cm2 for 17 s and LSV curves (f) of AuPt alloy electrodeposited in electrolytes containing 20 mmol  L1 HAuCl4 and H2PtCl6 totally at various ratio of a, a0 ) 3:1, b, b0 ) 2:1, c, c0 ) 1:1, d, d0 ) 1:2 and e, e0 ) 1:3 and 0.5 mol  L1 H2SO4.

3. Results and Discussion 3.1. Electrodeposition behaviors of Au, Pt and AuPt alloy In order to understand the electrodeposition process of AuPt alloy, the CV curves of Au, Pt and AuPt alloy were studied. The values of standard electrode potentials (vs. SHE, Standard Hydrogen Electrode) of Au and Pt are given in the followed. AuCl4 + 3e $ Au(s) + 4Cl

u

f = +0.93 V

(5)

PtCl62 + 2e $ PtCl42 + 2Cl fu = +0.726 V

(6)

PtCl42 + 2e $ Pt(s) + 4Cl fu = +0.758 V

(7)

In the acid electrolytes, the electrodeposition potential of Au is more positive than that of Pt, which can be seen from Fig. 1a and Fig. 1b. The deposition peak potential of Au is 0.66 V (peak M), while that of Pt is -0.16 V (peak H). Because Au and Pt are not dissolved during the anodic period, there are no dissolution peak on their anodic curves. However, there is a peak S at -0.08 V during the anodic period in Fig. 1b. Because the surface of the GCE was covered by Pt coating in the CV measurement, the actual working electrode was Pt. Thus, adsorption and desorption of hydrogen atoms can take place on the working electrode. The weak desorption of hydrogen occurs on a Pt electrode in the range of 0
0.2 V vs. SHE (or -0.24
-0.04 V vs. SCE) in 0.5 mol  L1 H2SO4 of 25  C, so, peak S is thought as a desorption peak of hydrogen atoms. In Fig. 1c, there are three peaks at about 0.66 V, 0.21 V and -0.16 V, respectively. In order to identify these peaks, constant potential electrodeposition of AuPt alloys were carried on, and AuPt alloys' contents are shown in Table 1. The peak M in Fig. 1c corresponds to the deposition of Au, because there is no Pt existing in the coatings electrodeposited at 0.7 V, 0.6 V and 0.4 V. At about 0.3 V, the cathodic current of curve c increases, and Pt element is also determined existing in the deposit. With the deposit potentials moving to a negative direction, the atomic percentages of Pt

increase, which show that peak N and peak H in Fig. 1c are caused by the co-deposition Au and Pt. The rate controlling step of AuPt deposit was also studied by CV curves. It can be seen in Fig. 2a that an increase in the scan rate shifts the reduction peaks (peak M, peak N and peak H) of CV to a more negative potential. The higher current contribution means that the electrodeposition of AuPt in the electrolyte containing HAuCl4, H2PtCl6 and H2SO4 is irreversible. When the electrode reaction is controlled by diffusion processed, the relationship between the current intensity of the peak and the scan rate of CV can be described as: Ip = 0.4958 (nF)3/2 (aDv)1/2 (RT)1/2 Ac*

(8)

Where Ip is the current intensity of the peak, v is the potential scan rate, n is the number of electrons, F is the Faraday constant, a is the charge transfer coefficient, D is the diffusion coefficient, R is the gas constant, T is the temperature, A is the surface area of the working electrode, and c* is the bulk concentration of the reducible ions [25].

0.2

Current/mA

working electrodes in constant potential electrodepositions. Without special description, the scan rates of CV and LSV were 50mV  s1 and 1mV  s1, respectively.

0.0

-0.2 e) 1:3 -0.4

a) 3:1 b) 2:1

-1.5

-1.0

d) 1:2 c) 1:1 -0.5

0.0

0.5

1.0

1.5

Potential / V vs. Ag Fig. 6. CV curves of AuPt/C oxygen electrode in oxygen-saturated electrolyte of EMI-TFSI mixing Li-TFSI at mass ratio of 1 to 0.4. The AuPt catlysts were electrodeposited on carbon paper at 10 mA  cm2 for 17 s in electrolytes containing 20 mmol  L1 HAuCl4 and H2PtCl6 totally at various ratio of a) 3:1, b) 2:1, c) 1:1, d) 1:2 and e) 1:3 and 0.5 mol  L1 H2SO4.

J. Zhang et al. / Electrochimica Acta 151 (2015) 415–422 Table 2 The contents of Au and Pt in the AuPt alloy electrodeposited from the electrolytes containing various main salts concentrations. [HAuCl4]:[H2PtCl6]

3:1

2:1

1:1

1:2

1:3

xAuin electrolyte/% Au/at% Pt/at%

75.0 84.19 15.81

66.7 75.44 24.56

50.0 58.54 41.46

33.3 42.94 57.06

25.0 38.02 61.98

There is a linear relationship between Ip and v1/2 in Eq. (8). As shown in Fig. 2a, peak M is the deposition peak of Au, while peak N and peak H are the co-deposition peaks of AuPt alloy. Moreover, Fig. 2b indicates that the deposition of Au and the co-deposition of AuPt alloy are both irreversible diffusion controlled processes [26]. 3.2. Micromorphologies and catalytic activities of Au and Pt deposits Au and Pt catalysts were loaded on carbon paper at first to compare with AuPt alloy. Their morphologies are shown in Fig. 3a and Fig. 3b. The Au crystallizations are clusters in diameter of 0.5
1 mm, while the Pt grains are long rice-like stripes in length of 0.3
0.5 mm. The catalytic abilities of Au and Pt were measured by CV shown in Fig. 3c. During the forward scan towards to the negative potentials, there are cathodic peaks of ORR at about -0.1 V (Ga1) and -0.4 V (Ga2) by the catalysis of Au, and at about -0.2 V (Pa1) by the catalysis of Pt, respectively. The ORR by Au occurs at a relatively more positive potential, which shows that the ORR is easier to be catalyzed by Au. The current of peak Ga2 is higher than that of peak Pa1, which indicates that the Au catalyst has a bigger active specific surface area causing a higher reaction current. During the forward scan towards

419

to the positive potentials, the current of anodic peak of Pt (Pc1) is higher than that of anodic peak of Au (Gc1), which shows that Pt has a better catalytic activity on OER [10]. 3.3. Effect of main salts concentrations on AuPt catalysts AuPt catalysts were electrodeposited in the electrolytes containing HAuCl4 and H2PtCl6 at different concentration ratio. The contents of Au and Pt in the deposits are listed in Table 2. Compared with Pt, Au is easier to be deposited because the reductive potential of Au is more positive. The atomic percentages of Au in the AuPt deposits are all a little higher than that in the electrolytes, however do not increase sharply with the increasing of golden salts concentration, which states that the co-deposition type of the AuPt alloy is an irregular co-deposition of normal codeposition [27,28]. As seen from Fig. 4, the peaks at about 40 , 46 , 67 and 81 are AuPt alloy peaks [29], which are similar with peaks of Pt, not Au. The interplanar distance of AuPt (111) is 2.2742 Å, which is bigger than 2.2650 Å of Pt (111) (PDF card 04–0802). The diameter of gold atom is 1.44 Å that is larger than 1.39 Å of platinum atom. The increase of the interplanar distance of AuPt alloy demonstrates that gold atoms enter the platinum’s crystal lattice. This structure is uniform and different with the AuPt particles prepared by CV scanning [30]. Because gold ions are reduced before platinum ions during the CV scanning, a gold core forms at first, and then Au and Pt atoms are deposited on the surface of this Au core to form AuPt bimetallic clusters. In the case of the direct-current electrodeposition, Au and Pt atoms are reduced at the same time and co-deposited homogeneously on the substrate. AuPt (111) belongs to common stable

Fig. 7. SEM images of AuPt catalysts electrodeposited at 10mA  cm2 in electrolytes containing 6.67 m mol  L1 HAuCl4, 13.3 mmol  L1 H2PtCl6 and 0.5 mol  L1 H2SO4 for various time of a) 4 s, b) 8 s, c) 34 s and d) 68 s.

420

J. Zhang et al. / Electrochimica Acta 151 (2015) 415–422

Table 3 The contents of Au and Pt in the AuPt alloy electrodeposited for various time. Time/s

4

8

17*

34

68

Au/at% Pt/at%

47.16 52.84

42.99 57.01

42.94 57.06

41.88 58.12

38.16 61.61

*

Table 4 The contents of Au and Pt in the AuPt alloy electrodeposited at different current densities. Current density/mA  cm2

2.5

4.5

10*

20

40

Au/at% Pt/at%

47.16 52.84

42.99 57.01

42.94 57.06

41.88 58.12

38.16 61.84

The sample is same with that in the fourth column of Table 2. *

planes whose catalytic activities are less than those of high-index planes. One of the merits of catalysts prepared by electrochemical method is that the crystal surface and particle size can be reproduced. The AuPt particles prepared by the direct-current electrodeposition could be further treated by a square-wave potential to get high-index planes and smaller particles as Tian's research reported [20]. The effect of main salts concentrations on the morphologies of AuPt catalysts can be seen from Fig. 5. When the concentration ratio of gold salt to platinum salt is 3:1, the content of Au in the deposit is relatively higher at about 84at%. In that case, the gold atoms cannot entirely enter the platinum's crystal lattice causing the micromorphology of the AuPt deposit (Fig. 5a) similar with the Au deposit (Fig. 3a). When the concentration ratio of gold salt to platinum salt is 2:1, the morphology of AuPt catalyst become flat and round in range of 300
500 nm despite of newborn small grains on the surface of carbon fiber (Fig. 5b). As seen in Fig. 5c, when the concentrations of gold salt and platinum salt are equal, the AuPt particles are pancake-like clusters range from 200 nm to 500 nm combined with dozens of grains. With the further increase of the platinum salt concentration ratio, the clusters are spherical with the diameters decreasing to about 150
250 nm as shown in Fig. 5d and Fig. 5e. Obviously, the concentration ratio of the main salts influent the electrochemical crystallization of AuPt particles. As shown in Fig. 5f, when the current density is 4 mA  cm2, the corresponding potentials of curves move to the negative direction with the increasing of H2PtCl6 in the electrolyte, which means the alloyco-depositions become difficult and the crystallization prefer to form new grains, rather than grow existing grains. So, the grain sizes of AuPt particles decrease with the ratios raise of platinum salts. According to the LSV curves and the composition analysis of constant potential deposition, the co-deposition process of AuPt alloy can be divided to four stages. Stage I is pre-deposition process, where potential is in range of OCP (open circuit potential) to 0.75 V. There is no any deposition taking place, so the current density is almost near zero. Stage II is the deposition of Au, where potential range is about 0.75
0.3 V. Stage III is the co-deposition of AuPt alloy, where

The sample is same with that in the fourth column of Table 2.

potential is from about 0.3 V to -0.25 V. Stage IV is the co-deposition of AuPt alloy combined with hydrogen evolution, where potential is from -0.25 V to more negative. Hydrogen bubbles can be seen attaching on the cathode when the current density is over 6 mA  cm2 during the actual deposition experiments. There is a need to explain that when a piece of carbon paper is used as cathode, the actual current density is not as high as that applied according to its apparent area because the porous carbon paper has a bigger real area. The currents of the reductive peaks at about -0.6 V during the forward scan towards to the negative potentials in Fig. 6 are used to evaluate the catalytic activities of the AuPt catalysts [31,32]. The peak currents of curve b, c and d are obviously higher than that of curve a and e, which probably are caused by the cooperative effect of AuPt alloy. When the ratio of Au to Pt in the deposit is closer to 1:1, the catalytic cooperative effect is stronger [10]. By comprehensive comparison on the properties of the AuPt catalysts electrodeposited in the electrolytes with different main salts concentrations, when the concentration ratio of HAuCl4 and H2PtCl6 is in range of 1:1 to 1:2, the AuPt catalysts are proper to illustrate good catalytic activities. 3.4. Effect of electrodeposition time on AuPt catalysts AuPt catalysts were electrodeposited with various time. As seen from Table 3, the electrodeposition time has little effect on the alloy contents in range of 8
34 s. When the electrodeposition time is 4 s, Au is preferably deposit in such a short time, so the content of Au is the highest. After a long time electrodeposition, the consumption of HAuCl4 is larger than that of H2PtCl6, however HAuCl4 can’t be supplied in time, so the percentages of Pt in the deposits increase and the ratios of Au to Pt are closer to their ratios in the electrolytes. The micromorphologies of these AuPt catalysts are shown in Fig. 7 and Fig. 5d. The grain sizes are about 300 nm without obvious difference with the extension of the electrodeposition time. Thus, the optimized electrodeposition time is in range of 8
34 s. Obviously, the preparation time of AuPt catalysts by direct-current

Fig. 8. SEM images (a-d) of AuPt catalysts electrodeposited for 17 s in electrolytes containing 6.67 m mol  L1 HAuCl4, 13.3 mmol  L1 H2PtCl6 and 0.5 mol  L1 H2SO4 at different current densities of a) 2.5mA  cm2, b) 4.5mA  cm2, c) 20mA  cm2 and d) 40mA  cm2, and CV curves (e) of these AuPt/C oxygen electrodes in oxygen-saturated electrolyte of EMI-TFSI mixing Li-TFSI at mass ratio of 1 to 0.4.

J. Zhang et al. / Electrochimica Acta 151 (2015) 415–422

421

electrodeposition is much less than that by chemical synthesis methods [13,17], which is benefit to the industry produce.

Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (To Dr. Jinqiu Zhang).

3.5. Effect of current density on AuPt catalysts

References

The effect of current density on the contents of AuPt alloy can be seen from Table 4. With the increase of the current density, the contents of Pt in the catalysts also rise. Two reasons accelerate Pt depositing more on the carbon paper. One is that the consumption rate of HAuCl4 is faster at a higher current density and HAuCl4 can't be supplied soon, then Pt could be deposited more accordingly. The other is that the deposition of Pt is easier at a higher current density because of the rising polarization potential. The current density also has obvious influence on the micromorphologies of AuPt catalysts. By comparing Fig. 8a and Fig. 8b, it can be seen that the bigger AuPt particles are round in sizes of 300
450 nm despite their aggregation, however the amount of the smaller particles in diameter of around 80 nm rises with the increasing of the current density. With the further increase of the current density to 20mA  cm2, the AuPt particles become more rough. Their clusters are in range of 150
250 nm, which are composed by grains in size of 50
100 nm as seen in Fig. 8c. When the current density is 40mA  cm2, the aggregation of grains is more obviously, and the size of the clusters increases to 300 nm as shown in Fig. 8d. The catalytic activities have relationship with the contents and the morphologies of the catalysts. Therefore, the AuPt catalyst electrodepositied at 20mA  cm2 with proper compositions, small grain size and big roughness has a relatively highest catalytic activity among the samples, as seen in Fig. 8e.

[1] Z. Huang, B. Chi, J. Pu, J. Li, New development of key materials for highperformance lithium-air batteries, Prog. Chem. 25 (2013) 260. [2] T. Ogasawara, A. Débart, M. Holzapfel, P. Novák, P.G. Bruce, Rechargeable Li2O2 electrode for lithium batteries, J. Am. Chem. Soc. 128 (2006) 1390. [3] J.J. Xu, D. Xu, Z.L. Wang, H.G. Wang, L.L. Zhang, X.B. Zhang, Synthesis of perovskite-based porous La0.75Sr0.25MnO3 nanotubes as a highly efficient electrocatalyst for rechargeable lithium oxygen batteries, Angew. Chem. Int. Ed. 52 (2013) 3887. [4] Y.S. Mekonnen, K.B. Knudsen, J.S.G. Mýrdal, R. Younesi, J. Højberg, J. Hjelm, P. Norby, T. Vegge, Communication: The influence of CO2 poisoning on overvoltages and discharge capacity in non-aqueous Li-Air batteries, J. Chem. Phys. 140 (2014) 121101. [5] M.K. Song, S. Park, F.M. Alamgir, J. Cho, M. Liu, Nanostructured electrodes for lithium-ion and lithium-air batteries: The latest developments, challenges, and perspectives, Mater. Sci. Eng. R. 72 (2011) 203. [6] Y. Cao, Z.K. Wei, J. He, J. Zang, Q. Zhang, M.S. Zheng, Q.F. Dong, a-MnO2 nanorods grown in situ on graphene as catalysts for Li-O2 batteries with excellent electrochemical performance, Energy Environ. Sci. 5 (2012) 9765. [7] X. Han, Y. Hu, J. Yang, F. Cheng, J. chen, Porous perovskite CaMnO3 as an electrocatalyst for rechargeable Li-O2 batteries, Chem. Commun. 50 (2014) 1497. [8] Z. Guo, D. Zhou, X. Dong, Z. Qiu, Y. Wang, Y. Xia, Ordered hierarchical mesoporous/macroporous carbon: A high-performance catalyst for rechargeable Li-O2 batteries, Adv. Mater. 25 (2013) 5668. [9] Y. Yang, Q. Sun, Y.S. Li, H. Li, Z.W. Fu, Nanostructured diamond like carbon thin film electrodes for lithium air batteries, J. Electrochem. Soc. 158 (2011) B1211. [10] Y.C. Lu, Z. Xu, H.A. Gasteiger, S. Chen, K. Hamad-Schifferli, S.H. Yang, Platinumgold nanoparticles: A highly active bifunctional electrocatalyst for rechargeable lithium-air batteries, J. Am. Chem. Soc. 132 (2010) 12170. [11] J. Yin, B. Fang, J. Luo, B. Wanjala, D. Mott, R. Loukrakpam, M.S. Ng, Z. Li, J. Hong, M.S. Whittingham, C.J. Zhong, Nanoscale alloying effect of gold-platinum nanoparticles as cathode catalysts on the performance of a rechargeable lithium-oxygen battery, Nanotechnology 23 (2012) 305404. [12] C. Terashima, Y. Iwai, S.P. Cho, T. Ueno, N. Zettsu, N. Saito, O. Takai, Solution plasma sputtering processes for the synthesis of PtAu/C catalysts for Li-air batteries, Int. J. Electrochem. Sci 8 (2013) 5407. [13] F.S. Ke, B.C. Solomon, S.G. Ma, X.D. Zhou, Metal-carbon nanocomposites as the oxygen electrode for rechargeable lithium-air batteries, Electrochimica Acta 85 (2012) 444. [14] L. Wang, M. Ara, K. Wadumesthrige, S. Salley, K.Y.S. Ng, Graphene nanosheet supported bifunctional catalyst for high cycle life Li-air batteries, J. Power Sources 234 (2013) 8. [15] A.K. Thapa, Y. Hidaka, H. Hagiwara, S. Ida, T. Ishihara, Mesoporous b-MnO2 air electrode modified with Pd for rechargeability in lithium-air battery, J. Electrochem. Soc. 158 (2011) A1483. [16] Z. Guo, G. Zhu, Z. Qiu, Y. Wang, Y. Xia, High performance Li-O2 battery using g-MnOOH nanorods as a catalyst in an ionic-liquid based electrolyte, Electrochem. Commun. 25 (2012) 26. Leger, L. Dubau, S. Rousseau, F. Vigier, [17] C. Coutanceau, S. Brimaud, C. Lamy, J.-M.  Review of different methods for developing nanoelectrocatalysts for the oxidation of organic compounds, Electrochim. Acta 53 (2008) 6865. [18] B. Sun, H. Liu, P. Munroe, H. Ahn, G. Wang, Nanocomposites of CoO and a mesoporous carbon (CMK-3) as a high performance cathode catalyst for lithium-oxygen batteries, Nano Res. 5 (2012) 460. [19] Z. Chen, L. Christensen, J.R. Dahn, Comparison of PVDF and PVDF-TFE-P as binders for electrode materials showing large volume changes in lithium-ion batteries, J. Electrochem. Soc. 150 (2003) A1073. [20] N. Tian, Z.Y. Zhou, S.G. Sun, Y. Ding, Z.L. Wang, Synthesis of tetrahexahedral platinum nanocrystals with high-index facets and high electro-oxidation activity, Science 316 (2007) 732. [21] S. Woo, I. Kim, J.K. Lee, S. Bong, J. Lee, H. Kim, Preparation of cost-effective Pt-Co electrodes by pulse electrodeposition for PEMFC electrocatalysts, Electrochim. Acta 56 (2011) 3036. [22] Y. Zhao, Y.E.L. Fan, Y. Qiu, S. Yang, A new route for the electrodeposition of platinum-nickel alloy nanoparticles on multi-walled carbon nanotubes, Electrochim. Acta 52 (2007) 5873. [23] J.Q. Zhang, G.L. Chen, M.Z. An, P. Wang, Preparation of PtAu catalytic particles on positive electrode of Li/air battery using pulse electroplating, Int. J. Electrochem. Sci. 7 (2012) 11957. [24] J.B. Park, J. Lee, C.S. Yoon, Y.K. Sun, Ordered mesoporous carbon electrodes for Li-O2 batteries, ACS Appl. Mater Inter. 5 (2013) 13426. [25] A.J. Bard, L.R. Faulkner, Electrochemical Methods Fundamentals and Applications (second version in Chinese), Chemical Industry Press, Beijing, 2005. [26] Z. Chen, M.L. Zhang, W. Han, Z.Y. Hou, Y.D. Yan, Electrodeposition of Li and electrochemical formation of Mg-Li alloys from the eutectic LiCl-KCl, J. Alloy. Comp. 464 (2008) 174. [27] A. Brenner, Electrodeposition of alloys; principles and practice, Academic, New York, 1963.

4. Conclusions In an acid electrolyte, AuPt catalysts were electrodeposited on carbon paper successfully in order to prepare AuPt/C oxygen electrodes for lithium air batteries. When the electrolyte contains 6.7
10 m mol  L1 HAuCl4, 10
13.3 mmol  L1 H2PtCl6 and 0.5 mol  L1 H2SO4, the electrodeposition time is in range of 8
34 s, and the direct current density is 20mA  cm2, AuxPty (x = 40
60, y = 100-x) alloy particles with high catalytic abilities on ORR and OER in EMI-TFSI/Li-TFSI electrolyte can be electrodeposited. The co-deposition of AuPt alloy is an irregular co-deposition controlled by diffusion, while gold atoms enter the platinum's crystal lattice causing the interplanar distance of AuPt (111) bigger than that of Pt (111). Au is easier to be electrodeposited because the electrodeposition potential of Au is more positive than that of Pt. The increase of the concentration of H2PtCl6 in the electrolyte, the extension of the electrodeposition time or the rise of the current density can improve the content of Pt in the deposit. The cluster sizes of AuPt catalysts decrease to 150
250 nm at a proper current density. The AuPt alloy’s composition and morphology that can be influenced by the electrodeposition process have effects on the catalytic ability. Because there exist aggregations of AuPt catalysts deposited in such an electrolyte containing simple salts, the future work will focus on controllable electrodeposition of non-aggregate nano scale AuPt catalysts with stable and proper contents by adding proper additives and using special power supply mode, and discuss the effect of catalysts' diameters, contents and surfaces on their catalytic properties. Acknowledgments Thanks for the supports by National Natural Science Foundation of China (51304056), China Postdoctoral Science Foundation (2013M531049), Fundamental Research Funds for the Central Universities” (HIT. NSRIF. 2011021) and Scientific Research

422

J. Zhang et al. / Electrochimica Acta 151 (2015) 415–422

[28] V.D. Jovi c, B.M. Jovi c, M.G. Pavlovi c, V. Maksimovi c, Morphology and composition of Ni-Co alloy powders electrodeposited from ammoniacal electrolyte, J. Solid State Electrochem. 10 (2006) 959. [29] Z.H. Yu, M. Tian, Q.Y. Jiao, D.G. Xia, Self-assembling nanobimetallic particles of Au-Pt on glassy carbon electrode, Acta Phys. -Chim. Sin. 22 (2006) 1015. [30] Y. Song, Y. Ma, Y. Wang, J. Di, Y. Tu, Electrochemical deposition of goldplatinum alloy nanoparticles on an indium tin oxide electrode and their electrocatalytic applications, Electrochim. Acta 55 (2010) 4909.

[31] J. Hassoun, F. Croce, M. Armand, B. Scrosati, Angew. Chem. Int. Ed. 50 (2011) 2999. [32] M.J. Trahan, S. Mukerjee, E.J. Plichta, M.A. Hendrickson, K.M. Abraham, Studies of li-air cells utilizing dimethyl sulfoxide-based electrolyte, J. Electrochem. Soc. 160 (2013) A259.