Polyoxometalate-decorated graphene nanosheets and carbon nanotubes, powerful electrocatalysts for hydrogen evolution reaction

Accepted Manuscript Polyoxometalate-decorated graphene nanosheets and carbon nanotubes, powerful electrocatalysts for hydrogen evolution reaction Ali A. Ensafi, E. Heydari-Soureshjani, M. Jafari-Asl, B. Rezaei PII:

S0008-6223(15)30525-X

DOI:

10.1016/j.carbon.2015.12.045

Reference:

CARBON 10584

To appear in:

Carbon

Received Date: 12 August 2015 Revised Date:

7 December 2015

Accepted Date: 15 December 2015

Please cite this article as: A.A. Ensafi, E. Heydari-Soureshjani, M. Jafari-Asl, B. Rezaei, Polyoxometalate-decorated graphene nanosheets and carbon nanotubes, powerful electrocatalysts for hydrogen evolution reaction, Carbon (2016), doi: 10.1016/j.carbon.2015.12.045. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Polyoxometalate-decorated graphene nanosheets and

1

carbon nanotubes, powerful electrocatalysts for hydrogen

3

evolution reaction

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2

4

Ali A. Ensafia, E. Heydari-Soureshjani, M. Jafari-Asl, B. Rezaei

5 6

Department of Analytical Chemistry, Faculty of Chemistry, Isfahan University of

SC

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Technology, Isfahan 84156–83111, Iran

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Abstract

11

The present study is an attempt to prepare new nanocomposites based on Pt[PW11NiO39]5‒@reduced

12

modified

13

[PW11NiO39]5‒@multiwall

14

diallyldimethylammonium chloride (PDDA) modified-rGO and PDDA-CNT are

15

prepared, the surfaces of which are then decorated with polyoxometalate

16

([PW11NiO39]5‒). Finally, [PW11NiO39]5‒@PDDA-rGO and [PW11NiO39]5‒@PDDA-

17

CNT are decorated with platinum nanoparticles to fabricate [PW11Pt-NiO39]‒@PDDA-

18

CNT/GCE and [PW11Pt-NiO39]‒@PDDA-rGO/GCE. The amounts of the noble metal

oxide

nanotubes

(CNT).

(rGO) For

and

this

Pt-modified

purpose,

poly

AC C

EP

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carbon

graphene

required for the modification of the electrodes is reduced by using the replacement

19

reaction, as a simple and effective method. Polarization measurement, cyclic

20

voltammetry and electrochemical impedance spectroscopy are used to investigate the

21

electrochemical

22

a

properties

of

[PW11Pt-NiO39]‒@PDDA-rGO

and

Corresponding Author: Phone: (98) 31–33913269. Fax: (98) 31–33912350. E–mail: [email protected]; [email protected]; [email protected].

1

[PW11Pt-

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NiO39]‒@PDDA-CNT in a solution of 0.5 mol L‒1 H2SO4. The nanocomposites are then

24

characterized in detail using scanning electron microscopy, transmission electron

25

microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction pattern, Brunauer–

26

Emmett–Teller surface area analysis and infrared spectroscopy. It is found that the

27

nanocomposites exhibit a high catalytic activity for hydrogen evolution reaction with

28

low overpotentials, high current densities and long-term stability.

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Keywords: Polyoxometalate; Pt-nanoclusters; Modified reduced graphene oxide;

31

Electrocatalysis; Hydrogen evolution reaction.

32 33

1. Introduction

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Solar cells, wind energy, global thermal energy and bio-mass are the key concepts

35

in the world’s future [1,2]. Hydrogen is the most plentiful element in nature, which is an

36

appropriate substitute for nonrenewable and environmentally destructive fossil fuels due

37

to its advantages such as renewability, storability and portability; more importantly, it is

38

non-pollutant [1,2] as its main advantage since it is environmentally friendly given that

39

water is the by-product of its combustion without any greenhouse gases emitted [2–7].

41 42

EP

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In addition, hydrogen has an excellent energy density by weight [8]. Presently, platinum and its alloys are the best catalysts used for hydrogen evolution reaction (HER), but their application is often hampered by such limitations as high preparation costs. This

43

has encouraged recent research aimed at developing new methods of reducing the

44

loaded platinum in HER catalysis [8‒12]. Galvanic replacement reaction provides a

45

simple and effective method to prepare noble metal particles including Pt, Pd, and Au. 2

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46

One advantage of this method is the very low noble metal required for the modification

47

of an electrode [13,14]. HER is one of the electrochemical processes in both acidic and alkaline solutions that

49

has been most often studied at the surface of different electrode materials including

50

platinum, tungsten, mercury, gold, silver and copper [15‒17]. The mechanism has been

51

shown to behave differently depending on the electrode material used. For example, the

52

reaction at the surface of a Pb electrode is very slow whereas it is fast at the surface of

53

Pt. This is due to the importance of the adsorbed hydrogen (H•) as an intermediate

54

[18,19].

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Selection of catalytically active materials for HER is based on their three important

56

properties, namely, the actual electrocatalytic characteristics of the material, its long-

57

term stability and its ability to offer a high specific area [1]. Polyoxometalates (POMs), a large family of soluble anionic metal oxide clusters of

59

d-block transition metals in high oxidation states, are ideal candidates for designing

60

catalysts because their chemical properties can be desirably modified by choosing the

61

proper constituent elements. Transition metal-substituted derivatives have been,

62

especially, employed frequently in homogeneous and heterogeneous catalysis [20,21].

63

One of the most important properties of these metal oxide clusters that makes them very

65 66

EP

AC C

64

TE D

58

useful in the preparation of modified electrodes is their capability for reversible multivalence reduction and formation of mixed-valence species [22–24]. Electrodeposition of POM on electrode surfaces has been reported by Keita and

67

Nadjo [25]. Bidan et al. entrapped POM in various conducting polymer films [26] and

68

Dong’s group described the spontaneous adsorption after soaking the electrode in an

3

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aqueous POM solution [27]. Although most of these methods have recorded feats of

70

success, some have suffer such drawbacks as complicated preparation process or poor

71

long-term stability. Thus, development of simple, fast and convenient methods to

72

immobilize polyoxometalates at electrode surfaces still remains to be a challenge.

73

In

the

present

study,

[PW11NiO39]5‒

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69

was

decorated

on

poly

diallyldimethylammonium chloride‒reduced graphene oxide (PDDA‒rGO) and on

75

PDDA‒multiwall carbon nanotubes (PDDA‒CNT). Then, Pt nanoclusters were

76

decorated

77

[PW11NiO39]5‒@PDDA‒rGO to fabricate [PW11Pt-NiO39]‒@PDDA‒CNT and [PW11Pt-

78

NiO39]‒@PDDA‒rGO as new catalysts. The electrocatalysts thus obtained were

79

structurally characterized in detail by scanning electron microscopy (SEM), energy

80

dispersive X‒ray spectroscopy (EDX), transmission electron microscopy (TEM), X‒ray

81

diffraction pattern (XRD), Brunauer-Emmett-Teller (BET) surface area analysis, field-

82

emission scanning electron microscopy (FE‒SEM) and Fourier transform infrared

83

spectroscopy (FT‒IR). Electrochemical studies were also performed to investigate the

84

capabilities of the catalysts for HER. The results revealed the high efficiency and long-

85

term stability of the electrocatalysts in acidic solutions.

87 88

surface

of

[PW11NiO39]5‒@PDDA‒CNT

and

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the

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at

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2. Experimental 2.1. Reagents

89

High quality grade chemicals were used without further treatment. Sodium

90

tungstate dihydrate extrapure (Na2WO4.2H2O), tetra n-butyl ammonium chloride

91

monohydrate (C16H36ClN), nickel(II) nitrate hexahydrate (Ni(NO3)2.6H2O), phosphoric

4

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acid (H3PO4), hydrofluoric acid (HF), sulfuric acid (H2SO4), Triton X‒100, and

93

acetonitrile were purchased from Merck. PDDA (low molecular weight), MWCNTs,

94

nitric acid (HNO3) and potassium hexachloroplatinate (K2PtCl6) were purchased from

95

Sigma-Aldrich.

96

purchased from Fluka.

hexafluorophosphate

97

2.2. Apparatus

was

SC

98

(C16H36F6NP)

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Tetrabutylammonium

The surfaces of the nanocomposites were explored using different techniques.

100

TEM was made on a Philips CM120, FE‒SEM was performed using NOVA

101

NANOSEM 230 equipped with EDX, and X‒ray diffractometry (XRD: D/MAX‒255)

102

was conducted at an accelerating voltage of 20 kV. A high performance volumetric

103

physisorption apparatus operated at 77 oK, Brunauer-Emmett-Teller, and Barrett-Joyner-

104

Halenda (BJH) were employed to investigate the real surface areas of the

105

nanocomposites. FT‒IR spectra were recorded with a JASCO FT‒IR (680 plus)

106

spectrometer using KBr pellets. Atomic force microscopy (AFM) was performed using

107

BrukerNanos instrument (Germany). Atomic absorption spectrometric (AAS) analysis

108

was conducted using Perkin Elmer A Analyst 700. The pore size distributions of the

109

porous

111

TE D

EP

[PW11NiO39]5‒@PDDA‒CNT,

AC C

110

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[PW11NiO39]5‒@PDDA‒rGO,

[PW11Pt-

NiO39]‒@PDDA‒CNT and [PW11Pt-NiO39]‒@PDDA‒rGO were determined using mercury porosimeter method (Poremaster GT‒60). Electrochemical characterization of

112

[PW11NiO39]5‒@PDDA‒CNT and [PW11NiO39]5‒@PDDA‒rGO modified glassy carbon

113

electrodes (GCE) was carried out in a 0.5 mol L‒1 H2SO4 solution using cyclic

114

voltammetry,

cathodic

polarization

5

method

and

electrochemical

impedance

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115

spectroscopy. The electrochemical measurements were carried out in a conventional

116

three-electrode system, using a µ-Autolab electrochemical analyzer (Model PGSTAT 30

117

potentiostat/galvanostat

118

microcomputer. The working electrodes were prepared by immobilization of

119

[PW11NiO39]5‒@PDDA‒CNT and/or [PW11NiO39]5‒@PDDA‒rGO at the surface of

120

GCE. Replacement was used to substitute the divalent metal cations or W with Pt to

121

fabricate [PW11Pt-NiO39]‒@PDDA‒CNT and/or [PW11Pt-NiO39]‒@PDDA‒rGO. A

122

large Pt foil was used as the counter electrode and an Ag/AgCl (KClst’d) as the reference

123

one. Electrochemical impedance spectroscopy measurements at different cathodic

124

overpotentials were performed at the steady-state in a frequency range of 100 kHz to 0.1

125

Hz with a perturbation amplitude of 10 mV.

Netherlands)

controlled

by

a

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2.3. Synthesis of exfoliated graphene oxide

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The

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(Eco–Chemie,

GO was prepared using a modified Staudenmaier method [28]. Natural graphite

129

powder (with a particle size of 70 µm and a purity of 99.999%) was chemically oxidized

130

at room temperature to form graphite oxide. For this purpose, 1.00 g of graphite was

131

added into a mixed acid solution containing 20 mL conc. sulfuric acid (98% w/w), 10

132

mL conc. nitric acid (63% w/w) and 10.0 g potassium chlorite. The mixture was

134 135

AC C

133

EP

128

continuously stirred for approximately 100 h. The resulting GO was rinsed with 5.0 wt% HCl aqueous solution and repeatedly washed with deionized water until the pH of the filtrate was neutral before the product was dried at room temperature. Finally, the

136

GO thus obtained was dispersed in water (0.5 mg mL–1), which was put in an ultrasonic

137

bath for 2 h for final conversion into exfoliated graphene oxide (EGO) [28].

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138 139

2.4. Preparation of PDDA‒CNT and/or PDDA‒rGO Into a conical flask containing 200 mL of water and 80 mg of MWCNTs and/or

141

rGO, 0.030 g of Triton X‒100 and 1.0 mL of 0.5 wt% PDDA were added. The mixture

142

was then ultrasonicated for 120 min at room temperature (10 min on and 3 min off).

143

Upon ultrasonication, a homogeneous black suspension was obtained, indicating the

144

complete dispersion of PDDA‒CNT and/or PDDA‒rGO. In order to separate

145

PDDA‒CNT and/or PDDA‒rGO from the mixture, the mixture was multiply washed

146

with water via centrifugation (8 times at 4000 rpm for 15 min at each run). The retrieved

147

PDDA‒CNT and PDDA‒rGO were then dried at room temperature using a vacuum

148

pump [29].

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149

2.5. Synthesis of [(n-C4H9)4N]3[PW12O40].nH2O

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150

A quantity of 5.00 g of Na2WO4.2H2O was mixed with 10 mL of H2O and

152

stirred for 12 h at room temperature to get completely dissolved. Then, 2.0 mL conc.

153

H3PO4 (98% w/w) and 5.0 mL conc. HCl (37% w/w) were added to the solution and

154

stirred for 12 h at room temperature. To the resulting suspension, 30 mL of H2O was

155

added until a clear solution emerged. Immediately, 1.44 g of tetra n-butylammonium

157

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EP

151

chloride was added to the mixture and stirred for 15 min. The resulting solid was filtered and washed with water, ethanol and finally with ether. It was then allowed to dry at

158

room temperature overnight. The product was purified by adding 50 mL of acetonitrile

159

to the sediment. The mixture was finally filtered, washed with 50 mL of H2O and dried

160

overnight in an oven at 90 °C.

7

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161 162

2.6. Synthesis of [(n-C4H9)4N]5H2[PW11O39] and [(n-C4H9)4N] 4[PW11NiO39] A quantity of 20.0 mmol (6.60 g) of Na2WO4.2H2O was dissolved in 13 mL of

164

H2O. Then, 1.82 mmol (0.266 g) of sodium hydrogen phosphate was added to the

165

solution before pH reached about 4.8, by adding nitric acid (1.0 mol L-1) while the

166

solution was being stirred. The mixture was then heated up to 80‒85 °C. Another

167

solution was prepared by dissolving 9.0 mmol (2.90 g) of tetra n-butylammonium

168

chloride monohydrate in 10 mL of water. The second solution was then added drop by

169

drop to the above reaction mixture. The resulting sediment was filtered, dried, and

170

purified as described in the previous section.

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The above procedure was repeated to synthesize [(n-C4H9)4N]4[PW11NiO39], except

172

that in this case Ni(NO3)2.6H2O was added in addition to sodium hydrogen phosphate. If

173

the color of the mixture turned to green as acetonitrile was being added, a few drops of

174

nitric acid were added until the color changed from green to yellow.

176

2.7. Preparation of [PW11NiO39]5‒@PDDA‒CNT and/or [PW11NiO39]5‒@PDDA‒rGO

EP

175

TE D

171

POM was synthesized as described in Section (2.6) above. Independent

178

suspensions of PDDA‒CNT and PDDA‒rGO were prepared by dispersing each into 10

179 180

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177

mL of water. The suspensions were then added drop by drop to POM reaction mixtures and refluxed for 12 h at 100 oC. The final products were then collected through

181

filtration, dried in a vacuum oven, and denoted as [PW11NiO39]5‒@PDDA‒CNT and

182

[PW11NiO39]5‒@PDDA‒rGO.

183

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184

2.8. Preparation of the modified electrodes A GCE was polished with emery paper followed by alumina (0.05 µm) for 3 min

186

before it was thoroughly washed with water, sonicated in ethanol, washed with water

187

again, and finally dried. To prepare the modified electrodes, 2.50 mg of

188

[PW11NiO39]5‒@PDDA‒CNT and/or [PW11NiO39]5‒@PDDA‒rGO was dispersed in 1.0

189

mL of water under ultrasonic agitation to form an ink. Then, 10 µL of the suspension

190

was dropped onto the surface of the GCE. Upon drying of the electrode surface at room

191

temperature, 10 µL of Nafion solution (2.0%) was dropped onto the surface of the

192

modified-GCE and allowed to dry at room temperature.

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The replacement mechanism was used for decorating [PW11NiO39]5‒@PDDA‒CNT

193

[PW11NiO39]5‒@PDDA‒rGO

194

and/or

195

[PW11NiO39]5‒@PDDA‒CNT–GCE

196

immersed into a 5.0 mmol L–1 of K2PtCl4 in 0.1 mol L–1 of H2SO4 solution over

197

different times followed by rinsing with distilled water. Then, cyclic voltammograms

198

were recorded in the potential range from ‒0.30 to 1.50 V in 0.5 mol L–1 H2SO4

199

solution.

200

NiO39]‒@PDDA‒CNT/GCE and [PW11Pt-NiO39]‒@PDDA‒rGO/GCE.

202 203

206

For

this

purpose,

[PW11NiO39]5‒@PDDA‒rGO–GCE

or

TE D modified

Pt.

electrodes

were

designated

as

was

[PW11Pt-

EP

3.

Results and discussion

3.1. Physical

characterization

of

[PW11NiO39] 5‒@PDDA‒CNT

and

[PW11NiO39] 5‒@PDDA‒rGO

204 205

final

AC C

201

The

with

Different methods including FT‒IR, XRD, TEM, FE‒SEM, BET and EDX were used

to

investigate

the

characteristics 9

of

[PW11NiO39]5‒@PDDA‒CNT

and

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207

[PW11NiO39]5‒@PDDA‒rGO. In addition, BET, TEM and EDX were used to

208

characterize [PW11Pt-NiO39]‒@PDDA‒CNT and [PW11Pt-NiO39]‒@PDDA‒rGO. Fig. 1A shows the FT‒IR spectra of [PW12O40]3‒, [PW11O39]7‒, and [PW11NiO39]5‒.

210

The peaks at 2852 and 2925 cm‒1 are assigned to the symmetric and asymmetric

211

stretching vibrations of the –CH2 group of [(n–C4H9)4N] while the one at 1467 cm‒1 is

212

attributed to the C–H scissoring vibrations of CH3–N+ moiety. The bands at 1078 and

213

1043 cm‒1 are attributed to the asymmetric vibrations of P–O of the tetrahedral PO4,

214

while those at 964, 896, and 804 cm‒1 are attributed to the stretching modes of the

215

terminal W–Ot, edge-sharing W‒Ob‒W, and corner sharing W‒Oc‒W units, respectively

216

[31]. All these bands are characteristic of a Keggin-type structure and are present in the

217

FT‒IR spectrum of [PW11NiO39]5‒ [31]. In addition, the FT‒IR spectra of PDDA‒rGO

218

and PDDA‒CNT presented absorption bands corresponding to carbonyl C=O stretching,

219

aromatic C=C stretching, epoxy C–O stretching, and alkoxy C–O stretching vibrations,

220

respectively

221

[PW11NiO39]5‒@PDDA‒rGO,

222

[PW11NiO39]5‒, PDDA‒CNT, and PDDA‒rGO are observed in the spectra of these

223

hybrid compounds while the broad peak centered at about 3469 cm‒1 is attributed to the

224

O–H stretching vibration of rGO. Furthermore, the absorption bands below 700 cm‒1 are

226 227

SC

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TE D shown

here).

all

the

For

[PW11NiO39]5‒@PDDA‒CNT

characteristic

peaks

corresponding

and to

EP

(not

AC C

225

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209

ascribed to the characteristic peaks of Ni–O and W–O vibrations in the POM lattice (not shown here).

XRD patterns of [PW12O40]3‒, [PW11O39]7‒, and [PW11NiO39]5‒ are presented in Fig.

228

1B. [PW11NiO39]5‒ exhibits a sharp peak at 2θ = 16, 24, and 28, demonstrating the

229

oxidation of [PW11NiO39]5‒. After decoration of [PW11NiO39]5‒ on PDDA‒CNT and/or

10

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230

on PDDA‒rGO, the one peak at 2θ = 24 is due to CNT and the one at 2θ = 26.3 is due to

231

rGO. These results demonstrate the successful decoration of polyoxometals on both

232

PDDA‒CNT and PDDA‒rGO.

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233 234

Fig. 1. A): FT‒IR spectra, and B): XRD patterns of a): [PW12O40]3‒, b): [PW11O39]7‒ and

235

c): [PW11NiO39]5‒.

237

The

morphologies

and

structures

of

SC

236

[PW11NiO39]5‒@PDDA‒CNT

and

[PW11NiO39]5‒@PDDA‒rGO were investigated using FE‒SEM (Fig. 2). The FE‒SEM

239

images show that the [PW11NiO39]5‒ prepared on the surface of PDDA‒CNT and/or

240

PDDA‒rGO roughly consisted of plate-like shapes [PW11NiO39]5‒ stacked on the

241

surface of each CNTs (Fig. 2a) and/or rGO sheets (Fig. 2b). The incorporation of

242

[PW11NiO39]5‒ resulted in an appreciable change in the morphologies, making them

243

more porous.

244

Fig.

246

[PW11NiO39]5‒@PDDA‒rGO.

249 250

images

of

a):

AC C

248

SEM

[PW11NiO39]5‒@PDDA‒CNT,

and

b):

EP

245

247

2.

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238

Fig. 3A (a, b c) shows TEM images of PDDA‒rGO, [PW11NiO39]5‒@PDDA‒rGO,

and [PW11Pt-NiO39]‒@PDDA‒rGO whereas Fig. 3B (a, b c) shows those of PDDA‒CNT,

[PW11NiO39]5‒@PDDA‒CNT,

and

[PW11Pt-NiO39]‒@PDDA‒CNT,

251

respectively. These images reveal typical small nanoparticles of [PW11NiO39]5‒ attached

252

at the surface of PDDA‒rGO and/or PDDA‒CNT. On the other hand, the TEM images

11

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of

254

NiO39]‒@PDDA‒CNT (Figs. 3A-c and 3B-c) confirm the decoration of non-

255

agglomerate Pt-nanoparticles at the surface of [PW11NiO39]5‒@PDDA‒rGO and/or

256

[PW11NiO39]5‒@PDDA‒CNT. Moreover, decoration of Pt nanoparticles at the surface

257

of [PW11NiO39]5‒@PDDA‒rGO and/or [PW11NiO39]5‒PDDA‒CNT by the replacement

258

mechanism was observed to decrease the amount of Pt required while it also increased

259

the electrochemical performance of the nanoparticles for electrochemical HER (based

260

on the electrochemical study). Moreover, Fig. 3 shows that nanoparticles 10–15 nm in

261

size are formed at the CNT and rGO surfaces after the replacement process.

[PW11Pt-

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262 263

and

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post-replacement

[PW11Pt-NiO39]‒@PDDA‒rGO

253

Fig. 3. TEM images of A): a): PDDA-rGO, b): [PW11NiO39]5‒@PDDA‒rGO, and c):

264

[PW11Pt-NiO39]‒@PDDA‒rGO;

265

[PW11NiO39]5‒@PDDA‒CNT, and c): [PW11Pt-NiO39]‒@PDDA‒CNT.

B):

a):

PDDA-CNT,

b):

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266

The surface areas and the porosities of [PW11NiO39]5‒@PDDA‒CNT and

268

[PW11NiO39]5‒@PDDA‒rGO were determined via nitrogen adsorption-desorption

269

isotherms at 77 K using a surface area analyzer. The surface areas, pore volumes, and

270

pore diameters of the porous catalysts produced are reported in Table 1. The results

271

indicate that [PW11NiO39]5‒@PDDA‒CNT has pore characteristics notably better than

273

AC C

272

EP

267

those

of

[PW11NiO39]5‒@PDDA‒rGO.

The

BET

surface

areas

of

[PW11NiO39]5‒@PDDA‒CNT and [PW11NiO39]5‒@PDDA‒rGO were 273 and 171 m2

274

g‒1, respectively. The external surface area (the sum of the surface areas of both

275

mesopores and macropores), the micropore surface area (the total surface area minus the

276

external surface area), and the micropore volume of [PW11NiO39]5‒@PDDA‒CNT were

12

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estimated as 273 m2 g‒1, 346 m2 g‒1, and 0.12 cm3 g‒1, respectively. The pore sizes were

278

classified on the basis of the International Union of Pure and Applied Chemistry

279

(IUPAC) classification into micropores, mesopores, and macropores with pore

280

diameters of up to 2, 2 to 50, and > 50 nm, respectively [32]. The average pore diameter

281

(Dp) of [PW11NiO39]5‒@PDDA‒CNT was calculated to be 12.2 nm, so the

282

[PW11NiO39]5‒@PDDA‒CNT isotherm mainly presented pores in the confines of the

283

mesopores. After galvanic replacement between Ni and Pt, the pore of the surface, due

284

to the presence of Pt, was filled and the specific surface area reduced [32]. The results

285

are given in Table 1.

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286

Table 1: Surface area and the porosities of the porous synthesized catalysts.

287 288

After the modified electrodes had been prepared, EDX was used to detect the

290

presence of W, Ni, O, and C before and after the replacement process. These studies

291

(Table 2) revealed that [PW11NiO39]5‒@PDDA‒CNT and [PW11NiO39]5‒@PDDA‒rGO

292

had been successfully synthesized. Replacements among Ni, W, and Pt after the process

293

revealed that 2.43% and 1.43% w/w of nickel decreased whereas 3.95% and 3.12% w/w

294

of

296 297

EP

platinum

AC C

295

TE D

289

increased

in

[PW11NiO39]5‒@PDDA‒CNT

[PW11NiO39]5‒@PDDA‒rGO, respectively. It was concluded that the spontaneous displacement between them was successful and that [PW11Pt-NiO39]‒@PDDA‒CNT and [PW11Pt-NiO39]‒@PDDA‒rGO had been successfully synthesized.

298 299

and

Table 2: EDS results of the synthesized catalysts.

13

ACCEPTED MANUSCRIPT

300

3.2. Pt mechanism adsorption on the surface of POM The optimum time for the modification of [PW11NiO39]5‒ with Pt was 20 min. The

302

results of the reduction peaks of Pt nanoclusters proved that Pt-nanoclusters were

303

fabricated at the surface of the modified electrodes (Fig. 4) [30]. This process could be

304

explained with recourse to the following three probabilities:

306

[PW11NiO39]5‒ + PtCl6 2- + 2K+

[PW11PtO39]3– + Ni2+ + 6Cl– + 2K+ (1)

2) Replacement between W and Pt [33]:

311

M AN U

307

1) Replacement between Ni and Pt [33]:

SC

305

RI PT

301

312

Fig. 4. Cyclic voltammograms of a): [PW11NiO39]5‒@PDDA‒CNT and b):

313 314 315 316 317 318 319

(2)

3) The likely adsorption of PtCl62- on the surface of the positively charged PDDA [34].

TE D

310

[PW10PtNiO39]7– + WCl6 + 2K+

[PW11NiO39]5‒@PDDA‒rGO in 0.5 mol L–1 H2SO4 from ‒0.30 to +1.50 V.

EP

309

[PW11NiO39]5‒ + PtCl6 2- + 2K+

3.3. Electrochemical HER at the surface of the modified electrodes Figs. 5A, 5B, 5C, 5D, and 5E show hydrogen evolution at the surface of the

AC C

308

modified electrodes in the H2SO4 solution. Current density increased in the following order: [PW12O40]3‒ < [PW11O39]7‒ < [PW11NiO39]5‒ < [PW11Pt-NiO39]–. [PW11O39]7‒ has more negative charges than [PW12O40]3‒ so that it was able to adsorb more H+ leading to

320

a higher hydrogen evolution observed at the surface. After Pt (rather than Ni or W) was

321

added at the electrode surface and Ni or W was replaced with Pt in the Keggin

322

structures, the capability of the nanocomposites for hydrogen evolution increased due to 14

ACCEPTED MANUSCRIPT

the addition of an empty d-orbital, which allowed for the acceptance of the hydrogen

324

electron pair. When [PW11NiO39]5‒ was decorated on PDDA‒CNT and/or PDDA‒rGO,

325

the current densities increased and the real surface areas increased because of the porous

326

morphologies. Due to their super acidity and good proton conductivity, POMs also have

327

a certain degree of impact on the electrocatalytic reaction so that it would become

328

capable of adsorbing more H+ in its structure leading to a higher hydrogen evolution

329

kinetic [35].

SC

RI PT

323

330

Fig. 5. A): Polarization curves of a): [PW12O40]3‒, b): [PW11O39]7‒, c): [PW11NiO39]5‒,

332

and d): [PW11Pt-NiO39]‒. B): Polarization curves of a): PDDA‒CNT and b):

333

[PW11NiO39]5‒@PDDA‒CNT. C): a): PDDA‒rGO and b): [PW11NiO39]5-

334

@PDDA‒rGO. D): Polarization curves of a): [PW11NiO39]5‒, b):

335

[PW11NiO39]5‒@PDDA‒CNT, and c): [PW11Pt-NiO39]‒@PDDA‒CNT. E): Polarization

336

curves of a): [PW11NiO39]5‒, b): [PW11NiO39]5‒@PDDA‒rGO, and c):

337

[PW11NiO39]5‒@PDDA‒rGO. Conditions: 0.5 mol L‒1 H2SO4 at a scan rate of 25 mV

TE D

M AN U

331

338 339

342 343 344 345

EP

341

3.4. Linear polarization

The polarization curves, obtained in 0.5 mol L‒1 H2SO4, illustrate the electrocatalytic

AC C

340

s‒1.

activities

[PW11NiO39]5‒@PDDA‒rGO,

of

[PW11NiO39]5‒@PDDA‒CNT,

[PW11Pt-NiO39]‒@PDDA‒CNT,

and

[PW11Pt-

NiO39]‒@PDDA‒rGO at GCE for HER. The plots were corrected for the IR‒drop (R determined by EIS) and all of the electrocatalysts exhibited a typical Tafel region. The

346

linear parts of the steady‒state polarization curves indicate that HER (over the

347

electrodes) was kinetically controlled by charge transfer. The Tafel equation is

348

expressed as: 15

ACCEPTED MANUSCRIPT

349

η = (2.303RT/βnF)log(j0) – (2.303RT/βzF)log(j) = α + blog(j)

(3)

The related electrochemical parameters (i.e., Tafel slopes and exchange current

351

densities) were derived from the Tafel plot using Eq. (3), where η(V) represents the

352

applied overpotential, j(A cm‒2) is the resulting current density, b(V dec‒1) is the Tafel

353

slope, and α is the intercept related to the exchange current density, j0(A cm‒2). β, n, and

354

F are the symmetric factor, number of electrons exchanged, and the Faraday constant,

355

respectively [36]. The apparent exchange current density, j0, provides information about

356

the catalytic activity of the electrodes. As shown in Table 3, the values for i0 clearly

357

indicate that [PW11Pt-NiO39]‒@PDDA‒CNT has a higher apparent activity for HER

358

than [PW11Pt-NiO39]‒@PDDA‒rGO does, which is related to the higher exposed

359

surface area of [PW11Pt-NiO39]‒@PDDA‒CNT. After replacement of Pt in the Keggin

360

structure, the amount of α for both nanocomposites increased, indicating that the kinetic

361

reaction of HER was improved and that the overpotential of the hydrogen evolution

362

decreased.

TE D

M AN U

SC

RI PT

350

In acidic solutions, hydrogen evolution at a metal surface mainly involves three

364

reactions (Eqs. 4 to 6). The common first step is the discharge reaction (4), which is

365

followed by either the combination or the ion‒atom reaction to produce H2. Tafel

366

analysis has been used to distinguish the different pathways [37]. The discharge reaction

368

AC C

367

EP

363

may be expressed as follows (Volmer step): H3O+ + e‒ + cat

cat–H + H2O

(4)

369

The combination reaction (Tafel step) is given by:

370

cat–H + cat–H

371

And, the ion + atom reaction (Heyrovsky step) is as follows:

2cat + H2

(5)

16

ACCEPTED MANUSCRIPT

372

H3O+ + e‒ + cat–H

373

The correlation between the Tafel slope and the HER mechanism was developed for

374

metal surfaces, on which the Volmer‒Heyrovsky could be observed when Pt was

375

present at the surface of the electrodes [37].

(6)

RI PT

cat + H2 + H2O

376

Table 3: Kinetic parameters obtained from the steady state Tafel curves for the HER in 0.5 mol L-1 H2SO4 solution.

378

SC

377

379

3.5. Long-term stability

381

The most important finding of the present study is that the modified nanoparticles

382

exhibit long-term stability. Figs. 6A and 6B show the long-term stabilities of

383

[PW11NiO39]5‒@PDDA‒rGO and [PW11Pt-NiO39]‒@PDDA‒rGO electrocatalysts as

384

investigated in 0.5 mol L−1 H2SO4 solution and using cyclic voltammetry. It can be

385

observed that the current density (with consideration of Langmuir surface area) of HER

386

increased moving from scan cycle 1 to 500. These results confirm the claim that the

387

long

388

NiO39]‒@PDDA‒CNT‒GCE

389

[PW11NiO39]5‒@PDDA‒rGO‒GCE and [PW11Pt-NiO39]‒@PDDA‒rGO‒GCE. This

391 392

TE D

stabilities

EP

term

of

[PW11NiO39]5‒@PDDA‒CNT‒GCE are

the

same

as

and

[PW11Pt-

those

of

AC C

390

M AN U

380

may be due to the increasing porosity on the surfaces as we move from the 1st to the 500th scan cycle. In acidic solutions, nickel is unstable since it may leak slowly and increase the surface porosity of the electrode [38]. To confirm this result, the effective

393

surface areas of [PW11NiO39]5‒@PDDA‒CNT‒GCE, before and after 500 scan cycles,

394

were calculated using the cyclic voltammetric results of the modified electrode in 5.0

395

mmol L‒1 [Fe(CN)6]3‒/4‒ containing 0.1 mol L-1 KNO3. In addition, AFM images were 17

ACCEPTED MANUSCRIPT

prepared for [PW11NiO39]5‒@PDDA‒CNT‒GCE before and after 500 scan cycles (Fig.

397

7). The results confirmed that the surface porosity of the electrode increased after 500

398

scans. To calculate the surface areas of the electrode before and after 500 scan cyclic

399

runs, Randles-Sevcik equation (Ip = 2.69×105n3/2AD1/2ν1/2C) was used, in which Ip is

400

the peak current, n is the number of electrons, A is the surface area of the working

401

electrode, D is the diffusion coefficient of the electroactive species, C is the bulk

402

concentration of the electroactive species, and ν is the scan rate. The surface areas of the

403

electrode before and after 500 scans were calculated as 9.71 and 12.08 (cm2),

404

respectively. These results confirm that the porosity of the nanocomposite increased

405

during the scan runs.

SC

M AN U

406

RI PT

396

Moreover, atomic absorption spectrometry was used to check the replacement of Pt

408

[PW11NiO39]5‒@PDDA‒CNT‒GCE into a 3.0 mL of 5.0 mmol L–1 solution of PtCl62–.

409

The results showed a Ni concentration of 0.225 µg mL–1 in the solution, confirming the

410

replacement of Pt with Ni in the nanocomposite.

414 415 416

dipping

EP

413

after

Fig. 6. Long‒term electrochemical stability test for the nanocomposites in 0.5 mol L‒1 H2SO4. A): [PW11NiO39]5‒@PDDA‒rGO, and B): [PW11Pt-

AC C

412

in

TE D

with

411

Ni

[PW11NiO39]5‒@PDDA‒CNT‒GCE

407

NiO39]‒@PDDA‒rGO; a) 1st cycle linear polarization curve; and b) linear

polarization curve after 500 cycles (taking Langmuir surface area into account) at a scan rate of 25 mV s‒1.

417

18

ACCEPTED MANUSCRIPT

418

Fig. 7. A): Cyclic voltammograms of [PW11NiO39]5‒@PDDA‒CNT in a solution containing 5.0 mmol L-1 [Fe(CN)6]3‒/4‒ in 0.1 mol L‒1 KNO3, a): before, and b):

420

after 500 scan cycles; B): AFM images of [PW11NiO39]5‒@PDDA‒CNT‒GCE,

421

a): before, and b): after 500 scan cycles.

422

RI PT

419

3.6. Electrochemical impedance spectroscopy

424

Electrochemical impedance spectroscopy (EIS) is a useful tool for studying the

425

kinetics of electrodes in electrochemical HER. EIS was used for further characterization

426

of [PW11NiO39]5‒@PDDA‒CNT‒GCE, [PW11NiO39]5‒@PDDA‒rGO‒GCE, [PW11Pt-

427

NiO39]‒@PDDA‒CNT‒GCE, and [PW11Pt-NiO39]‒@PDDA‒rGO‒GCE. The EIS

428

results were analyzed using Nyquist plots. Fig. 8 (A and B) shows the Nyquist and Bode

429

plots obtained from the EIS responses of [PW11NiO39]5‒@PDDA‒rGO‒GCE at various

430

overpotentials before and after the open circuit potential (‒250 mV vs. Ag/AgCl) in 0.5

431

mol L‒1 H2SO4. All the impedance results were normalized based on the specific area of

432

the modified electrodes (the BET results) as presented in Fig. 8 (Nyquist plots). Clearly,

433

semicircles are observed in the Nyquist plots of all the electrodes at higher

434

overpotentials (η > 450 mV), whereas the values of η are greater than 150 mV when Pt

436 437

M AN U

TE D

EP

AC C

435

SC

423

is present on the surface of the electrode. The equivalent circuits for the electrocatalysts are characterized by a one-time constant because only one semicircle is observed in each Nyquist plot. Thus, there is one unit of capacitor and resistor in parallel. The

438

absence of Warburg impedance indicates that mass transport is rapid enough so that the

439

reaction is kinetically controlled. The dependence of phase angle, Φ, on frequency

440

(Bode plot) suggests an additional resistor element in series with the above‒mentioned 19

ACCEPTED MANUSCRIPT

two elements. As shown in Fig. 8, the catalytic system can be captured by a simple

442

equivalent electrical circuit. The resistance element, R1, is attributed to the

443

uncompensated solution resistance, Rs, whereas the resistance element, R2, is attributed

444

to the charge transfer resistance, Rct. The fitting includes a constant phase element

445

(CPE), which represents the double‒layer capacitance under HER conditions C*dl [37].

446

The

Nyquist

and

Bode

plots

from

RI PT

441

the

EIS

responses

for

[PW11NiO39]5‒@PDDA‒CNT‒GCE,

[PW11Pt-NiO39]‒@PDDA‒CNT‒GCE

and

448

[PW11Pt-NiO39]‒@PDDA‒rGO‒GCE

were

for

449

[PW11NiO39]5‒@PDDA‒rGO‒GCE.

SC

447

same

as

those

M AN U

the

450

Fig. 8. Nyquist (A) and Bode (B) plots from the EIS responses of

452

[PW11NiO39]5‒@PDDA‒rGO‒GCE in 0.5 mol L‒1 H2SO4 at various HER overpotentials

453

in order of ‒200, ‒300, ‒400, ‒450, ‒475, ‒500, ‒525, ‒550, ‒570, and ‒600 mV (from up to down).

454 455

4. Conclusion

EP

456

TE D

451

This article reports on the successful preparation of new polyoxometalate catalysts

458

employed for HER. The synthetic catalysts exhibited porous morphologies, excellent

459 460

AC C

457

activities, and low overpotentials toward HER. The main contribution to the apparent activities of the modified electrodes was found to be due to the increasing intrinsic

461

activities and real surface areas. However, it is difficult to know whether surface

462

roughness or intrinsic activity contributes more to the apparent electrode activity.

463

Studies of several nickel‒based materials have revealed that the main contribution to

20

ACCEPTED MANUSCRIPT

electrocatalytic activity is due to the increase in real surface roughness. On the other

465

hand, the Ni and W nanocomposite in Keggin structures produces an evident

466

electrocatalytic effect. In this work, the replacement reaction was used to reduce the

467

noble metal quantities required for the modification of nanoparticles. The high

468

electrocatalytic

469

NiO39]‒@PDDA‒rGO towards HER may be directly related to their nanostructure

470

morphologies and the submonolayers nature of Pt coated on the surfaces of

471

[PW11NiO39]5‒@PDDA‒CNT and [PW11NiO39]5‒@PDDA‒rGO. Finally, the porous

472

nanocatalysts showed good stability in the long‒run experiments. The nanocatalysts

473

prepared are, therefore, suitable for hydrogen generation as advancements in green

474

chemistry.

480 481 482 483

TE D

479

and

[PW11Pt-

The authors wish to thank Iran National Science Foundation (INSF, project No. BN073) and National Elites Foundation for their support.

EP

478

Acknowledgments

AC C

477

[PW11Pt-NiO39]‒@PDDA‒CNT

M AN U

475 476

of

SC

activities

RI PT

464

484 485 486

21

ACCEPTED MANUSCRIPT

487

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489

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and

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EP

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M AN U

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Pb(II)

AC C

593

of

RI PT

575

oxidative degradation of azo dye, J. Sol‒Gel Sci. Technol. 63 (2012) 194‒199. [37] Sh. Mohamadi, N. Mirghaffari, Optimization and comparison of Cd removal from aqueous solutions using activated and non-activated carbonaceous adsorbents prepared by pyrolysis of oily sludge, Water, Air & Soil Pollut. 226 (2015) 1‒11.

26

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[38] A J. Bard, L.R. Faulkner, Electrochemical Methods and Application, John Wiley

598

and Sons, New York, 2001.

599

[39] D. Merki, H. Vrubel, L. Rovelli, S. Fierro, X. Hu, Fe, Co, and Ni ions promote the

600

catalytic activity of amorphous molybdenum sulfide films for hydrogen evolution,

601

Chem. Sci. 3 (2012) 2515‒2525.

602

[40] S.C. Lin, J.Y. Chen, Y.F. Hsieh, P.W. Wu, A facile route to prepare PdPt alloys

603

for ethanol electro-oxidation in alkaline electrolyte, Materials Lett. 65 (2010) 215‒218.

SC

RI PT

597

M AN U

604

605

606

610

611

612

EP

609

AC C

608

TE D

607

613

614

27

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615

Table 1

616

Surface area and the porosities of the porous synthesized catalysts. Surface area (m2 g‒1)

Sample

BJH des

Langmuir

BJH ads

BJH des

BJH ads

1

273

369

420

208

0.89

0.91

2.76

2.18

2

162

182

198

47

0.31

0.32

2.46

2.20

3

116

174

199

91.

0.89

0.91

2.76

2.18

4

69

86

94

20

0.31

2.46

2.20

628 629 630

SC

EP

627

AC C

626

TE D

622

625

0.32

1: [PW11NiO39]5‒@PDDA‒CNT; 2: [PW11NiO39]5‒@PDDA‒rGO; 3: [PW11Pt-NiO39]‒@PDDA‒CNT, and 4: [PW11Pt-NiO39]‒@PDDA‒rGO.

621

624

BJH des

RI PT

BJH ads

620

623

Pore diameter (nm)

BET

M AN U

617 618 619

Pore volume (cm3 g‒1)

631 632 633

28

634

Table 2

635

EDS results of the synthesized catalysts.

636

A): [PW11NiO39]5‒@PDDA‒CNT and [PW11Pt-NiO39]‒@PDDA‒CNT. Element

Intensity (c s‒1)

Line 2

1

2

1

2

C

Ka

Ka

27.63

19.12

74.15

O

Ka

Ka

4.70

8.45

10.52

P

Ka

Ka

3.09

0.75

1.25

Ni

Ka

Ka

4.14

0.86

3.39

W

La

La

4.78

1.74

Pt

‒

La

‒

0.70

1

2

64.53

27.33

25.99

24.92

5.17

13.37

0.36

1.19

0.38

0.96

6.11

1.89

10.67

5.28

60.21

32.53

‒

3.95

‒

25.85

100.0

100.0

100.0

100.0

M AN U ‒

5‒

TE D

1: [PW11NiO39] @PDDA‒CNT, and 2: [PW11Pt-NiO39] @PDDA‒CNT.

B): [PW11NiO39]5‒@PDDA‒rGO and 2: [PW11Pt-NiO39]‒@PDDA‒ rGO. Line 1 Ka

O

Ka

P Ni W Pt

Atomic (%)

Conc. (wt%)

1

2

1

2

1

2

Ka

24.32

16.52

67.14

63.27

23.83

18.62

Ka

5.38

8.63

14.82

18.95

7.22

10.15

AC C

C

2

Intensity (c s‒1)

EP

Element

Ka

Ka

2.89

0.83

1.12

0.96

1.28

1.17

Ka

Ka

3.74

1.06

5. 05

3.62

7.11

4.89

La

La

3.98

2.12

11.87

10.08

60.56

43.83

‒

La

‒

0.64

‒

3.12

‒

21.34

100.0

100.0

100.0

100.0

Total 641

Conc. (wt%)

SC

1

Total 637 638 639 640

Atomic (%)

RI PT

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1: [PW11NiO39]5‒@PDDA‒rGO, and 2: [PW11Pt-NiO39]‒@PDDA‒ rGO. 29

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Table 3

645

Kinetic parameters obtained from the steady state Tafel curves for the HER in 0.5 mol L-1 H2SO4

646

solution. Tafel slope (V dec‒1)

1

0.55‒0.70

5.54

2

0.55‒0.70

3.98

3

0.26‒0.27

16.45

4

0.26‒0.27

15.28

α

io (µA)

0.33

0.114

0.24

0.089

M AN U

SC

Range of –η (V)

0.97

0.032

0.90

0.024

EP

TE D

1: [PW11NiO39]5‒@PDDA‒CNT; 2: [PW11NiO39]5‒@PDDA‒rGO; 3: [PW11Pt-NiO39]‒@PDDA‒CNT, and 4: [PW11Pt-NiO39]‒@PDDA‒rGO.

AC C

647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675

Electrocatalyst

RI PT

642 643 644

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Legend for the figures:

678

Fig. 1. A): FT‒IR spectra, and B): XRD patterns of a): [PW12O40]3‒, b): [PW11O39]7‒ and

679

c): [PW11NiO39]5‒.

680

Fig.

681

[PW11NiO39]5‒@PDDA‒rGO.

682

Fig. 3. TEM images of A): a): PDDA-rGO, b): [PW11NiO39]5‒@PDDA‒rGO, and c):

683

[PW11Pt-NiO39]‒@PDDA‒rGO; B): a): PDDA-CNT, b): [PW11NiO39]5‒@PDDA‒CNT,

684

c): [PW11Pt-NiO39]‒@PDDA‒CNT.

685

Fig. 4. Cyclic voltammograms of a): [PW11NiO39]5‒@PDDA‒CNT, and b):

686

[PW11NiO39]5‒@PDDA‒rGO. Conditions: in 0.5 mol L‒1 H2SO4 with a scan rate of 25

687

mV s‒1.

688

Fig. 5. A): Polarization curves of a): [PW12O40]3‒, b): [PW11O39]7‒, c): [PW11NiO39]5‒

689

and d): [PW11Pt-NiO39]‒. B): Polarization curves of a): PDDA‒CNT and b):

690

[PW11NiO39]5‒@PDDA‒CNT. C): Polarization curves of a): PDDA‒rGO and b):

691

[PW11NiO39]5- @PDDA‒rGO. D): Polarization curves of (a): [PW11NiO39]5‒, (b):

693 694

images

of

a):

[PW11NiO39]5‒@PDDA‒CNT,

and

b):

EP

TE D

M AN U

SC

SEM

AC C

692

2.

RI PT

676 677

[PW11NiO39]5‒@PDDA‒CNT,

and

(c):

[PW11Pt-NiO39]‒@PDDA‒CNT.

E):

Polarization curves of (a): [PW11NiO39]5‒, (b): [PW11NiO39]5‒@PDDA‒rGO, and (c): [PW11NiO39]5‒@PDDA‒rGO. Conditions: in 0.5 mol L‒1 H2SO4 with a scan rate of 25

695

mV s‒1.

696

Fig. 6. Long‒term electrochemical stability test for the nanocomposites in 0.5 mol L−1

697

H2SO4. A): [PW11NiO39]5‒@PDDA‒rGO, and B): [PW11Pt-NiO39]‒@PDDA‒rGO; a) 1st 31

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cycle linear polarization curve; and b) linear polarization curve after 500 cycles (with

699

consideration of Langmuir surface area) at a scan rate of 25 mV s‒1.

700

Fig. 7. A): Cyclic voltammograms of [PW11NiO39]5‒@PDDA‒CNT‒GCE in 5.0 mmol

701

L–1 [Fe(CN)6]3–/4– containing 0.1 mol L–1 KNO3 a): before, and b): after 500 scan, B):

702

AFM images of [PW11NiO39]5‒@PDDA‒CNT‒GCEs, a): before, and b): after 500 scan.

703

Fig.

704

[PW11NiO39]5‒@PDDA‒rGO‒GCE in 0.5 mol L‒1 H2SO4 at various HER overpotentials

705

in order of ‒200, ‒300, ‒400, ‒450, ‒475, ‒500, ‒525, ‒550, ‒570, and ‒600 mV (from

706

up to down).

Nyquist

(A)

and

Bode

(B)

plots

from

the

AC C

EP

TE D

M AN U

SC

8.

RI PT

698

32

EIS

responses

of

AC C

EP

TE D

M AN U

SC

RI PT

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AC C

EP

TE D

M AN U

SC

RI PT

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AC C

EP

TE D

M AN U

SC

RI PT

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AC C

EP

TE D

M AN U

SC

RI PT

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AC C

EP

TE D

M AN U

SC

RI PT

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AC C

EP

TE D

M AN U

SC

RI PT

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AC C

EP

TE D

M AN U

SC

RI PT

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AC C

EP

TE D

M AN U

SC

RI PT

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