porous-RGO nanosheets for high-performance asymmetric supercapacitor

Journal Pre-proof Synthesis of a novel hybrid anode nanoarchitecture of Bi2O3/porous-RGO nanosheets for high-performance asymmetric supercapacitor Lakshmanan Gurusamy, Sambandam Anandan, Na Liu, Jerry J. Wu PII:

S1572-6657(19)30757-X

DOI:

https://doi.org/10.1016/j.jelechem.2019.113489

Reference:

JEAC 113489

To appear in:

Journal of Electroanalytical Chemistry

Received Date: 25 April 2019 Revised Date:

12 September 2019

Accepted Date: 12 September 2019

Please cite this article as: L. Gurusamy, S. Anandan, N. Liu, J.J. Wu, Synthesis of a novel hybrid anode nanoarchitecture of Bi2O3/porous-RGO nanosheets for high-performance asymmetric supercapacitor, Journal of Electroanalytical Chemistry (2019), doi: https://doi.org/10.1016/j.jelechem.2019.113489. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Elsevier B.V. All rights reserved.

Graphical Abstract

1

Synthesis of a Novel Hybrid Anode Nanoarchitecture of Bi2O3/Porous-RGO

2

Nanosheets for High-Performance Asymmetric Supercapacitor

3 Lakshmanan Gurusamya, Sambandam Anandanb, Na Liuc, and Jerry J. Wua,*

4 5 6

a

Department of Environmental Engineering and Science, Feng Chia University, Taichung,

7

Taiwan b

8 9

c

Department of Chemistry, National Institute of Technology, Trichy, India

College of New Energy and Environment, Jilin University, Changchun 130021, China

10

*Correspondence, E-mail: [email protected], Fax: +886-4-24517686; Tel: +886-4-

11

24517250 Ext. 5206

12 13 14

Abstract

15

Novel bismuth oxide (Bi2O3) nanoparticles dispersed on porous reduced graphene oxide

16

nanosheets is prepared using the facile hydrothermal reaction followed by a calcination process

17

in the air atmosphere. At electrochemical study, the electrode materials of Bi2O3/porous-RGO

18

display the capacitance retention to be 81.1% at a current density of 0.5 Ag-1 and α-MnO2-NRs

19

exhibit about 80.7 % at a scan rate of 10 mVs-1 for 3000 cycles in 6 M KOH electrolytes of

20

three-electrode configuration. Moreover, the outstanding capacitance retention of anode and

21

cathode materials mainly due to the porosity (porous-RGO), thermal stability with maximal

22

weight loss rate temperature T(mwlr) reach of 623 oC, a smaller size of Bi2O3 (~7.5± 0.5 nm), and

23

aspect ratio of α-MnO2 nanorods for 5.1 ± 0.9 nm. The assembled asymmetric supercapacitor

24

(ASC) achieves the specific capacitance of 84 Fg-1 at a scan rate of 5 mVs-1 and capacitance 1

25

retention of 91.4% at a current density of 1 Ag-1 by the Bi2O3/porous-RGO//α-MnO2-NRs in

26

PVA/KOH gel electrolyte of two-electrode configuration. Notably, the ASC delivers an energy

27

density of 86 Wh kg-1 (787 mF cm-2) at a power density of 9000 W kg-1. As a result,

28

Bi2O3/porous-RGO and α-MnO2-NRs is considered as a promising candidate for future

29

anode/cathode material in ASC energy storage device.

30 31

Keyword: Nanosheets, Hydrothermal, Porous-RGO, Carbon gasification, Anode materials

32 33

1. Introduction

34

Globally, it needs more efforts in the development, demonstration, and utilization of

35

electrochemical energy storage technology. The worldwide energy crises would occur because

36

the problems have been arisen through the insufficient renewable energy storage and conversion

37

technologies [1]. Among all energy storage technologies, the supercapacitor is the state-of-the-

38

art device, which fulfills next-generation energy demands for developing portable electronic

39

devices, such as camera, mobile phones, memory backup systems, and electrical and hybrid

40

vehicles [2,3]. Furthermore, it bridges the gap between high-energy chemical batteries and high

41

power traditional dielectric capacitor. Generally, supercapacitor electrode materials can be

42

classified into two types, namely electrical double layer electrode materials (EDLEM) and

43

Faradaic electron-transfer electrode materials (pseudocapacitors) [4, 5]. Development of EDLEM

44

mechanisms is based on the Helmholtz electrical double layers (HEDL) upon polarization. For

45

example, carbon nanofibers (CNFs), activated carbons, graphene, and porous-reduced graphene

46

oxide (porous-RGO) are often selected as electrode materials for supercapacitors. The

47

pseudocapacitive electrode stores the electrical charges through the electron transfer between the

2

48

electrodes by the faradic reversible redox reaction, such as transition-metal oxides/hydroxides

49

and conductive polymers [6-8]. However, the pseudocapacitive electrode materials have some

50

disadvantages, such as lower rate performance and electrical conductivity, and capacity loss

51

upon cycles [9-10]. To overcome these drawbacks, we need to prepare a hybrid nanoarchitecture

52

of metal oxide nanoparticles decorated on the carbon framework [11-14].

53

Supercapacitors were more dominating in the commercial market because of the porous

54

carbon materials employed as both anode and cathode electrode for electrical double layer

55

capacitors (EDLCs). Usually, EDLCs can provide fast charging/discharging rate, excellent

56

electrochemical kinetics, and good electrical conductivity. However, they still suffer from the

57

low specific capacitance with decreased energy density to hinder asymmetric supercapacitor

58

(ASC) technology [15, 16]. To overcome this obstacle, people need more efforts by exploring

59

new pseudocapacitive electrode materials, such as Mn3O4, Fe2O3, MoO3, Bi2O3, and so forth.

60

Among those, nanoscale bismuth (Bi)-based materials possess a low environmental impact,

61

cheap cost, and rich in natural resources. Additionally, the implementation of Bi2O3

62

nanoparticles has a suitable voltage window with high theoretical specific capacitance (690 mAh

63

g-1) for the negative side, which makes them promising candidates for anode materials in ASC

64

technology. Moreover, it has been widely used for different kinds of applications, such as

65

catalysts, optical materials, gas sensors, and supercapacitors, etc. Meanwhile, the recent reports

66

on Bi2O3 nanoparticles displayed the pseudocapacitance properties, but exhibited the poor

67

capacitance, lower accessible area, and difficulty for the ion and electron transport from the

68

electrolyte to the electrode surface. To overcome these shortcomings, several studies have

69

rationally prepared and fabricated nano-architecture of Bi2O3 nanoparticles decorated carbon

3

70

nanocomposites to succeed the high surface area for boosting the electrochemical performance

71

[17, 18].

72

Among all the carbon materials, graphene as a novel member of carbon morphology has a

73

freestanding atom-thick monolayer of sp2 hybridized carbon atoms with covalently bonded 2D

74

basal planes. In addition, graphene has been suggested as versatile electrode materials due to its

75

excellent electrical conductivity (5×10-3 S/cm) for 2.6% weight of RGO mixed with 97.4% of

76

H2O, thermal conductivity (~ 5000 Wm-1k-1), and strong young’s modulus (~1 TPa) [19, 20].

77

The restacking layers of graphene nanosheets are remarkably agglomerated due to the impaired

78

van der Waals force of attraction or (π-π) conjugation. The ion diffusion path is perpendicular to

79

the basal plane graphene owing to the ion-accessible surface areas, which have been

80

disintegrated between the adjacent layers of graphene. It is essential to improve the diffusion

81

kinetics of electrolyte ions into the electrode surface. Therefore, the penetration of these

82

electrolyte ions mostly belongs to the porous architectures of interconnected pores of RGO [21,

83

22].

84

The porous-RGO electrodes have been synthesized by several methods, such as self-

85

assembly [23], template approach [24] and activation method [25]. Among these, the hard

86

template technique is one of the most common, inarguable, and effective established methods of

87

preparing porous graphene. Nevertheless, it still possesses many disadvantages, such as

88

expensive cost, multiple steps, lack of structural robustness, massive use of template agents, and

89

inherent difficulty. To solve this problem, the porous-RGO nanosheets were prepared by the

90

effective technique of catalytic carbon gasification (CO, CO2) method without using any

91

template assistance [26, 27]. Hence, the synthetic strategy of the porous-RGO electrode was

92

adopted through the restacked layer behavior of lamellar microstructure of graphene. The

4

93

porous-RGO is more contributing to pseudocapacitive performances (capacity and cyclic

94

stability) of metal oxide nanoparticles due to possessing multiple pore channels with mesoporous

95

architecture. This evidence is the most favorable for interfacial charge transfer, thus facilitating

96

onto superior contact occurring in between the electrolyte and porous graphene materials.

97

Therefore, the porous-RGO architectures are well suitable for supporting redox species of metal

98

oxide nanoparticles [28-30].

99

In this work, the development of high-performance ASC has been conducted with much

100

effort. In the negative side, the Bi2O3/porous-RGO was prepared using catalytic carbon

101

gasification method in the air atmosphere. The anode materials exhibited a highest specific

102

capacitance and capacitance retention due to the nanosized Bi2O3 nanoparticles which were

103

strongly decorated on the porous-RGO nanosheets. Mesoporous morphology with the high

104

specific surface area (SSA) has promoted the full utilization of the transport both ion and

105

electron. In the positive side, the α-MnO2-NRs (nanorods) were prepared using the hydrothermal

106

process. It exhibited the higher electrochemical performance due to nanosized α-MnO2-NRs with

107

the higher aspect ratio of 5.1 ± 0.9 nm and enhanced interfacial charge transfer reaction between

108

electrode/electrolyte surfaces. Inspired by these features, the Bi2O3/porous-RGO and α-MnO2-

109

NRs electrodes could play a decisive role in the overall electrochemical performance of ASC.

110 111

2. Materials and Methods

112

2.1. Materials

113

Graphite powder (200 mesh, ≤ 74 µm), sodium nitrate (NaNO3), sulfuric acid (H2SO4,

114

95%), potassium permanganate (KMnO4, ≥99.0 %), hydrogen peroxide (H2O2), bismuth nitrate

115

Bi (NO3)3), aqueous ammonia (NH3.H2O), manganese sulfate dihydrate (MnSO4.2H2O), ultra-

5

116

pure water (18.2 mΩ), ethanol (C2H5OH), nickel foam (1.6 mm thickness), carbon black, and

117

polytetrafluoroethylene (PTFE), were purchased from Sigma Aldrich. The entire analytical grade

118

chemicals were used without further purification.

119

2.2. Preparation of graphene oxide from graphite powder

120

According to Hummer’s method, we have prepared graphene oxide (GO) from purified

121

natural graphite powders [31, 32]. In a typical synthesis, 1 g of graphite powder was mixed with

122

0.5 g of sodium nitrate (NaNO3) and dissolved in the minimum amount of water. After that, 23

123

ml of concentrated sulfuric acid (H2SO4) was added under a uniform stirring for 2 hours.

124

Furthermore, 5 g of KMnO4 was gradually added to the mixture of above solution then mixed for

125

overnight. The temperature is kept at below 5 oC by adding ice cubes to prevent overheating and

126

explosion. Then, the mixture was continuously stirred for 12 hours and the resulting solution was

127

diluted by adding 500 ml water under vigorous stirring. After that, the suspension was further

128

treated with 30% H2O2 solution (5 ml), realizing a color change of the suspension to light brown

129

to yellow, to confirm the completion of the reaction. The resultant products are washed with HCl

130

and water for several times until to remove the excess of salt impurities. The dry graphene oxide

131

obtained by drying the final product at 60 oC for 24 hrs.

132

2.3. Catalytic carbon gasification synthesis of Bi2O3 nanoparticles (NPs) dispersed on

133

Porous-RGO nanosheets

134

The Bi2O3/porous-RGO electrodes were synthesized via the following process. Concisely,

135

250 mg of graphene oxide (GO) was dispersed in 100 ml of water under sonic-probe irradiation

136

for 10 min. The resulting suspension was subjected to centrifugation at 5000 rpm for 10 min, and

137

thus obtained GO suspension. Then, 200 mg of Bi(NO3)3 was added into GO suspension

138

followed by intense agitation for 10 min. Then, 6 ml of NH3.H2O (30%) dropwise was added to

6

139

the above suspension for 30 min; finally, the resultant mixture light yellow viscous suspension

140

was transferred into a stainless steel vessel and subjected to the hydrothermal reduction at 180 oC

141

for 12 hrs, then cool to room temperature naturally. Afterward, the obtained bulk-Bi2O3/RGO

142

(Intermediate product) was washed with ultra-pure water (18.2 mΩ) and ethanol until to remove

143

the unnecessary impurities. This intermediate product was thermally treated in the furnace at 400

144

o

145

final product of Bi2O3/porous-RGO electrode. The same procedure was followed to prepare the

146

Bi2O3 and RGO under the similar condition but without calcination.

147

2.4. Preparation of α-MnO2 nanorods (NRs)

C for 4 hrs in air. Subsequently, the furnace has to reach the room temperature and collect the

148

The α-MnO2 nanorods were prepared by adding 15 ml of 0.002 M MnSO4.H2O dissolved

149

in 50 ml distilled water with stirring for 10 minutes. Then, 15 ml of 0.005 M KMnO4 was added

150

into the above mixture solution with stirring about 20 minutes. After 30 minutes, the entire

151

solution became clear. Subsequently, the transparent solution was transferred into Teflon-lined

152

stainless steel autoclave (100 ml) of 80 % capacity of the total volume. The autoclave was locked

153

and placed into the muffle furnace and maintained at 140 °C for 12 h. When the reaction was

154

completed, the autoclave temperature was automatically cooled down to room temperature. The

155

obtained precipitate of α-MnO2 nanorods were washed several times by water and ethanol and

156

dried at 70 °C for 6 hours.

157

2.5. Sample characterization

158

The morphology and structural characterization of the entire samples were observed by the

159

field emission scanning electron microscopy (FE-SEM; JEOL, JSM-7610F) and high-resolution

160

transmission electron microscopy (HRTEM; JEOL, JEM2010) at an acceleration voltage of 200

161

kV. The presence of carbon, bismuth, and oxygen element in the nanocomposite of

7

162

Bi2O3/Porous-RGO were evaluated by energy dispersive X-ray spectrum (EDS) and elemental

163

mapping analysis equipped within the HRTEM; JEOL, JEM2010. The XRD pattern was

164

recorded on a Philips XPertPro X-ray diffractometer with Cu Kα radiation (λ=1.5418) of

165

operating on 40 kv, 60 mA at a scanning speed of 5 oC/min. Furthermore, the FT-IR spectra were

166

used to determine the stretching frequency of all as-prepared electrode materials through the KBr

167

pellet technique (Nicolet FT-IR 380 Spectrometer). Nitrogen adsorption/desorption isotherm was

168

measured by the micromeritics ASAP 2020 instrument at temperature 77K. The specific surface

169

area and pore size distribution curves (PSD) of all electrodes was calculated by the BET and BJH

170

method, and the weight of the samples is 0.110 g. Thermogravimetric analysis was carried out

171

(AutoTGA2950-V54A) by flowing the air gas condition for applying the temperature starting

172

from room temperature to 1000 oC under a heating rate of 10 oC min-1.

173

2.6. Electrochemical Measurements

174

2.6.1. Three-electrode fabrication process The working electrodes were prepared as follows: about (80:10:10) % of Bi2O3/porous-

175 176

RGO

active

materials

(8

mg),

conductive

177

polytetrafluoroethylene (PTFE) (1 mg) were mixed together in a test tube containing 1 ml

178

ethanol and vigorously stirred for an overnight. After that, the resulting slurry was coated

179

uniformly onto the nickel foams (about 10 mg of active material over a geometric surface area of

180

1 cm2). Finally, the electrodes were dried for 10 hrs at 80 oC in the open-air atmosphere

181

condition. All the electrochemical measurements were carried out by an electrochemical

182

workstation (Auto lab PGSTAT128N) in a three-electrode system in 6 M KOH aqueous

183

electrolyte solution at room temperature. A Pt-wire and saturated Hg/HgO acted as the counter

184

and reference electrodes, respectively. Cyclic Voltammetry (CV), Galvanostatic Charge and

8

carbon

black

(1

mg),

and

binder

185

Discharge (GCD), and cyclic durability test were performed under an ambient condition within

186

the range of potential -1.2 to 0.2 V (vs Hg/HgO). Electrochemical impedance spectroscopy (EIS)

187

measurements were carried out in the frequency range from 0.01 Hz to 100 kHz. Moreover, the

188

specific capacitance is calculated from the CV curve using the following equation as given

189

below:        =



  

 

1

190

where V and V0 are the initial and final potential of the CV curve, I is the current density

191

(Ag-1), m is the mass of the electrode materials (gm) on the nickel foam, and ‘s’ is representing

192

the scan rate (mVs-1). Subsequently, the specific capacitance (Csp, Fg-1) of the single electrodes

193

was calculated from the non- linear galvanostatic charge/discharge curve (GC/GD) at different

194

current density based on the formula as follows:  & 

    /! = " #

&'(

$ % 2

195

The discharging energy can be calculated from integrated area of the GD curves, charging

196

energy /! from GC. Where /! is the average discharge capacitance or claimed

197

capacitance when C is the function of U, ‘I’ is the specific discharge current (Ag-1), and ‘t’ is the

198

discharge time (s) [33].

199

2.6.2 Asymmetric supercapacitor fabrication process

200

A solid-state asymmetric supercapacitor fabrication process is presented in supporting

201

information Fig. S1, where α-MnO2 NR and Bi2O3/porous-RGO NS are functioned as positive

202

and negative electrodes, respectively. First, anode or cathode material was crushed for 10

203

minutes then became a fine powder. Then, the slurry was prepared by the weight of anode or

204

cathode material (80:10:10) %, PTFE and carbon black mixed with ethanol (10 mg material into

205

1 ml ethanol). The obtained slurry was added on cutting surface 12 cm-2 of the nickel foam (8 mg 9

206

cm-2) and dried at 60 oC for 8 hours. Next, the polymeric electrolyte was prepared by adding 6 g

207

KOH with 6 g PVA dissolved into the 60 mL distilled water under stirring for 1 hour at 85 oC.

208

The polymeric electrolytes were obtained by the more transparent solution changed in the gel

209

form at increased time and constant temperature. After that, gel electrolytes were added into

210

already coated nickel foams surfaces and dried at 80 oC for overnight. Finally, the cellulose

211

papers were used as a separator and inserted between the anode (+) and cathode (-). The two

212

ends were wrapped with copper tap and connected with the electrochemical workstation (Auto

213

lab PGSTAT128N) in a two-electrode system. Areal capacitances of the electroactive materials

214

were calculated from their GCD curves according to the following equation:

215

Areal capacitances 56  = " × ∆ /5 × ∆V (3)

216

where Ac is the areal capacitance (F/cm2), I is the discharging current density (A/g), ∆ is

217

the discharging time, and ∆V (V) is the potential window. The electrode materials were coated on

218

nickel foam with surface area A (cm2) of 12 cm2 Bi2O3/porous-RGO NS and 12 cm2 α-MnO2

219

NR.

220

The energy density and power density was calculated from the following equations: (4) and (5).

221

Energy density > = 0.5 ×  × ∆B C 4

222 223

where ED is the energy density (Wh kg-1), Cs is the specific capacitance (Fg-1), and ∆E is the change in the potential window (V), respectively. Power density I> = > × 3600/∆ 5

224 225

where PD is the power density (W kg-1) and ∆ is the discharge time from the GCD curve, respectively.

226 227

3. Results and discussion 10

228

The synthetic procedure of Bi2O3/porous-RGO anode material is displayed in Scheme. 1.

229

First, GO powders were dispersed in water under the ultrasonic assistance, then the Bi(NO3)3

230

solution was added to the GO dispersion during the stirring process. The Bi(NO3)3 was

231

uniformly dispersed on the surface of GO suspension with the help of the sp2 carbon atoms

232

(vacant ‘p’ orbitals), greater surface energy also fewer functional groups [11, 34]. During the

233

hydrothermal reaction, the Bi(NO3)3 was dissociated into the Bi3+ cation and NO3- anionic

234

species at the temperature above 130 oC. The cationic species of Bi3+ is converted into Bi(OH)3

235

by the addition of aqueous ammonium hydroxide. Then, the anionic species of NO3- was further

236

decomposed into NO2- and O2. Furthermore, upon increasing the hydrothermal temperature

237

gradually, reduced size of the Bi2O3 NPs was obtained by the dehydration of H2O molecules.

238

Reduced size of Bi2O3 NPs was uniformly located on the RGO nanosheets because the RGO has

239

excellent physical properties, such as chemisorption and strong electrostatic force of interaction

240

between metal to carbon bond. However, the Bi2O3/porous-RGO anode was prepared by

241

applying calcination temperature at 400 oC for precise time of 4 hours in air atmosphere on bulk-

242

Bi2O3/RGO electrodes. The Bi2O3/porous-RGO Ns were prepared from through catalytic carbon

243

gasification using Bi2O3-NPs (nanocatalysts) obtained in situ to generate mesoporous structures

244

as revealed in Scheme. 1. However, the porous morphology created by the O2 (air oxidation)

245

atoms can react with carbon (RGO-Ns), which was selectively decomposed, in the presence of

246

adjacent Bi2O3 NPs at the temperature lower than carbon combustion temperature. The remaining

247

Bi2O3 NPs and unreacted carbon atoms retained in these composites. The CO2 can also react with

248

H2 (intercalation reaction) according to the reverse water gas shift reaction: CO2 + H2 ↔ H2O +

249

CO (Scheme. 1). The reaction process is expressed as follows in equation (6). Therefore, the

250

formation of porous morphology depends on the number and average size distribution of the

11

251

Bi2O3 NPs. The creation of mesopores on RGO nanosheets can be confirmed by FE-SEM (Fig.

252

S2 (d)) and HR-TEM images in Fig. 1d. 6'TU'V

LMN + P QR + SC WXXXXXXXY Porous − RGO + CO/ SC + P QR 6 253 254

3.1. Morphology and structural analysis of anode materials

255

The FE-SEM image of RGO, Bi2O3, bulk-Bi2O3/RGO (before calcination), and

256

Bi2O3/porous-RGO (after calcination) of as-synthesized materials are as shown in Fig. S2 (a-d).

257

The formation of well-defined Bi2O3 nanoparticles (NPs) can uniformly agglomerate on the

258

surface of porous reduced graphene oxides framework as displayed in Fig. S2 (d). More

259

importantly, the hybrid Bi2O3/porous-RGO architectures formed by the conductive matrix of

260

porous-RGO nanosheets act as a nucleation site for growing a Bi2O3 NPs due to numerous

261

interconnected pores. Besides, the well-aligned Bi2O3 NPs are able to provide plenty of redox

262

active sites on the surface of porous-RGO. Owing to the benefit to facilitation, the quick

263

migration of electrolyte ion into electroactive sites was done at higher charge/discharge current

264

density. Finally, it efficiently improves the electrochemical reaction kinetics during buffer

265

volume change of electrochemical measurements and thus leads to supercapacitor performance.

266

On the other hand, Fig. S2 (c) shows that Bi2O3 NPs were irregularly dispersing on the RGO

267

nanosheets to yield bulk-Bi2O3/RGO (before calcination), whose morphology is not suitable for

268

the penetration of electrolyte ions for capacitive performance. Besides, for comparison, the

269

porous morphology of Bi2O3/porous-RGO electrode materials was prepared at various time

270

intervals (2, 3, and 4 hours) with a constant temperature of 400 oC and the results are presented

271

in Fig. S3.

12

272

The detailed morphological structure of as-synthesized electrode materials, such as the

273

bulk-Bi2O3/RGO and Bi2O3/porous-RGO, was further confirmed by TEM and HR-TEM

274

analysis. The bulk-Bi2O3/RGO nano-hybrid material was successfully prepared by the one-pot

275

hydrothermal method, which shows that the Bi2O3 nanoparticles are anchoring thoroughly on the

276

RGO sheets as displayed in Fig. 1a. Moreover, the bulk-Bi2O3/RGO was converted into the

277

Bi2O3/porous-RGO through the calcination temperature and it is shown in Fig. 1c. Before

278

calcination, the average size of the Bi2O3 nanoparticles (10.6 ± 0.4 nm, Fig. S4 (a)) was

279

calculated from the bulk-Bi2O3/RGO. After calcination, the average size of the Bi2O3

280

nanoparticles (7.5 ± 0.5 nm, Fig. 1b and Fig. S4 (b)) was also calculated from the Bi2O3/porous-

281

RGO (Fig. 1c). As shown in Fig. 1d, the composite of Bi2O3/porous-RGO had the well-defined

282

porous channel, where the porous structure not only increases the surface active site, but also the

283

electrolyte ions easily penetrate both intercalation/de-intercalation processed at higher current

284

density. As can be seen in Fig. 1e, the Bi2O3 nano-particles are well enclosed by the porous

285

structure of graphene. The HR-TEM measurement (Fig. 1f) provided detailed information about

286

crystal lattice of Bi2O3 nanoparticles chemically bonded on the RGO layer in Bi2O3/porous-RGO

287

composite. The different diameter of selected Bi2O3 nanoparticles was calculated in

288

Bi2O3/porous-RGO composite as shown in Fig. S5a. The Bi2O3 nanoparticles encircled (Fig. S5

289

(b-c)) about 1.5 nm thickness on RGO layer (measured by Image-J software) and this is because

290

the size of the Bi2O3 crystallinity increased due to the movement from hydrothermal temperature

291

180 oC to annealing temperature at 400 oC as presented in Fig. 1f. Moreover, the clear lattice

292

fringes with an interplanar distance of 0.32 nm correspond to the Bi2O3 crystal plane (Fig. S5d)

293

of the nanoparticles facet (021), which values were measured from the FFT and IFFT spectrum

294

(Gatan Microscopy suite software) as depicted in Fig. 1g-i.

13

295

The elemental mapping analysis (Fig. S6 (a-c)) of Bi2O3/porous-RGO obviously confirms

296

the homogeneous distribution and coexistence of Bi, O, and C element. Such observations

297

unambiguously confirm that the Bi2O3 nanoparticles have been successfully anchored on RGO

298

nanosheets to form the Bi2O3/porous-RGO material. These results are in good concordance with

299

the XPS spectrum in Fig. S10b. In addition, the EDX spectrum of the corresponding

300

Bi2O3/porous-RGO material (Fig. S6 (d)) was further confirmed in the presence of C, O, and Bi

301

element with increasing weight % and decreasing the atomic %.

302

The crystallographic structure of the Bi2O3/porous-RGO, bulk-Bi2O3/RGO, RGO, and

303

Bi2O3 electrode materials were further determined by the X-ray diffraction (XRD) measurements

304

as shown in Fig. 2a. The crystalline structure of the Bi2O3 electrode was observed in the cubic

305

crystal systems of hydrothermally formed bulk-Bi2O3/RGO and tetragonal crystalline structures

306

of calcinated Bi2O3/porous-RGO as displayed in Fig. 2a. This phase transformation may occur

307

because Bi2O3 NPs was able to create the voids morphology of supporting materials as

308

mentioned in the previous publication [35]. Remarkably, two sharp hump RGO diffractions

309

peaks were clearly detected at the position of 2θ=26.48 and 54.73, belonging to the typical (120)

310

and (023) reflection planes. As can be seen that the many typical sharp diffraction peaks intensity

311

of Bi2O3 in Bi2O3/porous-RGO and bulk- Bi2O3/RGO increased gradually due to the presence of

312

graphene, also fewer oxygen-containing functional groups (Fig.S10c) with strong electrostatic

313

force of attraction between Bi2O3 and porous-RGO [36]. Among the diffraction peaks, the

314

leading sharp diffraction peak appears at the position of 2θ=26.5, 27.3, 30.5, and 32.78, which

315

can be indexed to the (120), (021), (222), and (132) crystal planes of enlarging pattern as shown

316

in Fig. S7. It is confirmed that the calculated interlayer spacing and crystallite size of the sharp

317

diffraction peak of Bi2O3 in Bi2O3/porous-RGO calculated at the position of d (021) is 0.32 nm and

14

318

7.3 nm, respectively, according to the Braggs and Scherer equation as presented in supporting

319

information (Table-S1 and Table-S2). These values were further compared in the HR-TEM

320

image, where almost all are consistent with Bi2O3 NPs size and lattice spacing values of FFT

321

spectrum.

322

The porous structure of as-prepared electrode materials was further explored by the

323

nitrogen adsorption−desorption techniques as depicted in Fig. 2b. The corresponding pore-size-

324

distribution (PSD) curved of all four-electrode materials displays that pore distribution ranging

325

from 2 to 7 nm and resultant pore volumes of 0.071, 0.050, 0.008, and 0.0037 cm3/g,

326

respectively. As a result, it is obviously shown the wide PSD curve of Bi2O3/porous-RGO

327

electrode material, i.e., it exhibited a high capacitance due to the high specific surface area and

328

plentiful mesoporous (2-50 nm), which may be favorable for electrical double layer capacitance

329

(EDLCs) of porous graphene. Further, it can also improve the electrical conductivity upon Bi2O3

330

NPs, combining with the porous RGO to developing pseudocapacitive performance.

331

Furthermore, the mesoporous RGO morphology was created by the catalytic effect of Bi2O3 NPs,

332

which were done by simultaneous pyrolysis and the etching of RGO NPs. This kind of pore

333

nanostructure was very important to be employed for improving the rate capability at higher

334

discharge/charge current density. Moreover, the application of mesoporous RGO structures was

335

further utilized for electrolyte ion to rapidly diffuse into the accessible observed porous surface

336

area and their satisfactory amount of pseudocapacitive performance was improved on Bi2O3 NPs

337

involved in the redox reaction mechanisms. Additionally, the N2 adsorption isotherms data was

338

presented in Fig. S8. All of them belong to type IV isotherms with a distinct type of H3

339

hysteresis loop at a high relative pressure (P/P0) between 0.4 to 1.0, which suggests the presence

340

of mesoporous solids characteristics. The calculated specific surface area (SBET) is 40.4, 23.8,

15

341

10.8, and 7.9 m2/g for Bi2O3/porous-RGO, bulk-Bi2O3/RGO, RGO, and bare Bi2O3, respectively.

342

As a result, the high specific surface area of Bi2O3/porous-RGO is a potential electrode material

343

for supercapacitor applications.

344

The stretching frequency of functional groups moieties in all the as-prepared materials was

345

determined from FT-IR spectra as seen in Fig. 2c. GO exhibited series of characterized

346

absorption bands, which are obviously related to the different types of oxygen-rich functional

347

groups of both surfaces and edges of the GO nanosheets, such as located at 3427 cm-1 (broad

348

absorbance band of O–H stretching vibration in hydroxyl or carboxyl groups), 3736 cm-1 (sharp

349

deformation peak of O-H distinctive vibration), and 1631 cm-1 (C=C stretching vibrations and

350

breathing vibration of epoxy groups), respectively. The weak peak arising from 1319 cm-1

351

corresponds to a strong C-O stretching frequency of carbonyl groups, while the 1065 cm-1

352

characteristic frequency can be assigned to a robust C-O bond between alcoholic functional

353

group presented on an RGO nanosheet. The weak bands centered around 1371 cm-1 could be

354

ascribed to N-O bond between nitro groups of the RGO nanosheets (‘N’ comes from the

355

precursor of aqueous ammonia or Bi(NO3)3) [37]. These results are good agreements with XPS

356

spectrum of Fig. S10d. Furthermore, the strong narrow peak centered at 831 cm-1 is assigned the

357

predictable vibration of the Bi-O-Bi bonds of Bi2O3 materials. The very strong intense peak at

358

534 cm-1 is absorption band due to the vibration of Bi-O bonds (Fig. S10e) between [BiO6] units.

359

These are the typical vibrations present in Bi2O3/porous-RGO and Bi2O3 electrode materials that

360

confirmed the incorporation of Bi2O3 NPs on the RGO nanosheets [38, 39]. Meanwhile, the bulk-

361

Bi2O3/RGO did not appear metal-oxygen bond [Bi-O] stretching vibration at the position of 534

362

cm-1, which means that hydrothermal temperature is not enough for the formation of (Bi) metal

363

to oxygen bond between these electrode materials and thus hinders the pseudocapacitive

16

364

performance. Therefore, it is necessitated to follow the calcination temperature for formation of

365

metal to oxygen bond and simultaneously to generate the pores on RGO nanosheets. These

366

results are good concordance with the XPS survey spectrum of Fig. S9a and S10a.

367

Generally, the weight loss, thermal stability, and pyrolysis behavior of all as prepared

368

electrode materials have been investigated by the thermogravimetric analysis (TGA) under the

369

flowing of air gas atmospheric condition with a heating rate of 10 oC min-1 as presented in Fig.

370

2d. From the TGA curve, the first weight loss was carried out between 30 oC to 200 oC, which

371

were attributed to the evaporation of physically adsorbed/intercalated water molecules or may be

372

due to moisture in these samples [40, 41]. The second weight loss was located in the temperature

373

range of 200 oC to 350 oC due to the formation of Bi to Bi2O3. More importantly, the third major

374

significant weight loss of the Bi2O3, Bulk-Bi2O3/RGO, RGO and Bi2O3/porous-RGO electrode

375

materials were observed (Table S3) for temperatures belonging to (i) 355 oC to 545 oC, (ii) 415

376

o

377

determine the maximal weight loss rate temperature T(mwlr) around 355 oC and followed by

378

weight loss occurred for 6.2%. Hence, we decided to prepare Bi2O3/porous-RGO anode material

379

for suitable calcination temperature at 400 oC, which may ascribe to the phase transformation of

380

Bi2O3 from bulk-Bi2O3/RGO to Bi2O3/porous-RGO [42]. Furthermore, the content of the case (ii)

381

was observed with weight loss at 33% of RGO (retain) and 67 % of Bi2O3 for bulk-Bi2O3/RGO.

382

Case (iii) can be designated for 91.6% weight loss occurring for RGO nanosheets, which is due

383

to the bulk pyrolysis of the carbon skeleton (RGO sheets). We can also evaluate the mass content

384

of the case (IV) with 27% of RGO (retain) and 73% of Bi2O3, respectively, which are almost in

385

good agreement with an EDX spectrum. Finally, the thermal stability of the Bi2O3/porous-RGO

386

is greater upon compared with other electrodes because the unexceptional T

C to 847 oC, (iii) 522 oC to 734 oC, and (iv) 623 oC to 901 oC. In case (i), it represented to

17

(mwlr)

were attained

387

at the temperature of 623 oC and it was completely decomposed from C to CO or CO2. These

388

explanations designate that the Bi2O3 nanoparticles can catalyze the oxidation of RGO

389

nanosheets that were in contact with them. For 6 hrs at 400 oC, the thermal stability of the

390

Bi2O3/porous-RGO electrode materials exhibits (Fig. S11) T (mwlr) to be 638 oC.

391

3.2. Morphology and structural analysis of cathode material

392

The TEM images of α-MnO2-NRs are shown in Fig. S12 (a). As can be seen in Fig. S12,

393

the pure and uniform α-MnO2-NRs were observed with the length of 165 ± 5 nm (Fig. S13 (a))

394

and its width of 32.7 ± 0.3 nm (Fig. S13 (b)) (calculated in image J software). The SAED pattern

395

(inset) exhibited a line pattern of single crystalline nature from the single nanorods as displayed

396

in Fig. S12 (b). The shape of the α-MnO2-NRs clearly shows a blind stick with an aspect ratio

397

(length/width) of 5.1 ± 0.9 nm as seen in Fig. S12 (c-d). The HR-TEM images of well-defined

398

lattice line are displayed in Fig. S12 (e-f). The FFT and IFFT spectrum and d-spacing values

399

were analyzed using digital microscopy software as shown in Fig. S12 (g-i). The interlayer

400

spacing of α-MnO2-NRs is about d = 0.2845 nm, which corresponds to the (211) crystal planes

401

of the tetragonal structure. The EDX spectrum was used to confirm the elements composed in

402

electrode materials. Therefore, the EDX peaks have demonstrated Mn and O elements present in

403

α-MnO2-NRs and resultant atomic/weight % as shown in Fig. S13 (c). No other impurities were

404

detected, which indicated the high quality of α-MnO2-NRs was obtained from the preparation

405

process.

406

As depicted in Fig.3a, the high yield of α-MnO2-NRs was observed by FE-SEM images.

407

The blind stick shape of the α-MnO2-NRs was obtained and the maximum size of the NRs length

408

and width are 180 nm and 23 nm, respectively. Subsequently, the crystallographic structure and

409

phase purity of α-MnO2-NRs has been measured after hydrothermal time of 12 hours by the

18

410

XRD pattern. As shown in Fig. 3b, the diffraction peaks angles, appeared at 2θ = 12.7°, 15.4°,

411

24.8o, 28.8°, 37.4°, 42.2°, 49.9°, 57.1°, 60.1°, 65.5o, 69.7o, and 73.1o, are assigned to crystal

412

plane (110), (200), (220), (310), (211), (301), (411), (600), (521), (022), (541), and (312)

413

reflection, respectively [43, 44]. The intense and sharp peaks were observed in α-MnO2-NRs,

414

where those exhibited a highly crystalline form of tetragonal structure (JCPDS card PDF file no.

415

44-0141) and no other any impurity peaks detected. According to the Braggs equation, the lattice

416

spacing of d = 0.2855 nm value was calculated at the angle 2θ = 37.4. The pure form of α-MnO2

417

NRs was used to calculate the average crystal size of 14.6 nm along the growth of (211) direction

418

(inset), enlarged pattern in Fig. 3b, according to Scherrer’s equations.

419

Fourier transforms infrared (FTIR) spectroscopy of α-MnO2-NRs was shown in Fig. 3c.

420

The broad peaks appeared at 3480.6 cm-1, which corresponds to stretching vibration of absorbed

421

(H-O-H) water molecules. The peak position of 1619.3 cm-1 is assigned to the bending mode of

422

hydroxyl groups of Mn-OH. The band located at about 522 cm-1 is the characteristic vibration of

423

metal-oxygen (Mn-O) bond in [MnO6] octahedra. The peaks appear at around 711.9 and 1382.7

424

cm-1, which are given to the stretching vibration of an O-Mn-O bond between α-MnO2-NRs [45,

425

46].

426

As presented in Fig. 3d, the shape of the plots shows α-MnO2-NRs have a typical type

427

II/IV isotherm with a discrete hysteresis loop obtained in the pressure range (P/PO) = 0.5-1.0 of

428

the region at 77 K. BET surface areas reveal the α-MnO2 NRs have gravimetric surface areas of

429

44.2 m2/g [47]. The BJH pores size distribution curve (PSD) is depicted (inset) in Fig. 3d. The

430

PSD curves to reach maximum range is 45.6 nm within the average pore size of 3.7 nm and the

431

corresponding pore volume is 0.13 and 0.05 cm3/g. The available surface area of mesopore and

19

432

micropore exists on α-MnO2 NRs and it provides electrolyte ion easily to reach the electrode

433

surfaces [48].

434

3.3. Electrochemical characterization of anode materials

435

The electrochemical properties of Bi2O3/porous-RGO, bulk-Bi2O3/RGO, RGO, and Bi2O3

436

anode materials were evaluated by the cyclic voltammetry (CV) test in three-electrode

437

configuration at room temperature as discussed in Fig. 4a-d. Furthermore, the typical CV profile

438

concerning all of the corresponding anode materials is involved in the same potential window of

439

-1.2 to 0.2 V at various scan rate (100 to 5 mVs-1) of the electrolyte solution containing 6 M

440

KOH. The electrochemical performances mainly depended on the concentration and hydrous

441

ionic radius of the electrolyte. Therefore, smaller ionic radius K+ (3.31Ao) is more suitable to

442

enhance ionic mobility on the electrode surface. Based on the CV data, the quasi-reversible

443

faradic redox peaks were observed from Bismuth-metal ion nanocomposites anode materials in

444

an alkaline electrolyte solution. The excellent pseudocapacitive properties were ensured as

445

presented in those nanocomposites (as shown in Fig. 4a-b, d). Without Bismuth-metal ion on

446

RGO nanosheet, it displays the electrical double layer peak, instead of redox peak (as shown in

447

Fig. 4c). More details of Fig. 4a, the low scan rate at 5 mVs-1 shows the negative direction of

448

reduction peaks, which could be seen at the potential range of -0.7 to -1.2 V, representing a

449

reduction of Bismuth-metal ion (+3 oxidation) to the metallic bismuth (0 oxidation). In addition,

450

the positive direction of oxidation peak appears at the potential range of +0.07 to -0.1085 V,

451

which indicates the oxidation of bismuth metallic state (0 oxidation) to the bismuth-metal ion

452

(+3 oxidation). Moreover, the unique Bi2O3/porous-RGO nanocomposites displayed a larger

453

redox peak current density with a higher area of CV curve, which may be because the electrode

454

materials possess the rapid electronic and ionic transport as well to enhance the kinetics of a

20

455

faradic reversible redox reaction. Besides, the more negative reduction current density was

456

observed due to the hydrogen intercalation in the oxide and formation of BiO2– (2 Bi2O3 + 3 OH-

457

+ e- → 4BiO2- + H2O + Had). From the above consequence, the unique Bi2O3/porous-RGO

458

nanocomposites is considered as a better anode material for electrochemical energy storage

459

devices. To the best of our knowledge, the possible conversion of redox mechanism of Bi-based

460

electrode materials had been proposed and described in the following equation given as below:

461

PC SQ /porous − RGO + 3`C S + 6  ↔ 2P + 6S`  [49-53]

462

The galvanostatic charge and discharge (GCD) behaviors were carried out under the designated

463

voltage window of -1.2 to 0.2 V for the electrode materials, such as Bi2O3/porous-RGO, bulk-

464

Bi2O3/RGO, RGO, and Bi2O3 in the aqueous 6 M KOH electrolyte solution at a different current

465

density of 1~20 Ag-1, respectively. The GCD profile of the Bismuth-metal ion containing

466

nanocomposites is totally differing from that without Bismuth-metal ion of RGO nanosheets

467

GCD curve, which demonstrated that enough valance state has been transferred between

468

electrode surfaces under the same potential as shown Fig. S14 (a-b), S14 (d). This credit goes to

469

the fast kinetic faradaic redox reaction involved in the pseudocapacitive behavior of Bi2O3

470

nanocomposites. Meanwhile, the RGO shows that the GCD profiles appeared regular triangular

471

shape of the asymmetric GCD curve, which is designated that typical triangular shape of the

472

peak exhibited by electrical double layer mechanisms occurred on the RGO electrode surface as

473

shown in Fig. S14 (c). As can be seen from these experimental results, the lengthy discharging

474

tail-time of the Bismuth-metal ion present anode nanomaterials could be divided into the two-

475

discharge region: (i) horizontal discharge and (ii) vertical discharge region. A horizontal

476

discharge named gradual voltage drops, which clearly shows that discharging time is

477

substantially greater than charging time due to the faradaic reaction occurring to continue over

21

478

the long period of discharging time for the presence of discharging plateau. This can be

479

attributed to the improved pseudocapacitance with EDLC properties containing pore volume of

480

mesoporous RGO nanosheets. Besides, the vertical discharge called for steep voltage drop upon

481

the first few seconds of discharge, which has represented for IR drop (non-optimized electrolyte

482

interface) as presented in the corresponding non-symmetrical peaks [54]. For instances, the same

483

type of the GCD curves has been already reported as a different kind of bismuth present in

484

nanocomposites electrodes, such as BiVO4/SWCNT [55], Bi2O3 [56], and BiVO4: Ag [57].

485

However, the extended discharging tail of the Bi2O3/porous-RGO showed the larger specific

486

capacitance at low current density.

487

According to the equation (1), the specific capacitances of Bi2O3/porous-RGO composites

488

are calculated as 226, 158, 120, 49, and 32 Fg-1 at a scan rate of 5, 10, 20, 50, and 100 mVs-1,

489

respectively. The higher value of capacitance was produced at lower scan rate due to the fast

490

diffusion of electrolyte ion, which had enough time to interact with mesoporous nanocomposite

491

electrode surface, while at high scan rate the ion would not have enough time to approach the

492

electrode surface as depicted in Fig. 5a. According to the equation (2), the specific capacitances

493

of Bi2O3/porous-RGO electrodes materials can reach up to 285.1 Fg-1 at the current density of 1

494

Ag-1. Meanwhile, the composites can still deliver 243, 238, 234, and 123 Fg-1 at the different

495

current density of 5, 10, 15, and 20 Ag-1, respectively. At low current density, it exhibits the

496

higher specific capacitance, which could attribute to the mesoporous morphology with a high

497

electrical conductivity of graphene network. Therefore, the electrolyte ion can rapidly diffuse

498

into the accessible surface area of electroactive materials. For comparative studies, the calculated

499

specific capacitance value of Bi2O3/porous-RGO is much better than that of bulk-Bi2O3/RGO,

500

RGO, and Bi2O3 electrodes at the different current density. Additionally, the rate capability of all

22

501

the prepared electrode materials were compared, such as 285.1, 217.4, 145, and 116 Fg-1 at 1 Ag-

502

1

503

Bi2O as showed in Fig. 5b. At a current density of 20 Ag-1, the rate performance exists in lower

504

status (specific capacitance reduces) due to the greater IR drop and unsatisfactory redox process

505

in the active material. Furthermore, the diffusion-controlled of redox reactions occur in the bulk

506

of pseudocapacitive materials (bulk-Bi2O3/RGO), resulting in low coulombic efficiency and

507

sluggish of reaction kinetics. Moreover, the bulk-Bi2O3/RGO, RGO, and Bi2O3 displayed the low

508

rate performance at the higher current density of 20 Ag-1, which can be attributed to the larger

509

resistance of the electrode material, limited ion-diffusion rate in the electrolyte solution, low

510

electrical conductivity, and large interfacial resistance between electroactive materials and the

511

conducting substrate. The cyclic durability of supercapacitor electrode material was evaluated by

512

the GCD curve under the potential range of -1.2 to 0.2 V at a current density of 0.5 Ag-1, the

513

Bi2O3/porous-RGO anode revealed a higher capacitance, excellent rate capability, and

514

outstanding capacitance retention. Furthermore, the calculated specific capacitances of first

515

cycles are 522.5, 396.2, 287.9, and 216 Fg-1 and final cycles are 425, 274, 193, and 113 Fg-1 for

516

Bi2O3/porous-RGO, bulk-Bi2O3/RGO, RGO, and Bi2O3 under the potential range of -1.2 to 0.2 V

517

at a current density of 0.5 Ag-1 as displayed in Fig. 5c. On the other hand, the order of

518

capacitance retention of Bi2O3/porous-RGO, bulk-Bi2O3/RGO, RGO, and Bi2O3 anode materials

519

(81.1% > 73.2% > 68.5% > 55.9%) after 3000 cycles presented in Fig. 5d. Additionally, the

520

cyclic durability of Bi2O3/porous-RGO also confirmed to the higher number of cycles, which

521

exhibits (Fig. S15) 77% capacitance retention after 10000 cycles. According to the capacitance

522

retention, the Bi2O3/porous-RGO has exhibited the most outstanding cyclic performance. This

and 123, 98.5, 81, and 39 Fg-1 at 20 Ag-1 for Bi2O3/porous-RGO, bulk-Bi2O3/RGO, RGO, and

23

523

result mainly depends on the strong electrical conductivity of porous-RGO nanosheets, which

524

enhances the electrolyte ion penetration on the surface of a working electrode.

525

The electrochemical impedance spectroscopy (EIS) of all the prepared electrode materials

526

were further studied for the solution resistance (Rs), charge to transfer resistance (Rct) (or)

527

polarization resistance, and Warburg impedance (Zw). The impedance data of the Nyquist plots

528

were obtained between the real (Z’) and an imaginary axis (-Z’’) of the impedance spectrum at

529

the electrolyte solution containing 6 M KOH with the frequency range of 10 KHz to 0.1 Hz. Fig.

530

S16 (a) shows the typical Nyquist plots of Bi2O3/porous-RGO, bulk-Bi2O3/RGO, RGO, and

531

Bi2O3 electroactive materials, where the EIS curve could be divided into three frequency range.

532

At higher frequency range, the Bi2O3/porous-RGO shows the intersection point of the real axis

533

solution resistance (Rs) value at 0.54 Ω, which indicates that the superior conductivity is higher

534

than that of the bulk-Bi2O3/RGO, RGO, and Bi2O3. However, the medium frequency ranges of

535

Bi2O3/porous-RGO displays a semicircle arc of the diameter, which is able to measure the value

536

of Rct at 0.10 Ω and it demonstrates the faster electrochemical reaction in between the

537

electrode/electrolyte surface due to exhibiting the higher current density of with

538

pseudocapacitance [58]. Owing to the present advantage of the porous-RGO network, those can

539

offer direct current pathways for good electrical transport, thus effectively reducing the

540

resistance of the electrode materials. Nevertheless, the low-frequency ranges of Bi2O3 /porous-

541

RGO electrode exhibited Warburg impedance (Zw) of a diagonal line of vertical slope spike,

542

which precisely started from 45o as depicted in Fig. S16 (b). It indicates that the ion

543

diffusion/transportation processes took place from an electrolyte solution to electrode surfaces

544

with the porous-RGO network to ensure the resultant capacity of the rate capability behavior

545

predominantly. Meanwhile, the ion/electron penetration process of bulk-Bi2O3/RGO is slower

24

546

than that of the Bi2O3/porous-RGO, which may occur due to the poor electrostatic interaction

547

between the metal to carbon bond and the inaccessible area of RGO nanosheets. Above this

548

electrochemical characterization, it can also confirm the lower value of solution resistance (Rs),

549

charge to transfer resistance (Rct), and higher ion diffusion process (Zw) of the Bi2O3/porous-

550

RGO anode material can possess an excellent supercapactive performance.

551

3.4. Electrochemical characterization of cathode material

552

The α-MnO2-NRs cathode material was investigated in 6 M KOH electrolyte in the three-

553

electrode configuration by cyclic voltammograms (CVs) recorded at a different scan rate of 100-

554

1 mVs-1 within the potential window of 0-0.9 V (vs. Hg/HgO) as depicted in Fig. 6a. As can be

555

seen from the data, α-MnO2-NRs exhibit a pseudo-capacitive behavior via redox reaction,

556

suggesting a good reversibility and greatest electron transfer reaction take place on electrode

557

materials at all potential scan rates. Furthermore, the charge storage mechanisms of α-MnO2-

558

NRs can be understood from the intercalation/deintercalation ions. The K+ ions enter into the

559

electrode surface (α-MnO2-NRs) based on the intercalation/deintercalation phenomenon, which

560

occurs through the reversible reaction between Mn-ionic states during redox phenomenon as the

561

following reaction (MnO2 + K+ + e- ↔ MnO.OK). On the other hand, the electrode surface

562

involves the redox reaction in terms of adsorption/absorption of electrolyte cations (K+) as the

563

following reaction (MnO2 surface + K+ + e- ↔ MnO2- K+ surface) [59, 60]. The α-MnO2-NRs show

564

the specific capacitance of 142, 177, 195, 223, 249, and 369 Fg-1 at a scan rate 100, 50, 20, 10, 5,

565

and 1 mVs-1. At very low scan rate (1 mVs-1), α-MnO2-NRs exhibited the highest specific

566

capacitance due to a good reversibility through anodic peak current density (-ipa) divided by

567

cathodic peak current density (+ipc) equal to 1, (i.e.)

568

rate (100 mVs-1) exhibited for the lowest specific capacitance due to its quasi-reversibility or

–cd .e

Rcf g.h

25

= 1. Nevertheless, the faster scan

cd ij

≠ 1 n61, 62p. Furthermore, the capacitance retention for the α-MnO2-

569

irreversibility

570

NRs was found to be about 80.7%. In addition, initial (1st) and final (3000 th) cycles exhibited the

571

223 and 180 Fg-1, respectively, at a scan rate of 10 mVs-1 as presented in Fig. 6b. The GCD

572

measurements were used to calculate the specific capacitance of α-MnO2 NRs electrode within

573

the potential window of 0 - 0.9 V and depicted in Fig. 6c. A comparison of GCD profiles was

574

made at various current densities from 1 to 10 Ag-1, where the specific capacitance was found

575

755 Fg-1 at current density of 1 Ag-1. However, while the current density increased from 1 to 10

576

Ag-1, the specific capacitance values decreased from 755 to 366 Fg-1 due to quasi-reversible

577

redox reaction increasing from lower to higher current density, indicating a decreased electron

578

transfer rate from 1 to 10 Ag-1. Besides, the resistance behavior of α-MnO2-NRs can be

579

determined by EIS spectra and displayed in Fig. 6d. The charge transfer resistance (Rct = 4.5 Ω)

580

and solution resistance (Rs = 0.4 Ω) were found at higher frequency region, designating rapid

581

charge transfer kinetics and higher electrical conductivity between the electrode material and

582

electrolyte [63]. The Warburg resistance (WR = 5.6 Ω) was originated at lower frequency region,

583

which indicates the kinetics for fast diffusion of electrolyte ion into electrode materials. The Rs,

584

Rct, and WR values were obtained based on the equivalent circuit fit inset in Fig. 6d.

585

3.5. Asymmetric supercapacitor assembled in two-electrode configuration

Rcf kl.Q

586

The solid-state ASC device was assembled by α-MnO2 NRs as a positive electrode (8 mg

587

cm-2) and Bi2O3/porous-RGO NS as a negative electrode (8 mg cm-2) using gel electrolyte of

588

PVA/KOH. As depicted in Fig. 7a, the corresponding cathode (+) and anode (-) materials

589

displayed within different potential for (0 to 0.9 V) and (0.2 to -1.2 V) ranges of three-electrode

590

configuration at a constant scan rate of 50 mVs-1. The results confirm the stable cell voltage up to

591

1.8 V using such an assembled ASC device. The CV data onto the solid-state ASC device was 26

592

measured at various scan rate (100 - 5 mVs-1) under 1.8 V, suggesting operating voltage 1.8 V

593

can readily achieve without decomposition of gel-electrolyte as revealed in Fig. 7b. The ASC

594

device achieved the specific capacitance values equation (1) of 84, 75, 67, 61, and 50 Fg-1,

595

respectively, at a scan rate of 5 to 100 mVs-1. The superior performance of ASC device was

596

further confirmed from GCD curve at a various loading current density (10-1 Ag-1) as shown in

597

Fig. 7c. The ASC device achieved the specific capacitance values of 6, 7, 22, 42, 47, and 53 Fg-1,

598

respectively, at a current density of 10-1 Ag-1. The highest specific capacitance was obtained at 1

599

Ag-1 due to a good reversibility (redox reaction) at 1 Ag-1, which agrees well with CV curves.

600

The GCD measurements of ASC device were further performed within different potential

601

windows within 1.0-1.8 V at a current density of 1 Ag-1 and shown in Fig. 7d, indicating the

602

stable electrochemical behavior of assembled ASC within potential voltage from 0-1.8 V.

603

After 3000 cycles, the ASC devices still exhibited an excellent capacitance retention of

604

91.4 % and loss of capacitance 8.6 % at a constant current density of 1 Ag-1. According to

605

equation (3), the first and final cycles achieved to the areal capacitance of 784 and 717 mF cm-2

606

and depicted in Fig. 8a. The capacitance of ASC devices was calculated as a function of current

607

density as presented in Fig. 8b. A high areal capacitance of 787 mF cm-2 (54 Fg-1, based on the

608

mass of electrode material coated on nickel foam surface at an approximate loading of 8 mg cm-

609

2

610

Ragone plot of energy and power density of ASC devices was calculated according to equation

611

(4) and (5). Furthermore, the ASC has exhibited a volumetric capacitance of 74 Fcm-3 (54 Fg-1)

612

at a current density of 1 Ag-1 as seen in Fig. S17. The solid-state ASC displays the maximum

613

energy density of 86 Wh kg-1 at a power density of 9000 W kg-1 and this remains delivered 10

614

Wh kg-1 at a power density of 11000 W kg-1 calculated at a current density of 1 and 10 Ag-1,

) can be achieved at a current density of 1 Ag-1 and shown in Fig. 8b. As depicted in Fig. 8c, the

27

615

respectively. These values are compared and higher than that of recent reports of Bi-based ASC

616

in reported literature as shown in Table S4. The inset in Fig. 8c has shown the red light emitting

617

diode illuminated by the fabricated Bi2O3/porous-RGO Ns)//α-MnO2-NRs devices, indicating

618

promising electrode materials in the practical application of energy storage.

619 620

4. Conclusions

621

In summary, the unique structure of the Bi2O3/porous-RGO (catalytic carbon gasification)

622

and α-MnO2-NRs (hydrothermal) materials was successfully prepared. In the three-electrode

623

system, the unique morphology of Bi2O3/porous-RGO Ns and α-MnO2-NRs achieved the

624

maximum specific capacitance of 285 and 755 Fg-1 at a current density of 1 Ag-1. Besides,

625

significant rate capability and excellent cyclic durability were observed at a potential window of

626

the anode (-1.2 to 0.2 V) and cathode (0 to 0.9 V) in 6 M KOH electrolyte. This excellent

627

electrochemical performance can be attributed to the synergetic effect of the highly conducting

628

porous-RGO nanosheets possessing mesoporous morphology, nanosized of α-MnO2-NRs, and

629

ultra-small Bi2O3 nanoparticle. In the two-electrode system, the ASC delivered an outstanding

630

areal capacitance of 787 mF cm-2 and excellent capacitance retention of 91.4 % obtained after

631

3000 cycles within the working voltage reaching 1.8 V at a constant current density of 1 Ag-1.

632

The ASC achieved a remarkable performance of the maximum energy density of 86 Wh kg-1 at a

633

power density of 9000 W kg-1 and maintained 10 Wh kg-1 at a power density of 11000 W kg-1 in

634

PVA/KOH gels electrolyte. This work has demonstrated the excellent electrochemical

635

performance of Bi2O3/porous-RGO and α-MnO2-NRs composites, which has laid the foundation

636

for a new class of electrode materials for high-performance aqueous asymmetric supercapacitor.

637

28

638 639

ACKOWLEDGEMENT

640

The authors wish to thank for the financial support by the Ministry of Science and

641

Technology (MOST) in Taiwan under the contract number of 105-2221-E-035-002-MY2. In

642

addition, the support of Taiwan-India joint project (NSC-102-2923-E-035-001-MY3 and DST-

643

GITA/DST/TWN/P-50/2013) is also acknowledged. The support in providing the fabrication and

644

measurement facilities from the Precision Instrument Support Center of Feng Chia University is

645

also acknowledged.

646 647 648

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36

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37

650

Figure Caption

651 652

Scheme 1. Schematic illustration of the synthesis routes towards the Bi2O3/porous-RGO

653

supercapacitor electrode

654

Fig. 1. Morphological characterization of supercapactive electrode materials for (a) bulk-

655

Bi2O3/RGO (inset part shows the SAED pattern), (b) the average size of the Bi2O3 nanoparticles

656

calculated from the Bi2O3/porous-RGO (c) The TEM image of obvious porous nanostructures of

657

Bi2O3/porous-RGO electrode at different magnification shows in (c, d). The HR-TEM image of

658

Bi2O3 nanoparticles encircled by the porous-RGO nanosheets is depicted as (e, f). The lattice

659

distance was calculated from the FFT and IFFT spectrum of Bi2O3/Porous-RGO electrode is

660

clearly illustrations of (g-i)

661

Fig. 2. (a) XRD, (b) Pore size distribution curve (PSD), (c) FTIR, and (d) TGA (inset: maximum

662

weight loss rate temperature T (mwlr) calculated from Bi2O3) for the Bi2O3/porous-RGO, bulk-

663

Bi2O3/RGO, RGO, and Bi2O3 electrode materials

664

Fig. 3. (a) FE-SEM images, (b) XRD and enlarged pattern (inset) in (211) planes, (c) FTIR, and

665

(d) BET (inset pore size distribution curve) of α-MnO2- NRs

666

Fig. 4. Typical CV results in (a) Bi2O3/porous-RGO, (b) bulk-Bi2O3/RGO, (c) RGO and (d)

667

Bi2O3 within the potential range of -1.2 to 0.2 V at a scan rate of 5 mVs-1 to 100 mVs-1

668

Fig. 5. (a) and (b) the Specific capacitance versus scan rate and a current density of all electrode

669

materials, (c) and (d) the calculated specific capacitance and capacitance retention for 3000 cycle

670

number of prepared electrode materials

671

38

672

Fig. 6. (a) CV curves at a different scan rate (100-1mVs-1), (b) cyclic stability at 10 mVs-1, (c)

673

GCD curves at a different current density (10-1Ag-1), and (d) EIS spectra (inset) equivalent

674

circuit fit of α-MnO2-NRs

675

Fig. 7. (a) CV curves compared to anode/cathode materials within their respective stable

676

potential window at a scan rate of 50 mVs-1, (b) CV curves of the ASC at a different scan rate

677

(100-1 mVs-1), (c) GCD curves of ASC at a different loading current density (10-1 Ag-1), and (d)

678

GCD curve of Bi2O3/porous-RGO Ns//α-MnO2-NRs at a different applied potential for 1.8 to 1.0

679

V

680

Fig. 8. (a) Areal capacitance and capacitance retention vs. cycle number of (inset) first and final

681

5 cycles of corresponding ASC devices at a 1 Ag-1, (b) Areal and specific capacitance calculated

682

at a different current density (1-10 Ag-1), and (c) The energy density vs. power density of ASC

683

device and (inset) connected with LED bulb illustration lighting

684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 39

699 700

Scheme. 1

701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 40

723 724

Fig. 1

725 726 727 728 729 730 731 732

41

733

Fig. 2

734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755

42

756

Fig. 3

757 758 759

(a)

760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 43

785

Fig. 4

786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805

44

806

Fig. 5

807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828

45

829

Fig. 6

830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851

46

852 853

Fig. 7

854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874

47

875

Fig. 8

876 877 878 879 880

48

Highlights




Bismuth oxide nanoparticles were successfully dispersed on porous reduced graphene oxide nanosheets by catalytic carbon gasification method




The capacitance retention of Bi2O3/porous-RGO and α-MnO2-NRs has been achieved up to 81.1 % at 0.5 Ag-1 and 80.7 % 10 mVs-1




The ASC delivered an energy density of 86 Wh kg-1 (787 mF cm-2) at a power density of 9000 W kg-1