Field-Enhanced Nanoconvection Accelerated Electrocatalytic

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Field-enhanced Nanoconvection Accelerates Electrocatalytic Conversion of Water Contaminants and Electricity Generation Qinghua Ji, Gong Zhang, Huijuan Liu, Ruiping Liu, and Jiuhui Qu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b06620 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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Environmental Science & Technology

Field-enhanced Nanoconvection Accelerates Electrocatalytic Conversion of Water Contaminants and Electricity Generation Qinghua Ji†, Gong Zhang†, Huijuan Liu†, Ruiping Liu†, ‡, *, Jiuhui Qu † ‡ †

Center for Water and Ecology, State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China

‡

Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

Corresponding author：Prof. Dr. Ruiping Liu Tel: +86-10-62849160, Fax: +86-10-62923558, E-mail: [email protected]

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ACS Paragon Plus Environment


1

ABSTRACT

2

The development of high-performance electrocatalytic systems for the extraction of

3

energy from contaminants in wastewater are urgently needed in emerging renewable

4

energy technologies. However, given that most of contaminants are present in low

5

concentrations, the heterogeneous catalytic reactions often suffer from slow kinetics

6

due to mass transfer limitations. Here, we report that localized free convection induced

7

by enthalpy change of the reaction can enhance interfacial mass transport. This

8

phenomenon can be found around high-curvature nano-sized tips. The finite-element

9

numerical simulation shows that the heat of reactions can produce temperature

10

gradients and subsequently lead to fluid motion at the interfaces, which facilitates the

11

rate-limiting step (mass transfer). To demonstrate the effects of localized field-

12

enhanced mass transport in electrocatalytic conversion of aqueous dilute species, a

13

galvanic cell is constructed with vertically-aligned polyaniline array with sharp tips (as

14

cathode) for the detoxification of low concentration carcinogenic chromate and

15

synchronous electricity generation, which show lower overpotential (0.17 V

16

decreased), higher reaction rate (increased by 28%) and power density (22.3 W m−2 in

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2 mM chromate). The power output can be scaled up (open voltage of ~3.7 V and

18

volumetric power density of 840.1 W m−3) by using a continuous flow-through cell with

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stacked electrodes for further improve the mass transport.

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KEYWORD: Energy harvesting; electrocatalytic; environmental decontamination;

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field-enhanced effect; finite element simulations

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INTRODUCTION

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Increasing industrial chemical pollution in worldwide water systems is one of largest

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environmental problems facing humanity 1. Although most of these contaminants are

25

present at relatively low concentrations, many can cause great challenges to ecological

26

sustainability and human health 2. To help mediate the increasing burdens being placed

27

on energy and the environment, the water industry is moving toward resource and

28

energy recovery from wastewater because the chemicals in wastewater represent a large

29

potential source of energy and valuable substances 3-5. The chemical energy stored in

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energy-rich chemical bonds of high enthalpy molecular can be released through

31

efficient energy conversion systems 5. Unfortunately, due to their low concentrations,

32

the ability to convert these substances into electricity is limited. Chemical reactions

33

often suffer from slow kinetics, particularly in heterogeneous and multiphase processes

34

6,

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electrode reaction depends on the nature of the electrode surface and interfacial mass

36

transfer 7, it is critical to optimize electrocatalysts with particular characteristics to

37

facilitate electron and interfacial mass transfer.

38

resulting in low energy harvesting efficiency. Considering that the rate of the

The geometrical morphology of nanomaterials is related to their subsequent activity

39

8, 9.

40

catalysis due to their efficient charge transport and high electrochemically active

41

surface area

Recently, 1D nanostructure arrays have received attention for photo- or electro-

10, 11.

Theoretical calculations show that the electric field surrounding 3



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nanowires or nanorods can be enhanced, which can then lower the overpotentials of

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electrode reactions. Additionally, needle-like tips can concentrate the reactants and

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locally accelerate the reactions around nanometer-sized tips

45

the phase, enthalpy, or temperature induced by chemical reactions and their influences

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on the local reaction processes have never been explored, which are of both

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fundamental and practical importance. Here, we report that local enthalpy changes of

48

reaction near the high-curvature tips induces temperature change, which causes

49

nanoscale free convection, subsequently accelerates mass transport and improves

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energy harvesting efficiency from contaminates in wastewater.

12.

However, changes in

51

High enthalpy contaminants account for a large proportion of discharged pollutants

52

among various industrial chemicals 13, 14. Furthermore, higher oxidation states, such as

53

high valence radioactive metals [U(VI)15, Tc(VII)16, Np(V)17, Pu(VI)18] and

54

carcinogenic metals [Cr(VI)19, V(V)20], tend to show higher potential for migration and

55

greater threat to the environment21, 22. Therefore, their reduction to lower oxidation

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states can minimize the potential threats. During reduction processes, redox reactions

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can provide electrical energy through rationally designed redox-based electrochemical

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systems 23. As a particularly example, hexavalent chromium (chromate) is a worldwide

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water contaminant, which can produce carcinogenic effects on the liver, kidney and

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immune systems, and considers to have 100-fold more toxicity than trivalent chromium

61

13.

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Polyaniline (PANI) has been previously used to reduce chromate due to its high

63

activity and unique redox chemistry 24, 25. The example here we used is a localized field-

64

enhanced redox cell (FRC) based on a PANI nanoarray as the cathode and iron as the

65

anode (denoted as a Fe/PANI cell), which demonstrated efficient detoxification of

66

chromate and synchronously derived electricity from spontaneous redox reactions of

67

chromate (HCrO4-/Cr3+) taking place on the electrodes. The reduction of chromate to

68

trivalent chromium is an exothermic reaction and the localized enthalpy change-

69

induced free convection accelerates the interfacial mass transport. We constructed a

70

Fe/PANI flow-through FRC capable of sufficient high detoxification efficiency, while

71

maintaining stable electricity generation.

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MATERIALS AND METHODS

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Material preparation. The PANI nano array was obtained through electrodeposition

74

using 0.5 M aniline (in 1 M HClO4) solution as the electrolyte. The indium tin oxide

75

(ITO) substrates were sequentially ultrasonicated in deionized water, acetone, and

76

isopropyl alcohol. A graphite fiber felt (5 mm thick) was treated with 10% HNO3 for 2

77

h and washed with deionized water. The electrodeposition processes were carried out

78

in a three-electrode system (graphite fiber felt or ITO as working electrode, Pt foil as

79

counter electrode, and Ag/AgCl as reference electrode) with the galvanostatic method.

80

Using ITO as the substrate, the procedure involved: 0.08 mA cm-2 for 10 min, followed

81

by 0.04 mA cm-2 for 2 h, and 0.02 mA cm-2 for another 2 h. Using graphite fiber felt as 5



82

the substrate, the procedure involved: 2.0 mA cm-2 for 10 min, followed by 1.0 mA

83

cm-2 for 2 h, and 0.5 mA cm-2 for another 2 h. After electrodeposition, the as-prepared

84

electrodes were taken out and washed with deionized water.

85

Fabrication of the Fe/PANI flow-through cell. Six pairs of 3 mm-thick Fe plates and

86

PANI@GF separated by 2 mm-thick nonconductive porous separators were stacked

87

and loaded into an airtight flow-through cell (denoted as the Fe/PANI flow-through

88

cell). Each pair of Fe and PANI@GF electrodes were connected in series. Pt wires were

89

used to connect the end electrodes to a LED array. The influent was injected into the

90

Fe/PANI flow-through cell through two inlets by a peristaltic pump and the effluent

91

was sampled at regular intervals.

92

Characterization. The materials were characterized by field-emission scanning

93

electron microscopy (SEM, SU8020, Hitachi), attenuated total reflectance-Fourier

94

transform infrared spectroscopy (ATR FT-IR, Tensor 27 Spectrometer, Bruker), and

95

confocal Raman microscopy (inVia-Reflex, 532 nm incident laser, Renishaw). Total Fe

96

and Cr concentrations were analyzed by inductively coupled plasma optical emission

97

spectrometry (ICP-OES, 710, Agilent Technologies). The Cr(VI) was measured on the

98

basis of the conventional 1,5-diphenylcarbazide spectrophotometric methods using a

99

UV-vis spectrophotometer (U-3010, Hitachi) at 540 nm.26 Electrochemical assessments

100

of the Fe/PANI (flow-through) cell were conducted using an electrochemical

101

workstation (Interface1000, Gamry Instruments). 6


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Numerical simulation. The surface current density on the PANI array and heat transfer

103

were simulated using the COMSOL Multiphysics® 5.2. The “Electrochemistry”

104

module was used to solve the electric current density distribution on the tip under an

105

average current density of -20 A m-2. The “Heat transfer in fluids” and the “Laminar

106

flow” modules were combined to solve the thermal convection driven by temperature

107

gradient.

108 109

As the reduction product of Cr(VI) was Cr(III), the heat change was determined by calculating the enthalpy change in the following reaction.25

110

HCrO4-+7H++3e→Cr3++4H2O

111

Standard enthalpy change of the above reaction was calculated according to:

112

ΔHº =Σ (νΔHfo) (products) – Σ (νΔHfo) (reactants)

113

where, v is the stoichiometric coefficient and ΔHfo is the standard enthalpy of

114

formation.

115

The standard enthalpies of formation of each compound were 27:

116

ΔHfo (HCrO4-) = -878.22 kJ mol-1, ΔHfo (H+) = 0 kJ mol-1, ΔHfo (Cr3+) = -251 kJ mol-1,

117

ΔHfo (H2O) = -285.83 kJ mol-1

118

Thermodynamic analysis of the above reaction showed an exothermic process.

119

Assuming the electrochemical reaction occurred at constant pressure under standard

120

conditions, the enthalpy change (ΔHºcal) was calculated to be -516.1 kJ/mol.

121

RESULTS AND DISCUSSION 7



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Nanoscale field-enhanced effects on high-curvature surface. To obtain the desired

123

PANI array, we used a stepwise polymerization method that included three current

124

density steps (see in Materials and Methods). The PANI nanoarray can easily grow on

125

various substrates using this simple electrochemical method. Through this procedure, a

126

densely-aligned PANI array with high-curvature cone-shaped tips was readily obtained.

127

The high-curvature structure can concentrate electric fields and affect ion distributions,

128

further influencing the efficiency of heterogeneous processes 12. Here, a finite-element

129

numerical simulation was performed to explore field enhancement by the vertically-

130

aligned PANI array at the nanometer scale. Cone-shaped tips, with height of ~150 nm

131

and average diameter of ~60 nm, were used to represent the PANI array (Figure 1a and

132

Figure S1). When a negative bias was applied, the free electron density on the array

133

and cation concentration around the high-curvature tip were enhanced

134

concentrated free electrons on the PANI tips further allowed the enhancement of local

135

current density (Figure 1b). Based on the numerical simulation, it can be predicted that

136

the vertically-aligned array with a high-curvature tip is a promising structure for

137

creating high electric fields to overcome potential barriers.

8


12.

The

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b

a

x103 2.0

x104 2.0

d

1.0

1.0

0

0

Surface current density (A m-2)

c 1.0 0.5

Bare ITO PANI fiber PANI array

Cr(VI) concentration (C/C0)

Current density (mA cm-2)

e

0.0 -0.5 -1.0

0.17 V

-1.5 -2.0

-0.4 -0.2 0.0

138

Temperature gradient (K m-1)

0.2

0.4

0.6

0.8

1.0

Bare ITO PANI fiber (0.6 mg cm-2) PANI fiber (0.3 mg cm-2) PANI array

0.8 0.6 0.4

k=

0.2

-9.0

k=7 .0 8

9E 4 /s

E-4

/s

0.0 0

400

Potential (V vs. Ag/AgCl)

800 1200 Time (s)

1600

139

Figure 1. High-curvature tip of the PANI array. a, Morphology of the PANI

140

nanoarray imaged by SEM. b, Computed current density distributions on the surface of

141

cone-shaped PANI tip. Tip radius is 3 nm. c, Polarization curves of the PANI array and

142

PANI fiber network (see SEM image in Figure S2) in 1 mM chromate (pH = 2.0). d,

143

Color map of computed temperature gradient distribution around the tip. Contours show

144

the motion of fluid. e, Chromate reduction kinetics of the Fe/PANI cell using the PANI

145

array and PANI fiber network as the cathode (1 mM chromate, pH = 2.0).

146

To prove this prediction, we prepared two electrodes with either a PANI nanofiber

147

network (no high-curvature tip, Figure S2) or vertical PANI nanoarray on ITO

148

substrates, respectively. Figure 1c shows the chromate reduction current densities

149

versus applied potential for the PANI nanofiber network and vertical PANI nanoarray.

150

The linear sweep voltammetry curves show clear reduction peaks for both electrodes,

9



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151

although the PANI nanoarray exhibited more positive onset potential (0.17 V higher

152

than the PANI fiber network). This result confirms that the high-curvature tips of the

153

PANI array can lower the activation overpotential for chromate reduction.

154

It is well-known that redox reactions involve the breaking and forming of chemical

155

bonds, through which energy (in the form of heat) can be produced or consumed 27. In

156

the PANI nanoarray model, the non-uniform distributed local current density induced

157

by the geometrical morphology resulted in the non-uniform distribution of redox

158

reactions at the electrode/solution interfaces. Once thermal energy is produced (or

159

consumed) upon the detoxification of chromate, temperature gradients can be generated

160

and the local solution can undergo a free convection motion

161

quantitative impact of reaction heat on the temperature gradients and mass transfer, we

162

calculated the enthalpy change (∆H) in chromate reduction and mapped the temperature

163

gradient magnitude and velocity magnitude contour of the solution around the PANI

164

array (Figure 1d). As the reaction is an exothermic process (ΔHºcal = -516.1kJ mol-1,

165

see in Materials and Methods), the maximum temperature gradient in the interfacial

166

region reached 5.7 × 104 K m-1. This temperature non-uniformity was partially

167

responsible for solution motion (maximum velocity magnitude: 2.06 × 10-8 m s-1, dark

168

line in Figure 1d) because of local change in the fluid density (known as Rayleigh-

169

Bénard effect)29. According to the Stokes–Einstein equation (Di=RT/6πμRi), the

170

diffusion constant (Di) is strong temperature(T) dependence.7 Thus, the increased 10


28.

To evaluate the

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171

temperature can also lead to fast ion diffusion and subsequently enhance mass transport.

172

For heterogeneous reactions in dilute species, interfacial mass transport is a critical

173

factor and can greatly influence reaction kinetics and efficiency 25. The lower activation

174

overpotential and enhanced interfacial mass transport enable the fast catalytic reaction.

175

The kinetics study of the Fe/PANI cell for chromate reduction demonstrated that the

176

PANI nanoarray had a 28% higher k value (k= -9.09E-4) than that of the PANI

177

nanofiber network (Figure 1e). PANI nanofiber network with different loading

178

capacities (0.3 and 0.6 mg cm-2) were prepared and used in Fe/PANI cell for chromate

179

reduction. However, there was no obviously different in reaction kinetics. This result

180

demonstrated that although the loading capacity of PANI nanofiber network was high

181

enough, it still showed poorer performance than PANI nanoarray. These results support

182

the superiority of the high-curvature tip in accelerating the electrochemical reaction by

183

lowering the overpotential and enhancing the localized mass transfer in the

184

electrode/electrolyte interface.

185

Study on the working principle of the Fe/PANI cell. Attenuated total reflectance-

186

Fourier transform infrared (ATR FT-IR) measurement was performed to study the

187

transition of PANI in the Fe/PANI FRC (Figure 2a), which confirmed the reversible

188

transformation of PANI during chromate reduction. Two peaks found at 1600 cm-1 and

189

1500 cm-1 are associated with benzenoid ring stretching (reduced states of PANI) 30.

190

The C-N stretching absorption of the quinoid-benzenoid-quinoid unit at 1377 cm-1 and 11



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191

quinone ring-stretching deformation at 1568 cm-1 appeared with peak intensities further

192

increasing during reactions with chromate, indicating that the content of the quinoid

193

structure increased (oxidized states of PANI) 31, 32. In addition, the band shifts upon Fe

194

connection reflected the recovery of the reduced states of PANI. a

b Raman

PANI+Cr(VI) PANI+Fe

Intensity (A. U.)

PANI+Fe

PANI+Cr(VI)

Intensity (A. U.)

PANI

PANI

ATR FT-IR

1800

1600 1400 Wave number (cm-1)

c

1200

1600

1400

1200

Raman shift (cm-1)

Electricity generation

HCrO-4

Electron donor

PANIRe Fe2+

PANIOx

Fe

Cr3+ Electron acceptor

195

Electron

196

Figure 2. Working principle of the Fe/PANI cell on chromate reduction and

197

electricity generation. a, ATR FT-IR study of PANI transformation. b, Raman study

198

of PANI transformation. c, Proposed mechanism of the Fe/PANI cell for chromate

199

reduction and electricity generation. Blue dots represent electrons.

12


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200

Raman spectroscopy confirmed the reversible transformation of PANI during

201

chromate reduction (Figure 2b). The key bands for p-disubstituted benzene rings

202

(reduced states of PANI) at 1620 cm-1 and 1190 cm-1 diminished upon injection of

203

chromate; new bands located at 1167 cm-1 (C–H deformation in quinoid rings), 1221

204

cm-1 (C–N stretching in amino site), 1493 cm-1 (C=N stretching in quinoid rings), and

205

1582 cm-1 (C–C stretching in quinoid rings) characterized the formation of semiquinone

206

radical cations and a quinoid-like structure, reflecting the oxidation of PANI 33, 34. The

207

band changes were reversed upon the connection of Fe with PANI, as the typical bands

208

for the reduced form of PANI (1620 cm-1 and 1190 cm-1) were recovered.

209

The above observations suggest that PANI underwent reversible changes between

210

reduced and oxidized states in the Fe/PANI FRC. We propose that the working

211

principle of the Fe/PANI FRC can be written as in Figure 2c. Overall, a net reduction

212

of chromate took place and PANI acted as an electrocatalyst, while Fe spontaneously

213

donated electrons, through which electricity was generated.

214

Electrochemical assessments of the Fe/PANI field-enhanced redox cell. To assess

215

the localized field-enhanced effects on the energy extraction performance, the Fe/PANI

216

FRC was constructed using iron foil (thickness, 200 µm) as the anode and the densely-

217

aligned PANI nanoarray (on carbon fiber) as the cathode (Figure 3a, b). As discussed

218

above, PANI has a continuum of oxidation states ranging from reduced

219

leucoemeraldine to oxidized pernigraniline forms, and thus has multifunctional 13



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220

electrochemical properties. To illustrate the advantage of this redox-conducting

221

polymer in a galvanic cell, different oxidation states of PANI were used as electrodes

222

(cathodes). The Fe/PANIRe (reduced state of PANI) and Fe/PANIOx (oxidized state of

223

PANI) cells (5 mM sulphuric acid as electrolyte) generated average open-circuit

224

voltages (VOC) of ~0.62 V and ~0.88 V, respectively, demonstrating that the output

225

voltage of the Fe/PANI FRC was greatly affected by the oxidation state of the PANI

226

(Figure 3c). Due to the reduction of PANIOx during discharge, the VOC of the

227

Fe/PANIOx cell started at ~1.02 V and slowly decreased to 0.88 V. This decrease was

228

caused by the reduction of PANI by Fe. Assuming there were oxidants with moderate

229

oxidizing ability to keep the PANI in oxidized state, the VOC of the Fe/PANI FRC could

230

maintain stable. a

b

c

1 um

Open circuit voltage (V)

1.2 1.0 0.8 0.6 0.4

Fe/PANIOx (1 mM HCrO4-+5 mM H2SO4) Fe/PANIRe (5 mM H2SO4) Fe/PANIOx (5 mM H2SO4)

0.2 0.0

15

5

10 15 20 25 30 Current density (A m-2)

35

40

0.8 0.6 0.4

k=

0.2

k=

-0 .

00

12

-0.0

008

/s

/s

/s

0

PANI Fe Fe/G cell Fe/PANI FRC

23

5

1.0

00

1 mM 2 mM 5 mM 10 mM

10

Time (s)

. -0

Cr(VI) concentration (C/C0)

20

0

231

e

25

500 1000 1500 2000 2500 3000

k=

Power density (W m-2)

d

0

0.0 0

200

14


400

600 800 Time (s)

1000 1200

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232

Figure 3 Schematic and electrochemical performance of the Fe/PANI FRC. a,

233

Schematic of the Fe/PANI FRC for chromate reduction and electricity generation. b,

234

SEM image of the large-scale densely-aligned PANI array. c, Open circuit voltages of

235

the Fe/PANI cell influenced by the oxidation states of PANI and electrolytes. d, Power

236

densities of the Fe/PANI FRC with different initial chromate concentrations (pH = 2.0).

237

e, Chromate reduction kinetics with PANI, Fe, Fe/G cell, and Fe/PANI FRC (1 mM

238

chromate, pH = 2.0).

239

The VOC of the Fe/PANI FRC (1 mM chromate with 5 mM sulphuric acid as

240

electrolyte) starts at ~1.09 V and climbs to a stable value of ~1.15 V, indicating that the

241

Fe/PANI cell with chromate as the electrolyte is capable of working with a relatively

242

high voltage and suggesting that chromate can act as an efficient oxidant to maintain

243

PANI in an oxidized state through the reduction of chromate to chromic 25. Furthermore,

244

the Fe/PANI FRC was capable of extracting energy from aqueous chromate, with the

245

areal power density influenced by the initial chromate concentration (Figure 3d). The

246

cell using 2 mM and 5 mM chromate as the electrolytes exhibited high maximum power

247

densities (∼22.3 and 21.6 W m−2, respectively).

248

The advantage of the Fe/PANI FRC on the reduction of chromate is shown in Figure

249

3e. The kinetics of all setups were pseudo-zero-order reactions in chromate reduction,

250

and thus the reaction rates were independent of chromate concentration. Among all the

251

setups, the Fe/PANI FRC achieved higher reaction kinetics (k = -0.0023/s) than that of

252

the Fe/G (graphite fiber) cell (k = -0.0012/s) or Fe alone (k = -0.0008/s). This result

253

demonstrates that PANI nanoarray exhibited high electrocatalytic activity toward 15



254

chromate reduction due to the lowed overpotential and enhanced interfacial mass

255

transfer. This high activity helps to increase the energy harvesting efficiency from

256

chromate. The utilization of redox conductive polymer instead of carbon material

257

represents a promising direction for realizing efficient energy extraction from aqueous

258

redox species. In the above processes, Fe can be considered as an electron donor and

259

reduce Cr(VI) directly, because iron-based materials had strong reactivity towards

260

reducing chromate by providing and transferring electrons (Fe/Fe2+ = -0.440 V vs NHE)

261

35, 36.

262

electrode, which also influences the power generation of the cell. As the VOC of the

263

FRC cell was stable during the experiments, the Fe electrode maintained a constant

264

dissolution rate, resulting in a pseudo-zero-order reactions of chromate reduction. This

265

inference was experimentally confirmed by measuring the total iron ions in the

266

electrolyte (Figure S3). The dissolved Fe can be removed by chemical precipitation.

267

Practical application. To demonstrate the possible application of the Fe/PANI FRC,

268

we prepared a PANI nanoarray supported on graphite fiber felt (GF) (Figure S4) and

269

stacked six pairs of Fe/PANI(GF) in a single flow-through cell to improve power

270

generation and chromate detoxification performance (Figure 4a and Figure S5). This

271

flow-through configuration provided porous electrodes with enhanced mass transfer

272

and allowed an efficient and more controllable electrochemical process 25. Therefore,

273

the generated voltage was influenced by the flow velocity through the cell (Figure 4b).

Thus, chromate reduction rates depend on the dissolution of Fe2+ from an Fe

16


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274

The Voc increased exponentially with increasing flow velocity through the cell. For 1

275

mM chromate solution (pH = 2.0), the Voc increased from ~1.31 V at 0.3 cm min-1 to

276

~3.70 V at 10 cm min-1 and fitted perfectly to an exponential association model (Figure

277

S6). Interestingly, due to the high conductivity of PANI nanoarray, the chromate

278

reduction efficiency remained as high as nearly 100% even when the flow velocity

279

reached 10 cm min-1 (Figure 4c). This phenomenon indicates that energy harvesting

280

and chromate decontamination were simultaneously achieved with the high efficiency

281

flow-through Fe/PANI FRC. c

b 4.5 1 mM + 5 mM H2SO4 5 mM H2SO4

Cr(VI)


4.0 Cr(III)

100

HCrO4-

3.5

10 cm min-1

3.0 2.5

5.0

2.0

2.5 1.7

1.5 1.0

1.0

0.5

0.3

Cr(VI) residual (%)

a

Inflow

10

1

0.5 0.0

2000 3000 Time (s)

0.1

4000

2.5 cm min-1 5.0 cm min-1 10 cm min-1

800 600 400 200 0

0

100 200 300 400 500 600 700 Current density (A m-2)

0.3

0.5 1.0 1.7 2.5 5 Flow velocity (cm min-1)

10

f 3.5


1000

Power density (W m-3)

1000

e

d

282

0

3.0 2.5

Cell 1 Cell 2 Cell 1+2 (In series)

2.0 1.5

Voc

1.0 0.5 0.0

Cell 1 Cell 2 0

100

200 300 400 Time (s)

500

600

283

Figure 4. Performances of the Fe/PANI flow-through FRC. a, Schematic of the

284

Fe/PANI flow-through FRC. b, Open circuit voltages versus flow velocity of the

285

Fe/PANI flow-through FRC in different electrolytes. Flow velocity ranged from 0.3 to

286

10 cm min-1. c, Chromate reduction performance of the Fe/PANI flow-through FRC at 17



287

different flow velocities. (1 mM chromate, pH = 2.0). d, Power densities of the Fe/PANI

288

flow-through FRC at different flow velocities (2 mM chromate, pH = 2.0). e, Open

289

circuit voltages versus time curves of two individual cells and their series connections.

290

Inset: circuit diagram. (electrolyte: 1 mM chromate, pH = 2.0). e. Photograph of LED

291

array driven by two flow-through FRCs connected in series (electrolyte: 1 mM

292

chromate, pH = 2.0).

293

The volumetric power density of the Fe/PANI flow-through FRC can be controlled

294

by tuning the flow velocity of the electrolyte (Figure 4d). A maximum volumetric

295

power density of 840.1 W m−3 was produced at 10 cm min-1. From a practical

296

perspective, it is important that the Voc of the flow-through FRC can be scaled up

297

through series connections of multiple cells (Figure 4e). When two cells (Voc = 1.5 V)

298

were connected in series, the Voc was the sum of the two cells (~3.0 V), which was high

299

enough to power a 3 × 3 LED array (Figure 4f). Furthermore, the brightness of the

300

LEDs can be controlled by changing the flow velocity through the cell (Figure S7,

301

Figure S8, and Movie S1).

302

ENVIRONMENTAL IMPACT

303

The localized field-enhanced cell using densely-aligned 1D redox polymer nanoarray

304

and active metal presents a potentially high efficiency energy conversion method to

305

recover energy from industrial discharged high enthalpy species. Furthermore, as the

306

power and energy output from the flow-through cell can be notably enhanced, it is

307

possible to design devices with sufficient electricity for practical application, such as

18


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Page 19 of 24


308

self-driven water purification and desalination processes. However, extractions of

309

energy from various compounds involving both pollutants and electrode materials are

310

more complex than this demonstration. For practical applications, the electrochemical

311

cell design and operation need to be carefully evaluated and optimized. A good

312

understanding of the micro-reaction or micro-process on a single tip could be a

313

significant challenge 37. Nevertheless, it is an important step towards the development

314

of a localized field-enhanced micro-reactor integrated system with compact and small

315

equipment and better performance than conventional bench-scale systems.

316

We report that localized enthalpy change-induced fluidic motion near high-curvature

317

tips accelerates interfacial mass transport and electrocatalytic reactions. Control

318

experiments and finite-element numerical simulations have indicated that this field-

319

enhanced effects reduces the over potential and mass transfer limitations, thereby

320

improving the conversion and energy extraction efficiency for diluted species. This

321

work provides new insights into the understanding of the geometry effect of nanosized

322

catalysts, and offer a promising approach for the electrocatalytic conversion of diluted

323

species and capturing decentralized energy from diverse low concentration high

324

enthalpy contaminants (e.g., high valence metal compounds and strong oxidizing

325

agents) in wastewater. With the demonstration of localized field-enhanced effects in

326

this study, the principle can be extended to general electrochemistry for the design of

327

efficient catalysis and broader application. 19



328

ASSOCIATED CONTENT

329

Supporting Information. Additional figures (Figure S1 to S8). This material is

330

available free of charge via the Internet at http://pubs.acs.org.

331

ACKNOWLEDGEMENTS

332

This work was supported by the National Natural Science Foundation of China (Grant

333

No. 51738013, 51438011 and 51608516) and National Key R&D Program of China

334

2016YFC0400502.

335

REFERENCES

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