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Synthesis and Stabilization of Blue-Black TiO2 Nanotube Arrays for Electrochemical Oxidant Generation and Wastewater Treatment Yang Yang, and Michael R Hoffmann Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03540 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 26, 2016

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Environmental Science & Technology

Synthesis and Stabilization of Blue-Black TiO2 Nanotube Arrays for Electrochemical Oxidant Generation and Wastewater Treatment

Yang Yang and Michael R. Hoffmann*

Linde + Robinson Laboratories California Institute of Technology 1200 E. California Blvd. MC 131-24 Pasadena, California 91125, USA

A manuscript submitted to Environ. Sci. Technol.

*Corresponding author: Email: [email protected]

ACS Paragon Plus Environment


Graphical abstract


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1

Abstract

2

Efficient, inexpensive, and stable electrode materials are key components of commercially viable

3

electrochemical wastewater treatment system. In this study, blue-black TiO2 nanotube array

4

(BNTA) electrodes are prepared by electrochemical self-doping. The 1-D structure, donor state

5

density, and Fermi energy level position are critical for maintaining the semi-metallic

6

functionality of the BNTA. The structural strength of the BNTA is enhanced by surface crack

7

minimization, reinforcement of the BNTA-Ti metal interface, and stabilized by a protective over-

8

coating with nano-particulate TiO2 (Ti/EBNTA). Ti/EBNTA electrodes are employed as both

9

anodes and cathodes with polarity switching at a set frequency. Oxidants are generated at the

10

anode, while the doping levels are regenerated along with byproduct reduction at the cathode.

11

The estimated maximum electrode lifetime is 16895 h. Ti/EBNTA has comparable hydroxyl

12

radical production activity (6.6×10-14 M) with boron-doped diamond (BDD, 7.4×10-14 M)

13

electrodes. The chlorine production rate follows a trend with respective to electrode type of

14

Ti/EBNTA > BDD > IrO2. Ti/EBNTA electrodes operated in a bipolar mode have a minimum

15

energy consumption of 62 kWh/kg COD, reduced foam formation due to less gas bubble

16

production, minimum scale formation, and lower chlorate production levels (6 mM vs. 18 mM

17

for BDD) during electrolytic wastewater treatment.



18

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Introduction

19

Electrochemical oxidation (EO) can be utilized in small-scale decentralized wastewater

20

treatment systems. Electrolytic treatment systems are relatively efficient, compact in design, and

21

can be easily automated for remote controlled operation.1,

22

electrochemical wastewater treatment can be hindered by several challenges, which include: 1)

23

high energy consumption costs per kilogram of COD treated in units of kWh/kg COD3,

24

depending on the composition of the electrodes 2) foam formation, and scale deposition on the

25

electrode surfaces,5-7 3) lack of control of undesirable byproduct formation,8,

26

relatively high cost of semiconductor electrode production due to the use of platinum group

27

metals a primary ohmic contact materials for transfer of electrons to the base metal.10

2

However, applications of

9

4

,

and 4) the

28

The energy consumption of EO processes (50-1000 kWh/kg COD)11-17 are significantly

29

higher than aerobic biological treatment (3 kWh/kg COD; assuming 320 g/m3 of inlet COD, 50%

30

of removal efficiency, and 0.45 kWh/m3 of energy consumption per volume).18 Foaming, which

31

is due both to the gas evolution and the presence of naturally-occurring and artificial surfactants

32

in wastewater reduces electrochemical treatment efficiency by blocking active sites on the

33

electrode surfaces. In addition, the accumulation of foam in the reactor headspace above the

34

electrochemical arrays may result in corrosion of the electrical connections. The spillover of

35

foam may also cause secondary pollution of the treatment site. Scaling, which is due to the

36

cathodic forcing of the precipitation of Ca2+ and Mg2+,1 is also undesirable since it also reduces

37

treatment efficiency and reduces the reactive interfacial surface areas. Electrolysis of chloride-

38

containing wastewater produces chlorination byproducts such as chlorate (ClO3-) and perchlorate

39

(ClO4-).8, 9, 12, 19, 20 Anodes operating at higher oxidative levels are often able to eliminate organic

40

compound byproducts at longer reaction times, however with the tradeoff of higher yields of


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41

ClO3- and ClO4-.9, 21 Commercially available electrodes are relatively expensive due to the need

42

to provide a low Schottky-barrier semiconductor in direct contact with the base-metal support.

43

For active electrodes, IrO2 or RuO2 are employed as ohmic contacts, and for nominally inactive

44

electrodes, boron-doped diamond electrodes (BDD) are employed.1, 2

45

In order to lower the cost of electrode production, research has been focused on modification

46

of Ti metal base to produce an anode that would be active for wastewater treatment. However,

47

the surface of Ti metal is easily oxidized to produce a passive layer of TiO2 during anodic

48

polarization. Titanium base-metal surfaces that are oxidized into nanotube arrays (NTA) have

49

been shown to be relatively inactive as anodes.22 However, the conductivity of NTA can be

50

improved by cathodization in an aqueous electrolyte at room temperature.23,

51

cathodization, the color of NTA turns from gray to blue-black. The chlorine evolution activity of

52

blue NTA (BNTA) has been reported to be comparable to that of IrO2 and BDD anodes.25, 26 The

53

production of hydroxyl radical (·OH) on BNTA is supported by the electrochemical degradation

54

of p-nitrosodimethylaniline (though the direct electron transfer mechanism can not be excluded).

55

25-27

56

Moreover, the previously reported active lifetimes of BNTA anodes range from a few minutes to

57

several hours before inactivation.25, 26, 28

24

After

However, the mechanism contributing to improved electrochemical activity is unclear.

58

Herein, we report on the mechanisms of activation and deactivation of BNTA and the

59

development of methods to improve the structural stability of BNTA. An operational method is

60

designed for increasing the lifetime of BNTA in electrochemical oxidant generation and

61

wastewater treatment.



62

Experimental Section

63

TiO2 NTA films were synthesized by anodic oxidation of titanium foil (6 cm2) at a constant

64

voltage of 42 V in an ethylene glycol (EG) electrolyte containing 0.25 wt% NH4F and 2 wt%

65

H2O.29 NTA with tube lengths of 10 and 16 µm were prepared after 3 h and 6 h of anodization,

66

respectively. Unless noted otherwise, the fabrication and tests are based on 16 µm NTA. Samples

67

was thoroughly rinsed by DI water followed by calcination in 450 °C for 1 h. In order to prepare

68

BNTA, the NTA was cathodized in 1 M NaClO4 solution at a current density of 5 mA/cm2 for 10

69

min. Structurally enhanced BNTA (EBNTA) was prepared by as follows: After anodization in

70

the NH4F electrolyte, the product NTA was subjected to second anodization in 5 wt% H3PO4/EG

71

electrolyte at an applied potential of 42 V for 1 h.30 After this step, the NTA was ultrasonically

72

cleaned with ethanol for 10 min and then dried under vacuum for 1 h before calcination.

73

Following calcination, a TiO2 layer was deposited on the NTA by spray pyrolysis as described

74

previously.31 EBTNA with a TiO2 over-coating layer is denoted as Ti0.5/EBTNA or Ti1/EBTNA,

75

where the subscript represents the mass loading (mg/cm2). BDD electrodes were obtained from

76

Neocoat®.

77

pyrolysis. It has been proved that TiO2 overcoating is able to enhance the chlorine evolution

78

activity of IrO2.31 Electrodes were characterized by field emission scanning electron microscope

79

(FESEM, ZEISS 1550VP), X-ray photoelectron spectroscopy (XPS, Surface Science M-Probe

80

ESCA/XPS), and Diffuse reflectance UV-Vis spectrophotometer (UV-Vis, SHIMADZU UV-

81

2101PC). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were

82

measured using a Biologic VSP-300 potentiostat. To obtain Mott-Schottky plots, EIS analyses

83

were conducted at anodic potentials of 0-1.1 VRHE with frequency ranges from 1 to 100 kHz.

IrO2 electrodes with a TiO2 overcoating (Ti/Ir) were also prepared by spray-


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84

Electrolysis was performed under constant current conditions. In the monopolar (MP) mode,

85

an anodic potential was applied in order to test the BNTA electrodes, which were coupled with

86

Pt foil cathodes. In the bipolar (BP) mode, BNTA electrodes were used as both anodes and

87

cathodes. The polarity was reversed at a given interval. Chemical Oxygen Demand (COD) levels

88

were determined using dichromate digestion (Hach Method 8000) and Total Organic Carbon

89

(TOC) concentrations were determined using an Aurora TOC analyzer. Anions and cations were

90

quantified by ion chromatography (ICS 2000, Dionex, USA).

91

Hydroxyl radical production was measured by using benzoic acid (BA) and p-benzoquinone

92

(BQ) as probe molecules. The second-order rate constants for ·OH with BA (kBA, ·OH) and BQ

93

(kBQ, ·OH) are 5.9 × 109 and 1.2 × 109 M-1 s-1, respectively.32 The quasi steady-state concentration

94

of ·OH ([·OH]ss) in the electrolysis reaction is estimated according to the pseudo first-order rate

95

constant for BA decay (kBA) or BQ decay (kBQ) in a 30 mM NaClO4 electrolyte. (eq 1-2).

d[BA] = kBA,⋅OH [BA][⋅OH]ss = kBA [BA] dt

96

[⋅OH]ss =

97

98 99

kBA kBA,⋅OH

(1)

(2)

BA and BQ concentrations were determined by HPLC (1100) using a Zorbax XDB column with 10% acetonitrile and 90% 26 mM formic acid as an eluent.

100

Free chlorine concentrations ([FC]) were measured using the DPD (N,N-diethyl-p-

101

phenylenediamine) reagent (Hach method 10102). The current efficiency was estimated by the

102

following equation

103

η =

2VFd[FC] Idt


(3)


104

where V is electrolyte volume (25 mL), F is the Faraday constant 96485 C mol-1, I is the current

105

(A).

106

Results and Discussion

107

Band Structure Analyses

108

During cathodization a variable number Ti(IV) sites within NTA are electrochemically

109

reduced to Ti(III). The effective loss of charge is compensated by H+ intercalation.23, 24 Valence-

110

band XPS measurements (Figure 1a) show that cathodization of the NTA creates conduction

111

band tail states (a relative 0.1 eV shift) in the BNTA. This shift will be more accurately

112

determined by ultraviolet photoelectron spectroscopy with higher resolution (0.055 eV)33 in our

113

ongoing evaluations. This effect appears to lead to a disordered TiO2 structure.34 DRUV-Vis

114

characterization (Figure 1b) shows that the BNTA has a stronger red and infrared absorption

115

level than NTA, but the band-gap of BNTA (3.3 eV) is slightly larger than that of the NTA (3.2

116

eV). Therefore, the cathodization-induced color change cannot be explained simply by band gap

117

narrowing, but could be attributed to the formation of continuous dopant states. The resulting

118

dopant states can be assigned to the Ti(III) centers located at energies between 0.3-0.8 eV below

119

conduction band.35

120

(Figure 1)

121

It is clear that the increase of conductivity of BNTA is not due to band gap narrowing. In

122

contrast, the position of Fermi energy level (EF) actually determines the conductivity of

123

semiconductor. If the donor state densities (ND) are very high, then the EF will be located above

124

the conduction band edge (EC), resulting in a degenerately-doped n-type semiconductor with a

125

semi-metallic character.36 Flat-band potentials (EFB) were measured as an indirect measure of

126

EF.37 As seen in Figure 1c and d, the EFB shifts from 0.35 V for NTA to -0.29 V for BNTA,


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accompanied with the sharp increase of ND (4.43 × 1019 and 2.79 × 1026 cm-3 for NTA and

128

BNTA, respectively). The shift of EFB implies the shift of EF. Calculations show that the EF of

129

BNTA is above the EC (Scheme 1a, calculations are presented in Text S1), thus BNTA can be

130

classified as a degenerately-doped TiO2. In this case, the states between EF and EC are mostly

131

filled with electrons, thus the conduction band has relatively large electron concentration,36

132

resulting in the marginal increase of conductivity. The 1-D structure of BNTA nanotubes is

133

found to be crucial for the maintaining the degenerate state. Typical TiO2 films do not yield a

134

current response in the anodic branch of CV even after cathodization (Figure S1). While BNTA

135

with tube lengths of 10 µm and 16 µm have a significant current response above 2.7 VRHE for

136

which the current densities are proportional to the tube length. In the case of the TiO2 films, the

137

excited-state hole most likely oxidizes the bulk-phase Ti(III) centers as a relaxation pathway.

138

After excitation, the BNTA structure allows for facile hole transport from the bulk-phase to the

139

surface of tube walls.38 This feature preserves the bulk Ti(III) centers for longer periods of time.

140

(Scheme 1)

141

CV analyses (Figure S1) show that the BNTA electrode has higher overpotentials for oxygen

142

evolution and hydrogen production than the reference state Ti/Ir electrodes. The onset potential

143

of BNTA (2.81 VRHE) are similar to that of BDD (2.88 VRHE), except that the maximum current

144

response of the former is ten-fold higher. This feature indicates a higher electrochemical activity

145

for the BNTA.

146

However, the lifetimes of the initial BNTA were determined to be 3 h at 10 mA/cm2 (Figure

147

2a) and 30 min at 20 mA/cm2 (Figure 2b). Deactivation was observed when anodic potentials

148

exceeded 5 VSHE. Thus, the deactivation of the unprotected BNTA can be ascribed to the

149

oxidation of Ti(III) centers at high applied anodic potentials.


This interpretation is also


150

supported by the reduced infrared absorption (Figure 1b). However, aged BNTA maintained a

151

considerable doping level of ND = 3.84 × 1025 cm-3 and an EF located above EC (Figure 1d).

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152

In order to explain the electrochemical activity of the BNTA, an electron tunneling

153

mechanism can be invoked. At an anodic potential of +2.7 VSHE, which is sufficient potential for

154

hydroxyl radical generation, on an n-type semiconductor, band bending will produce a space

155

charge layer at the solid-water surface (Scheme 1b). The width of space charge layer (dSC),

156

which is a function of anodic potential, EF, and ND, is calculated to be 1349, 0.6, and 1.5 nm for

157

NTA, BNTA, and aged BNTA, respectively (Scheme 1b, calculation is shown in Text S2). The

158

dSC for NTA is too large for electron penetration as tunneling can only happen at dSC < 1-2 nm.39

159

Given this limit, BNTA has the highest electron tunneling probability, while that of aged BNTA

160

is significantly lower due to a longer dSC. The electron tunneling mechanism is consistent with

161

experimental observations reported herein. This mechanism also explains the importance of

162

maintaining a high value of ND.

163

Stability Enhancements

164

It may be possible to enhance the lifetime of BNTA by periodically increasing the depleted

165

levels of Nd. For example, the BNTA could be used both as anodes and cathodes and operated in

166

BP mode, in which the polarity is reversed at a given interval. Consequently, this approach

167

requires BNTA to have sufficient stability in both anodic and cathodic cycles. Although

168

operating in BP mode can extend the lifetime of BNTA from 3 h to 4 h at 10 mA/cm2 (Figure 2a),

169

deactivation is still observed due to spalling off of some of the BNTA from the Ti base. It is clear

170

that the structural strength of the attached BNTA is a critical factor determining the lifetime.

171

(Figure 2)


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172

In order to improve the structural strength of the BNTA, three nano-fabrication strategies

173

were explored. First, cracks in the surface of the NTA films were minimized. Cracks are visible

174

on surface of freshly prepared NTA (Figure 3a). Cracks were formed as the result of capillary

175

forces generated by the high surface tension (72 mN/m) of water during synthesis, rinsing and

176

drying processes.40 Cracks on the surface expose the bottom portion of the attached NTA to gas

177

evolution reactions, which most likely lead to erosion and the subsequent detachment of NTA

178

films. A reduction in the occurrence of cracks in the NTA films (Figure 3d) was achieved by

179

replacing water with ethanol (22 mN/m) for rinsing. The film was then vacuum dried instead of

180

heat dried. Second, the bottom attachment points of NTA were enhanced. The presence of a

181

fluoride-rich bottom layer of BNTA most often results in poor adhesion to the metal substrate.41

182

Extended anodization in fluoride-free electrolyte results in the formation of a dense, compacted

183

layer near the bottom attachment points of the nanotubes to the titanium substrate (Figure 3e).30

184

Third, the tops of the NTA were capped with a protective TiO2 layer that was deposited using a

185

spray-pyrolysis coating procedure (Figure 3c vs f) with precise control of the loading of the

186

amount of TiO2 deposited as protective top layer. At a loading level of 0.5 mg/cm2, the deposited

187

TiO2 formed a porous layer, while at loading level of 1 mg/cm2 produced compact film (Figure

188

S2). Comparison of the CV profiles of Ti0.5/EBNTA to BNTA shows that the Ti0.5/EBNTA had a

189

higher current response (Figure S2) than the untreated BNTA. The capping of NTA tips by the

190

porous TiO2 layer appears to prevent charge leakage at the tube tips during the cathodic doping

191

process.42 More electrons were guided to reduce the Ti(IV) sites of tube wall, instead of being

192

consumed by proton reduction at the tube tips. This resulted in a heavier doping of Ti0.5/EBNTA

193

than that of uncapped BNTA. The doping of Ti(IV) with TiO2 to Ti(III) is accompanied by H+

194

intercalation to maintain charge neutrality. However, the compact TiO2 layer of Ti1/EBNTA



195

appeared to block the access of bottom NTA to H+ intercalation. Thus, the current response of

196

Ti1/EBNTA was found to be even lower than that of untreated BNTA.

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197

(Figure 3)

198

Overall, the stability of BNTA at 10 mA/cm2 was improved by crack minimization and

199

bottom layer enhancement (Figure 2a). A lifetime test carried out at 20 mA/cm2 showed that

200

capping the nanotubes with a protective overcoat of TiO2 could further increase the stability of

201

the EBNTA (Figure 2b). Even though Ti0.5/EBNTA was deactivated after 4h, layer detachment

202

was not observed. Deactivation of Ti0.5/BNTA is likely due to an increase in disorder of the

203

tubular structure, which was induced by polarity switching. More defects in the structure may

204

result in internal recombination and a loss of conductivity. The deactivated Ti0.5/BNTA can be

205

partially regenerated by re-annealing at 450 °C (Figure S3). Reducing the regenerative self-

206

doping frequency from 10 to a 30 min/cycle prolongs the operational lifetime (Figure 2b). On the

207

basis of the 7 h lifetime of Ti0.5/BNTA measured at 20 mA/cm2, the lifetime at actual operational

208

current of 5 and 1 mA/cm2 can be estimated as 257 and 16895 h, respectively. The specific

209

calculations are presented in Text S3. Long-term tests under actual time are needed to support

210

these estimated values.

211

Oxidant Generation

212

BA and BQ were chosen as a ·OH probes. Given that direct electron transfer (DET) might

213

also contribute to organic degradation, CV analyses were performed. If DET take places, an

214

increase of current should be observed at the same anodic potential.43, 44 However, this pathway

215

is excluded on EBNTA as its CV is barely affected by the presence of BA (Figure S4). In

216

contrast, DET by BA and BQ is observed on BDD. This could lead to an overestimation of

217

[·OH]ss.


Page 13 of 29


218

The Ti/Ir anode is unable to produce ·OH, since loss of BA was not observed. As shown in

219

Figure 4a, EBNTA has the highest value of [·OH]ss. The Ti0.5/EBNTA is less active for ·OH

220

production than EBNTA but comparable to BDD electrode. The existence of ·OH was confirmed

221

again using BQ as a probe molecule. The [·OH]ss as measured by BQ degradation should be

222

commensurate with that measured by BA degradation (eq 4), which is the case observed for the

223

Ti0.5/EBNTA anode.

[⋅OH]ss =

224

kBA kBA,⋅OH

≈

kBQ k BQ,⋅OH

(4)

225

It is also important to note that Ti0.5/EBNTA is able to produce ·OH at a very low current density

226

(1 mA/cm2). At a current density of 1 mA/cm2, the gas evolution reactions (water splitting) were

227

reduced significantly. The reduced gas formation rate results in a lower foam formation potential

228

during wastewater electrolysis (vide infra).

229

As shown in Figure 2b, the EBNTA electrode array has the highest selectivity and activity

230

with respect to chlorine generation. Ti0.5/EBNTA has lower chlorine evolution rate (CER) than

231

EBNTA but outperformed both the Ti/Ir and BDD electrodes in terms of CER. Even though a

232

decrease in the current density to 1 mA/cm2 resulted in the decrease in the CER of Ti0.5/EBNTA,

233

the current efficiency was much less impacted. This result indicates that Ti0.5/EBNTA has good

234

selectivity for chlorine evolution. The selectivity for the CER could be further enhanced by an

235

increased [Cl-].

236

(Figure 4)

237

In spite of the higher activity for oxidant production observed with the EBNTA electrode, the

238

Ti0.5/EBNTA, which is more durable, could be better for practical engineering applications. In

239

Figure S5, four-hour NaCl electrolysis tests with Ti0.5/NTA at current densities of 5 and 10

240

mA/cm2 and operational modes, MP and BP, are presented. Free chlorine production and ClO3-



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241

evolution are observed, while the formation of ClO4- (detection limit: 1 ppb) was not observed.

242

The concentrations of FC and ClO3- are proportional to current density. Electrolysis in the BP

243

mode produces less Cl2 and ClO3- than in the MP mode. These results indicate that use of the

244

Ti0.5/BNTA cathode may contribute to the loss FC and ClO3-. The Ti(III) centers are suspected to

245

be the active sites for ClO- and ClO3- reduction as follows:45

246

Ti3+ + ClO- + 2H + → Ti4+ + Cl- + H 2O

(5)

247

6Ti3+ + ClO3- + 6H + → 6Ti4+ + Cl- + 3H 2O

(6)

248

The reduction of ClO3- to Cl- on Ti0.5/NTA cathode is confirmed by the data presented in Figure

249

S5.

250

Potential Applications for Human Wastewater Treatment

251

The various electrodes preparations were tested for possible applications for human

252

wastewater treatment on a small scale. The observed trend for chemical oxygen demand (COD)

253

reduction had the following order: BDD > Ti0.5/EBNTA > Ti/Ir (Figure 5a). This trend matches

254

the corresponding ·OH radical production activity. With respect to NH4+ removal (Figure 5b),

255

BDD and Ti0.5/EBNTA were found to be more active than Ti/Ir; this observation is in agreement

256

with CER activity. Ammonium ion removal is achieved via breakpoint chlorination involving the

257

self-reactions of the intermediate chloramines.12, 46 BDD had the highest activity toward organic

258

compound mineralization (i.e., conversion to CO2 and H2O); 80% of the initial TOC was

259

removed from the wastewater (Figure 5c). However, a substantial amount of ClO3- (18 mM) and

260

ClO4- (3 mM) were formed during electrolysis with the BDD anode after 4 h (Figure S6). BDD

261

electrodes were operated in BP mode to take advantage of the reduction activity of BDD cathode.

262

Enhanced COD and TOC removal were found instead of the significant reduction of ClO3- and

263

ClO4-. This result implies that the hydrogen evolution and reduction of oxygen prevail at BDD


Page 15 of 29


264

cathode. The latter reaction most likely produces H2O2,47, 48 which may contribute to organic

265

removal. In the case of the Ti0.5/EBNTA electrodes operated in the BP mode, less ClO3- (6 mM)

266

was produced and no ClO4- was found after 4 h. This implies that Ti0.5/EBNTA operated in BP

267

mode could effectively reduce the formation of chlorate. Further studies on the reduction of

268

organic chlorination byproduct with Ti0.5/EBNTA cathode are underway.

269

As shown in Figure 5d, using BDD as both anode and cathode requires a higher cell voltage.

270

As a result, BP mode did not show any advantages over MP mode in terms of specific energy

271

consumption. Ti0.5/EBNTA was found to be the most energy efficient among the electrodes

272

tested. When the Ti0.5/EBNTA anode was operated at 1mA/cm2, COD could be gradually

273

removed even though the removal of NH4+ was insignificant due to the lower levels of chlorine

274

production (Figure 5a and b). Nonetheless, the lowest energy consumption for COD (62 kWh/kg

275

COD) was achieved with Ti0.5/EBNTA. This value is also among the lowest value for

276

electrochemical treatment processes (Table S1). These results are promising given that COD

277

removal can be enhanced by increasing the electrode area/ reactor volume ratio, while energy

278

consumption can be further reduced by increasing the conductivity of wastewater and by

279

reducing the electrode separation distance.

280

(Figure 5)

281

Operation in the BP mode appears to minimize depositional scaling. IC analysis (Figure S7)

282

shows that concentrations of Ca2+ and Mg2+ were constant in Ti0.5/BNTA and BDD (BP)

283

electrolysis system. While approximately 50% of Ca+ and Mg2+ were removed in the form of a

284

mixed Ca, Mg-hydroxyapatite precipitate on the cathode surface in both the Ti/Ir and BDD (MP)

285

systems. The COD and NH4+ removal efficiency of the Ti0.5/EBNTA anode operated at 5

286

mA/cm2 is commensurate with that of Ti/Ir anode operated at 25 mA/cm2 (data not shown). The



287

lower current input required by Ti0.5/EBNTA results in less gas evolution. Therefore, less visible

288

foaming is produced (Figure S8).

289

The TiO2/EBNTA dual anode-cathode system is a promising alternative for oxidant

290

generation and wastewater treatment. It is an inexpensive material to prepare at moderate

291

temperature (≤ 450 °C) under a normal atmospheric environment. In contrast, BDD can only be

292

prepared by energy assisted (plasma or hot filament) chemical vapor deposition (CVD),49 which

293

requires high temperatures and vacuum conditions. In addition, the dimensions of CVD chamber

294

limit the size of BDD electrodes. Ongoing studies of the EBNTA electrodes are focused on

295

improvement of anodic stability, utilization for water disinfection50, and for the removal of trace

296

organic pollutants.51

Page 16 of 29

297 298

ASSOCIATED CONTENT

299

Supporting information involves BNTA characterizations such as cyclic voltammeties, and

300

FESEM images. Results of electrochemical BA degradation, the electrolysis of 30 mM NaCl

301

solution, and wastewater are also enclosed. The SI text includes a calculation of the band

302

structure, width of the space charge layer, and lifetime estimation.

303

ACKNOWLEDGEMENTS

304 305

The authors gratefully acknowledge the financial support of Bill and Melinda Gates Foundation (BMGF-RTTC Grant, OPP1111246).


Page 17 of 29


306 307

308 309

Scheme 1. A schematic illustration of the positions of conduction band (CB), valence band (VB),

310

and Femi energy level (EF) at a) the flat band potential. b) Band bending at 2.7 VNHE creates

311

space charge layers with the width of dSC.

312



313

314 315

Figure 1. a) Valence-band XPS of NTA and BNTA. Triplicate analyses were performed on

316

NTA and BNTA prepared by three different batches. Data are overlapped in the figure. b)

317

DRUV-Vis spectrum of NTA, BNTA, and aged BNTA. Insert figure shows the Kuberka-Munk

318

function, from which band gap can be obtained. c-d) Mott-Schottky plots of NTA, BNTA and

319

aged BNTA obtained in a 0.1 M phosphate buffer solution at pH 7.2.

320


Page 18 of 29

Page 19 of 29


321 322

323

Figure 2. Accelerated lifetime test of pristine and enhanced BNTA operated in MP and BP

324

modes at a) 10 and b) 20 mA/cm2. Electrolyte is 1 M NaClO4. Electrode is considered as

325

deactivated when cell voltage reaches 10 V.

326



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327

328 329

Figure 3. SEM images of a-c) pristine NTA, d) crack miminized NTA, e) bottom enhanced

330

NTA, and f) Ti0.5/EBNTA. The area marked by dash lines in figure 3e illustrates the compact

331

layer produced by a second anodization step.

332


Page 21 of 29


333 334

(b)

4 2

N TA Ti N TA 0.5 /E * B N TA **

N TA

Ti

0.5

/E B

/E B

Ti

0.5

EB

BD D

0

0.10

40

0.08

30

0.06 20

0.04 0.02

10

0.00

0 D D

6

50

B

Chlorine evolution rate (mmol/m2/s)

8

0.12

Current efficiency (%)

[OH]ss (10-14 M)

10

Ti /I EB r Ti N TA 0. 5 /E B Ti N 0. TA 5 /E B N TA **

(a)

335 336

Figure 4. a) [·OH]ss estimated from 1 mM BA electrocatalytic degradation in 30 mM NaClO4 at

337

5 mA/cm2. [·OH]ss = kBA/ k·OH. b) Chlorine evolution measured in 30 mM NaCl electrolysis at 5

338

mA/cm2. *: BQ was used as probe molecule in the electrolysis conducted at 5 mA/cm2. **:

339

Ti0.5/EBNTA was operated at 1 mA/cm2.

340



Page 22 of 29

341 342

(a)

(b)

350

25

Ti/Ir (MP)

Ti0.5/EBNTA* 20

Ti0.5/EBNTA*

250

NH4+ (mM)

COD (mg/L)

300

Ti/Ir (MP)

15

200 150

Ti0.5/EBNTA

100

5

50 BDD (MP) BDD (BP) 0 0 1

Ti0.5/EBNTA

0 2

3

0

4

1

2

3

4

Time (h)

Time (h)

(c)

(d)

100

Energy consumption (kWh/kg COD)

2500 3.3 V

80 60 40 20 0

8.0 V 9.0 V 200

2000

4.0 V

1500

5.2 V

100

200 150

50

100 50

.5 / E

(M P) B Ti N TA 0. 5 /E B N TA B * D D (M P B DD ) (B P)

0

Ti 0

Ti /Ir

P) D

D B

D D B

(B

(M P)

TA N EB

Ti 0

.5 /

r(

M P)

0

Ti /I

150

Energy consumption (kWh/kg NH4+)

TOC removal (%)

BDD (MP)

BDD (BP)

10

343 344

Figure 5. The removal of a) COD, b) NH4+, c) TOC, and d) energy consumption during

345

wastewater electrolysis at 5 mA/cm2. Ti0.5/EBNTA* was operated at 1 mA/cm2. The cell voltage

346

is marked above each bar in figure 5d. Energy consumption is calculated based on the first two

347

hours of treatment, except that for NH4+ removal with Ti/Ir electrode is based on the last two

348

hours NH4+ removal data. The Ti/Ir (MP) and BDD (MP) electrodes were coupled with Pt foil

349

cathode and operated in MP mode, while Ti0.5/BNTA and BDD (BP) were operated in BP mode

350

at polarity reversal interval of 30 min.

351


Page 23 of 29


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