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Sep 20, 2016 - In this study, blue-black TiO2 nanotube array (BNTA) electrodes are prepared by electrochemical self-doping. The 1-D structure, donor s...
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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]

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Graphical abstract

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Abstract

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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for BDD) during electrolytic wastewater treatment.

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Introduction

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

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

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can be easily automated for remote controlled operation.1,

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

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

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

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

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

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

2

However, applications of

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4

,

and 4) the

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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25-27

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

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several hours before inactivation.25, 26, 28

24

After

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

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

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

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

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wastewater treatment.

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Experimental Section

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

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

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

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

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

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

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

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

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

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

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

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

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

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Neocoat®.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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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

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[⋅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.

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

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

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following equation

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η =

2VFd[FC] Idt

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(3)

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

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(A).

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Results and Discussion

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Band Structure Analyses

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

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

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

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

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

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

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

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

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

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

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

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conduction band.35

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(Figure 1)

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

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

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

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

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

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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

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

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

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

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

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

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

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

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

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

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

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

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

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(Scheme 1)

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

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

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

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

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for the BNTA.

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

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

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

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

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This interpretation is also

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

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

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

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

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

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

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

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

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

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

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

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

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maintaining a high value of ND.

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Stability Enhancements

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

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

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

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

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

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

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

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(Figure 2)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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(Figure 3)

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

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

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

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

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

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

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

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

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

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

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

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

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these estimated values.

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Oxidant Generation

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

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

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

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

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

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[·OH]ss.

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

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

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

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

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

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Ti0.5/EBNTA anode.

[⋅OH]ss =

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kBA kBA,⋅OH



kBQ k BQ,⋅OH

(4)

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

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

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

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during wastewater electrolysis (vide infra).

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

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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

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

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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-].

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(Figure 4)

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

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

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

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

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

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

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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)

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

(6)

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

249

S5.

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Potential Applications for Human Wastewater Treatment

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The various electrodes preparations were tested for possible applications for human

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

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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

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

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

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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

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

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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

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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.

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

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

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

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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

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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

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

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

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

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reducing the electrode separation distance.

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(Figure 5)

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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)

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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)

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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

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

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foaming is produced (Figure S8).

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

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

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

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

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

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

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

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organic pollutants.51

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ASSOCIATED CONTENT

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Supporting information involves BNTA characterizations such as cyclic voltammeties, and

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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).

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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.

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313

314 315

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

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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

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

320

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321 322

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

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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.

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328 329

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

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

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layer produced by a second anodization step.

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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

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

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

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

340

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(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

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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

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