Sunlight-driven electrodesalination, and electrodesalination-boosted wastewater

remediation coupled with molecular hydrogen production: A Novel Solar

Water-Energy NexusSeonghun Kim,1,2 Guangxia Pio,1,2 Dong Suk Han,3 Ho Kyong Shon,4

and Hyunwoong Park1,2,*1School of Energy Engineering and 2School of Architectural, Civil, Environmental, and

Energy Engineering, Kyungpook National University, Daegu 41566, Korea3Chemical Engineering Program, Texas A&M University at Qatar, Education City, P.O. Box

23874, Doha, Qatar4School of Civil and Environmental Engineering, University of Technology Sydney, Post Box

129, Broadway, NSW, 2007, Australia

*To whom correspondence should be addressed (H. Park):

School of Energy Engineering, Kyungpook National University, Daegu 41566, Korea

E-mail: hwp@knu.ac.kr; Tel: +82-53-950-8973

Keywords

Photoelectrocatalytic desalination; Water treatment; Hydrogen production; Oxygen vacancy;

Hydrogen treatment; Urea decomposition

Graphical Abstract

1

Abstract

A novel solar water-energy nexus technology is presented with photoelectrocatalytic (PEC)

desalination of saline water and desalination-boosted wastewater remediation coupled with

production of molecular hydrogen (H2) from water. For this, morphologically tailored TiO2

nanorod arrays (TNR) and hydrogen-treated TNR (H-TNR) photoanodes are placed in an

anode cell and Pt foils are located in a cathode cell, while a middle cell containing saline

water (0.17 M NaCl) faces both the cells through anion and cation exchange membranes,

respectively. Upon irradiation of simulated sunlight (AM 1.5G, 100 mWcm2), the

photogeneration of charge carriers initiates the transport of chloride and sodium in the middle

cell to the anode and cathode cells, respectively, leading to desalination of saline water. The

chloride in the anode cell is converted to reactive chlorine species (RCS) which effectively

decompose urea to N2 as a primary product (>80%) while the sodium in the cathode cell

accelerates the H2 production from water at the Faradaic efficiency of ~80%. The H-TNR

photoanodes exhibit the superior PEC performance to the TNR for the anodic and cathodic

processes owing to the reduced charge transfer resistance and sub-nanosecond charge transfer

kinetics (~0.19 ns), leading to the specific energy consumption of 4.4 kWhm3 for 50%

desalination with energy recovery of ~0.8 kWhm3. The hybrid system is found to operate

over ~60 h with natural seawater and virtually all photoanodes are demonstrated to be

capable of driving the hybrid process. Although tested as a proof-of-concept, the present

technology opens up a novel field of sunlight-water-energy nexus, promising high efficiency

desalination and desalination-boosted remediation of water with simultaneous H2 production.

2

1. Introduction

Water and energy are the most essential elements for sustainable human society.

They are strongly interdependent in that the energy production requires a significant amount

of water, while the production, processing, distribution, and end-use of water requires large

energy inputs.1-3 For example, ~3% of the U.S. electrical energy production has been spent

only for wastewater (WW) treatment while approximately 40% of freshwater withdrawals

has been used for cooling thermoelectric power plants (~37 and ~53 m3s1 for 1 GWh coal-

fired and nuclear power plants, respectively).3-5 The state-of-the art seawater reverse osmosis

desalination process needs 34 kWm3 while emitting CO2 of 1.41.8 kgm3 for produced

water.2 On the other hand, WW contains a large amount of energy in diverse forms, including

kinetic, potential, thermal, and internal chemical energy.6,7 Particularly, the last accounts for

6.37.6 kJL1, a three-fold the electrical energy required for the treatment.7

Several water-energy nexus technologies have emerged,8-14 among which microbial

desalination is the most representative.15 In principle, it treats WW via the microbial activity

in the anode cell while desalinating saline water (in the middle cell) and producing electricity.

The process is similar to electrodialysis in terms of system configuration (i.e., three-cell

stack) and desalination mechanism, whereas the microbial desalination is exo-electrogenic

and the electrodialysis needs electrical energy input. Unfortunately, the microbial desalination

process is rather limited in application because of the environmentally sensitive microbial

activity. For example, WW must be neutral in pH (57) during entire process to maintain the

microbial activity. However, even under highly buffered conditions (e.g., use of 100 mM

phosphate buffer), a pH decrease below 5 is inevitable owing to the production of organic

acids.15 In addition, the WW substrate is limited and expensive acetate is often used to boost

the microbial activity. Much worse, the enriched chloride in the anode cell adversely affects

the bacterial community as well as the microbial activity.3

Herein, we propose, for the first time, a hybrid process that is driven by sunlight and

boosted by electrodialysis (Scheme 1). Sunlit inorganic photoanode initiates the desalination

of saline water in the middle cell. As chloride in the middle cell moves to the anode cell, the

WW treatment is boosted by reactive chlorine species (RCS) generated via the reaction with

photogenerated holes. An increase in the electrical conductivity in the anode cell further

enhances the photoelectrocatalytic (PEC) WW treatment. Upon a potential bias, molecular

hydrogen (H2) is produced in the cathode cell and the H2 production is enhanced as the

desalination proceeds with sodium enrichment (i.e., conductivity increase). The produced H2

can alleviate the electrical energy input via fuel cells. The uniqueness and advantage of this

hybrid process are a broad operation condition (virtually entire pH range) and a wide variety

of inorganic and organic substrates (i.e., aquatic pollutants), while desalination boosts the

overall operation and chloride catalyzes the anodic reaction which in turn facilitates the

desalination. It should be noted that the key in this loop is to efficiently harness sunlight and

effectively produce RCS in the WW treatment.16

With this in mind, high efficiency TiO2 nanorod arrays were synthesized via a

hydrothermal process followed by a thermal hydrogen treatment to increase the oxygen

vacancies. While one-dimensional rod configurations induce a radial directional transfer of

minority charge carriers,17-19 the oxygen vacancies created via the hydrogen treatment

enhance overall charge transfer owing to the improved conductivity.20 Urea was employed as

a model WW substrate16 because of an enormously large amount in natural discharge (240 Mt

per day; for comparison, fossil fuel daily production at 0.5 Mt), possible cause of algal

bloom, and a high gravimetric hydrogen content (~6.71 wt%).21 For practical utilization WO3

was tested as a photoanode for RSC-mediated decomposition of urea where natural seawater

was tested as a saline water over 60 h.

2. Experimental4

2.1. Fabrication of photoanodes

TiO2 nanorod (TNR) arrays were fabricated on fluorine-doped SnO2 glass substrates

(FTO, Pilkington, ~500 nm thick FTO layer) via a hydrothermal reaction. The FTO substrates

were cleaned with ultra-sonication for 15 min in aqueous methanol (50 vol. %). After drying

in N2 stream, they were placed in a Teflon-lined stainless steel autoclave containing

hydrochloric acid (20 mL, Junsei, 35%), titanium butoxide (0.6 mL, Aldrich, 97%), and

deionized water (20 mL, >18 Mcm). The autoclave was subjected to heat treatment at 150

C for 4 h followed by being cooled to room temperature. As-synthesized samples were

rinsed with a copious amount of deionized water and dried at 60 C overnight, followed by

annealing at 450 C for 30 min. For synthesis of hydrogen-treated TNR (H-TNR), as-obtained

TNR samples were placed in a tube furnace at 350 C for 2 h through which H2 gas (H2:Ar =

5:95) flowed at 1 bar. WO3 electrodes were synthesized on FTO via a spin-coating process.

Tungsten powder (1 g, Aldrich) was dissolved in aqueous H2O2 (4 mL, 30 wt% in H2O,

Sigma-Aldrich) to obtain peroxytungstic acid, to which 2-propanol (Sigma-Aldrich) and

polyethylene glycol 400 (PEG 400, Sigma-Aldrich) were sequentially added. Then the

precursor solution was spread on FTO substrates followed by annealing at 550 C for 10 min.

After repeated 14 cycles, the samples were annealed at 550 C for 1 h.

2.2. Photoelectrocatalytic activity tests

As-synthesized TNR and H-TNR electrodes (working electrodes), saturated calomel

electrode (SCE, reference electrode), and a Pt foil (counter electrode) were immersed in

aqueous sodium sulfate (20 mM, Duksan, 99%) or sodium chloride (20 mM, Daejung, 99%)

at pH ~6 in a single (undivided) cell. Linear sweep voltammograms were obtained from 1 to

+1.5 V vs. SCE at the scan rate of 5 mVs-1 using a potentiostat (Ivium) in the dark and under

simulated sunlight (AM 1.5G, 100 mWcm2) (ABET Tech). The PEC activity of as-

5

synthesized TNR and H-TNR samples was examined for the decompositions of N,N-

dimethyl-4-nitrosoaniline (RNO, Aldrich),22 phenol (Aldrich),23 and formate (HCOONa,

Aldrich),24 and for oxygen evolution from water oxidation25 while applying +0.5 V vs. SCE to

the titania samples in the dark or under simulated sunlight. The incident photon-to-current

efficiency (IPCE) of TNR and H-TNR electrodes were estimated using the following

equation (eq. 1).

IPCE(%) = 1239.8 (Vnm) Iph 100% / Plight (eq. 1)

where, Iph, Plight, and refer to photocurrent density (mAcm2) at +0.5 V vs. SCE, photon

flux (mWcm2), and wavelength (nm), respectively.

To examine the sunlight-driven anodic and simultaneous cathodic reactions, an air-

tight, two-divided cell with a proton-exchange membrane (Nafion 117, Chemours) was

designed. The TNR and H-TNR electrodes were immersed in the anode cell containing

aqueous solution of urea (CH4N2O, 2 mM, Junsei), and the SCE and Pt foil were placed in the

cathode cell to avoid any possible interference of chloride leaching out from the reference

electrode with the anodic reaction. The anolyte and the catholyte were the same with 20 mM

Na2SO4 or 20 mM NaCl. A constant potential (+0.5 V vs. SCE) was applied to the TNR and

H-TNR electrodes in the dark or under simulated sunlight, while the decomposition of urea

and the productions of intermediates (nitrate and ammonia) in the anode cell, and the

molecular hydrogen (H2) evolved from water in the cathode cell (N2 purged through the

catholyte for 1 h prior to the PEC reaction) were measured.

Finally, an air-tight, threefold cell-stack was designed to combine the electrochemical

redox reactions and desalination process. The stack was comprised of an anode cell with

photoanodes (i.e., TNR, H-TNR, or WO3) and aqueous urea solution (2 mM or 10 mM, 15

mL), a middle cell with aqueous NaCl (0.17 M, 10 mL) or natural seawater (10 mL) derived

from the East Sea, Korea (latitude: 36.13, longitude: 129.40),26 and a cathode cell with a Pt

6

foil and aqueous solution of K2SO4 (2 mM, 15 mL). An anion exchange membrane (AEM,

AMI-7001S, Membrane International) and a cation exchange membrane (CEM, CMI-7000S,

Membrane International) were placed between the anode cell/middle cell and the middle

cell/cathode cell, respectively (Scheme 1). While applying a potential bias at +0.5 V vs. SCE

to the photoanodes under simulated sunlight, the photocurrent (Iph), cathode potential (Ec),

and overall stack voltage (Estack) were recorded. Simultaneously, chloride and sodium in the

three cells were quantified. In addition, the concentrations of urea, NO3, and NH4

+ in the

anode cell, and the H2 produced in the cathode cell were estimated.

The concentration of urea was determined by the difference of NH4+ produced from

the reaction with urease (Worthington, 55.1 unitmg1, for dry weight; 1 unit referring to the

decomposition rate of urea to NH4+ at 1 molmin1) and NH4

+ present before reaction with

the urease using the following equation 2:16

(NH2)2CO + H2O + 2H+ + urease CO2 + 2NH4+ (eq. 2)

Anions (Cl, NO2, and NO3

) and cations (Na+ and NH4+) were analyzed with ion

chromatographs (IC, Thermo scientific, DIONEX ICS-1100) that were equipped with a

conductivity detector, IonPac AS-11HC (4250 mm) column for anions, and IonPac CS-12A

(4250 mm) column for cations. The concentrations of total and free chlorines were

estimated using DPD (N,N-diethyl-p-phenylenediamine) reagent (Hach method 10101 and

10102).27 H2 in the cathode cell headspace was quantified using gas chromatograph (GC,

Younglin, ACME 6100GC) that was equipped with a thermal conductivity detector (TCD)

and Porapac Q column.25 The Faradaic efficiency for H2 production was estimated by the

following equation (eq. 3).

Faradaic efficiency = H2 (mol) 2F (100%) / (I t) (eq. 3)

where I, t, and F are the current (A), time (s), and the Faraday constant (96,485 Cmol1),

respectively. The ion transport efficiency was estimated by the following equation (eq. 4).

7

Ion transport efficiency = Amount of monovalent ion (mol) for t / (I t) (eq. 4)

The specific energy consumption (SEC) for 50% desalination was estimated by the following

equation (eq. 5).28

SEC (kWhm3) = Estack (V) Iph (A) time (h) / saline water volume (m3)

2.3. Surface characterization

The top and cross-sectional morphologies of as-synthesized TNR and H-TNR

samples were examined using a field emission SEM (FE-SEM, Hitachi SU8200). To examine

the crystallite structures of the samples and the binding states of the component elements, X-

Ray diffraction (XRD, PANalytical EMPUREAN) with Cu-Kα radiation (40 kV and 50 mA)

and X-ray photoelectron spectroscopy (XPS, Thermo Fisher scientific) with Al-Kα (һν =

1486.6 eV) were employed, respectively. A UV-Vis spectrometer (Shimadzu, UV-2450) was

used to obtain the absorbance of the samples. Time-resolved photoluminescence lifetime

decays were measured using an inverted-type scanning confocal microscope (Microtime-200,

Picoquant, Germany) with a 40 objective. The measurements were performed by

employing a single-mode pulsed diode laser (379 nm with a pulse width ~200 ps and a laser

power of ~30 μW) as the excitation source. A dichroic mirror (Z375RDC, AHF), long-wave

pass filter (HQ405lp, AHF), and an avalanche photodiode detector (PDM seriesm MPD) were

used to collect emissions from the samples. The measurements were performed at the Korea

Basic Science Institute (KBSI), Daegu Center, South Korea. Time-correlated single-photon

counting technique was used to obtain fluorescence decay curves with a time resolution 8 ps.

Exponential modeling of the obtained fluorescence decays was performed by iterative least

squares deconvolution fitting using the SymPhoTime software(version 5.3). The details of the

measurements and data analysis are found elsewhere.17,19

8

3. Results and Discussion

3.1. PEC behavior of hydrogen-treated titania NR arrays

As-synthesized TiO2 samples via an hydrothermal process for 4 h followed by

hydrogen treatment at 350 C for 2 h (denoted as H-TNR) exhibited the configuration of ~1

μm-tall and ~120 nm-wide nanorod arrays vertically aligned on the FTO substrate (Figure 1a

and b). The overall morphology of H-TNR was nearly the same as that of non-hydrogen-

treated TiO2 nanorods (TNR) (Figure S1), indicating that the post-hydrogen treatment is mild

and causes insignificant damage of nanorod frameworks. The XRD spectra of TNR and H-

TNR samples showed the identical rutile structure with predominant 101 planes at 2 =

36.08 (ICDD #01-077-0441) (Figure 1c). A longer hydrothermal treatment intensified the

XRD peaks (Figure S2). The hydrogen treatment changed the color of TNR from white to

yellowish gray and the absorption onset slightly shifted from 407 nm to 415 nm (Figure 1d).

This optical change was trivial compared to literature which reported the significant color

change to dark blue and/or black upon the harsh reductive treatment.29-31 Accordingly, the

XPS Ti2p band shift was not found in the H-TNR (Figure S3), suggesting only the partial

reduction of the Ti(IV) to Ti(III) via the hydrogen treatment.25

Despite these insignificant changes, the H-TNR exhibited superior

photoelectrochemical performance to the TNR under AM 1.5G light (100 mWcm2). As

shown in Figure 1e, the linear sweep voltammograms of the TNR and H-TNR showed the

similar value of photocurrent onset potential (Eon = ca. 0.1 V vs. SCE) in sodium sulfate

electrolyte whereas the photocurrent density (Iph) of the H-TNR was significantly larger than

that of the TNR by a factor of ~2 (Figure S4). The IPCE profiles further revealed that the H-

TNR was far more effective for the conversion of high energy photons (i.e., < 390 nm)

(Figure 1e inset). No significant difference of the IPCE onset wavelength (~420 nm) between

TNR and H-TNR samples confirmed the absence of contribution of visible light to the 9

photocurrent.

Time-resolved photoluminescence emission decay spectra for TNR and H-TNR were

compared particularly for the emission over 500 nm (excitation = 379 nm) (Figure 1f).

Obviously, the H-TNR sample exhibited a faster decay and the emission was homogeneous

over the sample (Figure 1f inset). The decay curves were fitted with two exponential

components and the intensity-weighted average lifetimes () were estimated using a curve

fitting method reported elsewhere.19 of H-TNR was estimated to be ~0.19 ns, which was

~2.7 times-shorter than that of TNR ( ~0.51 ns). In contrast, the blue emission (i.e., near-

bandgap emission) decay kinetics of the two samples were similar with ~0.22 ns (Figure S5).

This indicates that H-TNR possesses faster charge transfer pathways particularly via the

surface traps (e.g., oxygen vacancies) created by the hydrogen treatment. The Mott-Schottky

plots further revealed that the hydrogen treatment increased the donor density (ND) of TNR

by a factor of 2 (Figure S6a). This could be attributed to the partial reduction of Ti(IV)

accompanying an increase in the oxygen vacancies,25,32 which is the phenomena usually found

with TiO2 doped with penta-valence metals (e.g., Nb5+).33 Therefore, the partially reduced

states of TiO2 (i.e., the existence of Ti(4-x)+ and/or oxygen vacancy) should have an increased

electrical conductivity while possessing altered charge transfer pathways.34 The impedance

(Nyquist plots) confirmed that the charge transfer resistances at the NR/solution interface and

inside the NR are significantly reduced by hydrogen treatment (Figure S6b).

With knowledge in mind, the PEC performance of TNR and H-TNR electrodes was

compared with various chemical redox reactions in single (undivided) cells. The detailed

experimental results were discussed in Supporting Information. As summarized in Table 1, H-

TNR electrodes exhibited superior PEC activity for O2 production via multi-electron transfer

water oxidation (R1 in Supporting Information). This enhanced PEC performance of H-TNR

should be attributed to larger photocurrents (Iph). However, the enhancement factor (i.e., PEC

10

activity of H-TNR with respect to TNR ~1.9) was smaller than the Iph ratio of H-TNR/TNR

(~1.2). The reduced Faradaic efficiency of H-TNR (~63% for H-TNR and ~80% for TNR.

Figure S7a) confirmed the low PEC activity for O2 production, suggesting the existence of

other electron transfer oxidation pathways. As the first candidate among them, the hydroxyl

radical (OH) mediated oxidation was examined using well-known OH quenchers (RNO and

phenol, See R2). However, the enhancement factor (1.21.3) was comparable to that for O2

production (Figure S7b and S8). Alternatively, we speculated the direct electron (i.e., valence

band hole) transfer oxidation using formate as a probe substrate to examine the pathway

(R3).24 The enhancement factor for the formate decomposition was enhanced to ~1.56 (Figure

S7b), closer to the Iph ratio. This indicates that H-TNR is active particularly for direct electron

transfer oxidation processes.

3.2. RCS-mediated oxidation of urea and simultaneous H2 production

The oxidation of chloride (an inorganic hole scavenger) was further tested because it

proceeds predominantly with the direct electron transfer.22,23,35 With 20 mM chloride, the total

amount of reactive chlorine species (RCS, including Cl, Cl2, HOCl/OCl) produced using H-

TNR (see R4R7) was ~1.8 fold-larger than that using TNR (Table 1). The amounts of RCS

reached plateau (Figure S9), attributed to the back reactions usually occurring in the

undivided cell. Hence, as-synthesized TNR and H-TNR arrays were coupled to Pt foils, the

pairs of which were separately placed via a proton-exchange membrane (PEM) (Figure 2a).

The PEC performance of the couples was compared with the degradation of urea in two

different electrolytes (sulfate vs. chloride) because urea and chloride are two major

components in human urine-contaminated water.16,27 With a TNR photoanode at +0.5 V vs.

SCE, stable Iph of 11.2 mAcm2 was generated over 12 h in both the electrolytes (Figure 2a).

Despite a more positive Eon in the sulfate (see Figure 1e), Iph in the sulfate was slightly larger

11

than that in chloride due to a higher conductivity (1.79 and 2.88 mScm1 for 20 mM NaCl

and Na2SO4, respectively),36 which is in good agreement with the literature.16,22,23,35,37

However, the decomposition of urea in the sulfate was limited (kapp ~0.04 h1) whereas it was

significantly enhanced by a factor of ~6 in the chloride owing to the RCS-mediated

decomposition of urea (R7, Figure 2b, and Table 1). Nitrate (NO3) was found to be a primary

degradation product of urea for the TNR in both the electrolytes with the similar production

rate (~54 Mh1) (Figure 2c). On the other hand, the concentration of ammonia (NH4+) was

trace and reached plateaus in 4 h (Figure 2d). A larger production of ammonia in the chloride

electrolyte suggests the existence of chloroamines as intermediates in the decomposition

pathway (See Figure S13).27

H-TNR was further compared with TNR in the sulfate and chloride electrolytes, the

primary feature of which was as follows. Firstly, a larger and more stable Iph was generated

over 12 h. Secondly, kapp for urea decomposition (4.72 101 h1) was ~2 and ~11 times

higher than those with TNR in the chloride and sulfate electrolytes, respectively. The amount

of nitrate was doubled while the ammonia concentration increased significantly. Thirdly, the

H2 production rate (13 molh1) was the highest because of the enhanced Iph (Figure 2e).

These results clearly suggest that H-TNR is superior to TNR in not only generating

photocurrent but also oxidizing chloride to RCS, enhancing the urea decomposition kinetics

and H2 production. Finally, when the urea concentration increased 5 times (i.e., 10 mM), H-

TNR exhibited a similar PEC performance in terms of Iph, urea decomposition kinetics (kapp

~3.33 101 h1), and H2 production (Figure S11). This similarity was attributed to the unique

feature of the RCS mediated reaction which does not need any direct interaction of urea and

H-TNR.

3.3. PEC desalination and desalination-boosted PEC decomposition of urea and

12

simultaneous H2 production

To utilize the RCS-mediated oxidation using H-TNR, we have designed a three-cell

stack (Scheme 1). Upon irradiation to the H-TNR held at +0.5 V vs. SCE, Iph was trace in the

initial phase and linearly increased for ~6 h, followed by reaching plateau with 2 2.5

mAcm2 (Figure 3a). The TNR sample exhibited a similar profile yet with a smaller Iph. The

operational stack voltage (Estack) between the H-TNR photoanode and the Pt cathode couple

increased gradually and reached plateau of ~2.5 V after 2 h. These behaviors of Iph and Estack

were significantly different from that observed in the two-cell configuration (Figure 2a). The

difference could be attributed to low initial concentrations of anolyte and catholyte in the

three-cell stack (each 2 mM).

With such the low electrolyte concentration, the transfers of the photogenerated holes

and electrons at the H-TNR/anolyte and the Pt/catholyte interfaces, respectively, should be

limited. The build-up of the charge carriers in the H-TNR and the Pt can induce the transport

of chloride and sodium ions from the middle cell to the anode and cathode cells, respectively

(Figure 3b). An increase in the ion concentration of the both electrolytes increases the

electrical conductivity, facilitating the separation and transfer of the photogenerated charge

carriers (i.e., increasing Iph) and, as a result, accelerating the ion transport from the middle

cell (vice versa). However, once the ion concentrations in the anolyte and catholyte reach a

certain level (e.g., 0.3 mmol, corresponding to ~20 mM for chloride in the anolyte at 6 h), the

electrical conductivity of the electrolyte is high enough that the overall process becomes

limited by the PEC performance of H-TNR and hence Iph reaches the plateau thereafter (2

2.5 mAcm2). As shown in Figure 3b, the amount of chloride in the anolyte increased rather

slowly in the initial phase (~4 h) and linearly later on, confirming the non-linear ion transport

kinetics. The amount of sodium in the catholyte followed the same pattern, indicating the

equimolar ion transport. In addition, the amounts of ions in the middle cell decreased slowly

13

in the initial phase and linearly afterwards, the tendency of which was similar to those in the

electrolytes. Furthermore, ions (difference of #ions for a certain period of time) in the

middle cell were similar to ions in the electrolytes. The ion transport efficiency

(ions/flowed charges) was estimated to be ~100% for chloride and sodium (Figure S11).

Once transported to the anolyte, chloride is quickly oxidized to RCS by the

photogenerated holes, accompanying a decrease in pH (Figure S12 and R6) while initiating

the oxidation of urea (R4R7). As shown in Figure 3c, a trace amount of urea (~0.1 mM) was

decomposed in the initial phase (~2 h) due to the small amount of chloride in the anolyte

(<0.04 mmol, i.e., ~3 mM as shown Figure 3b), whereas most of the produced RCSs (0.3

mM) was used for the urea decomposition (Figure S13). Urea can be directly oxidized by the

photogenerated holes; however, such the direct oxidation pathway was found to be limited

(urea ~10% for 2 h in 20 mM sulfate solution as shown in Figure 2b). With increase in the

amount of chloride in the anolyte, the urea decomposition with chlorination was accelerated

and reached the peak at 4 h, being completed in 8 h (compare Figure 3c and S13 for H-TNR).

Nitrate and ammonia were produced as the decomposition products of urea in the liquid

phase (Figure 3c), whereas the nitrogen deficiency gradually increased and reached over 80%

in 8 h because of N2 production (Figure S14). It is noteworthy that the time-profiled

productions of nitrate and ammonia were slightly different from those in the two-cell

configuration (Figure 2c and d), presumably due to time-transient chloride concentration and

pH in the three-cell stack (Figure S12). However, it is evident that there was usually the peak

production of ammonia whereas the nitrate production continued with time, indicating that

chloroamines (NHxCl3-x, where x = 0, 1, 2, 3) intermediates were decomposed to nitrate and

N2 in the later stage. The gradual decomposition of chloroamines led to increases in the free

chlorine concentration (Figure S13). The H2 production in the cathode cell followed a similar

pattern with the urea decomposition of the anode cell. It was insignificant in the initial phase

14

for 2 h and increased linearly with time (Figure 3d). This behavior could be understood in

terms of the cathode potential (Ec), which was transient during the first 2 h and then stabilized

(Figure S14). Nevertheless, the slowed H2 production in the later stage was attributed to the

accumulation in the headspace and highly alkaline condition (Figure S12). The Faradaic

efficiency of H2 production was estimated to be ~80%.

With a 5-fold larger urea concentration (10 mM), similar time-dependent changes in

Iph and Estack for TNR and H-TNR were obtained (Figure S15). The primary difference from

the case of low urea concentration (2 mM, Figure 3) was gradual decreases of Iph after the

peak (~12 h), finally reaching the minimum level in ~36 h (particularly for H-TNR).

Similarly to this profile, the amount of ions in the middle cell decreased linearly with time

and reached the minimum level in ~36 h. The time-profiled changes in the amount of ions in

the anolyte and the catholyte were the same as the ion transports in the middle cell. This

behavior indicates that virtually all the ions in the middle cell could be transported to the

neighboring cells. Meanwhile, the decomposition of urea was completed in 12 h. Ammonia

was concomitantly produced over 12 h and further decomposed to nitrate, confirming it being

the intermediate. Even after complete removal of urea and a peak production of ammonia, the

continuous production of nitrate indicates that nitrate is one of the final products. Instead of

potentiostat, a DC power supply (2 V) was employed and the similar system performance

was obtained (Figure 3 vs. S16). Based on the DC-powered result, the specific energy

consumption (SEC) of the present three-cell stack for 50% desalination was estimated to be

~4.4 kWhm3 (Table 1). This SEC value is comparable to those of electrodialysis and

reverse osmosis,28 indicating the present system operates efficiently although tested as a

proof-of-concept. The obtained SEC value could be further reduced when the evolved H2

feeds back to the stack as an electrical energy via a fuel cell. In addition, the energy input

required for the remediation of urea using conventional treatment processes could be saved. If

15

these saved energies are combined to the SEC value, the overall operational energy input for

the desalination ~3.6 kWhm3.

We applied this system to seawater desalination while utilizing desalted chloride for

remediation of urea (10 mM) using H-TNR (Figure 4a). The cathode cell was open to

atmosphere to avoid the H2 pressure buildup. While Estack (~2.2 V) was nearly constant over

60 h, Iph was stabilized to ~1.5 mAcm2 after a peak at ~3.5 mAcm2. Urea was completely

decomposed in ~10 h and the nitrate production continued over 60 h. This demonstrates that

as-fabricated PEC system successfully operates with seawater. Instead of TiO2 (TNR and H-

TNR), WO3 with a smaller bandgap (2.6 2.8 eV) was employed as a photoanode (Figure

4b). With brackish water (0.17 M NaCl) in the middle cell, a similar PEC behavior (i.e., a

nearly constant Estack of ~2.3 V and increasing Iph) was observed. Equimolar chloride and

sodium ions were transported to the anode and cathode cells, respectively, while urea was

decomposed and nitrate and ammonia were produced. The slower kinetics of the urea

decomposition and the smaller amount of nitrate were attributed to the lower PEC

performance of WO3 compared to TNR and H-TNR. This indicates that the RCS mediated

decomposition of urea with simultaneous desalination can be performed with nearly all

photoanodes and the overall performance is determined by a photoanode employed.

Conclusions

This proof of concept of the operation of photoelectrochemical desalination and the

desalination-boosted water treatment coupled with hydrogen production from water was

successfully demonstrated. Morphology-tailored, thermochemically-treated TiO2 nanorod

arrays were active for oxidation of chloride to reactive chlorine species. For coupling the

photoinduced process and the conventional electrochemical desalination process, a three-cell

stack was fabricated, composed of an anode cell with TiO2 photoanodes (TNR and H-TNR), 16

an cathode cell with Pt foils, and a middle cell with saline waters including natural seawater.

The stack initiated the hybrid process with photogenerated charge carriers, followed by ion

transportation from the middle cell to the neighboring cells. As the desalination proceeded,

the generation of reactive chlorine species by the photogenerated holes drove the oxidation of

urea in the anode cell while H2 was produced in the cathode cell. The kinetically facile

generation of reactive chlorine species against the O2 evolution from water led to the effective

remediation of aquatic pollutants, and further widened the applicability of diverse

photoanodes including WO3. The specific energy consumption for desalination of the present

system was ~4.4 kWhm3, which was shown to be further reduced upon including the H2

energy and the saved energy required for the anodic treatment. Further modification of

cathodes can expand the applicability of the present system, particularly for other chemical

conversions including O2 and CO2 (e.g., H2O2 and CO, respectively).

Acknowledgements

HKS acknowledges the ARC Future Fellowship (FT140101208).

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Scheme 1. Illustration on the sunlight-driven photoelectrochemical desalination, desalination-boosted water treatment, and simultaneous H2 production.

20

2 Degree)25 35 45 55 65

Intensity (a. u.)

(101) (111)

(002) **

*

(110)

*

FTO

TNR

H-TNR

c d

Wavelength (nm)350 400 450 500 550

Absorbance (a. u.)

TNR

H-TNR

e

E (V vs. SCE)-0.5 0.0 0.5 1.0 1.5

I (mA

/cm2)

0

1

2

3

4

TNR / SulfateH-TNR / SulfateTNR / ChlorideH-TNR / Chloride

Wavelength (nm)360 400 440 480

IPCE (%

)

0

20

40

60

80

H-TNR

TNR

Time (ns)0.0 0.5 1.0 1.5 2.0 2.5

Intensity (normalized)

0.0

0.2

0.4

0.6

0.8

1.0TNR ( = 0.51 ns)H-TNR ( = 0.19 ns)

f

Figure 1. (a – d) Surface characterizations of as-synthesized and post-hydrogen-treated TiO2

nanorods (TNR and H-TNR, respectively. Grown on FTO substrates). (a and b) SEM images of H-TNR. (c) XRD patterns of FTO (non-indexed peaks), TNR, and H-TNR (indicated with asterisks). (d) UV-vis absorption spectra of TNR and H-TNR (inset: photos). For more SEM images, see Figure S1. (e) Linear sweep voltammograms of TNR and H-TNR electrodes in aqueous sodium sulfate or sodium chloride solutions (0.1 M) under AM 1.5 light (100 mWcm2). The inset shows the IPCE profiles of TNR and H-TNR at +0.5 V vs. SCE in aqueous sodium sulfate solution. (f) Time-resolved photoluminescence emission spectra of TNR and H-TNR. The inset shows the 2D images for lifetime (left: TNR, right: H-TNR). See Figure S5 for more information.

21

Figure 2. Photoelectrocatalytic urea decomposition by TNR and H-TNR in the anode cell and simultaneous H2 productions in the cathode cell. Both cells were filled with the same solutions (sulfate or chloride at 20 mM) and separated by a proton-exchange membrane (dashed line). (a) Time-profiled photocurrents at +0.5 V vs. SCE under AM 1.5 light (100 mWcm2). (b) Urea (C0 = 2 mM) decompositions. (c and d) Productions of nitrate and ammonia. (e) Concomitant H2 productions. For legend, see Figure 2c and d.

22

Figure 3. Photoelectrocatalytic urea (C0 = 2 mM, 15 mL) decomposition using TNR and H-TNR in the anode cell, desalination in the middle cell (NaCl 170 mM, 10 mL) equipped with anion and cation exchange membranes (dashed lines), and simultaneous H2 productions in the cathode cell containing K2SO4 (C0 = 2 mM, 15 mL). Iph refers to photocurrent density produced from TNR and H-TNR held at +0.5 V vs. SCE under AM 1.5 light (100 mWcm2) and Estack indicates the overall three-cell stack voltage across the anode, middle, and cathode cells. (a) Changes in Iph and Estack. (b) Changes in the amounts of ions in the anode, middle, and cathode cells. (c) Time-profiled changes in urea concentration and TOC (upper panel), and concomitant productions of ammonia and nitrate (lower panel). (d) H2 productions in the cathode cell and Faradaic efficiency for the produced H2.

23

Figure 4. Photoelectrocatalytic urea (C0 = 2 mM, 15 mL) decomposition using photoanodes (a & b: H-TNR, c & d: WO3) in the anode cell, desalination in the middle cell containing saline waters (a & b: natural seawater, c & d: 170 mM NaCl. Each 10 mL), and simultaneous H2 productions in the cathode cell containing K2SO4 (2 mM, 15 mL). The photoanodes were held at +0.5 V vs. SCE under AM 1.5 light (100 mWcm2). (a and c) Changes in Iph and Estack. (b and d) Changes in the amounts of ions in the anolyte (AN) and catholyte (CA). Time-profiled changes in urea concentration and concomitant productions of ammonia and nitrate were shown as well.

24

Table 1. Photoelectrocatalytic performance of TNR and H-TNR for conversion of (in)organic substrates and desalination in aqueous solution of sodium sulfate and sodium chloride.a

Single (undivided) cellb Two-divided cellc Three-divided celld

Iph

(mA cm2)O2

(mol h1)RNO

(min1)PhOH(min1)

Formate(min1)

[Free Cl]SS

(M)Urea

in NaCl(h1)

Ureain Na2SO4

(h1)

Specific energy(kWh m3)

Input BalancedTNR 1.7 14.9 1.12102 6.16103 3.97103 41 0.25 0.041 - -H-TNR 3.3 17.8 1.49102 7.12103 6.17103 75 0.47 0.13 4.36 3.58Factore 1.9 1.19 1.33 1.16 1.56 1.83 1.88 3.2 - -aA constant potential (+0.5 V vs. SCE) was held at TNR and H-TNR under AM 1.5 (100 mW cm2)bPerformed in aqueous sodium sulfate (0.1 M), except for [Free Cl]SS which refers to the free chlorine concentration (Cl2 + HClO + ClO) at steady state in 20 mM NaCl. cSeparated by proton-exchanged membranes. [Urea]0 = 2 mM in the anode cell. The anolyte and the catholyte were the same with 20 mM Na2SO4 or 20 mM NaCl.dSeparated by an anion exchange membrane and a cation exchange membrane. [Urea]0 = 2 mM in the anolyte; [K2SO4] = 2 mM in the catholyte; [NaCl] = 170 mM or natural seawater in the middle cell. Input energy refers to applied electrical energy (Iph E time) per volume of saline water (10 mL) required for 50% desalination (~12 h). Balanced energy refers to the difference of the applied electrical energy and the energy recovered from produced H2 (~0.66 kWh m3 assuming the chemical-to-electrical energy conversion efficiency of 50% using a fuel cell) and required for the treatment of urea (~0.12 kWh m3, estimated from Ref xx). ePEC activity of H-TNR with respect to TNR.

25