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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
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