Electrocatalytic Conversion of Carbon Dioxide and Nitrate Ions to Urea by a Titania–Nafion Composite Electrode

Article abstract of DOI:10.1002/cssc.201701448

CO₂ and nitrate ions were successfully converted to urea by a TiO₂–Nafion nanocomposite electrode under ambient conditions. The composite electrode was constructed by dropcasting the mixture of P25 titania and Nafion solution on an indium-doped tin oxide (ITO) electrode. When the electrode was electrolyzed in CO₂-saturated 0.1 m KNO₃ (pH 4.5) solution at ?0.98 V versus Ag/AgCl, urea was formed with a Faradaic efficiency of 40 %. The other reduced products obtained were NH₃, CO, and H₂.

Full text of DOI:10.1002/cssc.201701448

Accepted Article
Title: Electrocatalytic Conversion of CO2 and Nitrate Ions to Urea
below -1.0 V vs Ag/AgCl by a TiO2-Nafion® Composite Electrode
Authors: Saravanakumar Duraisamy, Jieun Song, Sunhye Lee,
Namhwi Hur, and Woonsup Shin
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10.1002/cssc.201701448
ChemSusChem
Electrocatalytic Conversion of CO2 and Nitrate Ions to Urea below -1.0 V vs
Ag/AgCl by a TiO2-Nafion® Composite Electrode
Dr. Duraisamy Saravanakumar[a] [b]*, Dr. Jieun Song[b], Sunhye Lee[b], Prof. Namhwi Hur[b] and
Prof. Woonsup Shin[b]*
[a]Department of Chemistry, School of Advanced Sciences, VIT University,
Vellore 632014, India.
[b]Department of Chemistry and Korea Center for Artificial Photosynthesis, Sogang University,
Seoul 121-742, Korea.
E-mail: duraisarav@gmail.com ; shinws@sogang.ac.kr
Abstract
CO2 and nitrate ions were successfully converted to urea by a TiO2/Nafion®
nanocomposite electrode under ambient conditions. The composite electrode was constructed by
drop-casting the mixture of P-25 titania and Nafion® solution on an ITO electrode. When the
electrode was electrolyzed in CO2-saturated 0.1 M KNO3 (pH 4.5) solution at -0.98 V vs. Ag/AgCl,
urea was formed with a Faradaic efficiency of 40%. The other reduced products obtained were
ammonia, CO, and H2.
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10.1002/cssc.201701448
ChemSusChem
Table of Contents
CO2 and nitrate ions were converted to urea with a Faradaic efficiency of 40% using a TiO2-
Nafion® nanocomposite electrode as an electrocatalyst at -0.98 V vs. Ag/AgCl at room temperature
and atmospheric pressure. The CO and ammonia precursors resulting from the simultaneous
conversion of CO2 and nitrate ions were adsorbed on TiO2 and subsequently converted to urea.
Keywords: CO2 conversion · electrocatalyst · modified ITO electrode · P-25TiO2 · urea
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10.1002/cssc.201701448
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There have so far been few investigations of the photochemical conversion of CO2 and nitrate
to urea using TiO2 and its derivatives at ambient conditions [1] . Shibata et al. have demonstrated
the possibility of urea synthesis by electrochemical conversion of carbon dioxide and nitrate/nitrite
ions by gas diffusion electrodes modified with variety of electroactive materials at ambient room
temperature and atmospheric pressure [2]. The efficiency of urea formation on gas diffusion
electrodes loaded with Cu or Zn was about 22 % and 37 % at -2.25 V and −1.75 V vs. SHE,
respectively [2b]. These results imply the possibility of urea synthesis from CO2 and ammonia at
ambient conditions, in contrast to the temperatures of 150–200 C and pressure of 100–200 atm
used in the highly energy-intensive industrial urea synthetic process [3]
.
The chemical conversion of CO2 to valuable products such as hydrocarbons, syngas, formate,
and other chemical commodities is crucial to mitigate the greenhouse effect caused by CO2 in the
atmosphere [4]. Increasing concentrations of CO2 in the atmosphere are a key cause of
environmentally damaging phenomena such as global climate change and ocean acidification.[4]
Electrochemical CO2 conversion processes that can operate at room temperature present clean and
sustainable methods to utilize CO2, but systems that use metallic electrodes require a large
overpotential [5]. Electrodes can be modified with suitable materials like inorganic nanomaterials,
metal complexes with suitable ligand systems to reduce the required overpotential, increase their
catalytic activity for CO2 conversion, and allow kinetically fast and thermodynamically stable
conversion[4].
As like CO2, the pollutant nitrogen oxide also has significant impacts on the environment, and
is also responsible for many human health problems, particularly in infants. Nitrogen oxide
accumulates in water supplies from both the intensive use of synthetic fertilizers and industrial
activity, and several technologies, including electro-reduction, have been developed to decrease
the levels of nitrate contamination [6]. The electrochemical synthesis of urea from environmental
CO2 and nitrate in water under mild conditions would be a fascinating method for the utilization
of CO2 and nitrate pollutants. Urea is one of the most valuable industrial products derived from
CO2 because of its use as an important nitrogen source for fertilizers, which are produced
worldwide at a scale of 100 million ton per year [4d]. In this study, we used commercially available
P-25 TiO2 nanoparticles with Nafion® as an electrocatalyst for the simultaneous conversion of CO2
and nitrate to urea in aqueous solution at room temperature and pressure. The use of aqueous
reaction conditions for the electrochemical conversion of CO2 to urea is a significant improvement
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in this study, since the large-scale application of previous CO2 conversion processes is limited by
their use of undesirable expensive organic solvents.
We have recently demonstrated the successful conversion of CO2 to CO and formate without
any overpotential in neutral aqueous solution using bio- and synthetic electrocatalysts. We reported
the highly selective electrocatalytic conversion of CO2 to CO in aqueous solutions using the
enzyme carbon monoxide dehydrogenase/acetyl-CoA synthase (CODH/ACS) and a synthetic
Ni(cyclam) complex attached poly(allylamine)- catalysts [7]. We also converted CO2 to formate
with a current efficiency of 80–100ꢀ% by the acetogenic bacteria Moorella thermoacetica and
Clostridium formicoaceticum in a neutral aqueous solution [8]. Encouraged by these works and
related works on electrocatalytic conversion of CO2, we extended our efforts to the conversion of
CO2 to CO and urea using nanoparticulate (P-25) titania. TiO2 is a well-known catalyst for many
chemical transformations because of the strong reducing power of its redox couple and its high
electro-stability, chemical stability, excellent optical and electronic properties. TiO2 is also readily
prepared in different morphologies [9], which are used as the catalysts in several chemical
processes. Recently, Kamat et al. reported the electrocatalytic reduction of CO2 to methanol by
nanostructured TiO2 deposited on a glassy carbon electrode in acetonitrile [10]. TiO2 and its
[11]
derivatives are also already utilized for the photochemical activation of CO2
and nitrate
reduction [12]. TiO2 nanocrystals embedded in SiO2 matrices were used as a photocatalyst for the
-
conversion of CO2 in different solvents and also for simultaneous reduction of CO2 and NO3 for
the formation of urea [1d&1e]. We are not aware of any existing reports on the electrochemical
synthesis of urea using TiO2, and thus report here the first instance of its use in urea synthesis. The
industrial process for urea synthesis requires a significant amount energy to produce the required
ammonia from hydrogen and nitrogen. The advantage of the system reported herein is that the urea
synthesis is achieved by an energetically favorable process. In this study, we used commercially
available P-25 TiO2 nanoparticles with Nafion® as an electrocatalyst for the simultaneous
conversion of CO2 and nitrate to urea in aqueous solution at room temperature and pressure. The
use of aqueous reaction conditions for the electrochemical conversion of CO2 to urea is important
since the large-scale application of previous CO2 conversion processes is restricted by their use of
costly organic solvents. The formation of urea from the simultaneous electrocatalytic conversion
of CO2 and nitrate on the P-25 TiO2- modified electrode occurs at ambient temperature and
pressure.
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Nafion®, a negatively charged perfluorinated polymer, can bind positively charged TiO2, and
forms a stable porous film on electrode surfaces. 5 % (v/v) Nafion® in water was used to make a
composite with TiO2. A planar tin-doped indium oxide (ITO) plate has been selected as an
electrode to deposit TiO2-nafion composite due to their promising characteristics such as good
electrical conductivity, low background current over wide range of potentials at different pH
values and its feasibility for size variations and durability. ITO electrode is inert up to -1.6 V vs
SCE [13] (-1.55 V vs Ag/AgCl(3M KCl)) in 0.3 M Na2SO4 and -0.48 V vs RHE [14] (-1.091 V vs
Ag/AgCl(3M KCl)) in 0.1 M NaAc at neutral pH 7. It should be noted that, in the acidic pH range,
ITO electrode surface would be altered substantially due to segregation of indium or dissolution
of tin during anodic potential sweeping [15]. The TiO2-Nafion® composite (30 µL) was drop-cast
on a plasma-treated indium tin oxide (ITO)-coated glass electrode. The modified electrode named
as TiO2-Nafion® modified ITO electrode, was then utilized for characterization and employed for
further electrocatalytic CO2 conversion.
A stable porous TiO2-Nafion® film was observed on the ITO surface by scanning electron
microscopy (SEM) as shown in Figure 1a. An X-ray diffraction (XRD) pattern of the TiO2-Nafion®
film on ITO is presented in the Supporting Information (Figure S1). The SEM image shows that
the composite electrode exhibits a homogeneous porous structure.
Cyclic voltammograms of the modified electrode with TiO2-Nafion® modified ITO were
obtained in 0.1 M KNO3 (pH 4.5) under both Ar and CO2 atmospheres (Figure 2). Under Ar, an
irreversible reduction wave is observed between −0.9 to −1.0 V vs. Ag/AgCl. The increase in the
reduction current observed under CO2 shows the occurrence of a catalytic reaction.The catalytic
current observed for the TiO2-Nafion®-modified ITO electrode suggests that TiO2 can act as an
electrocatalyst for CO2 conversion, in addition to the known roles of TiO2 and TiO2-doped
materials in the photocatalytic conversion of CO2 [11]. The cyclic voltammogram of the blank ITO
(Figure 2, blue) under CO2 showed an electrochemical response at -0.8 V, which may be due to
the reduction of Sn4+ or In3+ [15a]
.
CO2-saturated solution containing 0.1M KNO3 at pH 4.3, was electrolyzed under 1 atm CO2
using the TiO2-Nafion® modified ITO electrode at -0.98 V vs. Ag/AgCl for ~2 hours at RT, 25 C.
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The electrolysis cell consisted of an H-type cell with two gas-tight compartments separated by a
Nafion® 117 membrane (see the experimental section for full details). Since the maximum catalytic
current was observed at -0.98V and this working potential has been chosen for electrolysis. The
CO2 saturation didnot cause any drastic change in the local pH of the solution, where the pH of
the solution was altered from 4.5 to 4.3 after saturation of CO2. The charge accumulation during
electrolysis is shown in Figure 3. As shown in Figure 3, the TiO2-Nafion®-modified ITO electrode
affords a robust and linear charge build-up without significant loss of activity over the period.
In the presence of 0.1 M nitrate ions, the major CO2 conversion products are found to be CO,
ammonia, and urea. The formation of liquid products like formate was not found for this electrode
at the potentials investigated. The Faradaic efficiency of the urea formation was ~40 %, and the
Faradaic efficiencies for the formation of CO and ammonia were 27 % and 30 %, respectively.
The total Faradaic efficiency was 99 %, including 2 % of an H2 byproduct. The conversion of CO2
by the TiO2-Nafion modified electrode was found to occur at a rate of 1.95, 0.55 and 0.33
micromoles per hour for CO, ammonia and urea, respectively. When, TiO2 modified electrode was
subjected to electrolysis under the Ar atmosphere, the only product formed was ammonia at 0.30
micromole/h. However, the bare ITO electrode did not produce any of the above said products in
the presence of CO2. The modified electrode exhibited a current density of 0.18 and 0.02 mA/cm2
in the presence of CO2 and Ar atmospheres respectively. The pH of the solution after the
electrolysis was checked and was found to be 5.2. The increase in pH from 4.3 to 5.2 after
electrolysis did not cause any degradation in the electrode performance within the electrolysis
time, as the repeated experiments with the same electrode retained almost the same efficiency and
catalytic current. Already, Shchukin et al. reported the photocatalytic synthesis of urea at pH 5.5
(0.1 M NaNO3) aqueous solution using TiO2−PVA−PAH/PSS capsules [16]. These observations
show that TiO2 based catalysts exhibit a good stability even at pH 5.5. Also, it has been already
demonstrated that the physically adsorbed TiO2 on glass is stable up to pH 9 [17]
.
A blank test under CO2 atmosphere was carried out using neat ITO plate at the same
electrolysis potential as that of the modified electrode, -0.98 V vs Ag/AgCl(Figure S2). During the
electrolysis, the ITO electrode turned opaque indicative of change in the In:Sn ratio by the
reduction of either Sn4+ or In3+ [15]. The possible reduction product, ammonia was not found after
electrolysis at the blank ITO electrode, which further evidences that the charge consumed during
the electrolysis could be due to the reduction of Sn4+ or In3+. Upon coating TiO2-nafion composite
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on the ITO surface, the modified electrode exhibited excellent electrocatalytic properties along
with remarkable stability. The ITO electrode was not found to undergo any deterioration after
coating with TiO2, which is attributed to the complete coverage of ITO with active TiO2 particles.
Control experiments showed that no CO formation occurred when CO2 was electrocatalytically
reduced with the nano TiO2-Nafion® electrode in 0.1 M KHCO3 buffer(Figure S3), and the
reduction of nitrate ions in the absence of CO2 by bulk electrolysis at -0.98 V vs. Ag/AgCl with
the TiO2-Nafion® nanocomposite electrode in 0.1 M KNO3 produced ammonia as the sole product
(Figure 2 and 3; green). These results indicate that electrons in the conduction band of TiO2 have
the capability to catalyze CO2 and nitrate simultaneously. The corresponding Faradaic efficiencies
are presented in Table 1.
The Faradaic efficiency for urea production of 40 %, achieved by the TiO2-Nafion® modified
electrode at −0.98 V vs. Ag/AgCl with an electrode surface area of 1.75 cm2 and the other
conversion products are CO, ammonia and hydrogen. In the present system produced urea with
40 % efficiency is better than the system reported by Shibata et al. [2b]. Gas diffusion electrodes
with surface area of 12.6 cm2 loaded with various metal catalysts were employed for the conversion
[2b]
CO2 to urea, in which Zn and Cu-loaded gas diffusion electrodes
are the best and the urea
production was reported as, 37 % and 22 % at −1.75 V and −2.25 V vs. SHE (corresponding to
−1.95 and −2.45 V vs. Ag/AgCl(3M KCl)) respectively. Along with urea, the Zn and Cu loaded
gas diffusion electrodes produced CO, HCOOH, and ammonia. While comparing the performance
of the TiO2-Nafion® modified electrode in terms of potential with Zn and Cu loaded gas diffusion
electrodes, TiO2- Nafion® electrode produced urea at more positive potential, requiring an
overpotential that was 1 V lower than the Zn gas diffusion electrode. The onset potential of the gas
diffusion electrodes for urea formation is about -1.25 V Ag/AgCl(3M KCl) with current efficiency
of around 15% whereas the Faradaic efficiency of 40 % was obtained at lower potential of -0.98 v
vs Ag/AgCl(3M KCl) with our electrode. The cyclic voltammogram of TiO2-Nafion® electrode in
the presence of CO2 reveals that the onset potential is -0.75 V and it is still possible to lower the
overpotential. The Faradaic efficiency for CO by TiO2-Nafion® electrode is 37 %. From the
previous work [2], it is clear that the formation of urea is connected to the ability of formation of
ammonia-like precursor by catalysts. To check the ability of the TiO2-Nafion® modified electrode
for the formation of ammonia-like precursor, the nitrate ions were reduced at -0.98 V vs. Ag/AgCl
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10.1002/cssc.201701448
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in the absence of CO2 by the TiO2-Nafion® nanocomposite electrode which produced ammonia
with a Faradaic efficiency of 98 %.
Further, the electrocatalytic conversion of CO2 to urea was carried out by following the same
conditions adopted in the literature for gas diffusion electrode, i.e., in 0.2 M KHCO3 containing
0.02M KNO3 to check the performance of the modified electrode. The electrolysis was carried out
using TiO2-Nafion® modified electrode at a potential of -0.98 V vs Ag/AgCl in the presence of
CO2 for about 1 h (Figure S4). The current density and the charge consumed were similar to that
of our present report (in 0.1 M KNO3) and the conversion of CO2 to urea was also found to be
36 %. This shows that our TiO2 electrocatalyst exhibits reasonable activity towards the
electrocatalytic conversion of CO2 to urea at lesser potential of -0.98 V (compared to −1.95 and
−2.45 V vs. Ag/AgCl of Zn and Cu gas diffusion electrodes respectively), both in the presence and
absence of KHCO3.
Urea formation depends on the capability of the catalysts which reduce CO2 and nitrate
simultaneously and produce CO and ammonia. Urea was formed by the in situ-generated
intermediate precursors CO- and ammonia, which were the products of the concomitant conversion
of CO2 and nitrate on the TiO2-Nafion® electrode [2]. TiO2 is highly active for the production of
CO- and ammonia-like precursors, and consequently, for the formation of urea. The CO- and
ammonia precursors could adsorb on the TiO2 surface as CO and NH2 molecules, which could
[2d]
then combine to form urea. A suggested pathway
shown below:
for urea formation on the TiO2 surface is
One molecule of the two-electron reduction product CO can combine with two molecules of
the eight-electron reduction product NH2 to form urea as one possible product. The overall reaction
requires 18 electrons and 18 protons to form urea in a high-energy consuming reaction. The nano
TiO2-Nafion® modified electrode presented in this paper can supply 18 electrons to effect the
conversion of CO2 and nitrate ions simultaneously and efficiently to obtain the urea product. This
system is the first TiO2-based electrocatalyst that shows successful electrocatalytic conversion of
CO2 to urea. We are optimizing the system to achieve maximum efficiency by varying the nitrate
ion concentration, solution pH, and electrolysis potential to obtain a highly stable electrode to
construct a CO2 conversion system that shows good long-term stability. After optimising the
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10.1002/cssc.201701448
ChemSusChem
conditions, a photoelectrochemical cell will be constructed with our composite electrode along
with a suitable anode.
In conclusion, a combination of commercially available TiO2 and Nafion® forms a stable film
on ITO and is capable of converting CO2 to urea with a combined reduction of nitrate in aqueous
solution. This system shows a Faradaic efficiency of 40%, at lower overpotential than that
observed in previous reports of similar systems.
Experimental Section
Materials. TiO2 P-25 (anatase- 80 %, rutile 20 % with 50 m2 g-1 surface area and 27 nm
particle size, Degussa Corporation, Germany), Nafion® solution (5 wt.% ), Nafion® 117 film
(thickness 0.007 in), urease and Indium Tin Oxide (ITO) coated glass were purchased from Sigma-
Aldrich and used without further purification. Potassium nitrate, potasium bicarbonate were
purchased from Alfa Aesar. Deionized water (DI) water collected from Millipore Milli-Q plant
were used for cyclic voltammetry and other studies.
Electrode modification. The ITO glass (0.5 × 5 cm) plates were cleaned by ultra-sonication
for 30 min with 10% mucasol solution in deionized water, and treated with oxygen plasma for 10
min. A plasma cleaner module (Harrick Plasma Corporation PDC-32G) was used for the plasma
cleaning of ITO. P-25 TiO2 powder (2 mg) was dispersed in water (200 µL) and mixed with 80
µL of a 5 % v/v Nafion® solution and finally 120 µL water to make up the net volume 400 µL.
The amount of TiO2 present in the composite is 0.5 wt %. 30 µL of the TiO2-Nafion®
nanocomposite solution was drop-cast on ITO over 0.5 x 3.5 cm area (the apparent working area
of the composite modified electrode was 1.75 cm2), and the solvent was evaporated at RT. The
final amount of TiO2 on the electrode surface will be 0.15 mg (0.0375 wt %). This uniformly
coated ITO electrodes were characterized and used for electrochemical experiments.
Characterization. The surface morphology of the as-prepared film on the ITO electrode was
measured by field-emission SEM (JEOL LTD. JSM 890). X-ray diffraction (XRD) pattern for the
TiO2-Nafion@ electrode was performed on a BRUCKER D8 Advance diffractometer, with
monochromatic Cu Kα radiation (λ = 1.5418 Å) source.
Cyclic voltammetry and bulk electrolysis. Cyclic voltammetry measurements and bulk
electrolysis experiments were carried out with a CHI 900 electrochemical analyzer (CH
Instruments) in a three-electrode system comprising a modified ITO electrode as a working
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electrode, an Ag/AgCl (3 M KCl) reference electrode, and a coiled platimun wire auxiliary
electrode. Ag/AgCl(3M KCl) was calibrated using potassium ferrocyanide and the half-wave
potential of Ag/AgCl, E°Ag/AgCl = 0.197 V vs NHE. All redox potentials were measured versus an
Ag/AgCl electrode.
For bulk electrolysis, the TiO2-nafion@ composite electrode was used in a custom-made gas-
tight H-type cell ( SI figure S5), in which coiled platinum wire electrode was placed in one
compartment and connected through the Nafion@ film to the working electrode compartment. The
working electrode compartment contained 7.0 mL of 0.1 M KNO3 (pH 4.5) with 7 mL of gas
space. The electrolyte solution in the cathode compartment was purged with high purity CO2 gas
for 20 mins before the electrolysis. The oxygen produced at anode compartment was quenched by
adding potassium ferrocyanide. The solubility of CO2 in water is 36 mM / 1 atm at 25 °C.[18]
Potassium ferrocyanide was added in the anode compartment in order to quench the oxygen. The
electrolysis was perfomed at room temperature and ambient pressure. Nafion® 117 film was
pretreated to remove organic impurities by reported procedure [19]. First, the membrane was boiled
in H2O2 ( 3%, v/v) for 1 h and then boiled in deionized water for 1 h. Again, the membrane was
boiled in 0.5 M H2SO4 for 1 h and finally once again boiled in deionized water for 1 h. The
pretreated membrane was stored in deionized water.
Product analysis. Gas sampling was done through a rubber septum on the gas sampling port,
and the interior pressure was adjusted by injecting CO2 gas equivalent to the withdrawn sample
volume before sampling. The effect of dilution on the gaseous products was included in
calculations of the amount of the products. The gaseous products were quantified by
chromatographic techniques, and the Faradaic efficiency was calculated by a method similar to
those previously reported [7,8]. Carbon monooxide and hydrogen were analyzed by gas
chromatography (HP 5890 SeriesⅡ) using Porapak R80/100 packed column(Altech) with thermal
conductivity detector. The operating conditions were: injection volume, 100 microlitre; oven
temperature, 35 °C; carrier gas, nitrogen with flow rate of 10 mL/min. Ammonia produced was
estimated by the indophenol method [20] using a spectrophotometer (Agilent 8453). The amount of
urea produced was measured using the above method for ammonia after decomposition of the urea
by urease into CO2 + 2NH3 (Murease). The number of moles of urea produced (Murea) was equal to
(Murease − Mammonia)/2.
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Faradaic efficiency. The Faradaic efficiency of the TiO2-Nafion@ electrode was calculated
in a custom made gas-tight H-type cell in 0.1 M KNO3 (pH 4.5) at an applied potential of -0.98 V.
The amount of products produced during constant potential bulk electrolysis for about 2 h, was in
accordance with charge consumption, assuming that almost all of the current was used for the
simultaneous conversion of CO2 and nitrate. The bare ITO electrode did not produce any product
at -0.98 V in 0.1 M KNO3 (pH 4.5).
Acknowledgements
This work was supported by the Korea Institute of Energy Technology Evaluation and
Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of
Korea (No. 20142010201800). D.S. acknowledges the Science and Engineering Research Board
(SERB), Government of India (SB/FT/CS-168/2013) for the financial assistance.
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Figure Captions
Figure 1. SEM image of the TiO2-Nafion® modified ITO.
Figure 2. Cyclic voltammograms of a TiO2-Nafion® modified ITO electrode (0.5 × 3.5 cm) in
0.1M KNO3 (pH 4.5) under Ar (green),1 atm CO2 (purple) and blank ITO plate under 1 atm CO2
(blue) at scan rate of 50 mV s-1.
Figure 3. Electrolysis of 0.1 M KNO3 solution (pH 4.5) at -0.98 V vs. Ag/AgCl in the presence of
1 atm CO2-saturated (purple) and absence of CO2 (green), at RT with TiO2-Nafion® modified ITO
electrode; (a) plot of time vs current density and (b) plot of time vs charge density.
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Fig.1.
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Fig.2.
0.0
-0.2
-0.4
-0.6
-1.2
-1.0
-0.8
-0.6
-0.4
Potential (V vs. Ag/AgCl)
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Fig.3a & 3b.
-0.30
-0.25
-0.20
-0.15
-0.10
-0.05
0.00
a)
0
2000
4000
6000
8000
Time (sec)
-1.8
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
b)
0
2000
4000
6000
8000
Time (sec)
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Table
Table 1. Faradaic efficiencies (%)/concentrations(micromoles)/h of the conversion products[a]
+
Gas
atmosphere
Ar
Urea
-
CO
-
NH4
H2
98/0.30
-
2
-
CO2
40/0.33 27/1.95 30/0.55
Bare ITO
-
-
-
[a]Reaction conditions: Electrolyte : 0.1 M KNO3, pH = 4.5 (under argon) and 4.3 (under CO2
saturation), pressure = 1 atmosphere, temperature = RT (25 C), electrolysis potential = -0.98V vs
Ag/AgCl(3M KCl) and time = 2.15 h, current density = 0.18 mA/cm2 under CO2.
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Mechanism
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