Photoelectrochemical reduction of dissolved carbon dioxide over Ni(OH)2 into organic oxygenates

Article abstract of DOI:10.1007/s13738-020-02117-4

Abstract: The hydrothermal method has been used to prepare Ni(OH)₂ photocathode. The photoelectrochemical (PEC) reduction of CO₂ over Ni(OH)₂ has been conducted in 0.2?M LiClO₄ in aqueous and N,N-dimethylformamide (DMF) medium under visible light irradiation. The thin film was characterized by XRD, UV–Vis, FTIR, FESEM-EDX, BET analysis, and electrochemical method for the determination of phases, bandgap energy, chemical bonding, surface morphology, elemental compositions, surface area, and electrochemical properties, respectively. Based on UV–Vis spectroscopy, the bandgap energy of Ni(OH)₂ was 1.8?eV which enabled efficient visible light absorption for the photoreaction. The photocurrent density in aqueous and DMF solution at 0.2?V (vs. Ag/AgCl) was 24?mA?cm^?2 and 5?mA?cm^?2, respectively. Acetaldehyde and methanol are the products in aqueous solution, while formic acid and methanol are the products in DMF, after 6?h of photoelectrolysis. The product formations from the photoelectrochemical reduction of dissolved CO₂ were 612 and 854?ppm in aqueous and DMF, respectively, where the Faradaic efficiency in aqueous and DMF is 24 and 33%, respectively. Furthermore, throughout the PEC study, the transformation of Ni(OH)₂ to NiO plays a significant role in the formation of organic oxygenates from the reduction reaction of CO₂. Graphic abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s13738-020-02117-4

Journal of the Iranian Chemical Society (2021) 18:1363–1372
https://doi.org/10.1007/s13738-020-02117-4
ORIGINAL PAPER
Photoelectrochemical reduction of dissolved carbon dioxide
over Ni(OH)2 into organic oxygenates
Ahmad Nazeer Che Mat1 · Wan Jefrey Basirun1,2 · Muhammad Mehmood Shahid3
Received: 14 June 2020 / Accepted: 26 November 2020 / Published online: 2 January 2021
© Iranian Chemical Society 2021
Abstract
The hydrothermal method has been used to prepare Ni(OH)2 photocathode. The photoelectrochemical (PEC) reduction of CO2
over Ni(OH)2 has been conducted in 0.2 M LiClO4 in aqueous and N,N-dimethylformamide (DMF) medium under visible
light irradiation. The thin flm was characterized by XRD, UV–Vis, FTIR, FESEM-EDX, BET analysis, and electrochemical
method for the determination of phases, bandgap energy, chemical bonding, surface morphology, elemental compositions,
surface area, and electrochemical properties, respectively. Based on UV–Vis spectroscopy, the bandgap energy of Ni(OH)2
was 1.8 eV which enabled efcient visible light absorption for the photoreaction. The photocurrent density in aqueous and
DMF solution at 0.2 V (vs. Ag/AgCl) was 24 mA cm−2 and 5 mA cm−2, respectively. Acetaldehyde and methanol are the
products in aqueous solution, while formic acid and methanol are the products in DMF, after 6 h of photoelectrolysis. The
product formations from the photoelectrochemical reduction of dissolved CO2 were 612 and 854 ppm in aqueous and DMF,
respectively, where the Faradaic efciency in aqueous and DMF is 24 and 33%, respectively. Furthermore, throughout the
PEC study, the transformation of Ni(OH)2 to NiO plays a signifcant role in the formation of organic oxygenates from the
reduction reaction of CO2.
Supplementary Information The online version of this
article (https://doi.org/10.1007/s13738-020-02117-4) contains
supplementary material, which is available to authorized users.
* Ahmad Nazeer Che Mat
hmd_naz@yahoo.com
1
Department of Chemistry, Faculty of Science, University
of Malaya, 50603 Kuala Lumpur, Malaysia
2
Nanotechnology and Catalysis Centre (NanoCat), Institute
of Post-Graduate Studies Building, University of Malaya,
50603 Kuala Lumpur, Malaysia
3
Micro-Nano System Center, School of Information Science
and Technology (SIST), Fudan University, Shanghai 200433,
China
Vol.:(0123456789)
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Graphic abstract
Keywords Ni(OH)2 · Photoelectrocatalysis · CO2 reduction · C2 products · Electrolyte medium
reaction (HER) as a competitive and favorable reaction as
Introduction
compared to CO2 reduction reaction. Additionally, most
of the metal oxide photocatalysts as aforementioned suf-
fer from wide bandgap energy making them to be more
active in ultraviolet rather than visible light. Hence, new
strategies such as doping with metal [4], nonmetal [5], and
organic–inorganic dye sensitizations [6] have been imple-
mented in order to enhance the efciency of the photocata-
lytic reaction. Additionally, developing photocatalysts with
the Z-scheme mechanism to enhance photogenerated charge
carriers separation has to turn out to be research inclina-
tions in recent times in order to optimize the photocatalytic
activity. This is because the less energetic electrons from
the conduction band from one of the semiconductors will
recombine with photogenerated holes from the valance band
of another semiconductor leaving strongly reductive electron
and strongly oxidative holes in diferent semiconductors.
The strongly reductive electrons may react with CO2 for fur-
ther reduction reaction, while strongly oxidative holes will
undergo oxidation reaction of water into H2 and O2. Hence,
the z-scheme mechanism is seen to be a better strategy to
minimize competitive reaction between water oxidation and
CO2 reduction where the photogenerated charge carriers can
The major issues which need to be addressed nowadays are
regarding energy and environmental crisis where the photo-
conversion of CO2 into carbohydrates in the manifestation
of sunlight by plants could be one of the exit plans which
inspired researchers to exploit atmospheric CO2 as a feed-
stock for the production of synthetic fuels. The focus toward
artifcial photosynthesis such as photocatalytic water split-
ting into H2 and O2, in addition to the photoelectrocatalytic
conversion of CO2 into fuels such as methanol and formic
acid or even higher carbon content products have gained
momentum in the past decades. One of the recompenses of
photocatalytic and photoelectrocatalytic conversion of CO2
into valuable organic products is that these processes convert
atmospheric CO2 directly into the fuel rather than converted
into carbohydrates, which is then converted into short-chain
alcohols as biofuels by fermentation.
Numerous semiconductor photocatalysts have been uti-
lized for the photo-conversion process such as TiO2 [1],
Co3O4 [2], and Cu2O [3], but the efciency of CO2 conver-
sion is still far beyond expectation for industrial and practi-
cal applications due to the existence of hydrogen evolution
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be utilized as maximum as possible for both reduction and
oxidation reactions.
Ni(OH)2 for the formation of CO from the photoreduction
of CO2 has not been discussed so far. Another investigation
of photoreduction of CO2 into CO also has been reported
by Lei et al. [24] over Cu/Ni(OH)2 in 0.5 M KHCO3 against
the reversed hydrogen electrode (RHE). Even though the
formation of CO reached the Faradaic efciency of 92% with
photocurrent density of 4.3 mA/cm2, least courtesy has been
rewarded to the function of Ni(OH)2 and reaction mechanism
of photocatalytic reduction of CO2 instead of the importance
of Cu has been acknowledged a lot. The most recent report
by Akple et al. [25] regarding the photocatalytic reduction
of CO2 that has been performed over honeycomb-like gra-
phitic carbon nitride (g-C3N4) modifed Ni(OH)2 suggests
the formation of methane and methanol with photocatalytic
activity of 1.48 and 0.73 μmolh−1g−1, respectively, which
is 3.5 and 4.3 times higher than bare g-C3N4 and Ni(OH)2,
correspondingly. However, the photocatalytic reaction con-
dition, the reaction mechanism, the importance of Ni(OH)2,
and the quantum yield for the formation of the photocatalytic
reduction of CO2 were not clearly emphasized.
Metals with low hydrogen evolution reaction (HER) over-
potentials such as Ni, Ti, Pt, and Fe are efcient electrocata-
lysts for the HER process but have strong CO adsorption
which is important for the reduction of CO2 [7–11]. Lately,
Ni-based catalyst such as Ni(OH)2 as a p-type semiconduc-
tor has been utilized in various applications because of its
naturally abundance, cheap, and less toxic [12]. Ni(OH)2
has received increasing attention as batteries [13], direct
oxidation of carbohydrates [14], supercapacitor [15], water
splitting [16], and sensor [17]. However, Ni(OH)2 is among
the most prevalent photocatalyst for water oxidation or H2
evolution reaction due to the remarkable ability of Ni(OH)2
to chemisorb H2O molecule thus facilitating the cleavage
of HO-H bond which is conductive to Volmer step of H2
evolution reaction [18]. The use of Ni(OH)2 as H2 evolu-
tion photocatalyst can be obtained from several studies in
the literature; for example, Ioannis et al. [19] demonstrated
that the high surface area mesoporous network containing
10 wt% of Ni(OH)2 modifed with CdS was suitable for
the generation of H2 in 5 M NaOH+10 v/v % ethanol as a
sacrifcial agent with an apparent quantum yield of 72%. A
better H2 evolution rate of 1.4 mmol/h over the as-prepared
catalyst was reported due to the strong electronic coupling
and charge transfer states at CdS/Ni(OH)2 heterojunctions.
Furthermore, instead of water splitting, the photodecomposi-
tion of organic dyes has been reported by Abdallah et al. [20]
by using Ni(OH)2/BiVO4. The author reported that incorpo-
ration of 0.5 wt% Ni(OH)2 exhibits the optimum quantum
yield of 96% with a reaction rate constant of 1.9572 h−1
within 90 min. The fabrication of ternary photocatalyst,
namely Ni(OH)2/GO/TiO2, has been prepared by Barakat
et al. [21] for the degradation of 2-chlorophenol which suc-
cessfully removed 80% of the pollutant from the wastewater
of 25 mg/L concentration within 4 h at pH 6.0.
Thus, motivated by the above literature on photocatalytic
reduction of CO2 over Ni(OH)2, this manuscript intends to
investigate further the importance of using Ni(OH)2 in pro-
ducing valuable organic oxygenates products for the energy
generation from CO2 gas. The mechanism in the formation
of higher hydrocarbon than mainly CO as reported before
such as formic acid, methanol, ethanol, and acetaldehyde in
aqueous and DMF will be discussed with better photocur-
rent density is also reported due to alkalinity of Ni(OH)2
which plays signifcant role in capturing acidic CO2, thus
reducing the gas into valuable chemicals. Besides, the trans-
formation of Ni(OH)2 into NiO after photoelectroreduction
of CO2 shows an important part in constructing valuable
organic product also will be acknowledged as an innova-
tive and novel strategy in this current manuscript. Therefore,
from the author’s point of view and knowledge, based on
the previous publications on Ni(OH)2 for the photoreduction
of CO2, the signifcant of the present manuscript highlights
more on the importance of the transformation of Ni(OH)2 to
NiO in assisting the reduction reaction, the efect of reaction
medium on the fnal product(s), and reaction mechanisms
which are not clearly discussed by the previous report as
aforementioned above.
However, only a few research works on the photoreduc-
tion of CO2 have been done by implementing Ni(OH)2
that can be found from literature. For example, a hybrid
nanocomposite of Ni(OH)2/TiO2 in an aqueous medium
leads to the formation of CH4 with a photoreduction rate
of 2.20 μmol/gcat.h with 0.5 wt% Ni(OH)2 loading, while
the photoreduction rate decreased to 0.58 and 0.37 μmol/
gcat.h for methanol and ethanol, respectively, with 15 wt%
Ni(OH)2 loading. The presence of Ni(OH)2 in the nanocom-
posites increases the selectivity of the product besides act-
ing as the co-catalyst [22]. Another recent study of photore-
duction of CO2 over Ni(OH)2/graphene was conducted by
Siva et al. [23] over diferent types of electrolytes, namely
NaHCO3, KHCO3, and NaCO3 in an aqueous medium to
produce CO. Their report stated that NaHCO3 is a better
choice of electrolyte for the formation of CO as compared
to other electrolytes. However, details report on the role of
The ·CO2− radical anion is the product between the reac-
tion of the excited electron in the conduction band and the
dissolved CO2. This ·CO2− radical anion is the precursor for
the initial formation of formic acid. We propose a mecha-
nism involving the hydroxyl radical ·OH in contrast to the
mechanism proposed by Habisreutinger et al. [26] which
·
·
−
involves the hydrogen radical H and CO2 radical anion
which is responsible for the formation of formic acid. The
second driving force for the reduction of dissolved CO2 is
the presence of OH in the Ni(OH)2. It was proven from the
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XRD, the conversion of Ni(OH)2 into NiO after 6 h of elec-
trolysis in the presence of light illumination. Again we pro-
posed a mechanism for the reduction of dissolved CO2 with
the loss of H2O from the Ni(OH)2 involving the hydroxyl
Xe lamp as the light source. The method of calculating the
amount of dissolved CO2 is given in the ESI.
·
radical OH for the initial formation of the formic acid
Results and discussions
precursor. While the 6 h of photoelectrolysis of dissolved
carbon dioxide in an aqueous solution produces methanol
(from the further reduction of formic acid) as an expected
outcome, the formation of acetaldehyde as the major com-
ponent is somewhat rare. In contrast, the photoelectrolysis
of dissolved carbon dioxide in DMF only produces formic
acid as the major product, while methanol is the minor
product which is due to the lower concentration of reducing
agents, which is confrmed by the low pH after 6 h of reac-
tion. Unlike the results of this paper, most photocatalytic
and photoelectrocatalytic reactions involving the reduction
of dissolved CO2 in aqueous solution only report the con-
version into single-carbon products such as methanol and
formic acid.
Material characterization
X-ray powder difraction (Fig. 1) shows the 2θ of Ni(OH)2
at 19.1°, 33.1°, 38.5°, 51.6°, 59.1°, 62.6°, 69.1°, and 72.4°
which correspond to (001), (010), (002), (012), (110), (111),
(020), and (112) planes, respectively. The crystallite size
measured by the Debye–Scherrer’s formula D=Kλ/β cos θ
gives the value of 11.2 nm, where D is the crystallite size;
K is constant (0.94); λ is 1.54056 Å; β is FWHM; and θ is
the difraction angle. The difraction pattern matches closely
with the theophrastite mineral, with a crystal system and
space group of hexagonal and P-3 m1, correspondingly. The
other parameters of β-Ni(OH)2 that can be attained from
powder X-ray difraction pattern are a= b = 3.1300 Å and
c=4.6300 Å, while α=β=90° and γ=120° [ICDS code no.
98-002-8101]. After 6 h of photoelectrochemical reduction
of CO2, some Ni(OH)2 are converted into NiO (Fig. 1), with
the presence of 2θ peak at 45° attributed to the (200) of NiO.
The optical characteristics of the Ni(OH)2 flm (Fig. 2a)
shows a broad visible absorption range from 550 to 750 nm
with the calculated bandgap energy of 1.8 eV from the
Experimental methods
Lithium perchlorate (LiClO4) was purchased from Sigma-
Aldrich, N,N-dimethylformamide (DMF) was procured from
Merck, and nickel nitrate hexahydrate (Ni(NO3)2·6H2O) was
attained from Fisher Scientifc. Nickel (II) hydroxide was
prepared by dissolving 0.60 g Ni(NO3)2·6H2O and 0.25 g
urea in 100 ml distilled water under continuous stirring for
two hours. The mixture was autoclaved at 100 °C for 12 h.
The green powder was washed with distilled water several
times and dried in an oven at 90 °C. The Ni(OH)2 electrode
flms were fabricated by pressing the Ni(OH)2 powders with
acetylene black and poly(vinylidene) fuoride (PVDF) binder
with the ratio of 8:1:1.
XRD was attained between 2θ of 5° and 90° using PANa-
lytical EMPYREAN over Cu Kα1 radiation with a step size
of 0.01°. PL was acquired by the Micro-Raman PL sys-
tem (Renishaw, UK) with a wavelength of 325 nm from a
He-Cd laser source. FTIR was recorded on a Perkin Elmer
400FTIR/FT-FIR spectrometer from 4000–200 cm−1. The
surface analysis was obtained using FESEM-EDX SU8220
Hitachi. The products were analyzed using Shimadzu GCMS
with DB-WAX column (30 m length × 0.25 μm thick-
ness×0.25 mm diameter).
The PEC performance was studied by PGSTAT30 Auto-
lab potentiostat/galvanostat (Ecochemie, Netherlands) in an
aqueous and DMF solution of 0.2 M LiClO4 after CO2 bub-
bling for an hour where the counter and reference electrodes
were Pt wire and Ag/AgCl, respectively. The potential range
of LSV was from − 1.0 to + 0.2 V vs. Ag/AgCl at 50 mV s−1
and chronoamperometry at − 0.5 V vs Ag/AgCl with 150 W
Fig. 1 XRD spectrum of Ni(OH)2 flm before and after 6 h reaction
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Journal of the Iranian Chemical Society (2021) 18:1363–1372
1367
which give the value of 2.99, − 2.41, and − 0.6 eV for χ,
ECB, and EVB of Ni(OH)2, respectively.
The PL spectrum (Fig. S1, electronic supporting infor-
mation, ESI) of Ni(OH)2 microparticles shows three peaks
centered at 412, 547, and 677 nm. These peaks can be
assigned as the electronic transition of Ni2 + ion, namely
1T2g(D)→3A2g(F) and 1T2g(D)→3T2g(F). These values are
close to the bandgap value of 1.73 eV by Qi et al. from
ultraviolet–visible measurements [29], which was due to
3
3
the electronic transition from the A2g to T2g level. They
also reported a larger bandgap value of 2.95 and 3.22 eV
for nanoplates of Ni(OH)2 with a thickness between 20 and
50 nm. The larger bandgap is due to the quantum confne-
ment efect when the particle sizes become smaller [29–31].
The FTIR of Ni(OH)2 is indicated in Fig. S2, ESI. The
sharp peak at 3622 cm−1 is allocated to the stretching vibra-
tion of the O–H stretching mode, which is characteristic of
the free O–H group of the brucite-like structure [32]. Mean-
while, a peak at 1631 cm−1 indicates the bending vibration
mode of water molecules [33]. The asymmetric and sym-
−
metric stretching of NO3 ion can be observed at around
1384 cm−1 and 1000 cm−1, correspondingly [34–36]. While
the peaks at 503 and 524 cm−1 are attributed to the bending
vibration of the water molecule and Ni–O(–H) stretching
vibration which are the characteristics of Ni(OH)2, respec-
tively [37–40]. Additionally, a weak peak at 450 cm−1 is
attributed to the lattice vibration of the Ni–O bond which is
in accordance with the published literature [41].
Fig. 2 a UV–Vis spectrum and b Tauc’s plot of Ni(OH)2
Tauc’s plot (Fig. 2b). Additionally, based on the calcu-
lated bandgap of Ni(OH)2 as aforementioned above, the
conduction band edge (ECB) and valance band edge (EVB
)
The FESEM image (Fig. 3) of Ni(OH)2 reveals a spheri-
cal and smooth morphology. The morphological image of
Ni(OH)2 shows agglomeration of many crystallites with
good dispersion of the spherical Ni(OH)2 particles. The
better dispersion of the particles provides adequate sur-
face area for the adsorption of CO2 for further photoelec-
trochemical reduction reaction to take place which might
enhance the efciency of the CO2 reduction reaction. Fur-
thermore, the spherical and compact structure of Ni(OH)2
probably could be able to help in refecting the incident light
of Ni(OH)2 can be determined by using the formula given
below as indicated in Eqs. 1 and 2 [27]:
ECB = ꢀ−Ee−0.5Eg
(1)
EVB = ECB + Eg
(2)
where χ is electronegativity of semiconductor, Ee is constant
(4.5 eV) [28], and Eg denotes the bandgap of semiconductors
Fig. 3 FESEM images of
Ni(OH)2
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thus maximizing the utilization of visible light for further
improvement of photocatalytic reduction of CO2. The ele-
mental composition of Ni(OH)2 is shown from EDX analysis
(Fig. S3, ESI). The N2 adsorption–desorption behavior is
shown in (Fig. S4a, ESI). The sample presents a type IV
isotherm with an apparent hysteresis loop between 0.5–1.0
P/P°, according to the Brunauer–Deming–Deming–Teller
(BDDT) classifcation, indicating the presence of mesopores
(2–50 nm). The hysteresis loop of Ni(OH)2 represents a type
of H1 behavior, which suggests the presence of a narrow
distribution of uniform cylindrical pores [42]. (Figure S4b,
ESI) shows the BET plot of spherical Ni(OH)2 nanoparti-
cles. The total surface area of the Ni(OH)2 spherical parti-
cles is 18.9 m2 g−1, from the multipoint BET equation. The
pore width distribution from the desorption branch of N2 iso-
therms by the Barrett–Joyner–Halenda (BJH) method shows
a pore width and pore volume of 14.7 nm and 0.02 cm3 g−1,
respectively, confrming the presence of mesopores (Fig.
S4c, ESI).
S5, ESI) without the presence of CO2 gas in aqueous and
DMF solution. The results showed that without the pres-
ence of CO2, no reduction reaction takes place, indicating
that the products of the photoelectrochemical reaction were
generated from the reduction of CO2 and not from any other
sources of carbon (Fig. 4). The high currents at the positive
region are due to the conversion of the Ni(OH)2 electrode
into NiOOH, as seen in the positive electrode in NiMH bat-
teries as specifed in Eq. 3:
Ni(OH)2 + OH− → NiOOH + H2O + e−
(3)
Nevertheless, the release of electrons by the Ni(OH)2
electrode at a higher positive region is also due to the for-
mation of the ·CO2− radical anion in solution after accepting
an electron from the conduction band upon excitation.
Based on our previous study [43], the fuctuation in the
voltammetric analysis is indicative of the lower selectivity
of the products. Hence, methanol, ethanol, and acetalde-
hyde were detected in this study during chronoamperometry
(Fig. 5) in an aqueous solution (Fig. S6, ESI). Meanwhile,
the steady photocurrent in DMF (Fig. 6) is indicative of
the high selectivity of reduction products, which is mainly
formic acid (Fig. S7, ESI). The summary of the results is
tabulated in Table 1.
Photoelectrochemical reduction of dissolved CO2
In an aqueous solution, the photoelectrochemical oxidation
against Ag/AgCl as a reference electrode begins at − 0.48 V
with a maximum photocurrent density of 24 mA cm−2
(0.2 V) under visible light irradiation as compared to the
dark condition, while in DMF solution, the photoelectro-
chemical oxidation starts at − 0.61 V. The photocurrent
density under visible light irradiation in DMF is maximum
at 5 mA cm−2 (0.2 V) which is higher than in the dark condi-
tion, but still lower than in the aqueous solution. The con-
trol experiments were conducted on acetylene black (Fig.
The amount of dissolved CO2 in aqueous and DMF is
determined before and after the experiment by using the
titration method as mentioned in the experimental section.
Before the photoelectrochemical reduction of dissolved CO2,
the concentration of CO2 in aqueous and DMF is 1076 and
1454 ppm, respectively. The concentration of CO2 after
Fig. 4 LSV of Ni(OH)2 in aque-
ous (AQ) and DMF solution of
0.2 M LiClO4 in dark (D) and
light (L)
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Journal of the Iranian Chemical Society (2021) 18:1363–1372
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formic acid into alcohols such as methanol and acetaldehyde
in aqueous solution (Fig. S6, ESI), while the major product
formed during the photoelectrochemical reduction of CO2
in DMF medium is formic acid, detected by GCMS (Fig.
S7, ESI).
After 6 h of photoelectrochemical reduction of CO2, the
major reduction product in aqueous solution is acetaldehyde
with a Faradaic efciency of 24% (471 ppm) at retention
time, Rt of 1.5 and 1.7 min. However, the formation of for-
mic acid at Rt of 1.3 was also detected. The transformation
of formic acid into methanol as the minor product was sug-
gested as the main reason for the slight increment of pH
value after the reaction. The Faradaic efciency (F.E) was
calculated from Eq. 4 [44]:
Fig. 5 Chronoamperometry of Ni(OH)2 in an aqueous solution of
0.2 M LiClO4 after 6 h in light
[
(
)]
F ⋅ E = (n × z × F)∕ Jphoto × A × t × 100%
(4)
where n is the number of moles of product generated; F is
the Faraday constant with a value of 96 485 C mol−1; z is
the number of electrons involved; Jphoto is the photocurrent
density (A cm−2) generated during the measurement time
(t) (seconds); andA is the illumination area of the photo-
electrode (cm2).
Meanwhile, formic acid appears at Rt of 1.487 min during
the photoelectrochemical reduction of CO2 in DMF (Fig.
S7, ESI) with the Faradaic efciency of 33%. The formation
of methanol in DMF is almost not detected even after 6 h
reaction due to the low amount of water (3%) content in the
reaction medium, in addition to the competition with the
hydrogen evolution reaction (HER) from water oxidation
and the re-oxidation of product into CO2. Therefore, formic
acid is regarded as the major product from the photoelectro-
chemical reduction of CO2 in DMF.
Fig. 6 Chronoamperometry of Ni(OH)2 in DMF solution of 0.2 M
LiClO4 +3% H2O after 6 h in light
6 h of photoelectrochemical reduction (at − 0.5 V vs Ag/
AgCl) in aqueous and non-aqueous medium decreased to
464 and 600 ppm, respectively. This suggests that 612 ppm
and 854 ppm of products are formed during the photoelec-
trochemical reduction of CO2 in aqueous and non-aqueous
medium, respectively, after 6 h. It must be mentioned that
the pH of the electrolyte also changed after the experiment
in both reaction media. After 1 h of bubbling the solution
with CO2, the pH in aqueous and DMF is 3.82 and 8.83,
respectively. However, after 6 h of photoelectrochemical
reduction, the pH of the solution changed to 7.47 and 5.83,
in aqueous and DMF solution, respectively. This indicates
that the increment in pH reading is due to the conversion of
The photochemical reduction mechanism of CO2 to pro-
duce formic acid and methanol is still unclear. Two mecha-
nisms describe the reduction of CO2, namely the formalde-
hyde pathway [45] and the carbene pathway [46]. Ji et al.
[45] proposed that formic acid and formaldehyde could be
formed according to the formaldehyde pathway which is
thermodynamically feasible [47] and these products could
be desorbed from the surface of the catalyst into the solu-
tion upon the reduction reaction [26, 48]. Upon light illu-
mination, photogenerated charge carriers will be produced
Table 1 Chronoamperometry
Conditions
Aqueous 0.2 M
LiClO4 solution
0.2 M LiClO4 in DMF
+ 3% H2O solution
results of 6 h of
photoelectrochemical reduction
at Ni(OH)2 the electrode at
Concentration of CO2 after 1 h bubbling
1076 ppm
464 ppm
612 ppm
Acetaldehyde
Formic acid Methanol
1454 ppm
600 ppm
854 ppm
Formic acid
Methanol
− 0.5 V (vs. Ag/AgCl) of
Concentration of CO2 after 6 h reduction
Major product concentration
Major product
dissolved CO2 in aqueous and
DMF solution
Minor product
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where the electrons with higher energy than the bandgap of
Ni(OH)2 will be promoted to the conduction band leaving
the photogenerated holes in the valence band. In most cases,
the initial step of activation of dissolved CO2 in the solution
is the establishment of ·CO2− radical anion after accepting
an electron from the conduction band [49]. The adsorption
Ni(OH)2 was transformed into NiO (JCPDS #47-1049)
with a distinct peak at 44° in Fig. 1. The water content in
Ni(OH)2 is an important source for the CO2 reduction to
release formic acid (Eqs. 10 and 16). Hence, Ni(OH)2 in the
present study can be suggested as a suitable candidate for
CO2 reduction reaction into valuable organic oxygenates.
Furthermore, we also proposed the formation of NiO from
Ni(OH)2 through Eqs. 18 and 19 driven by the reaction from
Eqs. 14 and 16 as supported by the XRD analysis in Fig. 1.
·
−
of CO2 through monodentate binding is through one of
the oxygen atom or carbon atom which favors the produc-
tion of the carboxylate radical (·COOH) [50]. The hydrogen
radical (·H) initiated from the reduction of the proton will
VB+h − Ni(OH)2 → NiO + H+ + OH
⋅
·
(18)
further react with COOH to yield HCOOH [51, 52]. The
proposed reaction mechanism for formic acid production is
via the formation of ·CO− and ·COOH as the intermediate
species. This is a favorable reaction pathway as reported
by Habisreutinger et al. [26], since the CO2 reduction in
this study proceeds in a high dielectric medium such as an
aqueous solution. The proposed reaction mechanism for the
establishment of HCOOH is shown in [26].
⋅
Ni(OH)2 + COOH → NiO + HCOOH + OH
⋅
(19)
The production of acetaldehyde is unclear, but the com-
·
bination of formic acid and CO2− radical anion could be
the major step in the formation of acetaldehyde. Further-
more, the production of acetaldehyde is proposed due to the
reduction of acetic acid according to Eq. 20, although unde-
tected by GCMS. The presence of reductants such as the
conduction band electrons (CB−e ), H+, water, and Ni(OH)2
could play a major role in the further reduction to eliminate
the oxygen atoms to produce acetaldehyde in an aqueous
solution.
Ni(OH)2 + hꢀ → CB−e + VBh+
(5)
CO2 + CB−e → CO−2
⋅
(6)
H2O → H+ + OH−
(7)
HCOOH + CO−2 → HCOO− +∙ COOH → HC(O )(O−) − COOH
⋅
⋅
(20)
H+ + CB−e → H
⋅
(8)
CO−2 + H+ → COOH
⋅
⋅
(9)
Conclusions
⋅
⋅
COOH + H → HCOOH
(10)
In summary, the photoelectrochemical reduction of CO2 in
the visible region has been studied in the aqueous and non-
aqueous medium by using Ni(OH)2 photocathode which
has been fabricated by using the hydrothermal method. The
optimum ratio for the fabrication of the photoelectrode of
Ni(OH)2 is 8:1:1 ratio of Ni(OH)2/PVDF/acetylene black.
The prepared Ni(OH)2 photocathode showed excellent pho-
tocatalytic properties over CO2 reduction with a high photo-
current density of 24 mA cm−2 was obtained in the aqueous
solution of 0.2 M LiClO4 as compared to 5 mA cm−2 in
DMF solution. The Faradaic efciencies were 24 and 33%
in the aqueous and DMF, respectively. Better absorption
of visible light irradiation of the prepared Ni(OH)2 pho-
tocathode might be due to low bandgap energy of 1.8 eV
which led to better photocatalytic properties toward CO2
reduction into organic oxygenates. Apart from the formation
We proposed also an alternate pathway which involves
the ·OH intermediate:
Ni(OH)2 + hꢀ → CB−e + VBh+
(11)
(12)
(13)
(14)
(15)
(16)
(17)
CO2 + CB−e → CO−2
⋅
VB+h + H2O → H+ + OH
⋅
CO−2 + H2O → COOH + OH−
⋅
⋅
⋅
⋅
COOH + H2O → HCOOH + OH
CB−e + OH → OH−
⋅
·
−
of CO2 radical anion as the intermediate species for the
dissolved CO reduction in both aqueous and non-aqueous
medium, the presence of OH-group from the transforma-
tion of Ni(OH)2 was paramount important for the production
of organic oxygenates such as methanol, formic acid, and
acetaldehyde. Therefore, this study revealed that Ni(OH)2
OH− + H+ → H2O
On the other hand, from the XRD results (Fig. 1) after
6 h of the photoelectrochemical reduction reaction, some
1 3

Journal of the Iranian Chemical Society (2021) 18:1363–1372
1371
14. V. Ganesh, S. Farzana, S. Berchmans, Nickel hydroxide deposited
indium tin oxide electrodes as electrocatalysts for direct oxida-
tion of carbohydrates in alkaline medium. J. Power Sources 196,
9890–9899 (2011)
could be a suitable candidate for green energy generation
especially for the photoelectrochemical reduction of CO2
into organic chemicals.
15. S. Ramesh, K. Karuppasamy, H.M. Yadav, J.-J. Lee, H.-S.
Kim, H.-S. Kim, J.-H. Kim, Ni(OH)2-decorated nitrogen doped
MWCNT nanosheets as an efcient electrode for high perfor-
mance supercapacitors. Sci. Rep. 9, 1–10 (2019)
Acknowledgments The authors would like to thank University of
Malaya and the Ministry of Higher Education Malaysia for funding
this research through Grants RP020D-16SUS and FP039-2016.
16. X. Zhou, Z. Xia, Z. Zhang, Y. Ma, Y. Qu, One-step synthesis of
multi-walled carbon nanotubes/ultra-thin Ni(OH)2 nanoplate com-
posite as efcient catalysts for water oxidation. J. Mater. Chem. A
2, 11799–11806 (2014)
Compliance with ethical standards
Conflict of interest All authors declare that there is no conficting in-
17. H. Zhou, J. Peng, X. Qiu, Y. Gao, L. Lu, W. Wang, β-Ni(OH)2
nanosheets: an efective sensing platform for constructing nucleic
acid-based optical sensors. J. Mater. Chem. B 5, 7426–7432
(2017)
terest and look forward to your positive response.
18. Q. Xu, H. Jiang, H. Zhang, Y. Hu, C. Li, Heterogeneous inter-
face engineered atomic confguration on ultrathin Ni(OH)2/Ni3S2
nanoforests for efcient water splitting. Appl. Catal. B 242, 60–66
(2019)
References
1. Z. He, J. Tang, J. Shen, J. Chen, S. Song, Enhancement of photo-
catalytic reduction of CO2 to CH4 over TiO2 nanosheets by modi-
fying with sulfuric acid. Appl. Surf. Sci. 364, 416–427 (2016)
2. Q. Zhang, J. Du, A. He, Z. Liu, C. Tao, Low overpotential electro-
chemical CO2 reduction to formate on Co3O4–CeO2/low graphitic
carbon catalyst with oxygen vacancies. J. Solid State Chem. 279,
120946–120954 (2019)
19. I. Vamvasakis, I.T. Papadas, T. Tzanoudakis, C. Drivas, S.A.
Choulis, S. Kennou, G.S. Armatas, Visible-light photocatalytic
H2 production activity of β-Ni(OH)2-modifed CdS mesoporous
nanoheterojunction networks. ACS Catal. 8, 8726–8738 (2018)
20. N.I.M. Abdallah, Y. Li, X.-T. Wang, X. Li, C.-W. Wang, Design
and fabrication of Ni(OH)2/BiVO4 heterostructured nano-pho-
tocatalyst for high-efcient removal of organic dyes, J. Alloys
Compd., (2020) 154828-154836
3. E. Szaniawska, K. Bienkowski, I.A. Rutkowska, P.J. Kulesza, R.
Solarska, Enhanced photoelectrochemical CO2-reduction system
based on mixed Cu2O–nonstoichiometric TiO2 photocathode.
Catal. Today 300, 145–151 (2018)
21. M. Barakat, M. Anjum, R. Kumar, Z. Alaff, M. Oves, M.O.
Ansari, Design of ternary Ni(OH)2/graphene oxide/TiO2 nano-
composite for enhanced photocatalytic degradation of organic,
microbial contaminants, and aerobic digestion of dairy wastewa-
ter. J. Clean. Prod. 258, 120588–120597 (2020)
4. N.T.T. Truc, L.G. Bach, N.T. Hanh, T.-D. Pham, N.T.P. Le Chi,
D.T. Tran, M.V. Nguyen, V.N. Nguyen, The superior photocata-
lytic activity of Nb doped TiO2/g-C3N4 direct Z-scheme system
for efcient conversion of CO2 into valuable fuels. J. Colloid
Interface Sci. 540, 1–8 (2019)
22. A. Meng, S. Wu, B. Cheng, J. Yu, J. Xu, Hierarchical TiO2/
Ni(OH)2 composite fbers with enhanced photocatalytic CO2
reduction performance. J. Mater. Chem. A 6, 4729–4736 (2018)
23. S. Palanisamy, S. Srinivasan, Electrochemical reduction of CO2 on
Ni(OH)2 doped water dispersible graphene under diferent elec-
trolyte conditions. SN Appl. Sci. 1, 837–843 (2019)
5. L. Yu, X. Ba, M. Qiu, Y. Li, L. Shuai, W. Zhang, Z. Ren, Y. Yu,
Visible-light driven CO2 reduction coupled with water oxidation
on Cl-doped Cu2O nanorods. Nano Energy 60, 576–582 (2019)
6. D.H. Apaydin, E. Tordin, E. Portenkirchner, G. Aufscher, S.
Schlager, M. Weichselbaumer, K. Oppelt, N.S. Sariciftci, Pho-
toelectrochemical Reduction of CO2 Using Third-Generation
Conjugated Polymers. ChemistrySelect 1, 1156–1162 (2016)
7. S. Kaneco, H. Katsumata, T. Suzuki, K. Ohta, Electrochemical
reduction of CO2 to methane at the Cu electrode in methanol with
sodium supporting salts and its comparison with other alkaline
salts. Energy Fuels 20, 409–414 (2006)
24. L. Dai, Q. Qin, P. Wang, X. Zhao, C. Hu, P. Liu, R. Qin, M.
Chen, D. Ou, C. Xu, Ultrastable atomic copper nanosheets for
selective electrochemical reduction of carbon dioxide. Sci. Adv.
3, e1701069–e1701078 (2017)
25. M.S. Akple, T. Ishigaki, P. Madhusudan, Bio-inspired honey-
comb-like graphitic carbon nitride for enhanced visible light
photocatalytic CO2 reduction activity, Environ. Sci. Pollut. Res.,
(2020) 1-15
8. J. Qu, X. Zhang, Y. Wang, C. Xie, Electrochemical reduction of
CO2 on RuO2/TiO2 nanotubes composite modifed Pt electrode.
Electrochim. Acta 50, 3576–3580 (2005)
26. S.N. Habisreutinger, L. Schmidt-Mende, J.K. Stolarczyk, Pho-
tocatalytic reduction of CO2 on TiO2 and other semiconductors.
Angew. Chem. Int. Ed. 52, 7372–7408 (2013)
9. N. Furuya, S. Koide, Electroreduction of carbon dioxide by metal
phthalocyanines. Electrochim. Acta 36, 1309–1313 (1991)
10. A.A. Peterson, J.K. Nørskov, Activity descriptors for CO2 elec-
troreduction to methane on transition-metal catalysts. J. Phys.
Chem. Lett. 3, 251–258 (2012)
27. L. Kong, Z. Jiang, H.H. Lai, R.J. Nicholls, T. Xiao, M.O. Jones,
P.P. Edwards, Unusual reactivity of visible-light-responsive
AgBr–BiOBr heterojunction photocatalysts. J. Catal. 293, 116–
125 (2012)
28. X.-J. Wen, C.-G. Niu, M. Ruan, L. Zhang, G.-M. Zeng, AgI
nanoparticles-decorated CeO2 microsheets photocatalyst for the
degradation of organic dye and tetracycline under visible-light
irradiation. J. Colloid Interface Sci. 497, 368–377 (2017)
29. Y. Qi, H. Qi, C. Lu, Y. Yang, Y. Zhao, Photoluminescence and
magnetic properties of β-Ni(OH)2 nanoplates and NiO nanostruc-
tures, J. Mater. Sci.: Mater. Electron, 20 (2009) 479-483
11. T. Yamamoto, D.A. Tryk, A. Fujishima, H. Ohata, Production of
syngas plus oxygen from CO2 in a gas-difusion electrode-based
electrolytic cell. Electrochim. Acta 47, 3327–3334 (2002)
12. Y. Zhang, L. Sun, K. Lv, Y. Zhang, One-pot synthesis of Ni(OH)2
fakes embedded in highly-conductive carbon nanotube/graphene
hybrid framework as high performance electrodes for supercapaci-
tors. Mater. Lett. 213, 131–134 (2018)
30. G. Lakshminarayana, S. Buddhudu, Spectral analysis of Mn2+
,
Co2+ and Ni2+: B2O3–ZnO–PbO glasses. Spectrochim. Spectro-
chimica Acta Part A 63, 295–304 (2006)
13. H. Wu, Y. Li, J. Ren, D. Rao, Q. Zheng, L. Zhou, D. Lin, CNT-
assembled dodecahedra core@ nickel hydroxide nanosheet shell
enabled sulfur cathode for high-performance lithium-sulfur bat-
teries. Nano Energy 55, 82–92 (2019)
31. A. De Moura, R. Lima, E. Paris, M. Li, J.A. Varela, E. Longo,
Formation of β-nickel hydroxide plate-like structures under mild
1 3

1372
Journal of the Iranian Chemical Society (2021) 18:1363–1372
43. M.R. Hasan, S.B.A. Hamid, W.J. Basirun, S.H.M. Suhaimy,
A.N.C. Mat, A sol–gel derived, copper-doped, titanium dioxide–
reduced graphene oxide nanocomposite electrode for the photo-
electrocatalytic reduction of CO2 to methanol and formic acid.
RSC Adv. 5, 77803–77813 (2015)
conditions and their optical properties. J. Solid State Chem. 184,
2818–2823 (2011)
32. Z. Liu, R. Ma, M. Osada, K. Takada, T. Sasaki, Selective and con-
trolled synthesis of α-and β-cobalt hydroxides in highly developed
hexagonal platelets. J. Am. Chem. Soc. 127, 13869–13874 (2005)
33. D. Dubal, V. Fulari, C. Lokhande, Efect of morphology on super-
capacitive properties of chemically grown β-Ni(OH)2 thin flms.
Microporous Mesoporous Mater. 151, 511–516 (2012)
34. M. Chatterjee, D. Enkhtuvshin, B. Siladitya, D. Ganguli, Hol-
low alumina microspheres from boehmite sols. J. Mater. Sci. 33,
4937–4942 (1998)
44. C. Jiang, J. Wu, S.J. Moniz, D. Guo, M. Tang, Q. Jiang, S. Chen,
H. Liu, A. Wang, T. Zhang, Stabilization of GaAs photoanodes
by in situ deposition of nickel-borate surface catalysts as hole
trapping sites. Sustain. Energy Fuels 3, 814–822 (2019)
45. Y. Ji, Y. Luo, Theoretical Study on the Mechanism of Photore-
duction of CO2 to CH4 on the Anatase TiO2 (101) Surface. ACS
Catal. 6, 2018–2025 (2016)
35. S. Tamura, A. Mori, N. Imanaka, Li+ ion conduction in (Gd,
La)2O3–LiNO3 system. Solid State Ion. 175, 467–470 (2004)
36. M. Sulaiman, A. Rahman, N. Mohamed, Efect of water-based
sol gel method on structural, thermal and conductivity properties
of LiNO3–Al2O3 composite solid electrolytes. Arab. J. Chem. 10,
1147–1152 (2017)
46. S.t. Neatu, J.A. Maciá-Agulló, P. Concepción, H. Garcia, Gold–
copper nanoalloys supported on TiO2 as photocatalysts for CO2
reduction by water, J. Am. Chem. Soc., 136 (2014) 15969-15976
47. J. Mao, K. Li, T. Peng, Recent advances in the photocatalytic CO2
reduction over semiconductors. Catal. Sci. Technol. 3, 2481–2498
(2013)
37. T. Ramesh, P.V. Kamath, Synthesis of nickel hydroxide: efect of
precipitation conditions on phase selectivity and structural disor-
der. J. Power Sources 156, 655–661 (2006)
48. N. Zhang, R. Long, C. Gao, Y. Xiong, Recent progress on
advanced design for photoelectrochemical reduction of CO2 to
fuels. Sci. China Mater. 61, 771–805 (2018)
38. P. Jeevanandam, Y. Koltypin, A. Gedanken, Synthesis of nano-
sized α-nickel hydroxide by a sonochemical method. Nano Lett.
1, 263–266 (2001)
49. J.L. White, M.F. Baruch, J.E. Pander III, Y. Hu, I.C. Fortmeyer,
J.E. Park, T. Zhang, K. Liao, J. Gu, Y. Yan, Light-driven hetero-
geneous reduction of carbon dioxide: photocatalysts and photo-
electrodes. Chem. Rev. 115, 12888–12935 (2015)
39. D. Yang, R. Wang, M. He, J. Zhang, Z. Liu, Ribbon-and boardlike
nanostructures of nickel hydroxide: synthesis, characterization,
and electrochemical properties. J. Phys. Chem. B 109, 7654–7658
(2005)
50. M. Gattrell, N. Gupta, A. Co, A review of the aqueous electro-
chemical reduction of CO2 to hydrocarbons at copper. J. Electroa-
nal. Chem. 594, 1–19 (2006)
40. E. Shangguan, H. Tang, Z. Chang, X.-Z. Yuan, H. Wang, Efects
of diferent Ni(OH)2 precursors on the structure and electrochemi-
cal properties of NiOOH. Int. J. Hydrog. Energy 36, 10057–10064
(2011)
51. M. Subrahmanyam, S. Kaneco, N. Alonso-Vante, A screening for
the photo reduction of carbon dioxide supported on metal oxide
catalysts for C1–C3 selectivity. Appl. Catal. B 23, 169–174 (1999)
52. N. Sasirekha, S.J.S. Basha, K. Shanthi, Photocatalytic perfor-
mance of Ru doped anatase mounted on silica for reduction of
carbon dioxide. Appl. Catal. B 62, 169–180 (2006)
41. M.K. Motlagh, A. Youzbashi, F. Hashemzadeh, L. Sabaghzadeh,
Structural properties of nickel hydroxide/oxyhydroxide and oxide
nanoparticles obtained by microwave-assisted oxidation tech-
nique. Powder Technol. 237, 562–568 (2013)
42. M. Zhou, Z. Wei, H. Qiao, L. Zhu, H. Yang, T. Xia, Particle size
and pore structure characterization of silver nanoparticles pre-
pared by confned arc plasma. J. Nanomater. 2009, 3 (2009)
1 3