Ammonium ion-promoted electrochemical production of synthetic gas from water and carbon dioxide on a fluorine-doped tin oxide electrode

Article abstract of DOI:10.1039/d0cc08040c

In situgenerated Sn nanoparticles on fluorine-doped tin oxide act as an electrocatalyst for the CO₂reduction reaction to efficiently and stably produce synthetic gas from water and carbon dioxide with the reaction rate drastically enhanced by the addition of ammonium ions.

Full text of DOI:10.1039/d0cc08040c

Showcasing research from Professor Hiroaki Tada’s
laboratory, Department of Applied Chemistry,
Faculty of Science and Engineering, Kindai University,
Osaka, Japan
As featured in:
Volume 57
Number 12
11 February 2021
Pages 1403–1528
ChemComm
Ammonium ion-promoted electrochemical production
of synthetic gas from water and carbon dioxide on a
fluorine-doped tin oxide electrode
Chemical Communications
rsc.li/chemcomm
Sn nanoparticles on fluorine-doped tin oxide act as an
electrocatalyst for CO2 reduction to efficiently and stably
produce synthetic gas from water and carbon dioxide with
the reaction rate drastically enhanced by the addition of
ammonium ions.
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Ammonium ion-promoted electrochemical
production of synthetic gas from water and
carbon dioxide on a fluorine-doped tin oxide
Cite this: Chem. Commun., 2021,
57, 1438
Received 11th December 2020,
Accepted 11th January 2021
electrode†
b
Shin-ichi Naya,a Hisayoshi Yoshiokab and Hiroaki Tada
*
DOI: 10.1039/d0cc08040c
rsc.li/chemcomm
In situ generated Sn nanoparticles on fluorine-doped tin oxide act
as an electrocatalyst for the CO2 reduction reaction to efficiently
and stably produce synthetic gas from water and carbon dioxide
with the reaction rate drastically enhanced by the addition of
ammonium ions.
Further, H2CO3 can be ionized in two steps (eqn (2) and (3)).
H2CO3(aq.) + H2O(l) 2 HCO3À(aq.) + H3O+(aq.), pKa1 = 6.37
(2)
HCO3À(aq.) + H2O(l) 2 CO32À(aq.) + H3O+(aq.), pKa2 = 10.32
(3)
The CO2 reduction reaction (CO2-RR) to hydrocarbons and
oxygenated compounds is the key technology to solving the
Since only molecular CO2 undergoes reduction,8 the electro-
issues of climate change and energy deficiency through carbon chemical CO2-RR should be carried out in a near-neutral
recycling.1 However, these multi-electron reduction processes electrolyte. On the other hand, the electrochemical CO2-RR
are usually accompanied by high overpotential with low needs protons and electrons, and is usually carried out under
selectivity.2 On the other hand, synthetic gas consisting of H2 acidic conditions, where the solubility of CO2 in water
and CO is a basic feedstock for various important chemical decreases. In this study, to solve this trade-off, the electroche-
processes including C1 chemistry and the Fischer–Tropsch mical CO2-RR was performed on a fluorine-doped tin oxide
process. At present, synthetic gas is mainly produced by gasi- (FTO) electrode using NH4+ ions with a great affinity for FTO as
fication of fossil fules at a high temperature with a large a proton donor under near-neutral conditions, while highly
quantity of energy consumed. The electrochemical production active bulk-metal electrodes including mainly Ag and Cu
of synthetic gas from water and CO2 by utilizing renewable besides Au, Ni, Sn, Zn have been developed so far (Table S1,
electricity can be a fascinating green process,3 while surplus ESI†). Recent theoretical work has shown that the optimal bulk
electricity can also be effectively used as the energy source. pH of the electrolyte solution for CO2-RR should be close to 7.9
Since the pioneering work by Fujishima and co-workers,4 the A CO2-RR on a metal oxide electrode was performed for the first
research on the electrochemical production of synthetic gas is time using an FTO electrode in acidic electrolyte solution, and
currently in rapid progress.5–7 Fortunately for this purpose, the the transient generation of CO and steady generation of H2 were
standard electrode potential of CO2 reduction to CO (standard confirmed.10 Also, an Sn–SnOx-coelectrodeposited titanium elec-
reduction potential E0 = À0.104 V, eqn (1)) is close to that of trode was reported to show much higher faradaic efficiency for
water reduction (E0 = 0 V, eqn (2)), where the potential is shown CO2 reduction to CO and HCOOH than an SnO2 electrode with a
with respect to the standard hydrogen electrode potential native oxide layer.11 On the contrary, the electrochemical CO2-RR
(SHE). CO2 dissolved in water is partly hydrated to generate at the metal Sn electrode12–15 and the Sn–Cu bimetallic
H2CO3 (K = 1.7 Â 10À3 at 25 1C, eqn (1)).
electrode16 yields HCOOH as a major product. Although Li, Sun
and co-workers have reported that the electrochemical CO2-RR
using a carbon supported Cu(core)–SnO2(shell) nanocatalyst
yields H2 and HCOOH at the SnO2 thickness = 1.8 nm and CO
and H2 at SnO2 thickness = 0.8 nm,17 the selective synthetic gas
production has not been achieved.
CO2(aq.) + H2O(l) 2 H2CO3(aq.)
(1)
a Environmental Research Laboratory, Kindai University, 3-4-1, Kowakae,
Higashi-Osaka, Osaka 577-8502, Japan
b Graduate School of Science and Engineering, Kindai University, 3-4-1, Kowakae,
Higashi-Osaka, Osaka 577-8502, Japan. E-mail: h-tada@apch.kindai.ac.jp;
Fax: +81-6-6727-4301; Tel: +81-6-6721-2332
Here, we report the stable and efficient electrochemical
production of synthetic gas from water and CO2 on an FTO
+
electrode in a near-neutral electrolyte solution containing NH4
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cc08040c
ions by using a three-electrode two-compartment cell with the
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electrolysis, new peaks appear at 2y = 30.651 and 32.021
assignable to the diffraction from the (200) and (101) planes
of the metallic Sn crystal (ICDD 00-004-0673).
As shown in the Sn3d-X-ray photoelectron (XP) spectra
(Fig. 2B), FTO possesses two signals observed at binding
energies (EB) at 487.6 eV and 496.1 eV due to the emissions
from the 3d5/2 and 3d3/2 orbitals of SnO2. The electrolysis in
0.1 M Na2SO4–0.1 M NaHCO3 without (NH4)2SO4 at E = À0.9 V
vs. RHE for 30 min induces new signals at EB,3d = 485.5 eV and
5/2
Fig. 1 (A) LSVs for FTO electrode: (a) in 0.1 M Na2SO4 aq. after 30 min CO2
bubbling (pH 5.5), (b) in 0.1 M Na2SO4–0.1 M NaHCO3 aq. (pH 8.0), (c) in
0.1 M Na2SO4–0.1 M NaHCO3 aq. containing 0.1 M (NH4)2SO4 after 30 min
CO2 bubbling (pH 7.7). (B) LSVs for FTO electrode in 0.1 M Na2SO4–0.1 M
NaHCO3 aq. containing various concentrations of (NH4)2SO4 after 30 min
CO2 bubbling.
EB,3d = 493.9 eV assignable to the corresponding emissions
3/2
from metallic Sn,18 which are intensified by the electrolysis
with 0.1 M (NH4)2SO4 aq. After the CO2-RR, the O1s-XP signal at
527.7 eV significantly weakens, while the F1s-XP signal around
685 eV disappears (Fig. S2, ESI†). These results are also con-
sistent with the partial reduction of the FTO surface to form
metallic Sn NPs on the surface. As a result, the FTO electrode
changed from colourless to brown (Fig. S3, ESI†). Scanning
electron microscopy (SEM) images show that particles with a
size of B200 nm are observed on the surface of FTO after
electrolysis at E = À0.9 V vs. RHE for 30 min (Fig. S4, ESI†).
A two-compartment three-electrode cell having the structure
of GC | 0.1 M (NH4)HCO3 aq. (pH 6.7) | Nafion | 0.1 M
(NH4)HCO3 aq. (pH 6.7) | FTO, Ag/AgCl was constructed, and
for comparison, the electrolyte solution was replaced by 0.1 M
NaHCO3 aq. The flow cell was driven at a CO2 flowing rate of
100 mL minÀ1. Only H2 and CO (or synthetic gas) were detected
as the gaseous products by gas chromatography, and quantified
under a CO2 flow with a constant rate of 100 mL minÀ1 with the
current traced simultaneously. In the 0.1 M NaHCO3 electrolyte
cell, the current density (J) gradually increases from
0.7 mA cmÀ2 at the electrolysis time (tel) = 0 to 1.3 mA cmÀ2
at tel = 1 h (Fig. 3A). Also, CO and H2 are produced with constant
rates of 28 mmol hÀ1 and 45 mmol hÀ1, respectively. The faradaic
efficiency (Z) was calculated by eqn (4) as a function of tel.
Ð
structure of glassy carbon (GC, anode) | 0.1 M (NH4)HCO3 aq.
(pH 6.7) | Nafion | 0.1 M (NH4)HCO3 aq. (pH 6.7) | FTO
(cathode), Ag/AgCl (reference).
Liner sweep voltammograms (LSVs) were measured for the
FTO electrode in the range of electrode potential (E) from 0 to
À0.9 V vs. reference hydrogen electrode (RHE) (Fig. 1A). In the
0.1 M Na2SO4 aq. with CO2 bubbling, only a small current flows
in the potential range. In 0.1 M Na2SO4–0.1 M NaHCO3 aq. after
30 min of CO2 gas bubbling, the cathodic current due to
reduction(s) involving the CO2-RR flows at E o ca. À0.7 V.
Strikingly, the addition of (NH4)2SO4 to the electrolyte solution
causes a drastic increase in the current.
Further, LSVs were recorded for the FTO electrode in 0.1 M
Na2SO4–0.1 M NaHCO3 aq. containing various concentrations
of (NH4)2SO4 ([(NH4)2SO4]) after 30 min of CO2 bubbling
(Fig. 1B and Fig. S1, ESI†). The cathodic current increases with
increasing [(NH4)2SO4] in the range below 100 mM. Evidently,
+
the NH4 ion acts as a promoter to remarkably enhance the
electrochemical reduction reaction(s).
Z = 2N(t)/( I(t)dt/F)
(4)
X-ray diffraction measurements were carried out for the FTO
electrode before and after electrolysis in 0.1 M Na2SO4–0.1 M
NaHCO3 aq. after 30 min of CO2 bubbling (Fig. 2A). FTO
possesses diffraction peaks at 2y = 33.891, 38.971, and 51.781
indexed as the diffraction from the crystal planes of SnO2(101),
(111), and (211), respectively (ICDD 01-075-2893). After the
where N(t) is the mole number of CO or H2 produced at tel = t,
and F is the Faraday constant.
Thus, the metallic Sn NPs formed on FTO (Sn/FTO) at the
initial stage of electrolysis catalyze the electrochemical
reduction reactions. The Z value of CO is almost constant
B30%, while the value of H2 decreases from 58% at tel = 0 to
44% at tel = 1 h (Fig. 3B). Interestingly, the replacement of the
electrolyte with 0.1
M (NH4)HCO3 aq. gives rise to a
drastic increase in the current. The J slowly increases from
1.7 mA cmÀ2 to reach a constant of 2.4 mA cmÀ2 at tel 430 min
(Fig. 3C). Also, CO and H2 are produced with constant rates of
51 mmol hÀ1 and 95 mmol hÀ1, respectively. The Z values of CO
and H2 generation are almost constant with B30% and 55%,
respectively (Fig. 3D). In each electrolyte cell, the total Faradaic
efficiency remains in the range of 80–90% probably because of
the existence of liquid-phase product(s) such as formic acid.15
However, 1H-NMR measurements failed to detect no product of
which the concentration may be too low. In this manner, a
drastic enhancement of synthetic gas from water and CO2 can
Fig. 2 (A) XRD patterns of FTO electrode before and after 30 min
electrolysis at E = À0.9 V vs. RHE in 0.1 M Na2SO4–0.1 M NaHCO3 aq.
(B) XP spectra of FTO electrodes before (black), after 30 min electrolysis in
0.1 M Na2SO4–0.1 M NaHCO3 aq. (blue), and after 30 min electrolysis in
0.1 M Na2SO4–0.1 M NaHCO3–0.1 M (NH4)2SO4 aq. (red).
+
be induced by the addition of NH4 ions to the electrolyte
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Fig. 4 (A) Adsorption isotherm of NH4+ ions on FTO particles at 25 1C. (B)
z-potential and particle size of ATO particles in aqueous solutions as a
function of [(NH4)2SO4] (pH 5.8 Æ 0.3).
particle size increases with increasing [(NH4)2SO4] due to the
aggregation of the particles. Clearly, NH4+ ions are adsorbed on
the surface of FTO particles by the Coulomb interaction.
Fig. 3 (A) Chronoamperometry curves and time evolution of gaseous
products in a three-electrode cell with the structure of GC | 0.1 M NaHCO3
aq. (pH 6.8) | Nafion | 0.1 M NaHCO3 aq. (pH 6.8) | FTO, Ag/AgCl under CO2
flow with a constant rate of 100 mL minÀ1. The potential of FTO electrode
was maintained at À0.9 V vs. RHE. Each pH is the value at the initial state.
(B) Faradaic efficiency of CO and H2 generation under the same condi-
tions. (C) Chronoamperometry curves and time evolution of gaseous
products in a three-electrode cell with the structure of GC | 0.1 M
(NH4)HCO3 aq. (pH 6.7) | Nafion | 0.1 M (NH4)HCO3 aq. (pH 6.7) | FTO,
Ag/AgCl under CO2 flow with a constant rate of 100 mL minÀ1. The
potential of FTO electrode was maintained at À0.9 V vs. RHE. (D) Faradaic
efficiency of CO and H2 generation under the same conditions.
+
Further, the adsorption state of NH4 ions on FTO from an
aqueous solution of (NH4)HCO3 was studied by diffuse reflec-
tance Fourier-transform infrared (DRIFT) spectroscopy (Fig. S7A,
ESI†). After the adsorption, a broad absorption appears around
+ 20
1440 cmÀ1 due to the deformation of NH4 . In addition, two
absorptions with the peaks at 1695 cmÀ1 and 1362 cmÀ1
assignable to the anti-symmetric and symmetric stretching
vibrations of HCO3À-ion co-adsorbed with a monodentate struc-
ture are observed.21 Importantly, the absorption peaks for the
+
HCO3À ions adsorbed on FTO intensifies in the presence of NH4
ions (Fig. S7B, ESI†). Clearly, the NH4+ ions on FTO promote the
solution, while steady generation of O2 at the GC anode is also
confirmed. The pHs of the anolyte and the catholyte after
tel = 4 h were 6.9 and 6.8, respectively. This is ascribable to
the buffering action of HCO3À ions since the pHs of the anolyte
and the catholyte lowered to 2.7 and 5.4, respectively, after
À
adsorption of HCO3 ions.
On the basis of these results above, a possible action
mechanism of the NH4+ ion-promoted electrochemical CO2-RR
is proposed (Scheme 1). At the GC cathode, water is oxidized
to yield O2 and H+ (eqn (5)). On the other hand, the NH4 ions
+
tel = 4 h without HCO3 ions. Further to check whether SO42À
À
are strongly adsorbed on the FTO surface through Coulomb
interaction. At the initial stage of electrolysis, the partial
reduction of FTO yields Sn NPs on the surface (eqn (6)).
Sn/FTO can act as the active sites for water reduction to
generate hydrogen atoms on the surface (eqn (7)) of which
coupling produces H2 (eqn (8)). In parallel with the process,
ions affect the CO2-RR properties or not, (NH4)2SO4 was
used instead of (NH4)HCO3 as the supporting electrolyte
(Fig. S6, ESI†). Remarkable effects by the NH4 ion addition
+
similar to those shown in Fig. 3 with (NH4)HCO3 are observed,
indicating that the influence of SO24À ions on the cell perfor-
mances is small. Recently, the electrochemical production of
synthetic gas on a boron-doped diamond electrode has been
reported with the faradaic efficiencies of CO B28% and H2
B42% in KClO4 aq.19
+
To gain insight into the action of NH4 ions on the electro-
chemical CO2-RR on FTO, FTO particles were synthesized
(see the ESI,† for details), and the adsorption properties for
+
NH4 ions were studied. The adsorption behaviour apparently
follows the Langmuir model (Fig. 4A). From the Langmuir plot,
the saturated adsorption amount (Gs) and the adsorption
equilibrium constant (Kad) were calculated to be 0.248 mmol gÀ1
and 1.23 Â 103 MÀ1, respectively. The large Kad value indicates
+
that there is strong interaction between the NH4 ion and
the FTO surface. Further, the z-potential of FTO particles
was measured as a function of [(NH4)2SO4] (Fig. 4B). In water
(pH 6.1), the FTO particles have a z-potential of À10.4 mV,
Scheme 1 Action mechanism of the ammonium ion on the electro-
which increases with an increase in [(NH4)2SO4]. Also, the chemical reduction of CO2 on the FTO electrode.
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À
HCO3 ions or CO2 (eqn (1) and (2)) can be concentrated near Seamless Technology Transfer Program through Target-driven
+
the FTO surface through the hydrogen bonds with NH4 ions. R&D, JSPS KAKENHI Grant-in-Aid for Scientific Research (C) no.
The subsequent electrochemical H+-coupled electron transfer and 18K05280 and 20K05674, the Futaba Foundation, and the
to CO2 yields CO and H2O (eqn (9)).
GC anode:
Nippon Sheet Glass Foundation for Materials Science and
Engineering.
2H2O - O2 + 4H+ + 4eÀ
(5)
FTO cathode:
Conflicts of interest
SnO2+ 4H+ + 4eÀ - Sn/FTO + 2H2O
Sn/FTO + H+ + eÀ - Sn-H/FTO
2Sn-H/FTO - H2 + 2Sn/FTO
(6)
(7)
(8)
There are no conflicts to declare.
Notes and references
CO2Á Á ÁNH4 + Sn-H/FTO + eÀ - CO + H2O + NH3 + Sn/FTO
+
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