Further study of CO2 electrochemical reduction on palladium modified bdd electrode: Influence of electrolyte

Article abstract of DOI:10.1002/asia.201901669

The study of CO₂ electrochemical reduction to useful compounds using bare or modified BDD electrode attracts numerous attentions. Meanwhile, the efficiency of products obtained from CO₂ electrochemical reduction is known to be determined by the electrode material and the electrolyte. Formic acid as main product and CO as a minor product, have also been known on the CO₂ reduction using BDD electrode. Recently, we reported the successful improvement of CO production from the reduction of CO₂ by decorating the surface of BDD electrode with palladium particles. Following this, herein, we present further inves-tigation on electrolyte dependence, including cation and anion dependence and also concentration effect in order to understand deeply the CO₂ reduction on surface of palladium modified BDD electrode. The results suggest the use of NaCl and KCl as a catholyte for optimum performance, in addition to the improvement of CO₂ reduction product in higher electrolyte concentration.

Full text of DOI:10.1002/asia.201901669

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Further study of CO2 electrochemical reduction on palladium
modified BDD electrode: influence of electrolyte
Authors: Prastik Krisma Jiwanti and Yasuaki Einaga
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Chemistry - An Asian Journal
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Further study of CO electrochemical reduction on palladium
2
modified BDD electrode: influence of electrolyte
[a]
[b]
Prastika Krisma Jiwanti and Yasuaki Einaga*
[
a]
Dr. Prastika Krisma Jiwanti
Faculty of Science and Technology, Airlangga University
Surabaya 60115, Indonesia
E-mail: prastika.krisma@fst.unair.ac.id
Prof. Yasuaki Einaga
[
b]
Department of Chemistry
Faculty of Science and Technology, Keio University
3-14-1 Hiyoshi, Yokohama 223-8522, Japan
Email: einaga@chem.keio.ac.jp
Abstract: The study of CO
compounds using bare or modified BDD electrode attracts numerous
attentions. Meanwhile, the efficiency of products obtained from CO
2
electrochemical reduction to useful
on PdBDD electrode was carried out, specifically on electrolyte
dependence study, that is believed to promote more active
catalytic reaction. As a significance, CO is an important feedstock
2
and intermediate for the production of methanol [ , acetic acid
and ethanol [17,18]
There have been many reports on enhancing the efficiency
of CO reduction by changing the electrolyte. However, to the best
15]
[16]
,
electrochemical reduction is known to be determined by the electrode
material and the electrolyte. Formic acid as main product and CO as
.
a minor product, have also been known on the CO
BDD electrode. Recently, we reported the successful improvement of
CO production from the reduction of CO by decorating the surface of
2
reduction using
2
2
of our knowledge, study of electrolyte dependence on PdBDD
electrode is the first time. Our group has been successfully
enhanced the production of HCOOH up to 95% efficiency using
BDD electrode with palladium particles. Following this, herein, we
present further investigation on electrolyte dependence, including
cation and anion dependence and also concentration effect in order
bare BDD by changing the electrolyte to RbCl aqueous solution,
and also by changing the selectivity using different anions [
19,20]
.
to understand deeply the CO
modified BDD electrode. The results suggest the use of NaCl and KCl
as catholyte for optimum performance, in addition to the
2
reduction on surface of palladium
Y. Huang et al [21] studied the effect of anion for CO
reduction to
2
a
ethylene and ethanol on Cu (111) and Cu (100) electrode, and
found that the production of ethylene was improved as the anion
improvement of CO
concentration.
2
reduction product in higher electrolyte


changed from ClO to I . Therefore, we believe that studying the
electrolyte dependence is in a high interest to deeply understand
2
the behavior of CO reduction on PdBDD electrode specifically,
also in intension to possibly improve the production of CO. In this
report, we report a detail study of electrolyte dependence,
including various cations and anions, different concentrations,
and potential dependence study.
Introduction
In the recent years, the electrochemical reduction of CO
useful chemicals necessities is noteworthy, by considering the
2
into
2
rising emissions of CO gas in the atmosphere, which leads to the
experimental evidence of adverse environmental impact [1]. Many
simultaneous approaches have been proposed, whether reducing
Results and Discussion
the emission in atmosphere through CO
CO
gas to useful and valuable chemicals [2-5]. Among many
methods to convert CO gas into valuable compound,
electrochemical reduction has widely been employed by many
researchers around the world due to its easy controllable
operation.
2
scrubbing or converting
Firstly, the study of cation dependence on PdBDD electrode was
evaluated by performing linear sweep voltammetry in 0.1 M MCl
solutions in the presence of CO gas. As we can see in Fig. 1, the
2
current density of hydrogen evolution was decreased as the
cation size increases. It is suggested that the hydrogen evolution
could be restrained in the solution containing larger cation size,
2
2
Meanwhile, a suitable electrode is noted to be an important
part for a maximum system performance. Many experiments have
and thus, promoting more electrons in the solution for CO
electrochemical reduction.
2
been carried out and reported, by modifying and finding suitable
The electrolyte concentration dependence was then
evaluated by performing the CO electrochemical reduction in
electrode for CO
2
electrochemical reduction system [ . Recently,
6- 8]
2
boron-doped diamond (BDD) electrode, which has been known to
NaCl aqueous solution as catholyte, with various electrolyte
concentrations, ranging from 0.1 M, 0.3 M, 0.5 M, and 1 M (Fig.
have low background current and wide potential window, is
[
studied for CO
2
electrochemical reduction, either on bare 9,10], or
p
2A). FE of the products and its partial current density (j ) were
modified surface [11-13]. In addition, it was suggested that the high
overpotential of BDD electrode could be decreased by decorating
its surface with metal particles. Our previous report used
palladium (Pd) particles modified on surface of BDD (PdBDD)
electrode, in which, it could improve the production of CO, in
comparison with bare BDD electrode at the same applied
increased as the electrolyte concentration increased. A CO partial
current density (jCO) value of 0.45 mA/cm2 was achieved in
catholyte concentration of 0.5 M NaCl. Whereas, a 46.9% FE of
CO was achieved at catholyte concentration of 1 M NaCl. As it
can be seen in Fig. 2, the efficiency of all products does not show
a significant different value, after reaching concentration of 0.5 M,
potential [14]. Therefore, a further study on CO
reduction to CO
2
1
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Chemistry - An Asian Journal
10.1002/asia.201901669
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explaining that it has attained a maximum value. The rise of the
FE as well as the current density at higher electrolyte
concentration is suggested, due to the higher ionic conductivity in
accordance to the more positive value of the potential at outer
Helmholtz plane, leading to the improvement of driving force for
the reaction [23]
.
Figure 2. FE and j
p
of CO
2
electrochemical reduction products in correlation
with various concentrations of (A) NaCl, (B) KCl, (C) RbCl, and (D) CsCl
aqueous solution as catholyte. The electrochemical reduction was performed at
potential -1.6 V for 1 h. Linear sweep voltammograms of PdBDD electrode in
Figure 1. Linear sweep voltammograms of PdBDD electrode in 0.1 M MCl
solution with scan rate of 100 mV/s.
0.1 M MCl solution with scan rate of 100 mV/s.
Furthermore, concentration dependence was studied in
+
+
+
electrolyte with other cations (K , Rb , and Cs ) (Fig.2B-2D). The
FE of CO in NaCl, KCl, and CsCl shows similar trend, in which,
the increasing of electrolyte concentration could improve the
production of CO. Therefore, it is assumed that there is no direct
correlation between cation type and electrolyte concentration.
However, overall results show that by increasing the
To study in detailed about cation effect, Fig. 3 shows a plot
+
+
+
+
between different cation (Na , K , Rb , Cs ) in correlation with FE
and j of products. In a concentration of 0.1 M, production of CO,
p
explained by the FE and the jCO, is increased as the cation size is
bigger. Study by Ringe S. et al [25] has proved that the larger the
cation, the smaller hydrated cation is. Thus, resulted in a high
concentration of cation near surface of electrode, leading to larger
surface charge density and stronger interfacial electric field, in
concentration of the electrolyte, the CO
efficiency could be improved and
2
reduction products
evolution could be
H
2
suppressed. A special phenomenon was observed while applying
RbCl as an electrolyte, in which, formic acid dominates the
products rather than the CO, in spite of the similar total current
density achieved (data not shown). As only small part of BDD
surface covered by Pd particles, it is obvious that the behavior of
BDD electrode contributes to overall performance, in which, our
2
which, the adsorption of CO on the surface of electrode could be
improved. Besides, H evolution relatively could be suppressed
2
as cation size is bigger, in agreement to the result reported by
Thorson M. R. et al [26]. Nevertheless, as describes above, an
exception using PdBDD electrode was found only in RbCl solution,
in which, formation of formic acid was dominant, therefore
selectively suppressing the formation of CO. This is in contrast
with the result on Cu (111) and Cu (100) electrode reported by
Resasco J. et al [27], in which, rate of CO production is less
affected by cation size.
previous report of CO
2
reduction on bare BDD electrode reported
that the maximum production of formic acid achieved while using
Rb cation as a catholyte [19,24]. It is said that Rb hydrated with
water easily and cover the surface of electrode, enable the
decomposition of water or hydrogen evolution production.
+
+
Following the different behavior in RbCl solution, it was
considered that to produce a maximum efficiency of CO, the
applied potential could be shifted to more positive potential.
Thereafter, we performed a potential dependence varied from -
1
.4 V to -1.8 V (Fig. 4). The result shows that at more negative
potential, faradaic efficiency of formic acid was increased in
contrast with the suppression of H evolution. Meanwhile, CO
2
production was maintained in similar efficiency. In point of fact,
faradaic efficiency of CO was decreased at potential -1.4 V, and
H
2
evolution dominated. Therefore, it is concluded that on bare or
+
Pd modified BDD electrode, cation Rb is specifically a formic acid
catalyst, and is not a suitable support for CO production.
2
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Figure 3. FE of products in electrolyte with different cation, in various concentrations of 0.1 M (A), 0.3 M (B), 0.5 M (C), and 1.0 M (D). j
p
of products in electrolyte
with different cation, in various concentration of 0.1 M (a), 0.3 M (b), 0.5 M (c), and 1.0 M (d). All measurements were performed at potential -1.6 V for 1 h.


In Fig. 6, shows that Cl and ClO
4
anions could improve the
production of CO, and CO 2

shows a highest hydrogen evolution.
It is known that several anions, such as SO
Cl ) could specifically adsorbed on electrode and restrained CO
Moreover, to study the effect of anion, CO
reduction in electrolytes of NaCl, Na SO , Na CO
were performed. The study using CV from potential of -0.5 V to
.2 V was performed after N and CO gas purging, in which, the
peak shift could be observed (Fig. 5). As an example, in 0.1 M
NaCl, a cathodic peak of Pd (known as a deposition step of Pd)
2
electrochemical
3
2

and halides (i.e.
2
4
2
3
, and NaClO
4
4

2
1
2
2
strongly on surface of electrode, promoting products, and
2
suppressing H .
evolution [
30]
2
at around -0.1 V was shifted to 0.2 V after CO gas bubbled into
the solution. It could be explained that as the pH of the solution
decreased, the potential window is narrower, and therefore the
peak shift to more positive potential could be observed [28,29], in
agreement with our previous work [14]. Similar peaks could also be
observed in NaCl, Na
2 4 4
SO , and NaClO catholyte. On the other
hand, Na CO shows a different behavior. The peak shift was
2
3
relatively in more negative potential, in which, it could be
explained by the buffering effect described later.
Figure 5. CV of PdBDD electrode in 0.1 M NaCl (A), Na
and Na CO (D) solution after N (dash line) and CO (solid line) gas purging
with scan rate of 100 mV/s from potential -0.5 V to 1.2 V vs. Ag/AgCl.
2 4 4
SO (B), NaClO (C),
2
3
2
2
Figure 4. Various potentials vs. faradaic efficiency of products on the CO
2
electrochemical reduction in 0.5 M RbCl.
3
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The result present here is also linear to the recent report,
and current density of the CO
concentration of the electrolyte will favor the hydrogen evolution.
2
reduction products, whereas lower
mentioning that the presence of Cl favoring CO production [31]

.
2 2 3
Meanwhile, a high H evolution in an electrolyte of Na CO is
suggested due to buffering effect, as described by the following
reactions (Eq (1-2)):
Conflict of Interest
CO2(aq) + H
2
O
(l) ⇆ H+(aq) + HCO 
(aq) ⇆ 2H+(aq) + CO
3
(aq)
pK
1
2
=
6.35 (1)
The authors declare no conflict of interest.
Experimental Section
H+(aq) + HCO
2
pK
= 10.33 (2)
3
3
(aq)
After CO
2
purging into the Na
2
CO
3
solution, the pH was around

neutral, and the concentration of HCO
amount of HCO
3
is higher. The high

3
could drive the increasing of hydrogen evolution,
as following reaction (Eq. (3-4)):
Chemicals
All chemicals were purchased from Wako Pure Chemical Industries except
H
2
O + e  *H + OH
(3)
(4)
2
PdCl (99%) was purchased from Sigma Aldrich. All reagents were used
without any further purification. Ultra-pure water was obtained from a
Simply-Lab water system (Direct-Q UV3, Millipore) to prepare aqueous
solutions.

+ e  *H + CO
2
HCO
3
3
In addition, the partial current density of each products is following
the trend of products faradaic efficiency as shown in Fig. 6.
Among all electrolytes, Na CO shows highest total current
2 3
density, which can be interpreted to have a higher electron
transfer, and thus promoting hydrogen evolution.
Preparation of BDD and PdBDD electrode
Boron-doped diamond (BDD) electrode was prepared by depositing it on
the surface of Si (111) wafers for 6 hours using micro-wave plasma-
assisted chemical vapor deposition system (Model AX-5400, CORNES
Technology Corp.). A mixture of trimethoxyborane and acetone was used
as carbon and boron sources to achieve a B/C ratio of 1%. A detailed
procedure was explained in previous report [22]. To prepare palladium
2
modified on surface of BDD (PdBDD) electrode, 10 mL of 1 mM PdCl /0.1
M HCl was prepared and electrodeposition with chronoamperometric
technique was performed during 300 s at potential of -0.15 V. All modified
electrodes were treated as described in previous report [14], and
characterization using field emission scanning electron microscopy-energy
dispersive X-ray (SEM-EDX INSPECT F50, FEI) was performed to
convinced the well-prepared modified electrode for analysis (Fig. S1). The
Pd particles were dispersed homogeneously all over the surface of BDD
electrode with the percent weight of 4.73 % and average particle size of
around 50 nm. In addition, the detailed study on PdBDD characterization
was explained in our previous report [14]
.
CO
Two cells separated by Nafion membrane was used to conduct the
4
electrochemical reduction of CO . The anolyte side was 0.1 M Na SO .
2
electrochemical reduction and products analysis
2
2
Various MCl (M= Na, K, Rb, and Cs) electrolytes with different
concentrations of (0.1, 0.3, 0.5, and 1.0 M) were used as catholyte. Pt
mesh and PtBDD were used as counter and working electrode respectively.
2
Figure 6. Anion effect on the CO electrochemical reduction products
efficiency performed at potential -1.6 V during 1 h.
All potentials were measured against Ag/AgCl (3 M NaCl). N
purging for 15 min were performed prior to each reduction process. CO
electrochemical reduction was carried out for 1 h under mild CO gas flow
rate (<50 sccm) to maintain the saturation of CO gas in the electrolyte. N
2 2
and CO gas
2
2
Conclusion
2
2
gas was then purged into the catholyte side at about 50 sccm to collect the
gas product in the solution. All the electrochemical measurements were
recorded using potentiostat (Autolab PGSTAT204, Metrohm Autolab B.V.).
In this work, the effect of electrolyte, including cation and anion
effects on the CO
PdBDD electrode were studied. Finally, to improve the production
of CO specifically on CO reduction using PdBDD or BDD based
2
electrochemical reduction on the surface of
2
+
+
material as an electrode, the used of cation Na or K is suggested,
in addition to the used of Cl anion. Moreover, higher
concentration of the electrolyte could also improve the efficiency
4
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Chemistry - An Asian Journal
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[
[
[
[
2]
3]
4]
5]
J. Li, L. Wang, Y. Cao, C. Zhang, P. He, H. Li, Chinese J. Chem. Eng.
2018, 26, 2266–2279.
J. Wei, Q. Ge, R. Yao, Z. Wen, C. Fang, L. Guo, H. Xu, J. Sun, Nat.
Commun. 2017, 8, 1–8.
D. Ren, Y. Deng, A. D. Handoko, C. S. Chen, S. Malkhandi, B. S. Yeo,
ACS Catal. 2015, 5, 2814–2821.
M. Alvarez-guerra, A. Del Castillo, A. Irabien, Chem. Eng. Res. Des.
2013, 92, 692–701.
[
[
6]
7]
S. Lee, G. Park, J. Lee, ACS Catal. 2017, 7, 8594–8604.
W. Tang, A. A. Peterson, A. S. Varela, Z. P. Jovanov, L. Bech, W. J.
Durand, S. Dahl, J. K. Nørskov, I. Chorkendorff, Phys. Chem. Chem.
Phys. 2012, 14, 76–81.
[
[
[
8]
9]
M. Azuma, J. Electrochem. Soc. 1990, 137, 1772.
P. K. Jiwanti, K. Natsui, Y. Einaga, Electrochemistry 2019, 2, 109–113.
10] K. Natsui, H. Iwakawa, N. Ikemiya, K. Nakata, Y. Einaga, Angew.
Chemie - Int. Ed. 2018, 57, 2639–2643.
[
11] N. Roy, N. Suzuki, Y. Nakabayashi, Y. Hirano, K. Katsumata, K.
Nakata, A. Fujishima, C. Terashima, H. Ikari, ChemElectroChem 2018,
5, 2542–2550.
[
[
[
12] S. A. Yao, R. E. Ruther, L. Zhang, R. A. Franking, R. J. Hamers, J. F.
Berry, J. Am. Chem. Soc. 2012, 134, 15632–15635.
13] P. K. Jiwanti, K. Natsui, K. Nakata, Y. Einaga, Electrochim. Acta 2018,
2
66, 414–419.
14] P. K. Jiwanti, Y. Einaga, Phys. Chem. Chem. Phys. 2019, 21, 15297–
5301.
1
[
[
[
[
15] W. K. Lewis, P. K. Frolich, Ind. Eng. Chem. 1928, 20, 285–290.
16] A. D. Singh, N. W. Krase, Ind. Eng. Chem. 1935, 27, 909–914.
17] M. R. Wilkins, H. K. Atiyeh, Curr. Opin. Biotechnol. 2011, 22, 326–330.
18] M. Köpke, C. Mihalcea, J. C. Bromley, S. D. Simpson, Curr. Opin.
Biotechnol. 2011, 22, 320–325.
Figure S1. SEM-EDX of PdBDD electrode.
One liter of collected gas was then analyzed using gas chromatography
with a flame ionization detector and a thermal conductivity detector (GC-
[
[
19] M. Tomisaki, K. Natsui, N. Ikemiya, K. Nakata, Y. Einaga,
ChemistrySelect 2018, 3, 10209–10213.
2014, Shimadzu Corp.). Whereas the liquid sample from catholyte side
20] M. Tomisaki, S. Kasahara, K. Natsui, N. Ikemiya, Y. Einaga, J. Am.
Chem. Soc. 2019, 141, 7414–7420.
was analyzed using high performance liquid chromatography (HPLC) with
an electroconductivity detector (Prominence, Shimadzu Corp.). The
analysis detail was explained in our previous report [19]. Faradaic
[21]
Y. Huang, C. W. Ong, B. S. Yeo, ChemSusChem 2018, 11, 3299–
306.
22] T. Watanabe, Y. Honda, K. Kanda, Y. Einaga, Phys. Status Solidi Appl.
3
efficiencies (FE) were calculated by this equation (Eq. (1)):
[
[
[
Mater. Sci. 2014, 211, 2709–2717.
23] J. Wu, F. G. Risalvato, F.-S. Ke, P. J. Pellechia, X.-D. Zhou, J.
Electrochem. Soc. 2012, 159, F353–F359.
FE = (n F M)/(I t) x 100%
(1)
24] N. Ikemiya, K. Natsui, K. Nakata, Y. Einaga, RSC Adv. 2017, 7, 22510–
2
2514.
where n is electron involved in reduction, F is faraday constant (96.485 C
mol−1), M is concentration of sample, I is current and t is reduction time.
Meanwhile, current density of each product was measured by multiplying
[25] S. Ringe, E. L. Clark, J. Resasco, A. Walton, B. Seger, A. T. Bell, K.
Chan, Energy Environ. Sci. 2019, 17–25.
the FE and the total current density for CO
2
reduction process.
[26] M. R. Thorson, K. I. Siil, P. J. A. Kenis, J. Electrochem. Soc. 2013, 160,
DOI 10.1149/2.052301jes.
[
27] J. Resasco, Y. Lum, E. Clark, J. Z. Zeledon, A. T. Bell,
ChemElectroChem 2018, 5, 1064–1072.
[
28] Z. Zhang, T. Cserfalvi, M. Gratzl, J. Electroanal. Chem. 2019, 845, 1–5.
Keywords: CO
2
reduction • palladium • boron-doped diamond •
[29] T. R. Soreta, J. Strutwolf, C. K. O’Sullivan, Langmuir 2007, 23, 10823–
electrolyte
10830.
[
[
30] K. Ogura, J. R. Ferrell, A. V. Cugini, E. S. Smotkin, M. D. Salazar-
Villalpando, Electrochim. Acta 2010, 56, 381–386.
31] Y. C. Hsieh, S. D. Senanayake, Y. Zhang, W. Xu, D. E. Polyansky,
ACS Catal. 2015, 5, 5349–5356.
[
1]
J. Hansen, P. Kharecha, M. Sato, V. Masson-delmotte, F. Ackerman,
D. J. Beerling, P. J. Hearty, O. Hoegh-guldberg, S. Hsu, C. Parmesan,
et al., Plos One, 2013, 8, e81648.
5
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Entry for the Table of Contents
Pd
CO2
Pd
B
D
D
Na
K
Rb
Cs
+
Pd
Pd
+
+
+
Pd
Pd
Pd
CO
HCOOH
2
The CO electrochemical reduction to CO and HCOOH on PdBDD electrode was studied, showing the dependence of different
catholyte and anolyte. The result suggests the use of electrolyte NaCl or KCl with higher electrolyte concentration to improve the
production of CO on PdBDD electrode.
6
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