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Electrochimica Acta 96 (2013) 18–22
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Monolayers of choline chloride can enhance desired electrochemical reactions
and inhibit undesirable ones
Wei Zhu a , Brian A. Rosen a,b , Amin Salehi-Khojin a,c , Richard I. Masel a,∗
a
Dioxide Materials Inc., 60 Hazelwood Drive, Champaign, IL 61820, United States
Department of Chemical and Biomolecular Engineering, University of Illinois-Urbana Champaign, 600 South Mathews Avenue, Urbana, IL 61801, United States
c
Department of Mechanical and Industrial Engineering, University of Illinois-Chicago, 842 West Taylor Street, Chicago, IL 60607, United States
b
a r t i c l e
i n f o
Article history:
Received 11 December 2012
Received in revised form 8 February 2013
Accepted 12 February 2013
Available online 19 February 2013
Keywords:
Choline chloride
Hydrogen evolution reaction
Formic acid electro-oxidation
Carbon dioxide reduction
a b s t r a c t
The effects of choline chloride on the hydrogen evolution reaction, formic acid electrooxidation, and carbon dioxide conversion are investigated on a platinum cathode. We find that choline chloride suppresses
hydrogen formation, but by contrast formic acid electrooxidation is enhanced and the overpotential of
carbon dioxide reduction is reduced dramatically. We also find that choline chloride can be used to
extend the window where electrochemical experiments can be performed. These results demonstrate
that monolayers of species such as choline can have significant effects that need to be explored in more
detail.
© 2013 Elsevier Ltd. All rights reserved.
1. Introduction
This paper is a continuation of our efforts to determine whether
monolayers of organic molecules could be used as “helper catalysts” to speed desired electrocatalytic reactions and inhibit
undesirable reactions. The idea of a “helper catalyst” is described
in Rosen et al. [1], Masel [2] and Oh et al. [3]. When a monolayer
of an organic compound adsorbs on a metal surface, the presence
of the organic compound can change the binding energy of key
intermediates [1,2]. That can lead to changes in rate. In a recent
paper [4], we reported in situ sum frequency generation (SFG)
spectroscopy that indicates that in the presence of EMIM–BF4 , CO2
conversion goes via an EMIM–CO2 complex rather than through
CO2 − . The EMIM–CO2 complex forms near 0 V with respect to SHE,
so it provides a low energy pathway for CO2 conversion. These
results suggest that the adsorption of a cationic species such as a
quaternary amine tends to stabilize anionic intermediates. If the
amine binds too strongly, it will simply poison the surface, but if
the binding strength is modest, or the amine can accept electrons,
then rate enhancement is possible.
At this point there is very little experimental evidence outside of
our laboratory showing that adsorbed molecules can act as helper
catalysts although we have started to publish results [1,2]. There
are also a few papers in the literature that show that the idea of an
∗ Corresponding author. Tel.: +1 2172391400; fax: +1 2173334050.
E-mail address: [email protected] (R.I. Masel).
0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2013.02.061
adsorbed helper catalysts extends to photocatalysis [3]. We previously reported an electrocatalytic system that reduces CO2 to
carbon monoxide (CO) at overpotentials below 0.2 V. The system
relies on an ionic liquid electrolyte to lower the energy of the CO2
reduction intermediate, and thereby lower the initial reduction barrier. In addition, previous workers had found tetrabutylammonium
hydrogen sulfate and dibutyl ammonium hydrogen sulfate [5,6],
can suppress the hydrogen evolution in batteries, so there is the
possibility of significant drop in hydrogen production.
The objective of this work was to determine whether a simple quaternary amine, choline chloride, [(CH3 )3 NCH2 CH2 OH]+ –Cl−
could be used as a helper catalyst. Choline chloride is a common
food additive for livestock. It is also sold as a dietary supplement
for humans. It is inexpensive since it is a waste product of soybean
oil production. Thus, it is an attractive “helper catalyst” candidate.
Importantly, quaternary amines do not bind strongly on platinum
[7] and dissociate completely in water under all of the conditions
reported here. Therefore there was the possibility that choline chloride could be an effective helper catalyst.
In this work we examine the effect of choline chloride on
three different reactions: the hydrogen evolution reaction (HER),
formic acid electrooxidation and carbon dioxide reduction. The
HER can occur via cationic intermediates, so if the arguments
in Masel [2] are correct, HER should be inhibited. Formic acid
electrooxidation can occur by two pathways: a direct pathway
which has been theorized to occur via a formate intermediate
or related species [8–13], and an indirect pathway leading to an
adsorbed CO. The direct pathway should be enhanced by choline
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W. Zhu et al. / Electrochimica Acta 96 (2013) 18–22
chloride. In addition, we would like to see whether overpotential
of carbon dioxide reduction will be reduced.
Fortunately, all the predictions above are desirable results.
The HER is undesirable during CO2 conversion in aqueous media,
because HER competes with the main reaction, CO2 conversion
[14–17]. It is also a side reaction in formic acid fuel cells. Therefore inhibition of the HER would be desirable. Moreover, formic
acid electrooxidation is the main reaction in formic acid fuel cells
[18–24]. Enhancements could improve the stability of the fuel cell
and lower the needed catalyst loading. Finally, the energy barrier of
carbon dioxide conversion can be reduced to improve the efficiency
of carbon dioxide conversion.
The objective of the work here was to determine whether
choline chloride changes the rate of the hydrogen evolution reaction, formic acid electrooxidation and carbon dioxide conversion. In
detail, we examined the effect of 0.5 M choline chloride (pH 8.3) on
the rate of all reactions on platinum using mainly cyclic voltammetry (CV). We also compared three standard solutions: 0.5 M sodium
bicarbonate (pH 8.5), 0.5 M sulfuric acid (pH 1.2) and a borax buffer
solution (pH 8.3). Sulfuric acid was an internal standard. Sodium
bicarbonate and the borax buffer have a similar pH, ion-strengths
and conductivities to our choline chloride solutions, so they were
good comparison cases. In addition, we investigated the HER effect
in choline chloride with different concentration of sulfuric acid.
For formic acid electrooxidation, we utilized CV and chronoamperometry to analyze 5 M formic acid with small amount of choline
chloride. Finally, we took bicarbonate solution as carbon dioxide
source and obtained the overpotential of carbon dioxide by CV. Our
results show a significant suppression of the HER, enhancement
of formic acid electrooxidation and reduction of overpotential of
carbon dioxide conversion in the presence of the choline chloride.
2. Experimental
2.1. Materials
The catalyst platinum black ink is prepared by mixing 5.6 mg of
metal black (Alfa Aesar 99.9% metal basis) with 1 mL deoxygenated
Millipore water. The counter electrode is made by attaching a
25 mm × 25 mm platinum mesh (size 52) to a 5 in. platinum wire
(99.9%, 0.004 in. diameter). The reference electrode (BASi MF-2052)
is a silver–silver chloride electrode with Flexible Connector. Four
kinds of electrolyte are used for comparing HER: 0.5 M choline
chloride, 0.5 M sodium bicarbonate, 0.5 M sulfuric acid and buffer
solution. Later, HER is also investigated by gradually adding sulfuric acid in choline chloride electrolyte. Formic acid electrooxidation
is conducted with 5 M formic acid solution with different amount
of choline chloride. Sodium bicarbonate is used as carbon dioxide
source for electro-reduction. The solutions are prepared with triple
distilled water. Measurements are taken at 25 ◦ C under argon gas
(99.999% purity) bubbling at 1 atm.
19
and solution purity [25,26]. Following this procedure, the platinum
was adherent and did not fall of upon rotation.
2.3. Cyclic voltammetry
The electrolytes are first loaded into the glass cell and then
purged with dry argon (99.99%) for 2 h in order to remove oxygen
from the electrolytes. Prior to each experiment, a 20–40 linear
sweep cyclic voltammogram at 75 mV/s is taken with the range
between −1.5 V and +1 V vs. Ag/AgCl in order to condition the
electrodes and remove oxides from the surfaces. The potential
window is different for different electrolytes. Then several cycles
are performed at 10 mV/s before taking the final cycle to insure
that the CV had stabilized (i.e., any “dirt” or other material is
removed from the surfaces). Finally, cleaning and stabilizing CV
cycles are performed at 10 mV/s.
2.4. Chronoamperometry
Chronoamperometry was performed in 5 M formic acid solution
with different concentration of choline chloride. In all cases, the
measurement was performed by stepping from open cell potential
to +0.1 V vs. Ag/AgCl.
3. Results
3.1. Hydrogen evolution reaction suppression
Our first experiments were to determine whether choline chloride would inhibit the HER. The first experiment was to do cyclic
voltammetry in each of the solutions and see how the hydrogen
evolution reaction changed.
Fig. 1 presents the cyclic voltammogram of the hydrogen evolution reaction (HER) on platinum catalyst in 0.5 M solutions
containing sulfuric acid, bicarbonate, borax buffer and choline chloride. In each case we plot the potential versus the measured value
of RHE to avoid the issues with the drift in the Ag/AgCl reference
electrode due to the high chlorine concentration in the choline chloride. The sulfuric acid data looks similar to those from the previous
literature with hydrogen adsorption peaks at 0.11 V and 0.27 V,
and hydrogen desorption peaks at 0.14 V, 0.21 V and 0.28 V. The
hydrogen evolution starts at around 0 V. In sodium bicarbonate
electrolyte, the peaks related to hydrogen reactions are at almost
the same potentials as in sulfuric acid. There are hydrogen adsorption peaks at 0.16 V and 0.30 V, and hydrogen desorption peak at
0.20 V and 0.30 V. The hydrogen evolution reaction begins at 0 V as
2.2. Instruments
The measurements were made with a Solartron SI 1287 potentiostat in a standard three-electrode electrochemical cell. The
working electrode is prepared by applying the metal black ink onto
a gold surface of a rotating electrode. Most of the measurements
were made without rotation, although the data in Fig. 4 was
taken at a rotation rate of 1000 rpm. The catalyst is applied on the
surface of the rotating electrode by adding 12.5 ␮L of the ink to
the surface and allowing the water to evaporate under ambient
temperature for 60 min. In order to ensure the quality of the
measurements, special attention is paid to the material cleaning
Fig. 1. Cyclic voltammetry of platinum in 0.5 M choline chloride (pH 8.3), 0.5 M
sodium bicarbonate (pH 8.5), 0.5 M sulfuric acid (pH 1.2) and a borax buffer solution
(pH 8.3). The data were taken at a scan rate of 10 mV/s and at room temperature.
The data is plotted against RHE.
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W. Zhu et al. / Electrochimica Acta 96 (2013) 18–22
well. The same situation happened in buffer solution, which shows
the hydrogen adsorption peaks at 0.17 V and 0.27 V, and hydrogen
desorption peak at 0.14 V and 0.31 V. In this case, the hydrogen evolution reaction starts at 0 V, but proceeds to bulk reaction slower
than in sulfuric acid and sodium bicarbonate due to slow removal
of hydrogen bubbles from the electrode surface.
Everything changes in the choline chloride electrolyte. We do
not observe the characteristic hydrogen adsorption and desorption
peaks. There is a peak at 0.33 V (RHE) that we attribute to the interaction between choline ion and catalyst surface and a hydrogen
reduction peak at about −0.4 V vs. RHE. Clearly, hydrogen production has been suppressed.
In the second experiment, we started with a platinum catalyst in
a 0.5 M choline chloride solution in water, and slowly added sulfuric
acid and measured the behavior of the system with CV. In this case,
we used Ag/AgCl in 3.5 M KCl as the reference electrode. Notice
that when the concentration of sulfuric acid is less than or equal
to 0.01 M (pH = 1.8), we observe almost no for hydrogen formation
near RHE. After the concentration of sulfuric acid is increased to
0.02 M (pH = 1.8), the HER is not stable between −0.2 V to −0.8 V.
When concentration is increased again to 0.1 M, hydrogen evolution reaction starts immediately around −0.25 V vs. Ag/AgCl (Fig. 2).
Therefore, with sulfuric acid in choline chloride solution, HER is
also suppressed even the pH demonstrates strong acid environment. Later, the increased concentration of sulfate will result in the
beginning of HER. Overall, choline chloride demonstrates strong
HER suppression even in the presence of 0.02 M of sulfuric acid.
Fig. 3. Chronoamperometric measurements of formic acid electrooxidation on a
choline coated platinum catalyst at 0.1 V relative to a Ag/Ag+ reference electrode.
Notice that additions of choline chloride enhance the rate without changing the pH
of the solution.
chloride, the current density for formic acid electrooxidation is
largest when choline chloride concentration is 0.1 M. Later, the current density decreases when more choline chloride is added in the
electrolyte, but still stays relatively higher than pure formic acid
after 2-h of operation. These results demonstrate that formic acid
oxidation can be enhanced by small amount of choline chloride.
3.3. Carbon dioxide reduction in choline chloride
3.2. Tests to examine the effect of choline chloride on the formic
acid electro-oxidation
The results in the previous section indicate that hydrogen formation is strongly suppressed in the presence of choline chloride.
The next question we wanted to address is whether we have completely poisoned the catalyst, or whether we have instead had
a positive effect of formic acid electrooxidation. In addition, we
would like to know whether formic acid electrooxidation would
be enhanced in the presence of choline chloride. Fig. 3 shows
chronoamperometric scans for Pt held at 0.1 V vs. Ag/AgCl in 5 M
formic acid with different concentration of choline chloride. We
chose 0.1 V because this potential is similar to that used in formic
acid fuel cells [12,18–24].
For all chronoamperometric curves, the current density starts
out high on the Pt surface but the current rapidly drops as the
surface charges. Then the curves diverge. In the presence of choline
Fig. 2. The effect of sulfuric acid concentration on hydrogen evolution from a choline
chloride coated platinum catalyst as measured by cyclic voltametry. The data were
taken at a scan rate of 10 mV/s and at room temperature. The potentials are referenced to an Ag/Ag+ electrode. Notice that hydrogen evolution reaction is suppressed
at sulfuric acid concentrations up to 0.02 M (pH 1.8).
Fig. 4 illustrates the effect of choline chloride on the conversion of bicarbonate. In this case, we regard bicarbonate as a carbon
dioxide source. Notice that at bicarbonate concentrations between
0.03 M and 0.2 M there is a peak at about −0.83 V with respect to
Ag/AgCl. That peak is associated with bicarbonate conversion to
yield CO and/or H2. There is also an oxidation peak near 0 V with
respect to Ag/AgCl.
Fig. 5 shows the result of an experiment where we held the
potential at −0.83 V with respect to a Ag/AgCl reference electrode,
and then did a CV at 5 mV/s. Notice the CO peak at 0.68 V (Ag/AgCl).
This result shows that bicarbonate can be converted to CO, and
perhaps other products at −0.83 V with respect to Ag/AgCl.
To put these results in perspective, Hori [27] reports that at pH
7, the equilibrium potential for the reaction.
CO2 + H2 O + 2e− CO + 2OH−
(1)
Fig. 4. The effect of sodium bicarbonate concentration on the CV of bicarbonate on
a choline coated platinum catalyst. Notice that the conversion of bicarbonate starts
at about −0.8 V with respect to an Ag/Ag+ electrode.
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W. Zhu et al. / Electrochimica Acta 96 (2013) 18–22
Fig. 5. CO stripping with 0.5 M choline chloride and 0.3 M sodium bicarbonate. These
measurements were taken by holding the potential of the platinum electrode at
−0.83 V with respect to Ag/Ag+ for 20 min, and then scanning at 5 mV/s. Notice the CO
reduction peak at 0.68 V (Ag/AgCl). CO2 is being converted to CO under the conditions
of our experiment.
is −0.52 V with respect to SHE at pH 7. This is equivalent to
−0.81 V with respect to Ag/AgCl at a pH of 8.3. We observe CO2
conversion starting at about −0.83 V with respect to Ag/AgCl. Thus
the results in Fig. 4 show that CO2 conversion is occurring with a
low overpotential in the choline chloride solution.
4. Discussion
This work clearly demonstrates that choline chloride can act as
a helper catalyst. Previous workers had shown that a different quaternary amine could suppress the HER in batteries [5,6] and poison
ethanol decomposition [7], but what is special about our work is
that we have observed suppression of an undesirable reaction and
enhancement of desirable ones.
From the data here we do not know whether it is the choline
or the chloride that is enhancing the reaction, but in related
21
work we found that other choline compounds (choline–BF4 and
choline–acetate) give similar effects. Therefore we assert that the
choline ion is enhancing the reaction.
Fig. 6 shows a schematic of our proposed mechanism of the
process. Fig. 6(a) shows the effect of choline ions on the electrooxidation of formic acid, while Fig. 6(b) shows the suppression of
the hydrogen evolution reaction by choline. Without a suppression
agent such as choline ion, when the electrode potential is more
negative than the potential of zero charge (PZC), the surface will
be negatively charged and the protons interact with the surface
easily. However, when choline cations are added in the solution,
a thin layer of choline ion forms on the catalyst surface, as seen
in SERS. The thin layer of choline causes the surface to be positive
charged, which will decrease the concentration of protons on or
near the surface. The adsorbed layer also acts to sterically hinder
the adsorption of protons. This decreases the rate of reaction until
the potential is negative enough that protons can finally reach the
catalyst surface, and go through the reduction process to release
hydrogen. As a result there is an increased overpotential for proton
reduction.
By adding more sulfuric acid in the solution, the increased concentration of sulfate will replace the choline ions adsorbed on the
catalyst. There is a transition state where sulfate is competing with
choline ions to be adsorbed on the surface.
Formic acid electrooxidation shows the opposite effect. This is
harder to explain. Formic acid goes by two pathways on platinum:
a direct and indirect pathway leading to buildup of CO on the surface [8,9,11,12,22,28]. The CO poisons the reaction. We observe two
effects in our data, a decrease in CO poisoning and an increase in
the absolute rate of formic acid conversion even before there is
significant CO poisoning.
It is hard to explain how both processes could occur. A simple
steric hindrance would decrease CO buildup, but could not explain
the increase in rate.
We can explain the data if we assume that the mechanism of
formic acid electrooxidation follows a direct pathway such as
HCOOH → HCOO
HCOO
(ad)
(ad)
+ H+
+
→ CO2 + H + 2e
(2)
(3)
And an indirect pathway such as
HCOOH + H+ → HCO+ (ad) + H2 O
+
HCO
(ad)
→ CO(ad) + H
+
(4)
(5)
+
CO(ad) + H2 O → CO2 + 2H + 2e
(6)
The indirect pathway would be suppressed since the amine
lowers the concentration of H+ near the surface, while the direct
pathway would be enhanced due to stabilization of the formate
either by complexation with the positive charges in the choline,
[(CH3 )3 NCH2 CH2 OH]+ or hydrogen bonding to the hydroxyl proton in the choline. Admittedly, while we have observed the formate
and formyl species in ultra high vacuum (UHV) [13,29–31] in the
presence of adsorbed water, we have not observed these species
during formic acid electrooxidation on our most active electrocatalysts. Thus, this mechanism is speculative. Still, it is a reasonable
hypothesis given our observations.
5. Conclusions
Fig. 6. A possible mechanism of (a) formic acid reduction, and (b) hydrogen evolution reaction at some negative potential level on catalyst surface with choline
chloride.
In summary then we find that choline chloride has fascinating effects on the rates of electrocatalytic reactions. On one hand,
it suppresses two undesirable reactions: the hydrogen evolution
reaction, and CO formation during formic acid electrooxidation.
Yet, it enhances other reactions, the direct electrooxidation of
formic acid on platinum, and carbon dioxide conversion. Further, it
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W. Zhu et al. / Electrochimica Acta 96 (2013) 18–22
extends the region where electrochemical experiments can be done
to lower potentials. These results demonstrate that organic additions can have unexpected influences on electrocatalytic reactions,
and therefore need to be explored.
Acknowledgments
This work was supported by the US Department of Energy under
grant DE-SC0004453. The views expressed in this paper are those
of the authors and do not necessarily represent the views of the US
Department of Energy.
R.M. is the founder and principal owner of Dioxide Materials,
a company that is working to commercialize the findings in this
paper. R.M., W.Z., B.R. and A.M. have also submitted patents related
to the work here.
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