Monitoring potassium metal electrodeposition from an ionic liquid using in situ electrochemical-X-ray photoelectron spectroscopy

Article abstract of DOI:10.1016/j.cplett.2011.04.071

The real time electrodeposition of potassium has been monitored for the first time in an ionic liquid using in situ electrodeposition-X-ray photoelectron spectroscopy (XPS). The ionic liquid used was N-butyl-N- methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C₄mpyrr] [NTf₂]), and electrodeposition occurred at a nickel mesh electrode. Potassium electrochemistry was monitored at the ionic liquid-vacuum-electrode interface using a novel cell design.

Full text of DOI:10.1016/j.cplett.2011.04.071

Chemical Physics Letters 509 (2011) 72–76
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Monitoring potassium metal electrodeposition from an ionic liquid using in situ
electrochemical-X-ray photoelectron spectroscopy
Rahmat Wibowo , Leigh Aldous , Robert M.J. Jacobs , Ninie S.A. Manan c,d, Richard G. Compton a,
a
a
b
⇑
a
Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom
Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Mansfield Road, Oxford OX1 3TA, United Kingdom
School of Chemistry and Chemical Engineering, The QUILL Centre, Queen’s University Belfast, Belfast BT9 5AG, United Kingdom
Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
The real time electrodeposition of potassium has been monitored for the first time in an ionic liquid using
in situ electrodeposition-X-ray photoelectron spectroscopy (XPS). The ionic liquid used was N-butyl-N-
methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C mpyrr][NTf ]), and electrodeposition
4 2
occurred at a nickel mesh electrode. Potassium electrochemistry was monitored at the ionic liquid–vac-
uum–electrode interface using a novel cell design.
Received 10 April 2011
In final form 22 April 2011
Available online 28 April 2011
Ó 2011 Elsevier B.V. All rights reserved.
1
. Introduction
Ionic liquids (ILs) generally possess extremely low vapour pres-
interface; this represented the first, and to the authors knowledge
only reported application to date of a three-electrode electrochem-
ical set-up for in situ electrochemical-XPS measurements on ILs
[25].
We have recently investigated the electrodeposition of the Alkali
Group I metals at Pt and Ni electrodes in the IL N-butyl-N-meth-
sures [1], as well as inherent conductivity and the ability to sup-
port relatively reactive metal deposits [2]. These characteristics
have facilitated their application in a number of relatively unique
in vacuo techniques [3,4], including in situ X-ray photoelectron
spectroscopy (XPS) of ionic liquids and various solutes [3,5–22].
This has recently been expanded to in situ electrochemical-XPS
measurements, a form of in vacuo spectroelectrochemical mea-
surement [9,22–24].
ylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C
4
mpyrr]-
[NTf ]), allowing determination of a range of fundamental kinetic
2
and thermodynamic properties [26–29]. These metals are of partic-
ularinterestwithregards to energystoragedevices such asbatteries,
and our work highlights the similarity in the trends within the Alkali
Foelske-Schmitz et al. investigated the electrochemistry of
Highly Orientated Pyrolytic Graphite (HOPG) in the ionic liquid
[C mim][BF ], which was performed in the XPS chamber, the ex-
4 4
cess liquid removed from the carbon surface, the carbon trans-
ferred to the sample holder and XPS performed [9]. Silvester
performed XPS measurements in conjunction with ex situ electro-
chemical measurements in order to probe the bromide content of
4 2
metal group with [C mpyrr][NTf ] and the common battery electro-
lyte propylene carbonate. The tendency of these metals to form
dendritic electrodeposits is of both interest and concern, as it can
result in harmful short-circuiting of battery devices [30].
In this Letter we report for the first time in situ metal electrode-
position-XPS measurements in an ionic liquid, achieved with a un-
ique cell design which allowed real time measurement of
[
C
4
mpyrr][NTf
2
] [20].
4 2
potassium metal electrodeposition from [C mpyrr][NTf ] at the
Licence et al. have carried out in situ electrochemical-XPS mea-
three-phase ionic liquid–electrode–vacuum boundary.
surements using two-electrode systems containing IL droplets, and
monitored the oxidation of a copper wire to Cu(I) [23], reduction of
Fe(III) to Fe(II) [24] and the XPS induced charging of the IL itself
2
. Experimental
[
22].
Weingarth et al. have also recently accomplished in situ electro-
N-Butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)-
imide ([C mpyrr][NTf ]) was prepared in-house using standard
methods [31]. The K[NTf ] salt was prepared by the reaction of
aqueous solutions of the appropriate potassium hydroxide salt
with a 5% molar excess of H[NTf ], followed by repeated dissolu-
4
2
chemical-XPS measurements, whereby the X-ray beam was fo-
cused on the interface between the IL and a Pt electrode, a
potential difference applied and spectra recorded at the polarised
2
2
tion in water then drying under vacuum, until pH neutral.
XPS measurements were performed using a VG ESCALAB MkII
⇑
Corresponding author. Fax: +44 0 1865 275 410.
E-mail address: richard.compton@chem.ox.ac.uk (R.G. Compton).
spectrometer equipped with a monochromatic Al K
(photon energy of 1486.6 eV). All XPS experiments were recorded
a X-ray source
0
009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2011.04.071

R. Wibowo et al. / Chemical Physics Letters 509 (2011) 72–76
73
with a constant analyser pass energy of 100 eV for survey scans
and 20 eV for detailed scans. All XPS spectra were referenced to
the cations C 1s peak, using a binding energy of 285.0 eV. The cell
was covered with a specially fashion lid to prevent ionic liquid loss
during degassing, placed upright in a desiccator and put under high
vacuum overnight before use. The cell was then rapidly transferred
to the entry chamber and evacuated for a further 2 h before being
moved into the sample chamber.
around and anchored in place in a traditional cell holder by a mod-
ified XPS stub on the underside (g), allowing electrochemical mea-
surements to be performed in addition to traditional samples. The
cell was lowered from above onto two pogo spring pins (d and e),
which entered the tapered holes and made electrical contact with
the working and reference electrodes, respectively, via the copper
foil. Much of the counter electrode (c) passed through a hole in
the holder before coming to rest on a copper o-ring set in the
holder (f). Three shielded wires (from d to f) ran from the holder
to the potentiostat, via an electrical feed-through in the XPS
housing.
All electrochemical measurements were carried out using a
computer-controlled PGSTAT-12
Chemie, Netherlands).
l-Autolab potentiostat (Eco-
3
.2. XPS measurement
3
. Results and discussion
.1. Experimental set-up
A custom electrochemistry-XPS cell and cell holder were fabri-
XPS spectra of the electrochemical set-up was recorded, and
3
Figure 2 displays a wide scan for both 0.1 M and 0.5 M K[NTf
solved in [C mpyrr][NTf ]. Clear signals were observed for the IL,
corresponding to C, N, O, F and S. The stoichiometric ratios agreed
well (within ca. 5%) of those expected for the pure IL. Signals for K
2
2
] dis-
4
2
cated in house, and are depicted in Figure 1. The cell consisted of
two hollow cylinders of PEEK, as well as a modified stainless steel
XPS stub. These pieces were precisely machined and then firmly
pressed together. A shallow well ca. 1 mm deep was made across
the surface of the top cylinder of PEEK, and the hollow cylinder
p and K 2s at 295 and 377 eV were not observed for solutions con-
taining 0.1 M K[NTf ], but could be observed for 0.5 M K[NTf ].
2
2
In both spectra only weak signals were observed for Ni 2p3/2 at
55 eV, despite the presence of a significant quantity of Ni in the
8
and shallow well filled with IL (ca. 150 ll). Two pieces of copper foil
form of the Ni mesh. The weak signal was found to be related to
the angle of the sample with respect to the detector. The sample
was kept horizontal when the cell contained IL, with a correspond-
ing take off angle of 75°. This resulted in strong signals for the
surface of the IL, but could only detect the top surface of the Ni mesh,
emission from the side of the Ni mesh being largely unable to reach
the detector. Tilting the sample to change the take off angle to 90°
were sandwiched between the two cylinders of PEEK, and two ta-
pered holes drilled into the bottom cylinder of PEEK. One piece of foil
was joined to a Pt wire, which was brought into contact with the IL to
act as a quasi-reference electrode (depicted as ‘a’ in Figure 1). The
second piece of foil was joined to a Ni mesh via a Pt wire to act as
working electrode (b). This mesh was allowed to float on the surface
of the IL, forminga three-phase boundary between the IL, Ni and vac-
uum. The stainless steel stub (c) was used as counter electrode. A
removable lid was fashioned to prevent loss of the IL during its vig-
orous bubbling when initially in vacuo.
(
e.g. with the mesh adhered to a carbon tab) resulted in a significant
increase in the Ni signal. However, measurement at this angle was
not possible when IL was present in the cell, and it is also noted that
analysis of the Ni mesh was not required, interest instead being
focused on the surface of the IL in the vicinity of the Ni mesh.
Electrical connection from the potentiostat to the cell was pro-
vided by a removable cell holder. This cell holder was moved
3.3. Cyclic voltammetry performed in the XPS chamber
2
Figure 3 displays cyclic voltammetry recorded for 0.5 M K[NTf ]
in [C
4
mpyrr][NTf
2
] in the in situ electrochemical-XPS cell, scanning
À1
at 10 mV s between 0 and À3.6 V vs. Pt wire. The observed re-
sponse bears strong resemblance to the previously reported data
Figure 2. X-ray photoelectron spectra recorded for the in situ electrochemistry-XPS
cell when filled with (a) 0.1 M K[NTf ] and (b) 0.5 M K[NTf ] in [C mpyrr][NTf ].
Figure 1. In situ XPS-electrochemical cell and associated holder.
2
2
4
2

7
4
R. Wibowo et al. / Chemical Physics Letters 509 (2011) 72–76
Figure 3. Cyclic voltammogram recorded for 0.5 M K[NTf
Ni mesh electrode inside the XPS chamber. Scan rate = 10 mV s
2 4 2
] in [C mpyrr][NTf ] at a
À1
.
recorded under similar conditions at a Ni microdisk electrode [26],
namely K underpotential deposition features occurring in the re-
gion of ca. À1.5 to À2.5 V with bulk electrodeposition of K metal
from ca. À3 V onwards. Stripping of the bulk metal deposit is ob-
served on the reverse scan. Stripping efficiency was less that
1
00% on the Ni mesh (ca. 33% efficiency in Figure 3), which has
been previous noted for K on Ni microdisk electrodes [26], as well
as for a range of other metals on a range of substrates [32]. If this
low stripping efficiency was due to reaction of K metal with trace
impurities present in the IL, stripping efficiency would decrease as
the concentration of K[NTf ] decreased (due to a higher ratio of
2
possible reactants to K electrodeposited). However, experiments
at a Ni microdisk electrode demonstrated that a maximum striping
2
efficiency was observed at ca. 0.1 M K[NTf ] and stripping effi-
ciency decreased with increasing K[NTf ] concentration, therefore
2
this trend can tentatively be assigned to dendritic metal deposits
forming. During oxidation the material in contact with the elec-
trode is oxidised first, and the remainder loses good electrical con-
tact with the electrode resulting in less than 100% stripping
efficiency [32].
3.4. In situ electrodeposition-XPS measurements
Electrodeposition of K metal at the Ni mesh was performed by
Figure 4. XPS high resolution scans (pass energy of 20 eV, 10 scans) for the K 2s
region taken during electrolysis at À3.2 V vs. Pt quasi-reference at a Ni mesh
holding the potential at À3.2 V vs. the Pt wire quasi-reference.
XPS measurements were performed periodically, both in wide scan
mode and by performing detailed scans in the K 2s region; atten-
2 4 2
electrode floating on 0.1 M K[NTf ] in [C mpyrr][NTf ]. The scans were recorded
after 12, 15, 20, 25, 32 and 36 h of electrolysis.
tion was focused on the K 2s region as the K 2p partially over-
À
À11 À1
m2
s ) [26] is two orders of magnitude slower
lapped with the C 1s signal from the [NTf
2
]
anion.
(D = 1.02 Â 10
À9
2 À1
For the first ca. 12 h, the K 2s region was essentially featureless
Figure 4), although with extended electrolysis time (up to 36 h)
than that observed in aqueous media (D = ca. 3 Â 10 m s ) [33].
After 36 h the electrolysis was ceased and all connections run-
ning from the potentiostat to the XPS unplugged. The system
was allowed to remain like that for 1 h before another XPS mea-
surement recorded. The measurement demonstrated no significant
change in the various peaks relative to that recorded before elec-
trolysis was ceased, showing that the system was stable for this
period. After this the cell was reconnected, and the K deposit oxi-
dised at an applied potential of 0 V vs. Pt wire quasi-reference
for 30 min. Figure 6 displays the XPS spectra recorded (a) after
holding À3.2 V for 36 h and (b) after the subsequent application
of 0 V for 30 min. There is clearly a significant decrease in the size
of the K peak, corresponding to oxidation of K (in contact with the
(
increasing quantities of K were detected at the surface of the IL,
corresponding to electrodeposited K metal. From 15 h onwards
the quantity of K detected was found to increase in an approxi-
mately linear manner with time (Figure 5). The preceding 15 h ap-
peared to correspond to an induction period, likely corresponding
to the formation of K electrodeposits growing on and close to the
Ni surface which due to the take off angle could not be detected.
After 36 h a total charge of 4.5 C was passed, corresponding to
approximately 2 mg of K metal deposited (assuming 100% colum-
bic efficiency). An electrolysis extending over 36 h is expected
when dealing with IL systems, due to the relatively high viscosity
+
+
and low diffusion coefficients typically experienced in IL systems.
Ni) to K , with much of the resulting K , as well as K metal no long-
er adhered to the Ni surface diffusing away.
+
For example, the diffusion coefficient of K in [C
4
mpyrr][NTf
2
]

R. Wibowo et al. / Chemical Physics Letters 509 (2011) 72–76
75
of the peak. Additionally, Villar-Garcia et al. have recently high-
lighted that the charging of the ionic liquid surfaces during X-ray
irradiation, as well as day-to-day variations can lead to binding
energy values shifting by at least 0.8 eV unless internal referencing
to unaffected moieties is performed (e.g. long-chain aliphatic
carbon peaks) [22], which was not possible in this current work.
Therefore given the minor uncertainties present, elements can be
accurately identified and quantified but detailed speciation
+
information (such as differentiation between K and K ) cannot be
accurately assigned to the system.
4
. Conclusion
A novel cell design designed for application in conventional XPS
chambers has been presented. Using this design it has been clearly
been demonstrated that the growth of (reactive) metal deposits
can be monitored at the three-phase ionic liquid–electrode–vac-
uum boundary using in situ XPS measurements.
Figure 5. Plot of the absolute XPS peak area recorded under the K 2s peak as a
function of electrolysis time.
Acknowledgements
3
.5. Is it possible to distinguish between potassium metal and
Professor Christopher Hardacre is thanked for the kind donation
of the ionic liquid, which was synthesized by N.S.A.M. N.S.A.M.
acknowledges the financial support of the Ministry of Higher Edu-
cation Malaysia and University of Malaya via a SLAI fellowship.
R.W. thanks the Directorate of Higher Education, The Ministry of
National Education, Republic of Indonesia for funding. John Free-
man is thanked for assistance with Figure 1, and Charlie Jones for
the construction of the cell and holder.
potassium ions?
During this work, it was observed that when the electrochemi-
cal cell was polarised between applied potentials of À2.0 V to
+0.5 V vs. Pt quasi-reference (no Faradaic current passed), the
À1
2
binding energy of all elements shifted by À0.82 eV V (R = 0.99
for linear fit, n = 5) implying minor distortion of the IL-vacuum
interface due to the polarised Ni mesh. This shift is distinct from
À1
the recently highlighted À1.0 eV V ‘electrochemical shift’ experi-
enced by IL molecules in the electrochemical double layer [25], as
our focus was extended over an area considerably larger than the
double layer. However, this was accounted for by referencing all
spectra to the ionic liquid C 1s peak.
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