AFM Determines Key EDL
Properties in Future Ionic
Liquid Electrolytes

ASYLUM RESEARCH

Introduction“…while countless breakthroughs have beenannounced over the last decade, time and again
these advances have failed to translate into commercial
batteries with anything like the promised improvements incost and energy storage.”
The above quote was taken from an article
on the future of battery technology, publishedFebruary 2015.1 While the article mentions
batteries specifically, it sums up the problems of
all electrochemical storage devices, including
electrochemical capacitors, as power limitations andsafety concerns hinder their development.2 Adding tothis problem is the continued miniaturization of device
structures, as this needs the development of methods
that allow in situ studies of nanoscale dynamics inside
an electrochemical cell.
For improving device performance, new materials forboth the electrolyte and the electrodes are of interest.
For the electrolyte, ionic liquids (pure salts with melting
points below 100oC) have received much attention
as candidates for electrochemical storage.3, 4 This isbecause ionic liquids offer many desirable properties,+

One key area of interest is the interface between
ionic liquid electrolytes and the electrode itself. The
dynamics occurring at these interfaces determine
the performance of devices such as electrochemicalcapacitors (ECs). Specifically, the electrical double
layer (EDL) is of interest, as this consists of strong
interactions occurring between the ions close to
the electrode surface and the surface itself. These
interactions are different from ion interactions insolution, meaning the EDL is a critical factor in
determining the performance of any electrochemicalstorage device. However, while the EDL ofconventional electrolytes is well understood, the samecannot be said of ionic liquids.
A unique property of ionic liquids is they form
alternating anion and cation layers at solid-liquidinterfaces, making them distinctively different fromother electrolytes. With increasing distance from theinterface, the ion ordering diminishes, resulting in awell-defined EDL structure near the surface. Molecular
dynamic (MD) simulations can predict how the ionsarrange as a function of electrode material andsurface charge, providing a means to help interpret
experimental data.
Atomic force microscopy (AFM) can be used toprobe the structure of ionic liquids at higher spatial
resolutions than other methods, as well as thesuitability of these ionic liquids for future energy
storage applications. Additionally, AFM offers scientists
a more flexible experimental environment than othertechniques.

++

including large electrochemical windows,5 highthermal stability,6, 7 high conductivities 8 and low vapor
pressure.9-

High-Resolution AFM Imagingof the Ionic Liquid EDL
Ball and stick molecular models of two commonly used roomtemperature ionic liquids. (Top) propylammonium nitrate(abbreviated PAN), and (Bottom) 1-ethyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide (abbreviated EMIm TFSI here, also
known as [EMIm+][Tf2N-]). In all models, carbon atoms are gray,hydrogen atoms are white, nitrogen atoms are blue, sulfur atoms
are yellow, oxygen atoms are red, and fluorine atoms are green.

An understanding of the interface between solids
and liquids is critical in many scientific fields. This isespecially true in charge storage systems, where
the interface between the electrolyte and electrodeaffects device performance. The structure of thisinterface, known as the electrical double layer, isof particular interest due to the strong interactions
taking place between the ions and the charged© Oxford Instruments Asylum Research Inc. EDL Properties in Ionic Liquid Electrolytes 1

electrode surface. Upon charging and discharging,the ions within the EDL redistribute to compensate
for the surface charge. However, unlike the EDL ofconventional solvents, the structure of this ionic liquid
EDL is not well understood.

Molecular resolution images revealed the presence of
sharp rows formed by an anion-cation-cation-anion
unit cell on the surface with open-circuit potential,with like ions nearby (Figure 2). Critically, the ion
layer nanostructure changed with applied surface

Traditionally, scanning tunneling microscopy (STM)
has been used to study the lateral structure ofionic liquid layers. However, in liquids, the strong
interfacial nanostructure can interfere with the STM
signal, making high-resolution imaging difficult. To
overcome this limitation, the ionic liquid is often
frozen and imaged under ultrahigh vacuum (UHV)
conditions. However, this is also problematic because
the nanostructure of the interfacial layer in UHV andfreezing conditions doesn’t represent the true stateof the ionic liquid structure under realistic operating
conditions.
AFM offers a way to study ionic liquids in situ underrealistic conditions. Recently, amplitude modulation
AFM (AM-AFM, also commonly called tapping
mode AFM) was used to determine the interfacial
structure between highly ordered pyrolytic graphite(HOPG) and bulk propylammonium nitrate (PAN),
near room temperature.10 The AFM cantilever was
oscillated close to its resonant frequency, with a freeamplitude of less than 1 nm. The imaging amplitude
set-point ratio was kept greater than 0.7 to minimize
the tip-sample force and possible perturbation of
the EDL structure. These conditions resulted in the
AFM tip probing the interfacial ions adsorbed on theelectrode surface with sub-nanometer resolution(Figure 1), without contacting the electrode itself.
These conditions could be verified by reducing the setpoint ratio, increasing the tip-sample force, until the tip
probed beyond the ion layer and made contact withthe electrode surface.
This study marked the first time that molecular levelstructures have been determined within a viscous
liquid using AM-AFM. It was also the first investigationof its kind to achieve molecular resolution of theinterfacial layer between the electrode (graphite) andthe ionic liquid. Significantly, the resolution obtainedin this study using AFM was comparable to that
achieved for frozen ionic liquid monolayers using
STM.11
A second study, also at 25oC, involved high-resolution
imaging of the interfacial ion layer between 1-ethyl3-methylimidazolium bis(trifluoromethylsulfonyl)
imide (EMIm TFSI) and a HOPG surface.12 The
researchers varied the bias applied to the electrodeand added 0.1 wt/wt% Li TFSI or EMIm Cl to theionic liquid. AFM allowed them to study the effect ofthese modifications on the ion arrangement at theelectrode-electrolyte interface.

Figure 1: Height image of a PAN–HOPG interface obtained using
tapping mode with a scan amplitude of 1 nm. Two-dimensional
Fourier analysis confirmed that PAN adsorbs on the HOPG surface
with a rhomboidal structure with a lattice spacing of 0.48±0.02 nm,
approximately twice that of the underlying HOPG. Adapted withpermission from Ref. 10.

Figure 2: Tapping mode phase images of the IL Stern layer
adsorbed to a graphite (HOPG) substrate for both, (Left) the pure IL(EMIm TFSI) and (Right) for an IL solution with added chloride anions(EMIm TSFI + 0.1 wt/wt % EMIm Cl). These images were taken at
open circuit potential (OCP) (i.e. with no applied bias), corresponding
to 0.26 V vs. Pt for the pure IL and 0.42 V vs. Pt for the solution
with added chloride anions. For the pure IL (Top), the phase image
reveals alternating pattern of two rows of stiffer regions (lighter
areas- example rows highlighted in red) followed by two rows ofmore compliant regions (darker areas- example rows highlighted
in gray). Addition of chloride ions (Bottom) changes this structure,
resulting in a pattern of two stiffer rows, one compliant row, one
stiff row, then one compliant row. Additional work at applied
potentials above and below the OCP (not shown here) ascertainedthat the stiffer rows must correspond to the IL cations and themore compliant rows correspond to the IL anions. Adapted withpermission from Ref. 12.

2 EDL Properties in Ionic Liquid Electrolytes © Oxford Instruments Asylum Research Inc.

potential, or when low concentrations of Cl- or Li+ ionswere present. This study also found experimental
evidence to suggest there was an additional layerof cations on top of the ion layer in contact with thegraphite electrode surface. This outer layer may
screen the effect of the surface potential on the EDL
nanostructure.
For the first time, in situ AFM allowed the researchers
to distinguish between cations and anions on theelectrode surface, as well as determine their exact
positions within the ionic liquid layer. As expected,
the structure of the ion layer at room temperature
was much different from that of frozen monolayers
determined by UHV STM.11, 13 This work shows that
AFM may prove to be a critical technique in defining
the EDL of ionic liquids, an important step in their use
in future electrochemical systems.

Probing Ion Layering with Force Curves
Electrochemical capacitors (ECs) have advantages
over batteries for energy storage, making theman important area of scientific investigation.14
For example, they have higher power densities(10 kW kg-1) than batteries, and also charge anddischarge faster. Unfortunately, their energy density islower, so much of the work with ECs over recent years
has focused on improving their energy density.15, 16
One approach is to increase the operating voltage of
the EC using ionic liquid electrolytes, thus increasing
the stored energy. Using ionic liquids is an advantage
when compared to aqueous electrolytes, as applying
too high a voltage to an aqueous electrolyte cancause electrolysis and form dangerous hydrogengas. Ionic liquids can withstand much higher voltages
without breaking down. These higher voltages
translate into higher energy densities.
The capacitance of ECs is affected by the electrical
double layer, which will ultimately be a key factor
determining EC performance in future energy storage
devices. Further analysis of the EDL will enable thedevelopment of models that explain electrochemical
behavior and how ionic liquids can be used forelectrochemical storage. As an example, scientists
might get a better idea of defects in the ion layering.
The ion layer is not perfectly uniform, so we need to
study this and how these imperfections improve or
degrade performance.
The structure of the EDL of an ionic liquid as a functionof distance to the electrode surface can be studied
using force distance (F-D) curves, which was doneon HOPG as a model system for carbon-based EC
electrodes.17 The bias evolution of the ion layering
was probed by plotting F-D curves using AFM, shown

Figure 3: Details of single force−distance curves. (a) Single approachand retract curve showing multiple ion layers. (b) Single force
separation plot of curve shown in (a), and 2D histogram of 50
consecutively measured force curves (c). Adapted with permission
from Ref. 17.

in Figure 3. The authors gave particular attention to thereproducibility of the F-D curves, something that is often
neglected in other AFM studies.
By applying a potential, the authors were able tostudy the reconstruction of the ion layers using a
combination of molecular dynamics (MD) simulations
and AFM experiments. The researchers highlight the© Oxford Instruments Asylum Research Inc. EDL Properties in Ionic Liquid Electrolytes 3

excellent agreement between the MD simulations and
AFM experiments.
Specifically, the team used AFM to perform forcedistance spectroscopies in a droplet of EMIm TFSI ionicliquid on HOPG. In this case, F-D curves were measured
at rates between 0.1 and 0.3 Hz, and experiments were
performed with uncoated silicon nitride cantilevers
with a spring constant of 0.38 N/m. For comparison,
curves were aligned along the y-axis to be at zero force
far away from the sample.
The F-D curves were used to determine the ion
layer positions within the EDL of EMIm TFSI. The
first layer was measured to be 0.37 nm from the
HOPG surface, with subsequent layers spaced 0.7
nm apart. Two-dimensional histograms were also
calculated using 50 F-D curves and showed impressive
reproducibility. These statistics were compared withmolecular dynamics simulations to obtain a thoroughunderstanding of the ionic liquid EDL on biased andunbiased graphite surfaces. This step is necessary to
fully interpret the F-D curves and identify the ion layers.
This combination of MD simulation and experimental
AFM force-distance spectroscopy allowed the authors
to determine the individual ion layers as anion or cation.
Results of the MD simulations suggested the ion layers
closest to the electrode surface consisted of anions andcations in two distinct orientations. They also found thearrangement of cations and anions in the layer closest
to the surface differed at +1 V and -1 V, when comparedwith the potential at zero charge. This was attributed
to the existence of a Coulombic force between the ion
and the electrode. Finally, the ions in subsequent layers(after the first layer) have a characteristic anion-cation
pair spacing of about 0.7 nm.
Significantly, the authors suggest this study may
allow the future analysis of the ion layering inelectrochemically relevant systems for ECs. Theysuggest that all aspects of the EDL may be investigatedusing AFM techniques, further improving ourunderstanding of charge storage mechanisms in thefield of energy storage.

Using photothermal excitation to improve
AFM cantilever response in ionic liquids
As shown by examples here, AFM is a powerful toolfor high-resolution imaging of the ionic liquid EDL
structure. However, one instrumentation c hallenge
in these experiments is that ionic liquids are highly
viscous compared to water, a more common

medium used in AFMs. The high viscosity increases
hydrodynamic drag on the cantilever, which means
more energy must be directed to the cantilever to drive
its oscillation in AC modes (e.g. tapping mode). Most
AFMs use a piezoelectric actuator near the cantileverto drive its oscillation (called piezoacoustic excitation).
However, the high drive amplitude needed in ionicliquids tends to excite other mechanical resonancesin the AFM besides the cantilever. These contaminate
the cantilever frequency response with outside
resonances, complicating operation and interpretation.
For the best results, it’s important the cantileverresponse is free of these extraneous peaks andthat it remains stable with time. A solution is touse photothermal excitation to drive the cantileverdirectly, instead of the more common piezoacoustic
excitation. Photothermal excitation uses a secondary
laser focused near the base of the cantilever. The
power of this laser is modulated at the resonant
frequency of the cantilever, exciting the oscillation
by creating a small thermal expansion/contraction.
Other mechanical resonances are not affected, so
the resulting cantilever response is clean and stable.
In fact, photothermal excitation has been shown to
improve the signal when imaging in viscous ionicliquids using AM-AFM.18, 19
The use of photothermal excitation in AFM to study
the ion layering was demonstrated with the ionicliquid EMIm TFSI on both mica and HOPG surfaces.20
The researchers used AFM to carry out dynamic
force-distance and small-amplitude force-distance(SAFD) spectroscopies within the aforementioned
systems, with the goal of determining how thesubstrate (surface) influences the components ofresistance to squeeze.
Using F-D spectroscopy, the group was able tomeasure and compare the layered structureof EMIm TFSI on mica versus HOPG (Figure 4).
Significantly, they found stronger and more numerous
nanostructure layers on mica, which the groupattributed to the stronger surface charge. Significant
reproducibility was obtained in the F-D data setsusing photothermal excitation, with the authors using
50 data sets for their analysis.
The authors noted two important differences between
this study and one of their previous investigations
that used piezoacoustic excitation instead ofphotothermal excitation.21 The most fundamental
difference is the previous work was unable to resolve

4 EDL Properties in Ionic Liquid Electrolytes © Oxford Instruments Asylum Research Inc.

Conclusion
Mica

HOPG

Ionic liquids are promising electrolyte materials
that may enhance the performance of future
electrochemical storage devices. A necessary step
toward this becoming a reality is to develop anunderstanding of the EDL and the dynamics between
the electrode and ionic liquid. To that end, AFM
offers experimental capabilities not found with othertechniques. It also provides the resolution and insights
necessary to understand the EDL structure andcorresponding interfacial dynamics in ionic liquids.
The Asylum Research Cypher AFM offers unmatched
resolution and stability for scientists wishing to perform
imaging and spectroscopy in ionic liquids. And whencombined with blueDrive photothermal excitation for amore accurate cantilever response, it’s the ideal choice
for studying the ionic liquid EDL.

Figure 4: (from top to bottom) Amplitude, phase, kts, and bts forthe cantilever response in EMIm TFSI on mica (left) and HOPG
(right). The charge-mediated layers on mica are stronger than thetemplated layers on HOPG. This leads to an increase in resistanceto local flow on mica as the preferentially oriented moleculesinterlock with each other. On HOPG, the opposite effect is observed,
wherseby the ionic liquid has a slight reduction in flow resistance inthe presence of the wall. Adapted with permission from Ref. 20.

the ionic liquid structure at a HOPG interface due tothe aforementioned contamination of the cantileverby outside resonances. A more subtle benefit ispiezoacoustic excitation must drive the base of thecantilever with a large amplitude to achieve evena small tip amplitude in the viscous ionic liquids.
In that case, interpreting the tip motion requires
subtracting the cantilever base movement, whichitself is difficult to measure accurately. But whenphotothermal excitation is used, motion of the baseis nonexistent and so this makes the analysis simpler
and more exact.

© Oxford Instruments Asylum Research Inc. EDL Properties in Ionic Liquid Electrolytes 5

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6 EDL Properties in Ionic Liquid Electrolytes © Oxford Instruments Asylum Research Inc.

ASYLUM RESEARCH