Spontaneous Ionic Polarization in Ammonia-Based Ionic Liquid - ACS

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Spontaneous ionic polarization in ammonia-based ionic liquid Ki-Jeong Kim, Hongtao Yuan, Hoyoung Jang, Bongju Kim, Donghoon Seoung, Yasuyuki Hikita, Harold Y. Hwang, and Jun-Sik Lee ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00383 • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 26, 2018

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ACS Applied Energy Materials

Spontaneous Ionic Polarization in Ammonia-Based Ionic Liquid Ki-jeong Kim1,2,‡,*, Hongtao Yuan3,‡,*, Hoyoung Jang2, Bongju Kim4, Donghoon Seoung2, Yasuyuki Hikita3, Harold Y. Hwang3,4, and Jun-Sik Lee2

1

Beamline Research Division, Pohang Accelerator Laboratory, Pohang 37673, S. Korea

2

Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park,

California 94025, USA 3

Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo

Park, California 94025, USA 4

Geballe Laboratory for Advanced Materials, Department of Applied Physics, Stanford University,

Stanford, California 94305, USA

KEYWORDS. Ionic gel, in situ bias, Self-polarization, Surfaces and interface property, Photoemission spectroscopy, DEME-TFSI

ABSTRACT Ionic liquids and gels have attracted attention for a variety of energy storage applications, as well as for high performance electrolytes for batteries and super-capacitors. Although the electronic structure of ionic electrolytes in these applications is of practical importance for device design and improved performance, the understanding of the electronic structure of ionic liquids and gels is still at an early stage. Here we report soft x-ray

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spectroscopic measurements of the surface electronic structure of a representative ammoniabased ionic gel (DEME-TFSI with PS-PMMA-PS copolymer). We observe that near the outermost surface, the area of the anion peak (1s N- core level in TFSI) is relatively larger than that of the cation peak (N+ in DEME). This spontaneous ionic polarization of the electrolyte surface, which is absent for the pure ionic liquid without copolymer, can be directly tuned by the copolymer content in the ionic gel, and further results in a modulation in work function. These results shed new light on the control of surface electronic properties of ionic electrolytes, as well as a difference between their implementation in ionic liquids and gels

TEXT Utilization of ionic liquids (ILs) – molten room temperature salts composed mostly of organic ions – have led to innovations in various energy storage/conversion applications, such as batteries, supercapacitors, and fuel cells, as well as electric-double layer transistors 1-6. Ionic gel (IG) electrolytes, a mixture of IL and copolymer, have been employed as a substitute for ILs in some device applications 7, 8 to access the controllable viscosity and adhesion of IGs by varying the weight percentage of copolymer, and the resulting ease of processing in device fabrication. In particular, the intrinsic properties of ILs such as high ionic conductivities and large specific capacitances are retained in IGs 8-11. Furthermore, the ionic molecule distributions near the electrode are crucial in practical applications using IL12-16 and IG-based electrolytes 4, 17 for their functionality and for tailoring novel ionic devices 5, 18-22, so called “iontronics” 23. Nevertheless, the understanding of ion distribution and its relationship with the IL/IG electronic structure still remain elusive. Although many experimental studies, such as those using high-energy x-ray reflectometry 24, atomic force microscopy 25, scanning tunneling microscopy 26, and x-ray


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photoelectron spectroscopy (XPS) 2, 16, 27, 28, have made significant progress in characterizing the ion profiles of IL/metal interfaces in the depth range of 10–50 nm, the molecular-scale structures of ionic liquid/gels and the ion distribution near the free surface remain unknown. Here, we present an XPS study on an ammonia based ionic gel (N, N-diethyl-N-(2-methoxyethyl)-Nmethylammonium bis(trifluromethylsulfonyl)imide, DEME-TFSI) and show for the first time the finding of a spontaneous ionic polarization at the surface of the DEME-TFSI based IG. Figure 1A upper panel shows the molecular structure of DEME and TFSI. Note that their ionic molecular states (i.e., cation and anion) are determined by one nitrogen ion (either N+ or N-) in molecules that are bonded with several functional groups, namely -CH3, -CH2CH3, and CH2CH2OCH3 for the DEME cation and -SO2CF3 for the TFSI anion. The IGs were prepared by mixing a triblock copolymer (-PS-PMMA-PS-) which consists of soluble poly(methyl methacrylate) (PMMA) and insoluble polystyrene (PS) blocks with the IL (i.e., DEME-TFSI A)

B) F

DEME Et

Intensity

Et

N+ OMe

O

XPS Eph = 840 eV N

C

S IL

Me

IG

TFSI O F3C

S N- S O

700

O

300

100

Binding Energy (eV)

IL

C)

CF3

XAS N K-edge

O

Copolymer PMMA

500

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


IL IG

IG

390 400 410 420 430 440

Photon Energy (eV)

Figure 1 (a) Schematic diagrams of the ionic DEME and TFSI molecular structures, as well as the copolymer PSPMMA-PS. Inset photos contrast the viscosity and adhesion of the IG with the flow of the IL when the container is inverted. XPS (b) and XAS (c) measurements on the IL and IG.


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composition) (see Materials in Supporting Information). In comparison with the mechanical properties of the IL, in particular, the IG is more viscous and adheres to the container, as shown in the two inset pictures in the figure – the IG remains fixed at the top of the bottle when inverted. Since the copolymer molecules are not chemically-bonded with DEME or TFSI molecules, and occupy a small volume fraction, the overall electronic structure of the IG would be expected to be similar with that of the IL. To probe this we performed XPS and x-ray absorption spectroscopy (XAS) measurements of both the IG and IL (Figure 1B and 1C. The difference between the IG and IL samples are generally negligible, except the carbon (C) spectral area, due to the contribution from the copolymer. In particular, N K-edge XAS spectra indicate that overall N-configurations in the IL and the IG are similar within the probing depth of ~ 50 Å of the TEY mode (see Supporting Information). Considering these spectroscopic findings, the cation and anion distributions in the IG and IL seem to be nearly identical on large length scales, despite the presence of the copolymer in the IG. To probe this distribution more precisely at the surface, we examined the photon energy dependence of the XPS measurement. Following the universal curve of the photoelectron escape depth versus electron kinetic energy

26, 29-31

, the probing depth of XPS measured at an incident

photon energy Eph = 500 eV can be tuned down to ~5 Å from the surface, which is comparable to the ion size. On the other hand, XPS measured at Eph = 840 eV can reach an escape depth of about 10 Å. Therefore, a comparison between XPS spectra at these two different photon energies can provide contrasting information for the surface N ion distribution in the IL and IG.


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Figure 2 shows a comparison in the N 1s XPS spectra measured at Eph = 840 (Fig. 2A) and 500 eV (Fig. 2B). Since each molecule (DEME and TFSI) contains one nitrogen atom per ion (i.e., N+ and N-, respectively), the ratio of the integrated areas between the N+ and N- core level peaks in the XPS spectrum would be 1:1 for equal populations

2, 16

. For measurements with Eph = 840

eV, this is the case for both the IL and IG. By contrast, measurements at Eph = 500 eV clearly indicate that the integrated area of the N- peak is notably larger than that of N+, only for the IG. This indicates that the cation and the anion distribution in the IG are not uniform within the XPS probing depth of 500 eV, but rather there is a preferred anion polarization at the surface of the IG, which is absent for the IL (Fig. 2B inset).

DEME

TFSI

N+

N-

B)

IL

Intensity (arb. unit)

A)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ionic liquid

IG

432

436

440

444

Kinetic energy (eV)

Ionic gel

90

93

96

99

Kinetic energy (eV)

Figure 2 Nitrogen 1s core-level spectra of the IL and IG, measured at Eph = 840 eV (A) and 500 eV (B). Open circles are raw data and the solid lines show fitting curves. The red and blue colored shadeareas denote the cation (N+) in the DEME molecule and the anion (N-) in the TFSI molecule, respectively. The arrow in the IG indicates the difference between the cation and anion peak intensities. The insets (right panel) represent a schematic distribution of the ionic molecules in both the IL and IG.

In order to further examine the development of this surface polarization in the IG, we prepared several IGs with varying copolymer content (0, 1, 3, 5, 10 and 15 wt. %) and performed XPS measurements at Eph = 500 eV – here, 0 wt. % corresponds to the pure IL. We observed a


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systematic evolution of the ratio of the integrated areas between the N+ and N- core level peaks with increasing copolymer content (see Supporting Information). These results are summarized in Fig. 3A, giving the surface polarization (P) as a function of the copolymer content. Here P (%) = (AN- - AN+)/(AN- + AN+) × 100, where AN- and AN+ are the area of the XPS nitrogen peaks assigned to the anion and cation, respectively. With increasing copolymer content, P increases linearly up to 5 wt. % and saturates, at a value of P ~ 10 %. This behavior clearly indicates that surface polarization is the result of the addition of copolymer to the IL. As a next step, to understand the consequence of this correlation between P and the polymer content, the work functions of all samples were measured. Figure 3B shows the difference of work function (∆ϕ) between the IG and IL. Note that the work function changes were determined from the variation in the secondary edge under a sample bias of –50 V (Fig. 3B inset). Up to 5 wt. %, the ∆ϕ variation appears negligible. However, when the copolymer content is above 5 wt. %, the measured work function of the IG becomes smaller than that for the IL (4.3 eV

32

), resulting in ∆ϕ ~ -0.3 eV. Since a change in work function is related with a surface

dipole moment 33, this ∆ϕ behavior with varying copolymer content indicates that the P change originates from a change of internal electronic structure in the IG, and the resulting surface dipole. Therefore, electrons in the anions require less energy to escape from the IL when the surface is anion-rich, due to the additional positive electrostatic force acting on them. It is still an open question that how microscopically the work function modulated with the surface polarization and further detailed study is needed.


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B)

10

Liquid

5

Eph = 500 eV

0 0.2

10 %

Intensity

Polarization (%)

A)

0.0

(eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5% Liquid

54

55

56

57

KE (eV)

-0.2

-0.4 0

5

10

15

Copolymer content (%)

Figure 3 (a) Spontaneous surface polarization with varying (0 – 15 wt. %) copolymer. Note that 0 wt. % indicates the IL. All values were estimated from N 1s core level peaks at Eph = 500 eV. The colored thick lines are guides-to-the-eye. (b) The measured work function difference (∆ϕ). The inset shows the secondary edge spectra for representative samples (0, 5, and 10 wt. %).

In summary, we observed a spontaneous ionic polarization at the IG surface, and found that this polarization effect strongly depends on the copolymer content of the gel. We further observed a modulation of the work function of the IG, a key parameter for the design and functionality of novel ionic devices that take advantage of the unique properties of IGs of known interest in the energy sciences. While the correlation between spontaneous surface polarization and work function can be simply understood, their distinct functional dependencies on copolymer content suggest a complex evolution of the surface electronic structure. This provides a fascinating area for further experimental investigation and theoretical development34, with potential to develop new methods to optimize IGs in a wide range of applications, such as, ionic conductivity, create the possibility of designing ideal electrolytes for batteries, super-capacitors, actuators, dye sensitized solar cells and thermo-electrochemical cells.


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EXPERIMENTAL PROCEDURES The ionic liquid, DEME-TFSI (N, N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis(trifluromethylsulfonyl)imide), was purchased from Kanto Chemical Co. IGs were synthesized by mixing and stirring an ethyl propionate solution with a triblock copolymer, PSPMMA-PS,

(styrene-block-methylmethacrylate-block-styrene)

and

DEME-TFSI11,14.

The

copolymer content used in the main text is the weight percentage and defined as (weight of copolymer)/[(weight of copolymer) + (weight of IL)]. The solution was spin-coated onto Au/SiO2/Si substrates at a speed of 6000 RPM for 10 minutes, after which the sample was annealed at ~ 80˚C for 60 mins to evaporate the solvent. Each sample was introduced into a UHV chamber and pumped down for 10 hours until the base pressure reached 5 x 10-10 Torr. XPS was performed at the beamline 8-2, Stanford Synchrotron Radiation Lightsource (SSRL) equipped with a CMA electron analyzer. We used the incident photon energy of 500 eV and 840 eV to measure the surface sensitive properties. The binding energies and the total spectral resolutions were calibrated by recording the Au 4f7/2 core-level spectrum. Secondary electron emission spectra (–50 V sample bias) were recorded at photon energies of 500 eV. All spectra were obtained at an emission angle of 45˚. The photoemission spectra were carefully analyzed by using a standard nonlinear least-squares fitting procedure with Voigt functions . To measure the XAS spectra, we monitored both the total electron yield (TEY) mode signal via the drain current and Auger electron yield (AEY) mode signal via CMA electron analyzer. Since these two modes (AEY and TEY) have different probing depth (~ 5 nm and 1 nm respectively),


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we could get effective information from both the bulk and the surface sensitive regions. We note that these measurements were performed at the XPS chamber at BL8-2/SSRL, using the (horizontal) linearly polarized X-ray beam. All measurements were performed at room temperature.

Supporting Information. Supporting information is available.

AUTHOR INFORMATION Corresponding Author *[email protected] and [email protected]. ‡These authors contributed equally.

Author Contributions K.J.K., H.T.Y., J.S.L., H.Y.H. conceived the ideas for this research project. H.T.Y. developed the condition for the IG layer deposition and fabricated the sample. K.J.K., H.T.Y., H. J., B.K., D.S. carried out PES experiments. K.J.K., H.T.Y.,Y.H., S.J.L. wrote the manuscript. H.Y.H. and J.S.L. supervised the project. All authors discussed the manuscript and agreed on its final content.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT Synchrotron studies were carried out at the SSRL (BL 8-2), a Directorate of SLAC and an Office of Science User Facility operated for the US DOE Office of Science by Stanford University. J.S.L., H.T.Y., Y.H., and H.Y.H. acknowledge support by the Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, under contract DE-AC0276SF00515. K.-j.K acknowledges that the experiments at PLS-II were supported in part by MSICT and POSTECH.

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TABLE OF CONTENTS

DEME

TFSI

N+

N-

B)

IL


A)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Ionic liquid

IG

432

436

440

444

Kinetic energy (eV)

Ionic gel

90

93

96

99

Kinetic energy (eV)


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Spontaneous Ionic Polarization in Ammonia-Based Ionic Liquid - ACS

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