Potential-Dependent Structure of the Ionic Layer at the Electrode

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Potential-Dependent Structure of the Ionic Layer at the Electrode Interface of an Ionic Liquid Probed Using Neutron Reflectometry Naoya Nishi, Junya Uchiyashiki, Yoichi Ikeda, Seiji Katakura, Tatsuro Oda, Masahiro Hino, and Norifumi L Yamada J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01151 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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The Journal of Physical Chemistry

Potential-Dependent Structure of the Ionic Layer at the Electrode Interface of an Ionic Liquid Probed Using Neutron Reflectometry Naoya Nishi,∗,† Junya Uchiyashiki,† Yoichi Ikeda,† Seiji Katakura,† Tatsuro Oda,‡ Masahiro Hino,‡ and Norifumi L. Yamada¶ †Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan ‡Institute for Integrated Radiation and Nuclear Science, Kyoto University, Kumatori, Osaka 590-0494, Japan ¶Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japan E-mail: [email protected]

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Abstract Neutron reflectometry (NR) has been performed at the electrochemical interface between a Nb film and an ionic liquid, trihexyltetradecylphosphonium tetrakis(pentafluorophenyl)borate ([THTDP+ ][PFPB− ]).

THTDP+ and PFPB− have large sizes and a

large difference in the scattering length density, both of which are suitable to sensitively probe the potential-dependent structure of the electric double layer (EDL) at the electrode interface of ionic liquids using NR. The neutron reflectivity profiles as a function of momentum transfer have shown a clear potential dependence that is ascribable to the ionic composition change in the EDL: the accumulation of PFPB− and the depletion of THTDP+ at positive potentials and vice versa at negative potentials. The analysis of the reflectivity profiles has revealed that the EDL is composed of one ionic layer as in the Helmholtz model and is fully occupied by either of THTDP+ or PFPB− at potentials far from the potential of zero charge. Further electrochemical analysis suggests that the differential capacitance decreases at the fully occupied conditions, which agrees with the camel or bell shape behavior of the differential capacitance predicted previously due to ion crowding in the EDL in ionic liquids.

1

Introduction

Ionic liquids (ILs) are liquids that are composed of cations and anions without neutral molecules and have several characteristics in bulk and at the interfaces which molecular liquids do not have. 1–3 ILs have attracted the interest of researchers in possible applications in electrochemistry, 4,5 such as batteries, 6 super capacitors, 7 dye-sensitized solar cells, 8 electrodeposition, 9,10 and electrochemical CO2 fixation. 11 For such electrochemical applications, the behavior of IL ions at the interface plays a critical role for the performance of the whole system. Therefore, it is of crucial importance to investigate the interfacial structure of ILs. At the interface of ILs, ions may be spontaneously ordered and form ionic multilayers. This layering occurs even at the free surface of ILs 12–19 without the help of a solid substrate and 2


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strong electric field at the interface. At the free surface each ionic layer is electrically neutral and composed of the same number of cations and anions. In contrast, at the electrode interface with ILs, the surface charge on the electrode changes the composition in adjacent ionic multilayers, which was proposed theoretically 20,21 and found experimentally by using atomic force microscopy (AFM) 22,23 and x-ray reflectometry (XR). 24–26 Two peculiarities have been revealed for the electric double layer (EDL) in ILs, which are not observed for that in conventional electrolyte solutions. One is “ion crowding”, due to excluded volume between ions, which is assumed to be negligible in the conventional Gouy-Chapman model. 27,28 As the electrode is charged up, counter ions are enriched (co ions are depleted) in the first ionic layer.

The counter ion occupation in the first layer

makes it difficult for following counter ions to enter the layer for further charging. Therefore, the differential capacitance decreases as the electrode is charged up. This decrease is different from the prediction of two famous conventional EDL models: constant capacitance from capacitor-like Helmholtz model 29 and increasing capacitance from the Gouy-Chapman model. 27,28 This ion crowding was proposed in lattice gas mean-field theory of ILs 30 and later confirmed in simulation 31 and experiments. 18,32–34 The other peculiarity is “overscreening”, due to strong local electrostatic interaction between neighboring ions. The charge on the electrode induces the enrichment of counter ions in the first ionic layer and makes the first layer charged. The charged first layer makes the second ionic layer oppositely charged due to the ion-ion electrostatic interaction. In this way, alternately charged layers, which was first revealed at the negatively-charged sapphire interface of ILs by XR, 35 are formed on the charged electrode interface of ILs. 26 In such a overscreening condition, the potential profile is expected to show damped oscillatory decay, which is characteristic to fluids beyond the Kirkwood line 36,37 and in contrast to monotonic decay that conventional electrolyte solutions typically show. Beside the above two peculiarities, another interesting phenomenon recently found is a long-range force between two charged substrates in ILs, which is distinctively longer than the Debye length for the EDL screening. 38–40 In spite of these previous 3



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findings, more studies would be highly desirable to further elucidate the EDL structure in ILs by using a high-sensitivity method. Neutron reflectometry (NR) is a powerful tool to study the interfacial structure in the surface normal direction with angstrom resolution. 41 free surface 12,42–44 and buried interfaces 45–48 of ILs.

NR has been applied to the

Lauw et al. developed a sophis-

ticated electrochemical cell for NR 45 and studied the gold electrode interface of 1-butyl1-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide. 45,46 They observed the cation adsorption even at positive potentials, indicating strong chemical interaction between the cation and gold.

A recent NR study by Griffin et al. was on the mica interface of 1-

decyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide. Although the potential at the IL/mica interface is not controllable, they observed a peak in the reflectivity profile originated from at least two ionic bilayers induced by high surface charge on the mica surface. 47 Very recently, Pilkington et al. reported a NR study on the gold interface of trihexyltetradecylphosphonium bis(mandelato)borate and its d-acetone solution. 48 The solvent was added to match the scattering length density (SLD) contrast between the liquid and the gold electrode to increase the NR sensitivity to the EDL structure. Although the pure IL did not show potential dependence, the d-acetone solution showed a EDL layer whose ionic composition varied with the electrode potential. As described above, there still exists a limited number of NR studies and it would be worth to further explore the electrochemical interface of ILs by using NR. To do so, it is of importance to design the system components such as IL ions and electrode material, to maximize the NR sensitivity to the EDL structure and its potential dependence. For the IL ions, high SLD contrast between the cation and anion is desirable to sensitively detect the ionic accumulation and depletion that occur in the EDL structure depending on the electrode potential. Another aspect is ionic size. Large ions are beneficial because a thicker EDL layer affects the reflectivity profiles at smaller momentum transfer, qz , allowing us to detect the reflectivity change easier. Furthermore, larger IL ions are also expected to show ion crowd4


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ing behavior at more moderate and experimentally attainable potentials within the potential window, as was suggested by Ivaniˇstˇsev and Fedorov in their molecular dynamics (MD) study on the electrode/IL interface. 49 From the above aspects, we selected an IL, trihexyltetradecylphosphonium tetrakis(pentafluorophenyl)borate ([THTDP+ ][PFPB− ]), whose structure is shown in Fig.S1.

[THTDP+ ][PFPB− ] is a highly hydrophobic ILs 15,18,50–53 composed

of large and bulky ions and was used for the electrochemistry at the liquid-liquid interface between IL and water by us 54 and Stockmann et al. 55–57 THTDP+ and PFPB− have high SLD contrast as listed in Table 1. The ionic diameters are large, which were estimated to be 11 and 10 Å for THTDP+ and PFPB− , respectively, from quantum chemical calculations by assuming that their shapes are spherical. We selected Nb as the electrode material and deposited Nb film on a polished Si substrate that has small surface roughness. Nb electrode has been utilized in electrochemical studies. 58–60 The SLD of Nb, 3.92 (×10−6 Å−2 ), is comparable to and smaller than 4.50, that of Au used in the previous NR studies described above, 45,46,48 and more contrast matched to those of [THTDP+ ][PFPB− ] (1.57) and Si (2.07), two bulk materials in the present study (Table 1). In the present study, we will present the EDL structure at the electrochemical [THTDP+ ][PFPB− ]/Nb/Si interface probed using NR. Quantitative discussion has been performed for the thickness and ionic composition of the EDL structure, which has been possible because of the clear potential dependence found in the NR data.

2 2.1

Experimental Materials and experimental cell

The IL used, [THTDP+ ][PFPB− ], was prepared via metathesis reaction from [THTDP+ ]Cl− (Aldrich) and Li+ [PFPB− ] (TCI) and purified similarly to the methods described elsewhere. 14,61 The cell used for NR is shown in Fig.S2.

Nb film with a thickness of 360

Å was used as the working electrode (WE). The Nb film was sputter-deposited onto the 5



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polished surface of a Si block (50×50×10 mm, width/depth/thickness; the neutron beam is incident in the depth direction). [THTDP+ ][PFPB− ] was sandwiched between the WE and the counter electrode (CE). CE was Al4 Ti film with a thickness of 400 Å, which was sputter-deposited on the as-sliced (non-polished) surface of a Si wafer (50×50×0.7 mm). The Nb and Al4 Ti films were fabricated by ion beam sputtering technique. 62 Al4 Ti was chosen because the SLD, 1.0 (×10−6 Å−2 ), is close to that of [THTDP+ ][PFPB− ], 1.57 (Table 1). The use of the rough non-polished surface as well as the low SLD contrast enabled us to prevent specular reflection of the neutron beam from the CE surface. The neutron beam was incident on the center area (50×30 mm) of the [THTDP+ ][PFPB− ]/Nb/Si interface from the side of the Si block. Two sheets of a porous PTFE film (Mitex LCW, Merck, thickness: 130 µm) were placed as a spacer between the WE and CE on the two edge areas (50×5 mm each) that are outside the beam-irradiated center area (Fig.S2). Ag foil with a thickness of 50 µm was inserted between the two PTFE sheets as quasi reference electrode (QRE). The cell was not sealed because [THTDP+ ][PFPB− ] is highly hydrophobic and highly viscous. In this three-electrode electrochemical cell, the electrode potential of WE with respect to QRE, denoted by E, was controlled using a potentiostat (CompactStat, Ivium).

The

potentials investigated by NR are −2, −1, 0, +1, +2 V within the potential window (cyclic voltammogram (CV) is shown in Fig.S3). A constant potential was set at least for 10 min before starting the neutron reflectivity measurement at the potential, to avoid the influence of ultraslow relaxation of the EDL structure in ILs. 25,34,54,63–68

2.2

Neutron reflectometry

NR was performed using a horizontal-type neutron reflectometer, SOFIA, 69,70 at Beamline 16 (BL16) of the Materials and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC). The sample interface was irradiated with neutron beam at an incident angle, θ, of either of 0.3, 0.75, and 1.80 degrees and the reflected beam was detected with a 2-D detector. The wavelength of the neutron beam, λ, was evaluated 6


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using time-of-flight method, which enabled to measure the intensity of the reflected neutron beam at each momentum transfer along the surface normal, qz (= 4π sin θ/λ). The reflected intensity data at the three incident angles were normalized with a transmitted intensity data and combined as a whole profile of the reflectivity, R, as a function of qz .

2.3

Data analysis

The reflectivity profiles were analyzed by fitting model functions based on the Parratt formalism 71 including the N´ evot-Croce roughness factor. 72 2% resolution of qz was taken into account in the fitting.

All the reflectivity profiles at the five electrode potentials were

simultaneously fitted using three-slab model containing potential-dependent and potentialindependent parameters.

The three-slab model describes the SLD, ρ, as a function of a

displacement along the surface normal, z, as follows 

 1 + erf  ρ(z) =ρ0 + (ρi − ρi−1 )    i=1 4 X

where erf(z) =

√2 π

Rz 0

exp (−t2 )dt.

"

Pi−1

z− √

2

j=1

dj

2σi−1,i

#   ,  

(1)

ρi and di are the SLD and the thickness of the slab

i (i = 1 ∼ 3). ρ0 and ρ4 correspond to the SLD for the two bulk materials, Si and [THTDP+ ][PFPB− ] in the present study, and are fixed to 2.07 and 1.57 (×10−6 Å−2 ), respectively (Table 1).

All the other parameters were optimized (not fixed).

σi−1,i is the

roughness at the interface between the slabs i − 1 and i. In the three-slab model, the slabs 1 and 2 are used for the niobium electrode film, which are composed of the Nb and nativeoxide Nb2 O5 layers, and the slab 3 for the EDL on the IL side of the IL/WE interface (see Results and Discussion for the details). We did not include native-oxide SiO2 layer between Si and Nb in the fitting because it did not improve the fitting quality (chi-square).

It is

probably due to the SLD and the thickness for the SiO2 layer; the former, 3.47 (×10−6 Å−2 ), is in between those of Si and Nb (Table 1) and the latter is significantly smaller than that 7



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of the Nb film (360 Å). We also tried the four-slab model, by adding the second EDL layer between the EDL layer and the IL bulk. The chi-square value certainly decreased because of the larger number of fitting parameters but the fitted parameters for the EDL layers had significantly large standard errors due to their strong mutual correlations, which hindered us to discuss the results.

In the four-slab model, we did not find characteristics of the

alternately charged layers, which was proposed in a XR study on the electrode interface of an IL. 26

3

Results and Discussion

The reflectivity profiles (logR vs qz plots) for the IL/Nb/Si interface obtained at several electrode potentials are shown in Fig.1a with vertical offset for the clarity. All the profiles exhibited fringes with a single periodicity, 0.0175 Å−1 . The periodicity corresponds to 360 Å (= 2π/0.0175), which is the thickness of the niobium film (see below and Table 2 for the film thickness). Fig.1b shows magnified view of the profiles at relatively high qz region without the vertical offset. One can notice that the reflectivity profiles change with respect to the electrode potential.

At positive potentials, the fringe periodicity becomes shorter.

This

behavior is more prominent in the results of the reflectivity simulations where we fixed the EDL layer thickness to be the same (17 Å) for all the potentials (see Fig.S4). In contrary, at negative potentials, the reflectivity becomes higher. We note that the potential dependence in the reflectivity profiles in the present study is significantly larger compared with previous NR studies on the electrode interface of ILs. 46,48 This is probably due to the combination of large ionic sizes, high SLD contrast between the cation and anion, and low SLD of the electrode material. Both of these trends can be qualitatively explained with the potential dependence of the EDL structure. The former trend, shorter periodicity at positive potentials, indicates that the effective (apparent) thickness of the Nb film becomes greater at more positive potentials,

8


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with the help of the interfacial structuring of ions in the EDL. Because the SLD of Nb (and Nb2 O5 on the surface, see below) is higher than that of the IL bulk (Table 1), the “thickening” of the niobium film suggests that the EDL also has a SLD higher than the IL bulk. Such a high SLD is realized by the accumulation of PFPB− and the depletion of THTDP+ because PFPB− has higher SLD than THTDP+ (Table 1).

The anion accumulation and cation

depletion at the electrode interface at positive potentials agree with a simple electrochemical expectation.

The latter trend, which is higher reflectivity at negative potentials, will be

discussed below in detail but originates from the same electrochemical principle of the EDL behavior: the cation accumulation and anion depletion at negative potentials. To quantitatively assess the potential-dependent EDL structure, we performed simultaneous fitting of the model function to these experimental reflectivity profiles. The experimental reflectivity profiles are well reproduced by using the three-slab model (eq 1), in which two slabs are used for the niobium film and one slab for the EDL structure.

The obtained

SLD profiles are shown in Fig.2 and the fitted parameters are listed in Table 2 and 3 for potential-independent and potential-dependent ones, respectively. The niobium film electrode was represented as two layers: low-SLD layer with 307 Å thickness and high-SLD layer with 57 Å. The former and the latter layers are likely to correspond to Nb and Nb2 O5 , respectively. This composition agrees with previous surface studies on niobium metal films where researchers found that the Nb surface is naturally oxidized in air to form stable Nb2 O5 layer on the surface with a thickness of 6 nm. 73–76 The SLD values for the two layers, 4.17 and 5.64 (×10−6 Å−2 ), are both greater than 3.92 and 4.37, the calculated values for bulk Nb and Nb2 O5 , respectively (Table 1).

The higher SLD values are probably due to the

existence of other niobium oxides having higher SLD, such as NbO2 (5.31), NbO (5.19), and Nb2 O3 (6.09) in the two layers, as was suggested in previous reports. 73–77 The EDL structure exhibited clear potential dependence as shown in Fig.2b. The SLD of the EDL layer becomes higher at positive potentials and vice versa at negative potentials. For quantitative discussion, Fig.3a shows the potential dependence of the SLD value of the 9



EDL layer. At E = 0 V, the EDL layer exhibited the SLD almost same as that for the IL bulk (horizontal dotted line). This indicates that the potential, 0 V, is close to the potential of zero charge where the cation and anion in the EDL are at the same local concentration as in the IL bulk. In Fig.3b shown is the thickness of the EDL layer as a function of E. The thickness of the EDL layer at 0 V was determined to be 24 Å but without evaluation of the standard error, due to experimental difficulty with little SLD contrast between the EDL and IL bulk. Actually, even when we removed the EDL layer in the fitting, i.e., used two-slab model instead of three-slab model for the 0 V data, the residual sum of squares did not change. On the other hand, when the potential was switched to more positive or negative potentials from 0 V, the EDL layer clearly appeared in the SLD profiles (Fig.2b). The EDL thickness shown in Fig.3b is 17 Å on average at +2, −1, and −2 V, although at +1 V almost twice value 35 Å (with larger standard error) was obtained. This 17 Å thickness is similar to 15 Å, one ionic layer thickness found in our previous XR study for the ionic multilayers at the free surface of an IL, [THTDP+ ] bis(nonafluorobutanesulfonyl)amide, whose cation is common to the present [THTDP+ ][PFPB− ] and anion is smaller than PFPB− . 15 The 17 Å value is larger than 11 and 10 Å, the diameters for THTDP+ and PFPB− , respectively, estimated from quantum chemical calculations, but this discrepancy is acceptable because the calculation does not take into account inter-ionic volume and is known to underestimate the ionic sizes in ILs by about one third. 14,15,78 Therefore, the EDL structure observed in the present study is likely to be composed of one ionic layer. The SLD for the EDL structure becomes high (low) when the potential is more positive (negative) as shown in Fig3a.

As discussed qualitatively above, the potential-dependent

change in the SLD is ascribable to the accumulation and depletion of the cation and anion in the EDL. For quantitative discussion, we can estimate the upper and lower limits of the SLD, ρAL and ρCL , where the EDL is composed of the anion and cation layer, respectively. By assuming the cation and anion has the same size (the quantum chemical calculations provided 10% larger diameter for THTDP+ , see above), their values (in the unit of 10−6 Å−2 ) 10


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are: ρAL = 2ρPFPB− = 3.6 and ρCL = 2ρTHTDP+ = −0.5 (Table 1). These equations come from the SLD definition in the present study; ρPFPB− and ρTHTDP+ are defined per ion-pair volume, calculated using ρ =

P

j bj /VIP

where bj is the scattering length of atom j in the ion,

VIP = MIP /(NA σIL ) is the ion-pair volume, MIP is the molar mass of the ion pair, NA is the Avogadro constant, and σIL is the mass density of [THTDP+ ][PFPB− ] (1.253 g cm−3 ). We adopted this definition because the ionic volume (especially inter-ionic volume) is difficult to evaluate whereas the ion pair volume can be rigorously calculated as shown above. It should also be noted that if densification of ions 18,32,33,79 occurs in the EDL the absolute values for these parameters would increase. The SLD value of 3.2 at +2V is close to and slightly lower than ρAL , illuminating that at this potential the EDL layer is almost fully occupied with the anion. Oppositely, at −2 V the SLD value of 0.0 is comparable to ρCL , indicating that the cation layer is formed at the potential. These saturation of the ionic layer with either of the cation or anion is clear experimental evidence of ion crowding in the EDL in ILs, discussed in previous theoretical studies. 30,80 After we obtained the SLD profiles shown in Fig.2, now we can propose an explanation why the reflectivity increases at negative potentials (Fig.1b).

At negative potentials, the

EDL layer is enriched with the cation and therefore has SLD lower than both of IL and Si, the two bulk materials sandwiching the interfacial layers (Fig.2a).

In principle, when an

interfacial layer has SLD that is outside of the range between two bulk values, the reflectivity profile shows positive fringes (periodical increase in reflectivity) whose periodicity is determined by the layer thickness. In the present case, the cation-rich EDL layer with a thickness of 17 Å will cause such positive fringes with a long periodicity of 0.37 Å−1 (= 2π/17). The reflectivity increase at negative potentials in Fig.1b can be regarded as the onset of the long-period fringe. A simple simulation for the reflectivity and SLD profiles support this explanation (Fig.S5). In electrochemistry of ILs, among the characteristic parameters are the differential capacitance Cd and the surface charge density on the electrode Q. We evaluated Q from the 11



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SLD and the thickness of the EDL layer by using the following equation, 18 which was used in our previous analysis of XR data at the IL/water interface: Q = −Qfull

dEDL ρEDL − ρIL dion ρCL − ρIL

(2)

where Qfull = e/d2ion is the absolute upper limit of Q when the EDL layer is fully covered by the cation or anion and dion is the thickness of the one ionic layer (17 Å in the present study). Fig.3c shows Q as a function of E estimated using eq 2. The potential-dependence of Q is almost linear (dashed line), meaning that Cd is almost potential-independent (Cd = ∂Q/∂E), as proposed in the Helmholtz model 29 that assumes one-layer ion gathering at the electrode surface. The x intercept of the dashed line is at −0.14 V and close to 0 V, agreeing with the fitting results at 0 V where the SLD is almost the same as in the bulk. A closer look at Fig.3c allows us to notice that, at ±2 V, the |Q| values deviate from the linearity to lower (Fig.3a), indicating that Cd is decreasing at these extreme potentials where the EDL layer is fully occupied by either of the cation or anion. The decrease in Cd at potentials far from the potential of zero charge (close to 0 V in the present case as discussed above) is a trend opposite to the conventional Gouy-Chapman model 27,28 for the EDL in electrolyte solutions. On the other hand, this Cd trend was theoretically proposed 30 to be unique to the EDL in ILs and confirmed experimentally. 32,33 The EDL structure composed of one ionic layer found in the present study reminds us of the capacitor-like Helmholtz EDL model. 29 The Helmholtz model-like one ionic layer screening of the electrode potential agrees with previous suggestion by Baldelli et al. by using sum frequency generation (SFG) and electrochemical impedance spectroscopy (EIS) for the EDL in ILs. 81,82 A recent NR study also found one ionic layer at the electrode interface of a solution of an IL, whose cation is THTDP+ . 48 On the other hand, the EDL composed of one ionic layer cannot be explained with the diffuse layer from the conventional GouyChapman model. 27,28 This is also different from a feature for the EDL in ILs, alternately

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charged layers, found in previous studies using theory, 80 MD simulation, 31 and XR. 35 A main reason for the discrepancy would be the difference in the IL ions used. We used an IL composed of large and bulky ions to enhance the sensitivity of NR to the EDL structure. The charge on large ions is delocalized in the ionic structure. Hence, electrostatic interaction between neighboring ions, which is the major source of the formation of alternately charged layers, is relatively weak in such ILs. Thus, for large-ion ILs, even when the first ionic layer is highly charged, e.g., at ±2 V in the present study, the induction of the oppositely-charged second ionic layer should not be strong. Furthermore, other interactions such as π-π and alkyl-alkyl interactions, may even attract like charge ions to the second ionic layer. Actually, in our previous XR study on the EDL structure in a large-ion IL at the IL/water interface, 18 we observed cationic bilayer, which is an ion crowding phenomenon 30,80 and seems to be caused by the interdigitation of the alky chains between the IL cations. As the ionic size decreases, the ion crowding type structure revealed in the present study will switch to the other structure, alternately charged layers at a certain size of IL ions. To further clarify the EDL structure in ILs, it would be desirable to focus on the ionic size dependence.

4

Conclusions

Neutron reflectometry clearly probed structural change of the EDL responding to the electrode potential at the electrode interface of an IL. The EDL structure depending on the potential was able to be quantitatively analyzed and discussed thanks to the large ionic sizes and high SLD contrast between ions. The EDL is composed of one ionic layer whose ionic composition vary with the potential.

An experimental evidence was obtained on the full

occupation of the ionic layer by either of the cation or anion at potentials far from the potential of zero charge, where ion crowding occurs. Ion size dependence will be an interesting subject for further study. The present study will also be extended to metal ion-containing ILs because studies for the applications of ILs to lithium ion batteries 83 and electrodepo-

13



sition electrolytes 84 found that their performances are strongly IL-dependent and IL ions coexisting with metal ions at the electrode interface seem to play a crucial role.

Acknowledgement This work was partly supported by Grant-in-Aids for Scientific Research (Nos. 26410149, 26248004, 16H04216, and 18K05171). The neutron reflectivity experiment was performed at the Materials and Life Science Experimental Facility in J-PARC (Proposal Nos. 2013B0104, 2014A0172, and 2015A0054). This work has been carried out in part under the Visiting Researcher Program of Institute for Integrated Radiation and Nuclear Science, Kyoto University.

Supporting Information Available The following files are available free of charge. • NR21SupInfo.pdf: Structures of ions, cell sketch, cyclic voltammogram, reflectivity profile simulations.

References (1) Fedorov, M. V.; Kornyshev, A. A. Ionic Liquids at Electrified Interfaces. Chem. Rev. 2014, 114, 2978–3036. (2) Hayes, R.; Warr, G. G.; Atkin, R. Structure and Nanostructure in Ionic Liquids. Chem. Rev. 2015, 115, 6357–6426. (3) Zhang, S.; Zhang, J.; Zhang, Y.; Deng, Y. Nanoconfined Ionic Liquids. Chem. Rev. 2017, 117, 6755–6833.

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Table 1: Scattering length density calculated for the used materials. Material

a

SLD (10−6 Å−2 ) [THTDP+ ][PFPB− ] 1.57 THTDP+ −0.24a PFPB− 1.80a Si 2.07 Nb 3.92 Nb2 O5 4.37 Ionic scattering length per ion-pair volume in [THTDP+ ][PFPB− ] (see text for the detail).

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Table 2: Fitting parameters independent of the electrode potentials.a SLD (10−6 Å−2 ) ρSi ρNb ρNb2 O5 ρEDL ρIL a

Thickness (Å)

Roughness (Å) σSi−Nb 307 ± 4 σNb−Nb2 O5 57 ± 4 σNb2 O5 −EDL -e σEDL−IL

2.07 b 4.17 ± 0.15 dNb 5.64 ± 0.22 dNb2 O5 -e dEDL b 1.57 For potential-dependent parameters, see Table 3. b Fixed (see Table 1). c Constrained with a lower limit of 3 Å. d Constrained to be the same value. e Potential dependent and listed in Table 3.

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3 9 8 8

+6

c

±9 ±3 ±3

d d

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Table 3: Fitting parameters depending on the electrode potentials.a

b

E ρEDL dEDL (ρEDL − ρIL )dEDL Q −6 −2 (V) (10 Å ) (Å) (10−6 Å−1 ) (µC cm−2 ) +2 3.2 ± 0.7 18 ± 5 +29 ± 7 +5.6 ± 1.5 +1 2.1 ± 0.3 35 ± 15 +20 ± 8 +3.7 ± 1.5 b b 0 1.5 ± 0.4 (24) (−2) (−0.4) b −1 0.7 ± 0.6 17 ± 9 −16 ± 3 −2.8 ± 0.6 −2 0.0 ± 0.6 16 ± 5 −25 ± 3 −4.7 ± 0.6 a For potential-independent parameters, see Table 2. Optimized but the standard error was too large to be evaluated.

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Fig.1.

(a,b) Neutron reflectivity profiles (as a function of momentum transfer) at the

IL/Nb/Si interface. Electrode potential: −2 (purple inverse triangle), −1 (blue diamond), 0 (green circle), +1 (orange square), and +2 (red triangle) V. The curves are obtained from the fitting with the three-slab model. In (a), profiles are vertically offset for clarity.

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Fig.2. (a,b) Scattering length density profiles (as a function of surface-normal displacement) at the IL/Nb/Si interface. Electrode potential: −2 (purple), −1 (blue), 0 (green), +1 (orange), and +2 (red) V. The vertical dotted lines are located at the interface between layers. The dashed lines in (b) are hypothetical profiles with a zero roughness for clarity.

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Fig.3. Potential dependence of (a) the scattering length density and (b) the thickness of the EDL layer, and (c) the surface charge on the electrode. In (a), the horizontal dotted and upper and lower dashed lines are located at the SLD for the IL bulk, the anionic layer, and the cationic layer, respectively. In (b), horizontal dotted lines are at the thickness of one ionic layer, dion multiplied by 0, 1, 2, and 3. In (c), dashed line is the line connecting the points at ±1 V. 30


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Graphical TOC Entry

Neutron reflectometry Ionic liquid Electrode

Positive E

Negative E

Substrate

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Potential-Dependent Structure of the Ionic Layer at the Electrode

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