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Proton-Rocking-Chair-Type Redox Capacitors Based on ITO Electrodes with Multilayer Films Containing Ru Complexes Kai Yoshikawa, Daisuke Motoyama, Yusuke Hiruma, Hiroaki Ozawa, Shusaku Nagano, and Masa-aki Haga ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05907 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

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ACS Applied Materials & Interfaces

Proton-Rocking-Chair-Type Redox Capacitors Based on ITO Electrodes with Multilayer Films Containing Ru Complexes Kai Yoshikawa,1 Daisuke Motoyama,1 Yusuke Hiruma,1 Hiroaki Ozawa,1 Shusaku Nagano,2 and Masa-aki Haga*1 1

Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-

27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan 2

Nagoya University Venture Business Laboratory, Furo-cho, Chikusa, Nagoya, 464-8603,

Japan

KEYWORDS: Ru complex, proton-coupled electron transfer, layer-by-layer method, multilayer structure, redox capacitor

ACS Paragon Plus Environment

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ABSTRACT: A rechargeable proton-rocking-chair-type redox capacitor was fabricated using scalable layer-by-layer-(LbL)-assembled films composed of two dinuclear Ru complexes that exhibit proton-coupled electron transfer (PCET) reactions with different Ru(II/III) redox potentials (RuNH-OH and RuCH-OH). RuNH-OH and RuCH-OH contain different coordination environments that involve two phosphonate linker ligands at both ends and bridging 2,6,2’,6’tetrakis(benzimidazol-2-yl)-4,4’-bipyridine or 1,3,1’,3’-tetrakis(benzimidazol-2-yl)-5,5’-biphenyl ligands, respectively. The molecular units were assembled onto ITO electrodes by complexation between the phosphonate groups and zirconium(IV) ions. The LbL growing process of these multilayer films was monitored by electrochemical or UV-Vis spectroscopic measurements. The thus obtained LbL films on the ITO electrodes showed PCET reactions at different potentials, depending on the bridging ligands. The introduction of cyclometalated Ru-C bonds in the bridging ligand of RuCH-OH led to the stabilization of the ruthenium(III) oxidation state, and therefore, RuCH-OH exhibited lower pKa values for the imino N-H protons in the bridging benzimidazole groups compared to those of the corresponding RuNH-OH complex. The proton movements that accompany the redox reaction in the Ru multilayer films on the ITO electrode were confirmed using a pH indicator probe. For the performance test of a proton-rocking-chair-type redox capacitor, a two-electrode system composed of RuNH-OH and RuCH-OH multilayer films on ITO electrodes was examined in an aqueous solution of NaClO4. Under galvanostatic conditions, stable, reversible, and repeatable charging-discharging processes occurred. The capacitance increased with increasing number of LbL layers. For a comparison, a similar redox capacitor composed of two RuNMe-OH and RuCMe-OH analogues, in which all four imino N-H protons on the benzimidazole moieties are protected by N-Me groups, was constructed and examined. In these complexes, the capacitance decreased by 77% compared to the PCET-type capacitor


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composed of a cell containing RuNH-OH and RuCH-OH; this result strongly suggests that the proton movement plays a more important role for the charge storage than the anion movement. In such LbL films composed of Ru complexes that exhibit PCET-type redox reactions, the capacitance is drastically improved with increasing number of layers and using protons as charge carriers.

Introduction Energy-storage devices have recently received much attention as they are indispensable in storing the energy harvested from renewable power sources, and releasing the energy that supports especially the now ubiquitous portable electronic devices. Numerous energy-storage devices such as rechargeable batteries or supercapacitors have been developed so far.1 Among these, the electric double layer capacitor (EDLC) exhibits some attractive advantages that include high capacitance, quick charging/discharging, and stable reusability, which are due to the simple cation/anion moving processes.2 In addition, pseudocapacitors, which represent the extended version of supercapacitors, employ redox-active materials such as metal oxide nanoparticles,3-4 framework structures,5-6 organic or inorganic compounds,7-9 or nanocarbon materials.10-11 As the faradaic reactions on the surfaces of electrodes are responsible for the charging capacity, the performance of pseudocapacitors can be improved not only by increasing the surface area of the electrodes, but also by tuning the functions of the redox-active species.12 For example, the rocking-chair-type redox capacitor is widely used in lithium-ion batteries.13-15 In the charging/discharging process for the lithium-ion batteries, Li ions were released upon the oxidation of lithium cobalt (III) oxide for the discharging process, and under the charging process the Li ions were reinserted into the layered electrode. This type of charging/discharging process is compared to a swing of “rocking-chair”


3


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on the floor, and the device has been often called as “rocking-chair-type” battery. Similar to the Li ion, proton-based rocking-chair-type redox capacitors have aroused wide-spread interest in recent years, especially with regard to their environmental sustainability. For rocking-chair redox capacitors or all-organic proton batteries, the dissociable proton sites of the quinone/hydroquinone couple have been used.16-19 Other studies have reported capacitors that use proton-conductive polymers as electrolytes or electrodes.20-21 However, examples for such studies, which focus on proton-based energy devices, remain limited. As the concentration gradients of protons between biomembranes, which can be generated by the simultaneous movement of protons and electrons, is the driving force for the production of energy in biological systems,22-23 the proton-coupled electron transfer (PCET) reaction should promise great potential in a variety of chemical and biological processes.24-26 The PCET reaction of biological systems have been mimicked using artificial molecular systems, for example in the context of artificial photosynthesis using protonresponsive molecules.27-30 We have reported PCET reactions of redox-active benzimidazoleligated ruthenium complexes in solution and on solid surfaces.31-37 The tridentate 2,6bis(benzimidazol-2-yl) pyridine (H2bimp) ligand contains two imino NH groups and thus behaves as a dibasic acid in solution. In addition, mono- or dinuclear Ru complexes of the H2bimp ligand show different pKa values, depending on the oxidation state of the Ru center. For example, [Ru(H2bimp)(Mebimp)] (Mebimp = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine; for the chemical structure, see Scheme 1) exhibits pKa values of 6.3 and 7.9 in CH3CN/Britton-Robinson (BR) buffer (1/1, v/v) for the Ru(II) state.36 However, when [Ru(H2bimp)(Mebimp)] is oxidized to the Ru(III) state, the pKa values decrease to 11.0

-

-

a11

RuNHOEt RuCHOEt

a12

a13

a14

pK

pK

pK

pK

-

< 2.3

2.5

3.8

5.2

7.3

9.1

9.8

(4.9)

(7.2)

(8.9)

(9.2)

a21

a22

a23

a24

a)

The notations for the pKa values are based on the PCET equilibria in Scheme 3. The pKa values in parentheses were determined by spectrophotometric titrations (Figures S2 and S3). Two clear trends can be observed in Table 1: firstly, the pKa values decrease with increasing the oxidation state from Ru(II)-Ru(II) to Ru(III)-Ru(III); secondly, the pKa values for RuCH-OEt are higher than those for RuNH-OEt, indicating that RuCH-OEt is more basic than RuNH- OEt. Recently, we have reported the construction of energy-storage redox batteries based on the mononuclear Ru complexes [Ru(H2bimp)(Mebimp)] and [Ru(H2bimb)(Mebimp)] (Scheme 1), in which a charging process in neutral unbuffered solution induces a potential gradient between [Ru(H2bimp)(Mebimp)] and [Ru(H2bimb)(Mebimp)] and keeps the energy as a proton gradient between two cells.36 Immobilizing these two Ru analogues on two electrodes thus represents a continuation of this work, i.e., new proton-rocking-chair-type redox capacitors based on multilayer films of RuNH-OH and RuCH-OH can be constructed. As each dinuclear Ru complex involves two electrons per molecular unit, the capacitance is expected to increase.


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Fabrication of Ru multilayer films on ITO substrates using the Layer-by-Layer (LbL) method In order to generate well-defined LbL films, the formation of the first layer is crucial. From previous studies on the surface assembly of analogous Ru complexes with tetraphosphonic acid anchors, we gained optimal experimental conditions for the fabrication of the homogeneously dense packed monolayer films on ITO electrodes. The protocol for the immobilization of the primer layer is based on the immersion of the ITO electrode into a solution of the Ru complex for at least 3 h, followed by a gentle heating-drying treatment. Subsequently, for the formation of the 2nd layer, the modified ITO substrate was immersed for 30 min in an aqueous solution of ZrOCl2 (20 mM) to promote the binding between the Zr(IV) ions and the free phosphonic acid groups on the surface of the primer layer. After washing with copious amounts of water and drying under a flow of N2, another Ru layer was deposited on the outmost Zr-site on the ITO|(RuNH-OH or

b

a

Figure 2. The change of UV-Vis absorption spectra (a) and cyclic voltammograms in 0. 1 M HClO4 aqueous solution (b) for RuNH-OH upon the increase of the number of layers from 1, 3, 5, and 10 on ITO substrates.


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RuCH-OH)/Zr/(RuNH-OH or RuCH-OH) bilayer film. By repeating this alternating immersion treatment using solutions of ZrOCl2 or the Ru complex, multilayer LbL films were fabricated in scalable manner.37, 39 The growth of these multilayers using the LbL method was monitored by UV-Vis spectroscopy and cyclic voltammetry. Figures 2a and 2b show the changes of UV-Vis absorption spectra and cyclic voltammograms for RuNH-OH upon increasing the number of layers from 1 to 10. The UV-Vis spectra for RuNH-OH showed a ligand-centered p-p* transition at 380 nm and a metal-to-ligand charge transfer (MLCT) band at 555 nm, which is characteristic for the Ru(II) oxidation state. As shown in the insert of Figure 2a, the linear plot of the absorbance at 555 nm as a function of the number of layers indicated the formation of a lamellar structure on the ITO substrate. A similar linear relationship for the plot of absorbance as a function of the number of layers at 430 nm was observed for RuCH-OH (see supporting information in Figure S5), although the spectral feature for RuCH-OH is significantly different from those of RuNH-OH given the Ru(III)-Ru(III) oxidation state in RuCH-OH. The absorption maximum at 430 nm was assigned to the ligand-centered p-p* transition of Ru(III)CH-OH. Comparing the UV-Vis spectra in solution to those in the solid films revealed little change for RuNH-OH and RuCH-OH under the acidic condition, which indicates that specific interactions between the Ru complex do not occur in the solid state (Figure S4). X-ray photoelectron spectroscopy (XPS) measurements of the ITO substrates decorated with the Ru complexes revealed the C 1s, Ru 3d, N 1s, Zr 3d, and P 2p peaks, which arise from the ITO electrode with Ru-complex-modified surfaces (Figures S7 and S8). The intensity of the XPS signals strongly depends on the depth of the element of interest relative to the sample surface. The relative XPS signal ratios for C(1s)/In(3d) and C(1s)/Sn(3d) from the bottom ITO substrate increased with increasing number of layers. Therefore, the observed intensity ratios


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reveal that the LbL multilayer films containing Ru complexes grow in a well-defined manner on the ITO surface. In order to evaluate the thickness and homogeneity of the films, we conducted Xray reflectivity (XRR) measurements on LbL multilayer films that were deposited on a sapphire substrate. The experimental and fitted XRR plots for one, three, and five layers of RuNH-OH are shown in Figure 3. The number of fringe peaks increased upon increasing the number of layers. The fitting results for simulating the multilayered films as a uniform single layer are shown in Table 2. Based on the simulation of the XRR curve, the density of the LbL multilayer films should be approximately 1.3-1.4 g cm-1, which suggests that homogeneous films were formed on the sapphire surface. The thickness of the LbL multilayer films was estimated to be approximately 1.1 (monolayer), 6.9 (three layers), and 8.7 nm (five layers). The film thickness increased almost linearly with the number of layers. These results indicate that the LbL method affords homogeneous films whose thickness depends on the number of molecular layers of Ru complexes. Furthermore, AFM measurements have been performed in order to get the information of surface morphology. The AFM images for the original ITO surface after the RCA treatment and the modified ITO by five layered RuNH-OH are shown in Figures S9a and S9b. Although the roughness of the original ITO surface was relatively small within ±1 nm, the densely-packed spherical domain morphology was observed after the immobilization of five layered RuNH-OH films on the ITO surface.


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Figure 3. XRR profiles of monolayer or multilayer (three or five layers) films of RuNHOH that were deposited onto a sapphire substrate. Experimental and fitted data are shown as black dots and red curves, respectively.

Table 2. Simulated parameters obtained from XRR experiments of monolayer or multilayer (three or five layers) films of RuNH-OH on a sapphire substrates. Number of layers

Density / g cm

Thickness / nm

Roughness / nm

substrate

3.97

–

0.100

1

1.3 ± 0.3

1.1 ± 0.1

0.44 ± 0.15

3

1.38 ± 0.03

6.98 ± 0.03

0.684 ± 0.03

5

1.35 ± 0.03

8.69 ± 0.03

0.703 ± 0.03

-3


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Redox properties of RuNH-OH and RuCH-OH immobilized on ITO electrodes Cyclic voltammograms of monolayer films of RuNH-OH and RuCH-OH deposited on ITO electrodes in aqueous solutions of NaClO4 (0.1 M) and HClO4 (0.1 M) are shown in Figure 4. In an aqueous solution of NaClO4 (0.1 M), the reversible two-electron Ru(II)-Ru(II)/Ru(III)-Ru(III) redox waves for RuNH-OH and RuCH-OH were observed at 0.22 V (DEp = 123 mV) and -0.13 V (DEp = 35 mV) vs. Fc+/Fc, respectively. In an aqueous solution of HClO4 (0.1 M), these redox potentials shifted to 0.38 V (DEp = 29 mV) and -0.06 V (DEp = 38 mV), respectively, whereby the half-width of the CV waves sharpened.

Figure 4. Cyclic voltammograms for monolayers of (a) RuNH-OH and (b) RuCH-OH deposited on an ITO electrode in aqueous NaClO (0.1 M; dotted line) or HClO (0.1 M; 4

4

solid line); scan rate: 100 mV/s.

From the cyclic voltammograms of the ITO-immobilized RuNH-OH and RuCH-OH monolayers, the surface coverage for both Ru complexes [(4.0 ± 0.2) × 10+,, 𝑚𝑜𝑙 𝑐𝑚+1 ] was obtained, which is consistent with the calculated values ( 4.4 × 10+,, 𝑚𝑜𝑙 𝑐𝑚+1 ) for ITO


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electrodes fully covered with the Ru complexes. This surface coverage is comparable to previously reported values.37, 39 Under neutral conditions, a broad redox wave was observed for the Ru center, which suggests that chemical equilibria between the different protonated species of the Ru complex immobilized on the ITO surface are present in neutral aqueous solution, and that the addition of acid induces a shift of the equilibrium in favor of the protonated forms. However, the phosphonate-immobilized Ru complexes can be easily desorbed from the ITO surface under basic conditions (pH > 8). With increasing number of layers, the total amount of electric charge increased linearly, which indicates that homogenous, well-defined multilayer films were formed on the ITO electrode.

pH Dependence of the redox potentials and the proton movement in ITO-immobilized multilayers of RuNH-OH and RuCH-OH As described in the previous section, the peak potentials of the immobilized Ru complexes on the ITO electrode strongly depend on the pH value of the solution. The ITO-immobilized Ru complexes are generally stable at pH < 8, even in an aqueous solution of HClO4 (0.1 M). Furthermore, a pH dependence of the E1/2 value of ITO-immobilized RuNH-OH was observed at 2 < pH < 7.5, which indicates that the oxidation of RuNH-OH is accompanied by a proton movement in this region, considering the Pourbaix diagram (Figure S10). We have previously reported the spectroelectrochromic response for the redox reaction of Ru complexes immobilized on ITO electrodes with two Pt wires as the quasi-reference and the counter electrodes.35 The oxidation of immobilized RuNH-OH induced a proton release from the imino-N-H groups in neutral unbuffered solution, which was monitored using pH indicator UV probes such as 5(6)carboxyfluorescein (pKa = 6.3). In order to confirm the reversibility of the proton-release/-capture


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in RuNH-OH upon oxidation, we measured the electrochromic response of a film, consisting of ten layers of RuNH-OH immobilized on an ITO electrode, with 5(6)-carboxyfluorescein in solution. The monitored wavelengths were carefully chosen as 480 nm and 555 nm, given that i) the former represents the isosbestic point for the Ru(II)-Ru(III) process of ITO-immobilized RuNH-OH, i.e., the pH change can be monitored exclusively using the pH indicator UV probe 5(6)-carboxyfluorescein (Figure S11); ii) the latter represents the spectral change for the MLCT transition, which is accompanied by the Ru(II)-Ru(III) redox process. As 5(6)-carboxyfluorescein does not absorb any UV-Vis irradiation at 555 nm, the redox reaction of the immobilized Ru complex is monitored exclusively. When a potential pulse (30 s interval) from -0.4 V to +0.3 V vs Pt (open circuit potential vs. Pt) was applied, the MLCT absorbance at 555 nm decreased due to the oxidation of Ru(II) to Ru(III) in RuNH-OH. At the same time, a small decrease of the absorbance at 480 nm was observed, which corresponds to the increase of the proton concentration, which arises from the proton release from ITO-immobilized RuNH-OH (Figure 5).

Figure 5. Changes of the UV-Vis absorbance at 555 nm (blue) and 480 nm (red) upon the application of continuous potential pulse sequence for RuNH-OH (10 layers) in 0.1 M NaClO in the presence of the pH indicator probe 5(6)-carboxyfluorescein (16 µM). 4


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Setting the original potential to -0.4 V vs. Pt for 30 s, the absorbance at both 555 and 480 nm returned to their original values. The continuous application of potential pulses revealed a reproducible absorbance change for both wavelengths, suggesting that proton release/capture on RuNH-OH is associated with the Ru(II)/Ru(III) redox reaction. From the UV titration curve at 480 nm for 5(6)-carboxyfluorescein, the pH change, i.e., the change of the proton concentration, was estimated ( 3.3 × 10+,3 mol cm+1 ) . Considering the surface coverage of RuNH-OH (5.7 × 10+,3 mol cm+1 ) obtained from the CV measurements, the release/capture of one proton was involved with the redox reaction of the immobilized RuNH-OH in neutral unbuffered solution. Similar experiments were carried out for RuCH-OH LbL films. The electrochromic response in the UV-Vis spectra revealed isosbestic points at 480 nm and 555 nm, and these wavelengths were selected as monitoring wavelengths for the pH change; alas, only insubstantial changes were observed, which suggests that the proton movement from RuCH-OH was not detected in this pH probe system probably due to the higher pKa value of RuCH-OH (Figure S9). To gain a better understanding of the PCET reaction in the ITO-immobilized RuNH-OH multilayer films, electrochemical quartz crystal microbalance (EQCM) experiments were carried out. For comparison, RuNMe-OH films, in which the N-H imino group on benzimidazole is protected with a methyl group, were also studied. The EQCM technique is a useful and sensitive method for detecting small mass changes of redox-active materials on the electrode surface as a result of redox process.44, 45 Figure 6 shows the EQCM frequency responses of ITO-immobilized multilayer (ten layers) films of RuNH-OH and RuNMe-OH as a function of the potential together with cyclic voltammograms in aqueous NaClO4 solution. For RuNMe-OH, the resonant frequency of the RuNMe-OH film decreased upon Ru(II)/Ru(III) oxidation (Figure 6a). Conversely, only a relatively small frequency change was observed for the RuNH-OH films (Figure 6b).


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Figure 6. EQCM frequency responses of ITO-immobilized multilayer (10 layers) films of (a) RuNMe-OH and (b) RuNH-OH together with their cyclic voltammograms in aqueous NaClO (0.1 M) solution. 4

The change of frequency (DF) corresponds predominantly to the mass changes on the crystal surface, which can be quantified by the Sauerbrey equation: ∆𝐹 = −

1 A B ∆D >?@ C E

= −2.2640 × 10+G 𝐹 1

∆D E

(1)

wherein m is the mass per unit area, n the harmonic number of the oscillation (n = 1), µ the shear modulus of quartz ( 𝜇 = 2.947 × 10,, g cm+, s+1 ), and r the density of quartz ( 𝜌 = 2.648 g cm+N ). The EQCM measurements of the RuNMe-OH multilayer films revealed a decrease of frequency by 12.7 Hz on the EQCM surface, which is consistent with a mass change of 13.3 ng according to equation 1. The Ru(II)/Ru(III) oxidation of the immobilized RuNMe-OH film induced the formation with ion pairs with ClO4- anions on the surface of the films, and the concomitant increase in weight led to the decrease of the resonant frequency. This change is fully reversible during the entire potential scan. On the other hand, the ITO-immobilized multilayer film of RuNH-OH exhibited a PCET reaction, in which the charge imbalance was cancelled out by the proton release/capture on the redox reactions of protonated Ru(II)-NH/deprotonated Ru(III)-N.


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Consequently, the resonance frequency of the RuNH-OH multilayer film in 0.1 M NaClO4 showed a smaller change (5.4 Hz), which corresponds to mass change of 5.3 ng upon oxidizing Ru(II) to Ru(III). Interestingly, for EQCM measurements of the RuNH-OH multilayer film under acidic conditions (0.1 M HClO4), a higher DF value was observed compared to that under neutral conditions (0.1 M NaClO4) (Figure S13). For example, for the multilayer film of RuNH-OH, the frequency decrease (16.7 Hz) was almost comparable to that for the RuNMe-OH multilayer film. Considering the Pourbaix diagram, the multilayer film of RuNH-OH under acidic conditions (pH = 1; 0.1 M HClO4) exhibits predominantly a two-electron redox reaction with no proton transfer, which stands in sharp contrast to its behavior under neutral conditions (0.1 M NaClO4). Therefore, in 0.1 M HClO4, the oxidation of the RuNH-OH film requires the insertion of the counter anions for the charge compensation on the ITO surface.

Fabrication of a redox capacitor based on multilayer films of RuNH-OH and RuCH-OH Subsequently, we fabricated a proton-rocking-chair-type redox capacitor for energy devices based on two proton-responsive LbL films of RuNH-OH and RuCH-OH (Figure 7). A neutral aqueous electrolyte solution (0.1 M NaClO4) was deposited between two electrodes that consisted of (RuNH-OH)n and (RuCH-OH)n multilayer films (n: number of layers; n = 10, 20, 30, 40, or 50 layers) using silicon spacers (ID: 10 × 10 × 1 mmN ). Chronopotentiometric measurements at a constant current of 10 µA cm-2 were then carried out on the two electrode cells (Figure 7).


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(a)

(b)

Figure 7. Schematic illustration of the (a) components and (b) two-electrode cell for the proton-rocking-chair-type redox capacitor in neutral unbuffered aqueous solution.

For the charging process, a constant current was applied at the anode in order to induce the oxidation of the original Ru(II)NH-OH multilayer, which should release protons via the PCET reaction on RuNH-OH and thus provide the driving force for the proton movement to the counter electrode. At the same time, the Ru(III)CH-OH multilayer films on the cathode is reduced to Ru(II) under galvanostatic conditions, which leads to the capture of protons in the RuCH-OH multilayer film. Figure 8(a) exhibits the galvanostatic charge-discharge cycles of the two-electrode redox capacitor at a loading current density of 10 µA cm-2. Between 0 and 0.8 V, nonlinear charging and discharging curves were observed, indicating that not only the movements of anions or cations in the electrolyte solution, but also the faradaic reactions on the immobilized Ru complexes occur simultaneously during the charging/discharging processes.


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Figure 8. (a) Effect of the number of layers on the galvanostatic charge/discharge curves of ITO|(RuNH-OH) ||(RuCH-OH) |ITO (n = 10~50), and (b) plot of the capacitance as a n

n

function of the number of layers at a constant current density of 10 µA cm . -2

Based on the galvanostatic charge-discharge curve, the capacitance (C) can be calculated by: PQ

𝐶 = ∆R where C, I, t, and density (I = 10

(2)

V represent the area-specific capacitance (in F cm-2), the discharge current A cm-2), the discharge time (in s), and the potential window (V = 0.8 V),

respectively. The dependence of the capacitance in the present redox capacitors on the number of layers in ITO|(RuNH-OH)n||(RuCH-OH)n|ITO for the galvanostatic charge/discharge curves are shown in Figure 8. Increasing the number of layers of the ITO-immobilized multilayer films of the Ru complex led to higher capacitance values (Table S2). Interestingly, the capacitance of the multilayer films increased linearly with increasing number of layers. This result strongly indicates that all the layers of Ru-complex-containing multilayer films are redox active and contribute to the charging/discharging. This proton-rocking-chair-type redox capacitor is stable between 0 and 0.8 V with 86% coulombic efficiency. Furthermore, the cycle test showed repeatable charging/discharging retaining 60% of capacitance and 80% coulombic efficiency after 1000 cycles (Figure S14). In order to confirm the effect of the PCET reaction on the performance of the


22

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redox capacitor, the combination of methyl-protected RuNMe-OH and RuCMe-OH, which have no dissociative proton sites, were chosen as electrodes in a control experiment.

Figure 9. Comparative galvanostatic charging/discharging curves for ITO|(RuNHOH) ||(RuCH-OH) |ITO (red) and ITO|(RuNMe-OH) ||(RuCMe-OH) |ITO (green) 20

20

20

20

(current density: 10 µA cm ). -2

As shown in the Figure 9, the capacitance for the charging/discharging process of ITO|(RuNMeOH)20||(RuCMe-OH)20|ITO decreased by 77% compared to that of ITO|(RuNH-OH)20||(RuCHOH)20|ITO (Figure S15 and Table S2). Thus, the protons should behave as more efficient charge carriers than the sodium ions in the electrolyte solution. With increasing charge-current density under galvanostatic conditions for ITO|(RuNH-OH)20||(RuCH-OH)20|ITO, the capacitance decreased


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Figure 10. (a) Galvanostatic charging/discharging curves for ITO|(RuNH-OH) ||(RuNH10

OH) |ITO (current density: 1 µA cm ) and (b) the comparison for the capacitance values -2

10

of ITO|(RuNH-OH) ||(RuCH-OH) |ITO (red line) and ITO|(RuNH-OH) ||(RuNH10

10

10

OH) |ITO (purple line). 10

(~100 µA cm-2) with increasing number of layers in the multilayer films (n = 10, 30, or 50) (Figure S16). As the resistance increases with increasing number of layers in the multilayer films, the proton movement was restricted inside the confined space in the LbL multilayer nanostructure. On the other hand, the symmetric capacitor composed of ITO|(RuNH-OH)10||(RuNHOH)10|ITO showed much lower capacitance than that of ITO|(RuNH-OH)10||(RuCH-OH)10|ITO in 1 µA cm-2 (Figure 10). The almost linear charging/discharging curve for ITO|(RuNHOH)10||(RuNH-OH)10|ITO suggested the possibility of charging without any redox reactions for Ru(II)NH-OH on the cathode. Furthermore, the capacitance for the symmetric capacitor was a little too small to be measured at 10 µA cm-2. The Ragone plot, i.e., the plot of the values of the specific energy density (in W h kg-1) as a function of the specific power density (in W kg-1), affords a clear chart used for the performance comparison of various energy-storage devices (Figure 11).2, 46


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Figure 11. The Ragone plot of various energy storage devices.

This chart shows the energy density E (W h kg-1) and power density P (W kg-1), both of which are important performance parameters for energy-storage devices and calculated by: 𝐸=

TU (∆R)B 1

T

PQ

(Capacitance values: 𝐶D = D = D∆R) 𝑃=

W Q

(3.1) (3.2)

where Cm is the mass-specific capacitance (in F kg-1), m the mass of the active material (in kg). In general, the electric double layer capacitor (EDLC) is located at the upper left region in the Ragone plot given its high power and low energy density. Conversely, the battery is located at the lower right side in this plot, considering its low power and high energy density. The supercapacitor, whose charging system consists of the redox reaction and the electric double layer, is positioned between the capacitor and battery. In the present 20-layer proton-rockingchair-type redox capacitor, E and P values of 1.84 W h kg-1 and 311 W kg-1 were calculated respectively, and therefore, the capacitor is placed in the middle of the supercapacitor region in the Ragone plot.


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Experimental Section. Preparation of the multilayer films of RuNH-OH and RuCH-OH on the ITO electrodes Aqueous solutions of RuNH-OH and RuCH-OH (50 µM; pH = 5), an aqueous solution of ZrOCl2 (20 mM), and an indium-tin oxide (ITO) electrode (Kuramoto Co.; surface resistance < 10 W square-1; surface roughness, Ra < 0.5 nm) were used for the preparation of the multilayer films on the ITO electrodes. The homolayer films ITO|(RuNH-OH)n and ITO|(RuCH-OH)n were fabricated by controlling the number of immersions according to previous reports from our group.37, 39 Fabrication of the redox capacitors A silicon spacer (ID : 10 ´ 10 ´ 1 mm3) was sandwiched between two ITO electrodes with (RuNH-OH)n and (RuCH-OH)n, or (RuNMe-OH)n and (RuCMe-OH)n multilayer films (n: number of layers), followed by injection of a neutral aqueous electrolyte solution of NaClO4 by micro syringe through a needle tube in order to release packed air. For the cell performance test, the two modified ITO electrodes were connected to the ALS/CH Model 660A electrochemical analyzer. Physical measurements UV-Vis absorption spectra (300-800 nm) were obtained from a Hitachi U-4000 spectrometer. NMR spectra were recorded on a 500 MHz JEOL JNM-ECX500 spectrometer. Mass spectra for the Ru complexes were measured using a Micromass LCT electrospray mass spectrometer equipped with an electrospray interface (ESI) system. X-ray photoelectron spectra were measured using a Shimadzu/Kratos Axis HSi spectrometer with monochromated Al Ka radiation as an


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excitation source (l = 0.8339 nm). Binding energies were calibrated using the C 1s binding energy (285.0 eV) as a reference. All electrochemical measurements were carried out using an ALS/CH Model 660A electrochemical analyzer. The cyclic voltammetry measurements were performed in a standard one-compartment cell at 25 °C. A glassy carbon electrode or homolayer ITO|(RuNHOH or RuCH-OH)n films were used as the working electrode, while a Pt wire was used as the auxiliary electrode. The reference electrode was Ag/AgCl. All potentials are referenced against the Fc+/Fc couple. The spectro-electrochemistry was studied using a custom-made thin-layer cell, in which the multilayer-film-modified ITO electrodes were employed as the working electrode, while two Pt wires were used as the reference and counter electrodes. The galvanostatic charging/discharging measurements were carried out in the two-electrode configuration using homolayer ITO|(RuNH-OH or RuNMe-OH)n as the anode and ITO|(RuCH-OH or RuCMeOH)n as the cathode, as well as a constant current of 10 µA cm-2 and a potential window of 0.8 V. The EQCM measurements were conducted using an AT-cut quartz crystal resonator coated with thin ITO electrode pads (polished for mirror finishing; SEIKO EG&G QA-A9M-ITO(M) chip) that were used to immobilize RuNH-OH or RuNMe-OH on the ITO-coated quartz crystal resonator. The thickness and density of the films were measured by X-ray reflectivity (XRR) using a Rigaku high-resolution X-ray diffractometer (ATX-G). The diffractometer was equipped with a Cu rotating anode (50 kV, 300 mA) that generated Cu-Kα radiation (λ = 0.1542 nm). The reflective oscillation fringes from the experiment were fitted using Rigaku GXRR software and a single layer model (incident-angle range: 0.1-2.1°). Conclusions


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Two dinuclear Ru complexes with tetraphosphonate groups, RuNH-OH and RuCH-OH, which possess different redox potentials and exhibit inherent PCET reactions, were used as molecular components for the LbL growth of multilayer films. These LbL films were fabricated by the successive coordination bonding between four phosphonate groups on the Ru complexes at both ends and Zr(IV) ions. XPS and XRR measurements revealed the formation of well-ordered LbL films on solid ITO surfaces. The proton movement on multilayer-film-modified ITOs was proved indirectly by monitoring the absorbance change for a pH indicator in solution and by the mass change that accompanies the oxidation on the multilayer-film-modified ITO EQCM chips. A rechargeable proton-rocking-chair-type redox capacitor was constructed from two electrodes that consist of RuNH-OH or RuCH-OH LbL multilayer films immobilized on ITO and a neutral aqueous NaClO4 electrolyte solution. The capacitance of the multilayer films increased linearly with increasing number of layers. On the other hand, when methyl-protected RuNMe-OH and RuCMe-OH, which have no dissociative proton sites, were used as molecular components for the LbL films, the capacitance decreased by 77%. It should thus be the protons that play an important role for the improvement of the capacitance in this proton-rocking-chair-type redox capacitor. A Ragone plot revealed that a capacitor with 20 layers exhibits a relatively high energy density relative to conventional supercapacitors; furthermore, the energy density is scalable by increasing the number of the layers through a simple immersion method for LbL growth. Therefore, this new PCET-type redox capacitor may potentially be used for the construction of robust energy-storage systems that employ neutral aqueous electrolyte solutions.

ASSOCIATED CONTENT


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Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Synthesis; Electrochemistry of dinuclear Ru complexes; Pourbaix diagrams of RuNH-OEt and RuCH-OEt for two one-electron processes, together with those for two-electron oxidation process; pH dependence of UV-Vis absorption spectra, spectrophotometric titration data for ethylprotected RuNH-OEt and RuCH-OEt; the monitoring of LbL growth by UV-Vis spectra and CV measurements for RuNH-OH; XPS measurements of RuNH-OH and RuCH-OH (1-4 layers); AFM images for bare original ITO and the modified LbL ITO surface; Pourbaix diagrams for RuNH-OH and RuCH-OH on ITO electrodes; spectral responses to potential pulses for RuCHOH (10 layers) using a pH indicator probe; EQCM response of RuNH-OH (10 layers) under neutral or acidic conditions; results of cyclic charging/discharging tests for RuNH-OH/RuCHOH capacitors for 1000 times; capacitance data for RuNH-OH/RuCH-OH and RuNMeOH/RuCMe-OH capacitors; current-density dependence of the capacitance of RuNHOH/RuCH-OH capacitors. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (M. H.) Author Contributions The manuscript was written by contributions from all authors. All authors have approved of the final version of the manuscript. Funding Sources


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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI grant JP17H05383 (Coordination Asymmetry), the Science Research Promotion Fund from the Promotion and Mutual Aid Corporation for Private Schools of Japan, as well as the Institute of Science and Engineering at Chuo University (M. H.).

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