Charge Storage Mechanism of RuO2

Article 2

Charge Storage Mechanism of RuO/water Interfaces Eriko WATANABE, Hiroshi Ushiyama, Koichi Yamashita, Yusuke Morikawa, Daisuke Asakura, Masashi Okubo, and Atsuo Yamada J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 05 Jun 2017 Downloaded from http://pubs.acs.org on June 7, 2017

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The Journal of Physical Chemistry

Charge Storage Mechanism of RuO2/water Interfaces Eriko Watanabe1, Hiroshi Ushiyama1, Koichi Yamashita1, Yusuke Morikawa1, Daisuke Asakura2, Masashi Okubo1, Atsuo Yamada1* 1

Department of Chemical System Engineering, School of Engineering, The University of Tokyo,

Bunkyo-ku, Tokyo 113-8656, Japan 2

Research Institute for Energy Conservation, National Institute of Advanced Industrial Science

and Technology, Tsukuba, Ibaraki 305-8568, Japan

1

ACS Paragon Plus Environment


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ABSTRACT

Capacitive energy storage at the electrochemical double layer formed on a particle surface can enable efficient devices that deliver high power and exhibit excellent reversibility. However, even with state of the art nanocarbons with highly controlled morphology to maximize the specific surface area, the available energy density remains far below that of existing rechargeable batteries. Utilizing nanoparticles of transition metal oxides is a viable option to alleviate the conflict between energy and power densities by accommodating additional electrons around the surface transition metal sites, called “pseudocapacitance”. However, an understanding of pseudocapacitive surfaces has been limited due to a lack of suitable analysis methodology. Here, we focus on the RuO2/water interface and elaborate on a reaction scheme including charge transfer into related surface orbitals using density functional theory calculations based on interfacial structures determined under a given electrode potential at a fixed pH of 0. The extensive contributions of the surface oxygen atoms and their surface-site dependence are revealed through the Ru-O orbital hybridization and/or O-H bond breaking/formation, largely deviating from the general explanation based only on the nominal valence states (penta-, tetra-, or trivalent) of Ru atoms.

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1. Introduction Electrochemical energy storage is of paramount importance for the broad implementation of sustainable energy technologies in society, such as electric vehicles and smart grids.1 Even among cutting-edge technologies, no solution can satisfy the two major requirements for energystorage devices: high energy and power densities. Although lithium ion batteries, which are among the most widespread energy-storage devices, provide a high energy density, their power density is limited due to the sluggish bulk lithium diffusion through the electrode material. To compensate for their low power density, electrochemical double layer capacitors, which employ rapid ion adsorption/desorption at carbon surfaces, have garnered attention as a supplementary or alternative energy-storage device that offers high power density.2–7 As the firm trade-off between energy and power density is an essential constraint based on the difference in charge storage mechanisms (i.e., bulk vs. surface), one strategy to enhance the energy density is to utilize additional rapid Faradaic reactions on the surface of a transition metal oxide,8–12 referred to as a pseudocapacitor. The specific capacitance of these pseudocapacitive materials far exceeds those of conventional carbon-based double layer electrodes. Thereby, the charge storage mechanisms of transition metal oxides have been investigated by experiments and theoretical calculations.13–17 However, a full understanding has not been achieved due to a lack of suitable analytical methods: (i) the atomic scale metal oxide/water interfacial structure strongly depends on pH and electrode potential, but these factors cannot be considered in standard density functional theory (DFT) calculations, and (ii) conventional photoemission and absorption spectroscopy provides limited information on surface redox chemistry.18,19

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We selected the RuO2/water interface as a model system to explore the details of pseudocapacitive redox surfaces because its electrochemical behavior as a pseudocapacitor electrode has been widely studied and well established. It utilizes the fast-redox reaction of Ru accompanied by the electroadsorption of protons to generate a very large capacitance of approximately 800 F g-1 far exceeding that of the best-performing carbon (ca. 150 F g-1).20–30 This surface redox occurs via the following reaction, RuOa(OH)b + δH+ + δe- ⇄ RuOa-δ (OH)b+δ .

(R1)

The general explanation of this reaction has been that surface protonation can add one or two electrons to the Ru 4d orbitals and change the nominal valence from 5+ to 3+.31 However, conflicts with this intuitive description can be found in the literature based on the hypothetical bulk RuOOH compound. Appealingly, electron transfer to O and H should also occur, considering that the metallic character of RuO2 forms a delocalized wide band at the Fermi level with strong Ru4d – O2p hybridization and a polar covalent O-H surface bond.32,33 However, detailed discussions on the interfacial protonation have been hindered thus far by the lack of a reliable interface model. To overcome the undeveloped methodologies and illuminate the details of surface redox chemistry, we decided to adopt the advanced surface Pourbaix diagram recently determined by Watanabe et al. that simultaneously provides a reliable interfacial structure for RuO2 and water at any given pH as well as the electrode potential vs. reversible hydrogen electrode (RHE).34 The diagram was established by applying the ab initio methodology developed by Rossmeisl et al., wherein they enabled the independent treatment of the pH and electrode potential in DFT calculations for the first time and determined the interfacial structure of Pt and water.35 4


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In this article, based on the reliable RuO2/water interfacial structures, the Bader charge (i.e., the charge enclosed in space whose boundary is defined as the charge density minimum between atoms) and the local density of states (LDOS) of a protonated RuO2 surface were calculated as a function of electrode potential at a fixed pH of 0 to outline the electron orbitals that contribute to the pseudocapacitive reaction.

2. Methods 2.1 Scheme to incorporate the pH and electrode potential effects into DFT First, we briefly review a theoretical scheme that can incorporate the effects of pH and the electrode potential into DFT calculations. The details are given in Ref 34 and 35. This method is based on a survey of several probable interfacial structures, and it calculates the interfacial Gibbs free energies at a given electrode potential and pH for each structure. Initially, the interfacial Gibbs free energy per surface atom was determined under standard hydrogen electrode (SHE) conditions, 𝜇H++𝑒 − = 0, 𝑛

𝐺int (𝜇H++𝑒 − = 0, Φ𝑒 − ) = {𝐺𝑁,𝑛 − 𝐺𝑁,0 − 2 𝐺H2 }⁄𝑁

(1)

where n and N represent the number of hydrogen and oxygen atoms on a certain unit surface, respectively, Φ𝑒 − is the work function of the system, and 𝐺𝑁,𝑛 is the Gibbs free energy of a structure with n hydrogen atoms on N surface oxygen atoms. Then, the free energy at given potential and pH conditions can be introduced by varying 𝜇H+ +𝑒 − as follows:

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𝑛

(2)

𝐺int (𝜇H++𝑒 − , Φ𝑒 − ) = 𝐺int (𝜇H++𝑒 − = 0, Φ𝑒 − ) − 𝑁 𝜇H++𝑒 −

where 𝜇H++𝑒 − is a function of pH and 𝑈SHE that can be determined independently by considering the Born-Haber cycle in Figure 1(a), i.e., 𝜇H++𝑒 − = Φ𝑒 − (SHE) − 2.3𝑘𝐵 𝑇 ⋅ pH − Φ𝑒 −

𝑈SHE =

Φ𝑒− −Φ𝑒− (SHE) 𝑒

.

(3)

(4)

Thus, calculating the work function yields unique values of 𝑈SHE . Then, 𝜇H++𝑒 − can also be uniquely determined once the pH value is provided. Finally, the Gibbs free energies of the RuO2/water interface (𝐺𝑁,𝑛 , 𝐺𝑁,0) for the given structure can be derived directly from the DFT total energies by including thermal effects in an empirical manner.36 The scheme to derive 𝐺int (𝜇H++𝑒 − , Φ𝑒 − ) as a function of pH and USHE is summarized in Figure 1(b). Once a set of probable interfacial structures is given, we can identify the most stable interfacial structures as functions of pH and the electrode potential. Note that the method enables us to include the effects of dipoles at the interface and hence explicitly account for the influence of the electric field.

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Figure 1 Theoretical schemes to determine the interfacial structures as functions of pH and the electrode potential by considering (a) the Born-Haber cycle for hydrogen oxidation and (b) 𝐺int (𝜇H++𝑒 − , Φ𝑒 − ) projected onto iso-pH-planes (set of blue planes). The red and purple lines are examples of 𝐺int (𝜇H++𝑒 − , Φ𝑒 − ) . Once the work functions (or USHE) of these structures are calculated, 𝐺int (𝜇H++𝑒 − , Φ𝑒 − ) can be calculated by projecting them onto the iso-pH planes. The Born-Haber cycle is considered with the hydrogen dissociation energy in gas ( Δd 𝐺 ), the ionization energy of a hydrogen atom in gas (Δi 𝐺), the solvation energy of a hydrogen ion (ΦH+ ), and the work function of an electron (Φe− ).

2.2 Structures of RuO2/water interfaces The structures of RuO2/water interfaces have been determined as functions of pH and electrode potential using the methodology explained in 2.1;34 the RuO2 (110) surface was 7



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selected because it has the lowest surface energy.37 Figure 2 shows the structures of bulk RuO2 and a stoichiometric RuO2 (110) surface. The RuO2 (110) has two distinct surface sites, a bridge (br) site and a coordinatively unsaturated (cus) site (Figure 2(b)). In the following discussion, the notation Xbr/Xcus (X = O, OH, H2O) is employed to represent the topmost surface structures at the ‘br’ and ‘cus’ sites. Figure 3 and Figure 4 show the detected surface structures at a pH of 0 and the calculated surface Pourbaix diagram of RuO2(110)/water, respectively. The (1 × 2) surface unit cells with four layers thick slabs were used for all the calculations. Although the previous paper determined the structures of RuO2(110)/water with the outer water layers to include the effect of water affinity in a potential region above -0.1 V vs. RHE, surface models without outer water layers are employed for simplicity in the present study. This simplification is justified because the electronic structures of the topmost RuO2(110) surface were calculated to be identical for both cases, i.e., with and without outer water layers (Figure S1). At a pH of 0, the interfacial structures were found to be H2Obr/H2Ocus, OHbr/H2Ocus, Obr/H2Ocus, Obr/(0.5H2O+0.5O)cus and Obr/Ocus in the potential ranges of below -0.5 V, -0.5 – -0.1 V, -0.1 – 1.1 V, 1.1 – 1.5, and above 1.5 V vs. RHE, respectively, and the respective nominal valences of Ru atoms on these surfaces are +3.0, +3.5, +4.0, +4.5 and +5.0 as shown in Figure 4. Note that the transition potentials slightly deviate from those of the experimentally determined potentials, presumably due to the difference between the averaged surface in polycrystalline films (experiment) vs. the sole (110) plane surface (the present model), limited sampling of probable interfacial models, and the accuracy limit of the DFT calculations. For example, at 0.4, 1.0, and 1.4 V vs. RHE, the averaged formal valences of +3.0, +4.0 and +5.0 are suggested by the experimental work, respectively,38 whereas +4.0, +4.0 and +4.5 are determined in the DFT calculations, respectively,34 as shown in Figure 4(b). Nevertheless, there is a reasonable 8


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agreement between the DFT calculations and the experiments, particularly in a region of higher potential. Thus, the present study provides valuable insights into the overall orbital contributions to the surface redox reactions based on reliable interfacial models.

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Figure 2 Structures of (a) bulk RuO2 and (b) stoichiometric RuO2(110) from a side view, where only the two topmost layers are shown for simplicity. The green and red balls represent Ru and O atoms, respectively.

Figure 3 The potential dependent RuO2(110)/water interfacial structures based on Ref 34. The green, red, and white balls represent Ru, O, and H atoms, respectively. Surface structural notations at each potential region are (a) H2Obr/H2Ocus (U < -0.5 V vs. RHE), (b) OHbr/H2Ocus (0.5 < U < -0.1 V vs. RHE), (c) Obr/H2Ocus (-0.1 < U < 1.1 V vs. RHE), (d) Obr/(0.5H2O+0.5H2O)cus (1.1 < U < 1.5 V vs. RHE) and (e) Obr/Ocus (1.5 V < U vs. RHE). 10


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Figure 4 (a) Pourbaix diagram of RuO2(110)/water calculated in Ref 34 and (b) a comparison of Ru nominal valence at a pH of 0 determined by DFT for the (110) surface in Ref 14 and by experiments for an averaged surface in polycrystalline RuO2 films.38

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2.3 DFT details Based on the interfacial structures determined as functions of pH and electrode potential in Ref 34, the spin-polarized DFT calculations were performed with the GPAW software together with the ASE simulation package.39–41 The GGA/RPBE functional and PAW method were employed,42,43 and the gpaw-setup version 0.9.11271 was used for all elements. The k-points were sampled using a (4 × 4 × 1) mesh and electronic structures were calculated by the finite difference method with a grid parameter of 0.20 ± 0.01 Å. Dipole corrections were applied in all calculations.44 The Bader charges and Wigner-Seitz local density of states were calculated to analyze the amount of charge transfer during the surface redox reactions.

3. Charge distribution analysis To verity the atomic orbitals (Ru 4d and O 2p) contributing to the surface redox reactions, we first analyzed the Bader charges of the surface Ru and O atoms at br and cus sites as a function of the electrode potential, which are summarized in Table 1 and Figure 5. Changes in the Bader charge upon redox reactions are not uniform but rather proceed in a sequential manner at each specific atomic site/orbital according to the variation in the surface structure induced by the external electrode potential. Based on our results, major characteristic features of charge transfer can be confirmed roughly in two regions; the Ru redox region below -0.1 V vs. RHE (Region I) and the O redox region above 1.1 V vs. RHE (Region II). Region I

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In a lower potential region below -0.1 V vs. RHE, charge transfer is mainly observed on the surface Ru atoms rather than on the surface O atoms. In this region, major interfacial structural changes of RuO2(110)/water occur at br sites, H2Obr ⇄ OHbr + H+ +e-

(U = -0.5 V vs. RHE)

OHbr ⇄ Obr + H+ +e- (U = -0.1 V vs. RHE) .

(R2) (R3)

Overall changes in the Bader charge of Ru and O atoms at br sites are 0.52e and 0.26e, respectively, which are much larger than those at cus sites, i.e., 0.13e and 0.0e, respectively, indicating that the charge transfer is dominant at surface sites where there are changes in the local coordination structure. Region II In a higher potential region above 1.1 V vs. RHE, charge transfer is, in contrast, observed on surface O atoms rather than on surface Ru atoms. As evidenced in Figure 5, large changes occur at 1.1 V and 1.5 V vs. RHE, which reflect the surface redox reactions, respectively, H2Ocus ⇄ 0.5H2Ocus+0.5Ocus+H++e- (U=1.1 V vs. RHE)

(R4)

0.5H2Ocus+0.5Ocus ⇄ Ocus+H++e-

(R5)

(U = 1.5 V vs. RHE) .

During the structural changes from H2Ocus to Ocus, changes in the Bader charge are 0.32e and 0.65e for Ru and O atoms at cus sites, respectively. Similar to Region I, changes in the Bader charge are so localized that only small changes are observed at br sites, i.e., 0.09e and 0.17e, respectively. Notably and somewhat surprisingly, many more electrons are extracted from O atoms than from Ru atoms. 13



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To summarize, our results reveal the contribution from the O 2p orbital in the overall potential region, especially above 1.1 V vs. RHE, and the phenomena show clear surface-site dependence. Having verified the contribution of the orbitals to the pseudocapacitive surface redox reaction, it should be noted that the general explanation based only on the nominal average valence of transition metals, e.g., Ru5+, Ru4+, and Ru3+, is just a convenient reference. Although the contribution from the surface oxygen orbital has been recognized and asserted in the capacitor community, there has been no methodology to prove or verify this contribution due to a lack of experimental/theoretical probes that can exclusively extract surface information under electrochemical conditions. The present DFT approach incorporating the effect of pH and electrode potential provides the first platform toward an overall understanding of the pseudocapacitive redox surface.

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Table 1 Bader charge (Bader q) and formal charge (Formal q) of Ru and O atoms at br and cus sites. (a)H2Obr/H2Ocus, (b)OHbr/H2Ocus, (c)Obr/H2Ocus, (d)Obr/(0.5H2O+0.5O)cus and (e)Obr/Ocus. br site

Potential region (vs. RHE) (a)

U < -0.5 V

(b) -0.5 V < U < -0.1 V

(c)

-0.1 < U < 1.1 V

(d)

1.1 V < U < 1.5 V

(e)

1.5 V < U

cus site

Bader q /e Formal q /e

Bader q /e

Formal q /e Formal q /e (averaged)

Ru

1.25

2.67

1.52

3.33

+3.0

O

-1.10

-2.0

-1.11

-2.0

-2.0

Ru

1.61

3.67

1.59

3.33

+3.5

O

-1.09

-2.0

-1.14

-2.0

-2.0

Ru

1.77

4.67

1.65

3.33

+4.0

O

-0.84

-2.0

-1.11

-2.0

-2.0

Ru

1.81

4.67

1.89/1.73

5.33/3.33

+4.5

O

-0.71

-2.0

-1.08/-0.62

-2.0

-2.0

Ru

1.86

4.67

1.97

5.33

+5.0

O

-0.67

-2.0

-0.46

-2.0

-2.0

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Figure 5 The Bader charges of surface Ru and O atoms at (a) br sites and (b) cus sites as a function of electrode potential. Blue and red lines represent the charges of Ru and O atoms, respectively. The vertical gray dashed lines indicate the potentials where structural changes occur at the surface.

4. Mechanism of the contribution of surface O atoms The contribution of the surface O atoms to the pseudocapacitive reactions originates from two distinctive features at the interface: (i) the notable hybridization of Ru 4d and O 2p orbitals below -0.1 V vs. RHE (Region I), and (ii) the critical change of the character of O from “O in molecule-like H2O” to “O on the RuO2 surface” at high potentials above 1.1 V vs. RHE (Region II). 16


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The first well-known feature in the lower potential region below -0.1 V vs. RHE is the large hybridization of Ru 4d and O 2p orbitals. The Bader charges of Ru atoms in bulk RuO2 are inherently known to be smaller than the values of Ti atoms in bulk TiO2 with an identical rutiletype structure due to the higher electronegativity of Ru compared to that of Ti.32 This implies that Ru-O bonds exhibit a more covalent character, which is consistent with a certain amount of contribution of the oxygen orbital over the wide potential range. Indeed, as seen from pDOS in Figure S2(a) and the visualizations of the Kohn-Sham orbitals near the Fermi level, the Ru dz2 and O p orbitals (Figure S2(b)-(c)) are strongly hybridized. In our calculations, the amount of electron extracted from O atoms at br sites is half of that from Ru atoms at br sites, i.e., 0.26e from O and 0.52e from Ru in Region I. The previous DFT calculations on hypothetical bulkRuOOH also showed the same ratio, 2(Ru):1(O), of electron extractions (0.15e from O and 0.30e from Ru). Thereby, the hybridization with the Ru orbital is the dominant mechanism of the O contribution to the surface redox reactions in Region I, reflecting the intrinsic character of bulk RuO2. The second unique feature, which is dominant at the higher potential region above 1.1 V vs. RHE (Region II), is in O states at cus sites. As mentioned in Section 3, a surprisingly large change in the Bader charge is observed in O compared to that in Ru, specifically at cus sites, with a ratio of approximately 1(Ru):2(O), i.e., 0.32e for Ru and 0.65e for O, as a result of the transition between H2Ocus and Ocus. This feature cannot simply be explained by the hybridization of Ru and O orbitals because the hybridization causes electron extraction from Ru and O atoms with a ratio of approximately 2(Ru):1(O). The large extra contribution from O can be explained by the transition of O character from “O in molecule-like H2O” to “O on the RuO2 surface” as the potential increases in Region II. Figure 6 shows the LDOS of Ru and O atoms at br and cus


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sites. A characteristic feature of the LDOS in “O in molecule-like H2O” (Figure 6(c)) is the negligible states around the Fermi level, indicating that 2p states are fully occupied to form O-H bonds. Indeed, the Bader charge of O in H2Ocus is almost identical to that of an isolated H2O molecule, i.e., -1.11e for O in H2Ocus and -1.04e for O in isolated H2O molecule. Conversely, from the LDOS in “O on the RuO2 surface”, large localized unoccupied states appear just above the Fermi level (Figure 6(e)), implying the formation of empty dangling bonds. This situation reduces the electron density around Ocus, thereby increasing the Bader charge of Ocus to -0.46e. The value is considerably larger than those of O in bulk RuO2 (-0.87e) or of O in Obr (-0.84e – 0.67e). To summarize, the transition of O-states from “O in molecule-like water” to “O on the RuO2 surface” upon oxidation causes the large contribution of O atoms during surface redox reactions in the high potential region above 1.1 V vs. RHE (Region II). The overall pseudocapacitive reactions are summarized in Figure 7. We identified and quantified the significant contribution from surface O atoms in the entire potential range (throughout Region I to Region II) as a result of the hybridization of Ru 4d and O 2p orbitals. A large extra contribution from the O 2p orbital occurs in Region II by the formation of large unoccupied 2p states at Ocus together with the empty dangling bond during the transition from H2Ocus to Ocus.


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Figure 6 LDOS of Ru and O atoms at br and cus sites in (a) H2Obr/H2Ocus, (b) OHbr/H2Ocus, (c) Obr/H2Ocus, (d) Obr/(0.5O+0.5H2O)cus, and (e)Obr/Ocus. The blue solid, blue dashed, red solid, and red dashed lines represent the LDOS of Ru at cus, Ru at br, O at cus, and O at br sites, respectively. For clarity, the LDOS of O at cus sites is shaded red. The vertical black dashed lines indicate the Fermi level.


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Figure 7 Schematic derivation of the pseudocapacitive reactions on the RuO2 (110) surface in the representative potential regions, (a) Region I (U < -0.1 V vs. RHE) and (b) Region II (1.1 V < U vs. RHE). The nominal valence of the Ru atoms is denoted for reference.

5. Conclusions In summary, atomic-scale details in pseudocapacitive surface reactions were revealed by applying DFT calculations using the RuO2/water interface as a model system. Such an approach has been hindered so far due to the lack of suitable interfacial model. Based on the reliable interfacial structures determined by the ab-initio methodology that incorporate the effects of the pH and electrode potential, the potential-dependent surface electronic structures were analyzed by calculating the Bader charges and the LDOS for each component atom. Contrary to the conventional intuitive descriptions with formal average valences of Ru in penta-, tetra-, or


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trivalent states, we revealed the extensive and unique contribution of oxygen orbitals at specific surface sites to pseudocapacitive reactions depending on the electrode potential. The contribution of surface O atoms can be divided into two features; (i) the well-known hybridization of Ru and O orbitals at lower potential region, and (ii) the transition of O characters between “O in RuO2 surface” and “O in molecule-like water” at higher potential region. This approach is not to be limited to pseudocapacitor systems but can be extended to any water/solid electrochemical interface, which could lead to a much more accurate/deeper understanding of charge transfer processes in important systems such as solar cells, fuel cells and/or water-splitting catalysis, and aqueous rechargeable batteries for a sustainable society.

ASSOCIATED CONTENT Supporting Information. LDOS of atoms at the topmost surface layer with and without outer water layers (Figure S1) and electronic structures of bulk RuO2 (Figure S2).

AUTHOR INFORMATION Corresponding author [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This work was supported by a JSPS Grant-in-Aid for Specially Promoted Research (No. 15H05701). The authors acknowledge Prof. Wataru Sugimoto from Shinshu University for his helpful discussions. RFERENCES (1)

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TOC Graphic


28

(a) ΔiG + H(g) Page The 29 Journal of 35 of PhysicalHChemistry (g) + e (g)

-ΦH -Φe ΔdG 1 2 1/2 H2(g) H+(s) + e-(m) 3 µH +e 4 5 (b) 6 pH=0 7 pH=14 8 9 10 11 12 13 14 ACS Paragon Plus Environment 15 µH +e 16 +

-

Gint

+

-

+

USHE

-

(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

(b)


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Ru ACS Paragon Plus Environment

O

(a) U < -0.5 V vs. RHE

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1 2 3 4 5 6 (d) 1.1 < U < 1.5 V vs. RHE 7 8 9 10 11 12 13

(b) -0.5 < U < -0.1 V vs. RHE

(c) -0.1 < U < 1.1 V vs. RHE


(e) 1.5 V < U vs. RHE


Ru O

(a) The Journal of Physical Chemistry Page 32 of 35

1.5

Ru5+

USHE (eV)

1 1.0 Ru4.5+ 2 3 4 0.5 5 Ru4+ 6 7 0.0 8 9 10 0.5 Ru3.5+ 11 Ru3+ 12 13 1.0 0 2 4 6 8 10 12 14 14 15 pH 16 17(b) DFT: (110) surface 18 Ru5+ Ru3+ 19 Ru3.5+ Ru4+ Ru4.5+ 20 21 22 USHE (eV) 23 24 exp.: polycrystalline film 25 3+ 26 Ru2+Plus Ru4+ Ru5+ RuEnvironment ACS Paragon 27 0.5 0.0 0.5 1.0 1.5 28 USHE (eV) 29

(a) br site 3.0

Page 33 The of 35Journal of Physical Chemistry

Bader charge /e

Bader charge /e

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

2.5

Ru

2.0 1.5 1.0 0.0 0.5

O

1.0 1.5

2.0 1.0 0.5 0.0 0.5 1.0 1.5 2.0 Electrode potential V vs. RHE (b) cus site 3.0 2.5

Ru

2.0 1.5 1.0 0.0 0.5

O

1.0 1.5

2.0 ACS Paragon Plus Environment 1.0 0.5 0.0 0.5 1.0 1.5 2.0 Electrode potential V vs. RHE

(a) U < -0.5 V vs. RHE 5 4

(b) -0.5 < U < -0.1 V vs. RHE The5 Journal of Physical Chemistry 4

(c) -0.1 < U < 1.1 V vs. RHE Page 34 of 35

5 4 3 2 1 0

4

2

Ru at cus

Ru at br

O at cus

0

2

Energy (eV)

LDOS

LDOS

13 3 2 32 2 4 1 51 6 0 70 4 2 0 2 4 4 2 0 2 4 8 Energy (eV) Energy (eV) 9 (e) 1.5 V < U vs. RHE 10(d) 1.1 < U < 1.5 V vs. RHE 5 115 12 4 134 143 3 15 162 2 17 1 181 19 0 200 4 2 4 2 0 2 4 0 2 4 ACS Paragon Plus Environment 21 Energy (eV) Energy (eV) 22 23

O at br

4

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Schematic derivation of the pseudocapacitive reactions on the RuO2 (110) surface in the representative potential regions, (a) Region I (U < -0.1 V vs. RHE) and (b) Region II (1.1 V < U vs. RHE). The nominal valence of the Ru atoms is denoted for reference. 271x357mm (96 x 96 DPI)


Charge Storage Mechanism of RuO2

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