Promoted Electrochemical Performance of β-MnO2 through Surface

Apr 11, 2017 - Copyright © 2017 American Chemical Society. *E-mail: [email protected] (L.M.)., *E-mail: [email protected] (J.J.)...
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Promoted Electrochemical Performance of β‑MnO2 through Surface Engineering Chi Chen, Kui Xu, Xiao Ji, Ling Miao,* and Jianjun Jiang* School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China ABSTRACT: Different crystal facets with different surface atomic configurations and physical/chemical properties will have distinct electrochemical performances during their surface/near-surface redox reactions, and it is important to realize the controllable synthesis of high active surfaces for electrode materials. Herein, using first-principles calculations, the electrochemical performances of different surfaces of βMnO2 were investigated. Higher surface adsorption pseudocapacitance and lower ion diffusion barrier from the surface to the near surface make the {001} surface of β-MnO2 superior to other surfaces when acting as an electrode material. Moreover, β-MnO2 with a large percentage of the {001} surface was predicted to be obtained through surface F-termination. Ftermination decreases the surface energy of the {001} surface while suppressing the growth of {110} surface, which demonstrated as the surface with a much lower electrochemical performance. This work might provide a feasible strategy to synthesize anticipated surfaces with a high electrochemical performance for transition metal oxides. KEYWORDS: adsorption, diffusion, β-MnO2, F-termination, surface energy, DFT

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electrochemical performances.11 The surfaces with a larger pseudocapacitance should possess a larger density of adsorption sites on the surfaces and effective ion diffusion channels from the surface to the near surface.12 To conquer the limited electrode kinetics of β-MnO2, it is necessary to figure out their different surface properties and investigate their influences on pseudocapacitance. Several researches have reported the surface-dependent catalytic properties13−15 and battery behaviors16 of β-MnO2, but few researchers have focused on the surface-dependent performances of SCs. In addition, it could be noted that the 1D tunnel vertical to the {001} surface of βMnO2 plays an important role in the migration of ions from the surface to the near surface.16,17 Therefore, it is meaningful to understand the pseudocapacitance behaviors of these surfaces and propose feasible strategies to synthesize β-MnO2 that has more superior surfaces with a higher pseudocapacitance, including a larger percentage of the adsorption sites on the surfaces and a better exposure of the 1D tunnel from the surface to near surface. Recently, surface structure controlling has been studied for high-performance catalyst and energy storage.18−20 Surfacecontrolled noble metal catalysts, such as Pt, Pd, and Au, attract a great deal of researchers’ attention. Tian et al. synthetized tetrahexahedral Pt nanocrystals with high-index facets through a

INTRODUCTION Supercapacitors (SCs) store energy using either physical ion adsorption onto the electrode/electrolyte interface, in which case they are called electric double-layer capacitors (EDLCs), or fast surface/near-surface reversible redox reactions, in which case they are termed pseudocapacitors.1 Compared with EDLCs, pseudocapacitors can provide 10−100 times higher specific capacitance to fulfill wider applications.2 Various transition metal oxides have been investigated for pseudocapacitor electrode materials, such as RuO2,3 MnO2,4,5 NiO,6 V2O5,7 and so forth. Because of its low cost, abundance, and multiple valences, MnO2 is considered to be a promising candidate for the pseudocapacitor electrode material with a high theoretical capacitance of 1370 F·g−1.8 For kinds of crystallographic structures that exist for MnO2, the obtained capacitances of these crystals are pretty low almost below 300 F·g−15,9 especially for pristine β-MnO2 with a capacitance below 10 F· g−1.9,10 Therefore, it is necessary to find a method to improve the experimental capacitance of MnO2, especially for β-MnO2. Compared with the theoretical capacitance, the practical capacitance of β-MnO2 could be negligible. The high potential on specific capacitance of β-MnO2 is hindered by its limited electrode kinetics. Great efforts should be made to promote the electrochemical performance of β-MnO2 on account of its promising application prospect. Different crystal facets with different surface atomic configurations and physical/chemical properties will influence the surface/near-surface reactions greatly, resulting in various © 2017 American Chemical Society

Received: November 14, 2016 Accepted: April 11, 2017 Published: April 11, 2017 15176

DOI: 10.1021/acsami.6b14601 ACS Appl. Mater. Interfaces 2017, 9, 15176−15181

Research Article

ACS Applied Materials & Interfaces square-wave potential treatment.21 Subsequently, a few other metal oxides were also synthetized with active facets, such as TiO2,22 Cu2O,23 ZnO,24 and α-MnO2.25 Yang et al. obtained anatase TiO2 single crystals with a large percentage of active {001} facets by decreasing the surface energy using fluorinetermination.22 This method has been adopted to improve the photoreactivity26−28 and the performance of lithium-ion batteries29,30 for TiO2. Similarly, the termination-tunable surface energy might also be used on other oxides such as βMnO2, and it is also important to explore the influences of its controlled surfaces on pseudocapacitor behaviors. In this work, first-principles calculations were performed to investigate the electrochemical performances of different surfaces of β-MnO2. It was found that the {001} surface shows the largest density of H adsorption sites with the channel for easier diffusion of H ions from the surface to the near surface. Then, the {001} surface was obtained by F-termination to decrease the surface energy, and the origin of surface rebuilding was discussed. This work provides a possible route to synthesize a selected surface with a high electrochemical performance for transition metal oxides.

of adsorbed F atoms, and EF = 1/2 EF−F, EF−F indicating the total energy of the dimer F2.

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RESULTS AND DISCUSSION Pristine Surface Properties. Bulk. The pristine rutile βMnO2 belongs to the tetragonal space group P4/mnm (no. 136), as shown in Figure 1. Our predicted DFT + U lattice

Figure 1. (a) Top view and (b) lateral view of crystal structure for βMnO2.

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parameters are a = b = 4.399 Å and c = 2.902 Å, with errors within 1.01%, compared with experimental results.38 The presented structural block, as shown in Figure 1b, is the MnO6 octahedra with Mn−O bonds of 1.899 and 1.890 Å. Simultaneously, antiferromagnetic (AFM) is the preferred ground state for β-MnO2 crystals, and the total energy per formula unit in the AFM state is 0.09 and 2.33 eV lower than in the ferromagnetic (FM) and nonmagnetic (NM) states, respectively, similar to ref 39. Pristine Surface. Surface atomic structures are important to their electrochemical performances. Therefore, figuring out the surface properties is essential to have a deep understanding of pseudocapacitor behavior of β-MnO2 as an electrode material. Here we carried out a systematic investigation of four low-index stoichiometric surfaces: {110}, {101}, {100}, and {001}. Figure 2a illustrates the top and side views of the pristine surfaces and the calculated surface energies, γ, are exhibited in Figure 2b. The surface energies of both FM and AFM states are ordered as follows: {110} < {100} < {101} < {001}, similar to ref 15. That is, the {110} surface is the most stable, whereas the {001} surface is unstable. Therefore, during the crystal growth of βMnO2, the {110} surface will dominate the largest area, whereas the {001} surface will represent the least area. The ground states of these surfaces prefer FM states, and the total energy per formula unit in FM states are 0.02−0.05 eV lower than that in AFM states. With regard to the lowest energy principle, the following discussions will focus on FM states. Electrochemical Properties on Different Pristine Surfaces. The charge storage mechanism of β-MnO2 is reversible redox reaction between the III and IV oxidation states of Mn ions

COMPUTATIONAL DETAILS Our first-principles calculations were performed using the VASP code, 31 based on the density-functional theory (DFT).32,33 The exchange−correlation energy was calculated using the general gradient approximation (GGA)34 with Hubbard U corrections (GGA + U). Also, in our calculations we used the GGA + U approach with U − J = 3.9.35,36 A plane wave cutoff of 400 eV and k-point meshes of 9 × 9 × 1, 9 × 11 × 1, 9 × 9 × 1, and 11 × 7 × 1 in the Monkhorst−Pack37 sampling scheme were used for {001}, {100}, {101}, and {110} surfaces, respectively. To calculate the migrations of hydrogen atoms from the surface to bulk, the k-point meshes were chosen as 3 × 3 × 1 and 3 × 2 × 1 for {001} and {110} surfaces with supercells of 2 × 2 × 1 and 3 × 2 × 1, respectively. The c axis was set as 28 Å to ensure enough vacuum to avoid interactions between two periods. The structural relaxation was performed until the forces on all of the atoms were less than 0.01 eV/Å. In addition, spin polarization was considered in the calculations. In the solution, fluorine will bond to the MnO2 surface preferentially because of its highest electronegativity. Besides, H2O and other big acid radicals either have a weak van der Waals interaction with the surfaces or are too big to be anchored onto the surface-unsaturated metal atoms. Therefore, to reduce the computational amount and complexity, we adopted the computational approximation without a solvent, similar to the strategy outlined in the literature.14,15,22 Binding energies Eb are defined as E b = Esub + ada − (Esub + Eada)

where Esub+ada is the total energy of the substrate with an adsorbed atom, Esub is the total energy of the substrate, and Eada is the total energy of the adsorbed atom. Surface energies γ are defined as22 γ=

MnO2 + H+ + e− ↔ MnOOH

At the initial stage, because there will be few H+ diffusing into the bulk, the H+ adsorption onto the surface sites and the H+ diffusion to the near surface will be the main factors for pseudocapacitance. During this electrochemical process, H+ will adsorb onto the surface O2− or diffuse to the near-surface O2− and then an e− will be stored with the reduction of Mn4+ to Mn3+. Therefore, the surface reaction sites (surface O2−) and diffusion barriers from the surface to the near surface are two critical factors for a reversible redox. Thus, different facets with

Eslab − NEunit − NFE F 2A

where Eslab is the total energy of the slab, Eunit is the total energy per unit of β-MnO2, N is the total number of MnO2 primitive units of β-MnO2 contained in the slab model, NF is the number 15177

DOI: 10.1021/acsami.6b14601 ACS Appl. Mater. Interfaces 2017, 9, 15176−15181

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) Surface structures of a pristine surface of β-MnO2. (b) The surface energies for AFM and FM states and ref 15, and the maximum adsorption pseudocapacitance, Cϕmax.

Figure 3. H migration routes for (a) the {110} surface, (b) the {001} surface, and (c) the corresponding migration energy barrier.

On the other hand, easier diffusion from the surface to the near surface might provide more adsorption sites, leading to a higher adsorption pseudocapacitance for β-MnO2. On the basis of the former discussion, the {110} surface is the dominant surface for β-MnO2, whereas the {001} surface is the smallest. However, the {001} surface performs the highest theoretical adsorption pseudocapacitance, whereas the capacitance from the {110} surface is the lowest. It will be interesting to compare the H diffusion behavior along the {001} direction with that in the {110} direction. To calculate the diffusion energy barriers, constrained energy minimization was used to fix the depth of the H ions, while the remaining degrees of freedom were relaxed except for the first adsorption site, which is relaxed in all degrees of freedom. The different H adsorption sites along the diffusion path and their relative energies are shown in Figure 3. The barrier from the surface to the near surface of the {001} direction, namely, the first diffusion energy barrier in Figure 3c, is much lower than that in the {110} direction, which are 1.11 and 3.18 eV, respectively. Similarly, it is reported that the {001} surface of β-MnO2 is the most beneficial for the diffusion of the Li ion to the bulk with the lowest barrier.16 Moreover, the diffusion coefficient D is sensitive to the diffusion energy barrier Ebr. Their relationship can be given by Arrhenius equation

various surface atomic structures will affect the pseudocapacitance greatly. First of all, the facet with a higher concentration of oxygen atoms will provide more active sites for surface redox, resulting in larger adsorption pseudocapacitance. On these four investigated surfaces, the first-layer oxygen atoms are considered to be effective active sites for adsorption pseudocapacitance. According to the thermodynamic approach from Conway’s theory, the adsorption pseudocapacitance could be defined as12 Cϕ =

QF eρ ·F ·ℜ(1 − ℜ) = ·ℜ(1 − ℜ) RT RT

where Q is the charge density associated with the total oxidizable or reducible sites, e is the charge of an electron (1.602 × 10−19 C), ρ is the concentration of the first-layer oxygen atoms, F is the Faraday constant (96485.34 C·mol−1), R is the ideal gas constant (8.31 J·mol−1·K−1), T is the temperature (we chose room temperature = 298.15 K), and ℜ is the coverage percentage of all reaction sites. The maximal adsorption pseudocapacitance Cϕmax for each surface is shown in Figure 2b, when ℜ = 1/2. It could be found out that the {001} surface will provide the maximum theory adsorption pseudocapacitance of 1.61 mF·cm−2, followed by {100}, {101}, and {110} surfaces. The capacitance of the {001} surface is approximately double the capacitance of the {110} surface. Thus, the pseudocapacitance of β-MnO2 will be promoted if a larger percentage of the {001} surface could be produced.

⎛ −E ⎞ D = D0 ·exp⎜ br ⎟ ⎝ RT ⎠ 15178

DOI: 10.1021/acsami.6b14601 ACS Appl. Mater. Interfaces 2017, 9, 15176−15181

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ACS Applied Materials & Interfaces where D0, R, and T are the scale factor, ideal gas constant, and temperature, respectively. It can be calculated that D{001}/D{110} is 1.01 × 1035, indicating that it is conducive for H to diffuse along the {001} direction, whereas it is almost impossible for H to diffuse along the {110} direction. That is, the large percentage of the {001} surface will enhance the pseudocapacitance of β-MnO2 possibly because of an easier H diffusion channel from the surface to the near surface. After long-time cycling, enough H+ will diffuse into the bulk.40 Therefore, besides the influences from surface H+ adsorption and diffusion to the near surface, bulk H+ will also affect the pseudocapacitance. All of these three parts will devote to the total pseudocapacitance. With regard to bulk H+ contribution, the lower bulk diffusion barrier will benefit the bulk redox reactions. The second diffusion barriers in Figure 3c are considered to be bulk diffusion barriers, which are 0.78 and 2.75 eV for the {001} and {110} surfaces, respectively. It implies that the {001} surface will also have an enhanced pseudocapacitance after long-time cycling. In summary, to promote the electrochemical performance of β-MnO2, one of the feasible strategies is controlling the crystal growth to obtain as large a {001} surface as possible. Then, the larger surface adsorption pseudocapacitance and the expected diffusion rates from the surface to the near surface and in the bulk can be achieved. Surface Rebuilding. F-Terminated Surfaces. Utilizing the strong interaction between F and Mn can provide an effective method for crystal growth controlling to stabilize the active surfaces. Table 1 lists the bonding energies and bond lengths of

lower the surface energies for these four surfaces, but also make the {001} surface more stable than other surfaces. The surface energy of the {001} surface is decreased from 1.38 to −0.91 J· m−2, whereas that of the {110} surface is changed from 0.29 to −0.71 J·m−2. With this transformation, a large percentage of the {001} facet will be produced by F-terminations during the crystal growth of β-MnO2. The transformation scheme of the equilibrium crystal morphology for β-MnO2 after F-termination is shown in Figure 5b, according to Wulff construction theory.42 The resulting morphology of β-MnO 2 by F − treatment will be a thin film with a large {001} surface, instead of a rod-like structure for pristine β-MnO2, resulting in enhanced pseudocapacitance. For a thin film, the proportion of the surface and the near surface will increase in the whole crystal. As a result, the contributions from the {001} surface and the near surface to the pseudocapacitance will increase and the contribution difference between the {001} surface and other surfaces will be prominent. Origin of Surface Rebuilding. The origin of the abovediscussed surface rebuilding caused by F-terminations could be ascribed to the weakening of surface reconfiguration. The changes in local structures will give a visual comprehension, which can be seen from Figures 2a and 4. Also, the related Mn−O bond lengths and the relevant O atom displacements are listed in Table 2. It is obvious that the surface reconfiguration occurs on the four pristine surfaces after surface structural relaxation, as marked by the green circle in Figure 2a, especially the outward buckling of oxygen atoms next to the unsaturated Mn atoms, owing to orbital splitting from unpaired 3d electrons of surface Mn atoms. In bulk β-MnO2, Mn-3d orbitals consist of three energetically lower t2g orbitals (xy, xz, and yz) and two higher eg orbitals (z2 and x2 − y2). After surface cleaving and relaxation, these energetically equivalent t2g or eg orbitals will split.15 Then, the orientation of Mn−O bonds will be distorted and surface reconfiguration will occur. Nevertheless, surface reconfiguration could be weakened apparently after the saturation of unsaturated Mn atoms by F-terminations because of the attenuation of the influence from Mn-3d orbital splitting on bonding, as shown in Figure 4. Also from Table 2, we can find out that the length changes and displacements of O atoms almost decrease after Ftermination, except the displacement of O atoms on the {101} surface. However, the relative displacements between Mn and O atoms on the {101} surface is diminished to keep Mn atoms closer to the square-plane center of the four O atoms. Therefore, the surface energies could be changed by Ftermination because of less surface reconfiguration.

Table 1. Bonding Energies Eb and Bond Lengths l of F−F and Mn−F on Different Surfaces Eb/eV l/Å

F−F

Mn−F{110}

Mn−F{101}

Mn−F{100}

Mn−F{001}

−1.32 1.423

−2.54 1.761

−2.67 1.795

−2.46 1.780

−2.70 1.770

F−F and Mn−F bonds on different surfaces. The bonding energy of F−F is −1.32 eV, close to the experimental result,41 which is much higher than the bonding energies of Mn−F, approximately −2.46 to −2.70 eV on all surfaces. Therefore, it can be ensured that F atoms will bond to the surfaceunsaturated Mn instead of forming F2. Hence, the dissociative F ions in the solution can migrate to the MnO2 surface and bond to the surface-unsaturated Mn to form Mn−F bonds with bond lengths of approximately 1.76−1.79 Å, as shown in Figure 4. To understand the effects of the adsorbed F on these surfaces, surface energies γ of F-terminated surfaces are calculated, as shown in Figure 5a. F-Terminations not only

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CONCLUSIONS In this work, first-principles calculations were performed to discuss the electrochemical performances of different surfaces for β-MnO2. Our work found out that the {001} surface provides the highest adsorption pseudocapacitance of 1.61 mF· cm−2, which is almost double the capacitance of the dominant {110} surface. Simultaneously, the H diffusion energy barrier along the {001} direction is much lower than that in the{110} direction. The rate of diffusion coefficients for the {001} and {110} surfaces D{001}/D{110} is 1.01 × 1035, indicating that it is easier for H to diffuse along the {001} direction, which might provide more H adsorption sites to enhance the pseudocapacitance of the {001} surface. Immediately, a feasible strategy is adopted to obtain the highly active {001} surface of β-MnO2 with higher electrochemical performances. F-terminations

Figure 4. Relaxed structures of F-terminated surfaces of β-MnO2. 15179

DOI: 10.1021/acsami.6b14601 ACS Appl. Mater. Interfaces 2017, 9, 15176−15181

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ACS Applied Materials & Interfaces

Figure 5. (a) Surface energies of F-terminated β-MnO2. (b) Transformation scheme of equilibrium crystal morphology after F-termination.

Table 2. Mn−O Bond Lengths lMn−O and Relevant O Atom Displacements δ for Different Surfaces with Different Situationsa unrelaxed

F-terminated

surface

site

lMn−O

lMn−O

δ

lMn−O

δ

{110}

Mn(5) Mn(6) Mn(5) Mn(5) Mn(4)

1.899/1.890 1.890 1.899/1.890 1.890 1.899

1.924/1.905 1.880 1.852/1.859 1.866 1.828

0.333 0.106 0.125 0.299 0.198

1.892/1.899 1.879 1.895/1.872 1.889 1.835

0.019 0.054 0.258 0.084 0.081

{101} {100} {001} a

relaxed

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could lower the surface energies for these four surfaces, ascribed to the weakening of the surface reconfiguration by Fterminations. More importantly, they will make the {001} surface more stable than other surfaces with the lowest surface energy of −0.91 J·m−2. Consequently, the large percentage of the {001} surface with higher electrochemical performances is anticipated through surface F-terminations.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.M.). *E-mail: [email protected] (J.J.). ORCID

Chi Chen: 0000-0003-2479-9657 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was supported by the National Natural Science Foundation of China (grant nos. 51302097 and 51571096). The authors would like to acknowledge the help from Dr. Zimi Hu and Dr. Lin Lv.

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DOI: 10.1021/acsami.6b14601 ACS Appl. Mater. Interfaces 2017, 9, 15176−15181