Hollandite Structure Kxâ0.25IrO2 Catalyst with Highly Efficient

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Hollandite Structure Kx≈0.25IrO2 Catalyst with Highly Efficient Oxygen Evolution Reaction Wei Sun,† Ya Song,‡ Xue-Qing Gong,*,‡ Li-mei Cao,† and Ji Yang*,† †

State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Processes, School of Resources and Environmental Engineering and ‡Key Laboratory for Advanced Materials, Center for Computational Chemistry and Research Institute of Industrial Catalysis, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P.R. China S Supporting Information *

ABSTRACT: Oxygen evolution reaction (OER) catalysts with high activity are of particular importance for renewable energy production and storage. Here, we prepare Kx≈0.25IrO2 catalyst that exhibits an excellent OER activity compared to IrO2, which is univerally acknoweledged as a state-of-the-art OER catalyst. The prepared catalyst reflects a small overpotential 0.35 V at a current density of 10 mA cm−2 and a lower Tafel slope (65 mV dec−1) compared to that for IrO2 (74 mV dec−1). The performed X-ray photoelectron spectroscopy (XPS) and X-ray adsorption (XAS) experiments indicate that the Ir-site of Kx≈0.25IrO2 has a lower valence and more Ir-5d occupied states, suggesting more electrons on the Ir site. The extra electrons located on the Ir site and distorted IrO6 octahedral symmetry have a significant effect on the 5d orbital energy distribution which is verified by our DOS calculation. The performed DFT calculations state that the Kx≈0.25IrO2 essentially obtains good OER performance because it has a lower theoretical overpotential (0.50 V) compared to IrO2 (0.61 V). KEYWORDS: OER, hydrothermal, hollandite iridate, electronic structure, DFT

1. INTRODUCTION

However, detailed understanding of the effect with alkali elements (Na and K) is quite limited. Despite that, alkali elements are often found in some complex oxides, such as hollandite-type,24,25 cryptomelane,26−28 and birnessite29,30 oxides. In these complex oxides, the transition metal presents mixed valence and different crystal structure differ with their mono-oxides due to Na or K presence. This mixed valence and special lattice structure give rise to a vital change in its electronic structure, such as Mn3+ appearance in KxMn8O16 (cryptomelane) and NaxMnO2 (birnessite). Thus, it inspires us to introduce Ir into this special crystal structure to form stable homogeneous oxide in order to tune the electronic structure of Ir to obtain different OER activity. Here, Kx≈0.25IrO2 a hollandite structure oxide31,32 is an efficiently OER catalyst and its OER performance even better than iridium dioxide (IrO2). The Kx≈0.25IrO2 and IrO2 are prepared by hydrothermal methods. This approach has a lot of superiorities, such as simplicity, short synthesizing time, and low temperature over solid state strategy. Another highlight is the fact that the materials morphology can be controlled under different conditions, such as precursors, handling temperature, and dwell time. The IrO2 is chosen due to it is univeral acknowledgment as a state-of-the-art OER catalyst.33−36 We

Global concerns about increasing energy demands have stimulated extensive studies on the production of renewable energy sources and energy storage, including processes such as water splitting,1,2 direct solar,3 and rechargeable metal-air batteries.4,5 The oxygen evolution reaction (OER) in particular is often a critical step in these energy production/storage options, which is limited by the sluggish kinetics associated with the O−H bond breaking and OO bond formation.6−8 A key way of overcoming this problem is to find effective OER catalysts. The metal oxides have been studied for decades9−12 and proposed to be the promising catalysts for overcoming this bottleneck. It is believed that the OER activity of metal oxide catalysts is governed by the types of metal−oxygen (M−O) bonds (covalent or ionic bonds) and the electronic/orbital structure of the active sites.4,13−16 Fortunately, these properties can be tuned by doping transition metals into the structure or forming mixed oxides, because of the electronic structure varies with changing components in electrode materials. For example, the catalysts of Co doped RuO2,17 Ti doped Fe2O3,18 Zn doped RuO2 and Co3O4,19,20 Sb doped SnO2,21 and Sn22 and Bi23 doped IrO2 have been successfully prepared. These materials not only focus on different component effects on the morphology and surface but also elucidate how the foreign metal elements tune the host phase electronic structure to further enhance its OER activity. © XXXX American Chemical Society

Received: October 23, 2015 Accepted: December 22, 2015

A

DOI: 10.1021/acsami.5b10159 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

scanning electron microscope (SEM) images of the prepared Kx≈0.25IrO2 are shown in Figure 1a,b with different resolution. The morphology of the obtained materials is the needle-like nanorods, and some of them look like sea urchins aggregated by the nanorods which are similar to previous reports.40 Figure 1c shows the transmission electron microscope (TEM) image of Kx≈0.25IrO2, revealing that the needles have widths of approximately 10 nm and lengths from 50 to 300 nm. The high-resolution TEM (HRTEM) image in Figure 1d shows the lattice fringes of the strongest diffraction face (1̅12) of Kx≈0.25IrO2 with a characteristic spacing of 2.57 Å. 2.2. Electrochemical Performance of Kx≈0.25IrO2 and IrO2. Cyclic voltammetry (CV) scans were used to evaluate the properties of Kx≈0.25IrO2/Ti and IrO2/Ti electrodes in 0.1 M HClO4 solution. Below the OER potential window, the voltammetric charge q* of a material is in fact related with the active surface area of the coating on the electrode.9,41−43 The voltammetric charge q* (mC cm−2) is obtained as shown in the following equation:

also incorporate some relevant electronic structure experiments (XPS and XAS) with the DFT calculations to elucidate why Kx≈0.25IrO2 obtained better OER activity than IrO2.

2. RESULTS AND DISCUSSION 2.1. Physical Characterization of Kx≈0.25IrO2. Kx≈0.25IrO2 is prepared via a hydrothermal synthesis approach (see the Supporting Information). The KxIrO2 is commonly prepared via solid state synthesis31,32,37 through mixing IrO2 with sylvite (K2CO3, KCl, etc.) under high temperature (usually higher than 1000 °C) and dwelling for a long time due to the fact that IrO2 is a quite stable oxide. The hydrothermal synthesis method is very simple, without using any bulky capping agents, and could be employed at relatively low temperature (150 °C) and is not time-consuming. The precursors are only iridium chloride (IrCl3) and KOH solution. The XRD pattern (Figure 1a) reveals that the as-prepared material represents a

q* =

∫ i dE/(Aν)

where i is the voltammetric current (mA); E is the potential (mV); A is the geometry area of the electrode (cm2), and ν is the scan rate (mV s−1). Also, the q* can also be presented as follows proposed by Ardizzone et al.:41 q* = q0 + kν−1/2

where q0 is the double layer charge, which can be a useful tool for evaluating the relative surface area of an elctrode; k is a constant value. Thus, it can plot the q* vs the ν−1/2 to obtain the q0. From Figure 2a, it can clearly be found that the q* for Kx≈0.25IrO2 and IrO2 decrease with scan rate. This could be explained by two main approaches: S. Trasatti et al.41 proposed

Figure 1. SEM images of (a) Kx≈0.25IrO2 and (b) the selected area in a higher resolution showing a sea-urchin like morphology. (c) TEM image and (d) HRTEM image of the nanorods. (e) XRD pattern and (f) EDX spectra of Kx≈0.25IrO2.

Hollandite-type crystal structure,31,37,38 and it is consistent with the card number ISCD no. 80336. The Hollandite structure is a monoclinic structure while IrO2 gives a tetragonal one. Therefore, there is a remarkable difference between KxIrO2 and IrO2 (see Figure S1) at low angle positions as well as the intensities and positions of the strongest peaks. The EDX measurement (Figures 1f and S1b) confirms that poisonous39 Cl− does not exist on the obtained crystal, and the atomic ratio of K, Ir, and O is rather close to 1:4:8. Thus, according to the XRD and EDX results, the x ≈ 0.25 is adopted in here. The

Figure 2. (a) Voltammetric charge q* at the different scan rate for Kx≈0.25IrO2 and IrO2. (b) The voltammetric charge q* plot as a function with ν−1/2. The double layer charge q0 is determined at the v−1/2 = 0 corresponding to the intercept of the fitting line. (c) Cyclic voltammograms of Kx≈0.25IrO2/Ti at different scan rates in 0.1 M HClO4. The solid arrows indicate the varying direction of the peak potential. (d) The relationship of the anodic peak current ip vs the scan rate ν. The catalyst loading were 0.2 mg cm−2 for both Kx≈0.25IrO2 and IrO2. B


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H2SO4) and catalyst mass loading (0.2 mg cm−2 vs 4−6 mg cm−2). In here, the Ir site provided more efficiently catalytic activity in Kx≈0.25IrO2 comparable to IrO2. The IrO2 is very stable through OER process even in strong acidic conditions (see Figure S3), so the stability of Kx≈0.25IrO2 is worthy of attention. The chronoampermetric tests are carried out at a constant potential (1.68 V vs SCE) as shown in Figure 3c for 17 500 s, the current slowly declined for both Kx≈0.25IrO2 and IrO2, which is caused by oxygen bubbles gathered on the electrode surface. Figure 3d describes the OER polarization curves of Kx≈0.25IrO2 before and after chronoampermetric tests. The CV curves almost overlap showing that the catalyst remains stable during OER experiments. 2.3. Effects of the Preparing Conditions. In this work, there is a transformation from Kx≈0.25IrO2 to IrO2 with increasing hydrothermal temperatures and dwelling time observed from the SEM, TEM, and XRD measurements. As shown in Figure S4, one can distinctly find that the morphology of the prepared materials largely changes from needle-like to graininess. The XRD patterns (Figure 4a) confirm the transformation, accompanied by disappearance of the characteristic peaks at (1̅01), (002), and (4̅11) of Kx≈0.25IrO2, indicating that the catalyst gradually changes from Kx≈0.25IrO2 to a mixture of Kx≈0.25IrO2 and IrO2 and finally to IrO2. The transformation is mainly affected by temperature and is captured by TEM images. As shown in Figure 4b, the nanorods maintained a needle structure at 150 °C for 12 h, while they start to melt at 200 °C after 6 h (Figure 4c) and the melting is obviously severe after 12 h (Figure 4d). The mixed materials then further evolve to a grain structure with only little Kx≈0.25IrO2 at 250 °C after 6 h (Figure 4e). The polarization curves (Figure S5) obtained at different transformation stages further illustrate that Kx≈0.25IrO2 shows a higher performance of OER. 2.4. XPS and XAS Spectroscopy. We performed X-ray photoelectron spectroscopy (XPS) measurements (more detailed XPS spectra in Figures S6 and S7) and X-ray adsorption spectroscopy (XAS) to obtain valence information on Ir in Kx≈0.25IrO2 and IrO2. The XPS is very sensitive for characterizing the complex nature of oxides surface.50 Here, as shown in Figure 5a, the binding energies of Ir-4f7/2 and 4f5/2 in IrO2 are 61.78 and 64.73 eV, respectively, very close to the IrO2 single crystal values of 61.7 and 64.7 eV, respectively.51−53 While, for Kx≈0.25IrO2 a significant feature is that a shift to lower binding energy is clearly observed compared to IrO2, and the shift value is approximately 0.35 and 0.4 eV at 4f7/2 and 4f5/2, respectively. This suggests a lower valence of Ir and a higher electron density at the Ir site. The O-1s core level spectra are presented in Figure 5b, and the binding energy of lattice oxygen in Kx≈0.25IrO2 is 529.54 eV, which is lower than that of IrO2 (529.76 eV). This is mainly due to the different structures of these two oxides. Figure 5c is the unit cell of Kx≈0.25IrO2, dipicting that the K atom is bonded with the O atom. As the electronegativity of K is critically low and thus lattice oxygen gains an electron from the K atom more readily, leading to this lower binding energy. The higher electron density of the Ir site in Kx≈0.25IrO2 is further confirmed by XAS as shown in Figure 5d. The Ir-LIII edge XANES primarily corresponds to electron transition of 2p → 5d, which is allowed by dipole selection rules. An obvious feature is that a significant decrease of intensity could be observed in the so-called “white line region” in Kx≈0.25IrO2 sample compared with IrO2, which means that more 5d states of Ir in Kx≈0.25IrO2 are occupied than that of IrO2. The energy of the adsorption edge has been proven

that the slow diffusion of protons through the inner part of porous electrode will give rise to a decreasing Faradaic response. The second explanation is proposed by Sugimoto et al.,44 who show that it is mainly due to the sluggish behavior of the surface redox process, since Faradaic current only contributes at slow scan rate. In Figure 2b, plotting q* as a function of ν−1/2 gives a approximately straight line, and the intercept of the fitting lines gives the q0 of Kx≈0.25IrO2 and IrO2, which are 10.4 mC cm−2 and 8.6 mC cm−2, respectively. There is another feature of Kx≈0.25IrO2 is that one anodic peak in the forward scan and one cathodic peak in the backward scan are observed (see Figure S2), which might be caused by the low valency state of Ir45 because it was not observed for IrO2 under the same scan conditions (see Figure S2). In order to investigate the redox reaction, the voltammetric property is acquired at different scan rates between −0.2 and 0.4 V as shown in Figure 2c. The peak current for the anodic scan varied linearly with scan rates (see Figure 2d), which is primarily attributed to a surface couple.46 The as-measured OER activity of Kx≈0.25IrO2 and IrO2 prepared catalysts is capacity-corrected and iR-corrected in acidic solution and shown in Figure 3a. The Kx≈0.25IrO2

Figure 3. (a) Polarization curves with iR correction for OER of Kx≈0.25IrO2 and IrO2. The resistance of the solution was tested by EIS with about 33Ω. The vertical line indicated the theoretical potential for OER, the horizontal dash indicated the j = 10 mA cm−2. (b) Tafel curves of Kx≈0.25IrO2 and IrO2. (c) Chronoampermetric curves for Kx≈0.25IrO2 and IrO2 at the constant potential 1.68 V vs RHE. (d) Polarization curves for Kx≈0.25IrO2 at initial and after chronoampermetric experiments. The catalysts loading were 0.2 mg cm−2 on a Ti plate.

possesses a higher current density (j) than IrO2. The potential at j = 10 mA cm−2 is a very meaningful index to evaluate the OER activity of catalysts, because it is relevant to solar fuel synthesis. As shown in Figure 3b, the Kx≈0.25IrO2 reached such current density with a small overpotential η = 0.35 V while at the same η, IrO2 only afforded 3.15 mA cm−2, the Tafel data of Kx≈0.25IrO2 and IrO2 are 65 mV dec−1 and 74 mV dec−1, respectively. Many investigations on IrO2 show a Tafel slope around 60 mV dec−1, much lower than that in our work.47−49 Actually, the Tafel slope is critically susceptible to the electrolyte, the loading mass, particle size, and effective area of electrode. This difference in Tafel slope of IrO2 is mainly due to the different acidity of electrolyte (0.1 M HClO4 vs 0.5 M C


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three short bonds and three long bonds giving rise to a distorted octahedron. The second peak is mainly from Ir−Ir which corresponds to the c axis length of 3.158 Å for IrO2. While for Kx≈0.25 IrO 2, it is between two neighboring octahedron IrO6, 3.07 and 3.149 Å, which is obvious that the Ir−Ir distance of K x≈0.25IrO2 is shorter than that in IrO2. Therefore, the XPS and XAS data reveal that more electrons occupy the 5d states and the IrO6 octahedron is distorted further in Kx≈0.25IrO2. 2.5. DFT Calculations. This distorted octahedron and more occupied 5d states are the special properties of Kx≈0.25IrO2 and will make a significant effect on the energy distribution of the d-orbitals of Ir sites. The density of states (DOS) is a good descriptor for the bonding character and occupancy of the orbital states. Figure 6a is the PDOS of Kx≈0.25IrO2 and IrO2 using the general gradient approximation (GGA) calculation; details of this analysis are shown in the Calculation section of the Supporting Information. As in an ionic model, the five 5d electrons of Ir4+ in IrO2 in IrO6 octahedral coordination symmetry will spilt into two types of degeneration including occupied low energy t2g triplet (i.e., dxz, dyz, and dx2−y2) and leave the high energy eg doublet (i.e., dxy and dz2) empty;56,57 thus, its electron configuration can be given as t2g5eg0. In the Kx≈0.25IrO2 structure, the 5d electrons of Ir have a similar orbital degenerations as a result of having close octahedral coordination symmetry. In Figure 6a, the lowermost of conduction band in these two oxides are mainly arising from dxy and dz2 orbital and with bandwidth of 4 eV. However, we can find that the dz2 in Kx≈0.25IrO2 is pushed toward to a higher energy and broader relative to that in IrO2. This is because the apical Ir−O bond length is bigger which decreases repulsion of the apical fraction. We see that dxz and dyz give almost all of the electron density at the Fermi level (EF) for both oxides. However, the π bonding part (3−6 eV below EF) of dyz is reduced and in contrast its π antibonding has more states. This is mainly due to the four Ir−O bonds in the plane are unequal and within the yz orientation have less overlap with oxygen compared to the xz orientation. This distorted plane will also affect the dx2−y2 orbital energy distribution. The dx2−y2 in IrO2 is a very narrow one below EF and with ∼1.2 eV bandwidth, while it is broader and pushed toward the EF in Kx≈0.25IrO2. These broaden bands no doubt promote the O-2p (oxygen adsorbate intermediates) bonding with Ir-5d states. In short, the extra electrons in the Ir site (∼0.25 e/per Ir) and distorted octahedral symmetry of Kx≈0.25IrO2 give rise to these unique orbital energy distribution, and further these electronic structure changes have a significant effect on OER activity. During the OER process, two water molecules adsorbed on oxide activated sites go through four steps involving four proton-coupled electrons transfer processes and are ultimately converted to O2. For extensive metal oxides, the OER activity with the metal−oxygen (M−O) bond has a typical Volcano trend.58 This gives the principle that the most powerful catalyst must possess the ability to form the M−O bond neither too strong nor too weak. As previous studies reveal that the interaction between metal states and oxygen intermediates has a strong correlation with metal d-band states, the closer the dband center to EF, the stronger the chemical bond with oxygen adsorbates.59,60 In our calculation, the d-band of Kx≈0.25IrO2 is 2.07 eV which is closer to EF compared to IrO2 of 2.13 eV. This could also be observed by the PDOS where the antibonding states are pushed further away from EF. We also perform the density function calculation (DFT) to elucidate the nature of

Figure 4. (a) XRD spectra of the catalysts prepared at different hydrothermal temperatures and dwell times, along with the complete IrO2 phase spectrum for comparison. Again, the evolution of the conversion of Kx≈0.25IrO2 into IrO2 was distinctly confirmed by the disappearance of the diffraction peak of the a, b, c, and d regions denoted by the rectangles. (b−e) TEM images of prepared catalysts at 150 °C with 360 min, 200 °C with 360 min, 200 °C with 720 min, and 250 °C with 360 min, respectively.

extremely useful in determining the oxidation state of oxides. Although, the Ir-LIII edges in these two samples are almost overlapped, their first derivative (see the inset of Figure 5d) gives the change in oxidation state. The maximum adsorption energy of Kx≈0.25IrO2 is lower than that of IrO2, which indicates that the lower valence of Ir in Kx≈0.25IrO2 and this is consistent with our XPS revelation. The lower valence of Ir promises a longer bond of Ir−O in Kx≈0.25IrO2 than in IrO2. This phenomenon is confirmed by the extended X-ray adsorption fine structure (EXAFS). Figure 5d is the Fourier transform of k3χ(k) of the Ir-LIII edge of Kx≈0.25IrO2 and IrO2, and the phase correction is applied and the k-range is 3−16 Å. The first peak mainly arises from IrO6 coordination, and it can be clearly found that Kx≈0.25IrO2 has a larger R value than IrO2 implying that the Ir−O bond in Kx≈0.25IrO2 is longer. The Ir−O bonds in IrO2 are four long plane bonds of 1.998 Å (4L) and two short apical bonds of 1.96 Å (2S),54,55 while in Kx≈0.25IrO2 there are D


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Figure 5. (a) Ir-4f core level XPS of Kx≈0.25IrO2 and IrO2. (b) O-1s core level XPS of Kx≈0.25IrO2 and IrO2. (c) Crystal structure of Kx≈0.25IrO2, the right picture indicated in the cell by the dotted oval describes the electron transfer to oxygen from potassium and further makes the Ir−O bond more covalent. (d) The Ir LIII-edge XANES spectra for Kx≈0.25IrO2 and IrO2. The orbital energy diagram depicts how the white line comes when the 2p core−shell adsorbs the X-ray. The insert is the first derivative of normalized μ(E). (e) Fourier transforms of k3-normalized Ir-LIII edge EXAFS of Kx≈0.25IrO2 and IrO2. The dotted rectangles correspond to different coordination are indicated in parts c and f. (f) Crystal structure of IrO2.

ΔGOH* = 3.2 ± 0.2 eV, but this relationship for the perfect one is ΔGOOH* − ΔGOH* = 2.44 eV.61 In here, we find that ΔGOOH* − ΔGOH* is 3.22 eV for IrO2, while is really low for Kx≈0.25IrO2 with a value of 2.91 eV. It indicates that Kx≈0.25IrO2 has a priority to be a good OER catalyst in theoretical calculations. The third reaction step is generally regarded as the ratedetermining step (RDS) due to the rupture of the surfaceoxygen bonds; thus, the theoretical over potential is defined as ηtheory = (ΔGOOH* − 1.23) V. According to our calculation results, the free energy of the third step in Kx≈0.25IrO2 (ΔGOOH* = 1.73 eV) is really lower than IrO2 (ΔGOOH* = 1.84 eV), similarly, giving rise to a smaller theoretical overpotential of Kx≈0.25IrO2 (ηtheory = 0.50 V; IrO2, ηtheory = 0.61 V). It implies that the Kx≈0.25IrO2 has a better ability to reduce the energy of RDS.

3. CONCLUSION In summary, we have reported the hydrothermally synthesized Kx≈0.25IrO2 exhibiting a higher OER activity compared to IrO2. Kx≈0.25IrO2 can afford j = 10 mA cm−2 with lower over potential of η = 0.35 V, and the Tafel slope is only 65 mV dec−1. The XPS and XAS data show that a lower valence and more occupied Ir-5d states of Ir site in Kx≈0.25IrO2 give rise to a higher electron density at the Ir site. The extra electrons located on the Ir site and the distorted octahedral symmetry have a significant effect on 5d orbital energy distribution. The special electronic structure leads to a stronger interaction between Ir5d states and oxygen intermediates. The performed DFT calculations state that Kx≈0.25IrO2 possesses natural OER activity with a lower theoretical overpotential of ηthe = 0.50 V. Our studies suggest that hollandite iridium oxide, a new type of crystal structure, has a great potentiality for using in water oxidation due to its unique electronic structure.

Figure 6. (a) Calculated PDOS of 5d Ir of Kx≈0.25IrO2 and IrO2. The right pictures indicate the different IrO6 coordination symmetry of Kx≈0.25IrO2 and IrO2. The bonds length are marked, and we can find the IrO6 is a distorted octahedron in Kx≈0.25IrO2. The dotted line crossing 0 eV is the EF. (b, c) The calculated free-energy diagrams for the OER of IrO2 and Kx≈0.25IrO2. The RDS is indicated by the red vertical line.

the OER activity of K. Parts b and c of Figure 6 are the binding free energy diagrams of IrO2 and Kx≈0.25IrO2, respectively. For a wide class of metal oxides, a linear relationship was found in the binding free energy between OOH* and OH* with ΔGOOH* − E


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10159. Details about materials, experimental procedures, physical characterizations and theoretical calculations; figures including SEM, TEM images, EDS data, XPS spectra, and CVs for Kx≈0.25IrO2 and IrO2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is based on work supported by the National Natural Science Foundation of China (Grants 21177037, 21277045, and 21322307). We thank beamline BL14W1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.

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REFERENCES

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