Nanotubular IridiumâCobalt Mixed Oxide Crystalline Architectures

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Nanotubular Iridium-Cobalt Mixed Oxide Crystalline Architectures Inherited from Cobalt Oxide for Highly Efficient Oxygen Evolution Reaction Catalysis Areum Yu, Chongmok Lee, Myung Hwa Kim, and Youngmi Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12247 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017

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ACS Applied Materials & Interfaces

Nanotubular Iridium-Cobalt Mixed Oxide Crystalline Architectures Inherited from Cobalt Oxide for Highly Efficient Oxygen Evolution Reaction Catalysis Areum Yu,a Chongmok Lee,a Myung Hwa Kim,*a Youngmi Lee*a a

Department of Chemistry and Nano Science, Ewha Womans University, Seoul, 03760,

Korea *Co-corresponding authors: [email protected] (Y.L.); [email protected] (M.H.K.)

KEYWORDS: Electrospinning; Mixed Metal Oxide Nanotubes; Iridium; Cobalt; Electrocatalyst; Oxygen Evolution Reaction

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Abstract We report the unique transformation of one-dimensional tubular mixed oxide nanocomposites of iridium (Ir) and cobalt (Co) denoted as IrxCo1−xOy, where x is the relative Ir atomic content to the overall metal content. The formation of a variety of IrxCo1−xOy (0 ≤ x ≤ 1) crystalline tubular nanocomposites was readily achieved by electrospinning and subsequent calcination process. Structural characterization clearly confirmed that IrxCo1−xOy polycrystalline nanocomposites had a tubular morphology consisting of Ir/IrO2 and Co3O4; and Ir, Co and O were homogeneously distributed throughout the entire nanostructures. The facile formation of IrxCo1−xOy nanotubes was mainly ascribed to the inclination of Co3O4 to form the nanotubes during the calcination process, which could play a critical role in providing a template of tubular structure and facilitating the formation of IrO2 by being incorporated with Ir precursors. Furthermore, the electroactivity of obtained IrxCo1−xOy nanotubes were characterized for oxygen evolution reaction (OER) with rotating disk electrode (RDE) voltammetry in 1 M NaOH aqueous solution. Among diverse IrxCo1−xOy, Ir0.46Co0.54Oy nanotubes showed the best OER activity (the least positive onset potential, greatest current density and low Tafel slope) which was even better than commercial Ir/C. The Ir0.46Co0.54Oy nanotubes also exhibited a high stability in alkaline electrolyte. Expensive Ir mixed with cheap Co at an optimum ratio showed a greater OER catalytic activity than pure Ir oxide, one of the most efficient OER catalysts.


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Introduction In recent years, electrolytic water splitting has been investigated to generate pure hydrogen as a fuel for sustainable energy system.1 The oxygen evolution reaction (OER) is the process of generating molecular oxygen through electrolysis of water. This anodic half reaction of water splitting, OER, is a kinetically sluggish reaction with a significant overpotential, which lowers the overall efficiency of water splitting.2 As a result, efficient electrocatalysts providing a low overpotential for OER are highly demanded to reduce the energy barrier. Iridium dioxide (IrO2) is utilized as one of the most efficient electrocatalysts for the OER due to its activity and stability.3-5 However, iridium (Ir) is a quite expensive and scarce element, and therefore the scalable production and application are not practical. Hence, enormous researches on development of Ir composite electrocatalysts have been performed to reduce Ir content in electrocatalysts with enhanced efficiency: Ir composites with other transition metals (e.g., Ru,6-9 RuCo,10 RuMo,11 RuTa,12 Sn,13-14 Au,15-16 Ni,17-20 Ta,6, 21-23 Si,24 Co25-27 and Bi28) have been investigated. Among these transition metals, cobalt (Co) is nonprecious and has been reported to have relatively good OER activity.29-34 In this paper, we demonstrate the synthesis of one-dimensional (1D) nanomaterials consisted of mixed Ir and Co oxides aiming at developing a cost-effective and highly active electrocatalyst for OER. 1D nanomaterials are promising candidates for energy conversion, electronics, and biologic sensors due to their fascinating physical, optical, mechanic and electronic properties.35-37 Therefore, the fabrication of 1 D nanostructures with uniform size and morphology has been studied in recent years. Electrospinning is a versatile and efficient technique to synthesize 1D nanomaterials and has many advantages such as



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diversity of the electrospinning materials and simplicity. 1D Ir and Co oxides nanocomposites with various Ir to Co atomic ratios are prepared via a simple electrospinning of Ir and Co precursors mixed at different ratios, followed by calcination. Particularly, we explore the important roles of Co3O4 crystalline phase as an effective template for the tubular formation of mixed Ir and Co oxides; and as a favorable transformation of Ir metal into IrO2 crystalline phase. The OER activities of these mixed oxide nanocomposites are then investigated systematically as a function of the Co to Ir composition ratios and morphologies, and the optimum composition for the best OER activity is suggested.

Experimental Materials. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O), iridium chloride (IrCl3), poly(vinyl

pyrrolidone)

(PVP,

MW

≈

1,300,000),

ethanol

(99.9

%),

N,N-

dimethylmethanamide (DMF, 99.9 %), sodium hydroxide (NaOH), potassium nitrate (KNO3) and Nafion (5 wt% solution) were purchased from Sigma-Aldrich (St.Louis, MO,USA). Commercial Ir/C (20 wt% metal loading on Vulcan XC-72) was purchased from Premetek Company. All other chemicals used were of analytical grade, and all solutions were prepared with deionized water (resistivity ≥ 18 MΩ·cm). Synthesis of IrCo Composite Nanotubes. IrCo oxide nanocomposites with various compositions were synthesized by electrospinning and subsequent calcination. 187 mg of each metal precursor (Co(NO3)2·6H2O or IrCl3) was individually dissolved in 4.5 mL of a


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mixed solvent (ethanol:DMF = 1:1) and sonicated for 30 min. Ir precursor solution was further stabilized by being stirred for 1 day at room temperature. Then, Ir precursor solution and Co counterpart were mixed appropriately to achieve the desired relative ratios. PVP was then added to the mixed precursor solution to reach a final concentration of 87 mg mL−1 and the solution was stirred continuously overnight to be completely homogenized. As prepared precursor solutions with various Ir to Co ratios were placed into a plastic syringe and ejected from a needle with a flow rate of 10 µL min−1 using an electrospinning system (NanoNC ESR200R2). The distance from the metal needle to an aluminum plate where electrospun nanofibers were collected was set to 15 cm, and the applied field was 15 kV. The collected electrospun nanofibers were dried in a vacuum oven to remove the remaining solvent. Metal precursors/PVP nanofibers were calcined at 500 °C for 1 h with a continuous flow of O2 gas at 10 sccm and He gas at 80 sccm. The morphologies and compositions of as obtained IrCo oxide nanocomposites were characterized by fieldemission scanning electron microscopy (FESEM; JEOL JSM-7610F) equipped with an energy dispersive X–ray spectrometer (EDS), high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F), X-ray photoelectron spectroscopy (XPS; ESCA Lab 250 XPS, Al Kα radiation) and high resolution X-ray diffraction (XRD; Rigaku D/Max-2000/PC X-ray diffractometer using Cu K radiation) Electrodes and Electrochemical Measurements. IrCo oxide nanocomposites were dispersed in deionized water with a concentration of 2 mg mL−1. A glassy carbon (GC) disk electrode (3 mm in diameter) was polished with 0.3 µm aluminum powder slurry on a polishing cloth and then sonicated in distilled water for 5 min to remove any alumina



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residue. For electrochemical characterization, 6 µL of a well-dispersed nanocomposite solution was placed onto the cleaned GC disk electrode and then the electrode was dried in an oven at 60 °C for 10 min. These loading and drying steps were repeated five times to load 60 µg of each catalyst in total. Then, 10 µL of 0.05 wt% Nafion (diluted in ethanol) was applied onto the modified GC electrode. All electrochemical measurements were performed with a three-electrode cell system. A GC electrode loaded with each nanotube catalyst was used as the working electrode. A saturated calomel electrode (SCE) and a coiled Pt wire were employed as the reference electrode and counter electrode, respectively. Teflon cells (BASi) were used. First, cyclic voltammetry (CV) experiments to investigate the general electrochemical properties of the nanocomposites were conducted in an Arsaturated 1 M KNO3 aqueous solution for a potential range from 0.3 V to 0.6 V using a CHI 720C electrochemical workstation (CH Instruments, USA). Electrochemical activities of these nanocomposites toward OER were characterized with rotating disk electrode (RDE) voltammetry in 1 M NaOH aqueous solution. RDE voltammetry was carried out using a RDE-1 rotor/Epsilon electrochemical analyzer (BASi). iR drops were compensated for all the voltammetry results.

Results and Discussion Synthesis and Characterization of IrCo Oxide Nanocomposites. Figure 1 shows a scheme of IrCo oxide nanocomposites formation. Firstly, the precursor solution was delivered to syringe and electrospun onto an aluminum plate, and the collected


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calcined IrCo oxide nanocomposites

Figure 1. Synthetic scheme of IrCo oxide nanocomposites.

nanofibers comprising of PVP and metal precursors (IrCl3 and Co(NO3)2) were thermally annealed at 500 °C under flowing the mixed He (80 sccm) and O2 (10 sccm) gas of atmospheric pressure for 1 h. During this thermal annealing process, PVP was removed and IrCo oxide nanocomposites were produced. Depending on the metal precursor mixing ratio, IrCo oxide nanocomposites with seven different Ir to Co contents were prepared. The nanocomposites are denoted as IrxCo1−xOy where x is the relative Ir atomic content to the overall metal content determined with EDS. Figures 2A-B show the SEM images of the nanofibers prepared via electrospinning a solution containing Ir and Co precursors at 1:1 atomic ratio and PVP before calcination. From the SEM measurements, it was confirmed that as-spun Co(NO3)2+IrCl3/PVP nanofibers possessed smooth surface and their diameters are relatively uniform with the estimated average diameter of 176.3 (± 27.0) nm. PVP is



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one of the most commonly used polymers for electrospinning techniques due to its advantages characteristics.38-39 In fact, PVP shows low toxicity, high hydrophilic property, and high solubility in water as well as diverse organic solvents;40-41 and consists of C, N, H and O atoms, leading to facile decomposition via forming CO2, NO2 and H2O during calcination.39 Figures 2C-I shows the representative SEM images of the IrxCo1−xOy nanocomposites with 0 ≤ x ≤ 1 obtained after calcination at 500 °C. It was evident that as the Ir content increased, the surfaces of the nanocomposites became rougher, indicating a higher porosity; and the overall structures looked like rather nanofibers than nanotubes. On the contrary, the materials with higher Co content had smoother surfaces and clearly nano-tubular structures with thin walls, making them appear translucent. At first glance, this structure difference depending on the metal relative ratio is induced presumably due to the different metal precursor decomposition temperature. During the calcination process, the decomposition of a metal precursor can take place and a metal oxide starts to form, along with PVP combustion. Previous work36 strongly suggested that the rigid shell formation of metal oxide aggregates on the surface of nanofibers prior to the complete combustion of PVP polymer could play an essential role in the formation of the final product with a tubular morphology. It was thus proposed that once the oxide shell was made, further the continuous removal of PVP in the core could induce the migration of remaining metal ions onto the surface. This transformation indeed leads to the well-defined formation of tubular metal oxide crystalline structures after completing the oxidation of PVP. In our study, Co3O4, generated from a precursor Co(NO3)2, fulfills this condition and therefore tends to


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Figure 2. SEM images of (A,B) electrospun Co(NO3)2+IrCl3/PVP nanofibers, IrxCo1−xOy nanotubes prepared with different compositions of (C) Co3O4, (D) Ir0.22Co0.78Oy, (E) Ir0.33Co0.67Oy, (F) Ir0.46Co0.54Oy, (G) Ir0.60Co0.40Oy, (H) Ir0.71Co0.29Oy and (I) IrOy. form a tubular morphology after calcination, as previously reported.

38-39

In contrast, the

fabric morphology is much more favorable than the tubular morphology in the case of pure IrO2 formation as shown in Figure 2I. The reason is likely that the decomposition of the precursor, IrCl3, requires relatively higher temperature to form IrO2 crystalline structure, the formation of the core-shell structure prior to the complete PVP removal could not be feasible under such an environment.42,41



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Figure 3. (A) TEM image and (B) high-resolution TEM image of Ir0.46Co0.54Oy nanotubes. (inset: SAED patterns). (C) TEM image used for elemental mapping of Ir0.46Co0.54Oy nanotubes. (D–F) Elemental mapping analysis of Ir, Co, and O atoms in Ir0.46Co0.54Oy nanotubes. The relative ratios of Ir and Co were determined through the EDS results which were analyzed at more than 20 places. After being calcined, Co(NO3)2+IrCl3/PVP nanofibers with 1:1 precursor atomic ratio was transformed to Ir0.46Co0.54Oy nanotubes of which diameters were decreased considerately to 76.0 (± 12.5) nm due to the combustion of PVP polymer and decomposition of meta l precursors. We further explored the morphology and distribution of elements in the Ir0.46Co0.54Oy nanotubes by TEM. The hollow morphology with a thin wall of Ir0.46Co0.54Oy nanotubes was apparently shown in Figure 3A. To determine the crystalline structure of Ir0.46Co0.54Oy nanotubes, the high resolution TEM (HRTEM) image was analyzed as seen in Figure 3B. HRTEM image and the fast Fourier transform (FFT) image confirm the existence of


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Figure 4. XRD spectra of various IrxCo1−xOy nanocomposites with 0 ≤ x ≤ 1. The peaks marked with * and ■ are XRD peaks of Co3O4 and Ir metal. The peaks without mark are XRD peaks of IrO2. specific crystalline planes corresponding to both IrO2 and Co3O4 crystal structures such as (110) for IrO2 rutile phase and (311) for cubic Co3O4 phase, indicating of the polycrystalline nature of the nanotubes. Furthermore, the EDS elemental mapping images illustrate that Ir, Co and O elements are homogeneously distributed throughout the Ir0.46Co0.54Oy nanotubes (Figures 3D–F). The crystal phases of the various IrxCo1−xOy nanocomposites were investigated with the XRD spectra in Figure 4. The XRD spectrum of pure Co3O4 nanotubes showed five distinct



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peaks at 31.1°, 36.7°, 44.8°, 59.3° and 65.2°, being assigned to (220), (311), (400), (511) and (440) planes of the Co3O4 cubic structure consistent with reference values (JCPDS-431003).39 For the XRD spectrum of the IrOy nanofibers, prepared from only Ir precursor without Co precursor, exhibited small peaks at 27.8°, 34.5° and 53.9°, representing the peaks of IrO2, but it had strong intense peaks at 40.5°, 47.2° and 69.0° corresponding to the (111), (200) and (220) plane of Ir(0) metal.43 This clearly indicates that IrOy sample exists mostly in the form of Ir(0) metal. On the other hand, the XRD pattern of Ir0.46Co0.54Oy nanotubes represents the mixed metal oxides consisted of both Co3O4 cubic phase corresponding to (311) and (400) planes and IrO2 rutile phase corresponding to (110), (101) and (211) planes. Interestingly, the majority of Ir in Ir0.46Co0.54Oy nanotubes is present as the metal oxide phase of IrO2 rather than the pure metal phase of Ir(0). This is sharply contrasted to the case of pure IrOy nanofibers. In our previous works, a lower calcination temperature than 750 °C leads to the favorable formation of the pure metal phase of Ir(0) with the fiber-like morphology.44 As aforementioned, it is reasonably understood that the decomposition temperature of IrCl3 precursor is relatively high to form iridium oxides and further the surface migration kinetics of Ir ions is not favorable so that the complete oxidation of Ir precursor would be only achieved by increasing temperature up to 900 °C.44 In contrast, the facile initial formation of Co3O4 on the nanotube shell provides oxygen-rich environment and therefore possibly facilitates Ir oxide formation and the surface migration of Ir ions, resulting in the production of the mixed homogeneous metal oxide composites with a tubular morphology. Compared to the XRD peaks of pure IrO2 and pure Co3O4, the various mixtures of Ir and Co represented the formation of two distinct alloy structures:


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IrxCo1−xO2 in the tetragonal phase and CoxIr3−xO4 in the cubic phase. Particularly, IrxCo1−xOy nanotube structures (0 < x < 1) exhibited the characteristic IrO2 peaks at higher angles and the characteristic Co3O4 peaks at lower angles. In fact, when Ir content is greater than Co, the nanotubes showed rutile crystalline structures predominantly, suggesting Co incorporation into IrO2 tetragonal lattice structure as IrxCo1−xO2.25 In contrast, when Ir content is lower than Co, the nanotubes had both rutile and cubic structures, indicating both Co incorporation into IrO2 rutile structure (IrxCo1−xO2) and Ir incorporation into Co3O4 cubic structure (CoxIr3−xO4).45 The results, thus, suggest that the mixed oxide of the tetragonal crystal structure is much more favorable in the high content of Ir due to the smaller ionic radius of Co in the tetravalent ionic state. However, the binary mixture systems under the high content of Co become also favorable to have the mixed oxide formation of CoxIr3−xO4 in the cubic phase. Therefore, the nanotubes are considered to be quite homogeneous solid solution over the partial compositional range of two metal oxides where the actual crystalline structures are highly dependent of the relative composition ratios. Ir0.46Co0.54Oy nanotubes were further analyzed by XPS. As shown in Figure 5A, the high-resolution Ir 4f XPS spectrum exhibited two peaks of 61.4 and 64.4 eV, corresponding to the 4f7/2 and 4f5/2 of 4+ oxidation state of Ir.44 In Figure 5B, the Co 2p XPS spectrum showed two distinct peaks at 780.4 and 796.1 eV, attributed to Co 2p3/2 and Co 2p1/2, respectively, confirming the oxidation state of Co as Co3O4.46 Two peaks of Ir0.46Co0.54Oy nanotubes observed at 61.4 and 64.4 eV in Figure 5A are slightly shifted from those of pure IrO2 nanofibers at 61.1 and 64.2 eV. 44 It implies that the binding energy of Ir4+ in the



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Figure 5. (A) High resolution Ir 4f XPS spectrum, and (B) high resolution Co 2p XPS spectrum of Ir0.46Co0.54Oy nanotubes. mixed oxide form could be affected by both the Co4+ merged into the sites of Ir4+ of the IrO2 tetragonal lattice and Ir3+ merged into the sites of Co3+ of the Co3O4 cubic lattice. Furthermore, this phenomenon is also observed in the region of Co 2p XPS spectrum of Ir0.46Co0.54Oy nanotubes compared to those of the pure forms of two oxides (at the binding energies of 780.3 and 794.8 eV) reported in our previous work.38 Although the XPS data reasonably suggest the existence of the mixed oxide formation in consistent with the results of XRD, it is necessary to confirm the actual chemical states of Ir and Co for Ir0.46Co0.54Oy nanotubes in two distinct crystal lattices by X-ray absorption spectroscopy.


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Figure 6. (A) Cyclic voltammograms of the Ir0.46Co0.54Oy nanotubes in 1 M KNO3 aqueous solution at various scan rates of 10, 20, 50, 100, 150 and 200 mV s−1 in the range of 0.3 V – 0.6 V vs. SCE. (B) Plots of current difference at 0.45 V against scan rate. (C) Cyclic voltammograms of various IrxCo1−xOy nanocomposites with 0 ≤ x ≤ 1 in 1.0 M KNO3 at scan rate of 0.1 V s−1.



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Electrocatalytic Activity of IrCo oxide Nanocomposites. In order to evaluate the electrochemical properties of the catalysts, CV was performed in 1 M KNO3 aqueous solution. Figure 6A shows the CV curves for Ir0.46Co0.54Oy nanotubes with various scan rates of 10, 20, 50, 100, 150 and 200 mV s−1 in a range of 0.3 V – 0.6 V vs. SCE. The CV curves had a rectangular shape, indicating the fast charging/discharging processes. In Figure 6B, difference between charging/discharging currents at 0.45 V of IrxCo1−xOy nanocomposites were plotted as a function of scan rate. In CV of Ir0.46Co0.54Oy nanotubes, the current at 0.45 V was linearly proportional to scan rate (R2 = 0.9998), even at a high scan rate, showing a typical electrical double-layer behavior without any faradaic processes observed. Whereas the IrxCo1−xOy nanocomposites (x = 0 and 0.22) did not exhibit a linear dependence. In addition, various IrxCo1−xOy nanocomposites with 0 ≤ x ≤ 1 were studied with CV in 1 M KNO3 at a scan rate of 100 mV s−1 for the same potential range as seen in Figure 6C. As the content of Ir increases from Co3O4 to Ir0.46Co0.54Oy, the capacitive current increases and CV curve shape becomes more like a rectangle. In fact, both Ir0.46Co0.54Oy and Ir0.60Co0.40Oy had the largest capacitive current in a similar extent but Ir0.46Co0.54Oy showed a CV curve in the most ideal rectangular shape. However, the further increase in Ir content caused the decreased capacitive current. In other words, Ir0.46Co0.54Oy composite nanotubes approached to an optimum mixing ratio of Ir and Co, showing the highest capacitance and fastest charging/discharging processes among the IrxCo1−xOy nanocomposites. It could be attributed to the both morphology effect of tube shape and synergetic effect between Co and Ir oxides. According to previous report, the incorporation of Co in the IrO2 framework influenced the conductivity and electrochemical properties of


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the electrode.25 As substituted atoms play as defects in each structure, the conductivity is increased.47 It has been also reported that Co mixed oxide composites exhibit superior capacitive performance than single metal oxides, Mn,48-50 Ni,51 etc. Likewise, the incorporation of Co (or Ir) into the other framework possibly induces the increased conductivity of IrxCo1−xOy nanocomposites. In addition, the tube morphology resulting in the increased electrochemical surface area (ECSA) also affect the capacitive current. The electrocatalytic performances of IrxCo1−xOy nanocomposites with 0 ≤ x ≤ 1 for OER in an alkaline electrolyte were tested with RDE voltammetry in 1 M NaOH aqueous solution at an electrode rotation speed of 1600 rpm. Electrode rotation was conducted to remove the generated oxygen bubbles from the electrode surface. RDE voltammetric curves of the catalyst materials are shown in Figure 7A. Each polarization curve was normalized by the corresponding electrode geometric surface area (GSA), which was obtained with chronocoulometry (CC) experiment carried out in 10 mM of K3Fe(CN)6 solution containing 0.1 M KCl. The GSA was estimated from the slope of the linear plot of the measured charge (Q) vs. time1/2.52 Figure 7B shows the Tafel plots for Figure 7A, OER at the tested various electrocatalysts, based on Tafel equation (equation 1). The calculated Tafel slopes are presented in Table 1. η = a + b log ()

(1)

where η is the overpotential, j is the measured current density, and b is the Tafel slope. Tafel plot is generally applied to evaluate the activity of catalytic reaction.53 While Co3O4 nanotubes had the most positive OER onset potential, the onset potential shifted to less



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positive direction, indicating the lowered OER overpotential, with the increase of Ir content upto Ir0.46Co0.54Oy nanotubes (Figure 7A).


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Table 1. Comparison of the Tafel slopes and potentials achieving 10 mA cm−2 for various IrxCo1−xOy nanocomposites with 0 ≤ x ≤ 1. The average and standard deviation were obtained from five measurements for each sample. Materials Tafel slope (mV dec−1) Potential at 10 mA cm−2 (V vs. RHE)

Co3O4

Ir0.22Co0.78Oy

Ir0.33Co0.67Oy

Ir0.46Co0.54Oy

Ir0.60Co0.40Oy

Ir0.71Co0.29Oy

IrOy.

66.2

65.3

69.9

58.6

57.4

55.8

67.4

(±11)

(±5.5)

(±4.9)

(±1.9)

(±2.5)

(±2.3)

(±4.4)

1.660

1.565

1.557

1.540

1.563

1.561

1.622

(±0.005)

(±0.011)

(±0.011)

(±0.003)

(±0.002)

(±0.003)

(±0.009)



j / mA cm

-2

A

Co3O4 Ir0.22Co0.78Oy Ir0.33Co0.67Oy Ir0.46Co0.54Oy Ir0.60Co0.40Oy Ir0.71Co0.29Oy IrOy

80

60

40

20

0 1.1

1.2

1.3

1.4

1.5

1.6

1.7

Potential /V vs RHE

B Potential /V vs RHE

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1.70 Co3O4 Ir0.22Co0.78Oy Ir0.33Co0.67Oy Ir0.46Co0.54Oy Ir0.60Co0.40Oy Ir0.71Co0.29Oy IrOy

1.65

1.60

1.55

1.50 -1.0

-0.5

0.0

0.5

1.0

1.5

Log (j)

Figure 7. (A) iR-compensated RDE voltammograms for the OER in Ar-saturated 1 M NaOH aqueous solution at a scan rate of 10 mV s−1 with a rotating speed of 1600 rpm. (B) Tafel plots for the OER obtained from the RDE voltammograms show in


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The additional Ir content increase affected adversely causing the onset potential shift toward a more positive direction, and pure IrOy nanofibers had the highest onset potential except Co3O4 nanotubes among the catalysts tested. Ir0.46Co0.54Oy nanotubes exhibited the highest current density and lowest onset potential among the catalysts compared. The Tafel slope of Ir0.46Co0.54Oy nanotubes was measured to be 58.6 mV dec−1 in 1 M NaOH. These values were close to the ones for Ir-base materials reported in the literatures.5, 8, 14, 17, 23-24 The measured low Tafel slopes of IrxCo1−xOy nanocomposites with 0.46 ≤ x ≤ 0.71 suggest the favorable OER kinetics at the materials having Ir, in contrast to Co3O4 nanotubes having a relatively large Tafel slope, 66.2 mV dec−1. Table 1 also presents the potentials achieving 10 mA cm−2 for the catalysts tested in current study. Given that the thermodynamic potential of OER is 1.23 V vs. RHE, Ir0.46Co0.54Oy nanotubes reach a current density of 10 mA cm−2 at an overpotential of 0.31 V. Overall, Ir0.46Co0.54Oy nanotubes showed the lowest onset potential and overpotential at 10 mA cm−2, low Tafel slope and highest current density compared to IrOy nanofibers and Co3O4 nanotubes, suggesting the highest electroactivity. In fact, Ir0.46Co0.54Oy nanotubes showed a relatively low Tafel slope and the lowest potential at 10 mA cm−2 among the tested samples and the differences from the data values of the other samples were significant based on paired t-test (Figure S1 in Supporting Information). It is very appreciable that expensive Ir mixed with cheap Co at a certain ratio exhibits a greater OER catalytic activity than pure Ir oxide, one of the most efficient OER catalysts.3-5 The efficient electrochemical and catalytic properties of Ir0.46Co0.54Oy nanotubes could be ascribed to the synergetic effect of Ir and Co in the composite and unique tubular morphology. As aforementioned, the highest activity of Ir0.46Co0.54Oy among IrxCo1−xOy



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nanocomposites is possibly attributed to the presence of distorted rutile and cubic phases in the greatest degree as observed in the XRD results (Figure 4). In Table 2, the OER activity of Ir0.46Co0.54Oy nanotubes was compared with that of other Ir-composite materials which have been recently reported. Considering both overpotential at 10 mAcm−2 and Tafel slope, Ir0.46Co0.54Oy nanotubes, synthesized in a simple electrospinning, show comparably noticeable activity to other Ir-composite catalysts. To verify the stability of Ir0.46Co0.54Oy nanotubes as an OER catalyst, RDE voltammetry for OER was carried out. As seen in Figure 8A, the shapes of voltammetric curves were retained during the 1000-iterative potential sweeps, confirming the stability of the catalyst. Comparing the first and last scans, the polarization curves were virtually identical after 1000 scans. Figure 8B is a graph showing the potentials at which current is 10 mA cm−2 for every 100th scan. The potential generating 10 mA cm−2 was not changed during the 1000 repetitive scans. The Ir0.46Co0.54Oy nanotubes shows excellent stability in an alkaline solution. In general, transition metal oxides show a higher stability than the metals during OER because the metal oxides are already oxidized forms without possible changes via oxidation.2 In this aspect, the high stability of Ir0.46Co0.54Oy nanotubes could be ascribed to their oxide natures: both Ir and Co were present as the oxides (vide supra, Figures 4 and 5 and the related discussion). Finally, the OER activity of the most active Ir0.46Co0.54Oy nanotubes was compared with that of commercial Ir/C (Figure 9). Tafel slope of Ir/C was 54.0 mV dec−1, slightly smaller than that of Ir0.46Co0.54Oy nanotubes. It is notable that Ir0.46Co0.54Oy nanotubes


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exhibit a lower onset potential and also attain a current density of 10 mA cm−2 at a less positive potential (i.e., lower overpotential), compared to Ir/C, indicating the good OER catalytic efficiency.



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Table 2. Comparison of the OER catalytic performance of Ir0.46Co0.54Oy nanotubes with other Ir-base materials. Catalysts

Solution

Ir0.46Co0.54Oy nanotubes

1 M NaOH

Overpotential (V) at 10 mA cm−2 vs. RHE 0.31

RuIrCoOx

0.5 M H2SO4

AuIr/C

Tafel slope (mV dec−1)

Reference No.

58.6

This work

0.394

70

10

0.1 M NaOH

0.394

-

16

Ir0.7Ni0.3O2−y

0.5 M H2SO4

0.335

57

17

Ir/Ni oxide

1 M KOH

0.264

44

20

IrO2–Ta2O5 oxide

0.5 M H2SO4

0.362

59

23

0.5IrO2-0.5SiO2

0.5 M H2SO4

0.322

80

24

Leached-Ir0.7Co0.3Ox

0.5 M H2SO4

0.33

40

25

Bi2Ir2O7

1 M H2SO4

0.365

45

26

nc-IrOx/nc-CoOx

1 M NaOH

0.18a

29−34

IrNi0.125Ox

0.1 M H2SO4.

0.28a

-

18

Ru0.7Ir0.3O2

0.5 M H2SO4

0.35b

45

8

Overpotentials are obtained at 10 mA cm−2, aat 1 mA cm−2, bat 100 mA cm−2, given that the thermodynamic OER potential is 1.23 V vs. RHE.


Page 25 of 34

Figure 8. (A) OER polarization curves of Ir0.46Co0.54Oy nanotubes subjected to iterative potential cyclings in Ar-saturated 1 M NaOH aqueous solution for the initial, 100th, 200th, 500th, 800th, and 1000th scans. (B) A graph showing the potentials producing 10 mA cm−2 at every 100th scans.

Ir0.46Co0.54Oy Ir/C

80

Current /mA

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 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0 1.1

1.2

1.3

1.4

1.5

1.6

Potential /V vs RHE

Figure 9. Comparison between Ir0.46Co0.54Oy nanotubes and Ir/C for the OER in Arsaturated 1 M NaOH aqueous solution at a scan rate of 10 mV s−1 with a rotating speed of 1600 rpm



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Conclusions Diverse 1D IrxCo1−xOy nanocomposite materials with 0 ≤ x ≤ 1 for OER were prepared via a simple electrospinning of Ir and Co precursors mixed at different ratios, followed by calcination. From a pure Co precursor, Co3O4 nanotubes having thin and smooth walls were prepared. As the Ir content increases, the tube surface became rougher and more porous and pure IrOy exhibited a nanofiber-like morphology rather than a nanotube. This morphology difference depending on the mixing ratio is considered to be originated from the different decomposition temperatures of Ir and Co precursors. In addition, the facile formation of IrxCo1−xOy nanotubes was ascribed to the properties Co3O4 to form the nanotubes during the calcination process. The facile formation of Co3O4 shell in IrxCo1−xOy possibly provides a template of the tubular structure, facilitates the formation of IrO2 by being incorporated with Ir precursors, and eventually produces the mixed IrCo oxide nanotubes. It was confirmed that IrxCo1−xOy nanocomposites with 0 < x < 1 were composed mainly of IrO2 and Co3O4.

As-prepared 1D IrxCo1−xOy nanocomposites were

studied systematically toward OER activity. Among diverse IrxCo1−xOy catalysts, Ir0.46Co0.54Oy nanotubes showed the best OER activity (the most negative onset potential, greatest current density and low Tafel slope). The OER performance of the Ir0.46Co0.54Oy nanotubes was significantly more efficient than pure IrOy nanofibers and Co3O4 nanotubes, which could be ascribed to the synergetic effect of combining Ir and Co oxides. In addition, Ir0.46Co0.54Oy nanotubes exhibited a high durability in alkaline electrolyte during 1000 iterative OER scans.


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Acknowledgements This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT and Future Planning (NRF2017R1A2A2A14001137 for YL) and (2016R1D1A1B03934962 for MHK).

Supporting Information The averaged Tafel slopes and potentials at a currend density of 10 mA cm−2 obtained for five repetitive measurements of various IrxCo1−xOy nanocomposites (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.



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ToC Graphic


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