IrO2 Nanofibrous Structures Decorated with

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Single-Step Electrospun Ir/IrO Nanofibrous Structures Decorated with Au Nanoparticles for Highly Catalytic Oxygen Evolution Reaction Sinyoung Moon, Yun-Bin Cho, Areum Yu, Myung Hwa Kim, Chongmok Lee, and Youngmi Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14563 • Publication Date (Web): 24 Dec 2018 Downloaded from http://pubs.acs.org on December 26, 2018

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ACS Applied Materials & Interfaces

Single-Step Electrospun Ir/IrO2 Nanofibrous Structures Decorated with Au Nanoparticles for Highly Catalytic Oxygen Evolution Reaction Sinyoung Moon,† Yun-Bin Cho,† Areum Yu, Myung Hwa Kim, Chongmok Lee, Youngmi Lee* Department of Chemistry and Nano Science, Ewha Womans University, Seoul, 120-750, Korea *Corresponding author: (Fax) +82-2-3277-2384, (E-mail) [email protected]. †These

authors contributed equally to this work.

KEYWORDS: Iridium oxide; Gold; Fibrous Nanocomposite; Electrospinning; Oxygen evolution reaction; Electrocatalysis

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ABSTRACT

Nanocomposites of gold (Au) and iridium (Ir) oxide with various compositions (denoted as AuxIr1−xOy, x = 0.05, 0.10 or 0.33, Au precursor molar ratio to Ir precursor) were synthesized via electrospinning and subsequent calcination method with two different solvent composition ratios of ethanol to N,N-dimethylformamide (DMF) in the electrospinning solution (ethanol:DMF = 70:30 or 50:50 v/v%). Simple single-step electrospinning successfully fabricated a hierarchical nanostructure having Au nanoparticles formed on fibrous main frames of Ir/IrO2. Different solvent composition in the electrospinning solution induced the formation of main frames with distinct nanostructures; nanoribbons (AuxIr1−xOy-70) with ethanol:DMF = 70:30; and nanofibers (AuxIr1−xOy-50) with ethanol:DMF = 50:50. The pure Ir or Au counterparts (IrOy and Au) were also prepared by the same synthetic procedure as AuxIr1−xOy. Oxygen evolution reaction (OER) activities of as synthesized AuxIr1−xOy were investigated in 0.5 M H2SO4 and compared with those of IrOy, Au and commercial iridium (Ir/C, 20% Ir loading on Vulcan carbon). Among them, Au0.10Ir0.90Oy-50 exhibited the best OER activity, even better than previously reported catalysts containing both Ir and Au. The high OER activity of Au0.10Ir0.90Oy-50 was mainly attributed to the fiber frame structure and the optimal interfacial areas between Au and Ir/IrO2 which are electrophilic OER active sites. The stability of Au0.10Ir0.90Oy-50 was also evaluated to be much higher than those of Ir/C during OER. Current study suggests that the presence of Au on the Ir/IrO2 surface improves the OER activity of Ir/IrO2.


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1. INTRODUCTION In the face of an energy crisis, efficient energy conversion/storage systems such as water splitting have been attracting a great attention. Electrochemical water splitting involves oxygen evolution reaction (OER) at an anode and hydrogen evolution reaction (HER) at a cathode.1 HER at the cathode proceeds with a relatively insignificant overpotential using platinum-based catalysts particularly in acidic environments.1-8 Overall performance of water electrolysis, therefore, is limited by the coupled OER at the anode due to the sluggish reaction rate having a high overpotential.9 The reason for the slow kinetics of OER is that the reaction consists of multiple elementary reaction steps involving 4-electron transfer and generating many intermediate species.10 Due to its complexity, several different OER mechanisms have been suggested.10 The following mechanism is the generally accepted one on metal oxide surface in acidic media:11 (I)

S + H2O → S-OHads + H+ + e−

(II)

S-OHads → S-Oads + H+ + e−

(III)

S-Oads + H2O → S-OOHads + H+ + e−

(IV)

S-OOHads → S + O2 + H+ + e−

where S is the surface active site of a heterogeneous catalyst, and -OHads, -Oads, and -OOHads are hydroxyl, oxygen, and hydroperoxyl species adsorbed on the surface active site, respectively. The overall OER kinetics is determined by the rate-determining step (RDS), an elementary reaction step having the highest activation energy, and RDS depends on catalytic materials.9 Therefore, to improve the performance of water splitting in acidic environment, efficient and stable OER electrocatalysts, which can accelerate the reaction rate of the RDS with lowering the overpotential,



need to be developed. High OER activity has been reported for the materials consisting of noble metals, in particular their oxide forms, due to the practical and stable use for a long-term period.12 One of the state of the art catalyst materials for OER in acidic medium is iridium dioxide (IrO2) possessing high activity and stability.9,13 On IrO2-based catalysts, reaction (I) above is divided into two concerted reactions as follows and the combination of these subreactions is known as the RDS:14 (I-1)

S + H2O → S-OH*ads + H+ + e-

(I-2)

S-OH*ads → S-OHads

where S-OH*ads and S-OHads have the same chemical structure, but different energy states due to a different bond strength to the surface active site. Considering the suggested RDS above, it is reasonably inferred that the more electrophilic the catalyst surface is, the more facilitated RDS. It has been reported that Ir complex with higher oxidation states has a better ability for water oxidation.13 It might be because more electrophilic surface of Ir at higher oxidation state promotes RDS compared to that at lower oxidation state. Among transition metals, Au has the highest electronegativity.15 Au present contiguously to IrO2 can act as an electron sink and therefore the catalytic surface of IrO2 may become more electrophilic.16 It would make the progress of RDS easier improving its OER activity. There are only a few previous studies on iridium oxide-gold (IrOx-Au) nanocomposites for OER activities. In these works, the catalysts were prepared by reduction of aqueous precursor solution using reducing agent (nanoflower),16 electrooxidation of electrodeposited materials (dendrite),17 and electroless deposition (nanoparticle).18


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Fibrous nanostructured materials are promising electrocatalysts due to their advantages of high electrical conductivity and stability,19-21 and electrospinning is a useful tool for the synthesis of fibrous nanomaterials.20,22-24 Furthermore, compared to other synthetic methods, such as wet synthesis, chemical vaper deposition or electrodeposition, this synthetic tool has powerful advantages of simplicity, up-scalability and high versatility in terms of compositions and morphologies. In the light of the advantages of fibrous structured catalysts, in this study, we demonstrate the synthesis of a series of fibrous Au/Ir oxide nanocomposites with diverse Au and Ir contents via electrospinning and post-calcination method. By varying the solvent composition in the electrospinning solution (the volumetric ratio of ethanol to N,N-dimethylformamide), Au/Ir oxide nanocomposites with two different structures were synthesized, i.e., nanoribbon- and nanofiberstructures. Their OER activities depending on metal composition and solvent composition (i.e., nanostructure) were carefully investigated and compared in 0.5 M aqueous sulfuric acid. The precise onset potential for OER were also determined using SECM. SECM has been utilized for the determination of the reaction onset potentials,25,26 or the screening the catalyst’s activity at a constant current or a constant potential.27,28 To our best knowledge, this is the first study on OER activity of fibrously-structured IrOx-Au composites prepared by electrospinning method.

2. EXPERIMENTAL SECTION 2.1. Materials Iridium chloride hydrate (IrCl3•xH2O, 99.9% trace metals basis), polyvinylpyrrolidone (PVP, average MW ~1,300,000), ethanol (ACS reagent, ≥ 99.5%), N,N-dimethylformamide (DMF, anhydrous, 99.8%), sulfuric acid (H2SO4, 95.0-98.0%), hexaammineruthenium(III) chloride



([Ru(NH3)6]Cl3), potassium nitrate (KNO3) and Nafion (5 wt% solution) were purchased from Sigma-Aldrich (St. Louis, MO). Hydrogen tetrachloroaurate hydrate (HAuCl4•xH2O, 99.9% metals basis, Au 49% min) was from Alfa Aesar. Commercial Ir/C (20 wt% Ir loading on Vulcan XC-72) was purchased from Premetek Co. All the chemicals were of analytical-reagent grade and used as received without further purification. All aqueous solutions were prepared using deionized water (resistivity ≥ 18 MΩ•cm). 2.2. Synthesis of the Nanocomposites The electrospinning solution consisted of 0.21 g of total metal precursor, 0.3909 g of PVP and 4.5 mL of a solvent mixture (ethanol:DMF = 70:30 or 50:50 in v/v%). The detailed method to make the solution is as follows. IrCl3•xH2O was dissolved in the solvent mixture by sonication for 30 min and the solution was stirred for 24 h. Next, the corresponding amount of the solution containing HAuCl4•xH2O dissolved in the solvent mixture was added to the Ir precursor solution to obtain the molar concentration ratios of HAuCl4•xH2O:IrCl3•H2O = 0.05:0.95, 0.10:0.90 or 0.33:0.67. This mixed solution was stirred for an hour followed by the addition of 0.3909 g of PVP. Then, the final solution was agitated for 48 h at room temperature. The prepared electrospinning solution was poured into a syringe and emitted through a metal needle (21 gauge) connected to a voltage power supplier with an applied voltage of 16 kV and a flow rate of 10 μL min−1 in an electrospinning system (NanoNC ESR200R2). An aluminum foil was placed at a distance of 15 cm from the needle to collect the emitted electrospun liquid jet. The collected products were put into a calcination chamber and the temperature was increased to 400 C at a heating rate of 1 C min−1 and maintained at 400 C for 3 h in a mixed gas atmosphere consisting of 80 sccm of He and 10 sccm of O2. The synthesized materials are denoted as AuxIr1−xOy-a, where x is the molar ratio of Au precursor to Ir counterpart (in fact, x = 0.05, 0.10 or 0.33) and a is the


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volume % of ethanol in the mixed solvent of ethanol and DMF (a = 50 or 70). For instance, Au0.10Ir0.90Oy-70 is the one prepared with an electrospinning solution consisting of Au and Ir precursors at their molar ratio of 10:90 and a solvent mixture of ethanol and DMF at 70:30 v/v%. For comparison, the nanomaterials were also synthesized using the solutions containing only Ir or Au precursor (0.21 g of either IrCl3•H2O or HAuCl4•xH2O) via the same procedure as that of AuxIr1−xOy-a. The prepared materials from pure Ir and Au precursors are denoted as IrOy-a and Au-a, respectively, where a is also the volume % of ethanol in the solvent mixture. 2.3. Physical Characterization The structure and composition of as prepared IrOy, Au and AuxIr1−xOy nanocomposites were characterized by field-emission scanning electron microscope (FE-SEM, JEOL JSM-6700F) equipped with an energy dispersive X-ray spectrometer (EDS) operating at an accelerating voltage of 10 kV, high-resolution transmission electron microscope (HR-TEM, JEOL JEM-2100F), an Xray photoelectron spectroscope (XPS, ESCALAB 250, Al Kα radiation), and high resolution Xray diffraction (XRD, Rigaku D/Max-2000/PC X-ray diffractometer using Cu Kα radiation). To obtain the average values of the diameters or widths, 50 samples were investigated. The average atomic % values of Ir and Au were calculated by analyzing 20 spots of each sample. XPS was measured after loading the catalyst on a Si wafer (Silicon Materials Inc.). 2.4. Electrochemical Characterization A glassy carbon (GC) disk electrode (3 mm in diameter. Bioanalytical Systems, Inc.) was polished with 0.3 μm alumina slurry on a wet polishing cloth (Mark V Lab) and then sonicated in fresh water for 5 min. As prepared IrOy, Au and AuxIr1−xOy were individually dispersed in deionized water (2.0 mg mL−1). 60 μg of each sample was loaded on a GC electrode by repeating 6-μL loading of a sample-dispersed solution five times and drying at 60 C for 10 min between



each loading. To affix the sample to the GC electrode surface, 10.0 μL of 0.05 wt% Nafion solution was drop-casted on the sample modified-GC electrode. Electrochemical measurements were performed with electrochemical analyzer (CHI 730D), using a three electrode cell with a platinum wire and a saturated calomel electrode (SCE) as the counter and reference electrodes, respectively. OER activities were measured in 0.5 M H2SO4 at a scan rate of 10 mV s−1 with rotating disk electrode (RDE) voltammetry at a rotation rate of 1600 rpm. For stability test, cyclic voltammetry (CV) was performed in a potential range of 1.16 V to 1.51 V vs. RHE at a scan rate of 100 mV s−1 and linear sweep voltammograms (LSVs) were obtained repetitively at a scan rate of 10 mV s−1 every 100 CV cycles. The potentials corresponding to 10 mA cm−2 of LSVs were plotted on a potential-cycle graph. All the potentials were compensated for iR drop and referred to RHE. Regarding all the experimental results including SECM described below, the current densities were obtained from the measured current values normalized to the electrode geometric surface areas (GSAs). GSAs were determined using chronocoulometry (CC) experiment in 10 mM K3Fe(CN)6 solution including 0.1 M H2SO4.29 2.5. SECM Tip Electrode Fabrication A tip electrode was prepared by sealing Pt microwire (25 μm in dia.) under vacuum in a glass capillary (outer dia. = 1.5 mm, inner dia. = 0.5 mm) followed by vertical polishing to expose a Pt microdisk at the end plane, as previously reported.30 The glass sheath of the prepared Pt disk ultramicroelectrode (UME) was reduced with grinding to make the RG = 5-6. RG is the ratio of the overall tip electrode radius including a glass sheath to the Pt disk radius. 2.6. SECM Measurements Electrochemical measurements using SECM were performed in a four-electrode setup using an electrochemical analyzer (CHI 920C SECM) with a Pt wire and a SCE with saturated-KNO3


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double junction as the counter and reference electrodes, respectively. The Pt tip electrode was approached to the vertical distance of 10 μm above a catalyst-loaded GC electrode using a positive feedback mode operated in 10 mM [Ru(NH3)6]Cl3 dissolved in 0.1 M KNO3 solution with the tip potential (Etip) and the substrate potential (Esub) held at –0.4 V and 0 V (vs. SCE), respectively. For the comparison of OER performances, LSV was carried out for Au0.10Ir0.90Oy- and Ir/Cmodified GC substrates in Ar-saturated 0.5 M H2SO4 at a scan rate of 10 mV s−1 while the tip potential was held at 0 V to reduce O2 generated from OER at the substrates.

3. RESULTS AND DISCUSSION 3.1. Physical Characterization Figure 1 is representative SEM images of IrOy, Au and AuxIr1−xOy nanomaterials prepared with various metal precursor and solvent compositions of the electrospinning solution. As seen in Figures 1A and 1B, IrOy-70 and IrOy-50 have distinct nanostructures each other: IrOy-70 has a nanoribbon structure (width of 296 ± 91 nm, n = 50), but IrOy-50 is nanofiber-shaped (diameter of 116 ± 15 nm, n = 50). It is reasonably described that different structure formation is originated by the dissimilar evaporation rates of the two solvents in the electrospinning solution. It has been well established that nanostructures of electrospun materials are significantly affected by the form of emitted electrospun liquid jets.31 During the electrospinning process, solvent present in the liquid jet is evaporated. In case of the rapid solvent evaporation rate, the evaporation occurs unevenly between the inner and outer parts of the liquid jet stream, leading to the formation of a dry polymer skin on the jet. Then, this tube-like skin is collapsed under unbalanced lateral stress around the jet, producing a flattened ribbon structure. On the contrary, with a slower solvent evaporation rate, the solid skin forms slowly and evenly and therefore more cylindrical jet formation is favorable.32



Figure 1. Representative SEM images of (A) IrOy-70, (B) IrOy-50, (C) Au0.05Ir0.95Oy-70, (D) Au0.05Ir0.95Oy-50, (E) Au0.10Ir0.90Oy-70, (F) Au0.10Ir0.90Oy-50, (G) Au0.33Ir0.67Oy-70, (H) Au0.33Ir0.67Oy-50, (I) Au-70 and (J) Au-50. Scale bar = 100 nm.


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Since the boiling point of ethanol (78 C) is much lower than that of DMF (153 C), the solvent evaporation rate becomes faster with increasing the content of ethanol in the electrospinning solution. Thus, the flattened nanoribbon structure of IrOy-70 can be attributed to the faster solvent evaporation rate of its electrospinning solution than that of IrOy-50 nanofibers. Figures 1C to 1H show that AuxIr1−xOy-70 and AuxIr1−xOy-50 are also nanoribbon and nanofiber structures, respectively, in line with IrOy-70 and IrOy-50. It proves that the overall frames of the prepared materials are only affected by the solvent composition of the electrospinning solution. However, the addition of Au precursor in the electrospinning solution led to the formation of Au nanoparticles on the mainframe as seen in the high-resolution TEM images with each elemental mapping (Figure 2). The interplanar spacing of 0.238 nm on the nanoparticles is referred to the (111) plane of face-centered cubic Au, while lattice fringes with spacings of 0.318 and 0.259 nm on a nanofiber match well with the interplanar spacings of the (111) and (101) planes of facecentered cubic Ir. Figure 2B shows that Au nanoparticles are locally distributed on the surface of a nanofiber frame which evenly consists of Ir element. Therefore, it confirms that nanoparticles are composed of Au and the principal element of the fibrous main frame is Ir. Decreased Ir concentration along with increased Au concentration in the electrospinning solution produced smaller lateral dimensions, i.e., smaller widths or diameters of the overall frames of AuxIr1−xOy-70 and AuxIr1−xOy-50; and larger nanoparticles formed on them (Figure S1). The nanoparticle sizes seemed to be rarely affected by the solvent compositions. Actually, the sizes of the nanoparticles in AuxIr1−xOy-70 and AuxIr1−xOy-50 with the same value of x were not statistically different each other. Figure S2 shows that the atomic ratios of Ir to Au from the EDS results are similar between AuxIr1−xOy-70 and AuxIr1−xOy-50 with the same x value.



Figure 2. (A) Representative high resolution TEM image of Au0.10Ir0.90Oy-50 and (B) elemental mapping analysis of Ir and Au in a single Au0.10Ir0.90Oy-50 nanofiber. The inset shows the corresponding FFT images obtained in each area presented in dashed line.


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According to Figures 1I and 1J, even though it was fabricated via the same procedure as IrOy and AuxIr1−xOy, the Au nanomaterial formed clusters of Au nanoparticles whose average diameters were estimated over 100 nm (Figure S1). The agglomeration observed in the synthesized Au could be ascribed to the calcination at high temperature (400 C) without any stabilizers. Figure 3 is XRD spectra of IrOy-50, AuxIr1−xOy-50 (x = 0.05, 0.10 and 0.33) and Au-50, proving that all the samples have highly crystalline structures. Figure S3 presents the XRD spectra of Au0.10Ir0.90Oy-

Figure 3. X-ray diffraction patterns of (A) IrOy-50, (B) Au0.05Ir0.95Oy-50, (C) Au0.10Ir0.90Oy-50, (D) Au0.33Ir0.67Oy-50, and (E) Au-50. IrO2 (*), Ir (•), and Au (#).



70 and Au0.10Ir0.90Oy-50, representatives of AuxIr1−xOy series electrospun from two different solvent compositions. It shows that IrOy-50 consists of IrO2 and metallic Ir, while AuxIr1−xOy-50 (x = 0.05, 0.10 and 0.33) and Au0.10Ir0.90Oy-70 are composed of Au, IrO2 and metallic Ir. In particular, Au, Ir and IrO2 peaks appeared separately and any peak shifts were not observed in Figures 3 and Figure S3. This supports that Au, Ir and IrO2 tend to exist individually rather than to form an alloy. The lattice parameters were calculated using Bragg’s law (Table S1). The calculated lattice parameter values were nearly the same regardless of the composition: Negligible differences even compared with pure IrOy-50 and Au-50. This additionally supports no alloy formation. The mixture of IrO2 and metallic Ir on the bodies of AuxIr1−xOy-50 (x = 0.05, 0.10 and 0.33) is denoted here as Ir/IrO2 (vide infra). To investigate the chemical states for the element, we obtained high-resolution XPS spectra at the surface and inner part of IrOy-50, AuxIr1−xOy-50 (x = 0.05, 0.10 and 0.33) and Au-50 after the surface etching with Ar+ ion physical sputtering (Figure 4 and Figure S4). There were no XPS peak shifts in Au 4f region for Au-50 and AuxIr1−xOy-50 (x = 0.05, 0.10 and 0.33) before and after the surface etching. Au 4f spectra show two peaks corresponding to Au metal (83.8 eV for Au 4f7/2

Figure 4. High-resolution XPS spectra of Au-50, Au0.10Ir0.90Oy-50 and IrOy-50 for (A) Au 4f region after 60-s surface etching with Ar+ ion sputtering, (B) Ir 4f region at the surface before etching, and (C) Ir 4f region after 60-s surface etching.


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and 87.5 eV for Au 4f5/2).33 In contrast, Ir 4f peaks observed after the surface etching were shifted to lower binding energies compared to the ones at the composite surface,: Peaks at 61.4 eV and 64.4 eV before Ar+ ion sputtering vs. peaks at 60.6 eV and 63.6 eV after Ar+ ion sputtering. This indicates that the element of Ir exists mostly as IrO2 on the surface while presents mainly as metallic Ir at the inner part of the nanofiber.34 The corresponding peak positions of the XPS spectra were almost identical, independent of the element composition ratios as shown in Figure 4 and Figure S4.

3.2. Oxygen Evolution Reaction Figure 5 shows RDE voltammograms of IrOy, Au and AuxIr1−xOy nanomaterials depending on metal precursor and solvent compositions of the electrospinning solution for OER obtained in Arsaturated 0.5 M H2SO4 at a rotation rate of 1600 rpm and a scan rate of 10 mV s−1. OER activities showed the same trend regardless of the solvent composition: with increasing Au content, the current-potential curve gradually shifts toward less positive potential from IrOy to Au0.10Ir0.90Oy, but then Au0.33Ir0.67Oy is lagged behind Au0.10Ir0.90Oy with further increase in the Au content (Figures 5A and 5B). The insets of Figures 5A and 5B show that Au material requires a much larger overpotential for OER than IrOy and AuxIr1−xOy nanocomposites, indicating that Au is quite inert for OER within a given potential range between 1.3 and 1.7 V compared with IrOy and AuxIr1−xOy.35,36 The potential values generating a current density of 10 mA cm−2 were summarized for all the tested samples in Table S2. Since Ir with high oxidation states is efficient for OER37 and Au has high electronegativity,15,38 Au existing contiguous to Ir/IrO2 might tend to draw electrons from Ir/IrO2 and make the active surface of Ir/IrO2 more electrophilic.



Figure 5. RDE voltammograms of as prepared (A) IrOy-70, AuxIr1−xOy-70 (x = 0.05, 0.10 and 0.33), Au-70; (B) IrOy-50, AuxIr1−xOy-50 (x = 0.05, 0.10 and 0.33), Au-50; and (C) the comparison of RDE curves of Au0.10Ir0.90Oy-70, Au0.10Ir0.90Oy-50 and Ir/C for OER obtained in Ar-saturated 0.5 M H2SO4 with a rotation rate of 1600 rpm at a scan rate of 10 mV s−1. The insets are the magnified RDE curves of Au-70 (in (A)) and Au-50 (in (B)) for OER.


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In this aspect, the generation of enlarged Au and Ir/IrO2 interfacial areas is anticipated to be a critical factor in the improvement of OER activity of the Ir oxide. However, in this study, increasing Au concentration in the electrospinning solution induced the formation of larger Au nanoparticles which expanded the surface areas electrochemically inert for OER. According to the results in Figures 5A and 5B, it is inferred that the amount of Au and Ir/IrO2 interfaces increases even with the size growth of the Au nanoparticles when Au precursor concentration increased up to the production of Au0.10Ir0.90Oy. With further increase in relative Au concentration, however, the Au and Ir/IrO2 interfaces did not seem to be enlarged presumably due to the favorable formation of much larger Au nanoparticles, generating catalytically inert sites. Current observation suggests that Au0.10Ir0.90Oy produces the most appropriate amount of Au and Ir/IrO2 interfaces and therefore induces a significant enhancement of OER activity by the generation of more electrophilic Ir/IrO2 active surface. Figure S5 is Tafel plots of the OER RDE voltammetric results shown in Figures 5A and 5B. Tafel slope is a useful tool to estimate the RDS of an electrochemical reaction.39 The adsorption strength of the intermediate and mixed mechanism of the reaction can alter the Tafel slope.40,41 Based on the OER mechanism described in the Introduction section, the Tafel slope of 120 mV dec−1 predicts that the RDS is Reaction I, 40 mV dec−1 suggests the RDS of Reaction II. 30 mV dec−1 proposes that the RDS is the direct recombination of oxygen atoms of S-Oads to generate O2, which cannot occur with the large activation barrier of that reaction,42,43 E. Antolini reported that Tafel slope of 60 mV dec−1 was obtained on IrO2-based catalysts due to the combination of Reactions I-1 and I-2.15 In Table S2, Tafel slopes of IrOy and AuxIr1−xOy were estimated to be 50 to 60 mV dec−1, showing that the RDS for OER at these catalysts is likely to be the combination of Reactions I-1 and I-2.



Figure 5C is the comparison of the OER activities of Au0.10Ir0.90Oy-70, Au0.10Ir0.90Oy-50 and Ir/C. It presents that Au0.10Ir0.90Oy-50 shows a sharper increase of the OER current density than Au0.10Ir0.90Oy-70, while both Au0.10Ir0.90Oy-70 and -50 have much greater activity over Ir/C. Compared to nanofibers, flat nanoribbons are likely to be stacked more compactly, blocking their active surface sites. Thus, it is thought to be that a larger area of the active sites of Au0.10Ir0.90Oy50 would participate in OER compared with that of Au0.10Ir0.90Oy-70. Turnover frequency (TOF) and mass activity of the catalysts at 1.5 V (vs. RHE) were calculated and compared in Table S3. Among them, Au0.10Ir0.90Oy-50, which was the most active as shown in Figure 5, was also found to have the largest values of both TOF and mass activity. In comparison with the Ir/C, the OER current of Au0.10Ir0.90Oy-50 at 1.5 V (vs. RHE) was much greater. Therefore, Au0.10Ir0.90Oy-50 produced the highest values of both TOF and mass activity even though the amount of Ir in Au0.10Ir0.90Oy-50 was greater than that in Ir/C. Figure 6 presents OER stability tests of Ir/C and Au0.10Ir0.90Oy-50, the most active one among a series of AuxIr1−xOy materials synthesized. During a period of 5000 CV cycles, the potential of Au0.10Ir0.90Oy-50 corresponding to the current density of 20 mA cm−2 shifted by only 5 mV toward a more positive direction. In contrast, the potential of Ir/C to generate 20 mA cm−2 changed more significantly (19 mV) during 1500 cycles, indicating the increased overpotential required for OER. This observation implies that Ir/IrO2 mainframe has a high stability and also acts as a great supporting material for adsorbed Au nanoparticles. IrO2 is well-known for its high stability for OER.9,13 In general, nanoparticles tend to aggregate one another to reduce their surface energy.44 The Ir/IrO2 frames of Au0.10Ir0.90Oy-50, as good supporting materials, might prevent the aggregation of Au nanoparticles maintaining the total number of Au and Ir/IrO2 interfaces. In fact, the SEM image obtained after OER stability test of 5000 CV cycles shows that the morphology


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1.56

Potential /V vs. RHE

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


1.54 1.52 1.50 1.48 1.46 1.44

cIr Au0.10Ir0.90Oy-50

1.42 1.40 0

1000

2000

3000

4000

5000

Cycle number Figure 6. Potentials at 20 mA cm−2 for Au0.10Ir0.90Oy-50 and Ir/C measured from linear sweep voltammograms obtained every 100 CV cycles in a potential range of 1.16 V to 1.51 V. and structure of Au0.10Ir0.90Oy-50 are well maintained (Figure S6A). XPS spectra of Au0.10Ir0.90Oy50 after OER stability test of 5000 CV cycles were also obtained (Figure S6B-D). The peaks for Au 4f region were not noticeably changed before and after OER. The peaks for Ir 4f region measured at the inner part of the catalyst exposed with the surface etching also did not change. However, the Ir 4f peaks measured at the surface of the catalyst shifted slightly to higher binding energies by 0.1 eV compared to the ones before OER. This suggests that OER induces a little oxidation of Ir metals only at the catalyst surface. Table 1 is the comparison of the overpotential (vs. RHE) for OER between Au0.10Ir0.90Oy-50 and previously reported OER catalysts containing both Ir oxide and Au.16-18 Our Au0.10Ir0.90Oy-50 exhibited a smaller overpotential than IrO2 ED-1000 and IrOx/Au which were fabricated by complex sequential synthetic processes. Furthermore, the overpotential value of Au0.10Ir0.90Oy-50



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Table 1. Comparison of the OER catalytic performance of Au0.10Ir0.90Oy-50 with that of other catalysts composed of Ir and Au previously reported. Overpotential (vs. RHE)a

Electrolyte

Ref. No.

Au0.10Ir0.90Oy-50

0.241 V

0.5 M H2SO4

This work

IrOx[0.05]-Au nanoflowersb

0.481 V

0.1 M PBS

[16]

IrO2 ED-1000c

0.280 V

0.5 M H2SO4

[17]

IrOx/Aud

0.370 V

0.1 M NaOH

[18]

Potentials at 10 mA cm−2 were converted to the values vs. RHE. IrOx[0.05]-Au nanoflowers were formed by boiling the aqueous mixture of IrCl3·xH2O, HAuCl4, and sodium-citrate. OER activity was measured at a scan rate of 20 mV s−1. c IrO ED-1000 was electrodeposited on gold dendrite using CV in a 2 mM IrCl + 0.5 M H SO aqueous 2 3 2 4 solution, followed by oxidation of the deposited iridium using CV in 0.5 M H2SO4 aqueous solution. OER activity was measured at a scan rate of 5 mV s−1. d IrO /Au was prepared by the electroless deposition in 0.1 M IrCl ·3H O dissolved in 0.1 M NaOH (pH x 3 2 13) on the highly reactive gold electrode which was pre-anodized in 0.5 M H2SO4 at 2.68 V (vs. RHE). OER activity was measured at a scan rate of 10 mV s−1. a

b

was twice as small as that of wet-synthesized IrOx[0.05]-Au nanoflowers. This proves the excellent OER activity of Au0.10Ir0.90Oy-50 compared to other IrOx-Au containing materials even in highly acidic condition.

3.3. SECM Study of OER at Au0.10Ir0.90Oy-50 and Ir/C Figure 7 shows the investigation of OER at our best catalyst, Au0.10Ir0.90Oy-50 compared with Ir/C using substrate generation-tip collection (SG-TC) mode of SECM. Voltammetric curves of OER at Au0.10Ir0.90Oy-50 and Ir/C-modified GC substrate electrodes were obtained while a tip electrode collects O2 molecules generated from the substrate electrodes at a close tip-to-substrate separation (d = 10 μm). The tip electrode utilized was a 25-μm Pt disk UME of which electrode


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Figure 7. SECM experimental results for OER in SG-TC mode. The experiments were carried out in an Ar-saturated 0.5 M H2SO4 solution with Au0.10Ir0.90Oy-50- and Ir/C -modified GC substrate electrodes. The substrate potential was scanned from +1.03 V up to the potential at which the substrate current density (jsub) reaches 50 mA cm–2 with a scan rate of 10 mV s–1. Generated O2 molecules were collected simultaneously at a 25-μm Pt disk tip electrode to which 0.259 V (vs. RHE) was applied. Tip-to-substrate vertical distance (d) was 10 μm. The dashed horizontal line indicates jsub equal to 5 mA cm–2. size was reported to reduce O2 to H2O without any by-product (i.e., the number of electron transfer is four).45 The tip potential (Etip) was held at 0.259 V (vs. RHE) which is sufficiently negative to



reduce O2 (Figure S7). Accordingly, the measured tip current (itip) can be considered to be directly proportional to the amount of O2 produced from the substrate. In Figure 7, the substrate current density (jsub) of Au0.10Ir0.90Oy-50 started to increase from a less positive potential than that of Ir/C, as also observed in Figure 5. The substrate potential (Esub) of Au0.10Ir0.90Oy-50 (1.447 V vs. RHE), generating jsub of 5 mA cm–2, was ca. 30 mV less than that of Ir/C (1.476 V vs. RHE) at the same jsub of 5 mA cm–2. As the Esub was swept toward anodic direction, the itip increased responding to the observed jsub. In the same manner of the jsub, the itip measured for Au0.10Ir0.90Oy-50 started to increase more rapidly from a less positive potential than that for Ir/C. From the itip responses, the OER onset potentials at Au0.10Ir0.90Oy-50 and Ir/C were estimated to be 1.345 V and 1.437 V, respectively. The noisy behavior of the itip at Au0.10Ir0.90Oy-50 above 15 nA is attributed to a great amount of O2 generation from Au0.10Ir0.90Oy-50. The collection efficiency in this SECM experimental setup was estimated to be less than ~0.01 % due to the relatively small tip dimension compared to the substrate (Figure S8). Therefore, the absolute quantity of total O2 molecules generated from the substrate was hard to be measured.

4. CONCLUSIONS Mixed AuxIr1−xOy nanostructures (x = 0.05, 0.10 and 0.33) and pure Ir or Au counterparts (IrOy and Au) were synthesized by electrospinning and post-calcination method. By varying the solvent composition in the electrospinning solution (ethanol:DMF = 70:30 or 50:50 v/v%), two different nanostructures were fabricated: nanoribbons (IrOy and AuxIr1−xOy-70) and nanofibers (IrOy-50 and AuxIr1−xOy-50). The addition of Au precursor in the electrospinning solution led to the formation of Au nanoparticles on Ir/IrO2 fibrous main frames and the nanoparticles grew in the size with increasing Au precursor concentration. Among a series of as prepared AuxIr1−xOy materials with


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various metal compositions, Au0.10Ir0.90Oy-50 showed the best OER activities in 0.5 M H2SO4, closely followed by Au0.10Ir0.90Oy-70. This was ascribed to that Au0.10Ir0.90Oy was optimal to generate the most appropriate amount of Au and Ir/IrO2 interfaces, active sites for the facilitation of RDS of OER: Highly electronegative Au adjacent to Ir/IrO2 makes the Ir/IrO2 more electrophilic and thus more active for OER. The cylindrical fiber structure of Au0.10Ir0.90Oy-50 resulted in the slightly better OER activity than easily stackable Au0.10Ir0.90Oy-70 having a flat ribbon-like structure. Au0.10Ir0.90Oy-50 exhibited much better stability than Ir/C during repetitive CV measurements for OER. Compared to previously reported materials composed of both Ir and Au, Au0.10Ir0.90Oy-50 presented the lowest overpotential presenting the best OER activity. From the SECM measurements, Au0.10Ir0.90Oy-50 was found to require much less onset potential than Ir/C for OER. In conclusion, nanofiber-like Au0.10Ir0.90Oy-50 exhibits a good OER activity and stability and the improved activity is from the interfacial areas between Au and Ir/IrO2. Moreover, this material can be easily synthesized by simple single-step electrospinning method.

ASSOCIATED CONTENT Supporting Information ; The supporting Information is available free of charge on the ACS Publications website at DOI: . Ir and Au diameter information on AuxIr1−xOy materials (Figure S1). Atomic % of Ir in AuxIr1−xOy materials (Figure S2). XRD spectra of Au0.10Ir0.90Oy-70 and Au0.10Ir0.90Oy-50 (Figure S3). Lattice parameter of AuxIr1−xOy-50 materials (Table S1). High-resolution XPS spectra of Au0.33Ir0.67Oy-50 and Au0.05Ir0.95Oy-50 (Figure S4). The Tafel plots of AuxIr1−xOy series for OER obtained in Ar-saturated 0.5 M H2SO4 (Figure S5). Comparison of



potentials achieving 10 mA cm−2 and Tafel slopes (Table S2). Comparison of TOF and mass activity (Table S3). XPS comparison before and after OER stability test (Figure S6). Linear sweep voltammogram for O2 reduction in O2-saturated 0.5 M H2SO4 at a Pt tip electrode (Figure S7). Experimental results of the SG-TC mode of SECM in a solution of [Ru(NH3)6]3+ (Figure S8).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions †These

authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT and Future Planning (NRF-2017R1A2A2A14001137) and by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018R1A6A1A03025340).


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