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Fundamental Study of Facile and Stable Hydrogen Evolution Reaction at Electrospun Ir and Ru Mixed Oxide Nanofibers Yun-Bin Cho, Areum Yu, Chongmok Lee, Myung Hwa Kim, and Youngmi Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14399 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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ACS Applied Materials & Interfaces

Fundamental Study of Facile and Stable Hydrogen Evolution Reaction at Electrospun Ir and Ru Mixed Oxide Nanofibers Yun-Bin Cho, Areum Yu, Chongmok Lee, Myung Hwa Kim*, Youngmi Lee* 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: Hydrogen Evolution Reaction; Mixed Iridium-Ruthenium Oxide Nanofibers; Cathodic Activation; Electrospinning; Hydrogen adsorption Energy

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ABSTRACT

Electrochemical hydrogen evolution reaction (HER) has been an interesting research topic in terms of the increasing need of renewable and alternative energy conversion devices. In this paper, IrxRu1−xOy (y = 0 or 2) nanofibers with diverse compositions of Ir/IrO2 and RuO2 are synthesized by electrospinning and calcination procedures. Their HER activities are measured in 1.0 M NaOH. Interestingly, the HER activities of IrxRu1−xOy nanofibers improve gradually during repetitive cathodic potential scans for HER, and then eventually reach the steady-state consistencies. This cathodic activation is attributed to the transformation of the nanofiber surface oxides to the metallic alloy. Among a series of IrxRu1−xOy nanofibers, the cathodically activated Ir0.80Ru0.20Oy shows the best HER activity and stability even compared with IrOy and RuOy, commercial Pt and commercial Ir (20 wt% each metal loading on Vulcan carbon), where the superior stability is possibly ascribed to the instant generation of active Ir and Ru metals on the catalyst surface upon HER. Density functional theory (DFT) calculation results for hydrogen adsorption show that the energy and adsorbate-catalyst distance at metallic Ir0.80Ru0.20 are close to those at Pt. This suggests that mixed metallic Ir and Ru are significant contributors to the improved HER activity of Ir0.80Ru0.20Oy after the cathodic activation. The present findings clearly demonstrate that the mixed oxide of Ir and Ru is a very effective electrocatalytic system for HER.


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1. INTRODUCTION As the world faces energy crisis, hydrogen (H2) has received attention to be a promising energy source. Recently, loads of studies on water electrolysis, i.e., hydrogen evolution reaction (HER), have been performed due to the growing importance of the development of renewable and alternative energy conversion devices.1-3 HER is a multi-step electrochemical process which undergoes to produce H2 as a final product from H+ (in acidic condition) or H2O (in basic condition) via 2-electron transfer.4 The rate determining step of HER can be inferred from a slope of Tafel plot.5 Platinum (Pt) has been reported as the best HER catalytic material since it exhibits high exchange current density (j0) and Tafel slope of 30 mV dec−1 in acidic condition.6-8 On the other hand, the reaction rate in alkaline medium decreases by more than one order of magnitude showing a higher Tafel slope compared with that in acid (50~150 mV dec−1)9-11 due to the strong Pt-H binding energy and the low solubility of H2 in concentrated NaOH or KOH solutions.12-15 In addition, Pt is known to be susceptible to poisoning by underpotential deposition (UPD) in the presence of trace metal ions in the electrolytes.16 Oxidative treatment of the cathode material has been reported to show high activity for a long period of time without UPD problem.17 In this regard, metal oxides could be a promising material for HER. Iridium oxide (IrO2)18-23 and ruthenium oxide (RuO2)18-20, 24-27 have been studied for their great abilities for HER. Due to the stability of IrO218 and the resistance of RuO2 to UPD poisoning in the presence of metal ions,16, 23 there are some efforts to make a composite of IrO2 and RuO2 as HER electrocatalysts via only solid-state synthetic methodology.28-29 According to these studies, in acidic condition, the activity differences depending on the composition of IrO2-RuO2 composites were highly related to the ability of hydrogen adsorption/desorption, and the


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improvement of HER activity was interpreted due to H-chemisorption within the oxide layer. However, the HER activity in alkaline condition, having low proton concentration, would be understandably governed by different factors from that in acidic media. The activity differences among the composites in alkaline solution were presumed to be caused by the local pH increase on the cathodes and the weak hydrogen adsorption on IrO2 without any physical characterization.28 One dimensional (1D) nanostructures of metal/metal oxide have attracted attention as solid catalyst materials for their distinct properties of high electrical conductivity and chemical stability.30-32 Electrospinning is a simple and practical synthetic method for 1D nanomaterials with diverse compositions.31, 33-35 In this study, we synthesized nanofibers of Ir and Ru mixed oxide (IrxRu1−xOy, y = 0 or 2) with various Ir to Ru relative ratios via electrospinning method. The HER activities of IrxRu1−xOy nanofibers depending on the composition were compared in alkaline solution (1.0 M NaOH). The HER activities of IrxRu1−xOy were enhanced during a course of HER event, and conclusively, Ir0.80Ru0.20Oy showed the best HER activities with the highest current density and lowest Tafel slope among the IrxRu1−xOy nanofibers with various x values, even compared with Pt. Current work is devoted to find the reason for the improved activity of Ir0.80Ru0.20Oy with physical analysis and density functional theory (DFT) computation. Its physical change during HER is mainly investigated with X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). To further understand the relation between their physical change and HER activity, hydrogen adsorption energy is calculated using DFT calculation.

2. EXPERIMENTAL SECTION


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2.1. Materials. Iridium chloride hydrate (IrCl3•xH2O, 99.9 % trace metals basis), ruthenium chloride hydrate (RuCl3•xH2O, 99.98 % trace metals basis), polyvinylpyrrolidone (PVP, average MW ~ 1,300,000 by LS), sodium hydroxide (NaOH, reagent grade, ≥ 98 %, pellets), Nafion (5 wt% solution), ethanol (ACS reagent, ≥ 99.5 %), and N,N-dimethylformamide (DMF, anhydrous, 99.8 %) were all purchased from Sigma-Aldrich. Commercial Pt/C (cPt, 20 wt% metal loading on Vulcan XC72), and commercial Ir/C (cIr, 20 wt% metal loading on Vulcan XC-72) were supplied by Fuel Cell Store and Premetek Co., respectively. All solutions were prepared with 18 MΩ·cm deionized water. 2.2. Synthetic Procedure for IrxRu1−xOy Nanofibers. IrxRu1−xOy nanofibers were synthesized according to the procedure as follows. The electrospinning

solution

was

prepared

by

adding

0.187

g

of

metal

precursors

(IrCl3•xH2O:RuCl3•xH2O = 5:1 for Ir0.80Ru0.20Oy, 1:1 for Ir0.45Ru0.55Oy, and 1:3 for Ir0.23Ru0.77Oy) in 4.5 mL of a mixture of ethanol and DMF (V:V = 1:1). The nanofibers prepared using pure Ir precursor or pure Ru precursor were named as IrOy and RuOy, respectively. The solution was sonicated for 30 min and then stirred for a day. 0.3909 g of PVP was added to the solution and stirred for 16 h to make a homogeneous mixture. Thereafter, the mixture was poured into a plastic syringe. The needle was connected to a voltage power supply and 14 kV was applied to the needle at a flow rate of 10 µL min−1 using an electrospinning system (NanoNC ESR200R2). The spinning distance between the needle tip and grounded aluminum plate was 15 cm. The obtained electrospun nanofibers were dried in a vacuum oven under room temperature for longer than 1 h to eliminate the remaining solvent, and then placed in a furnace for the removal of PVP and the decomposition of metal precursors. The temperature was increased at a rate of 3 °C


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min−1 upto 450 °C, and maintained at 450 °C for 6 h with a flowing of O2 gas at 10 sccm and He gas at 80 sccm. 2.3. Electrochemical and Physical Characterizations. IrxRu1−xOy, IrOy, and RuOy nanofibers were dispersed separately in deionized water (2 mg mL−1). 5 µL of each sample was loaded on a glassy carbon (GC) rotating disk electrode (RDE) and dried at 60 °C. This process was repeated for 6 times. 10 µL of 0.05 wt% Nafion (in ethanol) was dropped onto the modified GC electrode and dried at room temperature. Electrochemical measurements for HER were performed using a RDE (BASi) and an electrochemical analyzer (730D, CH Instrument. Inc.) in Ar-saturated 1.0 M NaOH aqueous solution at a rotation rate of 1600 rpm with iR compensation. A Pt wire and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. All potential values are reported with respect to reversible hydrogen electrode (RHE). The structures and compositions of IrxRu1−xOy nanofibers were characterized by field-emission scanning electron microscope (FE-SEM; JEOL JSM-6700F) equipped with an energy dispersive X-ray spectrometer (EDS), high-resolution transmission electron microscope (HRTEM, Cscorrected STEM, JEOL JEM-2100F), high resolution XRD (Rigaku D/Max-2000/PC X-ray diffractometer using Cu Kα radiation), and XPS (ESCALAB250 XPS system, Theta Probe XRS System; Al Kα radiation). Raman scattering measurements were directly carried out on 80 µg of IrxRu1−xOy, IrOy, and RuOy nanofibers. 2.4. DFT Calculation. Vienna ab initio simulation package (VASP) based on the density functional theory (DFT) was used in the present article to obtain hydrogen adsorption energy by solving the Kohn-Sham equation of a manybody system with an iterative approach. Electron-ion core interactions were


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represented by the projected augmented wave (PAW) approach, while the generalized gradient approximation (GGA) of Perdew, Burke and Ernzerhof (PBE) was used as the exchangecorrelational functional to describe the interactions among electrons. Plane-wave basis set with an adequate cutoff energy of 400 eV and accurate precision were employed. For geometry optimizations, a Monkhorst-Pack mesh of 8 × 8 × 8 k-points was used. All of the cells presented in this study were fully relaxed until forces acting on all atoms were below 0.02 eV Å–1 using a total energy convergence of 10–6 eV before calculations of hydrogen adsorption energy. The structures of Ir0.80Ru0.20 and Ir0.80Ru0.20O2 were constructed by extending Ir and IrO2 unit cells into 2 × 2 × 2 cells, respectively, and exchanging a relevant portion of Ir atoms with Ru atoms to become the final content of Ir:Ru = 3:1 assuming ~80:20 ratio of Ir:Ru by using the substitutional search utility in materials exploration and design analysis (MedeA). Among all combinations having the relevant composition (Ir:Ru = 3:1), the systems having the farthest minimum distance between Ru-Ru atoms and the lowest energy after relaxation were chosen as the regular crystal structures of Ir0.80Ru0.20 and Ir0.80Ru0.20O2 for calculation (Figure S1). 2 × 2 surface cells of Pt(111), Ir(111) and Ru(111) were utilized for the calculation of hydrogen adsorption energy. Hydrogen adsorption energy is calculated as below36: Eads(H) = Esubstrate-H – (Esubstrate + EH2/2) where Esubstrate-H, Esubstrate and EH2 is the total energies of substrate adsorbed with a H atom, pure substrate and H2, respectively. Each surface was modeled by using a slab of six atomic layers of Pt(111), Ir(111), Ir80Ru20(111) and Ir80Ru20O2(110) surface cells with a vacuum layer of 1.5 nm. A hydrogen atom was initially located slightly above these surfaces (2 Å) and converged to within 0.02 eV Å–1. The adsorption free energy (△GH) was calculated by adding 0.24 eV to the calculated binding energies.37-38


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3. RESULTS AND DISCUSSION 3.1. Structural Characterization of IrxRu1−xOy nanofibers. The structure and the content of IrxRu1−xOy nanofibers depending on the atomic ratio of Ir to Ru were confirmed by SEM and EDS (Figure 1). The corresponding content is denoted as subscripts of IrxRu1−xOy (x = 0.80, 0.45 and 0.23). The nanofibers made from only Ir precursor, IrOy has a fiber structure with vertically grown nanorods (fiber thickness = 99.1 ± 18.7 nm in average, n = 40). With the increase of Ru content up to ~55 % in the nanofibers (Ir0.80Ru0.20Oy and Ir0.45Ru0.55Oy), the structure and fiber thickness were similar to the ones of IrOy: 94.3 ± 21.3 nm for Ir0.80Ru0.20Oy and 94.9 ± 18.4 nm for Ir0.45Ru0.55Oy (n = 40). The thickness of nanorods grown on the fibers was quite small dimension of ca. 16 nm and their length was various from 7

Figure 1. Representative SEM images of (a) IrOy, (b) Ir0.80Ru0.20Oy, (c) Ir0.45Ru0.55Oy, (d) Ir0.23Ru0.77Oy, and (e) RuOy. Scale bar = 100 nm.


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nm to 98 nm for all of IrOy, Ir0.80Ru0.20Oy, and Ir0.45Ru0.55Oy. When the Ru content was much greater than Ir content in the nanofibers (Ir0.23Ru0.77Oy), however, nanorods disappeared and the nanofibers became porous and thicker (thickness = 118.9 ± 20.7 nm in average, n = 40). RuOy was as thick as Ir0.23Ru0.77Oy (thickness = 104.3 ± 15.9 nm in average, n = 40) but more porous. Since the atomic weight of Ir (192.22) is heavier than that of Ru (101.07), the molar concentration of the total precursor in the electrospinning solution increases with increasing Ru content, producing thicker fibers. This distinct morphology difference depending on the relative composition ratio might be partly due to the different growth kinetics between IrO2 and RuO2 for the formation of one-dimensional nanostructures. The nucleation of RuO2 occurs at a lower temperature than that of IrO2.39,40 At our calcination temperature of 450 °C, RuO2 is seemingly shifted to the main growth stage but IrO2 still increases the nucleus number density. Thus, in the case of IrOy, the nanorods grow vertically from the nuclei formed on the surface of the fibers as previously reported.39,41 IrO2 nanofibers calcined at 900°C have simple nanofiber structures without any nanorods on them presumably because IrO2 is inferred to be at the growth stage.42 In current study, the IrxRu1−xOy mixed oxide with higher Ir content follows the growth characteristics of IrO2 and therefore leads to grow vertical nanorods on the nanofiber surfaces and vice versa. The representative TEM image and EDS mapping data of Ir0.80Ru0.20Oy are shown in Figure 2. It shows that Ir and Ru are evenly dispersed throughout both nanorods and

Figure 2. (a) TEM image and (b) the corresponding EDS mapping data of Ir0.80Ru0.20Oy.


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nanofiber and their atomic contents on the nanorods and nanofiber are virtually the same (Table S1). While the Raman bands at 498, 620, and 678 cm−1 can be assigned to the first order Eg, A1g, and B2g phonon bands of the pure rutile RuO2, the Raman bands measured at 548, 688, and 723 cm−1 are assigned to the first order Eg, B2g, and A1g phonon bands of the pure rutile IrO2 structure, respectively (Figure 3a).43 Although A1g, and B2g phonon bands for a pure IrO2 nanofibers are not resolved clearly in our spectrum, the Raman bands for pure RuO2 nanofibers and IrO2 nanofibers are good agreement with those observed in bulk single crystal. Particularly, the Raman spectra show that the Eg peak shifts linearly to a higher Raman shift direction with increasing Ir content from RuOy to IrOy, passing through IrxRu1−xOy (Figure 3b). The Eg peaks were not divided into two different peaks for IrxRu1−xOy, indicating that Ir and Ru atoms are dispersed in a rutile structure. For a binary mixture system of IrO2-RuO2, interestingly, it has been well described that a continuous solid solution over the whole compositional range can be

Figure 3. (a) Raman spectra measured at the laser excitation wavelength of 633 nm and (b) maximum peak positions of Eg modes of IrOy, IrxRu1−xOy (x = 0.80, 0.45, and 0.23) and RuOy.


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readily formed. It is most likely that Ru4+ and Ir4+ ions can share the same site on the cationic sub-lattice of a tetragonal phase in the crystal structure due to the similar ionic radius, 0.0625 nm for Ir4+ and 0.0620 nm for Ru4+, respectively. The XRD patterns are shown in Figure 4, from which the highly crystalline structure of IrxRu1−xOy nanofibers is evidenced. The diffraction peaks for RuOy could be mainly indexed to the pure RuO2 since the calcination temperature (450 °C) was enough to transform Ru into RuO2.44 In contrast, both pure Ir metal and IrO2 co-existed in IrOy because the calcination temperature was not enough for Ir oxidation (900 °C)45 unlike Ru case. Along with the addition of the Ru content in IrOy, the XRD peaks of rutile phase RuO2 began to appear and increase. An

Figure 4. X-ray diffraction patterns of IrOy, IrxRu1−xOy (x = 0.80, 0.45, and 0.23) and RuOy: IrO2 (◇), Ir (◆), and RuO2 (*).


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interesting point is that the peaks corresponding to Ir metal are observed for Ir0.80Ru0.20Oy, but suddenly disappear for both Ir0.45Ru0.55Oy and Ir0.23Ru0.77Oy. It is likely because the existence of higher content of Ru could facilitate the oxidation of Ir metal or the insertion of Ir into the rutile structure. For further analysis of these materials, the high resolution TEM images were obtained (Figures S2). As shown in Figure S2a, two different interplanar distances are observed clearly for IrOy: 0.230 nm corresponds to the crystallographic plane of (111) in pure Ir metal; and 0.322 nm is close to the (110) plane of the tetragonal structures of IrO2. It suggests that the nanorods are IrO2 but the nanofibers have both phases of Ir metal and IrO2. In the case of Ir0.80Ru0.20Oy, the nanorods have the (110) plane of the tetragonal structures in the mixed oxide form, while the (111) planes of pure Ir metal were additionally observed in the nanofibers (Figure S2b). In contrast, the high resolution TEM images (Figure S2c) and XRD (Figure 4) of RuOy clearly confirm the existence of the pure RuO2 phase with the crystal plane of (110). The obtained lattice parameters, a and c, of IrxRu1−xOy nanofibers are found in Table S2. Figure 5 represents that the XRD peak positions as a function of x in IrxRu1−xOy. The peak of the (101) plane shifts more significantly depending on the composition than that of the (110) plane. This is ascribed to the c-

Figure 5. XRD peak positions of (110) and (101) planes depending on x of IrxRu1-xOy.


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axis lattice parameters of pure IrO2 (0.317 nm) and pure RuO2 (0.311 nm) are different with a greater extent than the a- and b-axis parameters of pure IrO2 (0.453 nm) and pure RuO2 (0.452 nm). In fact, the c-axis lattice parameter of IrxRu1−xOy linearly varies with the Ru content increase, indicating continuous substitutional solid solution formation. This is consistent with Raman Eg peak shifts (Figure 3b), closely following the prediction of Vegard’s rule46 for ideal solid solution. Combining the results of Raman spectra (Figure 3), it could be presumed that Ir and Ru atoms are quite evenly and regularly distributed in IrxRu1−xOy crystal lattices.

3.2. Hydrogen Evolution Reaction Activity and Physical Analysis of IrxRu1-xOy Nanofibers. RDE polarization curves for HER were repetitively obtained with an Ir0.80Ru0.20Oy nanofiberloaded GC electrode in 1.0 M NaOH at a rotation rate of 1600 rpm with a scan rate of 10 mV s−1 (Figure 6). The onset potential moved toward a more positive direction and the current density

Figure 6. The first and fifth scans of linear sweep voltammograms of Ir0.80Ru0.20Oy for HER obtained in Ar-saturated 1.0 M NaOH with a rotation rate of 1600 rpm. Scan rate 10 mV s−1.


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inclined during repetitive potential scans for HER and the consistent curves were observed after 5 scans. With increasing Ru content, higher number of repetitive scans was required to obtain the consistent curves (~20 times for RuOy) (not shown). This phenomenon was referred to “cathodic activation” in this study. Figure 7 shows the XPS spectra before and after the cathodic activation of Ir0.80Ru0.20Oy. The

Figure 7. (a) Ir 4f and (b) Ru 3d XPS spectra of Ir0.80Ru0.20Oy before and after the cathodic activation.


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binding energies of Ir 4f7/2 and Ir 4f5/2 shift by ~0.3 eV toward the low energy direction after the cathodic activation (61.5 and 64.6 eV for Ir 4f7/2 and Ir 4f5/2 before the cathodic activation and 61.4 and 64.3 eV after the cathodic activation), indicating that the surface Ir was mostly present as IrO2 before activation and slightly reduced after activation.47-48 Meanwhile, the Ru 3d5/2 peak shifted by 0.4 eV to a lower binding energy (from 281.2 eV to 280.8 eV) after the cathodic activation, showing the transformation from RuO2 to metallic Ru (Figure 7b).49-50 XRD spectra of Ir0.80Ru0.20Oy-modified GC electrode before and after the cathodic activation were also obtained (Figure S3a). The peaks located at 40.7 and 47.3 degrees before activation represents to face-centered cubic (fcc) structure of metallic Ir (JCPDS-6-598) indicating the original existence of metallic Ir in Ir0.80Ru0.20Oy nanofibers (Figure S3b). After activation, these peaks became broad and slightly shifted by ca. 0.1 degree toward the position of fcc structure of metallic Ru (JCPDS-88-2333). That implies that the surface oxide of Ir0.80Ru0.20Oy is reduced to the fcc structure of mixed metallic Ir and Ru during the cathodic polarization for HER measurement. Figure 8a shows the resulting linear sweep voltammograms (LSVs) of IrOy, a series of IrxRu1−xOy, RuOy, cPt and cIr for HER obtained in Ar-saturated 1.0 M NaOH at a rotation rate of 1600 rpm. All the data presented are the consistent ones obtained after the repetitive LSV experiments, i.e., after the stable cathodic activation. Compared with IrOy, Ir0.80Ru0.20Oy showed a higher current density within HER region. Further increase of Ru content (Ir0.45Ru0.55Oy and Ir0.23Ru0.77Oy), however, decreased the current density compared with that of Ir0.80Ru0.20Oy; and RuOy showed more declined HER activity than Ir0.23Ru0.77Oy. Ir0.80Ru0.20Oy exhibited the best HER activity among IrOy, IrxRu1−xOy and RuOy; and its current density was much rapidly increased even compared with cPt and cIr.


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Figure 8. (a) Linear sweep voltammograms and (b) the corresponding Tafel plots of IrOy, IrxRu1-xOy (x = 0.80, 0.45, and 0.23), RuOy, cPt and cIr for HER obtained in Ar-saturated 1.0 M NaOH with a rotation rate of 1600 rpm. Scan rate 1.0 mV s–1. Figure 8b shows the Tafel plots of the corresponding LSVs for HER in Figure 8a. The Tafel plot becomes curvier with increasing Ir content. This could be attributed to the local pH increases near the catalyst’s surface during HER and much weaker hydrogen adsorption on IrO2 than that on RuO2.28 The resulting Tafel slopes of IrOy, Ir0.80Ru0.20Oy, Ir0.45Ru0.55Oy, Ir0.23Ru0.77Oy, and RuOy between 0 V and −0.05 V are −32.9, −31.5, −45.5, −47.4, and −47.8 mV dec−1, respectively: the Tafel slope decreases from IrOy to Ir0.80Ru0.20Oy, but then increases with further


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increasing Ru content due to the strong hydrogen adsorption on RuO2.51 Among all the samples, Ir0.80Ru0.20Oy shows the lowest Tafel slope even compared with those of cPt (−39.8 mV dec−1) and cIr (−34.9 mV dec−1), indicating that Tafel reaction is the rate-determining step of HER at its surface.5 The slightly low Tafel slope of cPt in this work compared with previous reports14, 52 is ascribed to the relatively larger sample loading amount.5 The potential change of Ir0.80Ru0.20Oy, IrOy, RuOy, cPt, and cIr at 10 mA cm−2 during HER was measured in 1.0 M NaOH with a rotation rate of 1600 rpm for 10000 s (Figure 9). The potentials of cPt and cIr shifted toward a more negative direction, and declined by 64.1 mV and 134 mV, respectively, after applying 10 mA cm−2 for 10000 s. In contrast, Ir0.80Ru0.20Oy, IrOy and RuOy showed potential shifts toward a more positive direction during a constant current supply, indicating their cathodic activation of HER activities. The full cathodic activation of the catalysts takes longer with increased Ru content in the order of IrOy (500 s) < Ir0.80Ru0.20Oy (2000 s) < RuOy (3700 s). After the cathodic activation, the potentials reached the steady-state levels and

Figure 9. Chronopotentiograms of Ir0.80Ru0.20Oy, IrOy, RuOy, cPt, and cIr with a constant applied current 10 mA cm−2 at a rotation rate of 1600 rpm.


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then decreased, implying the increased overpotential. The potential shift of Ir0.80Ru0.20Oy (3.54 mV) was less than those of IrOy (35.8 mV) and RuOy (9.44 mV), exhibiting a highly stable HER activity of Ir0.80Ru0.20Oy within ~3 h with a constant 10 mA cm–2 application. Cherevko et al. have reported that metallic Ru and Ir have better HER activities than RuO2 and IrO2, but they are less stable than the oxides due to the dissolution of their native oxide during HER.18 In this work, however, the active metallic Ir and Ru are generated directly from the durable oxide form, Ir0.80Ru0.20Oy, when they are needed for HER via the cathodic polarization, and therefore the HER activity and stability could be improved without problematic dissolution. A summary of HER performance for Ir0.80Ru0.20Oy, IrOy, RuOy, cPt and cIr is described in Table S3.

3.3. DFT Calculation. Table S4 shows the lattice parameters obtained from DFT calculation after relaxation of IrO2, Ir0.80Ru0.20O2 and RuO2 cells. The calculation results were within ~1 % variation of the values obtained from XRD spectra (Table S2), indicating the reliability of the calculation. For a better understanding of the higher HER activity of Ir0.80Ru0.20Oy after the cathodic activation, DFT calculation was performed for the most stable crystal phases of the mixed metal and metal oxide of Ir and Ru with 80:20 ratio. The hydrogen adsorption energy of Ir0.80Ru0.20(111) and Ir0.80Ru0.20O2(110) were calculated and compared with those of Pt(111), Ir(111) and Ru(111). The resulting structures and information of hydrogen adsorption on their surfaces are presented in Figure S4 and Table 1. The Eads(H) values of Ir0.80Ru0.20(111) and Ir0.80Ru0.20O2(110) were calculated to be –0.42 and –0.87 eV, respectively, while it was –0.45 eV for Pt(111). Figure 10 is free energy diagram for HER at equilibrium potential for Ir0.80Ru0.20(111), Ir0.80Ru0.20O2(110), Pt(111), Ir(111) and Ru(111). Compared with Ir0.80Ru0.20O2(110), the Eads(H) and free energy of


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Table 1. Hydrogen adsorption energies, adsorption sites and distance (dM-H) from the results of hydrogen adsorption on Ir0.80Ru0.20(111), Ir0.80Ru0.20O2(110), Pt(111), Ir(111) and Ru (111). The dM-H is the distance between H and neighboring metal atoms on their adsorption sites.

Figure 10. Free energy diagram for HER at equilibrium potential for Ir0.80Ru0.20(111), Ir0.80Ru0.20O2(110) , Pt(111), Ir(111) and Ru(111). Ir0.80Ru0.20(111) were much closer to the corresponding values of Pt(111) known as the best HER catalyst. This implies that metallic Ir0.80Ru0.20(111) has more favorable hydrogen adsorption energy than Ir0.80Ru0.20O2(110). Meanwhile, pure Ir(111) weakly binds H atom but pure Ru(111) overstabilizes adsorbed H on its surface, compared with Pt(111) standard. Therefore, it could be thought that the existence of ~20 % Ru in Ir0.80Ru0.20(111) induces favorable hydrogen adsorption. In addition, distance between a stabilized H atom adsorbate on


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the catalyst surface (dM-H, where M is a corresponding metal atom) was also calculated. H atom on pure Ir(111) (dIr-H = 1.90 Å) and on pure Ru(111) (dRu-H = 1.88 Å) are located slightly farther from the metal atoms than that on Pt(111) (dPt-H = 1.86 Å). With the addition of Ru, however, Ir0.80Ru0.20(111) draws H atom near the surface at Ir-Ir bridge site and dIr-H (1.87 Å) becomes close to dPt-H (1.86 Å) on Pt(111). Thus, it is expected that Ru (~20 %) on Ir0.80Ru0.20(111) acts as a significant role of improving HER activity of Ir0.80Ru0.20Oy by optimizing hydrogen adsorption energy and the distance dIr-H.

4. CONCLUSIONS IrxRu1−xOy nanofibers with various compositions were synthesized by electrospinning and following calcination procedure. As-prepared IrxRu1−xOy nanofibers were composed of Ir, IrO2 and RuO2 since the calcination temperature was enough for the formation of RuO2, but not for IrO2. Raman spectra and XRD showed that Ir and Ru atoms were evenly dispersed in the same rutile oxide structure, and HRTEM proved that vertically grown rods on IrOy and IrxRu1−xOy nanofibers consisted of the oxides while pure Ir metal was also observed at fibers. The repetitive LSV voltammograms showed that HER activities of IrxRu1−xOy measured in 1.0 M NaOH were enhanced via the cathodic polarization. According to XPS and XRD spectra, metallic Ir and Ru alloy formed on the surface of IrxRu1−xOy during HER. Among a series of IrxRu1−xOy, Ir0.80Ru0.20Oy showed the best HER activity even better compared with cPt, cIr, IrOy and RuOy. The high stability was acquired presumably due to the direct generation of active metallic Ir and Ru on the surface of Ir0.80Ru0.20Oy during HER. DFT calculation proved that the metallic Ir0.80Ru0.20(111), rather than Ir0.80Ru0.20O2(110), has closer adsorption energy and dM-H to Pt(111), known as the best HER catalyst. This suggests that the existence of Ru (~20 %) on Ir0.80Ru0.20


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which is generated during cathodic sweep acts as a significant role of improving HER activity of Ir0.80Ru0.20Oy by optimizing hydrogen adsorption energy and the dIr-H. In summary, Ir0.80Ru0.20Oy is a promising HER catalyst with superior activity and stability and this study suggests a new strategy of HER catalysts’ fabrication.

ASSOCIATED CONTENT Supporting Information. ; The Supporting Information is available free of charge on the ACS Publications website at DOI: . Crystal structures of Ir0.80Ru0.20 and Ir0.80Ru0.20O2. Averaged EDS atomic ratios of Ir and Ru in the fibers and rods of Ir0.80Ru0.20Oy. Lattice-resolved HRTEM images of IrOy, Ir0.80Ru0.20Oy and RuOy. Lattice parameters of IrOy, IrxRu1−xOy and RuOy. XRD patterns of Ir0.80Ru0.20Oy before and after the cathodic activation. A summary of HER performance for Ir0.80Ru0.20Oy, IrOy, RuOy, cPt and cIr. Lattice parameters from DFT calculation. The resulting structures of hydrogen adsorption on Pt(111), Ir0.80Ru0.20(111), Ir(111), Ru(111) and Ir0.80Ru0.20O2(110).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Youngmil Lee) *E-mail: [email protected] (Myung Hwa Kim)


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Author Contributions The manuscript was written through contributions of all authors. 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 for YL) and the NRF grant funded by the Ministry of Education (2016R1D1A1B03934962 for MHK).

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