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Comparative Study on Hydrogen Evolution Reaction Activity of Electrospun Nanofibers with Diverse Metallic Ir and IrO Composition Ratios 2

Su-jin Kim, Hyeseung Jung, Chongmok Lee, Myung Hwa Kim, and Youngmi Lee ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00402 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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ACS Sustainable Chemistry & Engineering

Comparative Study on Hydrogen Evolution Reaction Activity of Electrospun Nanofibers with Diverse Metallic Ir and IrO2 Composition Ratios Su-jin Kim†, Hyeseung Jung†, Chongmok Lee, Myung Hwa Kim*, Youngmi Lee* Department of Chemistry and Nano Science, Ewha Womans University, 52, Ewhayeodae-gil, Seodaemun-gu, Seoul, 03760, Republic of Korea

*Co-corresponding authors: [email protected] (YL), [email protected] (MHK) †Authors

equally contributed to this work.

Keywords: Hydrogen evolution reaction; Nanofibers; Iridium metal; Iridium oxide; Electrospinning; DFT calculation

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ABSTRACT Hydrogen evolution reaction (HER) is of importance in energy conversion processes. This paper reports the facile synthesis of iridium/iridium oxide nanofibers (Ir/IrO2NFs) with diverse Ir and IrO2 relative composition ratios and their electrocatalytic HER activities. Highly porous Ir/IrO2NFs are simply synthesized via electrospinning and the following calcination at various temperatures (300 to 900°C). Different calcination temperature alters the actual composition of Ir/IrO2NFs: The higher temperature, the greater IrO2 content exists. The HER activity of Ir/IrO2NFs is examined in 1 M H2SO4. Ir/IrO2NF calcined at 300°C exhibits the best HER activity in terms of the onset potential, overpotential generating 50 mA cm−2, turnover frequency, Tafel slope along with the decent stability for 5 h; and the HER performance of this material even exceeds that of platinum, a benchmark HER catalyst. As the IrO2 content in Ir/IrO2NFs increases with a higher calcination temperature, the HER activity decreases. Ir/IrO2NF calcined at 900°C consists of only IrO2, and presents the worst activity. DFT calculations show that hydrogen atom adsorption on metallic Ir (not IrO2) resembles that on Pt: similar adsorption energy and adsorbatesubstrate distance. Both the experimental and theoretical results clearly demonstrate that metallic Ir rather than IrO2 is a good HER catalytic platform.


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INTRODUCTION In recent years, hydrogen has been increasingly drawing attention as a viable energy source, replacing fossil-based fuels in the future.1,2 Hydrogen is a very attractive energy carrier, considering its large specific energy density and eco-friendly oxidation reaction.2,3 Electrochemical reduction of water is an advantageous method for the molecular hydrogen production because it can generate pure hydrogen without any contaminants, such as carbon monoxide (CO) in reformate hydrogen.4,5 Thus, sustainable hydrogen production from water splitting has become recognized more importantly.2,4,6 Hydrogen evolution reaction (HER), a cathodic half reaction of electrolytic water splitting, has been studied extensively for renewable energy conversion and storage systems.2-4,7,8 In acidic condition, HER is described as 4H+ + 4e− → 2H2

E = 0 V (vs. SHE)

(1)

An ideal catalyst for this proton reduction process is required to minimize the overpotential to zero and consequently to maximize the reaction efficiency.4 The most effective HER electrocatalyst is known to be platinum (Pt) metal with near zero overpotential and high exchange current density (j0).4,7,9 However, Pt has disadvantages such as high cost, scarcity of supply,10 and susceptibility to poisoning by underpotential deposition with trace metal ions present in the electrolytes.11 Efficient HER utilizing metal alloy,12,13 enzyme,14 metal oxides,15,16 and metal dichalcogenides electrocatalysts9 has been studied as alternatives to that using Pt. Binding energy between a catalyst and a reaction intermediate is generally accepted as a catalytic descriptor. Pt has a moderate adsorption energy to Hads intermediate (Pt-Hads) in HER, linked to its high HER activity. In particular, iridium (Ir) has the adsorption energy of Ir-Hads very close to that of Pt-Hads;17 and high



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corrosion resistivity in acidic condition.18 Thus, Ir-based electrocatalysts are of great interest as potential catalysts for HER in acidic media.17,19-26 The related research hitherto has been mainly focused on the oxide forms of Ir rather than metallic one. Cherevko et al. reported the comparison study for the HER activity of Ir metal and IrO2 using their sputtered thin films17 but still lacks the systematic fundamental study regarding the HER activity dependency of Ir oxidation state. Elongated thin fiber-like nanostructures of metal/metal oxide have received much attention as promising catalyst materials due to the positive properties of high electrical conductivity and durability. There are various methods to synthesize nanofibers such as drawing,27 template-based synthesis,28

phase

separation,29

self-assembly,30

and

electrospinning.31

In

particular,

electrospinning for generating fibrous nanomaterials has beneficial features of easiness, high versatility and practical large scale production.32-35 In this study, we investigate the catalytic effect of HER of Ir/IrO2 nanofibers (Ir/IrO2NFs) synthesized with simple electrospinning of Ir precursor/polymer solution and subsequent calcination at various temperatures. The resultant Ir/IrO2NFs have diverse relative composition ratios of metallic Ir and Ir oxide depending on the calcination temperature, ranged from 300C to 900C. The morphologies, compositions and structures of these Ir/IrO2NFs samples are characterized by field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HRTEM) X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The HER performance of the Ir/IrO2NFs with diverse Ir to IrO2 content ratio is systematically assessed and compared with that of Pt in acidic media (1.0 M H2SO4) by linear sweep voltammetry (LSV) using rotating disk electrode (RDE) measurement. For a better understanding of the experimental results, HER catalyzed by Ir/IrO2NFs is further studied using density functional theory (DFT) calculations based on d-band theory and hydrogen adsorption


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energy.

EXPERIMENTAL SECTION Chemicals and Materials. Iridium (III) chloride hydrate (IrCl3·xH2O, 99.8%) was from Alfa Aesar (Haverhill, MA), and sulfuric acid (H2SO4, 95.0-98.0%), polyvinylpyrrolidone (PVP, MW ≈ 1,300,000) and Nafion (5 wt % solution) were purchased from Sigma-Aldrich (St. Louis, MO). Ethanol (97%) was supplied by Daejung (Korea). Commercial Pt/C (cPt/C, 20 wt % metal loading on Vulcan XC-72) was the product of E-TEK Company. All other chemicals used were of analytical grade, and all solutions were prepared with deionized water (resistivity ≥ 18 MΩ·cm).

Synthesis of Ir/IrO2 Nanofibers. First, IrCl3/PVP nanofibers were synthesized by electrospinning as follows. 100 mg of IrCl3·xH2O and 500 mg of PVP were mixed and dissolved in 1.8 mL of ethanol and agitated for 5 h. The homogeneously mixed solution was loaded in a syringe connected to a stainless needle of gauge 23. Then, the solution was emitted through the needle connected to a voltage power supply (applied voltage, 8.0 kV) at a flow rate of 3.0 μL min−1 via an electrospinning device (NanoNC ESR200R2). An aluminum plate was placed at a distance of 15 cm apart from the needle tip to collect the nanofibers electrospun. The obtained electrospun IrCl3/PVP nanofibers were thermally annealed at various temperatures (300, 500, 700 and 900 °C) for 2 h in air. After the calcination, the Ir/IrO2NFs were obtained in the form of black powders.

Characterizations. Morphologies of the synthesized nanofibers were examined by FE-SEM (JEOL JSM-6700F, Japan, operated at an accelerating 10 kV) and HRTEM (JEOL JEM-2100F). For SEM analysis, the samples were prepared by attaching the Ir/IrO2NFs powders to a carbon



tape. The phase structures were examined by high-resolution XRD (Rigaku diffractometer) with Ni filtered Cu Kα radiation, λ = 0.15418 nm at 25 °C. It was measured at a rate of 3 ° min-−1 from 20° to 80°. XPS was carried out with Theta Probe AR-XPS System (Thermo Fisher Scientific). Xray source was monochromated Al Kα, hν = 1486.6 eV and the energy was 15 kV at 100 W. Spot size was 400 µm.

Electrodes and Electrochemical Measurements. Ir/IrO2NFs calcined at various temperatures and commercial Pt supported on carbon (cPt/C) were loaded on a glassy carbon (GC) disk electrode (3 mm in diameter) independently for electrochemical characterization. 2 mg of each of the catalysts was dispersed in 1 mL of deionized water via at least 30 min sonication to form a homogeneous solution. Then, 8 μL of this suspension was dropped five times (40 μL loading in total) on a GC electrode. The catalyst-loaded GC electrode was dried in oven at 60 °C, and then the electrode surface was coated with 10 μL of 0.05 wt% Nafion solution (diluted from 5 wt% Nafion with ethanol) to anchor the catalyst material on the electrode surface. The electrochemical experiments for HER were performed in a standard three-electrode system controlled by RDE-1 rotor (BASi)/CHI 760C (CH Instruments, Inc.) electrochemical workstation. The catalyst loaded on the GC electrode was used as the working electrode, and saturated calomel electrode (SCE) and coiled Pt wire were used as the reference electrode and the counter electrode, respectively. The potentials reported in this study were all calibrated and converted to the ones vs. RHE. Unless otherwise noted, the voltammograms were recorded with iR drop compensation. The HER measurements were carried out with RDE voltammetry at a scan rate of 5 mV s−1 in Ar-saturated 1.0 M H2SO4 aqueous solution. The chronopotentiometry tests for HER were measured with continuous rotation of the electrode at 900 rpm to remove the produced hydrogen


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bubbles from the electrode surface. The working electrode was held at a constant current density of 10 mA cm−2 for a desirable time, and the operating potentials were monitored as a function of time. Electrochemical impedance spectroscopy (EIS) was conducted in 1 M H2SO4 solution at 0.1 V (vs. SCE) in the frequency range from 1 Hz to 10 kHz.

Computational Methods. DFT calculation with Vienna ab initio simulation package (VASP)36 was carried out to further interpret the experimental data for the HER activity on the Pt(111), Ir(111) and IrO2(110) surfaces by solving the Kohn-Sham equation of a manybody system with an iterative approach. The system was under the periodic boundary condition and computation was done in a reciprocal space. Plane-wave basis set with an adequate cutoff energy of 450 eV and accurate precision were employed. Electron-ion core interaction was represented by the projected augmented wave (PAW) approach,37 while the generalized gradient approximation (GGA) and Perdew, Burke and Ernzerhof (PBE) were used as the exchange-correlation functional to describe the interactions among electrons. A 14  14  1 Monkhorst-Pack k-point sampling was used with a Gaussian smearing parameter (σ) of 0.1 eV and a self-consistence-filed (SCF) convergence criterion of 1  10−6 eV. The electronic density of states (DOS) projected onto metal d-bands and the corresponding d-band center energies at Pt(111), Ir(111) and IrO2(110) reciprocal unit cells were obtained from the DFT calculation. The systems were modeled using 2  2  4 cell of Pt, Ir, IrO2 when the hydrogen adsorption energy were calculated. We used 15 Å of vacuum in the z direction over the topmost layer to model the adsorption surface. Two supercells were constructed from the fully relaxed Ir and IrO2 bulk structures with the a = 5.404 Å, b = 5.404 Å for Ir metal and a = 6.344 Å, b = 6.409 Å for IrO2. The topmost layer with the adsorbed H atom was relaxed in our calculations, whereas the



intercalated atoms and the bottom layer were kept fixed.

RESULTS AND DISCUSSION Physical Characterization. Electrospinning followed by calcination was used to synthesize various composites of Ir/IrO2NFs. Figure S1 shows the representative FE-SEM images of as-spun nanofibers composed of Ir(III) and PVP, before the calcination. These nanofibers were found to have smooth surfaces with a mean diameter of 374.9 ± 43.3 nm (n = 50). Figure 1 shows FE-SEM images of Ir/IrO2NFs produced after the calcination at various temperatures. The resultant Ir/IrO2NFs are denoted as Ir/IrO2NF-x (x = 300, 500, 700 and 900, the corresponding calcination temperature in °C). The nanofiber morphologies dramatically changed after 2-h annealing in air. The post-calcined Ir/IrO2NFs, regardless of the calcination temperature, had a porous nanofiberlike morphology inducing the increased surface roughness. In addition, the fiber diameters was greatly decreased, caused by PVP removal during the calcination process. In fact, the mean diameters of Ir/IrO2NF-300, -500, -700 and -900 decreased to 112.6 ± 17.3 nm, 119.6 ± 15.4 nm, 116.6 ± 11.4 nm and 114.6 ± 15.4 nm (n = 50), respectively; and the measured diameters were rarely dependent of the calcination temperature. A closer inspection of the SEM images reveals the nanofiber morphology difference depending on the annealing temperature. Ir/IrO2NFs calcined at relatively lower temperatures (300°C and 500°C) exhibit highly porous lumpy structures formed from the networks of individual distinctive nanopebbles (Figure 1A and 1B). As the calcination temperature increased, Ir/IrO2NFs had still rugged structures but more densely packed morphology composed of less distinguishable pebble building blocks (Figure 1C and 1D). This morphological difference is seemingly induced from that the initially formed nanopebbles tend to melt and coagulate one another at high temperature.


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Figure 1. Low- and high-magnification FE-SEM images of Ir/IrO2NFs annealed at (A) 300 °C, (B) 500 °C, (C) 700 °C and (D) 900 °C for 2 h in air.



The HRTEM images of various Ir/IrO2NFs are presented in Figure S2. It shows that only cubic Ir metal phase exists in Ir/IrO2NF-300 and pure tetragonal IrO2 phase is found in Ir/IrO2NF-900. On the other hand, both phases of metallic Ir and IrO2 were present in Ir/IrO2NF-500 (Figure S1B). This indicates that the ratio of Ir metal to IrO2 in Ir/IrO2NFs is varied depending on the annealing temperature. The Ir/IrO2NFs were further analyzed with XRD technique. As shown in Figure 2, the XRD patterns of Ir/IrO2NF-300 present three distinct peaks at 40.6, 47.2 and 69.1, being assigned to (111), (200) and (110) planes of metallic Ir cubic structure.38 Likewise, for Ir/IrO2NF500, the same three major peaks of Ir metal cubic structure were observed. In addition, small satellite peaks also appeared at 28.0, 34.6, and 54.1, being assigned to (110), (101) and (211) planes corresponding to IrO2 tetragonal phase.38 For the XRD patterns of Ir/IrO2NF-700, peaks

Figure 2. XRD patterns of Ir/IrO2NFs annealed at different temperatures (300, 500, 700, 900 °C).


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corresponding to IrO2 crystalline structure was predominant while peaks related to metallic Ir structure were much weaker. Ir/rO2NF-900 exhibited only IrO2 peaks without any noticeable metallic Ir peaks. This observation suggests that the crystal structure and composition of Ir/IrO2NFs change from metallic Ir to IrO2 with increasing calcination temperature. It is well known that metallic Ir is sensitive to oxygen, especially at the medium temperature between 850K and 1100K as metal would oxidize to IrO2. This tendency is in line with the TEM results (vide supra), albeit unobservable metallic Ir in Ir/IrO2NF-700. As prepared various Ir/IrO2NFs were also examined with XPS for Ir 4f region (Figure S3). Two XPS peaks were observed for Ir/IrO2NF-300 at 60.8 eV (Ir 4f7/2) and 63.7 eV (Ir 4f5/2) of binding energy, corresponding to metallic Ir peaks.39 These peaks shifted toward higher binding energies as the calcination temperature increased; and those for Ir/IrO2NF-900 eventually reached to 61.4 eV (Ir4f7/2) and 64.4 eV (Ir4f5/2), relevant to the ones of IrO2.39

HER at Ir/IrO2NFs. Electrocatalytic HER activities of as prepared Ir/IrO2NFs were investigated with RDE voltammetry and compared to that of cPt/C, state-of-the-art catalyst, under acidic condition. Figure 3A shows the cathodic RDE polarization curves obtained at a scan rate of 5 mV s−1 and electrode rotation speed of 900 rpm in 1.0 M H2SO4. To exclude the effects of different sizes and surface areas of electrocatalysts on the observed HER activity, the measured currents were normalized to the geometric surface areas (GSA) of the electrodes and converted to current density (j). The measured GSAs were 0.071, 0.071, 0.072, 0.089 and 0.072 cm2 for the Ir/IrO2NF-300, -500, -700, -900 and cPt/C, respectively. Very similar GSAs were obtained for the Ir/IrO2NFs series, regardless of the calcination temperature. The HER activity, on the overpotential basis, was estimated to be in the following order: Ir/IrO2NF-900 < Ir/IrO2NF-700 < Ir/IrO2NF-500



< cPt/C < Ir/IrO2NF-300. In fact, Ir/IrO2NF-300 catalyst exhibited the most positive HER onset potential, ca. 0.00 V and the onset potential became more negative with increasing the calcination temperature: Ir/IrO2NF-500 (−0.004 V), Ir/IrO2NF-700 (−0.081 V) and Ir/IrO2NF-900 (−0.128 V). In addition, Ir/IrO2NF-300 was synthesized with various calcination times (from 30 min to 6 h). The most positive HER onset potential was observed for Ir/IrO2NF-300 calcined for 2 h while that became more negative for Ir/IrO2NF-300 calcined for shorter or longer times (Figure S4). This suggests that 2-h calcination is an optimized condition: the shorter calcination time seems to be not sufficient to combust PVP polymer completely, and the longer calcination may increase the relative content of less active IrO2. As seen in Figure 3A, Ir/IrO2NF-300 required a much lower overpotential (−27 mV) to achieve the same current density of 50 mA cm−2 than both cPt/C (−53 mV) and Ir/IrO2NF-500 (−54 mV). Meanwhile, Ir/IrO2NF-700 exhibited very low HER activity followed by worse Ir/IrO2NF-900. In other words, the j value generated at the same applied potential was greater at Ir/IrO2NF-300 than the other catalysts: At −0.03 V, j values were measured to be −50.78, −18.34, −0.77, −0.17 and −24.94 mA cm−2 for Ir/IrO2NF-300, -500, -700, -900 and cPt/C, respectively (Figure 3B). The Ir/IrO2NF-300, composed of only metallic Ir, exhibited the best HER activity compared to the other Ir/IrO2NFs materials, and even outperformed cPt/C. It is inferred that metallic Ir is superior to crystallized IrO2 in terms of the HER activity. For the repetitive RDE scans for HER, the polarization curve of Ir/IrO2NF-300 was not changed while that of Ir/IrO2NF-500 was shifted toward less negative potential direction and eventually reached to the consistent steady one at the seventh scan which was comparable to that of cPt/C. This event, named as cathodic activation, is discussed in details (vide infra).


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Figure 3. (A) RDE polarization curves in a cathodic potential window for the four different Ir/IrO2NFs and cPt/C obtained with a potential sweep rate of 5 mV s−1 and electrode rotation speed of 900 rpm in 1.0 M H2SO4. (B) Current density attained −0.03 V vs. RHE extracted from (A). (C) Tafel plots. (D) TOF values estimated at −0.03 V vs. RHE.

To explore the electrochemical kinetics of the HER at each material, Tafel plots were developed for the kinetic-controlled region near the onset potential of the RDE voltammetric results (Figure 3A). The Tafel plots (overpotential vs. log j) shown in Figure 3C provide an important parameter, Tafel slope, which implies the elemental reaction mechanism.18,40,41 The Tafel slope of cPt/C was measured to be 30.2 mV dec−1, very similar to the literature value (30 mV dec−1),42-45 confirming



the reliability of our analysis. The Tafel slope for Ir/IrO2NFs increased as the calcination temperature increased: 30.1, 35.0, 75.9 and 74.7 mV dec−1 for Ir/IrO2NF-300, -500, -700 and -900, respectively. Among the series of Ir/IrO2NFs, Ir/IrO2NF-300 was found to have the smallest Tafel slope comparable to cPt/C. The HER at Pt surface is known to proceed through the Volmer-Tafel mechanism, where the Tafel step (recombination of chemisorbed hydrogen atoms) is the ratedetermining step (RDS).46 The similar Tafel slope values at both Ir/IrO2NF-300 and cPt/C suggest that the HER at Ir/IrO2NF-300 proceeds via the same mechanism as that of Pt, i.e., Volmer-Tafel mechanism with the Tafel recombination RDS. The observed Tafel slope of 30.1 mV dec−1 for Ir/IrO2NF-300 is the lowest value reported to date regarding Ir- or IrO2-based catalysts.20,47 The HER catalytic efficiency of the Ir/IrO2NFs was also evaluated with the turnover frequency (TOF), representing the number of H2 produced per second per active site.48,49 TOF was calculated using the RDE voltammetric curve like Figure 3A as follows: The j value of each material measured at a certain potential (j at −0.03 V in current study) was normalized to the estimated number of active sites. The number of active sites was calculated from the real surface area of Ir/IrO2NFs, assuming that the surface Ir atoms were possible active sites.49 As presented in Figure 3D, the TOF value for HER was the greatest for Ir/IrO2NF-300 and it became smaller in the order of Ir/IrO2NF-300 > Ir/IrO2NF-500 > Ir/IrO2NF-700 > Ir/IrO2NF-900. Conforming to the other electrochemical parameters determined above (e.g, onset potential and j value), TOF value also corroborates the highest HER efficiency of Ir/IrO2NF-300 composed mainly of metallic Ir. TOF of Ir/IrO2NF-300 (3.92 H2 s−1 per active site) was estimated to be ca. 7.7 times greater than even that of cPt/C (0.51 H2 s−1 per active site). Impedance measurements were carried out at 0.1 V (Figure S5). The Ir/IrO2NF-300 exhibited the lowest faradaic impedance (Zf) of 14 Ω among the Ir/IrO2NFs series:50 Zf values were 20, 45


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Figure 4. Chronopotentiometric curves of Ir/IrO2NFs catalysts and cPt/C in 1.0 M H2SO4 solutions at a current density of 10 mA cm−2 (electrode rotation speed, 900 rpm).

and 67 Ω for Ir/IrO2NF-500, -700 and -900, respectively. The highest Zf of Ir/IrO2NF-900 is still less than 70 Ω, indicating the good conductivity even for Ir/IrO2NF-900. The smallest Zf of Ir/IrO2NF-300 suggests that the most facile HER kinetics is achieved at the Ir/IrO2NF-300 and the HER electron transfer at Ir/IrO2NFs becomes much less effective as the calcination temperature increases. The durability of all the catalysts was assessed by chronopotentiometric measurement during the continuous HER at a current density supply of 10 mA cm−2 in 1.0 M H2SO4 for 5 h without iRcompensation (Figure 4). As in the RDE voltammetry results in Figure 3A, the potential to attain the same 10 mA cm−2 was the most positive for Ir/IrO2NF-300; and insignificant potential shifts were monitored for the Ir/IrO2NFs series during the HER for 5 h. This implies the high acid resistivity of Ir. In particular, the measured potentials of Ir/IrO2NF-700 and -900 at 10 mA cm−2 were moved toward less negative direction during the initial period of HER and then attained the



stable levels. This phenomenon is attributed to cathodic activation.51 In fact, the RDE voltammetric curves for Ir/IrO2NF-700 and -900 samples became to have more positive onset potentials and higher current densities; and eventually reached to the consistent polarization curves while the voltammetric scans were repeated many times. To obtain the fully consistent RDE curves, Ir/IrO2NF-700 and -900 samples required the repetitive potential scans of 200 and 500 times, respectively (Figure S6A). During the continuous scans in cathodic potential region for HER, some of IrO2 in Ir/IrO2NFs is likely to be reduced to metallic Ir, causing the observed RDE curve changes and potentiometric potential shifts. Ir/IrO2NF-900, having the highest IrO2 content, needed more repetitive RDE scans or loner time to attain the constant curve or potential level (Figure 4). It should be noted that the trend of the HER activity was not changed even after the cathodic activation, supporting the best HER activity of Ir/IrO2NF-300. The cathodic activation of Ir/IrO2NFs was also confirmed with pH calibration, taking into account of the potentiometric pH sensing ability of IrO2. Initially, the pH sensitivity of Ir/IrO2NF-900 was measured to be 62.67 mV pH−1 but decreased to 45.08 mV pH−1 after 500-times RDE scans (Figure S6B and S6C). The repetitive RDE scans seemingly induce the reduction of IrO2 to metallic Ir which is less sensitive to proton. In view of the currently investigated characteristics, i.e., onset potential, overpotential and Tafel slope, Ir/IrO2NF-300 showed a better HER activity compared to other Ir-based or carbon supported metal catalysts recently reported (Table S1).

DFT Calculation. As described in the prior sections, all the experimental results persistently support that metallic Ir is better than IrO2 in the aspect of HER electrocatalysis. For a better understanding of these experimental results, DFT calculations were performed regarding Pt(111), Ir(111) and IrO2(110), the most stable correspondent crystal phases. Two indicative parameters to


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Figure 5. (A) Electronic density of states projected onto the d-band for different metal reciprocal cell (εd − εF; relative to the Fermi level). (B) Kinetic potentials (Ek) at −10 mA cm−2 for HER on electrodes presented in Figure 3A as functions of the calculated each metal d-band center energy at metal reciprocal cell (d − F: relative to the Fermi level).

the catalytic activity were calculated: (1) Metal d-band center energy relative to the Fermi level (d − F), commonly accepted as a measure of the reactivity of the transition metals;52 and (2) surface adsorption state of H atom intermediate, such as adsorption energy (Eads), adsorption site and distance between a H atom adsorbate and the catalyst M surface (dM-H). Figure 5A shows the electronic density of states (DOS) projected onto the metal d-bands and the d-band center energies for Pt, Ir and IrO2. The calculated values of (d − F) were very close between Pt(111) and Ir(111). In contrast, the metal d-projected DOS and d-band center energy of IrO2(110) were upshifted toward the Fermi level, resulting a smaller absolute (d − F) value. This proposes that a relatively stronger H-Ir binding in IrO2 phase than in pure metallic Ir. In Figure 5B, the kinetic potential attaining −10 mA cm−2 of HER (denoted as Ek, estimated from Figure 3A) are plotted vs. (d − F) value. The Ek value tends to be less negative as the absolution value of (d



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− F) increases. Ir(111) having the highest (d − F), closer to that of Pt, shows the most positive Ek, indicating the best HER activity. It is inferred that Ir in IrO2(110) can adsorb hydrogen atom too strongly due to the decreased (d − F) values, leading the declined HER activity. 2  2 surface cells of Pt(111), Ir(111) and IrO2(110) were utilized for the calculation of Eads of H atom based on the following equation: Eads = Ecatalyst-H – (Ecatalyst + EH2/2)

(2)

where Ecatalyst-H, Ecatalyst and EH2 are the total energies of catalyst adsorbed with a H atom, pristine catalyst and H2, respectively. The resulting structures and calculation data of hydrogen adsorption on the catalyst surfaces are presented in Figure S7 and Table 1. The Eads values of H atom on Pt(111), Ir(111) and IrO2(110) were calculated to be –0.43 eV, −0.36 eV and −0.96 eV, respectively. Compared with the case of IrO2(110), both Eads of H atom and dM-H values of Ir(111) were much closer to the corresponding values of Pt(111). In addition, the H atom adsorption sites on Pt(111) and Ir(111) were also identical. On the other hand, H atom adsorbs at Ir-Ir bridge site on IrO2(110) plane, causing Table 1. Hydrogen adsorption energies (Eads), adsorption sites, and distance (dM−H)* from the hydrogen adsorption on Pt(111), Ir(111), and IrO2 (110). Eads(H) / eV

Adsorption site

Distance / Å

Pt(111)

−0.43

fcc

dPt−H = 1.858

Ir(111)

−0.36

fcc

dIr−H = 1.896

IrO2(110)

−0.96

Ir-Ir bridge

dIr−H = 1.912

* The dM−H is the distance between H and neighboring metal atoms on their adsorption sites.


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relatively longer dM-H. This theoretical calculation results propose that metallic Ir rather than IrO2 is a a good HER electrocatalytic platform via adsorbing H atom, resembling Pt, the best HER material to date, which is well matched with the experimental observations discussed above. Various composites of Ir/IrO2NFs were synthesized via simple electrospinning process and subsequent calcination conducted at four different temperatures of 300, 500, 700 and 900 °C. The morphology and relative composition ratio between Ir and IrO2 of Ir/IrO2NFs changed depending on the calcination temperature. Ir/IrO2NF-300 had a more porous and lumpy structure mainly composed of metallic Ir; and the amount of IrO2 was increased with a higher annealing temperature. Finally, only IrO2 phase was found in Ir/IrO2NF-900, the one calcined at the highest temperature. The HER activity of as prepared Ir/IrO2NFs series was also different one another. Ir/IrO2NF-300 exhibited excellent HER activity with the least onset potential of ~0.0 V, the largest cathodic current, the highest TOF and a Tafel slope as small as 30 mV dec−1. This is a similar Tafel slope to that reported for Pt, suggesting Tafel recombination step as the RDS of the HER at Ir/IrO2NF300. Ir/IrO2NF-300 also exhibited a good durability, with keeping the most positive potential to generate the same current density of 10 mA cm−1 for the 5-h continuous electrolytic HER. DFT calculation also showed that the (d − F) value, Eads of H atom and dM-H of Ir(111), rather than IrO2(110), were evaluated to be very close to those of Pt(111). The higher HER activity of Ir/IrO2NF-300 compared to the other Ir/IrO2NFs could be ascribed to its high content of metallic Ir resembling Pt, the best HER material to date.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI:



AUTHOR INFORMATION Corresponding Author *Y. Lee. E-mail: [email protected] *M. H. Kim. E-mail: [email protected] Author Contributions †These

authors contributed equally to this work.

ACKNOWLEDGEMENTS 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 2016R1D1A1B03934962) 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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TABLE OF CONTENTS AND SYNOPSIS

HER activity of Ir/IrO2 nanofibers enhances as the metallic Ir content increases, providing the hydrogen atom adsorption energy similar to Pt.


Comparative Study on Hydrogen Evolution ... - ACS Publications

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