IrO2âZnO Hybrid Nanoparticles as Highly Efficient Trifunctional

Subscriber access provided by Warwick University Library

Article 2

IrO-ZnO Hybrid Nanoparticles as Highly Efficient Trifunctional Electrocatalysts In Hye Kwak, Ik Seon Kwon, Jun Dong Kim, Kidong Park, JaePyoung Ahn, Seung Jo Yoo, Jin-Gyu Kim, and Jeunghee Park J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03844 • Publication Date (Web): 22 Jun 2017 Downloaded from http://pubs.acs.org on June 25, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

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

The Journal of Physical Chemistry

IrO2-ZnO Hybrid Nanoparticles as Highly Efficient Trifunctional Electrocatalysts Inhye Kwak,† Ik Seon Kwon,† Jundong Kim,† Kidong Park,† Jae-Pyoung Ahn,‡ Seung Jo Yoo,≠ Jin-Gyu Kim,≠ and Jeunghee Park*,† † ‡ ≠

Department of Chemistry, Korea University, Jochiwon 339-700, Korea

Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 136-791, Korea Division of Electron Microscopic Research, Korea Basic Science Institute Daejeon 305-806,

Korea

ACS Paragon Plus Environment

1


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

Page 2 of 29

ABSTRACT

Development of high-performance catalysts is very crucial for the commercialization of sustainable energy conversion technologies. Searching for stable, highly active, and low-cost multifunctional catalysts has become a critical issue. In this study, we report the synthesis of IrO2-ZnO hybrid nanoparticles and their highly efficient electrocatalytic activities toward oxygen/hydrogen evolution (OER/HER) as well as oxygen reduction reactions (ORR). For comparison, we synthesized RuO2-ZnO, showing a smaller catalytic activity than IrO2-ZnO, which provides robust evidence for the unique synergic effect of these hybrid structures. IrO2ZnO and RuO2-ZnO exhibit excellent OER catalytic performance with the Tafel slope of 57 and 59 mV dec-1, respectively. For HER, IrO2-ZnO shows a higher catalytic activity than RuO2-ZnO. The number of electrons involved in the ORR was 3.7 and 2.8, respectively, for IrO2-ZnO and RuO2-ZnO. The remarkable catalytic performance of IrO2-ZnO would be ascribed to the abundant oxygen vacancies and the metallic states of Ir, which ensure excellent catalytic activity and stability.

KEYWORDS: IrO2, RuO2, ZnO, hybrid nanoparticles, trifunctional electrocatalysts


2

Page 3 of 29

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


INTRODUCTION There is an urgent need to search for sustainable and renewable energy sources because of fossil fuel depletion, increased environmental concerns, and global warming. The new generation of energy-conversion technologies incudes fuel cells, metal-air batteries, hydrogen production from water electrolysis, and solar fuel synthesis. Development of high-performance catalysts is very crucial for the commercialization of sustainable energy conversion technologies. For instance, hydrogen-generation water-splitting devices require efficient catalysts to increase the oxygen and hydrogen evolution reaction (OER/HER) rates with the lower overpotentials. The catalytic OER and oxygen reduction reaction (ORR) have been regarded as the most important processes in fuel cells and metal-air battery technologies. To date, Pt is the best electrochemical HER and ORR catalyst, while RuO2 and IrO2 are the best OER catalyst.1-10 However, the scarcity and high cost of Pt limit its widespread technological use. RuO2 and IrO2 suffer from limited stability in hostile electrochemical environments, and their high costs make them less desirable. As a consequence, the search for a robust and efficient alternative non-noble metal catalyst based on earth abundant metals has been vigorously pursued. More recently, developing the stable, highly active, and low-cost multifunctional (e.g., HER/OER or OER/ORR bifunctional, HER/OER/ORR trifunctional) catalysts has become a critical issue.11-24 Zn-air batteries with high energy densities were demonstrated using OER/ORR bifunctional catalysts.14-16,19,20,23,24 A powerful strategy to lower the catalyst loading and thus reducing cost at improved device efficiency, is the utilization of the catalyst supports, which helps to increase the dispersion of the active catalysts. Carbon is probably most popular and widely used support material because of


3


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

Page 4 of 29

many advantages such as large surface area and high electrical conductivity. Metal oxides could be secondly prevailing support materials, since they usually have a good durability toward the dissolution or corrosion, sufficient electrical conductivity, and strong binding interaction with the catalyst nanoparticles, which promotes the catalytic activity.25,26 In the present work, we chose earth-abundant ZnO support for the IrO2 and RuO2 nanoparticle (NPs), to develop the low-cost multifunctional catalysts with the high reaction rates at a minimized kinetic overpotentials. We used a novel one-step sol-gel reaction to produce uniform size IrO2 NPs (1-2 nm) and RuO2 NPs (3-4 nm) that are highly dispersive on the ZnO support with a strong binding interaction. We found that IrO2-ZnO hybrid NPs exhibit excellent trifunctional electrocatalytic activities toward OER, HER, and ORR in alkaline electrolytes, which are more efficient compared to RuO2-ZnO. This result is distinctive from the fact that both IrO2 and RuO2 have only moderate activities toward HER and ORR. In order to understand the remarkable catalytic efficiency of IrO2-ZnO, the electronic structures were thoroughly investigated using synchrotron X-ray photoelectron spectroscopy in combination with various electrochemical analyses. The comparison of two materials provides robust evidence for the unique synergic effect of these hybrid nanostructures. Experimental Synthesis of IrO2-ZnO, RuO2-ZnO, and ZnO NPs using a sol-gel method.27 ZnCl2, RuCl3, and IrCl3 hydrate were purchased from Aldrich. ZnCl2 (1.4 mmol) was dissolved in ethanol (30 mL), and IrCl3⋅xH2O (or RuCl3⋅xH2O) was added with 0.028−0.54 mmol to control the composition. For ZnO, only ZnCl2 was used. An ammonia (5 mL) aqueous solution (5%) was added dropwise under magnetic stirring, and the solution becomes colloidal. The colloidal solution was


4

Page 5 of 29

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


centrifuged, and the gel precipitate was washed several times with water and ethanol. After collecting of the precipitate and drying in air, the calcination was performed under O2 (100 sccm)/Ar (100 sccm) flow at 400 °C for 2 h, producing the oxide nanoparticles. Characterization. The structures and compositions of the products were analyzed by scanning electron microscopy (SEM, Hitachi S-4700), field-emission transmission electron microscopy (TEM, FEI TECNAI G2 200 kV), high voltage TEM (HVEM, Jeol JEM ARM 1300S, 1.25 MV), and energy-dispersive X-ray fluorescence spectroscopy (EDX). For the identification of composition, scanning transmission electron microscopy (STEM) and energy-dispersive X-ray fluorescence spectroscopy (EDX) with elemental maps were acquired using TEM (FEI Talos F200X) operated at 200 keV that equipped with high-brightness Schottky field emission electron source (X-FEG) and Super-X EDS detector system (Bruker Super-X). High-resolution X-ray diffraction (XRD) patterns were obtained using the 9B and 3D beam lines of the Pohang Light Source (PLS) with monochromatic radiation (λ=1.54595 Å). XPS data were collected using the 8A1 beam line of the PLS with a photon energy of 630 eV. Inductively coupled plasma atomic emission spectroscopy (ICP-AES, Jobin Yvon Ultima 2) was also used to analyze the composition. Electrochemical measurements. Electrochemical experiments were performed in a threeelectrode cell connected to an electrochemical analyzer (CompactStat, Ivium Technologies). A Ag/AgCl electrode was used as reference electrode, and a Pt wire was used as counter electrode. The preparation of catalysts for electrochemical characterizations: catalyst (5 mg) was dispersed in a mixture of water (500 µL), ethanol (500 µL) and 5 wt% Nafion (10 µL) by sonication for 30 min to form a homogeneous ink. The catalyst ink (10 µL) was loaded onto a glassy carbon (GC)


5


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

Page 6 of 29

electrode (diameter = 5 mm, area = 0.1963 cm2) that was purchased from Pine Instruments. The density of the loaded catalyst on GC electrode was 0.2547 mg/cm2. For OER, a Ag/AgCl electrode (saturated KCl) and a Pt wire were used as reference and counter electrodes, respectively. The GC rotating disk electrode (RDE) was used with a speed of 1600 rpm. The applied potentials (E) reported in our work were referenced to the reversible hydrogen electrode (RHE) through standard calibration. In 1 M KOH electrolyte (pH 14), E (vs. RHE) = E (vs. Ag/AgCl) + EAg/AgCl (= 0.197 V) + 0.0592 pH = E (vs. Ag/AgCl) + 1.026 V. In 0.1 M KOH electrolyte (pH 13), E (vs. RHE) = E (vs. Ag/AgCl) + 0.967 V. The reference was also calibrated in the same electrolyte by measuring hydrogen oxidation/evolution currents on a Pt wire and defining the potential of the zero current as the RHE. The potential shift of the Ag/AgCl electrode was confirmed as 0.197 V vs. RHE. The overpotential (η) was defined as E (vs. RHE) − 1.229 V. The linear sweep voltammetry (LSV) curve was measured using linear sweeping from 1.0 to 1.8 V (vs. RHE) with a scan rate of 1–20 mV s-1. Before electrochemical measurement, the electrolyte was purged with O2 (ultrahigh grade purity) for at least 0.5 h to ensure saturation of the electrolyte. We determined the electrochemical capacitance using two different methods: (1) measurement of the frequency dependent impedance of the system using electrochemical impedance spectroscopy (EIS), and (2) measurement of the non-Faradaic capacitive current associated with double-layer charging from the scan-rate dependence of cyclic voltammograms (CVs). EIS measurements were carried out for the electrode in an electrolyte by applying an AC voltage of 10 mV in the frequency range of 100 kHz to 0.1 Hz at a bias voltage of 1.53 V (vs. RHE). To measure double-layer charging via CV, a potential range in which no apparent Faradaic processes occur was determined from static CV. This range is typically a 0.2 V potential window


6

Page 7 of 29

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


centered at the open-circuit potential of the system. All measured current in this non-Faradaic potential region is assumed to be due to double-layer charging. The charging current, ic, is then measured from CVs at multiple scan rates. The working electrode was held at each potential vertex for 10 s before beginning the next sweep. The double-layer charging current density (J) is equal to the product of the scan rate (ν) and the electrochemical double-layer capacitance (Cdl), as given by equation J = ν Cdl, Thus, a plot of J as a function of ν yields a straight line with a slope equal to Cdl. The scan rates were 20, 40, 60, 80 and 100 mV s-1. HER electrocatalysis (in 1 M KOH electrolyte) was measured using a linear sweeping from +0.2 to −0.6 V (vs. RHE) with a scan rate of 2–10 mV s–1. A Ag/AgCl electrode was used as reference electrode, and a Pt wire was used as counter electrode The electrolyte was purged with H2 (ultrahigh grade purity) during the measurement. For ORR, the CV measurements were conducted in an O2-saturated 1 M or 0.1 M KOH solution with a scan rate of 50 mV s-1 in the potential range varying from -1.0 to +0.3 V. A Ag/AgCl electrode and a Pt wire were used as reference and counter electrodes, respectively. The O2 (ultrahigh grade purity) gas was bubbled with a flow rate of 20 sccm (mL min-1) during the measurement. RDE measurements were performed at rotation speeds varying from 400 to 2500 rpm with a scan rate of 5 mV s-1 from 1.2 to 0 V (vs. RHE). The number of electrons involved in the ORR was calculated using the Koutecky–Levich (K–L) equation: J-1 = JL-1 + JK-1 = (Bω1/2) -1 + JK-1, B = 0.62nFC0(D0)2/3 n-1/6, and JK = nFkC0, where J is the measured current density, JK and JL are the kinetic- and diffusionlimiting current densities, ω is the angular velocity of the disk (=2πN = 2πf/60, where N is the linear rotation speed, and f is the RDE rotation rate in rpm), n is the overall number of electrons transferred in oxygen reduction, F is the Faraday constant (= 96485 C mol-1), C0 is the bulk concentration of O2 (=1.2 ×10-6 mol cm-3), n is the kinematic viscosity of the electrolyte


7


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

Page 8 of 29

(=1.01×10-2 cm2 s-1), and D0 is the diffusion coefficient of O2 at room temperature (=1.97×10-5 cm2 s-1). The number of electrons transferred (n) and JK can be obtained from the slope and intercept of the K–L plots (J-1 vs. ω-1/2), respectively. RESULTS AND DISCUSSION Pure ZnO NPs, and IrO2-ZnO and RuO2-ZnO hybrid NPs were synthesized using a sol−gel method, and the composition of Ir or Ru ([Ir]/([Ir]+[Zn]) or [Ru]/([Ru]+[Zn]) was controlled to be 5-30 atomic % (at%). The X-ray diffraction (XRD) patterns showed that IrO2 (or RuO2) peaks appeared as the Ir (or Ru) content increased (Supporting Information, Figure S1). We assumed that the miscibility of IrO2 (or RuO2) and ZnO in our samples was far below 5%. We focused on 10% Ir or Ru samples, which showed the best catalytic performances, and hereafter refer to the IrO2-ZnO and RuO2-ZnO hybrid NPs as IZO and RZO, respectively. Figure 1a shows a high-resolution transmission electron microscopy (HRTEM) image for IZO. The average size of the ZnO NPs is 10 nm. The inset shows the small size of the IrO2 NPs embed homogeneously in the ZnO support. The lattice-resolved and corresponding fast-Fourier transform (FFT) images (insets) of IZO and RZO are shown in Figures 1b and 1c, respectively. The size of the IrO2 and RuO2 NPs are 1-2 nm and 3-4 nm, respectively. The d-spacing between the neighbouring (200) planes (d200) of IrO2 is 2.3 Å, corresponding to that of tetragonal phase (JCPDS No. 15-0870, a = 4.498 Å, c = 3.154 Å). For ZnO, d100 = 2.8 Å, corresponds to the wurtzite phase (JCPDS No. 80-0075, a = 3.253 Å, c = 5.209 Å). For RuO2, d110 = 3.3 Å is consistent with the tetragonal phase (JCPDS No. 43-1027, a = 4.499 Å, c = 3.107 Å). High-angle annular dark-field (HAADF) imaging scanning transmission electron microscopy (STEM) mages and energy-dispersive X-ray fluorescence (EDX) maps of the M shell of Ir, L shell of Ru, L shell


8

Page 9 of 29

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


of Zn, and K shell of O show that Ir and Ru are distributed homogeneously over the entire ZnO support (Figure 1d). The EDX spectra show that the Ir and Ru contents over the marked area are about 10% (Figure 1e). The ICP-AES analysis also provides about 10 % of Ir or Ru. X-ray photoelectron spectroscopy (XPS) was performed using a photon energy of 630 eV, as shown in Figure 2 (also see the survey scan in Supporting Information, Figure S2, confirming the Ir and Ru content is about 10%). The fine-scan XPS peaks were resolved by a Voigt function. IrO2 and RuO2 (purchased from Aldrich) were used as reference. Their HRTEM images and XRD patterns are shown in Figure S3 (Supporting Information). Figure 2a shows the Zn 3p peaks for ZnO, IZO, and RZO. All three samples show two resolved bands (PZ1 and PZ2) at 88 and 91 eV, which are separated by 3 eV. The 3p3/2 and 3p1/2 peaks for the neutral state (Zn0) should appear at 89 and 91 eV (separated by 2 eV), respectively, but they could not be resolved in these spectra. The PZ2 band at 91 eV, which is blue-shifted (3 eV) from that of the neutral state (Zn0 at 89 eV for the 3p3/2 peak), is assigned to Zn2+ in the ZnO lattices. The lower energy band (PZ1) at 88 eV is indicative of non-stoichiometry at the surface oxygen (O) vacancies that is reflected by highly oxidative oxygen species (O22-/O-).28 Figure 2b shows the O 1s peak. The peak of ZnO NPs was resolved into three bands (PO1PO3): 530.3 eV (PO1) for the O vacancies (O22-/O-) with the lattice oxygen (O2- of ZnO), 532.0 eV (PO2) for adsorbed O2 or OH- ions (close to neutral O (O0) at 531 eV), and 533.6 eV (PO3) for adsorbed H2O. The resolved bands of IZO consisted of PO1 at 530.8 eV, PO2 at 532.1 eV, and PO3 at 533.4 eV. The peak of RZO was resolved into four bands: PO1 at 531.0 eV, PO2 at 532.4 eV, PO3 at 534.0 eV, and PO4 (O2- of RuO2) at 529.4 eV. The PO1 bands of IZO and RZO are notably blue-shifted from that of ZnO by 0.5 and 0.7 eV, respectively, because of the


9


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

Page 10 of 29

enriched O vacancies.25,29-33 As the metallic Ir and Ru are formed (see below), the surface O vacancies could be increased by charge compensation. Figure 2c shows the Ir 4f7/2 and 4f5/2 peaks for IrO2 and IZO. The 4f7/2 (at 61.2 eV) and 4f5/2peaks (at 64.2 eV) of IrO2 are 0.2 eV-blue-shifted from the Ir0 position (61 and 64 eV, respectively). The 4f7/2 peak was resolved into PI1 at 61.2 eV and PI2 at 61.8 eV. We assigned them to Ir3+ and Ir4+, respectively. Hara et al. suggested that commercial IrO2 contains a large amount of Ir3+ at the outerlayers.34 IZO shows PI1 at 60.7 and PI2 at 61.6 eV. The 0.3 eV red shift of the PI1 band from Ir0 indicates the formation of lower oxidation states, such as the metallic form.26 The Ru 3d5/2 peak of RuO2 and RZO is shown in Figure 2d. The Ru 3d3/2 peak overlaps with the C 1s peak (284.5 eV), hindering a detailed analysis. The peak of RuO2 was resolved into the PR1 band at 280.3 eV and the PR2 band at 280.7 eV, assigned to Ru4+ and Ru5+ ions, respectively.35 The Ru0 peak is located at 280.1 eV. RZO shows the PR1 band at 280.05 eV and the PR2 band at 280.7 eV. The PR1 band was red-shifted by 0.05 eV from that for Ru0 and corresponds to the metallic state of Ru. The red shift from the position of neutral state is more significant for Ir (of IZO) than Ru (of RZO), 0.3 eV vs. 0.05 eV, suggesting more metallic Ir than Ru. To evaluate the electrocatalytic OER (4OH- → O2 + 2H2O + 4e-) activities, we performed linear sweep voltammetry (LSV) using a glassy carbon (GC) rotating disk electrode (RDE) at a rotation speed of 1600 rpm. A typical three-electrode setup was used. Figure 3a shows the LSV curves (scan rate: 2 mV s-1) for IZO, RZO, and ZnO in an O2-saturated, 1 M KOH electrolyte. The potentials reported here were referenced to the reversible hydrogen electrode (RHE) by standard calibration. The overpotential (η) was defined by the applied potential (vs. RHE) minus


10

Page 11 of 29

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


the water oxidation potential (1.229 V). The overpotential that were required to deliver a current density (J) of 10 mA cm-2 (ηJ=10) were 0.36, 0.40, and >0.8 V for IZO, RZO, and ZnO, respectively. The catalysis kinetics for OER was examined using Tafel plots (η vs. log J) based on the Tafel equation: η = b log(J/J0), where η is the overpotential, b is the Tafel slope (mV dec-1), J is the current density, and J0 is the exchange current density (mA cm-2) (Figure 3b). The Tafel slope (b) was obtained from the linear portion in the low-potential region, which corresponds to the activation-controlled current density region. The higher catalytic activity is represented by the smaller b. The linear fit gave b = 57, 59, and 130 mV dec-1 (dec = decade) for IZO, RZO, and ZnO, respectively. Figure 3c shows the chronoamperometric responses, confirming the high stability of these materials. Current attenuations of ~4% (at η = 0.4 V) during 6 h were observed for both samples. We measured the LSV curves for 5% and 30% Ir (or Ru) samples, confirming that the best performance occurred for the 10% samples (Supporting Information, Figure S4 and Table S1). Electrochemical impedance spectroscopy (EIS) showed significantly lowered charge-transfer resistance (Rct) for IZO and RZO than for ZnO, as shown in Figure 3d. A frequency range of 100 kHz to 0.1 Hz and an amplitude of 10 mV at η = 0.3 V were used for the EIS measurements. In the limit of high frequency and under non-Faradaic conditions, the electrochemical system is approximated by the modified Randles circuit as shown the inset, where Rs is the solution resistance, CPE is a constant-phase element related to the double-layer capacitance, and Rct is the charge-transfer resistance from any residual Faradaic processes. The lower Rct value means that the recombination of carriers (electron–hole pairs) at the interface with the electrolyte is


11


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

Page 12 of 29

decreased. A semicircle in the Nyquist plot at high frequencies represents the charge transfer process, with the diameter of the semicircle reflecting the charge-transfer resistance. The x- and y-axes are the real part (Z′) and negative imaginary part (–Z′′) of the impedance, respectively. The simulation of EIS spectra using an equivalent circuit model yielded the Rct values (see the fitting parameters in Supporting Information, Table S2). The charge transfer resistance, Rct, is a key parameter for characterizing the semiconductor–electrolyte charge transfer process. The value of Rct is 23, 44, and 1.0×104 Ω, respectively, for IZO, RZO, and ZnO. The Rct value of IZO and RZO is much smaller than that of ZnO, indicating that the IrO2 (or RuO2) incorporation reduces significantly the charge-transfer resistance. We performed double-layer capacitance (Cdl) measurements, showing Cdl = 0.30, 0.12, and 0.053 mF cm-2 in 1 M KOH, for IZO, RZO, and ZnO, respectively (Supporting Information, Figure S5). The capacitance of IZO and RZO is much larger than that of ZnO, indicating that IZO and RZO have a higher surface roughness and thus exposes more active sites toward OER, compared to ZnO. All these data showed that the incorporation of IrO2 and RuO2 greatly reduced the charge-transfer impedance and increased the double-layer capacitance, which are responsible for the enhanced OER catalytic activities of IZO and RZO. The OER data, and the Rct and Cdl values consistently shows that IZO is more efficient OER catalyst than RZO. We investigated the OER catalytic performance of commercial IrO2 and RuO2, as shown in Figure S6, and Tables 3 and 4 (Supporting Information). IrO2 and RuO2 show that ηJ=10 = 0.32 and 0.39 V; b = 53 and 51 mV dec-1, respectively. Their ηJ=10 value and Tafel slope are comparable to those of IZO and RZO. The data consistently show that IrO2 behaves as a better OER catalyst than RuO2, which is similar to that observed for IZO and RZO. We further


12

Page 13 of 29

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


performed the measurements of OER catalytic performance in 0.1 M KOH, confirming that IZO and RZO exhibit comparable activities with those of IrO2 and RuO2 (Supporting Information, Figure S7 and Table S5). The data showed that RZO and RuO2 behave as better catalysts than IZO and RuO2. Rasiyah and Tseung reported that IrO2 exhibits the higher OER activities than RuO2 in 5 M KOH.4 The Shao-Horn group observed slightly higher activities for RuO2 in 0.1 M KOH.5 We observed consistent pH-dependent behaviors for IrO2 and RuO2 NPs, probably due to the higher stability of IrO2 in the stronger basic electrolyte. Remarkably, the relative catalytic OER activities of IZO and RZO are determined by those of IrO2 and RuO2. The excellent catalytic activity of IZO and RZO was achieved by using only 10 at% of IrO2 and RuO2, corresponding to 20 wt% and 14 wt%, respectively. We evaluated the catalytic activities of IZO and RZO toward HER, using LSV measurements in a H2-saturated 1 M KOH electrolyte with a durability test. Table 1 summarizes the results. The overpotential (η), defined by the applied potential vs. RHE, needed to deliver J = -10 mA cm-2 was found to be ηJ=10= 0.25, 0.27, >0.8, and 0.090 V for IZO, RZO, ZnO, and Pt/C, respectively (Figure 4a). Commercial 20 wt% Pt/Vulcan carbon (Pt/C) was selected as a reference material. We checked that IrO2 and RuO2 showed negligible catalytic performance, similarly to ZnO. The catalysis kinetics was examined using Tafel plots based on the Tafel equation: η = b log(J/J0). The Tafel slope (b) was obtained from the linear portion in the low-potential region, which corresponds to the activation-controlled current density region. The IZO, RZO, ZnO, and Pt/C show b = 72, 96, 210, and 56 mV dec–1, respectively (Figure 4b). Both IZO and RZO exhibit great enhancements in catalytic performance relative to ZnO. Chronoamperometric responses of IZO and RZO showed that the current attenuation is about 20% and 25%, respectively, for 8 h, and the same HER performance was also observed using the graphite rod as a counter electrode


13


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

Page 14 of 29

(Supporting Information, Figure S8). Therefore the catalytic activities and stabilities of IZO toward HER are higher than those of RZO. Generally, two mechanisms are involved in the HER process, namely the Volmer step (electrochemical hydrogen adsorption; H3O+ + e– → Hads), followed by either a Heyrovsky (electrochemical desorption; Hads + H3O+ + e– → H2 + H2O) or Tafel process (chemical desorption; Hads + Hads → H2). In low overpotential region, Tafel slope is expected to be 40 and 30 mV dec–1 for the Volmer-Heyrovsky and Volmer-Tafel mechanisms, respectively. The b values of IZO and RZO suggest that the reaction kinetics would rather follow the VolmerHeyrovsky mechanism, similarly to that of Pt/C. The electrocatalytic ORR activities of IZO, RZO, ZnO, and Pt/C were examined in an O2saturated, 1 M KOH electrolyte. Figure 5a shows the LSV curves of IZO, RZO, ZnO, and Pt/C using a GC RDE at a rotation speed of 1600 rpm and a scan rate of 5 mV s-1. The current density and onset potential indicate superior ORR catalytic activities for IZO, which are comparable to those of Pt/C. IZO and Pt/C exhibit onset potentials at 0.87 and 0.92 V, diffusion-limited current densities of 4.0 and 5.2 mA cm−2, and half-wave potentials (E1/2) of 0.81 and 0.87 V, respectively. The cyclic voltammogram curves show a distinct ORR peak at around 0.8 V vs. RHE for IZO (Supporting Information, Figure S9). Figure 5b displays the LSV curves for IZO at rotation speeds of 400–2500 rpm. The current density gradually increased with the rotation rate. The number of electron transfer (n) involved in the ORR was calculated using the Koutecky-Levich (K-L) equation, as shown in Figure 5c. The LSV curves of RZO, ZnO, and Pt/C, at various rotation speeds and the K-L plot are shown in Figure S9 (Supporting Information). The n values were approximately 3.7, 2.8, 2.6, and 3.9 at


14

Page 15 of 29

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


potentials ranging from 0.2 to 0.6 V for IZO, RZO, ZnO, and Pt/C, respectively. The catalytic activities of IZO were also examined in O2-saturated 0.1 M KOH electrolyte, showing n = 3.6 (Supporting Information, Figure S10). The n value is expected to become 4 if the ORR follows a four-electron transfer reaction (O2 + 2H2O + 4e-→ 4OH-). If n = 2, two-electron transfer reaction occurs with hydrogen peroxide as intermediates species; O2 + H2O + e- → HO2- + OH- and HO2+ H2O + e-→ 3OH-. The reliably estimated n value of IZO indicates a four-electron transfer reaction rather than chemical regeneration of oxygen from hydrogen peroxide. The OER and ORR polarization curves (scan rate: 10 mV s-1) of the IZO, RZO, and Pt/C catalysts confirm that IZO acts as a highly efficient bifunctional oxygen catalyst (Figure 5d). The overall activity as a bifunctional catalyst can be evaluated by the potential difference (∆E) between the applied potential at J = 10 mA cm-2 (EJ=10) for OER and E1/2 for ORR (with a smaller ∆E for the better reversible oxygen electrode). Pt/C exhibits ∆E = 1.07 V with EJ=10 = 1.94 V and E1/2 = 0.87 V. Remarkably, IZO exhibited a smaller ∆E of 0.78 V with EJ=10 = 1.59 V and E1/2 = 0.81 V. Our data were compared with other reported values for bifunctional OER/ORR electrocatalysts, which showed that the ∆E value is similar to the state-of-the-art values (Supporting Information, Table S6). We conclude that IZO is an excellent electrocatalyst for all of OER, HER, and ORR. Many previous works indicated that the O vacancies acted as active sites for OER, HER, and ORR by promoting the adsorption of H2O and O2.26,29-33,36,37 The enriched O vacancies (mainly in ZnO support) facilitates the adsorption of H2O or O2, and thus provide the more active sites for the catalytic reaction of IrO2 (or RuO2) NPs, which results in the higher current densities at lower potentials for IZO (or RZO). The metallic state of IrO2 and RuO2 NPs, as shown by XPS


15


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

Page 16 of 29

analysis, can increase the electrical conductivities. The metallic state of IrO2 NPs is more prominent than that of RuO2, which could make IZO the more competitive HER and ORR electrocatalyst. It has been reported that Ir metal is a more efficient ORR catalyst than Ru metal, which is consistent with our results.38 The O vacancies can also suppress the oxidation of Ir or Ru to very high valence states during the OER process. Strasser group reported that antimonydoped tin oxide support sustained lower Ir oxidation states of Ir/IrOx particles and mitigated the corrosive dissolution of Ir ions.26 We suggest that more metallic state of IrO2 (than that of RuO2) NPs in the ZnO support guarantees excellent catalytic activities and stabilities of IZO toward OER. CONCLUSIONS We synthesized IrO2–ZnO and RuO2–ZnO hybrid NPs (IZO and RZO) using a sol-gel method. IrO2 (size = 1-2 nm) and RuO2 (size = 3-4 nm) nanoparticles are distributed homogeneously over the entire ZnO support (size = 10 nm). XPS analysis revealed the abundant O vacancies and the more metallic state of Ir than that of Ru. Both IZO and RZO exhibit excellent catalytic performances toward OER: the Tafel slopes are 57 and 59 mV dec-1 (in 1 M KOH), respectively. For HER catalytic reaction, IZO shows a better performance than RZO; their respective Tafel slopes are 72 and 96 mV dec-1 (in 1 M KOH). The number of electron transfer involved in the ORR were approximately 3.7, 2.8, and 2.6, for IZO, RZO, and ZnO, respectively. Remarkably, a four-electron transfer pathway was observed for IZO, which is comparable to that of Pt/C. Overall IZO exhibits excellent catalytic activities and stabilities toward OER, HER, and ORR. A synergic effect of the abundancy of O vacancies and more metallic nature of Ir could make IZO become a highly efficient catalyst. Therefore, IZO is a promising material for developing efficient multifunctional electrocatalysts to compete with state-of-art ones.


16

Page 17 of 29

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


ASSOCIATED CONTENT Supporting Information. Tables S1−S6, Figures S1−S10. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected] ACKNOWLEDGMENTS This study was supported by 2014R1A6A1030732 and 2016K000295, funded by the Ministry of Science. The HVEM measurements were performed at the KBSI. The experiments at the PLS were partially supported by MOST and POSTECH. The TEM measurement was supported by Nano Material Technology Development Program (2009-0082580). REFERENCES (1) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of Electrocatalysts for Oxygen- and Hydrogen-Involving Energy Conversion Reactions. Chem. Soc. Rev. 2015, 44, 2060-2086. (2) Shao, M.; Chang, Q.; Dodelet, J. P.; Chenitz, R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116, 3594-3657. (3) Trasatti, S. Electrocatalysis by Oxides–Attempt at a Unifying Approach. J. Electroanal. Chem. 1980, 111, 125-131. (4) Rasiyah, P.; Tseung, A. C. C. The Role of the Lower Metal Oxide/Higher Metal Oxide Couple in Oxygen Evolution Reactions. Electrochem. Soc. 1984, 131, 803-808.


17


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

Page 18 of 29

(5) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett. 2012, 3, 399-404. (6) Reier, T.; Oezaslan, M.; Strasser, P. Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt Catalysts: A Comparative Study of Nanoparticles and Bulk Materials. ACS Catal. 2012, 2, 1765-1772. (7) McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977-16987. (8) Antolini, E. Iridium as Catalyst and Cocatalyst for Oxygen Evolution/Reduction in Acidic Polymer Electrolyte Membrane Electrolyzers and Fuel Cells. ACS Catal. 2014, 4, 1426-1440. (9) Danilovic, N.; Subbaraman, R.; Chang, K. C.; Chang, S. H.; Kang, Y.; Snyder, J.; Paulikas, A. P.; Strmcnik, D.; Kim, Y. T.; Myers, D., et al. Using Surface Segregation to Design Stable Ru-Ir Oxides for the Oxygen Evolution Reaction in Acidic Environments. Angew. Chem. Int. Ed. 2014, 53, 14016-14021. (10) Pi, Y.; Zhang, N.; Guo, S.; Guo, J.; Huang, X. Ultrathin Laminar Ir Superstructure as Highly Efficient Oxygen Evolution Electrocatalyst in Broad pH Range. Nano Lett. 2016, 16, 4424-4430. (11) Zhang, C.; Antonietti, M.; Fellinger, T. P. Blood Ties: Co3O4 Decorated Blood Derived Carbon as a Superior Bifunctional Electrocatalyst. Adv. Funct. Mater. 2014, 24, 7655-7665. (12) Masa, J.; Xia, W.; Sinev, I.; Zhao, A.; Sun, Z.; Grützke, S.; Weide, P.; Muhler, M.; Schuhmann, W. MnxOy/NC and CoxOy/NC Nanoparticles Embedded in a Nitrogen-Doped


18

Page 19 of 29

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


Carbon Matrix for High-Performance Bifunctional Oxygen Electrodes. Angew. Chem. Int. Ed. 2014, 53, 8508-8512. (13) Meng, Y.; Song, W.; Huang, H.; Ren, Z.; Chen, S. –Y.; Suib, S. L. Structure−Property Relationship

of

Bifunctional

MnO2

Nanostructures:

Highly

Efficient,

Ultra-Stable

Electrochemical Water Oxidation and Oxygen Reduction Reaction Catalysts Identified in Alkaline Media. J. Am. Chem. Soc. 2014, 136, 11452-11464. (14) Zhang, J.; Zhao, Z.; Xia, Z.; Dai, L. A Metal-Free Bifunctional Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions. Nat. Nanotech. 2015, 10, 444-452. (15) X. Liu, X.; Park, M.; Kim, M. G.; Gupta, S.; Wu, G.; Cho, J. Integrating NiCo Alloys with Their Oxides as Efficient Bifunctional Cathode Catalysts for Rechargeable Zinc–Air Batteries. Angew. Chem. Int. Ed. 2015, 54, 9654-9658. (16) Jung, J. I.; Risch, M.; Park, S.; Kim, M. G.; Nam, G.; Jeong, H. Y.; Shao-Horn, Y.; Cho, J. Optimizing Nanoparticle Perovskite for Bifunctional Oxygen Electrocatalysis. Energy Environ. Sci. 2016, 9, 176-183. (17) Dou, S.; Tao, L.; Wang, S.; Dai, L. Etched and Doped Co9S8/Graphene Hybrid for Oxygen Electrocatalysis. Energy Environ. Sci. 2016, 9, 1320-1326. (18) Aijaz, A.; Masa, J.; Rösler, C.; Xia, W.; Weide, P.; Botz, A. J. R.; Fischer, R. A.; Schuhmann, W.; Muhler, M. Co@Co3O4 Encapsulated in Carbon Nanotube-Grafted NitrogenDoped Carbon Polyhedra as an Advanced Bifunctional Oxygen Electrode. Angew. Chem. Int. Ed. 2016, 55, 4087-4091.


19


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

Page 20 of 29

(19) Fu, G.; Chen, Y.; Cui, Z.; Li, Y.; Zhou, W.; Xin, S.; Tang, Y.; Goodenough, J. B. Novel Hydrogel-Derived Bifunctional Oxygen Electrocatalyst for Rechargeable Air Cathodes. Nano Lett. 2016, 16, 6516-6522. (20) Chen, P.; Zhou, T.; Xing, L.; Xu, K.; Tong, Y.; Xie, H.; Zhang, L.; Yan, W.; Chu, W.; Wu, C., et al. Atomically Dispersed Iron–Nitrogen Species as Electrocatalysts for Bifunctional Oxygen Evolution and Reduction Reactions. Angew. Chem. Int. Ed. 2017, 56, 610-614. (21) Hou, Y.; Wen, Z.; Cui, S.; Ci, S.; Mao, S.; Chen, J. An Advanced Nitrogen-Doped Graphene/Cobalt-Embedded Porous Carbon Polyhedron Hybrid for Efficient Catalysis of Oxygen Reduction and Water Splitting. Adv. Funct. Mater. 2015, 25, 872-882. (22) Liu, X.; Liu, W.; Ko, M.; Park, M.; Kim, G. M.; Oh, P.; Chae, S.; Park, S.; Casimir, A.; Wu, G., et al. Metal (Ni, Co)-Metal Oxides/Graphene Nanocomposites as Multifunctional Electrocatalyst. Adv. Funct. Mater. 2015, 25, 5799-5808. (23) Jia, Y.; Zhang, L.; Du, A.; Gao, G.; Chen, J.; Yan, X.; Brown, C. L.; Yao, X. Defect Graphene as a Trifunctional Catalyst for electrochemical Reactions. Adv. Mater. 2016, 28, 95329538. (24) Hua, B.; Li, M.; Sun, Y. F.; Zhang, Y. Q.; Yan, N.; Chen, J.; Thundat, T.; Li, J.; Luo, J. L. A Coupling for Success: Controlled growth of Co/CoOx Nanoshoots on Perovskite Mesoporous Nanofibers as High-Performance Trifunctional Electrocatalysts in Alkaline Condition. Nano Energy. 2017, 32, 247-254. (25) Liang, F.; Yu, Y.; Zhou, W.; Xu, X.; Zhu, Z. Highly Defective CeO2 as a Promoter for Efficient and Stable Water Oxidation. J. Mater. Chem. A 2015, 3, 634-640.


20

Page 21 of 29

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


(26) Oh, H. S.; Nong, H. N.; Reier, T.; Bergmann, A.; Gliech, M.; de Araújo, J. F.; Willinger, E.; Schlögl, R.; Teschner, D.; Strasser, P. Electrochemical Catalyst-Support Effects and Their Stabilizing Role for IrOx Nanoparticle Catalysts during the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2016, 138, 12552-12563. (27) Jang, D. M.; Kwak, I. H.; Kwon, E. L.; Jung, C. S.; Im, H. S.; Park, K.; Park, J. Transition-Metal Doping of Oxide Nanocrystals for Enhanced Catalytic Oxygen Evolution. J. Phys. Chem. C. 2015, 119, 1921-1927. (28) Kundu, P.; Singhania, N.; Madras, G.; Ravishankar, N. ZnO-Au Nanohybrids by rapid Microwave-Assisted Synthesis for CO Oxidation. Dalton Trans. 2012, 41, 8762-8766. (29) Zhu, Y.; Zhou, W.; Chen, Y.; Yu, J.; Liu, M.; Shao, Z. A High-Performance Electrocatalyst for Oxygen Evolution Reaction: LiCo0.8F0.2O2. Adv. Mater. 2015, 27, 7150-7155. (30) Bao, J.; Zhang, X.; Fan, B.; Zhang, J.; Zhou, M.; Yang, W.; Hu, X. ; Wang, H.; Pan, B.; Xie, Y. Ultrathin Spinel-Structured Nanosheets Rich in Oxygen Deficiencies for Enhanced Electrocatalytic Water Oxidation. Angew. Chem. 2015, 127, 7507-7512. (31) Song, F.; Schenk, K.; Hu, X.; A Nanoporous Oxygen Evolution Catalyst Synthesized by Selective Electrochemical Etching of Perovskite Hydroxide CoSn(OH)6 Nanocubes. Energy Environ. Sci. 2016, 9, 473-477. (32) Zhu, Y.; Zhou, W.; Yu, J.: Chen, Y.; Liu, M.; Shao, Z. Enhancing Electrocatalytic Activity of Perovskite Oxides by Tuning Cation Deficiency for Oxygen Reduction and Evolution Reactions. Chem. Mater. 2016, 28, 1691-1697.


21


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

Page 22 of 29

(33) Xu, L.; Jiang, Q.; Xiao, Z.; Li, X.; Huo, J.; Wang, S.; Dai, L. Plasma-Engraved Co3O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen Evolution Reaction. Angew. Chem. Int. Ed. 2016, 55, 5277-5281. (34) Hara, M.; Asami, K.; Hashimoto, K.; Masumoto, T. An X-ray Photoelectron Spectroscopic Study of Electrocatalytic Activity of Platinum Group Metals for Chlorine Evolution. Electrochim. Acta. 1983, 28, 1073-1081. (35) Schaefer, M.; Schlaf, R. Electronic Structure Investigation of Atomic Layer Deposition Ruthenium (Oxide) Thin Films using Photoemission Spectroscopy. J. Appl. Phys. 2015, 118, 065306. (36) Li, Y. H.; Liu, P. F. L.; Pan, F.; Wang, H. F.; Yang, Z. Z.; Zheng, L. R.; Hu, P.; Zhao, H. J.; Gu, L.; Yang, H. G. Local Atomic Structure Modulations Activate Metal Oxide as Electrocatalyst for Hydrogen Evolution in Acidic Water. Nat. Commun. 2015, 6, 8064 1-7. (37) Luo, Z.; Miao, R.; Huan, T. D.; Mosa, I. M.; Poyraz, A. S.; Zhong, W.; Cloud, J. E.; Kriz, D. A.; Thanneeru, S.; He, J., et al. Mesoporous MoO3-x Material as an Efficient Electrocatalyst for Hydrogen Evolution Reactions. Adv. Energy Mater. 2016, 6, 1600528 1-11. (38) Gnanamuthu, D. S.; Petrocelli, J. V. Generalized Expression for the Tafel Slope and the Kinetics of Oxygen Reduction on Noble Metals and Alloys. J. Electrochem. Soc. 1967, 114, 1036-1041.


22

Page 23 of 29

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


Table 1. Data summary for the electrocatalytic performance of IZO, RZO, and ZnO in 1 M KOH.

0.25

72

3.7

23

59

0.27

96

2.8

44

> 0.8

130

> 0.8

210

2.6

1.0×105

0.58

93

0.090

56

3.9

Hydrogen evolution reaction

ηJ=10 a

bb

ηJ=10a

IZOe

0.36

57

RZOe

0.40

ZnO Pt/C

Catalysts

a

Rctd

bb

Oxygen reduction reaction nc

Oxygen evolution reaction

Overpotential (V) at │J│=10 mA cm-2, bParameters of the Tafel equation, cNumber of electrons

involved in ORR, obtaining using K-L plots, dCharge transfer impedance (Ω) measured by a Nyqiust plot; e10% IrO2 or RuO2


23


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

Page 24 of 29

Figure 1. (a) HRTEM and (b) lattice-resolved and corresponding FFT images for IZO, and (c) RZO, showing IrO2 and RuO2 NPs embedded in the ZnO support. (d) HAADF STEM images and EDX mapping of the M shell of Ir, L shell of Ru, L shell of Zn, K shell of O, and (e) EDX spectra of IZO and RZO.


24

Page 25 of 29

0

(a) Zn 3p

3p1/2 3p3/2(Zn )

0

(b) O 1s

O

RZO

Intensity (arb.units)

RZO PZ2

PO3 PO2 PO1 PO4

PZ1

IZO

IZO

ZnO PZ2 94

Intensity (arb.units)

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


92

(c) Ir 4f

90

PZ1 88

86 0

0

Ir

Ir

IZO

84

4f7/2

536

PO3

PO2

534

532

ZnO

530

528

0

(d) Ru 3d5/2

4f5/2

Ru

RZO PI2

PR2

PI1

IrO2

RuO2

PI1

PR2

PI2 66

PO1

64

62

60

Binding Energy (eV)

282

281

PR1

PR1 280

279

Binding Energy (eV)

Figure 2. Fine-scan XPS peaks of (a) Zn 3p, (b) O 1s, (c) Ir 4f, and (d) Ru 3d5/2 for IZO, RZO, ZnO, IrO2, and RuO2. The data (open circles) are fitted by Voigt functions (colored lines), and the sum of the resolved bands is represented by black lines. The peak positions of the neutral states (Zn0, O0, Ir0, and Ru0) are marked to delineate the shift.


25


0.8

(b)

(a)

-2

-1

50

E (H2O/O2) = 1.23 V

40 30 20

0.4

R

c de

V

)

(b -1 O ) c Zn e d mV 9 5 = ZO (b -1 dec ) V m 7 5 = b ( IZO

0.2

0

10

m 30 1 =

0.6

IZO RZO ZnO

η (V)

Current Density (mA cm )

60

0

1.2

1.4

1.6

0.0 -1.5

1.8

-1.0

Potential (V vs. RHE) 30 -2

-0.5

0.0

(c)

(d)

30

CPE Rs

ZnO IZO RZO

10

10

IZO

5

0

1

2

3

4

5

6

0

Rct

RZO

20 15

0

1.0

-2

25 20

0.5

log J (mA cm )

-Z" (Ω)


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

Page 26 of 29

0

10

Time (h)

20

30

40

50

Z' (Ω)

Figure 3. (a) LSV curves of IZO, RZO, and ZnO for OER (in O2-saturated 1 M KOH) and (b) corresponding Tafel plots. The water oxidation potential (E0=1.23 V) is marked. (c) Chronoamperometric responses at a constant potential of η = 0.4 V (1.63 V vs. RHE). (d) Nyquist plots for EIS experiments in the range from 100 kHz to 0.1 Hz at η = 0.3 V (1.53 V vs. RHE).


26

0

(a)

0.8

ZnO

-10

(b)

-1

0.6

RZO

-20

IZO

Z nO

Pt/C η (V)

-2

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



Page 27 of 29

-30

0.4

RZ O 0.2

IZO

-40 0.0 -50 -0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

(b

-1.0

-0.5

Potential (V vs. RHE)

10 =2

m

ec Vd

)

-1 ec ) d V 6m (b = 9

mV d (b = 72

-1 ec )

-1 ec ) d V m 6 5 Pt/C (b =

0.0

0.5

1.0

1.5

2.0

-2

log J (mA cm )

Figure 4. (a) LSV curves (scan rate: 2 mV s–1) for IZO, RZO, ZnO, and commercial Pt/C as catalysts toward HER in H2-saturated 1 M KOH. (b) Tafel plots derived from the LSV curves.


27

-2

(a) ZnO

-2

RZO Pt/C

IZO

-4

-6 0.0


-2

0

0.2

0.4

0.6

0.8

1.0

0 -1

2 -1

-1

0.4 0.3

-2

(c) 0.2 V 0.3 0.4 0.5 0.6

0.2

IZO (n = 3.7)

Pt/C (n = 3.9)

0.1 0.02

0.03

ω

−1/2

0.04

(rpm

-1/2

)

0.05

400 rpm 600 900 1225 1600 2500

-2 -3 -4 -5 0.0

0.2

0.4

0.6

0.8

1.0

Potential (V vs. RHE) Current Density (mA cm )

0.5

Page 28 of 29

(b) IZO


J (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



60

(d) IZO RZO Pt/C

40

OER

20 ∆E = 0.78 V

0

ORR 0

E (H2O/O2)

0.6

0.8

1.0

1.2

1.4

1.6

1.8


Figure 5. (a) LSV curves of IZO, RZO, ZnO, and Pt/C for ORR in O2-saturated 1 M KOH. (b) LSV curves of IZO at different rotation speeds, and (c) K–L plots of IZO and Pt/C at potentials varying from 0.2 to 0.6 V. (d) ORR and OER polarization curves of IZO, RZO, and Pt/C.


28

Page 29 of 29

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


Table of Contents Graphic

IrO2-ZnO hybrid nanoparticles exhibit excellent multifunctional electrocatalytic activities toward oxygen evolution, hydrogen evolution, and oxygen reduction reactions.


29

IrO2âZnO Hybrid Nanoparticles as Highly Efficient Trifunctional

Recommend Documents

IrO2âZnO Hybrid Nanoparticles as Highly Efficient Trifunctional