Dynamic Structure Evolution of Composition Segregated Iridium

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Dynamic Structure Evolution of Composition Segregated Iridium-Nickel Rhombic Dodecahedra towards Efficient Oxygen Evolution Electrocatalysis Yecan Pi, Qi Shao, Xing Zhu, and Xiaoqing Huang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04023 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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Dynamic Structure Evolution of Composition Segregated Iridium-Nickel Rhombic Dodecahedra towards Efficient Oxygen Evolution Electrocatalysis Yecan Pi,†,# Qi Shao,†,# Xing Zhu,‡ and Xiaoqing Huang*,† †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu, 215123, China. ‡

Testing & Analysis Center, Soochow University, Jiangsu, 215123, China. #

These authors contributed equally.

*

Address correspondence to: [email protected]

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Abstract: The anodic oxygen evolution reaction (OER) is central to various energy conversion devices, but the investigation of the dynamic evolution of catalysts in different OER conditions remains quite limited, which is unfavorable for the understanding of the actual structure-activity relationship and catalyst optimization. Herein, we constructed monodispersed IrNix nanoparticles (NPs) with distinct composition-segregated features and captured their structural evolution in various OER environments. We decoded the interesting self-reconstruction of IrNix NPs during the OER, in which an Ir-skin framework is generated in an acidic electrolyte, while a Ni-rich surface layer is observed in an alkaline electrolyte owing to Ni migration. Benefiting from such self-reconstruction, considerable OER enhancements are achieved under both acidic and alkaline conditions. For comparison, IrNix nanoframes with Ir skins prepared by chemical etching show a similar structural evolution result in the acidic electrolyte, but a total different phenomenon in the alkaline electrolyte. By tracking the structural evolution of IrNix catalysts and correlating them with OER activity trajectories, the present work provides a significant understanding for designing efficient OER catalysts with controlled compositional distributions.

Keywords: Iridium, composition segregation, electrocatalysis, structural evolution, oxygen evolution reaction

Energy conversion and storage is the critical issue in the development of renewable energy.1-4 The oxygen evolution reaction (OER) is an enabling process for many promising energy conversion devices, such as electrolyzers, regenerative fuel cells, rechargeable metal-air batteries, photoelectrochemical cells for CO2 reduction.5-11 Unfortunately, the sluggish OER kinetics imposes a considerable overpotential and thus results in obviously energy loss, greatly hampering potential applications.12-17 Therefore, the


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design of OER catalysts with largely reduced overpotentials are highly needed for device efficiency and cost effectiveness and are however very complex.18-20 A major challenge is the prevalent structural evolution of catalysts under electrochemical conditions, especially in the acidic or alkaline aqueous solutions as well as strongly oxidative OER environments.21-24 The facts raise questions regarding the real structure responsible for efficient OER catalysis. A substantial investigation of such an evolution is highly imperative to understand the real relationship between the structure and OER property and provides critical guidance for the OER catalyst design. In fact, in-depth studies of the structural evolution of catalysts for several classical electrocatalytic processes, for example, the oxygen reduction reaction (ORR), has occurred. It has been demonstrated that the formation of a Pt skin “surface-sandwich” structure for Pt-bimetallic nanoparticles (NPs) during ORR electrocatalysis is beneficial for their ORR activity.25-28 Such refined characterizations for catalyst restructuring offers different insights into the ORR mechanism, thereby revealing fresh perspectives in the design of advanced catalysts.29-32 For the OER, related research is still in its infancy. Jaramillo et al. introduced more active IrOx/SrIrO3 by surface rearrangement via the leaching of Sr from a SrIrO3 surface during the OER in an acidic electrolyte.22 Other work by Schmidt et al. demonstrated the surface selfreconstruction of the perovskite OER catalyst, in which they proved the growth of a self-assembled metal oxy(hydroxide) active layer under OER conditions.23 Nevertheless, previous studies have been mainly focused on thin films or irregular materials, while the subtle structural evolution of catalysts is difficult to observe directly. In addition, investigations of the structural evolution of catalysts under different OER conditions are significant for the design of efficient catalysts, however, these studies are still far from consideration. Thus, the development of monodispersed nanomaterials that allow the direct decoding of the structural behavior under different OER conditions and correlate this behavior with their OER activities is imperative to fill the knowledge gap.


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Herein, we report the creation of a class of composition-segregated IrNix (x = 1, 2 and 3) rhombic dodecahedron (RDH) NPs and their distinct structural evolution during OER electrocatalysis in different electrolytes (i.e., H2SO4 and KOH solutions) and correlate them with OER activities. The pristine IrNix NPs feature Ir-rich frames along their edges and corners, whereas the segregation degree varies with their initial components. Distinct morphology and composition changes have been decoded in different electrolytes during the OER. Under acidic conditions, Ir-skin nanoframes were formed due to Ni leaching, leading to the dramatic enhancement of OER activity. A quite different result was observed under alkaline conditions, in which Ni oxide species were involved on the surface to form a Ni-rich shell structure, also resulting in a distinct contribution to OER enhancement. For the Ir-skin nanoframes obtained by acid etching, slight structural and activity changes during acidic OER electrocatalysis were revealed, indicating the similarity between the chemical etching and electrochemical etching processes. Nevertheless, under alkaline conditions, the Ir-skin nanoframes showed distinct OER enhancements during electrocatalysis, which is benefited by the formation of the Ir-Ni mixed oxide surface.


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Figure 1. Morphology and composition profile analysis of IrNix-P. (a, b) HAADF-STEM images and (c) HRTEM image of IrNi2-P. The inset in (c) shows the corresponding TEM image, and the enlarged area is marked with a dashed box. (d) Line scan analysis of IrNi2-P and (e) EDS elemental mappings of IrNi1-P. (f) Ball schematic sketch of IrNi1-P.

RESULTS AND DISCUSSION


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An efficient wet-chemical route was developed to prepare the IrNix NPs with different compositions (i.e., IrNi1 NPs, IrNi2 NPs and IrNi3 NPs, see Supplementary Information for details). The obtained monodispersed IrNix NPs have uniform morphology and size, as revealed by the transmission electron microscopy (TEM) images (Figure S1). The IrNi3 NPs have a clear RDH shape, as observed along the different typical projections (Figure S2). With an increase in the Ir content, the NP surfaces become rough and are associated with an increase in size (24.3 ± 2.3 nm for IrNi3 NPs, 26.1 ± 2.9 nm for IrNi2 NPs and 30.6 ± 4.4 nm for IrNi1 NPs), while their RDH shapes are visible. The crystal phases of the IrNix NPs were revealed by X-ray diffraction (XRD, Figure S3), in which two sets of distinct facecentered cubic (fcc) patterns were observed. One group of the diffraction peaks correspond to fcc Ni. The other diffraction peak group is located between those of Ir and Ni, which can be regarded as an Ir-Ni alloy phase. With an increase in the reaction temperature, the peaks of the alloy phase shift to a lower angle, suggesting that the phase has changed from Ni-rich to Ir-rich alloys. The component distributions were further evidenced by high-angle annular dark-field scanning TEM energy-dispersive X-ray spectroscopy (HAADF-STEM-EDS) elemental mappings, which exhibit Ir-enriched edges and corners of the NPs (Figure S4). Such elemental distribution is very close to the previously reports, likely being caused by preferential etching of low-coordinated Ni by the trace O2 in solution during the synthesis.29 To gain in-depth understanding of the formation mechanism, we have carefully investigated the growth process of IrNi1 NPs (Figure S5). The results show that the formation of IrNi1 NPs has gone through the initial formation of Ni-rich rhombic dodecahedron nanostructures. Thereafter, the reduction of Ir was gradually accelerated, and the formation of IrNi alloy phase became the dominant process finally. As a comparison, we have tried to adjust the feeding ratios to achieve the different components of the IrNix NPs, while the products became irregular and aggregated with the increasing of nickel precursor, indicating that the synthesis of well-defined IrNix NPs depended highly on the concentration of Ir and Ni precursors (Figure S6).


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The obtained IrNix NPs with composition-segregated features could be ideal catalysts to study the structural evolution during an OER. Before the electrochemical study, the IrNix NPs were loaded onto carbon black, followed by controlled thermal treatment to remove the organic surfactants. The morphology and composition profiles of the obtained pristine catalysts (referred to as “IrNix-P”) were initially investigated. Figure 1a-b demonstrates an HAADF-STEM image of IrNi2-P, where the NPs are highly dispersed on the support without aggregation. The high-resolution TEM (HRTEM) image shows a lattice spacing of 0.220 nm (close to that of the (111) Ir lattice spacing of 0.222 nm) at the corner of IrNi2-P, while the facet region shows a lattice spacing of 0.210 nm (near to that of the (111) Ni lattice spacing of 0.203 nm), suggesting the Ir-rich corner and Ni-rich facet regions, respectively (Figure 1c). Figure 1d shows line scans along the diagonal direction of the hexagon of IrNi2-P. The results clearly show that Ir is mainly distributed on the corners, whereas Ni has nearly symmetrical profiles. The elemental segregation characteristics of IrNix-P were also demonstrated by the EDS elemental mappings, where IrNi3-P showed clear Ir enrichment at the edges and corners (Figure S7a). With an increase in the Ir content, more Ir atoms are deposited on the facets (Figure S7b). The highest Ir content of IrNi1-P results in a nearly all Ir overlayer on the surface (Figure 1e-f).


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Figure 2. Electrochemical studies of IrNix-P and the morphology and composition profile analysis of IrNi2-PE (H+). (a) Polarization curves of IrNi2-P with increasing potential cycles in 0.05 M H2SO4. (b) Current densities obtained at a potential of 1.55 VRHE of IrNix-P and IrNix-PE (H+). (c) HAADF-STEM image of IrNi2-PE (H+). (d) Line scan analysis and (e) EDS elemental mappings of IrNi2-PE (H+). (f) Ball schematic sketch of IrNi2-PE (H+).

OER catalysis reactions of IrNix-P were first carried out in a 0.05 M H2SO4 solution. The activity was assessed by using alternating cyclic voltammetry (CV) and polarization curves (Figure S8). Figure 2a shows the polarization curves of IrNi2-P after increasing potential cycles. To eliminate the oxidation current of surface species during the initial cycles, the polarization curve after 3 potential cycles is


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referred to as “initial”.33 As shown in Figure 2a, IrNi2-P requires the overpotential of 356 mV to reach 10 mA cm-2 and improves to 340 mV after 5 potential cycles. The activity gradually increases with increasing potential cycles and finally reaches its maximum value after 25 potential cycles (electrochemical activated state, referred to as “IrNi2-PE (H+)”), which requires an overpotential of only 315 mV to reach the same current density, showing a significant overpotential decrease of 41 mV. The electrochemical performances of IrNi3-P and IrNi1-P were also investigated under the same conditions, where obvious catalytic enhancements were also observed in both IrNi3-PE (H+) and IrNi1-P (H+) (Figure 2b). To understand how the above OER behavior of the IrNix catalysts correlates with their structural evolution, detailed investigations of IrNi2-P after OER were carried out. Both the STEM image of IrNi2PE (H+) (Figure 2c) and TEM images of IrNix-PE (H+) (Figure S9) show that the initial NPs have transformed into nanoframes, providing visual evidence of morphological transformation. The XRD pattern reveals the Ir-rich alloy phase of IrNi2-PE (H+), consistent with the HRTEM result (Figure S10). Line scans and EDS elemental mapping confirm that most of the Ni has been leached out from the particle’s facet, leaving an Ir-rich nanoframe after OER electrocatalysis (Figure 2d-e). Furthermore, IrNi2-PE (H+) has an Ir-rich shell in the final nanoframe, demonstrating the formation of an Ir-skin during the OER under acidic conditions (Figure 2f).


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Figure 3. Morphology and composition profile analysis of IrNi2-C and electrochemical studies of IrNix-C. (a) HAADF-STEM image and (b) line scan analysis of the IrNi2-C. (c) Polarization curves of IrNi2-C with increasing potential cycles in 0.05 M H2SO4. (d) Current densities obtained at a potential of 1.55 VRHE of IrNix-C and IrNix-CE (H+).

The above results indicate obvious Ni leaching during the OER under an acidic condition, along with distinct activity enhancement. To obtain a deeper understanding of such an evolution, we simulated this evolution by chemical etching (referred to as the “IrNix-C”, Figure S11-S13) and compared the differences between IrNix-P and IrNix-C. As shown in Figure 3a, for IrNi2-C, the nanoframes are highly dispersed on the support without aggregation. The line scans (Figure 3b) further confirmed that inner Ni was leached, resulting in the Ir-rich frame, showing a similar phenomenon between the chemical etching


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and electrochemical etching processes. Figure 3c shows the polarization curves of IrNi2-C with increasing potential cycles. There is a slight activity increase after 25 potential cycles, implying that the surface structure has quickly evolved into a steady state. Figure 3d presents the current density of IrNixC before and after 25 potential cycles at a potential of 1.55 V, which show limited enhancements. Therefore, it is conceivable that the chemical etching process has resulted in a relatively stable surface structure with limited change during electrocatalysis (Figure S14). In addition, the OER activities of IrNix-PE (H+) and IrNix-CE (H+) after the OER are similar (Figure S14g), mainly due to the formation of a similar surface structure, as confirmed by the morphology and composition profiles (Figure S15S17). X-ray photoelectron spectroscopy (XPS) was further carried out to study the surface compositions and chemical states of IrNix catalysts before and after the OER electrocatalysis. As shown in Figure S18, most of the surface Ni in IrNi2-P and IrNi2-PE (H+) is oxidized. Compared to IrNi2-P, there is an obvious decrease in the Ni signal in IrNi2-PE (H+), while the Ir signal remains largely constant. This agrees with the formation of the Ir-rich surface under acidic OER conditions. The Ir 4f spectra shows that Ir in IrNi2P has both metallic and oxidized states with an Irx+/Ir0 ratio of 0.24/1, while the ratio of Irx+/Ir0 in IrNi2PE (H+) increases to 3.7/1, implying the oxidation of surface Ir during the OER. For IrNi2-C, there is an imperceptible Ni signal, yet the Ir signal remains strong, indicating the serious erosion of the surface Ni composition along with the formation of an Ir skin after chemical etching. Interestingly, a weak Ni signal can be observed in IrNi2-CE (H+), proving Ni migration into the particle’s surface during the OER. The surface Ir is mainly in the oxidation state, where the ratio of Irx+/Ir0 in IrNi2-C has increased from 0.44/1 to 2.0/1 for IrNi2-CE (H+). These results indicate that the Ni atoms tend to migrate to the catalyst’s surface and dissolve into the solution during acidic OER electrocatalysis, facilitated by abundant H+ as well as the high oxidizing potential. The resulting nanoframes with both high surface area and active site availability are related to


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increased OER activity. The electrochemical activity of the IrNix catalysts reached their maximum value finally, indicating that the evolution was finished after the formation of the Ir-skin structure. Inspired by the previous reports on the Pt-Ni system, we inferred that the Ir surface layer in combination with Ir-Ni subsurface core structure might critically impact OER catalysis.29

Figure 4. Electrochemical studies of IrNix-P and IrNix-C in 0.1 M KOH. Polarization curves of (a) IrNi2-P and (c) IrNi2-C with increasing potential cycles. Current densities obtained at a potential of 1.55 VRHE of (b) IrNix-P and (d) IrNix-C before and after 25 potential cycles.

Compared to the acidic condition, the OER under an alkaline condition has been commercially available for more than a century.34-37 Despite this, substantial investigations regarding the evolution of the catalyst under working conditions remain scarce.23 To this end, OER measurements for different


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IrNix catalysts were further performed in a 0.1 M KOH solution. The alternating CV and polarization curves were used to investigate their electrochemical behavior (Figure S19). Figure 4a illustrates the polarization curves of IrNi2-P with increasing potential cycles. Initially, IrNi2-P shows poor activity with a current density of only ~1.7 mA cm-2 at a potential of 1.55 V (vs. RHE). However, with an increase in potential cycling, its activity increases gradually and finally reaches its maximum value after 25 potential cycles. Particularly, at the same potential of 1.55 V, IrNi2-PE (OH-) can deliver a current density of 6.2 mA cm-2, revealing more than a 3.6-fold improvement. As shown in Figure 4b, IrNi3-P and IrNi1-P also exhibit a distinct increase in the OER activity, while the degree of such an enhancement varies with their initial component. The OER performance of IrNix-C was also investigated under the same conditions for comparison. As shown in Figure 4c, IrNi2-C shows higher initial activity than that of IrNi2-P, which can be attributed to its larger surface area. The activity of IrNi2-C continues to increase with prolonged potential cycling and tends to be stabilized over the course of potential cycles. Interestingly, we found that all the IrNix-C samples exhibit distinct OER activity increases during electrocatalysis (Figure 4d & Figure S20), indicating that the structural evolution of IrNix-C is highly beneficial for the OER enhancement.


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Figure 5. Morphology and composition profile analysis of IrNi2-PE (OH-) and IrNi2-CE (OH-). HAADF-STEM image and line scan analysis of (a, c) IrNi2-PE (OH-) and (b, d) IrNi2-CE (OH-). (e) EDS elemental mapping of IrNi2PE (OH-). (f) Ball schematic sketches of IrNi2-PE (OH-) and IrNi2-CE (OH-).

To gain deep insight into the surface-evolution process under the alkaline condition, we collected the catalysts after the OER for deep characterizations. Interestingly, while the initially solid NPs have transformed into hollow structures (Figure S21), the EDS analysis indicates negligible composition


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changes for both IrNi2-P and IrNi2-C after the electrochemical process (Table S1), implying a totally different evolution in the alkaline electrolyte compared with that in the acidic electrolyte. Figure S22a demonstrates the XRD patterns of IrNi2-PE (OH-) and IrNi2-CE (OH-), where two weak diffraction peaks at approximately 37° and 63° are observed in the XRD pattern of IrNi2-PE (OH-), which can be identified as the NiO phase. In contrast, there is no obvious change for IrNi2-CE (OH-). HRTEM images further prove the formation of NiO on the surface of IrNi2-PE (OH-) (Figure S22b-c). The HAADFSTEM image of IrNi2-PE (OH-) clearly shows that the hollow NPs have a bright nanoframe associated with the dark shell in their external surface (Figure 5a), suggesting a compositional segregation in the surface of the hollow particles. For comparison, IrNi2-C exhibited a negligible morphology change after OER electrocatalysis (Figure 5b). Interesting, the line scan analyses and EDS elemental mapping of IrNi2-PE (OH-) proved that the Ni in the inner region has diffused outwards, which results in a hollow structure with a NiO shell after OER electrocatalysis in the KOH electrolyte (Figure 5c&e). For IrNi2CE (OH-), the line scan analysis indicates an increase in surface Ni, in which the Ir skin has disappeared (Figure 5d). In addition, the Ni 2p of IrNi2-CE (OH-) shows a significantly enhanced signal compared to that of IrNi2-C (Figure S23a), which also provides evidence of the migration of Ni during OER electrocatalysis under the alkaline condition. The Ir 4f regions of IrNi2-PE (OH-) and IrNi2-CE (OH-) show that surface Ir has changed into an oxidation state during the OER (Figure S23b). Therefore, there is a tendency for Ni migration to the catalyst “surface” for the formation of Ni oxide species on the surface under the alkaline OER condition. The surface Ni of IrNi2-CE (OH-) is obviously lower than that of IrNi2-PE (OH-), showing that the Ir skin can suppress the migration of Ni to the surface. Moreover, according to the dramatic enhancement in IrNix-PE (OH-) and IrNix-CE (OH-), the presence of Ni oxide species on the surface seems to benefit the OER under alkaline conditions.


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Figure 6. Surface structural evolution of the IrNix catalysts. Schematic representation of the surface structure changes of IrNix-P and IrNix-C after OER electrocatalysis under acidic and alkaline conditions.

Figure 6 illustrates our interesting findings of the above structural evolution of the compositionsegregated IrNix catalysts during the OER. For all the catalysts (IrNix-P and IrNix-C) in the acidic or alkaline electrolytes, Ni tends to migrate onto the NP surfaces, although the migration process can be


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affected by the different initial surface components (ratio of Ir/Ni) or structures (Ir-skin nanoframe). The difference is that this migration will eventually result in various surface structures due to different external environments. Specifically, the dissolution of the Ni component in the acidic electrolyte results in an Ir-skin structure with Ni in the subsurface, which is very similar to the effect of chemical etching. The resulting nanoframes with high surface areas and active sites show obvious advantages to the OER. In addition, the Ir surface layers in combination with the Ir-Ni subsurface core structure give rise to an OER enhancement in the acidic condition. On the other hand, the Ni atoms tend to migrate to the surface in the alkaline electrolyte, in which a NiO layer is formed for IrNix-PE (OH-), which benefits the OER. For all IrNix-C, the migration of Ni is decelerated by the initial Ir skin surface and results in the formation of an Ir-Ni mixed oxide surface, which shows a distinct contribution to the OER enhancement in the alkaline condition as well.

CONCLUSIONS In summary, we have revealed the structural evolution of the composition-segregated IrNix NPs under diverse OER conditions and correlated this evolution with their OER activity trajectories. The distinct structural evolution phenomena under different conditions were clearly observed. In the acidic electrolyte, the leaching of surface Ni leads to the formation of Ir-rich surface structure, while the migration of Ni atoms results in an increase Ni content in the surface under the alkaline electrolyte. More importantly, such in-situ structural evolutions both show a self-optimizing electrocatalysis activity. The present work provides deep insight into the structural evolution of OER catalysts and offers a fundamental understanding of the real relationship between structure and properties, which provides significant guidance for the design of efficient OER catalysts.


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EXPERIMENTAL SECTION Chemicals. Iridium (III) chloride hydrate (IrCl3, 99.9%, Alfa Aesar), nickel (II) acetylacetonate (Ni(acac)2, C10H14NiO4, 96%) and oleylamine (C18H37N, 70%) were purchased from J&K Scientific Ltd. Oleic acid (C18H34O2, 99%) was purchased from Tokyo Chemical Ind. Phloroglucinol anhydrous (C6H6O3, ≥99.0%) was purchased from Aladdin Industrial Corp. Cyclohexane (C6H12, ≥99.5%), ethanol (C2H6O, 95%), nitric acid (HNO3, 65%~68%) and potassium hydroxide (KOH, ≥85%) were all purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Nafion solution (~5 wt% in a mixture of lower aliphatic alcohols and water) was purchased from Sigma-Aldrich. All the chemicals were used without further purification. The water (18 MΩ/cm) used in all experiments was prepared by passing through an ultra-pure purification system (Aqua Solutions). Synthesis of IrNix (IrNi1, IrNi2 and IrNi3) NPs. In a typical synthesis of the IrNix NPs, 10 mg IrCl3, 3.2 mg Ni(acac)2, 36 mg phloroglucinol, 4 mL oleylamine and 1 mL oleic acid were added into a glass vial (volume: 20 mL). The vial was then capped and ultrasonicated for approximately 1 h to form a homogeneous mixture. The vial was then heated to a desired temperature (i.e., 200 °C, 190 °C and 180 °C) and maintained for 5 h in an oil bath. After cooling to room temperature, the resulting products were washed with ethanol/cyclohexane mixture and collected by centrifugation. The IrNi1, IrNi2 and IrNi3 NPs were obtained with synthesis temperatures of 200 °C, 190 °C and 180 °C, respectively. Preparation of the IrNix catalysts. To the IrNix NPs redispersed in cyclohexane, carbon support (Vulcan XC-72) was added with Ir loading of 11~14 wt%. The actual loadings were determined by ICPAES (considering Ir cannot be fully dissolved by aqua regia, the loading of Ir is calculated by the combination of ICP and EDS). The mixture was sonicated at room temperature for approximately 30 min. Ethanol (10 mL) was then added to the mixture, followed by sonication for another 30 min to


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complete the loading process. The catalyst was washed with ethanol and collected by centrifugation. The resulting catalyst powder was heated at 250 °C in air for 1 h to remove organic surfactants. Chemical etching of the IrNix catalysts. One milliliter HNO3 (~65%) was added to the IrNix catalysts redispersed in 5 mL water. The mixture was then heated at 60 °C for 8 h under stirring. After chemical etching, the products were washed with ethanol/water mixture and collected by centrifugation. Characterizations. Low-magnification transmission electron microscopy (TEM) was conducted on a Hitachi HT7700 transmission electron microscope at an accelerating voltage of 120 kV. High-resolution TEM (HRTEM), high-angle annular dark-field scanning TEM (HAADF-STEM) and HAADF-STEM energy-dispersive X-ray spectroscopy (HAADF-STEM-EDS) were conducted on an FEI Tecnai F20 transmission electron microscope at an acceleration voltage of 200 kV. The samples were prepared by dropping the cyclohexane (or ethanol) onto the carbon-coated copper TEM grids using pipettes and then dried under ambient conditions. Powder X-ray diffraction (PXRD) patterns were collected on a Shimadzu XRD-6000 X-ray diffractometer. The concentrations of the catalysts were determined by inductively coupled plasma atomic emission spectroscopy (710-ES, Varian, ICP-AES). X-ray photoelectron spectra (XPS) were collected with an SSI S-Probe XPS Spectrometer. The carbon peak at 284.8 eV was used as a reference to correct for charging effects. Electrochemical measurements. Electrochemical measurements were conducted in a three-electrode setup with a saturated calomel electrode (SCE) as the reference electrode and a graphite rod as the counter electrode. The working electrode was a modified glassy-carbon electrode (GCE) (diameter: 5 mm, area: 0.196 cm2) from the Pine Instrument Co. To prepare the working electrode, the IrNix catalysts were mixed with ethanol, water, and Nafion (5%) (v: v: v = 4: 1: 0.01) and sonicated for 30 min to form a homogeneous catalyst ink. A certain volume of the catalyst ink was then cast on a GCE and dried under


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ambient conditions. The loading amount of Ir on working electrode was controlled to be 24.5 µgIr/cm2. All the potentials were calibrated to the RHE. The cell was purged with N2 prior to each set of experiments. Cyclic sweep voltammetry was carried out between +0.05 V to +1.7 V with a scan rate of 20 mV s-1. Linear sweep voltammetry was recorded between +1.0 V to +1.7 V with a scan rate of 5 mV s-1. The current densities were calculated by normalizing to the geometric value of the GCE. All the potentials and voltages are iR corrected unless noted.

ASSOCIATED CONTENT Supporting Information. Experimental details and data. Figure S1-23&Table S1. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author [email protected] ACKNOWLEDGMENT This work was financially supported by the Ministry of Science and Technology (2016YFA0204100, 2017YFA0208200), the National Natural Science Foundation of China (21571135), Young Thousand Talented Program, Jiangsu Province Natural Science Fund for Distinguished Young Scholars (BK20170003), the project of scientific and technologic infrastructure of Suzhou (SZS201708), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the startup supports from Soochow University.

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Dynamic Structure Evolution of Composition Segregated Iridium

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