Highly Efficient Acidic Oxygen Evolution Electrocatalysis Enabled by

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Highly Efficient Acidic Oxygen Evolution Electrocatalysis Enabled by Porous Ir-Cu Nanocrystals with 3D Electrocatalytic Surfaces Yecan Pi, Jun Guo, Qi Shao, and Xiaoqing Huang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03620 • Publication Date (Web): 04 Nov 2018 Downloaded from http://pubs.acs.org on November 5, 2018

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Chemistry of Materials

Highly Efficient Acidic Oxygen Evolution Electrocatalysis Enabled by Porous Ir-Cu Nanocrystals with 3D Electrocatalytic Surfaces Yecan Pi,† Jun Guo,‡ Qi Shao,† and Xiaoqing Huang*,† †College

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

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

*Address

correspondence to: [email protected]

ABSTRACT: The design of efficient Ir-based catalyst for oxygen evolution reaction (OER) in acidic condition is the key for realizing the commercialization of polymer electrolyte membrane (PEM) technology. This study reports a class of highly porous Ir-Cu nanocrystals (P-IrCux NCs) through a facile chemical dealloying strategy. Featuring the increased active surface area and plenty of defects, the newly-generated P-IrCux NCs are highly active and stable towards OER in acidic condition. The optimized P-IrCu1.4 NCs exhibit the current density of 12.8 mA cm−2 at the potential of 1.55 V (vs. RHE), presenting 3.5-fold improvement in mass activity and 1.8-fold improvement in specific activity over the pristine solid IrCu1.4 NCs (S-IrCu1.4 NCs). Moreover, the obtained P-IrCu1.4 NCs also show excellent OER stability with negligible potential shift after continuous electrolysis in 0.05 M H2SO4 for 10 h. Further analysis reveals that the coordinatively unsaturated atoms at defects result in the higher oxidation state of iridium, which promotes the optimized intrinsic activity of P-IrCux NCs. This work

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highlights a promising strategy to create 3D porous Ir-based catalysts for enhancing OER electrocatalysis.

INTRODUCTION Hydrogen (H2) is considered as one of the most crucial energy sources of the 21st century, whose demand is even growing with the deteriorating energy crisis and environment issues.1-3 Compared to many H2 production strategies, water electrolysis (2H2O → O2 + 2H2) provides a room-temperature, fossil fuel-free path for the large scale production of H2, and is compatible with sustainable energy sources such as wind and sunlight.4-7 However, the substantial overpotential of the anode half reaction, oxygen evolution reaction (OER), is the major bottleneck for commercializing the water electrolysis technology.8-13 Even the best catalysts require hundreds millivolts of OER overpotential to reach the satisfied current density of ≥ 10 mA cm-2, which is an order of magnitude higher than that of cathode half reaction, hydrogen evolution reaction (HER).14-16 While various catalysts have been explored for alkaline OER, Ir and Ru are still the only materials with satisfied activity and stability in acidic condition.17,18 Considering the better stability than Ru-based catalysts, the development of Ir-based catalysts is of significant for improving polymer electrolyte membrane (PEM) electrolyser, the most promising electrolysis technology performing in the corrosive acidic condition that can overcome the disadvantages of conventional alkaline electrolyser.19,20,21 To this end, more and more efforts are devoting into the design of efficient Ir-based catalysts, in which the previous works have focused on the introduction of second components to enhance the OER catalysis.2224,25

However, the severe leaching of the second components is inevitable in the corrosive acidic OER

condition, largely limiting their practical applications.26 From the structural point of view, the creation of porous structure is expected to be an attractive way to largely enhance the electrocatalysis, in which the porous structure can reduce the amount of precious metals and expose more active sites for


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catalysis.27,28 In addition, accompanied by the creation of porous structure, many defects can be introduced, in which the unsaturated coordination atoms around the defects may further enhance the electrocatalysis.29-31 Therefore, the design of unique Ir-based nanocrystals (NCs) with porous structure is highly desirable, while it is still a grand challenge.

EXPERIMENTAL SECTION Chemicals. Iridium (III) chloride hydrate (IrCl3·xH2O, 99.9%, Alfa Aesar), copper (II) acetylacetonate (Cu(acac)2, C10H14CuO4, 99%), hexadecyltrimethylammonium bromide (CH3(CH2)15N(Br)(CH3)3, CTAB, 99%) and oleylamine (C18H37N, 70%) were purchased from J&K Scientific Ltd. Phloroglucinol anhydrous (C6H6O3, ≥99.0%) was purchased from Aladdin Industrial Corp. Cyclohexane (C6H12, ≥99.5%), ethanol (C2H6O, 95%), nitric acid (HNO3, 65%~68%) and sulfuric acid (H2SO4, 95%~98%) 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).

Preparation of solid IrCux nanocrystals (S-IrCu0.9, S-IrCu1.4 and S-IrCu2.3 NCs). In a typical


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synthesis of S-IrCu1.4 NCs, 5 mg IrCl3·xH2O, 6 mg Cu(acac)2, 72 mg phloroglucinol, 27 mg CTAB and 5 mL oleylamine were added into a vial (volume: 30 mL). After the vial had been capped, the mixture was ultrasonicated for around 1 h. The resulting homogeneous mixture was then heated at 200 oC for 5 hours in an oil bath, before it was cooled to room temperature. The resulting colloidal products were collected by centrifugation and washed three times with an ethanol/cyclohexane mixture. The preparation of S-IrCu0.9 and S-IrCu2.3 NCs was similar to the preparation of S-IrCu1.4 NCs except that 4 mg and 8 mg Cu(acac)2 were used for S-IrCu0.9 and S-IrCu2.3 NCs, respectively. In order to investigate electrochemical properties, the S-IrCux NCs were incorporated onto carbon black (Cabot, Vulcan XC-72).

Preparation of porous IrCux nanocrystals (P-IrCu0.9, P-IrCu1.4 and P-IrCu2.3 NCs). The P-IrCux NCs were obtained by etching S-IrCux NCs with nitric acid. In a typical preparation of P-IrCu1.4 NCs, the S-IrCu1.4 NCs were redispersed in 5mL cyclohexane (with 0.2 mL oleylamine, and sonicated for 10 min) and 1mL HNO3 was then added into the solution. Thereafter, the mixture was kept at 60 oC for 5 h under stirring. The P-IrCu1.4 NCs were collected by centrifugation and washed three times with an ethanol/cyclohexane mixture. P-IrCu0.9 and P-IrCu2.3 NCs were obtained by etching S-IrCu0.9 and S-IrCu2.3 NCs with the same method. For the synthesis of P-


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IrCux NCs loaded on carbon black (P-IrCux/C), 20mg S-IrCux/C was redispersed in 10 mL water and 1 mL HNO3 was then added into the mixture. Thereafter, the mixture was kept at 60 °C for 5 h under stirring. The resulting P-IrCux/C were collected by centrifugation and washed with ethanol.

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 accelerating voltage of 200 kV. The samples were prepared by dropping the cyclohexane (or ethanol) dispersion of samples onto carbon-coated copper TEM grids using pipettes and dried under ambient. Energy dispersive X-ray spectroscopy (EDS) was conducted on a HITACHI S-4700 cold fieldemission scanning electron microscope (SEM) operated at 15 kV. The samples were prepared by dropping the cyclohexane (or ethanol) dispersion of samples onto a piece of silicon substrate. Powder X-ray diffraction (PXRD) patterns were collected on a Shimadzu XRD-6000 X-ray diffractometer with a Cu Kα X-ray source (λ = 1.540598 Å). X-ray photoelectron spectra


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(XPS) were collected with an SSI S-Probe XPS Spectrometer. The fitting of Ir 4f peaks was performed after subtraction of a Shirley background using the XPS Peak 4.1 program for asymmetric peak line shapes. The peak separation and the peak area ratios between the Ir 4f7/2 and the Ir 4f5/2 components were constrained to 2.98 eV and 4 :3, respectively. The charge-correction was based on the carbon species with minimal binding energy (assumed to be 284.8 eV). The concentration of catalysts was determined by inductively coupled plasma atomic emission spectroscopy (710-ES, Varian, ICP-AES) (considering that Ir cannot be fully dissolved by aqua regia, we calculated the Ir loadings by the combination of ICP-OES and SEM-EDS).

Electrochemical measurements. A typical three-electrode cell was used to perform the electrochemical measurements. The working electrode was a glassy-carbon electrode (GCE) (diameter: 5 mm, area: 0.196 cm2) from the Pine Instrument. The saturated calomel electrode (SCE) and the graphite rod were used as reference and counter electrodes, respectively. To prepare the working electrode, the S-IrCux/C or P-IrCux/C was mixed with ethanol, water, and Nafion (5%) (v: v: v=4 : 1 : 0.05) and sonicated for 30 min to form catalyst ink. After that, 10 to 20 μL catalyst ink was deposited on RDE and dried under ambient condition to form uniform


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thin film (loading amount of 60 μgIr cm−2). All the potentials are versus reversible hydrogen electrode (RHE).

The ECSAs were determined by integrating the hydrogen adsorption charge on the CV between the potentials of +0.05 V and +0.5 V in N2-saturated 0.05 M H2SO4 solution. The potential scan rate was 50 mV s-1 for the CV measurement. OER measurement was conducted in 0.05 M H2SO4 solution with scan rate of 5 mV s-1. Before the electrochemical activity test, the catalyst was scanned at the potential between 0.05 V and 1.55 V with the sweep rate of 50 mV s−1 for 6 cycles. This process was to eliminate the effect of the oxidation current from the catalysts themselves. The OER current densities were corrected by ohmic iR drop compensation during the measurements.

The Faradaic efficiency (FE) was calculated from the total charge Q (C) passed through the working electrode and the total amount of O2 produced n (mol). Q = i • t, where t is the time (s) under constant oxidation current i (A). The gas product was analyzed by gas chromatography (Agilent 7890B) equipped with molecular sieve 5A capillary column and thermal conductivity detector (TCD). Ar was used as the carrier gas. The oven temperature


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and detector temperature are 80 °C and 250 °C, respectively. The amount of O2 was quantified by the integral area of TCD signal. Assuming that four electrons are needed to produce one O2, the Faradaic efficiency can be calculated as follows:

2 • FE (%) = 4F n(O ) Q

where F is the Faraday constant, and n (O2) is the mole number of O2.

RESULTS AND DISCUSSION Herein, we designed an advanced porous Ir-based electrocatalyst with unique 3D pores through a facile dealloying strategy. The pristine solid IrCux alloy nanocrystals (S-IrCux NCs) were initially synthesized. Subsequently, a nitric acid treatment was introduced, in which the Cu was selectively leached away to yield porous IrCux NCs (P-IrCux NCs). The unique porous structure endows P-IrCux NCs with improved surface area as well as abundant unsaturated sites around the pores, which are highly beneficial for OER catalysis. Consequently, the optimized P-IrCu1.4 NCs show much higher OER activity with 3-fold improvement current density than that of S-IrCu1.4 NCs at 1.55 VRHE (vs. reversible hydrogen electrode, RHE), and require only 311 mV of overpotential to reach 10 mA cm-2 in 0.05 M H2SO4. The P-IrCu1.4 NCs are also stable with negligible activity decay and structure change after continuous electrolysis for 10 h.


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Figure 1. (a) HAADF-STEM image, (b) TEM image, (c) EDS spectrum and (d) XRD pattern of S-IrCu1.4. Inset of a shows corresponding size distribution. (e) HRTEM image of S-IrCu1.4. (f) Magnified HRTEM image recorded from region marked in e. (g) STEM-EDX mappings of an individual S-IrCu1.4 NC.

A colloidal-chemical approach was developed for the preparation of S-IrCux NCs (x = 0.9, 1.4 and 2.3) with the iridium (III) chloride hydrate (IrCl3·xH2O) and copper (II) acetylacetonate (Cu(acac)2) as the metal precursors, oleylamine as solvent and surfactant, hexadecyltrimethylammonium bromide (CTAB) as surfactant and phloroglucinol as reducing agent. The compositions of the S-IrCux NCs can


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be readily tuned by simply adjusting the Ir and Cu precursor ratios (see Experimental Section for details). The resulting S-IrCux NCs were initially characterized by transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) (Figure 1a,b and Figure S1), where uniform NCs with average diameters of 20-25 nm were clearly observed. The energy dispersive X-ray spectroscopy (EDS) confirms their Ir/Cu ratios of 1/0.9, 1/1.4 and 1/2.3, respectively (Figure 1c and Figure S2). X-ray diffraction (XRD) pattern reveals the distinct face-centred cubic (fcc) patterns associated with Ir-Cu alloy structure (Figure 1d and Figure S3), where the diffraction peaks shift more significantly with more Cu content. In addition, the peak shoulders at low diffraction angle suggest the composition-segregated feature. Figure 1e shows the high-resolution TEM (HRTEM) image of SIrCu1.4 NC, in which the continuous lattice fringe indicates its single crystallinity. The magnified HRTEM image shows lattice fringes with an interplanar spacing of 0.211 nm, corresponding to the (111) plane of Ir-Cu alloy (Figure 1f). The elemental distribution was further evidenced by the element mappings of S-IrCu1.4 NC (Figure 1g), where Ir and Cu distribute evenly throughout the NC with slightly rich Ir on the surface, agreeing well with the XRD result.


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Figure 2. (a) HAADF-STEM image, (b) TEM image, (c) EDS spectrum and (d) XRD pattern of P-IrCu1.4. Inset of a shows the corresponding size distribution. (e) HRTEM image of P-IrCu1.4. The yellow dashed curve highlights the pores in the particle. (f) Magnified HRTEM image recorded from region marked in e. (g) STEM-EDX mappings of an individual P-IrCu1.4 NC.

To obtain the P-IrCux NCs, pristine S-IrCux NCs were treated with the concentrated nitric acid to etch away Cu from S-IrCux NCs (see Experimental Section for details). Figure 2a shows the typical HAADF-STEM image of obtained P-IrCu1.4 NCs. The NCs remain high uniformity and monodispersity


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after the acid treatment with an average diameter of 21.5 nm. Figure 2b shows the TEM image of PIrCu1.4 NCs with higher magnification, where highly porous structures with rough outline and a high density of pores distributed across the particles are clearly observed, showing that the initial solid NCs have been successfully transformed into highly porous structures. The porous feature of P-IrCu0.9 NCs and P-IrCu2.3 are also clearly presented in Figure S4, in which the distinct pores can be observed in the P-IrCux NCs with different initial components. EDS analyses display the obvious loss of Cu component, indicating the Ir-rich feature of the porous structures (Figure 2c and Figure S5). While XRD shows the Ir-Cu alloy phase of the obtained P-IrCux NCs (Figure 2d and Figure S6), the peaks slightly shift to smaller values, indicating the increased d-spacing after the leaching of Cu. In addition, the peaks intensity of P-IrCux NCs decreases significantly compared to S-IrCux NCs, and their half-peak breadth is also obviously wider than that of S-IrCu1.4 NCs, implying the lower crystallinity and smaller crystallite size of P-IrCux NCs.32 We calculated the crystallite size of S-IrCux NCs and P-IrCux NCs through the Scherrer equation from XRD (XRD-size), and compared with their size distribution counted though the TEM images (TEM-size) (Figure S7). It is found that the size of S-IrCux NCs based on the TEM images have a little decrease after the dealloying, while their crystallite size shows a huge reduction. Therefore, it is conceivable that the leaching of Cu has resulted in crystal defects in the P-IrCux NCs, leading to smaller crystallite size, although their outlines are largely remained. Furthermore, the degree of “reduction” varies with the initial Cu content, where P-IrCu2.3 NCs shows smaller crystallite size than that of P-IrCu0.9 NCs. The trend of the crystallite size of the dealloyed NCs is plausible considering that defect densities increase with more Cu loss. To directly visualize the structure after chemical dealloying, we performed the HRTEM image of P-IrCu1.4 NCs. As shown in Figure 2e, many pores (< 5 nm) can be observed within the particles, leading to the interrupted lattice fringe in the particle. More importantly, such porous structure endows many defects (Figure S8), which are beneficial for their catalytic applications. The magnified HRTEM image shows lattice fringes with interplanar spacing of 0.222 nm,


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approaching to the (111) plane of Ir (Figure 2f). Furthermore, the elemental distribution is further described by the element mappings, where the Ir and remaining Cu distribute uniformly within the whole particle (Figure 2g).

Figure 3. (a) Schematic illustration of the transformation from S-IrCu to P-IrCu NCs via chemical dealloying. Comparison of (b) Ir 4f and (c) Cu 2p XPS spectra of S-IrCu1.4 and P-IrCu1.4 NCs.


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Figure 3a illuminates the transformation of the chemical dealloying process based on the above characterizations. Leaching of Cu results in the unique porous structures, accompanied by the introduction of defects, in which the unsaturated coordination atoms around the defects may result in different electronic properties.33-34 To confirm this hypothesis, the chemical states of the S-IrCux NCs and P-IrCux NCs were further investigated by X-ray photoelectron spectroscopy (XPS) (Figure 3b,c and Figure S9). We found that the S-IrCux NCs have the distinct surface Ir/Cu ratios of 52.2/47.8 for SIrCu0.8, 47.2/52.8 for S-IrCu1.4 and 39.5/60.5 for S-IrCu2.3, respectively. After the chemical dealloying, there was negligible Cu remains in the surface, as confirmed by the Cu 2p spectra of P-IrCux NCs (Figure 3c and Figure S9b,d). For the Ir 4f spectra, Ir0 and Ir4+ were observed in the S-IrCux NCs with similar content of Ir0 (~60%). After chemical dealloying, the content of Ir0 decreases to 44.4%, 32% and 30.6% for P-IrCu0.9, P-IrCu1.4 and P-IrCu2.3 respectively, indicating the surface oxidation during the dealloying process (Figure S9a,c and Figure S10). More importantly, compared to the S-IrCux NCs, PIrCux NCs exhibits two new peaks centered at about 62.2 eV and 65.2 eV, which is 0.7 eV higher than the binding energy (BE) values for Ir4+ (61.5 eV and 64.5 eV) (Figure 3b and Figure S9a,c). The higher BE value usually indicates an increase in the oxidation state. However, the BE increase of the new chemical state is obviously smaller than the reported BE shift of 1.4 eV between Ir4+ and Ir6+.35,36 In addition, the ratio of such new oxidation state increases with the initial Cu content, which is 19.4% for P-IrCu0.9, 30.4% for P-IrCu1.4 and 33.3% for P-IrCu2.3, respectively. To get better understand such higher Ir oxidation state, O 1s XPS spectra of the P-IrCux NCs was measured (Figures S11). Fits of the O 1s spectra reveals two different oxygen species at about 530.1 eV and 531.6 eV, which can be related to the lattice oxygen and hydroxyl groups, respectively.25,35 In addition, the content of hydroxyl species in the P-IrCux NCs increases with the increased initial Cu content. Since the porous structure with abundant defects is promoted by the Cu leaching during the dealloying process, the higher Ir oxidation


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state can be attributed to the formation of surface hydroxyl species (Ir-OH) at the low coordinated Ir around the defects, which are considered to possess high adsorption with oxygen species.37-39

Figure 4. (a) Polarization curves of S-IrCux NCs and P-IrCux NCs. Inset shows the Tafel plots. Scan rate was 5 mV s-1. The electrolyte was 0.05 M H2SO4. (b) Current densities at the potential of 1.55 VRHE, (c) overpotential required for 10 mA cm-2 and (d) Tafel slopes of different catalysts. Dash lines and columns: S-IrCux NCs; Solid lines and columns: PIrCux NCs.

Ir-based materials are considered to be the state-of-the-art OER electrocatalysts in acidic condition for their outstanding activity and stability. Before the electrochemical measurements, the NCs were first deposited on carbon support to improve the dispersion and conductivity (Figure S12). The OER


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performances of S-IrCux NCs and P-IrCux NCs were then performed under a standard three-electrode system in 0.05 M H2SO4 solution at room temperature. All the potentials were calibrated against and converted to reversible hydrogen electrode (RHE) for the comparisons. Figure 4a shows the polarization curves of S-IrCux NCs and P-IrCux NCs at the scan rate of 5 mV s−1. Compared to the SIrCux NCs, the OER curves shifted to much lower potentials after dealloying, clearly indicating the improved activities of P-IrCux NCs. Figure 4b shows the current densities of different catalysts at 1.55 VRHE, where the activity is in order of S-IrCu0.9 > S-IrCu1.4 > S-IrCu2.3. Significantly, the current densities were enhanced by 1.9-, 3.2- and 3.8-folds after dealloying of S-IrCu0.9, S-IrCu1.4 and S-IrCu2.3, respectively. Figure 4c shows the overpotentials of different catalysts at 10 mA cm-2, where the P-IrCux NCs show significant decreased overpotentials by 20-47 mV than those of S-IrCux NCs, indicating their outstanding activities among the recently reported OER catalysts in acidic conditions (Table S1). The improved activity of P-IrCux was also reflected by their lower Tafel slopes (Figure 4d). The Tafel slopes of P-IrCu0.9, P-IrCu1.4 and P-IrCu2.3 are 50.7, 53.9 and 49.7 mV dec-1, respectively, while those of the corresponding S-IrCux NCs are 67.7, 69.2 and 79.9 mV dec-1, respectively. The smaller Tafel slopes of the P-IrCux NCs suggest their enhanced reaction kinetics.


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Figure 5. (a) ECSA-corrected polarization curves of S-IrCu1.4 and P-IrCu1.4. Inset shows corresponding voltammograms. (b) OER current normalized to the Ir mass (mass activity) and the Ir-based ECSA (specific activity), respectively, at the potential of 1.55 VRHE. (c) Nyquist plots of S-IrCu1.4 and P-IrCu1.4. (d) Chronopotentiometric curve obtained with the P-IrCu1.4 at constant current densities of 10 mA cm−2, and the corresponding Faradaic efficiency from gas chromatography measurement of evolved O2.

To disclose the crucial factors in affecting the performances, the Ir-based electrochemical surface areas (ECSAs) were calculated from the hydrogen adsorption region based on the charge densities of 218 µC cm-2. Obviously, the P-IrCux NCs exhibit the increased ECSAs compared with the S-IrCux NCs, which benefits from their porous structure and can account for the enhanced activity. To this end, the intrinsic activities of different catalysts were evaluated by normalizing the current densities to the Ir


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mass (defined as “mass activity”) and the Ir-based ECSAs (defined as “specific activity”), respectively (Figure 5a,b and Figure S13). It is found that P-IrCux NCs show both the enhanced activities compared to the S-IrCux NCs. Taking the S-IrCu1.4 NCs and P-IrCu1.4 NCs as the examples, at the potential of 1.55 VRHE P-IrCu1.4 NCs represents the 3.5-fold improvement in mass activity and 1.8-fold improvement in specific activity over the S-IrCu1.4 NCs, respectively. The improved activity for P-IrCux NCs could be further verified by their much lower interfacial charge-transfer resistance (Figure 5c), implying the fast electron transport during the electrocatalytic process. These results indicate that the enhanced OER performance of P-IrCux NCs is not only due to the increased ECSAs, but also the introduced defects, which can induce the electronic structure modification via Jahn-Teller distortion.25,40-41 Significantly, the P-IrCux NCs also exhibit excellent stability. As shown in Figure 5d, after continuous electrolysis under constant current density of 10 mA cm−2 for 10 h, no obvious increase in potential was observed for PIrCu1.4 NCs, with the porous structure largely maintained (Figure S14). In addtion, the Ir 4f XPS reveals that higher Ir oxidation state associated with the defective structures can still be observed after OER stability test, confirming the high stability of such porous structure (Figure S15). The high stability was also confirmed by measuring the evolution of O2 during the stability test, where almost 100 % Faradaic efficiency was observed (Figure 5d).

CONCLUSIONS In summary, we have demonstrated an efficient strategy for creating porous IrCux NCs via chemical dealloying of solid IrCux NCs. Owing to the increased surface area as well as defects within the 3D pores, the obtained porous NCs exhibit highly efficient performance for the OER catalysis. The optimized P-IrCu1.4 NCs can reach current density of 10 mA cm−2 at the lowest overpotential of 311 mV, and exhibits excellent stability during 10 h continuous electrolysis in 0.05 M H2SO4. XPS studies reveal that the coordinatively unsaturated atoms at defects tend to show higher BE, which likely lead to the


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increased intrinsic activity of P-IrCux NCs. The present work demonstrates a new approach to optimize the Ir-based OER electrocatalysts via chemical dealloying.

ASSOCIATED CONTENT Supporting Information Additional TEM images, EDS spectra, XRD patterns and XPS spectra; voltammetry, polarization curves and Nyquist plots; table for comparison of OER activities. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author E-mail: [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, the start-up supports from Soochow University, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

REFERENCES 1. Zou, X.; Zhang, Y. Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44, 5148-5180. 2. Dresselhaus, M. S.; Thomas, I. L. Alternative Energy Technologies. Nature 2001, 414, 332-337. 3. Crabtree, G. W.; Dresselhaus, M. S.; Buchanan, M. V. The Hydrogen Economy. Phys. Today 2004, 57, 39-44.


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Highly Efficient Acidic Oxygen Evolution Electrocatalysis Enabled by

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