Synthesis of Ni–Ir Nanocages with Improved Electrocatalytic

Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9787-9792

pubs.acs.org/journal/ascecg

Synthesis of Ni−Ir Nanocages with Improved Electrocatalytic Performance for the Oxygen Evolution Reaction Chao Wang,† Yongming Sui,*,‡ Man Xu,‡ Chuang Liu,‡ Guanjun Xiao,‡ and Bo Zou*,‡ †

Shandong Provincial Key Laboratory of Laser Polarization and Information Technology, Department of Physics, Qufu Normal University, 57 Jingxuan West Road, Qufu 273165, China ‡ State Key Laboratory of Superhard Materials, Jilin University, 2699 Qianjin Street, Changchun 130012, China S Supporting Information *

ABSTRACT: Design and fabrication of inexpensive and efficient oxygen evolution reaction (OER) catalysts are of great importance for polymer electrolyte membrane water electrolyzers (PEMWEs). Although the best electrocatalyst is IrO2 for OER in the PEMWEs, the practical application of Ir has been impeded because of its high cost and limited activity. Herein, a galvanic replacement reaction mechanism was developed for the preparation of polycrystalline Ni−Ir nanocages (NCs) by using Ni nanoparticles as templates. The formation of Ni−Ir NCs achieves the enhancement of OER catalytic performance, accompanied by the reduction of Ir loading but significantly increasing the efficiency of Ir atoms. The as-prepared Ni2.53Ir NCs exhibit improved catalytic activity toward OER in acid solution, which requires only an overpotential of 302 mV to deliver a current density of 10 mA/cm2. At an overpotential of 300 mV, the Ir-based mass activity of Ni2.53Ir catalysts reaches 114.7 mA/mgIr, which is 2.1 times higher than that of commercial Ir black. The obtained Ni2.53Ir NCs could be potentially applied for industrial scale PEMWE systems. KEYWORDS: Ni−Ir nanocages, Hollow porous nanostructure, Electrocatalyst, Oxygen evolution reaction, Water splitting

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INTRODUCTION Increasing fossil fuel crisis and air pollution have stimulated intense research on renewable sources with high efficiency as well as environmental friendliness.1,2 Among many sustainable alternative energy carriers, H2, as a clean and efficient energy carrier, has received increasing attention due to its potential in solving these problems.3−5 Electrocatalytic water splitting with the help of renewable electricity is considered to be a promising way to produce H2. Owing to the intrinsically very slow reaction kinetics caused by the involved multielectron transfer steps, the efficiency of electrocatalytic water splitting absolutely relies on the efficient catalysts of the oxygen evolution reaction (OER) on the anode.6−8 It is well-known that Ir oxides are one type of the best OER electrocatalysts in acidic electrolytes with low overpotential and good stability.9,10 However, the widespread applications of Ir-based OER electrocatalysts suffer from high cost, low reserves, and limited catalytic activity. Therefore, it is highly urgent to develop Ir-based OER electrocatalysts with enhanced catalytic performance and high usage efficiency. Density functional theory studies indicate that the OER activity of catalyst is mainly determined by the bond strength of the OER intermediates (*OH, *O, and *OOH) to the catalyst surface.11−13 A careful tuning of the bond strength was predicted to maximize the OER rate. It is well-established that element doping is one of the most effective methods to modulate the bond strength by tuning the electronic structure and thus improving the electrocatalytic activity.14−17 For © 2017 American Chemical Society

example, the activity of the Ru oxide could be improved by Ni doping.18 Strasser et al. reported that adding Ni to Ir in the form of electrochemically oxidized IrNiOx core−shell nanoparticles (NPs) could improve the intrinsic OER activity and utilization efficiency of Ir.19,20 However, these IrNiOx core− shell NPs are still confronted with challenges in taking full advantage of the inner Ir atoms in the solid NPs, which is why hollow porous Ir-based nanostructures are critically needed to further improve the utilization efficiency and catalytic performance of Ir. Although some noble metal-based hollow porous nanostructures have been prepared,21,22 preparation of Ir-based counterparts remains a bigger challenge. Therefore, synthesis of Ir-based alloy hollow porous nanostructures is highly desirable. Herein, we report a facile solvothermal approach to synthesize Ni−Ir hollow porous nanostructures which concentrate Ir in a thin shell. Due to the advantageous hollow porous characteristic as well as synergetic effect between Ni and Ir, the as-synthesized Ni2.53Ir nanocages (NCs) exhibit improved OER activity and stability in acid solution. Our work paves a new way for the preparation of highly efficient OER electrocatalysts. Received: May 23, 2017 Revised: September 11, 2017 Published: September 24, 2017 9787

DOI: 10.1021/acssuschemeng.7b01628 ACS Sustainable Chem. Eng. 2017, 5, 9787−9792

ACS Sustainable Chemistry & Engineering

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EXPERIMENTAL SECTION

Research Article

RESULTS AND DISCUSSION Morphological and Structural Characterization. On the basis of a developed galvanic replacement mechanism, Ni2.53Ir NCs were prepared by Ni NPs as templates. The morphology and structure of the as-synthesized samples were first characterized by TEM. As shown in Figure 1a and S1, the

Reagents. Iridium(III) chloride hydrate (IrCl3 99.9%) and oleylamine (OLA ≥ 70%) were acquired from Alfa Aesar. Nickel(II) acetylacetonate (Ni(acac)2 ≥ 99.99%) were obtained from Aldrich. Analytical grade toluene and methanol were commercially available. All available chemicals were used as received. Synthesis of Ni−Ir Nanocages. The IrCl3−OLA solution was prepared in a Schlenk line by dissolving 0.2 mmol of IrCl3 in 1 mL of OLA at 120 °C for 30 min. In a typical synthesis of Ni2.53Ir NCs, 0.2 mmol of Ni(acac)2 was added to 5 mL of OLA in a 50 mL three-neck flask in a glovebox filled with nitrogen gas. Next, the flask was sealed and connected to a Schlenk line which was equipped with a condenser. The mixture was heated to 270 °C gradually under nitrogen protection and energetic magnetic stirring and maintained at this temperature for 15 min to prepare Ni templates. After that, the temperature was immediately decreased to 250 °C, and then the premade IrCl3−OLA solution was added rapidly to the reaction mixture. The reaction mixture was kept at 250 °C for 2 h and then cooled to room temperature. Finally, after 10 mL of toluene was added, the black precipitate from the reaction mixture was centrifuged at 13 000 rpm for 5 min. The resulting precipitates were washed with toluene and methanol three times and then were further purified twice more by washing with chloroform and an excess amount of acetone. Finally, stable colloidal solutions were formed by dissolving the products in toluene. Characterization. Powder X-ray diffraction (XRD) patterns were recorded using a Shimadzu XRD-6000 X-ray diffractometer operating at 40 kV and 40 mA with Cu Kα radiation. For transmission electron microscopy (TEM) characterization, samples were first suspended in toluene, and then several drops of the suspension were added onto a carbon coated 300-mesh copper grid. The morphology and microstructure of the samples were further examined by TEM (JEM-2200FS working at 200 kV). Composition of the samples was investigated by energy-dispersive X-ray (EDX) analysis and scanning transmission electron microscopy energy dispersive X-ray spectroscopy elemental mapping (STEM-EDX mapping) attached to the JEM-2200FS. Compositional analysis was performed by inductively coupled plasma atomic emission spectrometry (ICP-AES; PerkinElmer OPTIMA 3300 DV). Electrocatalytic Performance Measurement of Oxidation Evolution Reaction. The electrocatalytic performance investigations were performed using a CHI 660E electrochemical analyzer (Shanghai Chenhua Apparatus, China). Catalyst-covered glassy carbon electrode 3 mm in diameter was used as working electrode; Pt wire 1 mm in diameter was used as counter electrode, and saturated Hg/HgSO4 was used as reference electrode. Prior to use, the glassy carbon electrode was polished using alumina slurries 0.05 μm in size, ultrasonically cleaned with deionized water for 3 min to remove the alumina residue from the electrode, and then allowed to dry. For Ni2.76Ir, Ni2.53Ir, Ni1.43Ir, Ni0.93Ir, and homemade Ir NPs, 10 mg of catalysts was dispersed in 2 mL of toluene and sonicated for 3 min. For Ir black, 10 mg of catalysts was dispersed in 2 mL of aqueous solution. With a microliter syringe, 2 μL of the above dispersion was deposited on a prepolished glassy carbon electrode surface, leading to a catalyst loading of 10 μg. The working electrodes were obtained by drying in air at room temperature. H2SO4 aqueous solution (0.05 M) was used as the supporting electrolyte. All potentials reported in this paper were normalized with respect to the reversible hydrogen electrode (RHE). To prepare catalytic active water splitting catalyst, the electrochemical dealloying and oxidation process was performed for these Ni−Ir nanoparticles, commercial Ir black, and Ir NPs, respectively, within the potential range from 0 to 1.46 V at a scan rate of 100 mV/s for 20 cycles. The electrocatalytic activity of these catalysts was studied by using linear sweep voltammetry (LSV) between 1.1 to 1.56 V with a scan rate of 5 mV/s. Compensation for iR drop was used for catalytic activity measurements. Chronopotentiometry curves were recorded at 0.07 mA, resulting in a current density of 1 mA/cm2 in 0.05 M H2SO4 solution under magnetic stirring at 2000 rpm for 10 h to investigate the stability of these electrocatalysts.

Figure 1. (a) TEM image and (b) XRD pattern of Ni2.53Ir NCs, (c) HRTEM image and (d) its FFT pattern of a representative Ni2.53Ir NC displayed in panel c. (e) STEM image and elemental mapping of Ni2.53Ir NCs.

TEM and high-magnification TEM images demonstrate clearly that the as-synthesized samples consist of hollow porous NPs with size range from 40 to 90 nm. In addition, the STEM image also indicates their hollow porous feature, as displayed in Figure S2. Powder XRD was used to examine the crystallographic structure and alloy nature of the as-synthesized Ni2.53Ir NCs. As exhibited in Figure 1b, all the diffraction peaks could be clearly assigned to face-centered cubic (fcc) structured Ni−Ir alloy with diffraction peaks between the standard peaks of Ni (JCPDS no. 87-0712) and Ir (JCPDS no. 87-0715), confirming the alloy feature of Ni2.53Ir NCs. The weak peak at 44.5° could be assigned to (111) reflection of elemental Ni, indicating that there is a little pure Ni in the Ni2.53Ir NCs. High-resolution TEM (HRTEM) image of a single Ni2.53Ir NC further confirmed the hollow porous feature, as shown in Figure 1c. The interplanar spacings of 0.215 and 0.212 nm are in the range of 0.203 nm (Ni (111) planes) to 0.222 nm (Ir (111) planes), which also reveals the alloy characteristic of Ni2.53Ir and is in agreement with the XRD results. Meanwhile, the polycrystalline feature of the Ni2.53Ir NC is demonstrated by the HRTEM image (Figure 1c) and corresponding fast Fourier transform (FFT) pattern (Figure 1d). EDX was used to investigate the elemental compositions of the samples, 9788

DOI: 10.1021/acssuschemeng.7b01628 ACS Sustainable Chem. Eng. 2017, 5, 9787−9792

Research Article

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also take place at higher temperature, resulting in a more homogeneous chemical composition of Ni2.53Ir NCs. Therefore, the formation of Ni2.53Ir NCs involves a number of processes, including the preparation of Ni templates via decomposing Ni(acac)2, oxidation and etching of Ni with reduction and deposition of Ir, and coreduction of Ni and Ir by OLA followed by backfilling of the NCs. To further understand the formation mechanism and tailor the composition and crystal structure, we investigated the products obtained with different Ir/Ni precursor molar ratios. As shown in Figure 3a, with the precursor molar ratio of Ir to

indicating the fact that Ni and Ir are the only elements composing the Ni2.53Ir NCs (Cu and C signals come from Cu grid), as displayed in Figure S3. The atomic ratio of Ni to Ir was measured to be 2.53:1 by ICP-AES, in good agreement with the EDX results. STEM-EDX elemental mapping images displayed in Figure 1e demonstrate that Ni and Ir are in homogeneous distribution within the shell, further confirming the hollow characteristic. All results validate that hollow porous Ni2.53Ir NCs were well-synthesized. Investigation of the Formation Mechanism. Timedependent experiments were performed to understand the structural evolution from solid NPs to NCs. Figure 2 shows the

Figure 3. (a) TEM image, (b) HRTEM image, and (c) STEM image and corresponding elemental mapping of Ni@Ni−Ir nanoparticles obtained by adjusting the precursor molar ratio of Ir:Ni to 2:3 under otherwise the same conditions.

Ni of 2:3, Ni−Ir NPs with a few small pinholes on some NPs were obtained under otherwise the same conditions. The formation of pinholes on these templates also reveals the preferential etch of the high-energy facets of Ni templates. The lattice spacing of 0.212 nm and XRD pattern indicate the formation of Ni−Ir alloy (Figure 3b and Figure S5). Furthermore, elemental mapping reveals that Ir and Ni are enriched in the surface and the core, respectively (Figure 3c). On the basis of the above analysis, we could conclude that a core−shell architecture is obtained with Ni and Ni−Ir alloy as the core and the shell, respectively. Obviously, sufficient Ir ions are necessary for hollowing out a cavity. However, with surplus Ir species, Ir branches begin to grow on the surfaces of NCs. Figure S6 shows the TEM and HRTEM images of Ni1.43Ir and Ni0.93Ir samples synthesized by adjusting the precursor molar ratios of Ir:Ni to 3:2 and 2:1, respectively. It is obvious that raising the content of Ir precursor would result in the excessive growth of Ir branches on the NC surfaces. In addition, temperature, as an important reaction parameter, plays a key role in the formation of Ni−Ir NCs. At a lower reaction temperature of 230 °C, Ni and Ir ions cannot be coreduced by OLA, and thus, Ni2.53Ir NCs cannot be formed as expected. However, at a higher reaction temperature of 270 °C, Ir ions were reduced quickly by OLA, resulting in the rapid growth of Ir branches on the Ni templates, as displayed in Figure S7. Reaction temperature has an important effect on the reduction rate of Ir ions, whereas the concentration of Ir ions and reaction temperature play an important role together in the rate of replacement reaction. In comparison with lower concentration,

Figure 2. (a−d) Temporal TEM images of the reaction intermediates obtained at (a) 5, (b) 15, (c) 30, and (d) 120 min after injecting the IrCl3−OLA solution into the reaction mixture at 250 °C.

TEM images of the intermediates taken at different reaction stages after injecting the IrCl3−OLA solution into the reaction mixture. Due to the standard redox potential of Ir3+/Ir (Eo = 1.156 V vs SHE) higher than that of Ni2+/Ni (Eo = −0.257 V vs SHE), a galvanic replacement reaction takes place between Ir and Ni species. As displayed in Figures 2a and b, pinholes on some NP surfaces and a few hollow NPs can be found after the injection for 5 and 15 min. Meanwhile, EDX investigation reveals the existence of Ir in these samples, indicating the proceeding of galvanic replacement reaction (Figure S4). With the reaction going on, the Ni templates were dissolved in part, thus leading to generation of voids inside some NPs and then the formation of hollow structure (Figure 2c). When the reaction proceeded for 120 min, almost all the solid NPs vanished, developing into hollow porous NPs which are uniform in wall thickness (Figure 2d). In galvanic replacement reaction, the deposition of Ir that preferentially occurs on the high-energy facets of Ni templates is the main reason for the structure evolution from solid Ni nanoparticle to Ni2.53Ir NCs, which was confirmed by our previous work and Xia et al.16,23 It was demonstrated that by a noble-metal-induced reduction method, noble and non-noble metal ions could be coreduced by octadecylamine solvent at higher temperature.24 Thus, along with the dissolution of Ni templates, alloying occurs between Ni and Ir. Moreover, interdiffusion between Ni and Ir could 9789

DOI: 10.1021/acssuschemeng.7b01628 ACS Sustainable Chem. Eng. 2017, 5, 9787−9792

Research Article

ACS Sustainable Chemistry & Engineering higher concentration of Ir ions facilitates the fast dissolution of Ni templates and thus the formation of hollow structures. Therefore, Ni−Ir NCs are favored with proper reaction temperature and feeding molar ratio of Ir to Ni. Electrocatalytic Oxygen Evolution Reaction. Alloy NCs are anticipated to enhance the utilization efficiency of noble metal atoms and improve catalytic performance, in the field of catalysis.22,25,26 Herein, we convert the as-synthesized Ni−Ir NPs to Ni doped IrOx NCs by simple electrochemical oxidation. Strasser et al. reported that Ir could be oxidized irreversibly by voltammetric cycling in H2SO4 solution.27,28 Hence, the Ni0.93Ir, Ni1.43Ir, Ni2.53Ir, and Ni2.76Ir NPs are electrochemically oxidized by potential cycling up to 1.46 V where surface Ir atoms form irreversible oxide species. As shown in Figure S8, the formation of irreversible Ir oxide is clearly confirmed by the gradually disappearing peaks of hydrogen adsorption/desorption (0−0.4 V) as well as the gradually appearing peaks of Ir(III)/Ir(IV) redox (0.7−1.1 V).29,30 Upon potential cycling, unstable Ni in the Ni−Ir NCs is leached. However, due to the lower standard redox potential of Ni2+/Ni (Eo = −0.257 V vs SHE), oxidation peaks of Ni cannot be detected during potential cycling in all the investigated samples. After 10 cycles, the peaks associated with the Ir(III)/Ir(IV) tend to be unchanged, and subsequent cycles do not lead to any variation in the CV profile, which indicates that a stable OER electrocatalyst is formed. As benchmark catalyst, commercial Ir black and homemade Ir NPs (Figure S9) were also processed in the same way. The amount of active Ir sites for each catalyst was evaluated by the characteristic Ir(III)/Ir(IV) redox couple (Figure 4), as given in Table 1.19

Figure 5. OER polarization curves of Ni2.76Ir, Ni2.53Ir, Ni1.43Ir, Ni0.93Ir, Ir black, and Ir NPs normalized against geometric area of the glass carbon electrode (a) and against the mass of Ir (b). All measurements were carried out in 0.05 M H2SO4 at a scan rate of 5 mV/s with iR compensation.

overpotential of 302 mV, whereas overpotentials as high as 314, 309, 312, 310, and 309 mV are required for Ni2.76Ir, Ni1.43Ir, Ni0.93Ir, Ir black, and Ir NPs, respectively, also evidencing the enhanced electrocatalytic performance of Ni2.53Ir for OER. As shown in Figure S10, the error range of overpotentials to deliver a current density of 10 mA cm−2 are less than 2 mV. TEM observation of the Ni−Ir catalysts after the catalytic performance tests shows that all the Ni−Ir catalysts keep the anticipated hollow porous structure in the Ni−Ir catalysts, as shown in Figure S11. Widely available active sites of the inherent hollow porous structure can account for the enhanced OER activity. Note that, on the NCs surfaces, the growth of Ir branches can give rise to the reduction of porosity as well as active sites, leading to a loss of catalytic performance. In addition, the Ni2.53Ir catalysts outperform all Ni−Ir catalysts, commercial Ir black, and Ir NPs on Ir mass basis, as depicted in Figure 5b. At an overpotential of 300 mV, the Ir-based mass activity of Ni2.53Ir is 114.7 mA/mgIr, which is 2.1 times higher than that of Ir black (54.3 mA/mgIr). After catalytic performance tests, elemental compositions and elemental mapping analysis of these Ni−Ir NCs indicate that a trace amount of Ni element does exist and distributes homogeneously in the electrochemically treated Ni−Ir NCs (Figures S12 and S13). The improved activity is usually caused by some synergetic effects, for example, geometric and electronic effects which originate from the lattice contraction and shift of the dband center of Ir in the bimetallic structures. It was reported that the d-band structure of IrOx surfaces could be tuned by doping non-noble transition-metal atoms, which is also confirmed by Yang et al. and our previous work in the case of Cu doped IrOx.14,16,33 Therefore, we conclude that the enhanced Ir-based mass activity of Ni2.53Ir comes from the modification of the electronic structure of IrOx by Ni atoms, which is regarded as an practical method to enhance the catalytic activity of noble metal atoms. Moreover, the Tafel plots of the Ni−Ir NCs, Ir black, and Ir NPs are compared to achieve additional insight into the OER catalytic activity (Figure 6a). The Tafel slope of Ni2.53Ir is 46.6 mV/decade, which is smaller than that of Ni2.76Ir (51.8 mV/decade), Ni1.43Ir (48.9 mV/decade), Ni0.93Ir (51.7 mV/decade), Ir black (55.1 mV/decade), and Ir NPs(46.9 mV/decade), indicating the favorable OER kinetics over Ni2.53Ir NCs. Obviously, the data suggest that the Ni2.53Ir NC is the most active OER catalyst investigated in this study. As far as we know, such catalytic performance is among more efficient OER electrocatalysts (Table S1).9,28,34−38

Figure 4. Ir (III−IV) redox peaks of Ni2.76Ir, Ni2.53Ir, Ni1.43Ir, Ni0.93Ir, Ir black, and Ir NPs.

Table 1. Evaluated Amount of Active Ir Sites of the Investigated Catalysts in This Work (nmol)

Ni2.76Ir

Ni2.53Ir

Ni1.43Ir

Ni0.93Ir

Ir black

Ir NPs

0.49

0.78

0.61

0.55

0.39

0.47

We then evaluated the electrocatalytic performance of all the electrochemically treated catalysts toward OER by LSV in 0.05 M H2SO4 in a standard three-electrode system. Figure 5a shows the polarization curves of all the investigated catalysts. It can be seen that the Ni2.53Ir catalysts are more active for OER in comparison with other Ni−Ir catalysts, commercial Ir black, and homemade Ir NPs catalysts at higher potential. To obtain a current density of 10 mA cm−2, which is a metric correlated with solar fuel synthesis,31,32 the Ni2.53Ir catalysts need an 9790

DOI: 10.1021/acssuschemeng.7b01628 ACS Sustainable Chem. Eng. 2017, 5, 9787−9792

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]; Tel: 86-431-85168882; Fax: 86431-85168883. ORCID

Chao Wang: 0000-0002-0344-4964 Guanjun Xiao: 0000-0002-7013-1378 Bo Zou: 0000-0002-3215-1255 Notes

Figure 6. (a) Tafel plots of Ni2.76Ir, Ni2.53Ir, Ni1.43Ir, Ni0.93Ir, Ir black, and Ir NPs from the iR-compensated polarization curves. (b) Chronopotentiometry curves of Ni2.76Ir, Ni2.53Ir, Ni1.43Ir, Ni0.93Ir, Ir black, and Ir NPs in 0.05 M H2SO4 with a steady current density of 1 mA/cm2.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NFSC (Grants 21673100, 11404135, and 11504126), Changbai Mountain Scholars Program (Grant 2013007), and the China Postdoctoral Science Foundation (Grants 2015T80295 and 2014M550171).

Finally, the stability of the catalysts, which is also critical for practical water splitting systems, was investigated by using a chronopotentiometric strategy under the current load conditions of 1 mA/cm2 for 10 h. As displayed in Figure 6b, no obvious loss of catalytic activity was observed during the 10 h stability test for all Ni−Ir NCs and Ir NPs. Furthermore, Ni2.53Ir NCs could deliver a current density of 1 mA/cm2 at a lower overpotential during the stability test period. In contrast, the catalytic activity of Ir black gradually decays because of the aggregation of Ir black NPs and thus the loss of the active sites. The outstanding physical stability of Ni−Ir catalysts is also evidenced by TEM. As shown in Figure S11, the hollow porous structure of the Ni−Ir NCs is well-retained, confirming the excellent structural stability. On the basis of the above analysis, Ni2.53Ir outperforms all of the other catalysts investigated here, suggesting the advantages of alloy NCs.

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CONCLUSION In conclusion, polycrystalline Ni−Ir NCs were prepared using Ni NPs as templates based on a developed galvanic replacement reaction mechanism. The Ni2.53Ir NCs, which maintain the primary hollow porous alloy characteristic after electrochemical treatment, benefit the enlargement of active surface area, utilization efficiency of Ir, adjustment of the electronic structure of IrOx, and thus the improvement of the catalytic performance. In acid electrolytes, electrochemically treated Ni2.53Ir NCs exhibit improved catalytic performance toward OER with a small overpotential of 302 mV to deliver a current density of 10 mA/cm2, a small Tafel slope of 46.6 mV/ decade, and prominent durability. It is anticipated that Ni2.53Ir NCs could promote the practical application of Ir in acid PEMWEs.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01628. TEM, STEM images, and EDX results of Ni2.53Ir NCs; XRD pattern of Ni@Ni−Ir nanoparticles; TEM and HRTEM images of Ni1.43Ir and Ni0.93Ir NCs; cyclic voltammetry surface oxidation curves of the Ni−Ir nanoparticles and Ir black; TEM images, EDX spectra, and elemental mapping of the Ni−Ir NCs after catalytic performance tests (PDF) 9791

DOI: 10.1021/acssuschemeng.7b01628 ACS Sustainable Chem. Eng. 2017, 5, 9787−9792

Research Article

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DOI: 10.1021/acssuschemeng.7b01628 ACS Sustainable Chem. Eng. 2017, 5, 9787−9792