Ultrathin Laminar Ir Superstructure as Highly Efficient Oxygen

Jun 1, 2016 - The SAED pattern (Figure 1e) of an individual 3D Ir superstructure shows a series of concentric circles, indicating their polycrystallin...
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Ultrathin Laminar Ir Superstructure as Highly Efficient Oxygen Evolution Electrocatalyst in Broad pH Range Yecan Pi,† Nan Zhang,† Shaojun Guo,*,‡ Jun Guo,§ and Xiaoqing Huang*,† †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu, 215123, China Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China § Testing and Analysis Center, Soochow University, Jiangsu, 215123, China ‡

S Supporting Information *

ABSTRACT: Shape-controlled noble metal nanocrystals (NCs), such as Au, Ag, Pt, Pd, Ru, and Rh are of great success due to their new and enhanced properties and applications in chemical conversion, fuel cells, and sensors, but the realization of shape control of Ir NCs for achieving enhanced electrocatalysis remains a significant challenge. Herein, we report an efficient solution method for a new class of three-dimensional (3D) Ir superstructure that consists of ultrathin Ir nanosheets as subunits. Electrochemical studies show that it delivers the excellent electrocatalytic activity toward oxygen evolution reaction (OER) in alkaline condition with an onset potential at 1.43 V versus reversible hydrogen electrode (RHE) and a very low Tafel slope of 32.7 mV decade−1. In particular, it even shows superior performance for OER in acidic solutions with the low onset overpotential of 1.45 V versus RHE and small Tafel slope of 40.8 mV decade−1, which are much better than those of small Ir nanoparticles (NPs). The 3D Ir superstructures also exhibit good stability under acidic condition with the potential shift of less than 20 mV after 8 h i-t test. The present work highlights the importance of tuning 3D structures of Ir NCs for enhancing OER performance. KEYWORDS: Iridium, nanosheets, three dimensional, oxygen evolution reaction, electrocatalysis

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study of the relationship between the structure of Ir nanomaterials and properties for achieving the OER catalysis optimization.23,24 Herein, we report the first example of using an efficient wetchemical route to prepare 3D Ir superstructure composed of ultrathin Ir nanosheets as subunits (simplified as 3D Ir superstructure). Such interesting superstructure with 3D accessible sites, maximized surface area and proper layer distance is highly beneficial for enhancing the electrochemical energy conversion.25,26 As a result, the obtained 3D Ir superstructures deliver very high OER performance in alkaline condition with a sharp onset potential at 1.43 V vs RHE and very low Tafel slope of 32.7 mV dec−1. Particularly, it also shows superior performance for OER in acidic solution with low onset overpotential and small Tafel slopes of 40.8 mV dec−1. The 3D Ir superstructures also exhibit outstanding stability with limited overpotential change, as revealed by longterm chronopotentiometry measurement. To the best of our knowledge, our 3D Ir superstructure is one of the best OER electrocatalysts in both alkaline and acidic conditions reported to date.

arth-abundant materials, such as nickel-based and cobaltbased compounds, are widely explored as enhanced catalysts for oxygen evolution reaction (OER).1−7 Despite significant progress having been achieved, the biggest challenge issue of non-noble metal-based nanostructured materials developed so far is that they still underperform the Ir and Ru benchmarks for OER.8−10 Another issue related to non-noble metal catalysts for OER that should be specifically mentioned is that they are generally not stable in strongly acidic condition, which largely hinders their application in proton exchange membrane water electrolyzer (PEMWEs) in which the corrosive acidic environment has to be used.11−13 Only a few noble metals with good OER activity, such as Ir and Ru, can endure such a harsh condition.8,14 Therefore, Ir and Ru are still considered as the state-of-the-art OER electrocatalysts with relatively low overpotential and Tafel slope and particularly superior stability.15 Compared to Ru, Ir is supposed to be a more ideal catalyst candidate for OER due to its slightly lower activity but higher stability.16,17 Nanocatalysts have obvious advantage in boosting electrochemical catalysis, because of the reduced usage of noble metals but increased catalytically active sites achieved by their high surface area to volume ratio.18,19 However, to our best of knowledge there are few reports on the control of Ir-based nanostructures.20−22 The majority of Irbased nanostructures obtained to date are limited to NPs with very limited OER activity, which severely hinder the in-depth © XXXX American Chemical Society

Received: April 14, 2016 Revised: May 18, 2016

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DOI: 10.1021/acs.nanolett.6b01554 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters

Figure 1. Morphological and structural characterizations for 3D Ir superstructure. (a) HAADF-STEM image, (b) TEM image, (c) enlarged TEM image, and (d) TEM-EDX spectrum of 3D Ir superstructure. (e) SAED pattern of an individual 3D Ir superstructure. (f) HRTEM image of the edge part of an individual 3D Ir superstructure. (g) PXRD pattern of 3D Ir superstructure.

An efficient wet-chemical approach was developed to synthesize 3D Ir superstructure with high yield. In a typical preparation of 3D Ir superstructure, iridium(III) chloride (IrCl 3 ), citric acid (CA), glyoxal (GO), and poly(vinylpyrrolidone) (PVP) were dissolved in benzyl alcohol under magnetic stirring for 0.5 h. The resulting homogeneous mixture was transferred to a Teflon-lined stainless-steel autoclave and then heated at 200 °C for 5 h before it was cooled to room temperature. The resulting colloidal nanocrystals were collected by centrifugation and washed by an ethanol (1 mL) + acetone (8 mL) mixture three times (see Supporting Information for details). Detailed characterizations of as-prepared 3D Ir superstructure are shown in Figures 1 and S1. Typical transmission electron microscopy (TEM) (Figure 1b,c and S1a,b) and highangle annular dark-field scanning TEM (HAADF-STEM) images (Figure 1a) of the 3D Ir superstructure show that it has nearly spherical profile at the first glance. These nanospheres are monodisperse with an average diameter of 60 ± 20 nm. If we take a closer look at the detailed structure, each nanosphere is essentially a 3D structure comprised of many building blocks of ultrathin nanosheets (Figure 1b,c).

The ultrathin nanosheet is measured to be 430 358 34

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2 34 2

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Overpotential at10 mA cm−2. DOI: 10.1021/acs.nanolett.6b01554 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters the surface area of catalyst) were measured for more comprehensive comparison. From Figure S13, we can see that the specific activities of Ir catalysts herein follow the order of surface-clean 3D Ir > 3D Ir > Ir NPs. Apparently, the surfaceclean 3D Ir shows the highest specific activity, further confirming their extremely high electrocatlytic activity for OER in acidic condition. Furthermore, the primary OER study on 3D Ir superstructure intermediate (Figure 2a,b) shows that the activity of 3D Ir superstructures can be further enhanced if the active sites can be exposed more (Figure S14). The catalytic stability of 3D Ir superstructures was evaluated by using chronopotentiometry technique (Figure 5c). During the measurements, the working electrode was continuously rotating at 1600 rpm to remove the constantly generated oxygen bubbles. There is a slight potential shift of less than 0.02 V after an 8 h chronopotentiometry test at a constant current density of 2.5 mA cm−2 in both 0.1 M HClO4 and 0.5 M HClO4, while the Ir NPs have an obvious overpotential increase during the measurement. This indicates that 3D Ir superstructures exhibit the enhanced durability in acidic condition. To further evaluate the stability of the 3D Ir superstructure, chronopotentiometry tests were carried out in 0.1 M HClO4 solutions at constant current density of 5 and 10 mA/cm2. It was revealed that in all cases the 3D Ir exhibits smaller overpotential change compared with that of Ir NPs, although deactivation became more obvious with the increase of current density (Figure S15). Considering the harsh corrosive conditions as well as the oxidation environment of OER, the deactivation is probably due to the inevitable etching of Ir under the OER condition. The 3D Ir after the OER test were carefully characterized by means of TEM, SAED, and XPS. As revealed by TEM (Figure S16a,b), we can see that the morphology of the 3D structure can be mainly maintained after a long time of electrolysis process in the harsh corrosive conditions. As shown by the SAED and XPS (Figure S16c,d), the partial oxidation of Ir after the OER test had taken place, which is consistent with previously the reported result.35 To conclude, we have demonstrated an efficient wetchemical approach for the creation of a new class of 3D Ir superstructure composed of ultrathin Ir nanosheets as subunits. In the developed approach, the combined use of CA and GO is essential for the successful creation of the layered assembly structure. This approach leads to favorable 3D Ir superstructure with many active sites fully exposed. When applied as the electrocatalyst of OER in alkaline medium, 3D Ir superstructures exhibit very high catalytic activity with the current density of 10 mA cm−2 at overpotential of 0.24 V and Tafel slope of 32.7 mV dec−1. In acidic medium, it shows very high electrocatalytic activity and much better stability for OER with an impressively small overpotential of 0.24 V and very low Tafel slope of 40.8 mV dec−1, which are the best OER performances among all the developed OER electrocatalysts to date (Table 1, Tables S1 and S2). Given its excellent performance in both alkaline and acidic media, the 3D Ir superstructure may find broad applications in various water splitting, chemical conversions, and beyond.

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Experimental details and data. Figures S1−18 and Tables S1 and S2.(PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the start-up fundings from Soochow University and Peking University, Young Thousand Talented Program, the National Natural Science Foundation of China (21571135), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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DOI: 10.1021/acs.nanolett.6b01554 Nano Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.nanolett.6b01554 Nano Lett. XXXX, XXX, XXX−XXX