Etching approach to hybrid structures between PtPd nanocages and graphene towards efficient oxygen reduction reaction

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1 Nano Research DOI /s Nano Res 1 Etching approach to hybrid structures between PtPd nanocages and graphene towards efficient oxygen reduction reaction Song Bai 1, Chengming Wang 1 ( ), Wenya Jiang 1, Nana Du 1, Jing Li 1, Junteng Du 1, Ran Long 1, Zhengquan Li 2, and Yujie Xiong 1 ( ) Nano Res., Just Accepted Manuscript DOI /s on March 26, 2015 Tsinghua University Press 2015 Just Accepted This is a Just Accepted manuscript, which has been examined by the peer-review process and has been accepted for publication. A Just Accepted manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides Just Accepted as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these Just Accepted manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI ), which is identical for all formats of publication.

2 TABLE OF CONTENTS (TOC) Etching approach to hybrid structures between PtPd nanocages and graphene towards efficient oxygen reduction reaction Song Bai 1, Chengming Wang 1, *, Wenya Jiang 1, Nana Du 1, Jing Li 1, Junteng Du 1, Ran Long 1, Zhengquan Li 2, Yujie Xiong 1, * 1 Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Chemistry for Energy Materials, School of Chemistry and Materials Science, Laboratory of Engineering and Material Science, and National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui , P. R. China 2 Department of Materials Physics, Zhejiang Normal University, Jinhua, Zhejiang , P. R. China A method has been developed to synthesize hybrid structures between PtPd bimetallic nanocages and graphene by employing selective epitaxial growth of single-crystal Pt shells on the Pd nanocubes that are supported on reduced graphene oxide (rgo), followed by Pd etching. The hollow nature, {100} surface facets and bimetallic compositions of PtPd nanocages, together with the good conductivity and stability of graphene, enable high electrocatalytic performance in oxygen reduction reaction (ORR). Provide the authors website if possible. Yujie Xiong,

3 Nano Research DOI (automatically inserted by the publisher) Research Article Etching approach to hybrid structures between PtPd nanocages and graphene towards efficient oxygen reduction reaction Song Bai 1, Chengming Wang 1 ( ), Wenya Jiang 1, Nana Du 1, Jing Li 1, Junteng Du 1, Ran Long 1, Zhengquan Li 2, and Yujie Xiong 1 ( ) Received: day month year Revised: day month year Accepted: day month year (automatically inserted by the publisher) Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2014 KEYWORDS Platinum, nanocage, oxygen reduction reaction, electrocatalysis, graphene ABSTRACT Cathodic oxygen reduction reaction (ORR) is a highly important electrochemical reaction in renewable-energy technologies. In general, the surface area, exposed facets and electrical conductivity of catalysts all play important roles in determining their electrocatalytic activities, while their performance durability can be improved by integration with supporting materials. In this research article, we have developed a method to synthesize hybrid structures between PtPd bimetallic nanocages and graphene by employing selective epitaxial growth of single-crystal Pt shells on the Pd nanocubes that are supported on reduced graphene oxide (rgo), followed by Pd etching. The hollow nature, {100} surface facets and bimetallic composition of PtPd nanocages, together with the good conductivity and stability of graphene, enable high electrocatalytic performance in ORR. The obtained PtPd nanocages-rgo structures exhibit mass activity (0.534 A mgpt -1 ) and specific activity (0.482 ma cm -2 ) with value 4.4 times and 3.9 times greater than those of Pt/C, respectively. 1. Introduction Cathodic oxygen reduction reaction (ORR), O2 + 4H + + 4e - 2H2O, is one of the most important electrochemical reactions due to its prominent role in renewable-energy technologies, such as fuel cells and metal-air batteries [1-5]. The electrocatalyst involved in the ORR plays a vital role in determining the performance of the energy devices, including power output, charge-discharge rate, energy efficiency, cycling life, etc [6, 7]. Pt has thus far shown the greatest potential as an electrocatalyst material for the ORR, mainly because the complex four-electron reaction pathway and the acidic environment surrounding the electrode pose barriers to the substitution of other materials for Pt [3, 6]. However, the high material cost and limited source of Pt greatly impede its Address correspondence to Yujie Xiong, yjxiong@ustc.edu.cn; Chengming Wang, chmwang@ustc.edu.cn.

4 2 Nano Res. large-scale commercial applications. To date, various approaches have been developed to minimize the usage of Pt material in electrodes while maintaining high electrocatalytic activities. The first typical approach is to enlarge the exposed surface area of Pt nanostructures or maximize the number of active Pt reaction sites on surface, as only active surface atoms can participate in the catalytic reactions. To this end, M-Pt (M: other metals) core-shell structures have been widely used to reduce the usage of Pt [8, 9]. The second route is to combine Pt with less expensive metals such as Pd, Fe, Ni and Co to form bimetallic or alloy PtM catalysts [10, 11]. The exposure of other metal atoms can prevent the Pt oxidation erosion and increase the number of active sites accessible to oxygen. Thirdly, the ORR reaction is surface-structure sensitive, and as a result, reaction kinetics and mechanisms strongly depend on the crystallographic orientation of catalyst surfaces [12, 13]. Different exposed crystallographic facets with various step or terrance sites show differentiated abilities in molecular or species adsorption/desorption [14, 15]. For this reason, shape-controlled synthesis for forming suitable exposed facets on Pt nanostructures represents an alternative way to tune their ORR activities. For instance, high ORR catalytic performace has been observed on Pt nanocubes covered by {100} facets [3, 16]. The last method is to load Pt nanostructures on the supporting materials (e.g., graphene nanosheets) with low material cost, high surface area as well as good conductivity and stability [6, 17]. The addition of graphene cannot only maximize the availability of surface area for electron transfer and decrease the aggregation of Pt nanostructures, but also provide better mass transport of reactants to the Pt catalyst and avoid Pt loss into the electrolyte during the ORR reactions [17-19]. Despite the tremendous efforts above, it remains an open question whether Pt-based hybrid structures with well-controlled surface and interface can be designed by taking advantages of several different approaches, in efforts to enable high ORR activity and durability that can hardly be achieved by a single approach. Inspired by this idea, we demonstrate a class of novel hybrid structures between PtPd bimetallic nanocages and reduced graphene oxide (rgo) towards improved ORR catalytic performance in this article. In the hybrid structures, the single-crystal PtPd nanocages do not only maximize the exposed Pt surface through providing both the inner and exterior catalytic sites, but also offer the well-defined {100} surface facets for ORR studies. Meanwhile, the positive roles of the graphene substrates and the small amount of Pd as described above may also favor the enhanced catalytic activity and durability. Differently from the previously reported Pt nanocages [20, 21], the novel structures of PtPd nanocages presented here are enclosed by five metal faces and one graphene face on the bottom, further reducing the Pt usage. 2. Experimental 2.1 Synthesis of Pd nanocubes-rgo structures. In a typical synthesis [22], 12 mg of graphene oxide (GO) (see Electronic Supplementary Information (ESM) for the synthetic method of GO) was dispersed in 10 ml of DI water to form a 1.2-mg ml -1 GO aqueous suspension with probe sonication (Scientz-IID, China) for 1 h. PVP (55,000, 105 mg), ascorbic acid (AA, 60 mg), and KBr (300 mg) were dissolved in 8 ml of GO aqueous suspension. The reaction mixture was injected into a 50-mL three-neck flask, and pre-heated under magnetic stirring at 80 o C for 10 min. 3.0 ml of an aqueous solution containing K2PdCl4 (63 mg) was then added into the flask, and the reaction was allowed to proceed at 80 o C for 3 h. The product was collected by centrifugation, and washed with acetone once, water ten times, and ethanol three times to remove excessive PVP. The obtained Pd-rGO hybrid structures were redispersed in water for further use. 2.2 Synthesis of Pt-Pd-rGO structures. In a typical synthesis [22], 1.5 ml of Pd-rGO structures (suspension concentration: 1 mg ml -1 Pd in water), H2PtCl6 6H2O (20 mg ml -1 in N,

5 3 N-dimethylformamide (DMF), 0.5 ml), PVP (K30, mg), and 0.1-mL methylamine solution (30%) were mixed in 10 ml of DMF. The resulting homogeneous solution was transferred to a Teflon-lined stainless-steel autoclave with capacity of 15 ml and heated at 160 o C for 10.5 h. After the autoclave had cooled down to room temperature, the resultant product was separated by centrifugation, and washed with water and ethanol for several times. 2.3 Synthesis of PtPd nanocages-rgo. In a typical synthesis, the obtained Pt-Pd-rGO structures in Section 2.2 were mixed with 100-mL concentrated nitric acid, and then the mixture solution were stirred for 4 days at room temperature to selectively dissolve the Pd cores. The obtained product was separated by centrifugation, and washed with water and ethanol for several times. voltammetry curves (CVs) stay unchanged. Then the blank CVs were obtained between 0.05 and 1.05 V at a scan rate of 20 mv s -1. The electrochemically active area (ECA) was estimated by measuring the charge associated with the Hupd adsorption between 0.05 V and 0.40 V, and assumed 210 µc cm -2 for the adsorption of a monolayer of hydrogen on a polycrystalline platinum surface. For the ORR, a fresh electrolytic solution was deaerated by O2 bubbling for 30 min, and the ORR curves were then recorded in 0.1-M HClO4 with continuous O2 gas at a sweep rate of 20 mv s -1 and a rotation speed of 1,600 rpm. Electrochemical measurements were controlled by a CHI 760E electrochemical station (Shanghai Chenhua, China). 3. Results and discussion 2.4 Electrochemical measurements. The catalysts dispersed onto a glassy carbon (GC) rotating disk electrode (RDE, PINE, PA, USA, geometric surface area: cm 2 ) were used as working electrodes (WE). To prepare the WE, 20 μl of an aqueous suspension of the catalysts was transferred to the GC RDE. The loading amounts of the Pt-based catalysts were kept constant in terms of Pt (51 µg cm -2 ) to assess their electrochemical performance (see Table S1 for detailed information). For the loading of Pd-rGO catalyst, the loading weight of Pd was kept in 42 µg cm -2 consistent with the Pd loading in Pt-Pd-rGO catalyst. A reversible hydrogen electrode (RHE) and a platinum foil were used as the reference and counter electrodes, respectively. Prior to electrocatalytic measurements, the WE was cleaned with RF plasma (Plasma Cleaner pdc-002, Harrick, NY, USA) at a power level of 10.5 W for 1.5 min to remove residue organics, and then covered with 10 μl of Nafion dispersed in water (0.5%). Hundreds of potential cycles were conducted in 0.1-M HClO4 solution with saturated CO and then in another fresh 0.1-M HClO4 solution with continuous N2 gas in the potential region from 0.05 to 1.0 V at a sweep rate of 50 mv s -1 till the cyclic Figure 1 Schematic illustration for the synthesis of hybrid structures between PtPd bimetallic nanocages and rgo (namely, PtPd nanocages-rgo). The hybrid structures between PtPd nanocages and graphene have been synthesized through a three-step process with Pd nanocubes as templates. As illustrated in Fig. 1, in the first step, Pd nanocubes are in-situ grown on rgo by co-reducing K2PdCl4 and GO nanosheets (see Fig. S1 in the ESM) with ascorbic acid. As shown by transmission electron microscopy (TEM) (Fig. 2(a)), Pd nanocrystals with a well-defined cubic profile have been uniformly sustained on rgo nanosheets to form hybrid structures (namely, Pd nanocubes-rgo Nano Research

6 4 Nano Res. Figure 2 TEM and HRTEM images of (a-c) Pd nanocubes-rgo, (d-f) Pt-Pd-rGO, and (g-i) PtPd nanocages-rgo hybrid structures. ). The edge lengths of the Pd nanocubes on the rgo are in the range of nm (Fig. 2(b)). High-resolution TEM (HRTEM) image shows that the obtained Pd nanocubes are single crystals enclosed by Pd {100} facets (Fig. 2(c)). In the next step, Pt shells are selectively coated on the Pd surface through an epitaxial growth process (Fig. 1) [22]. As shown in Fig. 2(d), the Pd-Pt core-shell nanocrystals dispersed on rgo nanosheets (namely, Pt-Pd-rGO) inherit the cubic profile from their precursor Pd nanocubes-rgo. In particular, the image contrast between inner grey black Pd and exterior deep black Pt, resulting from the higher electron density of Pt, can be clearly observed in the Pd-Pt core-shell nanocubes (Fig. 2(e)). The HRTEM image (Fig. 2(f)) shows that the Pt shell is also a piece of single crystal enclosed by {100} facets with a thickness of ca. 1.5 nm. In this structure, there are five Pt{100}-Pd{100} interfaces and five exposed Pt{100} faces, as one face of Pd nanocubes is in intimate contact with rgo (Fig. S2(a), ESM). The last step is the removal of the Pd nanocubes from the Pt-Pd-rGO structures through chemical

7 5 etching by nitric acid. It has been reported that concentrated nitric acid can selectively remove Pd from PtPd bimetallic porous structures [23]. However, the seamless single-crystal Pt shells in our Pt-Pd-rGO structures make it impossible for nitric acid to initiate etching at the Pd cores by permeating through the shells. Alternatively, the rgo in contact with the Pd cores opens a new window to nitric acid, as the defects and grain boundaries in the rgo allow the penetration of ions and small molecules (Fig. S3, ESM) [24]. As shown in Figs. 2(g) and 2(h), the interior grey black Pd nanocubes disappear, transforming the solid structures into hollow nanocubes on rgo. From the image contrast, one can identify that some pinholes should exist on the walls, edges or corners of hollow nanocubes. In other words, the resulted hollow nanostructures are nanocages rather than nanoboxes. It has been reported that metal nanoboxes became unstable when exposed to corrosive environment [25, 26]. Particularly the surface charges accumulated at the corners may increase the instability of these sites during the corrosion process. Thus slight Pt dissolution may occur on surface (preferentially at the corners) when the Pd cores are etched by nitric acid. Nevertheless, the continuous fringes with the same orientation in HRTEM image (Fig. 2(i)) imply that each nanocage is a piece of single crystal with both outer and inner surfaces enclosed by {100} facets. The wall thickness of ca. 1.7 nm for the nanocages is approximately consistent with Pt shell thickness in the Pt-Pd-rGO structures. Inherited from the five Pd-Pt interfaces in Pt-Pd-rGO, only five interior faces are formed in the obtained cages after etching (Fig. S2(b) in ESM). It should be noted that a trace amount ( 5 wt%) of Pd content has been detected by inductively-coupled plasma mass spectrometry (ICP-MS) in the Pt-based nanocages (Table S1, ESM), as a small quantity of Pd cores is not completely etched most likely due to the strong Pd-Pt bonding as well as the protection of rgo. Thus we name the hybrid structure as PtPd nanocages-rgo here despite its very low Pd content. This is the first report to obtain single-crystal nanocages (shells) with nanocubes (cores) as templates as far as we know, and represents a different approach from the reported galvanic replacement and corrosion systems for Au and Pd nanocages [13, 25, 26]. The evolution from Pd nanocubes-rgo to PtPd nanocages-rgo structures has been further verified by X-ray diffraction (XRD, Fig. 3(a)). In the pattern of Pd nanocubes-rgo hybrid structures, all the diffraction peaks can be indexed to face-centered cubic (fcc) Pd (JCPDS No ). For the Pt-Pd-rGO and PtPd nanocages-rgo structures, the diffraction peaks slightly shift towards those for standard Pt (JCPDS No ) due to the Pt coating on Pd surface and then the removal of the Pd. The composition and valence states of the obtained PtPd nanocages-rgo are determined by X-ray photoelectron spectroscopy (XPS, Fig. 3(b-d)). The survey XPS spectrum indicates the presence of C, O, N, Pt and Pd elements in PtPd nanocages-rgo structures (Fig. S4, ESM). In the C1s deconvolution spectrum (Fig. 3(b)), four peaks centered at 284.5, 285.5, 287.1, and ev are observed, which can be assigned to C C, C OH, C=O, and O=C O groups, respectively [27]. The intensities of all C1s peaks that correspond to the C binding to O are very low, suggesting that most of the O-containing functional groups have been removed from graphene in the PtPd nanocages-rgo sample. The weak N peak may be ascribed to the slight N-doping of rgo induced by the DMF involved in the synthesis process [28]. In the high-resolution spectrum of Pt4f (Fig. 3(c)), the peaks at 74.4 ev (Pt4f5/2) and 71.0 ev (Pt4f7/2) are in good agreement with the zero valence of Pt. On the other hand, the peaks at 75.3 ev (Pt4f5/2) and 72.0 ev (Pt4f7/2), corresponding to a trace amount of Pt(II) species, have been observed, which are typical features for solution-phase synthesized Pt nanocrystals [29]. In comparison with Pt peaks, the peaks responsible for Pd are significantly weaker, indicating that most of the Pd has been removed in the etching process (Fig. 3(d)). The elemental distributions in the PtPd nanocages-rgo structures are further revealed by scanning transmission electron microscopy (STEM) and energy-dispersive spectroscopy (EDS) mapping (Fig. 3(e-h)), indicating Nano Research

8 6 Nano Res. the existence of trace Pd in the sample. Figure 3 (a) XRD patterns of the as-obtained Pd nanocubes-rgo, Pt-Pd-rGO, and PtPd nanocages-rgo hybrid structures; (b-d) XPS high-resolution spectra of the obtained PtPd nanocages-rgo sample: (b) C1s, (c) Pt4f, and (d) Pd3d; (e) STEM image and (f-h) EDS mapping profiles of PtPd nanocages-rgo in elements of (f) Pt, (g) Pd, and (h) C, respectively. From the viewpoint of structure characteristics, each synthetic step in the sample evolution may lead to promising enhanced catalytic performance. In the second step, the transformation from Pd nanocubes-rgo to Pt-Pd-rGO changes the surface structure from Pd{100} to Pt{100}. It is well known that Pd surface shows dramatically lower activity towards ORR in comparison with Pt despite the lower price of Pd [30], and as such, this transformation creates high-activity surface for ORR. As compared with the solid Pt nanocubes supported on rgo (namely, Pt nanocubes-rgo, Fig. S5 in the ESM), the thin Pt coating on Pd substantially lowers the Pt consumption in the Pt-Pd-rGO structures (1/1.5 of Pt/Pd atom ratio determined by ICP-MS, Table S1, ESM). Note that the Pt nanocubes-rgo structures possess the same Pt{100} facets on surface so as to serve as a reference sample to assess the performance of our samples. In the third step from Pt-Pd-rGO to PtPd nanocages-rgo, Pt surface structure is not changed while the exposed Pt area is almost doubled. As a result, more Pt atoms can participate in the catalytic reaction, further boosting the ORR activity.

9 7 Figure 4 (a) CV curves of as-obtained catalysts recorded in N2-saturated 0.1-M HClO4 solution; (b) ORR polarization curves of as-obtained catalysts in O2-saturated 0.1-M HClO4 solution; (c) Tafel plots of Pt-based catalysts; and (d) mass activity and specific activity of Pt-based catalysts at 0.9 V versus RHE. To assess the ORR catalytic activity, the obtained samples are first loaded onto GC electrodes for CV at a scan rate of 20 mv s -1 in N2-purged 0.1-M HClO4 solution. The loading weights of Pd and Pt in all the samples are kept the same according to the Pd/Pt ratio in Pt-Pd-rGO. Note that the small amount of residual Pd in PtPd nanocages-rgo is not taken into account. As shown in Fig. 4(a), the CV curves exhibit two distinctive potential regions: the one between 0.05 and 0.4 V that can be attributed to the adsorption/desorption processes of underpotentially deposited hydrogen (H + + e - = Hupd) on the surface, and the other beyond 0.6 V that corresponds to the formation of a layer of adsorbed hydroxyl species (2H2O = OHad + H3O + + e - ) [17, 31-33]. The Pt ECA values of PtPd nanocages-rgo, Pt-Pd-rGO and Pt nanocubes-rgo are determined as 11.10, 9.50 and 8.78 cm 2, respectively. Given the average edge lengths of Pt-Pd-rGO and Pt nanocubes-rgo at 14 and 18 nm, respectively, this result mainly reflects the size effect that the smaller particle size should show larger ECA [34, 35]. As for the PtPd nanocages-rgo hybrid structures, their hollow structures make the particle size effect not the decisive factor of ECA as their exterior flat surfaces are not the only sites for hydrogen adsorption. In other words, the high ECA of PtPd nanocages-rgo can be ascribed to the hollow nature. Note that the ECA value of Pd-rGO cannot be estimated through the same criterion as hydrogen adsorption on Pd surface is not in monolayer. ORR measurements are further performed in O2-saturated 0.1-M HClO4 solution using a GC RDE at a rotation rate of 1,600 rpm. Fig. 4(b) shows the Nano Research

10 8 Nano Res. Table 1 Half-wave potentials (E1/2) of the ORR on the electrodes prepared with various catalysts. Catalysts E1/2 initial E1/2 after 10,000 cycles (V vs. RHE) (V vs. RHE) Pd nanocubes-rgo Pt-Pd-rGO PtPd nanocages-rgo Pt nanocubes-rgo ORR polarization curves for the synthesized samples. The half-wave potential (E1/2) of the PtPd nanocages-rgo structure has been determined to be V, significantly higher than those of other catalysts (see Table 1), demonstrating its superb catalytic performance. Notably, the remarkably lower E1/2 (0.829 V) of Pd nanocubes-rgo in comparison with those of Pt-based catalysts confirms that Pd surface generally possesses low ORR catalytic activity. Thus the Pd should make a negligible contribution to the ORR activity of Pt-Pd-rGO structure, given the small fraction of exposed Pd ( 11.4% for the bottom surface in area if not considered the presence of rgo substrates) as well as its further coverage by rgo nanosheets. consideration of the low ORR activity of Pd, the further ORR performance comparison is performed based on the three Pt-based catalysts exposed with the same Pt{100} facets to exclude the surface composition factor. The kinetic current can be calculated based on the Koutecky Levich equation: (1) i i i k d where i is the experimentally measured current, id is the diffusion limiting current, and ik is the kinetic current. By normalizing the ik with Pt loading and ECA, the mass activity and specific activity of Pt based catalyst can be acquired, respectively. In As a result, the Tafel plots based on the relationship between the specific activity and the potentials are obtained. As shown in Fig. 4(c), the plots of all the Pt-based catalysts show slopes of about -120 mv dec -1 between V and -60 mv dec -1 between V, respectively. The slopes of different catalysts at a fixed potential are approximately equal at potentials below 0.98 V, indicating that they follow the same reaction mechanism over the majority of the potential range. From the comparison of Tafel plots, one can see that the PtPd nanocages-rgo exhibits higher kinetic currents than others, further demonstrating the superb activity of this catalyst [36, 37]. The ORR performance of the Pt-based catalysts can be better assessed by calculating mass activity and specific activity whose values at 0.9 V are shown in Fig. 4(d). The mass activity of A mgpt -1 and specific activity of ma cm -2 have been achieved by PtPd nanocages-rgo structures, which are 2.5 and 2.2 times higher than those of Pt-Pd-rGO, respectively, demonstrating the importance of creating hollow structures to ORR activity. The performance comparison with solid structures (i.e., 5.0 and 4.0 times greater than the mass and specific activities of Pt nanocubes-rgo, respectively) clearly shows that our synthetic strategy avoiding use of Pt in the interior can greatly reduce Pt usage. In our PtPd nanocages-rgo sample, the hollow structures create more active sites for the ORR reaction. The pores on the nanocages may also facilitate the O2 adsorption and mass transport. Meanwhile, Pd etching may lead to more dangling bonds on the interior surface of the PtPd cages in comparison with those on outside surface, providing more active sites to promote the catalytic reaction. In addition to the activity, performance durability is another factor that one has to take into account for future applications. The durability tests have been performed by applying cyclic potential sweeps between 0.60 and 1.05 V (which causes surface oxidation/reduction cycles of Pt) at a sweep rate of 50 mv s -1 and a rotation speed of 1,600 rpm in O2-saturated 0.1-M HClO4. After 10,000 cycles, the CV measurement shows minimal ECA loss (11.3%) for PtPd nanocages-rgo (Fig. 5(a)). Moreover, the

11 9 Figure 5 Durability tests of (a,b) PtPd nanocages-rgo sample by running the ORR reaction for 10,000 cycles in comparison with those of (c,d) Pt/C: (a,c) CV curves; (b,d) ORR polarization curves. half-wave potential of the PtPd nanocages-rgo exhibits a negative shift of only 9 mv in the mixed kinetic-diffusion control region after the durability test (Fig. 5(b) and Table 1). All the results indicate the excellent ORR stability of the PtPd nanocages-rgo. The high durability of the PtPd nanocages-rgo may be attributed to rgo nanosheets with large surface area. The rgo can strongly bind to Pt, thereby preventing Pt dissolution and agglomeration in the cyclic reaction [17, 38, 39]. However, the relatively low durability of other catalysts supported on rgo (especially for Pt nanocubes-rgo), indicated by both CV and polarization curves (Table 1 and Fig. S6 in ESM), suggests that the rgo is not the only contributor to the high durability of PtPd nanocages-rgo. Another possible attribution is that the small amount of Pd binding to Pt on the inside cage surface may prevent the Pt dissolution as well as Pt oxidation erosion as reported in the Pt-on-Pd nanostructures [40, 41]. After 10,000 cycles, the mass activity and specific activity of PtPd nanocages-rgo are still as high as A mgpt -1 and ma cm -2, which are particularly 11.0-fold and 7.2-fold greater than those of Pt nanocubes-rgo (free of Pd), respectively (Fig. S7 in ESM). This finding further suggests the enhanced durability of the PtPd nanocages-rgo structure that is most likely related to the presence of the small amount of Pd. The excellent ORR performance of the PtPd nanocages-rgo in both activity and durability is also benchmarked against commercial Pt/C catalyst at the same Pt loading (E-TEK, Pt: 40 % wt., average particle size: 10 nm) (Fig. 5 and Fig. S8, ESM). The PtPd nanocages-rgo shows apparently higher electrocatalytic durability than Pt/C (Fig. 5). The E1/2 values of Pt/C are 49 and 67 mv lower than those of PtPd nanocages-rgo before and after 10,000 cycles, respectively. Moreover, the PtPd nanocages-rgo exhibits mass activity and specific activity with Nano Research

12 10 Nano Res. values 4.4 and 3.9 times higher than those of Pt/C at 0.9 V, whose differences increase to 6.1 and 5.0 times after 10,000 cycles, respectively (Fig. S8, ESM). Furthermore, the excellent electrocatalytic performance of PtPd nanocages-rgo can be well appreciated when compared with the recently reported Pt nanodendrites (mass activity of A mgpt -1 and specific activity of 0.44 ma cm -2 at 0.9 V vs. RHE) as well as other catalysts [42, 43]. 4. Conclusion A novel hybrid structure between PtPd nanocages and rgo has been developed using a three-step synthetic process with Pd nanocubes as templates. The first and second steps form two intermediate products Pd nanocubes-rgo and Pt-Pd-rGO, respectively. Each synthetic stage represents one step towards enhanced catalytic performance in ORR, as the catalytic activity increases in the order: Pd nanocubes-rgo < Pt-Pd-rGO < PtPd nanocages-rgo. The performance improvement is mainly enabled by two factors: (1) the change of surface structure from Pd{100} to Pt{100} with higher activity in the second step; and (2) the further enlargement of exposed Pt{100} surface area and active sites in the third step. With maximal surface-to-volume ratio and minimal Pt usage, the final product of PtPd nanocages-rgo exhibits excellent catalytic activity and durability. A mass activity of A mgpt -1 and a specific activity of ma cm -2 have been achieved by the PtPd nanocages-rgo, which are 4.4 times and 3.9 times greater than those of Pt/C, respectively. This work represents a straightforward concept to design the structural parameters of catalytic materials step by step towards catalytic performance enhancement, and it is anticipated that this strategy can be extended to the design and synthesis of other catalysts. Acknowledgements This work was financially supported by the 973 Program (No. 2014CB848900), the NSFC (No , ), the Recruitment Program of Global Experts, the CAS Hundred Talent Program, the Anhui Provincial Natural Science Foundation (No MB24), and the Fundamental Research Funds for the Central Universities (No. WK , WK ). Electronic Supplementary Material: Supplementary material (further details of synthesis and characterizations, as well as XPS, TEM imaging and electrochemical measurements) is available in the online version of this article at (automatically inserted by the publisher). References [1] Zhang. J.; Sasaki, K.; Sutter, E.; Adzic, R. R. Stabilization of platinum oxygen-reduction electrocatalysts using gold clusters. Science 2007, 315, [2] Steele, B. C. H.; Heinzel, A. Materials for fuel-cell technologies. Nature 2001, 414, [3] Guo, S.; Zhang, S.; Sun, S. Tuning nanoparticle catalysis for the oxygen reduction reaction. Angew. Chem. Int. Ed. 2013, 52, [4] Cheng, F.; Chen, J. Metal air batteries: from oxygen reduction electrochemistry to cathode catalysts. Chem. Soc. Rev. 2012, 41, [5] Li, Y.; Dai, H. Recent advances in zinc-air batteries. Chem. Soc. Rev. 2014, 43, [6] Zhu, C.; Dong, S. Recent progress in graphene-based nanomaterials as advanced electrocatalysts towards oxygen reduction reaction. Nanoscale 2013, 5, [7] Cao, R.; Lee, J. S.; Liu, M.; Cho, J. Recent progress in non-precious catalysts for metal-air batteries. Adv. Energy Mater. 2012, 2, [8] Choi, R.; Choi, S. I.; Choi, C. H.; Nam, K. M.; Woo, S. I.; Park, J. T.; Han, S. W. Designed synthesis of well-defined Pd@Pt core shell nanoparticles with controlled shell thickness as efficient oxygen reduction electrocatalysts. Chem. Eur. J. 2013, 19, [9] Zhang, L.; Lyyamperumal, R.; Yancey, D. F.; Crooks, R. M.; Henkelman, G. Design of Pt-shell nanoparticles with alloy cores for the oxygen reduction reaction. ACS Nano 2013, 7, [10] Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.;

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15 Electronic Supplementary Material Etching approach to hybrid structures between PtPd nanocages and graphene towards efficient oxygen reduction reaction Song Bai 1, Chengming Wang 1 ( ), Wenya Jiang 1, Nana Du 1, Jing Li 1, Junteng Du 1, Ran Long 1, Zhengquan Li 2, and Yujie Xiong 1 ( ) Supporting information to DOI /s12274-****-****-* (automatically inserted by the publisher) Experimental Chemicals. K2PdCl4 (Aladdin, P106044), HClO4 (Sigma-Aldrich, ) and poly(vinyl pyrrolidone) (PVP, M.W. 55,000, Aldrich, ) were used in our synthesis. All the other chemicals were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd.. The water used in all experiments was de-ionized (DI). All chemicals were used as received without further purification. Synthesis of graphene oxide (GO) sheets. GO was synthesized from natural graphite flake (~325 mesh, Alfa Aesar) by a modified Hummers method [S1]. In a typical procedure, 2.0 g of graphite powder was added into concentrated H2SO4 (80 ml) in a 500-mL flask under ice bath. Under vigorous stirring, KMnO4 (10.0 g) and NaNO3 (4.0 g) were gradually added, while the temperature of the mixture was kept below 10 C for 4 h. Subsequently, the reaction mixture was stirred at 35 C for another 4 h until it became pasty green. The mixture was then diluted with DI water (200 ml), after which its color changed into brownish. The addition of DI water was performed in an ice bath to keep the temperature below 100 C. The mixture was then stirred for 30 min, and 15 ml of 30 wt% H2O2 was slowly added to the mixture to reduce the residual KMnO4, after which the color of the mixture changed to brilliant yellow. The mixture was redispersed in water and then was dialyzed for one week to remove residual salts and acids. The resulted solid was centrifuged and dried at 45 C for 12 h. Synthesis of Pt nanocubes-rgo. In a typical synthesis, bare rgo was first prepared under the same experimental conditions as those for Pd-rGO except the absence of K2PdCl4. Then Pt nanocubes-rgo was

16 synthesized under the same experimental conditions as those for Pt-Pd-rGO except the use of bare rgo instead of Pd-rGO structure as precursors. Sample Characterizations. Prior to electron microscopy characterizations, a drop of the aqueous suspension of particles was placed on a piece of carbon-coated copper grid and dried under ambient conditions. TEM images were taken on a JEOL JEM-2010 LaB6 high-resolution transmission electron microscope operated at 200 kv. HRTEM/STEM images and EDS mapping profiles were taken on a JEOL JEM-2100F field-emission high-resolution transmission electron microscope operated at 200 kv. X-ray powder diffraction (XRD) patterns were recorded by using a Philips X Pert Pro Super X-ray diffractometer with Cu-Kα radiation (λ = Å ). X-ray photoelectron spectra (XPS) were collected on an ESCALab 250 X-ray photoelectron spectrometer, using nonmonochromatized Al-Kα X-ray as the excitation source. The concentrations of metal elements were measured as follows: the samples were dissolved with a mixture of HCl and HNO3 (3:1, volume ratio) which was then diluted with 1% HNO3. The concentrations of metals were then measured with a Thermo Scientific PlasmaQuad 3 inductively-coupled plasma mass spectrometry (ICP-MS). The weight ratios of metals to rgo were determined by sample weighing prior to the dissolution of metals for the ICP-MS measurements. Nano Research

17 Figure S1 TEM image of graphene oxide nanosheets.

18 Figure S2 Cross-section view for the structures of (a) Pt-Pd-rGO and (b) PtPd nanocages-rgo. Nano Research

19 Figure S3 Schematic illustration showing the etching mechanism for PtPd nanocages-rgo synthesis.

20 Figure S4 Survey XPS spectrum of the obtained PtPd nanocages-rgo sample. Nano Research

21 Figure S5 (a) Schematic illustration, (b,c) TEM and (d) HRTEM images of Pt nanocubes-rgo hybrid structures as a reference sample.

22 Figure S6 Durability tests for (a,b) Pd nanocubes-rgo, (c,d) Pt-Pd-rGO and (e,f) Pt nanocubes-rgo hybrid structures by running the ORR reaction for 10,000 cycles. The left column shows their CV curves, and the right column contains their ORR polarization curves. Nano Research

23 Figure S7 Mass activity and specific activity of Pt-based catalysts at 0.9 V versus RHE after durability tests.

24 Figure S8 Comparison of mass activity and specific activity of PtPd nanocages-rgo with Pt/C catalyst (a) before and (b) after 10,000 cycles, respectively. Nano Research

25 Table S1 Loadings of as-synthesized Pt/Pd-based catalysts (relative to rgo supports) and their Pt/Pd composition ratios. Catalysts Metals/rGO ratio Pt/Pd ratio (wt. %) (wt. %) Pd nanocubes-rgo 79.8 : 20.2 Pt-Pd-rGO 89.9 : : 45.0 PtPd nanocages-rgo 83.2 : : 4.7 Pt nanocubes-rgo 81.4 : 18.6 Reference S1 Hummers, W. S.; Offeman, R. E. Preparation of graphitic oxide. J. Am. Chem. Soc. 1958, 80, Address correspondence to Yujie Xiong, yjxiong@ustc.edu.cn; Chengming Wang, chmwang@ustc.edu.cn.