PdPtRu nanocages with tunable compositions for boosting the methanol oxidation reaction

PtRu/C is a well-known commercial electrocatalyst with promising performance for the methanol oxidation reaction (MOR). Further improving the MOR properties of PtRu-based electrocatalysts is highly desirable, especially through structure design. Here we report a facile approach for the synthesis of PdPtRu nanocages with different components through a seed-mediated approach followed by chemical etching. The Pd@PtRu nanocubes were first generated using Pd nanocubes as the seeds and some Pd atoms were subsequently etched away, leading to the nanocages. When evaluated as electrocatalysts for the MOR in acidic media, the PdPtRu nanocages exhibited substantially enhanced catalytic activity and stability relative to commercial Pt/C and PtRu/C. Specifically, PdPt2.5Ru2.4 achieved the highest specific (8.2 mA cm−2) and mass (0.75 mA mgPt−1) activities for the MOR, which are 2.2 and 4.2 times higher than those of commercial Pt/C. Such an enhancement can be attributed to the highly open structure of the nanocages, and the possible synergistic effect between the three components.


Introduction
Faced with the aggravation of environmental pollution and continuous increase of energy demand, direct methanol fuel cells (DMFCs) are regarded as alternative energy conversion devices due to their high conversion efficiency, high energy density, and environment friendly characteristics. [1][2][3][4][5] In DMFCs, the methanol oxidation reaction (MOR) at the anode is conducted through multiple steps, resulting in slower reaction kinetics than electro-oxidation of hydrogen. Currently, Pt is proved to be the most effective single-metallic electrocatalyst with excellent activity and durability for the MOR. [6][7][8] However, scarce abundance and high cost of Pt severely limit its widespread use in DMFCs. Furthermore, Pt-based anodes are vulnerable to the poisoning effect associated with the CO-like intermediates during the MOR, leading to a dramatic loss of catalytic activity. [9][10][11][12] As such, designing highly active and strongly CO-tolerant Pt-based MOR electrocatalysts is critical to commercialize DMFCs widely.
Recently, tremendous efforts have been devoted to exploiting Pt-based nanocrystals through various strategies such as composition optimization, shape control, and structure design. [13][14][15][16][17] Combining Pt with other cheap metals to form binary or multimetallic alloys is considered as a promising method to improve the utilization efficiency of Pt, and thus reduce the loading of Pt as well. Simultaneously, the introduction of a second metal is demonstrated to tune the electron structure of Pt, thereby improving the activity and durability of Pt-based electrocatalysts. [18][19][20][21] Up to now, there are a rich variety of reports on the synthesis of PtM (M ¼ Pd, Ru, Cu, Co, Ni, etc.) bimetallic alloys serving as electrocatalysts for the MOR. [22][23][24][25] Of them, PtRu alloys have received great attention as electrocatalysts for the MOR in acidic electrolytes, showing superior catalytic properties. [26][27][28] As an oxophilic metal, Ru can dissociate water to form the oxygenated species (e.g., OH ads ) during the MOR. Such oxygenated species on Ru sites can oxidize and subsequently remove the CO ads intermediates on Pt sites, resulting in the strong anti-poisoning capability of electrocatalysts. [29][30][31] For instance, Wang and coworkers reported the synthesis of the PtRu nanodendrites, showing substantially enhanced catalytic activity and stability towards the MOR as compared to PtRu nanocrystals and commercial Pt/C. 32 Despite the huge success in PtRu-based electrocatalysts, there is still a large room to improve the catalytic properties for the MOR, especially through the design of hollow nanostructures.
Hollow nanostructures have emerged as a new class of electrocatalysts due to the more abundant active sites and higher utilization efficiency of noble-metals compared to their solid counterparts. Among them, nanoframes and nanocages have received more attention due to the accessibility of reactants to both interior and exterior surfaces of electrocatalysts. [33][34][35][36][37][38][39] For example, Zhang et al. reported that PtCu nanoframes exhibited a MOR mass activity of 2.26 A mg Pt À1 in alkaline media, which was much higher than that of PtCu nanoparticles and commercial Pt/C. 40 In another study, AuPt bipyramid nanoframes were generated by a facile seedmediated approach, showing substantially enhanced catalytic activity for the MOR relative to commercial Pt/C. 41 Recently, Xia et al. developed a promising approach for the synthesis of Ptbased nanocages with Pd nanocubes as the seeds. 42,43 However, synthesizing Pt-based hollow nanostructures with multimetallic components as MOR electrocatalysts is still challenging, in particular for those with well-dened exposed facets.
Here we report the synthesis of PdPtRu nanocages with different atomic ratios of Pt/Ru by a seed-mediated approach in combination with chemical etching. The PdPtRu nanocages exhibited substantially enhanced catalytic properties for the MOR compared to commercial Pt/C and PtRu/C. The PdPt 2.5 -Ru 2.4 nanocages achieved the highest activity and durability because of the highly open structure and appropriately strong electron coupling arising from the three components.

Synthesis of Pd nanocubes
Pd nanocubes with diameters of 13.8 nm were produced according to our previously reported method. 44 In a typical synthesis, an aqueous solution containing water (8 mL), KBr (300 mg), AA (60 mg), and PVP (105 mg) was added into a 20 mL vial, and then pre-heated at 80 C in a magnetically stirred oil bath for 10 min. Subsequently, an aqueous solution consisting of water (3 mL) and Na 2 PdCl 4 (57 mg) was added into the vial. Aer that, the vial was capped and heated at 80 C for 3 h. Finallythe product was collected through centrifugation, washed three times with water, and then dispersed in EG at a concentration of 1.3 mg mL À1 for further use.

Synthesis of Pd@Pt-Ru core-shell nanocubes
In a standard synthesis, PVP (50 mg), AA (30 mg), KBr (50 mg), the suspension of Pd nanocubes (0.23 mL), and EG (3.7 mL) were mixed in a 20 mL vial and pre-heated at 110 C. Aer 15 min, the temperature was raised rapidly to 200 C within 15 min. Aer that, a solution containing EG (7 mL), Na 2 PtCl 6 -$6H 2 O (0.75 mg), and RuCl 3 (0.3 mg) was added into the vial at a rate of 1.0 mL h À1 . The vial was kept at 200 C for 1 h. The nal product was collected by centrifugation, washed once with acetone and twice with water, and then dispersed into water (1 mL) for further use.

Synthesis of PdPtRu nanocages
In a typical process, PVP (50 mg), KBr (300 mg), HCl (0.18 mL), FeCl 3 (30 mg), and water (6.5 mL) were mixed in a 20 mL vial, and heated at 95 C. 1.0 mL of the Pd@Pt-Ru nanocube suspension was added into the vial using a pipette. Aer 2.5 h, the product was collected by centrifugation, and washed with water three times.

Morphological, structural, and compositional characterization
The morphology of the obtained sample was characterized by transmission electron microscopy (TEM) using a HITACHI HT-7700 microscope operated at 100 kV. X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB 250Xi (Thermo, U.K.). The corresponding binding energies were calibrated with a C-C 1s peak of 284.5 eV. High-resolution TEM (HRTEM), high-angle annular dark-eld scanning TEM (HAADF-STEM) and energy dispersive X-ray (EDX) mapping analyses were performed using a FEI Tecnai F20 G2 microscope operated at 200 kV. The atomic ratios of Pd, Pt and Ru in these samples were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES, IRIS Intrepid II XSP, TJA Co., USA).

Electrochemical measurement
Electrochemical measurements were carried out in a threeelectrode system at room temperature using a rotating disk electrode (RDE, Pine Research Instrumentation, USA) connected to an electrochemical workstation (CHI 760E). A reversible hydrogen electrode (RHE) was used as the reference electrode. The counter electrode was a Pt wire. To prepare the catalyst ink, 5 mg of PtRu/C or Pt/C catalysts were dispersed in 5 mL of a mixed solution and sonicated for 10 min. The solution contained a mixture of de-ionized water, isopropanol, and 5% Naon on 117 solution at the volumetric ratio of 8 : 2 : 0.05. Aer that, 20 mL of the PtRu/C or Pt/C catalyst ink was added onto the RDE and dried under an air ow to make the working electrode. The geometric area of the RDE was 0.196 cm 2 . The electrochemical active surface area (ECSA) was determined from the cyclic voltammogram (CV) curves, calculating the amount of charges by integrating the hydrogen desorption region aer double layer correction. The CV measurement was performed in Ar-saturated 0.

Results and discussion
The PdPtRu nanocages were synthesized through a seedmediated growth in combination with chemical etching (Scheme 1). First, the Pd@Pt x Ru (x ¼ 0.5, 1, 2) nanocubes were generated by the co-deposition of Pt and Ru atoms using Pd nanocubes with an average diameter of $13.8 nm as the seeds (Fig. S1 †). Fig. S2 † shows the TEM images of Pd@Pt x Ru nanocrystals, displaying a uniform cube-like shape (Fig. S2a, c and e †) and size distributions ($18 nm, Fig. S2b, d and f †). In the XRD patterns (Fig. S3 †), all the diffraction peaks of the Pd@Pt x Ru nanocubes can be attributed to Pd or Pt with a facecentered cubic (fcc) structure. Taking Pd@PtRu as an example, the HRTEM, EDX mapping and line-scan analyses are shown in Fig. S4, † demonstrating the core-shell structure. Subsequently, the Pd core was selectively removed using HCl and FeCl 3 mixed solution. According to previous results, 42 the incorporation of Pd atoms into the PtRu overlayers could occur during the synthesis because of the interdiffusion between them as well as the co-reduction of Pd 2+ ions resulting from the weak dissolution of Pd nanocubes at elevated temperature. Some Pd atoms embedded in the shell came in contact with the etchant and were etched away easily. As such, the removal of these embedded Pd atoms provides a pathway for the complete dissolution of the Pd core, leading to the formation of PdPtRu nanocages.
The morphology, structure, and composition of the Pt-based nanocages were characterized by the TEM, HRTEM, EDX mapping and line-scan analyses, as shown in Fig. 1. Clearly, most of the nanocrystals display a highly open structure with a cube-like shape and uniform size ( Fig. 1a and S5 †), indicating the formation of nanocages. The typical HRTEM image (Fig. 1b) of an individual nanocage shows well-resolved, ordered fringes in the same orientation, suggesting that the nanocage is a single crystal. The fringes with a lattice spacing of 0.19 nm can be indexed to the {100} planes of Pt-based alloys with a facecentered cubic (fcc) structure. In addition, the difference in contrast in the HRTEM image also conrms the hollow structure. The distribution of Pt, Ru and Pd in the nanocage is determined using EDX analysis ( Fig. 1c and d). Clearly, the edges of the nanocages are composed of Pt, Ru, and Pd, while some Pd still exists in the inner side of the edges (Fig. 1c). This demonstration is further supported by the EDX line-scan spectra (Fig. 1d). This result indicates that Pd atoms were not totally etched but partly kept instead. As such, the interior of the nanocage is Pd rich, while Pt and Ru were distributed uniformly at the surface. The atomic ratio of Pt/Ru was about 1 : 2.5 : 2.4 as quantitatively determined by ICP-AES analysis (Table S1 †), which is close to the EDX result (Table S2 †). For simplicity, this sample was dened as standard one and labelled as PdPt 2.5 Ru 2.4 . This method can be extended to synthesize other PdPtRu nanocages with different compositions by tuning the amount of Pt and Ru precursors. When the feed ratio of Pt/Ru was varied to 2 or 0.5, hollow nanostructures with a cubic shape and similar size were still achieved ( Fig. S6 and S7 †). The big difference of these two samples from the standard one is the atomic ratio of Pt/Ru according to EDX analysis (Table S2 †). The quantitative composition of these two samples was measured by ICP-AES analysis (Table S1 †). Accordingly, these two samples were labelled as PdPt 7.3 Ru 3.6 and PdPt 3.7 Ru 8.1 .
The valence and electronic states of the three PdPtRu nanocages were analyzed using the XPS technique, as shown in Fig. S8. † As observed, Pt, Ru and Pd exist mainly in the zerovalent state for such three samples, indicating that the metallic state is the majority. In addition, there is an obvious positive shi of Pt 4f and Pd 3d and an obvious negative shi of Ru 3p in these three nanocages compared to those of the corresponding bulk materials. Taking PdPt 2.5 Ru 2.4 nanocages as an example (Fig. S8a †), the binding energies of Pt 0 are located at 71.6 (4f 7/2 ) and 74.9 (4f 5/2 ) eV, respectively, displaying a positive shi compared with bulk Pt 4f peaks at 71.20 (4f 7/2 ) and 74.53 eV (4f 5/2 ). Simultaneously, the binding energies of Ru 0 are located at 462.8 (3p 1/2 ) and 485.1 (3p 5/2 ) eV, respectively, Scheme 1 Schematic illustration of the synthesis of the PdPtRu nanocages, including the co-deposition of Pt and Ru atoms on Pd nanocubes, and the selective removal of the Pd core via chemical etching. showing a negative shi compared with bulk Ru 3p peaks at 463.3 (3p 1/2 ) and 485.6 eV (3p 3/2 ). This result demonstrates the occurrence of electron transfer between Pt, Pd and Ru. Due to the possible interaction between the three elements in the PdPtRu nanocages, this effect might weaken the adsorption of partial oxidation intermediates (e.g., CO) on Pt, thereby improving the catalytic activity for the MOR by alleviating the CO poisoning. 45,46 These three PdPtRu nanocages were then loaded on a carbon support (Vulcan XC-72R, Fig. S9 †), and then evaluated as the catalysts for the MOR with commercial Pt/C and PtRu/C as references. Fig. S10 † shows the cyclic voltammetry (CV) curves of these ve catalysts including commercial Pt/C and PtRu/C recorded in Ar-purged 0.1 M HClO 4 solution at a sweep rate of 100 mV s À1 . As determined by the hydrogen desorption region  (Table S3 †). This demonstration is also supported by CO-stripping measurement ( Fig. 3 and Table S3 †). Fig. 2a and b compare the CV curves of these catalysts for the MOR performed in the solution containing 0.1 M HClO 4 and 0.5 M CH 3 OH at a sweep rate of 50 mV s À1 . Obviously, the nanocages exhibited higher specic and mass activities relative to commercial Pt/C and commercial PtRu/C due to their unique structure and synergetic effect of multiple components. In addition, the MOR activities of the nanocages followed the sequence: PdPt 3.7 Ru 8.1 < PdPt 7.3 Ru 3.6 < PdPt 2.5 Ru 2.4 (Fig. S11 †). Specically, the PdPt 2.5 Ru 2.4 nanocages showed the highest mass (0.75 mA mg Pt

À1
) and specic (8.2 mA cm À2 ) activities towards the MOR, which are 4.2 and 2.2 times higher than those of commercial Pt/C (0.18 mA mg Pt À1 and 3.8 mA cm À2 ), respectively (Fig. 2c). In addition, the PdPt 2.5 Ru 2.4 nanocages outperformed most of the previously reported Ptbased MOR electrocatalysts in acid media (Table S4 †). The superior catalytic activities of the nanocages can be understood based on two reasons. (i) The bifunctional mechanism and ligand effect. As is well known, Pt is the active site for methanol adsorption and dissociation during the MOR, causing the formation of intermediates mainly including CO ads . Such CO ads intermediates can severely poison active sites of Pt through strong adsorption, and nally deactivate Pt-based electrocatalysts. The incorporation of Ru and Pd can cause a down shi of the d-band center of Pt (ligand effect), 47 thereby reducing the binding strength of Pt with the adsorbed intermediates (e.g., CO) and enhancing the tolerance to CO poisoning. The addition of Ru and Pd with the oxophilic capability also promotes the removal of CO ads intermediates during the catalytic reaction, and thus substantially enhances the activity by inhibiting the CO poisoning (bifunctional mechanism). 48 (ii) The structural effect. ECSA data (Table S3 †) and HRTEM image (Fig. 1b) indicate that the nanocages usually contain lots of low-coordinated sites on the surface. These lowcoordinated sites are active for the MOR, which might be responsible for the better MOR activity. 49 To further conrm the alleviation of CO poisoning in the PdPtRu nanocages, the CO stripping of the three catalysts including PdPt 2.5 Ru 2.4 /C, PtRu/C, and Pt/C was tested in 0.1 M HClO 4 with CO ow, as shown in Fig. 3. As observed, the peak potentials for CO oxidation on PdPt 2.5 Ru 2.4 /C, PtRu/C, and Pt/C are 0.60, 0.61, and 0.88 V, respectively, showing a negative shi of 28 mV for PdPt 2.5 Ru 2.4 /C relative to Pt/C. This result indicates that the addition of Ru and Pd effectively promotes the oxidation of CO and dramatically alleviate the CO poisoning.
The electrocatalytic stability of the PdPtRu/C including commercial PtRu/C and Pt/C catalysts for the MOR was investigated by the long-term current-time (I-t) measurement technique, as shown in Fig. 2d and S11d. † As can be seen, the PdPtRu/C catalysts show higher steady current density relative  to commercial PtRu/C and Pt/C for the MOR over the entire time range, especially the PdPt 2.5 Ru 2.4 /C catalyst. In order to further conrm the excellent MOR activity and durability of PdPtRu/C, the electrocatalytic test was conducted in 0.1 M HClO 4 and 1 M CH 3 OH, as shown in Fig. S12. † Compared to Pt/C and PtRu/C, the catalytic activity of PdPt 2.5 Ru 2.4 /C was improved signicantly. This data also showed that the PdPt 2.5 Ru 2.4 nanocages had better tolerance for CO poisoning independent of the methanol concentration. Such superior durability can be attributed to weaker CO adsorption on the nanocages and better CO tolerance due to the addition of oxophilic Ru and Pd. Aer the stability test, the three PdPtRu/C catalysts were characterized by the TEM technique, as shown in Fig. S13. † As observed, the nanocages were kept unchanged and still well-dispersed on the carbon support, indicating that their high structural stability is also responsible for the enhanced catalytic durability. However, the Pt/C and PtRu/C catalysts exhibited some degree of aggregation, which was also responsible for the degradation of MOR activity during the stability test.

Conclusions
In summary, we have developed a facile approach to synthesize PdPtRu nanocages with tunable compositions. Such nanocages were generated through two steps including the formation of Pd@PtRu nanocrystals and partial etching of Pd. The PdPtRu nanocages exhibited substantially enhanced catalytic activity and stability for the MOR relative to commercial PtRu/C and Pt/ C, with the PdPt 2.5 Ru 2.4 nanocages being the best catalyst. This enhancement can be attributed to the bifunctional mechanism and ligand effect arising from incorporation of Ru and Pd as well as the unique hollow structure. This work not only provides a facile method for the synthesis of PtRu-based MOR catalysts with a highly open structure, but also offers a great opportunity to design catalysts with enhanced performance for a rich variety of potential applications.

Conflicts of interest
The authors declare that they have no conict of interest.