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In-situ Assembly Ultrathin PtRh Nanowires to Graphene Nanosheets as Highly Efficient Electrocatalysts for the Oxidation of Ethanol Yi Shen, Bin Gong, Kaijun Xiao, and Lei Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09573 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 9, 2017

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ACS Applied Materials & Interfaces

In-situ Assembly Ultrathin PtRh Nanowires to Graphene Nanosheets as Highly Efficient Electrocatalysts for the Oxidation of Ethanol Yi Shen,a* Bin Gong,a Kaijun Xiao,a Lei Wang*b a

School of Food Science and Engineering, South China University of Technology Guangzhou 510640, P. R. China. b

Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060

Abstract One-dimensional (1D) anisotropic Pt-based nanowires are promising electrocatalysts in polymer electrolyte membrane fuel cells owing to the inherent structural merits. Herein, we report an insitu growth of ultrathin PtRh nanowires (diameters 2~3 nm) on graphene nanosheets via the oriented attachment pathway. Mechanistic studies reveal that graphene nanosheets play a critical role in the nucleation and growth of PtRh nanowires. The resulting hybrid of PtRh nanowire decorated graphene nanosheets shows outstanding activity and durability toward ethanol electrooxidation. It exhibits a specific current density of 2.8 mA cm-2 and a mass-normalized current density of 1 A mg-1 metal, which are 5.4 and 3.1 times of those of state-of-the-art Pt/C catalyst, respectively. After 2000 cyclic tests, it maintains 86% of the initial electrochemical active surface area, which is larger than that of 63% obtained from the Pt/C catalyst. The superior performance is attributed to the combination of the advantageous 1D morphological motif with the synergistic effects of PtRh alloys and graphene nanosheet support.

Keywords: PtRh Nanowires; polymer electrolyte membrane fuel cells; graphene nanosheets; ethanol electro-oxidation; graphene-mediated synthesis

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Introduction Being a high-efficient and environmentally-friendly device to convert chemical energy stored in fuels to electrical power, polymer electrolyte membrane fuel cells (PEMFCs) have recently stimulated intense research.1 Despite notable progress in PEMFCs, the widespread commercialization of PEMFCs is still impeded by the lack of high-performance electrocatalysts which show sufficient activity and durability for the oxidation of fuels at the anode and reduction of oxygen at the cathode. Currently, state-of-the-art catalysts in PEMFCs primarily consist of dispersive nanosized Pt-based particles (2~5 nm) supported on carbon materials. The critical issue of these nanoparticulated catalysts is the poor durability. Zero-dimensional (0D) Pt-based nanoparticles (NPs) are susceptible to dissolution, migration, coalescence and Oswald ripening due to the high surface energy, which leads to a significant loss of electrochemical active surface areas (ECSAs) and subsequent activity decay during the long-term operation of PEMFCs.2-5 To address this issue, one promising strategy is to develop one-dimensional (1D) nanostructured catalysts.6-9 Compared with their 0D counterparts, 1D catalysts exhibit many inherent structural characteristics, i.e., high aspect ratios, high stability, preferential exposure of active crystal facets, and fast electron transport, which are highly beneficial to the performance of the catalysts.10-12 So far, numerous efforts have been directed in searching for 1D catalysts in PEMFCs.13-28 For instance, Wong and his co-workers conducted a significant amount of work to optimize the performance of 1D catalysts via delicate tailoring the elemental composition, surface structures and sizes of the anisotropic architectures.29-32 They demonstrated that 1D anisotropic morphology was highly favorable to activity and durability of catalysts in PEMFCs. Apart from the morphology of the active metallic component, the carbon support also affects the durability of catalysts. Carbon black, which is widely utilized as catalyst supports in PEMFCs, suffers from serious electrochemical corrosion, thus accelerating the segregation, dissolution and detachment 2 ACS Paragon Plus Environment

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of metallic components.33 In contrast, graphene nanosheet (GNS) exhibits much better resistance to electrochemical corrosion because of its high conductivity and unique graphitized basal planes, rendering it to be an ideal alternative support in PEMFCs.34 Ethanol is considered to be one of promising fuels in PEMFCs because of its sustainable production from fermentation processes, high energy density and non-toxicity. To completely oxidize ethanol to carbon dioxide, the cleavage of C-C bonds is requisite, which, however, remains difficult on single Pt surface.35 To this end, several metallic elements such as Ru,36 Rh,37 and Ir38 were coupled with Pt to promote the oxidation of ethanol via the so-called bi-functional and ligand effects. Of these elements, Rh is regarded as one of most efficient components to facilitate the cleavage of the C-C bonds at lower potentials. It was reported that Rh was more effective to stabilize the metal–CH2CH2O intermediates during ethanol oxidation in comparison with Pt because of the high-lying d-band and more unoccupied d-states.35 In addition, the alloying Rh with Pt was accompanied by an electron transfer from Rh to Pt, resulting in more dstates of Rh and a downshift in the d-states of Pt. Such synergistic effects afforded moderate bonding to ethanol, intermediates and products, which facilitated C–C bond breaking.35 Previous studies demonstrated that PtRh alloys showed exceptional efficiency toward ethanol oxidation.39 Herein, we report a simple approach to in-situ assembly PtRh nanowires (NWs) to GNS as a high-performance electrocatalyst for the oxidation of ethanol. The prominent advantage of the PtRh catalyst reported in this work lies in its structural features, which consist of ultrafine 1D PtRh nanowires supported by two-dimensional (2D) graphene nanosheets. The as-prepared PtRh NW decorated GNS (denoted as PtRh NW/GNS) exhibits outstanding activity and durability toward ethanol oxidation because of the combination of the structural merits of the two components. 3 ACS Paragon Plus Environment


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Experimental Section Synthesis of catalysts The GNS support was derived from chemical vapor deposition of methane and further purified by acid leaching and sedimentation separation as previously reported.40 The resulting GNS mainly consisted of few-layered graphene with a Brunauer-Emmett-Teller surface area of 247 m2 g-1. The PtRh NW/GNS catalyst was synthesized via a polyol-assisted reduction process. Typically, 100 mg of GNS, 2 g of polyvinylpyrrolidone (PVP Mw = 4 × 105) and 1.2 g NaOH were thoroughly dispersed into 150 mL of ethylene glycol by ultrasonication to form a stable suspension. Metal precursors, i.e., H2PtCl6 aqueous solution (8 wt. %) and Rhodium (III) chloride hydrate with a molar ratio of Pt: Rh = 75:25 were added into the suspension. The total metal loading in the feedstock was 40 wt.%. The mixture was transferred to a pre-heated oil bath and refluxed at 180ºC for 30 min in N2 atmosphere. After cooling to ambient temperature, the mixture was separated by centrifuge and thoroughly washed with ethanol. The product was dried at 80ºC overnight for further use. To explore the formation mechanism of PtRh NWs, parallel experiments were conducted. Another two catalyst samples were synthesized in the absence of the GNS support or surfactant PVP. Structural Characterization The morphology of the samples was observed by transmission electron microscopy (TEM JEM2010, JEOL). High-resolution TEM images were obtained from an alternative JEM-2010F (JEOL) microscope. An energy dispersive X-ray (EDX) analyzer equipped in the TEM and an axis-ultra X-ray photoelectron spectrometer (Kratos-Axis Ultra System) with monochromatized Al-Kα radiation were used to analyze the elemental composition of the samples. The actual Pt loading in the electrode was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES) analysis. X-ray diffraction (XRD) analyses were performed on a high4 ACS Paragon Plus Environment

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resolution X-ray diffractometer (Philips X’pert Pro) equipped with Cu-Kα radiation of 1.54 Å, an X-ray mirror and X’celerator detector for high-speed data collection. Electrochemical measurements Electrochemical measurements were carried out using a home-made set-up and the details of the experimental procedures were reported in the authors’ previous work.41-44 In brief, an Au plate (effective area 1 × 1 cm) coated with a thin layer of catalyst ink was used as working electrode. To prepare work electrodes, 0.5 mL of catalyst ink was transferred to one side of the Au plate by a pipette and dried at 80ºC under nitrogen flow. The loading of the catalyst is ca. 200 µg cm-2. A saturated calomel electrode (SCE) and Pt gauze were used as reference and counter electrodes, respectively. Before the test, the electrolyte was bubbled with N2 gas for 30 min. The electrochemical measurements were conducted at 25 ºC and all potentials in this study were referenced to the SCE.

Results and Discussion Structural properties The hybrid of PtRh NW/GNS was synthesized via a one-pot synthesis protocol. The metal precursors, i.e., H2PtCl6 and Rhodium (III) chloride hydrate were reduced in the presence of GNS support and polyvinylpyrrolidone (PVP) via the well-established polyol reduction process (see the details in the Experimental Section). The in-suit nucleation, growth and anchoring of PtRh NWs to GNS can lead to strong coupling between the two components, resulting in enhanced activity and durability of the hybrid.45 Figure 1 displays the morphological and compositional properties of the as-prepared PtRh NW/GNS. As shown in Figure 1a, the hybrid mainly consists of worm-like NWs intimately adhered to the GNS support (denoted by the arrow). The serpentine morphological feature of the PtRh NWs can be related to the variation in 5 ACS Paragon Plus Environment


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growth direction, which is beneficial to their contact with the GNS support, thereby facilitating the electron transport during the oxidation of ethanol.23 High-resolution TEM images (Figure 1b, c) reveal that the PtRh NWs are interconnected with diameters of 2~3 nm and lengths of several dozen nanometers. Such ultrathin diameters of PtRh NWs are highly advantageous for the oxidation of ethanol because of the large number of available active sites and enhanced activity arisen from the size effects.32 Further inspection (see Figure 1d, e) reveals that these ultrathin NWs are segmented and composed of numerous single crystal building blocks. The orientated attachment growth pathway is also verified by the presence of a number of twin boundaries as denoted by the arrows in Figure 1f, g. The lattice fringes with a spacing value of 0.226 nm are well matched to the inter-distance of (111) planes of PtRh alloyed crystals with a face-centered cubic structure.39 The prevailing (111) facets in the NWs can be related to the close lattice match of PtRh(111) and C(110) planes.41 Figure 1h shows the EDX result of the PtRh NW/GNS. The copper signal in the spectrum is attributed to the TEM grid. The PtRh NW/GNS consists of Pt (33.6 wt.%), Rh (5.7 wt.%), C (57.1 wt.%) and O (3.6 wt.%), indicating the high purity of the sample. The metal loading of the PtRh NW/GNS was also analyzed by ICP-AES. The weight percentages of Pt and Rh are determined to be 32.4 and 5.9 wt.%, respectively, which are close to the EDX results. The crystallographic properties of the PtRh NW/GNS were examined by XRD as shown in Figure 2a. For comparison, the diffraction peaks of standard Pt and Rh references were included in the XRD profile. A single set of peaks are present in the XRD profile, thereby indicating the formation of uniform PtRh alloys.44 The strongest (111) peak is located at 40.1º, which lies exactly between those of Pt (39.7º) and Rh (41º), manifesting slight lattice contraction of PtRh crystals arisen from the incorporation of Rh atoms into Pt lattice. It is also worth noting that the 6 ACS Paragon Plus Environment

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intensity of the (111) peak is so strong that the (002) peak located at 46.2º is almost indiscernible from the XRD pattern, which is consistent with the dominant (111) facets in the PtRh NWs as observed from TEM images. No distinct peaks ascribable to any possible impurities are observed, suggesting the high purity of the sample. The broadness of the peaks is probably related to the ultrathin diameters of the PtRh NWs. The elemental composition of the PtRh NW/GNS was studied by XPS as shown in Figure 2b-d. The survey spectrum shown in Figure 2b verifies that the sample exclusively consists of Pt, Rh, O and C elements. The high-resolution Pt 4f spectrum shown in Figure 2c consists of two doublets of 4f 5/2 and 7/2 of Pt.41 The spectrum was carefully deconvoluted. Two species including metallic Pt0 and Pt oxides were identified. The component of Pt oxides corresponds to peaks with binding energies of 73.2 and 76.5 eV in the doublets of 4f 3/2 and 5/2, respectively. The metallic Pt0 exhibits binding energies of 72 and 75.4 eV in the doublets of 4f 3/2 and 5/2, respectively. Such binding energies of Pt0 are larger than the reported value of 71.1 eV,46 which was related to size effects as well as the electronic effects arisen from the incorporation of Rh atoms. Since the XPS spectrum of Rh 3d is overlapped with Pt 4d 5/2, the spectrum was carefully deconvoluted as shown in Figure 2d. The resulting peaks with bind energies of 307.3, 314 and 315.5 eV can be well ascribed to Rh 3d 5/2, 3/2 and Pt 4d 5/2, respectively.23, 44 Formation mechanism of PtRh NWs To explore the formation mechanism of the ultrathin PtRh NWs, the morphologies of the products which were taken at different reaction times during the synthesis were analyzed by TEM as shown in Figure 3. At 2 min, nanoclusters with sizes less than 1 nm are preferentially anchored to the edges of the GNS support as shown in Figure 3a. At 5 min, the sizes of these clusters increase. Meanwhile, some elongated nanorods of several nanometers are formed 7 ACS Paragon Plus Environment


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(Figure 3b). With increasing reaction time, the nanorods continue to grow to form NWs by attachment of NPs as shown in Figure 3c. After 30 min, the NWs reach to lengths of dozens of nanometers and interweave with each other to form a 2D network (see Figure 3d). Notably, the diameters of the NWs show very limited variations throughout the growth stage (5~30 min), which is consistent with the oriented attachment pathway. Such time-dependent evolution of PtRh NWs is similar to that of Pd NWs as reported by Wang et al.,47 which involves a simultaneous rapid nucleation within a very short period of time and an attachment growth process over a relatively long period of time. It was reported that controlling the reaction kinetics during nucleation stage played a critical role in the formation of anisotropic NWs. To this end, the burst nucleation strategy via hot rejection was widely utilized in the literature.47 In contrast, in this work, the precursors, i.e., H2PtCl6 and Rhodium (III) chloride hydrate were pre-mixed with the GNS support in the polyol solution before heating. We surmised that the formation of PtRh NWs in this work could be related to the synergistic role of GNS and PVP. To verify this, control experiments were conducted. Another two batches of PtRh samples were synthesized without adding the PVP surfactant or GNS support during the polyol-assisted reduction process. Instead of 1D NWs, 0D quasi-spherical PtRh NPs with an average size of 2.0 ± 0.5 nm are uniformly dispersed in the GNS support when PVP is not added (denoted as PtRh NP/GNS) as shown in Figure S1. Similarly, larger spherical PtRh NPs with an average size of 3.5 ± 0.6 nm are obtained in the absence of the GNS supprt (denoted as PtRh NP) as shown in Figure S2. The control experiments illustrate that both the GNS and PVP are dispensiable for the formation of PtRh NWs. PVP is known as a surfactant, which can serve as a structure-directing agent by selectively adsorb onto specific crystal facets and thus regulate the growth kinetics of crystals.39 Such a structure-directing role of PVP is also possibly embodied in this work for the formation


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of PtRh NWs. Nevertheless, it is worth noting that single PVP is not capable of directing the formation of PtRh NWs in the absence of the GNS support. The GNS utilized in this work was derived from chemical vapor deposition of methane and contained numerous edges and defects as reported in the authors’ previous work.40-44 It was demonstrated that these high-energy edges and defects could sever as effective heterogeneous sites for the nucleation of crystals.41-44, 48-50 As a result, the metal precursors can be reduced fast and simultaneously to produce a large number of nanoclusters (nuclei). The fast depletion of precursors makes the conventional growth mode of atomic addition impossible.47 Instead, owing to the high surface energy, the resulting primary NPs show a strong tendency to form an 1D structure via oriented attachment.39 In addition, the GNS support can also function as templates for the horizontal growth of PtRh NWs. Similar to the formation of Pt NWs in the surface of highly ordered pyrolytic graphite,47 the GNS is capable of guiding the attachment of PtRh NPs to form 1D NWs through slight adjustment of their crystallographic orientations. The role of GNS also is analogous to those of the single-layered Ni(OH)251 and graphene oxide nanosheets52 in the growth of ultrathin Pt and Au NWs, respectively. We also find that the formation of PtRh NWs is dependent on the ratio of metal precursors. We varied the ratio of the precursors in the feedstock. When a single H2PtCl6 aqueous solution or Rhodium (III) chloride hydrate was used, the resulting products of Pt100 and Rh100 are quasispherical NPs as shown in Figure S3a, d. Using a precursor ratio of 50:50, the resulting Pt50Rh50 product contains considerable PtRh NPs and a small quantity of 1D NWs (Figure S3b). Decreasing the ratio to 25:75, the obtained Pt25Rh75 product mainly consists of short nanorods and elongated NPs (Figure S3c). These results clearly illustrate that the formation 1D anisotropic PtRh NWs is composition-dependent, which is similar to the results reported by 9 ACS Paragon Plus Environment


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Yuan et al39 and Peng et al.53 This could be related to the variation in the reduction kinetics of metal precursors with different ratios.53 Electrochemical activity The electrochemical characterization of the catalysts was first conducted by cyclic voltammetry (CV) in 1 M H2SO4 with a scan rate of 50 mV s-1 as shown in Figure 4a. For the sake of comparison, a state-of-the-art Pt/C catalyst with an average size of 2.6 ± 0.6 nm (see Figure S4) was also tested in the identical conditions. It can be seen that all the catalysts exhibit the typical CV curves, consisting of hydrogen adsorption-desorption, oxidation-reduction of metallic surface and double-layer regions. Compared with the Pt/C, the PtRh-based catalysts exhibit a less-resolved broad peak for the hydrogen adsorption-desoprtion regions owing to the alloying of Pt with Rh.44 To study the electrochemical stability of the catalysts in 1 M H2SO4, the CV test was conducted for 2000 cycles. The resulting CV curves are selectively shown in Figure S5. It clearly illustrates some features including i) the hydrogen adsorption-desoprtion regions shrinks with increasing cycle numbers; ii) the onset potentials for the formation of Pt-O species continuously shift to high potentials and iii) the reduction peaks of these Pt-O species shift to positive potentials. These variations are closely related to the surface structures of the catalysts. The diminishment in the number of active sites in the catalysts is responsible for the shrink of the hydrogen adsorption-desoprtion regions while the disappearance of high-energy facets and sites such as defect, edge and dislocation atoms attributes to the shifts of the onset potentials of the formation of Pt-O species. On the basis of the hydrogen desorption charge, the ECSAs were calculated using the method reported in the authors’ previous work.41-44 The initial ECSAs are determined to be 63.5, 62, 46.1 and 48.7 m2 g-1 Pt, for the PtRh NP/GNS, Pt/C, PtRh NP and PtRh NW/GNS, respectively. To illustrate the variation in the ECSAs, the normalized ECSAs are


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plotted against cycle number as shown in Figure 4b. The ECSAs of the catalysts decrease with increasing cycle number except those of the Pt/C. The Pt/C catalyst shows a slight increase in ECSA after 200 cyclic tests, which was also noted by Liang et al.54 This is probably due to the activation process of the catalyst, which removed the impurities adsorb onto Pt NPs and facilitated the diffusion of reactants. After 2000 cyclic tests, the residue ECSAs of PtRh NP/GNS, Pt/C, PtRh NP and PtRh NW/GNS are 74, 63, 58 and 86% of the initial values, respectively. The largest sustained ECSA of the PtRh NW/GNS suggests the superior electrochemical stability in 1 M H2SO4. One prominent structural feature of the PtRh NW/GNS lies in the hierarchical assembly of 1D NWs to 2D nanosheets. The combination of the inherent structural merits of 1D PtRh NWs and their strong interaction with the GNS support can lead to low mobility of the NWs and therefore better resistance to migration and coalescence. To verify this, the catalysts were subjected to a heat treatment at 600ºC in N2 atmosphere (purity >99.999%) for 24 h. The ECSAs and morphology of the annealed samples are also characterized by CV and TEM, respectively, as shown in Figure 5. The ECSAs of the annealed catalysts are determined to be 23.3, 18.2, 10.1 and 34.9 m2 g-1 Pt, for the PtRh NP/GNS, Pt/C, PtRh NP and PtRh NW/GNS, corresponding to 36.6, 29.3, 21.9 and 71.7% of their initial values, respectively. The decreases in ECSAs are mainly attributed to the sintering of the metallic components and atomic migration. The TEM image (the inset in Figure 5a) indicates that the PtRh NWs stabilized by the GNS support well maintain the 1D morphology. In contrast, The sizes of metallic NPs in the annealed PtRh NP/GNS and Pt/C samples are significantly increased (Figure 5b, d), indicating the occurrence of sintering. Because of the absence of stabilization of carbon support, the sintering in the PtRh NP sample is more serious, which leads to severe aggregation of the catalyst (Figure 5c). These 11 ACS Paragon Plus Environment


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results clearly suggest that the PtRh NP/GNS possesses much better resistance against sintering in the heat treatment owing to the 1D anisotropic structure of the PtRh NWs and their interaction of the GNS support. The activity of the catalysts toward ethanol electro-oxidation was evaluated by CV in 1 M H2SO4 + 1 M ethanol solution. Shown in Figure 6a, the CV curves consist of typical well-define two peaks. The inset presents an enlarge view of the spot marked by a rectangle. It clearly illustrates the onset potentials of ethanol oxidation over the catalysts. The PtRh NW/GNS exhibits an onset potential of 0.2 V, which is much less than those of PtRh NP/GNS (0.32 V), PtRh NP (0.34 V) and Pt/C (0.39 V). For the sake of comparison, the current is also normalized by metal mass as shown in Figure 6b. It can be noted that the PtRh NP/GNS catalyst exhibits both largest specific and mass-normalized current densities. Figure 6c summarizes the specific and mass-normalized forward peak current densities of the catalysts. Notably, the PtRh NP/GNS exhibits a specific current density of 2.8 mA cm-2 and a mass-normalized current density of 1 A mg-1 metal, which are 5.4 and 3.1 times of those of the Pt/C catalyst, respectively. The stability of the catalysts was further evaluated by accelerated durability test. Figure S6 shows the CV curves of the selective cyclic numbers. The peak current densities of the forward scan are recorded as a function of cycle number as shown in Figure 6d. It reveals that the PtRh NP/GNS possesses the smallest decay rate among the four catalysts. After 2000 cycles, the PtRh NW/GNS maintains 86% current density, which is larger than those of PtRh NP/GNS (73%), PtRh NP (58%) and Pt/C (62%). The morphology of the spent catalysts is also characterized by TEM as shown in Figure S7. It reveals that the 1D anisotropic structure is well preserved in the PtRh NW/GNS. In contrast, the PtRh NP/GNS, PtRh NP and Pt/C catalysts show clear structural deteriorations such as segregation, detachment and coalescence. The activity of the catalysts was 12 ACS Paragon Plus Environment

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also evaluated by chronoamperometric tests as shown in Figure S8. It reveals that after 1200 s, the current densities of the catalysts follow the sequence of PtRh NW/GNS > PtRh NP/GNS > PtRh NP > Pt/C, which is consistent with the CV results. We believe that the combination of the advantageous 1D morphological motif with the beneficial structural properties of PtRh alloys and GNS support accounts for the superior performance of the PtRh NW/GNS catalyst. The 1D structural morphology together with the ultrathin diameters of PtRh NWs can be the major contributors to the outstanding activity and durability. The dominant active (111) facts exposed on PtRh NWs provide more active sites for the oxidation of ethanol.23 Meanwhile, the 1D structure of PtRh NWs results in better resistance to dissolution, migration and coalesces, thus greatly improving the durability of the catalyst. In addition, the anisotropic nanostructures of ultrathin PtRh NWs could induce a favorable downshift in the Pt d band because of size effects, which contributes to a weaker d−π* interaction with the adsorbed intermediates, thereby facilitating the oxidation of poisoning species at low potentials.32 It should be noted that the GNS support is also beneficial to the performance of the PtRh NW/GNS. The in-situ assembly of 1D PtRh NWs to 2D GNS support maximizes the contact and induces strong interaction, which not only enhances the electron transfer but also prevents PtRh NWs from aggregation, coalescence, and Ostwald ripening, thus leading to superior long-term catalytic stability. Conclusion We assembled ultrathin PtRh NWs to GNS support via a one-pot synthesis protocol. The growth of PtRh NWs follows the oriented attachment pathway, which involves the formation of primary PtRh NPs and subsequent growth to NWs. Our mechanistic studies reveal that the GNS support play a dispensable role in the formation of PtRh NWs by providing heterogeneous nucleation 13 ACS Paragon Plus Environment


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sites and regulating the kinetics of crystal growth. The as-prepared PtRh NW/GNS exhibits remarkable activity and durability toward the oxidation of ethanol. The superior catalytic performance is attributed to the synergistic effects of the two components, in which the 1D PtRh NWs function as active component to catalyze the oxidation of ethanol while the GNS support serves as an excellent support to stabilize the active metals as well as highway to transport electrons. This study demonstrates that the assembly of 1D anisotropic NWs to 2D nanosheets could result in novel architectures with remarkable properties. The strategy disclosed in this work could be possibly generalized to obtain other multifunctional materials by varying 1D active components and 2D supports. This method could provide a new route to synthesize highperformance electrocatalysts in fuel cells and batteries. ASSOCIATED CONTENT Supporting Information Morphological and structural characterizations of PtRh NP/GNS, PtRh NP and Pt/C catalysts, TEM images of catalysts with varying composition, cyclic voltammetric curves and TEM images of the spent catalysts. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected]. (Y. Shen) E-mail: [email protected]. (L. Wang) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The project was financially supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, Natural Science Foundation of Guangdong Province, China (2014A030310315), and Guangdong Research Center for Interfacial Engineering of Functional Materials, College of Materials Science and Engineering, Shenzhen University, Shenzhen (G1605) REFERENCE 1. Wang, Y.; Chen, K. S.; Mishler, J.; Cho S. C.; Adroher, X. C. A Review of Polymer Electrolyte Membrane Fuel Cells: Technology, Applications, and Needs on Fundamental Research. Appl. Energy 2011, 88, 981–1007 14 ACS Paragon Plus Environment

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Figures

Figure 1 (a-e) TEM images, (f, g) magnified views, and (h) EDX results of PtRh NW/GNS.



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Figure 2 (a) XRD pattern, (b) survey XPS spectrum, (c) Pt 4f XPS spectrum and (d) Rh 3d XPS spectrum of PtRh NW/GNS.


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Figure 3 Evolution of PtRh NWs as a function of reaction time (t), (a), t=2, (b), t= 5, (c) t= 10, and (d) t= 30 min.



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Figure 4 (a) CV curves of the catalysts in 1 M H2SO4, (b) normalized ECSA as a function of cycle number, CV of the catalysts in 1 M H2SO4 + 1 M C2H5OH.


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Figure 5 Comparison on the CV curves of the fresh and annealed catalysts after the heat treatment (HT) recorded in 1M H2SO4 with a scan rate of 50 mV s-1. (a) PtRh NW/GNS, (b) PtRh NP/GNS, (c) PtRh NP and (d) Pt/C catalysts. The insets in the graphs are the corresponding TEM images of the annealed catalysts.



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Figure 6 CV of the catalysts in 1 M H2SO4 + 1 M C2H5OH (a) normalized by ECSA (inset is the magnified view of the spot marked with a rectangular) and (b) normalized by metal mass, (c) comparison of peak current densities of the catalysts, and (d) normalized peak current density as a function of cycle number.


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TOC graph


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