Bimetallic PlatinumâRhodium Alloy Nanodendrites as Highly Active

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Bimetallic Platinum–Rhodium Alloy Nanodendrites as Highly Active Electrocatalyst for the Ethanol Oxidation Reaction Juan Bai, Xue Xiao, Yuan-Yuan Xue, Jia-Xing Jiang, Jing-Hui Zeng, Xifei Li, and Yu Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05422 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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ACS Applied Materials & Interfaces

Bimetallic Platinum–Rhodium Alloy Nanodendrites as Highly Active Electrocatalyst for the Ethanol Oxidation Reaction Juan Bai,a Xue Xiao,a Yuan-Yuan Xue,a Jia-Xing Jiang,a Jing-Hui Zeng,a Xi-Fei Lib and Yu Chen*a a Key Laboratory of Macromolecular Science of Shaanxi Province, Key Laboratory of Applied Surface and Colloid Chemistry (MOE), Shaanxi Key Laboratory for Advanced Energy Devices, School of Materials Science and Engineering, Shaanxi Normal University, Xi'an 710062, PR China. b Institute of Advanced Electrochemical Energy, Xi’an University of Technology, Xi’an 710048, PR China.

ABSTRACT: Rationally designing and manipulating composition and morphology of precious metal-based bimetallic nanostructures can markedly enhance their electrocatalytic performance, including selectivity, activity, and durability. We herein report the synthesis of bimetallic PtRh alloy nanodendrites (ANDs) with tunable composition by a facile complex-reduction synthetic method under hydrothermal conditions. The structural/morphologic features, formation mechanism, and electrocatalytic performance of PtRh ANDs are investigated thoroughly by various physical characterization and electrochemical methods. The preformed Rh crystal nuclei effectively catalyze the reduction of Pt2+ precursor, resulting in PtRh alloy generation due to the catalytic growth and atoms interdiffusion process. The Pt atoms deposition distinctly interferes in Rh atoms deposition on Rh crystal nuclei, resulting in dendritic morphology of PtRh ANDs. For the ethanol oxidation reaction (EOR), PtRh ANDs display the chemical composition and solution pH co-dependent electrocatalytic activity. Because of the alloy effect and particular morphologic feature, Pt1Rh1 ANDs with optimized composition exhibit better reactivity and stability for the EOR than commercial Pt nanocrystals electrocatalyst. KEYWORDS: fuel cells, PtRh alloy, nanodendrites, ethanol oxidation reaction, electrocatalytic performance

1. INTRODUCTION Clean energies have attracted comprehensive attention during the past decades due to increasing of fossil fuel consumption and environmental pollution. At present, fuel cells have been extensively regarded as an environmental-friendly energy conversion equipment for the portable electronics, automobiles and stationary applications.1 Among various fuel cells, direct alcohol fuel cells are achieving increasing interest due to the convenient storage and transportation of alcohol.2 As a fuel, ethanol has higher specific energy densities, lower permeability in proton exchange membrane, and less toxic than methanol, making it a promising candidate in direct alcohol fuel cells.3-5 So far, Pt nanoparticles are commonly used as anodic electrocatalyst for the ethanol oxidation reaction (EOR).6-7 Unfortunately, Pt-catalyzed EOR is usually incomplete because of the hardness of C-C bond cleavage, resulting in a number of byproducts such as acetaldehyde and acetic acid.6-7 Meanwhile, monometallic Pt electrocatalysts suffer from the surface deactivation due to the poisoning by reaction intermediates, especially COads.6-7 Thus, developing electrocatalysts that promote the full oxidation of ethanol molecule and improve poison tolerance is crucial for the commercial promotion of direct ethanol fuel cells (DEFCs). Over the past 30 years, large efforts are underway to synthesize the highly active EOR electrocatalysts for DEFCs application. So far, various bimetallic Pt-based nanocrystals (including PtRh, PtNi, PtCu, PtAu, and PtPd, etc.) have been synthesized by great diversity of methods (including seed growth, co-chemical

reduction, hard-templating synthesis, and thermal decomposition, etc.), which generally exhibit higher electrocatalytic activity than monometallic Pt nanocrystals due to the synergistic effect between two metal atoms, including electronic effect, bi-functional mechanism, and lattice strain effect.8-23. In particular, the introduction of Rh can effectively break C-C bond at low overpotential, which enhance the reaction pathway of ethanol to CO2.8-13 The electrocatalytic performance of precious metal nanostructures depends on their chemical composition and morphology. Compared with conventional nanocrystals, nanodendrites with branched structure generally show better activity and durability for various electrochemical reactions due to their big real surface area, numerous defect atoms with high energy, fast mass transfer, and self-supported interconnected structure.24-43 For example, PtFe nanodendrites with high-index facets show higher reactivity for the oxygen reduction than PtFe nanospheres and PtFe nanocubes.24 Bimetallic Au@Pt nanocrystals on graphene show better reactivity and durability for the methanol electrooxidation than Pt nanocrystals on graphene.25 Accordingly, the rational design of bimetallic Pt-based nanostructures with controlled morphology is no longer an artwork but it became a science issue. Herein, we used a simple one-pot reduction strategy to synthesize bimetallic PtRh alloy nanodendrites (PtRh ANDs) with different Pt/Rh molar ratio. During the synthesis, the preformed Rh crystal nuclei effectively catalyzed the reduction of Pt2+ precursor, which resulted in the alloy perporty and dendritic morphology of PtRh ANDs. For the EOR, bimetallic PtRh ANDs

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3.1. Characterization of Bimetallic Pt1Rh1 ANDs. The chemical composition of Pt1Rh1 ANDs was characterized by ICP-AES, showing a Pt/Rh molar ratio of 48/52. The Pt/Rh molar ratio is very close to the initial feed ratio and the result from EDX analysis (Figure 1A). To explore the structure of Pt1Rh1 ANDs, PXRD measurement was performed. PXRD pattern shows the four diffraction peaks at 2θ = 40.2, 47.2, 68.6, 82.6 (Figure 1B), suggesting a face-centered cubic (fcc) structure. Four diffraction peaks of Pt1Rh1 ANDs lie on between monometallic Pt (JCPDS 04-0802) and monometallic Rh (JCPDS 05-0685), which is indicative of PtRh alloy generation. Based on Vegard's law,46 the alloying Rh content in Pt1Rh1 ANDs is calculated to be 35%, whereas EDX and ICP-AES data show that Pt/Rh molar ratio of Pt1Rh1 ANDs is 48:52. This fact indicates that Pt1Rh1 ANDs are not ordered solid solution alloy. Using Debye–Scherrer formula, the crystalline size of Pt1Rh1 ANDs is estimated to be 3.4 nm. The surface composition and chemical state of Pt1Rh1 ANDs were analyzed by XPS. XPS measurement shows the Pt/Rh molar ratio is 51:49 (Figure S1), which is consistent with EDX and ACP-IES results. The similarity between bulk composition and surface composition confirms the formation of PtRh alloy, again.47-48 The Pt 4f XPS analysis reveal the amount of Pt(0) and Pt(II) species are ca.80.1% and 19.9%, respectively (Figure 1C). The existence of Pt(II) species originates from air oxidation. Due to the interference of Pt 4d to Rh 3d3/2 signal, the Rh 3d5/2 XPS peak is used to analyze the chemical state. The percentage of Rh(0) species is calculated to be 77.3% (Figure 1D), indicating the metallic Rh0 is another predominant species in Pt1Rh1 ANDs.49

displayed composition and solution pH co-dependant electrocatalytic activity. Compared with commercial Pt nanocrystals electrocatalyst, Pt1Rh1 ANDs with optimized composition showed improved reactivity and durability for the EOR.

2. EXPERIMENTAL SECTION 2.1. Reagents and Chemicals. Potassium hydroxide (KOH), formaldehyde (HCHO, 40%) solution, potassium tetrachloroplatinate(II) (K2PtCl4), and rhodium(III) chloride hydrate (RhCl3·3H2O) were supplied from Aladdin Industrial Co. Polyallylamine hydrochloride (Mw = 5000) was obtained from Nitto Boseki Co., Ltd. Commercial Pt nanocrystals electrocatalyst was obtained from Johnson Matthey Corporation. 2.2. Synthesis of Bimetallic PtRh Alloy Nanodendrites (PtRh ANDs). Typically, 20.9 mg RhCl3 and 41.5 mg K2PtCl4 were added into 8 mL of 0.25 M polyallylamine solution. After adjusting solution pH to 5 with KOH solution, 4.0 mL of HCHO solution as reducing agent was added to the mixture. The mixed solution was heated to 80 °C without stirring. After 7 h, bimetallic Pt1Rh1 ANDs were collected by centrifugation 10 min at 15000 rpm. Pt1Rh1 ANDs were dispersed in 30 mL of 1 M CH3COOH aqueous solution and stirred for 10 h,44 and then centrifugated and dried at 50 oC for 15 h in a vacuum dryer. Using similar synthetic method, Rh3Pt1 and Rh1Pt3 ANDs could be also obtained. 2.3. Electrochemical Characterization. All electrochemical tests were carried on CHI 760 D electrochemical workstation with three-electrode system at 30 ± 1 oC. The electrocatalyst modified glassy carbon (GC) electrode with the diameter of 3 mm was used as the working electrode, a saturated calomel electrode was used as the reference electrode, and a carbon rod was used as the counter electrode. The electrolyte is N2-saturated 0.5 M H2SO4 solution (pH = 0.3) or 1 M KOH solution (pH =13.5). The electrocatalyst suspension was obtained by mixing 4 mg electrocatalyst sample, 2 mL water, and 10 µL of 5 wt % Nafion. Then, 4 µL of electrocatalyst suspension was dropped on the GC electrode surface and allowed to dry at room temperature. All electrode potentials in this work were quoted versus the reversible hydrogen electrode (RHE). 2.4. Instruments. Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, energy-dispersive X-ray (EDX) maps, and selected area electron diffraction (SAED) patterns were obtained from a TECNAI G2 F20 transmission electron microscopy. The composition and structure of sample were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES, M90), X-ray photoelectron spectroscopy (XPS, AXIS ULTRA spectrometer) and power X-Ray diffraction (PXRD, DX-2700 X-ray diffractometer) spectroscopy. The crystalline size of samples was calculated by Debye–Scherrer equation (i.e., D = kλ/ βcosθ). Where D was the average size of nanoparticles, k was the constant (0.89), λ was the wavelength of the X-ray source, β was the half peak width of the (111) diffraction peak, and θ was the diffraction angle.45

Figure 1. (A) EDX, (B) PXRD, (C) Pt 4f XPS, and (D) Rh3d XPS spectra of Pt1Rh1 ANDs. The size and morphology of Pt1Rh1 ANDs were investigated by TEM (Figure 2A-B). Low-resolution TEM image shows Pt1Rh1 ANDs are well-dispersed (Figure 2A), which have the mean particle size of 30 nm (Insert in Figure 2A). Middle-magnification TEM image shows Pt1Rh1 ANDs have dendritic-like structure, which are composed of subunits with diameters of ca. 1.8 nm. SAED pattern reveals obvious diffraction rings (Fig. 2C), showing its polycrystalline property. The surface structure of

3. RESULTS AND DISCUSSION 2


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Pt1Rh1 ANDs was investigated by HRTEM images (Figure 2D and E). The lattice spacing distance of ca. 0.223 nm and 0.224 nm are clearly observed at the surface of Pt1Rh1 ANDs, which is between the interplanar spacing value (0.2265 nm) of Pt (111) plane and that (0.2196 nm) of Rh (111) plane. To visualize the chemical composition of Pt1Rh1 ANDs, HAADF-STEM-EDX elemental mapping measurement was performed (Figure 2F). Both EDX mapping patterns and line scanning profiles reveal the complete overlapping of Rh and Pt elements, which is also a strong evidence for the generation of PtRh alloy.

can be confirmed by linear sweep voltammetry measurements (Figure S5). As observed, the onset reduction potential (−0.054 V) of polyallylamine-Pt2+ complex is lower than that (0.046 V) of polyallylamine-Rh3+ complex, resulting in the easier reduction of polyallylamine-Rh3+ complex than polyallylamine-Pt2+ complex. Thus, Rh3+ precursor can be preferentially reduced to generate Rh crystal nuclei during Pt1Rh1 ANDs synthesis. Subsequently, the reduction of Pt2+ precursor is accelerated by Rh crystal nuclei due to the autocatalytic growth mechanism.48, 57 The newly generated Pt atoms deposit on Rh surface and then uniformly mix with performed Rh atoms via an interdiffusion process, accompanying the simultaneous reduction of Rh3+ precursor, and PtRh alloy generates ultimately. Scheme 1. The reaction precursors dependent morphological evolution.

Figure 2. (A) low-resolution TEM image and (B) middle-magnification TEM image. (C) SAED pattern of Pt1Rh1 ANDs. (D and E) Enlarged HRTEM image of Pt1Rh1 ANDs. (F) HAADF-STEM-EDX mapping images of Pt1Rh1 ANDs. Insert: the line scanning profiles on the single Pt1Rh1 nanodendrite.

3.2. The Formation Mechanism of Bimetallic Pt1Rh1 ANDs. Polyallylamine, a polyelectrolyte with abundant primary amine groups, has good hydrophilic property and excellent coordination ability for metal ions, which can effectively serve as complexant and surfactant to synthesize various precious metal nanostructures, such as Pt nanocubes, Rh nanosheets, and Pd tetrahedrons, etc.50-54 According to thermodynamic principle, the ions/molecules with high electrode potential will be preferentially reduced during redox reaction. The electrode potential of RhCl3/Rh pairs (φ=0.76 V)55 is smaller than that of PtCl42-/Pt pairs (φ= 1.18 V).56 Under the present expermential conditions (i.e., 80 oC and 7 h), the single-component RhCl3 precursor in polyallylamine solution can be reduced to generate ultrathin Rh nanosheet nanoassemblies (Figure S2). However, the single-component PtCl42- precursor in polyallylamine solution cannot be reduced (Figure S3). Until evaluating reaction temperature to 120 °C, the single-component PtCl42- precursor in polyallylamine solution can be reduced to generate Pt nanocubes (Figure S4). Our previous works have demonstrated that polyallylamine molecule can easily coordinate to Rh3+ and Pt2+ species to generate polyallylamine-Rh3+ and polyallylamine-Pt2+ complexes due to its intrinsic coordination capability.50-53 Thus, we conjecture that polyallylamine molecules have stronger coordination capability to Pt2+ species than Rh3+ species, which

Figure 3. (A) TEM images of intermediates at (a) 30 min, (b) 1 h, (c) 4 h, and (d) 7 h. (B) Schematic illustration on the formation process of Pt1Rh1 ANDs. As shown in Scheme 1, the change of reaction precursors results in the morphological diversity of products. Considering the preferential reduction of Rh3+ precursor and autocatalytic growth mechanism of Pt on preformed Rh crystal nuclei, we can infer that the introduction of Pt2+ precursor interfere in the deposition of Rh atom on preformed Rh crystal nuclei, resulting in the disappearance of ultrathin nanosheets and the generation of nanodendrites. For the formation of nanodendrites, the fast aggregation-based growth, selective oxidation etching, and slow 3



kinetic epitaxial overgrowth are three kinds of main mechanisms.58-62 In our synthesis, Pt1Rh1 ANDs are obtained at 7 h. Consequently, the, fast aggregation-based growth process is definitively excluded. Furthermore, the selective oxidation etching mechanism can also be ruled out because no transition metal ions or oxidation etching agent exist in reaction system. To understand the generation mechanism of PtRh nanodendrites, we monitor the morphologies of reaction intermediates by TEM. At 30 min, a large number of tiny nanocrystals generate (Figure 3Aa). EDX result shows the Rh/Pt molar ratio is 85:15 (Figure S6), which is much higher than that (52:48) of final Pt1Rh1 ANDs. This fact further confirms that Rh3+ precursor is preferentially reduced during Pt1Rh1 ANDs synthesis. At 1 h, both tiny nanocrystals and nanocrystals with dendritic structure are observed (Figure 3Ab), indicating branches began to grow from the tiny nanocrystals. At 4 h, the nanodendrites with obvious branched structure (∼13 nm) generate (Figure 3Ac). At 7 h, the diameter of nanodendrites increases to ∼30 nm (Figure 3Ad). Further increasing reaction time, the morphology and size of nanodendrites retain constant (Figure S7), indicating the complete reduction of Pt2+ and Rh3+ precursors with 7 h. The time-dependent morphology evolution gives an experimental evidence on how PtRh nanodendrites formed from the tiny nanocrystals. As a result, we reasonably deduced that the generation of Pt1Rh1 ANDs originates from the kinetic epitaxial overgrowth mechanism, as shown in Figure 3B. 3.3. EOR Activity of Bimetallic PtRh ANDs. Considering that the activity of electrocatalysts evidently relate to their chemical composition, bimetallic Pt3Rh1 and Pt1Rh3 ANDs were synthesized under same conditions. TEM images show the sizes and morphologies of Pt3Rh1 and Pt1Rh3 ANDs are very similar to Pt1Rh1 ANDs (Figure S8). The EOR performance of PtRh ANDs with different chemical composition were characterized by cyclic voltammetry (CV) measurements in an acidic medium. Herein, the current is normalized with metal mass to obtain the mass activity of PtRh ANDs for the EOR. As observed, Pt1Rh1 ANDs show biggest EOR peak current at ca. 0.88 V among three PtRh ANDs (Figure 4A), showing the best EOR activity. Meanwhile, EOR peak current at Pt1Rh1 ANDs is also bigger than that at Pt nanocubes (Figure S9), which in turn indicates the introduction of Rh enhance EOR kinetics. The EOR activities of PtRh ANDs were further evaluated by chronoamperometry tests. At 0.88 V potential, the EOR current at Pt1Rh1 ANDs is much bigger than those at Pt3Rh1 ANDs and Pt1Rh3 ANDs, also showing the chemical composition dependent EOR activity (Figure 4B). Mainly, the introduction of Rh can effectively break C-C bond and enhance the reaction pathway of ethanol to CO2,8-13 resulting in high activity of Pt1Rh1 ANDs. Meanwhile, we observed that Rh nanocrystals have very low activity for the EOR (Figure S10). After increasing Rh content to Pt1Rh3, Pt active sites decrease, resulting in low activity of Pt1Rh3 ANDs. Under the alkaline conditions, CV measurements demonstrate that PtRh ANDs still show the chemical composition dependant EOR activity (Figure S11). Specifically, Pt1Rh1 ANDs exhibit the best EOR activity, similar to the results in an acid medium. In additional, PtRh ANDs also show the solution pH dependant EOR activity (Figure S12). For instance, the EOR peak current at Pt1Rh1 ANDs in KOH

electrolyte is 2 times higher than that in H2SO4 electrolyte. Meanwhile, the EOR peak potential at Pt1Rh1 ANDs in KOH electrolyte negatively shift ca. 230 mV compared to that in in H2SO4 electrolyte. The lower EOR peak potential and higher EOR peak current indicate that the EOR at Pt1Rh1 ANDs in alkaline solution has faster reaction kinetics than that in acidic solution.

Figure 4. (A) Metal mass normalized CV curves of Pt3Rh1, Pt1Rh1 and Pt1Rh3 ANDs in N2-saturated 0.5 M H2SO4 electrolyte containing 0.5 M CH3CH2OH at 50 mV·s−1. (B) Metal mass normalized chronoamperometric curves of Pt3Rh1, Pt1Rh1, and Pt1Rh3 ANDs in N2-saturated 0.5 M H2SO4 electrolyte containing 0.5 M CH3CH2OH at 0.88 V potential. To evaluate the potential application of Pt1Rh1 ANDs, we compared their electrochemical performance with commercial Pt nanocrystals electrocatalyst. Initially, the electrochemical properties of Rh1Pt1 ANDs and Pt nanocrystals electrocatalyst were evaluated by CV in N2-saturated 1 M KOH electrolyte. CV curves reveal that the onset potential of metal oxide formation at Rh1Pt1 ANDs negatively shifts by ca. 144 mV relative to Pt nanocrystals electrocatalyst (Figure 5A). The electrode potential of Rh3+/Rh pairs (φ=0.76 V) 55 is smaller than that of PtCl2+/Pt pairs (φ= 1.18 V).56 Thus, the introduction of Rh is responsible for the low onset oxidation potential of Pt1Rh1 ANDs. Additionally, CV curves show that Pt1Rh1 ANDs have bigger hydrogen adsorption/desorption peak area than Pt nanocrystals electrocatalyst (Figure 5A), suggesting Pt1Rh1 ANDs have a bigger electrochemically active surface area (ECSA). Generally, the ECSA of electrocatalysts relates to their real surface area. In principle, the smaller particle size generally results in bigger real surface area.63 Indeed, PXRD measurements show the crystalline size of PtRh nanocrystal subunits at Pt1Rh1 ANDs is much smaller than that of Pt nanocrystals electrocatalyst (3.4 vs. 8.8 nm, Figure S13). Thus, the small crystalline size of PtRh nanocrystal subunits is responsible for high ECSA of Pt1Rh1 ANDs. In order to exactly measure ECSA value, CV measurements are further performed in acidic solution (Figure S14). According to the charge of hydrogen desorption peak in the range of 0 to 0.4 V, ECSA values of Rh1Pt1 ANDs and Pt nanocrystals electrocatalyst are measured to be 66.3 and 16.5 m2 gmetal-1, respectively. ECSA values of electrocatalysts were also measured by CO-stripping CV test (Figure 5B). According the charge of CO oxidation peak, ECSA values of Pt1Rh1 ANDs and Pt nanocrystals electrocatalyst are measured to be 64.1 and 15.8 m2 gmetal-1, respectively. Thus, both measurement methods show the ECSA value of Pt1Rh1 ANDs is ∼4-fold bigger than that of Pt nanocrystals electrocatalyst, which can be attributed to tiny particle size and distinctive 3D porous architecture of Pt1Rh1 ANDs. Additionally, it is observed that Rh1Pt1 ANDs exhibit a obvious negative shifts in both onset oxidation potential and peak potential for the COad oxidation 4


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reaction relative to Pt nanocrystals electrocatalyst, suggesting that Rh1Pt1 ANDs have weak CO affinity. As shown in Figure 6A, the introduction of Rh results in low onset oxidation potential of Rh1Pt1 ANDs. This fact indicates that Pt1Rh1 ANDs easily produce the OHads at lower electrode potential than Pt nanocrystals electrocatalyst, which accelerates the COads electrooxidation due to the bi-functional mechanism.

superior activity than that at Pt nanocrystals. Meanwhile, EOR current at Pt nanocrystals electrocatalyst disappears completely at 320 s. In contrast, EOR current at Pt1Rh1 ANDs preserve 20.1 A g-1 at 2000 s, indicating Pt1Rh1 ANDs have superior durability for the EOR. The COads stripping CV tests have demonstrated that Pt1Rh1 ANDs have better COad tolerance than Pt nanocrystals electrocatalyst, which contributes to high electrocatalytic stability of Pt1Rh1 ANDs for the EOR. Although the crystalline size of Pt1Rh1 ANDs is much samller than that of Pt nanocrystals electrocatalyst, the self-stability of Pt1Rh1 ANDs is comparable to that of Pt nanocrystals electrocatalyst. After 1000 CV cycles in 1M KOH electrolyte, Pt1Rh1 ANDs retain 86.7% of the initial ECSA value (Figure 6D), which is bigger than Pt nanocrystals electrocatalyst (42.4%, Figure S15). After continuous CV cycling, TEM image shows the morphology and chemical composition of Pt1Rh1 ANDs are maintained well (Figure S16). Mainly, the extreme chemical inertness of Rh 65 and the interconnected structure of nanodendrites and can effectively suppress the dissolution of Rh and Ostwald ripening of Pt1Rh1 ANDs, respectively. Accordingly, the good self-stability of Pt1Rh1 ANDs is also responsible for high electrocatalytic stability of Pt1Rh1 ANDs for the EOR.

Figure 5. (A) Metal mass normalized CV curves of Pt1Rh1 ANDs and commercial Pt nanocrystals electrocatalyst in N2-saturated 1 M KOH electrolyte at 50 mV·s−1. (B) Pre-absorbed COads stripping CV curves of Pt1Rh1 ANDs and commercial Pt nanocrystals electrocatalyst in N2-saturated 1 M KOH electrolyte at 50 mV·s−1. The electrocatalytic performance of Pt1Rh1 ANDs and Pt nanocrystals electrocatalyst for the EOR were evaluated by CV (Figure 6A). Metal mass normalized CV curves show the EOR peak potential at Pt1Rh1 ANDs negatively shifts ca. 80 mV relative to Pt nanocrystals electrocatalyst. At 0.65 V potential, the EOR current of Pt1Rh1 ANDs is 462.1 A·g–1, which is 8 times higher than that at Pt nanocrystals electrocatalyst (57.5 A·g–1). So, the lower EOR onset potential and bigger EOR current reflect that Pt1Rh1 ANDs show better electrocatalytic performance for the EOR than Pt nanocrystals electrocatalyst. After converting metal mass normalized CV curves in ECSAPt-normalized CV curves (Figure 6B), the EOR current density at Pt1Rh1 ANDs is still greater than that at Pt nanocrystals. In particular, at 0.65 V potential, EOR current density value at Pt1Rh1 ANDs is also bigger than that at previous reported Pt-based electrocatalysts (Table S1), also suggesting high activity of Pt1Rh1 ANDs. Thus, high mass activity of Pt1Rh1 ANDs for the EOR originates from their big ECSA value and intrinsic activity. In CV curve of the EOR, the ratio of the positive scan oxidation peak current (If) to negative scan oxidation peak current (Ib), If/Ib, is generally used to estimate the anti-poisoning ability of electrocatalyst. As observed, the If/Ib value (5.86) at Pt1Rh1 ANDs is ∼4.76-fold bigger than that (1.23) at Pt nanocrystals electrocatalyst, revealing the superior anti-poisoning ability of Pt1Rh1 ANDs during the EOR.21, 64 The previous investigation have proved that the introduction of Rh can efficiently break C-C bond, which results in high CO2 selectivity.8-13 Meanwhile, the introduction of Rh decreases the CO affinity at Pt1Rh1 ANDs surface (Figure 5). These two factors may be responsible for excellent anti-poisoning ability of Pt1Rh1 ANDs.

Figure 6. (A) Metal mass normalized CV curves of Pt1Rh1 ANDs and commercial Pt nanocrystals electrocatalyst in N2-saturated 1 M KOH electrolyte containing 1 M CH3CH2OH at 50 mV·s−1. (B) ECSAPt-normalized CV curves of Pt1Rh1 ANDs and commercial Pt nanocrystals electrocatalyst in N2-saturated 1 M KOH electrolyte containing 1 M CH3CH2OH at 50 mV·s−1. (C) Chronoamperometric curves of Pt1Rh1 ANDs and commercial Pt nanocrystals electrocatalyst in N2-saturated 1 M KOH electrolyte containing 1 M CH3CH2OH at 0.69 V potential. (D) CV curves of Pt1Rh1 ANDs in N2-saturated 1 M KOH electrolyte before and after repeating CV scans in N2-saturated 1 M KOH electrolyte at 50 mV·s−1.

4. CONCLUSIONS

The activity and durability of Pt1Rh1 ANDs and Pt nanocrystals for the EOR were further evaluated by chronoamperometry tests at 0.6 V potential (Figure 6C). Over the whole time range, EOR current at Pt1Rh1 ANDs is higher than that at Pt nanocrystals electrocatalyst, further confirming that Pt1Rh1 ANDs have

In summary, 3D bimetallic PtRh alloy nanodendrites with tunable composition were synthesized successfully via a facile complex-reduction method. During the synthesis, the difference in the coordination interaction strength between polyallylamine and different metal precursors changed the original reduction order of 5



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metal precursors. During the growth of PtRh ANDs, the preformed Rh crystal nuclei effectively catalyzed the reduction of Pt2+ precursor, which played a vital role for the alloy formation and dendritic morphology. Considering the difference in reduction rate of different metal precursors was generally real during the synthesis of bimetallic nanostructures, the catalytic growth mechanism could be used to explain the alloy formation when certain metal ion couldn't be reduced solely under the same experimental conditions. When as-prepared PtRh ANDs with tunable chemical compositions were explored as the EOR electrocatalysts in acidic or alkaline solutions, which revealed the composition and solution pH co-dependant EOR activity. Compared with commercial Pt nanocrystals electrocatalyst, Pt1Rh1 ANDs displayed improved activity and durability due to the composition effect and morphological effect. Thus, the super activity and durability of Pt1Rh1 ANDs makes it a highly promising anodic EOR elecctrocatalyst in DEFCs.

ASSOCIATED CONTENT Supporting information Experimental section and characterization details are available in Supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. Additional information for TEM and SEM images of ultrathin Rh nanosheet nanoassemblies. LSV curves of polyallylamine + K2PtCl4 and polyallylamine + RhCl3 mixture at the glassy carbon electrode in 0.1 M KCl electrolyte. TEM images of Pt3Rh1, Pt1Rh3, reaction intermediates of Pt1Rh1 ANDs collected at 9 h and Pt1Rh1 ANDs after continuous CV cycling. Metal mass normalized CV curves of Pt3Rh1, Pt1Rh1, and Pt1Rh3 ANDs in N2-saturated 1 M KOH electrolyte containing 0.5 M CH3CH2OH at 50 mV·s−1. PXRD patterns of Pt1Rh1 ANDs and commercial Pt nanocrystals electrocatalyst. XPS survey spectrum of the Pt1Rh1 ANDs.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] (Y. Chen)

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was supported by the Fundamental Research Funds for the Central Universities (GK201602002 and GK201701007), and the 111 Project (B14041).

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