Au Nanowires@Pd-polyethyleneimine Nanohybrids as Highly Active

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Au Nanowires@Pd-polyethyleneimine Nanohybrids as Highly Active and Methanol-tolerant Electrocatalysts towards Oxygen Reduction Reaction in Alkaline Media Qi Xue, Juan Bai, Congcong Han, Pei Chen, Jia-Xing Jiang, and Yu Chen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03447 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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Au Nanowires@Pd-Polyethyleneimine Nanohybrids as Highly Active and Methanol-Tolerant Electrocatalysts towards Oxygen Reduction Reaction in Alkaline Media Qi Xue, Juan Bai, Congcong Han, Pei Chen, Jia-Xing Jiang, and Yu Chen* Key Laboratory of Macromolecular Science of Shaanxi Province, Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), Shaanxi Key Laboratory for Advanced Energy Devices, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, PR China. ABSTRACT: Optimizing the shape, structure, and interface of noble metal bimetallic nanostructures significantly improve their catalytic properties. Recently, Au@Pd core-shell bimetallic nanostructures have attracted considerable interest for fuel cell applications. Regrettably, most of the research mainly focus on the morphological and structural control whereas interfacial control is neglected. In this work, polyethyleneimine (PEI) functionalized Au nanowires@Pd core-shell bimetallic nanohybrids (Au-NWs@Pd@PEI) are synthesized by chemical reduction method using Au nanowires (Au-NWs) as seeds in the presence of PEI, and demonstrate their application towards oxygen reduction reaction (ORR) in an alkaline solution. Electrochemical results display that Au-NWs@Pd@PEI with optimal component show enhanced ORR activity, selectivity, and durability compared to Pt/C electrocatalyst. Physical characterizations demonstrate that the electronic property of Pd shell mainly is affected by PEI polymer rather than Au-NWs core. Relative to PEI functionalized Pd nanowires without Au core, ORR activity enhancement of Au-NWs@Pd@PEI is exclusively ascribed to the surface strain effect. Meanwhile, loosepacked PEI polymer on Pd surface can work as barrier nanosieves to exclusively prevent the access of methanol to Pd sites, which imparts Au-NWs@Pd@PEI with enhanced methanol tolerance towards ORR in an alkaline solution. KEYWORDS: alkaline direct methanol fuel cells, inorganic-organic nanohybrids, core-shell structure, oxygen reduction reaction, surface strain

1.

INTRODUCTION

Oxygen reduction reaction (ORR) is an extremely pivotal half-reaction in metal-air batteries and fuel cells.1-6 At present, bi-/tri-metallic Pt-based nanostructured materials are durable and active cathode electrocatalysts towards ORR.6-12 However, limited supply and high cost seriously restrict their wide commercial applications. Thus, various low-cost non-Pt nanostructured materials are widely explored to work as Pt-alternative cathode electrocatalysts. Although transition metal based nanostructures and nanostructured carbon materials reveal high reactivity towards ORR in an alkaline solution, they lack sufficient stability during the fuel cells operation due to their irreversible oxidation.13 Among various non-Pt electrocatalysts, Pd-based nanostructured materials are promising Pt-alternative cathode ORR electrocatalysts due to their comparable activity to Pt in an alkaline solution.14-18 Generally, the chemical component, shape, and interface property of noble metal based nanostructured materials strongly affect their electrocatalytic performance.19-25 For example, Yang group reported bimetallic Au@Pd core-shell nanospheres displayed enhanced ORR activity relative to their

monometallic counterparts due to the geometric and electronic effect.26 Meanwhile, Au element could effectively inhibit the electrochemical dissolution of Pd, which imparted spherical Au@Pd core-shell nanospheres with excellent durability towards ORR.26 Wang and Lou groups reported three-dimensional (3D) Pt nanostructures consisting of one-dimensional (1D) Pt nanowires revealed improved ORR activity compared to zero-dimensional (0D) Pt nanocrystals due to the fast electron communication and mass transport.27 Meanwhile, Pt nanoassemblies with 3D interconnected structure displayed a better resistance to Ostwald ripening, dissolution, and aggregation compared to Pt nanocrystals, resulting in high ORR durability.27 In our previous works, we demonstrated polyallylamine-functionalized Pt nanostructures had excellent ORR activity in acidic media due to electronic effect driven from strong interaction between polyallylamine and Pt nanostructures.28 Meanwhile, loosepacked polyallylamine layers on Pt nanodendrites worked as "molecular window screen" to physically impeded the accessibility of methanol on Pt surface, imparting Pt nanodendrites with especial methanol tolerance.28

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Similar to Pt-based nanostructured materials, Pd-based nanostructured materials also have high electrocatlytic activity towards methanol oxidation reaction (MOR) in an alkaline solution.29, 30 Thus, Pd-based nanostructured materials also easily suffer from CO intermediates poisoning when them are applied as cathode electrocatalysts in alkaline direct methanol fuel cells, originating from inevitable methanol crossover phenomenon. As a result, the design and preparation of Pd-based nanostructured materials with excellent ORR activity and especial methanol tolerance are important subjects for the practical application of alkaline direct methanol fuel cells. Herein, we successfully synthesized polyethyleneimine (PEI, Scheme S1) functionalized Au nanowires@Pd core-shell bimetallic nanohybrids (AuNWs@Pd@PEI) by a simple seeded growth method using Au nanowires (Au-NWs) as seeds in the presence of PEI and hydrazine hydrate. Au-NWs@Pd@PEI exhibited enhanced ORR activity, durability as well as methanol tolerance in an alkaline solution. 2.

EXPERIMENTAL SECTION

2.1 Reagents and Chemicals. Palladium chloride (PdCl2), chloroauric acid (HAuCl4), α-naphthol (C10H8O), and hydrazine hydrate (N2H4·H2O, 85%) purchased from Sinopharm Chemical Reagent Co., Ltd. PEI (Scheme S1, MW=20 000) was supplied by Aladdin Co. Commercial Pt black was supplied by Johnson Matthey Corporation. 2.2 Synthesis of Au-NWs@Pd@PEI. Au-NWs were prepared by a chemical reduction method.31 Typically, AuNWs were obtained by adding 5 mL of 0.1 M α-naphthol ethanol solution into 5 mL of 0.01 M HAuCl4 aqueous solution for 5 min. For the synthesis of [email protected]@PEI, 1 mL of 0.5 M PEI solution and 0.1 mL of 0.05 M PdCl2 solution were added in 1 mL of 9.85 mg mL-1 Au-NWs solution. Then, 0.1 mL of N2H4·H2O solution was added in the mixture solution (pH = 6) to reduce the PdII precursor at 60 °C under the stirring conditions. After 2 h, [email protected]@PEI were obtained by centrifugation and washing with water. For Au-NWs@Pd@PEI, we try to adjust the Pd shell thickness by changing the volume of PdCl2 solution. When volumes of 0.05 M PdCl2 solution are 0.05, 0.1, 0.25, and 0.5 mL, the obtained Au-NWs@Pd@PEI were named as [email protected]@PEI, [email protected]@PEI, [email protected]@PEI, and [email protected]@PEI, respectively. 2.3 Electrochemical Instruments. All electrochemical tests were performed at 30 ± 1 oC using CHI 760D workstation with Gamry RDE710 rotating disk electrode. A carbon rod and saturated calomel electrode were used as the counter electrode and reference electrode, respectively. A glassy carbon electrode (GCE) modified by electrocatalyst was used as working electrode, which was achieved through a conventional thin-film method.32 Briefly, 5 mg of electrocatalyst was dispersed in 2.5 ml of

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water containing 12.5 μL of 5 wt % Nafion by sonication. Then, 10 μL of amount of such electrocatalyst ink was attentively pipetted on GCE surface, yielding electrocatalyst loading of 0.0102 mg cm−2 at room temperature. 2.4 Characterization. The bulk component, shape, crystal structure, and architecture of the sample were characterized by inductively coupled plasma atomic emission spectrometer (ICP-AES, X Series 2, Thermo Scientific USA), field emission scanning electron microscope (FE-SEM, SU8020) with an energy dispersive Xray (EDX) analysis accessory, transmission electron microscope (TEM, Tecnai G2 F20), nitrogen physical adsorption instrument (ASAP 2400), and X-ray diffraction (XRD, DX-2700) spectrometer. The surface charge and surface chemical component of samples were investigated by a Malvern Zetasizer Nano ZS90 system and an X-ray photoelectron spectroscopy (XPS, AXIS ULTRA). 3.

RESULTS AND DISCUSSION

3.1 Shape and Structure of [email protected]@PEI. Due to extreme chemical inertness of Au metal, Au based nanostructured materials have prominent electrochemical self-stability.26, 33-35 Meanwhile, 1D Au-NWs with high aspect ratio can provide the big surface area, fast electron communication, easy mass transport, and slow Ostwald ripening.36, 37 Consequently, in this work, 1D Au-NWs are chosen as substrate to construct Au@Pd core-shell nanostructures. In the typical synthesis, Au-NWs were obtained by adding α-naphthol ethanol solution into HAuCl4 solution for 5 min. As the starting synthetic material for [email protected]@PEI, the initial 1D Au-NWs were investigated in detail. EDX, TEM, and XRD characterization confirm the high-quality Au-NWs with 5.47 nm diameter can been obtained by the simple αnaphthol reduction method (Figure S1). [email protected]@PEI were obtained by adding consecutively 1 mL of PEI, 0.1 mL PdCl2, and 0.1 mL N2H4·H2O aqueous solutions into an Au-NWs solution and then heated at 60 °C for 2 h under stirring conditions. The shape of [email protected]@PEI is first investigated by FESEM and TEM. FE-SEM and TEM images show that the product is composed of high-aspect-ratio NWs (Figure 1ab), which is almost same as initial Au-NWs (Figure S1). The average diameter of product is ca. 6.73 nm (Inset in Figure 1b), which is slightly bigger than that (5.47 nm) of initial Au-NWs (Figure S1). The slight diameter increment suggests the only ultrathin Pd shell form on Au-NWs surface. The obvious Pd signal is observed at EDX spectrum (Figure 1c), confirming the deposition of Pd atoms on AuNWs surface. EDX data reveals the Au/Pd atomic ratio of [email protected]@PEI is 89:9, well matching the result (87:8) of ICP-AES. Herein, Al signal in EDX spectrum is derived from the Al film substrate. Besides the characteristic diffraction peaks of Au-NWs, no obvious characteristic diffraction peak of Pd crystal is observed at XRD pattern of

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[email protected]@PEI (Figure 1d), further confirming the formation of ultrathin Pd shell on Au-NWs surface. The deposition of Pd atoms on Au-NWs surface is visualized by high-angle annular dark-field scanning TEM (HAADFSTEM)-EDX maps, which reveals that ultrathin Pd shell uniformly coat on Au-NWs surface (Figure 1e). The fine structure of [email protected]@PEI was observed by HR-TEM (Figure 1f). The obvious Au(111) plane with 0.230 nm dspacing is observed at core position, whereas Pd shell with ~1 nm thickness reveals the obvious Pd(111) plane with 0.223 nm d-spacing at edge position.

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Figure 1. Shape and structure of [email protected]@PEI. (a) FE-SEM image, (b) TEM image, (c) EDX spectrum, (d) XRD pattern, (e) HAADF-STEM-EDX maps, and (f) HR-TEM image of [email protected]@PEI. Inset in Figure 1b: NW diameter distribution. 3.2 Surface Chemical Component of [email protected]@PEI. The surface component and electronic property of [email protected]@PEI were analyzed by XPS. Typically, both Au 4f and Pd 3d binding energies of [email protected]@PEI reveal 0.53 eV and 0.28 eV negative shift relative to the standard binding energies of bulk Pd and Au crystals, respectively (Figure 2ab).38 Due to very similar electronegativity (2.2 vs. 2.4) and work function (5.12 vs. 5.1 eV) between Au and Pd, the simultaneous negative shift of Au 4f and Pd 3d binding energies can't be attributed to the interaction between Au and Pd elements.39-41 Indeed, we also synthesized Au-Pd alloy nanocrystals using conventional NaBH4 reduction, which reveal negligible binding energy shift for Au and Pd (Figure S2). Due to strong interaction between -NH2 functional group and noble metal, PEI possesses a strong binding affinity for the noble metal.28, 42-44 An obvious N1s XPS signal is observed for Au-NWs@Pd0. @PEI (Figure 2c), implying PEI adsorbs at [email protected]@PEI surface during the synthesis. To

understand the function of PEI during the synthesis of [email protected]@PEI, Pd0.1/Au-NWs without PEI were synthesized in the absence of PEI. The zeta potential (+ 36 mV at pH 7) of [email protected]@PEI is much bigger than that (+ 5 mV at pH 7) of Pd0.1/Au-NWs without PEI, which originates from the protonation of -NH2 functional group at PEI. PEI functionalization imparts [email protected]@PEI with excellent water-solubility (Figure S3), which is propitious to uniform coverage of [email protected]@PEI on galssy carbon electrode. The distribution of PEI on [email protected]@PEI surface was visualized by HAADF-STEMEDX maps (Figure 2d). The N pattern is similar to the shape of [email protected]@PEI, suggesting the uniform adsorption of PEI on [email protected]@PEI surface. Lone pair electrons of N atoms at -NH2 functional group can donate efficiently electrons to Au atoms and Pd atoms, which results in the simultaneous negative shift in Au 4f binding energy and Pd 3d binding energy.28, 42-44 TEM image of Pd0.1/Au-NWs without PEI shows the obvious phase separation between Pd nanoparticles and Au-NWs (Figure S4), which indirectly suggests that PEI effectively works as a linker agent for the deposition of Pd atoms on Au-NWs during the synthesis of [email protected]@PEI. To understand the function of PEI, we further synthesized PdNWs@PEI without Au-NWs core by adding hydrazine hydrate to an aqueous solution containing PEI and K2PdCl4 according to our previous work44 (Figure S5). XPS analysis show that the percentage of Pd0 species (90%) at [email protected]@PEI is higher than that (84%) at Pd-NWs@PEI (Figure S6), indicating that the ligand effect of Au atoms heightens the antioxidation capacity of Pd atoms.45

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Figure 2. Electronic property and surface component of [email protected]@PEI. (a) Au 4f, (b) Pd 3d, and (c) N1s XPS spectra of [email protected]@PEI. The black dot lines in abc stand for standard values of Au 4f, Pd 3d, and N 1s standard values, respectively. (d) HAADF-STEM-EDX maps of Au-

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[email protected]@PEI. Black dot lines in Figure (a), (b), and (c) represent the standard binding energies of Pd, Au, and N element, respectively. 3.3 Component Control of Au-NWs@Pd@PEI. EDX spectra show that the Pd signal increases with amount of PdCl2 (Figure 3a). XRD patterns show the characteristic diffraction peaks of Pd crystal increases with amount of PdCl2 (Figure 3b). TEM images show that all AuNWs@Pd@PEI have wire-like shape and smooth surface (Figure 3c), indicating uniform coverage of Pd shell on AuNWs surface. The average diameters of [email protected]@PEI, [email protected]@PEI, [email protected]@PEI, and [email protected]@PEI are measured to be 6.02 nm, 6.73 nm, 7.35 nm, and 8.02 nm respectively (Figure 3d). Specifically, Pd shell thickness of AuNWs@Pd@PEI increases from 0.275 to 1.275 nm (i.e., from 1 to 6 layers of Pd atoms) with increasing PdCl2 amount, revealing a shell thickness-adjustable synthesis. The increase in Pd shell thickness was further confirmed by HR-TEM images (Figure 3e). As observed, Pd shell thicknesses of Au-NWs@Pd@PEI gradually increase with Pd content. At the same time, HAADF-STEM-EDX maps show the diameter of Au-NWs core retain constant while the thickness of Pd shell increases with Pd content (Figure 3f), further confirming the shell thickness-adjustable synthesis.

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Figure 3. Shell thickness-adjustable synthesis. (a) EDX spectra, (b) XRD patterns, (c) TEM images, (d) wire diameter, (e) HR-TEM images of [email protected]@PEI, [email protected]@PEI, [email protected]@PEI, and [email protected]@PEI. (f) HAADF-STEM-EDX maps of [email protected]@PEI, [email protected]@PEI, [email protected]@PEI, and [email protected]@PEI. 3.4 Component-Dependent ORR activity of AuNWs@Pd@PEI. Electrochemical properties of a series of Au-NWs@Pd@PEI were analyzed by cyclic voltammetry (CV).26 All potentials in this work were reported on a reversible hydrogen electrode (RHE) scale. As observed, the reduction peak of Au oxide at Au-NWs locates at ca. 1.1 V (Figure 4a).46, 47 For [email protected]@PEI, a clear reduction peak at 1.1 V is still observed (Figure 4a), indicating an incomplete Pd shell on Au-NWs surface. After increasing Pd feed, only a very weak reduction peak is observed at 1.1 V for [email protected]@PEI, indicating Pd shell almost completely wrap the Au-NWs. The reduction

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Au-NWs core, resulting in ORR activity degradation.54-56

Meanwhile, CV curves (Figure 4a) show that [email protected]@PEI have highest potential value for the Pd oxide formation (i.e, more available metallic Pd active sites towards ORR), which is also responsible for the high ORR activity.

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peak of Au oxide is hardly found for [email protected]@PEI and [email protected]@PEI when further increasing the amount of Pd (Figure 4a), indicating a complete Pd shell on Au-NWs surface. Meanwhile, the reduction peak potential of Pd oxide at Au-NWs@Pd@PEI negatively shifts with increasing Pd amount (i.e., Pd shell thickness). For example, the reduction peak potential of Pd oxide at [email protected]@PEI positively shifts ca. 75 mV relative to [email protected]@PEI (Figure 4b). The positive shift in the reduction peak potential of Pd oxide suggests that the hydroxyl adsorption/desorption on [email protected]@PEI surface occurs at a higher potential than that on [email protected]@PEI. The present electrochemical results display the Pd shell thickness on Au surface strongly affects the formation potential of Pd oxide. Since hydroxyl species on Pd surface inhibit ORR activity,48-51 such the hydroxyl adsorption/desorption at higher potential be propitious to ORR activity enhancement. The ORR activity of a series of Au-NWs@Pd@PEI were investigated by linear sweep voltammetry (LSV) using rotating disk electrode (RDE) technique (Figure 4c). The ORR current density at electrocatalysts was normalized to the geometric area of electrode. LSV curves show that ORR half-wave potentials (E1/2) of Au-NWs@Pd@PEI change dramatically with Pd amount (i.e., Pd shell thickness). We further compare the mass activity and specific activity of Au-NWs@Pd@PEI with different Pd content towards ORR at 0.9 V potential (Figure 4d). Herein, the electrochemically active surface areas (ECSA) of AuNWs@Pd@PEI with different Pd content were measured by CO stripping method (Figure S7).52, 53 As observed, both mass activity and specific activity of Au-NWs@Pd@PEI towards ORR initially increase and then decrease gradually with increasing Pd shell thickness. For example, [email protected]@PEI have the highest mass activity and specific activity at 0.9 V, implying best ORR performance among them. The lattice constants of Au and Pd are 2.30 and 2.23 Å, respectively. The lattice mismatch (∼4.7%) between Au and Pd lattice inevitably cause a surface strain effect for the ultrathin Pd shell, which improves ORR activity of Pd atoms due to the reduction for the adsorption strength of O2 intermediates on Pd surface. The different Pd shell thickness on Au-NWs core results in the different Pd lattice strain, which results in Pd shell thickness dependent ORR activity. Specifically, [email protected]@PEI with Pd shell thickness of 0.63 nm (about 3 layers of Pd atoms) show the best ORR activity. When Pd shell thickness increases to 1.275 nm (about 6 layers of Pd atoms), the corresponding [email protected]@PEI show a weak ORR activity. Likely, the outer Pd atoms are hardly affected by the

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Figure 4. Component-dependent ORR activity. (a) CV curves of Au-NWs and various Au-NWs@Pd@PEI in N2saturated 0.1 M KOH solution at 50 mV s−1. (b) The histogram of reduction peak potential values of Pd oxide at Au-NWs@Pd@PEI. (c) ORR polarization curves of AuNWs@Pd@PEI in O2-saturated 0.1 M KOH solution at 5 mV s−1 and 1600 rpm. (d) ORR mass activity and specific activity of Au-NWs@Pd@PEI at 0.9 V. 3.5 ORR Activity of [email protected]@PEI and Pt/C Electrocatalyst. Since [email protected]@PEI exhibit the highest ORR activity among a series of Au-NWs@Pd@PEI, we further compared their ORR performance with the state-of-art Pt/C electrocatalyst. According to CV curves in N2-saturated 0.1 M KOH solution (Figure 5a), the reduction peak potential of surface oxide at [email protected]@PEI positively shifts ca. 100 mV compared to Pt/C. The ECSA of [email protected]@PEI and Pt/C electrocatalyst are calculated to be 74 m2 g-1Pd and 61.5 m2 g-1Pt by CO stripping method (Figure S8).52, 53 Obviously, ultrathin Pd shell layer on AuNWs contributes to the high ECSA value of [email protected]@PEI. Furthermore, we remove the PEI layers of the [email protected]@PEI through oxidation treatment (See Supporting Information for details) and name it as [email protected], the ECSA of [email protected] only increase 10% compared to [email protected]@PEI (Figure S9), indicting PEI only cover partly Pd surface. PEI has branched molecular structure (Scheme S1), which results in loose-packed layer on Pd surface. Such a loose-packed layer can retain the most of Pd active sites for electrocatalytic reaction. XPS measurements (Figure S6) have demonstrated that Pd atoms on [email protected]@PEI have improved antioxidation capacity due to the ligand effect of Au-NWs core,45 which contributes the high onset oxidation potential of Pd shell. Since surface Pd oxide species are inactive towards ORR, the antioxidation capacity of [email protected]@PEI favors ORR activity improvement. The ORR activities of [email protected]@PEI and Pt/C electrocatalyst were investigated by LSV (Figure 5b). [email protected]@PEI show a 61 mV E1/2 increase towards ORR

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compared to Pt/C electrocatalyst. The ORR mass activity of [email protected]@PEI and commercial Pt/C electrocatalyst are 0.51 A mg-1 and 0.072 A mg-1 at 0.85 V potential and 0.29 A mg-1 and 0.025 A mg-1 at 0.90 V potential, respectively (Figure 5c), which suggests [email protected]@PEI have higher ORR activity than Pt/C electrocatalyst. To understand the function of Au-NWs core, ORR activity of Pd-NWs@PEI without Au-NWs core was also investigated. Pd-NWs@PEI without Au-NWs core represent 40 mV decrease over E1/2 of [email protected]@PEI (Figure S10). This result suggests that the interaction between Au-NWs core and Pd shell play a crucial role towards ORR activity enhancement of [email protected]@PEI. XPS (Figure S6) and CV (Figure S11) measurements reveal [email protected]@PEI have weaker oxophilicity than Pd-NWs@PEI without AuNWs core, which contributes to ORR activity enhancement due to the more metallic Pd active sites. For the bimetallic core-shell nanostructures, the surface strain effect and electronic effect induced by the interaction between the core metal and shell metal generally affect their electrocatalytic activity. However, the two effects are difficult to distinguish due to the unavoidable coexistence. In this work, the electronic effect caused by Au-NWs core can be ruled out because Pd 3d binding energies in [email protected]@PEI and Pd-NWs@PEI without Au-NWs core are almost identical (Figure S6), originating from strong electron donation form -NH2 functional group on PEI to Pd atoms.27, 41-43 In addition, ORR E1/2 value of Au-NWs@Pd without PEI negatively shifts only 23 mV compared with Au-NWs@Pd@PEI (Figure S12). Thus, ORR activity enhancement of [email protected]@PEI can be mainly ascribed to the surface strain effect relative to PdNWs@PEI without Au-NWs core. Meanwhile, [email protected]@PEI also show higher E1/2 value and mass activity at 0.9 V towards ORR compared to various Pdbased electrocatalysts reported previously 10, 34, 44, 57-73 (Table 1), further confirming high ORR activity of [email protected]@PEI. Electrocatalytic ORR is mainly carried out in two ways. One is that O2 directly passes through the 4e− transfer path to finally generate H2O; the other is that O2 generates H2O2 through 2e− pathway. The 4e− pathway of ORR is highly desired for metal-air batteries and fuel cells due to the high energy conversion efficiency.14, 74 ORR kinetics at AuNWs@Pd@PEI were investigated by using RDE technique at different rotation rates (Figure 5d). Because of the accelerated O2 diffusion, limiting current density of ORR at Au-NWs@Pd@PEI increases with rotation rate. The transferred electron number per O2 molecule in ORR was determined by the Levich-Koutecky equation.57, 75-77 Based on Levich-Koutecky plot (insert in Figure 5d), the number of transferred electron of ORR at Au-NWs@Pd@PEI is calculated to be 3.8 at 0.65 V potential, suggesting a four electron reduction process.

j (A mg )

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)

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0.9 V

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Figure 5. ORR activity comparison. (a) CV curves of [email protected]@PEI and Pt/C electrocatalyst in N2-saturated 0.1 M KOH solution at 50 mV s−1. (b) ORR polarization curves of [email protected]@PEI and Pt/C electrocatalyst in O2-saturated 0.1 M KOH solution at 5 mV s−1 and 1600 rpm. (c) ORR mass activity of [email protected]@PEI and Pt/C electrocatalyst at 0.85 V and 0.9 V. (d) ORR polarization curves of [email protected]@PEI in O2-saturated 0.1 M KOH solution at different rotation rates. Insert in Figure 5d: Koutecky-Levich plot (j−1 vs ω1/2) towards ORR at 0.65 V. Table 1. E1/2 values and ORR mass activity at 0.9 V of Pdbased nanocrystals reported previously towards ORR in an alkaline solution10, 34, 44, 57-73 ORR mass Catalysts

E1/2 (V)

activity

Ref.

(at 0.9 V, A/mg) [email protected]@PEI

This

0.90

0.295

~ 0.86

~ 0.1

201510

0.759

----

201434

0.865

----

201544

0.875

0.216

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0.88

----

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0.86

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----

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PdMn/C nanocatalysts

0.87

~ 0.09

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PdFe/C nanocatalysts

0.85

~ 0.08

201661

Pd@PtNi core-shell

0.873

0.073

201762

work

AuPd alloyed flowerlike-assembly nanochains Au@Pd/reduced graphene oxide Pd nanowires Pd tetrahedron/tungsten oxide PtPd@Pt core-satellite nanoassemblies Ni-Pd core-shell nanoparticles Au@Pd core-shell nanothorns

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catalyst

----

201863

~ 0.9

0.097

201564

~ 0.85

----

201765

0.81

----

201666

0.86

----

201667

0.884

~ 0.26

201668

~ 0.82

----

201869

~ 0.93

0.168

201670

palladium nanoparticles/nitrogendoped graphene Pd nanodots/nitrogendoped titania B-doped Pd particle Pd@Pt core-island shell nanocrystals Pd/TiO2 catalysts Pd3Pb intermetallic compound Pd@PdO-Co3O4

0.90

----

201871

Pd-Ni icosahedra

~ 0.88

0.22

201472

0.83

----

201473

Pd-N2H4 nanoparticle

3.6 ORR Selectivity of [email protected]@PEI and Pt/C Electrocatalyst. Recently, direct methanol fuel cell in an alkaline solution is attracting more and more attention due to its distinct advantages over acidic media, including enhanced MOR kinetics, improved ORR kinetics, less CO poisoning effect, and simplified water management.29, 30 However, methanol can penetrate through Nafion membrane, resulting in an undesirable competitive reaction between MOR and ORR at cathode. An ideal ORR electrocatalyst should have satisfactory tolerance to methanol crossover. Since methanol diffusion form solution from metal surface is mandatory requirement towards MOR, physically preventing methanol diffusion has been applied to achieve ORR selectivity.78-80 The electrocatalytic performance of [email protected]@PEI and Pt/C electrocatalyst towards MOR were investigated by CV. No obvious MOR peak is observed at [email protected]@PEI (Figure 6a) whereas a big MOR peak at 0.89 V is found at Pt/C electrocatalyst (Figure 6b), revealing [email protected]@PEI almost are inactive towards MOR in an alkaline solution. Meanwhile, ECSA normalized CV curves show [email protected] without PEI have super activity towards MOR compared to [email protected]@PEI (Figure S13), which indicates PEI layer on [email protected] plays a pivotal role for physically preventing methanol diffusion. Solubility tests display that PEI is methanol phobic (Figure S14). Thus, PEI layer on [email protected]@PEI may prevent methanol diffusion from electrolyte to Pd sites due to its methanol-phobic property. As already mentioned, PEI polymer on [email protected]@PEI forms a loose-packed layer. Considering that size discrepancy between O2 molecule and methanol molecule (3.4 Å vs. 4.3 Å)28, loosepacked PEI layer can work as barrier nanosieves to exclusively prevent the access of methanol to Pd sites.28, 44 Furthermore, ORR selectivity of [email protected]@PEI and Pt/C electrocatalyst were investigated in the presence of

(a)2.7 1.8 0.9

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Pd3Fe intermetallic

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without CH3OH with CH3OH

1.8

1.5

Porential vs. RHE (V)

-2

doped graphene

j (mA cm )

PdFe@Pd/nitrogen-

methanol in an alkaline solution. ORR polarization curve at [email protected]@PEI in the presence of methanol is very similar to that in the absence of methanol, only accompanying a very small 18 mV shift in E1/2 (Figure 6c). In contrast, an obvious MOR peak at 0.76 V appears at ORR polarization curve of Pt/C in the presence of methanol (Figure 6d), similar to that case of [email protected] without PEI (Figure S15). Thus, ORR polarization measurements indicate the PEI functionalization imparts [email protected]@PEI with unordinary ORR selectivity in the presence of methanol due to the physical prevention of methanol accessibility. Current (mA)

nanoflower

-2

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0.4

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Figure 6. ORR selectivity comparison. CV curves of (a) [email protected]@PEI and (b) Pt/C electrocatalyst in N2saturated 0.1 M KOH solution with or without 0.1 M CH3OH at 50 mV s-1. ORR polarization curves of (c) [email protected]@PEI and (d) and Pt/C electrocatalyst in O2saturated 0.1 M KOH electrolyte with or without 0.1 M CH3OH at 5 mV s-1 and rotation rate of 1600 rpm. 3.7 ORR Durability of Au-NWs@Pd@PEI and Pt/C Electrocatalyst. Besides activity and selectivity, durability is vital parameter for evaluating ORR performance of electrocatalysts. The accelerated durability test (ADT) was performed to investigate the electrocatalytic durability of [email protected]@PEI and Pt/C electrocatalyst towards ORR.57, 58, 62 ADT was conducted by repeating potential scans between 1.4 and 0 V at 50 mV s-1. After 10,000 cycles, [email protected]@PEI reveal 38 mV degradation in E1/2 value towards ORR (Figure 7a). In contrast, Pt/C electrocatalyst reveals 60 mV shifts in E1/2 value towards ORR after same cycles (Figure 7b). The ORR mass activity of [email protected]@PEI at 0.9 V after ADT is 0.088 A mg-1 (Figure S16), which is still better than Pt/C (0.00259 A mg-1). After ADT, TEM images display that the shape of [email protected]@PEI completely remains (Figure S17) whereas Pt/C electrocatalyst generates serious aggregation (Figure S18). These results suggest that [email protected]@PEI are more stable than Pt/C electrocatalyst. As shown in SEM

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ACS Catalysis

image (Figure 1a), [email protected]@PEI on electrode surface display 3D interconnected structure, which can effectively suppress Ostwald ripening and particle aggregation. Meanwhile, we observed that [email protected]@PEI are also more stable than Pd-NWs@PEI without Au-NWs core (Figure S19). As shown in Figure 4a and 4b, the introduction of Au-NWs core can effectively elevate the onset potential of Pd oxide formation due to ligand effect of Au atoms, which also reduces the dissolution of Pd shell. Additionally, the methanol tolerance of [email protected]@PEI still retains well after ADT (Figure S20). The fact suggests that PEI polymer on [email protected]@PEI is highly stable, resulting in the long-term ORR selectivity of [email protected]@PEI. -2

-2

(b)

Au-NWs@Pd 0.1@PEI

j (mA cm )

(a) 0 j (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Initial 10000 cycles

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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. Supporting figures related to the preparation details of [email protected] without PEI; the calculation detail of ECSA; additional XPS, XRD SEM, TEM, HAADF-STEM-EDX maps, CV, and ORR data (PDF)

AUTHOR INFORMATION *Email for Y. C: [email protected] Notes The authors declare no competing financial interest.

Initial 10000 cycles

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ASSOCIATED CONTENT

Corresponding Author

PtC

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Figure 7. ORR durability comparison. ORR polarization curves of (a) [email protected]@PEI and (b) and Pt/C electrocatalyst before and after ADT in O2-saturated 0.1 M KOH electrolyte at 5 mV s–1 and rotation rate of 1600 rpm. CONCLUSIONS In the assistant of PEI, highly-quality Au-NWs@Pd@PEI with tunable shell thickness were synthesized by simple chemical reduction method. During the synthesis, PEI simultaneously worked as surfactant, linker agent, and functional agent. When using as cathode ORR electrocatalysts, Au-NWs@Pd@PEI displayed shell thickness dependent electrocatalytic activity towards ORR in an alkaline solution. Relative to Pd-NWs@PEI without Au-NWs core, the component optimized [email protected]@PEI revealed enhanced ORR activity, which indicates that the surface strain effect induced by Au-NWs core played an important role towards ORR activity enhancement. Compared to [email protected] without PEI, [email protected]@PEI displayed particular methanol tolerance towards ORR in an alkaline solution because loose-packed PEI layer at [email protected]@PEI could prevent the methanol diffusion from electrolyte to Pd sites due to its methanol phobic property and steric effect. Additionally, Pd-Au interaction improved the antioxidation capacity of Pd atoms due to ligand effect, which contributed to durability of [email protected]@PEI towards ORR. As a result, [email protected]@PEI even had the better activity, methanol tolerance, and durability compared to Pt/C electrocatalyst, indicating [email protected]@PEI were highly active Pt-free electrocatalyst towards ORR in alkaline direct methanol fuel cells.

ACKNOWLEDGMENT This research was sponsored by National Natural Science Foundation of China (21473111), Fundamental Research Funds for the Central Universities (GK201602002 and GK201703030), and the 111Project (B14041).

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Table of Contents Artwork H2O O2

C2H5OH Au

Pd

ORR activity Pt/C Δ =61 mV

H2O CO2

N

PEI

ORR selectivity Pt/C Δ =234 mV

[email protected]@PEI

[email protected]@PEI

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