Polyethyleneimine modified [email protected] alloy nanocrystals as advanced electrocatalysts towards the oxygen reduction reaction

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Polyethyleneimine modified AuPd@PdAu alloy nanocrystals as advanced electrocatalysts towards the oxygen reduction reaction Q1

Qi Xue1, Guangrui Xu1, Rundong Mao, Huimin Liu, Jinghui Zeng, Jiaxing Jiang, Yu Chen∗

Q2

Key Laboratory of Macromolecular Science of Shaanxi Province, Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, Shaanxi, China

a r t i c l e

i n f o

Article history: Received 13 May 2017 Revised 22 June 2017 Accepted 27 June 2017 Available online xxx Keywords: Fuel cells PdAu alloy Surface modification Oxygen reduction reaction Alcohol tolerance

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a b s t r a c t Designing the low cost, active, durable, and alcohol-tolerant cathode catalysts towards the oxygen reduction reaction (ORR) is significant for the large-scale commercialization of direct alcohol fuel cells. Recently, Pd-based nanocrystals have attracted attention as Pt-alternative cathode catalysts towards the ORR in the alkaline electrolyte. Unfortunately, the pristine Pd-based nanocrystals lack the selectivity towards the ORR due to their inherent activity for the alcohol molecule oxidation reaction in the alkaline electrolyte. In this work, polyethyleneimine (PEI) modified AuPd alloy nanocrystals with Au-rich AuPd alloy cores and Pd-rich PdAu alloy shells (AuPd@PdAu-PEI) are successfully synthesized using a traditional chemical reduction method in presence of PEI. The rotating disk electrode (RDE) technique is applied to evaluate the ORR performance of AuPd@PdAu-PEI nanocrystals. Compared with commercial Pd black, AuPd@PdAu-PEI nanocrystals show significantly enhanced activity and durability towards the ORR, and simultaneously exhibit particular alcohol tolerance towards the ORR in the alkaline electrolyte. © 2017 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

1. Introduction The alkaline direct alcohol fuel cells (ADAFCs) have received intensive attentions as highly efficient power generators for portable electric devices, owing to the diversity of fuel molecules, high energy conversion efficiency, improved alcohol molecules (such as methanol, format, ethanol, and is opropanol) oxidation reaction kinetics in the alkaline electrolyte, and elevated oxygen reduction reaction (ORR) kinetics in the alkaline electrolyte [1–9]. At present, Pt-based nanocrystals have been proved to be best cathode catalyst towards the ORR, nevertheless, they suffer from the high cost due to the limited reserves in nature. Recently, a lot of investigations have shown that Pd-based nanocrystals have become highly promising Pt-alternative cathode catalysts towards the ORR in the alkaline electrolyte due to their high catalytic activity and the significantly lower price [10–20]. Although significant progress has been made in developing Pdbased ORR cathode catalysts in recent years, they still have some serious disadvantages, including the poor durability and low selectivity towards the ORR [10,12–19]. On the one hand, Pd has much lower intrinsic electrochemical stability relative to Pt due to the

∗

1

Corresponding author. E-mail address: [email protected] (Y. Chen). These authors contributed equally to this work.

low redox-potential of Pd element, resulting in poor durability in electro-catalyzing ORR. Thus, enhancing the electrochemical stability of Pd nanocrystals in the alkaline electrolyte is still required. On the other hand, the alcohol crossover generally occurs in the exchange membrane, which debases the performance of the ADAFCs due to the mixed potential and CO-poisoning of cathode catalysts. Like Pt nanocrystals, Pd nanocrystals also show the outstanding activity towards the alcohol molecules oxidation reaction in the alkaline electrolyte. Thus, improving the ORR selectivity of cathode Pd nanocrystals is also a very important subject for the ADAFCs. In order to improve the ORR durability, many strategies have been used to suppress the dissolution/aggregation of Pd nanocrystals under dynamic electrochemical conditions, including composition control, morphology control, and chemical encapsulation [21]. Among them, bimetallic Pd–Au nanocrystals generally display the improved durability towards the alcohol molecules oxidation reaction and ORR because Au can effectively restrain the electrochemical dissolution of Pd. Meanwhile, bimetallic Pd–Au nanocrystals also exhibit improved ORR activity compared to monometallic Pd nanocrystals due to electronic and geometric effects [22–26].Unfortunately, bimetallic Pd–Au nanocrystals lack the alcohol tolerance towards the ORR due to high catalytic activity towards the alcohol molecules oxidation reaction in the alkaline electrolyte [27–32]. Very recently, the surface modification of metal nanocrystals with functional inorganic/organic molecules has become a highly effective method for achieving their selectivity for certain

http://dx.doi.org/10.1016/j.jechem.2017.06.007 2095-4956/© 2017 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

Please cite this article as: Q. Xue et al., Polyethyleneimine modified AuPd@PdAu alloy nanocrystals as advanced electrocatalysts towards the oxygen reduction reaction, Journal of Energy Chemistry (2017), http://dx.doi.org/10.1016/j.jechem.2017.06.007

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electrochemical reaction due to the ensemble effect of functional molecules [33–39]. For instance, in the presence of dioxygen, the calix[4]arene modified Pt nanocrystals can catalyze selectively the hydrogen oxidation reaction [34,35]. In the presence of methanol, the cyanide (CN– ) modified hollow PtNi alloy nanospheres exhibit the particular catalytic selectivity towards the ORR [39]. In this work, we report the one-step synthesis of polyethyleneimine (PEI, Scheme S1) modified AuPd alloy nanocrystals with Au-rich AuPd alloy cores and Pd-rich PdAu alloy shells (AuPd@PdAu-PEI) and investigate their catalytic activity and selectivity towards the ORR in the alkaline electrolyte.

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2. Experimental

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2.1. Synthesis of AuPd@PdAu-PEI nanocrystals

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PEI modified AuPd alloy nanocrystals with Au-rich AuPd alloy cores and Pd-rich PdAu alloy shells (AuPd@PdAu-PEI) were prepared by a simple one-step chemical-reduction method. In a typical synthesis, 0.2 mL of 0.242 M HAuCl4 solution, 0.5 mL of 0.5 M PdCl2 , 2 mL of 0.5 M PEI (Mw ≈ 10,0 0 0) were added into 6.8 mL of deionized water and stirred for 5 min at room temperature. After adjusting pH of the mixture solution to 9.0, 0.5 mL of N2 H4 ·H2 O solution was added into the mixture and then stirred for 3 h at room temperature. Finally, the products were isolated by centrifugation (15,0 0 0 rpm) for 10 min, washed with distilled water, and dried in a vacuum oven at 60 °C for 12 h. For comparison, the AuPd alloy nanocrystals (AuPd-ANC) were prepared in the absence of PEI. PEI modified Pd nanocrystals (Pd@PEI) without Au were prepared in the absence of HAuCl4 .

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2.2. Electrochemical measurements

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Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) tests were performed on an electrochemical workstation (CHI 660D) with or without a rotating disk electrode (RDE, Gamry RDE710) using a standard three-electrode cell. A catalyst modified glassy carbon electrode, a Pt plate, and a saturated calomel electrode were used as working, counter, and reference electrodes, respectively. All the electrochemical tests were performed at 30 °C and all the potentials were reported with respect to the reversible hydrogen electrode (RHE). The working electrode was prepared according to the previous literatures [40]. Typically, the catalyst ink was prepared by ultrasonicating 10 mg of catalyst in 5 mL of isopropyl alcohol/Nafion® solution (20% isopropyl alcohol and 0.02% Nafion®) for 1 h. Then, 10 μL of the homogeneous ink was then drop-casted the glassy carbon electrode and dried at room temperature to form a thin catalyst film with a metal loading of 101.9 μg/cm2 .

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2.3. Instruments

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The structural characterization was performed on DX-2700 powder X-ray diffraction (PXRD) diffractometer with Cu-Kα radiation source. The surface charge and surface chemical composition were analyzed by Nano ZS90 zeta potential analyzer and AXIS ULTRA X-ray photoelectron spectroscopy (XPS) with Al Kα radiation source. The binding energy was calibrated with respect to adventitious carbon (C1s, 284.6 eV). The bulk chemical composition, morphology and surface structure of the catalysts were characterized using Hitachi SU-8020 energy dispersive X-ray (EDX) analyzer, inductively coupled plasma atomic emission spectrometer (ICP-AES), and TECNAI G2 F20 transmission electron microscopy (TEM).

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3. Results and discussion

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3.1. Characterization of AuPd@PdAu-PEI nanocrystals

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AuPd@PdAu-PEI nanocrystals were easily obtained by adding 105 N2 H4 ·H2 O into the mixture solution of HAuCl4 , PdCl2 , and PEI 106 at room temperature, as illustrated in Fig. 1a. The composition 107 and structure of the products were analyzed by EDX, ICP-AES, 108 and PXRD. EDX spectrum reveals the products contain Pd and 109 Au elements (Fig. 1b), indicating the successful reduction of AuIII 110 and PdII precursors. The atomic ratio of Pd/Au in the products 111 is quantitatively determined to be about 4.6:1 using ICP-AES 112 technique, in consistent with theoretical stoichiometric ratio. PXRD 113 pattern of AuPd@PdAu-PEI nanocrystals reveals the four obvious 114 diffraction peaks located at 2θ = 39.4°, 45.5°, 67.0°, 81.0° (Fig. 1c), 115 indicating AuPd@PdAu-PEI nanocrystals have a face-centered 116 cubic (fcc) structure. The characteristic diffraction peaks of 117 AuPd@PdAu-PEI nanocrystals lie between those of monometallic 118 Au (JCPDS 04-0784) and monometallic Pd (JCPDS 46-1043), indicat- 119 ing the formation of AuPd alloy. Relative to Pd metal, all diffraction 120 peaks of AuPd@PdAu-PEI nanocrystals negatively shift to lower an- 121 gles, originating from the lattice expansion due to the alloying of 122 Pd atom with bigger Au atom. The broadening of diffraction peaks 123 suggests the generation of nanomaterials. The Debye–Scherrer 124 equation was used to calculate the average crystallite size of 125 AuPd@PdAu-PEI nanocrystals using the strongest (111) diffraction 126 peak. The calculated average crystallite size is about 9.6 nm. 127 The morphology and particle size of AuPd@PdAu-PEI nanocrys- 128 tals were characterized by TEM. A large-area TEM image reveals 129 that the AuPd@PdAu-PEI is monodisperse and uniform (Fig. 2a). 130 The particle size distribution histogram shows that the mean di- 131 ameter of AuPd@PdAu-PEI nanocrystals is about 10.5 nm (Fig. 2b), 132 which is very close to the value calculated from PXRD data. 133 The selected area electron diffraction (SAED) image displays the 134 discontiguous concentric rings with dot characteristic, which areQ3 135 indexed to (111), (200), (220) and (311) facets of fcc noble metal, 136 indicating that AuPd@PdAu-PEI nanocrystals have high crystal- 137 lization (Fig. 2c) [41]. HRTEM image shows that AuPd@PdAu-PEI 138 nanocrystals have the quasi-spherical shape (Fig. 2d), in similar 139 to Pd@PEI nanocrystals without Au (Fig. S1). This fact indicates 140 that Au cores have negligible impact on the shape of AuPd@PdAu- 141 PEI. The magnified HRTEM images display the clear and continu- 142 ous lattice fringes with a space value of 0.233 nm and 0.223 nm 143 at center and edge of nanocrystals, corresponding to (111) facets 144 of Au and Pd metal, respectively (Fig. 2e). The corresponding fast 145 Fourier transform (FFT) pattern reveals the hexagonally symmetri- 146 cal spot pattern, confirming that the AuPd@PdAu-PEI nanocrystals 147 are enclosed by (111) facets (Fig. 2f). EDX maps and line scanning 148 were carried out to investigate the distribution of Pd and Au ele- 149 ments in AuPd@PdAu-PEI nanocrystals (Fig. 2g). The size of Au pat- 150 tern is smaller than that of Pd pattern, indicating AuPd@PdAu-PEI 151 nanocrystals have a core-shell structure (Fig. 2h). EDX line scan- 152 ning profile on the center of single AuPd@PdAu-PEI nanocrystal 153 displays that the Au is abundant, which is close to Pd content. In 154 contrast, the Au content is much lower than that of Pd element 155 on the edge of single AuPd@PdAu-PEI nanocrystal (Fig. 2h). Thus, 156 EDX line scanning profiles on the two different regions definitely 157 demonstrate that AuPd@PdAu-PEI nanocrystals have Au-rich AuPd 158 alloy cores and Pd-rich PdAu alloy shells. It is clear that the stan- 159 dard reduction potential of AuIII (AuCl4 – /Au, +1.002 V) is slightly 160 higher than that of PdII (PdCl4 2– /Pd, 0.951 V) [42]. In the presence 161 PEI, LSV measurements show that the reduction potential of AuIII 162 precursor is still slightly higher than that of PdII precursor (Fig. S2). 163 Notably, AuIII precursor is more easily reduced compared with PdII 164 precursor. Therefore, the preferential reduction of the AuIII reac- 165 tion precursor leads to the generation of the AuPd@PdAu core-shell 166

Please cite this article as: Q. Xue et al., Polyethyleneimine modified AuPd@PdAu alloy nanocrystals as advanced electrocatalysts towards the oxygen reduction reaction, Journal of Energy Chemistry (2017), http://dx.doi.org/10.1016/j.jechem.2017.06.007

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Fig. 1. (a) A schematic illustration of AuPd@PdAu-PEI synthesis, (b) EDX spectrum and (c) PXRD pattern of AuPd@PdAu-PEI nanocrystals.

Fig. 2. (a) TEM image, (b) histogram of size particle distribution, and (c) SAED pattern of AuPd@PdAu-PEI nanocrystals; (d) HRTEM image of single AuPd@PdAu-PEI nanocrystal; (e) the magnified HRTEM images and (f) corresponding FFT patterns; (g) STEM and EDX maps of Pd and Au elements; (h) line scanning profiles of center and edge regions at single AuPd@PdAu-PEI nanocrystal.

Please cite this article as: Q. Xue et al., Polyethyleneimine modified AuPd@PdAu alloy nanocrystals as advanced electrocatalysts towards the oxygen reduction reaction, Journal of Energy Chemistry (2017), http://dx.doi.org/10.1016/j.jechem.2017.06.007

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Fig. 3. (a) N 1s XPS spectrum of AuPd@PdAu-PEI nanocrystals, (b) STEM and EDX maps of N element, (c) Pd 3d XPS and (d) Au 4f XPS spectra of AuPd@PdAu-PEI nanocrystals.

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nanocrystals with Au-rich AuPd alloy cores and Pd-rich PdAu alloy shells. XPS, zeta potential and EDX maps were carried out to analyze the surface composition of AuPd@PdAu-PEI nanocrystals. The previous works have indicated the amino groups can strongly adsorb the noble metal surface because of the strong N-metal bond [43–46]. For AuPd@PdAu-PEI nanocrystals, the N 1s XPS signal is detected (Fig. 3a). Meanwhile, the zeta potential of AuPd@PdAu-PEI nanocrystals is about +41 mV at pH 7, which is ascribed to the protonation of amino groups at PEI. In contrast, the zeta potential of AuPd-ANC nanocrystals is only +2.6 mV, which in turn confirms the high zeta potential of AuPd@PdAu-PEI nanocrystals (+41 mV at pH 7) originates from the protonation of amino groups at PEI. Thus, both XPS and zeta potential measurements indicate PEI molecules strongly adsorb at AuPd@PdAu nanocrystals surface. The distribution of PEI on AuPd@PdAu nanocrystals was visualized by EDX elemental maps (Fig. 3b). The shape of N element pattern is very close to the morphology of AuPd@PdAu nanocrystals, revealing the uniform distribution of PEI on AuPd@PdAu nanocrystals surface. This fact indicates that PEI can effectively act as a capping agent during the synthesis due to the bulky molecular size of PEI and the strong electrostatic repulsion between protonated PEI molecules [43–46]. Indeed, the AuPd alloy nanocrystals (AuPdANC) seriously aggregate in the absence of PEI (Fig. S3), which in turn indicates that PEI plays a key role for the well-mono dispersity of AuPd@PdAu-PEI nanocrystals. The chemical state and electronic structure of the Au and Pd element were analyzed by XPS (Fig. 3c and d). High resolution Pd 3d spectrum and Au 4f spectrum are deconvoluted with the doublet peak, respectively. As observed, the metallic Pd0 and Au0 are dominant species, indicting the complete reduction of PdII and AuIII precursors. Meanwhile, Pd 3d and Au 4f binding energies of AuPd@PdAu-PEI nanocrystals simultaneously shift to negative values compared with the standard values of Pd and Au elements, respectively [47]. In contrast, Pd 3d and Au 4f binding energies of AuPd-ANC exhibit a negligible shift compared with the standard values of Pd and Au elements, respectively (Fig. S4). These experimental data demonstrate that the negative shift in Pd 3d and Au

4f binding energies originates from the binding of PEI on PdAu surface because the lone pair electrons of –NH2 groups can strongly donate electrons to Au and Pd atoms [46], which induces the negative shift in Pd 3d and Au 4f binding energies.

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3.2. ORR activity of AuPd@PdAu-PEI nanocrystals

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The electrochemical performance of AuPd@PdAu-PEI nanocrystals was first evaluated by CV in N2 -purged 0.1 M NaOH electrolyte. To explore the potential practical application, commercial Pd black was also tested under the same experimental condition (Fig. 4a). Our previous works have demonstrated that PEI layer on metal surface is highly loose-packed due to its particularly branched molecular structure, which effectively retains the most of metal active sites [20,43,45]. The strong reduction peak of palladium oxide at 0.687 V is observed at CV curve of AuPd@PdAu-PEI nanocrystals, confirming that PEI molecules only partly cover the Pd surface. According to the Coulombic charge (Q) associated with the reduction peak of palladium oxide at CV curve, the electrochemically active surface area (ECSA) of cathode Pd catalysts was calculated from the reduction peak of monolayer palladium oxide using the reduction charge of 420 μC/cm2 [48,49],

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ECSA =

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Q m ·C

where m is the Pd mass on the electrode surface, and C is the theoretical reduction charge of monolayer palladium oxide on Pd nanocrystals surface (420 μC/cm2 ). The ECSA of AuPd@PdAu-PEI nanocrystals is about 18.3 m2 /g, which is 1.9 times higher than that of Pd black (9.5 m2 /g). Mainly, the well dispersion and small particle size effectively increase the available surface for an electrochemical reaction. The pervious works have demonstrated that the surface composition strongly affect the reduction peak of the bimetallic Pd-based nanocrystals. For Pd–Au alloy nanocrystals, the reduction peak of surface oxide positively shifts with increasing atomic ratio of Au [50,51]. In our work, the reduction peak of palladium oxide at AuPd@PdAu-PEI nanocrystals positively shift about 14 mV compared to the commercial Pd black. This slight shift

Please cite this article as: Q. Xue et al., Polyethyleneimine modified AuPd@PdAu alloy nanocrystals as advanced electrocatalysts towards the oxygen reduction reaction, Journal of Energy Chemistry (2017), http://dx.doi.org/10.1016/j.jechem.2017.06.007

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Fig. 4. (a) CV curves of AuPd@PdAu-PEI nanocrystals and commercial Pd black in N2 -saturated 0.1 M NaOH electrolyte at a scan rate of 50 mV/s, (b) ORR polarization curves of AuPd@PdAu-PEI nanocrystals and commercial Pd black in O2 -saturated 0.1 M NaOH electrolyte at a scan rate of 5 mV/s and rotation rate of 1600 rpm. Inset in (b): jk at AuPd@PdAu-PEI nanocrystals and commercial Pd black at 0.90 V.

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indicates that the surface of AuPd@PdAu-PEI nanocrystals is Pdrich PdAu alloy, in consistent with EDX line scanning profiles. The ORR kinetics at AuPd@PdAu-PEI nanocrystals and commercial Pd black were evaluated by LSV using RDE technique in O2 purged 0.1 M NaOH electrolyte at a rotation rate of 1600 rpm. The ORR current densities were achieved by normalizing the current to the geometric area of glassy carbon electrode. For all ORR polarization curves, the diffusion-limited current density region occurs between 1.0 and 0.3 V, and the mixed kinetics-diffusion control region occurs between 0.98 and 0.63 V. The ORR onset potential (Eonset = 0.982 V) and ORR half-wave potential (E1/2 = 0.869 V) at AuPd@PdAu-PEI nanocrystals are higher than those at Pd black (Eonset = 0.921 V and E1/2 = 0.824 V) under the same experimental conditions, indicating improved ORR activity (Fig. 4b). Also, the E1/2 at AuPd@PdAu-PEI nanocrystals (0.869 V) also has obvious positive shift as compared to various Pd-based cathode catalysts in the literature, including hollow PdCu nanospheres (0.850 V vs. RHE) [10], Pd–B nanoparticles (0.860 V vs. RHE) [12], Pd nanoparticles (0.830 V vs. RHE) [12], Pd/TiO2 −x :N (0.810 V vs. RHE) [14], Pt-on-Pd/C (0.862 V vs. RHE) [16], Pd nanosheets (0.834 V vs. RHE) [17], Pd nanowires (0.865 V vs. RHE) [20], confirming the high ORR activity of AuPd@PdAu-PEI nanocrystals. The intrinsic kinetic activity of cathode Pd catalyst towards the ORR was calculated from the corresponding ORR polarization curves using Levich–Koutecky equation [52],

1 1 1 1 1 = + = + j jk jd jk 0.62nF AgeoD2/3 ω1/2 v−1/6Co2 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282

where j was the measured current density, jk was the kinetic current density, jd was the diffusion limited current density, n was electron transferred number per O2 molecule, F was the Faraday constant (96,500 C/mol), Ageo was the electrode area, D was the diffusion coefficient of O2 molecule, ω was the rotation rate, ν was the kinematic viscosity of the NaOH electrolyte, and CO2 was the concentration of O2 molecule in NaOH electrolyte. At 0.90 V potential, the jk values at AuPd@PdAu-PEI nanocrystals and commercial Pd black are 1.553 A/m2 and 0.260 A/m2 , respectively (Insert in Fig. 4b). On the one hand, the introduction of Au elevates the onset oxidation potential of Pd (Fig. 4a), resulting in more available active sites (i.e., metallic Pd) towards the ORR [53,54]. On the other hand, –NH2 groups at PEI strongly donate electrons to Pd atoms, which is responsible for the negative shift in Pd 3d binding energies (Fig. 3c). The Nørskov d-band model theory has demonstrated that the electronic structure of metal strongly affects the adsorption energy of intermediates [55,56]. For the ORR at Pd surface, proton first interact with oxygen to generate the HO2 · and/or HO2 – intermediates, which is prior to the O–O bond cleavage [57]. The downshift of Pd 3d binding energies effectively

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weakens the adsorption energy of HO2 · and/or HO2 – intermediates [58,59], resulting in the improved ORR activity.

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3.3. ORR durability of AuPd@PdAu-PEI nanocrystals

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The long-term durability of an electrocatalyst is a very important issue for their practical application. Herein, an accelerated durability test is conducted by applying repeated potential scans between 1.0 and 0.3 V (vs. RHE) in O2 -saturated 0.1 M NaOH electrolyte. After 10 0 0 cycles, AuPd@PdAu-PEI nanocrystals display about 12 mV degradation value in E1/2 towards the ORR (Fig. 5a), which is much smaller than 22 mV degradation value at commercial Pd black under same experimental conditions (Fig. 5b). These results indicate the improved stability of AuPd@PdAu-PEI nanocrystals towards the ORR. On the one hand, AuPd@PdAu-PEI nanocrystals have a uniform size, which potentially decreases the aggregation/ripening of nanocrystals [60]. On the other hand, Au can effectively stabilize Pd nanocrystals by raising the onset oxidation potential of Pd (Fig. 4a), which also restrain ripening/dissolution of nanocrystals [61].

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3.5. ORR selectivity of AuPd@PdAu-PEI nanocrystals

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As mentioned, the alcohol crossover from the anode to the cathode though exchange membrane is one of the major problems in ADAFCs. The selectivity of cathode catalysts towards the ORR was explored by CV and RDE experiments in the presence of ethanol. Compared to the ORR curve in NaOH electrolyte without ethanol, the ethanol addition caused a negative shift of 160 mV in E1/2 of Pd black towards the ORR (Fig. 6a), originating from the simultaneous methanol oxidation during the ORR. Similarly, AuPdANC without PEI lacks the electro-catalytic selectivity towards the ORR in the presence of ethanol. As observed, the introduction of ethanol seriously decreases the ORR activity of AuPd-ANC (Fig. S5). Under the same experimental conditions, only a very small shift of 17 mV in E1/2 is observed for AuPd@PdAu-PEI nanocrystals (Fig. 6b), showing an excellent ethanol tolerance. Further CV measurements show that the peak current density of ethanol oxidation reaction at AuPd@PdAu-PEI nanocrystals is 32 times smaller than that at commercial Pd black (Fig. S6), revealing the decreased catalytic activity for ethanol oxidation reaction. Mainly, loose-packed PEI layer efficiently act as barrier networks to physically block access of ethanol molecules on Pd sites, originating from the lager molecular size ˚ than that of the oxygen molecule of ethanol molecule (size: 5.1 A) ˚ (size: 3.4 A).

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Fig. 5. ORR polarization curves of (a) AuPd@PdAu-PEI nanocrystals and (b) Pd black before and after 10 0 0 cycles in O2 -saturated 0.1 M NaOH electrolyte at 5 mV/s and rotation rate of 1600 rpm.

Fig. 6. ORR polarization curves of (a) commercial Pd black and (b) AuPd@PdAu-PEI nanocrystals in O2 -saturated 0.1 M NaOH electrolyte with or without 0.1 M CH3 CH2 OH at a scan rate of 5 mV/s and rotation rate of 1600 rpm.

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4. Conclusions

Supplementary materials

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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2017.06.007.

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References

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In this work, AuPd@PdAu-PEI nanocrystals with high selectivity, activity, and durability towards the ORR in the alkaline electrolyte were synthesized by a facile one-step synthesis pathway in the presence of PEI. The E1/2 value (0.869 V) at AuPd@PdAu-PEI nanocrystals towards the ORR is much higher than that (0.824 V) at Pd black under the same experimental conditions, originating from the change in the binding energies of Pd atoms. After accelerated durability tests, AuPd@PdAu-PEI nanocrystals display a less degradation value in E1/2 towards the ORR compared with Pd black (12 mV vs. 22 mV), indicating that the introduction of Au can effectively improve electrochemical stability of Pd. In particular, loosepacked PEI layer on AuPd@PdAu-PEI nanocrystals efficiently acted as barrier networks to physically block access of ethanol molecules on Pd sites, which imparted AuPd@PdAu-PEI nanocrystals with particular alcohol tolerance towards the ORR in the alkaline electrolyte. The present work demonstrated that the rational composition control and surface modification are highly efficient strategies for improving the activity, selectivity, and durability of Pd-based nanocrystals towards the ORR.

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Acknowledgments

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This research was sponsored by the National Natural Science Foundation of China (21473111) and Fundamental Research Funds for the Central Universities (GK201602002 and GK201701007).

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