PdBi Composite Nanochains as Highly Active and Durable

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Letter Cite This: Nano Lett. 2019, 19, 4752−4759

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Bi(OH)3/PdBi Composite Nanochains as Highly Active and Durable Electrocatalysts for Ethanol Oxidation Xiaolei Yuan,†,∥ Yong Zhang,†,∥ Muhan Cao,†,∥ Tong Zhou,‡ Xiaojing Jiang,† Jinxing Chen,†,§ Fenglei Lyu,† Yong Xu,† Jun Luo,‡ Qiao Zhang,*,† and Yadong Yin*,§

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†

Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, SWC for Synchrotron Radiation Research, Soochow University, 199 Ren’ai Road, Suzhou 215123, Jiangsu, People’s Republic of China ‡ Institute for New Energy Materials and Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China § Department of Chemistry, University of CaliforniaRiverside, Riverside, California 92521, United States S Supporting Information *

ABSTRACT: Developing high-performance electrocatalysts for the ethanol oxidation reaction (EOR) is critical to the commercialization of direct ethanol fuel cells. However, current EOR catalysts suffer from high cost, low activity, and poor durability. Here we report the preparation of PdBiBi(OH)3 composite nanochains with outstanding EOR activity and durability. The incorporation of Bi can tune the electronic structure and downshift the d-band center of Pd while the surface decoration of Bi(OH)3 can facilitate the oxidative removal of CO and other carbonaceous intermediates. As a result, the nanochains manifest an exceptional mass activity (5.30 A mgPd−1, 4.6-fold higher than that of commercial Pd/C) and outstanding durability (with a retained current density of ∼1.00 A mgPd−1 after operating for 20 000 s). More importantly, the nanochain catalyst can be reactivated, and negligible activity loss has been observed after operating for 200 000 s with periodic reactivation, making it one of the best EOR catalysts. KEYWORDS: PdBi nanochains, bismuth hydroxide, necklacelike structure, EOR, long-term stability

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optimizing the binding strength among the reactant, intermediates, and product with the active sites through tuning the electronic structure of Pd.20−23 Much less attention has been paid to addressing the durability issue, although it has been reported that improvement can be made by fabricating intermetallic alloys or metal oxides/hydroxides composites.24,25 In the composite system, the synergy between Pd and other metals or metal oxides/hydroxides is believed to weaken the binding strength of the poisoning species on Pd. For example, Li et al. reported a composite system by depositing Pd nanoparticles on graphene/Ni(OH)2 to form hybrid electrocatalysts. The presence of Ni(OH)2 was found to provide abundantly adsorbed OH (OHad) species, which can facilitate the oxidative removal of carbonaceous species on Pd sites and improve the EOR stability.26 In this work, we report the development of a highperformance Pd-based nanocomposite electrocatalyst that takes advantage of the synergistic effect between nanoscale oxides and metal particles to significantly boost durability. The design of the catalyst configuration is also driven by our

eveloping alternative energy sources to replace fossil fuels has been a vital task in modern society. Direct ethanol fuel cells (DEFCs), a reliable alternative source, have been regarded as a promising candidate because of their high energy density, low toxicity, convenient storage, and transportation of the liquid fuel.1−4 However, the commercial application of DEFCs has been greatly limited by the challenges in the development of catalysts. Platinum (Pt)based materials have been the most widely used catalysts for anode operation, while their high cost (>55% of the total cost) and poor durability have impeded the practical applications.5 It is thus highly desirable to develop non-platinum-based electrocatalysts. Palladium (Pd) has drawn much attention as an alternative catalyst for the ethanol oxidation reaction (EOR). Although much effort has been devoted to exploring high-performance Pd-based catalysts, their practical applications have been hampered by the relatively low activity and severe loss of activity caused by the poisoning of intermediates (especially CO) during the operation.6−8 Making Pd into nanostructures has been attempted to expose more active sites and efficiently increase the mass activity.9−11 Furthermore, integrating Pd with a second metal, such as Pt,12,13 Au,14,15 Ag,16 Sn,17 Co,18 or Cu,19 has been found to boost the intrinsic activity of Pd by © 2019 American Chemical Society

Received: May 5, 2019 Revised: June 10, 2019 Published: June 12, 2019 4752

DOI: 10.1021/acs.nanolett.9b01843 Nano Lett. 2019, 19, 4752−4759

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Figure 1. Structural characterizations of the PdBi-Bi(OH)3 nanochains. (a) Typical TEM image, (b) HRTEM image, (c) SAED pattern, (d−f) aberration-corrected HAADF images, and (g) EDS elemental mapping images.

nanocrystals were electrochemically treated through a cyclic voltammetry (CV) process in an alkaline solution (1 M NaOH) which could oxidize metallic Bi to Bi2O3 and Bi(OH)3 when the potential reached 0.54 V vs the reversible hydrogen electrode (RHE).27 In addition, a series of samples with different Pd/Bi ratios were prepared (more details in the Experimental Section). Figures 1a and S1 showed the transmission electron microscope (TEM) images of the as-prepared PdBi-Bi(OH)3 nanostructure, which exhibited necklacelike chain morphology with a diameter of 11.3 ± 2.2 nm. From energy-dispersive Xray spectroscopy (EDS) elemental mapping results (Figure 1g and S2a), Pd and Bi elements were distributed uniformly in which Pd acted as the primary host while a small amount of Bi could be observed. From EDS data, the elemental ratio of Pd/ Bi was ca. 81.5:18.5 (Figure S2b), which was in good agreement with the ICP−OES (inductively coupled plasma− optical emission spectrometry) data (Pd/Bi = 84.2:15.8). X-ray diffraction (XRD) characterization was employed to investigate the crystal structure of PdBi-Bi(OH)3 nanochains (Figure S3). The diffraction peaks at 2θ = 38.8, 45.1, 65.8, 79.0, and 83.4° could be indexed as the {111}, {200}, {220}, {311}, and {222} facets of Pd with face-centered cubic (fcc) phases (JCPDS card no. 46-1043), respectively.28,29 No characteristic peak of Bi (JCPDS 85-1329) or Bi(OH)3 (JCPDS 01-898) could be observed. Compared to pure Pd, the peak position of PdBi-Bi(OH)3 was shifted to lower angles, indicating the incorporation of larger Bi atoms (atomic number 83) into the Pd (atomic number 46) lattice. From the highresolution transmission electron microscopy (HRTEM) image of PdBi-Bi(OH)3 nanochains (Figure 1b), the lattice spacing was 0.23 nm, which could be indexed as the (111) plane of Pd. The selected-area electron diffraction (SAED) pattern (Figure 1c) showed the polycrystalline nature of the nanochains, in which the diffraction patterns were indexed as Pd, such as {111}, {200}, and {220}.30,31

understanding that the promotion of both the activity and stability of Pd materials can be achieved by alloying Pd with other metals, especially 3d transition metals to optimize the Pd−Pd interatomic distance and neighboring environment and tune the d-band center of Pd. Specifically, here we demonstrate the preparation of PdBi-Bi(OH)3 composite nanochains and their application as a highly efficient and durable electrocatalyst for EOR in an alkaline environment. Different from previous reports where metal particles were physically mixed with oxides, the as-prepared PdBi-Bi(OH)3 nanochains were strongly coupled, in which Bi(OH)3 was converted from PdBi and used to decorate the surface of PdBi nanochains. Because of the incorporation of Bi and the decoration of surface Bi(OH)3 species, the nanocomposite catalyst manifests not only high EOR activity (5.30 A mgPd−1), which was 4.6fold higher than that of commercial Pd/C, but also high durability, with the ability to maintain 1.00 A mgPd−1 even after a chronoamperometric (CA) test for 20 000 s. More importantly, the catalysts could be reactivated by simply replacing the used electrolyte with a new one, indicating superior stability. No obvious activity loss has been observed after the CA test for 200 000 s. Through systematic analysis using X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), and CO-stripping tests, we attribute the dramatically improved activity and stability to the strong interaction between Bi species and Pd that can tune the electronic structure of Pd and enhance the CO tolerance. The PdBi-Bi(OH)3 nanochains were successfully synthesized through a two-step approach. In a typical process, palladium(II) acetylacetonate (Pd(acac)2), bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), and poly(vinylpyrrolidone) (PVP) were dissolved in diethylene glycol (DEG) in a threenecked flask. The solution was then held at 120 °C for 15 min. The product was separated from the solution by centrifugation and washed three times using a mixture of acetone and ethanol (volume ratio of 1:1). Subsequently, the obtained PdBi 4753

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Figure 2. XPS spectra of (a) Pd 3d and (b) Bi 4f of PdBi-Bi(OH)3 nanochains. XANES spectra at (c) the Pd K3 edge and (d) the Bi L3 edge.

Figure 3. CV curves obtained from PdBi-Bi(OH)3 nanochains and commercial Pd/C in N2-saturated (a) 1 M NaOH and (b) 1 M NaOH solution containing 1 M ethanol solution at a scan rate of 50 mV s−1. (c) CA curves of PdBi-Bi(OH)3 nanochains and commercial Pd/C measured in N2saturated 1 M NaOH solution containing 1 M ethanol, recorded at 0.86 V vs RHE. (d) Long-term curves of PdBi-Bi(OH)3 nanochains and commercial Pd/C in N2-saturated 1 M NaOH solution containing 1 M ethanol, recorded at 0.86 V.

intensity ratios than the other ones (red circles). In general, the brighter atoms had larger average atomic numbers because of the high sensitivity of high-angle annular dark-field imaging (HAADF) for variations in the atomic number of elements.33,34 Therefore, the atom projections in the green and red circles could be assigned to atom columns containing Bi and pure Pd, respectively. Obviously, the continuous Pd surface was interrupted by the incorporation of Bi species, which may also bring about enhanced catalytic performance.22

An aberration-corrected atomic-resolution scanning transmission electron microscope (STEM) was used to further investigate the surface structure of PdBi-Bi(OH)3 nanochains. As shown in Figure 1d,e, abundant defects, such as vacancies, twins, grain boundaries and low-coordination-number atoms (edges, terraces, and steps), could be clearly identified in the surface regions. According to the previous reports, these defects might provide more active sites and boost the electrochemical performance of catalysts.5,32 From Figure 1f, some atoms (green circles) have much higher brightness and 4754

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electrocatalyst.41 The If/Ir value of PdBi-Bi(OH)3 nanochains could reach 2.5, which was much higher than that of Pd/C (0.9), suggesting a much stronger tolerance against poisoning by intermediate species.26 The current density of PdBiBi(OH)3 nanochains was 5.30 A mgpd−1, which was 4.6 times higher than that of Pd/C. Meanwhile, we evaluated the influence of composition and shape. PdBi-Bi(OH)3 samples with different Pd/Bi ratios were tested (Figures S5 and S6). As depicted in Figure S7, the PdBi-Bi(OH)3 nanochains with a Pd/Bi ratio of ∼81.5:18.5 (feeding ratio 1:1) had the best performance. Compared to the nanoparticles, nanochains with different Pd/Bi ratios had higher activity and stability as a result of their unique property against aggregation.10 Moreover, nanochains with more coverage of Bi species will result in lower activity due to the reduced active areas. As discussed above, an important challenge in the commercialization of EOR electrocatalysts is their poor operation durability. In some cases, the electrocatalysts became inactive within several minutes. To evaluate the long-term EOR stability of the samples, we conducted the test through a chronoamperometric method operating at 0.86 V for 20 000 s. The commercial Pd/C was used as the reference. As shown in Figure 3c, the current density of Pd/C dropped to almost zero after 20 000 s, suggesting the poor EOR stability. In contrast, the current density of PdBi-Bi(OH)3 nanochains could remain >1.00 A mgPd−1 (or 36.0% of the initial current density) after the same treatment (operating at 0.86 V for 20 000 s). More impressively, the PdBi-Bi(OH)3 could be reactivated electrochemically in a fresh electrolyte (Figure 3d). After each 20 000 s stability test, the PdBi-Bi(OH)3 catalyst was first reactivated by four CV cycles in 1 M NaOH solution, followed by the CA test in a fresh 1 M NaOH electrolyte containing 1 M ethanol. After reactivation, PdBi-Bi(OH)3 could retain its original EOR activity. After 10 consecutive cycles (200 000 s), there was no significant drop in the EOR activity, indicating the outstanding durability of PdBi-Bi(OH)3. As a reference, the EOR stability of commercial Pd/C has also been evaluated. During each cycle, the current density of Pd/C decreased rapidly in the first 300 s, with a loss of more than 60% of their initial current density, and further dropped to almost zero after 20 000 s, indicating poor durability. To determine the possible mechanism for the enhanced EOR durability of the PdBiBi(OH)3 sample, we carried out a control experiment by simply mixing commercial Pd/C and commercial Bi(OH)3 nanoparticles (Figure S8). Compared to Pd/C catalysts, no enhancement in the EOR activity or stability has been observed for the physically mixed sample (Figure S9). Moreover, to further understand the role of Bi(OH)3, we employed an acid-etching method to obtain PdBi by removing the surface Bi(OH)3 from the PdBi-Bi(OH)3. After being immersed in 0.5 M H2SO4 for 12 h, PdBi particles maintained their chainlike morphology (Figure S10). As shown in Figure S11a, the mass activity of PdBi was 2.18 A mg−1, which was much lower than that of initial PdBi-Bi(OH)3 nanochains. The current density of PdBi dropped quickly during the CA test, suggesting the poor EOR stability (Figure S11b). Moreover, the If/Ir value of PdBi dropped to 1.13, which was lower than that of PdBi-Bi(OH)3 nanochains (2.5), suggesting a much lower tolerance to intermediate species. Therefore, we concluded that Bi(OH)3 played an important role in enhancing the activity and stability in EOR. The high EOR activity and excellent stability made PdBi-Bi(OH)3 one of the best-reported EOR electrocatalysts (Table S1).

X-ray photoelectron spectroscopy (XPS) was carried out to obtain more information about the binding states of PdBiBi(OH)3 nanochains. Figure 2a showed that the Pd signal consisted of a low-energy band (Pd 3d5/2 at 335.7 eV) and a high-energy band (Pd 3d3/2 at 340.9 eV), suggesting the metallic Pd0 states.35 In the nanochains, binding energies of 163.5 and 158.1 eV were assigned to Bi 4f5/2 and Bi 4f7/2 of Bi0, respectively, as plotted in Figure 2b. In addition, a considerable quantity of Bi3+ could be observed, which was ascribed to the contributions from Bi(OH)3 (159.0 and 164.3 eV) and Bi2O3 (159.9 and 165.1 eV).27,36 The O 1s signal further confirmed the existence of Bi−O (530.7 eV) in Bi2O3 and O−H in Bi(OH)3 (531.8 eV) (Figure S4a).37 The XPS result allowed us to conclude that the resulting product had a multicomponent structure, including bimetallic PdBi, Bi2O3, and Bi(OH)3, while Bi2O3 and Bi(OH)3 arose from the electrochemical treatment.36 To further verify the local structure and possible interaction between Pd and Bi, X-ray absorption nearedge structure (XANES) analysis was performed at the Pd K3 edge and the Bi L3 edge. Pd foil, Bi foil, commercial PdO, Bi2O3, and Bi(OH)3 were used as references to calibrate the energy of the absorption edge. The Pd K3 edge spectrum of the as-prepared product had features very similar to those of the Pd foil, indicating the metallic state of Pd in the nanochains (Figure 2c). In contrast, XANES results confirmed that the Bi species was a mixture of metallic Bi, Bi2O3, and Bi(OH)3 (Figure 2d). Further analysis indicated that the Pd K3 edge of nanochains slightly shifted to lower energy compared to that of the Pd foil (Figure S4b), indicating that electrons transferred from metallic Bi to Pd, which made the Pd in the product electron-rich.38,39 The electron-rich structure of Pd would contribute to its downshift of the d-band center, which could adjust the adsorption/desorption capacities of reactant molecules or intermediates, leading to enhanced EOR performance.19,39 CV curves of both PdBi-Bi(OH)3 and commercial Pd/C were recorded in N2-saturated 1 M NaOH (Figure 3a). For Pd/C, peaks from 0.1 to 0.3 V were attributed to the hydrogen adsorption/desorption, while the pronounced cathodic peak from 0.7 to 0.9 V corresponded to the reduction of PdO to Pd. In contrast, the hydrogen adsorption/desorption peaks disappeared almost completely for the PdBi-Bi(OH)3 sample, indicating a high coverage of Bi species on the surface of Pd and the strong interaction between Pd and Bi. In addition, the anode peak of PdBi-Bi(OH)3 between 0.8 and 1.0 V can be ascribed to the formation of the PdO monolayer and Bi3+. The pronounced cathode peaks at around 0.75 V can be assigned to the reduction of Pd2+ to Pd and Bi3+ to Bi.27 The EOR performance of PdBi-Bi(OH)3 and Pd/C was evaluated in 1 M NaOH containing 1 M ethanol, as shown in Figure 3b. For both catalysts, there were two peaks: a peak with higher density in the range of 0.8 and 1.0 V in the forward scan and a peak with lower density in the range of 0.6 and 0.8 V in the reverse scan. The peak in the forward scan at 0.84 V could be assigned to the oxidation of ethanol into intermediates (mainly acetate), while the peak in the reverse scan at 0.75 V was attributed to the further oxidation of CHx and CO to CO2 (C1 pathway).40 On the basis of the previous reports, the main pathway was the C2 pathway and the dominating product was acetate.26 The ratio between the intensity of the forward scan peak (If) and the reverse scan peak (Ir) reflected the degree of oxidation of ethanol to final product CO2 and the tolerance of poison such as CO in an 4755

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Figure 4. CO stripping measurements of (a) commercial Pd/C and (b) PdBi-Bi(OH)3 nanochains. (c) Enlarged area of CO-stripping curves at potentials from 0.65 to 0.85 V. Response of (d) commercial Pd/C and (e) PdBi-Bi(OH)3 nanochains to bubbling CO in 1 M NaOH containing 1 M ethanol during the CA tests at 0.86 V. (f) Schematic illustration of the oxidative removal mechanism of CO on PdBi-Bi(OH)3 nanochains sites in which yellow, blue, red, green, and gray spheres represent Pd, Bi, O, H, and C, respectively.

the Pd actives, which provided antipoisoning sites for the Pd electrode. It has been reported that the rate-determining step of EOR on Pd in alkaline media was the oxidative removal of adsorbed intermediates (e.g., CO) in the presence of OH species.6 There were two widely accepted reaction mechanisms: the Eley−Rideal mechanism,44,45 in which free OH− ions from the alkaline solution could help to remove intermediates, and the Langmuir−Hinshelwood mechanism, in which adsorbed OH (OHad) species around Pd sites acted as the main component (COad + OHad → CO2 + H2O).46−48 For efficient electrocatalysts, the Langmuir−Hinshelwood mechanism could be the dominant mechanism if sufficient OHad could be provided around Pd sites and the ratedetermining step could be accelerated dramatically. To gain more in-depth insight into the high EOR stability of PdBi-Bi(OH)3, the CO stripping experiment was employed as a model reaction for evaluating the CO tolerance of catalysts.26,49 An ideal EOR electrocatalyst should be capable of rapidly removing the COad intermediate at lower oxidation potentials. For the commercial Pd/C sample, peaks ranging from 0.8 to 0.9 V in the first scan could be attributed to the oxidation of adsorbed CO into CO2 on Pd (Figure 4a).50 For the PdBi-Bi(OH)3 sample, the peak at around 0.9 V in the first scan was identified as the combination of CO oxidation on the Pd surface and the surface oxidation of PdBi-Bi(OH)3 (Figure 4b). A new peak range from 0.7 to 0.8 V might stem from the oxidation of CO adsorbed on Pd adjacent to Bi(OH)3. As shown in Figure 4c, the onset potential of CO oxidation on PdBi-Bi(OH)3 nanochains shifted 100 mV in the negative direction compared to that of Pd/C, which may arise from the downshift of the Pd d-band center.51 The appearance of the negative peak indicated that the presence of Bi(OH)3 facilitated the oxidative removal of CO adsorbed on the adjacent Pd.26 To further demonstrate the enhanced CO tolerance of PdBi-Bi(OH)3, CO was purged into the electrolytes at 200 s during the EOR test (Figure 4d,e). For commercial Pd/C, upon the purging of CO, the current

The poor stability of nanomaterials used in electrocatalysis might be due to two main reasons. First, the high surface energy of nanoparticles may cause agglomeration or a morphology change, leading to the loss of active sites and low activity and selectivity. Second, Pd-based electrocatalysts were suffering from the rapid loss of activity because of the complex catalytic process and the poisoning of intermediate species. In this work, the unique morphology of onedimensional nanochains may better conserve the initial shape and overcome possible aggregation.42,43 The morphology of the PdBi-Bi(OH)3 sample after 10 consecutive EOR cycles (200 000 s) was characterized with TEM. As shown in Figure S12a, the morphology of PdBi-Bi(OH)3 was well-maintained. From the HRTEM image, the 0.23 nm lattice spacing of Pd can be clearly observed, and the SAED pattern also showed the polycrystalline nature of Pd (Figure S12b,c). Moreover, elemental mapping and EDS line scans confirmed that Pd and Bi atoms were distributed homogeneously over the whole nanochains (Figure S12d,f). The average Pd/Bi ratio quantified by EDS was ca. 85.6:14.4 (Figure S12e,f), which could well match the initial results (Pd/Bi = 81.5:18.5). These postcatalytic characterizations verified the structural robustness of the PdBi-Bi(OH)3 nanochains, which enabled high EOR stability. As a reference, the morphology of commercial Pd/C catalysts has also been characterized. As shown in Figure S13, a severe agglomeration of Pd nanoparticles can be observed. After treatment at 0.86 V for 80 000 s, the average particle size of Pd nanoparticles increased from 5.3 to 6.4 nm. Figure S14 showed the CV curves of PdBi-Bi(OH)3 nanochains and Pd/C before and after testing for 80 000 s, from which the exceptional durability of PdBi-Bi(OH)3 has also been confirmed. Moreover, the continuous Pd surface has been interrupted by the incorporation of Bi, which weakened the contiguous adsorption of intermediate species and enhanced the resistivity against poisoning.22 In addition, the obvious enhancement in the stability could also be attributed to strongly coupled Bi(OH)3 content near 4756

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Pd(acac)2 and Bi(NO3)3·5H2O were predissolved in 3 mL of DEG (the molar ratio of Pd2+ to Bi3+ was 1:1), and the precursor solution was rapidly injected into the flask under a N2 atmosphere by syringe. The reaction mixture was heated to 120 °C and held for another 15 min to ensure the complete reaction. The final product was collected by centrifugation after cooling to room temperature. The black product was washed with an ethanol and acetone mixture (1:1) three times to remove excess PVP. The resulting sample was first treated in N2-saturated 1 M NaOH by cyclic voltammetry (CV) cycling between 0 and 1.2 V vs the reversible hydrogen electrode (RHE) at a scan rate of 50 mV s−1 for 20 cycles. Then, the final activated process was performed using chronoamperometry (CA) tests at 0.86 V in 1 M NaOH + 1 M ethanol for 3600 s. To remove the surface Bi(OH)3, the sample was immersed in 0.5 M H2SO4 for 12 h. In the same way, different ratios of Pd/ Bi nanocrystals were also obtained by adjusting the addition molar ratios of Pd/Bi precursors as 2:1, 1:2, and 1:3. Characterization. Transmission electron microscope (TEM) images were taken on a TECNAI G2 LaB6 operated at 200 kV, and high-resolution TEM images (HRTEM) and elemental mapping information were obtained on a TECNAI G2F20 operated at 200 kV. Aberration-corrected STEM images with a probe corrector were obtained on Titan Cubed Themis G2 300 at 200 kV. X-ray diffraction (XRD) was performed on an Empyrean diffractometer with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Kratos AXIS Untraded ultrahigh vacuum (UHV) surface analysis system, and the binding energy of C 1s (285.4 eV) was used as the reference. X-ray absorption near-edge structure (XANES) analysis was performed at the Pd K3 edge and the Bi L3 edge and collected at beamline 17C1 at the National Synchrotron Radiation Research Center (NSRRC, Hsinchu, Taiwan, China) with 1.5 GeV energy and a 360 mA storage ring current in top-up mode which was a constant current, from 200 eV below to 800 eV above the edge in fluorescence mode with a step size of 0.25 eV in the near-edge region and a dwell time of 2 s. The Pd K3 edge and Bi L3 edge were calibrated against their metallic foils, commercial PdO, Bi2O3, and Bi(OH)3. To identify the composition of the samples, inductively coupled plasma− atomic emission spectroscopy (ICP−AES) was performed on a Varian 710-ES. Electrochemical Measurements. Electrochemical experiments were performed in a standard three-electrode system on a CHI660E workstation. A glassy carbon electrode (3 mm in diameter), Pt gauze, and saturated calomel electrode (SCE) were used as the working, counter, and reference electrodes, respectively. All of the potentials reported were referenced vs RHE on the basis of the equation with IR compensation: E(SCE) + 0.242 V + 0.0592(pH) = E(RHE). The ink was made as follows: the as-obtained catalysts and Vulcan XC-72 carbon (20% weight of the Pd−Bi catalysts) were dispersed in 1 mL of ethanol, water, and Nafion mixing solution (1:1:0.06 v/v/v) and then ultrasonicated for 40 min to form a uniform ink. As-prepared ink solution was drop cast onto the glassy carbon electrode and then dried at room temperature. The loading amount of Pd for all the samples was kept at 28 μg cm−2 (determined by ICP−AES). For CV curves, the electrochemical reactions were conducted at a scan rate of 50 mV s−1 in N2-saturated 1 M NaOH with or without 1 M ethanol. The long-term stability of the as-prepared samples was estimated by CA measurements at 0.86 V vs RHE in 1 M

density suddenly dropped to zero, implying their poor CO tolerance. In contrast, for the PdBi-Bi(OH)3 sample, more than 50.9% of the original current density could be maintained after 3600 s, suggesting the dramatically enhanced CO tolerance. As depicted in Figure 4f, the OHad species formed on Bi(OH)3 provided an efficient way to accelerate the oxidative removal of CO or other intermediates on Pd active sites. The strongly coupled Bi(OH)3 played an important role in determining the antipoisoning property. On the basis of the above discussion and analysis, we can attribute the outstanding electrocatalytic performance of the as-prepared necklacelike PdBi-Bi(OH)3 nanostructure to the following factors. (i) It has been widely accepted that defects can boost the catalytic performance of metal nanoparticles. Here, the unique necklacelike structure can provide abundant defects, including vacancies, twins, grain boundaries, and lowcoordination-number atoms (edges, terraces, and steps), leading to significantly improved catalytic activity. (ii) The electronic structure and the d-band center of Pd have been tuned by the incorporation of Bi. As a result, the catalytic performance has been dramatically enhanced because the adsorption/desorption properties of reactants and intermediates could be varied accordingly. (iii) The antipoisoning capability of the product has been boosted by the decoration of surface Bi(OH)3 that can provide abundant OHad species and accelerate the oxidative removal of CO or other carbonaceous intermediates through the Langmuir−Hinshelwood mechanism. In summary, we report here the successful synthesis of necklacelike PdBi-Bi(OH)3 nanochains that exhibit superior electrocatalytic activity and durability for EOR application. Benefiting from the tuned electronic structure of Pd and abundant defects brought about by the incorporation of Bi as well as the surface decoration of Bi(OH)3, the PdBi-Bi(OH)3 nanochains manifested a high mass activity of 5.30 A mgPd−1. More importantly, the surface decoration of Bi(OH)3 greatly facilitated the oxidative removal of intermediate poisoning (especially CO) via the Langmuir−Hinshelwood mechanism. More than 36.0% of the original current density (ca. 1.00 A mgPd−1) could be maintained after 20 000 s CA tests. More impressively, the catalyst could be reactivated simply. Negligible activity loss could be observed after 200 000 s CA tests, indicating a high potential for practical applications. This work highlighted the importance of the rational incorporation of a non-noble metal with Pd for improved electrocatalytic EOR activity and stability, providing excellent opportunities for the commercial application of DEFCs.

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EXPERIMENTAL METHODS Chemicals. Palladium acetylacetonate (Pd(acac)2, 99%), bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, 99%), polyvinylpyrrolidone (PVP, MW ∼ 40,000, 99%) and commercial Pd/C (40% wt.% of Pd) were purchased from Sigma-Aldrich. Commercial bismuth hydroxide (Bi(OH)3, 90%) was purchased from Aladdin. Diethylene glycol (DEG, 98%), ethanol (99%) and acetone (99%) were obtained from Sinopharm Chemical Reagent. Nafion alcohol solution (5 wt. %) was purchased from Alfa Aesar. All chemicals were used as received without further purification. Synthesis of PdBi-Bi(OH)3 nanochains. The synthesis was carried out by using a standard Schlenk line. In a typical synthesis, 17 mL DEG containing 0.55 g PVP was heated at 60 °C to remove the impurity under vacuum condition. Then, 4757

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(7) Monyoncho, E. A.; Steinmann, S. N.; Michel, C.; Baranova, E. A.; Woo, T. K.; Sautet, P. ACS Catal. 2016, 6, 4894−4906. (8) Rizo, R.; Aran-Ais, R. M.; Padgett, E.; Muller, D. A.; Lazaro, M. J.; Solla-Gullon, J.; Feliu, J. M.; Pastor, E.; Abruna, H. D. J. Am. Chem. Soc. 2018, 140, 3791−3797. (9) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Norskov, J. K.; Jaramillo, T. F. Science 2017, 355, No. eaad4998. (10) Wu, H.; Li, H.; Zhai, Y.; Xu, X.; Jin, Y. Adv. Mater. 2012, 24, 1594−1597. (11) Chen, X.; Wu, G.; Chen, J.; Chen, X.; Xie, Z.; Wang, X. J. Am. Chem. Soc. 2011, 133, 3693−3695. (12) Jiang, B.; Li, C.; Qian, H.; Hossain, M. S. A.; Malgras, V.; Yamauchi, Y. Angew. Chem., Int. Ed. 2017, 56, 7836−7841. (13) Hu, C.; Cheng, H.; Zhao, Y.; Hu, Y.; Liu, Y.; Dai, L.; Qu, L. Adv. Mater. 2012, 24, 5493−5498. (14) Liu, J.; Zheng, Y.; Hong, Z.; Cai, K.; Zhao, F.; Han, H. Sci. Adv. 2016, 2, No. e1600858. (15) Chen, Y.; Fan, Z.; Luo, Z.; Liu, X.; Lai, Z.; Li, B.; Zong, Y.; Gu, L.; Zhang, H. Adv. Mater. 2017, 29, 1701331. (16) Qi, J.; Benipal, N.; Liang, C.; Li, W. Appl. Catal., B 2016, 199, 494−503. (17) Luo, Z.; Lu, J.; Flox, C.; Nafria, R.; Genç, A.; Arbiol, J.; Llorca, J.; Ibáñez, M.; Morante, J. R.; Cabot, A. J. Mater. Chem. A 2016, 4, 16706−16713. (18) Wang, A. L.; He, X. J.; Lu, X. F.; Xu, H.; Tong, Y. X.; Li, G. R. Angew. Chem., Int. Ed. 2015, 54, 3669−3673. (19) Jiang, K.; Wang, P.; Guo, S.; Zhang, X.; Shen, X.; Lu, G.; Su, D.; Huang, X. Angew. Chem., Int. Ed. 2016, 55, 9030−9035. (20) Zhang, L.; Chang, Q.; Chen, H.; Shao, M. Nano Energy 2016, 29, 198−219. (21) Luo, M.; Sun, Y.; Wang, L.; Guo, S. Adv. Energy Mater. 2017, 7, 1602073. (22) Gao, F.; Goodman, D. W. Chem. Soc. Rev. 2012, 41, 8009− 8020. (23) Zhang, L.; Xie, Z.; Gong, J. Chem. Soc. Rev. 2016, 45, 3916− 3934. (24) Xu, C.; Tian, Z.; Shen, P.; Jiang, S. P. Electrochim. Acta 2008, 53, 2610−2618. (25) Antolini, E. Energy Environ. Sci. 2009, 2, 915−931. (26) Huang, W.; Ma, X. Y.; Wang, H.; Feng, R.; Zhou, J.; Duchesne, P. N.; Zhang, P.; Chen, F.; Han, N.; Zhao, F.; Zhou, J.; Cai, W. B.; Li, Y. Adv. Mater. 2017, 29, 1703057. (27) Zalineeva, A.; Serov, A.; Padilla, M.; Martinez, U.; Artyushkova, K.; Baranton, S.; Coutanceau, C.; Atanassov, P. B. J. Am. Chem. Soc. 2014, 136, 3937−3945. (28) Li, X.; Wang, X.; Liu, M.; Liu, H.; Chen, Q.; Yin, Y.; Jin, M. Nano Res. 2018, 11, 780−790. (29) Liu, Z.; Fu, G.; Li, J.; Liu, Z.; Xu, L.; Sun, D.; Tang, Y. Nano Res. 2018, 11, 4686−4696. (30) Zhu, Q.-L.; Tsumori, N.; Xu, Q. Chem. Sci. 2014, 5, 195−199. (31) Ding, Y.; Fan, F.; Tian, Z.; Wang, Z. L. J. Am. Chem. Soc. 2010, 132, 12480−12486. (32) Lim, B.; Jiang, M.; Camargo, P. H.; Cho, E. C.; Tao, J.; Lu, X.; Zhu, Y.; Xia, Y. Science 2009, 324, 1302−1305. (33) Browning, N. D.; Chisholm, M. F.; Pennycook, S. J. Nature 2006, 444, 235. (34) Liu, X. Z.; Ge, B. H.; Xu, Z.; Gu, L. Science 2018, 360, 19−22. (35) Tiwari, J. N.; Lee, W. G.; Sultan, S.; Yousuf, M.; Harzandi, A. M.; Vij, V.; Kim, K. S. ACS Nano 2017, 11, 7729−7735. (36) Casella, I. G.; Contursi, M. Electrochim. Acta 2006, 52, 649− 657. (37) Su, Y.-Z.; Xiao, K.; Li, N.; Liu, Z.-Q.; Qiao, S.-Z. J. Mater. Chem. A 2014, 2, 13845−13853. (38) Yuan, X.; Jiang, X.; Cao, M.; Chen, L.; Nie, K.; Zhang, Y.; Xu, Y.; Sun, X.; Li, Y.; Zhang, Q. Nano Res. 2019, 12, 429−436. (39) Chen, Z.; Zhang, J.; Zhang, Y.; Liu, Y.; Han, X.; Zhong, C.; Hu, W.; Deng, Y. Nano Energy 2017, 42, 353−362. (40) Brodsky, C. N.; Young, A. P.; Ng, K. C.; Kuo, C.-H.; Tsung, C.K. ACS Nano 2014, 8, 9368−9378.

NaOH + 1 M ethanol. CO stripping experiments were conducted in 1 M NaOH. High-purity gaseous CO was first bubbled into the cell at a potential of 0.21 V (vs RHE) for 30 min. Then, the electrolyte was bubbled with N2 for 15 min to eliminate the dissolved CO in the solution. Finally, two complete cycles of CO-stripping voltammograms were recorded at a scan rate of 50 mV s−1. For comparison, commercial Pd/C (20 wt % Pd nanoparticles supported on Vulcan XC-72 carbon) was employed as a reference in our system.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.9b01843.

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Additional characterization, electrocatalytic testing curves, and a comparison of various previously reported catalysts (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.Z.). *E-mail: [email protected] (Y.Y.). ORCID

Fenglei Lyu: 0000-0002-9019-1909 Qiao Zhang: 0000-0001-9682-3295 Yadong Yin: 0000-0003-0218-3042 Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Key R&D Program of China (2016YFE0129600 and 2017YFA0700104), the National Natural Science Foundation of China (21673150 and 21703146), and the Natural Science Foundation of Jiangsu Province (BK20180097). We acknowledge financial support from the 111 Project, the Collaborative Innovation Center of Suzhou Nano Science and Technology (NANO-CIC), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). We also acknowledge Dr. Luoyue Zhang for collecting the XANES data. Y.Y. is grateful for support from the UC-KIMS Center for Innovation Materials for Energy and Environment, which is jointly funded by UC Riverside and the Korea Institute of Materials Science (research program POC2930).

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REFERENCES

(1) Debe, M. K. Nature 2012, 486, 43−51. (2) Kamarudin, M. Z. F.; Kamarudin, S. K.; Masdar, M. S.; Daud, W. R. W. Int. J. Hydrogen Energy 2013, 38, 9438−9453. (3) Hunt, S. T.; Milina, M.; Alba-Rubio, A. C.; Hendon, C. H.; Dumesic, J. A.; Román-Leshkov, Y. Science 2016, 352, 974−978. (4) Sapi, A.; Liu, F.; Cai, X.; Thompson, C. M.; Wang, H.; An, K.; Krier, J. M.; Somorjai, G. A. Nano Lett. 2014, 14, 6727−6730. (5) Li, C.; Yuan, Q.; Ni, B.; He, T.; Zhang, S.; Long, Y.; Gu, L.; Wang, X. Nat. Commun. 2018, 9, 3702. (6) Chen, L.; Lu, L.; Zhu, H.; Chen, Y.; Huang, Y.; Li, Y.; Wang, L. Nat. Commun. 2017, 8, 14136. 4758

DOI: 10.1021/acs.nanolett.9b01843 Nano Lett. 2019, 19, 4752−4759

Letter

Nano Letters (41) Liu, Z.; Ling, X. Y.; Su, X.; Lee, J. Y. J. Phys. Chem. B 2004, 108, 8234−8240. (42) Vinod, M.; Gopchandran, K. G. Curr. Appl. Phys. 2015, 15, 857−863. (43) Zhan, X.; Tang, H.; Du, Y.; Talbi, A.; Zha, J.; He, J. RSC Adv. 2013, 3, 15966−15970. (44) Matsushima, T.; White, J. M. J. Catal. 1975, 39, 265−276. (45) Masel, R. I. Principles of Adsorption and Reaction on Solid Surfaces; John Wiley & Sons: New York, 1996; Vol. 3, p 488. (46) Conrad, H.; Ertl, G.; Küppers, J. Surf. Sci. 1978, 76, 323−342. (47) Spendelow, J. S.; Goodpaster, J. D.; Kenis, P. J. A.; Wieckowski, A. J. Phys. Chem. B 2006, 110, 9545−9555. (48) Camara, G. A.; Iwasita, T. J. Electroanal. Chem. 2005, 578, 315− 321. (49) Farias, M. J. S.; Herrero, E.; Feliu, J. M. J. Phys. Chem. C 2013, 117, 2903−2913. (50) Wang, A. L.; Xu, H.; Feng, J. X.; Ding, L. X.; Tong, Y. X.; Li, G. R. J. Am. Chem. Soc. 2013, 135, 10703−10709. (51) Garcia, A. C.; Paganin, V. A.; Ticianelli, E. A. Electrochim. Acta 2008, 53, 4309−4315.

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DOI: 10.1021/acs.nanolett.9b01843 Nano Lett. 2019, 19, 4752−4759