Monodisperse Palladium Sulfide as Efficient Electrocatalyst for

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Monodisperse Palladium Sulphide as Efficient Electrocatalyst for Oxygen Reduction Reaction Cheng Du, Peng Li, Fulin Yang, Gongzhen Cheng, Shengli Chen, and Wei Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16359 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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ACS Applied Materials & Interfaces

Monodisperse Palladium Sulphide as Efficient Electrocatalyst for Oxygen Reduction Reaction Cheng Du, Peng Li, Fulin Yang, Gongzhen Cheng, Shengli Chen*, and Wei Luo* a

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei

430072, P. R. China, Tel.: +86-27-68752366 *Corresponding author. E-mail addresses: [email protected], [email protected]

Abstract: In this paper, we report a colloidal synthesis of palladium sulphides (including Pd16S7, Pd4S, and PdS) via a facile one-pot hot-solution synthetic route and their promising application as electrocatalyst for the oxygen reduction reaction (ORR). Among the different palladium sulfides tested, monodisperse Pd4S nanoparticles exhibit the best electrocatalytic activity toward ORR in alkaline medium, with the half-wave potential which is ca.47 mV more positive than that of the state-of-the-art Pt/C catalyst. Density functional theory (DFT) calculations indicate the existence of oxygen absorption sites in Pd4S surface result in optimized oxygen binding ability for the 4-electron oxygen reduction.

Keywords: palladium sulfides, ORR, colloidal synthesis, monodisperse, DFT

1. Introduction 1

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The oxygen reduction reaction (ORR) plays a key role in a wide range of renewable energy technologies, such as fuel cells and metal-air batteries. 1-3 Nonetheless, searching for efficient catalysts to overcome sluggish oxygen reduction kinetics is still the key issue for the upcoming clean-energy based economy. 4 To date, metallic platinum (Pt) and Pt-based alloys are considered as the state-of-the-art catalysts for the ORR. 5, 6 However, their widespread commercialization has been hindered by the high price and scarcity, insufficient long term stability and vulnerability to fuel (e.g., methanol-poisoning effects). 7, 8 Therefore, developing highly efficient ORR catalysts with improved catalytic activity and superior durability is highly desirable, but still a great challenge. Recently, with relatively lower cost, palladium (Pd) based electrocatalysts have attracted great attention aiming to replace the costly Platinum group metals, including oxygen reduction reaction (ORR), 9, 10 hydrogen evolution reaction (HER), 11, 12 formic acid oxidation reaction (FAOR), 13, 14 and ethanol oxidation reaction (EOR).15, 16

However, development of doping non-metallic elements into Pd as highly efficient

catalysts for ORR, to the best of our knowledge, has not been widely explored. Sato’s group reported that B-doped Pd (Pd-B) could weaken the absorption of ORR intermediates with nearly optimal binding energy by lowering the barrier associated with O2 dissociation.17 The same group also reported a stepwise electroless deposition method for synthesizing P-doped Pd, which exhibited very high catalytic activity toward ORR owing to its amorphous structure and tunable mass activities of Pd nanoparticles after P doping.18 On the other hand, transition metal sulphides, 2


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including CoS2,19 Ni3S2,20 NiS2,21 MoS2,22 and Rh2S3,23 have been investigated as promising catalysts for HER. Researches indicated that their high HER activities might be caused by decreasing the free energy of H adsorption from the S-edge side.24 However, the catalytic activity of the metal sulfides toward ORR has not been widely studied.25 Moreover, unlike the well-studied transition metal sulfides toward HER, their ORR mechanism remain ambiguous. It has been reported that palladium sulphides (Pd4S), with the Pd-Pd distance being close to that of Pd metal, exhibit superior catalytic performance on selective hydrogenation and Suzuki coupling.26,27 In this regard, we expect that Pd4S may show potential application in the field of ORR. However, palladium sulfides are generally synthesized through presulfidation of Pd precursors using H2S, Na2S, or organic sulfur-containing molecules, which suffered from high environmental pollution and scaling cost.28,29 Thermal decomposition of complexes containing Pd-chalcogen bonds into palladium chalcogenides has been developed as another strategy to obtain palladium sulfides, but it involves the complicated and cumbersome Pd-chalcogen precursors formation.30,31 In this work, for the first time, we reported the colloidal synthesis of palladium sulphides (including Pd16S7, Pd4S, and PdS) via a facile one-pot hot-solution synthetic route. Among the different palladium sulfides tested, monodisperse Pd4S nanoparticles (NPs) exhibited much enhanced electrocatalytic activity toward ORR in alkaline medium, with the half-wave potential of 0.877 V, ca.47 mV more positive than that of commercial Pt/C. The as-prepared Pd4S catalyst also demonstrated enhanced tolerance against methanol and significant improvement 3



in terms of stability in comparison to Pt/C catalyst. Density functional theory (DFT) calculations indicated the existence of oxygen absorption sites in Pd4S surface was able to trap atomic oxygen moderately and desorb O2 facilely, and thus led to its superior ORR activity. 2. Experimental Section Chemicals and materials. Palladium acetylacetonate was purchased from Wuhan Greatwall Chemical Co. Ltd. (China). Oleylamine was purchased from Aladdin Industrial Co. (China). Sulfur was purchased from Sinopharm Chemical Reagent Co. Ltd. (China). Pt/C (Johnson Matthey Hispec 3600) was purchased from Shanghai Hesen Electric Co. Ltd. (China). 5 wt. % Nafion solution was purchased from Sigma-Aldrich Co. (China). All chemicals were used as received. The water used throughout all experiments was purified by a UP water purification system. Synthesis of Pd4S nanoparticles (NPs). 0.2 mmol palladium acetylacetonate (Pd(acac)2) and 0.1 mmol S powder were mixed with 5 mL oleylamine (OAm). To remove water and oxygen, we moderately stirred the mixture and heated it to 110°C under vacuum for 20 min. The solution was then put under a blanket of Ar gas. Then the reaction mixture was heated to 320°C for 1 h. The solution began to darken at ∼200°C and turned black at 260°C. After 1 h at 320°C, the reaction was slowly cooled down to room temperature by turning off the heating mantle. Ethanol and hexanes were added to precipitate the NPs, and the product was collected by centrifugation at 8500 rpm for 5 min. The obtained NPs were further washed twice in ethanol and dried by oil pump vacuum at 50 °C for 3 h to give Pd4S NPs as a dark 4


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gray powder. Other types of PdxSy nanostructures were also prepared using the same procedure except for the different amount of S powder. For example, when S powder was used as 0.2 mmol and 0.4 mmol, the mixture of Pd16S7 and Pd and pure PdS nanocrystal were prepared, respectively. When reducing the amount of S powder to 0.05, 0.025 and 0.01mmol, Pd4S and mixed Pd4S and Pd nanocrystals were obtained. Synthesis of 20 wt% PdxSy/C. 6 mg PdxSy and 24 mg carbon (C, Vulcan XC-72R) were mixed in 10 mL hexane in a 25 mL vial. This colloidal mixture was sonicated for 1 h to ensure complete adherence of PdxSy onto the C support. After evaporation of hexane, 15 mL of acetic acid was added to the PdxSy/C dispersion and heated for 6 h at 70°C. The reaction mixture was cooled down to room temperature. 30 mL of ethanol was added and the mixture was centrifuged at 8500 rpm for 8 min. This procedure was repeated twice. For comparison, the one-step Pd4S/C nanoparticles were also synthesized though an in situ approach by adding XC-72 during the colloidal synthesis. Characterization. TEM images were obtained using a FEI Tecnai G20 U-Twin TEM instrument operating at 200 keV. HAADF-STEM images were operated using Titan G2 60 at 300 keV. Powder X-ray diffraction (XRD) patterns were measured by a Bruker D8-Advance X-ray diffractometer using Cu Ka radiation source (λ=0.154178 nm) with a velocity of 6o min-1. X-ray photoelectron spectroscopy (XPS) measurement was performed with a Thermo Fischer ESCALAB 250Xi spectrophotometer. The electrochemical measurements were performed on CHI760E electrochemical workstation (Shanghai Chenhua Co.). Elemental composition was 5



determined by an inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was performed on IRIS Intrepid II XSP. Electrochemical Measurements. For alkaline testing conditions in 0.1 M KOH, characterization was performed in a standard three-electrode cell, with glassy carbon (GC, ϕ = 5 mm) was used as the working electrode to test the ORR activity of the as-synthesized catalysts, a Pt foil counter electrode (1 cm*1 cm*0.2 mm) and a Hg/HgO reference electrode. The actual value of the potential vs. reversible hydrogen electrode (RHE) of the Hg/HgO reference electrode was calibrated by using hydrogen electrode reaction with the 20 wt% Pt/C as working electrode. Linear sweep voltammetry (LSV) measurements were carried out with scan rate of 5 mV s-1 and 1600 rpm rotating speed. All the measurements were measured at ambient condition. Before measurements, the GC electrode was polished using 5 µm, 500 nm and 50 nm alumina slurry, respectively. The samples (5 mg) were dispersed in isopropanol solvent (1 mL, 0.1% Nafion) for preparing the catalytic ink. 6 µL sample ink suspension was deposited onto the GC rotating disk electrode (RDE) with an overall catalyst loading of 0.15 mg cm-2, and then dried naturally. For RRDE experiments, the ORR polarization curves were obtained by using the same conditions as RDE measurements. The H2O2% and the electron transfer number (n) were determined by the following equations: H2O2% = 200×Ir/(N×Id+Ir); n=4×N×Id/(N×Id+Ir), where Id is the disk current, Ir is the ring current and N is the current collection efficiency of the Pt ring. In our system N was determined to be 0.37. Electrochemical impedance

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spectroscopy measurement was conducted at 0.91 V (vs. RHE), while the voltage frequency ranged from 100 kHz to 0.01 Hz at an amplitude of 5 mV. Theoretical Methods. All calculations are performed on periodic supercells with use of first-principles density functional theory (DFT) and the generalized gradient approximation of Perdew-Burke-Enzerhoff (PBE) functional for exchange-correlation and the ultrasoft pseudopotentials for nuclei and core electrons. The Kohn-Sham orbitals are expanded in a plane-wave basis set with a kinetic energy cutoff of 30 Ry and the charge-density cutoff of 300 Ry. The Fermi-surface effects has been treated by the smearing technique of Methfessel and Paxton, using a smearing parameter of 0.01 Ry. All calculations are carried out with spin polarization. The PWSCF codes contained in the Quantum ESPRESSO distribution were used to implement most of the calculations. For the calculations of pure-metal Pd and Pt with experimental equilibrium lattice constants of 3.89 Ǻ (Pd) and 3.92 Ǻ (Pt), four-layer slabs of face centered cube (fcc) structure are used to model the Pd (111) and Pt (111) surfaces. A Slab of (2√ 3×2√3) is used and the Brillouin-zones are sampled with a 5×5×1 k-point mesh. For the primitive tetragonal bulk cell of Pd4S (containing two formula units of Pd4S, see Figure S18), Brillouin-zones sampling is performed on a k-point mesh of 4×4×4 points. Then we use four-layer slabs with (2×1) supercell size to model the Pd4S (110) surface (Figure S19) and 4×6×1 k-point mesh is employed for the surface calculations. To simulate both Pd (111), Pt (111) and Pd4S (110) surface, we use slabs with periodic boundary conditions in three directions and employ 15 Ǻ thick vacuum 7



region to ensure the decoupling of the adjacent slabs. The surface structure optimization is performed with the bottom two layers fixed and the top two layers allowed to relax with the adsorbates during the calculations until the Cartesian force components acting on each atom is below 10-3 Ry/Bohr and the total energy converge to within 10-5 Ry. To describe the ORR activity of Pd4S, Pt and Pd, we use the so-called volcano curve which relates the oxygen reduction activity to the adsorption energy of atomic oxygen, established by Nørskov and co-workers.32,33 And the method to calculate adsorption energy of atomic oxygen developed by Nørskovis also employed here.31 3. Results and Discussion The palladium sulfides were synthesized by a facile one-pot hot-solution sulfurization process. Typically, under a nitrogen flow, 0.2 mmol Pd(acac)2 and 0.1 mmol sulfur in 5 mL oleylamine were heated to 320 °C, after kept the temperature for about 1 h, Pd4S NPs were obtained by certification. The morphology and structure of the Pd4S NPs are analyzed by transmission electron microscope (TEM) and high-resolution TEM (HRTEM). As shown in Figure 1a and 1b, the as-prepared highly monodisperse Pd4S NPs have a narrow size distribution with a diameter of 10 nm (measured by Nano Measure software with about two hundred nanoparticles). The lattice spacing of parallel fringes are 0.21 nm and 0.36 nm, corresponding to the (211) planes and (110) planes of tetragonal Pd4S, respectively. High-angle annular dark-field scanning TEM (HAADF-STEM) images (Figure S1a) combined with elemental mapping and line scans (Figure S1b) further reveal the uniform distribution of Pd and S within the Pd4S 8


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NPs. In addition, Pd16S7/Pd mixture, PdS, and Pd catalysts were also synthesized through the similar synthetic procedure by changing the initial amount of sulfur. Their corresponding TEM and HRTEM images are shown in Figure S2. As seen in Figure S2a and S2b, the mixture of Pd16S7 and Pd, aggregated PdS NPs are obtained. The lattice spacing of parallel fringes are 0.32 nm, corresponded to the (004) planes of Pd16S7 (Figure S2d), and 0.29 nm related to the (321) planes of PdS (Figure S2e), respectively. Without sulfur doping, aggregated Pd nanodendrites with the size of ~150 nm are obtained. (Figure S2c, f) To get more information about the as-prepared palladium sulfides and palladium, energy-dispersive X-ray (EDX) analyses are performed. The results of EDX spectra further confirm the presence of Pd and S (Figure 1c, Figure S2g-i). The atomic ratios of Pd to S are estimated to be 2.2, 3.3 and 1.1, which matched well with Pd16S7/Pd mixture, Pd4S and PdS, respectively. (Table S1) The powder X-ray diffraction pattern (XRD) of the Pd4S NPs and the corresponding standard patterns are shown in Figure 1d. The peaks at 35.1˚, 36.6˚, 38.7˚, 39.4˚, 40.7˚, 42.7˚, 51.6˚, 59.4˚, 63.9˚and 68.1˚, are assigned to the (200), (102), (201), (210), (112), (211), (212), (311), (302) and (321) planes, corresponding to the Pd4S tetragonal structure based on the data of the standard PDF file (PDF #73-1387). Additionally, Pd16S7, PdS and Pd are characterized as cubic-phase Pd16S7, tetragonal-phase PdS and the cubic-phase Pd, respectively (the XRD pattern matched with PDF nos. 75-2228, 25-1234 and 87-0645, respectively; see Figures S3-S5).The weight percentage of the phases Pd16S7 and Pd was measured to be 78% and 22% by Jade 6 software using the whole pattern fitting and Rietveld refinement. In addition, 9



the accurate molar ratio of Pd16S7 and Pd was further detected to be 81 % and 19 % by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), consistent with EDX and XRD results. To get further insight of the effect of sulfur addition on the morphology and composition of the synthetic products, we further reduced the contents of S powder to 0.05, 0.025 and 0.01 mmol, respectively. As shown in Figure S6, decreasing the amount of S powder, results in the aggregation and low uniformity of palladium sulphides. Moreover, as shown in the XRD patterns (Figure S7), the mixture phases of Pd4S and Pd are observed when reducing the content of S powder. These results further indicate the content of S powder precursor has a significant influence on the final composition, morphology, and dispersion of palladium sulphides. Furthermore, X-ray photoelectron spectroscopy (XPS) is carried out to ascertain the surface compositions and chemical states of the as-prepared Pd4S NPs. As shown in Figure 2a, the presented Pd 3d peaks indicate two types of valence state (Figure 2a), while the strong peaks located at 335.6 eV and 340.8 eV could be attributed to Pd0 and the weak peaks located at 336.3 eV and 341.5 eV could be assigned to PdII. The spectra of the Pd-3d region of the Pd4S NPs is further compared with the as-synthesized Pd. As shown in Figure 2a, the deconvolution of the Pd-3d region in Pd4S displays a shift of about 0.6 eV towards higher binding energy in comparison to that of Pd (335.0 eV), indicating the presence of a partial positive charge on Pd in Pd4S, consistent with the previous reports.34 The binding energy (BE) difference between Pd 3d5/2 and 3d3/2 is found to be 5.2 eV due to the spin-orbit coupling. In the 10


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S spectra (Figure 2b), the peaks located at 163.2 eV (2p3/2) and 164.4 eV (2p1/2) could be assigned to S0, which indicates the presence of Pd-S bond. Another doublet located at 162.0 eV (2p3/2) and 163.3 eV (2p1/2) could be indexed to S2-. The splitting of the S 2p peak due to spin orbit coupling is about 1.2 ~ 1.3 eV, which is similar to the reported value (1.0 ~ 1.3 eV).35 Compared with the binding energy of S 2p3/2 of pure sulfur (163.9 eV),35 a shift of about 0.7 eV towards lower binding energy side could be observed in our sample, which is similar but inverse to Pd 3d region (~0.6 eV to higher binding energy side), and indicates a portion of charge transfer from Pd to S. In addition, a broad peak located at about 168.8 eV is attributed to the oxidized forms of sulfur (SOx2-, x= 3, 4),

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which is usually observed in metal sulfides. 37 The atomic

ratio of Pd and S at the particle surface of the sample was measured to be 1.64. To explore the application of the synthesized palladium sulphides as ORR electrocatalyst in 0.1 M KOH solution, they were first loaded on carbon support (Vulcan XC-72R) by sonication. Prior to electrocatalytic measurements, the carbon-supported samples were washed with acetic acid at 70 °C for 6 h to remove the surface organics,38 and characterized by TEM as shown in Figure S8. The polarization curves of Pd4S/C, (Pd16S7 +Pd)/C, PdS/C, Pd/C, Pd4S without carbon support and commercial Pt/C are shown in Figure 3a. Among all the catalysts tested, Pd4S exhibits the highest catalytic activity, with the onset potential of 1.00 V, and half-wave potential of 0.877 V (ca.47 mV more positive than that of commercial Pt/C), respectively. To the best of our knowledge, the outstanding performance toward ORR makes the as-synthesized Pd4S among the best catalysts ever reported in 11



an alkaline solution (Table S3). On the other hand, the polarization curves of Pd4S/C exhibits well-defined diffusion-limited current near -5.3 mA cm-2 in the region from 0 to 0.7 V, much close to that of commercial Pt/C. This value is better than the other palladium sulfides, suggesting Pd4S/C has more excellent electrode kinetics. The electrochemically active surface areas (ECSA) of PdxSy/C electrocatalysts were measured by cyclic voltammetry in an Ar-saturated 0.1 M KOH solution at a 50 mV s-1 sweep rate (Figure S10a). The value is evaluated by integrating the reduction charge of surface palladium oxide.39, 40 As shown in Figure S10b, the ECSA of Pd4S/C(44.0 m2 g-1) is larger than those of (Pd16S7 +Pd)/C (13.3 m2 g-1), PdS/C (14.7 m2 g-1) and Pd/C (34.2 m2 g-1), which might be ascribed to the smallest particle size of monodisperse Pd4S nanoparticles with narrow size distribution. For comparison, one-step Pd4S/C catalyst was also synthesized through an in-situ approach by adding XC-72 during the colloidal synthesis. As shown in Figure S11, the nanoparticles with low uniformity are ready to aggregate. In addition, the ORR performance of the one-step Pd4S/C is much inferior to that of monodisperse Pd4S/C, with a half-wave potential of 0.862 V (Figure S12). Moreover, the ORR activity of palladium sulphides obtained by further reducing the content of S powder to 0.05, 0.025, and 0.01 mmol were investigated. As shown in Figure S13, half-wave potentials are measured to be 0.829, 0.822, and 0.812 V, respectively, inferior to that of Pd4S/C. Furthermore, the Tafel plot of monodisperse Pd4S/C is measured to be 45.9 mV dec−1, which is smaller than those of Pt/C (60.5 mV dec−1), (Pd16S7 + Pd)/C (58.3 mV dec−1), PdS/C (63.8 mV dec−1), Pd/C (67.2 mV dec−1) and Pd4S (82.7 mV dec−1), 12


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indicating a more desirable kinetic process for Pd4S toward ORR (Figure 3b). The stability of Pd4S/C is assessed by continuous cyclic voltammetry (CV) sweeps between 0.72 and 1.02 V vs. RHE in O2-saturated KOH (0.1 M) with a sweep rate of 100 mV s-1. The 10000th cycle exhibits negligible change compares to the first cycle as indicated in Figure S14. In addition, after 10000 cycles, the linear sweep voltammetry (LSV) curve of Pd4S/C almost appear overlapping with the initial curve (Figure 3c). Moreover, it is interesting to note that the dispersion of Pd4S/C was maintained well (Figure S15a) and the result of XRD indicates that the composition of Pd4S/C has no change (Figure S15b) even after long-time ORR test, demonstrates the high stability of the as-synthesized Pd4S/C. To further understand the ORR catalytic ability, the Pd4S/C catalyst after ORR stability test is characterized by XPS (Figure S16). It is clear to observe that there is almost no difference of the surface compositions and chemical states of the Pd element in comparison to the initial sample. Compared with the S 2p XPS before stability test, the peaks attributed to the S2- species (seen before stability test, Figure 2b) disappear, which might be due to the oxidation of S2- species under the ORR potential window. However, the binding energies of S 2p of S0 species (163.3 eV and 164.6 eV) have scarcely changed. From the results of XRD and XPS in Figure S15b and Figure S16, we speculate that the ORR catalytic capability could be originated from Pd4S. The stability of the Pd4S/C and Pt/C electrocatalyst was also investigated by current-time (i-t) chronoamperometric measurements in O2-saturated KOH (0.1 M). As depicted in Figure 3d, the relative current of the Pd4S catalyst still remains 91.4% after 40000 13



seconds of continuous operation, whereas Pt/C clearly decreased to 50.2% at the same condition. Additionally, the methanol tolerance of the Pd4S/C was also studied. The original ORR current of Pd4S/C under 0.8 V exhibits only a slight change after the introduction of 2 M methanol (Figure 4a). In contrast, the corresponding current of Pt/C shifts from a cathodic current to a reversed anodic current in a short time after the addition of methanol, indicating a conversion of the dominant ORR to the methanol oxidation reaction. These results indicate superior stability and tolerance to methanol of the Pd4S/C catalyst. Figure 4b demonstrates the LSV curves of Pd4S/C at various rotating speeds to study the kinetics of the ORR. According the Koutecky-Levichequation,18 the electron transfer number for ORR was calculated to be ~ 4, suggesting a four-electron pathway on reducing the O2 to H2O in the ORR catalytic process (Figure 4c).We performed rotating ring-disk electrode (RRDE) technique to further monitor the formation of intermediate species during the ORR process. As shown in Figure S17a, the corresponding H2O2 yield of Pd4S/C is 3.5%-11.4% in the range of testing, slightly higher than that of Pt/C (3.0%-11.4%), but much lower than those of Pd/C (10%-29%), PdS/C (15%-27%) and (Pd16S7 + Pd)/C mixture (29%-46%). Figure S17b displays the number of electron transfer are 3.8-3.9, 3.9-4.0, 3.4-3.8, 3.5-3.7, 3.1-3.4 for Pd4S/C, Pt/C, Pd/C, PdS/C and (Pd16S7 + Pd)/C mixture, respectively, suggesting four-electron O2 reduction pathway and the two-electron O2 reduction pathway were both presented during ORR process for Pd/C, PdS/C and (Pd16S7 + Pd)/C mixture. Electrochemical impedance analysis was measured to study the 14


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mechanism of the superior ORR activity of the Pd4S/C over Pt/C, (Pd16S7 +Pd)/C and PdS/C. As shown in Figure 4d, the Pd4S/C exhibits the lowest impedance, suggesting the highest electrical conductivity, and fastest electrode kinetics of Pd4S/C. To gain further insights into the origin of the high ORR activity of Pd4S, we performed density functional theory (DFT) calculations to explore the ORR activity trends on different material surfaces. Herein, we employ the volcano model established by Nørskov and co-workers.32 This model has been established based on the well-known Brønsted-Evans-Polanyi (BEP) relation, which shows that the kinetic barriers (activation energy) of surface reactions usually scale linearly with the corresponding thermodynamic reaction energy. For ORR, it has been well-established that the rates of the rate-determining step well satisfy the Volcano model with the O adsorption energy as the thermodynamic descriptor. The O adsorption energy (∆‫ܧ‬ைெ∗ ) is calculated according to ∆‫ܧ‬ைெ∗ = ‫ܧ‬ைெ∗ − ‫ ܧ‬ெ , where ‫ܧ‬ைெ∗ and ‫ ܧ‬ெ are the DFT total energies of a slab bearing a certain surface site with and without addition of an O ad-atom at the site. Although the TEM images may suggested that there were also Pd4S (211) facets exposed, their fraction should be much less. In addition, the Pd4S (110) surface is more stable than the (211) surface. Based on the considerations, the oxygen adsorption energies on Pt (111), Pd (111), and Pd4S (110) surfaces have been calculated respectively. However, the sites on (211) surface might have also contributed to the enhanced ORR activity of Pd4S. As seen in Figure 5a, the Pd atoms on Pd4S (110) surfaces can be divided into three groups, which are in different horizontal planes respectively (also see Figures 15



S19 and S20 for details). The plane of group 3 is the highest, then group 1, and the plane of group 2 is the lowest. We denote the adsorbed O atoms with the group numbers of Pd atoms involved in each adsorption structure. For example, O133 refers to an O atom adsorbed at the hollow site involving one group-1 Pd atoms and two group-3 Pd atoms. Various possible adsorption configurations for different O coverages have been calculated (Figure S20). We found that the O atoms prefered to adsorb at the hollow sites involving one group-1 Pd atom and two group-3 Pd atoms (Pd1-Pd3-Pd3 sites), regardless of the O coverage (Figure 5a). The corresponding adsorption energies are summarized in Figure 5b. We noted that the difference in O adsorption energy between the most and the second most stable configurations at each O coverage is substantial, which suggests that the Pd1-Pd3-Pd3 sites will be fully occupied by O atoms. In the other words, these O atoms act as spectators on the surface. Therefore, we investigated the further adsorption of O on Pd4S surface with all the Pd1-Pd3-Pd3 sites pre-adsorbed by O atoms (Figure S21). The corresponding adsorption energy of the most stable configuration (Pd4S-4O133-O112) were used to compare with those on Pd and Pt surfaces to build a volcano plot (Figure 5c). For simplicity, the adsorption energy values are given with respect to that on pure Pt surface. It can be seen that the oxygen adsorption on such a pre-adsorbed surface is less strong than that on Pt (111) surface, suggesting that the stably adsorbed O133 on Pd4S surface can significantly promote the ORR activity of the adjacent surface sites. 4. Conclusions

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In summary, nearly monodisperse Pd4S NPs have been successfully synthesized through a facile one-pot hot-solution colloidal synthetic approach, and further used as electrocatalysts for catalyzing the ORR in alkaline solutions for the first time. As expected, the as-prepared Pd4S electrode exhibits superior activity toward ORR with high onset potential, high half-wave potential, low Tafel slope, and good stability and methanol tolerance. DFT calculations suggest that the superior performance might be attributed to the more optimal oxygen adsorption free energy derived from the existing of oxygen absorption sites on Pd4S surface. This facile synthetic method for preparation monodisperse palladium sulfides, as well as their superior catalytic performances toward ORR may promote the intensive investigations of transition metal sulfides for more applications.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21571145, 21633008), the Fundamental Research Funds for the Central Universities and Large-scale Instrument and Equipment Sharing Foundation of Wuhan University.

Supporting information Figures S1-S21, TableS1-S4, showing HAADF-STEM, TEM, HRTEM, EDX, XRD, chemical compositions of the as-prepared palladium sulphide; the detail of DFT

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studies, the comparison of ORR activity of Pd4S with other reported electrocatalysts. These materials are available free of charge via the Internet at http://pubs.acs.org.

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Figure captions:

Figure 1. Morphology and structural characterization of Pd4S nanoparticles. Representative (a) low-magnification and (b) high-magnification TEM images of Pd4S nanoparticles, the inner figure is the particle distribution; (c) EDX image and (d) XRD patterns of the Pd4S. Figure 2. XPS spectrum of the (a) Pd 3d for the as-synthesized Pd4S/C and Pd/C, (b) S 2p for the as-synthesized Pd4S/C. Figure 3. (a) LSV curves of Pd/C, PdS/C, (Pd16S7 + Pd)/C , Pd4S/C, Pt/C and Pd4S in 0.1 M KOH; (b) The corresponding Tafel slope of Pd/C, PdS/C, (Pd16S7 + Pd)/C, Pd4S/C, Pt/C and Pd4S; (c) LSV curves for Pd4S/C before and after 10000 CV cycles; 24


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(d) Current-time curves of Pd4S/C (red) and Pt/C (black), the currents were converted to current percentage. The catalyst loading in all cases was 0.15 mg cm-2. Figure 4. (a) Chronoamperometric responses of Pd4S/C (red) and Pt/C (black) to the addition of 2 M methanol;(b) LSV curves of Pd4S/C for ORR at rotating speeds range from 625 to 2500 rpm; and (c) corresponding Koutecky-Levich plots; (d) Nyquist plots of the as-prepared PdxSy/C recorded at 0.91 V in 0.1 M KOH. Figure 5. (a) Adsorption configurations on Pd4S (110), and the names of associated structures are shown below; (b) The O adsorption energies of the most stable adsorption configurations and the sub-stable adsorption configurations at different O coverages; (c) ORR activities of Pd4S (110), Pd (111), Pt (111) and Pt3Ni (111) predicted by the calculated O adsorption energies (∆‫ܧ‬ைெ∗ ) based on the theoretical Volcano model. The ∆‫ܧ‬ைெ∗ values are given with respect to ∆‫ܧ‬ை௉௧∗ . Blue spheres represent Pd atoms, yellow spheres represent S atoms, and red spheres represent O atoms.

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Figure 1

Figure 2 26


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Figure 3

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Figure 4

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Figure 5

TOC 29



Nearly monodisperse Pd4S NPs have been successfully synthesized through a facile one-pot hot-solution colloidal synthetic approach. The as-prepared Pd4S electrode exhibits superior activity toward ORR with the half-wave potential which is ca.47 mV more positive than that of the state-of-the-art Pt/C catalyst.

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Monodisperse Palladium Sulfide as Efficient Electrocatalyst for

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