CoP Nanosheet Hybrids: Highly Electroactive and

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Pd Nanoparticle/CoP Nanosheet Hybrids: Highly Electroactive and Durable Catalysts for Ethanol Electrooxidation Sheng-Hua Ye, Jin-Xian Feng, and Gao-Ren Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02263 • Publication Date (Web): 12 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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Pd Nanoparticle/CoP Nanosheet Hybrids: Highly Electroactive and Durable Catalysts for Ethanol Electrooxidation Sheng-Hua Ye, Jin-Xian Feng, and Gao-Ren Li* MOE Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China E-mail: [email protected]

ABSTRACT Herein, we develop the strongly coupled Pd nanoparticles/CoP nanosheets attached on the cloth of carbon fibers (CFC) (Pd@CoP NSs/CFC) to enhance the catalytic activity and durability of Pd nanoparticles by in-situ nucleation and growth of Pd nanoparticles on the fabricated CoP nanosheets. Compared with Pd/C and Pd/CFC catalysts, the Pd/CoP NSs exhibit not only high electroactivity but also excellent stability for the electrooxidation of ethanol in basic solution. The improved electrocatalytic activity of Pd@CoP NSs can be attributed to number increase and activity improvement of active sites, which can be attributed to the strong interactions between CoP nanosheets and Pd nanoparticles. The high stability of Pd/CoP NSs is ascribed to the direct oxidation of intermediate species to carbonate by OHad that was produced from the intermediate species of Co(OH)2. This study will open up exciting opportunities for the development of catalysts with enhanced electroactivity and stability via the fabrication of hybrids with strong electrical coupling between metal nanoparticles and co-catalysts.

Keywords: Electrocatalyst; Pd nanoparticle; CoP nanosheet; ethanol electrooxidation; synergic effect

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INTRODUCTION The shortages of fossil-fuel and environmental pollution have resulted in a great desire for green energy devices that can produce power at high efficiency with almost no emission. Recently, direct ethanol fuel cells (DEFCs) have attracted increasing interest as ethanol is an abundant and inexpensive liquid fuel.1-5 Although DEFCs hold a great potential for green energy devices, the precious metal Pt has hindered the widespread application of such devices.6-8 Recently, Pd has attracted great interest because of its relatively low-cost, high electrocatalytic activity and superior stability.9-12 Many studies have demonstrated that the Pd-based catalysts showed the electrocatalytic activity similar to Pt/C for ethanol oxidation in basic media. However, certain technical challenges, such as catalyst poisoning and poor durability, limit the usefulness of Pd electrocatalysts for DEFCs.13-14 Regarding electrocatalysis at the DEFC anode, the following issue is a huge challenge: during ethanol oxidation, CO is a very stable intermediate while simultaneously being a strong poison for Pd electrocatalysts and thus leading to low electrocatalytic activity and poor durability.15-20 To improve the electrocatalytic activity and stability of Pd electrocatalysts, the change of the electronic structure of Pd is indispensable.21 An appropriate method is to find suitable supports that will enhance the catalytic reactivity of Pd electrocatalysts by strong electronic interactions between support and Pd. The main requirements for the supports are high conductivity, large surface area, strong resistance to corrosion and good dispersion of Pd.22 Various carbon, for instance, graphene and multiwalled carbon nanotubes (MWCNTs), have been widely utilized as supports for electrocatalysts, such as PtIrNi/C,23 Pd/graphene/Ni,24

Pd-CeO2@C,25 PdNi/C,26 Pd-PEDOT/graphene,27

Pd/rGO/CFP,28

Pd/CNTA,29

Pd@Au/C,30 MWCNTs-Pd,31 MWCNTs-PdNi32 and so on. However, the carbon supports are known to anode corrosion in acid or alkaline electrolyte, which will lead to aggregation and sintering of Pd.33 To avoid this intrinsic defect, great interest recently has been attracted for robust metal oxides, metal nitrides and metal carbides as supports such as Pd/CeO2-x,34 Pd/CoAl-LDHs,35 Pd/NbRuyOz,36 Pd/β-MnO2,37 Pd/TiO2,38 Pd/VOx,39 Pd/TiN,40 Pd@Pt DNC/rGO,41 PtPd NFs/rGO/GCE42 and Pd/WC43. Indeed, the catalyst/metal compounds exhibited the enhanced catalytic activity and stability because of the synergetic 2 Environment ACS Paragon Plus

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effect between the support and catalyst. Unfortunately, the ability of above hybrid catalysts to eliminate or mitigate CO poisoning is very limited. Therefore, the design of heterogeneous electrocatalysts with highperformance still remain a great challenge. In this study, we firstly developed a facile method for the design of Pd nanoparticles/CoP nanosheets (NSs) supported on carbon fiber cloth (CFC) (Pd@CoP NSs/CFC) with 3D hierarchical structure. The unique CoP NSs supported on CFC provide a high specific surface area, which will benifit the penetration and transport of active species, and stable supports for nucleation and growth of Pd nanoparticles, which can efficiently prevent the agglomeration of Pd nanoparticles and accordingly enhance the utilization of catalysts. Compared with individual Pd or CoP, Pd@CoP NSs/CFC showed the enhanced catalytic activity and stability for ethanol oxidation. Here the synergistic effects promoted by the electronic interactions between Pd nanoparticles and CoP nanosheets play a significant role for the unusually high catalytic activity of hybrid catalysts. The hydroxyl (OHad) resulting from CoP is the crucial active species for excellent durability of Pd@CoP NSs/CFC because of the efficient oxidation of adsorbed carbon monoxide (COad). In addition, the enhanced durability of Pd@CoP NSs/CFC is attributed to the excellent chemical stability, high conductivity and the anti-aggregation of Pd nanoparticles because of the strong coupling between Pd and CoP. This study highlights a novel strategy for the synthesis of hybrid electrocatalysts with superior catalytic activity and high stability for ethanol oxidation.

EXPERIMENTAL SECTION Synthesis of Electrocatalysts: The chemical reagents we used all were analytical grade, and they were utilized directly without any purification. Electrochemical deposition was carried out by galvanostatic electrolysis in a two-electrode cell. The counter electrode was a graphite electrode (1.8 cm2, spectral grade), and the working electrode was a carbon fiber cloth (CFC) (2 cm × 0.5 cm) that was purchased from Phychemi Company in Hong Kong. The fabrication procedures of Pd@CoP NSs/CFC are given in the following: (1) Co(OH)2 NSs/CFC was electrodeposited on CFC in 10 mL 0.02 M Co(NO3)2 + 0.1 M NH4Cl solution 3 Environment ACS Paragon Plus

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at 1.0 mA cm-2 for 20 minutes. (2) CoP NSs/CFC was then fabricated by low-temperature phosphorization using PH3 and the experimental details were shown as following: Co(OH)2 NSs/CFC and NaH2PO2 were put at the separated positions in a porcelain boat with NaH2PO2 at the upstream side of the furnace. The porcelain boat was heated at 300 0C for 1h with nitrogen atmosphere and then cooled to room temperature. (Caution: NaH2PO2 releases PH3 during the heating process, and this reaction is considered as corrosive and flammable. So the gas tightness of the furnace is high-level and the concentrated CuSO4 solution is utilized as tail gas absorber.) (3) Pd@CoP NSs/CFC was prepared by the electrochemical deposition of Pd nanoparticles on CoP NSs/CFC in the solution of 20 mM K2PdCl4+50 mM NaCl+2.5 mM sodium citrate at 0.5 mA cm-2 for 30 minutes. Structural characterizations: The surface morphologies of the catalysts were studied by transmission electron microscopy (TEM, JEM-2010HR) and field emission scanning electron microscopy (FE-SEM, JSM-6330F). The chemical-state of samples was analyzed via X-Ray photoelectron spectroscopy (XPS) (ESCAKAB 250). The C 1s line at 284.8 eV was used as a standard to correct all XPS peaks. All the curves were fitted and the background was subtracted. The inductively coupled plasma-atomic emission spectrometry (ICP-AES) was utilized to determine the chemical component of catalysts. The catalysts were also characterized via Fourier transform infrared spectroscopy (FT-IR, Nicolet 300) with the attenuated total reflection technique (ATR) (ATR-FTIR). Electrochemical characterizations: The electrochemical properties of the fabricated catalysts were investigated in a three-electrode electrolytic tank. The Pd@CoP NSs/CFC, Pd/CFC or Pd/C served as working electrodes. The counter electrode was a carbon electrode, and the reference electrode was a saturated calomel electrode (SCE). In this study, 20% Pd/C (Johnson Matthey, Vulcan XC-72) with nanoparticle size of 4 nm was used. Pd loadings of Pd@CoP NSs/CFC, Pd/CFC, and commercial Pd/C are 13.54, 17.22 and 70.77 µg/cm2, respectively. Cyclic voltammogram (CV) and chronoamperometry curves were measured by an electrochemical workstation (CHI 660D, CH instruments, Inc.). Ethanol electrooxidation was studied by cyclic voltammograms (CVs) that were recorded at 50 mV/s in 1.0 M 4 Environment ACS Paragon Plus

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ethanol+1.0 M KOH. Chronoamperometry curves were measured for ethanol electrooxidation at -0.3 V. Before experiments, Ar gas with high purity was purged into electrolyte solution for 10 minutes. All the electrochemical tests were performed at 25 0C.

RESULTS AND DISCUSSION The fabrication procedure of Pd@CoP NSs/CFC is shown in Scheme 1. SEM images of CFC is exhibited in Figure S1, and CFC is consisted of carbon fibers. Co(OH)2 nanosheets were deposited on CFC and 3D hierarchical porous structure is clearly observed in Figure S2. The unique porous structure exhibits high surface area and large available space that will favor fast electrolyte penetration or diffussion. Then CoP NSs/CFC was fabricated by low-temperature phosphorization of Co(OH)2 NSs/CFC. Figure 1a shows SEM image of CoP NSs/CFC, and 3D hierarchical porous structure is clearly seen. EDS result of CoP NSs shows that the mole ratio of Co to P is close to 1 (Figure S3). Figure 1b shows TEM image of CoP NSs, and it clearly shows that the CoP nanosheet is porous because of the dehydration of Co(OH)2 during the heat-treatment. The magnified TEM images are shown in Figure 1c-d, which shows the polycrystalline structure of CoP nanosheet. Figure 1d shows HRTEM image that indicates the lattice space is 2.76 Å, which is identical with the lattice spacing of CoP(002) (PDF 29-0497). SAED parttern in Figure 1e also demonstrates the polycrystalline structure of CoP nanosheet. After the electrodeposition of Pd nanoparticles on CoP NSs/CFC, the Pd@CoP NSs/CFC was fabricated. SEM image of Pd@CoP NSs/CFC is exhibited in Figure 1f, which clearly shows the 3D hierarchical porous structure is kept well. TEM images of Pd@CoP NSs/CFC are shown in Figure 1g-h, which clearly show that the Pd nanoparticles disperse uniformly on the surfaces of CoP NSs. The sizes of Pd nanoparticles are 3~5 nm (Figure S4). HRTEM image shown in Figure 1i demonstrates that Pd nanoparticles are amorphous. SAED pattern in Figure 1j only shows the pattern of CoP and the signal of Pd is not seen, further indicating the amorphous structure of Pd nanoparticles. The element mappings of Pd@CoP NSs are shown in Figure 1l-n, and the results demonstrate that the Pd nanoparticles disperse on CoP nanosheets uniformly. XRD patterns of CoP NSs/CFC and Pd@CoP NSs/CFC are shown in Figure 5 Environment ACS Paragon Plus

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2a, and CoP peaks such as (011), (111), (211), (020) are clearly seen (JCPDS 29-0497). However, the diffraction peaks of Pd is not seen for Pd@CoP NSs/CFC, and this also indicates that the Pd nanoparticles are amorphous. This result is consistent with above HRTEM and SAED results. For contrastive study, Pd nanoparticles (NPs) supported on CFC (named as Pd NPs/CFC) were also fabricated, and its SEM images with different magnifications are exhibited in Figure S5, which clearly shows that Pd nanoparticles uniformly disperse on carbon fibers. Pd loadings of Pd@CoP NSs/CFC and Pd/CFC catalysts are 0.01354 and 0.01722 mg, respectively. XPS was employed to study the influence of CoP on the electronic state of Pd, and survey spectrum of Pd@CoP NSs/CFC is exhibited in Figure S6, which indicates the existences of Pd, Co, P and C. The C comes from CFC substrate. In Pd 3d spectrum, compared with those of Pd/CFC, the Pd 3d peaks of Pd@CoP NSs/CFC positively shift ~0.3 eV as shown in Figure 2b. The Pd 3d peak shift represents the electrons transfer from Pd to CoP, and it confirms the electronic states of Pd were altered by the electronic interactions between Pd and CoP. The electronic state change of Pd will possibly improve the electrocatalytic performance of Pd@CoP NSs/CFC.44 Importantly, the previous studies reported that the binding energy of Pd 3d peaks usuallly negatively shifted when Pd catalysts were supported on transition metal oxides and carbon materials.45 However, the opposite phenomenon is seen in our study, and it implies that the unique interactions between CoP NSs and Pd nanoparticles are quite different from those of conventional Pd-metal oxides and carbon materials. The electrochemical behaviors of Pd@CoP NSs/CFC, Pd/C and Pd/CFC in 1.0 M KOH solution were studied as shown in Figure 3a. The Pd@CoP NSs/CFC exhibits a couple of oxidation and reduction peaks among -0.1~0.2 V, which corresponds to the reversible redox reactions of Co(II) - e ↔ Co(III).46 A peak appearing at -0.38 V corresponds to PdO reduction. The electrochemical active area (ECSA) of Pd catalysts is calculated by the reduction peak area of PdO and it is often utilized to evaluate the number of electroactive sites. The ECSA (m2/gPd) is usually determinated by the formula: ECSA = Q / (0.405×mpd), here Q represents coulombic charge corresponding to the reduction peak area of PdO (mC), mpd is the mass of Pd loading (mg), 0.405 is the charge for the electroreduction of PdO single-layer (mC/cmPd2).47 6 Environment ACS Paragon Plus

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Herein, the ECSA of Pd@CoP NSs/CFC is calculated to be 40.3 m2/g, which is much larger than those of Pd/C (23.1 m2/g) and Pd/CFC (26.95 m2/g). The obvious ECSA enhancement of Pd@CoP NSs/CFC may be ascribed to 3D hierarchical porous structure, well dispersion of Pd nanoparticles and electronic effect of CoP. The electrocatalytic activities of Pd@CoP NSs/CFC and CoP NSs/CFC toward ethanol oxidation were firstly evaluated in solution of 1.0 M ethanol + 1.0 M KOH at the scan rate of 50 mV/s as shown in Figure 3b. The ethanol oxidation peak appears at the range of -0.6 ~ -0.1 V. The redox peak at the range of 0~0.2 V corresponds to the reversible redox reaction of Co(II) - e ↔ Co(III), and they are separate from ethanol oxidation peak. So CoP almost has no effect on ethanol oxidation peak. Here Pd@CoP NSs/CFC shows much enhanced catalytic activity for ethanol electrooxidation. The catalytic activity of Pd@CoP NSs/CFC for ethanol electrooxidation was compared with those of Pd/C and Pd/CFC in 1.0 M KOH + 1.0 M ethanol solution at scan rate of 50 mV/s as shown in Figure 3c. The mass peak current densities of forward scanof Pd@CoP NSs/CFC is 1413.3 mA/mg (normalized to Pd mass loading), which is 2.85 and 4.12 times larger than those of Pd/CFC(495.8 mA/mg) and commercial Pd/C (342.7 mA/mg), respectively, and this activity is also much better than those appeared in literatures (summarized in Table S1). In addition, the current density of Pd@CoP NSs/CFC normalized to ECSA also shows ~1.91 and ~2.35 times higher than those of Pd/CFC and Pd/C, respectively (please see Figure S7). Various CVs of Pd@CoP NSs/CFC with cycle number increase in 1.0 M KOH+1.0 M C2H5OH solution at the scan rate of 50 mV s-1 is exhibited in Figure S8, and the results are shown in the Figure 3d. The peak current density of Pd@CoP NSs/CFC shows an ascent during the initial stage, and peak current density reaches maximum at 110th cycle. Subsequently, the peak current density keeps stable and remains ~91.1% of the maximum value after 250 cycles. Oppositely, Pd/CFC and Pd/C catalysts remain much lower peak current densities retention (56.35% and 42.2%, respectively) after 250 cycles (Figure 3d-e). Therefore, Pd@CoP NSs/CFC exhibits much improved cycling durability compared with Pd/C and Pd/CFC. In addition, Figure 3d and 3e show that the Pd@CoP NSs/CFC owns much improved catalytic activity compared with Pd/CFC and Pd/C. After 250 cycles, the surface morphology of Pd@CoP 7 Environment ACS Paragon Plus

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NSs/CFC is kept well (please see Figure S9), and Pd@CoP NSs/CFC owns superior structure stability. Chronoamperometric experiments were also performed in solution of 1.0 M ethanol + 1.0 M KOH to further study the catalytic activity and durablity of Pd@CoP NSs/CFC as shown in Figure 3f. The potential was kept at -0.3 V. The Pd@CoP NSs/CFC exhibited much higher current density and slower current attenuation rate compared with Pd/CFC and commercial Pd/C. FT-IR spectroscopy with the attenuated total reflection technique (ATR) (ATR-FTIR), which can sensitively identify the intermediate species adsorbed on the surface of electrode, was utilized to investigate the role of CoP for durability enhancement of Pd@CoP NSs/CFC. ATR-FTIR spectra of Pd@CoP NSs/ CFC and Pd/CFC after electrocatalysis are shown in Figure 4a. For Pd@CoP NSs/CFC, a sharp band appears at 3624 cm-1, which corresponds to the stretching mode of hydroxyl in Co(OH)2.48 However, for Pd/CFC, the peak of hydroxyl is not seen, suggesting Co(OH)2 has been generated on the surface of CoP during the catalysis process. To further demonstrate this fact, XPS measurements were performed and it is only limited to ~5 nm thickness. As shown in Figure 4b(i), for Pd@CoP NSs/CFC before electrocatalysis, the main peak located at 778.9 eV is assigned to Co of CoP,49 and the peaks at 780.7 and 782.6 eV correspond to Co(II) of CoO and Co(OH)2, respectively, which are originated from surface oxidation. In addition, the peak intensity of CoP obviously decreases after the electrocatalytic reaction along with the formations of CoO and Co(OH)2 as shown in Figure 4b(ii). After Ar+ sputtering Pd@CoP NSs/CFC for 120 s, the peak intensity of CoP recovers and those of CoO and Co(OH)2 visibly decrease as exhibited in Figure 4b(iii). The above results indicate the formation of Co(OH)2 on the surface of CoP during the catalysis process. Previous studies revealed that the transition metal hydroxides like Ni, Co, Fe hydr-(oxy)oides which can greatly improve catalytic stability of noble metal because they can release active OH to promote the oxidation of intermediate products.50 In our case, the in-situ formation of Co(OH)2 on the surface of CoP shares the similiar mechanism to promote the electrocatalytic activity and durability of Pd@CoP NSs/CFC. In addition, the electrocatalytic avtivity comparisons of Pd@CoP NSs/CFC and Pd@Co(OH)2 NSs/CFC were performed and the results are exhibited in Figure 4c and Figure S10. Apparently, the catalytic activity and durability of Pd@Co(OH)2 NSs/CFC are inferior to those of Pd@CoP NSs/CFC, and this result demonstated that the CoP support is 8 Environment ACS Paragon Plus

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a crucial factor for the electrocatalytic activity enhancement. Electrochemical impedence spectra (EIS) in a frequency range from 0.01 Hz to 0.1 MHz were performed to evaluate the electron transfers in Pd@CoP NSs/CFC and Pd@Co(OH)2 NSs/CFC as shown in Figure 4d. The semicircular Nyquist plots are obtained for both samples at potentials of -0.3, -0.35 and -0.4 V. The diameter of semicircular profile represents the charge transfer resistance (Rct) for ethanol oxidation on the prepared electrodes.51 The Rct values of Pd@CoP NSs/CFC are much smaller than those of Pd@Co(OH)2 NSs/CFC at all potentials, suggesting Pd@CoP NSs/CFC have much higher electron transfer ability than Pd@Co(OH)2 NSs/CFC. Obviously, the CoP NSs as supports can increase the electronic conductivity of electrode due to its high conductivity,52 thereby leading to the enhanced electrocatalytic performance. To demonstrate the improved antipoisoning ability of Pd@CoP NSs/CFC, CO stripping studies were carried out at room temperature in 1.0 M KOH solution. Here the solution of 1.0 M KOH was purged by N2 for 30 minutes and was subsequently bubbled by CO (99.99%) for 15 mintues. To obtain the maximum CO coverage at the active sites of catalysts, the potential was controlled to be -0.8 V. The superfluous CO in 1.0 M KOH solution was purged by N2 for 20 minutes. Three consecutive CVs were measured on Pd@CoP NSs/CFC with CO maximum coverage in 1.0 M KOH as shown in Figure 5a, which clearly shows that there is a typical oxidation peak of CO between -0.2 to 0.2 V at the first scan. On the second and third forward scans, only the redox peaks of Co(II) - e ↔ Co(III) were seen and CO oxidation peak disappeared because of the complete CO removal in Pd active centers, indicating high ability of CO antipoisoning of Pd@CoP NSs/CFC (here the redox peaks of Co(II) - e ↔ Co(III) are in agreement with those exhibited in Figure 3b). However, for Pd/CFC, CO oxidation peak gradually weakens with increasing cycle as exhibited in Figure 5b, which indicates that CO was stripped completely at least 7 cycles (After 7th scan, the remainder redox peaks are attributed to the oxidation and reduction of Pd). The above results demonstrate CO antipoisoning ability of Pd was improved obviously by introducing CoP NSs.

CONCLUSIONS In conclusion, a robust route was developed for the design and fabrication of Pd@CoP NSs/CFC with 9 Environment ACS Paragon Plus

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high catalytic activity and superior durability for ethanol electrooxidation compared with Pd/C catalysts in basic media. Although Pd/CFC or CoP NSs/CFC shows low catalytic performance for ethanol electrooxidation, the Pd@CoP NSs/CFC showed not only superior catalytic activity but also high stability compared with commerical Pd/C catalysts for ethanol electrooxidation in basic solution. These results suggest that the enhancement of electrocatalytic performance can be ascribed to the electronic structure change of Pd upon its synergistic interaction with CoP nanosheets. CoP plays a crucial role for high CO antipoisoning ability of Pd@CoP NSs/CFC because of COad can be directly oxidized to carbonate by OHad that was produced from the intermediate species of Co(OH)2. The strategy presented in this paper will bring new hope for the development of new hybrid electrocatalysts with enhanced catalytic activity and stability for DEFCs.

Associated Content Supporting information SEM, TEM, XPS and EDS results of Pd@CoP NSs/CFC and Pd/CFC; electrochemical data of EOR. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This work was supported by National Basic Research Program of China (2016YFA0202603 and 2015CB932304), Natural Science Foundation of Guangdong Province (S2013020012833 and 2016A010104004), and Fundamental Research Fund for the Central Universities (16lgjc67).

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Guo, S. Dong, S. Adv. Mater. 2012, 24, 2326-2331. (5) (a) Suntivich, J.; Xu, Z.; Carlton, C.; Kim, J.; Han, B.; Lee, S. W.; Bonnet, N.; Marzari. N.; Allard, L. F.; Gasteiger, H. A.; Hamad-Schifferi, K.; Shao-Horn, Y. J. Am. Chem. Soc. 2013, 135, 7985-7991. (b) Li, M.; Cullen, D.; Sasaki, K.; Marinkovic, N. S.; More, K.; Adzic, R. R. J. Am. Chem. Soc. 2013, 135, 132-141. (6) Lim, B.; Kobayashi, H,; Ty, T.; Wang, J.; Kim, M. J.; Li, Z. Y.; Rycenga, M.; Xia, Y. J. Am. Chem. Soc. 2010, 132, 2506-2507. (7) (a) Kibsgaard, J.; Chen, Z.; Reinecke, B. N.; Jaramillo, T. F. Nat. Mater. 2012, 11, 963-969. (b) Suntivich, J.; Gasteiger, A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; Shao-Horn, Y. Nat. Chem. 2011, 3, 546-550. (8) Zhao, D.; Xu, B. Q. Angew, Chem. Int. Ed. 2006, 45, 4955-4959. (9) (a) Liu, W.; Herrmann, K.; Geiger, D.; Borchardt, L.; Simon, F.; Kaskel, S.; Gaponik, N.; Eychmüller, A. Angew. Chem. Int. Ed. 2012, 51, 5743-5747. (b) Huang, X.; Zhang, H.; Guo, C.; Zhou, Z.; Zheng, N. Angew. Chem. Int. Ed. 2009, 48, 4808-4812. (c) Jin, M.; Zhang, H.; Xie, Z.; Xia, Y. Angew. Chem. Int. Ed. 2011, 50, 7850-7854. (10) (a) Mazumder, V.; Chi, M.; Mankin, M.; Liu, Y.; Metin, O.; Sun, D.; More, K.; Sun, S. Nano Lett. 2012, 12, 11021106. (b) Xu, J.; Wilson, A. R.; Rathmell, A. R.; Howe, J.; Chi, M.; Wiley, B. J. ACS Nano 2011, 5, 6119-6127 (11) (a) Wang, L.; Nemoto, Y.; Yamauchi, Y. J. Am. Chem. Soc. 2011, 133, 9674-9677. (b) Zhang, H.; Jin, M.; Wang, J.; Li, W.; Camargo, P. H. C.; Kim, M. J.; Yang, D.; Xie, Z.; Xia, Y. J. Am. Chem. Soc. 2011, 133, 6078-6089. (12) (a) Wu, H.; Li, H.; Zhai, Y.; Xu, X.; Jin, Y. Adv. Mater. 2012, 24, 1594-1597. (b) Yin, S.; Cai, M.; Wang, C.; Shen, P. K. Energy Environ. Sci. 2011, 4, 558-563. (13) (a) Baber, A. E.; Tierney, H. L.; Sykes, E. C. H. ACS Nano 2010, 4, 1637-1645. (b) Lee, Y. W.; Kim, M.; Kang, S. W.; Han, S. W. Angew. Chem. Int. Ed. 2011, 50, 3466-3470. (14) (a) Markovic, N. M. Nat. Mater. 2013, 12, 101-102. (b) Tian, N.; Zhou, Z.; Yu, N. F.; Wang, L.Y.; Sun, S. G. J. Am. Chem. Soc. 2010, 132, 7580-7581. (15) Liang, H. W.; Liu, S.; Gong, J. Y.; Wang, S. B.; Wang, L.; Yu, S. H. Adv. Mater. 2009, 21, 1850-1854. (16) Chen, Z.; Waje, M.; Li, W.; Yan, Y. Angew. Chem. Int. Ed. 2007, 46, 4060-4063. (17) Cui, C. H.; Li, H. H.; Yu, J. W.; Gao, M. R.; Yu, S. H. Angew. Chem. Int. Ed. 2010, 49, 9149-9152. (18) Mazumder, V.; Chi, M. F.; Mankin, M. N.; Liu, Y.; Metin, O.; Sun, D. H.; More, K. L.; Sun, S. H. Nano Lett. 2012, 12, 1102-1106. (19) Chen, L. Y.; Guo, H.; Fujita, T.; Hirata, A.; Zhang, W.; Inoue, A.; Chen, M. W. Adv. Funct. Mater. 2011, 21, 43644370. (20) Antolini, E. Energy Environ. Sci. 2009, 2, 915-931. (21) (a) Hammer, B.; Nørskov, J. K. Surf. Sci. 1995, 343, 211-220. (b) Sumanta, S.; Rajkumar, J.; Suchitra.; Umesh, V. W.; Balamurugan, K.; Sampath, S.; Sebastian, C. P. Chem. Mater. 2015, 27, 7459-7467. (22) Bianchini, C.; Shen, P. K. Chem. Rev. 2009, 109, 4183-4206. (23) Shen, S. Y.; Zhao, T. S.; Xu, J. B.; Li, Y. S. Energy. Environ. Sci. 2011, 4, 1428-1433. (24) Tsang, C. H, A; Hui, K. N.; Hui, K. S.; Ren. L. J. Mater. Chem. A. 2014, 2, 17986-17993. (25) Tan, Q.; Du, C. Y.; Sun, Y. R.; Du, L.; Yin, G. P.; Gao, Y. Z. Nanoscale 2015, 7, 13656-13662. (26) Lee, K. W.; Kang, S. W.; Lee, S. U.; Park, K. H.; Lee, Y. W.; Han, S. W. ACS, Appl. Mater. Interface 2012, 4, 4208-4214. (27) Yue, R. R; Wang, H. W; Bin, D.; Xu, J. K.; Du. Y. K.; Lu. W. S.; Guo. J. J. Mater. Chem. A. 2015, 3, 1077-1088. (28) Sawangphruk, M.; Krittayavathananon, A.; Chinwipas, N. J. Mater. Chem. A 2013, 1, 1030-1034. (29) Hu, F. P.; Cui. X. W.; Chen, W. X. J. Phys. Chem. C 2010, 114, 20284-20289. (30) Zhu, L. D.; Zhao, T. S.; Xu, J. B.; Liang, Z. X. J. Power Sources 2009, 187, 80-84. (31) Sun, Z. P.; Zhang, X. G.; Liu, R. Li.; Liang, Y. Y.; Li, H. L. J.Power Sources 2008, 185, 801-806. (32) Singh, R. N.; Anindita, A. S. Carbon 2009, 47, 271-278


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ACS Catalysis

Scheme 1. Schematic illustration of the fabrication of Pd@CoP NSs/CFC.


ACS Catalysis

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

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CoP/CFC

CoP(112)

Pd@CoP/CFC Pd CoP(112)

Pd

Pd nanoparticles

(m)

(l)

(k)

500 nm

Pd La1

(n)

Co Ka1

P Ka1

Figure 1. (a) SEM image, (b) TEM image, (c-d) HRTEM images and (e) SAED pattern of CoP NSs/CFC; (f) SEM image, (g) TEM image, (h-i) HRTEM images, (j) SAED pattern of Pd@CoP NSs/CFC; (k) SEM image of a typical Pd@CoP nanosheet; (l) Pd elemental mapping in Pd@CoP nanosheet; (m) Co elemental mapping in Pd@CoP nanosheet; (n) P elemental mapping in Pd@CoP nanosheet.


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ACS Catalysis

(a)

∆=0.3 eV

(b)

CoP ♦ CFC

*

Pd@CoP NSs/CFC

♦ (011)

* *

(111)

♦

Pd 3d

(211) (020)

*

(011) (111)

* *

CoP NSs/CFC

*

3d5/2

3d3/2

(211) (020)

*

* Pd@CoP NSs/CFC

20

30

40

50

60

70

Degree / 2theta

80

90 332

Pd/CFC 334

336

338

340

342

Binding Energy / eV

344

346

Figure 2. (a) XRD patterns of Pd@CoP NSs/CFC and CoP NSs/CFC; (b) XPS spectra of Pd@CoP NSs/ CFC and CoP NSs/CFC in Pd 3d region.


ACS Catalysis

1800

-1

(a)

Current density / mA mgPd

-1

Current desity / mA mgPd

400

Pd@CoP NSs/CFC Pd/CFC commercial Pd/C

300

1600

Pd@CoP NSs/CFC CoP NSs/CFC

1200 1000

100 0

-100

Pd

PdO

-200

800 600 400 200 0

-200

-0.8

-0.6

-0.4

-0.2

0.0

0.2

-0.8

Potential / V vs SCE 1600

-0.6

0.2


(d)

1800

91.1%

1200

1000

1000

800 600 400 200 0 -200

-0.8

-0.6

-0.4

-0.2

0.0

800 600 400

56.35%

200

42.20%

0

0.2

0

50

Potential / V vs SCE

3

1200 1000

2

800 600

1

400 200 Pd@CoP Pd/CFC NSs/CFC

Pd/C

Pd@CoP Pd/CFC NSs/CFC

Pd/C

0

Current density / mA mg

1400

4

-2

Maximum peak current density Peak current density after 250 cycles

Current density / mA cmPd

(e)

-1 Pd

1400

-1

0.0

1400

1200

0

-0.2

1600

1400

1600

-0.4


2000

Pd@CoP NSs/CFC Pd/CFC Commercial Pd/C

(c)

-1

2000 1800


Current density / mA mg-1 Pd

(b)

1400

200


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

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100

(f)

1200

150

Cyclic number / n

200

250


1000 800 600 400 200 0

0

100

200

300

400

Time / s

Figure 3. (a) CVs of Pd@CoP NSs/CFC, Pd/CFC and commercial Pd/C in 1.0 M KOH at 50 mV/s; (b) CVs of Pd@CoP NSs/CFC and CoP NSs/CFC in 1.0 M KOH + 1.0 M ethanol at 50 mV/s; (c) CVs of Pd@CoP NSs/CFC, Pd/CFC and commercial Pd/C in 1.0 M KOH+1.0 M ethanol at 50 mV/s; (d) The cycling stabilities of Pd@CoP NSs/CFC, Pd/CFC and commercial Pd/C; (e) The comparisons of maximum peak current densities with the peak current densities after 250 cycles of Pd@CoP NSs/CFC, Pd/CFC and commercial Pd/C catalysts for ethanol oxidation; (f) Chronoamperometry curves of Pd@CoP NSs/CFC, Pd/CFC and commercial Pd/C measured in 1.0 M KOH+1.0 M ethanol held at -0.3 V.


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(a)

CoP

(b)

(ii) Pd/CFC

4000

-1

1500

3600

3200

2800

2400

2000

Wavenumber / cm-1

(c)

after electrocatalysis (ii) +

after Ar sputtering for 120s

(iii)

(i) Pd@CoP NSs/CFC

1800

Co(OH)2 shake-up

(i) CoP@Pd NSs/CFC

Intensity

Transimittence

CoO


1600

772

1200

(d)

500

1200 900 600 300 0

776

780

784

788

792

Binding Energy / eV 600

Pd@CoP NSs/CFC Pd@Co(OH)2 NSs/CFC

-Z" / ohm

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

ACS Catalysis

400

Pd@CoP NSs/CFC -0.30 V -0.35 V -0.40 V Pd@Co(OH)2 NSs/CFC -0.30 V -0.35 V -0.40 V

300 200 100

-300 0 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0

100

200


300

400

500

600

700

Z' / ohm

Figure 4. (a) ATR-FTIR spectra of (i) Pd@CoP NSs/CFC and (ii) Pd/CFC after electrocatalysis; (b) XPS spectra of (i) as-prepared Pd@CoP NSs/CFC, (ii) Pd@CoP NSs/CFC after electrocatalysis, and (iii) Pd@ CoP NSs/CFC after Ar+ sputtering for 120s; (c) CVs of Pd@CoP NSs/CFC and Pd@Co(OH)2 NSs/CFC in 1.0 M KOH+1.0 M ethanol at 50 mV/s; (d) EIS curves of Pd@CoP NSs/CFC and Pd@Co(OH)2 NSs/ CFC in 1.0 M KOH + 1.0 M ethanol in a frequency range of 0.1 Hz to 0.1MHz.


ACS Catalysis

-2

-2

0.003

(a) 1st 2nd 3rd

0.010

1 st 2 nd 3 rd 4 th 5 th 6 th 7 th 8 th

(b)

0.002 0.001

0.005

0.000

0.000

-0.005 -0.8

Current density / mA cmgeo

0.015

Current density / mA cmgeo

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

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-0.001

-0.6

-0.4

-0.2


0.0

0.2

-0.002 -0.8

-0.6

-0.4

-0.2

0.0

0.2


Figure 5. CO stripping voltammograms of (a) Pd@CoP NSs/CFC and (b) Pd/CFC measured in solution of 1.0 M KOH at 50 mV/s.


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ACS Catalysis

TOC Graphic


CoP Nanosheet Hybrids: Highly Electroactive and

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