Polymer-Embedded Fabrication of Co2P Nanoparticles Encapsulated

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Polymer-embedded Fabrication of Co2P Nanoparticles Encapsulated in N,P-Doped Graphene for Hydrogen Generation Minghao ZHUANG, Xuewu Ou, Yubing DOU, Lulu Zhang, Qicheng Zhang, Ruizhe Wu, Yao DING, Minhua Shao, and Zhengtang Luo Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02203 • Publication Date (Web): 07 Jun 2016 Downloaded from http://pubs.acs.org on June 8, 2016

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Polymer-embedded Fabrication of Co2P Nanoparticles Encapsulated in N, P-Doped Graphene for Hydrogen Generation

Minghao Zhuang, Xuewu Ou, Yubing Dou, Lulu Zhang, Qicheng Zhang, Ruizhe Wu, Yao Ding, Minhua Shao* and Zhengtang Luo* Department of Chemical and Biomolecular Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong E-mail: [email protected], [email protected]

Abstract: We developed a method to engineer well-distributed dicobalt phosphide (Co2P) nanoparticles encapsulated in N, P-doped graphene (Co2P@NPG) as electrocatalysts for hydrogen evolution reaction (HER). We fabricated such nanostructure by absorption of initiator and functional monomers, including acrylamide and phytic acid on graphene oxides, followed by UV-initiated polymerization, then by adsorption of cobalt ions and finally calcination to form N, P-doped graphene structures. Our experimental results show significantly enhanced performance for such engineered nanostructure due to the synergistic effect from nanoparticles encapsulation and nitrogen and phosphorus doping on graphene structures. The obtained Co2P@NPG modified cathode exhibits small overpotentials of only -45 mV at 1 mA cm-2, respectively, with a low Tafel slope of 58 mV dec-1 and high exchange current density of 0.21 mA cm-2 in 0.5 M H2SO4. In addition, encapsulation by N, P-doped graphene effectively prevent nanoparticle from corrosion, exhibiting nearly unfading catalytic performance after 30 hours testing. This versatile method also opens a door for unprecedented design and fabrication of novel low-cost metal phosphide electrocatalysts encapsulated by graphene.

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Introduction With increasing demand for energy consumption, the searching for sustainable and highly efficient energy to substitute finite fossil fuels has been attracting globally attention. Molecular hydrogen (H2), which owns the highest gravimetric energy density and also produce only water as the by-product, is highly regarded as the promising candidate for the future energy supply.1-3 And the abundant demand of H2 production provokes the intensive research inputs on the water splitting, in which highly efficient electrocatalysts are crucial to help lower the overpotential meanwhile improve the energy transfer efficiency in hydrogen evolution reaction (HER). Up to now, platinum (Pt) still gives the best performance in HER, but its broad utilization is greatly limited by the high cost and scarcity.4 Therefore, developing highly active HER alternative catalysts based on non-precious metals is essential to satisfy the hydrogen generation industry. Towards this end, great progresses have been achieved owing to the development of series of electrocatalysts with various components and nanostructures, such as transition-metal based carbide, sulfide, nitride, selenide and phosphide.5-11 On the other hand, transition metal coated with carbon or heteroatoms doped carbon, as highly stable electrocatalyst, also has been experimentally proved to be high-performance HER catalysts with extraordinary durability.12-20 Recently, transition metal phosphides (TMPs) based electrocatalysts have been broadly applied for HER and other electrocatalytic reactions, especially for cobalt phosphide and its derivatives. Most recent reports indicated that cobalt phosphides give better performance towards HER than the other non-noble TMPs, such as iron, nickel, copper, tungsten and molybdenum phosphides. (See more details and the comparison in Table S1).21-37 It has been proved that the phosphorus atoms doping into cobalt crystal lattices play a crucial role for HER, because of the P atoms of more electronegativity can grab electrons from metal atoms, and play a role as Lewis base to work with positively charged proton in HER process. However, different P content doping will induce various bonding energy with proton in HER process, in order to obtain optimal binding between cobalt phosphide and protons, more work is 2


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underway to fine-tune the P content and obtain optimized performance.38,39 To further improve their electrocatalytic performance, especially the durability in extreme environments, one possible solution is to package the TMPs nanoparticles with carbon layers, to combine both advantages of non-metal elements neutralization and carbon cage protection. Nevertheless, limit success was obtained to construct such architecture due to lack of feasible method to in situ wrap the TMPs nanoparticles, meanwhile enable a conductive connection between TMPs nanoparticles. Herein, we developed a one-pot strategy for the fabrication of dicobalt phosphide nanoparticles encapsulated in N, P-doped graphene, and then used it as electrocatalyst for HER. By absorbing functional monomers on graphene oxide, we allowed the acrylamide monomers to form electrostatic interaction with phytic acid molecules, and anchor onto the surface of graphene oxide by in situ UV-initiated polymerization. The grafted phytic acid enables high-efficient cobalt ion adsorption, and more importantly allows the incorporate of P atoms in the graphene lattice, similar to N atoms from acrylamide monomers. Final calcination step renders the formation of N,P-doped graphene which wrap around the dicobalt phosphide nanoparticles (Co2P@NPG). Our experimental results demonstrated that the Co2P@NPG composite gave an excellent electrocatalytic activity for HER, with a small overpotential of only -45 mV at 1 mA cm-2 and long-term unfading catalytic process, signifying the synergistic effect of Co2P nanoparticles and encapsulating N, P-doped graphene layers, that lead to high and stable catalytic performance towards HER. Moreover, this method

is

versatile

and

can

be

easily

adopted

for

other

N,P-doped

graphene-encapsulated transition metal phosphides structures (TMPs@NPG) by simple replacing the metal precursors, demonstrated by the fabrication of Fe2P@NPG, Ni2P@NPG and Pd5P2@NPG, which can promote us investigate deeper and broader on the application of TMPs in the future work based this versatile method.

Results and Discussion As presented in Scheme 1, the Co2P@NPG hybrids were prepared by the following 3


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steps: 1) anchoring of Poly(acrylamide-phytic acid) onto graphene oxide by in situ UV-initiated polymerization; 2) full adsorption of Co ion, followed by calcination to form Co2P@NPG nanostructure. In the pre-polymerization solution, the aromatic photo-initiator molecules will be absorbed onto the surface of GO via π-π stacking, concurrently with the electrostatically attracted the amino groups on acrylamide (AM) and the phosphate groups on phytic acid (PA), followed by UV-initiated polymerization. The formed polymer/GO complex was confirmed by AFM result (Figure S1), attested by increasing of average thickness from 1.1 nm for GO, to 4.8 nm for the PAM-PA modified GO. The film uniformity was also verified by TEM, Raman, XPS, FTIR and SEM investigations, as shown Figure S2. Abundant uniformly dispersed polymer structures (Figure S2b) were observed, corresponding to domains of functional polymer chains, in contrast to the clean surface on the pristine GO (Figure S2a). The Raman spectra (Figure S2c) show ID/IG increases from 0.69 to 0.83, verifying the successful polymer functionalization on the GO surface. Furthermore, the XPS spectra (Figure S2d) reveals sharp and strong peaks of N 1s (398.4 eV) and P 2p (130.6 eV), with N and P atom percentages of 3.41% and 2.56%, absented in pristine GO samples. The FTIR spectra of GO, PAM-PA and GO-PAM-PA are given in Figure S2e, with all the characteristic peaks in GO and PAM-PA can be identified in GO-PAM-PA composite. In addition, the SEM and EDS images as depicted in Figure S3 indicates that both C, O, N and P elements uniformly dispersed on the surface of GO-PAM-PA. The as-prepared GO-PAM-PA with abundant phosphate groups from PA, enables capture of large amount of metal ions by chelation or electrostatic interaction.40 All these characterization results help to verify that we have successfully obtained the Poly(acrylamide-phytic acid) grafted graphene oxide template for further utilization. After fully adsorption of cobalt ions, followed by calcination and acid treatment to remove unstable impurities, we obtained a black powder-like product. Their structures and morphologies were firstly examined by SEM and HRTEM measurements, shown in Figure 1. It can be found in Figure 1a that the whole hybrids structure exhibits a surface wrinkled morphology, and Co2P nanoparticles are well dispersed inside and 4


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partially outside the graphene layers, more clearly as depicted in 1b, with no serious metal sintering even when annealed under 900℃ ℃, in contrast to bulk structure (>500 nm in diameter) from typical sintering.23,41,42 We believe that aggregation of metal ions are efficiently prevented in our experiments for the following reasons: 1) during the material ions adsorption process, the uniformly dispersed functional polymer (PAM-PA) chains preclude the metal ions to aggregate; and 2) during the annealing process, carbonized polymer will further separate and encapsulate the metal particles, protecting them from agglomeration at high temperature. The HRTEM images presented in Figure 1c confirm that Co2P nanoparticles are well dispersed in the graphene layers. High magnification image (Figure 1d) further verify the crystal structure of Co2P nanoparticle where the Co2P (111) crystal planes can be clearly seen with the lattice spacing of 0.270 nm, with the particle diameters in 30±5 nm. This is consistent to the 27 nm value obtained by fit using Scherrer formula to XRD peak at 40.72o, corresponding to average thickness vertical to the lattice plane.43 Such uniform Co2P@NPG nanostructure takes advantage of the prefer binding of Cobalt ions with phytic acid by chelation interaction in the adsorption process. More importantly, high annealing temperature (900 ℃ ) allows the formation of high crystalline structure of Co2P nanoparticles, as confirmed in selected area electron diffraction (SAED) pattern shown inset Figure 1d, as well as high quality graphene structure with layer distance of 0.347nm, consistent with 0.34nm van der Waal distance of graphene.44 The essence of the doping in the composition can be disclosed by the TEM elemental mapping images of Co, P, C, N and O (Figure 2b-f). As expected, N and partial P atoms are distributed homogeneously over the graphitic carbon sheet, as well as in graphene shell. XPS measurements were then carried out to identify the elemental compositions and valence states of the Co2P@NPG hybrid. As depicted in Figure 3, the high-resolution scan of the Co 2p electrons (Figure 3a) yielded four peaks at 781.1 eV (Co 2p3/2) and 792.7 eV (Co 2p1/2), followed by two satellite peaks at 788.3 and 797.6 eV.45 For the N 1s electrons (Figure 3b), the asymmetric N 1s peak could be fitted into five peaks at binding energy of 398.4, 400.5, 401.2 and 404 eV 5


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corresponding to the pyridinic N, pyrrolic N, graphitic N, and oxidized N, respectively.46,47 Figure 3c shows that P 2p electrons had three separated peaks at 132.4 eV for P-C, 129.2 and 103.1 eV for P-Co.46,48 Moreover, the Co2P loading value is calculated to be 11.46 wt% from XPS result (Figure S11), close to 12.45 wt% obtained from SEM-EDS result (Figure S12) and 9.85 wt% from TGA result (Figure S13). The heteroatoms doping could help improve the surface catalytic activity towards HER, which has been verified previously.46 XRD measurements in panel 3d illustrates a broad peak at 26.1°, corresponding to the graphene (002) crystal planes, with additional peaks at 40.7°, 41.0°, 43.3° and 44.1°, indexed to the (121), (201), (211) and (130) planes of Co2P (data from JCPDS no. 32-0306), respectively. To investigate the versatility of our method, we have also successfully fabricated other kinds of TMPs@Carbon nanostructures, including Fe2P@NPG, Ni2P@NPG and Pd5P2@NPG. As shown in Figure S4, both Fe2P, Ni2P and Pd5P2 nanoparticles could be wrapped by few-layers of graphene, and the corresponding XRD spectra (Figure S4d-f) confirms that crystalline structures were obtained. The electrocatalytic activities of the as-synthesized Co2P@NPG toward HER were examined in 0.5 M H2SO4. Firstly, as shown in the control experiments of Figure S4a, nonzero cathodic currents can be seen at the electrode modified by Co2P@NPG with various Co content range from 2 to 20wt%. The lowest onset overpotential (-45 mV at 1 mA cm-2) of Co2P@NPG was observed with 10wt% Co loading. Further increasing metal content will cause aggregation in sintering, as shown in Figure S7, the TEM images of Co2P@NPG where one order of magnitude larger nanoparticles is obtained with 20wt% Co content. Such aggregation will adversely decrease the number of active sites per mass unit, and thus lower the integral catalytic activity. In addition, we found the obtained overpotentials were -224, -129, and -194 mV to reach the current density of -20 mA cm-2, when for the Co2P@NPG samples were annealed at 800, 900 and 1000℃, respectively (Figure S4b). The fact that the lowest overpotential obtained for sample annealed at 900℃, contested that although higher annealing temperature enables stable crystal structure but adversely reduce the relative content of nitrogen, phosphorous and Co. We also performed a series of control experiments 6


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with different electrode materials including Co2P@NPG, Co2P clusters, N, P-doped graphene (NPG), P doped graphene (PG), N doped graphene (NG) and reduced graphene oxide prepared under same condition, depicted in Figure 4a. We observed that the Co2P@NPG sample presents the lowest overpotential at any certain current density. To obtain the current density of 1, 10, 20 and 100 mA cm-2, the Co2P@NPG here needs overpotential of 45, 103, 128 and 220 mV, which is superior or comparable to most of recently reported TMPs towards HER (as seen in Table S1). Figure 4b illustrates the fit to the linear portions of the plots using Tafel equation (η=blog j + a, where j is the current density and b is the Tafel slope), yielding Tafel slopes of 58, 76, 118, 132 and 231 mV dec-1, for Co2P@NPG, Co2P clusters, NPG, NG and PG catalysts, respectively. Noted that the rGO shows nearly non-activity towards HER, but the Tafel slopes of PG, NG and NPG shown an increasing trend, which is consist with the previously reported observation on N, P dopants act as active sites towards HER.46 By extrapolation to the Tafel plots, the exchange current density (j0) of Co2P@NPG was obtained to be 0.21 mA cm-2, comparable to the 20% Pt/C (0.30 mA cm-2). On the other hand, as shown in Fig. 4c, there was only slight decreasing performance observed in HER activity even after 10000 cycles for Co2P@NPG electrocatalyst. We further examined the durability by electrolysis at the fixed overpotentials over extended periods. Figure 4d presents the current density level off when measured at two overpotentials of -100 mV and -150 mV over continuous 30h testing, suggesting extraordinary durability of the Co2P@NPG composite for HER. The extreme conditions durability is also measured in the media with pH range from 0 to 14 (see in Figure 4e). We observed that even in the extreme alkaline media 1M KOH with pH≈14, it still exhibited rather small onset overpotential (-61 mV at 1 mA cm-2) with rather low Tafel slope (96 mV dec-1), significantly improved comparing to other reported HER catalysts (See the comparison in Table S2).12-15,46,49 Moreover, we also investigated the morphologies of Co2P@NPG catalyst before and after 1000 cycles testing in 1M KOH (see in Figure S13), the distribution and size of Co2P nanoparticles, as well as the surrounded graphene-like layers were not altered, indicating that Co2P@NPG could perform excellent durability even in extreme 7


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environment. Cyclic voltammetry (CV) was used to further assess the electrochemical performance. As shown in Figure 5a and 5b, we observe no faradaic reaction for voltammetric scan within +0.1 to +0.2 V, and obtained the double-layer capacitance value of 66.8 mF cm-2 for the Co2P@NPG (loading 0.5 mg cm-2), remarkably comparable or higher than recently reported literature on Co-based electrocatalysts for hydrogen production.11,12,47

We have also characterized the interfacial reaction and electrode

kinetics in HER process by electrochemical impedance spectroscopy (EIS). Figure 5c and 5d illustrate the typical Nyquist and Bode plots of the Co2P@NPG-modified electrode at various overpotentials from 100 to 200 mV. Inset Figure 5c given the equivalent circuit, which is applied as a circuit model to fit the impedance data. This equivalent circuit is composed of three elements, i.e. the solution resistance (Rs), constant phase element (CPE) and charge transfer resistance (Rct). The Rs value is obtained to be 24Ω and kept constant at arbitrary overpotential, the large Rs was observed probably due to the N and P dopants on graphene sheet, which inevitably lower the conductivity of the whole composite. On the other hand, the charge transfer resistance (Rct) decreased sharply with the increasing of overpotentials, from 347Ω at 100 mV to only 28Ω at 200 mV, showing a faster HER process happened at a higher overpotential.

Conclusions In summary, we report a design and construction of uniformly dispersed Co2P nanoparticles encapsulated with N,P-doped multilayers graphene by using GO-PAM-PA (graphene oxide-poly(acrylamide)-phytic acid) as template and precursor. The N, P-doped graphene layers protect the Co2P nanoparticles from degrading, as well as function as the conductive support for electrons transport. Moreover, the coupling of N and P dopants with abundant defects on the surface of graphene produces more active sites. HER evaluation revealed that this catalyst exhibited excellent activity with large current density with small Tafel slope, as well 8


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as outstanding electrochemical durability, to the first approximation, comparable to the commercial Pt-group electrocatalysts. Notably, this method is versatile and cab be easily apply to preparation of other TMPs, including Fe2P@NPG, Ni2P@NPG and Pd5P2@NPG, as demonstrated. Such facile method to fabricate heterogeneous catalysts with implanted transition metal phosphide-based composites reported here provides a new vision for the design of earth-abundant catalysts applying for hydrogen generation and other electrochemical processes.

Supporting Information Available: Characterization of GO-PAM-PA and Co2P@NPG composites, including AFM, TEM, SEM, Raman, XPS, FTIR, BET, TGA and additional electrocatalytic analysis. This material is available free of charge via the internet at http://pubs.acs.org.

Acknowledgment This project is supported by the Research Grant Council of Hong Kong SAR (Project numbers 16204815 and 26026115). We appreciate support from Center for 1D/2D Quantum Materials. Technical assistance from the Materials Characterization and Preparation Facilities is greatly appreciated.

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(30) Popczun, E. J.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. Highly Active Electrocatalysis of the Hydrogen Evolution Reaction by Cobalt Phosphide Nanoparticles**. Angew. Chem. Int. Edit., 2014, 53, 5427-5430. (31) Pu, Z. H.; Liu, Q.; Jiang, P.; Asiri, A. M.; Obaid, A. Y.; Sun, X. P. CoP Nanosheet Arrays Supported on a Ti Plate: An Efficient Cathode for Electrochemical Hydrogen Evolution. Chem Mater. 2014, 26, 4326-4329. (32) Tian, J. Q.; Liu, Q.; Cheng, N. Y.; Asiri, A. M.; Sun, X. P. Self-Supported Cu3P Nanowire Arrays as an Integrated High-Performance Three-Dimensional Cathode for Generating Hydrogen from Water. Angew. Chem. Int. Edit., 2014, 53, 9577-9581. (33) Yang, H. C.; Zhang, Y. J.; Hu, F.; Wang, Q. B. Urchin-like CoP Nanocrystals as Hydrogen Evolution Reaction and Oxygen Reduction Reaction Dual-Electrocatalyst with Superior Stability. Nano Lett. 2015, 15, 7616-7620. (34) Hellstern, T. R.; Benck, J. D.; Kibsgaard, J.; Hahn, C.; Jaramillo, T. F. Engineering Cobalt Phosphide (CoP) Thin Film Catalysts for Enhanced Hydrogen Evolution Activity on Silicon Photocathodes. Adv Energy Mater. 2016, 6, 175-185. (35) Kibsgaard, J.; Tsai, C.; Chan, K.; Benck, J. D.; Norskov, J. K.; Abild-Pedersen, F.; Jaramillo, T. F. Designing an improved transition metal phosphide catalyst for hydrogen evolution using experimental and theoretical trends. Energy Environ Sci. 2015, 8, 3022-3029. (36) Saadi, F. H.; Carim, A. I.; Verlage, E.; Hemminger, J. C.; Lewis, N. S.; Soriaga, M. P. CoP as an Acid-Stable Active Electrocatalyst for the Hydrogen-Evolution Reaction: Electrochemical Synthesis, Interfacial Characterization and Performance Evaluation. J Phys Chem C. 2014, 118, 29294-29300. (37) Wang, J. M.; Yang, W. R.; Liu, J. Q. CoP2 nanoparticles on reduced graphene oxide sheets as a super-efficient bifunctional electrocatalyst for full water splitting. J Mater Chem A. 2016, 4, 4686-4690. (38) Shi, Y.; Zhang, B. Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction. Chem Soc Rev. 2016, 45, 1529-1541. (39) Ha, D. H.; Moreau, L. M.; Bealing, C. R.; Zhang, H. T.; Hennig, R. G.; Robinson, R. D. The structural evolution and diffusion during the chemical transformation from cobalt to cobalt phosphide nanoparticles. J Mater Chem. 2011, 21, 11498-11510. (40) Martin, C. J.; Evans, W. J. Phytic Acid Metal-Ion Interactions .2. The Effect of Ph on Ca(Ii) Binding. J Inorg Biochem. 1986, 27, 17-30. (41) Xing, Z. C.; Liu, Q.; Asiri, A. M.; Sun, X. P. High-Efficiency Electrochemical Hydrogen Evolution Catalyzed by Tungsten Phosphide Submicroparticles. Acs Catal. 2015, 5, 145-149. (42) Li, D.; Senevirathne, K.; Aquilina, L.; Brock, S. L. Effect of Synthetic Levers on Nickel Phosphide Nanoparticle Formation: Ni5P4 and NiP2. Inorg Chem. 2015, 54, 7968-7975. (43) Patterson, A. L. The Scherrer formula for x-ray particle size determination. Phys Rev. 1939, 56, 978-982. (44) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-based composite materials. Nature. 2006, 442, 282-286. (45) Kaspar, T. C.; Droubay, T.; Heald, S. M.; Engelhard, M. H.; Nachimuthu, P.; Chambers, S. A. Hidden ferromagnetic secondary phases in cobalt-doped ZnO epitaxial thin films. Phys Rev B. 2008, 77, 201-210. (46) Zheng, Y.; Jiao, Y.; Li, L. H.; Xing, T.; Chen, Y.; Jaroniec, M.; Qiao, S. Z. Toward Design of 12


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Synergistically Active Carbon-Based Catalysts for Electrocatalytic Hydrogen Evolution. Acs Nano. 2014, 8, 5290-5296. (47) Duan, J. J.; Chen, S.; Chambers, B. A.; Andersson, G. G.; Qiao, S. Z. 3D WS2 Nanolayers@Heteroatom-Doped Graphene Films as Hydrogen Evolution Catalyst Electrodes. Adv Mater. 2015, 27, 4234-4241. (48) Liyanage, D. R.; Danforth, S. J.; Liu, Y.; Bussell, M. E.; Brock, S. L. Simultaneous Control of Composition, Size, and Morphology in Discrete Ni2-xCoxP Nanoparticles. Chem Mater. 2015, 27, 4349-4357. (49) Staszak-Jirkovsky, J.; Malliakas, C. D.; Lopes, P. P.; Danilovic, N.; Kota, S. S.; Chang, K. C.; Genorio, B.; Strmcnik, D.; Stamenkovic, V. R.; Kanatzidis, M. G.; Markovic, N. M. Design of active and stable Co-Mo-Sx chalcogels as pH-universal catalysts for the hydrogen evolution reaction. Nat Mater 2016, 15, 197-203.

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Scheme 1. Schematic of the fabrication of Co2P nanoparticles encapsulated by N,P-Doped Graphene (Co2P@NPG). The following steps are used: a) the photo initiator (2-hydroxy-2-methlpropiophenone) were anchored to graphene oxide, meanwhile the phytic acid electrostatically absorbed onto acrylamide monomers; b) UV-initiated in situ polymerization; c) embedding of cobalt ions in the polymer matrix; d) the prepared composites are calcined under the Argon gas to form Co2P@NPG structure.

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Figure 1. Morphology Co2P@NPG strcuture. a, b) SEM images of Co2P@NPG, Co2P nanoparticles dispersed on graphene layers, with diameter of 30±5 nm. The solid and dash circle in 1b point to the nanoparticles on and underneath graphene layer; c, d) HRTEM images of Co2P@NPG, higher magnification shown in (d) Co2P nanoparticle encapsulated in few-layer graphene. Inset is the SAED pattern of Co2P nanoparticle, showing the single crystal structure of Co2P.

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Figure 2. Transmission electron microscopy (TEM) images of Co2P@NPG. a) TEM image and b-f) Co, P, C, N and O element mapping images of Co2P@NPG, confirming the distribution of Co and P, and N and P elements on the graphene support layer.

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Figure 3. X-ray photoelectron spectroscopy (XPS) spectra and X-ray diffraction (XRD) result of Co2P@NPG. High-resolution scans of a) Co 2p, b) N 1s and c) P 2p. d) XRD spectra of Co2P@NPG in comparison to the standard data.

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Figure 4. Characterization of the HER electrocatalytic activity and durability. a) Polarization curves for HER in 0.5 M H2SO4 at a glassy carbon electrode modified with G-900, NG-900, PG-900, NPG-900, Co2P clusters, Co2P@NPG-900 and 20wt% Pt/C, respectively. b) Corresponding Tafel plots derived from a. c) The polarization curves of Co2P@NPG-900 of CV cycles. d) Time dependence of cathodic current density over Co2P@NPG-900 during electrolysis at overpotentials of 100 mV and 150 mV. e) Polarization curves of Co2P@NPG-900 in 0.5M H2SO4 (pH = 0.30 ± 0.05), 1M PBS (pH = 7.23 ± 0.07) and 1M KOH (pH = 13.94 ± 0.04) electrolytes, respectively, and f) the corresponding Tafel plots of e. 18


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Figure 5. Electrochemical properties of Co2P@NPG. a) Cyclic voltammograms within the range of no faradaic reactions. b) Variation of double-layer charging currents at +0.15 V as a function of scan rate. Symbols and the red solid line are experimental data from a and the linear fit respectively. c) Nyquist and d) Bode plots of the Co2P@NPG-modified electrode at various HER overpotentials. Inset c is the equivalent circuit and inset d is the zoomed-in image. All the data here obtained from Co2P@NPG-900 with 10wt% Co content.

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Polymer-Embedded Fabrication of Co2P Nanoparticles Encapsulated

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