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Heterostructured arrays of NixP/S/Se nanosheets on CoxP/S/Se nanowires for efficient hydrogen evolution Weiqiang Tang, Jianying Wang, Lixia Guo, Xue Teng, Thomas J. Meyer, and Zuofeng Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14466 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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ACS Applied Materials & Interfaces

Heterostructured arrays of NixP/S/Se nanosheets on CoxP/S/Se nanowires for efficient hydrogen evolution Weiqiang Tang,1 Jianying Wang,1 Lixia Guo,1 Xue Teng,1 Thomas J. Meyer,2 and Zuofeng Chen*,1 1 Shanghai Key Lab of Chemical Assessment and Sustainability, Department of Chemistry, Tongji University, Shanghai 200092, China. 2 Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States ABSTRACT: Development of efficient electrocatalysts for hydrogen evolution reaction (HER) is of increasing importance in energy conversion schemes. The earth abundant transition-metal phosphides, especially CoP and Ni2P, have emerged as promising catalysts for HER. We describe here the preparation and characterization of a hybrid catalyst of Ni2P nanosheets@CoP nanowires on carbon cloth (CC) for the reaction. The heterostructure and synergistic effects of the Ni2P and CoP components result in an extremely low overpotential of 55 mV for achieving a catalytic current density of 10 mA cm‒2, which is remarkable for transition-metal phosphide electrocatalysts. The synthetic procedure could be readily extended to related, heterostructured bimetallic sulfides or selenides for HER. KEYWORDS: transition-metal phosphides (TMPs); chalcogenides; chemical bath deposition; heterostructures; hydrogen evolution reaction (HER) Introduction. Future energy demands point to an increasing use of renewable energy resources with hydrogen a promising fuel in future applications.1 Electrochemical water splitting offers a simple way to produce highly pure hydrogen. However, the process is rarely applied at the industrial level because the two half reactions, i.e., anodic oxygen evolution (OER) and cathodic hydrogen evolution (HER), require electrocatalysts to improve their efficiencies and lower overpotentials.2,3 Currently, Pt and Pt-group metals are the most efficient electrocatalysts with near-zero overpotentials and small Tafel slopes but the use of Pt suffers from its scarcity and high price which hinder its large-scale application.4-8 Over the past few years, major efforts have been devoted to developing efficient noble-metal-free HER electrocatalysts, including sulfides, selenides, phosphides, and other nonprecious transition metal compounds.9-15 Transition-metal phosphides (TMPs) have been extensively utilized to catalyze the hydrodesulfurization (HDS) reactions.16 Based on the similarity in mechanism between HDS and water reduction, phosphides are also known as excellent water reduction catalysts arising from their moderate affinity for hydrogen - their ability to adsorb H atoms and desorb H2.17-19 Phosphides are formed by the alloying of metal and phosphorus and possess metalloid characteristics with good electrical conductivity,20 both of which are beneficial to their electrochemical performance as catalysts. In recent years, TMPs such as Ni2P, CoP, MoP and WP have been of increasing interest due to their relatively high electrocatalytic activity toward water reduction.21-28 However, compared with noble metals such as Pt, there is still a considerable challenge in enhancing catalytic activity and stability for practical applications. Generally, the hydrogen adsorption energy and kinetic energy barrier for the hydrogen evolution pathway are key elements in HER reactivity. They can be tuned by varying the electronic structure of the catalyst, especially the crystal phase exposed on the surface.29,30 In has been shown that electron transfer from Ni or Co to P in the phosphides leads to the formation of active sites consisting of positively charged Ni or Co and negatively charged P.31 It is also well-known that electrocatalysts composed of dualor multi-components usually exhibit higher HER activity by taking advantage of synergistic effects between individual

components.32-37 Given the difference in electronegativity and bonding strength to H between Ni and Co, a reasonable approach to improving catalyst design should be to integrate the two in mixtures of CoP and Ni2P. We describe here the synthesis of heterostructured, Ni2P nanosheets@CoP nanowires (Ni2P NS@CoP NW) supported on carbon cloth substrates. The procedure involves a facile threestep approach with a hydrothermal reaction and chemical bath deposition followed by phosphorization. The carbon cloth as the support material improves the conductivity of the hybrid catalyst and enhances dispersion in electroactive phases. By virtue of the heterostructure of nanosheets grown on nanowires, the asprepared catalyst provides a highly effective surface area with multiple active sites and the combination of CoP and Ni2P enhances the activity by electronic interactions and a synergistic effect. The enhancement in reactivity compared with individual CoP nanowires and Ni2P nanosheets on carbon cloth is notable. In 0.5 M H2SO4, the onset overpotential is as low as 30 mV and the Tafel slope is 48 mV dec‒1. To achieve a current density of 10 mA cm‒2 requires an overpotential of only 55 mV. The results of this study offer a promising hydrogen evolution cathode superior to the best TMP-based catalysts reported so far. There is also generality to the approach with a procedure for fabricating hetero-structured electrocatalysts readily applied to bimetallic sulfides or selenides. Electrocatalyst preparation and characterization. As shown in Scheme 1, the Ni2P NS@CoP NW-CC electrode was prepared by facile hydrothermal and chemical bath deposition followed by phosphorization (More details in Supporting Information). In the procedure, a cobalt carbonate hydroxide (Co2(OH)2CO3) nanowire arrays were hydrothermally developed on the highly conductive carbon cloth electrode. In a second step, Ni(OH)2 nanosheets were grown on the Co2(OH)2CO3 nanowire arrays in an aqueous chemical bath containing appropriate amounts of NiSO4, K2S2O8 and ammonia. The reactions include the formation of Ni(OH)2 prcipitate followed by conversion to NiOOH nanosheets through reaction with persulfate in the solution:

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Scheme 1. Schematic diagram illustrating the procedure for preparing Ni2P NS@CoP NW-CC. CBD: chemical bath deposition.

[Ni(H2O)6-x(NH3)x]2+ + 2OH‒ → Ni(OH)2 + (6-x)H2O + xNH3 (1) 2Ni(OH)2 + S2O82‒ + 2OH‒ + H2O → 2NiOOH·H2O + 2SO42‒ + 2H2O

(2)

During the drying process, NiOOH in the shape of nanosheets was degraded to Ni(OH)2: NiOOH·H2O

‒

e‒

→

Ni(OH)2

+

CC and Fig. S2 an XRD pattern which features diffraction peaks at 33.5° and 60.0°, respectively, indexed to the (101) and (110) planes of coated Ni(OH)2 (JCPDS No.38-0715). Fig. S3 shows the Co 2p3/2 and Ni 2p3/2 XPS of Ni(OH)2 NS@Co2(OH)2CO3 NW-CC, which provide additional evidence for the co-existence of Co2(OH)2CO3 and Ni(OH)2 at the hybrid electrode.38

OH‒

(3)

Finally, the Ni2P NS@CoP NW-CC electrode was obtained by gas-solid phosphorization of the hybrid precursor electrode with NaH2PO2. A plausible operating mechanism for the heterostructured Ni2P NS@ CoP NW electrode is illustrated on the lower left of Scheme 1. The CoP nanowires tightly grown on carbon cloth serve as a conductive backbone substrate for electron transport. The direct growth of Ni2P nanosheets on CoP nanowires forms a robust heterostructure, increasing the specific surface area and providing abundant electrochemically active sites. The interconnected CoP nanowires and Ni2P nanosheets form a highly porous structure allowing for electrolyte penetration. Electronic interactions and synergistic effects between the Ni2P and CoP sites may also lead to the formation of catalytically active sites toward HER. The morphology, structure, and compositions of the asprepared precursor and catalyst electrodes were investigated in further detail. The SEM image of a carbon cloth is shown in Fig. 1A. It features a 3D intricate network structure with woven carbon fibers. After a hydrothermal reaction for Co2(OH)2CO3 growth, the surface of the carbon cloth was uniformly covered by nanowires having sharpened tips with diameters of approximately 150 nm, Fig. 1B. The nanowire arrays serve as the backbone for the subsequent growth of Ni(OH)2 nanosheets. As shown in Fig. 1C, the SEM image reveals that the Ni(OH)2 nanosheets were uniformly and perpendicularly grown on the nanowire arrays by CBD treatment. Fig. 1D shows the XRD patterns of these electrode samples. The dominant diffraction peaks are consistent with the standard XRD pattern for Co2(OH)2CO3 (JCPDS No.48-0083). After CBD treatment, the diffraction peaks of Co2(OH)2CO3 become weaker with barely visible evidence for coated Ni(OH)2. The presence of Ni(OH)2 can be seen on the CBD-treated CC in the absence of pre-coated Co2(OH)2CO3. Fig. S1 shows the SEM image of Ni(OH)2 NS-

Fig. 1. SEM images of (A) bare CC, (B) Co2(OH)2CO3 NW-CC, and (C) Ni(OH)2 NS@Co2(OH)2CO3 NW-CC; (D) XRD patterns of the samples in the figure. SEM images of Ni2P NS@CoP NW-CC of different magnifications are shown in Fig. 2A and 2B. The nanosheetdecorated nanowires preserve the heterostructured morphology after phosphorization although the surface is roughened. The rough surface and abundant open spaces in the Ni2P NS@CoP NW-CC electrode provide active sites and are in favor of electrocatalysis. The energy-dispersive X-ray (EDX) spectrum reveals the co-existence of Co, Ni and P (Fig. S4). The result of inductively coupled plasma optical emission spectroscopy (ICPOES) reveals an approximate atomic ratio of 1: 1.1 for Ni: Co (Fig. S5). The corresponding EDX elemental mapping images of Ni2P NS@CoP NW-CC in Fig. 2C reflect the expected distribution of Ni, Co and P elements. These observations indicate a conformal transformation of Ni(OH)2 NS@Co2(OH)2CO3 NW into Ni2P NS@CoP NW. The hybrid catalyst was evaluated by TEM following removal from the surface. Fig. 2D shows that the CoP nanowire was decorated with a number of Ni2P nanosheets, consistent with the SEM image. In Fig. 2E, the high-resolution TEM (HRTEM) image taken on the nanosheet part demonstrates fringe spacings of 0.192, 0.203 and 0.221 nm in certain domains, which matches


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ACS Applied Materials & Interfaces well with the (210), (201) and (111) crystal planes of Ni2P. In Fig. 2G, the measured lattice fringe focused on the nanowire part was 0.247 nm, consistent with the (111) crystal plane of CoP. Fig. 2F and 2H show the selected-area electron diffraction (SAED) patterns, which confirm further the crystalline nature of both Ni2P nanosheets and CoP nanowires as indicated by the discrete spots.

that the morphologies were retained after phosphorization. In Fig. S8, the XRD patterns confirm the successful preparation of CoP NW-CC and Ni2P NS-CC and the generality of the technique for preparing hybrid electrodes.

Fig. 3. (A) XRD patterns for Ni2P NS@CoP NW-CC; (B) Ni 2p3/2, (C) Co 2p3/2, and (D) P 2p XPS of Ni2P NS@CoP NW-CC.

Fig. 2. (A,B) SEM images of Ni2P NS@CoP NW-CC at different magnifications; (C) EDX elemental mapping images of Co, Ni, and P in Ni2P NS@CoP NW-CC; (D) TEM image of Ni2P NS@ CoP NW; (E,G) HRTEM images of Ni2P NS and CoP NW of the hybrid electrode; (F,H) Corresponding SAED patterns. Fig. 3A shows an XRD pattern for Ni2P NS@CoP NW-CC. At the hybrid electrode, the diffraction peaks could be indexed to the indicated crystal planes of Ni2P (JCPDS No.03-0953) and CoP (JCPDS No.29-0497), confirming the co-existence of Ni2P and CoP in the electrocatalyst. An X-ray photoelectron (XPS) analysis was carried out to probe the surface chemistry of Ni2P NS@CoP NW-CC. The survey spectrum further corroborates the presence of Ni, Co and P (Fig. S6). There are three main Ni 2p3/2 XPS peaks (Fig. 3B) at binding energies of 853.0, 856.2 and 861.1 eV arising from Ni-P, Ni-POx and its satellite, respectively. The peak located at 853.0 eV is slightly higher than that for metallic Ni (852.6 eV) pointing to Ni in Ni-P with a partially positive charge.39 Similarly, in Co 2p3/2 XPS (Fig. 3C), the peaks at 778.8, 781.9 and 786.4 eV arise from Co-P, Co-POx and its satellite peak. The binding energy of 778.8 eV also suggests the presence of partially charged Co in Co-P since the binding energy of metallic Co is 778.2 eV.40,41 In addition, given the binding energy (779.1 eV) of Co in CoP,42 this value also points to a lower binding energy, presumably arising from electron interactions between Co and Ni in the hybrid electrode.43,44 The P 2p spectrum (Fig. 3D) was fit with two spin-orbit doublets at 129.7 and 134.2 eV. The lower binding energy is consistent with a metal phosphide and the second from an oxidized metal phosphate site on the surface, presumably from exposure to air.45,46 In control experiments, the CoP NW-CC and Ni2P NS-CC electrodes were prepared individually by phosphorization of the precursor electrodes. In Fig. S7, the SEM and TEM images show

Electrocatalytic HER performance. The electrocatalytic activity of Ni2P NS@CoP NW-CC toward hydrogen evolution was investigated in 0.5 M H2SO4 in a typical three-electrode setup. Fig. 4A shows linear sweep voltammetry (LSV) curves for Ni2P NS@CoP NW-CC, CoP NW-CC, Ni2P NS-CC, commercial Pt/C (20 wt% Pt/XC-72), and bare CC at a scan rate of 2 mV s‒1. As expected, 20% Pt/C exhibits the highest HER activity with an almost zero onset overpotential, whereas the bare CC electrode exhibits a poor HER activity with a severely retarded catalytic onset. Under the same conditions, CoP NWCC and Ni2P NS-CC show onset overpotentials of 84 mV and 97 mV, respectively. In comparing these electrodes with a single component, the hybrid Ni2P NS@CoP NW-CC electrode shows a significantly reduced onset overpotential of 30 mV and an overpotential of only 55 mV to reach a current density of 10 mA cm‒2. Fig. 4B shows Tafel plots. Consistent with the LSV results, Ni2P NS@CoP NW-CC exhibits a small Tafel slope of 48 mV dec‒1. This value is relatively close to 31 mV dec‒1 for Pt/C and lower than 69 mV dec‒1 for CoP NW-CC, 71 mV dec‒1 for Ni2P NS-CC and 179 mV dec‒1 for the bared CC electrode. The low overpotential and small Tafel slope for Ni2P NS@CoP NW-CC point to its superior HER activity. As listed in Table S1, as a measure of catalytic performance, it is competitive or superior to other well-known, non-noble-metal HER catalysts in acidic solution. The electrocatalytic activity of Ni2P NS@CoP NW-CC toward the hydrogen evolution was also investigated in 1 M KOH and the results are shown in Fig. S9. To probe the stability of the electrode, the long-term electrolysis experiments were conducted at an overpotential of 65 mV for 12 h in 0.5 M H2SO4. As shown in Fig. 4C, the catalytic current was sustained with a negligible current loss during a 12 h electrolysis period. At the end of the electrolysis test, the recorded polarization curves overlay the initial curve showing that the Ni2P NS@CoP NW-CC electrode is also highly stable for extended periods. High electrical conductivity and favorable electrolyte penetration are important features that favor catalyst reactivity. Electrochemical impedance spectroscopies (EIS) of Ni2P NS@CoP NW-CC at varied overpotentials and EIS of different


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electrodes at a constant overpotential were measured in 0.5 M H2SO4. Nyquist plots of the EIS response and the equivalent electrical circuit are shown in Fig. 4D. In general, the Nyquist plots exhibit two time constants. In the high-frequency region, the semi-circle corresponds to electrolyte diffusion within the electrode. The second semi-circle, in the low-frequency region, arises from charge transfer resistance in the electrode.31,33,34,47 As can be seen, the charge transfer resistance decreases as the overpotential increases. In addition, the semi-circle diameters at high- and low-frequencies for Ni2P NS@CoP NW-CC are much smaller than for either CoP NW-CC and Ni2P NS-CC. The decrease in magnitude confirms that the combination of Ni2P nanosheets and CoP nanowires do enhance electrocatalyst conductivity and electrolyte penetration into the hybrid electrode. To further probe the mixed catalyst, we measured doublelayer capacitance (Cdl) to estimate electrochemically active surface areas (ECSAs) by cyclic voltammetry. Based on the data in Fig. 4E and 4F, the capacitance of Ni2P NS@CoP NW-CC was 86.8 mF cm‒2 compared to 39.4 mF cm‒2 for CoP NW-CC and 25.1 mF cm‒2 for Ni2P NS-CC, showing a significantly enhanced surface area for the mixed catalyst.

Fig. 4. (A) LSV curves and (B) Tafel plots for the labelled electrodes. (C) LSV curves of Ni2P NS@CoP NW-CC before and after 12 h of electrolysis at an overpotential of 65 mV with the inset showing the electrolysis curve. (D) Nyquist plots for Ni2P NS@CoP NW-CC as function of overpotential and for a series of electrodes at a constant overpotential of 110 mV, together with the equivalent electrical circuit. (E) CVs of Ni2P NS@CoP NW-CC at potentials between 0 - 0.2 V at scan rates from 10 - 50 mV s–1. (F) Capacitive currents at 0.105 V as a function of scan rate for different electrodes in 0.5 M H2SO4. Extension to heterostructured NiCo chalcogenides. Inspired by the composition and structure of hydrogenases in which metal sulfur clusters buried in the proteins are the active catalytic sites,18 transition metal sulfides and selenides have attracted increasing attention for HER electrocatalysis.48 To demonstrate the versatility of the synthetic method, we also fabricated Ni3S4 NS@Co3S4 NW-CC and Ni0.85Se [email protected] NW-CC electrodes based on the heterostructured NiCo precursor electrode and used them for HER. As shown in Fig. 5A and 5B, the heterostructures also appeared for the new catalysts after

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addition of sulfur and selenium, although the surfaces were rougher. From the data in Fig. 5C and 5D, the XRD patterns are consistent with conversion of the precursor electrodes into (NiCo)3S4 (JCPDS No.11-0068) and (NiCo)0.85Se (JCPDS No.52-1008 and 18-0888), respectively. Fig. 5E shows LSV curves for Ni3S4 NS@Co3S4 NW-CC and Ni0.85Se [email protected] NW-CC at a scan rate of 2 mV s‒1 in 0.5 M H2SO4. From the data, both electrodes are highly active toward HER with onset overpotentials of 103 and 92 mV, respectively, followed by sharply rising cathodic currents at more negative potentials. The insets show Tafel plots obtained from polarization curves (0.1 mV s‒1) for which the Tafel slopes are 94 mV dec−1 for Ni3S4 NS@Co3S4 NW-CC and 61 mV dec−1 for Ni0.85Se [email protected] NW-CC, slighly inferior to that of Ni2P NS@CoP NW-CC.49 The electrodes are also highly durable. At an applied potential of -0.15 V, catalytic currents were sustained at 40 - 55 mA cm‒2 for at least 12 h, Fig. 5F. Based on these results, as HER electrodes, Ni3S4 NS@Co3S4 NW-CC and Ni0.85Se [email protected] NW-CC are among the best for transition-metal chalcogenides.43,50-52

Fig. 5. SEM images and XRD patterns for (A,C) Ni3S4 NS@Co3S4 NW-CC and (B,D) Ni0.85Se [email protected] NW-CC. (E) LSV curves for Ni3S4 NS@Co3S4 NW-CC and Ni0.85Se [email protected] NW-CC at 2 mV s‒1 with insets showing Tafel plots. (F) Electrolysis curves for Ni3S4 NS@Co3S4 NW-CC and Ni0.85Se [email protected] NW-CC at overpotentials of 150 mV for 12 h in 0.5 M H2SO4. Conclusions. We have demonstrated here fabrication of heterostructured Ni2P nanosheets@CoP nanowires electrodes on carbon cloth and their application as hydrogen evolution catalysts in acidic solutions. The unique heterostructure of the electrode in Ni2P NS@CoP NW-CC results in an impressive level of electrocatalytic activity and excellent durability. We have also shown that the synthetic procedure could be readily extended to NiCo chalcogenides and that they are also promising as water reduction catalysts. The results that we have obtained


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ACS Applied Materials & Interfaces provide a monolithic catalyst for large-scale production of hydrogen, a new synthetic route to related catalysts, and perhaps a new approach to creating new transition-metal structures for other applications in catalysis.

combining experiment and theory. Angew. Chem.-Int. Edit. 2015,

54, 52-65. (10) Faber,

M.

S.;

Jin,

S.

Earth-abundant

inorganic

electrocatalysts and their nanostructures for energy conversion

ASSOCIATED CONTENT

applications. Energy Environ. Sci. 2014, 7, 3519-3542.

Supporting Information

(11) Hong, W. T.; Risch, M.; Stoerzinger, K. A.; Grimaud, A.;

As noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

Suntivich, J.; Shao-Horn, Y. Toward the rational design of nonprecious transition metal oxides for oxygen electrocatalysis.

AUTHOR INFORMATION

Energy Environ. Sci. 2015, 8, 1404-1427.

Corresponding Author

(12) Carenco, S.; Portehault, D.; Boissiere, C.; Mezailles, N.,

*E-mail: [email protected]

Sanchez, C. Nanoscaled metal borides and phosphides: recent developments and perspectives. Chem. Rev. 2013, 113, 7981-

Notes The authors declare no competing financial interest.

8065. (13) Park, S. K.; Kim, J. K.; Kang, Y. C. Metal-organic

ACKNOWLEDGMENTS

framework-derived

We thank the National Natural Science Foundation of China (21573160, 21405114), The Recruitment Program of Global Youth Experts by China, and Science & Technology Commission of Shanghai Municipality (14DZ2261100) for support.

nanocubes with enhanced electrochemical properties for sodium-

CoSe2/(NiCo)Se2

box-in-box

hollow

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