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Surface Roughening of Nickel Cobalt Phosphide Nanowire Arrays/Ni Foam for Enhanced Hydrogen Evolution Activity Xina Wang, Rui Tong, Yi Wang, Hualong Tao, Zhihua Zhang, and Hao Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07226 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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ACS Applied Materials & Interfaces

Surface Roughening of Nickel Cobalt Phosphide Nanowire Arrays/Ni Foam for Enhanced Hydrogen Evolution Activity Xina Wang,a Rui Tong,a Yi Wang,b Hualong Tao,c Zhihua Zhang,c Hao Wanga,* a

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Laboratory of Ferro & piezoelectric

Materials and Devices, Faculty of Physics and Electronic Science, Hubei University, Wuhan, Hubei 430062 b

Stuttgart Center for Electron Microscopy, Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart,

Germany c

Liaoning Key Materials Laboratory for Railway, School of Materials Science and Engineering, Dalian Jiaotong University, Dalian

116028, P.R. China

Graphical abstract

Ternary NiCoP nanowires decorated by homogeneous nanoparticles were obtained on Ni foam for high efficient HER property in neutral and basic conditions.

ACS Paragon Plus Environment


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Germany c


116028, P.R. China

Abstract: Developing earth-abundant, efficient and stable electrocatalysts for hydrogen evolution reaction (HER) in alkaline or even neutral pH value electrolyte is very important for hydrogen production from water splitting. Constructing bi-metal phosphides via tuning the bonding strength to hydrogen, and increasing effective active sites through nanostructuring and surface engineering should lead to high HER activity. Here, ternary NiCoP nanowires (NWs) decorated by homogeneous nanoparticles have been obtained on Ni foam for high efficient HER property via long-term cyclic voltammetric (CV) sweeping. The electron density transfer between the positively charged Ni and Co and negatively charged P atoms, one-dimensional electron transfer channel of the NWs, and the abundant active sites supplied by the nanoparticles and NWs, endow the catalyst with low overpotential of 43 mV and 118 mV to achieve the respective current density of 10 and 100 mA cm-2, together with long durability for at least 33 hours in 1 M KOH. A cycled anodic dissolution-redeposition mechanism is disclosed for the formation of the NiCoP nanoparticles during the CV sweeping process. Such a surface roughening method is found to be adaptable to enhance the HER property of other phosphides including Ni2P nanoplates/NF, NiCoP nanoparticles/NF and CoP NWs/NF. Keywords: NiCoP; Nanowires; Surface roughening; CV sweeping; Hydrogen evolution

*Corresponding author. Email: [email protected]

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1. Introduction Hydrogen is one of the most promising clean and renewable energy which can replace future fossil fuels. The generation of hydrogen via water electrolysis, especially powered by renewable energy, requires highly efficient and robust electrocatalysts.1 When considering the large-scale applications of H2 production, it’s very important to develop inexpensive and earth-abundant catalytic materials with high hydrogen evolution reaction (HER) activity at low overpotentials with long durability. In recent years, extensive efforts have been made on transitional metal compounds including Ni-Mo alloy, MoSx, WS2, MO2, WC, CoS2, CoSe2, FeP, MoP, Ni-P and Co-P to replace the most efficient but expensive catalyst, Pt.2-14 Among them, transition metal phosphides such as CoP, CoP2, Ni5P4, Ni2P, NiP2 and Ni12P5 show excellent HER activity in acidic medium.14-24 When considering the potential applications in alkaline electrolyzers, developing high efficient HER catalysts with high stability in alkaline or even neutral pH value electrolyte is also desirable. Owing to the advantages including large surface area, exposure of reactive lattice planes, enhanced diffusion of electrolyte species, and quick release of formed gas bubbles from the catalysts surfaces, constructing nanostructured electrocatalysts is another strategy for efficient HER activity.25 Combined with the problems such as surface oxidation and the performance degradation with CV sweeping cycles reported for many phosphides, surface engineering should be extensively important for the enhancement of HER property with acceptable stability. Similar with the hydrodesulfurization (HDS) reaction that relies on both the adsorption of organosulfur and H2 molecules on the catalysts and desorption of H2S, the HER behavior is found to relate to the moderate interaction with hydrogen, involving the bonding of protons (in acidic medium) or water molecules (in a base case) to the active sites and the competition between the adsorption and removal of adsorbed H atoms.21,26,27 Meanwhile, theoretical study shows that the high HER activity of Ni2P is associated with the “ensemble effect”, where a moderate bonding strength to hydrogen by P ligand exists with active proton-acceptor (negative charged nonmetal atoms) and hydride-acceptor (metal atoms) sites.28,29 In fact, recent experimental studies exhibit the transfer of electron density from Ni or Co to P in the phosphides, resulting in the formation of active sites of positively charged Ni or Co and negatively charged P, which should be responsible for the high HER property.30,31 Considering the difference in electronegativity and bonding strength to H2 between Ni and Co atoms, ternary phosphides or doping of binary ones should be highly expected for the improvement of catalytic property. Indeed, W-doped and Fe-doped phosphides have been reported,29 and a pyrite-type CoPS with η10 of 75mV and high stability in acid has been established through tuning the hydrogen adsorption free energy by changing the component of the negatively charged atoms.32 Though Ni2-XCoXP has been reported as more active HDS and 2



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hydrogenitrogenation (HDN) catalysts than Ni2P,33-35 there are still few reports on ternary NiCoP compounds for HER applications especially in alkaline or even neutral pH value media. In this work, nanocrystal decorated NiCoP NW arrays with efficient HER property in both alkaline and neutral pH value medium were prepared on Ni foam. A drastic enhancement in the electrocatalytic behavior is found after surface roughening of the NWs via long-term CV sweeping, of which a η10 of 43 mV and η20 of 60 mV with durability for at least 33 hours have been achieved in 1M KOH. A mechanism has been systematically investigated for the hydrogen evolution catalytic enhancement via CV-sweeping. Moreover, it is found the enhancement strategy can be widely applied to other phosphides with different morphology.

2. Experimental section 2.1. Synthesis of Ni-Co-salt NWs on Ni foam (NF). Ni-Co-salt (chemical formula: Ni2/3Co4/3CO3(OH)2) NWs were prepared as the following hydrothermal method.36 In a typical synthesis, 0.1M CoCl2·6H2O (1.43 g), 0.05M NiCl2·6H2O (0.71 g) and urea (1.08 g) were dissolved in 60 mL water under vigorous stirring for 10 min, then the solution was transferred into a 100 ml Teflon-lined stainless-steel autoclave and a piece of Ni foam (4 cm ×7 cm) was immersed into the solution. Before the immersion, the Ni foam was sonicated in 3 M HCl solution for 20 min to remove the surfacial NiOx layer, then rinsed subsequently with water and ethanol, and dried in air. Ni-Co-salt NWs were then hydrothermally grown on the NF substrate after holding the autoclave at 120 ºC for 8 h in an electric oven. Ni-Co-salt NWs with other Ni/Co precursor concentration (0.05M/0.15M, 0.07M/0.1M, and 0.08M/0.1M) were also prepared for comparison. 2.2. Synthesis of NiCoP NWs/NF electrode. By using NaH2PO2 powder as phosphor source, the phosphorization of Ni-Co-salt NWs into NiCoP NWs was carried out in a quartz tube furnace with length, outer and inner diameter of 20 cm, 15 mm and 12 mm, respectively. Both the source and Ni-Co-salt NWs/NF sample were loaded into an one-close-end quartz tube (Φinner:12 mm) with a source-substrate distance of 5 cm, where the powder was positioned at the end of the small tube (center of the furnace) and the sample at the upstream side. The phosphorization process was held at 400 ºC for 2 h under N2 atmosphere, and then naturally cooled down to room temperature. 2.3. Synthesis of Ni2P Nanoflakes/NF electrode. The preparation of Ni2P nanoplates includes two steps. Firstly, the hydrothermal growth of Ni-salt nanoflakes was carried out by loading the cleaned NF into an autoclave at 120 ºC for 8 h, in which the precursor was composed of 2.13 g NiCl2·6H2O and 2.52 g hexamethylenetetramine dissolved in 60 mL water. Then the phosphorization of Ni-salt nanoflakes into Ni2P nanoflakes was undertaken in 3


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the quartz tube furnace, in which the annealing temperature and source-substrate distance were kept the same as those of NiCoP NWs. 2.4. Synthesis of NiCoP Nanoparticles Film/NF electrode. For synthesis of NiCoP nanoparticles film, the as-prepared Ni-Co-salt NWs/NF sample was placed just above the NaH2PO2 powder, both of which were located at the center of the quartz tube furnace. Then the phosphorization process was held at 400 ºC for 2 h under N2 atmosphere before naturally cooled down to room temperature. 2.5. Synthesis of CoP NWs/NF electrode. The preparation of CoP nanowires includes two steps. Firstly, the growth of Co-salt NWs was carried out by immersing clean NF substrate in the aqueous solution of 1.43 g CoCl2·6H2O and 0.72 g urea holding in an autoclave at 120 ºC for 8 h. Then the Co-salt NWs was phosphorized into CoP NWs in the quartz tube furnace similar with that of NiCoP samples. 2.6. Synthesis of Pt/C electrode. Firstly, 2 mg mL-1 Pt/C ink was prepared by dispersing 2 mg of Pt/C (20 wt% Pt on Vulcan XC-72) in a 2 ml mixture containing water, isopropanol, and Nafion (5%) with volume ratio of 4:1:0.05. Pt/C electrode was then obtained by coating 20 µL Pt/C ink onto a freshly polished glassy carbon electrode with subsequent natural drying in the air.37 2.7. Morphology, composition and structure characterizations. The crystallinity and morphology of the samples were characterized by X-ray diffraction (XRD, Bruker D8 Advance X-ray diffractometer) and field emission scanning electron microscopy (SEM, JSM-7100F, JEOL), respectively. The microstructure and compositional distribution were studied by transmission electron microscopy (TEM) equipped by scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS) (JOEL 2100F). The electronic structure and surfacial chemical composition of the samples were studied by X-ray photoelectron spectroscopy (XPS) measurements (Escalab 250Xi) with Al Kα radiation. 2.8. Electrochemical measurements. The electrochemical properties of the samples were performed with cyclic voltammetry (CV) and Linear Sweep Voltammetry (LSV) technique in a typical three-electrode system using a CHI 760E electrochemical analyzer (CH Instruments, Inc., Shanghai). The as-prepared NiCoP NWs/NF, platinum foil and a saturated Ag/AgCl electrode were used as working, counter and reference electrode, respectively. The effective area of the electrode was 1 cm × 1 cm. The CV sweepings adopt two typical rates, i.e., 10 mV s-1 and 100 mV s-1 in the range of 0.1~-0.3 V. During LSV tests, the scan rate was kept at 2 mV s-1. In addition, electrochemical impedance spectroscopy (EIS) was measured with the same configuration at a bias of -0.08 VRHE with an amplitude of 5 mV in a frequency range from 100 kHz to 0.1 Hz. The spectra were analyzed 4



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by Z-View program (Scribner Associates Inc.). According to high frequency AC impedance measurements, all potentials were IR compensated, where I and R denote current and uncompensated Ohmic electrolyte resistance. All the experiments were carried out at ambient temperature. The potentials were converted to those vs reversible hydrogen electrode (RHE) using the following equation. E(vs RHE)= E(vs Ag/AgCl) + EAg/AgCl (ref) + 0.0591V×pH (EAg/AgCl (ref)= 0.1976 V vs NHE at 25 oC)

(1)

3. Results and discussion 3.1. Materials Characterization. The fabrication process of the samples includes three steps of 1) hydrothermal growth of Ni-Co-salt, 2) phosphorization and 3) CV sweeping as presented by Figure S1. From the low-magnification SEM images (Figure S1), high-density large-scale NiCoP NW arrays can be obtained on NF by the phosphorization of Ni-Co salt NWs on Ni foam. As shown by the different magnified SEM images of Ni-Co-salt (chemical formula: Ni2/3Co4/3CO3(OH)2) NWs (Figure 1a),36 pencil-like Ni-Co-salt NWs with average diameter of ~140 nm were uniformly grown on the Ni foam. After phosphorization, the NWs surface becomes much rougher, and flake-like nanoparticles with size ranged in 40±10 nm can be found to sparsely distribute on the surface (Figure 1b). The side-view SEM image (Figure S1) indicates that the phosphide nanoarrays have average length of ~3.2 µm. After CV sweeping at 100mV s-1 for 4000 cycles (Figure 1c), the large nanoflakes were replaced with many smaller nanoparticles with size less than 20 nm on the surface of the nanowires. Simultaneously, from the corresponding EDS spectrum (Figure 1d) equipped with the SEM images, elemental Ni, Co and P can be clearly found for both the as-prepared and post CV processing samples, suggesting the ternary nature of the production. From XRD patterns for both samples (Figure 1e), except the diffraction peaks (marked by number signs) from NF substrate at 44.5°, 51.8° and 76.4°, the peaks at 40.7°, 47.3°, 54.2°, 55.0° can be indexed to the diffractions of (111), (210), (002) and (211) planes of hexagonal NiCoP (JCPDS No. 71-2336) with space group of P-62m. No significant signals of Ni or Co-oxides have been detected, indicating the sufficiency of the phosphorization process. From the above results, long-term CV processing has much more effect on the morphology rather than on crystallinity and elemental composition of the sample.

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Figure 1. Low- and high (inset)-magnification SEM images of (a) Ni-Co-salt NWs/NF, (b) as-prepared NiCoP NWs/NF, (c) post-CV sweeping NiCoP NWs/NF. (d) EDS spectra of NiCoP NWs/NF before and after CV sweeping. (e) XRD patterns of as-prepared and post-CV sweeping NiCoP NWs/NF (top), with the simulated spectra of NiCoP and NF provided for comparison (bottom).

To further understand the microstructure dependence on the long-term CV processing, differently magnified TEM images of single NiCoP nanowire are given in Figure 2(a-c) and (d-f) for the as-prepared and post-CV processing samples, respectively. Before CV-processing, sparse nanoflakes with size of 40 ± 10 nm form on the surface of NiCoP nanowire, the distance between every two adjacent nanoflakes is larger than 50 nm. The NW is polycrystalline with grain size typical larger than 30 nm, in which the lattice spacing of 2.23 Å corresponds to the (111) plane of hexagonal NiCoP. After CV-sweeping for 4000 cycles, the NW becomes much looser and still remained pollycryalline structure, and many dense nanoparticles with size ranged in 5-20 nm have uniformly grown on the surface of the NW (Figure 2d-e). The STEM image and the corresponding EDS mappings (Figure 2g-j) further confirm the uniform distribution of the elemental Ni, Co and P in the surface roughened NW. From the magnified image (Figure 2f) of the purple square region in Figure 2e, the nanograins show lattice fringes with d spacing of 2.02 Å and 2.23 Å, consistent with the (201) and (111) planes of NiCoP. The small nanograins, with a 6



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size of several nanometers, are accumulated to each other and form dense nanoclusters closely attached to the NWs, which ensures the high electronic conductivity to the NF substrate. Simultaneously, plenty of the exposed facets may supply more active sites for the following HER behavior than the relative smooth nanowires before CV processing.

Figure 2. The microstructure and composition distribution of single NiCoP NW before and after long-term CV sweeping. TEM images with different magnifications for as-prepared (a-c) and post CV-sweeping NWs (d-f), respectively. (f) Magnified image from the purple square region in image (e). (h-j) Corresponding EDS mappings of elemental P, Co and Ni from the purple rectangular area in the STEM image (g).

3.2. HER Property. The HER catalytic property of the NiCoP NWs/NF electrodes before and after CV sweeping was measured in 1 M KOH (pH= 13.6) and 1M PBS (pH= 6.9) aqueous electrolyte respectively using a three-electrode setup. The HER activity of commercial Pt/C and pure NF electrode was also evaluated for comparison. IR compensation was adopted for all the polarizations including LSV and CV curves in this work. As shown by the polarization curves in Figure 3a and 3c, Pt/C shows the highest catalytic behavior in 0.5 M H2SO4, much better than the activity in 1M KOH, and a Tafel slope, b of 30.9 mV dec-1 can be obtained, very close to the reported value of commercial Pt.21 Compared with the pure NF and Pt/C, the NiCoP NWs show superior HER 7


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activity in alkaline media, and remarkable property enhancement can be found after processed by fast-speed CV sweeping. The overpotentials (vs RHE) to reach the current density of 10, 20, and 100 mA cm-2 (labeled as η10, η20, and η100, respectively) for the initial sample are 101(η10), 124(η20), and 204(η100) mV, respectively. After CV sweeping for 2000 cycles, the values greatly deceased to 61(η10), 81(η20), and 152(η100) mV, respectively. The electrocatalytic behavior will reach an optimum after sweeping for 4000 cycles, and little change can be found with further CV cycles to 5000, indicating the high stability of the rough NiCoP NWs. The optimized overpotentials are 43(η10), 60(η20), and 118(η100) mV, comparable to the best electrocatalysts of phosphides, such as CoP nanosheets (η10= 49 mV, η20= 59 mV, η100= 94 mV) and CoPS nanoplates (η10= 48 mV, η20= 65 mV) in acid and nanocrystalline Ni5P4 (η10= 49 mV, η100= 202 mV) in base (see the summarization in Table S1).18,33 The activity is also superior than the performance of the Ni2P nanoparticles/N-doped RGO (η10= 102 mV, η20= 122 mV), Ni12P5 nanoparticles/Si NWs (η10= 107 mV, η20= 141 mV) and Ni5P4-Ni2P nanosheet arrays/NF (η10= 120 mV, η20= 140 mV, η100= 200 mV) in acid.20,22,24 The effect of CV-processing on the HER activity is found to be applicable for neutral electrolytes. As shown by the HER behavior in PBS solution (Figure 3b), similar activity enhancement via CV processing can also be achieved for the NiCoP NWs/NF electrocatalyst, the potentials η10 and η20 could be lowered from 152 mV and 206 mV to 76 mV and 129 mV respectively after CV sweeping for 3000 cycles, which are almost the lowest among the metal phosphides working in neutral pH value solution.

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Figure 3. (a) LSV curves of NiCoP NWs/NF before and after CV sweeping for different cycles, NF and Pt/C in 1 M KOH, along with Pt/C in 0.5 M H2SO4. (b) Polarization data for NiCoP NWs/NF in 1 M PBS initially and after 1000, 2000, 3000 CV sweeps. The insets in (a) and (b) summarize the overpotential-cycle number plots at different current densities. (c) Tafel plots of the corresponding NiCoP NWs/NF after CV processed for different cycles, Pt/C in 1 M KOH and Pt/C in 0.5 M H2SO4 are also provided for comparison. (d) Tafel plots of the corresponding NiCoP NWs/NF after CV processed for different cycles in 1 M PBS. (e) Time dependence of the overpotential for the CV-processed NiCoP NWs/NF at a static current density of 10 mA cm-2 for 33 hours in 1 M KOH. (f) Nyquist plots of the three electrodes in 1M KOH at -0.08 VRHE.

The CV processing-dependent Tafel plots of overpotential versus log J in 1 M KOH (Figure 3c) and 1 M PBS (Figure 3d) are respectively given for the NiCoP NWs/NF. Tafel slopes of 69.1, 68.9, 62.5 and 59.4 mV dec-1 can be obtained after the sample underwent fast-speed CV scanning for 0, 2000, 3000 and 4000 cycles in 1M KOH. Besides, Tafel slopes of 182.6, 122.4, 120.3 and 103.9 mV dec-1 can be acquired after fast-speed CV scanning for 0, 1000, 2000 and 3000 cycles in 1M PBS. Considering the theoretical Tafel slopes of 30, 40, and 120 mV dec-1 correlated to the three rate limiting steps in typical HER process,21,26 our data suggest that the HER activity of NiCoP NWs/NF follows a Volmer-Heyrovsky mechanism. The reliability and durability of the HER property were studied for the NiCoP NWs/NF. Once the activity reaches the optimum values by CV cycling, they will remain stable even after 15 days’ exposure to the air or 5 9


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days’ immersion in KOH aqueous solution (Figure S2), indicating that the CV cycling effect on the HER activity is irreversible due to its roles on the surface roughening of the nanowires. Moreover, the time dependence of the overpotential at 10 mA cm-2 (Figure 3e) shows an increase of less than 10 mV in overpotential after working in base for 33 hours. Such stability seems superior to the CoP nanocrystals/carbon cloth, nanocrystalline Ni5P4, FeP nanorod arrays in alkaline media (see the summarization in Table S1). To further study the effect of CV sweeping on the kinetics of HER, the EIS analysis is presented as the Nyquist plots (Figure 3f) fitted with equivalent circuit (the inset). The equivalent series resistance (ESR) can be obtained from the intersection of the curve and the real resistance (Z') axis, while the charge-transfer resistance (Rct) corresponds to the width of the semicircle plotted at higher frequencies. The NF, as-prepared and post-CV sweeping NiCoP samples have the same ESR value of about 0.9 Ω, indicating the whole system has very low internal resistance. Compared with the Rct (6.4 Ω) of the as-prepared NWs, the charge transfer resistance is greatly lowered to 3.4 Ω after CV processing for 4000 cycles, suggesting that CV sweeping can help to speed up the Faradaic process and kinetics of the HER.38 3.3. Chemical States Evolved with CV Processing. XPS measurements were carried out to examine the elemental type and valence state of the NiCoP NWs before and after CV sweeping. Figure 4(a) and (b-d) show the XPS survey spectra and fine scans for P (2p), Ni (2p), and Co (2p) region, respectively. As shown by the survey spectra, Ni, Co, P, C and O are present for both the as-prepared and CV sweeping for 4000 cycles samples. For accuracy, all the data have been corrected by C 1s peak at 285.0 eV. For the as-prepared sample, the Ni 2p3/2 peaks at 853.4 and 855.9 eV can be attributed to Niδ+ in NiCoP and Ni2+,39-41 respectively. Another two Co 2p3/2 peaks at 778.9 and 781.1 eV originate from Coδ+ in NiCoP and Co2+.17,39,42 The peaks at higher binding energies such as 861.7 eV and 786.0 eV can be assigned to the satellites of Ni 2p3/2 and Co 2p3/2 respectively. In the high resolution XPS spectrum of P, the peaks of 129.23 and 130.07 eV can be attributed to P 2p3/2 and 2p1/2 from NiCoP, respectively.40-42 Compared with the binding energy of the elemental ones, such as metal Co 2p3/2 region of 778.1~778.2, metal Ni 2p3/2 of 852.5~852.9eV and P 2p peak at 130.2 eV,43 after formed ternary alloy, both Co and Ni peak are positively shifted whereas P 2p shifts to much lower energy, suggesting that there exists electron density transfer from Ni and Co to P, finally making Ni and Co positively charged (δ+) while P is negatively charged (δ-) in NiCoP NWs. Accordingly, a moderate bonding strength to hydrogen may be realized through a similar “ensemble effect” between the Pδ-, Niδ+ and Coδ+ sites, which contributes to the good HER property.

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Figure 4. (a) XPS survey scans for the NiCoP NWs/NF electrodes before and after CV sweeping for 4000 cycles at a scan rate of 100 mV s-1. (b-d) High-resolution XPS spectra of P, Ni, Co 2p peaks from the as-prepared and post-CV sweeping NiCoP NWs/NF.

It should be noted that for both the as-prepared and post-CV sweeping samples, the orthophosphate XPS peaks such as PO42- and HPO3H- can be found at 134.3 and 133.1 eV, respectively.39 Combined with the Ni2+ and Co2+ XPS peaks, the result strongly implies the existence of Ni and/or Co orthophosphate at the surface of the NiCoP NWs. In the depth-dependent fine XPS spectra (Figure S3) obtained after Ar sputtering for 2 minutes, the intensity of NiCoP XPS peaks greatly surpasses the orthophosphate, indicating the thinness of the orthophosphate layer. Indeed, Ni or Co-related orthophosphate can be readily formed during the sample transfer of Ni2P and/or CoP from air to the XPS chamber.39,43 Nonetheless, it is believed that the orthophosphate will be cathodic reduced when performed in HER process under negative bias,44 suggesting that the effect of orthophosphate on the HER activity can be ignored, which can also be confirmed by the XPS result. Though the intensity of orthophosphate surpasses that of NiCoP, the CV-processed NiCoP NWs still exhibit higher HER activity. Additionally, high-resolution XPS results of Pt and O (Figure S4) show that the CV-processed electrocatalysts are free of Pt impurities and the oxygen signal from nickel and cobalt oxides can also be excluded, which suggests that the enhanced HER property is primarily originated from the surface roughening of NiCoP NWs rather than Pt contaminations and metal oxide. 3.4. Mechanism Aspect. To find out the origin for both the surface roughening and CV-processing modification effect, the electrochemical behavior of the NiCoP NWs was studied by changing the technique type, 11


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CV potential range and sweeping rate for the NiCoP NWs/NF electrodes. Figure 5a shows the LSV curves of the NiCoP NWs before and after holding at constant -50mA/cm2 in 1M KOH for 22h in a chronoamperometry mode (the inset). No obvious change can be found in either the LSV curves or morphology of the samples, indicating that constant cathodic current processing can not induce any change in the electrocatalytic behavior and morphology. When applied in CV mode involving cyclic anodic and cathodic sweeping, it was found that the electrocatalytic property is highly dependent on sweeping rate and potential range. Figure 5b shows the CV curves of the NiCoP NWs in a range of -0.3~1.0V vs RHE after processed for 0 to 800 cycles with a constant rate of 100 mV/s in 1M KOH. A high anodic current of 70 mA/cm2 can be produced by the wide positive potential for the first CV cycle. Though the anodic current generally decreases with the cycles, the cathodic current also drops obviously, leading to heavy decay in the electrocatalytic behavior as seen from the LSV curves (the inset in Figurre 5b). In the SEM image of the sample after 800 cycled CV sweeping, many nanoplates with size of several hundred nanometers have been formed along the skeletons of the previous NWs (Figure S5), which maybe caused by the severe dissolution of nickel and cobalt cations under the long-term high anodic current processing.

Figure 5. (a) LSV curves of the NiCoP NWs before and after being held at constant -50 mA cm-2 in 1M KOH for 22h in a chronoamperometry mode (the inset). (b) CV curves of the NiCoP NWs after 800 cycled CV sweeping at 100 mV s-1 ranged in -0.3~1.0V vs RHE in 1M KOH. The consecutive CV curves at various cycles with sweeping rates of 100 mV s-1 (c) and 10 mV s-1 (d) in a range of -0.3~0.1V. The insets in (b), (c) and (d) show the corresponding LSV curves before and after CV process with final cycles.

When the potential range of CV sweepings was shortened to -0.3V~0.1V at a constant rate of 100mV/s (Figure 5c), the anodic current for the 1st cycle can be greatly decreased to 20 mA cm-2, and a simultaneous increase in both the anodic and cathodic current can be found with the cycle numbers. After sweeping for 4000 12



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cycles, the anodic current increases to 38 mA cm-2, and the electrocatalytic property is also greatly enhanced along with the formation of NiCoP nanocrystals on the NWs. However, when the anodic current was decreased to 3 mA cm-2 by reducing the sweep rate to 10 mV s-1 (Figure 5d), both the anodic and cathodic currents changed slightly with the cycle numbers, and the morphology of the NiCoP NWs also remained unchanged. Accordingly, the LSV curves (inset d) show little change in the electrocatalytic property of the sample before and after CV processing. The results suggest that the moderate anodic current and alternating anodic-cathodic voltage processing are essential factors for the formation of the NiCoP nanocrystals on the NWs. Similarly, with potential reversal (PR) deposition, a formation mechanism is proposed here for the growth of NiCoP nanocrystals on NiCoP NWs accordingly (Figure 6). PR deposition, in which an anodic voltage is periodically applied following the cathodic deposition in the electrolyte containing precursors, has been recently adopted to deposit nickel phosphide and/or sulphide nanostructure films. Unlike potentiostatic (PS) deposition, in PR process, the anodic dissolution of Ni cations will occurred at ~ 0.1V after the cathodic deposition semicycle, which may restrict the growth of the nanograins and result in more tiny size for the nanocrystals.45-47 In our case, the anodic dissolution process of Ni and Co cations can be found due to the obvious anodic current presented in Figures 5b and 5c, during which P anions are released around the NiCoP NWs. At the reversal potentials, re-deposition of the dissolved ions may occur for the formation of the NiCoP nanocrystals on the surface of NWs (Figure 6). By periodically applying positive and negative potentials, the components are carried from the NWs to the surface, which is different with the cathodic NiP or CoP deposition from the Ni2+ and PO43- precursors in the electrolyte.44 As the CV deposition time extends, the number and size of the grains will grow. Since the deposition process usually happens with hydrogen evolution independently of the pH value of electrolyte, the nascent hydrogen produced via HER and large surface area provided by the NWs may play key roles for the re-deposition process, such as reducing the dissolution overpotential and reaction energy barriers. Though it is found too high or too low anodic current would cause excess or insufficient dissolution of the cations, an in-depth electrochemical study is under way to disclose the intermediate reaction during the dissolution-redeposition process of the NiCoP nanoparticles.

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Figure 6. Schematic illustrating the formation mechanism of NiCoP nanoparticles from the homogeneous NWs via CV processing.

Besides the NiCoP NWs with a Ni/Co precursor concentration of 0.05M/0.1M as discussed above, the LSV curves of the NWs with various Ni/Co composition were shown in Figure S6 by changing the precursor concentration (0.05M/0.15M, 0.07M/0.1M and 0.08M/0.1M). Compared with 0.05M/0.1M, the increase in the relative concentration of Ni or Co results in the deterioration of HER property. The hydrogen evolution catalytic enhancement via CV sweeping can also be applied to other phosphide systems. From the polarization property as summarized in Table 1, for Ni2P nanoplates/NF electrode in 1 M KOH, the η10 and η100 can be decreased from 135 mV and 166 mV to 46 mV and 68 mV after CV sweeping for 5000 cycles (for detail see Figure S7). An η10 of 53 mV and η20 of 76 mV can be achieved for NiCoP nanoparticles film on NF after sweeping for 5000 cycles, greatly lower than the 116 mV (η10) and 142 mV ( η20 ) without CV processing (for detail see Figure S8). The η10 and η20 of CoP NWs can be decreased from 120 mV and 142 mV to 54 mV and 77 mV after CV sweeping for 5000 cycles (for detail see Figure S9). The Tafel plots are also decreased by CV-sweeping for all electrodes. In comparison, the surface roughened NiCoP NWs/NF electrode shows superior electrocatalytic property due to the following contributions. Firstly, the high specific surface area of NiCoP NWs decorated by numerous nanoparticles can provide plenty of active lattice planes for the HER. Secondly, the one-dimensional NWs structure and their tight attachment with the nanoparticles can supply good electrical conductivity, promote fast mass transport of reactants and products, and accelerate the release of the formed H2 gas bubbles from the surface. Thirdly, the electrical environment correlated with the negatively charged P and positively charged Ni and Co can effectively tune the bonding strength to hydrogen responsible for the native HER activity. Table 1. The enhancement of CV sweeping on the HER property of NiCoP NWs, NiCoP nanoparticles and Ni2P nanoplates on NF in 1 M KOH

Catalyst

Initial η10

η20

Post-CV sweeping Tafel slop

η10

14


η20

Tafel slop


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Ni2P nanoplates NiCoP nanoparticles CoP nanowires NiCoP nanowires

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(mV)

(mV)

(mV dec-1)

(mV)

(mV)

(mV dec-1)

135 116 120 101

166 142 142 124

95.3 82.3 79.1 69.1

46 53 54 43

68 76 77 60

64.9 75.6 72.8 59.4

4. Conclusions Hexagonal NiCoP NWs decorated by homogeneous nanoparticles with size of 5-20 nm have been obtained for high efficient HER property through surface roughening processes via long-term CV sweeping. An anodic dissolution and cationic deposition process during CV sweeping is found to be responsible for the formation of the NiCoP nanoparticles. Due to the large surface areas provided by the nanoparticles and the NWs, the one-dimensional electron tunnel role of the NWs, and the electron density transfer among Niδ+, Coδ+ and Pδ-, the rough NiCoP NWs/NF electrode exhibits overpotentials as low as 43(η10), 60(η20), and 118(η100) mV, with high durability for at least 33 h in 1 M KOH. Moreover, the surface roughening strategy can be applied to other phosphides with different morphologies to enhance the HER activity.

Supporting Information Side-view SEM images; Optical photograph; LSV curves of CV-processed samples after 15 days’ exposure in air; XPS spectra etched by Ar+ for 0, 1 min and 2 min; XPS spectra of Pt 4f and O 1s; SEM images of NiCoP/NF samples after 800 cycled CV sweeping ranged in -0.3~1.0 VRHE; LSV curve of Ni2P nanoplates/NF, NiCoP nanoparticles/NF and CoP NWs/NF before and after 5000 cycles sweeping; SEM images of post-CV processing Ni2P nanoplates, NiCoP nanoparticles and CoP nanowires; Summary of HER performance of recently reported phosphides. The material is available free of charge via Internet at http://pubs.acs.org.

Acknowledgements We thank Dr. Zhi Li (Institute of Physics, Chinese Academy of Sciences) for the kind help in the TEM characterizations. This work was supported by the National Natural Science Foundation of China (No. 11574077, 51472080), Research Fund for the Doctoral Program of Higher Education of China (RFDP, No. 20104208120004), and Open Research Fund Program of the State Key Laboratory of Low-dimensional Quantum Physics (No. KF201411).

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Germany c


116028, P.R. China

Graphical abstract

Ternary NiCoP nanowires decorated by homogeneous nanoparticles were obtained on Ni foam for high efficient HER property in neutral and basic conditions.

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Ni

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