Large-Area Synthesis of a Ni2P Honeycomb Electrode for Highly

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Large-Area Synthesis of Ni2P Honeycomb Electrode for Highly Efficient Water Splitting Xu-Dong Wang, Yang Cao, Yuan Teng, Hong-Yan Chen, Yang-Fan Xu, and Dai-Bin Kuang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10893 • Publication Date (Web): 06 Sep 2017 Downloaded from http://pubs.acs.org on September 7, 2017

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ACS Applied Materials & Interfaces

Large-Area Synthesis of Ni2P Honeycomb Electrode for Highly Efficient Water Splitting Xu-Dong Wang, Yang Cao, Yuan Teng, Hong-Yan Chen,* Yang-Fan Xu, and Dai-Bin Kuang*

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, P. R. China.

KEYWORDS:

nickel

phosphide,

macroporous,

large

area,

bifunctional

electrocatalyst, water splitting

ABSTRACT

Transition metal phosphides have regarded as robust, inexpensive electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) recently. Thus far, tremendous scientific efforts have been applied to improve the catalytic

activity

of

catalyst,

whereas

the

scale-up

fabrication

of

morphology-controlled catalysts while maintaining their desired performance remains a great challenge. Herein, we present a facile and scalable approach to fabricate the macroporous Ni2P/nickel foam (mp-Ni2P/Ni foam) electrode. The obtained electrocatalyst exhibits superior bifunctional catalytic activity and durability as

ACS Paragon Plus Environment


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evidenced by low overpotential of 205 and 300 mV required to achieve a high current density of 100 mA cm-2 for HER and OER, respectively. Such spray-based strategy is believed to widely adapt for the preparation of electrodes with uniform macroporous structures over a large area (e.g. 100 cm2), which provides a universal strategy for mass fabrication of high performance water splitting electrodes.

1. INTRODUCTION

With the ongoing concerns regarding energy and environmental issues, increasing interest has been devoted to the development of renewable energy resource and utilization technology. One of the fast-developing technologies for storing energy and further converting it to fuel is electrolysis of water to hydrogen.1 Pt/C and RuO2 currently are recognized as the state-of-the-art electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively.2 Nonetheless, the scarcity and the consequential high cost considerably impede their practical application on a large scale. Accordingly, substantial scientific communities have been driven to seek for alternative low cost and efficient nanostructured water splitting electrocatalysts, such as transition metal sulphides,3 nitrides4, phosphides5-12 and selenides.13, 14 Recently, nickel phosphide is found to act as efficient alternative catalyst,15,

16

by virtue of its unique electronic properties and superior corrosion

resistance. Tailoring the surface morphology can facilitate to expose more active sites of catalysts, which is pivotal to develop materials with outstanding performances for water splitting.17-20 Therefore, a number of chemical transformation strategies have


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been carried out to prepare various nanostructured nickel phosphides, such as solvothermal method and phosphidation of nanostructured metal oxides/hydroxides.16, 21-32

Among the diverse micro/nano architectures, macroporous structured catalyst is a

good choice since the interior hollow space can not only provide a large specific surface area but also facilitate the infiltration and diffusion of electrolyte.33-35

In terms of the practical applications of water splitting, high quality large-area electrode with catalysts directly anchored on the current collector is required.36 Devices that combine flexibility with high mechanical stability can effectively reduce the disintegration tendency of the catalyst layer, remitting the impact of bubble accumulation. Although stunning progress has been made on metal phosphides, most of the previous researches were mainly focused on small-scale catalyst in laboratory scale.37 Sandwiching the active particles between two current collector sheets (such as Ni foam and Ti foil) has been proposed to fabricate large-area electrodes,38, 39 which, however, can not satisfy the water spliiting requirements such as high stability and good performance. Thus, there is a pressing need for a universal and facile fabrication strategy to prepare morphology-controlled catalysts in large area while maintaining the desired performance.37, 40

Here, we report a three-dimensional (3D) honeycomb monolithic catalysts consisting of macroporous Ni2P (mp-Ni2P) coated on nickel foam by a facile and easily scalable method, where Ni precursors and polystyrene spheres (PS) are spray-coated on nickel foam, followed by annealing and phosphidation process at 300 °C. Such hollow macroporous structure possesses many unique merits, including



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facilitating the infiltration and diffusion of electrolyte,33-35 exposing more active sites and reducing the internal interfacial resistance. Thus, the obtained 3D mp-Ni2P honeycomb based electrode delivers superior bifunctional catalytic activity and durability as evidenced by low overpotential of 205 and 300 mV required to achieve 100 mA cm-2 for HER and OER, respectively.

2. EXPERIMENTAL SECTION

Materials. Nickel chloride hexahydrate (NiCl2·6H2O) and citric acid monohydrate (C6H8O7·H2O) were all obtained from Tianjin Baishi Chemical Industry Co. Ltd. Ni foam was purchased from Shanxi LIZHIYUAN battery materials Co. Ltd. 5 wt % Nafion solution was purchased from Sigma-Aldrich. Styrene was purchased from Tianjin Fu Chen Chemical Reagent Factory. Sodium hypophosphite (NaH2PO2) was purchased from Aladdin Industrial Corporation. All the chemicals used in this experiment were of analytical grade and used as received without further purification.

Catalysts synthesis. Coating NiCl2 layers on the polystyrene spheres (PS) latex with diameter of ~600 nm to obtain the core-shell structure PS@NiCl2 particles was performed by a simple mixing and spray coating process. Briefly, 10 mL polystyrene spheres (PS)34 latex was added to 50 mL ethanol solution of 40 mM citric acid and 40 mM NiCl2. The resultant solution was deposited onto Ni foam (100 cm2) by the spray coating method. During the coating process, the substrate was heated to 100 °C on a hot plate to accelerate the solvent evaporation. Subsequently, the as-prepared sample


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was calcined at 300 °C for 30 min to obtain the macroporous NiO on Ni foam (mp-NiO/Ni foam). To synthesize mp-Ni2P/Ni foam sample, appropriate NaH2PO2 was placed at front zone of the tube furnace, and the as-prepared mp-NiO/Ni foam was put at back zone. Subsequently, mp-NiO/Ni foam was heated up to 300 °C and the NaH2PO2 was heated up to 250 °C, holding at the temperatures for 90 min under static N2 atmosphere. The similar phosphidation processes for bulk Ni2P were carried out by direct phosphorization of commercial nickel foam as aforementioned. Next, the final products were washed thoroughly with 0.5 M H2SO4 and deionized water to remove the impurities. The average mass loading of mp-Ni2P is about 2 mg cm-2, which was determined by weighting the Ni foam before and after catalyst loading using an analytical balance with a nominal precision of 0.1 mg. For comparison, approximately 4 mg cm-2 commercial Pt/C and RuO2 on Ni foam electrode was prepared using Nafion as a binder.

Structural and electronic property characterization. The phase structure of samples were analyzed by X-ray powder diffraction diffractometer (XRD, Rigaku Co.). The morphology images were examined on field emission scanning electron microscope

(Hitachi

SU8010

SEM).

High-resolution

transmission

electron

microscopy (HRTEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images were obtained from the transmission electron microscopy (TEM, Titan G2 60-300). Elemental mapping images were recorded by energy-dispersive X-ray spectroscopy (EDX) attached to Titan G2 60-300. The surface chemical composition of samples was analyzed by X-ray photoelectron



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spectroscopy (XPS, ESCALAB 250, Thermo Fisher Scientific).

Electrochemical measurements. The electrochemical measurements were conducted on a CHI660E potentiostat in a three-electrode cell, using graphite rod as counter electrode and saturated Ag/AgCl (EθAg/AgCl=0.1976 V) as a reference electrode. The polarization curves were obtained by linear sweep voltammetry (LSV) in N2 saturated aqueous solution of 1.0 M KOH or 0.5 M H2SO4 with a scan rate of 2 mV s-1. Unless otherwise noted, all the potentials were iR corrected. The determination of double layer capacitance (Cdl) of mp-Ni2P and bulk Ni2P was performed by cyclic voltammetry at different scan rates (10 to 100 mV s-1) in the potential range of -0.1 to 0.1 V vs. RHE. Electrochemical impedance spectroscopic (EIS) measurements were conducted at a fixed voltage -100 mV vs. RHE, the frequency range of which was 0.05 Hz to 100 kHz. Long-term durability test was conducted by potential cycling from +1.2 to +1.5 V, -0.2 to +0.1 V, vs. RHE at scan rates of 100 mV s-1 for the OER and HER stability respectively. Prior to the experiments, all electrodes were sealed by epoxy resin to define the electrode area (0.3 cm2). Chronopotentiometry experiments were conducted on FL-9790 II.

3. RESULTS AND DISCUSSION

Physical Structure and Morphological Characterizations. The Ni precursor ink was first prepared by mixing 40 mM NiCl2, 40 mM citric acid and 10 mL polystyrene spheres (PS) latex (with diameter of ~600 nm) in 50 mL ethanol solution, which was then deposited onto the nickel foam by automatic spray coating, as illustrated in


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Scheme 1. Such simple approach provides a valid strategy for large-area fabrication of the electrode on different conductive substrates such as Ni foam, FTO and Ti foil, and herein a typical sample of 100 cm2 using Ni foam substrate was prepared as an illustration (Movie S1). A subsequent annealing treatment at 300 °C for 30 min in air was applied to convert the Ni based precursor into NiO macropores (mp-NiO), after which an in situ P/O exchange process at 300 °C was carried out to obtain mp-Ni2P. As shown in Figure S1, the color of Ni foam turned in brown from grey after NiO coating, and further changed to black after phosphidation process, indicating the formation of Ni2P. Such simple procedure is highly desirable for the realization of large-area catalyst electrode for hydrogen production.

Scheme 1. Schematic illustration of the synthesis procedure of Ni2P honeycomb monolithic electrode.

With the aid of the scanning electron microscope (SEM) characterization (Figure 1a-c), spheres of PS@NiCl2 with diameter of about 600 nm were clearly observed which distributed uniformly on the surface of Ni foam. After removing the PS template via an annealing process, macroporous structured NiO (mp-NiO) with



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smooth surface was obtained (Figure 1d-f). The diameter of the open-pore from top view SEM image is ~400 nm, however, the real pore size of the same micropore would be larger than 400 nm, which has also been observed by other reports.41

Figure 1. Typical FE-SEM images of PS@NiCl2/Ni foam (a, b, c) and mp-NiO/Ni foam (d, e, f).

Interestingly, the macroporous structure of NiO was well maintained after phosphidation as confirmed by SEM and TEM images (Figure 2a-d). The XRD pattern (Figure S2) confirmed that the resulted sample was Ni2P phase with hexagonal structure (JCPDS No. 03-0953). From the HRTEM image (Figure 2e), interconnected nanoparticles (~16 nm, Figure S3) were observed on the macroporous skeleton of mp-Ni2P. Comparing with the pore-free bulk Ni2P electrode (bulk Ni2P/Ni foam, Figure S4) fabricated by directly phosphorizing commercial nickel foam current collector, such rough and loose structure would greatly enlarge the active surface area. The high crystallinity of Ni2P particle was confirmed with the HRTEM image (Figure 2f) and the observed interplanar distance of 0.209 nm was consistent with (201) plane


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of Ni2P. Energy-dispersive X-ray (EDX) mappings (Figure 2g-i) verified that Ni and P distributed homogeneously across the macroporous structure.

Figure 2. General characterization of the mp-Ni2P nanostructures. (a-c) SEM images, (d-f) TEM and HRTEM images, (g) HAADF-STEM image of mp-Ni2P; (h, i) element mapping images of Ni, and P in the area shown in (g).

The chemical composition and elemental chemical states of the mp-Ni2P was investigated by X-ray photoelectron spectroscopy (XPS). As plotted in Figure S5, the XPS peaks at 853.0 and 870.4 eV suggested the presence of Niδ+ (0

Large-Area Synthesis of a Ni2P Honeycomb Electrode for Highly

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