Free-standing ternary NiWP film for efficient water oxidation reaction

Accepted Manuscript Title: Free-standing ternary NiWP film for efficient water oxidation reaction Authors: Yunpeng Yang, Kuo Zhou, Lili Ma, Yanqin Liang, Xianjin Yang, Zhenduo Cui, Shengli Zhu, Zhaoyang Li PII: DOI: Reference:

S0169-4332(17)32972-0 https://doi.org/10.1016/j.apsusc.2017.10.049 APSUSC 37392

To appear in:

APSUSC

Received date: Revised date: Accepted date:

16-6-2017 7-9-2017 7-10-2017

Please cite this article as: Yunpeng Yang, Kuo Zhou, Lili Ma, Yanqin Liang, Xianjin Yang, Zhenduo Cui, Shengli Zhu, Zhaoyang Li, Free-standing ternary NiWP film for efficient water oxidation reaction, Applied Surface Science https://doi.org/10.1016/j.apsusc.2017.10.049 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Free-standing ternary NiWP film for efficient water oxidation reaction Yunpeng Yang

a, #

, Kuo Zhou

a, #

, Lili Ma a, Yanqin Liang

a, b

, Xianjin Yang

a, b

, Zhenduo Cui a,

Shengli Zhu a, b, Zhaoyang Li a, b a School

of Materials Science and Engineering, Tianjin University, Tianjin 300072, China Key Laboratory of Composite and Functional Materials, Tianjin 300072, China # These authors contributed equally to this work and should be considered co-first authors b Tianjin

Graphical abstract

Highlights   

Free-standing ternary NiWP films are fabricated by template method. The OER performance is optimized by controlling the element ratio. NiWP films show higher catalytic current density than commercial RuO2.

Abstract High-efficient catalysts for oxygen evolution reaction (OER) is of great concern in improving energy efficiency for water splitting. Here we report a high-performance OER electrocatalyst of nickel-tungsten-phosphorus (NiWP) film prepared by template method.

This

free-standing

ternary

electrocatalyst

*Corresponding author: [email protected] (Y. Q. Liang)

exhibits

a

remarkable

electrocatalytic activity of OER in alkaline medium due to the synergetic effect among these elements and the good electrical conductivity. The reported NiWP composite catalyst has an overpotential of as low as 0.4 V (vs. RHE) at 30 mA cm-2, better than that of the commercial RuO2 catalyst. Moreover, a small charge transfer resistance of 4.06 Ω and a Tafel slope of 68 mV dec-1 demonstrate the outstanding catalytic activity. Keywords: NiWP free-standing film; Oxygen evolution reaction; Template method; Electrocatalytic properties 1. Introduction Clean and sustainable hydrogen energy has drawn increasing attention in the past few years. Electrochemical water splitting is considered as an environmental friendly, high efficient and convenient method. [1] However, the efficiency of hydrogen evolution is dependent on the anodic half-reaction of oxygen evolution reaction (OER) as well as the cathodic half-reaction of hydrogen evolution reaction (HER). [2, 3] In principle, OER is kinetically demanding because it is 4-electron and 4-proton process. OER requires noble metal based catalysts due to its intrinsically sluggish kinetics, however, these catalysts are insufficient earth-abundance to be utilized on a global scale, such as IrO2 and RuO2 etc. [4-7] Therefore, it is highly desirable to develop substituted elements with low cost and commensurate activity compared with precious metal catalysts. Numerous earth-abundant metal catalysts have been identified as good OER catalysts in alkaline conditions. Nickel has emerged as an attracting non-precious metal electrocatalyst toward OER [8-11] due to its good corrosion resistivity, long-term stability and relatively good oxygen electrocatalytic property. [12-14] It is

because the Ni-O bonds can be readily formed through the interaction between Ni atoms with water molecules, consequently accelerating OER processes. To further enhance the OER activity, Ni element has been coupled with other non-precious metal to increase the amount of active sites. Up to date, many nickel-based materials have been reported, such as Ni-Al, Ni-Ti, Ni-Fe [15-17] or Ni alloys combined with a third element such as Fe, Co, P, V, Mo or Pd. [18-21] Among the Ni alloys, Ni-W catalysts have unique combination of good corrosion resistance and electrocatalytic behaviors. [22] A synergism between nickel and tungsten aroused by the increased adsorption of OH- ions has a positive effect on OER performance. [23] However, the conventional nickel-based electrocatalysts are usually in the form of fine powders, which usually requires polymeric binders to anchor catalysts onto electrodes. The undesirable interface caused by extra conductive additives will in turn limit active surface areas and hamper the electron transport. Besides, violent gas evolution (i.e. H2, O2) at higher potential during OER makes the coated catalysts easy to peel off from electrodes and which greatly hinders their practical application in water splitting. Recently, the free-standing films electrocatalysts exhibit superior performance for the electrochemical activity with many intrinsic advantages in comparison with their powdery counterparts. [21, 24] In the present work, we present a new strategy, i.e., a “self-supporting” approach, [6] for preparation of a tungsten&phosphorus co-decorated Ni-based alloy by the hydrothermal method. Herein, the NiWP free standing film was formed by using anodic aluminum oxide (AAO) template as sacrificial substrate in a low temperature

water bath. The effect of various element ratio (Ni:W:P) on the OER performance also investigated in detail. The free-standing NiWP film directly acted as a conductive substrate to assure the sufficient contact between electroactive species and electrolyte, which may facilitate the electronic transmissions and give rise to a prolonging life time of catalyst. 2. Results and discussion The fabrication process is demonstrated in Fig. 1. In this experiment, NiWP composites were prepared by the template method. A thin AAO sheet was used as the template and the sensitized AAO sheet was deposited in the mixed solution. First, the AAO sheet was soaked into SnCl2 solution for 60 seconds, washed by deionized water and then immersed into PdCl2 solution for 30 seconds. The purpose of this step was to sensitize the AAO sheet by introducing reduced metallic Pd so that Ni, W and P can be more easily deposited onto AAO sheet. The concentration of Pd atoms is so low that no trace of Pd was seen on the EDS spectrum. The operating mechanism of this process can be represented by the following reaction: SnCl2 + PdCl2 → SnCl4 + Pd↓

(1)

Second, the sensitized AAO sheet was placed in a mixed solution composed by NiSO4 • 6H2O, Na2WO4 • 2H2O, NaH2PO2 • H2O and sodium citrate. And then it was treated at 80 °C for 2.5 hours. The product was transferred into NaOH solution to dissolve the AAO sheet template. Involved reactions are displayed as the follows: Ni2+ +2e- → Ni,

(2)

WO2− 4 + 6e + 4H2O → W+ 8OH ,

(3)

− 2− H2 PO− 2 + 3OH → HPO3 + 2H2O + 2e ,

(4)

+ H2 PO− 2 + 2H + e → P + 2H2O,

(5)

Finally, a flexible free-standing NiWP film with a thickness of 20 μm can be easily obtained and are bearing high pH for electrochemical measurement. It is known that the catalytic performance is largely determined by the proportion of different active elements in the material. In order to get the best OER performance, a series of samples with following element ratio (Table 1) were prepared (denoted as sample N1, N2, N3, N4 and N5). As shown in Table 1, the concentration of P is fixed, the influence of Ni (N1, N2, N3) and W (N1, N4, N5) on the OER performance were investigated. Fig. 2 shows the low and high magnification SEM images, indicating that overall morphologies of the samples with different elements proportion are basically same, the entire surface of the film was composed of “crystalline grains” with varying dimensions. It is notable that the size of the grains in N2 and N4 are larger than that in other samples. The probable reason should be that the molar ratio of Ni:W is higher than a threshold of 2:5, which indicates that the grain size of NiWP film is principally dependent on the concentration of Ni. Higher content of Ni may favor larger grain dimensions. It can be seen from the high magnification image that the sample surface is made of fine mesh structures. When the reaction occurred, the cations first nucleated at each active site, and then grew up slowly until evolved into the grain-like structure and connected together. The Pd atoms, as a catalyst, could greatly reduce the reaction energy barrier to make this reaction proceed easily. [24] As the ion

concentration of the solution decreased with the processing of the reaction, the rate of reduction on the surface of the sample also slowed down, resulting in the loose construction formation in the top. When the sample was socked in the sodium hydroxide solution to remove the template, the unstable surface was dissolved preferentially, leaving the mesh structure of the sample. Fig. 2f is the image of sample N2 after 1000 cycles of LSV test. As shown in the figure, the mesh structure of sample N2 became a nanoporous structure, indicating the surface of sample was further peeled off under the continuous anodic polarization. EDS was carried out to determine the content of the sample N2 (Figure 3a) and Ni, W and P were all detected. Furthermore, the corresponding EDS mapping analyses (Figure 3b) reveal the homogeneous distribution of nickel, tungsten and phosphorus in the whole structures. XRD measurements were conducted on these samples (Figure 4a). It can be clearly observed that, except N2 film, the other samples all show the broad patterns at 45° due to the amorphous nature of the materials. The particularly sharp peak of N2 is corresponding to the typical peak of metallic nickel. The occurrence of these patterns should be attributed to the highest content of nickel in N2. In the synthesis process of sample N2, large amounts of nickel nucleate and aggregate by energy fluctuation, formed the nickel crystals during the reduction process. TEM images were presented to get more detail information in Figs. 4b and c. The incomplete crystallization of sample N2 was observed with coexistence of metallic nickel and amorphous NiWP nanostructures. The HR-TEM image shows the lattice fringe with a spacing of 0.17

nm and 0.20 nm corresponds to the (2 0 0) and (1 1 1) plane of Ni. The electron diffraction patterns (Figure 4d) can be indexed to (1 1 1), (2 0 0) and (2 2 0) planes, which are consistent with the result of XRD. To better characterize the surface morphology of sample N2, AFM images of the sample N2 before and after the 1000 cycles of LSV test were performed. AFM images in Figs. 5a, d show consistent morphology with SEM images, the surface of the sample was made up of a number of continuous protrusions corresponding to the grain-like structures. Figs. 5b, e, c and f demonstrate the increase of surface roughness as a result of the electrochemical measurement. Additionally, we use X-ray photoelectron spectra (XPS, Figure 6) to further determine the composition of the NiWP sample. The overall survey spectrum can be seen in Fig. 6a, confirming the existence of Ni 2p, W 4f and P 2p in the composite catalyst. The high-resolution Ni 2p XPS spectrum shows two intensity increases at about 862 and 857 eV, which can be assigned to either the Ni2+ or Ni3+, due to the surface of metallic Ni was partially oxidized. Two peaks of the high-resolution W 4f XPS spectrum located at 37.9 and 36 eV can be attributed to metallic W and W6+.The signal at 133 eV in high-resolution P 2p XPS spectrum is corresponding to P3+. Electrocatalytic properties of the NiWP were investigated with respect to OER in a typical three electrode configuration in 1M KOH. The obtained NiWP self-supporting sheet was directly used as a working electrode. For comparison, five samples with different elements ratio, RuO2 and Ni foam were all measured under identical conditions.

As shown in Fig. 7, the OER performance of the sample N1, N2, N3, N4 and N5 are superior to that of the nickel foam. NiWP has a lower onset potential and a higher current density in the higher overpotential region than nickel foam. This is due to the incorporation of tungsten and phosphorus in Ni matrix which improves the oxygen adsorption capacity of nickel itself, thereby enhancing the OER performance significantly. [25, 26] Moreover, a sample with the same nickel-phosphorus ratio as N2 (denoted as NiP) was synthesized to evaluate the OER activity for comparison. The current density of NiP is much lower than that of the series of NiWP films, indicating that tungsten plays an essential role in improving the catalytic activity and need further investigation. The potential to reach the current density of 30 mA cm-2 of these samples (N1, N2, N3, N4 and N5) are 1.65, 1.63, 1.66, 1.66, and 1.67 V vs. RHE, respectively, where N2 exhibits the lowest potential. The remarkable OER performance of sample N2 is attributed to the partial crystallized feature. In the amorphous matrix, the partial crystallization of Ni improves the electron transport efficiency of the catalyst, [27] while the amorphous portion can provides abundant active sites for water oxidation reaction. The synergistic effect of the amorphous and crystalline structures contributes to the optimal oxygen adsorption energy, thus giving rise to the excellent electrocatalytic performance of N2. Notably, N2 film even shows a higher OER activity than RuO2 (with the overpotential of 0.4 V and 0.42 V at 30 mA cm-2, respectively). A detailed comparison of Ni-based catalysts with various electrode configurations is shown in Table 2, which shows relatively low overpotential at a specific current density.

Cyclic voltammetry (CV) curves of NiWP were performed to further study the active area of the catalysts (Figs. 8a-e). The capacitance area of the electrode surface determines the active area of the material, and the size of the active area is positively correlated with the number of active sites, so the capacitance area of the electrode surface can reflect the size of the active area of the material. [28, 29] The plot of current density (at 1.17 V vs. RHE) against scan rate (5-200 mV s-1) has a linear relationship and the slope is equivalent to twice of the double layer capacitance. [30] As shown in Fig. 8f, the slope of sample N2 is much higher than the slopes of other samples, suggesting sample N2 has the largest number of active sites. In order to further characterize the intrinsic mechanism of the oxygen evolution of the NiWP, the Tafel measurement was carried out. A smaller slope of the Tafel curve demonstrates a lower overpotential in the catalytic process at the same kinetic current density or apparent current density. [31, 32] The slopes of the Tafel curves of the samples are in the following order: N2
samples N1, N2, N3, N4 and N5 are 12.38, 4.06, 61.69, 26.38 and 24.69 Ω, respectively. The lowest charge transfer resistance of sample N2 suggests the best kinetics performance. It can be presumed that part of the crystallization of sample N2 makes its own conductivity improved. [36, 37] Water splitting should be performed in either strongly acidic or alkaline solution to minimize the overpotential. Consequently, good corrosion resistance and stability is an essential requirement for an excellent oxygen evolution catalytic electrode. The durability of the sample N2 was analyzed by chronopotentiometry measurement, which was carried out by applying a constant anodic current density of 10 mA cm−2 (Figure 9c), and there was no significant potential increase over 10 hours, suggesting an excellent stability and corrosion resistance of the sample. Sample NiP was also measured under the same condition and showed an obvious potential increase compared with N2 film. In order to further investigate the durability of NiWP, the sample was subjected to continuous LSV scanning for 1000 cycles (Figure 9d). The sample after the LSV cycling shows lower onset potential and higher current density than before. It could be attributed to the surface dissolution effect. The unstable parts of the sample dissolved during the measurement and form a mesh surface structure, which had already been confirmed in the SEM images. This mesh structure enabled more active sites exposed, resulting in OER performance greatly improved. The elemental change caused by the surface dissolution effect was also confirmed by ICP analysis. Sample N2 had a mass fraction of Ni:W:P=87.67 Wt%:6.67 Wt%:5.66 Wt% before the LSV polarization and

Ni:W:P=86.91 Wt%:6.65 Wt%:6.44 Wt% after the LSV polarization, indicating a small amount of Ni dissolved into the solution. Moreover, a slight peak emerged at around 1.45 V, which can be ascribed to the oxidation of Ni2+ to Ni3+. This oxidation may be attributed to the exposure of Ni as some of the surface substance dissolved during cycling. 3. Conclusion In summary, the free-standing nickel-tungsten-phosphorus (NiWP) film has been developed successfully on AAO surface using a template-scarified hydrothermal method. By controlling the element ratio of Ni, W and P, we get the optimized NiWP film (Ni:W:P=87.67 Wt% : 6.67 Wt% : 5.66 Wt%, denoted as N2), which behaves as a highly efficient and durable catalytic electrode for OER in alkaline media. It was found that partially crystallized sample of N2 exhibited excellent performance, owning to its largest number of active sites and kinetic property. This work is believed to be a promising application for electrochemical preparation of hydrogen and oxygen. Taking advantages of the synergistic effect of Ni, W and P, the as-prepared NiWP catalyst (Catalyst N2) shows a satisfying OER performance due to the faster charge transfer as well as the increasing of the effective surface areas by combining metallic and non-metallic elements in one self-supported catalyst. We believe that this research will inspire new avenue to explore the use of non-precious metal alloys in the large-scale alkaline water splitting industry. 4. Experimental section Synthesis of NiWP samples

All chemicals were of research purity and used without any further treatment. In a typical synthesis process, a piece of thin AAO sheet was soaked into tin chloride solution (SnCl2, 10 g/L) for 60 seconds and then rinsed with deionized water. Next it was soaked into palladium chloride solution (PdCl2, 1 g/L) for 30 seconds. After that, the treated AAO sheet was placed in a mixed solution composed by NiSO4 • 6H2O, Na2WO4 • 2H2O, NaH2PO2 • H2O and sodium citrate (40 g/L) and then treated at 80 ℃ for 2.5 hours. According to the different element proportion of nickel, tungsten and phosphorus, the samples were denoted as sample N1, sample N2, sample N3, sample N4 and sample N5, respectively. Another treated AAO sheet was placed in a mixed solution composed by NiSO4 • 6H2O (0.075 mol/L), NaH2PO2 • H2O (0.208 mol/L) and sodium citrate (40 g/L) and then treated at 80 ℃ for 2.5 hours, denoted as sample NiP. When the above steps were finished, the AAO sheets were dissolved by NaOH solution and final products were obtained. Physical characterization The morphologies and structures of samples were characterized by scanning electron microscopy (SEM, Hitachi-S4800), transmission electron microscopy (TEM, Jem-2100f) and atomic force microscope (AFM, Keysight-AFM5500). X-ray diffraction (XRD) patterns were recorded on a D8 Advanced X-ray diffractometer. Electrochemical characterization All of the electrochemical measurements were performed in a three-electrode system on an electrochemical workstation (Gamry Reference 600). The as-prepared samples were used as self-supporting working electrodes (0.5 cm×0.5 cm) directly. A

silver chloride electrode was used as the reference electrode and a Pt wire was used as the counter electrode. The RuO2 electrode ink was prepared by 4 mg RuO2, 600 μL deionized water, 400 μL ethanol and 80 μL Nafion solution. All the electrochemical tests were performed in 1M KOH solution at room temperature. The reference was calibrated against and converted to reversible hydrogen electrode (RHE). Linear sweep voltammetry (LSV) was carried out from 0.8 to 2 V vs. RHE. Cyclic voltammetry (CV) measurement was carried out over a range of 1.13 V-1.23 V vs. RHE. Chronopotentiometry measurement was carried out at a constant anodic current density of 10 mA cm−2 for 10 hours. The Nyquist plots were measured with frequencies ranging from 100 kHz to 0.01 Hz. Tafel plots were fitted from the corresponding LSV curves. The conversion of the Ag/AgCl reference electrode to the reversible hydrogen electrode (RHE), the applied external potential and the effect of the pH of the electrolyte can be converted into the potential vs. RHE: E(RHE) = E( Ag/AgCl) + EAgCl +0.059 ×pH, where EAgCl = 0.197 V at 25 °C. Acknowledgments This work is Financial supported by the National Natural Science Foundation of China (51402211) and Natural Science Foundation of Tianjin (15JCQNJC03600). References [1] A. Pendashteh，J. Palma，M. Anderson，R. Marcilla, NiCoMnO4 Nanoparticles On N-doped Graphene: Highly Efficient Bifunctional Electrocatalyst for Oxygen Reduction/Evolution Reactions，Appl. Catal. B- Environ. 201 (2017) 241–252 [2] Y. Feng, X.Y. Yu, U. Paik, N-doped graphene layers encapsulated NiFe alloy

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Figure captions Fig. 1. Schematic illustration of the synthesizing procedures of the NiWP catalyst. Fig. 2. a-e) SEM images of samples N1, N2, N3, N4 and N5 f) SEM images of sample N2 after 1000 cycles of LSV durability test. Fig. 3. a) EDS patterns of sample N2 b) EDS elemental mapping images for sample

N2. Fig. 4. a) XRD patterns of sample N1, N2, N3, N4 and N5 b) Low-resolution TEM image of sample N2 c) High-resolution TEM image of sample N2 d) Selected area electron diffraction image of sample N2. Fig. 5. a) AFM topography, b) 3D AFM image and c) SGM image of sample N2 before 1000 cycles LSV durability test. d) AFM topography, e) 3D AFM image and f) SGM image after 1000 cycles LSV durability test. Fig. 6. a) XPS spectrum of sample N2. High resolution spectra of b) Ni 2p, c) W 4f and d) P 2p. Fig. 7. Experimental linear sweep voltammetry (LSV) plots of sample N1, N2, N3, N4, N5, NiP, RuO2 and Ni foam in 1M KOH solution. Fig. 8. a-e) Cyclic voltammetry (CVs) curves of sample N1, N2, N3, N4 and N5 measured at different scan rates from 5 to 200 mV s-1 in 1M KOH solution. f) Current density at 1.17 V (vs. RHE) plotted versus scan rate. Fig. 9. a) Tafel plots of sample N1, N2, N3, N4 and N5. b) EIS plots of sample N1, N2, N3, N4 and N5. The inset of (b) shows the corresponding equivalent circuits diagram. c) Chronopotentiometry plots of sample N2 and sample NiP with a current density of 10 mA cm-2 for 10 hours. d) LSV plots for sample N2 before and after 1000 cycles of LSV durability test.

Fig. 1. Schematic illustration of the synthesizing procedures of the NiWP catalyst.

Fig. 2. a-e) SEM images of samples N1, N2, N3, N4 and N5 f) SEM images of sample N2 after 1000 cycles of LSV durability test.

Fig. 3. a) EDS patterns of sample N2 b) EDS elemental mapping images for sample N2.

Fig. 4. a) XRD patterns of sample N1, N2, N3, N4 and N5

b) Low-resolution TEM

image of sample N2 c) High-resolution TEM image of sample N2 d) Selected area electron diffraction image of sample N2.

Fig. 5. a) AFM topography, b) 3D AFM image and c) SGM image of sample N2 before 1000 cycles LSV durability test. d) AFM topography, e) 3D AFM image and f) SGM image after 1000 cycles LSV durability test.

Fig. 6. a) XPS spectrum of sample N2. High resolution spectra of b) Ni 2p, c) W 4f and d) P 2p.

Fig. 7. Experimental linear sweep voltammetry (LSV) plots of sample N1, N2, N3, N4, N5, NiP, RuO2 and Ni foam in 1M KOH solution.

Fig. 8. a-e) Cyclic voltammetry (CVs) curves of sample N1, N2, N3, N4 and N5 measured at different scan rates from 5 to 200 mV s-1 in 1M KOH solution. f) Current density at 1.17 V (vs. RHE) plotted versus scan rate.

Fig. 9. a) Tafel plots of sample N1, N2, N3, N4 and N5. b) EIS plots of sample N1, N2, N3, N4 and N5. The inset of (b) shows the corresponding equivalent circuits diagram. c) Chronopotentiometry plots of sample N2 and sample NiP with a current density of 10 mA cm-2 for 10 hours. d) LSV plots for sample N2 before and after 1000 cycles of LSV durability test.

Table captions Table 1. The composition of precusor in different samples (mol/L). Table 2. Comparison of different Ni-based catalysts for OER with recently reported work.

Table 1. The composition of precusor in different samples (mol/L). NiSO4•6H2O Na2WO4•2H2O NaH2PO2•H2O

N1

N2

N3

N4

N5

0.045 0.156 0.208

0.075 0.156 0.208

0.027 0.156 0.208

0.045 0.069 0.208

0.045 0.226 0.208

Table 2. Comparison of different Ni-based catalysts for OER with recently reported work.

Catalyst

Electrolyte

Potential @ 10 mA

Reference

cm-2 (V vs. RHE) NiWP

1M KOH

1.57

This Work J. Mater. Eng. Perform.

Ni-W

1M KOH

1.74 2015, 24, 4182-4191 ChemNanoMat.

Ni-P/NF

1M KOH

1.54 2016, 1, 558-561

NiO

1M KOH

1.68

RSC Adv. 2015, 5, 86713–86722

Ni2P/Ni/NF

1M KOH

1.43

ACS. Catal. 2016, 6, 714−721