Understanding Structure-Dependent Catalytic Performance of Nickel

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Understanding Structure-Dependent Catalytic Performance of Nickel Selenides for Electrochemical Water Oxidation Kun Xu, Hui Ding, Haifeng Lv, Shi Tao, Pengzuo Chen, Xiaojun Wu, Wangsheng Chu, Changzheng Wu, and Yi Xie ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02884 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on November 29, 2016

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ACS Catalysis

Understanding Structure-Dependent Catalytic Performance of Nickel Selenides for Electrochemical Water Oxidation Kun Xu1§†, Hui Ding1§†, Haifeng Lv2, Shi Tao3, Pengzuo Chen1, Xiaojun Wu 2, Wangsheng Chu3, Changzheng Wu1*, and Yi Xie1 1.

2. 3.

Hefei National Laboratory for Physical Sciences at the Microscale, and iChEM (Collaborative Innovation Centre of Chemistry for Energy Materials), CAS Key Laboratory of Mechanical Behavior and Desigh of Materials, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China CAS Key Laboratory of Materials for Energy Conversion and Department of Material Science and Engineering University of Science and Technology of China National Synchrotron Radiation Laboratory University of Science & Technology of China, Hefei, Anhui 230029, P. R. China.

KEYWORDS: water oxidation catalysis, nickel based materials, non-oxide precatalysts, conductivity, surface oxidation ABSTRACT: In this study, we systematically explore the connection between electrical conductivity and catalytic activity of OER catalyst, and disclose the association between the structure of non-oxide based catalyst and corresponding OER activity, using a category of Ni-based material as a model of system, i.e. the serial Ni-based compounds (NiO, NiSe, Ni3Se2 and Ni) with a wide range of continuously adjustable band gap ranging from insulator to metallic state. The X-ray Photoelectron Spectroscopy (XPS) and High Resolution Transmission Electron Microscopy (HRTEM) revealed that structural rearrangement occurs (forming electro-catalytic active species) on the surface of these catalysts during electrochemical water oxidation. Extended X- ray Absorption Fine Structure (EXAFS) curve fitting suggested the trend of surface oxidation facility for these investigated catalysts. Benefiting from the synergetic effect of intrinsic metallic state and more facile surface reorganization enabled by anion incorporated in metal matrix, Ni3Se2 taken on the higher catalytic activity for electrochemical water oxidation compared with NiO, NiSe and Ni. Our work suggests that both electrical transport and active species forming on the surface of pre-catalysts which are highly correlated with the structure of pre-catalysts are critical factors to determine the OER performance.

Introduction The efficient storage of solar energy, wind power and other sorts of renewable energy is an important and challenging issue.1 Water splitting to produce H2 and O2 shows great promise towards the storage of renewable energy and present , a potential alternative to traditional fossil fuels.2 3 However, the efficiency of electrochemical water splitting was, to a large extent, limited by the sluggish oxygen evolution reaction (OER) which involves multi proton-coupled electron , transfer.4 5 High performance OER catalysts are thus required to reduce the overpotential that applied to drive the sluggish oxygen evolution reaction. At present, the state of the art OER catalysts are Ir, Ru and their oxides which are expensive , and rare, making it impractical for large scale application.6 7 Therefore, extensive efforts have been devoted to develop highly active, cost-effective and earth abundant transition metal oxides based electrocatalysts.8 Recently, various economical metal non-oxide based com, pounds including metal chalcogenides, 9 10 nitrides, 11 13 14-17 18 phosphides and carbides have also been well studied and show great potential as catalysts for OER. For instance, Ni3N nanosheets have exhibited a decent catalytic performance for OER.12 Moreover, Ni2P nanoparticles which often

used as hydrogen evolution catalyst have also been reported to exhibit high OER activity.17 In fact, for most of metal nonoxide compounds, it was proved that surface reorganization would take place during OER process, forming an oxide or , hydroxide layer covered on the bulk phase.11 16 21 In this sense, due to the exposure to electrolyte, the in-situ formed oxide or hydroxide/metal non-oxide core-shell heterostructure is considered as actual effective species for OER process. Metal non-oxide compounds with higher conductivity could accelerate the electron transport between electrode and metal oxide or hydroxide shell, which is beneficial for OER process, and thus it is reasonable for OER activity enhancement compared with their corresponding insula- , tor/semiconductor oxide counterparts.10 12 18 Therefore, it arouse us an open question whether higher electrical conductivity of catalysts lead to higher catalytic activity for OER. Besides, despite substantial progress has been made for better design of advance catalysts, disclosing the association between the structure of non-oxide based catalyst and OER activity is still highly desirable but remains a big challenge. As well known, the electronegativity of elemental selenium (2.55) is smaller than that of elemental oxygen (3.44). Based on above basic knowledge, it is expectable that the electrical conductivity of nickel selenides would be higher than that of

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nickel oxide phase. Moreover, serial nickel selenides compounds with a wide range of continuously adjustable electronic behavior could be realized by changing Se content in Ni framework. The DOS results, calculated according to the corresponding crystalline structure information of the four catalysts which will be discussed later, are in consistence with the above analysis results. As shown in Figure 1a and b, NiSe is found to have a smaller band gap compared with NiO which was evidenced by the calculated density of states (DOS). Furthermore, decreasing Se content in Ni framework, the nickel selenide compound (Ni3Se2) could become metallic state (Figure 1c). The DOS of Ni near the Fermi level shown in Figure 1d is more intense than that of Ni3Se2, suggesting the higher electrical conductivity of Ni compared with Ni3Se2 resulting from further decreasing the Se content in Ni framework. In this regard, the nickel selenide compounds together with nickel oxide and pure Ni could provide an ideal platform to investigate the connection between the structures of non-oxide based catalyst and OER activity. Herein, we for the first time study the in-depth physicochemical correlation between the conductivity of non-oxide precatalysts and their OER catalytic capabilities, using four Ni-based materials (NiO, NiSe, Ni3Se2 and Ni) with different electrical behavior. By comparing the catalytic performance of four different Ni-based materials, we experimentally demonstrated that Ni3Se2 displays the more decent OER catalysis. Furthermore, we propose that Ni3Se2 taken on the higher catalytic activity for electrochemical water oxidation compared with NiO, NiSe and Ni arising from the synergetic effect of intrinsic metallic state and more facile surface reorganization enabled by anion incorporated in metal matrix, which was evidenced by XAFS, XPS and HRTEM results. Our work may provide new insights to better understand the association between the conductivity of pre-catalyst and OER activity. a

b

c

d

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ture was stirred for 1 h. Afterwards, 6.5 ml 50%wt NaOH solution was poured into above mixture, the pink solution finally turn to black within several minutes.22 Preparation of Ni3Se2. Ni3Se2 was prepared with a modified method based on literature. In a typical procedure, 1.6 mmol NiSO4·7H2O was dissolved in 24 ml deionized water, then put into a 50 ml Teflon-lined autoclave. 2.4 mmol EDTA was + added into the NiSO4 solution to chelate with Ni2 . After EDTA dissolved, 1 g NaOH was added into the above solution and keep stirring for 5 min. 0.8 mmol Na2SeO3 was then added to the solution. After forming homogeneous solution, 6 ml 85% hydrazine hydrate was poured into the mixture. The mixture kept stirring for 10 min, and then the Teflonlined autoclave autoclave was carefully sealed and keep at 180 °C for 7 h. The product was centrifuged, washed with deionized water and ethanol, and keep at 60 °C in a vaccum drier overnight.23 Preparation of NiSe. In a typical procedure, 0.5 mmol NiSO4·6H2O and 0.5 mmol Na2SeO3 were added into 20 mL glycol and keep stirring for 30 min. After these reagents dissolved, the mixture was put into a 50 ml autoclave. After carefully sealed, the autoclave was keep at 180°C for 24 h.24 Preparation of NiO. The NiO nanoparticles were prepared according to previous literature. In a typical procedure, 0.87 g Ni (NO3)2·6H2O was dissolved in 6 ml H2O formed solution A. Then 0.3 g NaOH and 0.1 g PVP were added into 15 ml H2O under magnetically stirred to form solution B. After that, solution A was added dropwise into solution B. The precursor was collected by centrifuge, washed with water and ethanol for several times, and dried under vacuum overnight. The final NiO nanoparticles were obtained by calcined the precursor powder at 600°C for 2 h in air.25 Materials Characterization. X-ray powder diffraction (XRD) patterns were recorded on a Philips X’Pert Pro Super diffractometer with Cu Kα radiation (λ=1.54178 Å). The nitrogen adsorption-desorption isotherms were performed using a Micromeritics ASAP 2000 system at 77 K. X-ray photoelectron spectra (XPS) measurements were obtained from an ESCALAB MK II X-ray photoelectron spectrometer with Mg Kα as the excitation source. The field emission scanning electron microscopy (FE-SEM) images were taken on a JEOL JSM-6700F SEM. The high-resolution TEM (HR-TEM) were performed on a JEM-2100F field emission electron microscope at an acceleration voltage of 200 KV. The absorption spectra of Ni K-edge were collected in transmission mode using a Si (111) double-crystal monochromator at the X-ray absorption fine structure (XAFS) station of the 1W1B beamline of the Beijing Synchrotron Radiation Facility (BSRF, Beijing) and the beamline 14W1 of the Shanghai Synchrotron Radiation Laboratory (SSRL, Shanghai) at room temperature.

Figure 1. Density of states diagrams of (a) NiO, (b) NiSe, (c) Ni3Se2 and (d) Ni.

EXPERIMENTAL SECTION Preparation of Ni. 10 ml 1.585 M NiCl2 solution was dropwise added into 12 ml N2H4 at 60°C, and the resulting mix-

Electrochemical Measurement. To prepare working electrodes, 3 mg catalyst powder and 1 ml mixture solution of isopropyl alcohol and water (volume ratio 1:3) were mixed with 40 µl 5%wt Nafion solution. The mixture was ultrasonicated for 30 minutes to form a homogenous ink. Afterwards, 5 µl suspension was transferred onto the polished 3 mm glass carbon electrode, forming mass loading of 0.2 mg/cm2. Electrochemical measurements were conducted in 0.1 M KOH

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electrolyte (purged with O2 for 30 min before test) at room temperature using a three electrode system on CHI 760 Work Station. All potentials were measured versus Ag/AgCl reference electrode (3.3 mol/L KCl), and Pt foil was used as counter electrode. Several cyclic voltammetry cycles were taken from 0-0.8 V vs Ag/AgCl to stabilize the OER performance of catalyst before polarization curves were recorded. The post catalysis samples were collected after 1000 CV cycles from 0-0.8 V vs Ag/AgCl at 50 mV/s. Note: recently, some literatures showed that trace amount of iron incorporation could alter the electronic structure of NiOx and thus improving the OER catalytic performance.26-28 In our case, the presence of a trace amount of Fe in KOH solution may also enhanced the catalytic performance of the Ni-based material investigated herein. Therefore, the Ni(Fe)Ox/M complex species may be the actual active species during electrochemical water oxidation. Calculation method. During calculations, the structure and cell optimizations of pure Nickel and Nickel Selenides are carried out within the Perdew-Burke-Ernzerhof generalized gradient approximation (GGA) 29 implemented in the Vienna Ab initio Simulation Package (VASP).30 The projector augmented wave (PAW) potential 31 and the plane wave cutoff energy of 350 eV are employed. Both the lattice constants and positions of all atoms are relaxed until the force is less than 0.01 eV/Å. The criterion for the total energy is set as 1 × − 10 5 eV. During geometric optimization, the Brillouin zone are sampled with 7 × 7 × 7 mesh, which is dense enough for pure Nickel and all the three kinds of Nickel Selenides. To count the electron correlation effects of d orbitals and obtain accurate electronic and magnetic properties, we further ap, ply the screened hybrid HSE06 functional, 32 33 which includes the accurate Fock exchange and usually performs – much better than the GGA and GGA+U methods.34 36 For Ni3Se2, scissor operation is applied based on the experimental value, which is proved to be effective for a variety of systems.37 40 Spin-orbital coupling (SOC) effect is beyond consideration since it does little influence to the density of states. Though the GW approximation (GWA) of Hedin 41 is generally believed to accurately predict electronic structure, it is unnecessary based on results of our calculation since the hybrid functional bandgap is consistent with experimental data.

RESULTS AND DISCUSSION

a

b

c

d

e

Figure 2. Crystal structure characterizations. (a) XRD patterns of Ni, Ni3Se2, NiSe and NiO. HRTEM images of (b) Ni, (c) Ni3Se2, (d) NiSe and (e) NiO. Ni, Ni3Se2, NiSe, and NiO nanoparticles were all prepared according to previous literatures, as described in experimental section. The crystal structures of these materials were firstly characterized by X-ray powder diffraction (XRD). As shown in Figure 2a, results from XRD patterns suggested the high purity and crystallinity of all these Ni-based compounds (see Figure S1; Ni, JCPDS Card No. 04-0850 Space group Fm3m (225); Ni3Se2, JCPDS Card No. 19-0841, Space group R32 (155); NiSe, JCPDS Card No. 02-0892, Space group P63/mmc (194); NiO, JCPDS 44-1159, Space group R-3m (166)). HRTEM further confirm the crystal phase of four catalysts, as shown in Figure 2b-e. Furthermore, scanning electron microscopy (SEM) highlighted the particle morphology of all the materials (Figure S2). To investigate their geometrical surface area, BET measurements were conducted. The value of the specific BET surface area of all samples are in the same level of magnitude and are 10.47 m2/g, 4.39 m2/g, 6.09 m2/g, 9.22 m2/g for Ni, Ni3Se2, NiSe and NiO (Figure S3), respectively. In comparison to other well studied high surface area nickel materials such as Ni3N nanosheets, 12 NiSe nanowires , array42 and Nickel based 3D Porous Hierarchical structure43 44, 45 , the four different catalysts in this research possess similar nanoparticle morphology and same level BET surface area, excluding the impact arisen from great disparity of surface area. And thus the four Ni-based materials with different electronic structures, similar morphology and geometrical surface area provided the opportunity to study the relationship between crystal structure and OER activity. To explore the OER catalytic capabilities of four Ni-based materials with different electrical behavior, cyclic voltammetry (CV) measurements were conducted in 0.1 M KOH electrolyte with a three electrode system. Before recorded CV, 50 cyclic voltammetry cycles were taken to stabilize performance of catalysts. To make a fair comparison, CV curves using all the catalysts which have been normalized by the BET surface area as exhibited in Figure 3a. A pair of redox peaks for all the Ni-based materials locate at 1.45 V vs RHE are observed, which can be assigned to the redox couple of Ni(II)/Ni(III). Specifically, Ni3Se2 exhibited a current density of 1.32 mA/cm2 at 1.70 V vs RHE, which is significantly larger

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than those of Ni (0.25 mA/cm2), NiSe (0.40 mA/cm2), and NiO (0.06 mA/cm2) under identical conditions (Table S1). Additionally, the Tafel slope

a

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component of electrocatalysts, which is favorable for OER , , process.11 12 18 In this regard, the trend of catalytic capabilities order Ni3Se2 > NiSe > NiO in our experiments may stem from their electronic structure disparity. However, the catalytic activity of Ni3Se2 is higher than that of Ni, although DFT results suggest that the electrical conductivity of Ni is higher than that of Ni3Se2. This seeming anomaly result from the diverse facility of surface oxidation between Ni and Ni3Se2. Unlike the case of the pure metal Ni, the incorporation of Se atoms into the Ni-based framework will cause a certain degree of atomic displacement and alteration of coordination numbers and bond length. And thus from surface oxidation reaction viewpoint, lower coordination numbers and longer bond length would weaken the corresponding metal Ni-Ni bonds strength in nickel selenides to facilitate subsequent partial surface oxidation (forming the necessary OER catalytic active species) during water oxidation process. Consequently, it is more facile for Ni3Se2 to generate electrocatalytic active species on the surface during electrolysis in comparison to Ni, thereby inducing higher OER capability. Thus, we believed that Ni3Se2 exhibit the higher catalytic activity for electrochemical water oxidation compared with NiO, NiSe and Ni, arising from the synergetic effect of intrinsic metallic state and more facile surface reorganization by the anion incorporated in metal matrix.

a

b

Figure 3. Electrochemical characterizations. (a) The normalized CV curves of Ni, Ni3Se2, NiSe and NiO by the BET surface area of electrocatalyst in 0.1 M KOH electrolyte. (b) Corresponding Tafel plots of Ni, Ni3Se2, NiSe, and NiO. Note: the Tafel slopes were derived from cathodic sweep to avoid the interference of oxidation peak in Nibased materials. of Ni3Se2 (46 mV/dec) was smaller than that of Ni (71 mV/dec), NiSe (72 mV/dec) and NiO (136 mV/dec), further suggesting its more rapid OER reaction kinetics. Together with CV results and Tafel slope data, we can safely draw the conclusion that Ni3Se2 was the most efficient OER electrocatalyst among the Ni-based materials investigated herein. The higher OER activity of Ni3Se2 compared to other Nibased materials and the OER capability trend investigated herein is intriguing, and some key points about the structure of Ni3Se2 may ascribe to it. The OER catalytic investigations on Co4N nanowires and Ni3C nanoparticles revealed that Co4N/CoOx and Ni3C/NiOx are actual electro-catalytic active , species, correspondingly. 11 18 These works suggest that surface oxidation is imperative to form an active structure for non-oxide transition metal compounds. Therefore, in this research, we assume the core-shell structure of NiOx/M (M denoted as Ni-based materials investigated herein) complex species may be actual active phase for OER. On the other hand, previous studies have demonstrated that the metallic core could ensure the rapid electron flowing inside the bulk

Figure 4. (a) X ray photoelectron spectroscopy (XPS) of Ni, Ni3Se2, NiSe, NiO and corresponding after 1000 CV cycles sample. (b) Ni K-edge X ray absorption near edge structure (XANES) of Ni, Ni3Se2, NiSe, NiO and corresponding after 1000 CV cycles sample. To verify our proposed assumption of conductivity and surface oxidation, X-ray Photoelectron Spectroscopy (XPS) and ex-situ X-ray Absorption Fine Spectroscopy (XAFS) characterizations were carried out. It is well known that XPS is one of the most powerful tools to probe the surface composition for materials. Figure 4a shows the XPS results of the raw catalysts and corresponding post-catalysis catalysts, clearly demonstrating the surface reorganization (surface oxidation) occurs during the water oxidation. Taking the example of Ni3Se2, the peaks locate at 852.4 eV and 869.5 eV corresponding to Ni 2p3/2 and Ni 2p1/2 peaks of Ni3Se2 (simi, lar to Ni3S246 47). Meanwhile, peaks at 855.4 eV and 873.1 eV could be attributed to Ni 2p3/2 and Ni 2p1/2 peaks in NiO or , Ni(OH)2 arisen from air exposure.46 48 Notably, the characteristic peaks of post-catalysis Ni3Se2 decreased sharply while

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relative intensity of nickel oxide or hydroxide is strengthened, suggesting the existence of strong surface oxidation after catalysis. Therefore, based on XPS analysis results, it is confirmed that NiOx/M hetero-structure complex species are actually active phase for water oxidation in this work. As shown in Figure 4b, the X-ray Absorption Near-edge Spectroscopy (XANES) spectrums of the four different Ni-based materials before and after catalytic reaction show no significant change, which illustrates that no obviously bulk phase transformation occurred. Accordingly, long-term OER catalysis would not lead to electronic structure alteration of the bulk phase in our experiments. Hence, the discussions on the electronic structure are still valid for post catalysis materials. We then fit the Extended X-ray Absorption Fine Structure (EXAFS) data of pristine catalysts in the r space (Figure S6), obtained corresponding parameters about crystal structure (Table 1). The results indicate the distinct difference on crystal structure among the four nickel based catalysts. Compared with Ni3Se2 and NiSe, Ni show higher coordination numbers and shorter Ni-Ni bond length. For metal non oxide catalysts, oxygen atom incorporation into the crystal lattice will be more difficult when it comes to crystal compounds with relatively higher coordination numbers and shorter bond length. Thus, surface oxidation facility (forming electro-catalytic active species) follow the order NiSe ~ Ni3Se2 > Ni.

the surface of Ni-based materials. To evaluate the electrochemical active area of these catalysts, double layer capacitance (Cdl) characterization was conducted (Figure S5). The double layer capacitance of these post catalysis Ni-based materials was estimated to use cyclic voltammetry at several different scan rates. The Cdl of the Ni, Ni3Se2, NiSe and NiO were 13.9 µF, 22.1 µF, 19.1 µF and 32.2 µF, respectively. The analysis of Cdl is consistent with the consequence of HRTEM characterization and confirm the NiOx was the eventual active species. Generally, the exposure active sites and conductivity are the two critical factors to determine the electrocatalytic performance of materials. Of note, the Ni3Se2 presents the best OER catalytic in our investigated system due to the balance of active sites exposure and conductivity.

a

b

c

d

Table 1 Curve-Fitting EXAFS Data for Ni, NiO, Ni3Se2, NiSe 2

-3

2

Figure 5. HRTEM images of post catalysis samples of (a) Ni, (b) Ni3Se2, (c) NiSe, and (d) NiO.

sample

bond

N

R (Å)

σ (×10 Å )

Ni

Ni-Ni

11.0 ± 1.1

2.49 ± 0.02

6.4 ± 0.6

Conclusion

Ni-O

6.0 ± 0.6

2.09 ± 0.02

5.1 ± 0.5

Ni-Ni

12.0 ± 1.5

2.95 ± 0.02

6.3 ± 0.6

Ni-Se

6.0 ± 0.6

2.47 ± 0.02

9.2 ± 1.0

Ni-Ni

2.0 ± 0.3

2.67 ± 0.02

6.2 ± 0.6

Ni-Se

4.0 ± 0.4

2.37 ± 0.02

8.2 ± 0.8

Ni-Ni

2.9 ± 0.3

2.64 ± 0.03

10.7 ± 1.2

In conclusion, we have demonstrated both bulk conductivity and facility of active species formation on the surface of pre-catalysts are critical factors for trigger electrochemical water oxidation. Our work disclosed correlation between conductivity of non-oxide pre-catalysts and their OER catalytic performance. Taking Ni-based material (NiO, NiSe, Ni3Se2 and Ni) as a model, OER catalytic activity of these materials with different structure has been investigated in detail. In-depth characterizations revealed the existence of surface reorganization for all the investigated catalysts during OER catalysis. Due to the synergetic advantages of metallic character and facile surface reorganization, Ni3Se2 exhibits optimal OER catalytic activity among the investigated Nibased materials, while the others with poor conductivity or having difficulty over the formation of active species are significantly inferior to that of Ni3Se2. Our work may supply a guidance to design highly efficient catalysts.

NiO

NiSe

Ni3Se2

To have a deeper insight into the OER catalytic mechanism on these materials, High Resolution Transmission Electron Microscope (HRTEM) measurements were also conducted. Obviously, as exhibited in Figure 5, amorphous oxide layer formed on the surface of all those Ni-based materials after catalysis, also confirming the existence of surface oxidation. More interestingly, the thickness of in-situ formation oxide layer of the four catalysts varied from each other. The amorphous oxide layer thickness of post-catalysis NiO, NiSe and Ni3Se2 were 7-8 nm, 4-6 nm and 4-5 nm respectively, and 1-3 nm for Ni, indicating the surface oxidation was relatively facile for NiSe and Ni3Se2 in comparison to that of Ni. This result is consistent with the analysis of coordination number and bond length from EXAFS data. Also, the electrochemical active area is related to the thickness of amorphous layer on

ASSOCIATED CONTENT Supporting Information Supporting Information Available: Additional structural and morphology characterization S1, Electrochemical characterization S2. X-ray adsorption fine structure S3. This material is available free of charge via ACS Publication website at http://pubs.acs.org

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AUTHOR INFORMATION Corresponding Author * C. Z. Wu (E-mail: [email protected])

Author Contributions §†

These authors contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was financially supported by the National Basic Research Program of China (2015CB932302), the National Natural Science Foundation of China (U1432133, 21331005, 21601172, U1532265, J1030412), National Program for Support of Top-notch Young Professionals, the Chinese Academy of Sciences (XDB01020300), the China Postdoctoral Science Foundation (2015M580539, 2016T90571) and the Fundamental Research Funds for the Central Universities (WK2060190027, WK2060190061).

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