Co0.85Se heterostructure catalysts for efficient oxygen evolution

Journal of Alloys and Compounds 825 (2020) 154073

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Constructing FeCoSe2/Co0.85Se heterostructure catalysts for efficient oxygen evolution Ke Zhang *, Menglin Shi , Yu Wu , Chuanyi Wang ** College of Environmental Science and Engineering, Shaanxi University of Science and Technology, Xi’an, 710021, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 November 2019 Received in revised form 20 January 2020 Accepted 26 January 2020 Available online 1 February 2020

The slow kinetics and high cost of noble metal catalysts greatly hinder the oxygen evolution reaction (OER) in the application of large-scale oxygen production, as well as water splitting. In order to overcome this burgeoning issue, worldwide research efforts have been carried out to find alternative, and efficient electrocatalysts. Due to its cheap and abundant sources, FeCo-based materials are considered as better candidate electrocatalysts for OER. Herein, a nanocomposite catalyst consisting of FeCoSe2 nanoparticles and Co0.85Se nanosheets was synthesized via a one-pot hydrothermal method. The resultant FeCoSe2/ Co0.85Se heterostructure catalysts can further used as a self-supported catalytic electrode to drive the OER, which exhibits the overpotential of 0.33 V at a current density of 10 mA cm
2 and Tafel slope of 50.8 mV dec
1. Raman spectroscopy was employed to explore the intrinsic active species of as-prepared hybrid catalyst during the OER process, and test result shows that metal selenides was acting as the precursor of the real reaction species. This work provides a possibility to develop cheap and effective OER electrocatalysts to replace the costly noble metal catalysts for OER in electrochemical devices. © 2020 Elsevier B.V. All rights reserved.

Keywords: Co0.85Se nanosheet FeCoSe2 nanoparticles OER Real active species

1. Introduction Oxygen evolution reaction (OER) is a crucial half-reaction in water splitting, but the complicated four-electron oxidation process which involves HeO bond cleavage and OeO bond formation at anode requires high overpotential to overcome the reaction barrier [1e3]. Efficient electrocatalyst development is critically important to expediting the OER process and thus significantly improving the overall water splitting. Currently, noble metal-based materials (such as Ru and Ir) are often used as the OER catalysts in commercial water electrolyzers, but the high price and scarcity of the noble metal materials like Ru and Ir seriously impede their practical applications [4]. In order to resolve this issue, tremendous efforts have been devoted to design and synthesize high performance and cheap alternative electrocatalysts. Up to now, the low-cost and earth-abundant materials, such as transition-metal hydroxides [5e8], selenides [9,10] and nitrides [11,12] have been widely studied for OER. As compared with hydroxides and oxides, selenides with much more active sites and better conductivity may have a better

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Zhang), [email protected] (C. Wang). https://doi.org/10.1016/j.jallcom.2020.154073 0925-8388/© 2020 Elsevier B.V. All rights reserved.

prospect of application [13]. Co0.85Se (CS) is one of transition metals-based selenides, and possesses better electrochemical property for its 2D structure. Many previous reports pointed out that CS nanosheet could afford significant OER performance in alkaline solutions [14]. Doping additional transition-metal is an effective method to improve the electrocatalyst performance. Recent investigations indicate that Co-based electrocatalysts doped with Fe ions can exhibit better catalytic properties, although the detailed mechanism, especially involved catalytic active sites, is still unclear [15e17]. Wang et al. synthesized FeSe/CoSe nanocomposites and observed the synergistic effect between Fe and Co in OER. They concluded that higher oxidation states of Co3þ and Fe3þ are the active sites in catalytic process [18]. Kim et al. systematically studied the role of Fe in perovskite catalysts with respect to OER performance. The activity and stability of catalyst could be improved by introducing only 5% Fe, but the oxidation state of Fe in catalyst does not undergo obvious change [19]. Actually, in some cases, the structure or phase of catalysts will reconstruct or transform during the OER processes. For example, Trzesniewski et al. investigated the active oxygen species of Ni and Fe-based catalysts by using the in-situ characterization methods. The Ni(OH)2 is activated by the formation of oxyhydroxide species in alkaline conditions [20]. It’s important to characterize and

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understand the surface of catalysts thus to ascertain the reactive species, providing theoretical and factual basis to design and synthesize high-performance catalysts. Currently, in-situ technology has received a lot of attention, which can obtain the intermediate information of catalysts during reaction process. To further improve the OER activity and understand the real reaction species, we synthesized a heterogeneous bimetallic catalyst consisting of FeCoSe2 (FCS) nanoparticles and layered CS nanosheets via a one-step hydrothermal route. FCS nanoparticles are dispersed on the CS nanosheets to form FeCoSe2/Co0.85Se (FCS/ CS) catalyst. By manipulating iron content, the electrocatalyst with 5% iron doped exhibits the best OER performance. Compared with CS nanosheets, all the Fe doped catalysts possess better catalytic property, indicating a strong synergistic effect between Co and Fe. Raman spectroscopy was also employed to explore the real reaction species during the OER processes. 2. Experimental section 2.1. Chemicals and materials Cobalt acetate [Co(CH3COO)2$4H2O] and sodium selenite (Na2SeO3) were purchased from Sinopharm Chemical Reagent Co.,Ltd. Diethylenetriamine (DETA) was acquired from Shanghai Macklin Biochemical Co., Ltd. Ferric nitrate [Fe(NO3)3$9H2O] was acquired from Tianjin Beilian Fine Chemicals Development Co., Ltd. Nafion (5 wt %) was bought from Alfa Aesar (china) chemical Co., Ltd. RuO2 was bought from Shanghai Aladdin Bio-Chem Technology Co., LTD. All chemical reagents were used as received without any further purification. 2.2. Synthesis of FCS/CS composite In a typical procedure, stoichiometric Fe(NO3)3$9H2O and 1 mmol Co(CH3COO)2$4H2O were dissolved in 13.3 mL of deionized water under ultrasonic about 30 min, then 1 mmol of Na2SeO3 and 26.7 mL of DETA added to the solution, respectively. The reaction solution was transferred into a 50 mL Teflon-lined autoclave at 180  C for 16 h. The power was centrifugation and washed with H2O repeatedly. For comparison, other catalysts were synthesized via the same method by just tuning the quantity of metal precursors. 2.3. Characterization X-ray powder diffraction (XRD) was carried out on a Bruker D8 Advance diffractometer with Cu Ka radiation (l ¼ 0.15418 nm). Scanning electron microscope (SEM) images were employed on ZEISS Sigma 500. Transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) were taken with FEI Tecnai G2 f20 s-twin with an acceleration voltage of 200 kV. X-ray photoelectronic spectra (XPS) were recorded on a Shimadzu-Amics instrument with aluminum Ka radiation. The actual Fe/Co molar ratio of 5% FCS/CS was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES). The Raman spectra of the materials were carried out on a LabRAM HR Evolution spectrometer with 532 nm laser excitation. 2.4. Electrochemical characterizations The electrochemical measurements were carried out in a CHI 660e electrochemistry workstation (Shanghai Chenhua Instruments Co., Ltd.) with a standard three-electrode cell at room temperature. A glassy carbon electrode (GCE, diameter: 3 mm), Pt foil and saturated calomel electrode (SCE) were used as working electrode, counter electrode and reference electrode, respectively.

In this work, all the potentials are converted to the correlation values with respect to the reversible hydrogen electrode. The catalysts contain deionized water, 1% Nafion and catalysts power were evenly added onto the top of GCE for OER test. For comparing and analyzing the activity of catalyst, commercial RuO2 was also tested, and the loading amount of RuO2 is 20 mg. Linear sweep voltammetry (LSV) was employed with 95% iR compensation to investigate OER activity of as-prepared samples in 1.0 M KOH solution at 5 mV s
1. Chronopotentiometric (CP) measurement was carried out to evaluate the catalysts durability. Turnover frequency (TOF) Calculation. TOF value was calculated according to the followed equation [18]:

TOF ¼

jS 4F n

where j is the current density at 95% iR-corrected overpotential, S represents the geometric area of GCE (0.07 cm2), F is Faraday constant (96485.3C mol
1), and n represents the molar of metal ions. In this work, the molar of Co and Fe was determined by ICP. 3. Results and discussion SEM and TEM measurements were employed to observe the morphology of 5% FCS/CS composites. From Fig. 1a, it can be seen that the as-prepared 5% FCS/CS is composed of irregular nanoparticles and thin nanosheets, the FCS nanoparticles with diameter ranging from a few nanometers to hundreds of nanometers anchored on the surface of CS nanosheet. Furthermore, the CS catalyst exhibits ultrathin nanosheets with rich channels and fold judging from the nearly transparent TEM image (Fig. 1b). The addition of Fe ions reduces the formation of CS nanosheet, and lead to the generation of CoSe2 particles. Fig. 1c is the magnified image of a part in Fig. 1b. The HRTEM image (Fig. 1d) shows well-defined lattice fringes with the d-spacing of 0.25 nm and 0.20 nm, which are corresponding to the (120) and (220) crystal planes of orthorhombic CoSe2, respectively. The crystal structures and phase information of the as-prepared samples were performed by XRD. As shown in Fig. 2, bare CS displays six diffraction peaks at about 33.37, 45.00 , 50.71, 60.40 , 62.12 and 70.15 , which are attributed to (101), (102), (110), (103), (112) and (202) crystal facets of hexagonal CS (PDF card No. 52e1008), respectively [21]. The peaks of CoSe2 in FCS/CS catalysts are greatly assigned to the orthorhombic phase with a space group of Pnnm (JCPDS card no. 53e0449). Compared with bare CS, a little shift can be noticed in FCS/CS catalysts, attributing to the reduction of DETA in the interlayer. Upon the addition of Fe ions, the peak intensity of CoSe2 increased gradually. This phenomenon indicates the addition of Fe can change the structure of catalysts. The XRD results demonstrate that FCS/CS nanocomposites have been successfully synthesized by a one-step hydrothermal method. To further complete definition of FeCoSe2, the FCS/CS nanocomposite with 20% Fe was synthesized, On the basis of XRD (As shown in Fig. S2), we can see that neither new diffraction peak nor crystalline FeSe2 phase appeared. Therefore, FeCoSe2 means Fe-doped CoSe2 in this manuscript. The detailed structural change of CS nanosheet and FCS/CS nanocomposite was evaluated by Raman spectrum to revealed the chemical synergistic effect. As shown in Fig. 2b, the five characteristic peaks at 188, 466, 507, 602 and 670 cm
1 are indexed to the Ag, Eg, F12g , F22g and A1g modes of hexagonal CS, respectively [22]. The Raman peak at 165 cm
1 is assigned to the SeeSe stretching mode [23]. It’s worth noting that there is a little change in A1g characteristic peak, suggesting the structure or size variation of hybrid catalysts [24].

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Fig. 1. SEM and TEM of 5% FCS/CS composite.

Fig. 2. (a) XRD patterns and (b) Raman spectrum of the as-prepared catalysts.

XPS characterization was employed to investigate the elemental composition and surface valances of FCS/CS. The full spectra of CS and FCS/CS catalyst are shown in Fig. S3, in which five significant characteristic peaks at 790, 715, 532, 284 and 58 eV clearly confirm the existence of Co, Fe, O, C and Se elements, respectively. Fig. 3a shows the high-resolution XPS spectra of Co 2p. The peaks at about 777.6 and 792.7 eV are related to the Co 2p3/2 and Co 2p1/2,

verifying the existence of CoeCo bonds. The Co 2p3/2 and Co 2p1/2 peak at the binding energy around 780.5 and 796.3 eV are associated with cationic Co. According to the relevant research [25], the electronic properties of Co can be modified by Fe through charge transfer. The binding energy of Co in FCS/CS is lower than that of CS, indicating the electron transfer from Fe to Co. In addition, two satellite peaks at the binding energy of 784.9 and 801.7 eV are

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K. Zhang et al. / Journal of Alloys and Compounds 825 (2020) 154073

Fig. 3. High-resolution XPS spectra of CS and 5% FCS/CS catalysts in Co 2p, Se 3d, Fe 2p, and O 1s region.

related to the antibonding orbital between Co and Se. The Co cations in the obtained materials exhibit a d7 electronic structure in the manner of t62ge1g. This structure is near the optimal value for eg filling with regard to paramagnetic configuration, which is desired for high-activity catalysts [26]. Fig. 3b shows the XPS spectra of CS and FCS/CS catalysts in Se 3d region. The fitted peaks at the binding energy of 53.9 and 54.7 eV are correspond to the Se 3d5/2 and Se 3d3/2 orbitals, respectively, relating to the metaleSe bonds [27]. The Co 2p3/2 signal of FCS/CS can be fitted into four chemical states, which are ascribed to some intermediate valence generated in the conversion process from CS to FCS/CS [28]. Note that the binding energy of CS is higher than that of FCS/CS, which can be attributed to the formation of orthorhombic CoSe2, thus resulting in changed coordination number of Se [18]. The peaks at binding energy of 58.8 and 61.0 eV are related to the Co 3p3/2 and Co 3p 1/2, respectively. The high-resolution spectrum of Fe 2p region in FCS/CS (Fig. 3c) features Fe 2p 3/2 peak at about 713.1 eV and a shake-up satellite peaks at about 718.5 eV [29], confirming the presence of Fe cation. Fig. 3d shows the high-resolution XPS spectra for the O 1s region of catalysts, and the peak of O 1s can be divided into three characteristic peaks at 530.4, 531.5 and 532.7 eV, which correspond to the

Co-oxygen bond, CoOOH, and chemical adsorbed H2O, respectively. The existed oxygen indicates the partial oxidation or hydroxylate of catalysts. The hydroxylated area of 5% FCS/CS catalyst is more than that of CS, implying the nanocomposite possesses good hydrophilia, superior ionic permeability and more active sites [27]. The OER performance of the synthesized catalysts and commercial RuO2 is investigated in 1 M KOH solution via a standard three-electrode system. As shown in Fig. 4a, the 5% FCS/CS catalyst demonstrates the lowest onset potential than those of referential catalysts. Remarkably, when the current density achieved 10 mA cm
2, the 5% FCS/CS catalyst exhibits the overpotential at about 0.33 V, much smaller than bare CS nanosheet and commercial RuO2. Note that all the Fe doped catalysts exhibit much higher catalytic activities than bare CS, demonstrating there is a strong synergistic effect between Co and Fe. The over doped Fe changed the morphology obviously, and the increased volume of nanoparticles would decrease catalyst surface, thus leading to the decrease in catalytic activity. To analyze the OER kinetics, IRcorrected Tafel plot was derived from LSV curves by the Tafel equation. 5% FCS/CS catalyst delivers the lowest Tafel slope of 50.8 mV dec
1 followed by 2% FCS/CS (51.9 mV dec
1), 10% FCS/CS

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Fig. 4. Electrochemical properties of FCS/CS catalyst. (a) iR-corrected polarization curves of FCS/CS catalysts for OER. (b) iR-corrected Tafel plot derived from (a). (c) Nyquist plots of FCS/CS catalysts at 1.6 V. (d) Chronopotentiometry curves of FCS/CS at a constant current density of 10 mA cm
2 for 12 h. All the measurements were performed in 1.0 M KOH.

(59.2 mV dec
1), CS (70.5 mV dec
1) and RuO2 (124.1 mV dec
1), suggesting that 5% FCS/CS nanocomposite possesses superior OER kinetics. EIS test is introduced to reveal interior electron transfer kinetic [30]. The relevant Nyquist plots is seen in Fig. 4c. Obviously, the charge-transfer resistance (Rct) of 2% FCS/CS, 5% FCS/CS, 10% FCS/CS and CS is 20, 12.5, 28 and 39.5 U, respectively. CS nanosheet displayed the highest electrochemical impedance, and the conductivity of sample catalysts was enhanced by addition of Fe. The improved conductivity of catalysts could result in the superior OER performance. Stability is an important parameter for catalysts to be applied in large scale oxygen production. To evaluate this key property of FCS/CS composite, CP measurement at a constant current density of 10 mA cm
2 in 1.0 M KOH was conducted. As shown in Fig. 4d, FCS/CS composite exhibits lower overpotential and better stability than commercial RuO2 during the continuing reaction, demonstrating that FCS/CS nanocomposite possesses superior stability for OER. The double-layer capacitance (Cdl) shows positive correlation

with the electrochemical active surface area (ECSA) value of catalysts, thereby the Cdl is often used to compare the catalysts’ active area [31]. The CV curves of RuO2, CS and 5% FCS/CS catalysts with the potential range from 1.21 to 1.31 V at different scan rates are illustrated in Fig. S4. The values of Cdl were calculated through the linear relationship between the current density and the scan rate. As presented in Fig. 6a, 5% FCS/CS catalyst (16.7 mF/cm2) shows a higher Cdl than those of CS (6.4 mF/cm2) and RuO2 (4.0 mF/cm2), suggesting that the 5% FCS/CS nanocomposite possesses much more OER active sites. TOF is an important factor for intrinsic catalytic activity evaluation [32]. In this work, the TOF values at various potential were calculated. To avoid overestimate, it was assumed all metal ions acted as active sites. As shown in Fig. 5b, the 5% FCS/CS catalyst exhibits the highest TOF value than those of RuO2 and CS at different overpotentials. In-situ Raman spectroscopy is a powerful technique to explore the surface structure and composition change of the catalysts during the reaction process. Fig. 6a shows the LSV of 5% FCS/CS nanocomposite, the peak at around 1.11 V is related to the oxidation

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Fig. 5. (a) Plots of the current density at 1.26 V vs scan rate for RuO2, CS and 5% FCS/CS catalyst. (b) TOF of RuO2, CS and 5% FCS/CS calculated at various potentials.

Fig. 6. (a) LSV curves of 5% FCS/CS catalyst, (b) Raman spectra of 5% FCS/CS catalyst during the LSV measurement.

Fig. 7. The surface of FCS/CS transform to active species during OER process.

of Co2þ [27]. The Raman spectrum was collected at 0.92, 1.11, 1.40, 1.54 and 1.63 V, respectively. As shown in Fig. 6b, all the peaks of catalyst are decreased with the increasing of potential, and there is a wide peak appeared, which can be ascribed to the formation of CoOOH [33]. This phenomenon is consistent with the results of

previous studies that most Co-based electrocatalysts will be oxidized to CoOOH during OER process (see Fig. 7) [34]. As the active species, the most transformation from CS to CoOOH was achieved at ca. 1.11 V and 1.54 V.

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4. Conclusion In summary, layered CS nanosheets supported FCS nanoparticles were designed and synthesized via a one-pot hydrothermal method. The test results demonstrate that the as-prepared 5% FCS/ CS catalyst possesses superior OER activity with an overpotential of 330 mV at the current density of 10 mA cm
2. Moreover, 5% FCS/CS catalyst has a large ECSA values and better stability than those of CS and commercial RuO2. This result can be ascribed to the following reactions: (1) Because of the ultrathin nanosheet structure, the catalysts can provide more active surface area, short ion and electron diffusion path distances for oxygen evolution reaction [35]. (2) the electronic structure and surface property can be modulated in the presence of Fe element, which is more favorable for water molecules adsorption and oxidation on FCS/CS surface [36,37]. Raman spectrum reveals that the active species are not from the asprepared original materials; the active species actually in charge of OER are formed during the oxygen evolution progress in alkaline electrolyte. The present work will facilitate the design of efficient electrocatalysts and understanding the surface changes of transition metal-based hybrid catalysts for OER. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Ke Zhang: Conceptualization, Methodology, Writing - review & editing. Menglin Shi: Data curation, Investigation. Yu Wu: Validation, Resources. Chuanyi Wang: Supervision. Acknowledgement This work is financially supported by the scientific research startup fund of Shaanxi University of Science and Technology. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.154073. References [1] A. Bergmann, T.E. Jones, E. Martinez Moreno, D. Teschner, P. Chernev, M. Gliech, T. Reier, H. Dau, P. Strasser, Nat. Catal. 1 (2018) 711e719. [2] X.C. Min-Rui Gao, Qiang Gao, Yun-Fei Xu, Ya-Rong Zheng, Jun Jiang, ShuHong Yu, ACS Nano 8 (2014) 3970e3978.

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