CoO encapsulated in carbon nanorod

Accepted Manuscript Enhancing oxygen evolution reaction electrocatalytic performance with vanadium-doped Co/CoO encapsulated in carbon nanorod

Huiyong Huang, Yanqiang Li, Wubo Li, Siru Chen, Chao Wang, Ming Cui, Tingli Ma PII: DOI: Reference:

S1387-7003(19)30116-9 https://doi.org/10.1016/j.inoche.2019.02.041 INOCHE 7299

To appear in:

Inorganic Chemistry Communications

Received date: Revised date: Accepted date:

29 January 2019 27 February 2019 28 February 2019

Please cite this article as: H. Huang, Y. Li, W. Li, et al., Enhancing oxygen evolution reaction electrocatalytic performance with vanadium-doped Co/CoO encapsulated in carbon nanorod, Inorganic Chemistry Communications, https://doi.org/10.1016/ j.inoche.2019.02.041

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ACCEPTED MANUSCRIPT Enhancing Oxygen Evolution Reaction Electrocatalytic Performance with Vanadium-doped Co/CoO Encapsulated in Carbon Nanorod Huiyong Huang, a Yanqiang Li, *a Wubo Li, a Siru Chen, *c Chao Wang, a Ming Cui, a Tingli Ma *b State Key Laboratory of Fine Chemicals, School of Petroleum and Chemical

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a.

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Engineering, Dalian University of Technology, Panjin Campus, Panjin 124221, China

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E-mail: [email protected]; [email protected]; [email protected] b. Graduate School of Life Science and Systems Engineering, Kyushu Institute of

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Technology, 2-4 Hibikino, Wakamatsu, Kitakyushu, Fukuoka 808-0196, Japan

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c. Center for Advanced Materials Research, Zhongyuan University of Technology, Zhengzhou, 450007, China

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Abstract

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Oxygen evolution reaction (OER) plays a key role in electrochemical splitting of water and it is urgent to develop high-performance and cost-effective OER catalysts.

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In this work, we report the synthesis of V-Co/CoO@C nanorod as highly efficient

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OER catalysts. The V2O5 nanowire and metal organic framework composite leads to the successful preparation of the catalyst. The V2O5 nanowire not only induces the formation of Co/CoO species, but also facilitates uniform V doping. When used as OER catalyst, the V-Co/CoO@C nanorod only needs a low overpotential of 320 mV to achieve a high current density of 10 mA cm-2. Key words: Electrocatalysis; Oxygen Evolution Reaction; Non-precious metal catalysts; Co/CoO; V doping

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1 Introduction Electrocatalytic water splitting is one of the most effective strategies to generate clean and renewable hydrogen fuel [1-3]. However, the kinetically sluggish oxygen

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evolution reaction (OER) half reaction greatly decrease the efficiency of water

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splitting devices and efficient OER catalysts are required [4-5]. Noble metal oxides

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such as IrO2 and RuO2 could efficiently catalyze the OER process, but the scarcity and high cost of noble metal prohibit them from being applied on a large-scale [6-7].

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Therefore, low-cost and highly efficient OER catalysts must be developed before the

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coming of hydrogen energy era. Co-based materials are promising candidates for OER and various of Co-based compounds such as cobalt oxide, cobalt hydroxide,

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cobalt sulfide and cobalt selenide are investigated [8-14]. However, the

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electrocatalytic activity of cobalt based compound is not very high for OER, especially when their crystalline structures are perfect [15]. Moreover, the low

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conductivity of the semi-metallic Co-based compounds such as cobalt selenide and

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cobalt phosphide will also decrease their catalytic activity [16]. Thus, it is needed to increase their catalytic activity by means of introducing oxygen vacancies in the form of heteroatom doping or increase their conductivity by coupling them with conductive substrates. Recently, researches found that V doping is an effective method to improve the OER performance of metal-based catalysts [17-22]. For example, Sun’s group reported iron–vanadium composite spheres with high OER performance. Theory

ACCEPTED MANUSCRIPT calculation reveals that V site exhibits the lowest overpotential, indicating great potential of V included catalysts for efficient water oxidation [21]. By designing amorphous cobalt-vanadium hydr(oxy)oxide catalysts, Yang’s group found that the V sites are the inferior active site compared with the Co sites [19]. Bao’s group point out

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that the oxygen defect concentration increased greatly in vanadium-doped perovskite,

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and enhanced CO2 electrolysis performance was achieved [22]. Porous carbon can be

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coated on to the electrocatalysts to further improve their conductivity. In this communication, we report the electrocatalytic activity of vanadium doped

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Co/CoO encapsulated in carbon nanorod (V-Co/CoO@C nanorod) as highly efficient

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OER catalyst. The synthesis strategy was schemed in Figure 1 A. The use of V2O5@ZIF-67 composite lead to the successful formation of the catalyst. We found

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that the use of V2O5 not only induce the formation of Co/CoO species, but also lead to

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uniform V doping. The prepared-Co/CoO@C nanorod displays an overpotential of 320 mV at a current density of 10 mA cm-2 in 1 M KOH with very high stability. Experimental section

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2

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2.1 Synthesis of V2O5 nanowire V2O5 nanowire was synthesized according to a published literature [23]. In a typical procedure, 300 mg of ammonium metavanadate and 500 mg of triblock copolymer (P-123) and were added to a mixture of deionized water (30 mL) and 2 M HCl (1.5 mL). The mixed solution was then transferred to a Teflon-lined autoclave and heated at 120 ℃ for 24 h. The obtained V2O5 nanowires were washed with deionized water for several times.

ACCEPTED MANUSCRIPT 2.2 Synthesis of V2O5@ZIF-67 and V-Co/CoO@C nanorod To synthesize V2O5@ZIF-67, 25 mg of V2O5 and 800 mg of PVP (Mw = 40,000) were dispersed in 40 ml of methanol and sonicated for 40 min. Then 291 mg of Co(NO3)2·6H2O and 328 mg of 2-Meththylimidazole was added to the above solution

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under constant stirring. Two hours later, the formed purple solid was collected by

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centrifugation, washed with methanol three times and dried at room-temperature. The

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as-prepared V2O5@ZIF-67 was calcined in Ar at 600 ℃ for 2 h, and the obtained samples were denoted as V-Co/CoO@C nanorod. For comparison, ZIF-67 was also

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synthesized and calcined using the same procedure, and the obtained samples was

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found to be Co/C. 2.3 Materials characterization

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Power X-ray diffraction patterns (XRD) were collected using a Riguku

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D/MAX2550 diffractometer. The morphology of the catalysts was observed using a Nova NanoSEM 450 (FEI) field emission scanning electron microscopy (FE-SEM)

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and a FEI, Tecnai G2 F20Transmission electron microscopy (TEM). X-ray

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photoelectron spectroscopy (XPS) measurement was conducted with an ESCALAB 250Xi (ThermoFisher). The pore structure was characterized by N2 sorption at -196 °C (Quantrachrome Quadrasorb Si-MP). 2.4 Electrochemical measurements The catalytic activity measurements were conducted using a three-electrode system, with a glassy carbon, Pt wire, and Ag/AgCl electrode as the working electrode, counter electrode and reference electrode. The electrolyte is O2-saturated 1 M KOH

ACCEPTED MANUSCRIPT solution. The catalyst loading was 0.24 mg cm- 2. The current densities were normalized to the geometric area of the glassy carbon electrode and the scan rate for the electrochemical measurements was 10 mV s-1. All potentials were referenced to reversible hydrogen electrode (RHE) scale by E (RHE) = E (Ag/AgCl) + 0.059 pH + 0.197

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V.

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3 Results and discussions

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XRD analysis was used to monitor the formation of catalysts. As shown in Figure S1, the XRD pattern of the prepared V2O5 matches with the literature results [23].

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After adding cobalt nitrate and 2-methylimidazole into the V2O5 solution, ZIF-67 can

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be successfully formed on the surface of the V2O5 nanowires. This is demonstrated by the XRD pattern of V2O5@ZIF-67, where typical peaks of ZIF-67 can be observed

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(Figure 1B, S1). The SEM image of V2O5@ZIF-67 also indicates that ZIF-67 were

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assembled on V2O5, though the length V2O5@ZIF-67 becomes shorter than V2O5 (Figure 1C, D). After thermal annealing at 600 oC, the morphology of the product

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does not change significantly (Figure 1E), and the rod-like structure can be still

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maintained. XRD pattern analysis shows that Co/CoO species was formed (Figure 1B and S2). Though no obvious peaks associated with V species were detected, we can see that V element was homogeneous distributed in the samples from the elemental mapping images, indicating that V are doped into the sample (Figure 1F). XPS measurements of V-Co/CoO@C nanorod reveal that the Co and V contents in the material are 9.34 % and 0.67 % (atomic ratio). In addition, C and N were also detected with the contents of 61.38 % and 9.63 %, respectively (Figure S3). In the Co

ACCEPTED MANUSCRIPT 2p XPS spectra (Figure 2A), two main peaks located at 780.23 eV (Co 2p3/2) and 796.03 eV Co (2p1/2) are accompanied by two shake-up satellite peaks (785.26 and 802.56 eV), which are characteristic features of Co2+ [24-25].

The V 2p XPS in

Figure 2B of V-Co/CoO@C nanaorod has binding energies of 516.23 eV (2p3/2) and

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523.73 eV (2p1/2) with a spin-energy separation of 7.5 eV, corresponding to the V5+

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phase [26]. The N2 adsorption isotherm of V-Co/CoO@C nanorod exhibit a sharp

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uptake in the low-pressure region (10-5 to 10-3 atm), which is a typical feature of microporous materials; and a hysteresis loop at the pressure of 0.6-0.9, which

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indicates the presence of mesopores (Figure 2C). The bimodal pore distribution is also

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confirmed by the pore size distribution, where pores located at 0.5 nm and 5 nm are observed (Figure 2D). The pores can also be observed from high resolution TEM

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image (Figure S4), where many pores exist in the carbon matrix. This also

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demonstrated that the high surface mainly comes from the carbon in V-Co/CoO@C. It should be noted that specific surface area of V-Co/CoO@C nanorod is as high as 298

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m2 g-1, which is much higher than most of the reported metal-based catalysts such as

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metal oxides, metal sulfides and metal oxides/C composites [27-29]. The high specific surface area can provide more active sites and facilitate mass transport. Thus, the catalytic activity of V-Co/CoO@C nanorod is expected to be very high. The OER catalytic activity of V-Co/CoO@C nanorod was investigated in three-electrode electrochemical cell. V2O5 and Co/C (synthesized by calcined ZIF-67 at 600 oC for 2h, Figure S5) were also measured for comparison. From the polarization curves shown in Figure 3A, we can see that the V-Co/CoO@C nanorod

ACCEPTED MANUSCRIPT exhibits a much lower onset potential than V2O5 and Co/C. The overpotential of the different samples at a current density of 10 mA·cm-2 (a metric related to solar fuel synthesis) is compared in Figure 3B. It can be seen that V-Co/CoO@C nanorod has the lowest value of 320 mV, which is considerably smaller than the value of 431 mV

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for Co/C, 1330 mV for V2O5. Both the small ŋ (onset) and ŋ (10 mA·cm-2) values

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highlight the high OER catalytic activity of V-Co/CoO@C nanorod. It should be

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noted that the OER catalytic activity of Co/CoO@C nanorod is also comparable with commercial RuO2 and many other Co-based catalysts [14, 30-33]. To reveal the

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reaction kinetics of the catalysts, the Tafel slopes were calculated from their

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corresponding polarization curves (Figure 3C). Compared with V2O5 (265 mV dec-1) and Co/C (162 mV dec-1), the Tafel slope of V-Co/CoO@C nanorod (143 mV dec-1) is

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obvious smaller, indicating its fast OER kinetics. Furthermore, the electrochemical

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impedance spectroscopy (EIS) measurement was conducted to the charge transfer resistance at the electrode/electrolyte interface. The Nyquist plots prove the much

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lower charge-transfer resistance of V-Co/CoO@CNR, probably due to the carbon in

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the catalyst and its rod-like structure (Figure 3D). The electrochemically active surface area (ECSA) of the catalysts is an important factor for their catalytic activity, since a higher ECSA usually means faster mass transport and more active sites. Current densities are measured between the non-Faradic region with different scan rates using cyclic voltammetry (CV), and the differences of the charging current density was plotted against scan rates (Figure 3E and S6). The slope of the plot was twice of the double-layer capacitances (Cdl). The Cdl of V-Co/CoO@C nanorod (16.9

ACCEPTED MANUSCRIPT mF·cm-2) is calculated to be 8 times of that for Co/C (2.18 mF·cm-2) and hundreds of times of that for V2O5 (0.02 mF·cm-2). This demonstrates that V-Co/CoO@C nanorod has a larger catalytic surface area. The durability of V-Co/CoO@C nanorod was evaluated at a constant current density of 10 mA·cm-2 for 20 h. It can be seen that the

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overpotential remains largely unchanged, suggesting that V-Co/CoO@C nanorod is

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highly stable.

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To reveal the high catalytic activity of V-Co/CoO@C nanorod for OER. We think the following aspects are important. First, the usage of V2O5 nanowire induces the

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formation of CoO at a high temperature of 600 oC and the role of CoO for OER is

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significant. This can be demonstrated by the poor performance of Co@C obtained form calcinating ZIF-67 at 600 oC as shown above. Second, the role of Co is also

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non-negligible. When the V2O5@ZIF-67 was thermal annealed at 500 oC, only CoO

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was detected, and the OER performance of the V/CoO@C nanorod is much lower that of the V-Co/CoO@C nanorod (Figure S7 and S8). Thus, we infer that there may be a

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synergistic effect between Co and CoO. Last and most significantly is that the use of

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V2O5 can leads to uniform V doping. It has been demonstrated that V doping can enhances oxygen defect concentration and lower the overpotential at the Co sites, and lead to enhanced electrocatalysis activity [19, 22]. The the O 1s XPS was measured to investigate the oxygen defect in our samples (Figure S9). It can be seen that four kinds of oxygen were detected, corresponding to the lattice oxygen (529.2 eV, Olattice, O2-), oxygen adsorbed (Oads) in the form of highly oxidative O-/O2 2− species (529.7 eV) and surface adsorbed oxygen (530.9 eV) or water (532.2 eV, OH). The

ACCEPTED MANUSCRIPT relative atomic concentration of each oxygen state estimated from the fitting curves. The oxygen adsorbed (Oads) in the form of highly oxidative O-/O22− species (529.7 eV) in V-Co/CoO@C is obvious higher than that in Co@C (16.7% for V-Co/CoO@C vs 6.0% for Co@C), indicating there is more oxygen defect in the V-Co/CoO@C (Table

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S1).

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Conclusion

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In conclusion, we reported the synthesis of V-Co/CoO@C nanorod using V2O5@ZIF-67 as the precursor and demonstrate its high OER catalytic activity. In 1.0

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M KOH solution, the V-Co/CoO@C nanorod only require an overpotential of 320 mV

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to achieve a current density of 10 mA·cm-2. The use of V2O5 nanowire not only induces the formation of Co/CoO, but also lead to uniform V doping. Both the

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Co/CoO active species and V doping contributed greatly to the OER performance of

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the catalyst. This work provides an effective method to synthesize V doped materials for energy storage and conversion application.

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Conflict of interest

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Authors declare there is no conflict of interest. Acknowledgements This work is supported financially by the National Natural Science Foundation of China (Grant No. 51772039), the Fundamental Research Funds for the Central University (DUT18LK13) and the Research Center for Solar Light Energy Conversion, Kyushu Institute of Technology, Japan. Reference

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Figure 1 (A) Scheme illustration of the synthesis of V-Co/CoO@C. (B) XRD patterns

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of the samples. (C) SEM image of V2O5. (D) SEM image of V2O5@ZIF-67. (E) SEM

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image of V-Co/CoO@C. (F) TEM and element mapping of V-Co/CoO@C.

Figure 2 (A) Co 2p spectra of V-Co/CoO@C. (B) V 2p spectra of V-Co/CoO@C. (C) N2 sorption isotherms of V-Co/CoO@C at 77 K. (D) Pore size distributions of V-Co/CoO@C.

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Figure 3 (A) LSV curves of V2O5, Co/C, V-Co/CoO@C and RuO2 in O2-saturated 1.0 M KOH solution at a scan and rotation rates of 10 mV s-1. (B) Overpotential of V2O5,

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Co/C, V-Co/CoO@C and RuO2 at a current density of 10 mA cm-2. (C) Tafel plots for V2O5, Co/C and V-Co/CoO@C. (D) Nyquist plots for V2O5, Co/C and V-Co/CoO@C. (E) The capacitive current densities at 1.2 V as a function of the scan rate for V2O5, Co/C and V-Co/CoO@C. (F) Stability of V-Co/CoO@C at a constant current density of 10 mA cm-2.

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Graphical abstract

ACCEPTED MANUSCRIPT Highlights  Porous V-Co/CoO@C nanorod was synthesized by using a V2O5@ZIF-67 as precursor.  The V-Co/CoO@C nanorod shows high OER activity of 10 mA cm-2 at a η value of 320 mV.  The use of V2O5 lead to the formation of Co/CoO and V doping.

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 The Co/CoO species and uniform V doping contributed to its OER performance.