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First insight on Mo(II) as electrocatalytically active species for oxygen evolution reaction
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First insight on Mo(II) as electrocatalytically active species for oxygen evolution reaction Zailun Liu, Chen Yuan, Fei Teng*, Maoyuan Tang, Zain Ul Abideen, Yiran Teng Jiangsu Engineering and Technology Research Center of Environmental Cleaning Materials (ECM), Collaborative Innovation Center of Atmospheric Environment and Equipment Technology (CICAEET), Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control (AEMPC), Jiangsu Joint Laboratory of Atmospheric Pollution Control (APC), School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, 219 Ningliu Road, Nanjing 210044, China
article info
abstract
Article history:
The replacement of noble metals with earth-abundant metals is still a big challenge for the
Received 28 September 2018
practical application of electrocatalysis. In this work, we have developed the MoxC-modi-
Received in revised form
fied alloy@nitrogen-doped carbon hybrid electrocatalysts (MoxC-alloy@NC, alloy: FeCo,
9 November 2018
NiCo) for oxygen evolution reaction (OER) by a simple thermolysis method. Compared with
Accepted 13 November 2018
FeCo@NC and NiCo@NC, the OER performances of MoxC-FeCo@NC and MoC-NiCo@NC are
Available online xxx
greatly enhanced, mainly due to the improved electrical conductivity by the introduce of
Keywords:
overpotential (318 mV) at 10 mA cm2 in 1 M KOH solution, compared with MoC-NiCo@NC
Alloy
(186 mV/dec, 352 mV). In consideration of a lower BET area (6.6 m2 g1) of MoxC-FeCo@NC
Mo(II) species
than those of MoC-NiCo@NC (25.4 m2 g2), the remarkable electrocatalytic activity of MoxC-
MoxC
FeCo@NC is mainly attributed to the presence of Mo(II) acting as the OER active species.
Oxygen evolution reaction
Although Mo as hydrogen evolution reaction (HER) active species is well known, Mo(II) as
Electrocatalyst
the OER active species has not been reported before.
MoxC. Moreover, MoxC-FeCo@NC exhibits a smaller Tafel slope (80 mV/dec) and a lower
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Today, the hydrogen and oxygen production of water splitting have attracted an increasing attention [1e3]. As a half reaction of water splitting, oxygen evolution reaction (OER) is kinetically sluggish and requires a high overpotential [2,4,5]. Although precious metals electrocatalysts including ruthenium and iridium oxides have outstanding OER activities, the resource scarcity and high cost have seriously limited their widespread applications [6,7]. Thus, it is needed to explore
efficient, non-precious electrocatalysts. In relative studies, tremendous efforts have focused on 3d transition metals including metal oxides [8e11], phosphates [12e14], perovskites [2,15], oxyhydroxides [16,17] and composites [18e20]. The 3d transition metals have many intriguing advantages, such as low cost, natural abundance and environmental friendliness. However, 3d transition metals usually have unsatisfactory OER activity, which is closely relative to the atomic configuration, surface area and surface states [5,21e23]. For example, the different alloys can present
* Corresponding author. E-mail address: [email protected] (F. Teng). https://doi.org/10.1016/j.ijhydene.2018.11.105 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Liu Z et al., First insight on Mo(II) as electrocatalytically active species for oxygen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.105
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different electrocatalytic activities [21]. Thus, it is highly desirable to develop efficient, inexpensive OER electrocatalysts. Recently, Bao et al. [21] have reported the singlelayer graphene-encapsulated alloy electrocatalyst with a high surface area for water oxidation; Feng et al. [24] have reported that the N-doped graphene layers-encapsulated NiFe alloy nanoparticles show an excellent OER activity; Oh et al. [25] have developed a conductive 3D carbon-encapsulated NiCo nanowire network as a highly efficient OER electrocatalyst; Fu et al. [26] have also reported the synthesis of Ni3Fe nanoparticles embedded in N-doped 2D porous graphitic carbon sheets as an OER electrocatalyst; Chen et al. [27] have reported non-precious ternary alloys encapsulated in graphene layers electrocatalyst. Nevertheless, their OER activities are still unsatisfactory. It is still a big challenge to develop transition metals electrocatalysts with efficient OER activity. Due to the unique d-band electronic structure, Group VI transition metals show a nearly similar catalytic property to Pt. Typically, molybdenum carbide has been intensively investigated as an active, stable electrocatalyst for hydrogen evolution reaction (HER) [28e33]. Furthermore, it has been demonstrated that Mo(II) and Mo(III) are the active centers for HER [34,35], and the HER activity of MoCx is dependent on the Mo(II) and Mo(III) active centers exposed on catalyst surface [29,35,36]. However, minimal attention has been paid towards the exploration of its OER activity, only a few efforts were devoted to the development of Mo2C for OER [33,37,38]. In addition, the effects of molybdenum carbide in composite materials as OER electrocatalyst is scarcely reported. Moreover, the research of Mo species (Mo(II), Mo(III), Mo(IV), Mo(VI)) as the active centre for OER is not systematic so far. Herein, we have found, for the first time, that Mo(II) can act as the OER active centre. In this work, molybdenum carbide is introduced to the alloy@NC catalysts through a simple thermal treatment method. The OER performance of MoxC-FeCo@NC and MoCNiCo@NC catalysts is greatly enhanced, compared with FeCo@NC and NiCo@NC catalysts, respectively. Compared with MoC-NiCo@NC, MoxC-FeCo@NC exhibits a smaller charge transfer resistance of 15.67 U, a smaller Tafel slope (80 mV/dec) and a lower overpotential (318 mV) at 10 mA cm2 in 1 M KOH solution. The current density for MoxC-FeCo@NC is 21 mA cm2 at h ¼ 350 mV, which is about 2.1 times as high as that for MoC-NiCo@NC. Further, there are only two oxidation states for Mo (Mo(II), Mo(VI)) on the MoxC-FeCo@NC surface, while four oxidation states (Mo(II), Mo(III), Mo(IV), Mo(VI)) on the MoC-NiCo@NC surface. It is found that Mo(II) is the main OER active species, which favors to develop new, efficient OER electrocatalysts.
Experimental Materials preparation: MoxC modified alloy@nitrogen-doped carbon (NC) (denote alloy represents FeCo, NiCo) were synthesized as follows. First, 0.0002 mol (NH4)6Mo7O24·4H2O and 0.02 mol C2H2O4·2H2O were dissolved in 40 ml of distilled water, after 0.01 mol of melamine was added, and the mixed solution was stirred at 70 C for 6 h by magnetic stirrer. Then 10 ml of 0.5 M FeCl3·6H2O (0.005 mol) and 0.5 M CoCl2·6H2O
(0.005 mol) (or 10 ml of 0.5 M CoCl2·6H2O (0.005 mol) and 0.5 M NiCl2·6H2O (0.005 mol)) mixture solution was added into the above mixed solution, stirring at room temperature for 12 h. After that the mixture solution was dried at 80 C for 24 h. Subsequently, the homogeneous mixture was placed in a quartz tube and calcined at 900 C in a tube furnace for 2 h in N2 atmosphere with a heating rate of 10 C min1. The preparation method of FeCo@NC and NiCo@NC are similar as the MoxC modified alloy@nitrogen-doped carbon, except the addition of (NH4)6Mo7O24·4H2O. Materials characterization: The fine surface structures of the samples were characterized by high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F), equipped an electron diffraction (ED) attachment with an acceleration voltage of 200 kV and an energy dispersive X-ray spectrometer (EDS). The crystal phases of the samples were characterized by X-ray diffraction (XRD, Rigaku D/max2550VB) with graphite monochromatized Cu Ka radiation (l ¼ 0.154 nm), operating at 40 kV and 50 mA at a scan rate of 7 min1. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a VG ESCALAB MKII XPS system used Mg Ka X-rays as the excitation source with a voltage of 12.5 kV and power of 250 W, the C1s peak at 284.8 eV of the surface adventitious carbon used as the reference standard for all the binding energies. Nitrogen sorption isotherms were performed at 77 K and <104 bar on a Micromeritics ASAP2012 gas adsorption analyzer, each sample was degassed at 160 C for 6 h before measurements, and the surface area was calculated by the Brunauer-Emmett-Teller (BET) method. Elecrochemical measurements: Electrochemical measurements were carried out on a standard three-electrode electrochemical cell using CHI 660D electrochemical working station at room temperature. Pt wire was used as counter electrode and Ag/AgCl (saturated KCl-filled) as the reference electrode. A commercial glass carbon with a diameter of 3 mm covered by a thin catalyst film was used as the working electrode. Typically, the working electrode was prepared as follows, 5 mg of measured catalyst materials and 50 mL Nafion solution (5 wt-%, Du Pont) were dispersed in 1 ml waterethanol mixture solution with the volume rate of 3:1, sonicated for 1 h to form a homogene ink. Then 5 mL of the homogeneous ink was pipetted onto the glass carbon electrode and dried under room temperature. And final the catalyst loading on the surface of the glass carbon electrode is 0.28 mg/ cm2. The oxygen evolution activities of different catalyst materials were recorded in 1 M KOH (PH 13.6) electrolyte at room temperature with a scan rate of 5 mV s1. All of the electrochemical measurements were iR-compensated. The potentials of LSV curves and Tafel plots are reported vs the reversible hydrogen electrode (RHE), based on the Nernst equation. According to the following calculation: E (vs RHE) ¼ E (vs Ag/AgCl) þ EºAg/AgCl þ 0.0592 pH. The electrochemical active surface area of different catalysts was estimated by measuring the double-layer capacitances based on the electrochemical method reported on literature [19]. The cyclic voltammorgrams (CVs) were obtained with different scan rates from 20 to 120 mV s1 in the potential range of 0.1e0.2 V (vs. Ag/AgCl). The stability of catalysts was measured at a scan rate of 100 mV s1 for 2000 cycles. The electrochemical impedance spectroscopy (EIS) data were
Please cite this article as: Liu Z et al., First insight on Mo(II) as electrocatalytically active species for oxygen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.105
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measured with frequencies range from 100 kHz to 0.1 Hz, and the impedance data were fitted to a simplified equivalent circuit, obtained the series and charge-transfer resistances.
Results and discussion The MoxC-alloy@NC catalysts were synthesized by a simple one-step thermolysis method in N2 atmosphere, in which the homogeneous mixture of (NH4)6Mo7O24·4H2O, oxalic acid dihydrate (C2H2O4·2H2O), melamine and metal chlorine are used as the precursors. The details of preparation and characterization are provided in (seeing electronic supporting information (ESI)). The use of oxalic acid dihydrate and melamine are vital in the formation of MoxC, NC and alloy, since oxalic acid dihydrate and melamine undergo the carbonization, which helps the deoxygenation of (NH4)6Mo7O24·4H2O and reduces metal ions to form alloy. Fig. 1a shows the powder X-ray diffraction (XRD) patterns of the MoxC-alloy@NC samples. The diffraction peaks at 26.2 and 42.2 are assigned to the graphite carbon (JPCDS: 75-1621). The peaks at 34.5 , 38.1 , 39.6 , 52.2 , 61.8 , and 69.6 can be assigned to Mo2C (JPCDS: 01-1188), and the peaks at 32 , 35.8 , 48.7 , and 64.1 are ascribed to MoC (JPCDS: 45-1015). The characteristic peaks at 45 and 65.4 can be attributed to FeCo alloy (JPCDS: 49-1567), and the peaks at the 44.3 and 51.6 are ascribed to NiCo alloy (Co JPCDS: 89-4307, Ni JPCDS: 89-7128), the XRD result of NiCo alloy agree well with the previous reports [21,39]. The XRD patterns demonstrate that the MoxCalloy@NC samples are composed of alloy (FeCo or NiCo), MoxC (MoC, Mo2C) and graphite carbon. In addition, the XRD patterns of FeCo@NC and NiCo@NC are shown in Fig. S1 (seeing ESI). For FeCo@NC sample, the peaks at 45 and 51.6 can be
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attributed to FeCo alloy (JPCDS: 49-1567) and Co (JPCDS: 894307), respectively. For NiCo@NC sample, the peaks at 44.3 and 51.6 are ascribed to NiCo alloy (Co JPCDS: 89-4307, Ni JPCDS: 89-7128) [21,39]. To further identify the compositions and chemical states, their XPS spectra of MoxC-alloy@NC samples have been analyzed in detail (Fig. 1bed, Fig. S2-S3, seeing ESI). Fig. 1b shows the survey XPS spectrum of MoxC-FeCo@NC. The Fe 2p XPS spectrum (Fig. 1c) shows two major peaks at a low combining energy of 710.7 eV and a high combining energy of 724.2 eV, corresponding to Fe 2p3/2 and Fe 2p1/2 of Fe2þ, respectively. The formation of Fe2þ ions on the surface of FeCo may be due to the exposure to air [40,41]. For the Co 2p XPS spectrum (Fig. 1d, Fig. S2b, seeing ESI), the peaks at 781.3 eV and 796.8 eV could be caused by the formation of Co-C bond on the surface of FeCo or NiCo alloy. Similarly, for the Ni 2p XPS spectrum (Fig. 1d, Fig. S2c, seeing ESI), the peaks at 854.8 eV and 872.8 eV could be caused by the formation of Ni-C bond on the surface of NiCo alloy [42]. In addition, the fitting peaks of Mo 3d reveal that there are different oxidation states for Mo in MoxC-alloy@NC, namely, there are four oxidation states (Mo(II), Mo(III), Mo(IV), Mo(VI)) in MoC-NiCo@NC (Fig. S2d, seeing ESI), and two oxidation states (Mo(II), Mo(VI)) in MoxCFeCo@NC (Fig. S3a, seeing ESI). The Mo(II) and Mo(III) can be attributed to the Mo-C bonds in MoxC, whereas Mo(Ⅳ) and Mo(Ⅵ) are assigned to MoO2 and MoO3, respectively [34,43,44]. Wan et al. [34,45] and Xiang et al. [34,45] have reported that the surface of MoxC can be contaminated with molybdenum oxides (MoO2 and MoO3) when they are exposed to the air. For N XPS spectrum (Fig. S2f and Fig. S3c), two types of nitrogen species are present: pyridinic and quaternary nitrogen.[30] Fig. 2a shows that the MoxC-FeCo@NC sample consists of graphene-like NC layers and FeCo alloy nanoparticles. Further
Fig. 1 e (a) XRD patterns of MoxC-alloy@NC (alloy: FeCo and NiCo; NC: nitrogen-doped carbon) samples; (bed) XPS spectra of MoxC-FeCo@NC sample: (b) Survey spectrum; (c) Fe 2p spectrum; (d) Co 2p spectrum. Please cite this article as: Liu Z et al., First insight on Mo(II) as electrocatalytically active species for oxygen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.105
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Fig. 2 e (a,b) HRTEM images of MoxC-FeCo@NC; (c,d) HRTEM images of MoC-NiCo@NC.
HRTEM analysis (Fig. 2b) reveals that the FeCo alloy nanoparticle are supported by the graphite carbon, and the lattice spacing of 0.178 nm is ascribed to the (200) plane of FeCo alloy; the lattice spacings of 0.183 nm and 0.225 nm correspond to the (101) plane of MoC and the (101) plane of Mo2C, respectively. Likewise, Fig. 2c and d shows the HRTEM images of the MoC-NiCo@NC sample. The d spacings of 0.217 nm correspond to the (111) planes of NiCo alloy, and the d spacings of 0.249 nm is ascribed to the (100) plane of MoC. In order to evaluate the electrocatalytic activities of MoxCalloy@NC catalysts, the oxygen evolution reaction performances are investigated in 1 M KOH solution using a typical three-electrode system. Fig. 3a presents the polarization curves of MoxC-FeCo@NC and MoC-NiCo@NC. Generally, the potential at the current density of 10 mA cm2 is often used to evaluate the electrocatalytic properties of an electrocatalyst [5,12]. At 10 mA cm2, MoxC-FeCo@NC has a lower potential (1.548 V vs. RHE) than MoC-NiCo@NC (1.582 V vs. RHE), FeCo@NC (1.616 V vs. RHE) and NiCo@NC (1.712 V vs. RHE). At the same current density (10 mA cm2), MoxC-FeCo@NC has an obviously smaller overpotential (318 mV) than MoC-NiCo@NC (352 mV), FeCo@NC (390 mV), and NiCo@NC (480 mV) (Fig. 3b), illustrating that MoxC-FeCo@NC catalyst has a highest OER activity amongst. At 10 mA cm2, the MoxC-FeCo@NC catalyst has a smaller overpotential than those of the precious reported catalysts, e.g., RuO2 (~405 mV, 1 M KOH) [27], IrO2/C (360 mV, 1 M KOH) [46]. In addition, at the same overpotential (h ¼ 350 mV), the current density is 21 mA cm2 for MoxC-FeCo@NC, which is about 2.1, 6.56 and 32 times as high as those of MoC-NiCo@NC, FeCo@NC and NiCo@NC, respectively (Fig. 3b). To understand the different OER activities, their BET areas are measured and compared (Fig. S4, seeing ESI). Although the BET area of MoC-NiCo@NC is obviously higher than that of
MoxC-FeCo@NC, MoC-NiCo@NC has a lower overpotential than MoxC-FeCo@NC at 10 mA cm2. Therefore, the BET area is not the affecting factor for the OER activity. Bao et al. [21] have demonstrated that the electrocatalytic activity difference of alloy@NC is mainly attributed to the different alloy components. According to the XPS results, there are two oxidation states for Mo (Mo(II), Mo(VI)) in MoxC-FeCo@NC (Fig. S3a, seeing ESI), while four oxidation states for Mo (Mo(II), Mo(III), Mo(IV), Mo(VI)) in MoC-NiCo@NC (Fig. S2d, seeing ESI). The alloy as the OER active species has been widely reported [21,24e27]. And it has been widely reported that Mo(II) and Mo(III) are the electrocatalytic active centers for hydrogen evolution reaction (HER) [34,35], and the HER activity of MoCx is mostly dependent on the Mo(II) and Mo(III) active centers exposed on catalyst surface [29,35,36]. Because of the unique d-band structure, MoxC can exhibit the electronic and catalytic properties analogous to Pt-group metals [34,42,47,48]. For example, Wan et al. [34,49] and Michalsky et al. [34,49] have attributed the excellent HER performance to the electron configuration around Fermi level (EF) of Mo2C [34,49]; Through DFT calculation, Liu et al. [30] have confirmed that the excellent HER performance of Mo2C is mainly ascribed to the low adsorption free energy of H (DG(H*)). Mo(II) and Mo(III) as the HER active centers has been widely reported [29,35,36], however, Mo(II) and Mo(III) as the OER active centers has not been reported so far. The introduction of MoxC has greatly improved the OER performance, because MoxC can be hydroxylated at alkaline condition, then it can undergo an electrochemical oxidation process, resulting in the formation of MoOx as the OER active species [38,50,51]. It is still a big challenge to avoid the oxidation of MoxC during electrochemical process [52,53]. To insight into the nature of Mo(II) and Mo(III) as the OER active centers, their electron configurations were mainly investigated. The different
Please cite this article as: Liu Z et al., First insight on Mo(II) as electrocatalytically active species for oxygen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.105
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Fig. 3 e The electrocatalytic properties of MoxC-alloy@NC catalysts: (a) Polarization curves; (b) Current density at h ¼ 350 mV (i) and Overpotentials at 10 mA cm¡2 (ii); (c) Tafel plots; (d) Cycle stability for 2000 CV sweeps between 100 mV and 200 mV (vs. Ag/AgCl).
electronic configurations (4d4 5s0/Mo(II) and 4d35s0/Mo(III)) could result in different electron densities around Mo(II) and Mo(III). The electron density around Mo(III) is more positive than that around Mo(II), thus Mo(III) has a stronger adsorbing ability to OH than Mo(II). It is well known that the too strong or too weak adsorption does not favor for the catalytic reaction. Therefore, we could think that the OH adsorption on Mo(III) centers may be too strong to benefit the OER activity, and the OH adsorption on Mo(II) centers may be moderate to boost the OER activity of MoxC-FeCo@NC. To further understand the OER activity, Tafel plots are shown in Fig. 3c, and the slopes are obtained by the Tafel equation [16]. The Tafel slope is 80 mV/dec for MoxCFeCo@NC, which is obviously smaller than the MoC-NiCo@NC (186 mV/dec), FeCo@NC (85 mV/dec), and NiCo@NC (125 mV/ dec), respectively. The smaller Tafel slope of MoxC-FeCo@NC illustrates that the OER process can occur rapidly. It is wellknown that the smaller Tafel slope can result in a remarkable increment OER rate with overpotentials [11], which is beneficial for practical applications. Besides the OER activity, the durability is another crucial parameter for an excellent electrocatalyst. Fig. 3d shows the OER polarization curves for 2000 sweeps. After 2000 sweeps, the polarization curve of MoxC-FeCo@NC does not change nearly, compared with the initial one, whereas the polarization curve of MoC-NiCo@NC decays quickly, which may be due to the formed Ni-C bond on the surface of NiCo alloy that can cause an irreversible electrochemical reaction [42]. The results confirm a high stability of MoxC-FeCo@NC. To further investigate the effect of capacitance, the electrochemical double-layer capacitance (Cdl) of MoxC-alloy@NC catalysts are estimated by using a simple cyclic voltammetry method [17,54,55]. Fig. S5(a-d) (seeing ESI) shows the cyclic
voltammorgrams (CVs) at different scanning rates (20e120 mV s1) in the potential range of 0.1e0.2 V (vs. Ag/AgCl). The differences in current density (DJ ¼ Ja-Jc) at a potential of 0.15 V (vs. Ag/AgCl) against scanning rates are plotted to get the linear slope that is equivalent to twice of Cdl. As shown in Fig. 4a, MoxC-FeCo@NC has a Cdl of 1.03 mF cm2, which is ca. 2, 8 and 14 times higher than those of FeCo@NC (0.56 mF cm2), MoCNiCo@NC (0.2 mF cm2) and NiCo@NC (0.12 mF cm2), respectively. Usually, a large electrochemical active area favors for a high Cdl value and the ratio of current density to Cdl can reflect the intrinsic electrochemical activity [36]. As a result, MoxCFeCo@NC has a higher intrinsic electrochemical activity. Usually, the electrocatalytic activity is closely relative to the electrocatalytic kinetics [56], which can be revealed by electrochemical impedance spectroscopy (EIS). Fig. 4b and Fig. S6 (seeing ESI) shows the Nyquist plots of MoxC-alloy@NC catalysts, and the inset of Fig. 4b and Fig. S6 (seeing ESI) presents the simulated equivalent electric cicuit (EEC). The simulated EEC consists of an electrolyte solution resistance (Rs), a constant phase element (CPE) accounting for the electrical double-layer capacitance, a charge transfer resistance (Rct) and a Warburg impedance (W) accounting for the diffusion resistance. In the Nyquist plots, the semicircle in the lowfrequency range represents Rct, while Rs appears in the highfrequency range [56]. On one hand, the Rs values were measured to be 2.56 U, 3.59 U, 29.09 U, and 14.25 U for MoxCFeCo@NC, MoC-NiCo@NC, FeCo@NC, and NiCo@NC electrodes, respectively. A low Rs value indicates that the resistance of electrolyte solution is not susceptible to the electrode materials. On the other hand, the Rct values were calculated to be 15.67 U, 28.21 U, 29.47 U and 6848 U for MoxC-FeCo@NC, MoC-NiCo@NC, FeCo@NC and NiCo@NC electrodes, respectively. A low Rct value of MoxC-FeCo@NC indicates a faster OER
Please cite this article as: Liu Z et al., First insight on Mo(II) as electrocatalytically active species for oxygen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.105
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Fig. 4 e (a) Capacitive current density differences (DJ ¼ Ja-Jc) against scan rates at a potential of 0.15 V (vs. Ag/AgCl). (b) Nyquit plots (the inset of equivalent electric circuit).
et al. [59] have reported that the Mo(IV) (MoO2) has excellent OER activity; and Dong et al. [60] have also reported that the Mo(III) (MoC) can enhance the OER activity. Thus, the enhanced OER activity are mainly attributed to Mo(II), Mo(III) and Mo(IV) species, rather than Mo(VI) species. In addition, the electron density around Mo(III) and Mo(IV) is more positive than that around Mo(II). As a result, Mo(III) and Mo(IV) have a stronger adsorption ability to OH than Mo(II), which may not favor for the OER reaction; Mo(II) has a moderate adsorption ability to OH, leading to an improved OER reaction. Summarily, FeCo or NiCo alloy and MoxC (e.s.p. Mo(II)) are the main OER active sites, which synergistically enhanced the OER activity, and NC mainly enhances the conductivity and makes the alloy nanoparticles distribute uniformly. Fig. 5 e A plausible OER mechanism on MoxC-alloy@NC catalysts in alkaline medium.
Conclusions
rate. Obviously the Rct value of MoxC-FeCo@NC is smaller than the others, consistent with the OER activity. On base of the results above, we have proposed a plausible OER mechanism on MoxC-alloy@NC electrocatalyst in alkaline solution. The OER process proceeds through four singleelectron transfer steps (Fig. 5). The overall OER mechanism on MoxC-alloy@NC in alkaline medium involves the following steps [57]. Several intermediates including *OH, *O and *OOH could be involved in the charge-transfer process. * þ OH /*OH þ e
(1)
*OH þ OH /H2 O þ *O þ e
(2)
*O þ OH /*OOH þ e
(3)
*OOH þ OH /*O2 þ H2 O þ e
(4)
*O2 /O2 þ e
(5)
For MoxC-FeCo@NC, there are two oxidation states for Mo (Mo(II), Mo(VI)), and its OER activity is higher than that of the FeCo@NC (Fig. 3a). While four oxidation states (Mo(II), Mo(III), Mo(IV), Mo(VI)) for MoC-NiCo@NC, it also shows a better OER activity than that of the NiCo@NC (Fig. 3a). Yang et al. [58] have demonstrated that the Mo(VI) (MoO3) has no OER activity; Cui
In summary, MoxC-alloy@NC electrocatalysts can be synthesized by a simple thermolysis method. MoxC-FeCo@NC electrocatalysts shows a small charge transfer resistance, a small Tafel slope and a small overpotential in KOH solution. The introduction of MoxC has greatly improved the OER performance and the Mo(II) species could act as the OER active site.
Acknowledgements This work is financially supported by National Science Foundation of China (21377060), the Project of Science and Technology Infrastructure of Jiangsu (BM201380277), the Key Project of Environmental Protection Program of Jiangsu (2013005), Jiangsu province graduate training innovation project (SJCX17_0262, SJZZ16_0153, KYCX17_0894, KYCX17_0895), Jiangsu province undergraduate practice and innovation project (201810300043Z, 201810300240), Six Talent Climax Foundation of Jiangsu (20100292), “333” Outstanding Youth Scientist Foundation of Jiangsu (20112015).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.11.105.
Please cite this article as: Liu Z et al., First insight on Mo(II) as electrocatalytically active species for oxygen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.105
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Please cite this article as: Liu Z et al., First insight on Mo(II) as electrocatalytically active species for oxygen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.105
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First insight on Mo(II) as electrocatalytically active species for oxygen evolution reaction Zailun Liu, Chen Yuan, Fei Teng*, Maoyuan Tang, Zain Ul Abideen, Yiran Teng Jiangsu Engineering and Technology Research Center of Environmental Cleaning Materials (ECM), Collaborative Innovation Center of Atmospheric Environment and Equipment Technology (CICAEET), Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control (AEMPC), Jiangsu Joint Laboratory of Atmospheric Pollution Control (APC), School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, 219 Ningliu Road, Nanjing 210044, China
article info
abstract
Article history:
The replacement of noble metals with earth-abundant metals is still a big challenge for the
Received 28 September 2018
practical application of electrocatalysis. In this work, we have developed the MoxC-modi-
Received in revised form
fied alloy@nitrogen-doped carbon hybrid electrocatalysts (MoxC-alloy@NC, alloy: FeCo,
9 November 2018
NiCo) for oxygen evolution reaction (OER) by a simple thermolysis method. Compared with
Accepted 13 November 2018
FeCo@NC and NiCo@NC, the OER performances of MoxC-FeCo@NC and MoC-NiCo@NC are
Available online xxx
greatly enhanced, mainly due to the improved electrical conductivity by the introduce of
Keywords:
overpotential (318 mV) at 10 mA cm2 in 1 M KOH solution, compared with MoC-NiCo@NC
Alloy
(186 mV/dec, 352 mV). In consideration of a lower BET area (6.6 m2 g1) of MoxC-FeCo@NC
Mo(II) species
than those of MoC-NiCo@NC (25.4 m2 g2), the remarkable electrocatalytic activity of MoxC-
MoxC
FeCo@NC is mainly attributed to the presence of Mo(II) acting as the OER active species.
Oxygen evolution reaction
Although Mo as hydrogen evolution reaction (HER) active species is well known, Mo(II) as
Electrocatalyst
the OER active species has not been reported before.
MoxC. Moreover, MoxC-FeCo@NC exhibits a smaller Tafel slope (80 mV/dec) and a lower
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Today, the hydrogen and oxygen production of water splitting have attracted an increasing attention [1e3]. As a half reaction of water splitting, oxygen evolution reaction (OER) is kinetically sluggish and requires a high overpotential [2,4,5]. Although precious metals electrocatalysts including ruthenium and iridium oxides have outstanding OER activities, the resource scarcity and high cost have seriously limited their widespread applications [6,7]. Thus, it is needed to explore
efficient, non-precious electrocatalysts. In relative studies, tremendous efforts have focused on 3d transition metals including metal oxides [8e11], phosphates [12e14], perovskites [2,15], oxyhydroxides [16,17] and composites [18e20]. The 3d transition metals have many intriguing advantages, such as low cost, natural abundance and environmental friendliness. However, 3d transition metals usually have unsatisfactory OER activity, which is closely relative to the atomic configuration, surface area and surface states [5,21e23]. For example, the different alloys can present
* Corresponding author. E-mail address: [email protected] (F. Teng). https://doi.org/10.1016/j.ijhydene.2018.11.105 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Liu Z et al., First insight on Mo(II) as electrocatalytically active species for oxygen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.105
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different electrocatalytic activities [21]. Thus, it is highly desirable to develop efficient, inexpensive OER electrocatalysts. Recently, Bao et al. [21] have reported the singlelayer graphene-encapsulated alloy electrocatalyst with a high surface area for water oxidation; Feng et al. [24] have reported that the N-doped graphene layers-encapsulated NiFe alloy nanoparticles show an excellent OER activity; Oh et al. [25] have developed a conductive 3D carbon-encapsulated NiCo nanowire network as a highly efficient OER electrocatalyst; Fu et al. [26] have also reported the synthesis of Ni3Fe nanoparticles embedded in N-doped 2D porous graphitic carbon sheets as an OER electrocatalyst; Chen et al. [27] have reported non-precious ternary alloys encapsulated in graphene layers electrocatalyst. Nevertheless, their OER activities are still unsatisfactory. It is still a big challenge to develop transition metals electrocatalysts with efficient OER activity. Due to the unique d-band electronic structure, Group VI transition metals show a nearly similar catalytic property to Pt. Typically, molybdenum carbide has been intensively investigated as an active, stable electrocatalyst for hydrogen evolution reaction (HER) [28e33]. Furthermore, it has been demonstrated that Mo(II) and Mo(III) are the active centers for HER [34,35], and the HER activity of MoCx is dependent on the Mo(II) and Mo(III) active centers exposed on catalyst surface [29,35,36]. However, minimal attention has been paid towards the exploration of its OER activity, only a few efforts were devoted to the development of Mo2C for OER [33,37,38]. In addition, the effects of molybdenum carbide in composite materials as OER electrocatalyst is scarcely reported. Moreover, the research of Mo species (Mo(II), Mo(III), Mo(IV), Mo(VI)) as the active centre for OER is not systematic so far. Herein, we have found, for the first time, that Mo(II) can act as the OER active centre. In this work, molybdenum carbide is introduced to the alloy@NC catalysts through a simple thermal treatment method. The OER performance of MoxC-FeCo@NC and MoCNiCo@NC catalysts is greatly enhanced, compared with FeCo@NC and NiCo@NC catalysts, respectively. Compared with MoC-NiCo@NC, MoxC-FeCo@NC exhibits a smaller charge transfer resistance of 15.67 U, a smaller Tafel slope (80 mV/dec) and a lower overpotential (318 mV) at 10 mA cm2 in 1 M KOH solution. The current density for MoxC-FeCo@NC is 21 mA cm2 at h ¼ 350 mV, which is about 2.1 times as high as that for MoC-NiCo@NC. Further, there are only two oxidation states for Mo (Mo(II), Mo(VI)) on the MoxC-FeCo@NC surface, while four oxidation states (Mo(II), Mo(III), Mo(IV), Mo(VI)) on the MoC-NiCo@NC surface. It is found that Mo(II) is the main OER active species, which favors to develop new, efficient OER electrocatalysts.
Experimental Materials preparation: MoxC modified alloy@nitrogen-doped carbon (NC) (denote alloy represents FeCo, NiCo) were synthesized as follows. First, 0.0002 mol (NH4)6Mo7O24·4H2O and 0.02 mol C2H2O4·2H2O were dissolved in 40 ml of distilled water, after 0.01 mol of melamine was added, and the mixed solution was stirred at 70 C for 6 h by magnetic stirrer. Then 10 ml of 0.5 M FeCl3·6H2O (0.005 mol) and 0.5 M CoCl2·6H2O
(0.005 mol) (or 10 ml of 0.5 M CoCl2·6H2O (0.005 mol) and 0.5 M NiCl2·6H2O (0.005 mol)) mixture solution was added into the above mixed solution, stirring at room temperature for 12 h. After that the mixture solution was dried at 80 C for 24 h. Subsequently, the homogeneous mixture was placed in a quartz tube and calcined at 900 C in a tube furnace for 2 h in N2 atmosphere with a heating rate of 10 C min1. The preparation method of FeCo@NC and NiCo@NC are similar as the MoxC modified alloy@nitrogen-doped carbon, except the addition of (NH4)6Mo7O24·4H2O. Materials characterization: The fine surface structures of the samples were characterized by high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F), equipped an electron diffraction (ED) attachment with an acceleration voltage of 200 kV and an energy dispersive X-ray spectrometer (EDS). The crystal phases of the samples were characterized by X-ray diffraction (XRD, Rigaku D/max2550VB) with graphite monochromatized Cu Ka radiation (l ¼ 0.154 nm), operating at 40 kV and 50 mA at a scan rate of 7 min1. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a VG ESCALAB MKII XPS system used Mg Ka X-rays as the excitation source with a voltage of 12.5 kV and power of 250 W, the C1s peak at 284.8 eV of the surface adventitious carbon used as the reference standard for all the binding energies. Nitrogen sorption isotherms were performed at 77 K and <104 bar on a Micromeritics ASAP2012 gas adsorption analyzer, each sample was degassed at 160 C for 6 h before measurements, and the surface area was calculated by the Brunauer-Emmett-Teller (BET) method. Elecrochemical measurements: Electrochemical measurements were carried out on a standard three-electrode electrochemical cell using CHI 660D electrochemical working station at room temperature. Pt wire was used as counter electrode and Ag/AgCl (saturated KCl-filled) as the reference electrode. A commercial glass carbon with a diameter of 3 mm covered by a thin catalyst film was used as the working electrode. Typically, the working electrode was prepared as follows, 5 mg of measured catalyst materials and 50 mL Nafion solution (5 wt-%, Du Pont) were dispersed in 1 ml waterethanol mixture solution with the volume rate of 3:1, sonicated for 1 h to form a homogene ink. Then 5 mL of the homogeneous ink was pipetted onto the glass carbon electrode and dried under room temperature. And final the catalyst loading on the surface of the glass carbon electrode is 0.28 mg/ cm2. The oxygen evolution activities of different catalyst materials were recorded in 1 M KOH (PH 13.6) electrolyte at room temperature with a scan rate of 5 mV s1. All of the electrochemical measurements were iR-compensated. The potentials of LSV curves and Tafel plots are reported vs the reversible hydrogen electrode (RHE), based on the Nernst equation. According to the following calculation: E (vs RHE) ¼ E (vs Ag/AgCl) þ EºAg/AgCl þ 0.0592 pH. The electrochemical active surface area of different catalysts was estimated by measuring the double-layer capacitances based on the electrochemical method reported on literature [19]. The cyclic voltammorgrams (CVs) were obtained with different scan rates from 20 to 120 mV s1 in the potential range of 0.1e0.2 V (vs. Ag/AgCl). The stability of catalysts was measured at a scan rate of 100 mV s1 for 2000 cycles. The electrochemical impedance spectroscopy (EIS) data were
Please cite this article as: Liu Z et al., First insight on Mo(II) as electrocatalytically active species for oxygen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.105
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measured with frequencies range from 100 kHz to 0.1 Hz, and the impedance data were fitted to a simplified equivalent circuit, obtained the series and charge-transfer resistances.
Results and discussion The MoxC-alloy@NC catalysts were synthesized by a simple one-step thermolysis method in N2 atmosphere, in which the homogeneous mixture of (NH4)6Mo7O24·4H2O, oxalic acid dihydrate (C2H2O4·2H2O), melamine and metal chlorine are used as the precursors. The details of preparation and characterization are provided in (seeing electronic supporting information (ESI)). The use of oxalic acid dihydrate and melamine are vital in the formation of MoxC, NC and alloy, since oxalic acid dihydrate and melamine undergo the carbonization, which helps the deoxygenation of (NH4)6Mo7O24·4H2O and reduces metal ions to form alloy. Fig. 1a shows the powder X-ray diffraction (XRD) patterns of the MoxC-alloy@NC samples. The diffraction peaks at 26.2 and 42.2 are assigned to the graphite carbon (JPCDS: 75-1621). The peaks at 34.5 , 38.1 , 39.6 , 52.2 , 61.8 , and 69.6 can be assigned to Mo2C (JPCDS: 01-1188), and the peaks at 32 , 35.8 , 48.7 , and 64.1 are ascribed to MoC (JPCDS: 45-1015). The characteristic peaks at 45 and 65.4 can be attributed to FeCo alloy (JPCDS: 49-1567), and the peaks at the 44.3 and 51.6 are ascribed to NiCo alloy (Co JPCDS: 89-4307, Ni JPCDS: 89-7128), the XRD result of NiCo alloy agree well with the previous reports [21,39]. The XRD patterns demonstrate that the MoxCalloy@NC samples are composed of alloy (FeCo or NiCo), MoxC (MoC, Mo2C) and graphite carbon. In addition, the XRD patterns of FeCo@NC and NiCo@NC are shown in Fig. S1 (seeing ESI). For FeCo@NC sample, the peaks at 45 and 51.6 can be
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attributed to FeCo alloy (JPCDS: 49-1567) and Co (JPCDS: 894307), respectively. For NiCo@NC sample, the peaks at 44.3 and 51.6 are ascribed to NiCo alloy (Co JPCDS: 89-4307, Ni JPCDS: 89-7128) [21,39]. To further identify the compositions and chemical states, their XPS spectra of MoxC-alloy@NC samples have been analyzed in detail (Fig. 1bed, Fig. S2-S3, seeing ESI). Fig. 1b shows the survey XPS spectrum of MoxC-FeCo@NC. The Fe 2p XPS spectrum (Fig. 1c) shows two major peaks at a low combining energy of 710.7 eV and a high combining energy of 724.2 eV, corresponding to Fe 2p3/2 and Fe 2p1/2 of Fe2þ, respectively. The formation of Fe2þ ions on the surface of FeCo may be due to the exposure to air [40,41]. For the Co 2p XPS spectrum (Fig. 1d, Fig. S2b, seeing ESI), the peaks at 781.3 eV and 796.8 eV could be caused by the formation of Co-C bond on the surface of FeCo or NiCo alloy. Similarly, for the Ni 2p XPS spectrum (Fig. 1d, Fig. S2c, seeing ESI), the peaks at 854.8 eV and 872.8 eV could be caused by the formation of Ni-C bond on the surface of NiCo alloy [42]. In addition, the fitting peaks of Mo 3d reveal that there are different oxidation states for Mo in MoxC-alloy@NC, namely, there are four oxidation states (Mo(II), Mo(III), Mo(IV), Mo(VI)) in MoC-NiCo@NC (Fig. S2d, seeing ESI), and two oxidation states (Mo(II), Mo(VI)) in MoxCFeCo@NC (Fig. S3a, seeing ESI). The Mo(II) and Mo(III) can be attributed to the Mo-C bonds in MoxC, whereas Mo(Ⅳ) and Mo(Ⅵ) are assigned to MoO2 and MoO3, respectively [34,43,44]. Wan et al. [34,45] and Xiang et al. [34,45] have reported that the surface of MoxC can be contaminated with molybdenum oxides (MoO2 and MoO3) when they are exposed to the air. For N XPS spectrum (Fig. S2f and Fig. S3c), two types of nitrogen species are present: pyridinic and quaternary nitrogen.[30] Fig. 2a shows that the MoxC-FeCo@NC sample consists of graphene-like NC layers and FeCo alloy nanoparticles. Further
Fig. 1 e (a) XRD patterns of MoxC-alloy@NC (alloy: FeCo and NiCo; NC: nitrogen-doped carbon) samples; (bed) XPS spectra of MoxC-FeCo@NC sample: (b) Survey spectrum; (c) Fe 2p spectrum; (d) Co 2p spectrum. Please cite this article as: Liu Z et al., First insight on Mo(II) as electrocatalytically active species for oxygen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.105
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Fig. 2 e (a,b) HRTEM images of MoxC-FeCo@NC; (c,d) HRTEM images of MoC-NiCo@NC.
HRTEM analysis (Fig. 2b) reveals that the FeCo alloy nanoparticle are supported by the graphite carbon, and the lattice spacing of 0.178 nm is ascribed to the (200) plane of FeCo alloy; the lattice spacings of 0.183 nm and 0.225 nm correspond to the (101) plane of MoC and the (101) plane of Mo2C, respectively. Likewise, Fig. 2c and d shows the HRTEM images of the MoC-NiCo@NC sample. The d spacings of 0.217 nm correspond to the (111) planes of NiCo alloy, and the d spacings of 0.249 nm is ascribed to the (100) plane of MoC. In order to evaluate the electrocatalytic activities of MoxCalloy@NC catalysts, the oxygen evolution reaction performances are investigated in 1 M KOH solution using a typical three-electrode system. Fig. 3a presents the polarization curves of MoxC-FeCo@NC and MoC-NiCo@NC. Generally, the potential at the current density of 10 mA cm2 is often used to evaluate the electrocatalytic properties of an electrocatalyst [5,12]. At 10 mA cm2, MoxC-FeCo@NC has a lower potential (1.548 V vs. RHE) than MoC-NiCo@NC (1.582 V vs. RHE), FeCo@NC (1.616 V vs. RHE) and NiCo@NC (1.712 V vs. RHE). At the same current density (10 mA cm2), MoxC-FeCo@NC has an obviously smaller overpotential (318 mV) than MoC-NiCo@NC (352 mV), FeCo@NC (390 mV), and NiCo@NC (480 mV) (Fig. 3b), illustrating that MoxC-FeCo@NC catalyst has a highest OER activity amongst. At 10 mA cm2, the MoxC-FeCo@NC catalyst has a smaller overpotential than those of the precious reported catalysts, e.g., RuO2 (~405 mV, 1 M KOH) [27], IrO2/C (360 mV, 1 M KOH) [46]. In addition, at the same overpotential (h ¼ 350 mV), the current density is 21 mA cm2 for MoxC-FeCo@NC, which is about 2.1, 6.56 and 32 times as high as those of MoC-NiCo@NC, FeCo@NC and NiCo@NC, respectively (Fig. 3b). To understand the different OER activities, their BET areas are measured and compared (Fig. S4, seeing ESI). Although the BET area of MoC-NiCo@NC is obviously higher than that of
MoxC-FeCo@NC, MoC-NiCo@NC has a lower overpotential than MoxC-FeCo@NC at 10 mA cm2. Therefore, the BET area is not the affecting factor for the OER activity. Bao et al. [21] have demonstrated that the electrocatalytic activity difference of alloy@NC is mainly attributed to the different alloy components. According to the XPS results, there are two oxidation states for Mo (Mo(II), Mo(VI)) in MoxC-FeCo@NC (Fig. S3a, seeing ESI), while four oxidation states for Mo (Mo(II), Mo(III), Mo(IV), Mo(VI)) in MoC-NiCo@NC (Fig. S2d, seeing ESI). The alloy as the OER active species has been widely reported [21,24e27]. And it has been widely reported that Mo(II) and Mo(III) are the electrocatalytic active centers for hydrogen evolution reaction (HER) [34,35], and the HER activity of MoCx is mostly dependent on the Mo(II) and Mo(III) active centers exposed on catalyst surface [29,35,36]. Because of the unique d-band structure, MoxC can exhibit the electronic and catalytic properties analogous to Pt-group metals [34,42,47,48]. For example, Wan et al. [34,49] and Michalsky et al. [34,49] have attributed the excellent HER performance to the electron configuration around Fermi level (EF) of Mo2C [34,49]; Through DFT calculation, Liu et al. [30] have confirmed that the excellent HER performance of Mo2C is mainly ascribed to the low adsorption free energy of H (DG(H*)). Mo(II) and Mo(III) as the HER active centers has been widely reported [29,35,36], however, Mo(II) and Mo(III) as the OER active centers has not been reported so far. The introduction of MoxC has greatly improved the OER performance, because MoxC can be hydroxylated at alkaline condition, then it can undergo an electrochemical oxidation process, resulting in the formation of MoOx as the OER active species [38,50,51]. It is still a big challenge to avoid the oxidation of MoxC during electrochemical process [52,53]. To insight into the nature of Mo(II) and Mo(III) as the OER active centers, their electron configurations were mainly investigated. The different
Please cite this article as: Liu Z et al., First insight on Mo(II) as electrocatalytically active species for oxygen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.105
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Fig. 3 e The electrocatalytic properties of MoxC-alloy@NC catalysts: (a) Polarization curves; (b) Current density at h ¼ 350 mV (i) and Overpotentials at 10 mA cm¡2 (ii); (c) Tafel plots; (d) Cycle stability for 2000 CV sweeps between 100 mV and 200 mV (vs. Ag/AgCl).
electronic configurations (4d4 5s0/Mo(II) and 4d35s0/Mo(III)) could result in different electron densities around Mo(II) and Mo(III). The electron density around Mo(III) is more positive than that around Mo(II), thus Mo(III) has a stronger adsorbing ability to OH than Mo(II). It is well known that the too strong or too weak adsorption does not favor for the catalytic reaction. Therefore, we could think that the OH adsorption on Mo(III) centers may be too strong to benefit the OER activity, and the OH adsorption on Mo(II) centers may be moderate to boost the OER activity of MoxC-FeCo@NC. To further understand the OER activity, Tafel plots are shown in Fig. 3c, and the slopes are obtained by the Tafel equation [16]. The Tafel slope is 80 mV/dec for MoxCFeCo@NC, which is obviously smaller than the MoC-NiCo@NC (186 mV/dec), FeCo@NC (85 mV/dec), and NiCo@NC (125 mV/ dec), respectively. The smaller Tafel slope of MoxC-FeCo@NC illustrates that the OER process can occur rapidly. It is wellknown that the smaller Tafel slope can result in a remarkable increment OER rate with overpotentials [11], which is beneficial for practical applications. Besides the OER activity, the durability is another crucial parameter for an excellent electrocatalyst. Fig. 3d shows the OER polarization curves for 2000 sweeps. After 2000 sweeps, the polarization curve of MoxC-FeCo@NC does not change nearly, compared with the initial one, whereas the polarization curve of MoC-NiCo@NC decays quickly, which may be due to the formed Ni-C bond on the surface of NiCo alloy that can cause an irreversible electrochemical reaction [42]. The results confirm a high stability of MoxC-FeCo@NC. To further investigate the effect of capacitance, the electrochemical double-layer capacitance (Cdl) of MoxC-alloy@NC catalysts are estimated by using a simple cyclic voltammetry method [17,54,55]. Fig. S5(a-d) (seeing ESI) shows the cyclic
voltammorgrams (CVs) at different scanning rates (20e120 mV s1) in the potential range of 0.1e0.2 V (vs. Ag/AgCl). The differences in current density (DJ ¼ Ja-Jc) at a potential of 0.15 V (vs. Ag/AgCl) against scanning rates are plotted to get the linear slope that is equivalent to twice of Cdl. As shown in Fig. 4a, MoxC-FeCo@NC has a Cdl of 1.03 mF cm2, which is ca. 2, 8 and 14 times higher than those of FeCo@NC (0.56 mF cm2), MoCNiCo@NC (0.2 mF cm2) and NiCo@NC (0.12 mF cm2), respectively. Usually, a large electrochemical active area favors for a high Cdl value and the ratio of current density to Cdl can reflect the intrinsic electrochemical activity [36]. As a result, MoxCFeCo@NC has a higher intrinsic electrochemical activity. Usually, the electrocatalytic activity is closely relative to the electrocatalytic kinetics [56], which can be revealed by electrochemical impedance spectroscopy (EIS). Fig. 4b and Fig. S6 (seeing ESI) shows the Nyquist plots of MoxC-alloy@NC catalysts, and the inset of Fig. 4b and Fig. S6 (seeing ESI) presents the simulated equivalent electric cicuit (EEC). The simulated EEC consists of an electrolyte solution resistance (Rs), a constant phase element (CPE) accounting for the electrical double-layer capacitance, a charge transfer resistance (Rct) and a Warburg impedance (W) accounting for the diffusion resistance. In the Nyquist plots, the semicircle in the lowfrequency range represents Rct, while Rs appears in the highfrequency range [56]. On one hand, the Rs values were measured to be 2.56 U, 3.59 U, 29.09 U, and 14.25 U for MoxCFeCo@NC, MoC-NiCo@NC, FeCo@NC, and NiCo@NC electrodes, respectively. A low Rs value indicates that the resistance of electrolyte solution is not susceptible to the electrode materials. On the other hand, the Rct values were calculated to be 15.67 U, 28.21 U, 29.47 U and 6848 U for MoxC-FeCo@NC, MoC-NiCo@NC, FeCo@NC and NiCo@NC electrodes, respectively. A low Rct value of MoxC-FeCo@NC indicates a faster OER
Please cite this article as: Liu Z et al., First insight on Mo(II) as electrocatalytically active species for oxygen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.105
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Fig. 4 e (a) Capacitive current density differences (DJ ¼ Ja-Jc) against scan rates at a potential of 0.15 V (vs. Ag/AgCl). (b) Nyquit plots (the inset of equivalent electric circuit).
et al. [59] have reported that the Mo(IV) (MoO2) has excellent OER activity; and Dong et al. [60] have also reported that the Mo(III) (MoC) can enhance the OER activity. Thus, the enhanced OER activity are mainly attributed to Mo(II), Mo(III) and Mo(IV) species, rather than Mo(VI) species. In addition, the electron density around Mo(III) and Mo(IV) is more positive than that around Mo(II). As a result, Mo(III) and Mo(IV) have a stronger adsorption ability to OH than Mo(II), which may not favor for the OER reaction; Mo(II) has a moderate adsorption ability to OH, leading to an improved OER reaction. Summarily, FeCo or NiCo alloy and MoxC (e.s.p. Mo(II)) are the main OER active sites, which synergistically enhanced the OER activity, and NC mainly enhances the conductivity and makes the alloy nanoparticles distribute uniformly. Fig. 5 e A plausible OER mechanism on MoxC-alloy@NC catalysts in alkaline medium.
Conclusions
rate. Obviously the Rct value of MoxC-FeCo@NC is smaller than the others, consistent with the OER activity. On base of the results above, we have proposed a plausible OER mechanism on MoxC-alloy@NC electrocatalyst in alkaline solution. The OER process proceeds through four singleelectron transfer steps (Fig. 5). The overall OER mechanism on MoxC-alloy@NC in alkaline medium involves the following steps [57]. Several intermediates including *OH, *O and *OOH could be involved in the charge-transfer process. * þ OH /*OH þ e
(1)
*OH þ OH /H2 O þ *O þ e
(2)
*O þ OH /*OOH þ e
(3)
*OOH þ OH /*O2 þ H2 O þ e
(4)
*O2 /O2 þ e
(5)
For MoxC-FeCo@NC, there are two oxidation states for Mo (Mo(II), Mo(VI)), and its OER activity is higher than that of the FeCo@NC (Fig. 3a). While four oxidation states (Mo(II), Mo(III), Mo(IV), Mo(VI)) for MoC-NiCo@NC, it also shows a better OER activity than that of the NiCo@NC (Fig. 3a). Yang et al. [58] have demonstrated that the Mo(VI) (MoO3) has no OER activity; Cui
In summary, MoxC-alloy@NC electrocatalysts can be synthesized by a simple thermolysis method. MoxC-FeCo@NC electrocatalysts shows a small charge transfer resistance, a small Tafel slope and a small overpotential in KOH solution. The introduction of MoxC has greatly improved the OER performance and the Mo(II) species could act as the OER active site.
Acknowledgements This work is financially supported by National Science Foundation of China (21377060), the Project of Science and Technology Infrastructure of Jiangsu (BM201380277), the Key Project of Environmental Protection Program of Jiangsu (2013005), Jiangsu province graduate training innovation project (SJCX17_0262, SJZZ16_0153, KYCX17_0894, KYCX17_0895), Jiangsu province undergraduate practice and innovation project (201810300043Z, 201810300240), Six Talent Climax Foundation of Jiangsu (20100292), “333” Outstanding Youth Scientist Foundation of Jiangsu (20112015).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.11.105.
Please cite this article as: Liu Z et al., First insight on Mo(II) as electrocatalytically active species for oxygen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.105
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Please cite this article as: Liu Z et al., First insight on Mo(II) as electrocatalytically active species for oxygen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.11.105