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Co0.85Se on three-dimensional hierarchical porous graphene-like carbon for highly effective oxygen evolution reaction
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Co0.85Se on three-dimensional hierarchical porous graphene-like carbon for highly effective oxygen evolution reaction Qi-Sui Zhong a, Wei-Yan Xia b, Bo-Cai Liu a, Chang-Wei Xu a,*, Nan Li a,** a b
School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 51006, China The Experimental School Affiliated to Guangzhou College, Guangzhou 510430, China
article info
abstract
Article history:
Designing an efficient, cheap and abundant catalyst for oxygen evolution reaction (OER) is
Received 7 January 2019
crucial for the development of sustainable energy sources. A novel catalyst which could be
Received in revised form
a promising candidate for such electrocatalysts is described. Co0.85Se supported on three-
27 February 2019
dimensional hierarchical porous graphene-like carbon (HPG) exhibits outstanding catalytic
Accepted 2 March 2019
performances for OER in alkaline medium. It is found that the onset overpotential is
Available online 31 March 2019
311 mV on the Co0.85Se/HPG electrode, which is more 28 and 41 mV negative than that on the Co/HPG and Co3O4/HPG electrodes. What's more, the value of Tafel slope is 61.7 mV
Keywords:
dec1 and the overpotential at the current density of 10 mA cm2 is 385 mV on this elec-
Water splitting
trode. The Co0.85Se/HPG of this work is an appealing electrocatalyst for OER in basic
Oxygen evolution
electrolyte.
Co selenides
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Co0.85Se Graphene
Introduction As fossil fuels continuously devoured and environmental pollution increasingly worsened, researching alternative energy has attracted more and more attention. Hydrogen, a clean and renewable energy carrier, is expected to be an alternative energy carrier due to its high energy density, rich source as well as simple and clean production after perfect combustion. However, hydrogen is widely produced by the methane reforming reaction which also uses fossil fuels as raw materials, so the energy challenges which are similar to other non-renewable sectors still exist [1,2]. By contrast, hydrogen generated from electrochemical water splitting can
effectively ameliorate this problem, especially when the renewable energy such as solar and wind energy systems are used to drive the reaction [3,4]. Therefore, hydrogen with electrochemical water splitting is an appealing and promising method to continually produce hydrogen. Electrochemical water splitting in alkaline solution consists of two half reactions: the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER). However, the OER requires four-electron transfer process [5], which leads to kinetically sluggish for this reaction. Therefore, the OER needs high overpotential for the water splitting that is the main reason of the high energy consumption. Hence, many researchers make great efforts to find
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C.-W. Xu), [email protected] (N. Li). https://doi.org/10.1016/j.ijhydene.2019.03.003 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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electrocatalysts which can effectively decrease the overpotential of OER. Recently the non-noble metal oxides such as nickel oxide [6] and cobalt oxide [7] have been extensively investigated, due to their high earth-abundance and small overpotential of the OER, so they are good substitutes of RuO2 and IrO2 catalysts in commercial electrolyzers. Meanwhile, metal (Fe, Co, Ni and their mixed versions) phosphides, sulfides, and selenides [8e13], especially of the Ni and Co selenides such as NiSe2 [14,15], Ni3Se2 [16,17], NiSe [18], Ni0.85Se [19], Ni3Se4 [20], CoSe2 [21e23], CoSe [24,25], Co0.85Se [26e29] and (Ni,Co)Se2 [30,31], have also engaged increasing attention. Co selenides show outstanding catalytic performances for OER in alkaline medium, and moreover, the elements of Co and Se are abundant on earth. For instance, Liu et al. [32] have reported that CoSe2 nanosheets, with low overpotential of 320 mV at a current density of 10 mA cm2, are the best catalytically active material among the Co-based electrocatalysts. For another example, the Co0.85Se nanosheets, with hexagonal structure and half-metallic characterization [33,34], exposing rich active sites are better for absorbing the OH through the strong electrostatic affinity with Co2þ and Co3þ [29], so their catalytic activity can be improved. Pure Co selenides have poor active site exposure yield. Co selenides using carbon materials as a carrier can give more active sites to improve charge and mass transfer for reaction [26,27]. Yang et al. [35] reported that the CNTs as conductive substrates can raise the electrical conductivity of CoS2, so that the electrochemical activity of CNT-CoS2 is comparable or even better than that of the RuO2 and IrO2 for OER. Currently, graphene is reported to be a new-generation supporting material due to its ultrathin nanosheet structure and high electrical conductivity [36]. It is reported that there is a positive coordinated effect between Co0.85Se and graphene [34]. Yang et al. [37] reported that CoSe2 supported on the graphene exhibits higher catalytic performance than CoSe2 without graphene for OER. Compared with RuO2, CoSe2 nanobelts anchored on nitrogen-doped graphene (NG) appear more remarkable electrocatalytic performances for OER with a small Tafel slope of 40 mV dec1 and a low overpotential of 366 mV at the current density of 10 mA cm2 [38]. Nevertheless, the strong van der Waals force between individual graphene layer leads to irreversibly aggregate, so the activity of graphene will be decreased gradually. At present, the threedimensional hierarchical porous architecture graphene-like carbon (3D HPG) relying on its self-supporting structure can effectively prevent the aggregation of graphene. In this paper, the activity of Co0.85Se supported on the 3D HPG for OER will be reported. It has been researched that the HPG as a support, has the large specific surface area and the strong cohesive force with catalyst nanoparticles [39e41]. Additionally, the 3D HPG also exhibits great electrical conductivity and distribution effect [42].
Experiment All chemical reagents were of analytical purity from SigmaAldrich. The HPG was prepared by the reported method [40,41]. The Co0.85Se/HPG was synthesized as following method [43,44]. CoCl2$6H2O (0.238 g) and 0.263 g
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Na2SeO3$5H2O were dissolved in 60 mL distilled water. Then 8 mL N2H4$H2O (80%) and 0.26 g HPG were added in turn under stirring. These precursors were added to 100 mL Teflon-lined stainless autoclave and maintained at 140 C for 24 h. After being restored at room temperature, the products were centrifugated, washed with distilled water and absolute ethanol and dried at 60 C for 12 h under vacuum. The Co/HPG and Se/ HPG were prepared by the same method just without Na2SeO3$5H2O and CoCl2$6H2O. The Co3O4/HPG was synthesized by the hydrothermal method [45]. CoCl2$6H2O (5.0 mmol), 1.0 mmol CO(NH2)2 and 1.0 mmol NH4F were dissolved in 40 mL distilled water to form homogeneous solution and put into a 50 mL Teflon-lined stainless autoclave. The Teflon-lined stainless autoclave was maintained at 120 C for 6 h, and then cooled at room temperature. The obtained precipitate was centrifugated, washed with distilled water and absolute ethanol and dried at 60 C for 12 h. After being dried, the precipitate was calcined at 400 C for 2 h. X-ray diffraction (XRD) patterns were recorded on the Xray diffractometer (PW3040/60, PANalytical) using Cu Ka radiation (l ¼ 0.15418 nm). Scanning electron microscopy (SEM) images were carried out on the Quanta 400 FEG microscope (FEI Company). Transmission electron microscopy (TEM) images were obtained from the JEOL JEM-2010 microscope (JEOL Ltd.). X-ray photoelectron spectroscopy (XPS) spectra was performed by using the ESCALAB 250 spectrometer (ThermoFisher Scientific, USA). All electrochemical measurements were carried out in a three-electrode cell by the CHI 760D electrochemical workstation in a temperatureecontrolled water bath at 25 C. The working electrodes were prepared by the electrocatalyst powders with 5 wt% PTFE (polytetrafluoroethylene) loaded on the surface of a graphite rod with a geometric area of 0.33 cm2. The loading of electrocatalyst powders on the electrodes was accurately controlled at 0.1 mg cm2. A platinum foil (3.0 cm2) electrode and a saturated calomel electrode (SCE, 0.241 V versus NHE) were used as counter electrode and referee electrode.
Results and discussions XRD patterns of the Se/HPG, Co/HPG, Co0.85Se/HPG and Co3O4/ HPG are depicted as shown in Fig. 1. The diffraction peak at around 26.4 shown in the four materials is indexed as the (002) face of graphene [46]. Diffraction peaks of the Co0.85Se/ HPG at 33.7, 45.2, 51.5, 60.4, 62.9 and 70.4 are corresponding to the (101), (102), (110), (103), (112) and (202) faces of the hexagonal phase Co0.85Se (JDCPDS 52-1008) [47]. In addition, the diffraction peaks of Se/HPG, Co/HPG and Co3O4/HPG are corresponding to their own XRD standard cards. The 3D HPG exhibits a unique three-dimensional interconnected porous structure with the 4e6 nm thickness graphene nanosheets (Fig. 2a) and that is further revealed in the TEM image of Fig. 2b. As shown in Fig. 2c and d, the Co0.85Se with a nanosheet structure like graphene is consistent with the previous report [43,44], and it is well dispersed on the surface of graphene nanosheets in the HPG. In addition, the parallel fringe with a spacing of 0.33 and 0.27 nm are assigned to the (002) face of graphene and the (101) face of the
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Fig. 1 e XRD patterns of Se/HPG, Co/HPG, Co0.85Se/HPG and Co3O4/HPG. hexagonal phase Co0.85Se respectively. From the HRTEM, the Co0.85Se has a high crystalline nature and the ultrathin Co0.85Se nanosheets are evenly distributed on the graphene nanosheets. Furthermore, the diffraction rings of selectedarea electron diffraction (SAED) pattern are corresponding to the (101) and (102) crystalline faces of Co0.85Se (inset figure of Fig. 2d), proving it with high crystallinity. Chemical bonding states of the elements in the Co0.85Se/ HPG were tested by XPS (Fig. 3). The characteristic peaks in the survey spectrum are assigned to Se 3d, C 1s, O 1s and Co 2p (Fig. 3a). The binding energy of C 1s peak centered at 284.7 eV is attached to the graphitic carbon of HPG. The higher
resolution C 1s peak of Co0.85Se/HPG is shown in Fig. 3b. The peaks at 284.7, 285.2 and 286.2 eV are related to the CeC, epoxy CeO and carbonyl C]O groups [48,49]. Compared with the C 1s peak of initial HPG, the peak intensity of CeO and C]O species decreasing is due to the oxygen atoms of oxygencontaining groups stabilizing Co2þ on the surface of HPG. In Fig. 3c, the XPS spectrum of Co 2p can be corresponding to Co 2p1/2, Co 2p3/2 and two satellite peaks (marked as “Sat.”) [49,50]. By performing a peak fitting, there are six peaks including three peaks for the Co 2p3/2 and three peaks for the Co 2p1/2. The binding energy values of 778.6, 782.4, 793.4 and 798.0 eV are assigned to Co3þ 2p3/2, Co2þ 2p3/2, Co3þ 2p1/2 and Co2þ 2p1/2 respectively, so both Co3þ and Co2þ are in the material [51]. In addition, the Co2þ/Co3þ ratio is 8.4 by calculating correlative peaks area [52,53]. From XPS spectrum of Se 3d, the binding energy values of 55.1 and 59.4 eV are assigned to the Se 3d5/2 and Se 3d7/2 [47,50,54,55] as shown in Fig. 3d. The results of XRD and XPS demonstrate that the Co0.85Se is successfully composed on the surface of 3D HPG. In order to illustrate the electrochemical performances of Co0.85Se/HPG and the OER activity of Se/HPG, Co/HPG, Co0.85Se/HPG and Co3O4/HPG, these catalysts are tested by linear sweep voltammetry (LSV) in 0.1 mol L1 KOH (a sweep rate of 0.001 V s1). The loading of catalyst is 0.1 mg cm2. It is reported that the Co0.85Se favors a desirable four electron pathway for OER (4OH / O2 þ 2H2O þ 4e) [56]. The OER performance of Co0.85Se/HPG is sketched as Fig. 4. Here, the LSV curves are obtained after the IR compensation using the corresponding solution resistance (Rs) for all the electrodes [57]. Compared with the Co/HPG, Co0.85Se/HPG and Co3O4/ HPG, the Se/HPG shows a very low performance of OER (Fig. 5a), so the activity of Se/HPG will not be discussed. The
Fig. 2 e a) SEM image of 3D HPG，b) TEM image of 3D HPG, c,d) TEM images of Co0.85Se/HPG. SAED pattern in the inset figure of Fig. 2d.
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Fig. 3 e XPS of a) survey, b) C 1s, c) Co 2p and d) Se 3d for Co0.85Se/HPG.
Fig. 4 e Graphical sketch that illustrates the Exceptional OER Performance of Co0.85Se/HPG. onset overpotential (h) of the Co0.85Se/HPG is 311 mV, which is more 28 and 41 mV negative than that of the Co/HPG (339 mV) and Co3O4/HPG (352 mV) (Fig. 5b). As shown in inset figure of Fig. 5b, the value of Tafel slope is 61.7 mV dec1 on the Co0.85Se/HPG electrode, which is obviously lower than that on the Co/HPG electrode (80.1 mV dec1) and close to that on the
Co3O4/HPG electrode (50.1 mV dec1). This performance is also better than the performance of reported EG/Co0.85Se (73 mV dec1) [58], Co0.85Se@NC (75 mV dec1) [56] and Co0.85Se (85 mV dec1) [59] as shown in Table 1. Moreover, the value of Tafel slope on the Co0.85Se/HPG is close to that on the Pt (20 wt%)/C (60 mV dec1) [21], Co3O4@Co/NCNT (61 mV dec1) [60], distinctly lower than that on the mesoporous Co3O4 (72.2 mV dec1) [7], NiCo-selenide urchin-like microspheres (89 mV dec1) [9] in basic electrolyte. The current density at 0.7 V (overpotential 480 mV) is 17.1 mA cm2 on the Co0.85Se/HPG electrode, which is 1.9 and 3.2 times as high as that on the Co/ HPG electrode (9.1 mA cm2) and Co3O4/HPG electrode (5.3 mA cm2). It is brief and convenient to evaluate the electrocatalyst properties by the overpotentials at the current density of 10 mA cm2 [31]. At that current density, the overpotential of Co0.85Se/HPG is 385 mV, which is obviously lower than that of the Co/HPG (442 mV). The activity of support material of HPG is studied by the Co3O4/HPG and Co3O4/C (Carbon Vulcan XC-72) [7,61]. It is notable that the OER performance on the Co3O4/HPG electrode is much higher than that on the Co3O4/C electrode in Fig. 5b. Although the overpotential of the Co3O4/HPG (480 mV) is close to that of the Co3O4/C, the current density at 0.7 V on the Co3O4/HPG electrode is 5.3 mA cm2, higher than that on the Co3O4/C (3.9 mA cm2) [7,58]. The reason of high activity is that the ultrathin graphene nanosheets has large enough surface area to touch catalyst nanoparticles, and thereby the interfacial electron transfer is promoted. Moreover, Co0.85Se
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Fig. 5 e LSV curves on Se/HPG, Co/HPG, Co0.85Se/HPG and Co3O4/HPG electrodes in 0.1 mol L¡1 KOH with a sweep rate of 0.001 V s¡1. Tafel plots in the inset figure of Fig. 5b.
nanosheets are uniformly dispersed on the graphene nanosheets, so more active sites are exposed and electrical conduction paths are increased. The electrochemical impedance responses (EIS) of Co0.85Se/HPG, Co/HPG, Co3O4/HPG and Se/ HPG are shown in Fig. S1. The solution resistance (Rs, ohmic resistance) and the charge transfer resistances (Rct) are obtained from the fitted equivalent circuit (Fig. S2) [62,63] and listed in Table S1. The Co0.85Se/HPG shows less charge transfer resistance (6.8 U). The stability for OER was researched by chronoamperometry in 0.1 mol L1 KOH at 0.7 V (overpotential 480 mV) shown in Fig. 6. The Co0.85Se/HPG exhibits a slow current decay compared with the other electrocatalysts. During the OER test, small oxygen bubbles aggregating on the electrode surface and occupying the active sites where the catalyst contacts with electrolyte result in slow decrease of oxidation current. The active sites of electrode surface are exposed again until the oxygen bubbles have grown large enough to leave the surface of electrode. The generation and release of oxygen lead to perturbation and ‘current waves’ in the chronoamperometry curves. After testing, the oxidation current density on the Co0.85Se/HPG electrode is 6.1 mA cm2, which is larger than that on the Co/HPG electrode (3.5 mA cm2) and the Co3O4/HPG electrode (2.6 mA cm2). The 3D HPG shows great electrical conductivity, distribution effect and strong cohesive force with nanoparticles. Meanwhile, its ultrathin nanosheets can stabilize other nanoparticles to remain high activity and good stability for OER. The Co0.85Se nanosheets dispersed on graphene nanosheets give the remarkable OER performance as following: (i) the interconnected porous network structure exposing
Table 1 e Comparison of this work with reported Co0.85Se in the Tafel slope. Tafel slope/(mV dec1) Co0.85Se/HPG (this work) [58] EG/Co0.85Se [56] Co0.85Se@NC [27] Co0.85Se/NF [59] (Ni, Co)0.85Se [59] Co0.85Se
61.7 73.0 75.0 78.9 79.0 85.0
Fig. 6 e Chronoamperomety curves on Co/HPG, Co0.85Se/ HPG and Co3O4/HPG electrodes in 0.1 mol L¡1 KOH at a potential of 0.7 V (overpotential 480 mV).
abundant active sites and promoting the oxygen and electrolyte diffusion and the charge transfer; (ii) Co0.85Se uniformly dispersed on the graphene nanosheets and increasing electrical conduction paths [55].
Conclusions The Co0.85Se using 3D HPG as the carrier has large specific surface area and exhibits outstanding electrocatalytic activity and stability for OER. The 3D HPG with 4e6 nm thickness graphene nanosheets shows a unique three-dimensional interconnected porous structure. The Co0.85Se, the structure of which is similar to the graphene nanosheet, is homogeneously dispersed on the graphene nanosheets surface. In addition, the Co0.85Se/HPG shows the lowest onset overpotential of 311 mV for OER. On the Co0.85Se/HPG electrode, the value of Tafel slope is 61.7 mV dec1, which is lower than that on the Co/HPG electrode and close to that on the Co3O4/ HPG electrode. Meanwhile, the current density at 0.7 V (overpotential 480 mV) is 17.1 mA cm2, which is higher than that on the other electrodes. The overpotential at the current density of 10 mA cm2 is 385 mV on the Co0.85Se/HPG electrode. In the HPG, the ultrathin graphene nanosheets has large
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enough surface area to touch catalyst nanoparticles, and thereby the interfacial electron transfer is promoted. The main reason of improvement of activity and stability is that Co0.85Se nanosheets are uniformly dispersed on the graphene nanosheets, which are better for exposing active sites and increasing electrical conduction paths. The Co0.85Se/HPG of this work is an appealing electrocatalyst for OER in alkaline medium, which could be the candidate of efficient, cheap and abundant catalysts.
Acknowledgments This work was financially supported by the Natural Science Foundation of Guangdong Province (2014A030313521), Scientific Research Foundation for Yangcheng Scholar (1201561607).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.03.003.
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Co0.85Se on three-dimensional hierarchical porous graphene-like carbon for highly effective oxygen evolution reaction Qi-Sui Zhong a, Wei-Yan Xia b, Bo-Cai Liu a, Chang-Wei Xu a,*, Nan Li a,** a b
School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 51006, China The Experimental School Affiliated to Guangzhou College, Guangzhou 510430, China
article info
abstract
Article history:
Designing an efficient, cheap and abundant catalyst for oxygen evolution reaction (OER) is
Received 7 January 2019
crucial for the development of sustainable energy sources. A novel catalyst which could be
Received in revised form
a promising candidate for such electrocatalysts is described. Co0.85Se supported on three-
27 February 2019
dimensional hierarchical porous graphene-like carbon (HPG) exhibits outstanding catalytic
Accepted 2 March 2019
performances for OER in alkaline medium. It is found that the onset overpotential is
Available online 31 March 2019
311 mV on the Co0.85Se/HPG electrode, which is more 28 and 41 mV negative than that on the Co/HPG and Co3O4/HPG electrodes. What's more, the value of Tafel slope is 61.7 mV
Keywords:
dec1 and the overpotential at the current density of 10 mA cm2 is 385 mV on this elec-
Water splitting
trode. The Co0.85Se/HPG of this work is an appealing electrocatalyst for OER in basic
Oxygen evolution
electrolyte.
Co selenides
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Co0.85Se Graphene
Introduction As fossil fuels continuously devoured and environmental pollution increasingly worsened, researching alternative energy has attracted more and more attention. Hydrogen, a clean and renewable energy carrier, is expected to be an alternative energy carrier due to its high energy density, rich source as well as simple and clean production after perfect combustion. However, hydrogen is widely produced by the methane reforming reaction which also uses fossil fuels as raw materials, so the energy challenges which are similar to other non-renewable sectors still exist [1,2]. By contrast, hydrogen generated from electrochemical water splitting can
effectively ameliorate this problem, especially when the renewable energy such as solar and wind energy systems are used to drive the reaction [3,4]. Therefore, hydrogen with electrochemical water splitting is an appealing and promising method to continually produce hydrogen. Electrochemical water splitting in alkaline solution consists of two half reactions: the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER). However, the OER requires four-electron transfer process [5], which leads to kinetically sluggish for this reaction. Therefore, the OER needs high overpotential for the water splitting that is the main reason of the high energy consumption. Hence, many researchers make great efforts to find
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C.-W. Xu), [email protected] (N. Li). https://doi.org/10.1016/j.ijhydene.2019.03.003 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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electrocatalysts which can effectively decrease the overpotential of OER. Recently the non-noble metal oxides such as nickel oxide [6] and cobalt oxide [7] have been extensively investigated, due to their high earth-abundance and small overpotential of the OER, so they are good substitutes of RuO2 and IrO2 catalysts in commercial electrolyzers. Meanwhile, metal (Fe, Co, Ni and their mixed versions) phosphides, sulfides, and selenides [8e13], especially of the Ni and Co selenides such as NiSe2 [14,15], Ni3Se2 [16,17], NiSe [18], Ni0.85Se [19], Ni3Se4 [20], CoSe2 [21e23], CoSe [24,25], Co0.85Se [26e29] and (Ni,Co)Se2 [30,31], have also engaged increasing attention. Co selenides show outstanding catalytic performances for OER in alkaline medium, and moreover, the elements of Co and Se are abundant on earth. For instance, Liu et al. [32] have reported that CoSe2 nanosheets, with low overpotential of 320 mV at a current density of 10 mA cm2, are the best catalytically active material among the Co-based electrocatalysts. For another example, the Co0.85Se nanosheets, with hexagonal structure and half-metallic characterization [33,34], exposing rich active sites are better for absorbing the OH through the strong electrostatic affinity with Co2þ and Co3þ [29], so their catalytic activity can be improved. Pure Co selenides have poor active site exposure yield. Co selenides using carbon materials as a carrier can give more active sites to improve charge and mass transfer for reaction [26,27]. Yang et al. [35] reported that the CNTs as conductive substrates can raise the electrical conductivity of CoS2, so that the electrochemical activity of CNT-CoS2 is comparable or even better than that of the RuO2 and IrO2 for OER. Currently, graphene is reported to be a new-generation supporting material due to its ultrathin nanosheet structure and high electrical conductivity [36]. It is reported that there is a positive coordinated effect between Co0.85Se and graphene [34]. Yang et al. [37] reported that CoSe2 supported on the graphene exhibits higher catalytic performance than CoSe2 without graphene for OER. Compared with RuO2, CoSe2 nanobelts anchored on nitrogen-doped graphene (NG) appear more remarkable electrocatalytic performances for OER with a small Tafel slope of 40 mV dec1 and a low overpotential of 366 mV at the current density of 10 mA cm2 [38]. Nevertheless, the strong van der Waals force between individual graphene layer leads to irreversibly aggregate, so the activity of graphene will be decreased gradually. At present, the threedimensional hierarchical porous architecture graphene-like carbon (3D HPG) relying on its self-supporting structure can effectively prevent the aggregation of graphene. In this paper, the activity of Co0.85Se supported on the 3D HPG for OER will be reported. It has been researched that the HPG as a support, has the large specific surface area and the strong cohesive force with catalyst nanoparticles [39e41]. Additionally, the 3D HPG also exhibits great electrical conductivity and distribution effect [42].
Experiment All chemical reagents were of analytical purity from SigmaAldrich. The HPG was prepared by the reported method [40,41]. The Co0.85Se/HPG was synthesized as following method [43,44]. CoCl2$6H2O (0.238 g) and 0.263 g
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Na2SeO3$5H2O were dissolved in 60 mL distilled water. Then 8 mL N2H4$H2O (80%) and 0.26 g HPG were added in turn under stirring. These precursors were added to 100 mL Teflon-lined stainless autoclave and maintained at 140 C for 24 h. After being restored at room temperature, the products were centrifugated, washed with distilled water and absolute ethanol and dried at 60 C for 12 h under vacuum. The Co/HPG and Se/ HPG were prepared by the same method just without Na2SeO3$5H2O and CoCl2$6H2O. The Co3O4/HPG was synthesized by the hydrothermal method [45]. CoCl2$6H2O (5.0 mmol), 1.0 mmol CO(NH2)2 and 1.0 mmol NH4F were dissolved in 40 mL distilled water to form homogeneous solution and put into a 50 mL Teflon-lined stainless autoclave. The Teflon-lined stainless autoclave was maintained at 120 C for 6 h, and then cooled at room temperature. The obtained precipitate was centrifugated, washed with distilled water and absolute ethanol and dried at 60 C for 12 h. After being dried, the precipitate was calcined at 400 C for 2 h. X-ray diffraction (XRD) patterns were recorded on the Xray diffractometer (PW3040/60, PANalytical) using Cu Ka radiation (l ¼ 0.15418 nm). Scanning electron microscopy (SEM) images were carried out on the Quanta 400 FEG microscope (FEI Company). Transmission electron microscopy (TEM) images were obtained from the JEOL JEM-2010 microscope (JEOL Ltd.). X-ray photoelectron spectroscopy (XPS) spectra was performed by using the ESCALAB 250 spectrometer (ThermoFisher Scientific, USA). All electrochemical measurements were carried out in a three-electrode cell by the CHI 760D electrochemical workstation in a temperatureecontrolled water bath at 25 C. The working electrodes were prepared by the electrocatalyst powders with 5 wt% PTFE (polytetrafluoroethylene) loaded on the surface of a graphite rod with a geometric area of 0.33 cm2. The loading of electrocatalyst powders on the electrodes was accurately controlled at 0.1 mg cm2. A platinum foil (3.0 cm2) electrode and a saturated calomel electrode (SCE, 0.241 V versus NHE) were used as counter electrode and referee electrode.
Results and discussions XRD patterns of the Se/HPG, Co/HPG, Co0.85Se/HPG and Co3O4/ HPG are depicted as shown in Fig. 1. The diffraction peak at around 26.4 shown in the four materials is indexed as the (002) face of graphene [46]. Diffraction peaks of the Co0.85Se/ HPG at 33.7, 45.2, 51.5, 60.4, 62.9 and 70.4 are corresponding to the (101), (102), (110), (103), (112) and (202) faces of the hexagonal phase Co0.85Se (JDCPDS 52-1008) [47]. In addition, the diffraction peaks of Se/HPG, Co/HPG and Co3O4/HPG are corresponding to their own XRD standard cards. The 3D HPG exhibits a unique three-dimensional interconnected porous structure with the 4e6 nm thickness graphene nanosheets (Fig. 2a) and that is further revealed in the TEM image of Fig. 2b. As shown in Fig. 2c and d, the Co0.85Se with a nanosheet structure like graphene is consistent with the previous report [43,44], and it is well dispersed on the surface of graphene nanosheets in the HPG. In addition, the parallel fringe with a spacing of 0.33 and 0.27 nm are assigned to the (002) face of graphene and the (101) face of the
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Fig. 1 e XRD patterns of Se/HPG, Co/HPG, Co0.85Se/HPG and Co3O4/HPG. hexagonal phase Co0.85Se respectively. From the HRTEM, the Co0.85Se has a high crystalline nature and the ultrathin Co0.85Se nanosheets are evenly distributed on the graphene nanosheets. Furthermore, the diffraction rings of selectedarea electron diffraction (SAED) pattern are corresponding to the (101) and (102) crystalline faces of Co0.85Se (inset figure of Fig. 2d), proving it with high crystallinity. Chemical bonding states of the elements in the Co0.85Se/ HPG were tested by XPS (Fig. 3). The characteristic peaks in the survey spectrum are assigned to Se 3d, C 1s, O 1s and Co 2p (Fig. 3a). The binding energy of C 1s peak centered at 284.7 eV is attached to the graphitic carbon of HPG. The higher
resolution C 1s peak of Co0.85Se/HPG is shown in Fig. 3b. The peaks at 284.7, 285.2 and 286.2 eV are related to the CeC, epoxy CeO and carbonyl C]O groups [48,49]. Compared with the C 1s peak of initial HPG, the peak intensity of CeO and C]O species decreasing is due to the oxygen atoms of oxygencontaining groups stabilizing Co2þ on the surface of HPG. In Fig. 3c, the XPS spectrum of Co 2p can be corresponding to Co 2p1/2, Co 2p3/2 and two satellite peaks (marked as “Sat.”) [49,50]. By performing a peak fitting, there are six peaks including three peaks for the Co 2p3/2 and three peaks for the Co 2p1/2. The binding energy values of 778.6, 782.4, 793.4 and 798.0 eV are assigned to Co3þ 2p3/2, Co2þ 2p3/2, Co3þ 2p1/2 and Co2þ 2p1/2 respectively, so both Co3þ and Co2þ are in the material [51]. In addition, the Co2þ/Co3þ ratio is 8.4 by calculating correlative peaks area [52,53]. From XPS spectrum of Se 3d, the binding energy values of 55.1 and 59.4 eV are assigned to the Se 3d5/2 and Se 3d7/2 [47,50,54,55] as shown in Fig. 3d. The results of XRD and XPS demonstrate that the Co0.85Se is successfully composed on the surface of 3D HPG. In order to illustrate the electrochemical performances of Co0.85Se/HPG and the OER activity of Se/HPG, Co/HPG, Co0.85Se/HPG and Co3O4/HPG, these catalysts are tested by linear sweep voltammetry (LSV) in 0.1 mol L1 KOH (a sweep rate of 0.001 V s1). The loading of catalyst is 0.1 mg cm2. It is reported that the Co0.85Se favors a desirable four electron pathway for OER (4OH / O2 þ 2H2O þ 4e) [56]. The OER performance of Co0.85Se/HPG is sketched as Fig. 4. Here, the LSV curves are obtained after the IR compensation using the corresponding solution resistance (Rs) for all the electrodes [57]. Compared with the Co/HPG, Co0.85Se/HPG and Co3O4/ HPG, the Se/HPG shows a very low performance of OER (Fig. 5a), so the activity of Se/HPG will not be discussed. The
Fig. 2 e a) SEM image of 3D HPG，b) TEM image of 3D HPG, c,d) TEM images of Co0.85Se/HPG. SAED pattern in the inset figure of Fig. 2d.
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Fig. 3 e XPS of a) survey, b) C 1s, c) Co 2p and d) Se 3d for Co0.85Se/HPG.
Fig. 4 e Graphical sketch that illustrates the Exceptional OER Performance of Co0.85Se/HPG. onset overpotential (h) of the Co0.85Se/HPG is 311 mV, which is more 28 and 41 mV negative than that of the Co/HPG (339 mV) and Co3O4/HPG (352 mV) (Fig. 5b). As shown in inset figure of Fig. 5b, the value of Tafel slope is 61.7 mV dec1 on the Co0.85Se/HPG electrode, which is obviously lower than that on the Co/HPG electrode (80.1 mV dec1) and close to that on the
Co3O4/HPG electrode (50.1 mV dec1). This performance is also better than the performance of reported EG/Co0.85Se (73 mV dec1) [58], Co0.85Se@NC (75 mV dec1) [56] and Co0.85Se (85 mV dec1) [59] as shown in Table 1. Moreover, the value of Tafel slope on the Co0.85Se/HPG is close to that on the Pt (20 wt%)/C (60 mV dec1) [21], Co3O4@Co/NCNT (61 mV dec1) [60], distinctly lower than that on the mesoporous Co3O4 (72.2 mV dec1) [7], NiCo-selenide urchin-like microspheres (89 mV dec1) [9] in basic electrolyte. The current density at 0.7 V (overpotential 480 mV) is 17.1 mA cm2 on the Co0.85Se/HPG electrode, which is 1.9 and 3.2 times as high as that on the Co/ HPG electrode (9.1 mA cm2) and Co3O4/HPG electrode (5.3 mA cm2). It is brief and convenient to evaluate the electrocatalyst properties by the overpotentials at the current density of 10 mA cm2 [31]. At that current density, the overpotential of Co0.85Se/HPG is 385 mV, which is obviously lower than that of the Co/HPG (442 mV). The activity of support material of HPG is studied by the Co3O4/HPG and Co3O4/C (Carbon Vulcan XC-72) [7,61]. It is notable that the OER performance on the Co3O4/HPG electrode is much higher than that on the Co3O4/C electrode in Fig. 5b. Although the overpotential of the Co3O4/HPG (480 mV) is close to that of the Co3O4/C, the current density at 0.7 V on the Co3O4/HPG electrode is 5.3 mA cm2, higher than that on the Co3O4/C (3.9 mA cm2) [7,58]. The reason of high activity is that the ultrathin graphene nanosheets has large enough surface area to touch catalyst nanoparticles, and thereby the interfacial electron transfer is promoted. Moreover, Co0.85Se
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Fig. 5 e LSV curves on Se/HPG, Co/HPG, Co0.85Se/HPG and Co3O4/HPG electrodes in 0.1 mol L¡1 KOH with a sweep rate of 0.001 V s¡1. Tafel plots in the inset figure of Fig. 5b.
nanosheets are uniformly dispersed on the graphene nanosheets, so more active sites are exposed and electrical conduction paths are increased. The electrochemical impedance responses (EIS) of Co0.85Se/HPG, Co/HPG, Co3O4/HPG and Se/ HPG are shown in Fig. S1. The solution resistance (Rs, ohmic resistance) and the charge transfer resistances (Rct) are obtained from the fitted equivalent circuit (Fig. S2) [62,63] and listed in Table S1. The Co0.85Se/HPG shows less charge transfer resistance (6.8 U). The stability for OER was researched by chronoamperometry in 0.1 mol L1 KOH at 0.7 V (overpotential 480 mV) shown in Fig. 6. The Co0.85Se/HPG exhibits a slow current decay compared with the other electrocatalysts. During the OER test, small oxygen bubbles aggregating on the electrode surface and occupying the active sites where the catalyst contacts with electrolyte result in slow decrease of oxidation current. The active sites of electrode surface are exposed again until the oxygen bubbles have grown large enough to leave the surface of electrode. The generation and release of oxygen lead to perturbation and ‘current waves’ in the chronoamperometry curves. After testing, the oxidation current density on the Co0.85Se/HPG electrode is 6.1 mA cm2, which is larger than that on the Co/HPG electrode (3.5 mA cm2) and the Co3O4/HPG electrode (2.6 mA cm2). The 3D HPG shows great electrical conductivity, distribution effect and strong cohesive force with nanoparticles. Meanwhile, its ultrathin nanosheets can stabilize other nanoparticles to remain high activity and good stability for OER. The Co0.85Se nanosheets dispersed on graphene nanosheets give the remarkable OER performance as following: (i) the interconnected porous network structure exposing
Table 1 e Comparison of this work with reported Co0.85Se in the Tafel slope. Tafel slope/(mV dec1) Co0.85Se/HPG (this work) [58] EG/Co0.85Se [56] Co0.85Se@NC [27] Co0.85Se/NF [59] (Ni, Co)0.85Se [59] Co0.85Se
61.7 73.0 75.0 78.9 79.0 85.0
Fig. 6 e Chronoamperomety curves on Co/HPG, Co0.85Se/ HPG and Co3O4/HPG electrodes in 0.1 mol L¡1 KOH at a potential of 0.7 V (overpotential 480 mV).
abundant active sites and promoting the oxygen and electrolyte diffusion and the charge transfer; (ii) Co0.85Se uniformly dispersed on the graphene nanosheets and increasing electrical conduction paths [55].
Conclusions The Co0.85Se using 3D HPG as the carrier has large specific surface area and exhibits outstanding electrocatalytic activity and stability for OER. The 3D HPG with 4e6 nm thickness graphene nanosheets shows a unique three-dimensional interconnected porous structure. The Co0.85Se, the structure of which is similar to the graphene nanosheet, is homogeneously dispersed on the graphene nanosheets surface. In addition, the Co0.85Se/HPG shows the lowest onset overpotential of 311 mV for OER. On the Co0.85Se/HPG electrode, the value of Tafel slope is 61.7 mV dec1, which is lower than that on the Co/HPG electrode and close to that on the Co3O4/ HPG electrode. Meanwhile, the current density at 0.7 V (overpotential 480 mV) is 17.1 mA cm2, which is higher than that on the other electrodes. The overpotential at the current density of 10 mA cm2 is 385 mV on the Co0.85Se/HPG electrode. In the HPG, the ultrathin graphene nanosheets has large
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enough surface area to touch catalyst nanoparticles, and thereby the interfacial electron transfer is promoted. The main reason of improvement of activity and stability is that Co0.85Se nanosheets are uniformly dispersed on the graphene nanosheets, which are better for exposing active sites and increasing electrical conduction paths. The Co0.85Se/HPG of this work is an appealing electrocatalyst for OER in alkaline medium, which could be the candidate of efficient, cheap and abundant catalysts.
Acknowledgments This work was financially supported by the Natural Science Foundation of Guangdong Province (2014A030313521), Scientific Research Foundation for Yangcheng Scholar (1201561607).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.03.003.
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