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Three-dimensional porous graphene supported Ni nanoparticles with enhanced catalytic performance for Methanol electrooxidation
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Three-dimensional porous graphene supported Ni nanoparticles with enhanced catalytic performance for Methanol electrooxidation Hai Zhu, Juntao Wang, Xiaoling Liu, Xiaoming Zhu* R & D Center for Non-power Nuclear Technology, School of Nuclear Technology and Chemistry & Biology, Hubei University of Science and Technology, Xianning, 437100, China
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abstract
Article history:
Three-dimensional porous graphene (3D-G) is prepared by template-assembly method and
Received 26 December 2016
employed as catalyst support for Ni nanoparticles for methanol electrooxidation.
Received in revised form
Morphology characterization confirm that Ni nanoparticles with sizes around 20 nm are
10 February 2017
uniformly scattered on the pore wall surface of the three-dimensional graphene without
Accepted 26 February 2017
apparent agglomeration. Electrochemical measurements indicate that the Ni/3D-G pro-
Available online xxx
cesses higher electrocatalytic activity for methanol oxidation reaction than that of the Ni nanoparticles supported on two-dimensional graphene (Ni/2D-G) and Ni nanoparticles
Keywords:
without graphene. The peak current density on Ni/3D-G is 64.6 mA cm2, which is 1.5 times
Nickel
higher than that on Ni/2D-G. The remarkable electrocatalytic performance of the Ni/3D-G
Porous
catalyst are mainly derived from the 3D graphene. As a carrier for methanol oxidation, the
Graphene
3D-G with abundant pore architecture not only hinder the agglomeration of Ni particles
Methanol electrooxidation
that is beneficial to accelerating the efficient charge transport through the whole catalyst,
Fuel cells
but also offer readily accessible channels for the diffusion of CH3OH to the active sites of catalyst surface. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Induction Direct methanol fuel cells (DMFCs) have recently aroused great concern as a sustainable power source for dealing with energy shortage and environmental pollution, due to their fairly high energy density, low operating temperature, and very low environmental intrusion [1e3]. However, the traditional precious metal catalysts (e.g. Pt, Pd and Au), are facing great challenges as the finite natural resources and high costs restrict their widely utilization [4e7]. Therefore, considerable efforts has been recently focused on fabrication of easily
available and high active electrocatalysts for the DMFCs. As an effective candidate, Ni-based catalysts have provoked extensive interests because of the low cost and good stability for fuel cells [8e10]. For instance, Ni-based catalysts showed good electrocatalytic toward ethanol oxidation [11]. Nickel nanowires as an efficient catalyst for urea peroxide electrooxidation in alkaline media [12]. Ni/C exhibited high performance during the electrooxidation of hydrogen peroxide and sodium borohydride [13]. Ni-based catalysts also have been employed as the catalysts for CH3OH oxidation and achieved high performance, such as Ni/TiO2 [14], Ni-Co [15] and NiO [16]. However, the catalytic activity of nickel
* Corresponding author. Fax: þ86 715 8265933. E-mail address: [email protected] (X. Zhu). http://dx.doi.org/10.1016/j.ijhydene.2017.02.189 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhu H, et al., Three-dimensional porous graphene supported Ni nanoparticles with enhanced catalytic performance for Methanol electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.02.189
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catalysts is lower than that of precious metal catalysts, and the morphology and composition have great influence on the performance of Ni catalysts. Accordingly, much effort has been made to improve the electrochemical performance of Ni catalysts, including alloying nickel with other metals [17e19], loading nanoparticles on a high surface area of the substrate to prevent aggregation [20e22]. Of these methods, the catalyst supported on carbonaceous materials is the most effective and facile method for improving the electrocatalytic activity of Ni. As the high surface area, high flexibility, excellent electrical conductivity, good chemical and thermal stability, twodimensional (2D) graphene have attracted worldwide attention as a support for the modification of various electrode materials [23,24]. However, in the process of preparation of electrode materials, the 2D graphene nanosheets are prone to restacking and agglomeration arise from the strong interlayer interactions, which can greatly reduce accessible surface area of the active materials. Recently, three-dimensional (3D) graphene is considered more suitable for electrode material modification and has been used as the robust matrix in energy storage and conversion applications (e.g. lithium ion batteries [25], fuel cells [26], supercapacitors [27] and other applications [28]) due to the advantages of both high conductivity and abundant holes. Huang et al. [29] reported the Pt nanoparticles loaded on 3D graphene aerogel by one-step co-reduction method and as a durable electrocatalyst for oxygen reduction. Zhou et al. [30] showed that 3D graphene networks could be employed as the support material for Pd catalyst for the electrocatalytic oxidation of formic acid. Jang et al. [31] reported three-dimensional graphene decorated with PteAu alloy nanoparticles as high stable electrocatalyst for methanol electrooxidation. However, for improving the electrocatalytic activity of Ni based electrocatalysts, there is still no scientific report in literature about the usage of 3D graphene. In this work, we report the designed synthesis of 3D porous graphene supported Ni nanoparticles (Ni/3D-G) and compare its electrocatalytic performances with a 2D graphene supported Ni nanoparticles (Ni/2D-G) when used as electrode material for the oxidation of methanol in alkaline medium. With the interconnected 3D porous architecture and superior conductive network, the Ni/3D-G catalyst exhibits remarkably higher electrocatalytic activity and stability in methanol oxidation than the Ni/2D-G at the same Ni loading.
Experimental Preparation of 3D graphene The 3D graphene was prepared by a templated-assembly method according to our previous work [32]. The first step was the synthesis of graphene oxide (GO) and (3aminopropyl)-triethoxysilane (APTES) modified silica (ASiO2) spheres. GO was obtained by a modified Hummers' € ber method [33]. A-SiO2 nanospheres were synthesized by Sto method [34]. The second step was the self-assembly process of GO and A-SiO2 spheres. In a typical synthesis, the 140 mg of GO and 340 mg of A-SiO2 dispersions were dispersed in 240 mL of deionized water and ultrasonicated for 30 min. Then, the
mixed solution of the pH value is adjusted to 2 by hydrochloric acid, followed by filtering, washing and drying the brown cake was obtained. The third step was an annealing and etching process. The GO was thermally reduced into graphene at 800 C, and then the obtained black powder added to 10% HF to remove the SiO2 sphere. Finally, the 3D graphene was collected after washing by filtration and dried at 80 C overnight.
Preparation of 2D graphene The 2D graphene was obtained by chemical reduction method [35]. Typically, the 200 mL of hydrazine (50%) solution was added into 100 mL of GO (1 mg mL1) solution, the mixed solution was heated to 50 C for 12 h under continuously stirring. After that, the products were isolated by filtration, washed and then dried at 80 C.
Preparation of Ni/3D-G catalyst Typically, 50 mg of 3D graphene and 570 mg of NiCl2$6H2O were first added in 60 mL of deionized water and ultrasonicated for 30 min. Then, 20 mL of hydrazine (50%) and 7 mL NaOH (0.25 mol L1) solution was dropwisely added into the above mixed solution with continuous stirring. The resulting mixtures were then heated to 80 C for 5 h. Finally, the products were collected after washed several times with deionized water, followed by drying in a vacuum oven at 80 C overnight. The actual loading of Ni in the as-prepared samples was measured by inductively coupled plasma spectroscopy analysis. The content of nickel in the product obtained by the above process was 70.08 wt.%, which was very close to the amount of nickel precursor during the preparation step.
Preparation of Ni/2D-G and pristine Ni catalysts In the control experiment, the Ni/2D-G catalyst was prepared similarly to the synthesis procedure of Ni/3D-G catalyst in addition to the use of 2D graphene nanosheets instead of 3D graphene. As for the pristine Ni catalyst, the synthesis steps were similar to the Ni/3D-G and Ni/2D-G catalysts except that the graphene omitted.
Characterization The crystalline structures of the samples were characterized by the X-ray diffractometer (XRD, Bruker D8 Advance, Cu Ka radiation, l ¼ 0.15418 nm). The morphologies of the materials were observed with a scanning electron microscopy (SEM, SIRION, FEI, USA) and transmission electron microscopy (TEM, JEM-2001F, JEOL, operating at 200 KV). The value of the Ni contents in the samples were measured by an inductively coupled plasma analyzer (ICP, IRIS Intrepid II XSP). N2 adsorptionedesorption analysis was performed on a Quadrasorb (Quantachrome) instrument, and the specific surface area of as-obtained samples was calculated with the BrunauereEmmetteTeller (BET) method. All electrochemical measurements were carried out through the standard threeelectrode cell using a CHI660D electrochemical workstation over a voltage range of 0.0e0.8 V at room temperature. A
Please cite this article in press as: Zhu H, et al., Three-dimensional porous graphene supported Ni nanoparticles with enhanced catalytic performance for Methanol electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.02.189
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platinum foil (1.0 cm2) was used as the counter electrode and a Hg/HgO (1.0 mol L1 KOH) as the reference electrode. The working electrode adopted a catalysts-modified glassy carbon electrode (GCE, 3 mm in diameter). To fabricate the catalysts-modified GCE electrode, 10 mg of the as-prepared sample was added to 4.5 mL isopropanol and 0.5 mL 0.5 wt.% Nafion (DuPont, USA) mixed solution and then ultrasound for 10 min to form a uniform electrocatalyst ink. Meanwhile, the GCE electrode was first polished and then washed by acetone and deionized water. After that, 17 mL of the electrocatalyst ink was dropped onto the polished GCE working surface and dried under an infrared lamp.
Results and discussion The microstructures of 3D and 2D graphene are illustrated in TEM and SEM, as shown in Fig. 1. In the TEM images of graphene-encapsulated silica spheres (Fig. 1a), the crinkled and rough textures of graphene can be observed on the edge of the silica spheres, indicating that the regular silica spheres are uniformly and tightly coated by the flexible and ultrathin graphene sheets. From the TEM (Fig. 1b) and SEM (Fig. 1c) images, we can clearly observe that the 3D graphene has continuous interconnected porous morphology, deriving from the removal of SiO2 template. These bubble-like porous with a diameter of about 200 nm are the foundation of 3D structure. While the TEM image (Fig. 1d) of 2D graphene reveals that the graphene sheets with micrometer scale are crumpled and agglomerated.
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The XRD patterns of the two kinds of graphene are shown in Fig. 2a. As can be seen, the XRD diffraction peaks of the two materials are the same, and both of them exhibit a strong diffraction peak at 2q values of about 26.6 , corresponding to the (002) plane of the hexagonal graphene structure. The surface structure characteristics of 3D graphene and 2D graphene were further confirmed by N2 adsorptionedesorption isotherms and shown in Fig. 2b. Both 3D graphene (red line) and 2D graphene (blue line) display a type IV isotherm curve with a hysteresis loop characteristic of uniform mesopores, which confirms the mesopores in the two samples, and the hysteresis loop of 3D graphene is obviously larger, demonstrating a more developed porous structure. Moreover, the BrunauereEmmetteTeller (BET) specific surface area of 3D graphene is calculated to be 883 m2 g1, which is much higher than that of 2D graphene (463 m2 g1). The structure and composition of the as-synthesized of Ni/ 3D-G, Ni/2D-G and pristine Ni composites are presented in Fig. 3. In addition to the characteristic peaks positioned at around 26 correspond to the graphene in Ni/3D-G and Ni/2DG composites. All the XRD patterns of the three composites can be characterized by three strong diffraction peaks located at 2q values of 44.50, 51.83 and 76.46 , corresponding to (111), (200) and (220) reflections of face-centered cubic (fcc) crystalline nickel (JCDPS, NO. 04-0850), respectively. Furthermore, no significant diffraction peaks of nickel oxide or other impurities can be observed in the as-obtained composites. Accordingly, the above XRD patterns demonstrate that the hybridization of Ni particles on graphene has been successfully realized.
Fig. 1 e TEM images of graphene-encapsulated silica spheres (a) and 3D-G (b); SEM images of 3D-G (c) and 2D-G (d). Please cite this article in press as: Zhu H, et al., Three-dimensional porous graphene supported Ni nanoparticles with enhanced catalytic performance for Methanol electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.02.189
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Fig. 2 e XRD patterns of the 3D-G and 2D-G (a); N2 adsorptionedesorption curves of the 3D-G and the 2D-G (b).
Fig. 3 e The XRD patterns of the Ni/3D-G, Ni/2D-G and Ni catalysts.
The morphology and structure of the Ni/3D-G, Ni/2D-G and pristine Ni catalysts were characterized by SEM and TEM, as shown in Fig. 4. As can be seen, the SEM image (Fig. 4a) of the Ni/3D-G catalyst indicate that the macropores of 3D porous
graphene frameworks are well retained after hybridization with Ni nanoparticles, which will be conducive to improve the effective contact between electrolyte and active material in the fuel cell. The TEM image (Fig. 4b) of Ni/3D-G catalyst confirms that the nickel nanoparticles with high density are uniformly anchored on the pore wall surface of the 3D graphene without agglomeration, and the average sizes of Ni nanoparticles in the composite are 15e20 nm. Fig. 4c gives the SEM images of the Ni/2D-G catalyst, we can clearly see that the most Ni nanoparticles aggregated together, and the graphene sheets are not very flat and tightly wrapped Ni nanoparticles, which can be ascribed to the 2D graphene have restacked together during the synthesis stage. The TEM image of the Ni/ 2D-G catalyst (Fig. 4d) exhibits that nickel nanoparticles with size around 100 nm are embedded on the graphene sheets, but the average size of Ni nanoparticles are larger than that of nanoparticles in Ni/3D-G composite, and dispersion of nanoparticles is not well. Both the SEM image (Fig. 4e) and the TEM image (Fig. 4f) of the pristine Ni catalyst display that the irregular nickel nanoparticles with size of 100e150 nm are agglomerated together. Based on the morphology and structure results of the Ni/3D-G, Ni/2D-G and pristine Ni catalysts, suggesting that the adding of graphene into the precursor solution could decrease the particle size and inhibit the agglomeration of Ni during the crystallization process. Besides, in contrast to Ni/2D-G, the Ni nanoparticles with smaller size and distribute more uniformly on the surface of graphene, which will make the reaction more easily between the electrolyte and the catalyst particles. The cyclic voltammograms of Ni/3D-G (70 wt.%, Ni loading), Ni/2D-G (70 wt.%, Ni loading) and pristine Ni catalysts in 1.0 M KOH solution at a scanning speed of 50 mV s1 are represented in Fig. 5. In order to activate the Ni-based catalysts by creating NiOOH layer on the catalysts surface, the cyclic voltammetry was tested in the voltage range of 0.0e0.8 V (vs. Hg/HgO) after scanning this electrode for 10 cycles [9]. As can be seen, the Ni/3D-G catalyst presents a strong anodic peak at 0.47 V can be related to the oxidation of Ni2þ to Ni3þ, and an obvious cathodic peak at 0.38 V is attributed to the reduction of Ni3þ to Ni2þ, respectively. The oxidation and reduction process of electrochemical about Nibased catalysts can be explained as follows [36e38]:
Ni þ 2OH / Ni(OH)2 þ 2e
(1)
Ni(OH)2 þ OH / NiOOH þ H2O þ e
(2)
Compared with the Ni/3D-G catalyst, Ni/2D-G catalyst also exhibits a pair of redox peaks, however, it has a much broader separation and lower current densities than the Ni/3D-G electrocatalyst, indicating that it may have a poorer cycling performance. For the pristine nickel catalyst, almost no significant redox couple is seen, indicating that the electrochemical activity is the worst. Based on the above comparison, it can be reasonably conclude that the 3D-graphene can greatly promote the charge transfer during the redox process of catalysts, which benefit from the 3D graphene with high specific surface area and conductivity.
Please cite this article in press as: Zhu H, et al., Three-dimensional porous graphene supported Ni nanoparticles with enhanced catalytic performance for Methanol electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.02.189
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Fig. 4 e Morphology and microstructure of the as-synthesized catalysts: SEM (a) and TEM (b) images for Ni/3D-G; SEM (c) and TEM (d) images for Ni/2D-G; SEM (e) and TEM (f) images for Ni catalyst.
methanol oxidation, and the oxidation mechanism is attributed to the following reaction [36,39]: NiOOH þ CH3OH / Ni(OH)2 þ product
Fig. 5 e Cyclic voltammograms of Ni/3D-G, Ni/2D-G and Ni catalysts in 1.0 M KOH solution at a scan rate of 50 mV s¡1.
The effect of methanol concentration on the electrocatalytic activity of Ni/3D-G catalyst is given in Fig. 6. The KOH concentration was fixed at 1.0 M and the CH3OH concentration varied from 0.00 to 2.00 M with scan rate of 50 mV s1. As exhibited in the CV curves (Fig. 6a), a pair of well defined anodic and cathodic peaks can be detected at about 0.47 V and 0.38 V, respectively, which is related to the transformation of the Ni(OH)2/NiOOH. After increasing the concentration of methanol in the electrolyte, the anodic current densities of Ni/ 3D-G increase with the formation of NiOOH and finally obtain an obvious anodic peak locate at 0.67 Ve0.72 V, corresponding to the methanol electrooxidation process. Besides, the current densities recorded in the electrolyte containing methanol are much higher than that in electrolyte only containing KOH, suggesting that NiOOH plays a catalytic role in the process of
(3)
As shown, the peak current densities increase with increasing methanol content up to 0.75 M, indicating that the Ni/3D-G processes good electrocatalytic activity towards methanol electrooxidation reaction. As seen in Fig. 6b, when the CH3OH concentration is higher than 0.75 M, the oxidation peaks become much broader and appear at the higher positive potential values, which can be attributed to increasing of the concentration of the unoxidized organic residue with increasing the concentration of methanol which requires higher potential. Moreover, although the oxidation peak current continues to increase, the corresponding cathodic peak current is gradually reduced, this phenomenon confirms that the reversibility of the reaction becomes poor. To investigate the kinetic characteristics of methanol electrooxidation on the surface of Ni/3D-G, Fig. 7 displays the cyclic voltammograms of Ni/3D-G at various scan rates of 10e200 mV s1 in 1 M KOH electrolyte containing 0.75 M CH3OH. As seen from Fig. 7a, with the increase of scanning rate, the anodic peak gradually shifts to more positive potentials, while the cathodic peak shifts to more negative potentials. This displacement of redox peaks are caused by irreversible oxidation reaction process. Besides, the oxidation peak also appeared in the negative scan, which is associated with the further oxidation of CH3OH and the desorption of intermediate products. The relationship of the methanol oxidation peak current densities and the square root of scan rates is presented in Fig. 7b. As can be seen, a positive linear relationship between the methanol oxidation peak current densities and the square root of the scan rates, demonstrating that the electrocatalytic reaction process on the surface of the Ni/3D-G is controlled by the diffusion of methanol.
Please cite this article in press as: Zhu H, et al., Three-dimensional porous graphene supported Ni nanoparticles with enhanced catalytic performance for Methanol electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.02.189
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Fig. 6 e Cyclic voltammograms of the Ni/3D-G (70 wt.%, Ni loading) at a scan rate of 50 mV s¡1, and 1.0 M KOH at different concentration of methanol (a). The effect of higher methanol concentration (1 M and 2 M) on the electrocatalytic activity of Ni/3D-G catalyst in the voltage range of 0.0e1.2 V (vs. Hg/HgO) (b).
The electrocatalytic performance of Ni/3D-G with different nickel loadings (Ni/3D-G with 50 wt.%, 60 wt.%, 70 wt.% and 80 wt.% Ni loading) were also studied by cyclic voltammograms and the results are presented in Fig. 8. By comparision, the electrocatalytic activity of Ni/3D-G catalyst increased with the increase of loading amount of nickel in the catalyst up to 70 wt.%. However, more increase of nickel content than 70 wt.% lead to a decline in the electrocatalytic activity of Ni/3D-G. With 50 wt.%, 60 wt.%, 70 wt.% and 80 wt.% Ni loading, the anodic peak current densities of Ni/3D-G catalyst are 44.3, 52.4, 64.6 and 55.5 mA cm2, respectively. Generally, the anodic peak current densities increase as the amount of the nickel increase, but eventually the anodic peak current densities become lower due to the excess nickel on the surface of the 3D-G can negatively affect the materials, reducing the catalytic activity, which results from that the too much nickel particles can reduce the exposed area of catalyst surface active sites, and thus suppressing the transfer path of electron. Hence, the optimal
Fig. 7 e Cyclic voltammograms of Ni/3D-G catalyst at various scan rate (a) and plots of the peak current density against the square root of the scan rate for Ni/3D-G catalyst (b) in 1.0 M KOH solution containing 0.75 M methanol.
Fig. 8 e Study effect of nickel loading on electrocatalytic activity of Ni/3D-G toward methanol oxidation in 1.0 M KOH þ 0.75 M methanol solution at a scan rate of 50 mV s¡1.
Please cite this article in press as: Zhu H, et al., Three-dimensional porous graphene supported Ni nanoparticles with enhanced catalytic performance for Methanol electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.02.189
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loading of nickel is 70 wt.% and the peak current density is 64.6 mA cm2 at 0.70 V for Ni/3D-G catalyst. To examine the influence of the introduction of graphene sheets on the electrochemical features of the Ni-based electrocatalysts, the electrocatalytic methanol activity of Ni/3D-G catalyst was compared with that of Ni/2D-G and pristine Ni catalysts. Fig. 9a displays the CV results of the Ni/3D-G, Ni/2DG and Ni catalysts in 1 M KOH electrolyte containing 0.75 M CH3OH at a scan rate of 50 mV s1 ranging from 0.0 to 0.8 V (vs. Hg/HgO). As shown in the CV curves, both the Ni/3D-G and Ni/ 2D-G catalysts show a relatively high electrocatalytic activity when compared with the pristine Ni catalyst. More explicitly, the peak current density for CH3OH electrooxidation on Ni/3DG catalyst is 64.6 mA cm2, which is about 1.5 times as high as that on Ni/2D-G catalyst at the same Ni loading percentage under otherwise identical conditions. For the Ni/2D-G catalyst, although exhibits much better performances than Ni catalyst, has lower electrocatalytic activity than Ni/3D-G catalyst is attributed to the stacking of graphene sheets and larger particle sizes in the Ni/2D-G catalyst lead to insufficient contact between the active sites and electrolyte. It is demonstrated that the graphene as a support in the electrode
Fig. 9 e Cyclic voltammograms of the Ni/3D-G, Ni/2D-G and Ni catalysts in 1.0 M KOH þ 0.75 M methanol solution at a scan rate of 50 mV s¡1 (a). The Tafel plots of the prepared Ni-based catalysts (b).
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strongly enhanced the electrochemical performance of nickel catalyst, and the facilitating role of the 3D-G is more obvious than the 2D-G. In order to determine of kinetic parameters of the electrooxidation of methanol at the Ni/3D-G, Ni/2D-G and pristine Ni catalysts, the Tafel plots of the three catalysts are recorded in 1 M KOH electrolyte containing 0.75 M CH3OH solution at a scan rate of 1 mV s1. Fig. 9b depicts the obtained plots of the Tafel region and the Tafel plots were fitted in to the Tafel equation within the linear regions. The Tafel slope value could be calculated using the following equation for the anodic reactions: logI ¼ logIo þ ½ð1 aÞnF=2:303RTE
(4)
where the constants R and F denote the universal gas constant and the Faraday constant, respectively, T is the temperature (K), a is the charge transfer coefficient of the reaction n is the number of electrons transferred during the oxidation reaction, Io is the exchange current density and a is the charge transfer coefficient of the reaction. The slope of Tafel plots are 151.7, 197.6 and 373.5 mV dec1 for methanol electrooxidation at the Ni/3D-G, Ni/2D-G and pristine Ni, respectively. In the electrochemical oxidation of methanol, the rate-determining step is a one-electron process, thus, a values should be 0.12, 0.48 and 0.65 for methanol electrooxidation at the Ni/3D-G, Ni/2D-G and pristine Ni, respectively. Furthermore, the Io value of Ni/ 3D-G is 4.72 106 A cm2, which is much higher than that of Ni/2D-G and pristine Ni catalysts. Both the larger charge transfer coefficient and the higher exchange current density confirm again the enhanced methanol electrocatalytic oxidation reaction kinetics obtained on the prepared Ni/3D-G over Ni/2D-G and pristine Ni catalysts. To estimate the electrocatalytic stability of the as-prepared catalysts toward the methanol oxidation reaction, the results of chronoamperometric experiments on the Ni/3D-G and Ni/ 2D-G catalysts under the same loading are presented in Fig. 10. The test was carried out by holding the potential at 0.70 V (vs. Hg/HgO) in 1 M KOH electrolyte containing 0.75 M CH3OH for about 3750 s. As shown in the curves, the oxidation
Fig. 10 e Chronoamperometric curves for Ni/3D-G and Ni/ 2D-G at an oxidation potential 0.70 V (vs. Hg/HgO) in 1.0 M KOH þ 0.75 M methanol solution.
Please cite this article in press as: Zhu H, et al., Three-dimensional porous graphene supported Ni nanoparticles with enhanced catalytic performance for Methanol electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.02.189
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current densities decrease rapidly for both of the catalysts in the first 150 s, then slowly and gradually decreased to achieve a quasi-equilibrium steady state. However, The decreasing degree of Ni/3D-G is smaller than that of Ni/2D-G, and the steady-state current density of Ni/3D-G after 3750 s is 30.8 mA cm2, which is approximately 1.5 times higher than that of Ni/2D-G catalyst. By comparison, the initial and steadystate current densities on Ni/3D-G catalyst are obviously higher than that on Ni/2D-G, demonstrating that the Ni/3D-G catalyst possessed excellent catalytic durability during the methanol oxidation reaction. The superior electrocatalytic performance of the Ni/3D-G catalyst demonstrated above may be originate from the unique structure advantages offered by the 3D graphene. Firstly, 3D graphene provides more growth sites for Ni to limit the particle size and agglomeration in the crystallization process, the uniformly dispersed particles with smaller size can expose more active sites for the methanol oxidation reaction. Secondly, 3D graphene with porous structure serve as a fast three dimensional channel for electrolyte diffusion can enlarge the contact area between the catalyst and the electrolyte. Moreover, with inherent characteristics of graphite, 3D graphene constructs a interconnected conductive network for accelerating the electron transfer on the Ni catalyst surface during the electrooxidation processes.
Conclusion In summary, we use 3D porous graphene as an excellent support for the growth of Ni nanoparticles, successfully obtained Ni/3D-G composite. The small-sized Ni nanoparticles with high density are uniformly distributed on the pore wall surface of the 3D graphene and the Ni/3D-G catalyst exhibits outstanding electrocatalytic performance toward the methanol oxidation reaction. The maximum obtained oxidation peak current density is 64.6 mA cm2 at 0.70 V in 0.75 M CH3OH þ 1.0 M KOH for Ni/3D-G (70 wt.% Ni loading), which is much higher than Ni/2D-G and pristine Ni catalysts. 3D graphene plays an important role in improving the electrochemical performance of catalyst. In the composite, 3D graphene not only limits particle size and agglomeration is beneficial to increase the catalyst active site, but also provides a effective three dimensional passageway for facilitating electrolyte diffusion and electron transfer on the catalyst surface. Based on the above results, the catalyst structure and preparation method mentioned in this study is easy to realize and the 3D graphene serve as a promising catalyst support can be widely applied to the construction of other threedimensional structure materials and prepare good electrocatalyst materials in fuel cell applications.
Acknowledgments This study was the financially supported by Science and Technology Research Project of Education Department of Hubei Province (Q20162804) and Start up foundation of Hubei University of Science and Technology (BK1519).
references
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[20] Yi Q, Huang W, Zhang J, Liu X, Li L. Methanol oxidation on titanium-supported nano-scale Ni flakes. Catal Commun 2008;9:2053e8. [21] Thamer BM, El-Newehy MH, Barakat NAM, Ali Abdelkareem M, Al-Deyab SS, Kim HY. Influence of nitrogen doping on the catalytic activity of Ni-incorporated carbon nanofibers for alkaline direct methanol fuel cells. Electrochim Acta 2014;142:228e39. [22] Abdel Hameed RM. Optimization of manganese oxide amount on Vulcan XC-72R carbon black as a promising support of Ni nanoparticles for methanol electro-oxidation reaction. Int J Hydrogen Energy 2015;40:13979e93. [23] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Electric field effect in atomically thin carbon films. Science 2004;306:666e9. [24] Huang CC, Li C, Shi GQ. Graphene based catalysts. Energy Environ Sci 2012;5:8848e68. [25] Chang YH, Li J, Wang B, Luo H, He HY, Song Q, et al. Synthesis of 3D nitrogen-doped graphene/Fe3O4 by a metal ion induced self-assembly process for high-performance Li-ion batteries. J Mater Chem A 2013;1:14658e65. [26] Zhao SL, Yin HJ, Du L, Yin GP, Tang ZY, Liu SQ. Three dimensional N-doped graphene/PtRu nanoparticle hybrids as high performance anode for direct methanol fuel cells. J Mater Chem A 2014;2:3719e24. [27] Cao XH, Yin ZY, Zhang H. Three-dimensional graphene materials: preparation, structures and application in supercapacitors. Energy Environ Sci 2014;7:1850e65. [28] Mao S, Wen Z, Kim H, Lu G, Hurley P, Chen J. A general approach to one-pot fabrication of crumpled graphene-based nanohybrids for energy applications. ACS Nano 2012;6:7505e13. [29] Huang Q, Tao F, Zou L, Yuan T, Zou Z, Zhang H, et al. Onestep synthesis of Pt nanoparticles highly loaded on graphene aerogel as durable oxygen reduction electrocatalyst. Electrochim Acta 2015;152:140e5.
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[30] Zhou Y, Hu XC, Fana Q, Wen HR. Three-dimensional crumpled graphene as an electro-catalyst support for formic acid electro-oxidation. J Mater Chem A 2016;4:4587e91. [31] Jang HD, Kim SK, Chang H, Choi JH, Cho BG, Jo EH, et al. Three-dimensional crumpled graphene-based platinum-gold alloy nanoparticle composites as superior electrocatalysts for direct methanol fuel cells. Carbon 2015;93:869e77. [32] Zhu H, Wu XZ, Zan L, Zhang YX. Three-dimensional macroporous graphene-Li2FeSiO4 composite as cathode material for lithium-ion batteries with superior electrochemical performances. ACS Appl Mater Interfaces 2014;6:11724e33. [33] Hummers WS, Offeman RE. Preparation of graphene oxide. J Am Chem Soc 1958;80. 1339e1339. [34] Stober W, Fink A, Bohn E. Controlled growth of monodisperse silica spheres in micron size range. J Colloid Interface Sci 1968;26:62e9. [35] Zhou Y, Bao Q, Zhong Y, Loh KP. Hydrothermal dehydration for the “green” reduction of exfoliated graphene oxide to graphene and demonstration of tunable optical limiting properties. Chem Mater 2009;21:2950e6. [36] Fleischmann M, Korinek K, Pletcher D. The oxidation of organic compounds at a nickel anode in alkaline solution. J Electroanal Chem Interfacial Electrochem 1971;31:39e49. [37] Vukovic M. Voltammetry and anodic stability of a hydrous oxide film on a nickel electrode in alkaline solution. J Appl Electrochem 1994;24:878e82. [38] Enea O. Molecular structure effects in electrocatalysis-II. Oxidation of d-glucose and of linear polyols on Ni electrodes. Electrochim Acta 1990;35:375e8. [39] Fleischmann M, Korinek K, Pletcher D. The kinetics and mechanism of the oxidation of amines and alcohols at oxidecovered nickel, silver, copper, and cobalt electrodes. J Chem Soc Perkin Trans 1972;2:1396e403.
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Three-dimensional porous graphene supported Ni nanoparticles with enhanced catalytic performance for Methanol electrooxidation Hai Zhu, Juntao Wang, Xiaoling Liu, Xiaoming Zhu* R & D Center for Non-power Nuclear Technology, School of Nuclear Technology and Chemistry & Biology, Hubei University of Science and Technology, Xianning, 437100, China
article info
abstract
Article history:
Three-dimensional porous graphene (3D-G) is prepared by template-assembly method and
Received 26 December 2016
employed as catalyst support for Ni nanoparticles for methanol electrooxidation.
Received in revised form
Morphology characterization confirm that Ni nanoparticles with sizes around 20 nm are
10 February 2017
uniformly scattered on the pore wall surface of the three-dimensional graphene without
Accepted 26 February 2017
apparent agglomeration. Electrochemical measurements indicate that the Ni/3D-G pro-
Available online xxx
cesses higher electrocatalytic activity for methanol oxidation reaction than that of the Ni nanoparticles supported on two-dimensional graphene (Ni/2D-G) and Ni nanoparticles
Keywords:
without graphene. The peak current density on Ni/3D-G is 64.6 mA cm2, which is 1.5 times
Nickel
higher than that on Ni/2D-G. The remarkable electrocatalytic performance of the Ni/3D-G
Porous
catalyst are mainly derived from the 3D graphene. As a carrier for methanol oxidation, the
Graphene
3D-G with abundant pore architecture not only hinder the agglomeration of Ni particles
Methanol electrooxidation
that is beneficial to accelerating the efficient charge transport through the whole catalyst,
Fuel cells
but also offer readily accessible channels for the diffusion of CH3OH to the active sites of catalyst surface. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Induction Direct methanol fuel cells (DMFCs) have recently aroused great concern as a sustainable power source for dealing with energy shortage and environmental pollution, due to their fairly high energy density, low operating temperature, and very low environmental intrusion [1e3]. However, the traditional precious metal catalysts (e.g. Pt, Pd and Au), are facing great challenges as the finite natural resources and high costs restrict their widely utilization [4e7]. Therefore, considerable efforts has been recently focused on fabrication of easily
available and high active electrocatalysts for the DMFCs. As an effective candidate, Ni-based catalysts have provoked extensive interests because of the low cost and good stability for fuel cells [8e10]. For instance, Ni-based catalysts showed good electrocatalytic toward ethanol oxidation [11]. Nickel nanowires as an efficient catalyst for urea peroxide electrooxidation in alkaline media [12]. Ni/C exhibited high performance during the electrooxidation of hydrogen peroxide and sodium borohydride [13]. Ni-based catalysts also have been employed as the catalysts for CH3OH oxidation and achieved high performance, such as Ni/TiO2 [14], Ni-Co [15] and NiO [16]. However, the catalytic activity of nickel
* Corresponding author. Fax: þ86 715 8265933. E-mail address: [email protected] (X. Zhu). http://dx.doi.org/10.1016/j.ijhydene.2017.02.189 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhu H, et al., Three-dimensional porous graphene supported Ni nanoparticles with enhanced catalytic performance for Methanol electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.02.189
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catalysts is lower than that of precious metal catalysts, and the morphology and composition have great influence on the performance of Ni catalysts. Accordingly, much effort has been made to improve the electrochemical performance of Ni catalysts, including alloying nickel with other metals [17e19], loading nanoparticles on a high surface area of the substrate to prevent aggregation [20e22]. Of these methods, the catalyst supported on carbonaceous materials is the most effective and facile method for improving the electrocatalytic activity of Ni. As the high surface area, high flexibility, excellent electrical conductivity, good chemical and thermal stability, twodimensional (2D) graphene have attracted worldwide attention as a support for the modification of various electrode materials [23,24]. However, in the process of preparation of electrode materials, the 2D graphene nanosheets are prone to restacking and agglomeration arise from the strong interlayer interactions, which can greatly reduce accessible surface area of the active materials. Recently, three-dimensional (3D) graphene is considered more suitable for electrode material modification and has been used as the robust matrix in energy storage and conversion applications (e.g. lithium ion batteries [25], fuel cells [26], supercapacitors [27] and other applications [28]) due to the advantages of both high conductivity and abundant holes. Huang et al. [29] reported the Pt nanoparticles loaded on 3D graphene aerogel by one-step co-reduction method and as a durable electrocatalyst for oxygen reduction. Zhou et al. [30] showed that 3D graphene networks could be employed as the support material for Pd catalyst for the electrocatalytic oxidation of formic acid. Jang et al. [31] reported three-dimensional graphene decorated with PteAu alloy nanoparticles as high stable electrocatalyst for methanol electrooxidation. However, for improving the electrocatalytic activity of Ni based electrocatalysts, there is still no scientific report in literature about the usage of 3D graphene. In this work, we report the designed synthesis of 3D porous graphene supported Ni nanoparticles (Ni/3D-G) and compare its electrocatalytic performances with a 2D graphene supported Ni nanoparticles (Ni/2D-G) when used as electrode material for the oxidation of methanol in alkaline medium. With the interconnected 3D porous architecture and superior conductive network, the Ni/3D-G catalyst exhibits remarkably higher electrocatalytic activity and stability in methanol oxidation than the Ni/2D-G at the same Ni loading.
Experimental Preparation of 3D graphene The 3D graphene was prepared by a templated-assembly method according to our previous work [32]. The first step was the synthesis of graphene oxide (GO) and (3aminopropyl)-triethoxysilane (APTES) modified silica (ASiO2) spheres. GO was obtained by a modified Hummers' € ber method [33]. A-SiO2 nanospheres were synthesized by Sto method [34]. The second step was the self-assembly process of GO and A-SiO2 spheres. In a typical synthesis, the 140 mg of GO and 340 mg of A-SiO2 dispersions were dispersed in 240 mL of deionized water and ultrasonicated for 30 min. Then, the
mixed solution of the pH value is adjusted to 2 by hydrochloric acid, followed by filtering, washing and drying the brown cake was obtained. The third step was an annealing and etching process. The GO was thermally reduced into graphene at 800 C, and then the obtained black powder added to 10% HF to remove the SiO2 sphere. Finally, the 3D graphene was collected after washing by filtration and dried at 80 C overnight.
Preparation of 2D graphene The 2D graphene was obtained by chemical reduction method [35]. Typically, the 200 mL of hydrazine (50%) solution was added into 100 mL of GO (1 mg mL1) solution, the mixed solution was heated to 50 C for 12 h under continuously stirring. After that, the products were isolated by filtration, washed and then dried at 80 C.
Preparation of Ni/3D-G catalyst Typically, 50 mg of 3D graphene and 570 mg of NiCl2$6H2O were first added in 60 mL of deionized water and ultrasonicated for 30 min. Then, 20 mL of hydrazine (50%) and 7 mL NaOH (0.25 mol L1) solution was dropwisely added into the above mixed solution with continuous stirring. The resulting mixtures were then heated to 80 C for 5 h. Finally, the products were collected after washed several times with deionized water, followed by drying in a vacuum oven at 80 C overnight. The actual loading of Ni in the as-prepared samples was measured by inductively coupled plasma spectroscopy analysis. The content of nickel in the product obtained by the above process was 70.08 wt.%, which was very close to the amount of nickel precursor during the preparation step.
Preparation of Ni/2D-G and pristine Ni catalysts In the control experiment, the Ni/2D-G catalyst was prepared similarly to the synthesis procedure of Ni/3D-G catalyst in addition to the use of 2D graphene nanosheets instead of 3D graphene. As for the pristine Ni catalyst, the synthesis steps were similar to the Ni/3D-G and Ni/2D-G catalysts except that the graphene omitted.
Characterization The crystalline structures of the samples were characterized by the X-ray diffractometer (XRD, Bruker D8 Advance, Cu Ka radiation, l ¼ 0.15418 nm). The morphologies of the materials were observed with a scanning electron microscopy (SEM, SIRION, FEI, USA) and transmission electron microscopy (TEM, JEM-2001F, JEOL, operating at 200 KV). The value of the Ni contents in the samples were measured by an inductively coupled plasma analyzer (ICP, IRIS Intrepid II XSP). N2 adsorptionedesorption analysis was performed on a Quadrasorb (Quantachrome) instrument, and the specific surface area of as-obtained samples was calculated with the BrunauereEmmetteTeller (BET) method. All electrochemical measurements were carried out through the standard threeelectrode cell using a CHI660D electrochemical workstation over a voltage range of 0.0e0.8 V at room temperature. A
Please cite this article in press as: Zhu H, et al., Three-dimensional porous graphene supported Ni nanoparticles with enhanced catalytic performance for Methanol electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.02.189
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platinum foil (1.0 cm2) was used as the counter electrode and a Hg/HgO (1.0 mol L1 KOH) as the reference electrode. The working electrode adopted a catalysts-modified glassy carbon electrode (GCE, 3 mm in diameter). To fabricate the catalysts-modified GCE electrode, 10 mg of the as-prepared sample was added to 4.5 mL isopropanol and 0.5 mL 0.5 wt.% Nafion (DuPont, USA) mixed solution and then ultrasound for 10 min to form a uniform electrocatalyst ink. Meanwhile, the GCE electrode was first polished and then washed by acetone and deionized water. After that, 17 mL of the electrocatalyst ink was dropped onto the polished GCE working surface and dried under an infrared lamp.
Results and discussion The microstructures of 3D and 2D graphene are illustrated in TEM and SEM, as shown in Fig. 1. In the TEM images of graphene-encapsulated silica spheres (Fig. 1a), the crinkled and rough textures of graphene can be observed on the edge of the silica spheres, indicating that the regular silica spheres are uniformly and tightly coated by the flexible and ultrathin graphene sheets. From the TEM (Fig. 1b) and SEM (Fig. 1c) images, we can clearly observe that the 3D graphene has continuous interconnected porous morphology, deriving from the removal of SiO2 template. These bubble-like porous with a diameter of about 200 nm are the foundation of 3D structure. While the TEM image (Fig. 1d) of 2D graphene reveals that the graphene sheets with micrometer scale are crumpled and agglomerated.
3
The XRD patterns of the two kinds of graphene are shown in Fig. 2a. As can be seen, the XRD diffraction peaks of the two materials are the same, and both of them exhibit a strong diffraction peak at 2q values of about 26.6 , corresponding to the (002) plane of the hexagonal graphene structure. The surface structure characteristics of 3D graphene and 2D graphene were further confirmed by N2 adsorptionedesorption isotherms and shown in Fig. 2b. Both 3D graphene (red line) and 2D graphene (blue line) display a type IV isotherm curve with a hysteresis loop characteristic of uniform mesopores, which confirms the mesopores in the two samples, and the hysteresis loop of 3D graphene is obviously larger, demonstrating a more developed porous structure. Moreover, the BrunauereEmmetteTeller (BET) specific surface area of 3D graphene is calculated to be 883 m2 g1, which is much higher than that of 2D graphene (463 m2 g1). The structure and composition of the as-synthesized of Ni/ 3D-G, Ni/2D-G and pristine Ni composites are presented in Fig. 3. In addition to the characteristic peaks positioned at around 26 correspond to the graphene in Ni/3D-G and Ni/2DG composites. All the XRD patterns of the three composites can be characterized by three strong diffraction peaks located at 2q values of 44.50, 51.83 and 76.46 , corresponding to (111), (200) and (220) reflections of face-centered cubic (fcc) crystalline nickel (JCDPS, NO. 04-0850), respectively. Furthermore, no significant diffraction peaks of nickel oxide or other impurities can be observed in the as-obtained composites. Accordingly, the above XRD patterns demonstrate that the hybridization of Ni particles on graphene has been successfully realized.
Fig. 1 e TEM images of graphene-encapsulated silica spheres (a) and 3D-G (b); SEM images of 3D-G (c) and 2D-G (d). Please cite this article in press as: Zhu H, et al., Three-dimensional porous graphene supported Ni nanoparticles with enhanced catalytic performance for Methanol electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.02.189
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Fig. 2 e XRD patterns of the 3D-G and 2D-G (a); N2 adsorptionedesorption curves of the 3D-G and the 2D-G (b).
Fig. 3 e The XRD patterns of the Ni/3D-G, Ni/2D-G and Ni catalysts.
The morphology and structure of the Ni/3D-G, Ni/2D-G and pristine Ni catalysts were characterized by SEM and TEM, as shown in Fig. 4. As can be seen, the SEM image (Fig. 4a) of the Ni/3D-G catalyst indicate that the macropores of 3D porous
graphene frameworks are well retained after hybridization with Ni nanoparticles, which will be conducive to improve the effective contact between electrolyte and active material in the fuel cell. The TEM image (Fig. 4b) of Ni/3D-G catalyst confirms that the nickel nanoparticles with high density are uniformly anchored on the pore wall surface of the 3D graphene without agglomeration, and the average sizes of Ni nanoparticles in the composite are 15e20 nm. Fig. 4c gives the SEM images of the Ni/2D-G catalyst, we can clearly see that the most Ni nanoparticles aggregated together, and the graphene sheets are not very flat and tightly wrapped Ni nanoparticles, which can be ascribed to the 2D graphene have restacked together during the synthesis stage. The TEM image of the Ni/ 2D-G catalyst (Fig. 4d) exhibits that nickel nanoparticles with size around 100 nm are embedded on the graphene sheets, but the average size of Ni nanoparticles are larger than that of nanoparticles in Ni/3D-G composite, and dispersion of nanoparticles is not well. Both the SEM image (Fig. 4e) and the TEM image (Fig. 4f) of the pristine Ni catalyst display that the irregular nickel nanoparticles with size of 100e150 nm are agglomerated together. Based on the morphology and structure results of the Ni/3D-G, Ni/2D-G and pristine Ni catalysts, suggesting that the adding of graphene into the precursor solution could decrease the particle size and inhibit the agglomeration of Ni during the crystallization process. Besides, in contrast to Ni/2D-G, the Ni nanoparticles with smaller size and distribute more uniformly on the surface of graphene, which will make the reaction more easily between the electrolyte and the catalyst particles. The cyclic voltammograms of Ni/3D-G (70 wt.%, Ni loading), Ni/2D-G (70 wt.%, Ni loading) and pristine Ni catalysts in 1.0 M KOH solution at a scanning speed of 50 mV s1 are represented in Fig. 5. In order to activate the Ni-based catalysts by creating NiOOH layer on the catalysts surface, the cyclic voltammetry was tested in the voltage range of 0.0e0.8 V (vs. Hg/HgO) after scanning this electrode for 10 cycles [9]. As can be seen, the Ni/3D-G catalyst presents a strong anodic peak at 0.47 V can be related to the oxidation of Ni2þ to Ni3þ, and an obvious cathodic peak at 0.38 V is attributed to the reduction of Ni3þ to Ni2þ, respectively. The oxidation and reduction process of electrochemical about Nibased catalysts can be explained as follows [36e38]:
Ni þ 2OH / Ni(OH)2 þ 2e
(1)
Ni(OH)2 þ OH / NiOOH þ H2O þ e
(2)
Compared with the Ni/3D-G catalyst, Ni/2D-G catalyst also exhibits a pair of redox peaks, however, it has a much broader separation and lower current densities than the Ni/3D-G electrocatalyst, indicating that it may have a poorer cycling performance. For the pristine nickel catalyst, almost no significant redox couple is seen, indicating that the electrochemical activity is the worst. Based on the above comparison, it can be reasonably conclude that the 3D-graphene can greatly promote the charge transfer during the redox process of catalysts, which benefit from the 3D graphene with high specific surface area and conductivity.
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Fig. 4 e Morphology and microstructure of the as-synthesized catalysts: SEM (a) and TEM (b) images for Ni/3D-G; SEM (c) and TEM (d) images for Ni/2D-G; SEM (e) and TEM (f) images for Ni catalyst.
methanol oxidation, and the oxidation mechanism is attributed to the following reaction [36,39]: NiOOH þ CH3OH / Ni(OH)2 þ product
Fig. 5 e Cyclic voltammograms of Ni/3D-G, Ni/2D-G and Ni catalysts in 1.0 M KOH solution at a scan rate of 50 mV s¡1.
The effect of methanol concentration on the electrocatalytic activity of Ni/3D-G catalyst is given in Fig. 6. The KOH concentration was fixed at 1.0 M and the CH3OH concentration varied from 0.00 to 2.00 M with scan rate of 50 mV s1. As exhibited in the CV curves (Fig. 6a), a pair of well defined anodic and cathodic peaks can be detected at about 0.47 V and 0.38 V, respectively, which is related to the transformation of the Ni(OH)2/NiOOH. After increasing the concentration of methanol in the electrolyte, the anodic current densities of Ni/ 3D-G increase with the formation of NiOOH and finally obtain an obvious anodic peak locate at 0.67 Ve0.72 V, corresponding to the methanol electrooxidation process. Besides, the current densities recorded in the electrolyte containing methanol are much higher than that in electrolyte only containing KOH, suggesting that NiOOH plays a catalytic role in the process of
(3)
As shown, the peak current densities increase with increasing methanol content up to 0.75 M, indicating that the Ni/3D-G processes good electrocatalytic activity towards methanol electrooxidation reaction. As seen in Fig. 6b, when the CH3OH concentration is higher than 0.75 M, the oxidation peaks become much broader and appear at the higher positive potential values, which can be attributed to increasing of the concentration of the unoxidized organic residue with increasing the concentration of methanol which requires higher potential. Moreover, although the oxidation peak current continues to increase, the corresponding cathodic peak current is gradually reduced, this phenomenon confirms that the reversibility of the reaction becomes poor. To investigate the kinetic characteristics of methanol electrooxidation on the surface of Ni/3D-G, Fig. 7 displays the cyclic voltammograms of Ni/3D-G at various scan rates of 10e200 mV s1 in 1 M KOH electrolyte containing 0.75 M CH3OH. As seen from Fig. 7a, with the increase of scanning rate, the anodic peak gradually shifts to more positive potentials, while the cathodic peak shifts to more negative potentials. This displacement of redox peaks are caused by irreversible oxidation reaction process. Besides, the oxidation peak also appeared in the negative scan, which is associated with the further oxidation of CH3OH and the desorption of intermediate products. The relationship of the methanol oxidation peak current densities and the square root of scan rates is presented in Fig. 7b. As can be seen, a positive linear relationship between the methanol oxidation peak current densities and the square root of the scan rates, demonstrating that the electrocatalytic reaction process on the surface of the Ni/3D-G is controlled by the diffusion of methanol.
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Fig. 6 e Cyclic voltammograms of the Ni/3D-G (70 wt.%, Ni loading) at a scan rate of 50 mV s¡1, and 1.0 M KOH at different concentration of methanol (a). The effect of higher methanol concentration (1 M and 2 M) on the electrocatalytic activity of Ni/3D-G catalyst in the voltage range of 0.0e1.2 V (vs. Hg/HgO) (b).
The electrocatalytic performance of Ni/3D-G with different nickel loadings (Ni/3D-G with 50 wt.%, 60 wt.%, 70 wt.% and 80 wt.% Ni loading) were also studied by cyclic voltammograms and the results are presented in Fig. 8. By comparision, the electrocatalytic activity of Ni/3D-G catalyst increased with the increase of loading amount of nickel in the catalyst up to 70 wt.%. However, more increase of nickel content than 70 wt.% lead to a decline in the electrocatalytic activity of Ni/3D-G. With 50 wt.%, 60 wt.%, 70 wt.% and 80 wt.% Ni loading, the anodic peak current densities of Ni/3D-G catalyst are 44.3, 52.4, 64.6 and 55.5 mA cm2, respectively. Generally, the anodic peak current densities increase as the amount of the nickel increase, but eventually the anodic peak current densities become lower due to the excess nickel on the surface of the 3D-G can negatively affect the materials, reducing the catalytic activity, which results from that the too much nickel particles can reduce the exposed area of catalyst surface active sites, and thus suppressing the transfer path of electron. Hence, the optimal
Fig. 7 e Cyclic voltammograms of Ni/3D-G catalyst at various scan rate (a) and plots of the peak current density against the square root of the scan rate for Ni/3D-G catalyst (b) in 1.0 M KOH solution containing 0.75 M methanol.
Fig. 8 e Study effect of nickel loading on electrocatalytic activity of Ni/3D-G toward methanol oxidation in 1.0 M KOH þ 0.75 M methanol solution at a scan rate of 50 mV s¡1.
Please cite this article in press as: Zhu H, et al., Three-dimensional porous graphene supported Ni nanoparticles with enhanced catalytic performance for Methanol electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.02.189
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loading of nickel is 70 wt.% and the peak current density is 64.6 mA cm2 at 0.70 V for Ni/3D-G catalyst. To examine the influence of the introduction of graphene sheets on the electrochemical features of the Ni-based electrocatalysts, the electrocatalytic methanol activity of Ni/3D-G catalyst was compared with that of Ni/2D-G and pristine Ni catalysts. Fig. 9a displays the CV results of the Ni/3D-G, Ni/2DG and Ni catalysts in 1 M KOH electrolyte containing 0.75 M CH3OH at a scan rate of 50 mV s1 ranging from 0.0 to 0.8 V (vs. Hg/HgO). As shown in the CV curves, both the Ni/3D-G and Ni/ 2D-G catalysts show a relatively high electrocatalytic activity when compared with the pristine Ni catalyst. More explicitly, the peak current density for CH3OH electrooxidation on Ni/3DG catalyst is 64.6 mA cm2, which is about 1.5 times as high as that on Ni/2D-G catalyst at the same Ni loading percentage under otherwise identical conditions. For the Ni/2D-G catalyst, although exhibits much better performances than Ni catalyst, has lower electrocatalytic activity than Ni/3D-G catalyst is attributed to the stacking of graphene sheets and larger particle sizes in the Ni/2D-G catalyst lead to insufficient contact between the active sites and electrolyte. It is demonstrated that the graphene as a support in the electrode
Fig. 9 e Cyclic voltammograms of the Ni/3D-G, Ni/2D-G and Ni catalysts in 1.0 M KOH þ 0.75 M methanol solution at a scan rate of 50 mV s¡1 (a). The Tafel plots of the prepared Ni-based catalysts (b).
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strongly enhanced the electrochemical performance of nickel catalyst, and the facilitating role of the 3D-G is more obvious than the 2D-G. In order to determine of kinetic parameters of the electrooxidation of methanol at the Ni/3D-G, Ni/2D-G and pristine Ni catalysts, the Tafel plots of the three catalysts are recorded in 1 M KOH electrolyte containing 0.75 M CH3OH solution at a scan rate of 1 mV s1. Fig. 9b depicts the obtained plots of the Tafel region and the Tafel plots were fitted in to the Tafel equation within the linear regions. The Tafel slope value could be calculated using the following equation for the anodic reactions: logI ¼ logIo þ ½ð1 aÞnF=2:303RTE
(4)
where the constants R and F denote the universal gas constant and the Faraday constant, respectively, T is the temperature (K), a is the charge transfer coefficient of the reaction n is the number of electrons transferred during the oxidation reaction, Io is the exchange current density and a is the charge transfer coefficient of the reaction. The slope of Tafel plots are 151.7, 197.6 and 373.5 mV dec1 for methanol electrooxidation at the Ni/3D-G, Ni/2D-G and pristine Ni, respectively. In the electrochemical oxidation of methanol, the rate-determining step is a one-electron process, thus, a values should be 0.12, 0.48 and 0.65 for methanol electrooxidation at the Ni/3D-G, Ni/2D-G and pristine Ni, respectively. Furthermore, the Io value of Ni/ 3D-G is 4.72 106 A cm2, which is much higher than that of Ni/2D-G and pristine Ni catalysts. Both the larger charge transfer coefficient and the higher exchange current density confirm again the enhanced methanol electrocatalytic oxidation reaction kinetics obtained on the prepared Ni/3D-G over Ni/2D-G and pristine Ni catalysts. To estimate the electrocatalytic stability of the as-prepared catalysts toward the methanol oxidation reaction, the results of chronoamperometric experiments on the Ni/3D-G and Ni/ 2D-G catalysts under the same loading are presented in Fig. 10. The test was carried out by holding the potential at 0.70 V (vs. Hg/HgO) in 1 M KOH electrolyte containing 0.75 M CH3OH for about 3750 s. As shown in the curves, the oxidation
Fig. 10 e Chronoamperometric curves for Ni/3D-G and Ni/ 2D-G at an oxidation potential 0.70 V (vs. Hg/HgO) in 1.0 M KOH þ 0.75 M methanol solution.
Please cite this article in press as: Zhu H, et al., Three-dimensional porous graphene supported Ni nanoparticles with enhanced catalytic performance for Methanol electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.02.189
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current densities decrease rapidly for both of the catalysts in the first 150 s, then slowly and gradually decreased to achieve a quasi-equilibrium steady state. However, The decreasing degree of Ni/3D-G is smaller than that of Ni/2D-G, and the steady-state current density of Ni/3D-G after 3750 s is 30.8 mA cm2, which is approximately 1.5 times higher than that of Ni/2D-G catalyst. By comparison, the initial and steadystate current densities on Ni/3D-G catalyst are obviously higher than that on Ni/2D-G, demonstrating that the Ni/3D-G catalyst possessed excellent catalytic durability during the methanol oxidation reaction. The superior electrocatalytic performance of the Ni/3D-G catalyst demonstrated above may be originate from the unique structure advantages offered by the 3D graphene. Firstly, 3D graphene provides more growth sites for Ni to limit the particle size and agglomeration in the crystallization process, the uniformly dispersed particles with smaller size can expose more active sites for the methanol oxidation reaction. Secondly, 3D graphene with porous structure serve as a fast three dimensional channel for electrolyte diffusion can enlarge the contact area between the catalyst and the electrolyte. Moreover, with inherent characteristics of graphite, 3D graphene constructs a interconnected conductive network for accelerating the electron transfer on the Ni catalyst surface during the electrooxidation processes.
Conclusion In summary, we use 3D porous graphene as an excellent support for the growth of Ni nanoparticles, successfully obtained Ni/3D-G composite. The small-sized Ni nanoparticles with high density are uniformly distributed on the pore wall surface of the 3D graphene and the Ni/3D-G catalyst exhibits outstanding electrocatalytic performance toward the methanol oxidation reaction. The maximum obtained oxidation peak current density is 64.6 mA cm2 at 0.70 V in 0.75 M CH3OH þ 1.0 M KOH for Ni/3D-G (70 wt.% Ni loading), which is much higher than Ni/2D-G and pristine Ni catalysts. 3D graphene plays an important role in improving the electrochemical performance of catalyst. In the composite, 3D graphene not only limits particle size and agglomeration is beneficial to increase the catalyst active site, but also provides a effective three dimensional passageway for facilitating electrolyte diffusion and electron transfer on the catalyst surface. Based on the above results, the catalyst structure and preparation method mentioned in this study is easy to realize and the 3D graphene serve as a promising catalyst support can be widely applied to the construction of other threedimensional structure materials and prepare good electrocatalyst materials in fuel cell applications.
Acknowledgments This study was the financially supported by Science and Technology Research Project of Education Department of Hubei Province (Q20162804) and Start up foundation of Hubei University of Science and Technology (BK1519).
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Please cite this article in press as: Zhu H, et al., Three-dimensional porous graphene supported Ni nanoparticles with enhanced catalytic performance for Methanol electrooxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.02.189