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NiCo2O4 nanostructures with various morphologies as the high-performance electrocatalysts for H2O2 electroreduction and electrooxidation
Journal of Electroanalytical Chemistry 729 (2014) 103–108
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
NiCo2O4 nanostructures with various morphologies as the high-performance electrocatalysts for H2O2 electroreduction and electrooxidation Xue Xiao a,c, Fan Yang a,b, Kui Cheng a, Xin Wang a, Jinling Yin a, Ke Ye a, Guiling Wang a, Dianxue Cao a,⇑ a Key Laboratory of Superlight Material and Surface Technology of Ministry of Education, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China b College of Science, Northeast Agricultural University, Harbin 150030, China c College of Materials and Chemical Engineering, Heilongjiang Institute of Technology, Harbin 150050, China
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Article history: Received 19 February 2014 Received in revised form 5 June 2014 Accepted 8 July 2014 Available online 17 July 2014 Keywords: Nickel cobaltite Nanostructures Hydrogen peroxide Electroreduction Electrooxidation
a b s t r a c t Ni foam supported-NiCo2O4 nanostructures with various morphologies are successfully prepared by a facile template-free method. The synthesis involves the electrodeposition of the bimetallic (Ni, Co) film on Ni foam support. The NiCo2O4 nanostructures are formed by immersing the bimetallic film in an oxalic acid solution, followed by a calcination progress. The control over NiCo2O4 nanostructures with different morphologies is achieved by adjusting the electrodeposition time and immersing time. The electrode is characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. H2O2 electrooxidation and electroreduction in KOH solution on the NiCo2O4 nanostructures are studied by cyclic voltammetry and chronoamperometry. Results show that NiCo2O4 nanostructures exhibit high performance and superior stability for both H2O2 electrooxidation and electroreduction, resulting from hierarchical porous structure inside their architectures. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The ever worsening energy depletion and global warming issues call for urgent development of clean alternative energies. Low-temperature fuel cells provide efficient and clean power generation from a range of fuels for application in both stationary and portable devices. Lately, special attention has been given to the use of hydrogen peroxide (H2O2) as the oxidant in fuel cells instead of O2, notably, for several types of low temperature liquid-based fuel cells [1–7]. In fact, H2O2 is not only as the major stable oxidizing species, but also as the energy storage material (H2O2 = H2O + 1/2O2, DG = 120 kJ/mol), thus it can be as the fuel in fuel cells (electron donor) (H2O2 = 1/2O2 + 2H+ + 2e). On the above theoretical basis, a novel type of fuel cell using H2O2 both as the fuel and the oxidant has been built, namely direct peroxide-peroxide fuel cell (DPPFC) [8–16]. To boost the cell performance of H2O2-based fuel cell, the preparation of electrocatalysts for H2O2 electroreduction and electrooxidation needs to be in great development. In general,
⇑ Corresponding author. Tel./fax: +86 451 82589036. E-mail address: [email protected] (D. Cao). http://dx.doi.org/10.1016/j.jelechem.2014.07.010 1572-6657/Ó 2014 Elsevier B.V. All rights reserved.
noble metals [10] exhibit high electrocatalytic activity towards both H2O2 electroreduction and electrooxidation, however, also have the high catalysis to H2O2 decomposition. Non-noble metals [12] only catalyzed H2O2 electrochemical oxidation rather than resulting in H2O2 reduction. In recent years, transition metal oxides (Co3O4, NiCo2O4, La1xSrxMnO3) are attracting great interest due to their enormous applications [17–27]. In general, they have been demonstrated to be the high-performance electrocatalysts for H2O2 electroreduction [20,28–33]. Furthermore, recent study [34] has been reported that transition metal oxides also have catalytic activity for H2O2 electrooxidation. Among the transition metal oxides, NiCo2O4 is widely studied due to low cost, abundant resources, controllable size and shape. In addition, NiCo2O4 possesses much better electrical conductivity, at least two orders of magnitude higher, and higher electrochemical activity than Co3O4 and also possesses excellent redox property due to the contributions from both nickel and cobalt ions [20]. It is well known that the size and morphology of materials has received increased attention due to the fact that they play very important roles in determining electrical, optical, and other properties [35–38]. In general, special morphology possesses the exciting performance. Therefore, developing facile, morphology-controlled approaches to building novel self-generated architecture of
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NiCo2O4 is the hotspot lately. Solid templates or structure-directing agents are commonly used to fabricate NiCo2O4 materials with hierarchical structures [39–44]. However, impurities can be introduced from the templates or agents that can affect the properties adversely. In this work, we reported the first successful preparation of morphology-controlled NiCo2O4 nanostructures supported on Ni foam substrate towards H2O2 electroreduction and electrooxidation through a simple template-free method. Various morphologies including nanowires, nanothorns, and nano-honeycombs had been readily prepared. Ni foam was used as the substrate due to its three-dimensional (3D) open structure, ensuring the full utilization of catalyst surfaces and also enabling O2 produced by H2O2 electrooxidation or hydrolysis to diffuse away from the electrode. Electrochemical experiments revealed that the as-prepared samples exhibited excellent catalytic activity and superior stability towards H2O2 electroreduction and electrooxidation in alkaline medium.
2. Experimental Three steps should be included in a typical synthesis of NiCo2O4 nanostructures. Firstly, the bimetallic (Ni, Co) film was electrodeposited onto Ni foam substrate (110 PPI, 320 g m2, 1.1 mm in thickness, Changsha Lyrun Material Co. Ltd., China) in a threeelectrode electrochemical cell with Ni foam working electrode, platinum foil counter electrode and saturated calomel reference electrode using containing 51.2 g L1 CoSO47H2O (>99.0%), 22.6 g L1 NiSO46H2O (>99.0%)and 70 mL L1 triethanolamine as the deposition solution. The electrode was obtained by applying a constant current of 10 mA cm2 for 20–40 min via a computerized potentiostat (Autolab PGSTAT302, Eco Chemie) controlled by GPES software. Prior to the electrodeposition, the Ni foam was pretreated following the procedure reported in literature [25]. Secondly, the obtained bimetallic film was immersed into a mixed solvent containing 0.3 mol L1 oxalic acid ethanol solution with 5% H2O, and then was kept at 45 °C for 1.5–5 h without stirring. Finally, after being washed with ethanol, the precursor film was calcined at 250 °C for 3 h in a muffle furnace to obtain the resulting NiCo2O4 nanostructures. The electrode was characterized using a scanning electron microscope (SEM, JEOL JSM-6480), a transmission electron microscopy (TEM, FEI Teccai G2S-Twin, Philips) and an X-ray diffractometer (Rigaku TTR III) with Cu Ka radiation (k = 0.1514178 nm). H2O2 electroreduction and electrooxidation were carried out in KOH solution at ambient temperature (20 ± 2 °C) under N2 atmosphere using the same electrochemical cell mentioned above. The reported current densities were calculated using the geometrical area of the electrode. All solutions were made with analytical grade chemical reagents and ultra-pure water (Milli-Q 18 MX cm). All potentials were referred to the reference electrode unless specified.
3. Results and discussion Fig. 1 shows the XRD pattern of the as-prepared NiCo2O4 nanostructures supported on Ni foam substrate. For the sake of eliminating the influence from the Ni foam, we scraped the sample powder from the Ni foam substrate after calcination. As observed in Fig. 1, seven well-defined diffraction peaks are observed at 2h values of 18.9°, 31.1°, 36.6°, 44.6°, 55.4°, 59.1° and 64.9°. All of these peaks can be successfully indexed to (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) plane reflections of the spinel NiCo2O4 crystaline structure (JCPDF card No. 20-0781; space group: F ⁄ 3 (2 0 2)).
Fig. 1. XRD pattern of the as-prepared NiCo2O4 nanostructures.
Fig. 2 shows the SEM images of NiCo2O4 nanostructures supported on Ni foam in various morphologies obtained by adjusting the preparation parameters. The NiCo2O4 nanowire arrays can be always obtained at the same electrodeposition time, and the immersing time has an effect not on the morphology, but on the size (Fig. 2A, C and E). The lengths of these nanowires are in the range of 500 nm1 lm (Fig. 2B, D and F). Clearly, the skeletons of Ni foam were completely covered by nanowires, which grew densely and almost vertically from the substrate. It can be seen that the morphology of NiCo2O4 nanostructures is dependent on the electrodeposition time. When the electrodeposition time was extended to 30 min, the morphology of products was transformed to nanothorns, and several thorns seemed to have a common or a joint basis forming a flower-like structure (Fig. 2G and H). With the further increase of electrodeposition time to 40 min, the products display honeycomb-like superstructures, and the entire structure is built from numerous nanowires (Fig. 2I and J). It is worth noting that after oxidative conversion into spinel NiCo2O 4, the basic morphology of the samples is perfectly conserved without calcination-induced significant alterations. The effect of electrodeposition time and immersing time on the size of NiCo2O4 nanostructures was slight and the increase of electrodeposition time and immersing time would result in the waste of resources and energy. So, the best electrode of various NiCo2O4 nanostructures supported on Ni foam in this study was obtained by the electrodeposition of bimetallic (Ni, Co) film at 10 mA cm2 for 20 min and then be immersed in 3.0 mol L1 oxalic acid ethanol solution with 5% H2O for 1.5 h. The electrocatalytic performances of the best electrode for H2O2 electroreduction and electrooxidation were investigated by cyclic voltammetry. Transmission electron microscopy (TEM) measurements were carried out to further investigate the structure of the as-synthesized NiCo2O4 nanowire array electrode, and the corresponding SEM image is shown in Fig. 2A and B. As observed, NiCo2O4 film exhibits similar nanowire array morphology with the diameter and length of approximatively 40–50 nm and 500–600 nm, respectively. In addition, numerous inter-particle mesopores with a size ranged from 2 to 5 nm in these nanowires can be clearly seen (Fig. 3A and B), which results from the thermal decomposition of the mixed metal (Ni, Co) oxalate precursor releasing CO2 and H2O gases [45]. The porous structure can greatly increase the surface area and also facilitate the mass transport of electrolytes within the electrodes. The crystal orientation and growth direction of the NiCo2O4 nanowires were studied by high resolution transmission electron microscopy (HRTEM) (insert of Fig. 3B). It can be seen that the tip shows clear lattice fringes with the same
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Fig. 2. SEM images of various NiCo2O4 nanostructures supported on Ni foam obtained by the electrodeposition of bimetallic (Ni, Co) film at 10 mA cm2 for 20 min (A and B), 30 min (G and H) and 40 min (I and J) and then be immersed in 3.0 mol L1 oxalic acid ethanol solution with 5% H2O for 1.5 h; 3 h (C and D); 5 h (E and F).
orientation and spacing. The spacing between adjacent fringes is ca. 0.47 nm, close to the theoretical interplane spacing of spinel NiCo2O4 (1 1 1) planes, consistent with other NiCo2O4 nanostructures in the literature [20].
The catalytic performance of a series of NiCo2O4 nanostructures electrodes toward H2O2 electroreduction and electrooxidation were investigated under alkaline conditions. Fig. 4A and B shows the curves for H2O2 electroreduction and electrooxidation recorded
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Fig. 3. Different-magnification TEM images of NiCo2O4 nanowire array electrode (A and B). Insert is the corresponding HRTEM image.
low valence cobalt and nickel species (including Co2+, Ni2+) also be oxidated to high valence cobalt species (Co3+, Co4+, Ni3+). Therefore, it is reasonable to believe that H2O2 is reduced or oxidated by the interconversion among different valence nickel and cobalt species. It can be seen from Fig. 4 that various morphologies of NiCo2O4 nanostructures have different catalytic activity for H2O2 electroreduction and electrooxidation. The characterization of the surface of nanostructures is a very important issue in the applications of nanostructures such as catalysts and sensors. In order to examine characteristic properties of the NiCo2O4 nanostructure surface, we use the NiCo2O4 nanowire array electrode (the corresponding SEM images are shown in Fig. 2A and B) as an example, and the surface of Ni foam substrate and NiCo2O4 nanowire array electrode characterized electrochemically by cyclic voltammograms (CV) in a 3.0 mol L1 KOH solution at a scan rate of 50 mV s1 is shown in Fig. 5. The CV curve of Ni foam substrate only showed a redox couple, which is due to the reversible reactions of Ni(II)/Ni(III) formed on the nickel surface in the alkaline electrolyte [10]. The CV measurement of NiCo2O4 nanowire array electrode displayed the characteristic response of NiCo2O4 in alkaline solution. The anodic peaks were caused by the convertion of Co(II)/Co(III) (Eq. (1)) and further oxidated to Co(IV) (Eq. (2)), also included the oxidation reaction of Ni(II)/Ni(III) (Eq. (3)). In the reversible sweep, the cathodic peaks belonged to the reduction of Co(IV)/Co(III), then to Co(II), as well as the reaction of Ni(III)/Ni(II) [46–48]. Since the redox potentials of Ni(II)/Ni(III) and Co(III)/Co(IV) are in close proximity, the two reactions exhibited a large anodic peak at 0.43V in common [46].
Fig. 4. The cyclic voltammetry curves of the as-prepared NiCo2O4 nanostructures for H2O2 electroreduction (A) and H2O2 electrooxidation (B) at a scanning rate of 10 mV s1.
in the solution of 3.0 mol L1 KOH + 0.4 mol L1 H2O2, and 3.0 mol L1 KOH + 0.75 mol L1 H2O2 respectively, at a scanning rate of 10 mV s1. In order to distinguish NiCo2O4 nanostructures prepared under different conditions from each other, we will name NiCo2O4 nanostructures shown in Fig. 2A, C, E, G and I as S1, S2, S3, S4 and S5. It can be observed from Fig. 4, the as-prepared NiCo2O4 nanostructures all exhibit high catalytic performance towards H2O2 electroreduction and electrooxidation. Generally, high valence cobalt and nickel species (including Co3+, Ni3+) in the compound of NiCo2O4 can be reduced to low valence (Co2+, Ni2+) and
Fig. 5. Cyclic voltammograms of Ni foam substrate and NiCo2O4 nanowire array electrode in 3.0 mol L1 KOH at a scan rate of 50 mV s1.
X. Xiao et al. / Journal of Electroanalytical Chemistry 729 (2014) 103–108
Co3 O4 þ OH þ H2 O () 3CoOOH þ e
ð1Þ
CoOOH þ OH () CoO2 þ H2 O þ e
ð2Þ
NiðOHÞ2 þ OH () NiOOH þ H2 O þ e
ð3Þ
Fig. 6 shows the effects of H2O2 concentration on the electrocatalytic performance for H2O2 electroreduction at NiCo2O4 nanowire array electrode (the corresponding SEM images are shown in Fig. 2A and B). The CV curves were recorded in 3.0 mol L1 KOH containing different concentrations of H2O2 at a scan rate of 10 mV s1. The potential scans started from the open circuit potential and go negatively. As seen, the cathodic currents are much larger in the presence of H2O2 than that without H2O2. When H2O2 concentration is lower than 0.2 mol L1, diffusion control can be obviously observed. As H2O2 concentration increased, the reduction current density remarkably increased approximately linearly. During the test at high H2O2 concentration, there are a lot of gas bubbles appear on the electrode surface due to the instability of H2O2 in alkaline medium. A current density of 330 mA cm2 was achieved at 0.6V in the solution containing 0.4 mol L1 H2O2, of course at the expense of significant H2O2 decomposition, and a current density of 190 mA cm2 was achieved at 0.4V in 0.4 mol L1 H2O2 and 3.0 mol L1 KOH by Ni foam supported-NiCo2O4 nanostructures with various morphologies, which exhibited much better catalytic activity than NiCo2O4 nanowire electrodes in the literature [49,50]. Electrodes with porous construction led to large surface area, which can result in excellent catalytic activity and good mass transport. The stability of electrode is a key parameter to determine the quality of electrode, and further influences the cell stability. The chronoamperometric curves for H2O2 electroreduction at NiCo2O4 nanowire array electrode were also investigated (Fig. 7), at three different potentials determined by the CV curves in Fig. 6. Each chronoamperometric curve was tested using the same NiCo2O4 nanowire array electrode. The higher the reduction potential, the larger the reduction current density, which is in agreement with CV curves in Fig. 6. At low reduction potential (0.25V and 0.3V), the curves remained smooth without any fluctuation during the 1200s test period, indicating that the NiCo2O4 nanowire array electrode has superior stability. At high reduction potential (0.4V), there can be seen the obvious fluctuations in the curve, and a lot of gas bubbles appeared on the surface of electrode. H2O2 concentration also has a great influence on the electrochemical behavior of H2O2 electrooxidation. It can be seen from Fig. 8, the starting oxidation potential is around 0.18V (vs. SCE), which is independent of H2O2 concentration. At 0.2V, the oxidation current densities increased significantly from 245 mA cm2 to
Fig. 6. Cyclic voltammograms of NiCo2O4 nanowire array electrode in 3.0 mol L1 KOH + x mol L1 H2O2 at a scanning rate of 10 mV s1 (x = 0, 0.2, 0.4, 0.6 and 0.8).
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Fig. 7. Chronoamperometric curves for H2O2 electroreduction at different potentials in 3.0 mol L1 KOH + 0.2 mol L1 H2O2.
Fig. 8. Cyclic voltammograms of NiCo2O4 nanowire array electrode in 3.0 mol L1 KOH + x mol L1 H2O2 at a scanning rate of 10 mV s1 (x = 0.25, 0.5, 0.75 and 1.0).
520 mA cm2 with the increase of H2O2 concentration from 0.25 mol L1 to 1.0 mol L1. Each curve has the same characteristic, namely line-like without any peaks. Results revealed that NiCo2O4 nanowire array electrode can be conceived as a promising costeffective and scalable alternative for H2O2 electrooxidation since it offers many advantages such as low cost, abundant resources and environmental friendliness. Fig. 9 displayed the chronoamperometric curves for H2O2 electrooxidation at different potentials in 3.0 mol L1 KOH + 0.75 mol L1 H2O2. There is a decrease in currents within the first
Fig. 9. Chronoamperometric curves for H2O2 electrooxidation at different potentials in 3.0 mol L1 KOH + 0.75 mol L1 H2O2.
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seconds, and the oxidation currents at each potential remained nearly constant during the test period. After 1200s test, the oxidation currents at 0V, 0.1V and 0.2V were 135 mA cm2, 290 mA cm2 and 368 mA cm2, respectively. 4. Conclusions In summary, we have developed a novel preparation of morphology-controlled NiCo2O4 nanostructures supported on Ni foam through a facile template-free method. Ni foam was used as the substrate due to its three-dimensional (3D) porous structure, ensuring the full utilization of catalyst surfaces and also enabling O2 produced by H2O2 electrooxidation or hydrolysis to diffuse away from the electrode. By adjusting the preparation parameters including electrodeposition time and immersing time, nanowires, nanothorns and nano- honeycomb with various sizes have been successfully prepared. The resulting electrodes all exhibit catalytic performance for H2O2 electroreduction and electrooxidation due to the interconversion among different valence nickel and cobalt species in the compound of NiCo2O4. As NiCo2O4 nanowire array electrode an example, a reduction current density of 330 mA cm2 was achieved at -0.6V in the solution containing 3.0 mol L1 KOH and 0.4 mol L1 H2O2, and an oxidation current density of 520 mA cm2 was achieved at 0.2V in the solution containing 3.0 mol L1 KOH and 1.0 mol L1 H2O2, respectively. Results revealed that NiCo2O4 nanostructures can be conceived as a promising cost-effective and scalable alternative for H2O2 electroreduction and electrooxidation since it offers many advantages such as low cost, abundant resources and environmental friendliness. Conflict of interest There is no conflict of interest. Acknowledgements We gratefully acknowledge the finance supported by the National Nature Science Foundation of China (21306033), the Fundamental Research Funds for the Central Universities (HEUCF201403018) and the Heilongjiang Postdoctoral Fund (LBH-Z13059). References [1] D. Cao, Y. Gao, G. Wang, R. Miao, Y. Liu, Int. J. Hydrogen Energy 35 (2010) 807. [2] C. Shu, E. Wang, L. Jiang, Q. Tang, G. Sun, J. Power Sources 208 (2012) 159. [3] D.M.F. Santos, P.G. Saturnino, R.F.M. Lobo, C.A.C. Sequeira, J. Power Sources 208 (2012) 131. [4] E. Kjeang, A.G. Brolo, D.A. Harrington, N. Djilali, D. Sinton, J. Electrochem. Soc. 154 (2007) B1220. [5] C. Ponce de León, F.C. Walsh, C.J. Patrissi, M.G. Medeiros, R.R. Bessette, R.W. Reeve, J.B. Lakeman, A. Rose, D. Browning, Electrochem. Commun. 10 (2008) 1610. [6] D.J. Brodrecht, J.J. Rusek, Appl. Energy 74 (2003) 113. [7] C.P. de León, F.C. Walsh, A. Rose, J.B. Lakeman, D.J. Browning, R.W. Reeve, J. Power Sources 164 (2007) 441.
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NiCo2O4 nanostructures with various morphologies as the high-performance electrocatalysts for H2O2 electroreduction and electrooxidation Xue Xiao a,c, Fan Yang a,b, Kui Cheng a, Xin Wang a, Jinling Yin a, Ke Ye a, Guiling Wang a, Dianxue Cao a,⇑ a Key Laboratory of Superlight Material and Surface Technology of Ministry of Education, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China b College of Science, Northeast Agricultural University, Harbin 150030, China c College of Materials and Chemical Engineering, Heilongjiang Institute of Technology, Harbin 150050, China
a r t i c l e
i n f o
Article history: Received 19 February 2014 Received in revised form 5 June 2014 Accepted 8 July 2014 Available online 17 July 2014 Keywords: Nickel cobaltite Nanostructures Hydrogen peroxide Electroreduction Electrooxidation
a b s t r a c t Ni foam supported-NiCo2O4 nanostructures with various morphologies are successfully prepared by a facile template-free method. The synthesis involves the electrodeposition of the bimetallic (Ni, Co) film on Ni foam support. The NiCo2O4 nanostructures are formed by immersing the bimetallic film in an oxalic acid solution, followed by a calcination progress. The control over NiCo2O4 nanostructures with different morphologies is achieved by adjusting the electrodeposition time and immersing time. The electrode is characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. H2O2 electrooxidation and electroreduction in KOH solution on the NiCo2O4 nanostructures are studied by cyclic voltammetry and chronoamperometry. Results show that NiCo2O4 nanostructures exhibit high performance and superior stability for both H2O2 electrooxidation and electroreduction, resulting from hierarchical porous structure inside their architectures. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The ever worsening energy depletion and global warming issues call for urgent development of clean alternative energies. Low-temperature fuel cells provide efficient and clean power generation from a range of fuels for application in both stationary and portable devices. Lately, special attention has been given to the use of hydrogen peroxide (H2O2) as the oxidant in fuel cells instead of O2, notably, for several types of low temperature liquid-based fuel cells [1–7]. In fact, H2O2 is not only as the major stable oxidizing species, but also as the energy storage material (H2O2 = H2O + 1/2O2, DG = 120 kJ/mol), thus it can be as the fuel in fuel cells (electron donor) (H2O2 = 1/2O2 + 2H+ + 2e). On the above theoretical basis, a novel type of fuel cell using H2O2 both as the fuel and the oxidant has been built, namely direct peroxide-peroxide fuel cell (DPPFC) [8–16]. To boost the cell performance of H2O2-based fuel cell, the preparation of electrocatalysts for H2O2 electroreduction and electrooxidation needs to be in great development. In general,
⇑ Corresponding author. Tel./fax: +86 451 82589036. E-mail address: [email protected] (D. Cao). http://dx.doi.org/10.1016/j.jelechem.2014.07.010 1572-6657/Ó 2014 Elsevier B.V. All rights reserved.
noble metals [10] exhibit high electrocatalytic activity towards both H2O2 electroreduction and electrooxidation, however, also have the high catalysis to H2O2 decomposition. Non-noble metals [12] only catalyzed H2O2 electrochemical oxidation rather than resulting in H2O2 reduction. In recent years, transition metal oxides (Co3O4, NiCo2O4, La1xSrxMnO3) are attracting great interest due to their enormous applications [17–27]. In general, they have been demonstrated to be the high-performance electrocatalysts for H2O2 electroreduction [20,28–33]. Furthermore, recent study [34] has been reported that transition metal oxides also have catalytic activity for H2O2 electrooxidation. Among the transition metal oxides, NiCo2O4 is widely studied due to low cost, abundant resources, controllable size and shape. In addition, NiCo2O4 possesses much better electrical conductivity, at least two orders of magnitude higher, and higher electrochemical activity than Co3O4 and also possesses excellent redox property due to the contributions from both nickel and cobalt ions [20]. It is well known that the size and morphology of materials has received increased attention due to the fact that they play very important roles in determining electrical, optical, and other properties [35–38]. In general, special morphology possesses the exciting performance. Therefore, developing facile, morphology-controlled approaches to building novel self-generated architecture of
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NiCo2O4 is the hotspot lately. Solid templates or structure-directing agents are commonly used to fabricate NiCo2O4 materials with hierarchical structures [39–44]. However, impurities can be introduced from the templates or agents that can affect the properties adversely. In this work, we reported the first successful preparation of morphology-controlled NiCo2O4 nanostructures supported on Ni foam substrate towards H2O2 electroreduction and electrooxidation through a simple template-free method. Various morphologies including nanowires, nanothorns, and nano-honeycombs had been readily prepared. Ni foam was used as the substrate due to its three-dimensional (3D) open structure, ensuring the full utilization of catalyst surfaces and also enabling O2 produced by H2O2 electrooxidation or hydrolysis to diffuse away from the electrode. Electrochemical experiments revealed that the as-prepared samples exhibited excellent catalytic activity and superior stability towards H2O2 electroreduction and electrooxidation in alkaline medium.
2. Experimental Three steps should be included in a typical synthesis of NiCo2O4 nanostructures. Firstly, the bimetallic (Ni, Co) film was electrodeposited onto Ni foam substrate (110 PPI, 320 g m2, 1.1 mm in thickness, Changsha Lyrun Material Co. Ltd., China) in a threeelectrode electrochemical cell with Ni foam working electrode, platinum foil counter electrode and saturated calomel reference electrode using containing 51.2 g L1 CoSO47H2O (>99.0%), 22.6 g L1 NiSO46H2O (>99.0%)and 70 mL L1 triethanolamine as the deposition solution. The electrode was obtained by applying a constant current of 10 mA cm2 for 20–40 min via a computerized potentiostat (Autolab PGSTAT302, Eco Chemie) controlled by GPES software. Prior to the electrodeposition, the Ni foam was pretreated following the procedure reported in literature [25]. Secondly, the obtained bimetallic film was immersed into a mixed solvent containing 0.3 mol L1 oxalic acid ethanol solution with 5% H2O, and then was kept at 45 °C for 1.5–5 h without stirring. Finally, after being washed with ethanol, the precursor film was calcined at 250 °C for 3 h in a muffle furnace to obtain the resulting NiCo2O4 nanostructures. The electrode was characterized using a scanning electron microscope (SEM, JEOL JSM-6480), a transmission electron microscopy (TEM, FEI Teccai G2S-Twin, Philips) and an X-ray diffractometer (Rigaku TTR III) with Cu Ka radiation (k = 0.1514178 nm). H2O2 electroreduction and electrooxidation were carried out in KOH solution at ambient temperature (20 ± 2 °C) under N2 atmosphere using the same electrochemical cell mentioned above. The reported current densities were calculated using the geometrical area of the electrode. All solutions were made with analytical grade chemical reagents and ultra-pure water (Milli-Q 18 MX cm). All potentials were referred to the reference electrode unless specified.
3. Results and discussion Fig. 1 shows the XRD pattern of the as-prepared NiCo2O4 nanostructures supported on Ni foam substrate. For the sake of eliminating the influence from the Ni foam, we scraped the sample powder from the Ni foam substrate after calcination. As observed in Fig. 1, seven well-defined diffraction peaks are observed at 2h values of 18.9°, 31.1°, 36.6°, 44.6°, 55.4°, 59.1° and 64.9°. All of these peaks can be successfully indexed to (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) plane reflections of the spinel NiCo2O4 crystaline structure (JCPDF card No. 20-0781; space group: F ⁄ 3 (2 0 2)).
Fig. 1. XRD pattern of the as-prepared NiCo2O4 nanostructures.
Fig. 2 shows the SEM images of NiCo2O4 nanostructures supported on Ni foam in various morphologies obtained by adjusting the preparation parameters. The NiCo2O4 nanowire arrays can be always obtained at the same electrodeposition time, and the immersing time has an effect not on the morphology, but on the size (Fig. 2A, C and E). The lengths of these nanowires are in the range of 500 nm1 lm (Fig. 2B, D and F). Clearly, the skeletons of Ni foam were completely covered by nanowires, which grew densely and almost vertically from the substrate. It can be seen that the morphology of NiCo2O4 nanostructures is dependent on the electrodeposition time. When the electrodeposition time was extended to 30 min, the morphology of products was transformed to nanothorns, and several thorns seemed to have a common or a joint basis forming a flower-like structure (Fig. 2G and H). With the further increase of electrodeposition time to 40 min, the products display honeycomb-like superstructures, and the entire structure is built from numerous nanowires (Fig. 2I and J). It is worth noting that after oxidative conversion into spinel NiCo2O 4, the basic morphology of the samples is perfectly conserved without calcination-induced significant alterations. The effect of electrodeposition time and immersing time on the size of NiCo2O4 nanostructures was slight and the increase of electrodeposition time and immersing time would result in the waste of resources and energy. So, the best electrode of various NiCo2O4 nanostructures supported on Ni foam in this study was obtained by the electrodeposition of bimetallic (Ni, Co) film at 10 mA cm2 for 20 min and then be immersed in 3.0 mol L1 oxalic acid ethanol solution with 5% H2O for 1.5 h. The electrocatalytic performances of the best electrode for H2O2 electroreduction and electrooxidation were investigated by cyclic voltammetry. Transmission electron microscopy (TEM) measurements were carried out to further investigate the structure of the as-synthesized NiCo2O4 nanowire array electrode, and the corresponding SEM image is shown in Fig. 2A and B. As observed, NiCo2O4 film exhibits similar nanowire array morphology with the diameter and length of approximatively 40–50 nm and 500–600 nm, respectively. In addition, numerous inter-particle mesopores with a size ranged from 2 to 5 nm in these nanowires can be clearly seen (Fig. 3A and B), which results from the thermal decomposition of the mixed metal (Ni, Co) oxalate precursor releasing CO2 and H2O gases [45]. The porous structure can greatly increase the surface area and also facilitate the mass transport of electrolytes within the electrodes. The crystal orientation and growth direction of the NiCo2O4 nanowires were studied by high resolution transmission electron microscopy (HRTEM) (insert of Fig. 3B). It can be seen that the tip shows clear lattice fringes with the same
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Fig. 2. SEM images of various NiCo2O4 nanostructures supported on Ni foam obtained by the electrodeposition of bimetallic (Ni, Co) film at 10 mA cm2 for 20 min (A and B), 30 min (G and H) and 40 min (I and J) and then be immersed in 3.0 mol L1 oxalic acid ethanol solution with 5% H2O for 1.5 h; 3 h (C and D); 5 h (E and F).
orientation and spacing. The spacing between adjacent fringes is ca. 0.47 nm, close to the theoretical interplane spacing of spinel NiCo2O4 (1 1 1) planes, consistent with other NiCo2O4 nanostructures in the literature [20].
The catalytic performance of a series of NiCo2O4 nanostructures electrodes toward H2O2 electroreduction and electrooxidation were investigated under alkaline conditions. Fig. 4A and B shows the curves for H2O2 electroreduction and electrooxidation recorded
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Fig. 3. Different-magnification TEM images of NiCo2O4 nanowire array electrode (A and B). Insert is the corresponding HRTEM image.
low valence cobalt and nickel species (including Co2+, Ni2+) also be oxidated to high valence cobalt species (Co3+, Co4+, Ni3+). Therefore, it is reasonable to believe that H2O2 is reduced or oxidated by the interconversion among different valence nickel and cobalt species. It can be seen from Fig. 4 that various morphologies of NiCo2O4 nanostructures have different catalytic activity for H2O2 electroreduction and electrooxidation. The characterization of the surface of nanostructures is a very important issue in the applications of nanostructures such as catalysts and sensors. In order to examine characteristic properties of the NiCo2O4 nanostructure surface, we use the NiCo2O4 nanowire array electrode (the corresponding SEM images are shown in Fig. 2A and B) as an example, and the surface of Ni foam substrate and NiCo2O4 nanowire array electrode characterized electrochemically by cyclic voltammograms (CV) in a 3.0 mol L1 KOH solution at a scan rate of 50 mV s1 is shown in Fig. 5. The CV curve of Ni foam substrate only showed a redox couple, which is due to the reversible reactions of Ni(II)/Ni(III) formed on the nickel surface in the alkaline electrolyte [10]. The CV measurement of NiCo2O4 nanowire array electrode displayed the characteristic response of NiCo2O4 in alkaline solution. The anodic peaks were caused by the convertion of Co(II)/Co(III) (Eq. (1)) and further oxidated to Co(IV) (Eq. (2)), also included the oxidation reaction of Ni(II)/Ni(III) (Eq. (3)). In the reversible sweep, the cathodic peaks belonged to the reduction of Co(IV)/Co(III), then to Co(II), as well as the reaction of Ni(III)/Ni(II) [46–48]. Since the redox potentials of Ni(II)/Ni(III) and Co(III)/Co(IV) are in close proximity, the two reactions exhibited a large anodic peak at 0.43V in common [46].
Fig. 4. The cyclic voltammetry curves of the as-prepared NiCo2O4 nanostructures for H2O2 electroreduction (A) and H2O2 electrooxidation (B) at a scanning rate of 10 mV s1.
in the solution of 3.0 mol L1 KOH + 0.4 mol L1 H2O2, and 3.0 mol L1 KOH + 0.75 mol L1 H2O2 respectively, at a scanning rate of 10 mV s1. In order to distinguish NiCo2O4 nanostructures prepared under different conditions from each other, we will name NiCo2O4 nanostructures shown in Fig. 2A, C, E, G and I as S1, S2, S3, S4 and S5. It can be observed from Fig. 4, the as-prepared NiCo2O4 nanostructures all exhibit high catalytic performance towards H2O2 electroreduction and electrooxidation. Generally, high valence cobalt and nickel species (including Co3+, Ni3+) in the compound of NiCo2O4 can be reduced to low valence (Co2+, Ni2+) and
Fig. 5. Cyclic voltammograms of Ni foam substrate and NiCo2O4 nanowire array electrode in 3.0 mol L1 KOH at a scan rate of 50 mV s1.
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Co3 O4 þ OH þ H2 O () 3CoOOH þ e
ð1Þ
CoOOH þ OH () CoO2 þ H2 O þ e
ð2Þ
NiðOHÞ2 þ OH () NiOOH þ H2 O þ e
ð3Þ
Fig. 6 shows the effects of H2O2 concentration on the electrocatalytic performance for H2O2 electroreduction at NiCo2O4 nanowire array electrode (the corresponding SEM images are shown in Fig. 2A and B). The CV curves were recorded in 3.0 mol L1 KOH containing different concentrations of H2O2 at a scan rate of 10 mV s1. The potential scans started from the open circuit potential and go negatively. As seen, the cathodic currents are much larger in the presence of H2O2 than that without H2O2. When H2O2 concentration is lower than 0.2 mol L1, diffusion control can be obviously observed. As H2O2 concentration increased, the reduction current density remarkably increased approximately linearly. During the test at high H2O2 concentration, there are a lot of gas bubbles appear on the electrode surface due to the instability of H2O2 in alkaline medium. A current density of 330 mA cm2 was achieved at 0.6V in the solution containing 0.4 mol L1 H2O2, of course at the expense of significant H2O2 decomposition, and a current density of 190 mA cm2 was achieved at 0.4V in 0.4 mol L1 H2O2 and 3.0 mol L1 KOH by Ni foam supported-NiCo2O4 nanostructures with various morphologies, which exhibited much better catalytic activity than NiCo2O4 nanowire electrodes in the literature [49,50]. Electrodes with porous construction led to large surface area, which can result in excellent catalytic activity and good mass transport. The stability of electrode is a key parameter to determine the quality of electrode, and further influences the cell stability. The chronoamperometric curves for H2O2 electroreduction at NiCo2O4 nanowire array electrode were also investigated (Fig. 7), at three different potentials determined by the CV curves in Fig. 6. Each chronoamperometric curve was tested using the same NiCo2O4 nanowire array electrode. The higher the reduction potential, the larger the reduction current density, which is in agreement with CV curves in Fig. 6. At low reduction potential (0.25V and 0.3V), the curves remained smooth without any fluctuation during the 1200s test period, indicating that the NiCo2O4 nanowire array electrode has superior stability. At high reduction potential (0.4V), there can be seen the obvious fluctuations in the curve, and a lot of gas bubbles appeared on the surface of electrode. H2O2 concentration also has a great influence on the electrochemical behavior of H2O2 electrooxidation. It can be seen from Fig. 8, the starting oxidation potential is around 0.18V (vs. SCE), which is independent of H2O2 concentration. At 0.2V, the oxidation current densities increased significantly from 245 mA cm2 to
Fig. 6. Cyclic voltammograms of NiCo2O4 nanowire array electrode in 3.0 mol L1 KOH + x mol L1 H2O2 at a scanning rate of 10 mV s1 (x = 0, 0.2, 0.4, 0.6 and 0.8).
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Fig. 7. Chronoamperometric curves for H2O2 electroreduction at different potentials in 3.0 mol L1 KOH + 0.2 mol L1 H2O2.
Fig. 8. Cyclic voltammograms of NiCo2O4 nanowire array electrode in 3.0 mol L1 KOH + x mol L1 H2O2 at a scanning rate of 10 mV s1 (x = 0.25, 0.5, 0.75 and 1.0).
520 mA cm2 with the increase of H2O2 concentration from 0.25 mol L1 to 1.0 mol L1. Each curve has the same characteristic, namely line-like without any peaks. Results revealed that NiCo2O4 nanowire array electrode can be conceived as a promising costeffective and scalable alternative for H2O2 electrooxidation since it offers many advantages such as low cost, abundant resources and environmental friendliness. Fig. 9 displayed the chronoamperometric curves for H2O2 electrooxidation at different potentials in 3.0 mol L1 KOH + 0.75 mol L1 H2O2. There is a decrease in currents within the first
Fig. 9. Chronoamperometric curves for H2O2 electrooxidation at different potentials in 3.0 mol L1 KOH + 0.75 mol L1 H2O2.
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seconds, and the oxidation currents at each potential remained nearly constant during the test period. After 1200s test, the oxidation currents at 0V, 0.1V and 0.2V were 135 mA cm2, 290 mA cm2 and 368 mA cm2, respectively. 4. Conclusions In summary, we have developed a novel preparation of morphology-controlled NiCo2O4 nanostructures supported on Ni foam through a facile template-free method. Ni foam was used as the substrate due to its three-dimensional (3D) porous structure, ensuring the full utilization of catalyst surfaces and also enabling O2 produced by H2O2 electrooxidation or hydrolysis to diffuse away from the electrode. By adjusting the preparation parameters including electrodeposition time and immersing time, nanowires, nanothorns and nano- honeycomb with various sizes have been successfully prepared. The resulting electrodes all exhibit catalytic performance for H2O2 electroreduction and electrooxidation due to the interconversion among different valence nickel and cobalt species in the compound of NiCo2O4. As NiCo2O4 nanowire array electrode an example, a reduction current density of 330 mA cm2 was achieved at -0.6V in the solution containing 3.0 mol L1 KOH and 0.4 mol L1 H2O2, and an oxidation current density of 520 mA cm2 was achieved at 0.2V in the solution containing 3.0 mol L1 KOH and 1.0 mol L1 H2O2, respectively. Results revealed that NiCo2O4 nanostructures can be conceived as a promising cost-effective and scalable alternative for H2O2 electroreduction and electrooxidation since it offers many advantages such as low cost, abundant resources and environmental friendliness. Conflict of interest There is no conflict of interest. Acknowledgements We gratefully acknowledge the finance supported by the National Nature Science Foundation of China (21306033), the Fundamental Research Funds for the Central Universities (HEUCF201403018) and the Heilongjiang Postdoctoral Fund (LBH-Z13059). References [1] D. Cao, Y. Gao, G. Wang, R. Miao, Y. Liu, Int. J. Hydrogen Energy 35 (2010) 807. [2] C. Shu, E. Wang, L. Jiang, Q. Tang, G. Sun, J. Power Sources 208 (2012) 159. [3] D.M.F. Santos, P.G. Saturnino, R.F.M. Lobo, C.A.C. Sequeira, J. Power Sources 208 (2012) 131. [4] E. Kjeang, A.G. Brolo, D.A. Harrington, N. Djilali, D. Sinton, J. Electrochem. Soc. 154 (2007) B1220. [5] C. Ponce de León, F.C. Walsh, C.J. Patrissi, M.G. Medeiros, R.R. Bessette, R.W. Reeve, J.B. Lakeman, A. Rose, D. Browning, Electrochem. Commun. 10 (2008) 1610. [6] D.J. Brodrecht, J.J. Rusek, Appl. Energy 74 (2003) 113. [7] C.P. de León, F.C. Walsh, A. Rose, J.B. Lakeman, D.J. Browning, R.W. Reeve, J. Power Sources 164 (2007) 441.
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