Spherical clusters of β-Ni(OH)2 nanosheets supported on nickel foam for nickel metal hydride battery
Highlights
• Facile one-step preparation of Ni foam supported β-Ni(OH)2 nanosheets. • Nanosheets have porous structure and large electrochemical active surface area. • Nanosheets have large high rate discharge capacity and superior cycling stability.
Introduction
Nickel metal hydride (Ni–MH) rechargeable batteries have high specific energy, high specific power, long cycling life, and superior safety. They are competitive power sources for all-electric plug-in vehicles and hybrid vehicles [1]. However, when Ni–MH batteries are frequently charged and discharged at high rates or high temperatures, their performances are reduced due to capacity fading, increase of internal resistance and decrease of cycle stability [2]. It is generally recognized that the performance of a Ni–MH battery is mainly limited by properties of the positive electrode, i.e. Ni(OH)2 electrode. During charge and discharge processes, solid-state proton intercalation/deintercalation reactions take place at the Ni(OH)2 electrode where both electrons and protons are exchanged; this leads to continuous changes in both composition and phase structure of the electrode active materials [1], [3].
The charge and discharge reactions of a nickel hydroxide electrode in alkaline electrolytes are usually considered to be a one-electron process involving oxidation and reduction between β-Ni(OH)2 and β-NiOOH and can be simply described as:
This reaction mechanism involves an equivalent proton diffusion through solid-state lattices of β-Ni(OH)2 and β-NiOOH [4]. Therefore, the specific charge–discharge capacity of β-Ni(OH)2 depends on the magnitude of the resistance for the transportation of equivalent protons within the solid lattices as well as the electronic conductivity of Ni(OH)2, particularly at high charge and discharge rates. Decreasing the particle size and changing the morphology of Ni(OH)2 have been proved to be effective ways for reducing the resistance of equivalent proton transportation [5], [6], [7], [8], [9]. Nanostructured Ni(OH)2 particles with various morphologies that have been recently prepared by different methods, indeed exhibit improved capacity and cycling reversibility. These nano-sized Ni(OH)2 particles include nanorods, nanowires, nanotubes, nanoribbons, and nanosheets [10], [11], [12], [13], [14], [15], [16], [17]. However, nano-sized Ni(OH)2 particles are generally prepared in a powder form and need to be pressed onto a current collector (e.g. nickel foam) with conducting additives (e.g. carbon black) and binders to obtain the electrode. This conventional electrode preparation procedure may lead to the reduction of utilization efficiency of active materials due to ineffective contact between Ni(OH)2 particles and the current collector. The addition of ancillary materials also decreases the specific energy of electrodes. Moreover, these conventional electrodes lack buffer ability in response to volume change of the active material particularly when overcharged.
In this study, spherical clusters of Ni(OH)2 nanosheets were directly grown onto nickel foam current collector via a template-free self-growth method, and the obtained electrode, free of conducting additives and binders, was evaluated as the positive electrode of the Ni–MH battery. Even though nano-structured Co3O4, NixCoyO4 and CuO have been grown on Si wafer, Ti foil or Ni foam substrates by the template-free growth method used in this work, the grown of Ni(OH)2 nanosheets on Ni foam has not been reported in literatures as far as we know.
Section snippets
Preparation and characterization of clusters of Ni(OH)2 nanosheets
Clusters of Ni(OH)2 nanosheets attached on nickel foam were prepared via a template-free growth method [18], [19], [20], [21]. 10 mmol Ni(NO3)2 and 6 mmol NH4NO3 were dissolved in 35 cm3 H2O + 15 cm3 ammonia (30 wt%). The solution was magnetically stirred for 10 min at room temperature and heated at 90 ± 1 °C for 2 h (growth ready solution). Nickel foam (10 mm × 10 mm × 1.1 mm, 110 PPI, 320 g m−2, Changsha Lyrun Material Co., Ltd. China) was degreased with acetone, etched with 6.0 mol dm−3 HCl for 10 min, rinsed with
Characterization of β-Ni(OH)2/Ni-foam electrode
Fig. 1 shows the XRD patterns of the Ni(OH)2 powders scratched from nickel foam. The XRD pattern matches well with the standard crystallographic spectrum of hexagonal β-Ni(OH)2, with a space group of P3m1 (JCPDS card No. 14-0117), and with no peaks detected from other phases. The lattice constants are calculated to be a = b = 0.3123 nm, c = 0.4610 nm. These values are closely consistent with the standard values (a = b = 0.3126 nm, c = 0.4605 nm, JCPDS 14-0117) of β-Ni(OH)2 in the range of errors. The XRD
Conclusions
A facile one-step method for the preparation of nickel foam supported β-Ni(OH)2 nanosheets electrode was demonstrated. β-Ni(OH)2 nanosheets wrap together to form spherical clusters and directly attach to the skeletons of nickel foam, which serves as the current collector of the electrode. The use of ancillary conducting materials and binders was eliminated. The nanosheets have porous structure giving the electrode a large surface area. The open spaces within β-Ni(OH)2 allow for easy penetration
Acknowledgements
We gratefully acknowledge the financial support of this research by National Nature Science Foundation of China (20973048) and Harbin Engineering University (HEUFT08008).
References (34)
- et al.
J. Power Sources
(2007) - et al.
J. Power Sources
(2007) - et al.
Electrochim. Acta
(2006) - et al.
Electrochim. Acta
(2005) - et al.
J. Power Sources
(1997) - et al.
J. Power Sources
(2003) - et al.
Int. J. Hydrogen Energy
(2010) - et al.
Int. J. Hydrogen Energy
(2009) - et al.
Int. J. Hydrogen Energy
(2002) - et al.
Chem. Mater.
(2008)