Nickel oxide nanoparticles supported onto oriented multi-walled carbon nanotube as electrodes for electrochemical capacitors
Introduction
Supercapacitors, also known as ultracapacitors or electrochemical capacitors, are electric devices commonly classified into subcategories as electrical double-layer capacitors (EDLCs), pseudocapacitors (PCs), and hybrid capacitors [1]. These devices can be applied in equipment involving pulsed technology aiming for storing energy purposes, and in sine-current technology where the capacitive element is used to represent an impedance to an alternating current with a negligible energy loss. There are several different technological niches of applications involving supercapacitors, as is the case of hybrid electric vehicles, etc. [[2], [3], [4]].
Advanced electrode materials composed of carbon such as carbon nanotubes (CNT) and graphene (GR), as well as several different types of composite materials (e.g., CNT/NiO and GR/NiO), are very promising candidates to substitute traditional activated carbon (AC) commonly used in the fabrication of supercapacitors [[5], [6], [7], [8]]. The significant features of these materials are: (i) high electronic conductivity; (ii) chemical inertness; (iii) fast (reversible) charge-transfer reactions involving the oxygenated active surface groups and/or the metallic sites (e.g., Ni(II)/Ni(III)), and (iv) high specific surface area. Different surface morphologies with a high degree of porosity can be prepared during growth or functionalization of meso- and nano-structured carbon-based electrode materials [[9], [10], [11]].
According to the literature [12,13] important gains in the specific capacitance and specific power can be achieved in practice using composite carbon-based electrode materials. In fact, while only moderate specific capacitance of ∼50 F g−1 can be obtained in aqueous solutions using multi-walled carbon nanotubes (MWCNT) [14], the use of conductive metallic oxides (e.g., MnO2, RuO2, NiO, etc.) anchored on the nanostructured carbon scaffold can yield specific capacitance higher than 1000 F g−1 (e.g., NiCo2O4) [9,15,16]. In addition, some studies revealed that the use of conductive metallic oxides in conjunction with graphene strongly avoids the restacking process involving the graphene layers, which results in the formation of unstable porous structures with an inefficient geometry for the adequate conduction of electrons and mass-transfer of solvated ions [17,18]. It is worth mentioning that the electrochemical behavior of oxides containing transition metals is always pseudocapacitive since the oxidation/reduction of the active metals constitutes a reversible solid-state surface Faradaic reaction accompanied by the injection/ejection of hydrated ions [19,20].
Several works showed that the use of carbon-based materials containing a large surface concentration of micropores (e.g., d < 1 nm) is not recommended for supercapacitors since the hydrated ions from the electrolyte cannot penetrate in these pores to form an electrical double-layer where the energy is electrostatically stored [[21], [22], [23]]. Therefore, there is a trade-off between the specific surface area and the dimension of the active pores. In this sense, the use of vertically oriented CNT scaffolds supported on metallic substrates containing pores with appropriate dimensions (e.g., pore radius and length) is very attractive for manufacturing new composite electrodes. In fact, their intrinsic nanoarchitecture strongly facilitates the anchoring process of the redox-active material (e.g., pure or mixed oxides, and conducting polymers) resulting in a surface morphology with enhanced characteristics for the charge storage process [[24], [25], [26], [27]]. Several different studies [15,[27], [28], [29]] showed that significant gains in performance for supercapacitors could be achieved in practice using composite carbon-based electrode materials where, for instance, the surface of CNT or GR is modified using metal oxides [4,[30], [31], [32]]. Therefore, the fabrication of well-designed CNT-based scaffolds can be very helpful for the development of composite materials where the nanostructured carbon is used for tailoring the pseudocapacitive properties of redox-active materials, such as NiO, Co3O4, NiCo2O4, MnO2, etc.
Another important issue in the case of supercapacitors is the development of new electrode materials that exhibit high overvoltage for the water electrolysis (ΔV > 1.23 V) permitting to obtain in aqueous media a very high specific capacitance (C) in conjunction with a large voltage window since the energy (E) stored in these devices depends on the square of the voltage window [33], according to the relation E = 0.5C(ΔV)2.
From the above considerations, the synthesis of nickel oxide (NiO) nanoparticles supported onto oriented multi-walled carbon nanotubes can result in promising electrode materials for electrochemical capacitors. This work reports the fabrication of electrodes composed of nickel oxide (NiO) nanoparticles supported on oriented multi-walled carbon nanotubes using stainless-steel fine-mesh substrates. Several different characterization studies were performed to investigate the surface and electrochemical properties of the electrode material. The energy storage mechanism at the electrode/solution interface in the presence of reversible solid-state surface Faradaic reactions (RSFRs) was carefully investigated. Using a symmetric coin cell filled with a 1.0 M Li2SO4 aqueous solution, cyclic voltammetry experiments were carried out to determine the morphology factor (φ) representing the fraction of the electrochemically active area confined to the inner surface regions of the porous electrode. In addition, the impedance behavior of the porous electrodes was studied using a single-channel R-C transmission line model according to the De Levie's model.
Section snippets
Fabrication of NiO nanoparticles on MWCNT supported on stainless-steel
It was previously optimized the CVD conditions to produce the most appropriate CNTs density on the stainless-steel substrate, as well as the amount of NiO deposited on the metallic substrate covered by CNTs. We have used different concentrations of the nickel nitrate solution to soak the CNTs forest. As a result, experimental findings and conditions reported in this work were selected after carrying out several different laborious experimental works. In this sense, the present CVD conditions
SEM and TEM analyses
Fig. 1 shows SEM data obtained for the AISI:CNT-NiO samples.
From SEM data, we can observe carbon nanotubes coating the stainless-steel fine-mesh in Fig. 1(a–c) and the carbon nanotubes changing their morphology after incorporation of NiO nanoparticles in Fig. 1(d–f). Fig. 1 (b & c) evidence highly packed nanotubes which are radially oriented in relation to mesh wires. Fig. 1 (d & e) reveal that after the dipping process using the nickel nitrate ethanol solution, CNT tips to stick together to
Conclusions
Highly packed carbon nanotube (CNT) structures radially oriented in relation to mesh wires of the stainless-steel support were synthesized which after a dipping process using a nickel nitrate ethanol solution followed by a thermal treatment resulted in the decoration of CNT with NiO nanoparticles (AISI:CNT-NiO). TEM analysis revealed that CNTs have a turbostratic multi-walled structure containing a few dozen walls with an interplanar spacing of 0.32 ± 0.02 nm and a diameter ranging from 20 to
Acknowledgements
The authors are very grateful to LNNano/CNPEM for SEM & HRTEM support, to LNLS/CNPEM for the XPD beamline and staff (specially Dra. Cristiane Rodela) and also the financial support from the Brazilian funding agencies CNPq (301486/2016-6), FAEPEX (2426/17), FAPESP (2016/25082-8, 2017/19222-4, 2014/02163-7) and CAPES (1740195). L.M. Da Silva wishes to thank the “Fundação ao Amparo à Pesquisa do Estado de Minas Gerais – FAPEMIG” (Project CEX-112-10), “Secretaria de Estado de Ciência, Tecnologia e
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