Pseudo-capacitive behavior of multi-walled carbon nanotubes decorated with nickel and manganese (hydr)oxides nanoparticles
Graphical abstract
Introduction
One of the main strategies explored to enhance the capacitance properties of electrode materials used in supercapacitors (electrochemical capacitors) has been decorating carbon materials with transition metal oxides to obtain high-performance composite electrodes. In this sense, nickel [1], [2], [3], [4], manganese [5,6], niobium [7], [8], [9], and iron [6,10] (hydr)oxides have been extensively studied as alternative to ruthenium oxide since the latter exhibits very high pseudocapacitance but its use is restricted due to cost and environmental issues. In this work, we explored a simple, facile, and cost-effective synthesis of manganese oxides (e.g., MnOx, with x ranging from 1 to 2) and nickel hydroxides onto a high-surface area nanostructured carbon support. Considering that all Mn atoms are electrochemically active and an one-electron redox reaction per active site, the theoretical specific capacitance for a monolayer of MnO2 is ~1370 F g−1 [11]. In a similar fashion, Ni(OH)2 can even exhibits a higher theoretical specific capacitance of ~ 2300 F g−1 [12]. On the experimental level [13], [14], [15], the surface morphology and structural phase present a secondary role while the specific capacitance is affected by the film thickness for manganese oxides. This behavior can be understood considering that the solid-state redox transition reactions (SSRTRs) involving the Mn(III) and Mn(IV) ionic species occur at the near-surface regions of the electrode/electrolyte interface. With that in mind, we reduced average size of manganese oxide particles to nanosized domains, improving electrical conductivity. These nanoparticles were synthesized onto a scaffold composed of radially-oriented multiwalled carbon nanotubes (ROMWCNTs) anchored on a stainless-steel AISI mesh. A similar electrode material having a different metallic composition was reported elsewhere [16]. As verified, the nanostructured carbon-based material worked as an efficient current drain, which quickly extracts electrons from the oxide's surface thus improving the overall conductivity of the composite electrodes containing transition metals. Also, the nanoarchitecture of the porous carbon-based support facilitates the ionic transport of electrolyte species during the charge-discharge processes occurring into the channels where the electrode/electrolyte interface is established [14].
In addition, in the current work, we also observed the migration/diffusion of the encapsulated Ni0-catalyst particles used in the CVD process towards the carbon's surface during MnOx nanoparticles syntheses, i.e., nickel particles moved to the outside regions of the carbon material resulting in the formation of the Ni(OH)2 species. We extensively characterized the composite material using the ex situ XPS, Raman, and XRD techniques. We also explored this material as electrodes on symmetric coin cells (e.g., supercapacitor prototypes) which exhibited high specific capacitance using CV and GCD techniques, thus revealing promising electrochemical characteristics for the charge-storage process.
The operando Raman techniques helped to observed different near-surface structural changes during the dynamic polarization of the electrodes. In this sense, the findings found the mechanism of the active stretching mode and pseudocapacitive process involving a solid-state redox reaction during the charge-discharge process of the supercapacitor. The operando Raman study also permitted to elucidate different aspects concerned with the reversible intercalation-deintercalation of the electrolyte ions, as well the verification of the reversible adsorption of solvated ions on the surface electrode.
A remaining issue regarding the use of metal oxides in the electrochemical capacitors is their lifespan, i.e., it is well-known that several types of metal oxides containing transition metals that exhibit a high specific capacitance did not show an acceptable lifespan during the charge-discharge cycles (e.g., less than 5,000 cycles), and commonly present a poor capacitance retention [5,6,14,[17], [18], [19]]. In this work, we also overcame this drawback and showed that more than 70,000 cycles with nearly 100% of capacitance retention can be achieved using the present composite electrode material (e.g., Ni(OH)2:MnOx@CNT). Additionally, our promising experimental findings were obtained using a neutral aqueous-based electrolyte, which is environmentally friendly in comparison to the organic electrolytes. Interesting findings are found in this work regarding the charge and energy storage processes using a symmetric energy storage device.
Section snippets
MWCNTs growth onto the AISI mesh
The synthesis of MWCNTs onto the stainless-steel mesh (AISI) was recently described by us elsewhere [16]. In short, MWCNTs were grown onto an AISI 304L M100 fine-mesh substrate (e.g., mesh wire diameter of 0.1 mm and mesh pore size of 0.15 mm × 0.15 mm) using a homemade CVD furnace and using nickel as the catalyst. The metallic samples were inserted into the furnace, which was purged with 1000 sccm of N2 for 5 min to reduce the oxygen content to a negligible level. With the inert N2 atmosphere,
Material characterization: ex situ studies
Fig. 1 shows SEM data contrasting (a & b) carbon nanotubes support and (c & d) its decoration with Ni(OH)2 and MnOx particles. From micrographs in Fig. 1 (a & b), one can observe an average length of ~ 30 µm for nanotubes, which grew radially oriented onto a fine-mesh stainless-steel support. In the Fig. 1 (c & d), many particles appear mixed and decorating carbon nanotubes on the interception regions. In addition, Ni(OH)2 and MnOx bigger particles are mostly close together on the top surface
Conclusions
This work reported the fabrication and characterization of Ni(OH)2-MnOx (hidro)oxides supported on carbon nanotubes and their application as the electrodes in symmetric supercapacitors devices using an aqueous electrolyte. The nanoparticles containing Ni and Mn were anchored onto the carbon nanotubes used as the scaffold while the latter was supported on a fine-mesh stainless-steel substrate. TEM data showed that dozens of Ni(OH)2-MnOx nanoparticles are kept closed together and firmly connected
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are very grateful to LNNano/CNPEM for SEM & HRTEM support, to LNLS/CNPEM for the XPD beamline and staff and also the financial support from the Brazilian funding agenciesCNPq (301486/2016-6), FAPESP (2018/20756-6, 2016/25082-8, 2017/11958-1, 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 Ensino Superior de Minas Gerais -
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