Highly stable nickel-aluminum alloy current collectors and highly defective multi-walled carbon nanotubes active material for neutral aqueous-based electrochemical capacitors
Graphical abstract
Introduction
Electrochemical capacitors (ECs), also known as supercapacitors, are generally applied on short-term energy storage and supply systems, handling medium-to-high currents inputs and outputs. ECs present better power density, higher cycle life over a wider operating temperature range and lower equivalent series resistance (ESR) than Li-ion batteries [1,2]. ECs and Li-ion battery devices are not competitors since they are complementary technologies with specific niches of applications [1,2]. Batteries have a longer capability for storing energy while ECs can permit the rapid release of the stored energy (e.g., very high power densities and consequently high gravimetric currents).
The improvement of the storage energy capability of ECs while keeping their intrinsic properties (e.g., high power and long service life) is very attractive from the technological viewpoint since the niche of applications would increase considerably. In order to increase the energy density (E) for ECs, an approach is to improve the specific capacitance (C) and/or the operational voltage (V), since the energy is given by E = CV2/2. So, the energy density is proportional to the square of operational voltage of the cell, i.e., the use of a large window voltage is crucial for enhancing the energy storage capability of ECs. That explains why several types of research in the field of ECs keep their focus on the use of organic electrolytes since in these cases an EC could work using a voltage window of up to ∼2.5 V (or higher), while in most aqueous electrolytes the counterpart is typical of ∼1.2 V (or less) due to the electrochemical stability of the electrolyte (e.g., water splitting). However, in many cases, the 1.2 V voltage window can be larger due to kinetic constraints [1,2]. In the special case of the ionic liquids, one can reach even higher voltages, but these substances are more expensive and harder to handle when compared to organic and aqueous electrolytes [1,2].
Up to now, the most cost-effective substances for ECs are the organic electrolytes. However, to work with organic electrolytes and ensure a long lifespan of the device, one needs to be sure that the electrolyte is water-free, which is experimentally challenging and add further costs to the device. Furthermore, organic electrolytes are flammable and highly toxic, therefore, not suitable for wearable applications. In this scenario, recent advances have resulted in higher working window voltages for ECs in aqueous neutral electrolytes, thus renewing the interest of the scientists. Most used electrolytes are Li2SO4 [3], LiNO3 [4], Na2SO4, K2SO4 [5], which allows carbon-based electrode materials to work at voltages higher than 1.2 V. In addition, in the case of Li2SO4, the conductivity can reach values in the range of 110–115 mS cm−1 (at 1.0 mol L−1) [4], which is higher than that obtained for organic electrolytes (e.g., ∼8 mS cm−1 (at 1.0 mol L−1 LiClO4 in DMSO). The reduced conductivity exhibited by organic electrolytes leads to a high equivalent series resistance (ESR) in supercapacitor devices thus causing serious ohmic losses. Therefore, one can see the advantages and disadvantages for both types of electrolytes.
In this scenario, an important issue is concerned with the use of aluminum current collectors, which is a conventional material used for manufacturing supercapacitor devices (e.g. cost-effective and easy to weld) [1,2]. Aqueous electrolytes are known to promote corrosion of these collectors, limiting their use in aqueous-based systems [[6], [7], [8], [9]]. An alternative to aluminum current collectors on aqueous-based systems is nickel foil, which however is one order of magnitude more expensive [10]. More cost-effective than Ni foils are coated aluminum foils. Chromate-conversion-coated aluminum is an alternative to avoid corrosion of aluminum current collector [8,11,12]. However, the replacement of chromates is shadowed by other environmental concerns [13].
In the present work, we report the modification of aluminum foils into nickel-aluminum alloy (NixAl) used as current collectors, which is electrically conductive, very resistant to corrosion in neutral aqueous solutions, and potentially a better solution for the issues described above. The formation of nickel-aluminum alloy occurs just before the deposition of multi-walled carbon nanotubes (MWCNTs) onto the aluminum substrate by the chemical vapor deposition process. The present work shows that a symmetric coin cell containing the MWCNT:NixAl electrodes soaked with a neutral aqueous electrolyte have a very long service life (e.g., more than 85,000 cycles) with a negligible loss of specific capacitance and/or specific energy for an operating voltage of 1.5 V. In addition, due to its intrinsic properties the present MWCNT:NixAl electrodes are promising candidates as scaffolds for supporting redox-active materials, such as mixed-metal oxides (MMOs) and conducting polymers.
Section snippets
Electrode synthesis
Before synthesis, 10 cm × 15 cm rectangular aluminum foils were etched using hydrochloric acid (1:4 (v/v)) solution for 30 s. Afterward, nickel thin-film was electrodeposited on the aluminum foil in 2.0 mol dm−3 Ni(NO3)2 aqueous solution (conditions: 14.3 mA cm−2 at 60 °C for 1.0 min) and then dried. Ni-coated Al foils were placed into a thermal CVD furnace. 1000 sccm N2 flow creates an O2-free atmosphere after 5 min and then furnace temperature started to increase from room temperature to
Nickel-aluminum alloy
SEM analysis was accomplished to monitor changes in surface morphology during electrode preparation. Fig. 1(a–f) presents micrographs from aluminum transformation step-by-step: (a) as-purchased; (b) acid etched, (c) nickel covered aluminum, and (d–f) surface morphology after annealing but without carbon growth. As-purchased aluminum is a flat surface with lines from manufacturing and rolling processes (Fig. 1(a)). Contrasting with the as-purchased aluminum surface, the HCl etched aluminum
Conclusions
A uniform Ni layer containing few cracks was electrodeposited onto the aluminum foil to obtain after the thermal treatment a nickel-aluminum alloy very resistant to corrosion in neutral aqueous solutions. XRD findings proved the formation of Ni3Al and NiAl. Dense carbon nanowire structures were grown onto the nickel-aluminum current collector. The turbostratic multi-walled tubes (MWCNTs) exhibited a diameter in the range of 10–40 nm. MWCNTs are a highly crystalline sp2 based material full of
Author contribution statement
Rafael Vicentini has performed mainly materials production and characterization. Also he has performed electrochemistry measurements and analysis. He has prepared Figs. 3, 6, 7, 8, 9 & 11
Hudson Zanin and Leonardo Morais da Silva wrote the main text, coordinate experimental design and analysis of raw data.
Hudson Zanin mainly focuses on materials characterization and Leonardo Morais da Silva mainly focuses on electrochemistry of devices.
Willian G Nunes has helped with XPS and electrochemistry
Acknowledgements
The authors are very grateful to LNNano/CNPEM for SEM, HRTEM, XPS and XRD support, to CCS Nano/FEEC for SEM support, and to the financial support from the Brazilian funding agencies CNPq (301486/2016-6), FAPESP (2016/25082-8, 2017/19222-4, 2014/02163-7, 2017/11958-1) FAEPEX (2426/17) and CAPES (1740195). L.M. Da Silva wishes to thank “Fundação do Amparo à Pesquisa do Estado de Minas Gerais – FAPEMIG” and National Council for Scientific and Technological Development – CNPq (PQ-2 grant). The
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