Microsoft Word - 3Galluzzi_Revised Manuscript.docx DOI: 10.3303/CET2184032 Paper Received: 30 July 2020; Revised: 5 January 2021; Accepted: 18 February 2021 Please cite this article as: Ponticorvo E., Galvagno S., Portofino S., Borriello C., Tammaro L., Iovane P., Rametta G., Sarno M., 2021, Alumina Based Electrode for Stable and Improved Supercapacitor Applications, Chemical Engineering Transactions, 84, 187-192 DOI:10.3303/CET2184032 CHEMICAL ENGINEERING TRANSACTIONS VOL. 84, 2021 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Paolo Ciambelli, Luca Di Palma Copyright © 2021, AIDIC Servizi S.r.l. ISBN 978-88-95608-82-2; ISSN 2283-9216 Alumina Based Electrode for Stable and Improved Supercapacitor Applications Eleonora Ponticorvoa*, Sergio Galvagnob, Sabrina Portofinob, Carmela Borriellob, Loredana Tammarob, Pierpaolo Iovaneb, Gabriella Ramettab, Maria Sarnoa a Department of Physics "E.R. Caianiello" and Centre NANO_MATES (Research Centre for Nanomaterials and Nanotechnology at the University of Salerno) University of Salerno, Via Giovanni Paolo II, 132 - 84084 Fisciano (SA), Italy b Nanomaterials and Devices Laboratory (SSPT-PROMAS-NANO), ENEA, Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Piazzale E. Fermi 1, 80055 Portici, NA, Italy eponticorvo@unisa.it This work describes the preparation of an Al2O3 electrode material for a performing and stable supercapacitor. The electrode was prepared by a DC thermal plasma plant installed at ENEA Research Centre of Portici. The structures and morphologies of samples were characterized by SEM, XRD, TG, and FT-IR. Al2O3 electrode electrochemical characteristics were performed in a sulphuric acid solution electrolyte. Cyclic Voltammetry (CV) technique was carried out. The results show that Al2O3 exhibits a good specific capacitance. Furthermore, after 10000 cycles, 98% of the initial capacitance was maintained. 1. Introduction Energy availability and greenhouse gasses emission has become a problem related to the excessive consumption of traditional non-renewable sources (Wang et al., 2013). The development and the search for renewable green energy resources and, at the same time, better energy storing systems are urgently needed. Supercapacitors have attracted broad interest (Wang et al., 2007; Sarno et al., 2015) due to their high energy density, excellent cycle stability, high specific capacitance, and long life (Xia et al., 2012). According to the different energy storage mechanisms, supercapacitors can be classified into two main categories (Yang et al., 2012): the double-layer capacitors and the pseudocapacitors. In the double-layer capacitors (e.g., carbon materials), electrodes store energy by using the electrostatic capacitance of the interface double-layer. The pseudocapacitors, which has higher capacitance than double-layer capacitors, keep charge by a rapid and reversible redox reaction. As electrode materials, the metal oxides have attracted great interest due to their high capacitance characteristics in redox reactions. Numerous transition metal oxides and conducting polymers have been used. Aluminum oxide has many unique and attractive properties, such as large specific surface area, good thermal conductivity, inertness to most acids and alkalis, mechanical strength and stiffness, wear resistance, high adsorption capacity, and thermal stability. Moreover, Al2O3 is also non-toxic, highly abrasive, and inexpensive (Mallakpour and Khadem, 2015; Mirjalili et al., 2011; Gunday et al., 2019). These properties make Al2O3 suitable for various applications such as catalysts, sensors, and supercapacitors. In particular, the supercapacitor performance of a ternary electrode, constituted by γ-Al2O3 nanoparticles, polyindole, and reduced graphene oxide, was reported (Azizi et al., 2020). The beneficial effect of Al2O3 in improving and enhancing the conductive polymer electrochemical stability and capacitance, thanks to catalyzing redox reactions ability, was demonstrated. However, to the best of our knowledge, the electrochemical properties of an electrode constituted by the only alumina with high purity and morphological uniformity have never been reported. In order to form a stable, cheap, and performing electrode, here we report the use of Al2O3 powders, prepared by thermal plasma technique, for supercapacitor applications. Among high purity and fine powders synthesis procedures, the vapor phase reactions that avoid complex and expensive preparative steps, usually required in chemical processes, i.e., precipitation and purification, are particularly helpful for the production of deagglomerated particles with a narrow size distribution (Iovane et al., 2019; Hong and Yan, 2018). Scanning Electron Microscopy (SEM), thermogravimetric analysis (TG), Fourier 187 Transform Infrared Spectroscopy (FTIR), and X-ray diffraction (XRD) were used for characterization. The electrochemical performances of the electrode were investigated by cyclic voltammetry tests. The results of CV measurements proved that this kind of supercapacitor has good electrochemical capacitance performance within a wide potential range and improved electrochemical stability. 2. Experimental 2.1 Sample preparation Aluminum oxide powder (particle size 0,5-1mm) was milled by Retsch PM100 (planetary ball mill) using a 500 ml steel jar and 1 cm diameter balls and 1/3 balls, 1/3 empty and 1/3 powder ratio. The powder was sieved in 40-63 µm range by Retsch AS basic 200 after milling and drying in an oven at 105 °C. This powder was treated by a DC thermal plasma plant installed at ENEA Research Centre of Portici (Iovane et al., 2019). The system comprises a powder feeder system, a DC non-transferred plasma torch, a power supply (80 kW), a cyclone/bag filter, and a dry scroll pump for vacuum. The plasma operates under a light vacuum (up to 20 kPa). The torch consists of a water-cooled tungsten cathode and a copper anode nozzle. It is fitted in the upper part of a jacketed-cylindrical stainless steel reactor of 13 cm inner diameter, and 185 cm length cooled with circulating cold water. The reactor is equipped with a collection tank, where the produced powders are collected along with the unreacted materials. At the top of the reactor, one nozzle feeds the powder directly into plasma fame (internal feed). The test has been conducted using argon (Ar) as the main gas (about 40 slpm) to light the plasma and helium (He) (about 10 slpm) as secondary gases to improve the flame conditions. Other experimental conditions were fixed: the plasma torch power at 20 kW, the gas flow rate (Ar) at 0,5 slpm, the powder feeding rate at 2 g/min, the pressure of the reaction chamber 87 kPa. The powder produced was sieved under 25µm by Retsch AS basic 200. 2.2 Characterization methods The combined use of different techniques obtained the sample characterization. Scanning electron microscopy (SEM) images were received using a LEO 1530 equipped with a filament of tungsten at 4 kV and working under high vacuum conditions, equipped with an energy dispersive X-ray (EDX) probe. A Bruker D8 X-ray diffractometer with a monochromatic CuK radiation was used for measurements of powder diffraction profiles. The thermal behavior of the samples was investigated using TG and derivative thermogravimetric (DTG) analysis (Mettler Toledo TGA, Model) between 25 and 1000 °C under airflow (60 mL min −1) with a heating rate of 10 °C min−1. FTIR was recorded on the Fourier transform infrared spectrometer BRUKER VERTEX-70, (Bruker Corporation). The surface area was obtained by N2 adsorption-desorption at 77 K (Kelvin 1042 V3.12, COSTECH Instruments). Electrochemical tests to evaluate the synthesized samples' capacitance performances, in particular cyclic voltammetry (CV) tests, were carried out by means of an Autolab PGSTAT302N potentiostat. In detail, cyclic voltammetry measurements were run at different scan rates (10, 20, and 50 mV/s) in the potential range 0 ÷ 0.8 V and in a 0.5 M H2SO4 electrolyte solution. Starting from the CV, the Eq (1) was used in order to evaluate the specific capacitance of the samples: CSP = I ∙ V ΔV ∙ V (1) ∆V is the potential range chosen, I is the current density, V is the potential, v is the CV scan rate. Before performing the tests, 4 mg of synthesized material were dispersed into 80 μl of a 5 wt% Nafion solution, 200 μl of ethanol, and 800 μL of water. The resulting mixture consists of a homogeneous suspension which, after being sonicated for 30 mins and then air-dried, was partially deposited dropwise onto a DRP-110 Screen Printed Electrode (SPE). SPE consisted of a carbon working electrode, a platinum counter electrode, and a silver reference electrode and was preferred over commonly used carbon electrodes thanks to their excellent characteristics (Rowley-Neale et al., 2015; Sarno and Ponticorvo, 2018; Sarno et al., 2019). 3. Results and discussion 3.1 Chemical and morphological characterization As shown in Figure 1, the XRD pattern of the prepared sample consists of α-Al2O3 phase, corresponding to (012), (104), (110), (113), (024), (116), (211), (018), (214), (300) and (119) planes at 24.84°, 34.65°, 37.27°, 43.90°, 52.01°, 57.10°, 58.81°, 60.83°, 66.04°, 67.84°, and 76.90°, according to ICDD (The International Centre for Diffraction Data) JCPDS No. 00-010-0173. The powders morphology was evaluated by SEM. The plasma synthesis takes place in the gaseous phase, where new particles are nucleated in the tail of the fame. Applying fast quenching, the particles tend to form a uniform spherical shape in order to minimize their specific surface areas and surface energy (Iovane et al., 2019), which enable effective electrolyte access into the 188 electrode and is favorable for better supercapacitor performance (Xiao et al., 2017). SEM characterization (Figure 2) of the sample shows good results in terms of spherical shape, where the powder displays aggregate with roundness (RN) higher than 80%. Figure 1: XRD patterns of Al2O3 powder Figure 2: SEM image of Al2O3 powder Thermogravimetric analysis (TG) thermograph (Figure 3) with soft loss and without sharp peaks proofs single- phase and no impurity in the sample. As shown in Figure 3, the weight loss (~ 1 %) occurs at temperatures lower than 400 ºC due to the evaporation of volatile components, such as water residues, including adsorbed water, free water, crystal water. The FTIR spectra of the α-Al2O3 samples are shown in Figure 4. The intense bands between 500 and 1000 cm−1 are related to the vibrational frequencies of O–Al–O bonds (Aman et al., 2013). This band was divided into two bands, which are the most characteristic FTIR feature. 20 40 60 80 (1 1 9 ) (3 0 0 ) (2 1 4 ) (0 1 8 ) (2 1 1 ) (1 1 6 ) (0 2 4 ) (1 1 3 ) (1 1 0 ) (1 0 4 ) In te n si ty a .u . 2 ° (0 1 2 ) 189 Figure 3: TG profile of Al2O3 powder Figure 4: FTIR spectrum of Al2O3 powder 3.2 Electrochemical characterization CV curves of the prepared sample at different scan rates (10, 20, and 50 mV/s) were reported in Figure 5. From the figure, we can see that the Al2O3 electrode show quasi-rectangular current–voltage response curves. No detectable redox peaks can be found in the chosen potential window, which indicates a nearly ideal capacitive behavior (Wang et al., 2015; Du et al., 2009; Yoo et al., 2011; Zhu et al., 2010; Sarno et al., 2016; Karandikar et al., 2012; Nishihara et al., 2012; Sarno et al., 2019). A capacitance of ~ 10 F/g was obtained. Moreover, long-term cycling tests were carried out in 0.5 M H2SO4 over 10000 cycles at 20 mV/s, proving that 98 % of the initial capacitance was retained (Figure 6). However, the presence of highly conductive material to promote charge transport and accumulation is the most required feature for supercapacitors electrodes (Pandolfo et al., 2006). Therefore, the obtained encouraging results suggest the possibility to achieve even higher capacitances further by adding more conductive double-layer supercapacitive materials, such as graphene. The loading of graphene will be the subject of future dedicated works. 200 400 600 800 1000 70 80 90 100 M a s s r e s id u e ( % ) Temperature (°C) 1800 1500 1200 900 600 T ra n s m it ta n c e ( a .u .) Wavenumber (cm -1 ) 190 Figure 5: Cyclic voltammograms measured at different scan rates in a 0.5 M H2SO4 solution. Figure 6: Capacitance retention as a function of cycle number. 4. Conclusions In summary, an efficient procedure was adopted in order to prepare Al2O3 powder by a DC thermal plasma plant installed at ENEA Research Centre of Portici. SEM, FT-IR, XRD, and TG analyses confirmed the formation of the above-mentioned sample. Cyclic voltammograms, run at different scan rates in a 0.5 M H2SO4 solution, revealed that the Al2O3 powder exhibits quasi-rectangular current–voltage response curves without any detectable redox peaks in the analyzed potential window, which suggests a nearly ideal capacitive behavior thanks to the accessibility and wettability of the electrode for ion adsorption. Moreover, the synthesized Al2O3 powder exhibited very high capacitance retention, showing high stability as well as promising capacitive performance as robust electrode material. Acknowledgments This research was supported by MiSE-ENEA bilateral program agreement "Research on Electric System" – Project 1.3 "Materiali di frontiera per usi energetici-Frontier materials for energy uses" (ADP MiSE-ENEA Piano Triennale di Realizzazione 2019-2021) – ENEA-NANO_MATES cooperating agreement “Materiali e componenti per additive manufacturing, con impatto sul sistema elettrico - Materials and components for additive manufacturing, with an impact on the electrical system” . 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