Electric charging/discharging characteristics of super capacitor, using de-alloying and anodic oxidized Ti-Ni-Si amorphous alloy ribbons
© Fukuhara and Sugawara; licensee Springer. 2014
Received: 11 February 2014
Accepted: 22 April 2014
Published: 21 May 2014
Charging/discharging behaviors of de-alloyed and anodic oxidized Ti-Ni-Si amorphous alloy ribbons were measured as a function of current between 10 pA and 100 mA, using galvanostatic charge/discharging method. In sharp contrast to conventional electric double layer capacitor (EDLC), discharging behaviors for voltage under constant currents of 1, 10 and 100 mA after 1.8 ks charging at 100 mA show parabolic decrease, demonstrating direct electric storage without solvents. The supercapacitors, devices that store electric charge on their amorphous TiO2-x surfaces that contain many 70-nm sized cavities, show the Ragone plot which locates at lower energy density region near the 2nd cells, and RC constant of 800 s (at 1 mHz), which is 157,000 times larger than that (5 ms) in EDLC.
KeywordsElectric storage devices Amorphous alloys with TiO2-x surfaces Nanometer sized cavity
Titanium oxide (TiO2) is of considerable interest for wide range of applications, including photocatalysis , optovoltaics , solar energy conversion , chemical sensors , bioprobes  and environmental pollution control . Although the majority of the applications of TiO2 are generally controlled by the crystalline phase , we report distinguished amorphous material supercapacitors, devices that store electric charge on their amorphous titanium oxide surfaces that contain many 70-nm sized cavities.
Following the capacitance studies of Ni-Nb-Zr-H glassy alloys with femtofarad capacitance tunnels [8, 9], we have found that the capacitance of nanocrystalline de-alloyed Si-Al [10, 11] or Si-Al-V , and de-alloyed and anodic oxidised amorphous Ti-Ni-Si alloy ribbons  show prompt charging/discharging of 102 μF (0.55 F/cm3) at a frequency of 1 mHz, from 193 to 453 K, and with a high voltage variation from 10 to 150 V. Especially, the de-alloyed and anodic oxidized Ti-Ni-Si alloy one displayed a capacitance of ~ 4.8 F (~52 kF/cm3) in discharging behaviors for voltage after 1.8 ks charging at DC current of 100 mA . We assume that the surface structure of the oxide consists of a distributed constant equivalent circuit of resistance and capacitance, analogous to active carbons in electric double-layer capacitors (EDLCs). The amorphous materials of interest are completely different from the conventional “wet” cells such as EDLC and secondary cells which are controlled by diffusivity of ions. We termed this device a “dry” electric distributed constant capacitor (EDCC).
In this study, we report DC and AC charging/discharging characteristics for anodic oxidized-amorphous Ti-15 at.% Ni-15 at.% Si alloy supercapacitors with higher narrow cavity densities and higher electric resistivities, with an aim to obtain further wide behaviors for Ti-Ni-Si ones, in comparison with those of the de-alloyed Si-Al alloy one [10, 11].
The rotating wheel method under an He atmosphere was used for preparing Ti-15 at.% Ni-15 at.% Si alloy ribbons of 1 mm width and a thickness of about 50 μm, using a single-wheel melt-quenching apparatus (NEV-A05-R10S, Nisshin Gikken, Saitama, Japan) with rotating speed of 52.3 m/s. De-alloying and anodic oxidation of the specimens were carried out for 288 ks in 1 N HCl solution and for 3.6 ks in 0.5 Mol H2SO4 solution at 50 V and 278 K, respectively. The densities of the specimen before and after surface treatment were 4.424 and 3.878 Mg/cm3, respectively.
The phase transformations upon heating were studied by differential scanning calorimetry (DSC) at a heating rate of 0.31 K/s using 10-mg specimens. The structure of specimens was identified by X-ray diffraction with Cu Kα radiation in the grazing incidence mode. Topography images were observed using a noncontact atomic force microscope (NC-AFM, JSPM-5200, JEOL, Akishima, Tokyo, Japan). A scanning Kelvin probe force microscopy (SKPM) based on the measurement of electrostatic force gradient was applied to measure an absolute electrical potential between the cantilever tip coated with Pt at 0 eV and TiO2 surface as the work function difference.
The specimen (1 mm wide, 50 μm thick, and 10 mm long) with double–oxidized surface was sandwiched directly by two copper ribbons beneath two pieces of glass plates using a clamp. Capacitances were calculated as a function of frequency between 1 mHz and 1 MHz from AC electric charge/discharge pulse curves of 10 V applied at 25 ns ~ 0.1 s intervals, using a mixed-signal oscilloscope (MSO 5104, Tektronix, Beaverton, OR, USA) and 30 MHz multifunction generator (WF1973, NF Co, Yokohama, Japan) on the basis of a simple exponential transient analysis. The charging/discharging behavior of the specimen was analyzed using galvanostatic charge/discharge on a potentiostat/galvanostat (SP-150, BioLogic Science Instruments, Claix, France) with DC’s of 10 V, 10 pA ~ 100 mA for ~900 s at room temperature. The details of the procedure have been described in previous paper . Experimental inspection for electric storage was carried out by swing of reflected light of DC Galvanometer (G-3A, Yokogawa Electric, Tokyo, Japan) after charging at 1 mA for 20 s.
Results and discussion
Thermoanalysis and phase analysis of anodic oxidized alloys
Morphological and dielectric analysis of anodic oxidized alloys
DC charging/discharging activity of EDCC
AC electric measurement of EDCC
Figure 5b shows a frequency dependent RC constant in input voltage of 10 V at room temperature for the former and the latter . The former’s RC decreases parabolically from around 800 s (13.1 min) to around 5 ms with increasing frequency up to 1 kHz at 100 ms-15 ns intervals, before becoming saturated in the frequency region from 1 kHz to 1 MHz. The 800 s (13.1 min) at 1 mHz is 157,000 times larger than that (5 ms) in the conventional EDLC . However, it needs larger ones from 0.1 s to few hours for practical use. On the other hand, the latter’s RC constants are 4–5 times smaller than those of the former in whole frequency region.
Electric storage inspection of EDCC
Additional file 1: Movie 1. The swinging movie of the reflected light spot at seven rounds for around 60 s as an evidence of the electric power charged. (MP4 8 MB)
Amorphous Ti-15 at.% Ni-15 at.% Si alloys prepared by the rotating wheel method were leached out for 288 ks in 1 N HCl solution at room temperature and anodically oxized for 3.6 ks in 0.5 M H2SO4 solution at 50 V and 278 K, respectively. AFM images showed a large numbers of volcanic craters with round pores approximately 70 nm in diameter on amorphous TiO2-x surface. The line profiles of the NC-AFM revealed spots ca. 7 nm in size with higher work functions of 5.53 eV in volcanic craters and at the bottom of ravines, indicating storage of electric charges.
DC discharging behaviors of the EDCC devices for voltage under constant currents of 1, 10 and 100 mA after 1.8 ks charging at 100 mA show parabolic decrease, demonstrating direct electric storage without solvents. In comparison of the power density and energy density for EDCC, the Ragone plot is hardly much for the 2nd cells. In sharp contrast to the de-alloyed Si-20at%Al specimen, frequency dependent capacitance and RC constant in input voltage of 10 V at room temperature for the Ti based one show 30 times larger in frequency region from 1 kHz to 1 MHz and 4–5 times larger in whole frequency region, respectively. The 800 s of the Ti based one at 1 mHz is 157,000 times larger than that (5 ms) in the conventional EDLC, lying in practical use region from 0.1 s to few hours. The 65 s-swing of reflected light spot in Movie clearly demonstrates electric storage of EDCC used in this study.
This work was supported by a Grant-in-Aid for Science Research in a Priority Area, “Advanced Low Carbon Technology Research and Development Program”, from the Japan Science and Technology (JST) Agency under the Ministry of Education, Culture, Sports Science, and Technology, Japan.
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