**1. Introduction**

High-pulsed electrical power is required for lasers, flash photography, spark ignition, spot welders, fusion reactors, kinetic weapon systems, rapid acceleration of electric vehicles, etc. Capacitors, generally electric double layer capacitors (EDLC), also known as supercapacitors,

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

are preferred for pulse power applications, because they provide far higher power electric pulses, per weight/volume, than batteries [1, 2]. Moreover, unlike batteries, capacitors are not damaged by providing pulsed power. This robust feature leads to their employment as power load levelers to extend battery life. For example, in satellites, systems designed to transfer high power demand from batteries to a parallel capacitor system can significantly increase battery and satellite, lifetime.

law relation was found. Still, even with the noted degradation, the performance at the time periods of interest (e.g. 0.05 s discharge) was better than any commercial supercapacitor and possibly any EDLC prototype. Notably, comparison with EDLC prototype performance was

Performance of Aqueous Ion Solution/Tube-Super Dielectric Material-Based Capacitors as…

http://dx.doi.org/10.5772/intechopen.71003

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The NP supercapacitors were constructed of anodized titania foils filled with various aqueous salt solutions. The remaining metal of the original titania was one electrode and a graphitic material served as the other electrode. The performance of these capacitors was characterized using standard galvanostat constant current protocols. All procedures are described below.

Anodization process–Titanium foil anodes (99.99% Sigma Aldrich), approx. 0.05 mm thick, were anodized, as described elsewhere [8, 9, 21–23], in an ethylene glycol solution containing small quantities of ammonium fluoride (0.25% w/w) and water (2.75% w/w), using a titanium cathode (2 cm distant from the anode) at a constant DC voltage of 40 V for 46 min. This process created a layer of cylindrical hollow titania tubes on the parent titanium, average length measured to be 7.7+/−0.4 μm [24], but for purposes of conservative computation of energy density and all other parameters, assumed to be 8 μm in length. The tube diameter was found to be approximately 90 nm, but that figure does not enter the computations. In prior studies employing a nearly identical protocol, but using different anodization time periods, the intent was to create anodized layers/tubes of various lengths in order to test the impact of tube length on dielectric value and energy density. In this study, the intent was to focus only on the impact of the liquid phase composition, thus all the matrix material, that is the anodized titania, was produced using a single protocol and produced nearly identical anodized layers. Typical tubes formed from this process are very regular in structure and densely packed together [8, 9, 23, 24]. They are all oriented with the long axis perpendicular to the surface of

the parent foil. No effort was made to crystallize the tubes via a thermal treatment.

Assembly of capacitors—All the capacitors employed were a standard parallel plate construction, consisting of an electrode composed of the unanodized section of the original titania foil, the dielectric consisting of the anodized section (2 × 1 cm) filled with an free ion containing aqueous solution, and a positive electrode of Grafoil, a form of compressed graphite. The tubes in the anodized layer were filled with solution simply by placing them in a beaker filled with the solution for 50 min at room temperature. Three different ion precursors

), ammonia chloride (NH<sup>4</sup>

(KOH). Capacitors were constructed from aqueous solutions of the three salts, specifically three weight percent concentrations of each salt, 10, 20, and 30%, for a total of nine capacitors.

After the salt solution saturation, the capacitor had one electrode, the metallic component of the anodized titanium foil, and a compound dielectric in the form of the titania tubes filled with aqueous solution. Placing a Grafoil sheet (2 × 1 cm) on top of the open tube end of the anodized film completed the capacitor. Specifically, a rectangle of Grafoil (compressed natural graphite, 99.99% carbon [25, 26]) 0.3 mm thick was placed on top. The metallic part of the

Cl), and potassium hydroxide

difficult as fitted data on time response of these capacitors are apparently nonexistent.

**2. Experimental**

were used: sodium nitrate (NaNO<sup>3</sup>

A 'control study' employing distilled water was run as well.

Most research into increasing capacitor energy density is focused on developing graphene, the conductive material with the highest surface area (~2600 m<sup>2</sup> /g), electrodes for the next generation EDLC [3–7]. The theory suggests that capacitors with graphene electrodes could have an ultimate energy density of ~800 J/cm<sup>3</sup> , a value far less than the current generation commercial lithium ion battery (~2200 J/cm<sup>3</sup> ). Notably, current commercial supercapacitors have an energy storage rating of <50 J/cm<sup>3</sup> .

Recently, a new type of 'supercapacitor' was invented, NP supercapacitors (NPS) with energy density rivaling the best prototype EDLC, but based on an entirely different paradigm [8–14]. Unlike EDLC that gain energy density through the use of high surface area electrodes with low dielectric value, NPS use low surface area electrodes and dielectrics with remarkably high dielectric constants, specifically super dielectric materials (SDM), that is materials with dielectric values greater than 10<sup>5</sup> , although values >1011 are reported. SDM are composed of an 'active phase', such as salt dissolved in a liquid, and an 'inactive' mechanical phase such as anodized titania, T-SDM [8, 9], high surface area porous refractory oxides, Powder-SDM [10–12], or even simple fabrics, Fabric-SDM [13], that hold the active phase in place. The theoretical basis of SDM [8, 9] is that in an electric field the ions in solution travel to create dipoles, which are far longer (ca. 1 μm) than those found in solid dielectrics (ca. 10−4 μm). It is the 'field canceling' effect of dipoles, proportional to length, which leads to increased capacitance, as per the classic model of dielectric behavior [15–17].

It is reasonable to label NPS, a new type of supercapacitor based on the energy storage values achieved, approx. 400 J/cm<sup>3</sup> for T-SDM with aqueous NaCl solutions at very slow discharge rates, rivaling, perhaps surpassing, the best graphene-based EDLC prototypes [18–20]. One unresolved issue: NPS performance as a function of frequency. Given the theory of NPS requires micron scale ionic migration in a liquid to form giant dipoles, there should be significant performance degradation ('roll-off') with increasing frequency. That is, if not enough time is available in a charge cycle for dipoles to fully form via ion travel, the dielectric value, energy density, etc. will be reduced. Thus, it is important to directly test the performance of NPS as a function of frequency. Given the most likely application, power release over very short times, ca. 0.05 s, special attention should be paid to discharge rate dependence of power and energy.

The one study of NPS performance as a function of discharge time was on F-SDM, a variety not found to have particularly high energy density. Significant roll-off of all parameters with decreasing discharge period (roughly equivalent to increasing frequency) was documented. The roll-offs, all parameters, were well fit by simple power law relations over orders of magnitude of discharge time. In the present study, we employed the same method used in the earlier study to characterize performance as a function of discharge period of a variety of high energy density NPS, those employing anodized titania saturated with various aqueous free ion solutions. Once again, significant roll-off was observed as expected and the power law relation was found. Still, even with the noted degradation, the performance at the time periods of interest (e.g. 0.05 s discharge) was better than any commercial supercapacitor and possibly any EDLC prototype. Notably, comparison with EDLC prototype performance was difficult as fitted data on time response of these capacitors are apparently nonexistent.
