Surfactant effects on metal oxides/hydroxides as a supercapacitor electrode materials: Improved performance and possible mechanisms

Supercapacitors are electrochemical energy storage devices with very high power rates which perfectly fill the gap between traditional batteries and conventional capacitors ^[1,2]. Supercapacitors have already found applications in portable devices such as laptops, cameras or cell phones as well as in lifts, cranes or even hybrid electric vehicles (HEV) ^[3]. In general, the electrode materials of ES can be categorized into three types:

(1) Carbon materials with high specific surface area,

(2) Conducting polymers

(3) Metal oxides, such as RuO₂, Co₃O₄, SnO₂, V₂O₅, IrO₂, MoO_x.MnO₂, NiO, Ni(OH)₂ Co(OH)₂,

Metal oxides/hydroxides: In general, metal oxides can provide higher energy density for ES than conventional carbon materials and better electrochemical stability than polymer materials. They not only store energy like electrostatic carbon materials but also exhibit electrochemical faradaic reactions between electrode materials and ions within appropriate potential windows. The general requirements for metal oxides in ES applications are:

(1) The oxide should be electronically conductive,

(2) The metal can exist in two or more oxidation states that coexist over a continuous range of composition and electrode potential with no phase changes involving irreversible modifcations of a 3-dimensional structure, and

(3) The protons can freely intercalate into the oxide lattice during reduction (and out of the lattice on oxidation), allowing facile interconversion of MeO + H O +e^- ⇔MeOOH + OH ^-.To date, those investigated include ruthenium oxide, manganese oxides, cobalt oxides, nickel oxides, and vanadium oxides.

Selection of electrolyte and modification of electrolyte by surface active additives is attractive for commercial use and allows to reach better capacitance performance at low cost. To obtain a better wettability small amounts of three kinds of surfactants (anionic, cationic and non-ionic) as electrolyte additives were used.

This approach is based on the surfactant physical properties - due to their amphiphilic structure (hydrophobic "tail" and hydrophilic "head") they can effectively lower the surface tension of liquids in which they are solubilised ^[4]. Surfactants, with their amphiphilic properties arising from the molecular structure, i.e., hydrophilic 'head' and hydrophobic 'tail', may effectively reduce such high surface tension. Hence, they enable better electrolyte penetration into the electrode structure, therefore, utilized electrode/electrolyte interface can be significantly enlarged.

For this study, metal oxides/hydroxides will be synthesized by electrochemical deposition method, their structure, electrochemical characterization and supercapacitor performance will be studied with and without added surfactant. Electrochemical data will be obtained by cyclovoltammetry (CV), charge-discharge measurements and electrochemical impedance measurements (EIS). The samples will be characterized by SEM, TEM, FTIR, and Raman spectroscopy.

     REFERENCES:

1. A. Burke, J. Power Sources 91 (2000) 37.

2. R. Kötz, M. Carlen, Electrochim. Acta 45 (2000) 2483.

3. J.R. Miller, A. Burke, Electrochem. Soc. Interface 17 (2008) 53.

4. M.J. Rosen, Surfactants and Interfacial Phenomena, 3rd ed., John Wiley & Sons, Inc., New Jersey, 2004.