Multifunctional CNT-Polymer Composites for Ultra-Tough Structural Supercapacitors and Desalination Devices (2014)

Multifunctional materials have attracted attention for the reduction of weight and volume on a system wide level by combining the functions of multiple components. The most immediate benefits come from combining structural functions, such as strength and stiffness, with non-structural functions, such as electrical conductivity and energy storage. Here we report on flexible multifunctional nonwoven composite fabric exhibiting a combination of remarkable mechanical properties and very high specific and volumetric capacitance for use in supercapacitors and capacitive desalination/deionization devices. The achieved specific tensile strength and toughness was higher than that of aluminum matrix composites, titanium and aluminum alloys, steels and many other common structural materials. Stable electrochemical performance has been demonstrated with specific capacitance of the polymer component stabilized at ~800 F·g -1 after 30,000 cycles. A carbon nanotube (CNT) - based fabric was produced using a commercial-scale continuous chemical vapor deposition process that allows rapid manufacturing of high-strength CNT sheets with tunable mechanical properties. In addition to providing a platform for achieving multifunctionality, these CNT substrates also allow the elimination of non-electroactive materials such as binders and heavy foil current collectors which further increase the energy density. Polyaniline (PANI) was chosen as the electrochemically active material due to its high conductivity, good environmental stability, tailorable nanostructure, and mechanical properties. [1] Compared to other supercapacitor active materials PANI is unique in that the ion exchange process by which the polymer equilibrates with acid solutions also imposes the anion into the polymer. This has been the basis for the use of PANI as an anion exchange polymer for mixtures of halide ions such as those found in salt water and can be used for desalination applications using capacitive deionization (CDI). [2] Pulsed electrodeposition of the PANI was utilized to allow control over the morphology, uniformity, and amount of PANI deposited. [3] When tested for supercapacitor and CDI applications in 1M HCl and NaCl solutions, the composites showed rapid ion adsorption and high specific and volumetric capacitances up to ~300 F•cm -3 in low-cost aqueous electrolytes and salt solutions exceeding that of the state of the art activated carbon electrodes. The high power characteristics of these composites are demonstrated by retaining more than 80% the capacitance when the current density is increased to 20 A·g -1 or when the supercapacitor operates at a very high frequency of 1 Hz. The mechanical properties of these materials were studied using tensile tests and dynamic mechanical analysis to determine room temperature static mechanical properties and temperature dependent dynamic mechanical performance from 30-200°C. The specific tensile strengths of PANI-CNT fabrics are comparable to or exceed that of lightweight titanium alloys and many metal matrix composites. Additionally, the PANI-CNT fabrics demonstrate outstanding modulus of toughness higher than the majority of other light-weight structural materials. When combined with the low-cost active materials and ambient temperature processing conditions, PANI-CNT composites may offer a great solution for weight and volume reduction for systems which require a combination of structural and energy storage functions as well as potential applications in desalination via CDI.