Energy Storing Electrical Cables: Integrating Energy Storage and Electrical Conduction

Abstract

DOI: 10.1002/adma.201400440 approach proposed by Simon and Gogotsi is depositing pseudocapacitive material (like MnO 2 ) onto a nanostructured current collector. [ 9 ] Despite some achievements that have been made by using similar methods, such as depositing MnO 2 onto nanowires, [ 10 ] nanotubes, [ 11 ] and nanopillars, [ 12 ] these arrays usually suffer from structural collapse, resulting in a decrease of useful surface area, reduction of deposited active materials and limited accessibility of electrolyte. Furthermore, these nanostructured current collectors are either derived from expensive and environment-unfriendly template methods or tedious fabrication processes. Therefore, it is still a challenge to readily and simply fabricate nanostructured electrodes with large area, template-free, and high aspect ratio arrays without nanostructures clumping together. Here we developed a large area, template-free, high aspect ratio, and freestanding CuO@AuPd@MnO 2 core-shell nanowhiskers (NWs) design. Our electrochemical measurements show that these CuO@AuPd@MnO 2 NWs exhibit remarkable properties including high specifi c capacitance, excellent reversible redox reactions, and fast charge-discharge ability. Moreover, a novel coaxial supercapacitor cable (CSC) design which combines electrical conduction and energy storage by modifying the copper core used for electrical conduction was demonstrated. For accomplishing large surface area necessary for high supercapacitor performance, we developed NWs on the outer surface of the electrical copper wire. The NWs structure is developed by just heating the inner core and therefore is practical to upscale the process to make extended lengths. An attractive advantage of using the coaxial design is that electricity can be conducted through the inner conductive metal wire and electrical energy can be stored in the nanostructured concentric layers added to this inner metal wire with an oxide layer in between. It is always a vital task for many applications including aviation to fi nd better methods to save weight and space, while maintaining the intended purpose. Therefore, integration of electrical cable and energy storage device into one unit offers a very promising opportunity to transmit electricity and store energy at the same time. In addition, CSC built from these NWs exhibits excellent fl exibility and bendability, superior long-term cycle stability, and high power and energy densities. The development of this innovative lightweight, fl exible, and space saving CSC will be very attractive for many applications including hybrid and allelectric vehicles, electric trains, heavy machineries, aircrafts and military. Fabrication of electrode involves three steps: (1) growth of CuO NWs from a pure copper wire by heat treatment; (2) deposition of a conducting metal layer by sputter-coating; (3) electrodeposition of active material onto the nanostructures ( Figure 1 a). Scanning electron microscope (SEM) image (Figure 1 b) clearly Currently, millions of miles of electrical cables have been used for providing electrical connections in machineries, equipment, buildings and other establishments. Energy storage devices are completely separated from these electrical cables if used. However, it will revolutionize energy storage applications if both electrical conduction and energy storage can be integrated into the same cable. Coaxial cable, also called coax, is one of the most common and basic cable designs that is used to carry electricity or signal. It has an inner conductor enclosed by a layer of electrical insulator, and covered by an outer tubular conducting shield (see Supporting Information, SI, Figure S1). The term “coaxial” is used because both the inner and outer conductors share the same geometric axis. In a coaxial design, it is possible to combine two different functional devices into one device that can still perform the original functions independently. Supercapacitors, also known as electrochemical capacitors, have become one of the most popular energy storage devices in recent years. Compared to other energy storage devices like batteries, supercapacitors have faster charge-discharge rates, higher power densities, and longer life times. [ 1 ] As a signature of their performance, safety, and reliability, they have recently been employed in the emergency doors of Airbus A380. Supercapacitors store energy by Faradaic or non-Faradaic reactions. Supercapacitors which utilize the Faradaic reactions are also called pseudocapacitors. Metal oxides/hydroxides like ruthenium oxide (RuO 2 ), [ 2 ] manganese dioxide (MnO 2 ), [ 3 ] cobalt oxide (Co 3 O 4 ), [ 4 ] nickel oxide (NiO), [ 5 ] and nickel hydroxide (Ni(OH) 2 ), [ 6 ] have been extensively studied as electrode materials in pseudocapacitors. Among them, MnO 2 has stood out due to its outstanding characteristics such as high theoretical specifi c capacitance (∼1,400 F g −1 ), natural abundance, and environmental friendliness. [ 7 ] However, the poor electrical conductivity of MnO 2 (∼10 −5 – 10 −6 S cm −1 ) is a limiting factor in achieving its theoretical specifi c capacitance. [ 7b , 8 ] One feasible