Zenan Yu

Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions

Supercapacitors have drawn considerable attention in recent years due to their high specific power, long cycle life, and ability to bridge the power/energy gap between conventional capacitors and batteries/fuel cells. Nanostructured electrode materials have demonstrated superior electrochemical properties in producing high-performance supercapacitors. In this review article, we describe the recent progress and advances in designing nanostructured supercapacitor electrode materials based on various dimensions ranging from zero to three. We highlight the effect of nanostructures on the properties of supercapacitors including specific capacitance, rate capability and cycle stability, which may serve as a guideline for the next generation of supercapacitor electrode design.

Highly Ordered MnO2 Nanopillars for Enhanced Supercapacitor Performance

This report demonstrates a simple, but efficient method to print highly ordered nanopillars without the use of sacrificial templates or any expensive equipment. The printed polymer structure is used as a scaffold to deposit electrode material (manganese dioxide) for making supercapacitors. The simplicity of the fabrication method together with superior power density and energy density make this supercapacitor electrode very attractive for the next-generation energy storage systems.

Energy Storing Electrical Cables: Integrating Energy Storage and Electrical Conduction

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

Flexible, sandwich-like Ag-nanowire/PEDOT:PSS-nanopillar/MnO2 high performance supercapacitors

We demonstrate the design and fabrication of a Ag/PEDOT:PSS/MnO2 layer by layer structure for high performance flexible supercapacitors (SCs). This is a unique design combining Ag-nanowires and PEDOT:PSS-nanopillars as current collectors in SCs. In this sandwich-like structure, electrons can be easily transferred through a networked structure of Ag-nanowires to PEDOT:PSS-nanopillars and finally to the MnO2 layer and vice versa. This scheme provides excellent specific capacitances of 862 F g−1 (based on MnO2) at a current density of 2.5 A g−1 and robust long-term cycling stability. Moreover, SCs fabricated based on this structure exhibit exceptional flexibility and bendability (less than 2% loss in specific capacitance even after 100 bends) and high energy and power densities. All wet processing methods of fabrication along with superior performance make these SCs very attractive for the next generation flexible energy storage systems.

Coil‐Type Asymmetric Supercapacitor Electrical Cables

Cable-shaped supercapacitors (SCs) have recently aroused significant attention due to their attractive properties such as small size, lightweight, and bendability. Current cable-shaped SCs have symmetric device configuration. However, if an asymmetric design is used in cable-shaped supercapacitors, they would become more attractive due to broader cell operation voltages, which results in higher energy densities. Here, a novel coil-type asymmetric supercapacitor electrical cable (CASEC) is reported with enhanced cell operation voltage and extraordinary mechanical-electrochemical stability. The CASECs show excellent charge-discharge profiles, extraordinary rate capability (95.4%), high energy density (0.85 mWh cm(-3)), remarkable flexibility and bendability, and superior bending cycle stability (≈93.0% after 4000 cycles at different bending states). In addition, the CASECs not only exhibit the capability to store energy but also to transmit electricity simultaneously and independently. The integrated electrical conduction and storage capability of CASECS offer many potential applications in solar energy storage and electronic gadgets.