ELECTROSPUN NANOFIBER ELECTRODES: A PROMISING PLATFORM FOR SUPERCAPACITOR APPLICATIONS

With the ongoing global energy crisis and environmental concerns, the development of sustainable and renewable energy storage and conversion systems, such as electrochemical capacitors, lithium ion batteries, and proton-exchange membrane fuel cells, has evolved into an urgent task (Lu et al.,2017; Goodenough, 2014). Electrochemical capacitors, also called supercapacitors (SCs), are considered highly complementary to batteries due to their high power density, extremely long cycle life, low maintenance cost, and safe operation features. Their excellent performance makes SCs promising for portable electronics, power back-up devices, hybrid electric vehicles, and other electronic products.

 According to the charge storage mechanism, SCs can be classifified into electrochemical doublelayer capacitors (EDLCs) and pseudocapacitors (Wang et al., 2012; Simon and Gogotsi, 2008). An EDLC usually consists of two activated carbon electrodes, which are separated by a porous membrane separator and soaked in an aqueous or nonaqueous electrolyte. Through reversible ion adsorption/desorption processes, the electric charges are stored on the surface of the electrode while the oppositely charged ions are adsorbed on the electrolyte side to form electrical double layers. With a high specifific surface area and abundant nanopores of the activated carbon electrodes, EDLCs generally exhibit high power density up to 105 W/kg and a superlong cycle lifetime. There have been extensive studies in the past few decades on various types of carbonaceous materials for use as EDLC electrodes, such as graphene, carbon nanotubes (CNTs), carbon nanofifibers (CNFs), etc. (Zhang et al., 2016). However, the tortuous microporosity and nongraphitic structure of these carbonaceous materials easily lead to long diffusion distances of electrolytes and relatively high ionic resistance, so they fail to meet the requirement for high energy density of electrochemical capacitors at a high power output. It is thus essential to increase both the specifific capacitance (Csp) of the electrodes and the working voltage of EDLCs to increase energy density of EDLCs signifificantly. Unlike EDLCs, pseudocapacitors store charges through reversible redox reactions between the electrode materials and electrolytes. Since the electrochemical processes happen not only at the electrode surface regions but also inside the nanodomains of the electrode materials, more charges are stored in pseudocapacitors compared to EDLCs. Consequently, pseudocapacitors have much higher Csp and energy density than EDLCs. To date, a variety of pseudocapacitive materials with designed architectures, such as metal oxides, conducting polymers, and heteroatom-doped carbons, have been developed as the working electrodes for pseudocapacitor applications due to their distinct multiple reversible redox states or electrochemical doping/dedoping characteristics (Deng et al., 2016; Simotwo and Kalra, 2016). But structural deterioration of the electrodes easily occurs as a result of the large volume changes associated with repeated redox reactions or doping/dedoping processes at high potentials; and other drawbacks, including low conductivity, high cost, and remarkably poor stability during cycling, have severely hindered the practical applications of pseudocapacitive materials. Thus current research efforts are directed toward many novel hybrid systems based on a combination of electrochemical double-layer capacitive and pseudocapacitive characteristics.

Paper link：https://www.sciencedirect.com/book/9780323512701/electrospinning-nanofabrication-and-applications