A High‐Energy‐Density Asymmetric Microsupercapacitor for Integrated Energy Systems

Abstract

pseudocapacitors have been explored quite extensively to meet the growing demand for fl exible and wearable electronics. [ 14,15 ] Redox materials, [ 10,14,16 ] such as MnO 2 , [ 17 ] Co 3 O 4 , [ 18 ] poly(3,4-ethylenedioxythiophene) (PEDOT), [ 12,13 ] MoS 2 , [ 14,19 ] etc., have been used to enhance the electrochemical performance. However, it is still a major challenge to simultaneously realize the delivery of high energy density at high power density and the supply of a wide potential window in symmetric pseudocapacitors. Therefore, the discovery of new asymmetric micro-pseudocapacitors [ 18,20 ] that demonstrate enhanced energy-supply performance and increased energy density without sacrifi cing high power density is much needed and vital for satisfying the demands that are crucial for developing practical applications. Furthermore, designing planar-type micro-SCs makes their integration into a circuit possible so that both the output operating voltage and total energy density can be controlled by the circuit design.Herein, we describe tough and fl exible yarns of carbon fi bers (CFs) with smaller diameters (≈50 µm) for asymmetric micro-pseudocapacitors that provide a wide potential window and high energy density and power density based on the total cell volume. The micrometer-sized carbon fi bers, which are obtained through mechanical extraction from commercial CFs, are coated with uniform WO 3 nanowires (NWs). The hierarchical architecture based on V 2 O 5 and polypyrrole (PPy) ultrathin fi lms fabricated on the WO 3 -coated carbon fi bers acts as positive and negative electrode, respectively. The ultrahigh energy density of the asymmetric micro-SC is about three times higher than the highest value of 4 V/500 µAh thin-fi lm lithium battery. Signifi cantly, the as-assembled integrated system featured improved energy storage and power delivery without compromising cyclability and tenacity.The micrometer-sized carbon fi bers (micro-CFs) with a diameter of only ≈50 µm are mechanically extracted from the commercial CFs (diameter of 800 µm). These micro-CFs demonstrate excellent strength, stiffness, and light weight ( Figure 1 a,b and Figure S1a and S1b, Supporting Information). In one of our previous studies, [ 21 ] we showed that uniform WO 3 NWs grown on micro-CFs (without sacrifi cing the properties of the CFs) are essential for fabricating a hierarchical architecture and enhancing the availability factor of active materials. The micro-CFs with a diameter of ≈70 µm remain stretchable and knottable after coating with WO 3 NWs (Figure 1 c). They can be bent into different shapes without any damage. Scanning electron microscopy (SEM) images in Figure 1 d and Figure S1c and S1d, Supporting Information show vertical WO 3 NWs arrays with a length of ≈20 µm on the CFs. The high mechanical strength and fl exibility of the micro-CFs enables the fabrication The ongoing demand for miniaturized electronics has recently promoted the development of energy storage devices with large capacitance and volumetric energy dens...

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