Fast Ion and Electron Transport in a Supercapacitor Based on Monolithic Nanowire‐Array Electrodes Prepared from a Defect‐Free Anodic Aluminium Oxide Mold

Abstract

anodic aluminum oxide (AAO) molds. Our model electrodes are based on thin gold (Au) fi lms on which AuNWs are grown directly from, and aligned vertically to the fi lm surface. The 1D structure and alignment of the NWs give short path lengths for ion and electron transport. The NWs are seamlessly integrated with the underlying fi lms in an attempt to prevent electrons from scattering at the NW-fi lm interface. Moreover, the proposed method does not require current collectors, binders, and conductive additives. The monolithic feature of the NW-array electrodes effectively circumvents the interfacial resistance typically observed for agglomerated electrodes composed of granular materials. Due to the high electrical conductivity of the metal (>10 5 S cm −1 [ 9 ] , NW-array electrodes designed in this way can suppress internal resistance involved in charge transport, leading to SCs with superior rate performance. More importantly, defects in the NW-array electrode considerably infl uence the internal resistance of the SCs, so device performance is enhanced by creating the electrode structure utilizing a homemade, defect-free AAO mold, effectively reducing the internal resistance and increasing the electroactive area. Figure 1 a illustrates the method used to fabricate the NWarray electrodes. Thin Au fi lms with ≈1.8 µm thickness were fi rst prepared on one side of AAO molds via Au sputtering, followed by electrodeposition of Au. Following this procedure, AuNWs were electrodeposited and grown from the pre-formed Au fi lms through cylindrical pores in the AAO molds, and the height of the NW arrays was uniform at ≈19 µm. After removing the AAO, this method produced fl exible and free-standing monolithic NW-array electrodes as displayed in Figure 1 b. In our approach, the pore structure of the AAO molds can be directly transferred to the structure of the NW-array electrodes. Using this method, we tested two different molds in order to design the electrodes with a desirable structure for SCs. Figure 1 c shows the surface and cross-sectional scanning electron microscope (SEM) images of a commercial AAO mold we tested. The pores had a diameter d = 37.5 ± 9.9 nm (relative standard deviation, RSD = 0.26), and were randomly distributed with the center-to-center spacing of nearest-neighbor pores, r = 129.3 ± 27.4 nm (RSD = 0.21). The large RSDs of d and r refl ect an irregularity in pore size and pore spacing. Additionally, the mold contained defects, both inside the pores and on its surface. The resulting NW-array electrode (henceforth, AuNW D ) had analogous defects (e.g., branched, bottlenecked, and disconnected NWs shown in the top portion of Figure 1 e). We also prepared and tested a homemade, defect-free AAO Incorporation of ionic and electronic charges with nanostructured materials can give rise to interesting conduction phenomena and new device properties. [ 1 ] Supercapacitors (SCs), also called electrochemical capacitors, store energy via the formation of electric double layers (EDLs) within porous electrodes presentin...

Metal–Organic Coaxial Nanowire Array Electrodes Combining Large Energy Capacity and High Rate Capability

“…(iii) The NWs are bound directly to a thin metal film through an electrochemical technique; this method creates a robust bonding of the NWs to the film, which would reduce the resistance of electron flow across the NW–film interface. (iv) The electrodes are prepared using an in‐house AAO with a defect‐free pore structure; Such molds yield high‐quality NW arrays, which facilitate charge transport …”

“…(iv) The electrodes are prepared using an in-house AAO with ad efect-free pore structure;S uch molds yield high-qualityN Warrays, which facilitate chargetransport. [19] Figure 1a shows the procedure used to prepare the coaxial Ppy/AuNW array electrode:( i) One side of AAO is sputtercoatedw ith Au, followed by the electrodepostion of Au, forming a1 .8 mmt hick conductive underlayer. ( ii)Byu sing an analogous electrodeposition technique, the AuNWs are directly grown on the thus prepared Au underlayer through the cylindrical pores of AAO.…”

“…Electrode Fabrication:T he defect-free AAO mold was prepared by am ethod previously reported, [19] with slight modification. The electrodeposition of Au and electropolymerization of pyrrole were performed using aV ersaSTAT 4p otentiostat/galvanostat (Prinston Applied Research) equipped with ap latinum mesh counter electrode and aA g/AgCl reference electrode saturated with potassium chloride solution.…”

Graphite‐Aligned Ni/Ni(OH)2 Nanowire‐Based Aqueous Asymmetric Supercapacitors Exhibiting Excellent Cycle Stability, High Rate Performance, and Wide Operation Voltage

“…In metallic nanowires, shortened ion diffusion lengths result in improved rate performance ( t=l 2 /D ), where t and l are the time required for ions to diffuse through the electrode material and diffusion length, respectively, and D is the diffusion coefficient . These features result in high charge/discharge rates . However, the metal nanowires that store charge via the formation of electric double layers have relatively low capacitance when compared to electrodes involving faradaic charge transfer processes during charge storage.…”

“…Previously, we have shown that using a gold nanowire array‐based architecture in a symmetric supercapacitor electrode can be beneficial for achieving very high charge transfer rates . In order to increase capacity, these nanowires were coated with a conducting polymer shell to introduce pseudocapacitance in the device .…”