Research team of Chinese Academy of Sciences and University of Delaware developed EDLC, carbon tube based, supercapacitor with significantly improved frequency performance enabling miniaturization of AC line-filter and power devices by replacement of large aluminum capacitors.
A research team led by Prof. Meng Guowen from the Institute Solid State Physics, Hefei Institutes of Physical Science (HFIPS) of Chinese Academy of Sciences (CAS), cooperating with Prof. Wei Bingqing of the University of Delaware, Newark, U.S., successfully developed structurally integrated, highly-oriented carbon tube (CT) grids as electrodes of electric double-layer capacitors (EDLCs) to significantly improve in the frequency response performance and the areal and volumetric capacitances at the corresponding frequency. It is expected to be used as a high-performance small-sized alternating current (AC) line-filtering capacitor in electronic circuits, providing the essential materials and technology for the miniaturization and portability of electronic products.
EDLCs, usually with carbon materials as electrodes, are considered potential candidates for AC line-filtering to replace AECs due to their higher specific capacitance, in line with the trend of device miniaturization, but restricted by their low operating frequency (~1 Hz). Although the operating frequency can be enhanced by using highly-oriented carbon nanomaterials as electrodes, the specific capacitance is very limited. Meanwhile, the physical contacts between adjacent carbon nanotubes or graphene sheets would not only increase the resistance, further slowing the frequency response, but also make it difficult to increase the mass loadings of the carbon nanomaterials and thus obtain a large capacitance. There is an urgent need to develop newly-structured materials to increase the fast frequency response while maintaining high specific capacitance.
Filter capacitors are used in filter circuits to convert alternating current into direct current by smoothing out ripples in the incoming supply. They have been dominated by aluminum electrolytic capacitors and are typically the largest component in an electronic circuit because of their low areal capacitances, thus limiting the potential for miniaturization. Using three-dimensional porous anodic aluminum oxide (AAO) templates, Han et al. constructed a network of carbon tubes in which they deposited nickel catalyst nanoparticles and grew vertically aligned carbon nanotubes using chemical vapor deposition. After removal of the AAO, a flexible film was obtained. The films showed a 25% improvement in areal capacitance at 120 hertz and can be connected in series without affecting their electrochemical performance.
Since 2015, the research team has been working on this topic. After unremitting efforts, a new three-dimensional (3D) structure-integrated and highly-oriented CT array with laterally interconnected CTs by chemical bonds has been successfully developed. The 3D CT grid with truly-interconnected and structurally-integrated vertical and lateral CTs (denoted as 3D-CT) can provide highly oriented, high structural stability, superior electrical conductivity, and effective open porous structure, which is expected to meet the requirements of the electrode materials of the small-sized high-performance AC line-filtering EDLCs.
In order to obtain this unique structure, the researchers firstly anodized an aluminum sheet containing a small amount of Cu impurity, to obtain the highly ordered vertical porous anodic aluminum oxide (AAO) template containing Cu-impurity nanoparticles on the pore walls. Subsequently, a 3D interconnected porous AAO template was obtained by selectively etching the Cu-containing nanoparticles on the pore walls with phosphoric acid.
The 3D-CT grid was synthesized by a chemical vapor deposition (CVD) method using the 3D-AAO template. To increase the specific surface area, and further enhance the specific areal and volumetric capacitance, the 3D-CTs can be modified, as exemplified by filling with much-smaller-diameter carbon nanotubes (CNTs) within the vertical and lateral CTs via the Ni catalyst-assisted CVD method, or surface-treated with KMnO4.
The researchers directly used the 3D-CT grids as the electrodes to construct a series of symmetric EDLCs. It was found that such capacitors have good frequency response performance and very high specific areal capacitance.
More importantly, to reach high operating voltage, six 3D-CT grid-based EDLCs were connected in series, which also exhibited an excellent frequency-dependent performance, and a promising filtering performance like a single EDLC. It is largely due to the slight rise of the equivalent series resistance is compromised by a corresponding augmentation in capacitive reactance, ultimately leading to its fast frequency response. This proves that high voltage AC line-filtering capacitors can be achieved by means of connecting multiple EDLCs in series.
Furthermore, the 3D-CT grid-based EDLCs exhibit significant volumetric advantages over the comparably rated AECs in low-voltage operations (below 25 volts).
The findings provide a sound technological basis and key materials for developing EDLCs for miniaturizing AC line-filter and power devices, which would be helpful to replace the bulky AECs and realizes the miniaturization of portable electronics, mobile power supply, electrical appliances, and distributed energy harvesting and power supply on the Internet of Things, greatly promoting the development of high-performance digital circuits and emerging electronic technologies.