Three-Dimensionally Ordered Mesoporous (3DOm) Carbon Materials as Electrodes for Electrochemical Double-Layer Capacitors with Ionic Liquid Electrolytes

Abstract

Compared to rechargeable batteries, electrochemical double-layer capacitors (EDLCs) are normally considered to be higher power but lower electrical energy density charge storage devices. To increase the energy density, one can enlarge the interfacial area between electrodes and electrolyte through the introduction of nanopores and employ electrolytes that are stable over wider voltage ranges, such as ionic liquids. However, due to the relatively high viscosity of ionic liquids and large ion sizes, these measures can result in diminished power performance. Here, we describe the synthesis of carbon electrodes that overcome these limitations and simultaneously provide high specific energies and high specific powers in EDLCs using the ionic liquid EMI-TFSI as an electrolyte. A colloidal crystal templating method was optimized to synthesize three-dimensionally ordered mesoporous (3DOm) carbons with well-defined geometry, three-dimensionally interconnected pore structure and tunable pore size in the range from 8 to 40 nm. To achieve precise control over the pore sizes in the carbon products, parameters were established for direct syntheses or seed growth of monodisperse silica nanospheres with specific sizes, using L-lysine-assisted hydrolysis of silicon alkoxide precursors. Porous carbons were then templated from these materials using phenol−formaldehyde (PF) or resorcinol− formaldehyde (RF) precursors. The pore structures of the nanoporous carbon products were characterized in detail, and the materials were tested as electrodes for EDLCs. Optimal pore sizes were identified that provided a large interface between the electrode and the electrolyte while maintaining good ion transport through the relatively viscous electrolyte. 3DOm PF-carbons with pore diameters in the 21−29 nm range exhibited similar high specific capacitance values (146−178 F g −1 at 0.5 A g −1 , with respect to the mass of carbon in a single electrode) as typical large-scale activated-carbon-based EDLCs but showed significantly better high-rate performance (80−123 F g −1 at 25 A g −1 ), a result of the more accessible pore space in which ion diffusion was less restricted.

Unbiased Quantification of the Electrochemical Stability Limits of Electrolytes and Ionic Liquids

“…They were chosen because they are all commonly used in electroanalytical devices 22,23,[29][30][31] and electrochemical capacitors 3,6,7,32 due to their wide electrochemical windows. The electrochemical limits were determined based on J cut-off values of 0.5, 1.0, and 5.0 mA/cm 2 and are listed in Table I.…”

“…The electrochemical limit of the ionic liquid EMI TFSI (commonly used in electrochemical devices with high-surface-area carbon electrodes 22,23 ) at electrodes consisting of non-porous glassy carbon or of a high-porosity carbon (Black Pearl carbon, 3DOm carbon, or CIM carbon) are shown in Table III for several J cut-off values. While the onset of reduction occurs at approximately −2.5 V (see Figure 6B), the most commonly used J cut-off value of 1.0 mA/cm 2 predicts for all highly porous carbons unreasonably small cathodic limits in the range of −0.5 to −1.0 V. This is readily explained by the large capacitive current at these high-surface area electrodes.…”

“…25,26 Three-dimensionally ordered mesoporous (3DOm) carbon was prepared through infiltration of a phenol-formaldehyde resol into an ordered silica template made from sedimented silica spheres, as described previously. 23 A final pore size of 37 nm was determined through nitrogen sorption, and an ordered structure was observed through transmission electron microscopy and small-angle X-ray scattering. CIM and 3DOm carbon films were prepared by finely grinding the carbon powder with an agate mortar and pestle, followed by addition of an aqueous suspension of PTFE (19:1:20 carbon:PTFE:water by mass) and grinding until a homogenous paste was formed.…”

Mesoporous materials for energy conversion and storage devices

“…In a typical EDLC, the capacitance is directly related to the surface area of the conductive electrodes in contact with the electrolyte. As a result, the high surface area and mesopore channels of mesoporous carbons may enhance the energy density of EDLCs [155][156][157][158] . In principle, increasing the surface area and electrical conductivity of mesoporous carbon would be a perfect way to boost the EDLC performance; however, in practice, this is difficult to achieve 159 .…”

Design Considerations for Unconventional Electrochemical Energy Storage Architectures

“…While there are many explored hard templating techniques offering high structural ordering, [ 481 ] colloidal self-assembly lithography is seemingly the preferred approach due simplicity in implementation and large variety of size and colloid chemistries. Numerous colloidal crystal template electrode materials including anode, [ 327,[482][483][484][485][486][487] cathode, [ 326,482,488,489 ] separators and electrolytes, [ 490 ] mesoporous carbon for supercapacitor applications, [ 491 ] as well as more complex electrochemical supercapacitors and full-cell architectures [ 492,493 ] have been realized and tested so far. Electrochemical properties of the so-formed nanocomposites were found to be superior to their bulk counterpart owing to peculiar morphology.…”