Carbon Aerogels and Their Applications in Supercapacitors and Electrosorption Processes
Tri D. Tran*, Joseph C. Farmer* and Richard W. Pekala**
*Chemistry & Materials Science Department
Lawrence Livermore National Laboratory, Livermore, CA 94550
**PPG Industries, Inc., Monroeville, PA 15146
Carbon aerogels are unique porous materials consisting of interconnected nanometer-sized particles (3-30 nm) with small interstitial pores (< 50 nm). This monolithic (continuous) structure leads to very high surface area (400-1100 m2/g) and high electrical conductivity (25-100 S/cm). The aerogel chemical composition, microstructure, and physical properties can be controlled at the nanometer scale, giving rise to unique optical, thermal, acoustic, mechanical and electrical properties. Among their many applications, carbon aerogels have found use as electrode materials in several electrochemical devices, most notably, aerocapacitors and electrosorption processes.
The aerocapacitor is a high power-density, high energy-density electrochemical double-layer capacitor with carbon aerogels as electrodes. This device stores electrical energy by electrostatic charge separation at the electrode/electrolyte interface. It is considered for use as loading leveling power sources and for electric vehicle power-trains. An innovative aerogel-based electrosorption process has been used successfully to remove heavy metals and inorganic salts from aqueous waste solutions. A waste stream with various anions and cations is passed through a stack of electrically charged carbon aerogel electrodes. Impurities are separated from the electrolyte by the imposed electric field. This non-polluting separation technology is energy-efficient and potentially cost-effective alternative to conventional deionization processes such as reverse osmosis and ion-exchange.
Carbon aerogel synthesis and properties
Carbon aerogels are a special class of aerogels (air-filled foams) developed at Lawrence Livermore National Laboratory (LLNL). Many common characteristics include a tortuous open-cell structure, an ultrafine particle (cell) and pore size (<50 nm), and high surface area (400-1000 m2/g). The aerogel solid matrix is composed of interconnected colloidal-like carbon particles or polymeric carbon chains depending on the precursor formulation and processing conditions. Carbon aerogels are usually formed from the sol-gel polymerization of resorcinol and formaldehyde, followed by supercritical or evaporative drying, and subsequent pyrolysis at an elevated temperature (circa. 1050#161#C) in an inert atmosphere [1, 2]. The resulting carbon aerogels are electrically-conductive in contrast to all other types of organic and inorganic aerogels which are generally insulating materials. Carbon aerogels can be produced as monoliths, composites, thin films, powders, or microspheres.
The chemistry and physics of carbon aerogels have been studied extensively in the last decades. Unique optical, thermal, acoustic, mechanical, and electrical properties of these and related organic precursors have been investigated [3-7]. The structure and properties of carbon aerogels are largely controlled by three factors: (1) the molar ratio of the monomer building block (resorcinol) to the catalyst (sodium carbonate) (R/C ratio) of the starting solution, (2) the pyrolysis temperature, and (3) activation procedures. The R/C ratio affects the number of the resorcinol-formaldehyde cluster and the size to which they grow. At low R/C values(< 100), the clusters tend to be small (< 50A) and there is high degree of cross-linking, resulting in high surface areas and better interconnectivity. At large R/C ratios (i.e., 300), the resulting structure resembled strings of colloidal particles (large clusters > 100A) with less connectivity. At the same R/C ratio, materials showed similar particle sizes. The aerogel density can then be varied by starting with a different resorcinol concentration. In these cases, higher-density materials simply have more interconnected particles per unit volume than their low-density counterparts. This unique feature allows synthesis of high-density electrode materials (circa 1.2 g/cc) with exceptionally large surface areas (600 m2/g). These properties are necessary for achieving large capacitance per unit volume in energy storage devices.
The ability to control the structure and properties of porous carbon aerogels has led to their increased use as electrode materials in advanced energy storage devices and other electrochemical devices. Aerocapacitors and electrosorption processes, including the process that has become known as carbon aerogel capacitive deionization (CDI), have been successfully developed. Both are currently under commercial development. Selected features of these technologies are discussed below. Carbon aerogels provide an almost ideal electrode materials because of their high electrical conductivity (10-100 S/cm), high surface areas (400-1000 m2/g), efficient (interconnected) and open pore structure with controllable pore size (30-500A).
Thin carbon aerogel/carbon paper composites are developed for use in experiments with electrochemical double-layer capacitors and electrosorption processes. The porous and thin electrode structure improves ion transports and reduce ohmic resistance. The composite material was synthesized by impregnating a resorcinol-formaldehyde (RF) solution into a porous commercial carbon paper. This RF/carbon paper structure was then cured between two glass plates in a closed vessel which prevented evaporation. The cured composite was then soaked in acetone and subsequently dried at room temperature. Finally, the RF/carbon paper was heat treated at 1050#161#C in a nitrogen atmosphere for three hours to carbonize the resorcinol-formaldehyde (RF) component. Thin electrodes with high densities (~ 0.8 g/cm3) and large BET surface areas (eg. 600 m2/g) have been prepared. While commercial activated carbons and activated carbon fibers have shown large surface areas (#163# 3000 m2/g), these tend to have a much lower density (#163# 0.2 g/cm3). Therefore, on a volumetric basis, the BET surface area of carbon aerogel (500 m2/cm3) is comparable to those obtained from the highest-surface area commercial materials . Equally important, the pore size, pore size distribution and the microstructure of aerogels is expected to be more advantageous for double-layer formation than those from commercial sources. The pore volume distribution in aerogels  shows a Gaussian behavior with an average pore size around 5 nm (50 A) while the pore size analysis for commercial materials (i.e., activated fibers) shows a preponderance of pores having diameters less than 1 nm (10A). Since it is unlikely that double-layer formation occurs in these small pores, the electrochemically-active (useful) area represents only a small fraction of the BET surface area. Capacitance measurements are consistent with this understanding [6-8].
Electrochemical capacitors with carbon aerogel electrodes
Electric and hybrid vehicles of the future will need some additional power for a quick acceleration or hill-climbing. The development of energy/power storage technology for electric drivetrains remains one of the biggest challenges in commercializing low-emission vehicles. What is needed is an efficient and low-cost system that combines high specific energy (i.e., energy per unit weight or volume) with high specific power. One feasible solution is a hybrid concept where a high-energy density battery is coupled with a high-power density device such as a supercapacitor. Supercapacitors are electrochemical energy storage devices that are designed for a rapid storage and release of large quantities of energy. They are often referred to as "double layer capacitors" since they store charge at a polarized solid/electrolyte interface. This phenomenon is driven by available surface area of the electrode material and the proper pore size distribution. Carbon aerogels provide almost ideal electrode materials for this application because of their low electrical resistivity (< 25 mohm-cm), controllable pore size distribution (5-500 A), and high specific and volumetric surface areas (~ 1000 m2/g and 500 m2/cm3, respectively). Researchers at Lawrence Livermore National Laboratory have developed aerocapacitors based on carbon aerogel electrodes .
The aerocapacitor is composed of a positive and a negative wafer-thin (0.125 mm) carbon aerogel electrodes. The electrodes are separated by a microporous separator wetted with electrolyte. Prototype devices have been fabricated in both aqueous (1V/cell) and organic electrolytes (3V/cell). The energy and power densities for aqueous aerocapacitors were 2 Wh/Kg and 8 KW/Kg, respectively. These performance values are about two-order-of-magnitude higher than conventional electrolytic capacitors. PolyStor Corp. (Dublin, CA) is commercializing the LLNL-patented aerocapacitors with both aqueous and organic electrolytes for power electronic applications at the present time.
Electrosorption on carbon aerogel electrodes for deionization
A novel electrochemical separation process with carbon aerogel electrodes has been recently developed for removing ionic impurities from aqueous streams . A unit (single) cell consists of two carbon aerogel electrodes. It is undivided and uses no membranes During operation, the waste stream containing dissolved ions is passed through a stack of coupled carbon aerogel electrodes. A potential (#163# 1.2 V for aqueous systems) is applied across these high-surface-area electrodes. In some simple cases, the charged impurities are transported to the electrode surface and may be electrostatically held within the electric double layer, which is typically 1 to 10 nm (10 to 100A) thick. However, in most cases, such as those involving large polyvalent oxyanions or heavy metals, the means of separation appears more complex. For example, physisorption, chemisorption, electrodeposition, and/or electrophoresis may be the dominant mechanisms. Conclusive experimental data with the Cr system and extensive results accumulated at LLNL support this understanding. After the stack is saturated, regeneration is accomplished by discharging the cells at 0 V, or by reverse polarization at -1.2V. Reverse polarization can increase the regeneration effectiveness and/or reactivate the carbon aerogel electrodes.
The electrosorption process on carbon aerogel electrode materials have been demonstrated to be superior to previous cells containing electrodes consisted of activated carbon powders or packed carbon particles. In these latter cases, an inert polymeric binder was usually used. Such systems are troubled by limited liquid accessibility to small pores (average diameter ~ 10 A) resulting in sluggish mass transports. The non-conducting polymeric binders occlude significant fractions of the carbon surface and prevent intimate particle-particle electrical contact, thereby causing high electrical resistance. Polymer binders are also susceptible to both chemical attack and radiation-induced degradation, a problem that would be encountered during the removal of dissolved radionuclides. Porous flow-through electrodes made of finely-divided particles in packed beds require membranes as separators to prevent electrical shorting and particulate entrainment in the flow. Such systems are plagued by a relatively large pressure drop, which increases the overall energy required by the process. The advantages and efficient operation associated with carbon aerogel processes have been demonstrated and identified in many recent studies [10-15].
Aerogel-based electrosorption studies have been performed to treat a variety of cations and anions at the laboratory scale (10-40 GPD, 50-500 ppm total dissolved solids, TDS). This selection of ions is representative of the major species in underground aquifers, seawater, and storage tanks (e.g., Na+, NH4+, Cl-, ClO4-, NO3-). Additionally, many heavy metals (copper, zinc, nickel, cadmium, chromium, lead and uranium) have been removed from aqueous streams.
A large electrochemical system composed of 18 stacks, each stack having 150 cells (double-sided electrodes) was recently built. This scaled-up version, using about 1200 sq. ft of 4 in x 8 in electrodes, has been assembled as a mobile unit for performing field tests. It was designed to treat about 1000 GPD of aqueous wastes of medium ions levels ( < 1000 ppm TDS). Collaborative efforts have been planned for evaluating this technology at military installations and commercial sites to treat waste streams.
This electrosorption process with carbon aerogels is non-polluting, energy-efficient, and potentially competitive alternative to deionization technologies such as reverse osmosis, ion-exchange, and evaporation. The proof-of-concept study and subsequent results provide an extensive data base for further applications of this process to treat low-level radioactive wastes and contaminated ground water. Potential applications include recycled boiler-water for fossil fuel-fired and nuclear power plants, ultra pure water for biotech and semiconductor processing, softening domestic water, and desalinating brackish and seawater.
The aerogel synthesis and the development of both aerocapacitors and electrosorption cells represent several key technologies for the next decade. Recent advances in these areas offer much potentials and unique challenges. The development of a low-cost and efficient aerocapacitor will play a key role in the development of hybrid electric vehicles. Of great significance is the availability of the electrosorption process that would be competitive to current deionization technologies. This technology at the present state of development provides a unique solution to treat hazardous waste and remediate contaminated groundwater. Potential applications in a wide range of areas deserves special considerations and further development. The ability to tailor and provide carbon aerogels with special properties is the underlying challenge.
Funding for the fundamental studies of carbon aerogels was provided by the Office of Basic Energy Sciences-Division of Advanced Energy Projects under the direction of Dr. Walter Polanski. Support for the development of the carbon aerogel electrosorption process was provided by the Strategic Environmental Research and Development (SERDP) Program, under the direction of Dr. John Harrison. This work was done under the auspices of the U.S. Department of Energy (DOE) by LLNL under Contract No.W-7405-Eng-48.