John Chmiola

Anomalous Increase in Carbon Capacitance at Pore Sizes Less Than 1 Nanometer

Carbon supercapacitors, which are energy storage devices that use ion adsorption on the surface of highly porous materials to store charge, have numerous advantages over other power-source technologies, but could realize further gains if their electrodes were properly optimized. Studying the effect of the pore size on capacitance could potentially improve performance by maximizing the electrode surface area accessible to electrolyte ions, but until recently, no studies had addressed the lower size limit of accessible pores. Using carbide-derived carbon, we generated pores with average sizes from 0.6 to 2.25 nanometer and studied double-layer capacitance in an organic electrolyte. The results challenge the long-held axiom that pores smaller than the size of solvated electrolyte ions are incapable of contributing to charge storage.

Relation between the Ion Size and Pore Size for an Electric Double-Layer Capacitor

The research on electrochemical double layer capacitors (EDLC), also known as supercapacitors or ultracapacitors, is quickly expanding because their power delivery performance fills the gap between dielectric capacitors and traditional batteries. However, many fundamental questions, such as the relations between the pore size of carbon electrodes, ion size of the electrolyte, and the capacitance have not yet been fully answered. We show that the pore size leading to the maximum double-layer capacitance of a TiC-derived carbon electrode in a solvent-free ethyl-methylimmidazolium-bis(trifluoro-methane-sulfonyl)imide (EMI-TFSI) ionic liquid is roughly equal to the ion size (approximately 0.7 nm). The capacitance values of TiC-CDC produced at 500 degrees C are more than 160 F/g and 85 F/cm(3) at 60 degrees C, while standard activated carbons with larger pores and a broader pore size distribution present capacitance values lower than 100 F/g and 50 F/cm(3) in ionic liquids. A significant drop in capacitance has been observed in pores that were larger or smaller than the ion size by just an angstrom, suggesting that the pore size must be tuned with sub-angstrom accuracy when selecting a carbon/ion couple. This work suggests a general approach to EDLC design leading to the maximum energy density, which has been now proved for both solvated organic salts and solvent-free liquid electrolytes.

Monolithic Carbide-Derived Carbon Films for Micro-Supercapacitors

Microcapacitors for Manufacture Capacitors can store small amounts of charge, and as they can charge and discharge quickly, they work well with batteries for recovering power, such as in regenerative braking in hybrid cars. For very small power requirements, capacitors have not been competitive with microbatteries, but using monolithic carbon films to store the charge, Chmiola et al. (p. 480) demonstrate the feasibility of such applications. The small pores in the carbon films are sufficiently large to allow electrolyte transport and can be made using a processing technique compatible with current chip manufacturing. Such microcapacitors can thus be integrated with electronics to make autonomous sensors or implantable devices. The power density of small-scale capacitors can be increased by using monolithic carbon films. Microbatteries with dimensions of tens to hundreds of micrometers that are produced by common microfabrication techniques are poised to provide integration of power sources onto electronic devices, but they still suffer from poor cycle lifetime, as well as power and temperature range of operation issues that are alleviated with the use of supercapacitors. There have been a few reports on thin-film and other micro-supercapacitors, but they are either too thin to provide sufficient energy or the technology is not scalable. By etching supercapacitor electrodes into conductive titanium carbide substrates, we demonstrate that monolithic carbon films lead to a volumetric capacity exceeding that of micro- and macroscale supercapacitors reported thus far, by a factor of 2. This study also provides the framework for integration of high-performance micro-supercapacitors onto a variety of devices.

Desolvation of Ions in Subnanometer Pores and Its Effect on Capacitance and Double-Layer Theory†

The study of charged solid–liquid interfaces, manifested as “double layers”, represents a problem of both practical and scientific importance. Double layers are present in all electrolyte solutions and have been traditionally studied using planar noble-metal electrodes and mercury drops. However, in the ionic channels in cells or the small-diameter pores of electrochemical double-layer capacitors (EDLCs),ions are in a very confined situation, which is different from that of a planar solid/electrolyte interface. By using nanoporous carbon with pores smaller than the size of an ion and a single associated solvent molecule, we show that the implicit assumption that double layers are governed only by ion/ electrode charge separation may be short-sighted. Other factor may play a more dominant role than previously thought, for example, increasing the confinement of the ions leads to an increase in the capacitance. Including the effect of partially desolvating ions in the current double-layer theory could lead to a better understanding of the behavior of ions in confined environments.

Microelectrode Study of Pore Size, Ion Size, and Solvent Effects on the Charge/Discharge Behavior of Microporous Carbons for Electrical Double-Layer Capacitors

The capacitive behavior of TiC-derived carbon powders in two different electrolytes, NEt4BF4 in acetonitrile AN and NEt4BF4 in propylene carbonate PC, was studied using the cavity microelectrode CME technique. Comparisons of the cyclic voltammograms recorded at 10–1000 mV/s enabled correlation between adsorbed ion sizes and pore sizes, which is important for understanding the electrochemical capacitive behavior of carbon electrodes for electrical double-layer capacitor applications. The CME technique also allows a fast selection of carbon electrodes with matching pore sizes different sizes are needed for the negative and positive electrodes for the respective electrolyte system. Comparison of electrochemical capacitive behavior of the same salt, NEt4BF4, in different solvents, PC and AN, has shown that different pore sizes are required for different solvents, because only partial desolvation of ions occurs during the double-layer charging. Squeezing partially solvated ions into subnanometer pores, which are close to the desolvated ion size, may lead to distortion of the shape of cyclic voltammograms.

Double-Layer Capacitance of Carbide Derived Carbons in Sulfuric Acid

Nanoporous carbons obtained by selective leaching of Ti and Al from Ti2AlC, as well as B from B4C, were investigated as electrode materials in electric double-layer capacitors. Cyclic voltammetry tests were conducted in 1 M H2SO4 from 0-250 mV on carbons synthesized at 600, 800, 1000, and 1200°C. Results show that the structure and pore sizes can be tailored and that the optimal synthesis temperature is 1000°C. Specific capacitances for Ti2AlC CDCs and B4C CDCs were 175 and 147 F/g, respectively, compared to multiwall carbon nanotubes and two types of activated carbon, measured herein to be 15, 52, and 125 F/g, respectively.