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

Abstract

The capacitive behavior of TiC-derived carbon powders in two different electrolytes, NEt 4 BF 4 in acetonitrile ͑AN͒ and NEt 4 BF 4 in propylene carbonate ͑PC͒, was studied using the cavity microelectrode ͑CME͒ 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 electrodes͒ for the respective electrolyte system. Comparison of electrochemical capacitive behavior of the same salt, NEt 4 BF 4 , 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. Global warming, as well as the increasing price and decreasing availability of fossil fuels, all highlight the need to move toward a sustainable development where renewable energy and electric/ hybrid vehicle engines are widely used. Renewable energy sources such as sunlight, wind, or hydro ͑rivers, waves, etc.͒ generate electrical energy which must be stored for use in autonomous systems such as transportation or electronics. Therefore, the development of high-performance electrical-energy storage devices is required.There are two major types of electrical-energy storage devices, batteries and electrical double-layer capacitors ͑EDLCs͒. Batteries are electrochemical-energy storage devices that can be fully discharged in a few minutes or hours efficiently. Li-ion batteries, for example, exhibit high specific energy ͑ϳ150 Wh/kg͒ but are today limited in high-power delivery/uptake as well as in low-temperature operation ͑failure below −20°C͒ and cycle life.1 EDLCs, also known as supercapacitors or ultracapacitors, can be reversibly charged/discharged at higher rates as compared to batteries, thus enabling energy recovery ͑harvesting͒ at much higher rates as well.2,3 Their cycle life ͑Ͼ500,000 cycles͒ as well as lowtemperature operation ͑down to −40°C͒ positively compare to batteries. However, the specific energy of supercapacitors is about 20 times less than that of Li-ion batteries. EDLCs thus have to be used complementary with batteries in many of the applications where high-power delivery/uptake is needed, such as in hybrid electric vehicles, photo and video cameras, cell phones, etc. 3Recent publications suggested that a real breakthrough in improving the specific energy of EDLCs could be obtained using tailored microporous carbons 4 such as carbide-derived carbons ͑CDCs͒.5 CDCs are obtained by the extraction of a metal from its carbide at high temperatures.6-8 Using TiC as the precursor and chlo...

Double‐Layer Capacitors

“…With the reduced distance (d) between the ion and the carbon surface, the capacitance increases according to equation (2.1). Recent results obtained in organic electrolytes confirm this hypothesis of the partial desolvation of the ions when entering small pores (Aurbach et al 2008;Chmiola et al 2008;Lin et al 2009). The study of the electrochemical behaviour of these CDCs in ionic liquids revealed that in these solvent-free electrolytes, the maximum capacitance was achieved when the pore size was in the range of the ion size .…”

“…Three different pore groups are defined by IUPAC (Sing et al 1985): micro, meso and macro pores with diameters less than 2 nm, between 2 and 50 nm, and more than 50 nm, respectively (figure 2b). These pores must be neither too big, to provide sufficient SSA (m 2 g −1 ), nor too small to host the solvated ions from the electrolyte (average size between 1 and 2 nm in organic electrolytes; Lin et al 2009) during the capacitor charge-discharge. The first progress in this area was to formulate carbon materials with a pore structure adapted to the size of the solvated ions in order to optimize charge storage.…”

Easy approach to synthesize N/P/K co-doped porous carbon microfibers from cane molasses as a high performance supercapacitor electrode material

“…The cathodic and anodic limiting potentials of neat TEABF 4 , EMIBF 4 and EMIDCA ILs are found to be À1.3/+1.0 V, À2.0/+2.4 V and À2.0/+1.5 V, respectively. [43][44][45] Thus, the capacitive energy storage in ILs/ACN mixtures should be higher and supercapacitors could be charged and discharged to higher operating cell potential (up to anodic limiting potential of 3.0 V) in comparison to aqueous K 2 SO 4 solution with anodic limiting potential of 1.0 V without sacricing the rate performance. Similar observation was reported by Lei and colleagues.…”

Design of Activated Carbon/Activated Carbon Asymmetric Capacitors

“…The available surface area is related to the wettability of the porosity of the electrode, which comes as a result of the combination of, on the one hand, the polarity and size of the molecules of the solvent and the ions that constitute the electrolyte, and on the other hand, the pore size and surface chemistry of the electrode. The pores of the electrode must show a similar size to that of the ions of the electrolyte to avoid ion sieving effects and to enhance the surface capacitance (Chmiola et al, 2006;Pandolfo and Hollenkamp, 2006;Raymundo-Piñero et al, 2006;Simon and Burke, 2008) and, in addition, a surface chemistry of the electrode that matches that of the solvent will help in delivering a high wettability of the surface of the porosity (Lin et al, 2009). Besides, faradaic reactions involving the surface groups of the electrodes can also contribute to the capacitance by means of the so-called "pseudocapacitance" contribution (Conway et al, 1997).…”