Development of Design Model of Electric Double-Layer Capacitors as All-Purpose Energy Recovery Devices for Reducing COィイD22ィエD2 Emission

1999 Fiscal Year Final Research Report Summary

• Project/Area Number: 10555266
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 展開研究
• Research Field: 化学工学一般
• Research Institution: KYOTO UNIVERSITY
• Principal Investigator: MIYAHARA Minoru Kyoto University, Graduate School of Engineering, Associate Professor, 工学研究科, 助教授 (60200200)
• Co-Investigator(Kenkyū-buntansha): HIGASHITANI Ko Kyoto University, Graduate School of Engineering, Professor, 工学研究科, 教授 (10039133)
• Project Period (FY): 1998 – 1999
• Keywords: Activated Carbon / Electric Double Layer Capacitor / Charge / discharge / Energy Recovery / Nernst-Planck Equation / Nanopores / Electric Capacity

Research Abstract

Electric double-layer capacitor (EDLC) is expected to stand as an all-purpose energy recovery device for reducing carbon dioxide emissions. For reasonable and reliable use of the EDLC with activated carbon electrodes, a model for describing both equilibrium and kinetic characteristics of the EDLC must be established. This research project pursued charging/discharging behavior of EDLC and obtained the following results.
1.Characterization : Employing nitrogen adsorption, we determined pore-sizes, micropore volumes, micropore surface area of high-surface-area activated carbons with different degree of activation. Pore sizes ranged form 0.6-1.0 nm, which compare with the sizes of hydrated ions in aqueous solution.
2.Equilibrium : Static capacities -- not the apparent ones by constant current charge measurement -- of EDLC's composed of activated carbons with various pore characteristics and with various surface characteristics were measured successfully by monitoring electric current after p otential disturbance. The carbons employed were found to have significantly larger capacities than those of activated carbon fibers with similar surface areas. This characteristics are thought to come from the random nature of the interior surface of the alkali-activated carbons used.
3.Kinetics : Transient processes of charging/discharging EDLC was successfully measured with on-line based experimental system constructed in this study. We succeeded in modeling the charging/discharging behavior. The model employed the Nernst-Planck equation for describing anions and cations, and included electric resistance of the carbon electrodes. The time constants obtained through the analysis showed that a small difference in pore diameter near the size of hydrated ions drastically changes the mobility of the ions probably because of steric disturbance in the movement of ions. Further, it was made possible to predict the effect of various resistances onto the total performance of the charge/discharge ability of capacitors.
Through all the above study, a model for predicting performance of EDLC based both on equilibrium and kinetic characteristics has been successfully established, and showed usefulness as a design tool for activated carbon EDLC.