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Open Access Publications from the University of California

Classical Density Functional Theory for Ionic Liquids in Amorphous Nanoporous Electrodes

  • Author(s): Neal, Justin Nathaniel
  • Advisor(s): Wu, Jianzhong
  • et al.

The electric double-layer capacitor (EDLC), also known as the supercapacitor, offers superior power density, however, improved energy density is needed to expand its current scope of application. Based on a coarse-grained model using classical density functional theory, we study the structure, ion partitioning, and selectivity of symmetric and asymmetric ionic liquid mixtures for both zero and non-zero applied potential. At zero applied potential, narrow pores favor adsorption of small anions due to the distribution of asymmetric ions which induce a net surface charge. This further enriches the smaller anions, producing a self-amplified selectivity. We find a similar selectivity for non-zero applied potential, that the small pore is selective to the small anion. For both zero and non-zero potentials, the selectivity decreases as the pore size increases. We developed an idealized model to study the capacitance with respect to changes in electrolyte composition, electrode potential, and pore size. The energy density depends on the ion distribution within the pores of an electrode. The ion layer near the surface of the electrode is always selective to the small counterion, which increases the capacitance appreciably as the thickness of the superficial layer decreases, versus that of the larger counterion from the symmetric ionic liquid. For mesoporous carbons containing an asymmetric ionic liquid, the capacitance is on par with the anomalous capacitance seen for a symmetric ionic liquid in ultranarrow pores. We move on to predict the capacitance of electrode materials based on a linear combination of the capacitance from ideal slit-pores with the weights of the probability distribution given by the experimentally measured pore size distribution. We find good agreement with measured experimental data, though, with experimental guidance, refinement of the weighting function could account for ion enrichment in the pores and the overestimate of BET surface area for electrode materials. We propose a four-component ionic liquid mixture would increase the device capacitance of the EDLC by 30% over symmetric ionic liquids in the same material. We offer this work as guidance in future design and development of electrodes and electrolytes for the next generation of EDLCs.

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