Ion Dynamics and Electrosorption in Carbon Electrodes with Bimodal Porosities and Heterogeneous Interfaces

Abstract

Electrochemical capacitors offer high reliabilities and power densities and demonstrate exceptional promise in a broad range of energy storage and management applications. However, research efforts have not, to date, properly explored the influence of electrode interfaces’ heterogeneity and disorder on charge storage and dynamics of electrosorbed ions. Surface functional groups and structural carbon defects contribute quantum capacitance.[1] Specific surface functional groups, which must appropriately match the chemistries of electrosorbed ions, develop favorable interfaces that reduce impedance and improve cyclabilities.[2] Finally, ionophilic surfaces draw ions away from pore surfaces and improve electrosorption during charge and discharge processes.[3] However, these effects strongly depend on electrode pore dimensions and behaviors of ions in confinement. Furthermore, standard materials characterization and electrochemical testing techniques cannot properly assess interactions at electrode/electrolyte interfaces. Subsequently, in-depth studies must rely on combinations of well-tailored experimental model systems, in-depth X-ray and neutron scattering techniques, and computational modeling to assess the influences of disordered interfaces on capacitance. Our research investigated the effect of electrode surface heterogeneity and disorder in complex pore architectures and electrolytes with large, non-idealized electrolytes. The approach used carbide-derived carbons (CDCs) synthesized from a Mo2C precursor at 800 °C. This electrode material offered bimodal pore size distributions (0.8 nm and 2.6 nm diameter pores, shown in Figure 1a), and 1300 °C vacuum annealing or 400 °C air oxidation yielded, respectively, defunctionalized or oxidized pore surfaces. We filled these pores with 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([OMIm+][TFSI-]) room-temperature ionic liquid (RTIL). QENS measurements on these systems determined that the ions were significantly more mobile in oxidized pores than in defunctionalized pores (Figure 1b). A Cole-Cole model derived mobility information, and the RTIL diffusion coefficient in oxidized pores (2.71*10-10 m2 s-1) was more than twice as large as the one for the same RTIL in defunctionalized pores (1.05*10-10 m2 s-1) due to lower ion densities in oxidized pores. Electrochemical impedance analysis showed that electrochemically driven ionic mobility was higher in oxidized pores. On the other hand, defunctionalized pores demonstrated contained higher ion densities, and, subsequently, featured higher charge storage densities (Figure 3f). However, cyclic voltammograms suggested charge saturation and pore de-filling behaviors at low potentials; surface chemistry influenced the magnitude of this effect. Molecular dynamic (MD) simulations showed similar strong affinities of oxidized surfaces for RTIL ions, which lowered their charge storage densities in 2.6 nm diameter pores. X-ray Pair Distribution Function (PDF) analysis showed differences in ion-ion correlations between bulk and confined states and further underscored the influence of pore and ion diameter on electrode/electrolyte interactions at heterogeneous interfaces. [1] Dyatkin, B. & Gogotsi, Y. Effects of Structural Disorder and Surface Chemistry on Electric Conductivity and Capacitance of Porous Carbon Electrodes. Faraday Discussions172, 139-162, (2014). [2] Dyatkin, B., et al. Capacitance, Charge Dynamics, and Electrolyte-Surface Interactions in Functionalized Carbide-Derived Carbon Electrodes. Progress in Natural Science - Materials International25, 631–641, (2015). [3] Dyatkin, B., et al. Influence of Surface Oxidation on Ion Dynamics and Capacitance in Porous and Non-Porous Carbon Electrodes. The Journal of Physical Chemistry C, (2016). Figure 1