Effect of Salt Depletion on Charging Dynamics in Nanoporous Electrodes

Electrochemical double-layer capacitors built from nanoporous electrodes can have such a high ratio of electrode surface area to pore volume that charging the capacitor can deplete the salt from the liquid volume. This can result in increased resistance, resulting in a slow, nonlinear charging rate of which quantitative understanding is limited. In some cases, this effect is masked by an external solution resistance or by the transport of salt into the pore from an external reservoir. However, in forms relevant to a compact energy storage device, the phenomenon can have an important effect on charging time and linearity, and understanding it is important for such design. We have observed salt depletion effects by using dealloyed gold, which has well-defined 10 nm pores and a chemically well-understood surface, and by minimizing the amount of external salt within range of diffusion. Good correspondence is observed with a modified de Levie model that accounts for reduced local conductivity due to salt depletion. The model's assumption that the Stern layer (ions closely bound to the pore wall) makes a low contribution to conductance in the pore is validated by experimental data. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3416905] All rights reserved.