Impact of the Channel Length in Nanoporous Electric Double-Layer Capacitors on the Charge Transport Explored by Metal-Organic Framework Films

For enhancing the performance of electric double-layer capacitors, the porous electrodes must be further optimized. While many studies on electrolyte and electrode structures enable detailed insights, the length of the pore channels of the electrode has been overlooked. Here, we use films of two-dimensional conductive metal-organic frameworks, where the film thickness (and thus the pore channel length) is rationally tuned over a wide range. Cyclic voltammetry experiments with two different electrolytes were conducted, revealing the charge transport kinetics in the porous electrodes. For the highly mobile electrolyte, the kinetics is not limited by ion transport (i.e., diffusion) even for thick films, exhibiting mainly surface-controlled kinetic behavior. In contrast, for the less mobile electrolyte, the kinetics is primarily limited by ion diffusion, and the pore channel length has a severe impact, where long channels result in strongly decreased capacitances, highlighting the importance of adjusting the channel length.