Revising the Concept of Pore Hierarchy for Ionic Transport in Carbon Materials for Supercapacitors

• Published in: Advanced energy materials 2018-08, Vol.8 (24), p.n/a
• Main Authors: Borchardt, Lars, Leistenschneider, Desirée, Haase, Jürgen, Dvoyashkin, Muslim
• Format: Article
• Language: English
• Subjects: Carbon; Catalysis; Diffusion; Electrodes; Electrolytes; hierarchy; Pore size; Porosity; porous carbon; pulsed field gradient NMR; Solvents; Sorbates; supercapacitor; Supercapacitors; Transport
• ISSN: 1614-6832; 1614-6840
• DOI: 10.1002/aenm.201800892

Rapid motion of electrolyte ions is a crucial requirement to ensure the fast charging/discharging and the high power densities of supercapacitor devices. This motion is primarily determined by the pore size and connectivity of the used porous carbon electrodes. Here, the diffusion characteristics of each individual electrolyte component, that is, anion, cation, and solvent confined to model carbons with uniform and well‐defined pore sizes are quantified. As a result, the contributions of micropores, mesopores, and hierarchical pore architectures to the overall transport of adsorbed mobile species are rationalized. Unexpectedly, it is observed that the presence of a network of mesopores, in addition to smaller micropores—the concept widely used in heterogeneous catalysis to promote diffusion of sorbates—does not necessarily enhance ionic transport in carbon materials. The observed phenomenon is explained by the stripping off the surrounding solvent shell from the electrolyte ions entering the micropores of the hierarchical material, and the resulting enrichment of solvent molecules preferably in the mesopores. It is believed that the presented findings serve to provide fundamental understanding of the mechanisms of electrolyte diffusion in carbon materials and depict a quantitative platform for the future designing of supercapacitor electrodes on a rational basis. Diffusion of ions and solvent molecules of an electrolyte solution confined to micro‐, meso‐, and hierarchical model carbons is traced by high‐gradient nuclear magnetic resonance. Unexpectedly, presence of mesopores in addition to smaller micropores—the concept of hierarchy, widely used in heterogeneous catalysis to promote diffusion—can slow down ionic transport. The mechanisms leading to this counterintuitive phenomenon are discussed.