Hierarchically Porous Carbon Colloidal Aerogels for Highly Efficient Flow Cells

Electrodes with high active areas often compromise with limited ion transport kinetics in flow electrochemical devices. Herein, hierarchically porous carbon colloidal aerogels (HPCCAs) are constructed with multiscale porosities to meet the tradeoff between highly active areas and efficient mass transfer behavior. It is realized by introducing multiphase co-separation in a sol-gel transition process of aramid nanofibers/polyvinylpyrrolidone/carbon nanotubes followed by subsequent freeze-drying and carbonization. The resulting HPCCA possesses a high volumetric electrochemically accessible surface area (3.27 × 10⁷ m⁻¹) and excellent mass transfer efficiency, 2–3 times higher permeability than commercial Toray carbon paper and 9.86 times higher than bare aerogel. An all-vanadium single cell with HPCCAs as electrodes possesses a high energy efficiency of 83.18% under the current density of 100 mA cm⁻², which is 10–31% higher than most of the state-of-the-art carbon electrode materials including commercial carbon papers. In addition, the cell with HPCCAs shows outstanding long-term stability up to 1000 cycles. Notably, HPCCAs are applicable to more flow battery systems, such as iron/chromium (Fe/Cr), iron/vanadium (Fe/V), zinc/bromine (Zn/Br), vanadium/methylene blue (V/MB), sodium salt of flavin mononucleotide/potassium ferrocyanide (FMN-Na/K₄[Fe(CN)₆]), and methyl viologen/4-hydroxy-2,2,6,6-tetramethyl-piperidin-1-oxyl (MV/4-HO-TEMPO). This work offers a new chemistry paradigm for developing advanced nanoporous aerogel materials and paves the way toward highly efficient flow electrochemical devices.