Enhancing TiO2+/Ti3+ redox kinetics by nitrogen-doped carbon nanofiber composite electrodes for titanium-cerium flow batteries

Titanium-cerium flow batteries have garnered significant attention in the energy storage field due to the high potential of the Ce⁴⁺/Ce³⁺ redox pair, the higher potential of the TiO²⁺/Ti³⁺ redox pair compared to the hydrogen evolution reaction potential, and the abundant reserves of titanium and cerium. However, traditional carbon felt (CF) electrodes have limited active sites and poor hydrophilicity, which restrain the redox kinetics of the TiO²⁺/Ti³⁺ redox pair. To address this, we built a 3D nitrogen-doped carbon nanofiber composite electrode (N-CNF/CF) based on carbon felt. The nitrogen-doped functional group of N-CNF/CF electrode with high specific surface area significantly improves contact between electrolyte and electrode and electron transfer, thus enhances the TiO²⁺/Ti³⁺ reaction kinetics and exhibits excellent electrochemical stability. Also, DFT calculations uncover that N-doped modulates the electron transfer and enhances the titanium adsorption. After 200 cycles, the average voltage efficiency and energy efficiency remain at 85.9 % and 84.06 %, respectively, representing a significant improvement over the CF electrode; the average discharge capacity also increases by 14 %. Additionally, under conditions of 0.75 M active material, the capacity and efficiency decay over 30 days is negligible. This study demonstrates that the N-CNF/CF composite electrode holds great promise for practical applications in titanium-cerium flow batteries.