Cobalt-Bridged Polyoxoniobate Framework for High Reversibility and Capacity Retention Polysulfide Redox Flow Batteries.

Abstract

In this work, we report for the first time a cobalt-bridged polyoxoniobate framework (Co-PONbs) featuring a 3D coordination architecture, constructed by linking {PNb₁₂O₄₀} clusters through the coordinatively versatile Co²⁺ ions. This unique inorganic framework exhibits exceptional stability in strongly alkaline electrolytes, helping to address the challenge of catalyst degradation under operating conditions. Acting as an efficient electrocatalyst, Co-PONbs significantly enhances the redox conversion kinetics of polysulfides in redox flow batteries. Electrochemical in situ Raman spectroscopy confirms the preferential adsorption of polysulfides on Co-PONbs/SP-modified electrodes. First-principles calculations demonstrate that embedding cobalt centers in {PNb₁₂O₄₀} clusters generates asymmetric dual-active sites. This unique configuration drives S─S bond cleavage in S₄ ^2- via a cooperative electron-transfer mechanism, kinetically promoting conversion to S₂ ^2- intermediates. When implemented in aqueous polysulfide-iodine redox flow batteries, the Co-PONbs/SP composite-modified carbon felt cathode sustains remarkable energy efficiency (82.7%) and Coulombic efficiency (99.5%) over 530 h of continuous operation. The system demonstrates unprecedented cycling stability with 99.96% capacity retention after 1500 cycles at 60 mA cm^-2.