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

Abstract

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 {PNb 12 O 40 } clusters through the coordinatively versatile Co 2+ 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 {PNb 12 O 40 } clusters generates asymmetric dual‐active sites. This unique configuration drives S─S bond cleavage in S 4 2− via a cooperative electron‐transfer mechanism, kinetically promoting conversion to S 2 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 .