From Keggin to oxide: Tailoring a new class of Fe-based molybdophosphate catalysts for reversible Li‒CO2 batteries

The development of efficient earth-abundant catalysts through scalable synthesis routes remains a critical challenge for advancing rechargeable lithium-CO 2 (Li‒CO 2 ) batteries. Herein, a facile synthesis strategy was developed by directly introducing Fe atoms into phosphomolybdic acid (PMA), followed by calcination at temperatures between 300 and 600 °C to optimize catalyst structure and performance. This approach produced two distinct catalyst families: Fe phosphomolybdate (FePMA), which preserves the Keggin ion structure, and FeMoO, a molybdenum trioxide-iron phosphate (MoO 3 /FePO x ) composite formed at higher temperatures. Benefiting from abundant redox-active centers, the FeMoO-500 catalyst exhibited a high discharge capacity of 26,110 mAh g −1 , with a coulombic efficiency of 93% and high stability of 92 cycles at a current density of 50 mA g −1 . It also effectively facilitated the CO 2 reduction/evolution reactions, displaying a very low overpotential of 0.8 V. Meanwhile, FePMA-300 also demonstrated good catalytic activity, achieving a high discharge capacity of 20,300 mAh g −1 and an overpotential of 0.9 V, thanks to its intact Keggin structure that hosts a rich network of Fe and Mo redox sites. This study introduces FePMA and FeMoO as cathode catalysts in Li‒CO 2 batteries for the first time and sheds light on their potential applications in other CO 2 conversion technologies.