Carbon dioxide in the cage: Manganese metal-organic frameworks for high performance CO₂ electrodes in Li-CO₂ batteries

Improving the reversibility and energy efficiency of Li-CO₂ electrochemistry would help in developing practical Li-air batteries capable of providing stable power supply in the presence of CO₂. However, it is hard for most existing electrodes to convert CO₂ effectively (high discharge capacity) and efficiently (low charge potential). Herein, we have, for the first time, identified the potential of metal-organic frameworks (MOFs) as porous catalysts in CO₂ electrodes, taking advantage of their high capability for CO₂ capture and monodispersed active metal sites for Li₂CO₃ decomposition. In particular, eight porous MOFs (Mn₂(dobdc), Co₂(dobdc), Ni₂(dobdc), Mn(bdc), Fe(bdc), Cu(bdc), Mn(C₂H₂N₃)₂, and Mn(HCOO)₂) and two nonporous materials (MnCO₃ and MnO) were investigated. Among them, Mn₂(dobdc) achieves a remarkable discharge capacity of 18022 mA h g^-1 at 50 mA g^-1, while Mn(HCOO)₂ retains a low charge potential of ∼4.0 V even at 200 mA g^-1 for over 50 cycles. The Li-CO₂ electrochemistry on MOF electrodes was investigated with an arsenal of characterization techniques, including X-ray diffraction, scanning electron microscopy, electrochemical impedance spectroscopy, Raman spectroscopy and in situ differential electrochemical mass spectrometry. The findings provide useful design principles for improving the reversibility and energy efficiency of Li-CO₂ electrochemistry and will herald the advent of practical technologies that enable energy-efficient utilization of CO₂ for electrochemical energy storage and supply.