Fast Energy Storage in Two-Dimensional MoO₂ Enabled by Uniform Oriented Tunnels

While pseudocapacitive electrodes have potential to store more energy than electrical double-layer capacitive electrodes, their rate capability is often limited by the sluggish kinetics of the Faradaic reactions or poor electronic and ionic conductivity. Unlike most transition-metal oxides, MoO₂ is a very promising material for fast energy storage, attributed to its unusually high electronic and ionic conductivity; the one-dimensional tunnel is ideally suited for fast ionic transport. Here we report our findings in preparation and characterization of ultrathin MoO₂ sheets with oriented tunnels as a pseudocapacitive electrode for fast charge storage/release. A composite electrode consisting of MoO₂ and 5 wt % GO demonstrates a capacity of 1097 C g^-1 at 2 mV s^-1 and 390 C g^-1 at 1000 mV s^-1 while maintaining ∼80% of the initial capacity after 10,000 cycles at 50 mV s^-1, due to minimal change in structural features of the MoO₂ during charge/discharge, except a small volume change (∼14%), as revealed from operando Raman spectroscopy, X-ray analyses, and density functional theory calculations. Further, the volume change during cycling is highly reversible, implying high structural stability and long cycling life.