Hydronium Intercalation Enables High Rate in Hexagonal Molybdate Single Crystals.

Rapid proton transport in solid-hosts promotes a new chemistry in achieving high-rate Faradaic electrodes. Exploring the possibility of hydronium intercalation is essential for advancing proton-based charge storage. Nevertheless, this is yet to be revealed. Herein, a new host is reported of hexagonal molybdates, (A2O)x center dot MoO3 center dot(H2O)y (A = Na+, NH4+), and hydronium (de)intercalation is demonstrated with experiments. Hexagonal molybdates show a battery-type initial reduction followed by intercalation pseudocapacitance. Fast rate of 200 C (40 A g-1) and long lifespan of 30 000 cycles are achieved in electrodes of monocrystals even over 200 mu m. Solid-state nuclear magnetic resonance confirms hydronium intercalations, and operando measurements using electrochemical quartz crystal microbalance and synchrotron X-ray diffraction disclose distinct intercalation behaviours in different electrolyte concentrations. Remarkably, characterizations of the cycled electrodes show nearly identical structures and suggest equilibrium products are minimally influenced by the extent of proton solvation. These results offer new insights into proton electrochemistry and will advance correlated high-power batteries and beyond. A new mechanism of hydronium (H+(H2O)n) intercalation is demonstrated in a new host of hexagonal molybdates by experiments. Thanks to hydronium intercalation, fast-rate and long lifespan are achieved in electrodes of monocrystals with sizes even over 200 mu m.image