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, we report a new host of hexagonal molybdates, (A 2 O) x ·MoO 3 ·(H 2 O) y (A = Na + , NH 4 + ), and demonstrate hydronium (de)intercalation 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 30000 cycles are achieved in electrodes of monocrystals even over 200 μm. Solid-state NMR confirms hydronium intercalations, and operando measurements using electrochemical quartz crystal microbalance and synchrotron XRD 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. This article is protected by copyright. All rights reserved.