Redox Chemistry of Molybdenum Trioxide for Ultrafast Hydrogen‐Ion Storage

Hydrogen ions are ideal charge carriers for rechargeable batteries due to their small ionic radius and wide availability. However, little attention has been paid to hydrogen-ion storage devices because they generally deliver relatively low Coulombic efficiency as a result of the hydrogen evolution reaction that occurs in an aqueous electrolyte. Herein, we successfully demonstrate that hydrogen ions can be electrochemically stored in an inorganic molybdenum trioxide (MoO3 ) electrode with high Coulombic efficiency and stability. The as-obtained electrode exhibits ultrafast hydrogen-ion storage properties with a specific capacity of 88 mA hg-1 at an ultrahigh rate of 100 C. The redox reaction mechanism of the MoO3 electrode in the hydrogen-ion cell was investigated in detail. The results reveal a conversion reaction of the MoO3 electrode into H0.88 MoO3 during the first hydrogen-ion insertion process and reversible intercalation/deintercalation of hydrogen ions between H0.88 MoO3 and H0.12 MoO3 during the following cycles. This study reveals new opportunities for the development of high-power energy storage devices with lightweight elements.