Effects of interlayer confinement and hydration on capacitive charge storage in birnessite

Nanostructured birnessite exhibits high specific capacitance and, while an important electrode material for high-power energy storage devices, its capacitive mechanism remains unclear. Capacitive charge storage in birnessite is now shown to be governed by interlayer cation intercalation. Nanostructured birnessite exhibits high specific capacitance and nearly ideal capacitive behaviour in aqueous electrolytes, rendering it an important electrode material for low-cost, high-power energy storage devices. The mechanism of electrochemical capacitance in birnessite has been described as both Faradaic (involving redox) and non-Faradaic (involving only electrostatic interactions). To clarify the capacitive mechanism, we characterized birnessite's response to applied potential using ex situ X-ray diffraction, electrochemical quartz crystal microbalance, in situ Raman spectroscopy and operando atomic force microscope dilatometry to provide a holistic understanding of its structural, gravimetric and mechanical responses. These observations are supported by atomic-scale simulations using density functional theory for the cation-intercalated structure of birnessite, ReaxFF reactive force field-based molecular dynamics and ReaxFF-based grand canonical Monte Carlo simulations on the dynamics at the birnessite-water-electrolyte interface. We show that capacitive charge storage in birnessite is governed by interlayer cation intercalation. We conclude that the intercalation appears capacitive due to the presence of nanoconfined interlayer structural water, which mediates the interaction between the intercalated cation and the birnessite host and leads to minimal structural changes.

Automated summary

Nanostructured birnessite demonstrates high specific capacitance and ideal capacitive behavior in aqueous electrolytes, making it a promising electrode material for low-cost, high-power energy storage devices. Capacitive charge storage in birnessite is governed by interlayer cation intercalation, facilitated by the presence of nanoconfined interlayer structural water mediating interactions between the intercalated cation and the birnessite host, resulting in minimal structural changes.