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

Nanostructured birnessite (δ-MnO2) exhibits high specific capacitance and nearly ideal capacitive behavior 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 microscopy dilatometry to provide a holistic understanding of its structural, gravimetric, and mechanical response. These observations are supported by atomic-scale simulations using density functional theory for the cation-intercalated structure of birnessite and ReaxFF-based molecular dynamics, as well as 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.