Metal pre-intercalation promotes water-mediated proton-coupled electron transfer in layered δ-MnO₂ for aqueous...

The charge storage capacitance of δ-MnO₂-based pseudocapacitors stems from a combination of bulk cation intercalation/deintercalation and surface proton chemisorption/desorption. Here, we investigate the mechanistic origins of the enhanced capacitance in δ-MnO₂ with pre-intercalated Cu²⁺. To this end, we synthesize Au-core/δ-MnO₂-shell nanostructures with and without Cu²⁺ pre-intercalation, enabling real-time in situ spectroscopic monitoring of structure-function relationships during electrochemical cycling. Transition metal pre-intercalation preserves interlayer-confined water, which in turn supports proton-coupled charge storage via the reversible reaction of MnO₂ + H₂O + e^- ⇌ MnOOH + OH^-. This confined water forms a hydrogen-bonded network that lowers the energy barrier for proton transport within the interlayer space. Similar mechanistic transition is also evident in δ-MnO₂ systems pre-intercalated with other transition metal ions, such as Co²⁺ and Mg²⁺. By tuning the MnO₂ shell thickness, we decouple the relative contributions of proton- and cation-driven processes, revealing that proton intercalation delivers a markedly higher specific capacitance than cation intercalation. Electrolyte-dependent studies further reveal that Cu²⁺ pre-intercalation promotes OH^- transport within the interlayer space while preserving proton accessibility at active sites. These findings suggest that proton-coupled transport may offer further increases in charge storage performance in pseudocapacitors.