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

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.