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Metal pre-intercalation promotes water-mediated proton-coupled electron transfer in layered δ-MnO2 for aqueous pseudocapacitive energy storage
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DOI：10.1038/s41467-026-76022-4.png)
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Abstract
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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. Electrochemical charge storage in MnO2 is limited by slow ion transport and structural instability. Here, the authors show that metal pre-intercalation enables water-mediated proton-coupled charge storage, enhancing capacitance and reversibility in layered MnO2.
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