Designing Proton-Selective Pathway on MnO₂ Surface Toward Sustainable Protonation Electrochemistry

Manganese dioxide (MnO₂), known for its low-cost, high-theoretical capacity, and environmental friendliness, has garnered great attention in developing mild aqueous Zn-MnO₂ batteries (AZMBs). However, Mn²⁺ dissolution severely compromises cycling stability and practical viability; moreover, the ambiguity of charge storage mechanisms (e.g., Zn²⁺ vs. H⁺) makes the direction of strategic engineering uncertain. Herein, via advanced electron microscopy, we show that the charge storage in MnO₂ is dominated by H⁺ intercalation rather than Zn²⁺ insertion. Inspired by this finding and by referring to the recipe of proton exchange membrane, we uniformly coat individual MnO₂ particles with a proton-selective surface, that is, Nafion, which successfully suppresses Mn²⁺ dissolution as an "ion filter" and simultaneously facilitates reversible H⁺ insertion/extraction as a "proton channel". Therefore, the MnO₂@Nafion cathode exhibits an outstanding specific capacity (277 mAh g^-1 after 100 cycles at 0.2 A g^-1) and remarkable cycling stability, retaining 91.7% of its capacity after 3000 cycles at 2 A g^-1. These results outperform previously reported manganese-based cathodes, demonstrating the potential of MnO₂@Nafion as a high-performance and durable cathode material for AZMBs. This work rationalizes the rising endeavors in the mechanism understanding of MnO₂-based aqueous battery systems and provides new insights for developing more sustainable aqueous battery materials.