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.