Stable colloid-in-acid electrolytes for long life proton batteries

The emerging proton electrochemistry offers opportunities for future energy storage of high capacity and rate. However, the development of proton batteries is hindered by low working-potentials of electrodes and poor cycle life of full-cells (e.g., tens-of-hours). The high-potential MnO2/Mn2+ redox couple presents a facile and competitive cathode choice, typically via electrodepositing solids on substrates for energy storage. Herein, we show the formation of homogeneous and stable MnO2 colloids from the Mn2+ electrolysis in H2SO4 (≥ 1.0 M), and their application to achieve long life proton batteries. Colloid electrolytes enable prolonged cycling of a MnO2//MoO3 cell from 11.7 h to 33 days, and a MnO2//pyrene-4,5,9,10-tetraone cell for 489days, which is the longest duration ever reported for proton batteries. Comprehensive analysis shows the colloid particle is mainly a MnO2 nucleus coordinated by electrolyte ions of hydrated Mn2+, Mn3+, H+ and SO42-. Through water dilution, solid components of colloids precipitate into hierarchical nanosheet spheres; Further characterizations of the precipitates and deposited substrates reveal ε-MnO2 as the major electrolytic product regardless of electrolytes used. Different colloids could reform from precipitates depending on presence/absence of Mn2+ in acids, suggesting colloid balances include both physical and chemical interactions. Our findings of the new chemistry for the MnO2/Mn2+ electrolysis are also anticipated to underlie a range of novel aqueous energy storage.