Sodium manganese hexacyanoferrate as ultra-high rate host for aqueous proton storage

Abstract Rechargeable aqueous proton batteries (APBs) are a fairly promising electrochemical storage system that deserves to be developed vigorously due to their unique Grotthuss proton conduction, potential performance, and abundant resources. Therefore, it is requisite to find suitable cathode materials for proton storage. Herein, sodium manganese hexacyanoferrate (Na2Mn[Fe(CN)6]•2H2O) cathode was applied in rechargeable APBs, which demonstrates desirable electrochemical properties and structural stability. At room temperature, NaMnHCF delivers a high reversible specific capacity of 96.3 mAh g − 1 at the current rate of 10 C with negligible redox polarization, and Coulomb efficiency can reach 97.1%. More prominently, minor capacity attenuation appears during the current rate increasing from 10 to 120 C, and it still maintains ultra-long cycle life at these rates, indicating the outstanding ultra-high rate performance. For example, capacity retention can attain 43.3% and 40.0% after 20,000 cycles at the current rate of 60 and 85 C, respectively. Besides, in-situ pH test verifies the successful protons insertion/extraction process in NaMnHCF, and ex-situ XPS, FTIR, and XRD tests reveal the reversible insertion/extraction process of proton and the stable structure of NaMnHCF. These satisfied electrochemical properties are ascribed for the Grotthuss mechanism of proton inside NaMnHCF structure, which could provide a meaningful reference for the cathode exploration of rechargeable APBs.