Electrochemical and scanning transmission X-ray microscopy studies of MnO2 and Mn3O4 for supercapacitor cathodes: Influence of fabrication method and electrochemical activation on charge storage

This investigation is motivated by increasing interest in Mn3O4 as a promising alternative to MnO2 for supercapacitors with high active mass and the need for better understanding of charging and electrode activation mechanisms. High energy ball milling (HEBM) of chemically precipitated MnO2 and Mn3O4 in the presence of quercetin resulted in significant capacitance increase. The use of Mn2+ salts for Mn3O4 synthesis facilitated the application of quercetin as a new chelating capping agent for synthesis of Mn3O4, which showed higher capacitance, compared to HEBM Mn3O4. The capacitance of Mn3O4 prepared using quercetin was 6.0F cm−2 (149.50 F g−1) for cyclic voltammetry at 2 mVs−1 and 8.03 F cm−2 (200.93 F g−1) for chronopotentiometry at 3 mA cm−2, which is on-par with the capacitance of MnO2. The time-consuming activation procedure, which limits Mn3O4 applications, was significantly accelerated for HEBM Mn3O4 and practically eliminated for Mn3O4 prepared using quercetin as a capping agent. Soft X-ray scanning transmission X-ray microscopy (STXM), an advanced synchrotron based analytical microscopy, was used at the O 1s and Mn 2p edges to identify and quantitatively map the Mn oxidation states present in Mn3O4 materials. Variable and fixed sweep rate electrochemical cycling procedures were used for the analysis of oxidation state of Mn and charging mechanism using STXM analysis coupled with electrochemical testing. The combination of STXM and electrochemistry provided valuable insights into the activation kinetics and charging mechanism. The STXM results showed that the tested materials contained mixtures of Mn2+, Mn3+ and Mn4+ oxides. A small amount of MnO phase in the tested samples indicated partial reduction. The higher content of MnO2 phase in the tested Mn3O4 prepared using quercetin as a capping agent, compared to HEBM Mn3O4, correlated with higher capacitance and the ability to eliminate the activation procedure.