Pseudocapacitive Hausmannite Nanoparticles with (101) Facets: Synthesis, Characterization, and Charge‐Transfer Mechanism

Hausmannite Mn 3 O 4 octahedral nanoparticles of 18.3±7.0 nm with (101) facets have been prepared by an oxygen‐mediated growth. The electrochemical properties of the Mn 3 O 4 particles as pseudocapacitive cathode materials were characterized both in half‐cells and in button‐cells. The Mn 3 O 4 nanoparticles exhibited a high mass‐specific capacitance of 261 F g −1 , which was calculated from cyclic voltammetry analyses, and a capacitive retention of 78 % after 10 000 galvanostatic charge–discharge cycles. The charge‐transfer mechanisms of the Mn 3 O 4 nanoparticles were further studied by using synchrotron‐based in situ X‐ray absorption near edge spectroscopy and XRD. Both measurements showed concurrently that throughout the potential window of 0–1.2 V (vs. Ag/AgCl), a stable spinel structure of Mn 3 O 4 remained, and a reversible electrochemical conversion between tetrahedral [Mn II O 4 ] and octahedral [Mn III O 6 ] units accounted for the redox activity. Density functional theory calculations further corroborated this mechanism by confirming the enhanced redox stability afforded by the abundant and exposed (101) facets of Mn 3 O 4 octahedra.