Microstructural characterization of porous manganese thin films for electrochemical supercapacitor applications

An in-depth microstructural characterization was performed on manganese oxide materials that have been produced for electrochemical supercapacitor applications using a novel physical vapor deposition process. Manganese was e-beam evaporated and deposits as a combination of the cubic forms of Mn and MnO with a porous zigzag structure. The electrochemically oxidized sample that is used as the supercapacitor base material is tetragonal Mn₃O₄. An apparent active layer with increased sodium levels was imaged by STEM, lending some credence to the argument that the pseudocapacitance effect is based entirely on a surface layer of adsorbed sodium. Upon furnace annealing the zigzag structure near the free surface is destroyed and replaced with a columnar oxide layer of cubic MnO and tetragonal Mn₃O₄. This capping effect ultimately reduces the usable surface area and is thought to account for the reduction in capacitance seen on annealing.