High supercapacitive performance of sol-gel ZnO-doped manganese oxide coatings

In the present study, ZnO-doped manganese (Mn) oxide coatings were prepared as a function of ZnO content (≤ 40 at.%) by sol-gel process. After post heat treatment at 300 °C, the influences of ZnO on the microstructural characteristics and pseudocapacitive performance of the Mn-oxide films were investigated. The structural analyses identified the sol-gel ZnO-doped Mn-oxide powder as a tetragonal Mn₃O₄ phase with a nanocrystalline structure. The formation of Zn_xMn_{3 - x}O₄ spinel occurred when ZnO content was ≥ 20 at.%. The crystallite size of Mn-oxide powder increased when a small amount of ZnO was added, and then decreased when larger amounts were added. The cyclic voltammetry (CV) data showed that the specific capacitance (SC) of the Mn-oxide film in 1 M Na₂SO ₄ electrolyte can be increased from 236 F/g to 301 F/g at 25 mV/s by doping with 10 at.% ZnO. The formation of spinel tended to inhibit the SC value of the films. After activation of the Mn-oxide film, however, a relatively high cycling efficiency of > 85% was obtained for all compositions after 1200 CV cycles.