Colloidal Processing of Mn3O4-Carbon Nanotube Nanocomposite Electrodes for Supercapacitors

Abstract

This investigation addresses the challenges in the development of efficient nanostructured Mn3O4 cathodes for supercapacitors. A high areal capacitance and the ability to avoid a time-consuming activation procedure for electrodes with high active mass loading of 40 mg cm−2 are reported. This facilitates practical applications of Mn3O4 based electrodes. The highest capacitance of 6.11 F cm−2 (153 F g−1) is obtained from cyclic voltammetry at a scan rate of 2 mV s−1 and 6.07 F cm−2 (151.9 F g−1) from the chronopotentiometry at a current density of 3 mA cm−2 in a potential window of 0.9 V in a neutral Na2SO4 electrolyte. The new approach is based on the application of rhamnolipids (RL) as a capping agent for the synthesis of Mn3O4 particles and a co-dispersant for Mn3O4 and carbon nanotubes, which are used as conductive additives. The size and shape of the Mn3O4 particles are influenced by RL. The enhanced performance of the electrodes is linked to the chemical structure and properties of RL molecules, which exert influence on Mn3O4 particle size and shape during synthesis, reduce agglomeration, facilitate RL adsorption on Mn3O4 and carbon nanotubes, and influence their co-dispersion and mixing at the nanometric scale.

Scanning transmission X-ray microscopy studies of electrochemical activation and capacitive behavior of Mn3O4 supercapacitor electrodes

“…17,19,22 Higher capacitances can be achieved using special techniques, such as electrostatic heterocoagulation, 21 liquid–liquid extraction 16 and synthesis in the presence of a capping agent. 20 However, to avoid the influence of different factors on Mn 3 O 4 particles and with the goal of developing the STXM methodology, a traditional approach was used for the synthesis of Mn 3 O 4 and electrode fabrication. This was important for monitoring the chemical modifications of Mn 3 O 4 during activation and obtaining better understanding of the charging mechanism.…”

“…Quercetin (QC), Mn(NO 3 ) 2 $4H 2 O, isopropanol, NaOH, Na 2 SO 4 , poly(vinyl butyral) binder (PVB, MilliporeSigma, Canada), multiwalled carbon nanotubes (MWCNT, ID 4 nm, OD 13 nm, length 1-2 mm, Bayer, Germany) and industrial Ni foam current collectors (95% porosity, 1.6 mm thickness, Vale, Canada) were used. A chemical precipitation method 16,20 was used for the synthesis of Mn 3 O 4 nanoparticles from 0.07 M Mn(NO 3 ) 2 solution in DI water. The pH of the solution was adjusted to pH ¼ 10 by aqueous 0.01 M NaOH for the synthesis of Mn 3 O 4 nanoparticles.…”

“…2,14,15 A signicant increase in capacitance was observed for electrodes with high active mass loading. [16][17][18][19][20] The activation process was linked to in situ modi-cation of the electrode microstructure. 17,19,21,22 Previous investigations showed that capacitance variations during activation process can be diminished using mixed rhamnolipids as a capping agent for the synthesis of Mn 3 O 4 particles and modication of particle morphology.…”

“…17,19,21,22 Previous investigations showed that capacitance variations during activation process can be diminished using mixed rhamnolipids as a capping agent for the synthesis of Mn 3 O 4 particles and modication of particle morphology. 20 However, the origin of the capacitance variation during the activation process and charging mechanism are not well understood. Further analysis of activation behavior is critically important for better understanding of charging mechanism and applications of Mn 3 O 4 electrodes in asymmetric supercapacitors.…”

Influence of Capping Agents on the Synthesis of Mn3O4 Nanostructures for Supercapacitors

“…The subsequent charging behavior involved the following reaction: Na δ MnO italicx · normalH 2 normalO ↔ MnO italicx · normalH 2 normalO + δ Na + + δ normale − Other investigations demonstrated Mn 3 O 4 activation at the beginning of cycling, which resulted in a significant capacitance increase. ,, A significant activation effect was reported for high-AM electrodes, − and it was related to the changes of the material morphology and oxidation state of Mn. ,,, Numerous X-ray photoelectron spectroscopy (XPS) studies ,,, demonstrated an increase of the oxidation state of Mn after long cycling. Recent scanning transmission X-ray microscopy (STXM) investigations analyzed Mn 3 O 4 activation by cycling at various potential sweep rates and at a constant sweep rate.…”