2, Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada
The growing interest in using manganese dioxide (MnO2) for supercapacitor (SC) electrodes is attributed to its high theoretical specific capacitance (1400F g-1), relatively large voltage window, natural abundance and environmental friendliness. However, the low electronic conductivity of MnO2 impedes its further development in commercial applications. Normally, the conductive additives such as multiwalled carbon nanotubes (MWCNT) are introduced to enhance the electronic conductivity of MnO2 for the fabrication of SC electrodes. However, it is critical to achieve a high electrochemical performance of SC with high active mass loading at high charge-discharge scan rates. The important task is to avoid agglomeration of MnO2 nanoparticles and MWCNT. New strategies are developed to address this problem. In this work, liquid-liquid extraction method (LLEM) is developed to prepare non-agglomerated small sized MnO2 nanoparticles with porous surface and improved mixing of MnO2 and MWCNT. Head-tail (HT) surfactants, containing amine, phosphonate or carboxylic groups, are used as extractors for extraction of MnO2 nanoparticles. Moreover, it was found that conceptually new multifunctional head-tail-head (HTH) surfactants containing one phosphonate end and one carboxylic end can be used as efficient extractors for LLEM. Furthermore, HTH surfactants can be used as dispersing and charging agents to prepare stable MnO2 suspension for electrophoretic deposition (EPD) of thin film. The influence of molecular structure of different extractor molecules such as hexadecylamine (HDA), hexadecylphosphonic acid (HDPA), palmitic acid (PA) and 16-phosphonohexadecanoic acid (16PHA) on the electrochemical performance of SC electrodes have been investigated systematically. Various interactions between nanoparticles and extractor molecules such as covalent, ion-pair or electrostatic interactions play role in preparing the non-agglomerated MnO2 nanoparticles. Compared to other HT surfactants such as HDA, HDPA and PA, the HTH surfactant 16PHA used as extractor for fabrication of SC electrodes with active mass loading of 37mg cm-2 showed highest capacitance of 5.7 F cm-2 (157 F g-1) and 2.5 F cm-2 (67 F g-1) at a scan rate of 2 mV/s and 100mV/s, respectively. Additionally, MnO2 particles extracted by 16PHA, are negatively charged and deposited on the anode surface by EPD forming a relatively smooth, dense and agglomerate-free thin film. The conceptually new strategy using multifunctional HTH surfactants as extractor for preparation of non-agglomerated nanoparticles by LLEM paves the way for the fabrication of different functional nanomaterials for advanced applications.