New Colloidal Techniques for the Fabrication of Manganese Dioxide-Carbon Nanotube Electrodes of Supercapacitors

Abstract

Electrochemical supercapacitors based on MnO2 electrode materials are currently attracting significant interest due to the high specific capacitance obtained using environmentally friendly aqueous electrolytes and low cost of MnO2. A complicating factor in the application and commercialization of MnO2 electrodes for electrochemical supercapacitors is low electronic and ionic conductivity of MnO2. This problem is usually addressed by the fabrication of porous nanocomposite electrodes, containing MnO2 nanoparticles and carbon nanotubes (CNT) or other conductive additives. Despite the impressive progress achieved in the fabrication of MnO2-CNT electrodes, there is a need for simple and versatile methods for the fabrication of MnO2 nanoparticles, efficient dispersion of MnO2 and CNT, and fabrication of porous electrodes. We report new strategies for the fabrication of MnO2-CNT composites, which are based on the use of new dispersing agents and new techniques for mixing of MnO2 and CNT. The electrochemical performance of the composites, prepared by different methods was compared. The results were used for the fabrication of advanced electrodes and devices. Efficient dispersing agents are of critical importance for the fabrication of MnO2 nanoparticles and formation of stable suspensions for colloidal processing. An important property of a dispersant is its adsorption on the particle surface. Therefore, there is a need in the development of charged dispersing agents with strong interfacial adhesion. New methods have been developed for the synthesis, surface modification and dispersion of MnO2 nanoparticles using advanced dispersing agents, which provide strong bidentate, polydentate or chelating bonding to the particle surface. Various dispersants were developed and compared, such as molecules from catechol, chromotropic acid, gallic acid, salicylic acid and bicinchoninic acid families. The influence of the nature of functional groups, number of aromatic rings, length of the hydrocarbon chain on the adsorption, dispersion and electrostastic charging in the suspensions was investigated. New dispersants were used for the synthesis of non-agglomerated MnO2 particles. Various aromatic and steroid dispersants were investigated for the dispersion of CNT. It was found that steroid dispersants outperform other dispersants in the dispersion of CNT. The polyaromatic molecules, containing chelating and charging functional groups can be used as co-dispersants for MnO2 and CNT. Further progress in the application of new dispersants was achieved by the development of liquid-liquid interface synthesis and extraction method, which allowed agglomerate free synthesis of MnO2 and their mixing with CNT. In this strategy, the problems related to particle agglomeration during the drying stage were avoided. As an extension of these investigations, chelating polymers were used for dispersion. The unique feature of this strategy is that chelating aromatic ligands of the monomers provide multiple adsorption sites for attachment on MnO2 and CNT and impart electrical charges for electrosteric dispersion. In another strategy, complexes of polymers and chelating agents were used for co-dispersion of MnO2 and CNT. As a result, we achieved an outstanding efficiency in dispersion. Heterocoagulation methods have been developed for the nanotechnology of MnO2 - CNT composites. The methods are based on selective strong adsorption of dispersants with specific adsorption ligands on MnO2 or CNT and heterocoagulation of the materials using electrostatic forces, related to opposite charges of individual dispersants or click chemistry methods. Proof of concept studies in aqueous and non-aqueous suspensions showed significant improvement in component dispersion and mixing. The capacitive performance of MnO2-CNT composites, prepared by different methods was compared. Ni foams were used as current collectors for the fabrication of electrodes with active mass loading of 30-50 mg cm-2 and mass ratio of active material to current collector of 0.3-0.42. The capacitive behavior of the composite electrodes was studied in Na2SO4 electrolyte using cyclic voltammetry, chronopotentiometry and impedance spectroscopy. The use of new dispersants and mixing techniques allowed for significant improvement in electrode performance. The composites, prepared in the presence of dispersants using liquid-liquid extraction and mixing methods showed the highest specific capacitance of 8 F cm-2. The liquid-liquid extraction method for the synthesis of MnO2 particles and their mixing with CNT allowed good capacitance retention at high charge-discharge rates. Significant improvement in capacitive behavior was achieved using heterocoagulation methods for the fabrication of MnO2-CNT composites. The new strategies allowed significant reduction in electrode resistance due to the use of efficient colloidal dispersion and mixing techniques. The capacitance retention of above 70% was achieved in the scan rate range of 2-100 mV s-1. The electrodes showed cyclic stability above 90% after 5000 cycles. The composite electrodes were used for the fabrication of asymmetric capacitors with voltage window of 1.6V. We report capacitances, power-energy characteristics and cyclic behavior of electrodes, prepared using different methods.