Study on Scaling up of MnO2 Supercapacitor
Date Issued
2012
Date
2012
Author(s)
Chen, Chiung-Hung
Abstract
First, aqueous gel electrolytes have been successfully applied to the MnO2‧nH2O supercapacitors. Each gel polymer electrolyte consists of polymer, salt, and water. The polymers, Potassium polyacrylic acid (PAAK), potassium polyacrylic acid-co-polyacrylamide (PAAK-co-PAAM), and polyacrylamide (PAAM) were used in the gel electrolytes. All the gel electrolytes still maintain high ionic conductivities in the order of 10-1 Scm-1. Compared to the MnO2 in liquid electrolytes, the capacitance can be enhanced by the interactions between the polymers and the MnO2.
Second, for the MnO2 supercapacitor, the problem of capacitance reduction with increasing oxide loading can be solved to a great extent by introducing superabsorbent polymer, namely polyacrylic acid (PAA), to form new composite powders composed of MnO2, carbon black, and PAA. Besides, the capacitance of oxide in the composite electrode is also much higher than that of the electrode without PAA. One way to increase the geometric capacitance density is to increase the oxide loading per unit area. As mentioned previously, the capacitance is reduced with increasing oxide loading because the inner electrode is not enough wetted. The specific capacitance of the composite electrode is about 250 F/g with oxide loading of 4.4 mg. On the other hand, the specific capacitance of the electrode without PAA suffers from severe reduction with increasing oxide loading.
In order to further increase the geometric capacitance, Nickel foam is used as current collector which has high porosity of 95%. Therefore, large amount of active materials can be put into the porous current collector. The largest amount of active materials per unit area in Nickel foam is 35 mg/cm2 while that is 4 mg/cm2 on Titanium current collector. From the preliminary results, it indicates that although PAA can help to improve the diffusion of electrolyte ions within thick electrodes, the porosity of electrodes becomes critical in Nickel foam-based electrodes. In this part, the excellent capacitance retention can be realized by replacing the XC72 carbon black with the pearl2000 porous carbon black. Regardless of the fact that the higher porosity leads to higher charge transfer resistance, the factor that inner electrode can get enough wetted in Nickel-foam based electrodes is more important.
The last part of this research is to improve the stability of MnO2 under low potential. The capacitance fading of MnO2 supercapacitor under negative polarization below 0 V (versus Ag/AgCl) is due to the formation of electrochemically inactive Mn(II) and limits the potential window no greater than 0.8 V. Adding the metal redox couple, Ti(IV)/Ti(II), as a charge meditator into electrolyte increases the charge transfer efficiency of some specific reactions and suppressed the formation of Mn(II). Therefore, the stability of the MnO2 symmetric supercapacitor can be improved after 3000 cycles over an operating voltage window of 1.2 V.
Subjects
Supercapacitors
MnO2
Aqueous Electrolyte
Gel Electrolyte
Nickel Foam
Type
thesis
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