Abstract
Supercapacitors (SCs) have attracted tremendous attention due to their high specific power and long cycle life. The two major issues with SCs are low specific energy and self-discharge. The benefits of doping mediators or redox species in electrolytes include: 1) providing extra pseudocapacitance out of bulk of electrode materials, 2) increasing the electronic and ionic conductivities of electrolyte in the composite electrodes; and 3) releasing the ions into electrolyte to promote the capacity of electrolyte. Redox species or mediators as dopants in electrolytes are reported to realize the specific capacitance up to 400-500 F/g and the specific energy up to 30-50 Wh/kg. For the issue of self-discharge, adding primary cell materials into the mediator SC is proposed because the materials can provide a micro-current to compensate the self-discharge current. In this research, Cu powder/CuSO
4
and Zn powder/ZnSO
4
were added into the positive and negative electrodes respectively. The positive electrode also contained PVDF/LiTFS polymer electrolyte doped with K
4
Fe(CN)
6
as mediator whereas the negative electrode contained PVDF/LiTFS doped with (VO)SO
4
as mediator. A mixture of epoxy/PVDF/LiTFS thin film was used as the separator between the electrodes also to provide adhesion. Figure 1 shows the galvanostatic charge/discharge (GCD) curves of a structural hybrid SC. A cyclic voltammetry (CV) curve obtained at a scan rate of 10 mV/s of this hybrid SC is illustrated in Figure 2. The broad peak from 0.4V to 0.8V is an indication of pseudocapacitance behavior related to the redox reactions of the mediators. The asymmetric shape of CV curve is related to the electrochemical reactions of Cu and Zn during charging/discharging. It was found that the voltage of the SC can be withheld at 1V for more than 3 months. The mechanical strength of the SC was tested using a MTS machine. It was found that the elastic constant as 20 MPa and ultimate tension strength was 2 MPa.
Figure 1