Energy Storage and Management in Supercapacitors for application in Piezoelectric Energy Harvesting Systems

Electrical double layer capacitors (supercapacitors) were fabricated using activated carbon as the active material and polyvinylidine fluoride (PVDF) as a binder with a suitable conductive additive (MWCNTs) together in an optimized ratio. The supercapacitor cells were assembled using an aqueous solution of 0.5M Na2SO4 as the electrolyte. These cells had an average capacitance of 1.7F each as measured by the constant current charging method. The two electrode symmetric cell had a specific capacitance of 23.05 F/g. The fabrication methodology has been discussed as well as the potential applications of the supercapacitor in piezoelectric element based energy harvesting systems have been elucidated. Introduction Electrical double layer capacitor (EDLC), sometimes called a supercapacitor or an ultracapacitor, stores electric charge in the electrical double layer at a surface-electrolyte interface, primarily in high-surface-area carbons. Because of the high surface area and the thinness of the double layer, these devices can have very high specific and volumetric capacitances and essentially unlimited charge/ discharge cycle life 1 . Supercapacitors have a much higher (at least one order of magnitude higher) power density and higher shelf-life than batteries. At the same time they have a considerably lower energy density 2 . Conventional electrolytic capacitors on the other hand have a much higher power density and much lower energy density compared to supercapacitors. This makes supercapacitors a bridge between capacitors and batteries as illustrated by the Ragone plot in Figure 1. Energy harvesting circuits have a very low power output; sufficient energy must be accumulated to make the setup feasible for any practical use. Also, on inclusion of an energy storage device, it is important to ensure shortercharging duration so that the system doesn’tremain dormant for longer durations. These factors interlocked with sufficiently long cycle life are the essential elements of an effective energy harvesting system. Rechargeable batteries are usually rated with a cycle life of about 300-1000 cycles 3 while supercapacitors are rated higher than 100,000 cycles. Supercapacitors can be charged at any voltage up to their rated voltage while batteries have to be charged at a small voltage window around their rated voltage. This aspect defines the charging characteristics of a storage device in an energy harvesting circuit. Anjana Jain et al /Int.J. ChemTech Res.2014-2015,7(3),pp 1118-1124. 1119 Figure 1. The Ragone plot given above illustrates how supercapacitors form a bridge between capacitors and batteries with intermediate energy density and power density 4 . Energy harvesting systems which employ piezoelectric elements as micropowergenerators utilize the direct piezoelectric effect wherein certain materials transform mechanical strain to electrical charge 5 .They have three essential parts namely the micropowergenerator, the voltage rectifier and the energy storage/ management device. Such systems are known to have low power and a varying voltage depending on the mechanical input and often require several adjustments in circuitry like adjusting the rectifier output voltage to obtain maximum efficiency 6 . The presence of mesopores in electrodes based on CNTs, due to the central canal and entanglement enables east access of ions from electrolyte. For electrodes built from multiwalled carbon nanotubes (MWCNTs), specific capacitance is in a range of 3-135 Fig 7, 8 . The following study includes, firstly, a simple procedure to fabricate activated carbon based supercapacitors, secondly, a basic testing scheme for the supercapacitor cell and lastly, application of the supercapacitor cells in a piezoelectric microgenerator based energy harvesting system. The objective of this study is to study feasibility of using supercapacitors along with piezoelectric micropower generators to form an effective energy harvesting setup.