Printed power source: Supercapacitor with an enzymatic bio-fuel cell

Inexpensive power sources are needed e.g. in RFID applications. By applying printing techniques the manufacturing costs can be minimized. To widen the applicability range the materials should be easily disposable. Enzymatic bio-fuel cells are an alternative for printable primary batteries. Since bio-fuel cells can provide only relatively low power, we have developed supercapacitors that can be combined with enzymatic bio-fuel cells to facilitate the power peaks demanded in the applications. The materials for the supercapacitors have been chosen to be compatible with the fuel cell and with printing methods, e.g. the activated carbon powder in the electrodes was bound with chitosan. As substrates we have used paperboards and polymer foils. The current collectors have been made of graphite and metal inks. Since the voltage requirement is limited to approximately 1 V, aqueous electrolytes have been used. Supercapacitors of various sizes have been prepared. The geometrical electrode areas have been between 0.5 and 2 cm2. The maximum feasible output current has been in the order of 50 mA corresponding to about 50 mW power. When the capacitor is used together with an enzymatic power source, the leakage current must be as low as possible in order to avoid forming an excess load for the bio-fuel cell. Typical leakage current values have been in the order of 10 µA. Some general conclusions concerning the electrical properties of supercapacitors have been done. Larger geometrical electrode area leads to lower equivalent series resistance since both the ionic conductivity and electrode conductivity are increased. Also making the activated carbon electrode layer thinner decreases the resistance. The same applies to thinning the separator. The capacitance itself is not dependent on the geometrical electrode area but almost completely on the mass of activated carbon. Leakage current depends also on the geometrical area but in some cases it seems to be even more dependent on capacitance and thus the activated carbon surface area. The majority of the leakage current is probably consumed to maintain the double layer.