A biofuel cell converts chemical energy stored in the chemical bonds of a biofuel, usually glucose, into electrical energy using enzymes. In the anode, glucose is oxidized to produce gluconolactone and give out electrons that are then routed through an external circuit to power a device. In the cathode, the electrons reduce oxygen to form water. When the fuel supply ends, the fuel cell stops working. Now, Pankratov et al. describe a charge-storing biofuel cell, a device in which the charges produced at the anode and the cathode form an integrated supercapacitor.
Like a biofuel cell, the device is composed of an anode where glucose is oxidized and a separated cathode where oxygen is reduced. Electrons produced in the anode reduce an osmium(III) compound to osmium(II), whereas the reduction of oxygen in the cathode is tied to the oxidation of the same osmium couple. As a result, a concentration (or, better, an activity) difference and therefore a voltage difference is generated between the two electrodes, akin to a supercapacitor. To discharge the supercapacitor one connects the anode and the cathode through an external circuit when needed.
The device proposed by Pankratov et al. can generate an open-circuit voltage of 0.45 V, meaning that virtually all chemical energy is stored as charges in the supercapacitor (no leakage currents). At discharge, the power density generated by this charge-storing biofuel cell exceeds that of the corresponding biofuel cell by a factor of 8.