Single-walled carbon nanotubes (SWCNTs) covalently phenylated, naphthylated or terphenylated were used for the construction of cathodes in a biobattery, and in a biofuel cell. Zn is the anode in the biobattery and single-walled carbon nanotubes are covalently modified with glucose oxidase/catalase (SWCNT-GOx/Cat) on the biofuel cell anode. The cell parameters were determined and the potentials of each of the electrodes under cell working conditions were simultaneously measured vs. the Ag/AgCl reference electrode. This allowed to evaluate the changes of potential under changing loads of the fuel cell or biobattery. A power density of ca. 1 mW/cm(2) was achieved using the biobattery with phenylated nanotubes at the cathode and the open circuit potential was 1.5 V. The fully enzymatic fuel cell studied had power density of 40 mu W/cm(2) at 20 k Omega loading. The open circuit potential for the biofuel cell was 0.4 V. The power densities and working potentials are larger for the biobattery which is especially useful for testing novel cathodes. (C) 2012 Elsevier Ltd. All rights reserved.