Application of Modified Carbon Nanotube Materials for Enzymatic Biofuels Cells Based on Direct Enzyme-Electrode Contacts

Carbon nanotubes (CNTs) arranged in 3 dimensional structures represent an interesting material for the development of biocatalytic electrodes. Due to their architecture they can provide docking places for enzymes and after modification of the surface properties often a direct electrochemistry can be observed. The direct electron transfer where the catalytic starts near the E⁰ of the enzymes redox center can avoid a loss of in cell potential and allows the development of efficient enzymatic biofuel cells (EBFC). Also the membrane-less construction of the EBFCs is advantageous for high power output. With this respect the pyrroloquinoline quinone dependent glucose dehydrogenase ((PQQ) GDH) is an interesting enzyme since it is insensitive towards oxygen which is the terminal electron acceptor at the cathode.

Here two types of carbon nanotubes materials - bucky paper (BP) and vertically aligned carbon nanotubes (vaCNTs) - are used for the development of glucose/oxygen biofuel cells. For the anode development these materials are modified with poly(3-aminobenzoic acid-co-2-methoxyaniline-5-sulfonic) acid (PABMSA) for covalent coupling of the glucose oxidizing (PQQ) GDH. The cathode is based on the oxygen reducing Bilirubin oxidase (BOD) which is covalently coupled to PQQ modified BP and vaCNTs electrodes.

For the electrochemical characterisation of the individual electrodes voltammetric measurements are performed. The voltammograms for the different anode preparations show that the modification of both carbon nanotube materials with an aniline-based polymer film (PABMSA) and covalent enzyme coupling result in an direct enzyme-electrode contact. The influence of the polymer concentration during the electrode preparation and the impact of the buffer composition on the current density are investigated. Both electrode materials show the highest current density in 100 mM citrate phosphate (CiP) buffer containing 10 mM glucose and applying 5 mg/ml PABMSA for CNTs modification. For the BP-based anode current densities up to 0.75 mA/cm² can be detected while electrodes made of vaCNTs reveal a maximum current density of 1.3 mA/cm² at +0.1 V vs. Ag/AgCl. The cathode construction with PQQ as interlayer and a covalent attachment of the BOD to the carboxylic groups shows the highest electrocatalytic activity under air saturated conditions – for bucky paper about 1 mA/cm² at 0.1 V vs. Ag/AgCl. Applying vaCNTs for the BOD-cathode development a local maximum current of 1.3 mA/cm² and a steady-state catalytic current of 0.55 mA/cm² at +0.1 V vs. Ag/AgCl can be obtained.

A combination of the BP-based  (PQQ)GDH/PABMSA and BOD/PQQ electrodes in a biofuel cell application achieves a power output of 107 µW/cm² at a cell potential of 490 mV. The same modification procedures and enzymes applied in a vaCNTs fuel cell lead to a power density of 122 µW/cm² at cell potential of 540 mV.

The separate evaluation of both carbon nanotubes based materials reveal a better stability of the vaCNTs-based enzyme electrodes.