Oxygen Reduction by Enzyme-Carbon Nanotube Composites for the Development of Biofuel Cells
Authors: Cosnier Serge 
Published: February 2020

Glucose biofuel cells are promising candidates to replace lithium batteries in implanted devices one day. In fact, the possibility to convert the chemical energy of glucose and oxygen into electric energy is particularly interesting for in vivo applications because glucose and oxygen are present in body fluids at relatively low but constant concentration [1].
Carbon nanotubes (CNTs) have great advantages as electrode material in biofuel cell design due to their high conductivity and shape which allow enhanced DET or MET with the biocatalysts. Furthermore, CNTs can be shaped to pellets, fibers, or films (Buckypapers, BPs) that makes them easy to process as mesoporous pure CNT bioelectrodes in biofuel cell designs. Moreover, the possibility to add appropriate functionalities via organic functionalization enabled optimal tuning of such nanostructured electrodes by attaching specific anchoring sites for enzymes.
In this context, we report the non-covalent functionalization of multiwalled carbon nanotubes (MWCNT) with pyrene monomers bearing a boronic acid function [2]. This elegant technique consists in the formation of boronic acid ester through the reaction of a boronic acid function on the electrode surface with the sugar residues of glycosylated proteins like glucose oxidase (GOx) or horseradish peroxidase (HRP). By attaching at the same time GOx, a bi-enzymatic system was elaborated and showed efficient bioelectrocatalytic oxygen reduction in physiological conditions. To address the efficiency of this covalent binding, we also performed the same experiments using 1-pyrenebutyric acid N-hydroxysuccinimide ester.
We also report the functionalization of MWCNT electrode by oxidative electropolymerization of pyrrole monomers bearing pyrene and N-hydroxysuccinimide groups [3]. Both polymers were applied to the immobilization and electrical wiring of laccase. A “pseudo” host-guest interaction of polymerized pyrene with a hydrophobic cavity of laccase was used for the oriented enzyme immobilization on MWCNT electrodes. The latter leads to higher catalytic current for oxygen reduction (1.85mA cm-2).
Finally, we present here an efficient and easy way of designing free standing redox buckypaper electrodes via the elegant combination of MWCNT and a bis-pyrene derivative. This bis-pyrene 2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid (bis-Pyr-ABTS) that acts as a cross-linker between nanotubes, assures the formation of mechanically reinforced buckypaper obtained by a classical filtration technique of MWCNT suspension in presence of bis-Pyr-ABTS. In addition, the ABTS derivative assumes the mediated electron transfer to laccase. The buckypaper electrodes were applied to the mediated electron transfer using laccase in solution. For this setup, the developed mediator electrodes demonstrate high performances with maximum currents up to 2 mA ± 70 µA [4].
Taking into account that carbon nanotubes became highly appropriate candidates for the elaboration of biofuel cells and electrochemical supercapacitors, we report here the original combination of these two devices. The possibility to recharge supercapacitors with an internal energy source could thus represent a significant improvement for the performance of biofuel cells. In this context, we propose an original hybrid battery-capacitor system using a compression of enzymes-carbon nanotubes as supercapacitors and electrodes for a biofuel cell setup. This hybrid supercapacitor/biofuel cell enables high power discharge cycles, the carbon nanotube disks being continuously recharged through the biocatalytic energy conversion under physiological conditions [5].
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