Stable ‘Floating’ Air Diffusion Biocathode Based on Direct Electron Transfer Reactions Between Carbon Particles and High Redox Potential Laccase

Successful realisation of many important electrochemical processes, including development of fuel cell (FC) technology, requires generation of novel highly effective catalysts, which are active at ambient temperatures and physiological pH values. Optimal technological solutions can be achieved by using biocatalysts heterogeneously arranged in an electroconductive matrix, e.g. redox enzymes immobilised on properly selected highly dispersed materials. For cathodic processes occurring in FCs, blue multicopper oxidases (BMCO) have been identified as very promising bioelements. BMCO efficiently catalyse the reduction of O2, a very common electron acceptor because of its high-redox potential and its ready availability, at very low overpotentials [1–5]. At the end of the 1970s, direct electron transfer (DET) based bioelectrocatatalytic reduction of O2 by a fungal laccase (Lc), an enzyme from the BMCO family, was discovered [6, 7]. Later this occurrence was identified in many other redox enzymes including different BMCO, e.g. ascorbate oxidase [8, 9] and bilirubin oxidase [10, 11], as well as fungal [12–15], plant [12, 16] and bacterial [17, 18] Lcs. BMCO contain four copper ions which are historically classified into three types according to their spectroscopic characteristics, viz. the T1, T2 and T3 sites (Figure 1, bottom) [19].