Achieving oxygen reduction at high positive potentials with fast heterogeneous electron transfer is desirable for the biocathode of fuel cells based on enzymes. Here, we present an effective interface for obtaining direct electron transfer from a laccase (Lac)-based cathode for O2 reduction, starting from a potential very close to the redox equilibrium potential of the oxygen/water couple. The interface between Lac and single-walled carbon nanotubes was improved by modification with the steroid biosurfactant sodium cholate. The heterogeneous electron-transfer rate between the type-1 Cu site of Lac and the modified electrode was determined to be 3000 s(-1). The electron-transfer rate was sensitive to the side chain of the steroid biosurfactant, and the rate decreased more than 10-fold when sodium deoxycholate was used as the side chain.

The rate of heterogeneous direct electron transfer of laccase immobilized on single-walled carbon nanotube (SWCNT) and carbon paper electrodes was evaluated by cyclic voltammetry and background-current-corrected steady-state linear voltammetry. These rates indicated that the molecular orientation of laccase immobilized on the SWCNT electrode was more favorable for direct electron transfer, than that of laccase immobilized on the carbon paper electrode. The inhibition of the bioelectrocatalytic O 2 reduction current of the two electrodes by chloride and fluoride were tested. The results indicated differing inhibition mechanisms by these two halides. Laccase immobilized on the SWCNT electrode exhibited high stability and high resistance to chloride inhibition.

Nondoped and N-doped SWCNTs (N-SWCNTs) were used to clarify the effect of N-doping on a direct electron transfer (DET) reaction of laccase (Lac, from Trametes sp.). The level of N-doping in the carbon phase of the N-SWCNTs, which were synthesized by a CVD method, was determined to be 0.1, 2.4, and 4.1% from X-ray photoelectron spectroscopy measurements. The N-SWCNTs were also carefully characterized using electron microscopy, Brunauer-Emmett-Teller (BET) specific surface area measurements, Raman spectroscopy, and electrochemistry. The bioelectrocatalytic current for the DET reaction of Lac immobilized onto the N-SWCNTs tended to decrease with increasing N dopant ratio, whereas the amount of Lac adsorbed per BET surface area of the N-SWCNTs did not depend on the N dopant ratio. There were two main explanations for this behavior. First, an electrostatic interaction between the positively charged interface of the N-SWCNTs due to nitrogen species surface functional groups and the negative charges of carboxylate residues surrounding the T1 site. Second, the surface potential of the N-SWCNTs during Lac modification, because the slope value of the surface potential versus N dopant ratio of the N-SWCNTs was about 53 mV/N%. From additional investigations into the surface potential effect and thermodynamic investigations, we carefully concluded that the above behaviors may be due to denaturation and/or decreasing of the DET reaction rate caused by the strong electrostatic interaction between Lac and the N-SWCNTs surface.

The capacitive behaviour of iron reduced graphite oxide in alkaline media is reported.

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