The Preparation Of Laccase-based Electrode And The Study Of Its Catalytic Effect On Oxygen Reduction

Enzymatic biofuel cells (EBFCs) are devices which convert chemical energy directly into electrical energy using enzyme as bioelectrical catalyst, organic compounds as fuel, and oxygen as oxidant. Due to the relatively simple structure of EBFCs and the high catalytic activity and selectivity of the enzymes towards specific substrate, EBFCs are promising to be clean energy-generating device, which can operate under moderate working conditions close to physiological ones. Furthermore, sensors based on the principle of (EBFCs) are also effective tools for biochemical analysis. To construct enzymatic electrodes which allow efficient electron transfer between the active centers of the enzyme and the conducting substrate is one of the prerequisite for the commercialization of such devices. Meanwhile, systematic studies of on catalytic reactions of substrates at such enzymatic electrodes will be of great help for optimization such devices, e. g., improve its energy efficiency. On the other hand, such studies can also provide information on how the substrates turnover in corresponding physiological processes and how the chemical conversion are correlated with electrical behavior.The preferable bioclectrocatalyst for oxygen reduction reaction(ORR) at the cathode for EBFCs is laccase for its relatively high formal potential (close to reversible potential of ORR) and its high catalytic performance toward ORR. However, at laccase based bioeathodes reported so far, direct electron transfer between the active centers of laccase and the conducting electrode substrate can only be realized under some specific rigorous conditions. Furthermore, at such laccase based bioeathodes, ORR overpotential is quite high (>500mV) and the output current density remains rather low (～10’-102μAcm-2). These drawbacks remain the bottle neck for the improving the efficiency of EBFCs.To solve above problems, in the present work several strategies were employed to construct laccase based bioeathodes with improved efficiency between laccase active site and conducting carrier, and systematic studies on ORR at such electrodes were carried out, from which the relationship between the catalytic activity of laccase cathode toward ORR, electrode stability and the structure of the carrier for laccase loading, morphology of the matrix for laccase entrapment was derived. The main conclusions are listed below:1. Laccase catalyzed ORR mediated by ABTS:Rotating disk electrode technique and cyclic voltammetry method were used to investigate the kinetics of ABTS2-/ABTS·-at glassy carbon electrode(GCE) and its mass transport in solution. Kinetic parameters of the redox of ABTS2-/ABTS·-and the diffusion coefficient have been achieved. After that, ORR at free laccase in solution and at immobilized laccase at GCE with ABTS as electron relay was evaluated. We found that the in such system, the onset potential of ORR is limited by formal potential of ABTS2-/ABTS·-; catalytic current is controlled by the diffusion of ABTS and oxygen toward the active center of laccase, in contrast, the electrode reaction and enzymatic chemical transformation are relatively fast.2. Direct electrochemistry of laccase modified electrode immobilized by Multiwall carbon nanotubes(MCNT)-poly aryl amide(PAA) composite and its catalytic effect on ORR:in laccase modified electrode immobilized by Multiwall carbon nanotubes(MCNT)-poly aryl amide(PAA) composite. PAA is used as electrical wire. PAA modified MCNT is used as support and electron carrier for laccase. The direct electrochemistry of laccase T1site and electrode is achieved and results revealed more than67%of the immobilized enzyme molecules are efficiently electrically wired. However, the enzyme modified electrode showed relative poor oxygen reduction activity with onset potential for oxygen reduction at0.55V, E1/2=425mV and j1/2=75.3μA cm-2(normalized by geometrical area of GCE). The low output current density and high overpotential is found to due to the hindered mass transport of oxygen molecule through the catalyst layer. As an oxygen sensor, the laccase based biochatode has detection limit of0.57μmol L-1, the sensitivity of74.22μA L mmol-1Michelis-Menten constant for oxygen of55.8μmol L-1.3. Direct electrochemistry of laccase modified electrode supported by aitrogen-doped mesoporous carbon(MPCN)-Chitosan(CTS) composite and O-carboxymethylated Chitosan(O-CMCH) and their catalytic effect on ORR: Composite composed of MPCN and CTS or O-CMCH are used as support and electron carrier for farbricating laccase based electrodes. Efficient electron transfer between the T1active center in laccase and conducting matrix are realized. Such laccase cathode shows good catalytic performance toward ORR with onset potential for ORR of0.86V and E1/2=0.7and j1/2=170μAccm-2. Between them, the mechanical stability of composite made up of O-carboxymethylated Chitosan and mesoporous carbon-nitrogen material with entrapped laccase modified cathode is better. But both of laccase cathodes display excellent catalytic activity reproducibility and long-term usability on ORR. In the case of with O-CMCH as film formation reagent, TOF for ORR at such electrode is found to be4.0moelcule catalyst site-1s-1and with the mid-wave current density (0.93V,170μAcm-2).