Thylakoid membranes have previously been used for electrochemical solar energy conversion, but the current output and open circuit voltage are low, in part due to limitations of the cathode. In this paper, a thylakoid bioanode and laccase biocathode were combined in the construction of a bio-solar cell capable of light-induced generation of electrical power. This two-compartment cell showed a greater than 5-fold increase in short circuit current density and an open circuit voltage 0.275 V larger than that of a thylakoid bio-solar cell incorporating an air-breathing Pt cathode. The electrodes were then tested in several solutions of varying pH to evaluate the possibility of constructing a compartment-less bio-solar cell. This membrane-less cell, operating at pH 5.5, generated a short circuit photocurrent density of 14.0 ± 1.8 μA cm(-2) which is 25% larger than the two-compartment cell and a similar open circuit voltage of 0.720 ± 0.018 V.

Laccase, a blue multicopper oxidoreductase enzyme, is a robust enzyme that catalyzes the reduction of oxygen to water and has been shown previously to perform improved direct electron transfer in a biocathode when mixed with anthracene-modified multi-walled carbon nanotubes. Previous cathode construction used crude laccase enzyme isolated as a brown cell extract powder containing both active and inactive proteins. Purification of this enzyme, yielding a blue solution, resulted in greatly improved enzyme activity and removed insulating protein that competed for docking space in this cathodic system. Cyclic voltammetry of the purified biocathodes showed a background subtracted limiting current density of 1.84 (±0.05) mA/cm 2 in a stationary air-saturated system. Galvanostatic and potentiostatic stability experiments show that the biocathode maintains up to 75% and 80% of the original voltage and current respectively over 24 hours of constant operation. Inclusion of the biocathode in a glucose/O 2 biofuel cell using a mediated glucose oxidase (GOx) anode produced maximum current and power densities of 1.28 (±0.18) mA/cm 2 and 281 (±50) μW/cm 2 at 25 • C and 1.80 (±0.06) mA/cm 2 and 381 (±33) μW/cm 2 at 37 • C, respectively. Enzymatic efficiency of this glucose/O 2 enzymatic fuel cell is among the highest reported for a glucose/O 2 enzymatic fuel cell.Laccase is an oxidoreductase enzyme from a class of multicopper oxidases (MCO) that catalyzes the four-electron reduction of molecular oxygen to water. Laccase has four copper atoms integrated into its two catalytic active sites: a tri-nuclear cluster responsible for the reduction of molecular oxygen, and a mononuclear Cu atom responsible for scavenging electrons from a variety of nonspecific aromatic substrates through one-electron oxidation and radical product formation. 1,2 Laccase is relatively thermostable and has a high turnover rate, making it an ideal target in the field of bioelectrocatalysis. 3,4 Biofuel cells allow for the harnessing of electrical energy that is available from a chemical reaction through the use of bioelectrocatalysts, and oxidoreductase enzymes are common bioelectrocatalysts considered for efficient energy conversion. For this purpose, a large amount of research effort has been put forth developing materials and methods to enhance the electrical connection of catalytic oxidoreductase enzyme active sites to electrode surfaces. 5-8 Two primary methods of electron transfer exist for connecting the enzyme active sites to a conductive electrode surface: mediated electron transfer (MET) and direct electron transfer (DET). MET focuses on using a reversible redox species as a shuttle for electrons from the active site of the enzyme to the electrode surface. This method is suitable for enzymes whose active sites are buried deep inside the insulating protein shell and are not very accessible to pass electrons directly to a conductive surface. Typically, MET employs a polymer matrix to immobilize the enzyme on the electrode surface while the mediator c...

One of the main issues of enzymatic biofuel cells is the need to develop stable bioelectrodes. However, there is no standard method to study bioelectrode stability. In this study, laccase and anthracene modified multiwall carbon nanotubes (anth-MWCNTs) were used in conjunction with different immobilization polymers in order to increase the stability of the biocathodes. A series of stability assays were used to understand the operational stability of the biocathode in a biofuel cell environment. The immobilization matrices used in this study were tetrabutyl ammonium bromide modified Nafion (TBAB modified Nafion), octyl modified linear polyethylenimine (C 8 -LPEI), and vapor deposited tetramethyl orthosilicate (TMOS) gels. The decrease in activity of the galvanostatic and potentiostatic measurements over a 24 hour period of the TBAB modified Nafion were 4.0 ± 0.6% and 4.1 ± 0.4%; C 8 -LPEI were 0.7 ± 0.1% and 9.6 ± 0.5%; and TMOS were 4.1 ± 0.1% and 10 ± 2.0%, respectively. This data show that potentiostatic measurements provide a harsher environment for the enzyme and result in lower stability, as well as showing that the TBAB modified Nafion offers a more stabilizing immobilization strategy compared to the C 8 -LPEI or TMOS matrices. The increasing interest in producing electrical energy from renewable resources has caused the field of biofuel cells to flourish. Enzymatic biofuel cell systems are capable of producing energy from renewable fuels. Basically, enzymatic biofuel cells (EBFC) generate electricity from the oxidation of the fuel at the anode and the reduction of the oxidant at the cathode using biological catalysts (enzymes) instead of traditional metal catalysts. More detailed descriptions of such biofuel cells can be found in reviews that discuss the fundamental concepts of this technology.1,2 Applications of EBFCs range from energy sources for small electronic devices, 3 self-powered sensors 4 and more recently as implantable power sources. [5][6][7] To improve this technology, much research has been focused on engineering and modifying the cathode half of these biofuel cells. [8][9][10][11][12][13][14][15][16][17][18] One such engineered bioelectrode system, previously developed by our group, employs anthracene modified multi-walled carbon nanotubes (anth-MWCNTs) for docking of the oxidoreductase enzyme laccase to the current collector. This biocathode uses anthracene covalently modified to the ends of the multi-walled carbon nanotubes (MWCNTs) in order to favorably orient the laccase enzyme to the ends of the carbon nanotubes, since laccase has a hydrophobic binding site for anthracene and the aromatic structure allows for improved conductivity between the active site and the carbon surface. The steric orientation provides increased catalytic reduction of oxygen to water and favorable distances such that the laccase performs direct electron transfer (DET). 8In biofuel cells, DET occurs through the enzyme's ability to oxidize or reduce a substrate while transferring the necessary electrons to or fro...