Air-Breathing Enzymatic Cathode for Portable Biofuel Cells Carolin Lau & Plamen Atanassov Department of Chemical & Nuclear Engineering, UNM Center for Emerging Energy Technologies, … University of New Mexico, Albuquerque, NM 87131 Vojtech Svoboda & Sameer Singhal CFD Research Corporation, 215 Wynn Drive, Huntsville, AL 35805 Enzymatic biofuel cells (BFC) employ enzymes as catalysts for anodic and/ or cathodic processes, and use biofuels that are already available in nature like sugars and alcohols [1]. Additionally to the …

Enzymatic biofuel cells (BFC) employ enzymes as catalysts for anodic and/ or cathodic processes, and use biofuels that are already available in nature like sugars and alcohols [1]. Additionally to the benefit of using sustainable and logistically easily accessible fuels with high energy density, BFCs can be made for portable applications. This presentation is part of an industrial cooperation to develop an enzymatic, membraneless and platinum free biofuel cell as power extender in milli-watts range. Herein our research is focused on the development of an enzymatic cathode based on the reduction oxygen supplied in a passive mode from ambient air. The air-breathing biocathode (Fig.1 left) consists of two layers: a hydrophobic air-breathing layer facing the ambient air and a second hydrophilic, catalytic layer in contact to the aqueous fuel solution. Both layers are made of teflonized carbon blacks (XC72) that are pressed together onto a current collector (Nickel mesh) under high pressures. That method guaranties a porous matrix for fuel and oxygen supply (Fig.1 right). To prevent fuel solution form leaking out, the hydrophobic layer has higher Teflon content, whereas oxygen reducing enzymes are immobilized in the carbon supported catalytic layer. Fig. 1: Stacked fuel cell with air-breathing cathode, hydrophobic layer facing outside (a); SEM image of catalytic layer surface (b). Enzymes involved in direct oxygen activation are oxidases and oxygenases. Multi-copper oxidases are an important and very well identified and characterized class of oxidases reducing O 2 in a four- electron reduction to water: O 2H 4e 4H O 2 2 ? + + ? + All off the three different copper centers are involved into that four-electron process. The T1 site is the primary electron acceptor and about 7 Ã… below the enzyme surface. For fungal laccase the T1 is connected to the trinuclear T2/ T3 cluster by a His- Cys-His tripeptide, providing a 1.3 nm pathway for electrons [1,2]. Therefore this enzyme should be easily accessible for direct electrical communication with the electrode. The theoretical maximum open circuit potential of a oxygen reduction cathode, given by O 2 formal potential, is +820 mV (vs. SHE). The redox potential of blue copper oxidases varies from species to species and depends on the pH of the surrounding matrix. Enzymes of interest for this project are listed in Table 1. Table 1: Redox potentials of T1 Copper Site in some copper-containing Enzymes ( E0 in mV vs. SHE) and the actual pH of the study Enzyme E0, mV (pH) Laccases Trametes versicolor Rhus Vernicifera 780-800 (pH 4.0) 394-434 (pH 7.0) Ascorbate Oxidase Cucurbita pepo medullosa 344 (pH 7.4) Bilirubin Oxidase Myceliophthora thermophila 450-480 (pH 7.0)

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