Sabina Topcagic, Becky L. Treu, and Shelley D. Minteer

Department of Chemistry, Saint Louis University, Missouri

CONTENTS

Introduction…………………………………………………………………………………………………… 216

Portable Electrical Energy Sources………………………………………………………………… 216

Batteries……………………………………………………………………………………………. 216

Fuel Cells…………………………………………………………………………………………… 217

Biofuel Cells……………………………………………………………………………………… 218

Enzyme Immobilization Techniques…………………………………………………………….. 220

Wired Technique………………………………………………………………………………. 221

Sandwich Technique………………………………………………………………………… 222

Entrapment Technique…………………………………………………………………….. 222

Nafion® Modification……………………………………………………………………… 223

NAD+-Dependent Alcohol Dehydrogenase Biofuel Cells…………………. 225

PQQ-Dependent Alcohol Dehydrogenase Biofuel Cells……………………. 226

Membraneless Biofuel Cells……………………………………………………………… 227

Conclusions…………………………………………………………………………………………………… 229

References…………………………………………………………………………………………………….. 230

Abstract There are three types of batteries: primary, secondary, and fuel cells. A fuel cell is an electrochemical device that converts chemical energy into electrical energy via catalysts. Fuel cells have many advantages over the two other types of batteries due to the fact they can be regenerated with the addition of fuel specific to the system. Traditional fuel cells employ heavy metal or precious metal catalysts, whereas biofuel cells employ biological catalysts (enzymes). Enzymes are highly specific catalysts, so they allow for the simplifi­cation of the fuel cell by eliminating the need for a polymer electrolyte membrane, which is one of the mostly costly parts of a fuel cell. Dehydrogenase enzymes have been employed at the anode of biofuel cells to oxidize alcohols. Methanol, ethanol, propanol, and butanol are examples of alcohols that can be used in biofuel cells. Long-term goals include investigating a variety of power applications for this technology ranging from portable electronics to sensors.

INTRODUCTION

Previous chapters of this book detail methods for producing ethanol from agri­cultural products and biomass. Although many of these methods are efficient, it is crucial to be able to efficiently convert energy to electrical power. As detailed in an earlier chapter, researchers have been attempting to develop direct ethanol fuel cells (DEFCs), but there have been problems because traditional precious metal catalysts (Pt-based catalysts) are unable to efficiently catalyze the oxidation ethanol and maintain an electrode with minimal fouling at low temperatures. However, living organisms are capable of efficiently catalyzing the oxidation of ethanol at 20-40°C. Living organisms, such as Pseudomonas aeruginosa [1], acetobacter [2], and gluconobacter [3], contain enzymes that can oxidize a variety of alcohols, including ethanol. Over the last 40 years, researchers have been working on employing living organisms and/or their enzymes in a fuel cell to convert chemical energy to electrical energy. This type of battery or fuel cell is referred to as a biofuel cell. The early research was plagued with enzyme stability problems and low power densities, but those issues are being overcome with current research. In this chapter, we will discuss the brief history, techniques, and applications of alcohol-based biofuel cells.