Nanobiocatalysis For Biofuel Cells

Research Objectives:

The objectives of this research are to understand, characterize, and improve enzymatic biofuel cell (BFC) systems with the long term goal being to eventually develop the knowledge necessary to commercialize miniature systems for in vivo applications. In order to realize this long term goal, improving enzyme immobilization techniques to increase its direct electron transfer (DET) efficiency and/or mediated electron transfer (MET) efficiency as well as it long term stability are our primary focus.

What is a Biofuel Cell?

A biofuel cell is simply a fuel cell which uses some biological product to catalyze the anode and/or cathode reactions. Two major classes are enzyme based cells and microorganism based cells. We are focused on cells which utilize glucose oxidase (GOx) as a model enzyme to catalyze the glucose oxidation and generate the electrical power from sugars. We have chosen GOx because glucose, the fuel which GOx oxidizes, is abundant in nature, including plants and animals. GOx is also one of the most studied enzymes; a breadth of knowledge about this molecule is currently available. To your right is an image a BFC with a footprint smaller than a US penny which we have created and optimized. The design is quite simple, consisting of a fuel storage reservoir, anode and cathode current collectors, and a membrane electrode assembly (MEA). A schematic is also shown below.

Figure 2: (A) Biofuel Cell Assembly Diagram and (B) Its VI and Power Density Plots

How does a Biofuel Cell Work?

The biofuel cells (BFCs) shown above convert sugar (glucose) and atmospheric oxygen to gluconolactone and water, while producing electrical power in the process. At the anode, immobilized GOx reacts with glucose to release two electrons and two protons. These electrons are then captured by benzoquinone (BQ) which shuttles the electrons to the anode current collector. Meanwhile, the protons diffuse through the membrane portion of the MEA. At the cathode, the protons combine with electrons and oxygen to form water, which then evaporates away. A schematic of this process is shown below.

The two largest obstacles with BFCs which must be overcome are increasing the power density and increasing the enzyme stability. In collaboration with Dr. Kim of Korean University (Seoul, South Korea), we have developed and characterized a novel enzyme immobilization technique which we have named “crosslinked enzyme clusters” or CEC. Through the procedure shown to the right we are able to precipitate GOx and then crosslink them to create enzyme aggregates which show an unprecedented level of stability and also an impressive activity.

Figure 4: Nanobiocatalysis

Applications of Biofuel Cells:

The most probable application of BFCs will be to use miniature cells to derive power from plants/animals to power small devices. It is believed that miniature biofuel cells could be placed within a human patient to power micro sensor/transmitter devices (such as glucose sensors for diabetics) to provide a doctor with pressure, temperature, concentration, etc. data or to power a pacemaker or bladder control valve. It is also believed that these miniature BFCs could be used by the military to derive power from either trees or insects to power chemical or biological agent sensing devices.

Current and Future Work:

Our research is now focused on improving enzyme immobilization methods and electron transport mechanisms to increase enzyme stability, activity, and electron transport rates. We will first examine immobilization and electron transfer techniques which have been reported by others as well as our CEC/MET technology to assess stability, activity, mass transport characteristics, electron transport resistances and enzyme morphology in a uniform way. This will allow multiple immobilization methods and electron transfer mechanisms to be compared side by side for the first time because the same strict characterization procedure will be used to compare all methods and mechanisms. Through this characterization we will be able to draw some conclusions and develop correlations which will tell us how different broad categories of immobilization techniques and electron transfer mechanisms affect enzyme loading, enzyme activity, enzyme dispersion, enzyme stability, mass transport resistance, and electron transfer rates. This knowledge will then allow us to create enzymatic BFC electrodes which will produce more power and which will be more stable than those currently available. It will also help us to understand and report the science behind how enzyme and immobilization characteristic correlate to electron transfer rates, thus allowing us to suggest methods in which enzymes or mediators might be altered to improve BFCs. The techniques which will be used to characterize these systems are illustrated below. A three pronged approach will be used: a potentiostat will be used to asses performance and activity, and impedance analyzer will be used to asses resistance to mass and charge transfer, and either TEM and/or confocal imaging will be used to determine enzyme loading and dispersion.

Figure 5: Schematic Diagram of Biofuel Cell Powering Glucose Sensor in Human Blood Stream. (A) Schematic Diagram and (B) AFM Image of Our Nanobiocatalysis (GOx on CNT via CEC method)

Figure 6: Overview of Biofuel Cell Research at WSU