Portland, Ore. - As R&D proceeds apace on hydrogen-based fuel cells for products as diverse as automobiles and laptops, an obscure research movement is looking at an alternative that would tap known biological processes to generate electricity.
One project has prototyped what is said to be an environmentally benign power source that consumes a few milliliters of ethyl alcohol and oxygen from the air and, using enzymes rather than heavy metals as catalysts, can produce electricity for months without recharging. The only by-products of the reaction are vinegar, water and carbon dioxide vapors, said Shelley Minteer, a professor of chemistry at Missouri's St. Louis University, who presented her findings in New Orleans last week at the 225th national meeting of the American Chemical Society.
"Regular hydrogen fuel cells have hydrogen coming in one electrode and oxygen from the air coming in to the other electrode. There is a chemical reaction between the hydrogen and oxygen, giving you electrical energy as hydrogen becomes oxidized and oxygen gets reduced," Minteer said. "We have basically the same setup, but we are reducing ethanol [ethyl alcohol] with a living enzyme, and the oxygen is being oxidized." Minteer's assistant on the project is St. Louis graduate student Nick Akers.
Bio-fuel cells have been the subject of scientific speculation since the 1950s, but until now they have mainly been researched for medical purposes. Notably, University of Texas at Austin professor Adam Heller has built prototypes for the Defense Advanced Research Projects Agency. Heller's implantable micron-size bio-fuel cells oxidize glucose as their energy source at their anode and reduce oxygen at their cathode, but only to produce a fraction of a microwatt of electricity inside the body, for pacemakers or similar ultralow-power devices.
Minteer's prototype, by contrast, is designed to provide enough energy to power cell phones, personal digital assistants and perhaps even laptop computers someday. The prototype achieves power densities ranging from 1.16 milliwatts per square centimeter to 2.04 mW/cm2, depending on the ratio of alcohol to aldehyde in the polymer layer. Those results, according to Minteer, represent a 32x increase over the power densities of competing bio-fuel cell designs.
When the researchers used methanol as fuel with Minteer's bio-anodes (methanol has been the choice for most bio-fuel cell designs), power densities of 1.55 mW/cm2 and an open-circuit potential of 0.71 volt were achieved.
"There is no limit really to how much power we can produce with bio-fuel cells, but I have a feeling that they will be used for small devices," said Minteer. "We are not going to see 'living automobiles' anytime soon."
As the catalyzing agent in bio-fuel cells, enzymes are complex proteins that initiate the specific biochemical reactions that make the fuel cell work. The enzymes for Minteer's prototype fuel cell were extracted from yeast.
Enzymes are not alive, in the sense that they can live independently of a host, but neither are they inert agents. Rather, they are active agents that living cells use to direct their metabolic activities. Enzymes are present in all living things.
Ordinarily, enzymes die quickly when removed from their living host, via a process called "denaturing." Structurally, enzymes are long protein molecules that are folded into a configuration that defines their catalytic function. Denaturing-unraveling the molecules from their original, folded form-usually terminates their catalytic properties within a few days.
Implantable bio-fuel cells, such as those produced by Heller at the University of Texas, remain within the body, simplifying the problem of keeping them from denaturing. Previous attempts to produce bio-fuel cells-for example, methanol-based systems pursued by the Office of Naval Research-have used enzymes that were "glued" to an electrode, which shortened their lifetime as they denatured.
Minteer has been able to extend the lives of her enzymes by months via the creation of an environment favorable to keeping them both nurtured and contained. Minteer's novel approach uses polymers with natural pores. She uses wet chemistry-precisely formulated solutions-to open the pores of quaternary ammonium bromide salt-treated Nafion, yielding a habitat that is proportioned to accommodate a single enzyme molecule when folded. The treatment also modifies the polymer by reducing its acidity to a neutral pH.
The prepared enzymes are implanted in the pores, from which they cannot unravel as they age, because of their confinement. The pores also insulate the micro-environment of the enzyme from the fluctuations in the outside environment.
Ordinarily, enzymes are extremely sensitive to changes in pH and temperature. Even slight departures from ideal conditions often lead to denaturing, resulting in a dead fuel cell. The pores, however, maintain a relatively constant average internal pH and temperature, an effect important for applications such as cell phones, which experience a wide range of temperatures in the course of a day.
Yeast in, vinegar out
Minteer's current prototype uses a naturally occurring, and already commercially available, yeast enzyme. A reaction by-product is vinegar, which can be syphoned into a disposable cartridge. It is also possible to use different kinds of yeast to get even more-inert by-products, such as gaseous CO2, which would not even require a disposable waste-cartridge.
"Our fuel cell is a battery that never runs down," Minteer said. "As long as you keep putting in alcohol and don't cut off the air, it will continue to produce electricity on demand. [And] instead of an environmentally unfriendly heavy metal, the catalysts for the bio-fuel cell are harmless living enzymes."
Hydrogen fuel cell research has already been explored by General Motors, DaimlerChrysler, Ford and Honda. It also got a boost this year from the Bush administration, which proposed earmarking $1.2 billion in research funding so that, as the president said in announcing the plan to the public, "America can lead the world in developing clean, hydrogen-powered automobiles."
In a nod to the hydrogen fuel cell's momentum, Minteer said her team is "trying to re-engineer the high-surface-area electrodes designed for hydrogen fuel cells [to make them usable with] our particular application." The larger the electrode surface area, the higher the capacity of the resulting bio-fuel cell. She gives herself five years to reverse-engineer the hydrogen fuel cell for bio-fuel apps.
In the meantime, she is going ahead full steam on a new prototype for a cell phone, using the disposable vinegar cartridge. Once in a cell phone, the cartridge could be "charged" by dropping it in just a few milliliters of ethanol-and the solution doesn't have to be chemically pure. "We've tested probably 30 to 50 of the ethanol cells-with vodka, gin, white wine and beer," said Minteer. It liked the hard stuff best.
The cell phone prototype should be ready in six to nine months, said Minteer. It will be about the size of a postage stamp-5 centimeters on a side.