An enzymatic biofuel cell is a specific type of fuel cell that uses enzymes as a catalyst to oxidize its fuel, rather than precious metals. Enzymatic biofuel cells, while currently confined to research facilities, are widely prized for the promise they hold in terms of their relatively inexpensive components and fuels, as well as a potential power source for bionic implants.
Operation
Enzymatic biofuel cells work on the same general principles as all fuel cells: use a catalyst to separate electrons from a parent molecule and force it to go around an electrolyte barrier through a wire to generate an electric current. What makes the enzymatic biofuel cell distinct from more conventional fuel cells are the catalysts they use and the fuels that they accept. Whereas most fuel cells use metals like platinum and nickel as catalysts, the enzymatic biofuel cell uses enzymes derived from living cells (although not within living cells; fuel cells that use whole cells to catalyze fuel are called microbial fuel cells). This offers a couple of advantages for enzymatic biofuel cells: Enzymes are relatively easy to mass-produce and so benefit from economies of scale, whereas precious metals must be mined and so have an inelastic supply. Enzymes are also specifically designed to process organic compounds such as sugars and alcohols, which are extremely common in nature. Most organic compounds cannot be used as fuel by fuel cells with metal catalysts because the carbon monoxide formed by the interaction of the carbon molecules with oxygen during the fuel cell's functioning will quickly “poison” the precious metals that the cell relies on, rendering it useless. Because sugars and other biofuels can be grown and harvested on a massive scale, the fuel for enzymatic biofuel cells is extremely cheap and can be found in nearly any part of the world, thus making it an extraordinarily attractive option from a logistics standpoint, and even more so for those concerned with the adoption of renewable energy sources.
Enzymatic biofuel cells also have operating requirements not shared by traditional fuel cells. What is most significant is that the enzymes that allow the fuel cell to operate must be “immobilized” near the anode and cathode in order to work properly; if not immobilized, the enzymes will diffuse into the cell's fuel and most of the liberated electrons will not reach the electrodes, compromising its effectiveness. Even with immobilization, a means must also be provided for electrons to be transferred to and from the electrodes. This can be done either directly from the enzyme to the electrode (“direct electron transfer”) or with the aid of other chemicals that transfer electrons from the enzyme to the electrode (“mediated electron transfer”). The former technique is possible only with certain types of enzymes whose activation sites are close to the enzyme's surface, but doing so presents fewer toxicity risks for fuel cells intended to be used inside the human body. Finally, completely processing the complex fuels used in enzymatic biofuel cells requires a series of different enzymes for each step of the ‘metabolism’ process; producing some of the required enzymes and maintaining them at the required levels can pose problems.