Enzymatic biofuel cell
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.
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.
Early work with biofuel cells, which began in the early 20th century, was purely of the microbial variety. Research on using enzymes directly for oxidation in biofuel cells began in the early 1960s, with the first enzymatic biofuel cell being produced in 1964. This research began as a product of NASA’s interest in finding ways to recycle human waste into usable energy on board spacecraft, as well as a component of the quest for an artificial heart, specifically as a power source that could be put directly into the human body. These two applications – use of animal or vegetable products as fuel and development of a power source that can be directly implanted into the human body without external refueling – remain the primary goals for developing these biofuel cells. Initial results, however, were disappointing. While the early cells did successfully produce electricity, there was difficulty in transporting the electrons liberated from the glucose fuel to the fuel cell’s electrode and further difficulties in keeping the system stable enough to produce electricity at all due to the enzymes’ tendency to move away from where they needed to be in order for the fuel cell to function. These difficulties led to an abandonment by biofuel cell researchers of the enzyme-catalyst model for nearly three decades in favor of the more conventional metal catalysts (principally platinum), which are used in most fuel cells. Research on the subject did not begin again until the 1980s after it was realized that the metallic-catalyst method was not going to be able to deliver the qualities desired in a biofuel cell, and since then work on enzymatic biofuel cells has revolved around the resolution of the various problems that plagued earlier efforts at producing a successful enzymatic biofuel cell.
However, many of these problems were resolved in 1998. In that year, it was announced that researchers had managed to completely oxidize methanol using a series (or “cascade”) of enzymes in a biofuel cell. Previous to this time, the enzyme catalysts had failed to completely oxidize the cell’s fuel, delivering far lower amounts of energy than what was expected given what was known about the energy capacity of the fuel. While methanol is now far less relevant in this field as a fuel, the demonstrated method of using a series of enzymes to completely oxidize the cell’s fuel gave researchers a way forward, and much work is now devoted to using similar methods to achieve complete oxidation of more complicated compounds, such as glucose. In addition, and perhaps what is more important, 1998 was the year in which enzyme “immobilization” was successfully demonstrated, which increased the usable life of the methanol fuel cell from just eight hours to over a week. Immobilization also provided researchers with the ability to put earlier discoveries into practice, in particular the discovery of enzymes that can be used to directly transfer electrons from the enzyme to the electrode. This process had been understood since the 1980s but depended heavily on placing the enzyme as close to the electrode as possible, which meant that it was unusable until after immobilization techniques were devised. In addition, developers of enzymatic biofuel cells have applied some of the advances in nanotechnology to their designs, including the use of carbon nanotubes to immobilize enzymes directly. Other research has gone into exploiting some of the strengths of the enzymatic design to dramatically miniaturize the fuel cells, a process that must occur if these cells are ever to be used with implantable devices. One research team took advantage of the extreme selectivity of the enzymes to completely remove the barrier between anode and cathode, which is an absolute requirement in fuel cells not of the enzymatic type. This allowed the team to produce a fuel cell that produces 1.1 microwatts operating at over half a volt in a space of just 10 cubic micrometers.
While enzymatic biofuel cells are not currently in use outside of the laboratory, as the technology has advanced over the past decade non-academic organizations have shown an increasing amount of interest in practical applications for the devices. In 2007, Sony announced that it had developed an enzymatic biofuel cell that can be linked in sequence and used to power an mp3 player, and in 2010 an engineer employed by the US Army announced that the Defense Department was planning to conduct field trials of its own "bio-batteries" in the following year. In explaining their pursuit of the technology, both organizations emphasized the extraordinary abundance (and extraordinarily low expense) of fuel for these cells, a key advantage of the technology that is likely to become even more attractive if the price of portable energy sources goes up, or if they can be successfully integrated into electronic human implants.
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