Enzymatic Biofuel Cell - History

History

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 which 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 which plagued earlier efforts at producing a successful enzymatic biofuel cell.

1998 saw the resolution of many of these problems. 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. Previously 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 more importantly, 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, particularly the discovery of enzymes which 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 which 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 which 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 which 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 which 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.