Enzymatic biofuel cells are a potentially clean and renewable technology since they only produce water as the by-product of combustion, but they do suffer from poor electron transfer between the enzyme catalysts and the electrode surface employed in the cells. A new strategy that makes use of both direct electron transfer and mediated electron transfer processes in the same device could help overcome this problem. The result? Fuel cells that have much higher power density than those that rely on direct or mediated electron transfer processes alone.
In an enzymatic biofuel cell (BFC), enzyme electrocatalysts convert the chemical energy of biofuels, such as sugars, alcohols and hydrogen, into electrical energy. Redox enzymes oxidize biofuels at the anode and oxygen is reduced at the cathode, producing electric power as a result.
BFCs are better than conventional fuel cells in many ways. For one, they are cheaper, since enzymes are much less expensive than precious metal catalysts. They are also specific (they catalyse only one biofuel), can be miniaturized and operate at room temperature. They do produce little power, however, do not last very long and are relatively inefficient. These problems are thought to come from the fact that it is difficult to electrically wire the enzymes and the electrode surface.
To improve this electron transfer, researchers have designed BFCs so that they work by either direct electron transfer (DET) or mediated electron transfer (MET). In DET, an enzyme is electrically connected to an electrode surface so that electrons directly tunnel between it and the surface. In MET, a mediator, such as a metal complex-based polymer is employed. This mediator not only undergoes redox reactions at an electrode surface, it can also exchange electrons with the enzyme, which results in electrons being transported via the mediator (electron shuttle).
Combining MET and DET
A team led by Jong-Min Lee of Nanyang Technological University in Singapore has combined these two mechanisms for the first time in a single device.
The researchers used the multi-copper oxidase enzyme laccase, known to have a high activity for the oxygen reduction reaction (ORR). This is a key chemical reaction that takes place at the cathode of fuel cells. They also designed an electron transfer system connected to a multi-walled carbon nanotube surface. This system contains three parts: a 2,2’-Azino-bis(3- ethylbenzthiazoline-6-sulfonic acid) (ABTS) compound in the middle with a pyrene group at one end and a polypyrrole group at the other end.
Although researchers have used ABTS as a MET mediator for ORR enzymes before now, in this work the pyrene group appears to orient itself towards the copper redox active site of the laccase. This improves electron transfer from the ABTS to the laccase. The polypyrrole group, for its part, also helps to attach the ABTS to the electrode for better transfer of electrons.
Thanks to these combined mechanisms, the maximum ORR current density in a fuel cell based on this system can be as high as 2.45 mA/cm2. What is more, the MWCNTs/polypyrrole-ABTS-pyrene/laccase bioelectrode keeps 50% of its initial ORR current even after 120 days.
Although the researchers say that their system still needs to be optimized (its working cell voltage is relatively low at just 0.19 V), their DET/MET coupling technique might come in handy for making other types of bioelectrochemical devices. These include glucose biosensors and photobioelectrochemical cells, in which enzymes are wired to electrode interfaces too.
The research is detailed in Nature Energy 10.1038/s41560-018-0166-4.
