A fuel cell consists of a negative (anodic) electrode and a positive (cathodic) electrode immersed in an electrolyte. In a hydrogen fuel cell a catalyst at the anode separates hydrogen molecules into protons and electrons. The electrons travel through an external circuit to the cathode, generating a flow of electricity while the protons migrate through the electrolyte to the cathode, where they combine with oxygen and the electrons to produce water and heat.

Fuel cells are considered as being green since they only produce water as the by-product of combustion. They do nevertheless require rare metals like platinum as catalysts to oxidize hydrogen and reduce oxygen.

Redox enzymes

In recent years, researchers have started to look at employing redox enzymes, such as hydrogenases and multi-copper oxidases for oxidizing hydrogen and reducing oxygen into water, as biocatalysts in fuels cells. Such enzymatic fuel cells (EFCs) would be environmentally friendlier and cheaper than fuel cells that rely on expensive platinum and other rare metal catalysts. Despite much progress, however, EFCs still only produce moderate levels of power and are not very stable.

A team of researchers led by Elisabeth Lojou from CNRS/Aix-Marseille University has now built a new-generation EFC in which the platinum catalyst is replaced by bacterial enzymes. The bioelectrodes in this device consist of macroporous carbon felt modified by aminomethylpyrene-functionalized carbon nanotubes to entrap two thermostable enzymes, nickel-iron-hydrogenase and bilirubin oxidase, at the anode and cathode electrodes, respectively. These enzymes are stable between 25° and 80°C

The porous carbon felt acts as a host structure for the enzymes, say Lojou and colleagues, but it also serves as a protection against chemical species generated during oxygen reduction, which can adversely affect the enzymes. The cell can thus operate for several days without any loss in performance. Indeed, it can produce 15.8 mWh of energy after 17 hours of continuous operation.

Current of around 1 A/mg of enzymes

Thanks to this controlled architecture and the intrinsic properties of the enzymes, the researchers were able to quantify, for the first time, the proportion of enzymes effectively participating in the current produced. They also found that the currents produced by these biocatalysts are as high as those expected from platinum catalysts. Finally, finite element simulations allowed them to determine that an electrode that is less than 100 microns thick and which has a porosity of 60% was the best and produces a current of around 1 A/mg of enzymes at 50°C.

The new enzymatic fuel cell is detailed in Energy and Environmental Science DOI: 10.1039/C7EE01830D.