In a typical hydrogen fuel cell, a membrane separates the device's anode and cathode into side-by-side chambers. Hydrogen reaching the anode electrode splits into protons, which migrate across the membrane through the liquid medium, and electrons, which travel over a wire to the cathode, thereby generating electricity. Meanwhile, oxygen in the cathode combines with the protons to form water. A heavy metal such as platinum plates the anode to catalyze the reaction.
By contrast, Squier's proposed biofuel cells would use a low-cost porous hematite electrode in which the bacteria's purified protein could be bound. The coated electrodes would catalyze the reaction, enabling electricity to flow from the anode to the cathode using nothing more than the biological agents in the biomass as fuel.
The metallic hematite acceptor drains electrons shuttled by the protein in a manner similar to respiration, with the electrons taking the place of oxygen. Respiration in the cell depends on the protein-draining electrons to maintain a steady metabolism. When the protein is purified, it continues performing the same function, essentially allowing the hematite electrode to "breathe" electrons.
In the lab, PNNL has successfully performed charge transfers from the protein to the hematite electrodes both directly and from biomass fuel sources, and the team has characterized the current flow by fluorescent correlation spectroscopy and confocal microscopy. "The fluorescence shows that our current peaks after a few seconds," Squier said, "but the flux is about what you would expect to get from bioreactors using living bacteria."
Squier performed the work with PNNL staff scientists Yijia Xiong, Liang Shi and Uljana Mayer, with assistance from the lab's William R. Wiley Environmental Molecular Sciences Laboratory biogeochemistry grand challenges program.