Escherichia coli (E. coli) is not something anyone wants to come into contact with; after all, this bacteria is responsible for fairly potent, sometimes dangerous, food poisoning. A team of researchers from Singapore have found a way to engineer E. coli to perform a very different role: It's now a catalyst for a new hybrid fuel cell that splits hydrogen from water with remarkable efficiency. Its curious workings are detailed in a study published in the journal NPJ Biofilms and Microbiomes.
Photoelectrochemical (PEC) cells harness solar energy to send an electrical current through a sample of water. This current reacts with the water molecules and causes a chemical “split,” producing several hydrogen molecules and groups of oxygen-hydrogen ions. The circuit’s negatively charged electrode attracts the positively charged hydrogen towards it, removing it from the water. This process is called electrolysis, and sometimes is referred to as artificial photosynthesis.
If this hydrogen can be stored, it can be ignited and burned at a future date in order to produce energy. Thus, PEC cells represent the key component of hydrogen fuel cells. Unfortunately, PEC cells tend to consume more energy generating the hydrogen gas than is released by burning it.
A microbial fuel cell (MFC) is somewhat similar, but instead of using solar power, the electrical current is generated using the electricity produced by a bacteria. The MFC is divided between an oxygen-depleted (anoxic) and oxygen-rich (oxic) region. A fuel – in this instance a chemical substrate that the bacteria would naturally use in order to produce energy – is mixed with a specific bacteria in the anoxic section.
As the bacteria uses it up, carbon dioxide is emitted, along with free electrons and protons. The protons pass through a membrane to one side of the cell, and the electrons are attracted towards the oppositely charged electrode on the other side of the cell. This difference in charge sets up a voltage, which produces a current within an electrical circuit. When the electrodes encounter the protons in the oxic side of the cell, water is produced.
E. coli acts as the “driver,” or catalyst, to the MFC. anyaivanova/Shutterstock
This team of researchers, from Nanyang Technological University in Singapore, realized that a PEC cell and a MFC could be used in conjunction with each other. First off, slightly modified versions of their own PEC cells and MFCs were created. A custom gold-titanium oxide electrode system was used for the PEC cell, increasing its ability to conduct electricity.
E. coli was chosen to be the MFC’s bacteria, and their membranes were injected with conjugated oligoelectrolytes (COEs), a class of compounds that improve the ability of the bacteria to transfer electrical charge. Within the MFC, COEs essentially carry charge away from the bacteria and towards the appropriate electrodes more efficiently than unmodified bacteria are able to.
The MFC is allowed to operate as it normally does, but this charge is transferred to the PEC cell, where it supplements the current already being produced by incoming solar energy. All things considered, the enhanced current generated by this PEC-MFC hybrid is reported to be 70 times greater than that of a PEC operating on its own.
This powerful current is able to electrolyze the water in the PEC extremely efficiently, creating far more hydrogen than any previous PEC of a comparable size. Although this hybrid is not yet ready to be made commercially available, it’s certainly a novel stepping stone in the right direction.