Trillions of microbes live on us, in us, and around us, quietly sharing our bodies and our cities. But in the future, some microbes may have to work for their real estate. In fact, they may quite literally become the generators that power our lives. How? By putting them to work inside electricity-generating fuel cells.
The concept of using microbes to generate electricity, otherwise known as a microbial fuel cell, has been around for decades. In essence, a microbial fuel cell is a system that converts chemical energy into electrical energy by taking advantage of the natural oxidation (i.e., electron-release) that occurs as microbes digest organic matter. In this fuel cell, electrons released from microbial digestion are transferred to an electrode. As electrons travel along a charge gradient, they pass through an external electrical connection that harvests some of their energy in a battery or resistor.
Scientists have envisioned many ways in which microbial electricity may one day power our lives, from small household electronics to automobiles to self feeding robots. The reality of the matter, however, is that the technology is not yet developed enough to produce substantial quantities of power in a cost-effective way. Most fuel cells today are so large that they can’t fit inside the electronics they are intended to power.
In working to improve microbial fuel cell technology, a lot of effort has been focused on electrochemical engineering. Many scientists are working to improve the efficiency of the electrode: making it better at grabbing and transferring electrons. However, equally important to the development of effective microbial fuel cells is understanding the microorganisms that power them.
In this aspect, one discovery in particular has given scientists hope: electrigens. These are organisms that can harvest energy by directly growing on electrodes. (This is in contrast to the many other organisms that have been scouted for fuel cell application, most of which are simply going about their business digesting organic matter, unaware that some of their precious electrons are being siphoned away to a battery). The most well-studied of the electrigens is Geobacter, an iron-breathing organism that lives in oxygen-free environments. Several species of Geobacter use electrically conductive pili (small antenna-like appendages) to transfer electrons from organic matter directly to iron oxides in the environment. In essence, the process by which Geobacter acquires energy can be co-opted for electricity generation by replacing iron oxides, which occur naturally in soils, with an electrode.
Electrigens represent a promising step towards the development of a sustainable fuel cell, one that can generate a high enough power outputs to be useful. That these bugs naturally thrive on electrodes cuts out one of the largest hurdles associated with microbial fuel cell development. One of the earliest examples of an electrigen-powered fuel cell is the Benthic Unattended Generator (BUG). BUGs live at the bottom of the ocean, producing electric current from organic matter. Their design includes a piece of graphite buried in oxygen-free sediments which serves as an anode. Electrons are collected on this anode when microbes break down sedimentary organic matter. Electrons are transferred to another piece of graphite (the cathode) sitting in the overlying water.
BUGs may one day be used to power monitoring devices and other electronic equipment at the bottom of the ocean. Using a similar concept, scientists have speculated on the possibility of generating electricity from oil spill remediation. Microbes munching away on oil liberate oodles of electrons. Perhaps some of these electrons could be used to power, say, the equipment needed for oil spill remediation.
My personal favorite future use of a fuel cell? Compost generators. Imagine, if you will, an in-home compost bucket that doubles as a generator, allowing you to literally extract power from your trash. That’s a future I’m ready for.
Lovley, D. (2006). Bug juice: harvesting electricity with microorganisms Nature Reviews Microbiology, 4 (7), 497-508 DOI: 10.1038/nrmicro1442