Determining PMFCs' environmental impact will require further research into a variety of areas, including how electrodes affect the root environment. They could potentially reduce nutrient availability, for example, or reduce a plant's ability to fight off infection [source: Deng, Chen and Zhao].
Moreover, because they work best in some of our most protected lands -- wetlands and croplands -- PMFCs could face a steep environmental approval process. On the other hand, wastewater MFCs can oxidize ammonium and reduce nitrates, so it is possible that plant-based MFCs could balance the risk by protecting wetlands from agricultural runoff [sources: Deng, Chen and Zhao; Miller; Tweed].
There's No Place Like Loam
Soil, as it turns out, is full of untapped (electrical) potential.
As green plants go about the business of photosynthesis -- converting energy from sunlight to chemical energy, then storing it in sugars like glucose -- they exude waste products through their roots into a soil layer known as the rhizosphere. There, bacteria chow down on plants' sloughed-off cells, along with proteins and sugars released by their roots [source: Ingham].
In PMFC terms, this means that, as long as the plant lives, the bacteria have a meal ticket and the fuel cell generates power. The first law of thermodynamics, which some translate as "there's no such thing as a free lunch," still applies because the system receives energy from an external source, namely the sun.
But how on Earth, or under it, do microbes generate electricity simply by consuming and metabolizing food? As with love or baking, it all comes down to chemistry.
Broadly speaking, MFCs work by separating two halves of an electro-biochemical process (metabolism) and wiring them together into an electrical circuit. To understand how, let's look at cell metabolism in detail.
C6H12O6 + 6O2 → 6CO2 + 6H2O
But within individual cells -- or single-celled organisms like bacteria -- this broad statement glosses over a series of intermediate steps. Some of these steps temporarily release electrons which, as we all know, are handy for generating electricity. So, instead of glucose and oxygen reacting to produce carbon dioxide and water, here glucose and water produce carbon dioxide, protons (positively charged hydrogen ions (H+)) and electrons (e-) [sources: Bennetto; Rabaey and Verstraete].
C6H12O6 + 6H2O → 6CO2 + 24H+ + 24e-
In a PMFC, this half of the process defines one half of the fuel cell. This portion is located in the rhizosphere with the plant roots, waste and bacteria. The other half of the cell lies in oxygen-rich water on the opposite side of a permeable membrane. In a natural setting, this membrane is formed by the soil-water boundary [sources: Bennetto; Rabaey and Verstraete; Deng, Chen and Zhao].
In the second half of the cell, free protons and electrons combine with oxygen to produce water, like so:
6O2 + 24H+ + 24e- → 12H2O
Protons reach this second half by flowing across the ion exchange membrane, creating a net positive charge -- and an electrical potential that induces electrons to flow along the external connecting wire. Voila! Electric current [sources: Bennetto; Rabaey and Verstraete; Deng, Chen and Zhao].
But how much?