The idea is not new but the output is low. Ultimately, the researchers achieved an output of up to 1.5 microwatts (μW). This is sufficient for applications such as sensors that monitor soil conditions in agriculture, thereby avoiding the use of chemical batteries and solar panels. As long as the soil has microbes and the setup is intact, it will work.
The fuel cell is basically an anode (carbon felt) and a cathode (conductive metal) buried in the ground. The researchers tested different configurations. See the figure reproduced below.
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