Soil-based microbial fuel cells (MFCs) are not widely used, but show some promise as clean-energy options for low-energy sensors used in precision agriculture (soil moisture and health), infrastructure monitoring (water pipes leakages) or environmental monitoring (concentration of pollutants). This latest evolution of an MFC uses a non-traditional design & readily available components to deliver a performance improvement of over 120%. Importantly, the fuel-cell was able to generate 68 times more energy than needed by the sensors - so plenty to spare for more sensors!
https://scitechdaily.com/scientists-develop-new-dirt-powered-fuel-cell-that-runs-forever/