Plant Microbial Fuel Cells: Energy Sources and Biosensors for battery-Free Smart Agriculture

Smart sensors used for intensive crop monitoring require minimal maintenance and should prioritize ecological sustainability. Consequently, battery-free energy harvesting represents a key aspect of sustainable development in smart agriculture. Plant microbial fuel cells (PMFCs) introduce a cutting-edge renewable energy source that scavenges energy from the symbiotic relationship between a plant and electron-generating bacteria in the soil, potentially supplying power as long as the plant lives. Characterizing PMFCs' power production is challenging, as it depends on many factors, such as soil impedance and plant condition. Electrochemical impedance spectroscopy (EIS) is often used in laboratory tests, but it is inefficient to deploy in off-grid contexts. This article introduces an ultralow power EIS biosensor architecture that utilizes PMFCs as an energy source and for the EIS measure. We prove that ultralow-power EIS is compatible with PMFCs' mW-level power production through an implementation that integrates an EIS analog frontend and PMFC-tailored harvesting electronics. The architecture also facilitates PMFC unloading periods, crucial for PMFC recovery and durability. Experimental results show that a full-range EIS sweep (21.3 mHz–21.8 kHz, 19 points) executed with the proposed architecture requires only 3.64 J. We highlight the potential of cost-effective, self-powered EIS in assisting PMFCs' development into reliable energy sources for battery-free nodes. We also demonstrate that plant state, as well as maximum power point could be monitored through ultralow power EIS measurements.