Practical and Thermodynamic Limits of Dual-Use, Potentiometric Energy Harvesters for Self-Powered Sensors

Energy-autonomous ion-selective electrode (ISE) sensors (that would not require any battery or secondary energy harvester for signal acquisition and transmission) will find broad application in wearable and implantable devices for continuous health monitoring, smart agriculture, and environmental sensing. Although ISEs have been used as sensors for more than 50 years, their practical and thermodynamic potential as an energy harvester has not been explored. An ISE measures target ion concentration by the selective uptake of ions in the membrane. The concentration gradient across the membrane–solution boundary produces harvestable Gibbs free energy of mixing. The power harvested so far has been in the nW/cm² range, raising questions about the viability of the approach. Here, we present a model to calculate the theoretical and practical power harvest limit of an ISE. We show that the thermodynamic performance limit should approach mW/cm² in the limit of zero sensor and solution resistance. Indeed, we demonstrate experimentally that simple improvements in sensor electrodes can improve power harvest by 3 orders of magnitude, i.e., 100 s of nW/cm². This power is sufficient to support a low-powered wearable, implantable device.