Aramid Nanofiber Membranes for Energy Harvesting from Proton Gradients

Harvesting osmotic energy from industrial wastewater is an often-overlooked source of electricity that can be used as a part of the comprehensive distributed energy systems. However, this concept requires, a new generation of inexpensive ion-selective membranes that must withstand harsh chemical conditions with both high/low pH, have high temperature resilience, display exceptional mechanical properties, and support high ionic conductance. Here, aramid nanofibers (ANFs) based membranes with high chemical/thermal stability, mechanical strength, toughness, and surface charge density make them capable of high-performance osmotic energy harvesting from pH gradients generated upon wastewater dilution. ANF membranes produce an averaged output power density of 17.3 W m⁻² for more than 240 h at pH 0. Taking advantage of the high temperature resilience of aramid, the output power density is increased further to 77 W m⁻² at 70 °C, typical for industrial wastewater. Such output power performance is 10× better compared to the current state-of-the-art membranes being augmented by Kevlar-like environmental robustness of ANF membranes. The improved efficiency of energy harvesting is ascribed to the high proton selectivity of ANFs. Retaining high output power density for large membrane area and fluoride-free synthesis of ANFs from recyclable material opens the door for scalable wastewater energy harvesting.