Carbonized Cotton Fibers for Ultrahigh Power-Density Electrokinetic Energy Harvesting

The prospect of harvesting "clean" electricity from water by harnessing the interaction between an intrinsically charged material interface and fluid flow offers ever-increasing possibilities in diverse areas of applications ranging from natural calamity forecasting and wastewater treatment to smart healthcare. However, despite the phenomenal advancements in developing materials and their miniaturized fabrication procedures with ultrahigh precision, the resulting electrical power density in practice could not surpass a meager limit of even a few milliW/sq m of area thus far, restricting its practical value proposition largely. Herein, we demonstrate an unprecedented amplification in the established experimental limits of electrokinetic energy production via exploiting ion–water interactions in carbonized fibrous plugs that are optimally processed by annealing pristine plant-derived cotton materials at favorable activation temperatures. Massive elevation in the ionic and fluidic conductance of the processed material, acting in tandem, culminates in giant amplifications in the charge mobilization so that water flow at a modest speed of around 0.1 m/s is shown to result in open-circuit voltages of tens of volts and short-circuit currents of tens of microamperes, resulting in power density of the order of several Watts per square meter of the exposed surface area. Being different from the fabrication-intensive paradigm of nanofluidic energy conversion, our methodology offers a unique means of achieving a delicate combination of surface-governed charge transport and ion selectivity that may otherwise be difficult to engineer by using the other commonly used functional materials. These findings not only rationalize a gross deficit in the fundamental understanding of electrokinetic pumping in interlaced fibrous porous materials but also open up the prospects of emerging inexpensive functionalized materials for clean energy harvesting with an efficacy that could not hitherto be realized.