Nanofluidic Ion Transport through Reconstructed Layered Materials

Electrolytes confined in nanochannels with characteristic dimensions comparable to the Debye length show transport behaviors deviating from their bulk counterparts. Fabrication of nanofluidic devices typically relies on expensive lithography techniques or the use of sacrificial templates with sophisticated growth and processing steps. Here we demonstrate an alternative approach where unprecedentedly massive arrays of nanochannels are readily formed by restacking exfoliated sheets of layered materials, such as graphene oxide (GO). Nanochannels between GO sheets are successfully constructed as manifested by surface-charge-governed ion transport for electrolyte concentrations up to 50 mM. Nanofluidic devices based on reconstructed layer materials have distinct advantages such as low cost, facile fabrication, ease of scaling up to support high ionic currents, and flexibility. Given the rich chemical, physical, and mechanical properties of layered materials, they should offer many exciting new opportunities for studying and even manufacturing nanofluidic devices

Energy from the Nanofluidic Transport of Water through Nanochannels between Packed Silica Spheres

Efficient harvesting of electrokinetic-streaming-potential requires a trade-off between high flow-rate and nanofluidic confinement. To attain the best out of these parameters, we have developed a periodic network of tetrahedral and octahedral voids interconnected through fine biconical nanofluidic channels by close-packing nearly monodisperse silica spheres of diameters 285, 620, 1000, 1750, and 2900 nm. The interstices of close-packed silica spheres (diameter 285 to 1750 nm) simultaneously exhibit surface-charge-governed ionic conductivity and fast flow of water. The power density harvested from streaming water was found to be increasing with increased diameter of the close-packed spheres up to 1750 nm (151 mWm–2), and to be decreasing with further rise in the sphere diameter. The power density was found to be dependent on the mass loading of the silica spheres, contact area of the electrodes, and pH of the streaming water. Pretreatment of the silica spheres with concentrated nitric acid further enhanced the efficiency of the energy harvesting through streaming potential. Harvesting of streaming potential from packed silica spheres was found to be a convenient way of obtaining energy from water flowing through the household water taps, as they can be connected in a series to add up energy generated in multiple devices

Energy from the Nanofluidic Transport of Water through Nanochannels between Packed Silica Spheres

Efficient harvesting of electrokinetic-streaming-potential requires a trade-off between high flow-rate and nanofluidic confinement. To attain the best out of these parameters, we have developed a periodic network of tetrahedral and octahedral voids interconnected through fine biconical nanofluidic channels by close-packing nearly monodisperse silica spheres of diameters 285, 620, 1000, 1750, and 2900 nm. The interstices of close-packed silica spheres (diameter 285 to 1750 nm) simultaneously exhibit surface-charge-governed ionic conductivity and fast flow of water. The power density harvested from streaming water was found to be increasing with increased diameter of the close-packed spheres up to 1750 nm (151 mWm–2), and to be decreasing with further rise in the sphere diameter. The power density was found to be dependent on the mass loading of the silica spheres, contact area of the electrodes, and pH of the streaming water. Pretreatment of the silica spheres with concentrated nitric acid further enhanced the efficiency of the energy harvesting through streaming potential. Harvesting of streaming potential from packed silica spheres was found to be a convenient way of obtaining energy from water flowing through the household water taps, as they can be connected in a series to add up energy generated in multiple devices

Electrical Power Generation from the Contrasting Interfacial Activities of Boron- and Nitrogen-Doped Reduced Graphene Oxide Membranes

Water, the medium of life, has also served as a sustainable source of energy for hundreds of years. However, most of the water-based energy harvesting techniques relies either on rapid flow or on the fast evaporation rate of the molecules. Here, the complementary charge transfer activities of boron (B-r-GO) and nitrogen (N-r-GO) doped reduced graphene oxide (r-GO) flakes are exploited to extract energy from serene water resources. B-r-GO and N-r-GO samples prepared by annealing graphene oxide sheets with boric acid and urea were individually coated on cellulose membranes to fabricate B-r-GO/N-r-GO devices, which produces open-circuit voltages up to 570 mV when dipped in water. The power-output were found to be tunable by varying parameters like coating area, dopant amounts, annealing temperature, and ionic conductivity. The potential-drops due to the prolong soaking of B-r-GO/N-r-GO devices (for a few days) can be completely recovered through vacuum drying. In order to open-up the possibility of fabricating wearable energy devices the B-r-GO/N-r-GO samples are also coated on arbitrary substances like jeans cloths. The DFT calculations indicate that compared to N-r-GO, the B-r-GO structure is more stable and has considerably higher charge transfer activity with water molecules

Control of Selective Ion Transfer across Liquid–Liquid Interfaces: A Rectifying Heterojunction Based on Immiscible Electrolytes

The current rectification displayed by solid-state p–n semiconductor diodes relies on the abundance of electrons and holes near the interface between the p–n junction. In analogy to this electronic device, we propose here the construction of a purely ionic liquid-state electric rectifying heterojunction displaying an excess of monovalent cations and anions near the interface between two immiscible solvents with different dielectric properties. This system does not need any physical membrane or material barrier to show preferential ion transfer but relies on the ionic solvation energy between the two immiscible solvents. We construct a simple device, based on an oil/water interface, displaying an asymmetric behavior of the electric current as a function of the polarity of an applied electric field. This device also exhibits a region of negative differential conductivity, analogous to that observed in brain and heart cells via voltage clamp techniques. Computer simulations and mean field theory calculations for a model of this system show that the application of an external electric field is able to control the bulk concentrations of the ionic species in the immiscible liquids in a manner that is asymmetric with respect to the polarity or direction of the applied electric field. These properties make possible to enhance or suppress selective ion transport at liquid−liquid interfaces with the application of an external electric field or electrostatic potential, mimicking the function of biological ion channels, thus creating opportunities for varied applications

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